Performance of GaN-Based LEDs with Nanopatterned Indium Tin Oxide Electrode

1School of Electronic and Information, Guangdong Polytechnic Normal University, Guangzhou 510665, China 2State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China 3College of Electromechanical Engineering, Guangdong Polytechnic Normal University, Guangzhou 510635, China 4School of Electronics and Information Technology, Sun Yat-Sen University, Guangzhou 510275, China

Compared to the other techniques, NSL has the advantages of low cost and high throughput, which is very suitable for surface patterning.In the previous works, the twodimensional photonic crystal structure was fabricated on an InGaN/GaN multiple quantum well structure by a silica nanosphere lithography, and several times enhancement in photoluminescence intensity was observed [21,22].However, there was no research on the electroluminescence.Comparing to the silica nanosphere, the size of polystyrene (PS) nanosphere will be decreased with increasing the etching time as an etching mask.Through the PS NSL method, nanopillars with different diameters can be obtained and the LEE of LEDs can be optimized [23].In addition, the nanopillar structure will take the characteristic of small top and big bottom, and this is helpful for the light to escape due to the gradient refractive index.In order to reduce the influence of etching on the electrical characteristics of the LED device, the nanostructures should be prepared on the ITO transparent electrode layer.However, this research has rarely been reported.In this work, by PS NSL technology, GaN-based blue LEDs with surface patterned ITO electrode were fabricated and the optical and electrical performances of the LEDs with nanopatterned ITO electrode were analyzed and discussed.The electroluminescence intensity of the ITO patterned LEDs is increased by 17% at 100 mA injection current compared to that of conventional LEDs.Finally, the light output enhancements are simulated based on three-dimensional finite difference time domain (3D-FDTD) method to verify the experimental results.

Experimental Methods
The GaN-based ( = 465 nm) epitaxial wafer was grown on a 2-inch sapphire (Al 2 O 3 ) substrate using a metal-organic chemical vapor deposition.After the growth of a 2 m undoped GaN (u-GaN) buffer layer and 3 m n-GaN layer, an active layer of five-period InGaN/GaN MQWs and a 150 nm p-GaN layer were deposited.The device fabrication process was as follows.A transparent ITO electrode with a thickness of about 400 nm was first deposited on the p-GaN surface.Then, the LED chips with dimensions of 300 m × 300 m were formed by mesa-etching the exposed n-type GaN via standard lithography, ITO wet etching, and subsequent inductively coupled plasma (ICP) etching.Cr/Pt/Au (200/400/2000 nm) was finally deposited on the top of ITO surface as well as the exposed n-GaN layer as a contact metal for both the p-and the n-GaN layers.
After the formation of the contact metal electrodes on the p-and the n-GaN layers, the nanopatterned ITO layer was fabricated through the NSL, and the process flow was shown in Figure 1.First, a hexagonal close-packed monolayer of PS nanospheres with a 450 nm diameter was formed on the ITO layer, as shown in Figure 1(a).Second, the ITO layer with a monolayer mask of PS spheres was etched by an ICP etching machine via the gas flow of BCl 3 /Cl 2 /Ar, as shown in Figure 1(b).Finally, the PS spheres were removed by trichloromethane solvent with sonication and the periodic ITO nanopillar arrays were obtained, as shown in Figure 1(c).Figure 1(d) presented the schematic of the modified LED structure with a nanopillar patterned ITO electrode.

