Tunable GaN Photonic Crystal and Microdisk on PDMS Flexible Films

Article Cite This: ACS Appl. Electron. Mater. 2019, 1, 1112−1119

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Tunable GaN Photonic Crystal and Microdisk on PDMS Flexible Films Kwai Hei Li, Yuk Fai Cheung, and Hoi Wai Choi*

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Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, P. R. China

ABSTRACT: Flexible micro-/nanoscale photonics has shown great promise in a wide range of applications, especially those that cannot be addressed by traditional photonics based on rigid materials and structures. Flexible photonics are typically implemented by transferring compact optical devices made in high-quality crystalline semiconductors onto plastic substrates. However, success in developing flexible optical devices based on GaN micro-/nanostructures has been extremely limited. In this work, we target to overcome this bottleneck by forming GaN photonic crystals and microdisks on flexible PDMS films using a combination of nano-/microsphere lithography and laser lift-off techniques. The GaN−PDMS configurations not only endow the devices with mechanical flexibility but also enable optical tuning of the photonic bandgaps from stretchable photonic crystals and whispering-gallery-mode laser emission from bendable microdisks over a remarkably large range. Their optical properties of the devices are extensively studied through a range of spectroscopy techniques and simulations. The present demonstrations verify the feasibility of the proposed GaN−PDMS platform for forming compact flexible devices, which could pave the way toward emerging applications of flexible photonics technology. KEYWORDS: gallium nitride, flexible, laser lift-off, photonic crystal, microdisk laser

1. INTRODUCTION Recently, there has been increasing interest in the development of micro-/nanostructured optical devices that can stretch and bend, especially in the fields of chemical and biochemical sensing, such as skin-mounted devices that could directly sense optical signals.1−4 The excellent optoelectronic properties of GaN-based semiconductors, including wide wavelength coverage from ultraviolet to visible, strong light−matter interaction, high sensitivity, and fast response time, make them superior candidates for those applications.5−8 However, existing demonstrations of GaN micro-/nanostructured devices are constructed based on GaN epitaxial layers which have to be grown on a lattice-matched substrate, the most common of which are sapphire, silicon, and silicon carbide.9−11 Owing to the high intrinsic stiffness, those substrates cannot be deformed nor stretched without causing damage to the crystal structure, thereby limiting the mechanical flexibility of devices. This bottleneck should be overcome to broaden the scope of practical applications in flexible and wearable optoelectronics. To enable flexibility in the GaN-based material system, one possible approach is direct heteroepitaxial growth on graphene films and metallic substrates.12,13 However, the material quality achieved through such deposition is far from ideal due to the © 2019 American Chemical Society

highly lattice and thermal mismatch. A more promising approach presented in this work is heterogeneous integration by transferring the high quality GaN films from sapphire substrates to flexible plastic platform. Compared with typical chemical−mechanical polishing (CMP) process,14 the laser lift-off (LLO) process adopted in this study is a simple yet effective method to instantly detach GaN layer from its sapphire substrate by rapid thermal decomposition at the interface accomplished by pulsed-laser irradiation through the transparent sapphire.15 The LLO technique has been applied to demonstrate fully suspended GaN-based flexible light emitters and microdisplays with high bending ability and significant strain relaxation.16 To mechanically tune the microand nanostructured devices in a controllable manner, the subsequent transfer to a flexible polydimethylsiloxane (PDMS) film through the optimized LLO process will be implemented in this work. In addition, a low-cost and high-efficiency patterning techniquenano-/microsphere lithography (NSL)17−19is employed to define 2D photonic crystal Received: February 24, 2019 Accepted: June 4, 2019 Published: June 4, 2019 1112

DOI: 10.1021/acsaelm.9b00114 ACS Appl. Electron. Mater. 2019, 1, 1112−1119

Article

ACS Applied Electronic Materials

Figure 1. (a) Schematic diagram and (b) optical image of GaN PhCs on PDMS films. FE-SEM images showing (c) ordered hexagonal array of GaN nanopillars, bird’s eye view of nanopillars in (d) unstretched and (e) stretched conditions, and close-up views of nanopillar arrays in (f) unstretched and (g) stretched conditions.

