The ground state of substitutional Cu in Ge undergoes a transformation from a highly localized 1s³ to a much more extended 1s²2s¹ configuration at the uniaxial pressure of 0.42 GPa along the [100] direction. The transformation is associated with a dramatic reduction of the resistivity. The observed metal-insulator transition is explained with the formation of extended Hubbard bands well separated from the valence band. It is shown that the hole mobility in the upper Hubbard band is limited by scattering from ionized and neutral Cu centers in the delocalized 1s²2s¹ configuration. The concentration of the scattering centers and thus also the hole mobility can be controlled by uniaxial pressure. Hole mobilities as high as 10⁷ cm²/Vs have been observed at 16 K for intermediate uniaxial stresses. These results indicate a very weak coupling between acoustic phonons and the holes in the upper Hubbard band.

Temperature, electric current and magnetic field dependences of dc resistivity and Hall coefficient have been measured for the Si:Sb sample whose donor concentration is 0.80N_c, where N_c is the critical concentration of the metal-insulator transition. From the observation of ε₂ conduction the hard Coulomb gap seems to open at low temperatures, and we have found that the resistivity, Hall coefficient and magnetoresistance sensitively depend on the electric current density. These data have been analyzed using a two-band model including the upper and lower Hubbard bands. In particular, below N_c positive magnetoresistance is observed in the low electric current density regime. As the electric current density increases, a crossover to negative magnetoresistance is observed. It is concluded that these phenomena originate from the rise of the electron temperature and the transfer of electrons from strongly localized to weakly localized states in the upper Hubbard band.

Topological interaction of shallow impurities with dislocations is studied. This problem is related with the violation of the topological structure of the crystal lattice by dislocations. The interaction results in a topological gauge field that is confined inside the dislocation cores but influences the shallow impurity states in vicinity of the dislocations via an Aharonov-Bohm effect-like way. It is found that this effect reduces the binding energies of shallow centers localized at unsplit dislocations. For split dislocations, the interaction of shallow-level centers with the stacking faults existing between partials is important and is taken into account on the base of the δ-potential quantum well model of the stacking faults.

Dynamical properties of several configurations of B₁₂ clusters in Si crystal are studied by means of the first-principles calculations. The cubo-octahedral B₁₂ cluster in Si is found to be unstable against infinitesimal displacement, decaying into an icosahedral B₁₂ cluster or a novel B₁₂ cluster with S₄ symmetry. The calculated infrared absorption active modes of the icosahedral B₁₂ cluster in Si are in reasonable agreement with the experimental results, which strongly suggests the existence of the icosahedral B₁₂ cluster in Si after heavily dosed B ion implantation.

The equilibrium geometries and the electronic structures for different locations of H atoms and ions in GaAs have been evaluated by first-principle local density functional methods. Discrete near-edge levels are induced in the bandgap by the interactions of hydrogen with the GaAs lattice, as well as by charge effects in the case of H ions. Radiative transitions between those conduction-band and valence-band near-edge states account for the main features of the emission bands observed in the photoluminescence spectra of hydrogenated III-V compounds.

The Zeeman spectra of the neutral acceptor Zn-H in Ge have been studied in the Voigt geometry with B along the three principal crystallographic directions <100>, <111> and <110> using plane-polarized radiation with the samples held near 4.2 K. The results for B ∥ <111> are described in detail. The interpretation of the data supports the model proposed for this complex, viz., that the axis is along a <111> bond direction of the host. This lowers the symmetry of the hole states resulting in a split ground state consisting of two Kramers' doublets. These are sufficiently separated in energy for the upper one to be depopulated at the sample temperatures used. The spectra provide a "template" for the identification of other such complexes.

In this paper the temperature dependences of resistivity ρ and Hall constant R_H of Pb_{1 - x}Ge_xTe (x ≤ 0.04) doped with Yb were investigated both under atmospheric pressure and under hydrostatic compression. It was shown that the ρ(1/T) dependences revealed low temperature activation regions of impurity conductivity, indicating the formation of impurity states near the valence band top. Under pressure the slope of the activation regions decreases and insulator-metal transition occurs. On the base of the obtained results a reconstruction diagram of the energy spectrum of Pb_{1 - x}Ge_xTe<Yb> under pressure was proposed and the main parameters of the Yb-induced level were estimated.

The characteristics of nitrogen-oxygen (N-O) complexes in nitrogen-doped Czochralski (NCz) silicon annealed at the temperature of 450 to 1150 °C up to 24 h have been studied by infrared transmission absorption spectroscopy (FTIR) at room temperature and at low temperature (8 K). The relations of N-O complex absorption lines in the middle and in the far infrared range have been built up. Only during annealing at 550 °C, the two weak 1018 and 810 cm ^{- 1} lines were found in FTIR spectra. It is considered that the five infrared absorption lines of N-O complexes have different thermal stability.

The resistivity and Hall coefficient of n-ZnSe monocrystals were investigated in the region of the metal-insulator (M-I) transition induced by variation of shallow donor impurity Al concentration from 2 x 10¹⁶ to 2 x 10¹⁸ cm ^{- 3}. It has been found that the transition from an activated conduction to a metallic one occurs at donor concentration 1.3 x 10¹⁷ cm ^{- 3} and takes place within the impurity band before it merges with the bottom of conduction band. On the dielectric side of the M-I transition the conduction with an activation energy ϵ₁ induced by electron excitation from the impurity states into the conduction band is replaced by the hopping conduction with a constant activation energy ϵ₃ which, in the lowest temperature region, is changed to a hopping conduction with variable activation energy.

IR absorption due to electronic excitations of a hydrogen-intrinsic defect-related double donor (H-donor) in proton-implanted silicon is studied. It is shown that a broad feature at about 600 cm ^{- 1} is due to the ionization of the E_s ≈ E_c - 0.1 eV and E_t ≈ E_c - 0.06 eV levels while the 375 cm ^{- 1} band is related to the E_s ---> E_t excitation. It is concluded that the E_s and E_t levels are connected with ground spin-singlet and excited spin-triplet neutral states of the H-donor. An analysis of Si-H bond-stretching and bond-bending local vibrational modes allows us to suggest that the H-donor is an interstitial-type complex with two possible configurations with C₂/C_s- or C_2v-symmetry.

Magnesium has been introduced into silicon containing oxygen by diffusion. The low-temperature absorption spectrum observed clearly demonstrates that magnesium can pair with oxygen to form magnesium-oxygen complex impurities in silicon. Similar to an isolated magnesium donor, the magnesium-oxygen complex is also an interstitial donor in silicon. The ionization energy of neutral magnesium-oxygen complex donor is 124.66 meV, which indicates that it is a slightly deeper donor compared with neutral magnesium impurity in silicon.

The photoluminescence properties of heavily doped GaAs:Zn (100) layers grown by liquid phase epitaxy from gallium and bismuth solutions at various temperatures have been studied. It was shown that a line at 1.35 eV connected with the novel defect appears in photoluminescence spectra of the layers doped at a level above 10¹⁹ cm ^{- 3}. It has been found that the defect is a neutral complex consisting of native point defects of GaAs.

Models of the nitrogen-hydrogen defect in GaP, which contain one and two H atoms, are investigated using ab initio density functional cluster theory. We find that a single H atom binding to N possesses two infrared absorption frequencies close to those attributed to an NH₂ defect. The modes shift with its charge state consistent with the photo-sensitivity found for the defect. A third mode observed for this centre is assumed to be an overtone of the bend mode. The isotope shifts of the calculated modes are in excellent agreement with experiment in contrast with the model which contains two H atoms.

In this paper recent results of optical studies of bound excitons in wide-gap II-VI and nitride semiconductors are surveyed. In the first part latest results about self-compensation mechanisms will be presented. Then, the dynamical characteristics of excitonic transitions are evaluated for various impurities, dopants, and dopant concentrations, and for excitation via particular resonant excitation channels. Relaxation and conversion channels between excitonic systems are analyzed, especially in strained heteroepitaxial systems which show splitting effects of the bands from which the carriers stem. The dynamical behavior of excitons in wide-gap material gives evidence of nonradiative decay channels. Calorimetric spectroscopy at mK temperatures was used to investigate the nonradiative processes and to determine quantum efficiencies. Recent results of transient four-wave mixing experiments at bound-exciton complexes will be shown to demonstrate that shallow defects influence the dephasing in the investigated systems which gives information about the homogeneous and inhomogeneous broadening mechanisms. In the last part negatively charged excitons (trions) were observed in unintentionally donor doped Zn_0.90Mg_0.10Se/ZnSe single quantum wells by magneto-luminescence experiments. The binding energy of the trion singlet ground state is about 2.7 meV.

