I. Kamiya

Imaging and probing electronic properties of self-assembled InAs quantum dots by atomic force microscopy with conductive tip

Atomic force microscopy with a conductive probe has been used to study both the topography and the electronic properties of 10-nm-scale self-assembled InAs quantum dots (QDs) grown by molecular beam epitaxy on n-type GaAs. The current flowing through the conductive probe normal to the sample surface is measured for imaging local conductance, while the deflection of cantilever is optically detected for disclosing geometrical structure. The conductance on InAs QDs is found to be much larger than that on the wetting layer, allowing imaging of QDs through measurements of local current. We attribute this change in conductance to the local modification of surface band bending associated with surface states on InAs QD surface. Mechanisms of electron transport through QDs are discussed based on current–voltage characteristics measured on QDs of various sizes.

Resonant tunneling through a single self-assembled InAs quantum dot in a micro-RTD structure

Abstract By forming a micron-scale Schottky diode on a GaAs/n+–GaAs wafer with low density ( ∼4×10 8 cm −2 ) self-assembled InAs quantum dots (QDs) embedded in the top GaAs layer, we have studied resonant tunneling transport of electrons via single QDs. As the diode defined by electron-beam lithography and chemical etching has a size of about 1 μm 2 and the edges of each diode are depleted, the number of active InAs QD contained in each microdiode can be reduced to less than unity in average. Current–voltage measurements were performed using conductive probe atomic force microscope, and current peaks caused by electrons resonantly tunneling through quantum levels of single QDs have been observed at temperatures as high as ∼130 K .

Local capacitance measurements on InAs dot-covered GaAs surfaces by scanning capacitance microscopy

Capacitance images responsible for surface depletion were observed on an InAs dot-covered GaAs surface by scanning capacitance microscopy. We performed local capacitance versus bias voltage measurements on quantum dots (QDs) and a wetting layer (WL) as well as conductance versus bias voltage (G–V) measurements. Both results indicate that the surface depletion is more suppressed beneath the QDs than under the WL. In addition, the conventional thermionic equation theory fitted to the measured G–V curves shows that the interface barrier height between the GaAs and the InAs QD increases as the QD size is reduced. We ascribe this result to the influence of the surrounding WL, whose surface Fermi level is strongly pinned at the midgap of the n-GaAs.

Electrostatic force characterization on InAs dot-covered n-type (001) GaAs surfaces by contact-mode atomic force microscopy with a conductive tip

We performed atomic force microscopy in contact mode while applying an ac bias voltage between a conductive tip and a sample to characterize near surface band structures of InAs-covered n-type (001) GaAs, where self-assembled dot structures were formed. Electrostatic force of less than 10 pN was detectable, and clear electrostatic force images and topographic images were simultaneously obtained with lateral resolution higher than 20 nm. The electrostatic force images from single and double frequency components reveal that the gap width between the tip and the conductive region under the dot-covered area of the sample is smaller and is less modulated by the bias voltage than under the wetting layer. The results indicate that surface depletion is more suppressed beneath the dots.

UHV-AFM study of MBE-grown 10 nm scale ridge quantum wires

We evaluate the shape of ridge quantum wires (RQWIs) with nm-scale resolution by using SEM and ultra-high-vacuum (UHV) AFM system which is connected to MBE chamber. This MBE-AFM system provides us a detailed information about the evolution of size and uniformity of ridges grown under various conditions. In this report, we investigate systematically how the growth temperature T s and As flux affect the width and morphology of GaAs ridge structure. The results show that a very sharp and uniform GaAs ridge structures (W < 10 nm) can be obtained. On the basis of this understanding, we fabricated the ridge quantum wire of 10 nm width.

Current Images of CdSe Colloidal Nanodots Observed by Conductive-tip Atomic Force Microscopy

Abstract : We have fabricated submonolayer-thick films of CdSe colloidal nanodots in order to investigate electronic properties of individual nanodots by conductive-tip atomic force microscopy (AFM). Topographic and current images of isolated single CdSe colloidal dots on single crystalline Au (111) surface which was covered with dodecanethiol self-assembled monolayer were obtained by AFM operating in contact mode with a conductive tip under appropriate bias voltages. In the current image, it is found that the dot regions have higher electric resistance due to tunneling resistance through the CdSe dots. We also found 10 nm-scale electric inhomogeneity around the dots, which may corresponds to the previously reported etch-pits of Au (III) surfaces formed during the deposition of the dodecanethiol molecules.

Characterization of InAs dots on n-GaAs by AFM with a conductive tip

We performed atomic force microscopy (AFM) on an InAs-covered GaAs surface on which InAs dot structures are formed surrounded by a wetting layer. In order to characterize its surface band structures, we applied an a.c. bias voltage (f = 25 kHz) between a conductive AFM tip and a sample. By means of a lock-in detection technique, clear electrostatic force images as well as topographic images were obtained with high resolution. The electrostatic force images of the different frequency components (f and 2f) reveal that the gap width between the tip and the conductive region of the sample is modulated very little by the bias voltage in the dot-covered area, whereas the gap width is obviously modulated in the wetting layer area. These results indicate that the surface depletion is more suppressed in the dot-covered area than in the wetting layer area.

Terahertz Excitation, Transport and Spectroscopy of an AFM-Defined Quantum Dot

We have investigated the terahertz photoresponse of a single semiconductor quantum dot, electrostatically defined by a sharp conducing Atomic Force Microscope tip in contact with a resonant tunneling diode structure. The quantum dot is excited by radiation from a Free Electron Laser in experiments both at room temperature and at cryogenic temperatures. Pronounced resonant tunneling features and classical rectification at frequencies from 0.3 to 3THz are observed in the I-V curves of these devices. These results demonstrate a novel approach to achieving terahertz excitation and studying transport in quantum dots.