Epitaxial layers superlattice

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... 

We now assume that we have properly recorded rocking curves available, and that a substrate with a single epitaxial layer >0.5 //m thick (and less than, say, 5 m) is measured. This will result in two peaks, one each from the substrate and layer. The same analysis will apply to multiple peaks (other than those from superlattices) provided that they are well separated so that interference effects are minimised. The basic parameters are derived as follows, with the symmetric reflection used unless otherwise specified. The examples are given for (001) substrates and layers, but are quite general (with the caution that if the Poisson ratio is required, the value appropriate to the crystal orientation should strictly be used). 

In Chapter 3 we went as far as we could in the interpretation of rocking curves of epitaxial layers directly from the features in the curves themselves. At the end of the chapter we noted the limitations of this straightforward, and largely geometrical, analysis. When interlayer interference effects dominate, as in very thin layers, closely matched layers or superlattices, the simple theory is quite inadequate. We must use a method theory based on the dynamical X-ray scattering theory, which was outlined in the previous chapter. In principle that formrrlation contains all that we need, since we now have the concepts and formtrlae for Bloch wave amplitude and propagatiorr, the matching at interfaces and the interference effects. 

Thin epitaxial layers display a rich variety of X-ray optical phenomena which can be exploited for materials characterisation. Superlattice structures in... 

MBE MD-SLS MESFET molecular beam epitaxy modulation doped strained-layer superlattice metal-semiconductor field effect transistor... 

Nevertheless few results have been obtained concerning their epitaxial growth. Electrodeposition of epitaxial CdSe quantum dots on gold single crystals has been reported [218, 219]. In the same way epitaxial layers of CdTe [220] and CdSe [221] have been electrodeposited on (111) InP. Using cyclic voltammetry K. Rajeshwar [222] has electrosynthesized CdSe/ZnSe superlattices (non epitaxial). XPS depth profiles have clearly demonstrated the modulation in the Cd and Zn content. 

A periodic arrangement of many epitaxially grown thin layers with lattice mismatch constitutes a strained-layer superlattice. An example of such a superlattice structure can be found in the vertical-cavity surface-emitting laser (VCSEL). As discussed by Choquette (2002) and Nurmikko and Han (2002), the control of layer thickness, elastic strain due to LAN to us mismatch, stress-driven crack formation and processing induced defects in the superlattice presents major scientific and technological challenges in the development of these devices. 

Bean, J. C., Feldman, L. G., Fiory, A. T., Nakahara, S. and Robinson, I. K. (1984), Gea Sii a /Si strained layer superlattice grown by molecular beam epitaxy, Journal of Vacuum Science and Technology A2, 436-440. 

Perovic, D. D., Weatherly, G. G., Baribeau, J. M. and Houghton, D. C. (1989), Heterogeneous nucleation sources in molecular beam epitaxy grown Gea,Sii a /Si strained layer superlattices, Thin Solid Films 183, 141-156. 

Besides its high sensitivity even at low material concentrations, Raman spectroscopy provides the advantage of simple applicability, high lateral resolution in the range of micrometers, and the variation of the information depth. Furthermore, in the fabrication of quantum wells and superlattices, Raman spectroscopy can be used for on-line growth control of the crystallinity and thickness of the epitaxial layers. 

T Karasawa, K Ohkawa, T Mitsuyu. Molecular-beam epitaxial-growth and characterization of ZnS-Zn Cdi S strained-layer superlattices. J Appl Phys 69 3226-3230, 1991. 

YH Wu, H Yang, H Ishida, H Fujiyasu, S Nakashima, K Tahara. High-quality ZnTe-ZnSe strained-layer superlattice with buffer layer prepared by hot wall epitaxy. Appl Phys Lett 54 239-241, 1989. 

Strained-layer superlattice An epitaxial thin film where the lattice spacing of the crystalline structure of the film material has been strained but not to the point of creating dislocations. 

The proposed technique seems to be rather promising for the formation of electronic devices of extremely small sizes. In fact, its resolution is about 0.5-0.8 nm, which is comparable to that of molecular beam epitaxy. However, molecular beam epitaxy is a complicated and expensive technique. All the processes are carried out at 10 vacuum and repair extrapure materials. In the proposed technique, the layers are synthesized at normal conditions and, therefore, it is much less expansive. The presented results had demonstrated the possibility of the formation of superlattices with this technique. The next step will be the fabrication of devices based on these superlattices. To begin with, two types of devices wiU be focused on. The first will be a resonant tunneling diode. In this case the quantum weU will be surrounded by two quantum barriers. In the case of symmetrical structure, the resonant... 

Summarizing, it is possible to conclude that the technique of forming ultrasmall semiconductor particles turned out to be a powerful tool for building up single-electron junctions, even working at room temperature, as well as thin semiconductor layers and superlattices with structural features, reachable in the past only via molecular beam epitaxy. 

Zou S, Weaver MJ (1999) Surface-enhanced Raman spectroscopy of cadmium sulfide/cadmium selenide superlattices formed on gold by electrochemical atomic-layer epitaxy. Chem Phys Lett 312 101-107... 

A second application of current interest in which widely separated length scales come into play is fabrication of modulated foils or wires with layer thickness of a few nanometers or less [156]. In this application, the aspect ratio of layer thickness, which may be of nearly atomic dimensions, to workpiece size, is enormous, and the current distribution must be uniform on the entire range of scales between the two. Optimal conditions for these structures require control by local mechanisms to suppress instability and produce layer by layer growth. Epitaxially deposited single crystals with modulated composition on these scales can be described as superlattices. Moffat, in a report on Cu-Ni superlattices, briefly reviews the constraints operating on their fabrication by electrodeposition [157]. 

In general, unlike for the perfect epitaxial structures of fully strained materials, for nitride heteroepitaxial layers it is essential to perform not a single scan for a symmetrical reflection, but a set of two- or even three-dimensional maps of symmetrical and asymmetrical reflections. Additionally, for some applications, an intense beam is needed and therefore low-resolution X-ray diffractometry can be sometimes a preferable technique to the commonly used high-resolution XRD. For example, if we examine a heterostructural nitride superlattice, low resolution diffractometry will give us a broader zeroth-order peak (information on the whole layer) but more satellite peaks (information on the sublayers). Therefore, multipurpose diffractometers with variable configurations are the most desirable in nitride research. 

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