Kelvin Probe Force Microscopy Measurements Research Articles

Kelvin probe force microscopy on corona charged oxidized semiconductor surfaces

We present results demonstrating that Kelvin probe force microscopy (KPFM) can be used with high accuracy for corona charge aided oxide thickness mapping. In our experiments, corona charge layers of varying density were deposited onto the surface of thermally oxidized silicon wafers with different oxide thicknesses. After deposition, the surface potentials of the samples were characterized using both the standard Kelvin probe method and KPFM. Comparison of the surface potentials measured by both techniques showed that the values are in excellent agreement, and that only insignificant discharging of the corona charge layer occurred during the topography scan necessary in KPFM measurements.

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.

Investigations of Nanoparticles by Scanning Near-Field Optical Microscopy Combined with Kelvin Probe Force Microscopy Using a Piezoelectric Cantilever

A microfabricated cantilever with a lead zirconate titanate (PZT) piezoelectric thin film used as an integrated deflection sensor was applied to dynamic-mode atomic force microscopy (AFM). This probe can be used for scanning near-field optical microscopy (SNOM) and Kelvin-probe force microscopy (KFM). We adopted a frequency modulation (FM) detection method not only to stabilize imaging conditions in dynamic-mode AFM but also to enhance sensitivity for the detection of electrostatic forces in KFM measurement. We investigated local optical and electrical properties of gold and polystyrene nanoparticles dispersed on an indium tin oxide (ITO) substrate. We successfully identified the species of each particle by different contrasts obtained in SNOM and KFM images.

Tip-to-sample distance dependence of an electrostatic force in KFM measurements

In most scanning probe methods like an atomic force microscopy, a cantilever is mechanically vibrated in order to obtain topographies. Therefore, a tip-to-sample distance periodically changes during the scanning. Since the electrostatic force, which is a long-range force, is used for potential feedback in Kelvin probe force microscopy (KFM), such mechanical vibration leads to the fluctuation of the electrostatic force between the tip and the sample. In this study, firstly, we performed two-dimensional simulations of the electric fields between surfaces of the tip and the sample and evaluated the tip-to-sample distance dependence of the electrostatic force. Secondly, we experimentally confirmed the existence of the fluctuation of the electrostatic force and the tip-to-sample distance dependence of the electrostatic force was evaluated. Both the simulations and the experiments on the tip-to-sample distance dependence showed the importance of considering the tip sidewall effect in the KFM potential determination.

Microstructured π-Conjugated Organic Monolayer Covalently Attached to Silicon

A phenylacetylene (PA) monolayer was prepared on hydrogen-terminated silicon. Through vacuum ultraviolet (VUV) lithography, micropatterned PA-monolayer/SiO2 samples were subsequently fabricated. The surface potential and electrical conductivity of the microstructured PA monolayers were examined by Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (C-AFM), respectively. KPFM measurements showed that the PA-monolayer surface regions were negatively charged. C-AFM measurements demonstrated that, in the microdomains, the PA-monolayer regions had a higher conductivity than the SiO2 regions fabricated by VUV lithography. This is the first report of electronic property images of a monolayer directly attached to silicon being demonstrated in microregions.

The Imaging Mechanism of Atomic-scale Kelvin Probe Force Microscopy and its Application to Atomic-Scale Force Mapping

Kelvin probe force microscopy (KPFM) using noncontact atomic force microscopy (NC-AFM) has been utilized for years and has achieved true atomic-scale resolution in the measurement of contact potential difference (CPD). However, the meaning of atomic-scale CPD images has not been clearly explained. In this paper, we propose a novel model which explains the imaging mechanism of atomic-scale KPFM. Our model shows that the cantilever oscillation induced by an AC electric field, which is applied for KPFM measurement, brings about the atomic resolution in CPD images. We show experimental evidence of our model by measuring the dependence of the cantilever oscillation and the frequency-shift signal on the tip-sample distance. We conclude that the CPD images reflect the interaction strength between the tip and the sample surface rather than the electrostatic force. Finally, we propose a novel method for force mapping based on our model.

Kelvin probe force microscopy on III–V semiconductors: the effect of surface defects on the local work function

The application of Kelvin probe force microscopy (KPFM) in ultra high vacuum (UHV) allows to determine the absolute work function of surfaces with a very high energy (<5 meV) and lateral (<20 nm) resolution. We present measurements on different UHV cleaved III–V compound semiconductors. The (110)-surface shows work function variations due to defect states at step edges. We observed band bending on the (110)-surface of GaAs from surface photovoltage measurements. Finally, we discuss the influence of the previous effects on KPFM measurement of a UHV cleaved GaP pn-homojunction. Due to the long range nature of the electrostatic forces the geometry of the tip, cantilever and sample plays an important role in KPFM.

