| Wide bandgap semiconductor,Ga2O3,has a direct energy-band gap(Eg)of 4.5-5.2 eV and a Baliga's figure of merit(BFOM)of more than 3000,allowing its strong absorbance in solar-blind ultraviolet(UV)region of the electromagnetic spectrum and low direct current losses in low frequency applications.For applications in solar-blind detection,compared to the alloying AlGaN and MgZnO materials,Ga2O3 has more stable and controllable structure,and it also show lower cost than diamond.Currently,Ga2O3 thin films could be prepared by using magnetron sputtering,pulsed laser deposition(PLD),molecular beam epitaxy(MBE),atomic layer deposition(ALD)and metal-organic chemical vapor deposition(MOCVD)methods;and in addition,its bulk substrate may be grown by conventional melting techniques,such as floating zone(FZ),edge defined film fed(EFG)and Czochralshi methods.In the view of device structures,the low noise(dark current)and high photo-response could be obtained in a Schottky photodiode,and the gate voltages in a phototransistor could control the currents in the active channel layer,leading to large gain and high photo-response.In this doctoral thesis,based on the Ga2O3 thin films,Ga2O3-based Schottky photodiodes and Schottky-gated field-effect phototransistors are fabricated and characterized.The main research achievements are stated as follows:(1)The ?-Ga2O3-based Schottky photodiode is constructed by magnetron sputtering,UV i-line photolithography and lift-off techniques.Its solar-blindness is performed and discussed.It shows the dark current(Idark)of 1.32×10-11 A.Under 254 nm UV light illumination,the photo-to-dark current ratio(PDCR)is 2.83 × 105,responsivity(R)is 144.46 A/W,specific detectivity(D*)is 7.29×1014 Jones and external quantum efficiency(EQE)is 64711%.At zero bias,the R is 0.73 mA/W and the D*is 3.35×1010Jones,suggesting a self-powered operation.In addition,it could also operate stablely at voltage as high as 180 V.(2)Based on the above results,we discuss the effects of device scales on the device performances.Using the MOCVD-grown ?-Ca2O3 thin film,the ?-Ca2O3-based Schottky photodiode detectors with different device scales,i.e.different on-state resistances(Ron),are fabricated.Owing to the high quality and excellent uniform film,all the devices display R of 175 A/W-1372 A/W,EQE of 85700%-671500%and D*of ?1014 Jones,superior to most of the similar devices.Smaller device scales lead to higher Idark,lphoto and R,suggesting that the device performances have a significant and disciplinary relationship with the device scales.(3)In the view of the device structure design,the MOCVD-grown ?-Ca2O3 metal-oxide-semiconductor(MOS)structured photodiode is demonstrated for the first time.Owing the decreasing reverse leakage current by the used high-k oxide dielectric,compared to the metal-semiconductor Schottky photodiode,it has lower reverse leakage current and higher PDCR.Dual-mode operation,both the photoconductive mode and the depletion mode governed by the electron tunneling and thermionic electron theory respectively,is achieved in the fabricated device.Moreover,self-powered operation is realized at zero bias.(4)The gate voltage could control the carrier transports in channel,large gain and high photo-response coule be achieved.The enhancement-mode(E-mode)Schottky-gated Si-doped ?-Ca2O3 metal-semiconductor field-effect phototransistor is fabricated and characterized for the first time,in which the Si-doped ?-Ca2O3 thin film is grew by MOCVD equipment.The device has a turn-on voltage(Von)of 4.04 V lower than the other Ca2O3 field-effect transistor(FET)-based photodetectors,and a subthreshold swing(SS)of 1.4 V/dec.As a photodetector radiated by 254 un 245 ?W cm-2 intensity UV light,it shows PDCR of 4.85×104,linear dynamic region(LDR)of 93.7 dB,R of 74 A/W,D*of 2.15×1014 Jones and EQE of 3.6×104%. |
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