High-temperature Scanning Tunneling Microscopy Research Articles

Layer-by-Layer Etching of Si(111) Surface by Oxygen at Elevated Temperature

The kinetics of Si surface etching with gas-phase O2 was examined using scanning tunneling microscopy (STM), which was intended for the application of gas-phase etching by O2 for Si surface flattening. High-temperature STM observation showed the recession of the step edges with the O2 flux of 5×10-8 Torr on Si(111) at an elevated temperature of ∼800° C in real time. The high-resolution image clearly showed that the terrace was disordered during the exposure to O2 gas. The recession of step edges is considered to be the result of the detachment of Si atoms from the step edges to fill the defects created by the desorption of SiO.

Dynamic process of structural phase transitions on the Sb-induced Si(111) reconstruction surface observed by high-temperature scanning tunneling microscopy

In situ high-temperature scanning tunneling microscopy was performed on the Sb-induced Si(111) reconstruction surface during Sb desorption. The dynamic process of three structural phase transitions: from (√3 × √3)R30°-Sb structure to (2 × 1)-Sb structure, from (2 × 1)-Sb to (5√3 × 5√3)R30°-Sb and from (5√3 × 5√3)R30°-Sb to (5 × 5)-Sb have been investigated. It has been found that the (5√3 × 5√3)R30°-Sb structure changed into the (5 × 5)-Sb structure, which was not observed by previous studies, and that the (5 × 5)-Sb structure subsequently changed into the (7 × 7)-Sb structure. The atomically resolved Si(111)-(5√3 × 5√3)R30°-Sb structure was observed for the first time. A structure model of the (5√3 × 5√3)R30°-Sb surface has been proposed, which can explain the behavior of the boundary between (2 × 1)-Sb and (5√3 × 5√3)R30°-Sb regions.

Intrarow diffusion of Au atoms in the Si(111)-(5 x 2)Au structure.

Intra-row diffusion of Au atoms in the Si(111)-(5\ifmmode\times\else\texttimes\fi{}2)Au reconstructed structure has been observed by using high-temperature scanning tunneling microscopy. Loosely bonded Au atoms, which are arranged with 2(n+1)a spacings at room temperature, diffuse in the twofold direction at higher temperatures among adsorbed sites arranged at 2a intervals. These diffusing Au atoms seem to be the reason there is a nonintegral number of Au atoms in the (5\ifmmode\times\else\texttimes\fi{}2) unit cell. \textcopyright{} 1996 The American Physical Society.

Dynamic observation of Si-island growth on a Si(111)-7 × 7 surface by high-temperature scanning tunneling microscopy

Si-island growth on a Si(111)-7 × 7 surface was studied at 350°C in situ using high-temperature scanning tunneling microscopy. At the beginning of growth, deposited Si atoms formed small amorphous clusters within each triangular subunit of the 7 × 7 structure. The amorphous clusters grew, and crystallized islands also appeared as the quantity of deposited Si atoms increased. At the domain boundaries of the 7 × 7 structure, islands tended to begin forming on the unfaulted half of the 7 × 7 structure. These phenomena indicate that cancellation of the stacking fault on the substrate surface dominates island growth.

Exchanges between group-III (B, Al, Ga, In) and Si atoms on Si(111)- sqrt(3) x sqrt(3) surfaces.

High-temperature scanning tunneling microscopy (HT-STM) observation of exchanges between group-III and Si atoms on Si(111)-\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3 surfaces is reported. The exchange rates are obtained from the HT-STM images as a function of the temperature. The activation energies of the exchange are 1.4, 1.7, and 1.4 eV for Al, Ga, and In adatoms, respectively. The prefactors are within the range ${10}^{10}$--${10}^{13}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. These prefactors are close to that for the exchange between Pb and Si adatoms on a Si(111)-\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3 surface and much larger than that for the exchange between Pb and Ge adatoms on a Ge(111)-c(2\ifmmode\times\else\texttimes\fi{}8) surface. The adatom exchange mechanisms on Si(111)-\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3 and Ge(111)-c(2\ifmmode\times\else\texttimes\fi{}8) are different. We also investigated exchanges between B and Si atoms in the second layer of the Si(111)-\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3 surface. The obtained activation energy and prefactor are 3.5 eV and 1\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and the activation energy is close to that of B diffusion in bulk Si. \textcopyright{} 1996 The American Physical Society.

