Article pubs.acs.org/JPCC
Maskless Surface Patterning of AlGaInP Light-Emitting Diodes by Photochemical Laser Interference Etching Guanjun Lin, Zhiyuan Zuo, Duo Liu,* Zhaobin Feng, Qian Zhang, Xiaoyu Lin, and Xiangang Xu State Key Laboratory of Crystal Materials, Shandong University, 27 South Shanda Road, Jinan, Shandong 250100, P. R. China ABSTRACT: Laser nanofabrication has attracted great interest for mass production of microstructures with desired or improved properties. We report here a facile, maskless method for fabricating two-dimensional (2D) quasiperiodic patterns on AlGaInP light-emitting diodes (LEDs) by photochemical laser interference etching. The interference pattern produced by two 532 nm laser beams is transferred to the top GaP window layer of the AlGaInP LEDs by photochemical etching of GaP in an etchant composed hydrofluoric acid (HF) and hydrogen peroxide (H2O2). The etched GaP surface contains 2D quasiperiodic pyramidal structures with a period of 150 ± 10 nm, which can dramatically increase the light extraction efficiency of the AlGaInP LEDs by 70%. This maskless patterning method is general and can be applied to fabricating patterns on optoelectronic devices based on other semiconductor materials.
1. INTRODUCTION Light-emitting diodes (LEDs) are in their ways to illuminate the world, with extensive applications ranging from solid state lighting and traffic signals to digital display and optical communication.1,2 Although the performances of LEDs have been greatly improved in the past decade owing to the adoption of more sophisticated thin film growth techniques and multiple quantum well (MQW) structure,3,4 regular LEDs still suffer from low light extraction efficiency due to total internal reflection at the air and device interface. The high refractive index of III−V semiconductors permits only a small percent of the photons generated at the MQWs to escape from planar LED chips, while most photons are trapped inside the chips as guided modes until being absorbed and dissipated as heat. For example, Snell’s law predicts that only 2.2% of the electroluminescent photons can be extracted from conventional AlGaInP LEDs.5 As a result, many techniques have been developed to increase the light extraction efficiency of LEDs. These techniques usually involve creating new optical channels for light extraction by fabricating surface textures,6−10 photonic crystals,11−15 or plasmonic nanostructures16,17on LEDs through usage of microfabrication techniques, such as optical lithography, e-beam lithography, or focused ion beam milling. Recently, laser nanofabrication has attracted great interests for mass production of two-dimensional (2D) or three-dimensional (3D) microstructures at submicrometer or nanometer scale with desired or improved properties.18−21 For example, direct laser writing enables the inscription of predefined patterns on target materials through selectively removing some materials by laser irradiation.22,23 Laser interference lithography has already been used to fabricate submicrometer patterns on LEDs through the exposure and etching processes.24,25 It was recently found that laser-assisted etching can be used to fabricate SiO2 microlens and roughened GaP.26,27 In this study, we report a photochemical © 2013 American Chemical Society
laser interference etching method for creating 2D quasiperiodic structure on AlGaInP LEDs. Our results show that the light extraction efficiency of AlGaInP LEDs can be dramatically improved.
