Photovoltaic effect in the impurity absorption region of Si-structures with blocked impurity conductivity

A study is made of the field dependence of the photoconductivity in two-layer Si:Sb-and Si:Bstructures with blocked impurity-band conductivity and different thicknesses of the undoped (blocking) layer. The impurity concentration in the doped (active) layer was ≈10¹⁸ cm⁻³. Measurements were made at temperatures T=4–15 K for high (Φ∼10¹⁶ photons/cm² · s) and low (Φ<10¹⁴ photons/cm² · s) incident photon fluxes. A photovoltaic effect is observed in the Si:B structures with a thin (3 μm) blocking layer. It is found that a photovoltage develops for photons with energies exceeding the ionization energy of boron and its magnitude is independent of the photoexcitation intensity (for Φ>10¹³ photons/cm² · s) and, in the limit of low temperatures, it is close to the activation energy ɛ ₃ for jump conductivity in the active layer. The photovoltaic effect is explained by ballistic transit of the blocking layer by holes emitted from the contact which are then cooled in the active layer, as well as by the presence of a potential barrier ≈ɛ ₃ between the active and blocking layers. These factors are taken into account in a model for describing the major features of the dependence of the photovoltage on temperature and on the photon intensity and energy. Fiz. Tekh. Poluprovodn. 33, 456–463 (April 1999)     

Nature of the donor action of Gd impurity in crystals of lead and tin telluride

Electron spin resonance and the Hall effect are investigated in n-Pb_1−x Sn_xTe:Gd crystals grown from melt. It is found that there is no direct correlation between the free electron density and the density of the Gd³⁺ impurity in these crystals. The conclusion is drawn that the the electron conductivity of Pb_1−x Sn_xTe:Gd crystals is not caused by the Gd impurity but by intrinsic defects of the crystal lattice which have zero activation energy due to the Gd impurities. Fiz. Tekh. Poluprovodn. 32, 1331–1333 (November 1998)     

Effect of copper doping on kinetic coefficients and their anisotropy in PbSb<Subscript>2</Subscript>Te<Subscript>4</Subscript>

In anisotropic PbSb₂Te₄ and PbSb₂Te₄:Cu single crystals, nine main independent components of the Hall, electrical-conductivity, thermopower, and Nernst-Ettingshausen effects and their anisotropy in the range 77–450 K have been studied. PbSb₂Te₄ single crystals exhibit a high hole concentration (p ≈ 3 × 10²⁰ cm⁻³). Copper exhibits a donor effect and significantly (approximately by a factor of 2) reduces the hole concentration in PbSb₂Te₄. The temperature dependences of the kinetic coefficients, except for the Hall effect, have a form typical of the one-band model. The significant anisotropy of the Hall coefficient R ₁₂₃/R ₃₂₁ ≈ 2 at low temperatures corresponds to the multi-ellipsoid model of the energy spectrum of holes in PbSb₂Te₄. An important feature of the data on transport phenomena is the high thermopower anisotropy (ΔS ≈ 60–75 μV/K) in the mixed conductivity region caused by the mixed scattering mechanism. Data on the anisotropy of the transverse Nernst-Ettingshausen effect confirm the mixed mechanism of hole scattering; in the cleavage plane, scattering at acoustic phonons dominates, while in the trigonal axis direction, impurity scattering appears significant. Doping with copper enhances the role of impurity scattering in the direction of the trigonal axis c ₃; as a result, two components of the Nernst-Ettingshausen tensor Q ₃₂₁ and Q ₁₃₂ in the PbSb₂Te₄:Cu single crystal are positive at low temperatures, whereas, in the undoped crystal, only the Q ₃₂₁ component is positive.     

