U.S. Air Force Research Laboratory Sensors Directorate

The authors used depth-resolved cathodoluminescence spectroscopy and current-voltage measurements to probe metal-ZnO diodes as a function of native defect concentration, oxygen plasma processing, and metallization. The results show that resident native defects in ZnO single crystals and native defects created by the metallization process dominate metal-ZnO Schottky barrier heights and ideality factors. Results for ZnO(0001¯) faces processed with room temperature remote oxygen plasmas to remove surface adsorbates and reduce subsurface native defects demonstrate the pivotal importance of crystal growth quality and metal-ZnO reactivity in forming near-interface states that control Schottky barrier properties.

The authors develop a rigorous theory of the enhancement of spontaneous emission from a light emitting device via coupling the radiant energy in and out of surface plasmon polaritons (SPPs) on the metal-dielectric interface. Using the GaN∕Ag system as an example, the authors show that using SPP pays off only for emitters that have a low luminescence efficiency.

The proper selection of electrical contact materials is one of the critical steps in designing a metal contact microelectromechanical system (MEMS) switch. Ideally, the contact should have both very low contact resistance and high wear resistance. Unfortunately this combination cannot be easily achieved with the contact materials currently used in macroswitches because the available contact force in microswitches is generally insufficient (less than 1mN) to break through nonconductive surface layers. As a step in the materials selection process, three noble metals, platinum (Pt), rhodium (Rh), ruthenium (Ru), and their alloys with gold (Au) were deposited as thin films on silicon (Si) substrates. The contact resistances of these materials and their evolution with cycling were measured using a specially developed scanning probe microscope test station. These results were then compared to measurements of material hardness and resistivity. The initial contact resistances of the noble metals alloyed with Au are roughly proportional to their resistivities. Measurements of contact resistance during cycling of different metal films were made under a contact force of 200–250μN in a room air environment. It was found that the contact resistance increases with cycling for alloy films with a low concentration of gold due to the buildup of contamination on the contact. However, for alloy films with a high gold content, the contact resistance increase due to contamination is insignificant up to 108cycles. These observations suggest that Rh, Ru, and Pt and their gold alloys of low gold content are prone to contamination failure as contact materials in MEMS switches.

The authors propose a Ge∕Ge0.76Si0.19Sn0.05 quantum cascade laser using intersubband transitions at L valleys of the conduction band which has a “clean” offset of 150meV situated below other energy valleys (Γ,X). The entire structure is strain-free because the lattice-matched Ge and Ge0.76Si0.19Sn0.05 layers are to be grown on a relaxed Ge buffer layer on a Si substrate. Longer lifetimes due to the weaker scattering of nonpolar optical phonons reduce the threshold current and potentially lead to room temperature operation.

The combined use of GaAs wires with Ohmic contacts formed from bulk wafers, soft lithographic transfer printing techniques, and optimized device designs enables mechanically flexible transistors to be formed on low-cost plastic substrates, with individual device speeds in the gigahertz range and with high degrees of mechanical bendability. These high-speed devices incorporate materials in simple layouts that can be fabricated with modest lithographic patterning resolution and registration. This letter describes their electrical and mechanical characteristics. The results have the potential to be important to certain large-area, “macroelectronic” systems that can provide for high-speed communication and processing capabilities.

A scattering model for rice canopy based on Monte Carlo simulations is applied to interpret RADARSAT data and to predict the temporal response of rice growth. The model takes into account the coherent wave interactions among vegetative elements which usually occur in clusters with closely spaced elements. The model was also used to analyze the structural effect of rice fields on the scattering

Design and simulation results are presented for an ultralow switching energy, resonator based, silicon-on-insulator (SOI) electro-optical modulator. The nanowire waveguide and Q ~8500 resonator are seamlessly integrated via a high-transmission tapered 1D photonic crystal cavity waveguide structure. A lateral p-n junction of modulation length L(m) ~λ is used to alter the index of refraction and, therefore, shift the resonance wavelength via fast carrier depletion. Differential signaling of the device with ΔV ~0.6 Volts allows for a 6 dB extinction ratio at telecom wavelengths with an energy cost as low as 14 attojoules/bit.

For light impinging normally on the surface of a double-side-polished sample of thickness d, the sample's absorption coefficient α can be determined from the well-known formula for fractional transmittance: Tmeas = (1 − R)2exp(−αd)/[1 − R2exp(−2αd)]. Here, R is a fundamental property of the air/sample interface and is known as the “reflectance coefficient.” Often R in this equation is equated to the measured top-surface reflectance Rmeas, but such an approximation can lead to serious error. In fact, the authors explicitly show that Rmeas = R + R(1 − R)2exp(−2αd)/[1 − R2exp(−2αd)] and then further develop an easily solvable transcendental equation that determines both R and α from Tmeas and Rmeas. In strongly absorptive regions (αd ≫ 1), it turns out that R ≈ Rmeas, but in the opposite limit (αd ≪ 1), R ≈ Rmeas/(2 − Rmeas). Formulation by the authors enables accurate determinations of: (1) ε∞, the high-frequency dielectric constant; and (2) relatively weak absorbances, such as those related to defects or impurities with energy levels in the bandgap. The authors also compare the exact calculations of α in semi-insulating GaN:Fe with those obtained from commonly used approximations.

