Article Cite This: ACS Photonics 2018, 5, 2624−2630
Epitaxial Aluminum-on-Sapphire Films as a Plasmonic Material Platform for Ultraviolet and Full Visible Spectral Regions Chang-Wei Cheng,† Yun-Jhen Liao,‡ Cheng-Yen Liu,† Bao-Hsien Wu,‡ Soniya S. Raja,§ Chun-Yuan Wang,† Xiaoqin Li,∥ Chih-Kang Shih,∥ Lih-Juann Chen,*,‡ and Shangjr Gwo*,†,§,⊥ Department of Physics, ‡Department of Materials Science and Engineering, and §Institute of Nanoengineering and Microsystems, National Tsing-Hua University, Hsinchu 30013, Taiwan ∥ Department of Physics, The University of Texas at Austin, Austin, Texas 78712, United States ⊥ National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan Downloaded via UNIV OF READING on August 9, 2018 at 14:56:41 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
†
S Supporting Information *
ABSTRACT: In comparison to noble metals (gold and silver), aluminum is a sustainable and widely applicable plasmonic material owing to its abundance in the Earth’s crust and compatibility with the complementary metal−oxide−semiconductor (CMOS) technology for integrated devices. Aluminum (Al) has a superior performance in the ultraviolet (UV) regime with the lowest material loss and good performance in the full visible regime. Furthermore, aluminum films can remain very stable in ambient environment due to the formation of surface native oxide (alumina) acting as a passivation layer. In this work, we develop an epitaxial growth technique for forming atomically smooth aluminum films on transparent c-plane (0001) sapphire (Al-on-Sapphire, ALOSA) by molecular-beam epitaxy (MBE). The MBE-grown ALOSA films have small plasmonic losses and enable us to fabricate and utilize high-quality plasmonic nanostructures in a variety of optical configurations (reflection, transmission, and scattering). Here, the surface roughness and crystal orientation of ALOSA films are characterized by atomic force microscopy (AFM) and X-ray diffraction (XRD). Moreover, the formation of smooth native oxide layer and abrupt heterointerfaces are investigated by transmission electron microscopy (TEM). We have also measured the optical dielectric function of epitaxial aluminum films by using spectroscopic ellipsometry (SE). These results show that the structural and optical properties of epitaxial aluminum films grown by MBE are excellent compared to polycrystalline aluminum films grown by other deposition methods. To illustrate the capability of device applications for the full visible spectrum, we demonstrate clear surface plasmon polarition (SPP) interference patterns using a series of double-groove surface interferometer structures with varied groove−groove separations under white-light illumination. Finally, we show the device performance of zinc oxide (ZnO) nanowire (UV) and indium gallium nitride (InGaN) nanorod (blue and green) plasmonic lasers prepared by using the epitaxial Al films. The measured lasing thresholds are comparable with the best available data obtained on the Ag films. According to these result, we suggest that epitaxial ALOSA films are a versatile plasmonic material platform in the UV and full visible spectral regions. KEYWORDS: surface plasmon polariton (SPP), molecular-beam epitaxy (MBE), aluminum film, sapphire substrate, SPP interferometer, plasmonic nanolaser small imaginary part of optical dielectric function (ε′′ or ε2). However, the presence of interband transitions (ITs) in gold and silver band structures prohibit their application in the ultraviolet (UV) regime.6−9 For noble metals, ITs have a threshold behavior: When the incident photons have a photon energy (wavelength) higher (shorter) than ∼2.5 eV (∼500 nm) for gold and ∼4.0 eV (∼300 nm) for silver, they predominantly excite electron−hole pairs, instead of surface plasmons. This threshold behavior means that surface plasmon resonances can
etal films and nanostructures exhibit fascinating optical phenomena for the applications of plasmonics, which can be understood by collective oscillations of conduction electrons relative to the static ionic background (surface plasmon polariton (SPP) and localized surface plasmon). The surface plasmon resonances associated with optical excitation of metal films and nanostructures offer unique possibilities for light concentration beyond the diffraction limit, which can lead to strong field localization and enhancement in deep subwavelength regions.1−5 At present, gold (Au) and silver (Ag) are two most used metals for plasmonic applications due to their low material losses in the optical frequency regime related to their low resistivity (high electric conductivity) and
