indium-tin-oxide for transparent low-resistance [PDF]

tion in sequence of deposition): Ni/Au (5 nm∕ 5 nm) with a 1 min at 500 °C anneal in oxygen ambient;. NiZn/Ag (5 nmâˆ

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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

Our society’s drive for energy efficiency places high relevance on the identification of low-resistance ohmic contacts to wide bandgap group-III nitrides for use in light-emitting diodes [1,2] (LEDs) and third-generation solar cells [3]. In particular, transparent ohmic contacts to p-type GaN are of topical concern. Throughout the group-III nitrides, hole conduction is the more limited one compared to electron transport [4–6]. The high binding energy of the common Mg acceptor [7], its propensity for structural defect generation, and low hole mobility require large-area hole injection close to the optically active region. This can best be achieved by p-contact schemes that provide both low contact resistance and high optical transparency in a specified spectral region. Furthermore, the large binding energy of electrons at the valence band maximum of GaN limits suitable p-contact metals to those with the highest work function [8].

While contact resistance and current spreading are known to improve with increasing thickness of the metal stack, the transmittance decreases. For p-type GaN, semitransparent Ni/Au contacts are most commonly used. Reported specific contact resistance values mostly range from 4.4 × 10−3 to 5 × 10−4 Ω · cm2 [9,10], with the exception of only one report of a value as low as 4 × 10−6 Ω · cm2 [11]. A spectral transmittance around 70% to 80% in the visible spectrum has been reported [9,10]. ITO layers, on the other hand, have shown very high transparency with transmittance above 90% in the visible spectrum, but non-ohmic contact behavior [12], with specific contact resistance in the range of 10−1 Ω · cm2 [13]. In an attempt to improve both contact resistance and transparency, we here jointly study both in several metal/ITO contacts schemes— namely Ag/ITO, AgCu/ITO, Ni/ITO, NiZn/ITO—to pGaN and compare them to standard Ni/Au, NiZn/Ag and ITO contacts. 2. Experiments

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(0001) Ga-face p-type GaN layers, 200 nm thick, were grown by metal-organic vapor epitaxy on

The specific contact resistance (ρc ) of lateral contacts is generally measured using the transmission line method [14]. In this model, the ohmic contact is deposited on a thin film layer, whose total resistance is much lower than that of the supporting substrate underneath [14]. The resistance values of contact pairs with different pad spacing Rpp are analyzed as a function of spacing, and ρc and the sheet resistance can be extrapolated from the data. Typically, these 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 GaN layer makes shunt currents through the n-type layers underneath very likely and so distorts the interpreted value of ρc. Under low enough applied test voltage, however, the pn-junction between the parallel layers does not turn on and blocks parallel conduction through the n-side of the device [15,16]. We therefore limit our analysis of ρc to the 1 V range. Furthermore, inaccuracies in the layer and contact geometry have a strong effect on values derived from contacts on poorly conducting layers, such as p-type GaN [17]. We therefore paid particular attention to accurately account for the actual contact geometry within 5%. This results in an error of ρc less than 10% for our results.

Ni/Au (5 nm/5 nm) NiZn/Ag (5 nm/200 nm) ITO (200 nm) Ag/ITO (3 nm/67 nm)

Current (mA)

0.5

0.0

-0.5

-1.0

-2

-1

0

1

2

Voltage (V) Fig. 1. (Color online) Current-voltage characteristics of different contact schemes on p-GaN.

The current-voltage characteristics between a pair of the respective contact schemes to p-GaN are shown in Fig. 1 (5 μm contact spacing). The relative optical transmittance of the same contact layers on glass is shown in Fig. 2 and Fig. 3. We find that after annealing, the Ni/Au contacts (Fig. 1, black solid squares) readily show ohmic behavior with ρc  2.2 × 10−3 Ω · cm2 . In our experience, however, this value can vary strongly with minor details of the deposition and annealing process, source metal purity, and surface roughness of the p-GaN. For example, minor C contamination of the

Relative Transmittance (%)

3. Results

1.0

Ni/ITO 3 nm/130 nm 0 annealed 550 C 1 min

100

200 nm ITO

80

Ni/ITO 1 nm/60 nm

60

Ni/ITO 3 nm/130 nm 0 Ni/Au after anneal annealed 450 C 1min

40 Ni/Au before anneal 20 on glass 0 300

400

500

600

700

800

Wavelength (nm)

Fig. 2. (Color online) Transmittance of contact films on 0.2 mm glass slides for various film stacks.