Effect of In doping on the kinetic coefficients in solid solutions of the system PbzSn1−z )0.95Ge0.05Te

In the quaternary solid solutions (Pb_zSn_1−z )_0.95Ge_0.05Te (z=0.35 and 0.40) the effect of addition of indium (in amounts of 5–20 at. %) on the temperature dependence of the electrical conductivity σ, Hall coefficient R, Seebeck coefficient S, and Hall mobility u is investigated on samples prepared using powder technology. We found a monotonic dependence of the hole density p on the indium content N _In with a tendency toward saturation at a level p _max≈3×10²¹ cm⁻³, an abrupt drop in the mobility in samples with p≈p _max, and changes in the character of the temperature dependences R(T) and σ(T). We show that these peculiarities in the behavior of the kinetic coefficients can be interpreted in terms of quasilocal indium impurity states against the background of the valence band spectrum (with energy ɛ _In∼0.3 eV) and resonance hole scattering into these states. Fiz. Tekh. Poluprovodn. 32, 1190–1193 (October 1998)     

Lorentz number and Hall factor in degenerate semiconductors during resonance scattering of charge carriers

In highly degenerate semiconductors, the Lorentz number L and Hall factor A _R differ significantly from universal constants π²/3 and 1, respectively, due to the pronounced energy dependence of the relaxation time during resonance scattering of carriers. The values of L and A _R are calculated for various values of the resonant impurity bandwidth, band filling with current carriers, and the relative contribution of resonance scattering. The available published data on the bandwidth of thallium impurity states in lead telluride, where intense resonance scattering of holes was observed, are discussed. The previously obtained data on the energy of Tl in PbTe resonant states were corrected taking into account the calculated Hall factor. An analysis of the experimental data on the dependence of the PbTe:Tl thermal conductivity on the temperature and content of additional Na dopant both showed that phonon scattering by polarized regions around charged impurity atoms is negligible and confirmed the results of theoretical calculations of the Lorentz number at dominant resonance scattering of holes.     

Doping of the Bi<Subscript>1.9</Subscript>Sb<Subscript>0.1</Subscript>Te<Subscript>3</Subscript> solid solution with Sn impurity

In (Bi_1.9Sb_0.1)_{1 − x} Sn _x Te₃ solid solution with different contents of Sn, the electrical conductivity (σ₁₁) and the Hall (R ₁₂₃ and R ₃₂₁), Seebeck (S ₁₁ and S ₃₃), and Nernst-Ettingshausen (Q ₁₂₃ and Q ₃₂₁) coefficients have been measured. It is shown that doping with tin strongly modifies temperature dependences of the kinetic coefficients. The effect of tin on electrical homogeneity of the samples has been studied: with increasing number of Sn atoms embedded, crystals become more homogeneous. These features indicate the presence of the quasi-local states of Sn in the valence band of Bi_1.9Sb_0.1Te₃. Within a one-band model, we estimated the effective mass of the density of hole states (m _d ), the energy gap extrapolated to 0 K (E _g0 = 0.20–0.25 eV), the energy of impurity states (E _Sn ≈ 40–45 meV), and the scattering parameter (r ≈ 0.1–0.4). Numerical values of the scattering parameter indicate a mixed mechanism of scattering in the samples under investigation with dominant scattering at acoustic phonons. With increasing content of tin in the samples, the contribution of impurity scattering increases.     

Self-compensation in PbSe:Tl thin films

Processes of defect formation are investigated in epitaxial PbSe:Tl films prepared by vacuum evaporation from molecular beams at various condensation temperatures from mixtures with a thallium content of 0–1.6 at. %. It is established that an increase in the content of the acceptor impurity in the film is accompanied by a significant increase in the number of selenium donor vacancies through the self-compensation mechanism. The thallium concentrations in the films are determined, along with the impurity transport coefficients, which vary from 0.82 to 0.44 as the condensation temperature varies from 250 °C to 350 °C. The carrier densities are calculated theoretically as a function of the thallium content in the films. The noticeable discrepancy between theory and experiment for thallium concentrations in the film N _Tl<0.3 at.% is attributed to the presence of growth-induced nonequilibrium donor defects in the sample, whose influence is taken into account by simply substituting their concentration into the electroneutrality equation. Estimates based on self-compensation theory lead to the conclusion that the films must be evaporated at T _K >400 °C to obtain films having a low carrier density. Fiz. Tekh. Poluprovodn. 33, 27–30 (January 1999)     

