RHEED information

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. 

Because defects limit the order on a surface, they will alter the dif action pattern, primarily by broadening diffracted beams. Methods have been developed, mostly in the LEED literature, to analyze the shape of diffracted beams to gain information on step distributions on surfaces. These methods apply equally well to RHEED. 

RHEED is a powerful tool for studying the surface structure of crystalline samples in vacuum. Information on the surface symmetry, atomic-row spacing, and evidence of surfece roughness are contained in the RHEED pattern. The appearance of the RHEED pattern can be understood qualitatively using simple kinematic scattering theory. When used in concert with MBE, a great deal of information on film growth can be obtained. 

Chemical differences in the Auger peaks are also present in the a-WCl a-W2C sample. Although the usual sputter profile does not give information about the distribution of chemical states in the sample, analysis of the W and C components suggests that besides the surface component, two different carbide components are present. The behaviour of the two bulk components of W and C is the same the first is located near the surface and decreases deeper into the sample the second increases to become the most important in the deeper part of the sample. RHEED analysis indicates a-WC but the depth sensitivity of this method is about 5-10 nm. XRD analysis indicates the presence of a-WC,a-W2C and W with a depth sensitivity of a few pm. Thus, the first bulk component of W and C can be identified as a-WC and the second as a-W2C. The overall sample consists of a contamination layer, followed by a thin layer of a-WC on top of the a-W2C phase. 

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. 

Reflection high-energy RHEED composition. The momentum transfer in backscattering collisions between nuclei is used to identify the nuclear masses in the sample, and the smaller, gradual momentum loss of the incident nucleus through electron-nucleus interactions provides depth-profile information. Monoenergetic electrons of 1 -20keV are elastically scattered from a Atomic structure... 

An electron reflected from a surface carries diffraction information if it has experienced elastic scattering or information on the excitation of phonons, plasmons, and electronic or vibrational transitions as the result of inelastic events. The major effort in diffraction studies has concentrated on the use of low energies ( 200 eV) but reflection high-energy electron diffraction (RHEED 20—40keV) is suitable also for surface work (see ref. 2 for a description of pattern interpretation). RHEED... 

Different characterization techniques, for example X-ray diffraction,EXAFS, SEELFS, electron diffraction (LEED, RHEED, i SAED, and CBED ), and electron microscopy (HRTEM, WBDF ) give information about crystal structure, lattice distances, or morphology. All these techniques give average information about the shapes and the lattice distances of particle collections and of isolated particles. To obtain information about the particles at the atomic level, HRTEM is necessary. Other information about the surface structure of small particles can be obtained by Most HRTEM studies have been devoted to... 

The details of thin-film formation by PVD or CVD on the atomic and molecular scale are unknown in most cases, but such knowledge would be very helpful to design new processes and to tailor film properties. Information is lacking due to the high reactivities, short lifetimes, and low concentrations of the relevant transient gas-phase species. Furthermore, many of the thin-film properties in statu nascendi are unknown due to the experimental difficulties of thin-film characterization during deposition, particularly with non-crystalline films and if established methods such as RHEED and FEED cannot be applied. Among the film properties, mechanical stress in thin films can lead to unwanted (uncontrolled) instabilities and peel-off phenomena. Therefore, in situ diagnostic methods have been developed to... 

Both LEED and RHEED are probes of surface structure that depend on the existence of long-range order of periodic structure in the surface. They do not provide compositional information and are always used along with other techniques such as AES that can analyze for composition. 

While the spatial resolution of AES, XPS and SIMS continues to improve, atomic scale analysis can only be obtained by transmission electron microscopy (TEM), combined with energy dispersive X-ray spectroscopy (EDX) or electron energy loss spectroscopy (EELS). EDX detects X-rays characteristic of the elements present and EELS probes electrons which lose energy due to their interaction with the specimen. The energy losses are characteristic of both the elements present and their chemistry. Reflection high-energy electron diffraction (RHEED) provides information on surface slmcture and crystallinity. Further details of the principles of AES, XPS, SIMS and other techniques can be found in a recent publication [1]. This chapter includes the use of AES, XPS, SIMS, RHEED and TEM to study the composition of oxides on nickel, chromia and alumina formers, silicon, gallium arsenide, indium phosphide and indium aluminum phosphide. Details of the instrumentation can be found in previous reviews [2-4]. 

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