Studying epitaxial films

When the crystals that comprise the films are all oriented the same way, the first study that should be conducted is, naturally, to determine this common orientation. The evaluation of the disorientation with respect to the normal to the film-substrate interface is performed with the use of rocking curves, whereas the orientation within the plane is determined by ( )-scans, which consist of measuring the evolution of the intensity diffracted by planes that are not parallel to the interface, while the sample is rotating aroimd the normal to this interface. These techniques have become common and most manufacturers sell diffractometers adapted to this kind of study. 

More recently, measurements have been developed based on the complete evaluation of the intensity distribution over the entire diffraction spot. This reciprocal space mapping [FEW 97] can be used to give an account of all the microstractural effects that have an influence on the diffraction signal. Thus, it is possible to obtain the distribution functions of orientation, size, morphology and the size distribution of the diffracting domains, as well as the microstrains. 

We will first describe the methods for accurately determining the orientation of the crystals and then, in a second step, we will ejq)lain in detail how reciprocal space mapping can be used to quantitatively analyze the microstracture of epitaxial films. 

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Froment M, Bernard MC, Cortes R, Makili B, Lincot D (1995) Study of CdS epitaxial films chemically deposited from aqueous solutions on InP single crystals. J Electrochem Soc 142 2642-2649... 

Cachet H, Cortes R, Froment M, Mamin G (1999) Epitaxial growth of electrodeposited cadmium selenide on (111) gallium arsenide. Philos Mag Lett 79/10 837-840 Muthuvel M, Stickney JL (2006) CdTe Electrodeposition on InP(lOO) via Electrochemical Atomic Layer Epitaxy (EC-ALE) Studies Using UHV-EC. Langmuir 22 5504-5508 Streltsov EA, Osipovich NP, Ivashkevich LS, Lyakhov AS (1999) Effect of Cd(ll) on electrodeposition of textured PbSe. Electrochim Acta 44 2645-2652 Beaunier L, Cachet H, Cortes R, Froment M (2000) Electrodeposition of PbSe epitaxial films on (111) InP. Electrochem Commun 2 508-510... 

The monomer Ga(AsBut2)3 has been used in a number of deposition studies. In an initial study, films were grown at 480 °C under reduced pressure (5-10 torr) and gave epitaxial films with low carbon incorporation. The nonstoichiometric As Ga ratio (3 1) in the precursors gave rise to -type films, whereas the films are generally p-type when prepared using the dimers mentioned... 

This study also reported that films deposited on carbon membranes at temperatures >80°C were of hexagonal (wurtzite) structure, with a high density of planar defects, in contrast to the zincblende obtained from both hydroxide and ion-by-ion mechanisms at lower temperatures and to the epitaxial films on InP at all temperatures. 

Studies of semiconductor films have shown many facets. The properties of epitaxial films have mainly been investigated on Ge and Si, and to a lesser degree on III—V compounds. Much work, lias been done on polycrystalline II-VI films, particularly with regard to the stoichiometry of the deposits, doping and post-deposition treatments, conductivity and carrier mobility, photo-conductance, fluorescence,electroluminescence, and metal-semiconductor junction properties. Among other semiconductors, selenium, tellunum. and a few transition metal oxides have found some interest. 

Other systems can be treated in a similar manner such as the Ti-B-CI-H system,10 and of course the Si-H-CI system has been exhaustively studied because of the commercial importance of silicon epitaxial films used for integrated circuit fabrication.11... 

Although cracking is often observed in GaN epitaxial films, there have been very few publications on this subject [20,21]. Hiramatsu and co-workers studied residual stress in thick GaN films grown by HVPE [21]. For films <4 pm thick, compressive strain was observed in the films by an increase in the c lattice constant. However for films 4 - 20 pm thick, microcracks were observed near the film/substrate interface [21], Nakamura has suggested that a 0.1 pm thick In0.05Ga0.9jN layer below the AlGaN clad layer in laser devices is needed to suppress cracking in nitride films [22],... 

This last case occurs, for example, when studying epitaxial thin films. This matter will be discussed in Part 2 of this book. 

The study by X-ray diffraction of epitaxial films is a field of its owm Because they are comprised of a set of crystals all of which have practically the same crystallographic orientation, epitaxial thin films ate close to being single crystals. However, they are polycrystalline samples, so they deserve their own section in this book. Note, however, that the development of instruments designed for the study of these films owe a lot to the diffractometers intended for single crystals, and particularly those that focus on the study of defects in single crystals. 

The prodnction of epitaxial thin films was developed particularly by the microelectronics indnstry, based on semi-condnctor materials [FEW 01], The continuons decrease in the thickness of films, combined with the miniatiuization of the systems, has led to the development of epitaxial films which are sometimes only a few nanometers thick. Fnrthermore, the development of new devices with magnetic, optical or electronic properties has enconraged the introdnction of oxide crystals in active components. The stracture of these oxides and the microstractures of the films prodnced are often complex, and improving the properties requires adequate characterization capabilities. X-ray diffraction, probably along with electronic or scanning probe microscopes, is one of the key techniques in the study and constant improvement of the characteristics of epitaxial films... 

In this context, diffractometers designed for the study of epitaxial films first became available on the market about 20 years ago. The enthusiasm for nanotechnologies has led to very significant developments in this field. Several books specific to X-ray diffraction on epitaxial films have been published over the past few years [BOW 98, PIE 04]. In the second part of this book, we will describe in detail the methods for quantitative analysis of the microstructure of epitaxial films. The objective in the following sections is to give a description of the basic systems that make it possible, beyond the simple determination of the crystallographic orientation, to achieve this type of analysis. 

Figure 2.69. Schematic drawing of a complete sample holder designedfor the study of epitaxial films, which enables the user to make a three-dimensional measurement of the intensity distribution...

The most thoroughly characterized surface diffraction study at a single crystal surface is that of underpotentially deposited lead on a silverfl 11) single crystal electrode (epitaxial film on mica). " These investigators began by measuring the CTR of the silver substrate since, unless these were clearly observed, it would be unlikely that diffraction from an electrodeposited monolayer would be detected. 

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