We introduce a class of unidirectional lasing modes associated with the frozen mode regime of nonreciprocal slow-wave structures. Such asymmetric modes can only exist in cavities with broken time-reversal and space inversion symmetries. Their lasing frequency coincides with a spectral stationary inflection point of the underlying passive structure and is virtually independent of its size. These unidirectional lasers can be indispensable components of photonic integrated circuitry.

Controlling the propagation of optical fields in three dimensions using arrays of discrete dielectric scatterers is an active area of research. These arrays can create optical elements with functionalities unrealizable in conventional optics. Here, we present an inverse design method based on the inverse Mie scattering problem for producing three-dimensional optical field patterns. Using this method, we demonstrate a device that focuses 1.55-μm light into a depth-variant discrete helical pattern. The reported device is fabricated using two-photon lithography and has a footprint of 144 μm by 144 μm, the largest of any inverse-designed photonic structure to date. This inverse design method constitutes an important step toward designer free-space optics, where unique optical elements are produced for user-specified functionalities.

The path to achieving integrated RF and power conversion circuitry using the β-Ga2O3 material system is described with regard to the materials high Johnson's RF figure of merit. Recent results, including large signal data at VD = 50 V, are provided, showing progress in achieving high-voltage RF operation. Additionally, progress in achieving high-gain devices through gate length scaling is also benchmarked by a record RF power device with a gate length of 0.5 μm achieving a 2.1 GHz μm fT−LG product. These results are compared with state-of-the-art RF devices, and the expectations for β-Ga2O3 at this point in its maturity throughout this Letter with future milestones laid out to measure progress. The conclusion includes near- and long-term projections for β-Ga2O3 devices for RF based on the results and projected milestones presented.

We report the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$W$ </tex-math></inline-formula> -band large-signal power and efficiency performance of Ga-polar graded-channel (GC) AlGaN/GaN high-electron-mobility transistors (HEMTs) with a 50-nm gate-length mini-field-plate (FP) T-gate. The pre-matched GC GaN HEMT devices with on-chip pre-matching networks show a peak power-added efficiency (PAE) of 45% at 94 GHz at 2.1 W/mm associated power density. With de-embedding of the metal losses in the matching networks, the peak PAE at the GC GaN HEMT itself was estimated to be 50% at 94 GHz with a 2.2 W/mm power density. These efficiencies are the highest reported at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$W$ </tex-math></inline-formula> -band frequencies among all GaN HEMT technologies, including both Ga-polar and N-polar orientations. The power performance was facilitated by high extrinsic <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{T}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$f_{\mathrm {MAX}}$ </tex-math></inline-formula> of 170 and 347 GHz, respectively, with a GC GaN HEMT structure and a mini-FP T-Gate.

We develop and demonstrate a multiwatt highly strained InGaAs∕GaAs vertical-external-cavity surface-emitting laser with a free lasing wavelength of around 1170nm. This laser can be tuned from ∼1147to∼1197nm. This low-cost compact wavelength agile laser can potentially provide high-power coherent light in a wide yellow-orange band by the intracavity frequency doubling.

The authors demonstrate the multiwatt linearly polarized dual-wavelength operation in an optically pumped vertical-external-cavity surface-emitting laser by means of an intracavity tilted Fabry-Perot etalon and a Brewster window. The sum frequency generation from the lithium triborate crystal pumped by this laser confirms that these two wavelengths oscillate simultaneously. Over 30dB side-mode suppression can be achieved at dual wavelengths with a spectral spacing of 2.1nm. The output power is slightly reduced by the intracavity Fabry-Perot etalon and Brewster window.

DC, small, and large signal results are shown under continuous wave and pulsed conditions for a β-Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> metal-oxide-semiconductor field-effect transistor operating at 1 and 2 GHz. The device has a maximum transducer gain, maximum output power, and peak power added efficiency of 13 dB (15 dB), 715 mW/mm (487 mW/mm), and 23.4% (21.2%), respectively at 1 GHz (2 GHz). We observe the continuous wave output power is limited to 213 mW/mm by drain dispersion likely from surface or interface traps in the gate-drain region as indicated by pulsed IV measurements. High parasitic resistances, as indicated by high knee voltages, also limit the power performance under continuous and pulsed large signal conditions.

