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InGaN-based light-emitting diodes grown on a micro-/ nano-scale hybrid patterned sapphire substrate Wen-Cheng Ke, Fang-Wei Lee, Chih-Yung Chiang, Zhong-Yi Liang, Wei-Kuo Chen, and Tae-Yeon Seong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10226 • Publication Date (Web): 28 Nov 2016 Downloaded from http://pubs.acs.org on December 2, 2016

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ACS Applied Materials & Interfaces

InGaN-based light-emitting diodes grown on a micro-/nano-scale hybrid patterned sapphire substrate Wen-Cheng Ke*1, Fang-Wei Lee2, Chih-Yung Chiang1, Zhong-Yi Liang1, Wei-Kuo Chen2 and Tae-Yeon Seong3 1

Deptment of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan

2 3

Department of Electrophysics, National Chiao-Tung University, HsinChu 300, Taiwan

Department of Materials Science and Engineering, Korea University, Seoul 02841, Korea.

ABSTRACT A hybrid patterned sapphire substrate (hybrid-PSS) was prepared using an anodic aluminum oxide etching mask to transfer nano-patterns onto a conventional patterned sapphire substrate with micro-scale patterns (bare-PSS). The threading dislocations (TDs) suppression of light-emitting diodes (LEDs) grown on a hybrid-PSS (HP-LED) exhibit a smaller reverse leakage current compared with that of LEDs grown on a bare-PSS (BP-LED). The strain-free GaN buffer layer and fully strained InGaN active layer were evidenced by cross-sectional Raman spectra and reciprocal space mapping of the X-ray diffraction intensity for both samples. The calculated piezoelectric fields for both samples are close, implying that the quantum confined Stark effect was not a dominant mechanism influencing the electroluminescence (EL) peak wavelength under a high injection current. The bandgap shrinkage effect of the InGaN well layer was considered to explain the large red-shifted EL peak wavelength under high injection currents. The estimated LED chip temperatures rise from room temperature to 150°C and 75°C for BP-LED and HP-LED respectively at a 600-mA injection current. This smaller temperature rise of LED chip is attributed to the increased contact area between sapphire and the LED structural layer because of the embedded nano-pattern. Although the chip 1

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generates more heat at high injection currents, the accumulated heat can yet be removed outside the chip effectively. The high diffuse reflection (DR) rate of hybrid-PSS increases the escape probability of photons, resulting in an increase in the viewing angle of LEDs from 130° to 145°. The efficiency droop was reduced from 46% to 35%, effects, which can be attributed to the elimination of TDs and strain relaxation by embedded nano-patterns. In addition, the light output power of HP-LED at 360-mA injection currents exhibits a ~22.3% enhancement, demonstrating that hybrid-PSSs are beneficial to applicate in high-power LEDs.

KEYWORDS: light-emitting diode, dislocation, nano-patterns, Raman, electroluminescence, diffuse reflection, efficiency droop *Corresponding author e-mail: [email protected]

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1. INTRODUCTION InGaN-based light-emitting diodes (LEDs) have been employed widely in the indoor and outdoor lighting and display applications. To approach the high-power end of the lighting market, increasing the luminosity of commercialized LEDs is crucial. However, InGaN-based high-power LEDs generally exhibit an unsatisfactory characteristic: their efficiency monotonically declines upon increasing injection current density, known as efficiency droop. This pronounced efficiency-droop behavior has been considered to hinder efficient LED operation at high powers. Carrier overflow from the active layer due to the polarization field.1,2 Auger recombination,3 carrier injection,4 and junction heating5 are a few of the phenomena proposed to explain efficiency droop. Improvements made thus far in the luminous intensity include increasing the light output power of LEDs, for example, by use of low-resistivity indium-tin-oxide transparent conductive layer,6,7 thin p-GaN,8 surface roughing,9 photonic structure,10 flip-chip,11 and patterned sapphire substrate (PSS).12-15 PSSs, which improve light extraction efficiency and epitaxial crystal quality, have attracted much attention in the commercial LED market, as the key to high-brightness LEDs. Light output power is greatly enhanced by increasing pattern density (i.e., reducing the spacing between neighboring patterns). Unfortunately, the dramatic decrease of the C-plane area on PSSs has limited the LED epitaxial window, resulting in poor material quality of LED structure, which degrades LED chip performance, for example, electrostatic discharge and leakage current. Recent studies on substrate patterning in the nano-scale indicate a further reduction in the dislocation density and an enhancement in the light extraction efficiency of InGaN-based LEDs.16-17 LEDs grown on nano-patterned sapphire substrates (NPSSs) demonstrate improved light output power compared with those grown on micro-scale PSSs. Nano-imprinting is a mass-production technique for fabrication NPSSs.18 Until now, pattern loss issues, especially for small-sized and high-density nano-patterns, remain a technical bottleneck, because resin frequently sticks to the mold. 3



