The Study On Fabrication Of GaM-based Heterostructures And HEMT Devices

GaN-based materials have wide band gap,high electron saturation drift velocity,high breakdown voltage and excellent anti-irradiation.They are easy to form heterostructures with high concentration of two-dimensional electron gas(2DEG)at the interface produced by large spontaneous and piezoelectric polarization effects.Therefore,they are very suitable to fabricate new generation of microelectronic devices and circuits with high frequency and high power which are very promising for application in military and civilian.At present,GaN materials and devices are the global hot and difficult research spots in the field of semiconductors.This dissertation focuses on the research of some problems of GaN based heterostructures and high electron mobility transistors(HEMTs),such as charge carriers distribution in N-polar GaN/AlxGai-xN/GaN heterostuctures,growth of Fe-doped GaN epifilm with high resistivity and of GaN-based HEMT materials with high quality using metal organic chemical vapor deposition(MOCVD),and the fabrication of GaN-based HEMT devices with less or without current collapse.Some novel results are obtained in this work as in the following:1.Influences of structural parameters and doping concentration on the distribution of 2DEG and two-dimensional hole gas(2DHG)in N-polar GaN/AlxGai-xN/GaN heterostructures are systematically researched firstly to the best of our knowledge.The results provide a valuable guideline to design and analysis of N-polar GaN/AlxGai-xN/GaN HEMTs.Using a self-consistent solution of the one-dimensional Schrodinger and Poisson equations,a theoretical study of N-polar GaN/AlxGai-xN/GaN heterostructures is conducted systematically.The dependence of 2DEG and 2DHG sheet densities on variables,such as GaN top layer thickness,AlxGa1-xN back barrier thickness and Al content x and n-type doped concentration in AlxGa1-xN and GaN layers,is investigated.As for unintentionally doped N-polar GaN/AlxGa1_xN/GaN heterostructures,2DEG concentration at upper GaN/AlxGa1-xN interface will be gradually increase by increasing GaN top layer thickness,while 2DHG density at bottom AlxGa1-xN/GaN interface will be gradually decrease,until saturated.The increase of AlxGa1-xN back barrier thickness causes a simultaneous rise of 2DEG and 2DHG concentration.When larger Al content x in AlxGa1-xN back barrier is employed,the remaining 2DHG concentration at the bottom AlxGa1-xN/GaN interface is shown to be higher and 2DEG concentration at the upper GaN/AlxGa1-xN interface is also larger.For N-polar GaN/AlxGa1-xN/GaN heterostructures with AlxGa1-xN back barrier and bottom GaN layer n-type doped,we found that high sheet density of 2DEG is present while there is no 2DHG in n-type doped AlxGa1-xN back barrier structure,and that 2DEG concentration at upper GaN/AlxGa1-xN interface will increase by increasing n-type doping concentration.And for N-polar GaN/AlxGa1-xN/GaN heterostructures with bottom GaN layer δ-doped,we found that 2DHG concentration at bottom AlxGa1-xN/GaN interface will decrease until disappearance by increasing δ-doping concentration.There exists a doping concentration range without 2DEG at the upper interface and 2DHG at the bottom interface at the same time.With further increasing n-type δ-doping concentration at the bottom GaN layer,the concentration of 2DEG at the upper interface increases.Four possible physical origins of 2DEG in the upper interface of N-polar GaN/AlxGa1-xN/GaN heterostructures are proposed based on the calculated results.2.The effects of unintentionally-doped and Fe-modulation-doped GaN buffers with high resistivity on direct current characteristics of the corresponding HEMTs are researched and it is founded that HEMTs with Fe-modulation-doped GaN buffers have beter reliability.GaN buffers with different Fe-modulation-doped concentration were grown using MOCVD system and the effects of Fe-modulation-doped concentration on the properties such as resistivity,crystal quality and surface morforlogy of GaN buffers were studied.Unintentionally-doped and Fe-modulation-doped epitaxial GaN buffer layers with good surface morphology and higher resistivity than 107 Ω·cm were achieved on the 4-inch sapphire substrates.Based on this,GaN/AlxGa1-xN/AlN/GaN heterostructures with above two kinds of GaN buffers were grown and the corresponding HEMTs were fabricated.The sheet resistance and 2DEG mobility of the heterostucture with Fe-modulation-doped high resistive GaN buffer(sample A)were measured to be 348 Ω/sq and 2503 cm2/V·s,while for heterostructure with unintentionally-doped high resistive GaN buffer(sample B)were 373 Ω/sq and 1926 cm2/V·s.HEMTs fabricated from samples A and B have a gate length of 3 μm,a gate width of 100 μm,a gate-to-source distance of 5 μm,and a gate-to-drain distance of 20 μm.At VDS = lOV,the transfer characteristics and gate leakage current of both kinds of devices as a function of gate voltage were measured in the pulsed mode,and the results show that HEMTs from sample A have a higher maximum current density of 395 mA/mm and lower gate leakage current of 3.32×10-7 A at VGS=2V.Furthermore.when HEMTs from sample A and B were applied different VGS and VDS stesses,their IDS-VDS output and Gm-VGS transfer characteristics were measured in pulsed mode.The results indicate that devices with Fe-modulation-doped high resistive GaN buffer show more potential in improving reliability upon off-state stress.3.GaN/In0.17Al0.83N/AlN/GaN HEMTs with both source and gate dual field-plates(FPs)were proposed firstly and realized,and Impacts of bias stress on drain current degradation in GaN/Ino.17Al0.83N/AlN/GaN HEMTs without FPs,with gate FP and with both source and gate dual FPs were comparatively studied.The restlts reveal that HEMTs with both source and gate dual FPs show less current collapse.GaN/Ino.17Al0.83N/AlN/GaN HEMT structure material was grown by MOCVD on 2-inch sapphire substrate.Hall measurements at room temperature yield a 2DEG comcentration of 2.432x1013 cm-2 and mobility of 850 cm2/V·s,leading to a sheet resistance of 302 Ω/sq.To study on the effects of FPs on the drain current degradation,three types of devices without FPs(device A),with gate FP(device B)and with source and gate dual FPs(device C)were fabricated,and tested before voltage stress,after off-state stress and after on-state stress,respectively.Our experiment results indicated that an effective reduction of stressed-related drain current degradation obtained in the device with dual FPs.Measured output characteristics under VGS = 2 V after the off-state bias stess show 3.32%drain current reduction rate for devices with dual FPs,which is less than that in both the devices with a gate FP of 7.57%and devices without FPs of 14.63%.The transfer characteristics under VDS = 10 V of the three kinds of devices show that after the off-state bias stress,there was a positive shift in the threshold voltage in device A,and the variations of threshold voltage in device B and C were not obvious.The decrease of drain current and transconductance was more serious after the application of the on-state stress.The variations in device A are the largest,device B is next and device C is the least.Therefore,a dual FPs structure applied in AlGaN/GaN HEMTs to restrain the devices’degradation achieve excellent performance.