Scott A. Chambers

Epitaxial growth and properties of thin film oxides

Oxide film growth by molecular beam epitaxy for the purpose of preparing metal oxides for surface science studies is discussed and reviewed. Critical issues such as the choice of oxidizing gas, the selection of substrates, crystal symmetry and lattice match, interface chemistry, and the relationship of these issues to the materials science of oxide-on-metal and oxide-on-oxide film growth are discussed. Recent work on the growth of select, representative oxides that span the ranges of crystal structure, metal oxidation state, and stoichiometry is reviewed and used to illustrate the basic materials science.

Epitaxial growth and properties of ferromagnetic co-doped TiO2 anatase

We have used oxygen-plasma-assisted molecular-beam epitaxy (OPA-MBE) to grow CoxTi1−xO2 anatase on SrTiO3(001) for x=∼0.01–0.10, and have measured the structural, compositional, and magnetic properties of the resulting films. Whether epitaxial or polycrystalline, these CoxTi1−xO2 films are ferromagnetic semiconductors at and above room temperature. However, the magnetic and structural properties depend critically on the Co distribution, which varies widely with growth conditions. Co is substitutional in the anatase lattice and in the +2 formal oxidation state in ferromagnetic CoxTi1−xO2. The magnetic properties of OPA-MBE grown material are significantly better than those of analogous pulsed laser deposition-grown material.

Ferromagnetism in oxide semiconductors

Over the past five years, considerable work has been carried out in the exploration of candidate diluted oxide magnetic semiconductors with high Curie temperatures. Fueled by early experimental results and theoretical predictions, claims of ferromagnetism at and above room temperature in doped oxides have abounded. In general, neither the true nature of these materials nor the physical causes of the magnetism have been adequately determined. It is now apparent that these dilute magnetic systems are deceptively complex. We consider two well-characterizedn-type magnetically doped oxide semiconductors and explore the relationship between donor electrons and ferromagnetism.

Experimental determination of valence band maxima for SrTiO3,TiO2, and SrO and the associated valence band offsets with Si(001)

We address the issue of accurate determination of the valence band maximum (VBM) for SrTiO3(001) single crystals and epitaxial films, as well as TiO2(001) anatase and SrO epitaxial films. These measurements are of critical importance in determining valence band offsets in heterojunctions of these oxides with Si. Three different methods are analyzed: (1) fitting a Gaussian broadened theoretical density of states to the x-ray photoelectron valence band spectrum; (2) finding the intersection of a regression line that spans the linear portion of the x-ray photoelectron valence band leading edge with the background between the valence band maximum and the Fermi level; and (3) determining the energy at which high-resolution ultraviolet photoemission intensity at the leading edge goes to zero. We find that method 1 yields physically unreasonable results when used in conjunction with density functional theory because the latter does not predict the detailed shape of the valence bands in these oxides with sufficie...

Clusters and magnetism in epitaxial Co-doped TiO2 anatase

We show that under certain conditions, highly Co-enriched TiO2 anatase clusters nucleate on epitaxial TiO2 anatase grown on LaAlO3(001) by oxygen plasma assisted molecular beam epitaxy. In the most extreme cases, virtually all incident Co segregates to the clusters, yielding a nanoscale ferromagnetic phase that is not ferromagnetic in homogeneous films of the same Co concentration. The nucleation of this phase simultaneous with continuous epitaxial film growth must be carefully monitored in order to avoid drawing false conclusions about the film structure.

Measurement of the band offsets between amorphous LaAlO3 and silicon

The conduction and valence band offsets between amorphous LaAlO3 and silicon have been determined from x-ray photoelectron spectroscopy measurements. These films, which are free of interfacial SiO2, were made by molecular-beam deposition. The band line-up is type I with measured band offsets of 1.8±0.2 eV for electrons and 3.2±0.1 eV for holes. The band offsets are independent of the doping concentration in the silicon substrate as well as the amorphous LaAlO3 film thickness. These amorphous LaAlO3 films have a bandgap of 6.2±0.1 eV.

Epitaxial growth and properties of doped transition metal and complex oxide films.

