High Sensitive Photodetectors Based On Two Dimensional Semiconductors And The Photocurrent Generation Mechanism

Two dimensional(2D)materials exhibit some distinct physical properties and have drawn tremendous attention in recent years,because they can be atomically thin,stacked with van der Waals force and free of surface chemical dangling bonds.2D materials have great potential in energy storage,high frequency electrical devices,biological medicine,etc.High performance-photodetector based on 2D materials with different well-designed structures are extensively explored due to a wide and varied selection of 2D materials with various bandgap.The main contents of this thesis are as follows:1.In this work,photodetectors of In2Se3 have been fabricated on a ferroelectric field effect transistor structure.Research shows that the modulating effect of P(VDF-TrFE)as gate dielectric on the In2Se3 is obviously stronger than that of SiO2.The current ON/OFF ratio of In2Se3 with SiO2 back gate and with P(VDF-TrFE)top gate was 1.7 and 25,respectively,indicating that there are more traps at the interface between In2Se3 and SiO2 than that at the interface between In2Se3 and P(VDF-TrFE).In addition,the ohmic contact between In2Se3 and metal electrode Cr/Au is better than Cr/Ag.Finally,photodetectors gated by P(VDF-TrFE)with high sensitivity,fast and wide spectral response is acquired.The responsivity of the In2Se3 photodetector can reach 25 A/W,the response time is 165?s,and the detection range is from visible to short wavelength infrared(1550 nm).These are superior to traditional photoconductive photodetectors gated by oxide dielectrics.P(VDF-TrFE)plays a vital role in the In2Se3 photodetector.The stable remnant polarization can provide an ultra-high local electrostatic field(about 109 V/m within several nanometers scale)in In2Se3 channel which is higher than that produced by back gate bias in traditional field effect transistors.With the help of such an ultra-high electrostatic field,the In2Se3 channel is fully depleted,increasing the sensitivity of the In2Se3 detector even without gate voltage.More importantly,the strong electrostatic field derived from the remanent polarization of P(VDF-TrFE)can not only suppress the dark current but also reduce the bandgap of In2Se3,which broadens the detection range from near-infrared to short wavelength infrared(1550 nm).Generally speaking,ferroelectric field provides another approach to improve photodetector performance.2.Photodetectors based on MoS2 of different thickness are prepared on ITO/Al2O3 substrate in this work.The current ON/OFF ratio of single layer MoS2 could exceed 109,while that of multilayer MoS2 is 105.The responsivity of multilayer MoS2 photodetector can reach 2.7×104 A/W.The photocurrent generation mechanism is further analyzed.The relationship between the photocurrent and the incident optical power at a gate voltage of-2.5 V and 3 V is precisely fitted by the photocurrent formula of photoconductive effect and the photovoltaic effect,respectively.Then the fitting parameters determining the origin of photocurrent are figured out.The results show that when the gate voltage is smaller than the threshold voltage,the photoconductive effect should prevail,while the photovoltaic effect dominates when the gate voltage is greater than the threshold voltage.We further verify this conclusion through the response time.The response times at gate voltages of-2.5 V and 3 V are 90 ?s and 200 ?s,respectively.This is due to the trap states at the interface captures the photogenerated holes,while the release of holes from the trap states takes some time when the light is turned off.Although the trap states lead to a slow response time,they also result in a higher responsivity.The intrinsic reason is the trap induced photoconductivity gain.The above two research works are based on two structures,realizing fast and high sensitivity photoelectric response.They provide a new idea for designing novel photodetectors based on 2D materials.