Influence of substrate orientation on photoluminescence in InGaN/GaN multiple quantum wells
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Influence of substrate orientation on photoluminescence in InGaN/GaN multiple quantum wells P.Chen, S.J.Chua and W.Wang Institute of Material Research and Engineering, 3 Research Link, Singapore 117602 ABSTRACT In this study, the photoluminescence (PL) spectra of InGaN/GaN multiple quantum wells (MQWs) grown on sapphire substrates with different orientation at 720 oC were investigated at room temperature. Four different substrates were used, which have different surface step structures. Their orientations are: c-plane, a-plane, c-plane with off-set of 2 degree and c-plane with off-set of 6 degree towards a-plane. PL spectra from InGaN/GaN MQWs grown on c-plane substrates showed double-peak emission, while those from the MQWs grown on the others substrates showed single-peak emission. The strongest emission was found on the substrate with off-set of 2 degree towards a-plane. A Shift of PL peak position to long wavelength was observed on the substrates with an off-set angle. Meantime, all emissions were also investigated at different excitation power density. Experimental results indicate that the regular step structure on the substrates can seriously affect the growth of InGaN/GaN MQWs. Segregation effect in InGaN will lead to the high In composition regions (quantum-wire like structure) in the substrates with an off-set angle due to the regular steps on them, as observed by atomic force microscopy. It can be concluded that the surface steps on substrate play an important role in the formation of the In-rich InGaN quantum-wire like structure.
INTRODUCTION InGaN/GaN multiple quantum wells (MQWs) is the key structure for blue and green light-emitting diodes[1] and laser diodes[2]. Thus, the understanding of optical properties of the MQWs structure becomes highly important, however at present, the properties are not fully understood. In the MQWs, the exciton localization effect is generally considered to play an important role in the radiative recombination process. It has been known that potential fluctuations, originating from segregation effect in InGaN, is one of important reasons resulting in the exciton localization effect in InGaN/GaN MQW. Up to now, owning to the high-quality epilayers grown on the substrate [3-9], most results on the topic are based on the InGaN/GaN MQWs grown on exact c-plane sapphire substrate. There are several mechanisms for carrier localization in the MQWs: (a) monolayer thickness fluctuation, (b) spatial compositional (or strain) undulation, and (c) complete phase separation [10]. Case (a) and case (b) usually produce a weak potential undulation in the MQWs. A strong complete phase separation, case (c), produces well-defined quantum dots, disks, or segmented MQWs depending on the lateral size. The surface structure of substrate can play an important role in III-V alloy growth. AlGaAs/GaAs quantum-wire (QWR) laser diode grown on a V-grooved substrate has been achieved many years ago [11]. In that case, GaAs QWR formation occurred at the bottom of the grooves due to the different migration
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