Katja Kuitunen

Tensile strain in arsenic heavily-doped Si

In this paper we highlight the existence of tensile stress in heavily arsenic-doped epitaxial silicon (Si:As) prepared by low pressure chemical vapor deposition. Despite the large size of As atoms compared to Si ones, we demonstrate with x-ray diffraction and convergent electron beam diffraction that the heavily doped epitaxial layers show a tetragonal lattice with a reduced out of plane parameter. Using positron annihilation spectroscopy, we highlight the formation of arsenic-vacancies defects during the growth. We show that the tensile strain is related to this type of defects involving inactive As atoms and not to the As active concentration.

He implantation to control B diffusion in crystalline and preamorphized Si

We demonstrate that He can be a powerful tool to control B diffusion both in crystalline (c-Si) and preamorphized Si (PA-Si). By means of positron annihilation spectroscopy (PAS), we showed in He-implanted c-Si the formation after annealing of large open-volume defects at the implant projected range Rp of He (voids) and of smaller vacancy-type defects toward the surface (nanovoids). In particular, these nanovoids locally suppress the amount of self-interstitials (Is) generated by B implantation, as verified by PAS, eventually reducing B diffusion and leading to a boxlike shape of the B-implanted profile. On the other hand, for B implantation in PA-Si, the authors demonstrated that if He-induced voids are formed between the end-of-range (EOR) defects and the surface, they act as a diffusion barrier for Is coming from the EOR defects. Indeed, this barrier strongly reduces diffusion of B placed in proximity of the surface.

Defect Generation and Evolution in Laser Processing of Si

In this work concerted experiments and theoretical analysis are applied to conclusively demonstrate the vacancy generation during fast melting and re-growth of Si by laser irradiation. Experiments, based on the positron annihilation spectroscopy and designed to test the theoretical predictions, evidence a vacancy super-saturation after the laser process The dependence on the pulse energy and number of shots of the residual damage, after a multi shot laser irradiation process, is characterised by means of these measurements. Kinetic Monte Carlo simulations of the molten Si re-crystallization show trapping of vacancies in the re-crystallized region. The main outcome of this simulation is the dependence of the vacancys generation efficiency on under-cooling. The knowledge of this dependence allows us to implement a continuum model (based on kinetics equations for the phase, the free defect and clustered defects) aimed to simulate the damage evolution during the pulse duration and in the time interval between two successive pulses. These continuum kinetic simulations of the full process show a defect evolution in close agreement with the experiments.

Vacancy Clusters in Germanium

Fast neutron irradiation of germanium has been used to study vacancy reactions and vacancy clustering in germanium as a model system to understand ion implantation and the vacancy reactions which are responsible for the apparently low n-type doping ceiling in implanted germanium. It is found that at low neutron doses (~1011cm-2) the damage produced is very similar to that resulting from electron or gamma irradiation whereas at higher doses (> 1013cm-2) the damage is similar to that resulting from ion implantation as observed in the region near the peak of a doping implant. Electrical measurements including CV profiling, spreading resistance, Deep- Level Transient-Spectroscopy and high resolution Laplace Deep-Level Transient-Spectroscopy have been used in conjunction with positron annihilation and annealing studies. In germanium most radiation and implantation defects are acceptor like and in n-type material the vacancy is negatively charged. In consequence the coulombic repulsion between two vacancies and between vacancies and other radiation-induced defects mitigates against the formation of complexes so that simple defects such as the vacancy donor pair predominate. However in the case of ion implantation and neutron irradiation it is postulated that localized high concentrations of acceptor like defects produce regions of type inversion in which the vacancy is neutral and can combine with itself or with other radiation induced acceptor like defects. In this paper the progression from simple damage to complex damage with increasing neutron dose is examined.

Excimer Laser Annealing of Ion-Implanted Silicon: Dopant Activation, Diffusion and Defect Formation

Minimization of dopant diffusion during electrical activation is a crucial issue in developing sub-50 nm silicon technology. Excimer laser annealing (ELA) in the melting regime is capable of meeting the requirements on shallow junctions in terms of depth, doping concentration and abruptness. However, in order to be successfully employed it has to be demonstrated that ELA can be integrated in a device processing flow. Especially, the compatibility of ELA with other high temperature processing steps such as rapid thermal annealing (RTA) needs to be addressed. In this contribution, we report on phenomena observed for B redistribution that occur during ELA in B-implanted Si and after subsequent RTA. Specific topics to be covered include (i) B build-up at the maximum melt depth during ELA, and (ii) B activation and diffusion beyond the ELA melt depth.