Edward Likovich

Growth of ZnO nanowires catalyzed by size-dependent melting of Au nanoparticles.

We present a general approach to growing ZnO nanowires on arbitrary, high melting point (above 970 degrees C) substrates using the vapor-liquid-solid (VLS) growth mechanism. Our approach utilizes the melting point reduction of sufficiently small (5 nm diameter) Au particles to provide a liquid catalyst without substrate interaction. Using this size-dependent melting effect, we demonstrate catalytic VLS growth of ZnO nanowires on both Ti and Mo foil substrates with aspect ratios in excess of 1000:1. Transmission electron microscopy shows the nanowires to be single-crystalline, and photoluminescence spectra show high-quality optical properties. We believe this growth technique to be widely applicable to a variety of substrates and material systems.

High-Current-Density Monolayer CdSe/ZnS Quantum Dot Light-Emitting Devices with Oxide Electrodes

Films of semiconductor quantum dots (QDs) are promising for lighting technologies, but controlling how current flows through QD films remains a challenge. A new design for a QD light-emitting device that uses atomic layer deposition to fill the interstices between QDs with insulating oxide is introduced. It funnels current through the QDs themselves, thus increasing the light emission yield.

Narrow band defect luminescence from Al-doped ZnO probed by scanning tunneling cathodoluminescence

We present an investigation of optically active near-surface defects in sputtered Al-doped ZnO films using scanning tunneling microscope cathodoluminescence (STM-CL). STM-CL maps suggest that the optically active sites are distributed randomly across the surface and do not correlate with the granular topography. In stark contrast to photoluminescence results, STM-CL spectra show a series of sharp, discrete emissions that characterize the dominant optically active defect, which we propose is an oxygen vacancy. Our results highlight the ability of STM-CL to spectrally fingerprint individual defects and contribute to understanding the optical properties of near-surface defects in an important transparent conductor.

Epitaxial Catalyst-Free Growth of InN Nanorods on c -Plane Sapphire

We report observation of catalyst-free hydride vapor phase epitaxy growth of InN nanorods. Characterization of the nanorods with transmission electron microscopy, and X-ray diffraction show that the nanorods are stoichiometric 2H–InN single crystals growing in the [0001] orientation. The InN rods are uniform, showing very little variation in both diameter and length. Surprisingly, the rods show clear epitaxial relations with thec-plane sapphire substrate, despite about 29% of lattice mismatch. Comparing catalyst-free with Ni-catalyzed growth, the only difference observed is in the density of nucleation sites, suggesting that Ni does not work like the typical vapor–liquid–solid catalyst, but rather functions as a nucleation promoter by catalyzing the decomposition of ammonia. No conclusive photoluminescence was observed from single nanorods, while integrating over a large area showed weak wide emissions centered at 0.78 and at 1.9 eV.

Direct injection tunnel spectroscopy of a p-n junction

We demonstrate spectroscopic measurements on an InGaAs p-n junction using direct tunnel injection of electrons. In contrast to the metal-base transistor design of conventional ballistic electron emission spectroscopy (BEES), the base layer of our device is comprised of a thin, heavily doped p-type region. By tunneling directly into the semiconductor, we observe a significant increase in collector current compared to conventional BEES measurements. This could enable the study of systems and processes that have thus far been difficult to probe with the low-electron collection efficiency of conventional BEES, such as luminescence from single-buried quantum dots.

Observation of magnetoresistance polarity reversal in 3D to 2D tunneling in an asymmetric GaMnAs resonant tunneling diode

We investigate the magnetoresistance (MR) characteristics of a GaMnAs-based asymmetric resonant tunneling structure with a 3D GaMnAs top layer and a 2D GaMnAs quantum well (QW). The incorporation of a 2D layer distinguishes our device from the many conventional devices reported in the literature in that the MR characteristics of our device result from 3D to 2D tunneling, whereas MR in a conventional device results from 3D to 3D tunneling. By observing a shift of negative differential resistance (NDR) features as an in-plane magnetic field is varied, we infer that the 3D to 2D tunneling magnetoresistance (TMR) in our device has the opposite sign from conventional TMR in GaMnAs. We relate this difference to the effect of quantum confinement on the 2D density of states in the GaMnAs QW.