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Investigação teórica sobre possíveis aplicações na eletrônica de nanofios de AlN, GaN e InN: um estudo de primeiros princípios / Theoretical investigation of possible application of aln, gan and inn nanowires in the electonics: first principles studyColussi, Marcio Luiz 30 July 2012 (has links)
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Using the formalism of Density Functional Theory with spin polarization and the Generalized
Gradient Approximation for exchange and correlation term, we studied the stability
and electronic properties of substitutional impurities of C, Si and Ge in GaN, AlN and
InN nanowires and the variation of the band offset with the diameter variation in AlN/GaN
nanowires heterojunctions. For the study of substitutional impurities we use AlN, GaN and InN
nanowires in the wurtzite phase with diameter of 14.47 Å, 14.7 Å and 16.5 Å, respectively.
For the study of variation of the band offset with the diameter of the nanostructure, we use
nanowires in the wurtzite phase with a mean diameter ranging from 0.99 nm to 2.7 nm and the
zinc blende phase with an average diameter ranging from 0.75 nm to 2.1 nm.
The electronic structure calculations show that of GaN, AlN and InN nanowires are
semiconductors with direct band gap at point Γ. To study the substitutional impurities, we
consider that the impurity can occupy the cation or anion sites in non-equivalent positions that
are distributed from the center to the surface of the nanowire. For the C impurities, in GaN
nanowires, we find that when the C atom is substituted in the N site, it will be uniformly distributed
along the diameter of the nanowire. When substituted at the Ga site, it will be preferably
find on the surface of the nanowire. In this case, the formation energy of CGa is almost identical
to the CN, thus can occur formation of the auto-compensed CN-CGa pair. In AlN nanowires,
when the C atom occupying the N site, it is also observed an almost uniform distribution along
the diameter of the nanowire with a small preference (less energy formation) to the surface sites.
Since the formation energy of the CN is lower than CAl in all regions of the nanowires, taking
thus more likely to form CN. For InN nanowires, in the center sites, the formation energy of the CN and CIn is very similar, and the CN will have a uniform distribution along the diameter,
but on the surface of the CIn is more stable and band structure show that this configuration has
shallow donor levels.
For Ge substitutional impurities in GaN nanowires, we observed that the center of the
nanowire, the Ge atom is more likely to be found located in the Ga site, but in surface to find
the most likely of N site, this being the most stable configuration. For AlN nanowires, the
center of nanowire is possible to find the Ge atom at the N or Al sites, as the formation energy
is practically the same. On the surface the more likely it is to find the Ge atom of the N site,
which also is the most stable configuration. As for InN nanowires, the Ge atom will be found
preferably at the In site with uniform distribution along the diameter of the nanowire. Analyzing
the band structure of GeIn observed shallow donor levels. For the Si substitutional impurities,
we obtain that in GaN and InN nanowires of the most stable configuration, the Si atom is to be
found at the cation (Ga and In) sites in the central sites of the nanowire and analyzing the band
structure of SiGa and SiIn, we also observed shalow donor levels. However, for AlN nanowires
in the centerof the nanowire is greater the probability of finding the Si atom at the Al site, but
the surface is greater the probability of finding the Si atom at the N site which is the most stable
configuration.
Finally, we analyze the variation of the band offset to the change in diameter of the
nanowires forming the heterostructure. We consider heterostructure on yhe wurtzite and zinc
blende phases, therefore during the synthesis the two phases are obtained. We found that the
result is similar for the two phases and the extent that the diameter increases the value of the
band offset also increases, tending to the value obtained for the bulk. / Usando o formalismo da Teoria do Funcional da Densidade com polarização de spin e a
aproximação do gradiente generalizado para o termo de troca e correlação, estudamos a estabilidade
e as propriedades eletrônicas de impurezas substitucionais de C, Si e Ge em nanofios de
GaN, AlN e InN e a variação do band offset com o diâmetro em heteroestruturas da nanofios
AlN/GaN. Para o estudo de impurezas substitucionais utilizamos nanofios de AlN, GaN e
InN na fase da wurtzita e com diâmetros de 14,47 Å, 14,7 Å e 16,5 Å, respectivamente. Já para
o estudo da variação do band offset com o diâmetro da nanoestrutura, utilizamos nanofios que
formam a heteroestrutura na fase wurtzita com diâmetro médio variando 0,99 nm até 2,7 nm e
na fase blenda de zinco com diâmetro médio variando de 0,75 nm até 2,1 nm.