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Thus, reduction of the C-plane area in micro-scale PSS and pattern loss of small-sized nano-scale patterns in NPSSs, among other factors, limit LEDs grown on PSSs. To further improve the performance of LEDs using a PSS, a hybrid-PSS was proposed: embedding nano-pattern structures onto a conventional micro-scale pattern of PSS (bare-PSS).19,20 The application of metal Ni islands as an etching mask was reported in the preparation of a hybrid PSS. Using a Ni island mask to create nanopatterns on PSSs forms a convex nanostructure. In this study, anodic aluminum oxide (AAO) was employed as an etching mask to prepare a high-density concave nanopattern structure on a PSS. The optoelectronic properties of LEDs grown on hybrid-PSS and bare-PSS are demonstrated and compared. Possible mechanisms are proposed to explain the improved light output power and efficiency droop of InGaN-based LEDs grown on hybrid-PSS.

2. EXPERIMENTAL SECTION A pattern specification of 4-in bare-PSSs with diameter/spacing/height = 2.6/0.4/1.6 µm was used in this study. As shown in Figure 1, the hybrid-PSS was patterned using an AAO dry etching mask in the following fabrication steps: (a) deposition of an Al–Ti film with a thickness of 600/100 nm onto the bare-PSS prepared using E-beam evaporation system at room temperature. The purpose of the Ti interlayer is to improve the adhesion between the Al films and sapphire substrate. A deposition rate of 0.1 nm/sec was used for the Al film in order to achieve a mirror-like surface; (b) creation of a nano-hole array structure by the anodization process of Al–Ti film using 1.1M phosphoric acid under 80 V for 5min at 10°C. The oxide pore was widened in 6 wt% H3PO4 for 25 min at 25°C. Two-step anodization processes were employed to improve the size uniformity of nanopatterns created with the AAO etching mask; (c) substrate dry etching by an inductively coupled plasma reactive ion etching system with a BCl3 flow rate of 30 sccm, applied power of 600 W, RF bias power of 450 W, and etching time of 450 s. The etching selectively between the sapphire 4


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substrate and AAO mask was approximately 1:3; (d) removal of the remaining AAO mask by an acid-pickling process in a solution of HF:HNO3:CH3COOH (2:3:10) at room temperature. The anodization system is shown in Fig. 1(e). The sapphire substrate is insulator; therefore, a copper wire must be connected between the sample surface (i.e. Al film) and copper holder for the current conduction. The distance between the graphite plate and substrate was 10 cm; Next, blue InGaN LED epitaxial structures were grown using Aixtron’s commercial low-pressure metal–organic chemical vapor deposition system. As shown in Fig. 1(f), the LED structures comprised a 20-nm-thick AlN buffer layer on the PSS, a 3.8-µm-thick undoped GaN layer, a 29-nm-thick AlGaN layer, a 3.5-µm-thick n-GaN layer, an InGaN–GaN multiple quantum well (MQW) with emission wavelength in the blue region, a 10-nm-thick Mg-doped AlGaN cladding layer, a 40 nm-thick p-type AlGaN/GaN superlattice layer and a 75-nm-thick p-GaN layer. LED chips of size 26 mil × 30 mil were prepared for comparison of optoelectronic performance of LED epitaxial structures grown on bare-PSS and hybrid-PSS. The surface morphologies of hybrid-PSS were characterized by a scanning electron microscope (SEM) (JEOL JSM-7001F). The crystalline quality of LEDs was evaluated by X-ray diffraction (XRD) using Bruker-AXS D8-Advance diffractometer and transmission electron microscopy (TEM, FEI Tecnai Osiris). The electroluminescence (EL) spectra were measured using a light detector with high-resolution charge-couple device (CCD) spectrometer. The optical property of PSSs under the back polishing process were measured by a LAMBDA 750S UV/Vis/NIR spectrophotometer in the wavelength range 400–550 nm using a 60-mm integrating sphere. Raman scattering was used to characterize the strain distribution of the LED structural layer. Raman scattering experiments were carried out by an MRS-iHR550 modular Raman system in the confocal mode. The laser output power was set at 100 mW with an excitation wavelength of 532 nm. The focused beam size was ~10 µm by objective (Olympus) 50× visible lens with NA = 0.75 and WD = 0.37 mm. By controlling 5



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the sample x-y stage, the signal from different cross-sectional positions of LED epitaxial structure could be separated. The internal quantum efficiency (IQE) of LEDs was measured at the temperature of 300 K and 14 K by photoluminescence (PL) using a 325 nm He-Cd laser (Kimmon IK 5552R-F).

