0 and are the spherical coordinates, E V is the top of the valence band, and
I + 1 y[sin 4 6(cos 4 + sin 4 Cos 4 0 - 2/31 (2.4)
with
Y C 2 /2B' A ± B') (2.5)
Following the work of Barber [25], we have used the simplified model of the band structure illustrated in Figure 2.1. In this model the heavy-hole band is considered parabolic and thus the mass m* is a constant, equal to its value at 4.2 K. For energies within 0.02 eV the light-hole band is considered parabolic with a constant slope corresponding to the value of m* at 4.2 K. For higher energies the
2
light-hole band is assumed to take on approximately the same slope as that of the heavy-hole band, but remains separated from the heavy-hole band by A/3 eV [27]. The extrapolation of these two constant slopes creates the kink in the light-hole band at 0.02 eV. Because of the change in slope, the light-hole band has an energy-varying effective mass and in general can only be described in terms of partial FermiDirac integrals [25]. Although the split-off band is parabolically distributed, the apparent effective mass at the top of the valence band is a function of temperature due to the energy displacement at V = 0. Theoretical and experimental studies [33,34] have shown that at high temperatures the heavy-hole band is not parabolic and thus m* is not energy and temperature independent. However, within the range of