Structural characteristics of α-Ga2O3 films grown on sapphire by halide vapor phase epitaxy

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Abstract

While the importance of α-Ga2O3 crystals is increasingly gaining much interest, the structural behavior of halide vapor phase epitaxy (HVPE) α-Ga2O3 crystals in wet chemical etching has not been explored yet. In this study, we investigate the structural characterization of HVPE α-Ga2O3 materials via alkali KOH solution etching. Further, the activation energy of the etch rate for the materials, etch pit, and surface roughness after wet chemical treatment have been examined. The cross-sectional TEM analysis demonstrates that the triangular-shaped etch pits with (11 2 6) plane are caused by the propagation of threading dislocations in the HVPE α-Ga2O3 crystals.

Introduction

Ga2O3 compound semiconductors are the materials for the next-generation high-power and high-frequency devices owing to their unique physical properties, such as large breakdown voltage and Baliga's figure of merit [1,2]. They exhibit six different phases, namely, α, β, δ, ε, κ, and γ. Among them, meta-stable-phased α-Ga2O3 crystals with the largest bandgap of approximately 5.3 eV has been considered one of the most promising functional oxide semiconductor materials as their corundum structure can provide bandgap tunability with Al and In and heteroepitaxial growth on substrates with the corresponding crystal structure [3]. However, because α-Ga2O3 crystals cannot be achieved by melt growth techniques, heteroepitaxial growth methods on foreign substrates, including halide vapor phase epitaxy (HVPE), mist chemical vapor deposition, and molecular-beam epitaxy (MBE), have been investigated. Among them, HVPE is widely utilized for the growth of (AlxGa1-x)2O3 and Ga2O3 alloys owing to its high growth rate, large scalability, and relatively high crystallinity capability. Despite the importance of HVPE α-Ga2O3 semiconductor applications, their wet etching characteristics has not been investigated [[4], [5], [6]]. Study on wet etches of materials is necessary while estimating the structural defects and fabricating complete devices with low process cost, as well as high throughput and efficiency. In this study, we investigate the potassium hydroxide (KOH)-based chemical wet-etching characteristics of HVPE α-Ga2O3 crystals. Although it is well established that wet chemical etchants can alter the structural etching behavior of other Ga2O3 phases, there are no reported research studies on α-Ga2O3 single crystals [7,8]. This study investigates the structural and morphological nature of wet-etched HVPE α-Ga2O3 single crystals, including etch pit formation combined with specific dislocation components.

Section snippets

α-Ga2O3 deposition

α-Ga2O3 films were deposited on c-plane sapphire substrates (Hi-Solar Ltd., Korea) using a horizontal home-made HVPE system at atmospheric pressure. Metallic Ga, O2, and N2 were chosen as the group III precursor, reactive gas, and carrier gas, respectively. HCl gas was reacted with liquid Ga metal to synthesize GaCl or GaCl3 at 673 K. The O2, N2, and GaCl or GaCl3 gases were then transported into the growth zone to form an α-Ga2O3 crystal on the substrate at 723 K. The gas flow rate of O2 was

Results and discussions

X-ray diffraction (XRD) analysis was performed to investigate the crystal structure and quality of the grown Ga2O3 crystals, as shown in Fig. 1. It is evident that the grown material was purely composed of α-Ga2O3 crystals without any gallium oxide phase, such as β, δ, ε, and γ. In addition, undesirable polycrystalline and amorphous phases could not be detected in the measurement except for (110) plane peak with weak intensity. The present of (110) reflection in α-Ga2O3 crystals can be also

Conclusions

In summary, the structural behavior of HVPE α-Ga2O3 in chemical wet etching was investigated for the first time. The activation energy of the crystal in KOH solution was evaluated to be approximately 0.29 eV, indicative of the good potential for application in opto-electronic device fabrication; Chemical wet etching in KOH solution is a suitable method for estimating the crystallinity of HVPE. Wet chemical etching-induced triangular- and hexagonal-shaped pits with the basal plane of (1126) on

CRediT authorship contribution statement

Soo Hyeon Kim: Funding acquisition, Writing - original draft, acquisition of data, Drafting the manuscript, Approval of the version of the manuscript to be published. Mino Yang: Funding acquisition, acquisition of data, Approval of the version of the manuscript to be published. Hyun Uk Lee: Funding acquisition, acquisition of data, Approval of the version of the manuscript to be published. Un Jeong Kim: Conceptualization, Formal analysis, Conception and design of study, analysis and/or

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2020R1A4A4078674) and the Korea Basic Science Institute grant (No. C070300).

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