Photoluminescence Properties Of Gan/AlGaN Multiple Quantum Well Microdisks
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A great deal of research involving the group III-Nitrides has been directed toward the realization of practical blue laser devices, and recently great successes in the development of edge-emission lasers have been reported." 2 The anticipated commercial release of the edgeemission laser justifies and motivates the consideration of more exotic laser geometries for the III-Nitride system. Vertical cavity and microdisk cavity laser geometries offer several benefits over the edge-emitter. Such benefits include the ability to manufacture and test lasers without material cleaving, the inherent integratable nature of surface-emitting micro-cavities, and the possibility of greatly reduced laser threshold by reduction of the spontaneous emission rate for the lasing modes. Semiconductor microdisks are an example of a micro-cavity geometry which has been successfully demonstrated for the InGaAsP system.4 In addition to the expected optical resonance micro-cavity effects, the fundamental change from a large area plane to small discrete structures may also significantly affect carrier dynamics such as carrier relaxation and available recombination channels. We report the results of picosecond time-resolved photoluminescence (PL) emission studies carried out on a GaN/AlGaN multiple quantum well (MQW) microdisk structure. The results are compared with data obtained for the MQW structure prior to formation of the microdisks and significant changes in the behavior of the various optical transitions are discussed. The implications of our results to future III-nitride micro-cavity lasers are also discussed. EXPERIMENT The GaN/AlGaN MQW structure used for this study was grown by reactive molecular 649 Mat. Res. Soc. Symp. Proc. Vol. 482 ©1998 Materials Research Society
beam epitaxy (MBE) on a (0001) sapphire substrate. It consists of a 500 A AIN buffer layer followed by. growth of a 10 period MQW consisting of 50 A/50 A GaN/AlxGal-xN (x- 0.07) and a 200 A AIN cap layer. All layers were grown nominally undoped. Ion beam etching was used to pattern an array of micro-disks of approximate 9 jim diameter and 50 Ptm spacing. The sample was etched to an approximate depth of 250 nm and thus into the sapphire substrate so that no semiconductor material is present between micro-disks. Figure 1 shows schematics of (a) the MQW structure, (b) MQW micro-disks, and (c) a scanning electron microscopy (SEM) image obtained after micro-disk fabrication. As depicted in Figure l(b), each micro-disk is comprised of a 10 period MQW. The SEM image presented in Figure 1(c) clearly shows the array of disks with a measured diameter of 8-9 1m which is in good agreement with the target diameter of 9 1m. The regularity of the array and the quality of the disk shape are also evident in the SEM image. Time-resolved PL emission spectroscopy measurements were conducted both prior to The PL detection system consists of a 1.33 m and after micro-disk fabrication. monochrometer equipped with a fast micro-channel plate photomultiplier used in a time correlated single pho
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