Preparation And Application Of Amorphous Gallium Oxide Based Thin Film Photodetectors

Solar-blind photodetectors that detect the wavelength of 200-280 nm have advantages of anti-interference,high signal-to-noise ratio,and all-weather,and have great application potential in the fields including secure communication,missile warning,flame detection,biochemical detection,ozone-hole monitoring,etc.Compared with the traditional Si-based and vacuum multiplier tube solar-blind photodetectors,solar-blind photodetectors based on the wide bandgap semiconductor such as AlGaN,MgZnO,Ga2O3 have advantages of free of filter,high detection sensitivity,low driving voltage,small size,and easy integration,making it ideal for the new generation of solar-blind photodetectors.Binary compound Ga2O3 with a direct band gap of～4.9 eV,an absorption cutoff of 253 nm in the solar-blind range,large UV absorption coefficient of～105 cm-1,simple process,and high stability,is an ideal material for solar-blind photodetectors.Compared with crystalline Ga2O3 with high growth/process temperatures and lattice matching substrates,amorphous Ga2O3(a-Ga2O3)has great advantages such as simple preparation process,low process temperature,large-area process with high uniformity,and low cost,and thus has great application potential in large-area array imaging,flexible and wearable high-performance photoelectric devices.It has been reported that the a-Ga2O3 photodetectors still have problems of low responsivity(R),slow response speed due to low carrier mobility and defect density.To address these problems,in this thesis,the effects of fabrication conditions on a-Ga2O3 thin films and photoelectric performances were studied and high-performance a-Ga2O3 Schottky photodiodes,a-Ga2O3 thin-film transistors(TFTs),and a-Ga2O3/IGZO heterojunction TFT photodetectors were prepared and studied.The main contents of this thesis are as follows:1.Effects of fabrication conditions on a-Ga2O3 thin films and the performances of the photodetectors with Ti/a-Ga2O3/Ti structure.We systematically investigated the fabrication conditions of a-Ga2O3 thin films including sputtering oxygen partial pressures(Po),sputtering power,post-annealing,and their effects on the film surface morphology,optical bandgap(Eg),Urbach energy(Eu),trap density(Ntrap),and carrier mobility(u),oxygen vacancy concentration,and performances of the a-Ga2O3 based photodetectors to develop an optimal process.(1)Effects of Po on a-Ga2O3 thin films and the photodetector performancesWith Po increase from 0 to 1.5%,the root-mean-square(RMS)surface roughness of the a-Ga2O3 thin films was almost unchanged(～0.2 nm),the Eg increased from 4.98 to 5.21 eV,and Eu increased from 0.276 to 0.419 eV.The dark current(Idark)of the a-Ga2O3 phtotodetectors was all in the range of 10-11 A withour obvious change with Po increasing,but the photocurrent(Iphoto)decreased from 6.61×10-7 to 2.88×10-10 A,R decreased from 20 to 0.008 A·W-1,rise/fall time(τr/τd)reduced from 2.65/0.178 to 2.02/0.059 s.This is attributed to that the increase of Po decreases the oxygen vacancy density and increases the degree of disorder in a-Ga2O3 films.(2)Effects of sputtering power on a-Ga2O3 thin films and the photodetector performancesAs the sputtering power increased from 70 to 130 W,the RMS and Eu of a-Ga2O3 thin films had no clear change(～0.2 nm,～0.28 eV),and Eg increased from 4.88 to 5.03 eV.Meanwhile,the Idark of a-Ga2O3 photodetectors increased from 1.84×10-11 to 1.13×10-8 A,the Iphoto increased from 3.27×10-8 to 3.45×10-5 A,and the R increased from 1 to 1044 A·W-1,all increased by 3 orders of magnitude,and the τr/τa increased from 1.79/0.062 to 4.00/3.39 s.This is suggested to be due to the increase of the oxygen vacancy density caused by the increased sputting power in the pure argon atmosphere.