Letter pubs.acs.org/NanoLett
Epitaxial GaN Microdisk Lasers Grown on Graphene Microdots Hyeonjun Baek,†,§ Chul-Ho Lee,†,‡,§ Kunook Chung,† and Gyu-Chul Yi*,† †
National Creative Research Initiative Center for Semiconductor Nanostructures and Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea ‡ Department of Physics, Columbia University, New York, New York 10027, United States S Supporting Information *
ABSTRACT: Direct epitaxial growth of inorganic compound semiconductors on lattice-matched single-crystal substrates has provided an important way to fabricate light sources for various applications including lighting, displays and optical communications. Nevertheless, unconventional substrates such as silicon, amorphous glass, plastics, and metals must be used for emerging optoelectronic applications, such as high-speed photonic circuitry and flexible displays. However, highquality film growth requires good matching of lattice constants and thermal expansion coefficients between the film and the supporting substrate. This restricts monolithic fabrication of optoelectronic devices on unconventional substrates. Here, we describe methods to grow high-quality gallium nitride (GaN) microdisks on amorphous silicon oxide layers formed on silicon using micropatterned graphene films as a nucleation layer. Highly crystalline GaN microdisks having hexagonal facets were grown on graphene dots with intermediate ZnO nanowalls via epitaxial lateral overgrowth. Furthermore, whispering-gallery-mode lasing from the GaN microdisk with a Q-factor of 1200 was observed at room temperature. KEYWORDS: GaN, graphene, ZnO, heterostructures, laser, whispering-gallery-mode
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and thermal conductivity of graphene can enhance device performance.12 For example, transferrable and flexible LEDs have been demonstrated using nitride layers and nanostructures grown on graphene films, respectively.13,14 Nevertheless, better structural and optical qualities of the materials prepared on graphene films are still required for sophisticated devices such as a laser. For laser applications, here, we present a novel approach to grow high-quality GaN microdisks on graphene dots via epitaxial lateral overgrowth (ELOG). GaN microdisk arrays were grown on amorphous silicon oxide (SiO2)/Si substrates using micropatterned graphene dots as a local nucleation layer. This approach is shown schematically in Figure 1a. Graphene films, which were synthesized on Ni films by chemical vapor deposition (CVD), were transferred onto SiO2/Si substrates using a standard transfer method.15 Then, the large size graphene films were patterned using ebeam lithography with a negative resist (Micro Resist Technology, ma-N 2401). Using the resist as an etch mask, the remaining parts of the graphene films were then removed by oxygen plasma etching for 30 s at a plasma power of 30 mA to form regular hexagonal arrays of 3 μm diameter microdots separated by 10 μm (Figure 1b). Before GaN growth, zinc oxide (ZnO) nanostructures were grown on the graphene micropatterns using metal−organic vapor-phase epitaxy (MOVPE) to enhance nucleation and crystallization of GaN.
he fabrication of optoelectronic devices on unconventional substrates would permit the creation of large-sized flexible displays and complex optoelectronic circuits.1−3 In particular, monolithic integration of compound semiconductor photonic devices with silicon (Si)-based electronic circuits would create a new field combining electronics with photonics, facilitating intra- and interchip communications.4 Accordingly, tremendous efforts have been made related to direct growth of films on such substrates. However, those films have been of poor quality because of mismatches in key physical characteristics. This has made it difficult to fabricate sophisticated devices such as a laser that requires significantly better material quality. To circumvent this mismatch problem, various techniques based on lift-off of the films from the original substrates, including wafer bonding or fusion5,6 and transferprinting methods,7,8 have been explored. However, many challenges remain because of low yields and complicated fabrication processes. Moreover, single-crystal substrates must still be used for the epitaxial growth. The most challenging issue facing heteroepitaxial growth can be resolved using an appropriate intermediate layer. Recently, intermediate layers, such as titanium and graphene, were used to improve the quality of gallium nitride (GaN) thin films grown on amorphous substrates, which enabled the fabrication of light-emitting diodes (LEDs).3,9 Graphene films, the atomically thin crystalline layers, provide not only epitaxial relationship with inorganic semiconductors for the growth of high-quality materials but also mechanical transferability and flexibility to devices.10,11 In addition, the high current density © XXXX American Chemical Society
Received: March 19, 2013
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Figure 1. (a) Schematic illustration of fabrication process used to grow hexagonal GaN microcrodisk arrays on CVD graphene films. (b) Optical microscopic image of hexagonally patterned CVD graphene films. (c,d) Plane-view SEM images of ZnO nanowalls grown on patterned graphene films and hexagonal GaN microdisks grown on ZnO-coated graphene films, respectively. The insets in (c) and (d) are corresponding 30°-tilted SEM images.
