Research On Key Technology Of Breakdown Voltage Of Gallium Oxide Power Diodes

Power semiconductor devices,also known as power electronic devices,are mainly used for power conversion and circuit control of electric power equipment,as the core parts for the power system.Over recent years,the academic community of power semiconductor devices has witnessed the rapid development of ultra-wide bandgap semiconductor gallium oxide（Ga₂O₃）ascribing to its superior material properties.With an ultra-wide bandgap of 4.8 e V and a high critical breakdown field strength of 8 MV/cm,β-Ga₂O₃ will possess an excellent performance in high-voltage applications.Baliga’s figure of merit（B-FOM）is used to characterize the potential of a material in power application,as a high B-FOM indicates a low power consumption.The B-FOM ofβ-Ga₂O₃ is 3444,which is 3444 times that of silicon（Si）,4 times that of gallium nitride（Ga N）,and 10 times that of silicon carbide（Si C）.Moreover,high quality bulkβ-Ga₂O₃ substrate growth can be realized with Czochralski method,which significantly reduces the manufacturing cost.Together with the easily achieved n-type doping,β-Ga₂O₃ can be served as the major candidate for the next generation power semiconductor materials.Based on the aforementioned background,this paper carries out the research to extent breakdown voltage of gallium oxide power diodes,and proposes corresponding solutions to the challenges existing in the current research of gallium oxide power devices:limitation of breakdown voltage by the crowded electric field at the edge of the anode,the lack of p-type doping,and the experimental device breakdown voltage being lower than the theoretical.The positive results achieved are as follows:（1）The electric field concentration effect at the edge of the anode for the verticalβ-Ga₂O₃Schottky barrier diode（SBD）results in premature breakdowns of the device,limiting the breakdown voltage of theβ-Ga₂O₃ SBD.In order to take advantage of the excellent material properties ofβ-Ga₂O₃,an anode edge magnesium（Mg）ion implantation terminal structure was proposed for the first time.The structure forms a uniform distribution underneath the anode edge by Mg ion implantation at multiple different energy levels and doses.The high-energy Mg ion implantation will destroy the crystal lattice of theβ-Ga₂O₃ material,making the injection region a high-resistance area to withstand a large breakdown voltage.The piranha solution is adopted to repair the interface damage.Mg ion terminals with three different anode radius are fabricated forβ-Ga₂O₃ SBD devices.The turn-on voltage as low as 0.82 V,the current density of to 200 A/cm² at forward bias voltage of 2 V,and the on-resistance of 5.1 mΩ·cm² are delivered simultaneously by theβ-Ga₂O₃ SBD with an anode radius of 90μm.Moreover,the subthreshold swing is as low as 61 m V/dec,approaching the theoretical value of 60 m V/dec,indicating a high-quality interface between Schottky metal andβ-Ga₂O₃.The ideal factor is only 1.05,as the result of that the thermoelectric emission（TE）dominates the forward conduction.Based on the TE method,the barrier height is estimated to be 1.02 e V.For the reverse breakdown mesurement,the breakdown voltage ofβ-Ga₂O₃ SBD with an anode radius of 90μm increases from 500 V to 1550 V after the introduction of the Mg ion implantation terminal,which is enlarged more than the factor of three.At the same time,the electric field distribution of the device is simulated with Silvaco TCAD simulation software.The peak electric field at the anode edge decreased from 10.2MV/cm to 6.8 MV/cm after the introduction of Mg ion implantation terminal,verifying that the this terminal technique is indeed an effective way to alleviate the peak electric field crowded at the anode edge and increase the breakdown voltage.（2）Ga₂O₃ is difficult to achieve p-type doping because of its flat valence band and large effective mass of holes.For power devices,the lack of one type doping means that bipolar devices are difficult to be achieved,which severely limits device performance and application scenarios.With p-type nickel oxide（p-Ni O_x）materials grown by RF magnetron sputtering,p-Ni O_x/n-Ga₂O₃ heterojunction barrier Schottky（HJBS）diodes for the first time.The forward current densities reach 291 A/cm²,813 A/cm²,and 824 A/cm² at the forward bias voltage of 4.2 V for the HJBS diodes with fin-channel widths of 1.5μm,3μm and 5μm,respectively.Due to the conductivity modulation effect in the forward conduction,the on-resistance of the device is greatly reduced to 2.45 mΩ·cm²,1.94 mΩ·cm²,and 1.91mΩ·cm² for the above HJBS diodes with three different fin widths.For the reverseβ-Ga₂O₃epitaxial,the depletion by the pn heterojunction is larger than that caused by the Schottky metal,leading to an increased breakdown voltage of the HJBS diode when compared with the SBD.The breakdown voltage of HJBS diode reached 1340 V.With the on-resistance of1.94 mΩ·cm²,this device yields the power figure of merit to 0.93 GW/cm²,which is the highest inβ-Ga₂O₃ diodes at the same period of time.Additionally,since p-Ni O_x is depleted in different extent for different fin-channel widths,the reverse leakage current can be modulated by the design of the fin-channel size.（3）For theβ-Ga₂O₃ HJBS diodes,reduced p-Ni O_x fin width leads to a large device turn-on voltage,which resulting in the undesired high power consumption.In order to solve this problem and improve the breakdown voltage at the same time,the p-Ni O_x is implemented only at the edge of the anode to optimize the trade-off between the electric field concentration and the turn-on voltage.The field plate termination is adopted to optimize the edge electric field concentration effect further.The forward turn-on voltages of 0.80 V,0.85 V,and 0.90V,current densities of 862 A/cm²,913 A/cm²,and 997 A/cm²,and on-resistance of 3.12mΩ·cm²,3.00 mΩ·cm²,and 2.74 mΩ·cm² for the three devices with anode radius of 30μm,75μm,and 120μm,respectively.With the introduction of the p-Ni O_x junction extension terminal,the breakdown voltage increases from 300 V to 1130 V while the reverse leakage current is still relatively high.After the introduction of field plate,the breakdown voltage reaches 1860 V with the reduced reverse leakage pushing the power figure of merit of the device up to 1.11 GW/cm².Prepared large size devices achieve excellent performance of 7A/1200 V,offering a roadmap for future practical applications ofβ-Ga₂O₃ diodes.（4）Due to the ultra-wide bandgap of 4.8 e V and the high critical breakdown field strength of 8 MV/cm ofβ-Ga₂O₃,academic and industry community have high expectations on the application ofβ-Ga₂O₃-based power devices in the high-voltage field.In particular,devices developed based on wide bandgap semiconductor Si C and Ga N materials have been reported to have breakdown voltages exceeding 10 kV.But this breakdown volatge has not been achieved inβ-Ga₂O₃-based power devices nowadays,which is only about 8 kV forβ-Ga₂O₃diodes and transistors.To pursue the device limit,β-Ga₂O₃ SBD with a double-layer field plate combined with anode interface engineering process is achieved.The device relieves the electric field at the edge through the application of field plate structure,as the implementation of post annealing process raises the Schottky barrier height and enlarges the depletion width.At the same time,the anode interface engineering process also repairs the interface damage,reducing the trap-assisted electrons tunneling.Thus,the leakage current is suppressed and the breakdown voltage has been driven to more than 10 kV.This is the first time forβ-Ga₂O₃-based power devices to achieve this high-class breakdown,verifying the great potential in Ga₂O₃ high voltage applications.