This PDF file contains the front matter associated with SPIE Proceedings Volume 11281, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

We have used first-principles modeling, based on advanced hybrid functional calculations within density functional theory, to accurately predict properties of point defects and impurities in Ga2O3 and related materials. Point defects act as compensating centers, but they also assist diffusion of impurities. Accurate knowledge of formation energies and migration barriers then allows determining the doping profile. These results provide guidance for incorporating Ga2O3 into devices. Work performed in collaboration with J. Hwang, A. Janotti, J. L. Lyons, H. Peelaers, and J. B. Varley.

In this paper, we present an experimental study of hydrogen (H) and deuterium (D) in single crystal and polycrystalline b-Ga2O3. H or D was introduced into the specimens using ampoule passivation at elevated temperatures. The samples were characterized with infrared absorption and gas effusion measurements. As-grown single crystal b-Ga2O3 contains a residual H concentration of about 1019 cm-3 and exhibits a local vibrational mode at 3437.6 cm-1 that has been attributed to O – H. Polycrystalline b-Ga2O3 thin-films were grown by pulsed laser deposition. Hydrogen effusion measurements show that these samples contain H concentrations of up to about 1020 cm-3. From the hydrogen effusion spectra, the H chemical potential was determined as a function of the H concentration, which can be related to the H density-of-states distribution.

Recent developments in growing highly n-doped wide bandgap oxides like β-Ga2O3 and more recently ZnGa2O4 hint to important applications, such as transparent electrodes and ohmic contacts. Attempts to interpret the phonon contribution to the measured magnetoconductivity raise fundamental questions on the interplay between the large number of phonon modes in these lattices, electron-phonon scattering, and lattice disorder. Here, we present density functional theory (DFT) modeling of electron-phonon scattering for a wide range of n-carrier concentrations and temperatures in β-Ga2O3 and ZnGa2O4 . The calculated coupling strengths are corroborated with zone-unfolded phonon spectra extrapolated from a series of simulation supercells of different sizes and with calculated dielectric functions. The results provide a first-principles understanding of dominant low-field mobility features suggested by phenomenological models used traditionally for semiconductors with high lattice symmetry.

P-type doping of wide-band-gap oxides such as In2O3 and Ga2O3 would open vast opportunities in device in device design, ranging from transparent contacts for solar cells to high-power transistors. In this presentation we discuss the fundamental difficulties concerning p-type doping in these materials and opportunities offered by forming dilute alloys, with minimum disturbance in structural parameters yet bringing beneficial changes to their electronic structure.

Ga2O3 is a wide-band-gap oxide, with promising properties suitable for high-power devices and UV photodetectors. The latter requires combining transparency with conductivity, which are properties usually not occurring together. We use first-principles calculations to accurately define the fundamental transparency limits of Ga2O3, by considering both indirect and direct free-carrier absorption. These results also shed light on recent experimental observations. We will also discuss absorption in In2O3, as Sn-doped In2O3 (ITO) is the most widely used transparent conducting oxide, and explain what makes it such a good material. Work performed in collaboration with E. Kioupakis and C.G. Van de Walle.

Point defects are at the heart of the important properties of wide band-gap and oxide semiconductors for power electronics applications, and therefore understanding the details of point defects and their role in determining the properties becomes imperative. Beta-Ga2O3 has received significant attention recently due to its unique advantages, including high breakdown voltage and availability as bulk substrates, which make it a viable candidate for next-generation power device applications. Here we present the first direct microscopic observation of the formation of interstitial-divacancy complexes within beta-Ga2O3 lattice using atomic resolution scanning transmission electron microscopy. We directly observed that cation atoms are present in multiple interstitial sites, and each interstitial atom is paired with two adjacent vacancies. The observed structure of the complexes is consistent with the calculation using density functional theory (DFT), which predicts them to be compensating acceptors. The number of the observed complexes increase as a function of Sn doping concentration, which matches with the increase in the concentration of the trap state at Ec - 2.1 eV measured using deep level optical spectroscopy, which strongly suggests that the defects corresponds to that trap level. Our finding provides new crucial information on the exact origin of the properties of beta-Ga2O3 that has been unobtainable using other methods. The results also provide new important insight on the material’s unique response to the impurity incorporation that can impact their properties, which can ultimately guide the development of growth and doping of new-generation materials for power electronics.

β-Ga2O3 represents the emerging ultrawide bandgap semiconductor material, having a bandgap of 4.5-4.9 eV and estimated breakdown field of 6-8 MV/cm, promising for power electronic devices and solar blind photodetectors. High quality material growth and fundamental understanding of its ternary alloy (AlxGa1-x)2O3 are still lacking. Metalorganic chemical vapor deposition (MOCVD) growth of β-Ga2O3 and (AlxGa1-x)2O3 are studied with the focus on understanding the effects of growth conditions on the material quality, control of background and n-type doping, as well as transport properties. Record high room temperature and low temperature mobilities have been achieved in MOCVD β-Ga2O3.

Recently, immense interest in the semiconductor Ga₂O₃ arose due to its large bandgap and high predicted electrical breakdown field. By alloying Ga₂O₃ with In₂O₃ or Al₂O₃, its bandgap can be tuned over a large range, allowing possible applications in heterostructure devices or devices with an adjusted absorption energy. For this purpose, property screening over a large composition range is crucial. In this contribution, we present electrical, optical and structural properties of (Ga,In)₂O₃ and (Ga,Al)₂O₃ thin films grown by continuous composition spread pulsed laser deposition. The influence of growth parameters on phase boundaries were investigated and unipolar devices were fabricated.