3. RESULTS AND DISCUSSION The SEM images of bare-PSS and hybrid-PSS are shown in Figure 2a and 2b respectively. The average diameter, depth, and density of the nano-hole patterns on the C-plane sapphire area were ~100 nm, ~100 nm and 2.5 × 109 cm−2 respectively. In addition, nonregular-shaped islands with height of ~100 nm were formed on the side-wall of micro-scale pattern. The crystalline quality of GaN layer in LEDs was characterized by full widths at half maximum (FWHMs) of XRD rocking curves for the (002) and (102) planes (Figure 2c and 2d), which implies the density of screw-type and edge-type threading dislocations (TDs) in GaN epitaxial films.21,22 The FWHMs of (002)/(102) XRD spectra were decreased from 331/319 arcsec for bare-PSS LED (BP-LED) to 308/288 arcsec for hybrid-PSS LED (HP-LED). The estimated density of screw-type/edge-type TDs in GaN layer for BP-LED and HP-LED were 2.2 × 108/5.4 × 108 cm−2 and 1.9 × 108/4.4 × 108 cm−2 respectively. To characterize threading dislocations, cross-section [1-100] TEM two-beam bright-field (BF) images were taken from BP-LED and HP-LED samples using g=[0002] and [11-20] reflections (Figure 3). It is well known that the [0002] TEM BF images reveal screw-type dislocations, while [1120] TEM BF images display edge-type dislocations.23 The invisible criteria were employed to examine the types of TDs. It is noteworthy that for both samples, most of TDs were annihilated below an AlGaN layer (marked ‘A’), as indicated by the arrows. For BP-LED sample, there are three different types of TDs, namely, edge, mixed screw-type dislocations, as marked in Figure 3a and 3b, while HP-LED sample contains edge and mixed-type dislocations (Figure 3c and 3d). It is noted that BP-LED sample contain more edge-type dislocations than HP-LED sample. This indicates that the 6


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type of dislocations in the samples depends on the different PSS structures. It was known that nanopipes are mainly related to pure screw-type TDs.24 This implies that HP-LED may contain less nanopipes than BH-LED. In addition, it was found that the edge TDs played a major role in reduction in the PL intensity.25 Thus, the TEM results indicate that HP-LED would show better optical performance than BP-LED. The lateral growth mechanism of GaN thin films on nano-hole PSSs has been reported elsewhere.26 The TD density of GaN thin films can be reduced if the aspect ratio of nano-hole pattern and lateral growth rate of GaN thin films can be optimized well. In general, the reducing c-plane area on the PSS will limit the LED epitaxial windows, resulting in poor material quality of the LED chip. The nanohole patterns embedded on the PSS appear to markedly reduce the C-plane area for the hybrid-PSS. However, the AAO etching mask fully protected the PSS surface without a nanohole region. After removal of the AAO mask, the C-plane areas surrounding the nanohole retained the original sapphire surface. In addition, from the cross-sectional TEM images (Figure 3e), the GaN thin film grew not only on the bottom surface of nano-hole pattern but also in the surrounding area of nanohole pattern. Thus, the total C-plane area did not decrease for the HP-LED relative to the BP-LED. In this study, the aspect ratio of nanohole patterns are controlled ~1, GaN thin films can be fully filled on the bottom surface of nanohole pattern. The lateral growth mechanism will bend the TDs (white arrows) and reduce the TDs density effectively in the initial growth stage of GaN thin films on hybrid-PSS. Figure 3f shows the (0002) reflection high-resolution X-ray diffractometer (HRXRD) ω-2θ scans for both samples. The well/barrier width and In composition of the MQWs were estimated to be approximately 2.5/12.5 nm and 13%, respectively, according to the simulated fitting with the HRXRD spectrum. The FWHMs of first satellite peak were 98 and 90 arcsec, respectively, for the BP-LED and HP-LED, which provides compelling evidence of the abrupt interface and high structural quality of InGaN/GaN MQWs for HP-LEDs. The lateral growth 7



