Ultrahigh Degree of Optical Polarization above 80% in AlGaN-Based

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Ultrahigh degree of optical polarization above 80% in AlGaNbased deep-ultraviolet LED with moth-eye microstructure Shuai Wang, Jiangnan Dai, Jiahui Hu, Shuang Zhang, Linlin Xu, Hanling Long, Jingwen Chen, Qixin Wan, Hao-Chung Kuo, and Changqing Chen ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b00899 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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Ultrahigh degree of optical polarization above 80% in AlGaN-based deep-ultraviolet LED with moth-eye microstructure Shuai Wang,a Jiangnan Dai,a,* Jiahui Hu,a Shuang Zhang,a Linlin Xu,a Hanling Long,a Jingwen Chen,a Qixin Wan,a Haochung Kuo,b and Changqing Chen a a

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science

and Technology, Luoyu Road 1037, Wuhan 430074, China b

Department of Photonics and Institute of Electro-Optical Engineering, National

Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 300, Taiwan

ABSTRACT For the first time, an ultrahigh degree of optical polarization (DOP) of 81.8% in AlGaN-based deep ultraviolet LED (DUV-LED) operated at 286 nm has been experimentally demonstrated. The very high DOP was obtained by introducing the novel moth-eye microstructure fabricated on the backside of sapphire substrate. Compared with conventional DUV-LED with a DOP of 64.7%, a significant 1.26-fold enhancement was obtained. It was worth mentioning that the DOP was accurately measured via self-built full spatial transverse electric (TE) and transverse magnetic (TM) mode light intensity test system, which was mainly composed of angle resolution bracket, Glan-Taylor prism and spectrometer. For both TE and TM mode light, the extraction angle inside semiconductor was extended from conventional (-26°, 26°), (-52°, -41°), (41°, 52°) to (-52°, 52°). Combined with finite difference time domain simulation, it was further confirmed that the novel moth-eye microstructure could notably weaken the total internal reflection at sapphire/air interface and enlarge

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the light extraction angle. As a result, compared with the conventional one, DUV-LED with moth-eye microstructure approximately doubled the light extraction efficiency. Keywords: DUV-LED, moth-eye microstructure, degree of optical polarization, light extraction efficiency

AlGaN-based DUV-LEDs have attracted increasing attention for the potential applications in sterilization, water purification, medical phototherapy, secure communication and so on.1-3 But their further applications are still held back because of the low external quantum efficiency (EQE), which is the production of internal quantum efficiency (IQE) and light extraction efficiency (LEE). The EQE was currently with the typical values in single-digit percentage range.4 The state of the art EQE higher than 20% was reported by RIKEN, recently.5 As a crucial factor that limit the IQE, great efforts have been made to reduce the high threading dislocation density in AlGaN materials, including the adoption of NH3 pulsed-flow migration-enhanced epitaxy,6 lateral growth on nano-patterned sapphire substrates7 and epitaxy on AlN bulk substrates.8,9 Besides, the solutions to improve the carrier injection were also explored systematically in recent reports.10-12 Another bottleneck is the poor light extraction efficiency (LEE) which is typically below 8%.4 For conventional flip-chip DUV-LED, besides the sapphire/air interface, there are dozens of interfaces inside the epitaxial layers. Along the photons propagation direction from AlGaN multi-quantum wells (MQWs) to sapphire substrate, the refractive index decreases successively. It means that photons always propagate from optically denser medium to optically


