Conductivity Manipulation And Device Research Of Diamond Surface-channel Field Effect Transistors

Ultrawide bandgap semiconductor diamond has many advantages,such as large bandgap,high breakdown electric field,high carrier mobility,high thermal conductivity,and has a variety of different surface terminal properties,which has enormous application potential in power electronic devices.Hydrogenated（C-H）and silicon-terminated（C-Si）diamonds both have negative electron affinity theoretically,which can induce two-dimensional hole gas（2DHG）on the diamond surface and serve as conductive channels of field effect transistors（FETs）,and the FETs based on C-H/C-Si diamond are the research hotspot currently.However,the current electrical performance of these devices is far from fully utilizing the advantages of diamond materials.Further exploration and research are still needed in the aspects of surface terminal conductivity manipulation,device structure design,preparation process,and material and device mechanism.Specifically,there is still a significant gap between domestic and foreign research results in the preparation of normally-off diamond FETs,C-Si diamond materials and devices.Based on the above research status,this paper focuses on basic p-type conductivity manipulation in the preparation of C-H/C-Si diamond FET devices,and starts from surface termination treatment,ohmic contact,dielectric process,and dielectric/diamond interface properties.Various diamond materials with high conductivity and diamond FETs with high performance have been prepared.The specific research content and results are as follows:1.To optimize the surface conductivity manipulation and device characteristics of normally-on C-H diamond FETs,the low temperature atomic layer deposition（ALD）process which can protect the surface conductivity of C-H diamond and the optimized gate photolithography process were used to fabricated the high-performance C-H diamond FETs based on C-H diamond with good conductivity.The high-performance normally-on hafnium oxide（HfO₂）/C-H diamond metal-oxide-semiconductor field-effect transistor（MOSFET）device was prepared based on 200℃ALD HfO₂ dielectric and optimized development process.The 4.2-μm device achieves saturation drain output current(I_Dsat)of-223.1 mA/mm,on-resistance(R_on)of 36.8Ω·mm,and maximum transconductance(g_mmax)of 22.5 mS/mm,which are superior to the HfO₂/C-H diamond MOSFET devices reported to date.In addition,the double-layer photoresist process was developed to reduce charge accumulation effect in electron beam lithography process,improve line integrity and uniformity,and successfully prepared a 150℃ALD alumina/C-H diamond microwave device with a gate length of 200nm.I_Dsat of the device is-542.6 mA/mm,current gain cutoff frequency（f _T）and maximum oscillation frequency(f_max)are 8.3 GHz and 12.6 GHz respectively,and the output power density is 202 mW/mm at 2 GHz and input power of 0.92 dBm.The large off-state leakage current and the small gate voltage swing limit the microwave characteristics of the device,which points out the direction for the further performance optimization of the device.2.To solve the problem that the surface conductivity of normally-off C-H diamond FET device was degraded and the device properties were poor due to the traditional preparation methods such as surface termination modification and charge control of insulating dielectric,an enhanced-gate structure preparation method of fluoride/C-H diamond by electron beam evaporation at room temperature was proposed to obtain an normally-off metal-insulator-semiconductor field-effect transistor（MISFET）device with high mobility and low R_on.The BaF₂/C-H diamond MISFET device with a gate length of 4μm achieves threshold voltage(V_TH)of-0.9 V,I_Dsat of-96.5 mA/mm,and g_mmax of 30.0 mS/mm,which is the highest current and transconductance among normally-off diamond FET devices with conventional structure based on the surface transfer doping principle.There is no obvious degradation of the device after 20 consecutive output characteristic curve tests.The analysis demonstrates that the fixed charge and interfacial state density at the dielectric of BaF₂/C-H diamond MISFET are as low as 3.54×10¹² cm^-2 and 6.87×10¹¹ cm^-2·e V^-1,respectively.Besides,the effective mobility is constant at 225.0 cm²/V·s in the gate voltage range of-3 V≤V_GS≤-5 V.Excellent BaF₂/C-H diamond interface properties are the fundamental reason for obtaining high-performance devices.3.To solve the problem that novel C-Si diamond with theoretical conductive surface usually changed into a high resistance state by surface oxidation and was difficult to achieve depletion type conductivity,a new preparation process of normally-on hydrogen passivated C-Si diamond was proposed by sputtering silicon film on diamond and achieving surface diffusion doping of silicon in the process of removing silicon film by high temperature hydrogen plasma etching.In this way,a hydrogenated C-Si diamond with a square resistance of 5500Ω/sq was realized,and the conductivity reached the superior level of C-H diamond.The fabrication process of normally-on C-Si diamond FET was developed,which was compatible with existing fabrication process of C-H diamond FET devices.The V_TH of the device with gate length of 1.5μm is 1.4 V,and I_Dsat is-270.5 mA/mm,R_on is 39.6Ω·mm,g_mmax is 42.4 mS/mm.The gate breakdown field and drain breakdown field reach 8.4 MV/cm and 1.8 MV/cm,respectively.The results provide a new idea for the research and development of C-Si diamond devices and materials.4.To solve the problem of high ohmic contact resistance and poor adhesion of metal electrodes in diamond devices,a new surface diffusion borosilicate（B-Si）co-doping process using boron nitride as solid boron source and magnetron sputtered silicon layer as silicon source was proposed.The B-Si co-doped C-Si diamond with square resistance of 2724Ω/sq was realized.SIMS results show that the peak doping concentrations of boron and silicon on the diamond surface are 3.7×10²⁰ atom/cm³ and 1.1×10²⁰ atom/cm³,respectively,and the diffusion depths of silicon and boron are above 1μm.Based on the ohmic contact electrode of gold（Au）deposited on the material,the ohmic contact resistance as low as 0.46Ω·mm is achieved,and the ohmic contact characteristics of the surface terminal diamond are raised to a new level.The MOSFET with Al₂O₃ dielectric was prepared based on B-Si co-doped C-Si diamond with square resistance of 4201Ω/sq.The device with gate length of 6μm achieves the open state characteristics of I_Dsat of-321.2 mA/mm,R_on of 28.6Ω·mm,and g_mmax of 18.4 mS/mm.However,it is difficult to realize the true off-state characteristic by channel shutdown.In the future,it is necessary to further study the surface diffusion co-doping process to explore the possibility of optimizing ohmic contact and achieving high performance surface channel based on the B-Si co-doped diamond material.