mbelab-theses.bib
@mastersthesis{Lewis2008MASc,
abstract = {GaAs$_{1-x}$Bi$_x$ is an exciting new semiconductor material, which has been proposed as a new material for infrared light emitting devices. Recent advancements in the growth of GaAs$_{1-x}$Bi$_x$ films have made it possible to produce GaAs$_{1-x}$Bi$_x$ light emitting diodes for the first time. Throughout this research we have grown, fabricated and characterized GaAs$_{1-x}$Bi$_x$ light emitting diodes. Similarly structured In$_x$Ga$_{1-x}$As light emitting diodes were also produced and characterized for comparison to the GaAs$_{1-x}$Bi$_x$ devices. Strong electroluminescence was obtained from GaAs$_{1-x}$Bi$_x$ devices, showing two emission peaks, one corresponding to the GaAs$_{1-x}$Bi$_x$ layer and the other to the GaAs cladding. Emission from In$_x$Ga$_{1-x}$As devices was about 100 times brighter than from GaAs$_{1-x}$Bi$_x$ devices. Temperature dependent electroluminescence and photoluminescence measurements of a GaAa$_{1-x}$Bi$_x$ light emitting diode were made and showed some unusual results. The wavelength of the peak in the electroluminescence from the GaAs$_{1-x}$Bi$_x$ was independent of temperature in the range 100 K to 300 K while the GaAs peak shifted with temperature as expected. Photoluminescence measurements on the same structure show temperature dependence of the peak wavelength similar to the temperature dependence of GaAs.},
author = {Ryan B. Lewis},
school = {University of British Columbia},
year = {2008},
url = {http://hdl.handle.net/2429/5823},
title = {{GaAs$_{1-x}$Bi$_x$} light emitting diodes: a new long wavelength semiconductor light source}
}
@mastersthesis{Yazdi2007MSc,
abstract = {
I am reporting on lensless imaging of human red blood cell using
coherent x-ray scattering (CXS)technique. The successful microfabrication
of a sample-mask structure using focused ion beam (FIB) milling was the
key element in this imaging technique. The sample-mask structure is 600 - 800 nm
gold films deposited using sputtering or electron beam evaporation
on Si$_3$N$_4$ membrane windows. We used commercially
available 100 nm thick Si$_3$N$_4$ membranes held by 3 mm diameter
silicon frames that are designed for use in transmission electron microscopy.
The red blood cell (RBC) sample was mounted in front of a 3 $\mu$m hole
milled through both gold and Si$_3$N$_4$ layers on the opposite side. Three
smaller reference apertures with diameters 300, 250 and 200 nm on the gold
side were milled all the way through both layers at a distance of 9 $\mu$m
center-to center from the sample aperture. These holes are used for
holographic lensless x-ray imaging. It was found that a gold surface
roughens during ion milling due to a sputter instability and which
produces cup-like features with a characteristic length up to few hundred nm.
We found apertures milled through gold films deposited by sputtering
show good circularity and sidewall roughness of 20 nm.
We present result on CXS measurements in transmission geometry
near Fe L$_3$- and C K-absorption edges on a single RBC.
We captured high resolution images of the sample by simple
Fourier inversion of the recorded far-field scattered intensity. We found
8.5~\% reduction in the transmission intensity near Fe L$_3$-edge due to
presence of Fe in the form of hemoglobin molecules inside RBC.
This absorption agrees with estimated agrees well with the estimated
value of 9~\% within experimental uncertainty. From limited data measured
below C K-edge we measured a presence of at least 300 nm thick carbon
inside RBC which lies in the range of the estimated value of 1.8 $\mu$m.
