Journal of Alloys and Compounds 481 (2009) L15–L19 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom Letter GaN Schottky barrier photodiode on Si (1 1 1) with low-temperature-grown cap layer L.S. Chuah a,∗ , Z. Hassan a , H. Abu Hassan a , N.M. Ahmed b a School of Physics, Universiti Sains Malaysia, 11800 Penang, Malaysia b School of Microelectronic Engineering, Universiti Malaysia Perlis, 02600 Perlis, Malaysia article info Article history: Received 25 September 2008 Received in revised form 26 February 2009 Accepted 26 February 2009 Available online 17 March 2009 Keywords: AlN GaN Photodiode Schottky barrier height Thermal annealing abstract In this work, GaN films were grown on three-inch silicon substrates by plasma-assisted molecular beam epitaxy (PAMBE) with AlN (about 200nm) as the buffer layer. Finally, a thin AlN cap layer (50nm) was grown on the GaN surface. Current–voltage (I–V) measurements before and after heat treatment were carried out. Different annealing temperatures (500–700 ◦ C) were investigated. Under dark condition, the Schottky barrier height (SBH) derived by the I–V method is 0.48 eV for as-deposited Ni/AlN/GaN/AlN Schottky diode. On the other hand, the effective barrier heights of 0.52, 0.55, and 0.57eV were obtained for Schottky diodes annealed at 500, 600, and 700 ◦ C, respectively. We found that the SBHs of annealed Schottky diodes under dark and illuminated conditions were observed to be higher relative to the as- deposited Schottky diode. When annealed at 700 ◦ C, the resulting Schottky diodes show a dark current of as low as 5.05 × 10 -5 A at 10 V bias, which is about two orders of magnitude lower than that of as-deposited Ni/AlN/GaN/AlN Schottky diode (2.37 × 10 -3 A at 10V bias). When the sample was under illumination condition, the change of current was significant for the annealed samples as compared to the as-deposited sample. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Semiconductors of the III–V nitride group and their alloys have received much attention due to their contribution in optoelectron- ics. These nitrides form a continuous alloy system with direct band gaps ranging from 6.2 eV for AlN to 0.7 eV for InN [1]. Among all the III–V nitrides, gallium nitride (GaN) is the most extensively studied [2,3]. GaN normally crystallizes in the stable wurtzite structure, although it has also been observed to have the metastable zinc blende polytype when grown on a cubic sub- strate [4]. At room temperature, the band gap of wurtzite GaN is ∼3.4 eV, which corresponds with the blue-ultraviolet region. The common operating region for semiconductor optical devices is from the infrared to green. By extending this range into the blue, semi- conductor components would be able to emit and detect the three primary colors of the visible spectrum. Also, GaN possesses large intrinsic dielectric breakdown fields, good thermal conductivity and chemical stability at elevated temperatures [5]. This makes it a desirable material for high speed, high power visible-to-UV optoelectronic devices which can operate in high temperatures and caustic environments. ∗ Corresponding author. Tel.: +60 4 6533673; fax: +60 4 6579150. E-mail addresses: chuahleesiang@yahoo.com (L.S. Chuah), zai@usm.my (Z. Hassan). Silicon (Si) is one of the most common elements of the earth crust and the substrates are of very low price and are available in very large size due to its mature development and large-scale pro- duction. The thermal conductivity is higher than that of sapphire and is close to that of GaN. The crystal perfection of Si is better than that of any other substrate material and it has good thermal stabil- ity under GaN epitaxial growth condition. The growth of GaN on Si enables the possibility of integrating GaN optoelectronics devices with Si-based electronics. As GaN devices are usually made from hexagonal GaN epitaxial layers, Si (1 1 1) can provide the hexagonal template for AlN depo- sition. According to the literature, X-ray diffraction (XRD) patterns showed that full width at half maximum (FWHM) of AlN (0 0 0 2) peak grown on Si (1 1 1) substrates was smaller than that grown on Si (1 0 0) substrates. XRD results also indicate that the preferred orientation of AlN films on Si (1 1 1) substrates is more easily con- trolled than those on Si (1 0 0) substrates. It can be attributed to the more matched lattice template with hexagonal structures of AlN films provided by (1 1 1) plane of silicon. Vibrational characterization by Fourier transform infrared spec- troscopy (FTIR) revealed that the stress in the AlN films deposited on Si (1 1 1) substrates was also smaller than AlN films deposited on Si (1 0 0) substrates. The lattices in AlN (0 0 0 1) and Si (1 1 1) are both hexagonal, and thus Si (1 1 1) can provide matched template for AlN (0 0 0 1) plane. The lattice mismatch between these two planes is 19% (d Si (1 1 1) - d AlN (0 0 0 1) /d Si (1 1 1) , here d Si (1 1 1) refers to the Si 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.02.151