L. A. Boatner

Wide band gap ferromagnetic semiconductors and oxides

Recent advances in the theory and experimental realization of ferromagnetic semiconductors give hope that a new generation of microelectronic devices based on the spin degree of freedom of the electron can be developed. This review focuses primarily on promising candidate materials (such as GaN, GaP and ZnO) in which there is already a technology base and a fairly good understanding of the basic electrical and optical properties. The introduction of Mn into these and other materials under the right conditions is found to produce ferromagnetism near or above room temperature. There are a number of other potential dopant ions that could be employed (such as Fe, Ni, Co, Cr) as suggested by theory [see, for example, Sato and Katayama-Yoshida, Jpn. J. Appl. Phys., Part 2 39, L555 (2000)]. Growth of these ferromagnetic materials by thin film techniques, such as molecular beam epitaxy or pulsed laser deposition, provides excellent control of the dopant concentration and the ability to grow single-phase layers. T...

Ferromagnetism in Mn-implanted ZnO:Sn single crystals

We have investigated the magnetic properties of Mn-implanted n-type ZnO single crystals that are codoped with Sn. Theory predicts that room-temperature carrier-mediated ferromagnetism should be possible in manganese-doped p-type ZnO, although Mn-doped n-type ZnO should not be ferromagnetic. While previous efforts report only low-temperature ferromagnetism in Mn-doped ZnO that is n type via shallow donors, we find evidence for ferromagnetism with a Curie temperature of ∼250 K in ZnO that is codoped with Mn and Sn. As a 4+ valence cation, Sn should behave as a doubly ionized donor, thus introducing states deep in the gap. Hysteresis is clearly observed in magnetization versus field curves. Differences in zero-field-cooled and field-cooled magnetization persists up to ∼250 K for Sn-doped ZnO crystals implanted with 3 at. % Mn. Increasing the Mn concentration to 5 at. % significantly reduces the magnetic hysteresis. This latter observation is inconsistent with the origin for ferromagnetism being due to segreg...

Advances in wide bandgap materials for semiconductor spintronics

Existing semiconductor electronic and photonic devices utilize the charge on electrons and holes in order to perform their specific functionality such as signal processing or light emission. The relatively new field of semiconductor spintronics seeks, in addition, to exploit the spin of charge carriers in new generations of transistors, lasers and integrated magnetic sensors. The ability to control of spin injection, transport and detection leads to the potential for new classes of ultra-low power, high speed memory, logic and photonic devices. The utility of such devices depends on the availability of materials with practical (>300 K) magnetic ordering temperatures. In this paper, we summarize recent progress in dilute magnetic semiconductors (DMS) such as (Ga, Mn)N, (Ga, Mn)P, (Zn, Mn)O and (Zn, Mn)SiGeN2 exhibiting room temperature ferromagnetism, the origins of the magnetism and its potential applications in novel devices such as spin-polarized light emitters and spin field effect transistors. # 2003 Elsevier Science B.V. All rights reserved.

Radiation damage in zircon and monazite

Monazite and zircon respond differently to ion irradiation and to thermal and irradiation-enhanced annealing. Monazite cannot be amorphized by 800 keV Kr+ ions at temperatures greater than 175°C; whereas, zircon can be amorphized at temperatures up to 740°C. The damage process (i.e., elastic interactions leading to amorphization) in radioactive minerals (metamictization) is basically the same as for the ion-beam-irradiated samples with the exception of the dose rate which is much lower in the case of natural samples. The crystalline-to-metamict transition in natural samples with different degrees of damage, from almost fully crystalline to completely metamict, is compared to the sequence of microstructures observed for ion-beam-irradiated monazite and zircon. The damage accumulation process, representing the competing effects of radiation-induced structural disorder and subsequent annealing mechanisms (irradiation-enhanced and thermal) occurs at much higher temperatures for zircon than for monazite. The amorphization dose, expressed as displacements per atom, is considerably higher in the natural samples, and the atomic-scale process leading to metamictization appears to develop differently. Ion-beam-induced amorphization data were used to calculate the α-decay-event dose required for amorphization in terms of a critical radionuclide concentration, i.e., the concentration above which a sample of a given age will become metamict at a specific temperature. This equation was applied to estimate the reliability of U-Pb ages, to provide a qualitative estimate of the thermal history of high-U natural zircons, and to predict whether actinide-bearing zircon or monazite nuclear waste forms will become amorphous (metamict) over long timescales.

Strontium and barium iodide high light yield scintillators

Europium-doped strontium and barium iodide are found to be readily growable by the Bridgman method and to produce high scintillation light yields.

