J. A. Silberman

Band gap variation and lattice, surface, and interface ‘‘instabilities’’ in Hg1−xCdxTe and related compounds

This paper brings into focus the differences between Hg1−xCdxTe (MCT) and ‘‘ordinary’’ semiconductors such as column 4, 3–5, and most 2–6 compounds. The key characteristic of MCT which makes it both different and of practical interest is the variation of band gap with composition (0 to 1.5 eV). This paper shows that this benefit is not without its difficulties. In particular, the same unusual characteristic of the Hg atomic states which leads to the advantageous movement of  band gap with composition weakens the bonding of Hg in the lattice. This bonding weakness exhibits itself in many ways in the characteristics of MCT, both at surfaces and interfaces and in the bulk. Because of this there is a potential instability associated with the surface or interface; however, this instability depends critically on the crystalline perfection of the MCT to depths far below the surface. The reason for this is defects move from the bulk to the surface (or in the opposite direction) with surprising ease—even at room t...

Optically enhanced low temperature oxygen chemisorption on GaAs(110)

Optically enhanced oxidation of GaAs is promising both for insight into fundamental mechanisms of oxygen chemisorption and for possible application in GaAs direct write. For atomically clean GaAs(110), we have found that sample temperature is a key parameter in determining the increase of oxygen uptake under low‐intensity argon ion laser illumination (1 W/cm2 at 2.41 eV) relative to the ‘‘dark’’ uptake. Although cooling to 185 K suppresses the dark oxygen adsorption rate, the oxygen uptake with optical irradiation is the same at both 185 and 400 K. The ratio of laser‐assisted to dark uptake for 108 L exposure then approaches the enhancement necessary for spatially selective oxidation, showing an increase from 2.5 at room temperature to well over 10 at 185 K. After ruling out sample heating and direct oxygen gas excitation, we suggest that the light‐induced increase in uptake is due to electronic excitations in the GaAs. To explain the enhancement, one also needs to assume a weakly adsorbed precursor state...

Room temperature stability of cleaved Hg1−xCdxTe

X‐ray and ultraviolet photoemission spectroscopy (XPS and UPS) at 21.2 eV have been used to determine the stability against Hg evaporation of cleaved (110) surfaces of p‐Hg1−xCdxTe (x = 0.2, 0.31, 0.39) in vacuum at room temperature. No evidence of Hg loss was obtained for periods of observation from five to twenty hours. However, oxygen excited by an ionization gauge and Ne ion sputtering both produced preferential loss of Hg.

AES sputter profiles of anodic oxide films on (Hg,Cd)Te

Sputter profiling with Auger electron spectroscopy has been used to determine the depth profiles of Hg,Cd,Te, and O in anodic oxides on (Hg,Cd)Te in an effort to monitor the composition of the oxides and the properties of the oxide‐semiconductor interface. The measurements are straightforward, but the analysis and extraction of atomic concentration profiles are complicated due to nonstoichiometric erosion of the base material and possibly of the oxide. Conclusions from this and earlier investigations about the composition of the semiconductor past the interface are still dependent on a more basal understanding of the effects of low energy ion nombardment on (Hg,Cd)Te.

Comparative study of activated oxygen uptake on HgTe, Hg0.69Cd0.31Te, and CdTe

Soft‐x‐ray photoemission spectroscopy was used to monitor the uptake of oxygen on the (110) faces of HgTe, Hg0.69Cd0.31Te, and CdTe during the formation of roughly two monolayers of oxide. The oxidation was activated by a hot filament in line‐of‐sight of the sample surface. The oxides formed contain Cd and Te; whereas, Hg is lost from the surface during the initial stages of oxide formation. The rate of oxidation for the Hg0.69Cd0.31Te sample was found to be 69% as great as that for the binary HgTe, and lower than observed for CdTe. The scaling of oxidation rate with the number of Hg–Te bonds between HgTe and the x=0.31 alloy, together with the observed release of Hg upon oxidation, suggests the importance of breaking the Hg–Te bond as a step in the oxidation process. Evidence for an unstable character of the oxide on the Hg containing compounds is also presented.