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thinner medium, which will induce the considerable reflection and total internal reflection (TIR). Specifically, the topmost valence subband near Γ point is the heavy-hole band for GaN and the crystal-field splitting band for AlN, respectively, which is due to the spin–orbit and crystal-field splitting effects. Such effects lead to the predominant optical polarization of light emission is TE (E⊥c) mode for GaN and TM (E∥c) mode for AlN.13-16 Therefore, there will be a large proportion of TM mode light radiated from deep ultraviolet MQWs, leading to stronger reflection and TIR by multi-layer interfaces as well as the lower LEE.4,17 In recent years, significant investigations have been conducted to disturb the TIR so that the improved LEE for DUV-LEDs can be achieved, such as vertical DUV-LED structure with roughed n-AlGaN layer,18 high-reflectivity metal contacts,19 nano-patterned substrates and embedded

voids

in

epitaxial

layers,7

nanorods

DUV-LED

structure,20-22

two-dimensional photonic crystal on AlN substrates,23,24 sapphire sidewall roughing induced by laser stealth dicing25,26 and AlN-doped-silicone encapsulant on sapphire.27 Sapphire surface roughing has also been proved to be effective to help photons escape out of AlGaN-based DUV-LEDs.28,29 However, few efforts have been made on the systematic research into the interaction of roughening structure with TE/TM mode light radiated from MQWs experimentally. The effect of roughening structure on the light extraction angle is also pending an in-depth study to prepare better DUV-LED structures with higher LEE. In this paper, the novel moth-eye microstructure fabricated on the backside of sapphire substrate has been introduced to enhance the TE mode light intensity and


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DOP of AlGaN-based DUV-LED. A comparative study of DUV-LEDs with and without moth-eye microstructure was carried out in detail to explore the effect of moth-eye microstructure on TE/TM mode light and light extraction angle thoroughly. By using self-built full spatial TE and TM mode light intensity test system, DOP of DUV-LED with moth-eye microstructure was accurately measured to be 81.8%. The LEE was finally approximately doubled compared with conventional DUV-LED.

EXPERIMENTS The AlGaN-based DUV-LEDs were grown on 2-inch c-sapphire substrates in a vertical

cold

wall

MOCVD

reactor

under

50

torr.

Trimethylaluminum,

trimethylgallium and ammonia were used as precursors for Al, Ga and N sources, respectively. The n-type dopant was the 100 ppm SiH4/H2 mixture and the p-type dopant was the bis-cyclopentadienyl magnesium. Hydrogen was purified by palladium tube and served as the carrier gas. First, a 20-nm-thick AlN nucleation layer was grown at 670℃, followed by a 2.5-µm-thick AlN layer grown at 1050℃. Subsequently, 20 periods of AlN (7 nm)/Al0.65Ga0.35N (14 nm) superlattices (Sls) at 920℃, n-Al0.55Ga0.45N layer with the thickness of 2.5 µm at 840℃ were grown in sequence. Then the active region composed of 7 periods of Al0.5Ga0.5N (12 nm)/Al0.35Ga0.65N (2 nm) MQWs were deposited, whose growth temperature was 840℃. The Si doping concentration in the n-Al0.55Ga0.45N layer and MQWs was 4×1018 cm-3, which number was confirmed by van der Pauw Hall measurement. Then the 30-nm-thick Al0.7Ga0.3N (6 nm)/Al0.5Ga0.5N (1.5 nm) electron blocking layer (EBL)


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was grown at 840℃. The EBL was followed by the 200-nm-thick p-GaN layer, lastly annealed at 650℃ for 20 min in the nitrogen atmosphere to activate Mg acceptor. Then the epi-wafer was diced into two pieces to fabricate two types of 10 mil×21 mil DUV-LEDs. Next, the conventional flip-chip process was adopted to achieve the conventional DUV-LED with flat sapphire. To fabricate the DUV-LED with novel moth-eye microstructure, the sapphire substrate was grinded to 200 µm and polished by chemical mechanical polishing method, followed by laser stealth dicing at the depth of 70 µm and 130 µm along c-axis from the sapphire surface without any blasting point on sapphire surface. Then the novel moth-eye microstructure was fabricated on the backside of sapphire surface by step projection exposure process and modified inductively coupled plasma etching. Finally, the DUV-LED with novel moth-eye microstructure was obtained after chip splitting. Field-emission scanning electron microscope (Nova NanoSEM 450) and atomic force microscope (Veeco NanoScope MultiMode) in contact mode were used to perform the morphologies of moth-eye microstructure. As shown in Figure 1, the period, height and bottom length of moth-eye microstructure were 3 µm, 1.75 µm and 2.5 µm respectively. The full spatial TE/TM mode light intensity test system with a Glan-Taylor prism placed between DUV-LED fixed on an angle resolution bracket and spectrometer was illustrated in Figure 2.

