Electron scattering and transport phenomena in small-gap zinc-blende semiconductors

We give expressions for electrical conductivity, thermoelectric power and thermal conductivity of conduction band electrons in small-gap zinc-blende semiconductors, obtained by solving the Boltzmann equation by a variational procedure. The term resulting from the phonon-drag is included in the Boltzmann equation. The following electron scattering mechanisms are investigated: inter and intraband scattering by optical phonons via polar and nonpolar interactions, scattering by charged centers (ionized defects and heavy holes) and by neutral centers, as well as scattering by acoustic phonons. Particular attention is paid to the screening of the electron-optical phonon polar interaction by free carriers, which is particularly important in the case of a linear energy band. The formula for the intraband RPA dielectric function for the case of the linear band is given.The general formulation of all the problems investigated permits direct application of the results given in this paper to both intrinsic or n-type HgTe-type and InSb-type semiconductors, including mixed crystals, e.g. Cd_xHg_1?xSe near the cross point.     

Transport properties of photo-excited carriers in slightly compensated Hg0.785Cd0.215Te

Photo-Hall measurements are presented for slightly compensated n-type Hg_0.785Cd_0.215Te at 4 K where the electron mobility is limited by charged scattering centers. Using a 10.6 μm laser as the optical source, we observe that the electron mobility passes through a maximum as a function of photo-excited carrier density. Mobility expressions based on the Kane-band model are modified to incorporate neutralization of charged scattering centers by photo-excited carriers. Calculated mobility values are found to be in satisfactory agreement with the data. The observed decrease in mobility for high carrier densities is attributed to electron-hole scattering. The mobility enhancement at lower carrier densities is explained in terms of neutralization of charged acceptors.     

Electronic properties of epitaxial 6H silicon carbide

The electrical conductivity and Hall coefficient were measured in the temperature range from 78 to 900 K for n-type epitaxially grown 6H silicon carbide. A many-valley model of the conduction band was used in the analysis of electron concentration as a function of temperature. From this analysis, the density of states mass to the free electron mass ratio per ellipsoid was calculated to be 0.45. It was estimated that the constant energy surface of the conduction band consists of three ellipsoids. The ionization energy of the shallowest nitrogen donor was found to be 105 meV, when the valley-orbit interaction was taken into account. The electron scattering mechanisms in the epitaxial layers were analyzed and it was shown that the dominant mechanism limiting electron mobility at high temperatures is inter-valley scattering and at low temperatures (200 K), impurity and space charge scattering. A value of $\text{360}\text{cm}{}^2\text{V}$ sec was calculated for the maximum room temperature Hall mobility expected for electrons in pure 6H SiC. The effect of epitaxial growth temperature on room temperature Hall mobility was also investigated.     

Electron mobility and electron scattering in CdxHg1−xSe mixed crystals

We report a detailed study, both experimental and theoretical, of electron mobility and Hall coefficient in small-gap Cd_xHg_1?xSe mixed crystals. The electron mobility is calculated by the variational technique. The results obtained with no adjustable parameter are within 15% of the experimental values in the range of temperature 4.2–300 K, electron concentrations 7 × 10¹⁶?5 × 10¹⁸cm^?3 and composition 0 < x ? 0.25.The scattering of electrons by charged centres, optical phonons (polar and nonpolar interaction), acoustic phonons as well as disorder (alloy) scattering is taken into account. It is shown that the composition-dependent dielectric function and the band edge symmetry play an important role in the explanation of the experimental results.     

Light scattering from non-equilibrium electron excitations in semiconductors

Light scattering from non equilibrium single-particle electron excitations in direct gap n-doped semiconductors is considered. It is shown that, in quasi-equilibrium conditions, deviations from the Maxwellian profile of the scattering spectrum of hot electrons can be accounted for by exchange and correlation effects.     

