Role of lateral growth on the structural properties of high temperature GaN layer

Abstract

The role of lateral growth on the structural properties of high temperature (HT) GaN epitaxial layer has been investigated by means of transmission electron microscopy (TEM) and X-ray diffraction (XRD). Variations of the lateral growth rate of HT GaN in metal-organic chemical vapor deposition (MOCVD)can be obtained by changing the V/III ratio. It is found that under higher lateral growth rate, dislocation is easier to bend into subgrains away from c axis, and the position where bend occurs is closer to the buffer layer, however, dislocation density does not show to monotonically vary with increasing lateral growth rate. A model concerning the GaN growth dynamics and dislocation bending mechanics has been proposed to explain the correlation between lateral growth and the structural properties of GaN. GaN, lateral growth, dislocation, V/IIIDue to the high interfacial energy associated with GaN thin films on sapphire substrates, a two-step MOCVD growth process has been developed to produce a high-quality GaN films with a smooth surface [1,2] . This process involves the growth of GaN or AlN nucleation layers at low temperatures (LT) on the nitridated sapphire surface, followed by the growth of the device quality GaN film at high temperature. The buffer layer is thought to have an amorphous like structure at its deposition temperature. Before the growth of HT GaN, buffer is annealed at high temperature, crystallizes by solid phase epitaxy, and forms columnar fine crystals. GaN layer growth begins with the nucleation of islands on top of the fine crystals in the buffer. The size and density of these islands depend on the growth conditions of buffer layer [3][4][5] . During further growth, the GaN nucleation islands become bigger at both lateral and vertical directions as the result of geometric selection, then gradually coalesce and form well-developed columnar subgrain structure. Cross-sectional TEM has revealed that these islands are free of threading dislocation (TD) prior to coalescence [6,7] . However, since these islands often have small misorientations with respect to their neighbors, TD array are created during island-island coalescence to accommodate the slight misorientations between grains [3,6] . Previous works have demonstrated that control of the buffer layer growth conditions, such as growth time and annealing temperature, is essential for the film quality, since the buffer layer morphology determines the HT GaN nucleation island morphology. However, GaN microstructure characteristic could not be solely determined by the buffer layer. This indicates that besides the size and density of the initial nucleation islands, the role of the lateral growth of HT GaN cannot be ignored.It is well known that in the lateral epitaxial overgrowth (LEO) of GaN technique, TD will grow vertically to the top of the mask and bend laterally into regions parallel to the mask [8][9][10] . By contrast, synthesis of GaN nanorods, which are expected to greatly improve the photoelectronic device characteristics, requires precisely...

Optical and electrical properties of Si-doped in a-plane GaN grown on r-plane sapphire

“…The results showed that the XRC-FWHM value of the (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) plane along the c-axis direction and m-axis direction increased with increasing SiH 4 flow rate, which demonstrated that the crystalline quality along the c-axis direction slightly decreased. It means that the dislocation density increases with increasing SiH 4 flow rate.…”

“…The peak shape analyses of XRD were based on the separation of the Gaussian and Lorentzian broadening using Pseudo-Voigt function [13,14].…”

“…To circumvent this problem, a material growth with non-polar crystal orientation was developed [1][2][3]. Masayuki et al analyzed a non-polar type a-plane (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) AlGaN/GaN HFET free from polarization, and concluded that the polarization was not formed perpendicular to the heterointerface so the device could allow nearly normally-off operations [4]. However, the extended defect densities in non-polar a-plane (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) GaN were yet very high according to the state-of-the-art of material growth technology.…”

“…Masayuki et al analyzed a non-polar type a-plane (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) AlGaN/GaN HFET free from polarization, and concluded that the polarization was not formed perpendicular to the heterointerface so the device could allow nearly normally-off operations [4]. However, the extended defect densities in non-polar a-plane (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) GaN were yet very high according to the state-of-the-art of material growth technology. For the a-plane (11-20) GaN on r-plane (1-102) grown by metal organic chemical vapor deposition (MOCVD), there occurred a TD density of 3×10 10 cm −2 and a basal stacking fault density of 3.5×10 5 cm −1 [5].…”