Masashi Iwatsuki

High-resolution imaging of contact potential difference with ultrahigh vacuum noncontact atomic force microscope

An ultrahigh vacuum scanning Kelvin probe force microscope (UHV SKPM) utilizing the gradient of electrostatic force, was developed based on an ultrahigh vacuum noncontact atomic force microscope (NC-AFM) capable of atomic level imaging, and used for simultaneous observation of contact potential difference (CPD) and NC-AFM images. CPD images of a Si(111) surface with Au deposited, clearly showed the potential difference in phases between 7×7 and 5×2 structures. When Ag was deposited as a submonolayer on the Si(111) 7×7 reconstructed surface, the atomic level lateral resolution was observed in CPD images as well as in NC-AFM topographic images.

Observation of 7*7 Reconstructed Structure on the Silicon (111) Surface using Ultrahigh Vacuum Noncontact Atomic Force Microscopy.

Unlike ultrahigh vacuum (UHV) scanning tunneling microscopy (STM), the effectiveness of UHV in UHV atomic force microscopy (AFM) has not been verified. Intensive interaction between tip and sample in UHV often damages the sample surface in the contact mode. Although noncontact (NC) AFM is effective in protecting the sample surface, it has failed to provide atomic-level resolution. We used a stiff silicon cantilever ( ~40 N/m) capable of STM imaging, and succeeded in obtaining the first atomic-resolution images of Si(111)7×7 reconstruction in NC AFM at a tip-sample distance almost equal to that for STM imaging.

High resolution imaging of contact potential difference using a novel ultrahigh vacuum non-contact atomic force microscope technique

An ultrahigh vacuum scanning Kelvin probe force microscope (UHV SKPM) based on the gradient of electrostatic force was developed using the technique of a UHV non-contact atomic force microscope (NC-AFM) capable of atomic level imaging, and used for simultaneous observation of contact potential difference (CPD) and NC-AFM images. The CPD images with a potential resolution of less than 10 meV were observed in the UHV SKPM, demonstrating an atomic level resolution. The change of potential corresponding to the charges on the insulated surface of polypropylene have been observed in UHV SKPM. We also demonstrated a reliable method to obtain the CPD from the bias voltage dependence curves of the frequency shift in all of the scanning area. The results are consistent with comparing the barrier height images in that the work functions of adatoms are greater than the work function of corner holes.

Observation of Silicon Surfaces Using Ultrahigh-Vacuum Noncontact Atomic Force Microscopy.

Noncontact imaging has been developed for atomic force microscopy in ultrahigh vacuum in order to avoid the damage to the sample surface caused by the intense interaction between the clean surfaces of the sample and probe tip, which is a problem commonly observed in contact imaging. Initially it was thought that the noncontact (NC) mode might fail to achieve atomic level resolution due to the long distance between the tip and sample. Recently, it has been proved that this is not so. We used an FM detection system utilizing an oscillating cantilever with a constant-amplitude voltage supplied to the piezotransducer; and successfully obtained NC-mode stable atomic-resolution AFM images of the 2×1 structure of the Si(100) surface as well as the 7×7 structure of the Si(111) surface. We also compared AFM and STM images of the 7×7 structure in the same area of view.

Atomic-scale variations in contact potential difference on Au/Si(111) 7 × 7 surface in ultrahigh vacuum

Abstract The results of contact potential difference (CPD) imaging on Au-deposited p-type and n-type Si(111) 7×7 surfaces are discussed. The scanning Kelvin probe microscopy (SKPM) technique based on the gradient of the electrostatic force was used under ultrahigh vacuum (UHV) conditions to acquire the data presented. The CPD images of Au deposited on the Si(111) 7×7 surface show virtually identical features, irrespective of whether the Si is n- or p-type. In these images, it is believed that the atomically resolved potential difference does not originate from the intrinsic work function of the materials but reflects the local electron density on the surface. On the other hand, the average potentials corresponding to the DC levels in each CPD image reflects the work function value on the surface. The work function of p-type Si(111) 7×7 is found to be higher than that of n-type by about 0.45 eV, where both samples had the same resistivity of about 0.5 Ω cm and the same Au coverage. If the Au coverage is increased, the work function increases.