High temperature scanning tunneling microscopy during molecular beam epitaxy

We have built and tested a system that is capable of simultaneous molecular beam deposition and scanning tunneling microscopy (STM) imaging at T=300–1000 K. The STM is based on the beetle-type design. Active compensation of thermal drift reduces the residual drift in the images to ∼2 Å/min at 850 K sample temperature. Measurements at varying sample temperatures are also feasible. Due to the open design of the STM, a molecular beam from a commercial microevaporator is aimed between tip and sample. With our system, simultaneous molecular beam epitaxy (MBE) and STM experiments during growth at high temperature are feasible. This method (MBSTM) provides the possibility to follow the MBE growth process with the STM in a real in situ way. The operation characteristics and performance of this instrument will be shown using examples of silicon homoepitaxy and Ge/Si heteroepitaxy.

High-temperature scanning tunneling microscopy on vicinal Si(111) and formation of ordered facets

We have measured vicinal surfaces of Si(111) by using high-temperature scanning tunneling microscopy. Above the (7 × 7) (1 × 1) phase transition temperature T c we observe fluctuating monoatomic steps with uniform separation. Below T c the precipitation of 7 × 7 reconstructed terraces necessarily leads to a local reduction of the mean distance between the steps which, depending on the azimuthal orientation, finally gives rise to the formation of step bunches and higher-order facets. In the present work we concentrate on high-temperature observations (920 and 860 K) of (133) and (177) facets, respectively.

Stable phase boundaries between the 7 × 7 and the 5 × 2 Au structures on a Si(111) surface studied by high-temperature STM

Abstract Phase boundaries between the 7 × 7 and the 5 × 2 Au structures on a Si(111) surface, which were located along the two-fold direction of the 5 × 2 structure, were dynamically studied using high-temperature scanning tunneling microscopy (STM) during Au deposition at 500°C. Two types of stable phase boundaries were observed depending on the growth direction of the 5 × 2 domain. The stable phase boundaries had adatoms of the unfaulted half at the edge of the 7 × 7 side. The formation of an unstable phase boundary was quickly followed by the growth of new rows of the 5 × 2 structure next to the boundary which formed a stable phase boundary. These results suggest that the stacking fault of the 7 × 7 structure at a phase boundary is unstable.

Domain growth of Si(111)-5 × 2 Au by high-temperature STM

Si(111)-5 × 2 Au reconstruction was studied dynamically at 500°C by high-temperature scanning tunneling microscopy (STM). Transitions of the 5 × 2 domains during growth were observed as invasive erosions of one domain by another. Whenever this occurred, the domain with its 2-fold direction crossing the domain boundary eroded away. This phenomenon suggests that growth in the 2-fold direction has a stronger driving force. Comparison with STM images taken at room temperature shows that the gold atoms seem to diffuse in the 2-fold direction at higher temperatures. The driving force is believed to derive from repulsion between the gold atoms.

Dynamic observation of In adsorption on Si(111) surfaces by UHV high-temperature scanning tunneling microscopy

Indium adsorption and desorption processes on Si(111) surfaces were observed in situ by UHV high-temperature STM. By the deposition of In on the Si(111)7 × 7 surfaces at 380°C, the surface structure changed successively to 3 × 3 , 31 × 31 and 4 × 1. The number densities of silicon atoms in the restructuring layers for the 3 × 3 , 31 × 31 , and 4 × 1 structures were evaluated to be about 0, 1 and 2 ML, respectively. High-resolution STM images were also taken after the deposition.

Dynamic growth steps of n x n dimer-adatom-stacking-fault domains on the quenched Si(111) surface.

