Microstructure , optical properties , and optical resonators of Hf 1-xTixO 2 amorphous thin films

We report Hf1-xTixO2 (0< = x< = 1) thin films (HTO) as index tunable and highly transparent materials for ultraviolet to near infrared integrated photonic devices. By varying the Ti concentration, reactive cosputtered HTO thin films on thermal oxidized SiO2 on Si substrates show continuously tunable optical band gaps from 3.9 eV to larger than 5 eV. The film refractive index monotonically increases with Ti concentration, varying from 1.8 to 2.4 in the visible to near infrared wavelength range. Micro-disk amorphous HfO2 resonators on SiO2/Si substrates are fabricated using sputtering and lift-off method. A loaded quality factor of ~15800 at around 1580 nm wavelength is achieved in HfO2 disk resonators with diameters of 100 μm. The propagation loss of the HfO2 ridge waveguide is estimated to be 2.5 cm. The wide optical transparency range, variable index of refraction, low temperature, CMOS-compatible fabrication capability, and high optical transparency make amorphous HTO thin films promising candicates for integrated photonic applications. ©2016 Optical Society of America OCIS codes: (310.0310) Thin films; (310.6860) Thin films, optical properties; (230.0230) Optical devices; (230.5750) Resonators. References and links 1. B. Jalali and S. Fathpour, “Silicon photonics,” J. Lightwave Technol. 24(12), 1400–1415 (2006). 2. J. T. Choy, J. D. B. Bradley, P. B. Deotare, I. B. Burgess, C. C. Evans, E. Mazur, and M. Lončar, “Integrated TiO2 resonators for visible photonics,” Opt. Lett. 37(4), 539–541 (2012). 3. C. C. Evans, K. Shtyrkova, J. D. B. Bradley, O. Reshef, E. Ippen, and E. Mazur, “Spectral broadening in anatase titanium dioxide waveguides at telecommunication and near-visible wavelengths,” Opt. Express 21(15), 18582– 18591 (2013). 4. T. H. Loh, Q. Wang, K. T. Ng, Y. C. Lai, and S. 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Introduction
The fast development of integrated photonics enables application of optical materials and devices in a variety of emerging fields, such as optical telecommunication, data communication, biomedical sensing, quantum photonics and nonlinear photonics [1][2][3][4].For different applications, novel photonic materials with unique optical properties, such as wide transparency window, biocompatibility, low two-photon absorption (TPA) or multiphoton absorption processes are required [2,3], which cannot be fully achieved in existing silicon photonic materials such as Si, SiO 2 or Si 3 N 4 .On the other hand, optical thin film materials with different refractive index are highly desired for photonic integrated circuits, to function as key materials for optical waveguiding or cladding [5], graded index coupling [6], or index matching materials with functional photonic materials.In silicon photonics, the available optical material index range are 1.45-1.89(SiO x N y ) and 3.45-4.0(Si 1-x Ge x ) in the near infrared.A large index gap exists between 1.89 to 3.45 with no CMOS compatible material candidates, where a variety of functional photonic materials can be found, such as magnetooptcial materials Y 3 Fe5O 12 /Ce x Y 3-x Fe 5 O 12 (n~2.2) [7], electro-optic materials LiNbO 3 (n = 2.2) and BaTiO3 (n = 2.38) etc [8].Therefore, developing novel integrated photonic materials have garnered great interest in recent years.
Transparent metal oxide thin film materials, including ZrO 2 , TiO 2 and Ta 2 O 5 are studied in recent years for the above mentioned purposes [9,10,33].These CMOS-compatible materials known as high-K oxides in silicon microelectronics, are highly transparent in the visible to near infrared wavelength range.For example, crystalline TiO 2 features a bandgap of 3.2 eV [11], and high index of refraction of 2.4 at visible wavelengths [12].Amorphous TiO 2 waveguides with low propagation loss of 7.5 dB/cm around 633 nm and 1.2 dB/cm around 1550 nm have been achieved, while the propagation loss of anatase TiO 2 is 5.8 dB/cm at 1550 nm wavelength [13].Ring resonators with quality factor ranging from 2 × 10 4 to 1 × 10 5 have been demonstrated in previous studies [3,4,[14][15][16][17][18]. For Ta 2 O 5 , ring resonators have been reported with a Q value of 3.8 × 10 4 , ring loss of 2.1 cm −1 and material index of 2.1 in the near infared [19].These previous studies demonstrates the promising potential of metal oxides for integrated photonic applications.
Hafnium dioxide, a high-K oxide and novel waveguide material with optical band gap of 5.5 eV [20], is a good candidate for integrated photonic applications.It is transparent from ultraviolet to mid infrared wavelengths [21] showing refractive indices around 1.8 in the NIR [22].Moreover, HfO 2 can be alloyed to TiO 2 to form a full range solid solution of Hf 1-x Ti x O 2 (HTO) using CMOS compatible processes.Considering the index of TiO 2 is 2.4, this material may be a good candidate to fill the index gap from 1.89 to 2.4 range of silicon photonics.Several previous studies investigated the optical properties of this material for narrow concentration ranges [23,24].A further study on the full range index tunability, optical transparency and device integration is of interest for integrated photonic applications.
In this study, we report research on the structure and optical properties of Hf 1-x Ti x O 2 (HTO) thin films with 0 1 x ≤ ≤ .We demonstrate that high index and low loss amorphous HTO thin films with different Ti concentrations can be achieved by reactive co-sputtering.The material structure, morphology, valence state, optical band gap and refractive indices are systematically studied.An amorphous HfO 2 disk resonator is also fabricated using ebeam lithography and lift-off methods.High loaded quality factor of 1.58 × 10 4 and low optical loss of 2.5 cm −1 have been observed in the resonator device at around 1580 nm wavelength, demonstrating the potential application of these materials in low loss visible and NIR integrated photonic devices.

