Toyoko Arai

Tip cleaning and sharpening processes for noncontact atomic force microscope in ultrahigh vacuum

Abstract Tip cleaning and sharpening processes for noncontact atomic force microscope (AFM) operated in ultrahigh vacuum (UHV) were carried out and evaluated by a scanning Auger microscope (SAM) with a field emission electron gun and a noncontact AFM in UHV combined with a scanning tunneling microscope and a field emission microscope. The cantilever used in this study was piezoresistive, which can be heated by passing a current through the resistive legs of the cantilever. As a pretreatment, the tip was irradiated with ultraviolet light in oxygen to remove carbon contaminants. It was heated at about 750°C to form a clean oxide layer in oxygen of 5×10−5 Torr in an SAM chamber. The desorption of the layer can make a remained tip apex sharper by heating under electron beam irradiation. A thermally oxidized layer was also eliminated by HF etching to sharpen the tip apex. The procedures are useful to obtain a well-defined Si tip suitable for a noncontact AFM.

Bias dependence of Si 111 7= 7 images observed by noncontact atomic force microscopy

Abstract Noncontact atomic force microscopy (nc-AFM) imaging of a Si(111)7×7 surface has been done in order to examine the bias dependence of the contrast of Si adatoms. While the atomic corrugation depends upon the tip states, the contrast is found to be inverted by increasing the bias voltage at greater frequency shifts. Then, the term of the repulsive force between a Si adatom and a Si atom at the tip apex can play an important role in depicting the topography of the sample surface with atomic resolution. The difference in contrast between faulted and unfaulted halves and peculiar profiles near steps are also presented.

A Si nanopillar grown on a Si tip by atomic force microscopy in ultrahigh vacuum for a high-quality scanning probe

We grow a Si nanopillar on a commercial Si tip on an atomic force microscopy (AFM) cantilever using AFM in ultrahigh vacuum for a high-quality scanning force probe, and observe noncontact-AFM (nc-AFM) images of Si(111)7×7 and Ge deposited Si(111) with the nanopillar. We observe it ex situ by transmission electron microscopy to confirm its growth and crystallinity. The nc-AFM image clearly showed the high performance of the nanopillar as a probe with respect to the spatial resolution, image stability, and reproducibility. This nanopillar growth technique can elongate the lifetime of the cantilever and be applied to other materials.

Energy spectrum of backscattered electrons excited by a field emission scanning tunneling microscope with a build-up [111]-oriented W tip

A field emission scanning tunneling microscope (STM) combined with an electron energy analyzer was developed to acquire energy spectra of electrons backscattered from a sample surface impinged by a primary electron beam, which is field-emitted from an STM tip of build-up [111]-oriented W. Since the electron beam is field-emitted from the most protruding point of the [111] apex with a low work function and a sharp corner, the electron impinged area can be imaged with the STM by approaching the build-up W tip to the sample. Electron energy loss and Auger electron spectra were obtained for Si(111); an elastic backscattering peak, bulk plasmon loss peaks and a Si LVV Auger peak were detected. The origin of anomalous broad inelastic peaks was also discussed.

DNA molecules sticking on a vicinal Si(111) surface observed by noncontact atomic force microscopy

Abstract The DNA molecules on a vicinal Si(111) substrate with steps of single and double bi-atomic layers are imaged by noncontact atomic force microscopy (nc-AFM) in ultrahigh vacuum. The water solution containing pBR322 plasmid DNA molecules digested by Cla I is dropped on the substrate in a pure nitrogen atmosphere in a glove box, which is connected to the introduction chamber of the AFM. The ends of DNA molecules are frequently folded and pinned at the steps on the substrate, and the DNA strings often lie along the step. The chemical and dipole interactions between the DNA and the semiconductor substrate seem to play an important role in folding, pinning and sticking on the Si(111) substrate.

Atomic force microscope Si tip with Ge clusters with the capability of remoulding by heating

Commercially available Si cantilever tips for atomic force microscopy (AFM) are successfully modified to sharpen their tip apexes by depositing Ge on them in ultrahigh vacuum and subsequently heating. Even a slightly crashed Si tip is remoulded by this method of growing faceted Ge clusters on an expanded (001) facet at the apex. The AFM images are stably obtained over a few hours using the modified tips. These tips can be repeatedly remoulded by heating after breakage on scanning. This method is applicable to other materials, which are heteroepitaxially grown on Si in Stranski–Krastanov growth mode, to fabricate functionalized tips as per requirement.

Energy spectra of electrons backscattered from sample surfaces with heterostructures using field-emission scanning tunneling microscopy

We have improved the combined instrument of field-emission scanning tunneling microscopy (STM) with surface electron spectroscopy to directly identify the atomic species on a sample; we supplement an electric shield near the tip, the position of which can be adjusted by three-dimensional coarse stages, and optimize the grazing angle of a sample surface with respect to the entrance of an energy analyzer by placing the STM head on a rotation stage. We observe STM images and take the energy spectra of backscattered electrons from clean Si(111) and Al-deposited Si(111) using the improved instrument. The Auger peaks of Si LVV are found at a tip–sample separation of approximately 1 µm for Si(111), and Al LVV and Si LVV peaks are found for Al-deposited Si samples, depending on the deposited amount. The present study implies that submicron surface analysis of samples with heterostructures can be performed with the combined instrument.

Interaction measurements between a tip and a sample in proximity regions controlled by tunneling current in a UHV STM–AFM

Abstract The interaction force–distance curves between a tip and a sample surface in close proximity were measured by logarithmically changing a tunneling current passing through them with a ultrahigh vacuum scanning tunneling microscopy–atomic force microscopy (UHV STM–AFM). Since the tunneling current changes exponentially with the separation between the tip and the sample, the separation can be controlled precisely and linearly by modulating a logarithmic target value fed into the STM feedback circuit to be a triangular waveform. A piezoresistive cantilever with a conductive Si tip was used after cleaning the tip by heating it in the UHV chamber. As a preliminary result, force-separation curves with reversible and irreversible jumps in close proximity were presented.