Optical Properties and Morphology of GaN Grown by MBE on Sapphire Substrates
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ABSTRACT A series of GaN films grown by MBE on sapphire substrates with different nitrogen sources are characterized by optical transmission, spectroscopic ellipsometry (SE), photoluminescence (PL) and cross-sectional atomic force microscopy (AFM). The film thicknesses determined from broad spectral range transmission measurements and the AFM images are used in the analysis of the SE spectra. Interface roughnesses between the constituent layers, such as the substrate, the buffer and GaN layers are included in the modelling of the SE spectra and are also imaged by cross-sectional AFM. An effective medium type model is used for the modelling of interface and surface roughnesses in the SE spectra. The optical constants of the films in the band edge spectral range are determined in such a way as to simultaneously satisfy the transmission and the ellipsometry data. INTRODUCTION High crystalline quality nitride films have been difficult to grow because of a lack of suitable substrate materials. The growth, at lower temperatures, of a thin AIN buffer layer was shown to produce GaN films exhibiting crack free surfaces[l]. Although high defect concentrations are found to propagate from the substrate/film interface, a blue laser using InGaN quantum wells has been demonstrated[2]. Recent cathodoluminescence measurements have associated the luminescence emission from GaN films to the presence of crystallites in the layers[3]. Both hexagonal and cubic phases were found to coexist, producing emission at energies characteristic of each crystallographic structure. Imaging of the GaN surface by AFM, combined with cathodoluminescence measurements have also indicated the presence of hexagonal crystallites[4]. We report results obtained by multiple characterization techniques used to correlate the morphology of the GaN layers, as measured by cross-sectional AFM, with optical properties, such as the width of the absorption band edge and luminescence emission efficiency. In addition, spectroscopic ellipsometry (SE) and optical transmission measurements are combined to estimate the film optical constants. While optical transmission measurements provide information on the bulk sample properties, ellipsometry has great sensitivity to the quality of the reflecting surface and the epitaxial layer-substrate interface. Accurate reduction of ellipsometric data to the physical quantities of interest requires knowledge of the thicknesses and optical constants of the constituent layers. We have thus used the imaging of interfaces and surfaces by AFM together with optical transmission results in our SE analysis, with the aim of improving the modelling of the substrate/film and film/ambient interfaces. EXPERIMENT The GaN films are grown in our laboratory by molecular beam epitaxy using a solid source for gallium and various nitrogen sources. A thin AIN layer was grown by nitridizing the sapphire surface prior to the GaN growth. The substrates were (0001) oriented sapphire wafers. An ECR 681 Mat. Res. Soc. Symp. Proc. Vol. 423 0 1996 Materials Resear
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