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© 2018 American Chemical Society
Special Issue: Recent Developments and Applications of Plasmonics Received: November 11, 2017 Published: January 30, 2018 2624
DOI: 10.1021/acsphotonics.7b01366 ACS Photonics 2018, 5, 2624−2630
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variety of optical configurations for reflection, transmission, and scattering measurements. Then, we present structural and optical properties of MBEgrown ALOSA films, which were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), atomic force microscopy (AFM), and spectroscopic ellipsometry (SE). These results demonstrate that the as-grown aluminum films are atomically smooth single-crystalline materials with excellent optical properties. Besides, we confirm the presence of an ultrathin (∼3.5 nm) and ultrasmooth native oxide layer on top of the epitaxial aluminum film. Previous studies showed that a native oxide layer is formed with a thickness stabilizing at a few nanometers on a pristine Al surface within a few hours of air exposure. The oxide layer can remain stable for a very long period of time and acts as a passivation layer preventing further oxidation. To demonstrate plasmonic white-light applications, we fabricated a plasmonic structure (double-groove surface interferometer) by focused-ion beam (FIB) milling to measure SPP interference using a halogen lamp. The clear surface wave interference patterns observed on the ALOSA film illustrate the low-loss nature on atomically smooth single-crystalline aluminum. In addition, we are able to fabricate low-threshold ZnO (UV) and InGaN (blue and green) plasmonic nanolasers on the ALOSA film by simple drop-casting method. We achieve pulsed ZnO nanowire lasing at room temperature and continuous-wave (CW) InGaN nanorod lasing at cryogenic temperature. Due to the high-quality material and the presence of a smooth native oxide layer, the measured lasing thresholds are comparable with those obtained on the Ag films. These results indicate that epitaxial aluminum films as a plasmonic material platform have a vast potential to be applied in the ultraviolet and full visible range.
be observed only for incident photon energies lower than the IT energy. In contrast, aluminum (Al) has a narrow-band behavior in IT, such that it can exhibit surface plasmon resonances at wavelengths longer or shorter than the interband transition band at ∼1.5 eV (∼800 nm).10 It has been well-known that silver can be considered as the best plasmonic metal because the imaginary part of its dielectric function is very small the full visible and infrared wavelength range. However, in the UV regime (at wavelengths shorter than 300 nm), aluminum is a better plasmonic metal than silver. Therefore, aluminum is an ideal material for UV plasmonics.11−13 Furthermore, aluminum has a negative real part of optical dielectric function, that is, ε′(ε1) < 0, and a relatively small ε′′(ε2) in the full visible range. This is why aluminum is also an appealing material for visible plasmonics. As an enabling material for future plasmonic technology development, aluminum has additional advantages such as natural abundance, low cost, and compatibility with the complementary metal− oxide−semiconductor (CMOS) process. In the past decade, it has been reported that aluminum-based plasmonic applications performs very well in metal-enhanced fluorescence,14 deep-UV Raman scattering,15 nonlinear plasmonics,16,17 plasmonic nanoantennas,18 CMOS-compatible plasmonic nanoresonantors and color filters,19−22 and UV plasmonic nanolasers operating at room temperatre.23−26 The performance of plasmonic devices depends not only on the intrinsic properties of plasmonic materials, but also on their material quality. It has been found that material quality strongly depends on synthesis, deposition, and fabrication processes.27−33 For example, the deviation of recently measured silver28,29 and aluminum32 dielectric functions from the heavily quoted optical constant data