Bistable amphoteric centers with reverse order of electron levels in semiconductors

The effect of bistability of amphoteric centers with negative correlation energy on the free-carrier concentration is studied. It is found that, in the absence of doping compensation, in a certain temperature range, the free-carrier concentration does not depend on the concentration of bistable amphoteric U ^? centers. The conductivity activation energy and the Fermi level position in the band gap of the semiconductor are determined not only by the electronic characteristics of the centers, such as the center ionization energy, but also by the parameters of transitions between the two equilibrium states of the center. The results of the simulation of the temperature dependence of the free-carrier concentration in a semiconductor with bistable amphoteric U ^? centers are presented for different levels of compensations. It is shown that the observed activation energy of the dark conductivity depends on the compensation degree.     

Specific features of the generation-recombination properties of bistable defects in semiconductors: Manifestation in Hoffmann’s function

Features of the temperature dependence of the concentration of majority free charge carriers in semiconductors on temperature-dependent discharging of bistable centers differing in electrical activity are treated theoretically. Specifically, two-level acceptors, two-level donors, and amphoteric centers are considered. The effects of the level of compensation and the ratio between the binding energies of one and two electrons at a center upon the discharging are studied. The results are represented as features of the behavior of Hoffmann’s function.     

Effect of the initial doping level on changes in the free-carrier concentration in porous silicon during ammonia adsorption

Infrared spectroscopy is used to investigate the effect of ammonia adsorption on the concentration of equilibrium charge carriers in porous-silicon layers with various initial types of dopants at different concentrations. It is found that ammonia adsorption results in an increase in the number of free electrons in n-type samples up to a level exceeding 10¹⁸ cm^?3. In p-type samples, a nonmonotonic dependence of the charge-carrier concentration on ammonia pressure is observed. The obtained results are accounted for by the appearance of adsorption-induced shallow donor states that, along with the initial-dopant and surface-defect states, specify the charge-carrier type and concentration in the silicon nanocrystals of the porous layer after ammonia adsorption.     

Determination of the energy and concentration of amphoteric defects by differential processing of the temperature dependence of the concentration of free charge carriers

The use of the differential processing of the temperature dependence of the concentration of free charge carriers is analyzed for semiconductors containing amphoteric centers with positive and negative correlation energies under different conditions of compensation. It is found that, in the differential characteristic, the height and position of the peak corresponding to the acceptor state depend on the degree of compensation. This situation can be misinterpreted as a result of changes in the concentration and energy spectrum of the centers in the band gap of the semiconductor or as a result of the formation of new defects, as dictated by the conditions of compensation.     

Conductivity of Se95As5 chalcogenide glassy semiconductor layers containing the EuF3 rare-earth impurity in high electric fields

By investigating the I?CV characteristics of an Al-Se₉₅As₅:EuF₃-Te structure, it is established that, upon the application of a positive potential to Te, the current flows in it by the mechanism of space-charge-limited currents (SCLCs) of unipolar-injection, and the N-type I-V characteristic is observed for the opposite polarity. It is shown that these are processes of the thermal-field ionization of neutral and negatively charged U ^? centers, which play a dominant role in the current-flow mechanism in the investigated structures upon the application of an electric field with intensities exceeding 10⁵ V/cm. These are also processes of the electron-hole recombination and capture of charge carriers at U ^? centers. The energy position and concentration of the local states, which correspond to the indicated centers and characterize the effect of the electric field??activation length are determined. It is established that it is the EuF₃ impurities that predominantly affect the local-state concentration.     

Photoelectric C-V profiling of majority charge carriers and effective lifetimes of minority charge carriers in gettered GaAs wafers

The procedure for photoelectrochemical C-V profiling of the concentration of majority charge carriers and effective lifetimes of minority charge carriers in high-resistivity thick (1.6 mm) GaAs wafers subsequently to their gettering is described. Gettering was performed by both one-side and two-side coating of the wafers with a Y film and subsequent thermal treatment at 700 and 800°C. It was demonstrated that the concentration profile N _d-N _a and the effective lifetime for minority charge carriers throughout the wafer are rather uniform in both cases. This procedure makes it possible to measure the charge carrier concentration as low as 10¹² cm^?3.     