Experimental details
Hf 1-x Ti x O 2 (HTO) thin films were deposited by reactive co-sputtering.HfO 2 ceramic target (purity>99.95%)and Ti metal target (purity>99.95%)were used for radio frequency (RF) and direct current (DC) sputtering targets respectively.By varying the relative power of the two guns, a variety of Ti concentrations were obtained.Before deposition, the chamber was pumped to a background vacuum of 5 × 10 −4 Pa.Ar and O 2 mixture gas with 10% oxygen concentration was fed into the chamber for reactive sputtering, while oxygen concentration ranging from 10% to 60% was used for pure HfO 2 deposition.During the deposition process, the chamber was kept under a constant pressure of 0.5 Pa.Silicon, thermal oxidized SiO 2 (2 μm) on silicon or glass substrates were used for HTO deposition.The substrates were kept at room temperature.The sputtering power was kept below 100 W for both sputtering guns, with a deposition rate of 0.5~1.5 nm/min.
Phase identification was carried out using X-ray diffraction (XRD) on a Shimazu XRD-7000 X-ray diffractometer.Film thicknesses were characterized by a JEOL7600F field emission scanning electron microscopy (SEM).Atomic concentrations of Hf and Ti were analyzed using energy dispersive spectroscopy (EDS) on SEM.The resulting films showed Ti atomic concentrations of 0%, 19%, 38%, 64%, 78% and 100% in the HTO films.The valence state of Ti and Hf were measured by X-ray photoluminescence spectroscopy (XPS).Optical transmittance of thin films on glass substrates were measured using a Perkin-Elmer LAMBDA 750 UV/VIS/NIR spectrometer with an integrating sphere in the wavelength range from 200 nm to 2500 nm.The surface morphology was investigated by using atomic force microscopy (AFM).The film roughness was demonstrated by the root mean square value (RMS) in a 2 μm × 2 μm area.A Cauchy model was used to fit the thin film refractive index measured by spectroscopic ellipsometry (SE).
HfO 2 ridge waveguides and micro-disk resonators were fabricated by RF sputtering and lift-off method.The waveguide and resonator patterns were firstly defined by e-beam lithography (EBL) using a double layer resist of ZEP on PMGI SF9.In this process, PMGI resist was first spun on the substrate and baked at 180 °C for one minute to form a ~700 nm thick film.ZEP 520A electron beam resist is subsequently coated on top with a spin rate of 3000 rpm and thickness of 370 nm, and baked at the same temperature.Exposure of ZEP 520A resist contained a beam current of 10 nA at an aperture of 140 μm and a dose of 400 μC/cm 2 .After EBL completes, patterns were revealed by developing the resist in ZED-N50 for 1 min, and rinsed by MIBK and IPA.To achieve sufficient undercut to allow lift off, windows were opened in PMGI by developing in RD6 for 20 s. 200 nm thick HfO 2 thin films were subsequently deposited on the EBL patterns.A lift-off process was carried out by ultrasonic rinsing the samples in acetone to form the waveguide and resonator devices.Then another 200 nm thick HfO 2 thin film was deposited after the lift-off process to form a single mode ridge waveguide structure.Finally all the waveguides were cleaved to form facets on both sides for propagation test.

