Characterization of Al2O3/GaAs interfaces and thin films prepared by atomic layer deposition
Characterization of GaAs/Al(2)O(3) interfaces and thin (~1-10 nm) Al(2)O(3) films prepared by atomic layer deposition (ALD) is performed using variable angle spectroscopic ellipsometry (SE), x-ray photoelectron spectrometry (XPS), and stress measurements. The SE measurements reveal the presence in all samples of an interfacial layer around 1 nm-thick, though the layer originating from thermal ALD is slightly thinner than those from plasma ALD. The refractive index of the interfacial layer is found to be higher than that of a stoichiometric Al(2)O(3) film. The XPS measurements reveal higher O:Al ratios in films thinner than 4 nm, due to fact that the XPS technique probes oxygen in both the stoichiometric Al(2)O(3) films and the interfacial layers, because the mean free path length of the O1s photoelectrons is 4 nm. The perfectly symmetric Al signal in the XPS measurements suggests the absence of Al suboxides. The temperature cycle dependence of residual stress in the film reveals slightly different paths for the heating and cooling half cycles. The coefficient of thermal expansion is found to be much higher for thin films than thicker (~70 nm) films, which is attributed to the relatively large contribution of the interfacial layer to residual stress in the thin films.