Surface potential images of self‐assembled monolayers patterned by organosilanes: ab initio molecular orbital calculations

AbstractSelf‐assembled monolayers (SAMs) with different function terminal groups were prepared on substrates of n‐type silicon by chemical vapour deposition (CVD) from n‐octadecyltrimethoxysilane (ODS: H3C(CH2)17Si(OCH3)3), heptadecafluoro‐1,1,2,2‐tetrahydro‐decyl‐1‐trimethoxysilane (FAS: F3C(CF2)7 (CH2)2Si(OCH3)3), n‐(6‐aminohexyl)aminopropyltrimethoxysilane (AHAPS: H2N(CH2)6NH(CH2)3 Si(OCH3)3) and 4‐(chloromethyl)phenyltrimethoxysilane [CMPS: H2ClC(C6H4)Si(OCH3)3]. The sample surfaces covered with ODS‐SAM were irradiated under a reduced pressure of 10 Pa with vacuum ultraviolet (VUV) light through a photomask. Then, the irradiated areas with the ODS‐SAM removed were covered with another SAM by CVD. Surface potential images of the patterned areas were measured by Kelvin probe force microscopy (KPFM) and explained by ab initio molecular orbital (MO) calculations. The KPFM measurements showed that the surface potential contrasts of the FAS‐, AHAPS‐ and CMPS‐SAMs against the ODS‐SAM were about −170, +50 and −30 mV, respectively. The surface potentials as a function of area occupied by a molecule were predicted using the respective dipole moments perpendicular to substrates, which were obtained by the ab initio MO calculations. The predicted surface potentials of the FAS‐, CMPS‐ and AHAPS‐SAMs approximately agreed with the measured values at areas of about 1.5, 1.6 and 0.6 nm2 per molecule. The ab initio MO calculations also showed differences in surface potentials appearing due to the differences in Fermi levels of the respective SAMs. Copyright © 2002 John Wiley &amp; Sons, Ltd.

The decomposition mechanism of p-chloromethylphenyltrimethoxysiloxane self-assembled monolayers on vacuum ultraviolet irradiation

The decomposition mechanism of p-chloromethylphenyltrimethoxysiloxane (CMPS) self-assembled monolayers (SAMs) due to vacuum ultraviolet (VUV) irradiation at a wavelength of 172 nm has been studied based on water repellency, X-ray photoelectron spectroscopy (XPS) and Kelvin probe force microscopy (KPFM). The CMPS-SAMs were prepared through chemical vapor deposition at 100 °C. The water contact angle increased dramatically within the first 30 min, reaching a plateau at this point. The saturated water contact angle was 76°. Next, the CMPS-SAMs were irradiated at under 10 Pa from 0 to 30 min. The CMPS-SAMs irradiated for 15 and 30 min were covered with carboxylated hydrocarbon fragments and silanol groups, respectively. KPFM measurements for micro-patterned SAMs, i.e. CMPS/CO2H and CMPS/SiOH, showed that the surface potential of the carboxylated and silanol regions were −16 and +19 mV vs. CMPS-SAM, respectively. The former negative surface potential difference originated in the electronegativity of the CO2H fragments. These results provide us with a route to prepare an organic template for various fields such as bio-sensors, chemical sensors and electrolytes.

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.

Atomic Force Microscopy and Kelvin Probe Force Microscopy Measurements of Semiconductor Surface Using Carbon Nanotube Tip Fabricated by Electrophoresis

Carbon nanotube (CNT) tips were fabricated by attaching the CNTs directly to conventional Si cantilever tips by electrophoresis. Clear topographic and potential profiles have been obtained using the CNT tips in atomic force microscopy (AFM) and Kelvin probe force microscopy (KFM) measurements. It is important to place the CNT tip close to the sample to improve the spatial resolution in the KFM measurements.

Investigation of the cleaved surface of a p–i–n laser using Kelvin probe force microscopy and two-dimensional physical simulations

We have investigated the cross-sectional electric field and potential distribution of a cleaved n+-InP/InGaAsP/p+-InP p–i–n laser diode using Kelvin probe force microscopy (KFM) with a lateral resolution reaching 50 nm. The powerful characterization capabilities of KFM were compared with two-dimensional (2D) physics-based simulations. The agreement between simulations and KFM measurements regarding the main features of the electric field and potential is very good. However, the KFM yields a voltage drop between n- and p-doped InP regions which is 0.4 times the one simulated. This discrepancy is explained in terms of surface traps due to the exposure of the sample to the air and in terms of incomplete ionization. This hypothesis is confirmed by the 2D simulations.

Cross-Sectional Potential Imaging of Compound Semiconductor Heterostructure by Kelvin Probe Force Microscopy

The cross-sectional potential images of the GaAs/AlAs multiple quantum wells and InAlAs/InGaAs heterostructures have been successfully obtained by Kelvin probe force microscopy (KFM). It was shown that the smaller amplitude of the alternating voltage (Vac) applied to detect the electrostatic force during the KFM measurements gave a better potential profile of the heterostructures. The spatial resolution of the present KFM was investigated by measuring the InAlAs/InGaAs layered structures with various thicknesses. The minimum thickness of the InAlAs layer distinguished by the KFM was 40 nm.

Kelvin probe force microscopy for potential distribution measurement of semiconductor devices

This paper demonstrates that Kelvin probe force microscopy (KFM) is applicable to the characterization of semiconductor devices. The optimum operating conditions for KFM measurements are determined experimentally. Low potential deviation of less than several mV and high topographic resolution sufficient to display monolayer-height steps were obtained at tip-sample distances ranging from 40 to 60 nm. Potential distributions were measured on thin InGaAs resistors using KFM. The steep potential drops observed at the contact edges attributable to the contact resistance are verified by measuring the contact resistance using the transfer length method and the results of previously reported scanning tunneling potentiometry. The KFM results accurately explain the electrical properties of the metal/semiconductor interface.