Modulation spectroscopy study of a strained layer GaAs/GaAsP multiple quantum well structure

Using contactless electroreflectance (CER) and piezoreflectance at 300 K we have characterized a GaAs/GaAS_1-xP_x multiple quantum well (MQW) structure, `GaAs' (nominal) and GaAsP epilayers grown by chloride transport chemical vapor deposition on GaAs (001) substrates. From a detailed lineshape fit to the CER data from the epilayers we have determined the energies of the fundamental band gap and hence the phosphorous composition. The nominal `GaAs' epilayers were found to have phosphorous compositions of about 2.5-3.2%, a result of the phosphorous diffusion between growth chambers in the reactor. The GaAs_1-xP_x epilayer had x = 0.29. For the GaAs_0.97P_0.03/GaAs_0.71P_0.29 MQW comparison between the experimentally observed energies of a number of quantum transitions with a theoretical envelope function calculation, including the effects of strain in the barriers, made it possible to evaluate the unstrained conduction band offset parameter Q_c = 0.50±0.05. Our value for this parameter is discussed in relation to other works. Atomic force microscopy was employed to investigate the surface morphology of the 230 angstroms GaAsP top layer of the MQW in addition to a 2000 angstroms GaAsP epilayer. From the absence of any cross-hatch pattern associated with misfit dislocations on the former we concluded that the GaAsP in the MQW is pseudomorphic. On the other hand the 2000 angstroms epilayer exhibited signs of strain relaxation.