Here I will discuss first-principles calculations based on a hybrid functional that describe the properties of acceptor dopants in gallium oxide. These calculations now have predictive power, as will be demonstrated for the magnesium acceptor in gallium oxide. Acceptors are unlikely to lead to p-type conductivity in gallium oxide, they can compensate prevailing p-type conductivity, and I will compare the stability of acceptor impurities against native defects such as gallium vacancies. The stability of hole polarons in a set of ultrawide-bandgap oxides will also be examined and compared, both in pristine material and in the presence of acceptor impurities.

Monoclinic gallium oxide is a wide-bandgap (4.8 eV) semiconductor with a high breakdown field. To fully exploit the application in high power electronics, it is important to understand how the growth of gallium oxide affect the formation of planar defects. We use density functional theory calculations to explore the energetics and electronic structures of the planar defects including twin boundaries on the (001), (100), and (-102) planes and staking faults on the (001), (100), and (010) planes. We will also discuss the formation mechanism and how the choice of the growth surface can affect the formation of these planar defects. Work performed in collaboration with Sai Mu and Chris G. Van de Walle and supported by AFOSR.

Monoclinic β-Ga2O3 and its Al alloy (AlxGa1-x)2O3 are wide-bandgap materials and are of active interest due to their promising applications in power electronics. Several polymorphs (α,β,γ,κ) are known and are being pursued due their potential advantages for applications. A thorough understanding of the stability of the competing phases in Ga2O3 and (AlxGa1-x)2O3 is needed to promote the formation of the desired phase for device applications. We employ density functional theory to investigate the phase stability of Ga2O3 and (AlxGa1-x)2O3 at both zero temperature and finite temperature. We reveal that a unique configurational entropy is present in the γ phase due to cation vacancy disorder, and that it substantially contributes to stabilizing the γ phase in (AlxGa1-x)2O3 at finite temperature.

Ga2O3 is a wide-bandgap semiconductor with promising applications in high-power devices and UV photodetectors. The κ-polymorph is of interest as it possesses ferroelectric properties and exhibits large spontaneous electrical polarizations. Here we use hybrid density functional theory to elucidate how alloying with Al2O3 and In2O3 can be used to modify the structural and electronic properties [1,2]. We show how lattice constants, bandgaps, and conduction band offsets can be tuned as a function of Al/In concentration. These results can be used to guide experimental design of new devices. Work in collaboration with S. Seacat and J.L. Lyons [1] S. Seacat, J.L. Lyons, and H. Peelaers, Appl. Phys. Lett. 116, 232102 (2020). [2] S. Seacat, J. L. Lyons, and H. Peelaers, Appl. Phys. Lett. 119, 042104 (2021).

Designs of electronic devices using aluminum gallium oxide (ALGO) alloys as the barrier layer and gallium oxide (Ga2O3) as the active layer are being considered. The success of these devices is predicated in part on the ability to achieve controlled doping of the ALGO barrier layer. First-principles calculations using hybrid functionals can be instrumental in guiding this effort. I will highlight which dopants can lead to controlled doping in high-Al content ALGO alloys, identify the optimal doping conditions and discuss the impact of the different ALGO polymorphs on this effort. This work was performed in collaboration with J. Varley, J.L. Lyons, S.Mu, M. Wang, and C.G. Van de Walle and was supported by ONR/NRL 6.1 Basic Research Program.

Ga2O3 is an ultrawide bandgap semiconductor material platform for next-generation power electronics, largely owing to commercially-available single-crystal substrates that can be grown via industrially-scalable processes. Transition metals such as Ir, Cr, and Fe incorporated during growth can strongly influence the resulting optical and electrical properties of single crystals, with Fe now the de facto dopant for achieving semi-insulating substrates. Several other transition metals have been shown to exhibit diverse electronic behavior, acting as deep acceptors, deep donors, or even as efficient shallow donors, depending on how they incorporate into the lattice. Here we survey the current understanding of transition metal point defects in Ga2O3, focusing on their potential optical and electrical consequences from insights gained through first-principles-based calculations employing hybrid functionals.