The detailed science and technology of crystalline oxide film growth using vacuum methods is reviewed and discussed with an eye toward gaining fundamental insights into the relationships between growth process and parameters, film and interface structure and composition, and electronic, magnetic and photochemical properties. The topic is approached first from a comparative point of view based on the most widely used growth methods, and then on the basis of specific material systems that have generated very high levels of interest. Emphasis is placed on the wide diversity of structural, electronic, optical and magnetic properties exhibited by oxides, and the fascinating results that this diversity of properties can produce when combined with the degrees of freedom afforded by heteroepitaxy.

Carrier dynamics in α‐Fe2O3 (0001) thin films and single crystals probed by femtosecond transient absorption and reflectivity

Femtosecond transient reflectivity and absorption are used to measure the carrier lifetimes in α‐Fe2O3 thin films and single crystals. The results from the thin films show that initially excited hot electrons relax to the band edge within 300fs and then recombine with holes or trap within 5ps. The trapped electrons have a lifetime of hundreds of picoseconds. Transient reflectivity measurements from hematite (α‐Fe2O3) single crystals show similar but slightly faster dynamics leading to the conclusion that the short carrier lifetimes in these materials are due primarily to trapping to Fe d‐d states in the band gap. In the hematite single crystal, the transient reflectivity displays oscillations due to the formation of longitudinal acoustic phonons generated following absorption of the ultrashort excitation pulse.

Selective growth and characterization of pure, epitaxial α-Fe2O3(0001) and Fe3O4(001) films by plasma-assisted molecular beam epitaxy

Abstract Pure-phase, single-crystalline epitaxial films of α-Fe 2 O 3 (0001) and Fe 3 O 4 (001) have been grown on Al 2 O 3 (0001) and MgO(001) substrates, respectively, using oxygen-plasma-assisted molecular beam epitaxy. We discuss the growth conditions required to synthesize these phases, as well as the associated characterization by means of reflection high-energy electron diffraction, low-energy electron diffraction, and X-ray photoelectron spectroscopy and diffraction. The selective growth of these phases depends critically on the choice of substrate, the iron and oxygen fluxes, and the substrate temperature. MgO(001) and Al 2 O 3 (0001) were chosen as substrates for the growth of Fe 3 O 4 (001) and α-Fe 2 O 3 (0001), respectively, because of good lattice and crystal symmetry matching. The growth of α-Fe 2 O 3 is achieved using a low iron-to-oxygen flux ratio compared with that used to grow Fe 3 O 4 . Fe 3 O 4 must be grown at the relatively low substrate temperature of 250°C on MgO(001) to avoid interface reaction and Mg outdiffusion. The α-Fe 2 O 3 film surface is unreconstructed whereas the Fe 3 O 4 surface exhibits a (√2 × √2)R45° reconstruction. Application of a simple electron counting rule to the Fe 3 O 4 (001) surface suggests that the reconstruction is due to an ordered array of tetrahedral Fe vacancies.

Epitaxial growth and properties of MBE-grown ferromagnetic Co-doped TiO2 anatase films on SrTiO3(001) and LaAlO3(001)

Abstract We have investigated the heteroepitaxial growth and materials properties of pure and Co-doped TiO 2 anatase on SrTiO 3 (001) and LaAlO 3 (001), grown by oxygen plasma assisted molecular beam epitaxy. This material is a promising new diluted magnetic semiconductor that shows large magnetization and a Curie temperature well above room temperature. We have found that epitaxial films with the highest crystalline quality and most uniform distribution of Co result when a rather slow growth rate (∼0.01 nm/s) is used over a substrate temperature range of 550–600 °C on LaAlO 3 (001). These conditions result in layer-by-layer growth of single-crystal films and a very low density of extremely small nanocrystalline inclusions. In contrast, growth at a higher rate (∼0.04 nm/s) leads to extensive formation of secondary-phase rutile nanocrystals to which Co diffuses and segregates. The rutile nanocrystals nucleate on the evolving anatase film surface in such a way that lattice strain between the two phases is minimized. Cobalt appears to substitute for Ti in the lattice and exhibits a +2 formal oxidation state. Both pure and Co-doped films can be grown as n-type semiconductors by controlled incorporation of oxygen atom vacancies. Free electrons are required to couple the Co(II) spin to a ferromagnetic state.