Session C51: Applications Semiconductors & Dielectrics

Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$ nanoparticles for airborne organophosphate detection

Dire need for early detection of organophosphates (OP) exists in both civilian (pesticide/herbicide buildup) and military (G/V nerve agents) spheres. Nanoparticle materials are excellent candidates for the detection and/or decontamination of hazardous materials, owing to their large surface to volume ratios and tailored surface functionality. Within this category, metal oxides include structures that are stable with the range of normal environmental conditions (temperature, humidity), but have strong, specific reaction mechanisms (hydrolysis, oxidation, catalysis, stoichiometric reaction) with toxic compounds. In this talk, we will present on the suitability of Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$ nanoparticles as airborne organophosphate detectors. 23 nm particles were exposed to a series of organophosphate compounds (dimethyl methylphosphonate, dimethyl chlorophosphonate, diisopropyl methylphosphonate), and studied by x-ray magnetic circular dichroism and x-ray absorption spectroscopy to confirm the stoichiometric Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$ to FeO mechanism and determine magnetic sensor feasibility. AC Impedance Spectroscopy shows both high sensitivity and selectivity via frequency dependence in both impedance and resistivity, suggesting some feasibility for impedimetric devices.   

Towards a drift-free multi-level Phase Change Memory

For ultra-high density data storage applications, Phase Change Memory (PCM) is considered a potentially disruptive technology. Yet, the long-term reliability of the logic levels corresponding to the resistance states of a PCM device is an important issue for a stable device operation since the resistance levels drift uncontrollably in time. The underlying mechanism for the resistance drift is considered as the structural relaxation and spontaneous crystallization at elevated temperatures. We fabricated a nanoscale single active layer-phase change memory cell with three resistance levels corresponding to crystalline, amorphous and intermediate states by controlling the current injection site geometry. For the intermediate state and the reset state, the activation energies and the trap distances have been found to be 0.021 eV and 0.235 eV, 1.31 nm and 7.56 nm, respectively. We attribute the ultra-low and weakly temperature dependent drift coefficient of the intermediate state ($\nu =$0.0016) as opposed to that of the reset state ($\nu =$0.077) as being due to the dominant contribution of the interfacial defects in electrical transport in the case of the mixed phase. Our results indicate that the engineering of interfacial defects will enable a drift-free multi-level PCM device design.   

Sensing of NO$_{\mathrm{2}}$ with Zirconium Hydroxide via Electrical Impedance Spectroscopy

Nitrogen Dioxide (NO$_{\mathrm{2}})$ is a brown gas mainly produced as a byproduct of burning fossil fuels, such as automobiles and power plants. Nitrogen oxides can form acid rain and smog by reacting with air, can form toxic organic nitrates by reacting with soil, and can react with oxygen in water, destroying marine life due to a lack of breathable oxygen. Any concentration beyond 53 ppb (air quality standard) can cause irritation to the lungs and respiratory infections, and higher dosages can be fatal. As such, research in NO$_{\mathrm{2}}$ detection is incredibly important to human welfare. Zirconium hydroxide (Zr(OH)$_{\mathrm{4}})$ has been investigated as a candidate NO$_{\mathrm{2}}$ dielectric sensor using impedance spectroscopy analysis. Impedance changes of several orders of magnitude are seen down to our dosage minimum of 50 ppm\textbullet hr. Changes in impedance correlate with nitrogen and oxygen atomic ratio increases observed via X-ray photoelectron spectroscopy (XPS). The results indicate that Zr(OH)$_{\mathrm{4}}$ may be a strong candidate for use in impedance-based NO2 detection devices.   

