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Lasing in metal-coated GaN nanostripe at room temperature

Yow-Gwo Wang, Cheng-Chang Chen, Ching-Hsueh Chiu, Ming-Yen Kuo, M. H. Shih, and Hao-Chung Kuo

Citation: Applied Physics Letters 98, 131110 (2011); doi: 10.1063/1.3572023

View online: http://dx.doi.org/10.1063/1.3572023

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/98/13?ver=pdfcov Published by the AIP Publishing

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Lasing in metal-coated GaN nanostripe at room temperature

Yow-Gwo Wang,1Cheng-Chang Chen,1Ching-Hsueh Chiu,1Ming-Yen Kuo,2 M. H. Shih,1,2,a兲 and Hao-Chung Kuo1

1

Department of Photonics and Institute of Electro-optical Engineering, National Chiao Tung University, Hsinchu 300, Taiwan

2

Research Center for Applied Sciences (RCAS), Academia Sinica, Taipei 11529, Taiwan

共Received 17 January 2011; accepted 9 March 2011; published online 30 March 2011兲

This study demonstrated a metal-coated GaN nanostripe laser operable at room temperature. The ultraviolet lasing mode was observed at a wavelength of approximately 370 nm with a low threshold power density of 0.042 kW/cm2. The lasing mode of the metal-coated nanostripe was characterized

using finite-element method simulation. The results showed the significance of metal coatings in this nanocavity structure for lasing at room temperature. © 2011 American Institute of Physics. 关doi:10.1063/1.3572023兴

In recent years, the size of the semiconductor lasers has been reduced to the nanoscale, to integrate them into optical devices. Metal-coated cavities have been embraced as a promising candidate to achieve this goal, despite the lossy characteristics of the materials involved. The advantages of optical confinement and the field enhancement of surface plasmon effects enable metal-coated cavities to break the diffraction-limit constraining the size of lasers.1Moreover, to lower the absorption of metal, a thin dielectric layer was inserted between the gain material and metal, resulting in a higher quality factor and lower threshold gain, making lasing action possible.2In 2007, Hill et al.3demonstrated one of the smallest semiconductor nanolaser, with a nanorod size far below the lasing wavelength. To achieve lasing in metal-coated cavities at room temperature, Hill et al.4demonstrated a subwavelength metal-coated waveguide laser in 2009. Since that time, these devises have been widely studied from both the theoretical and experimental perspective,5,6 in an attempt to understand the effects of metals on subwavelength cavities. However, recent research has focused mainly on devices operating in the region of communication and infra-red wavelengths.7,8The combination of a metal-coated cavity using GaN-based materials has never been reported in previ-ous research. Although wide-bandgap GaN-based materials have found their way into many applications including light emitting diodes,9laser diodes,10and vertical surface emitting lasers,11,12the size of these devices remain at the micrometer scale.

In this paper, we demonstrated lasing in metal-coated GaN nanostripes at room temperature under optical pumping conditions. To make lasing action in this device possible, we employed SiO2 as an insertion layer between the metal and

GaN to improve the quality factor, in conjunction with alu-minum providing high reflectivity in the ultraviolet共UV兲 re-gion and high thermal conductivity. The lasing action was observed in GaN-based metal-coated nanocavities around UV wavelengths region at room temperature.

The GaN nanostripe was fabricated on an undoped GaN layer, acting as a gain medium for the device. A schematic diagram is presented in Fig. 1. The 2 ␮m thick undoped GaN layer was grown on a C-plane共0001兲 sapphire substrate using a low pressure metal-organic chemical vapor

deposi-tion 共MOCVD兲 system. To fabricate the device, we first de-posited a 300 nm Si3N4 layer as an etching mask, followed

by a 300 nm thick polymethylmethacrylate 共PMMA兲 layer. The nanostripe pattern was defined using E-beam lithogra-phy on the PMMA layer, whereupon the pattern was trans-ferred to the Si3N4layer through reactive ion etching 共RIE兲

with CHF3/O2 mixture. The nanostripe pattern was etched

down to the undoped GaN layer using inductively coupled plasma and reactive ion etching 共ICP-RIE兲 with a Cl2/Ar

mixture, and the mask layers were removed using wet etch-ing after these processes had been completed. A scannetch-ing electron microscope 共SEM兲 image of the GaN nanostripe prior to the deposition of SiO2and metal shielding layers is

shown in Fig.2共a兲. A 20 nm thick SiO2layer was deposited

on the nanostripe, and this layer was used to reduce the in-fluence of surface roughness caused by the previous dry etching process. Surface roughness increases the loss associ-ated with the absorption of metal, thereby lowering the qual-ity factor and increasing the lasing gain threshold. Such an increase in the lasing gain threshold could preclude the pos-sibility of achieving lasing at room temperature. A uniform aluminum 共Al兲 layer with a thickness of 60 nm was then deposited on the device using E-gun evaporation. Compare to metals widely applied in plasmonic devices such as Ag and Au, Al has a higher reflection in UV wavelength region 共Al: 92%, Ag: 25%, Au: 38% at 370 nm兲, though Al also has a higher absorption 共Al: 151 ␮m−1, Ag: 55 ␮m−1, Au: 65 ␮m−1 at 370 nm兲. Therefore a better optical confinement

a兲Electronic mail: mhshih@gate.sinica.edu.tw. FIG. 1.nanostripe.共Color online兲 Schematic diagram of the metal-coated GaN