Results and Discussion
By the method described above, we could fabricate the LEDs with periodic ITO nanopillar arrays.When the ITO was etched by ICP, the mask of PS spheres was also etched and the size of the PS spheres was altered.Therefore, various height and diameter of the top part of the ITO nanopillar could be obtained by changing the ICP etching time.In order to explore the different effects of various ITO nanopillar structure on LEE of the LED, three nanopatterned ITO samples were fabricated by different ICP etching time.The three samples with the nanopillar ITO layer etched by ICP for 60 s, 80 s, and 100 s were marked as samples A, B, and C, respectively.The three nanopatterned ITO samples and conventional LEDs were fabricated from the same InGaN/GaN LED wafer to eliminate the differences in the device characteristics.Figure 3(a) shows the light output intensity (LOI) as functions of injection current for the three nanopatterned LEDs and a conventional LED.At the same injection current, the light output intensity of the three nanopatterned LEDs is higher than that of the conventional LED.At an operating current of 100 mA, the light output intensity of the samples A, B, and C is approximately 3%, 14%, and 19% higher than that of the conventional LED, respectively.Figure 3(b) shows the forward current-voltage (I-V) characteristics of the four samples.It is clear that the LED with nanopatterned LEDs exhibits nearly the same I-V characteristics as the conventional LED.The forward voltages at 100 mA are 4.42, 4.46, 4.46 V, and 4.37 for the samples A, B, and C and conventional LED, which consequently indicates an acceptable electrical performance for the nanopatterned LEDs.In order to eliminate differences in input electric power (IEP) of the samples, the IEP-LOI curves were calculated from the curves in Figures 3(a conventional LED at the whole IEP range.At an operating current of 100 mA, the LOI of the samples A, B, and C are approximately 3%, 12%, and 17% higher than that of the conventional LED, respectively.Therefore, we believe that the proposed technology is an effective method to improve the LEE of GaN-based LEDs. The results showed that the LEE of the samples with nanopillar patterned structures were enhanced.With increasing the etching depth, the size of the nanopillar became smaller, and the light extraction efficiency of the sample would be relatively higher.With the nanopillar patterned ITO surface, photons should experience multiple scattering at the sample surface and could escape from the device easily, as shown in Figure 4.For the LEDs with the flat surface ITO structure, the emitted light having the incident angle smaller than the critical angle was the only light that can be extracted, as shown in Figure 4(a).According to Snell's law, the critical angle   of the total internal reflection satisfied the formula sin   =  2 / 1 , where  1 = 1.9 and  2 = 1 are the refractive indexes of ITO and air, respectively.Then, the critical angles of total reflection at air/ITO interface are around 31.8 ∘ .Thus, the majority of photons are reflected from the interface of conventional ITO LEDs [17].The nanopillar patterned LEDs have higher LEE, which can be explained from various points of view.Firstly, the nanopillar array played the role of roughness.Secondly, the periodic nanopillar array served as a two-dimensional grating.Such Bragg scattering assisted the waveguide modes to become the radiation mode; then  the photons have taken more opportunities to escape from the ITO surface into the air, as shown in Figure 4(b).The transmitted and reflected light can be expressed as [24]  This will make the photons that were originally emitted out of an escaping cone go back into the escaping cone and improve the LEE of the LEDs.
In order to verify the experiments, we also perform 3D-FDTD simulations to give the light extraction of the LEDs with nanostructures.The simulated LED structure is shown in Figure 5(a), which consisted of a 1000 nm sapphire substrate, a 5200 nm GaN layer (including the u-GaN, MQW, and p-GaN layer), and a 400 nm ITO layer.In simulation, the lateral space of simulation region is 8000 nm × 8000 nm, and there are about 18 × 20 nanopillars in the simulation region.To explore the effect of the diameters and heights of the nanopillars on the LEE, eight samples are simulated here.For samples 1, 2, and 3, the diameters are set as 400 nm, 390 nm, 380 nm, and the heights are set as 60 nm, 100 nm, and 140 nm, respectively.For samples 4-8, the diameters of the nanopillars are set as 380 nm and the heights are set as 180 nm, 220 nm, 260 nm, and 300 nm, respectively.The light extraction effect of LED with different nanostructures is simulated by the 3D-FDTD method.In the simulation, the wavelength of incident light is 465 nm, which corresponds to the center wavelength of emission spectrum.The refractive indices of GaN and ITO are approximately 2.49 and 1.9 at the wavelength of 465 nm.The simulation results are shown in Figure 5(b).The horizontal coordinate is the number of simulated samples, and the longitudinal coordinate is the increase ratio of LEE.The simulation results show that the LEE of samples 1-3 is increased, which is consistent with the experimental results.In addition, we also see that simulated sample 3 (height of 140 nm) is a local optimal value, and the structure of the optimal value is near sample 6 (height of 260 nm).This is consistent with the rough calculation [25] where  is the incident light wavelength and  ITO-pc is the effective refractive index of the ITO nanostructures layer.

