Evaluation of Metal/Indium-Tin-Oxide for Transparent Low-Resistance Contacts to P-Type Gan

Evaluation of metal/indium-tin-oxide for transparent low-resistance contacts to p-type GaN

Wenting Hou, Christoph Stark, Shi You, Liang Zhao, Theeradetch Detchprohm, and Christian Wetzel* Future Chips Constellation and Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180, USA *Corresponding author: [email protected]

Received 12 March 2012; revised 27 June 2012; accepted 8 July 2012; posted 16 July 2012 (Doc. ID 164558); published 2 August 2012

In search of a better transparent contact to p-GaN, we analyze various metal/indium-tin-oxide (ITO) (Ag/ITO, AgCu/ITO, Ni/ITO, and NiZn/ITO) contact schemes and compare to Ni/Au, NiZn/Ag, and ITO. The metal layer boosts conductivity while the ITO thickness can be adjusted to constructive transmission interference on GaN that exceeds extraction from bare GaN. We find a best compromise for an Ag/ITO (3 nm ∕ 67 nm) ohmic contact with a relative transmittance of 97% of the bare GaN near 530 nm and a specific contact resistance of 0.03 Ω ·cm2. The contact proves suitable for green light-emitting diodes in epi-up geometry. © 2012 Optical Society of America OCIS codes: 230.3670, 310.7005.

1. Introduction While contact resistance and current spreading Our society’s drive for energy efficiency places high are known to improve with increasing thickness of relevance on the identification of low-resistance the metal stack, the transmittance decreases. For ohmic contacts to wide bandgap group-III nitrides p-type GaN, semitransparent Ni/Au contacts are most commonly used. Reported specific contact for use in light-emitting diodes [1,2] (LEDs) and −3 third-generation solar cells [3]. In particular, trans- resistance values mostly range from 4.4 × 10 to p 5 × 10−4 Ω ·cm2 [9,10], with the exception of only parent ohmic contacts to -type GaN are of topical −6 2 concern. Throughout the group-III nitrides, hole con- one report of a value as low as 4 × 10 Ω ·cm [11]. duction is the more limited one compared to electron A spectral transmittance around 70% to 80% in the transport [4–6]. The high binding energy of the com- visible spectrum has been reported [9,10]. ITO mon Mg acceptor [7], its propensity for structural layers, on the other hand, have shown very high defect generation, and low hole mobility require transparency with transmittance above 90% in the large-area hole injection close to the optically active visible spectrum, but non-ohmic contact behavior p [12], with specific contact resistance in the range region. This can best be achieved by -contact −1 2 schemes that provide both low contact resistance of 10 Ω ·cm [13]. In an attempt to improve both and high optical transparency in a specified spectral contact resistance and transparency, we here jointly region. Furthermore, the large binding energy of study both in several metal/ITO contacts schemes— — p electrons at the valence band maximum of GaN namely Ag/ITO, AgCu/ITO, Ni/ITO, NiZn/ITO to - limits suitable p-contact metals to those with the GaN and compare them to standard Ni/Au, NiZn/Ag highest work function [8]. and ITO contacts.

