4 discloses that most of the singularities are clearly displayed, and furthermore, the frequency variations are in accordance with expectation. No singularities appear originating from points from which they were not expected. P,(0) at I' in branch five hardly demonstrates the expected square-root frequency variation, but the oscillations due to uncertainty are pronounced in this region of g(v). P2(0) on Q in branch two is clearly displayed, but g(v) falls off rapidly on the other side of the critical frequency. This makes it difficult to decide whether its contribution is of type 1 or 2. In branch five the point P,(0) on Q with frequency 8. 30 THz is hidden by Pa(0) in S, of frequency 8. 31 THz. This is because the contribution to g(v) from the latter exceeds that from the former considerably.Finally, three singularities expected to be seen are more or less absent. The first is the one of P,(0) at I. in branch three with frequency 6. 66 THz. This may be explained as follows. Branch three has a sharp maximum at t. , which means that the contribution from this region to g(v) is small compared to those of the majority of critical regions. An inspection of the frequency surfaces shows that there are three points in reciprocal space in the vicinities of which there are far fewer points having frequencies close to the critical ones than for all other critical points. These points are precisely those which are weakly or not at all displayed in Fig. 3, namely P3(0), P,(0), and P2(0) at I. , on Z, and at t. in branches three, four, and four, re-spectivelyy.
ACKNOWLEDGMENTSThe authors wish to express their gratitude to Professor I. %aller for clarifying discussions in the theory of topology and to Dr. S. Rolandson for providing parts of the improved computer program. L. van Hove, Phys. Rev. 89, 1189 (1953.The electronic contribution to the secondand third-order elastic constants of p-type lead telluride have been calculated as a function of temperature and carrier concentration. The results show that the electronic contribution to the elastic constants varies markedly with the carrier concentration if the nonparabolic, nonellipsoidal band model for p-PbTe is used.
Hydrogenated amorphous silicon ͑a-Si:H͒ has been deposited on n-and p-GaSb by the plasma glow discharge technique. The electrical characteristics of metal/a-Si:H/GaSb structures are presented. The current transport in these structures is dictated by the barriers at the metal/a-Si:H and a-Si:H/ GaSb interfaces and the series resistance of the bulk a-Si:H interfacial layer. Space charge limited current in the interfacial layer gives rise to a voltage dependent resistance and increases the forward ''turn-on'' voltage. Furthermore, these structures exhibit extremely low reverse leakage currents and high reverse breakdown voltages. Significantly, rectifying junctions of a-Si:H/p-GaSb have been achieved with barrier heights of ϳ0.4 eV.
Passivation of point and extended defects in GaSb has been observed as a result of hydrogenated amorphous silicon ͑a-Si:H͒ treatment by the glow discharge technique. Cathodoluminescence ͑CL͒ images recorded at various depths in the samples clearly show passivation of defects on the surface as well as in the bulk region. The passivation of various recombination centers in the bulk is attributed to the formation of hydrogen-impurity complexes by diffusion of hydrogen ions from the plasma. a-Si:H acts as a protective cap layer and prevents surface degradation which is usually encountered by bare exposure to hydrogen plasma. An enhancement in luminescence intensity up to 20 times is seen due to the passivation of nonradiative recombination centers. The passivation efficiency is found to improve with an increase in a-Si:H deposition temperature. The relative passivation efficiency of donors and acceptors by hydrogen in undoped and Te-compensated p-GaSb has been evaluated by CL and by the temperature dependence of photoluminescence intensities. Most notably, effective passivation of minority dopants in tellurium compensated p-GaSb is evidenced for the first time.
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