We report the improved performance of InGaN/GaN-based light-emitting diodes (LEDs) through Ag reflectors combined with a Zn middle layer. It is shown that the Zn middle layer (5 nm thick) suppresses the agglomeration of Ag reflectors by forming ZnO and dissolving into Ag. The Ag/Zn/Ag contacts show a specific contact resistance of 6.2 × 10−5 Ωcm2 and reflectance of ~83% at a wavelength of 440 nm when annealed at 500 °C, which are much better than those of Ag only contacts. Blue LEDs fabricated with the 500 °C-annealed Ag/Zn/Ag reflectors show a forward voltage of 2.98 V at an injection current of 20 mA, which is lower than that (3.02 V) of LEDs with the annealed Ag only contacts. LEDs with the 500 °C-annealed Ag/Zn/Ag contacts exhibit 34% higher output power (at 20 mA) than LEDs with the annealed Ag only contacts.
© 2012 OSA
For solid-state lighting application, the fabrication of high-power and high-efficiency GaN-based light-emitting diodes (LEDs) is essential. In this respect, vertical-injection GaN-based LEDs (VLEDs) have been widely investigated [1–6]. Compared to conventional lateral-geometry LEDs, VLEDs mounted on conducting supporters have several advantages, e.g. better current injection, excellent heat dissipation, and enhanced reliability with respect to electrostatic discharge [1–6]. VLEDs demand low-resistance ohmic contacts to both N-polar n-GaN and Ga-polar p-GaN. In addition, p-type contacts require high reflectance in order to maximize the extraction efficiency. Note that silver (Ag) is the most commonly used reflector because it exhibits reasonable ohmic with p-GaN [7–11]. However, Ag only contacts suffer from thermal degradation (e.g. agglomeration) when annealed at temperatures above 300 °C in air [7–11]. Thus, to improve the thermal instability, various approaches, such as the introduction of interlayers, capping layers, or Ag alloys, have been suggested until now. It was found that the use of interlayers (such as oxidized NiO/Au , a ZnNi solid solution , and Cu-doped indium oxide  was effective in improving both the reflectance and the contact resistivity of Ag contacts. For instance, Hibbard et al. , investigating the use of NiO/Au interlayers on the performance of GaN-based LEDs, showed that back surface light emission is ~70% higher than that did LEDs with the Ni/Au contact. Song et al. reported that the introduction of a transparent Cu-doped In2O3 (CIO) layer was fairly effective in improving the electrical properties of Ag contacts. It was shown that LEDs fabricated with the CIO/Ag contacts gave a forward voltage of ~3.0 V at 20 mA, which is much better than that (3.36 V) of LEDs made with Ag only contacts. Furthermore, the thermal stability was shown to be significantly improved when capping layers, such as NiZn , La films , and Ru , were used. For example, Chen et al. , investigating the electrical and optical properties of Ag contact with a La capping layer contacts, reported that the Ag/La bilayer contacts showed a reflectance of 91% at 460 nm and a contact resistivity of 1.6 × 10−4 Ωcm2 when annealed at 450 °C for 1 min. The smooth surface morphology of the annealed bilayer contacts was attributed to the formation of disordered La2O3. Furthermore, Son et al. , investigating the effect of a Ru overlayer on the electrical and optical properties of Ag-Cu alloy contact to p-GaN, reported that the AgCu alloy/Ru contacts produced low contact resistivity as well as high reflectance when annealed at 400 °C in air.
In this work, we introduced a 5-nm-thick Zn layer in between the two Ag layers. The Zn middle layer was employed to serve two functions: first a capping layer for the underlying Ag layer; second, an interlayer for the superjacent Ag layer. We investigated the effect of the Zn middle layer on the electrical and thermal properties of Ag reflectors to p-GaN and consequently on the performance of InGaN/GaN-based LEDs.
