Showing papers by "Kevin D. Leedy published in 2017"

$\beta$ -Ga2O3 MOSFETs for Radio Frequency Operation

Abstract: We demonstrate a $\beta $ -Ga2O3 MOSFET with record-high transconductance ( ${g}_{m}$ ) of 21 mS/mm and extrinsic cutoff frequency ( ${f}_{T}$ ) and maximum oscillating frequency ( ${f}_{\max }$ ) of 3.3 and 12.9 GHz, respectively, enabled by implementing a new highly doped ohmic cap layer with a sub-micron gate recess process. RF performance was further verified by CW Class-A power measurements with passive source and load tuning at 800 MHz, resulting in ${P}_{{OUT}}$ , power gain, and power-added efficiency of 0.23 W/mm, 5.1 dB, and 6.3%, respectively. These preliminary results indicate potential for monolithic or heterogeneous integration of power switch and RF devices using $\beta $ -Ga2O3.

Ge-Doped ${\beta }$ -Ga2O3 MOSFETs

Abstract: We report on MOSFETs fabricated on Ge-doped $\beta $ -Ga2O3 homoepitaxial material grown by molecular beam epitaxy on (010) Fe-doped semi-insulating substrates. The Ge-doped channel devices performed similar to previously reported devices with Sn- and Si-doped channels with the drain current ON/OFF ratios of $> 10^{8}$ and the saturated drain current of >75 mA/mm at $V_{G}=0$ V. Hall effect measurements showed a high carrier mobility of 111 cm2/( $\text{V}\cdot \text{s}$ ) with $4\times 10^{17}$ cm $^{-3}$ active carriers. A MOSFET with a gate-drain spacing of $5.5~\mu \text{m}$ had a three-terminal breakdown voltage of 479 V.

Highly conductive homoepitaxial Si-doped Ga2O3 films on (010) β-Ga2O3 by pulsed laser deposition

Abstract: Si-doped Ga2O3 thin films were fabricated by pulsed laser deposition on semi-insulating (010) β-Ga2O3 and (0001) Al2O3 substrates. Films deposited on β-Ga2O3 showed single crystal, homoepitaxial growth as determined by high resolution transmission electron microscopy and x-ray diffraction. Corresponding films deposited on Al2O3 were mostly single phase, polycrystalline β-Ga2O3 with a preferred (20 1 ¯ ) orientation. An average conductivity of 732 S cm−1 with a mobility of 26.5 cm2 V−1 s−1 and a carrier concentration of 1.74 × 1020 cm−3 was achieved for films deposited at 550 °C on β-Ga2O3 substrates as determined by Hall-Effect measurements. Two orders of magnitude improvement in conductivity were measured using native substrates versus Al2O3. A high activation efficiency was obtained in the as-deposited condition. The high carrier concentration Ga2O3 thin films achieved by pulsed laser deposition enable application as a low resistance ohmic contact layer in β-Ga2O3 devices.

Flyweight, Superelastic, Electrically Conductive, and Flame-Retardant 3D Multi-Nanolayer Graphene/Ceramic Metamaterial

Abstract: A ceramic/graphene metamaterial (GCM) with microstructure-derived superelasticity and structural robustness is achieved by designing hierarchical honeycomb microstructures, which are composited with two brittle constituents (graphene and ceramic) assembled in multi-nanolayer cellular walls Attributed to the designed microstructure, well-interconnected scaffolds, chemically bonded interface, and coupled strengthening effect between the graphene framework and the nanolayers of the Al2 O3 ceramic (NAC), the GCM demonstrates a sequence of multifunctional properties simultaneously that have not been reported for ceramics and ceramics-matrix-composite structures, such as flyweight density, 80% reversible compressibility, high fatigue resistance, high electrical conductivity, and excellent thermal-insulation/flame-retardant performance simultaneously The 3D well-ordered graphene aerogel templates are strongly coupled with the NAC by the chemically bonded interface, exhibiting mutual strengthening, compatible deformability, and a linearly dependent relationship between the density and Young's modulus Considerable size effects of the ceramic nanolayers on the mechanical properties are revealed in these ceramic-based metamaterials The designed hierarchical honeycomb graphene with a fourth dimensional control of the ceramic nanolayers on new ways to scalable fabrication of advanced multifunctional ceramic composites with controllable design suggest a great potential in applications of flexible conductors, shock/vibration absorbers, thermal shock barriers, thermal insulation/flame-retardant skins, and porous microwave-absorbing coatings