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mechanism blocks the dislocations propagating to the active layer region, leading to improved optoelectronic properties of LEDs. Figure 4 shows the current–voltage (I–V) characteristics of BP-LED and HP-LED at room temperature as well as a CCD image of the turn-on LED chip (inset). The forward voltages of both samples were ~2.75 V under an injection current of 20 mA. However, the reverse leakage currents showed a large discrepancy in these two samples, as shown in the inset of Figure 3. The leakage currents for BP-LED and HP-LED at a reverse voltage of 10 V were 5.01 and 3.07 µA respectively. A reduction in leakage current can be attributed to decreased TDs density of LED epitaxial structure grown on hybrid-PSS. Figure 5a and 5b show the room temperature EL spectra as a function of injection current for BP-LED and HP-LED. The emitting wavelengths at a current of 20 mA were 451.7 and 451.1 nm respectively for BP-LED and HP-LED. The peak wavelength difference between the two samples was 0.6 nm, which may be generated by a difference in the indium composition in the quantum well layer, nonuniform substrate temperature distribution, and active layer thickness that is generated from the epitaxial system. Because both samples were grown in the same reactor run, the abovementioned differences were considered negligible. Figure 5c shows plots changes in the EL peak wavelength with those in the injection current for the two samples. For BP-LED, with increase in injection current from 20 mA, the peak first blue shifted to a lower-limit wavelength of 450.7 nm for injection currents up to 100 mA and then red shifted to 466.3 nm as the injection current increased to 600 mA. For the HP-LED, the peak blue shifted to a lower-limit wavelength of 449.6 nm for injection currents up to 180 mA and then red shifted to 455.9 nm as the injection current increased to 600 mA. Quantum confined Stark effect (QCSE) arises from the polarization charges that are induced by a lattice mismatch of the InGaN well and GaN barrier layer.27,28 The polarization electric field drives the injection carriers to opposite sides of well layer. (That is, electrons in the InGaN well are driven toward the top interface and holes toward the bottom 8


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interface.) The wave function of electrons and holes separate out, resulting in reduced recombination efficiency. In addition, with a reduced overlap integral due to a strong polarization field, the effective bandgap of the quantum wells shrinks because of the tilted band edge; the electron–hole transition energy is in turn greatly reduced, which causes a red shifted emitting wavelength. With increasing injection current, the free carriers in MQWs are increased; the field induced by free electrons and holes in the well layer screen the polarized field and decrease the wavelength shifting by QCSE.29,30 These effects result in the peak wavelength shift to the short wavelength and the blue shift behavior. The peak wavelength blue shift and then red shift with increase in injection current exhibit a wavelength turning point (blue arrows in Figure 5c) at 100 and 180 mA for BP-LED and HP-LED respectively. In addition, when the injection current was increased from 20 mA to the turning point current, the blue shifted wavelengths were 1.0 and 1.5 nm respectively for BP-LED and HP-LED. The extended wavelength turning point and large blue-shift wavelength implies that more carriers can be injected into the InGaN well to screen the polarization field. Because the density of the TDs in the HP-LED is lower than that in the BP-LED, more carriers can be injected into InGaN MQWs with a lower probability of nonradiative recombination in TDs, which screen the polarized field; thus, a higher blue shift is observed in HP-LED compared with BP-LED. Besides material quality, screening of QCSE and band-filling of localized states should be considered as well as causes for the blue shift of the EL peak wavelength.31 If the tilted band edge of InGaN well can be reduced (i.e., weak QCSE), band-filling effect will also induce a higher blue-shifted wavelength. In addition to the relationship between the wavelength and injection current, luminous intensity is another concern. The integrated EL intensity and injection current (L–I) characteristic curves of both samples are shown in Figure 5d. The integrated EL intensity at 20 mA demonstrates a 31% enhancement for HP-LED compared with BP-LED. Luminous intensity is shown to increase with increase in injection current before intensity reaches a saturated point (green arrows in Figure 5d). 9