Conclusions
Benefiting from the excellent electrical conductivity and light transmittance, ITO transparent electrode has replaced nickel gold alloy as a transparent electrode of the LEDs.However, the refractive index of ITO is 1.9 and is much higher than the refractive index of air, which limits the light escaping from LEDs.In this paper, we fabricated LEDs with nanopillar patterned ITO layer via nanosphere lithography.The optical and electrical performances of the LEDs with nanopatterned ITO electrode were investigated.The results show that the LEE was enhanced with increasing the etching depth.The electroluminescence intensity of the ITO patterned LEDs was increased by 17% at 100 mA injection current compared to that of conventional LEDs.The enhancement of the LEE can be ascribed to the fact that the total reflection of the ITO surface is broken by the periodic nanopillars structure.The LEE may be further improved by optimizing the nanopillars structure.Therefore, this is a promising method for realizing high-efficiency LEDs.

Figure 1 :
Figure 1: Schematic fabrication process for nanopatterned ITO electrode.(a) Deposition of PS spheres on the ITO electrode surface.(b) ICP etching to nanopatterned ITO electrode.(c) Removal of the PS nanospheres.(d) Schematic illustration of the GaN-based blue LEDs with nanopatterned ITO electrode.

Figure 2 :
Figure 2: SEM images of the nanopillar patterned ITO layer.Scale bars are 400 nm.(a) The original surface of ITO layer.(b)-(d) 30 ∘ tilt SEM images of samples A, B, and C, respectively.(e)-(f) Cross-sectional image of samples B and C. The ITO layers of samples A, B, and C were etched by ICP etching for 60 s, 80 s, and 100 s, respectively.
Figure 2 shows scanning electron microscope (SEM) images of ITO layer of samples A, B, and C and the conventional LED.As shown in Figures 2(b)-2(d), when the ICP etching time is longer, the surface of ITO is etched more obviously and the height of the nanopillars becomes higher.The diameters of nanopillars for samples A, B, and C are found to be about 400 nm, 390 nm, and 380 nm from the SEM images, respectively.Figures 2(e) and 2(f) show cross-sectional images of samples B and C. The heights of the nanopillars of samples A, B, and C are about 60 nm, 100 nm, and 140 nm, respectively.

Figure 3 :
Figure 3: Electroluminescence curves of three nanopillar patterned LEDs and a conventional LED.(a) The optical output power versus injection current (O-I) characteristics.(b) The current versus voltage (I-V).(c) The optical output power versus power (O-P) characteristics.

Figure 4 :
Figure 4: Light emission of (a) conventional ITO LEDs and (b) LEDs with nanopatterned ITO electrode.

Figure 5 :
Figure 5: (a) Schematic illustration of LED simulation model.(b) The 3D-FDTD simulation result, in which the -axis is the serial number of the simulation samples, and the -axis is the increase ratio of LEE compared with reference sample.
2  sin   (cos   , sin   )  1 and  2 are refractive indices of ITO and air, respectively. is the wavelength of light in the vacuum.Λ  and Λ  are periods of the lattice in the  and  directions. and  are integers indicating the diffraction orders.  ,   , and   are the incident angle, transmitted angle, and reflected angle, respectively.  ,   , and   are the azimuthal angle of incident light, transmitted light, and reflected light, respectively.So, the waveguide modes can be coupled to the radiation modes.