Influence of the intensity of gg irradiation on the photoluminescence of GaAs:Te

The influence of gg irradiation (⁶⁰Co) of various intensities (P _γ≈1.7−7.5kGR/h) on the photoluminescence of GaAs:Te single crystals [n ₀=(1.2–2.3×10¹⁸ cm⁻³] is investigated. Together with the known photoluminescence impurity bands (hν _max≈1.2 eV and/or hν _max≈1.35 eV) and edge band (hν _max≈1.51 eV), new bands are also observed in the spectra at hν _max≈1.3 eV and hν _max≈1.48 eV. The observed effects are attributed to radiation-stimulated ordering of the donor impurity and deep impurity centers. Fiz. Tekh. Poluprovodn. 32, 38–39 (January 1998)     

Determination of donor and acceptor impurity concentrations in n-InP and n-GaAs

The Hall mobility of semiconducting materials is generally computed without accounting for the Hall factor,r _H. It is shown that for low magnetic fields this may lead to considerable errors in the calculation of the impurity concentrations,N _D andN _A, in both n-GaAs and n-InP of reasonable purity. Tables allowing the determination ofN _D andN _A under low magnetic field conditions are computed at room and liquid nitrogen temperatures. They account forr _H and for the inelastic nature of collisions with polar optical phonons without assuming Matthiessen's rule.     

The mechanism of generation of the donor- and acceptor-type defects in the <Emphasis Type="Italic">n</Emphasis>-TiNiSn semiconductor heavily doped with Co impurity

Temperature dependences of resistivity, thermoelectric coefficient, and structural characteristics of the n-TiNiSn intermetallic semiconductor heavily doped with Co impurity (the Co concentration N ^Co ≈ 9.5 × 10¹⁹−1.9 × 10²¹ cm⁻³) have been studied in the temperature range 80–380 K; the distribution of the electron density of states is also calculated. It is shown that, in TiNi^{1 − x} Co _x Sn with x < 0.03, the impurity atoms occupy crystallographic sites of atoms Ti and Ni simultaneously and in various proportions and generate the donor and acceptor defects, respectively. It is established that there is a relation between the impurity concentration, the amplitude of a large-scale fluctuation, and also the degree of filling of the potential well of a small-scale fluctuation (fine structure). The results are discussed in the context of the Shklovskiĭ-éfros model of a heavily doped and compensated semiconductor.     

Frenkel’-Poole effect for boron impurity in silicon in strong warming electric fields

A method based on measurement of the thermally stimulated conductivity of a weakly compensated semiconductor, which is doped with a deep impurity and which contains an impurity component that is shallower than the main component, has been developed for investigating the Frenkel’-Poole effect. The results of an investigation of the thermally stimulated conductivity of Si:Ga samples with gallium density N _A =(2–3)×10¹⁸ cm⁻³ and low accompanying impurity content (⩽10¹³ cm⁻³) are reported. The conductivity was measured after extrinsic photoexcitation of samples heated at a rate β=0.6 K/s in the temperature range T=4.2–24 K in electric fields E=20–1000 V/cm. It is shown that the maximum on the curves of the thermally stimulated conductivity is due to the thermally stimulated emptying of the boron impurity and shifts to lower values of T as E increases. The decrease of the ionization energy of impurity B in an electric field, which turns out to be somewhat weaker than the field according to the Frenkel’-Poole model for singly charged Coulomb centers, is found from the shift of the maximum. Fiz. Tekh. Poluprovodn. 31, 777–780 (July 1997)     

Pressure-induced insulator-metal transition in electron-irradiated Pb1−x SnxSe (x⩽0.03) alloys

