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Illumination & Displays

Enhancing light extraction from LEDs

LEDs with embedded donut-shaped air voids show great potential as highly efficient sources of next-generation solid-state lighting.
17 October 2012, SPIE Newsroom. DOI: 10.1117/2.1201209.004438

High-brightness LEDs based on gallium nitride (GaN), with luminescent wavelengths from infrared to ultraviolet (0.7–6.2eV), have already been extensively used in large full-color displays, traffic and signal lights, short-haul optical communication, backlighting for liquid crystal displays, and regular light fixtures.1 However, to fulfill the criteria of next-generation projectors, automobile headlights, and high-end light fixtures, further improvements to the optical power and external quantum efficiency are required.

The key issue limiting the development of GaN-based vertical-cavity surface-emitting lasers, which undergo a similar manufacturing process, is the lattice mismatch between GaN and sapphire substrates. This mismatch causes difficulty in growing a highly reflective structure. Due to this and a thermal expansion coefficient misfit, the GaN-based epilayer suffers high threading dislocation densities (TDDs, approximately 108–1010cm−2), which compromise the internal quantum efficiency. In addition, the high refractive index of GaN restricts the escape angle of emitted light, and this, at only 23°, results in poor light extraction efficiency (LEE). Various growth techniques have been proposed to improve the crystalline quality of a GaN-based epitaxial layer on a sapphire substrate. Examples include epitaxial lateral overgrowth (ELOG), microscale silicon nitride (SiNx) or silicon oxide (SiOx) patterned masks, and patterned sapphire substrates (PSS).

We have made LEDs on crown-shaped patterned sapphire substrates (CPSS). The fabricated LED has donut-shaped air voids between the PSS and GaN epilayer interface, which improve both epitaxial crystal quality and LEE. We have also analyzed the electro-optical properties of the LEDs in detail.

Figure 1. Scanning electron microscope images for (a) a tilted image of the surface of a crown-patterned sapphire substrate (CPSS), (b) the cross section of hemisphere-patterned sapphire substrate (HPSS), (c) the cross section of CPSS-LEDs and (d) the magnified view of air voids on top of the crown shape.

We prepared epitaxial structures for GaN-based LEDs on three differently shaped sapphire substrates. To fabricate CPSS, we first prepared sapphire substrates with periodic patterns (2μm diameter and 3μm spacing) by standard photolithography. A silicon dioxide (SiO2) film 200nm thick was applied by plasma-enhanced chemical vapor deposition and served as the dry-etching hard mask. We used the photoresist pattern as the mask and over-exposed the photoresist during photolithography, employing a buffer-oxide etching (BOE) solution to create the donut-shaped pattern. Similar photolithography processes were implemented for the hemisphere-shaped PSS (HPSS), but we omitted the over-exposure process. HPSS samples were then etched by reactive ion etching and dipped into the BOE solution to remove the SiO2 mask.

Scanning electron microscope (SEM) images of the CPSS are shown in Figure 1(a). For comparison, the cross section of a conventional HPSS is also shown in Figure 1(b). The diameter and interval of each crown-shaped pattern were 3μm and 2μm, respectively, while the height of the cone shape was about 1.17μm. We also grew LED structures on plane sapphire to make conventional LEDs (C-LEDs). Figure 1(c) shows the cross-sectional SEM image of the CPSS-LEDs. The air voids (with a refractive index n=1) were formed between the CPSS (n=1.7) and GaN (n=2.5) epilayer on top of the crown shape. These embedded donut-shaped air-voids can significantly increase the LEE due to enhanced scattering: see Figure 1(d).

Figure 2. Light output power and voltage as a function of current for the three fabricated LEDs. C-LED: Conventional LED.

