Improvement in Light Extraction Efficiency of High

Improvement in Light Extraction Efficiency of

High Brightness InGaN-Based Light Emitting Diodes

Tzer-Perng Chen, Ta-Cheng Hsu, Chuan-yu Luo, Ming-Chi Hsu, Tsung-Xian Lee Epistar Corporation, 5 Li-hsin 5th Rd., Science-based Industrial Park, Hsinchu, Taiwan 300, R.O.C.

ABSTRACT

Light extraction efficiency is important to the brightness of LEDs. In this study, various texturing and roughing schemes were formed on the surface or interface of InGaN-based LED structure grown on sapphire substrate to investigate their effects. Throughout the research, temperature-dependent PL measurement was used to calculate the internal quantum efficiency so as to derive the light extraction efficiency. The light extraction efficiency is around 60 to 65% while the epitaxy and substrate are flat. On the other hand, the light extraction efficiency reaches an optimal value of around 85% while the p-GaN surface is textured and the substrate is patterned. However, for LED having only one-side surface texturing structure optimized on either p- or n-side, the light extraction efficiency can be already as high as 75 to 80%. Methods for further enhancement, such as use of ZnO nanorod on chip surface, were also discussed.

Keywords: InGaN, LED, internal quantum efficiency, light extraction efficiency, surface texturing

1. Introduction

Light-emitting diodes (LEDs) are regarded as the most promising light source in next-generation solid-state lighting (SSL) due to the advantages in energy saving, long lifetime, vivid colors, high reliability, environmental protection, safety and compact size. Therefore, LEDs have been extensively studied in the past decades for lighting purpose [1]. Among various types of LED, InGaN-based LEDs emitting at blue light range draw the most attentions, which is the basis of forming white-light LEDs with yellow phosphors. Recently, white-light LED’s efficiency has been improved tremendously and has been superior to traditional light source, such as incandescent bulbs, halogen bulbs and even fluorescent lamps. This opens an opportunity for LEDs to be adopted into new applications such as backlight for LCD-TV, automotive headlamp, light source for projector and general lighting. However, for the applications mentioned above, it is very important to further enhance the external quantum efficiency (EQE) of LEDs. The EQE is approximately given by the product of internal quantum efficiency (IQE) and light extraction efficiency (LEE). The former is related to the quality and substrate of epitaxial layers; meanwhile, the latter is related to chip processing, die geometry and LED packaging. In order to improve the EQE of LEDs, it is necessary to improve both IQE and LEE. In this paper, we focus our study on the enhancement of the light extraction efficiency of InGaN-based

Gallium Nitride Materials and Devices IV, edited by Hadis Morkoç, Cole W. Litton, Jen-Inn Chyi,

Yasushi Nanishi, Joachim Piprek, Euijoon Yoon, Proc. of SPIE Vol. 7216, 72161T

© 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.808458LEDs. It is well known that the emitted photons from active region may have difficulty escaping from an LED chip owing to the total internal reflection on the boundary of the chip, especially at the surface/interface boundary between two media with a significant difference in the refractive index. This leads to lower light extraction efficiency due to the light being trapped inside the device. Accordingly, many approaches have been demonstrated to attain high extraction efficiency including die shaping, surface texturing, patterned sapphire substrate, transparent electrode, and omnidirectional reflector [2].

In this paper, several new surface/interface schemes were proposed to further improve the LEE of InGaN-based LEDs. However, there is no direct way to measure LEE experimentally. The estimated values of IQE in these textured-LEDs were obtained by temperature-dependent photoluminescence (PL) measurement. Besides, a detail analysis of light extraction by Monte Carlo ray tracing method was presented to investigate the effect of textured LEDs on improving LEE. Simultaneously, the optical simulation could be useful in maximizing extraction efficiency of LEDs.

