Photoreflectance and photoluminescence investigations of two-dimensional electron gas in pseudomorphic high electron mobility transistor structures

We present the photoreflectance (PR) and photoluminescence (PL) studies on three InxGa1−xAs/AlGaAs high electron mobility transistor structures with different indium composition and width of well. The indium composition of the InxGa1−xAs channel layer is x=0.195, 0.175, and 0.14; and the channel well width is 150, 150, and 160 Å for samples A, B, and C, respectively. These contactless techniques provide a convenient method to determine the carrier density, Fermi energy, subband energies, and alloy composition. For the three samples, the PL spectra present two features. The feature located at low-energy side is identified to be 11H transition and the higher energy one is attributed to 21H transition. The PL spectra of the 11 and 21H transitions are analyzed taking into account the effect of the subband filling. The 11 and 21H transitions shift to lower energy due to the increasing of indium composition, which affects the energy depth of the electron and hole quantum wells. In the PR spectra, not only the subband transitions coming from the channel well but also features associated with the GaAs and AlGaAs bulk layers are found. Two prominent features observed at about 1.42 and 1.8 eV are attributed to the interband absorption of GaAs and AlGaAs materials. Our detailed analysis gives the key parameters such as the subband energies, Fermi energy, and the carrier density. A good agreement is found between the experimental data and the theoretical simulation results.