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Light scattering measurements of surface morphology during molecular beam epitaxy growth of GaAs-based semiconductors Lavoie, Christian


Recent theoretical and experimental work has shown that the surface of epitaxial films is in general not atomically flat during growth due to the interplay between the random nature of the deposition process and the effects of surface diffusion. This developing roughness can be measured in-situ using elastic light scattering. The vertical sensitivity of the technique approaches the atomic scale while laterally the length scale probed depends on the wavelength of the light and the scattering geometry. We have performed in-situ elastic light scattering measurements simultaneously at various collection angles during molecular beam epitaxy using the 488 nm line of an argonion laser. At selected points in time during the growth, ex-situ angle-resolved light scattering measurements give the surface power spectral density function over the range 0.1< q < 20 µm-1 Using the time evolution of the in-situ measurements, together with the ex-situ angle-resolved scattering measurements on quenched epilayers, we investigated the changes in surface morphology during growth of lattice matched systems (GaAs on GaAs) and lattice mismatched systems (InGaAs on GaAs). Effects of crystallographic orientation and spatial frequency were studied. The light scattering results were compared with reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), scanning tunneling microscopy (STM), field emission scanning electron microscopy (FE-SEM) and Nomarski microscopy. Growth of GaAs on the rough thermally-cleaned surface shows smoothing at spatial frequencies greater than 5 µm and roughening for smaller spatial frequencies. Both roughening and smoothing behaviors are found to be anisotropic: the growing surface exhibits larger roughness along the [110] direction than along the [110] direction. The dependence on crystal orientation results from the fact that group III atoms diffuse faster along the [110] direction of a (2X4) reconstructed surface than along the [110]. In each direction, we determined that the time and spatial frequency dependence of the observed roughening behavior follow g(q,t)~(1-exp(-2aqt))/q² where g(q,t) is the power q spectral density of the surface morphology which depends on the spatial frequency q and the time t. As predicted for unstable growth on singular surfaces, we show that this time dependence is consistent with mounds forming on the surface that exhibit constant slope if the height of the mound is linearly dependent on the growth rate. In-situ and ex-situ elastic light scattering were also used to measure the evolution of the surface morphology of InxGa1-xAs films during molecular beam epitaxy growth on GaAs epilayers. The light scattering measurements are compared with atomic force microscopy (AFM) images of the surface morphology and x-ray measurements. The AFM results are in good agreement with the rms roughness obtained from light scattering and both techniques show the familiar cross-hatched pattern in the surface structure of the relaxed films. The effect of indium concentration and substrate temperature on the onset of relaxation were studied. In particular, we find that the growth of an InGaAs film with low indium content (~1.5%) at high substrate temperature (~600°C) provoke a drastic smoothing of the rougher surface morphology obtained from growing GaAs epilayers on GaAs substrates. [Scientific formulae used in this abstract could not be reproduced.]

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