The resolution of our lensless imaging technique is about 55 nm near
Fe L$_3$-edge and 78 nm near C K-edge.},
address = {Canada},
author = {Arvin N. Yazdi},
month = {March},
school = {University of British Columbia},
title = {Lensless Imaging of Red Blood Cells Using Coherent Soft X-ray Scattering},
url = {http://www.physics.ubc.ca/mbelab/papers/Yazdi_Thesis_2007.pdf},
year = {2007}
}
@phdthesis{Young2006PhD,
abstract = {
Semiconductor alloys that are lattice matched to GaAs but
have a smaller energy band gap are of interest for numerous
applications, including infrared lasers for telecommunications, high
efficiency solar cells, and high electron mobility transistors. For high
optoelectronic efficiency, these materials must be highly perfect single
crystals with low defect densities. In this thesis, two substitutional
GaAs-based alloy families, nitrides and bismides, are investigated
experimentally. In the first alloy, GaNAs, the addition of N results in
a large band gap reduction, though the small size of the N relative to
As introduces tensile strain into the lattice, and the high
electronegativity of N attracts electrons. The second alloy, GaAsBi,
also has a smaller band gap and is formed by the addition of the very
large Bi atom to GaAs, which introduces compressive strain and tends to
attract holes. The experimental investigations of these alloys focused
on elucidating the relationships between the growth process, atomic
structure, and electronic properties. Films were grown by molecular beam
epitaxy (MBE) with in-situ process monitoring and subject to post-growth
structural and electronic characterization. For GaNAs and a related
alloy, InGaNAs, degradation in luminescence efficiency, mobility and
structural integrity were observed as the nitrogen content of the alloy
was increased. A comprehensive study of strain relaxation in
compressively strained InGaNAs and InGaAs quantum wells revealed that
the nitrogen alloying did not have an effect on the critical thickness
for dislocation formation, or the dislocation density in relaxed
films. At large lattice mismatch, InGaNAs quantum wells were observed to
relax by means of unusually oriented pure edge-type misfit dislocations
aligned with h100i directions, likely due to the high stress associated
with the large misfit. Use of bismuth as a non-incorporating surfactant
during growth was successfully applied to improve the material quality
of the nitrides. The Bi surface layer during growth was investigated
using in-situ electron diffraction intensity measurements, and was found
to improve both the smoothness of nitride films, by promoting a
layer-bylayer growth mode, as well as increasing the photoluminescence
(PL) intensity by a factor of $2.4$. The improvement in PL is attributed
to a reduction in nitrogen Abstract iii clusters with the surfactant. In
addition, an increase in nitrogen content of up to $50\%$ was observed in
films grown with Bi over films grown without the surfactant. The
increase in the nitrogen incorporation scales with the Bi flux, and
saturates at full Bi coverage. A modified Langmuir model was applied to
describe the behaviour of Bi on the surface, as well as the increase in
nitrogen incorporation. The bismide alloy family requires atypical MBE
growth conditions such as low substrate temperature and low As
overpressure in order to achieve Bi incorporation. The conditions are
close to those where Ga and Bi droplets form. However, in- situ light
scattering was used to identify and avoid growth with droplets,
resulting in films with high structural quality as determined by x-ray
diffraction, and strong photoluminescence. $1\%$ Bi alloying in GaAs was
also found to result in a minimal $15\%$ decrease in electron mobility, as
compared with a $94\%$ reduction for $1\%$ N alloying. Finally, a preliminary
investigation was made into the concept of co-doping GaAs with both N
and Bi. GaNAsBi films showed room temperature PL at long wavelengths
commensurate with contributions to band gap reduction from both N and
Bi. Since lattice matching to GaAs can be achieved with a Bi/N ratio of
$1.7$, GaNAsBi and GaAsBi are promising new alloys for the applications
described.},
address = {Canada},
author = {Erin Christinia Young},
month = {December},
school = {University of British Columbia},
title = {{GaNAs} and {GaAsBi}: Structural and electronic properties of two resonant state semiconductor alloys},
url = {http://www.physics.ubc.ca/mbelab/papers/ecyoung_thesis_2006.pdf},
year = {2006}
}
@phdthesis{Adamcyk2002PhD,
abstract = {
In$_{y}$Ga$_{1-y}$As$_{1-x}$N$_{x}$ containing a small amount of
nitrogen (x<0.05) is a new narrow bandgap semiconductor
