M. A. Berding

Annealing conditions for intrinsic CdTe

Equilibrium native defect densities in CdTe are calculated from ab initio methods, and compared with experimental results. We find that CdTe is highly compensated p type under tellurium-saturated conditions, with the cadmium vacancy as the dominant acceptor and the tellurium antisite as the compensating donor. This finding is in agreement with recent experiments that find a much larger deviation from stoichiometry than would be predicted by the electrically active defects. Under cadmium-saturated conditions, cadmium interstitials are predicted to dominate and the material is found to be n type. Native defect concentrations and the corresponding carrier concentrations are predicted as a function of processing conditions, and can serve as a guide to postgrowth anneals to manipulate the conductivity of undoped material for applications in x- and γ-ray spectrometers. Furthermore, we show that by choosing appropriate annealing conditions and extrinsic dopants, one can increase the operating efficiency of nucle...

Structural properties of bismuth-bearing semiconductor alloys

Materials currently used for detection in the infrared spectral region have notoriously poor structural properties. In search of a better narrow‐gap material, we have addressed the structural properties of bismuth‐bearing III‐V semiconductor alloys theoretically. Because the Bi compounds are not known to form zinc‐blende structures, only the anion‐substituted alloys InPBi, InAsBi, and InSbBi are considered candidates as narrow‐gap semiconductors. We calculate the bond energies and lengths for the zinc‐blende Bi compounds and their diluted and concentrated alloys. Strain coefficients for the compounds are calculated, and predictions for the mixing enthalpies, miscibility gaps, and critical temperatures are made. Miscibility calculations indicate that InSbBi will be the most miscible, and because of the large lattice mismatch of the constituents, InPBi will be the most difficult to mix. Tendencies toward cluster formation and deviations from randomness in the alloys are considered. Calculations of the hardn...

AMPHOTERIC BEHAVIOR OF ARSENIC IN HGCDTE

The properties of arsenic in HgCdTe are predicted using ab initio calculations and a statistical theory. Predictions on the amphoteric nature of arsenic are in good agreement with experimental results on material growth both by liquid phase epitaxy and molecular beam epitaxy (MBE). The experimentally observed dependence of the arsenic diffusion on mercury partial pressure is also explained by our results. A microscopic model for activating the arsenic as an acceptor is suggested, and requirements of post-MBE-growth activation anneals are identified.

Defects in ZnTe, CdTe, and HgTe: Total energy calculations

Total energies for various impurities and defects in HgTe, CdTe, and ZnTe are calculated. Calculations were done using a self‐consistent linear muffin‐tin orbital (LMTO) method within the local density and atomic spheres approximation. We calculate the total energy for substitution on both lattice and interstitial sites, and estimate the lattice strain energies. Estimates of the variation with the alloy predict a linear variation of the substitution energy with the local concentration. We predict that the Te antisite will be more prevalent in all three of the compounds than previously thought. The problem of cross doping during heteroepitaxy on GaAs is predicted to be greater on the cation sublattice.

Deactivation in heavily arsenic-doped silicon

We have combined ab initio calculations with a general statistical theory to predict the properties of heavily arsenic-doped silicon. Although we find that a lattice vacancy surrounded by four arsenic (VAs4) is the dominant deactivating complex at high arsenic concentrations in equilibrium, vacancy clusters with fewer arsenic neighbors are present in significant quantities. These smaller complexes are essential not only to the establishment of equilibrium, since SiAs4 clusters are extremely rare, but can also explain deactivation even if VAs4 formation is kinetically inhibited. This suggests that materials with similar arsenic concentration and deactivation fractions can have different microscopic states, and therefore behave differently in subsequent processing. Good agreement is found between theory and experiment for the electronic concentration as a function of temperature and total arsenic concentration. We also show that for low arsenic concentrations, full activation is the equilibrium condition.

