David C. Look

Electron and hole conductivity in CuInS2

Abstract Single crystals of CuInS 2 have been grown from the melt and annealed in In or S to produce good n - or p -type conductivity, respectively. Two donor levels, one shallow and one deep (0.35 eV), and one acceptor level at 0.15 eV are identified. The hole-mobility data are best fitted with an effective mass m p ∗≅1.3m e , which can be explained by simple, two band k . p theory if the valence band has appreciable d character. Above 300°K, the hole mobility falls rapidly, evidently due to multiband conduction and/or interband scattering between the nondegenerate and degenerate valence bands. The conduction band mobility appears to be dominated, in many samples, by large concentrations ( >10 18 cm −3 ) of native donors and acceptors, which are closely compensated.

The Electrical and Photoelectronic Properties of Semi-Insulating GaAs

Publisher Summary This chapter discusses the electrical and pholectric properties of semi-insulating gallium arsenide (GaAs) and techniques required to measure these properties. Without a proper understanding of the techniques, it is impossible to critically evaluate the derived results. Because the theoretical bases of the various characterization methods are often not discussed at length in the relevant research papers, the chapter presents the attempt to do that for several cases of interest. The two most basic electrical parameters of interest, normally, are the carrier concentration and mobility. These quantities may be obtained from the proper measurements of current, electrical field, and applied magnetic field. The chapter also presents the block diagram of a computer-controlled apparatus that is designed to carry out the Hall-effect and photoelectronic measurements on high resistivity samples. If the sample conductivity is dominated by only one type of carrier, then a simple Hall-effect analysis is sufficient.

Semiconducting/Semi-Insulating Reversibility in Bulk GaAs

Bulk, liquid‐encapsulated Czochralski GaAs may be reversibly changed from semiconducting (ρ∼1 Ω cm) to semi‐insulating (ρ∼107 Ω cm) by slow or fast cooling, respectively, following a 5 h, 950 °C soak in an evacuated quartz ampoule. This effect has been studied by temperature‐dependent Hall‐effect, photoluminescence, infrared absorption, mass spectroscopy, and deep level transient spectroscopy measurements. Except for boron, the samples are very pure, with carbon and silicon concentrations less than 3×1014 cm−3. Donor and acceptor concentrations, on the other hand, are in the mid 1015 cm−3 range, which means that the compensation is primarily determined by native defects, not impurities. A tentative model includes a donor at EC−0.13 eV, attributed to VAs−AsGa, and an acceptor at EV+0.07 eV, attributed to VGa−GaAs.

Hole Transport in Pure and Doped GaAs

We have used a two‐band model (heavy and light holes) to calculate the transport properties of p‐type GaAs. The scattering mechanisms included are acoustic‐mode deformation potential, acoustic‐mode piezoelectric potential, polar‐ and nonpolar‐mode deformation potential, ionized impurity, and space charge. Interband scattering is included explicitly for the optical phonons and phenomenologically for the acoustic phonons. The intraband polar optical‐mode scattering, for which a relaxation time cannot be defined, was calculated by using the numerical method of Fletcher and Butcher. The acoustic deformation‐potential parameter and the coupling coefficient for interband scattering were calculated by fitting the theory to Hall‐mobility data for both pure and doped samples. We have determined ionized‐impurity and space‐charge contributions for two of our samples, one doped with Cr and exhibiting an 0.33‐eV activation energy (Cr4+/Cr3+), and the other heat‐treated and exhibiting an 0.14‐eV activation energy. The ...

The electrical characterization of semi‐insulating GaAs: A correlation with mass‐spectrographic analysis

The room‐temperature electrical properties of 28 semi‐insulating GaAs crystals have been determined by using a mixed‐conductivity analysis. It is shown that for most of these samples, such an analysis gives good accuracy for the electron mobility μn and electron concentration n, but poorer accuracy for the hole mobility μp, hole concentration p, and intrinsic concentration ni. The intrinsic concentration is determined at 296 °K to be ni? (1.7±0.4) ×106 cm−3, which compares favorably with the theoretical value deduced from the band gap and the effective masses. From a Fermi‐level analysis, the dominant Cr acceptor is found to lie at 0.69±0.02 eV from the valence band. For many of the samples, the ionized‐impurity concentrations NI have been estimated from spark‐source mass‐spectrographic measurements and are compared with the concentrations predicted from μn. In general, the expected inverse relationship between μn and NI is found to hold, but the scatter in the data is quite large, mainly due to the uncer...

