Monolithic Mid-Infrared Integrated Photonics Using Silicon-on

Jun 5, 2017 - ... platform will enable tunable mid-IR microphotonics that are desired for high-speed optical logic gates and chip-scale biochemical se...
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Monolithic Mid-Infrared Integrated Photonics Using Silicon-onEpitaxial Barium Titanate Thin Films Tiening Jin,†,‡ Leigang Li,†,§,∥ Bruce Zhang,†,§,∥ Hao-Yu Greg Lin,⊥ Haiyan Wang,*,§,∥ and Pao Tai Lin*,†,‡ †

Department of Electrical and Computer Engineering and ‡Department of Materials Science and Engineering, Texas A&M University, College Station, Texas 77843, United States § School of Electrical and Computer Engineering and ∥School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907, United States ⊥ Center for Nanoscale Systems, Harvard University, 11 Oxford Street, Cambridge, Massachusetts 02138, United States ABSTRACT: Broadband mid-infrared (mid-IR) photonic circuits that integrate silicon waveguides and epitaxial barium titanate (BTO) thin films are demonstrated using the complementary metal−oxide−semiconductor process. The epitaxial BTO thin films are grown on lanthanum aluminate (LAO) substrates by the pulsed laser deposition technique, wherein a broad infrared transmittance between λ = 2.5 and 7 μm is observed. The optical waveguiding direction is defined by the high-refractive-index amorphous Si (a-Si) ridge structure developed on the BTO layer. Our waveguides show a sharp fundamental mode over the broad mid-IR spectrum, whereas its optical field distribution between the a-Si and BTO layers can be modified by varying the height of the a-Si ridge. With the advantages of broad mid-IR transparency and the intrinsic electro-optic properties, our monolithic Si on a ferroelectric BTO platform will enable tunable mid-IR microphotonics that are desired for high-speed optical logic gates and chip-scale biochemical sensors. KEYWORDS: mid-infrared, microphotonics, barium titanate, ferroelectric thin films, pulsed laser deposition parency of up to λ = 8 μm, the strong absorption caused by silicon dioxide or sapphire after λ = 4 μm excludes Si photonics from applications such as label-free sensing utilizing mid-IR fingerprint absorption. On the other hand, ferroelectric materials such as lithium niobate (LN) and barium titanate (BaTiO3; BTO) have been developed for high-speed optical signal processing because of their exceptional intrinsic E-O property.13,14 In addition, IR transparent windows of LN and BTO extend beyond λ = 5 μm, which is wider than the windows of silicon dioxide and sapphire.15 As a result, they allow sensing of numerous gases, including carbon dioxide and nitrous oxide. For example, new platforms combining crystalline silicon and LN thin films have recently been created and applied for microdisk resonators and waveguide modulators.16−18 A high quality factor, Q, and low Vπ E-O modulation have been reported. However, challenges in LNthin-film processing remain. For example, preparation of LN thin films involves sophisticated fabrication processes, including oxygen implantation, crystal ion slicing, and wafer bonding, which prevent their widespread application in microphotonic devices. In addition, LN has a low E-O coefficient, reo, of merely 31 pm/V, which is considerably lower than that of other

1. INTRODUCTION Mid-infrared (mid-IR) microphotonics have attracted great attention because of their applications in broadband optical communication as well as label-free biochemical sensing.1−5 In an optical network, extending the present operational spectrum from the near-IR into the mid-IR range will provide additional optical channels and thus improve the data transmission rates. Meanwhile, for sensor applications, the mid-IR spectrum overlaps with the characteristic absorption bands and the fingerprint regime of numerous chemical functional groups, thereby enabling label-free biochemical detection. Several Sibased complementary metal−oxide−semiconductor (CMOS)compatible platforms have been explored and utilized for midIR microphotonics, including silicon-on-insulator, pedestal silicon configuration, silicon-rich silicon nitride, etc.6−9 Applications have also been demonstrated in various devices, including chip-scale infrared spectrometers, mid-IR directional couplers, and label-free glucose sensors.10−12 Clearly, mid-IR Si photonics has become an emerging platform for optical sensing applications. Nevertheless, the low optical nonlinearity (χ3) and absence of electro-optic (E-O) tunability (Pockets and Kerr effects) limit silicon’s application in nonlinear frequency conversion and high-speed optical signal modulation. In addition, the present Si photonics platform utilizes silicon dioxide or sapphire as the optical waveguide undercladding, both of which are mid-IR opaque. Thus, although silicon has a broad infrared trans© XXXX American Chemical Society