Modification of the properties of porous silicon on adsorption of iodine molecules

Infrared spectroscopy and electron spin resonance measurements are used to study the properties of porous silicon layers on adsorption of the I₂ iodine molecules. The layers are formed on the p-an n-Si single-crystal wafers. It is established that, in the atmosphere of I₂ molecules, the charge-carrier concentration in the layers produced on the p-type wafers can be noticeably increased: the concentration of holes can attain values on the order of ～10¹⁸?10¹⁹ cm^?3. In porous silicon layers formed on the n-type wafers, the adsorption-induced inversion of the type of charge carriers and the partial substitution of silicon-hydrogen bonds by silicon-iodine bonds are observed. A decrease in the concentration of surface paramagnetic defects, P _b centers, is observed in the samples with adsorbed iodine. The experimental data are interpreted in the context of the model in which it is assumed that both deep and shallow acceptor states are formed at the surface of silicon nanocrystals upon the adsorption of I₂ molecules.     

Effect of the levels of intrinsic defects in the CdP2 band gap on electrical characteristics of corresponding structures with the Schottky barrier

The electrical characteristics of Schottky barriers formed on n-type cadmium diphosphide are studied. It is established that the space-charge region at the metal-semiconductor interface represents in fact a Schottky layer formed owing to a high concentration of deep-level centers. The charge transport in the conducting direction for these structures is related to the above-barrier emission of electrons and is consistent with the diffusion theory for one or two types of charge carriers. The high concentration of ionized centers in the space-charge region gives rise to the tunneling mechanism of breakdown in the blocking direction. The frequency dependences of the complex conductance are governed by the exchange of charge carriers between the conduction band and donors that specify the conductivity type of the material and also by the recharing of the centers with a large depth of levels. Good agreement between the reported results and the theory is obtained.     

Mechanism of reverse current in the Al/p-InP schottky diodes

Reverse current-voltage characteristics of the Al/p-InP Schottky diodes based on Zn-doped InP epilayers were measured in relation to bias and temperature. Temperature dependence of reverse current is characterized by the activation energies of 0.75 and 0.51 eV in the high-temperature region and at temperatures T<280 K, respectively. Results are explained by the phonon-assisted tunneling generation of charge carriers from the surface states of a semiconductor with regard to the Frenkel emission mechanism. It is found that, in the low-temperature region, tunneling occurs via the centers with energy levels of 0.51 eV. Comparing experimental results with theory, we estimated electric-field strength in the barrier at (5–13)×10⁷ V/m and the surface density of the hole charge in the boundary layer of the semiconductor.     

Spin-lattice relaxation of <Superscript>113</Superscript>Cd and <Superscript>19</Superscript>F nuclear spins in the crystal lattice of CdF<Subscript>2</Subscript> semiconductor crystals with <Emphasis Type="Italic">DX</Emphasis> centers

Temperature dependences of spin-lattice relaxation rates of the ¹¹³Cd and ¹⁹F lattice nuclei in the CdF₂ semiconductor crystals containing bistable In and Ga impurity centers show that the relaxation mechanisms in the CdF₂:In and CdF₂:Ga crystals are different. The basic mechanism of spin-lattice relaxation of the ¹¹³Cd nuclei in the CdF₂:In crystal is the scalar contact’s interaction of nuclear spins with spins of mobile charge carriers of the conduction band. In the CdF₂:Ga crystal, relaxation of the ¹¹³Cd nuclei is controlled by the contact interaction with electrons moving within a narrow band of impurity states. The same mechanism is apparently responsible for relaxation of the ¹⁹F nuclei in this crystal. In the CdF₂:In crystal, the ¹⁹F nuclei relax by the dipole-dipole interaction with electron spins localized at the hydrogen-like orbits of shallow donors.     