Abstract We report the first demonstration of self-aligned gate (SAG) β -Ga 2 O 3 metal-oxide-semiconductor field-effect transistors (MOSFETs) as a path toward eliminating source access resistance for low-loss power applications. The SAG process is implemented with a subtractively defined and etched refractory metal, such as Tungsten, combined with ion-implantation. We report experimental and modeled DC performance of a representative SAG device that achieved a maximum transconductance of 35 mS mm −1 and an on-resistance of ∼30 Ω mm with a 2.5 μ m gate length. These results highlight the advantage of implant technology for SAG β -Ga 2 O 3 MOSFETs enabling future power switching and RF devices with low parasitic resistance.

We demonstrate Schottky barrier engineering using PtOx/thin Pt Schottky contacts combined with edge termination using a high permittivity dielectric (ZrO2) field-plate for high-voltage vertical β-Ga2O3 diodes. A systematic study of baseline bare Pt/β-Ga2O3, PtOx/thin Pt/β-Ga2O3, and PtOx/β-Ga2O3 Schottky diode characteristics was performed, which revealed that the PtOx/thin Pt/β-Ga2O3 contact can combine the advantages of both PtOx and Pt, allowing better reverse blocking performance than plain metal Pt/β-Ga2O3 Schottky diodes and lower turn-on voltage than plain oxidized metal PtOx/β-Ga2O3 ones. Moreover, the thin Pt interlayer in the PtOx/thin Pt/β-Ga2O3 anode contact configuration, deposited by e-beam deposition, also provides plasma-free interface at the Schottky junction as opposed to the direct sputter deposited PtOx contacts of the PtOx/β-Ga2O3 diodes. We further implemented a high permittivity dielectric (ZrO2) field plate in PtOx/thin Pt/β-Ga2O3 diodes that assisted in edge-field management and enabled a breakdown voltage to ∼2.34 kV. These results indicate that the PtOx/thin Pt/β-Ga2O3 Schottky contact, combined with a high permittivity field-plate, will be promising to enable Schottky barrier engineering for high-performance and efficient vertical β-Ga2O3 power switches.

Three dimensional (3D) imaging systems have been recently suggested for passive sensing and recognition of objects in photon-starved environments where only a few photons are emitted or reflected from the object. In this paradigm, it is important to make optimal use of limited information carried by photons. We present a statistical framework for 3D passive object recognition in presence of noise. Since in quantum-limited regime, detector dark noise is present, our approach takes into account the effect of noise on information bearing photons. The model is tested when background noise and dark noise sources are present for identifying a target in a 3D scene. It is shown that reliable object recognition is possible in photon-counting domain. The results suggest that with proper translation of physical characteristics of the imaging system into the information processing algorithms, photon-counting imagery can be used for object classification.

Undoped and Ga- and Al- doped ZnO films were synthesized using sol-gel and spin coating methods and characterized by X-ray diffraction, high-resolution scanning electron microscopy (SEM), optical spectroscopy and Hall-effect measurements. SEM measurements reveal an average grain size of 20 nm and distinct individual layer structure. Measurable conductivity was not detected in the unprocessed films; however, annealing in hydrogen or zinc environment induced significant conductivity (∼10−2 Ω.cm) in most films. Positron annihilation spectroscopy measurements provided strong evidence that the significant enhancement in conductivity was due to hydrogen passivation of Zn vacancy related defects or elimination of Zn vacancies by Zn interstitials which suppress their role as deep acceptors. Hydrogen passivation of cation vacancies is shown to play an important role in tuning the electrical conductivity of ZnO, similar to its role in passivation of defects at the Si/SiO2 interface that has been essential for the successful development of complementary metal–oxide–semiconductor (CMOS) devices. By comparison with hydrogen effect on other oxides, we suggest that hydrogen may play a universal role in oxides passivating cation vacancies and modifying their electronic properties.

As a promising three dimensional passive imaging modality, Integral Imaging (II) has been investigated widely within the research community. In virtually all of such investigations, there is an implicit assumption that the collection of elemental images lie on a simple geometric surface (e.g. flat, concave, etc), also known as pickup surface. In this paper, we present a generalized framework for 3D II with arbitrary pickup surface geometry and randomly distributed sensor configuration. In particular, we will study the case of Synthetic Aperture Integral Imaging (SAII) with random location of cameras in space, while all cameras have parallel optical axes but different distances from the 3D scene. We assume that the sensors are randomly distributed in 3D volume of pick up space. For 3D reconstruction, a finite number of sensors with known coordinates are randomly selected from within this volume. The mathematical framework for 3D scene reconstruction is developed based on an affine transform representation of imaging under geometrical optics regime. We demonstrate the feasibility of the methods proposed here by experimental results. To the best of our knowledge, this is the first report on 3D imaging using randomly distributed sensors.