2. Light Extraction Improvement by Surface/Interface Texturing

Surface/interface texturing methods have recently been introduced, in an attempt to reduce internal light reflection and scatter the light outward. One of the main approaches is to form a random texture on the top surface of the LEDs. It shows that the low-temperature growth conditions could result in many hexagonal pyramids on the p-GaN surface and thus increases the probability of photons escaping from the LED [3]. In addition, InGaN-based LEDs grown on patterned sapphire substrate (PSS) is also found to have a significant improvement on the light output power. The improvement may be attributed to two parts: one is the enhanced IQE by reducing the dislocation density, and the other is the improved LEE via the pattern structure on sapphire substrate [4]. Recently, by using the wafer-bonding, laser lift-off and chemical wet etching techniques, an n-side-up InGaN-based LED with hexagonal pyramid structure has been developed [5]. Moreover, there is a significant portion of the light coming out from the top-surface while the epitaxy being transferred from transparent sapphire to non-transparent silicon or metal substrate. Therefore, both the textured surface and the backside reflector are very important to enhance the LEE. In addition, there are other approaches can be used to improve the LEE effectively, such as photonics crystal, mesh contact and Microhole array on the surface/interface [6-8].

For the purpose of improving the LEE, we explored different surface/interface texturing methods in this study. First of all, the InGaN-based LED with top-side and inner surface texturing was fabricated. Fig. 1 shows the schematic cross section of three different double-side textured-LEDs. The first type, labeled as “ST-PSS-LED”, is the structure with textured p-side surface and PSS. The second one, labeled as “DT-LED”, is the structure with textured p- and n-side surface. The structure of the third type, “PDT -LED”, is almost the same as that in second one with only difference lies in the textured n-side, which is patterned surface instead of unpatterned surface. The first structure wasepitaxially grown onto c-plane (0001) patterned sapphire substrates by metal-organic chemical vapor deposition (MOCVD), and the surface-textured p-GaN is grown under elaborated low temperature condition. For the other two structures, a double transfer process was needed to form textured surface on u-GaN layer. First, an InGaN-based LED structure was grown on the normal sapphire substrate or patterned sapphire substrate, separately. The sapphire substrate was then removed by laser lift-off or chemical-mechanical polishing (CMP) methods and the epilayer was transferred to a temporal substrate with a glue bonding technique. Then, the u-GaN layer was etched by chemical wet etching or Inductively Coupled Plasma (ICP) dry etching to form different textured surfaces. Subsequently, the transferred epilayer with textured u-GaN layer was then re-bonded to a permanent transparent substrate and the temporal substrate was removed at final process step. As a result, the double-side textured LED with horizontal structure was obtained. Fig. 1 shows the scanning electron microscopy (SEM) image of the PSS, textured u-GaN and textured pattern u-GaN, respectively. For comparison, conventional ITO LED (C-LED), surface texturing LED (ST-LED) and patterned sapphire substrate LED (PSS-LED) were also prepared in the same chip size (10 x 23 mil2) with same emission wavelength (460nm) from the identical processing flow. The light output powers of these LEDs on bare chip and with packaging were measured by a calibrated integrating sphere. Apparently, the light output power of the double-side textured-LEDs was higher than those of the others. When comparing the efficiency of these three different double-side textured-LEDs, it was found that ST-PSS-LED was almost the same as the other two kinds of u-GaN textured-LEDs. This indicated that all of the double-side textured LEDs were good candidates for enhancing the LEE of InGaN-based LEDs.For example,at an injection current of 20 mA, the light output power of the ST-PSS-LED on bare chip was enhanced by 48%, 22% and 25% compared to that of the C-LED, ST-LED and PSS-LED, respectively. Moreover, the current-voltage characteristics of u-GaN textured LEDs were almost identical with that of other devices. This implied that the double-transfer and etching process would not result in any degradation in the electrical properties of LEDs.

InGaN-based LED with ZnO nanorod was also proposed and investigated. ZnO nanorod can be fabricated by a hydrothermal method [9], which is simple and inexpensive. For this reason, ZnO nanorod is worthy for further investigation and application for enhancing the LEE of LEDs. In this study, different size and density of nanorod arrays were formed by adjusting the process parameters of manufacturing including coating method, reaction time, growth temperature, and solution concentration. Fig. 2 shows two examples of ZnO nanorod made on top surface. From the experimental results, intense emissions can be seen along with the ZnO nanorod on the top p-GaN surface. At a driving current at 20mA, an increase of 15% in light output power of the ZnO-LED compared with C-LED was obtained.

