Nobutomo Uehara

Tip–sample distance dependency of non-contact atomic force microscopy images on a GaAs(110) surface

Tip–sample interactions are at the origin of atomic resolution in non-contact atomic force microscopy (NC-AFM) but also make it difficult to understand the meaning of atomic features observed as NC-AFM images. (110) surfaces of III–V semiconductors such as GaAs or InP have relaxed (1 × 1) surfaces with both anions and cations of different dangling bond states. Since these atoms are located at the surface where an atom attached to the tip apex can reach to interact with them and show significant relaxation via tip–sample interaction, NC-AFM images of the surface are expected to strongly depend upon the atomic species or the electronic state of the topmost atom at the tip apex. We have taken a number of NC-AFM images of GaAs(110) surfaces with a Si tip by means of room-temperature ultra-high vacuum NC-AFM and categorized them into two types of image which have different tip–sample distance dependencies. In comparison with a theoretical prediction of tip–surface interaction on the GaAs(110) surface, we have found that one of these images is due to a Si atom attached to the apex which reveals the tip–sample interaction with As and Ga surface atoms as individual peaks in the NC-AFM image.

Non-Contact Atomic Force Microscopy Observation on GaAs(110) Surface with Tip-Induced Relaxation

We investigate the tip-sample dependence of atomically resolved non-contact atomic force microscopy (NC-AFM) images of a GaAs(110) surface taken with a tip that can resolve the tip-sample interaction originating from the dangling bonds of Ga atoms and the valence charge distribution around As atoms. Comparing the NC-AFM images taken with various tip-sample distances with a theoretical investigation of tip-sample interactions on the surface, the tip-sample interaction near the As atoms and Ga atoms are experimentally distinguished, and it is suggested that observed NC-AFM images reflect the tip induced surface relaxation.

Tip-induced relaxation and amplitude of cantilever vibration observed on GaAs(110) surface

Based on topographic images of the GaAs(110) surface obtained by non-contact atomic force microscopy (NC-AFM) with different tip–sample distances, we discuss the tip–sample distance dependence of the cantilever vibration amplitude and the tip-induced surface Ga atom relaxation. In the case of a tip which reveals only As atoms as one kind of protrusion in the NC-AFM image, the damping of the cantilever vibration amplitude is small, and the frequency shift decreases gradually with decreasing tip–sample distance. On the other type of tip with which the bright and darker protrusions corresponding to both As and Ga atoms are observed, a large damping of cantilever vibration amplitude is measured with decreasing tip–sample distance. The frequency shift curve measured with this tip has a singular point. From this frequency shift curve, we conclude that the force acting between this type of tip and the sample surface is a hysteretic force. In this tip case, tip-induced surface relaxation of the topmost Ga atoms occurs. There is a relationship between the damping of the cantilever vibration amplitude and the tip-induced surface atom relaxation; that is, the energy dissipation due to the relaxation becomes remarkable.

Application of Photoacoustic Spectroscopy to Porous Silicon

We are investigating applicability of photoacoustic (PA) spectroscopy to porous silicon. Since PA spectroscopy is based on a non-radiative relaxation process, the measurement is of importance as a counterpart to photoluminescent spectroscopy. We studied a dependence of a PA amplitude on a chopping frequency and discussed the influence of a PA signal originated in a silicon substrate. The frequency dependence was elucidated with a two-layer model. Differences in PA spectra are correlated with a photoluminescent efficiency. From the correlation, we believe that non-radiative centers quench the efficiency.