The major equipment in this laboratory consists of two ultrahigh-vacuum-cluster-chamber systems to study and modify surfaces at atomic scale. Their capabilities are briefly described as follows:
Surface Preparation
Resistive heating, ion sputtering and annealing can clean sample surfaces in ultrahigh vacuum. Subsequent characterizations are essential because these processes could generate different surface reconstructions.
Physical Vapor Deposition
From sub-monolayer to multi-layer thin films can be deposited on sample surfaces at elevated temperatures in the 10-10 Torr vacuum by a Thermionic 5-pocket-e-beam evaporator. Multi-source co-deposition can be conducted by a three-single-pocket 3 kW e-beam evaporator and monitored by an Inficon Guardian EIES system.
Chemical Vapor Deposition
Ultrapure gases can be introduced into the chamber through leak valves connected to turbo-pumped gas manifolds. Gases can be ionized or dissociated though hot filaments onto sample surfaces at elevated temperatures.
Surface Characterization in K-Space
Low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED) are available to characterize surface reconstruction and to monitor layer-by-layer growth, respectively.
Near Surface Elemental Analysis
An Omicron CMA100 Auger electron spectroscopy system is available to provide elemental analysis of near-surface or deeper regions of the sample if coupled with ion sputtering.
Surface Characterization and Modification in Real-Space
A RHK variable-temperature scanning tunneling microscope is available to generate atom-scale images and tunneling spectroscopy of the surface. Atom-scale manipulation can also be achieved by bias pulses from the STM tip. Magnetic tips and carbon-nanotube attached tips can provide more information or higher resolutions, respectively.
Transport Characterization at Low Temperatures
A Janis variable-temperature cryostat can cool the sample down to 1.4 K by 4He or 0.3 K by 3He. Ports are available for optical or microwave illumination. The dewar is equipped with a 9 T superconducting magnet for Hall measurements.
Research Highlights: Atom-scale Si Surface Modification
A scanning tunneling microscopy study of PH3 adsorption on Si(111)-7x7 surfaces, P-segregation and thermal desorption
Surf. Sci. 601, 1768-1774 (2007)
(a) 10-L PH3 dosed Si(111)-7x7 surface at room temperature. The bright sites are unreacted. The darker sites of different shades are PH2 – , PH3 – and H-adsorption sites. (b) P/Si(111)-6√3 surface after a 30-L dose at 870 K. (c) P-desorption by a rapid thermal annealing at 1100 K for 1 s. The 7X7 domains nucleate from the upper step edge.
Electron transport in laterally confined phosphorus δ-layers in silicon
Phys. Rev. B 74, 153311 (2006)
(a) STM induced electron stimulated desorption creates dangling-bond patterns on H-terminated Si(100) surface. Subsequent PH3 exposure, annealing, and low-temperature Si-homoepitaxy define a quasi-1D P-δ layer embedded in crystal Si. (b) Conductance change of the 50-, 95-, and 200-nm wide P-donor nanowire and the As implanted (degenerately doped) electrodes as a function of temperature. Below a transition temperature, the wire conductance saturates.
Low temperature silicon epitaxy on PH3 dosed Si(100) Surfaces
Phys. Rev. B70, 115309 (2004)
To create an embedded P δ-doped layer, low-temperature Si homoepitaxy is needed. (a) The surface was quite rough after depositing 6.5 nm of Si at 468 K and annealed at 691 K for 5. (b) The surface is much smoother if annealed at 683 K for 15 s for every 1.3 nm Si deposition.
Electron-stimulated bond rearrangements on the H/Si(100)-3x1 surface
Surf. Sci. 446, 211-218 (2000)
The STM's electron beam can induce rearrangement of the H atoms from a dihydride-monohydride unit in (a) to a monohydride-dihydride unit in (b) on a H/Si(100)-3x1 surface. (c) The hydrogen desorption yield from the monohydride sites on the 3x1 surface has the same threshold as that on the 2x1 surface. As the electron energy is increased beyond 7.5 eV, the H-atom yield from the 3x1 surface continues to increase whereas that from the 2x1 surface levels off.
AI Nucleation on monohydride and bare Si(001) surfaces: atomic scale patterning
Phys. Rev. Lett. 78, 1271-1274 (1997)
Depositing 0.1 monolayer of Al on (a) H-terminated and (b) bare Si(100)-2x1 surface indicates that without dangling bonds, Al-Al sticking is fairly isotropic on the monohydride surface while 1-D Al chains perpendicular to the Si 2x1 rows form on the bare Si surface.
Atomic scale desorption through electronic and vibrational excitation mechanisms
Science 268, 1590-1592 (1995), Surf. Sci. 390, 35-44 (1997)
(a) Electrons from an STM tip can selectively desorb H atoms from a H/Si(100)-2x1 surface (blue) to form rows of individual Si dangling bonds (red) and Si dimers (yellow). (b) The isotope effect: desorption yields of H and D from a H- and a D-terminated Si(100)-2x1 surface, respectively.