Kelvin Probe Force Microscopy Measurements Research Articles

Quantitative 3D-KPFM imaging with simultaneous electrostatic force and force gradient detection

Kelvin probe force microscopy (KPFM) is a powerful characterization technique for imaging local electrochemical and electrostatic potential distributions and has been applied across a broad range of materials and devices. Proper interpretation of the local KPFM data can be complicated, however, by convolution of the true surface potential under the tip with additional contributions due to long range capacitive coupling between the probe (e.g. cantilever, cone, tip apex) and the sample under test. In this work, band excitation (BE)-KPFM is used to negate such effects. In contrast to traditional single frequency KPFM, multifrequency BE-KPFM is shown to afford dual sensitivity to both the electrostatic force and the force gradient detection, analogous to simultaneous amplitude modulated and frequency modulated KPFM imaging. BE-KPFM is demonstrated on a Pt/Au/SiOx test structure and electrostatic force gradient detection is found to lead to an improved lateral resolution compared to electrostatic force detection. Finally, a 3D-KPFM imaging technique is developed. Force volume (FV) BE-KPFM allows the tip–sample distance dependence of the electrostatic interactions (force and force gradient) to be recorded at each point across the sample surface. As such, FVBE-KPFM provides a much needed pathway towards complete tip–sample capacitive de-convolution in KPFM measurements and will enable quantitative surface potential measurements with nanoscale resolution.

Optical and surface probe investigation of secondary phases in Cu2ZnSnS4 films grown by electrochemical deposition

Cu2ZnSnS4 (CZTS) films were grown by electrochemical deposition, and we measured the work function of the as-grown and of the KCN-etched CZTS surfaces by using Kelvin probe force microscopy (KPFM) and micro-Raman scattering spectroscopy with incident laser wavelengths of 488.0 and 632.8nm, respectively, and the results indicate that a secondary phase formed at different depths. The KPFM measurements can discriminate phase uniformity at the nano-scale. Secondary phases, such as Cu2−xS (0<x<1), ZnS, and MoS2, were identified on the as-grown surface while Cu2−xS was removed from the KCN-etched surface. The KCN-etched CZTS absorption layer was measured to have a 5.0% conversion efficiency. Owing to the low cost of electrochemical deposition, it is desirable to obtain high tailored CZTS films can be obtained with the robust surface characteristics of a uniform work function with a single phase.

Interlayer Resistance and Edge-Specific Charging in Layered Molecular Crystals Revealed by Kelvin-Probe Force Microscopy

Organic field-effect transistors (OFETs) having an active channel of solution-processed 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8–BTBT) were investigated by Kelvin-probe force microscopy (KFM). We found step-like potential distributions in a channel region, suggesting that the interlayer resistance between the conjugated BTBT core layers is quite high and each conjugated layer is electrically isolated from one another by insulating alkyl chain layers. We also found a noticeable positive charging in the channel region especially at the step edges after the device operation. The observed charging was explained by long-lived positive charges on the trap sites, and the trap density at the step edge was estimated to be on the order of 1011 cm–2. The KFM measurements suggest that the device performance of the staggered C8–BTBT OFETs could deteriorate due to the considerably high access resistance, which stems from the high interlayer resistance and/or by the site-specific charge trapping at the contact/semiconductor interface which originates from step edge structures.

Interplay between surface preparation and device performance in CZTSSe solar cells: Effects of KCN and NH4OH etching

Despite the many similarities between Cu2ZnSn(S,Se)4 (CZTSSe) and Cu(In,Ga)(S,Se)2 materials and device architecture, open questions remain about the optimal surface preparation steps for CZTSSe absorbers, including whether differences exist for absorber layers deposited by different methods. In this work, we investigate KCN etching and NH4OH treatment as surface preparation methods for the absorber/CdS interface for two-stage processed CZTSSe and co-evaporated CZTSe absorber layers. Ambient-exposed, thus oxidized and contaminated, thin film absorbers are utilized to examine the effectiveness of these surface preparation methods and to elucidate their effects on device performance. Topography and surface potential images simultaneously obtained by Kelvin probe force microscopy (KPFM) show the existence of an overlayer on the ambient-exposed absorbers. Moreover, KPFM results also demonstrate that although NH4OH treatment removes much of the overlayer from the CZTSSe surface, KCN etching removes the overlayer completely. In addition, differences in the deposited CdS layer and depletion region width result depending on the surface preparation method, with the NH4OH-treated solar cells having narrower depletion region. This is reflected in device results in which KCN-etched solar cells outperform their NH4OH-treated counterparts due to increases in external quantum efficiency at long wavelengths and in open circuit voltage. KPFM measurements also demonstrate that grain boundaries (GBs) in the KCN-etched two-stage processed CZTSSe thin films are either negatively charged or neutral. On the other hand, KCN etching makes the surface almost equipotential in the co-evaporated CZTSe thin film by causing disappearance of positively charged GBs that existed before etching.