3. IQE Measurement and LEE Analysis

In order to investigate the effect of surface/interface texturing on improving light extraction, the IQE of epitaxialwafer was estimated experimentally by temperature-dependent PL measurement at different excitation power density [10]. Basically, this method is based on assuming that the IQE at low-temperature is equal to 100%, and then the IQE at room-temperature can be defined as the ratio between the efficiency of excited PL at room-temperature and that at low-temperature. In this study, a 390nm from a mode-locked Ti:sapphire laser was used as an excitation source. Fig. 3 shows the PL efficiency as a function of excitation power density at 15 and 300K. There are two samples, epitaxy grown on normal sapphire substrate and patterned sapphire substrate, were measured. As can be seen, corresponding to 20mA current injection, the IQE can be estimated to be around 65%~70% for both samples. Accordingly, if we assume that the IQE is 67%, the LEE can be further calculated and summarized in Table 1. It is found that the LEE of all double-side textured-LEDs is greatly improved and as high as 85%. However, for LEDs having optimized single-side surface texturing structure on either p- or n-side, the LEE is close to that of double-sided schemes, up to 75 to 80%.

The Monte Carlo ray tracing method was used to study the light extraction mechanism of textured-LEDs [11-12]. During the simulation, the photons are generated randomly within the active region and were emitted isotropically according to a distribution function describing the nature of the light. For each ray, the trajectory and the energy were determined by geometrical structure, Fresnel equations and material absorption in each layer. Both of the electrodes were omitted to enable clear understanding of the effect of textured surface. The shape and size of the textured structures such as PSS and hexagonal pyramid were set as close as possible to the actual values. The boundary condition for the bottom of the chip was assumed as 90% reflection. The refractive index of sapphire, GaN, ITO, and epoxy used in the simulation were 1.8, 2.4, 2.0, and 1.5, respectively. The absorption coefficient was set at 100 cm-1 for the active region. Fig. 4 shows the simulated LEE of the bare chip and packaged LED with an epoxy lens. From the simulation results, we find that the extraction efficiency of each face is different among these LEDs, and the total LEE of packaged LED was 1.3~1.5 times higher than that of bare chip. In comparison with experimental results, the trend was highly consistent except for the conventional bare chip LED. This was because of the actual LEE of conventional bare chip LED was dominated by the non-smooth surface, the LEE in simulation by use of smooth surface (~24%) was much lower then that in experiment (~45%). Besides, Fig. 5 shows the extraction efficiency with respect to the tracking number of photon passing through layer. The simulation results indicated that the increased LEE for the textured-LEDs was contributed by the effect of multiple reflections inside the device. Therefore, we expected that the LEE of textured-LEDs is more sensitive to the material absorption and bottom reflectivity, but its LEE is always higher than that of conventional planar LED. Nevertheless, the double-side textured-LEDs shown can provide slightly better light extraction for less time of multiple reflections than single-side textured-LEDs.

4. CONCLUSIONHigh-efficiency InGaN-based LEDs with surface/interface texturing has been demonstrated. The results from the experiments and simulations show that the LEE of double-side textured-LEDs with packaging is 1.3 and 1.1 times than that of conventional planar LED and single-side textured-LEDs, respectively. In other words, the LEE of double-side textured-LEDs can be improved to around 85%, while the IQE are estimated to be around 65%~70% by PL measurement. The LEE also has been estimated using Monte Carlo ray tracing method. The simulated results indicated that high LEE is achieved by surface texturing due to the multiple reflections. Therefore, In order to obtain higher LEE of textured-LEDs, the absorption of the active region and reflectivity of backside reflector have to be concerned.

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Acknowledgment

The authors would like to thank Prof. H. C. Kuo and C. H. Chiu of the Department of Photonics, National Chiao Tung University, Taiwan, R. O. C., for their valuable discussions and technical support.

Fig. 1 Schematic diagrams and SEM images of double-side textured-LEDs : (a) ST-PSS-LED, (b) DT-LED, and (c) PDT-LED.

Fig. 2 (a) Schematic diagrams of nano-rod structure on top of InGaN-based LED. (b) and (c) SEM images of the nano-rod which have a diameter of around 100 and 500nm , respectively.

Fig. 3 Relative PL efficiency as a function of excitation power density for flat epitaxy on normal and patterned sapphire structure measured at 15 and 300K.

Fig.4 The simulated light extraction efficiency of (a) the bare chip and (b) the packaged LED.

Fig.5 The extraction efficiency of different type of LED with respect to the tracking number of photon passing through layer.

Table 1 The estimated light extraction efficiency in different textured-LEDs with packaging.