Showing papers by "Kevin D. Leedy published in 2016"

3.8-MV/cm Breakdown Strength of MOVPE-Grown Sn-Doped $\beta $ -Ga 2 O 3 MOSFETs

Abstract: A Sn-doped (100) $\beta $ -Ga2O3 epitaxial layer was grown via metal–organic vapor phase epitaxy onto a single-crystal, Mg-doped semi-insulating (100) $\beta $ -Ga2O3 substrate. Ga2O3-based metal–oxide–semiconductor field-effect transistors with a 2- $\mu \text{m}$ gate length ( $L_{G})$ , 3.4- $\mu \text{m}$ source–drain spacing ( $L_{\textrm {SD}})$ , and 0.6- $\mu \text{m}$ gate–drain spacing ( $L_{\textrm {GD}})$ 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.

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

Abstract: 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.

Defect segregation and optical emission in ZnO nano- and microwires

Abstract: The spatial distribution of defect related deep band emission has been studied in zinc oxide (ZnO) nano- and microwires using depth resolved cathodoluminescence spectroscopy (DRCLS) in a hyperspectral imaging (HSI) mode within a UHV scanning electron microscope (SEM). Three sets of wires were examined that had been grown by pulsed laser deposition or vapor transport methods and ranged in diameter from 200 nm–2.7 μm. This data was analyzed by developing a 3D DRCLS simulation and using it to estimate the segregation depth and decay profile of the near surface defects. We observed different dominant defects from each growth process as well as diameter-dependent defect segregation behavior.

Induced conductivity in sol-gel ZnO films by passivation or elimination of Zn vacancies

Abstract: 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 s...

Ionic Metal–Oxide TFTs for Integrated Switching Applications

Abstract: Disordered ionic-bonded transition metal oxide thin-film transistors (TFTs) show promise for a variety of dc and RF switching applications, especially those that can leverage their low-temperature, substrate-agnostic process integration potential. In this paper, enhancement-mode zinc-oxide TFTs were fabricated and their switching performance evaluated. These TFTs exhibit the drain-current density of 0.6 A/mm and minimal frequency dispersion, as evidenced by dynamic current–voltage tests. A high-frequency power switch figure of merit $R_{{\mathrm{\scriptscriptstyle ON}}}Q_{G}$ of 359 $\text{m}\Omega \,\cdot \, $ nC was experimentally determined for 0.75- $\mu \text{m}$ long-channel devices, and through scaling 45.9 $\text{m}\Omega \,\cdot \, $ nC is achievable for 11 V-rated devices (where $R_{\mathrm{\scriptscriptstyle ON}}$ is ON-state drain–source resistance, and $Q_{G}$ is gate charge). An RF switch cutoff frequency $f_{c}$ of 25 GHz was measured for the same 0.75- $\mu \text{m}$ TFT, whereas $f_{c}$ exceeding 500 GHz and power handling in the tens of watts are projected with optimization.

Effects of Substrate and Post-Growth Treatments on the Microstructure and Properties of ZnO Thin Films Prepared by Atomic Layer Deposition

Abstract: Aluminum-doped zinc oxide (ZnO:Al) thin films were synthesized by atomic layer deposition on silicon, quartz and sapphire substrates and characterized by x-ray diffraction (XRD), high-resolution scanning electron microscopy, optical spectroscopy, conductivity mapping, Hall effect measurements and positron annihilation spectroscopy. XRD showed that the as-grown films are of single-phase ZnO wurtzite structure and do not contain any secondary or impurity phases. The type of substrate was found to affect the orientation and degree of crystallinity of the films but had no effect on the defect structure or the transport properties of the films. High conductivity of 10−3 Ω cm, electron mobility of 20 cm2/Vs and carrier density of 1020 cm−3 were measured in most films. Thermal treatments in various atmospheres induced a large effect on the thickness, structure and electrical properties of the films. Annealing in a Zn and nitrogen environment at 400°C for 1 h led to a 16% increase in the thickness of the film; this indicates that Zn extracts oxygen atoms from the matrix and forms new layers of ZnO. On the other hand, annealing in a hydrogen atmosphere led to the emergence of an Al2O3 peak in the XRD pattern, which implies that hydrogen and Al atoms compete to occupy Zn sites in the ZnO lattice. Only ambient air annealing had an effect on film defect density and electrical properties, generating reductions in conductivity and electron mobility. Depth-resolved measurements of positron annihilation spectroscopy revealed short positron diffusion lengths and high concentrations of defects in all as-grown films. However, these defects did not diminish the electrical conductivity in the films.

