Thesis (PhD (Physics))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Silicon/silicon dioxide (Si/SiO2) interface is widely used in microelectronics as the gate between
the drain and source of most metal oxide semiconductor field effect transistors (MOSFETs).
The functionality, reliability and electrical properties of such transistors are strongly dependent
on the quality of the Si/SiO2 structure forming the gate. Characterization of the Si/SiO2
interface is important in understanding device degradation therefore the Si/SiO2 interface is a
subject of intensive investigation. Research studies of Si/SiO2 interfaces using optical methods
have been reported by many groups around the world but to date many open questions still
exist. The physics of photoinduced trap or defect generation processes and the subsequent
trapping of charge carriers, the precise role of photoinduced interfacial electric field in altering
optical properties of the Si/SiO2 interface and its role in affecting the second harmonic (SH)
yield measurements are not well understood.
In this work a commercial near infrared femtosecond (fs) laser source [1.55 eV, 75 ± 5 fs, 10
nJ, 80 MHz] is used to study native Si/SiO2 interfaces of free standing single crystalline Si
membrane and bulk Si. Optical second harmonic (SH) generated at the Si/SiO2 interfaces
of a Si membrane in reflection and for the first time in transmission is demonstrated as well
as stationary, single colour, pump-probe reflectivity measurements from the Si/SiO2 interface
of bulk n-type Si. The experimental setups for the second harmonic generation (SHG) and
pump-probe techniques were designed and implemented, and measurements were recorded
by a computer controlled data acquisition system. Free standing Si membrane samples were
successfully produced at the Institut f¨ur Photonische Technologien (IPHT) in Jena, Germany
from bulk Si using a chemical etching process and were characterised using the z-scan technique.
The penetration depth of light with a photon energy of 1.55 eV in silicon allows transmission
of the fundamental fs laser pulses through the Si membrane (∼ 10 μm in thickness) and
this is exploited to generate a SH signal in transmission from the Si/SiO2 interfaces of the
Si membrane. In the presence of sufficiently intense fs laser light defects are created at the interfaces and populated by multiphoton transfer of charges from Si to SiO2 where they are
subsequently trapped. The transfer of charge establishes interfacial electric fields across the
interfaces of the Si membrane and this enhances SHG. This phenomenon is called electric field
induced second harmonic (EFISH) generation. To our knowledge, EFISH measurements from
interfaces of Si membrane performed in transmission are demonstrated for the first time in the
present study. The demonstration of EFISH in transmission revealed new results which allowed
us to provide additional perspectives on the EFISH generation process at Si/SiO2 interfaces
never reported before. The temporal response of SH signals from virgin spots were recorded
at different incident laser powers for both reflection and transmission geometries. The SH
responses measured in transmission were observed to be time dependent and show an increase
during irradiation of the sample corresponding to EFISH process.
A series of SH measurements were recorded at different laser powers to compare the magnitudes
of SH yield in each detection geometry for a single Si/SiO2 interface. The magnitude of the SH
yield measured in transmission was higher than expected and surpassed the SH yield measured
in reflection. The expectation is based on the fact that the local intensity of the fundamental
beam at the second interface where the SH in transmission is generated is low compared to
the local intensity at the first interface where the SH in reflection originates. A physical
model is developed to consistently interpret the experimental results obtained in this study.
In this model we established the origin of EFISH signals in each detection geometry, explain
the unexpected high SH signals measured in transmission and provide an analysis of the time
constants extracted from SH response in transmission and reflection.
In addition, we also report for the first time stationary pump-probe reflectivity measurements
from bulk n-type Si(111) samples with native oxide. A strong pump beam was focused on the
same spot as a weak probe beam from the same fs laser source. The change in reflectivity
of the Si(111)/SiO2 system was recorded by monitoring the change in intensity of the weak
reflected probe beam. The temporal evolutions of the reflectivity of the material were recorded
at different pump powers. The reflectivity of the material increases over several minutes of
irradiation and reaches steady-state after long time irradiation. The change in reflectivity of
the material is attributed to a nonlinear process called Kerr effect, and the temporal response arises from the photoinduced interfacial electric field across the Si(111)/SiO2 interface caused
by multiphoton charge transfer from bulk Si(111) to the SiO2 layer.