We report on the properties of the D₁ photoluminescence (PL) observed in electron irradiated 4H-SiC. We have investigated the temperature dependence of the PL and its excitation properties. At 2 K the D₁ PL spectrum in 4H-SiC consists of one no-phonon line, L₁ (2.9010 eV), and its phonon replicas. As the temperature is raised, two higher energy no-phonon lines, M₁ (2.9087 eV) and H₁ (2.9118 eV), appear while the L₁ intensity rapidly decreases. At temperatures between 100 and 200 K the D₁ PL is quenched. The associated activation energy is found to be approximately 57 meV. The PL excitation (PLE) spectrum of the L₁ no-phonon line at 2 K contains two strong peaks corresponding to the M₁ and H₁ lines. Furthermore, there is a series of sharp PLE peaks at higher energies. The energy positions of these lines can be fitted by a hydrogenic series of excited states with an associated binding energy of 53 meV. Our results can be explained by exciton recombination at an isoelectronic centre. One of the particles of the exciton is weakly bound, whereas the other is more tightly bound.

We have used Optically Detected Resonance (ODR) spectroscopy to probe the electronic properties of undoped and barrier-doped GaAs/Al_0.3Ga_0.7As multiple-quantum-well (MQW) samples with well widths between 12.5 and 20 nm in magnetic fields up to 15 T at low temperatures. The simultaneous observation of electron and hole CR along with several internal transitions of neutral excitons (IETs) verifies the symmetry-related energy differences of the internal transitions to differences between electron and hole CR. The observed degeneracy of the 1s ---> 2p⁺ IET from the two radiative magneto-excitons is due to the very small electron g-factor. ODR measurements on 20 nm wide MQWs (not-intentionally- and barrier-doped) exhibit transitions of the negatively charged excitonic complex.

Electronic Raman scattering was used to investigate excited states of shallow acceptors in Zn- and in Cd-doped p-type InP with acceptor concentrations in the range between 10¹⁶ cm ^{- 3} and 10¹⁸ cm ^{- 3}. Well resolved transition lines were obtained for acceptor concentrations of about 10¹⁷ cm ^{- 3} (Zn-doped) and 5 x 10¹⁷ cm ^{- 3} (Cd-doped), at higher concentrations the levels merge into broad bands. A linear correlation between the normalized intensity of the electronic spectrum and the acceptor concentration was found. The measured acceptor energy levels were used to prove which sets of published Luttinger parameters γ₁, γ₂ and γ₃ are compatible with our results.

Wafer bending was used to achieve externally controlled biaxial and uniaxial strains in high purity GaAs epilayers grown by MBE to study strain effects on bound exciton luminescence. Behavior of the energy levels under [001] and [110] strains confirms the validity of the crystal-field scheme for the acceptor bound excitons. The ratio of the deformation potentials d/b = 2.4 ± 0.1 was determined from the best fit to the experiment. Distinct anti-crossing under uniaxial stress of the two lowest energy levels was observed for the donor bound excitons. Component 3 of donor bound exciton luminescence, previously assigned to the state J = 1/2, splits under stress into a doublet. The complex behavior of the energy levels under stress may require a revision of the existing models of the donor bound exciton in GaAs.

We have investigated the optical properties of high-quality ZnSe crystals grown by the recrystallization method through optically detected cyclotron resonance (ODCR). ODCR signals are observed by scanning the photoluminescence (PL) wavelength or the applied magnetic field. ODCR as a function of the PL wavelength shows positive and negative signals, which means an increase and a decrease in PL intensity due to cyclotron resonance. It was found that some ODCR spectra obtained as a function of the magnetic field consist of two peaks. We compare the ODCR spectra with an ordinary cyclotron resonance spectrum and discuss the relationship between these spectra. It was made clear that the peak positions of ODCR signals are different from that of the ordinary cyclotron resonance.

ZnSe layers have been epitaxially grown by MOVPE on GaAs substrates and were doped with nitrogen plasma. Photoluminescence measurements at different light intensities and in the temperature range of 4.2 K to 80 K have been performed on the layers. It is shown that the presence of nitrogen leads to photoluminescence spectra revealing acceptor- and donor-bound exciton lines, together with a donor-acceptor pair (DAP) band at 2.69 eV and a free-to-bound (FB) transition accompanied by their longitudinal-optical (LO) phonon side bands. A detailed and coherent analysis of the position, shape and relative intensities of the spectral lines is carried out by means of an analytical model correlating the position of the zero-phonon lines to the relative intensities of the phonon side bands. The model includes central-cell correction and describes the effect of the charge carrier LO-phonon interaction in the framework of the adiabatic approximation within the envelope function approach. The same model is successfully used to analyse the excitation intensity and temperature dependence of the zero-phonon lines associated to the DAP and FB transitions. Comparison between experiment and theory leads to the following physical parameters: S = 0.5 ± 0.1 for the Huang-Rhys factor; E_A = 112 ± 2 meV, r_A = 5.8 ± 0.5 Å for the acceptor ionization energy and radius, respectively, and E_D = 25 ± 2 meV; r_D = 24 ± 0.5 Å for the donor ionization energy and radius, respectively.

Recent magnetoluminescence results obtained for homoepitaxial GaN layers are presented. The neutral impurity-bound excitons and donor-acceptor pair emission lines have been studied in magnetic fields up to 27 T. Low-temperature luminescence spectra have been measured with the magnetic field parallel and perpendicular to the hexagonal c-axis of the GaN layers. Experimental results allowed us to evaluate diamagnetic shifts, effective g-factors of electrons and holes involved in neutral donor and neutral acceptor complexes as well as the electron-hole exchange constant for close donor-acceptor pairs. Both the fine structure of the neutral acceptor-bound exciton emission and the specific properties of donor-acceptor pair spectra observed in magnetoluminescence experiments are tentatively attributed to the internal structure of the acceptor state.

We report a complete far-infrared investigation of shallow level centres in GaN. Transitions between the ground and excited states of shallow donors have been observed by means of transmission and far-infrared photoconductivity measurements - the latter for two configurations of the magnetic field relative to the crystal axis. The hitherto unobtainable high quality of the spectra allows, for the first time, a comparison with a theoretical model of shallow donors in a crystal with a wurtzite structure.

Electrically detected magnetic resonance (EDMR) measurements at 34 GHz were performed to obtain more information about the rate limiting transport processes in blue and green InGaN single quantum well light emitting diodes. With respect to g-factor and linewidth, two centers in both diodes were resolved, which could not be separated in earlier 9 GHz experiments. The position of an enhancing signal at g ≈ 2.008 (ΔB ≈ 15 mT) is found to shift to g ≈ 2.010 with increasing forward bias. Comparative EDMR and EPR (electron paramagnetic resonance) measurements on p-type GaN:Mg and Al_0.3Ga_0.7N:Mg films show that this feature is caused by spin-dependent hopping in the acceptor band of the p-type Al_0.3Ga_0.7N cladding layers in the diodes. In addition, a quenching line at g ≈ 2.002 (ΔB ≈ 18 mT) was observed at high forward biases as well as under reverse breakthrough conditions.

The diffusion and passivating effect of hydrogen (isotope ²H) in epitaxial p-type SiC is studied by secondary ion mass spectrometry and capacitance-voltage profiling on Schottky diodes. The incorporation of hydrogen is achieved by low-energy ion implantation. The influence of implantation energy, temperature and subsequent annealing is presented. Annealing experiments with an electric field applied reveal a reactivation of passivated acceptors and a H⁺ ion drift at a surprisingly low temperature of 530 K.

Stability and electronic properties of Be impurity in zincblende BN are investigated by means of first-principle full potential LAPW and LDA total-energy calculations. We find that Be replacing B in the lattice has very low formation energy and leads to a shallow acceptor level close to the crystal valence band edge. Be replacing the host N atom and Be at the interstitial T_d sites have higher formation energies and are not ascribed to any shallow acceptor level activity in the material.