alloy that has advantageous properties for the fabrication of
optoelectronic devices. In this thesis, we seek to improve the
material quality of InGaAsN and GaAsN by studying how the epitaxial
growth conditions affect both the structural and elec tronic
properties of the alloy. We describe a novel RF plasma source based
on a helical resonator design that was developed for the incorporation
of nitrogen into GaAsN and InGaA sN thin films grown by molecular beam
epitaxy. The plasma source is equipped with a baffle apparatus that
decreases the ion content of the flux. We show how the structural and
electronic properties of InGaAsN epilayers depend on the growth
conditions. \textit{In sit u} light scattering measurements and
atomic force microscopy show that a faceted surface morphology occurs
when growth conditions increase adatom surface diffusion: slow growth
rate, high substrate temperature and high V/III ratio. Large nitrogen
concentrations also favour the faceted growth mode. The residual
strain in relaxed InGaAsN films is found to be higher than in InGaA s
epilayers having the same lattice mismatch. \textit{In situ} substrate
curvature
measurements were used to monitor the strain state of the sample in
real time during the growth. \textit{Ex situ} transmission electron
microscopy and x-ray d iffraction measurements agree with the residual
strain determined with the \textit{in situ} monitor. These
characterization results also indicate that threading dislocation
glide is slower in InGaAsN than in InGaAs. Finally, we find that the
electronic properties of InGaAsN are generally degrade d with
increasing nitrogen content. However, by choosing appropriate growth
conditions, we demonstrate InGaAsN quantum wells with room temperature
photoluminescence efficiencies that are comparable to InGaAs
structures. These photoluminescence r esults may be related to the
faceting transition that was observed during GaAsN growth. In
contrast with the findings of other groups, rapid thermal anneals only
moderately improve the photoluminescence intensity and line shape of
InGaAsN single quantum wells. We observe peak intensity gains on the
order of 2 after one minute anneals at 785$^{\circ}$C. Hall
measurements ind icate that the electron mobility of Si-doped GaAsN is
inversely proportional to the nitrogen content. We conclude that
nitrogen-related neutral impurity scattering is the limiting fa ctor
in the electron mobility of GaAsN. The use of Bi as a surfactant
during growth is shown to improve the surface morphology of GaAsN
epilayers and the photoluminescence properties of InGaAsN single
quantum wells. This work provides insight into some of the key issues
that must be taken into account in the growth of dilute nitrides.
},
address = {Canada},
author = {Martin Adamcyk},
month = {April},
school = {University of British Columbia},
title = {EPITAXIAL GROWTH OF DILUTE NITRIDE-ARSENIDE COMPOUND SEMICONDUCTORS BY MOLECULAR BEAM EPITAXY},
url = {http://www.physics.ubc.ca/mbelab/papers/Adamcyk_PhD_2002.pdf},
year = {2002}
}
@mastersthesis{Strom2003MSc,
abstract = {
The band gap and optical absorption edge is measured in
semi-insulating and p-type GaNAs as a function of nitrogen content
using a photoconductivity technique. The band gap is found to decrease
with nitrogen, from 1.42 eV with 0\% nitrogen to 1.20 eV with 0.9\%
nitrogen, and to 1.14 eV with 1.73\% nitrogen content. The
characteristic energy of the exponentional absorption edge (Urbach
parameter) for p-type GaNAs is found to increase with nitrogen, from
6.7 meV with 0\% nitrogen to 14 meV with 0.8\% nitrogen content.
The mobility and carrier concentration is measured as a function of
nitrogen con tent in p-type and n-type GaNAs using Hall measurements.
The electron mobility decreases from 3000 cm$^{2}$/Vs with 0\%
nitrogen, to 650 cm$^{2}$/Vs with 0.1\% nitrogen, and to 300
cm$^{2}$/Vs with 1.0\% nitrogen content. The hole mobility is
relatively una ffected by nitrogen and stays constant around 300
cm$^{2}$/Vs for up to 1\% nitrogen con tent. The carrier
concentration in p-type GaNAs is found to decrease for highly doped
(2.5x10$^{16}$ cm$^{-3}$) GaNAs and increase for low doped
(4.5x10$^{14}$ cm$^{- 3}$) GaNAs with increasing nitrogen content. The
carrier concentration converges to 7x10$^{15}$ cm$^{-3}$ for both low
and high doping at greater than 0.8\% nitrogen content, which sugge
sts there is a trap that is pinning the Fermi level. This behaviour
is modeled using conservation of charge in the band gap, and a trap
level at 0.18 eV above the top of the valence band is found to explain
the experimental data.
