Piezoreflectance study of GaAs/AlGaAs digital alloy compositional graded structures

We have studied the piezoreflectance (PzR) spectra related to intersubband transitions from several (001) GaAs/AlGaAs structures fabricated by means of molecular beam epitaxy using the digital alloy compositional grading (DACG) method in a temperature range of 20 to 300 K. These samples include an asymmetric triangular quantum well (ATQW) and three symmetric rectangular quantum wells (SRQW) with different unit cells [50 angstroms (×2), 25 angstroms (×4) and 12.5 angstroms (×8)]. A comparison of the relative intensity of the heavy- and light-hole related features in the PzR spectra and those from photoreflectance measurement reveals the contribution of the strain dependence of the energies of the confined states that allows us to identify unambiguously the features associated with the heavy- and light-hole valence bands. Comparison of the observed intersubband transitions with the envelope function calculations provides a self-consistent verification that the DACG method generated the desired potential profiles for the ATQW. For SRQWs, the 12.5 angstroms unit cell sample essentially has the characteristics of the intended analog configuration while the other two samples displayed observable differences. Furthermore, detailed study of the temperature dependence of the excitonic transition energies indicates that the main influence of temperature on the quantized transitions is through the temperature dependence of the band gap of the constituent material in the well. An anomalous behavior of the temperature dependence of the linewidth of the fundamental conduction to the heavy-hole excitonic feature of the samples is observed, and the possible contributions of the broadening parameters are discussed.