Structure, morphology and valence states of Hf 1-x Ti x O 2 thin films
Figure 1(a) and 1(b) show the XRD spectra of Hf 1-x Ti x O 2 films.Figure 1(a) shows the XRD spectrum of pure HfO 2 thin films deposited on silicon at various oxygen partial pressures with 100 W deposition power.The diffraction peaks located at around 28°, 34° of 2θ correspond to the (111) and (200) crystal planes of monoclinic phase HfO 2 [25].The film structrure gradually changed from nanocrystalline to amorphous with decreasing the oxygen volume concentration from 66% to 10%, consistent with previous studies [26].Next, the oxygen volume concentration is fixed at 10%, whereas Ti concentration is increased by increasing the Ti sputtering power in HTO films.Amorphous HTO films are achieved as shown in Fig. 1(b), except for pure TiO 2 films, which shows emergence of nanocrystalline mixed anatase and rutile phases.No obvious diffractions from the crystalline HfTiO 4 phase was observed, which is a phase observed in high temperature deposited HTO films with Ti: Hf ratio around 50%: 50% [27].Surface roughness of these HTO films are analyzed by AFM, as shown in Fig. 1(c) and 1(d).These scans are taken from an area of 2 μm × 2 μm of a Hf 0.81 Ti 0.19 O 2 and TiO 2 thin film respectively.All HTO films with Ti concentrations ranging from 0 to 80% show low surface RMS roughness of less than 1 nm (data not shown).The pure TiO 2 films show RMS surface roughnesses of 4.2 nm due to the formation of nanocrystals, which is consistent with the XRD observations.
The valence states of Hf 1-x Ti x O 2 films are investigated using XPS.Full spectrum scan on the film surface indicates the existence of Hf, Ti, O, and C without obvious contaminations.High resolution scans were carried out around the 4f and 2p core level energies of Hf and Ti ions respectively, as shown in Fig. 1(e) and 1(f).For all film concentrations, Hf 4f 7/2 and Hf 4f 5/2 spectrum are centered at 16.3 eV and 17.9 eV respectively, matching with Hf 4+ valence state [28].Ti 2p 3/2 and Ti 2p 1/2 are observed at 457.9 eV and 463.6 eV respectively, matching with the Ti 4+ valence states [28].These observations excluded the presence of Ti 3+ or Hf 3+ ions.Minimal peak shift of the Hf 4f or Ti 2p core levels are observed for all the compositions studied, indicating a solid solution behavior of the HTO alloy system without short range ordering of compound (such as HfTiO 4 ) formation [28].