Al1-x ScxN Thin Film Structures for Pyroelectric Sensing Applications

Abstract: AlN thin film structures have many useful and practical piezoelectric and pyroelectric properties. The potential enhancement of the AlN piezo- and pyroelectric constants allows it to compete with more commonly used materials. For example, combination of AlN with ScN leads to new structural, electronic, and mechanical characteristics, which have been reported to substantially enhance the piezoelectric coefficients in solid-solution AlN-ScN compounds, compared to a pure AlN-phase material. In our work, we demonstrate that an analogous alloying approach results in considerable enhancement of the pyroelectric properties of AlN - ScN composites. Thin films of ScN, AlN and Al1-x ScxN (x = 0 – 1.0) were deposited on silicon (004) substrates using dual reactive sputtering in Ar/N2 atmosphere from Sc and Al targets. The deposited films were studied and compared using x-ray diffraction, XPS, SEM, and pyroelectric characterization. An up to 25% enhancement was observed in the pyroelectric coefficient (Pc = 0.9 μC /m2K) for Sc1-xAlxN thin films structures in comparison to pure AlN thin films (Pc = 0.71 μC/m2K). The obtained results suggest that Al1-x ScxN films could be a promising novel pyroelectric material and might be suitable for use in uncooled IR detectors.

Fourier transform infrared spectroscopy measurements of multi-phonon and free-carrier absorption in ZnO

Abstract: Fourier transform infrared (FTIR) measurements were carried out on thin films and bulk single crystals of ZnO over a wide temperature range to study the free-carrier and multi-phonon infrared absorptions and the effects of hydrogen incorporation on these properties. Aluminum-doped ZnO thin films were deposited on quartz substrates using atomic-layer deposition (ALD) and sol–gel methods. Hall-effect measurements showed that the ALD films have a resistivity of ρ = 1.11 × 10−3 Ω cm, three orders of magnitude lower than sol–gel films (ρ = 1.25 Ω cm). This result is consistent with the significant difference in their free-carrier absorption as revealed by FTIR spectra obtained at room temperature. By reducing the temperature to 80 K, the free carriers were frozen out, and their absorption spectrum was suppressed. From the FTIR measurements on ZnO single crystals that were grown by the chemical vapor transport method, we identified a shoulder around 3350 cm−1 and associated it with the presence of two or more hydrogen ions in a Zn vacancy. After reducing the hydrogen level in the crystal, the measurements revealed the multi-phonon absorption of ZnO in the range of 700–1200 cm−1. This study shows that the multi-phonon absorption bands can be completely masked by the presence of a large concentration of hydrogen in the crystals.

Gallium-doped zinc oxide plasmonic nanostructures for mid-IR applications (Conference Presentation)

Abstract: The mid-infrared (mid-IR) region of the electromagnetic spectrum has a range of applications in defense, sensing, and free space optical communications. However, most mid-IR sources, particularly incoherent emitters, are practically limited as a result of significant non-radiative losses such as Auger and Shockley-Read-Hall recombination as well as phonon-assisted scattering. Recently, plasmonic materials have been a topic of interest due to their ability to overcome traditional limitations of light confinement as well as enhance light-matter interactions. For inherently inefficient sources, such as many mid-IR emitters, coupling of the emitting element to a plasmonic structure could enhance emission efficiency. In this work, we propose and experimentally evaluate the use of plasmon-mediated photoluminescence as a potential method for improving efficiency in mid-IR emitters. We assess the effectiveness of 3% gallium-doped zinc oxide (G3ZO) as a mid-IR plasmonic material. We design, simulate, fabricate, and characterize a two-dimensional periodic array of bow-tie nanoantennas (nantennas). Our structures are designed to enhance the overlap of the nantenna optical field with underlying In(Ga)Sb/InAs quantum well structures emitting at λ ≈ 4.0μm. Thin films of G3ZO are grown by pulsed laser deposition and are characterized electrically and optically, with the extracted material parameters used as inputs in our simulations. G3ZO plasmonic nantennas are then fabricated by electron-beam lithography and dry-etching. The spectral response of the patterned nantennas is characterized using Fourier transform infrared reflection spectroscopy. Samples are then characterized by temperature and polarization dependent photoluminescence spectroscopy in order to determine the extent to which the emission efficiency improves as a result of coupling to the nanostructures.