Laser Processing of Metal Oxides for Plasmonic Applications

Noble metals such as Au and Ag have been used traditionally for plasmonic devices. However, conventional metals are not suitable for near infrared (IR) plasmonic applications due to their relatively large optical losses at these wavelengths. Metal oxides, on the other hand, have been considered for low loss metallic components in the near IR because they can provide a tunable carrier density by doping. The zero-cross-over permittivity values of these metal oxides, for example, can easily be tuned from 1.0 \textmu m to 3 \textmu m by adjusting doping levels. Optical losses in devices made from these metal oxide materials are generally found to be much lower than those obtained with conventional metals. We have investigated various laser processing techniques for synthesizing several types of metal oxides. First, pulsed laser deposition was used to grow metal oxide thin films such as, Al-doped ZnO, Sn-doped In2O3 and VO2. Second, a laser sintering technique was used to improve the properties of solution-processed VO2 coatings. Third, a laser printing technique was used to produce metal oxide films. We will present details on the use of laser processing techniques for synthesizing these metal oxides along with their electrical, optical, and structural properties.   

Controlled Growth of Copper Oxide Nano-Wires through Direct Oxidation

Copper oxides, both Cu$_{2}$O and CuO, have many applications in solar cells, sensors, and nano-electronics. The properties of the copper oxides are further influenced by the dimension of the materials, especially when made in nanoscale. In particular, the properties of the copper oxide nanowires could be tuned by their structures, lengths, and widths. While several methods have been reported to grow nanowires, direct oxidation is arguably the most economical one. This research examines the effects of oxidization duration and temperature in dry air environment on the development of copper oxide nanowires in order to achieve cost effective controllable growth. Using the direct oxidation method in dry air we have demonstrated growth of CuO nano-wires at temperatures as low as 300 \textdegree C and as short as 1hr. Furthermore we have observed that the lengths and diameters of the CuO NWs can be controlled by the duration and temperature of the oxidation process.   

Distinguishing the Photothermal and Photoinjection Effects in Vanadium Dioxide Nanowires

Vanadium dioxide (VO$_{\mathrm{2}})$ has drawn significant attention for its unique metal-to-insulator transition. The high electrical resistivity below the transition temperature is a result of the strong electron correlation with the assistance of lattice distortion. Theoretical calculations indicated that the strong inter-electron interactions might induce intriguing optoelectronic phenomena, such as the multiple exciton generation. However, the resistivity of VO$_{\mathrm{2}}$ is temperature sensitive. Therefore, the light-induced conductivity in VO$_{\mathrm{2}}$ has often been attributed to the photothermal effects. In this work, we distinguished the photothermal and photoinjection effects in VO$_{\mathrm{2}}$ nanowires by varying the chopping frequency of the optical illumination. In our VO$_{\mathrm{2}}$ nanowires, the relatively slow photothermal processes can be well suppressed when the chopping frequency \textgreater 2 kHz, whereas the fast photoinjection component (direct photo-excitation of charge carriers) remains constant at all chopping frequencies. By separating the photothermal and photoinjection processes, our work set the basis for further studies of carrier dynamics under optical excitations in strongly correlated materials.   

Electronic properties of epitaxial Ge/AlAs heterostructures on Si and GaAs

Ge, with high electron and hole mobilities, has advantages over Si for low-power high-speed nanoscale logic. We report on the MBE growth of Ge/AlAs/GaAs and Ge/AlAs/GaAs/Si structures, where the Ge/AlAs band offsets provide carrier confinement inside the Ge layer. We studied the confinement of carriers in the Ge layer, the effect of the AlAs buffer layer, and the effects of a growth pause and growth temperature, correlated to structural and morphological properties. Magnetotransport and quantum transport measurements were obtained down to 390 mK and in magnetic fields up to 9 T. A weak-localization signal, in contrast to antilocalization, indicates absence of spin-orbit interaction and hence electron confinement in the Ge rather than in the III-V layers. For the Ge/AlAs/GaAs/Si structure a low-temperature sheet carrier density 1.4 x 10$^{\mathrm{14}}$ cm$^{\mathrm{-2}}$ and mobility 390 cm$^{\mathrm{2}}$/Vs were obtained, with similar values at 290 K, while at 200 K a maximum in mobility is reached of 470 cm$^{\mathrm{2}}$/Vs. For the Ge/AlAs/GaAs structures a mobility up to 260 cm$^{\mathrm{2}}$/Vs was obtained at 2 x 10$^{\mathrm{13}}$ cm$^{\mathrm{-2}}$ at 290 K. The Ge/AlAs/GaAs structures have also shown phonon-limited scattering vs temperature, attesting to the quality of interfaces.   