Potential variation around grain boundaries in BaSi2 films grown on multicrystalline silicon evaluated using Kelvin probe force microscopy

Potential variations across the grain boundaries (GBs) in a 100 nm thick undoped n-BaSi2 film on a cast-grown multicrystalline Si (mc-Si) substrate are evaluated using Kelvin probe force microscopy (KFM). The θ-2θ X-ray diffraction pattern reveals diffraction peaks, such as (201), (301), (410), and (411) of BaSi2. Local-area electron backscatter diffraction reveals that the a-axis of BaSi2 is tilted slightly from the surface normal, depending on the local crystal plane of the mc-Si. KFM measurements show that the potentials are not significantly disordered in the grown BaSi2, even around the GBs of mc-Si. The potentials are higher at GBs of BaSi2 around Si GBs that are formed by grains with a Si(111) face and those with faces that deviate slightly from Si(111). Thus, downward band bending occurs at these BaSi2 GBs. Minority carriers (holes) undergo a repelling force near the GBs, which may suppress recombination as in the case of undoped n-BaSi2 epitaxial films on a single crystal Si(111) substrate. The barrier height for hole transport across the GBs varies in the range from 10 to 55 meV. The potentials are also higher at the BaSi2 GBs grown around Si GBs composed of grains with Si(001) and Si(111) faces. The barrier height for hole transport ranges from 5 to 55 meV. These results indicate that BaSi2 GBs formed on (111)-dominant Si surfaces do not have a negative influence on the minority-carrier properties, and thus BaSi2 formed on underlayers, such as (111)-oriented Si or Ge and on (111)-oriented mc-Si, can be utilized as a solar cell active layer.

Scanning probe microscopy investigations of the electrical properties of chemical vapor deposited graphene grown on a 6H-SiC substrate

Sublimated graphene grown on SiC is an attractive material for scientific investigations. Nevertheless the self limiting process on the Si face and its sensitivity to the surface quality of the SiC substrates may be unfavourable for later microelectronic processes. On the other hand, chemical vapor deposited (CVD) graphene does not posses such disadvantages, so further experimental investigation is needed. In this paper CVD grown graphene on 6H-SiC (0001) substrate was investigated using scanning probe microscopy (SPM). Electrical properties of graphene were characterized with the use of: scanning tunnelling microscopy, conductive atomic force microscopy (C-AFM) with locally performed C-AFM current–voltage measurements and Kelvin probe force microscopy (KPFM). Based on the contact potential difference data from the KPFM measurements, the work function of graphene was estimated. We observed conductance variations not only on structural edges, existing surface corrugations or accidental bilayers, but also on a flat graphene surface.

Study of heterostructures of Cu3BiS3–buffer layer measured by Kelvin probe force microscopy measurements (KPFM)

The interface formed between Cu3BiS3 thin films and the buffer layer is a potentially limiting factor to the performance of solar cells based on Al/Cu3BiS3/buffer heterojunctions. The buffer layers of ZnS and In2S3 were grown by co-evaporation, and tested as an alternative to the traditional CdS deposited by chemical bath deposition. From the Kelvin probe force microscopy measurements, we found the values of the work function of ZnS, In2S3, and CdS, layers deposited into Cu3BiS3. Additionally, different electronic activity was found for different grain boundaries (GBs), from studies under illumination, we also found the net doping concentration and the density of charged GB states for Cu3BiS3 and Cu3BiS3/CdS.