Metalorganic chemical vapor deposition was used to grow blue (440 nm) InGaN/GaN multiple quantum-well (MQW) LED structures on (0001) sapphire substrates. The LED structures consist of a 0.15-μm-thick p-type GaN:Mg (na = 5 × 1017 cm−3) layer, a 20-nm-thick AlGaN electron blocking layer, a 0.1-μm-thick active layer, and a 2.0-μm-thick spreading layer, a 4.0-μm-thick n-type GaN:Si (nd = 5 × 1018 cm−3) layer, and a 2.0-μm-thick undoped GaN layer on the sapphire substrate. Prior to the lithography, the GaN samples were treated with a buffered oxide etch solution for 1 min and rinsed in de-ionized (DI) water. Circular transfer length method (CTLM) patterns were defined by the standard photolithographic technique for measuring specific contact resistance. The outer radius of CTLM patterns was fixed to be 200 μm and the gap spacing between outer and inner radii was varied from 5 to 40 μm. Prior to metal deposition, all samples were treated with a diluted HCl (HCl: DI water = 1: 1) solution for 1 min, rinsed in DI water, and blown dry in a N2 stream. After that, Ag/Zn/Ag (100 nm/5 nm/100 nm) films were deposited on LED samples by electron beam deposition under a base pressure of 6 × 10−7 Torr. For comparison, Ag films (200 nm) were also prepared. Some of the samples were annealed at 500 °C for 1 min in air. For LED samples, Cr/Ni/Au (25 nm/25 nm/50 nm) layers were deposited as an n-type ohmic electrode. Current-voltage (I-V) measurement was carried out by a high-current source-measuring unit (Keithley 238). X-ray photoelectron spectroscopy (XPS, Sigma Probe model) was performed using an Al Kα X-ray source (1486.6eV) in an UHV system in order to characterize the surface characteristics and to understand ohmic mechanisms. In addition, LED chips (500 × 250 μm2 in size) were fabricated and their optical outputs were examined by a Newport dual channel powermeter.
3. Results and discussion
Figure 1 shows the typical I-V characteristics of Ag only (200 nm) and Ag(100 nm)/Zn(5 nm)/Ag(100 nm) contacts before and after annealing at temperatures of 300 and 500 °C in air. The two schemes exhibit similar temperature dependence of the electrical behavior. In other words, both the as-deposited contacts show non-ohmic behavior. Their electrical properties become improved upon annealing, although the 300 °C-annealed Ag/Zn/Ag contacts still show non-linear I-V behavior. Both of the schemes reveal good ohmic behavior when annealed at 500 °C. Measurement showed that the 500 °C-annealed Ag only and Ag/Zn/Ag contacts give specific contact resistances of 5.4 × 10−4 and 6.2 × 10−5 Ωcm2, respectively.
The reflectance of the Ag only (200 nm) and Ag(100 nm)/Zn(5 nm)/Ag(100 nm) contacts before and after annealing at 300 and 500 °C is show in the inset of Fig. 1. For the Ag only samples, their reflectance is significantly degraded upon annealing, which is related to the presence of voids formed at the Ag/GaN interface due to agglomeration (as will be shown later). However, the reflectance of the Ag/Zn/Ag samples is not seriously degraded after annealing, although the 500 °C-annealed sample shows somewhat lower reflectance in the range 400 – 530 nm than the 300 °C-annealed sample. The Ag only and Ag/Zn/Ag contacts reveal reflectances of 61% and 83% at 440 nm, respectively, after annealing at 500 °C.
Figure 2 exhibits the typical I-V characteristics of blue InGaN/GaN MQW LEDs fabricated with the Ag only and Ag/Zn/Ag contacts before and after annealing at 500 °C. The LEDs with the 500 °C-annealed Ag/Zn/Ag contacts show a forward-bias voltage of 2.98 V at an injection current of 20 mA. On the other hand, the LEDs with the annealed Ag only contacts give a higher forward-bias voltage of 3.02 V at 20 mA. The series resistance of the LEDs with the annealed Ag/Zn/Ag contacts was 5 Ω, whereas the LEDs with the annealed Ag only contacts exhibited a higher series resistance of 6.9 Ω.
The light output-current (L-I) characteristics of the LEDs fabricated with the Ag only and Ag/Zn/Ag contacts as a function of the forward drive current are shown in the inset of Fig. 2. The LEDs fabricated with the 500 °C-annealed Ag/Zn/Ag contacts exhibit 18% and 25% higher light output power (at 20 mA) than the LEDs with the Ag only contacts annealed at 300 and 500 °C, respectively. This is consistent with the emission images (at 0.2 mA) of the LEDs fabricated with the Ag only and Ag/Zn/Ag reflectors (Fig. 3 ). A comparison shows that unlike the 500 °C-annealed Ag only contacts (Fig. 3(a)), the 500 °C-annealed Ag/Zn/Ag contact serves as a superior reflector, preventing the leakage of the emitted light (Fig. 3(b)). The leakage is due to the voids formed by agglomeration, as clearly demonstrated by their surface morphologies (the insets in Fig. 3). Unlike the Ag only sample (the inset in Fig. 3(a)), the Ag/Zn/Ag sample reveals a smooth surface without voids (the inset in Fig. 3(b)).
