GaN Light-Emitting Diode

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High-brightness polarized green InGaN/GaN lightemitting diode structure with Al-coated p-GaN grating Guogang Zhang, Xu Guo, Fang-Fang Ren, Yi Li, Bin Liu, Jiandong Ye, Haixiong Ge, Zili Xie, Rong Zhang, Hark Hoe Tan, and Chennupati Jagadish ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00433 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 21, 2016

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High-brightness polarized green InGaN/GaN light-emitting diode structure with Al-coated p-GaN grating Guogang Zhang1,2,4, Xu Guo3, Fang-Fang Ren1,2,4 a), Yi Li1,4, Bin Liu1,4, Jiandong Ye1,2, Haixiong Ge3, Zili Xie1,4, Rong Zhang1,4 b), Hark Hoe Tan2, and Chennupati Jagadish2

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Jiangsu Provincial Key Laboratory of Advanced Photonic and Electronic Materials, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China

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Department of Electronic Materials Engineering, Research School of Physics and Engineering, The Australian National University, Canberra, ACT 2601, Australia

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College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China

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Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

ABSTRACT The potential of polarized light sources in liquid-crystal displays has been extensively pursued due to the large energy savings as compared to conventional light sources. Here, we demonstrate high-brightness polarized green light emission from an InGaN/GaN light-emitting diode (LED) structure by combining the strong coupling between surface plasmons (SPs) and multiple quantum wells and polarization effects of SPs. As compared to the as-grown LED structure, a significant enhancement is observed in the total light emission with a high polarization degree of 54%. This work might provide an efficient way to realize simple, compact, and high efficiency polarized light emission devices for the applications in electro-optical integration.

KEYWORDS: InGaN/GaN, light-emitting diodes, surface plasmons, polarized light

a) Author to whom correspondence should be addressed. Electronic mails: [email protected] and [email protected] ACS Paragon Plus Environment

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III-nitride based light-emitting diodes (LEDs) have attracted much attention due to their wide applications in solid state light, high resolution imaging, visible light communication, and so on. In particular, polarized light emitting diodes are key building blocks as back light sources in the application of liquid crystal display and three-dimensional projection display.1,

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However, current efforts on

GaN-based LEDs are focused on the enhancement of emission efficiency,3-5 and only very few reports on the simultaneous improvement of polarized light output and emission efficiency. The commercial nitride-based LEDs grown on c-plane sapphire substrates are generally considered to be unpolarized light sources due to the rotational symmetry of encapsulation, although the polarization characteristics are present in unpackaged chips.6 Although the output with desired polarization can be obtained through the use of a polarizer or reflector,7, 8 at least half of energy will be wasted due to absorption or reflection effects. Therefore, the development of highly efficient polarized LEDs becomes an important issue in the research of GaN-based LEDs. Several approaches have been previously proposed to achieve polarized light emission from III-nitride LEDs.9-15 For example, it has been widely reported that high degree of intrinsic linearly polarized light emission can be obtained from LED structures grown on nonpolar/semipolar substrates.9, 10

However, it suffers from the high cost of nonpolar/semipolar substrates, as well as the high density of

basal stacking faults and threading dislocations in the material.16 Other reports have demonstrated that the highly polarized luminescence can be obtained through the integration of nanostructures, such as metal gratings,11 combined dielectric/metal structures,12 and photonic crystals.13 However, from the perspective of energy saving, these techniques are less than acceptable since they have increased the degree of polarization at the price of reducing the luminous efficiency. To overcome this problem, surface plasmons (SPs) generated through resonance coupling between multiple quantum wells (MQW) and a one-dimensional (1D) metal grating structure was proposed to increase both the radiative recombination rate and the degree of polarization, since exciton energy of an unpolarized light can be transferred into SP modes for polarized emission as a result of intrinsic polarized property of SPs.14, 15

