Study On Photoelectric Properties Based On Graphene Heterojunction

Germanium based photodetectors have broad application prospects in light detection due to their unique communication bandwidth response characteristics and good CMOS compatibility.However,the response band range of commercial detectors is generally limited to a certain band,which is difficult to meet the detection requirements of multi-band fusion,miniaturization and low weight.Therefore,In this work,monolayer graphene films with complete surface morphology,high transmittance and excellent performance were prepared by chemical vapor deposition method,and thin Si O₂ interface layer was introduced between multilayer graphene and semiconductor germanium base to prepare prototype devices based on graphene heterojunction.The structure and performance of the device are improved by changing the thickness of Si O₂,the number of graphene layers,the conductive type of the semiconductor germanium base,and the annealing process.The spectral response range and I-V characteristics of the device were tested,and the photoelectric characteristics such as responsivity,switching ratio and response time were calculated.Finally,the response mechanism of photoelectron was analyzed based on the photoelectric characteristics of the device.The main research results are as follows:（1）Preparation and characterization of graphene films:The results show that when methane flow rate is 20sccm,reaction temperature is 1030℃,annealing time is 20min,reaction time is10min,graphene films with complete surface morphology,high nucleation quality and few defects can be obtained.Raman spectrum test results show that no D-peak appears at1360nm.The g-peak at 1580nm is sharp and the half-peak width is small,which indicates that the degree of crystallization of graphene is good.The 2D peak-to-peak value is high,I_2D/I_G=2.089>2,indicating that there is a single layer of graphene film on the surface.In addition,the transmittance of 1 layer,2 layer,4 layer,7 layer and 10 layer graphene films at200nm-2400nm was 98.65%,94.85%,93.08%,88.45%and 82.55%respectively.It can be concluded that the thicker the stacked graphene layers,the transmittance is the lower,and the transmittance in ultraviolet and visible bands is slightly lower than that in infrared bands.（2）the influence of different parameters on the graphene devices heterojunction:1)introducing the interface oxide layer can effectively increase the barrier height in order to reduce the dark current,but if too thick oxide layer interface,photoproduction carrier will not be able to effectively through the oxide layer interface,make the photocurrent is reduced,thus reduce the ratio and degree of response of switching devices.2)With the increase of the number of graphene layers,the peak values of 2D peak and G peak become larger,and the width of 2D peak also becomes wider;The increase in the number of graphene layers will lead to an increase in the concentration of graphene carriers and thus increase the light response generated by the device.However,if the graphene is stacked too thick,the graphene may no longer be two-dimensional and the transmittance may decrease,which affects device performance.3)The doping type of germanium also affects the device performance,which is reflected in the positive photoelectric direction between N-type germanium and weak P-type graphene,and the reverse photoelectric direction between P-type germanium and weak P-type graphene.The escape work difference between N-type germanium and weak P-type graphene is much larger than the escape work difference between P-type germanium and weak P-type graphene,so the device prepared by N-type germanium has better performance.4)Annealing process can make graphene and Si O₂ substrate close contact,Si O₂and Ge interface state reduction leads to the reduction of interface charge density.The effect of the surface state on the van der Waals junction is reduced,and the dark current generated by the device is reduced,thus improving the device performance.（3）Photoelectric response characteristics of the device:the test results show that the device responds in the wavelength range of 254nm-2200nm（u V-visible-near-infrared short-wave-near-infrared long wave）;The responsivity and switching ratio reach peak values of 78.36m A/W and 1.74×10~3at 980nm,respectively.The rising time and falling time were 1ms and 3ms,respectively.Finally,the response mechanism of photoelectron is discussed based on the photoelectric characteristics of the device.