Dislocations interfaces

The state-of-the-art analysis methods for the evaluation of structural, chemical and electrical properties of thin layers in processed Si substrates are discussed. The properties of inclanted p-n junctions, Si-SiO interface, Ge inplant amorphization of Si aid misfit dislocation interface in epitaxial Si are exenplified to illustrate the features and limitations of the techniques. 

Preferential nucleation sites (film growth) Positions on a surface where the mobile adatoms prefer to condense. Examples Atomic steps Charge sites emerging dislocations Interfaces Lattice defects such as grain boundaries Substitutional atoms. 

Extended defects range from well characterized dislocations to grain boundaries, interfaces, stacking faults, etch pits, D-defects, misfit dislocations (common in epitaxial growth), blisters induced by H or He implantation etc. Microscopic studies of such defects are very difficult, and crystal growers use years of experience and trial-and-error teclmiques to avoid or control them. Some extended defects can change in unpredictable ways upon heat treatments. Others become gettering centres for transition metals, a phenomenon which can be desirable or not, but is always difficult to control. Extended defects are sometimes cleverly used. For example, the smart-cut process relies on the controlled implantation of H followed by heat treatments to create blisters. This allows a thin layer of clean material to be lifted from a bulk wafer [261. 

When a mismatch is inevitable, as in the combination Gej-Sii j. — Si, it is found that up to a value of jc = 0.4, there is a small mismatch which leads to a strained silicide lattice (known as commensurate epitaxy) and at higher values of jc there are misfit dislocations (incommensurate epitaxy) at the interface (see p. 35). From tlrese and other results, it can be concluded that up to about 10% difference in the lattice parameters can be accommodated by commensurately strained thin films. 

The interface between the substrate and the fully developed film will be coherent if the conditions of epitaxy are met. If there is a small difference between the lattice parameter of the film material and the substrate, die interface is found to contain a number of equally spaced edge dislocations which tend to eliminate the stress effects arising from the difference in the atomic spacings (Figure 1.13). 

Interface mismatch between two solids compensated with an edge dislocation... 

Figure 1.13 The grain boundary and interface which can be formed between two crystals with the insertion of dislocations. In the grain boundary the two crystals are identical in lattice structure, but there is a difference in lattice parameters in the formation of the interface...

Fig. 7.5. Nucleation in solids. Heterogeneous nucleotion con take place at defects like dislocations, grain boundaries, interphase interfaces and free surfaces. Homogeneous nucleation, in defect-free regions, is rare.

Analysis of stress distributions in epitaxial layers In-situ characterization of dislocation motion in semiconductors Depth-resolved studies of defects in ion-implanted samples and of interface states in heterojunctions. 

ATOMIC STRUCTURE AND PROPERTIES OF DISLOCATIONS AND INTERFACES IN TWO-PHASE TlAl COMPOUNDS... 

The precursor of such atomistic studies is a description of atomic interactions or, generally, knowledge of the dependence of the total energy of the system on the positions of the atoms. In principle, this is available in ab-initio total energy calculations based on the loc density functional theory (see, for example, Pettifor and Cottrell 1992). However, for extended defects, such as dislocations and interfaces, such calculations are only feasible when the number of atoms included into the calculation is well below one hundred. Hence, only very special cases can be treated in this framework and, indeed, the bulk of the dislocation and interfacial... 

While the c/a ratio deviates only by about 2% from one, it is not ideal and this has significant consequences for the pseudotwin and 120° rotational fault. It results in a misfit at these interface which is compensated by a network of misfit dislocations (Kad and H2izzledine 1992). In contrast, the non-ideal c/a ratio does not invoke any misfit at ordered twins. However, the misfit dislocations present at interfaces are about fifty lattice spacings apart and thus there are large areas between them where the matching of the lamellae is coherent. The structures and... 

Similarly, in studies of lamellar interfaces the calculations using the central-force potentials predict correctly the order of energies for different interfaces but their ratios cannot be determined since the energy of the ordered twin is unphysically low, similarly as that of the SISF. Notwithstcinding, the situation is more complex in the case of interfaces. It has been demonstrated that the atomic structure of an ordered twin with APB type displacement is not predicted correctly in the framework of central-forces and that it is the formation of strong Ti-Ti covalent bonds across the interface which dominates the structure. This character of bonding in TiAl is likely to be even more important in more complex interfaces and it cannot be excluded that it affects directly dislocation cores. 

Another reaction mechanism, which is conveniently mentioned under this heading, is due to Hill [479] who suggested that ions (atoms or molecules) frorh the product may move through the dislocation network of the reactant and activate potential nuclei, particularly in the vicinity of the reaction interface. Thus a reaction zone, within which potential nucleusforming sites are activated, is developed in front of an advancing interface. With appropriate assumptions, this reaction model provides an alternative explanation of the exponential rate law, eqn. (8), which in Sect. 3.2 was discussed with reference to chain reactions. 

Hill et al. [117] extended the lower end of the temperature range studied (383—503 K) to investigate, in detail, the kinetic characteristics of the acceleratory period, which did not accurately obey eqn. (9). Behaviour varied with sample preparation. For recrystallized material, most of the acceleratory period showed an exponential increase of reaction rate with time (E = 155 kJ mole-1). Values of E for reaction at an interface and for nucleation within the crystal were 130 and 210 kJ mole-1, respectively. It was concluded that potential nuclei are not randomly distributed but are separated by a characteristic minimum distance, related to the Burgers vector of the dislocations present. Below 423 K, nucleation within crystals is very slow compared with decomposition at surfaces. Rate measurements are discussed with reference to absolute reaction rate theory. 

For superlattices with small modulation wavelength of several nanometres, the dislocation multiplication cannot occur, and the dislocation activity is demonstrated by the movement of individual dislocations from B layer into A layer by stress. The critical shear stress to move a dislocation from B layer into A layer (cta/b) can be given by the Lehoczky theory equation [108] as shown in Fig. 13. Figure 13 also gives the normalized oq as function of tglb. It can be seen that there is no strength enhancement as t Ab, which corresponds to very small layer thickness (< 1 nm), and the disappearance of interfaces due to the diffusion between layer A and layer B. The increases rapidly with the increase of... 

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