Selenium is a p-type photoconducting semiconductor with a band gap of 1.8 eV and a photoconversion efficiency more than 20%, and its amorphous form has long been used as a photoconductor for many photosensing applications because of its high multiplication gain and very low dark current density. We have previously shown that amorphous Se (a-Se) based photodetectors with a lateral metal-insulator-semiconductor-insulator-metal (MISIM) device structure have a high quantum efficiency and a high signal contrast in the visible range. However, its photoconduction performance degrades with increasing the wavelength as a result of the weak absorption in the red light region. In this study, crystalline Se (c-Se) was used to replace a-Se as the photoconductor material of the MISIM device because it has a strong absorption of red light and should exhibit improved photoconducting performance in the long wavelength region.

To fabricate the lateral MISIM detector, a 2 μm-thick SiO₂ layer was first deposited on the Si substrate by wet oxidation, followed by sputter-deposition of Al (400 nm)/Ti (15 nm) bilayer thin film, which was then patterned by photolithography and dry etch to form comb electrodes with a spacing of 10 μm. An ultrathin dielectric layer, such as Al₂O₃ and Ga₂O₃, was deposited on the electrodes as the hole blocking layer (HBL). To fabricate the c-Se layer, a 1-nm thick Te thin film was deposited on the HBL layer, followed by the thermal evaporation deposition of a 500 nm-thick a-Se thin film. To complete the photodetector, the a-Se thin film was thermally annealed at temperatures above 100^oC for the phase transformation to c-Se. Te deposition condition and the annealing thermal budget of the a-Se layer are needed to be optimized so that the c-Se photodetector has a high photoelectric conversion efficiency and a very low dark current.

Figure 1 shows the I–V plots of four different c-Se photodetectors: one without the HBL, one with the HfO₂ HBL, one with the Al₂O₃ HBL and one with the Ga₂O₃ HBL. Compared with the a-Se based photodetector, the c-Se photodectors with the HBL have a parallel photoconduction performance at a much lower applied bias. For instance, at a bias of 10 V, the dark current and photocurrent densities of the c-Se detector are in the order of 10^-12 and 10^-8 A/mm², respectively. However, it requires a bias of 100 V for the a-Se detector to achieve a similar result. The lower required bias is most likely due to the better conductivity of the c-Se thin film, which has a resistivity three orders smaller than the a-Se one. Because of the stronger absorption in the long wavelength region, the c-Se detector has a better photoconduction efficiency under the red light illumination than the a-Se detector. When the HBL is absent from the MISIM structure, the photodetector exhibits enormous dark current density (~1x10^-7 A/mm² at 15 V), demonstrating the importance of the dielectric HBL in suppressing the dark current. In general, the dielectric HBL requires to form a high junction barrier with the Al electrode and have a high density of deep hole traps. Since the c-Se detector can be operated at a very low bias, a high junction barrier is more crucial than a high trap density, which becomes important at a high applied bias for the dark current suppression.

In summary, the study presents the fabrication and the photoconduction performance of c-Se based lateral MISIM photodetectors in the visible range. The dielectric HBL can effectively suppresses the dark current. Moreover, compared with the a-Se based detector, the c-Se detector has a lower operation bias and better photoconduction efficiency over the visible spectrum, which are desirable characteristics for a low cost, simple design and reliable visible photodetector.