Reflection high energy electron diffraction RHEED

2 Reflection high energy electron diffraction (RHEED) 

The alternative electron diffraction technique is referred to as reflection electron diffraction and uses a comparatively high energy primary beam (typically 10—50 keV) at a grazing incidence of 1—3°. There is, however, very little difference between LEED and RHEED in the depth of material probed, since, for example, a 50 keV electron at 3° incidence angle will have approximately the same momentum perpendicular to the surface as a normally incident 150 eV electron. To obtain the complete space group symmetry with RHEED, however, it is necessary to use at least two primary beam directions. 

The geometric arrangement is illustrated in Fig. 3 and the information available from the angular distribution and intensity of the diffracted beams is similar in almost every respect to that obtained from LEED. The use of a grazing incidence beam does, however, reveal more details of surface topography, particularly if there are small asperities on the surface. 

When such features exist, they are penetrated by the electron beam so the material is represented by a three-dimensional point lattice and diffraction only occurs when the Ewald sphere intersects a point. This produces a transmission-type spot pattern. For smooth surfaces, the diffraction pattern appears as a set of streaks normal to the shadow edge on the fluorescent screen, due to the interaction of the Ewald sphere with the rods projecting orthogonally to the plane of the two-dimensional reciprocal lattice of the surface. The reciprocal lattice points are drawn out into rods because of the very small beam penetration into the crystal (2—5 atomic layers). We would emphasize, however, that despite contrary statements in the literature, the appearance of a streaked pattern is a necessary but not sufficient condition by which to define an atomically flat surface. Several other factors, such as the size of the crystal surface region over which the primary wave field is coherent and thermal diffuse scattering effects (electron—phonon interactions) can influence the intensity modulation along the streaks. 

The underlying principle of RHEED is that particles of matter have a wave character. This idea was postulated by de Broglie in (1924). He argued that since photons behave as particles, then particles should exhibit wavelike behavior as well. He predicted that a particle s wavelength is Planck s constant h divided by its momentum. The postulate was confirmed by Davisson and Germer s experiments in 1928, which demonstrated the diffraction of low-energy electrons from Ni.  

The analogy of a crystal surface as a diffraction grating also suggests how surface defects can be probed. Recall that for a diffraction grating the width of a diffracted peak will decrease as the number of lines in the grating is increased. This observation can be used in interpreting the shape of RHEED spots. Defects on a crystal surfr.ee can limit the number of atomic rows that scatter coherendy, thereby broadening RHEED features. 

In a difiraction experiment one observes the location and shapes of the diffracted beams (the diffraction pattern), which can be related to the real-space structure using kinematic diffraction theory. Here, the theory is summarized as a set of rules relating the symmetry and the separation of diffracted beams to the symmetry and separation of the scatterers. 

The dimensionality of the diffraction problem will have a strong effect on how the diffraction pattern appears. For example in a ID problem, e.g., diffraction from a single Une of atoms spaced apart, only the component ofS in the direction along the line is constrained. For a 2D problem, e.g., the one encountered in RHEED, two components of S in the plane of the surface are constrained. For a 3D problem, e.g., X-ray scattering from a bulk crystal, three components of S are constrained. 

For a given structure, the values of S at which in-phase scattering occurs can be plotted these values make up the reciprocal lattice. The separation of the diffraction maxima is inversely proportional to the separation of the scatterers. In one dimension, the reciprocal lattice is a series of planes, perpendicular to the line of scatterers, spaced 2Jl/ apart. In two dimensions, the lattice is a 2D array of infinite rods perpendicular to the 2D plane. The rod spacings are equal to 2Jl/(atomic row spacings). In three dimensions, the lattice is a 3D lattice of points whose separation is inversely related to the separation of crystal planes. 

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Fig. 4. Schematic of an ultrahigh vacuum molecular beam epitaxy (MBE) growth chamber, showing the source ovens from which the Group 111—V elements are evaporated the shutters corresponding to the required elements, such as that ia front of Source 1, which control the composition of the grown layer an electron gun which produces a beam for reflection high energy electron diffraction (rheed) and monitors the crystal stmcture of the growing layer and the substrate holder which rotates to provide more uniformity ia the deposited film. After Ref. 14, see text.

Reflection High-Energy Electron Diffraction (RHEED)... 

This chapter contains articles on six techniques that provide structural information on surfaces, interfeces, and thin films. They use X rays (X-ray diffraction, XRD, and Extended X-ray Absorption Fine-Structure, EXAFS), electrons (Low-Energy Electron Diffraction, LEED, and Reflection High-Energy Electron Diffraction, RHEED), or X rays in and electrons out (Surfece Extended X-ray Absorption Fine Structure, SEXAFS, and X-ray Photoelectron Diffraction, XPD). In their usual form, XRD and EXAFS are bulk methods, since X rays probe many microns deep, whereas the other techniques are surfece sensitive. There are, however, ways to make XRD and EXAFS much more surfece sensitive. For EXAFS this converts the technique into SEXAFS, which can have submonolayer sensitivity. 

In the process of MBE, the surface structure can be investigated by reflected high energy electron diffraction (RHEED). During MBE growth, one often observes an oscillation in the intensity of the specular reflected beam as a function of time. This is interpreted to be due to the layer-by-layer growth of a two-dimensional island. 

Surface morphology Reflection high-energy electron diffraction (RHEED) Atomic force microscopy (AFM)... 

Fig. 4.1 Reflection high-energy electron diffraction (RHEED) patterns corresponding to the best epitaxial conditions of CdSe deposited on a (111) InP face (thickness 85 nm). (a) Observation under the (112) azimuth (b) observation under the (110) azimuth. (With kind permission from Springer Science-l-Business Media [6])...

For single crystal substrates which are not in the form of thin films, the techniques of transmission microscopy and nanodiffraction can not be used. For such cases, the techniques of reflection electron microscopy (REM) or its scanning variant (SREM) and reflection high energy electron diffraction (RHEED), in the selected area or convergent beam modes, may be applied (18). 

Fig. 2. Reflection high energy electron diffraction (RHEED) patterns taken from [110] azimuth, (a) Low-temperature grown GaAs at 250°C. (b) (Ga,Mn)As at 250°C. (c) I70°C, and (d) 320°C (Shen ei al. 1997a).

The structure of the PLD grown ZnO thin films on c-plane, a-plane, and r-plane sapphire substrates will be explained by results of X-ray diffraction (XRD), transmission electron microscopy (TEM) with selected area diffraction patterns (SAD), and reflection high-energy electron diffraction (RHEED). 

In-situ growth monitoring, in particular by reflective high energy electron diffraction (RHEED), has provided some fundamental information on the surface and nucleation properties of nitrides. Early RHEED studies by Hughes et al [34] and Hacke et al [49] were completed by Smith et al [50], The observed surface reconstructions for (0001) and (0001) GaN surfaces have allowed the modelling of the quasi-equilibrium surface, which has been calculated to be preferentially Ga-terminated [51], Feuillet et al [52] have followed the evolution of surface lattice constants in RHEED for the nucleation of GaN on AIN or InN on GaN (and vice versa) and extracted a wide range of information on the character of nucleation and misfit relaxation. 

Before leaving the subtopic of electron diffraction, it should be noted that two other techniques known as low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED) may also be used to gleam structural... 

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