Gallium oxide (Ga2O3) is positively researched as one of the ultra-wide-bandgap semiconductor materials which are expected to realize cost-effective power devices. To demonstrate device performances, many efforts have been paid on the investigation of crystal growth methods to prepare high-quality drift layers. Among them, halide vapor phase epitaxy (HVPE) has advanced as a capable growth method for n-type conductivity-controlled β-Ga2O3 homoepitaxial layers with a wide range by Si doping. Recently, the fabrication of SBDs and FETs using the β-Ga2O3 homoepitaxial wafers have been reported by many research groups. In our group, the HVPE growth of Ga2O3 and In2O3 was investigated in an atmospheric pressure system based on thermodynamic analyses, using group-III monochlorides (GaCl and InCl) and oxygen (O2) as precursors and nitrogen (N2) carrier gas. It was found that high-purity single-crystal layers can be grown at around 1000°C. The growth rate was found to be controlled by the input partial pressure of group-III monochloride and reach above 10 μm/h. In the homoepitaxy on β-Ga2O3(001) substrates, the n-type carrier density in the range 1E15 - 1E18 cm-3 was achieved. For the layer with the carrier density of 3E15 cm-3, the highest room-temperature mobility of 149 cm2/Vs was confirmed. In the heteroepitaxy of c-In2O3(111) on sapphire (0001) substrates, the lowest n-type carrier density of 2.2E16 cm-3 with relatively high mobility of 235 cm2/Vs was achieved. These results indicate that HVPE-grown single-crystal sesquioxides can be applicable to the fabrication of power devices.

"HVPE growth of [beta]-Ga2O3 films for devices on bulk and thermally enhanced [beta]-Ga2O3 composite substrates" was recorded at Photonics West 2020 in San Francisco, California.

Thin polymorphic gallium oxide films were grown on c-plane sapphire using pulsed laser deposition. The stacked thin films (ε-G_a2O₃ and β-G_a2O₃) were sequentially grown under the same conditions but in a different ambience. Our X-ray diffraction measurements and transmission electron microscopy images confirmed a β-G_a2O₃/ε-G_a2O₃ polymorphic heterostructure with rocking-curve widths of 1.4° (β-G_a2O₃ (¯603)) and 0.6° (ε-G_a2O₃ (006)). The crystallographic orientation relationships between c-plane sapphire and the heterogeneously nucleated ε-G_a2O₃ buffer layer, as well as between the ε-G_a2O₃ and β-G_a2O₃ heterogeneous layers, were determined. Our study will aid in developing novel deep-ultraviolet optoelectronic devices, such as solar-blind and metal-insulator-semiconductor deep-ultraviolet photodiodes.

More than 30% of electrical energy passes through power electronics today with speculation that in the next decade this could grow to 80%. The wide bandgap semiconductor market is already approaching $1 billon USD in 2019 and is projected to be almost $7 billion USD in 2028. Even with its high cost, SiC, is starting to dislodge the incumbent Si technology in some applications, such as hybrid and electric vehicles, due to smaller size, and higher efficiency. We review and report the IHS Markit’s market predictions for wide bandgap semiconductor technologies, and highlight the technoeconomic analysis results for the manufacturing cost of Ga₂O₃ wafers. Specifically, we focus on the potential for Ga₂O₃ to be more economically advantageous than SiC using current manufacturing methods and then identify opportunities where research can further reduce the volume cost of Ga₂O₃ wafers.

There are continuing rapid developments in vertical geometry Ga₂O₃ for high voltage switching applications. Ga₂O₃ is emerging as a viable candidate for certain classes of power electronics with capabilities beyond existing technologies due to its large bandgap, controllable doping and the availability of large diameter, relatively inexpensive substrates. These include power conditioning systems, including pulsed power for avionics and electric ships, solid-state drivers for heavy electric motors and advanced power management and control electronics. There are already cases where the performance exceeds the theoretical values for SiC. Existing Si, SiC (vertical devices), and heteroepitaxial GaN (lateral devices) enjoy tremendous advantages in terms of process maturity, an advantage that is especially true for Si, where the ability to precisely process the material has resulted in devices such as super-junctions that surpass the unipolar “limit”. Continued development of low defect substrates, optimized epi growth and surface treatments and improved device design and processing methods for Ga₂O₃ are still required to push the experimental results closer to their theoretical values. Even 3 μm epi layers with doping concentration of 1016 cm-3 should have a theoretical breakdown voltage of ~1800V. The actual experimental value of VB is currently well below the theoretical predictions. Thermal management is a key issue in Ga₂O₃ power devices for practical high current devices and initial studies have appeared on both the experimental and theoretical fronts. We summarize progress in edge termination design, temperature measurement using thermoreflectance-based thermography to measure the thermal rise and decay of the active diodes, failure under forward bias and development of large current (up to 130A) arrays.

In this talk, I will highlight our recent efforts in modeling Ga2O3 and other oxide materials and devices for application in power electronics. First, I will show the results of techno-economic analysis of the manufacturing cost of Ga2O3 wafers, supporting their projected cost advantage compared to SiC and GaN [1] Next, I will describe finite element analysis of electrical and thermal performance of vertical Ga2O3 transistors reported in literature, comparing MOSFET to FinFET device architectures [2]. Finally, I will summarize the findings of first-principles computational search for wide band gap semiconductors with high figures of merit and large thermal conductivity, highlighting new oxide material candidates for power electronic applications [3]. [1] Joule 3, 1 (2019) [2] ECS Journ. Sol. St. Sci. Tech. 8 Q3202 (2019) [3] Energy Environ. Sci. (2019) DOI: 10.1039/c9ee01529a

Gallium oxide (Ga₂O₃) is an emerging material for power electronics. The final penetration in the market is limited by several issues, including a stable and effective isolation between different devices and between different regions of the same device. In this work, we analyze lateral and vertical isolation structures, obtained by Mg implantation and annealing at 1000°C in Halide Vapor Phase Epitaxy β-Ga₂O₃. By means of repeated current-voltage characterization, it is possible to detect a severe current collapse, which can be completely recovered by white light illumination. When a constant bias is applied, the current collapse increases in magnitude at higher bias, showing a stronger filling of the deep levels. The transients closely follow the stretched-exponential model, an indication that the charge trapping is originated by extended defects, mini-bands or surface states. From the recovery transients carried out at various temperatures, it is possible to extrapolate a dominant thermal activation energy of 0.34 eV. The results of the recovery transients under monochromatic illumination show gradual variation in a broad energy range, consistent with the presence of extended defects. Temperature-dependent current-voltage characterization highlights the good performance of the bulk isolation and the presence of a significant surface leakage. Long-term stability tests show that the lateral structure is able to withstand a higher voltage level before catastrophic failure, but is less stable and is affected by a time-dependent degradation process. Charge trapping at the surface may act as a field-limiting element and partially explain the experimental findings.

