1. Introduction

InGaN/GaN-based vertical-configuration light-emitting diodes (VLEDs) have attracted a great deal of interest because of their potential in solid-state lighting applications [1,2]. VLEDs were fabricated by different methods, such as laser lift-off (LLO) [3], chemical lift-off [4], and growth on SiC [5] and Si substrates [6], after which the substrates are etched away. To fabricate high-efficiency VLEDs, the formation of reliable N-face n-type ohmic contact as well as high-quality p-type reflectors [7] is indispensable. Ti/Al-based contacts to Ga-polar n-GaN yielded ohmic behavior upon annealing at temperatures above 600 °C [8–10]. However, this is not the case for N-polar n-ohmic contacts [11–16]. Ti/Al-based contacts to metalorganic chemical vapor deposition (MOCVD)-grown Ga-polar n-GaN were ohmic at 700 °C [13], while the same contacts on MOCVD-grown N-polar n-GaN were non-ohmic. Dependence of such polarity on the electrical characteristics was described by the opposite directions of the spontaneous and piezoelectric polarization fields [13]. Furthermore, Ti/Al ohmic contacts to LLO-prepared N-polar n-GaN were electrically degraded when annealed at 400 °C [14]. This temperature dependence was explained by the complex surface states of N-polar n-GaN. Jeon et al. [16], investigating the electrical properties of Ti(30 nm)/Al(200 nm) and TiN(30 nm)/Al(200 nm) contacts to LLO-prepared N-polar n-GaN, showed that the use of TiN resulted in better electrical performance as compared to Ti/Al contacts when annealed at 300 °C. The electrical degradation of the N-polar Ti/Al contacts annealed in the range of 300–500 °C was described by the generation of acceptor-like Ga vacancies near the n-GaN surface caused by the outdiffusion of Ga. On the basis of the possible mechanism, Jeon et al. [17] employed an Al-Ga solid solution layer to repress the outdiffusion of Ga atoms. It was shown that unlike Ti/Al contacts, the Al-Ga solid solution/Ti/Al contacts remained ohmic after annealing at 250 °C. Jeon et al. [18], investigating the effect of laser-annealing on the electrical properties of Ti/Al-based contacts to N-polar n-GaN, also reported that as opposed to the untreated samples, the laser-annealed samples were ohmic with a specific contact resistance of 2.6–3.9 × 10⁻⁴ Ωcm² after annealing at 250 °C. Furthermore, the laser-annealed samples remained electrically stable even after annealing for 60 min at 300 °C.

In this study, we investigated the effect of a Ta diffusion barrier on the thermal stability and electrical properties of Ti/Al-based contacts to N-polar n-GaN. VLED chips are also fabricated with the Ta-based and Ti/Al/Au contacts and their output performance is compared.

2. Experiment procedures

For the preparation of N-polar n-GaN samples, LED wafers, consisting of ~150-nm-thick p-GaN, multiple quantum-well layers, 4-μm-thick n-GaN (n_d = 5 × 10¹⁸ cm⁻³), 2-μm-thick undoped GaN and a sapphire substrate, were grown. Before LLO process, the wafer was bonded to a Si wafer using Au-Sn alloy by thermal compression at 300 °C. The LLO process was subsequently performed using a KrF excimer laser (248 nm) to separate the sapphire substrate, followed by HCl: DI water (1: 2) cleaning for 5 min to remove the Ga droplets. The undoped GaN layers were then etched by an inductively coupled plasma reactive ion etching (ICP-RIE) system to expose the n-GaN layer. To investigate the contact resistivity of the samples, the circular transfer length method (CTLM) patterns were defined using the standard photolithography and lift-off techniques. The outer dot radius of the CTLM patterns was 200 μm and the spacing between the inner and outer radii varied from 5 to 40 μm. Prior to metal deposition, the samples were cleaned by a HCl: DI water (1: 2) solution for 1 min, followed by rinsing in DI water for 5 min. Ti(10 nm)/Ta(10 nm)/Al(150 nm) films were deposited on the n-GaN layers by an RF sputtering system, on which a Au layer(30 nm) was deposited by electron-beam evaporation. For comparison, Ti(10 nm)/Al(150 nm)/Au(30 nm) films were also prepared. Some of the samples were rapid-thermal-annealed at 250 °C for 1 min in N₂ ambient, after which some of the annealed samples were annealed at 250 °C up to 600 min in an oven. This temperature was chosen because annealing at temperatures exceeding 300 °C causes the warping of the LLO-prepared LEDs/metal supporter. The current-voltage (I–V) data were measured at room temperature using a parameter analyzer. X-ray photoemission spectroscopy (XPS) examinations were performed to understand the electrical degradation mechanisms.