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Injection currents at intensity saturated points are 360 and 480 mA respectively for BP-LED and HP-LED. The EL peak intensities of the two samples increased linearly with increase in injection current over the range of 20–100 mA, caused by the EL emission in the tilted InGaN well layers with the band-filling effects. In general, this dependence can be characterized by L α IF, where F physically reflects the various recombination process.32 If F = 1, radiative recombination dominates. If F < 1, increase in nonradiative recombination rate is seen. The fitting values of parameter F for both samples under different injection currents are shown in Figure 5d. The F values of both samples in the current range of 20–100 mA approach 1, indicating that LED emission is dominated by radiative recombination. However, the F value of BP-LED decreases to 0.5 with increasing injection current in the range of 160–360 mA, indicating that an LED emission process transfer to nonradiative recombination. Over the same current range, a higher F value (i.e., 0.7) for HP-LED indicates a nonradiative recombination rate lower than that for BP-LED. Thus, low nonradiative recombination rates for LEDs can be achieved with LED epitaxial structure grown on hybrid-PSS with a low TD density and/or weak QCSE. In general, both spontaneous polarization and strain-induced polarization (i.e., piezoelectric effect [PZ effect]) produce large electric fields, which significantly affect the optical properties of LEDs. Nonpolar or semipolar sapphire substrates have been proposed to overcome the QCSE issue in InGaN-based LEDs.33-35 Recently, the relaxed InGaN template layer was inserted between the MQWs and a high-material-quality GaN layer to achieve a strain balanced status for improving the LED’s performance.36 As shown in Figure 5c, by increasing the injection current from the wavelength turning point to 600 mA, the red-shifted wavelengths are 15.6 and 6.3 nm respectively for BP-LED and HP-LED. The smaller red-shifting wavelength of the LED chip with increased in injection current is more beneficial for high-current-driving LEDs. Because the PZ effect is predominant in the tilted InGaN well band diagram, the effect of strain-induced PZ effect should be 10


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considered. To analyze the strain distribution for LED structures grown on bare-PSS and hybrid-PSS, micro-Raman system was employed to measure the spacing resolution Raman spectra along different positions of the cross-sectional LED structural layer (Figure 6). The strain in the LED structural layer causes changes in the Raman spectra and enables determination of relaxation in the crystal. Emission lines of the E1(TO), E1(LO), and E2(high) phonons are created by atom oscillations on the C-plane, so they are sensitive to lattice strains on the C-plane.37 Two E2(high) peaks at 567.7 and 574.5 cm−1 are observed in the Raman spectra of both samples. The Raman peak at 574.5 cm−1 indicates a highly compressive strain is generated in the initial growth of the LED structural layer on the sapphire substrate.38 As the laser spot moves toward the 1-µm-limit from the surface, the Raman peak at 567.7 cm−1 predominates for both samples. The FWHMs of the E2(high) peak are 7.3 and 6.1 cm-1, respectively, for the BP-LED and HP-LED, which implies that the LED structure on the hybrid-PSS possesses higher material quality. In addition, the Raman peak at 567.7 cm−1 represents strain-free GaN thin films.39 Thus, the GaN layer near the InGaN MQWs was strain-free for both samples. Furthermore, to study the strain states of InGaN MQWs, reciprocal space mapping (RSM) of the XRD intensity from the (105) reflection for both samples was performed. Figure 6c and 6d show the (105) RSM for the BP-LED and HP-LED, respectively. The center position of the satellite peaks and GaN buffer layer peak are aligned in a vertical line parallel to the Qy-axis for both samples.40 The RSM analysis results indicated that the lattice constant of the InGaN MQWs is identical to that of the GaN buffer layer (i.e., the InGaN MQWs is fully strained). The strain that exists in the LED structure will influences the red-shifted EL peak wavelength with increase in injection current. To further understand the relationship between the red-shifting peak wavelength and strain in the two samples, the strain-induced piezoelectric field in LED structural layer was estimated. To fit the evolution of the EL peak energy as a function of injection current, the combined effects of bandgap shrinkage and QCSE were taken in account (Figure 7). This 11



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effect may be expressed as follows. Epeak = Eg (T ) − ∆EQCSE

(1)

where Epeak is the energy of the transition between the electron and hole ground states, Eg(T) is the shrinkage bandgap of InGaN well layer, ∆EQCSE is the red-shifted wavelength induced by the electric field in the well layer (Ew):41,42

∆E

= C (me∗ + mh∗ )e 2 Ew2 Lw 4 / h 2

QCSE

(2)

Here, C is a constant, me* and mh* represent respectively electron and hole effective mass, LW is the well thickness, and h is Planck’s constant. Ew can be further expressed by

Ew = Epz +

Va L

(3)

Here Epz is the piezoelectric field (first given an initial value), Va is the applied voltage, and L is the active layer thickness. Notably, the piezoelectric filed was calculated in a current density of 1 A/cm2 to exclude the band-filling effect.43 In the inset of Figure 7, the best-fitting values of Epz are 1.59 and 1.57 MV cm−1, respectively, for the BP-LED and HP-LED. However, the injection current of BP-LED increased from 360 to 600 mA; the peak wavelength cannot be fit solely by the QCSE term. Considering the bandgap shrinkage effect,44,45 the large peak wavelength red shift in the high-injection-current range can be fit well by adding Varshni’s equation.46