Charge Loss Characteristics of Different Al Contents in a HfAlO Trapping Layer Investigated by Variable Temperature Kelvin Probe Force Microscopy

Kelvin probe force microscopy (KFM) technology is applied to investigate the charge storage and loss characteristics of the HfAlO charge trapping layer with various Al contents. The experimental results demonstrate that with the increase of Al contents in the HfAlO trapping layer, trap density significantly increases. Improvement of data retention characteristic is also observed. Comparing the vertical charge loss and lateral charge spreading of the HfAlO trapping layers, the former plays a major role in the charge loss mechanism. Variable temperature KFM measurement results show that the extracted effective electron trap energy level increases with increasing Al contents in HfAlO trapping layer, which is in accordance with the charge loss characteristics.

Multifunctional nanoanalytics and long-range scanning probe microscope using a nanopositioning and nanomeasuring machine

An interferometer-based metrological scanning probe microscope (SPM) is successfully integrated into our nanopositioning and nanomeasuring machine (NPM machine) for high-precision measurements with nanometre uncertainty over a range of 25 mm × 25 mm × 5 mm. Both devices were developed at the Institute of Process Measurement and Sensor Technology of Ilmenau University of Technology, Germany. Outstanding results were achieved for different measurement tasks. With the NPM machine, truly long-range and long-term measurements are possible. Due to the tip wear, an automatic SPM cantilever replacement is preferable. Such a tip replacement is also required for the integration of multifunctional nanoanalytics. For example, for Kelvin probe force microscopy (KPFM), the measurement of topography and surface potential with different SPM tips is necessary. For this purpose, an electromagnetic tip changer was designed. The tip changer comprises three SPM probes. In order to retrieve the previous tip positions, additional fiducial marks were developed. The repeatability of relocation is less than 10 nm. The automatic tip changer and fiducial marks are integrated into a sample holder. The tip changer in combination with fiducial marks allows scanning distances three times longer (with the same type of SPM probes) and multifunctional nanoanalytics (with different SPM probes with special properties). Sample KPFM measurements are demonstrated. The developed tip changer, including special fiducial marks, improves the performance and functionality of the NPM machine crucially.

Investigation of HfAlO Trapping Layer with Various Al Contents by Variable Temperature Kelvin Probe Force Microscopy

In order to explore the charge storage and loss characteristics of HfAlO charge trapping layer, Kelvin probe force microscopy (KFM) technology was applied. HfAlO trapping layer was deposited by an atom layer deposition (ALD) system with the precursors Hf[N(C2H5) CH3]4 (TEMAH), Al(CH3)3 (TMA) and H2O as source materials. Comparing with HfO2 trapping layer, the trap density of the HfAlO sample was significantly increased. And the data retention abilities were improved with the increased Al contents. The effective electron trap energies were extracted, by variable temperature KFM measurement. The results demonstrate that the trap energies of HfO2 layer increase with Al doping.

Spatial distribution of free carrier concentration in vertical GaN Gunn‐diode structures studied by confocal micro‐Raman spectroscopy and Kelvin probe force microscopy

AbstractScanning confocal micro‐Raman and PL spectroscopy, Kelvin probe force microscopy (KPFM) and electrostatic force gradient microscopy (EFGM) were applied to study the concentration profiles of free carriers by scanning the cleaved edge along the growth direction of vertical n+/n0/n+‐GaN Gunn‐diode structure. The vertical frequency profile of the L– branch of the plasmon‐LO‐phonon modes was used to evaluate the concentration of free charge carriers. Obtained significant contrast through the undoped n0‐GaN region validates the expected reduction in carrier concentration. Variation of the local contrast in cross‐section KPFM and EFGM electrical images reflects the topology of the device structure and makes it possible to estimate electronic properties of individual GaN layers as well as interface boundaries. The carrier profiles determined from the Raman data and KPFM measurements are in good agreement. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Microscopic and macroscopic characterization of the charging effects in SiC/Si nanocrystals/SiC sandwiched structures

Microscopic charge injection into the SiC/Si nanocrystals/SiC sandwiched structures through a biased conductive AFM tip is subsequently characterized by both electrostatic force microscopy and Kelvin probe force microscopy (KPFM). The charge injection and retention characteristics are found to be affected by not only the band offset at the Si nanocrystals/SiC interface but also the doping type of the Si substrate. On the other hand, capacitance–voltage (C–V) measurements investigate the macroscopic charging effect of the sandwiched structures with a thicker SiC capping layer, where the charges are injected from the Si substrates. The calculated macroscopic charging density is 3–4 times that of the microscopic one, and the possible reason is the underestimation of the microscopic charging density caused by the averaging effect and detection delay in the KPFM measurements.

