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Combining diamond with GaN can significantly improve the heat dissipation performance of GaN-based devices. However, how to avoid the destructive damage to the GaN epi-layer caused by high-temperature hydrogen plasma during the diamond growth is still a problem. This study employed a Si transition layer and double-substrate structure microwave plasma chemical vapor deposition (MPCVD) to prepare diamond film on GaN epi-layer. The effects of double-substrate structure on the diamond growth were studied. The microwave plasma parameters of both single-substrate structure and double-substrate structure MPCVD diagnosed by emission spectra were comparatively investigated. It has been found that the microwave plasma energy of double-substrate structure MPCVD is relatively more concentrated and has higher radicals activity, which is beneficial to the diamond growth. The impacts of the Si transition layer on the diamond growth were also investigated. It demonstrates that the Si transition layer can effectively protect the GaN epi-layer from being etched by hydrogen plasma and improve the diamond growth. The relationship between the thickness of the Si transition layer and the diamond growth and the relationship between diamond film thickness and adhesion has been studied in detail.

The high frequency H-diamond metal-oxide-semiconductor field effect transistors (MOSFETs) were fabricated on single diamond substrate using 300°C ALD grown Al₂O₃ as gate dielectric and passivation layer. The devices gate length, gate/drain spacing and dielectric thickness are 100 nm, 2 μm, and 10 nm, respectively. The direct-current and frequency characteristics were investigated. The device shows a maximum saturation drain current of −492.6 mA/mm and g_m of 135.2 mS/mm. The device shows good high temperature working performance, and the maximum saturation drain current only has a little decreasing of 7.6%. at 200°C. In addition, the device exhibits a maximum cut-off frequency of 36.2 GHz and maximum oscillation frequency of 70.5 GHz. The transient drain current response measurement indicates that the drain current can follow the changing of gate voltage at the frequency of 1 MHz. These results indicate that the Al₂O₃ dielectric is suitable for using in high frequency or the high-speed switching devices.

The response property and stability of diamond Schottky barrier photodiodes (SBPDs) were investigated for the monitor applications of deep ultraviolet (DUV) light and high-energy radiation particles. The SBPDs were fabricated on the unintentionally doped insulating diamond epilayer grown on a heavily boron-doped p+-diamond (100) conductive substrate by microwave plasma chemical vapor deposition. The vertical-type SBPDs were constructed of semitransparent tungsten carbide (WC) Schottky contact on the top of the device and a WC/titanium ohmic contact on the bottom. The SBPDs were operated to detect the DUV light and protons in zero-bias photovoltaic mode. The spectral response of the SBPDs showed that the peak wavelength was at 182 nm with a sensitivity of 46 ± 1 mA/W. The response speed was shorter than 1 sec, with a negligible charge-up effect and persistent photoconductivity. The SBPDs showed a stable response upon the irradiation by 172-nm xenon excimer lamp with 70 mW/cm2 for 200 hrs and 70 MeV protons for the dose of 10 MGy, corresponding to a non-ionizing energy loss of 1.4 × 1016 MeV neq/cm2.