High pulsed current density β-Ga2O3 MOSFETs verified by an analytical model corrected for interface charge

Abstract: We report on Sn-doped β-Ga2O3 MOSFETs grown by molecular beam epitaxy with as-grown carrier concentrations from 0.7 × 1018 to 1.6 × 1018 cm−3 and a fixed channel thickness of 200 nm. A pulsed current density of >450 mA/mm was achieved on the sample with the lowest sheet resistance and a gate length of 2 μm. Our results are explained using a simple analytical model with a measured gate voltage correction factor based on interface charges that accurately predict the electrical performance for all doping variations.

Gate-recessed, laterally-scaled β-Ga 2 O 3 MOSFETs with high-voltage enhancement-mode operation

Abstract: Beta-phase gallium oxide (Ga 2 O 3 ) is a promising wide bandgap semiconductor possessing a larger bandgap (∼4.8 eV) and critical electric field strength (∼8 MV/cm) than GaN and SiC [1]. Early metal-oxide-semiconductor field-effect transistors (MOSFETs) have shown promising depletion-mode operation with high critical field strength [2], high-current density [3] and high breakdown voltage [4]. On native substrates, enhancement-mode operation has been achieved with a gated unintentional doped channel [5] and fin-channels [6], the latter achieving 600-V normally-off breakdown voltage. However, both are limited to ∼1 mA/mm or less. Here, we report a new enhancement-mode device achieved by gate recess with improved drain-current > 20 mA/mm and near 200-V breakdown for laterally scaled 3-μm source-drain distance (L sd ).

Toward realization of Ga 2 O 3 for power electronics applications

Abstract: As a transparent conducting oxide with a large bandgap of ∼4.9 eV and associated large estimated critical electric field (E c ) strength of 8 MV/cm, β-Ga 2 O 3 (BGO) has been touted for its tremendous potential as a power switch. Power switch metrics such as Baliga's figure of merit (BFOM) estimating dc conduction losses and Huang's material figure of merit (HMFOM) incorporating dynamic switching losses are functions of E C 3 and E C respectively [1, 2]. BFOM for BGO is expected to exceed that of GaN by 400% and HMFOM for BGO is expected to be comparable to GaN. It can also be shown that for a given power loss during switching, the switch frequency (f) varies as E C 2 suggesting the potential of BGO power conversion in the GHz regime [1, 2]. The manufacturability and cost value proposition of BGO based on large area native substrate availability as shown by Huang's chip area manufacturing FOM (HCAFOM) indicate a disruptive cost advantage over GaN (330%). Additionally, the Johnson figure of merit (JFOM) representing the power-frequency product for RF amplification for BGO is similar to that of GaN indicating potential for integration of power conversion and RF applications in the same platform. Finally, Huang's high temperature figure of merit (HTFOM) shows that BGO has the lowest metric for all semiconductors compared here due to low thermal conductivity and high field strength. However, all wide bandgap power semiconductors, including diamond, face significant thermal engineering challenges relative to Si because of the inherent high energy densities of materials with large E c values. A summary of unipolar FET figure-of-merit comparisons for power semiconductors is shown in Table 1.

Model-free determination of optical constants: application to undoped and Ga-doped ZnO

Abstract: For single slabs of uniform material, such as bulk semiconductors, we derive closed-form expressions for absorption and reflection coefficients, α and R, respectively, in terms of measured reflectance and transmittance, Rm and Tm. The formula for α can replace the several commonly used approximations for α as a function of Tm and in particular does not require αd≫1, where d is the thickness. Thus, it can be applied to weak impurity absorptions, such as Fe absorption in Fe-doped GaN. Finally, the real (η) and imaginary (κ) parts of the index of refraction (n=η+iκ) can be obtained from α and R and agree well with η and κ results obtained from other experiments. For multilayer structures, “effective” values of α, R, η, and κ are obtained, but they can often be assigned to a particular layer. This technique has been successfully applied to many bulk and layered structures.

