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 functions of annealing temperature for the In/ITO contacts annealed 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 properties 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 temperatures: (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 annealed 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 observed 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. Also, the behavior of the contact resistivity could not be attributed to the formation of new phase. Figure 5 shows the variations of the resistance measured at a bias voltage of 0.1 V between two In/ITO contact pads deposited on Ga-face and two In/ITO contact pads deposited on N-face n-GaN as a function of the annealing temperature As shown in Fig. 5 the dependence of the measured resistances on the annealing temperature is very different for In/ITO contacts on Ga-face n- GaN substrate compared to those on N-face n-GaN. For the contacts on Ga-face n-GaN, the resistances decreased Fig. 5. Resistance measured at a bias voltage of 0.1 V with increasing annealing temperature from 200 to 500 between two In/ITO contact pads (80-um gap spacing) de- ◦C, and the minimum resistance of 73.9 Ω was measured posited on Ga-face or N-face n-GaN as a function of annealing after 60 s anneals at 500 ◦C by using the CTLM For temperature. the contacts on N-face n-GaN, the resistance decreased with increasing annealing temperature from 200 to 300 In/ITO Ohmic Contacts to Ga-face and N-face n-GaN for··· – Ki Man Kang et al. -321-

◦C, and the minimum resistance of 24.3 Ω was measured ACKNOWLEDGMENTS after 60 s anneals at 300 ◦C by using CTLM. A further increase in annealing temperature resulted in an increase This work was supported by the Korea Research Foun- in the measured resistance. In addition, the resistance dation grant funded by the Korean Government Ministry was 3.5 times lower than that obtained for the contacts of Education, Science and Technology (MEST, Basic Re- on Ga-face n-GaN substrate, as shown in Fig. 5 Kwak search Promotion Funds) (KRF-2007-314-D00168) and et al. [15] showed that the resistance changes in Ga-face the Ministry of Knowledge Economy (MKE) through Re- and N-face n-GaN with Ti/Al deposition were to be con- gional Innovation Center (RIC). trary each other. This can be explained as follows; At the Ga-face Ohmic contact was formed as the density of the 2-dimensional electron gas rose due to the piezoelectric increase in donors at the interface of AlN/GaN due to REFERENCES the piezoelectric field whereas at the N-face, the barrier height rose as the 2-dimensional electron gas offset be- [1] P. Schlotter, J. Baur, Ch. Hielscher, M. Kunzer, H. tween Ti/Al and GaN [16]. In this experiment, however, Obloh, R. Schmidt and J. Schneider, Materi. Sci. and in the indium and n-GaN generated InGaN, the energy Engin. B 59, 390 (1999). gap of indium was so low, i.e., 1.92 eV, that it showed [2] A Motayed, A. V. Davydov, L. A. Bendersky, M. C. characteristics of an Ohmic contact. Wood, M. A. Derange, D. F. Wang, K. A. Jones and S. N. Mohammad, J. Appl. Phys. 92, 5218 (2002). [3] S. R. Jeon, Y. H. Song, H. J. Jang, G. M. Yang, S. W. Hwang and S. J. Son, Appl. Phys. Lett. 78, 3265 (2001). IV. SUMMARY [4] J. W. Ju, J. zhu, H. S. Kim, C. R. Lee and I. H. Lee, J. Korean Phys. Soc. 50, 810 (2007). [5] H. G. Hong, H. Na, M. K. Bae, D. H. Shin, S. N. Yi, In summary, we investigated the characteristics of the S. H. Doh, J. H. Na, K. H. Lee, R. A. Taylor and S. H. Ohmic contact between In/ITO and n-GaN for various Park, J. Korean Phys. Soc. 51, 209 (2007). thicknesses of In and for various annealing temperatures [6] J. S. Kim, E. K. Kim, H. J. Kim and E. Yoon, J. Korean from 200 to 600 ◦C The resistivity of In (20 nm)/ITO Phys. Soc. 51, 1195 (2007). (200 nm) films showed a minimum value of 27.9 Ω af- [7] S. Nakamura, M. Senoh, N. Iwasa and S. I. Nagahama, ter in a anneal at 400 ◦C The sheet resistance was lower Appl. Phys. Lett. 67, 1868 (1995). than 2 Ω/sq after annealing at 300 ◦C. The XRD anal- [8] J. O. Song, J. S. Kwak, Y. Park and T.-Y. Seong, Appl. ysis showed amorphous characteristics up to an anneal- Phys. Lett. 86, 213505 (2005). ing temperature at 200 ◦C; however, above 300 ◦C, the [9] J. S. Kwak, J.-O. Song, T.-Y. Seong, B. I. Kim, J. Cho, amorphous In/ITO crystallized The In/ITO contacts be- C. Sone and Y. Park, J. Nanosci. Nanotech. 6, 3547 (2006). came Ohmic with a specific contact resistance of 1.8 × −3 2 ◦ [10] J. S. Kwak and Y. J. Park, J. Korean Phys. Soc. 45, 988 10 Ωcm when annealed at 300 C. Also, we investi- (2004). gated the effect of crystal polarity on the electrical prop- [11] B. P. Luther, S. E. Mohney, T. N. Jackson, M. A. Khan, erties of the In/ITO contacts to n-GaN substrate. The Q. Chen and J. W. Yang, Appl. Phys. Lett. 70, 57 (1997). electrical properties of the contacts prepared on samples [12] J. S. Kwak, S. E. Mohney, J.-Y. Lin and R. S. Kern, with different polarities were also measured. The resis- Semicond. Sci. Tech. 15, 756 (2000). tances of the In/ITO on Ga-face and N-face films was [13] K. O. Schweitz, P. K. Wang, S. E. Mohney and D. Got- measured to be 84.7 Ω and 24.3 Ω, respectively, after an- thold, Appl. Phys. Lett. 80, 1954 (2002). nealing at 300 ◦C for 60 s. The resistance of the In/ITO [14] Z. F. Fan, S. N. Mohammad, W. Kim, O. Aktas, A. E. contacts to N-face n-GaN was much lower than that of Botchkarev and H. Morkoc, Appl. Phys. Lett. 68, 1672 (1996). the contacts to Ga-face n-GaN. This result indicates that [15] J. S. Kwak, J. H. Cho and C. S. Sone, J. Korean Phys. the In/ITO scheme can serve as a highly promising n- Soc. 48, 1259 (2006). type Ohmic contact for vertical LEDs. [16] J. S. Kwak, K. Y. Lee, J. Y. Han, J. Cho, S. Chae, O. H. Nam and Y. Park, Appl. Phys. Lett. 79, 3254 (2001).