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Abstract
We demonstrate two types of GaN-based flip-chip light-emitting diodes (FCLEDs) with highly reflective Ag/TiW and indium-tin oxide (ITO)/distributed Bragg reflector (DBR) p-type Ohmic contacts. We show that a direct Ohmic contact to p-GaN layer using pure Ag is obtained when annealed at 600°C in N2 ambient. A TiW diffusion barrier layer covered onto Ag is used to suppress the agglomeration of Ag and thus maintain high reflectance of Ag during high temperature annealing process. We develop a strip-shaped SiO2 current blocking layer beneath the ITO/DBR to alleviate current crowding occurring in FCLED with ITO/DBR. Owing to negligibly small spreading resistance of Ag, however, our combined numerical and experimental results show that the FCLED with Ag/TiW has a more favorable current spreading uniformity in comparison to the FCLED with ITO/DBR. As a result, the light output power of FCLED with Ag/TiW is 7.5% higher than that of FCLED with ITO/DBR at 350 mA. The maximum output power of the FCLED with Ag/TiW obtained at 305.6 A/cm2 is 29.3% larger than that of the FCLED with ITO/DBR obtained at 278.9 A/cm2. The improvement appears to be due to the enhanced current spreading and higher optical reflectance provided by the Ag/TiW.
© 2017 Optical Society of America
1. Introduction
Over the last decade, GaN-based light-emitting diodes (LEDs) have attracted considerable attention due to the strong potential for applications including full-color displays, visible light communication, street lighting as well as automotive front lighting [1–9]. Driving LED with high current densities is imperative to enlarge the market of solid-state lighting for cost reduction, compact volume, and high illumination within a single chip [10,11]. Among the various chip architectures, flip-chip technology offers some unique advantages in high luminance applications, such as high current driving capability and efficient thermal management [12–21]. When operating FCLEDs at high current densities, however, the non-uniform current spreading is still a challenging problem hindering the further improvement in device performance [22–25].
On the other hand, achieving p-type contact with low specific contact resistance and high optical reflectance is of particular importance for the realization of highly efficient FCLEDs. Silver (Ag) is considered as a reflective Ohmic contact for FCLEDs because of its high reflectance and excellent electrical properties. However, the Ag contact suffers from poor adhesion, inferior Ohmic behavior, and thermal instability such as agglomeration during high temperature annealing process [26,27], leading to the degradation of device performance. For this reason, many research groups tentatively adopted Ag-based alloys and interlayers to overcome the above problem [28–32]. However, these schemes inevitably degrade optical reflectance of the Ag mirror contact. Recently, it has been reported by Hasanov et al. that the Ni/Ag/TiW multilayer can be used as a reflective Ohmic conctact for FCLEDs, which has higher reflectance and better Ohmic contact performance compared with the Ni/Ag due to the use of TiW barrier diffusion layer [33]. However, the presence of a thin Ni layer underneath the Ag/TiW layer limits the optical performance of FCLEDs due to strong absorption of light by the underlying Ni layer. To solve this problem, a direct Ohmic contact to p-GaN using pure Ag without interlayers should be obtained. In addition, the indium-tin oxide (ITO) transparent conductive electrode in combination with dielectric distributed Bragg reflector (DBR) has also been used as highly reflective Ohmic contact for FCLEDs in recent years [34]. However, current crowding occurring around the p-electrode is still severe in the reported FCLED with ITO/DBR because the sheet resistance of ITO is higher than that of n-GaN layer. A uniform current density distribution at high injection currents plays a critical role in both improving electronic and optical properties and enhancing reliability of FCLEDs, since the chip regions with the highest current density is expected to exhibit degradation. Therefore, it is imperative to find solution for uniform current spreading in order to unlock the full potential of FCLED with ITO/DBR when operated at high injection currents.
In this study, we demonstrate two types of FCLEDs with highly reflective Ag/TiW and ITO/DBR Ohmic contacts. We investigate the effect of annealing temperature on the Ohmic contact performance between Ag and p-GaN. In the FCLED with Ag/TiW, a direct Ohmic contact to p-GaN is obtained using pure Ag after annealed at 600°C, and the TiW diffusion barrier layer covered on the top of Ag is used to suppress the agglomeration of Ag during high temperature annealing process. In the FCLED with ITO/DBR, we develop a strip-shaped SiO2 current blocking layer (CBL) underneath the ITO/DBR to suppress the current crowding around p-electrode. To make contact to n-GaN in both FCLEDs, several vias are formed by etching a portion of p-GaN and active region until the n-GaN layer is exposed. Comparative study of the effect of Ag/TiW and ITO/DBR Ohmic contacts on optical and electronic properties of FCLEDs was carried out in detail.
