Preparation of 2-in.-diameter (001) β-Ga₂O₃ homoepitaxial wafers by halide vapor phase epitaxy

Beta gallium oxide (β-Ga₂O₃) has a large bandgap of 4.5–4.9 eV and at the same time can be made conductive by doping with donor impurities. It follows from these properties that β-Ga₂O₃ has a wide range of applications, such as being used as transparent and conductive substrates for nitride-based LEDs,¹⁾ a new material for deep-UV photodetectors,²⁾ or high-power devices.^3,4) On the other hand, bulk single crystals of β-Ga₂O₃ can be grown by using melt growth methods at atmospheric pressure.^5–9) Considering the cost-effectiveness, this is a big advantage of β-Ga₂O₃ over other wide-bandgap semiconductor materials such as SiC and GaN whose bulk crystal growth is limited to gas-phase methods, which means that the growth rates are relatively low. Our group has developed high-quality bulk single crystals of β-Ga₂O₃ by the edge-defined film-fed growth (EFG) method, and wafers based on these bulk crystals have already been commercialized.⁹⁾ Furthermore, the electrical conductivity of the bulk crystals can be controlled by doping with Sn/Si or Fe/Mg, making the material conducting or semi-insulating, respectively.

Research and development activities on power devices using β-Ga₂O₃ material have also shown much progress recently. Superior device performances have been reported for Schottky barrier diodes (SBDs)^10–13) and field-effect transistors (FETs).^14,15) For example, breakdown voltages larger than 1000 and 750 V have been achieved respectively in field-plated β-Ga₂O₃ SBDs¹¹⁾ and MOSFETs.¹⁴⁾ In vertical power devices such as SBDs where n⁻-β-Ga₂O₃ thick layers are used as the drift layers, a thickness of some micrometers and a doping concentration on the order of 10¹⁶ cm⁻³ are typically required. To this end, halide vapor phase epitaxy (HVPE) has proved to be an effective method of growing thick and high-purity epitaxial layers of β-Ga₂O₃ owing to its high growth rate and efficient impurity doping controllability. The growth rate that can be obtained is as high as 5–20 µm/h.^16,17) Moreover, the n-type carrier concentration can be controlled in the range of 10¹⁶–10¹⁸ cm⁻³ by introducing Si as the donor. The present lower limit of doping is nearly the same as that in MBE-grown β-Ga₂O₃ layers¹⁸⁾ but one order of magnitude lower than that in MOVPE-grown ones.¹⁹⁾

In this study, we report on the homoepitaxial growth of β-Ga₂O₃ on 2-in.-diameter (001) wafers by HVPE, which was successfully performed for the first time. Following the enlargement of the wafer sizes, this scaling-up technology for the epitaxial growth has always been desirable towards the commercialization of β-Ga₂O₃ devices.

650-µm-thick 2-in.-diameter (001) β-Ga₂O₃ wafers prepared by EFG with n-type conductivity [effective donor concentration, N_d − N_a, (3–4) × 10¹⁸ cm⁻³] by Sn doping²⁰⁾ were used as substrates. Prior to homoepitaxial growth, the wafers were subject to degreasing in organic solvents followed by cleaning in hydrofluoric acid and in a mixture of sulfuric acid and hydrogen peroxide solutions. The epitaxial layers were then grown in a horizontal hot-wall HVPE reactor with GaCl and O₂ as precursors, SiCl₄ as doping gas, and N₂ as a carrier gas. The choice of precursors was based on the thermodynamic analysis of the driving force for β-Ga₂O₃ growth.¹⁶⁾ The reactor was scaled up from that in Ref. 17, which was designed for smaller samples (10 × 22 mm²). The Ga precursor, namely, GaCl is generated in situ by the reaction between high-purity Ga metal (7N grade) and Cl₂ gas in the so-called source zone maintained at 850 °C. β-Ga₂O₃ is then grown by the reaction between GaCl and O₂ gas in the growth zone. The temperature of the growth zone was set at 1000 °C. Note that no wafer rotation mechanism is employed in the HVPE reactor used.

The structural quality of the grown layers was characterized by the full-widths at half-maximum (FWHMs) of X-ray rocking curves (XRCs) for symmetric 002 reflection with X-ray incident azimuth of [100]. The thickness and carrier concentration (N_d − N_a) of the grown layers were measured by Fourier transform infrared spectroscopy (FT-IR) and electrochemical capacitance–voltage (ECV) measurements. As for the relative dielectric constant of β-Ga₂O₃, a value of 10²¹⁾ was used for the estimation of carrier concentration in ECV measurement.

