High-Rate Homoepitaxial Growth of Heavily Boron-Doped Diamond (100) Films By Microwave Plasma-Enhanced Chemical Vapor Deposition Using High Microwave Power Density

Abstract

Diamond is expected as a next-generation semiconductor material for high-power devices because of its high breakdown electric field, mobility, and thermal conductivity. For the power devices with high current operation, vertical structures are effective because the large contact area is easily obtained by trench structures in the same way as silicon and silicon carbide power devices. The low resistivity diamond single-crystal substrates are required to fabricate the diamond vertical power devices and are realized by heavy boron doping. The highest growth rate of heavily boron-doped ([B]film > 1020 atoms/cm3) diamond films obtained by microwave plasma-enhanced chemical vapor deposition (MPECVD) is 6 μm/h [1]. The higher growth rate is required for manufacturing cost reduction of the low resistivity diamond substrates. However, the higher the boron concentration is, the lower the growth rate of the diamond films becomes [1]. It is known that there are mainly two methods for increasing the growth rate of the diamond films. One is a nitrogen addition to the plasma. However, incorporated nitrogen into diamond acts as a donor, which is compensator for boron-doped diamond [2]. The other is high density of the microwave power input into the plasma. Therefore, we tried to change the shape of a diamond substrate holder for MPECVD to improve the growth rate by increasing the power density near the surface of substrate. Figure 1 shows the plasma states of (a) the conventional substrate holder and (b) the improved substrate holder. The improved substrate holder has a shallower bottom than the conventional one. In the case of the improved substrate holder, the top of the diamond substrate is higher than that of the holder, as shown in Fig.1(b). As a result, the diamond growth rate and incorporation of boron atoms into the diamond film are enhanced at the same time because the electric field is concentrated at the top surface of the diamond substrate and the radical density on the diamond surface increases. High-pressure, high-temperature (HPHT) synthetic Ib(100) single-crystal diamonds were used as substrates for the homoepitaxial diamond growth by MPECVD with a spherical resonator structure (Arios Inc. in Japan). Before the CVD growth, the diamond substrates were treated with HF diluted with H2O for 15 min. Next, the substrates were heated in 2:1 mixture of H2O2 and H2SO4 at 400°C for 15 min. The growth was carried out using CH4 and B(CH3)3 diluted with H2. The microwave power, the total pressure and the methane concentration were maintained at 900 W, 50 kPa and 2.4 %, respectively. The boron/carbon (B/C) ratios in the gas phase were fixed at 14,000 ppm for the conventional holder and 20,000 ppm for the improved substrate holder, respectively. The growth of the heavily boron-doped diamond films was conducted for two hours in the above conditions. After the growth, the surface morphologies of the heavily boron-doped diamond films were observed by optical microscopy (OM). The resistivities of the films were measured by a van der Pauw contact configuration and the growth rates were estimated by the micrometer. Figure 2 shows the OM images of typical surface morphologies of the heavily boron-doped diamond films grown on diamond (100) substrates by (a) the conventional substrate holder and (b) the improved substrate holder. Although the diamond film grown by the conventional substrate holder had some non-epitaxial crystallites with density of 1.6 × 104 cm-2, which degrade the diamond device performances, the diamond film grown by the improved substrate holder had no non-epitaxial crystallites. The resistivities of the films were 3.3 × 10-2 Ωcm for the improved substrate holder and 2.3 × 10-2 Ωcm for the conventional substrate holder, respectively. The boron concentrations in these films were 1~3 × 1020 cm-3, which is estimated from the growth condition. The growth rates were 29.5 μm/h for the improved substrate holder and 13.0 μm/h for the conventional substrate holder, respectively. The growth rate of 29.5 μm/h is the highest growth rate for heavily boron-doped ([B]film> 1020 cm-3) diamond films. Acknowledgements This work was partially supported by Adaptable and Seamless Technology Transfer Program through Target-driven R&D, JST, and Kanazawa University SAKIGAKE Project.