Use of energy-filtered photoelectron emission microscopy and Kelvin probe force microscopy to visualise work function changes on diamond thin films terminated with oxygen and lithium mono-layers for thermionic energy conversion

Kelvin probe force microscopy (KPFM) and energy-filtered photoelectron emission microscopy (EF-PEEM) with vacuum UV (VUV) excitation have been used to study the work function of p -type diamond films treated to exhibit a negative electron affinity (NEA) surface. NEA was generated by a lithium-oxygen monolayer termination. This monolayer was achieved in two different ways: thermally evaporated films 50 nm thick were either treated by in situ vacuum annealing or by a subsequent water wash. The work function values obtained from these samples by EF-PEEM were compared with KPFM measurements to establish which of the two fabrication techniques was most effective in activating a NEA surface. The washing method was shown to be more effective and the work function values obtained by the two techniques were comparable, as they showed the same work function peaks at 4.54 eV in their respective histograms. It was found that neighbouring polycrystalline facets could show a large variation in work function of up to 400 meV.

Initiation of a polarized state in thin lithium niobate films synthesized on isolated silicon substrates

The distributions of induced state and surface potential in thin lithium niobate films obtained via the deposition onto a (100)-oriented Si-oxidized substrate by the method of high-frequency magnetron sputtering were mapped using piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM). Using an electrical field applied with a conducting cantilever, the possibility of the formation of an induced polarization state and of its subsequent visualization was shown. The induced state in KPFM measurements was shown to be retained for a much longer time than for PFM.

Characterizations of Ohmic and Schottky-behaving contacts of a single ZnO nanowire

Current–voltage and Kelvin probe force microscopy (KPFM) measurements were performed on single ZnO nanowires. Measurements are shown to be strongly correlated with the contact behavior, either Ohmic or diode-like. The ZnO nanowires were obtained by metallo-organic chemical vapor deposition (MOCVD) and contacted using electronic-beam lithography. Depending on the contact geometry, good quality Ohmic contacts (linear I–V behavior) or non-linear (diode-like) contacts were obtained. Current–voltage and KPFM measurements on both types of contacted ZnO nanowires were performed in order to investigate their behavior. A clear correlation could be established between the I–V curve, the electrical potential profile along the device and the nanowire geometry. Some arguments supporting this behavior are given based on technological issues and on depletion region extension. This work will help to better understand the electrical behavior of Ohmic contacts on single ZnO nanowires, for future applications in nanoscale field-effect transistors and nano-photodetectors.

Kelvin probe force microscopy and electrostatic force microscopy responses to the polarization in a ferroelectric thin film: Theoretical and experimental investigations

The ability of non-contact electric modes (Kelvin probe force microscopy (KPFM) and electrostatic force microscopy (EFM)) to provide quantitative information about the remnant polarization in a ferroelectric thin film is theoretically and experimentally investigated. The theoretical relationship between the microscopic signal and the polarization in the film is established: it is linear in the KPFM mode and parabolic in the EFM mode. The KPFM and EFM signals are predicted tip-shape, tip-surface distance, and poled area dimensions dependent so that an absolute value of the polarization cannot be extracted. Nevertheless provided these parameters are held constant, KPFM theoretically enables to investigate quantitatively the polarization dependence on any parameter relative to the film preparation conditions or to the poling process. It does not enable to investigate its dependence on poled area geometry. Based on these conclusions, experimental KPFM investigations have been carried out on a ferroelectric strontium barium niobate (SrxBa1−xNb2O6, SBN:100x) epitaxial thin film deposited on a platinum covered MgO substrate. Poled areas are created in the film by moving the biased tip in contact with the surface. KPFM images are dominated by the written polarization and not by injected or attracted extrinsic charges. The experimental decay of the KPFM signal with increasing tip-surface distance confirms the contribution of the volume of the lever/tip to the KPFM response. The spontaneous evolution of the written polarization as well as the polarization dependence on poling voltage and poling duration are investigated using KPFM, with the poled area dimensions as a parameter. For comparison, the macroscopic remnant polarization in the SBN film has been measured versus poling field from hysteresis cycles drawn using a macroscopic Pt dot as top contact. The significant difference observed between macroscopic and microscopic results can be accounted for by using a simple model of the inhomogeneous electric field generated by a poling tip. KPFM measurements do not alter the initial polarization of the film and can provide essential qualitative and quantitative information on the polarization in a ferroelectric thin film.

