is characterized by holes with an energy and direction-dependent effective mass. These two bands can be described by the E vs k relationship [28]
.-
E~) ___1 (k2 E 24 + C2 (k 2 k2 + k 2 k2 + k 2k 2 (21
E(k) _ Ak2 [B x y x z y z } (2.1)
where A, B, and C are the experimentally determined inverse mass band
2 + k2 + k)2 and the upper sign is associated with parameters, k z(k k
the holes in the light-hole band, while the lower sign is associated with the holes in the heavy-hole band. Values of A, B, and C are obtained by cyclotron resonance measurements at 4 K [22,29].
Although warped, the bands are parabolic for small values of k.
However, for larger values of k, the bands become nonparabolic, and along the <100> and <111> directions the heavy- and light-hole bands are parallel over most of the Brillouin zone. This situation, however, is not strictly valid for general directions [30]. The assumption of overall parallelism, while questionable in III-V compounds, is reasonable in the case of Ge and Si [27,31]. The split-off band is separated at = 0 by an energy A = 0.044 eV. [32], and is characterized by an effective mass which is independent of energy and direction. If the anisotropy is small, the square root in equation (2.1) may be expanded [20] and the energy surfaces may be expressed by
ii2k2
E = Ev k (A ± B')j(e,f) (2.2)
2m
where