Research On The Applications Of Thermally Evaporated SiO_x In Gallium Nitride High-electron-mobility Transistors

GaN high-electron-mobility transistors(HEMTs),with its high transconductance,high saturation current and high breakdown voltage,has become a hot research field in the field of microelectronic devices for more than ten years.It has a broad application prospect in the field of high-temperature,high-frequency and high-power electronic devices.Although a lot of research has been carried out on GaN HEMTs,there is still a certain distance towards the large-scale commercialization of GaN HEMT devices due to the effect of current collapse effect and large leakage current.In this thesis,firstly the ohmic contact process of GaN HEMTs was optimized.Then the effects of SiOx(x=1.1)thermally evaporated without adding oxygen and reactively evaporated SiOx(1.1?x?1.7)on the performance of GaN HEMT were studied respectively.We also studied the performance of GaN(metal-oxide-semiconductor HEMTs)(MOS-HEMTs)using thermally evaporated SiOx as gate dielectric.Finally,the GaN HEMTs with single field plate and double field plate structure were simulated by Silvaco,and the parameters of the field plate structure were optimized.The specific contents of this thesis are as follows:1.Optimization of ohmic contact for GaN HEMTsThe samples with the electrode spacing of 10 ?m were annealed in nitrogen.The parameters such as the ohmic contact resistance(Rc)and the specific contact resistivity(pc)were calculated by the transmission line method(TLM).The experimental parameters of Al/Ti ratio,annealing temperature,annealing time and alloy metal were optimized.For the annealing conditions of 850 ? and 40 s,Rc and ?c of Ti/AI/Ni/Au electrode first decreased and then increased with the increasing Al/Ti ratio.When Al/Ti ratio was 5:1,the best Rc and pc were 0.85 ?·mm and 2.2 ×10-5 ?·cm2,respectively.Both Rc and ?c of Ti/Al/Ni/Au electrode first decreased and then increased with the increasing temperature(or time).Because Au diffused into the semiconductor,ohmic contacts became worse when the temperature was too high or the time was too long.The ohmic contact of Al/Ti/Ni/Au(30/150/50/60 nm)would be best when annealed at 900 ? for 50 s in nitrogen,Rc and ?c,were 0.24 ?·mm and 1.3 × 10-6 ?·cm2,respectively.No obvious degradation was observed in the metal electrode under this condition annealed again at 300 ? for 10 minutes,which proved that the ohmic contact had good thermal stability.Under the same annealing conditions(900 ?,50 s),Rc and?c of Ti/Al/Ni/Au were much lower than those of Ti/Al/Ti/Au(0.53 ?·mm,6.7 × 10-6?·cm2).2.Study on SiOx passivation layer thermally evaporated without adding oxygen(1)Effect of thermally evaporated SiOx on properties of GaN HEMTsThe effect of SiOx passivation on the performances of GaN HEMTs was studied and compared with the PECVD-SiNx passivated devices.Due to the suppression of the passivation on the surface traps,the maximum drain current(Idmax)of the SiOx and SiNx passivated HEMTs increased by 19%and 25%respectively,and the peak transconductance(gm)increased by 18%and 24%,respectively.The off-state current(Ioff)of SiOx passivated devices was three orders of magnitude higher than that of SiNx passivated devices.After SiOx passivation,the trap density(Dit)of the devices(1.16×1012 cm-2·eV-1)decreased significantly,while the SiNx passivated devices(1.13 × 1013 cm-2·eV-1)increased significantly.This is probably due to the ion bombardment introduced by PECVD during the growth process of SiNx,but there is no such damage in thermal evaporation.The gate leakage current(Igleak)of SiOx passivated devices reduced by an order of magnitude,while SiNx passivated devices increased by an order of magnitude.The Schottky barrier heights(?b)of unpassivated,SiOx passivated and SiNx passivated HEMTs were 0.79,0.85 and 0.73 eV,respectively.SiOx passivation increased the ?b,resulting in a decrease in gate leakage,Igieak should also include tunneling current related to trap density and trap energy.The surface traps with an activation energy of 0.21 eV are closely related to the gate leakage of GaN HEMTs,and these traps will be affected by different passivation processes.The breakdown voltage(Vbr)of SiOx passivated devices increased from 175 V to 206 V,while the Vbr of SiNx passivated devices was only 111 V.Due to the different passivations and grown methods,the cut-off frequency of SiOx passivated and SiNx passivated HEMTs increased from 4 GHz to 5.2 and 5.8 GHz respectively.In the bias stress measurements,the gate electron injection and the reverse piezoelectric effect made the HEMTs obviously degenerate after bias stress,such as the drain current,the transconductance and the gate leakage,and the degree of degradation increased with the increase of the bias voltage(or time).Negative gate bias causes the inverse piezoelectric effect in AlGaN/AlN/GaN heterojunction,which makes lattices relax and produces new lattice defects,forming traps that can bind electrons.The current degradation of the device was significantly reduced after SiOx passivation.This indicates that the SiOx passivation can reduce the surface trap density,and eliminates the degradation of the virtual gate effect to a certain extent.At Vgs =-20 V,Vds=0 V,Idmax and gm of unpassivated devices were reduced by 36.4%and 36.8%respectively,while SiOx passivated devices were 13%and 12.8%,respectively.In addition,Igieak of SiOx passivated devices(1.4 × 10-4 mA/mm)was also two orders of magnitude lower than that of unpassivated devices(2.8 × 10-2 mA/mm).This is because the SiOx passivation can reduce the surface trap of the AlGaN barrier layer,thereby reducing the gate electron injection.In addition,the stress introduced by the SiOx passivation in the AlGaN barrier layer can also suppress the inverse piezoelectric effect.