Surface and bulk structural defects in Hg1−xCdxTe

The techniques of optical microscopy and transmission electron microscopy (TEM) have been used to study the structural characteristics of a surface of a solid state recrystallized Hg1−xCdxTe crystal (x=0.35) cleaved at room temperature. Such surfaces are of special interest because of their wide use in photoemission spectroscopy studies. HgCdTe cleaved surfaces were found to be much rougher than similar surfaces of GaAs crystals, containing numerous steps and regions where fracture was clearly ductile rather than brittle. TEM observations indicate that a step is associated with numerous dislocations where the crack presumably arrested prior to changing direction, implying that the room temperature cleaved surface has a high defect density compared to the bulk crystal. TEM has also been used to characterize the defects associated with etch pits developed using ‘‘Polisar etch 2’’ on the (111)A face of a Hg1−xCdxTe crystal grown by the solid state recrystallization process. While in some cases dislocations w...

Angle‐resolved photoemission spectroscopy of Hg1−xCdxTe

Angle‐resolved photoemission spectroscopy, a technique which provides direct information on the quantum numbers k and E which characterize the electron states in a periodic solid, has been used to study the electronic structure of the alloy Hg1−xCdxTe for x=1.0, 0.31, 0.23, and 0.0. Data were collected mainly for emission along the normal to the cleaved (110) face of these crystals as a function of photon energy with various directions of polarization. The structure observed in the spectra of the alloy are similar in dispersion and polarization dependence to that obtained for CdTe. Preliminary analysis of the observed features using a free electron final state reveals the bulk origin of the measured transitions by comparison to calculated valence bands and elucidates aspects of the composition dependence of the valence band states involved in the optical excitation.

Surface and interfaces of HgCdTe. What can we learn from 3–5’s? What is unique with HgCdTe?

Fundamental studies of the wide variety of the surfaces and interfaces of mercury cadmium telluride are really only in their infancy. Major developments have been made in the understanding of the free surfaces of closely related 3–5 compound semiconductors and, making use of this knowledge, on the mechanism of formation of Schottky barriers and MOS (or MIS) interface states on the 3–5 materials. Thus, the 3–5 work provides an important starting point for HgCdTe work. For 3–5’s, a unified defect model has been developed which explains both the Schottky barrier formation and the source of the MOS interface states. This work is briefly reviewed and related to HgCdTe surface and interface phenomena. Based on this and a wide range of practical work plus the early results of fundamental work on HgCdTe, it is clear that defects also play an important role at HgCdTe interfaces. Previously however, it has apeared that the interface may not be so strongly coupled to defects in the bulk (i.e., to defects, dislocatio...

TEM investigation of the differences in ion milling induced damage of Hg1−xCdxTe and CdTe heterojunctions

The lattice defects induced by ion milling at the liquid phase epitaxy (LPE) grown HgCdTe on CdTe heterojunction have been investigated using transmission electron microscopy (TEM). TEM analysis reveals that subjecting both materials to identical milling conditions results in much higher defect densities (2–3 orders of magnitude) in the CdTe than in the HgCdTe (x=0.30). In addition, high resolution lattice images of both regions reveal that the stacking fault widths in the HgCdTe are much narrower than in the CdTe, which implies that the stacking fault energy is much higher in the HgCdTe as compared to the CdTe. These observed phenomena are shown to support the weakness and metallicity of the Hg–Te bond as proposed by theoretical considerations.

Initial stages of oxide formation on HgCdTe exposed to activated oxygen

The changes in chemistry and electronic structure of the (110) cleavage surface of Hg0.65Cd0.35Te and properties of the film produced are determined by monitoring with photoemission spectroscopy the growth of thin (10 A) oxide layers on initially clean surfaces. Oxygen uptake, normally quite slow, is stimulated in this case by operating an ionization gauge in line of sight of the sample during the exposures so as to provide a stream of excited oxygen striking the surface. A complex Te oxide is formed which retains Cd but not Hg. The Cd content appears enhanced at the oxide surface. An explanation for the behavior observed is sought in terms of the species produced by the activating gauge and the chemistry of the alloy surface.