Low-temperature Scanning Force Microscope Research Articles

Current distribution and Hall potential landscape towards breakdown of the quantum Hall effect: a scanning force microscopy investigation

We present line- and area-scans of the Hall potential landscape of a two-dimensional electron system (2DES) in narrow (AlGa)As-based Hall bars under quantum Hall (QH) conditions, obtained by low-temperature scanning force microscopy. For several magnetic field values B in the regime of the QH plateau with Landau level filling factor , we measured the evolution of the Hall potential profiles and of the longitudinal voltage drop along the Hall bar as a function of increasing voltage/current bias, leading finally to the electrically induced breakdown of the quantum Hall effect (QHE). Basically two types of evolution were observed: for the low B-field side of the QHE plateau, two distinct Hall potential drops appear close to the two edges of a cross section, equally distributed at low bias but continuously developing to an asymmetrical distribution with increasing bias. At high bias, a steady increase of the longitudinal voltage drop is observed, accompanied by a rising slope of the Hall potential drop in the bulk. For the upper B-field side of the QH plateau, the Hall voltage drops are broadly distributed across the whole cross section, and the distribution remains almost unchanged until the bias reaches a critical value where the Hall potential profile changes rather abruptly, enhancing locally the Hall field. Beyond this, with further increase of the bias, a steep rise of the longitudinal voltage drop is detected. These findings are naturally explained in the microscopic picture of the QHE, based on the self-consistent evolution of the compressible and incompressible landscape inside the 2DES with increasing bias.

Electronic transport in graphene nanostructures on SiO2

We report two experiments on graphene nanostructures. The first was performed on a graphene nanoribbon, where the nature of electronic transport was investigated in detail. Electrons or holes are found to localize in pockets of the potential along the ribbon. Transport is governed by the joint action of localization and Coulomb interaction. The temperature-dependence of the conductance shows activated behavior at temperatures above a few Kelvin. The activation energy retraces the edges of Coulomb blockade diamonds found in nonlinear transport. In the second experiment the metallic tip of a low-temperature scanning force microscope was scanned above a graphene quantum dot. In addition to the familiar Coulomb blockade fringes, localized states are detected forming in the constrictions connecting the dot to source and drain.

Scanning gate microscopy measurements on a superconducting single-electron transistor

We present measurements on a superconducting single-electron transistor (SET) in which the metallic tip of a low-temperature scanning force microscope is used as a movable gate. We characterize the SET through charge stability diagram measurements and compare them to scanning gate measurements taken in the normal conducting and the superconducting states. The tip-induced potential is found to have a rather complex shape. It consists of a gate voltage-dependent part and a part which is independent of gate voltage. Further scanning gate measurements reveal a dependence of the charging energy and the superconducting gap on the tip position and the voltage applied to it. We observe an unexpected correlation between the magnitude of the superconducting gap and the charging energy. The change in EC can be understood to be due to screening, however the origin of the observed variation in Δ remains to be understood. Simulations of the electrostatic problem are in reasonable agreement with the measured capacitances.

Imaging a coupled quantum dot-quantum point contact system

We have quantitatively studied the effect of charge traps on the electrical conductance of a quantum dot and a capacitively coupled quantum point contact. Using the sharp metallic tip of a low-temperature scanning force microscope as a scanned gate, we could localize the traps. The quantum point contact served as a charge detector and allowed us to distinguish single electron charging events in several traps from charging events on the dot. We used the quantum dot to analyze the tip-induced potential quantitatively and found its shape to be independent of the voltage applied to the tip within a certain range of parameters. We estimate that the trap density is below 0.1% of the doping density, that the charging energy of a trap is three times higher than that of the quantum dot, and that the interaction energy between the quantum dot and a trap is a significant portion of the dot’s charging energy. Possibly, such charge traps are the reason for frequently observed parametric charge rearrangements.

