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In/ITO Ohmic Contacts to Ga-Face and N-Face N-Gan for Ingan-Based Light-Emitting Diodes

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In/ITO Ohmic Contacts to Ga-Face and N-Face N-Gan for Ingan-Based Light-Emitting Diodes Journal of the Korean Physical Society, Vol. 55, No. 1, July 2009, pp. 318∼321 In/ITO Ohmic Contacts to Ga-face and N-face n-GaN for InGaN-based Light-emitting Diodes Ki Man Kang, J. M. Jo and Joon Seop Kwak∗ Department of Printed Electronics Engineering (WCU), Sunchon National University, Chonnam 540-742 Hyunsoo Kim Department of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Chonju 561-756 Y. S. Kim, C. Sone and Y. Park Samsung LED Co., Suwon 443-743 (Received 26 August 2008) We have investigated the In/indium tin oxide (ITO) scheme for obtaining high-quality Ohmic contacts to Ga-face and N-face nGaN for InGaN-based light-emitting diodes (LEDs). The In/ITO contacts to Ga-face n-GaN become Ohmic with specific contact resistances of 1.8 × 10−3 Ωcm2 ◦ when annealed at 30 C for 1 min in a N2 ambient The resistance of the In/ITO contacts to N-face n-GaN is shown to be much lower than that of the contacts to Ga-face n-GaN. This result indicates that the In/ITO scheme can serve as a highly-promising n-type Ohmic contact for vertical LEDs. PACS numbers: 73.40.Cg, 73.30.+y, 72.80.Ey Keywords: LED ITO, Ohmic contact, Polarity I. INTRODUCTION of the same contact to N-face n-GaN. In order to develop vertical LEDs in which n-type Ohmic contacts should be produced on N-face n-GaN, are must fabricated Ohmic InGaN-based light-emitting diodes (LEDs) have been contacts having a low resistance on N-face n-GaN. intensively studied for use in fullcolor outdoor LED dis- In this study, we have investigated the effect of plays, LED lighting and LCD backlight units [1–6] Fab- an indium interlayer on the formation of low-contact- rication of high quality Ohmic contacts that have low resistance ITO contacts both on Ga-face and N-face n- resistance and excellent reliability is technologically crit- GaN Since the work function of the In layer is as low as ical for the development and further advance of GaN 4.12 eV, In can reduce the barrier height between ITO based optical and electronic devices [7–9]. and GaN, and decrease the contact resistivity of the con- A number of research groups have made efforts to im- tacts. prove the electrical properties of Ohmic contacts and hence to enhance the optical and the electrical perfor- mance of GaN-based devices [1, 8] Many investigations have been carried out using Ti- or Al-based metalliza- II. EXPERIMENTAL DETAILS tion schemes as Ohmic contacts to n-type GaN [11,12] These metals have been found to form nitride phases The n-type GaN wafers were grown on sapphire by us- during an annealing process, which play an important ing metal-organic chemical-vapor deposition (MOCVD). role in the formation of Ohmic contacts to n-GaN [13, Before the laser lift-off (LLO) process, a highly-reflective 14] Kwak et al. [15] showed that the formation of the thick metal contact was formed on the p-GaN side by nitride phase at the GaN/metal interface in the Ti/Al using an e-beam evaporator to deposit Ag as a p-contact contacts could produce a good Ohmic contact to Ga-face metal and by depositing a support layer after annealing n-GaN; meanwhile it deteriorated the I-V characteristics the p-contact metal. After the formation of the sup- porting layer, a LLO process was performed using a KrF ∗E-mail: [email protected]; excimer laser. To obtain a planar surface on the device, Author to whom correspondence should be addressed we etched the LLO wafers by using an inductively cou- -318- In/ITO Ohmic Contacts to Ga-face and N-face n-GaN for··· – Ki Man Kang et al. -319- Table 1. Deposition parameters for the In, In/ITO film by rf magnetron sputtering. Substrate temperature 25 ◦C Flow rate of Ar gas 20 sccm Working pressure 5 mTorr Plasma power 40 W, 160W Indium thickness 1, 5, 10, 20, 30, 50 nm Indium-tin-oxide thickness 200 nm Fig. 2. Contact resistivities and sheet resistances as func- tions of annealing temperature for the In/ITO contacts an- nealed in a N2 ambient. Current-voltage (I-V) measurements were carried out by using a HP 4145B semiconductor parameter analyzer and a four-point probe technique the structural prop- erties were investigated using a X’ PECT-PRO X-ray diffraction (XRD) system with Cu k-alpha radiation and measuring the variation in the composition distribution with depth in the direction of the In/ITO and the n-GaN Fig. 1. Resistance measured at a bias voltage of 0.1 V by using MICROLAB 350 (VG Scientific) auger electron between two In/ITO contact pads annealed at 300 ◦C and spectroscopy (AES). 