Figure 2. Simulated band diagrams showing normalized frequency versus in-plane wave vector for nanopillar arrays with (a) a = 250 nm and (b) a = 260 nm. (c) Plots of simulated TE PBG as a function of lattice pitch of PhC. The gray shaded region represents emission band of InGaN/GaN MQWs.

proposed GaN−PDMS platform for developing flexible microand nanostructured optical devices.

(PhC) and microdisk structures on GaN layers. The GaN− PDMS configuration enables optical tuning in photonic bandgap (PBG) from stretchable PhCs and whispering-gallery (WG)-mode laser emission from bendable microdisks. Their optical properties, being sensitive to the change in geometries, are comprehensively characterized, complemented by numerical simulations, aiming to demonstrate the feasibility of the

2. RESULTS AND DISCUSSION 2.1. Stretchable Photonic Crystals. 2.1.1. Fabrication of Photonic Crystals on PDMS. The major fabrication steps involve transferring GaN layer from sapphire onto a PDMS film by LLO and nanostructuring onto GaN by NSL. During 1113


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ACS Applied Electronic Materials dry etching, the self-assembled monolayer of silica nanospheres serves as a hard mask for pattern transfer onto the GaN layer, resulting in periodic nanopillar array, as schematically illustrated in Figure 1a. Figures 1c−e show field-emission scanning electron microscopy (FE-SEM) images of ordered hexagonal array of nanopillars formed on a PDMS film, with pillar heights of 750 nm, base diameters of 250 nm, and sidewall inclination of ∼2°. To demonstrate the device stretchability, the edges of the PDMS film are clamped into linear stages capable of providing precise and reproducible strain along lateral direction. When applying external strain on the PDMS, the pitch of nanopillars can increase from 250 to 300 nm while the pillar dimension remains unchanged, as illustrated in Figures 1f,g. 2.1.2. Photonic Bandgap Calculation. Because a PhC comprising closed-packed pillar structure as fabricated by NSL does not produce a PBG, finite spacing between pillars must be introduced.20 The PhC-on-PDMS configuration with high stretchability provides the possibility of fine-tuning of PBG by altering pitches into various extents. To predict the existences and positions of PBGs in nanopillar-based PhCs, band diagrams are computed using Rsoft BandSOLVE which implements the plane wave expansion (PWE) algorithm for band computations. The simulation models are constructed with identical dimensions based on the experimental structures, and the supercell technique is utilized during 3D PWE simulation. Because the transverse electric (TE) mode is dominant in emission of InGaN/GaN MQWs,21 the TE band structures with fixed pillar base diameter of 250 nm and varying pitch (a) are computed. The simulation begins with a close-packed structure with a = 250 nm, which corresponds to the sample at unstretched conditions and no PBGs exist in all propagation directions, as shown in band diagram in Figure 2a. When finite spacing between pillars is introduced and reaches 260 nm, a forbidden gap appears at the frequency range a/λ of 0.610−0.634, corresponding to spectral region of 410−426 nm, as illustrated in Figure 2b. The computed positions of the PBG for the TE modes as a function of lattice pitch (a) are summarized and plotted in Figure 2c. As the pitch is raised further up to 315 nm, the PBG position can be tuned possibly across the entire visible spectrum. To obtain an apparent effect, the PBG coincides with the emission from InGaN/GaN MQWs at 455 nm with fwhm of 37 nm, and the corresponding pitch should be in the range 260−300 nm. 2.1.3. Optical Transmission Properties. To experimentally determine the PBG position, an optical transmission measurement is conducted in the near-planar direction. The measured transmission spectra shown in Figure 3 show three pronounced dips centered at 420, 452, and 476 nm when lattice pitch is extended to 260, 280, and 300 nm, respectively. The reduced transmission is mainly attributed to the presence of PBG which prohibits propagation of the incident beam along the in-plane at certain frequency range. For wavelengths beyond the PBG region, laterally propagating light does not experience PBG effect, giving rise to relatively high transmission. The gradual red-shift with increasing lattice pitch is in good agreement with bandgap simulation results in Figure 2c. At wavelengths beyond the frequencies range covered by the PBG, relatively low transmission is observed in the short wavelength region as incident light rays with wavelength comparable to pillar dimension will suffer a higher degree of light scattering effect induced by the nanopillar array. It is also found that the transmission within the bandgap region does not fall to zero,