Eg (T ) =

E0 − α T 2 β +T

(4)

where α = 2 × 10−3 eV K−1 and β = 1740 K The results indicated that the estimated temperature increased from 55°C at 380 mA to 150°C at 600 mA for BP-LED. In contrast, the estimated temperature increased slightly from 45°C at 480 mA to 75°C at 600 mA for HP-LED. When LEDs operated in a high-current condition, the radiative recombination rate of carrier was 12


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smaller than the injected rate in the active layer. Because the excess injection in the active layer cannot recombine solely by radiative recombination, the nonradiative recombination becomes more severe, leading to more excess injected carrier transfer through heat generation.47 Thus, to ensure that the LEDs exhibit normal behavior in the high-injection-current region, the LED chips need to possess improved heat sink ability. A slower decade rate of EL intensity/injection current in the current range at saturation point to 600 mA can be found for HP-LED (Figure 5d). The luminous intensity droops of hybrid-PSS LED and bare-PSS LED, calculated using (Imax − I600°mA)/Imax, were 13% and 68% respectively. The drooping rate of luminous intensity was strongly related to the thermal effect of the chip at the higher injection currents. The area of GaN coverage on the sapphire substrate increased from 134 µm2 for BPLED to 170 µm2 for HPLED, due to the existence of nano-patterns, which cause more generated heat in LED structural layer transferring to sapphire substrate by conduction. Thus, the heat generated in the chip under higher injection currents can be removed

through the hybrid-PSS; this effect is believed to reduce the luminous intensity droop of InGaN-based LEDs. In addition, the photon re-absorption behavior in dislocation and defects within the LED structural layer boost heat generation and leads to loss of luminous intensity droop. To determine the optical influences of nano-patterns on light scattering behaviors, the diffuse reflection (DR) rate curves of both samples in the wavelength range of 400–550 nm were measured (Figure 8a). The average DR rate was found to increase from 39.1% for bare-PSS to 43.7% for the hybrid-PSS, which was expected to increase light scattering in the LED structure. The wavelength dependence of the DR rate was crucial for selecting the optimal pattern for various peak wavelength LEDs. The DR curve for NPSS with a pattern diameter/depth of 100/150 nm is shown in Figure 8b. The DR rate of the NPSS increased as the wavelength of the incident light decreased. In addition, no interference phenomenon was observed in the DR spectrum. By contrast, the interference was obvious for the bare-PSS sample. Considering the

13



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difference in the substrate structure, the nonperiodic and nonuniform size distribution of the nanohole pattern on the NPSS would prevent the interference phenomenon from being generated. Thus, the generation of interference for the bare-PSS was attributed to the periodic and uniform size distribution pattern. Through control of the pattern size, a high DR rate for the bare-PSS sample can be achieved. In addition, the nonperiodic and nonuniform pattern on the bare-PSS will diminish the interference intensity, resulting in a high average DR rate for the hybrid-PSS. Thus, the contribution of embedded nano-patterns on micro-scale PSS is the formation of the Lamberation pattern on the PSS surfaces,48 which increases the probability of photons into the escape cone (Figure 8c). This effect results in an increase in the light extraction efficiency of LEDs and reduction of the self-heating effect. The increase in light scattering on the hybrid-PSS also influences the far-field radiation pattern of LEDs. Both the emission patterns were

typical Lambertian distributions, which had a maximum intensity value in normal directions (Figure 8d). The bare-PSS LED at 20-mA injection current shows a view angle of 130°, whereas the hybrid-PSS LED exhibits a wider radiation pattern, that is, a view angle of 145°. Finally, the external quantum efficiency (EQE) of both samples at various injection currents for characteristic LED performance is shown in Figure 8e. The maximum EQE values of the BP-LED and HP-LED are 31.7% and 46.2% respectively. The internal quantum efficiency of both samples was measured by excitation power- and temperature-dependent PL.49 Assuming that the peak PL quantum efficiency at 14 K is 100% at an excitation power of 10 mW, the peak IQE at 300 K was measured to be 61.5% and 81.8%, respectively, for the BP-LED and HP-LED. Thus, the LEE, calculated by η LEE = η EQE / η IQE , can be increased from 51.5% for the BP-LED to 56.5% for the HP-LED. The enhanced LEE of the HP-LED can be attributed to the light scattering of guided photons by the embedded nanopatterns.

