Optical and electrical characterization of Al_xGa_1-xAs layers grown on GaAs obtained by a metallic-arsenic-based-MOCVD system

In this work we report results on the optical characterization of Al _xGa_1-xAs epitaxial layers. The layers were characterized using photoluminescence (PL) to 10 K and photoreflectance (PR) to 300 K. The Al_xGa_1-xAs layers resulted n-type with an electron concentration of 1×10¹⁷ cm^-3and a corresponding carrier mobility of about 2200 cm²/V*s. The studies of the chemical composition by SIMS exhibit the presence of silicon, carbon and oxygen as the main residual impurities. The silicon concentration of around 1×10¹⁷ cm^-3 is very close to the carrier concentration determined by the Hall-van der Pauw measurements. The 10 K photoluminescence response of the samples is strongly dependent on the growth temperature. Growth temperatures higher than 750°C were necessary to detect a reasonable photoluminescence signal. The residual oxygen detected on the samples could be responsible for the weak photoluminescence signal. Photoreflectance spectra present two transitions mainly associated to GaAs and Al_xGa_1-xAs. In addition, short period oscillations near the GaAs band-gap energy are observed, and are interpreted as Franz-Keldysh oscillations associated to the hole-ionized acceptor (h-A^-) pair modulations. Using the Al_xGa_1-xAs band-gap obtained by PR, we calculated the molar fraction of aluminum, giving x = 0.221 of molar fraction.