Article pubs.acs.org/JPCC
Wideband Absorbers in the Visible with Ultrathin Plasmonic-Phase Change Material Nanogratings Weiling Dong,‡,§ Yimei Qiu,† Joel Yang,‡ Robert E. Simpson,*,‡ and Tun Cao*,†,§ †
Department of Biomedical Engineering, Dalian University of Technology, Dalian 116024, China Singapore University of Technology and Design (SUTD), 8 Somapah Road, 487372, Singapore
‡
S Supporting Information *
ABSTRACT: The narrowband surface plasmon resonance of metallic nanostructures was once thought to limit the bandwidth of absorptance, yet recent demonstrations show that it can be harnessed using mechanisms such as multiple resonances, impedance matching, and slow-light modes to create broadband absorptance. However, in the visible spectrum, realization of absorbers based on patterned plasmonic nanostructures is challenging due to strict fabrication tolerances. Here we experimentally compare two different candidates for visible light broadband high absorptance. The first candidate is planar thin film dual layers of Ge2Sb2Te5 and aluminum (Al), while the second structure employs ultrathin Al grating/Ge2Sb2Te5 dual layers. In both cases, the absorbers yield a measured absorptance greater than 78% in the visible. A remarkably high-absorptance bandwidth of 120 nm was measured and associated with the large imaginary part of Ge2Sb2Te5 dielectric function. We find that the simple dual-layer planar structure is an effective absorber in the near-infrared, but its absorptance is less effective in the visible. However, for visible wavelengths the grating structure can blueshift the absorptance peak to 422 nm. The simple geometries of the plasmonic absorbers facilitate fabrication over large areas. It has practical applications in light harvesting, sensing, and high-resolution color printing. and many more.21−24 By integrating plasmonic (meta)materials with semiconductor thin films such as black silicon,4 GaAs,5 organic semiconductors,6,7 and SiO2,15 a wideband high absorption in the visible region has been observed. Although broadband perfect absorption can be achieved in the visible using plasmonic and metamaterial designs, this scheme requires a complicated resonant element that is difficult to realize using conventional plasmonic resonators with a simple geometry, such as metallic nanorods, nanoparticles, and nanogratings. This is due to the inherently narrow-band plasmon resonances associated with these structures.15 Therefore, despite their potential for next-generation solar cell technologies, these challenges remain to be addressed. For example, the most popular plasmonic materials applied in the semiconductor thin films are noble metals such as gold (Au) and silver (Ag). Meanwhile, the majority of the nanostructures employ a bottom Au/Ag layer acting as an optical mirror to eliminate the transmittance. Moreover, the high-precision nanofabrication that is required to pattern these complicated resonant elements (i.e., crossed trapezoid) and prevent high-order diffraction is expensive and does not scale well for large area fabrication. These factors, along with the high costs of both Au and Ag, hinder the commercial applicability. Aluminum (Al), which has a plasmonic resonance in the ultraviolet−visible region, is an
1. INTRODUCTION Recently, a variety of perfect absorbers that employ a range of different mechanisms have been proposed to efficiently convert solar energy into electrical energy. For example, planar Fabry− Perot (FP) stacks can exhibit near-unity absorptance across a narrow bandwidth by means of antireflection layers.1 The thickness of the dielectric layer in the FP cavity must provide a π phase shift to obtain antireflection. This requirement increases the thickness of the stacked FP absorber thus limiting its application in flexible ultrathin solar cells. Moreover, due to the multilayered microcavity structure, the FP resonances tend to have a strong angular dependence.2 Conversely, the active solar cell layer is usually tens of nanometers thick, which reduces the light absorption and in turn impedes the photocurrent generation.3 Therefore, absorption in the thick antireflection layers can compromise the efficiency of the solar cell. To overcome this limitation, plasmonic nanostructures are required. Combinations of plasmonic nanostructures and semiconductor absorbers are emerging as good candidates for achieving nearly complete light absorption in thin films, which allow efficient carrier collection.4−6 The plasmonic nanostructures consist of arrays of subwavelength metallic resonators where the macroscopic electromagnetic properties of the surface arise from the collective response of the resonating elements.7−15 This collective response is especially interesting in the design of metamaterials (MMs), which exhibit a broad range of applications, including negative refractive index,16 optical cloaking,17 color filters,18 biosensing,19 optical lithography20 © 2016 American Chemical Society
Received: February 1, 2016 Revised: June 1, 2016 Published: June 1, 2016 12713
DOI: 10.1021/acs.jpcc.6b01080 J. Phys. Chem. C 2016, 120, 12713−12722
Article
The Journal of Physical Chemistry C
film provide absorptance wavelength selectivity. We conceive that this structure can provide increased power from Sb2Te3based thermoelectrics using visible light as the heat source. Other applications may include ultrathin photodetectors, solar cells, optical filters, high-resolution color printing, labeling, optical data storage, and biosensors.
