The authors report high-efficiency GaN-based light-emitting diodes (LEDs) fabricated with identical Ag contact formed on both n- and p-layers. Ag contacts thermally annealed at optimized conditions yielded low specific contact resistances of 4.5 × 10−4 and 9.4 × 10−4 Ωcm2, and high optical reflectivity (at 450 nm) of 88.1 and 85.3% for n- and p-contact, respectively. LEDs fabricated with identical Ag contacts formed on both layers showed 31% brighter light output power and nearly the same forward voltages as compared to reference LEDs. This indicates that Ag contact can be used as a reflective electrode for both n- and p-layers, leading to enhanced extraction efficiency and fewer process steps.
© 2013 OSA
Ag contact is conventionally used as a p-type reflective electrode to fabricate high-efficiency and high-power GaN-based light-emitting diodes (LEDs) employing flip-chip or vertical-injection configurations, since it can produce excellent ohmic contact to p-GaN and can exhibit the best reflectivity among metallic reflectors in the visible wavelength . Indeed, a number of groups have investigated Ag contact on p-GaN in terms of achieving better ohmic contact, higher optical reflectivity, and enhanced thermal stability [1–11]. Notably, the optical reflectivity of Ag contact was found to be crucial for enhancing the light extraction efficiency of LEDs .
Despite the advantages of Ag metallic reflectors, little is known about the feasibility of Ag contact to use in n-type reflective electrode. This seems to be associated with the difficulty of obtaining Ag ohmic contact to n-GaN. For example, it is well known that, for Ag contact on p-GaN, ohmic contact can be easily obtained upon thermal annealing under oxygen containing ambient, since annealing leads to a generation of gallium vacancies (acting as acceptors) at near contacts via the formation of thermodynamically stable Ag-Ga solid solution [1,5–9]. More specifically, our group  reported that the concentration of gallium vacancies can be increased to as high as ~1019 cm−3 upon thermal annealing, which leads to enhanced field emission and hence the formation of ohmic contact. According to this well-known thermodynamic interfacial reactions, Ag ohmic contact to n-GaN is expected to be very difficult to achieve since generated gallium vacancies may compensate the majority electron carriers and so impede ohmic formation. However, there are no studies on the feasibility of Ag to n-GaN contact and its application in LEDs for enhanced light extraction.
The use of Ag contact as an n-type reflective electrode as well as a p-type electrode would result in distinctive features as following. First, the process would be simplified since Ag contact can be deposited on both n- and p-layers simultaneously. Second, the light extraction efficiency of LEDs would be improved due to minimized absorption of propagating light at the reflective electrodes. In this study, we report the first demonstration of GaN-based LEDs fabricated with identical Ag reflective electrodes formed on both n- and p-layers. For this purpose, Ag contacts formed on n- and p-GaN layers were optimized in terms of contact resistance and optical reflectivity by changing thermal annealing conditions. LEDs fabricated with identical Ag contact (referred here to as “Ag LEDs”) exhibited excellent device performance compared to reference LEDs, including 31% brighter light output power and nearly the same forward voltages.
2. Experimental procedure
To optimize the electrical characteristics of Ag contacts formed on n- and p-GaN layers, a transmission line model (TLM) method with circular geometry, which had an inner radius of 150 μm and gap spacing (d) of 20, 60, 100, and 150 μm, was used as shown in the inset of Fig. 1. The conventional photolithographic technique was used to define TLM patterns, on which a 100 nm-thick Ag layer was deposited using an e-beam evaporator. To test p-contact, a Ag layer was deposited on top of commercial LED wafers, i.e., on the heavily Mg doped p-GaN layer with [Mg] = ~1020 cm−3. To test n-contact, LED wafers were dry-etched to a thickness of 0.5 μm to expose the n-GaN layer, on which the Ag layer was deposited. Dry etching was performed using an inductively coupled plasma reactive ion etching system. The carrier concentration (N) of exposed n-GaN was estimated to be as high as ~5 × 1018 cm−3. To optimize Ag contact in terms of ohmic contact and optical reflectivity, rapid thermal annealing was performed at temperatures of 300−500 °C for 1 min in N2 or O2 ambient. The optical reflectivity (Rop) of the Ag/GaN interface was monitored at a peak wavelength of 450 nm using a reflectivity measurement system with a minimum resolution of 10 μm (Elli-RSc model). Note that, for the optical reflectivity measurements, LED wafers grown on both-side polished sapphire substrate were used. The measured Rop was normalized by assuming that Rop = 100% for the as-deposited Ag. Secondary ion mass spectroscopy (SIMS) depth profiling was performed using primary Cs+ ions (CAMECA-IMS 6F model) to understand the interfacial reaction between Ag contact and n-GaN after thermal annealing.
