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.
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.
ZnS-based materials have a long history of use as x-ray luminescent materials. ZnS was one of the first discovered scintillators and is reported to have one of the highest scintillator efficiencies. The use of ZnS for high energy luminescence has been thus far limited to thin powder screens, such as ZnS:Ag which is used for detecting alpha radiation, due to opacity to its scintillation light, primarily due to scattering. ZnS in bulk form (chemical vapor deposited, powder processed, and single crystal) has high transmission and low scattering compared to powder screens. In this paper, the performance of single crystalline ZnS is evaluated for low energy x-ray (<10 keV) imaging. For these applications, a scintillator needs to be thick enough to absorb the incoming x-rays and to provide sufficient gain, but thin enough to allow for a good spatial resolution. The scintillators also need to have a good radiation hardness, a fast decay time, and low levels of afterglow. We present a trade study which compares the calculated scintillation gain and absolute efficiency for low energy x-rays (<10 keV) comparing thin (<100 μm) ZnS to CsI:Tl, Bi4Ge3O12 (BGO), and Y3Al5O12:Ce (YAG:Ce). The study also gives insight into the spatial resolution of these scintillators. Further, photoluminescence (PL) and PL excitation (PLE) of several undoped ZnS single crystals is compared to their Radioluminescence (RL) spectra. It was found that the ZnS emission wavelength varies on the excitation source energy.
A large body of literature was reviewed with the aim of identifying binary and ternary systems for producing long-wave infrared transmitting glass-ceramics for window applications. Known optical and physical property data was summarized for many ternary sulfides as well as their constituent binary sulfides. Some phosphide and arsenide chalcopyrite structures were reviewed as well. Where available, data on the transmission range, energy gap, refractive index, and hardness were tabulated. Several glass-forming systems were identified containing Ga2S3, GeS2, or As2S3.
Zinc sulfide has shown unequaled utility for infrared windows that require a combination of long-wavelength infrared transparency, mechanical durability, and elevated-temperature performance. This book reviews the physical properties of chemical vapor deposited ZnS and their relationship to the CVD process that produced them. An in-depth look at the material microstructure is included, along with a discussion of the material's optical properties. Finally, because the CVD process itself is central to the development of this material, a brief history is presented.
It is often useful to obtain custom glasses that meet particular requirements of refractive index and dispersion for highend
optical design and applications. In the case of infrared glasses, limited experimental data are available due to
difficulties in processing of these glasses and also measuring refractive indices accurately. This paper proposes
methods to estimate refractive index and dispersion as a function of composition for selected infrared-transmitting
glasses. Methods for refractive index determination are reviewed and evaluated, including Gladstone-Dale, Wemple-
DiDomenico single oscillator, Optical basicity, and Lorentz-Lorenz total polarizability. Various estimates for a set of
PbO-Bi2O3-Ga2O3 (heavy metal oxide) and As-S (chalcogenide) glasses will be compared with measured values of
index and dispersion.
Significant anisotropy in as-deposited CVD ZnS at several length scales has been demonstrated through investigation of
structural and optical properties. Compressive strength of cylinders of CVD ZnS oriented in the growth direction is
~50% higher than cylinders taken perpendicular to the growth direction. Lattice parameter measurements of mandrel
side (first-to-grow) material is ~0.4% smaller than growth side (last-to-grow) material in a cored sample representing
~500 hours of CVD growth, indicating significant strain along the growth direction. X-ray diffraction also shows
evidence of preferred orientations for hexagonality which differ depending on position in the growth history. In crosssection,
the cored sample shows several large bands which are correlated with different degrees of infrared absorption
and BTDF scattering. However, no universal trend is found that applies to the whole length from the mandrel to the
growth side regarding optical properties. The extinction in the visible and infrared is lower for measurements
perpendicular to the growth axis than parallel to it, possibly due to scattering from the growth bands.
