G. L. Pearson

Properties of vacancy defects in GaAs single crystals

The electrical properties of GaAs single crystals were measured after annealing at various elevated temperatures in controlled atmospheres. Two types of defects were introduced as the result of annealing: (i) donors having a high concentration near the surface and (ii) acceptors having a lower surface concentration but extending further into the crystal. The As overpressure dependencies indicate that the donors are As vacancies and the acceptors are Ga vacancies since their respective concentrations are proportional to iPAs4−1/4 and iPAs4+1/4. The entropy and enthalpy changes for the vacancy formation reactions at the surface were evaluated, based on experimental data and thermodynamic analysis. The diffusion coefficients of vacancies follow the relations D (VGa) =2.1×10−3 exp(−2.1/kT) and D (VAs) =7.9×103 exp(−4.0/kT). Both types of vacancies form nonradiative centers as determined from photoluminescence experiments.

Ohmic contacts to solution-grown gallium arsenide

A study has been made on the properties of Ohmic contacts to single‐crystal n‐type GaAs wafers which were grown by liquid epitaxy techniques. Carrier concentration profiles at the n+‐n junctions were measured by the Schottky barrier capacitance technique for both Au‐Ge‐Ni alloyed contacts and liquid epitaxial n+ contacts. The frequently observed high resistance layers at the interface were eliminated by the contacting processes described here. The Ohmic contact problems and the results of other experiments are explained in terms of the GaAs binary phase diagram and nonstoichiometry considerations.

Annealing and Arsenic Overpressure Experiments on Defects in Gallium Arsenide

Previous experiments reported by the authors revealed a measurable expansion in the lattice constant of GaAs single crystals quenched from temperatures above 1000°C. These results led to the conclusion that a defect structure consisting of both Frenkel pairs and Schottky vacancies was produced in these crystals. In this paper we report experiments in which an arsenic overpressure, obtained by including metallic arsenic along with the GaAs crystal, depresses the defect concentration. These results are used to identify the primary defect species as arsenic monovacancies. Subsequent heat treatments at both room temperature and high temperature show that annealing proceeds in two stages. The first stage is quite rapid but does not have a well‐defined activation energy. The second stage is slower and has an activation energy of 1.0 eV. These results are interpreted to include the presence of arsenic interstitial atoms. A physical model is proposed to describe the defect structure as well as the annealing behav...

A simplified model for graded-gap heterojunctions

Abstract A simplified method for calculating the energy band profiles of graded-gap heterojunctions, based on the generalized model of Oldham and Milnes, is presented. The profiles are derived by superposing an energy band grading function and the electrostatic potential in the heterojunction. The latter is obtained by using the depletion layer approximation as for conventional p-n homojunctions. The energy band profiles of hypothetical p(GaAs)-n(Al0·4Ga0·6As) heterojunctions are calculated using the simplified method. For small grading layer widths, the results are in good agreement with the generalized model. The barrier lowering factor η as a function of the graded layer width l is calculated for such heterojunctions. It is found, for acceptor and donor densities of 1018 and 1016 cm−3 respectively, that the barrier height is reduced from 0·47 eV to zero as l increases from zero (abrupt case) to ≈300 A. The applications of these analyses to practical heterojunctions are discussed.

Properties of Gallium Arsenide Diodes between 4.2° and 300°K

The current‐voltage characteristics of zinc‐diffused gallium arsenide diodes have been measured between 4.2° and 300°K, with carrier concentrations in the n‐type region varying from 6×1015 to 9×1018 cm−3. In the forward direction it is shown that the current is due to tunneling at low temperatures and high carrier concentrations whereas it arises from thermal processes involving recombination in the space‐charge region at higher temperatures and low carrier concentrations. The temperature at which the current changes from predominantly tunneling to predominantly thermal in origin has been determined as a function of the n‐type carrier concentration. In the reverse direction the current in these diodes appears to be dominated by tunneling at all temperatures and doping densities tested.