In this work, the heterostructures of III-N materials/β-Ga2O3 based on the modern polarization theory and band alignment are systematically investigated. The nitrogen (N)-polar AlN/β-Ga2O3 heterojunction is found that can form the triangle channel and hold large 2DEG density on the interface through polarization engineering. Compared with GaN-based HEMT, the proposed N-polar AlN/β-Ga2O3 HEMT with polarization-induced property can realize much larger 2DEG density, better DC and transconductance performance, as well as higher breakdown voltage.

In this talk, we will present two elemental device technologies developed for lateral Ga2O3 metal-oxide-semiconductor field-effect transistors (MOSFETs). One is nitrogen doping to a Ga2O3 channel layer to realize normally-off operation of Ga2O3 MOSFETs, and the other is an (AlGa)2O3 back barrier to shift a threshold gate voltage of Ga2O3 MOSFETs with a Si-implanted channel toward the positive voltage side.

Ultrafast (femtosecond pulse) laser irradiation provides unique laser-material interactions associated with high instantaneous electric fields and short timescales that lead to a nonequilibrium state between excited electrons and phonons. These interactions result in material modifications that differ from conventional chemical, physical, and thermal processing at much longer timescales, and an opportunity to address material and device processing challenges associated with wide-bandgap materials such as Ga₂O₃. In this work, we explore ultrafast laser irradiation (Ti:sapphire, 150 fs pulse width) of bulk (010) Sn-doped β-Ga₂O₃ under two different wavelengths, fundamental (780 nm) and frequency-doubled (390 nm), and a range of laser fluences. We identified a regime for laser-induced damage threshold resulting in material ablation, thermally-induced straight crack formation, and recrystallization. Rastering on a β-Ga₂O₃ substrate created surface nanostructures including laser-induced periodic surface structures at a high spatial frequency (period ~250 nm). These highly aligned periodic structures can be controlled by laser polarization and wavelength, presenting a means for direct writing of surface nanostructures. Enhanced atomic movement associated with a transient metallic state can provide a means for intentional generation of point defects via the laser irradiation. These point defects may offer a means of electrical modification, which was demonstrated as more than five orders of the magnitude enhancement of lateral conductance on rastered β-Ga₂O₃. Moreover, a hydrophobic surface of β-Ga₂O₃was achieved by ultrafast laser irradiation.

A novel method to fabricate β-(AlGa)2O3 solar-blind photodetector has been demonstrated. The β-Ga2O3 thin film was first deposited on c-plane sapphire substrate by pulsed laser deposition (PLD) and following by high-temperature annealing (1000°C and above). With a proper annealing condition, the PLD deposited β-Ga2O3 film could be transformed from a binary to become a ternary β-(AlGa)2O3 film, which is resulted from the Al atoms of sapphire substrate diffused into the PLD deposited β-Ga2O3 layer by high-temperature driven, and the Ga atoms from β-Ga2O3 thin film to the substrate diffusion as well. By high-temperature driven interdiffusion method, β-(AlGa)2O3 thin film with designed Al composition and film thickness could be achieved, which could cover higher bandgap larger than 4.9 eV. With such a method, one can achieve β-(AlGa)2O3 Solar-blind photodetector with good crystal quality and surface morphology, which also could push the response wavelength even further to the DUV range.

Critical Dimensions Measurement is a principle Quality Control method for industrial semiconductor production. Though an array of instrument is available for characterization. These are often device specialized for narrow band of samples, expensive and lack throughput for many applications. We have developed an extensive toolbox of computer vision and deep learning approaches for inline calculation of spatial dimensions of imaged devices. We expand this technique for real time measurement during video faced focal search for critical depth measurement. We use active template matching to compare and track the object through the stack to the known surface profile of the object. This allows for compensation of environmental noise, multi-object tracking within a single XY field-of-view and defect detection to be performed on the same images. Combined, this toolbox offers higher accuracy and faster processing than conventional approaches.

We have employed the framework of quantum magnetoconductivity to develop an experimental method of directly measuring the mobility µph representing inelastic electron-phonon scattering at a given temperature T. Further, we relate µph to material parameters via an equation µph(T) = function(T,Tph,m*,eps-0,eps-inf), where m* is the effective mass, eps-0 and eps-inf are the static and high-frequency dielectric constants, and kTph is an effective phonon energy that represents all the phonon interactions contributing to µph at temperature T. We apply this methodology to an “old” material ZnO; an exciting “new” material, β-Ga2O3; and a combination of the two, ZnGa2O4.

Following demonstrations of bulk and epitaxial growth, zinc gallate ZnGa2O4 has been receiving increased attention for power electronics applications due to bandgap and high and high carrier concentration mobility comparable to those of beta-Ga2O3. Here we use first principles calculations to study stability of the direct and inverted spinel structures of ZnGa2O4 as a function of temperature and the formation energies of native defects: Zn, Ga, and O vacancies, the Zn/Ga, Ga/Zn antisites that provide insights into the nature of shallow donors from high temperature growth, and the higher-energy cation-anion antisites. In addition, we discuss aspects of the electronic structure in the presence of aluminum incorporation.

The ZnGa2O4 (ZGO) complex oxide with a wide bandgap of 5.1 eV has become one of the promising materials for deep ultraviolet sensing applications. However, the sputtered ZGO films always showed the disordered nanocrystalline structure resulting in the relatively poor performance. In this study, the solid-phase epitaxy method is used for crystallizing the ZGO structure via rapid thermal annealing (RTA) process. The disordered crystal-grains as incubated seeds are obtained in the as-deposited film at the substrate temperature of 400℃. By employing RTA at 700℃, the ZGO film structure approaches the quasi-single-crystalline structure, which is evidenced by checking the transmission electron microscopy. Suppression of the Zn diffusion out under the annealing will be discussed. As a result, the spectral responsivity of RTA-treated ZGO photodetector can reach 2.53 A W-1 at 240 nm and 5 V bias, indicating a relative enhancement of 256% as compared with the as-deposited one.