Galvanomagnetic effects (B⩽7 T) in electron-irradiated n-and p-type Pb_1−x Sn_xSe (x⩽0.03) alloys (T≈300 K, E=6 MeV, Φ⩽5.7×10¹⁷ cm⁻²) in the neighborhood of a pressure-induced insulator-metal transition (P⩽18 kbar) are discussed. The field dependences of the Hall coefficient calculated in terms of the two-band model are in satisfactory agreement with the experimental data, and the main parameters of the charge carriers in irradiated alloys are determined. It is shown that there is an increase in the hole concentration in the metallic phase under the action of pressure, associated with the motion of the energy bands at point L of the Brillouin zone, and that electrons overflow from the valence band into the band E _t1 of resonance states induced by electron irradiation; the parameters of this band are estimated. Fiz. Tekh. Poluprovodn. 32, 663–667 (June 1998)     

Fabrication of blocked impurity-band structures on gallium-doped silicon by plasma hydrogenation

The electrical characteristics of blocked impurity-band structures (BIB-structures) based on gallium-doped silicon (N _Ga≈5×10¹⁷ cm⁻³) are investigated. The blocking layers were formed by passivation of the gallium impurity by means of treatments in an rf-discharge hydrogen plasma at substrate temperatures T=20–220 °C. It is found that the activation energy E _a of the hopping conductivity with hopping between nearest gallium neighbors decreases from 8.7 meV (before hydrogenation) to 1.3 meV (after hydrogenation at T=220 °C). The current-voltage characteristics and temperature dependence of the dark current of the structures and their change after isochronal (t=20 min) annealing at temperatures T=220–400 °C are determined. The current-voltage characteristics of the structures at low temperatures are calculated. The results of calculations are found to agree with experimental data. Fiz. Tekh. Poluprovodn. 31, 311–317 (March 1997)     

Dependence of electrical properties of polycrystalline cvd si films on grain size and impurity doping concentrationa,b

The electrical properties of sets of simultaneously grown p-type polycrystalline Si films, deposited by SiH₄ pyrolysis on polycrystalline high-purity alumina substrates and B-doped during growth, were determined by Hall-effect measurements in the temperature range 77-420K as functions both of impurity doping concentration N (~10^l5 to ~10²⁰cm⁻³) and average grain size (≈1 to ≈125μm) in the film. Room temperature data showed rapidly increasing resistivities and rapidly decreasing free-carrier concentrations for doping below a critical concentration N_m and distinct mobility minima at that concentration, with the value of N_m being larger the smaller the average grain size. Measurements as a function of sample temperature showed the intergrain barrier height E_b, decreasing from a maximum value of ~0.4eV at the critical concentration to very small values (~0.01eV) for concentrations above 10¹⁹cm⁻³, with a functional dependence close to E_b ∝l/N^1/2 and E_b for any given concentration being larger the smaller the average grain size. Results are interpreted in terms of the grain-boundary trapping model. Trapped carrier densities in the grain boundaries were calculated to range from ~5×l0¹¹cm⁻² at N≈N_m to ~5×l0¹²cm⁻² for N>10¹⁹cm⁻³, the density being higher the smaller the grain size, and evidence was found for an energy distribution of traps in the Si bandgap, rather than a fixed density at a single discrete energy level. The observed relationship between N_m and average grain size nearly coincides with that of the model for films with ~lμm grain size but sharply departs from it for larger grain sizes, indicating probable applicability of the model for grain sizes up to that range. ^aThis work was supported by the U.S. Department of Energythrough its San Francisco Operations Office under Contract DE-AC03-79ET23045 and monitored by the Solar Energy Research Institute, Golden, CO. bThese results were first described at the 22nd Electronic Materials Conference, Ithaca, NY, June 21–27, 1980, Paper No. M4.     

Tin impurity centers in glassy germanium chalcogenides

Tin atoms produced by radioactive decay of ^119mm Sn and ¹¹⁹Sn impurity atoms in the structure of Ge _x S_{1 − x} and Ge _x Se_{1 − x} glasses are stabilized in the form of Sn²⁺ and Sn⁴⁺ ions and correspond to ionized states of the amphoteric two-electron center with negative correlation energy (Sn²⁺ is an ionized acceptor, and Sn⁴⁺ is an ionized donor), whereas the neutral state of the Sn³⁺ center appears to be unstable. ¹¹⁹Sn atoms produced by radioactive decay of ^119m Te impurity atoms in the structure of Ge _x S_{1 − x} and Ge _x Se_{1 − x} glasses are stabilized at both chalcogen sites (they are electrically inactive) and germanium sites.     

