R. T. Murray

Dielectric relaxation of lanthanum doped zirconium oxide

Lanthanum doped zirconium oxide (Lax–Zr1−xO2−δ) films, with La contents, up to x=0.35, were studied. Films were annealed at 900 °C to crystallize them into phases with higher κ-values. Increasing the La content suppressed the monoclinic phase and stabilized the tetragonal or cubic phase. The highest dielectric constant was obtained for a lightly doped film with a La content of x=0.09, for which a κ-value of 40 was obtained. This was accompanied by a significant dielectric relaxation, following a single Curie–von Schweidler power-law dependency with frequency, changing to a mixed Curie–von Schweidler and Kohlrausch–Williams–Watts relationships after annealing. The dielectric relaxation was most severe for lightly doped films, which had the highest κ-values. The dielectric relaxation appears to be related to the size of crystal grains formed during annealing, which was dependent on the doping level.

Structural and electrical characterization of amorphous lanthanum hafnium oxide thin films

This paper describes the structural and electronic characterization of lanthanum hafnium oxide thin films deposited by metal organic chemical vapor deposition using a novel precursor mixture. Structural characterization is obtained using x-ray diffraction and cross-sectional transmission electron microscopy and indicates that the as-deposited films and films annealed up to 900°C remain effectively amorphous. Capacitance voltage and current voltage measurements on metal-oxide-semiconductor capacitors made using the films as a gate dielectric show the films exhibit good electrical integrity following post deposition annealing at temperatures up to 950°C. A dielectric constant, k for the films of 20±1.5 is determined and a density of interface states (at midgap) Dit of 3.9×1011cm−2eV−1 was measured.

Fabrication of epitaxial III-nitride cantilevers on silicon (1 1 1) substrates

The molecular beam epitaxy of AlGaN/GaN epilayers on silicon (1 1 1) using an aluminum nitride buffer layer, and subsequent fabrication of free standing III-nitride cantilevers on Si(1 1 1) has been investigated. Transmission electron microscopy (TEM) of cross-section samples reveals a columnar structure consisting of the hexagonal gallium nitride polytype. Selected area diffraction indicates an epitaxial relationship between the gallium nitride and silicon substrate which is described by GaN[0 0 0 1]//Si[1 1 1] and GaN(1 1 0 0)//Si(1 1 1). Imaging of the electronic structure of an AlGaN/GaN interface has been investigated by mapping the variation in the plasmon frequency using an electron energy loss spectrometer on a dedicated scanning transmission electron microscope. Cantilevers were fabricated using a combination of etching processes. Nitride etch rates during inductively coupled plasma dry etch processing using a Cl2/Ar plasma etchant were obtained by monitoring the optical reflectivity of the nitride films in situ. A peak GaN etch rate of 250 nm/min was measured, the etch rate was found to be strongly dependent on the d.c. self-bias. Thin beams of GaN having a length of 7 μm and 0.7 μm thickness, were fabricated and mechanically released from Si(1 1 1) substrates using a combination of two dry ICP etch processes, using Cl2/Ar and CF4/Ar/O2 chemistries, and a potassium hydroxide (KOH) aqueous wet etch.

Oxide layers on the carbon face of 6H silicon carbide substrates

Electronic grade wafers of 6H SiC have been given a surface oxide layer on the carbon face by either conventional thermal oxidation or low-temperature plasma oxidation. Transmission electron microscopy, energy-dispersive x-ray and parallel electron energy loss, have been used to characterize the layer and its interface with the parent carbide. In both cases the oxide is close to Si2CO6 in composition but has a higher oxygen content at the surface. The plasma-grown oxide displays a higher proportion of p-bonded carbon than the SiC substrate does.

Chemical beam epitaxy of GaN on Si (111) using AlAs buffer layers

The chemical beam epitaxy of GaN on Si (111) using an AlAs buffer layer has been investigated. Transmission electron microscopy of cross-section samples reveals a nanoscale columnar structure consisting of the hexagonal GaN polytype. Selected area diffraction indicates an epitaxial relationship between the GaN and Si substrate which is described by GaN [0001] ∥ Si [111] and GaN (100) ∥ Si (11). Raman spectroscopy of the samples shows that the AlAs longitudinal optical (LO) and transverse optical (TO) modes are observed at 360 and 400 cm-1, respectively, and that the GaN E2 TO and A1 LO modes occur at 562 and 733 cm-1, respectively. Raman peaks between 616 and 673 cm-1 are attributed to an interlayer of AlN formed by the nitridation of the AlAs buffer layer.

EDX and linescan modelling for core/shell GaN/AlGaN nanowire analysis

The growth of gallium nitride nanowires (NW) using nickel seeds to promote axial growth on sapphire substrates has previously been demonstrated [1]. Furthermore it has been shown that control of preferential axial or radial growth can be achieved by changing the group III:V flux during growth using either N-rich or Ga-rich conditions respectively. The next challenge is controlled growth of NW heterostructures incorporating both GaN and AlGaN. This achievement would open the way to development of nanoelectronic devices such as single wire III-nitride based field effect transistors. A critical issue is the control of Al incorporation in GaN/AlGaN radial NW heterostructures.

Electron Microscopy Analysis of AlGaN/GaN Nanowires Grown by Catalyst-Assisted Molecular Beam Epitaxy

Scanning transmission electron microscopy has been used to investigate the composition of nickel seeds which promote the columnar growth of AlGaN / GaN nanowires deposited by molecular beam epitaxy (MBE) on sapphire. The nickel distribution along the nanowires was investigated by both X-ray and electron-energy-loss spectroscopy. Gallium was observed in nickel seeds at the nanowires growth tips. No aluminium was detected and a minimal presence of nitrogen was observed in the nickel seeds, which exhibit a nickel oxide surface attributed to oxidation following removal from the MBE growth system.

Erratum to: Analysis of the Reactive Element Effect on the Oxidation of Ceria Doped Nickel

The effects of external doping with CeO2 on the oxidation of nickel have been evaluated. The materials studied were pure Ni and Ni with the surface doped with CeO2 by pulsed laser deposition. The oxidation kinetics were measured using thermogravimetric analysis. The oxidation microstructures were observed by scanning electron microscopy and cross-sectional transmission electron microscopy. Compositional analysis was performed with energy dispersive X-ray analysis and sputtering neutrals mass spectrometry. Phase identification was performed using X-ray diffraction. Doping with CeO2 resulted in a significant decrease in the NiO growth rate at intermediate temperatures, e.g. 800 C. The scales on doped Ni grew primarily inward whereas those on the undoped Ni grew primarily outward. Deposition of the CeO2 dopant onto Ni with a thin, preformed NiO layer produced a similar reduction in the subsequent NiO growth rate. The CeO2 dopant did not reduce the growth rate at high temperature (1,300 C). The results indicate that the CeO2 dopant influences grain boundary transport in the NiO. Mechanisms are presented to attempt to describe the above observations.

Production of large metallic clusters by thermal evaporation

Abstract Cluster beam evaporation has been used to produce metallic and semiconductor thin films for microelectronic applications. Previous work showed that the deposition of large clusters of antimony (>Sb 2000 ) onto silicon substrates and carbon films can be performed by thermal evaporation through a nozzle orifice and has been modelled using molecular diffusion theory. The results here show that the model can be extended to include other metals (e.g., Ag). TEM shows that cluster sizes in the case of Ag are much smaller than for Sb, having a di-similar statistical spread. Photoluminescence of Mg, Sb and Ag films show decreasing PL intensity with temperature which correlates with the expected concentration of clusters.