Morphological Characteristics of a-Plane GaN Grown on r-Plane Sapphire by Metalorganic Vapor-Phase Epitaxy

Abstract

We report the morphological evolution of a-plane GaN thin films grown on r-plane sapphire substrates by atmospheric metalorganic vapor-phase epitaxy. The surface flatness is improved under optimized growth conditions which are different from those of c-plane epitaxy. The peak-to-valley height of surface roughness is reduced from 4 to 0.8 mm when GaN is grown at 1120 C on a 40-nm-thick low-temperature GaN (LT-GaN) buffer layer, as well as at 1150 C on a 20-nm-thick LT-GaN. These samples show their highest electron mobility of 220 cm 2 /(V s) at an electron concentration of 1:1 Â 10 18 cm À3 at room temperature.

Development and prospects of nitride materials and devices with nonpolar surfaces

“…Only recently, with the great improvement of the growth techniques and by the use of different nucleation schemes, has it become possible to grow nitrides with planar surfaces different from the (0001) plane [64][65][66][67][68][69][70][71][72][73][74][75][76][77]. Several approaches have been suggested for achieving a planar nonpolar morphology.…”

Effects of initial conditions and growth temperature on the properties of nonpolar a ‐plane AlN grown by LP‐HVPE

“…FWHM value is smaller along the [0001] directions than that along the [1-100] direction. These data indicate that the crystallographic mosaicity is anisotropic because of the anisotropic in-plane growth rate of (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) AlN. This difference is reduced with increasing temperature up to 1450 o C, partly due to the decreased difference in the lateral rates between two in-plane directions at enhanced temperature.…”

“…Ko et al [11] found that high temperature could lead to a fully coalesced a-plane GaN layer. Kusakabe and Ohkawa [12] found that the combination of buffer thickness and temperature is the main factor in obtaining a smooth surface because of the different effects of enhanced temperature on the lateral growth in the [1-100] and [0001] orientations. The growth rates of a-and c-planes were measured at different temperatures as shown in Fig.…”

“…The a-plane AlN layers exhibit a complex strain pattern because of the fact that the AlN is stressed in the [1-100] AlN direction , whereas it is stress-free in the [11][12][13][14][15][16][17][18][19][20] AlN growth direction. The hexagonal unit cell of AlN turns into an orthorhombic cell with three independent lattice parameters, a, b, and c. To determine the lattice parameters a, and b (the (1-100) plane-distance d 1-100 ) of the layers, various AlN reflections were measured by XRD 2θ/ω scanning under symmetric (11)(12)(13)(14)(15)(16)(17)(18)(19)(20), and (10-10) diffraction geometries. For every reflection (hkil) Bragg's law was applied to derive the lattice plane spacing d hkil from the Bragg angle θ hkil .…”

“…Thus, the lattice parameter a and b of strained a-plane AlN can be determined by the measured lattice plane spacing d [11][12][13][14][15][16][17][18][19][20] , and d . The strain in a(x)-and m(y)-axis is expressed as , ,…”