Relative Transmittance (%)

unintentionally doped GaN (u-GaN) templates, 4 μm thick, on c-plane sapphire. Free hole concentrations of 4 × 1017 cm−3 at room temperature were achieved using Mg doping at a concentration of 1019 cm−3. On top, a p -GaN, 10 nm thick, Mg-doped (up to 1020 cm−3 ) contact layer was grown. As substrates for transmittance measurements, float glass slides, 0.2 mm thick, and n-GaN on double side polished (DSP) sapphire were used. Contact metal stacks were prepared by means of an e-beam evaporation and subsequent rapid thermal annealing on both templates using each of the following schemes (notation in sequence of deposition): Ni/Au (5 nm ∕ 5 nm) with a 1 min at 500 °C anneal in oxygen ambient; 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 ambient; Ag/ITO (3 nm ∕ 130 nm, 3 nm ∕ 67 nm), AgCu/ITO (3 nm ∕ 130 nm), Ni/ITO (3 nm ∕ 130 nm, 1 nm ∕ 60 nm, 3 nm ∕ 100 nm, and 3 nm ∕ 67 nm), and NiZn/ITO (3 nm ∕ 130 nm), each with a 1 min at 550 °C anneal in oxygen ambient; Ni/ITO (3 nm ∕ 130 nm) with a 1 min at 450 °C anneal in oxygen ambient. We define relative transmittance of the layer stack on a substrate by the transmitted power in ratio with the power transmitted through a bare substrate. We determine the ratio experimentally by simultaneous measurement of both samples in parallel beams of a spectrophotometer. To properly account for multiple reflection effects in transmittance, we tested the metal stacks on both kinds of substrates, the common glass slide and GaN on DSP sapphire.

100

ITO (100 nm) Ni/ITO (3 nm/100 nm) 90

80

ITO (67 nm) Ag/ITO (3 nm/67 nm) Ni/ITO (3 nm/67 nm)

70

60 300

on glass 400

500

600

700

800

Wavelength (nm) Fig. 3. (Color online) Transmittance of contact films on 0.2 mm glass slides for ITO film with and without a thin Ni or Ag layer. 10 August 2012 / Vol. 51, No. 23 / APPLIED OPTICS

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line) again is only less than 5% smaller than for bare ITO 100 nm (black solid line) and 67 nm (blue, short dashed line) in the visible spectrum. Apparently, the additional metal layer of Ni or Ag can indeed significantly lower ρc below that of the bare ITO contact while maintaining a rather high transmittance. The specific contact resistance of an Ni/ITO (3 nm ∕ 130 nm) film decreases when lowering the annealing temperature from 550 °C to 450 °C. The relative transmittance of an Ni/ITO (3 nm ∕ 130 nm) film on glass after annealing at 550 °C (Fig. 2, red solid star) reaches 97% at 510 nm. When lowering the annealing temperature to 450 °C (Fig. 2, orange open star), the relative transmittance decreases to 67% at 510 nm. The increase in optical transmittance in ITO-based contacts with temperature can be attributed to the increase of structural homogeneity and crystallinity of ITO film [25,26]. As a function of ITO layer thickness on glass, the peak for relative transmittance is found to move from 716 nm at 200 nm (Fig. 2, black square) over 510 nm at 130 nm (with 3 nm Ni) (Fig. 2, red solid star) to 417 nm at 100 nm (Fig. 3, black solid line). For an ITO thickness of 67 nm on glass, no peak in the relative transmittance is observed in the visible wavelength range. Apparently, the maximum of the relative transmittance can be tuned in wavelength by a variation of the ITO thickness. In the next step, the relative transmittance of ITO on GaN and glass is directly compared to the respective bare substrates (Fig. 4). For 67 nm ITO on glass (pink line, see labels), a relative transmittance is found to be only 78% around 500 nm. For GaN on DSP sapphire (red line), the relative transmittance oscillates around 100%. This obviously is due to its direct comparison with a very similar layered structure in the reference beam path producing osciallations as well. For the 67 nm ITO film on GaN on DSP sapphire (blue line), the relative transmittance increases over that of the bare GaN on DSP sapphire (red line). At 530 nm it reaches as high as 107%. With the insertion of an additional 3 nm Ni layer (black line), the relative transmittance reduces somewhat to 103%, and to 97% with the insertion of a 3 nm Ag layer instead (green line). Overall, however, 110

Relative Transmittance (%)