2. Experiments

5596 APPLIED OPTICS / Vol. 51, No. 23 / 10 August 2012 unintentionally doped GaN (u-GaN) templates, 4 μm 1.0 Ni/Au (5 nm/5 nm) thick, on c-plane sapphire. Free hole concentrations 17 −3 NiZn/Ag (5 nm/200 nm) of 4 × 10 cm at room temperature were achieved 0.5 ITO (200 nm) 19 −3 using Mg doping at a concentration of 10 cm .On Ag/ITO (3 nm/67 nm) top, a p-GaN, 10 nm thick, Mg-doped (up to 1020 cm−3) contact layer was grown. As substrates for 0.0 transmittance measurements, float glass slides, n Current (mA) 0.2 mm thick, and -GaN on double side polished -0.5 (DSP) sapphire were used. Contact metal stacks were prepared by means of an e-beam evaporation and subsequent rapid thermal annealing on both -1.0 -2 -1 0 1 2 templates using each of the following schemes (nota- Voltage (V) tion in sequence of deposition): Ni/Au (5 nm ∕ 5 nm) Fig. 1. (Color online) Current-voltage characteristics of different with a 1 min at 500 °C anneal in oxygen ambient; contact schemes on p-GaN. NiZn/Ag (5 nm ∕ 200 nm) with a 1 min at 550 °C anneal in oxygen ambient; ITO (200 nm, 100 nm, and 67 nm), each with a 1 min at 550 °C anneal in oxygen The current-voltage characteristics between a pair ambient; Ag/ITO (3 nm ∕ 130 nm, 3 nm ∕ 67 nm), of the respective contact schemes to p-GaN are AgCu/ITO (3 nm ∕ 130 nm), Ni/ITO (3 nm ∕ 130 nm, shown in Fig. 1 (5 μm contact spacing). The relative 1 nm ∕ 60 nm, 3 nm ∕ 100 nm, and 3 nm ∕ 67 nm), and optical transmittance of the same contact layers on NiZn/ITO (3 nm ∕ 130 nm), each with a 1 min at 550 °C glass is shown in Fig. 2 and Fig. 3. anneal in oxygen ambient; Ni/ITO (3 nm ∕ 130 nm) We find that after annealing, the Ni/Au contacts with a 1 min at 450 °C anneal in oxygen ambient. (Fig. 1, black solid squares) readily show ohmic −3 2 We define relative transmittance of the layer stack behavior with ρc 2.2 × 10 Ω ·cm . In our experi- on a substrate by the transmitted power in ratio with ence, however, this value can vary strongly with min- the power transmitted through a bare substrate. We or details of the deposition and annealing process, determine the ratio experimentally by simultaneous source metal purity, and surface roughness of the measurement of both samples in parallel beams of a p-GaN. For example, minor C contamination of the spectrophotometer. To properly account for multiple reflection effects in transmittance, we tested the Ni/ITO 3 nm/130 nm metal stacks on both kinds of substrates, the common 0 200 nm ITO glass slide and GaN on DSP sapphire. 100 annealed 550 C 1 min

80 Ni/ITO 1 nm/60 nm 3. Results 60 Ni/ITO 3 nm/130 nm 0 The specific contact resistance (ρc) of lateral contacts Ni/Au after anneal annealed 450 C 1min is generally measured using the transmission line 40 method [14]. In this model, the ohmic contact is de- Ni/Au before anneal posited on a thin film layer, whose total resistance is 20 much lower than that of the supporting substrate un- (%) Transmittance Relative on glass derneath [14]. The resistance values of contact pairs 0 R 300 400 500 600 700 800 with different pad spacing pp are analyzed as a Wavelength (nm) function of spacing, and ρc and the sheet resistance can be extrapolated from the data. Typically, these Fig. 2. (Color online) Transmittance of contact films on 0.2 mm glass slides for various film stacks. prerequisites are not met when analyzing p-contacts to full LED structures. Due to the limitations of hole transport in GaN, the high resistance of the p-type n 100 GaN layer makes shunt currents through the -type ITO (100 nm) layers underneath very likely and so distorts the in- Ni/ITO (3 nm/100 nm) terpreted value of ρc. Under low enough applied test 90 voltage, however, the pn-junction between the parallel layers does not turn on and blocks parallel conduc- 80 tion through the n-side of the device [15,16]. We therefore limit our analysis of ρc to the 1 V range. ITO (67 nm) Furthermore, inaccuracies in the layer and contact 70 Ag/ITO (3 nm/67 nm) geometry have a strong effect on values derived from Ni/ITO (3 nm/67 nm) on glass p (%) Transmittance Relative contacts on poorly conducting layers, such as -type 60 GaN [17]. We therefore paid particular attention to 300 400 500 600 700 800 accurately account for the actual contact geometry Wavelength (nm) within 5%. This results in an error of ρc less than Fig. 3. (Color online) Transmittance of contact films on 0.2 mm 10% for our results. glass slides for ITO film with and without a thin Ni or Ag layer.