Indirect interactions between localized magnetic moments in intrinsic semiconductors

The indirect interaction between localized magnetic moments in insulators and semiconductors is investigated at T = 0°K. This interaction arises from virtual excitations of the valence electrons to the conduction band and is oscillatory as a function of distance. The passage from the case of small gap semiconductors to insulators is studied in detail, as a function of the energy gap and the effective masses. The Bloembergen-Rowland interaction is derived as a limiting case for insulators. Possible applications are also discussed.     

An infra-red study of defects produced in n-type silicon by electron irradiation at low temperatures

The infra-red local mode absorption produced by irradiation of n-type silicon by 2 MeV electrons at temperatures in the range 100–140°K has been investigated. A new band at 884 cm^-1 has been observed and interpreted as due to a vacancy— oxygen complex (A-centre) with a trapped electron.     

Temperature dependent energy relaxation time in AlGaN/AlN/GaN heterostructures

The two-dimensional (2D) electron energy relaxation in Al_0.25Ga_0.75N/AlN/GaN heterostructures was investigated experimentally by using two experimental techniques; Shubnikov-de Haas (SdH) effect and classical Hall Effect. The electron temperature (T_e) of hot electrons was obtained from the lattice temperature (T_L) and the applied electric field dependencies of the amplitude of SdH oscillations and Hall mobility. The experimental results for the electron temperature dependence of power loss are also compared with the current theoretical models for power loss in 2D semiconductors. The power loss that was determined from the SdH measurements indicates that the energy relaxation of electrons is due to acoustic phonon emission via unscreened piezoelectric interaction. In addition, the power loss from the electrons obtained from Hall mobility for electron temperatures in the range T_e > 100 K is associated with optical phonon emission. The temperature dependent energy relaxation time in Al_0.25Ga_0.75N/AlN/GaN heterostructures that was determined from the power loss data indicates that hot electrons relax spontaneously with MHz to THz emission with increasing temperatures.     

The metal insulator transition in two-dimensional systems with charged impurities

The motion of interacting electrons in an array of charged impurities in two dimensions is discussed within a generalized selfconsistent current relaxation theory. The ratio of impurity density n_i to electron density n_c, where the metal insulator transition takes place, is shown to depend on n_c and varies between 0.2 and 10. A comparison of the theory with mobility measurements of Na⁺ drift experiments in inversion layers is made.     

Hot electron faraday-effect and non-equivalent inter-valley transfer in n-type Ge

I.R. Faraday rotation measurements have been performed on n-type Ge at 200K and 300K up to electric field strength of $\text{5.5kV}\text{cm}$. The decrease in rotation is interpreted in terms of an increase in average effective mass of the electrons. Both, nonparabolicity of the conduction band and transfer of electrons to satellite valleys have to be considered for the interpretation of the results.     

Experimental confirmation by galvanomagnetic methods of a complex transport model in In0.53Ga0.47As layers deposited by MBE on SI-InP

At low temperatures In_0.53Ga_0.47As samples show an increase of carrier concentration, which can be explained in terms of a two carriers transport model. This type of problem exists since the beginning of the semiconductor era, dating back to monocrystalline germanium.We propose that in all the investigated layers, there are X atoms or charged dislocations in the region of the first monolayers, which are built in during epitaxial growth. The layers were intentionally undoped. They form an impurity band in which low mobility carriers dominate over the localised electron scattering due to the s-d exchange interaction. These carriers do not freeze out at liquid helium temperature and give rise to two transport media for electrons; a conduction band at higher temperatures and an impurity band at lower temperatures. The electron which fall down onto the previously ionised X atoms, then move by thermally activated hopping. We show that the two carriers model for In_0.53Ga_0.47As epitaxial layers are confirmed by the carrier concentration-temperature, carrier concentration-magnetic field, resistivity-magnetic field behaviour, and also by YKA theory also. The differences between the two transport models are so distinctive that observed phenomena may exist. This paper presents experimental results, which constitute comprehensive evidence for the complicated structure of the semiconductor epitaxial layers on the sample of n-type In_0.53Ga_0.47As/InP layer with n=2.2×10¹⁵/cm³.     

