Guidance and control research

Effective work function of metal contacts to vacuum cleaved photoconducting cadmium sulfide, Curie point switching experiments, and germanium diodes for noise measurement

Electron transport in semiconducting carbon nanotubes with hetero-metallic contacts

We present an atomistic self-consistent study of the electronic and transport properties of semiconducting carbon nanotube in contact with metal electrodes of different work functions, which shows simultaneous electron and hole doping inside the nanotube junction through contact-induced charge transfer. We find that the band lineup in the nanotube bulk region is determined by the effective work function difference between the nanotube channel and source/drain electrodes, while electron transmission through the SWNT junction is affected by the local band structure modulation at the two metal-nanotube interfaces, leading to an effective decoupling of interface and bulk effects in electron transport through nanotube junction devices.Comment: Higher quality figures available at http://www.albany.edu/~yx15212

Time-dependence of SrVO3_3 thermionic electron emission properties

Thermionic electron emission cathodes are critical components of various high power and high frequency vacuum electronic devices, electron microscopes, e-beam lithographic devices, and thermionic energy converters, which all demand an efficient and long-lasting low work function cathode. Single phase, polycrystalline perovskite oxide SrVO$_3$, with its intrinsic low effective work function and facile synthesis process, is a promising cathode candidate, where previous works have shown evidence of an effective work function as low as 2.3 eV. However, assessment of the stability over time under conditions relevant for operation and the related interplay of evolving surface chemistry with emission performance are still missing, and necessary for understanding how to best prepare, process and operate SrVO$_3$ cathodes. In this work, we study the vacuum activation process of SrVO$_3$ and find it has promising emission stability over 15 days of continuous high temperature operation. We find that SrVO$_3$ shows surface Sr and O segregation during operation, which we hypothesize is needed to create a positive surface dipole, leading to low effective work function. Emission repeatability from cyclic heating and cooling suggests the promising stability of the low effective work function surface, and additional observations of drift-free emission during one hour of continuous emission testing at high temperature further demonstrates its excellent performance stability

On the effective bare work function of bcc thermionic electrode materials

An equation is derived for the effective work function of a polycrystalline metal with a fiber texture. This equation contains two parameters: the temperature and the maximal tilt angle, i.e. the maximal deviation from the fiber axis. A linear relationship is assumed between the work function of a uniform lattice plane and the angle of a low index plane with respect to the uniform lattice plane. The proportionality constant D in the [100] zone is evaluated from experimental data for tungsten: D=0.035 * (eV/degree). It is expected that D has the same value in other bcc metals. For a given maximal tilt angle, a higher temperature results in a higher effective work function. A reasonable agreement is found for the calculated effective work functions of tungsten with 110 fiber textures of various sharpness and the experimentally determined work functions from the literature. Furthermore, the effective work function of texture-free polycrystalline tungsten is calculated. The agreement with the experimentally determined value reported in the literature is excellent

Impact of Sb and Na Doping on the Surface Electronic Landscape of Cu2ZnSnS4 Thin Films

Open-circuit voltage deficiency is the key limiting factor in Cu2ZnSnS4 (CZTS) thin-film solar cells, which is commonly associated with band tails and deep gap states arising from elemental disorder. The introduction of dopants such as Na and Sb has led to improvement in device performance, yet their effects on the optoelectronic properties of CZTS are yet to be fully elucidated. In this Letter, we unraveled the effect of Sb and Na:Sb co-doping on the surface energy landscape of solution-processed CZTS films employing energy-filtered photoelectron emission microscopy. In the absence of the additives, 150 nm resolution photoemission maps reveal oscillations in the local effective work function as well as areas of low photoemission energy threshold. The introduction of dopants substantially reshapes the photoemission maps, which we rationalize in terms of Cu:Zn and Sn disorder. Finally, we establish unprecedented correlations between the photoemission landscape of thin films and the performance of over 200 devices

Nanometer-resolution measurement and modeling of lateral variations of the effective work function at the bilayer Pt/Al/SiO2 interface

A ballistic electron emission microscopy (BEEM) comparison of the dependence on gate voltage of the average energy barrier of a metal bilayer Pt/Al/ SiO2 /Si sample and a Pt/ SiO2 /Si sample suggests that the metal/oxide interface of the Pt/Al/ SiO2 /Si sample is laterally inhomogeneous at nm length scales. However, BEEM images of the bilayer sample do not show significantly larger lateral variations than observed on a (uniform) Pt/ SiO2 /Si sample, indicating that any inhomogeneous "patches" of lower-energy barrier height have size smaller than the lateral resolution of BEEM, estimated for these samples to be ???10nm. Finite element electrostatic simulations of an assumed inhomogeneous interface with nm size patches of different effective work function can fit the experimental data of the bilayer sample much better than an assumed homogenous interface, indicating that the bilayer film is laterally inhomogeneous at the nm scale.open2