We present a combined experimental and theoretical investigation on the surface electronic structure of truly bulk ZnGa2O4, a transparent conducting oxide with an ultra-wide band gap of 4.6 eV. Angle-resolved photoelectron spectroscopy, X-ray photoelectron spectroscopy and low-energy electron diffraction were used to analyze the electronic band structure, band bending and surface reconstruction respectively. In combination with density functional theory, the experimental results will be discussed to provide the very first insights on the surface electronic properties of ZnGa2O4, to motivate future investigations.

Ga doped ZnO(GaZnO) nanostructures for the improved ultrafast nonlinear optical properties by selective electron beam irradiation were studied. The crystalline size of the films shows a decrement due to lattice mismatch effects arised owing to EBI induced stress. The occurrence of the E2H mode in Raman spectra indicates the quality of the grown nanostructures. Gaussian deconvolution fitting on photoluminescence spectra reveals defect related emission originated from intrinsic defects. The core level spectra of O1s from XPS analysis has shown a substantial changes upon EBI in which peak related to oxygen vacancy defect got suppressed The third harmonic generation studies in both femtosecond and nanosecond regime shows substantial enhancement in the signal intensity which outcomes the effect of EBI treatment

The variation of luminescent properties with temperature is one promising way in order to measure temperature. Luminescent nanomaterials allow a local and contactless temperature determination. Among the oxides based nanophosphors, ZnGa2O4:Cr3+ is a potential nanothermometer because of its high sensitivity on a large temperature range. Indeed, the luminescence lifetime of the 2E emitted state is drastically dependent on temperature since the 4T2 and 2E levels are in thermal equilibrium. In this study we optimized the luminescence properties of 10 nm in diameter nanoparticles in order to improve their capability as thermal nanosensor. Persistent luminescence decays and photoluminescence properties are investigated at several temperatures. The obtained sensitivity are about 2%C-1 for these optical measurements making this materials a good temperature sensor. Furthermore in order to generate a local temperature increase, gold nanorods are synthetized and tested with the NPs.

The use of NiO in low-dimensional devices requires appropriate synthesis methods allowing to control the morphology, size, and final composition of the as-grown samples in order to improve and broaden the applicability of this material in different fields of research. In this work, a vapor-solid and a hydrothermal method have been employed and evaluated for the fabrication of undoped and Sn doped NiO micro and nanostructures. The presence of Sn favours the growth of microwires, by means of thermal treatments at 1400 °C, while by using a hydrothermal process nanobars and nanoparticles with reduced dimensions were obtained. The chemical method leads to a higher control in the final concentration of Sn incorporated in the NiO lattice. X-ray diffraction confirms the crystalline quality of the obtained products, as well as a texture effects and peak-shift associated with the Sn doping process. Photoluminescence measurements demonstrate an increase in the luminescent signal promoted by Sn doping, related to the presence of Ni vacancies.

Photometric stereo is a common technique for 3D reconstruction by calculating the surface normals of an object from different illumination angles. The technique is effective to estimate height profile of static objects with large features, but often fails for objects with smaller features or in flat environments with small depressions. We propose a method using deep learning to perform 3D reconstruction of small features. Our method handles sample noise, uneven illumination, and surface tilt. We demonstrate decreased noise susceptibility on synthetic data and promising performance on experimental datasets. This approach enables rapid inspection and reconstruction of complex surfaces without the need to use destructive or expensive analysis methods.

The material class of transparent conductive oxides features a variety of properties that are interesting and useful for optical and opto-electronic applications. ITO belonging to this class has shown to be either processed (Gui et al, Nat. Sci. Rep. 2019), or electro-statically-biased (Sorger et al., Nanoph. 2012) into the ENZ regime. Here, I review our latest work including; (1) ITO ENZ material deposition control, (1) the first ITO-based phase-shifters in silicon photonics with VpL=0.5V-mm (Amin et al, APL 2018) and 0.06V-mm performance (Amin et al, arXiv 1907.11131 2019), (2) electro-optic nonlinear activation function in feed-forward photonic neural networks (Amin et al, APL Mat. 2019), (3), optical-phase array in silicon-ITO photonics for GHz-fast beam steering, (4) a metatronic circuit board towards an analog photonic computer for PDEs, and (5) strong nonlinearity in ENZ ITO showing ~10dB of optical limiting.

Transparent Conductive Oxides (TCOs) have been described as a promising alternative to metals for IR plasmonics. However, low propagation of surface plasmons is a major shortcoming limiting the potential of these materials in waveguiding-related applications. As a proof of concept, we propose using polar plasmonic substrates as a method to overcome this issue. In this study, we demonstrate the existence of hybrid phonon-plasmon surface waves in an air-CdZnO-sapphire system and characterize their properties, including an improved propagation distance when compared to a plasmonic-only equivalent system.

Transparent conductive oxides (TCOs) have attracted significant research interests for integrated photonics and silicon photonics due to their dramatic change of optical permittivity, which can be dynamically controlled by the gate voltage. In addition, TCO materials can be easily integrated with silicon photonic devices by sputtering and regular lithography process, which demonstrates great potentials for large-scale integration. We report our recent research progress of hybrid silicon-TCO photonic devices using TCO gate/high-K insulator (HfO2)/Si waveguides, which achieves both outstanding energy efficiency and high modulation bandwidth. The strategy for future on-chip optical interconnect systems will also be discussed in this presentation.

