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Improvement of Light Extraction Efficiency in Flip-Chip Light Emitting Diodes on SiC Substrate via Transparent Haze Films with Morphology-Controlled Collapsed Alumina Nanorods Seunghwa Baek,†,‡ Gumin Kang,†,‡ Dongheok Shin,† Kyuyoung Bae,† Yong Hyun Kim,*,§ and Kyoungsik Kim*,† †
School of Mechanical Engineering, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, South Korea LED Convergence Research Center, LED R&D Division, Korea Photonics Technology Institute, 9, Chumdan venture-ro, 108 beon-gil, Buk-gu, Gwangju 61007, South Korea
§
S Supporting Information *
ABSTRACT: We demonstrate GaN-based flip-chip light emitting diodes (FC-LEDs) on SiC substrate achieving high extraction efficiency by simply attaching the optically transparent haze films consisting of collapsed alumina nanorods. Through controlled etching time of alumina nanorods, we obtain four types of films that have different morphologies with different optical transmittance and haze properties. We show that the light output power of the FC-LEDs with film, which has 95.6% transmittance and 62.7% haze, increases by 20.4% in comparison to the bare LEDs. The angular radiation pattern of the LEDs also follows the Lambertian emission pattern without deteriorating the electrical properties of the device. The improvement of light extraction is mainly attributed to the reduced total internal reflection (TIR) via efficient out-coupling of guided light from SiC substrate to air by collapsed alumina nanorod structures in the film. The high transparency of film and reduced Fresnel reflection via graded refractive index transition between the film and SiC substrate also contribute to the extraction enhancement of the device. We systematically investigate the influence of haze film’s geometrical or optical properties on the extraction efficiency of FC-LEDs, and this study will provide a novel approach to enhance the performance of various optoelectronic devices. KEYWORDS: optical haze, capillary force, collapsed alumina nanorods, GaN-based flip-chip LEDs, light extraction efficiency
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INTRODUCTION Since the invention of the light emitting diodes (LEDs), the LEDs are replacing conventional light sources in many applications, for example, display, traffic lights, medicine and automobiles because of their long-lifetime, environmental benefits, and low power consumption.1−4 One of them, the GaN-based flip-chip LEDs (FC-LEDs) on a SiC substrate has been considered as a good candidate for high-power and electrical stability because the SiC substrate has a high thermal conductivity and small lattice mismatch with GaN.5,6 The efficiency of the LEDs is determined by the product of internal quantum efficiency (IQE) and light extraction efficiency (LEE).7 IQE is defined as the ratio of generated photons to injected electrons in the LED’s active region while LEE is defined as the ratio of radiated photons into free space to generated photons. Because IQE is intimately related to crystal quality and epitaxial layer structure, low defect density, and good crystal quality of SiC-based GaN LEDs enable high IQE, which are already reported in the literature.8,9 In contrast to IQE, LEE of the FC-LED on SiC has not been improved so much due to the small radiation angle without © 2015 American Chemical Society
total internal reflection and large Fresnel reflection loss, which are caused by the large difference in refractive index between the air (n = 1.0) and SiC (n = 2.72). Only 4% of the lights generated in active layer can be extracted from SiC into air through the narrow escape cone determined by the critical angle of 21.6°.10,11 Most of the light (about 96%) is trapped inside and this degrades performance of device by raising the internal temperature. Therefore, the improved LEE is very critical for the total efficiency enhancement of the FC-LEDs on SiC substrate. Several methods have been proposed to improve the LEE of conventional LEDs, including patterned substrates,12−14,27 plasmonic structure,15,16 texturing of extraction surface,17−20 and photonic crystals.21−24 Most studies have focused on how to create a micro and nano structures at light extraction surface or electrodes using developed technology such as imprint lithography, laser holographic, e-beam, and ionbeam lithography. Received: August 21, 2015 Accepted: December 21, 2015 Published: December 21, 2015 135
DOI: 10.1021/acsami.5b07783 ACS Appl. Mater. Interfaces 2016, 8, 135−141
Research Article
ACS Applied Materials & Interfaces
Figure 1. (i−iii) Fabrication flowchart of the collapsed alumina nanorods. Top-view SEM image of (a) as-grown and (b) pore widened alumina nanopore arrays by anodization and wet etching process. Scale bar is 2 μm. Upper side inset shows magnified view of images. (c−f) 40° tilt SEM images of collapsed alumina nanorods as a function of etching time. Scale bar is 10 μm. Schematic in each image shows the cross-sectional view of collapsed alumina nanorods (green: alumina nanorods).