sets34−37 mostly reflects the improved material quality. In the case of plasmonic nanostructures fabricated by lithographic methods based on granular or polycrystalline films, the presence of surface roughness and grain boundaries can lead to strong scattering losses and short SPP propagation length.38 Recognizing the limitations of polycrystalline materials prepared by conventional deposition methods, extensive research efforts have been devoted to chemical synthesis techniques31,32,38−45 to synthesize gold39,40 and silver,31,38,41 aluminum nanocrystals.42−45 Aluminum is extremely reactive and highly sensitive to residual gases in the film deposition chamber.30 Therefore, the purity and quality of aluminum films grown by conventional deposition techniques is typically not comparable to highly crystalline aluminum films grown by molecular-beam epitaxy (MBE) in ultrahigh vacuum environment.32,46−48 It has been shown that the behavior of plasmon resonance measured for aluminum nanostructures is strongly dependent on the purity and quality of the bulk aluminum composition of the nanostructure.11,13 In the past, aluminum nanostructures are fabricated by lithography or patterned growth using thermally evaporated aluminum.49−51 Recently, synthesis of high-purity aluminum nanocrystals has been successfully demonstrated, which open a new approach for aluminum plasmonics.43−45 In this work, we report on a simple low-temperature MBE growth approach to prepare aluminum films on transparent, doubleside-polished c-plane (0001) sapphire (Al-on-Sapphire, ALOSA). We choose this heteroepitaxial system because the hexagonal lattice parameter of Al (111) plane (2.863 Å) is closely matched to that (2.748 Å) of the c-plane sapphire substrate. Moreover, the ALOSA films enable fabrication and utilization of low-loss aluminum plasmonic nanostructures in a
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RESULTS AND DISCUSSION The aluminum films were grown by a plasma-assistance MBE system (DCA 600). This base pressure was kept about 1 × 10−10 Torr during growth. We used commercial 2 in. doubleside-polished c-plane (0001) sapphire wafers as substrates. First, the sapphire substrate was thermally cleaned at 800 °C for 3 h. In situ reflection high-energy electron diffraction (RHEED) pattern of the c-plane sapphire surface can be obtained after this cleaning step (Figure 1a.). Then, aluminum was evaporated by using a 100 c.c. Kundsen cell. The deposition rate (about 200 nm/h) was controlled by the cell temperature and the substrate temperature is maintained at room temperature (∼300 K). We confirmed that low-temperature deposition is necessary for ultrasmooth and single-crystalline aluminum growth.32 During aluminum growth, we observed smooth film morphology and epitaxial growth by streaky RHEED patterns taken from the aluminum film (Figure 1a.). In order to check the crystal orientation of epitaxial aluminum film, the as-grown aluminum film was measured by ex situ XRD. We can observe both the (111) peak of aluminum film and the (0006) peak of c-plane sapphire substrate at around 38° and 42°, respectively (Figure 1b). The XRD pattern shows the aluminum film is single crystalline grown along the [111] direction. The root-meansquare (RMS) roughness of film surface was measured by ex situ AFM. The AFM image (Figure 1c) shows an atomically smooth surface with the RMS roughness about 0.5 nm in a 5 × 5 μm2 area. Cross-sectional TEM imaging (Supporting Information, Figure S1) demonstrates that an abrupt interface is formed between the sapphire substrate and the aluminum 2625
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Figure 2. Optical properties of epitaxial aluminum film on c-sapphire. (a) Optical image of an aluminum epitaxial film grown on a doubleside polished c-plane sapphire wafer. (b) Reflectivity spectrum of the as-grown aluminum film with a native oxide layer (∼3.5 nm) shows a high reflectivity (∼0.9) in a wide spectral range. The interband transition is at about 800 nm. (c) Real part of the dielectric function (ε1) extracted from the spectroscopic ellipsometry data as a function of photon energy (black solid curve). (d) Imaginary part of the dielectric constant (ε2). For comparison, literature data of Al (red dotted curve) and Ag (blue dotted curve) are also shown in the plots.