Received: February 28, 2017 Accepted: June 5, 2017 Published: June 5, 2017 A

DOI: 10.1021/acsami.7b02681 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

films were characterized by X-ray diffraction (XRD; PANalytical Empyrean). Next, a 1 μm thick a-Si thin film was deposited on the BTO/lanthanum aluminate (LAO) substrate by plasma-enhanced chemical vapor deposition (PECVD). The precursor gas for the a-Si deposition was SiH4, and the deposition temperature was 200 °C. The use of a-Si thin film allows for the formation of a smooth interface between the crystalline BTO layer and Si device layer. The waveguide structure was defined by photolithography, wherein a 50 nm thick Cr mask was prepared by electron beam evaporation, followed by the lift-off process. The waveguide structure was then transferred to the a-Si layer by RIE. SF6 was used for selective Si etching, as it does not react with the BTO film and therefore prevents surface roughness of BTO due to ion damage. Previous studies that utilized HF solution for etching BTO have shown nonuniform surfaces and rough waveguide edges. It is vital to have sharp Si waveguide facets as well as smooth BTO surfaces to reduce waveguide propagation loss and scattering loss. Finally, the Cr mask and organic residue on the device surface were removed using ceric ammonium nitrate solution, followed by oxygen plasma ashing process. 2.2. Optical Characterization. To characterize the performance of the a-Si-on-BTO waveguides, a broad mid-IR test station was built and is shown in Figure 2a. The light source is a pulsed laser with a

ferroelectric oxides, such as BTO thin films, with a high reo of 150 pm/V.19,20 In this work, we demonstrate a monolithic mid-IR microphotonics platform consisting of amorphous Si (a-Si) with epitaxial BTO thin films. This platform reveals the following numerous advantages: (i) integration of functional materials, (ii) compatibility with the CMOS fabrication process, and (iii) multifunctionality by repositioning the waveguide modes within different layers. More specifically, (i) the demonstrated crystalline BTO films avoid the fabrication complexity of LN thin film preparation,21,22 involving crystal slicing, high-temperature annealing, and exfoliation. In addition, BTO thin films have a broad IR transparent spectrum up to λ = 7 μm compared to that of LN crystals, which become opaque after λ = 5 μm.23 Furthermore, (ii) mid-IR waveguides can be created on the a-Si layer using the CMOS process, which prevents the difficulties of direct patterning on the chemical insert and mechanical hard BTO film. Finally, (iii) the waveguide modes can be positioned in the ferroelectric BTO layer for high-speed E-O modulation, or alternatively be shifted to align with the a-Si layer for biochemical sensing. By engineering the device structure, we demonstrated the monolithic mid-IR platform to be capable of serving various applications, including reconfigurable photonic circuits and label-free chemical detections.

2. EXPERIMENTAL SECTION 2.1. Device Fabrication. The detailed fabrication process is illustrated in Figure 1, with film deposition and waveguide fabrication.

