Micro-Raman characterization of homo-epitaxial n doped GaN layers for vertical device applications

Abstract

N-doped homo-epitaxial GaN samples grown on freestanding GaN substrates have been investigated by micro-Raman spectroscopy. Quantitative analysis of the E2h 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 E2h 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.

Comprehensive characterization of vertical GaN-on-GaN Schottky barrier diodes

“…After curve fitting analysis of Raman phonon spectra, volume mapping of the epilayer was attained in the WiRE 5.4 software interfaced PC. Further details of the µ-Raman measurement procedure are given in our recent report [4].…”

“…The metal-organic chemical vapor deposition (MOCVD) technique was employed for growing the GaN epilayers. In a close coupled showerhead reactor, the carrier gases such as trimethylgallium, ammonia, and hydrogen were introduced to grow the GaN films at a temperature of 1020 °C with 2µm h -1 growth rate [2,4]. The doping of the GaN layer was accomplished through the inclusion of diluted silane in a vapor form.…”

“…The doping of the GaN layer was accomplished through the inclusion of diluted silane in a vapor form. Prior to the GaN active layer growth, a thin layer of n + -GaN buffer (doping in 10 18 cm -3 range) was deposited on the GaN substrate to improve the interface quality between the substrate and the epilayer [4]. Finally, a 5 µm thick lightly-doped, high-quality n -GaN epitaxial layer was grown on top of the buffer layer.…”

“…Due to the rapid advances in the hydride vapour phase epitaxy (HVPE) technique, free-standing GaN substrates are commercially available and open the possibility of developing GaN-based vertical power diodes and transistors like trench MOSFETs [1]. As compared to the high-electron mobility transistors (HEMTs) made-up on heterostructures grown on foreign substrates, the vertical Schottky barrier diodes (SBDs) on homoepitaxial GaN substrate offer several advantages such as latticed matched epilayer, absence of thermal expansion coefficient (TCE) mismatching, reduced dislocation density (< 10 7 cm -2 ) in the active volume, less sensitivity to the surface trapping effects (as the current flows through the vertical direction), and better device scalability [1][2][3][4]. The vertical GaN-on-GaN SBDs can deliver high on-state current, low leakage current density, and improved breakdown voltage than the lateral diodes.…”

“…Indeed, in this work, comprehensive experimental studies (physical, electrical, and trap characterizations) are carried out for the vertical GaN SBDs. The atomic force microscopy (AFM), secondary-ion mass spectrometry (SIMS), micro-Raman spectroscopy [4] and cathodoluminescence (CL) [2] measurements are conducted to inspect the crystalline quality of the Si-doped n-type homoepitaxial GaN layer. The electronic trap levels in the GaN epilayer are investigated by the deep level transient Fourier spectroscopy (DLTFS) technique [12].…”