Self-powered Photodetectors Based On Gallium Oxide Heterojunctions

Gallium oxide(Ga2O3)semiconductor has become one of the best candidate materials for deep-ultraviolet(DUV)photodetectors(PDs),high power electronic devices and radiation resistant optoelectronic devices,due to its wide band gap,high breakdown field strength and strong radiation resistance.Especially,the photoresponse spectrum of Ga2O3 covers the whole solar-blind region(200～280 nm),making it the prefect semiconductor material for fabrication of solar-blind UV PDs.However,the operation mechanism of traditional metal-semiconductor-metal(MSM)PDs is the photoconductive effect of active semiconductor materials.Although MSM PDs can gain better photodetection performance and higher external quantum efficiency,they are energy consumption devices for relying on the external power supply.In the new era of our country has set a clear goal to reach its carbon peak by 2030 and achieve carbon neutrality by 2060,the development of energy-conservation Ga2O3 photodetection technology is becoming very essential and necessary.As the next-generation photoelectronic devices technology,selfpowered PDs based on the heterojunction configurations provide the Ga2O3-based PDs a new strategy to meet with requirements of future practical applications.Additionally,the self-powered PDs can convert the light signals into electric signals without the power supply making the devices operating wirelessly,independently,sustainably and economically.The physical operation mechanism of self-powered PDs is the photovoltaic effect of heterojunction.The self-powered photodetection manner is the result of interactions between photons and materials with the assistance of built-in field at heterointerface,always including three steps:(1)semiconductor materials harvest the target light and transform photons into exciting electrons and electron-hole pairs;(2)electron-hole pairs in depletion layer are quickly separated by the built-in field and then move to opposite directions;(3)electrons migrate in electron-transport layer(ETL)and holes migrate in hole-transport layer(HTL),and they are finally collected by electrodes to produce electrical signals.In this doctoral thesis,we constructed a series of heterojunction photodetectors based on Ga2O3 single-crystal materials and thin films.The intrinsic physical mechanisms of self-powered behavior and the regulation technologies of self-powered performance were researched and explored with the aid of the compatibility,diversity and tunability of heterostructures.The main research r-esults are listed as follows:(1)Based on β-Ga2O3/CuSCN single crystal core-shell microwire heterojunction,a solar-blind PD with ultrasensitive photodetection and self-powered operation manner was constructed.With the mechanical exfoliation method,the single-crystal microwire was firstly separated from the(100)-preferred β-Ga2O3 substrate;then the β-Ga2O3/CuSCN core-shell heterostructure was fabricated by a simple immersion method.Upon the illumination of 254 nm DUV light,the fabricated In-β-Ga2O3/CuSCN-In heterojunction PD demonstrated typical rectification characteristic and photovoltaic effect,realizing the highly sensitive and self-powered performance.The self-powered responsivity and detectivity of the heterojunction PD at 0 V were 98 μA/W and 9.7×1010 Jones,respectively;its distinguishability to the DUV light can reach up to 1 μW/cm2 at 1 V.(2)The self-powered PD constructed on the β-Ga2O3/CuI single crystal core-shell microwire heterojunction achieved a full-ultraviolet spectrum photodetection.To extend the response spectrum to wider wavelength,CuI with a narrower band gap value of 2.98 eV was selected to serve as the wrapping layer in In-β-Ga2O3/CuI-In heterojunction PD.Profited from the expression of CuI photoresponse in photovoltaic effect and the efficient spatial and radial carrier transport in core-shell heterostructure,the β-Ga2O3/CuI heterojunction PD not only obtained a high self-powered photoresponsivity of 8.46 mA/W but also expanded its photoresponse spectrum to UVA/B/C broadband wavelength of 200～410 nm.Besides,the first-principle calculation was employed here to predict the result of that the built-in field was originated from the non-equilibrium carrier migration at the β-Ga2O3/CuI heterointerface.(3)Self-powered PDs were fabricated on the ε-Ga2O3/PEDOT:PSS organic-inorganic hybrid heterojunctions,and their energy-conversation photodetection performance could be effectively regulated by optimizing the oxygen-vacancy(VO)defect concentrations in ε-Ga2O3 thin films.The preliminary ε-Ga2O3 thin films were firstly fabricate by metal-organic chemical vapor deposition(MOCVD)and then annealed in Ar,Air and O2 atmospheres at high temperature of 700℃,respectively,to acquire high,middle,low VO concentrations.The VO regulation mechanism is the formation or desorption of the lattice oxygen atoms under an oxidizing or reducing atmosphere.By acting as the carrier killers,the VO defects could modulate the self-powered properties of heterojunction devices for the electron trapping effect,which can affect the transport of photogeneration carriers.As the decrease of VO defects,the self-powered performance ofε-Ga2O3/PEDOT:PSS heterojunction PD was correspondingly enhanced.Thus,the responsivity and external quantum efficiency(EQE)of the PD device annealed in O2 can be as large as 67.9 mA/W and 33.2%.(4)The PDs with self-powered manner were realized in a βGa2O3/VOx all-oxide heterojunctions.Their self-powered photodetection performance can be effectually enhanced by modulating the hole transport efficiency via the conductivity of VOx thin films.Primarily,the β-Ga2O3 thin films were deposited by MOCVD method,and then the VOx thin films were fabricated by solution method to serve as the HTLs in the constructed planer heterojunctions.Subsequently,the β-Ga2O3/VOx heterojunctions were annealed in Air,O2 and Ar atmospheres at 500℃,respectively,to change the crystallinities,phase structures and chemical valences of VOx layers,which would further exert influences on the hole-transport properties.The VOx layer annealed in Ar atmosphere demonstrated mixture phases and high conductivity as well as more efficient hole extraction/transportation characteristic.Compared with the original devices,the optimized heterojunction PD achieved better self-powered photodetection performance with one order magnitude promoted responsivity(28.9 mA/W),which on/off ratio and visible/ultraviolet rejection ratio(R245/R400)can reach up to 1.23±106 and of 3.12×104,respectively.