RESULTS AND DISCUSSION The current-voltage characteristics of two DUV-LEDs were measured using Keithley


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2420. As shown in Figure 3a, the forward voltage at 20 mA was about 5.5 V. The similar current-voltage characteristics revealed decent diode current-voltage performance without damage during the moth-eye microstructure fabrication process. It could be seen clearly from Figure 3c that the light output power (LOP) of both two DUV-LEDs reached saturation at 90 mA, which confirmed almost the same IQE. Therefore, the strong enhancement of normalized spectra in Figure 3b and LOP in Figure 3c compared with conventional DUV-LED should be attributed to the LEE enhancement. As shown in Figure 3d, the LEE enhancement factor was calculated to be around 2 by means of LOP of conventional DUV-LED divided by that of DUV-LED with moth-eye microstructure, meaning that the LEE of DUV-LED with moth-eye microstructure approximately doubled. Whereas, the LEE enhancement factor gradually decreased with the driving current increase. Due to the typical EQE of DUV-LED was below 5%,4 there would be amount of increasing accumulated heat in the chips with the driving current increase, resulting in the red-shift wavelength and the increase of TE mode light radiated from MQWs.30 A part of TE mode light propagated along the c-axis direction with a small angle could be strongly reflected by the moth-eye microstructure to some extent, inducing the slight decrease of LEE enhancement factor. Based on the light intensity test system without Glan-Taylor prism demonstrated in Figure 2, the far field distributions of two DUV-LEDs at the current of 90 mA were obtained in Figure 4. The LOP of DUV-LED with moth-eye microstructure was much stronger than that of conventional DUV-LED along any direction. It indicated that the


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moth-eye microstructure could strongly weaken the TIR with a critical angle of 34° at sapphire/air interface. Moreover, the enhanced value of LOP along the direction parallel to c-axis reached the maximum, implying that the moth-eye microstructure effectively changed the propagation direction of radiated photons and converged towards the direction along c-axis. It suggested that the proportion of TE mode light among the extracted light. The maximum intensity for the TE mode light emission profile inside MQWs was parallel to the c-axis and it decreased until the emission was perpendicular to the c-axis. Nevertheless, the opposite trend was obtained for the TM mode light, such that the maximum light intensity was perpendicular to the c-axis and the light intensity decreased until the beam was parallel to the c-axis. Compared with TM mode light, TE mode light can lead to the weaker reflection and TIR by multi-layer interfaces. Therefore, the enlarged DOP by increasing the TE mode light proportion can help decrease the reflection and TIR as well as enhancing the LEE.4,17 In order to go deep into the influence of moth-eye microstructure on the TE/TM mode light enhancement of DUV-LEDs, Figure 5a showed the orientation and scale relation of TE/TM mode light radiated from MQWs along any direction in the air ambient. The plane ABH represented the DUV-LED plane with a normal vector paralleled to c-axis and plane AGE represented the Glan-Taylor prism plane. Vector k was the wave number of light in air ambient. Hence, the TE mode electric field and TM mode electric field amplitude E could be obtained amplitude E according to the following equations: ur ur E TE = A0 ⋅ E

cos 2 φ cos 2 θ + sin 2 φ


(1)

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ur ur E TM = A0 ⋅ E cos φ sin θ

(2)