HVPE of β-Ga₂O₃ layers was successfully achieved on 2-in.-diameter wafers with a typical growth rate of 3–4 µm/h for a GaCl nominal input partial pressure of 1.3 × 10⁻³ atm and an input VI/III (2O₂/GaCl) ratio of 3.5. Figure 1 shows a photograph of a homoepitaxial wafer after HVPE growth for 3 h and subsequent surface planarization by chemical mechanical polishing (CMP). The FWHMs of (002) XRCs taken at five sampling points were 89–100 arcsec. These values are comparable to that of the wafer measured at the center, which is 125 arcsec, implying the high crystalline quality of the homoepitaxial layer. The distributions of thickness and carrier concentration of the same homoepitaxial wafer obtained before CMP are shown in Fig. 2. The data was taken at 25 sampling points and has been smoothened for the points in between as displayed in the figures. The upstream edge (left-hand side of the figure) is relatively thin (minimum thickness ∼4.5 µm) compared with the remaining part (9.3–12.3 µm). This is attributed to the nonoptimized gas flow condition (i.e., nonuniform gas velocity and/or distribution of precursors over the wafer surface). The mean values and standard deviations (mean ± σ) for thickness and carrier concentration were 10.9 ± 1.8 µm and (2.7 ± 0.5) × 10¹⁶ cm⁻³, respectively. The inhomogeneity (defined as σ/mean) is 16.5% for thickness and 19.7% for carrier concentration. These values are larger than desirable since a value of 2–3% inhomogeneity for thickness is obtained for the state-of-the-art 6-in.-diameter 4H-SiC²²⁾ and GaN²³⁾ homoepitaxial layers. However, the uniformity is expected to be improved by optimizing the gas flow and/or utilizing the wafer rotation mechanism in future studies.

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To examine the quality of the homoepitaxial wafers prepared, SBDs were fabricated on a 2-in.-diameter homoepitaxial wafer. The homoepitaxial layer was planarized by CMP prior to the device process to remove pits formed on the surface.¹²⁾ The nominal thickness and average carrier concentration of the homoepitaxial layer after the CMP procedure were 5.3 µm and 3.4 × 10¹⁶ cm⁻³, respectively. Ti (50 nm)/Au (200 nm) cathode metals were then deposited on the back of the substrate by electron-beam evaporation to form ohmic contact over the entire area. Finally, circular Schottky contacts consisting of Ni (50 nm)/Au (200 nm) with various diameters (50–1000 µm) were deposited on the homoepitaxial layer via a photolithography process. A photograph of SBDs fabricated on the homoepitaxial wafer is shown in Fig. 3.

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The forward and reverse characteristics of current density–voltage (J–V) measurements at room temperature of 17 SBDs with 200-µm-diameter contacts (marked by red circles in Fig. 3) are shown in Figs. 4 and 5. Results on the dependence of device performance on the contact size will be reported elsewhere. All the SBDs exhibited fairly good forward characteristics with low voltage ratings of 1.1–1.6 V at 100 A/cm² and on-resistances (R_ON's) of 2.9–7.7 mΩ cm². One exceptional diode (labelled as A in Figs. 3 and 4), which is positioned near the upstream edge showed clearly smaller threshold voltage and on-resistance than others. This result can be explained in terms of the thickness of the drift layer, which is very small if any in diode A (below the lower limit in our FT-IR measurement, i.e., <2 µm after CMP). The diodes other than A turned on at nearly the same voltage (∼0.9 V), implying a spatial homogeneity of Schottky barrier height over the wafer surface. The dispersion in on-resistance among these devices (3.8–7.7 mΩ cm²) can be simply attributed to the rather large in-plane distributions in thickness and carrier concentration of the homoepitaxial layer.

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On the other hand, most of the SBDs did not show breakdown up to −200 V, which is the limit for reverse bias in our measurement system. The typical reverse currents were of reasonable orders (10⁻⁵–10⁻⁴ A/cm² at −200 V) considering that the devices were fabricated without dielectric surface passivation. These currents can be further suppressed by passivation or utilizing our recently developed trench MOS structure.¹³⁾ There are some exceptions (labelled as A, B, and C in Figs. 3 and 5) with leaky behaviour. SBD A behaved almost like a conductor at reverse biases. Together with its forward characteristics, this implies that there was almost no drift layer, that is, the surface of the original wafer was almost exposed after CMP in the position of diode A. For diodes B and C, the breakdown occurring at lower voltages could be attributed to structural imperfections in the epitaxial layer such as surface defects or threading dislocations. The former could be intrinsic or due to mechanical damage²⁴⁾ during CMP. In SBDs fabricated on a $(0\bar{1}0)$ β-Ga₂O₃ single-crystal substrate, it was indicated that dislocation defects along the [010] direction acted as main paths for leakage current.²⁵⁾ However, there are a few studies on dislocation in (010)- or $(\bar{2}01)$-oriented β-Ga₂O₃ wafers^26,27) but none on dislocations in (001)-oriented ones. Therefore, further studies are required to determine the exact origin of the leakage currents observed here.

In summary, we demonstrated for the first time the growth of thick homoepitaxial layers of β-Ga₂O₃ on 2-in.-diameter (001) substrates by the HVPE method. An average growth rate of 3–4 µm/h was obtained. Ni SBDs formed on the homoepitaxial wafers after CMP of the HVPE-grown layer surface showed reasonable forward and reverse J–V characteristics. Typical breakdown voltage and on-resistance for SBDs with a drift layer of 5.3 µm thickness and a carrier concentration of 3.4 × 10¹⁶ cm⁻³ were >200 V and 3.8–7.7 mΩ cm², respectively. In practice, along with efforts in developing advanced device structures, the scalability of the wafer up to larger sizes is indispensable for a material to be applied in real devices. Therefore, the results obtained herein for the 2-in.-diameter homoepitaxial wafers imply an optimistic applicability of β-Ga₂O₃ at a practical level.