Figure 3. Measured optical transmission spectra of the stretchable PhCs with different pitches.

mainly due to out-coupling of scattering light induced by random disorder within the PhCs. Such disorders may originate from point and line defects naturally formed during the self-assembly process, arising from slight nonuniformities of dimensions and geometrical irregularities of nanospheres.19 2.1.4. Tuning Effect of Stretchable PhC Structures on InGaN/GaN MQW Emission. The optical effects of stretchable PhC structures on InGaN/GaN MQW emission are evaluated by room-temperature photoluminescence (PL) measurements. Figure 4a shows measured PL spectra from stretchable PhCs, compared with an unpatterned sample. Compared with an unpatterned sample, the stretched PhC with a = 260 nm exhibits relatively high intensity at 420 nm. When the pitch is enlarged to 280 and 300 nm, two spectral peaks appear at 454.1 and 471.5 nm, respectively. To have clear comparison, the enhancement ratios of the PL intensities of the stretchable PhCs, compared with the unpatterned sample, are determined and plotted in Figure 4b. Apparently, a significant increase in PL intensities is obtained from spectral ranges within the PBGs (shaded regions), revealing that the PBG structure plays a remarkable role in manipulating spontaneous emission by extracting guided modes to air. More pronounced enhancement is observed from the PhC with a = 280 nm because of the PBG overlapping optimally with the center emission region from InGaN/GaN MQWs. For wavelengths beyond PBGs, a lesser degree of enhancement is attributed to “weak” PhC effect originated from folding of dispersion curves of Bloch modes at the Brillouin zone boundaries.22 As the size and excitation power of laser beam remain unchanged, the effective area of MQWs being excited decreases with increasing pitch, leading to a relatively weak PL intensity and low enhancement ratio from the PhC with a = 300 nm. It is also observed that the spectral contents in the shorter wavelength region are enhanced to a greater extent, attributed to a spectral blue-shift of 1.5 nm due to strain relaxation of nanostructured quantum wells. To investigate how the stretchable PhCs can interact with the guided mode, monochromatic light at 455 nm generated from a broadband light source dispersed through a monochromator couples to the PDMS film through an optical fiber. The coupled light will behave as guided mode and propagate laterally within the PDMS. Upon reaching the GaN regions, the changes of light intensity from sample surfaces will be acquired by a Carl Zeiss LSM700 confocal microscope equipped with a 150× (N.A. = 0.95) objective lens and a photomultiplier tube (PMT). From the confocal image shown in Figure 5a, an unpatterned sample exhibits weak intensity, suggesting that majority of the guided light remain trapped 1114


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Figure 4. (a) PL spectra from stretchable PhC structures compared with an unpatterned sample. (b) Measured enhancement ratio normalized with respect to an unpatterned sample. The shaded regions represent the PBG regions.

Figure 5. Confocal intensity maps (1.4 × 1 μm2 along the x−y plane) of (a) unpatterned sample as well as (b) unstretched and (c) stretched PhCs. FDTD simulated emission patterns of (d) unpatterned sample as well as (e) unstretched and (f) stretched PhCs.

