HVPE Growth and Characterization of ε-Ga₂O₃ Films on Various Substrates

Gallium oxide layers were deposited in a hot wall HVPE reactor using GaCl and oxygen as precursors. The GaCl vapor was synthesized in situ upstream in the reactor by the reaction of metallic gallium (99.9999%) and gaseous hydrogen chloride (99.999%). The flow of HCl through the Ga source and the flow of oxygen were 50 sccm and 300 sccm, respectively. The growth runs were performed at atmospheric pressure and at the temperature of 500 °C–600 °C. These growth conditions provided reasonably good crystal quality on all substrates. The growth rate was in the range 0.02–0.15 μm min⁻¹. Some of the samples were doped with tin (Sn) in order achieve n-type electrical conductivity. Gallium oxide layers were grown on planar and patterned sapphire, 2H-GaN, and 4H-SiC substrates. Commercially available patterned sapphire substrates (PSS) with cone-like bumps arranged in a hexagonal lattice at 5 μm pitch were used. P-type GaN:Mg templates were grown on sapphire substrates by MOCVD technique. The specimens were characterized by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and cathodoluminescence (CL). SEM results were obtained at electron accelerating voltage of 10 kV.

The structural quality and phase composition of the Ga₂O₃ films were assessed by XRD analysis. Most of the Ga₂O₃ layers had hexagonal crystal structure corresponding to ε-polymorphic form. The results of growth experiments in this study in comparison with published works are presented in Table I. It can be seen that regardless of the employed deposition technique and substrate used, ε-Ga₂O₃ films have typical XRC FWHM in the range from 0.2° to 1°. In our experiments we observed that Sn doping repeatedly resulted in even broader rocking curves with FWHM from 1.5° to 5°. It should be noted that these values are significantly broader compared to XRC FWHM typically obtained for α-Ga₂O₃epitaxial layers on sapphire. For example, XRC FWHM as small as 40 arcsec has been reported for Sn-doped α-Ga₂O₃ epitaxial films grown by Mist CVD.²⁸ Poorer crystalline quality of the ε-polymorph can be ascribed to the fact that ε-Ga₂O₃ is a quasi-hexagonal crystal composed of rotational orthorhombic domains.

Table I.  Summary of own and reported in literature data on epitaxial growth of ε-Ga₂O₃.

						XRD FWHM
References		Method	Tg, °C	Substrate	Doping	(0006)	(0004)	$(10\bar{1}1)$	$(10\bar{1}3)$	Conductive
Oshima et al., 2015 [11]		HVPE	550	GaN (0001)		0,67		1,14
Oshima et al., 2015 [11]		HVPE	550	AlN (0001)		0,61		1,24
Oshima et al., 2015 [11]		HVPE	550	β-Ga2O3 $(\bar{2}01)$		0,21		0,84
Xia et al., 2016 [12]		MOCVD	500	6 H SiC (0001)		0.62
Tahara et al., 2017 [29]		Mist CVD	700	AlN (0001)			0.58
Park et al., 2019 [30]		MOCVD	650	Al2O3 (0001)		0.326	0.818
Park et al., 2019 [30]		MOCVD	650	GaN (0001)		0.378	0.802
Leone et al., 2020 [17]		MOCVD	610	GaN (0001)		n/a		n/a
This work	GO316	HVPE	500	patterned Al2O3		0.38	0.36		0.38
	GO356	HVPE	500	β-Ga2O3 $(\bar{2}01)$		0.67
	GO463-2	HVPE	500	Al2O3		0.43
	GO345	HVPE	600	HVPE GaN/Al2O3		0.38
	GO345	HVPE	600	GaN/SiC		0.53
	GO345	HVPE	600	AlN/Si		0.60
	GO345	HVPE	600	Patterned GaN		0.42
	GO 344	HVPE	550	Patterned GaN		0.40
	GO575-1	HVPE	600	GaN	Sn	1.50 (ε + β)				yes
	GO573-1	HVPE	600	SiC	Sn	5.00 (ε + β)				yes
	GO581-4	HVPE	500	Al2O3	Sn	1.50 (ε + β)				yes
	GO584-1	HVPE	600	GaN	Sn	>5

The surface morphologies of the Ga₂O₃ epitaxial films were characterized by SEM. Hexagonal features having a mean size in the order of 10 μm were observed in greater or less amount on most of the specimens. Figure 1 shows SEM plan views of Ga₂O₃ layers deposited in the same growth run on (0001) sapphire and $\left(\bar{2}01\right)$ bulk β-Ga₂O₃ substrates. On both substrates Ga₂O₃ layer forms a mosaic structure composed of confluent hexagonal tiles forming a continuous layer. Hexagonal islands have the same orientation, e.g. diagonals of the hexagons on the β-Ga₂O₃ substrate are oriented along [100] direction. It is interesting to note that the apparent hexagonal symmetry of the growth features does not originate from the substrate as both sapphire and β-Ga₂O₃ have a lower degree of symmetry (trigonal and monoclinic, respectively).