Defect modeling studies in HgCdTe and CdTe

We have used a quasichemical formalism to calculate the native point defect densities in x = 0.22 Hg1−xCdxTe and CdTe. The linearized muffin-tin orbital method, based on the local density approximation and including gradient corrections, has been used to calculate the electronic contribution to the defect reaction free energies, and a valence force field model has been used to calculate the changes to the vibration free energy when a defect is created. We find the double acceptor mercury vacancy is the dominant defect, in agreement with previous interpretations of experiments. The tellurium antisite, which is a donor, is also found to be an important defect in this material. The mercury vacancy tellurium antisite pair is predicted to be well bound and is expected to be important for tellurium antisite diffusion. We consider the possibilities that the tellurium antisite is the residual donor and a Shockley-Read recombination center in HgCdTe and suggestions for further experimental work are made. We predict that the cadmium vacancy, a double acceptor, is the dominant defect for low cadmium pressures, while the cadmium interstitial, a double donor, dominates at high cadmium pressures.

Ballistic transport in semiconductor alloys

The electronic structure of semiconductor compounds GaAs, InAs, and InP and alloys Ga0.5In0.5As, Ga0.7Al0.3As, and InP0.5As0.5, obtained in the coherent potential approximation, is used to calculate the group velocity and velocity relaxation time limited by longitudinal optical phonons, alloy disorder, and ionized impurities as a function of electron energy at 300 K. The nonparabolic nature of the band structure is found to severely limit the electron mean free path. With the types of interactions considered to date, the presence of L valleys does not limit the mean free path of electrons moving in the 〈100〉 direction. At 1018‐cm−3 doping, electron‐electron interactions reduce the mean free path by only 15% to 20%. InAs and GaInAs alloys offer advantages over all the other materials for devices with base widths greater than 500 A; however, for thinner devices, ∼100 A, no material is appreciably better than GaAs, the III‐V compound currently under best control. The ballistic device‐related properties of se...

HgCdTe status review with emphasis on correlations, native defects and diffusion

The authors review the current status of knowledge of fundamental properties of the alloy Hg1-xCdxTe. The most vexing questions are about its correlation state. Several different experiments now suggest it is highly correlated, but no theory predicts this result. They also discuss other properties, including dislocations at interfaces, the residual donor, worms, surface segregation and its impact on passivation, and concentration fluctuations. The forces driving these phenomena, where they are known, will be presented. Most of the paper focuses on the following: correlations; native defects, formation enthalpies and entropies; native defect equilibria with mercury gas and with tellurium inclusions; and self-diffusion coefficient activation energies including its contribution from migration energies. They will take advantage of new first-principles, high-accuracy calculations to help explain the experimental situation. The calculations predict that the main native defects found in alloys equilibrated at low Hg pressures are Hg vacancies, while at high Hg pressures they are Hg interstitials, and, surprisingly, Hg antisites.

Full-band-structure calculation of Shockley-Read-Hall recombination rates in InAs

We report a calculational procedure to obtain the rate of electron–hole recombination, mediated by the Shockley–Read–Hall (SRH) mechanism. Our method uses a combination of first-principles calculations and accurate empirical band structures. First, we use ab initio calculations to identify the point defects, their densities and energy levels in the gap. Then we parametrize the tight-binding interaction between defect and the host atoms in a Green’s function approach to obtain the defect levels as identified by the first-principles calculations. Finally, the resulting tight-binding Hamiltonian is used to obtain the dipole matrix element between the conduction and valence band states, mediated through the defect levels in the gap, in second-order perturbation theory. The states are integrated over the entire Brillioun zone, subject to energy and momentum conservation, to obtain the limiting lifetimes of the carriers. This method is applied to study the minority carrier lifetimes in n-doped InAs. The calcula...

Cleavage energies in semiconductors

We present a method for the calculation of the surface and cleavage energies, Eγ, for semiconductors, based on a tight‐binding Green’s function approach and a difference‐equation solution for the layered structure. Energies are calculated for a representative group of semiconductors, and cleavage energies are found to agree well with the available experimental data. We find ESiγ(111)=1360 ergs/cm2, and Eγ(110)=1000, 180, and 120 ergs/cm2 for GaAs, CdTe, and HgTe, respectively.