CHARACTERIZATION OF N-XALXAS (X-LESS-THAN-0.3) EPITAXIAL LAYERS BY PHOTOCURRENT, PHOTOVOLTAGE AND ELECTROLYTE ELECTROREFLECTANCE SPECTROSCOPIES( GA1)

Photocurrent spectroscopy (PCS), photovoltage spectroscopy (PVS) and e lectrolyte electroreflectance spectroscopy (EER) have been compared as techniques for compositional characterization of n-Ga1-xAlxAs epitaxi al layers (x less-than-or-equal-to 0.3, N(d) = 10(18) cm-3) in 0.1 mol l-1 KOH. The donor densities and flatband potentials of the samples i n 0.1 mol l-1 KOH have also been studied by admittance measurements in the frequency range 500 Hz to 10 kHz, both in the dark and under illu mination. The donor densities obtained in this way are compared with v alues derived from mercury probe capacitance measurements on the same samples. Values of the flatband potential (E(fb)) measured in the dark showed a systematic variation with aluminium content consistent with the changes in bandgap energy. The displacement of E(fb) caused by ill umination was found to switch from negative to positive as the Al cont ent was increased from 0% to 30%, demonstrating that the photoinduced surface charge is sensitive to bulk composition. The bandgaps (E(g)) d erived by analysis of photovoltage and photocurrent spectra are compar ed with E(g) values derived by three-point fits of the EER spectra at E0. The latter values were found to depend on the Dc bias applied to t he samples and this is attributed to the effect of the inhomogeneous f ield in the space-charge region.