Wavelength and time-resolved cathodoluminescence intensity distributions of the emission of quantum wells are demonstrated to provide a one-to-one image of the crestallographic island structure of the heterointerfaces which are the boundaries of the well. A detailed description of the cathodoluminescence system is given. Results on GaAs quantum wells are presented as a typical example. Direct images of growth islands differing by 0.28nm (1 monolayer) height at GaAs/AlGaAs heterointerfaces and of the columnar structure of GaAs quantum wells are observed. The dependence of the lateral extension of these islands on growth conditions is investigated. For fixed growth rate r≈0.5monolayer/s the mean island size decreases from 6-7μm upon an increase of growth temperature from Tg=600°C to 660°C. Apparently the growth process changes from a planar to a three-dimensional one. Time-resoloved images directly visualize the lateral diffusion of excitons. Their lateral diffusion velocity is 105cm/s.
Ion scattering in the sheath of a glow discharge used for reactive ion etching is discussed. Ions arriving at the target must cross the sheath; consequently, at higher pressures many inclined ion trajectories are generated due to collisions with ambient sheath molecules. To obtain a vertical etching profile, it is, therefore, necessary to know the amount of inclined ions. The scattering probability depends on the sheath thickness and the ion mean free path when considering the ion energy dependence of the collision cross section. The effect of ion energy on the scattering probability has been calculated for Ar and found to be significant in typical reactive ion etching conditions. Calculations based on the experimental data of CF4 plasma parameters show that the scattering probability decreases with decreasing pressure and power density. Therefore, for the condition that the etching reaction is induced by ion bombardment, a vertical etching profile can be predicted.