Effect of Thickness Variation in High-Efficiency Ingan/Gan Light Emitting Diodes
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EFFECT OF THICKNESS VARIATION IN HIGH-EFFICIENCY InGaN/GaN LIGHT EMITTING DIODES J. Narayan and H. Wang Department of Materials Science and Engineering North Carolina State University, Raleigh, NC 27695 Jinlin Ye, Schang-Jing Hon, Kenneth Fox, Jyh Chia Chen, H. K. Choi, and John C.C. Fan Kopin Corporation, 695 Myles Standish Blvd. Taunton, MA 02780 ABSTRACT We have found that InxGa(1-x)N/GaN multi-quantum-well (MQW) light emitting diodes (LEDs) having periodic thickness variation (TV) in InxGa(1-x)N active layers exhibit substantially higher optical efficiency than LEDs with uniform InxGa(1-x)N layers. In these nano-structured LEDs, the thickness variation of the active layers is shown to be more important than In composition fluctuation in quantum confinement of excitons (carriers). Detailed STEM-Z contrast analysis, where image contrast is proportional to Z2 (atomic number)2, was carried out to investigate the thickness variation as well as the spatial distribution of In. In the nanostructured LEDs, there are short-range (SR-TV, 3 to 4 nm) and long-range thickness variations (LR-TV, 50 to 100 nm) in InxGa(1-x)N layers. It is envisaged that LR-TV is the key to quantum confinement of the carriers and enhancing the optical efficiency. We propose that the LR-TV thickness variation is caused by twodimensional strain in the InxGa(1-x)N layer below its critical thickness. The SR-TV may be caused by In composition fluctuation. The observations on thickness variation are in good agreement with model calculations based upon strain effects. INTRODUCTION The AlN-GaN-InN and their alloys have assumed a special importance due to their tremendous potential for fabricating the light emitting devices operating in the red to ultraviolet (UV) energy range.1-5 The In incorporation in the active layer with a composition of InxGa(1-x)N has been the key to obtaining a high optical efficiency despite high dislocation density (~ 1010cm-2). However, its role has not been clarified. Some studies have suggested In composition fluctuation leading to a phase separation to be responsible for high optical efficiency. Since the In content controls the band gap in InxGa(1-x)N alloys, it is envisaged that the composition fluctuation leads to quantumconfined (QC) regions whose size is smaller than the dislocation separation (DS). These QC regions trap the bound excitons, and thus the recombination of excitons would not be affected by the presence of defects such as dislocations.3, 6, 7 The evidence for In composition fluctuation in InxGa(1-x)N layers in multiplequantum-well (MQW) structures has been largely circumstantial. Some authors found the evidence for phase separation (In-rich and In-poor phases) in InxGa(1-x)N (x > 0.3) only in relatively thick layers (3000-4000 A), which were grown by molecular beam epitaxy.8, 9 However, they did not observe any phase separation in thin GaN/InxGa(1-x)N/GaN double heterostructures with x > 0.3, grown under similar conditions. Similarly, in MOCVD (metalorganic chemical vapor deposition)-grown samples
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