Optical characterization of AlxGa1−xAs/GaAs modulation-doped heterostructures grown under As2 and As4 fluxes

Abstract

The authors have investigated the optical properties of AlxGa1−xAs/GaAs modulation-doped heterostructures grown under different experimental conditions, mainly by changing the molecular species of the As beam stemming from a cracker cell at different temperatures. By low-temperature photoluminescence (PL) spectroscopy, they observed that using As2 instead of As4, the crystal quality of the ternary alloy improves significantly since narrower lines in the near-band-gap energy of AlGaAs were obtained when the temperature of the As-cracker zone increased. The experimental position of the energy of the ternary band gap transition as a function of temperature fits the Varshni and Viña functional forms satisfactorily, while it is difficult to assess the data for those samples grown employing As4. It was found that the As2 molecular beam reduces the creation of donor acceptor levels in AlGaAs, as compared with As4. The PL spectra line shape in the GaAs band gap region shows lines associated with band-to-band transition, exciton-bound to neutral-point defects (d,X), and conduction-band to neutral carbon-acceptor (C,A0). The intensity of these transitions was found to be independent of the As molecular-beam species; nevertheless, the photoreflectance spectra of the samples show line shape changes close to the GaAs band gap energy. This region has been related to the electron mobility of two-dimensional electron-gas systems.

Determination of surface electric potential by photoreflectance spectroscopy of HEMT heterostructures

“…Some reports have been focused to determine the origin of Franz-Keldysh oscillations (FKO) that are usually observed in the PR spectra. Now we know that wide-period FKO above energy gap of GaAs are associated to surface electric fields and short-period FKO are originated by the internal AlGaAs/GaAs interface [8][9][10].…”