QUANTIFICATION OF DEFECTS IN EPITAXIAL METAL-FILM GROWTH - A HELIUM DIFFRACTION INVESTIGATION OF THE CU/W(110) SYSTEM

Growth of Cu epitaxial films on W(110) has been studied by monitoring the specular helium intensity during film deposition, We observe oscillations in the elastic scattering intensity with coverage under both out-of-phase and in-phase conditions. The positions of maxima and minima do not always occur on integral and half integral layers. The deviation is due to departure from layer-by-layer growth or increased defect density. Due to the large scattering cross sections for the scattering from adatoms, clusters, step edges and vacancies, our study gives new information on the defect density during film growth. Our results indicate that no more than 5% of the Cu atoms are at defect positions for a 0.5 ML film deposited at 300 K. Thick films (3-5 ML) are very smooth and we estimate the number of atoms at defect sites to be less than 10%. Although more intensity oscillations with coverage are observed at 170 than at 300 K, the film quality is lower due to the higher defect density which occurs for deposition at low temperatures. In agreement with earlier work, we observe that the first layer is pseudomorphic, the second layer is a strained Cu(111) plane and the third layer has an undistorted Cu(111) structure. Diffraction is observed from the buckling of the bilayer and trilayer. Whereas the bilayer buckling amplitude is consistent with simple models, the trilayer buckling is less than expected and this buckling decreases further for thicker films. Deposition for T > 600 K results in nucleation of three-dimensional needle-like crystallites which have a very small contact area with the bilayer on which they are formed.