2. Experimental methods
GaN epitaxial layers were grown on c-plane (0001) patterned sapphire substrate by metal-organic chemical vapor deposition (MOCVD). The GaN-based LED structure was composed of a 25-nm-thick low temperature GaN nucleation layer, a 3.0-µm-thick undoped GaN buffer layer, a 2.5-µm-thick Si-doped n-GaN layer, a 200-nm-thick InGaN/GaN superlattice serving as strain release layer, a 12-pair of In0.16Ga0.84N (3 nm)/GaN (12 nm) multiple quantum well (MQW) active region, a 50-nm-thick low temperature p-GaN layer, a 48-nm-thick p-AlGaN/GaN superlattice electron blocking layer, and a 110-nm-thick Mg-doped p-GaN layer.
A schematic of fabrication process for FCLED with Ag/TiW is shown in Fig. 1 and the process is composed of the following steps: formation of n-type contact vias by inductively coupled plasma (ICP) etching based on BCl3/Cl2 mixture gas [Fig. 1(a)], sputtering of Ag (120 nm)/TiW (80 nm) on the p-GaN as p-type Ohmic contact [Figs. 1(b) and 1(c)], evaporation of Ni/Al/Ti/Pt metallization layer on the n-GaN as n-electrode [Fig. 1(d)], deposition of SiO2 passivation layer by plasma enhanced chemical vapor deposition [Fig. 1(e)], and evaporation of Au/Sn as p- and n-pads [Fig. 1(f)].
Fig. 1 Schematic illustration of fabrication process for FCLED with highly reflective Ag/TiW Ohmic contact.
A schematic of fabrication process for FCLED with ITO/DBR is shown in Fig. 2 and the process consists of the following steps: fabrication of n-type contact vias by taking advantage of ICP etching based on BCl3/Cl2 mixture gas [Fig. 2(a)], formation of a strip-shaped 210-nm-thick SiO2 CBL on the p-GaN using plasma enhanced chemical vapor deposition followed by photolithography and buffer oxide etching [Fig. 2(b)], evaporation of 90-nm-thick ITO on the p-GaN and the strip-shaped SiO2 CBL [Fig. 2(c)], deposition of Cr/Al/Ti/Pt/Au metallization layer as p- and n-electrodes [Fig. 2(d)], sputtering of quarter-wavelength-thick Ta2O5/SiO2 DBR stacks with 14-pair on ITO by ion beam deposition followed by the opening of via through DBR using ICP etching based on CHF3/Ar/O2 mixture gas [Fig. 2(e)], and evaporation of Cr/Ti/Pt/Au as p- and n-pads [Fig. 2(f)].
Fig. 2 Schematic illustration of fabrication process for FCLED with highly reflective ITO/DBR Ohmic contact.
The FCLEDs with highly reflective Ni/Ag and ITO/DBR Ohmic contacts were fabricated from the identical epitaxial wafers. The size of FCLEDs is 508 × 889 μm2, and the peak wavelength of FCLEDs is 453 nm. The light output power (LOP) versus current (L-I) and current versus voltage (I-V) characteristics of FCLEDs were measured using an integrating sphere and a semiconductor parameter analyzer (Keysight B2901A).
3. Results and discussion
Figures 3(a) and 3(e) show a schematic illustration of FCLEDs with Ag/TiW and ITO/DBR Ohmic contacts, respectively. Detailed structural analysis of FCLEDs with Ag/TiW and ITO/DBR was performed by focused ion beam (FIB) milling in combination with cross-sectional scanning electron microscopy (SEM). Figures 3(c) and 3(d) demonstrate the cross-sectional SEM images of FCLED with Ag/TiW, which were obtained by FIB milling along A-A and B-B directions, respectively, as marked in Fig. 3(b). Figures 3(g) and 3(h) depict the cross-sectional SEM images of FCLED with ITO/DBR, which were obtained by FIB milling along C–C and D–D directions, respectively, as marked in Fig. 3(f).