Figure 1. Structural properties of epitaxial aluminum film on csapphire. (a) RHEED patterns of c-plane sapphire and epitaxial aluminum film during MBE growth. (b) X-ray diffraction pattern showing both aluminum (111) and c-plane sapphire (0006) peaks. The full width at half-maximum (fwhm) of the Al (111) peak is about 0.18°. (c) AFM image (area: 5 × 5 μm2) taken on the as-grown, native-oxide-protected aluminum film. The RMS roughness is about 0.5 nm. (d) High-resolution TEM image of the single-crystalline aluminum film shows clearly the abrupt interface region between the aluminum epitaxial film and the c-sapphire substrate.
film (Figure 1d). For comparison, we also analyze structural properties of epitaxial aluminum films grown on silicon (111) substrates (Supporting Information, Figures S2 and S3). The as-grown aluminum film is high specular (Figure 2a). The measured reflectivity of aluminum film is ∼90% in the ultraviolet and visible regions except the IT region at wavelength about 800 nm (Figure 2b). For comparison, the reflectivity of aluminum film on silicon substrate was also measured (Supporting Information, Figure S4). In addition, we have measured the optical dielectric function by using spectroscopic ellipsometry. The real and imaginary parts of the dielectric function are shown in Figure 2c and d, respectively (complete data files are included in Supporting Information). In Figure 2, we compare these results with available aluminum data listed in Palik’s Handbook of Optical Constants of Solids36 and silver data by Johnson and Christy.34 Besides the IT, scattering losses from rough surface and grainy structure contribute to ε2. These results demonstrate that epitaxial Al films grown by MBE at RT exhibit lower losses than Palik’s values from near-infrared to UV range (1.24−5 eV, Figure 2d). On the other hand, except the UV regime (>4 eV), silver exhibits lower loss in the full visible range. However, we will show below that visible plasmonic devices based epitaxial aluminum films perform comparably well due to the presence of high-quality native oxide layers on the film surfaces. The properties of propagating SPP waves on silver films and nanostructures can be measured by SPP interference patterns in the full visible range using a white light source.31,38,52 Here, we demonstrate that clear SPP interference patterns can be obtained by using the single-crystalline aluminum films. SPP interferometers were formed on the aluminum surface by fabricating a pair of nanogrooves by focused-ion beam (FIB)
milling (Figure 3a). We confirmed that interference fringe spacing can be tuned by varying distance (D) between double grooves. For the white-light interference experiments, we design a fixed groove length (10 μm) and width (70 nm) and vary the distance between grooves from 2.5 to 4 μm. We excited SPP waves by using a halogen white-light source with an incident angle around 75−80° (Figure 3a). A part of the incident photons couple to surface plasmon modes in aluminum grooves and propagate along the aluminum surface in the form of SPP waves. Due to the low-loss film properties (long propagation length), the propagating SPPs can be reflected back and forth between two grooves before decoupling at grooves into far-field radiation (photons). The decoupled photons are collected through a microscope objective (100×, numerical aperture N.A. = 0.8), and FabryPérot interference patterns by a spectrometer. It is important to note that the overall spectrum profile is determined by the spectra of groove localized plasmon resonance and halogen lamp. The groove localized plasmon resonance has a broadband spectrum, but we still can identify the green resonance peak (tunable by changing the groove width and depth) from the optical dark-field image (Figure 3c). The main information on SPP interference can be captured the positions of local peaks and dips in the interference spectra, which are used to fit the real part of SPP effective index by using the simple constructive and destructive interference equations:31 peaks, 2k rD = 2qπ
(1)
dips, 2k rD = (2q + 1)π
(2)
where kr is the real part of wavenumber, D is the separation between two grooves, and q is an integer number (q = 0, 1, 2, 3, 2626
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Figure 3. SPP interference patterns on epitaxial aluminum film (a) Schematic of the white-light interference setup for measuring the SPP interference patterns on an MBE-grown aluminum epitaxial film. The incident angle of white light from a halogen lamp is about 75−80°. (b) TEM image of the native oxide layer spontaneously formed on top of the epitaxial Al film. Additional TiO2 and platinum (Pt) layers were deposited in order to resolve the native oxide layer. (c) SPP interference patterns with different separation distances between two grooves. Right panels are corresponding optical dark-field images. (d) Simulated effective indices of SPP propagation on aluminum surface using the FDTD method (black dashed curve). The material data are determined from the spectroscopic ellipsometry measurement results. The experimental data are fitted by using the constructive and destructive interference equations (red dots) described in the main text.
