Structural and Optical Properties of InGaN/GaN Multi-Quantum Well Structures with Different Indium Compositions
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Structural and Optical Properties of InGaN/GaN Multi-Quantum Well Structures with Different Indium Compositions
Chang-Soo KIM∗, Sam-Kyu NOH, Kyuhan LEE 1 , Sunwoon KIM 2 and Jay P. SONG 3 National Research Lab., Materials Evaluation Center, Korea Research Institute of Standards and Science(KRISS), Taejon, Korea 305-600 1 Optronix Inc., Taejon, Korea 305-380 2 Photonic Device Group, Samsung Electro-Mechanics, Suwon, Korea 442-743 3 SongJee Industrial Corporation, Sungnam, Korea 463-500
ABSTRACT The structural and optical properties of InGaN/GaN multiple quantum wells (MQWs) grown on sapphire by MOCVD have been investigated using high-resolution XRD, PL and TEM. The samples consisted of 10 periods of InGaN wells with 6.5nm thickness. The designed indium compositions were 15, 20, 25 and 30% (samples C15, C20, C25, C30, respectively). The thickness of GaN barrier was 7.5nm. The MQW in sample C15 maintained lattice coherency with the GaN epilayer underneath, the MQWs in the other samples, however, experienced lattice relaxation. The crystallinity of the samples decreased considerably with In concentration. As In composition increased, PL peak energy showed a red-shift, and the FWHM of the peaks increased. The increase in the FWHM is attributed to the defects due to the lattice relaxation. For C25 the PL peak intensity increased sharply in spite of the defects due to the lattice relaxation of the sample. It is concluded that the results are related to the In-rich region due to indium phase separation which was observed by TEM image.
INTRODUCTION The III-V nitrides such as GaN, InN and AlN are candidate materials for light-emitting diodes (LEDs), laser diodes (LDs) and high power transistors due to their wide band gaps, thermal stability and strong bond strength [1]. InGaN/GaN multiple quantum wells (MQWs) have been used as active layers for blue LDs. For a GaN epilayer, heteroepitaxial growth on a sapphire substrate has commonly been employed. It has been very difficult to grow a high quality GaN film with a smooth surface because of the large lattice and thermal mismatches ∗
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between GaN and sapphire. The InGaN layer grown on GaN experiences a compressive biaxial strain. As the thickness of the strained layer exceeds a critical value, the accumulated elastic energy is relieved by the formation of misfit dislocations at the interface [2]. The induced strain can, also, cause additional piezoelectric fields and will influence the optical and electrical properties of films and devices. Moreover, for InGaN/GaN QW structures it has been reported that quantum dot-like features of In-rich regions due to the phase separation may induce the exciton localization to enhance the photoluminescence emissions [3,4]. High-resolution X-ray
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