A photoconductivity technique for measuring the band gap and Urbach
edge is pres ented. In this method the sample is illuminated with
monochromatic light, and the photoconductivity is measured as a
function of incident wavelength. Light with energy greater than th e
band gap is absorbed by the sample and increases the photoconductivity
signal. The absorption coefficient is determined from the
photoconductivity signal. The electrical properties, such as the
mobility and carrier concetration are obtaine d from Hall
measurements. },
address = {Canada},
author = {Eric Strom},
month = {January},
school = {University of British Columbia},
title = {Optical and electrical properties of dilute {G}a{N}${_x}${A}s$_{1-x}$},
url = {http://www.physics.ubc.ca/mbelab/papers/Strom_MSc_2002.pdf},
year = {2002}
}
@mastersthesis{Mar2004MSc,
abstract = {
The morphology of GaAs (001) single crystals during thermal chlorine
etching was studied in this work. Models that can predict the
evolution of 3~$\mu$m pitch, 100~nm amplitude wet etched gratings
were developed. A model was developed in which the etch rate and the
angle of the exposed crystal plane with respect to t he (001) plane
were associated. Additional effects such as the formation of chlor ine
adatoms and surface diffusion were included to provide better
agreement with the experimental data. The possibility of chlorine
sliding along the surface d ue to the initial momentum from the
incident beam was also considered. Experime ntal data measured using
atomic force microscopy compared with the simulated dat a found that
the average diffusion length was determined to be approximately 50~
nm. By including the sliding effect, better agreement with the
experimental dat a was observed when using an average sliding
distance of approximately 11~nm.
},
address = {Canada},
author = {Richard Elliot Mar},
month = {March},
school = {University of British Columbia},
title = {Modeling Surface Pattern Evolution During Thermal {C}l$_2$ Etching of {G}a{A}s (001)},
url = {http://www.physics.ubc.ca/mbelab/papers/Mar_MSc_2004.pdf},
year = {2004}
}
@mastersthesis{Beaton2003MSc,
abstract = {
The design of a closed cycle optical cryostat for semiconductor
crystal characterization is discussed. The system designed and
developed is capable of performing photoluminescence, resistivity, and
Hall measurements as a function of temperature from $10$~K to higher
than $300$~K. Preliminary photoluminescence experiments are carried
out as a test of the system. Results from the photoluminescence
measurements show evidence for the existence of nitrogen clusters in
GaN$_{x}$As$_{1-x}$. The clusters are shown to produce states with
energies inside the band gap. It has also been found that the
introduction of bismuth, as a surfactant, during the growth process
tends to reduce the density of the nitrogen clusters in the material.
},
address = {Canada},
author = {Daniel A. Beaton},
month = {October},
school = {University of British Columbia},
title = {Temperature Dependence of Photoluminescence of the Dilute Nitride Semiconductor {G}a{N}$_{x}${A}s$_{1-x}$},
url = {http://www.physics.ubc.ca/mbelab/papers/Beaton_MSc_2003.pdf},
year = {2003}
}
@mastersthesis{Whitwick2003MSc,
abstract = {
The photochemical reaction of Cl$_2$ gas with (100) GaAs was studied
in this work. Thermal effects due to light induced heating of the
GaAs substrate were isolated from photochemical effects. Light
induced heating was calculated and the corresponding photothermal
Cl$_2$ etching of GaAs was accounted for.
The temperature dependence of the photochemical etch rate was
examined. The light-induced etch rate potentially follows an Arrhenius
temperature dependence with an activation energy of 0.161 eV.
Near 200$^o$C the light-induced etch rate
does not increase with increasing temperature.
The photochemical etch rate
was found to depend linearly on intensity.
A possible explanation for the temperature dependence of the
photochemical etch rates is presented. It is proposed that the
desorption of GaCl$_3$ is the important mechanism in the
photochemical Cl$_2$-GaAs etch. GaCl$_3$ is photodesorbed with
illumination, which increases the rate of etching. },
address = {Canada},
author = {Michael Brian Whitwick},
month = {April},
school = {University of British Columbia},
title = {Light Activated {C}l$_2$ Etching of {G}a{A}s and Optical Holographic Pattern Formation},
url = {http://www.physics.ubc.ca/mbelab/papers/Whitwick_MSc_2003.pdf},
year = {2003}
}
@mastersthesis{Rahmim2001MSc,
abstract = {
We have explored the use of coherent resonant x-ray scattering as a
powerful technique to study, characterize and reconstruct magnetic
domains for antiferromagnetic (AFM) and ferromagnetic (FM) thin films.
