Films diamond, twins

C. Wild, R. Kohl, N. Hetres, W. Miiller-Sebert, and P. Koidl, Oriented CVD diamond films twin formation, structure and morphology. DiamondRelat Mater., 3 373-381 (1994)... 

C Wild, R Kohl, N Herres, W Muller-Sebert, P Koidl. Oriented CVD diamond films Twin formation... 

Formation of twin structures on the faces of diamond crystals and films have also been studied by Koidl s group in the early stage of diamond film research [84], So, we begin with reviewing their works, which is then followed by studies of other groups. To make heteroepitaxial diamond films, it is necessary to avoid the formation of twins, and thus the studies on the formation mechanism and morphology of twins are of great importance. 

Summarizing the results described so far, whether twins are formed on (100) or (111) faces of diamond crystals, depends on the value of the a-parameter under CVD conditions used. For practical applications, it is often necessary to make diamond films with flat surfaces, and thus one must determine CVD conditions... 

Effects of Si substrate orientation, (100), (111), and (110), on oriented diamond growth have been studied in Ref [288] (see also Section 11.16). Continuous diamond films with (100) and (111) orientations could be synthesized on Si(lOO) and Si(l 11) substrates, respectively, using the BEN process. The (110)-oriented continuous film was more random in orientation than (100) and (111), but discrete crystallites with (110) orientation were observed, In Ref [289], (100)- and (11 l)-oriented growth was achieved on Si(lOO) and Si(lll), respectively. In the former case, D 100 //Si(100 and D[110]//Si[l 10], while in the latter, D 111 //Si lll and D[Tl0]//Si[Tl0]. Note, however, that both (100)- and (11 l)-oriented growths were possible on Si(lOO) substrates. The FWHMs of polar and azimuthal angles in XPF measurements were 14° and 11-12", respectively, for the (lOO)-oriented film on Si(lOO). On the other hand, they were 8.5° and IT , respectively, for the (111)-oriented film. In this case, a twinning occurred between diamond grains that were oriented in such a way as D 111 // Si 111 and those that were 60" rotated from... 

Within the lattice as well numerous defects exist They include vacancies, dislocations, stacking disorders, and twinning. Normally, these defects are already generated during nucleation of the crystallites, which means that the density of defects in a diamond film can be influenced by scrupulously controlling the conditions of preparation. 

These defects are generally detrimental to electronic, thermal and mechanical properties of diamond films, although in some cases they may be neutral or beneficial. The frequently occurring defects appear to be stacking faults (twinned clusters with many re-entrant surfaces/comers), which are deemed to play a major role in enhancing diamond growth rates.t ... 

Contemporaneously with Vohra, Michler and co-workers [32] carried out a detailed study of microwave-assisted CVD diamond film growth in methane/carbon dioxide gas mixtures on silicon wafers at different substrate temperatures (560-275°C) by XRD, transmission electron microscopy (TEM), AFM, and Raman spectroscopy. At temperatures above 430°C, the films consisted of nearly defect-free near-(112)-oriented grains with smooth (111) facets, exhibiting steps and risers at the surface. Below a transition region of 340-385°C, the film quality was much deteriorated, as evidenced by much smaller crystal size, increased twin density, and amorphous inclusions at incoherent twin boundaries. The Raman spectra (514.5-nm excitation) in the high-temperature region contained no peaks, but above the transition temperature, peaks at around 1430 and 1540 cm were evident, which the authors attributed to amorphous inclusions (Fig. 9). These observations are consistent with those of Vohra et al. [31] just mentioned. 

The most important crystallographic surface of CVD diamond is the (100) surface. Quasi-heteroepitaxial diamond films with smooth (100) surfaces can meanwhile be deposited on siHcon [65], silicon carbide [66], or iridium [67]. Also, the homoepitaxial growth of diamond is much less susceptible to the incorporation of stacking faults or creation of twin crystals in case of (100) as compared to (111). The (100) surface as illustrated in Figure 10.6 reflects the cubic symmetry of the lattice along the 100 direction. The top three atomic layers are shown in the figure along with... 

Diamond films were made by plasma assisted chemical vapor deposition. Our experiments revealed and defined four types of twins in the diamond films. These include the E=3, E=9, E=27 and E=81 boundaries. We speculate upon a E=243 twin boundary but because of the very small probability for its formation, we did not find it in our films. We shall now outline the main crystallographic characteristics of these twins. 

We have studied and defined four basic twin boundaries that form during the growth of diamond film from the gas phase. The basic E=3 boundary which occurs apparently due to strain build-up in the growing diamond crystal is usually coherent but can form as an incoherent boundary when two parts of the crystal which have a E=3 relationship grow toward each other. E=9 and E=27 boundaries form in a similar way. 

The <111> penetration twins on both 100 and 111 faces of the CVD diamond crystals have been identified and they have been shown to play a major role in determining the film morphology. °°... 

In Ref. [310], an HRTEM study was carried out for an HOD film of less than 1-pm thickness, also made by the two-step process of Ref. [263]. A majority of diamond grains were (100)-oriented, and twins, which started from the interface, were frequently found in the grains. At the interface between diamond and Si, a few... 

In Fig. 3a, cubic and octahedral faces are evident, and in Fig. 3b the twinned crystals with pseudo-fivefold symmetry can be clearly seen. This twinned fivefold symmetry is prevalent in CVD diamond thin films and apparently never develops on homoepitaxially grown crystals.Balllike diamond crystals are grown at high supersaturationsf (Fig. 3c). 

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