To analyze the chemical bonding states of Ga, XPS examination was performed on the Ag/Zn/Ag samples. During the XPS examination, the sample surface was Ar + ion-sputtered, and Zn, Ag, and Ga photoelectron signals were carefully monitored. The Ga 2p core levels were finally collected when only the Ga photoelectron peak (i.e., from Ga-N bonding) was detected. Figure 4 shows the XPS Ga 2p core level spectra obtained from the Ag/Zn/Ag contacts on p-GaN before and after annealing at 500 °C. XPS core level peak fittings were carried out with a Shirley-type background and Lorentzian–Doniac–Sunsic curves convoluted with a Gaussian profile. The Ga 2p core levels are composed of components from the Ga-N and Ga-O bonds. It is noted that the Ga 2p core level for the 500 °C-annealed sample shifts toward the lower binding-energy side by 0.25 eV compared to that of the as-deposited sample. This indicates that annealing causes the surface Fermi level to shift toward the valence band edge , resulting in a reduction in the band-bending of p-GaN, namely, a lowering of the Schottky barrier height (SBH). The Ga 2p peak shift is indicative of a change of the band-bending since the N 1s core level spectra exhibit a similar shift behavior . All the samples are shown to contain a small amount of oxygen at the interface region.
XPS examination was also carried out to characterize interfacial reactions between the Ag/Zn/Ag layer and GaN. Figure 5 shows the XPS depth profiles obtained from the Ag/Zn/Ag contacts on p-GaN before and after annealing at 500 °C. For the as-deposited sample (Fig. 5(a)), a Zn layer is in between the top and bottom Ag layers. Upon annealing at 500 °C (Fig. 5(b)), most of Zn atoms were out-diffused toward the sample surface region, although a small amount of Zn atoms dissolved into the Ag layer (as indicated by the arrows) . It is noted that oxygen was introduced into the surface region during annealing, being indicative of the formation of ZnO. A comparison shows that for the 500 °C-annealed sample, some amount of Ga were out-diffused into the Ag layer to form an Ag-Ga solid solution , which results in the generation of Ga vacancies at the surface region.
The electrical and optical properties of the Ag/Zn/Ag contacts were significantly improved upon annealing at 500 °C for 1 min in air. The annealing-induced improvement can be explained in terms of reduction in the effective SBH due to the shift of the surface Fermi level toward the valence-band edge (Fig. 4) and the improved thermal stability (Fig. 3). The surface Fermi level shift is associated with the formation of a Ag-Ga solid solution, generating acceptor-like Ga vacancies near the GaN surface region and so increase in the carrier concentration at the surface region [14,17]. It was shown that the use of the Zn middle layer prevented the formation of voids. This increases the contact areas between the Ag/Zn/Ag reflector and GaN as compared to the Ag only contact/GaN. It was reported that an Ag only layer is agglomerated as a result of surface diffusion to reduce the total free energy and the bulk diffusion of Ag atoms by oxygen–vacancy interaction [18,19]. Thus, agglomeration could be reduced by suppressing the formation of the oxygen–vacancy cluster . The ZnO layer formed on the sample surface (Fig. 5) would serve as a diffusion barrier to oxygen atoms during annealing. In addition, the presence of Zn atoms within Ag (Fig. 5(b)) might also contribute to the prevention of agglomeration of Ag layer, although this remains to be confirmed. The fact that the LEDs with the Ag/Zn/Ag contacts showed higher output power than the ones with the Ag only contacts (Fig. 2) is consistent with the combined results of the electrical and optical properties. It is worth mentioning that the output power of LEDs is more dominantly dependent on the optical properties than the electrical properties.
4. Summary and conclusion
We investigated the effect of the 5-nm-thick Zn middle layer on the thermal and electrical properties of Ag-based contacts. It was shown that the use of the Zn middle layer significantly improved the thermal stability of the Ag contacts. LEDs fabricated with the 500 °C-annealed Ag/Zn/Ag reflectors produced 25% higher output power (at 20 mA) than did LED with the 500 °C-annealed Ag only contacts. These results imply that the use of the Zn middle layer could serve as a potentially important processing tool for the fabrication of high-power InGaN/GaN-based vertical LEDs.
This work was supported by the World Class University program through the National Research Foundation of Korea funded by MEST (R33-2008-000-10025-0) and the Industrial Technology Development Program funded by the Ministry of Knowledge Economy (MKE), Korea.
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