Figure 2. (a) Mid-IR test station to characterize the performance of our a-Si-on-BTO waveguides. The probe light from a tunable pulsed laser (λ = 2.4−3.8 μm) is collimated into a mid-IR fiber using a reflective lens and then butt-coupled into the waveguide. The mid-IR signals from the waveguides are focused by a calcium fluoride biconvex lens and then imaged by an InSb camera. (b) The core of the mid-IR fiber is lined up with the front facet of the Si waveguide. The fine alignment between the optical fiber and waveguide is monitored by an upper microscope. Figure 1. Fabrication process of monolithic mid-IR microphotonics using a-Si ridge waveguides on the BTO thin film. The epitaxial BTO film is deposited on a single-crystal LaAlO3 (001) substrate by the pulsed laser deposition (PLD) technique, and an a-Si thin film is then grown on the BTO/LAO substrate by the PECVD method. Using photolithography and lift-off, the waveguide structure is first defined by a Cr mask and then transferred to the a-Si layer by reactive ion etching (RIE). Finally, the Cr mask is removed using ceric ammonium nitrate solution, followed by oxygen plasma ashing to remove the organic residue.

wavelength tunable from λ = 2.4 to 3.8 μm and a linewidth of 3 cm−1. It has a pulse repetition rate of 150 kHz, a pulse duration of 10 ns, and an average power of 150 mW. Using a reflective lens, the probe light is first collimated into a fluoride fiber that has a 9 μm core and 125 μm cladding and is then butt-coupled into the waveguide. The core of the mid-IR fiber is lined up with the smoothly cleaved front facet of the waveguide, as shown in Figure 2b. The fine alignment between the optical fiber and waveguide was monitored by an upper optical microscope equipped with a long-working-distance 10× objective lens. The mid-IR signals from the waveguides were focused by a calcium fluoride biconvex lens with a 25 mm focal length and then imaged by a liquid nitrogen-cooled 640 × 512 pixel InSb camera.

A pure BaTiO3 target was prepared using a conventional ceramic sintering method. The BTO films were deposited on single-crystal LaAlO3 (001) substrates by a KrF excimer pulsed laser (λ = 248 nm, frequency of 10 Hz; Lambda Physik). The deposition temperature was 700 °C, and the O2 pressure during deposition was kept at 40 mTorr. After the BTO deposition, the film was annealed at 600 °C with 200 Torr O2 pressure for 1 h and then cooled to room temperature. The obtained microstructure and crystallographic orientation of the BTO

3. RESULTS AND DISCUSSION 3.1. Characterization of BTO. The microstructure of the as-grown BTO thin film on an LAO (001) substrate was characterized by XRD θ−2θ scanning. As shown in Figure 3a, the BTO thin film mainly grows along the (l00) direction, as B

DOI: 10.1021/acsami.7b02681 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 5. Morphology and composition of our mid-IR devices from SEM and EDX inspection. SEM images of a 10 μm wide a-Si-on-BTO waveguide captured from (a) the top and (b) cross-sectionally. A smooth waveguide surface and sharp waveguide edges are observed. EDX mapping from the top of the waveguide using (c) Si Kα and (d) Ba Lα emission lines. Another EDX mapping along the waveguide front facet using (e) Si Kα and (f) Ba Lα. The a-Si and BTO thin films both show homogeneous compositions across the film surface and along the film depth.

Figure 3. (a) XRD θ−2θ scan of our epitaxially grown BTO thin film on an LAO (001) substrate. The dominant (l00)-type diffraction peaks indicate that the BTO film mainly grows along the (l00) direction. When the BTO film thickness increases, the film becomes polycrystalline from the minor BTO (211) peak at ∼56.0°. (b) The phi scans of BTO (101) and LAO (101) indicate that the BTO thin film has cubeon-cube growth on the LAO substrate and has good crystallinity, without in-plane rotation.