Samples of CVD ZnS from the United States, Germany, Israel, and China were evaluated using x-ray diffraction,
transmission and Raman spectroscopy, and biaxial flexure testing. Visible and near-infrared scattering, 6 μm
absorption, and ultraviolet cut-on edge varied substantially in tested materials. Transmission cut-on (ultraviolet edge)
blue-shifts with annealing and correlates with visible color but not the 6 μm absorption. Raman scattering for CVD
ZnS, presented here for the first time, was similar for all ZnS tested. Crystallographic hexagonality and texture was
determined and correlated qualitatively with optical scattering. All CVD ZnS tested with biaxial flexure exhibit similar
fracture strength values and Weibull moduli. This survey suggests that despite over 30 years of production as an
infrared window, the optical properties of CVD ZnS and their variability still defy easy explanation.
A series of experiments was performed to ascertain the effects of various metals on the structure and properties of hot isostatic pressed (HIPped) chemical vapor deposited (CVD) ZnS. Samples were HIPped without metal and with Fe, Co, Ni, Cu, Pd, Ag, Pt, and Au foils. It was found that metals promote recrystallization of CVD ZnS to a greater or lesser extent. A processing temperature of 750 °C for 16 hours was chosen to assess the effect of the metals, since HIPping without metal under these conditions does not recrystallize ZnS. Fe and Co have little or no effect on recrystallization, Ni and Au have moderate effect, and Pt, Pd, Ag, and Cu have the greatest effect. Ag and Cu, however, seem to have problems with indiffusion of the metal. Recrystallization is correlated with improvement in transmission characteristic of multispectral ZnS. "Interrupted HIP" experiments were conducted at 900 °C for 1 hour and at 750 °C for 2 hours to assess the relative effects of temperature and metal on the recrystallization. At 900 °C recrystallization proceeded in the bulk even without metal, but was accelerated by certain metals. At 750 °C, recrystallization took place only on the
surface in contact with certain metals and not in the bulk. The role of contact of the metal to the ZnS surface was further explored by comparing Pt HIP experiments with foil fully in contact, foil with air gap, and sputtered metal Pt. Some possible mechanisms for the role of the metal in promoting recrystallization of ZnS are suggested.
A model has been created based on scattering that describes the λ-2 dependence of the extinction in bulk samples of
CVD ZnS. The model is a version of surface scattering from internal surfaces of layers with different refractive index.
The form of the model was inspired by observation of a lamellar nanostructure in CVD ZnS composed of alternating
layers of thickness on the order of 10 to 100 nm. The scattering model produces a family of solutions which depends on
the difference in refractive index (Δn), the layer thickness, and the roughness. Reasonable ratios of roughness to layer
thickness require Δn for CVD ZnS with higher values than can be explained solely by the Δn between sphalerite and
wurtzite phases of ZnS. Other evidence suggests a substantial oxygen component in CVD ZnS that could result in the
lower refractive index Zn(O,S) necessary for the model. Differences in transmission for CVD ZnS, elemental ZnS, and
multispectral ZnS can be explained simply by a different magnitude of Δn between the layers. Absolute transmission is
modeled satisfactorily from the band edge to 10 μm using this approach. Extracted Δn's from transmission
measurements of various samples correlate well with measured hexagonality from x-ray diffraction.
The materials community realized that zinc sulfide (ZnS) was an important optical material for infrared windows over
forty years ago. Chemical vapor deposition (CVD) quickly became the method of choice for producing large ZnS
windows and domes. In addition to the development initiated in the United States, several international efforts for
understanding the processing and properties of CVD ZnS are notable. This paper summarizes the published history of
non-U.S. CVD ZnS development including the significant efforts in the United Kingdom, the former Soviet Union,
Israel, China, and Japan.
Polycrystalline Yttrium Aluminum Garnet (YAG) is being considered as an attractive material candidate for IR transparent missile domes and reconnaissance windows, due to its superior optical clarity and mechanical properties compared to the incumbent material choices. YAG possesses a very uniform index of refraction with minimal variation. Its fracture strength, hardness, and toughness also rank high among various other optically transparent materials and can be optimized further through grain size minimization. Polycrystalline YAG has been in development for several years at Raytheon for laser gain and IR transparency applications. Recent advances in optical loss characterization and optimization, scale-up efforts, and the fabrication of non-planar geometries such as hemispherical domes will be presented. In addition, the YAG material trade study conducted to date on thermal, optical, mechanical properties are discussed.
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