(2)Effect of reactively evaporated SiOx on properties of GaN HEMTsThe effects of SiOx passivation thermally evaporated at different chamber pressures on the performances of GaN HEMTs were investigated,and the properties of the SiOx films were characterized.With the increased chamber pressure(the increase of oxygen content),the x value of oxygen component in SiOx increased.SiO was oxidized to SiOx during evaporation,and the value of x ranged from 1.1 to 1.71 in the range of 1×10-6-6 ×10-4 Torr The AFM images showed that the square root roughness of the SiOx films with different components was less than 1 nm,and the roughness increased with the increase of chamber pressure.The SiOx with different components increased the ldmax of the devices in varying degrees,which is caused by the different chemical components of the passivation layer,or the different stress induced by passivation in the AlGaN barrier layer.The pulsed Idmax of unpassivated HEMTs decreased by 28.2%,while the current collapse of film B passivated devices was only 11.5%.The inhibition effect of film C(SiO1.41),D(SiO1.54)and E(SiO1.71)on current collapse was not so obvious,the current collapse were 20.2%,25.7%and 28.1%,respectively.This may be due to the higher SiOx/AlGaN interface trap density in these samples.Except for the film E,Ioff of SiOx passivated HEMTs was reduced by one order of magnitude.This change in Ioff is related to the surface trap density or the activation energy of electrons hopping on the surface of the device.The Igleak of film B passivated HEMTs was the lowest,which was nearly 20 times lower than that of unpassivated devices.The film A,C and D also decreased the Igleak in varying degrees.However,the film E increased the Igleak by an order of magnitude.The interface trap between SiOx/AlGaN will affect the gate leakage of devices.The Vbr of film B passivated device(151V)was maximum when compared with the unpassivated devices(99 V),film A(138 V),C(143 V),D(124 V)and E(87 V)passivated devices.3.Study on GaN MOS-HEMTs using thermally evaporated SiOx as gate dielectricGaN MOS-HEMTs with thermally evaporated SiOx gate dielectric of different thicknesses(10-30 nm)have been investigated and compared with standard MES-HEMTs.The process of thermal evaporation without ion bombardment has no damage to the surface of the device.Moreover,the thermal evaporation process has no high temperature growth process.The gate dielectric prepared by the lift-off process avoids the damage to semiconductors by dry or wet etching.Compared with MES-HEMTs(317 mA/mm),the Idmax of MOS-HEMTs with 10,20 and 30 nm SiOx were 380,402 and 452 mA/mm respectively.This was owing to the SiOx suppressed the surface traps and reduced the virtual gate effect.Or the stress introduced by the SiOx into the AlGaN layer enhanced the polarization effect of the heterojunction and increased the 2DEG density.Therefore,the 2DEG density of MOS-HEMTs will increase with the increasd SiOx thickness.As the 2DEG density increased,the conductivity of the channel became better,and the on-state resistance of MOS-HEMTs decreased significantly.In addition,MOS-HEMTs had a lower Ioff and a higher on/off ratio.Especially in devices containing 30 nm SiOx,Ioff decreased by two orders of magnitude,and the on/off ratio was 1.4 × 108.The Dit of MOS-HEMTs(?1.0 × 1012 cm-2· eV-1)was obviously lower than that of MES-HEMTs(2.4 × 1012 cm-2·eV-1).This made the SiOx MOS-HEMTs greatly improve in Ioff,Vbr and current collapse.Compared with MES-HEMTs(6.4 × 10-5 mA/mm),the Iglcak of MOS-HEMT with 30 nm SiOx(9.6 × 10-7 mA/mm)reduced by two orders of magnitude,and Vbr increased by 100 V.This is because the increasd barrier height suppressed the injection of carriers between metal and semiconductor.In addition,the current collapse was 46%lower than that of MES-HEMT devices,and the current gain cut-off frequency and the maximum oscillation frequency were increased by 144%and 74%respectively.4.Silvaco simulation on the breakdown characteristics of the field-plate GaN HEMTsSilvaco was used to simulate the GaN HEMT devices with single field plate and dual field plate structure using SiOx as insulator.The breakdown characteristics of HEMTs were studied in detail.The structure parameters of two kinds of field plates(field plate length,Lfp,and SiOx thickness,t)were optimized to maximize the breakdown voltage of the device.The Vbr of devices with single field plate(Lfp = 1 ?m,t = 0.4 ?m)was 465 V,which was 270%higher than that of the devices without field plate(119 V).The Vbr of devices with dual field plate(Lfp1 = Lfp2= 1.5 ?m,t2=t2=0.2?m)was 566 V,which increased by 380%and 22%respectively relative to the devices with no field plate and single field plate.Compared with the devices with no field plate(42 V/?m),the breakdown strength(Vbr/Lgd)of HEMTs with single field plate and dual field plate increased to 88 and 105 V/?m,respectively.In addition,the electric field distribution in the channel and the influence of bias voltage on the electric field were studied,and the relationship between the electric field distribution and the breakdown voltage was also analyzed.The results show that the field plate is an effective way to improve the withstand voltage performance of HEMT devices,and the dual field plate is more effective than the single field plate.Silvaco simulation on the breakdown characteristics of CaN HEMT devices is of guiding significance for the design and preparation of practical devices in the future.