Ferromagnetism in cobalt-implanted ZnO

The magnetic and structural properties of cobalt-implanted ZnO single crystals are reported. High-quality, (110)-oriented single-crystal Sn-doped ZnO substrates were implanted at ∼350 °C with Co to yield transition metal concentrations of 3–5 at. % in the near-surface (∼2000 A) region. After implantation, the samples were subject to a 5 min rapid thermal annealing at 700 °C. Magnetization measurements indicate ferromagnetic behavior, with hysteresis observed in the M vs H behavior at T=5 K. Coercive fields were ⩽100 Oe at this measurement temperature. Temperature-dependent magnetization measurements showed evidence for ordering temperatures of >300 K, although hysteresis in the M vs H behavior was not observed at room temperature. Four-circle x-ray diffraction results indicate the presence of (110)-oriented hexagonal phase Co in the ZnO matrix. From the 2θ full width at half maximum (FWHM) of the Co (110) peak, the nanocrystal size is estimated to be ∼3.5 nm, which is below the superparamagnetic limit at ...

Nanocomposite Materials Formed by Ion Implantation

Ion implantation has become a versatile and powerful technique for synthesizing nanometer-scale clusters and crystals embedded in the near-surface region of a variety of hosts in order to create nanocomposite materials with often unique optical, magnetic, and other properties. Here we review some of the principal features of this nanophase materials synthesis technique as well as the materials properties that are exhibited by nanocomposites created by using ion beams. Outstanding difficulties and future research directions are also discussed.

Ion implantation and annealing of crystalline oxides

The technique of ion implantation is being investigated as a general method for altering the near-surface properties of insulating materials. The primary motivation behind these investigations is to develop ion implantation as a practical means of controlling and improving the near-surface mechanical, optical, or electronic properties of insulators. Changes in these properties depend on the microstructures and compositions developed in the material during the ion implantation process and subsequent thermal treatments. In many cases, structures and compositions can be produced by implantation and thermal annealing that cannot be achieved by conventional techniques. In this work, the response of a wide range of crystalline oxides to ion implantation and subsequent thermal processing will be reviewed. The materials treated here include Al 2 O 3 , LiNbO 3 , CaTiO 3 , SrTiO 3 , ZnO, and MgO, as well as the non-oxide materials Si 3 N 4 and SiC. The response of these insulators to ion implantation varies widely and depends on the specific material, the implantation species and dose, and the implantation temperature. Ion implantation produces displacement and other damage in the near-surface region, and in many cases, the surfaces of originally crystalline insulators are turned amorphous. Thermal annealing can often be used to restore crystallinity to the damaged near-surface region, and additionally, metastable solid solutions can be produced. For a number of oxide materials, the annealing behavior has been studied in detail using both Rutherford backscattering-ion channeling techniques and transmission electron microscopy. These studies show that, in some materials, the annealing behavior is quite simple and takes place by solid-phase epitaxial crystallization where the amorphous-to-crystalline transformation occurs at an interface that moves toward the free surface during the annealing process. In such materials, the regrowth kinetics have been measured, and the associated activation energies for crystallization have been determined. The formation of metastable solid solutions during crystallization of the amorphous phase will also be discussed.

Spectroscopic ellipsometry of thin film and bulk anatase (TiO2)

Spectroscopic ellipsometry (SE) measurements were made on thin-film and single-crystal TiO2 anatase using a two-modulator generalized ellipsometer. The TiO2 films were epitaxially stabilized on a LaAlO3 substrate in the anatase crystal structure using reactive sputter deposition. The films were highly crystalline, possessing a “stepped surface” morphology indicative of atomic layer-by-layer growth. The SE results for the anatase film indicate that the material is essentially oriented with the c axis perpendicular to the substrate, but there is some anisotropy near the interface and the surface. Corrugations of the film surface, as observed using atomic force microscopy, are consistent with a surface structure needed to create cross polarization. Accurate values of the optical functions of crystalline anatase were obtained above and below the band edge using SE. Above the band edge, both the ordinary and extraordinary complex dielectric functions exhibited two critical points.

Scintillators With Potential to Supersede Lanthanum Bromide

New scintillators for high-resolution gamma ray spectroscopy have been identified, grown and characterized. Our development efforts have focused on two classes of high-light-yield materials: europium-doped alkaline earth halides and cerium-doped garnets. Of the halide single crystals we have grown by the Bridgman method-SrI2, CaI2, SrBr2, BaI2 and BaBr2-SrI2 is the most promising. SrI2(Eu) emits into the Eu2+ band, centered at 435 nm, with a decay time of 1.2 mus and a light yield of up to 115,000 photons/MeV. It offers energy resolution better than 3% FWHM at 662 keV, and exhibits excellent light yield proportionality. Transparent ceramic fabrication allows the production of gadolinium- and terbium-based garnets which are not growable by melt techniques due to phase instabilities. The scintillation light yields of cerium-doped ceramic garnets are high, 20,000-100,000 photons/MeV. We are developing an understanding of the mechanisms underlying energy dependent scintillation light yield non-proportionality and how it affects energy resolution. We have also identified aspects of optical design that can be optimized to enhance the energy resolution.