Enhancing light extraction mechanisms of GaN-bad light-emitting diodes through the
integration of imprinting microstructures,
patterned sapphire substrates, and surface
roughness
Yeeu-Chang Lee*, Ching-Huai Ni, and Chih-Yeeu Chen
Department of Mechanical Engineering and Center for Nano-Technology, Chung Yuan Christian University, Chung
Li 32023, Taiwan
*yclee@cycu.edu.tw
Abstract:Analysis of the various light extraction efficiency enhancement
mechanisms for the GaN-bad light emitting diodes (LEDs) was
investigated. Experiments utilized the imprinting technique to fabricate
pyramid and inverted pyramid microstructures. Roughness treatment was
then integrated with the imprinting structures on patterned sapphire
substrate (PSS) LEDs. An approximate 33% improvement in light output
power was obtained using the pyramid profile when compared with the
planar LED. This was nearly 15% higher than that of the inverted pyramid
profile. The roughness effect provided an approximate 5% efficiency
enhancement. The total light enhanced efficiency incread to 85.9% by
integrating the imprinting pyramid structure, PSS, and surface roughness.
©2010 Optical Society of America
OCIS codes: (230.3670) Light-emitting diodes; (220.4000) Microstructure fabrication.
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#131066 - $15.00 USD Received 2 Jul 2010; revid 3 Sep 2010; accepted 3 Sep 2010; published 14 Sep 2010 (C) 2010 OSA8 November 2010 / Vol. 18, No. S4 / OPTICS EXPRESS A489
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1. Introduction
Although GaN-bad light-emitting diodes (LEDs) have received great interests due to their small size, energy efficiency, longevity, and environmental soundness [1–3], a need for attaining higher external quantum efficiency (EQE) of LEDS continues to exist becau of their wide applications. The EQE is decided by the internal quantum efficiency (IQE) and the light extraction efficiency (LEE). The IQE of GaN-bad LEDs has greatly improved becau of advances in crystal structure and quality in recent years, but the LEE of LED chips is still low becau of the Fresnel loss and total internal reflection (TIR) [4]. The poor LEE is becau of the large refractive index difference and a smooth interface between air (n = 1) and the GaN (n = 2.5). In order to improve the LEE, various methods were propod to increa the opportunity for the emitting photons to escape into free space. The methods include changes to the surface roughness on the LED surface layer [5,6], the u of a patterned sapphire substrate (PSS) [7–9] and the direct fabrication of perodical optical structures on a GaN surface [10–13]. The methods build micro- or nano-structures on the LED or the substrate surface. In this study, some factors influencing LEE were investigated. The included the u of the imprinting technique to fabricate micro-pyramid and inverted micro-pyramid arrays on a PSS LED surface that received etching treatment to produce nano scale roughness on the imprinted structures. Each experiment was analyzed to determine the influence of various factors on the LEE.ricky gervais
2. Experiments
The GaN-ba LED ud in this study were grown on c-face (0001), two-inch conventional sapphire substrates (CSS) and PSS using a metal-organic chemical vapor deposition (MOCVD) system. Figure 1 shows the process by which 300 × 300 µm2LED chips were fabricated.
Fig. 1. Process flow to prepare a microstructure with a roughness effect on PSS LEDs.
#131066 - $15.00 USD Received 2 Jul 2010; revid 3 Sep 2010; accepted 3 Sep 2010; published 1
4 Sep 2010 (C) 2010 OSA8 November 2010 / Vol. 18, No. S4 / OPTICS EXPRESS A490
2.1 PSS wafer preparation
The wet etching technique prepared the PSS. SiO2film with a two- dimensional triangular array of circles, a 3µm diameter, and a 5 µm periodicity as an etching mask was patterned onto the sapphire via plasma enhanced chemical vapor deposition (PECVD) and standard photolithography. The substrate was then placed in an etching solution of H2SO4:H3PO4 with a mixture ratio of 5:1 at 300 °C. Finally, dilute HF etched the SiO2 mask away to complete the PSS process.
2.2 LED epitaxy
The LED layer structure consisted of a low temperature GaN nucleation layer, a 2.5 µm thick unintentionally doped GaN layer, a 3 µm thick n-type GaN layer, an active region with 10 periods of InGaN/GaN multiple quantum wells (MQWs), a 30 nm thick Mg-doped p-Al0.15Ga0.85N cladding layer (p = 5 × 1017 cm−3), and a 200 nm thick Mg-doped first p+-GaN contact layer (n = 7 × 1017 cm−3). The active layer consists of a 2.3 nm thick InGaN-well layer and a 13 nm thick GaN-barrier layer for the InGaN/GaN MQW LED structures.
2.3 Conventional chip process
The fabricated LED sample had indium tin oxide (ITO) evaporated onto it as a transparent conductive layer. Inductively coupled plasma (ICP) partially etched the LED sample to expo the n-GaN. Optical lithography defined the ITO pattern, and wet etching expod the p-GaN layer. Thermal evaporation with rapid thermal annealing to create the p- and n-electrodes deposited Cr/Au on the p-GaN and n-GaN surfaces.
2.4 Surface structure imprinting
The imprinting technique built microstructure pyramids and inverted pyramids on the LED surface.