To demonstrate Ag LEDs, a circular mesa with a radius of 250 μm was defined by dry etching of 0.5 μm. 200 nm-thick Ag layers were then simultaneously deposited on both n- and p-layers, followed by thermal annealing at optimized conditions (500 °C for 1 min in N2 ambient). For a comparative study, reference LEDs were also fabricated by the following procedure, i.e., after mesa etching, a Ti/Al (30 nm/80 nm) layer was deposited on the n-layer, followed by thermal annealing at 550 °C for 1 min in N2 ambient. The Ag layer was then deposited on the p-layer and subsequently annealed at 500 °C for 1 min in O2 ambient. Note that, prior to metal deposition, the sample surface was always pretreated using buffered oxide etchant for 3 min, followed by a rinse using deionized water. Completed LEDs are shown in the inset of Fig. 5(b). The electrical and optical characteristics of LEDs and their TLM patterns were measured using an on-wafer testing configuration comprised of a parameter analyzer (HP4156A) and photodiode (883-UV) mounted beneath the LED chips. Thus, the light escaping through bottom sapphire substrate was collected from the photodiode. An optical ray-tracing simulation was also performed to elucidate the Rop effect of each n- and p-contact on the light extraction efficiency of LEDs.
3. Results and discussion
Figure 1(a) shows the current-voltage (I–V) characteristics of Ag contact to n-GaN as a function of annealing temperature and gas ambient, as measured from adjacent contact pads with a spacing of 20 μm. Note that the I–V curve of as-deposited samples was nonlinear. I–V curves of the annealed samples in N2 ambient became steeper and more linear with increased annealing temperature, except for that of the 300 °C-annealed sample (this will not be discussed in detail in our study). Accordingly, the specific contact resistance (ρsc) obtained by TLM method was found to decrease with increased annealing temperature, as shown in Fig. 1(b). Note that, to obtain reliable ρsc data, the total resistance between adjacent contact pads, which is one of the most important parameters in the application of TLM method, was obtained using the measured current at 0.1 V [13,14]. Interestingly, Rop was also found todecrease with increased annealing temperature. This indicates that the optimized annealing condition for Ag contact to n-GaN should be carefully chosen due to a tradeoff relationship between ρsc and Rop. In this study, thermal annealing performed at 500 °C for 1 min in N2 ambient was chosen as the best condition for n-type reflective contact, yielding that ρsc = 4.5 × 10−4 Ωcm2 and Rop = 88.1%. Conversely, the Ag contact annealed at 500 °C for 1 min in O2 ambient produced a very poor I-V curve and Rop, e.g., ρsc = 2.1 × 10−2 Ωcm2 and Rop = 74.0%. These results indicate that to optimize Ag ohmic contact to n-GaN, thermal annealing performed in O2 gas ambient should be avoided. To investigate the effect of ambient gas on interfacial reactions, SIMS depth profilings were performed as shown in Fig. 2.
Figure 2 shows SIMS depth profiles of the Ag/n-GaN interface before and after thermal annealing performed at 500 °C for 1 min in N2 and O2 ambient. Notably, thermal annealing performed in O2 ambient exhibited excessive interdiffusion between Ag and GaN, which even distrupts Ag/n-GaN interfaces. In this case, interfacial gallium vacancies can be generated substantially, which can degrade ohmic contact. In addition, significant intermixing of Ag and GaN is also expected to degrade Rop. In contrast, the sample annealed in N2 ambient showed limited interfacial reactions along with a slight reduction in interfacial oxygen. Therefore, the ohmic formation for samples annealed in N2 ambient is due to the effective removal of interfacial oxide, whereas the generation of gallium vacancies is effectively suppressed. Consistently, the Ag surface annealed in N2 ambient was observed to be clean and smooth, while that annealed in O2 ambient was significantly deteriorated (see Fig. 3). Indeed, a strong annealing ambient dependence is due to the thermodynamic stability of gallium oxide against GaN phase, e.g., the standard molar enthalpy (heat) of formation (at 298.15 K) for GaN and Ga2O3 phases are −110.5 and −1089.1 kJ/mol, respectively, thus accelerating the out-diffusion of Ga under O2 ambient-annealing . Therefore, it is concluded that, for the fabrication of Ag LEDs, thermal annealing should definitely be performed in N2 ambient for the best Ag n-contact. However, under this annealing condition, Ag ohmic contact to p-GaN maynot be made, since it was generally reported that the O2 annealing conditions were required for good ohmic contact to p-GaN [6,7].
