Feb 6 – 8, 2006
CERN
Europe/Zurich timezone

First observation of new diffusion phenomena in CdTe

Feb 7, 2006, 5:50 PM
20m
Council Chamber, 503/1-001 (CERN)

Council Chamber, 503/1-001

CERN

CH-1211 Geneva 23

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.

Primary 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)

Presentation materials