ratio) to achieve a smooth surface. Then, we performed the first-step anodization in oxalic acid (H2C2O4, 0.3 M) at 40 V for 15 h. As-grown alumina nanopore arrays were completely removed using a mixture of chromic and phosphoric acids (6 wt % H3PO4 and 1.8 wt % CrO3). This step generates ordered nanoconcaves on the aluminum surface and these concaves guide the growth of highly ordered alumina nanopore arrays. Then, the second-step anodzation was performed under the same conditions used for the first-step process. Finally, we obtained well-ordered alumina nanopore arrays with a the pore size and period of 30 and 100 nm. (SEM image in Figure 1a) Through wet-chemical etching of as grown anodic alumina nanopore arrays (Figure 1a) in phosphoric acid (H3PO4, 5 wt %) solution, the pore size gradually increases (Figure 1b). After a particular wet etching time, the pore walls start to break and alumina nanopore structure are changed to several pieces of nanorods with nearly triangular cross section. During the etchant rinsing and drying process, the nanorods are collapsed and self-aggregated due to the elasto-capillary force-induced clustering.34−37 SEM images in Figure 1c−f show collapsed and aggregated alumina nanorods structure with different morphologies produced by controlled wet etching time. As the wet etching time increases, the thickness of nanorods decrease thus the mechanical stiffness of nanorods against capillary force decreases. The decreased mechanical stiffness causes nanorods to be more collapsed and aggregated. As shown in SEM image of Figure 1c and Figure S1a, the thickest nanorods produced by shortest etching time (43 min) are partially collapsed because they have strong mechanical stiffness. In contrast, the thinnest nanorods produced by longest etching time (46 min), result in densely collapsed nanorods structure SEM image of Figure 1f and Figure S1d. As a result of collapsed nanorods, the films have lots of aggregated lumps of alumina structure at top surface. In the SEM images of Figure 1c−f, these structures are represented by the white lines look like ridges. The shape and number of ridges plays important roles in determining the optical property of the films, which will be discussed later.38 To implement them onto the top surface of FC-LEDs on the SiC substrate, we detached four types of collapsed alumina
However, these methods involve high cost and complicated fabrication processes which make them difficult to apply to large area applications. Moreover, texturing the electrode of device can cause the deterioration of electrical properties. Recently, several studies have been carried out to enhance the LEE of LEDs through depositing nanoparticles or scattering layers such as ZnO25−29 and TiO230 on complete LEDs, rather than a direct texturing of top electrode or surface of LEDs. These methods allow us to enhance the light output of LEDs through scattering effect without deteriorating electrical properties of device. In this paper, to enhance the performance of FC-LEDs, we employ optically transparent haze films made of morphologycontrolled collapsed alumina nanorods. Our haze films have excellent optical transparency and haze values and the optical properties are widely controlled by changing the etching condition of alumina nanorods. We achieved the substantial light extraction efficiency enhancement up to 20.4% in comparison with the bare LEDs by simply attaching the haze films instead of directly fabricating on the top surface of LEDs. The haze films can be applied to various types of devices and surfaces because the films are flexible, and thus easily attached to any desired surface. Moreover, the radiation pattern of LEDs with the haze films follow Lambertian emission pattern owing to the film’s enormously high optical hazes. The fabrication process of haze film is completely independent from that of LED module, thus we can avoid any adverse effect on the electrical properties of LEDs. So, these positive effects of films suggest a novel approach and provide great potential for various light emitting applications.
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RESULTS AND DISCUSSION
We fabricated collapsed alumina nanorods by chemical-wet etching, subsequently drying and rinsing of alumina nanopore arrays. Figure 1 shows a schematic illustration of fabrication flowchart of ultrahigh transparent haze films. To fabricate wellordered alumina nanopore arrays, we employed a conventional two-step anodization method.31−33 Prior to anodization, we electro-polished high purity aluminum substrate in a mixture of perchloric acid and ethanol (HClO4:C2H5OH = 1:4 in volume 136
DOI: 10.1021/acsami.5b07783 ACS Appl. Mater. Interfaces 2016, 8, 135−141
Research Article
ACS Applied Materials & Interfaces
Figure 2. Schematic and photographs showing the printed image as seen through the transparent haze films with different etching time of (a) 43 min, (b) 44 min, (c) 45 min, and (d) 46 min. Distance between the film and printed image is 4 cm. (e) Measured total transmittance (black square), diffuse transmittance (red circle), and (f) haze values (blue square) of each haze film at 450 nm.