PL emissions related to multiple bound exciton complexes (MBEC) have been observed in 3C SiC, using time-resolved photoluminescence spectroscopy. At low temperatures we observe transition from MBEC_m complexes with up to five (m = 5) electron-hole pairs. The observed emission is due to the recombination of one electron-hole pair in the complex. The time decay of these emissions decreases with increasing m, ranging from 160 ns to 21 ns. For phonon-assisted emissions we observe additional splitting of the MBEC due to excited electron states. The observed PL emissions and their time decay can be explained according to the shell model. All emissions within each shell have similar decay times, due to fast relaxation of the electrons inside each shell. Multiple bound excitons have not been clearly observed in the 4H or 6H polytype of SiC.

Nitrogen donors in 6H-, 4H- and 3C-SiC were investigated using conventional electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) and the experimental results are discussed. An attempt is presented to interpret the experimentally found large differences in hyperfine interactions of the ¹⁴N nuclei on the various inequivalent sites in the different polytypes of SiC in terms of valley-orbit splitting and "central-cell corrections" in the framework of the effective mass theory (EMT). Phosphorus doping by neutron transmutation in 6H-SiC resulted in various phosphorus-related EPR spectra previously associated with shallow phosphorous donors and phosphorus-vacancy complexes. In analogy to the new interpretation of the nitrogen donor spectra in various polytypes, it is proposed that all phosphorus-related spectra found hitherto in 6H-SiC are due to isolated phosphorus donors in ground and excited EMT states.

We propose a new valence control method, the "codoping method (using both n- and p-type dopants at the same time)", for the fabrication of low-resistivity p-type GaN, AlN, ZnSe, CuInS₂, and n-type diamond based upon ab initio electronic structure calculations. Based upon the calculation, we proposed the following codoping method to fabricate low-resistivity semiconductors: 1. GaN: Si_Ga + 2Mg_Ga (or Be_Ga), H + 2Mg_Ga (or Be_Ga), and O_N + 2Mg_Ga (or Be_Ga), 2. AlN: O_N + 2C_N, 3. ZnSe: In_Zn + 2N_Se, Cl_Se + 2N_Se, Li_Zn + Cl_Se (or I_Se), and Te_Se + 2N_Se, 4. CuInS₂: V_Cu + In_Cu + 2P, and 5. diamond: B + 2N, and H + 2P. We compare our prediction with the recent successful codoping experiments to fabricate the low-resistivity p- and n-type wide band-gap semiconductors.

Gallium nitride doped with oxygen (unintentionally), silicon and magnesium was grown by metalorganic chemical vapor deposition on the conductive single crystals of GaN grown at high hydrostatic pressure. The layers were examined using X-ray diffraction, photoluminescence and far-infrared reflectivity. It was found that the incorporation of silicon depends on the side used for deposition. For the two Si-doped layers grown in the same run, the one grown on the (00.1) side (gallium-terminated) had always smaller free electron concentration with respect to the (00._1) side (nitrogen-terminated). This conclusion could be drawn from the lattice expansion by free electrons, the photoluminescence peak shift by Burstein-Moss effect and the position of plasma edge in far-infrared reflectivity.

Mg doping of cubic GaN epilayers grown by rf plasma-assisted molecular beam epitaxy on (100) GaAs is investigated by secondary ion mass spectroscopy (SIMS), low temperature photoluminescence (PL) and Hall-effect measurements. SIMS measurements show that the amount of incorporation of Mg saturates and is limited to a value of about 5 x 10¹⁸ cm ^{- 3} at substrate temperatures of 720 °C. Besides a shallow acceptor level at E_A ≅ 0.230 eV PL spectra reveal an additional Mg-related deeper defect band indicating an incorporation of Mg at off gallium sites or as complexes. The thermal activation energy of the shallow Mg acceptor is measured by temperature dependent Hall effect to be 0.110 eV.

The extremely rare, nitrogen-free, boron-doped diamonds display a Lyman spectrum in the infrared associated with a substitutional group III acceptor. The Lyman spectrum reveals a unique example of "self-energy" corrections to the effective mass theory for the same substitutional acceptor but located in a host differing merely in its isotopic composition. The Lyman spectrum, observed as a function of temperature, shows the existence of a close pair of 1s ground states separated by ≈2 meV. The electronic Raman spectrum discloses a line at 2.07(1) meV in natural and 2.01(1) meV in ¹³C diamonds corresponding to the transition between the spin-orbit split 1s(p_3/2) and 1s(p_1/2) ground states. Polarization features show the transition to be dominantly Γ₅ in character, placing an upper limit on the ratio of the non-spherical Luttinger parameters of the valence band; Zeeman and piezospectroscopic effects fully support the assignment.

We have investigated the acceptor doping of CdTe and ZnSe to elucidate various aspects of self-compensation. Before doping, a well-defined deviation of the stoichiometry was adjusted by thermal pre-treatment of the bulk crystals. We used ion implantation for doping. Radiation damage could be largely removed by proper annealing. Besides stable elements, we implanted radioactive isotopes of acceptors decaying into inactive centers (¹¹¹Ag ---> Cd, ⁷³As ---> Ge). On the other hand, we implanted native or isoelectronic isotopes which transmute by radioactive decay into relevant dopants (¹⁰⁷Cd ---> Ag, ⁸⁵Sr ---> Rb). In this way it is possible to avoid self-compensation. The use of radioactive isotopes allows an unambiguous correlation of defect features occurring in electrical and photoluminescence measurements with the incorporated dopants. Under optimum pre- and post-implantation annealing conditions, we find an almost one-to-one doping efficiency with respect to the implanted dose.

We present time-, temperature- and intensity-dependent photoluminescence measurements of undoped, n-type and p-type GaN epilayers. In the nominally undoped samples we observe at low temperatures the trapping of free excitons by neutral donors and subsequent radiative recombination. In the n-type Si-doped samples bound-exciton luminescence is dominant over a wide range of temperatures. The luminescence from p-type Mg-doped samples is dominated by shallow-donor-shallow-acceptor pair recombination and by a deep blue centres at 3.0 eV. These two emission bands show identical temperature and linear intensity dependence.

We have performed a complete group-theoretical analysis of state symmetries for electrons and excitons bound to impurities or defects in hexagonal GaN. The spin-orbit interaction has been taken into account. The selection rules for the optical transitions involving bound states have been established. Transitions with light polarized along and perpendicular to the symmetry axis, respectively, are shown to generally obey different selection rules.

Photoluminescence and absorption spectra of undoped and Si-doped GaN films have been studied. It was shown that moderate Si doping improves the optical properties of GaN epitaxial layers. The further increase of doping concentration results in a red shift and broadening of bound exciton line. The broadening seems to be due to potential fluctuations caused by randomly distributed donor impurities. The analysis of temperature dependence of GaN photoluminescence spectra allowed us to reveal the E¹₂ optical-phonon replica of free exciton. The observed flattening of absorption edge with doping can be described in terms of Urbach rule.

Polarization effects of photoluminescence (PL) and absorption on silver thiogallate (AgGaS₂) distortion-free single crystals are investigated in detail near the fundamental absorption edge and the A exciton associated with it. Under low density excitation by a xenon or halogen lamp, PL is observed near 2.605 eV and its intensity shows a long decay time profile of 6.5 ms and thereafter shows longer decay time than 100 ms. Under high density excitation by pulsed laser light, the time profile of this PL intensity shows short decay time of the order of 10 μs in its initial part. The excitation spectra of this PL show distinctive features of anisotropy of the chalcopyrite single crystal.

Hall effect, electric conductivity and electron mobility in n-ZnSe crystals, annealed in Zn melt at temperatures from 500 to 950 °C, are studied in the temperature range from 55 to 500 K. The sharp increase of the shallow donor concentration is accompanied by a decrease of their compensation from K = N_A/N_D = 0.91 to 0.35, and of the activation energy E_D from 28 to 9 meV, as the annealing temperature of n-ZnSe crystals increases above 650 °C. At low annealing temperatures, the conductivity of n-ZnSe is determined by shallow non-controlling donors such as Al_Zn, Ga_Zn, In_Zn. The concentration of V_Zn native defects compensating the shallow donors is sharply decreased and the concentration of the V_Se shallow donors determining the conductivity of crystal is sharply increased, as the annealing temperature increases above 800 °C. The decrease of annealing temperature of the samples below 650 °C results in a drastic decrease of the conductivity of the crystal, a decrease of electron concentration, and abnormally low mobility values. The observed anomalies of transport effects are found to be caused by the presence of a random potential relief in n-ZnSe samples.