Low-temperature anomalies of the photoelectromagnetic effect in p-Cd x Hg1 − x Te due to recharging of surface states

A number of anomalies inexplicable in the context of available theoretical models are observed in the photoelectromagnetic effect in p-Cd _x Hg_{1 ? x} Te crystals. A general model of the photoelectromagnetic effect in semiconductors with a large ratio between the mobilities of electrons and holes is developed, taking into account the influence of space charge. The model allows the interpretation of all experimentally observed anomalies of the photoelectromagnetic effect, including the double sign inversion in magnetic field. Comparison of the theory with the experiment makes it possible to determine the parameters of the material. It is suggested that the space charge responsible for the anomalies of the photoelectromagnetic effect is associated with a high concentration of specific surface states (～10¹³ cm^?3) recharged when trapping nonequilibrium charge carriers.     

Special features of the sublimational molecular-beam epitaxy of Si and its potentialities for growing Si:Er/Si structures

The concentration of charge carriers and their Hall mobility in Si:Er/Si layers grown by sublimational molecular-beam epitaxy were investigated as functions of temperature in the range of 300–77 K. No electric activity of Er-containing luminescent centers was observed. The feasibility of precise control over impurity profiles in growing the p ⁺-n-n ⁺ electroluminescent structures is demonstrated.     

Parameters of ZnO films with <Emphasis Type="Italic">p</Emphasis>-type conductivity deposited by high-frequency magnetron sputtering

It is demonstrated that a reduction in the defect concentration in ZnO films formed by high-frequency magnetron sputtering allows effective doping with acceptor impurities both in the cation (Li) and anion (N⁺) sublattices and p-type conductivity with reproducible charge-carrier parameters (concentration and mobility) to be obtained. ZnO films are doped with nitrogen by annealing in a high-frequency gas discharge atmosphere. Hall measurements by the Van der Pauw technique show that the deposition of thin Eu layers on the ZnO film surface increases the concentration and mobility of majority charge carriers. The embedding of metal impurities with different ionic radii (Ag and Au) in the cation sublattice of the ZnO films to compensate misfit stresses makes it possible to enhance the concentration of radiative-recombination centers.     

Two-electron tin centers formed in lead chalcogenides as a result of nuclear transmutations

It is shown that the emission [^119mmSn(^119mSn)] and absorption (¹¹⁹Sn) Mössbauer spectra of impurity tin atoms in lead chalcogenides are identical. It is assumed that the mechanism of emergence of multi-charged tin ions as a result of conversion isomeric transition in the parent ^119mmSn atom is valid only for free atoms, whereas the final charge state of a stabilized daughter atom in condensed media with a high concentration of charge carriers is the charge state of the parent atom. The large value of recoil energy for ¹¹⁹Sb atoms after radioactive decay of parent ^119mTe atoms gives rise to a significant number of displaced ^119mSn atoms; however, a greater fraction of ^119mSn atoms after the ^119mTe → ¹¹⁹Sb → ^119mSn decay remains in the anionic sublattice and are antisite defects. In contrast to tin atoms in the cationic sublattice, the ^119mSn antisite defects are found to be electrically inactive centers.     

The charge accumulation in an insulator and the states at interfaces of silicon-on-insulator structures as a result of irradiation with electrons and gamma-ray photons

The accumulation of charge in an insulator and the states at interfaces in silicon-on-insulator structures irradiated with 2.5-MeV electrons and 662-keV gamma-ray photons were studied. It was found that an additional positive charge appears in the buried insulator of the structures as a result of irradiation. The concentration of hole traps generated by radiation in the oxide is higher at the boundary with substrate than at the bonding interface between a split-off silicon layer and oxide. It is shown that the presence of even a weak built-in field in the structures (F?5×10³ V/cm) gives rise to efficient separation of charge carriers. There is no generation of additional states at the Si/SiO₂ interfaces in the silicon-on-insulator structures for both irradiation types, although this generation is observed in the initial thermal oxide.     