=A·√P, P was the normalized LOP and A0 was a constant. The angle θ where E corresponded not only to the direction of extracted light in the air ambient but also the angle of DUV-LED plane and Glan-Taylor prism plane. Φ was the rotation angle of polarizer translucent shaft. For a certain θ in air ambient, the normalized TE/TM mode light intensity could be obtained by integrating the Φ. Based on the light intensity test system demonstrated in Figure 2, the full spatial distributions of TE/TM mode light at the current of 90 mA could be obtained in Figure 5b. It was declared that the moth-eye microstructure strongly helped to enhance the extraction of TE mode light by 1.01 times. As a result, the DOP =

could be calculated to be

81.8% and 64.7% for DUV-LED with or without moth-eye microstructure respectively, which further confirmed the LEE enhancement in Figure 3d. Nevertheless, for TM mode light, it should be noticed that the moth-eye microstructure contributed little to its extraction and there existed a maximum at 52° in the air ambient from Figure 5b. To figure out the different enhanced features of TE/TM mode light, the propagation characteristics of TE/TM mode light in two DUV-LEDs were explained in Figure 6. Considering the polarization by using dipole radiation, emission intensity profile inside MQWs, angular dependent I(θE) could be expressed as equation (3) for TE mode light and equation (4) for TM mode light: ITE ( θ E ) = I x 0 ⋅ ( 1 + cos 2 θ E ) ITM ( θ E ) = I z0 ⋅ sin 2 θ E

(3) (4)

where Ix0 and Iz0 are governed by the thermal occupation according to Fermi–Dirac


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distribution.4 Taking the DUV-LED with high Al content into account, it was reasonable to set the refractive indexes of AlN, AlN/AlGaN Sls, n-AlGaN and MQWs as the same of 2.2831 when the wavelength of is 286 nm. And the refractive indexes of air, sapphire were 1, 1.8132 respectively. Except for the absorption of radiated light propagated towards p-GaN layer, radiation region in semiconductor could be divided into three main areas: sapphire surface radiation area, sapphire sidewall radiation area, TIR area related to AlN/sapphire interface and sapphire/air interface. Based on the formula θ = arcsin

, where n1 was the refractive index of optically thinner

medium and n2 was that of optically denser medium, the critical angle of AlN/sapphire and sapphire/air could be calculated to be 52° and 34° respectively. In line with Snell's law, the detailed spatial angle inside semiconductor of three different areas was shown in table I. Meanwhile, (-26°, 26°), (-41°, -26°) and (26°, 41°), (-52°, -41°) and (41°, 52°), (-90°, -52°) and (52°, 90°) were corresponding to the red shadow region, yellow shadow region, blue shadow region, gray shadow region respectively in Figure 6. For conventional DUV-LED, the light extraction angle was (-26°, 26°), (-52°, -41°), (41°, 52°). For DUV-LED with moth-eye microstructure, the light extraction angle was extended to (-52°, 52°). Because the maximum point in TE mode light emission intensity profile inside MQWs was at θE=0° and ITE(θE) gradually decreased with the θE increase to 90°, it could be concluded that the apparent enhancement on the extraction of TE mode light was primary attributed to the extended extraction angle (-52°, -41°) and (41°, 52°), which could be scattered by moth-eye microstructure on sapphire surface and extracted into air. However, the


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ITM(0°) was equal to zero and ITM(90°) reached the maximum. For this reason, TM mode light was easily affected by the reflection of multi-layer interfaces inside semiconductor and sapphire/air interface, signifying that the extended extraction angle (-52°, -41°) and (41°, 52°) caused by moth-eye microstructure contributed little to the extraction of TM mode light. With θE increase, ITM(θE) as well as the reflectance at multi-layer interfaces both increased. Hence, there was bound existing a maximum at a special θE corresponding to 52° of air ambient as shown in Figure 5b. TABLE I. Detailed spatial angle of different areas inside semiconductor. DUV-LEDs