Figure 3. (a) Schematic of GaN-based FC-LEDs on SiC substrate with a haze film. (b) Photography of the on (right) and off (left) LEDs with (bottom) and without (up) films. (c) Electroluminescence spectra of the LEDs with and without the haze films. (d) Light output power as a function of injection current for the LEDs with and without the haze films. (e) Far-field radiation patterns of bare LED and LEDs with film 43m, 44m, 45m, and 46m.
nanorods from aluminum substrate using Scotch tape. We refer to the detached nanorods with etching time of 43, 44, 45, and 46 min as 43m, 44m, 45m, and 46m, films, respectively. Figure 2a−d shows the photograph of each detached film, when it is placed on 4 cm above of the printed image. Unlike other printed images, the image seen through films looks misty because the light is highly scattered when it passed the films. To investigate quantitatively the optical property of each film, we measured total and diffuse transmittance at 450 nm wavelength using spectrophotometer (UV3600, Shimadzu) equipped with an integrating sphere (60 mm inner diameter). Each film has different transmittance and optical haze properties, as shown in Figure 2e,f. Optical haze is defined as the ratio of diffuse transmittance to total transmittance. In Figure 2e, measured total transmittance values (black solid square) of the 43m, 44m, 45m, and 46m films are 85.6%,
89.4%, 93.1%, and 95.6%, respectively. In Figure 2f, the optical haze values of films 43m−46m are 97.8%, 89.0%, 80.9%, and 62.7%, respectively. These haze films exhibit high total transmittance because most of the film is composed of alumina nanorods and nanocavaties with subwavelength-scale, which make the entire structure regarded as an effective medium without index mismatching boundary.39 Total transmittance and optical haze of each film are closely related to the number of ridges and the size of microcavities formed between collapsed nanorods. Differing from the nanocavities, these ridges and microcavities make the spatial region of refractive index mismatch and thus act as light scattering center in the film. Therefore, the film having more ridges and microcavities leads to higher optical haze. In contrast, the film having less ridges and microcavities results in higher total transmittance owing to the reduced scattering effect. 137
DOI: 10.1021/acsami.5b07783 ACS Appl. Mater. Interfaces 2016, 8, 135−141
Research Article
ACS Applied Materials & Interfaces Figure 3a shows a FC-LEDs (CREE, DA1000) on SiC substrate integrated with a haze film used in the following experiments. The LED structure consists of SiC, n-GaN, InGaN/GaN multiple-quantum-wells (MQW), p-GaN, and p and n Au electrodes. To investigate the effect of haze films on the optical performance of device, we simply attached the films to the SiC substrate using UV-curable (NOA68) adhesive. The FC-LEDs on the SiC substrate without the haze film were also prepared for comparison. Figure 3b shows corresponding photographs of the LEDs under operation. In the case of FCLEDs on the SiC substrate, a considerable fraction of photons emitted from MQW layers are reflected back and guided into the semiconductor layer due to total internal reflection (TIR) and Fresnel reflection, which are caused by large refractive index difference between SiC (nSiC = 2.72) and air (nair = 1.0). To reduce both reflection losses, we introduce a haze film as a simple intermediate layer between SiC substrate and air. In haze films, triangular cross-sectional nanorods with 20−26 nm side length are linked to the vertices of a 58 nm side length hexagon.37,38 Thus, these films can be considered as an effective medium with the refractive index determined by the following equation:9
Table 1. Light Output Power (LOP) of Bare LED and LEDs with Each Haze Film at Injection Currents of 350 and 500 mA device bare LED with 43m with 44m with 45m with 46m
LOP (mW) at 350 mA
enhancement (%)
314.7 ± 0.53
LOP (mW) at 500 mA
enhancement (%)
413.1 ± 0.75
340.7 ± 0.98
8.2 ± 0.56
455.9 ± 2.59
10.4 ± 0.71
353.5 ± 0.16
12.3 ± 0.27
469.6 ± 1.20
13.7 ± 0.47
363.7 ± 0.25
15.6 ± 0.24
481.8 ± 1.80
16.6 ± 0.68
374.5 ± 0.26
19.0 ± 0.29
497.2 ± 2.16
20.4 ± 0.70
bare LED. We also measured the normalized far-field angular radiation patterns of a bare LED, a LEDs with each haze film at 350 mA (Figure 3e). As compared to a bare LED, the intensity of light output from LED with 46m was substantially enhanced in the vertical direction, which indicates that the light output from the top surface of LED is mostly enhanced. Moreover, as a result of the improved light extraction at vertical direction, the angular radiation characteristic is similar to the Lambertian emission pattern. To investigate the light extraction properties depending on the angle of emission from FC-LEDs on SiC substrate, we measured incident-angle resolved total transmittance of each haze film by attaching it to a hemicylindrical prism with an integrating sphere. A schematic of