...). To fit the experimental data, one needs to know the exact thickness of native oxide. For this purpose, we deposited a 5 nm thick TiO2 layer by atomic layer deposition (ALD) in order to measure the native oxide layer (AlOx) by TEM with a clear contrast. The cross-section TEM image reveals that the thickness of native oxide is about 3.5 nm (Figure 3b). Using the SE-determined dielectric function and the TEM-imaged native oxide thickness, we confirm that the effective index simulated by using the finite-difference time-domain (FDTD) method is in good agreement with experimental data (Figure 3d). Our results indicate that simple double-groove interferometers fabricated on epitaxially grown aluminum films can be used as surface plasmonic cavity in the full visible range because of improved material quality. It has been proposed that diffraction-unlimited nanolasers will be critical components for the future development of onchip optical communications and quantum information technology. In the past few years, there are several successful demonstrations of semiconductor plasmonic nanolasers based on surface plasmon amplification by stimulated emission of radiation (SPASER)53 and low-loss metal−oxide−semiconductor plasmonic waveguides structure.54,55 Most of these plasmonic nanolasers adopted silver films as the plasmonic material because of their intrinsic low-loss properties.56−63 Recently, due to the advantages in the UV regime and compatibility with semiconductor manufacturing processes, aluminum has attracted a great deal of attention as a plasmonic material for UV plasmonic nanolasers.23−26 Here, we experimentally demonstrate both ZnO nanowire and InGaN
nanorod plasmonic lasers to show that the epitaxial growth Al film is an appropriate plasmonic material platform for both UV and visible spectral regions. We incorporate quasi one-dimensional (1D) semiconductor nanostructures onto epitaxially grown aluminum films to form metal−semiconductor hybrid nanostructure lasers. The plasmonic cavity is formed in the dielectric gap layer (alumina) between the atomically smooth aluminum film and the 1D semiconductor nanostructure. Wurtzite ZnO and InGaN semiconductors have different advantages as the laser gain media. The ZnO nanowire lasers can be operated at room temperature because of a large exciton binding energy (∼60 meV). On the other hand, the InGaN (InxGa1−xN) semiconductor alloy is a full-visible-range emitter.64 Moreover, the MBE-grown InGaN nanorods have a self-organized core−shell nanostructure (i.e., the InGaN core is encapsulated by a thin GaN shell), which can reduce the surface recombination (nonradiative) rate at the nanorod boundary. As a result, the InGaN nanorod laser can be operated under continuous-wave (CW) pumping at low lasing thresholds.57,58 To fabricate these plasmonic nanolasers, we dispersed nanowires or nanorods in isopropyl alcohol using an ultrasonic cleaner and deposit them by drop-casting onto the epitaxially grown Al films covered with an ALD-grown 3 nm thick Al2O3 dielectric layer. The lasing characteristics of ZnO nanowires and InGaN nanorods were measured by microphotoluminescence (μ-PL) spectroscopy using two different semiconductor diode lasers (Figure 4). The ZnO nanowires were optically pumped at room temperature by using a 355 nm pulsed diode laser with a 1.0 ns 2627
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Figure 4. Plasmonic lasing characteristics measured for ZnO nanowire (UV) and InGaN nanorods (blue and green). (a) Bimodal lasing spectrum measured at room temperature for the ZnO plasmonic nanowire laser at 377 and 381 nm. The nanowire length is about 2.2 μm and the crosssectional side width (hexagon) is about 40 nm. (b) The light−light curve shows the lasing threshold is about 20 MW/cm2 (355 nm diode laser, pulsed pumping, pulse width: 1.0 ns, repetition rate: 1 kHz) and the lasing intensity (at 377 nm) shows a strong nonlinear behavior with respect to the pumping intensity (red dots). The line width of lasing mode (at 377 nm) is also measured with increasing pumping intensity from spontaneous to stimulated emission region (blue dots). (c, e) Emission spectra of blue and green InGaN@GaN core−shell plasmonic nanorod lasers at 486 and 551 nm, respectively. The length and side width of the blue nanorod is about 450 and 20 nm, respectively, while the length and side width of the green nanorod is 480 and 30 nm. (d, f) Light-light curve and line width measured with varying input optical power density. The lasing thresholds for blue and green lasers are about 60 and 10 W/cm2 (pumped by a continuous-wave, 405 nm diode-laser), respectively.
