Experimental roughness excitation of surface electromagnetic waves and their detection by ellipsometry

Abstract

The dynamic aspects of the growth process of the (0001) basal face of a large cadmium single crystal have been carefully investigated by real-time ellipsometry, over a large range of crystal temperature and supersaturation, during in situ vapour growth experiments. At the same time, the ellipsometric response can be correlated with the morphological evolution of the growing face observed with a differential interference optical system that is very sensitive to local surface slope variations. From numerous experiments, it has been proven that the face grows at low supersaturation by the birth and spread of spiral layers originating at the outcropping of screw dislocations. The behaviour of the resulting step-trains looks like an evolutive periodic roughness which induces a very sensitive ellipsometric response. Analysis of the fluctuating signals reveals growth instabilities at different scales (pyramid or step interaction) but some abnormal large variations ( δΔ ≈ 20°), often observed even at constant supersaturation and temperature, are interpreted in terms of a surface roughness excitation of surface electromagnetic waves (SEWs). It has been shown that an optical grating can be used for SEW production and the ellipsometric detection consists in plotting the parameters (cos Δ, tan Ψ) vs. the wavelength λ at a fixed incident angle Θ i. Such spectra exhibit a very large variation in the phase Δ and a deep minimum for the ellipticity tan Ψ (resonant response). In our case, a step-train looks like an optical grating surface with a variable periodicity and thickness depending on the growth conditions (temperature, supersaturation, step interaction, …); in this case, it is then possible to obtain the resonant condition with a fixed wavelength and a critical interstep distance because of the very large possible adjustment of the interstep distance.