There is a great interest in photonic substances with permittivity approaching zero, which are called the epsilon-nearzero (ENZ) materials. They have a potential for multiple applications in telecom industry. The newest ENZ materials based on transparent conductive oxides (TCOs) and transparent conductive nitrides (TCNs) still have limited spectral bands of the ENZ effect. We show with simulations based on the Effective Medium Theory that the limitation can be defeated by using nanocomposite films made of several TCOs/TCNs with the ENZ effect observed in different regions of optical spectrum that stand apart from each other. We proposed to make such composites with the concurrent multibeam multi-target pulsed laser deposition (CMBMT-PLD). The composite films of aluminum and gallium doped zinc oxide (AZO-GZO) at different proportions were made by concurrent PLD of AZO and GZO targets with two 532-nm laser beams from a frequency doubled Q-switched Nd:YAG laser in a 10^-5-Torr vacuum. The deposition time varied from 10 to 50 min. The high-resolution scanning electron microscopy revealed that the films deposited on glass substrates were composed of nano-grains of the constituents with a size in the range 10-300 nm. Energy dispersive X-ray spectroscopy showed the presence of all the major constituents in the films. Optical absorption and reflection spectroscopy of the films in the visible and near-infrared regions demonstrated that they had a minimum of reflectance corresponding to the ENZ effect in a broad band (~ 200 nm) around 1200 nm in the agreement with theoretical predictions.

Transparent conducting oxides, such as Ga-doped ZnO (GZO) and Al-doped ZnO (AZO) are attractive materials for high-performance plasmonic devices operating at telecommunication wavelengths. In this contribution, we compare the growth of epsilon-near-zero GZO and AZO films on sapphire by two different deposition techniques: molecular beam epitaxy (MBE) and atomic layer deposition (ALD). For MBE of GZO, a multiple buffer consisted of a high-temperature MgO layer, a low-temperature ZnO, followed by a high-temperature ZnO layer is employed to assure the crystalline quality of the GZO film. By controlling the growth parameters, including Ga doping level, VI/II ratio, substrate temperature, we are able to produce GZO films at 350 °C with electron mobility between 30 and 50 cm2/V.s, electron concentration up to 7×1020 cm-3, and resistivity down to 2.5×10-4 Ω.cm. For ALD of AZO, without using any buffer, by reducing the Al pulse duration, we are able to grow the AZO films under a large ratio of Al to Zn pulses of 1:6, which improves the activation of Al as an effective dopant. Hence AZO films with electron concentration above 7×1020 cm-3, electron mobility between 10 and 20 cm2/V.s, and resistivity below 6×10-4 Ω.cm have been obtained at 250 °C. The corresponding epsilon-near-zero point in the ALD-grown material was tuned down to 1470 nm. Our data indicate that the ALD method provides a low-temperature route to plasmonic TCOs for telecommunication wavelength range. Effect of electron mobility on optical loss and, therefore, plasmonic figure of merit is discussed.

Conducting oxide films on optical fibers offer significant advantages as sensing layers in high temperature, harsh environments (e.g., solid oxide fuel cells or power plant boiler systems), where many traditional sensors degrade rapidly. In particular, oxides exhibiting the perovskite crystal structure are highly stable under substitution of the constituent atoms as well as under high concentrations of cation and oxygen vacancies. Strontium titanate (STO), for example, maintains the perovskite crystal structure under high dopant concentrations both on the “A-site” (Sr substitution) and “Bsite” (Ti substitution). As a result, STO and other perovskite oxides form a rich family of materials whose unique electrical, magnetic, and optical properties can be continuously tuned based upon the concentrations of the constituent atoms. Structural stability, in conjunction with an interconnected vacancy defect chemistry and optical as well as electrical properties, suggests great promise for this material system in the realm of harsh environment sensing. In this work, donor-doped strontium titanate films containing gold nanoparticles are investigated for gas sensing characteristics at high temperature (up to 680°C) under oxidizing and reducing conditions. Using time resolved, in-situ optical transmission measurements in conjunction with room-temperature ex-situ measurements of planar films, the relationship between the Au localized surface plasmon resonance (LSPR) response in the visible and oxide free-carrier based response in NIR is investigated.

Memristor technology will strongly influence the architecture of computer systems in the near future. The initial motivation for the development of memristive technologies was the search for novel materials for next-generation memories. The potential application of memristors in novel domains, e.g. in-memory information processing, reconfigurable logics [1], neuromorphic computing [2], and hardware cryptography [3], makes it more than ever necessary to understand the underlying mechanisms underlying the nonvolatile reconfiguration of electrical properties of memristors and to integrate memristor technology into beyond Si oxide electronics. We have developed reconfigurable perovskite memristors, namely the bipolar BiFeO3 and the unipolar YMnO3, and report on their properties and potential applications. [1] T. You et al., Adv. Funct. Mat. 24, 3357-3365, 2014. [2] N. Du et al., Front. Neurosci. 9, 227, 2015. [3] N. Du et al., J. Appl. Phys. 115, 124501, 2014.

Metal oxide thin-film transistors (MOTFTs) are expected to play a vital role in enabling printed transparent, plastic electronics. Compatibility with plastics, however, requires MOTFTs to be scalably processed at low temperatures (T). Herein, we explore blade-coating of indium oxide (In2O3) TFTs via sol-gel and combustion chemistries. We find that the sol-gel process enables amorphous In2O3 TFTs at 200°C with moderate electronic mobility (ca. 1 cm2V-1s-1) which increases to 5 cm2V-1s-1 for 212°C. Combustion synthesis is found to bypass the electronically-active amorphous state leading to an early crystallization onset. Paradoxically, combustion TFTs are found to possess poor charge transport at low-T of 200-250°C. Early nucleation during combustion forms nanocrystalline domains that are deleterious to charge transport. Our results highlight that surprisingly it is not crystallization, rather the absence of it that is required to fabricate high-mobility, low-T bladed In2O3 TFTs.

