Preparation of a Periodic Polystyrene Nanosphere Array Using the Dip-Drop Method with Post-deposition Etching and Its Application of Improving Light Extraction Efficiency of InGaN/GaN LEDs
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In this study, we synthesized a periodic polystyrene nanosphere (PS NS) array using the dip-drop method with post-deposition etching to improve the light extraction efficiency (LEE) of InGaN/GaN light-emitting diodes (LEDs). The dip-drop method has advantages such as simple procedure, inexpensive equipment, room temperature deposition, and easy implementation in LEDs. The arrangement of PS NSs on an indium-tin-oxide (ITO)-coated glass substrate depends on the average dip-drop speed and the concentration of the PS NS suspension. The periodic PS NS array can modulate the in-plane wave vector of emission light from a semiconductor to free space and thus increase the escape probability. The calculated and experimental results indicated that the light output intensity of the InGaN/GaN LEDs can be improved by using the periodic PS NS array as a window layer; this array comprises PS NSs with a diameter of 100 nm separated with periods of 100 and 100 nm in the x and y directions. Because of the improved LEE, the InGaN/GaN LEDs with the optimal PS NS array window layers exhibited a 38% increase in light output intensity compared with the conventional InGaN/GaN LEDs under 20-mA driving current.
KeywordsPolystyrene nanospheres Dip-drop method Post-deposition etching InGaN/GaN LEDs
External quantum efficiency
Light extraction efficiency
Light output intensity-current
- PS NS
Scanning electron microscopy
Total internal reflection
Recently, photonic crystals (PCs) have been widely investigated to improve the efficiency of optoelectronic devices such as light-emitting diodes (LEDs) , solar cells , and photodetectors . PCs are structures in which a periodic variation in the refractive index occurs at the scale of the wavelength of light in one or more directions [4, 5]. The structure of PCs with a sufficiently large refractive index contrast can yield a photonic bandgap in which the frequency range of propagating light is forbidden. The light extraction efficiency (LEE) of LEDs can be improved by using the PCs through two methods. One approach is to design the PC structure with a bandgap to match the trapped waveguide modes within the LED. The waveguide light within the bandgap of the PC is blocked in the lateral direction in the structure and guided to the only external emission channel for the light to exit the device. However, this approach is difficult to realize because of the significant material processing problem of creating a planar structure with a sufficiently large refractive index contrast to open a full optical bandgap. Another approach is to utilize the periodic refractive index of the PC to diffract the waveguide mode above a certain cutoff frequency into externally propagating modes: k ‖m = k ‖ + nk pc , where k ‖m and k ‖ are the modified and original in-plane wave vectors, respectively; n is an integer; and k pc is the reciprocal wave vector depending on the PC lattice constant. When the periodicity is chosen correctly, the modified in-plane wave vector falls within the escape corn, resulting in extraction to air at an angle dependent on the specific lattice constant within this range. Several methods exist to define the periodic PC structures on indium-tin-oxide (ITO) or p-GaN, including electron beam lithography [6, 7, 8, 9], laser holographic lithography , focused ion beam technology , nanoimprint lithography , and self-assembled colloidal polystyrene nanosphere (PS NS) coating [13, 14]. The self-assembled PS NS coating method has advantages such as a large area arrangement with a gradually changing fill factor, simple process, sophisticated equipment, and etching damage.
Gallium nitride-based LEDs with wavelengths from ultraviolet to blue/green have attracted considerable research attention [15, 16]. GaN-based LEDs with high brightness can be used in applications such as large-size full-color displays, short-haul optical communication, traffic signal lights, and backlights for color liquid crystal displays [17, 18, 19]. The brightness of GaN-based LEDs depends on the external quantum efficiency (EQE), which is the product of internal quantum efficiency and LEE. Because of the inherently high refractive index contrast between free space and the semiconductor material, the calculated critical angle for the generated light to escape from the p-GaN layer into the air is approximately 23°. The small critical angle indicated that few photons can be extracted from the device due to the total internal reflection (TIR). Thus, the LEE of GaN-based LEDs is very low, leading to a low EQE for GaN-based LEDs. Several studies [20, 21, 22, 23] have employed textured or patterned sapphire as a back reflector to increase the number of escape photons. The LEE for GaN-based LEDs with textured or patterned sapphire can be improved by the high probability of photons reflected from sapphire. However, the mechanically and chemically strong nature of sapphire renders roughening and patterning a challenging task. In addition, achieving the small dimensions of scattering objects through photolithography is difficult because of the short wavelength of nitride-based LEDs. Studies [24, 25, 26] have reported that a textured GaN surface can be used to increase the critical angle to enhance the LEE. However, surface texturing of GaN-based LEDs is impeded by the thin p-GaN and the sensitivity of p-GaN to plasma damage and electrical deterioration. In addition to the textured GaN surface, some studies [27, 28] have attempted to roughen the mesa sidewalls through photochemical etching or create oblique mesa sidewalls through a reflowed photoresist and adjust the CF4 flow during dry etching to increase the LEE. However, the surface of the rough mesa sidewalls was nonuniform, and the improved LEE for oblique mesa sidewalls was restricted within the sidewall region .
