A Cluster Calculation of B and P Impurities in Amorphous Silicon
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A CLUSTER CALCULATION OF B AND P IMPURITIES IN AMORPHOUS SILICON
J.A. COGORDAN R.M.VALLADARES, L. ENRIQUE SANSORES, VALLADARES UNAM, Instituto de Investigaciones en Materiales, Postal 70-360, 04510 Mdxico D. F., Mexico.
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ABSTRACT The local density of states and charge density contours of clusters of the type ISi 20 H2 8, where I can be Si, B or P, was calculated using the well-known pseudopotential SCF Hartree-Fock It is found that the covalent Method (and the HONDO Program). nature of the bonding in pure silicon gets altered and gives rise to an ionic component when B and P are substituted in the Also, the local density of states in the center of the cluster. neighborhood of a Si atom, nearest neighbor to the center of the of the p-states at the top of the cluster, show a splitting valence band in pure silicon when B is substituted, and a new These sustituted. p-state appears in the band gap when P is results are analyzed in the light of the local changes and its relevance to the solid state properties. INTRODUCTION Since the discovery that hydrogenated amorphous silicon could be doped with impurities [1] there has been a lot of interest from scientists and technologists in understanding its properties. From the technological point of view, the fact that amorphous semiconductors can be doped has been of fundamental A lot of research importance for the construction of devices. has been devoted towards understanding the doping mechanism and the nature of the energy levels introduced by the impurities. There has been several studies on the doping efficiency (2) and impurity as a function of the energy of the activation Several spectroscopic techniques like DLTS, concentration (3]. PDS and deep level capacitance [4-8] have been used to measure and understand the nature of the states introduced by the Phosphorus and boron have been the most widely impurities. studied dopants since they are commonly used in devices for n and p layers. to deal with the effect of local It is quite difficult Fortunately impurities in a matrix which is not well defined. many of the electronic properties which are exhibited by the In a previous paper [9] the solid are of a local nature. usefulness of cluster calculations to determine the properties of impurities in solids with respect to aspects such as the effect of doping or the production of electron traps was shown.
Mat. Res. Soc. Symp. Proc. Vol. 209. 01991 Materials Research Society
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DESCRIPTION OF THE CLUSTER is shown in The cluster used in this calculations figure-I. The general formula is ISi 2 0H28 where I can be either Si,B or P. The cluster has a tetrahedral symmetry, with the impurity located at the center, and 4 silicon atoms surrounding it. In order to keep the cluster at a manageable size, 6-fold rings of the boat type were used. These type of rings are not present in crystalline silicon but in amorphous silicon they are The hydrogens are used to saturate the found frequently. outermost bonds of the silicon atoms, so that no dangling bond states a
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