APPLIED PHYSICS LETTERS 98, 131110共2011兲

0003-6951/2011/98共13兲/131110/3/$30.00 98, 131110-1 © 2011 American Institute of Physics

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from an Al-coated GaN cavity can be expected. The size of the GaN nanostripe was approximately 500 nm in width and 50 ␮m in length, and the etching depth is 500 nm. Figure 2共b兲shows a magnified image of one side of the GaN nanos-tripe. The complete structure of the device after processing is shown in Fig. 2共c兲.

The metal-coated GaN nanostripe was optically pumped using a frequency-tripled Nd: YVO4 355 nm pulsed laser at

room temperature with a pulse width of 0.5 ns, at a rate of 1 kHz. The spot size of the normal incident beam was approxi-mately 50 ␮m, covering part of the nanostripe. A 15⫻ ob-jective lens was used to collect the light emitted from the GaN nanostripe through a multimode fiber, which was coupled with a spectrometer employing a charge-coupled de-vice. Despite the fact that the absorption of the metal reduces the power of lasers, we opted to pump directly from the top of the device to avoid the enormous losses that would have resulted from the undoped GaN, had we chosen to pump from the back of the wafer.

To ensure that the lasing action originated in our struc-ture, we first used an He–Cd 325 nm continuous-wave laser to pump the undoped GaN layer, conducting this test with and without the metal and SiO2layers to observe the

photo-luminescence 共PL兲 spectrum at room temperature. The peak wavelength of the undoped GaN layer without shielding lay-ers was approximately 362 nm. However, the spectrum of the undoped GaN layer with aluminum and SiO2did not have a

peak at 362 nm because the signal from approximately 350 to 380 nm had been totally absorbed by the shielding layer. The PL spectrum shown in Fig.3共a兲confirms that our design enabled lasing action. Moreover, Fig. 3共b兲 shows the PL spectrum of the device below 共black兲 and above 共red兲 the threshold under room temperature pulsed conditions. This device operated with only a single lasing mode at 370 nm. Figure3共c兲shows the light-in light-out curves of this mode, with a threshold power density of approximately 0.042 kW/cm2, and a quality factor 共Q兲 of approximately

200, as estimated by the ratio of wavelength to linewidth 共␭/⌬␭兲 around transparency. Following the soft turn-on at approximately the threshold power density, the measured light output was linear, serving as proof of the lasing action in this device. Compared to the sample without a metal or SiO2 shielding layer, the high thermal conductivity and re-flectivity of the aluminum made measurement easier and

in-creased the possibility of lasing at room temperature. As ex-pected, the nanostripe without the coating layer showed higher optical loss and lower Q value. It was for these rea-sons that lasing action was not observed in this experiment. To garner a better understanding of the experimental re-sults, we employed finite-element-method 共FEM兲 for the simulation of optical modes using this metal-coated GaN nanocavity. The simulation model comprised a sapphire layer, an undoped GaN layer, a thin SiO2layer, and a metal

shielding layer. We adopted the refractive indexes of un-doped GaN and the aluminum layer established by Peng and Piprek13and Rakic et al.,14with the refractive index of SiO2

as 1.46. This model included a perfectly matched layer sur-rounded the nanocavity to absorb redundant signals, which would have reflected back to the metal-coated nanostripe, thereby influencing the optical mode. Figures 4共a兲 and4共b兲 show simulated optical mode profiles for a GaN nanostripe cavity with and without metal-coating. In Fig. 4共a兲, the model incorporating metal and SiO2 layers had an optical mode well confined within the nanostripe, demonstrating a clear standing wave pattern with 3.5 nodes. However, using the nanostripe without a metal shielding layer, the optical mode shown in Fig. 4共b兲leaked into the region of air with a standing wave that lacked uniformity, compared with the re-sults of the metal-coated device. The wavelength of the op-tical mode shown in Fig. 4共a兲was 367 nm, and the quality factor 共Q兲 extracted from the eigenvalue of the mode was approximately 150, which was ten times greater than the optical mode shown in Fig. 4共b兲. This illustrates the diffi-culty of achieving GaN nanostripe lasing without shielding layers. The small difference in wavelength between the simu-lation and experimental results can be attributed to imprecise fabrication processes and the imprecision of material indices used in the model. From these simulation results, we con-firmed that the metal shielding layer in this structure played an important role in lasing at room temperature. Vertical con-finement would the key to further increasing the quality fac-tor of the device. The adoption of a distributed Bragg reflec-tor共DBR兲 beneath the undoped GaN layer could confine the FIG. 2. 共a兲 SEM image 共top view兲, of a GaN nanostripe prior to the

depo-sition of metal and SiO2layers;共b兲 magnified SEM image of one side of GaN nanostripe from angled view;共c兲 magnified SEM image of one side of GaN nanostripe from angled view following the deposition of SiO2 and aluminum.