Ni source from the graphite crucible has shown to increase ρc by more than an order of magnitude. In addition, we find that Ni/Au contacts on p-GaN surfaces, that have been roughened to increase light extraction, show higher ρc and tend to induce current crowding. For such conventional contacts, a spectral transmittance around 70% to 80% in the visible spectrum has been reported [9,10]. We here find a relative transmittance of 40% before annealing (Fig. 2, blue open circles) and 75% after annealing (Fig. 2, magenta solid circles) at a wavelength of 500 nm. This is in line with the literature reports [9,10]. When contact transparency is not required, such as in vertical LED structures, the nontransparent NiZn/Ag contact to p-type GaN provides an attractive alternative [18]. Here we achieve reliable low-resistance ohmic contacts with ρc  1.6 × 10−3 Ω · cm2 (Fig. 1, red crosses). In an attempt to increase transparency, the Ag layer thickness was reduced to 5 nm, resulting, however, in much higher specific contact resistance of 0.1 Ω · cm2. ITO is a well-known transparent conducting film that has widely been employed as contact to pGaN [19,20]. Only rectifying contacts were achieved with ITO on p-GaN [19,20]. For 200 nm ITO contacts (Fig. 1, blue open triangles), we only find rectifying behavior at ρc as high as 0.1 Ω · cm2 . For the same contact structure on glass (Fig. 2, black solid squares), we found a relative transmittance above 80% across the entire visible spectrum from 380 to 780 nm, in line with the literature [19,20], and a peak of 98% near 716 nm. While such transparency is desirable, the poor electrical performance limits its application as a p-contact material. An improvement to ρc of ITO contacts to p-GaN has been reported by depositing first a thin metal layer [21–24]. For Ag/ITO (3 nm ∕ 67 nm) (Fig. 1, magenta open circles), we find ohmic contacts with ρc  0.03 Ω · cm2 , while Ag/ITO (3 nm ∕ 130 nm) reaches 0.012 Ω · cm2 , and AgCu/ITO (3 nm ∕ 130 nm) reaches 0.02 Ω · cm2. The improvement in contact resistance is reported to come from the formation of an Ag–Ga solid solution that produces deep-acceptor like Ga vacancies near the GaN surface region under the contacts [24]. The influence of the thin extra metal layer on the relative transmittance in turn is rather small. On glass, transmittance (Fig. 3) of Ag/ITO (3 nm ∕ 67 nm) (magenta, dash-dotted line) is less than 5% smaller than for bare ITO (67 nm) (blue, short dashed line) in the visible spectrum. The same trend can be found for a thin Ni layer underneath ITO. For Ni/ITO (3 nm ∕ 130 nm, annealed at 550 °C) we find ρc  0.2 Ω · cm2 ; for Ni/ITO (3 nm ∕ 130 nm, annealed at 450 °C) ρc  0.1 Ω · cm2 ; for Ni/ITO (3 nm ∕ 67 nm) ρc  0.3 Ω · cm2 ; and for NiZn/ITO (3 nm ∕ 130 nm) ρc  0.06 Ω · cm2 . The formation of a NiO interfacial layer is believed to be the reason for lowering the contact resistance [10,11]. On glass, transmittance (Fig. 3) of Ni/ITO 3 nm ∕ 100 nm (red, dashed line) and Ni/ITO 3 nm ∕ 67 nm (green, dash-dot-dotted

on GaN on DSP sapphire

ITO (67 nm)

Ni/ITO (3 nm/67 nm) 100

GaN only Ag/ITO (3 nm/67 nm)

90

80

400

67 nm ITO on glass

500

600

700

Wavelength (nm) Fig. 4. (Color online) Relative transmittance of ITO (67 nm), Ni/ITO (3 nm ∕ 67 nm), Ag/ITO (3 nm ∕ 67 nm) on GaN and glass.

relative transmittance values above 96% are obtained for wavelengths longer than 500 nm. Apparently, even with a conductivity-enhancing thin metal layer, it should become possible to enhance the light extraction from GaN at a desired wavelength by a proper choice of the ITO thickness. 4. Discussion