(3)Effects of post-annealing on a-Ga2O3 thin films and the photodetector performancesCompared with the as-deposited a-Ga2O3 thin films,the annealed a-Ga2O3 thin films showed that Eg and Eu remained basically unchanged(～5.0 eV,～0.28 eV),the RMS decreased from 0.243 to 0.168 nm,the ratio of oxygen vacancy increased from 44.8%to 49.1%,Ntrap increased from 1.0×1016 to 1.8×1016 cm-3,and μ decreased from 1.4×10-2 to 8×10-3 cm2·V-1·s-1.Due to the increase of oxygen vacancy density after annealing,both Iphoto(4.42×10-6 A)and R(133.71 A·W-1)of the annealed a-Ga2O3 photodetectors were higher than the as-deposited(6.61×10-7 A,20 A·W-1)by an order of magnitude,while the τr/τd of the annealed one(5.54/0.660 s)were higher than that of the as-deposited one(2.65/0.178 s)because of the increased Ntrap and the decreased μ.2.a-Ga2O3 Schottky photodiodes and imaging arraya-Ga2O3 Schottky photodiodes with Ti/a-Ga2O3/Au asymmetric electrodes were fabricated for the first time and achieved excellent performances with the R of 1021.8 A·W-1,PDCR of 2.3×106,Idark of 6.6×10-12 A,and τr/τd of 2.98/0.51 s.The high performances were attributed to the large concentration of oxygen vacancies,the possible photo released carriers from the deep-level acceptors,and the high film uniformity.The former two are propitious to form high concentration of photo carrier and thus result in high R and PDCR,and the latter one contributes to high quality Schottky contact to achieve the low Idark and fast response.In addition,large-area array imaging based on these a-Ga2O3 Schottky photodiodes was demonstrated.3.a-Ga2O3 TFT and applicationsa-Ga2O3 TFT photodetectors were fabricated and achieved the low Idark of 3×10-12 A,and high Iphoto of 1.8×10-3 A.Under positive gate bias,the high concentration oxygen vacancies acts as deep-level traps and lead to high resistant channel without gate modulation in the dark,and thus results in the low Idark.However,the photogenerated carriers fill the deep-level traps and eliminate or reduce the trap shielding effect under illumination,so that it realizes effective gate modulation,and thus leads to high Iphoto and fast response.The key parameters of the a-Ga2O3 TFT with the R,detectivity(D*),and PDCR of 2.0×105 A·W-1,4.9×1018 Jones,9.9×107%,and 2.9×108,respectively,are considerably superior to those of the reported a-Ga2O3 phototransistors,and were comparable to or even better than those of the reported high-performance crystalline β-Ga2O3 phototransistors.The UV-visible rejection ratio was up to 8.5×107,and is close to the highest value for the reported Ga2O3 photodetectors.Furthermore,imaging array and light control logic gate circuits with high sensitivity and high contrast were demonstrated based on these a-Ga2O3 TFTs,and the application of a-Ga2O3 TFTs in the neuromorphology was also explored.4.a-Ga2O3/IGZO heterojunction TFTTo address the problem of slow response due to high defect density and low carrier mobility of a-Ga2O3 thin films,a-Ga2O3/IGZO heterojunction TFT was fabricated at 170℃low temperature and achieved the mobility of up to 15.8 cm2·V-1·s-1.The energy band structure,X-ray photoelectron spectroscopy,and variable temperature Hall effect results indicate that quasi-two-dimensional electron gas is formed at the Ga2O3/IGZO heterointerface,and thus leads to the high detection performances.The a-Ga2O3/IGZO TFT photodetector exhibited high R、D*,PDCR,and short rise detection time of 8.35×103 A·W-1,2.56×1017 Jones,5.21×106,and 1.18s,respectively,even with a single long channel(without interdigital structure,channel length=60 μm).Based on these heterojunction TFTs,a 10 x 10 array was fabricated and an "N" pattern imaging with high contrast was realized.The results showed that Ga2O3/IGZO heterojunction is an effective way to realize high-performance photodetectors with low process temperature,and it has great potential for large-area array imaging and flexible,wearable high-performance photoelectric devices.