Researches On CVD Single Crystal Diamond Growth And Diamond Field-effect Transistor

Diamond is a new functional material with many excellent properties, which hasbeen widely applied in many fields. Recently, the chemical vapor deposition （CVD）high quality single crystal diamonds （SCDs） and high performance diamond-basedsemiconductor devices have become hot topic issues in diamond researches. Thegrowth of high quality SCDs is dependent on the deposition conditions. Moreover,the selected suitable gate material plays an important role in improving theperformance of diamond-based high-power electronic devices.In this thesis, we systemically investigate the growth of SCDs under variousreaction ambient. For the diamond field-effect transistor （FET）, the deposition,structure, and electronic properties of the high dielectric constant （k） materials oftantalum pentoxide （Ta₂O₅） and hafnium dioxide （HfO₂） thin films on SCD substratesare studied. Using Ta₂O₅as gate material, the prototype diamond FETs wereconstructed, which are favorable for realizing a new generation of high-performancediamond-based devices. The main content and innovation of this thesis are listed asfollowing:（1） The gaseous carbon dioxide （CO₂） was introduced into the traditionalreaction feeding gases of H₂/CH₄/N₂=500/60/1.8in sccm to examine the growthbehaviors of the SCDs with adding CO₂. The growth rate increases at lower flowrates of CO₂, and then decreases with a larger amount, which can be attributed to thecompetition between the increasing carbon source and enhanced oxygen etchinginduced by CO₂. The homoepitaxial SCDs layers show good crystalline quality.（2） The greenhouse gas nitrous oxide （N₂O） was introduced into the H₂/CH₄ambient to assist the growth of CVD SCDs. The combination of the N-and O-relatedradicals generating from N2O are favorable for not only improving the quality ofSCDs, but also increasing the growth rate. The optimized growth condition is of H₂/CH₄/N₂O=750/90/2in sccm to realize high quality and high-rate growth ofhomoepitaxial SCDs.（3） The growth of CVD diamond crystals has been investigated at different CH4flow rates （10,20,40,60sccm） in H₂（500sccm） plasma at a constant reactionpressure of200torr. The growth rates of SCDs are increased from2to15m/h withincreasing the flow rates of CH4. The surface of the high quality SCDs grown at lowCH4flow rates are smooth. However, at higher CH4flow rates, there are someunepitaxial grains appearing on the growth surface of the SCDs, which are notconducive to further fabricate high performance diamond-based devices.（4） The Ta₂O₅has been proposed as the gate material for diamond transistor.Synthesized by radio frequency sputtering, the as deposited Ta₂O₅films on p-typediamond substrate were amorphous having good insulating behavior and a dielectricconstant （k） of16. The corresponding C-V characteristic shows an obvious hysteresisphenomenon. After rapid thermal annealing in a nitrogen atmosphere at800oC, ahexagonal-Ta₂O₅film was obtained and the k value was markedly improved to be29. In this case, the hysteresis phenomenon of the C-V curve was eliminated,meanwhile, the leakage current of the MOS structure was increased. These variationscan be attributed to the annealing-induced quality improvement of Ta₂O₅film andincrease of trapped charge near the interface of Ta₂O₅/diamond. Experimentally, theprototype diamond FET with Ta₂O₅as the gate material shows obvious control ofdrain current by the gate voltage.（5） By the methods of atomic layer deposition and radio frequency sputtering,higher quality HfO₂films with a high K of²0have been deposited on diamondsubstrate. It is found that the hysteresis effect in the corresponding C-V curves of therelated MOS structures is strongly dependent on the deposition method.