J. Mater. Sci. Technol., 2010, 26(3), 223-227. Development and Characterization of Sol-gel Derived Al Doped ZnO/p-Si Photodiode Babita Gupta, Anubha Jain and R.M. Mehra † Department of Electronic Science, University of Delhi South Campus, New Delhi-110 021, India [Manuscript received April 8, 2009, in revised form August 8, 2009] The effect of Al doping on the J -V characteristics sol-gel derived of ZnO/p-Si photodiodes was investigated. The resistivity of Si was 0.1 Ω·cm. ZnO films annealed at 500 ◦ C were of the best quality. To investigate the spectral response of the photodiodes, the J -V characteristics were measured under different monochromatic lights at wavelength 420, 530, 570 and 630 nm. The diodes exhibit strong responsivity in the blue region at 420 nm. The responsivity is 0.22 A/W for Al doped (0.8 wt pct) photodiode, whereas for the undoped photodiode, it was much lower. An estimate of the responsivity as a function of wavelength has been made in terms of the width of depletion region of photodiodes. KEY WORDS: Sol-gel; Al doping; Photodiode; J -V characteristics; Responsivity 1. Introduction Photodiodes are photovoltaic devices used for measuring small light signals. As distinguished from solar cell, photodiodes must be sensitive in a desired spectral region, i.e., their responsivity should be high for the wavelength of interest. Semiconductor based ultraviolet (UV), visible (VIS) and infrared (IR) pho- todetectors have been continuously developed along with visible and infrared range detectors, evolving from Si to compound semiconductor such as GaN [1–4] . Photo detectors designed to sense both UV and visible photons are usually based on Si and fabricated using metal Schottky, p/n, p-i-n and metal-semiconductor- metal structures. Most of these diode structures have low responsivity of 0.1–0.15 A/W. Most extensive work has been done on Si photodiodes [5] . The main limitation of Si photodi- ode is that its spectral range is limited and that the technology of forming the p-n junction accurately is expensive. Because of these considerations, several groups have fabricated and characterized n-ZnO/p- † Corresponding author. Prof., Ph.D.; Tel.: +91 1124 115849; Fax: +91 1124 110876; E-mail address: ram- mehra2003@yahoo.com (R.M. Mehra). Si heterostructure photodiodes [6–8] . The n-ZnO/p-Si heterostructure photodiodes are generally deposited by different deposition techniques such as molecular beam epitaxy (MBE), radio frequency (r.f.) sput- tering, molecular organic chemical vapor deposition (MOCVD) and pulse laser deposition (PLD) [4–8] . In this paper we present fabrication and charac- terization of the Al-ZnO/p-Si photodiodes as visible range detector. The Al-ZnO/p-Si photodiodes have been fabricated using the sol-gel technique. The mea- surements of responsivity have been made in the vis- ible region. 2. Experimental For sol-gel deposition, 0.2 mol/L solutions were prepared by dissolving zinc acetate (Zn (CH 3 COO) 2 ·2H 2 O) (purity 99.95%) in anhydrous methanol in increasing concentration of Al doping (98.5% purity AlCl 3 ) ranging from 0 to 1 wt pct. To obtain clear solution, we added an equimolar amount of monoethanolamine (MEA). The result- ing solution was very clear, transparent and homoge- nous. For fabricating heterojunction ZnO/p-Si pho- todiode, solution was deposited on boron doped Si (100) substrate, which has a resistivity of 0.1 Ω·cm