substrate has excellent in-plane alignment, without in-plane rotation. However, with an increase in the BTO film thickness, the BTO film shows another minor BTO (211) peak at ∼56.0°. This indicates that the film starts to have a secondary orientation of BTO (211) besides that of the primary (l00) ones. The infrared absorption spectrum of the as-deposited BTO thin film was measured by attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) on an instrument manufactured by Shimadzu Corp. The spectrum was collected from 1000−4000 cm−1, and the measurements were performed at room temperature. From the result plotted in Figure 4, a high transmittance is found in a broad mid-IR spectrum between 4000 cm−1 (λ = 2.5 μm) and 1430 cm−1 (λ = 7.0 μm). The minor absorption at 2400 cm−1 (λ = 4.2 μm) is due to the CO2 in the atmosphere. The increased absorption after 1430 cm−1 (λ = 7.0 μm) is due to combinations of various fundamental vibration modes existing at longer wavelengths, such as absorption bands at 500−600 cm−1 (16.6−20 μm) and 350−400 cm−1 (25.0−28.5 μm), which are attributed to the TiO stretching and bending vibrations, respectively, as previously reported.24,25 Because Si is transparent up to 8.0 μm, the integrated a-Si on the BTO platform is capable of operations up to λ = 7.0 μm, which is broader than that in the present Si on LN devices limited to λ = 5.0 μm as well as that in the Si on sapphire devices limited to λ = 4.5 μm. 3.2. Characterization of Device Morphology and Composition. The morphology of the mid-IR device stack has been examined by scanning electron microscopy (SEM). To reduce the waveguide propagation loss caused by light scattering, it is critical to minimize structure defects that may

Figure 4. Infrared absorption spectrum of our deposited BTO thin film from ATR-FTIR measurements. A high transmittance is found between 4000 cm−1 (λ = 2.5 μm) and 1430 cm−1 (λ = 7.0 μm). The increased absorption after 1430 cm−1 (λ = 7.0 μm) is due to a combination of various fundamental vibration modes existing at longer wavelengths.

indicated by the dominant (l00)-type diffraction peaks. For the primary (l00) textured domains (a axis domains), the phi scans of BTO (101) and LAO (101) displayed in Figure 3b indicate that the cube-on-cube growth of the BTO thin film on the LAO C

DOI: 10.1021/acsami.7b02681 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 6. (a) Device configuration and refractive index profile applied in our FDTD modeling. The refractive indexes, nSi, nBTO, and nLAO, are 3.4, 2.4, and 2.0, respectively. (b) The optical fields of our mid-IR waveguides are calculated at λ = 2.6, 3.0, and 3.4 μm. Fundamental modes with similar ellipsoid intensity distributions are resolved in the Si layer in all three wavelengths. (c, d) Calculated 1D intensity profiles along the y and z axes, respectively. In the y direction, a sharp Gaussian profile is found inside the a-Si ridge waveguide. On the other hand, the optical field expands extensively into the BTO layer along the z direction.

possibly be created at the waveguide’s edges or on their surfaces. Figure 5a shows the top images of 10 μm wide a-Sion-BTO waveguide. It has a well-defined ridge profile without any bending or distortion found on the edge, nor cracks or indents on the waveguide surfaces and BTO film. From the cross-sectional image shown in Figure 5b, the facets and side walls of the waveguide are sharp and also lack bumps and indentations. In addition, the clearly resolved interface between the top Si waveguide and undercladding BTO layer indicates that no depletion damage has been introduced during the fabrication process. Meanwhile, the material composition of our monolithic a-Si on a BTO platform was characterized by energy-dispersive X-ray spectroscopy (EDX) using emission lines of Si Kα at 1.74 keV and Ba Lα at 4.47 keV. The elemental spatial distributions of Si and Ba also reveal the device profiles belonging to the a-Si ridge waveguide and the BTO layer. Figure 5c,d displays the EDX mapping results from the top of the device, where a Si waveguide and its neighboring BTO film are clearly resolved. Likewise, from the cross-sectional EDX mapping shown in Figure 5e,f, the waveguide height and BTO film thickness are determined to be 1 and 0.5 μm, respectively. From Figure 5c−f, it is clear that the as-grown a-Si and BTO thin films have homogeneous compositions across the film surface as well as along the film depth. The obtained uniformity of the material compositions prevents the optical scattering loss caused by variation of the refractive indexes. 3.3. Finite-difference Time Domain (FDTD) Simulation of the Device. On the basis of the device structure shown in Figure 5, the propagating modes of our a-Si-on-BTO waveguide was calculated over the spectrum from λ = 2.4 to 3.8 μm. The

