Speaker
Dr
Herbert Wolf
(Technische Physik, Universitaet des Saarlandes, D-66041 Sasarbruecken, Germany)
Description
Understanding and control of diffusion profiles of intrinsic and extrinsic defects in
semiconductors is of central importance for developing electronic and optoelectronic
devices.
Usually, diffusion profiles in all materials are characterized by a depth profile
decreasing monotonously from the source of the diffusing species. In some compound
semiconductors for certain dopant atoms, however, completely different diffusion
profiles can be obtained.
Depending on the external vapor pressure during Ag diffusion in CdTe, for identical
conditions of temperature and time very different concentration profiles can be
observed. E.g. after diffusion at 800 K for 60 min under Cd pressure a peak-shaped
profile and under Te pressure a U-shaped profile was observed [1]. In compound
semiconductors, like CdTe or ZnTe, large concentrations of intrinsic point defects
can be obtained by inducing slight deviations from stoichiometry by exposing the
respective material to external vapor pressures of one of its constituents. It turned
out that variations of the deviation from stoichiometry during the diffusion process
strongly influence the diffusion profile of fast diffusion elements like e.g. Ag or
Cu in CdTe.
The observed unusual diffusion profiles can be described quantitatively by a model
based on defect reactions of the Ag dopant with intrinsic defects [2]. In a
semiconductor, intrinsic as well as extrinsic defects can assume different charge
states and, in addition, an inhomogeneous distribution of charged defects causes an
internal electric field. The model, therefore, considers the presence of different
charge states of the participating defects and a drift force acting on the charged
defects due to the internal electric field. It turned out that, e.g. in case of Ag
diffusion in CdTe, the flux of the Ag atoms essentially is determined by its
positively charged interstitial fraction. In addition, the Ag profile essentially
reflects the actual distribution of intrinsic defects.
A principal question arising from the observation of the unusual diffusion profiles
described above is the possible formation of such profiles for other combinations of
compound semiconductor and dopant. The formation of diffusion profiles for different
dopant atoms in different II VI semiconductors was investigated at ISOLDE using the
short-lived isotopes 24-Na, 43-K, 65-Ni, and 56-Mn. In case of 24-Na in CdS and CdTe
some features of unusual diffusion profiles similar to Ag in CdTe have been observed
but for a quantitative description the model mentioned above might to be modified to
some extent. In case of 65-Ni in CdZnTe a different type of an unusual diffusion
profile has been observed, which most probably reflects some atypical inhomogeneities
of the used host crystal.
[1] H. Wolf, F. Wagner, Th. Wichert, and ISOLDE Collaboration, Phys. Rev. Lett. 94
(2005) 125901.
[2] H. Wolf, F. Wagner, Th. Wichert, R. Grill, and E. Belas, J. Electr. Mat. 35
(2006) 1350
Author
Dr
Herbert Wolf
(Technische Physik, Universitaet des Saarlandes, D-66041 Sasarbruecken, Germany)
Co-authors
Mr
Frank Wagner
(Technische Physik, Universitaet des Saarlandes, D-66041 Sasarbruecken, Germany)
Mr
Joerg Kronenberg
(Technische Physik, Universitaet des Saarlandes, D-66041 Sasarbruecken, Germany)
Dr
Manfred Deicher
(Technische Physik, Universitaet des Saarlandes, D-66041 Sasarbruecken, Germany)
Prof.
Thomas Wichert
(Technische Physik, Universitaet des Saarlandes, D-66041 Sasarbruecken, Germany)