Amorphous semiconducting transparent oxides like InGaZnO₄ (a-IGZO) have a broad distribution of metal and oxygen vacancy defects that determine thin film transistor (TFT) characteristics and impact device reliability metrics such as hysteresis. Here, we demonstrate how hydrogen modifies the density of states (DoS) through a novel on-chip method that spectrally resolves trap concentration in a-IGZO spanning the bandgap. Requiring laser energies continuously tunable from 0:26 to 3:1 eV, this method also employs difference frequency generation to access shallow states near the conduction band. We characterize the effect of hydrogen incorporation on the sub-gap peaks of the DoS of an a-IGZO TFT. Specifically, our data suggests hydrogen hybridizes with vacancy defects through metal-hydrogen (M-H) bonds that passivate oxygen vacancy sites and O-H bonds that passivate metal vacancy sites. These interactions result in a suppression of oxygen vacancy and metal vacancy- related trap states in the sub-gap and an enhancement of a metal-hydrogen bonding peak near the VBM. Temperature dependent, photon energy-dependent hysteresis, and transient defect lifetime measurements further reveal the strong impact of hydrogen concentration on a-IGZO TFT performance germane to current optical display technology.

Nowadays, there is a real need in all solid-state sources emitting a high energy and tunable coherent light covering Band II of transparency of the atmosphere (2-5 µm). The best alternative is frequency down-conversion in a nonlinear crystal of a monochromatic wavelength emitted by a commercial laser. The requested performances of the crystal are a high damage threshold and phase-matching conditions associated with large conversion efficiencies. It is the case of the oxyde crystals KTiOPO4 (KTP) and the periodically poled LiNbO3 (PPLN) that are mainly used. The goal of this talk is to give an overview of their full characterization by using the sphere method we developed many years ago. We will also report recent data of new promising oxyde crystals as La3Ga5.5Ta0.5O14 (LGT), NaI3O8 and periodically poled KTiOPO4 (PPKTP). All our data can be used per se. They also lead to the main parameters enabling calculations of the best use of oxyde crystals in optical parametric generators (OPG).

DUV and VUV light emitters have a variety of applications, such as in ozone cleaners, UV sterilization, etc. Rocksalt-Structured (RS) MgZnO alloys have attracted much attention as candidate materials for the solid-state DUV and VUV emitters. In this study, successful growths of atomically-flat single crystalline RS-MgZnO films on (001) MgO substrates by the mist chemical vapor deposition method and observation of DUV emission were demonstrated. Further improvements in its crystalline quality resulted in the predominate observation of cathodoluminescence (CL) peak at around 199-210 nm. The CL spectra showed relatively high equivalent internal quantum efficiencies of 2.5–11% for the DUV near-band-edge emission.

Vanadium dioxide (VO₂) is undergoing a reversible insulator-to-metal transition (MIT), subject to thermal, electrical or optical stimuli. The transition is accompanied by drastic changes in the material’s electrical and optical properties which, along the MIT broadband frequency response (from DC to microwaves, THz/ optical domains), triggered a plethora of interesting applications (DC to millimeter-waves switching, THz modulators, reconfigurable filters and antennas etc.). Here we report on optical switching of the VO₂ material between its two dissimilar states when integrated in planar two terminal electrical devices and submitted to laser pulses with different temporal lengths from a high-power laser diode operating at 980 nm. During optical irradiation of VO₂ films with pulses having mean powers between 15mW and 140mW at repetition rates up to 500 kHz, we monitored its resistance change, witnessing on the MIT onset. We demonstrate that the MIT in VO₂ is optically triggered for pulses as short as 25 ns and energies higher than 130 nJ/pulse, with insulator-to-metal response times in the range 10-15 ns. The process is highly stable and reliable; the devices are able to perform more than one billion switching cycles at frequencies up to 400 kHz without damaging the material nor the device integrity. This optical activation scheme of VO₂ emerges as a promising solution for reconfiguration applications at THz and millimeter-waves frequencies.

This paper demonstrates the impacts of light and ambient gas on the resistance of sputter-deposited non-doped ZnO films. Although the authors have already demonstrated the impact of light from various LEDs with a different wavelength on the resistance of such films, key results were not shown in detail. In this paper, the influence of ambient gas on the resistance and the influence of temperature on resistance are demonstrated in detail, and feasible physics drawn from analysis results are discussed. Physical images of the phenomena are proposed. It is also comprehensively revealed that the film surface condition significantly contributes to the transport characteristics of the film; this is supported by the impact of ambient gas in the dark on the current decay process in the film.

Laser-induced damage threshold (LIDT) studies have been carried out mostly in research labs and universities in the past decades. In this work, a systematic study, using industry equipment in a production environment, was carried out on the LIDT of production-type laser optical film products. Various past studies draw conclusions regarding laser damage threshold improvement. However, only a handful of studies have been done in industrial environments, and those that do often report inconsistent findings. We believe a more comprehensive, head-to-head comparison will clear any discrepancies and also benefit LIDT film product manufacturing. The study was carried out in multiple different aspects: materials, coating technologies, coating designs, coating processes, and post-coating sample annealing treatment processes. Two different structure sample films, anti-reflection (AR) coatings and high reflection (HR) mirrors at the 1064nm wavelength regime, were identically prepared albeit using different machines and materials. TiO₂, Nb₂O₅, Ta₂O₅, and HfO₂ were used for high index materials; SiO₂ and Al₂O₃ were used for low index materials. We used ion beam sputtering (Veeco Spector dual or single ion source) and Leybold E-beam evaporation (with and without ion assist - IAD). Samples with and without annealing after coating were also studied for damage threshold improvement. The LIDT measurement was done on a 1064 nm laser system with a 10 ns pulse width. In terms of material, HfO₂ had the best LIDT value, at 29.5 J/cm², compared with Ta₂O₅, Nb₂O₅, and TiO₂, listed in performance order from best to worst. With the same layer structure and high index material, replacing SiO₂ with Al₂O₃ increased LIDT by 15-20%. In some cases, Leybold E-beam evaporation exhibited better performance than ion beam sputtering. Furthermore, the post-process including annealing treatment had a significant impact on improving LIDT. With controlled annealing, the damage threshold nearly doubled. Detailed studies will be presented at the conference.

