Gallium oxide is being widely studied, mainly for high-power electronics applications. It is a very promising material for photonic/optoelectronic applications, such as solar-blind UV detectors and light emitters. In this work, we study the temperature-dependent behavior of the optical properties of microcavities based on luminescent β-Ga2O3:Cr nanowires that emit an intense red-infrared band. Two distributed Bragg reflectors (DBR), when milled with a focused ion beam (FIB) and separated some microns, result in an optical microcavity that confines the light longitudinally. Both chromium R lines and Fabry-Perot spectral resonances (FPSR) are observed to shift as temperature varies, making these DBRs a valuable thermometer in a wide temperature range, due to both luminescent and interferometric transducing mechanisms. The underlying origin of this shift, in the case of the FPSR, is mainly the variation of the refractive index with temperature and the thermal expansion of the cavity. Ellipsometry studies carried out at different temperatures in bulk β-Ga2O3 yielded the dispersion relations for the three main crystal axes, i.e. its temperature-dependent anisotropic refractive index. These results were implemented in finite-difference time-domain (FDTD) simulations to compare the expected spectral shift of the FPSR in the modelled system with the experimental shift in the DBR cavities, as obtained experimentally by micro-photoluminescence. The results from these two approximations, and a third one based on solving the relevant analytical equations, are compared.
Interest in nonlinear optical (NLO) materials for mid-infrared frequency conversion has exploded in recent years largely due to the emergence of new ultrafast laser applications ranging from frequency-comb-based spectroscopy to high harmonic and THz generation. Here we discuss how to advance the state of the art of the best commercially available materials in the mid-IR, including the bulk birefringent crystals CdSiP2, ZnGeP2, and GaSe, as well as quasi-phase-matched orientation-patterned GaAs and GaP, and we point to emerging materials that need further development to extend the performance of widely-used near-infrared pump sources deeper into the infrared (to 12 microns and beyond).
BaGa4S7 (BGS) and BaGa4Se7 (BGSe) are attractive new nonlinear optical (NLO) crystals notable for the rare combination of wide band gaps (3.54 eV and 2.64 eV), long phonon cut-off wavelengths (13.7 m and 18 m), and relative ease of growth from stoichiometric melts, making them ideal for shifting widely-available 1-micron laser sources deep into the mid-IR. Here we demonstrate high purity HGF growth along desired phase-matching directions for simplified fabrication and maximum yield of oriented frequency conversion devices, allowing apertures up to 15x25 mm2 and lengths greater than 20 mm, as well as compare seeded vs. unseeded growth.
Properties of monoclinic β-Ga2O3 are strongly dependent upon orientation. In the case of optical properties, polarization can reveal differences in optical bandgap and absorptions related to transition metal ions. This phenomenon is known as pleochroism and has been extensively studied in minerology. β-Ga2O3 bulk single crystals doped with Zn, Mn, Cr, or Cu were grown by the Czochralski and vertical gradient freeze methods. Ultraviolet-visible-near infrared spectroscopy and photoluminescence (PL) revealed polarization- and orientation-dependent optical absorptions in β-Ga2O3. Crystals were annealed in reducing and oxidizing environments in an attempt to alter the intensities of absorptions characteristic to the different transition metal oxidation states in a given ligand field. Visible pleochroism was strongest in (001) oriented Mndoped samples as shown by polarized optical microscopy. All samples were electrically insulating, indicative of acceptor doping, aside from Cr-doped samples where Cr acts as a deep donor.
The interaction of transparent oxide semiconductors with light is critically important for a range of applications. Persistent effects could be exploited for holographic memory or optically defined circuits. Conversely, they may also be detrimental to device operation. Large, room-temperature persistent photoconductivity (PPC) was discovered in strontium titanate (SrTiO3, STO) after annealing in a hydrogen-containing atmosphere. Barium titanate (BaTiO3, BTO), a ferroelectric material, was recently found to also exhibit PPC. Room-temperature photodarkening was observed in Cu-doped gallium oxide (β-Ga2O3) after exposure to sub-bandgap light. Hydrogen is believed to play a central role in these persistent phenomena. In the proposed model, a photon excites substitutional hydrogen (a proton inside an oxygen vacancy), making the defect unstable. The proton leaves and binds to a host oxygen atom, forming an O-H bond that is observed with infrared spectroscopy. An oxygen vacancy is left behind. Because oxygen vacancies in STO and BTO are shallow donors, this process results in PPC. In β-Ga2O3:Cu, however, the oxygen vacancy neighbors a Cu acceptor. In that case, photoexcitation results in the rare Cu3+ state, which absorbs visible light. The effect can be “erased” by annealing at 300-400°C.
Much excitement has surrounded the accelerating development of β-Ga2O3 for electronics due to its ultrawide band gap, high breakdown voltage, compatibility with many dopants, and comparative ease of producing large substrates via meltgrowth techniques. Our research has focused on growth and characterization of Czochralski (CZ) and vertical gradient freeze (VGF) single crystals of β-Ga2O3 with various dopants, including donors (Zr, Hf, Cr), acceptors (Mg, Zn, Fe, Ni, Cu), and alloying elements (Al). We find in general that doping in CZ and VGF materials can be different and sometimes non-uniform due to the interaction with crucible material (Ir), selective evaporation, and thermal profile. We have also explored the creation and identification of gallium vacancies (VGa) through annealing, by using positron annihilation spectroscopy (PAS), hydrogenated Fourier Transform Infrared (FTIR) spectroscopy, and electrical measurements. Different analysis techniques probe different spatial and depth averages, and thus careful consideration must be given to correctly interpret results and significance of defect concentrations determined. Insights from our work to date are offered, in terms of their applicability to devices.
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