Features of vanadium impurity states in lead telluride

Temperature dependences of resistivity, magnetic susceptibility, and charge-carrier concentration and mobility in single-crystalline PbTe:V samples with a varied impurity content are investigated. It is shown that vanadium forms a donor level located ∼20 meV below the conduction-band bottom. The electron mobility is as high as 10⁵ cm² V⁻¹ s⁻¹ in the samples with N _V ≤ 0.21 at % and proves to be more than an order of magnitude higher in the samples with the highest vanadium content N _V = 0.26 at %. In the same samples, the real part of the conductivity is characterized by a pronounced frequency dependence. An increase in vanadium concentration is accompanied by a decrease in the effective magnetic moment of impurity atoms. The features of electron transport in PbTe:V may be caused by a variable vanadium valence and by the effect of interimpurity correlations.     

Manifestation of light and heavy electrons in the galvanomagnetic characteristics of Te-doped <Emphasis Type="Italic">n</Emphasis>-Bi<Subscript>0.88</Subscript>Sb<Subscript>0.12</Subscript> single crystals

The components of resistivity (ρ _ij ), Hall coefficient (R _ijk ), and magnetoresistance (ρ _{ij, kl} ) of n-Bi_0.88Sb_0.12 single crystals doped with tellurium to 0.01, 0.1, and 0.2 at % have been measured in the temperature range of 77–300 K. It is concluded that light and heavy electrons are involved in transport processes. The energy spacing between the bands of light and heavy electrons is found to be 40 meV, and the ratios of the effective masses and electron mobilities are estimated as m ₂^*/m _l ^* = 3 and b ≈ 0.16, respectively.     

Oxygen and Erbium related donor centers in Czochralski grown silicon implanted with erbium

The Hall effect measurements were conducted on Czochralski-grown silicon after implantation of erbium and two-step annealing at 700 °C and 900 °C. After the first step the formation of oxygen-related shallow donors was observed at E _c in the range 20–40 meV and erbium-related donor centers at ≈E _c -70 meV and ≈E _c -120 meV. Along with the same oxygen-related shallow thermal donors and donor centers at ≈E _c -70 meV, other donor centers at ≈E _c -150 meV are formed following the 900°C anneal, instead of those at ≈E _c -120 meV. The new donor states are of particular interest because of their possible involvement in the photoluminescence process. The obtained results for erbium-implanted silicon are compared with some fragmentary DLTS data found in the current literature on the donors with ionization energies less than 0.2 eV. Fiz. Tekh. Poluprovodn. 33, 1192–1195 (October 1999)     

Properties of Bi-doped PbTe layers grown by liquid phase epitaxy under controlled Te vapor pressure

Effects of Bi doping in PbTe liquid-phase epitaxial layers grown by the temperature difference method under controlled vapor pressure (TDM-CVP) are investigated. For Bi concentrations in the solution, x_Bi, lower than 0.2 at.%, an excess deep-donor level (activation energy E_d≈0.03–0.04 eV) appears, and Hall mobility is low. In contrast, for x_Bi>0.2 at.%, Hall mobility becomes very high, while carrier concentration is in the range of 10¹⁷ cm⁻³. Inductive coupled plasma (ICP) emission analysis shows that, for x_Bi=1 at.%, Bi concentration in the epitaxial layer is as high as N_Bi=2.3–2.7 × 10¹⁹ cm⁻³. These results indicate that Bi behaves not only as a donor but also as an acceptor, and the nearest neighbor or very near donor-acceptor (D-A) pairs are formed, so that strong self-compensation of Bi takes place. Carrier concentration for highly Bi-doped layers shows a minimum at a Te vapor pressure of 2.2 × 10⁻⁵ torr for growth temperature 470°C, which is coincident with that of the undoped PbTe.