Michael Snure

Ferromagnetism in Cu-doped ZnO films: Role of charge carriers

We report the observation of room temperature ferromagnetism in Cu-doped (5%) ZnO films grown on c-plane sapphire substrates. Films were prepared by pulsed laser deposition technique and were thoroughly characterized using several state-of-the-art characterization techniques. Hall measurements showed that the films are of n-type with a carrier concentration of 3×1017cm−3. Magnetization measurements showed that the films exhibit room temperature ferromagnetism with a saturation magnetization of ∼1.45μB∕Cu atom. When additional carriers were introduced in the films, ferromagnetism was completely vanished. Our results show that the p-type nature of the film is not essential for realizing ferromagnetic characteristics; however, the concentration of n-type carriers should not exceed a critical value.

CuBO2 : A p-type transparent oxide

The authors report the synthesis of CuBO2, a p-type transparent oxide belonging to Cu-delafossite family. High quality thin films of CuBO2 were deposited on c-plane sapphire substrates by pulsed laser deposition technique. Detailed structural, optical, and electrical characterizations on these films were performed. Optical transmission measurement showed the films to be highly transparent in the visible range and having indirect and direct band gaps of 2.2 and 4.5eV, respectively. Room-temperature electrical conductivity of CuBO2 films was 1.65Scm−1 and exhibited semiconductorlike temperature dependence. The Hall and Seebeck coefficients of the film were positive, indicating the p-type nature of the material.

Ferromagnetism in Ni-doped ZnO films: Extrinsic or intrinsic?

Here we report a detailed study aimed on understanding the origin of ferromagnetism in Ni-doped ZnO films. A pulsed laser deposition technique was used to deposit Ni-doped (5 at. %) ZnO films on sapphire (0001) substrates under different oxygen pressures ranging from 10−6 to 0.1 Torr. Films were characterized using numerous characterization techniques including x-ray diffraction, x-ray photospectroscopy, energy dispersive x-ray spectroscopy, optical absorption spectroscopy, and electrical transport, magnetotransport, and magnetization measurements. A detailed structure-property correlation and analysis of our results revealed that the ferromagnetism in ZnO:Ni films is not an inherent property of the material but results due to a strong tendency of Ni to precipitate out in the ZnO matrix.

Structural, electrical, and optical characterizations of epitaxial Zn1- xGaxO films grown on sapphire (0001) substrate

In this paper we report the structural, electrical, and optical properties of epitaxial Zn1−xGaxO films (x=0–0.05) grown on single crystal sapphire (0001) substrate by pulsed laser deposition technique. Structural and elemental analysis was performed using high-resolution x-ray diffraction (θ-2θ and Φ scan) and energy dispersive x-ray spectroscopy. Temperature dependent electrical resistivity and thermoelectric power measurements were performed over the temperature range of 77–300K and 296–373K, respectively. Hall effect and optical transmission measurements were preformed at room temperature. All these studies showed that the structural, electrical as well as the optical characteristics of Zn1−xGaxO films depend very sensitively on the Ga contents. As the Ga doping concentration is increased, initially an increase in carrier concentration and optical band gap is observed (until x=0.04), which is followed by a decrease at higher concentrations. These features were attributed to the combined effect of band...

Band-gap engineering of Zn1−xGaxO nanopowders: Synthesis, structural and optical characterizations

We report the preparation and detailed structural and optical characterizations of single phase gallium doped ZnO nanopowders. A low temperature solution-based technique was developed to synthesize high-purity Zn1−xGaxO (x:0–0.05) nanopowders. Structural and optical characterization experiments were performed using x-ray diffraction (XRD), energy dispersive x-ray spectroscopy, scanning electron microscopy, photoluminescence spectroscopy, Raman spectroscopy, and optical transmission spectroscopy. Analysis of XRD data showed a maximum in the lattice volume for x=0.03 as the Ga concentration increases in the Zn1−xGaxO series. Optical transmission spectroscopy results also showed an initial increase in the band gap (Eg) followed by a decrease for x>0.04. A maximum band gap was observed for the Zn0.96Ga0.04O sample, which has a value of 3.42 eV compared to the band-gap value of 3.31 eV for the undoped ZnO sample. All Zn1−xGaxO nanopowder samples showed a broad photoluminescence spectrum with significant shift ...

