Motoshi Shibata

NANOMETER-SCALE SI SELECTIVE EPITAXIAL GROWTH ON SI SURFACE WINDOWS IN ULTRATHIN OXIDE FILMS FABRICATED USING SCANNING TUNNELING MICROSCOPY

Using scanning tunneling microscopy (STM), nanometer-scale Si(111) and Si(001) windows in ultrathin SiO2 films are fabricated by electron-beam-induced thermal decomposition. At 450–630 °C, the oxidized Si surfaces are irradiated with a field emission electron beam from a STM tip with an energy of 70–150 eV and a current of 10–50 nA. The smallest window size is about 40 nm. The shape of the Si crystals selectively grown on the Si(001) windows is that of a frustum of a quadrangular pyramid, while that on the Si(111) windows is an (111) oriented two-dimensional island. We discuss the influence of the field emission electrons on the fabrication and the selective growth.

Nanometer-scale germanium islands on Si(111) surface windows formed in an ultrathin silicon dioxide film

Three-dimensional Ge islands between 15 and 200 nm in size were found to grow only on Si(111) surface windows in ultrathin SiO2 film after Ge deposition and subsequent SiO2 decomposition. The size of Ge islands gradually decreased as the Ge thickness decreased. Pseudomorphic two-dimensional Ge layers with the 5×5 structure formed in surrounding areas of the windows. The windows were produced by selective thermal SiO2 decomposition induced by focused electron beam irradiation. These results suggest a new technique for nanometer-scale Ge island fabrication at given points on Si surfaces.

Nanometer-scale Ge selective growth on Si(001) using ultrathin SiO2 film

We performed nanometer-scale Ge selective growth using ultrathin silicon dioxide film on Si(001) surfaces. Growth was observed in real time by scanning tunneling microscopy (STM). Window areas with a size of 10–50 nm were fabricated using two different methods: void formation during thermal decomposition of the oxide and field-emission electron-beam irradiation from an STM tip. Selective epitaxial growth was achieved by introducing germane gas (GeH4). With the first method, 3D nucleations occurred near the periphery of the voids and several Ge clusters of irregular shape grew. With the second method, 3D nucleations occurred at the center of the window, and several clusters coalesced forming one hut cluster. The second method was used to form a nanometer-scale Ge dot array.

Formation of three-dimensional Si islands on Si(111) with a scanning tunneling microscope

Silicon islands up to 10 nm in base length and 3 nm in height were grown on a Si(111) surface at room temperature with a scanning tunneling microscope at constant tunneling currents. The islands grew with constant rates at earlier growth stages by accumulating Si atoms from the surface area around the islands. The growth rate decreased when the island height exceeded 3 nm. At negative tip biases above 7 V, the technique produced a highly reproducible formation of the islands whose growth rate increased as the bias voltage increased.

Formation of Ge nanoislands using a scanning tunneling microscope

Germanium islands were grown on a sample surface by accumulating atoms from the surrounding area through directional surface diffusion initiated by the electric field of a scanning tunneling microscope (STM). The Ge islands grew with a constant rate determined by the tip–sample bias voltage. The parameters of tip–sample interaction were estimated from the kinetic data for island growth by using a scaling relationship among the growth rate, the dipole moment of atoms on surfaces, and the tip–sample bias voltage. The results show that continuous atom transfer with a STM occurs with a rate significantly higher for Ge than for Si.

Continuous transfer of Ge by the tip of a scanning tunneling microscope for formation of lines

Nanostructures such as continuous Ge lines about 5 nm in width and 2 nm in height were created on Ge wetting layers on Si(111) substrates with a scanning tunneling microscope (STM). Postfabrication annealing initiated growth of the lines in the lateral dimension thereby improving their uniformity. STM and electron diffraction data obtained for lines after annealing showed that the lines have a nonepitaxial structure consisting of tiny particles. Continuous intersections of lines can be achieved when sharp tip apexes are used. The results demonstrate the possibility of using the STM for direct massive transfer of individual atoms in the fabrication of nanostructures.

Nanometer-scale Si selective growth on Ga-adsorbed voids in ultrathin SiO2 films

Abstract We examined nanometer-scale Ga selective doping by Si growth on Ga-adsorbed voids in ultrathin silicon-dioxide on Si(111) surfaces. The doping processes were observed by scanning tunneling microscopy (STM). Voids in ultrathin oxide films were plugged with a ( 3 × 3 )-Ga structure, and the selective growth was performed by introducing disilane gas (Si 2 H 6 ). Si crystals were selectively grown only in the voids at 460–550°C. Two-dimensional nucleation was found to start from the edge of the voids. Incorporated Ga atoms mostly segregated during the selective growth and were reconstructed to the ( 3 × 3 ) structure by annealing at 600°C. These results show that Ga doped dots of nanometer-scale can be formed by selective epitaxial growth using an ultrathin silicon-dioxide mask.

Effect of tunneling current on the growth of silicon islands on Si(111) surfaces with a scanning tunneling microscope

The early-stage growth rate of a silicon island on a Si(111) surface at the center of the interaction between a sample and the tip of a scanning tunneling microscope (STM) was defined as a function of the tunneling current. The tunneling current dependence of the rate has a maximum at 0.3 nA, and the decrease of the rate at tunneling currents above 0.3 nA was related to the effect of electron flow on atom transfer by field-induced directional diffusion. The early-stage growth rate was about three times higher than the late-stage growth rate, which was almost independent of the tunneling current. The results suggest that atom transfer by field-induced diffusion on the sample surface was gradually substituted by atom transfer from the STM tip by field-induced silicon atom re-evaporation as the island grew from 0 to about 12 nm in height.

Instability of 2D Ge layer near the transition to 3D islands on Si (111)

Abstract The formation of a stable surface morphology of Ge on Si(111) at coverages close to the transition from two-dimensional (2D) to threedimensional (3D) growth was studied using scanning reflection electron microscopy and energy dispersive X-ray spectroscopy. Regions of the stable surface morphologies of 2D layers and 3D islands are described by a phase diagram as a function of coverage and temperature. Temperatures were determined at which the unstable 2D Ge layer at coverages between 1.5 and 2.3 BL does not transform into islands longer than 10 min. It was found that irradiation by focused e-beam creates points in the unstable 2D layer where 3D islands appear after annealing. These 3D islands probably nucleate on structural defects introduced by the irradiation. This shows that the instability of the 2D layer can be used for controllable nucleation of islands.

Kinetics of tip-induced island growth on Si(111) with a scanning tunneling microscope

The kinetics of island growth on Si(111) with a scanning tunneling microscope (STM) is measured as a function of the tip–sample bias voltage. Two processes appear to be involved in the island growth in the center of the tip–sample interaction. Field-induced evaporation transfers atoms between the sample and the STM tip, and creates an area of incomplete surface structures with atoms mobile under the electric field. The second process is directional field-induced diffusion which transfers atoms along the surface. We derived a scaling relation for the initial island growth rate, the dipole moments of atoms on the surface, and the tip–sample bias voltage. This scaling relation was used to estimate the values of field–dipole interaction parameters from the kinetic data obtained for the initial island growth rate.