1. Introduction

As a solid-state lighting source for the next generation, GaN-based light-emitting diodes (LEDs) arouse great interest for their small sizes, long lifespan and energy-efficiency [1, 2 ]. The light output power of LEDs is greatly determined by the light extraction efficiency (η_LEE) and current injection efficiency (η_CIE) [3]. However, it is noted that the injection current does not spread uniformly due to the high resistivity of p-GaN [4]. For this reason, Ni/Au semi-transparent conducting electrodes had been extensively used to improve current injection efficiency despite their low transmittance for a long time. Recently, several emerging alternatives such as ITO [5], graphene [6], carbon nanotubes (CNTs) [7], and various metal wires [8] have been investigated as advanced transparent conducting electrodes [4]. Admittedly, these materials all share excellent optical properties, but there are some drawbacks when applying them into high performance LEDs. Graphene and CNTs electrodes may lead to several problems such as high forward voltage, low hole injection efficiency towards the active region and poor device reliability due to the high sheet resistance [7, 9 ]. Metal wires have additional issues such as oxidation and chemical stabilities upon long-term use [10]. As for ITO electrodes, the low flexibility and high cost impede their further progress [11]. Therefore, considering the drawbacks of those emerging alternatives and for universal purpose, the stable Ni/Au electrodes are among one of the ideal choices as long as the light extraction efficiency is improved.

To improve the light extraction efficiency of LEDs, various approaches are used, including surface patterning [12], patterned sapphire substrates [13] and photonic crystals [14]. Surface patterning has been adopted as an effective approach to extract photons trapped within LED chips due to the total internal reflection by many groups. They patterned the transparent conductive layer with nanoimprint lithography [15], holographic lithography [16] and electron beam lithography [17]. However, electron beam lithography and holographic lithography are limited by expensive equipment and low yield [18]. Nanoimprint lithography can be applicable for large-scale production, but the stamp used for imprinting process is very expensive and the technique is still in development [19].

In this study, we apply a unique patterning method, the multiple-exposure colloidal lithography, to pattern the Ni/Au semi-transparent conductive electrodes of GaN-based LEDs so as to further improve the extraction efficiency. Traditional colloidal lithography can only obtain single hole patterns at one time [20]. However, our novel colloidal lithography can achieve various patterns with different opening areas in the same situation. It is a low-cost and high-throughput method for the fabrication of periodic nanostructures for optical applications. With this technique, we investigated how the scattering and diffraction effects of different hole arrays influence the light output of patterned Ni/Au LEDs and obvious power enhancement of LEDs is achieved.

2. Experiments

The blue LEDs with InGaN/GaN multiple quantum wells were grown on c-plane (0001) sapphire substrates via metalorganic chemical vapor deposition (MOCVD). After the growth, p-electrode was patterned by photolithography and a thin Ni/Au (5 nm/10 nm) layer used as p-type semi-transparent conductive electrode was evaporated onto the wafers. Then, we performed multiple-exposure colloidal lithography to pattern Ni/Au layer, as shown in Fig. 1(a) . Firstly, we deposited a polymer resist (PR) layer onto the Ni/Au LED wafers using spin coating, and at the same time, polystyrene (PS) colloidal spheres with a diameter of 1.2 μm were self-assembled at the air/water interface to form a hexagonal closed-packed monolayer. Later on, we transferred the PS colloidal spheres monolayer onto the prepared polymer resist wafer surface and dried them in the air. Next, we put a wafer on a level base plate and performed ultraviolet (UV) exposure to get single hole array patterns, as shown in Fig. 1(b). It was the same process as traditional colloidal lithography. In order to obtain different geometry of patterns, we put another wafers onto the tilted base plate with an inclination angle of α. Here α was set to be 26.6° to avoid overplay of patterns produced by multiple-exposure. In addition, we rotated the wafers on the tilted base plate with different angles of φ every time when performing UV exposure. When a wafer was exposed at φ = 0° and 180°, twined hole regions would be exposed on the both sides of the central symmetric axis of each PS sphere. Similarly, When a wafer was rotated with φ = 0°, 120° and 240° and exposed every time while rotating, three equally spaced hole regions would be exposed around the central symmetric axis of each PS sphere as well, as shown in Fig. 1(c). After multiple exposure was finished, PS colloidal spheres were needless, so we removed them with a blue adhesive tape. Then we developed the samples for 3 s at room temperature. Inductively coupled plasma (ICP) with Ar and a bias power of 400W for 30 s was applied for dry etching and acetone was used to wet etch residual PR. Subsequently, samples were annealed at 600°C for 1 minute in the air to achieve Ni/Au p-ohmic contact. Lastly, the LED wafers were processed into chips with a square mesa of 45mil × 45mil in size and n-GaN layer was exposed by photolithography and inductively coupled plasma reactive ion etching (ICP-RIE). Metal contacts composed of Cr/Pt/Au (70/40/1440 nm) were evaporated onto the p-type Ni/Au hole array semi-transparent conductive layer and the n-type GaN layer by an e-beam evaporator. For comparison, an non-patterned Ni/Au LED was also fabricated as the reference sample.