modes and redirecting the trapped photons into the radiated mode, resulting in intense emission throughout the surface, as shown in Figure 5c. Based on the measured and simulated results shown in Figures 5b,c and 5e,f, respectively, the PhC is capable of extracting the guided modes to different extents, which strongly depends on the overlap between emission wavelength and PGB. This can explain why non-Gaussian emission profiles are observed from the measured PL spectra in Figure 4a. 2.2. Bendable Microdisk Laser. 2.2.1. Fabrication of Microdisks on PDMS. To define circular geometry well suited for supporting the whispering gallery mode, the silica spheres in the micrometer size range are chosen as etch mask and the spin-coating conditions are slightly modified to ensure that the self-assembled spheres are sparsely distributed. After dry etching, the microspheres can serve as an etch mask capable of being transferred into the GaN layer, forming excellent circular and smooth geometry with height of 300 nm and diameter of 1.2 μm, as illustrated in Figure 6a. After detachment of sapphire substrate through the LLO process, the undoped GaN layer is etched away to expose the n-GaN

within the PDMS and GaN layers due to total internal reflections at the air/GaN interface with a high refractive index contrast. To understand such a phenomenon, 3D finitedifference time-domain (FDTD) simulations are performed, and the models are constructed based on the experimental devices with identical structures by taking into account random disorder arising from variations in pillar size and position.20,23 The Fabry−Pérot interference fringes, resulting from multiple reflections within GaN layer, are observed from the intensity map in Figure 5d, signifying that strong optical confinement exists along the vertical direction. Such finite fringe patterns cannot be clearly resolved from the measured data due to the optical diffraction limit. Despite the absence of a PBG, the unstretched nanopillar array can still function as a “weak” PhC in which Bragg scattering leads to suppression of Fabry−Pérot oscillations to the in-plane propagating guided light, as evidenced through the sparsely distributed intense spots in Figure 5b. When the pitch is increased to be ∼280 nm, the wavelength of 455 nm for the guided mode fall within the bandgap and the nanopillar array becomes a “strong PhC”.22 The presence of the PBG is capable of prohibiting the guided 1115


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Figure 6. FE-SEM images of (a) microdisk after drying and (b) microdisk transferred on PDMS film. Photographs of microdisk-on-PDMS structures (c) bent in free space and (d) attached on curved surface.

Figure 7. (a) Room temperature PL spectra of microdisk on PDMS with increasing excitation energy densities from 0.8Eth to 1.6Eth. (b) Integrated PL intensity and line width versus pump energy density.

observed beyond the threshold value, confirming a transition process of spontaneous emission to stimulated emission with the microdisk. Moreover, the log−log plot of the integrated PL intensities exhibits a clear S-shaped increase trend for the resonant mode, as presented in Figure 7b. From this plot, the spontaneous emission coupling factor (β) can be further determined from the ratio of PL intensities below and above the threshold and estimated at ∼0.33. When the threshold is approached, the observation of a narrowing process in the spectral line width is also direct evidence of lasing action. 2.2.3. Lasing Mechanisms. FDTD simulations are performed to verify the resonant characteristics of the microdisk. The computational area and the mesh size are fixed at 1.5 × 1.5 μm2 and 5 nm, respectively, with the time step set to 0.00967 fs satisfy the Courant stability condition. Because of the absence of a pair of top and bottom mirrors along the vertical direction, Fabry−Pérot resonances are unlikely to be established within the microdisk. On the other hand, the GaN/PDMS boundaries provide a sufficiently large refractive index contrast of ∼1 in the lateral direction, capable of supporting resonance modes by the total internal reflection. Although WG modes in microdisk resonators experience increasing radiation loss with reducing radii, minimum bend

surface by ICP etching, as evident from the FE-SEM image in Figure 6b. Being formed on a PDMS with high flexibility, the resultant microdisks can be readily bent in free space, as shown in Figure 6c. Figure 6d shows the photograph of sample attached on a curved surface excited by a focused laser beam at 405 nm. 2.2.2. Optically Pumped Lasing Properties. To evaluate the optical properties of microdisk cavities, room-temperature microphotoluminescence (μ-PL) measurements are conducted using a 349 nm diode-pumped solid-state (DPSS) pulsed laser as an excitation source. The μ-PL signal is collected with an optical fiber in the near-horizontal direction to maximize collection of PL signals. Figure 7a shows PL spectra of a microdisk at increasing excitation energy densities. Under low excitation, the spectrum exhibits a weak broadband peak at center wavelength of ∼443 nm, corresponding to spontaneous emission from InGaN/GaN MQWs. As the excitation energy density exceeds the threshold of ∼7 mJ/cm2 (Eth), a sharp peak emerges at a center wavelength of 440.6 nm with a line width of 0.65 nm and a Q-factor of 678. Figure 7b plots integrated PL intensities and spectral line widths of the resonant peak as functions of excitation energy densities. A nonlinear increase in the integrated PL intensity is clearly 1116


