Graphene Photodetector Based On Photogating Effect

Photodetector is an optoelectronic device that converts optical signals into electrical signals.It is widely used in civil and military fields,including image sensing,optical communication,environmental monitoring and chemical/biological sensing.With the increasing demands on the miniaturization,on-chip integration and high performance of optoelectronic devices,the two-dimensional materials represented by graphene are considered as the most potential materials for the development of new generation optoelectronic devices due to their unique physical properties.Compared with traditional materials and other two-dimensional materials,graphene has unique properties such as high carrier mobility,zero band gap energy band structure,and physical flexibility,which make it very important for the integrated design of ultra-fast,wide band and tunable light detection.However,the low light absorption rate（2.3%）and the short carrier lifetime（1 ps）lead to the extremely low quantum efficiency of graphene,which makes it difficult to perform photodetection directly as a light absorbing material.It is also difficult for pure graphene photodetectors to obtain high photoelectric gain by simply relying on optical absorption enhancement methods such as optical resonators,metal gratings and plasma nanostructures.In recent years,graphene composite photodetectors based on the photogating gain mechanism have achieved ultra-high responsivity.The gain mechanism is derived from the extension of photogenerated carrier lifetime caused by defects and impurities or artificially designed composite structures.However,it will lead to problems such as slow response speed and uncontrollable interface coupling.In addition,the light absorption depends on the semiconductor material rather than graphene,so the absorption spectrum is limited to the narrow-fixed wavelength band of the light absorbing material.In view of the above-mentioned problems and challenges in the photoelectric detection of graphene composite structure at present,the following three parts of research work are carried out in this dissertation and the solutions are given:1.In view of the contradiction between response time and responsivity of graphene photodetector,a new mechanism called manipulating photogating effect（MPE）is proposed,and which is validated by using the graphene/silicon-on-insulator（GSOI）hybrid photodetector in photoconductive mode.By the MPE mechanism,enhanced built-in potential of heterojunction and carrier concentration gradient in silicon can be controllably manipulated to improve the responsivity and response speed of device simultaneously.The experimental results demonstrate that a high responsivity can be obtained without the sacrifice of responding bandwidth for the first time,namely,it does not slow down the photoresponse speed caused by the long-lived trapping of charges.The responsivity of the GSOI device by MPE can reach 10⁷A/W,which is 10 times higher than that without vertical voltage,and the response time can be shortened from1000μs to 90μs.A detectivity（D^*）of 1.46×10¹³ Jones was measured.Further application of this mechanism to the position detector has also achieved significant improvement.The position detection sensitivity is as high as 518μA/mm,the nonlinearity is not more than 2%,and the detection range is expanded to 1.2 mm.In addition,the process of MPE is modeled and simulated by Silvaco TCAD,and the mechanism of carrier manipulation is explained.2.In view of the narrow response band and gain limitation of graphene/silicon composite devices,it is proposed that graphene and Te-hyperdoped Silicon（Te-Si）are compounded to obtain a high optical gain infrared detector that can work in the medium wave infrared band（SWIR）at room temperature.The device has the advantages of wide band,high gain,low power consumption and easy manufacturing,and breaks through the limitation of absorption band of traditional silicon-based devices and the complex process dependence of narrow band gap material devices.The device integrates the graphene channel with Te-Si,in which the intermediate band is introduced due to Te supersaturation implantation,thus realizing wide spectrum absorption in the infrared band;Graphene acts as a carrier transport channel to provide cyclic gain for the device.Furthermore,the gain factor of the device is designed to obtain ultra-high responsivity（100 A/W at 1.55μm,16.3 A/W at 2.7μm）and low equivalent noise power(0.08 pW Hz^-1/2 at 80 K,0.71 pW Hz^-1/2 at 300 K)in the SWIR band at room temperature.Importantly,the fabrication of such heterostructure devices is fully compatible with the silicon-based CMOS process,which is of great value for realizing miniaturized high gain infrared detectors.3.In view of the inherent limitation of quantum efficiency and response band of graphene photodetector,it is reported that an enhanced photogating effect in monolayer graphene photodetector based on a structured substrate,where the built-in potential is established by a mechanism of potential fluctuation engineering（PFE）.The enhancement factor of device responsivity is related to a newly defined parameter,namely,fluctuation period rate（P_f）.Compared to the device without nanostructured substrate,the responsivity of the device with an optimized P_fis enhanced by 100 times,reaching a responsivity of 240 A/W and a specific detectivity,D^*of 3.4×10¹² Jones at1550 nm wavelength and room temperature.The experimental results are supported by both theoretical analysis and numerical simulation.Since our demonstration of the graphene photodetectors leverages the engineering of structures with monolayer graphene rather than materials with multi-layer complex structure it should be universal and applicable to other hybrid photodetectors.