Properties of GaN Epitaxial Layer Grown by MOVPE on MgAl 2 O 4 Substrate
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A. KURAMATA, K. HORINO, K. DOMEN, R. SOEJIMA, H. SUDO and T. TANAHASHI Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, Atsugi 243-01, Japan Tel: +81-462-48-3111, Fax: +81-462-48-5193 ABSTRACT We propose a MgAl 20 4 substrate for GaN-based laser diodes. We grew GaN epitaxial layers using metal-organic vapor phase epitaxy on a MgA120 4 substrate. The GaN on the MgAI20 4 showed a narrow X-ray diffraction peak of 310 arcsec, a photoluminescence dominated by the band-edge emission, and good electronic properties. The optical cavity was fabricated by cleaving the GaN on the MgAl20 4, and the stimulated emission from the cavity using optical pumping was observed. We also fabricated a light emitting diode (LED) with a AlGaN/InGaN double-heterostructure. The electroluminescence was dominated by the band-edge emission of InGaN with a narrow FWHM of 13.5 nm. The characteristics of LED on the MgAl O 4 were comparable to that on a Al 20 3 substrate. MgAl 2 O 4 is feasible as a substrate for laser applications. INTRODUCTION GaN and related alloy compounds of InGaN and AlGaN have a direct band-gap that is suitable for making optical devices. The band-gap energy can also be varied in a wide range, from the 1.9 eV of InN to the 6.2 eV of AIN. GaN devices are more tolerant of higher operating temperatures, due to their larger band-gap when compared with other Ill-V semiconductors and Si. The growth techniques for GaN have recently made progress, and high quality GaN devices can be fabricated [1]. For example, high-intensity blue to yellow light emitting diodes (LEDs) have been fabricated [2-3]. However, a laser diode using these materials has not yet been made. The important challenge for realizing GaN-based laser diodes is the fabrication of optical cavity. It is difficult to make a cavity by cleavage when epitaxial layers are grown on a sapphire (A120 3) substrate. This is because sapphire has little tendency toward cleavage and the cleavage direction of sapphire is different from that of GaN. One way to make a cavity mirror is polish, but it is not suitable for a commercial production. For this reason, it is necessary to develop a new technique such as the use of a different cleavable substrate, dry etching, and selective growth. We think that the use of a cleavable substrate is the best way, because the method is simple and the cleaved facet has an ideal flatness for a cavity mirror. If we use metal-organic vapor phase epitaxy (MOVPE) as the growth method, the substrate material requirements are as follows: 1) It must be stable both thermally and chemically, because MOVPE growth of GaN is conducted at about 1000°C in a reducing atmosphere of ammonia and hydrogen. 2) The lattice constant and thermal expansion coefficient should match GaN. 3) The cleavage direction must match with that of GaN. 6H-SiC is known to be a good material as the substrate for GaN growth [4-6]. It can be cleaved in the same direction as GaN, and high-quality GaN epitaxial layers have been grown on it. However, cracks are easily generated in the GaN
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