The piezoelectric photoacoustic (PPA) spectra were successfully observed between liquid nitrogen and room temperatures for undoped (n-type) and N-doped (p-type) ZnSe epitaxial layers on a GaAs substrate grown by molecular beam epitaxy (MBE). Four different nonradiative recombination centers due to lattice defects are obtained in the nondoped and the N-doped sample. In the N-doped ZnSe, the lattice defects with an activation energy of (180 ± 5) meV are considered to be N_Se-Zn-N_Se complex acceptors. Three other peaks of nonradiative centers are due to intrinsic defects in the N-doped sample, since they are also obtained in nondoped samples. We have obtained nonradiative recombination centers in N-doped p-type ZnSe thin films grown by MBE for the first time.

One of the major problems encountered for the elaboration of blue light emitting devices processed with either nitrides or II-VI compounds is the difficulty for p-type doping of the materials. It is shown that the unintentional introduction of hydrogen, issued for instance from the organometallic precursors during the process is responsible for at least part of the problem. Various experimental techniques for investigating hydrogen in semiconductors are reviewed. They show unambiguously that hydrogen forms complexes with the acceptors and neutralises them. The microscopic structure of the complexes is discussed. The processes which allow to reactivate the neutralised acceptors are discussed; their limitations are outlined.

The interaction between hydrogen (H) and Zn was studied from the measurements of optical absorption spectra of H and Zn-doped Si crystals. FZ Si crystals were doped with Zn by annealing the crystals in Zn vapor at 1200 °C followed by quenching. After Zn-doping, the specimens were doped with H and/or deuterium (D) by annealing them in a H and/or D ambient followed by quenching. Optical absorption spectra of the above specimens were measured at about 6 K with an FT-IR spectrometer. Hydrogen atoms are known to passivate Zn acceptors partially since the optical absorption intensity associated with electronic transitions becomes much weaker after hydrogenation. Instead, other absorption peaks appear at around 2130 cm ^{- 1}. From the comparison of separations of those peaks, they are identified to be due to electronic transitions associated with Zn-H acceptors. Optical absorption peaks associated with Zn-D acceptors have higher energies by about 12.1 cm ^{- 1} than those of Zn-H acceptors.

Nominally undoped CdTe was exposed to a hydrogen plasma at 160 °C. It has been shown that after this treatment, seven typical photoluminescence lines H₁ to H₇ are observed in the excitonic region. The dependence of the most intense of these hydrogen-related lines, H₃, on external magnetic fields up to |B| = 7 T is investigated. The diamagnetic shift of the H₃ line is proportional to |B|², showing that it is caused by the recombination of excitons. Since an electron-hole interaction is observed, the exciton is either bound to an isoelectronic defect or to an ionised donor. The anisotropy of the Zeeman splitting is explained in terms of a small uniaxial compressive strain in a [100] direction perpendicular to the [001] surface orientation. In contrast, the well-known copper-bound exciton line, used as a reference, reveals uniaxial tensile strain in the same [100] direction. Therefore, the defects associated with the H₃ line either have a [100] symmetry or are incorporated close to [100] oriented extended defects. The electron g-factor is g_e = - 1.50 ± 0.04, and the hole parameters are K = 0.40 ± 0.05 and L = 0.00 ± 0.04, describing the isotropic and the anisotropic Zeeman effect, respectively.

Three families of shallow thermal donors have been identified in annealed Czochralski silicon from measurements of their infrared electronic transitions using Fourier transform spectroscopy. Donors produced in Al-doped Si incorporate an Al impurity; centers produced in hydrogenated material contain an H (or D) atom; a third set of donors labeled STD(X)N produced in nitridated or irradiated pre-hydrogenated samples may incorporate a lattice vacancy rather than a nitrogen atom. A critical review of the literature is made and some rationalization has been effected. Comments are included about the formation of donor centers following anneals of heavily damaged float-zone Si that also contains hydrogen.

Czochralski-grown phosphorus-doped (≈2 x 10¹⁴ cm ^{- 3}) silicon wafers have been annealed in nitrogen, wet nitrogen, argon, oxygen, and vacuum ambients at 470 °C for times up to 500 h. Sample characterization was made with capacitance-voltage, four-point probe, DLTS, thermally stimulated capacitance, admittance spectroscopy, secondary ion-mass spectrometry, and Fourier transform infrared spectroscopy. This study finds a strong relation between the previously reported ultra-shallow thermal donors (USTDs) and shallow thermal donors (STDs), and it is shown that the net concentration of thermally formed donors is independent on annealing ambient within the experimental accuracy. It was found that the majority of formed donors for long anneals consisted of either STDs or USTDs, however, it was found that oxygen-containing ambient is indispensable for forming USTDs.

The annihilation kinetics of new oxygen donors (NDs) has been studied. The ND and interstitial oxygen concentration as well as the energy distribution of ND states are determined subsequent to annealing steps with various duration (2 s to 250 h) and at temperatures ranging from 875 to 1025 °C. The annihilation occurs in two steps: At the temperatures used, energetically shallow ND states (E_C - E ≤ 0.1 eV) annihilate within 15 min, while the deeper ND states require annealing times up to 250 h. The thermal activation energy ΔE_A for the annihilation of shallow ND states is determined to be equal to (3.03 ± 0.20) eV.

Thermal donors generated in p-type boron-doped Czochralski-grown silicon by a 450 °C heat treatment have been studied by high-field magnetic resonance spectroscopy. The experiments were conducted at a microwave frequency of 140 GHz and in a magnetic field of approximately 5 T. The symmetry of the g-tensor of four individually resolved Si-NL8 species was determined. It turned out that only one of the early species shows a true orthorhombic symmetry. Upon prolonged heat treatment times the relative concentration of the most anisotropic (early) species showed a clear decrease, while that of the later species increased.

An electron paramagnetic resonance (EPR) study of hydrogen-related radiation-induced shallow donors in silicon has been performed. Three species of this donor family (D1-D3) were observed earlier by means of infrared absorption measurements in hydrogenated Czochralski-grown Si (Cz-Si) crystals after irradiation with fast electrons and subsequent annealing in the temperature range of 300 to 550 °C. An EPR signal with isotropic g-factor of 1.9987, labeled TU1, has been found in hydrogenated Cz-Si crystals which were irradiated and annealed at 300 to 350 °C. It was shown that the TU1 signal is related to the shallow donor state of the D1 center. Annealing at temperatures higher than 350 °C resulted in transformation of the D1 to the D2 centers. This led to the transformation of the TU1 spectrum to another one with S = 1/2 and anisotropic g-tensor, the principal values of which were found to be very close to those for the Si-NL10 EPR spectra in silicon. The observed EPR spectra are compared with the Si-NL10 ones and possible atomic structures, and formation mechanisms of the defects which give rise to these spectra are discussed.

In LEC-grown GaP, two vibrational lines at 3250.9 cm ^{- 1} and 3106.0 cm ^{- 1} are measured and assigned to the stretching modes of two different O-H centres. In GaP containing both hydrogen and deuterium, two additional lines appear, both shifted to lower energies by the same factor r = 1.346. This result shows that both O-H centres contain only one hydrogen atom. The O-D line at 2415 cm ^{- 1} is split into two lines. This splitting possibly indicates a further example for resonant interaction of local modes with extended ones. The comparison with GaAs suggests the assignment of two lines to the analogous O-H centres in GaAs.

In order to investigate the gap states caused by the oxygen precipitation, we performed deep level transient spectroscopy measurements on silicon crystals containing various types of oxygen precipitates. For all types of the oxygen precipitates, a new signal in the gap-state density was observed at about E_c - 0.25 eV, as well as the previously reported peak at about E_v + 0.3 eV. Both peaks were attributed to the defect due to oxygen precipitation. This distribution of the gap-state density is very similar to that of the well-known P_b center originating from the dangling bond of a silicon atom at the interface between the silicon and silicon dioxide. These gap states were further found to be passivated by hydrogen, similarly to the case of the P_b center.