Negative capacitance (impedance of the inductive type) of silicon p +-n junctions irradiated with fast electrons

Silicon p ⁺-n junction diodes irradiated with 3.5-MeV electrons (with the dose of 4 × 10¹⁶ cm^?2) are studied. The diodes’ inductance (L) was measured at a frequency f = 1 MHz with the amplitude of alternating current equal to 0.25 mA. Simultaneously with measurements of L at alternating current, a direct current was passed through the forward-biased diode, which brought about the injection of minority charge carriers into the base. In order to identify both of the mechanisms that give rise to the inductive-type impedance in irradiated diodes with the p ⁺-n junction and the main radiation defects that are directly involved in the formation of this impedance, irradiated samples were annealed isochronously in the temperature range T _a = 225–375°C with sub-sequent study of the main characteristics of the defects by deep-level transient spectroscopy. It is shown that the inductive-type impedance in irradiated diodes is caused by the processes of capture and retention of charge carriers injected into the base at the trapping centers for a time ～1/2f, i.e., for a half-period of oscillations. It is also shown that the trapping centers are the vacancy-oxygen complexes introduced by irradiation with electrons.     

The effect of the concentration of the majority charge carriers and irradiation intensity on the efficiency of radiation-defect production in <Emphasis Type="Italic">n</Emphasis>-Si crystals

The effect of the concentration of the majority charge carriers (n) and the electron-flux density (?) on the efficiency of radiation-defect production (η) in n-Si samples was studied. It is shown that the dependence η(?) features a maximum, which shifts to larger values of ? as n increases. This effect is explained by assuming that there is an optimal relation between the concentrations of primary defects produced per unit time and those of the free charge carriers, which charge these defects.     

Electron levels and luminescence of dislocation networks formed by the hydrophilic bonding of silicon wafers

Local energy levels produced by dislocations at the interface between bonded n- and p-Si wafers are studied by deep level transient spectroscopy and by a new technique for the detection of impurity luminescence, induced by the occupation of electron states upon the application of electric pulses (the pulsed trap-refilling-enhanced luminescence technique). It is established that only the shallow levels of the dislocation network, with activation energies of about 0.1 eV, are responsible for the D1 dislocation-related luminescence band in both n- and p-type samples. The occupation of deep levels has no effect on the D1-band intensity. A model of coupled neutral trapping centers for charge carriers is proposed. In this model, the difference between the energy position of the D1 band (0.8 eV) and the corresponding interlevel energy spacing (0.97 eV) is attributed to the Coulomb interaction between charge carriers trapped at the levels.     

Scattering of conduction electrons at a spatially correlated system of charges in a heavily doped GaAs: Te semiconductor

The results of studying the absorption of infrared radiation by free charge carriers in the GaAs:Te single crystals, grown by the Czochralski method, had the electron concentration n ₀=5×10¹⁷?6×10¹⁸ cm^?3 are reported. An analysis of the spectral dependences of the absorption coefficient took into account the spatial correlation in the impurity-charge distribution. It is shown that the short-range correlation model makes it possible to account for the decrease in the absorption coefficient and a weakening of its spectral dependence, in the region of the impurity-mediated free-carrier absorption.     

Vacancy-donor pairs and their formation in irradiated n-Si

The present experiments using fast electron and gamma-irradiation are aimed at checking the model of defect formation in oxygen-lean n-Si(FZ) in a quantitative way. Electrical measurements are taken over a wide temperature range of 20 to 300 K. Analysis of equations of charge balance making use of the statistics of charge carriers in non-degenerate semiconductors demonstrates that group-V impurity atoms strongly interact with intrinsic point defects. As a result, the concentration of shallow donor states is markedly decreased. This loss of shallow donors, ?δN _D , is accompanied with an increase in the concentration of radiation-produced deep acceptors, +δN _A ^rad , being equal in magnitude but opposite in sign. Such behavior correlates quantitatively with the formation model of donor-vacancy pairs put forward earlier by Watkins and Corbett, what has been proved on the basis of electrical data for the first time. The formation kinetics of these complexes is discussed. Defects of interstitial type in irradiated material appear to be electrically neutral in n-Si. However, their production in the course of electron- and gamma-irradiation is believed to be responsible for drastic changes in the mobility of charge carriers at cryogenic temperatures.