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Figure 8. (a) Top view of FDTD simulated pattern of the first-order WG mode at 442 nm. (b) Cross-section view of the calculated profile around the edge of microdisk cavity. The region shaded in red represents the position of InGaN/GaN MQWs. (c) FDTD-computed resonance spectrum and measured PL spectrum.

Figure 9. (a) PL spectra measured from bendable microdisk with different curvatures. (b) Emission peaks and spectral line widths as functions of curvatures.

2.2.4. Curvature-Dependent Lasing Properties. The incorporation of flexible PDMS substrate can open up opportunities for bending microdisks into varying extents. To study curvature-dependent lasing properties, μ-PL measurements are conducted on the samples attached onto supporting rods with different radii (R), and the excitation energy density is fixed at 10 mJ/cm2, as illustrated in the inset of Figure 9a. From the PL spectra shown in Figure 9a, the emission wavelength decreases from 440.6 to 438.7 nm when the bending radius (R) reduces from flat to 12.7 mm. The spectral blue-shift with decreasing R is strongly related to the change of the optical path length for one round-trip in disk-shape cavity. Based on the relation of 2nπr = mλ, an increase in bend curvature results in reduced effective radius (r), thus leading to a decrease in emission wavelength (λ). In addition to the spectral blue-shift, the curvature-induced bending will induce a certain amount of optical loss in the cavity, resulting in a line width broadening from 0.65 to 0.68 nm as well as a slightly decrease in corresponding Q-factors from 678 to 645. Figure 9b plots the center wavelengths as a function of bending curvatures (1/R) and exhibits a linear relationship in emission wavelength over a wide bending range. This sensitive response to substrate deformation suggests that the bendable microdisk can not only behave as wavelength-tunable laser source but also function as an optomechanical microsensor. Its sensitivity, defined as the ratio of center wavelength to the curvature, can

radii of submicrometer dimensions are often acceptable, and the radiation loss is rarely a concern for developing WG mode cavity with radius larger than 400 nm.24,25 The simulation result in Figure 8a further confirms that the intense mode can be established around the periphery of a 1.2 μm microdisk in the lateral plane, corresponding to the fundamental first-order WG mode. It is worth noting that the thickness of cavity is designed to be in subwavelength scale, which facilitates the coupling between the confined mode and the QW gain medium, as described in Figure 8b. The simulated transverse electric (TE) mode spectrum in the wavelength range of 410−470 nm is plotted in Figure 8c. Throughout the violet-blue spectral band, only three distinct peaks centered at 421, 442, and 465 nm, corresponding to fundamental TE WG modes of m = 16, 17, and 18, respectively, are obtained as unobserved higher-radial-order WG modes experience a much larger radiation loss than the fundamental mode for a given bend radius. Because only the resonant mode at 442 nm overlaps InGaN/GaN MQWs emission, single WG mode lasing is observed, which is also correlated well with the calculated mode spacing of ∼21 nm evaluated by relation of ΔλWG = λ2/(2πRn). The small discrepancy could be due to the slight difference in parameters between the simulation and experiment, such as refractive index and microdisk dimension. 1117