Electrically active shallow centers in silicon obtained by low-temperature silicide mediated crystallization were investigated. In addition to the phosphorous related level (L1) a second shallow level (L2 ≈ 30 meV below the conduction band) was observed and attributed to oxygen related donors. L2 appears with a high concentration and dominates the electrical characteristics of the recrystallized silicon layers. Schottky barrier junctions formed on these layers display a pronounced tunneling behavior, attributed to the high concentration of L2 centers in the near surface region.

The spectra of optical absorption and photoconductivity of perfect CdAs₂ single crystals were investigated near the intrinsic edge and in the extrinsic absorption region. The existence of three donor levels created by structural defects is established. The mechanism of defect formation is suggested: the interstitial Cd_i atom in different charge states created the ϵ₁ < 0.02 eV and ϵ₃ ≅ 0.42 eV levels, the As_v vacancies created the ϵ₂ ≅ 0.26 eV level.

The behaviour of group III, IV and VI dopants in vertical Bridgman grown GaAs was studied as a function of the stoichiometry of the melt. The melt composition was abruptly changed from stoichiometric to Ga-rich by intentional injection of gallium during the growth. In GaAs co-doped with Si and B, the Ga injection causes a drastic decrease of the free electron concentration which is explained by an increased concentration of As-sublattice acceptors. Spatially resolved photoluminescence (PL) results confirm this interpretation qualitatively as (Si_As)- and (B_As)-related modes dominate the spectra from the Ga-rich area of the sample. Pronounced PL peaks related to As-sublattice acceptors also appear in samples doped with B and Ge after Ga injection. The assignment of the PL modes was possible by taking account of the data in the literature and with respect to the results of Hall, Raman and mass spectrometric measurements. In GaAs:Sn and in group VI-doped samples, however, the Ga injection causes PL modes not previously reported.

Optical absorption spectra of hydrogen (H)-doped Si crystals were studied to clarify the hydrogenrelated optical absorption peaks. Hydrogen was doped in Si crystals by annealing them in a hydrogen atmosphere at 1200 °C followed by quenching. Some specimens were irradiated with high energy electrons (3 MeV) or neutrons. Some specimens were treated with atomic hydrogen. Their optical absorption spectra were measured at about 7 K with an FT-IR spectrometer. In accordance with Pritchard et al. an absorption peak was observed at 3618 cm ^{- 1} in H-doped specimens. Its intensity became weaker after electron or neutron irradiation. Instead, many absorption peaks appeared around 2000 cm ^{- 1} and an absorption peak appeared at 3942 cm ^{- 1}. The responsible defect for the latter peak does not include H. Many absorption peaks around 2000 cm ^{- 1} were compared with those observed in proton-implanted Si.

Studies of shallow donor impurities confined in semiconductor quantum-well (QW) structures are reviewed, concentrating on recent experimental (far infrared spectroscopy) and theoretical investigations of negative donor ions (D ^- ) and the effects of excess electrons on the donor-related transitions. With increasing excess electron densities in the QWs the two-electron D ^- -singlet and -triplet transitions are blue-shifted substantially, and cusps in the normalized blueshift versus filling factor (ν) are observed at integer and fractional values of ν. For ν < 1 the many-electron (bound magnetoplasmon) transitions approach the isolated two-electron transitions. Exact numerical diagonalization calculations of a donor and confined electrons with ν varying from 2 to <1 show the importance of electron correlations and localization at high magnetic fields in understanding this behavior. Similarities between some of these results and recent studies of internal transitions of neutral and negatively-charged excitons are noted.

We investigate the effect of the position of a donor in the quantum well on the energy spectrum of the D ^- system in the presence of a perpendicular magnetic field. As a function of the magnetic field we find that when D ^- centers are placed sufficiently off-center they undergo singlet-triplet transitions which are similar to those found in many-electron parabolic quantum dots. The main difference is that the number of such transitions depends on the position of the donor and only a finite number of such singlet-triplet transitions is found as function of the strength of the magnetic field. For sufficiently large magnetic fields the two-electron system becomes unbound. For the near-center D ^- system no singlet-triplet transition and no unbinding of the D ^- is found with increasing magnetic field.

A simple model in which both electrons of the shallow-negative donor D ^- are instantaneously localized approximately at the same distance with respect to the ion nucleus is considered. Within this model, the D ^- Hamiltonian becomes separable, and the effect of the electron-electron interaction is essentially taken into account by the interaction of each of the electrons with a renormalized-nucleus effective charge which depends on the angle between the two electron vector positions. The trial function is taken as a product of two three-parameter one-particle wave functions to calculate the ground state energy as a function of the angle. Theoretical results for the D ^- energy, including zero-point energy corrections within the harmonic approximation, are obtained for D ^- states centered in GaAs-(Ga,Al)As quantum wells as a function of the well width, and for different values of the magnetic field, applied perpendicular to the interfaces of the heterostructure. Our results are in good agreement with experimental data and with theoretical calculations using variational or Monte Carlo techniques.

We study the effects of screening on the binding energy of positively and negatively charged impurities in the quasi-one-dimensional electron gas. We assume a parabolic confinement for the electron gas. Many-body effects beyond the random-phase approximation and the valley degeneracy are taken into account in the screening function. Our results should apply to Si/Si_{1 - x}Ge_iix quantum wires.

The optical properties of p-type modulation doped GaAs/AlGaAs quantum wells have been investigated for acceptor concentrations in the high doping regime by means of photoluminescence (PL) and PL excitation. Several novel physical effects have been observed as the hole density increases. The main effect is a significant red-shift of the main emission due to the bandgap renormalization caused by many-body effects. The observed renormalization is compared with theoretical predictions of the effective bandgap shrinkage, based on self-consistent calculations. Another apparent observation at high hole densities is the exciton quenching. The acceptor doping level required to quench the excitons is found to be significantly lower than for the case with acceptor doping inside the well, but on the other hand, considerably higher than that one in the bulk case, due to the inefficiency of screening in a 2D system. Finally, we report on the observation of a new feature, the Fermi edge singularity (FES), i.e. the transition between 2D holes at the Fermi edge and free electrons. The dynamical behavior of the FES has been investigated by means of time-resolved PL, showing a relatively short decay time as well as rise time for the FES emission.

We report for the first time how the optical transitions associated with localized levels of a rare-earth impurity can be modulated by quantum confinement. The effect of quantum confinement on impurity critically depends on the size of the host crystal. The variation of absorption and luminescence efficiency of Tb³⁺-ion doped Y₂O₃ is discussed within the framework of the quantum confined atoms.

Due to their extended electronic structure shallow donors are believed to be very sensitive to confinement effects. Our interest is to investigate the influence of size on the properties of shallow donor phosphorus in mechanically milled silicon microcrystals. The crystallinity of powders is confirmed by X-ray diffraction and the size as determined by scanning electron microscopy was found to be 0.25 μm for smallest to 4 μm for largest microcrystals. In the current experiment we use electron paramagnetic resonance to measure the hyperfine interaction between donor electron and P nucleus. The hyperfine constant and mutual overlap of donor wave functions exhibit significant dependence on grain size and subsequent annealing. By photoluminescence spectroscopy the appearance of a strong band around 1515 nm which shifts to higher energies with diminishing grain size has been established.

Interband photoluminescence (PL) is studied as a function of magnetic field in modulation-doped GaAs/Ga_0.67Al_0.33As heterostructures in the Quantum Hall regime. The oscillatory PL data exhibit free electron and donor (ground electric subband) to free hole transitions, as well as free and bound exciton (first excited subband) transitions. Pronounced intensity oscillations of all transitions are observed as the magnetic field varies between 0 and 12 T. In particular, we observe, for the first time, strong intensity oscillations of donor-hole and bound exciton transitions with exactly opposite phase from which we conclude that the observed bound excitons are bound to the donors at the GaAs/Ga_0.67Al_0.33As interface. Similarly, the intensities of free electron-hole and free exciton transitions oscillate with exactly opposite phase. Various mechanisms of the intensity oscillations are discussed, taking into account the available parallel behaviour of the free and bound carrier excitations. We exclude an oscillatory screening by 2DEG to be the cause of the intensity oscillations. We conclude that the intensity transfer between the electron-hole and the free exciton transitions occurs when, as a function of magnetic field, the interband electron-hole energy becomes larger than the corresponding exciton energy, so that it is advantageous for the pair to form an excitonic state. A similar mechanism is responsible for the intensity transfer between the donor-hole and the bound exciton transitions.