Fig. 3 (a) Schematic of FCLED with highly reflective Ag/TiW Ohmic contact. (b) Top-view SEM image of FCLED with Ag/TiW. The four etched n-contact vias and two n-contact fingers were designed to spread current. (c) Cross-sectional SEM image of FCLED with Ag/TiW milled by FIB along A-A direction. (d) Cross-sectional SEM image of FCLED with Ag/TiW milled by FIB along B-B direction. The sidewall of the etched vias is covered by SiO2 passivation layer. (e) Schematic of FCLED with highly reflective ITO/DBR Ohmic contact. (f) Top-view SEM image of FCLED with ITO/DBR. Three p-contact fingers and two n-contact fingers were used to enhance current spreading. Contact to ITO by drilling nine vias through DBR on the right side of the chip. (g) Cross-sectional SEM image of FCLED with ITO/DBR milled by FIB along C-C direction. The inset is the magnified SEM image showing p-GaN/SiO2 CBL/ITO/DBR multilayers. (h) Cross-sectional SEM image of FCLED with ITO/DBR milled by FIB along D-D direction. The sidewall of the etched vias is covered by Ta2O5/SiO2 DBR.
The n-electrode pattern including four n-contact vias and two n-contact fingers was observed in both FCLEDs [Figs. 3(b) and 3(f)]. Contact to n-GaN was made by etching n-contact vias through p-GaN and InGaN/GaN MQW active region until the n-GaN layer was exposed [Figs. 3(d) and 3(h)]. In the FCLED with Ag/TiW, a direct Ohmic contact to p-GaN was obtained using pure Ag, and a TiW diffusion barrier layer was coated onto the Ag to alleviate the agglomeration phenomena during the high temperature annealing process [Fig. 3(c)]. In the FCLED with ITO/DBR, contact to ITO was achieved by the opening of via through insulating DBR [Fig. 3(e)]. A strip-shaped SiO2 CBL underneath the ITO/DBR was applied to alleviate the current crowding around the p-electrode, and the strip-shaped fingers of the SiO2 CBL was well aligned with the p-contact fingers, as shown in Figs. 3(e) and 3(g), leading to an improvement in lateral current spreading.
Specific contact resistance and optical reflectance are crucial factors for realizing efficient current injection and light extraction in FCLEDs. To evaluate the contact ability of the ITO and Ag on the p-GaN, we first measured the I–V characteristics of the 90-nm-thick ITO and 100-nm-thick Ag films deposited on the p-GaN layers with different annealing temperatures. Figure 4(a) shows an optical microscopy image of the circular transfer length method (CTLM) pattern. Figures 4(b) and 4(c) show the I-V characteristics of Ag and ITO contacts on p-GaN as a function of the annealing temperature, respectively. In the annealing temperatures ranging from 300 to 500°C, the non-linear I-V characteristics at the current of 10−9 A at 1V for the Ag samples were attributed to the large Schottky barrier height between Ag and p-GaN. However, after annealed at 600°C, the Ag deposited on the p-GaN surprisingly demonstrated perfect Ohmic behavior at current levels of 10−7 A at 1 V, as shown in Fig. 4(b). It was worth noting that both 300°C-annealed ITO and 600°C-annealed ITO samples showed linear Ohmic behaviors, and that the 300°C-annealed ITO sample exhibited better contact performance at current levels of 10−5 A at 1 V.
Fig. 4 (a) Schematic and optical microscopy image of the Ag CTLM pads. (b) I–V characteristic curves measured for pure Ag contact to p-GaN layer when annealed under different temperatures. (c) I–V characteristic curves measured for ITO contact to p-GaN layer when annealed under different temperatures. (d) I–V characteristic curves measured for the different CTLM spacing of the pure Ag deposited on p-GaN layer after annealing at 600°C. (e) I–V characteristic curves measured for the different CTLM spacing of the ITO deposited on p-GaN layer after annealing at 300°C. (f) Reflectance spectra of the as-deposited Ag (100 nm)/TiW (80 nm) and ITO (90 nm)/DBR (2.6 μm) films on quartz substrates.
We measured the specific contact resistance of the Ag and ITO films deposited on the p-GaN layers by using CTLM method. Figures 4(d) and 4(e) show the I–V characteristics measured for the different CTLM spacing of the Ag and ITO contacts after annealed at 600°C and 300°C, respectively. The specific contact resistances were determined from the plots of the measured resistances versus the spacing between the CTLM. The least-square method was used to fit a straight line to the experimental data. Here, the measured specific contact resistances of 600°C-annealed Ag and 300°C-annealed ITO were 9.42 × 10−2 and 4.46 × 10−4 Ω cm2, respectively. Figure 4(f) shows reflectance spectra of the Ag (100 nm)/TiW (80 nm) and ITO (90 nm)/DBR (2.6 μm) contacts before and after thermal annealing process. The reflectance of the 600°C-annealed Ag/TiW and 300°C-annealed ITO/DBR on quartz substrates was measured to be 94.5 and 92.8% at 453 nm, respectively. It is worth pointing out that the reflectance of the Ag/TiW merely exhibited a slight decrease from 96.7 to 94.5% at 453 nm after annealed at 600°C, suggesting that the TiW covered onto the Ag can effectively suppress the agglomeration of Ag and thus maintain high reflectance of Ag during high temperature thermal annealing process.
