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GaN-Based Light-Emitting Diode Structure With Monolithically Integrated Sidewall Deflectors for Enhanced Surface Emission Jae-Soong Lee, Joonhee Lee, Sunghwan Kim, and Heonsu Jeon
Abstract—To improve the overall surface emission efficiency, the structure of a standard GaN light-emitting diode (LED) was modified; the mesa sidewalls were etched at an angle, and deep enough to reach the sapphire substrate. Photoexcitation experiments, including photoluminescence and near- and far-field emission patterns, were performed on LED-like test devices, and results indicated that the angled sidewalls efficiently deflect photons that are initially guided laterally within the GaN epilayer in the off-surface direction. For a sidewall angle of 30 , the total surface emission strength was improved by a factor exceeding three. Computer simulations produced results consistent with the experimental observations. Index Terms—GaN, integrated optics, light deflectors, light-emitting diodes (LED), surface emission, total internal reflection (TIR).
I. INTRODUCTION
T
HE importance of GaN-based light-emitting diodes (LEDs) cannot be overemphasized because their demand and applications are still growing, and range from traffic lights, outdoor full-color displays, and LCD backlight units, to general illumination and lighting aimed at the massive consumer market. As improvement in material quality is quickly being saturated, one of the challenges en route to developing high-brightness, high-efficiency GaN-LEDs is how to extract more photons that are otherwise trapped inside the chip from the device. Total internal reflection (TIR) is mainly responsible for the photon trapping; photons that strike the GaN–air (or GaN–sapphire) interface at angles exceeding the critical angle are reflected back, the majority of which are guided laterally through the air–GaN–sapphire waveguide structure. Several approaches have been proposed for resolving this problem, and include surface-roughening [1], GaN growth on a patterned sapphire substrate [2]–[4], and the integration of two-dimensional photonic crystal patterns [5]–[7]. The idea behind these approaches is to interfere with the TIR phenomenon using either the scattering or diffraction of light, which is accomplished by integrating micro- or nano-structures in one of the GaN interfaces. On the other hand, it is also possible to utilize the TIR process in a positive manner. Here, we propose and demonstrate an LED structure that is especially suitable for the GaN-LED structure grown on a sapphire substrate (or for Manuscript received February 7, 2006; revised May 12, 2006. This work was supported by the Seoul R&BD Program. The authors are with School of Physics and Inter-University Semiconductor Research Center, Seoul National University, Seoul 151-747, Korea (e-mail:
[email protected]). Digital Object Identifier 10.1109/LPT.2006.879559
Fig. 1. (a) Schematic of the SDI-LED structure. (b) SEM images of a fabricated SDI-LED sturcture. (Color version available online at http://ieeexplore. ieee.org.)
any LED structure in which the epitaxial layer itself serves as a strong in-plane waveguide). II. PROPOSITION OF A NEW GaN LED STRUCTURE The underlying idea of the proposed GaN-LED structure is to redirect at once the photons guided within the GaN epilayer by deflection, instead of partially scattering/diffracting them as in the examples mentioned above [1]–[7]. In terms of device structure, this involves modifying the LED mesa sidewalls; smooth mesa sidewalls are prepared at an oblique angle, all the way through the GaN epilayer, so that the air–GaN–sapphire waveguide structure is terminated in an angle at the mesa sidewalls. A schematic diagram of the LED structure is illustrated in Fig. 1(a). Hereafter, we call this altered physical shape the “sidewall-deflector-integrated” (SDI) LED structure. Upon encountering the slanted mesa sidewalls, the guided photons are deflected and emerge through the sapphire substrate. These deflected photons combine with photons that escape directly through the LED surface, enhancing the overall emission intensity of the device. III. DEVICE FABRICATION A conventional GaN-LED heterostructure, which is 5 m thick, was grown using a low-pressure metal–organic chemical vapor-deposition technique. From the substrate side, the LED epilayer structure consists of a GaN buffer, a bottom cladding,
1041-1135/$20.00 © 2006 IEEE
LEE et al.: GaN-BASED LED STRUCTURE WITH MONOLITHICALLY INTEGRATED SIDEWALL DEFLECTORS
five InGaN–GaN multiple-quantum-wells (MQWs), a 20-nmthick Al Ga N electron-blocking layer, and a 0.2- m-thick GaN top cladding layer. The room-temperature MQW emission peaked at nm. To avoid any spatial inhomogeneity effect, samples were taken from the central region of the wafer where the emission efficiency variation was less than 20%. The LED mesa structures with angled sidewalls were fabricated using a monolithic process. First, a 7- m-thick photoresist layer was spin-coated and patterned into an array of circular disks with a nominal diameter of m. The photoresist sidewalls were then deformed using a melt-and-reflow process at an elevated temperature. Using the altered photoresist pattern as a sacrificial mask, we etched the LED wafer until the sapphire substrate was reached. A high-density inductively coupled-plasma reactive-ion-etch process was used for GaN etching. Previously, we employed a similar etching process to fabricate microlenses on sapphire substrate [8] and also to demonstrate a monolithic GaN vertical-external-cavity surface-emitting laser [9]. The etched GaN sidewall angle differed from the angle of the reflown photoresist sidewall, because the etching rates of the two materials differ. The desired GaN sidewall angles could be obtained once the etching rates were calibrated. In this initial proof-of-concept experiment, we fabricated the SDI-LED structures with sidewall angles of 20 , 30 , and 40 . The atomic force microscopy measurements also provided information on the etched surface roughness, which was 2 nm over 1 1 m . Fig. 1(b) shows scanning electron microscopy images of an etched GaN mesa and its sidewall surface. IV. DEVICE CHARACTERIZATION To facilitate device fabrication and characterization, we photopumped the SDI-LED structures, instead of fabricating electrically driven LED devices in full. Optical excitation was achieved using a 325-nm He–Cd laser, which was intentionally defocused to excite the entire circular mesa so that we could mimic the LED operation condition. Devices