Os cálculos de estrutura eletrônica apresentam que os nanofios de AlN, GaN e InN
são semicondutores com gap direto no ponto Γ. Para o estudo das impurezas substitucionais,
consideramos que a impureza pode ocupar o sítio do cátion ou do aniôn, em posições não
equivalentes que estão distribuídas do centro até a superfície do nanofio. Para a impureza de
C, em nanofios de GaN, obtemos que, quando o átomo de C for substituído no sítio do N,
o mesmo vai estar distribuído uniformemente ao longo do diâmetro do nanofio. Já quando
substituído no sítio do gálio, o mesmo vai ser encontrado preferencialmente na superfície do
nanofio, sendo que, na superfície do nanofio a energia do formação do CGa é praticamente a
mesma do CN, assim pode ocorre a formação de pares autocompensados CN-CGa. Em nanofios
de AlN, quando o átomo de C ocupar o sítio do N, também vai ter uma distribuição quase uniforme ao longo do diâmetro do nanofio com uma pequena preferência (menor energia de
formação) para os sítios da superfície. Sendo que a energia de formação do CN é menor que
do CAl em todas as regiões do nanofios, tendo assim, probabilidade maior de formar CN. Para
nanofios de InN, nos sítios do centro, a energia de formação do CN e CIn é muito próxima,
sendo que o CN vai ter distribuição uniforme ao longo do diâmetro, mas na superfície o CIn
ser torna mais estável e a estrutura de bandas mostra que esta configuração apresenta níveis
doadores rasos.
Para impurezas substitucionais de Ge, em nanofios de GaN, observamos que no centro
do nanofio, o átomo de Ge tem uma probabilidade maior de ser encontrado no síto do Ga,
mas nos sítios da superfície a probabilidade é maior de encontrar no sítio do N, sendo essa a
configuração mais estável. Para nanofios de AlN, no centro do nanofio, é possível encontrar o
átomo de Ge no sítio do N ou Al, já que a energia de formação é práticamente a mesma. Na
superfície a probabilidade maior é de encontrar o átomo de Ge no sítio do N, sendo, também,
esta a configuração mais estável. Já para nanofios de InN, o átomo de Ge vai ser encontrado
preferencialmente no sítio do In com distribuição uniforme ao longo do diâmetro do nanofio.
Analisando a estrutura de bandas do GeIn observamos níveis doadores rasos. Para a impureza
substitucional de Si, obtemos que em nanofios de GaN e InN a configuração mais estável, é
o Si ser encontrado no sítio do cátion (Ga ou In) nos sítios centrais do nanofio e analizando a
estrutura de bandas do SiGa e do SiIn, também observamos níveis doadores rasos. Entratanto,
para nanofios de AlN, no centro do nanofio a probabilidade é maior de encontrar o átomo de
Si no sítio do Al, mas na superfície a probabilidade é maior de encontrar o átomo de Si no sítio
do N, sendo esta a configuração mais estável.
Por fim, analisamos a variação do band offset com a variação do diâmetro do nanofios
que forma a heteroestrutura. Consideramos heteroestruturas na fase wurtzita e blenda de zinco,
pois nos processos de síntese as duas fases são obtidas. Observamos que o resultado é similar
para as dias fases e, a medida, que o diâmetro aumenta o valor do band offset também
aumenta, tendendo para o valor obtido para o cristal.
