Emission Enhancement of GaN/AlGaN Single-Quantum-Wells Due to Screening of Piezoelectric Field
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ABSTRACT Photoluminescence (PL) enhancement due to the screening of piezoelectric field induced by Si-doping is systematically studied in GaN/AlGaN quantum wells (QWs) fabricated by metal organic vapor-phase-epitaxy (MOVPE). The PL enhancement ratio of QWs for Si-doped directly into the wells was much larger than that for doped only into the barrier layers. This result shows that the crystal quality of the quantum well is not so damaged by heavy Si-doping, which is different from the cases of GaAs or InP material systems. The PL intensity enhancement ratio was especially large for thick wells. The typical value of the enhancement ratio was 30 times for a 5 nm-thick single QW. The optimum Si-doping concentration was approximately 4×1018 cm-3. From the well width dependence of the PL enhancement ratio and PL peak shift under high excitation conditions, we determined that the dominant effect inducing the PL enhancement is screening of piezoelectric field in the QWs. These results indicate that Si-doping is very effective for the application of GaN/AlGaN QWs to optical devices. INTRODUCTION GaN and AlGaN are currently of great interest for application to ultraviolet (UV) laser diodes (LDs) and light-emitting diodes (LEDs) [1,2], because the wide band gap direct transition can be adjusted between 6.2eV (AlN) and 3.4eV (GaN). Recently, UV LDs or LEDs are expected to realize the large capacity optical memories, new chemical processes, medical applications and disposal facilities to industrial wastewater [3]. Some research groups reported on the approaches to the realization of UV optical devices using GaN or AlGaN materials [4,5]. We have already achieved 333 nm current injection emission from Al0.03Ga0.97N/Al0.25Ga0.75N quantum wells (QWs) and 234 nm efficient photoluminescence from AlN/AlGaN QWs. However there are some large problems preventing them from achieving UV optical devices. The most serious problems are difficulty to obtain efficient UV emission from AlGaN QWs, in contrast to InGaN QWs, and current injection through high Al content (more than 30%) AlGaN alloy. The emission enhancement in GaN/AlGaN QWs using Si- or In- [6] doping have been reported, and some mechanisms of the emission enhancement were discussed in previous papers [7-9]. However, the dominant mechanism of them was not yet clarified. In this paper, we systematically investigate the PL intensity enhancement induced by Si-doping into GaN/AlGaN single (S) and multi quantum well (MQW) structures and reveal that the dominant effect is due to screening of piezoelectric field in the QWs. At first, the PL intensity enhancement due to Si-doping is compared between well doping
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MQWs and barrier doping ones. Then, the dependencies of Si-doping concentration and well thickness on the PL enhancement for the SQWs are investigated. Finally, the maximum blue shift by changing the PL excitation power is compared between Si-doped and undoped SQWs.
SAMPLE PREPARATION The samples were grown at 76 Torr on the Si faces of on-axis 6H-SiC(0001) substrate
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