 

Fig. 1 (a) Schematic illustration of the process for the fabrication of Ni/Au LEDs with different hole array patterns by multiple-exposure colloidal lithography. (b) Schematic illustration of the formation of single (α = 0°; φ = 0°) hole array patterns. (c) Schematic illustration of the formation of twined (α = 26.6°; φ = 0° and 180°) and triplet (α = 26.6°; φ = 0°, 120° and 240°) hole array patterns.

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3. Results and discussion

Figures 2(a)-2(d) show the top view scanning electron microscope (SEM) images of Ni/Au LEDs p-type semi-transparent conductive layer surface without patterns, with single hole array patterns, with twined hole array patterns and with triplet hole array patterns, respectively. It can be noted that the Ni/Au layer are patterned with an ordered hexagonal distribution of hole arrays after multiple-exposure colloidal lithography. The periods of the arrays in Fig. 2(b)-2(d) are 1.2μm, and the hole diameters are almost 400 nm.

 

Fig. 2 Top view SEM images of Ni/Au LEDs p-type semi-transparent conductive layer surface (a) without pattern, (b) with single hole array patterns, (c) with twined hole array patterns, (d) with triplet hole array patterns.

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The relationship between normalized transmission and wavelength for Ni/Au electrodes with and without patterns are plotted in Fig. 3 . These experimental data are originated from the transmittance measurement of single, twined, triplet hole arrays patterned Ni/Au films and non-patterned Ni/Au film on quartz. The effect of the substrates has been excluded. For comparison purposes, we normalize the peak transmittance of non-patterned Ni/Au film as 1 and it can be calculated that the single, twined and triplet hole array patterned Ni/Au films are more transmissive than non-patterned Ni/Au film with a factor of 1.20, 1.33 and 1.50 at a wavelength of 514 nm, respectively. For the wavelength from 400 nm to 800 nm, the improvement in transmittance is obvious. The larger opening areas allow more photons emitting out from multiple quantum wells (MQWs) within LED chips, thus increasing the transmittance of the Ni/Au layer.

 

Fig. 3 The normalized transmittance of Ni/Au LEDs without patterns and with single, twined, triplet hole array patterns.