Deep trapping effects at the GaAs‐GaAs : Cr interface in GaAs FET structures

Transient capacitance measurements were made in order to study the interface properties of FET‐type devices with and without LPE buffer layers. No detectable traps were found in the buffered devices, whereas two deep hole traps, located 0.58 and 0.81 eV above the valence band, were detected in the unbuffered FET devices when the gate‐depletion region approached the substrate interface. The trap concentrations were 1×1016 and 2×1015 cm−3, respectively. An interface model was developed and used to show that the minority‐carrier trapping effects which appeared in the active layer are actually due to the image effect of majority‐carrier traps in the Cr‐doped substrate acting through the interface space‐charge region.

Diffusion Mechanism of Zn in GaAs and GaP Based on Isoconcentration Diffusion Experiments

Precise relationships between the diffusion coefficient D and the zinc concentration C are obtained from isoconcentration diffusion experiments performed at 900°C for zinc in GaAs and at 1000°C for zinc in GaP. It is found that D varies with C from a slightly less than cubic to a somewhat less than square dependence over the concentration range 1018 to 1020 cm−3.Possible mechanisms that result in concentration‐dependent diffusion are discussed. Under the conditions given above, it is concluded that diffusion occurs by an interstitial‐substitutional mechanism with the interstitial mode being dominant. The charge states of the various species involved in the diffusion are believed to be: (1) interstitial zinc atoms are doubly ionized donors; (2) substitutional zinc atoms are either neutral or singly ionized acceptors; and (3) gallium vacancies are neutral. A theoretical expression for D versus C, derived under these assumptions, fits the isoconcentration diffusion data over wide ranges of D and C through th...

Diffusion and Solubility of Zinc in Gallium Phosphide Single Crystals

Radiotracer Zn65 has been diffused into 0.2‐Ω cm sulfur‐doped GaP single crystals from constant pressure vapor sources. Solid solubilities have been measured at temperatures between 800° and 1100°C, the diffusion profiles have been obtained and these have been analyzed by the Matano method. Although retrograde solid solubility is expected at higher temperatures, the data below 1100°C are an excellent fit to the relation C=2.6×1023 exp(−0.85/kT). The diffusion coefficient is found to be concentration‐dependent. At temperatures below 900°C the diffusion coefficient can be represented by the empirical expression D=7.5×10−8 C0.45 exp(−2.50/kT). Above 900°C the profiles are similar to those of Zn in GaAs and the diffusion coefficient varies approximately as the square of the zinc concentration. The results are interpreted and the diffusion mechanism is discussed in terms of parallel‐mode diffusion of interstitial and substitutional zinc.

Isothermal LPE growth of thin graded band‐gap AlxGa1−xAs layers

Liquid‐phase‐epitaxial growth experiments, using saturated Ga‐Al‐As solutions and GaAs substrates, were carried out under isothermal conditions. It was found that graded band‐gap AlxGa1−xAs growth occurs under these conditions. The layer thicknesses were between 400 and 700 A and the Al composition in the layer increased monotonically toward the surface. When a saturated Ga‐As solution was brought into the contact with an AlxGa1−xAs solid and held at constant temperature, partial dissolution of the AlxGa1−xAs was observed. The resulting processes of compositional graded growth or partial dissolution follow from thermodynamic nonequilibrium between solid and liquid. It can be expected that similar processes will occur in other AIII‐BIII‐CV systems.

Gamma‐Radiation Damage in Epitaxial Gallium Arsenide

Electrical and photoluminescence measurements were carried out on liquid epitaxial GaAs single crystals after irradiation at room temperature by 1.25‐MeV 60Co γ rays. Following each of a series of irradiations, Hall measurements were made on both n‐ and p‐type crystals as a function of temperature between 77 and 300°K. The n‐type crystals exhibited energy levels 0.13, 0.16, and 0.30 eV below the conduction band minimum after irradiation. The 0.13‐ and 0.16‐eV levels were introduced during irradiation at roughly one‐half the rate at which compensating deep acceptors were introduced. The p‐type crystals showed energy levels 0.059 and 0.10 eV above the valence band maximum. Before irradiation, these p‐type crystals had energy levels in the lower 0.20 eV level of the band gap. It is, therefore, not possible to determine whether the 0.059 and 0.10 eV were produced by irradiation or were merely uncovered as the free‐hole density was reduced by radiation‐induced deep donors. The carrier removal rate at 77°K was ...