The EQE values of BP-LED and HP-LED at an injected

current of 360 mA are 17% and 30% respectively. In addition, efficiency droop was calculated by Efficiency droop =

( EQEmax − EQE360 mA ) EQE max 14


(5)

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where EQEmax is the highest EQE achieved and EQE360mA is the EQE at an injection current of 360 mA. Efficiency droop was reduced from 46% for the BP-LED to 35% for the HP-LED (normalized EQE shown in Figure 8e), which is mainly attributed to the reduction of the strain-induced polarization field in MQWs and/or reduction of TDs in LED structural layer. In addition, light output power at 360-mA injection current can be increased from 170 mW for BP-LED to 208 mW for HP-LED. The improvements of optoelectronic performance such as light output intensity, reduced efficiency droop, small EL wavelength shift and wider view angle demonstrates that hybrid-PSS is beneficial in the fabrication of high-power LEDs.

4. CONCLUSION This study demonstrated the optoelectronic performance of LEDs grown on a hybrid-PSS, which embedded nano-hole patterns onto the micro-scale PSS. The nonradiative recombination rate of LED grown on hybrid-PSS can be suppressed by reducing TDs density and strain in the LED structural layer, which was confirmed by XRD and TEM analysis. The lower nonradiative rate also inhibits chip temperature rise and decelerates the efficiency droop for HP-LED. In addition, the view angle of HP-LED was increased by about 15° because of increased contact area between the LED structural layer and sapphire substrate by the embedded nano-pattern, which induces light scattering behavior. Consequently, the light output power for HP-LED can be enhanced at an injection current, as compared with BP-LED. The improved optoelectronic performance of InGaN-based LEDs grown on a hybrid-PSS is crucial for further application in the manufacture of high-power solid-state lighting products.

15



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ACKNOWLEDGEMENTS This work was supported by the Ministry of Science and Technology of Taiwan under contract nos. MOST103-2221-E-011-178 and MOST104-2221-E-011-174. We show our great appreciation for help from Epistar Co., Ltd. and Kinik Co., Ltd. in Taiwan for supporting LED epitaxy, chip process and dry-etching process.

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FIGURE CAPTIONS Figure 1 Process flow chart of embedded nano-hole pattern onto the micro-scale PSS. (a) Ai-Ti films deposition; (b) AAO mask by anodization process; (c) nanopatterns transferred by ICPRIE dry-etching; (d) acid-picking process. (e) The schematic diagram of anodization system. (f) The cross-sectional diagram of InGaN based LEDs on hybrid-PSS. Figure 2 Top-view SEM images of (a) bare-PSS and (b) hybrid-PSS. XRD spectra of BP-LED and HP-LED of (c) (002) and (d) (102) planes. Figure 3 . Cross-section (a) and (c) [0002] and (b) and (d) [11-20] TEM two-beam bright-field images taken from BH-LED and HP-LED samples, respectively. (e) The cross-sectional TEM image of HP-LED (the bending TDs indicated by white arrows). (f) The HRXRD ω -2θ scans for the (0002) reflection from BP-LED and HP-LED. Figure 4 I–V characteristic curves of BP-LED and HP-LED. The inset shows the I–V characteristic curve in the reverse voltage zone and CCD image of HP-LED chip turn-on under 2-mA injection current. Figure 5 Electroluminescence spectra of (a) BP-LED and (b) HP-LED at various injection currents and evolution of (c) peak wavelengths and (d) intensities as a function of injection current. Inset table is the fitting parameter F at two injection current ranges. Figure 6 Micro-Raman spectra measured at different positions from the cross-sectional LED structural layer of (a) BP-LED and (b) HP-LED. The reciprocal space mapping of XRD intensity from the (105) reflection for (c) BP-LED and (d) HP-LED (Fully-strain line represented by red dashed line in the RSMs) Figure 7 A simulation of the relationship between peak energy and various injection currents by the combined QCSE and bandgap shrinkage effect for (a) BP-LED and (b) HP-LED. The insets show the estimated piezoelectric field by fitting the peak energy in the current range of 5-20 23



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mA for both samples. Figure 8 Diffuse reflection rate spectra of (a) bare-PSS and hybrid-PSS and (b) double-polishing sapphire and NPSS. (c) light racing schematic diagram and (d) far-field radiation patterns of the BP-LED and HP-LED. (e) EQE and normalized EQE of BP-LED and HP-LED at various injection currents.

