Thin epitaxial layers

From a practical point of view, the optical detection of possible X—H bonds in hydrogenated samples is performed at LHeT as a better sensitivity is obtained at this temperature because the features are sharper than the ones observed at ambient. The sensitivity of Fourier Transform Spectroscopy (FTS) allows usually a normal incidence geometry of the optical beam. Two kinds of samples are generally used in the hydrogenation studies. The first are thin epitaxial layers (1 to 5 in thickness) with dopant concentrations in the 1017-102° at/cm3 range on a semi-insulating... 

The scattered intensity is proportional to the volume of the crystal. This implies that the scattering from a thin epitaxial layer, large in area compared with the beam diameter, will be proportional to the layer thickness. 

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

Originated in the late 1970 s, ALE has become a widely used synthesis route for thin epitaxial layers. Many ALE coated materials are produced commercially today. 

F. Besenbacher, L. Pleth Nielsen and P. T. Sprunger, Surface alloying in heteroepitaxial metal-on-metal growth, in Growth and Properties of Ultra-thin Epitaxial Layers, Vol. 8 of The Chemical Physics of Solid, Surfaces and Heterogeneous Catalysis, eds. D. A. King and D. P. Woodruff (Elsevier Science, Amsterdam, 1997), p. 207. 

Pd particles (1-15 nm) were prepared by ultra high vacuum (UHV) condensation of the metal on clean MgO or ZnO microcrystals, on ZnO (00.1) thin epitaxial layers synthesized in situ and on clean (001) MgO single crystals, air cleaved and in situ cleaned. The metal was evaporated from a Knudsen cell at a calibrated flux b The temperatures of the substrates were between 400 °C... 

Pd and bimetallic PdCu and PdCu3 particles have been prepared, both under UHV conditions and by decomposition of organometallic compounds, on MgO and ZnO micro-crystals, single crystals or thin epitaxial layers. The shapes and structures of the particles and the structures of the interfaces, and the dependence of these on particle size and annealing conditions, were determined by HRTEM and WBDF imaging. 

Growth from the vapor is the preferred phase transition for the production of thin epitaxial layers, while growth of bulk crystals from the vapor is rather the exception, only applied if unavoidable. An example is the growth of semiconductor-grade Sic by the so-called modified Lely method (MLM), a sublimation technique. The physical reason for the avoidance of vapor growth techniques for bulk crystals is the huge difference in the particle densities between the two states of aggregation. 

Group 12 (II B) bis(diethyldithiocarbamate) complexes, [MISaCNEtala] (M = Zn, Cd), were first utilized toward the growth of thin films of ZnS and CdS by O Brien and co-workers in 1989 (1969). Thin films were obtained at pressures of 10 " Torr and between 370 and 420°C. The CdS films were polycrystalline and hexagonal on glass, but thin epitaxial layers were obtained on InP(lOO) and GaAs (100). The quality of ZnS films obtained by this method was lower than those of CdS. 

In a heterostRieture laser, the aetive region ean be defined by epitaxial layers and made eonsiderably thinner. In GaAs/Al Ga. As heterostRietures, the aetive region ean be made as thin as 100 nm, and the threshold euRent... 

Autodopiag occurs whea dopants are unintentionally released from a substrate through diffusion and evaporation, and subsequently reiacorporated during the deposition layer. Epitaxial layers are typically doped at concentrations of lO " -10 atoms/cm. The higher levels of doping are used in bipolar technology where the epilayer forms the transistor base. The epitaxial layer can be up to several hundred micrometers, and as thin as 0.05—0.5 p.m. Uniformities of 5% are common. 

The short penetration depth of UV/blue photons is the reason that frontside CCD detectors have very poor QE at the blue end of the spectrum. The frontside of a CCD is the side upon which the polysilicon wires that control charge collection and transfer are deposited. These wires are 0.25 to 0.5 /xm thick and will absorb all UV/blue photons before these photons reach the photosensitive volume of the CCD. For good UV/blue sensitivity, a silicon detector must allow the direct penetration of photons into the photosensitive volume. This is achieved by turning the CCD over and thinning the backside until the photosensitive region (the epitaxial layer) is exposed to incoming radiation. 

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. 

Various microstructures and configurations are possible for useful solid materials, including bulk single crystals and epitaxial layers, polycrystalline articles or thin films with controlled grain size (including micro- and nanocrystalline... 

Significant improvements in ECALE deposit morphology and quality were reported as achieved by switching from a thin layer cell to a thick layer H-form cell, integrated in an automated deposition system [46]. Thin epitaxial films of zinc blende CdTe, CdSe, and CdS with predominate (111) orientations were grown. 

Silicon-based pressure sensors are amongst the most common devices making use of this process. A thin low-n-doped epitaxial layer on the wafer determines an etch stop depth and thus the thickness of e.g. the pressure sensor membrane. 

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. 

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. 

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. 

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