incident angle-resolved transmittance measurement system is shown in Figure 4a. The haze films were attached to the flat face of the hemicylindrical prism using the UV-curable adhesive. A collimated UV laser beam of a 450 nm diode laser passed through the circular edge of the hemicylindrical prism and center of the haze films. A photodetector connected to an integrating sphere captured the total transmitted light through the haze films. Transmittance was measured as a function of incident angle with a step of 5°, as shown in Figure 4b. When there was no haze film, total internal reflection occurs at 41.8° because the refractive index of the prism was 1.50, thus transmittance dramatically decreased above the critical angle. On the other hand, when the haze films were attached to the prism, the total transmittance was significantly enhanced beyond the critical angle, which indicated that the ridges and microcavities in films indeed enable us to out-couple efficiently the total internal reflected light. The 46m (43m) film showed the highest transmittance below (above) the critical angle. As we mentioned above, the 43m film had the most ridges and microcavities structures acting as scattering centers of the incidence light. The scattered light by the structures propagated in all directions regardless of the incident angle. For the 43m film, the transmittance of light below the critical angle decreased due to the increase of backward directional scattering effect. On the other hand, the transmittance of light above the critical angle increased due to the increase of forward directional scattering effect. To improve light extraction efficiency of LEDs, we want to optimize haze film optical properties, such as scattering or transmittance. Scattering increases the effective critical angle of light extraction while high transmittance minimizes the reflection loss due to the additional haze film. Figure 4b shows that our haze films significantly enhance the light extraction above the critical angle.
neff 2 = f × nalumina 2 + (1 − f ) × nair 2
Each haze film layer has an effective refractive index value, which is determined by the volume fraction (f) of the collapsed alumina nanorods. The 43m film layer with the shortest etching time (26 nm side length) has the lowest effective refractive index. From the experimentally measured SEM images of Figure S1a, the length of alumina nanorods and the film thickness are 5 and 3.23 μm due to the partially collapsed nanorods. Thus, the volume fractions of alumina (nalumina = 1.78) and air (nair = 1.0) are 0.10 and 0.90, respectively. The effective refractive index value of 43m film layer is 1.11. On the other hand, the 46m film layer with the longest etching time (20 nm side length) has the highest effective refractive index because the triangular nanorods collapsed almost completely. The film thickness (1.46 μm) is also smaller than 43m film, as shown in Figure S1d. The volume fractions of alumina and air are 0.14 and 0.86, respectively. The effective refractive value of 46m film layer is 1.14. The effective refractive index of haze films (neff_films = 1.11−1.14) is intermediate refractive index value between SiC (nSiC = 2.72) and air (nair = 1.0). Therefore, the light experiences gradually varying index when it propagates from SiC substrate to air as shown in the Figure S3b,c. By simply attaching haze films, we can significantly reduce Fresnel reflection and enhance the light extraction efficiency of FCLEDs on the SiC substrate. Figure 3c shows the electroluminescence (EL) spectra of the bare LED, LED with haze films using an integrating sphere at a current density of 350 mA/cm2. No significant shift is observed in the EL peak position around 450 nm for any of the five types of LED, with the same full-width at half maximum (fwhm) of ∼26 nm. However, the EL peak intensities of LED are greatly enhanced by attaching the haze films. Figure 3d represents the typical light output power (LOP) curves as a function of forward current for the bare LEDs and LEDs with each haze film. LOP of the five types of LEDs at injection current of 350 and 500 mA are summarized in Table 1. Measured LOP of bare LED, LED with 43m, 44m, 45m, and 46m at 350 mA are 314.7 ± 0.53, 340.7 ± 0.98, 353.5 ± 0.16, 363.7 ± 0.25, and 374.5 ± 0.26 mW, respectively. The LOP of the LED with 46m is ∼20.4% ± 0.70% higher than that of the 138
DOI: 10.1021/acsami.5b07783 ACS Appl. Mater. Interfaces 2016, 8, 135−141
Research Article
ACS Applied Materials & Interfaces
Figure 4. (a) Schematic diagram of the experimental setup for measuring angle-resolved light transmittance of the haze film. (b) Experimental light transmittance curves vs incident angle for the bare substrate and with each haze film.
Figure 5. (a) Calculated light transmittance vs radiation angle from active layer to free space for each device. Schematic (b) and light escape cone (c) of bare LED (top), with adhesive polymer (middle), and with 46m film (bottom).