Hall Potential Profiles in Quantum Hall Samples Measured by a Low-Temperature Scanning Force Microscope

Our measurements of the Hall potential distribution in GaAs/AlGaAs quantum Hall samples using a scanning force microscope at a temperature of 1.4 K are reviewed. The results emphasize the important role of compressible and incompressible strips, running along the edges of the two-dimensional electron system (2DES), for the quantum Hall effect. At the interface line between the 2DES and its alloyed metal contact with good ohmic behavior, a partial depletion of the electron concentration in the 2DES in front of contact is identified, causing for certain magnetic field regimes an incompressible strip lying in parallel to the interface line and therefore isolating the compressible 2DES bulk from the 2DES edge with the contact.

Low-Temperature Scanning Force Microscopy Using a Tuning Fork Transducer

We developed a low-temperature scanning force microscope using a quartz tuning fork operating at 4.2 K. A silicon tip from a commercial cantilever was attached to one prong of the tuning fork. With a metallic coating, a potential could be applied to the tip to sense the charge distribution in a sample while with a magnetically coated tip, magnetic force imaging could be performed. For the coarse approach mechanism, we developed a reliable low-temperature walker with low material cost and simple machining. We obtained Coulomb force images of boron nanowires at room temperature and of magnetic nano-structures at low temperature. With the tuning-fork-based magnetic force microscopy (MFM) probe, low-temperature MFM was performed at 77 K and 4.2 K in a high magnetic field (up to 3 T) by using a superconducting.

Short-range electrostatic interactions in atomic-resolution scanning force microscopy on theSi(111)7×7surface

A low-temperature scanning force microscope (SFM) operating in ultrahigh vacuum was used to study the interaction between a sharp but unreactive oxidized silicon tip and a Si(111) $7\ifmmode\times\else\texttimes\fi{}7$ surface. Weakly corrugated but, nevertheless, atomically resolved images were obtained over a relatively large range of tip-sample distance. The measured short-range interaction potential and image contrast can be explained by an electrostatic interaction that results from a dipole moment induced in the tip by the distribution of electronic charge among the dangling bonds on the surface. The experiments presented here demonstrate the potential of SFM to quantitatively map even weak, physical interactions with atomic scale resolution. Such experiments should lead to a better understanding of surface reactivity and local charge-transfer effects induced by adsorbates or defects at surfaces.

Atomic resolution imaging and frequency versus distance measurements on NiO(001) using low-temperature scanning force microscopy

Atomically resolved scanning force microscopy images and force versus distance measurements at low temperatures on a cleaved NiO (001) single crystal are reported. The force-distance data are well modeled with a capacitive force in the distance range of 0.5 to 20 nm. The residual forces at smaller tip-sample distances show a maximum attraction of 2.3 nN and decay within 0.2 nm. They show a steplike behavior associated with an increase in dissipation signal. This steplike behavior can be explained by interaction of more than one atom of the tip or of the surface.

Atomic resolution imaging and force versus distance measurements on KBr (0 0 1) using low temperature scanning force microscopy

Atomic scale dynamic scanning force microscopy images and force versus distance measurements at low temperatures on a cleaved KBr (0 0 1) single crystal are reported. Two distinct forms of atomically resolved contrast were observed. In one case, a nanotip was formed through a tip change. In this case, a strong corrugation of 0.07 nm was measured. It was possible to reverse this tip change intentionally. In the second case, the observed contrast was only 0.025 nm. The force–distance measurements are well modelled with a van der Waals force in the distance range of 0.5–15 nm. The residual forces at smaller tip–sample distances show a maximum attraction of 0.3 nN and decay within 0.2 nm.

Hall potential profiles in the quantum Hall regime measured by a scanning force microscope

A low-temperature scanning force microscope sensitive to electrostatics is used to investigate the potential distribution of a two-dimensional electron system (2DES) under quantum Hall conditions at a temperature T=1.4K. We mapped out the Hall potential profiles for the Landau level filling factor range 1<ν<14 with submicron resolution. At integer filling factors, the potential drop is rather nonlinear, but depends strongly on the scan position. Obviously inhomogeneities of the sample strongly affect the Hall potential profile if the bulk of the 2DES is nonconductive and cannot screen. At filling factors slightly above an even integer value, the Hall potential drops at prominent positions at both edges which are associated with the positions of the incompressible strips as confirmed by comparison with theoretical predictions. With these broad incompressible strips electrically decoupling the edge region from the compressible bulk region, a nearly vanishing Hall potential drop is observed in the compressible bulk region, suggesting that most of the current is flowing close to the edges. In addition, potential profiles near the current injecting contacts reveal the well known hot spot region in the direct vicinity of the contact, but recover to the described shape within a distance of only 6μm.