In standard electron spin resonance (ESR) spectroscopy on modulation-doped SiGe/Si/SiGe quantum wells, we observe a very sharp ESR due to the 2d electron gas and also cyclotron resonance (CR) of free carriers in the well. Both signals increase persistently after illumination and the 2d CR becomes sharper by one order of magnitude. We explain these effects together with the lack of Shubnikov-de Haas oscillations in terms of potential fluctuations due to the ionized donors which are partially neutralized and effectively screened by photogenerated carriers.

Far-infrared photoconductivity spectra of strained multi-quantum well Ge/Ge_{1 - x}Si_x (≈ 0.1) heterostructures resulting from the excitation of shallow acceptors were investigated. The spectra are shown to be shifted to the long-wavelength end of the far-infrared range if compared with acceptors in bulk p-Ge due to the splitting of light and heavy holes subbands in Ge layers owing to the "built-in" deformation and size quantization. The shallow acceptor spectra in bulk germanium under uniaxial tension, that is "equivalent" to the deformation of Ge layers in the heterostructures, were calculated by the variational technique; the results were used for the interpretation of the experimental data. Ge/GeSi heterostructures are shown to be a sensitive photoelectric detector for the long wavelength end of FIR range.

We present a theoretical study of impurity-related photoluminescence of acceptor-doped GaAs-(Ga,Al)As superlattices under in-plane magnetic fields within the effective-mass approximation. An expansion in terms of sine functions is used in order to obtain electron and hole magnetic Landau levels whereas products of sine and hydrogenic-like variational functions are taken for the shallow-acceptor envelope wave functions. The magnetoluminescence lineshapes associated to transitions from the lowest Landau conduction subband to impurity states are calculated for an homogeneous distribution of acceptors throughout the semiconducting superlattice. We found quite good agreement between the theoretical acceptor-related magnetoluminescence peak positions for GaAs-(Ga,Al)As superlattices and available experimental data.

An analytical approach to the problem of an impurity electron (or hole) in a quantum well (QW) subject to electric and strong magnetic fields, both directed perpendicular to the heteroplanes, is described. It is shown that the combined potential acting on the electron resembles that of a double quantum well. When the levels associated with the two effective QWs anti-cross, the impurity single QW subject to external fields can be treated as a resonant structure. The explicit dependencies of the resonant splitting upon the width of the QW, the magnitudes of the electric and magnetic fields, and the position of the impurity centre are obtained. Estimates of the expected splitting and frequency of the emitted radiation relevant to the interwell oscillations of the electron are made using typical values associated with GaAs QWs.

The temperature dependences of conductivity and hole mobility in boron doped SiGe quantum well structures were studied. The conductivity at low temperatures is shown to be due to hopping over neutral B centers while at higher temperatures, it is due to two-stage excitation including thermal activation of holes from the ground to strain-split B states following by hole tunneling into the valence band.

We extend the dressed-atom approach to treat the interaction of a laser field with a semiconductor system in which the interaction with the laser is incorporated through the renormalization of the semiconductor effective mass. The semiconductor is modeled via a simple parabolic two-band isotropic scheme in the k · p approximation. We compare the effects of the laser dressing on the intradonor transitions in doped GaAs-(Ga,Al)As quantum wells with those produced by an applied external magnetic field. It is shown that these effects may be comparable and therefore experimentally observable. Such approach allows the study of the effects of laser fields on a variety of phenomena in semiconductor systems for which the effective-mass approximation provides a good physical description.

Few-electron systems have been studied by variational means for semiconductor spherical quantum dots both with and without a donor center. It is shown that the pair (2e) and triple (3e) of electrons as well as the donor center with two (D ^- ) and three (D^{2 -} ) electrons can form several bound states if the range and depth of the confinement potential are sufficiently large. It is found that dipole-allowed optical-transition energies are nearly the same for the two groups of systems: {D⁰ and D ^- } and {1e, 2e, 3e, and D^{2 -} }. For D⁰ and D ^- , the transition energies are significantly larger than for the systems of the second group.

We investigate the role of protrusions due to individual Si shallow donors on the electron transmission properties through GaAs/Al_xGa_{1 - x}As heterojunctions. Our theoretical calculations show that the protrusions can change remarkably the electron transmission properties, being even capable of producing resonances. The modifications on the electron transmission are stronger when the individual dopant is localized in the Al_xGa_{1 - iix}. As alloy side, but they also depend how far is the individual dopant from the GaAs/Al_xGa_{1 - x}As heterojunction interface. When the existence of two simultaneous individual dopants is considered, the electron transmission can present more than one resonance peak.

Using a variational approach we investigate the effect of localization of a biexciton in the plane of the quantum well on its binding energy and the structure of the biexciton. The well width fluctuations leading to trapping of the biexcitons are modeled by a parabolic potential. The results obtained for different well widths are compared with recent experimental data and good agreement is obtained. From the analysis of the probability distributions we find that the structure of a biexciton is similar to the one of the H₂ molecule.

Acceptors confined in strained In_xGa_{1 - x}As/Al_0.3Ga_0.7As quantum well (QW) structures are theoretically investigated. The impurity states are calculated using a four-band effective-mass theory, in which the valence-band mixing as well as the mismatch of the band parameters and the dielectric constants between well and barrier materials have been taken into account. The binding energies of acceptors and the oscillator strengths of the active infrared electronic transitions between the acceptor ground states and the different excited p-like states are calculated with different indium fractions x and different well widths. The results show clearly, that a built-in strain strongly influences the transition energies and the relative oscillator strengths. Due to the splitting of the acceptor ground states, the absorption spectra of the acceptors also show a strong temperature dependence. The calculated spectra of the acceptor electronic transitions are ready to be compared with the infrared absorption measurements.

Using atomic force microscopy (AFM), low-temperature photoluminescence (both CW and time-resolved) as well as Raman spectroscopy, we have performed a comparative investigation of as-grown and α-particle irradiated GaAs doping superlattices. From AFM no evidence of structural damages was found. On the opposite, from optical investigations and depending strongly on the dose of α-particle irradiation, the electronic properties were found to vary. At low dose, the main PL signal remained but decreased in intensity. At the same time it shifted towards low energy. At high dose, the PL answer completely disappeared. After annealing, partial recovery was found. All together this indicates that, upon bombardment and annealing, first one opens and next closes new (non-radiative) defect-induced recombination paths.

A new type of nonradiative process responsible for the excitonic luminescence quenching in (CdTe,CdCrTe)/CdMgTe heterostructures is identified and its efficiency is estimated. During this process, which belongs to the class of Auger-type nonradiative transitions, an immobilized exciton recombines nonradiatively, and the energy is transferred to a chromium center nearby, causing its ionization.

The effect of the interaction of electrons with polar optical phonons on the binding energy of shallow donors is investigated taking into account the phonon confinement in the cylindrical quantum wires. We have investigated the variation of the hydrogenic donor binding energy shift conditioned by both electron-confined-phonon and electron-interface-phonon interactions as a function of wire radius and donor location.

The binding energies of shallow hydrogenic donor impurities in GaAs-(Ga,Al)As quantum boxes are calculated as a function of the size of the structure and as a function of the intensity of an applied electric field. The calculations are performed within the effective-mass approximation and using a variational method. We have found that when the size of the quantum box is reduced, both the energy of the ground state and the binding energy increase. Likewise we found that when the electric field intensity is increased, both the energy of the ground state and the binding energy decrease.

Using a variational procedure within the effective-mass approximation we calculated the binding energy of a shallow hydrogenic impurity on-center located in cylindrical shape GaAS-(Ga,Al)As low-dimensional systems, such as: quantum-well, quantum-well wires, and quantum dots, considering an infinite confining potential in all surfaces of the structures. The binding energy of the ground state is calculated as function of length, radius of the structure, and intensity of a magnetic field applied in axial direction. We show that the binding energy of the impurity decreases with both the length and radius of the structure, whereas it increases with the applied magnetic field. Finally, we show that our results are in good agreement with those obtained in previous calculations in GaAs-(Ga,Al)As quantum wells, quantum-well wires and quantum dots of comparable dimensions. We consider future understanding of optical phenomena related with donor impurities in GaAs-(Ga,Al)As low-dimensional systems in which the effects of applied magnetic fields compete with the quantum confinement, must take into account these results.

