Determination of subband energy levels of double quantum well AlGaAs lasers by photoreflectance and self-excited electron Raman scattering

Abstract

It is important to determine the subband energy levels of electrons and holes in quantum well (QW) for design improvement of laser characteristics such as very low temperature sensitive threshold current. We have found the self-excited electronic Raman scattering (ERS) spectra due to intersubband transitions above threshold from InGaAs and InGaP QW lasers, and determined the subband levels of electrons and holes at room temperature [I], [2].In this paper, subband energy levels of an AlGaAs separate-confinement-heterostructure (SCH) double quantum well (DQW) layer are determined by photoreflectance (PR) and compared with those determined by the self-excited ERS of lasers fabricated from the same DQW structure but with different waveguide thickness. Subband energy levels determined by both measurements are in a good agreement.The layer structure of the SCH-DQW is grown on a GaAs substrate by MOVPE. Two 8nm-Alo ~GaosAs-QWs are formed in AI0 35Ga065As wavegideibanier layers. These layers are not intentionally doped and surrounded by n-and p-A10&@52As cladding layers (>lpm). The thickness of A I O~~G Q~~A S waveguides adjacent to the Al,~&a05& cladding layers is 12nm. The barrier layer between QWs is 8nm thick.Lasers are fabricated from the same DQW structure but with thicker waveguide thickness (300nm). They have a 5pm loss-guide stripe and 900pm cavity length. Lasing wavelength and threshold current are 787.3nm and 31mA, respectively at 300K. The laser emission is strongly TE polarized. Fabrication and characteristics of the laser is described elsewhere [3].The PR of the layer is measured using a stabilized 532nm SHG YAG laser modulated at 720Hz by an optical chopper to moddate electric field by generating photo excited carriers in p-cladding layers. Carriers in the waveguides, the barrier, and QW layers and nand p-cladding layers adjacent to the wavegude layers are depleted. F is determined from Frantz-Keldish oscillation frequencies in the waveguide layer and depleted cladding layers. Fig. 1 shows the photoreflectance spectrum at room temperature. The FKOs are clearly observed in A10 5zAs cladding layers and AI0 35Ga 65As waveguide layers. The electric field F is determined as 90kVhm in both waveguide and cladding layers. The subband energy levels and wave functions are calculated by a finite difference numerical method of the Schroedinger equation using the band offset ratio AEc/AEG = 0.64 determined from the ERS described bellow. It shows that there are the subband energy levels of two electron subbands, e l and e2, three heavy hole subbands hhl, hh2, hh3, two light hole subbands IhI and lh2 in the quantum wells under electric field F=90kVlcm by numerical calculation of the Schroedinger equation. Also due to the electric field, there is an electric subband e2hear e2 in the QW for the layer structure shown in Fig.1. Peaks are assigned as, A el-hhl, Ao: el-IhI, B: el-hh2, C : (e2'/ed)-hhl, CO: (e2'/e2)-lhI, and AFKI is the first Franz-Keldish oscillation of A, CFK~ is the first Franz-Keldish oscillation of C in th...