Characterization of GaN epitaxial films grown on SiN x and TiN x porous network templates

Abstract

We report on the structural, electrical, and optical characterization of GaN epitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) on SiN x and TiN x porous templates in order to reduce the density of extended defects. Observations by transmission electron microscopy (TEM) indicate an order of magnitude reduction in the dislocation density in GaN layers grown on TiN x and SiN x networks (down to ~10 8 cm -2 ) compared with the control GaN layers. Both SiN x and TiN x porous network structures are found to be effective in blocking the threading dislocation from penetrating into the upper layer. Supporting these findings are the results from X-Ray diffraction and low temperature photoluminescence (PL) measurements. The linewidth of the asymmetric X-Ray diffraction (XRD) (1012) peak decreases considerably for the layers grown with the use of SiN x and TiN x layers, which generally suggests the reduction of edge and mixed threading dislocations. In general, further improvement is observed with the addition of a second SiN x layer. The room temperature decay times obtained from biexponential fits to time-resolved photoluminescence (TRPL) data are increased with the inclusion of SiN x and TiN x layers. TRPL results suggest that primarily point-defect and impurity-related nonradiative centers are responsible for reducing the lifetime. The carrier lifetime of 1.86 ns measured for a TiN x network sample is slightly longer than that for a 200 µm-thick high quality freestanding GaN. Results on samples grown by a new technique called crack-assisted lateral overgrowth, which combines in situ deposition of SiN x mask and conventional lateral overgrowth, are also reported.

Robust Heteroepitaxial Growth of GaN Formulated on Porous TiN Buffer Layers

“…Naturally, nanoscale porous TiN structures have been reported to be prone to delamination due to their lower adhesion surface area compared to compact and layered TiN structures. 38,51 However, this study confirmed that the porous TiN structure remained firmly bonded to the substrate without delamination (Figure 2b), enabling the porous structure to be more effective at releasing the accumulated residual stress. Cross-section images of TiN formation on the sapphire substrate verified the claim that the porous TiN structure showed little defects caused by delamination, unlike the compact TiN layer (Figure S3).…”

“…This preliminary study revealed that most parts of the GaN/TiN epitaxy grown on foreign sapphire were destroyed after the delamination tests. Although the TiN buffer layer are well known to successfully reduce the lattice mismatch of the heteroepitaxy system, the low wettability of TiN with the nucleation site of the GaN may hinder the formation of a mechanically compatible structure due to the interfacial defects. , …”

“…To solve this issue, we prepared a sufficiently low thickness yet fully coated Ti layer (under 20 nm) on sapphire (Figure e). Nitridation was performed at 1080 °C under a N 2 atmosphere, and Ti tends to agglomerate at this temperature due to low wettability between Ti and sapphire, enhanced Ti atomic mobility, and the difference between surface and volume diffusivity. , The 10–20 nm Ti layer was too thin to endure this agglomeration behavior, and thus, the Ti layer yielded the formation of pores on the site of the grain boundary and misfit dislocation of the Ti layer. After nitridation, we monitored many porous structures when the thickness of the Ti deposition was 10 nm (Figure f), and even 20 nm (Figure S2).…”

“…Cross-section images of TiN formation on the sapphire substrate verified the claim that the porous TiN structure showed little defects caused by delamination, unlike the compact TiN layer (Figure S3). In addition, GaN nucleation gas can penetrate the TiN buffer layer via pores and the GaN nucleation gas can adhere onto the sapphire substrate to fill voids causing TiN layer delamination; the sapphire surface has higher wettability with the GaN nucleation layer than the TiN layer. ,, Therefore, this penetration and growth onto the sapphire substrate should be the preferred method during GaN growth that can be seen in Figure d bottom.…”

“…Although the TiN buffer layer are well known to successfully reduce the lattice mismatch of the heteroepitaxy system, the low wettability of TiN with the nucleation site of the GaN may hinder the formation of a mechanically compatible structure due to the interfacial defects. 38,39 We also investigated the stress mismatch information between the layers induced by intrinsic property disparity during fabrication. Each monolithic layer contains thermomechanical residual stress when heated or cooled, induced by the difference of coefficient of thermal expansion (CTE), Youngs modulus, and Poisson's ratio of the materials, ultimately leading to delamination between the layers.…”