Using a variational approach within the effective mass approximation we calculate the binding energy of the ground and some excited donor impurity states in quantum-well wires with rectangular and cylindrical transversal sections under the action of applied electric fields. We study the binding energy as a function of the geometry of the system, the applied electric field as well as the impurity position inside the structure. We found that the presence of the electric field breaks down the degeneracy of states for impurities symmetrically positioned within the structure, and that the geometric confinement and the electric field are determinant for the existence of bound excited states in these structures.

Participation of shallow states in the energy transfer processes between the rare-earth ions and a host semiconductor matrix is discussed. It is argued that shallow levels link the atomic-like states of the inner core of the rare-earth ion with the band structure of the host crystal. For the two most investigated systems InP:Yb and Si:Er the important role of these states at various stages of excitation and de-excitation mechanisms is shown. Models of the energy transfer process with a formation of a shallow intermediate state are discussed. It is shown that the formation of such a state is essential for the RE ion core activation. Experimental results supporting the involvement of shallow states are reviewed. Temperature dependencies of the photoluminescence activation and decay time constants are presented; these show an important role of shallow doping in the activation of Auger processes relevant to the mechanism of rare-earth ion photoluminescence. For the Si:Er system direct evidence for the two-stage excitation mechanism is discussed.

Electric-dipole spin resonance (EDSR) spectra at 24, 34, and 60 GHz of Si:Be show six absorption lines with characteristics similar to those previously observed for the Γ⁺₈ ground state of group-III single acceptors. From the dependence on Fermi level position we attribute these absorptions to the singly ionized substitutional double acceptor Be ^- _s. The observed strong nonlinearity of the Zeeman splitting can be approximately described by a Hamiltonian containing terms up to second order in the magnetic field. A much better description is achieved by including the interaction with the Γ⁺₇ state derived from the split-off valence band, if one assumes that it lies only about 0.6 meV above the ground state.

The structural and electronic properties of cadmium in p-type silicon were studied by means of EPR. Cd was diffused in one set of samples whereas in a second set we used high energy implantations in the MeV range for doping, followed by a high temperature annealing step at 1200 °C. The angular variations of the corresponding spectra show cubic symmetry and can be explained with an S = 3/2 orbital singlet ground state. From comparison of the EPR results obtained from samples with different shallow p-dopants (B, Al, Ga, In) and doping levels it can be concluded, that the spectra arise from the Cd⁺ donor state with an energy level above the In acceptor level. These experimental findings were compared with theoretical predictions of electronic structure calculations based on the Local Density Approximation of the Density Functional Theory using the LMTO-ASA scheme. The comparison shows that the donor state must be due to substitutional Cd.

The effects on the electrical properties of Al_xGa_{1 - x}Sb of a level of the Te impurity (D level) coexisting with the DX center were investigated through an extensive analysis of Hall data. The experiments were performed in MBE samples in a wide range of alloy compositions (0 ≤ x ≤ 0.50) and Te doping densities (10¹⁶ to 10¹⁸ cm ^{- 3}). The possibility of observing carrier freeze-out to a D level, without competition with the DX center, is restricted to lightly doped samples and to a narrow range of alloy fractions close to the Γ-L crossing of conduction band edges. Within this range the D level is most likely coupled to the L minima. At higher alloy fractions, freeze-out to a D level can be observed only at a low temperature where the DX center occupancy is frozen. An example of this procedure is given for x = 0.40.

In Al_0.1Ga_0.9As:Ge two photoconductivity (PC) effects related to the DX ^- (persistent PC) and A₁ (giant PC) states are studied under hydrostatic pressure. We found that the barrier for electron capture to the DX ^- state, responsible for the persistent PC, changes its character for p > 10⁹ Pa and becomes pinned to the conduction band (CB) edge. Another barrier linked with giant PC, responsible for electron capture on A₁ state is pinned to the CB edge for all pressures. A new universal configurational coordinate diagram of donor related levels including two types of lattice deformation, of C_3v symmetry for DX ^- and T_d symmetry for A₁, is proposed. It describes both shallow-deep A₁ and DX ^- level instabilities and explains e.g. why capture of electrons to the DX ^- state goes via an intermediate effective mass state, or why for extremely sharp shallow A₁ state anticrossing behavior is observed.

Deep level transient spectroscopy measurements were performed on Zn-doped Si_{1 - x}Ge_x with x between 0 and 0.48. Our investigations reveal two deep hole traps which are attributed to the acceptor states Zn^{0/ -} and Zn ^{- / - 2} of substitutional Zn. Taking into account band-offset data for Si_{1 - x}Ge_x, we have found that the energy level related to Zn^{0/ -} is horizontally aligned across the composition-dependent bandgap for x up to ≈30 at% Ge which gives evidence for a localized defect state in this composition range. For higher Ge concentrations the data suggest a continuous change-over to a shallow level center. The defect level related to the second hole trap decreases with increasing Ge content.

A method to calculate the exchange-correlation potential for the homogeneous and inhomogeneous heavy and light hole gases is described. We present the results for the homogeneous hole gas of the diamond and zincblende type semiconductors. Due to the spin-mixed character of hole states, the exchange interaction also takes place between holes of different types. The exchange potentials depend on the mixture ratio between heavy and light holes. We study a decomposition of the exchange potentials into two parts representing the interaction between the holes of different types and the interaction between the holes of the same type. Exchange potentials of heavy holes are plotted versus heavy hole densities for various light hole concentrations.

We present the results and analysis of magnetooptical measurements performed at LHe temperatures on thin n-CdTe layers doped with Indium or Iodine, grown on an undoped CdTe buffer and a semiinsulating GaAs substrate. We show the possibility of metastable population of shallow donor centers in undoped CdTe. We report the difference in chemical shifts of different species. The dependence of the shape of the obtained spectra on the location of donor centers relative to the interface is discussed. The analysis of the temperature and voltage dependence of the shape of spectra shows that non-Gaussian shape of the peaks should be considered.

Thermally induced currents (TIC) and instabilities of photoresponse in Pb_{1 - x - y}Sn_xGe_yTe(In) (0.06 < x < 0.2; 0.08 < y < 0.12; 0.5% < C_In < 1%) films on BaF₂ substrates have been investigated at temperatures 4.2 K < T < 30 K. The temperature T_M corresponding to maximum values of single TIC peaks appears to be extremely low varying from 6 K up to 14 K. The combined effect of lighting and slight heating of the sample from 4.2 K up to ≈ 6 K results in photoresponse instabilities depending on the experimental regime. The results are interpreted in terms of thermal excitation of charge carriers from metastable impurity level.

The theoretical results presented deal with the energy spectrum of shallow bound hole and electron states and corresponding optical transitions, spectral dependences and polarization of the interband dislocation absorption (DA) and luminescence (DL) in direct-gap semiconductors with edge and screw dislocations. A possible explanation of the experimentally observed predominant polarization of the interband DA and DL along the Burgers vector is proposed. A general view of the dislocation absorption spectrum in different polarizations of light is analyzed both for a screw dislocation and an edge dislocation. For a screw dislocation a significant optical circular dichroism in DA should be observed.

The kinetics of the electron concentration variation in n-Pb_{1 - x}Sn_xSe (x = 0.20, 0.25) alloys irradiated with electrons was investigated. By comparing theoretical and experimental dependences of the electron concentration on the radiation flux the main parameters of the defect formation process were determined assuming that the radiation defect introduction rate decreases exponentially with an increase in radiation flux. A model was proposed according to which the major mechanism of defect formation process is the generation of complexes including the primary radiation defects and the intrinsic structure defects typical of initial crystals.

In cubic semiconductors the light hole and heavy hole masses ratio is small. That is why there were some attempts to describe these crystals by the model of zero light hole mass. But in all papers investigations were limited by the ground state. We took a more general approach which yields an exact analytical solution for all acceptor states and establishes that the acceptor wave functions have some features at large radius. For example, the ground state wave function behaves predominantly as the d-like hydrogen function. This method is also suitable for a theory of the more actual case of the arbitrary mass ratio. We have adopted it also to electric-dipole spin resonance in GaAs with Mn ions being acceptors.

