Surface potential of chalcopyrite films measured by KPFM

Atomic force microscopy is widely used to characterize the surface topography of a variety of samples. Kelvin probe force microscopy KPFM additionally allows determining images of the surface potential with nanometer resolution. The KPFM technique will be introduced and studies on surfaces of chalcopyrite semiconductors for solar cell absorbers will be presented. It is shown that operation in ultra high vacuum UHV is required to obtain meaningful work function values. Different methods for obtaining UHV clean surfaces are presented and KPFM studies on these are compared. Surfaces where prepared by in vacuum deposition, inert gas transfer, in vacuum decapping of a protective Se cap and a peel off method. Finally, a sputter annealing cycle also allows to obtain well suited surfaces for KPFM studies. Employing KPFM, variations in the local surface potential at grain boundaries of polycrystalline CuGaSe2 films were observed. A potential drop indicates the presence of charged defects at grain boundaries. Furthermore, different electronic activity was found for different grain boundaries, as concluded from studies under illumination. Using laterally resolved surface photovoltage, a Cu2 amp; 8722;xSe impurity phase could be observed in CuGaSe

Microscopic characterization of individual grain boundaries in Cu III VI2 chalcopyrites

The role of grain boundaries in polycrystalline Cu III VI2 absorber material for thin film photovoltaics has not been fully understood and is currently under discussion. Recently, intensive efforts have been devoted to the characterization of the properties of individual grain boundaries using microscopic techniques, including Kelvin probe force microscopy KPFM . KPFM provides local electronic information by measuring the surface potential in addition to the topography. We introduce the KPFM method and present simulations assessing the technique s limitations with respect to spatial resolution regarding the measurement of grain boundary properties. KPFM studies of individual GBs in the Cu In,Ga Se2 materials system are reviewed and critically discussed, considering also results from other microscopic characterization technique

The Reliability of Selected Weather Beliefs

Thirty Kentucky weather beliefs--twenty of which were considered to be scientifically valid while ten were not--were annotated, documented and discussed. It was shown that the weather beliefs which had scientific explanations were usually concerned with forecasts associated with rain and that most often they were based on observable atmospheric conditions. It was further shown that those beliefs which were not considered scientifically valid usually had an element of truth but were not considered plausible for one of the following reasons. First, there was evidence that some of these weather beliefs had been garbled during the process of oral transmission. Second, some segments of these beliefs were found to be either reversed or the sequence of events had been mixed in such a way that inconsistencies resulted. Third, some of the beliefs were found to be the object of cultural transference. Thus they had been geographically displaced and were no longer applicable

MOS based nanocapacitor using C-AFM

Nanocapacitors are integral devices of nanoscale MOS based integrated circuits and have not yet been realised. We report in this article our results to date on the realisation of such a nanocapacitor through the use of Atomic Force Microscopy (AFM) anodic oxidation to isolate nano-sized squares of poly-silicon, titanium and aluminium on Si/SiO2. The focus of this work is on the Conductive AFM performed topographical and electrical characterization

Special cantilever geometry for the access of higher oscillation modes in atomic force microscopy

Employing higher oscillation modes of microcantilevers promises higher sensitivity when applied as sensors, for example, for mass detection or in atomic force microscopy. Introducing a special cantilever geometry, we show that the relation between the resonance frequencies of the first and second resonance modes can be modified to separate them further or to bring them closer together. In atomic force microscopy the latter is of special interest as the photodiode of the beam deflection detection limits the accessible frequency range. Using finite element simulations, we optimized the design of the modified cantilever geometry for a maximum reduction of the frequency of the second oscillation mode with respect to the first mode. Cantilevers were fabricated by silicon micromachining and subsequently utilized in an ultrahigh vacuum Kelvin probe force microscope imaging the surface potential of C60 on graphit

Evaluation of Kelvin probe force microscopy for imaging grain boundaries in chalcopyrite thin films

In view of the outstanding performance of polycrystalline thin film solar cells on the basis of Cu In,Ga Se2, the electrical activity at grain boundaries currently receives considerable attention. Recently, Kelvin probe force microscopy KPFM has been applied to characterize of the properties of individual grain boundaries, observing a drop in the surface potential in many cases. We present finite element simulations of the electrostatic forces to assess the experimental resolution of KPFM. Depending on the tip sample distance, the observed drop in the work function amounts to only a fraction of the real surface potential drop. The simulations are considered for different grain boundary models and consequences for the quantitative evaluation of experimental results are discusse

Electrostatic potentials at Cu In,Ga Se2 grain boundaries Experiment and simulations

In the present Letter, we report on a combined ab initio density functional theory calculation, multislice simulation, and electron holography study, performed on a amp; 931;9 grain boundary GB in a CuGaSe2 bicrystal, which exhibits a lower symmetry compared with highly symmetric amp; 931; 3 GBs. We find an electrostatic potential well at the amp; 931;9 GB of 0.8 V in depth and 1.3 nm in width, which in comparison with results from amp; 931;3 and random GBs exhibits the trend of increasing potential well depths with lower symmetry. The presence of this potential well at the amp; 931; 9 GB can be explained conclusively by a reduced density of atoms at the GB. Considering experimental limitations in resolution, we demonstrate quantitative agreement of experiment and theor