Morphological and microstructural evolution in the two-step growth of nonpolar a-plane GaN on r-plane sapphire

Abstract

In this paper, we report a detailed study on the evolution of surface morphology and microstructure of nonpolar a-plane GaN (a-GaN) through controlled growth interruptions. Microscopy imaging shows that the two-step a-GaN growth went through a roughening-recovery process. The first-step growth (under high V/III and high pressure) produced a rough surface with tall mesas separated by voids. The second-step growth (under low V/III and low pressure) promoted the lateral growth and filled up the voids. Striations that formed during the island coalescence persisted throughout the second-step growth, but could be relieved by an additional third-step growth. The morphological evolution was explained according to the kinetic Wulff plots. The microstructure of the a-GaN films was investigated by transmission electron microscopy (TEM) and x-ray rocking curve analysis. Most of the extended defects observed in the plan-view TEM images were I1 type basal-plane stacking faults (BSFs) and their associated partial dislocations (PDs). It is found that the bending of PDs (at the inclined/vertical growth fronts) within the basal plane toward the m-axes was responsible for the substantial reduction in threading PDs and the increase in BSF dimension. Based on a careful correlation between the morphological evolution and the microstructure development, we proposed a model explaining the possible mechanisms for the great reduction in defect density during the two-step growth process.

The structural and optical properties of selectively grown a-plane GaN with LT-GaN and HT-AlN buffer layers

“…The maximum and minimum FWHMs of the XRCs at an azimuth angle 01 (c-mosaic) and 901 (m-mosaic) were 864 and 1584 arcsec, respectively. This was attributed to anisotropy in mosaic tilt due to the different in-plane growth rate [17]. The XRCs exhibited a low angle shift in comparison with the diffraction pattern of GaN [20].…”

Effect of vicinal off-cut substrates on the basal stacking fault density in nonpolar a-GaN epilayers

“…Under the g ! ¼ 1100 two-beam condition, all BSFs existing on the basal plane are observed to be arrayed along the [1 1 0 0] direction [5,6,11,20], as shown as straight lines in Fig. 4.…”

“…Such surface steps lead to the variation in stacking sequence along the growth direction, which is closely related to the formation of stacking faults [10]. It is well known that the dominant defects in the a-GaN epilayers are BSFs aligned along the GaN [1 1 0 0] direction [5,6,11,20]. [10].…”

“…As an alternative method for reducing polarization effects, nonpolar growth, such as a-plane GaN and m-plane GaN growth, was demonstrated in the generally established LED devices [4]. However, nonpolar GaN films still suffer from high densities of structural defects, such as basal stacking faults (BSFs) and partial dislocations, compared to the polar c-GaN films with high densities of threading dislocations [5,6]. BSFs and partial dislocations alter the electric and optical properties of GaN-based devices acting as carrier scattering centers by forming nonradiative centers within the band gap [7][8][9].…”

Defect reduction in (112̄0) nonpolar a-plane GaN grown on r-plane sapphire using TiN interlayers

“…Given the insensitivity of (1 1 2 0) diffraction to BSFs, the anisotropic broadening of the on-axis (1 1 2 0) XRCs may be caused by the anisotropy in mosaic tilt and/or domain size [18]. According to the invisibility criterion, the 1 1 2 2) reflection of the sample without (a) and with (b) the 10 nm TiN interlayer.…”