In this paper, we present experimental data on hydrogen diffusion in single crystal and polycrystalline -Ga2O3 obtained from hydrogen diffusion and effusion experiments. The effective diffusion coefficients for H and D diffusion were obtained from diffusion experiments and amount to D  210-13 cm2/s at a temperature of 425 °C. Similar values were obtained from H effusion experiments. To obtain the energetic barrier for hydrogen migration in -Ga2O3 the heating rate of the effusion experiment was varied. The temperature at which the molecular H flux shows its maximum value increases with increasing heating rate. According to Beyer and Wagner [3] this method is suitable to determine the temperature dependence of the diffusion coefficient. The hydrogen effusion data were further analyzed in terms of a hydrogen density-of-states. State-of-the-art single crystal -Ga2O3 exhibits a well-defined peak at E – Etr ~ 1.5 eV.

We have used first-principles calculations based on density functional theory to accurately model the interaction of hydrogen in Ga2O3 and related alloys. Hydrogen is often present during growth or processing of the material, particularly in metal-organic chemical vapor deposition (MOCVD). Our comprehensive study of surface reconstructions establishes under which conditions hydrogen will be present on the surface, and how it affects the growth. For hydrogen inside the material, we calculate diffusion barriers, and the formation of complexes with intentional or unintentional impurities. Work performed in collaboration with J. L. Lyons, S. Mu, J. B. Varley, M. Wang, and D. Wickramaratne.

We use multiscale modelling to investigate the role of electron injection and hydrogen incorporation inside amorphous (a) gate oxide films of SiO2, HfO2, Al2O3 and at interfaces with Si and TiN in creation of new defects, oxide degradation and dielectric breakdown of electronic devices. Injected electrons localize in a-SiO2 and HfO2 in deep states below the mobility edge. Trapping of two electrons at intrinsic sites results in weakening of Si-O and Hf-O bonds and emerging of efficient bond breaking pathways for producing neutral O vacancies and interstitial O ions with low activation barriers. Creation of O vacancies facilitates trap-assisted tunnelling through oxide films and is responsible for oxide charging and leakage current. Hydrogen incorporation leads to creation of additional defects in the oxide. Atomistic simulations of defect creation in amorphous oxide films are combined with kinetic simulations of trap assisted tunnelling of electrons and field assisted ionic diffusion

In this study, we proposed a new crucible-free crystal growth method, oxide crystal growth from cold crucible (OCCC) method, and attempted to grow oxide single crystals such β-Ga2O3, Gd3Ga3Al2O12 and Y2O3 under a high oxygen partial pressure atmosphere. In the OCCC method, as in the cold crucible method, high frequency is applied directly to the oxide melts. The convection of the melt can be controlled by optimizing the frequency value according to the electric resistivity of the melts, and stable crystal growth through the process from seeding to crystal diameter expansion is a major feature of this method.

Tin-gallium oxide (TGO) epilayers have been characterized through the electron microscopy techniques of wavelength-dispersive X-ray spectroscopy (WDX) and cathodoluminescence. Tin incorporation was found to be highly dependent on growth conditions with (0001)-sapphire and (010)-Ga2O3 substrates enhancing tin incorporation. Cathodoluminescence measurements show that TGO luminescence consists of an enhanced blue emission and quenched UV when compared to Ga2O3.and the onset of new green emission originating from the TGO, further correlated through cross-sectional WDX and cathodoluminescence mapping. As well as luminescence intensity changes TGO films display redshifted luminescence bands associated with a bandgap reduction due to the alloying, confirmed through optical transmission measurements.

The growth of a-GaOx thin films by plasma-enhanced atomic layer deposition is studied by in-situ spectroscopic ellipsometry. A method is developed to extract the dielectric constant and thickness of ultrathin films (< 1 nm) unambiguously. The optical properties were calculated after each step and for all cycles, which allows to follow their evolution until a “bulk-like” gallium oxide film is reached. We show that a-GaOx films can be self-doped with the introduction of oxygen vacancies to obtain semiconducting properties. With additional information retrieved from the in-situ monitoring, a better understanding and faster development of growth processes are gained.