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However, the SP-MQW coupling is generally weak due to the large thickness of the sandwiched p-GaN layer, which is required to maintain the p-n junction.17 In addition, large-scale electron beam lithography process employed to fabricate this grating is costly and subject to a low production efficiency, indicating that it is not suitable for commercialization. In this work, we demonstrated a green InGaN/GaN LED structure with high brightness and high degree of polarization. To avoid the damage to the InGaN/GaN MQWs and to facilitate reciprocal vectors for the excitation of SP modes, Al-coated grating is only created within the p-GaN layer. As compared to the as-grown LED structure, the LEDs with Al grating structures exhibit significantly enhanced photoluminescence (PL) emission with a high polarization ratio of 54%, which corresponds to a great enhancement of the internal quantum efficiency (IQE) of polarized emissions. The finite-difference time-domain (FDTD) numerical simulation results have demonstrated excellent agreement with the experimental data and the physical mechanisms for improved performance are discussed in detail. A schematic diagram of the proposed polarized GaN-based LED structure (Sample A) is shown in Figure 1. It consists of a sapphire substrate, n-GaN layer, InGaN/GaN MQW, a p-GaN grating, and a bilayer Al metal grating. Here, p, w, and t represent the grating period, Al width, and thickness of Al layer, respectively. To ensure an efficient SP-MQW coupling, the distance between the lower layer of Al grating and the MQW is fixed to be 20 nm, which is smaller than the penetration depth of SP waves. The FDTD method is employed to simulate the optical properties of the samples with an incident plane wave being launched in the InGaN/GaN MQW region and propagating along the z-axis. The relative dielectric constants of Al and GaN are frequency dependent and can be taken from Adachi18 and Marvin19 respectively. We define the transverse magnetic (TM) or transverse electric (TE) mode when the electric field parallel with x or y-axis, respectively. To demonstrate how the Al gratings affect the device performance, we consider the grating LED sample without Al layers as Sample B and the as-grown planar sample without nano-pattern as Sample C. ACS Paragon Plus Environment

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The dispersion equation 20 of SP resonances induced at the Al/GaN interface is

2π

λ

ε mε d 2π = k0 sin θ + n , εm + εd p

(1)

Figure 1. Schematic diagram of the designed polarized LED structure (Sample A).

where λ is the free space wavelength and p is the period of grating. The parameters εm and εd are the relative dielectric functions of the metal and dielectric, respectively. The integer n represents the diffraction order of the lower Al grating at a specific incident angle, θ. In this work, the PL emissions from the as-grown LED wafers are in the green wavelength region. Therefore, the grating period p = 420 nm is designed in accordance with Eq. (1) to support a SP mode occurring around the resonance wavelength of λ = 509 nm. The reflection spectra of TM-polarized light under different widths are plotted in Figure 2a, where the thickness of Al grating, t, is chosen as 20 nm and the incident angle, θ, fixed at 0°. One can see that an obvious reflection dip varies with the width and gradually disappears when the width exceeds a certain range (100 ~ 250 nm). To identify the origin of these dips, we calculate the corresponding electric field distribution in Sample A as shown in Figures 2b-2e for w = 100, 130, 170, and 250 nm, ACS Paragon Plus Environment

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respectively. The typical petal-like profile indicates that the SP modes are excited at the interface of Al/GaN, which leads to the reflection minima as fitted by the dashed line in Figure 2a. The distinct localization of optical energy densities is expected to extend into the active region and as a result, the radiative recombination rate of MQW is enhanced due to Purcell effect. As compared to the case of w = 100 nm (λ =532.5 nm), the electric field at w = 130 nm (λ =511 nm), w = 170 nm (λ =509 nm), and w = 250 nm (λ =519.5 nm) exhibits a better overlap between the optical energy density and the MQW region because of a longer penetration depth (1/e decay length). 21 Figure 2f shows that, the intensity of Ez field in the case of w = 100 nm (λ =532.5 nm) suffers from the most dramatic distance decay and the penetration depth is as short as 30 nm. Thus, to ensure an effective coupling between SP modes and the MQW area, the width should be chosen between 130 ~ 250 nm.

Figure 2. (a) Dependence of the reflection spectra on the width, w in Sample A. (b-e) Electric field distribution in Sample A with w = 100 nm (λ =532.5 nm), w = 130 nm (λ =511 nm), w = 170 nm (λ


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=509 nm), and w = 250 nm (λ =519.5 nm), respectively. (f) The distribution of Ez-field intensity with distance along the dashed lines shown in (b-e).

Next, we investigate how the structural parameters of Al gratings affect both of the polarization properties and the light extraction enhancement. Here the polarization degree of the emitted light, ρ, is defined as

ρ=

I TE − I TM I TE + I TM

(2)