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Figure 10. Schematic diagrams of (a) the starting InGaN/GaN MQW wafer; fabrication process flow for (b−e) PhCs and (f−i) microdisks on PDMS films. spheres are sparsely scattered over GaN surface by spin-coating. The spherical mask pattern is transferred to GaN by ICP etching, as depicted in Figure 10f. The etched surface is covered by the PDMS film, followed by degassing and curing processes, as shown in Figure 10g. The sapphire substrate is detached from GaN by LLO, as shown in Figure 10h. The undoped GaN and n-GaN layers are then ICPetched away to expose the microdisk surface, as illustrated in Figure 10i. 4.2. Optical Characterization. The microphotoluminescence (μPL) spectroscopic experiments are performed at room temperature using a 349 nm diode-pumped solid-state (DPSS) pulsed laser (pulse duration of 4 ns, repetition rate of 1 kHz) and a continuous-wave 405 nm diode laser for excitation. The collimated laser beam is focused to a spot of 20 μm onto the sample placed on a translation stage. The μPL signal is collected with an optical fiber and coupled to a spectrometer comprising an Acton SP2500A 500 mm spectrograph and a Princeton Instrument PIXIS open-electrode CCD. The μ-PL signal is collected with an optical fiber in the near-horizontal direction to maximize collection of PL signals, dispersed by a 2400 mm−1 grating in a 500 mm spectrograph and detected by a cooled chargecoupled device (CCD) camera. The optical transmission measurement is performed by focusing the incident beam from a high power broadband solid-state plasma light source (Thorlabs HPLS-30-03) onto the sample while the transmitted signal is measured by an optical spectrometer via fiber signal collection.

be extracted from the linear slope from the plot and determined to be 24.5 nm/mm−1.

3. CONCLUSION In this work, we demonstrate the fabrication of photonic crystals and microdisks on PDMS films by a combination of NSL and LLO techniques. The GaN−PDMS platform endows the devices with mechanical flexibility, enabling optical tunings of PBGs from stretchable PhCs and WGM laser emissions from bendable microdisk. Their optical properties are comprehensively characterized through a range of spectroscopy techniques, which correlate well with simulation results. The present demonstrations verify the feasibility of the proposed GaN−PDMS platform for forming flexible micro-/ nanostructured devices. 4. METHODS 4.1. Device Fabrication. The epitaxial structure used in this study is grown on a c-plane sapphire substrate by metal−organic chemical vapor deposition (MOCVD), which comprises 3 μm of undoped GaN, 3 μm of n-doped GaN, 100 nm of 10 pairs of InGaN/ GaN quantum wells, and 150 nm p-doped GaN. The PDMS film is prepared by thoroughly mixing silicone elastomer with the curing agent (Sylgard 184, Dow Corning) at a 10:1 weight ratio, followed by degassing to remove air bubbles. The PDMS is cured in an oven at 65 °C for 4 h. A strong and reliable bonding between PDMS and GaN can be created by activating the bonding surfaces using an oxygen plasma treatment, as shown in Figure 10b. The collimated beam from a 266 nm Nd:YAG laser (Continuum Surelite) is uniformly irradiated onto the polished sapphire surface. Laser irradiation is immediately blocked once the GaN epilayer is separated from the underlying sapphire to avoid heat accumulation; details of the LLO process can in found in ref 16. After removal of the sapphire substrate, the GaN film is thinned down to a thickness of 750 nm by BCl3-based inductively coupled plasma (ICP) etching, as shown in Figure 10c. Silica nanospheres with mean diameters of 250 nm, suspended in deionized wafer, are mixed with sodium dodecyl sulfate at a volume ratio of 10:1, and 10 μL of colloidal suspension is dispensed and dispersed uniformly across the sample by spin-coating, as depicted in Figure 10d. The self-assembled monolayer of spheres serves as a lithographic mask, and the pillar pattern is transferred to GaN by ICP etching using Cl2/He gas mixtures. The spheres are subsequently removed by sonification in DI water, leaving behind the nanopillar array, as illustrated in Figure 10e. For the fabrication of microdisks on a PDMS film, the process begins with the formation of microdisk, as depicted in Figure 10f. Silica microspheres with nominal diameters of 1.2 μm are diluted with deionized water, and a droplet of the colloidal suspension is dispensed onto a sample using a micropipet. The

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel (852) 28592693; Fax (852) 25598738. ORCID

Kwai Hei Li: 0000-0002-0520-1632 Hoi Wai Choi: 0000-0002-7192-5233 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a GRF grant of the Research Grant Council of Hong Kong (Project No. 17260616). REFERENCES

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