Variable-range-hopping (VRH) in-plane transport is detected by Hall effect measurements in Si-doped orthorhombic κ-Ga2O3 epitaxial films. Columnar rotational domains in nominally undoped the layers have size of tens of nm, while the dimension increases up to hundreds nm in Si-doped samples [https://doi.org/10.1002/adfm.202207821]. Significant anisotropy between in- and out-of-plane conductivity suggests that such domains play a significant role in the disorder-induced VRH transport. We discuss the variation of isothermal Hall mobility, Hall concentration and conductivity considering:(i) spatial scale of domain boundary irregularity, (ii) Si-doping level and compensation effects.

Beta-Ga2O3 has emerged as a new semiconductor for high voltage diodes and transistors. Progress in this field has been rapid due to the availability of high quality melt grown substrates. In this talk we present the progress in epitaxial growth by MBE and MOCVD. We address materials purity, which now has demonstrated unintentional compensating impurity concentrations N_A < 10**14 cm-3, controlled donor doping from <10**16 cm-3 to ~10**19 cm-3; intentional compensation doping by Mg or Fe; controllable wet etching with phosphoric acid; outstanding ohmic contacts; Schottky contacts with near ideal diode behavior and barrier heights in excess of 2 eV. In this talk we also present work from our group on high voltage vertical Schottky diodes with punch-through structures with average electric fields >2 MV/cm.

The unique material properties of Gallium Oxide make it promising for a range of future applications, but innovative materials and device engineering are needed to translate these ultimate material limits to real technology. This presentation will discuss our recent work on epitaxy, heterostructure design, and electrostatics to achieve high-performance β-Ga2O3 lateral and vertical electronic devices. We will discuss some advances in materials growth and device design for lateral structures which enabled key transistor demonstrations including the first β-(Al,Ga)2O3/β-Ga2O3 modulation-doped structures with excellent transport properties, double-heterostructure modulation-doped structures, scaled delta-doped transistors with cutoff frequency of 27 GHz, and self-aligned lateral field effect transistors with > 900 mA/mm current density. We will discuss the use of a new damage-free epitaxial etching technique using Ga atomic flux that enables highly precise fabrication of 3-dimensional structures. We will also show some applications of atomic Ga-flux etching to realize excellent field termination in vertical diodes, and lateral FINFETs with enhanced performance. Finally, we will discuss promising results using high-permittivity dielectrics integrated with semiconductors that have enabled lateral transistors with > 5.5 MV/cm breakdown field, the highest for a field effect transistor in any material system. We acknowledge funding from DOE/NNSA under Award Number(s) DE-NA000392, AFOSR GAME MURI (Award No. FA9550-18-1-0479, project manager Dr. Ali Sayir), and NSF ECCS-1809682.

Recently, oxide semiconductors have attracted tremendous attention due to their applications in various fields from UV emitters, power electronics, nano-sensors and optoelectronics. Furthermore, global warming forces new solutions to limit greenhouse gas emissions and mitigate the negative effects of climate change. By performing thermodynamic calculations based on free-Gibbs energy and equilibrium constants of seven theoretical reactions for two oxides systems ZnO and Ga2O3, we confirm that such semiconductors could be grown by decomposition of carbon dioxide. In addition, we present a modified carbothermal method of growing ZnO nanowires. Nanowires obtained by us display excellent structural and optical properties with FWHM of photoluminescence at T = 8 K from donor bound exciton equal to 0.5 meV. We strongly believe that our modified carbothermal method is an analog of the growth process of large ZnO bulk crystals, which occurred previously in the Olawa foundry in Poland.

We studied electron transport and microwave noise in Zn-polar BeMgZnO/ZnO and O-polar ZnO/MgZnO heterostructures with 2-dimensional electron gas (2DEG) grown on c-sapphire substrates by molecular beam epitaxy. In a short-pulse (<5 ns) high-field experiment, the electron drift velocity reached 1.2E7 cm/s at an electric field up to 200 kV/cm. Pulsed microwave hot-electron noise temperature measurements near 10 GHz in O- polar channels indicate that the hot electron temperature is controlled by hot LO phonons, which increase electron temperature, whereas the presence of excess noise (over "thermal" hot-electron noise) in the Zn- polar channels suggests some inhomogeneity of BeMgZnO barriers.