Conformal spray-deposited fluorine-doped tin oxide for mid- and long-wave infrared plasmonics

Abstract: Nanocrystalline spray-deposited fluorine-doped tin oxide (FTO) was investigated for mid- and long-wave infrared plasmonics. Silicon lamellar gratings were conformally coated with FTO, and the excitation of surface plasmon polaritons (SPP) was investigated via their angle and wavelength-dependent reflectivity. Photon-to-SPP coupling efficiency as a function of grating parameters, and in comparsion to gallium-doped zinc oxide (GZO) gratings, was quantitatively analyzed based on a figure of merit related to the sharpness and depth of the coupling resonance. Conformal spray-deposited FTO would be useful in mid- and long-wave infrared plasmonic channel wave guides.

Gallium oxide technologies and applications

Abstract: In this work, we provide early insight into the combined tradespace for both power switching and RF applications afforded by the high critical, electric-field strength of β-Ga 2 O 3 . MOSFETs formed by homoepitaxial growth of β-Ga 2 O 3 films doped with Sn, Si, and Ge on bulk substrates have been characterized electrically. Several key milestones have been achieved such as enhancement-mode operation > 600 V, low ohmic contact resistance < 0.2 Ω·mm, and RF power gain in the GHz regime. These results show great promise for monolithic and hybrid integration of RF amplifiers and switch technologies.

Model-free determination of optical constants: application to undoped and Ga-doped ZnO

Abstract: For single slabs of uniform material, such as bulk semiconductors, we derive closed-form expressions for absorption and reflection coefficients, ∝ and R, respectively, in terms of measured reflectance and transmittance, Rm and Tm. The formula for α can replace the several commonly used approximations for ∝ as a function of Tm, and in particular does not require ∝d >> 1, where d is the thickness. Thus, it can be applied to weak impurity absorptions, such as Fe absorption in Fe-doped GaN. Finally, the real (η) and imaginary (κ) parts of the index of refraction (n = η + iκ) can be obtained from ∝ and R and agree well with η and κ results obtained from other experiments. For multi-layer structures, “effective” values of ∝, R, η, and κ are obtained, but they can often be assigned to a particular layer. This new technique has been successfully applied to many bulk and layered structures.

Heterogeneous integration of low-temperature metal-oxide TFTs

Abstract: The breadth of circuit fabrication opportunities enabled by metal-oxide thin-film transistors (MO-TFTs) is unprecedented. Large-area deposition techniques and high electron mobility are behind their adoption in the display industry, and substrate agnosticism and low process temperatures enabled the present wave of flexible electronics research. Reports of circuits involving complementaryMO-TFTs, oxide-organic hybrid combinations, and even MO-TFTs integrated onto Si LSI back end of line interconnects demonstrate this technology’s utility in 2D and 3D monolithic heterogeneous integration (HI). In addition to a brief literature review focused on functional HI between MO-TFTs and a variety of dissimilar active devices, we share progress toward integrating MO-TFTs with compound semiconductor devices, namely GaN HEMTs. A monolithically integrated cascode topology was used to couple a HEMT’s >200 V breakdown characteristic with the gate driving characteristic of an IGZO TFT, effectively shifting the HEMT threshold voltage from -3 V to +1 V.

ALD TiO x as a top-gate dielectric and passivation layer for InGaZnO 115 ISFETs

Abstract: The suitability of atomic layer deposited (ALD) titanium oxide (TiO x ) as a top gate dielectric and passivation layer for indium gallium zinc oxide (InGaZnO115) ion sensitive field effect transistors (ISFETs) is investigated. TiO x is an attractive barrier material, but reports of its use for InGaZnO thin film transistor (TFT) passivation have been conflicting thus far. In this work, it is found that the passivated TFT's behavior depends on the TiO x deposition temperature, affecting critical device characteristics such as threshold voltage, field-effect mobility and sub-threshold swing. An O2 annealing step is required to recover TFT performance post passivation. It is also observed that the positive bias stress response of the passivated TFTs improves compared the original bare device. Secondary ion mass spectroscopy excludes the effects of hydrogen doping and inter-diffusion as sources of the temperature-dependent performance change, therefore indicating that oxygen gettering induced by TiO x passivation is the likely source of oxygen vacancies and, consequently, carriers in the InGaZnO film. It is also shown that potentiometric sensing using ALD TiO x exhibits a near Nernstian response to pH change, as well as minimizes V TH drift in TiO x passivated InGaZnO TFTs immersed in an acidic liquid. These results add to the understanding of InGaZnO passivation effects and underscore the potential for low-temperature fabricated InGaZnO ISFETs to be used as high-performance mobile chemical sensors.