10 August 2012 / Vol. 51, No. 23 / APPLIED OPTICS 5597 Ni source from the graphite crucible has shown to in- line) again is only less than 5% smaller than for bare crease ρc by more than an order of magnitude. In ITO 100 nm (black solid line) and 67 nm (blue, short addition, we find that Ni/Au contacts on p-GaN sur- dashed line) in the visible spectrum. Apparently, the faces, that have been roughened to increase light additional metal layer of Ni or Ag can indeed signif- extraction, show higher ρc and tend to induce current icantly lower ρc below that of the bare ITO contact crowding. For such conventional contacts, a spectral while maintaining a rather high transmittance. transmittance around 70% to 80% in the visible spec- The specific contact resistance of an Ni/ITO trum has been reported [9,10]. We here find a rela- (3 nm ∕ 130 nm) film decreases when lowering the tive transmittance of 40% before annealing (Fig. 2, annealing temperature from 550 °C to 450 °C. The blue open circles) and 75% after annealing (Fig. 2, relative transmittance of an Ni/ITO (3 nm ∕ 130 nm) magenta solid circles) at a wavelength of 500 nm. film on glass after annealing at 550 °C (Fig. 2, red This is in line with the literature reports [9,10]. solid star) reaches 97% at 510 nm. When lowering When contact transparency is not required, such the annealing temperature to 450 °C (Fig. 2, orange as in vertical LED structures, the nontransparent open star), the relative transmittance decreases to NiZn/Ag contact to p-type GaN provides an 67% at 510 nm. The increase in optical transmittance attractive alternative [18]. Here we achieve reliable in ITO-based contacts with temperature can be at- low-resistance ohmic contacts with ρc 1.6 × tributed to the increase of structural homogeneity 10−3 Ω ·cm2 (Fig. 1, red crosses). In an attempt to in- and crystallinity of ITO film [25,26]. crease transparency, the Ag layer thickness was re- As a function of ITO layer thickness on glass, the duced to 5 nm, resulting, however, in much higher peak for relative transmittance is found to move from specific contact resistance of 0.1 Ω ·cm2. 716 nm at 200 nm (Fig. 2, black square) over 510 nm ITO is a well-known transparent conducting film at 130 nm (with 3 nm Ni) (Fig. 2, red solid star) to that has widely been employed as contact to p- 417 nm at 100 nm (Fig. 3, black solid line). For an GaN [19,20]. Only rectifying contacts were achieved ITO thickness of 67 nm on glass, no peak in the with ITO on p-GaN [19,20]. For 200 nm ITO contacts relative transmittance is observed in the visible (Fig. 1, blue open triangles), we only find rectifying wavelength range. Apparently, the maximum of the 2 behavior at ρc as high as 0.1 Ω ·cm . For the same relative transmittance can be tuned in wavelength contact structure on glass (Fig. 2, black solid by a