Speaker
Frank Wagner
(Technische Physik, Universität des Saarlandes, D-66041 Saarbrücken, 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. Common to all diffusion profiles in semiconductors reported so far is the
monotonously decreasing depth profile if the source of the diffusing species is
located at the surface of the crystal. In compound semiconductors, 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.
In CdTe, the diffusion of Ag was studied using the radiotracer 111-Ag implanted into
typically 500 - 800 µm thick CdTe crystals [1] with surprising results. After
implantation of 111-Ag into the front side of a CdTe crystal and a diffusion anneal
under vacuum, the diffusion profile shows the expected ‘normal’behavior, a
monotonously decreasing Ag concentration. But, performing the diffusion anneal under
external Cd pressure results in a total different diffusion profile: two depletion
layers of about 300 µm width appear at both surfaces of the crystal. In contrast,
layers of increased Ag concentration, situated about 20 µm below each surface, with
the Ag concentration in the interior of the crystal being significantly lower are
observed if the diffusion is performed under Te pressure. Since in normal diffusion
experiments the material flow has the inverse direction of the corresponding
concentration gradients, these results can not be explained by simple diffusion
according to Fick´s laws. The observed new type of diffusion profiles can be
understood in the framework of a model based on defect reactions of the Ag dopant
with intrinsic defects [2]. The flux of the Ag atoms essentially is determined by its
interstitial fraction Agi while Ag atoms incorporated on substitutional Cd lattice
sites are immobile. It can be shown that the flux of Ag consists of two
contributions: i) a normal diffusion flux, directed opposite to the gradient of the
Agi distribution and ii) a drift of the charged defects caused by the gradient of the
Fermi level, which essentially follows the distribution of the intrinsic defects.
Moreover, the diffusion behavior of Ag can be manipulated by the presence of the
group Ib elements Cu or Au. If Ag is diffused simultaneously with these elements a
replacement of Ag atoms in favor of Cu or Au takes place. After evaporating of 20 nm
Cu on the 111-Ag implanted side of a CdTe crystal and subsequent diffusion at 550 K
for 30 min nearly all Ag atoms are located within a thin layer of 10 μm beneath the
back of the sample. This phenomenon can be explained using the model outlined before
where Cu acts as a replacement for interstitial Cd defects.
[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., in press.
Author
Frank Wagner
(Technische Physik, Universität des Saarlandes, D-66041 Saarbrücken, Germany)
Co-authors
Herbert Wolf
(Technische Physik, Universität des Saarlandes, D-66041 Saarbrücken, Germany)
Jörg Kronenberg
(Technische Physik, Universität des Saarlandes, D-66041 Saarbrücken, Germany)
Manfred Deicher
(Technische Physik, Universität des Saarlandes, D-66041 Saarbrücken, Germany)
Muhammed Türker
(Technische Physik, Universität des Saarlandes, D-66041 Saarbrücken, Germany)
Thomas Wichert
(Technische Physik, Universität des Saarlandes, D-66041 Saarbrücken, Germany)