Spectroscopic and structural studies of energetically efficient transport in nanocontacts to NiSi nanowires

Understanding correlations between mechanical, thermal, structural and electronic transport properties of different nanocontact geometries to nanowires, such as Au/Cu-NiSi-Si, remain one of the major goals of nanodevices reliability and scalability research. Aiming to clarify the failure modes and processes that affect the energy efficiency of transport and switching in constrained nanocontact geometries, such as end contacts, we conducted the structural, spectroscopic, and noise correlation studies. We show how the spatial (in)homogeneity at and in the near vicinity of the interface affects the transport performance of the nanojunctions. Mobile Ni clusters are identified at the nanojunction interface via Raman spectromicroscopy and their influence on charge transport is analyzed. We also show that the noise correlation spectra and micro-X-ray stress-strain studies in the nanojunctions are effictive tools in predicting the energy efficiency of the nanojunctions. A computational study of the interfacial properties of metal/Ni-Si via DFT and MD simulations is implemented.   

Memristive switching of ZnO nanorod mesh

We present a combined experimental and theoretical study of memristive switching in a self-assembled mesh of ZnO nanorods. A ZnO nanorod mesh spans the area between Ag contacts in a device that exhibits hysteresis with large ON/OFF ratio, reaching ION/IOFF values of 104. We show that switching behavior depends critically on the geometry of the nanorod mesh. We employ density functional theory (DFT) calculations to deduce the mechanism for resistive switching for the nanorod mesh. Redistribution of Ag atoms, driven by an electrical field, leads to the formation and evolution of a conducting path through nanorods. Field-induced migration of Ag atoms changes the doping level of nanorods and modulates their conductivity. Using static DFT and nudged-elastic-band calculations, we investigate the energy of interaction between Ag clusters and a ZnO surface, including migration barriers of Ag atoms. Current-voltage (I-V) characteristics are modeled using percolation theory in a nanorod mesh. To describe the dynamics of SET/RESET phenomena, model parameters include the experimentally observed nanorod geometry and the energetics of Ag on ZnO surfaces, obtained from DFT calculations.   

Field effect in perovskite heterostructures based on BaSnO$_{\mathrm{3}}$ and BaHfO$_{\mathrm{3}}$

Perovskite La-doped BaSnO$_{\mathrm{3}}$ (BLSO) was reported to possess high electron mobility and excellent oxygen stability [1]. We fabricated a field effect transistor on SrTiO$_{\mathrm{3\thinspace }}$substrate using BLSO as a channel layer and BaHfO$_{\mathrm{3}}$ (BHO)$_{\mathrm{\thinspace }}$as a gate insulator. To reduce the threading dislocations and enhance the electrical properties of the channel, undoped BaSnO$_{\mathrm{3}}$ (BSO) buffer layer was grown on SrTiO$_{\mathrm{3}}$ substrates before the channel layer deposition. X-ray diffraction measurement confirms the epitaxial growth of BHO on BSO. We investigated optical and dielectric properties of the BHO gate insulator; the optical bandgap and the dielectric constant were measured to be 6.1 eV and 37.8, respectively. Using BHO as the gate insulator, we obtained the conductivity modulation in the channel by field effect. We will report on the electrical properties of the field effect transistor such as the output characteristics, the transfer characteristics, the $I_{\mathrm{on}}$/$I_{\mathrm{off}}$ ratio, the subthreshold swing and the field effect mobility. \textbf{[1]} H. J. Kim, U. Kim \textit{et al.}, Appl. Phys. Express \textbf{5}, 061102 (2012).   

