Papers by Keyword: Conductive AFM

Abstract: In this paper, we report on the growth of highly uniform MoS₂ films, mostly consisting of monolayers, on SiC surfaces with different doping levels (n- SiC epitaxy, ~10¹⁶ cm^-3, and n⁺ SiC substrate, ~10¹⁹ cm^-3) by sulfurization of a pre-deposited ultra-thin MoO_x films. MoS₂ layers are lowly strained (~0.12% tensile strain) and highly p-type doped (<N_h>≈4×10¹⁹ cm⁻³), due to MoO₃ residues still present after the sulfurization process. Nanoscale resolution I-V analyses by conductive atomic force microscopy (C-AFM) show a strongly rectifying behavior for MoS₂ junction with n^- SiC, whereas the p⁺ MoS₂/n⁺ SiC junction exhibits an enhanced reverse current and a negative differential behavior under forward bias. This latter observation, indicating the occurrence of band-to-band-tunneling from the occupied states of n⁺ SiC conduction band to the empty states of p⁺ MoS₂ valence band, is a confirmation of the very sharp hetero-interface between the two materials. These results pave the way to the fabrication of ultra-fast switching Esaki diodes on 4H-SiC.

Abstract: The electrical properties of the interface between quasi free standing bilayer graphene (QFBLG) and SiC(0001) have been investigated by nanoscale resolution current measurements using conductive atomic force microscopy (CAFM). I-V analyses were carried out on Au-capped QFBLG contacts with different sizes (from 200 down to 0.5 μm) fabricated on SiC samples with different miscut angles (from on-axis to 3.5° off-axis). The extracted QFBLG/SiC Schottky barrier height (SBH) was found to depend on the contact size. SBH values ∼0.9-1 eV were obtained for large contacts, whereas a gradual increase was observed below a critical (micrometer scale) contact size (depending on the SiC miscut angle) up to values approaching ∼1.5 eV. Nanoscale resolution current mapping on bare QFLBG contacts revealed that SiC step edges and facets represent preferential current paths causing the effective SBH lowering for larger contacts. The reduced barrier height in these regions can be explained in terms of a reduced doping of QFBLG from SiC substrate at (11-20) step edges with respect to the p-type doping on the (0001) terraces.

Abstract: A nanoscale electrical characterization of graphene (Gr) contacts to Al_xGa_1-xN/GaN heterostructures has been carried out using conductive atomic force microscopy. The impact of the AlGaN microstructure on the current transport at Gr/AlGaN interface was evaluated considering two Al_0.25Ga_0.75N/GaN heterostructures with very different quality in terms of surface roughness and defectivity, i.e. a uniform and defect-free sample and a sample with a high density of V-defects, that locally cause a reduction of the AlGaN thickness. Rectifying contacts were found on the bare (Gr-free) AlGaN surfaces of both samples, but with a more inhomogeneous and lower Schottky barrier height (Φ_B≈0.6 eV) in the presence of V-defects with respect to the case of the uniform AlGaN (Φ_B≈0.9 eV). Very different electrical behaviour was observed for Gr on the two AlGaN samples, i.e. a low barrier height Schottky contact (Φ_B≈0.4 eV) for the uniform AlGaN and an Ohmic contact for the defective AlGaN. Both Schottky and ohmic Gr/AlGaN contacts exhibit an excellent lateral uniformity, that can be ascribed to an averaging effect of the Gr electrode over the AlGaN interfacial inhomogeneities.

Abstract: Utilizing semiconductor nanowires for optoelectronics device requires exact knowledge of their current-voltage properties. In this report, we examine accurate on-top imaging and I-V characterization of individual vertical Gallium Arsenide Nanowires (GaAs NWs) using conductive atomic force microscopy without additional microscopy tools, thus allowing versatile application. The measured current-voltage characteristic of a single NW shows the typical performance of a Schottky contact, which caused by the contact between the metallic AFM tip and the top of NWs. The height of the Schottky barrier is dependent on the diameter of the nanowires. The linear part of the curve was used to calculate the differential resistance, which was found to be about 25 to 100 MΩ. Energy band gap for GaAs NW was found to be 1.5 eV by differential conductivity measurement.

