The Study Of4H-SiC Homoepitaxial Growth And Devices

Silicon carbide (SiC), as one of the third-generation wide-band-gap semiconductors, has a great potential in the application of electronic devices for a long time. The excellent physical and electrical properties of SiC such as high breakdown electric field (2.2×106V/cm)[1], high saturation electron drift velocity (2.0×10’cm/s), high thermal conductivity (4.9W/cm K) and chemical stability allow its applications in high frequency, high power, high temperatures and other harsh conditions. This can be widely used in the national economy and national defense science and technology fields. However, it is necessary to grow high-quality SiC epitaxial layers before the SiC devices can be applied extensively. There is still a serious lack of domestic research in4H-SiC homoepitaxial technique and device febrication due to later start than Japan, USA and European countries.In this dissertation, the homoepitaxial growth of4H-SiC films have been systematically studied in order to obtain a stable epitaxial growth technology, and use the products of epitaxial wafers for devices fabrication. Our research results are as follows:1. Horizontal hot-wall CVD system-VP508GFR manufactured by EPIGRESS AB (Sweden) was used in the epitaxy of4H-SiC. Silane (SiH4) and propane (C3H8) were used as the precursor for silicon and carbon respectively. Hydrogen (H2) was used as dilution and carrier gas while high purity nitrogen (N2) and trimethylaluminum (TMA) was used for N-type and P-type doping respectively. The source exhausting has been analyzed to determine the process parameters which will impact in the doping and thickness uniformity. The technological process has been confirmed through the physical and chemical reactions involved in4H-SiC epitaxial growth.2. In our study, it was observed the4H-SiC epilayers grown on8°off-axis substrate has a better surface morphology than4°off-axis, indicating step control growth technique plays a significant role in the epitaxy of SiC. Furthermore, the growth rate of SiC epilayer increased with the increasing S1H4flow while reached a maximum of12μm/h, meanwhile N type doping keep lowering. According to these datas, the growth window of silicon clusters has been found: when S1H4flow rate was10sccm-20sccm, the silicon clusters began to form, and they bumped up with carrier gas, which results in an increase of C/Si ratio. This will low N-doping efficiency and growth rate. The rate of gas phase nucleation is directly proportional to the concentration of SiH4. A small of SiH4concentration will suppress gas phase nucleation. As a result, the mole fraction of silicon clusters cannot reach the equilibrium value under a small S1H4flow rate. When silane flow increased to30sccm, gas phase nucleation of silicon clusters would reach the equilibrium value.3. The epitaxial growth process was optimized in order to obtain good surface morphology of epilayer grown on4°off-axis substrates based on the traditional process technology of8°off-axis substrates. The results showed that the epitaxial layer of4°off-axis substrate has high quality and low defects density. The step bunching and triangular was eliminated, and the surface roughness is only0.223nm in20μm×20μm, which is close to the thickness of SiC bilayer.4. The epitaxial layer has only4H-SiC phase through the investigation of HRXRD and Raman. The FWHM of XRD rocking curve was52arcsec, indicating the high-crystalline-quality epitaxial layer has been synthesized. Fourier transform infrared spectroscopy (FTIR) was used to measure the thickness of epitaxial layer, the wafer to wafer thickness uniformity is<0.09%, the thickness variation is<0.9%in different runs. Mercury probe CV (MCV) and Secondary ion Mass Spectroscopy (SIMS) were used to measure the doping level of epitaxial layer. The wafer to wafer doping uniformity of4.37%and the run to run doping variation of5.3%were obtained. Resistance per square uniformity used tor MESFET structure was:2%-5%. 5. Using the4H-SiC homoepitaxial wafer, the SiC MESFET and SBD devices have been manufactured, respectively.The structure of MESFET devices were P-type buffer layer with thickness of0.2μm, doping concentration of2×1015cm-3, N-type active layer with thickness of0.4μm, doping concentration of2×1017cm-3, N+-type ohmic contacts layer with thickness of0.2μm, doping concentration of2×1019cm-3.1mm gate width MESFETS, at fo=2GHz, Vds=64V, the output power was4.1W with the gain of9.3dB, PAE31.3%and the drain efficiency (η) was35.5%. Furthermore, at S band2GHz, the pulse output power is measured for packaged3.6mm,9mm,20mm SiC MESFETs and the results are18.3W,38W,80W respectively. At the same time, the power gain of all SiC MESFETs exceeded8.5dB.The structure of SBD devices were N+-type buffer layer with thickness of0.5μm, doping concentration of1×1018cm-3, N-type active layer with thickness of12μm, doping concentration of5×1015cm-3. The SBD devices with size of1.5mm×1.5mm have been measure forward and reverse characteristics. The forward turn-on voltage was1.1V, when the forward voltage reach to3.5V, the output current was7.47A, the current density can reached330A/cm2.