Thermal Double Donors and Quantum Dots

Abstract

Combined local mode spectroscopy and ab initio modeling are used to demonstrate for the first time that oxygen atoms in thermal double donors (TDD) in Si are in close proximity. The observed vibrational modes in (16)O, (18)O, and mixed isotopic samples are consistent with a model involving [110] aligned oxygen chains made up of an insulating core lying between electrically active ends. The model also explains the minute spin density observed on oxygen in TDD(+) as well as the piezospectroscopic tensors of the donors. The analogy between the thermal donors and quantum dots is emphasized.

Local vibrations of thermal double donors in silicon

“…The higher frequency of 978 cm Ϫ1 originates from the asymmetric stretching vibration of the flanking O i and agrees closely with the experimental value of 975 cm Ϫ1 for TDD1 ͑Table I͒. 7,8 This agreement is thus consistent with our assignment of the O 2i -O 2r chain to TDD1. 13 The Fig.…”

“…This is shown in Fig. 3͑b͒ 7,8 This agreement is consistent with our assignment of the O i -O nr -O i chains to TDDn (nϾ2). 13 However, the calculated frequency for the O i -O 3r -O i 2ϩ chain (988 cm Ϫ1 ) is lower than that for the O i -O 2r -O i 2ϩ chain (992 cm Ϫ1 ) whereas the corresponding experimental frequency for TDD3 (999 cm Ϫ1 ) is by 11 cm Ϫ1 higher than that for TDD2 (988 cm Ϫ1 ).…”

“…The calculated highest LVM frequencies for the O chains are given in Table I together with the experimental frequencies and the frequencies calculated by Coutinho et al 8 The calculated LVM frequencies related to TDD's are given for the doubly positive charge state of the corresponding O chain because almost all TDD's have donated two electrons to the conduction band at temperatures TϾ100 K ͑for the O i -O 2r chain TϾ200 K). We first note that by increasing the size of the supercell from 32 to the present 162 atom sites we find a closer agreement with the experimental LVM frequencies for monomers and dimers: for O Fig.…”

“…An important achievement in finding these structures as well as in clarifying the consecutive mechanism between the TDD's was the assignment of the infrared absorption local vibrational modes ͑LVM's͒ of 975, 988, 999, and 1006 cm Ϫ1 to TDD1, TDD2, TDD3, and TDD4, respectively. [6][7][8] Also the isotopic shifts when 16 O is replaced by 18 O have been measured: the shifts are Ϫ43-Ϫ44 cm Ϫ1 . 8 The LVM frequencies on one hand act as useful fingerprints in following the kinetics of TDD's, and on the other hand serve as a stringent test for structural models for TDD's.…”

“…However, more recent total-energy calculations indicate that oxygen-only structures for TDD's are those energetically most favorable. 8,[11][12][13] On the basis of accurate total-energy calculations we recently put forward an oxygen-chain-like model for TDD's. 13 This consists of adjacent four-member rings ͑R's͒ and flanking interstitial oxygens (O i 's).…”