Transparent conducting oxides (TCOs) are widely investigated in purview of potential applications as charge collectors in light-emitting devices and thin-film photovoltaics where optical transparency in the visible photon energy range are required. TCOs that have been investigated as charge collectors in optoelectronic devices include indium-doped and fluorine-doped tin oxides, zinc oxides, nickel oxide (NiO), and their alloys. The strong interest in NiO stems from the fact that no dopants are needed to turn on its electrical conductivity while keeping its transparency at acceptable levels, differently from tin oxide, where In or F doping is required, and zinc oxide where conductivity is enhanced by Al doping. Our work will present our defect engineering efforts to optimize NiO and (MgNi)O thin films for their applications as TCOs. In the final part of our talk, we will present our recent work where NiOx thin films are used as hole transport layers (HTL) in green organic photovoltaics, with water-soluble poly[2-(3-thienyl)-ethoxy-4-butylsulfonate]-sodium (PTEBS) and fullerenes as active layer, showing improved external quantum efficiency for UV light.

Understanding surface reconstructions influenced by hydrogen is essential for controlling growth and materials properties. We use density functional theory to explore the adsorption of hydrogen, gallium, and oxygen adatoms on the Ga2O3 (010) and (110) surface and generate a phase diagram, which shows surface reconstructions as a function of Ga and H chemical potentials. For “bare” surfaces, we find that the ideal unreconstructed surface is low in energy but that reconstructions with Ga and O adatoms can be favorable under more Ga-rich conditions. We question whether such “bare” surfaces can be experimentally observed, since hydrogen is inevitably present during growth, even in ultrahigh-vacuum growth systems. Hydrogen-containing reconstructions are found under H-rich conditions with H being more stable under O-rich conditions. Our comprehensive study of surface reconstructions establishes under which conditions hydrogen will be present on the surface, and how it affects the growth. Work performed in collaboration with M. Wang and S. Mu and supported by AFOSR.

Gallium oxide (Ga2O3) continues to rapidly develop as promising material platform for next-generation power electronics adopting wider (ultrawide) bandgap semiconductors beyond SiC and GaN. This is in large part due to the availability of high quality single crystal substrates and epilayers that have allowed for exploration of important material properties, processing and growth recipes, and device designs. While p-type doping remains out of reach, controllable n-type doping has now been demonstrated via a variety of approaches and dopants, yet has remained challenging in wider-band gap alloys such as those incorporating Al. One primary concern is the role of cation vacancies, which have been shown to be favorable in Ga2O3 and can act as compensating centers, as well as form complexes with a variety of defects. Here we survey the current understanding of these and other native point defects and their interactions with other common dopants and impurities in Ga2O3, focusing on their potential optical and electrical consequences from insights gained through first-principles-based calculations employing hybrid functionals. We discuss how vacancies can influence defect kinetics and how their incorporation may be influenced by growth and processing steps. These results provide guidance for controlling the conductivity in Ga2O3 for facilitating next-generation power electronics based on this ultra-wide bandgap semiconductor. This work was partially performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and partially supported by LLNL LDRD funding under Project No. 22-SI-003 and by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. DOE, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office.

The position of the localized states in the band gap often dictates the electrical and optical properties of wide band gap materials. Here we show that modulating the position of these states by external stimuli provide interesting ways to tailor the electrical properties of Ga2O3. Experiments showed that limited exposure to sub-bandgap light caused a permanent transition from an insulator state to a conductor state in an insulating Ga2O3 crystal with 9-orders of magnitude increase in electronic conduction. Photoexcitation by light induced modification in the charge state of defects and subsequent lattice distortion around them was shown to be the underlying mechanism behind this transition. Density functional theory calculations confirmed that modifying the charge state of defects lead to redistribution of the localized electrons and massive structural distortion in the lattice around them causing large shift in the density of states and introducing new states with shallower energy levels. Other examples on modulating the localized states such as hydrogen incorporation in vacancies will be discussed. Lastly a new spectroscopy method we recently developed to facilitate the measurement of the position and density of the localized states in the band gap will be presented.

Recent increasing interest in the research of gallium oxide is being driven by its potential applications for the next generation of power electronic devices. Its material properties including a wide bandgap, high breakdown electric field and large Baliga figure of merit value make it particularly attractive for these applications. Metalorganic Chemical Vapour Deposition (MOCVD) is the leading method for production of compound semiconductor electronic and optoelectronic devices, and for gallium oxide materials, MOCVD may also be advantageous compared to some other epitaxial growth techniques by allowing the controlled growth of alloys of (AlxGa1-x)2O3 and heterostructures. AIXTRON is applying its well-established Close-Coupled Showerhead (CCS) reactor technology to the MOCVD growth of gallium oxide materials and initial results will be presented here. Further results of MOCVD growth of gallium oxide on both sapphire and native Ga2O3 substrates will be discussed.