2.4.1 Pyramid structure
An imprinting technique using a silicon mold created the pyramid surface structures. The mold patterns reflected the mask layout: a two- dimensional triangular array of circles with a 3 µm diameter and 5 µm periodicity. The fabrication of the silicon molds ud photolithography and the wet etching process. Finally, the mold patterns became an inverted pyramid array.
Pyramid microstructures were then built on the LED samples. The polymethylmethacrylate (PMMA, (950 PMMA A4, Micro. Chem.)) was spun onto the substrate to produce a uniformly thick, 0.7 µm fil
m across the substrate. To reduce the friction between the PMMA and the mold and improve the replica quality during demolding, the polytetrafluoroethylene (PTFE, (601S1-100-6, Dupont)) was coated onto the mold as an anti-adhesive film. The mold and substrate were then nt to press at a temperature of 150 °C and a pressure of 1500 Nt/cm2. After imprinting, a partial etching process was ud to remove the imprinting structures on the p- and n-contacts.
2.4.2 Inverted pyramid structure
A reversal imprinting technique with a polydimethylsiloxane (PDMS, ((Sylgard TM 184, Dow Corning)) mold created the inverted pyramid surface structures. A PDMS elastomeric mold with a pyramid pattern was cast on the silicon mold as mentioned above. PDMS is flexible, thermally stable, and conforms in such a manner as to allow complete conformal contact with the substrate (Normally, sapphire wafers warp 0 ~10 µm). This mold could be ud for large area imprinting, as it is cost effective and a commonly ud soft mold material. Instead of spinning the PMMA onto the substrates, the PMMA was spun onto the PDMS mold after spraying the PTFE. Much less pressing force was required (27 NT/cm2 in this experiment) to fabricate microstructures on the LED surface since the structures had already been formed by the mold pattern. The necessary requirement of the reversal imprinting is the mold had a #131066 - $15.00 USD Received 2 Jul 2010; revid 3 Sep 201
0; accepted 3 Sep 2010; published 14 Sep 2010 (C) 2010 OSA8 November 2010 / Vol. 18, No. S4 / OPTICS EXPRESS A491
lower surface energy than the substrate, which allowed the polymer material to adhere better to the substrate and detach successfully from the mold.
2.5 Roughness
High-density plasma was ud to generate ion bombardment and create nano roughness on the surface of the imprinting microstructure. This gave the photons generated in the LEDs incread opportunities for escape into free space.
To demonstrate the light extraction enhancement effect of the PSS, imprinting surface structure, and roughness, conventional LED chips and LED chips with a planar PMMA covered layer were prepared.
Finally, a high current measure unit (HP4156C) measured the current–voltage (I–V) by injecting different amounts of DC current into the LEDs. An integrated sphere with a calibrated power meter measured the light output power of the LEDs.
3. Results and discussions
3.1 SEM morphology
Two groups of LED chips with different extraction mechanisms were prepared. The C group of Table 1 shows the LEDs that were grown on the CSS, and the P group of Table 2 shows the LEDs grown on the PSS. In Table 1, C1 is a conventional LED, C2 is a single sprayed lasyer of PMMA on the LED, C3 is same as C2 but has extra roughness treatment. C4 reprents the imprinting pyramid array applied onto the conventional LED, and C5 has roughness added to the pyramids. The imprinting of the inverted pyramid array with and without roughness onto the conventional LEDs are labeled as C6 and C7 respectively.
As in Table 1, Table 2 shows the LED grown on the PSS instead of a planar substrate.
Table 1. Overview of the planar PMMA, imprinting PMMA and roughness treatment for
conventional LEDs
Code C1 C2 C3 C4 C5 C6 C7
LED types Conventional
LED
巧取豪夺Planar
PMMA
+ C1
文登考研
Planar
PMMA +
roughness
wds是什么意思+ C1
Pyramid
PMMA
+ C1
Pyramid
PMMA +
roughness
+ C1
Inverted
pyramid
PMMA
+ C1
Inverted
pyramid
PMMA
英汉汉英词典
+
roughness太平洋英语
+ C1 Table 2. Overview of the planar PMMA, imprinting PMMA and roughness treatment for
PSS LEDskings of leon
Code P1 P2 P3 P4 P5 P6 P7
LED types PSS LED
Planar
PMMA
+ P1
Planar
PMMA +
roughness
+ P1
Pyramid
PMMA
+ P1
Pyramid
PMMA +
roughness
+ P1
Inverted
pyramid
在线法医
PMMA
+ P1
Inverted
pyramid
PMMA
+
roughness
+ P1
height of the pyramids and inverted pyramids are 2.2~2.3 µm. Figure 3 (a) and 3(b) are the imprinted patterns made by the molds of Fig. 2(a) and 2(b). In Fig. 3(a), the thickness of the residual layer is 0.23 µm, and the height of the imprinting pyramid is 1.91 µm shorter than the depth of the mold becau the PMMA did not completely fill the silicon mold. In Fig. 3(b), the depth of the imprinting inv
erted pyramidis 1.26 µm, which is much less than the height of PDMS mold. This is becau PDMS is an elastomeric material and the mold deforms during embossing even under light pressure.
#131066 - $15.00 USD Received 2 Jul 2010; revid 3 Sep 2010; accepted 3 Sep 2010; published 14 Sep 2010 (C) 2010 OSA8 November 2010 / Vol. 18, No. S4 / OPTICS EXPRESS A492