Surface radiation area

With

Sidewall radiation area

TIR area

-52°≤ θE < -41°

-90° < θE < -52°

41° < θE ≤ 52°

52° < θE < 90°

-41° ≤ θE ≤ 41° moth-eye

-41°≤ θE < -26° Without

-52°≤ θE < -41°

26° < θE ≤ 41°

41° < θE ≤ 52°

-90° < θE < -52°

-26° ≤ θE ≤ 26° moth-eye

52° < θE < 90°

To further highlight that the moth-eye microstructure could weaken TIR at sapphire/air interface and effectively extended light extraction angle, the transmission curves along different incident direction of moth-eye microstructure and flat sapphire were both computed by finite difference time domain method, as illustrated in Figure 7. In the simulation, the plane wave source was legitimately adopted accounting for 200-µm-thick much larger than the working wavelength of 286 nm. For conventional DUV-LED, there existed a TIR critical angle of 34° at the sapphire/air interface. When the angle between incident light and c-axis in sapphire ambient, θs was in (0°,


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34°), the flat sapphire presented a ultrahigh transmittance approximately equal to 100%, yet the transmittance rapidly dropped to zero once θs≥34° due to TIR. For DUV-LED with moth-eye microstructure, when θs was in (0°, 34°), the transmittance was lower than that of flat sapphire for the reason that ray A with specific small incident angle may be totally internally reflected by moth-eye microstructure. The duty cycle of moth-eye microstructure was around 63% and there existed 37% flat sapphire ambient, indicating that its transmittance was still affected by the TIR critical angle of 34° like ray C shown in the inset. As a result, the transmittance reached a minimum at θs=34°. When θs was in (34°, 90°), the transmittance was mostly between 10% and 30%, instead of zero of flat sapphire. This distinguished difference exactly confirmed the moth-eye microstructure could weaken the TIR at the sapphire/air interface and effectively extended light extraction angle. In addition, the size of moth-eye microstructure determined that the transmittance reached a maximum like ray B at θs=54°. In Figure 8, it showed the cross section distributions of electric field amplitude E at different incident angle in sapphire ambient. For conventional DUV-LED, the in air ambient was very strong at θs=0°, declaring a high electric field amplitude E transmittance. At the TIR critical angle of θs=34°, the electric field distribution was in accordance with the feature of evanescent wave. At θs=54°, it occurred strong TIR and electric field in the air ambient was zero. For DUV-LED with moth-eye microstructure, it showed obvious reflection at θs=0°. At the TIR critical angle θs=34°, there existed strong electric field distribution in air ambient though some power was


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reflected back into sapphire ambient. At θs=54°, it exhibited strong electric field in air ambient but very weak electric field in sapphire ambient, meaning that the moth-eye microstructure could effectively weaken TIR. The results matched well with the transmission curves in Figure 7.

CONCLUSIONS In summary, the novel moth-eye microstructure was fabricated on the back of sapphire substrate and proved to be effective in improving the LEE of DUV-LED. Based on self-built full spatial TE/TM mode light intensity test system, an ultrahigh DOP of 81.8% in DUV-LED has been firstly precisely measured, indicating the 1.26-fold enhancement than that of conventional one. The TE mode light intensity was significantly enhanced by 1.01 times but negligible change for TM mode light intensity was observed due to the reflection of multi-layer interfaces inside semiconductor and sapphire/air interface. The light extraction angle inside semiconductor was extended from conventional (-26°, 26°), (-52°, -41°), (41°, 52°) to (-52°, 52°). These promising results gave rise to approximately doubled LEE in DUV-LED with moth-eye microstructure.


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ASSOCIATED CONTENT

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Tel: +86 027 87793035.

ORCID Jiangnan Dai: 0000-0001-9805-8726 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by Key Project of Chinese National Development Programs (Grant No. 2016YFB0400804), the Key Laboratory of infrared imaging materials and detectors, Shanghai Institute of Technical Physics, Chinese Academy of Sciences (Grant No. IIMDKFJJ-17-09), the National Natural Science Foundation of China (Grant No. 61675079, 11574166, 61377034, 61774065), and the Director Fund of WNLO.


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with AlxGa1-xN/AlyGa1-yN/AlxGa1-xN(x

Ultrahigh Degree of Optical Polarization above 80% in AlGaN-Based

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