J. K. Grepstad

Preconditioning of c-plane sapphire for GaN epitaxy by radio frequency plasma nitridation

The crystalline quality of molecular beam epitaxy grown layers of GaN on sapphire is generally improved by nitridation of the substrate. In this study, we use x-ray photoelectron spectroscopy, low energy electron diffraction, and atomic force microscopy to examine the case for nitridation of c-plane sapphire upon exposure to rf plasma generated nitrogen radicals. We find that a monolayer of surface nitride is formed after ∼300 min exposure with the substrate at 400 °C. Extended exposure causes growth of protrusions from the c-plane sapphire and thus leads to a rough surface morphology. Moreover, we report removal of adventitious surface carbon upon heat treatment at 300 °C in nitrogen plasma, albeit with reduced efficiency compared with hydrogen plasma cleaning.

Effects of thin cerium overlayers on the oxidation of tantalum and aluminium

Abstract Oxidation of aluminium and tantalum with thin (4–15 A) overlayers of cerium has been studied with X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS) using synchrotron radiation. The cerium layer is found to induce a large enhancement in the oxidation of the Ta and Al substrates at room temperature. The catalytic oxidation of Ta exhibits growth characteristics identical with those previously reported for Nb. This enhanced growth was attributed to a mechanism in which a conversion from tetra- to trivalent Ce-oxide plays an important role. In contrast, the enhanced oxidation of Al is caused by formation of an intermetallic surface layer which enables increased oxygen uptake. The growth of bulk Ta- and Al-oxide follows logarithmic growth laws at room temperature. The absence of bulk oxide formation at liquid nitrogen temperature shows that the enhanced surface oxidation is limited by a thermally activated process and not by electron tunneling across the oxide barrier. Field assisted diffusion is assumed to be the rate determining mechanism for both systems investigated.

Preconditioning of c-plane sapphire for GaN molecular beam epitaxy by electron cyclotron resonance plasma nitridation

Nitridation of c-plane sapphire is commonly employed in molecular beam epitaxy of GaN, in order to improve the crystalline quality of the deposited layers. In this study, we use x-ray photoelectron spectroscopy, Auger sputter profiling, reflection high energy electron diffraction, low energy electron diffraction, and atomic force microscopy to examine chemical and structural properties of sapphire (0001) substrate upon exposure to nitrogen activated by an electron cyclotron resonance plasma source. Incorporation of nitrogen into the sapphire surface was verified with x-ray photoelectron spectroscopy, and a monolayer of surface nitride is formed after approximately 60 min of nitridation at a substrate temperature of 620 °C. The thickness of the surface nitride layer is on the order of 4.5–6 A for nitridation times ranging from 60 to 180 min, as estimated from the recorded nitrogen Auger sputter profiles. Reflection high energy electron diffraction gave clear evidence for formation of surface aluminum nitri...

X‐ray photoelectron spectroscopy study of thin Ag overlayers on YBa2Cu3O7−x: Effects of oxygen annealing

X‐ray photoelectron spectroscopy was used to examine vapor deposited Ag overlayers on YBa2Cu3O7−x thin films and cleaved single‐crystal surfaces (x=1), including the effects of subsequent heat treatment in oxygen. The core level spectra of the present tetragonal crystals were found to closely resemble those previously reported for nominally orthorhombic superconducting samples. Minor differences, notably an increased fraction of Cu in the +1 oxidation state and a slightly higher binding energy of the Ba 3d levels, may be attributed to the lower oxygen content of the tetragonal semiconducting phase. Deposition of Ag on clean monocrystal surfaces was found to cause some disruption of CuO bonds at the Ag/YBa2Cu3O7−x interface. We also find weak indications of surface band bending (i.e., Schottky barrier formation) from a uniform shift in the measured binding energies of the substrate core levels upon Ag deposition. Heat treatment in pure oxygen (1 h at 500 °C) of Ag‐coated superconducting thin films was foun...

Ferroelectric stripe domains in PbTiO3 thin films: Depolarization field and domain randomness

Observation of stripe domains in PbTiO3 thin films using standard x-ray diffraction analysis at room temperature is discussed. High-quality c-axis oriented thin films of varying thickness, from 6 to 210 unit cells, were grown on buffered NH4–HF etched SrTiO3(001) and Nb:SrTiO3(001) substrates using off-axis radio frequency magnetron sputtering. High-resolution linear Qx scans reveal a superstructure around the specular Bragg peaks, consistent with the presence of ferroelectric stripe domains. For thin samples, the stripe width is found to be proportional to the square root of the film thickness, with random in-plane orientation of domains. For films with a thickness of more than ∼100 unit cells, both monodomain samples and stripe domains were observed. We present evidence for the presence of a threshold depolarization field, above which there is a monotonically decreasing relationship between the domain width and the depolarization field. Furthermore, simulations show that random variations in size of the...

