The members of the JOB group, from 1985 to now, have all contributed in one way or another to this thesis. I also appreciate the efforts of the rest of my committee - Jack Beauchamp and Bill Goddard. Special thanks are due to Guy Duremburg, Tony Stark and Ray Garcia, who spent countless hours grinding, drilling and advising on both the construction of the atmospheric STM and the many heater iterations, to Tom Dunn for invaluable electronic advice.
Technical assistance was provided by Pat Koen of the Caltech Electron Microscope Facility, Carol Garland of the Caltech Material Sciences Department, and J. All XPS work in Chapter 4 was graciously provided by Richard Vasquez at the Jet Propulsion Laboratory and Ion Microprobe work was performed by Robert Wilson from Hughes Research Laboratories. This model of point-contact imaging provides an explanation for increased stability of the STM system, a mechanism for producing multiple tips or sliding graphite planes, and an explanation for the observed anomalies.
The results of radiatively and resistively heated samples are shown in addition to the comparison of topographic images and barrier height images of pine (-/3 x -/3) reconstructed surfaces. In ultrahigh vacuum, band bending is observed, but the strength of the electric field experienced by the diamond semiconductor is less than expected; the introduction of surface charges is shown to account for the field.
In trod uction
A careful analysis of the results is necessary to determine any ambiguities in the STM data obtained. The exquisite resolution of the technique is due to the exponential dependence of the tunnel current on the separation between the tip and the sample;. Since the current is exponentially dependent on the distance away from the surface, this measurement is very sensitive and provides a picture of the surface topography.
From the basic theory of equation I, it is clear that din I / d z is proportional to the square root of the height of the barrier. The STM technique requires a measurement of a change in tunnel current for a given, small modulation distance of the tip-sample separation. Further discussions on barrier height measurements are presented by Binnig and Rohrer9 and in the context of Chapter 3.
Spectroscopy with STM is often performed to further investigate the nature of the surface under investigation. Furthermore, the observation of the R300 ({3 x {3) surface reconstruction of boron on Si(111) is also presented as both topographic images and barrier height images.
Imaging and Spectroscopy of Graphite in Air by Scanning
Tip sharpness is believed to be one of the factors affecting STM7 image resolution. Further studies on the role of the tip in HOPG imaging are described in this chapter. Analyzes of the composition and structure of the spikes were performed to characterize them.
The end of the tip is quite blunt (figure 1 inset) and many times the diameter of electrochemically sharpened tungsten tips (see figure 3 inset). The composition of the pencil lead surface as determined by x-ray photoelectron spectroscopy (XPS) is given in Table I. The graphite colloidal coating at the bottom of the tip appears relatively featureless and much smoother than the pencil lead surface (compare Figs. 1 and 3 -- note the equivalent scales).
The tip position is reported as ~z relative to the point at which the tunnel is first detected (Ro=inf). The imaging in this regime is surprising because at least part of the tip is in ohmic contact with the surface.
Preparation and Analysis of the Silicon (111) Surface
Variables affecting the nature of surface cleanliness and surface saturation by boron atoms migrating from the bulk of the sample will also be discussed. Many discussions have been published on the stability of the (7x7) surface and the similar (SxS) and (9x9) surfaces 15. The sample deviation is obtained by contact with a sample stub that fits into a hole in the tongue of the sample. louse.
Second, the heating system must have a separate base and plug design that allows the stub to be transferred from the heater in the sample preparation chamber to the lice in the STM chamber. The surface temperature of the silicon could not be observed due to the relative brightness of the filament. Thus, the equilibrium heat transfer to and from the sample is Qs(in) = Qs(out).
The actual parameters used correspond to the experimental parameters of the heater constructed and schematically diagrammed in Figure 1. This arrangement facilitates the tip resting on the holder while tightening any clamping mechanism. The filament temperature calibration data as a function of filament power is shown in Figure 6 .
Furthermore, even if the sample has become clean and reconstruction has occurred, contamination of the sample after transfer from the sample preparation chamber to the STM chamber is likely, as will be discussed in more detail in the next section. As explained in the Introduction (Chapter 1), the barrier height image contains less noise than the corresponding topographic image due to the nature of the data collection. Silicon has a lower localized charge density than the boron atom, leaving more delocalized electrons in the dangling bond of boron-free sites.
These lines may be an artifact of heating because the sample often has "hot spots". With both valves closed, the STM chamber pressure is routinely less than 2 x 10-10 torr. If LEED or Auger analyzes were available in the sample preparation chamber, it would be possible to determine the state of cleanliness of the silicon surface after baking but before transfer.
The sample loading area is left open so that the filament temperature can be monitored by optical pyrometry. Since the sample and filament temperatures are raised to the fourth power, this small area of the curve appears linear.
Topology, Composition and Spectroscopy of Synthetic
The results of XPS in the ESC A mode (electron spectroscopy for chemical analysis) are summarized in Table IV. The oxygen peak observed in the XPS peaks obscured the direct identification of the Fermi energy. Furthermore, the exponential nature of the IV curves suggests that tunneling is the major current contribution.
Analysis of the step shown in Figure 6 shows a height of -2 A and is assumed to be a single step because the interplanar spacing on the (111) surface is 2.1 A 18. Unfortunately, Hall measurements cannot be obtained for diamond due to of its irregular geometry. The presence of continuous oxygen at the surface is thought to be responsible for some of the charge control observed in vacuum spectroscopy.
The diamond spectroscopy observed at atmospheric conditions does not show an exponential dependence of the current on the voltage. Surface conductivity and metal impurities near the surface can have a significant effect on the heterogeneous nature of the local electrical properties. The tip was tungsten, and electrical contact was made with the stub and the top of the diamond with indium.
The suppression of the transmission of these vibrations is accomplished by disconnecting the STM from it. Kuk7 has shown, using FIM (field ion microscope) imaging of STM tips, that the vertical resolution also depends on the size of the tip array. The intensity of the signal on the oscilloscope depends on the magnitude of the received voltage.
Bumps of the expected size for the HSA + anti-HSA complex are observed on the surface. After closer examination of the lump, the features of the complex can be better distinguished. Stability of the molecule on the surface and a smooth, flat background that provides sufficient contrast are also necessary.
The height varies from 28 A on the shoulders of the complex to 18 A in the middle valley.