Optical properties of Hf 1-x Ti x O 2 thin films
The optical band gap of HTO films grown on glass substrates are characterized by UV-Vis spectroscopy.Figure 2(a) shows the optical transmission spectrum of HTO thin films in the wavelength range of 250 nm to 2000 nm.All HTO films show high optical transparency of more than 80% in the visible to NIR.The optical band gaps of HTO films are estimated from the band edge absorption using the Tauc plot method [29].Equation (1) and Eq. ( 2) are used to calculate the band gap energies, E g , where hv is the incident photon energy, α is the absorption coefficient, T is the transmittance, R is the reflectance, t is the film thickness and B is a constant [30].Figure 2(b) shows the Tauc plots for HTO films, displayed as (αhv) 2 as a function of hv for the incident light.The optical band gap energies can be calculate by extrapolating the linear portion of the curve to the energy (hv) axis, which are determined to be 3.9 eV to 4.35 eV for Hf concentrations ranging from 0 to 80% as shown in the inset of Fig. 2(b).For pure HfO 2 films, the absorption edge is beyond the test range of our tool, and the optical band gap is larger than 5 eV.
(2) Spectroscopic ellipsometry (SE) measurements are performed in the visible wavelengths range of 300 nm to 800 nm, at 70° angles of incidence.ψ and Δ parameters, defined by Eq. ( 3), are measured as functions of the incident wavelength [31], where R P and R S are the complex reflection coefficients for the light polarized parallel (p) and perpendicular (s) to the plane of incidence, respectively.tan exp( Δ) The model structure used to fit the SE data for all HTO thin films is: air/HTO film/SiO 2 /Si.The optical constants of the crystalline silicon substrate and silicon dioxide (SiO 2 ) over layer are taken from the literature.Since the HTO thin films are optically transparent in the visible to near infrared spectral region, the Cauchy dispersion model is applied [32].In this model, the refractiveindex n λ and the extinction coefficient k λ as a function of the wavelength are given by Eq. ( 4) and Eq. ( 5), where A, B, C, D, E, and F and the thickness of the film (d HTO ) are used as fitting parameters.. .
E F k D Figure 2(c) and 2(d) shows the measured and model fitted SE data of the amorphous HfO 2 thin film sputtered onto a SiO 2 (2 μm)/Si substrate.The blue lines represent the experimental data, while the green lines represent the fitted data using Cauchy equations, which agree very well with the experimental spectra.Figures 2(e) and 2(f) show the fitted index of refraction of HfO 2 thin films fabricated with different oxygen partial pressures, and the HTO films respectively.Lower refractive index is observed in armorphous HfO 2 compared to crystallized HfO 2 films, agreeing with previous reports [29].The index dispersion curve of HTO films show similar trends in the visible to near infrared.As shown in Fig. 2(f) and the inset, the index of HTO films can be continuously tuned from 1.81 to 2.26 at 800 nm wavelength.(For TE polarized mode, similar transmittance floor line is observed, while the resonance is much weaker for this device possibly due to less efficient coupling of light into the resonantor.)Critical coupling is achieved at around 1580 nm wavelength for TM polarized light.The full width at half maximum (FWHM) of the resonance peak is measured as 100 pm, as shown in Fig. 4(d).A loaded quality factor (Qfactor) is therefore estimated as 1.58 × 10 4 at around critical coupling wavelength of 1580.2 nm.The intrinsic quality factor Q in is calculated to be 3.1 × 10 4 using Eq. ( 6), where the transmittance at resonance, T 0 , is determined as 0.2.The diameter of the micro-disk resonator is 100 μm.The free spectral range (FSR), defined as the spacing between adjacent resonant wavelengths, is 4.1 nm.The resonator propagation loss (α) is therefore calculated to be 2.5 cm −1 according to Eq. ( 7) and Eq. ( 8),where n g is the group index, λ r denotes the resonant wavelength and L is the circumference of the resonator .This low propagation loss of amorphous HfO 2 demonstrates the promising potential of HTO thin films for integrated photonic applications.

