1558-1748 (c) 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/JSEN.2017.2724061, IEEE Sensors Journal > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 1 Abstract—The in-situ growth of Al2O3 on TiO2 by ultrasonic spray pyrolysis deposition (USPD) is presented in this paper. Here, Al2O3 is used as the passivation and the antireflection (AR) layer. TiO2-based photodetectors (PDs) with Al2O3, SiO2, and no passivation layers were studied. It was found that the PD without the passivation layer has the highest dark current and photocurrent due to high internal photoconductive gain related to the reaction between the oxygen molecule and TiO2. The PDs with Al2O3 and SiO2 passivation layers suppress the internal photoconductive gain and show stable I–V characteristics after the PDs are exposed to air for 30 days. The device with the Al2O3 passivation layer showed the most stable I–V characteristics and the highest detectivity and the shortest response time among the three devices. Key words: Aluminum oxide, antireflection, passivation, titanium dioxide, ultraviolet detectors, ultrasonic spray pyrolysis deposition. I. INTRODUCTION LTRAVIOLET (UV) photodetectors (PDs) are employed in many fields such as flame detection [1], imaging [2], and optoelectronic integrated circuits [3]. Wide bandgap materials such as TiO2 have gained considerable research attention owing to their properties such as intrinsic visible-blindness, and chemical and thermal stability. Xue et al. used the sol-gel method to grow the TiO 2 film as the active layer and the Au/TiO2/Au metal-semiconductor-metal (MSM) UV PDs were fabricated [4]. After that, Huang et al. used the sputtering method to deposit the TiO2 film and the Au/TiO2/Au MSM UV PDs were fabricated [5]. The TiO 2 film deposited by the sputtering method has better quality than the one deposited by the sol-gel. Therefore, the lower dark current was obtained by the sputtering method. The sol-gel process is cost effective; however, it is difficult to use this method to grow thin films in wafer sizes. Sputtering is a mature technique used to deposit thin films in wafer size; however, it requires vacuum facilities, which increases the cost. In contrast, ultrasonic spray pyrolysis deposition (USPD) is a promising method to grow thin films in wafer size without vacuum facilities [6]. The TiO 2 film deposited by the USPD method and the Ni/TiO2/Ni MSM UV PDs were fabricated in our previous work [7]. Liu et al. found that the performance of the TiO2 UV PD made by the sol-gel can be improved by a surface modification process such as the Manuscript received March 21, 2016. This work was supported by the Ministry of Science and Technology, Taiwan, under Contract MOST 104-2218-E-035-009-. H.-Y. Liu, G.-J. Liu, and R.-C. Huang are with the Department of Electronic Engineering, Feng Chia University, Taichung, Taiwan, R.O.C. (e-mail: hyliu@fcu.edu.tw). W.-C. Sun, S.-Y. Wei, and S.-M. Yu are with the Material and Chemicall Research Laboratories, Industrial Technology Research Institute, Hsinchu, 31040, Taiwan, R.O.C. self-assembly process [8]. The experimental results suggest that surface modification (passivation) is helpful to enhance device performance. In addition, the refractive index between air (n = 1) and TiO 2 (n = 2.5) is large, and this provides a high optical reflectance between air and TiO2. An antireflection (AR) layer between air and TiO 2 can reduce the optical reflectance. However, using AR and passivation layers on TiO2 PDs has not attracted much attention from researchers to date, despite the fact that this would further improve the performance of the TiO 2 -based photodetectors. Therefore, this work used the USPD method to grow TiO2 and Al2O3 thin films. The similar thin film growth method was demonstrated in our previous work [9], but the ex-situ growth Al2O3 was used as the insulator layer of the MISIM UV PD. The thickness of the insulator is too thin to serve as an AR layer. Besides, the ex-situ grown Al 2 O 3 cannot protect TiO 2 during the process until the Al 2 O 3 was deposited on the TiO2. The in-situ grown Al2O3 was used as the passivation and AR layer for the TiO2 MSM PD. The responsivity and detectivity (D * ) values of the PD prepared in this way are approximately 0.14A/W and 1.39 × 10 11 Jones, respectively, both of which are close to those of SiC- and GaN-based photodetectors [10]. Further, the PDs with the Al 2 O 3 and SiO 2 passivation layers show stable I-V characteristics. II. EXPERIMENTS Before TiO2 deposition, the Si substrate was placed in the furnace to grow SiO 2 as the back-side insulator layer. The TiO 2 -based PD was grown by USPD on a 4-inch (100) p-type Si substrate. The precursor solution was prepared by dissolving Ti(C3H7O)4 in CH3OH, after which the solution was atomized by an ultrasonic generator at a frequency of 120 kHz. The atomized Ti(C 3 H 7 O) 4 and CH 3 OH mixture was sprayed onto the chips placed on the heater, the temperature of which was set to be 350 ° C to trigger the pyrolysis reaction. The pyrolysis reaction is described as Ti(C3H7O)4(aq) + 2H2O(g) → TiO2(s) + 4C3H8O(g) (1) Han-Yin Liu, Guan-Jyun Liu, Ruei-Chin Huang, Wen-Ching Sun, Sung-Yen Wei, and Sheng-Min Yu In-Situ Growth of Al 2 O 3 as a Passivation and Antireflection Layer on TiO 2 -based MSM Photodetectors U Ni/Au PD-A:Al 2 O 3 PD-B:SiO 2 PD-C:None PD-A  TiO 2 Deposition  Al 2 O 3 Deposition  Photolithography & Etching  Electrode Metallization PD-B  TiO 2 Deposition  Photolithography  Electrode Metallization  SiO 2 Deposition  Photolithography & Etching PD-C  TiO 2 Deposition  Photolithography  Electrode Metallization Fig. 1 Device structure and fabrication process flow of PD-A, PD-B, and PD-C.