RHEED

First of all, it should be stressed that RHEED allows the observation of the phase purity of the layers in real time. During nucleation, a spotty pattern appears, which in itself allows for a judgment of the success of the nucleation procedure, since the symmetries of M- and C-plane GaN are distinctly different. Further growth at Ga-stable conditions yields a streaky RHEED pattern, reflecting the progressive smoothening of the surface. Phase mixture manifests itself in the occurrence of additional reflections (typically spots), which are readily distinguishable from the clean M-plane pattern. In addition, RHEED patterns can also reveal the in-plane orientation relationship between epilayer and substrate. 

Triple-crystal high-resolution X-ray diffraction (HRXRD) provides an ex situ nondestructive tool to check the relationship of corresponding out-of-plane orientations between the epilayer and the substrate and simultaneously, the phase purity as well. In the text that follows, we describe the phase purity of the samples and their surface orientation as examined by XRD. 

High-energy electrons (up to 50 keV) from an electron gun impinge on the surface of the sample at a grazing angle typically less than 5°. The diffracted 

A simple interpretation/use of the RHEED data is essentially based on the RHEED intensity oscillation that allows monitoring the crystal layer thickness. 

A complete interpretation of the RHEED intensity oscillations is not so straightforward. In feet, when STO is grown block-by-block by PLD or evaporation, the oscillation intensity reproduce the number of unit cells but when STO is grown by 

RHEED oscillations are also particularly useful in monitoring the type of growth mode of the depositing material. Four different film growth modes are generally identified mainly related to the diffusion coefficient of the adatoms on the surface, the dimensions of the substrate terraces, and the lattice parameter mismatch between the substrate and the film  

The RHEED analysis allows identifying the different film growth modes. In the step-flow and layer-by-layer growth modes, the RHEED pattern maintains the 2D typical features, but only in the second case it is possible to observe the periodical RHEED spot intensity oscillations without any damping. In the island growth mode, there is a quite pronounced damping of the RHEED intensity and, consequentiy, a disappearing of the RHEED intensity oscillations. 

---

RHEED Reflection high-energy electron diffraction [78, 106] Similar to HEED Surface structure, composition... 

The 02-graphite system has been studied by means of quite a few techniques LEED, RHEED, EELS, and NEXAFS (see Ref. 101). It was concluded,... 

As the table shows, a host of other teclmiques have contributed a dozen or fewer results each. It is seen that diffraction teclmiques have been very prominent in the field the major diffraction methods have been LEED, PD, SEXAFS, XSW, XRD, while others have contributed less, such as NEXAFS, RHEED, low-energy position diffraction (LEPD), high-resolution electron energy loss spectroscopy (HREELS), medium-energy electron diffraction (MEED), Auger electron diffraction (AED), SEELFS, TED and atom diffraction (AD). 

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. 

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. 

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. 

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. 

Electrons having energies and incident angles typical of RHEED can be treated as nearly nonpenetrating. As a result, atoms in the outermost plane are responsible for most of the scattering, and the resulting reciprocal lattice will be an array of rods perpendicular to the surfrce plane. 

Figure 3 (c) Photograph of a RHEED pattern from cleaved GaAs(IIO) obtained using a... 

---