Mixed conduction in Cr-doped GaAs☆

Abstract Hall-effect and magnetoresistance measurements have been carried out in GaAs : Cr as functions of magnetic field strength (B = 0–18kG) and temperature (T = 125–420°K). Independent solutions for the mobilities, μn and μp, and the carrier concentrations, n and p, are obtained from the basic mixed-conductivity equations. These quantities, as well as the intrinsic carrier concentration, ni are then calculated as a function of temperature for one sample, and subsequent analysis yields the following values in the range T = 360–420°K: an acceptor (presumably Cr) energy EA = 0.69±0.02eV (from the valence band); the bandgap energy Eg = Eg0 + αT, with Ego = 1.48±0.02eV, α ≅ 3.2 × 10 −4 eV ° K ; μ n = 2700± 100 cm 2 V sec , decreasing slightly with temperature; = 350± 50 cm 2 V sec ; and an acceptor-to-donor concentration ratio, itNA/ND≅8. The electron mobility appears to be limited by neutral impurity scattering, with NA ≅ 2 × 1016cm−3. Several other samples were also investigated but as a function of temperature only (at B = 0). At room temperature both positive (p-type) and negative (n-type) Hall coefficients were observed.

On the Interpretation of Photoconductivity and Photo-Hall Spectra in Semi-Insulating GaAs:Cr

Abstract A mixed-conductivity analysis is used to separate the contributions of n , p . and μ n to photoconductivity and photo-Hall spectra in semi-insulating GaAs:Cr. It is shown that the spectral dependence of μ n is often important in the interpretation of these data. A room-temperature energy diagram is presented, and includes chromium-related centers at 0.73 and 0.52 eV from the conduction band.

Schottky-Barrier Profiling Techniques in Semiconductors - Gate Current and Parasitic Resistance Effects

The theory for obtaining mobility and carrier concentration profiles by the Hall‐effect, magnetoresistance, and capacitance‐conductance methods is developed in the relaxation‐time approximation. This theory is then applied to semiconductors in which a Schottky barrier is used to control a depletion region. Particular emphasis is given to field‐effect transistor structures which are ideally suited for geometric magnetoresistance measurements. A unique feature of the present model is the correction for finite gate (Schottky‐barrier) current, which can be very important under forward‐gate‐bias conditions. The ability to use forward‐bias makes the near‐surface region more accessible. Also, parasitic resistance effects are treated. We apply these results to GaAs conducting layers formed by direct implantation of 4×1012/cm2, 100‐keV Si ions into Cr‐doped GaAs.

Defect production in electron‐irradiated, n‐type GaAs

Temperature‐dependent Hall‐effect measurements have been performed on pure, n‐type, vapor‐phase epitaxial GaAs, irradiated by 1‐MeV electrons at room temperature. The energies and production rates of two dominant defect centers, C2 and C3, are as follows: E2=EC−0.148, E3=EC−0.295 ±0.002 eV, τ2=2.0, and τ3=0.5±0.2 cm−1, in good agreement with deep level transient spectroscopy (DLTS) data. However, the most important result of this study is a very high production rate, τAS ≂4±1 cm−1, for ‘‘shallow’’ acceptors (CAS) lying below E3. In fact, CAS is produced at a much higher rate than all of the DLTS traps observed in this energy range, proving that close to half of the primary defects in electron‐irradiated GaAs are evidently not seen by DLTS. The high CAS production rate has important implications for microscopic models of C1 and C2, rendering unnecessary the assumption that one of these centers must be an acceptor in order to explain the Hall‐effect results. Finally, we show that all available Hall‐effect a...

A study of the 0.1‐eV conversion acceptor in GaAs

Two semi‐insulating liquid‐encapsulated Czochralski GaAs cyrstals, one Cr‐doped and the other undoped, were annealed at 750 °C for 15 min in flowing H2. Each sample converted to conducting p type in the near‐surface region, due to the formation of acceptors at Ev+0.1 eV. We have studied this phenomenon by electrical, optical, and analytical profiling techniques, and have determined conclusively that the acceptors in our samples are not related to Mn accumulation, a commonly accepted explanation. It is argued that the 0.1‐eV center may arise from several possible sources, each exhibiting a VGa ‐like state at this energy.