Narrow Band Defect Luminescence from AI-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.Engineering and Applied Science

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 °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.Engineering and Applied Science

Weak Localization and Mobility in ZnO Nanostructures

We conduct a comprehensive investigation into the electronic and magnetotransport properties of ZnO nanoplates grown concurrently with ZnO nanowires by the vapor-liquid-solid method. We present magnetoresistance data showing weak localization in our nanoplates and probe its dependence on temperature and carrier concentration. We measure phase coherence lengths of 50–100 nm at 1.9 K and, because we do not observe spin-orbit scattering through antilocalization, suggest that ZnO nanostructures may be promising for further spintronic study. We then proceed to study the effect of weak localization on electron mobility using four-terminal van der Pauw resistivity and Hall measurements versus temperature and carrier concentration. We report an electron mobility of ∼100 cm2/V s at 275 K, comparable to what is observed in ZnO thin films. We compare Hall mobility to field-effect mobility, which is more commonly reported in studies on ZnO nanowires and find that field-effect mobility tends to overestimate Hall mobility by a factor of 2 in our devices. Finally, we comment on temperature-dependent hysteresis observed during transconductance measurements and its relationship to mobile, positively charged Zn interstitial impurities.Engineering and Applied Science

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.Engineering and Applied Science

Magnetoresistance in an Asymmetric GaMnAs Resonant Tunneling Diode

In a GaMnAs/AlGaAs resonant tunneling diode (RTD) structure, we observe that both the magnitude and polarity of magnetoresistance are bias dependent when tunneling from a three-dimensional GaMnAs layer through a two-dimensional GaMnAs quantum well. This magnetoresistance behavior results from a shift of negative differential resistance features to higher bias as the relative alignment of the GaMnAs layer magnetizations is changed from parallel to antiparallel. Our observations agree with recent predictions from a theoretical analysis of a similar n-type structure by Ertler and Fabian, and our results suggest that further investigation into ferromagnetic RTD structures may result in significantly enhanced magnetoresistance.Engineering and Applied Science

Full Visible Range Covering InP/ZnS Nanocrystals with High Photometric Performance and Their Application to White Quantum Dot Light-Emitting Diodes

Cataloged from PDF version of article.High-quality InP/ZnS core–shell nanocrystals with luminescence tunable over the entire visible spectrum have been achieved by a facile one-pot solvothermal method. These nanocrystals exhibit high quantum yields (above 60%), wide emission spectrum tunability and excellent photostability. The FWHM can be as narrow as 38 nm, which is close to that of CdSe nanocrystals. Also, making use of these nanocrystals, we further demonstrated a cadmium-free white QD-LED with a high color rendering index of 91. The high-performance of the resulting InP/ZnS NCs coupled with their low intrinsic toxicity may further promote industrial applications of these NC emitters

Epitaxial Catalyst-Free Growth of InN Nanorods onc-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

Multifunctional Materials: A Case Study of the Effects of Metal Doping on ZnO Tetrapods with Bismuth and Tin Oxides

Hybrid metal oxide nano‐ and microstructures exhibit novel properties, which make them promising candidates for a wide range of applications, including gas sensing. In this work, the characteristics of the hybrid ZnO‐Bi2O3 and ZnO‐Zn2SnO4 tetrapod (T) networks are investigated in detail. The gas sensing studies reveal improved performance of the hybrid networks compared to pure ZnO‐T networks. For the ZnO‐T‐Bi2O3 networks, an enhancement in H2 gas response is obtained, although the observed p‐type sensing behavior is attributed to the formed junctions between the arms of ZnO‐T covered with Bi2O3 and the modulation of the regions where holes accumulate under exposure to H2 gas. In ZnO‐T‐Zn2SnO4 networks, a change in selectivity to CO gas with high response is noted. The devices based on individual ZnO‐T‐Bi2O3 and ZnO‐T‐Zn2SnO4 structures showed an enhanced H2 gas response, which is explained on the basis of interactions (electronic sensitization) between the ZnO‐T arm and Bi2O3 shell layer and single Schottky contact structure, respectively. Density functional theory‐based calculations provide mechanistic insights into the interaction of H2 and CO gas molecules with Bi‐ and Sn‐doped ZnO(0001) surfaces, revealing changes in the Fermi energies, as well as charge transfer between the molecules and surface species, which facilitate gas sensing

Bulk and Local Electron Transport and Optical Properties of Aluminum-doped Zinc Oxide

ZnO is a promising transparent conducting oxide (TCO) because its components are naturally abundant and inexpensive; and ZnO can be synthesized by several methods as thin films and nanostructures. Doping ZnO with Al (to form what is called AZO) significantly increases electrical conductivity while retaining high optical transparency, making AZO ideal for use as transparent electrodes in optoelectronic devices. However, the electrical conductivity of AZO has not exceeded that of indium tin oxide (ITO), the most widely-utilized TCO. A systematic study of bulk and local electrical and optical properties of AZO is needed to improve conductivity while maintaining transparency. To this end, we conducted bulk magnetotransport measurements on AZO, which indicated that its electron mobility was significantly lower than that of single-crystal ZnO, primarily due to electron scattering at AZO grain boundaries. To further understand this detrimental effect, we directly probed these grain boundaries with a scanning tunneling microscope. These measurements are the first investigation of a broad spectrum of grain boundary traps in AZO, which include shallow states near the conduction band edge that may limit electron mobility, and deeper states that may deplete carriers. Because optical properties can affect transparency in devices, we characterized AZO through a combination of photoluminescence and scanning tunneling microscope cathodoluminescence (STM-CL). STM-CL, which probes only the surface, shows a dramatic narrowing of emission lines compared to bulk photoluminescence. We attribute this to different charge states of oxygen vacancies preferentially located near the surface. This observed difference is especially of interest in understanding transport across interfaces. Finally, we present one application of AZO: a monolayer quantum dot (QD) light-emitting device with AZO electrodes that uses atomic layer deposited insulating oxide to fill the interstices among QDs. This combination of conducting and insulating oxide structures forces tunnel injected hot carriers through QDs and allows for chemical treatment of ligands without QD agglomeration. This device serves as a model for a new class of all-oxide, high current density QD devices. These investigations further the understanding of carrier conduction and surface optical properties of AZO and will contribute to optimization for TCO device applications

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

Engineering and Applied Science