Photochemical switching of ultrathin PbTiO3 films

The ferroelectric domain ground state of PbTiO3 thin films on Nb:SrTiO3 substrates has been studied as a function of film thickness. High resolution linear Qx x-ray diffraction profiles were recorded in order to probe the presence of stripe domains, which were invariably observed for as-grown samples. After photochemical treatment, films thinner than 50 unit cells were switched to a monodomain state that could be reversed upon subsequent annealing. The stripe-domain state is linked to a Qx modulation of the Qz parameter in the reciprocal space, which disappears upon switching.

Capping and decapping of MBE grown GaAs(001), Al0.5Ga0.5As(001), and AlAs(001) investigated with ASP, PES, LEED, and RHEED

Abstract Arsenic capping and regeneration of MBE-grown GaAs(001), Al 0.5 Ga 0.5 As(001), and AlAs(001) epilayer surfaces were examined with Auger sputter profiling (ASP), synchrotron radiation and X-ray photoelectron spectroscopy (PES), LEED, and RHEED. It is found that clean, ordered surfaces of different As/Ga(Al) compositions and different surface reconstructions can be prepared in a controlled manner after long-term storage in air, by thermal desorption of the As cap at appropriate annealing temperatures. A protective film of amorphous arsenic was deposited in situ with both As 2 and As 4 molecular beams onto cold substrates. The recorded Auger depth profiles unveil capping layer thicknesses from 0.3 to 3 μm, the thicker for depositions using the As 2 dimer source. The As 3+ surface oxide, formed immediately upon exposure of the passivated wafers to air, remains on the order of 10Athick, even after storage in atmosphere for several months. Core level photoemission shows selective desorption of this oxide upon annealing in UHV at 250°C. Further heating at 350°C evaporates the protective arsenic cap, and clean, As-terminated Al x Ga 1− x As(001) surfaces with a regular arrayof chemisorbed excess As As dimers prevail. The recorded LEED and RHEED patterns show a c(4 × 4) surface reconstruction for GaAs(001) and Al 0.5 Ga 0.5 As(001), whereas this structural phase was observed with RHEED only for the highly reactive AlAs(001) surface. Subsequently annealing in UHV at 450°C causes desorption of the chemisorbed surface arsenic and a concurrent transition from c(4 × 4) to the (2 × 4)/c(2 × 8) surface of As stabilized MBE-grown Al x Ga 1− x As(001). With AlAs(001), surface Al oxidation was observed immediately after annealing at 450°C, in spite of carefully controlled UHV environments

Antiferromagnetic Domain Reconfiguration in Embedded LaFeO3 Thin Film Nanostructures

Using photoemission electron microscopy in combination with X-ray magnetic linear dichroism, we report reconfiguration upon nanostructuring of the antiferromagnetic domain structure in epitaxial LaFeO3 thin films. Antiferromagnetic (AFM) nanoislands were synthesized using a dedicated process, devised to define nanostructures with magnetic order embedded in a paramagnetic matrix. Significant impact on the AFM domain configuration was observed. Extended domains were found to form along edges parallel to the in-plane <100> crystalline axes of the cubic substrate, with their AFM spin axis parallel to the edge. No such edge-imposed domain configuration was found for nanoislands defined with the edges at 45° with the in-plane crystalline axes. Epitaxial constraints on the film crystalline structure appear to play an important role in the formation of the edge-bound extended AFM domains. The data indicate a magnetostatic origin of this domain reconfiguration.

GaAs(100) substrate cleaning by thermal annealing in hydrogen

The removal of residual surface contaminants from chemically polished GaAs wafers has been examined with x‐ray photoelectron spectroscopy. Al Kα‐excited As 2p3/2 and Ga 2p3/2 core level spectra with a shallow probing depth show complete desorption of As3+ surface oxide after annealing in a stream of pure hydrogen at atmospheric pressure and 400 °C, whereas a temperature of 600 °C is required for entire removal of the Ga3+ surface oxide. Carbonaceous surface impurities are also shown to be efficiently reduced by this hydrogen processing. The recorded core level peak intensities indicate formation of a fully stoichiometric substrate surface, with no depletion of arsenic. The chemical structure of the hydrogen processed surface compares favorably with that found for substrates annealed in an ultrahigh vacuum environment. In contrast, annealing at 600 °C in 1 atm of high‐purity inert gas (argon) is shown to promote formation of a thick Ga–oxide on the substrate surface, thus emphasizing the reactive nature of...

Crossover from Spin-Flop Coupling to Collinear Spin Alignment in Antiferromagnetic/Ferromagnetic Nanostructures

The technologically important exchange coupling in antiferromagnetic/ferromagnetic bilayers is investigated for embedded nanostructures defined in a LaFeO(3)/La(0.7)Sr(0.3)MnO(3) bilayer. Exploiting the element specificity of soft X-ray spectromicroscopy, we selectively probe the magnetic order in the two layers. A transition from perpendicular to parallel spin alignment is observed for these nanostructures, dependent on size and crystalline orientation. The results show that shape-induced anisotropy in the antiferromagnet can override the interface exchange coupling in spin-flop coupled nanostructures.