Self-assembled Ni nano mask for nano-textured LEDs Mumta Hena Mustary, Volodymyr V. Lysak*

Abstract— Light enhancement of GaN based light emitting diodes (LEDs) have been investigated by texturing the top p-GaN surface. Nano-textured LEDs have been fabricated using self-assembled Ni nano mask during dry etching process. Experimental results were further compared with simulation data. Three types of LEDs were fabricated: Conventional (planar LED), Surface nano-porous (porous LED) and Surface nano-cluster (cluster LED). Compared to planar LED there were about 100% and 54% enhancement of light output power for porous and cluster LED respectively at an injection current of 20 mA. Moreover, simulation result showed consistency with experimental result. The increased probability of light scattering at the nano-textured GaN-air interface is the major reason for increasing the light extraction efficiency.

Index Terms— Light emitting diode, Nano porous, Nano cluster, Rapid Thermal Annealing (RTA), Finite Difference Time Domain (FDTD).

Light emitting diodes have recently been attracted much interest and developed rapidly for numerous applications such as traffic signals, full color display, solid state lighting, backlighting in liquid crystal displays [1]. In recent years, GaN based blue LED together with yellow phosphor is one of the key component for high efficiency white LED. Large refractive index difference between GaN (n=2.5) and air (n=1) limits the light extraction efficiency to only a few percent [2]. Over the past few decades, extensive efforts have been addressed ways to improve light extraction efficiency including chip shaping, patterned substrate, photonic crystal, surface roughening with micro and nano sized texture, nano-porous p-GaN surface. Among them surface texture is most feasible and cost effective method. In addition, further improvement in light extraction is possible if the roughening approach is reduced to nanoscale [3].

To our best knowledge, an effective method of nano-texture p-GaN is still not available. Top p-GaN is usually very thin, thus texturing p-GaN is a great challenge and it is difficult to control plasma damage during etching process. Due to the very thin thickness of top p-GaN layer, roughening of p-GaN is a great challenge. It is not easy to control the etch depth by dry or wet etching, as well as the damage during the etching process, so that it is difficult to obtain a p-GaN surface that is rough enough for effective light scattering.

In this work, we have fabricated surface nano-textured LEDs using self-assembled Ni surface as an etching mask that is very cost effective process. A very thin Ni layer was deposited directly on the top p-GaN avoiding SiO2 deposition before Ni deposition and SiO2 etching process that leads to a complicated fabrication procedure. Ni layer of thicknesses 10nm and 3nm were deposited on p-GaN surface followed by rapid thermal annealing at 850o C under N2 ambient for 15s and 30s for porous and cluster surface respectively. Those surfaces were further used as an etching mask for p-GaN texture using inductively coupled plasma (ICP) dry etching process. Our work reported a significant improvement of light output power i.e. 100% and 54% for porous and cluster LEDs respectively at an injection current 20mA. Furthermore, experimental result was compared with Finite Difference Time Domain (FDTD) simulation method to study the efficiency enhancement trend. GaN based blue LEDs used in this study were grown on C-plane sapphire substrate by Metal Organic Chemical Vapor Deposition (MOCVD). The epilayers structure consist of 2µm thick undoped GaN layer, 2µm thick n-GaN layer, seven-periods multiple quantum wells (MQWs) of InGaN/GaN pairs and 0.2µm thick top p-GaN layer. First LED samples were cleaned by acetone, methanol and D.I water to remove the unexpected residual from the surface. We have fabricated three types of LEDs: A conventional LED (planar LED), p-GaN surface textured using nano-porous Ni surface as a mask during ICP etching (porous LED), p-type GaN surface textured using Ni cluster as a mask during ICP etching (Cluster LED). All the LEDs were fabricated using the same process steps except for the surface texturing steps [4]. Fig. 1 shows the top views scanning electron microscope (SEM) images of Ni surface after RTA. For 10nm thickness of Ni layer, after annealed at 850C for 15s under N2 ambient, nano sized polygonal pores with average size ranging from 300-400nm was appeared due to shrinkage of Ni film by driving force of surface tension [Fig 1(a)]. For 5nm thick Ni layer pore size increase and merge into one another after annealing at 850C for 30s under N 2 ambient [Fig. 1(b)]. However, perfect Ni cluster surface with average size of 150-250 nm was appeared for 3nm thick layer followed by RTA at 850C for 30s under N2 ambient [Fig. 1(c)]. The effect of annealing at higher temperatures allows mass transport of either individual Ni atoms or small clusters across the surface in which larger dispersive clusters could be formed out of smaller clusters in a process analogous to Ostwald ripening while coarsening is the evidence for natural self-organization of surface adatoms, where the driving force is the reduction in surface energy of the islands [5]. Surface

morphology was also analyzed at different annealing time with same Ni layer thickness but did not seem to be changed much with increasing annealing time.

