Surface segregation at fine oxide powders
Motivation and results
The dopant distribution is the key for the control of the electrical properties of semiconducting oxide powders, which are used for various electronic applications. The effect of surface segregation in Sb and In doped SnO2 fine grained powders (crystallite size 20 ... 100 nm) was investigated in comparison with single crystalline samples. The kinetics and thermodynamics of Sb and In segregation processes were studied as function of annealing temperature by X-ray photoelectron spectroscopy (XPS). Significant differences of diffusion and segregation were revealed for doped powders and single crystals, which are obviously caused by simultaneous diffusion and particle growth processes proceeding during annealing of powders. For doped single crystals the thermodynamic equilibrium is approached after 24 h annealing above 850°C and at 1000°C for Sb and In, respectively. Doped powders show higher effective activation energies of diffusion and the thermodynamic equilibrium is not achieved for technological relevant annealing conditions. Based on dopant profile measurements anomalies in the electrical resistivity at 300°C of Sb doped SnO2 powders annealed at 700 and 900°C are attributed to a Sb-depleted zone formed beneath the segregated surface during the kinetic regime. Such inhomogeneous doping of powders must be avoided by appropriate preparation steps in order to achieve optimal resistivity behavior for commercial application.
The experience can be used for investigation of other fine-sized material such as carbon nanotubes.
D. Szczuko, J. Werner, S. Oswald, G. Behr, K. Wetzig: XPS investigations of surface segregation of doping elements in SnO2, Appl. Surf. Sci. 179 [1-4] (2001) 301-306. (doi:10.1016/S0169-4332(01)00298-7)
D. Szczuko, J. Werner, G. Behr, S. Oswald, K. Wetzig, Surface-related investigations to characterize different preparation techniques of Sb-doped SnO2 powders, Surf. Interface Anal. 31  (2001) 484-491. (doi:10.1002/sia.1099)
D. Dobler, S. Oswald, K. Wetzig: Calibration of XPS - energy scale for determination of the oxidation states of doping elements in SnO2 powders, Analyt. and Bioanalyt. Chem. 374  (2002) 646-649. (doi:10.1007/s00216-002-1448-y)
D. Dobler, S. Oswald, J. Werner, W. Arabczyk, G. Behr, K. Wetzig: X-ray photoelectron spectroscopy investigation of segregation processes at Sb and In doped SnO2, Chem. Phys. 286 [2-3] (2003) 375-383. (doi:10.1016/S0301-0104(02)00970-9)
D. Dobler, S. Oswald, G. Behr, J. Werner, K. Wetzig, Changes of Auger parameter in doped SnO2 powders, Cryst. Res. Technol. 38  (2003) 956-961. (doi:10.1002/crat.200310120)
S. Oswald, G. Behr, D. Dobler, J. Werner, K. Wetzig, W. Arabczyk, Specific properties of fine SnO2 powders connected with surface segregation, Analyt. and Bioanalyt. Chem. 378  (2004) 411 - 415. (doi:10.1007/s00216-003-2277-3)
G. Behr, S. Oswald, D. Dobler, J. Werner, Dotierte Halbleiteroxidfeinstpulver und Verfahren zu ihrer Herstellung, Patentschrift DE 103 26 957 A1, Offenlegungschrift, 13.01.2005.