Quantum epitaxial growth

The fundamental issue of epitaxial growth on polycrystalline substrates has been addressed in a more refined manner in relation to the electrodeposition of CdSe on metals. Polycrystalline, lll -textured Au surfaces were shown [17] to promote the electrodeposition of coherent, epitaxial CdSe quantum dot films over areas micrometers in size, i.e., much larger than the polycrystalline Au grains, despite the numerous grain boundaries present in the substrate. The Au films (considered as... 

Mitsuru Sugawara, Theoretical Bases of the Optical Properties of Semiconductor Quantum Nano-Structures Yoshiaki Nakata, Yoshihiro Sugiyama, and Mitsuru Sugawara, Molecular Beam Epitaxial Growth of Self-Assembled InAs/GaAs Quantum Dots... 

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. 

Lincot, Froment, and Cachet review the chemical and mechanistic aspects of chemical bath deposition of chalcogenide compounds with special emphasis on structural properties associated with epitaxial growth. Present applications of chemically deposited films are reviewed and several characteristic advantages are identified that may be exploited in the future for applications such as small band gap semiconductors, large area electrochromic devices, electroluminescence, quantum sized films, and films with spatially modulated composition and structure. 

Confined electronic systems are quantum systems in which carriers, either electron or holes, are free to move only in a restricted number of dimensions. In the confined dimension, the sizes of the structural elements are of the order of a few de Broglie wavelengths of the carriers or less. Depending on their dimensionality, these structures can be quantum dots (0-D), quantum wires (1-D), or quantum wells (2-D). Quantum wells are typically produced by the alternate epitaxial growth of two or more different semiconductors. Quantum wires are less commonly encountered, since their fabrication procedures are much more complicated (Sect. 5.3.4). 

The effect of size quantization on the electronic properties of semiconductors, discussed in Section 9.2.2, demonstrates that semiconductor electrodes made of nanostructured particles are of great practical interest. Based on size quantization, these films can be categorized into (a) thin semiconductor films deposited or epitaxial growth on a substrate where the SQE is due to the space confinement in two dimensions (i.e., a quantum well) and (b) particulate films of size-quantized nanoparticles that may be several micrometers thick their properties are due to the combined effect of film and isolated size-quantized particles. Both the situations are illustrated in Figure 9.41. 

Figure 941 Types of nanostructured semiconducting films (a) quantum well prepared by epitaxial growth in nanometer thickness and (b) particulate film in micrometer thickness.

Nanocrystalline particulate films, which exhibit pronounced quantum size effects in three dimensions, are of great interest due to applications in solar cell (108-112) and sensor (57, 113-115) applications. They exhibit novel properties due to not only the SQE manifested by individual nanoparticles but also the total surface area. Unlike MBE and MOCVD methods used to prepare quantum well electrodes, these electrodes can be prepared by conventional chemical routes described in Section 9.5.2.2. For example, II-VI semiconductor particulate films were prepared by using low concentrations of precursors and by controlling the temperature of the deposition bath. Nodes demonstrated the SQE for CdSe thin films deposited by an electroless method (98). The blue shift in the spectra of CdSe films has been demonstrated to be a function of bath temperature. As described in Section 9.5.2.1, electrodeposition of semiconductors in non-aqueous solvents leads to the formation of size-quantized semiconductor particles. On a single-crystal substrate, electrodeposition methods result in epitaxial growth (116, 117), and danonstrate quantum well properties. 

The research on bulk crystallization of GaN by HVPE as well as on the epitaxial growth and studies of nonpolar GaN-based quantum structures was supported by Polish Committee for Scientific Research GRANT Nr 3T08A 033 29 and GRANT NrlP03B05329. 

In semiconductors, PL originates from the radiative recombination of pho-toexcited electron-hole pairs. Their nonradiative lifetime is determined by both bulk and surface recombination. Therefore, the major problem in observing PL from semiconductor surfaces and interfaces is to minimize signals which arise from defects and impurities in the bulk. This can be achieved with an improvement in material quality by means of an epitaxial growth together with the fabrication of special structures, such as semiconductor heterostructures and quantum wells. The control of the epitaxial growth process allows one to vary the relative contribution of bulk and surface recombination independently of each other. 

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