Shiano Ono

Kelvin probe force microscopy on InAs thin films grown on GaAs giant step structures formed on (110) GaAs vicinal substrates

Surface potential measurements on InAs thin films grown on GaAs giant steps were performed by Kelvin probe force microscopy. We found that the removal of the water-related layer from both surfaces on a sample and a tip was very effective to improve the reliability of the surface potential measurements. The measured potential distribution corresponds to the surface corrugation of the InAs thin films. In addition, the InAs layer thickness dependence of the surface InAs Fermi levels is investigated, indicating that the surface Fermi level shifts toward the vacuum level as the increase of the InAs layer thickness.

Lateral Averaging Effects on Surface Potential Measurements on InAs Dots Studied by Kelvin Probe Force Microscopy

Surface potential measurements on InAs quantum dots (QDs) were performed using Kelvin probe force microscopy (KFM) based on noncontact-mode atomic force microscopy. By the KFM measurements, the potential distribution well corresponding to the topography was successfully observed, and the observed potential difference between the InAs QD region and the surrounding wetting layer region was nearly proportional to the QD diameter. From two-dimensional simulations, in which we calculated the appropriate tip bias to minimize the electrostatic force working between the tip and the sample, we considered that the real dependence of the potential on the QD size still existed, although lateral averaging effects on the potential measurements should be considered to explain the potential observed by the actual KFM measurements. We also pointed out that a lateral averaging effect made the observed potential contrast indistinct, particularly when the size of the nanostructure was smaller than the tip radius.

Current and potential characterization on InAs nanowires by contact-mode atomic force microscopy and Kelvin probe force microscopy.

We fabricated InAs nanowires on GaAs giant step structures, and studied their surfaces by two methods; one was current detection by contact-mode atomic force microscopy, and the other was surface potential measurement by Kelvin probe force microscopy (KFM). In the current detection method, the regions where the large current flowed were distributed along the step edges, and these regions agreed well with the expected distribution of InAs. This result confirms that the InAs nanowires were formed along the GaAs giant step edges. The KFM measurements showed that the potential value became more negative along each step edge, where the InAs nanowire was expected to be formed. The surface potential of the InAs nanowires is more negative than that of the surrounding GaAs, which may result from the electron accumulation in the InAs nanowires.

Kelvin Probe Force Microscopy on InAs Thin Films on (110) GaAs Substrates

Surface potential distributions on InAs thin films grown on (110) GaAs substrates were studied by Kelvin probe force microscopy. The topography showed that their surfaces were covered by many terraces. The surface Fermi levels (EFS) were evaluated from the potential images, and EFS near the terrace edge was higher than that on the terrace top, while both of them were above the conduction band edge of bulk InAs. These results indicate that the electrons are more concentrated near the terrace edge. The film thickness dependence of EFS was also studied, but the quantum confinement effects in the InAs thin films were not confirmed experimentally.

Surface Potential Imaging on InAs Low-Dimensional Nanostructures Studied by Kelvin Probe Force Microscopy

We fabricated InAs thin films, wires and self-assembled quantum dots (QDs) on GaAs substrates, and studied their surface potentials by Kelvin probe force microscopy (KFM). The obtained potential images on InAs wires became less distinct as the tip-to-sample distance increased, which can be explained by the broadening of the electric field distribution between the tip and the sample. Such broadening was successfully confirmed by a two-dimensional simulation. We also found that the geometric effect does not mainly dominate the potential contrast on InAs QDs. The surface Fermi levels on the InAs-grown regions on InAs wires and InAs QDs shifted toward the vacuum level because of the electron accumulation in the InAs. Furthermore, the InAs QD size dependence of the potential values was successfully observed. The obtained potential difference between the InAs QDs and the surrounding wetting layer was larger than that obtained between the InAs wires and the surrounding GaAs surfaces, which can be explained by the strong electron confinement effect in the InAs QDs.

Sample-and-Hold Operation in Kelvin Probe Force Microscopy

To achieve a more accurate determination of surface potential in Kelvin probe force microscopy (KFM), we have proposed and demonstrated a sample-and-hold operation of KFM (SH-KFM), in which a sample-and-hold (S/H) circuit is inserted between an optical deflection sensor and a lock-in amplifier, to sample the deflection signal at a tapping frequency. Owing to the sampling operation, the cantilever bending due to the electrostatic force can be extracted at a desired phase in the tapping oscillation by intentional tuning of a time delay for sampling. We performed SH-KFM measurements on InAs quantum dots, and successfully observed a very clear site dependence of the potential with a large contrast. The results also indicated that the spatial resolution in the potential images was estimated to be better than 10 nm, and that the amplitude of an ac modulation bias in KFM could be reduced to 10 mVp–p without any remarkable degradation of the potential images.

Sample-and-Hold Imaging for Fast Scanning in Atomic Force Microscopy

We have proposed to use a sample-and-hold circuit for direct monitoring of cantilever deflection signals as a novel method for fast imaging in an intermittent contact mode atomic force microscopy (AFM). This method enables us to construct a quasi-topographic image from tip heights at moments when the tip taps on a sample surface. As a result, we obtained the tip height image well corresponding to the sample topography at a scanning rate above 30 Hz/line without any other customization on both a cantilever and a piezo scanner in a commercial AFM system.

Improvement of KFM performance by intermittent bias application method and by sampling detection of cantilever deflection.

We have proposed an intermittent bias application method as well as a sampling detection method of cantilever deflection in Kelvin probe force microscopy (KFM) to improve its performances for surface potential measurements. In the former method, spiky biases, instead of the bias in a sinusoidal waveform normally used in KFM, are intermittently applied to generate electrostatic force at exact moments when the tip approaches the closest position to a sample surface. The latter one, on the other hand, realizes very sensitive detection of the electrostatic force, which is preferable in KFM. Both the dependence of the electrostatic force on the dc offset bias and the observed potential images clearly indicate that these two methods are very effective to improve the KFM performance.

Intermittent Bias Application in Kelvin Probe Force Microscopy for Accurate Determination of Surface Potential

We propose an intermittent bias application method in Kelvin probe force microscopy (KFM) to suppress electrostatic force variation due to an oscillatory motion of a KFM tip. In this method, spiky biases, instead of the bias in a sinusoidal waveform, which is used in conventional KFM, are intermittently applied to generate electrostatic force at exact moments when the tip approaches the closest position to a sample surface. The observed potential distribution around self-assembled InAs quantum dots indicates that the intermittent bias application method improved the quality of the potential images and that the achieved spatial resolution was estimated to be better than 10 nm.