Self-terminating growth in hexagonal boron nitride by metal organic chemical vapor deposition

In this work, we demonstrate the growth of atomically smooth few-layer hexagonal boron nitride (h-BN) on sapphire substrates by metal organic chemical vapor deposition using triethylboron (TEB) and NH3 as precursors. Changing the V/III ratio in a certain temperature and pressure range was found to change the growth mode from random 3D nucleation to self-terminating growth. Infrared reflectance and Raman spectroscopy were used to identify the h-BN phase of these films. Atomic force microscopy measurements confirm that the surfaces are smooth and continuous even over atomic steps on the surface of the substrate. Using X-ray reflectance measurements, the thicknesses of films grown under self-terminating conditions were determined to be ~1.6 nm and independent of the growth time (1 to 60 min) and TEB flux rate. On the basis of the results of this study, a possible mechanism for the observed self-terminating growth behavior is discussed.

Mid- to long-wavelength infrared surface plasmon properties in doped zinc oxides

This work investigates properties of surface plasmons on doped metal oxides in the 2-20 μm wavelength regime. By varying the stoichiometry in pulse laser deposited Ga and Al doped ZnO, the plasmonic properties can be controlled via a fluctuating free carrier concentration. This deterministic approach may enable one to develop the most appropriate stoichometry of ZnAlO and ZnGaO in regards to specific plasmonic applications for particular IR wavelengths. Presented are theoretical and experimental investigations pertaining to ZnAlO and ZnGaO as surface plasmon host materials. Samples are fabricated via pulsed laser deposition and characterized by infrared ellipsometry and Hall-effect measurements. Complex permittivity spectra are presented, as well as plasmon properties such as the field propagation lengths and penetration depths, in the infrared range of interest. Drude considerations are utilized to determine how the optical properties may change with doping. Finite element simulations verify these plasmonic properties. These materials not only offer potential use as IR plasmon hosts for sensor applications, but also offer new integrated device possibilities due to stoichiometric control of electrical and optical properties.

Anderson localization enhanced ferromagnetism in Zn0.95Co0.05O

We report an enhancement in the ferromagnetic characteristics of Zn0.95Co0.05O thin films due to the localization of charge carriers. Epitaxial thin films of Zn0.95−xCo0.05GaxO (x=0–0.05) were grown on single-crystal sapphire (0001) substrates by pulsed laser deposition technique. The role of charge carrier localization on the electrical and magnetic properties of ZnO:Co was studied by introducing Ga into the system. It was observed that Ga plays a significant role in affecting both the electrical transport mechanism as well as the magnetization of the material. Electrical resistivity of Zn0.95Co0.05O at room temperature was ∼96 mΩ cm and exhibited metal-like temperature dependence, although strongly influenced by electron-electron (e-e) interactions. Strong e-e interaction was understood to arise because of the randomness introduced in the crystal potential of ZnO by the cobalt dopants. As the Ga dopants are introduced, randomness in crystal potential and hence the disorder further increases resulting in...

Probing phonon and electrical anisotropy in black phosphorus for device alignment

Black phosphorus has emerged as a promising two-dimensional semiconductor, which has a unique structure that is anisotropic in-plane. This structural anisotropy translates to some very interesting orientation dependent properties. In this paper we present directional characterization and analysis of the phonon and electrical properties in black phosphorus. Using polarization dependent Raman we show a simple method for estimating orientation. A complementary radially contacted field effect transistor (FET) was fabricated in order to measure orientation dependent electrical properties. Mobility and transconductance followed a sinusoidal like dependence on orientation with a 30% anisotropy. Correlating these results with Raman, we show how Raman methods might be used as a nondestructive technique to orient black phosphorus devices.

Effect of epitaxial strain on the magneto-electric coupling of YMnO3 thin films

We report synthesis of phase pure multiferroic YMnO3 thin films on sapphire (0001) with conducting ZnGaO buffer contact layer. Films were prepared by using pulsed laser deposition technique and characterized using x-ray diffraction, scanning electron microscopy, energy dispersive absorption spectroscopy, and magnetic field dependent dielectric measurement techniques. Structural characterizations indicated phase purity and epitaxial nature of the films. The dielectric response indicated an anomaly in dielectric constant e and tan δ in the vicinity of 30 K, well below the bulk Neel temperature ∼70 K. This anomaly in e and tan δ and its magnetic field dependence is explained as an influence of strain due to lattice mismatch between the substrate and YMnO3 film. A substantial enhancement in magnetocapacitance was also observed for magnetic field applied parallel to ab plane of the film. Our results show that it is possible to tune the multiferroic property of YMnO3 via changes in ferroelastic route.