No abstract

No abstract

We present a model to describe the Reflectance-difference (RD) spectra of InGaAs grown on GaAs (001) at T=500 o C, in the energy range from 2.3-3.5 eV. The model assumes the presence of an orthorhombic strain in the InGaAs epilayer that accounts for an anisotropic process of nucleation of InGaAs islands. We show that the developed model leads to accurate fits to the experimental RD spectra of InGaAs/GaAs for epilayer thickness both below and above of the critical thickness for the 2D-3D growth-mode transition. From the fitting of the theoretical model to the experimental RD line shapes we obtain quantitative information on changes in surface stoichiometry and morphology at the onset of the 2D-3D transition. Our results demonstrate that the RD line shape of InGaAs/GaAs in the 2.3-3.5 eV energy range can be explained entirely in terms of surface-modified InGaAs E 1 and E 1 +∆ 1 bulk transitions with no need to invoke surface states.

ABSTRACT

Carbon nitride films were prepared by reactive magnetron sputtering of a graphite target in a nitrogen atmosphere at different pressures and temperatures. The films were characterized by infrared, Raman and ultraviolet-visible spectroscopies, as well as by ellipsometry. We obtained hydrogen-free dark films when a negative bias was applied to the substrates; the samples prepared at room temperature without bias were soft and hydrogen could be detected in their infrared spectra. The most prominent features, between 1000 and 1700 cm-1, in both the Raman and infrared spectra were modelled using two Gaussian curves. As the nitrogen content was varied, the ratio between their areas followed opposite trends in the Raman and the infrared spectra. The presence of hydrogen caused the main feature to absorb more strongly at lower frequencies but did not alter the trend. The presence of six and five membered heteroaromatic rings is suggested to explain the increase of the infrared vibrations associated with double conjugated double bonds. The index of refraction can be correlated with features in the infrared spectra, suggesting that its value increases as the number of carbon and nitrogen double bonds increases. Ion bombardment reduces nitrogen incorporation; it is responsible for the densification of the films and the introduction of several defects that make films prepared under bias more absorbing in the low energy region.