Memristors, with proposed applications for logic gates, non-volatile memory, and neuromorphic computing, typically consist of an oxide sandwiched between two metal contacts. When a voltage is applied, oxygen vacancies diffuse through the material and form a conducting filament, causing the memristor to switch from a high resistance state (HRS) to a low resistance state (LRS). Using density functional theory with hybrid functionals, we show the atomistic mechanisms on how Mg doping of Al2O3 can increase the performance of memristors, and how we can tune the performance further by adding or alloying with Ga2O3.

Gallium oxide (Ga2O3) presents several advantages as a semiconductor material, predominantly in power electronics, given its wide bandgap, high breakdown voltage, increased thermal stability, scalability, electron mobility, affordability, and abundance. Ga2O3 possesses a wide bandgap of around 4.8-4.9 eV (depending on crystal structure), which enables the handling of higher voltages, reducing leakage current and enabling operation at higher temperatures compared to traditional semiconductor materials such as silicon (Si). Its wide bandgap translates into a high breakdown electric field strength (8 MV/cm or higher), making Ga2O3 suitable for applications requiring high breakdown voltages, such as power devices and switches. Ga2O3 demonstrates excellent thermal stability, enabling efficient operation at elevated temperatures, which is beneficial in power devices where heat dissipation is a concern. This stability contributes to the reliability and longevity of semiconductor devices. Ga2O3 offers potential for large-area, high-quality single-crystal growth, which is essential for large-scale semiconductor device fabrication. This scalability is crucial for industrial production and integration into electronic systems. Gallium oxide has exhibited high electron mobility, which is essential for achieving high-speed operation in power transistors and other electronic devices. It is relatively abundant compared to some other semiconductor materials like gallium arsenide (GaAs), which could contribute to cost-effectiveness in mass production once manufacturing techniques and process control models are optimized. The convergence of these attributes make gallium oxide a promising candidate for next-generation power electronics, contributing to improved efficiency, reduced energy losses, and enhanced performance in various high-power applications.

Obtaining uniform silicon concentration, especially with low concentrations (ranging from 1x10^16 cm^-3 to 1x10^18 cm^-3) by molecular beam epitaxy (MBE) has been challenging due to the oxidation of silicon solid source in the oxide environment. In this talk, I will present our recent results on Si doping of β-Ga2O3 (010) films by diluted disilane as the Si source is investigated using a hybrid plasma-assisted molecular beam epitaxy (hybrid-PAMBE). Uniform Si concentrations ranging from 3×10^16 cm^-3 to 2×10^19cm^-3 are achieved. We also investigated Ga sub-oxide as Ga source. The impact of using Ga metal vs Ga sub-oxide on Si doping was investigated.

β-Ga2O3 MOSFETs with various un-intentionally doped (UID) layer thicknesses of (2 ̅01) beneath the Si doped film were grown by metalorganic chemical vapor deposition on a (0001) sapphire substrate. The UID layer thickness gives rise to MOSFET turn-on current increasing, Ron decreasing and breakdown voltage decreasing. Through X-ray photoelectron spectroscopy (XPS) we ascribe this increased turn-on current and reduced breakdown voltage to oxygen vacancies.

Recently, Ga2O3 has garnered significant attention as an ultrawide bandgap semiconductor, possessing a bandgap of approximately 5 eV. Ga2O3 exists in five polymorphs: α, β, γ, δ, and κ phases. This presentation will discuss the control of these five polymorphs using mist chemical vapor deposition (CVD) and epitaxial techniques. From an epitaxial growth standpoint, maintaining a small lattice mismatch between the thin films of interest and the underlying layers, such as substrates and buffer layers, is of significant importance. For instance, among these five polymorphs, the existence of the δ-phase remains a subject of debate within the Ga2O3 research community. Nevertheless, the utilization of small lattice-mismatched β-Fe2O3 buffer layers, grown using mist CVD, has enabled the growth of δ- Ga2O3 epitaxial thin films. In addition to discussing the δ-phase, this presentation will also encompass the epitaxial growth of the other four polymorphs of Ga2O3.

In this paper, we focus on the electrical transport properties of undoped, and carbon doped b-Ga2O3 thin-films. For this purpose, b-Ga2O3 samples were grown on sapphire substrates by plasma assisted pulsed laser deposition. The samples were characterized by Raman scattering and temperature dependent dark conductivity measurements. Raman scattering shows that all samples are polycrystalline. The dark conductivity exhibits two distinctly different ranges in the measured temperature range. In the relaxed state the conductivity is activated with an energy of 0.68 eV for T > 250 K. For T < 250 K the conductivity approaches a constant value of 6x10-12 Scm-1. After illumination with sub band-gap light the dark conductivity increases up to 3 orders of magnitude. This is accompanied by a decrease of the activation energy to 0.36 eV. For C-doped samples the activation energy decreases. The state with the smaller activation energy is metastable . The time and temperature dependence of this relaxation reveals two process and can be described with two stretched exponential decays. From the temperature dependence of the time constants the energies of the traps are determined.