Dynamic low-temperature scanning force microscopy on nickel oxide (001)

We present atomically resolved images of nickel oxide (001) obtained with low temperature non-contact atomic force microscopy. Using iron coated silicon cantilevers, it is possible to distinguish defects with a vertical resolution of less than 10 pm and to obtain atomic resolution across step edges on the upper and lower terrace within 1 nm of the edge. The noise level in these images could be reduced to ≈1.5 pm.

Dynamic Scanning Force Microscopy at Low Temperatures

In this paper, we review the design and various applications of a low temperature scanning force microscope for ultrahigh vacuum. It has been adopted for dynamic mode measurements, a powerful method to image surfaces with a resolution similar to scanning tunneling microscopy, but without the limitation to conducting materials. With this instrument, we have studied semiconducting (InAs), conducting (HOPG) and insulating samples (xenon thin film). Finally, we discuss a new experimental method to determine the tip-sample interaction with high accuracy.

A low-temperature scanning force microscope for investigating buried two-dimensional electron systems under quantum Hall conditions

A low-temperature scanning force microscope sensitive to electrostatics has been constructed for investigating a two-dimensional electron system (2DES) under quantum Hall conditions. In order to cope with the highly resistive properties of the 2DES under these conditions, a low-frequency measurement technique is presented, based on the shift of the cantilever's resonance frequency induced by a small ac current modulation within the 2DES. Since the 2DES is buried in a GaAs/AlGaAs heterostructure, a special calibration technique has to be applied, which allows to map Hall-potential profiles with clearly submicron resolution.

Low temperature scanning force microscopy of the Si(111)-(7x7) surface

A low temperature scanning force microscope (SFM) operating in a dynamic mode in ultrahigh vacuum was used to study the Si(111)- (7x7) surface at 7.2 K. Not only the twelve adatoms but also the six rest atoms of the unit cell are clearly resolved for the first time with SFM. In addition, the first measurements of the short range chemical bonding forces above specific atomic sites are presented. The data are in good agreement with first principles computations and indicate that the nearest atoms in the tip and sample relax significantly when the tip is within a few A of the surface.

A low temperature ultrahigh vaccum scanning force microscope

This article describes the design of a versatile ultrahigh vaccum (UHV) low temperature scanning force microscope system. The system allows scanning probe microscopy measurements at temperatures between 6 and 400 K and in magnetic fields up to 7 T. Cantilevers and samples can be prepared in UHV and transferred to the microscope. We describe some technical details of our system and present first measurements performed at different temperatures and in various scanning force microscopy operation modes. We demonstrate distortion free and calibrated images at temperatures ranging from 8 to 300 K, atomic resolution on NaCl at 7.6 K and various magnetic force microscopy images of vortices in high transition temperature superconductors. It is demonstrated that our instrumentation reaches the thermodynamically determined sensitivity limit. Using standard cantilevers force gradients in the 10−6N/m range, corresponding forces of about 10−15N can be measured.

Low temperature magnetic force microscopy

We have built a low temperature scanning force microscope which is able to measure contact and noncontact forces using the dc modes of force microscopy. We demonstrate the capabilities of our instrument on a magneto-optical disk at room temperature and at 77 K. Using a ferromagnetic thin film tip, the topography and the micromagnetic stray field of the sample is measured using the dc modes of force microscopy. The topographic and magnetic data are precisely correlated. The circular bit structure and the natural domain structure between the homogeneously magnetized bits is clearly visible. A lateral resolution below 100 nm and a force resolution of 10−12 N is reproducibly achieved.