Raman studies of GaN/sapphire thin film heterostructures

Abstract

Using Raman spectroscopy, we have studied the optical phonon modes of GaN nucleation layers with the thicknesses of 7 and 45nm, grown on sapphire (0001) substrates by metal organic chemical vapor deposition at low temperatures (500–600°C). These layers consisted of mixed hexagonal and cubic phases. The Raman results from mixed phases were compared with those from pure hexagonal layers which were grown at higher temperatures over 1000°C. The E2H and A1(LO) phonon modes are observed at 548 and 733cm−1 for 45nm thick nucleation layer, while the silent low-frequency B1 mode which is forbidden in good quality hexagonal GaN is observed at 314cm−1. The presence of the strong hexagonal modes for GaN nucleation layers of 45nm thick confirms the crystalline nature of the GaN nucleation layer and dominant hexagonal phase in this mixed cubic-hexagonal nucleation layer. The observed frequencies are shifted with respect to the corresponding A1 and E2 phonon modes in hexagonal GaN. The decrease in mode frequency implies the presence of in-plane tensile strain in these GaN nucleation layers of 45nm thick. The Raman scattering spectra taken from different positions on the sample show similar spectral features, indicating that the GaN nucleation layers of 45nm thick are homogeneous in micron scale.

MOCVD Growth of GaN on Sapphire Using a Ga2O3 Interlayer

“…36 In regular GaN/sapphire epitaxial growth, the presence of in-plane tensile strain is normally observed. 37 Moreover, the effect of the Ga 2 O 3 interlayer on the overall stress of the grown wafer can be considered insignificant since the PL peak wavelengths of GaN/sapphire and GaN/Ga 2 O 3 /sapphire fabricated under N 2 environment were identical at 368 nm, as shown in Fig. 6b.…”

Transient stress characterization of AlGaN/GaN HEMTs due to electrical and thermal effects

“…Complicating the issue, this same region is usually at the junction of low coefficient of thermal expansion (CTE) SiN x , high coefficient gold (Au) gate metal, and middle valued III-N semiconductor. In addition, residual stresses exist due to fabrication processes [6,[12][13][14][15]. SiN x passivation, in particular, can be deposited in a highly stressed state [16,17] and the AlGaN layer has large amounts of intrinsic tensile stress on the order of 1-3 GPa [6,12] from pseudomorphic growth on GaN.…”

Mixed Phases at the Bottom Interface of Si‐Doped AlGaN Epilayers of Optoelectronic Devices

“…Therefore, it is of great importance to understand and control the occurrence of these crystalline structures. Previous works have demonstrated that, in not-intentionally doped (NID) GaN layers, an obvious lower substrate temperature is necessary to obtain a pure cubic phase rather than that of a pure hexagonal phase by metal organic chemical vapor deposition (MOCVD) or molecular-beam epitaxy (MBE) [8,[11][12][13]. Recently, uniformly distributed c-GaN and h-GaN subgrains have been simultaneously found in NID GaN nucleation layers grown on sapphire substrate.…”

“…Recently, uniformly distributed c-GaN and h-GaN subgrains have been simultaneously found in NID GaN nucleation layers grown on sapphire substrate. Both subgrains provide templates for the subsequent epitaxial growth, having important influence on the crystalline quality of GaN [9,12,14]. However, few studies have examined the possible mixed crystalline structures in doped AlGaN alloys with high aluminum content.…”