3. Results and discussion

Figure 1 exhibits the typical I-V characteristics of Ti(10 nm)/Al(150 nm)/Au(30 nm) and Ti(10 nm)/Ta(10 nm)/Al(150 nm)/Au(30 nm) contacts on N-polar n-GaN as a function of the annealing time at 250 °C. Both of the as-deposited Ti/Al/Au and Ti/Ta/Al/Au contacts display ohmic behavior. With an increase in the annealing time, the electrical properties of the Ti/Al/Au contacts become gradually degraded. On the other hand, the Ti/Ta/Al/Au contacts remain ohmic and only slightly changed. The specific contact resistances were determined from the plots of the measured resistance versus the spacing between the CTLM pads. The specific contact resistivities of the as-deposited Ti/Al/Au and Ti/Ta/Al/Au contacts were measured to be 3.3 × 10⁻⁴ and 1.2 × 10⁻⁴ Ωcm², respectively. After 600 min, however, the annealed Ti/Al/Au and Ti/Ta/Al/Au contacts showed contact resistivities of 2.2 × 10⁻³ and 2.8 × 10⁻⁴ Ω·cm², respectively. A comparison shows that the introduction of the Ta layer effectively improves the thermal stability of Ti/Al-based contacts to N-polar n-GaN.

 

Fig. 1 The typical I-V characteristics of Ti(10 nm)/Al(150 nm)/Au(30 nm) and Ti(10 nm)/Ta(10 nm)/Al(150 nm)/Au(30 nm) contacts on N-polar n-GaN as a function of annealing time at 250 °C.

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Figure 2 illustrates the typical light output power of LEDs fabricated with the Ti/Al/Au and Ti/Ta/Al/Au contacts (annealed for 600 min) as a function of the forward current. Unlike the LEDs with the Ti/Ta/Al/Au contacts, the LEDs with the Ti/Al/Au contact become saturated when the current exceeds 300 mA. It is noted that the LEDs with the Ta barrier layer exhibit 8.3% higher output (at 300 mA) than the LEDs without the Ta barrier layer. This result is consistent with the I-V characteristics of the contacts (Fig. 1).

 

Fig. 2 The light output of LEDs fabricated with Ti/Al/Au and Ti/Ta/Al/Au contacts as a function of the forward current.

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To investigate the chemical bonding states of Ga, XPS examinations were made of the Ti/Al/Au and Ti/Ta/Al/Au samples before and after annealing at 250 °C for 600 min. Figure 3 illustrates the Ga 2p core levels taken from the contact/GaN interface regions. XPS core-level peak fittings were performed with a Shirley type background and Lorentzian–Doniac–Sunsic curves convoluted with a Gaussian profile. For the Ti/Al/Au contact, the Ga 2p core level was shifted toward the lower binding-energy side by 0.11 eV after annealing [Fig. 3(a)]. This indicates that annealing caused the upward band bending to become increased, resulting in an increase in the Schottky barrier height (SBH) for the Ti/Al/Au contact. For the Ti/Ta/Al/Au contacts [Fig. 3(b)], the Ga 2p core levels show multi-peaks, because they were overlapped with the Ta 4f core levels. Deconvolution shows that the Ga 2p core levels were virtually unchanged after annealing for 600 min. This is indicative of an insignificant change in the band bending, thereby leading to a negligible change in the SBH for the Ti/Ta/Al/Au samples. Besides, the normalized N/Ga atomic ratios were obtained from the integral intensity of the XPS N 1s peak to that of the Ga 2p peak (Ga-N bond) with reference to that of the as-deposited samples. The normalized ratios for the Ti/Al/Au and Ti/Ta/Al/Au samples were measured to be 1.42 and 1.07, respectively. This implies that unlike the Ti/Ta/Al/Au sample, the N-face n-GaN surface region under the Ti/Al/Au contact is Ga-deficient, namely, Ga vacancies are generated in the surface region.

 

Fig. 3 The Ga 2p core levels taken from the contact/GaN interface regions of (a) Ti/Al/Au and (b) Ti/Ta/Al/Au samples before and after annealing at 250 °C for 600 min.

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Figure 4 shows XPS depth profiles obtained from the Ti/Al/Au and Ti/Ta/Al/Au contacts on GaN before and after annealing at 250 °C for 600 min. The XPS results show no intermixing in the as-deposited samples [Figs. 4(a) and 4(c)]. It is worth noting that for all of the samples, some amounts of oxygen are present at the Al/Au interface. This is due to the fact that after the sputter-deposition of the Ti/(Ta/)Al samples, they were exposed to air before loading into the e-beam deposition for depositing a Au layer. A comparison exhibits that for the Ti/Al/Au sample [Fig. 4(b)], annealing caused the outdiffusion of Ga atoms from GaN into the n-contacts, whereas the Ga outdiffusion was negligible in the Ti/Ta/Al/Au sample [Fig. 4(d)]. This is consistent with the normalized N/Ga atomic ratio results. It is noted that the Ta layer remained stable even after annealing for 600 min. In addition, for the annealed Ti/Al/Au sample, some amounts of oxygen were introduced into the Ti/GaN interface region after annealing for 600 min. The XPS depth profile results show that the Ta layer serves as an effective barrier to the indiffusion of Al and oxygen toward the GaN.