This method is capable of high-resolution imaging (as it is not limited
by optical aberrations), is able to probe buried interfaces and is
operational in the presence of other fields. Here we report the first
experimental observation of x-ray speckle patterns generated by AFM
domains. Resonant x-ray scattering was performed on LaFeO3 thin films
possessing two types of domains with their AFM orientations
perpendicular to each other. X-ray magnetic linear dichroism (XMLD) at
the Fe L3 absorption edge has been exploited in order to give rise to
modulations of the scattering amplitudes according to domain
distributions, resulting in magnetic speckle.
We also report resonant x-ray scattering in the transmission geometry
from FM domains of Co/Pt multilayers. Magnetic x-ray circular dichroism
(MXCD) has been utilized with the contrast arising from the dependence
of scattering amplitude on magnetization direction of FM domains, which
are oriented normal to the surface (i.e. parallel or antiparallel to
photon helicity) due to the perpendicular interfacial anisotropy
provided by the broken symmetry at the Co-Pt interface. By tuning the
energy to the Co L3 edge, magnetic speckle is very clearly demonstrated.
We have analytically shown that upon reversal of magnetic contrast
(tuning of the scattering energy to either of the two crystal field
split peaks of the Fe L3 edge in the first experiment, and changing the
photon helicity in the second experiment) changes in speckle patterns
will be observed solely arising from the interference between roughness
and/or pinhole scattering with magnetic scattering.
We have developed a new reconstruction technique, upon extension of
Fourier transform iterative algorithms previously utilized in other
reconstruction tasks, capable of reconstructing AFM and FM magnetic
structure from resonant x-ray scattering intensity measurements. This
technique is shown to be very successful upon application to noisy
simulated data. Using this method, experimental speckle data from the FM
domains of the Co/Pt multilayer have been inverted resulting in
magnetic domains showing a remarkable similarity to the worm-domain
structure of the actual domain distribution imaged using magnetic force
microscopy (MFM). This, to our knowledge, has been the first
reconstruction of magnetic domains from experimental data. Moreover,
direct (non-iterative) reconstruction of FM domains has been shown to be
possible upon using small pinholes and/or rough samples with roughness
scale comparable to the size of domains.
},
address = {Canada},
author = {Arman Rahmim},
month = {November},
school = {University of British Columbia},
title = {Analysis of Coherent Resonant X-Ray Scattering and Reconstruction of Magnetic Domains},
url = {http://www.physics.ubc.ca/mbelab/papers/Rahmim_MSc_2001.pdf},
year = {2001}
}
@phdthesis{Pinnington1999PhD,
abstract = {
The surface of a film grown epitaxially on a crystalline substrate is
generally rough, even if the initial growth surface is smooth on the
atomic scale. In the case of strained-layer epitaxy, in which the
composition of the film is such that it does not share the same
lattice constant as the substrate, the roughness often develops in
response to strain-relief processes occurring in the film during
growth. In this thesis we show that a careful analysis of the
time evolution of the surface morphology during strained-layer growth,
can reveal quantitative information about both the strain-relief
mechanisms acting within the film and the diffusion processes
occurring at the surface.
Two complementary measurement techniques, namely atomic force
microscopy (AFM) and elastic light scattering, are used to acquire the
surface morphology information necessary for the analysis. In this
work we demonstrate quantitative agreement between roughness
measurements obtained by both techniques. A major advantage of light
scattering over AFM is its suitability to real-time monitoring of the
surface during growth.
We consider the growth of In$_{0.18}\rm Ga_{0.82}As$ and InAs,
on $(001)$-oriented GaAs substrates, by molecular beam epitaxy (MBE).
In both cases the film is compressively strained owing to the 7\%
lattice mismatch between InAs and GaAs. In the case of
In$_{0.18}\rm Ga_{0.82}As$ growth, the strain is relieved
plastically as the film thickness increases, through the introduction
of misfit dislocations at the film/substrate interface. A
characteristic crosshatch pattern develops at the surface, consisting
of ridges aligned along the <110> crystal directions.
We present an analytical model to describe this roughening, in which
the ridges arise from surface diffusion in response to the dislocation
strain fields. Although it has only three fitting parameters, the
model is able to reproduce both the time dependence and the length
scale dependence of the surface morphology, as measured by light
scattering and AFM.