A new technique for the determination of parameters of trap levels in semiconductors is examined. This method is based on the dependence of the dynamic negative resistance (DNR) in absolute value on the concentration of traps in injection-transit-time diodes. It was established by us earlier that a maximum value of DNR should be observed at an appropriate transit angle the meaning of which is a definite function of concentration, energetic level and capture cross-section of minority charge carriers. Using corresponding analytical expressions and results of microwave measurements it is possible to obtain the values of the above-mentioned parameters of traps.

The kinetics and temperature dependences (5 to 50 K) of the extrinsic photoconductivity (PC) in microwave (MCW) and DC electric fields in GaP doped with Te ( 2 x 10¹⁷ cm ^{- 3}) have been studied. A slow relaxation (≈10 ^{- 2} s) of MCW PC, in contrast to a fast DC PC relaxation was observed at 5 K. The steady-state MCW PC exceeded the DC PC by 2 to 3 orders of magnitude at 5 to 16 K, and dropped by subsequent increase of temperature to 35 K. In the same temperature range a slow relaxation of absorption of the room-temperature background radiation, induced by GaP excitation was observed. In the range 45 to 90 meV it increased, but decreased in the higher photon energy range. The obtained results are explained by accumulation of nonequilibrium electrons in the long-living excited state 1S(Γ3) of the Te donor in GaP.

The photoconductivity (PC) spectra and the induced absorption of background radiation in the energy range 10 to 40 meV are investigated in relatively low compensated B-, Ga- and As-doped Si at 4.2 K. Dips, corresponding to the photoionization of long-living excited states of B and As, are observed in the PC spectra of the D ^- (A⁺) bands. It was found, that PC of the bands has a slow relaxation (≈10 ^{- 4} s). So, the localization of nonequilibrium carriers in long-living excited states of B and As and in the D ^- (A⁺) states leads to a competition of both slow relaxation processes in PC and absorption phenomena. The separation of the processes is possible in electric field. It leads to a decrease of PC due to D ^- (A⁺) states but weakly influences the long-living excited states absorption.

Detailed information on the impurity system GaAs:Be has been obtained by carrying out far-infrared spectroscopy in a magnetic field on high-quality samples grown by molecular-beam epitaxy. The g-factors of the ground state and of one excited state have been determined.

Results are given for the Zeeman behaviour of the 1Γ⁺₈ and 2Γ ^- ₈ states of Zn ^- in Ge for B ∥ <100> and <111>. Due to the larger energy spacings between the states of Zn ^- relative to the neutral acceptors, the splitting pattern of 2Γ ^- ₈ at high fields is very similar to that of group III impurities at low fields. In contrast to the shallow acceptors, the behaviour of the deep ground state of Zn ^- is like that of a j = 3/2 atomic state with the same splittings for both directions of field, these are determined by a single g-factor of value 0.5355.

Experimental results on field ionization of shallow impurities in GaAs:Be and Ge:Sb,P are presented. In both materials enhacement of the impurity-to-band tunnelling rate was observed when lattice temperature was raised. Acoustic phonon-assisted and -activated tunnelling mechanisms are employed to explain the observed results. In case of activated tunnelling the importance of excited impurity states in the field ionization is emphasized. The role of heavy and light hole masses in an acceptor-to-valence band tunnelling process is discussed, too.

A doublet observed for the G line of Zn⁽⁰⁾ without applied perturbation is attributed to relative shifts of the two Γ ^- ₄ states which are part of the compound excited state of this transition. This effect also increases the energy spacing between the G doublet and the rest of the lines of the Lyman series. The energies and widths of all the lines are temperature dependent at temperatures where group III lines no longer show these effects. Zeeman measurements have demonstrated conclusively that the low temperature Lyman series of Zn⁽⁰⁾ in Ge arises from the 1Γ⁺₁ ground state. These studies also establish that the Zeeman excited states of Zn⁽⁰⁾ are compounded from wave functions identical to those of the ground state of Zn ^- and the excited states of group III acceptors in Ge.

The in-situ cleaved n-InAs(110) surface is studied by low temperature scanning tunneling spectroscopy and microscopy in magnetic fields up to 6 T perpendicular to the surface. The dI/dV(V) curves exhibit characteristic oscillations in magnetic field, which are attributed to Landau levels in the conduction band. The energy dependence of the effective electron mass is determined. Dopants reduce the Landau level energy. As expected, the energy reduction decreases with increasing Landau level number.

Photoluminescence (PL) measurements on silicon samples implanted with radioactive Hg isotopes which decay fully or partly through the series Au/Pt/Ir are reported. Results obtained with ¹⁹³Au isotopes (half-life 17 h) confirm earlier work concerning the involvement of Au in the 735 meV centre first observed in Fe-diffused silicon. A PL spectrum consisting of a series of doublets originating at 780/782 meV is found to be Au-related. An important new result is that the 1066 meV trigonal centre, commonly attributed to Fe-B pairs, is found to involve Au atoms. For Pt-related centres, ¹⁹¹Pt isotopes (half-life 2.9 days) provide conclusive proof of Pt involvement in three PL centres with zero-phonon lines at 1026, 884 and 777 meV. There is not sufficient evidence from the changes observed in the spectra to make any conclusive deductions concerning the occurrence of Ir-related PL spectra in our samples.

Two mechanisms of the inverse population of shallow impurity states in silicon under optical pumping have been proposed and analyzed, using a procedure allowing to reduce the number of required matrix elements of transitions. The first mechanism is based on the resonance interaction of the 2p₀ state in Si:Bi with optical phonons. The other one is based on the suppression of acoustic-phonon-assisted relaxation from the 2p₀ state in Si:P due to the momentum conservation law. Spontaneous emission was registered from shallow donors in Si:P under photoionization by a CO₂ laser. The dependence of the spontaneous emission intensity on the intensity of pumping radiation confirms the possibility of amplification on impurity transitions.

Effective mass calculation of the acceptor ground state g-factor is presented for two limiting cases: infinite and zero spin-orbit coupling. Simple analytical expressions for the g-values are obtained within the framework of the zero-radius potential model. These results are in good agreement with experimental data for GaAs. Analytical expressions for the tensor g-factor are presented for strained wide-bandgap semiconductors with weak spin-orbit interaction (such as GaN). These results can be used for qualitative analysis of ODMR and ESR experimental data.

Heat treatment of heavily carbon doped AlAs and GaAs results in a loss of C_As shallow acceptors. In Raman scattering experiments on annealed CBE grown GaAs with ¹²C and ¹³C isotopes, and MOVPE grown AlAs it is found that the loss of carriers is accompanied by the appearance of two high frequency lines. These lie near to the stretch mode of an isolated C₂ molecule (1855 cm ^{- 1}). This is consistent with the formation of two types of di-carbon defects in these materials where the C atoms are bonded together and one or both of which act as a donor. Using a local density functional method to investigate the structure and dynamics of several di-carbon defects, we find that the dimer at an As site is bistable and aligned approximately in a [100] direction in the neutral charge state, and in a [110] direction when positively ionised. The calculated frequencies lie within 10% of the measured values in both materials. Other defects are investigated too with a view of determining the structures giving rise to the modes.

A theoretical consideration of resonant states in strained semiconductors is presented. Energy position, lifetime and wave functions of resonant states have been obtained for two approaches: (i) zero radius potential model and (ii) Dirac's approach for the consideration of Coulomb potential of acceptor impurities. The energy dependence of optical transitions between resonant and local states is studied. Numerical calculations are produced for uniaxially stressed Ge:Ga.

Energies of the ground state and of the excited states of an acceptor in a wire-like cylindrical heterostructure are calculated as a function of the cylinder radius R. When deriving the acceptor energy spectrum, the electrostatic potential for a cylindrical quantum wire is used. The interaction between a hole and the different kinds of phonons is taken into account. Resonant states of the bound hole polaron are studied.

Line narrowing has been observed in GaAs for the first time for transitions from the ground into resonant states of shallow donors near the point where the resonant state crosses the bottom of the nearest underlying Landau subband. The linewidth decreases in a very narrow energy interval of the order of the resonant state width. The narrowing is due to a reduction of the autoionization width of the resonant state. The decrease of the autoionization width is interpreted in terms of a model in which the resonant state is described by a wave packet.

The low-temperature mobility saturation found in a nondegenerate 3D electron gas at strong charged-impurity scattering was shown to be due to the existence of a quantum mobility edge. The negative magnetoresistance caused by magnetic field suppression of interference corrections to the conductivity was observed. The interference corrections were found to be small near the mobility edge.