ITE(TM) represents the intensity of TE or TM-polarized emission collected from the forward side of our samples. Figure 3a exhibits the polarization degree as a function of the Al grating width and wavelength when t = 20 nm and θ = 0°. As the groove width w increases from 130 to 250 nm, the calculated polarization degree of Sample A maintains roughly an invariable maximum around λ = 509 nm (see the dash line). It agrees very well with the reflection dips shown in Figure 2, indicating such a high polarization degree is a result of the intrinsic property of strong SP resonances. In Figure 3b, the dependence of light extraction efficiency of Sample A normalized by that of Sample B on the parameter w is shown for w ranging from 130 to 250 nm. It is found that the groove width w in the range of 170 ~ 180 nm is favorable to ensure a maximum enhancement of light extraction at λ=509 nm, where the high light extraction spectral region matches the PL spectra of the LEDs very well as shown in Figure 4b. In this study, we choose the width of Al grating to be 170 nm. The polarization properties of the LED structure also depends on the Al grating thickness, t. As illustrated in Figure 3c, the polarization degree at λ = 509 nm is enhanced as Al thickness increases; however, thicker Al layer will cause a lower light extraction efficiency due to increased metal absorption. Thus, the balance of polarization degree and light extraction efficiency should be considered and in this study. The thickness of Al grating is chosen as 20 nm to ensure the simultaneous enhancement in both the polarization degree and relative higher light extraction efficiency from the top side of the LED (p-side).


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Figure 3. (a) Polarization degree as a function of the Al grating width and wavelength. The period is fixed at 420 nm. (b) The light extraction enhancement of Sample A relative to Sample B in spectral regime when w is varied from 130 to 250 nm. (c) Polarization degree as a function of the Al grating thickness and wavelength when θ = 0°.

Based on the optimized structural parameters, we fabricated Samples A-C using the following process. The LED epitaxial structure was grown by metal organic chemical vapor deposition on c-plane sapphire substrates. The structure comprises a 2 µm un-doped GaN buffer, 4 µm n-type GaN layer, 9-period InGaN/GaN MOWs, and a 420 nm thick p-type GaN (see Supporting Information Figure S1). The indium composition of InGaN/GaN MQWs is about 0.28, and the well and barrier thicknesses are approximately 3 and 15 nm, respectively. The grating structures were fabricated on the as-grown LED wafer using our recent developed processes of wafer-scale soft UV-curing nano-imprint lithography (NIL)22, 23 (see Supporting Information Figure S2) and inductively coupled plasma (ICP) etching. In ACS Paragon Plus Environment

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order to remove plasma-induced surface damage, the samples were wet-etched in a hot KOH solution (90 °C).24 Finally, a 20 nm-thick Al film was deposited onto Sample A by high-vacuum e-beam evaporation to form SP LED structure. Figure 4a and the inset show the SEM images of InGaN/GaN grating in a top and cross-sectional view, respectively. From these images an Al grating period of 420 nm and a groove width of 170 nm were measured.

Figure 4. (a) Top view SEM image of polarized InGaN/GaN LED structure (Sample A). The inset shows the cross-sectional view. The PL intensities of Samples A-C excited and collected from the (b) top and (c) backside of the samples. (d) PL enhancement ratio that is obtained by subtracting the PL intensity of Sample B from Sample A. (e) Calculated TM-polarized reflection spectra of Sample A with different incident angle. The light is reflected from the backside of the sample. The red and black dash lines correspond to the ±1 order SP modes, respectively.


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Figure 4b shows the normalized time-integrated PL spectra of all the samples collected by using a semiconductor laser (375 nm) with front-side excitation at room temperature. As compared to Sample C, the forward emission of Sample B is enhanced 4 times due to the modulation of optical modes via photonic crystal patterns in Sample B. The 3 nm blueshift of the emission at peak wavelength can be attributed to the suppressed quantum confined Stark effect (QCSE) induced by partial relaxation of the piezoelectric field in the MQWs.25 The further blueshift of 3 nm from Sample B to A is caused by SP-MQW coupling effect. In the case of Sample A, despite the excitation beam being strongly attenuated and reflected (see Supporting Information Figure S3) by the Al gratings, a 1.7-fold enhancement is still observed as compared to the as-grown LED structure (Sample C), which directly proves that the plasmonics effects can used to enhance light emission efficiency. The attenuation and reflection due to metal can be avoided by changing the excitation means such as electric injection or excitation from the backside of LED wafers. To reduce the impact of metal absorption and reflection by Al grating, the PL spectra was excited and detected from backside of transparent sapphire substrate. It is found the PL emission from Sample A is enhanced by a factor of 3.1 and 11 as compared to Samples B and C, respectively. It can thus be concluded

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that such a large enhancement is attributed to the

mechanisms of IQE improvement as well as the metal reflection effect. The former effect is the result of combined contribution from SP-MQW coupling and optical mode modulation via p-GaN grating. The p-GaN grating structures are same in Samples A and B and thus have almost same contribution to the luminescent enhancement. To discriminate the contribution between SP-MQW coupling and photonic grating in Sample A, the PL emission of Sample B was subtracted from that of sample A and then normalized to Sample C, as shown in Figure 4d. One can see that there is an obvious resonance enhancement centered at 505 nm, which can only be due to the resonant coupling between the MQWs and SP waves, as well as the reflection effects. By investigating the dependence of the reflection on wavelength and incident angle θ in Sample A, the well-known dispersion relationship under TM polarization is simulated and plotted in Figure 4e. It is found that there are two dips which vary with


