Fabrication And UV Detection Of Gallium Oxide Thin Film Transistors

Wide bandgap semiconductors represented by silicon carbide(SiC)and gallium nitride(GaN)have great significance in the field of ultraviolet photodetection and power and radiation-resistant electronics,and it is the foundation for safeguarding national security and the development of the national economy.Ultra-wide bandgap semiconductor gallium oxide(Ga2O3)has larger band gap(4.5?4.9 eV)and higher breakdown field strength than those of SiC and GaN,and thus has great value in the fields of deep-ultraviolet(deep-UV)photodetectors,ultra-high power electronics,and extreme environmental applications.The band gap of Ga2O3 corresponds to the absorption cutoff edge at 250 nm,which meets the requirements of deep-UV"solar-blind"optoelectronics.Therefore,ideal deep-UV detectors and chip system can be prepared to missile warning in all weather,shipbome communication,fire detection and,etc.At present,Ga2O3 solar-blind detection technology is still in the very beginning stage,and there is no mature product to be applied yet.Researches on Ga2O3 solar-blind detectors will give significant guidanc,e to its applications and accelerate formation of its industry.This thesis work mainly studies the thin film transistors(TFTs)based on mechanically exfoliated Ga2O3 single crystal thin films,the adjustment of TFT threshold voltage,and their application and performance analysis in solar-blind photodetectors.The main contents are as follows:1.Fabrication of bottom-gate top-contact Ga2O3 TFTs based on mechanically exfoliated ?-Ga2O3 single crystalline films?-Ga2O3 single crystal is a layered material with two cleavage planes of(100)and(001),and thus high-quality two-dimensional single crystalline thin film can be obtained by mechanical exfoliation.The Ga2O3 used in this thesis work is a high-quality Cr-doped n-type 3-Ga2O3 single crystal grown by Edge-defined film-fed growth by Professor Tao Xu-tang's group,from the State Key Laboratory of Crystal Materials of Shandong University.The Ga2O3 single crystal shows a band gap of 4.72 eV and has a carrier concentration of approximately 1018 cm-3.The mechanically exfoliated Ga2O3 film with nanometer thickness(90?300 nm)were transferred to SiO2/Si wafer substrates,and the Ga2O3 surface under source and drain electrodes was treated by dry etching to optimize the ohmic contact,then the Ti/Au source and drain electrodes were deposited by electron beam evaporation,and finally the bottom-gate top-contact Ga2O3 TFTs were completed.2.Adjustment the threshold voltage of Ga2O3 TFTsCompared with depletion TFTs,enhanced TFTs has higher application value in high-speed and low-power circuits.Therefore,the design of devices with small and positive threshold voltage has always been one of the goals of Ga2O3 TFTs.This thesis work shows that the thickness of Ga2O3 active layer,double gate structure,and p-type SnOx coverage on Ga2O3 surface can effectively adjust the threshold voltage of Ga2O3 TFTs.(1)Ga2O3 TFTs with different active layer thicknesses(90?300 nm)were fabricated in this thesis work.The device performance study showed that:As the thickness of Ga2O3 thin film decreases,the TFT threshold voltage shifts significantly to the positive bias due to the decrease in the total number of carriers in the active layer,from-36.5 V(300 nm Ga2O3)to 0.4 V(90 nm Ga2O3),and the transition from depletion type to enhanced type of Ga2O3 TFT is successfully realized.(2)TFTs with bottom-gate,top-gate double-gate structures were fabricated based on Ga2O3 with a thickness of 140 nm.Our studies show that the threshold voltage of the double-gate devices(-2.71 V)shifts to positive bias direction compared to the single bottom-gate(-3.14 V)or top-gate(-3.34 V)devices,and the double-gate TFT has the optimal performance with the switching ratio of 1.54×105 and the mobility of 7.56 cm2/Vs.This is because that the double-gate TFT controls the carriers both at the upper and lower channel interfaces through the top-gate and the bottom-gate at the same time respectively,and thus it enhances the gate control ability.In addition,the top dielectric layer Al2O3 encapsulates the device to reduce the impact of the environment(moistures and oxygen,etc.)to a certain extent,therefore,improved device performance has been achieved.The results show the effectiveness of this method.By further improving the double gate regulation ability,such as increasing the capacitance of dielectric,and improving the channel interface quality,it is expected that the threshold voltage can be tuned to be more positive.(3)Capping SnOxthin film on the back-channel surface of bottom-gate Ga2O3 TFT(Ga2O3 thickness is 185 nm)realized obvious positive shift of the threshold voltage from-29.14 V(not covered)to-11.37 V(SnOx covered)based on the carrier depletion principle at p-n junctions.The contaction of between SnO,and Ga2O3 consume the electrons in the Ga2O3 active layer close to the back surface of the channel,thus,the TFT can be turn off under gate voltage with smaller absolute value.In addition,with the capping of SnOx layer,the dangling bonds in the back channel surface,which acts as carrier traps,were passivated,and the number of total channel carriers decrease,and thus it reduces the electron scattering in the active layer,increases the mobility of the device(from 15.94 to 59.78 cm2/Vs),and reduces the off state current(from 10-3 to 10-6 mA/mm)and subthreshold swing(from 0.82 to 0.35 V/dec).This study provides a new method for threshold voltage regulation and device performance improvement of Ga2O3 TFT.3.Fabrication and characterization of Ga2O3 TFT solar-blind photodetectorsThe current of photodetectors can be controlled by gate voltage in TFT solar-blind photodetectors.It can achieve very low off state dark current,which is beneficial to achieve high photocurrent to dark current ratio.The solar-blind detector based on Ga2O3 TFT structure shows a good response to solar-blind ultraviolet(254 nm)when the TFT is turned off.The ratio of the photocurrent to dark currentIphoto/Idark is up to 8 × 105,the responsivity is 4.79 x 105 A/W,the external quantum efficiency(EQE)is 2.34 x 106,and the response time is<25 ms.To our best knowledge,these performance are the relatively high for the same type of Ga2O3 deep-UV detectors.These studies confirm the great potential of Ga2O3 TFT solar-blind photodetector in the field of photoelectric detection applications.