variation of the ITO thickness. squares), we found a relative transmittance above In the next step, the relative transmittance of ITO 80% across the entire visible spectrum from 380 to on GaN and glass is directly compared to the respec- 780 nm, in line with the literature [19,20], and a peak tive bare substrates (Fig. 4). For 67 nm ITO on glass of 98% near 716 nm. While such transparency is (pink line, see labels), a relative transmittance is desirable, the poor electrical performance limits its found to be only 78% around 500 nm. For GaN on application as a p-contact material. DSP sapphire (red line), the relative transmittance An improvement to ρc of ITO contacts to p-GaN has oscillates around 100%. This obviously is due to its been reported by depositing first a thin metal layer direct comparison with a very similar layered struc- [21–24]. For Ag/ITO (3 nm ∕ 67 nm) (Fig. 1, magenta ture in the reference beam path producing oscialla- open circles), we find ohmic contacts with ρc tions as well. For the 67 nm ITO film on GaN on 0.03 Ω ·cm2, while Ag/ITO (3 nm ∕ 130 nm) reaches DSP sapphire (blue line), the relative transmittance 0.012 Ω ·cm2, and AgCu/ITO (3 nm ∕ 130 nm) rea- increases over that of the bare GaN on DSP sapphire ches 0.02 Ω ·cm2. The improvement in contact resis- (red line). At 530 nm it reaches as high as 107%. With tance is reported to come from the formation of an the insertion of an additional 3 nm Ni layer (black Ag–Ga solid solution that produces deep-acceptor line), the relative transmittance reduces somewhat like Ga vacancies near the GaN surface region under to 103%, and to 97% with the insertion of a 3 nm the contacts [24]. The influence of the thin extra Ag layer instead (green line). Overall, however, metal layer on the relative transmittance in turn is rather small. On glass, transmittance (Fig. 3)of 110 Ag/ITO (3 nm ∕ 67 nm) (magenta, dash-dotted line) on GaN on DSP sapphire ITO (67 nm) is less than 5% smaller than for bare ITO (67 nm) Ni/ITO (3 nm/67 nm) (blue, short dashed line) in the visible spectrum. 100 GaN only The same trend can be found for a thin Ni layer Ag/ITO (3 nm/67 nm) underneath ITO. For Ni/ITO (3 nm ∕ 130 nm, 2 90 annealed at 550 °C) we find ρc 0.2 Ω ·cm ; for Ni/ITO (3 nm ∕ 130 nm, annealed at 450 °C) ρc 2 0.1 Ω ·cm ; for Ni/ITO (3 nm ∕ 67 nm) ρc 0 3 Ω 2 3 ∕ 130 80 67 nm ITO on glass . ·cm ; and for NiZn/ITO ( nm nm) Relative Transmittance (%) 2 ρc 0.06 Ω ·cm . The formation of a NiO interfacial layer is believed to be the reason for lowering the con- 400 500 600 700 tact resistance [10,11]. On glass, transmittance Wavelength (nm) (Fig. 3) of Ni/ITO 3 nm ∕ 100 nm (red, dashed line) Fig. 4. (Color online) Relative transmittance of ITO (67 nm), and Ni/ITO 3 nm ∕ 67 nm (green, dash-dot-dotted Ni/ITO (3 nm ∕ 67 nm), Ag/ITO (3 nm ∕ 67 nm) on GaN and glass.