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incident angle in the reflection spectra, corresponding to the ±1 order SP modes of the Al grating according to equation (1). At θ = 0°, the resonant SP coupling corresponds to a wavelength of 509 nm. The slight difference in resonant wavelength of 505 (from experiments) and 509 nm (from simulation) is the result of deviation of the various device dimensions due to fabrication error. Note that the PL enhancement peak from Sample A corresponds to a reflection dip, and therefore, it can be concluded that SP-MQW coupling is the dominant mechanism for PL output rather than the grating modulation and metal reflection. To further demonstrate the plasmonic effects on improving the performance of the MQW LEDs, we carry out power-dependent PL measurements. Figure 5a shows the PL peak wavelength of the three samples as a function of excitation power density at room temperature. The excitation level is calibrated for diﬀerences in excitation efficiencies (see Supporting Information Figure S3). When the excitation power density is increased from 1 to 25 kW/cm2, the PL spectra of Samples A, B, and C show 2.5, 4, and 5-nm blueshift, respectively, as shown in Figure 5a. Such an excitation power-induced blueshift is due to the polarization-induced electric fields and QCSE, and the results confirm that there exists a strong QCSE in the samples as a result of the high indium composition and lattice mismatch-induced strain.27 As compared to Sample C, Sample B exhibits a smaller blueshift, which is ascribed to the relief of QCSE via partial strain relaxation. As discussed in our previous work,28 Sample A exhibits a smallest blueshift, which should be attributed to the weaker carrier screening due to the lower injection carrier density in the MQW induced by the larger spontaneous emission into SP modes at the same injection level.


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Figure 5. (a) PL peak wavelength as a function of excitation power density. (b) Excitation power density versus normalized integrated PL intensity. The points are experimental data and the dash lines are ABC model fittings. (c) Dependence of IQE on excitation power density extracted from the fittings in (b). (d) IQE enhancement ratio of Sample A relative to Sample B calculated from (c).

The value of IQE provides a quantitative evaluation of the luminescent qualities of LEDs. It can be extracted by fitting the power dependent PL results using the approach proposed by Yoo et al.29 The integrated PL intensity from all the samples as a function of excitation power density are shown in Figure 5b. The data is calibrated for diﬀerences in both excitation and collection efficiencies (see ACS Paragon Plus Environment

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Supporting Information Figures S3, S4, and S5). The solid line is fitted using the ABC model based on carrier rate equation, and the derived IQE is shown as a function of excitation power density in Figure 5c. It is found that the IQE of all the samples increases when excitation power density is varied from 1 to 25 kW/cm2, which can be attributed to a reduction in radiative lifetime of carriers with increasing photogenerated carrier density. The overall IQE of Sample B is higher than that of Sample C, and the value is further improved in Sample A. However, the IQE enhancement ratio of Sample A to Sample B is gradually reduced with increasing of excitation power density, which is in accordance with our recent theoretical results.30 As stated in ref. 30, the IQE can be significantly enhanced by SP-MQW coupling when the original IQE is very low. With increasing carrier injection density, polarization-induced electric field will be weaken due to the increased free carrier screening effect, and the enhancement of the proportion of electron-hole density near the lower Al grating structure will be gradually decreased. Thus, the enhancement ratio will approach saturation, which is determined by the intrinsic property of SP evanescent fields. To further demonstrate the Purcell enhancement and its effects on the performance improvement in LED lighting, the time-resolved photoluminescence (TRPL) measurements were performed at room temperature, as shown in Figure 6a. A two-exponential component model is used to study the excitonic dynamics, and thus, the TRPL traces [I(t)] can be described by 31

I (t ) = A1exp(−t / τ1 ) + A2 exp(−t / τ 2 )

(3)

where A1 and τ1 (A2 and τ2) are for the fast (slow) decay components. The fast decay lifetime corresponds to the rapid carrier recombination in InGaN/GaN MQWs, and the fitted values are 1.96, 2.95, and 4.20 ns for Samples A, B, and C, respectively. As compared to Sample C, the reduction lifetime of Sample B is mainly due to the increased wave function overlap between electrons and holes within the MQWs caused by a partially reduced piezoelectric field; while the faster decay time of Sample A can be attributed to the resonant coupling between MQWs and SP waves, which has a much