Dynamic growth steps of Si(111)-n\ifmmode\times\else\texttimes\fi{}n dimer--adatom--stacking-fault (DAS) domains in the 1\ifmmode\times\else\texttimes\fi{}1 matrix have been investigated on quenched Si(111) surfaces, with high-temperature scanning tunneling microscopy. It has been found that both growth and annihilation of n\ifmmode\times\else\texttimes\fi{}n DAS domains occur with a single n\ifmmode\times\else\texttimes\fi{}n stacking-fault (SF) half cell as the building unit. Not only 7\ifmmode\times\else\texttimes\fi{}7, but also 9\ifmmode\times\else\texttimes\fi{}9 DAS domains have been observed to gradually expand on the quenched surface above 450 \ifmmode^\circ\else\textdegree\fi{}C. Every type of n\ifmmode\times\else\texttimes\fi{}n DAS domains grows with a successive addition of single SF half cells to a n\ifmmode\times\else\texttimes\fi{}n DAS domain side. Structural transformation of 7\ifmmode\times\else\texttimes\fi{}7 and 11\ifmmode\times\else\texttimes\fi{}11 SF half cells into 9\ifmmode\times\else\texttimes\fi{}9 SF half cells occurs at a 9\ifmmode\times\else\texttimes\fi{}9 domain side, to form a large 9\ifmmode\times\else\texttimes\fi{}9 DAS domain on the quenched Si(111) surface. \textcopyright{} 1996 The American Physical Society.

High-temperature scanning tunneling microscopy study of the Li/Si(111) surface

A study of the topographical changes of Li/Si(111) during annealing has been conducted using a high-temperature scanning tunneling microscope. Depending on the Li coverage and postdeposition annealing temperature the surface exhibited a variety of structures replacing the initial (7×7) reconstruction during in situ investigations. In addition to the previously observed (3×1) structure a coexistence phase with domains of (n×n) (n=5,7,9), a (4×4) structure, and a faceting transition were imaged. Distinct differences were noticed on the Li/Si(111)-(3×1) surface compared to the corresponding Na structure. Moreover, the dynamic properties of the morphological transformations were revealed by a high-temperature scanning tunneling microscope.

Effect of the adatom presence on stabilizing Si(111)-n x n dimer-adatom-stacking-fault structures.

Si(111) surfaces quenched from 1100 \ifmmode^\circ\else\textdegree\fi{}C have been observed at around 470 \ifmmode^\circ\else\textdegree\fi{}C using high-temperature scanning tunneling microscopy. We closely investigated dynamic motion of Si adatoms in the metastable stacking fault (SF) half cells of 11\ifmmode\times\else\texttimes\fi{}11 and 13\ifmmode\times\else\texttimes\fi{}13 dimer adatom stacking fault (DAS) structures. On these SF half cells adatoms continually change their positions at high temperature of 470 \ifmmode^\circ\else\textdegree\fi{}C. Most of the interior adatoms escape from the SF half cells more easily than the corner or center adatoms. This indicates that the interior adatoms hardly contribute to the stabilization of n\ifmmode\times\else\texttimes\fi{}n DAS structures.

Step dynamics on highly oriented and vicinal surfaces of Si(111)

By using high-temperature scanning tunneling microscopy we have measured step fluctuations on highly oriented and vicinal surfaces of Si(111) in the temperature range up to the (7 × 7) (1 × 1) phase transition temperature (1100 K). On mono-layer steps these fluctuations mostly occur in a width corresponding to a 7 × 7 unit cell half. By an Arrhenius plot of the fluctuation rate an activation energy for 7 × 7 kink diffusion of 1.8 ± 0.3 eV was estimated. On vicinal surfaces and just below the phase transition temperature fluctuations of entire 7 × 7 domains are observed.

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, 5×5 and 9×9 structures have been observed at 600°C, as well as the reconstructed 7×7 and disordered 1×1 areas. The 5×5 and 9×9 areas shrink to disappear as a consequence of the growth of 7×7 domains. The smaller domain size and the faster shrinking speed of the 9×9 structure indicate its lower stability compared to the 5×5 structure. The 11×11 and 13×13 DAS structures were occasionally observed at above 500°C, however, suggesting lower stability than the (2n+1)×(2n+1) DAS structures with smaller n. The √3×√3 structures also appear at high-temperatures below 550°C, whereas the 2×2, c 2×4, and c 2×8 structures are observed only at room temperatures. Clear STM images of the √3×√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.

