Thesis (PhD (Physics))--Stellenbosch University, 2008. / The advent of high intensity short pulse lasers has opened the door to investigating
buried solid-solid interfaces through the technique of optical second
harmonic generation (SHG). This has led to extensive study of technologically
important systems such as the Si/SiO2 interface. In this study, SHG is
employed to study the interface between highly boron doped p+-type Si and
its native oxide layer (SiO2).
Previous studies from this laboratory have extensively investigated the
photo-induced charge transfer process across the Si/SiO2 interface in the
case of undoped natively oxidized Si by means of SHG, with initial SHG
measurements being performed on boron doped p+-type Si.
The natively oxidized p+-type Si/SiO2 sample was placed on a computer
controlled positioning system which allowed for translation of the sample and
rotation around the azimuth. The laser system employed was characterized
in terms of spectral composition, pulse duration, pulse repetition rate, spatial
pro le and pulse energy in order to ensure quantitative measurements. The
SHG signal generated from the sample interface was recorded in re ection.
Under the applied irradiation conditions, defects are created at the interface
by the near infra red (NIR) femtosecond radiation from the laser. These
defects are then populated via multi-photon processes by electrons and to
a lesser extent holes. The charge transfer across the interface induces an
interfacial electric eld. This photo-induced electric eld is in addition to
the built-in interfacial electric eld caused by positive ionization of naturally
occurring interfacial defects due to the strong doping of the bulk Si.
It is this interfacial electric eld, consisting of the built-in doping induced
eld and the photo-induced electron and hole elds, that is probed by SHG.
The SHG signal is strongly dependent on the magnitude of this interfacial
electric eld as the electric eld induced second harmonic (EFISH) signal
dominates all other contributions to the observed SHG signal in the case of
the Si/SiO2 system.
The temporal evolution of the SHG signal is recorded for di erent intensities
from virgin as well as the pre-irradiated samples. This yields information
about the time scales on which the charge separation occurs as well as the in-
uence of existing photo-induced trap sites on the charge separation process,
since the strength of the SHG signal is an indirect measure of the interfacial
electric eld strength. The angular dependence of the SHG signal (SH rotational
anisotropy measurements) for both the initial signal (when the doping
induced electric eld dominates) and the saturated signal (when the electron
induced electric eld dominates) is measured. Both these measurements show
a four fold symmetry but with a relative 45 phase shift between them. This
iii
is taken as con rmation of the reversal of the interfacial electric eld direction.
The initial SHG signal as a function of intensity is also recorded for
di erent incident wavelengths. The variation in the non-quadratic dependence
of the initial SHG signal on the incident intensity is attributed to a
resonant enhancement of two-photon absorption and subsequent screening of
the interfacial electric eld by charge carriers.
The measurement performed and the results obtained contribute to the
understanding of the photo-induced charge separation process across buried
solid-solid interfaces, speci cally as it applies to the important Si/SiO2 interface.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/1186 |
| Date | 12 1900 |
| Creators | Neethling, Pieter Herman |
| Contributors | Rohwer, E. G., Bergmann, H. M. von, Stellenbosch University. Faculty of Science. Dept. of Physics. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | Stellenbosch University |
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