2. EXPERIMENT The AlGaInP epitaxial wafers used in this study were grown on GaAs substrate by metal−organic chemical vapor deposition (MOCVD). The epitaxial layers contain in sequence a GaAs substrate, a distributed Bragg reflector (DBR), a 1 μm thick nAlGaInP, a 20-period GaInP/AlGaInP MQW, a 1 μm thick pAlGaInP, and a 8 μm thick Mg-doped p-GaP window layer. The etchant was a mixture of hydrofluoric acid (HF) and hydrogen peroxide (H2O2) with a molar ratio of HF:H2O2 being 6:1. Figure 1a shows the schematic representation of the optical apparatus used to generate the interference patterns. The light source is a 100 mW 532 nm semiconductor laser. The laser beam was first split into two parts by a visible broadband beam splitter. Then, the two beams were adjusted by a combination of convex lens, visible broadband reflector, and linear variable density filter with the aid of an optical power meter (Ophir Optronics, PD300UV-193) until two spots are coincided at the center with the same spot diameter and power. Finally, an interference pattern was formed on the AlGaInP LED wafer (Figure 1b). The wafer was immersed into a flat-bottomed polytetrafluoroethylene (PTFE) container filled with the etchant. The incident angles of laser beams can be tuned to change the period of the interference pattern, which is defined by d = λ/(sin θ + sin β) with θ and β being the incident angles of the two laser beams, Received: July 26, 2013 Revised: November 26, 2013 Published: December 9, 2013 27062
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where I0 is the intensity of the single laser beam and λ is the laser wavelength. In this present study, the power of each laser beam was maintained at 21.6 mW with a spot size of 8 mm, and the etch time was 10 min. After etching, the wafer was ultrasonically washed in sequence with deionized water, acetone, and anhydrous ethanol. The surface morphology of the etched wafer was characterized by scanning electron microscopy (SEM, Hitachi S-4800, Japan) and atomic force microscopy (AFM, Veeco Dimension Icon). The surface elements were studied by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250, Germany) equipped with a monochromatic Al Kα excitation source. The etched wafer was then processed according to the standard electrode preparation and cutting processes. The electrical and optical properties of the resulting LEDs were tested by a source-measure unit (Keithly 4200, USA), a fiber spectrometer (Avantes Spec-2048, Netherlands), and a probe station (Suss PM5, Germany).
3. RESULTS AND DISCUSSION Figures 2a and 2b show the SEM images of the samples after photochemical etching under single beam and interference laser irradiations, respectively. A more detailed description of the single beam etching experiments can be found elsewhere.27 It is evident that single beam laser irradiation led to the formation of V-shaped microterraces (Figure 2a), while interference laser irradiation creates additional 2D quasiperiodic structures on the microterraces (Figure 2b). The etching process of p-GaP window layer is discussed as follows. Upon laser irradiation of the outmost GaP layer, a large number of electrons transited from the valence band to the conduction band. At the GaP− etchant interface, the electrons were captured by hydrogen peroxide and hydrogen ions in the etchant. The holes in the valence band will oxidize the surface atoms of GaP, resulting in its dissolution into the solution. The gross chemical reactions are summarized as
Figure 1. (a) Schematic representation of optical apparatus used to generate laser interference patterns on the LED wafer (1: 532 nm laser; 2: visible broadband beam splitter; 3: visible broadband reflector; 4: linear variable density filters; 5: convex lens; 6: etchant; 7: PTFE container; 8: LED wafer). (b) The interference pattern formed on the LED wafer.
respectively. The intensity distribution of the interference pattern (Figure 1b) can be expressed as ⎡ ⎤ 2πx(sin α + sin β) + 1⎥ I(x) = 2I0⎢cos ⎣ ⎦ λ
(1)
Figure 2. SEM images of (a) the single laser beam irradiation etched, (b) interference laser irradiation etched, and (c) view after interference laser irradiation etched of the GaP window layer surface. (d) X-ray photoelectron spectra (XPS) of the GaP window layer surface. 27063
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hv
GaP + 2H 2O2 + 3HF → GaF3 + H3PO4 + 2H 2↑
(2)
Hydrofluoric acid can significantly increase the etching rate due possibly to its strong hydrogen bonding and permeation effect of fluoride ions.28 In addition, hydrofluoric acid can also dissolve the intermediate Ga2O3 to facilitate the reactions. It seems that the photochemical etching process initiated preferentially at dislocation cores, which leads to the formation of micropyramids.29,30 When the sample was subjected to wet chemical etching under laser interference irradiation, the periodic variation of the incident beam intensity resulted in selective etching of the top GaP window layer and the formation of 2D quasiperiodic patterns. Figure 2c shows a top-view lowmagnification SEM image of the LED chips after 10 min etching under laser interference irradiation. Figure 2d shows the XPS spectra of the samples before and after etching. The peaks located at 18.91, 105.03, and 160.09 eV can be attributed to Ga 3d5/2, 3p3/2, and 3s of GaP, respectively. The peak at 128.86 eV corresponds