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The XRD analysis of the Ga₂O₃ film on sapphire revealed strong peaks at ∼40.3° and ∼41.8°, corresponding to the (0006) planes of α-Ga₂O₃ and sapphire, respectively. This confirms that the epitaxial layer is pure α-Ga₂O₃ polymorph with corundum-like structure. Thus the hexagonal symmetry of islands is not related to the true symmetry of α-Ga₂O₃ crystal structure and can be explained by the formation of rotation domains. Despite the apparent mosaic structure accompanied with the formation of twin domains, the epitaxial layer exhibits high structural quality with XRC widths of 42 arcsec and 19 arcmin for (0006) and (1018) reflections, respectively.

In contrast, the specimen grown on β-Ga₂O₃ substrate exhibits strong X-ray diffraction peaks at 2θ = 59.06°, 59.9° and 112.7° corresponding to $\left(\bar{6}03\right),$ (0006) and (00010) reflections of β-Ga₂O₃ and ε-Ga₂O₃, respectively. This confirms that the epitaxial layer on bulk β-Ga₂O₃ is ε-Ga₂O₃ polymorph with pseudo-hexagonal structure.

Figure 2 shows SEM images of the Ga₂O₃ layer deposited on 4H-SiC substrate at 500 °C. SEM shows flat surface with hexagonal truncated pyramid-like features assembled in clusters. Regular orientation, uniform size and faceting of these pyramids indicate the epitaxial nature of their formation.

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Similar hexagonal features were also observed on plan view image of Sn-doped Ga₂O₃ on p-GaN template (Fig. 3). The surface of the specimen has numerous hexagonal plates surrounded by smaller crystallites. These plates are surrounded by smaller size irregular shaped crystallites. Larger hexagonal islands and smaller crystallites emit similar CL spectra with the main broad band centered at about ∼430 nm and ∼410 nm, respectively. Smaller crystallites emit stronger blue band and two additional not well resolved red bands at ∼690 nm and ∼760 nm. Hexagonal islands exhibit a lower integral CL intensity than surrounding areas. Red CL are possibly related to Sn doping because similar bands were observed in Sn-doped β- and α-Ga₂O₃.^31,32

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So far, we have been considering the formation of hexagonal features on planar substrates. The overall picture does not change much for the growth on artificially patterned substrates. We reported previously³³ that PSS can be used to control polymorphic composition of Ga₂O₃ epitaxial layers. Depending on the growth conditions, Ga₂O₃ deposition on the patterned substrates results in either pure α-Ga₂O₃, or ε-Ga₂O₃, or a columnar structure containing both α- and ε-phases. In this work the growth conditions were optimised for the deposition of pure ε-Ga₂O₃. Figure 4 shows SEM images of the Ga₂O₃ layer grown on PSS with cone-shape bumps arranged in a hexagonal lattice. As can be seen, Ga₂O₃ forms regular hexagonal islands on tops of the cones. The structural quality of ε-Ga₂O₃ layers grown on PSS is higher compared to that for the layers grown on planar substrates (see Table I). Deposition on PSS results in regularly distributed islands of uniform size and shape. In contrast to that, nucleation on planar substrates is more stochastic process and results in randomly distributed islands of various sizes. Consequently, coalescence of regular islands produces less number of grain boundaries and associated defects.

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Nominally undoped Ga₂O₃ layers were highly resistive. Conducting Ga₂O₃ films were produced by doping with Sn. The concentration of electrons in the doped specimens was estimated to be in the in the range from low 10¹⁷ to high 10¹⁹ cm⁻³. The heterojunctions composed of n-type ε-Ga₂O₃:Sn and p-type GaN:Mg demonstrated diode-like I-V characteristics. These structures exhibited broad whitish emission under forward bias. It is interesting to note that the electroluminescence (EL) spectrum is evidently different when compared with CL spectra acquired from GaN and Ga₂O₃ layers of the same structure (Fig. 5). The origin of this phenomenon can be related to the difference in the EL and CL generation regions. The CL emission is generated from nearly the entire layer thickness whereas the EL emission stems from GaN/Ga₂O₃ interface layer of a thickness of the order of the carrier diffusion length.