As shown in the scanning electron microscope (SEM) image in Figure 1c, highly networked 600 nm high and 20 nm thick ZnO nanowalls only grew inside the graphene micropatterns. This intermediate layer plays a critical role in heteroepitaxial growth of high-quality GaN films on graphene layers.13 Finally, GaN microdisks were selectively overgrown on the ZnO-coated graphene microdot arrays using a two-step process. First, a thin GaN layer was grown at 540−600 °C during 10 min in a nitrogen atmosphere to protect the ZnO nanowalls from degradation and reaction with GaN, which occurs at a higher temperature. Then, a GaN layer was immediately deposited at 1000−1100 °C in a hydrogen atmosphere. The details of the MOVPE growth of the GaN are described elsewhere.13 As shown in Figure 1d, hexagonal arrays of GaN microdisks with a flat top surface and sidewalls were grown on the micropatterned graphene films. Interestingly, the diameters of the individual hexagons expanded to 8.6 ± 0.4 μm from 3 μm for the original graphene patterns. This indicates that the crystalline GaN microdisks were overgrown on the ZnO nanowalls in both the lateral and vertical directions, strongly suggesting the ELOG of GaN (Supporting Information Figure S1). Moreover, hexagonal microdisks having flat side facets formed naturally without requiring further etching, which enhances lasing characteristics by reducing diffraction loss.16,17 We used cathodoluminescence (CL) spectroscopy to investigate the spatially resolved optical characteristics of the GaN microdisk grown on graphene dots. Figure 2a shows representative CL spectra obtained from the two different locations indicated by blue and red circles in the inset SEM image of the hexagonal GaN microdisk. Both CL spectra exhibited the dominant peak at 364 nm and a weak, broad emission around 560 nm, corresponding to near-band-edge (NBE) and deep-level emissions, respectively. However, the
Figure 2. (a) Representative CL spectra at the center GaN (blue solid line) and the ELOG GaN (red solid line) regions. The inset shows the SEM image of the hexagonal GaN microcrodisk, in which local excitation spots are indicated by red and blue circles and original pattern of graphene films is indicated by black dotted line. (b) Monochromatic CL images at the fixed wavelengths of 364 and 560 nm. The relative intensity scale is presented gradually from black to white color.
intensity ratio of the two emission peaks (INBE/ID) for the ELOG region was 30, much higher than 9 for the center of a GaN microdisk. This difference was more clearly shown in spatially resolved monochromatic CL images at the fixed B
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selectively grown on SiO2-masked GaN substrates.18 We believe that the excellent microstructural and optical quality of hexagonal ELOG GaN microdisks grown on graphene films warrant their application in high-performance microcavity lasers to take advantage of, for example, the low threshold for lasing emission. The observed lasing emission was attributed to WGM or quasi-WGM resonance. In our GaN microdisk with a hexagonal cross-section, light can travel around the resonator with little loss by total internal reflection, as depicted in the inset of Figure 4a. However, if the lasing action occurs from the Fabry−
wavelengths of 364 and 560 nm. As shown in Figure 3b, the strong NBE emission at 364 nm was observed mainly from the
Figure 3. (a) Representative power-dependent PL spectra of the GaN microcrodisk. (b) The plot of integrated PL intensity as a function of excitation density, exhibiting the lasing threshold at 250 kW/cm2. Figure 4. (a) Calculated wavelengths (red circle) by plane wave approximation using referenced refractive index (red solid line). (b) Plot of PL spectrum of the hexagonal GaN microdisk. The diameter of the microcrodisk was 8.7 μm for this particular data.