24


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(a)

Al

(e)

Ti

Sapphire Substrate

(b)

Cu wire Graphite

AAO

Sample Cu holder

Sapphire Sapphire Substrate Substrate Electrolyte

(c)

Teflon holder

Plasma

(f) p-GaN InGaN/GaN MQWs Sapphire Substrate Substrate Sapphire

n-GaN

(d) Sapphire SapphireSubstrate Substrate Sapphire Substrate

Fig. 1 1



(b)

(a)

2 µm

2 µm

(d)

(c) BP-LED HP-LED

(002)

-400 -200

0

Intensity (arb.units)

Intensity (arb.units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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200 400

(102)

-400 -200

BP-LED HP-LED

0

200 400

ω scan (arcsec)

ω scan (arcsec)

Fig. 2 2


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(a)

(b) e

m

m

s

e

m

m

A

(c)

m

m

m m

e

(d)

m m

m e

A

e

m

m

m

m

m

e e m

m

1 µm

3



(e)

(f)

HRXRD intensity (arb. Units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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BP-LED HP-LED

(0002) GaN 0 1

-1 -3

-4000

-2

2

-2000

0

ω-2θ (arcsec)

Fig. 3 4


2000

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100 80 60 40

Current (µA)

0

Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1 -2 -3 -4 -5

-10 -8 -6 -4 -2 0

Voltage (V)

20 BP-LED HP-LED

0 0

1

2

Voltage (V)

Fig. 4 5


3


(a)

(b) EL intensity (arb. units)

EL intensity (arb. units)

BP-LED

600 mA 560 mA 520 mA 480 mA 440 mA 400 mA 360 mA 320 mA 280 mA 240 mA 200 mA 160 mA 120 mA 80 mA 40 mA

400

420

440

460

480

Wavelength (nm)

HP-LED

600 mA 560 mA 520 mA 480 mA 440 mA 400 mA 360 mA 320 mA 280 mA 240 mA 200 mA 160 mA 120 mA 80 mA 40 mA

400

500

420

440

460

480

500

Wavelength (nm)

(d) Integrated EL intensity (arb. units)

(c) BP-LED HP-LED

468

Wavelength (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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464 460 Red shift ~15.6 nm

456

Blue shift ~ 1.0 nm

452 Red shift ~6.3 nm

Blue shift ~1.5 nm

448 0

200

400

600

Current (mA)

BP-LED HP-LED

L ∝ IF

0

Fig. 5 6


200

400

Current (mA)

600

600

700

-1

500

800

sapphure

E2(High)

sapphire E1(LO)

sapphire

LED layer

E1 (TO)

Intensity (arb. units)

LED layer E1(LO)

E2(High)

500

HP-LED

(b)

A1 (TO) E1 (TO)

Intensity (arb. units)

BP-LED

sapphure

(a)

600

700

-1

800

Raman shift (cm )

Raman shift (cm )

(d)

(c) 7600

A’

7600

Qy x 10000 (rlu)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A1 (TO)

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7400

7400

7200

7200 2600

2800

3000

2600

Qx x 10000 (rlu)

2800

3000

Qx x 10000 (rlu)

Fig. 6 7



Experiment QCSE QCSE + Gap shrinkage

BP-LED

2.76

(b) 2.76 2.72

2.64 2.60 0

Peak energy (eV)

2.72 2.68

Experiment QCSE QCSE+ Gap shrinkage

HP-LED

2.75

2.75

2.68 2.74

2.73 0

Experiment Simulation (EPZ = 1.59 MV/cm)

2.64

Peak energy (eV)

(a) Peak energy (eV)

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5 10 15 20 25

Current (mA)

150

300

450

2.60 0

600

Current (mA)

2.74

2.73 0

Experiment Simulation (EPZ = 1.57 MV/cm)

5 10 15 20 25

Current (mA)

150

300

450

600

Current (mA)

Fig. 7 8


(a) 50 Diffuse reflectance (%)

(b)

Bare PSS Hybrid PSS

45

Detector

40

35 400

450

Double-polishing sapphire NPSS

10

500

Wavelength (nm)

550

8 6 4 2 0

400

500

600

Wavelength (nm)

700

(c) HP-LED

BP-LED

(e)100 80

90 120 150

0.8

60 30

180

60

0.6

40

0.4 0.2

20

0 BP-LED

BP-LED HP-LED 1.0

0.0

HP-LED

0

0

150

300

450

Current (mA)

Fig. 8 9


600

Normolized EQE

(d)

EQE (%)

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Diffuse reflectance (%)

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TOC graphics

p-GaN MQWs n-GaN

Sapphire SapphireSubstrate Substrate

2 µm

Sapphire Substrate

1


Nanoscale

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