Enhancement of the electron mobility with infrared photoexcitation in Si: Te at low temperatures

Hall measurements on Te-doped silicon (N _Te 10¹⁶ cm^–3) have been performed in the temperature range between 10 K and 300 K with infrared photoexcitation of electrons into the conduction band. The samples exhibit electron Hall mobilities which are increased by approximately 50% compared to measurements in the dark. The increased electron mobility can be correlated with an increased electron population of shallow donor levels by photoexcitation. Coulomb scattering due to charged shallow donor centers is converted into less efficient dipole scattering (Te-acceptor pairs) by the light-induced redistribution of electrons.     

Galvanomagnetic properties of n-type Hg0.8Cd0.2Te

Galvanomagnetic properties of low and high mobility n-Hg_0.8Cd_0.2Te are reported. The experiments were carried out in magnetic fields up to 60 kG and between 1.8 and 77 K. The Hall coefficient does not show thermal and magnetic freeze-out of carriers. At 77 K the transversal magnetoresistance shows a proportionality ?⊥ ∝ B as was predicted by Gurevich and Firsov for the case of polar optical scattering in non-degenerated semiconductors. At 4 K where the mobility is governed by impurity scattering ?⊥ ∝ B^2.4 was observed in the extreme quantum limit. A negative longitudinal magnetoresistance was found at 77 K. The experimental results of high and low mobility samples show significant differences.     

Photoluminescence of heavily doped n-InP

Photoluminescence and excitation spectra of weakly compensated n-type InP layers doped with tin in the density range from 3 × 10¹⁷ cm^-3 to 2 × 10¹⁹ cm^-3 are measured at T=2 K in order to get experimental informations about the influence of doping on important material parameters such as the band gap energy and the position of the quasi-Fermi level of electrons. The results derived from these investigations are compared with those obtained of relevant many-body theories to heavily doped semiconductors. Using RPA and Klauder's best (fifth) approximation the doping induced gap shrinkage of uncompensated n-InP is calculated. The comparison between theory and experiment yields the conclusion that for standard direct gap AIII-BV compounds such as InP and GaAs the compensation in principle expected has to be taken into the theoretical considerations. Moreover, we first present a “semianalytical” treatment of Klauder's multiple scattering approach which makes this theory easily applicable to other problems as for instance calculations of the luminescence line-shape of doped semiconductors.     

Galvanomagnetic effects in n-type gallium antimonide

This paper reports the theoretical and experimental study performed on Hall mobility and free carrier Faraday rotation in a degenerate n-GaSb sample in the temperature range 77–300K. Following relaxation time approximation the mobility is estimated separately for the Γ- and L-valley taking into account the scattering of electrons due to ionised impurities, space charge, polar optical phonons, deformation potential, intervalley acoustic and optical phonons. The effective mobility is calculated considering a two valley model, degeneracy and non-parabolicity of the Γ-valley, and compared with these experimental results. Microwave Faraday rotation data at 77 and 300K is analysed generalising the d.c. magneto-conductivity tensor components as derived by Bordure and Savelli to the high frequency, and is used to confirm the scattering mechanisms and band parameters used in the analysis of d.c. galvanomagnetic results.     

Features of the electron mobility variance in homeopolar semiconductors

Statistical features of the mobility fluctuation of current carriers (electrons) in nondegenerated semiconductors are studied. An analytical expression is obtained for the mobility variance of conduction electrons, conditioned by random changes of the energy level occupation in the conduction band. Within the framework of the developed theory, the temperature dependence of the mobility variance of conduction electrons in homeopolar semiconductors is considered.     

