GZO/GaN Schottky barrier ultraviolet band-pass photodetector with a low-temperature-grown GaN cap layer Kuo-Hua Chang 1 , Jinn-Kong Sheu 1 , Ming-Lun Lee 2 , Tao-Hung Hsueh 1 , Chih-Ciao Yang 1 , Kai-Shun Kang 1 , Jing-Fong Huang 1 , Li-Chi Peng 1 , and Wei-Chih Lai 1 . 1 Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan City 70101, Taiwan. Phone: +886-6-2757575-65298-103 E-mail: l7895106@mail.ncku.edu.tw 2 Department of Electro-Optical Engineering, Southern Taiwan University, Tainan 71005, Taiwan Aluminum gallium nitride (AlGaN) alloys are most promising materials for the fabrication of high-sensitivity solar/visible-blind photodetectors (PDs), since it has a wide direct band gap (3.4~6.2 eV at room temperature) and a high saturation electron drift velocity. In the past years, various types of GaN-based photodetectors have been demonstrated, such as p-n junction photodiodes, p-i-n PDs, p-π-n PDs, Schottky barrier PDs, andmetal-semiconductor- metal (MSM) PDs [1-5]. The most important key point to fabricate Schottky barrier PDs with high responsivity and low leakage current is the performance of Schottky contacts (SCs). It has been reported that various metals and trans- parent conducting oxide films deposited on GaN could achieve high performance SCs. In addition to the choice of contact metals, leakage current of SCs also depends strongly on the threading dislocation (TD) density of the GaN layers. In this study, we grew a LTG GaN cap layer on conventional high-temperature-grown GaN layer associated with the use of GZO films deposited on the LTG GaN to form the SB PDs. In this work, the samples used in this work were grown on c-face sapphire (0001) substrates by metal organic chemical vapor deposition. As shown in Fig. 1, a 30-nm-thick GaN nucleation layer was grown first at 550 o C, and followed by a 3-μm-thick Si-doped n-GaN and a 1-μm-thick un-doped GaN layer grown at 1050 O C. Finally, the wafers were capped with a 30-nm-thick un-doped GaN layer grown at temperature of 550 o C. It should be noted that the LTG GaN cap layer behaves in a kind of insulator with a sheet resistivity larger than 10 9 Ω/□. To fabricate the GaN SB PDs, Cl 2 -based plasma dry etching was applied to expose the n-GaN underlying layer. A GZO film with a thickness of about 200 nm was deposited on top of the samples by DC magnetron sputtering. The GZO target used in the DC magnetron sputtering containes 97% ZnO and 3% Ga 2 O 3 , in terms of weight percentage. The deposited GZO thin films exhibited a typical transmittance of over 80% at an incident wavelength of 360 nm, which implies a potential alternative to replace the conventional transparent contact layers, such as Ni/Au bilayer metal, which are used to serve as a transparent Schottky and Ohmic contact on GaN-based PDs and LEDs [6], respectively. Samples were then annealed at 700°C for 1 min in N 2 ambient by rapid thermal annealing in order to reduce resistivity of the GZO films. The typical resistivity of the GZO films after anneal- ing was measured to be around 5×10 -4 Ω-cm determined by Hall-effect measurement [7]. Then, a Cr/Au (50/200 nm) bilayer was deposited on the exposed n-GaN and the GZO served by e-beam evaporator to serve as n-type Ohmic contacts and anode electrodes, respectively. The diameter of the circular devices fabricated in this work was main- tained at 500 μm. Here, PD-I and PD-II are corresponding to the sample with and without the LTG GaN cap layer, respectively. The current-voltage (I-V) characteristics were measured at room temperature using HP4156C semicon- ductor parameter analyzer. Spectral responsivity of these SB PDs was measured using a Xe arc lamp and a calibrated monochromator as the light source. The monochromatic light was calibrated by Si photodiode and then illuminated onto the front side of SB PDs. FIG. 1. Schematic structure of the GZO/LTG GaN/i-GaN Schottky barrier diode (PD-I). Fig. 2. Typical I-V characteristics of the PD-I and PD-II, respectively. The power and the wavelength of the incident light are 1μW and 360 nm, respectively. Figure 2 shows a typical rectified I-V characteristic of the PD-I and PD-II. Under reverse biasing, PD-I was nearly independent in bias voltage and well below 20 pA as the bias voltages was lower than -10 V. In contrast, the dark -506- Extended Abstracts of the 2009 International Conference on Solid State Devices and Materials, Sendai, 2009, pp506-507 P-6-17