Light scattering and luminescence of InGanN epilayers

Raman spectra of In_xGa_1_-_xN (x=0.15,0.25,0.5,0.75,1) layers grown by MBE have been measured. The samples do not have well-defined long range crystalline order, and the spectral lines are broad and asymmetrical. We interpret the measurements using a structural correlation model, in which the disorder in the material is expressed in terms of a single parameter L, the structural correlation length. The calculated line- shapes fit the measured spectra remarkably well in almost all cases. L has a value of around 25 lattice spacings at best, and in some cases as little as 15 lattice spacings. This points to the formation of micro-crystallites; the absence of polarisation of the Raman spectra suggests that these crystallites are randomly oriented. Only the pure InN showed a spectrum corresponding to a pure crystal while the behaviour of L with increasing Ga concentration pointed to a deterioration of sample quality as the amount of Ga is increased. The data obtained from samples grown at higher temperatures suggest that materials rich in Ga favoured higher growth temperatures. The model also shows that the zone-centre frequency (#omega#_0) is significantly different from the maximum in the spectrum. The behaviour of #omega#_0 as a function of composition in InGaN clearly shows one-mode behaviour and its values agree well with theory. Raman scattering by TO phonons is not expected in back-scattering from most semiconductors. However, TO back-scattering from layered structures is a well documented effect. We present a formalism for calculating the intensity of the scattered light for a given experimental configuration. We propose that the mechanism responsible for the effect is the scattering of the part of the incident light reflected at the interface between the layers. Raman experiments from wurtzite lnN were obtained at different angles of incidence and these were compared with the model showing a considerable agreement. Finally we present a photoluminescence study of the InGaN alloy. We report a remarkably low value of the energy gap for our samples. The bowing parameter of the energy gap of InGaN was large (b=3.4eV) compared with the accepted value (b=1eV). This suggests clustering of GaN and InN crystallites in the material, thus implying a structural disorder in the material as already quantified by Raman scattering. (author)Available from British Library Document Supply Centre-DSC:DXN043431 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo