Low‐dislocation density AlGaN layer by air‐bridged lateral epitaxial growth

Abstract

A high quality AlGaN layer with low dislocation density and low c-axis tilt angle in wing regions was demonstrated by the advanced ELO technique, namely air-bridged lateral epitaxial growth. An underlying GaN seed layer was grooved along the 〈1100〉 GaN direction to the sapphire substrate, whose sidewalls and etched bottoms were covered with silicon nitride masks, and regrowth of AlGaN was carried out by a low-pressure metalorganic vapor phase epitaxy system. Fabrication of air-bridged structures suppresses interference of nucleated poly-crystals on the silicon nitride masks during lateral growth, and the low dislocation density AlGaN layer was realized. The threading dislocation density in the wing regions was reduced to 2 × 10 7 cm -2 and the c-axis tilt angle was 0.19°.1 Introduction III -V nitrides have shown potential for use in shorter wavelength optical devices. These nitride devices are usually fabricated on sapphire substrates because of a lack of large-size GaN substrates. The threading dislocation (TD) density has been known between 10 8 cm -2 and 10 10 cm -2 due to the large lattice mismatch between GaN and sapphire. In order to reduce the TD density, epitaxial lateral overgrowth (ELO) techniques have been performed [1,2]. The ELO techniques have achieved to considerable success in reducing the TD density to 10 6 cm -2 range in wing regions. A c-axis tilting in the wing regions, however, was observed with a different direction on both sides of seed regions in an azimuth perpendicular to the stripe direction. The tilt angle of the wing regions is commonly 0.2°-1.0° for a thin GaN layer by using (0002) X-ray diffraction (XRD) rocking curve measurements [3,4]. In our previous work, we have reported an advanced ELO technique, namely "air-bridged lateral epitaxial growth (ABLEG)" [5,6]. By using the ABLEG technique, the TD density at the surface of the wing regions is reduced to less than 5 × 10 6 cm -2 and the tilt angle of the wing regions is lowered to be 0.1°. In conventional nitride based device structures, AlGaN layers are grown on underlying GaN layers. It is necessary that high power and low noise optical devices have crack-free and thick AlGaN cladding layers. It is not, however, easy to grow high-crystalline quality AlGaN/GaN heterostructures due to the lattice mismatch. In order to suppress the crack formation, growth of an AlGaN layer on an underlying AlGaN layer instead of the underlying GaN layer is effective, because the lattice mismatch is relatively small. But it is quite difficult to grow a low TD density AlGaN layer on sapphire substrate, and to reduce the TD density by ELO techniques because of nucleation of poly-crystals on the mask surface [7]. Several apporaches have reported a TD density reduction of AlGaN layer [8,9]. The ABLEG technique not only reduces the interfacial stress between the wing regions and the masks, but also suppresses interference of nucleated poly-crystals during the lateral growth. In this study, we have performed ABLEG of AlGaN and achieved a TD density reductio...

Spatially resolved cathodoluminescence study on AlGaN layer fabricated by air‐bridged lateral epitaxial growth

“…That is, the growth temperature was 990 °C and the pressure 200 Torr. Some details of the growth conditions are reported in ref [15]. The TD densities of the wing regions and the seed regions in the ABLEG-AlGaN layer are 2 × 10 7 cm -2 and 2 × 10 8 cm -2 , respectively [15].…”

“…Some details of the growth conditions are reported in ref [15]. The TD densities of the wing regions and the seed regions in the ABLEG-AlGaN layer are 2 × 10 7 cm -2 and 2 × 10 8 cm -2 , respectively [15]. The samples were characterized at a temperature of 77 K in the CL and secondary-electron (SE) modes, simultaneously detecting energy dispersive X-ray (EDX) signals, in a HITACHI S-4700 cold field emission scanning electron microscope (SEM) equipped with an Oxford Mono-CL3 system.…”

“…In order to fabricate low TD density AlGaN layer, air-bridged structure is effective in avoiding a bad influence from the poly-crystals on the mask surface. The air-bridged AlGaN layers are fabricated on patterned sapphire, patterned GaN on SiC, and patterned GaN on sapphire [12][13][14][15]. Though the fluctuations of Al content are important issues, there are few reports about ELO AlGaN layers.…”

AlGaN films grown on trenched sapphire substrates using a low‐temperature GaNP buffer layer by MOCVD

“…Generally the threading dislocation density of GaN films grown on the sapphire substrates with a low-temperature (LT) GaN or AlN buffer layer has a large dislocation density due to the large lattice mismatch between GaN and sapphire [1]. To improve the device performance, lowdislocation-density nitride films are desirable and necessary.…”

Al0.17Ga0.83N film with middle temperature-intermediate layer grown on trenched sapphire substrate by MOCVD

“…Generally, the threading dislocation density of GaN films grown on the sapphire substrates with a low-temperature (LT) GaN or AlN buffer layer has a large value due to the considerable lattice mismatch between GaN and sapphire [1]. To improve the device performance, low dislocation density nitride films are desirable and necessary.…”