Preparation Of Novel Wide-bandgap Oxide Semiconductor Thin Films And Exploration Of Related Ultraviolet Photodetectors

The novel ultraviolet(UV)photodetectors based on third-generation wide-bandgap oxide semiconductors have drawn much attention from academia and industry,due to its high quantum efficiency,simple filter structure,low cost and excellent thermal stability.Presently,the research on wide-bandgap oxide semiconductors is still in the early stage,and many key scientific problems have not been solved yet,such as controllable p-type doping,energy-band engineering,modulation of defects and carriers.The isovalent ions doping to form multi-component alloys is an effective way of tuning the physical properties in the oxide semiconductors.The main challenges in this subject are phase segregation,lattice distortion and solid solubility limitation.Besides,oxygen vacancy defects are known as inevitable intrinsic defects in oxide semiconductor materials,which have a significant impact on the properties of materials and devices.Thus,effective control of the oxygen vacancy defect is the key to realize the device application of wide-bandgap oxide semiconductors.As typical wide-bandgap oxide semiconductors,ZnO and Ga₂O₃ are popular materials in the field of ultraviolet photodetectors.In this thesis,innovative researches were carried out focusing on the common issues of the wide-bandgap oxide semiconductor materials.The novel ZnO quaternary alloy materials have been developed.The effects of growth process on the structure and photoelectric properties of the ZnO quaternary alloy and Ga₂O₃thin films were studied.Based on the ZnO alloys,Ga₂O₃,and ZnO alloys/Ga₂O₃ heterojunctions,ultraviolet photodetectors with different device structures have been explored.The main research contents and results are as follows:1?The polar c-and nonpolar m-plane BeMgZnO alloy films were grown by pulsed laser deposition(PLD),and the alloy-based photoconductive UV photodetectors with Au/BeMgZnO/Au planar structure have been constructed.The results show that the deposited films are single-phase hexagonal wurtzite structure accompany with excellent in-plane/out-of-plane orientations.The c-BeMgZnO alloy film exhibits a wider optical bandgap of?4.2 e V than that of m-BeMgZnO film,primarily due to its higher Be content.Compared with pure ZnO-based photodetectors,the persistent photoconductive effect in BeMgZnO alloy-based photodetector is significantly surpressed,which is mainly due to the reduction of oxygen vacancy-related trap centers caused by Be-Mg co-doping.At bias of 5 V,the c-BeMgZnO device exhibits a low dark current of 19.5 p A and short rise/decay time of2.81 s/0.22 s.The m-BeMgZnO device exhibits a high photoresponsivity of 135 m A/W at280 nm.Both of the two devices show high UV/Visible rejection ratio of 10³with photoresponse wavelength lies in range of 200-350 nm.Note that,the m-BeMgZnO device shows a self-powered UV photoresponse at 0 V bias,which is attributed to the spontaneous polarization electricfield in the nonpolar alloy film.In addition,the m-BeMgZnO device has higher photoresponsivity but slower recovery speed than that of c-BeMgZnO device.The former is caused by the synergistic effect of spontaneous polarization electricfield and external electricfield in m-BeMgZnO alloy film.The latter is due to the small grains and more grain boundaries in c-BeMgZnO alloy film,since the defects at the boundaries boundaries always act as recombination centers which can improve the recombination efficiency of photo-generated carriers.2?Considering the complementary effect between the two different substitutes may lead to the reduction of lattice distortion in the co-substituted ZnO quarternary alloys,we proposed to develop novel Be Cd ZnO alloys in this work.By PLD method,Be Cd ZnO quaternary alloy film was successfully synthesized for the first time.The modulation effects of growth oxygen pressure on the crystal structure,surface morphology,composition and optical bandgap of the alloy film was investigated.The results show that,simultaneous incorporation of Be and Cd does not change the lattice structure of ZnO.The deposited alloy films are smooth with surface roughness less than 0.5 nm.The bandgap of the alloy film can be tuned from 3.3 e V to3.52 e V by adjusting O₂ pressure.Under high O₂ pressure,the crystal quality of the alloy film becomes better accompany with increasing O content and decreasing Be(Cd)contents,indicating the reduction of oxygen vacancy defects.Then,the alloy-based photoconductive UV photodetectors with Al/BeCdZnO/Al planar structure have been constructed.With improving the crystal quality of alloy film under oxygen-rich growth conditions,the dark current of the device decreases from 1.79 n A to 16.2 p A,and the decay time decreases from14.46 s to 3.42 s.This can be attributed to the decrease of oxygen vacancy defects which lead to the decrease of intrinsic carrier concentration and trap centers.3?Enlightened by the quaternary alloy of BeMgZnO and BeCdZnO,we