| Detecting optical signal quickly,accurately and comprehensively is a major scientific problem,attracting great research interests.From the perspective of modern measurement technology,the conversion of light into electrical signals is one of the most effective and accurate means.As the key device to realize photoelectric conversion,photodetector is an indispensable part of optical communication,imaging,spectroscopy and other precision optical systems.Nowadays,with the growing of scale and diversity of application areas,the need for a photodetection platform with higher performance in terms of speed,responsivity or wavelength range is becoming more eminent.Two-dimensional material graphene shows many different physical properties from tradi-tional semiconductor optoelectronic materials which has a great potential in photodetection.The gapless energy band structure determines that graphene has a wide light absorption range covering the ultraviolet to terahertz,which makes graphene photodetector can realize light detection in an ultrawide wavelength range;Ultrafast carrier dynamics including short carrier lifetime and high carrier mobility make it capable of ultra-high speed optical detection;High strength and stable chemical properties make graphene compatible with highly mature silicon process platform and meet the basic requirements of large-scale integration.However,the atomically thickness（0.334 nm）seriously limits the absorption of graphene（2.3%）which leads to the unacceptable responsivity（μA/W）of intrinsic graphene photodetector.In this thesis,graphene field effect transistors（GFETs）are used for photodetection.To overcome the limitation of responsivity,we propose a solution to enhance the light-graphene interaction by using optical micro-structures based on Bragg stacks,which successfully en-hancing the absorption of graphene,leading to a higher responsivity（increased by one order of magnitude）.In addition,wavelength-selectivity and angle-sensitivity detection are achieved owing to the optical mode characteristics.The main research contents are as follows:（1）Design,fabrication and characterization of graphene field effect transistors（GFETs）:The photoelectric conversion mechanism of the GFETs is analyzed.The results further clarify that improving the absorption of graphene is an effective way to achieve higher responsiv-ity.In experiments,the design and fabrication of the micron-sized GFETs,the construction of customized optical and electrical test platforms,the characterization of the basic optoelectronic properties are carried out.The electrical test results show that:1.The average mobility of chan-nel carriers in back gate GFETs（300 nm silicon oxide gate dielectric layer）reach 800 cm2/V/s;2.The graphene is p-doped,and the average Dirac voltage is+30 V;The photoelectric test results show that:1.The device has self-driven light detection capability（under the condition of zero bias single side irradiation,the optical response to 532 nm wavelength reaches 0.68m A/W）;2.The photocurrent scanning results show that when the light is focused on the edge of the electrode,the device has the highest photocurrent which is nearly independent of the channel length.This phenomenon shows that photovoltaic effect（photogenerated carriers are separated by built-in electric field near the electrode）is the main mechanism of photocurrent generation for GFETs.This part provides an important reference for the later design and fabrication of the Bragg stacks integrated graphene photodetector.（2）Resonant cavity mode enhances the light-graphene interaction:To enhance the absorption of graphene more effectively,the physical triggers underneath for the transmittance/absorbance and quality factor of lossy optical resonator are systematically studied.Through the qualitative analysis of coupled mode theory and the quantitative calculation of transfer matrix,we revealed that the trade-off between radiation and dissipation rates of the system are crucial to the high performances.As an example,we experimentally demonstrate a Bragg filter composed of niobium pentoxide（Nb2O5）and silica（Si O2）stacks which enable high Q of 183 and high T of 91.3%.Further,with the principle the tunable Bragg filter is able to work in a similar way at optical wavelengths,maintaining almost unchanged FWHM（full width at half-maximum）and T values.Finally,we successfully apply this strategy to the design of Bragg resonator containing graphene.The numerical results show that the asymmetric Bragg stack structure can effectively improve the absorption of graphene at the resonant wavelength,up to100%.Accordingly,it is predicted that the responsivity of photodetector based on this structure can reach 22.6 m A/W at the resonance wavelength（533.5 nm）,which is 42 times higher than that of the intrinsic graphene photodetector.Furthermore,thanks to the selective absorption of resonator,the device has strong wavelength selective detection ability（the wavelength response range is 4.7 nm）.（3）Optical Tamm state enhances the light-graphene interaction:The optical Tamstate（OTS）in the semi-infinite metal/Bragg film stack structure is sys-tematically studied,and the corresponding excitation conditions are defined.Combined with the numerical results,several methods of modulating the characteristics of OTS including res-onance wavelength,absorbance and quality factor)are demonstrated,and the physical triggers underneath of different modulation methods are analyzed theoretically.On this basis,the com-posite structures of graphene and metal/Bragg stack are designed and optimized.The numerical results show that the light absorption of graphene inserted to the"hot spot"is close to 60%,which is 26 times higher than its intrinsic absorption,thanks to the localized field enhancement of the OTS（the maximum electric field at the local position is about 25 times that of the incident light|E/E0|2=25）.After properly adjusting the fabrication process,the integration of graphene photodetector with Bragg stacks is successfully realized.The photoelectric test results show that:1.The highest responsivity is 5.1 m A/W at the resonant wavelength（532 nm）with zero bias,and the actual absorption of graphene is nearly 29.4%correspondingly.2.At low light intensity levels（from 10-100μW）,the photocurrent shows good linear correspondence.3.The results of photocurrent scanning imaging（SPCM）show that the light response mechanism is photovoltaic effect as mentioned above.In addition,benefiting from the selective absorption and angle sensitivity of OTS,the device shows strong wavelength selectivity（wavelength response range is 520-540 nm）and angle sensitivity（6.5 n A/deg）. |
