Measurement of the grain-boundary states in semiconductors by deep-level transient spectroscopy.

Abstract

The paper deals with standing issues regarding the application of deep-level transient spectroscopy (DLTS) to the measurement of the electron states at grain boundaries in semiconductors: (i) The relationship between the density of interface states and the associated DLTS spectra is worked out quantitatively for the case in which the levels form a continuous distribution in energy, leading to simple analytical expressions for the emission rate, the density of states, and the capture cross section at the quasi-Fermi level of the trapped carriers. This treatment will also apply, with minor modifications, to different physical systems where interface states are present, e.g., the oxide-semiconductor interfaces. The effect of field-enhanced emission on the DLTS spectra of the boundary states is also considered. (ii) As a practical illustration, the grain-boundary parameters are determined for the \ensuremath{\Sigma}=25 twin boundary in n- and p-type doped silicon bicrystals. In n-type silicon, a single boundary level is found at ${E}_{c}$-0.66 eV, with a density of 2.6\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. In p-type silicon, the boundary levels are continuously distributed in energy, with a density on the order of ${10}^{12}$ ${\mathrm{eV}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ in the energy range (${E}_{v}$+0.2 eV, ${E}_{v}$+0.6 eV). The density of states shows a sharp maximum at ${E}_{v}$+0.18 eV, associated with a single trap level, the density of which is on the order of 2\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. The capture cross sections of the traps are on the order of ${10}^{\mathrm{\ensuremath{-}}14}$ to a few ${10}^{\mathrm{\ensuremath{-}}13}$ ${\mathrm{cm}}^{2}$. These results are consistent with the data obtained from complementary capacitance-voltage and thermally-stimulated-capacitance experiments. The densities of states appear to be highly dependent on the thermal history of the specimens. Microanalytical investigations are currently in progress, aimed at clarifying the dependence of the electronic properties on impurity segregation at the grain boundaries in silicon.