Cathodoluminescence Studies of InGaN Quantum Wells
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ABSTRACT Low temperature cathodoluminescence has been used to investigate the spatial characteristics of light emission in InGai.xN single quantum wells. High spatial resolution, narrow band pass imaging shows the luminescence to be strongly inhomogeneous in wavelength as well as in intensity on a sub-micron scale. Cathodoluminescence spectra correlate favorably with photoluminescence spectra. However, when spectra are recorded from different areas in spot mode, the quantum emission varies significantly in wavelength. The observed variations are consistent with composition inhomogeneities in the quantum well. INTRODUCTION Much interest exists currently in the optimization of the optical properties of InxGal.xN quantum wells with the purpose of increasing the performance of the material. Nitride-based laser diodes have been demonstrated, however their emission is in the short wavelength limit of the visible spectrum (violet) [1]. Although, blue and green LEDs are already commercially available, longer wavelengths from yellow to red are not easily achieved. These limitations would be overcome by the successful growth of layers with high indium content (0.30.2. With misfit dislocations expected at -4-20nm separations, the cross-sectional dislocation density in the quantum well region is of the order of 101_-1014 dislocations/cm 2 . The presence of such high misfit-dislocation densities should result in strain variations that in turn should be highly sensitive to the tilt of the columnar structure in the GaN film. These strain inhomogeneities should have a pronounced effect on the optoelectronic properties of these materials. Little is known about the characteristics of luminescence at such fine spatial resolutions. In this paper we present the results of a study of the variations in the optical properties of In•Ga 1 _xN single quantum wells for x=0.28 and 0.52, performed using lowtemperature cathodoluminescence techniques.
625 Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998Materials Research Society
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Fig. 1. Cathodoluminescence spectra from large scanned areas of InxGaj_,N quantum wells (x=0.28, 0.52). Data taken with an electron beam accelerated voltage of 4kV.
SPECIMENS The specimens were grown by metalorganic chemical vapor deposition on (0001) sapphire substrates, using trimethyl gallium and ammonia as sources, using conventional GaN deposition methods. A 2 pm GaN layer doped with Si (n - 3x10 18 cm3 ) was grown at 1050'C. The InxGa1 . xN quantum wells were grown at 800'C by addition of trimethyl indium. A GaN top layer of 30nm thick was grown on top of the quantum wells. The top layer was kept purposely thin in order to facilitate the optical and structural characterization of the samples. The quantum wells and the top GaN layer were not intentionally doped. CATHODOLUMINESCENCE Cathodoluminescence (CL) was performed in a JEOL 840 scanning electron microscope (SEM) with the specimen close to 10K, using a MonoCL2 system and CF302 cold stage from Oxford
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