Fabrication And Performance Investigation Of Gallium Oxide Photodiode

As an ultra-wide bandgap oxide semiconductor,gallium oxide(Ga2O3)has a larger bandgap and lower cost,higher voltage compared with gallium nitride(GaN)and silicon carbide(SiC)and other wideband semiconductors and has the advantages of high irradiation resistance.Therefore,Ga2O3 has attracted extensive attention from researchers at home and abroad in the field of power electronics and optoelectronic devices.In the area of optoelectronic devices,Ga2O3 has been most widely investigated in solar-blind UV photodetectors as a semiconductor with a bandgap(4.4-5.3 eV)that can cover most of the solar-blind UV band.Ga2O3 has five phases of α,β,γ,δ and e.Among these polymorphs,the βphase gets extensive attraction in solar-blind detection.Furthermore,other metastable phases(such as the ε-phase)of Ga2O3 have also attracted great concern because of their unique lattice structure and material properties.Among various types of Ga2O3 photodetectors,Schottky barrier photodiode,p-n photodiode,and p-i-n photodiode can take advantage of the photovoltaic effect and thus have the advantages of high responsivity,fast photo-response speed and self-powered mode,which meet the demand of pursuing efficient and low-carbon development in today’s world.Nevertheless,the previously reported Ga2O3 photovoltaic devices suffer low photoresponse performance and are still far from the demands of practical application.Therefore,we need to focus on the parameters of the material and structure aspects to optimize and enhance the performance of the device.In this thesis,ε-phase and β-phase sGa2O3 films were grown on sapphire substrates and silicon substrates using the metal organic chemical vapor deposition method,and then three photodiode structures of solarblind UV photodetectors were constructed.Their internal mechanisms were analyzed in-depth,and the performance of the photodetectors was optimized to obtain devices with excellent performance.The main work and research results are as follows:(1)Two-dimensional transition metal carbides,carbonitrides,and nitrides,so-called MXenes,have emerged as an excellent electrode material for optoelectronic devices due to their high conductivity and hydrophilic surface.Here,the titanium carbide(T3C2)based MXene was employed to construct the Ti3C2/ε-Ga2O3 Schottky barrier diode photodetector.The fabricated device demonstrated a self-powered operation model with an extremely low dark current(0.07 pA),an outstanding light on/off switch ratio(2.5×106),a remarkable photoresponse speed(43 ms/145 ms),a responsivity of 15.5 mA/W,an external quantum efficiency of 7.5%and a detectivity of 2.15×1011 Jones.(2)By epitaxially growing β-Ga2O3 thin films with high crystallinity on sapphire substrates using metal-organic chemical vapor deposition combined with p-type organic semiconductor 2,2’,7,7’-tetrakis(N,Ndi-4-methoxyphenylamine)-9,9’-spirobifluorene(spiro-MeOTAD),we successfully constructed p-spiro-MeOTAD/n-Ga2O3 p-n heterojunction solar-blind photodetectors.The prepared devices are capable of running in self-powered mode and can detect UV light in a narrow spectral range with good solar-blind spectral selectivity.The device has high responsivity(65 mA/W)and external quantum efficiency(EQE)(32%)at small light intensity(～1μW/cm2)under 0 V bias.In addition,the device has an excellent instantaneous pulse response:a rise time of 2.98 μs and a decay time of 28.49 μs,which is better than most Ga2O3 self-powered photodetectors.(3)Novel p-i-n self-powered solar-blind UV photodetector based on p-type spiro-MeOTAD(spiro),weak n-type Ga2O3,and n-type Si is fabricated.Benefitting from the superior carriers’ separation ability of the double build-in electric field,the effective carrier transport of the novel vertical structure,and high solar-blind UV transmittance of spiro film,the self-powered responsivity of p-i-n-type photodetector based on spiro/Ga2O3/Si increases to 4.43 mA/W under UV light,which is～54-fold for that of the Ga2O3/Si photodetector(0.22 mA/W).Meanwhile,the response speed(0.03/0.196 s)of the p-i-n device is also dramatically improved by introducing the spiro layer.(4)Based on the previous work,we also investigated in detail the influence of spiro-MeOTAD thickness on the photoelectronic performance of p-i-n photodiode including open-circuit voltage and short-circuit current,and finally obtained the optimum device.The peak responsivity of the device is 192 mA/W under weak light irradiation of 2 μW/cm2 at 254 nm at zero bias voltage.In addition,we observe a binary response characteristic in the instantaneous photoresponse(I-t)curve:the photocurrent changes from negative to positive values when the UV irradiation is turned on to off.This property can be used in weak signal detection,optical communication,or optical logic operations.In addition,a four-stage photocurrent dynamic characteristic is observed at a small forward bias and the mechanism is analyzed.