F.K. de Theije

Oxidative etching of diamond

Abstract Three different ways are used to etch diamond {001} and {111} faces by oxidative methods: gas phase etching in dry oxygen, gas phase etching in a mixture of oxygen and water vapour, and liquid etching in molten potassium nitrate. The surface topography of the etched surfaces is compared with the theoretically predicted morphology. It appears that in most cases the experimental results are in contradiction to theory. These discrepancies are explained by the reactions between the diamond surfaces and the adsorbed oxygen, which are able either to stabilise or to destabilise the surfaces. Of these three methods, etching in molten potassium nitrate was found to be the best for revealing dislocations.

Effects of nitrogen impurities on the CVD growth of diamond: step bunching in theory and experiment

Abstract The step flow dynamics during homoepitaxial diamond deposition on vicinal {001} diamond surfaces in the presence of nitrogen impurities in the gas phase is investigated by experiments as well as by computer simulations. The adsorption of impurities on the diamond surfaces leads to step bunching, whereby complex two-dimensional patterns are formed. A mesoscopic Monte Carlo model is used to study this effect, whereby line tension, anisotropy in step propagation, roughness of steps and impurity strength are introduced on a physical basis. The similarity between experimental results and the step configurations resulting from simulations is remarkable. Furthermore, the coexistence of both step bunching and an increased growth rate upon nitrogen addition is explained by the fact that nitrogen on the surface has a different effect on the diamond growth than sub-surface nitrogen. This study gives a demonstration of a fruitful co-operation between computer simulations and real world experiments, which leads to a better understanding of the physical processes occurring during crystal growth.

A surface topographic investigation of {001} diamond surfaces etched in oxygen

Abstract The mechanism of material removal of {001} diamond surfaces etched in a flow of 10% oxygen in argon at atmospheric pressure has been studied using ex situ and in situ differential interference contrast microscopy, atomic force microscopy and scanning electron microscopy. It is shown that shallow, square etch pits are formed and etching proceeds by a step mechanism, which implies that the {001} diamond surface is strongly stabilized. The possible carbo—oxygen complexes which might be responsible for the stabilization of the {001} diamond surfaces are discussed. At high etching temperatures the sides of the etch pits are parallel to the ⟨110⟩ directions. At etching temperatures below 750°C, the sides change from ⟨110⟩ to ⟨100⟩ directions, as a result of the development of ⟨100⟩ oriented {100} ‘walls’ on the sides of the pits. It is suggested that ketone complexes are responsible for stabilization of the steps in the ⟨110⟩ direction, whereas the ‘walls’ are formed due to stabilization of adjacent, colliding ⟨100⟩ steps. Furthermore, the pits bounded by ⟨110⟩ side faces often have a concave outline. This phenomenon is explained by step interlacing: at the pit corners, slowly advancing double steps split up into fast single steps due to the 41 symmetry operator perpendicular to the {001} diamond face.

Mosaic growth of diamond: a study of homoepitaxial flame deposition and etching of (001}-oriented diamond layers

This study describes homoepitaxial growth and etch phenomena observed on cubic diamond faces as obtained by the application of an acetylene-oxygen combustion flame. It is demonstrated that flame etching of the diamond substrates prior to growth decreases the number of crystallographic imperfections in the diamond layers especially at the edges. This enables a more uniform distributed step generation by the large number of individual dislocations present in the used natural type IIa diamond substrates. Due to this improvement it appears possible to grow one single crystal diamond layer on top of several closely spaced, well-aligned natural diamond substrates. In the present work the overgrowth of a three-piece mosaic structure with gaps along (100) as well as along (110) is reported for the first time. The single crystal nature of this flame-deposited diamond layer could be confirmed by the observation of continuous growth step patterns at the surface across the joint regions. The widths of the gaps between the substrates in the original mosaic structure varied from 7 to 25 ~m, which is more than an order of magnitude larger than the gaps along (100) in two-piece mosaic structures, of which the single crystal overgrowth by HFCVD was previously reported. Comparison of micro-Raman spectra obtained from the joint regions with those from other parts of the overgrown mosaics discussed in the present work, show a larger FWHM of the diamond peak in case of a (110) joint and a shift of the peak position to higher wave numbers in case of a (100) joint.

Simulation of step patterns on natural diamond {111} surfaces

Almost all {111} surfaces of natural diamond crystals show trigons: triangular etch pits centred on dislocations. The step patterns that arise when two trigons meet allow us to propose an analytical expression for the velocity of steps during etching as a function of their orientation. This step velocity function has a deep global minimum for 110 steps on a surface inclined towards {110} and a local minimum for 110 steps on a surface inclined towards {100}. Continuum simulations, based on the kinematic wave theory, of the evolution of step patterns using the step velocity function are able to reproduce quite satisfactorily intricate step patterns formed by multiple interacting trigons. This means that the assumption, implicit in the simulation, that bulk and surface diffusion are not rate limiting during etching is valid. Occasionally, growth hillocks are observed on a {111} surface of a natural diamond. The step patterns on these surfaces show that the step velocity function governing growth does not have the local minimum for 110 steps on a surface inclined towards {100}. This confirms that the occasionally observed growth patterns and the usually observed etching patterns are formed under fundamentally different conditions.

Temperature dependent morphology transition of GaN films

The temperature dependence of the surface morphology of GaN epilayers was studied with AFM. The layers were grown by low pressure MOCVD on (0001) sapphire substrates in the temperature range of 980--1,085 C. In this range the (0001) and {l_brace}1{bar 1}01{r_brace} faces completely determine the morphology of 1.5 {micro}m thick Ga-faced GaN films. For specimens grown at 20 mbar and temperatures below 1,035 C the {l_brace}1{bar 1}01{r_brace} faces dominate the surface, which results in matt-white layers. At higher growth temperatures the morphology is completely determined by (0001) faces, which lead to smooth and transparent samples. For growth at 50 mbar, this transition takes place between 1,000 C and 1,015 C. It is shown that the morphology of the films can be described using a parameter {alpha}{sub GaN}, which is proportional to the relative growth rates of the (0001) and the {l_brace}1{bar 1}01{r_brace} faces.