Abstract: Epitaxial graphene fabricated by thermal decomposition of the Si-face of silicon carbide (SiC) forms a defined interface to the SiC substrate. As-grown monolayer graphene with buffer layer establishes an ohmic interface even to low-doped (e. g. [N] ≈ 10¹⁵ cm^-3) SiC, and a specific contact resistance as low as ρ_C = 5.9×10^-6 Ωcm² can be achieved on highly n-doped SiC layers. After hydrogen intercalation of monolayer graphene, the so-called quasi-freestanding graphene forms a Schottky contact to n-type SiC with a Schottky barrier height of 1.5 eV as determined from C-V analysis and core level photoelectron spectroscopy (XPS). This value, however, strongly deviates from the respective value of less than 1 eV determined from I-V measurements. It was found from conductive atomic force microscopy (C-AFM) that the Schottky barrier is locally lowered on other crystal facets located at substrate step edges. For very small Schottky contacts, the barrier height extracted from I-V curves approaches the value of 1.5 eV from C-V and XPS.

Abstract: We report on the influence of titanium thickness on the structural and electrical properties of annealed Ti/Ni ohmic contacts on highly doped n-type 3C-SiC. Electrical analysis by means of circular transfer length method demonstrate that an interlayer of titanium with thickness in the range of 25-150 nm has no significant influence on specific contact resistance. However, from a structural point of view, the formation of nickel silicides as well as Ti₃SiC₂ is severely affected by the titanium thickness. Moreover, the Kirkendall effect due to the reaction between Ni and SiC is influenced by the titanium thickness. In fact, Scanning Electron Microscopy analysis demonstrates that the adjunction of titanium affects the distribution of Kirkendall voids in the contact. Current maps determined by conductive Atomic Force Microscopy reveal significant variation of uniformity according to the titanium thickness.

Abstract: Conductive AFM and in situ methods were used to determine contact resistance and resistivity of individual Sb2S3 nanowires. Nanowires were deposited on oxidized Si surface for in situ measurements and on Si surface with macroelectrodes for conductive AFM (C-AFM) measurements. Contact resistance was determined by measurement of I(V) characteristics at different distances from the nanowire contact with the macroelectrode and resistivity of nanowires was determined. Sb2S3 is a soft material with low adhesion force to the surface and therefore special precautions were taken during measurements.

Abstract: Defects in cubic silicon carbide (3C-SiC) epilayers, that were grown using different techniques and on different substrates, were studied in terms of electrical activity and device limiting implications. An electrical characterization by conductive atomic force microscopy (C-AFM) showed that stacking faults (SFs) are normally the predominant type of defects that are electrically active at the semiconductor surface and, therefore, the most important defects that can affect the contact properties on these epilayers. It is also shown that an ultraviolet (UV) irradiation process can be used to suppress detrimental leakage currents passing through SFs that are carbon terminated at the semiconductor surface. Indeed, current-voltage characterization of Au/3C-SiC diodes showed a subsequent improvement of the Schottky behavior.

Abstract: The electrical current-voltage (I-V) and capacitance-voltage (C-V) characteristics of Au/3C-SiC Schottky diodes were studied as a function of contact area. The results were correlated to defects in the 3C-SiC, which were studied and quantified by conductive atomic force microscopy (C-AFM). A method based on C-AFM was introduced that enables current-voltage characterization of diodes of contact radius down to 5 µm, which consequently allows the extraction of diode parameters for Schottky diodes of very small contact area.

Abstract: This paper reports on the macro- and nanoscale electro-structural evolution, as a function of annealing temperature, of nickel-silicide Ohmic contacts to 3C-SiC, grown on 6H-SiC substrates by a Vapor-Liquid-Solid (VLS) technique. The structural and electrical characterization of the contacts, carried out by combining different techniques, showed a correlation between the annealing temperature and the electrical characteristics in both the macro- and the nanoscale measurements. Increasing the annealing temperature between 600 and 950 °C caused a gradual increase of the uniformity of the nanoscale current-distribution, with an accompanying reduction of the specific contact resistance from 5 x 10-5 to 8.4 x 10-6 Ωcm2. After high temperature annealing (950 °C) the structural composition of the contacts stabilized, as only the Ni2Si phase was detected. A comparison with previous literature findings suggests a superior crystalline quality of the single domain VLS 3C-SiC layers.