Research On GaN High Electron Mobility Transistor With Recessed Gate Based On Self-terminated Thermal Etching Method

As one of the third-generation semiconductors,GaN has the excellent intrinsic material properties like wide band gap and high breakdown field.Meanwhile,two-dimensional electron gas(2DEG)with high electron density as well as high electron mobility can be formed in AlGaN/GaN heterojunction due to the unique piezoelectric effect Therefore,GaN-based high-electron-mobility transistors(HEMTs)have emerged as excellent devices for high-power applications In practice,for consideration of fail-safe and simplification of drive circuit,true enhancement-mode(e-mode)devices are more desirable.Several approaches have been proposed to realize e-mode operation,such as fluorine-plasma ion implantation,recessed gate,and p-GaN gate.For the recessed gate-based e-mode HEMT,the precise gate recess with high uniformity and low density of interface states has become one of the key problems that need to be solved urgently.Thus this thesis systematically developed a new method to prepare the recessed gate which could be applied successfully in the practical fabrication process.The main contents of the thesis are as follows:1.A novel gate recess method with high uniformity,low density of interface states and controllable depth was successfully realized.Based on the in-depth investigation on the traditional dry etching and wet etching,a self-terminated thermal decomposition carried out in the MOCVD chamber at high temperature for the fabrication of recessed was proposed.A composite barrier structure with "10-nm-thick GaN insertion"based on the polarization model was specially designed,making the self-terminated thermal decomposition possible experimentally.To remove the GaN residues completely in the recess region after decomposition,the gas flow model was proposed.Additionally,the effects of decomposition temperature,chamber pressure,NH3 flow rate and the relative direction between the recess and the gas flow were analyzed in detail.Finally,smooth step-flow morphology with an RMS roughness of 0,29 nm was obtained.Compared with the conventional dry etching-only method,this self-terminated gate recessing method features an improved uniformity of recess depth by～3 times,a remarkably reduced density of interface states of 1011 eV-1·cm-2 gas well as a precise control of etching depth.2.The source and drain ohmic contacts with high uniformity and low contact resistivity were fabricated by Si ion implantation method.The mechanism of traditional Ti/Al-based high temperature ohmic contact and the role of each layer of metal were studied.The TiN surface contact layer was used for Au-free ohmic contacts with low surface roughness.The contact resistivity of 0.84 Ω·mm was achieved by optimizing the thickness of AlGaN barrier,the thickness ratio of Ti/Al metal stack and the annealing temperature.Furthermore,Si-ion implantation-based ohmic contact was fabricated in which SiN/AIN layer was used for surface protection during the high-temperature Si-activation process as well as a passivation layer.In addition,through a careful optimization of the ion implantation dose,low ohmic contact resistivity of 0.3Ω was achieved finally.3.The GaN MIS HEMT device with high uniformity and low density of interface states was successfully fabricated by using self-terminated gate recessing method based on the thermal etching.Compared with the devices fabricated by traditional dry etching method,its performance was greatly improved,including a high failure gate bias of+13 V,a tiny thermal-induced ΔVTH of-0.4 V,a small threshold voltage shift of～0.08 V,and a high uniformity of VTH with an average of-6.03 ± 0.12 V.Moreover,dual channel composite barrier structure with 2 nm-thick AlGaN bottom barrier was designed and the MIS gate e-mode device was successfully fabricated.The device features the a positive threshold voltage of 1.2 V@10 μA/mm,a high switching ratio of 109,and a large output current of～300 mA/mm,showing a great potential for the reliable fabrication of e-mode MIS-HEMT through the as-developed gate recessing technique.4.By using the thermal etching method,a complete process for the fabrication of regrown p-GaN gate e-mode devices(p-GaN e-HEMT)was developed.The p-GaN e-HEMTs feature threshold voltage of 1.75 V@10 μA/mm,switching ratio of 1010,and threshold voltage shifts of 0.005 V.The gate leakage current was reduced and the output current as well as peak transconductance were improved.For practical applications,to reduce the undesired inductive/capacitive parasitic effect due to the bond wires/package leads for a faster switching speed,on-chip integration of power transistors and diodes was studied.Therefore,a p-GaN-gated hybrid anode lateral diode with thick AlGaN barrier layer was successfully developed,which is fully compatible with the process flow of regrown p-GaN gate e-mode HEMT.Compared with the traditional diode with thin barrier,the forward turn-on voltage(Von)and on-resistance(Ron)can be improved simultaneously,and the reverse breakdown voltage of 488 V can be achieved without field-plate structure.