simulations were performed by the two-dimensional FDTD method. Figure 6a illustrates the device configuration and refractive index profile applied in our device modeling. The a-Si waveguide is 10 μm wide and 1 μm thick. Underneath the Si are a 0.5 μm thick BTO layer and an LAO substrate. The refractive indexes, nSi, nBTO, and nLAO, are 3.4, 2.4, and 2.0, respectively. A 12 μm × 6 μm light source was chosen to excite the waveguide mode because its size is comparable to that of the mid-IR fiber, which has a core diameter of 9 μm. Figure 6b depicts the optical field of our mid-IR waveguides calculated at λ = 2.6, 3.0, and 3.4 μm. Fundamental modes with similar ellipsoid intensity distributions are clearly resolved in the Si layer, whereas the optical field inside the BTO layer (beneath z = −1.5 μm) is gradually increased as the probe light shifts to longer wavelengths. To better analyze the mode properties, the calculated one-dimensional (1D) intensity profiles along the y and z axes are plotted in Figure 6c,d, respectively. In the y direction, the optical field shows a sharp single Gaussian profile well confined by the a-Si ridge waveguide due to the high refractive index contrast between the nSi of 3.4 and nair of 1. On the other hand, along the z direction, the optical field expands extensively into the BTO layer and increases as the wavelength increases from λ = 2.6 to 3.4 μm. The field found within the BTO layer is attributed to the relevantly high refractive index of the BTO film, nBTO = 2.4, as well as its small thickness. Hence, for different applications, we are able to manipulate the light-intensity distribution across the multilayers by adjusting the height of the a-Si and the BTO layers. For instance, in biochemical sensing, a multilayer structure is created so that the waveguide mode is located in the upper a-Si D

DOI: 10.1021/acsami.7b02681 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

