Preparation of SiO₂ thin film by Photo-Chemical Vapor Deposition and application in MSM-structured Ultraviolet Photodetectors

Ultraviolet (UV) photodetectors (PDs) have been widely used in daily life, whether in the field of electronics, environmental detection, etc. It is very important that the photo-dark current ratio, sensitivity of photo-response and noise analysis of devices were discussed for photodetector. Renally, many research groups have studied the metal−semiconductor−metal (MSM) structure of UV PDs. To improve the performance of the MSM structure photodetectors, it can be improved the performance by an insulating layer inserted between the electrode and the active layer. In the study, it was used photo-chemical vapor deposition to prepare high-quality silicon dioxide as an insulating layer, and the ZnO epitaxial thin film was prepared by RF magnetron sputtering method to fabricate high-performance ultraviolet photodetector.

UV PDs have been widely used in daily life, such as electronic equipment, environmental monitors [1-4]. Wide bandgap material, such as SiC, Ga₂O₃, GaN, TiO₂, ZnO have been received the scientist attention in recent years. Among them, ZnO has excellent chemical stability and good optical properties [5-7]. Therefore, it is often used in optical sensors and chemical sensors (such as gas sensors and biosensors). For PD, the photo-dark current ratio, the sensitivity of the photo-response and the noise analysis are often discussed index. Renally, many research groups have studied the metal−semiconductor−metal (MSM) structure of UV PDs. To improve the performance of the MSM structure PDs, it can be improved the performance what by an insulating layer (Al₂O₃, SiO₂, Si₃N₄, etc) inserted between the electrode and the active layer [8-14]. SiO₂ film is the easiest to prepare in these materials by PECVD or sputter method. However, the deposition film quality is not ideal. To improve the quality of SiO₂ film, the study was used photo-CVD system to prepare the SiO₂ film. The principle mainly uses vacuum ultraviolet light as the light source (deuterium lamp) to effectively excite the synthesis gas in the process of depositing SiO₂, and then prepare a high-quality insulating layer. Therefore, this study uses photo-chemical vapor deposition to prepare high-quality SiO₂ thin films and applied to the ZnO PD.

Figure 1 shows the I-V characteristic curves and photoresponse measurement results of MSM and MIS structure ZnO PDs. The dark and photo currents of the ZnO based MSM structure were 9.23 × 10⁻⁵ and 1.35 × 10⁻⁴. After depositing the SiO₂ film, the dark current (1.05× 10⁻⁹ A) decrease significantly about 3~4 order, and the photo current (2.1 × 10⁻⁷) also decrease about 2~3 order. After calculating the ratio of photo to dark current, the ratio of MSM structure was 1.46 and MIS structure was 200 when applied bias at 1V. Deposition of an insulating layer on the ZnO MSM structure not only improves sensitivity but also reduces overall power consumption.

On the other hand, the thickness of the insulating layer will also directly affect the performance of the photosensor. In Figure 2, we deposited five different thickness (22, 45, 98, 132, 198 nm) of the SiO₂ film to discuss the photoelectric properties difference for ZnO photodetectors. The result shown that all data were significantly improved compared with the non-insulating layer, and the best results can be obtained when the thickness is 98 nm. These results

　　To confirm the trap distribution between the electrode and active layer, we measured the noise power spectra from 1 to 100 Hz in the ZnO-based PDs with MSM and MIS structures in figure 3. The measurement parameters included the varied bias voltages from 2 to 4 V in the dark environment. The noise spectral density was analyzed by Hooge-type equations [15]. When the bandwidth was at the maximum measurement frequency (100 Hz) with an applied bias of 2 V, the NEP and D* values of the MSM structure were 4.082 × 10⁻⁷ W and 4.9 × 10⁶ cm·Hz^−0.5·W⁻¹, respectively. When a silicon oxide layer was inserted, the NEP and D* of the device (MIS structure) reached 6.826 × 10⁻⁹ W and 2.93 × 10⁷ cm· Hz^−0.5·W⁻¹, respectively. These phenomena indicate the good performance of the insulating layer for PDs. The results show that inserting an insulating layer between the semiconductor and electrode can improve the performance of PDs.

Figure 1 Photoresponse performance of ZnO-based MSM and MIS structure PDs. (a, b) I-V characteristics, (c, d) Responsivity characteristics.

Figure 2 Photocurrent to dark current contrast ratios of PDs with different silicon oxide film thicknesses.

Figure 3 Noise characteristics of ZnO-based MSM and MIS structures operated at various applied biases. (a, b) Noise power density; (c, d) NEP and D*.

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