An-Ban Chen

Effects influencing the structural integrity of semiconductors and their alloys

The bond length and energy changes of the constituents of alloys relative to their pure crystal values are calculated from an extension of Harrison’s method. It is demonstrated that the already weak HgTe bonds are destabilized by adjacent CdTe, HgS, or HgSe, but are stabilized by ZnTe. It is also argued that dislocation energies and the hardness of semiconductors vary as a high inverse power of the bond length of the constituents. Hence, the shorter ZnTe bond as an additive should improve the structural properties of HgTe and CdTe. Experiments that support these predictions are noted. The electronic transport properties of 0.1 eV band gap HgZnTe are about the same as those of HgCdTe, and the structural properties of the Zn compound are superior; thus, we conclude that HgZnTe is likely to be the better material for IR devices.

InTlSb: An infrared detector material?

In1−xTlxSb is proposed as promising infrared material. A number of optical and structural properties are studied within local density‐functional theory. The alloy at x=0.09 is estimated to have a gap of 0.1 eV. Although TlSb is found to favor the CsCl structure, the zinc blende alloy is stable for low x values. A phase diagram is calculated to estimate the regions of stable phases and explore the conditions for growing narrow‐gap In1−xTlxSb alloys.

CPA band calculation for (Hg, Cd) Te

A calculation of the electronic structure of (Hg,Cd)Te alloys in the coherent‐potential approximation (CPA) is presented. The problem is divided into three tasks: determining accurate band structures including relativistic terms for the constituent compounds (HgTe and CdTe), characterizing the alloy disorder, and executing the alloy calculation. These tasks are facilitated through the use of a set of symmetrized, orthonormal, local orbitals. We shall introduce the calculational procedure and discuss the results obtained to date. These include the calculated band structures for HgTe and CdTe, the local alloy disorder parameters, the alloy band energies, and density of states calculated in the virtual crystal approximation and in a simplified version of CPA.

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.

Electronic and transport properties of HgCdTe and HgZnTe

Band structures are calculated for HgTe, CdTe, and ZnTe. A near band‐edge approximation to the coherent potential approximation is used to compute the alloy band‐gap variation and electron and hole effective masses in HgZnTe and HgCdTe. Both diagonal and off‐diagonal disorder are included. The alloy‐scattering contributions to the electron and hole mobilities are calculated and found to be unimportant for electrons but significant for hole transport. The consequences for hole mobility are found to be significant if anion disorder is present because of a valence‐band offset.

Phase diagrams and microscopic structures of (Hg,Cd)Te, (Hg,Zn)Te, and (Cd,Zn)Te alloys

A cluster theory based on the quasichemical approximation has been applied to study the local correlation, bond‐length distribution, and phase diagrams of the II–VI pseudobinary alloys Hg1−xCdxTe, Hg1−xZnxTe, and Cd1−xZnxTe. The cluster energy is calculated by letting it relax in some effective alloy medium and then considering the contributions from the strain and chemical energies. Two different models are presented to simulate the alloy medium. While both models show that all three alloys have nearly random distributions, the signs of the local correlation prove to be sensitive to the alloy medium chosen for the energy calculation. Good agreement is found between experiment and the bond lengths and phase diagrams in both models.

Relation between the electronic states and structural properties of Hg1−xCdxTe

Physical reasons are presented for the coherent potential approximation (CPA) calculation method’s abillity to predict little band gap bowing or alloy scattering, while at the same time predicting large deviation from virtual crystal behavior in other energy ranges. Accurate treatment of structural properties requires that calculations be doubly self‐consistent: a density functional method plus CPA is needed. Experimental values of bond energies are greater than those predicted by simple theories. Weakened HgTe bonds in alloys help to explain the loss of Hg when the materials undergo surface treatments.

Vacancy formation energies in II–VI semiconductors

Cation and anion vacancy formation energies are calculated for HgTe, ZnTe, CdTe, and their dilute alloys. Harrison’s tight‐binding theory is extended to a cluster embedded in an extended crystal. Only neutral vacancies have been considered and two final states for the removed atom have been addressed: a free atom in vacuum and an atom on an ideal (111) surface. Corrections caused by rehybridization of the dangling bonds and Coulomb energies resulting from charge redistribution have been included.

Correlations in pseudobinary alloys

Correlations between atoms of Cd and Zn in Cd1−xZnxTe and Hg and Zn in Hg1−xZnxTe are calculated within a generalized quasichemical approximation. A tetrahedron of alloy atoms is used for the microcluster unit. The statistical mechanics of the correlations is outlined for the present problem, an approximate method of solution is indicated. Microcluster energies are obtained from an approximation of a Keating model due to Martin, plus a correction due to Harrison to account for the coupling between the tetrahedral bonds. It is found that the correlations are significant at typical growth temperatures (300–600 K). Hg1−xZnxTe is predicted to undergo spinodal decomposition between 300 and 600 K.

On the determination of the energy band offsets in Hg1−xCdxTe heterojunctions

The energy band offsets play a critical role in the electrical properties of heterojunctions. We raise here serious questions regarding the precision and accuracy of the measured and calculated energy band offsets in HgCdTe heterostructures. We argue that the Anderson model prescription for the offsets is not useful and is not correct for abrupt heterojunctions when narrow band gap semiconductors are the constituents of the heterostructure. New values for the photoionization energy of Hg1−xCdxTe are given and the reliability of these values for determining the offset is discussed.