abundant and inexpensive material compared to the noble metals. Moreover, Al is stable against oxidation, and its spectra are less sensitive to the variation of the nanostructure owing to the lower Q-factor of Al compared to Au and Ag. This increases the tolerance to critical-dimension control, making it advantageous for mass production via high-throughput lithographic processes.25 Therefore, for practical applications, it is important to investigate how the optical properties of thin film semiconductor absorbers can be tailored using simple Al plasmonic nanostructures. Recently a dual-layer vanadium dioxide (VO2) film was developed to achieve ultrathin (∼λ/65) perfect absorption in the mid-infrared (MIR) spectral region.26 This was attributed to the high loss of VO2 dielectric layer. However, the absorber is unsuitable for visible light harvesting because VO2 is transparent in the visible to near-infrared (NIR) spectral region. More recently, ultrathin, absorptive germanium (Ge) films coated on flat metal mirrors have been demonstrated. This structure exploits strong interference effects and achieves a wideband, large absorptance in the infrared (IR) region.27 However, the absorption of the structure is low in the visible region, and it requires a metallic mirror to achieve the nontrivial interface phase shifts, which is necessary for strong resonances in films that are much thinner than the wavelength of light. Hence, a highly efficient, low-cost light-trapping scheme based on very thin planar semiconductor absorbers in the visible region is highly desirable. Herein we propose wideband visible plasmonic absorbers that exploit the large imaginary part of the dielectric constant of Ge2Sb2Te5, which satisfies these conditions. We compare continuous and unpatterned planar aluminum (Al)-Ge2Sb2Te5 dual-layer structures with simple Al grating-Ge2Sb2Te5 structures, which do not require a continuous planar metal layer27,28 and are, therefore, applicable to transmissive devices. For this study, the Ge2Sb2Te5 phasechange material (PCM) was selected as the active semiconductor because its dielectric function possesses a large imaginary part (highly absorbing) over the entire visible spectrum, which is due to direct electronic transitions appearing at energies greater than the optical bandgap, EG.29 The aim of this work is to show that simple Al gratings are required to create wideband strong absorption with Ge2Sb2Te5 semiconductor film in the visible region, because planar Al/Ge2Sb2Te5 dual-layer structure is an effective absorber in the near-infrared (N-IR), but its absorptance is less effective in the visible. In contrast with the thin-film optical coating absorbers in previous literature, we incorporate Al instead of Au or Ag for the metal nanostructures to obtain full compatibility with industry standard complementary metaloxide-semiconductor (CMOS) wafer-scale processing technology. Both of our structures do not require expensive optical back reflectors such as Au, Ag, or sapphire, and they are, therefore, significantly lower in complexity and cost. Furthermore, the thin plasmonic nanostructures can exhibit low sheet resistance and high optical transparency in the visible region that can be used as transparent electrodes in photovoltaic solar cells. Notably, Sb2Te3-based materials are thermoelectrics;30 indeed, the cubic phase of Ge2Sb2Te5, which is discussed here, exhibits a Seebeck coefficient of ∼40 μV/K at room temperature.31 We envisage that the proposed Al grating-Ge2Sb2Te5 structure also paves a new route toward low-cost and ultrathin thermoelectric devices that can be heated using visible light. The measured absorptance of a 100 nm thick bare Ge2Sb2Te5 film across the visible spectrum is ∼50%. We show that plasmonic structures consisting of Al gratings on top of a Ge2Sb2Te5 thin
