Epitaxial layers mismatch

Cathodoluminescence microscopy and spectroscopy techniques are powerful tools for analyzing the spatial uniformity of stresses in mismatched heterostructures, such as GaAs/Si and GaAs/InP. The stresses in such systems are due to the difference in thermal expansion coefficients between the epitaxial layer and the substrate. The presence of stress in the epitaxial layer leads to the modification of the band structure, and thus affects its electronic properties it also can cause the migration of dislocations, which may lead to the degradation of optoelectronic devices based on such mismatched heterostructures. This application employs low-temperature (preferably liquid-helium) CL microscopy and spectroscopy in conjunction with the known behavior of the optical transitions in the presence of stress to analyze the spatial uniformity of stress in GaAs epitaxial layers. This analysis can reveal,... 

RBS and channeling are extremely useful for characterization of epitaxial layers. An example is the analysis of a Sii-j Gejc/Si strained layer superlattice [3.131]. Four pairs of layers, each approximately 40 nm thick, were grown by MBE on a <100> Si substrate. Because of the lattice mismatch between Sii-jcGe c (x a 0.2) and Si, the Sii-j Ge c layers are strained. Figure 3.51 shows RBS spectra obtained in random and channeling directions. The four pairs of layers are clearly seen in both the Ge and Si... 

A variety of compound semiconductors have been successfully prepared by this technique. Much of the work concerning ECALE has been concentrated on the deposition of CdTe on An substrates. Notwithstanding the inherent problems of the system (for instance, a 10% lattice mismatch), the formation of CdTe epitaxial layers became a model example of ECALE synthesis. In their pioneering studies, Stickney and co-workers [27, 28] have focused on the deposition of the compound on... 

Adsorbate Substrate Atom density mismatch/ML (7o) Pseudomorphic/epitaxial layers Change in CO desorption T(K)... 

When the epitaxial layer thickness is quite high, typically of the order of one micrometre, we can apply the simple criteria discussed in Chapter 3 to determine the layer parameters from the rocking curve. The effective mismatch can be determined by direct measurement of the angular splitting of the substrate and layer peaks and the differential of the Bragg law. This simple analysis catmot be applied when the layer becomes thin, typically less than about 0.25 //m, where, even for a single layer, interference effects become extremely important. We consider these issues in section 6.2 below. 

An important featnre to note in double-axis topography experiments is that when the beam area is large, the measnred rocking curve widths are not necessarily intrinsic. For example, mismatched epitaxial layers curve substrate wafers by an amonnt which depends on the degree of mismatch and layer thickness. Topographs of snch curved wafers show bands of diffracted intensity. 

Figure 7.42 Illustration of (a) matched and (b) mismatched lattice spacings in epitaxial layers. Reprinted, by permission, from P. Ball, Made to Measure, p. 36. Copyright 1997 by Princeton University Press.

Experimentally, we can introduce a built-in strain in an epitaxial layer by growing it on a lattice mismatched substrate. As long as the mismatched epitaxial layer is below the critical thickness, the produced strain is uniform and no dislocations are induced. As a result, the in-plane lattice constant of the epitaxial layer is fitted to that of the substrate, and the out-of plane lattice constant is adjusted to a new lattice constant according to the Hook law. Then, the subband structure is modified by introducing a built-in strain, and the strain has a dramatic influence on the electronic properties of the system. Theoretically, we can easily include the strain effect in the k.p theory. 

According to x-ray studies, the A1 content in the />-AlGaN epitaxial layers was estimated to about 12 at.% and the lattice mismatch to the ZnO layer amounted to 2.2 %. At the same time, for both epitaxial layers x-ray measurements revealed a good crystallinity. This is corroborated by CL measurements. 

The mechanical interaction between the different epitaxial layers may result in the formation of misfit dislocations. Nucleation and propagation of cracks can ensue if the mismatch in thermal expansion coefficient is relatively large. The defects significantly influence the physical properties of the thin films. Examples from different material combinations and models of how to predict the numbers for critical thicknesses are provided in Section 14.4. 

II-VI semiconductor layers and bulk semiconductors like Si, GaAs, InP, etc. In particular, quantum wells are formed by thin epitaxial multilayered structures like (Zn, Cd)Se/ZnS. Nevertheless, the choice between bulk semiconductors and the layers deposited or between the multilayers is governed by the lattice mismatch between the two components as the lattice mismatch causes the formation of misfit dislocations. In the optical devices these defects are potential non-radiative centres and at worst they can cause the failure of injection lasers. Figure 29 is a map of energy gap versus lattice constants for a variety of semiconductors it can be used to select different heterostructures, not only for optoelectronics applications but also for photovoltaic cells. In the latter application the deposited films are generally polycrystalline and the growth of high-quality epitaxial layers has received little applications. 

The 3D growth mode has origins not only in the basic material property differences between the epitaxial layer and the substrate (e.g. lattice constant mismatch, polar or nonpolar effect, etc.) but also in the growth conditions. 

This section discusses the generation mechanisms for type-I and type-II dislocations. Although the lattice mismatch for GaAs/Si and GaAs/GaP are almost the same, the types of dislocations are different. Therefore, the type of dislocation cannot be explained alone by lattice mismatch. Furthermore, the thermal stress in the epitaxial layer cannot explain the difference of the dislocation type because the thermal stress for GaAs/Si and GaP/Si is almost the same although the types of dislocations are different. 

Perovskite-type rare earth aluminates are also prospective materials for deposition of GaN and AIN layers and manufacturing of blue light semiconductor lasers, as well as epitaxial layers of HTSC materials. LaAlOs and LSAT (Lai j Srj Ali yTay03) are well known in this respect. The application of a material as a substrate requires the knowledge of its physical characteristics and their temperature evolution in addition to the standard condition, which is the minimum mismatch between the cell parameters of the deposited film and the substrate. For details refer to O Bryan et al. (1990). 

Early observations of elastic strain relaxation during growth of epitaxial layers led to paradoxical results. An attempt to interpret the observations on the basis of the critical thickness theory in its most elementary form suggested that, once the thickness of a film exceeded the critical thickness, the final elastic strain of the film should be determined by the thickness of the film alone, independent of the original, or fuUy coherent, mismatch strain. This is implied by the result in (6.27), which states that that the mean elastic strain predicted by the equilibrium condition G(/if) = 0 is completely determined by hf beyond critical thickness, no matter what the value of Cni- However, it was found that the post-growth elastic strain as measured by x-ray diffraction methods did indeed vary with the initial elastic mismatch strain, and it did so in different ways for different film thicknesses (Bean et al. 1984). As a consequence, the critical thickness theory came under question, and various alternate models were proposed to replace it. However, further study of the problem has revealed the relaxation process to be much richer in physical phenomena than anticipated, with the critical thickness theory revealing only part of the story. 

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