R. S. Becker

Atomic-scale surface modifications using a tunnelling microscope

The desire to modify materials on the smallest possible scale is motivated by goals ranging from high-density information storage to the purposeful transformation of genetic material. Here we report an atomic-scale modification of the surface of a nearly perfect germanium crystal, effected by the tungsten tip of a tunnelling microscope. We believe this to be the smallest spatially controlled, purposeful transformation yet impressed on matter and we argue that the limit set by the discreteness of atomic structure has now essentially been reached.

Tunneling microscopy of silicon and germanium: Si(111)7×7, SnGe(111)7×7, GeSi(111)5×5, Si(111)9×9, Ge(111)2×8, Ge(100)2×1, Si(110)5×1

The tunneling microscope has been used to study the low‐index faces of silicon and germanium. Si(111) 7×7 and 9×9, GeSi(111) 5×5, and SnGe(111) 7×7 are discussed in the light of the dimer–adatom–stacking fault structural model. Ge(111) 2×8 is compared to Si(111), Ge(100) is shown and compared to Si(100), and new structures on the Si(110) 5×1 surface are shown.

Scanning tunneling microscopy of decagonal and icosahedral quasicrystals

The scanning tunneling microscope is used under ultrahigh vacuum conditions to image the clean surfaces of decagonal Al65Co20Cu15 and icosahedral Al65Cu20Fe15. The tunneling images show nonperiodic surfaces whose features are adequately described by pentagonal quasilattice for the case of the decagonal material, suggesting that a tiling model accounts for the surface features. The scanning tunneling microscope measurements of the scale of surface features is in good agreement with high resolution x‐ray diffraction measurements of the bulk.

Structure of Decagonal Quasicrystals

Recent discoveries made in synthesizing stable quasicrystals allows us to apply conventional techniques in growing macroscopic single grains and use conventional probes to study the complex structure of this novel form of matter. The decagonal quasicrystals in particular appears to have a far less complicated phase diagram and an extreme phase stability compared to their three dimensional icosahedral counterparts. We have grown several millimeter size single grains and studied the structure of stable A165Co20Cu15 Decagonal quasicrystals, using High Resolution X-ray Diffraction (HRXD) and Scanning Tunneling Microscopy (STM). Unlike diffraction probes which provide large volume reciprocal space information, STM is a local probe and provides direct real-space information with atomic resolution. The two techniques are therefore complimentary and when combined becomes very powerful in structure determinations of complex systems like quasicrystals. We have been able to prepare clean single grain surfaces in UHV and resolve individual atoms in quasiperiodic ordering with STM. Images of 10-fold quasiperiodic surfaces exhibit very well defined five sets of mass density lines, separated by multiples of 72 degree rotations. Near perfect quasiperiodic order seem to extend beyond thousands of angstroms, and can be modeled by a Penrose lattice. All atomic layers observed have identical local structure unlike the two dissimilar layers of the approximant Al13Fe4 phase. High resolution x-ray scattering from single grains finds 2000 angstroms size defect free quasicrystalline layers stacked periodically.

Determination of surface atomic positions by scanning tunneling microscope observations

The scanning tunneling microscope (STM) is used to study the clean surface of Ge(111)‐1×1:As. The surface is found to consist of hexagonal incommensurate 1×1 domains having a lateral extent of ∼ 150 A. The domain boundaries are determined to be one double‐layers deep ‘‘trenches’’ in the surface, terminated with arsenic. Measurement of the occupied sample state tunneling images indicates the domains are laterally contracted by 0.7% due to the substitution of arsenic for germanium in the top layer. The geometric structure of the domain boundaries is discussed in the light of simulated filled state tunneling images for several possible domain terminations. The reliability of the STM for these kinds of measurements is discussed.

5.2. Germanium

Publisher Summary This chapter discusses that while silicon has dominated the solid-state electronics industry, as the semiconductor of choice, for device applications for more than 30 years, it is interesting to recall that the first transistor was constructed of germanium. Both germanium and silicon are indirect gap materials with otherwise remarkably similar band structure. Germanium forms tetrahedrally coordinated covalent bonds and takes the diamond crystal structure in its solid phase, like both carbon and silicon. Germanium has a lower melting point than silicon, 937°C versus 1410°C. Scientific and commercial interest in germanium has been increasing as heteroepitaxial growth methods improve resulting in attractive Ge-Si alloy structures with applications in bandgap engineering for optoelectronic devices. The chapter discusses that since the introduction of the vacuum tunneling microscope, a wide variety of germanium surfaces have been imaged by a growing number of investigators. While the number of experiments has been less than that for silicon, as in other fields of surface science, the scope has been nearly as great, with the major features of both clean and adsorbate-covered germanium surfaces brought to light. Recent experiments in the heteroepitaxy of germanium on silicon show that interest in this material is increasing, after taking a second seat to silicon in basic investigations for many years.

5.1. Silicon

Publisher Summary This chapter discusses that the atomistic viewpoint of the vacuum tunneling microscope is a powerful tool for the examination of structures and processes on the scale of atoms. In barely ten years of existence allocated to the scanning tunneling microscopy (STM), several long-standing questions in silicon surface physics have been discussed. The chapter explains that the semiconductor industry, along with a large segment of the condensed matter physics community, devotes the lions share of its resources to element 14 in the periodic table, silicon. The stability of silicon oxide with respect to attack by common processing reagents, such as water and methanol, coupled with the abundance of this element in the earths crust, has made it pervasive in advanced technology, from semiconductor microelectronics to power rectifiers and solar cells. One of the properties of elemental semiconductors is that their clean surfaces typically undergo a process termed “reconstruction,” whereby the fundamental periodicity of the structure taken by the surface atoms is different from that of the underlying bulk material. This process is because of the covalent nature of their bonds; a simple bulk termination at the surface leaves a large number of unsatisfied bonds that result in large free energy.