Research On Reliability In Hydrogen-Terminated Diamond MOSFETs

Ultrawide-bandgap semiconductor diamond has excellent characteristics such as high breakdown electric field strength,high thermal conductivity,high carrier saturation velocity and low dielectric constant,which has great application potential in fields of power electronics and microwave power devices.Compared with field-effect transistors（FETs）based on doped diamond or silicon-terminated diamond,hydrogen-terminated diamond FETs have characteristics such as low activation energy and high hole concentration,leading to higher current and power density.Compared with hydrogen-terminated diamond metal–semiconductor FETs（MESFETs）,hydrogen-terminated diamond metal–oxide–semiconductor FETs（MOSFETs）have significant advantages in suppressing leakage current and improving power density,making them become a research hotspot in the field of diamond semiconductor devices.However,the current hydrogen-terminated diamond MOSFETs still suffer from a large gap between the actual electrical performances and theoretical values,as well as low reliability.Therefore,it is urgently need to conduct research on the carrier transport mechanism and device structure optimization of the device.This dissertation focuses on the reliability problems of hydrogen-terminated diamond MOSFETs and conducts research on the effects of the key structures and preparation processes of gate dielectrics,diamond substrates,and passivation layers.The specific research contents are as follows:1.Research on the impact of Al₂O₃ gate dielectric deposition temperature on device reliability.In view of the problem of high current density but poor reliability and unclear mechanism of hydrogen-terminated diamond MOSFETs with low-temperature deposited Al₂O₃ gate dielectrics,a reliability study based on gradient negative gate bias stress（NGBS）measurements with zero drain bias was conducted.This method first measures the initial transfer characteristics of the device with an Al₂O₃ deposited at a low temperature（90°C）,and then iteratively executes the measurement sequences of NGBS–measurement–NGBS and recovery–measurement–recovery under negative bias stress（-1 V or-2 V or-3 V）applied to the gate and the recovery state after removing the NGBS to measure the transfer characteristics of the device,thereby obtaining the time-domain current characteristics of the device in both the NGBS and recovery states.The results show that the threshold voltage(V_TH)of the device decreases first and then increases（bidirectional shift）under weak NGBS（-1 V and-2 V）but monotonically decreases under relatively strong NGBS（-3 V）.Then,a modified kinetic model considering the motion of hydrogen was proposed,revealing that under weak NGBS,the motion of hydrogen in Al₂O₃ generates unactuated oxygen-dangling bonds,and the time constant of generation is greater than the trapping time constant of the traps,resulting in bidirectional shift in V_TH.Finally,comparative experiments were conducted on devices with an Al₂O₃deposited at a high temperature（300°C）containing lower hydrogen concentration.The results showed that the stability of V_TH was improved by more than 35%,verifying the correctness of the proposed modified kinetic model considering the motion of hydrogen.2.Research on the impact of diamond substrate traps on device reliability.A characterization method combining pulse measurements and strong NGBS measurements was proposed to address the issue of the reported weak NGBS reliability research methods being unable to distinguish the impact of traps in Al₂O₃and diamond substrate on the reliability of hydrogen-terminated diamond MOSFETs.This method first uses drain-current–drain-voltage(I_D–V_DS)pulsed measurements with variable quiescent biases to preliminarily determine the location of the traps,and then uses the gradient strong NGBS measurements to activate traps in both Al₂O₃ and diamond substrate,thus obtaining the changes in V_TH related to the response of diamond substrate traps.Based on this method,strong NGBS measurements were conducted on the devices at NGBS=-4 V,-6 V,and-8 V,respectively.As a result,an unusual phenomenon was found that V_TH first decreases and then increases（bidirectional shift）under strong NGBS（-6 V and-8 V）,while monotonically decreases under relatively weak NGBS（-4 V）.Based on this phenomenon,a kinetic model considering the dynamic response of diamond substrate traps was proposed.This model reveals that the quantity of activated diamond substrate traps under-8 V NGBS is about 32%higher than that of activated Al₂O₃ traps,and the emission time constant of diamond substrate traps is greater than the capture time constant of Al₂O₃traps.Therefore,this study reveals that the impact of diamond substrate traps on device reliability is higher than that of Al₂O₃ traps.3.Research on the suppression of device leakage current by using a passivation layer.To solve the issue of high drain leakage current(I_Dleakage)caused by the low aspect ratio in hydrogen-terminated diamond MOSFETs,a method was proposed to suppress I_Dleakageby using an Al₂O₃/HfO₂ double passivation layer,which can increase the fringing capacitance to expand the depletion zone.This method introduces a double passivation layer in the gate–source/gate–drain access region of device,which can adjust the relative dielectric constant and thickness of the upper passivation layer to increase the fringing capacitance,thus expanding the depletion region and reducing the hole concentration under the gate region.As a result,the I_Dleakage was reduced.Then,the impact of the relative dielectric constant and thickness of the upper passivation layer on I_Dleakage and hole concentration distribution were analyzed.The simulation results show that increasing both the relative dielectric constant and the thickness of the upper passivation layer is beneficial for suppressing I_Dleakage.Finally,hydrogen-terminated diamond MOSFET with an Al₂O₃/HfO₂（40/100 nm）double passivation layer and gate lengths of0.3μm and 0.6μm were fabricated,respectively.Among them,the device with a gate length of 0.3μm achieved an I_Dleakage of～-3×10^-7 m A/mm,a cut-off frequency/maximum oscillation frequency of 6.1/11.1 GHz,and an on/off ratio of～1×10⁹.I_Dleakage of device with a gate length of 0.6μm was decreased to the order of 10^-9m A/mm and 10^-8 m A/mm for the first time at room temperature and 200°C,respectively.Therefore,a record on/off ratio of～1×10¹¹ and～5×10⁹ were realized at room temperature and 200°C,respectively,which are about one order of magnitude and two orders of magnitude higher than those of the diamond FETs that have been reported.4.Research on device reliability under off-state bias stress conditions.In view of the issue of reliability study of hydrogen-terminated diamond MOSFETs under off-state bias stress conditions has not been reported and the intrinsic mechanism is still unclear,a characterization method combining pulsed I-V measurements and transient on-resistance(R_ON)measurements was adopted to study the reliability issues of the proposed hydrogen-terminated diamond MOSFET with an Al₂O₃/HfO₂ double passivation layer.This method first uses pulsed I_D–V_DS and drain current gate voltage(I_D–V_GS)measurements with variable quiescent biases and pulse periods to determine the location of traps.Then,the dynamic response characteristics of the traps under gradient off-state bias stress and temperature conditions were obtained through the transient R_ON measurement.The results indicate that under the off-state bias stress condition,the increase in R_ON is mainly due to the trapping effects in the gate–drain access region caused by the drain quiescent bias,and the increase in V_TH is mainly due to the detrapping effects under the gate region caused by the gate quiescent bias.A kinetic model considering the trapping of holes by gate dielectric traps is proposed to address the unusual phenomenon of a sudden increase in R_ON during the initial stage of recovery duration after removing off-state bias stress.It shows that two different activation energies（0.28 e V and 0.24 e V）of traps response dynamically to the activation of electric field and/or heat,thus leading to device degradation.