More Like this

1. Exploration of co-sputtered Ta ₂ O ₅ –ZrO ₂ thin films for gravitational-wave detectors

   https://doi.org/10.1088/1361-6382/ac1b06  

   Abernathy, M ; Amato, A ; Ananyeva, A ; Angelova, S ; Baloukas, B ; Bassiri, R ; Billingsley, G ; Birney, R ; Cagnoli, G ; Canepa, M ; et al ( September 2021 , Classical and Quantum Gravity)

   Abstract We report on the development and extensive characterization of co-sputtered tantala–zirconia (Ta 2 O 5 -ZrO 2 ) thin films, with the goal to decrease coating Brownian noise in present and future gravitational-wave detectors. We tested a variety of sputtering processes of different energies and deposition rates, and we considered the effect of different values of cation ratio η = Zr/(Zr + Ta) and of post-deposition heat treatment temperature T a on the optical and mechanical properties of the films. Co-sputtered zirconia proved to be an efficient way to frustrate crystallization in tantala thin films, allowing for a substantial increase of the maximum annealing temperature and hence for a decrease of coating mechanical loss φ c . The lowest average coating loss was observed for an ion-beam sputtered sample with η = 0.485 ± 0.004 annealed at 800 °C, yielding φ ¯ c = 1.8 × 1 0 − 4 rad. All coating samples showed cracks after annealing. Although in principle our measurements are sensitive to such defects, we found no evidence that our results were affected. The issue could be solved, at least for ion-beam sputtered coatings, by decreasing heating and cooling rates down to 7 °C hmore »−1 . While we observed as little optical absorption as in the coatings of current gravitational-wave interferometers (0.5 parts per million), further development will be needed to decrease light scattering and avoid the formation of defects upon annealing.« less
2. In-vacuum measurements of optical scatter versus annealing temperature for amorphous Ta ₂ O ₅ and TiO ₂ :Ta ₂ O ₅ thin films

   https://doi.org/10.1364/JOSAA.415665  

   Capote, Elenna M. ; Gleckl, Amy ; Guerrero, Jazlyn ; Rezac, Michael ; Wright, Robert ; Smith, Joshua R. ( March 2021 , Journal of the Optical Society of America A)

   Optical coatings formed from amorphous oxide thin films have many applications in precision measurements. The Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) and Advanced Virgo use coatings of$\mathrm{S}\mathrm{i}{\mathrm{O}}_2$(silica) and$\mathrm{T}\mathrm{i}{\mathrm{O}}_2:\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$(titania-doped tantala) and post-deposition annealing to 500°C to achieve low thermal noise and low optical absorption. Optical scattering by these coatings is a key limit to the sensitivity of the detectors. This paper describes optical scattering measurements for single-layer, ion-beam-sputtered thin films on fused silica substrates: two samples of$\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$and two of$\mathrm{T}\mathrm{i}{\mathrm{O}}_2:\mathrm{T}{\mathrm{a}}_2{\mathrm{O}}_5$. Using an imaging scatterometer at a fixed scattering angle of 12.8°, in-situ changes in the optical scatter of each sample were assessed during post-deposition annealing to 500°C in vacuum. The scatter of three of the four coated optics was observed to decrease during the annealing process, by 25–30% for tantala and up to 74% for titania-doped tantala, while the scatter frommore »the fourth sample held constant. Angle-resolved scatter measurements performed before and after vacuum annealing suggest some improvement in three of the four samples. These results demonstrate that post-deposition, high-temperature annealing of single-layer tantala and titania-doped tantala thin films in vacuum does not lead to an increase in scatter, and may actually improve their scatter.

   « less
3. Unique prospects of phase change material Sb ₂ Se ₃ for ultra-compact reconfigurable nanophotonic devices

   https://doi.org/10.1364/OME.435979  

   Jia, Wei ; Menon, Rajesh ; Sensale-Rodriguez, Berardi ( August 2021 , Optical Materials Express)

   In this work, we explore inverse designed reconfigurable digital metamaterial structures based on phase change material Sb₂Se₃for efficient and compact integrated nanophotonics. An exemplary design of a 1 × 2 optical switch consisting of a 3 µm x 3 µm pixelated domain is demonstrated. We show that: (i) direct optimization of a domain containing only Si and Sb₂Se₃pixels does not lead to a high extinction ratio between output ports in the amorphous state, which is owed to the small index contrast between Si and Sb₂Se₃in such a state. As a result, (ii) topology optimization, e.g., the addition of air pixels, is required to provide an initial asymmetry that aids the amorphous state's response. Furthermore, (iii) the combination of low loss and high refractive index change in Sb₂Se₃, which is unique among all phase change materials in the telecommunications 1550 nm band, translates into an excellent projected performance; the optimized device structure exhibits a low insertion loss (∼1.5 dB) and high extinction ratio (>18 dB) for both phase states.
4. Growth and characterization of Sc ₂ O ₃ doped Ta ₂ O ₅ thin films

   https://doi.org/10.1364/AO.59.00A106  

   Fazio, Mariana ; Yang, Le ; Markosyan, Ashot ; Bassiri, Riccardo ; Fejer, Martin M. ; Menoni, Carmen S. ( December 2019 , Applied Optics)

