Abstract
The band structure and transport properties of the technologically important materials GaAs and InP and (GaIn)(AsP)/InP were studied as a function of temperature, pressure and magnetic field. The electron effective mass, m*, was measured as a function of energy gap, E[o], using hydrostatic pressure as a dummy variable to change the band structure. In InP it was found that the three band k-p model was sufficient to explain the energy gap dependence of the effective mass while in GaAs, a small contribution from higher bands was necessary. In Ga[x]In[1-x]As[y]P[1-y] alloys, dm* /dEo was found to be about 50% larger than in compound semiconductors. The variation, dm*/dE[o], as a function of composition follows the variation of alloy disorder. Based on the trends in band gap dependence of the effective mass in various alloy systems and compound semiconductors, a tentative explanation was offered to account for the large dm*/dE[o] value in these alloys. Photoconductivity measurements were made as a function of pressure, to study the variation of the pressure coefficient of the direct band gap with composition. The results indicate that dEo/dP is also influenced by alloy disorder. Measurements of the hole mobility, mu, as a function of pressure show that, while in GaAs and InP dmu/dP is positive, in the alloy dmu/dP is negative. These results were analysed in terms of the transport parameters and their pressure dependence to evaluate the pressure coefficient of the heavy hole effective mass, dm[ch]/dP. It was found that, while in InP and the alloys dm[ch]/dP is positive, in GaAs dm[ch]/dP is negative. Measurements of the activation energy of the Mn acceptor level in (GaIn)(AsP) and its pressure coefficient show that the closer the level is to the valence band the more it is influenced by it, contrary to the arguments that the deep levels are independent of the host material. This is explained on the assumption that the Mn impurity wave functions are somewhat delocalised and are influenced by the valence band. In Mn+Ge co-doped alloys, it was found that when the Mn activation energy is large, even in heavily doped, closely compensated p-type materials the mobility is dominated by phonon scattering. This anamoly is attributed to the formation of Mn+Ge dipoles assosiated with the diffusion of neutral Mn. This hypothesis was supported by the measurements of the diffusion profiles of Mn into various substrates.