This paper presents the advantages and disadvantages of using triethylgallium (TEGa) and trimethylgallium (TMGa) for realization of high-purity, carbon-free, epitaxial Ga2O3 films. Critical process conditions and MOCVD reactor geometries on achieving high purity β-Ga2O3 films with high electron mobility and low background carrier concentration, including doping control in this range, will be discussed. This paper will also discuss the MOCVD growth of high Al composition (up to 30%) high quality strained β-(AlGa)2O3/Ga2O3 heterostructures. To obtain semi insulating films, we will discuss Ga2O3 doping with nitrogen, comparing various nitrogen precursors. We will present the limitations of MOCVD technique for achieving heavily doped (>1020 1/cm3), highly conductive β-Ga2O3. We will present a new in-situ etching process for Ga2O3 in MOCVD using Ga and Cl based chemistry.

The use of near-zero index (NZI) materials for controlling radiative energy in thermal energy management has gained significant attention. However, conventional metal oxides suffer from notable changes in their optical properties at high temperatures, limiting their practicality in the near-infrared (NIR) range. This research investigates the potential of Lanthanum-doped Barium Stannate (LBSO) as a refractory NZI material suitable for high-temperature environments. By employing LBSO in a metal-insulator-metal (MIM) nano-cavity structure alongside Barium Titanate (BTO), a wavelength-selective thermal emitter was developed. The emitter exhibited impressive performance, with approximately 97% absorption at 2.2 µm, coinciding with the peak emission wavelength at 1000 ℃. Notably, the LBSO-based device demonstrated outstanding stability even under harsh high-temperature conditions.

Super-Eg excitation wavelength dependent Zni-VO complex defects related characteristic emissions of green, yellow, and orange-red from ZnO QDs are completely inhibited by reducing oxygen vacancies through the hybridization of Zn interstitials in ZnO QDs with antibonding O- states of graphene oxide (GO) QDs. Only λex independent violet-purple-blue (V-P-B) emission resulting from transitions among Frenkel pairs (Zni0-VZn) and the defects Zni0-Oi in ZnO- GO QDs is successfully achieved. White-light emission from PL QD-LEDs is achieved using a mixture of yellow emissive ZnO and blue emissive ZnO-GO QDs with CIE (0.32, 0.34) excited by a UV LEDs chip (λ = 365 nm). Further, ZnO-GO QD-based deep-blue LEDs (λ = 438 nm) with luminance of 1980 cd/m2, a luminous efficacy (LE) of 2.53 cd/A, and external quantum efficiency (EQE) of 2.78% with CIE (0.16, 0.11) are also realized.

Core–shell heterostructures have attracted extensive attention in the photoelectric field because of their improved optoelectrical properties. In this study, we report a novel self-powered ultraviolet (UV) photodetector based on In2O3/InN nanorods (NRs) core-shell heterostructure developed on the corrugated V-grooved Si (100) substrate. Under 365 nm UV light the hybrid heterostructure demonstrates remarkable photo-to-dark current ratio, high detectivity (64.85 A/W), responsivity (8.5 × 1012 Jones) excellent external quantum efficiency (9.15 × 103 %) and fast rising/falling times in a self-driven mode (0 V). This remarkable photoelectric detection capability is attributed to the strong absorption and abundant trapping of UV light by the multiple scattering of V-grooved texture and formation type-II heterojunction between the In2O3 and InN NRs core-shell structure resulting self-power mode, which enable notable charge separation and fast electron transfer. These accomplishments set the stage for future optoelectronic applications of III-nitride based core-shell heterostructure UV photodetectors that are self-powered.

