J. C. Tramontana

Epitaxial MgO on Si(001) for Y-Ba-Cu-O thin-film growth by pulsed laser deposition

Epitaxial MgO thin films were grown on Si(001) by pulsed laser deposition. In spite of a large (−22.5%) lattice mismatch, epitaxy occurs with alignment of all crystallographic axes. Epitaxial quality and deposition rate are both sensitive to temperature and oxygen pressure. We believe this is the first demonstration of epitaxial MgO on Si. We employ MgO intermediate layers for superconducting epitaxial YBa2Cu3O7−δ/BaTiO3 thin films on Si with a critical current density of 6.7×105 A/cm2 at 77 K.

Initial stages of epitaxial growth of GaAs on (100) silicon

Direct observations of early stages of molecular‐beam epitaxial growth of GaAs on oriented and vicinal (100) Si surfaces are presented. Cross‐sectional transmission electron microscopy and plan view scanning electron microscopy images directly reveal three‐dimensional island growth for substrate temperatures above 300 °C. Island size, island spacing, surface morphology, and stacking fault defect spacing all increase with substrate temperature for fixed Ga and As fluxes. Below 300 °C, 7‐nm‐thick films are continuous and uniform. Films deposited on surfaces tilted from (100) coalesce anisotropically with respect to the tilt axis.

Ion milled tips for scanning tunneling microscopy

Ion milling of electrochemically etched tungsten tips is shown to improve the characteristics for scanning tunneling microscopy. The primary mechanism for the enhancement of tip reliability is identified to be the removal of a residual oxide. A greatly decreased radius of curvature is also achieved without significantly changing the macrostructural geometry of the tip.

Reactions at the interfaces of thin films of Y‐Ba‐Cu‐ and Zr‐oxides with Si substrates

Thin films were deposited by pulsed uv‐laser (ablation) deposition of Y1Ba2Cu3O7−x (YBCO), and composite zirconia and yttria targets onto silicon wafers. These films were analyzed to ascertain the chemical and physical structure of the film interfaces and further the development of Si substrates for superconducting YBCO films. Substrates were Si(100) with either a high‐quality, thermal oxide (SiO2) film, or a spin‐etch processed, oxide‐free, hydrogen‐terminated surface (Si:H). X‐ray photoelectron spectroscopy (XPS) of Y, Ba, Cu, and Si core levels revealed adverse reactions for thin (nominally 2 nm) YBCO films deposited directly onto either substrate surface. The surfaces of thicker YBCO films (50–100 nm) and various oxide powders were compared with XPS results from these thin films. The thicker‐film surfaces are similar to those of fractured bulk YBCO, while the thin YBCO films decomposed, as evidenced by changes in the Ba and Cu XPS. The Si XPS on these films showed the formation of metal‐silicate compo...

Epitaxial BaTiO3/MgO Structure Grown on GaAs(100) by Pulsed Laser Deposition*

BaTiO3 thin films on MgO encapsulated GaAs(100) were prepared epitaxially, in condition ranging from 600°C to 800°C and from 2×10-4 Torr O2 to 1×10-2 Torr O2, by pulsed laser deposition. The epitaxial relationship between all materials was cube-on-cube. BaTiO3 thin films obtained were c axis oriented tetragonal phase and showed ferroelectric properties. Epitaxial BaTiO3 had quite smooth surface of less than 100 A in roughness. BaTiO3 grown on 40-A-thick epitaxial MgO was also epitaxial although defect density in BaTiO3 looked higher than that in BaTiO3 grown on 400-A-thick MgO. The interfaces of BaTiO3/40-A-thick MgO/GaAs structure were abrupt and free of secondary phases in spite of the quite thin MgO layer.

High critical current densities in epitaxial YBa2Cu3O7−δ thin films on silicon‐on‐sapphire

The use of silicon on sapphire (SOS) as a substrate for YBa2Cu3O7−δ allows the growth of thick (∼4000 A) films without the thermally induced cracking characteristic of epitaxial films on bulk Si substrate. Epitaxy is sustained and reaction is prevented by an intermediate buffer layer of yttria‐stabilized (YSZ). The transport critical current density is as high as 4.6×106 A/cm2 at 77 K, and surface resistance measurements at 4.2 K are reported. Microtwin propagation from Si into YSZ is shown not to occur.

Misfit dislocations in GaAs heteroepitaxy on (001) Si

Abstract Transmission electron microscopy (TEM) is used to analyse the atomic structure and morphology of misfit dislocations, stacking faults and microtwins at the (001) GaAs on Si interface. Common structural features of GaAs grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) are discussed rather than a series of samples grown under specific growth conditions. In addition to the two well-characterized misfit dislocation types with 1 2 a 〈110〉 Burgers vectors, part ial dislocations are shown to play a significant role in the mismatch relaxation process terminating stacking faults and microtwins.

Simple ion milling preparation of 〈111〉 tungsten tips

Ion milling of single‐crystal tungsten 〈111〉 wire results in a steady‐state faceted tip which is sharp on a near‐atomic scale. The self‐limiting nature of the process makes preparation particularly simple and therefore useful for applications such as scanning tunneling microscope tips, focused ion beam sources, etc.

Use of ZnSe as an interlayer for GaAs growth on Si

ZnSe has been used as an interlayer between Si substrates and GaAs layers in molecular beam epitaxial growth of GaAs on Si. It is found that thin GaAs layers are much more uniform and have fewer defects when grown on ZnSe interlayers than when they are grown directly on Si. The growth of GaAs on ZnSe is much more difficult than the more usual reverse sequence, and different growth modes for the epitaxy of GaAs on ZnSe are compared. Deposition of GaAs on ZnSe at room temperature followed by solid phase regrowth led to an epitaxial layer plus a polycrystalline layer. A slow ramping of the substrate temperature during the GaAs epitaxial growth was found to give the best crystal quality.

Graded‐thickness samples for molecular beam epitaxial growth studies of GaAs/Si heteroepitaxy

In this letter we discuss the technique of graded‐thickness sample deposition for studying the growth mechanisms of GaAs heteroepitaxy on Si. We can observe the continuous evolution from the initial clean surface, through nucleation, growth, and coalescence of the deposited material. We describe results for a sample typical of buffer layer growth in the two‐step molecular beam epitaxial deposition of GaAs directly on Si. We are led to a specific model for the three‐dimensional nucleation and growth mechanisms in which Ga atom diffiusion dominates the stable cluster formation, As capture from the vapor by the islands immobilizes the Ga, and island growth is limited by the binding of diffusing Ga.