Electronic properties of micro-n-i-p-i structures in silicon

The electronic properties of ultrathin, heavily doped n-i-p-i structures in silicon have been studied with use of the ab initio pseudopotential method. Three doping configurations are used to model n-i-p-i structures by replacing silicon atoms with aluminum and arsenic atoms. The doped silicon samples are found to retain an indirect energy band gap. The heavy doping levels result in the formation of impurity bands. The top of the valence band and the bottom of the conduction band are impurity-related bands. The electron-hole charge separation even occurs when the n- and p-type impurities are nearest neighbors. The heavy doping significantly increases the electron-hole recombination matrix elements as compared to the light-doping case in which the bulk silicon band structure can be used to determine the matrix elements. However, the matrix elements for the three n-i-p-i structures in Si which were investigated are still 5 orders of magnitude smaller than those for the direct-band-gap transition in GaAs. © 1989 The American Physical Society.