P. G. Schroeder

Energy level alignment and two-dimensional structure of pentacene on Au(111) surfaces

X-ray photoemission, ultraviolet photoemission spectroscopy (UPS), and scanning tunneling microscopy (STM) have been used to determine the energy level alignment and the molecular ordering of monolayer and submonolayer pentacene films on Au(111) in ultrahigh vacuum. Pentacene evaporated onto the van der Waals surface of SnS2 was used as a noninteracting substrate for comparison. A large interface dipole was measured for pentacene on Au(111) (0.95 eV) whereas pentacene on SnS2 showed a relatively small interface dipole (0.26 eV). The different interface dipoles are related to the different orientations of the pentacene molecules due to different pentacene substrate interaction energies. Differences in the UPS spectra also support changing molecular orientations of the two substrates. STM images of pentacene on Au(111) revealed that the molecules lay flat on the substrate and are oriented parallel to each other, forming striped structures that are commensurate with the Au(111) lattice. The pentacene coverag...

Observation of strong band bending in perylene tetracarboxylic dianhydride thin films grown on SnS2

Perylene tetracarboxylic dianhydride (PTCDA) thin films were grown in several steps on tin disulfide (SnS2) single crystals and characterized by combined x-ray and ultraviolet photoemission spectroscopy (XPS), (UPS) in order to characterize the frontier orbital line-up and the interface dipole at their interface. Due to the large difference between the work functions of PTCDA (4.26 eV) and SnS2 (5.09 eV) this experiment represents a model system for the investigation of band bending related phenomena in organic semiconductor heterojunctions. Our results show that the equilibration between the Fermi levels of both materials in contact is achieved almost solely by band bending (bulk charge redistribution) in the PTCDA layer. No significant interface dipole was detected which means that the PTCDA molecular orbitals and the SnS2 bands align at the vacuum level corresponding to the electron affinity rule. Our experiments clearly demonstrate the importance of an additional XPS measurement which (in most cases) ...

Orbital alignment at p-sexiphenyl and coronene/layered materials interfaces measured with photoemission spectroscopy

The energy level alignment at the interfaces between para-sexiphenyl/highly oriented pyrolytic graphite (HOPG), coronene/SnS2, and coronene/HOPG were determined using in situ thin film deposition in combination with x-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS) measurements. The organic thin films were grown in multiple steps by vapor deposition, then sequentially characterized in situ after each growth step. The vacuum cleaved single crystals of SnS2 and HOPG substrates provided clean, atomically flat, and chemically inert surfaces, allowing for the investigation of the phenomena of band bending and interface dipoles without the interference of chemical reactions or morphological problems. Due to the distinctly different work functions of the HOPG (Φ=4.65 eV) and SnS2 (Φ=5.45 eV) substrates, the observed shifts in the binding energies of the organic overlayer related XPS core level emission lines could be associated with band bending resulting from Fermi level eq...

Two-dimensional dopant profiling of patterned Si wafers using phase imaging tapping mode atomic force microscopy with applied biases

Tapping mode atomic force microscopy with applied bias was used to spatially resolve areas of different doping type on Si wafers patterned with photolithography and subsequent ion implantation. The application of a direct current bias between cantilever and sample during the measurement produces Coulomb (electrostatic) forces, whose magnitude depends on the spatial variation of the doping density. This effect was utilized to detect areas of different doping type by monitoring the phase angle between the driving frequency and the cantilever response while scanning areas of different doping density. In this article we present a series of measurements at various bias voltages demonstrating that the observed phase contrast between differently doped areas is directly connected to the bias induced surface potential (band bending) present on these areas. To investigate the contrast mechanism quantitatively, we also measured deflection (force), amplitude and phase versus distance curves for a typical cantilever w...

Two‐Dimensional Dopant Profiling of an Integrated Circuit Using Bias‐Applied Phase‐Imaging Tapping Mode Atomic Force Microscopy