Effects of adiabaticity of electrons and ions on dust-ion-acoustic solitary waves

The nonlinear propagation of dust-ion-acoustic (DIA) waves in an adiabatic dusty plasma (containing adiabatic inertial-less electrons, adiabatic inertial ions, and negatively charged static dust) is investigated by the pseudo-potential approach. The combined effects of adiabatic electrons and negatively charged static dust on the basic properties (critical Mach number, amplitude, and width) of small as well as arbitrary amplitude DIA solitary waves are explicitly examined. It is found that the combined effects of adiabatic electrons and negatively charged static dust significantly modify the basic properties (critical Mach number, amplitude, and width) of the DIA solitary waves. It is also found that due to the effect of adiabaticity of electrons, negative DIA solitary waves [which are found to exist in a dusty plasma (containing isothermal electrons, cold ions, and negatively charged static dust) for α=zdnd0/ni0>2/3, where zd is the number of electrons residing onto a dust grain surface, nd0 is the constant (static) dust number density and ni0 is the equilibrium ion number density] disappears, i.e. due to the effect of adiabatic electrons, one cannot have negative DIA solitary waves for any possible set of dusty plasma parameters [0?α<1 and 0?σ=Ti0/Te0?1, where Ti0 (Te0) is electron (ion) temperature at equilibrium].     

Cyclotron resonance studies on bulk and two-dimensional conduction electrons in InSe

Cyclotron resonance is reported for both bulk electrons and electrons bound to charged defect planes in the layer compound InSe. At temperatures below 20 K all carriers present are bound to defect planes and behave as two-dimensional accumulation layers. At higher temperatures the electrons are excited into bulk regions between the defects, and show three-dimensional, bulk behaviour. The conduction band is shown to exhibit an “anomalous anisotropy” with m_∥ = 0.08m₀ and m_⊥ = 0.14m₀. The bound, two-dimensional carriers exhibit a strong non-parabolicity, and show a band edge mass m_⊥ = 0.13 which is considerably lower than the bulk value possibly due to a reduction in the polaron constant in the degenerate electron gas.     

Simplified model of dislocations as a SRH recombination channel in the HgCdTe heterostructures

A simple model of dislocation band formed by the dangling bonds of atoms of a dislocation core has been presented and discussed. The parameters of this model, which could be verified experimentally, are the average energy of the dislocation band states and the average length of the dislocation as well as electron and hole emission coefficients. The formulas for statistical functions of distribution of electrons in these bands have been derived. Next, we have developed a model of the SRH recombination channel connected with dislocation band states and we have adopted it to determine an effective lifetime of electron-hole pairs including effect of dislocations. In addition, influence of the tunnelling current from and into dislocation band has been considered, which seems to be a serious issue in reverse biased heterostructue HgCdTe photodiodes. Exemplary results of calculations for HgCdTe structures show that the number of the ionized atoms of the dislocation cores is of the order of a few percent. Moreover, the electric potential distributions in the area of the dislocation core has been calculated. Some experimental I-V characteristics of near room temperature HgCdTe devices are presented and compared with numerical simulations, what indicate on contribution of dislocations as a SRH recombination channel.     

Subnanosecond transient photocurrents in a-Si:H: A probe of thermalization within shallow band-tail states

The measurement of transient photocurrents in amorphous semiconductors has been shown to be a powerful way of exploring the thermalization of excess carriers within the manifold of localized states adjacent to the band edges. The extent to which thermalization within the shallowest states can be resolved in controlled by the experimental observation time. Reported here are measurements of transient photocurrents, using high-speed stripline techniques, in glow-discharge a-Si:H carried out in the time regime (0.6 ? 100 nsec) and the temperature range 150 ? 320K. We find an effective electron drift mobility of <μ_D(t)> = 1–3 cm²/Vsec at t=0.6 nsec and T=300K and a time-dependent activation energy, _?(t), for the photocurrent magnitude. The initial photocurrent decay (t < 100 nsec) can be fit to a power-law, t^?n, and the temperature dependence of the dispersion parameter, α=1?n, is given by α=α₀+βT. In view of these results the nature of the band-tail near the conduction band edge is discussed.