We then placed the patterned substrates in a metalorganic chemical vapor deposition (MOCVD) device for regular LED structure growth, followed by standard fabrication and testing procedures. Compared with C-LEDs, the output powers of HPSS-LEDs and CPSS-LEDs increased by 20% and 32.1%, respectively. The enhanced light-current-voltage (L-I-V) characteristics, shown in Figure 2, can be attributed to two factors. Firstly, the TDDs are reduced by ELOG on the crown tops of the PSS. This also greatly reduces the number of non-radiative recombination centers and increases photon generation efficiency. This finding is similar to that reported for GaN grown on recess-patterned PSS by Wuu and colleagues.2 Secondly, the scattering effect caused by the CPSS air voids means that more photons can be extracted from the LEDs. It has been reported that the inclined facets of PSS can redirect photons back to the device surfaces, leading to higher LEE.3 As a result, there is an additional 9.8% output power enhancement for LEDs grown on CPSS compared to those grown on HPSS, which lack the donut-shaped air voids between the PSS and GaN epilayer.

To probe further, we calculated the LEE of total radiant flux (LEE_TRF) of the LED by varying the crater angle (θ) indicated in Figure 3. LEE_TRF considers the rays that escaped from every surface of the LEDs. A smaller crater angle indicates a steep slope with greater depth in the crater, whereas a larger angle implies a slant slope from the edge of the crater. From the simulation results, the patterned shape with a crater angle in the range of 30° to 35° induces the largest LEE. Using these results, it is possible to optimize the range of crater angles and predict the optical enhancement via optical ray-tracing. From previous results,4 air voids can be treated as scattering centers due to the difference of refractive indices between the void (n≈1) and the material (n≈2). HPSS, on the other hand, lack air voids as an extra scattering medium and are therefore macroscopically incapable of being as effective as the CPSS. This was confirmed using our ray-tracing simulation: the calculated result for the increase in power output was 25.8% for HPSS, compared to over 40% for CPSS.

Figure 3. Optical properties as a function of crater angle for the CPSS. Light extraction efficiency of total radiant flux (LEE_TRF) considers the rays that escape from every surface of the LED.

In summary, we have overcome problems previously encountered when pairing GaN and sapphire substrates by implementing CPSS for use in LEDs. The resulting donut-shaped air voids increase scattering and reduce non-radiative recombination centers, thereby increasing light extraction efficiency. Our next step will be to optimize crater angles for optical enhancement to further increase the efficiency of the device.

Ching-Hsueh Chiu
Department of Photonics
Institute of Electro-Optical Engineering
National Chiao Tung University
Hsinchu, Taiwan

Ching-Hsueh Chiu is currently a PhD student. His research work mainly focuses on III-V compound semiconductor material growth by MOCVD alongside its characteristics.

Chia-Yu Lee, Che-Yu Liu, Hao-Chung Kuo
Department of Photonics
National Chiao Tung University
Hsinchu, Taiwan

Chia-Yu Lee is a PhD student researching III-V compound semiconductor material growth by MOCVD.

Che-Yu Liu is an MSc student, whose research work mainly focuses on optical measurement and analysis, and nano-structure analysis for GaN-based light emitting diodes.

Hao-Chung Kuo is a professor. His research areas include device fabrication, III-V nitride compound semiconductor lasers and LED material growth, III-V nitride nanotechnology and high-efficiency nanostructured photovoltaics.

Bo-Wen Lin, Wen-Ching Hsu
Sino-American Silicon Products Inc.
Hsinchu, Taiwan

Bo-Wen Lin is a PhD candidate researching the patterned sapphire substrate process.

Wen-Ching Hsu is a vice general manager. His research work focuses on the design and processes of patterned sapphire substrate.

Chien-Chung Lin
Institute of Photonic Systems
Department of Photonics
National Chiao Tung University
Tainan County, Taiwan

Chien-Chung Lin is an assistant professor. His research work is focused on the design and fabrication of semiconductor optoelectronic devices including LEDs, solar cells, and lasers.

Jinn-Kong Sheu
Institute of Electro-Optical Science and Engineering
National Cheng Kung University
Tainan County, Taiwan

Jinn-Kong Sheu is a professor. His research work focuses on epitaxial growth and characterization of III-V compound semiconductors, fabrication and characterization of optoelectronic devices, and wide band gap compound semiconductor materials and devices.

Chun-Yen Chang
Department of Electronics Engineering
National Chiao Tung University
Hsinchu, Taiwan

Chun-Yen Chang is a professor. He has profoundly contributed to research into microwaves, microelectronics and optoelectronics, including the invention of the method of low-pressure MOCVD using triethylgallium to fabricate LED, laser, and microwave devices.

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