 

Fig. 4 XPS depth profiles obtained from (a) and (b) Ti/Al/Au, and (c) and (d) Ti/Ta/Al/Au samples on GaN before and after annealing at 250 °C for 600 min, respectively.

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The I-V measurements exhibited that both types of the as-deposited Ti/Al/Au and Ti/Ta/Al/Au contacts to N-polar n-GaN displayed ohmic characteristics. The non-alloyed ohmic contacts are believed to form due to the presence of donor-like defects, i.e., V_N or O_N, which were formed by ICP-RIE [19]. After annealing at 250 °C for 600 min, the Ti/Al/Au contact became non-ohmic, whilst the Ti/Ta/Al/Au contacts remained ohmic. The different behavior could be interpreted in the following manner. For the Ti/Al contacts to Ga-polar n-GaN annealed above 600 °C, the ohmic formation was ascribed to the occurrence of AlN and TiN [8], generating N vacancies (V_N), and the generation of two-dimensional electron gases (2DEG) at the AlN/GaN interface [8,13,20]. In this work, the N-polar contact samples were annealed at a temperature of 250 °C, which is too low to form TiN. Besides, the Ta layer blocked the indiffusion of Al toward the GaN. So AlN was not formed. Thus, the electrical degradation of the annealed Ti/Al/Au contacts could not be explained by the formation of AlN and TiN, and their associated features [8,13,20]. Recently, Jeon et al. [16] reported that the electrical degradation of Ti/Al contacts to N-polar n-GaN annealed below 300 °C could be related to the generation of acceptor-like Ga vacancies (V_Ga) [21] near the n-GaN surface region caused by Ga outdiffusion. Kim et al. [14] also showed that the degradation behavior of the Ti/Al contacts to N-polar n-GaN could be associated with the formation of acceptor-like V_Ga-O_N complex. It was reported that the outdiffusion of Ga atoms could be facilitated by the presence of oxygen during annealing because of the high reactivity of Ga with oxygen [22]. Thus, considering the fact that oxygen was introduced into the Ti/GaN interface region during annealing, the thermal degradation of the Ti/Al/Au contacts could be attributed to the generation of either Ga vacancies (V_Ga), V_Ga-O_N complex or both near the n-GaN surface region. (At the moment we are not sure which defects have more dominant effect on the degradation.) These defects cause an electrical compensation in the n-GaN surface region, thereby increasing the effective Schottky barrier height [14], as evidenced by the shift of the Ga 2p core level toward the lower binding energy side, as shown in Fig. 3. In addition, the formation of the interfacial oxide can also be responsible for the electrical degradation [23,24], as confirmed by high resolution transmission electron microscopy result (not shown here). On the other hand, the improved thermal stability of the Ti/Ta/Al/Au contacts could be explained in terms of the fact that the oxygen indiffusion was blocked by the Ta layer. Thus, the use of the Ta diffusion barrier layer limits the occurrence of either Ga vacancies (V_Ga), V_Ga-O_N complex, both types of the defects, or interfacial oxide by hindering the introduction of oxygen from the annealing ambience [21]. This is consistent with the difference in the N/Ga atomic ratios of the Ti/Al/Au and Ti/Ta/Al/Au samples.

4. Summary and conclusion

We employed a Ta diffusion barrier to develop thermally stable and low resistance Ti/Al-based ohmic contacts to N-polar n-GaN for high-power VLEDs. It was shown that both of the Ti/Al/Au and Ti/Ta/Al/Au contacts were ohmic before annealing. However, unlike the Ti/Al/Au samples, the Ti/Ta/Al/Au samples remain thermally stable even after annealing for 600 min. LEDs fabricated with the Ti/Ta/Al/Au contacts exhibited higher output power than LEDs with the Ti/Al/Au contact. Based on the XPS results, the improved electrical behavior of the Ti/Ta/Al/Au contacts was explained by the suppression of the generation of Ga vacancy and the formation of interfacial oxide by preventing the indiffusion of oxygen toward the GaN. The results demonstrate that the Ta diffusion barrier layer could be a useful processing parameter for the fabrication of high-quality N-polar n-ohmic contacts for high-power vertical LEDs.