W. Brezna

Ultrafast VLS growth of epitaxial β-Ga2O3 nanowires

Well-defined monoclinic nanostructures of beta- Ga(2)O(3) were grown in a chemical vapor deposition apparatus using metallic gallium and oxygen as sources. Stable growth conditions were deduced for nanorods, nanoribbons, nanowires and cones. The types of nanostructures are determined by the growth temperature. We suppose that the vapor-solid growth mechanism rules the growth of nanoribbons and rods. For the nanowires we observed catalytic gold droplets atop, characteristic for the VLS growth mechanism with an extremely high growth rate of up to 10 microm min(-1). Nanowires grown on Al(2)O(3) substrates showed an excellent tendency to grow epitaxially, mapping the hexagonal symmetry of Al(2)O(3)(0001).

Quantitative scanning capacitance spectroscopy

In this work, a setup for quantitative scanning capacitance spectroscopy is introduced, where an ultrahigh precision, calibrated capacitance bridge is used together with a commercially available atomic force microscope (AFM). We show that capacitance data measured with this setup are of comparable quality as data obtained on macroscopic metal oxide semiconductor capacitors. In addition, our setup is sensitive enough to resolve the energy distribution of interface traps with the spatial resolution of an AFM. This is an advantage compared to conventional scanning capacitance microscopes, which have a limited energy resolution and only yield qualitative results due to large modulation voltages.

Charge Transport Through Thin Amorphous Titanium and Tantalum Oxide Layers

Heterosystems of metal/insulator/gold type with titanium oxide and tantalum oxide as internal barriers are studied using internal photoemission (IPE), field induced current transport (current transients after voltage steps) and chemical reaction induced current transport (chemicurrent). IPE investigations over a broad energy range from 0.8 to 4.5 eV allow a determination of the interstitial layers band gap and the maximum height of the internal tunnel barrier. The built-in field of the heterosystem is derived by the evaluation of the slope in the photoyield versus photon energy plot. Current transients recorded after voltage steps allow the determination of the heterosystems time constants which generally have a value of some milli seconds. In titanium oxide systems additional time constants with values of several 100 s appear for bias voltages >0.5 V. These time constants are assigned to slow processes altering the height of the titanium oxide barriers.

Scanning capacitance microscopy with ZrO2 as dielectric material

In this article, we explore the properties of ZrO2 as dielectric material for scanning capacitance microscopy (SCM). The ZrO2 layers were grown by chemical vapor deposition (CVD) at T=450 °C. The low growth temperature together with the good reproducibility of the CVD process and the high dielectric constant make ZrO2 a very promising material for SCM applications. Compared with SiO2 as dielectric material, much thicker ZrO2 layers can be used resulting in reduced leakage currents and improved signal quality. For SiO2 and ZrO2 layers having the same thickness, the latter yields higher signals and therefore an enhanced sensitivity. Furthermore, ZrO2 was found to be quite insensitive to parasitic charging effects, which often disturb SCM measurements on samples with SiO2 layers.

Tip geometry effects in scanning capacitance microscopy on GaAs Schottky and metal-oxide-semiconductor-type junctions

In this work, the influence of the tip geometry 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. In contrast to that, the electric field from the tip penetrates into the semiconductor on a MOS-type junction, 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 further calibration measurements.

An intercepted feedback mode for light sensitive spectroscopic measurements in atomic force microscopy

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.

Mapping of local oxide properties by quantitative scanning capacitance spectroscopy

In this work, quantitative scanning capacitance spectroscopy was applied to investigate the local dielectric properties of a chemical vapor deposition grown ZrO2 layer on low-doped silicon. Due to self-organization effects during the growth process, the ZrO2 layer shows small, periodic thickness variations on micrometer length scales near the sample edges. The measured capacitance data and derived oxide charge densities show the same periodicity as the thickness variations. The magnitude of the change of the oxide charge density, however, cannot be explained by the small thickness variations and is attributed to a local periodic change of the growth dynamics.

High resolution photocurrent imaging by atomic force microscopy on the example of single buried InAs quantum dots

Conductive atomic force microscopy in combination with an optical setup is a useful tool for obtaining spectrally and spatially resolved photocurrent information on photoactive nanostructures. We have demonstrated photocurrent mapping under ambient conditions using sub-surface InAs quantum dots (QDs) as an example, whereby the QDs appear as dark areas in the photocurrent signal. Spectrally resolved measurements of on-dot and off-dot areas show distinct differences, confirming the existence of QDs at the detected position.

Investigation of contact-force dependent effects in conductive atomic force microscopy on Si and GaAs

In this work, we systematically investigated the force dependence of conductive atomic force microscopy on n-type Si and n-type GaAs. IV curves were recorded under different tip sample forces and the corresponding Schottky barrier heights were extracted. The force dependent barrier heights of the Si and GaAs samples showed distinct reproducible differences. At low forces, the Schottky barriers decrease with increasing force because the native oxide layer on both the Si and the GaAs sample surface has to be penetrated. At intermediate forces on the GaAs sample, the Schottky barrier showed a large forced dependent increase due to the pressure coefficient of the GaAs band gap. Furthermore, a sudden change in the Schottky barrier increase with tip force was interpreted as the Γ‐X band crossover in GaAs.

Scanning Capacitance Microscopy Investigations of Focused Ion Beam Damage in Silicon

Abstract In this article we explore the application of scanning capacitance microscopy (SCM) for studying focused ion beam (FIB) induced damage in silicon. We qualitatively determine the technologically important beam shape by measuring the SCM image of FIB processed implantation spots and by comparison of topographical and SCM data. Further, we investigate the question how deep impinging ions generate measurable damage below the silicon surface. For this purpose, trenches were manufactured using FIB and analyzed by SCM in cross-sectional geometry.