Figure 4 shows I–V curves of Ag contacts to p-GaN obtained after annealing at 500 °C for 1 min in N2 and O2 ambient. Unlike with n-contact conditions, better Ag ohmic contact was produced after annealing in O2 ambient, which is in good agreement with previous findings . For example, ρsc was 9.4 × 10−4 for the N2 annealed sample and 2.5 × 10−4 Ωcm2 for the O2 annealed sample. As mentioned above, this is due to the presence of a number of acceptor-like gallium vacancies in O2 annealed samples that occur as a result of significant outdiffusion of gallium. However, it is still worth noting that Ag contacts annealed in N2 exhibited favorable ohmic behavior. This might be attributed to the effective removal of interfacial gallium oxide and, more importantly, the use of a heavily Mg-doped p-GaN layer, i.e., the ohmic contacts with p-GaN are easily obtained due to hopping-involved carrier transport through Mg-related deep-level states [14,15]. Notably, the Rop of the Ag/p-GaN interface was much larger in the N2-annealed sample than the O2-annealed one, with 85.3% and 61.9%, respectively. These results indicate that Ag LEDs with reasonable ohmic contactand good reflectivity can be successfully fabricated using optimized thermal annealing conditions (500 °C for 1 min in N2 ambient).
Figure 5(a) shows the I–V curves of fabricated Ag LEDs and reference LEDs. Note that reference LEDs were fabricated using best optimized ohmic conditions for both n- and p-layers, with ρsc = 2.5 × 10−4 Ωcm2 for the Ag p-contact (annealed in O2) and ρsc = 3.0 × 10−4 Ωcm2 for the Ti/Al n-contact. Note that the ρsc of the Ti/Al n-contact is slightly lower than that of the Ag n-contact (not shown). All ρsc and Rop values used to fabricate reference and Ag LEDs are summarized in the inset of Fig. 5(a). It is noted that the I–V curve of Ag LEDs is comparable (not significantly degraded) to that of reference LEDs. The forward voltage measured at 20 mA was 3.02 V and 3.12 V for reference LEDs and Ag LEDs, respectively.
As expected, the very slight degradation in the forward voltage of Ag LEDs was due to slightly larger p- and n-contact resistances. On the one hand, the optical output power of Ag LEDs was 31% greater than that of reference LEDs, as shown in Fig. 5(b). This is attributed to the reduced absorption of propagating light at highly reflective Ag contacts formed on both n- and p-layers. Interestingly, the Ag p-contact was likely to be partly transparent, as shown in the electroluminescent image (see the inset of Fig. 5(b)). This might be attributed to the formation of Ag-oxide for the reference LEDs, and the micropits or microstructural defects for the Ag LEDs. This is presently under investigation.
To investigate the enhancement factor of light extraction associated with each n- and p-reflective contact, an optical ray-tracing simulation was performed as shown in Fig. 6. For a simulation, real chip geometry was designed and the corresponding optical constants were used. A detailed method of the optical simulation can be found elsewhere . Figure 6 shows the calculated extraction efficiency of LEDs fabricated with different n- and p-contact Rop values for reference LEDs, LED A (Ag p-contact annealed in O2 and Ag n-contact annealed in N2), LED B (Ag p-contact annealed in N2 and Ti/Al n-contact), and Ag LEDs. It is evident that the Ag LEDs show 15% larger extraction efficiency than reference LEDs. This improvement may be due to the combined effects of the increased Rop of both contacts. However, the calculated enhancement factor (15%) was smaller than the experimental value (31%), which might be attributed to a different number of LEDs because one LED chip on sapphire substrate was considered for the simulation while a number of LEDs were arrayed on sapphire substrate in practice. Actually, the effect of contact reflectivity on the extraction efficiency of LEDs can be increased since generated light can propagate through chips.
Notably, a comparison of LED A with the reference LEDs (or LED B with Ag LEDs) shows that the enhancement factor is 3.1% (2.7%). This indicates that the enhancement factor of n-contact reflectivity on extraction efficiency is around 3.0%, which is in good agreement with previous findings . On the one hand, the enhancement factor of reference LEDs over LED B (or LED A over Ag LEDs) was as high as 11.5% (11.6%). These values reveal that the use of Ag contacts on both n- and p-contacts under optimized thermal annealing conditions can significantly enhance the extraction efficiency of LEDs while simplifying process steps.
We demonstrated LEDs fabricated with identical Ag reflective contact formed on n- and p-layers by optimizing thermal annealing conditions. Interestingly, the Ag contact annealed at optimized condition showed excellent ohmic behavior on both n- and p-GaN, with which Rop can be maximized. Accordingly, fabricated Ag LEDs showed reasonable I−V curves along with greatly improved output power compared to reference LEDs. These results indicate that, if the optimization of reflective Ag contact on both layers is carefully considered, high-efficiency and cost-competitive LEDs can be fabricated.
This study was supported in part by a Priority Research Center Program through the National Research Foundation of Korea, funded by the Ministry of Education, Science and Technology of the Korean government (2011-0027956) and in part by Basic Science Research Programs through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011-0013708).
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