Orientation dependent surface stabilization on flame deposited diamond single crystals

In this study a model for the orientation dependent surface stabilization of diamond grown from the vapour phase is presented. The model is based on a first order broken bond analysis of the diamond crystal with additional considerations about surface reconstruction by dimer formation. Due to the formation of dimers, surface stabilization is expected for orientations corresponding to Miller indices {hhk} h < k in the (110) zones. It is demonstrated that this model gives a good description of growth phenomena like facets, curved bands and microfaceting, observed on homoepitaxially grown diamond samples. These specimens are obtained by flame deposition on cylindrical type IIa natural diamond substrates with {111}, {110} or {001} top faces. In conformation with the model the surfaces {hhk}h < k between {111} and {113} appear to be stabilized in one direction, while a discrete number of orientations between {113} and {001} are subject to two-dimensional stabilization. All other surfaces are stabilized by the development of microfacets which also have orientations close to {hhk} h <_ k, as was determined by laser reflectometry. In addition it is shown that differences in the orientation dependent incorporation of nitrogen as nitrogen-vacancy pairs in the diamond lattice is consistent with the described model. For this purpose a single crystalline diamond layer grown on top of a hemispherical diamond substrate by the flame technique was investigated by cathodoluminescence topography. I . I n t r o d u c t i o n Recent ly homoepitaxial d iamond growth by the ace ty lene-oxygen combust ion flame technique [1] was repor ted on cylindrically shaped type I Ia natural d iamond substrates with {001}, {110} and {111} top faces [2-7]. Infrared, R a m a n and luminescence spectroscopy demons t ra ted that the quality of the {001} and {111} samples is comparable to that of type IIa natural diamond, a l though * Corresponding author. crystallographic imperfections like cracks and twins were observed for the {111} samples [5,6]. The quality of the {110} specimens is slightly less, amongst others due to an enhanced incorporat ion of hydrogen and ni trogen [5]. The samples exhibit facets and curved bands in the { h h k } h <_ k parts of the (110) zones. Orientat ions between {111} and {113} in this zone appeared to be stabilized in one direction, while the stabilization for orientat ions between {113} and {001} has a more two-dimensional character. All o ther orientations, including the [ h h k } h > ~ parts of the (110) zones, like {110}, have rough, pit ted surfaces due to the develop0022-0248/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0022-0248(94)00958-9 J.Z Schermer et al. /Journal of Crystal Growth 148 (1995) 248-260 249 ment of microfacets. The observed stabilization of surfaces in the {hhk}h< k parts of the (110) zones instead of those in the {hhk} h > k parts, is not expected from a periodic bond chain analysis of the diamond crystal [8]. Gardeniers et al. used a first order broken bond analysis to describe the orientation dependent homoepitaxial deposition of silicon on hemispheres [9,10]. In order to explain the observed growth phenomena for orientations with Miller indices {hhk}h<k, they put forward additional considerations about surface reconstruction and adsorption of species. Although this model was derived for silicon chemical vapour deposition (CVD) under near equilibrium conditions, it is applicable to describe the surface stabilization of vapour phase grown crystals with a diamond structure in general. Even for diamond growth, which is considered to be a process far from equilibrium, the model proved to be. successful in the qualitative description of the temperature dependent occurrence of {113} facets on homoepitaxially grown single crystals [4]. In the present study a model will be derived according to the same principles which gives a more detailed description of the possible two-dimensional stabilization for a discrete number of surfaces with orientations between {113} and {001} in the (110) zones. This model will be used as a starting point to discuss the observed features like facets, curved bands and microfacets on the diamond single crystals mentioned above. In addition it will be demonstrated that there is a relation between the surface stabilization as described by the model and the orientation dependent incorporation of nitrogen as nitrogen-vacancy pairs into the diamond lattice. 2. Theory of surface stabilization In general, {hkl} facets on a crystal will develop for those orientations that have a connected network of atoms or molecules within the repeat distance dhk l [12]. Following the GibbsWulff theorem, the development of these facets is possible because they have a lower growth rate than all their neighbouring orientations. On such