FIG. 3. 共Color online兲 共a兲 PL spectrum of undoped GaN layer with and without the SiO2and aluminum shielding layers;共b兲 measured spectra from metal-coated GaN nanostripe laser below and above threshold. Lasing wavelength of the GaN nanostripe is 370 nm;共c兲 light-in light-out curve 共L-L curve兲 of the metal-coated GaN nanostripe laser.

131110-2 Wang et al. Appl. Phys. Lett. 98, 131110共2011兲

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optical mode and prevent scattering to the undoped GaN layer beneath the nanostripe as shown in Fig. 4共a兲. This would form a three-dimensional cavity with a combination of metal-coated sidewall and a distributed Bragg reflector,15 representing a promising means to further improve the per-formance of the device or even enable operation of the de-vice under electrically-pumped conditions. The lasing perfor-mance of the GaN metal laser could also be much improved by including GaN-based heterostructure and modulating stripe sidewall geometry.16 Moreover, the band diagram of this structure shown in Fig. 4共c兲helps to clarify this lasing action. The lasing mode of this metal-coated GaN stripe is a hybrid plasmonic waveguide mode which combines a TE waveguide mode of the GaN stripe and the surface plasmon modes from the Al/SiO2/GaN interfaces. The similar struc-ture had been studied for an InGaAs/InP stripe4and a dielec-tric cylindrical nanowire.17 A small Fabry–Perot resonant closed to the lasing mode was also observed in the spectrum of Fig.3共b兲. The Fabry–Perot mode spacing can be described by the formula ⌬␭=␭2/2n

gL, and the effective index of the lasing mode is approximately 1.5 extracted from the nearly 1 nm mode spacing in spectrum. In the wavelength range between 300 and 400 nm, there was only one waveguide band, and the lasing wavelength of our experiment was in agreement with the waveguide mode calculated in the band diagram. This provides further proof that this metal-coated GaN nanostripe could form an optical mode, with a clear standing wave pattern, thereby making lasing action pos-sible. It is worth to note that the SiO2dielectric layer benefits quality factor and electrical injection of the cavity. However, it might reduce the overlap between lasing modes and gain medium, especially for surface plasmon waves at metal– dielectric interface.

In summary, this study demonstrated UV lasing from a metal-coated GaN nanostripe at room temperature. Using an E-beam lithography technique, we fabricated a GaN nanos-tripe 500 nm in width, 500 nm in height, and 50 ␮m in length. We then deposited a thin SiO2and aluminum layer on

the nanostripe to form a metal-coated nanocavity. We ob-served lasing action at 370 nm wavelength with a low thresh-old power density of 0.042 kW/cm2. FEM simulation

re-sults show that a metal-coated nanostripe cavity could sustain a standing wave pattern, which would not occur in a nanostripe cavity without a metal shielding layer. The wave-lengths of the mode were quite close to those of the experi-mental results. The band diagram of this structure also sup-ports the notion that the nanostripe structure would form an optical mode. This is a clear indication that the metal coating used on the nanocavity played an important role in lasing results at room temperature. The results presented here rep-resent a breakthrough in the integration of metal-coated nanocavities with GaN materials. Future efforts will focus on improving the performance of the device by increasing ver-tical confinement of the opver-tical mode, enabling the device to be electrically-pumped. Such developments would provide enormous potential for applications in the very near future.

The authors would like to thank Professor S. L. Chuang and Dr. S. W. Chang from University of Illinois at Urbana-Champaign, and Professor T. C. Lu from National Chiao Tung University for insightful suggestions. We also are grateful to the Center for Nano Science and Technology at National Chiao Tung University, National Nano device Laboratory 共NDL兲, and the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract Nos. NSC 99-2112-M-001-033-MY3 and NSC 100-3113-E-009-001-CC2.

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Photonics 2, 496共2008兲. FIG. 4. 共Color online兲 共a兲 Calculated Ezmode profile of metal-coated GaN

nanocavity;共b兲 calculated Ezmode profile of the GaN nanocavity without metal and SiO2shielding layers;共c兲 band diagram of the metal-coated GaN nanostripe.

131110-3 Wang et al. Appl. Phys. Lett. 98, 131110共2011兲

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FIG. 3. 共Color online兲 共a兲 PL spectrum of undoped GaN layer with and without the SiO 2 and aluminum shielding layers; 共b兲 measured spectra from metal-coated GaN nanostripe laser below and above threshold

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