An efficient current spreading can lead to uniform light emission intensity and improved device reliability. Based on the current distribution theory, the lateral current distribution of FCLEDs can be given by [35–37]
where J0 is the injected current density at the electrode edge, x is the distance from a certain location to the electrode edge, and Ls is defined as the current spreading length. In general, Ls is used to evaluate the current crowding effect, which can be given by the formula provided below:where nideal denotes the ideality factor of FCLEDs, and ρs,p-contact and ρs,n-GaN are sheet resistances of p-contact layer and n-GaN layer, respectively. K, T and q represent the Boltzmann constant, temperature, and element charge, respectively. In terms of Eq. (2), the sheet resistance of p-contact layer plays a crucial role in determining the current spreading length.The electroluminescence (EL) distribution in the active region of the FCLEDs is closely related to both the current density distribution and the temperature distribution. To investigate these parameters in FCLEDs, three-dimensional simulations of the current density distribution coupled with a thermal analysis were performed using the SimuLED commercial software package from STR [38]. Figure 5 shows the simulation results of the current density distribution in the active region of FCLEDs with Ni/Ag and ITO/DBR contacts at 350 mA under 300 K ambient temperature. Table 1 summarizes the current density distributions in active regions of FCLEDs at 150, 250, 350, and 500 mA, respectively. By contrast, it was found that a smaller root-mean-square value of current density in the active region at 350 mA for the FCLED with Ag/TiW (65.71 A/cm2) was obtained compared to FCLED with ITO/DBR (95.62 A/cm2), indicating that the Ag/TiW can provide a more favorable uniformity of current spreading.
Fig. 5 Current density distribution in the active region of FCLEDs at 350 mA under 300 K ambient temperature. (a) Current density distribution in the active region of FCLED with Ag/TiW. (b) Current density distribution in the active region, while there are no p-/n-contact fingers or SiO2 CBL in the FCLED with ITO/DBR. (c) Current density distribution in the active region, while there are only p-/n-contact fingers in the FCLED with ITO/DBR. (d) Current density distribution in the active region, while there are p-/n-contact fingers and SiO2 CBL in the FCLED with ITO/DBR.
Table 1. Simulation results of current density distribution in the active region of FCLEDs with Ag/TiW and ITO/DBR at 150, 250, 350, and 500 mA, respectively
We investigated the effect of the p-/n-contact fingers and SiO2 CBL on current density distribution of FCLED with ITO/DBR by SimuLED. By comparing Fig. 5(b) with Fig. 5(c), it was found that the root-mean-square (RMS) value of current density in the active region at 350 mA decreased from 179.53 A/cm2 [Fig. 5(b)] to 121.91 A/cm2 [Fig. 5(c)] by adding p-/n-contact fingers in the FCLED with ITO/DBR. In Fig. 5(c), it was observed that the current density was the highest at the p-contact finger, and decreased along the longitudinal direction. We attribute this behavior to the current crowding at the edge of the p-contact. After implementing the strip-shaped SiO2 CBL underneath the ITO/DBR [Fig. 5(d)], the current crowding occurring at the edge of the p-contact was effectively suppressed, and the resulting root-mean-square value of current density in the active region was further decreased to 95.62 A/cm2, indicating the enhanced current spreading.
Figure 6 shows the light emission intensity distributions of FCLEDs driven by the injection current of 150, 250, 350, and 500 mA, respectively. As a higher current density results in a stronger light emission intensity, the spatial distribution of light emission intensity is closely correlated with the distribution of current density. It was worth noting that the current crowding effect occurring in both FCLEDs was not obvious at 150 mA. As the injection current was further increased, however, noticeable light emission concentrated around the p-contact fingers in the FCLED with ITO/DBR, which was in good agreement with the SimuLED simulation results in Fig. 5. The sheet resistance of Ag (0.16 Ω/sq) is approximately two orders of magnitude lower than that of ITO (38 Ω/sq). Accordingly, a larger current spreading length is achieved for the FCLED with Ag/TiW in terms of Eq. (2). As a result, the FCLED with Ag/TiW exhibited more uniform light emission across the entire active region as compared to the FCLED with ITO/DBR.