attempted the first study of BeCaZnO quaternary alloy materials in this work.We succeeded in the growth of single-phase BeCaZnO alloy films by PLD,and developed planar photoconductive-type UV photodetectors using BeCaZnO as the active layer.The properties of the alloy films and related devices were studied as a function of the growth temperature,which revealed 525? as the suitable growth temperature.The bandgap of the alloy film can be tuned from 3.3 e V to3.62 e V by adjusting growth temperature.Under a bias of 5 V,the dark current of the Be Ca ZnO-based ultraviolet photodetector is 0.49 n A,the responsivity can reach 0.34A/W@330 nm,the corresponding detectivity 1.53×10¹¹ Jones,and the response wavelength range is 200 nm-380 nm.Compared with ZnO-based photodetectors,the BeCaZnO alloy photodetectors possessed a significantly lower dark current(from m A to n A)and faster photoresponse speed.However,there still exists persistent photoconductive effect in the alloy-based devices,which may originate from deep trap center in the BeCaZnO alloys.4?Based on the recently discovered resistive switching effect in Ga₂O₃semiconductor materials,we performed the first attempt to modulate the perforamces of UV photodetectors by taking advantage of the resistive switching effect in this work.The Ga₂O₃ films were grown on(100)Nb:SrTiO₃(NSTO)substrates by magnetron sputtering(MS)and PLD,respectively.Based on the deposited films,high-performance Schottky type UV photodetectors with Pt/Ga₂O₃/NSTO/In vertical structure were then fabricated.The self-driven UV photodetection performance and resistive switching(RS)characteristics of the fabricated devices were studied in detail.The resistive switching mechanism and the modulation of RS effect on performances of the UV photodetectors have been revealed.The results show that:(1)The growth rate of Ga₂O₃film by MS is faster than that of the film by PLD,while the O/Ga atomic ratio of the MS-grown film is lower.This suggests that it is easier to cause oxygen deficiency during the MS growth process,and thus causing more oxygen vacancies defects in the Ga₂O₃ film.The MS-grown Ga₂O₃ film is polycrystalline,while the PLD-grown Ga₂O₃ film exhibits preferred orientation growth along(400)plane.Note that,the device based on the MS-grown Ga₂O₃film shows larger dark current and better electrical conductivity,which demonstrates more oxygen vacancies defects formed in the film(i.e.higher intrinsic carrier density);(2)Photogenerated carriers can be efficiently separated by the Schottky built-in electric fileld at Pt/Ga₂O₃ interface,resulting in excellent self-driven UV detective performances of the device.Under 0 V bias,the fabricated device shows a fast photo-response speed(rise/decay time is 0.05 s/0.1 s),a low dark current of 10 p A,a peak responsivity up to 65.7 m A/W at 240 nm,the corresponding detectivity is 4×10¹¹ Jones;(3)Under alternate pulse voltage of-5 V and+3 V(pulse time of 10 ms),the fabricated device exhibits a stable resistive switching effect.Note that,the resistive switching ratio is as high as10⁴,and the high/low resistance show excellent retention properties(no obvious degradation after 10⁴ s).In addition,multilevel resistive switching can be achieved as well.The physical mechanism of the resistive switching effect is considered to be the modulation of Schottky barrier caused by carriers trapping/detrapping;(4)By controlling the direction and magnitude of the pulse voltage,the dark current of the Pt/Ga₂O₃/NSTO/In ultraviolet photodetector can be easily modulated from?4.9 n A to?5 p A,and the light/dark current ratio from 2.0 to1.23×10³,the detectivity from 8.71×10⁹ Jones to 3.42×10¹¹ Jones,which provide a new route for performance control of UV photodetectors.5?With expect to improve the performances of UV photodetectors by taking advantage of semiconductor junction effect,the novel Be ZnOS/Ga₂O₃heterojunction-based UV photodetectors were constructed.By optimizing the device structure,excellent self-driven dualband UV photo-detective performance of the fabricated devices was obtained.The experimental results show that the energy band alignment between Be ZnOS alloy semiconductor and Ga₂O₃ semiconductor is staggered(i.e.type-?energy band configuration).Thus,the photo-generated carriers can be effectively separated by built-in electric field at the Be ZnOS/Ga₂O₃ interface,resulting in the self-driven photoresponse characteristics of the heterojunction devices.The Be ZnOS/Ga₂O₃ heterojunction UV photodetectors show dual-band detection with photoresponse peak at wavelength of?240 nm and?350 nm.Compared the device with Al/Be ZnOS/Ga₂O₃/Au structure,the Pt/Be ZnOS/Ga₂O₃/Al device possesses optimum performances:under 0 V bias,the dark current of the device is as low as2 p A,the peak responsivity R_{240 nm} is 23.5 m A/W,the rise/decay time is 0.09 s/0.1 s,and the detectivity D*is 2.3×10¹¹ Jones,which is mainly benefiting from the double-junction coupling enhancement effect of the heterojunction and Pt/Be ZnOS Schottky junction.