Near-infrared (1 to 4 μm) control of plasmonic resonance wavelength in Ga-doped ZnO

Abstract: The plasmonic resonance wavelength λres in ZnO doped with 3 wt% Ga2O3 can be controlled over the range 1 to 4 μm by simple furnace annealing in flowing Ar For each annealing temperature TA, the reflectance Rm and transmittance Tm are measured over a wavelength range, λ=185 to 3200 nm, (energy range, E=67 to 0387 eV), and the reflectance coefficient R is calculated from Rm and Tm The value of λres is then determined from a Drude-theory analysis of R versus E that yields fitting parameters nopt (optical carrier concentration), μopt (optical mobility), high-frequency dielectric constant ϵ∞, and thickness d at each annealing temperature TA The validity of this process is confirmed by comparison of ϵ∞ with literature values and comparison of nopt and μopt with analogous quantities n and μH measured by the Hall effect

Infrared photonic to plasmonic couplers using spray deposited conductive metal oxides

Abstract: In recent years, infrared plasmonics has turned towards materials that are wavelength and application tailorable, and which are geared towards CMOS processing. The transparent conductive oxides are very favorable towards infrared plasmonic applications for a number of reasons, one of which being the natural visible transparency due to their relatively large bandgap. Fluorine-doped tin oxide (FTO) is one such transparent and doping-tunable material that in addition is low cost due to spray deposition techniques that result in perfectly conformal coatings. In this work, a deposition recipe that gives high free carrier concentration was used to fabricate structures for demonstration of surface plasmon excitation. 1D gratings with a range of structural parameters were etched in silicon. Then the gratings were conformally coated with FTO by aqueous spray deposition. Excitation of surface plasmon polaritons (SPP) at mid- and long- wave infrared wavelengths on these gratings was demonstrated. The observed (SPP) excitation resonances agree will with analytical excitation calculations and numerical simulations. We show that grating heights of ~10-15% of the wavelength are optimum for achieving the strongest sharpest coupling to plasmonic resonances in the mid- and longwave infrared. The presented results are compared with similar etched silicon gratings coated with Ga-doped ZnO (GZO). The dominant difference between our FTO and GZO measurements is the free carrier concentration. The useful wavelength range is predicted for FTO based plasmonics and compared with other plasmonic host materials. The work presented here could play a key role in novel decreased-cost detectors, filters, and on-chip optoelectronics.

Effect of Compound Dielectric and Metal Thinning on Metal-Insulator-Metal Resonant Absorbers for Multispectral Infrared Air-Bridge Bolometers

Abstract: Addition of wavelength selective absorbers on microbolometers tends to increase their thermal mass and slow their infrared response times. Making the bolometric material an integral part of the absorber and minimizing layer thicknesses is one possible way to maintain high detector speeds. Here, we study experimentally the effect on permittivity of adding a layer of semiconducting VOx between two layers of SiO2. Additionally, we investigate theoretically the effect on resonance wavelength of thinning the metal in metal-insulator-metal plasmonic resonant absorbers.

Near-IR (1 – 4 μm) control of plasmonic resonance wavelength in Ga-doped ZnO

Abstract: The plasmonic resonance wavelength λres in ZnO doped with 3wt%Ga2O3 can be controlled over the range 1 – 4 μm by simple furnace annealing in flowing Ar. For each annealing temperature TA, the reflectance Rm and transmittance Tm are measured over a wavelength range, λ = 185 – 3200 nm, (energy range, E = 6.7 – 0.387 eV), and the reflectance coefficient R is calculated from Rm and Tm. The value of λres is then determined from a Drude-theory analysis of R vs E that yields fitting parameters nopt (optical carrier concentration), μopt (optical mobility), high-frequency dielectric constant e∞, and thickness d, at each annealing temperature TA. The validity of this process is confirmed by comparison of e∞ with literature values, and comparison of nopt and μopt with analogous quantities n and μH measured by the Hall-effect.

Recent Progress of B-Ga2O3 MOSFETs for Power Electronic Applications

Abstract: We review AFRL’s major device results in the fabrication of β-Ga2O3 MOSFETs over the past 2 years. This includes: (1) AFRL’s standard fabrication process, (2) improvement of current density, (3) improvement of contact resistance, (4) review of the critical field measurement and (5) review of enhancement mode operation of a β-Ga2O3 finFET.