For the case of InAs growth on GaAs, the strain is relieved
elastically through a morphological transition in which nearly
identically-sized three-dimensional islands form, known in the
literature as quantum dots. For typical growth conditions the islands
are too small and closely spaced to be detected using visible
wavelengths. An ultraviolet light scattering apparatus is described,
which we show can detect the onset of quantum dot formation. The UV
scattering signal increases linearly with time after the dots have
formed, which we interpret as evidence that the dots are diffusing on
the surface. During prolonged annealing we observe the emergence and
growth of larger islands that initially consume material from the
quantum dots and then compete with each other for material. The
\emph{in situ} light scattering measurements reveal that these processes are
sensitively dependent on annealing temperature and arsenic
overpressure.
},
address = {Canada},
author = {Thomas Henry Pinnington},
month = {December},
school = {University of British Columbia},
title = {Surface Morphology Dynamics in Strained-Layer Epitaxy},
url = {http://www.physics.ubc.ca/mbelab/papers/Pinnington_PhD_1999.pdf},
year = {1999}
}
@mastersthesis{Ballestad1998MSc,
abstract = {
Control over the surface structure of semiconductor films during
growth is critical for devices of recent technological importance.
Typically the length scales of interest range from nanometers to
micrometers.
Examples include the size and spacing of quantum dots in quantum dot
lasers, and the pitch and amplitude of grating structures for distributed
Bragg reflectors.
Elastic light scattering has atomic height sensitivity to this surface
structure, on lateral length scales as low as half the incident
wavelength, and is easily implemented for {\em in-situ} monitoring
during film growth [Pinnington \emph{et al.}, PRL 79, 1698-1701
(1997)].
For the smooth surfaces of interest here, the distribution of the
scattered light intensity as a function of scattering angle directly
maps out the power spectral density (PSD). The PSD gives the 'root
mean square' roughness of the surface structure as a function of
inverse length scale, or spatial frequency.
Here we present {\em in-situ} light scattering measurements performed
during III-V semiconductor film growth by molecular beam epitaxy (MBE).
We have used the technique to monitor the smoothing of one-dimensional
grating structures during regrowth.
For the regrowth experiments, the grating pitch was chosen such
that the detection angle of the {\em in-situ} measurement coincided with
the scattering peak associated with a harmonic of the grating periodicity.
Because the initial shape of the patterned surface is known, it is
possible to reconstruct the shape of the grating from the PSD as it
evolves in time during growth.
We find that for {\em homoepitaxy} of gallium arsenide ({\em GaAs}) on
textured substrates, the time evolution follows the Kardar-Parisi-Zhang
(KPZ) model [Kardar \emph{et al.}, PRL 56, 889-892 (1986)].
},
address = {Canada},
author = {Anders Ballestad},
month = {July},
school = {University of British Columbia},
title = {Smoothing of Patterned {G}allium {A}rsenide Surface During Epitaxial Growth},
url = {http://www.physics.ubc.ca/mbelab/papers/Ballestad_MASc_1998.pdf},
year = {1998}
}
@phdthesis{Ballestad2005PhD,
abstract = {
The problem of a complete theory describing the far-from-equilibrium
statistical mechanics of epitaxial crystal growth remains unsolved.
Besides its academic importance, this problem is also interesting from
the point of view of device manufacturing. In order to improve on the
quality and performance of lateral nanostructures at the lengthscales
required by today's technology, a better understanding of the physical
mechanisms at play during epitaxial growth and their influence on the
evolution of the large-scale morphology is required. In this thesis,
we present a study of the morphological evolution of GaAs (001) during
molecular beam epitaxy by experimental investigation, theoretical
considerations and computational modeling. Experimental observations
show that initially rough substrates smooth during growth and
annealing towards a steady-state interface roughness, as dictated by
kinetic roughening theory. This smoothing indicates that there is no
need for a destabilizing step-edge barrier in this material system.
