| Graphene,with high carrier mobility,good conductivity,and wide UV-to-NIR spectral response,exhibits very important applications in the fields of ultra-fast and broad-spectrum-responsive photodetectors and transparent conducting electrodes.As one of the main methods for the preparation of high-quality graphene,epitaxial graphene?EG?on SiC has the advantages of avoiding of transfer process of graphene from others to a substrate,high uniformity on wafer size,good thermal stability,and compatibility with modern semiconductor procedure.At the same time,the wide bandgap semiconductor SiC is one of the ideal materials for the preparation of high performance ultraviolet photodetector,which has very important applications both in military and civil uses.Concerning graphene-based photodetectors,researchers proposed a various device structures and material combinations in order to solve the problem that the low responsivity due to graphene's low light absorption.Among these designs,taking the supporting substrate of graphene as a light absorbing medium and providing photogenerated carriers to the graphene channel in a planar device structure are a simple and practical method.In addition,graphene can be used as a transparent conductive electrode material in a photodetector due to its high transmittance and good conductivity.Therefore,SiC and graphene,both with excellent physical properties,can be combined to develop high performance UV photodetector based on EG on Si C.Here,characteristics and performances of the ultraviolet photodetectors based on EG on SiC are studied.The main results are as follows:?1?A simple ITO-graphene-ITO planar structure device was proposed and demonstrated.A novel rectified photocurrent effect is observed while a 325 nm ultraviolet light is local irradiated on one of the edges between the graphene and ITO electrode.The photocurrent has similar characteristics as those of a vertical structure graphene/semiconductor junction photodiode,but is clearly different from those found in a planar structure metal–graphene–metal device.Furthermore,The device can work at two modes:as a planar structural UV photodiode with responsivity of 11.7 mA/W at a applied bias 0.3 V;it can also be used as a self-powered UV photodetector at zero bias with responsivity of 4.4 mA/W.The photocurrent is varied linearly with laser power by nearly three orders of magnitude in our measured light power,indicated it has a wide dynamic range of photoresponse.In addtion,the device has a response frequency near gigahertz.This paves a potential way to realize ultrafast graphene planar photodiodes for monolithic integration of graphene-based devices on the same SiC substrate.?2?Several planar structure devices with different aspect ratios of graphene channel and different electrode materials as metal and EG were fabricated and measured.The effects of the aspect ratio of EG channels on the optoelectronic properties of the device are studied.It is found that the resistance in the dark field monotonously increases with the increase of the aspect ratio.When the ultraviolet light was locally irradiated on the interface between the EG and the metal electrode,the photovoltage of the device at zero bias is increased with the increase of the aspect ratio.Due to the high light transmittance of graphene compared with metal,the photocurrent and responsivity of the EG electrode devices are higher than that of the devices with metal electrode.In addition,with the increase of the aspect ratio of the EG channel,the same rectified photocurrent effect as finding in the ITO-graphene-ITO device is observed in both graphene and metal electrodes devices.And the rectified effect becomes more obvious with the bigger aspect ratio.The novel photoelectric phenomena are well explained based on a proposed model.?3?An EG/n-type SiC Schottky ultraviolet photodiode with extremely widescale adjustability in its responsivity and response speed is demonstrated.It is found that the device has 3 and 7 orders of magnitude adjustability both in the responsivity and response speed?tens of nanoseconds to milliseconds?through changing the bias from0 to-5 V,respectively.The highest responsivity of 2.18 A/W is achieved under-5 V bias with a 50?W 325 nm UV light due to the trap induced internal gain mechanism.The wide adjustability of light detection performance enables the device potential in different UV detection fields.?4?The metal top-gate and ITO top-gate field effect transistors?FET?were fabricated based on EG on SiC,where the polymer polymethyl methacrylate was used as the dielectric layer.For metal top-gate device,the proportion of the top-gate electrode width to the graphene channel is about 0.38 in order to increase the transmitance of ultraviolet light to the SiC substrate.A field-effect mobility of 170cm2.V-1.s-1 is achieved for metal top-gate electrode devcie in our measured gate voltages.The 17 mA/W maximum responsivity is achieved under-50 V gate voltage and 0.5 V source-drain voltage with 1 mW UV light power.To overcome the absorption of metal top-gate to UV light,ITO top-gate electrode with a proportion of1 was fabricated,who reveals better field-effect characteristics.A maximum field-effect mobility of 718 cm2.V-1.s-1 is achieved in our measured gate voltages,which is far superior to the reported results on the top-gate EGFET with polymer gate-insulating layer.The 43 mA/W maximum responsivity is achieved under 42 V gate voltage and 0.2 V source-drain voltage with 1 mW UV light power.This responsivity is the best one among the devices studied in this thesis,under the low source-drain voltage and the same UV light power. |
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