The results reported in this study contribute to the understanding of the photoinduced interfacial
electric field caused by charge carrier separation across buried solid-solid interfaces.
They also reveal nonlinear optical processes such as the Kerr effect caused by charge dynamics
across the interface in addition to the well known SHG process. / AFRIKAANSE OPSOMMING: Die silikon/silikon dioksied (Si/SiO2) skeidingsvlak word algemeen gebruik in mikro-elektronika
as die hek tussen die put en die bron van die meeste metaaloksied halfgeleier veld-effek transistors
(MOSFETs). Die werkverrigting, betroubaarheid en elektriese eienskappe van sulke
transistors word grootliks bepaal deur die kwaliteit van die Si/SiO2 struktuur wat die hek
vorm. Karakterisering van die Si/SiO2 skeidingsvlak is belangrik om die degradering van die
transistor te verstaan en daarom is die Si/SiO2 skeidingsvlak die onderwerp van intensiewe ondersoek.
Ondersoek van die Si/SiO2 skeidingsvlak deur van optiese metodes gebruik te maak
is geraporteer deur verskeie internasionale groepe, maar daar bestaan tot vandag toe nog n
groot aantal onbeantwoorde vrae. Die fisika van die fotogenduseerde generering van defekte
en van posisies waarin ladings gevang kan word, asook die daaropvolgende vasvang van ladingsdraers,
die presiese rol van die fotoge¨ınduseerde elektriese veld oor die skeidingsvlak in die
verandering van die optiese eienskappe van die Si/SiO2 skeidingsvlak en die grootte van die
tweede harmoniek (SH) sein word nog nie goed verstaan nie.
In hierdie werk word n kommersile naby-infrarooi femtosekonde (fs) laserbron [1.55 eV, 75 ± 5
fs, 10 nJ, 80 MHz] gebruik om natuurlike Si/SiO2 skeidingsvlakke van vrystaande enkelkristallyne
Si membrane en soliede Si te bestudeer. Optiese tweede harmoniek (SH) wat by die
Si/SiO2 skeidingsvlakke van ’n Si membraan gegenereer word - in refleksie en vir die eerste
keer in transmissie - is gedemonstreer, asook stasionˆere, een-golflengte pomp-toets refleksiemetings
op die Si/SiO2 skeidingsvlak van soliede n-gedoteerde Si. Die eksperimentele opstellings
vir die tweede harmoniek generering (SHG) en pomp-toets tegnieke is ontwerp en uitgevoer
en metings is opgeneem deur ’n rekenaarbeheerde dataversamelingstelsel. Vrystaande Si membraan
monsters is suksesvol by die Institut f¨ur Photonische Technologien (IPHT) in Jena,
Duitsland vervaardig uit soliede Si deur ’n chemiese etsproses en is gekarakteriseer met behulp
van die z-skanderingstegniek as deel van hierdie studie.