   We present the optical and structural characterization of films of${\mathrm{T}\mathrm{a}}_2{\mathrm{O}}_5$,${\mathrm{S}\mathrm{c}}_2{\mathrm{O}}_3$, and${\mathrm{S}\mathrm{c}}_2{\mathrm{O}}_3$doped${\mathrm{T}\mathrm{a}}_2{\mathrm{O}}_5$with a cation ratio around 0.1 grown by reactive sputtering. The addition of${\mathrm{S}\mathrm{c}}_2{\mathrm{O}}_3$as a dopant induces the formation of tantalum suboxide due to the “oxygen getter” property of scandium. The presence of tantalum suboxide greatly affects the optical properties of the coating, resulting in higher absorption loss at$\mathrm{\lambda <\#comment/>}=1064\mathrm{n}\mathrm{m}$. The refractive index and optical band gap of the mixed film do not correspond to those of a mixture of${\mathrm{T}\mathrm{a}}_2{\mathrm{O}}_5$and${\mathrm{S}\mathrm{c}}_2{\mathrm{O}}_3$, given the profound structural modifications induced by the dopant.
5. Strategies to Enhance the Capability of Carrier Injection to the Effective Channel for Bottom-gated Amorphous Oxide Thin Films Transistors

   Liu, Mingyuan ; Kim, Hyeonghun ; Song, Han Wook ; Lee, Sunghwan ( November 2021 , Materials Research Society)

   Over the two decades, amorphous oxide semiconductors (AOSs) and their thin film transistor (TFT) channel application have been intensely explored to realize high performance, transparent and flexible displays due to their high field effect mobility (μFE=5-20 cm2/Vs), visible range optical transparency, and low temperature processability (25-300 °C).[1-2] The metastable amorphous phase is to be maintained during operation by the addition of Zn and additional third cation species (e.g., Ga, Hf, or Al) as an amorphous phase stabilizer.[3-5] To limit TFT off-state currents, a thin channel layer (10-20 nm) was employed for InZnO (IZO)-based TFTs, or third cations were added to suppress carrier generations in the TFT channel. To resolve bias stress-induced instabilities in TFT performance, approaches to employ defect passivation layers or enhance channel/dielectric interfacial compatibility were demonstrated.[6-7] Metallization contact is also a dominating factor that determines the performance of TFTs. Particularly, it has been reported that high electrical contact resistance significantly sacrifices drain bias applied to the channel, which leads to undesirable power loss during TFT operation and issues for the measurement of TFT field effect mobilities. [2, 8] However, only a few reports that suggest strategies to enhance contact behaviors are available in the literature. Furthermore, the previousmore »approaches (1) require an additional fabrication complexity due to the use of additional treatments at relatively harsh conditions such as UV, plasma, or high temperatures, and (2) may lead to adverse effects on the channel material attributed to the chemical incompatibility between dissimilar materials, and exposures to harsh environments. Therefore, a simple and easy but effective buffer strategy, which does not require any additional process complexities and not sacrifice chemical compatibility, needs to be established to mitigate the contact issues and therefore achieve high performance and low power consumption AOS TFTs. The present study aims to demonstrate an approach utilizing an interfacial buffer layer, which is compositionally homogeneous to the channel to better align work functions between channel and metallization without a significant fabrication complexity and harsh treatment conditions. Photoelectron spectroscopic measurements reveal that the conducting IZO buffer, of which the work function (Φ) is 4.37 eV, relaxes a relatively large Φ difference between channel IZO (Φ=4.81 eV) and Ti (Φ=4.2-4.3 eV) metallization. The buffer is found to lower the energy barrier for charge carriers at the source to reach the effective channel region near the dielectric. In addition, the higher carrier density of the buffer and favorable chemical compatibility with the channel (compositionally the same) further contribute to a significant reduction in specific contact resistance as much as more than 2.5 orders of magnitude. The improved contact and carrier supply performance from the source to the channel lead to an enhanced field effect mobility of up to 56.49 cm2/Vs and a threshold voltage of 1.18 V, compared to 13.41 cm2/Vs and 7.44 V of IZO TFTs without a buffer. The present work is unique in that an approach to lower the potential barrier between the source and the effective channel region (located near the channel/dielectric interface, behaving similar to a buried-channel MOSFET [9]) by introducing a contact buffer layer that enhances the field effect mobility and facilitates carrier supply from the source to the effective channel region.« less