ELOG GaN region, while the deep-level emission at 560 nm was mostly from the center region and the pits at the boundaries. These position-dependent optical characteristics indicated that the density of defect states associated with the nonradiative recombination process and the deep-level emission are much lower in the ELOG region than that in the center of GaN microdisks, consistent with the structural analysis by transmission electron microscopy (TEM) (Supporting Information Figure S2). Lasing characteristics of the GaN hexagonal microdisks grown on graphene films were further investigated using confocal microphotoluminescence (μ-PL) spectroscopy. The third harmonic of a Nd:YAG laser (355 nm) with a pulse width of 6 ns and a repetition rate of 10 Hz was used as the excitation source, and the laser beam was focused to a spot size of 10 μm by a UV objective (×39). Then, the emission from the local area was collected by the same objective and dispersed in a monochromator equipped with a charge-coupled device. Figure 3a shows room-temperature μ-PL spectra of the GaN microdisk measured at various excitation densities of 90−450 kW/cm2. The broad emission centered at 366 nm had no remarkable features at excitation densities below 260 kW/cm2. However, at 260 kW/cm2, sharp peaks with a regular spacing of 1.5 nm appeared at 370.4, 371.9, 373.3, and 374.9 nm. Furthermore, the peak intensities increased rapidly with further increase of the excitation density. The sharp, regularly spaced PL emissions and the nonlinear increase of PL intensities with excitation density indicate lasing action in the GaN microdisk. A plot of integrated intensity as a function of excitation density shown in Figure 3b shows that the lasing threshold was 250 kW/cm2, which is comparable to that of GaN hexagonal microdisks
Perot (F−P) mode, only 21% of the incident light is reflected at the interface of air and GaN according to the Fresnel equation. Thus, the WGM rather than the F−P mode must be dominant in the GaN hexagonal microdisks. The WGM characteristics of the lasing action were further confirmed by a simple calculation, as discussed below. Using a plane wave approximation, the resonant wavelength in a dielectric hexagonal cavity is given by 1 2 ⎛ 6 = tan−1(β 3n2 − 4) ⎜N + λ 3 3 nD ⎝ π
)
(1)
where N is the mode number, n is the refractive index, D is the diameter of the circle circumscribing the hexagon, and β is the polarization-dependent factor, which is n−1 for a transverse magnetic (TM) mode and n for a transverse electric (TE) mode.19 For a microdisk having a diameter of 8.7 μm, the experimentally obtained mode spacing (Δλ) is in good agreement with the 1.4−1.5 nm calculated using eq 1 as shown in Figure 4.20 We further investigated the quality (Q)-factor of the microcavity laser. According to the definition of Q = λ/Δλ, with λ of 370.4 nm and Δλ of 0.3 nm, the Q-factor was estimated to be 1200. This value is higher than the previously reported value of 740 for WGM lasing of a 4 μm diameter GaN microdisk grown on a SiO2-masked GaN substrate.18 The high Q-factor suggested that the observed lasing emissions resulted from WGM or quasi-WGM resonance in a hexagonal dielectric C
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(5) Van Campenhout, J.; Rojo-Romeo, P.; Regreny, P.; Seassal, C.; Van Thourhout, D.; Verstuyft, S.; Di Cioccio, L.; Fedeli, J. M.; Lagahe, C.; Baets, R. Opt. Express 2007, 15, 6744−6749. (6) Tanabe, K.; Watanabe, K.; Arakawa, Y. Sci. Rep. 2012, 2. (7) Justice, J.; Bower, C.; Meitl, M.; Mooney, M. B.; Gubbins, M. A.; Corbett, B. Nat. Photonics 2012, 6, 610−614. (8) Yang, H. J.; Zhao, D. Y.; Chuwongin, S.; Seo, J. H.; Yang, W. Q.; Shuai, Y. C.; Berggren, J.; Hammar, M.; Ma, Z. Q.; Zhou, W. D. Nat. Photonics 2012, 6, 615−620. (9) Chung, K.; Park, S. I.; Baek, H.; Chung, J. S.; Yi, G.-C. NPG Asia Mater. 