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Deep level impurities in semiconductors for nuclear radiation detection


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Pearton, S. J 1981 , 'Deep level impurities in semiconductors for nuclear radiation detection', PhD thesis, University of Tasmania.

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This thesis describes experiments on the behaviour of deep level
defects in the semiconductors Ge, Si and GaAs. High-purity samples
of these materials are often used for the fabrication of solid state
nuclear radiation detectors; it is within this context that the work
has been performed.
Deep level impurities may have considerable effect on the
performance of solid state devices. By trapping signal carriers
(electrons and holes) they may change the effective lifetimes of
these entities, and so affect parameters such as switching times
in photoconductors, the efficiency of solid state lasers and lightemitting
diodes, and the energy resolution of semiconductor radiation
detectors. Positive uses of their effects include the increased
switching times of certain Si diodes, higher quantum gain in some
photoconductors and the production of high resistance compound semiconductor
wafers. The deep level defects may be caused by contamination
of the material with other elements, by lattice line and point
defects, or associations of these imperfections and impurities. They
may be present in the as-grown semiconductor crystal, introduced
during processing of the material, or during the operation of the
final device (e.g._radiation damage).
No general formalism exists to explain or predict the properties
of these deep level impurities, particularly because of their nonhydrogenic
nature. A greater understanding of solid state physics
than currently exists will need to be achieved to produce a theoretical
treatment of their behaviour. The study of their properties is therefore
desirable from both a device and a fundamental point of view.
Chapter 1 describes experiments on radiation damage centres in Ge.
The y-irradiation of p-type Ge crystals grown from silica crucibles
under an H2 atmosphere always produces two deep acceptor levels. We
detail evidence showing these are most likely due to oxygen-vacancy
complexes. Heat treating samples before irradiation appears to
reduce the amount of oxygen available for production of the deep
levels, and so these samples are hardened to Y-radiation damage.
Similarly, Li ions drifted through the crystal cause radiation hardening
of this material, possibly by binding oxygen into stable Li-0
pairs, and also by direct passivation of the y-induced defect centres.
The thermal and electrical stability of these centres is also discussed.
Proton and neutron irradiation of Ge produced acceptor levels only,
whereas y-damage produced both donor and acceptor levels.
Chapter 2 deals with deep level defects found in single crystal
and polycrystalline GaAs. Many different defect species are present -
even the high purity epitaxial wafers used to fabricate radiation
detectors showed high trap densities. The Poole-Frenkel effect (field
enhanced emission) and a magnetic field sensitivity were observed in
one of the deep donor levels (Ec- 0.62 eV).
Chapter 3 deals with the fabrication of thin, highly doped contacts
to semiconductors by pulsed laser melting of an evaporated
dopant layer. The use of Li to produce n +
layers has met with the
most success, and good quality Si and Ge radiation detectors were
fabricated in this fashion. The advantage and problems of the technique
are discussed.
Chapter 4 deals with the measurement of the energy levels and
capture cross sections of defects related to over 25 different
elemental impurities deliberately introduced into Ge. There appears
to be a band of energies at approximately one-third the band gap of
the material (E = 0.66 eV) into which many deep metal-related states
(particularly donors) fall. The capture cross sections of these
states for majority carriers are generally 10 -16to 10- 18 cm2. These
facts may be related to the notion that many of the energy levels
measured for metal-related centres in Ge may be due to defects of
complicated nature, rather than simple subsitutional or interstitial
Chapter 5 discusses experiments in which point defects in Ge and
GaAs are neutralised by the incorporation of atomic hydrogen. Data
on the depth and efficiency o.f the passivation as a function of
hydrogen plasma exposure duration and temperature are presented.
Significantly in Ge, copper-related defects may be neutralised to a
depth of q., 100 pm for a 3 hour plasma exposure at 300 ° C.
Miscellaneous experiments on the behaviour of deep level defects
are described in Chapter 6. They fall into an 'interest only'
category and no detailed data is presented or conclusions drawn.

Item Type: Thesis - PhD
Authors/Creators:Pearton, S. J
Keywords: Semiconductors, Semiconductors, Detectors
Copyright Holders: The Author
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Thesis (Ph.D.)--University of Tasmania, 1983. Includes bibliographical references.

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