Barker, John R. (John Reginald) (1969) A theoretical study of non-equilibrium photoexcited carriers in semiconductors. PhD thesis, University of Warwick.

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Abstract

A theoretical study is made of the physics of photoexcited carriers in one
of the bands of a semiconductor. The emphasis is on photoexcited hot carrier
phenomena, for which the mean carrier energy deviates significantly from the
thermal equilibrium value in the steady state. Very little previous theoretical
work in this area has been reported.
Two situations are analysed. The first is an investigation of hot photoexcited
carriers in germanium and silicon at low temperatures. The carriers
are excited into the band by a model black-body excitation spectrum having
a mean energy in excess of the thermal energy. Full account is taken of the
interaction of the carriers with impurities and phonons, and recombination is
asswned to occur via a cascade mechanism. Significant carrier heating is
found for trapping densities of the order 1016 cm-3 at lattice temperatures
below about 300K. The steady-state carrier distribution functions are derived
numerically from the Boltzmann equation in the absence of external fields. The
low field transport and trapping parameters are then derived by a perturbation
theory. The assumption of a linear response to applied fields is checked by
an adaptation of tileHonte Carlo technique first employed by Kurosawa (1966) and
Boardman et al (1968) in high field studies of semiconductors. The technique
is extensively modified to suit our problem; in particular the concert of the
self-scattering device is enlarged.
The theory shows good agreement with the experimentally measured (Rollin
and Rowell 1960) temperature variation of the Hall mobility of photoexcited
holes in germanium. Agreement is also obtained with an experimental curve for
the temperature dependence of the capture cross section for electrons in silicon. experimentally a 'cut-off' is found in the temperature dependence below
Tile hot carrier model explains this phenomenon in terms of an
anomalous temperature dependent Hall number which arises from the severe non-
Maxwellian heating of the carriers. However, not all the experimental
results can be explained this way and a tentative alternative mecnanism is
suggested.
The second situation analysed involves monochromatic photoexcitation
leading to the oscillatory photoconductivity effect in many polar semiconductors.
Considerable controversy has existed previously as to the origin of this effect.
The distribution functions and photoconductivity are studied as a function
of photon frequency and electric field strength on the basis of an analytical
model and detailed Monte Carlo calculations. Good agreement is found with
experiment as regards tile field dependence of the overall spectral response,
confirming the assumptions of an earlier approximate analytical approach
(Stocker and Kaplan 1966).
For certain photon frequencies the photoexcited carriers can theoretically
exhibit both total and differential negative mobility for certain ranges of
applied electric field, confirming a previous approximate theory (Stocker 1967).
although tile effect nas not been observed experimentally. This leads to a
non-uniform field distribution in the semiconductor and the possibility of spacecharge
instabilities. The possibility of steady-state negative photoconductivity
is investigated with particular reference to the spatial distribution of electric
field and the stability of the carrier system. The evolution and form of the
instabilities and steady-states are evaluated numerically. Full account is
taken of the electron and trap dynamics. It is shown that the total negative
resistance state is unstable in the presence of injecting contacts. Instead
either a non-uniform field distribution showing bulk positive resistance is
established or there occur propagating instabilities leading to positive current
oscillations.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics
Library of Congress Subject Headings (LCSH):	Semiconductors, Germanium -- Thermal properties, Silicon -- Thermal properties, Equilibrium, Nuclear excitation
Official Date:	October 1969
Dates:	Date Event October 1969 Submitted
Institution:	University of Warwick
Theses Department:	Department of Physics
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Hearn, C. J.
Sponsors:	Great Britain. Ministry of Defence (Navy)
Extent:	163 leaves.
Language:	eng

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