This paper proposes a new technique to engineer the Fin channel in vertical GaN FinFET toward a straight and smooth channel sidewall. Consequently, the GaN wet etching in the TMAH solution is detailed; we found that the m-GaN plane has lower surface roughness than crystallographic planes with other orientations, including the a-GaN plane. The grooves and slope (Cuboids) at the channel base are also investigated. The agitation does not assist in Cuboid removal or crystallographic planes etching rate enhancement. Finally, the impact of UV light on m and a-GaN crystal plane etching rates in TMAH has been studied with and without UV light. Accordingly, it is found that the m-GaN plane etching rate is enhanced from 0.69 to 1.09 nm/min with UV light; in the case of a-GaN plane etching, UV light enhances the etching rate from 2.94 to 4.69 nm/min.

N-doped homo-epitaxial GaN samples grown on freestanding GaN substrates have been investigated by micro-Raman spectroscopy. Quantitative analysis of the ${\mathrm{E}}_2^{\mathrm{h}}$ and the A₁(LO) modes’ behavior has been performed while intentionally increasing the carrier density using silicon doping. We noticed that as the carrier concentration increases up to 1.8 × 10¹⁸ cm⁻³, the ${\mathrm{E}}_2^{\mathrm{h}}$ mode remains unchanged. On the other hand, when the doping gets higher, the A₁(LO) position shifts to a higher frequency range, its width becomes larger, and its intensity drastically diminishes. This change in the A₁(LO) behavior is due to its interaction and its coupling with the free negative charge carriers. Furthermore, we calibrated the A₁(LO) frequency position shift as a function of the n-carrier concentration. We found out that for low n doping, the change in the A₁(LO) position can be considered as a linear variation while in the overall doping range, a sigmoid growth trend with a Boltzmann fit can be tentatively applied to describe the A₁(LO) position shift. This calibration curve can also be used to describe the coupling strength between the carriers and the A₁(LO) phonon. Eventually, this study shows that micro-Raman spectroscopy is a powerful non-destructive tool to probe the doping concentration and the crystalline quality of GaN material with a microscopic spatial resolution.

In this work, the physical and the electrical properties of vertical GaN Schottky diodes were investigated. Cathodo-luminescence (CL), micro-Raman spectroscopy, SIMS, and current-voltage (I-V) measurements were performed to better understand the effects of physical parameters, for example structural defects and doping level inhomogeneity, on the diode electrical performances. Evidence of dislocations in the diode epilayer was spotted thanks to the CL measurements. Then, using 2D mappings of the ${\mathrm{E}}_2^{\mathrm{h}}$ and A₁ (LO) Raman modes, dislocations and other peculiar structural defects were observed. The I-V measurements of the diodes revealed a significant increase in the leakage current with applied reverse bias up to 200 V. The combination of physical and electrical characterization methods indicated that the electrical leakage in the reverse biased diodes seems more correlated with short range non-uniformities of the effective doping than with strain fluctuation induced by dislocations.

This paper introduces a novel technology for the monolithic integration of GaN-based vertical and lateral devices. This approach is groundbreaking as it facilitates the drive of high-power GaN vertical switching devices through lateral GaN HEMTs with minimal losses and enhanced stability. A significant challenge in this technology is ensuring electrical isolation between the two types of devices. We propose a new isolation method designed to prevent any degradation of the lateral transistor’s performance. Specifically, high voltage applied to the drain of the vertical GaN power FinFET can adversely affect the lateral GaN HEMT’s performance, leading to a shift in the threshold voltage and potentially compromising device stability and driver performance. To address this issue, we introduce a highly doped n⁺ GaN layer positioned between the epitaxial layers of the two devices. This approach is validated using the TCAD-Sentaurus simulator, demonstrating that the n⁺ GaN layer effectively blocks the vertical electric field and prevents any depletion or enhancement of the 2D electron gas (2DEG) in the lateral GaN HEMT. To our knowledge, this represents the first publication of such an innovative isolation strategy between vertical and lateral GaN devices.

In this work, using micro-Raman spectroscopy mapping, we propose a methodology to separate the stress effect from the n-doping effect on A1LO and E2H GaN phonon modes frequency for low n-doped (<1017 cm−3) GaN layers grown on various substrates (GaN, sapphire, and silicon). This methodology shows a linear relation between the two phonon modes, in which the slope corresponds to GaN biaxial stress coefficients ratio KA1LOB/KE2HB and is 0.76 ± 0.01. Our value may act as a useful guideline for selecting or refining KE2HB and KA1LOB values. Samples that are mainly biaxially stressed show good agreement with the linear relation independently of the substrate. As for GaN/GaN samples, the change of slope indicates that layers are predominantly under dislocation-induced stress. However, independently of the substrate, the y-intercept increases with the n-carrier concentration, which provides a qualitive estimation of the net doping.