to P 2p of GaP. The peak at 532.1 eV corresponds to O1s which arises from chemisorbed oxygen species. Both samples exhibit similar XPS features, implying that the etching process imposes negligible effects on the electrical properties of the LED samples. The surface morphology of the p-GaP after etching treatment was also studied by AFM, as shown in Figure 3. It is evident that the fringe texture was uniformly formed on p-GaP surface after interference laser irradiation etching for 10 min. Figures 3a and 3b show the AFM image of the p-GaP surface after etching; the inset of Figure 3a shows the two-dimensional fast Fourier transformation (2D-FFT) image. The average etching depths of pyramidal pit and triangular stepladder are ∼170 ± 10 and ∼20 ± 5 nm, respectively. The size of the 2D quasiperiodic pyramidal structure is ∼150 ± 10 nm, much less than half of the wavelength of the incident wave. A schematic description of the etching mechanism is shown in Figure 3c. The etching process started when the laser interference pattern was projected onto the GaP window layer. As the etching reactions proceeded, the thickness of the GaP layer gradually decreased. Similar to the single beam etching process, a pyramidal pattern was formed due to the faster etching rate in regions close to dislocations. The decrease of the sample thickness led to a lateral shift of the interference fringes on the GaP window layer, forming a new group of etching structure. The superposition of these etching processes eventually resulted in the formation of subwavelength microstructures as shown in Figure 3c. Figure 4a shows the forward current−voltage (I−V) curves for the original and etched samples, respectively. With a 20 mA dc injection current, the forward voltages of the original, single beam laser etched and interference laser etched samples are 2.26, 2.28, and 2.27 eV, respectively. The results reveal that the etching process has negligible influence on the electrical properties of the LED chips. The patterned surface provides multiple chances for photons to escape from the LED chips, thus effectively improving the light extraction efficiency. We also performed electroluminescent properties tests for both the original and patterned samples. Figure 4b shows the far-field radiation patterns for original and patterned AlGaInP LED samples without encapsulation at an injection current of 20 mA. Note that the far-field radiation patterns for both the original and interference laser etched samples are similar since the randomly distributed V-shaped microterraces play the dominant role on the determination of the radiation pattern. It is evident that the emission intensities of patterned AlGaInP LED were greatly
Figure 3. (a) AFM image of p-GaP after 10 min laser interference etching (scanning area: 5 μm × 5 μm); the inset shows the 2D-FFT image. (b) AFM height profiles for the samples of (a). (c) Schematic representation of the formation principles of 2D quasiperiodic pyramidal structure on p-GaP window layer. The red arrow indicates the movement direction of the interference pattern.
enhanced along all directions. The highest intensity enhancement was obtained at 45°−75° and 105°−135°. The overall enhancement of the single beam laser etched samples is ∼65%, equivalent to our previous result.27 It should be noted that the emission intensity of interference laser etched sample was improved by a factor of ∼70%, which is greater than the single beam laser etched sample. We believe that this extraction enhancement is mainly due to the reduction of the total internal reflection by the microscale patterns and nanoscale surface features can further increase the light extraction efficiency by scattering.31,32
4. CONCLUSIONS In summary, we have fabricated 2D quasiperiodic pyramidal structure on the GaP window layer of AlGaInP LEDs by a unique photochemical laser interference etching method. Our results show that the 2D quasiperiodic pyramidal structure have a small period size T = 150 ± 10 nm, which is far less than half of the source wavelength. The I−V tests show that this method has minimal impact on the electrical properties of AlGaInP LEDs. 27064
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Figure 4. (a) I−V curves of the original and the etched samples (inset: enlarged view of I−V curves around 20 mA). (b) Far-field emission patterns of the original and etched samples under 20 mA current injection.
However, the electroluminescent intensity can be dramatically improved by 70%. Moreover, we believe that this method provides a supplementary approach for fabricating patterns on semiconductor surfaces, which could be transferred to other semiconductor materials such as GaAs, GaN, Si, and InP to improve the performance of solar cells, sensors, thermal photovoltaics, and radiation management.
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AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected]; Tel 86-531-88363901 (D.L.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the National Science Foundation of China (NSFC) (Grants 91123007, 91233122, and 60974117), the Excellent Young Investigators Award Foundation of Shandong Province (Grant BS2009CL021), SRF for ROCS, State Education Ministry, and National Basic Research Program of China (973 Program) (Grant 2009CB930503) for financial support.
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