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It should be mentioned, however, that the electrical conductivity is achieved at the cost of deteriorated crystal quality. Consequently, it is difficult to make an unambiguous conclusion whether the electrical conductivity is related to Sn donors, or associated structural defects, or both. In contradistinction to published studies on Ga₂O₃ epitaxy,^9,23 where Sn acted as a stabilizing agent for the formation of the ε-Ga₂O₃, we observed a dramatic decrease of the structural quality accompanied with the formation of the competing β-Ga₂O₃ phase in Sn-doped samples. In some cases Sn doping resulted in polycrystalline texture-like material.

Figure 6 compares XRD ω-2θ scans for Ga₂O₃ layers grown on sapphire substrate and GaN/sapphire template. For Ga₂O₃/Al₂O₃ sample, peaks for the ε-Ga₂O₃ {0002} set of planes were observed at 19.18°, 38.89°, 59.90°, 83.48° and 112.63°, corresponding to the (0002), (0004), (0006), (0008) and (0, 0, 0, 10) reflections, respectively. Weak peaks at 40.25° and 87.05° correspond to (0006) and (0, 0, 0, 12) reflections of α-Ga₂O₃. The presence of the small amounts of the α-phase can be explained by the growth of semi-coherent α-Ga₂O₃ layer on the sapphire substrate, followed by island growth of ε-Ga₂O₃.

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In addition to ε-Ga₂O₃ {0002} diffraction peaks, the sample grown on GaN/sapphire template also exhibits diffraction peaks at 18.94°, 38.39°, 59.11°, 82.23°, and 110.52° corresponding to $\left(\bar{2}01\right),$ $\left(\bar{4}02\right),$ $\left(\bar{6}03\right),$ $\left(\bar{8}04\right)$ and $\left(\overline{10},0,5\right)$ reflections of β-Ga₂O₃. The origin of the coexistence of ε and β phases in this specimen could be attributed to heavy Sn doping.

The pole figures of these two Ga₂O₃ films are shown in Fig. 7. Diffraction spots ε-Ga₂O₃ $\left(10\bar{1}4\right)$ and $\left(10\bar{1}5\right)$ from Ga₂O₃/Al₂O₃ Ga₂O₃/GaN/Al₂O₃ exhibit expected six fold symmetry (Fig. 7a, 7b). Surprisingly, the pole figure for $\left(\bar{2}02\right)$ reflection of β-Ga₂O₃ also shows six-fold symmetry. This indicates that the β-Ga₂O₃ phase is strongly $\left(\bar{2}01\right)$ oriented and forms six crystal types of β-Ga₂O₃ rotated every 60° to each other about this direction. Similar results have been reported by other research groups who observed formation of β-Ga₂O₃ rotation domains in Ga₂O₃ epitaxial films on sapphire^22,34

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The present work demonstrates epitaxial growth of ε-Ga₂O₃ on Al₂O₃, GaN, SiC β-Ga₂O₃ and patterned sapphire substrates by the HVPE method. Epitaxial layers of ε-Ga₂O₃ of comparable quality can be produced on all tested substrates. The narrowest XRC were obtained for the ε-Ga₂O₃ films grown on patterned sapphire substrates. Most of the samples exhibited islands and other growth features of hexagonal symmetry. However, the sixfold symmetry of the surface feature does not always correlate with the true symmetry of the crystal structure and can be caused by the formation of rotation domains. Doping with Sn resulted in conductive Ga₂O₃ films; however, the crystalline quality was drastically decreased. Concurrent growth of β-phase was observed in the heavily doped samples. Heterojunctions composed of n-type ε-Ga₂O₃:Sn and p-type GaN:Mg demonstrated diode-like I-V characteristics and emitted white light under forward bias.

The authors would like to acknowledge I.A. Kasatkin for his extensive help with XRD measurements of Ga2O3 layers. V.I. Nikolaev, S.I. Stepanov, A.I. Pechnikov acknowledge support from the Russian Science Foundation, grant no. 19-19-00409.