Characterization of Quasi-Metallic Tunnel-Field-Effect-Transistors

Band-to-band tunneling mechanism has proven to be a promising alternative to thermionic diffusion for ultra-fast switching applications. Tunneling Field-Effect-Transistors (TFETs), which primarily operate based on tunneling current, can offer low turn-on voltage with low sub-threshold swing[1]. Here, we demonstrate TFETs based on suspended, ultra-clean, quasi-metallic carbon nanotube pn devices. These devices exhibit a subthreshold swing as low as 2mV/decade, with a current Ion/Ioff ratio in the order of 105 at cryogenic temperatures. At room temperature, however, the current is dominated by the diffusion of carriers, which degrades the Ion/Ioff ratio and the subthreshold swing. We also explore the effect of the schottky contacts on the tunneling current by adding two back-to-back diodes to the tunneling current model. Our results provide evidence that the effect of the schottky contacts can be significant when quasi-metallic nanotubes exhibit band-to-band tunneling. Our results show that quasi-metallic carbon nanotubes can be potential candidates for future nanoelectronics. References: [1] A. M. Ionescu and H. Riel, "Tunnel field-effect transistors as energy-efficient electronic switches," Nature, vol. 479, pp. 329-337, 2011.   

Structural and electronic properties of BAlN alloy

Designs of far-UV emitters using BAlN as the barrier layer and AlN as the active layer are being considered. Realization of BAlN alloys is complicated by the fact that BN is most stable in a hexagonal structure, which is different from the ground-state wurtzite structure of AlN. Enabling such designs requires a fundamental understanding of the composition dependent electronic structure of BAlN. Using first-principles simulations based on a hybrid functional, we investigate the band gaps, band-gap bowing, and miscibility of BAlN using explicit alloy calculations. The results from these calculations are used to determine the band offsets between AlN and BAlN that are essential to assess the performance of UV-emitting devices.   

Sources of Shockley-Read-Hall recombination in III-nitride light emitters

Group-III nitrides are the key materials for high efficiency light-emitting diodes in the blue part of the visible spectrum, and a large research effort is aimed at extending this success to the green and the yellow range, where nitride LEDs are significantly less efficient. Though it has been noted that the efficiency of III-nitride devices may be limited by Shockley-Read-Hall recombination at point defects, the microscopic mechanism and defects responsible are unknown. Based on first-principles calculations of defect formation energies, charge-state transition levels, and nonradiative capture coefficients, we describe a mechanism by which complexes between gallium vacancies and oxygen and/or hydrogen can act as efficient channels for nonradiative recombination in InGaN alloys. The dependence of these quantities on alloy composition is analyzed. We find that modest concentrations of the proposed defect complexes, around $10^{16}$cm$^{-3}$, can give rise to Shockley-Read-Hall coefficients $A=(10^7 - 10^9)$ s$^{-1}$. The resulting nonradiative recombination can significantly reduce the internal quantum efficiency of optoelectronic devices.   

A first-principles study of pyroelectricity in GaN and ZnO

We present first-principles calculations on the primary pyroelectric coefficients for wurtzite GaN and ZnO. The primary pyroelectricity is attributed to the anharmonic atomic displacements of the Born effective charges on the cations and anions. We show that the primary pyroelectricity is the major part of the total pyroelectricity at low temperatures, while the secondary pyroelectricity becomes comparable with the primary pyroelectricity at high temperatures. Our calculations show that contributions from the acoustic and the optical phonon modes to the primary pyroelectric coefficient can be well described by the corresponding Debye and Einstein functions respectively.   

Resistive switching in nanodevices

A nanoscale metal/insulator/metal sandwich structure device may exhibit multiple resistive states, and switching between these states can be controlled by bias voltage. However, the underlying physical mechanism is poorly understood. We present an electronic mechanism that explains multiple resistive states in such devices due to multiple solutions of Poisson's equation. These solutions describe spontaneously charged states characterized by different (convex and concave) `band bendings'. For an insulator with mainly donor type defects, the low-resistivity state is characterized by a negatively charged insulator due to convex band bending, and the high-resistivity state by a positively charged insulator due to concave band bending; vice versa for insulators with mainly acceptor type defects. We show that the coexistence of such states, and switching between them is determined by defect/impurity abundance, device size, and basic material properties.