Isotopically controlled semiconductors Page: 3 of 26
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converted to silane (XXSiH4) at Voltaix Corp., New Jersey. We built a silane pyrolysis
reactor at the Lawrence Berkeley National Laboratory in which the Si was deposited on
an electrically heated graphite rod [7]. Removing this graphite rod by drilling, grinding
and chemical etching yielded the poly Si cylinders ready for FZ crystal growth.
The growth of large diamond single crystals is a challenge in itself without the additional
problems related to isotope enrichment. Thomas Anthony and colleagues at General
Electric have grown large gem-quality bulk diamonds with varying isotopic composition
from highly enriched 12C to pure 13C. They found that enrichment occurs during bulk
growth from a metal solvent and attributed this effect to the difference in the attempt
frequency for a 12C or 13C atom to leave the solvent and deposit on the growing diamond
crystal [8]. Anthony has grown both undoped and boron doped (blue!) isotopically
enriched diamonds [9,10].
The growth of single and multiple thin crystalline films of enriched semiconductors
(isotope superlattice) has been achieved in a number of places. The small volume MBE
sources available today allow the economic growth of thin films. Single crystal Si layers
grown by vapor phase epitaxy (VPE) on Si wafers has been achieved commercially
(Lawrence Semiconductor Research Laboratory, Inc., Tempe, AZ).
In the following sections, an overview of studies on isotopically controlled
semiconductors is presented. The sections are arranged according to the effects produced
by the differences in isotopic mass, spin and thermal neutron absorption cross section.
The selection of the specific physical phenomena discussed in the present article naturally
reflect the studies in which the author has been closely involved. Authoritative detailed
accounts of a number of isotope-related phenomena are featured in the present issue of
Solid State Communications.
3. Isotope mass-related effects
3.1. Lattice dynamics in bulk crystals
3.1.1. Phonons
The quanta of energy of crystal lattice vibrations are called phonons. The simplest
dependence of the phonon frequency w on mass M can be described within the virtual-
crystal approximation (VCA) [11,12] by a "spring and ball" model and is given by
(1)
The phonon dispersion relationship describes the phonon frequencies o as a function of
the wave vector k for the various vibrational modes. Raman spectroscopy is ideally
suited for the study of optical phonons at k = 0. Photoluminescence in indirect band gap
semiconductors requires the assistance of k w 0 phonons. In germanium, the
recombination of an electron at the conduction band minimum
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Haller, E. E. Isotopically controlled semiconductors, article, November 15, 2004; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc780938/m1/3/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.