In recent decades, GaN and related compounds gained prominence as top semiconductor materials for high-power, high-temperature optoelectronics, electronics, and power conversion. However, better substrate materials are still sought after despite extensive nitride research. Commonly used substrates like sapphire and silicon have high lattice mismatches with GaN, leading to challenges in heteroepitaxial growth. A promising solution emerged with ScAlMgO4 (SCAM) proposed as a GaN growth substrate. SCAM offers several advantages: i) Smaller lattice mismatch with GaN than sapphire, reducing dislocation density in grown structures. ii) Matching thermal expansion coefficient with GaN along the a-axis, reducing residual strain. iii) Easy cleavage along the c-plane, yielding atomically flat substrates without polishing. iv) Ability to grow large SCAM crystals via the Chochralski method. In this study1, we present experimental and theoretical investigations on the optical, electronic, and structural properties of ScAlMgO4. Our experimental techniques include variable angle spectroscopic ellipsometry, optical transmission, X-ray diffraction, scanning electron microscopy, and Raman spectrosco

In the context of searching of new oxide materials for the application in the field of photonics sensors, this work is focused on the fabrication and characterization of tellurite thin films doped with erbium. For the deposition of amorphous TeO2-ZnO-(Bi2O3 or BaO or Na2O)-Er2O3 thin films with thickness around 700 nm, radio-frequency magnetron sputtering technique with Ar:O2 flow ratio = 40:10 sccm was exploited. The deposited films were characterized in detail via X-ray diffraction, atomic force microscopy, profilometry, transmission spectroscopy, variable angle spectroscopic ellipsometry, scanning eectron microscopy with energy-dispersive X-ray analysis and luminescence spectroscopy. The results are discussed in relation with the chemical composition of the fabricated thin films. The financial support from the European Union’s Horizon Europe Framework Programme under grant agreement No 101092723 is greatly acknowledged.

Perovskite solar cells (PSCs) have gained significant attention as a promising photovoltaic technology due to their excellent power conversion efficiency (PCE) and cost-effective fabrication methods. However, ensuring long-term stability and addressing environmental concerns are vital challenges for successful commercialization. In this study, we explore metal oxide-based thin film encapsulation methods to enhance the performance and durability of PSCs. We considered back surface modification for flexible PSCs, as well as top surface encapsulation applicable to both rigid and flexible devices. Our results reveal increased operational stability and device performance, effectively addressing critical challenges in PSC commercialization.

Optimization of the electron transport layer in quantum dots light emitting diodes by co-doping ZnO with gallium and magnesium

We have developed a new light emitting device named ZOGAN LED. It is composed with the p-layer comprising p-ZnO and p-GaN. ZOGAN LED and GaN LED are dramatically different in device performance. For example, it is well known for GaN LED that the EQE of GaN LED becomes worse, as either the device size is getting smaller, or the injection current is larger. However, unlike GaN LED, ZOGAN LED shows no EQE droop with current increase or size decrease, while keeping its EQE level exceptionally high. No EQE droop and no size-dependent EQE decrease are unique features of ZOGAN LED, which is never ever reported before. The non-radiative leakage current is almost negligible and not increased as the chip size decreases even below 10um for ZOGAN microLED. For these unique properties, ZOGAN microLED is critical to accomplish the high-efficient microLED displays with >5,000PPI resolution. In this presentation, we will report the features of ZOGAN microLED, based on experimental results of electrical/optical measurements. After brief discussion and review on physical properties of ZOGAN LEDs, ZOGAN microLEDs in different chip sizes (20, 10, and 5 microns in diameter) will be demonstrated.

Phonon polaritons are strongly coupled hybrid light-matter waves which emerge when infrared light interacts with infrared active phonons in polar dielectric crystals. These excitations have recently attracted much attention as a versatile tool for low-loss nanophotonic applications in a range spanning from mid- to far-infrared. Phonon polaritons exist inside the Reststrahlen band, i.e. the spectral region between the transverse and longitudinal optical phonon frequencies where the dielectric permittivity is negative. For anisotropic oxide crystals such as molybdenum trioxide, calcite, and beta-type gallium oxide, so-called hyperbolic polaritons emerge when the infrared permittivity has opposite signs along different principle directions, leading to highly directional and strongly confined polaritonic states. Here I will report on both momentum space and real space experimental observation of a new type of phonon polaritons entitled hyperbolic shear polaritons in beta-gallium oxide, which display frequency dependent nanoscale propagation direction and asymmetries emerging directly from the low crystal crystal symmetry.

This conference presentation was prepared for SPIE Photonics West, 2024.

The development of oxide semiconductor films is an essential process for the application of high-performance electronic devices and the development of next-generation neuromorphic systems. Recently, research has been focused on the development of photodetection sensors using indium gallium zinc oxide (IGZO), which shows excellent performance as an active layer in transistors and resistance-changing memories. Various methods have been proposed to improve absorbance in visible light areas by designing additional photo absorption layers or introducing inter-gap sites through chemical processes to improve photo-response properties of IGZO materials with wide band gaps. In this work, we deposited two layers of IGZO thin films and performed annealing processes at different temperatures to improve photo-response properties without additional materials or complicated manufacturing steps. Consequently, we propose a relatively simple method to improve the photo-response properties in high-performance oxide semiconductors and provide a detailed description of the photo-response behavior for future applications of photo sensors and neuromorphic AI devices.