Selective epitaxial growth of single crystal diamond and its application for high-performance diamond MOSFET fabrication

概要

Chapter Ⅰ is a review of synthetic methods and conductive methods for diamond semiconductors. With respect to other wide bandgap semiconductor materials, diamond (5.5 eV) exhibits excellent properties, the highest thermal conductivity (22 kW m-1 K-1) reduces the heat dissipation area of electronic devices; the breakdown voltage of up to 10 MV/cm makes it suitable for high-voltage-resistant devices; the carrier mobility (4500 cm2 V-1 s-1 for electron, 3800 cm2 V-1 s-1 for hole) and saturation carrier drift speed make it potential for high frequency devices. Due to the expensive price and uncontrollable crystalline qualities of natural diamonds, researchers prefer synthetic diamonds. Synthesized diamond using high pressure high temperature methods that simulate natural forming conditions are mostly used in industrial cutting and milling tools. And chemical vapor deposition (CVD) has emerged as semiconductor devices require single crystals diamond or those with fewer grain boundaries. The CVD is a method in which a carbon-containing gas (usually methane) and hydrogen are excited into carbon-containing active species and atomic hydrogen after being activated into a reaction chamber, the reactive species and diamond is deposited on a substrate. It can be divided into hot filament CVD (HFCVD) and microwave plasma CVD (MPCVD) according to different activation sources. Both types of CVD can synthesize single crystal diamond with good crystallinity. Since the intrinsic diamond is insulating, it is necessary to make it conductive to be applied to electronic devices. There are usually two ways of doping and surface hydrogen termination to make diamond conductive. However, doping faces a series of problems. The first is that all dopants of diamond are deep energy levels (compare to silicon doping), thus the activation rate at room temperature is very low, resulting limited carrier concentration. Taking boron as an example, lightly boron doped diamond is almost non-conductive at room temperature. So far, the heavy doping of diamond can only be achieved by ion implantation and high concentration methane CVD method. High dose ion implantation will cause diamond graphitization; and high concentration methane CVD will relatively reduce the proportion of hydrogen, resulting in increased sp2 carbon. Therefore, the doped diamond is mostly used to improve the ohmic contact between diamond and metal. The surface conductivity induced by hydrogen-terminated diamond surface synthesized by CVD was first discovered in 1989. After the hydrogen-terminated diamond MESFET was implemented, after nearly 30 years of development, most diamond semiconductor devices with excellent performance were achieved on hydrogen-terminated diamond.

In Chapter Ⅱ, we proposed a selective growth method of point-arc remote MPCVD and SiO2 mask to achieve high quality single crystal diamond synthesis. Selective growth is a method for site selective growth of diamond using a mask, and most are used to selectively growth doped diamond to improve the ohmic contact. So far, low resistivity n-type heavily doped contacts of (111) diamond and p-type heavily doped contacts of (100) have been achieved through selective growth. However, the deformation of the metal mask during the growth process and the contamination of diamond by the filament metal material when using HFCVD remain to be solved. The proposed CVD is a contactless plasma without a discharging electrode. The synthesis of carbon nanotubes and heteroepitxial growth of diamond by this apparatus have been reported before However, successful selective homoepitaxial growth of diamond by this technique has not been reported. After analyzing the results of the surface morphology (by scanning electron microscope and energy-dispersive X-ray spectroscopy), crystalline quality (Raman spectroscopy), and contamination concentration of the selectively grown diamond (SIMS), the selective growth was successfully achieved by the proposed apparatus and SiO2 mask. Additionally, the selectively grown diamond has been proved of high crystallinity and without contamination from mask or antenna.

In Chapter Ⅲ, the high performance silicon-terminated diamond MOSFET with by selectively grown diamond/ metal layer was electrically characterized. The success of selective growth in Chapter II is indicative of the stability of SiO2. The study of silicon-terminated diamond in metal oxide semiconductor field effect transistor (MOSFET) devices was derived by the previous reports on the negative electron affinity, thermal stability, and surface hole accumulation of silicon-terminated diamond. The heavily boron-doped diamond layer was achieved by selective growth and a silicon-terminated diamond channel was formed at the contact area of the SiO2 mask and diamond under the high temperature environment. The MOSFET shows a good field effect without doping and hydrogen termination treatment to the diamond channel. The drain current (IDS) was modulated with varying gate voltage (VGS), reaching maximum -170 mA/mm at VGS = -60 V. By calculating the surface state density and channel carrier mobility of the silicon-terminated MOS structure, it is found that the characteristics of silicon-terminated diamond in device applications is comparable with those based on hydrogen termination. This work is significant for the stable operation of diamond MOS power due to proposing direct connection of diamond with SiO2 which is the most reliable insulator, making it silicon-terminated diamond is expected to be an alternative to hydrogen termination as a more stable surface conductive treatment.

In Chapter Ⅳ, the initial growth of local epitxial of diamond (111) on Ru/ Al2O3 (0001) (c-sapphire) has been confirmed. Heteroepitaxial growth is critical for large scale synthesize of diamond (111) substrate. As a more economically viable as the iridium, 150 nm ruthenium thin film was sputtering deposited on the c-sapphire substrate by RF/DC magnetron sputtering system. The XRD analyses performed on the ruthenium film revealed (0001) orientation with high crystallite quality. Bias enhanced nucleation and (111) preferential growth were both performed by antenna-edge-type microwave plasma-assisted chemical vapor deposition. After 30 minutes preferential growth, the crystallite with diameter approximately 500 nm smooth surface observed scanning electron microscope, electron backscattering diffraction orientation mapping indicates they are highly-orientated diamond (111) crystallite. The EBSD pole figures pattern is indicative of formation of double positioning defect. Heteroepitaxial growth of diamond (111) on ruthenium provides the more economical viable approach to large vertical channel diamond MOSFET or the GaN on diamond power electronics.

Chapter Ⅴ is a summary of the previous chapters. Using point-arc remote MPCVD and SiO2 mask to achieve the selective growth of high-quality single crystal diamond and improve the quality of the selectively grown diamond film; MOSFET operation of silicon-terminated diamond provides a simpler device manufacturing method and more stable surface conductive treatment; The heteroepitaxial growth of diamond on the ruthenium can reduce the synthesis cost of large-area diamond substrates. The above work, from substrate production to surface modification to actual device application, is of great significance to promote the development of diamond devices and improve device performance.

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