In this study, we investigated the conditions for compact and periodic PS NS array on an ITO surface using the dip-drop method with post-deposition etching and performed parametric analysis to optimize the LEE of InGaN/GaN LEDs with the periodic PS NS array. The deposition parameters of the compact PS NS array are the dip-drop speed and the concentration of the PS NS suspension. The calculated results indicate that the LEE of InGaN/GaN LED is related to the PS NS diameter and period of PS NSs. The InGaN/GaN LEDs with and without an optimal periodic PS NS array on ITO are compared.
Fabrication of InGaN/GaN blue LEDs with a Periodic PS NS Array on an ITO Layer
The epi-wafers of InGaN/GaN blue LEDs were grown on a c-face (0001) sapphire substrate by using a metal-organic chemical vapor deposition system. The device structure consists of a GaN buffer layer grown at a low temperature, a highly Si-doped n-type GaN layer, an InGaN/GaN multiple quantum wells (MQWs) active region, and an Mg-doped p-type GaN layer. The ITO was deposited on the p-type GaN layer as a transparent conductive layer to spread the injection current. The wafer was then patterned using the standard photolithographic process to define square mesas as the emitting regions by partially etching the exposed ITO/p-GaN/InGaN/GaN MQWs/n-GaN. A Ti/Pt/Au alloy was used as the ohmic contact metal on the p- and n-GaN contact regions, and the wafer was then alloyed in an N2 atmosphere for 5 min at 450 °C. The size of the emission window for the InGaN/GaN LEDs with ITO was 300 × 300 μm2. The finished wafer was placed in the PS NSs suspension to deposit the compact PS NS array on ITO layer.
Results and Discussion
The average turn-on voltage and light output intensity for selected chips from different position of InGaN/GaN LED wafers with the optimal PS NS arrangement (PS NS diameter of 100 nm and period of 100 nm) on an ITO window layer under a 20-mA driving current
Turn-on voltage (V) at 20 mA
Light output intensity (mcd) at 20 mA
PS NS array window layers can improve the LEE of InGaN/GaN LEDs. A compact monolayer PS NS array was obtained by adjusting the average dip-drop speed and PS suspension concentration. The optimal average dip-drop speed and PS NS suspension concentration to obtain a compact monolayer PS NS array were 0.005 mL/s and 4.1 × 1011 sphere/cm−3, respectively, for 100-nm PS NSs; 0.005 mL/s and 5.1 × 1010 sphere/cm−3, respectively, for 200-nm PS NSs; and 0.005 mL/s and 3.2 × 109 sphere/cm−3, respectively, for 500-nm PS NSs. The calculated and experimental results indicated that the periodic PS NS array window layer with PS NS diameter of 100 nm and periods of 100 nm in the x and y directions effectively enhanced the LEE of the InGaN/GaN LEDs. The InGaN/GaN LEDs with the optimal periodic PS NS array window layer yielded a 38% increase in light output intensity compared with that of the conventional InGaN/GaN LED under a 20-mA driving current because of the high LEE.
The authors gratefully acknowledge the financial support of the Ministry of Science and Technology, ROC (103-2221-E-150-033-MY2).
Availability of Data and Materials
The datasets supporting the conclusions of this manuscript are included within the manuscript.
PHL carried out the experimental design and data analysis. CDY simulated the compact and periodic PC NS array by using FDTD. YSY carried out the experimental process and measurement. JHL carried out the LED simulation. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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