Effect of interface between neodymium fluoride thin film and oxide or fluoride substrate on vacuum ultraviolet photoconductivity

抄録

Introduction

Vacuum ultraviolet (VUV) radiation spans the wavelength range from 100 to 200 nm. Its high photon energy renders VUV radiation indispensable in a wide range of applications, including semiconductor lithography, sterilization of surfaces, optical cleaning, surface modification, and spectroscopy, to name a few, so the development of VUV detectors is very important. In order to detect high-energy light, it is necessary to use materials with a large bandgap. Therefore, wide bandgap (WBG) semiconductors such as nitrides and oxides, as well as diamond, are being investigated. On the other hand, we focused on fluoride materials, which have higher durability against UV light irradiation than these, and carried out a similar approach using this material. Unlike silicon-based detectors developed by relatively mature production technologies and WBG semiconductor-based detectors, which are widely investigated in current literature, the development of fluoride photodetectors is at its infancy and thus not much is known regarding the correlation between fabrication parameters and photodetector performance. In this work, we investigated the influence of annealing on the interface between neodymium trifluoride (NdF₃) thin film and quartz (SiO₂) substrate. In semiconductor device fabrication, annealing is the process of heating the semiconductor to a predetermined temperature, keeping it at that temperature for a fixed time, and then cooling to room temperature. During annealing, the fabricated NdF₃ film on SiO₂ substrate is heated to a temperature of 200, 400, and 600 ^o C inside a furnace for 3 hours and then subsequently cooled to room temperature.

Experimental and Result

The effect of annealing on the interface between NdF₃ thin film and SiO₂ substrate and hence on the photoconductivity of NdF₃ thin film VUV photodetector, was investigated. Thin films were deposited using a pulsed laser deposition (PLD) under two substrate conditions: unheated substrate at R.T. and heated substrate at 600 ^o C. The deposited films were then annealed in vacuum and their structure and photoconductivity were compared to those of the unannealed (as-grown) films. The films deposited on the unheated substrate exhibited an increase in dark current resistance and hence a decrease in dark current after annealing. Consequently, our findings revealed a seven-fold increase in the SN ratio of the detectors following post deposition annealing. Resistance to dark current of the films deposited on the 600 ^o C-heated substrate decreased, resulting in similar values to those of the photocurrent at high bias voltages and suggesting poor photoconductivity. Investigation of the NdF₃/SiO₂ interface revealed that fluorine diffused from the film to the substrate during deposition. Diffusion can be attributed to the energy deposited by PLD ablation; however, diffusion is exacerbated when heating the substrate during deposition. Post deposition annealing did not seem to significantly contribute to fluorine diffusion. Diffusion of fluorine was found to degrade NdF₃ crystallinity near the interface, explaining the decrease in dark current resistance and increase in dark current. Deposition of NdF₃ on a MgF₂ substrate heated at 600 ^oC did not result in fluorine diffusion. Consequently, the SN ratio of the NdF₃/MgF₂ detector that was deposited on a 600 ^oC-heated substrate without post deposition annealing was similar to that of the NdF₃/SiO₂ detector that was deposited on an unheated substrate with post deposition annealing at 600 ^oC.

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