Tapping mode atomic force microscopy was used to spatially resolve areas of different doping type and density on a static rando m access memory integrated circuit. The application of a dc bias applied between cantilever and sample during imaging results in a change in the tapping-mode phase contrast that depends on the doping density of the imaged area. Our experiments demonstrated that this method allows for distinguishing between p- and n-doped areas as well as distinguishing between regions of doping den sities ranging from 1016 to 1020 cm-3. Methods for characterizing doping patterns in submicrometerpatterned semiconductor circuits are becoming increasingly important as device structures continue to shrink. During the last several years, a variety of techniques for two-dimensional doping profiling were introduced. Currently, scanning capacitance microscopy (SCM), scanning Kelvin force microscopy (SKFM), and nanospreading resistance probe (nano-SRP)1-5 show the most promise among these techniques for fulfilling the requirements specified by the National Roadmap for Semiconductor Technology (NRST).6 These requirements include: 20 nm spatial resolution, 10% uncertainty in the dopant concentration determination and a sensitivity range for dopant concentrations from 1 x 1014 to 1 x 1020 cm-3. Furthermore, it is desirable that the measurements are reproducible and nondestructive. The above mentioned scanning probe methods are at least partially capable of fulfilling these specifications, however each technique falls short for at least one of the requirements in the NRST. The greatest problems with SCM and SKFM are associated with inconsistencies in sample preparation. In the case of nano-SRP, the measurement results in damage to the sample surface.7 Recently, we introduced a new tapping mode atomic force microscopy (TMAFM)-based technique that can laterally resolve regions of varying dopant density and type. 8,9 This method uses TMAFM with an additionally applied dc bias between cantilever and sample. Depending on its polarity and magnitude, the bias introduces Coulomb forces between the cantilever and the sample surface. The relative strength of these forces is a function of the doping density. These minute variations in the Coulomb forces can be monitored as a change in the phase of the cantilever oscillation relative to the cantilever driving frequency (TMAFM uses a cantilever excited into resonance with an oscillating piezoelectric driver for the imaging process).10 Topographic and doping level dependent phase images can be acquired simultaneously with this method by operating the microscope in the so-called interleave lift mode in which each line in an image is scanned twice. The first scan produces a standard TMAFM height image where the amplitude of the cantilever oscillation is monitored as a feedback signal. The second scan retraces the same line at a user defined height above the sample following the previously determined topography profile. This procedure results in the tip sample distance remaining constant during the second scan when the doping dependent phase image is measured. This procedure assures that the measured phase contrast depends exclusively on the electronic properties of the sample surface and avoids distance dependent Coulomb forces. It also eliminates short range dispersion forces that can dominate the tip-surface interactions at distances near contact.

Spatially resolved dopant profiling of patterned Si wafers by bias-applied phase-imaging tapping-mode atomic force microscopy

Tapping-mode atomic force microscopy was used to spatially resolve areas of different doping types on Si wafers patterned by photolithography and subsequent ion implantation. Application of a direct current dc bias between cantilever and sample during measurement induced a change in the tapping-mode phase contrast depending on the dopant type of the scanned sample area. This allowed the direct identification of areas of different doping types. Additional measurements on Au samples demonstrate a direct correlation between bias-induced Coulomb force and resulting phase change allowing the conclusion that the observed phase contrast results from dc bias-induced band bending changes.

Determination of the electronic structure of organic Schottky contacts by photoemission spectroscopy

The alignment of the highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO) of the organic luminescent semiconductor Gaq3 relative to the Fermi level of Au was determined by depositing a Gaq3 thin film in a multi-step growth procedure on an Au foil. Before growth and after each deposition step the sample was characterized by combined x-ray and ultraviolet photoemission spectroscopy (XPS, UPS) measurements. Such measurements offer a direct way to determine the electronic structure of the interface. Our experiments demonstrate that this method allows distinguishing between band bending, charging and interface dipole related shifts in the UP-spectra. The additional XPS measurements allow the precise determination of the band bending occurring across the interface while comparison between XPS and UPS work function measurements allows one to pinpoint the organic film thickness dependent onset of charging phenomena. Our results show that the interface dipoles at Gaq3 Schottky contacts with Au, Pt and Ag amount to 0.6 - 0.7 eV. Our experiments also show that final state screening shifts can be dismissed as insignificant in such orbital line-up measurements. This was shown at the chloroindium phthalocyanine (ClInPc)/highly oriented pyrolytic graphite (HOPG) interface where no such shifts were observed.

Influence of electrostatic forces on the imaging process in scanning tunneling microscopy

Abstract We performed scanning tunneling microscopy (STM) experiments on layered semiconductor compound surfaces which suggest a significant influence of electrostatic forces (EF) on the imaging process. We performed STM experiments at varying tunneling biases on plain MoTe 2 and ultrathin epitaxial WS 2 films on MoTe 2 substrates. We observed tunneling bias dependent height changes of up to several nanometers of layer edges and layer terraces. Both of the samples showed height increases of up to 8 nm for step edges depending on the tunneling bias. Only at certain biases the values known from X-ray diffraction for the layer thickness were approached. These observations cannot be explained solely by electronic effects and tunneling probability changes. Our evaluation of the results shows that mechanical changes of the morphology caused by varying EF interaction between tip and sample are likely to be the cause of these phenomena. In order to investigate the magnitude and influence of EF between tip and sample more closely we performed additional experiments with bias applied atomic force microscopy (BAAFM) which indicate a strong influence of EF on the imaging process. Atomic force curves with additionally applied bias yielded that the EF are in the range of several ten to several hundred nN depending on the tunneling bias. The corresponding charge density on tip and sample suggests the presence of single electrons and holes at the interface instead of homogeneous charge densities which might result in a pulsed EF interaction between tip and sample.