In most atomic force microscopes (AFMs), the motion of the tip is detected by the deflection of a laser beam shining onto the cantilever. AFM applications such as scanning capacitance spectroscopy or ...photocurrent spectroscopy, however, are severely disturbed by the intense stray light of the AFM laser. For this reason, an intercepted feedback method was developed, which allows to switch off the laser temporarily while the feedback loop keeps running. The versatility of this feedback method is demonstrated by measuring tip-force dependent Schottky barrier heights on GaAs samples.
In this work, we introduce a two color, low intensity photocurrent feedback method for photocurrent spectroscopy utilizing an atomic force microscope (AFM). In most applications, measurements with ...weak optical excitations are not feasible with an AFM because the powerful AFM feedback laser severely disturbs the measurements. Therefore, we have developed a feedback system based on the pressure dependent Schottky barrier height at the tip-sample interface. The versatility of the new feedback system is demonstrated by recording high resolution photocurrent spectra on GaAsInAs heterostructures.
In this work, the influence of the tip-geometry and an unusual low frequency behavior in scanning capacitance microscopy is investigated experimentally and theoretically on metal–oxide–semiconductor ...(MOS) and Schottky type junctions on gallium–arsenide (GaAs). Using a two-dimensional model we find that on Schottky type junctions the electric field around the tip is screened by the surface states and that the essential parameters entering the capacitance versus voltage
C(
V) characteristics are the doping level and the contact area only. On a MOS-type junction, the electric field from the tip penetrates into the semiconductor and the tip geometry effects are much larger.
C(
V) spectra are fitted to the experimental data and allowed a quantitative determination of doping levels, oxide thickness, and contact area without moreover calibration measurements. Furthermore, we show that the natural surface depletion field on GaAs generates a permanent minority carrier accumulation around the tip–apex. In contrast to thermally generated minority carriers in the space charge region of a large area device, the minority carrier distribution around the SCM tip–apex is detected up to much higher excitation frequencies. An analytic approach to estimate the transition frequency between the low and high frequency regime is also given.
There is an increasing interest in cell-based microelectronic biosensors for high-throughput screening of new products from the biotech pipeline. This requires fundamental knowledge of the ...biocompatibility of the materials used as the growing support for the cells. Using monolayer-forming Caco-2 cells of human origin, the biocompatibility of silicon wafers coated with various metals, dielectrics and semiconductors was assessed. Besides microscopic inspection, proliferation of cells indicating viability as well as brush border enzyme activity indicating differentiation of adherent growing cells were chosen as parameters to estimate biocompatibility. The type of wafer used for deposition of the coating initially influences the biocompatibility of the final product. Whereas p-doped silicon was fully biocompatible, n-doped silicon reduced the proliferation of cells. Among the different coatings, Al and Ti even increased the cell growth as compared to glass. Culturing the cells for 6
days on coated wafers demonstrated that the differentiation of adhering cells on Ti- and ZrO
2-coated wafers was comparable to glass, whereas coatings with Si
3N
4, Au, Al, and ITO reduced differentiation to 15–35%. In the cases of Au and Si
3N
4 this effect equilibrated with prolonged culturing. These results demonstrate the importance of a careful selection of the materials used for the production of cell-based biosensors.
In this paper we compare macroscopic C-V-measurements with (local) scanning capacitance (SCM) measurements to extract electrical parameters. In future microelectronic CMOS devices, high-kappa ...dielectrics, primarily as gate oxides, will play an important role. The characterization of their electrical behaviour is one key issue to evaluate their suitability. We deposited zirconium dioxide (ZrO2) thin films by metal-organic chemical vapor deposition (MOCVD) on (100)-silicon substrates to fabricate metal oxide semiconductor gate stacks. These devices were evaluated by conventional C(V) characterization using a probing station. Additionally, we applied a new C(V)-characterization method, allowing to determine electrical characteristics of nm-thick dielectric films on a nanoscopic scale. This method enabled us to directly compare the nanoscopic and macroscopic measurements. As a result, local and global measurements turned out to be consistent. A comparison of our ZrO2high-kappa layers with thermal SiO2 films indicate a superior behaviour of MOCVD grown ZrsO2 film.
The fabrication of nanostructured dielectrics of high purity produced by electron beam induced deposition (EBID) was investigated. Additionally, the process parameters were optimized towards high ...deposition rate. Silicon oxide was chosen as exemplary dielectric. The deposition was performed with an electron microscope with a self-designed nozzle system. An organo-silicon precursor was used to deposit silicon oxide. The increase of the oxygen content and the reduction of carbon contaminations of the depositions were preformed by adding molecular oxygen. Patterns of large areas as well as nm-scale arrays were fabricated. To optimize the deposition rate the correlation between beam parameters and deposition rate was studied. It has been shown that this direct-write method allows obtaining arbitrary structures with superior material quality.
Scanning capacitance microscopy (SCM) is known to be a valuable tool for carrier mapping and profiling on nanoscale semiconductor samples. Certain applications, however, such as quantitative ...capacitance microscopy on InAs quantum dots, e.g. require low modulation frequencies and complete darkness, which are requirements completely incompatible with the current commercial SCM systems relying on a laser feedback system. For this reason, an intercepted feedback method was developed, which allows to switch off the laser temporarily while the feedback loop keeps running. As an application, images of sub surface InAs self assembled quantum dots were recorded. The InAs dots are clearly visible as bright areas in a contrast-rich capacitance landscape, which we attribute to local thickness variations of the InAs wetting layer in our sample.
Photocurrent (PC) spectroscopy is employed to study the carrier escape from self-assembled InAs/GaAs quantum dots (QDs) embedded in a Schottky photodiode structure. As a function of the applied ...field, we detect a shift of the exciton ground-state transition due to the quantum-confined Stark effect (
S
=
4.3
meV
/
V
). The tunneling time, which is directly related to the observed photocurrent linewidth due to
τ
∼
ℏ
/
(
2
Γ
)
, changes by a factor of five in the photocurrent regime. The measured linewidth dependency on the electric field is modeled by a simple 1D WKB approximation for the tunneling process, which shows that the energetic position of the wetting layer is important for the measured tunneling time out of the dot. In addition to that we present cross-sectional atomic force measurements (AFM) of the investigated photodiode structure. The method needs a minimum of time and sample preparation (cleaving and etching) to obtain the dot density, dot distribution, and give an estimate of the dot dimensions. Etching only the cleaved surface of the sample opens up the opportunity to determine the properties of a buried dot layer before or even after device fabrication.
PDF