2. METHODS 2.1. Fabrication of Al/Ge2Sb2Te5 Dual-Layer Planar Structures. Aluminum (60 nm thick) was evaporated onto a Si (111) substrate in an e-beam evaporater. The Ge2Sb2Te5 layer was subsequently sputter-deposited on top of the Al with various thicknesses. Radiofrequency (RF) sputtering was performed from a 99.99% pure, 2 in. diameter, Ge2Sb2Te5 alloy target at an RF power of 30 W. The thickness of the Ge2Sb2Te5 was determined by the calibrated deposition time and confirmed by AFM measurements. 2.2. Fabrication of Nanogratings. Fused silica substrates were first coated with 100 nm thick Ge2Sb2Te5 film using RF sputtering. The chamber base pressure was 2.6 × 10−5 Pa. The Ge2Sb2Te5 was RF sputter-deposited in an Ar atmosphere at a pressure of 0.5 Pa. The deposition rate was 0.9 Å/s (30 W, 1005 s) from a Ge2Sb2Te5 sputtering target of diameter of 50.8 mm (2 in) and purity 99.99%. The target substrate separation was 140 mm. Subsequently, 5 nm thick layer of Si3N4 was deposited onto the Ge2Sb2Te5 film as buffer layer at a pressure of 0.5 Pa. The deposition rate was 0.16 Å/s from a Si target of purity of 99.99%. The Si3N4 composition was achieved by reactive sputtering in an Ar:N2 atmosphere of 8:2. Thereafter, we spun PMMA 950 K A4 electron beam resist at 4000 rpm and baked it at 180 °C for 2 min. E-beam lithography (EBL) was used to pattern the resist. The EBL-defined patterns were developed by a methyl isobutyl ketone/isopropyl alcohol (MIBK-IPA) 1:3 solution for 30 s. Finally, a Ti layer of 5 nm and an Al layer of 65 nm thickness were deposited on the patterned substrates using a e-beam evaporater. A lift-off process was performed using 65 °C using 1-methyl-2pyrrolidinone (NMP), followed by isopropyl alcohol and a deionized water rinse. 2.3. Optical Measurements. Spectral reflectivity measurements of the Al/GST planar were performed using a PerkinElmer lambda 750 dual beam spectrometer while the Al/Ge2Sb2Te5 nanogratings were measured using a QDI 2010 UV−visible-NIR range microspectrophotometer (CRAIC Technology Inc., San Dimas, CA). For both sets of measurements, the incident and collected light were at normal incidence to the samples. For the measurements of the nanogratings, the electric field was linearly polarized light and the sample was rotated to achieve both TE and TM polarization relative to the grating direction. The spectra of the Ge2Sb2Te5 thin films were normalized to an Al mirror, which was considered 100% reflective across the measured spectral range. Optical micrographs were obtained using an Olympus BX51 using a X20 lens with a numerical aperture of 0.45. 2.4. FDTD Simulations. Full 3D numerical simulations were performed using Lumerical Solutions, a commercially available finite difference time-domain (FDTD) simulation software package. A unit cell of the Al/Ge2Sb2Te5 gratings array was simulated using periodic boundary conditions along the x-axis, and perfectly matched layers along the propagation of the incident light (z-axis). The Al wires were assumed to be infinitely long along the y-axis. The structure was illuminated with normal incident plane waves along z direction, and reflectance was monitored with a power monitor placed behind the radiation 12714
DOI: 10.1021/acs.jpcc.6b01080 J. Phys. Chem. C 2016, 120, 12713−12722
Article
The Journal of Physical Chemistry C
Figure 1. (a) Schematic of an amorphous Ge2Sb2Te5/Al dual-layer system subsequently deposited on a silica substrate. (b) Measured absorptance spectra for the amorphous Ge2Sb2Te5/Al dual-layer structure with different Tamph. (c) Schematic of a crystalline Ge2Sb2Te5/Al dual-layer system subsequently deposited on a silica substrate. (d) Measured absorptance spectra for the crystalline Ge2Sb2Te5/Al dual-layer structure with different Tamph.
increased from 14 to 79 nm. For the thinner amorphous films, the resonances are in the visible region, Tamph = 14, 19, and 24 nm, but the absoptance less than perfect (