As a wide band gap semiconductor, ZnO have attracted much attention due to their opportunity of combining band gap engineering, with large excitonic binding energies in the UV-visible range. While many wonderful fundamental results (LED, polariton lasers, etc) have been obtained with this material platform thanks a huge improvement of the growth methods, the development has always been limited in terms of applications by a lack of reliable p-type doping. In this presentation I will address new opportunities of this material platform in a radically different range which does not require p-type doping: from IR to THz. After a deep optimization of the designs and the growth processes, quantum cascade detectors and emitters have been processed and characterized in the Mid-IR and the THz range demonstrating the huge potential of oxides to address the issue of efficient emitters in the THz range. In addition, new opportunities of these heterostructures in even more emerging fields such as hyperbolic metamaterials and multisubband plasmons will be explored.

Thanks to a temporary storage of optical energy at defects, persistent phosphors, i.e. transition metal- or rare earth- doped materials, feature unique long-lasting luminescence. Apart from their current industrial development as micro sized powders dispersed in polymers for design and night signalization, persistent phosphors rise interest for nanotechnologies as high-security-level labelling. However, their relatively low storage capacity combined to the difficulty to finely tune their properties hurdle their development. In this regard, processing persistent phosphors as low scattering coatings offers tremendous opportunities: in addition to facilitating energy storage, transparency allows playing with devised photonics architectures to unprecedently boost persistent properties.

Tm-based laser with emission at 2.3 μm could be used to detect atmosphere pollutants and various molecules. Within this work we investigated if oxide compounds could be efficient for this laser emission as complex energy transfers could occur such as non-radiative relaxation and upconversion. Several oxides hosts are investigated, namely CaGdAlO4 (CALGO), Y2O3, CaYAlO4 and CaYAl3O7 to host Tm3+ for laser effect at 2.3 μm. We focus on the optical characterizations and spectroscopic analysis of these materials, determining intrinsic optical features such as lifetime and broadening of the emission bands.

This presentation will discuss recent studies on the fabrication and characterization of b-Ga2O3 containing Ge donors or N acceptors. A plasma source was used to dope Ga2O3 nanowires with N by exploiting their nanoscale cross sections, while bulk crystals were uniformly doped with Ge by neutron irradiation. The dopant incorporation was confirmed by chemical analyses. We find defect-related luminescence is strongly enhanced in N-doped Ga2O3, which likely originates from defect compensation effects. With Ge doping, both the UV band due to self-trapped holes (STHs) and defect-related emission increase following neutron irradiation, suggesting STHs being localized close to a defect site.

Mid-infrared optical sensors integrating plasmonic waveguides and quantum cascade optoelectronics are an emerging field of research leading to promising results in chemical sensing, environmental monitoring, and biomedical diagnosis. In this work, we investigate TiO2 as waveguiding material for mid-infrared surface plasmon polariton waveguides and show its potential for integrated sensors. Simulations reveal suitable TiO2 dimensions and diffraction grating couplers for ~4.3 µm light. Following these theoretical considerations, we fabricated such devices monolithically integrated with quantum cascade detectors (QCDs) and present their characterization. We further discuss their application in innovative biosensing experiments including glucose detection.

