Research On Ga₂O₃ Solar Blind Photodetector

Due to the strong absorption of the atmospheric ozone layer,the wavelength band from the sun with a wavelength of 200-280 nm hardly exists on the earth’s surface,so this band is also called the solar-blind band.Compared with infrared,visible and near-ultraviolet photodetectors,due to the lack of background radiation,photodetectors working in the solarblind band have significant advantages in detection accuracy,sensitivity and anti-interference ability.Solar-blind photodetectors have great application value in military and civilian fields such as missile tracking,fire warning,medical imaging,and ozone hole detection.So far,solarblind photodetectors based on wide bandgap materials such as AlN,diamond,AlGaN,MgZnO,and Ga2O3 have been reported extensively.Among them,the forbidden band width of Ga2O3 is 4.4-5.3 eV,and the corresponding absorption cut-off wavelength is just in the solar-blind band.It has the advantages of large absorption coefficient,stable physical and chemical properties,low preparation cost and high breakdown field strength(≈8 MV/cm).As an ideal solar-blind photoelectric detection material,Ga2O3 has important research and application value.However,the solar-blind photodetector based on Ga2O3 has a mismatch between responsivity and response speed,which seriously restricts the application of Ga2O3 solar-blind photodetector.To address this issue,this thesis innovatively introduces the focused ion beam etching technology to thin down and fabricate square nanopore arrays on β-Ga2O3 single crystal microflakes obtained by mechanical exfoliation.A photodetector with high responsivity and fast response speed is obtained.The specific content is as follows:(1)For the first time,focused ion beam etching technology was introduced to thin downβ-Ga2O3 microflakes,which solved the problems of poor controllability and low repeatability of traditional mechanical exfoliation methods.At the same time,the effect of the thickness ofβ-Ga2O3 microflakes on the performance of photodetectors was further studied.It is found that when the β-Ga2O3 microflakes reach a certain thickness,the device channel will be completely depleted due to the influence of oxygen adsorption and negative interface charges.At this time,the phototransistor will change from depletion mode to enhancement mode,showing extremely low dark current without gate voltage.The enhancement mode phototransistor prepared based on this technique exhibits a photo-dark current ratio as high as 2.3×105,a responsivity of 6.3×104 A/W,and an external quantum efficiency of 3.1 × 107%under the illumination of 254 nm wavelength light with a power density of 8 μW/cm2.The device not only has excellent photoelectric conversion ability but also has a fast response time(rise and fall time of 43 ms and 28 ms,respectively).By introducing focused ion beam etching technique to thin down βGa2O3 microflakes,the mismatch problem between the responsivity and response speed of Ga2O3 solar-blind photodetectors is effectively solved.(2)In order to further improve the performance of β-Ga2O3 phototransistors,considering the decrease in the light absorption capacity of the β-Ga2O3 microflakes after thinning by etching,an enhancement mode phototransistor with square nanopore arrays were fabricated using the focused ion beam etching technique.Firstly,the effect of square nanopore width on the performance of β-Ga2O3 phototransistor was studied.It is found that when the square nanopore period is 500 nm and the width is 200 nm,the phototransistor will change from depletion mode to enhancement mode due to the depletion effect of oxygen adsorption on the channel.The enhancement mode phototransistor containing square nanopore arrays prepared on this basis exhibited an extremely low dark current of 8 fA.Compared with the enhancement mode device with thinning,due to the light absorption enhancement effect of the square nanopore structure,the device shows higher responsivity(1.8×105 A/W),detectivity(3.4×1018 cm·Hz1/2/W-1),external quantum efficiency(8.8×107%),and an ultra-high photo-dark current ratio(9.3 ×108).In addition,the rapid adsorption and desorption mechanism of oxygen enables the device to maintain a relatively fast response speed(rise and fall times are 130 ms and 50 ms,respectively).Second,the oxygen adsorption and desorption mechanism was experimentally verified.Through verification,it is found that as the ambient vacuum increases,the device gradually exhibits a persistent photoconductive effect,and the inhibition of the persistent photoconductive effect is restored after oxygen treatment.This proves that the oxygen adsorption and desorption mechanism is the key to the high responsivity and fast response speed of the device.Finally,the repeatability of the device preparation method was verified.Through verification,it is found that the method has good repeatability for realizing high-performance enhancement mode phototransistors.