5598 APPLIED OPTICS / Vol. 51, No. 23 / 10 August 2012 relative transmittance values above 96% are ob- 5. Conclusion tained for wavelengths longer than 500 nm. Appar- In conclusion, we jointly analyzed specific contact re- ently, even with a conductivity-enhancing thin sistance and relative transmittance of a wide range metal layer, it should become possible to enhance of contact schemes on p-type GaN in the green spec- the light extraction from GaN at a desired wave- tral region. The conventional Ni/Au (5 nm ∕ 5 nm) length by a proper choice of the ITO thickness. stack shows a relatively good ohmic behavior with −3 2 ρc 2.2 × 10 Ω ·cm , while the transmittance is 5 ∕ 200 4. Discussion only around 75%. In turn, NiZn/Ag ( nm nm) shows a very good ohmic contact with low resistance −3 2 The observations for transmission on glass and GaN of ρc 1.6 × 10 Ω ·cm but it is strongly absorbing. can be explained by the following reasoning. The Pure ITO films are found to be highly transparent absorption coefficient for bare ITO is small in the but only rectifying contacts can be achieved. The re- visible wavelength range [27]. For wavelengths lative transmittance spectra of ITO-based contacts shorter than 350 nm, strong absorption due to inter- are dominated by thickness interference fringes band absortion in ITO is expected [28]. ITO-based the boundary conditions of which differ between contacts show an oscillation of the transmittance GaN and standard glass. By adjusting the ITO layer with a period that resembles interference fringes in thickness to 67 nm on GaN, the relative trans- an optically thin layer. The spectral variations with mittance reaches 106% of that of the bare GaN layer the thickness of ITO suggest that an optical etalon is near 530 nm. This can be exploited to maximize formed by the thin ITO film between glass and air. the relative transmittance at a desired wavelength The data show that the insertion of the thin metal range. layer results in only a small reduction of transmit- To reduce contact resistance significantly a thin tance. This is explained by the thickness being much metal layer of Ni, Ag, or their related alloys of NiZn, smaller than the lights penetration depth. or AgCu is inserted before the ITO with only minor Applying the standard theory (Δφglass losses to the spectral transmittance. By jointly opti- 2nITOd ∕ λ × 2π) and refractive indices of n 1.5 mizing ITO thickness and metal layer stack we find a for glass, and a range of n 1.94 740 nm− best performance for a Ag/ITO (3 nm ∕ 67 nm) contact 2.18 410 nm for ITO [29] constructive interference with a relative transmittance of 97% of the bare GaN can be expected in the following wavelength: 780 nm layer near 530 nm and a specific ohmic contact resis- and 436 nm for an ITO thicknesses of 200 nm; tance of 0.03 Ω ·cm2. This contact seems ideally sui- 526 nm for 130 nm, and 434 nm for 100 nm ITO. This ted for high-performance GaN-based LEDs in the interpretation can well describe the observed varia- longer green, yellow, and red spectral regions. tions for wavelengths above 350 nm. From this we This work was supported by a DOE/NETL Solid- also find that for 67 nm ITO on glass, the interference State Lighting Contract of Directed Research under should be destructive in the visible range near DE-EE0000627. This work was also supported by the 500 nm leading to a low transmittance. This is in line National Science Foundation (NSF) Smart Lighting with our experimental findings in Fig. 4 (pink line). Engineering Research Center (# EEC-0812056). At shorter wavelength, optical absorption dominates the transmittance. References and Notes Contact transmittance is typically characterized on glass only [22,24]. The aspect of layer thickness 1. I. Akasaki and H. Amano, “Crystal growth and conductivity control of group III nitride semiconductors and their applica- interference, however, is strongly dependent on tion to short wavelength light emitters,” Jpn. J. Appl. Phys. the actual carrier itself and therefore should only 36, 5393–5408 (1997). be characterized on the GaN layer of an actual 2. I. Akasaki and C. Wetzel, “Future challenges and directions GaN-based LED. While the index of refraction of for nitride materials and light emitters,” Proc. IEEE 85, 1750–1751 (1997). glass is lower than that of ITO, that of GaN 3. Y. Kuwahara, T. Fujii, T. Sugiyama, D. Iida, Y. Isobe, Y. (n 2.5) is higher. This leads to a phase change of Fujiyama, Y. Morita, M. Iwaya, T. Takeuchi, S. Kamiyama, “ π at the interface between ITO and GaN. (ΔφGaN I. Akasaki, and H. Amano, GaInN-based solar cells using 2n d ∕ λ × 2π π). Therefore, wavelengths of strained-layer GaInN/GaInN superlattice active layer on a ITO ” 4 destructive interference on glass lead to constructive freestanding GaN substrate, Appl. Phys. Express , 021001 (2011). interference on GaN. The optimal ITO thickness for a 4. S. Nakamura, M. Senoh, and T. Mukai, “Highly p-typed transmittance peak is obtained when ΔφGaN is an Mg-doped GaN films grown with GaN buffer layers,” Jpn. J. integer multiple of 2π, which leads to a thickness of Appl. Phys. 30, L1708 (1991). 5. W. Götz, N. M. Johnson, J. Walker, D. P. Bour, and R. A. Street, “Activation of acceptors in Mg-doped GaN grown by meta- 1 λ lorganic chemical vapor deposition,” Appl. Phys. Lett. 68, d m − ; – 2 2n (1) 667 669 (1996). ITO 6. A. Mao, J. Cho, E. F. Schubert, J. K. Son, C. Sone, W. J. Ha, S. Hwang, and J. K. Kim, “Reduction of efficiency droop in m GaInN/GaN light-emitting diodes with thick AlGaN cladding where is an integer. For blue and green LEDs with layers,” Electron. Mater. Lett. 8,1–4 (2012). wavelength around 541 nm, the optimal ITO thick- 7. J. W. Orton, “Acceptor binding energy in GaN and related ness would lie around 67 nm on GaN. alloys,” Semicond. Sci. Technol. 10, 101–104 (1995).