In fact, generic surface growth models display a much better agreement
with experiments when a weak, negative barrier is used. We also
observe that surface features grow laterally, as well as vertically
during epitaxy. A growth equation that models smoothing combined with
lateral growth is the nonlinear, stochastic Kardar-Parisi-Zhang (KPZ)
equation. Simulation fits match the experimentally observed surface
morphologies quite well, but we argue that this agreement is
coincidental and possibly a result of limited dynamic range in our
experimental measurements. In light of these findings, we proceed by
developing a coupled growth equations (CGE) model that describes the
full morphological evolution of both flat and patterned starting
surfaces. The resulting fundamental model consists of two coupled,
spatially dependent rate equations that describe the interaction
between diffusing adatoms and the surface through physical processes
such as adatom diffusion, deposition, and incorporation and detachment
at step edges. In the low slope, small amplitude limit, the CGE model
reduces to a nonlinear growth equation similar to the KPZ equation.
From this, the apparent applicability of the KPZ equation to surface
shape evolution is explained. The CGE model is based on fundamental
physical processes, and can therefore explain the underlying physics,
as well as describe macroscopic pattern evolution during growth.
},
address = {Canada},
author = {Anders~ Ballestad},
month = {March},
school = {University of British Columbia},
title = {Epitaxial Growth Dynamics in {G}allium {A}rsenide},
url = {http://www.physics.ubc.ca/mbelab/papers/Ballestad_PhD_2005.pdf},
year = {2005}
}
@phdthesis{Schmid2004PhD,
abstract = {Many fabrication processes for semiconductor nanostructures rely on
the understanding of surface pattern evolution during crystal growth
and etching. In this thesis, the morphological evolution of GaAs
surfaces during thermal chlorine etching and molecular beam epitaxial
growth is investigated by atomic force microscopy and light
scattering. The experimental results are compared to numerical
simulations based on continuum models. For both etching and growth,
the evolution of flat surfaces and small amplitude ($<$30~nm) random
surface patterns can be modeled with excellent accuracy with
stochastic differential equations for the surface height as predicted
by kinetic roughening theory. For MBE growth this equation is the
Kardar-Parisi-Zhang (KPZ) equation while etching requires the
extension of the KPZ model with a fourth-order linear
term. Anisotropic etch rates with respect to crystal orientation are
found to be a major consideration for surface pattern transfer by
thermal chlorine etching. It is shown how pattern transfer of one- and
two-dimensional gratings can be predicted and optimized by varying the
orientation of the pattern and by the use of a directional molecular
beam to supply the chlorine. To describe the complex shapes evolving
from etching and growth on microfabricated gratings, models based on
two coupled differential equations for the surface concentration of
etchant or adatoms and the surface height are developed. Excellent
fits to the experimental shapes observed over a wide range of etching
and growth conditions can be obtained with these models and they
emerge as a powerful tool to understand the pattern evolution in terms
of the underlying microscopic physics such as surface diffusion,
spatial flux inhomogeneity, sticking coefficients, step edge
incorporation and diffusion bias.
},
address = {Canada},
author = {Jens H. Schmid},
month = {May},
school = {University of British Columbia},
title = {Evolution of Surface Texture in Thermal Chlorine Etching And Molecular Beam Epitaxy of {G}allium {A}rsenide},
url = {http://www.physics.ubc.ca/mbelab/papers/jens_PhD_2004.pdf},
year = {2004}
}
@mastersthesis{Webster2004MASc,
abstract = {
Optical coherence tomography (OCT) is an emerging medical imaging
technology based on the coherent interference of light. Current use
of OCT in clinical settings is limited by the lack of a suitable
light source. This thesis describes the design of a new type of
source for OCT, based on the GaInNAs semiconductor materials system.
A semiconductor heterostructure consisting of several different
quantum wells is discussed as a device for generating broadband
($>$100~nm) light in the near infrared (900-1500~nm). The use of
temperature to control spectral shape and intensity is examined.
Other aspects of device design are investigated, including models
for quantum well emission and for the band gaps of dilute nitride
semiconductors.
Photoluminescence measurements are presented, providing a proof of
principle demonstration of the source design. Emission centred at
1225~nm with a 195~nm bandwidth is achieved. The use of temperature
to control inter-well carrier transfer is demonstrated and
successfully modelled. Localization effects of nitrogen cluster
states are shown to be greatly reduced in quantum well structures,
as compared to bulk samples.
},
address = {Canada},
author = {Scott Webster},
month = {November},
school = {University of British Columbia},
title = {Semiconductor Light Source for Optical Coherence Tomography},
url = {http://www.physics.ubc.ca/mbelab/papers/Webster_MASc_2004.pdf},
year = {2004}
}