Reconstruction of surface potential from Kelvin probe force microscopy images

We present an algorithm for reconstructing a sample surface potential from its Kelvin probe force microscopy (KPFM) image. The measured KPFM image is a weighted average of the surface potential underneath the tip apex due to the long-range electrostatic forces. We model the KPFM measurement by a linear shift-invariant system where the impulse response is the point spread function (PSF). By calculating the PSF of the KPFM probe (tip+cantilever) and using the measured noise statistics, we deconvolve the measured KPFM image to obtain the surface potential of the sample.The reconstruction algorithm is applied to measurements of CdS-PbS nanorods measured in amplitude modulation KPFM (AM-KPFM) and to graphene layers measured in frequency modulation KPFM (FM-KPFM). We show that in the AM-KPFM measurements the averaging effect is substantial, whereas in the FM-KPFM measurements the averaging effect is negligible.

Surface screening of written ferroelectric domains in ambient conditions

We have combined Piezoresponse Force Microscopy and Kelvin Probe Force Microscopy (KPFM) to study screening charge dynamics in written domains on PbZr0.2Ti0.8O3 (PZT) thin film surfaces under a controlled environment and at variable temperature. The screening dynamics decayed exponentially on a timescale of tens of minutes, consistently with what we expected for water-mediated surface diffusion of ionic species. Variable-temperature KPFM measurements showed variations in surface potential due to temporary unbalanced surface screening charges. Low humidity experiments revealed gradual incorporation of positive charges onto the surface, even in a non-reactive environment (N2), as well as deceleration of the screening dynamics upon reversal of the temperature variation. Our work may serve as a guide for future studies on the dynamics and nature of adsorbates on polarized PZT thin films.

Interpreting Kelvin probe force microscopy under an applied electric field: local electronic behavior of vapor–liquid–solid Si nanowires

Kelvin probe force microscopy (KPFM) is used to characterize the electrical characteristics of vapor–liquid–solid (VLS) Si nanowires (NWs) that are grown in-place between two predefined electrodes. KPFM measurements are performed under an applied bias. Besides contact potential differences due to differing materials, the two other primary contributions to measured variations on Si NWs between electrodes are: trapped charges at interfaces, and the parallel and serial capacitance, which are accounted for with voltage normalization and oxide normalization. These two normalization processes alongside finite-element-method simulations are necessary to characterize the bias-dependent response of Si NWs. After applying both normalization methods on open-circuit NWs, which results in a baseline of zero, we conclude that we have accounted for all the major contributions to CPDs and we can isolate effects due to applied bias such as impurity states and charged carrier flow, as well as find open connections when NWs are connected in parallel. These characterization and normalization methods can also be used to determine that the specific contact resistance of electrodes to the NWs is on the order of μΩ cm2. Thus, the VLS growth method between predefined electrodes overcomes the challenge of making low-resistance contacts to nanoscale systems. Thereby, the experiments and analysis presented outline a systematic method for characterizing nanowires in parallel arrays under device operation conditions.

Influence of surface roughness on the electrochemical behavior of carbon steel

The effect of the surface roughness of carbon steel on corrosion properties was investigated using electrochemical tests, and surface and Kelvin probe force microscopy (KPFM) analyses. The results of electrochemical tests show that the corrosion rate of carbon steel is increased as the surface roughness increases. It was estimated using KPFM measurement that the difference in the Volta potential between the peak and the valley increased with increasing surface roughness. As the peak has a lower potential than that of the valley, the peak acts as an anode. The surface roughness affects the Volta potential, and the Volta potential difference is inversely proportional to electron work function (EWF). The larger difference in Volta potential between the peak and valley on the rougher surface and the smaller EWF accelerated the micro-galvanic corrosion between them. The surface analyses reveal that corrosion initiated along the peak lines. The results from this study suggest that an increase of surface roughness leads to a decrease of the corrosion resistance.