Epitaxial layers, electronic

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,... 

Electrochemical behavior of ultrathin Pd epitaxial layers deposited electrochem-ically on Au(lll) and Au(lOO) has been found to be strongly dependent on the surface structures and the thickness of the Pd thin films [435]. From the kinetic studies of Pd deposition on Au(lll) electrode from K2 PdCU in 0.1 M H2 S O4, it has been deduced [436] that this process proceeds via an instantaneous nucleation and two-dimensional (2D) growth. Initial stages of Pd deposition on Au(llO) have also been studied by Robach et al. [437], who have applied STM, low-energy electron diffraction, and Auger electron spectroscopy for this purpose. 

Details of building specific types of microelectronic devices are well described by Grovenor15 as an illustration, we consider only the chemical techniques involved in making a metallic contact to a silicon wafer surface. Preparation of a wafer of Si or other material and application of epitaxial layers of semiconducting or insulating materials, if required, were outlined in Section 19.2.1. The construction of shaped features on the wafer is usually done by photolithography, or ion- or electron-beam variants thereof. 

Fig. 8. Energy below the conduction band of levels reported in the literature for GaAs. Arrangement and notations are the same as for Figs. 4 and 5. Notations not defined there are epitaxial layer on semi-insulating substrate (EPI/SI), boat-grown (BG), vapor phase epitaxial layer on semi-insulating substrate (VPE/SI), melt-grown (M), molecular beam epitaxy (MBE), horizontal Bridgman (HB), irradiated with 1-MeV electrons or rays (1-MeV e, y), thermally stimulated capacitance (TSCAP), photoluminescence excitation (PLE), and deep level optical spectroscopy (DLOS).

PVD reactors may use a solid, liquid, or vapor raw material in a variety of source configurations. The energy required to evaporate liquid or solid sources can be supplied in various ways. Resistive heating is common, induction heating of the source bottle is sometimes used, and electron beams are also employed. Molecular-beam-epitaxy (MBE) systems are PVD-type reactors that operate at ultrahigh vacuum. Very low growth rates are used ( 1 xm/h), and considerable attention is devoted to in situ material characterization to obtain high-purity epitaxial layers (2). 

In molecular beam epitaxy (MBE) [317], molecular beams are used to deposit epitaxial layers onto the surface of a heated crystalline substrate (typically at 500-600° C). Epitaxial means that the crystal structure of the grown layer matches the crystal structure of the substrate. This is possible only if the two materials are the same (homoepitaxy) or if the crystalline structure of the two materials is very similar (heteroepitaxy). In MBE, a high purity of the substrates and the ion beams must be ensured. Effusion cells are used as beam sources and fast shutters allow one to quickly disrupt the deposition process and create layers with very sharply defined interfaces. Molecular beam epitaxy is of high technical importance in the production of III-V semiconductor compounds for sophisticated electronic and optoelectronic devices. Overviews are Refs. [318,319],... 

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. 

Nitride-based photoconductive detectors consist of a single undoped or lightly doped epitaxial layer with interdigitated electrodes deposited and patterned on the top surface. Optical absorption in the semiconductor layer produces electron-hole pairs which can be swept out by an applied bias voltage to detect a measurable current proportional to the incident photon flux. A summary of published photoconductive detector performance is shown in TABLE 1. 

Figure 8.1 Diffraction pattern obtained from a GaN/SiC plan-view sample showing no lattice rotation between the GaN epitaxial layer and the SiC substrate. Reflections from GaN and SiC are marked by horizontal arrows and vertical arrows, respectively. All other reflections arise from double diffraction. The (0110) reflection is forbidden in 6H-SiC. The GaN layer was grown by H. Morkof and his group at the Virginia Commonwealth University by MOCVD on (0001) SiC at 900°C. Most of the GaN grown layer and SiC substrate were polished and ion milled away until a thin (<50 nm thick) bi-layer remains for the transmission electron microscope observations...

The technique of Raman scattering (RS) to study vibrational spectra in the numerous polytypes of SiC will be described. An explanation of the various notations used to describe the stacking sequences in these polytypes will then be given. Section C discusses the various optical phonons studied by RS and the concept of a common phonon spectrum for all polytypes will be introduced. Raman studies are also used to assess crystalline structure and quality of epitaxial layers of SiC on Si and SiC substrates. Section D outlines several other excitations of interest, e.g. polaritons, plasmons, and electronic RS, as well as impurity and defect recognition in irradiated and ion implanted material. 

Sublimation is one of the main methods of growing silicon carbide. This method is employed for growth of the material for abrasive applications as well as for the growth of single crystals and epitaxial layers for use in semiconductor electronics. The idea of the method is fairly simple, and is based on material transport from a hot source of material to a substrate which rests at a somewhat lower temperature. The transport is performed by the intrinsic vapour of the material at a high temperature, usually in the range 1600-2700 °C. 

Channelling only requires a goniometer to include the effect in the battery of MeV ion beam analysis techniques. It is not as commonly used as the conventional backscattering measurements because the lattice location of implanted atoms and the annealing characteristics of ion implanted materials is now reasonably well established [18]. Channelling is used to analyse epitaxial layers, but even then transmission electron microscopy is used to characterize the defects. 

Fig. 27 A STM image (llxl3nm ) at the liquid-graphite interface of a mixture of PAH and EPPAH. It is an image of a second epitaxial layer with a donor acceptor stoichiometry of 2 1 on top of a first epitaxial layer of PAH. The large features are PAH, the smaller and brighter features are EPPAH. EPPAH packs only every second row prohahly because of its preferential adsorption on the electron donor disk of PAH, which is exposed in the underlying first PAH layer (see model B). (Reproduced with permission from [81])...

Another concept in synthesis is epitaxy. Epitaxy is the continuation of the crystal orientation of the monocrystalline substrate in the deposited crystalline product, which may be the same compound as the substrate or a different solid that has the same crystal orientation as the monocrystalline solid. Epitaxial layers are essential for microlithography in the electronic industry carefully formed epitaxial layers do not have localized electronic interface states, which are deleterious for the functioning of the device. The process conditions for epitaxy by molecular beam epitaxy (MBE) are very low process pressure, comparatively high temperatures, and a low growth rate. MBE is a form of CVD, which was described in Chapter 6. Liquid phase epitaxy (LPE) is a form of growth of single crystals from a melt. 

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