Strain in Thermally-Oxidized GaAs/AlGaAs Multiple Quantum Well Heterostructures as Investigated by micro-PL and micro-Raman Spectroscopy
Rommel Jagus1,3*, Horace Andrew Husay2, Neil Irvin Cabello2, John Paul Ferrolino1, Gerald Angelo Catindig2, Karl Cedric Gonzales2, Elizabeth Ann Prieto2, Kerphy Liandro Patrocenio2, Elmer Estacio2,1, Armando Somintac2, Arnel Salvador2
1Material Science and Engineering Program, University of the Philippines, Diliman Quezon City, Philippines
2National Institute of Physics, University of the Philippines, Diliman Quezon City, Philippines
3College of Science and Mathematics, Mindanao State University – General Santos City, General Santos City, Philippines
* Presenter:Rommel Jagus,
Thermally-oxidized Vertical Cavity Surface Emitting Lasers (VCSELs) are widely used for optoelectronic devices. It has been known that lateral confinement by oxide aperture leads to VCSELs with low current threshold density and high power conversion efficiency. Among the common oxidation layers in VCSELs are AlAs and AlGaAs. Ensuring the quality of this layer pre- and post-oxidation is of paramount importance as it is usually grown near the quantum well (QW) active region for effective current confinement. In this work, we aim to investigate the possible strain induced in GaAs/AlGaAs QW structures due to thermally-oxidized AlAs and AlGaAs. The GaAs/AlGaAs QWs were grown by Molecular Beam Epitaxy (MBE). The structure composed of a 0.2 μm AlAs oxidation layer and a system of GaAs/AlGaAs QWs with nominal GaAs well widths of 50, 90, and 120 Å. Mesa arrays with 250 um diameter were fabricated by photolithography and ammonium hydroxide (NH4OH) wet etching on two samples cut from the grown wafer. One sample was left unoxidized while the other sample was thermally oxidized at 450°C for one hour in order to oxidize the AlAs layer into Al2O3. The oxidation was done in order to induce strain on the grown QWs above the AlAs layer. Oxidation of the mesa structure resulted in the formation of a central region composed of as-grown wells and a concentric region of strained wells. The mesa arrays were characterized using spatially-resolved micro-photoluminescence (μ-PL) and micro-Raman (μ-Raman). The μ-PL showed that the energy peaks of the wells in the oxidized region were shifted relative to the as-grown energy values. This shift was attributed to the strain induced on the wells by the oxidized layer. The μ-Raman confirmed the presence of the GaAs TO and LO phonon modes at 271 and 290 cm-1, respectively. The results were consistent with the strain-induced shift observed in the AlAs Raman peak position as shown by μ-Raman line scans. The change in line shape of the AlAs Raman peak implied a change in the Al mole fraction as a result of the conversion of AlAs to Al2O3. Using the empirical relation reported by Solomon et al., the Al mole fractions in the as-grown and oxidized regions of the mesa arrays were computed.

Keywords: Oxidation, Strain, Quantum wells, Micro-PL, Micro-Raman