T. Ishimaru

High-Temperature Scanning Tunneling Microscopy(STM) Observation of Metastable Structures on Quenched Si(111) Surfaces.

High temperature scanning tunneling microscopy (STM) observation has been performed to investigate the energetic stability of metastable structures of Si(111) surfaces. Upon supercooling from 1100°C, 5 x 5 and 9 x 9 structures have been observed at 600°C, as well as the reconstructed 7 x 7 and disordered 1 x 1 areas. The 5 x 5 and 9 x 9 areas shrink to disappear as a consequence of the growth of 7 x 7 domains. The smaller domain size and the faster shrinking speed of the 9 x 9 structure indicate its lower stability compared to the 5 x 5 structure. The 11 x 11 and 13 x 13 DAS structures were occasionally observed at above 500°C, however, suggesting lower stability than the (2n + 1) x (2n + 1) DAS structures with smaller n. The √5 x √5 structures also appear at high-temperatures below 550°C, whereas the 2 xe 2, c2 x 4, and c2 x 8 structures are observed only at room temperatures. Clear STM images of the √3 x √3 structure can rarely be obtained at temperatures higher than 600°C, which is considered to be due to the thermally excited random motion of Si adatoms.

Dynamic features in generation and disappearance of Si(111)-7 × 7 domains

Abstract Dynamic features in the growth process of Si(111)-7 × 7 DAS domains have been investigated by in-situ scanning tunneling microscopy (STM) observations. Quenching of Si(111) surfaces from higher temperatures above the critical temperature for 7 × 7 → 1 × 1 phase transition results in the coexistence of small domains of nucleated 7 × 7 DAS structure and disordered ‘1 × 1’ matrix. After the quenching, although the specimens are under the supercooling conditions, all the 7 × 7 domains do not necessarily grow with monotonous increase in size, instead both generation and disappearance of the 7 × 7 domains occur during the growth of the reconstructed 7 × 7 surfaces. The features in domain disappearance have been observed as follows. (i) 7 × 7 domains smaller than the criticalnucleus size disappear. If a 7 × 7 domain size is below the critical nucleus size for domain growth, it tends to shrink and finally to disappear. (ii) When two 7 × 7 domains with different sizes have a collision, the smaller domain is swallowed by the larger domain. When the two domains have similar sizes, they do not disappear but the boundary region just fluctuates ceaselessly. This feature is very similar to the coalescence process found in thin film growth.

Consideration on the Quantitativeness of Reflection High Energy Electron Diffraction Intensity as a Tool to Monitor the Coverage of the Si(111) Surface by 7×7 Domains

The quantitativeness of reflection high energy electron diffraction (RHEED) intensity as a high-speed tool to monitor nucleation and growth of 7 ×7 dimer-adatom-stacking-fault (DAS) domains on Si(111) surface rapidly quenched from a high-temperature disordered 1 ×1 phase has been investigated by comparing the time dependence of both RHEED intensity and scanning tunneling microscope (STM) images. The superlattice reflection intensity of RHEED normalized with that of uniform coverage increased gradually as a function of time after the quenching. The increase rate was higher at higher substrate temperature. A series of STM images revealed the transient aspect of the Si(111)7 ×7 reconstruction that a number of nuclei were randomly formed at the initial stage of growth, and the domain density decreased due to coalescence as the growth proceeded. The coverage by 7 ×7 DAS domains obtained from STM images was always higher than the normalized intensity of superlattice reflection, which showed that the intensity was reduced due to interference between randomly located DAS domains.

Rearrangement of dimers in a dimer-adatom-stacking fault structure on an Si(111) surface

During in situ scanning tunneling microscopy (STM) observations of quenched Si(111) surfaces at a temperature around 400–500°C, the structure with the adatom arrangement which is not coincident with the normal dimer–adatom–stacking fault (DAS) structure was often observed. The flipping between this structure and the normal DAS stacking fault (SF) half unit frequently occurred. From the adatom arrangement and the strong preference of the adatoms for the sites adjacent to the dimers, it has been concluded that the dimers in the DAS structures are rearranged frequently during the flipping.

Stepwise change in Gibbs free energy curve observed in Si(111) DAS domain growth

The formation and the annihilation rates of stacking fault (SF) half-units were precisely determined from the high-temperature scanning tunneling microscopy (STM) observation of dimer–adatom–stacking-fault (DAS) domains grown on quenched Si(111) surface at 485°C, as a function of the number of corner holes shared by a preexisting large domain and a newly born single SF triangle. In contrast to the general nucleation and growth with a single atom as a building unit, in the nucleation and growth of a n×n DAS domain with a single SF half-unit as a building unit, Gibbs free energy as a function of the number of SF half-units has discrete values. This feature is reflected in the behavior of a newly born SF half-unit adjacent to a larger DAS domain. For the SF half-units sharing one corner hole, the formation rate was lower than the annihilation rate due to the greater contribution of periphery strain to the increase in the Gibbs free energy than that of area increase. For the formation of the SF half-unit sharing two corner holes, the annihilation rate was negligibly small, suggesting that the addition of this single SF triangle increases the domain area keeping the periphery length constant, which results in Gibbs free energy reduction.

Effect of environmental O2 on the dynamical process of the Si(111)‘1×1’→7×7 structural phase transition

Abstract The effect of environmental O 2 on the dynamical process of the Si(111)‘1×1’→7×7 structural phase transition has been investigated. In a macroscopic view, growth of the 7×7 DAS structure on the quenched surface after pulsed laser irradiation was monitored in-situ with RHEED under different O 2 pressures. In an atomic scale view, the n × n DAS domain growth was observed with high temperature STM. It has been found that the oxides formed in the ‘1×1’ matrix or phase boundary between n × n DAS-1×1 and the n × n domain boundary is responsible for the suppression of the 7×7 growth. Under the higher O 2 pressure, not only suppression of the 7×7 growth, but also erosion of the 7×7 took place. The threshold O 2 pressure, which suppresses the 7×7 domain growth but does not attack the 7×7 domains themselves, was higher for the specimen kept at 600°C than that at 465°C because of fewer phase boundaries in the former.