2012, 4. (10) Park, W. I.; Lee, C.-H.; Lee, J. M.; Kim, N. J.; Yi, G.-C. Nanoscale 2011, 3, 3522−3533. (11) Lee, C.; Wei, X. D.; Kysar, J. W.; Hone, J. Science 2008, 321, 385−388. (12) Balandin, A. A. Nat. Mater. 2011, 10, 569−581. (13) Chung, K.; Lee, C.-H.; Yi, G.-C. Science 2010, 330, 655−657. (14) Lee, C.-H.; Kim, Y. J.; Hong, Y. J.; Jeon, S. R.; Bae, S.; Hong, B. H.; Yi, G.-C. Adv. Mater. 2011, 23, 4614−4619. (15) Bae, S.; Kim, H.; Lee, Y.; Xu, X. F.; Park, J. S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I.; Kim, Y. J.; Kim, K. S.; Ozyilmaz, B.; Ahn, J. H.; Hong, B. H.; Iijima, S. Nat. Nanotechnol. 2010, 5, 574−578. (16) Czekalla, C.; Sturm, C.; Schmidt-Grund, R.; Cao, B. Q.; Lorenz, M.; Grundmann, M. Appl. Phys. Lett. 2008, 92, 241102. (17) Chen, R.; Ling, B.; Sun, X. W.; Sun, H. D. Adv. Mater. 2011, 23, 2199−2204. (18) Kouno, T.; Kishino, K.; Sakai, M. IEEE J. Quantum Electron. 2011, 47, 1565−1570. (19) Nobis, T.; Kaidashev, E. M.; Rahm, A.; Lorenz, M.; Grundmann, M. Phys. Rev. Lett. 2004, 93, 103903. (20) Yu, G.; Wang, G.; Ishikawa, H.; Umeno, M.; Soga, T.; Egawa, T.; Watanabe, J.; Jimbo, T. Appl. Phys. Lett. 1997, 70, 3209−3211. (21) Bhowmik, A. K. Appl. Opt. 2000, 39, 3071−3075.
resonator rather than from the F−P mode. For WGM in an mfaceted polygonal cavity, the Q-factor is given by Q=
⎛ 2π ⎞ mπnDRm /4 sin⎜ ⎟ m /2 ⎝m⎠ 2λ(1 − R )
(2)
where m is the number of facets, n is the refractive index of the dielectric material, D is the diameter of the circle circumscribing the polygon, and R is the reflectivity of the facet mirrors.21 Using eq 2, the reflectivity, R, was calculated as 87% for both WGM and quasi-WGM; these reflectivities are quite reasonable for WGM or quasi-WGM resonance. A much smaller reflectivity and Q-factor would be expected for the F−P mode, which is not consistent with the experimental results. Our controlled ELOG of GaN on graphene films by MOVPE opens up significant opportunities for the fabrication of sophisticated devices including WGM microdisk lasers. CVD graphene films transferred to SiO2/Si substrates and patterned to microdots play a critical role in the nucleation and crystallization of ZnO nanostructures, which can be used as an intermediate layer for GaN growth. ELOG of GaN enhanced the material quality, formed crystalline microdisks having hexagonal facets, and demonstrated WGM behavior with a Q-factor of 1200 at room temperature. The approach presented here for growing high-quality GaN microdisks, even on an amorphous SiO2 layer, using patterned, transferable graphene films offers a promising and general route to fabricating high-quality light sources and photovoltaic and electronic devices on various substrates.
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ASSOCIATED CONTENT
S Supporting Information *
Electron backscatter diffraction (EBSD) and TEM analysis of GaN microdisk arrays on graphene. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions §
H.B. and C.-H.L. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Creative Research Initiative Project (R16-2004-01001-0) and the Future-based Technology Development Program (Nano Fields, 20100029300) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology.
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REFERENCES
(1) Liang, D.; Bowers, J. E. Nat. Photonics 2010, 4, 511−517. (2) Park, S. I.; Xiong, Y. J.; Kim, R. H.; Elvikis, P.; Meitl, M.; Kim, D. H.; Wu, J.; Yoon, J.; Yu, C. J.; Liu, Z. J.; Huang, Y. G.; Hwang, K.; Ferreira, P.; Li, X. L.; Choquette, K.; Rogers, J. A. Science 2009, 325, 977−981. (3) Choi, J. H.; Zoulkarneev, A.; Il Kim, S.; Baik, C. W.; Yang, M. H.; Park, S. S.; Suh, H.; Kim, U. J.; Bin Son, H.; Lee, J. S.; Kim, M.; Kim, J. M.; Kim, K. Nat. Photonics 2011, 5, 763−769. (4) Miller, D. A. B. Proc. IEEE 2009, 97, 1166−1185. D
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