Jiankun Yang

Microstructure and Optical Properties of Nonpolar m-Plane GaN Films Grown on m-Plane Sapphire by Hydride Vapor Phase Epitaxy

Thick nonpolar (10 (1) over bar0) GaN layers were grown on m-plane sapphire substrates by hydride vapor phase epitaxy (HVPE) using magnetron sputtered ZnO buffers, while semipolar (10 (1) over bar(3) over bar) GaN layers were obtained by the conventional two-step growth method using the same substrate. The in-plane anisotropic structural characteristics and stress distribution of the epilayers were revealed by high. resolution X-ray diffraction and polarized Raman scattering measurements. Atomic force microscopy (AFM) images revealed that the striated surface morphologies correlated with the basal plane stacking faults for both (10 (1) over bar0) and (10 (1) over bar(3) over bar) GaN films. The m-plane GaN surface showed many triangular-shaped pits aligning uniformly with the tips pointing to the c-axis after etching in boiled KOH, whereas the oblique hillocks appeared on the semipolar epilayers. In addition, the dominant emission at 3.42eV in m-plane GaN films displayed a red shift with respect to that in semipolar epilayers, maybe owing to the different strain states present in the two epitaxial layers. [DOI: 10.1143/JJAP.47.3346]

Hydride Vapor Phase Epitaxy Growth of Semipolar (1013)GaN on Patterned m-Plane Sapphire

We have investigated the hydride vapor-phase epitaxy growth of (1013)-oriented GaN thick films on patterned sapphire substrates (PSSs) ( 1010). From characterization by atomic force microscopy, scanning electron microscopy, double-crystal X-ray diffraction, and photoluminescence (PL), it is determined that the crystalline and optical qualities of (1013) GaN epilayers grown on the cylindrical PSS are better than those on the flat sapphire. However, two main crystalline orientations ( 1013) and (1122) dominate the GaN epilayers grown on the pyramidal PSS, demonstrating poor quality. After etching in the mixed acids, these ( 1013) GaN films are dotted with oblique pyramids, concurrently lining along the (3032) direction, indicative of a typical N-polarity characteristic. Defect-related optical transitions of the (1013) GaN epilayers are identified and detailedly discussed in virtue of the temperature-dependent PL. In particular, an anomalous blueshift-redshift transition appears with an increase in temperature for the broad blue luminescence due to the thermal activation of the shallow level.

Direct growth of graphene on gallium nitride by using chemical vapor deposition without extra catalyst

Graphene on gallium nitride (GaN) will be quite useful when the graphene is used as transparent electrodes to improve the performance of gallium nitride devices. In this work, we report the direct synthesis of graphene on GaN without an extra catalyst by chemical vapor deposition. Raman spectra indicate that the graphene films are uniform and about 5–6 layers in thickness. Meanwhile, the effects of growth temperatures on the growth of graphene films are systematically studied, of which 950 °C is found to be the optimum growth temperature. The sheet resistance of the grown graphene is 41.1 Ω/square, which is close to the lowest sheet resistance of transferred graphene reported. The mechanism of graphene growth on GaN is proposed and discussed in detail. XRD spectra and photoluminescence spectra indicate that the quality of GaN epi-layers will not be affected after the growth of graphene.

Cross-stacked carbon nanotubes assisted self-separation of free-standing GaN substrates by hydride vapor phase epitaxy.

We report a novel method to fabricate high quality 2-inch freestanding GaN substrate grown on cross-stacked carbon nanotubes (CSCNTs) coated sapphire by hydride vapor phase epitaxy (HVPE). As nanoscale masks, these CSCNTs can help weaken the interface connection and release the compressive stress by forming voids during fast coalescence and also block the propagation of threading dislocations (TDs). During the cool-down process, thermal stress-induced cracks are initiated at the CSCNTs interface with the help of air voids and propagated all over the films which leads to full self-separation of FS-GaN substrate. Raman and photoluminescence spectra further reveal the stress relief and crystalline improvement of GaN with CSCNTs. It is expected that the efficient, low cost and mass-producible technique may enable new applications for CNTs in nitride optoelectronic fields.

Defect reduction in semipolar {10} GaN grown on m-sapphire via two-step nanoepitaxial lateral overgrowth

A method to obtain high quality semipolar {10} GaN grown on m-plane sapphire is presented. This method is similar to two-step nanoepitaxial lateral overgrowth (2S-NELOG) by combining a TiN interlayer and self-assembled SiO2 nanospheres. For the 2S-NELOG semi-GaN, the root-mean-square (RMS) roughness is 1.8 nm with a scan area of 5 × 5 μm2. The reduction of the defect density is demonstrated using high resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM). The full widths at half maximum (FWHMs) of the on-axis X-ray rocking curves (XRCs) are 381 and 524 arcsec, respectively rocking toward the [30] direction and the [110] direction. The anisotropy of the mosaic is lower compared to planar and TiN semi-GaN. In addition, Raman analyses also show the partial relaxation of the stress in the 2S-NELOG semi-GaN.