Title:

High pressure studies of the conduction of germanium and gallium bands

In order to understand fully and predict accurately the behaviour of transferred electron devices in materials like GaAs an exact knowledge is required of both the structure of the conduction band and of the magnitude of the intervalley parameters. High pressure techniques provide a powerful tool for the study of these parameters and details of their application are described in this thesis. Under pressure different band edges move in energy at different rates giving rise to electron transfer between states which leads to information about the relative strengths of the various scattering processes. By extrapolation back to atmospheric pressure the normal position of the various states in energy and kspace may be obtained. The band structure of germanium at high pressures becomes similar to that of silicon at atmospheric pressure, making Delta1c minima accessible to direct electrical investigation. Here the temperature dependence of the electron mobility has been measured in both the L1c, and Delta1c, valleys of germanium. The L1c mobility was found to vary with the characteristic T1.5 temperature dependence typical of acoustic deformation potential scattering. On the other hand the Delta1c electron mobility was found to vary from ≅ 800 cm2v1s1 at room temperature to ≅ 6400 cm2v1s1 at 120K, corresponding to a temperature dependence of T2.7, similar to that in silicon. The analysis of both the L1 and Delta1c mobilities show that the L1c valley mobility is dominated by the acoustic deformation potential scattering whereas the Delta1c valley mobility is dominated by the intervalley scattering. Using the effective mass ratios mt = 0.288, mt = 1.353 and an acoustic deformation potential of 3.61eV gave Delta1c intervalley coupling constants as 3.7 x 108eV cm1 and 2.9 X 10 8eV cm1 for 430KLO and 320KLA phonons respectively. A negligible coupling constant was found for the low energy, 100KLA phonons in agreement with the already published results for silicon and GaP. To obtain good agreement with the high pressure results the electron effective mass in the Delta1c valleys of germanium was found to be about 50% higher than the measured effective mass in silicon. Since the interpretation of the pressure dependence of the transport properties of semiconductors strongly dependent on the effective mass of electrons, attempts were made to measure this parameter by magnetophonon technique using ultrapure material. Although it was not possible to observe the magnetophonon oscillations in germanium as a function of pressure, nevertheless pressure effects on ultrapure material made it possible to determine with some accuracy the pressure coefficients for both the L1c and Delta1c valleys of germanium. The measured pressure coefficients are: dEL/dP= 4.8 +/ 0.2 X 106eV bar1. dEDelta/dP = 2.4 +/ 0.4 X 10 6 eV bar1. These values of pressure coefficients are independent of the density of states of the minima involved or their atmospheric pressure energy separation. However, taking the density of states effective mass ratios of mL= 0.50, mDelta= 1.58 and mV = 0.4 for the L1c , Delta1c minima of the conduction band and the valence band respectively, best fits to the experimental resistivity were obtained giving the energy separation DeltaELDelta = 0.21 +/ 0.01 eV at atmospheric pressure and (L1c  Delta1c) band crossover at 29 +/ 2 k.bar. The study of GaAs under uniaxial stress has confirmed the TLX conduction band ordering. The resistance of a number of samples with (100) stress was measured up to 10 k bar using four probe method. No exponential change beyond 5 kbar was found in contradiction to the observations of previous workers. The analysis of the (100) stress results appear to resolve the discrepancies between the stress and electroreflectance data and suggest that the large value of the shear deformation potential Sigma(xu)= (21 +/ 9)eV, obtained by other workers must be revised downwards and should have a value closer to 8.0 eV as observed for silicon. A possible explanation as to why those workers obtained such a large value of the deformation potential is also given. The measured electron effective mass in the T1c, minimum under uniaxial stress agrees well with the resistancestress results. It was found that the effective mass is proportional to the applied stress with the constants of proportionality as Deltam* (100) = (0.23 +/ 0.06)% k bar1 Deltam* (111) = (0.075 + 0.015)% k bar1.
