Fachbereich 11 - Physik
Institut für Materialphysik
Diffusion and Defects in Elementary and Compound Semiconductors

Electrical characterisation of impurities and defects in semiconductors

The electrical properties of foreign atoms in semiconductor crystals are of vital importance for the fabrication of semiconductor devices. Device performance relies on intentional doping with donor or acceptor elements but may be adversely affected by contamination with unwanted impurities. We have developed the concept of variable-temperature spreading-resistance profiling (VT-SRP) for the characterization of electrical active impurities or defects in semiconductor crystals. Unlike conventional SRP systems, which are exclusively operated at room temperature, our home-built VT-SRP device allows for measurements at different temperatures typically ranging from 150K to 400K. VT-SRP is able to combine the accurate resolution of an impurity depth profile with a determination of the predominant impurity-related electronic level in the semiconductor bandgap. This novel feature was exploited on germanium crystals with diffusion-induced gold distributions. Another application concerns the depth profile analysis of foreign elements that occur in various defect configurations. This was demonstrated on Si samples diffused with sulfur or selenium since these impurities may be present as isolated atoms as well as pairs. Given the well-known energy levels of the two S or Se configurations in Si we were able to resolve not only the shape and depth of the diffusion profile but also the ratio of isolated atoms to pairs in the diffusion zone.

We investigated by deep level transient spectroscopy a series of p-Si_1-XGe_X samples after in-diffusion of Zn. The measurements reveal two deep hole traps which are attributed to the Zn^0/- single- and Zn^-/2- double-acceptor states of isolated Zn atoms on substitutional sites. The corresponding transient peaks are broadened for X>0 in comparison to those in pure silicon due to statistical fluctuations of the SiGe alloy composition in the local environment of the Zn atoms. This effect is described quantitatively within the alloy-broadening model which furthermore allows to determine the number of atoms sampled by the defect wave function. The results are compared with ab-initio calculations for substitutional Zn in pure Si using the linear muffin-tin orbital method in the atomic-spheres approximation based on the local density approximation. To model the Ge alloying, we have also studied trigonal pairs of substitutional Zn in Si with a single Ge ligand at different pair distances. From these results, the broadening of the transients can be understood qualitatively.

Drittmittelgeber:

Deutsche Forschungsgemeinschaft

Beteiligte Wissenschaftler:

HDoz. Dr. H. Bracht (project leader), Prof. Dr. A. Nylandsted-Larsen (Univ. Aarhus), Prof. Dr. H. Overhof (Uni Paderborn), PD Dr. N.A. Stolwijk (project leader), Dr. S. Voß

Veröffentlichungen:

S. Voß, N.A. Stolwijk, H. Bracht: Spreading-resistance profiling of silicon and germanium at variable temperature, J. Appl. Phys. 92 (2002) 4809-4819

S. Voß, N.A. Stolwijk, H. Bracht, A. Nylandsted-Larsen, H. Overhof: Substitutional Zn in SiGe: Deep Level Transient Spectroscopy and Electron Density Calculations, Phys. Rev. B, accepted