Thin films 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. 

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). 

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). 

Figure 7.9 shows RHEED patterns obtained with 30keV electrons impinging on clean surfaces of optimized ZnO thin films grown on r-, a-, and c-plane sapphire. The azimuthal directions of the two types of RHEED images of the c-axis oriented ZnO films (on a- and c-sapphire) are [flOO] (top) and [2110] (bottom) [52]. 

Fig. 7.9. RHEED images of optimized ZnO thin film surfaces on r-plane, a-plane, and c-plane sapphire, in the two azimuthal orientations (top and bottom) separated by 45° (left) or 30° (middle and right), respectively. The RHEED patterns of the a-axis textured film on r-plane sapphire (left) indicate an epitaxial and three-dimensional, island-like growth. The ZnO films on a-plane (middle) and c-plane sapphire (right) exhibit a smoother surface structure, as indicated by the streaky RHEED patterns and the observation of additional weak reflections in the top images due to 3 x 3 surface reconstruction [51]...

The samples used for RHEED and FEED are single crystals with carefiilly prepared flat surfaces. For THEED of thin films, the observed areas of the samples must be electron transparent with thickness less than or comparable to the inelastic mean free path of electrons. The inelastic mean free path increases with the electron voltage. The typical sample... 

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... 

ESA leads to an obstruction, for example, the ovens, Knudsen cells, etc. needed for deposition of thin films prevent a direct view of the substrate. This is one reason why RHEED is in rather widespread use in thin-film deposition systems—the electron gun and screen are remote and grazing incidence does not interfere with film deposition. Therefore, in many cases RHEED is a real in situ technique. By comparison, the conventional front view LEED system blocks essentially the full space and the rear view system blocks half of the space available. In this respect MEIS and RBS are also remote systems that do not take much space around the substrate or target. In some systems transport is installed as a solution (it is the only solution when adding STM to the tool box). 

Different techniques are nowadays available for growing thin films and heterostructures, which are often integrated in modular systems for cutting-edge in situ characterizations. We have described only three of them, among the most common and advanced techniques, together with two standard in situ characterizations (RHEED and plume analysis). 

Haeni, J. et al (2000) RHEED intensity oscillations for the stoichiometric growth of SrTiOs thin films by reactive molecular beam epitaxy. 

Thin epitaxial films (less than 3 nm) of CrAs and CrSb with zinc-blende structure can be grown on GaAs substrates by MBE. Their 7c exceeds 400 K (Akinaga et al. 2000c Zhao et al. 2001b). A zinc-blende structure is confirmed by in-situ RHEED collected during the growth and ex-situ cross-sectional transmission electron microscopy (TEM). The... 

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