Leonid Bolotov

Simultaneous measurement of potential and dopant atom distributions on wet-prepared Si(111): H surfaces by scanning tunneling microscopy

The authors demonstrate the ability of scanning tunneling microscopy and spectroscopy to simultaneously measure the distributions of both the surface potential and the individual dopant atoms on the atomically flat hydrogen-terminated Si(111) surfaces prepared by an aqueous NH4F etching without disturbing the original dopant distribution. At the p-n junctions, the acceptor and donor atoms were detected distinctly, and the variation in the observed height reflected the surface potential under the biasing condition. Further, a strong correlation between the dopant fluctuations and the surface potential distributions was identified.

A two-step process for growth of highly oriented Sb2Te3 using sputtering

A two-step growth method is proposed for the fabrication of highly-oriented Sb2Te3 and related superlattice films using sputtering. We report that the quality and grain size of Sb2Te3 as well as GeTe/Sb2Te3 superlattice films strongly depend on the thickness of the room-temperature deposited and subsequently by annealing at 523 K Sb2Te3 seed layer. This result may open up new possibilities for the fabrication of two-dimensional electronic devices using layered chalcogenides.

Scanning tunneling microscopy detection of individual dopant atoms on wet-prepared Si(111):H surfaces

We have performed scanning tunneling microscopy (STM) observation of individual acceptor and donor atoms on hydrogen-terminated Si(111)-1×1 surfaces prepared by wet etching in a NH4F aqueous solution. Separate measurements of p- and n-type substrates showed that acceptors appear as protrusions in filled-state images and as depressions in empty-state images, while for donors the topography is reversed in both filled- and empty-state images. The same relation between the bias polarity and the dopant appearance is preserved for codoped substrates. These results demonstrate that the STM on the Si(111):H surface can detect acceptors and donors distinguishably, enabling us to measure dopant profiles across codoped areas such as p-n junctions.

Properties of three-dimensional structures prepared by Ge dewetting from Si(111) at high temperatures

The formation of three-dimensional (3D) structures during Ge deposition on Si(111) at about 800 °C is studied with scanning tunneling, Kelvin probe and electron microscopies, and scanning tunneling and Raman spectroscopies. The observed surface morphology is formed by dewetting of Ge from Si(111), since it occurs mainly by means of minimization of surface and interfacial energies. The dewetting proceeds through massive Si eroding around growing 3D structures, providing them to be composed of SiGe with about a 30% Ge content, and leads to the significant reduction of the SiGe/Si interface area. It is found that the SiGe top component of 3D structures forms sharp interfaces with the underlying Si. The minimization of interfacial and strain energies occurs on the way that the 3D structures appear to get the dendrite-like shape. The Ge distribution in the 3D SiGe structures is inhomogeneous in the lateral dimension with a higher Ge concentration in their central areas and Ge segregation on their surface.

Scanning tunnelling spectroscopy of atomic clusters deposited on oxidized silicon surfaces: induced surface dipole and resonant electron injection

We investigated the tunnelling conductance of atomic clusters (C60, Si6Hx, AsSi2Hx) deposited on oxidized p-type Si(100) surfaces using scanning tunnelling microscopy where the metal probe/vacuum barrier/cluster/oxide/silicon structures form an asymmetric double-barrier tunnel (ADBT) nanoscale junction. Atomic clusters were assembled onto ultra-thin (~0.3?nm) silicon oxide by either C60 sublimation or ion beam deposition of Si-clusters generated in an ion trap. Electron transfer to the clusters induced surface dipole and reduced junction current under reverse biasing conditions (negatively biased substrate) depending on cluster structure and composition, where the AsSi2Hx clusters created the strongest dipole. Conductance enhancement was observed for forward bias originating in resonance-like electron injection through the unoccupied orbital of the clusters, which was spatially localized within ~1?nm diameter for C60. The resonance peak positions and the weak surface dipole indicated that the orbital energies of C60 and Si6Hx were beyond the forbidden energy gap of Si and shifted with respect to the silicon Fermi energy for heavily doped substrates. In contrast, the orbital energy of doped AsSi2Hx clusters was below the silicon Fermi level. These results demonstrate that the ADBT junction configuration reveals electronic coupling of the clusters to the semiconductor surfaces.