Nonlinear materials responses to external perturbations - in particular when associated with a nonvolatile property that can be read out and manipulated by an applied electric field– are of great interest in today’s ‘More than Moore era’ to embed more functionalities into future optoelectronic devices. Here, ferroelectric oxides play a dominant role, not only because one can switch their permanent polarization, but also because one can find suitable material candidates with a large Pockels coefficient in its vast compositional space that are relevant for efficient electro-optical modulators. The question remains, how these functional materials can be synthesized with excellent stoichiometric control as thin films in a scalable fashion and in a way that their integration with existing material platforms, first and foremost silicon, can be achieved. In this talk the role of stoichiometry on the ferroelectric properties of strained perovskite thin films, namely CaTiO3, SrTiO3 and BaTiO3 will be discussed. Rather than utilizing commonly employed thin film synthesis approaches for the growth of these complex oxide thin films – namely pulsed laser deposition, sputtering, metal-organic chemical vapor deposition or molecular beam epitaxy – the advantages of a hybrid growth approach combining the advantages of chemical vapor deposition and molecular beam epitaxy are highlighted not only in view of its inherent ability to control cation stoichiometry via a self-regulated growth mechanism, but also because it allows for scalable growth rates while maintaining a self-regulated growth, and the epitaxial integration on silicon. These recent thin film synthesis breakthroughs can accelerate the integration of ferroelectric oxides into existing material platforms and devices.

MOCVD growth of high-quality β-(AlGa)2O3 on (010), (001), and (100) β-Ga2O3 substrates using nitrous oxide (N2O), TEGa, and TMAl as sources will be presented. Coherently strained β-(AlGa)2O3 layers with Al composition of ~40% and ~30% were realized on (100) and (010) substrates, which were coloaded during the growth. The films were smooth, but the layers grown on (100) substrates were smoother (~0.3 nm). The N2O can also dope the layers with nitrogen. β-(AlGa)2O3 films with [N] ranging from ~5x1017 to ~2×1019 cm-3 were achieved. The effects of substrate temperature and Al composition on N incorporation will be discussed.

We present a new-generation atomic layer deposition (ALD) technology that revolutionizes the production of conformal optical coatings: the spatial ALD. In spatial ALD, the substrate is rotated across successive process zones to achieve ultra-fast and high-precision thin film deposition. We present our latest results obtained with our novel C2R spatial ALD system, including the fabrication of SiO2, Ta2O5 and Al2O3 with deposition rates reaching > 1 µm/h. We also show that these materials exhibit low surface roughness (<1 Å RMS), low optical loss (<10 ppm @ 1064 nm), excellent uniformity (< 2% over 200 mm) and high damage threshold (up to 40 J/cm2).

Here we investigated the role of oxygen sensitization temperature on physical properties of PbSe thin films. Polycrystalline thin films were deposited on quartz substrates using CBD method and sensitized at different treatment temperatures in an oxygen atmosphere. High temperature sensitization induces fusions/aggregations of individual PbSe grains and forms PbO and SeO2 phases in PbSe matrix. Carrier mobility sensitized at a high temperature increased by ~500 times, in comparison with the sample sensitized at a lower temperature. This mobility improvement could be attributed to the surface, dislocation and defect passivation effects via oxygen diffusing into PbSe and occupying Se vacancies.

In this work we demonstrate the formation of tungsten oxide nanoparticles and aggregate structures through direct femtosecond laser ablation in air. Through selection of different ablation parameters, a degree of control over the formation of aggregate structures is achieved. This allows for the creation of web-like structures spanning large areas of the sample surface or long aggregate chains with lengths over 100 μm, corresponding to more than 1000 nanoparticles linking together. Characterization of nanoparticles is conducted using SEM, TEM and Raman spectroscopy.

This conference poster presentation was prepared for the Photonics West OPTO 2023 Symposium.

Here we will discuss the use of polaritonic strong coupling in the design and fabrication of infrared emitters and optical components and their implications for applications ranging from chemical sensing to on-chip photonics.

Due to relatively low losses, transparent conducting oxides (TCOs) attracted a great deal of attention as a viable alternative to metals for a variety of plasmonic applications targeting the near-infrared (NIR) spectral range. However, additional losses can be caused by the presence of defects caused by the nonequilibrium nature of methods used for depositing TCO thin films. To understand the origin of the additional loss in the ZnO-based TCOs, we have investigated ZnO thin films heavily doped with Al and Ga that were grown by atomic layer deposition and molecular beam epitaxy using spectroscopic ellipsometry, Raman spectrometry, and x-ray diffraction.