Kevin Leedy

3.8-MV/cm Breakdown Strength of MOVPE-Grown Sn-Doped

A Sn-doped (100) β-Ga₂O₃ epitaxial layer was grown via metal-organic vapor phase epitaxy onto a single-crystal, Mg-doped semi-insulating (100) β-Ga₂O₃ substrate. Ga₂O₃-based metal-oxide-semiconductor field-effect transistors with a 2-μm gate length (L_G), 3.4-μm source-drain spacing (L_SD), and 0.6-μm gate-drain spacing (L_GD) were fabricated and characterized. Devices were observed to hold a gate-to-drain voltage of 230 V in the OFF-state. The gate-to-drain electric field corresponds to 3.8 MV/cm, which is the highest reported for any transistor and surpassing bulk GaN and SiC theoretical limits. Further performance projections are made based on layout, process, and material optimizations to be considered in future iterations.

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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 a...

-Ga 2 O 3 MOSFETs

Hall-effect measurements have been performed on a series of highly conductive thin films of Ga-doped ZnO grown by pulsed laser deposition and annealed in a forming-gas atmosphere (5% H2 in Ar). The mobility as a function of thickness d is analyzed by a simple formula involving only ionized-impurity and boundary scattering and having a single fitting parameter, the acceptor/donor concentration ratio K=NA/ND. For samples with d=3–100 nm, Kavg=0.41, giving ND=4.7×1020 and NA=1.9×1020 cm−3. Thicker samples require a two-layer formulation due to inhomogeneous annealing.

Contact resistance study of noble metals and alloy films using a scanning probe microscope test station

This study presents a basic step toward the selection methodology of electric contact materials for microelectromechanical systems (MEMS) metal contact switches. This involves the interrelationship between two important parameters, resistivity and hardness, since they provide the guidelines and assessment of contact resistance, wear, deformation and adhesion characteristics of MEMS switches. For this purpose, thin film alloys of three noble metals, platinum (Pt), rhodium (Rh) and ruthenium (Ru) with gold (Au), were investigated. The interrelationship between resistivity and hardness was established for three levels of alloying of these metals with gold. Thin films of gold (Au), platinum (Pt), ruthenium (Rh) and rhodium (Ru) were also characterized to obtain their baseline data for comparison. All films were deposited on silicon substrates. When Ru, Rh and Pt are alloyed with Au, their hardness generally decreases but resistivity increases. This decrease or increase was, in general, dependent upon the amount of alloying.

Mobility Analysis of Highly Conducting Thin Films: Application to ZnO

Sn-doped gallium oxide (Ga2O3) wrap-gate fin-array field-effect transistors (finFETs) were formed by top-down BCl3 plasma etching on a native semi-insulating Mg-doped (100) β-Ga2O3 substrate. The fin channels have a triangular cross-section and are approximately 300 nm wide and 200 nm tall. FinFETs, with 20 nm Al2O3 gate dielectric and ∼2 μm wrap-gate, demonstrate normally-off operation with a threshold voltage between 0 and +1 V during high-voltage operation. The ION/IOFF ratio is greater than 105 and is mainly limited by high on-resistance that can be significantly improved. At VG = 0, a finFET with 21 μm gate-drain spacing achieved a three-terminal breakdown voltage exceeding 600 V without a field-plate.

Characterization of metal and metal alloy films as contact materials in MEMS switches

We have developed ZnO thin-film transistor design and fabrication techniques to demonstrate microwave frequency operation with 2-mum gate length devices produced on GaAs substrates. Using SiO2 gate insulator and pulsed laser deposited ZnO active layers, a drain-current ON/OFF ratio of 1012, a drain-current density of 400 mA/mm, a field-effect mobility of 110 cm2/V ldr s, and a subthreshold gate voltage swing of 109 m\/dec were achieved. Devices with Ti-gate metal had current and power gain cutoff frequencies of 500 and 400 MHz, respectively.

Enhancement-mode Ga2O3 wrap-gate fin field-effect transistors on native (100) β-Ga2O3 substrate with high breakdown voltage

High temperature stability of Al-doped ZnO transparent thin films in air has been improved by a combination of optimized growth parameters and postgrowth treatment. Optical transparency was better than 90% for wavelengths ranging from 380 to at least 2500nm with films that also had resistivities of 2×10−4Ωcm. Depending on the growth conditions, film resistivities showed different degrees of increase in resistivity after storing in air at elevated temperatures. Films grown at lower pressures were stable up to 400°C for short exposure times (2h) and exhibited virtually no change in resistivity at 260°C for over 2500h.

Microwave ZnO Thin-Film Transistors

Record microwave frequency performance was achieved with nanocrystalline ZnO thin-film transistors fabricated on Si substrates. Devices with 1.2-mum gate lengths and Au-based gate metals had current and power gain cutoff frequencies of fT = 2.45 GHz and fmax = 7.45 GHz, respectively. Same devices had drain-current on/off ratios of 5 times1010 exhibited no hysteresis effects and could be operated at a current density of 348 mA/mm. The microwave performances of devices with 1.2- and 2.1- mum gate lengths and 50- and 100-mum gate widths were compared.

High temperature stability of postgrowth annealed transparent and conductive ZnO:Al films

Ga-doped ZnO was deposited by pulsed laser deposition at 200 °C on SiO2/Si, Al2O3, or quartz in 10 mTorr of pure Ar. The as-grown, bulk resistivity at 300 K is 1.8×10−4 Ω cm, three-times lower than that of films deposited at 200 °C in 10 mTorr of O2 followed by an anneal at 400 °C in forming gas. Furthermore, depth uniformity of the electrical properties is much improved. Mobility analysis shows that this excellent resistivity is mostly due to an increase in donor concentration, rather than a decrease in acceptor concentration. Optical transmittance is approximately 90% in the visible and near-IR spectral regions.

High-Frequency ZnO Thin-Film Transistors on Si Substrates

Abstract. The dependences of the 294 and 10 K mobility μ and volume carrier concentration n on thickness (d=25 to 147 nm) are examined in aluminum-doped zinc oxide (AZO). Two AZO layers are grown at each thickness, one with and one without a 20-nm-thick ZnON buffer layer. Plots of the 10 K sheet concentration ns versus d for buffered (B) and unbuffered (UB) samples give straight lines of similar slope, n=8.36×1020 and 8.32×1020  cm−3, but different x-axis intercepts, δd=−4 and +13  nm, respectively. Plots of ns versus d at 294 K produce substantially the same results. Plots of μ versus d can be well fitted with the equation μ(d)=μ(∞)/[1+d*/(d−δd)], where d* is the thickness for which μ(∞) is reduced by a factor 2. For the B and UB samples, d*=7 and 23 nm, respectively, showing the efficacy of the ZnON buffer. Finally, from n and μ(∞) we can use degenerate electron scattering theory to calculate bulk donor and acceptor concentrations of 1.23×1021  cm−3 and 1.95×1020  cm−3, respectively, and Drude theory to predict a plasmonic resonance at 1.34 μm. The latter is confirmed by reflectance measurements.