Electron diffraction, from surfaces

C. IR/Raman is a computational instrument that predicts IR/Raman spectra. C. LEED/RHEED helps interpret low-energy electron diffraction patterns and reflection high-energy electron diffraction from surfaces. 

S. A. Lindgren, L. Wallden, J. Rundgren, and P. Westrin, Low-Energy Electron Diffraction from Cu(lll) Subthreshold Effect and Energy-Dependent Inner Potential Surface Relaxation and Metric Distances between Spectra, Phys. Rev. B 29 (1984), 576. 

Fig. 4.3. Experimental intensity vs. voltage (energy) curves for electron diffraction from at Pt(l 11) surface. Beams are identified by different labels (h,k) representing reciprocal lattice vectors parallel to the surface. An incidence angle of 4° from the surface normal is used...

The deBroglie wavelengths, A, of electrons traveling with kinetic energies, KE, in the range from 20 to 200 eV are slightly smaller than typical interatomic distances and thus appropriate for diffraction from surfaces... 

We shall in the following discuss the surface sensitive techniques involving electron diffraction from a common viewpoint. These techniques can all be fit within a four-step description, which will be very useful for comparison. Various techniques are illustrated in terms of this description in Figure 1, and the four steps are defined below. 

Figure 2 Schematic diagram of a video LEED system used to generate intensity versus voltage curves of electrons diffracted from ordered surfaces. The I -V curves are compared to theory in solving a surface structure...

Penetration of an incident low energy electron beam (say 100 eV) is only a few layers, unlike X rays with radiation so penetrating that the surface region has negligible effect on the diffraction pattern. A primary X-ray beam is hardly attenuated after passage through thousands of crystal planes. In LEED, the interaction with the uppermost layers is intense, and a diffraction pattern corresponds to interference of waves scattered by superficial planes only. Reciprocal space is diperiodic for slow electron diffraction from a regular surface, and is modulated in... 

Lee K-W, Park D-K, Kim Y-Y, Shin H-J (2005) Investigation of the interface region between a porous silicon layer and a silicon substrate. Thin Solid Films 478 183-187 Lee C, Kim H, Cho Y, Lee WI (2007) The properties of porous silicon as a therapeutic agent via the new photodynamic therapy. J Mater Chem 17(25) 2648-2653 Li W, Zhao D, Haneman D (2000) Low-energy electron diffraction from heated porous silicon surfaces. Surf Sci 448 40-48... 

Electrons interact with solid surfaces by elastic and inelastic scattering, and these interactions are employed in electron spectroscopy. For example, electrons that elastically scatter will diffract from a single-crystal lattice. The diffraction pattern can be used as a means of stnictural detenuination, as in FEED. Electrons scatter inelastically by inducing electronic and vibrational excitations in the surface region. These losses fonu the basis of electron energy loss spectroscopy (EELS). An incident electron can also knock out an iimer-shell, or core, electron from an atom in the solid that will, in turn, initiate an Auger process. Electrons can also be used to induce stimulated desorption, as described in section Al.7.5.6. 

One fiirther method for obtaining surface sensitivity in diffraction relies on the presence of two-dimensional superlattices on the surface. As we shall see fiirtlrer below, these correspond to periodicities that are different from those present in the bulk material. As a result, additional diffracted beams occur (often called fractional-order beams), which are uniquely created by and therefore sensitive to this kind of surface structure. XRD, in particular, makes frequent use of this property [4]. Transmission electron diffraction (TED) also has used this property, in conjunction with ultrathin samples to minimize bulk contributions [9]. 

Thin films of metals, alloys and compounds of a few micrometres diickness, which play an important part in microelectronics, can be prepared by die condensation of atomic species on an inert substrate from a gaseous phase. The source of die atoms is, in die simplest circumstances, a sample of die collision-free evaporated beam originating from an elemental substance, or a number of elementary substances, which is formed in vacuum. The condensing surface is selected and held at a pre-determined temperature, so as to affect die crystallographic form of die condensate. If diis surface is at room teiiiperamre, a polycrystalline film is usually formed. As die temperature of die surface is increased die deposit crystal size increases, and can be made practically monocrystalline at elevated temperatures. The degree of crystallinity which has been achieved can be determined by electron diffraction, while odier properties such as surface morphology and dislocation sttiicmre can be established by electron microscopy. 

Alternatives to XRD include transmission electron microscopy (TEM) and diffraction, Low-Energy and Reflection High-Energy Electron Diffraction (LEED and RHEED), extended X-ray Absorption Fine Structure (EXAFS), and neutron diffraction. LEED and RHEED are limited to surfaces and do not probe the bulk of thin films. The elemental sensitivity in neutron diffraction is quite different from XRD, but neutron sources are much weaker than X-ray sources. Neutrons are, however, sensitive to magnetic moments. If adequately large specimens are available, neutron diffraction is a good alternative for low-Z materials and for materials where the magnetic structure is of interest. 

---