were excited from the epilayer side while emitted light was detected through the backside-polished sapphire substrate. For comparison, a set of reference devices was also prepared from the same GaN-LED base wafer. Unlike the SDI-LED devices, the reference devices were etched using a hard SiO mask to form shallow ( 500 nm) vertical sidewalls, just deep enough to etch through the MQWs and to reach the bottom GaN cladding layer, as in conventional laterally injected GaN-LED devices. The diameter of the circular mesa was kept the same as in the SDI-LED devices. Fig. 2 shows photoluminescence spectra for both the SDI-LED and the reference device. Here we show the 30 sidewall angle case only because computer simulation results, which are discussed later, indicated that the 30 sidewall angle gives the best LED luminosity. In this experiment, the numerical aperture was 0.2. In terms of emission intensity, that of the SDI-LED was approximately 3.1 times stronger than the reference device. Considering the simple structural modification made to the mesa sidewalls, this enhancement was substantial. We examined the Fabry–Pérot oscillations seen in both spectra in Fig. 2; they are not scaled in proportion to the intensity enhancement, but rather have similar modulation depths on an absolute scale. Recall that the Fabry–Pérot oscillations are a characteristic of the surface-emitting photons from
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Fig. 2. Emission spectra from an SDI-LED structure with = 30 (thick solid line) and the reference (thin solid line). Shown in dotted line is the difference of the two, which is a good estimation for the spectrum of the guided-and-thendeflected photons. (Color version available online at http://ieeexplore.ieee.org.)
GaN-LEDs. Therefore, the emission spectrum of the SDI-LED structure is a result of the simple addition of two kinds of photons: direct surface-emitting photons and something else. The latter can only be those photons that are guided laterally and deflected by the sidewalls. In this context, the spectrum for the guided-and-then-deflected photons can be deduced by subtracting the spectrum of the reference device from that of the SDI-LED. The result is shown with the dotted line in Fig. 2. In order to investigate the origin of the photoluminescence enhancement further, we examined the emission patterns of the SDI-LED structure. Fig. 3(a) shows the near-field intensity distribution profiles for a typical SDI-LED and the reference along the line intersecting the mesa center. The corresponding nearfield device images are shown in the insets. In good contrast to the reference device for which a low-level intensity spread across the entire active area, the SDI-LED structure not only exhibited broad surface emissions but also an additional intense emission from along the circumference of the circular mesa. We attribute the intensity profile with two spikes to the integrated sidewall deflectors, which efficiently redirect the laterally propagating photons at the mesa edges. To confirm this interpretation, we intentionally focused the pump beam tightly so that only the central portion of the circular mesa was excited. The strong emission along the circular mesa edges was preserved, as shown in Fig. 3(b). Note that the area of surface emission was reduced to a spot in the center of the mesa in accordance with the decreased pump beam size. This near-field study provided clear evidence that a large number of photons are actually guided within the GaN epilayer and that the angled sidewall deflectors indeed redirect these guided photons. We also examined the far-field emission pattern. Fig. 4(a) compares the measured far-field patterns of the three SDI-LED structures (whose sidewall angles are 20 , 30 , and 40 ) and reference. The emission from the 30 SDI-LED was over three times more intense than from the reference. This enhancement factor was roughly consistent with the result from the emission spectra (Fig. 2). For theoretical confirmation, we simulated the far-field emission patterns of the LED structures using software based on the ray optics; the results are shown in Fig. 4(b). The
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vations. The discrepancies in detailed angular distributions between the experimental and simulation results are believed to be due to the uncertainty in the sidewall angles of the actual devices. The simulation results also suggest that our angled sidewall approach offers the possibility of controlling the emission patterns. For example, for a sidewall angle near 20 , a conical emission pattern overloads the Lambertian profile; yet the overall emission intensity is markedly higher than that of the reference. V. CONCLUSION We proposed and demonstrated a new GaN-LED device structure that produced higher photon emission through the device surface. The LED structure, which was characterized by angled mesa sidewalls, efficiently deflected the photons that would otherwise have been guided laterally along the GaN epilayer. Experimental results, including photoluminescence and near- and far-field patterns, indicate that the proposed LED structure enhanced the overall surface emission intensity via the proposed mechanism by up to 3.1 times for a sidewall angle of 30 . The computer simulation results were in good agreement with the experimental observations, supporting success of the proposed LED structure. ACKNOWLEDGMENT Fig. 3. (a) Near-field emission pattern profiles for the SDI-LED structure (thick solid line) and the reference (thin solid line). The insets are the near-field images of the corresponding devices. The donut-shaped emission patterns (instead of uniform or Gaussian profiles) for the broad mesa surface emission are due to higher-order mode operation of the laser excitation source. (b) Near-field image of the SDI-LED structure when pump beam is tightly focused at the center of the circular mesa. (Color version available online at http://ieeexplore.ieee.org.)
Fig. 4. (a) Far-field patterns of the SDI-LED structures (with sidewall angles of = 20 ; 30 , and 40 ) and reference. (b) Computer-simulated far-field patterns of the corresponding devices. (Color version available online at http:// ieeexplore.ieee.org.)
overall agreement between the experiments [Fig. 4(a)] and simulations [Fig. 4(b)] is very high. It should be especially noted that the simulation reproduced the strongest enhancement at the sidewall angle of 30 , consistent with the experimental obser-
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