Fabrication And Characterization Of Hydrogen-Terminated Diamond Field Effect Transistors And UV Detectors

Diamond is regarded as the "ultimate semiconductor material" due to its excellent properties,such as ultra-wide bandgap,high critical breakdown field,high electron mobility,and high thermal conductivity.These excellent properties make diamond widely used in fields such as power electronic devices,radiation detectors,blind light detectors,and biosensors.Doped or intrinsic diamond is difficult to ionize at room temperature,while hydrogen-terminated diamond（H-diamond）exposed to air can form a two-dimensional hole gas,exhibiting excellent conductivity.After decades of development,H-diamond devices have made a series of advances in device principles,preparation technology,device performance,and detectors.However,there are still some problems with the structure and process immaturity and insufficient stability research on H-diamond electronic devices.In the field of diamond ultraviolet detectors,the dominant traditional two-terminal structure exhibits problems such as single structure,immature preparation processes,and unclear optimization directions.Therefore,to further develop the potential of diamond materials,it is urgent to conduct research on the preparation processes,device structure,and stability of electronic devices.In addition,in the field of ultraviolet detection,it is necessary to break through the limitations of traditional two-terminal detectors by optimizing the structure and performance of new three-terminal detectors.Under the above research background,this dissertation utilized high-quality single crystal diamond materials grown by microwave plasma chemical vapor deposition（MPCVD）to obtain H-diamond materials through hydrogen plasma treatment.Highperformance diamond transistors with different structures were then prepared,and the stability of H-diamond transistors was tested.To solve the issue of low responsivity in two-terminal diamond detectors,phototransistors were employed.For the first time,diamond-enhanced phototransistors were successfully realized,providing an optimized solution for the system integration of UV detectors.The main research contents of this dissertation are as follows:1 In this dissertation,Au and Al metals were used to form ohmic contacts and schottky contacts with H-diamond,respectively.The Au mask etching process was explored to fabricate H-diamond MESFETs devices.The maximum output current of the devices was 123 mA/mm,the on-state resistance was 28.2 Ω·mm,the transconductance was 38 mS/mm,the switching ratio reaches 109,and the mobility was 195 cm2/V·s.It is worth noting that the MESFET device demonstrated good stability in multiple cycling and constant voltage tests.The effect of working temperature on the performance of the MESFETs device was investigated.The device performs relatively stably at-45℃～30℃.However,at high temperatures,particularly in the 200℃ test environment,the device experiences significant degradation in performance,which was a common phenomenon.Moreover,H-diamond MESFETs devices with forked and circular gate electrode structures were prepared.Compared with the single-gate electrode structure,the forked and circular gate electrode structures had a larger output current,which were 18 and 11 times that of the single-gate structure,respectively.These two types of devices have unique advantages in detecting weak signal sources with high sensitivity,and are the focus of future research.2 H-diamond MOSFET devices were fabricated based on low-temperature atomic layer deposition（ALD）-deposited Al₂O₃ and HfO₂ gate dielectrics.The effects of Al₂O₃ gate dielectric deposited at different temperatures（80℃,150℃,and 250℃）on the performance of H-diamond MOSFET devices were investigated.By testing the electrical performance and stability of the devices deposited at different temperatures,it was found that the MOSFET device with Al₂O₃ gate dielectric deposited at 150℃had better overall electrical performance,with a maximum saturation current of 156 mA/mm and a switch ratio greater than 10⁸.The H-diamond MOSFET device could still maintain good transistor characteristics at a working temperature of 200℃,with the saturation current decreasing from 103 mA/mm at room temperature to 90 mA/mm at 200℃ and the switch ratio decreasing from 10⁸ at room temperature to 106 at 200℃.Due to the passivation effect of the Al₂O₃ gate dielectric on the H-diamond surface,the electrical characteristics of the device remained almost unchanged after being stored in air for about a month.Furthermore,to address issues such as the high operating voltage of Al₂O₃ gate dielectric devices,it was proposed to use HfO₂ material with higher dielectric constant as gate dielectric to prepare MOSFET devices.Results showed that under the same gate dielectric growth conditions and thickness,the threshold voltage of the MOSFET device prepared with HfO₂ gate dielectric was 0.6 V,while that of the device prepared with Al₂O₃ gate dielectric was 6.6 V.HfO₂ gate dielectric reduced the operating voltage of the device by about 6 V.In addition,the device with HfO₂ gate dielectric could more effectively limit the leakage current and had excellent electrical performance.3 The preparation method of enhanced hydrogen-terminated diamond devices was investigated.Firstly,the enhanced H-diamond devices were fabricated based on thermal annealing and ultraviolet ozone treatment.By comparing the electrical characteristics of the two devices,we found that both processes caused significant damage to the Hdiamond surface,leading to poor device performance.The maximum saturation current of MESFET devices prepared by thermal annealing process was 21 mA/mm,while that of devices prepared by ultraviolet ozone treatment process was 19 mA/mm.Therefore,we turned to the study of enhanced H-diamond devices based on BaF2 gate dielectric.The maximum saturation current of the device is 53 mA/mm,the on-state resistance is 43.2 Ω·mm,the switching ratio is greater than 10⁸,and the threshold voltage is-1.2 V.4 Diamond-based ultraviolet detector was studied.First,photovoltaic and Schottky barrier detectors were fabricated,and the performance of these two diamond ultraviolet detectors was investigated.However,these two-terminal detectors demonstrated poor responsiveness and slow response speed.In order to solve these problems,threeterminal phototransistor was proposed for ultraviolet detection.The detection performance of three-terminal phototransistors with H-diamond MESFET and MOSFET structures was studied.The research results showed that different types of Hdiamond diamond phototransistors had good detection performance.Among them,the maximum light-to-dark ratio of the MESFET structure phototransistor was greater than 10⁸,the maximum responsivity was 2.48×104 A/W,and the fastest response time was 25.5 ms;the maximum light-to-dark ratio of the MOSFET structure phototransistor was greater than 10⁸,the maximum responsivity was 3.2×103 A/W,and the fastest response time was 48 ms.However,both H-diamond MESFET and MOSFET structure threeterminal phototransistors behaved as depletion-type devices,which was not conducive to practical applications.Therefore,this article proposed the use of BaF2 gate dielectricenhanced phototransistors for ultraviolet detection,which achieved 0 V gate bias detection and provided optimization ideas for future systematic integration and miniaturized circuit design.