Investigations of Electronic Structures of Defects Introduced by Ar Ion Bombardments on MoS 2 by Scanning Tunneling Microscopy

We have investigated the defects introduced by Ar ion bombardment on cleaved MoS2 surface by scanning tunneling microscopy (STM). The resultant defects have been observed at room temperature as dark regions typically 6–8 nm in diameter and their contrast is found to depend on the sample bias voltage (SB). The high-temperature STM observations at 600°C and 800°C show a reduction of the diameter to 2–4 nm. Based on these experimental results it is concluded that the S atom complex produced at the defect site is partially negatively ionized and a depletion region of conduction electrons is produced around it. The observed lateral extent of the depletion region is in good agreement with that calculated from the repulsive Coulomb potential due to the negatively charged complex. The calculated screening length, however, disagrees with the observed size of the dark region.

An in-situ high temperature scanning tunnelling microscopy study of the boron-induced √3 × √3 reconstruction on the Si(111) surface

In-situ high temperature scanning tunnelling microscopy (STM) measurements were performed on vacuum-annealed, boron-doped Si(111) wafers. The samples were p-type with an atomic boron concentration of ∼ 10 19 cm −3 which were chosen such that after ultra-high vacuum processing at up to 1250°C and annealing at 600°C they gave rise to co-existence of both 7 × 7 and √3 × √3R30° surface terminations. After the initial “flashing” of the sample to 1250°C, we observe pinning of a variety of reconstructions due to the random out-segregation of boron to the surface. Prolonged annealing at the measurement temperature of 600°C was observed to give rise to the agglomeration of the boron into well-defined √3 × √3 regions. In the measurements, we observe preferential directions for the in-plane boundaries between √3 × √3 and 7 × 7 regions, showing clearly the importance of dimerisation as a strain relief mechanism between these regions. Detailed observations reveal the lateral diffusion of boron and silicon in the √3 × √3 regions as a function of time. The data are discussed in terms of the various controlling processes at the surface.

Surface reconstruction of sulfur-terminated GaAs(001) observed during annealing process by scanning tunneling microscopy

The surface reconstruction of in-situ prepared sulfur-terminated (S-terminated) and sulfur-protected (S-protected) GaAs(001) is studied by scanning tunneling microscopy (STM) at high temperatures of up to 260°C. We demonstrate a new reordering process from the (4 × 6) Ga-stabilized surface to the (2 × 6) S-stabilized surface and a novel method of air protection using a S passivation layer. Moreover, the in-situ observation of the annealing process of this S-protection layer is performed by high-temperature STM, avoiding the adsorption from environments and verifying that the (2 × 6) structure still becomes dominant on the S-terminated GaAs(001) surface without the effects of As atoms.

Exchanges between Si and Pb adatoms on Si(111)

Real-time observation by high-temperature scanning tunneling microscopy of exchanges between Si and Pb atoms on a Si(111)-√3 × √3 surface is reported. The exchange rate is obtained as a function of the temperature. The activation energy of the exchange is about 1.2 eV, and the prefactor, shown to depend on the Pb coverage, is from 2 × 10 10 to 8 × 10 11 s −1. This prefactor is much larger than that for the exchange between Pb and Ge adatoms on a Ge(111)-c(2 × 8) surface, indicating that the adatom arrangement greatly influences the exchange mechanism. We also report that metastable 9 × 9 reconstruction appears during Pb desorption.

Kinetics of hydrogen desorption on a Si(111) surface

An in situ observation of hydrogen (H) desorption on a Si(111) surface was carried out by using high temperature scanning tunneling microscopy (STM). There appeared a disordered phase with no atomic images around 490°C at which the surface monohydride started to desorb. When the sample temperature was maintained at a certain moment above 490°C, the area of the disordered phase decreased with time and changed into a Si(111)-√3 × √3 R30° structure. This behavior of the surface structure can be explained by the migration of SiH species on an unreconstructed Si(111)-1 × 1 surface at elevated temperature. Based on this behavior, we propose a model for the H desorption.