CHARGED GRAIN-BOUNDARIES IN n-Ge BICRYSTALS

Germanium bicrystals with two types of low-energy symmetric tilt boundaries, Z 9 and Z 1 1 (CSL model classification) were grown. Current-voltage characteristics of grainboundaries were measured in the temperature range 77 K-420 K. Potential barriers were found only at Z 1 1 boundaries in n Ge which confirms the theoretical predictions of the atomic structure of these boundaries [I]. From the experimental data the conclusion is drawn that the electronic charge of t 11 boundaries is captured at the lattice defect with the energy level of about 0.2 eV above the top of the valence band. The analysis of the atomic structure of grain-boundaries in the diamond lattice given by Moller [I] predicts low energy "special" boundaries to have various concentrations of broken bonds. Namely, for the <011> symmetric tilt boundaries classified in the coincidence site lattice, model as 1 11 (rotation axis <110>, rotation angle 0 = 12g0) the concentration of broken should bg as high as l o q 5 while for E 9 (rotation axis <110>, O = 39 ) a perfect bond saturation occurs. In semiconducting materials this difference should lead to drastically different electric properties of grain boundaries. A verification of this prediction is the aim of present paper. To minimize the effect of decoration of the grain boundaries by impurities (mostly oxygen) germanium was chosen as a model material. Because of its relatively small energy gap and the high potential barriers developing at charged boundaries attention is given to the barrier screening. Experiment.Bicrystals with the common rotation axis <110> and two types of symmetric tilt boundaries Z 9 and Z 11 were grown using a two seed Crochalski method. The relative grain orientations in the bicrystals were more accurate than 2O. Both pand n-type bicrystals doped during growth were investigated. Bulk,rectangular samples (3.5 x 3.5 x 25 ma), with the long axis perpendicular to the grain'boundary were cut out. The carrier concentration was measured by Hall Ton leave from : Inst i tute of Metal Research, Academics Sinica, Shenyang, China on leave from : Inst i tute of Physics, Polish Academy of Sciences, Poland Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1982119