Achievement of polarity reversion from Al(Ga)-polar to N-polar for AlGaN film on AlN seeding layer grown by a novel flow‐modulation technology

Jin Zhang,Abbas Nasir,Shuai Chen,Zhe Zhuang,Yiping Cui,Guohua Hu,Xiong Zhang,Aijie Fan,Jiaqi He,Jiadong Lyu

doi:10.1007/s10854-021-05510-8

Abstract

N-polar AlGaN epi-layer was realized on AlN seeding layer grown with a novel flow-modulation method. The polarity reversion from Al(Ga)-polar to N-polar for AlGaN/AlN films was confirmed by KOH etching and subsequent observation with optical microscope as well as high-resolution X-ray diffraction (HR-XRD) measurement. In particular, the dependence of crystalline quality and defect size in the N-polar AlGaN epi-layers on the V/III ratio was investigated with HR-XRD and scanning electron microscopy (SEM). It was found that as the full width at half maximum (FWHM) value of the X-ray rocking curve (XRC) varied with the V/III ratio in a fluctuating mode for the N–polar AlGaN epi-layers, and a FWHM value as small as 450 arcsec was achieved for the sample grown with a V/III ratio of 988. Moreover, it was revealed by the SEM measurement that the maximum diagonal length of hexagonal cone on the surface of the N-polar AlGaN epi-layers decreased sharply when a V/III ratio of 1236 was used although the crystalline quality and the surface morphology of the N-polar AlGaN epi-layers were not improved simultaneously. The peculiar migration of the group-III atoms on the surface of the N-polar AlGaN epi-layer associated with the molar ratio of TMA/(TMA + TMG) was considered to be responsible for this result.

Highlights

High electron mobility transistors (HEMTs) based on group-III nitrides such as GaN have attracted extensive and intense researches owing to their superior promise in breakdown voltage, electron saturation rate, intrinsic mobility, and thermal conductivity [1, 2]
When only TMA flow and no NH3 flow was injected into the reaction chamber, which was equivalent to the reduction in the ammonia partial pressure simultaneously [16], there were just few N atoms remaining on the surface of AlN epitaxial layer
There was denser nucleation domains observed on the surface of the high temperature (HT)-AlN seeding layer of the sample grown under flow-modulation mode than that grown under continuous mode

Summary

Introduction

High electron mobility transistors (HEMTs) based on group-III nitrides such as GaN have attracted extensive and intense researches owing to their superior promise in breakdown voltage, electron saturation rate, intrinsic mobility, and thermal conductivity [1, 2]. The rough hexagonal hillock-like defects on the surface induced by the poor migration capability of the atoms will degrade the crystalline quality and surface morphology of the N-polar III-nitride films [10]. Some research groups have dedicated to improve the crystalline quality and surface morphology for N-polar GaN, AlN, and AlGaN films. The control of the polarity was achieved by using a patterned LT-AlN buffer layer [15] In spite of these efforts mentioned above, there are no reports yet on the polarity-reversion from Al(Ga)-polar to N-polar for the AlGaN film grown on the AlN seeding layer by using the flow-modulation technology. Significant improvements in crystalline quality and surface morphology of the N-polar AlGaN film were achieved by carefully optimizing the V/III ratio for the epitaxial growth process

Experimental

Results and discussion

Conclusions

Conflicts of Interest