Carrier relaxation and recombination in InGaN/GaN quantum heterostructures probed with time-resolved cathodoluminescence
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Spatially, spectrally, and temporally resolved cathodoluminescence (CL) techniques have been employed to examine the optical properties and kinetics of carrier relaxation in InGaN/GaN heterostructure and single quantum well (QW) samples. CL images of the QW sample revealed a spotty cellular pattern indicative of local In compositional fluctuations on a scale of < 100 nm. The compositional variations induce local potential fluctuations, resulting in a strong lateral excitonic localization at InN-rich regions in the InGaN QW layer. Time-resolved CL measurements revealed a lateral spatial variation in the luminescence decay time which correlates with the spatial variation in the luminescence efficiency. A reduced lifetime is observed at boundary regions between centers of excitonic localization. A detailed time-resolved CL study shows that carriers generated in the boundary regions will diffuse toward and recombine at the InN-rich centers. An electron beam induced modification of the emission spectra was observed for InGaN/GaN heterostructure samples. Exposure to the e-beam resulted in a shift in the near-band gap emission to higher energies with a simultaneous increase in the emission intensity. These result are interpreted as a modification of the surface passivation through e-beam exposure and carbidization of the surface. UNTRODUCTION
The study of InGaN/GaN heterostructures and QWs has received a great deal of interest recently because of rapid progress and realization of high quality blue and green InGaN single QW light emitting diodes' and room-temperature pulse oscillation of InGaN single2 and multiple 3 QW laser diodes. Time-resolved photoluminescence has been used to study the radiative recombination in InGaN single4'3 and multiple6 QW structures. Several authors observed that the emission results mainly from recombination of excitons localized at certain potential minima originating from a large InMGa,.N compositional fluctuation.s Direct evidence for exciton localization was shown in InGaN single QWs by Chichibu et al. 9 However, the details of the carrier diffusion and capture process, and the relaxation of localized excitons are far from being well understood, particularly in the boundary regions between centers of exciton localization. In this study, we have employed spatially, spectrally, and temporally resolved cathodoluminescence (CL) to investigate the carrier relaxation and recombination in InGaN/GaN heterostructues and single QW samples. The impact of the large InxGa1.1N compositional fluctuations on the spatial variation in the luminescence intensity and lifetime is explored. Quantitative lifetime information is obtained in connection with these spatial variations. The sensitivity of the optical behavior to electron beam bombardment is further explored in an IniaN/GaN heterostructure sample. The results reveal that details of the surface passivating layer can have large effects on the subsequent luminescence properties. 193 Mat. Res. Soc. Symp. Proc. Vol. 512 01998 Materials Research Society
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