Takhee Lee

Electrical characterization of single GaN nanowires

In this paper a statistically significant study of 1096 individual GaN nanowire (NW) devices is presented. We have correlated the effects of changing growth parameters for hot-wall chemically-vapour-deposited (HW-CVD) NW sf abricated via the vapour–liquid–solid mechanism. We first describe an optical lithographic method for creating Ohmic contacts to NW field effect transistors with both top and bottom electrostatic gates to characterize carrier density and mobility. Multiprobe measurements show that carrier modulation occurs in the channel and is not a contact effect. We then show that NW fabrication runs with nominally identical growth parameters yield similar electrical results across sample populations of >50 devices. By systematically altering th eg rowth parameters we were able to decrease the average carrier concentration for these as-grown GaN NWs ∼10-fold, from 2.29 × 10 20 to 2.45 × 10 19 cm −3 ,a nd successfully elucidate the parameters that exert the strongest influence on wire quality. Furthermore, this study shows that nitrogen vacancies, and not oxygen impurities, are the dominant intrinsic dopant in HW-CVD GaN NWs.

Electronic transport in self-assembled alkanethiol monolayers

Electron tunneling through self-assembled monolayers of alkanethiols is investigated. Temperature-dependent current- voltage measurements are performed to distinguish between dierent conduction mechanisms. Temperature-independent electron transport is observed, proving direct tunneling as the dominant conduction mechanism of alkanethiols. An exponential dependence of tunneling current on molecule length is observed. Inelastic electron tunneling spectroscopy results are reported.

Mechanism of electron conduction in self-assembled alkanethiol monolayer devices.

Abstract: Electron tunneling through self‐assembled monolayers (SAMs) of alkanethiols was investigated using nanometer scale devices that allow temperature‐dependent current‐voltage, I(V, T), measurements. The I(V, T) measurement results show, for the first time, temperature‐independent electron transport characteristics, proving direct tunneling as the transport mechanism in alkanethiol SAMs. The measured tunneling currents can be fitted with theoretical calculations using the modified rectangular barrier model of direct tunneling with a barrier height ΦB= 1.42 ± 0.04 eV and a non‐ideal barrier factor α= 0.65 ± 0.01 (that may correspond to effective mass of 0.42 m). From the length‐dependent conduction measurement on different alkanethiols of various lengths, the tunneling current exhibits exponential dependence on the molecular length, d, as I ∝ exp(−βd), where β is a decay coefficient that was found to be bias‐dependent and agrees with the existing theory of direct tunneling. A zero field decay coefficient β0 of 0.79 ± 0.01 Å−1 was obtained.

Inelastic electron tunneling spectroscopy of an alkane SAM

Inelastic electron tunneling spectroscopy (IETS) of an alkanedithiol self-assembled monolayer (SAM) is investigated using a nanometer scale device. The IETS spectrum of the octanedithiol device clearly shows vibrational signatures of an octanedithiolate, -SC8 H16S-, bonded to gold electrodes. The pronounced IETS peaks correspond to vibrational modes perpendicular to the junction interface, which include the stretching modes of Au-S (at 33 mV) and C-C (at 133 mV), and wagging mode of CH2 (at 158 mV). The observed peak intensities and peak widths are in good agreement with theoretical predictions.