Fig. 6 (a-d) Measured light emission intensity distributions in the FCLED with Ag/TiW at 150, 250, 350, and 500 mA, respectively. (e-h) Measured light emission intensity distributions in the FCLED with ITO/DBR at 150, 250, 350, and 500 mA, respectively.
The I-V curves of FCLEDs with Ag/TiW and ITO/DBR Ohmic contacts are shown in Fig. 7(a). It is well established that I-V characteristics are dependent on both the specific contact resistance and spreading resistance of p-type contact. Because the specific contact resistance between Ag and p-GaN was two orders of magnitude higher than that between ITO and p-GaN as shown in Fig. 4, the forward voltage of FCLED with Ag/TiW was slightly larger than that of FCLED with ITO/DBR in low injection current regime (less than 100 mA). For example, at 60 mA, the forward voltage of the FCLED with Ag/TiW and ITO/DBR was 2.75 V and 2.74 V, respectively. However, with the increase of injection current, the spreading resistance of p-type contact plays a dominant factor in determining the I-V characteristics of FCLEDs. As a result, as the injection current was increased to be above 100 mA, the forward voltage of FCLED with Ag/TiW was much smaller than that of FCLED with ITO/DBR due to negligibly small spreading resistance of Ag. For instance, at 350 mA, the forward voltage of the FCLED with Ag/TiW and ITO/DBR is 3.08 V and 3.16 V, respectively.
Fig. 7 (a) Current versus voltage for FCLEDs with Ag/TiW and ITO/DBR. (b) Light output power and external quantum efficiency versus current for FCLEDs with Ag/TiW and ITO/DBR. (c) Far-field radiation pattern of FCLEDs with Ag/TiW and ITO/DBR at 350 mA. (d) EL spectrum of FCLED measured at 350 mA.
Figure 7(b) shows LOP and external quantum efficiency (EQE) of FCLEDs as a function of injection current under continuous-wave operation mode. At 350 mA, the LOP of FCLEDs with Ag/TiW and ITO/DBR was 439.3 and 408.8 mW, respectively; the EQE of FCLEDs with Ag/TiW and ITO/DBR was 45.3 and 42.5%, respectively. The LOP of FCLED with Ag/TiW was 7.5% higher than that of FCLED with ITO/DBR. The maximum output power of the FCLED with Ag/TiW (949 mW) obtained at 305.6 A/cm2 was 29.3% larger than that of the FCLED with ITO/DBR (733.9 mW) obtained at 278.9 A/cm2. These improvements can be attributed to the enhanced current spreading and the higher light extraction efficiency when using Ag/TiW instead of ITO/DBR. We also measured the normalized far-field angular radiation patterns of FCLEDs with Ag/TiW and ITO/DBR at 350 mA [Fig. 7(c)]. As compared to the FCLED with ITO/DBR, the intensity of light output from the FCLED with Ag/TiW was substantially improved in the vertical direction, which revealed that the light output from the sapphire surface of FCLED was mostly enhanced. A lambertian emission pattern, which follows a cosine dependence on the angle of incidence, was observed in both FCLEDs due to the index contrast between the sapphire substrate and the surrounding air. The EL spectrum of the FCLED measured at 350 mA is shown in Fig. 7(d).
4. Conclusion
In summary, we have demonstrated experimentally that both the Ag with coated TiW diffusion barrier layer and strip-shaped SiO2 current blocking layer beneath the ITO/DBR schemes can act as a highly reflective Ohmic contact in FCLEDs, which also offer superior current spreading while maintaining excellent electronic and optical properties. We show that a direct Ohmic contact with p-GaN can be obtained using pure Ag after thermal annealing at 600°C for 20 min in N2 ambient, and that a TiW diffusion barrier layer covered onto Ag can effectively suppress the agglomeration of Ag and thus ensure the high reflectance of Ag during high temperature annealing process. Our demonstration of highly reflective pure Ag Ohmic contact with superior current spreading paves the way for the realization of highly efficient ultra-high power flip-chip LEDs.
Funding
National Natural Science Foundation of China (Grant Nos. U1501241, 51675386 and 51305266); National High-tech R&D Program of China (Grant No. 2015AA03A101).
Acknowledgments
We acknowledge the nanofabrication assistance from Center for Nanoscience and Nanotechnology at Wuhan University.
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