Abstract
Optical techniques are used to characterise materials designed to emit and/or detect in the infrared. Two tri-metal InGaAlAs quantum well (QW) laser structure samples giown on InP are compared at room temperature to a conventional benchmark InGaAsP QW structure also grown on InP. The spectroscopic methods used to characterise the samples include photo-modulated reflectance (PR), electro-modulated reflectance (ER) and surface photo voltage spectroscopy (SPS). The positions of the QW transitions are compared with a theoretical model of a QW to find the conduction band offset. The InGaAlAs material system is shown to have a much higher conduction band offset of ~66% compared to that of ~40% in the InGaAsP sample. Having measured the QW bandgap and several higher order transitions successfully using conventional spectroscopic methods, a comparison with a novel spectroscopy technique, Fourier transform infrared surface photovoltage spectroscopy (FTIR-SPS) is performed. The technique is designed to make use of the simplicity of SPS whilst avoiding the physical limitations of grating spectroscopy which occur towards the infrared. The technique is shown to be in good agreement with conventional, grating-based spectroscopic measurements on the same sample. Four GalnSb/AIGalnSb type I QW laser structure samples grown on GaAs, aimed at emitting in the mid-infrared at ~4 mum with increasingly strained QWs are analysed at room temperature using FTIR-SPS. FTIR-SPS detects both the barrier bandgap, the QW ground state and up to 5 excited state QW transitions. These results are compared to temperature dependent photoluminescence (PL) of the QWs, where possible, and to temperature dependent PR of the barrier. The behaviour of the laser structure samples is compared to similar measurements made on four analogous multi-QW samples designed to give clear PL. The FTIR-SPS results are compared to a theoretical prediction of the samples' transition energies made using k.p theory. The theory and the experiment are shown to agree reasonably well if the theoretical QW energies are blue shifted by between 15 and 37 meV. The room temperature SPS measurements are also in good agreement with the temperature dependent PR of the barrier and PL of the QW. A novel vertical cavity surface emitting laser (VCSEL) aimed at emitting at 2.3 mum at room temperature and a corollary sample with the top mirrors removed were studied using PR and reflectivity (R) measurements. Using PR it was possible to measure the position of the cavity mode (CM), QW and one higher order transition. These measurements were compared to device measurements made of the QW and CM temperature dependence. The behaviour of the QW was in good agreement with the device measurements; however, there was some discrepancy in the energetic position of the CM in the two measurements. This is believed to be due mainly to effects of annealing the sample on the QW and CM. The measurements show that the device is strongly detuned at room temperature which is thought to account for its relatively poor behaviour. Finally, an InAs QD LED and a dilute InAsN QD test structure were studied as a function of temperature using PL. The dilute nitride QD was found to give luminescence at up to 3.6 mum at room temperature. It is believed that this is the longest interband wavelength PL ever observed from QDs of this material system grown on InP. There is also a considerably reduced temperature dependence in the dilute nitride QD structure as compared to that of nitrogen-free InAs Qds.