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The measured I-V characteristics of the fabricated LEDs are shown in Fig. 4(a) . Under 350 mA current injection, it is found that forward voltages are 5.67, 6.00, 6.03, and 6.08 V for Ni/Au LEDs without patterns and with single, twined, triplet hole array patterns, respectively. The relatively high forward voltage of Ni/Au LED can be attributed to the fact that the annealing atmosphere and temperature of Ni/Au electrodes was not optimized. Besides, the patterning might also affect the electrical properties of Ni/Au electrodes [21]. As the opening area of patterned Ni/Au layer increases, the forward voltages of the corresponding patterned LEDs rise as well. The voltage of the triplet hole array patterned Ni/Au LED is highest under 350 mA for the fact that the electrical resistance of the film is inversely proportional to its cross-sectional area and the voltage is in direct proportion to the electrical resistance. The turn-on voltages of the Ni/Au LEDs without patterns and with single / twined hole array patterns are almost same, about 2.5 V, however, that of the Ni/Au LED with triplet hole array patterns is higher for about 2.9 V, suggesting that the carrier transport characteristics of the triplet hole array patterned Ni/Au LED are not as good as those without and with single / twined hole array patterns. Figure 4(b) shows electroluminescence (EL) spectra of the fabricated Ni/Au LEDs without and with single, twined, triplet hole array patterns. EL was measured as a function of the wavelength at 350 mA. There are no significant differences in EL peak positions (at 466 nm) of the Ni/Au LEDs without and with different patterns. Nevertheless, the EL intensity of the LEDs with patterns is higher than that of the LEDs without patterns, especially of the twined hole array patterned LEDs. This result indicates that the EL intensity of twined hole array patterned LEDs is an optimized balance between enhanced light extraction and degenerated current spreading.

 

Fig. 4 (a) The I-V characteristics of Ni/Au LEDs without patterns and with single, twined, triplet hole array patterns. (b) EL spectra of Ni/Au LEDs without patterns and with single, twined, triplet hole array patterns.

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Figure 5 shows the light output power as functions of current for the Ni/Au LEDs. At a driving current of 350 mA, the light output power of LEDs are 43.92, 63.93, 80.63, 50.73 mW for the Ni/Au LEDs without patterns and with single, twined, triplet hole array patterns, respectively. In other words, we can achieve 45.6%, 83.6% and 15.5% improvement in light output by patterning Ni/Au electrodes with single, twined, triplet hole arrays. We can achieve the highest light output from the twined hole arrays patterned Ni/Au LED mainly due to the diffused scattering effect. However, the decrease in light output of triplet hole arrays patterned Ni/Au LED may be attributed to a poor current spreading profile when the hole opening area is too large, damaging the electrical characteristics of the devices. It is also found that output power increases with the injection current and no obvious intensity saturation is observed up to 400 mA for all these LEDs.

 

Fig. 5 The light output performance of Ni/Au LEDs without and with different patterns.

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The light output radiation patterns of these LEDs at 200 mA current are demonstrated in Fig. 6 . It can be noted that the light emission from the patterned Ni/Au LEDs is enhanced, mostly in the vertical direction. It indicates that the hole arrays of patterned Ni/Au LEDs increase the probability of photons emitting directly out of the top electrode surface from the MQWs region compared with that of the non-patterned Ni/Au LEDs. It was also found that Ni/Au LEDs with single, twined, triplet hole array patterns possess much larger view angles of 51.4°, 52.6°, 53.0°, respectively, compared to that of 49.6° for Ni/Au LEDs without patterns. This implies that the periods of these hole arrays are so large that there exist many diffraction orders in the blue light region, resulting in light leakage along many directions [22]. What’s more, the scattering effects of light around the hole arrays make it more obviously, especially for the triplet hole array patterned Ni/Au LEDs [23].

 

Fig. 6 Far-field emission patterns of Ni/Au LEDs without and with different patterns at a driving current of 200 mA.

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4. Conclusion

In summary, we propose a simple, universal, low-cost, and high-throughput multiple-exposure colloidal lithography method to pattern Ni/Au semi-transparent conductive electrodes of nitride-based LEDs. It is found that we have achieved 45.6%, 83.6% and 15.5% improvement in light output at 350 mA by patterning Ni/Au electrode with single, twined and triplet hole array, respectively. The novel lithography technique will not obviously degrade the electrical properties of the fabricated LEDs only if the opening area of the electrode is not too large. In addition, patterned Ni/Au LEDs possess much larger view angles than non-patterned ones. This work offers a promising potential technique to increase the light output power of LEDs.