Die diepte waartoe lig met ’n fotonenergie van 1.55 eV in silikon indring laat die transmissie
van die fundamentele fs laserpulse deur die Si membraan (met ∼ 10 μm dikte) toe en dit word
ontgin om ’n SH sein van die Si/SiO2 skeidingsvlakke van die Si membraan in transmissie te
meet. In die teenwoordigheid van fs laserlig met voldoende intensiteit word defekte by die skeidingsvlakke geskep en bevolk deur meer-foton ladingsoordrag van die Si na die SiO2 waar
die ladings daaropvolgens vasgevang word. Die oordrag van ladings skep elektriese velde oor
die skeidingsvlakke van die Si membraan en dit versterk die SHG. Hierdie verskynsel word
elektriese veld ge¨ınduseerde tweede harmoniek (EFISH) generering genoem. Sover ons kennis
strek is die meting van EFISH seine van skeidingsvlakke van Si membrane in transmissie vir
die eerste keer in hierdie studie gedemonstreer. Die demonstrasie van EFISH in transmissie
het nuwe resultate opgelewer wat ons toegelaat het om bykomende perspektiewe op die EFISH
genereringsproses by Si/SiO2 skeidingsvlakke te verskaf waaroor nog nooit vantevore verslag
gedoen is nie. Die tydafhanklike gedrag van die SH seine van voorheen onbestraalde posisies is
gemeet by verskillende drywings van die inkomende laserbundel vir beide die refleksie en transmissie
geometrie¨e. Die gedrag van die SH sein in transmissie is waargeneem om tydafhanklik
te wees en ’n toename te toon gedurende bestraling van die monster in ooreenstemming met
EFISH prosesse.
’n Reeks van SH metings is opgeneem by verskillende laserdrywings om die groottes van die SH
opbrengste in elke meetgeometrie vir ’n enkele Si/SiO2 skeidingsvlak te vergelyk. Die grootte
van die SH opbrengs wat in transmissie gemeet is was ho¨er as verwag is en het die grootte
van die SH opbrengs in refleksie oortref. Die verwagting is gebaseer op die feit dat die lokale
intensiteit by die tweede skeidingsvlak waar SH in transmisie gegenereer word relatief laag is in
vergelyking met die lokale intensiteit by die eerste skeidingsvlak waar SH in refleksie ontstaan.
’n Fisiese model is ontwikkel om die eksperimentele resultate wat in hierdie studie verkry is
op ’n konsekwente wyse te interpreteer. In hierdie model het ons die oorsprong van EFISH
seine in elke meetgeometrie vasgestel, die onverwagte ho¨e SH seine wat in transmissie gemeet
is verklaar en ’n analise van die tydkonstantes wat uit die SH gedrag in transmissie en refleksie
afgelei is gedoen.
Verder rapporteer ons ook vir die eerste keer stasionˆere pomp-toets reflektiwiteitsmetings van
soliede n-gedoteerde Si(111) monsters met ’n natuurlike oksied. ’n Sterk pompbundel is gefokus
op dieselfde posisie as ’n swak toetsbundel van dieselfde laserbron. Die verandering in reflektiwiteit
van die Si(111)/SiO2 stelsel is gemeet deur die verandering in die intensiteit van die
swak weerkaatste toetsbundel te monitor. Die tydevolusie van die reflektiwiteit van die mate riaal is gemeet by verskillende pompdrywings. Die reflektiwiteit van die materiaal neem toe
gedurende etlike minute van bestraling en bereik ’n stasionˆere toestand na ’n lang tyd van
bestraling. Die verandering in reflektiwiteit van die materiaal word toegeskryf aan ’n nielini
ˆere prosess, naamlik die Kerr effek, en die tydafhanklike gedrag ontstaan as gevolg van die
fotoge¨ınduseerde elektriese veld oor die Si(111)/SiO2 skeidingsvlak wat veroorsaak word deur
meer-foton ladingsoordrag van die soliede Si(111) na die SiO2 laag.
Die resultate wat in hierdie studie gerapporteer word dra by tot die verstaan van die fotoge
¨ınduseerde elektriese veld oor die skeidingsvlak wat veroorsaak word deur die skeiding
van ladingsdraers oor die bedekte kristal-kristal skeidingsvlak. Dit lˆe ook nie-liniˆere optiese
prosesse soos die Kerr effek bloot wat veroorsaak word deur die dinamika van ladings oor die
skeidingsvlak, bykomend tot die bekende SHG proses.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/5317 |
| Date | 12 1900 |
| Creators | Nyamuda, Gibson Peter |
| Contributors | Rohwer, E. G., Steenkamp, C. M., Stafast, H., University of Stellenbosch. Faculty of Science. Dept. of Physics. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Format | 117 p. : ill. |
| Rights | University of Stellenbosch |
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