10 August 2012 / Vol. 51, No. 23 / APPLIED OPTICS 5599 8. Y. Koide, H. Ishikawa, S. Kobayashi, S. Yamasaki, S. Nagai, J. 19. T. Margalith, O. Buchinsky, D. A. Cohen, A. C. Abare, M. Umezaki, M. Koike, and M. Murakami, “Dependence of elec- Hansen, S. P. DenBaars, and L. A. Coldren, “Indium tin oxide trical properties on work functions of metals contacting to contacts to gallium nitride optoelectronic devices,” Appl. Phys. p-type GaN,” Appl. Surf. Sci. 117/118, 373–379 (1997). Lett. 74, 3930–3932 (1999). 9. J. K. Sheu, Y. K. Su, G. C. Chi, P. L. Koh, M. J. Jou, C. M. 20. D. W. Kim, Y. J. Sung, J. W. Park, and G. Y. Yeom, “A study of Chang, C. C. Liu, and W. C. Hung, “High-transparency transparent indium tin oxide (ITO) contact to p-GaN,” Thin Ni/Au ohmic contact to p-type GaN,” Appl. Phys. Lett. 74, Solid Films 398/399,87–92 (2001). 2340–2342 (1999). 21. R.-H. Horng, D.-S. Wuu, Y.-C. Lien, and W.-H. Lan, “Low- 10. Y. C. Lin, S. J. Chang, Y. K. Su, T. Y. Tsai, C. S. Chang, S. C. resistance and high-transparency Ni/indium tin oxide ohmic Shei, C. W. Kuo, and S. C. Chen, “InGaN/GaN light emitting contacts to p-type GaN,” Appl. Phys. Lett. 79, 2925–2927 diodes with Ni/Au, Ni/ITO and ITO p-type contacts,” Solid- (2001). State Electron. 47, 849–853 (2003). 22. S.-M. Pan, R.-C. Tu, Y.-M. Fan, R.-C. Yeh, and J.-T. Hsu, “En- 11. J. K. Ho, C. S. Jong, C. C. Chiu, C. N. Huang, K. K. Shih, L. C. hanced output power of InGaN-GaN light-emitting diodes Chen, F. R. Chen, and J. J. Kai, “Low-resistance ohmic con- with high-transparency nickel-oxide-indium-tin-oxide ohmic tacts to p-type GaN achieved by the oxidation of Ni/Au films,” contacts,” IEEE Photon. Technol. Lett. 15, 646–648 (2003). J. Appl. Phys. 86, 4491–4497 (1999). 23. S. W. Chae, K. C. Kim, D. H. Kim, T. G. Kim, S. K. Yoon, B. W. 12. T. Margalith, O. Buchinsky, D. A. Cohen, A. C. Abare, M. Oh, D. S. Kim, H. K. Kim, and Y. M. Sung, “Highly transparent Hansen, S. P. DenBaars, and L. A. Coldren, “Indium tin oxide and low-resistant ZnNi/indium tin oxide ohmic contact on contacts to gallium nitride optoelectronic devices,” Appl. Phys. p-type GaN,” Appl. Phys. Lett. 90, 181101 (2007). Lett. 74, 3930–3932 (1999). 24. J.-O. Song, D.-S. Leem, J. S. Kwak, Y. Park, S. W. Chae, and 13. D. W. Kim, Y. J. Sung, J. W. Park, and G. Y. Yeom, “A study of T.-Y. Seong, “Improvement of the luminous intensity of light- transparent indium tin oxide (ITO) contact to p-GaN,” Thin emitting diodes by using highly transparent Ag-indium tin Solid Films 398/399,87–92 (2001). oxide p-type ohmic contacts,” IEEE Photon. Technol. Lett. 14. H. H. Berger, “Models for contacts to planar devices,” Solid- 17, 291–293 (2005). State Electron. 15, 145–158 (1972). 25. I. Hambergend and C. G. Granquist, “Evaporated Sn-doped 15. A. Weimar, A. Lell, G. Brüderl, S. Bader, and V. Härle, “Inves- In2O3 films: Basic optical properties and applications to en- tigation of low-resistance metal contacts on p-type GaN using ergy-efficient windows,” J. Appl. Phys. 60, R123–R160 (1986). the linear and circular transmission line method,” Phys. Stat. 26. M. J. Alam and D. C. Cameron, “Optical and electrical proper- Sol. (a) 183, 169–175 (2001). ties of transparent conductive ITO thin films deposited by sol– 16. L. Lewis, P. P. Maaskant, and B. Corbett, “On the specific con- gel process,” Thin Solid Films 377/378, 455–459 (2000). tact resistance of metal contacts to p-type GaN,” Semicond. 27. S. Boycheva, A. K. Sytchkovab, and A. Piegari, “Optical and Sci. Technol. 21, 1738–1742 (2006). electrical characterization of r.f. sputtered ITO films devel- 17. Deepak and H. Krishna, “Measurement of small specific oped as art protection coatings,” Thin Solid Films 515, contact resistance of metals with resistive semiconductors,” 8474–8478 (2007). J. Electron. Mater. 36, 598–605 (2007). 28. J. Szczyrbowski, A. Dietrich, and H. Hoffmann, “Optical and 18. J. O. Song, D. S. Leem, J. S. Kwak, O. H. Nam, Y. Park, and electrical properties of RF-sputtered indium–tin oxide films,” T. Y. Seong, “Low-resistance and highly-reflective Zn–Ni solid Phys. Status Solidi A 78, 243–252 (1983). solution/Ag ohmic contacts for flip-chip light-emitting diodes,” 29. As determined by spectroscopic ellipsometry on a 67 nm ITO Appl. Phys. Lett. 83, 4990–4992 (2003). layer on Si.

5600 APPLIED OPTICS / Vol. 51, No. 23 / 10 August 2012