Conclusion
In summary, Hf 1-x TixO 2 (0 ≤ x ≤ 1) thin films were deposited by reactive co-sputtering at room temperature.The HTO films show crystalline to amorphous phase transition as the oxygen partial pressure in the spuettering gas decreases.With increasing Ti concentration, the films exhibit continuously tunable optical band gaps from larger than 5.0 eV to 3.9 eV, increaseing index of refraction from 1.8 to 2.26, and consistently high optical transparency in the visible to near infrared wavelength range.Low loss amorphous HfO 2 disk resonators are demonstrated using sputtering and lift-off methods.A loaded quality factor of 1.58 × 10 4 and optical loss coefficient of 2.5 cm −1 are achieved at around 1580 nm wavelength.The wide optical transparency window, variable refractive index, low loss, low temperature fabrication capability and proven CMOS compatibility make HTO thin films highly attractive for integrated photonic applications.

Fig. 1 .
Fig. 1.(a) XRD spectrum of ZrO 2 films deposited under different oxygen partial pressures (b) XRD spectrum of Hf 1-x Ti x O 2 thin films (c) AFM image of TiO 2 -doped HfO 2 films.(d) AFM image of TiO 2 films.Binding energy of (e) Hf 4f and (f) Ti 2p of Hf 1-x Ti x O 2 thin films with different Ti concentrations.

Fig. 2 .
Fig. 2. (a)(b)Optical transparency of ZTO thin films deposited on glass substrates.Measured and fitted ellipsometric parameters, (c) Ψ and (d) Δ, for HTO thin film.Optical constants of (e) pure HfO2 and (f) HTO films.films of different oxygen partial pressure,3.3Amorphous HfO 2 disk resonatorsFigure 3(a) and 3(b) shows the cross-sectional SEM and AFM image of amorphous HfO 2 disk resonators fabricated using lift-off method.The SEM image shows the ridge HfO 2 resonator with 0.7 μm ridge width, 0.25 μm slab height and 0.21 μm ridge height.The AFM image shows smooth waveguide sidewalls, the RMS roughness of top surface is 3.7 nm, which is larger than the pure HfO 2 thin films possibly due to the fabrication process.The modal profile of the rigde waveguide is simulated using finite element analysis method (FEM) by applying COMSOL Multiphysics commercial software.The waveguide shows single mode

Fig. 3 .
Fig. 3. (a) Cross-sectional SEM image of an amorphous HfO 2 disk resonator (b) AFM image of the amorphous HfO 2 rigde waveguide (c) E x field of the fundamental TM mode simulated in a single mode HfO 2 ridge waveguide at 1550 nm wavelength.

Figures 4 (
Figures 4(a) and 4(b) show the optical microscope and SEM images of HfO 2 disk resonators respectively.Transmittance of the resonator device at around 1550 nm wavelength is characterized based on butt-coupling configuration using a NIR tunable laser and lens tipped fibers.Clear optical resonances are observed at the NIR wavelength range as shown in Fig.4(c) for the TM polarized mode.(For TE polarized mode, similar transmittance floor line is observed, while the resonance is much weaker for this device possibly due to less efficient coupling of light into the resonantor.)Critical coupling is achieved at around 1580 nm wavelength for TM polarized light.The full width at half maximum (FWHM) of the resonance peak is measured as 100 pm, as shown in Fig.4(d).A loaded quality factor (Qfactor) is therefore estimated as 1.58 × 10 4 at around critical coupling wavelength of 1580.2 nm.The intrinsic quality factor Q in is calculated to be 3.1 × 10 4 using Eq.(6), where the transmittance at resonance, T 0 , is determined as 0.2.The diameter of the micro-disk resonator is 100 μm.The free spectral range (FSR), defined as the spacing between adjacent resonant wavelengths, is 4.1 nm.The resonator propagation loss (α) is therefore calculated to be 2.5 cm −1 according to Eq. (7) and Eq.(8),where n g is the group index, λ r denotes the resonant wavelength and L is the circumference of the resonator .This low propagation loss of amorphous HfO 2 demonstrates the promising potential of HTO thin films for integrated photonic applications.

Fig. 4 .
Fig. 4. (a) Optical microscope image and (b) SEM image of HfO 2 micro-disk resonators (c) Transmission spectrum of a HfO 2 micro-disk resonator for the fundamental TM mode (d) The enlarged optical resonance peak near critical coupling at around 1580.2 nm wavelength.