Surface diffusion of adsorbed Si atoms on the Si(111)7×7 surface studied by atom-tracking scanning tunneling microscopy

The initial adsorption process of Si atoms deposited on a Si(111)7×7 surface has been investigated at 80 to 500 K using a variable-temperature scanning tunneling microscopy. At room temperature, adsorbed Si atoms spontaneously formed tetramers over the center dimers in the dimers adatoms and stacking fault model of a 7×7 structure. Many other adsorbed Si atoms, which were not used for the formation of tetramers, were observed to diffuse within each half of the 7×7 unit cell. The diffusion of Si atoms across the surface was examined directly by using an atom-tracking technique. At low temperatures, the adsorption position of the Si atom was found to depend only on the potential energy. At high temperatures, the activation energy of an Si atom beyond the boundary between the half-unit cells was measured as Ea=1.14 eV.

Observation of SrTiO3 step edge dynamics by real-time high-temperature STM

Abstract We have used high-temperature scanning tunneling microscopy (STM) to study in real time SrTiO 3 step edge dynamics in the 600°C to 800°C temperature range. We observed a dramatic transformation of the step edge structure above 690°C, involving step edge straightening due to rapid migration of surface atoms. The STM study shows that if a NH 4 F-HF-etched substrate is annealed at 800°C, a well-ordered surface, suitable for high-T c superconductor thin film deposition, can be produced.

Scanning tunneling microscopy observation of hydrogen‐terminated Si(111) surfaces at room temperature

Scanning tunneling microscopy has been applied to observe hydrogen‐terminated Si(111) surfaces at room temperature. A clear image was easily observed for a Si surface prepared by rinsing in pure water with very low dissolved oxygen after removal of native oxide by 1% HF solution dipping. A smooth surface in an atomic scale was exhibited in a 50×50 nm area. Completely triangular‐shaped holes were observed on the surface. The holes were surrounded by steps which were very likely directed toward 〈112〉. The treatment of the surface was remarkably stable even after a 3 h air exposure. Furthermore, nm size pits were found at the bottom part of the triangular‐shaped holes. The results imply that the nm size pits appeared to be due to microdefects and that the pits might be the origin of surface etching at the Si surface.

Scanning Tunneling Microscopy of Clean Silicon Surfaces at Elevated Temperatures

In this paper, we shall review scanning tunneling microscopy on clean Si(111) and Si(001) surfaces at elevated temperatures of as high as 950?C. Various problems occurring at elevated temperatures, such as the thermal drift, the low Curie temperature of the tip-scanning elements and outgassing, are overcome by paying special caution to the design of the STM unit, the sample dimensions, and the operational methods. Several interesting phenomena appearing at elevated temperatures are presented: the appearance of the dimer rows and step edges on the Si(001) surface at temperatures up to 920?C, the dynamic behavior of the (7?7)-(1?1) phase transition on the Si(111) surface at temperatures around 860?C, the step shifting appearing on the Si(111) surface under dc electric fields (electromigration effect) around the phase transition temperature (869?C), and the surface modification of pyramids and craters created on both Si(111) and Si(001) surfaces at elevated temperatures.

Real-time observation of the Si(111):(7×7)−(1×1) phase transition by scanning tunneling microscopy

Abstract The initial stage of the (1×1)−(7×7) phase transition of the Si(111) surface was directly observed by scanning tunneling microscopy. On cooling down to a transition temperature Tc, the (7×7) domain nucleated from the step edges and expanded towards the inner region of the terraces continuously, and the steps became straight [ 1 1 2] steps. The (7×7) domains took the shapes of triangles with acute top angles and fluctuated in size around Tc. The sizes of the observed domains were more than 6 units of (7×7) on the terraces. Based on free-energy considerations just at Tc as well as the energy difference between faulted and unfaulted triangles of (7×7) units, we have found that the real shapes of the (7×7) domains are equilateral triangles which are surrounded by unfaulted triangles and there exists a critical size around 6 units of (7×7) below which the domains disappear rapidly.