Previously we have conducted researches on the molecular beam epitaxy (MBE) of Zn-polar BeMgZnO/ZnO heterostructure field effect transistor (HFET) structures on GaN templates, which exhibit both high two-dimensional gas (2DEG) density and high electron mobility with relatively low Mg contents via the strain modulation of Be and Mg co-incorporation. In this contribution, we report on the growth of the HFET structures directly on sapphire substrate by employing a buffer consisting of a rock-salt structure MgO layer, a low-temperature (LT) ZnO layer and a high-temperature (HT) ZnO layer. Compared with growth of O-polar on sapphire, in which a thin and wurtzite MgO buffer is deposited at high temperatures above 700 °C, the MgO buffer for Zn-polar growth has to be reduced to 450 °C, in order to obtain smooth interface and surface for the BeMgZnO/ZnO HFETs. The residual electron sheet concentration in Zn-polar ZnO layers is ~2×10¹² cm^-2 on GaN while semi-insulating Zn-polar ZnO layers on sapphire have been obtained via controlling the buffer growth conditions, which is vital to the realization of HFET device structures.

Magnesium oxide (MgO) is a promising dielectric for use with GaN due its similar crystal structure and lattice constant, large bandgap, and high dielectric constant. We report on the structural properties of MgO films deposited on GaN templates on sapphire substrates via the atomic layer deposition (ALD) technique. Analysis of the crystal quality and structure as a function of surface treatment and growth temperature are presented. I-V and C-V measurements of MgO/GaN metal-oxide-semiconductor capacitance structures are also presented.

Recently Zinc Oxide has received a renewed attention for the realization of intersubband devices such as quantum cascade lasers (QCLs). Indeed this material is predicted to be able to tackle the main limitation of current terahertz (THz) QCLs: the limited operation temperature. We report the observation of electronic coupling within ZnO/(Zn, Mg)O asymmetric quantum wells (QWs), first step towards this goal. Samples were grown by molecular beam epitaxy (MBE) with surfaces down to 0.4 nm. X-ray reflectivity (XRR) was used for thickness measurements checking and for the investigation of the interface quality. Atomic resolution scanning transmission electron microscopy (STEM) images reveals that we are able to grow 2 monolayers (MLs) thin (Zn, Mg)O barriers in a reproducible way while keeping abrupt interfaces. Room temperature (RT) photoluminescence (PL) spectra show that QWs are still coupled when separated by a 1.0 nm thick barrier. On the contrary, a 4.0 nm thick barrier allows no coupling. Doped samples were investigated by absorption experiment. Absorption spectra were successfully fitted by a theoretical model, proving a clear electronic coupling in our heterostructures. This demonstration allows us to seriously envision the realization of ZnO based intersubband devices.

The electro-optic performance in free space at the 2 μ𝑚 wavelength range of potassium lithium tantalate niobate (KLTN) crystal operated at the paraelectric phase close to the phase transition temperature (Tc) is explored. An electrically induced change in the refractive index approaching 0.01 was demonstrated close to the phase transition temperature, while maintaining high optical quality and transparency at a wavelength of 1.85 μ𝑚. A special crystal geometry was employed in order to suppress unwanted acousto-optic oscillations that were superimposed on the generated pulse due the strong electrostriction of KLTN close to Tc.

Highly mismatched alloys are a class of materials whose fundamental properties are dramatically modified through the substitution of a relatively small fraction of host atoms with an element of very much different electronegativity. In ZnTe, the incorporation of a small amount of isoelectronic O leads to the formation of a narrow, O-derived intermediate band (IB, E-) located well below the conduction band (CB, E+) edge of the ZnTe through an anticrossing interaction between localized states of O and the CB of the ZnTe matrix. Therefore, ZnTe_1-xO_x (ZnTeO) alloy is one of the potential candidates for an absorber material in a bulk intermediate band solar cell (IBSC). We have previously demonstrated the generation of photocurrent induced by two-step photon absorption (TSPA) in ZnTeO IBSCs using n-ZnO window layer. Here, we review our recent progress on the development of ZnTeO based IBSCs using n-ZnS window layer and Cldoped ZnTeO. With n-ZnS window having a small conduction band offset with ZnTe, the open circuit voltage of ZnTeO IBSC was improved. Cl-doping was performed to introduce electrons into the IB of ZnTeO that is required to be halffilled with electrons for the efficient operation of an IBSC. Low temperature photoluminescence spectra indicated that the doped Cl atoms act as donors in ZnTeO. The improved photovoltaic properties were demonstrated in the IBSC using Cl-doped ZnTeO.

Recent progress on reactively sputtered metal oxide PV interlayers is presented. A strong correlation between initial material composition and annealing condition to the microstructure of the films is given, leading to pronounced device improvements. A new crystalline MoOx system employed for efficient hole extraction is shown to lead to prolonged OPV lifetimes1, and a new crystalline TiOx layer is shown to lead to efficient electron extraction. In order to meet the requirements on scalable OPV development, the up-scaling of the metal oxides from Roll-to-Roll (R2R) vacuum processing is discussed. 1 Ahmadpour et al, ACS Appl. Energy Mater. 2, 420 (2019)

We experimentally investigate how the static and dynamic optical properties of cadmium oxide evolve with yttrium doping, for the design of optical and plasmonic devices spanning the near-infrared to the mid-infrared wavelengths. The metallicity is seen to increase and the epsilon-near-zero point blue-shifts with increasing yttrium-concentrations. We demonstrate broadband, optical-pump-induced reflection and transmission modulation ((up to 135% near ENZ), with picosecond response-times controlled by doping-concentration.

