Geoffrey W. Brown

Towards the fabrication of phosphorus qubits for a silicon quantum computer

The quest to build a quantum computer has been inspired by the recognition of the formidable computational power such a device could offer. In particular silicon-based proposals, using the nuclear or electron spin of dopants as qubits, are attractive due to the long spin relaxation times involved, their scalability, and the ease of integration with existing silicon technology. Fabrication of such devices, however, requires atomic scale manipulation-an immense technological challenge. We demonstrate that it is possible to fabricate an atomically precise linear array of single phosphorus bearing molecules on a silicon surface with the required dimensions for the fabrication of a silicon-based quantum computer. We also discuss strategies for the encapsulation of these phosphorus atoms by subsequent silicon crystal growth.

The influence of energetic bombardment on the structure and properties of epitaxial SrRuO3 thin films grown by pulsed laser deposition

SrRuO3 epitaxial thin films were prepared by pulsed laser deposition (PLD) under a range of growth conditions to study the impact of bombardment on properties. Growth conditions favoring energetic bombardment resulted in SrRuO3 films with expanded in-plane and out-of-plane lattice constants. In particular, SrRuO3 films with pseudocubic out-of-plane lattice constants as large as 4.08 A were deposited (3.8% larger than the bulk value). Those films with expanded lattices had greater resistivities and depressed Curie transition temperatures. The relative lattice mismatch between film and substrate was found to temper the effect of bombardment such that as the mismatch increased, a higher degree of bombardment was required to produce extended lattice parameters. The pressure-dependent energetic species inherent to PLD and their interaction with the ambient are believed to be the source of the bombarding flux. Further experiments confirmed that in the range of 20–200 mTorr, oxygen/ozone partial pressure had a n...

Adsorption-controlled growth of PbTiO3 by reactive molecular beam epitaxy

Epitaxial lead titanate films have been grown on (001) SrTiO3 substrates by reactive molecular beam epitaxy (MBE). Growth of stoichiometric phase-pure epitaxial PbTiO3 films is achieved by supplying a continuous excess of lead and ozone to the surface of the depositing film. Results obtained from RBS composition analysis, measured film thicknesses and flux measurements using atomic absorption spectroscopy (AA) indicate that the film growth rate is completely determined by the incident titanium flux supplied to the surface. Atomic force microscopy (AFM) results show that films grow smoothly in a layer-by-layer fashion with an RMS roughness of <0.5 nm. The sticking coefficient of titanium is determined to be approximately unity while the excess lead, lead oxide and ozone desorb. Lead and ozone beam equivalent pressures have been measured in the MBE environment. Thermodynamic analysis is used to help describe the processes that prevent the incorporation of PbO into films under adsorption-controlled growth conditions.

Optimization of the growth of epitaxial SrBi2Ta2O9 thin films by pulsed laser deposition

Abstract The growth of epitaxial (001)-oriented SrBi 2 Ta 2 O 9 thin films by pulsed laser deposition (PLD) has been investigated. The effects of target composition, substrate temperature, oxidant pressure, laser pulse rate, laser fluence, and sample cooling have been studied. Variation of deposition parameters significantly affects the phase purity and epitaxial perfection of these ferroelectrics, and individual and combinatorial effects of the aforementioned growth variables as related to the adsorption-controlled growth regime are discussed. X-Ray diffraction and Rutherford backscattering spectrometry (χ min =12%) analyses indicate that the (001)-oriented epitaxial films grown under the optimized conditions identified in this study have the highest structural perfection reported to date.

Adsorption-controlled growth of Bi4Ti3O12 by reactive MBE

~Received 30 June 1997; accepted for publication 27 March 1998!Adsorption-controlled conditions have been identified and utilized to grow epitaxial bismuth titanatethin films by reactive molecular beam epitaxy. Growth of stoichiometric, phase pure, c-axisoriented, epitaxial films is achieved by supplying a large overabundance of bismuth and ozonecontinuously to the surface of the depositing film. Titanium is supplied to the film in the form ofshuttered bursts each containing a three monolayer dose of titanium to grow one formula unit ofBi

Solid-State Quantum Computer Based on Scanning Tunneling Microscopy

We propose a solid-state nuclear-spin quantum computer based on application of scanning tunneling microscopy (STM) and well-developed silicon technology. It requires the measurement of tunneling-current modulation caused by the Larmor precession of a single electron spin. Our envisioned STM quantum computer would operate at the high magnetic field (approximately 10 T) and at low temperature approximately 1 K.

Measurement of single electron and nuclear spin states based on optically detected magnetic resonance

A novel approach for measurement of single electron and nuclear spin states is suggested. Our approach is based on optically detected magnetic resonance in a nano-probe located at the apex of an AFM tip. The method provides single electron spin sensitivity with nano-scale spatial resolution.

Imaging charged defects on clean Si(100)-(2×1) with scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been used to image charged defects on the clean Si(100)-(2×1) surface. Previous studies have shown that, in the absence of “C”-type defects, the surface does not pin the Fermi level, allowing near surface charge to influence the state density contributing to the tunneling current. As in the case of cleavage faces of III–V semiconductor crystals, the charge-induced band bending produces long-range enhancements superimposed on the periodic surface lattice. This is observed in empty-state STM images taken on n-type material. No band bending signature is seen in the filled-state images. This can be understood by considering the band structure at the surface, which has surface states within the band gap. The charged defects observed in this work are of the types commonly observed in clean Si(100)-(2×1) STM studies, however, not all defects of a given type appear charged. This would indicate subtle differences in structure or the influence of impurities. Predictions for p...

H-dependent magnetic domain structures in La0.67Sr0.33MnO3 thin films

Abstract Maze- and bubble-like magnetic domain structures have been observed under ambient conditions in La 0.67 Sr 0.33 MnO 3 (LSMO) films by magnetic force microscopy (MFM) for films grown on a compressive (LaAlO 3 ) lattice-mismatched substrate. The substrate-induced stress in this soft magnetic material appears to be a necessary but not sufficient condition for the appearance of these structures, i.e. no magnetic structure has been seen for other films grown under similar conditions on the same substrate with similar distortions to the unit cell. In this study, in order to understand the stability of these structures, we have examined two films grown at 750°C and three films grown at 800°C by pulsed-laser deposition by scanning tunneling and magnetic force microscopy to obtain microstructure and domain structures as a function of in-plane magnetic field strength. The latter films consisted of pitted coalesced layers with some surface 3D growth. Although the films were nominally grown under the same conditions, there were subtle differences in the domain structure, wall spacing, the latter due in part to differences in film thickness, and response to an applied in-plane magnetic field. As the field was increased, the maze-like structures became stripe domains with reduced out-of-plane magnetization and decreased wall spacing, consistent with parallel rather than antiparallel alignment of in-plane spin polarization. After removal of the field, the stripe domains remained but the wall spacing and z polarization component returned to the original value. For one 800°C film the field required for changes in and disappearance of the domains paralleled the slope in the field-dependent magnetization. In contrast, the magnetic structure for one film with a thin insulating cap layer was reversible, suggesting that the walls were pinned by the cap.

Observation of buried phosphorus dopants near clean Si(100)-(2×1) surfaces with scanning tunneling microscopy

We have used scanning tunneling microscopy to identify individual phosphorus dopant atoms near the clean silicon