Scanning Tunneling Microscopy Observation of Individual Boron Dopant Atoms beneath Si(001)-2× 1 Surfaces

Individual B dopant atoms residing beneath Si(001)-2×1 surfaces have been detected by scanning tunneling microscopy (STM). A subsurface B atom appears as a broad protrusion in filled-state images, while it appears as either a broad depression or a localized protrusion in empty-state images depending on the STM tip condition. This variation in dopant appearance is attributed to the different work function (WF) of the tip, i.e., the amount of tip-induced band bending differs depending on the tip WF, resulting in a different tunneling path and an opposite dopant appearance under the same bias voltage.

Formation of ErP Islands on InP(001) Surface by Organometallic Vapor Phase Epitaxy

First observation of ErP islands formed on InP(001) during Er-exposure by organometallic vapor phase epitaxy is demonstrated for 0.7–0.8 ML coverage. Different features of the surface morphology for Er-exposed InP are observed depending on substrate temperatures. While large islands (200–300 nm) are grown on the InP(001) surface at a substrate temperature of 530° C, small dots (17–30 nm) with the density of about 5×109 cm-2 are formed at 580° C. ErP islands are preferably grown along the [010] and [100] directions to decrease the lattice distortion.

Measurements of Electrostatic Potential Across p--n Junctions on Oxidized Si Surfaces by Scanning Multimode Tunneling Spectroscopy

We investigated the variation in contact potential difference (CPD) voltage across p–n junctions on oxygen-passivated Si(110) surfaces by scanning multimode tunneling spectroscopy, which detects probe–sample interaction force simultaneously with tunneling current. The enhancement of sensitivity to electrostatic force was achieved with a small amplitude of probe vibration (0.3 nm) when the tip–sample gap was adjusted to reduce short-range interactions by maintaining the tunneling current at a specified bias voltage. At the optimal tip–sample gap, the CPD voltage, derived from force gradient spectra, agrees with the expected built-in potential across the p–n junction. The CPD voltage showed a standard deviation of ~30 mV on atomically flat terraces. Larger fluctuations were ascribed to structural and charge variations on the oxidized surfaces.

Low-energy electron irradiation of fullerene films formed on Si(111)-(7×7) surfaces

C60 films of 4–6 ML formed on Si(111)-(7×7) substrates were irradiated by field-emission (FE) electrons extracted at 15–80 V from probe tips of a scanning tunneling microscope (STM), and resulting evolution of film morphology was observed by the STM. At low extraction voltage, FE electrons stimulated polymerization of adjacent molecules and long-lasting migration of C60. At extraction voltage above ∼40 V, carbon spheroids less than 2 nm in height were created as a result of diffusion and coalescence of C60 fragments produced by electronic excitation.

Morphology and Electric Conductance Change Induced by Voltage Pulse Excitation in (GeTe)2/Sb2Te3 Superlattices.

Chalcogenide superlattice (SL) phase-change memory materials are leading candidates for non-volatile, energy-efficient electric memory where the electric conductance switching is caused by the atom repositioning in the constituent layers. Here, we study the time evolution of the electric conductance in [(GeTe)2/(Sb2Te3)1]4 SLs upon the application of an external pulsed electric field by analysing the structural and electrical responses of the SL films with scanning probe microscopy (SPM) and scanning probe lithography (SPL). At a low pulse voltage (1.6–2.3 V), a conductance switching delay of a few seconds was observed in some SL areas, where the switch to the high conductance state (HCS) is accompanied with an SL expansion under the strong electric field of the SPM probe. At a high pulse voltage (2.5–3.0 V), the HCS current was unstable and decayed in a few seconds; this is ascribed to the degradation of the HCS crystal phase under excessive heating. The reversible conductance change under a pulse voltage of opposite polarity emphasised the role of the electric field in the phase-transition mechanism.