Research On AlGaN/GaN HEMT Based On Advanced Ohmic Contact Process With Laser Annealing Technique

As an outstanding representative of the third-generation semiconductor materials,Gallium Nitride(Ga N)can be widely used in lighting,mobile communications,electric vehicles and consumer electronics due to its wide band gap and high critical electrical field.It is one of the frontier research in the semiconductor field all over the world.Different from conventional Sibased power devices,Ga N-based High Mobility Electron Transistor(HEMT)utilizes the polarization effect of the AlGaN/GaN heterojunction structure to provide a two dimensional electron gas(2DEG)channel.The excellent electrical characteristics make AlGaN/GaN HEMT a promising candidate for high frequency and high power applications.Although with vast prospects in the field of power devices,currently AlGaN/GaN HEMT still has many scientific and technical issues.Ohmic contacts are fundamental building blocks of power and RF devices.The conventional ohmic contact scheme consists of Ti/Al/Ni/Au evaporation followed by a rapid thermal annealing(RTA)at above 800 °C.However,high temperature annealing is responsible for a rough surface morphology of the metal and thermal damage of the epitaxial materials.Several approaches have been reported to solve these problems.However,these approaches need additional processing steps,which increase fabrication complexity and cost and damage other electrical properties of the device.In addition,breakdown voltage and dynamic resistance performance are the key characteristics of AlGaN/GaN HEMT.Several approaches have been proposed to optimize them including material growth,device design and process improvement.However,the impact of the critical process such as ohmic contact formation process on the other electrical properties of AlGaN/GaN HEMT is also important.In this thesis,the ohmic contact formation process and critical electrical characteristics were studied,including:(1)A new ohmic contact formation process for undoped AlGaN/GaN HEMT based on laser annealing(LA)was reported.The smooth surface morphology of ohmic metal and smooth interface of ohmic contact were achieved.The feasibility of the new ohmic contact formation process based on laser annealing was verified by the experiment design and equipment construction of laser annealing.Due to the high energy density of the selective laser annealing,the rough surface morphology of the metal and thermal damage of the epitaxial materials were improved compared with conventional rapid thermal annealing.Electrical measurements and a range of physical analysis tools including focused ion beam(FIB),transmission electron microscopy(TEM)and energy dispersive x-ray spectroscopy(EDS)were used to investigate the current transport mechanism of the ohmic contacts.(2)The AlGaN/GaN HEMT was fabricated based on advanced ohmic contact process with laser annealing technique.Breakdown voltage and dynamic resistance of AlGaN/GaN HEMT were improved.The mechanism of device performance improvement was revealed.Compared with conventional rapid thermal annealing,the formation of metal spikes at the ohmic contact interface was suppressed by laser annealing.Thus,the leakage current of the source and drain terminals was reduced under a high-voltage blocking state.The breakdown voltage of the LA device is 442 V with the gate-drain distance of 5?m,which is 37.3% higher than the 322 V breakdown voltage of the RTA device.At the same time,the dynamic resistance of AlGaN/GaN HEMT was also improved by laser annealing.For the LA device,the ratio of the dynamic resistance to the static resistance is only 1.35 times with the drain voltage of 200 V.For the RTA device,the ratio of the dynamic resistance to the static resistance is 8.66 times with the drain voltage of 125 V.The mechanism of the improvement of dynamic resistance characteristics by laser annealing was further studied by X-ray photoelectron spectroscopy.(3)A new Au-free ohmic contact formation process for undoped AlGaN/GaN HEMT based on laser annealing was reported.The CMOS-compatible and high reliability device was developed.In order to reduce the manufacturing cost of Ga N devices,Au-free process was studied in this thesis.Pre-ohmic recess etching of the Al Ga N barrier has been demonstrated to be an effective method to achieve Au-free ohmic contact in un-doped AlGaN/GaN HEMT.However,the accuracy and uniformity of etching were hard to control.Thus,a new Au-free ohmic contact formation process for undoped AlGaN/GaN HEMT based on laser annealing was reported.The current transport mechanism of ohmic contacts were studied.A fabrication process of AlGaN/GaN HEMT with Au-free process was designed.The impact of high-temperature annealing on the isolated and active region of the device was studied.Laser annealing manifests a new solution of ohmic contact fabrication in commercial AlGaN/GaN HEMT.