Cross-sectional cathodoluminescence of GaN epitaxial films
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M. Herrera Zaldivara), P. Femnndez and J.Piqueras Departamento de Fisica de Materiales, Facultad de Fisicas, Universidad Complutense, 28040 Madrid, Spain. a)Permanent address: instituto de Fisica, B. Universidad Aut6noma de Puebla, 72570 Puebla. Mdxico.
Abstract Cathodoluminescence (CL) in the scanning electron microscope has been used to investigate the variation along the growth axis direction of luminescence emission from epitaxial GaN films. CL spectra recorded at different positions of the sample cross-section as well as monochromatic CL images show strong spatial variations of the different luminescence emissions along the growth axis. At the buffer layer-substrate interface and at the top part of the sample, which corresponds to a Si doped epilayer, enhanced CL emission is observed as compared with the relatively low emission in the central region of the cross-section. The nature of the defects responsible for the observed CL distribution is discussed. INTRODUCTION GaN thin films grown on sapphire substrates have very high dislocation densities as compared with films of other semiconductors used for optoelectronic applications [ 1 ]. Although the dislocations in GaN are not so efficient non-radiative recombination centers as in other III-V compounds, it is important to know their influence on the luminescence emission in order to understand the emission mechanisms in GaN based heterostructures. Since dislocations and other extended defects are generated as a consequence of the lattice mismatch between the layer and the substrate, the study of the luminescence at the interface, as compared with the emission in other regions of the film, would provide information on the effect of mismatch defects and their spatial distribution. Defects along cross-sections of GaN films have been studied by transmission electron microscopy [ 1]. Stacking faults appeared in the vicinity of the substrate interface and dislocations were observed propagating in a direction normal to the substrate. The density of dislocations reaching the top surface was the same as in the rest of the film. In order to study the effect of defects generated at different depths in GaN films on their luminescent properties we have performed in the present work cross-sectional CL microscopy observations. The use of cross-sectional CL microscopy enables a direct study and observation of the emission across the film at constant excitation conditions. EXPERIMENTAL METHOD The samples used were GaN films grown on (0001) sapphire by VPE. The films consisted of an 8 [im thick GaN buffer layer and two 2 jtm thick epilayers. The upper layer was Si doped with a carrier concentration of 1018 cnf3 . The films were observed in the secondary electron and CL modes of SEM by using either a Hitachi S-2500 or a Leica S-440 microscopes with an 703 Mat. Res. Soc. Symp. Proc. Vol. 482 0 1998 Materials Research Society
electron beam energy of 15 keV. The samples were mounted in the specimen holder of the SEM either with the upper surface or with a lateral fracture surface
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