For application in flexible, electrochromic batteries, transparent, highly conductive and long lifetime electrolytes are necessary to achieve fast coloration and a maximum contrast. Based on a dimethyl-sulfoxide (DMSO) modified polyacrylamide (PAM) hydrogel, we developed a dual-ion Zn²⁺/Al³⁺ electrochromic battery consisting of a Zn anode and WO₃ cathode. To overcome shortcomings of conventional hydrogel electrolytes, which experience low solvent retention, we introduce a DMSO:H₂O mixed solvent polymerization process, which significantly increases the electrolyte retention of the hydrogel and therefore its lifetime for ionic conduction. The DMSO-modified electrolyte exhibits ionic conductivities up to 27 mS/cm at room temperature, while the formation of DMSO:H₂O nanoclusters enables ionic conduction even at temperatures as low as -15°C and retention of ionic conduction over more than 28 days. The electrochromic battery based on the modified hydrogel exhibits a specific charge capacity of 16.9 μAh/cm² at current densities of 200 μA/cm² with 100% coulombic efficiency retention over 200 charge-discharge cycles. Based on a double layer architecture, the flexible battery shows a contrast of over 80% trough electrochromic coloration.

In this contribution we report on a low cost plasmonic electrode for light-sensing applications. The electrode combines a conducting nonstoichiometric indium oxide (InO_x) layer with an ultrathin (~5 nm) discontinuous Au layer. The InO_x and Au layers were deposited on glass substrates by plasma enhanced reactive thermal evaporation and thermal evaporation, respectively. Several device configurations with one or two Au layer(s) sandwiched between InO_x layers were fabricated and characterized. The morphological and structural properties of both Au and InO_x layers were analyzed using AFM and XRD techniques. In particular, the effect of thermal annealing (673 K, 15 min) on the surface morphology of Au layers grown on bare glass and InO_x-coated substrate was investigated. It has been also found that the oxide film grown above an underlying nanostructured Au layer is amorphous, while a reference InO_x film on glass is nanocrystalline with a smooth surface. The electrical properties of InO_x grown on the Au surface are worsened due to Au-induced structural disorder. The observed difference in transmission spectra of the glass/InO_x/Au and glass/Au/InO_x structures indicates the difference in the morphology of the metal layer. Thus, the optical and morphological properties of the InO_x electrode can be varied in a wide range by incorporating several Au layers.

Zinc magnesium oxide has emerged as a potential candidate for sensing and optoelectronic applications due to its structural advantages over ZnO. However, especially for UV-optoelectronic device applications, suppression of defects bound emissions in thin films and nanorods are a challenging task. In this report, we show comparison of enhancement in near-band emission and suppression in the defects-band emissions in ZnMgO thin film and nanorods using UV-Ozone (UVO) annealing. Thin films were deposited using RF sputter system and hydrothermal route was used to grow nanorods on the rapid thermal annealed ZnMgO seed layer (as-grown) followed by UVO annealing for 10, 30, 50, 70 and 90 min. Field-emission gun scanning electron microscopy confirmed growth of high density nanorods. High-resolution x-ray diffraction pattern exhibited <002> peak for all samples and a gradual increase in grain size. Room temperature photoluminescence (PL) spectra showed highest NBE emission for 10 min in thin films and 50 min for nanorods. Calculated NBE to DBE integrated area ratio increased to 1.2 for 10 min in thin films and 7.8 times for 50 min nanorods sample as compared to respective as-grown samples. Activation energy calculated from NBE integrated area of nanorods temperature-dependent PL confirmed that 50 min annealed sample showed the highest activation energy. Authors would like to acknowledge DST, India and IITBNF.

ZnO thin films and nanorods are being used for UV optoelectronic applications such as UV-detectors, UVLEDs and Laser diodes because of its large bandgap (3.37 eV) and high excitonic binding energy at room temperature (60 meV). However native point defects in as grown ZnO films need to be suppressed before its device application. Here in this work, a comparative study on the effect of UV-Ozone annealing on the optical properties of ZnO thin films and nanorods have been carried out. Thin films were deposited using RF sputter system and hydrothermal route was used for nanorods growth, followed by UVO annealing for 50 min. Field emission gun scanning electron microscopy (FEG-SEM) confirmed formation of high density nanorods. High resolution X-ray diffraction (HRXRD) results exhibited (002) crystal orientation as the dominant peak for all samples. Calculated grain size for as-grown thin films and nanorods were 27 nm and 37 nm respectively. After UVO annealing it increased to 35 and 47 nm respectively. Room temperature photoluminescence showed enhancement in near band emissions (NBE) for both thin films and nanorods. Maximum enhancement in NBE as compared to as-grown for thin films and nanorods were found to be 6.6 and 3.6 times, respectively. Maximum NBE to DBE integrated area ratio for thin films and nanorods were 0.17 and 0.70 respectively.

SrAl₂O₄: Eu²⁺, Dy³⁺ nanoparticles prepared by pulsed laser ablation in liquids (PLAL) could be a green and versatile technique to obtain inorganic nanoparticles with persistent luminescence and different morphologies and sizes. Laser ablation technique could deliver large amounts of energy highly concentrated into one point of a material and has been carried out in vacuum, in air and in liquids. Because of the unique confinement effect from liquid environment, there is many advantages when the laser ablation occurs in liquid. In this paper, we focus on the control of the size through proper laser parameters, choice of the reaction solution. Persistent luminescent SrAl₂O₄: Eu²⁺, Dy³⁺ phosphor was obtained by laser ablation in liquids.

In this work, an individual nanowire of zinc oxide (ZnO-NW), decorated with gold nanoparticles (Au-NPs/ZnO-NW), was integrated in a nanophotodetector using a dual beam focused electron/ion beam (FIB/SEM) system. Au-NPs/ZnO-NW arrays were synthesized by one-step electrochemical deposition at relative low-temperatures (90 °C). The nanodevice fabricated with a single nanowire Au-NPs/ZnO-NW demonstrated fast detection of UV radiation up to the operating temperature of 120 °C. The improved UV sensing properties of an individual Au-NPs/ZnO-NW compared to a single, undecorated, ZnO NW was explained based on the formation of Schottky barriers at the Au/ZnO NW interface, which resulted in a much more narrowed conduction channel and a lower dark current. These results prove that high-performance hybrid nanomaterials may possess superior electrical, optical and sensing properties and are of great interest for further fundamental studies.