EFFECT OF STRAIN ON A 2ND-ORDER VAN-HOVE SINGULARITY IN ALXGA1-XAS INYGA1-YAS QUANTUM-WELLS/

We have performed low-temperature photoluminescence and photoluminesce nce excitation (PLE) measurements on highly degenerate p-type GaAs and InyGa1-yAs quantum wells. In the PLE spectrum of the GaAs well, evide nce of a second-order van Hove singularity in the joint density of sta tes of the ground-state light-hole and electron bands is found. This s ingularity results from the equality of ground-state light-hole and el ectron effective masses near the Gamma point, being a much more restri ctive demand than the usual condition for a van Hove singularity, whic h requires only the equality of first derivatives of the subband dispe rsions. The second-order van Hove singularity gives rise to a power-la w divergence at the singular point, whereas the corresponding usual va n Hove singularity results in a steplike discontinuity in the joint de nsity of states. The observed singularity could be described extremely well by a simple analytical model. The increased energy gap between l ight- and heavy-hole ground states in the compressively strained InyGa 1-yAs well enhances the valence-band parabolicity, resulting in the di sappearance of the van Hove singularity. Furthermore, it is shown that the anisotropic character of the heavy-hole ground state in GaAs is s trongly suppressed in the InyGa1-yAs system. All experiments are in go od agreement with our numerical modeling, based on an exact solution o f the 4 x 4 Luttinger Hamiltonian.