A. Villegas

Work function change caused by alkali ion sputtering

Abstract In the presented work we performed an experimental study of the work function decrease for a silicon sample caused by Cs+ ion bombardment. We varied the energy of primary Cs+ ions as well as the angle of incidence in order to reach a different concentration of implanted cesium ions and to find a dependence of the work function change on the cesium surface concentration. A “surface dipoles” model was developed. The model based on the electronegativity concept considers formation of Si–Cs dipoles with corresponding dipole moment. An electric field of these dipoles results in a decrease of the work function, whereas a partly ionic character of Si–Cs bond increases the surface binding energy of cesium and, as a consequence, its surface concentration. A good correlation between the model and the experimental data was found.

Comparative secondary ion mass spectroscopy analysis of solar cell structures grown by pulsed laser ablation and ion sputtering

We performed a complex secondary ion mass spectroscopy (SIMS) 3D analysis of solar cell structures based on II–VI semiconductors. The chemical composition analysis, as well as the depth distribution of the main elements and contamination were done for AuCu/CdTe/CdS/conducting glass structures. A structure where the II–VI compounds were grown by pulsed laser ablation (PLA) was compared with another structure grown by ion sputtering deposition (ISD). In both cases contamination due to O, C and Hw as found at high concentrations, particularly at the boundaries between crystallites. In addition to the SIMS depth profiling, the surface roughness (SR) was analysed by atomic force microscopy (AFM). Poor SIMS depth resolution was correlated to high surface roughness. The root-mean-square of the surface roughness (Rrms )w as found to be higher for ISD than for PLA structures. In addition, the lateral distribution of the main components and contamination were observed in the microscope mode with a resolution of about 1 µm. A larger lateral contamination was correlated to a larger Rrms of the analysed surface. Experimental ‘diffusion’ tails of Cu and Au from the ohmic contacts on the CdTe layer are also explained by a high Rrms for this layer.

Depth-profile analysis of nanostructures by SIMS: Depth resolution function

A new model of the depth-resolution function for secondary-ion mass spectrometry, which takes into account recoil implantation, ion mixing, and surface roughness formation under ion irradiation, is considered. A simple three-parameter equation is proposed to describe the depth-resolution function. Analytical expressions are obtained for two parameters.

Redistribution of ytterbium and oxygen in annealing of silicon layers amorphized by implantation

The redistribution of ytterbium and oxygen was studied in silicon layers that were implanted with 1-MeV Yb+ ions at a dose of 1×1014 cm−2, which exceeds the amorphization threshold, and 135-keV O+ ions at a dose of 1×1015 cm−2 and that were subsequently annealed at 620 and 900°C. The redistribution of Yb is due to segregation at the interface between the amorphous and single-crystal layers in solid-phase recrystallization of the buried amorphized layer. The redistribution of oxygen and its accumulation in regions with the highest concentration of Yb is associated with oxygen diffusion and the formation of YbOn complexes with n varying from 1 to 6. The parameters characterizing the dependence of the Yb segregation coefficient on the thickness of the recrystallized layer and the formation of YbOn complexes were determined.

SIMS Study of Modern Semiconductor Heterostructures.

In this presentation we considered difficulties and perspective technique for SIMS depth profiling analysis of modern semiconductor heterostructures. Experimental measurements were performed with specially prepared samples: MBE grown delta AlGaAs layers in GaAs. It was found that primary ion induced atomic mixing in a near surface layer, and surface roughness, formed under ion irradiation, defines the final depth resolution. We performed an experimental study of parameters of the noticed physical phenomenon and found their dependence on the primary ion energy. The found results make it possible to perform deconvolution of experimental profiles and reproduce the original elemental distribution in these and similar heterostructures