Cathodoluminescence Study of Diffusion Length and Surface Recombination Velocity in III-V Multiple Quantum Well Structur
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U. S. Army Research Laboratory, Ft. Monmouth, NJ 07703 ABSTRACT We describe studies of luminescence and lateral transport properties of excited carriers in GaAsAIGaAs multiple quantum well (MQW) structures by cathodoluminescence measurements in a scanning electron microscope. We examine the effect of in-plane, etch-defined feature size on MQW luminescence efficiency and variability, and determine the diffusion length and its temperature dependence from - 8K to 250K. Our measurements also provide information about nonradiative surface recombination velocity at the side walls of etch-defined MQW structures. INTRODUCTION Semiconductor quantum well structures have attracted considerable interest because of their applications in a wide variety of microelectronic and optoelectronic devices. Two-dimensional patterning to form quantum wires and quantum dots raises questions about sidewall damage during patterning by lithography and etching. Whereas luminescence in unpatterned quantum well materials depends upon the basic properties of these materials and any defects which are present, luminescence by quantum wires and dots also depends upon the effects of sidewalls on radiative and nonradiative carrier recombination, particularly when the wire width or dot diameter is smaller than, or on the order of, the diffusion length for excited carriers in the quantum wells. Photoluminescence (PL) measurements are often used to investigate the effect of patterning on luminescence efficiency in quantum well structures, which can be modeled in terms of the diffusion length L, nonradiative surface recombination velocity s, and minority carrier diffusion coefficient D [I]. Measurements with submicron spatial resolution are possible with cathodoluminescence (CL), in contrast with more spatially averaged PL measurements. With CL measurements the variability in luminescence among features in quantum dot structures can be characterized. Also, the minority carrier diffusion length can be measured, and information about the nonradiative surface recombination velocity can be obtained more directly than by PL measurements. EXPERIMENTAL
Sample preparation and description For these experiments, a test pattern consisting of arrays of squares was fabricated in a multiplequantum-well (MQW) material. The MQW material consisted of three 5nm GaAs quantum wells, separated by 25nm Al0 . 3Ga 0 .7 As barriers, and with a 5nm GaAs layer cap layer; three quantum wells were used to enhance the luminescence signal. All epitaxial layers were undoped and were grown on a semi-insulating GaAs (100) wafer by MOCVD. The test pattern consisted of arrays of uniform size microsquares of Igm and 2jim edge lengths, which were used to characterize feature size effects on luminescence strength and variability, and larger 250gtm x 1000gim pad areas, which were used to investigate nonradiative recombination velocity. To form the test pattern, electron-beam lithography and lift-off were used to pattern a 25nm Au / 5nm Cr thin film on the MQW material covering the areas to be
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