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pulse width and a 1 kHz repetition rate. The light−light curves measured by varying the laser power density (MW/cm2) show a clear nonlinear behavior above the lasing threshold and the lasing threshold is about 20 MW/cm2 (Figure 4b). The laser threshold is comparable to the best result reported on the Al film (12.2 MW/cm2).25 We have also measured the polar plot of emission intensity at 377 nm which shows the polarization of lasing mode is parallel to the c-axis of the wurtzite ZnO nanowire with a polarization ratio of 83% (Supporting Information, Figure S5a). The observed emission wavelength and polarization, as well as group indices (see Supporting Information), are in good agreement with plasmonic lasing under the exciton-surface plasmons coupling mechanism,63 instead of electron−hole-plasma lasing in the photonic mode.63 The lasing characteristics of InGaN nanorod plasmonic lasers measured by using a 405 nm CW diode laser show nonlinear light−light curve at 8K with low lasing thresholds (Figure 4d,f). The measured thresholds are comparable or even lower than those obtained on silver epitaxial films.57,58 Here, we demonstrate blue and green InGaN nanorod plasmonic lasers, and the polarization direction of lasing mode are parallel to the c-axis of the wurtzite InGaN nanorod with high polarization ratios of 89% and 95%, respectively (Supporting Information, Figures S5b,c). It has been known that the quality of dielectric gap layer plays an important role in plasmonic nanolaser with a metal−oxide−semiconductor structure.58 Based on our experimental results, we provide direct evidence that epitaxially grown aluminum films have low material losses and the smooth native oxide layer is critical in approaching their device performance toward those based on high-quality silver films.
CONCLUSIONS
We have demonstrated the epitaxial growth of high-quality aluminum films on c-plane sapphire wafers (ALOSA films). The measured film roughness by AFM indicates an atomically smooth surface and the XRD result shows that the aluminum films are single-crystalline. Furthermore, the high-resolution TEM images indicate that the epitaxial aluminum film grows along the [111] direction on c-plane sapphire because of a good in-plane lattice match. In terms of optical properties, the reflectivity spectra measured for as-grown aluminum films show a high reflectivity over a wide spectral range and the imaginary part of aluminum dielectric function measured by spectroscopic ellipsometry demonstrate the intrinsic material loss is lower than the literature data from UV to near-infrared region. Using the ALOSA films as a plasmonic material platform, we are able to demonstrate low-loss plasmonic devices from UV to visiblelight spectral region. The white-light SPP interferometers show clear Fabry-Pérot interference patterns and the effective indices of SPP can be fitted by simple constructive and destructive SPP interference relations. Finally, we show the lasing characteristics measured for ZnO nanowire and InGaN nanorod plasmonic lasers on epitaxial aluminum films in the UV and visible (blue and green) region, respectively. The measured lasing thresholds are as low as with those obtained on the Ag films. We attribute this to the presence of a smooth native oxide layer on top of the aluminum epitaxial film, which plays an important role in plasmonic nanolaser with a metal−oxide−semiconductor structure. 2628
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b01366. High-resolution TEM images, structural properties of epitaxial aluminum films grown on sapphire (0001) and silicon (111) substrates, optical properties of aluminum film grown on silicon (111) substrate, emission polar plots of ZnO (UV) nanowire and InGaN (blue and green) nanorod plasmonic lasers, time-resolved photoluminescence spectra, tables of literature results reported for nanowire/nanorod plasmonic lasers, group indices measured for ZnO nanowire lasers with different wire lengths, dielectric functions of epitaxial Al film measured by spectroscopic ellipsometry (PDF).
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Chih-Kang Shih: 0000-0003-2734-7023 Shangjr Gwo: 0000-0002-3013-0477 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge financial supports by the Ministry of Science and Technology (MOST) of Taiwan through Research Grant MOST-105-2112-M-007-011-MY3.
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REFERENCES
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DOI: 10.1021/acsphotonics.7b01366 ACS Photonics 2018, 5, 2624−2630