400 ◦C as a function of indium thickness for In/ITO contacts. III. RESULTS AND DISCUSSION pled plasma (ICP). The contact resistivities of the Ohmic contacts were measured by using the circular transfer length method (CTLM). Samples were degreased in ace- Figure 1 shows the variation of the resistance measured tone and methanol, rinsed in de-ionized (DI) water and at a bias voltage of 0.1 V between the two In/ITO contact photolithographically patterned. The CTLM patterns for annealing temperatures of 300 ◦C and 400 ◦C as a with gap spacings (20, 40, 80, 120, 160, 200 um) be- function of the In thickness for the In/ITO contacts. As tween the inner and the outer diameters, in which the shown in Fig. 1, the resistance between two In/ITO outer diameter was fixed 300 um, were patterned by us- contact pads decreased with increasing In thickness from ing a photolithographic technique Prior to In, In/ITO 1 to 20 nm/ITO (200 nm). However it increased when deposition by using a radio-frequency magnetron sput- the In thickness was increased from 20 to 30 nm. tering system, all the samples were treated in a boiling Figure 2 shows the variation of the contact resistivi- buffer oxide etch (BOE) solution for 30 s, rinsed in DI wa- ties and the sheet resistances as functions of annealing ter and dried with nitrogen after which the samples were temperature for the In ( 20 nm)/ITO ( 200 nm) contacts immediately loaded into a sputtering chamber. The sub- annealed in a N2 ambient. The annealing time was 60 strate temperature, the flow rate of Ar gas, the working s. The In/ITO contacts produced linear I-V character- pressure, and the power during the rf-magnetron sput- istics after being annealed at 300 ◦C, as shown in Fig. tering are listed in Table 1 Each sample was placed in 2 The contact resistivity in the as-deposited state was the rf-magnetron sputter system with a working pressure 1.05 × 10−1 Ωcm2 for the In/ITO contacts, and it did of 5 m Torr and a power of 40 W at the 2inch target. not change for temperatures up to 200 ◦C. After being ◦ Metal films were then deposited to a thickness of 1, 5, annealed at 300 C in N2 ambient however, the contact 10, 20, 30, and 50 nm for indium and of 20 nm/200 nm resistivity of the In/ITO contacts markedly decreased to for In/ITO An annealing process to investigate the effect 1.8 × 10−3 Ωcm2 which was two orders of magnitude of annealing temperature on the sheet resistance and the lower than that obtained for as-deposited In/ITO con- contact resistivity was performed with a rapid thermal tacts. The sheet resistance in the as-deposited state was annealing (RTA) system in N2 at temperatures from 200 65 Ω/sq for the In/ITO contacts, and it did not change to 600 ◦C for 60 s. up to a temperature of 200 ◦C. After being annealed at -320- Journal of the Korean Physical Society, Vol. 55, No. 1, July 2009 Fig. 3. XRD patterns of an In(20 nm)/ITO(200 nm) film on a Ga-face surface of n-type GaN annealed at various tem- peratures: (a) as-dep., (b) 200 ◦C, (c) 300 ◦C, (d) 400 ◦C, (e) 500 ◦C, and (f) 600 ◦C. ◦ 300 C in N2 ambient however, the sheet resistance of the In/ITO contacts significantly decreased to 19 Ω/sq. Figure 3 shows the XRD profiles of the In(20 nm)/ITO(200 nm) films on the Ga-face surface of n-type GaN annealed at various temperatures. As shown in Fig. 3, the In/ITO contacts at the as-deposited state and an- nealed at 200 ◦C produced broad peaks near 30◦ indi- cating the formation of amorphous-like structures in the ITO films. However, after annealing at 300 ◦C, XRD peaks corresponding to ITO were clearly observed, as shown in Fig. 3. Fig. 4. AES depth profiles of the In(20 nm)/ITO(200 nm) Figure 4 shows the AES depth profile of the In (20 contacts on n-GaN after being annealed at 300 ◦C and (b) ◦ nm)/ITO (200 nm) contacts on n-GaN after annealing 500 C for 1 min in a N2 ambient. ◦ ◦ at (a) 300 C and (b) 500 C for 1 min in a N2 ambient. Behaviors similar to that of unannealed In/ITO were ob- served as shown in Fig. 4 The composition practically did not vary not even after annealing at 500 ◦C, which means that no intermixing at the interface between the n-GaN and In and between In and ITO and no inter- action occurred with increasing temperature.
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