For our light extraction enhancement, we employed FCLEDs (CREE, DA100) as a reference LED because it is one of the most efficient commercial LEDs. Because FC-LEDs have a beveled structure in the light extraction surface, light emissions are dominant in the vertical direction, as experimentally supported in Figure 3e, black line. Therefore, the higher transmittance below the critical angle is crucial to enhance the light extraction of FC-LEDs. For these reasons, the 46m film improves the light extraction most efficiently because it shows the highest transmittance in the lower angular range (Figure 4b). To confirm the enhancement of light extraction from the LEDs with and without haze film, in Figure 5a, we calculated the transmittance of radiation light from active layer to air using Snell’s law and angle resolved transmittance measurement data. The bare LED (Figure 5b top) has a lower transmittance (∼0.78) and smaller critical angle (23.7°) than the LED with an adhesive polymer and 46m transparent haze film due to the large refractive index difference between air and SiC substrate. LED with adhesive polymer (Figure 5b middle) increases transmittance (∼0.87) within the same critical angle owing to the graded index effect. Finally, the LED with 46m film (Figure 5b bottom) also increases transmittance within the critical angle by graded index effect. Moreover, outside the critical angle, it significantly enhances the transmittance from zero due to the scattering effect of the haze film structure. The haze property of the film strongly suppresses the total internal reflection losses outside 23.7°, leading to an effective increase in the critical angle of radiation light. The escape cone of each device is described in Figure 5c, and the LED with 46m film has a wider
escape cone and higher transmittance caused by scattering and haze effects.
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CONCLUSIONS In summary, we investigated the light extraction efficiency enhancement of FC-LEDs on the SiC substrate based on the haze films consisting of collapsed alumina nanorods. Uniform and transparent haze film was successively fabricated by anodization of aluminum and chemical wet etching. Through careful control of wet etching time, we modify the morphological and optical characteristic of the films. LED simply attached with 46m film showed the highest extraction enhancement of 20.4%. The 46m film has the highest total transmittance and moderate optical haze. Our film fabrication process is very simple and it is also easy to control the optical properties of haze films. As a result, our approach can be applied to various types of LEDs and other lighting sources for enhancing the light extraction efficiency.
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EXPERIMENTAL SECTION
Optical Characterization of Haze Films. Total/diffuse transmittance spectra of each haze film was measured using a UV−vis-NIR spectrophotometer (UV3600, Shimadzu) with an integrating sphere (60 mm inner diameter). The incident angle-resolved light extraction of each haze film was measured using a blue laser and hemicylindrical prism with an integrating sphere (Ocean optics, 50 mm inner diameter). The blue laser beam with various incident angles, resolution of 5°, passed through the circular-edge of the hemicylindrical prism and was transmitted or reflected by the attached films to the flat-edge of the prism. The transmitted light through the films was collected and measured by an integrating sphere equipped with a photodetector (Thorlabs, DET 100A/M). 139
DOI: 10.1021/acsami.5b07783 ACS Appl. Mater. Interfaces 2016, 8, 135−141
Research Article
ACS Applied Materials & Interfaces Light Extraction Efficiency of LEDs. Luminous view angles were measured using a View Angle measurement device (VA-3000, J&C tech). The far-field beam distributions (radiation patterns) of bare LED and LED with 43m, 44m, 45m, and 46m transparent haze films were analyzed and compared. We followed the CIE 127 Conditions A and B. The measurement part is an integrating sphere (0.5 in. inner diameter) with a 2048 CCD array detector. The far-field beam distribution was measured every 5° in the range of 0−180°.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b07783. Haze films cross section SEM image to present proof of the sketches, locally zoomed total/diffuse transmittance, and haze property of each film with error bars at 450 nm, detailed explanation of graded refractive index effect and light output power data with error bar (PDF).
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AUTHOR INFORMATION
Corresponding Authors
*K. Kim. E-mail:
[email protected]. *Y. H. Kim. E-mail:
[email protected]. Author Contributions ‡
S.B. and G.K. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Low Observable Technology Research Center program of Defense Acquisition Program Administration and Agency for Defense Development.
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ABBREVIATIONS FC-LEDs, flip-chip light emitting diodes (LEDs) IQE, internal quantum efficiency LEE, light extraction efficiency LOP, light output power MQW, multiquantum wall TIR, total internal reflection
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REFERENCES
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DOI: 10.1021/acsami.5b07783 ACS Appl. Mater. Interfaces 2016, 8, 135−141
Research Article
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DOI: 10.1021/acsami.5b07783 ACS Appl. Mater. Interfaces 2016, 8, 135−141
