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Optical studies of the internal acceptor states in boron-doped silicon Parsons, Robert Raymond

Abstract

The introduction of boron into a silicon crystal lattice produces two sets of acceptor impurity states: (i) the "external" states (which include the acceptor ground state) which are associated with the P₃/₂ valence band maxima and are the usual impurity states lying in the forbidden gap, (ii) the "internal" states which are associated with the P₁/₂ valence band maximum and are degenerate with P₃/₂ continuum states. The energies of the transitions from the ground state to the so-called 2p', 3p' and 4p' internal acceptor states are such that the 2p', 3p' and 4p' internal absorption peaks are seen in the infrared. Three different experiments are performed to study the "internal" spectrum of boron-doped silicon. (i) The stress-perturbed behavior of the 2p' and 3p' absorption peaks is studied with the use of calibrated uniaxial stress (≤ 10⁹ dynes/cm² ) and polarized light. From these data it is deduced that the ground state has a ⌈8 symmetry while the 2p' and 3p' internal states each have a ⌈6 symmetry. In addition the deformation potential parameters of the ground state are calculated to be: b' = -0.66 ± 0.04 eV. and d' = -2.1 ± 0.2 eV. These calculated parameters for the ground state are used to obtain indirectly some information about the deformation potential parameters of the P₃/₂ valence band edge. (ii) For the temperature range 5°K ≤ T ≤ 60°K the temperature dependences of the breadths of the 2p' and 3p' peaks are measured. These temperature dependences are attributed to phonon broadening. The phonon broadening mechanism for the 2p' and 3p' peaks is shown to be primarily due to a lifetime effect caused by electron-phonon coupling of the 2p' and 3p' states to the P₃/₂ valence band states. (iii) For the impurity concentration range 2.6 x 10¹⁵ boron/cm³ ≤ N ≤ 4.5 x 10¹⁷ boron/cm³ the 2p' absorption peak is measured. With increasing concentration this peak is observed to broaden and become very asymmetrical. Explanations to the above data are presented.

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