ALD/ALE 2022 Session AA1-WeM2: ALD for Optical Applications

Photonic integrated circuits (PICs) have advanced significantly over the last decade with a view towards applications in communications, sensing, and computing in the classical and quantum domains.The fabrication of silicon PICs in particular has transitioned from university cleanrooms to foundry supported process design kits by leveraging mature fabrication techniques from the microelectronics industry.The silicon material system exhibits optical properties that are enabling for compact PICs, such as high index contrast and low optical propagation loss at telecommunications wavelengths, allowing for the demonstration of low power nonlinear optical devices in waveguides based on third order susceptibility.An approach to advance the state-of-the-art performance of nonlinear optics in silicon PICs is to pursue heterogeneous integration. A promising material for this purpose is potassium tantalate niobate (KTN).KTN is a solid solution of KTaO₃ and KNbO₃ perovskites with a Curie temperature (T_c) that is defined by the Ta:Nb ratio.Bulk KTN crystals show record high Kerr nonlinearity when thermally tuned near T_c.KTN thin films can be grown with controllable Ta:Nb ratios via atomic layer deposition (ALD) using potassium tert-butoxide (KOtBu), tantalum ethoxide (Ta(OEt)₅), niobium ethoxide (Nb(OEt)₅) and H₂O precursors.To date, deposition of KTN films on silicon substrates has not been reported.In this work, we explore several approaches for obtaining perovskite KTaO₃ and KTN thin films on silicon using KOtBu, Ta(OEt)₅, Nb(OEt)₅, and H₂O precursors. Our approaches include varying the ALD process parameters for the metalorganic and H₂O co-precursors, chemically modifying the substrate surface, deposition of interfacial layers, and implementing ALD equipment modifications.We report the K uptake as a function of ALD process parameters using both refractive index obtained from spectroscopic ellipsometry and quantification from X-ray photoelectron spectroscopy (XPS).For a subset of samples, we report the crystal structure of our RTA annealed ALD films using X-ray diffraction (XRD). For the first time, we demonstrate perovskite KTaO₃ and KTN films on silicon by ALD.

P-type Sb₂Te₃ thin film was deposited by atomic layer deposition (ALD) at 80 °C using (Et₃Si)₂Te and SbCl₃ as precursors. The good crystal quality, low defect, and excellent uniformity of low-temperature ALD processed Sb₂Te₃ was supported by Raman, transmission electron microscopy, and XPS. Furthermore, high performance self-powered broad-band photodetectors based on Sb₂Te₃/Si heterostructure were fabricated. The photodetector has a wide detection range of 405 to 1550 nm. Meanwhile, a high responsivity of 4287 mA/W at 405 nm, and a quick response speed of 98 μs (t_rise) were obtained under 0 bias voltage, which could be ascribed to the strong built in electric field between p-type Sb₂Te₃ thin film and n-type Si. The temperature dependent performance of Sb₂Te₃/Si photodetector was thoroughly examined. Resistivity, conductivity, and carrier concentration of Sb₂Te₃ were carried out at temperature ranging from 273 K to 473 K to reveal intrinsic mechanism. The Sb₂Te₃/Si heterostructure self-powered photodetector with excellent performance based on ALD process shows the great potential application in optoelectronic devices.

At present, the industrialization of metal mesh and tin doped indium oxide (ITO) film cannot meet the high transmittance requirements of infrared band. Therefore, how to achieve high carrier mobility has become a hot topic. In order to solve the above problems, In₂O₃:H film is selected, which is a special transparent conductive oxide film. On the one hand, hydrogen has a very small volume and mass, and will not produce impurity scattering in the process of electron transport; On the other hand, hydrogen is distributed in the gap of the indium oxide lattice, resulting in almost no lattice distortion, so it does not increase the number of grain boundaries and reduce the grain boundary scattering. The combined effect of these two factors makes the hydrogen doped indium oxide (In₂O₃:H) thin films have high carrier mobility, which can achieve high infrared transmittance by reducing the carrier concentration on the basis of ensuring the electrical properties.

In this paper, the growth rate, microstructure and photoelectric properties of In2O3:H films were investigated by changing the deposition process parameters. When the deposition temperature is 100 ℃, incomplete reaction occurs during deposition, and the growth rate is slow. The film is amorphous. At this time, the carrier mobility is low, which is only 30.18 cm²/Vs, and the optical properties are good, the transmittance can reach 80% at 4 μm; With the increase of deposition temperature, the growth rate gradually stabilized at 0.1nm/cycle, the amorphous state gradually changed to crystalline state, the carrier mobility increased sharply, and the highest was 64.05 cm²/Vs, and the carrier concentration and the optical properties decreased. The ratio of deposition time of InCp / (O₂ + H₂O) and the reaction time of InCp have little effect on the electrical and optical properties. With the increase of deposition period, the carrier mobility can reach up to 80 cm²/Vs, but the optical performance is very poor, the transmission rate at 4 μm is only 39.5%.

We report on the use of atomic layer deposition (ALD) for the development of metal fluoride thin films relevant to optical coatings operating at far ultraviolet wavelengths (FUV, 90-200 nm). The use of metal fluoride materials like MgF₂, AlF₃, LiF, CaF₂, and LaF₃ is relevant to all FUV optical systems for applications like anti-reflection coatings, dichroic beam splitters, bandpass filters, and reflective mirror coatings. However, many metal fluorides have associated concerns with environmental stability, primarily related to water vapor exposure. For example, LiF-protected aluminum mirror coatings can experience reflectance-losses if stored in modest relative humidity environments (RH ~40%). Initial space-based demonstrations of ALD coatings have shown improved stability by depositing a thin 1–2 nm capping layer of MgF₂ or AlF₃ onto the LiF-protected mirrors. In this work, we have conducted a broader study of the moisture sensitivity of ALD materials. This includes physical vapor deposition (PVD) of AlF₃ mirror coatings capped with ALD MgF₂ layers on Si, as well as stand-alone ALD coatings of MgF₂, AlF₃, CaF₂, and LiF on Si subjected to both long-duration and accelerated aging tests and then characterized by FUV reflectance measurements, spectroscopic ellipsometry, atomic force microscopy, x-ray photoelectron spectroscopy, x-ray diffraction, and secondary-ion mass spectrometry. To the best of our knowledge, this work also reports the first instance of CaF₂ and LaF₃ coatings deposited by ALD using anhydrous HF.

The observed changes in optical properties, surface morphology, and film composition can provide guidelines on storage conditions for these materials for future space instrumentation, and serves as a baseline for the direct comparison of material produced by PVD versus ALD.Finally, we also describe a new test reactor used in the work above that has enabled the deposition of PVD aluminum mirror coatings and ALD metal fluorides within the same vacuum chamber, to prevent oxidation of the aluminum surface and minimizing the time between metal evaporation and ALD encapsulation.