Fig. 1: SEM images of Ni film onto p-GaN surface after RTA at (a) 850C-N2-15s, 10nm (nano porous surface) (b) 850C-N2-30s, 5nm and (c) 850C-N2-30s, 3nm (nano cluster surface) thickness.

Fig. 2 demonstrates the current-voltage (I-V) characteristics of planar and surface nano-textured LEDs. It has been shown that electrical property degrades in surface textured LEDs.

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Fig. 2: Current-Voltage (I-V) characteristics of planar, porous and cluster LED. The inset shows the light emission images at an injection current of 15 mA.

For our nano-textured LEDs fabrication ICP dry etching process was applied which may cause plasma damage. The p- GaN is more sensitive than the n-GaN to plasma damage [6]. Future research is still necessary to carry out a better-controlled plasma etching process for nano-roughening. Nano-roughening could leads to poor Ohmic contact, which may cause the increasing of thermal heat generation [7] and as a result, the degradation of device performance at high input currents. Forward voltage is measured at 20mA are 3.0V, 3.9V and 3.6V for planar, porous and cluster LED respectively. This result indicates that the nano-textured LEDs are affected by plasma damage during short dry etching process. In the porous structure, the pore dimensions can be too small to fully fill-up by ITO. It allows increasing the resistance on p-GaN-ITO interface comparing to cluster one. In same time, higher interface resistance leads to more uniform hole distribution in p-GaN contact [8] that improves the output optical power as shown in the inset of Fig.2.Schematic cross-sectional view of the FDTD computational domain is shown in Fig.3a. Simulation model consist of sapphire substrate, n-GaN, MQW and p-GaN and thickness of each layer is similar to our experimental sample as mentioned before. The refractive indices of the sapphire substrate, the GaN layers, the InGaN MQW and ITO layers are set to be 1.78, 2.52, 2.58 and 2.0 respectively. The perfectly-matched layer (PML) boundary surrounds the simulation window and allows waves to exit without producing any unwanted reflections. Two observation line was placed; one on the top on MQW and other on the top of LED surface. Light extraction efficiency is calculated by the ratio of Poynting vectors integrated over the extraction LED surface to integrated Poynting vectors integrated over the MQW and only top emission is calculated. After that efficiency enhancement nano-textured LEDs was calculated by comparing with planar LED.

From the experimental L-I measurement in fig.2, slope efficiency or overall efficiency enhancement calculated from injection current 20 mA to 100mA was 34% and 23% for porous and cluster LED, whereas simulation result showed an efficiency enhancement of 43% and 25% for porous and cluster LED respectively. Fig. 3b shows the comparison of percentage of light intensity improvement between experimental and simulation results. Simulation result showed that efficiency enhancement trend is close and consistent to our experimental data. However, there is considerable difference for porous LED between the simulated and the experimental data. This difference was expected because of the differences in the structural design specially ITO surface is not perfect hemisphere in case of porous LED.

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(b)Fig. 3: a) FDTD simulation model; b) Light intensity enhancement of different samples and comparison between experimental and simulated data.

We have fabricated surface nano-textured LED using self-assembled Ni nano porous and nano cluster surface as an etching mask. In addition, extraction efficiency of LED is simulated by 2D-FDTD simulation method for comparison with experimental results. Nano porous and nano cluster LEDs were fabricated using 10nm and 3 nm thickness of Ni layer respectively followed by rapid thermal annealing. Nano- textured LEDs showed a great enhancement of light output power. Our experimental nano-porous and nano-cluster LED showed 100% and 54% enhancement of light output power at an injection current of 20mA respectively. From the experimental result, overall efficiency enhancement was 34% and 23% for porous and cluster LED, whereas simulation result showed an enhancement of 43% and 25% for porous and cluster LED respectively. Thus, simulation results showed consistency with the experimental results.The Work is partly supported by the Government of the Russian Federation (Grant 074-U01) through ITMO Early Carrier Fellowship scheme.

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