The observations for transmission on glass and GaN can be explained by the following reasoning. The absorption coefficient for bare ITO is small in the visible wavelength range [27]. For wavelengths shorter than 350 nm, strong absorption due to interband absortion in ITO is expected [28]. ITO-based contacts show an oscillation of the transmittance with a period that resembles interference fringes in an optically thin layer. The spectral variations with the thickness of ITO suggest that an optical etalon is formed by the thin ITO film between glass and air. The data show that the insertion of the thin metal layer results in only a small reduction of transmittance. This is explained by the thickness being much smaller than the lights penetration depth. Applying the standard theory (Δφglass  2nITO d ∕ λ × 2π) and refractive indices of n  1.5 for glass, and a range of n  1.94740 nm− 2.18410 nm for ITO [29] constructive interference can be expected in the following wavelength: 780 nm and 436 nm for an ITO thicknesses of 200 nm; 526 nm for 130 nm, and 434 nm for 100 nm ITO. This interpretation can well describe the observed variations for wavelengths above 350 nm. From this we also find that for 67 nm ITO on glass, the interference should be destructive in the visible range near 500 nm leading to a low transmittance. This is in line with our experimental findings in Fig. 4 (pink line). At shorter wavelength, optical absorption dominates the transmittance. Contact transmittance is typically characterized on glass only [22,24]. The aspect of layer thickness interference, however, is strongly dependent on the actual carrier itself and therefore should only be characterized on the GaN layer of an actual GaN-based LED. While the index of refraction of glass is lower than that of ITO, that of GaN (n  2.5) is higher. This leads to a phase change of π at the interface between ITO and GaN. (ΔφGaN  2nITO d ∕ λ × 2π  π). Therefore, wavelengths of destructive interference on glass lead to constructive interference on GaN. The optimal ITO thickness for a transmittance peak is obtained when ΔφGaN is an integer multiple of 2π, which leads to a thickness of   1 λ d m− ; 2 2nITO

(1)

where m is an integer. For blue and green LEDs with wavelength around 541 nm, the optimal ITO thickness would lie around 67 nm on GaN.

5. Conclusion

In conclusion, we jointly analyzed specific contact resistance and relative transmittance of a wide range of contact schemes on p-type GaN in the green spectral region. The conventional Ni/Au (5 nm ∕ 5 nm) stack shows a relatively good ohmic behavior with ρc  2.2 × 10−3 Ω · cm2 , while the transmittance is only around 75%. In turn, NiZn/Ag (5 nm ∕ 200 nm) shows a very good ohmic contact with low resistance of ρc  1.6 × 10−3 Ω · cm2 but it is strongly absorbing. Pure ITO films are found to be highly transparent but only rectifying contacts can be achieved. The relative transmittance spectra of ITO-based contacts are dominated by thickness interference fringes the boundary conditions of which differ between GaN and standard glass. By adjusting the ITO layer thickness to 67 nm on GaN, the relative transmittance reaches 106% of that of the bare GaN layer near 530 nm. This can be exploited to maximize the relative transmittance at a desired wavelength range. To reduce contact resistance significantly a thin metal layer of Ni, Ag, or their related alloys of NiZn, or AgCu is inserted before the ITO with only minor losses to the spectral transmittance. By jointly optimizing ITO thickness and metal layer stack we find a best performance for a Ag/ITO (3 nm ∕ 67 nm) contact with a relative transmittance of 97% of the bare GaN layer near 530 nm and a specific ohmic contact resistance of 0.03 Ω · cm2. This contact seems ideally suited for high-performance GaN-based LEDs in the longer green, yellow, and red spectral regions. This work was supported by a DOE/NETL SolidState Lighting Contract of Directed Research under DE-EE0000627. This work was also supported by the National Science Foundation (NSF) Smart Lighting Engineering Research Center (# EEC-0812056). References and Notes 1. I. Akasaki and H. Amano, “Crystal growth and conductivity control of group III nitride semiconductors and their application to short wavelength light emitters,” Jpn. J. Appl. Phys. 36, 5393–5408 (1997). 2. I. Akasaki and C. Wetzel, “Future challenges and directions for nitride materials and light emitters,” Proc. IEEE 85, 1750–1751 (1997). 3. Y. Kuwahara, T. Fujii, T. Sugiyama, D. Iida, Y. Isobe, Y. Fujiyama, Y. Morita, M. Iwaya, T. Takeuchi, S. Kamiyama, I. Akasaki, and H. Amano, “GaInN-based solar cells using strained-layer GaInN/GaInN superlattice active layer on a freestanding GaN substrate,” Appl. Phys. Express 4, 021001 (2011). 4. S. Nakamura, M. Senoh, and T. Mukai, “Highly p-typed Mg-doped GaN films grown with GaN buffer layers,” Jpn. J. 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 metalorganic chemical vapor deposition,” Appl. Phys. Lett. 68, 667–669 (1996). 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 GaInN/GaN light-emitting diodes with thick AlGaN cladding layers,” Electron. Mater. Lett. 8, 1–4 (2012). 7. J. W. Orton, “Acceptor binding energy in GaN and related alloys,” Semicond. Sci. Technol. 10, 101–104 (1995). 10 August 2012 / Vol. 51, No. 23 / APPLIED OPTICS

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