Vacuum Ultraviolet Photodetectors with MSM Structure Based on Hexagonal Boron Nitride Films via Magnetron Sputtering(Invited)
High performance Vacuum Ultraviolet(VUV)photodetectors are of great significance for space science,radiation monitoring,electronics industry,and basic science.The ultra-wide bandgap semiconductor material of hexagonal Boron Nitride(hBN)has high band edge absorption coefficient,thermal conductivity,and excellent thermal and chemical stability,making it very suitable for application in the field of VUV detection.At present,there are still some challenges in the preparation and transfer of large-scale high-quality hBN films.In this work,two-inch wafer-level hBN films were successfully deposited on silicon and sapphire substrates using radio frequency magnetron sputtering technique.Characterization of the surface morphology by scanning electron microscope and atomic force microscope confirmed the production of uniformly continuous and dense films with an root meam square value of 4.04 nm(the film thickness was 154 nm);Raman peaks were located in the 1 366 cm-1,and the full width at half maxima was about 33 cm-1,which confirmed that the hBN films had hexagonal phase,and the quality of the crystalline was relatively high;X-ray photoelectron spectroscopy verified B-N bonding with a B/N ratio of 1.05∶1,indicating that the films had a small amount of nitrogen vacancies;the UV-vis absorption spectrum indicated that the films only had strong absorption of deep ultraviolet light below 220 nm,with the intrinsic absorption edge located at 212.5 nm,the absorption coefficient was as high as 1.52×105 cm-1,and the optical bandgap of hBN was about 5.95 eV.Furthermore,the MSM-type photodetectors based on hBN films were fabricated,and the effects of electrode material,film thickness,and interdigital electrode width and spacing on performance were investigated.According to the Schottky contact model,the higher the work function of the metal,the Schottky barrier height formed with the hBN material would be higher,and the junction current density would be lower.During the preparation of electrodes,the actual barrier height was difficult to analyze and determine due to the combined effects of the presence of interface states,the introduction of impurities,and the quality of the Schottky contacts.The length,width,and spacing of the interdigital electrode were 1.5 mm,25 μm,and 25 μm,respectively,with a logarithm of 25 and an electrode thickness of 70 nm.When Ni was used as the electrode,the detector had the minimum dark current and the maximum photocurrent,and its performance was stable and not easily oxidized.As the thickness of the hBN films increased,the dark current slowly increased,while the photocurrent first increased and then decreased.The optimal thickness was 135 nm.In the thickness range of tens of nanometers,films were still in the process of island growth connection and not dense enough,which inhibited the transmission of the current signal,and the light and dark currents were weak.As the film continued to grow,the islands connected with each other to form dense films.The signal transmission ability was improved,and the number of photogenerated carriers increased,which resulted in a large increase in the photocurrent and a small increase in the dark current.Due to this growth mechanism,the polycrystalline defects became more numerous and the resistance of the whole bulk material was greater with the further increased of thickness,leading to a gradual decrease in the value of the detector photocurrent.The influence of interdigital electrode width on the dark current and photocurrent was very weak,while the interdigital electrode spacing affected the device photocurrent significantly.As the interdigital electrode spacing increased,the active area of the hBN films became larger and more photogenerated carriers were generated,which contributed to the increase of photocurrent(Mechanism Ⅰ).On the other hand,when the interdigital electrode spacing increased,the electric field strength between the neighboring interdigital electrodes decreased,the transit distance of the carriers increased,and the number of carriers collected by the electrodes decreased,which contributed to the decrease of photocurrent(Mechanism Ⅱ).The ultimately selected hBN detectors had an extremely low dark current(<3 pA@100 V)and significant photoresponse to 185 nm wavelength light.When the applied bias voltage was 25 V,the responsivity and specific detectivity of the detectors were 0.697 mA/W and 7.47×108 Jones,respectively;when the voltage was increased to 100 V,the responsivity and specific detectivity were 2.769 mA/W and 2.969×109 Jones,respectively.
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