Joseph Salfi

Electrical properties of Ohmic contacts to ZnSe nanowires and their application to nanowire-based photodetection

Multilayer Ti∕Au contacts were fabricated on individual, unintentionally doped zinc selenide nanowires with 80nm nominal diameter. Four-terminal contact structures were used to independently measure current-voltage characteristics of contacts and nanowires. Specific contact resistivity of Ti∕Au contacts is 0.024Ωcm2 and intrinsic resistivity of the nanowires is approximately 1Ωcm. The authors have also measured the spectral photocurrent responsivity of a ZnSe nanowire with 2.0V bias across Ti∕Au electrodes, which exhibits a turnon for wavelengths shorter than 470nm and reaches 22A∕W for optical excitation at 400nm.

Room temperature single nanowire ZnTe photoconductors grown by metal-organic chemical vapor deposition

Single nanowire ZnTe photoconductors prepared by metal-organic chemical vapor deposition are presented. These photodetectors exhibit the highest reported visible responsivity of 360 A/W (at 530 nm) and gain of 8640 (at 3 V bias). The high gain reflects a long carrier lifetime (i.e., ∼1 μs) and the role of fast selective trapping of one carrier is presented to explain this. These results reveal that such single ZnTe nanowires are excellent candidates for applications requiring high performance visible nanoscale photoconductive detectors.

Direct observation of single-charge-detection capability of nanowire field-effect transistors

A single localized charge can quench the luminescence of a semiconductor nanowire, but relatively little is known about the effect of single charges on the conductance of the nanowire. In one-dimensional nanostructures embedded in a material with a low dielectric permittivity, the Coulomb interaction and excitonic binding energy are much larger than the corresponding values when embedded in a material with the same dielectric permittivity. The stronger Coulomb interaction is also predicted to limit the carrier mobility in nanowires. Here, we experimentally isolate and study the effect of individual localized electrons on carrier transport in InAs nanowire field-effect transistors, and extract the equivalent charge sensitivity. In the low carrier density regime, the electrostatic potential produced by one electron can create an insulating weak link in an otherwise conducting nanowire field-effect transistor, modulating its conductance by as much as 4,200% at 31 K. The equivalent charge sensitivity, 4 × 10(-5) e Hz(-1/2) at 25 K and 6 × 10(-5) e Hz(-1/2) at 198 K, is orders of magnitude better than conventional field-effect transistors and nanoelectromechanical systems, and is just a factor of 20-30 away from the record sensitivity for state-of-the-art single-electron transistors operating below 4 K (ref. 8). This work demonstrates the feasibility of nanowire-based single-electron memories and illustrates a physical process of potential relevance for high performance chemical sensors. The charge-state-detection capability we demonstrate also makes the nanowire field-effect transistor a promising host system for impurities (which may be introduced intentionally or unintentionally) with potentially long spin lifetimes, because such transistors offer more sensitive spin-to-charge conversion readout than schemes based on conventional field-effect transistors.

Influence of growth temperature on the stoichiometry of InSb nanowires grown by vapor phase transport

We report on the influence of growth parameters on the stoichiometry of indium antimonide nanowires grown by vapor phase transport. Using electron microscopy and composition analysis, we show that there is an optimum growth temperature window for growing stoichiometric indium antimonide (InSb) nanowires. The choice of the metal catalyst, evaporation and growth temperature are all critical parameters affecting the morphology and stoichiometry of the growing crystal. By controlling the growth temperature, it was possible to grow either stoichiometric InSb nanowires or In nanowires that contained no Sb within detectable limits. Electrical transport measurements of single InSb nanowires with two ohmic contacts demonstrate n-type conduction persisting from room temperature to 15 K.

Electron transport in degenerate Mn-doped ZnO nanowires

The authors have performed variable-temperature electrical measurements on individual single-crystalline, Mn-doped ZnO nanowires. Using a back-gated field-effect transistor structure fabricated with electron-beam lithography, they have established that nanowires exhibit n-type conduction. At a temperature of 225K, the field-effect mobility and free electron concentration are ≈35cm2V−1s−1 and ≈3.6×1017cm−3, respectively. Carrier concentration varies weakly with temperature down to 12K, signifying that the material is degenerate. Mobility decreases with decreasing temperature down to 12K, in a manner consistent with ionized impurity scattering in a degenerate semiconductor.

Antenna-enhanced and polarization sensitive photoresponse in arrays of silicon P–i–N nanowires

We analyze a novel antenna effect that resonantly enhances the photocurrent response of end-contacted P-i-N junction nanowire gratings, due to coupling of incident radiation into the gratings multiple-scattering electromagnetic modes. Quantitative characterization of these resonances was performed by spectral and polarization-resolved photocurrent measurements on gratings with N = 500, 200 and 100 nanowires, aided by electron beam- induced current measurements, and in excellent agreement with electromagnetic scattering theory. Despite the small scattering cross-section of each nanowire, with triangular cross-section (height 8nm, width 6nm), the measured quality