This review presents the available experimental and theoretical understanding on the structure and electronic properties of grain boundaries in semiconducting materials. High-resolution electron microscope images of interfaces are interpreted within the framework of the structural unit model of grain boundaries, and the electronic properties of the grain boundaries discussed with relation to the popular symmetric Schottky barrier model for charge trapping and potential barrier formation. It is shown that these models give some limited understanding of the physical processes that occur at grain boundaries in elemental semiconductors, but that in compound semiconducting materials the effects of non-stoichiometry at the boundary regions must also be considered. Segregation of impurity and dopant species to the boundaries can have significant influence on their electrical properties, and the question of what structural or chemical features are responsible for the observed properties is posed. Diffusion at semiconductor grain boundaries is also discussed, and finally the electrical properties of zinc oxide varistor material are presented in the light of the models of carrier interactions with grain boundaries.

http://iopscience.iop.org/article/10.1088/0022-3719/18/21/008/pdf

Grain boundaries in semiconductors

The diffusion of phosphorus in crystallized amorphous Si layers was studied with secondary‐ion mass spectroscopy. A two‐dimensional diffusion model is used to find effective grain (Dg) and grain‐boundary (Dgb) diffusion coefficients. This simplified model leads to Dgb ≤ 10Dg, which is significantly lower than what has been deduced from conventional, larger grained polysilicon. Our result is consistent with specific‐gravity measurements, which found a significantly lower ‘‘mass defect’’ for layers deposited amorphous and subsequently crystallized as compared to initially polycrystalline layers.

Phosphorus diffusion in polycrystalline silicon

The dopant (P and B) diffusion path in n- and p-types polycrystalline-Si gates of trench-type three-dimensional (3D) metal-oxide-semiconductor field-effect transistors (MOSFETs) were investigated using atom probe tomography, based on the annealing time dependence of the dopant distribution at 900 °C. Remarkable differences were observed between P and B diffusion behavior. In the initial stage of diffusion, P atoms diffuse into deeper regions from the implanted region along grain boundaries in the n-type polycrystalline-Si gate. With longer annealing times, segregation of P on the grain boundaries was observed; however, few P atoms were observed within the large grains or on the gate/gate oxide interface distant from grain boundaries. These results indicate that P atoms diffuse along grain boundaries much faster than through the bulk or along the gate/gate oxide interface. On the other hand, in the p-type polycrystalline-Si gate, segregation of B was observed only at the initial stage of diffusion. After f...

Phosphorus and boron diffusion paths in polycrystalline silicon gate of a trench-type three-dimensional metal-oxide-semiconductor field effect transistor investigated by atom probe tomography

Boron diffusion into nitrogen doped silicon films for P+ polysilicon gate structures

The use of the transmission‐line‐matrix (TLM) method for analyzing the grain boundary and interfacial diffusion problems in thin films is demonstrated. The method employed has a variable mesh and automatic time‐stepping capability, and is highly versatile in treating complex structures and incorporating various boundary conditions. The present model takes account of separate diffusion coefficients in grain boundaries, grains, interfaces, and the substrate. The combined diffusion problems are solved as a function of position and time, and the concentration distributions are presented as clearly visible isometric projections and contour plots. The results are compared with those of semi‐infinite samples (Whipple’s analysis [Philos. Mag. 45, 1225 (1954)]) and idealized thin‐film systems (Gilmer and Farrell’s analysis [J. Appl. Phys. 47, 3792 (1976)]), with the emphasis being placed on the differences due to the treatment of concentration flux along the interface between the film and the substrate and into the substrate. The TLM method is shown to have considerable potential in practical applications for diffusion processes in thin films.

A.D. Buonaquisti

Papers

Diffusion characteristics of boron and phosphorus in polycrystalline silicon