the waveguide. Then, by correlating the spectral attenuation of the mode intensity with characteristic absorption spectra from various chemicals, we are able to identify the analyte composition and concentration. On the other hand, to implement E-O modulation, we adjust the multilayer structure so that the waveguide mode can extensively overlap with the ferroelectric BTO layer. The images of mode profiles displayed in Figure 7a demonstrate our methodology for varying the positions of the waveguide mode. Here, the thickness of the aSi layer, Ta‑Si, increases from 0.25 to 1.5 μm but the BTO layer remains at 0.5 μm. The mode profiles were calculated at λ = 3 μm with TM polarization, wherein the electric field is along the z direction. At Ta‑Si = 0.25 μm, most of the optical field resides within the BTO layer because the thin a-Si layer was not able to support a waveguide mode. When Ta‑Si increases to 0.5 μm, the mode shifts upward into the a-Si layer and even further into the background air, while the optical field remaining in the BTO layer significantly decreases. At Ta‑Si = 0.75 μm, the center of the waveguide mode lines up with the middle of the a-Si ridge. As a result, the evanescent fields are then observable in the top air cladding as well as in the lower BTO layer. Once Ta‑Si reaches 1 μm or higher, the optical field becomes substantially confined inside the a-Si layer. Figure 7c shows the dependence of a-Si film thickness on the confinement of the optical field within the a-Si layer. For a thin a-Si film with a Ta‑Si of 0.25 μm, only 11% of the optical field is observed within the a-Si layer, indicating that most of the optical field resides in the BTO layer. The field gradually shifts from the BTO layer to a-Si as Ta‑Si increases. When Ta‑Si reaches 0.75 μm, 65% of the optical field is already reallocated to the a-Si layer. Once Ta‑Si increases beyond 1 μm, the majority of the optical field is confined to the a-Si layer and only the evanescent field can be found in the BTO layer. To better visualize the variation in the optical field when the a-Si thickness changes, Figure 7b shows the calculated 1D intensity profiles parallel to the z direction at y = 0 μm. At Ta‑Si = 0.25 μm, the peak intensity exists in the interface between the a-Si and the BTO layers, and the field gradually decays toward the LAO substrate along the −z direction. At Ta‑Si = 0.50 μm, the mode center is positioned inside the a-Si layer, whereas the two additional intensity peaks belonging to the evanescent fields are found on the top and bottom edges of the a-Si layer. On the other hand, fundamental modes can be clearly found when Ta‑Si is 0.75 μm or thicker. The center of the waveguide mode moves along the +z direction, and the evanescent fields beyond and underneath the a-Si film decrease sharply as the aSi film becomes thicker. 3.4. Optical Characterization of the Device. The performance of the integrated a-Si film on BTO waveguides has been evaluated, and the results of the TM mode images and optical loss characterizations are shown in Figure 8. From Figure 8a, a fundamental mode is clearly seen over a broad spectral range from λ = 2.6 to 3.4 μm. The mode profiles remain the same at different wavelengths, whereas minor scattering is observed at a longer wavelength of 3.4 μm. No distortion in the captured mode images indicates that the waveguides have flat sidewall and a smooth interface between the a-Si and BTO layer. The high refractive indexes of a-Si and BTO are also attributed to the observed efficient guiding of the mid-IR lightwave. The intensity profiles of the waveguide modes are then extrapolated and are illustrated in Figure 8b. A well-resolved Gaussian profile corresponding to a fundamental mode is found over λ = 2.6−3.4 μm, and the result is consistent

Figure 7. (a) Calculated optical field at λ = 3 μm when the thickness of the a-Si layer, Ta‑Si, increases from 0.25 to 1.5 μm but the BTO layer remains at 0.5 μm. (b) The calculated 1D intensity profiles parallel to the z direction at y = 0 μm. (c) The calculated optical field confined inside the a-Si layer at different Ta‑Si. The optical field shifts upward from the BTO layer to the a-Si layer as the Ta‑Si increases.

layer, while its evanescent field extends into the external medium that eventually is absorbed by the analytes surrounding E

DOI: 10.1021/acsami.7b02681 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 8. (a) Waveguide mode images captured at λ = 2.6−3.4 μm. A fundamental mode is clearly observed over a broad spectral range. (b) The intensity profiles of the waveguide modes extrapolated along the y direction. A Gaussian profile corresponding to a fundamental mode is found over λ = 2.6−3.4 μm. (c) Mode images and (d) relative optical powers measured from the waveguides with different lengths. An optical loss of 4.2 dB/cm is obtained by fitting the mode intensity attenuation at λ = 3.0 μm.

cm at λ = 1.55 μm in the near-IR region.26−29 The low optical loss of the a-Si-on-BTO waveguide can be explained by the high mid-IR transparence of the epitaxial BTO thin film as well as the smooth interface between the a-Si and BTO layers. In addition, scattering loss caused by surface roughness significantly reduces at a longer wavelength because the Rayleigh scattering coefficient is proportional to 1/λ4. Thus, the integrated a-Si-on-BTO waveguides are experimentally demon-

with the simulated mode profiles in Figure 6c,d. In addition, the constant shape of the mode over a broad spectrum suggests a low dispersion of the waveguides. The optical loss, mode images, and optical powers from waveguides with different lengths have been measured and are displayed in Figure 8c,d. By fitting the mode-intensity attenuation, an optical loss of 4.2 dB/cm is obtained at λ = 3.0 μm, which is comparable to that in previous studies, with 3.8 dB/cm at λ = 5.2 μm and