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faster carrier recombination than others32. Therefore, the emission enhancement of Sample A are mainly attributed to the MQW-SP coupling effect. Figure 6c illustrates the polarization properties of Samples A, B, and C measured with the experimental setup illustrated in Figure 6b. The emitted light with different polarizations can be detected by rotating the linear polarizer from 0° to 180°. The measured normalized PL intensity varies as a function of rotation angle, revealing the degree of polarized light emission from the samples. As the maximum and minimum of the PL intensities correspond to the relative magnitude of TM- and TE-polarized emission, respectively, the polarization degree can be easily calculated based on equation (2). For Sample A, the output of TM mode is about 3.45-times higher than the TE mode, which results in a polarization degree of about 54%. While for Samples B and C, the polarization degree is as low as 22% and 0. The experimental results are in good agreement with FDTD simulation, which strongly suggests that the polarization degree of LED structures can be effectively enhanced based on the proposed design.

Figure 6. (a) Time-resolved photoluminescence decay curves of Samples A, B, and C at room temperature. (b) Schematic illustration of the setup for measuring polarization. (c) Measured LED light intensity as a function of orientation angle of the rotating linear polarizer.

In conclusion, a linearly polarized InGaN/GaN green LED with Al-coated p-GaN grating is demonstrated. FDTD analysis is implemented to optimize the structural parameters and predict the polarization degree and emission enhancement of the samples. The proposed LED design exhibit ACS Paragon Plus Environment

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increased PL intensity and significant enhancement of IQE as compared to a standard LED without grating and Al layer, which are due to strong SP-MQW coupling. The experimental results are in good agreement with the theoretical analysis. Our observation may offer an effective way to fabricate polarized emission devices with low-cost and high-throughput with UV-curing NIL technique.

ASSOCIATED CONTENT Supporting Information Available: [Device fabrication process, estimating quantum efficiency enhancement, and power dependence of photoluminescence intensity (PDF).] The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.XXXXXXX.

ACKNOWLEDGMENTS This work is supported by Special Funds for Major State Basic Research Project (2011CB301900), Hi-tech Research Project (2014AA032605, 2015AA033305), National Nature Science Foundation of China (61274003, 61422401, 51461135002, 61334009, 61274058, 61322403), the nature science foundation of Jiangsu province (BK20161401, BK2011010, BY2013077, BK20141320, BK20130013), Solid state lighting and energy-saving electronics collaborative innovation Center, a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Research Funds from NJU-Yangzhou Institute of Opto-electronics, and the Fundamental Research Funds for the Central Universities under Grant No. 021014380033. We acknowledge the Australian Research Council (ARC) for financial support, and the Australian National Fabrication Facility (ANFF) for access to facilities. We also thank Kaushal Vora for valuable technical support. Guogang Zhang acknowledges the financial support from the China Scholarship Council for his joint PhD Scholarship No. 201306190034 and Scientific Innovation Research of College Graduate in Jiangsu Province (CXZZ12_0052).


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(8). Montali, A.; Bastiaansen, G.; Smith, P.; Weder, C. Polarizing Energy Transfer in Photoluminescent Materials for Display Applications. Nature 1998, 392, 261-264. (9). Zhao, Y. J.; Yan, Q. M.; Feezell, D.; Fujito, K.; Van de Walle, C. G.; Speck, J. S.; DenBaars, S. P.; Nakamura, S. Optical Polarization Characteristics of Semipolar (30 3 1) and (30 3 1 ) InGaN/GaN Light-Emitting Diodes. Opt. Express 2013, 21, A53-A59. (10). Liu, B.; Kong, J. Y.; Zhang, R.; Xie, Z. L.; Fu, D. Y.; Xiu, X. Q.; Chen, P.; Lu, H.; Han, P.; Zheng, Y. D.; Zhou, S. M. Polarization and Temperature Dependence of Photoluminescence of m-Plane GaN Grown on γ-LiAlO2 (100) Substrate. Appl. Phys. Lett. 2009, 95, 061905. (11). Ma, M.; Meyaard, D. S.; Shan, Q. F.; Cho, J.; Schubert, E. F.; Kim, G. B.; Kim, M.-H.; Sone, C. Polarized Light Emission from GaInN Light-Emitting Diodes Embedded with Subwavelength Aluminum Wire-Grid Polarizers. Appl. Phys. Lett. 2012, 101, 061103.


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For Table of Contents Use Only

High-brightness polarized green InGaN/GaN light-emitting diode structure with Al-coated p-GaN grating Guogang Zhang, Xu Guo, Fang-Fang Ren, Yi Li, Bin Liu, Jiandong Ye, Haixiong Ge, Zili Xie, Rong Zhang, Hark Hoe Tan, and Chennupati Jagadish


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GaN Light-Emitting Diode

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