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Growth of InAs/InP Nanowires by Molecular Beam EpitaxyHaapamaki, Christopher M. 04 1900 (has links)
<p>InP nanowires with short InAs segments were grown on InP (111)B substrates by Au assisted vapour-liquid-solid growth in a gas source molecular beam epitaxy system. Nanowire crystal structure and morphology were investigated by transmission electron microscopy as a function of temperature, growth rate, and V/III flux ratio. At 370C predominantly kinked nanowires with random morphology and low areal density were observed with a rough parasitic 2D film. At 440C, nanowire density was also reduced but the 2D film growth was smoother and nanowires grew straight without kinking. An optimum temperature of 400C maximized areal density with uniform nanowire morphology. At the optimum temperature of 400C, an increase in V/III flux ratio changed the nanowire morphology from rod-shaped to pencil like indicating increased radial growth. Growth rate did not affect the crystal structure of InP nanowires. For InAs nanowires, changing the growth rate from 1 to 0.5 μm/hr reduced the presence of stacking faults to as low as one per nanowire. Short InAs segments in InP nanowires were found to grow through two mechanisms for nanowires of length L and diameter D. The first mechanism described the supply of In to the growth front via purging of In from the Au droplet where L was proportional to D. The second mechanism involved direct deposition of adatoms on the nanowire sidewall and subsequent diffusion to the growth front where L was proportional to 1/D. For intermediate growth durations, a transition between these two mechanisms was observed. For InP and InAs nanowires, the growth mode was varied from axial to radial through the inclusion of Al to form a core shell structure. Al<sub>x</sub>In<sub>1-x</sub>As(P) shells were grown on InAs cores with Al alloy fractions between 0.53 and 0.2. These nanowires were examined by transmission electron microscopy and it was found, for all values of x in InAs-Al<sub>x</sub>In<sub>1-x</sub>P structures, that relaxation had occurred through the introduction of dislocations. For InAs-Al<sub>x</sub>In<sub>1-x</sub>As structures, all values except x=0.2 had relaxed through dislocation formation. A critical thickness model was developed to determine the core-shell coherency limits which confirmed the experimental observation of strain relaxation. The effects of passivation on the electronic transport and the optical properties were examined as a function of structural core-shell passivation and chemical passivation. The mechanisms for the observed improvement in mobility for core-shell versus bare InAs nanowires was due to the reduction in ionized impurity scattering from surface states. Similarly an increase in photoluminescence intensity after ammonium sulfide passivation was explained by the reduction of donor type surface states.</p> / Doctor of Philosophy (PhD)
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Estudo de primeiros princípios de nanofios em arseneto de índio e fosfeto de índio / First principles study of indium arsenide and indium phosphide nanowiresSantos, Cláudia Lange dos 29 July 2011 (has links)
Conselho Nacional de Desenvolvimento Científico e Tecnológico / In this work we used the density functional theory to study InAs and InP nanowires
and InAs/InP nanowire heterostructures. Initially we studied the structural, electronic
and mechanical properties of InAs and InP nanowires as a function of the diameter and the
influence of external mechanical stress on the electronic properties of these systems. Our
results show that all analyzed properties change with increasing quantum confinement.
Further, the application of an external stress along the nanowire axis reveals a direct to
indirect band gap transition for compressive strain in very thin nanowires.
We have also studied the quantum confinement effects on the effective masses of
charge carriers in InAs nanowires grown in different crystallographic directions. We found
the electron and hole effective masses increase with decreasing diameter independently of
the growth direction. However, in the range of the studied diameters, the hole effective
mass is significantly smaller to the corresponding one at the bulk system.
From the study of the stability and electronic properties of the cadmium and zinc
doped InAs nanowires, we show that the Cd impurity prefers to be at the core region,
whereas Zn impurity is found to be equally distributed along the nanowire diameter. The
analysis of the electronic properties of these systems show that these impurities introduce
shallow acceptor levels in the band gap, enabling a p-type behavior of these nanowires.
Finally, we determined (i) the structural, electronic and mechanical properties of
axially and radially modulated InAs/InP nanowire heterostructures for a specific diameter
and (ii) the structural and electronic properties of radial InAs/InP nanowire heterostructures
as a function of the diameter and composition. From (i), our calculations showed
the analyzed properties have an intermediate value between those for the pure InAs and
InP nanowires with similar diameters. In particular, the presence of an InP shell covering
the InAs nanowires enhances the InAs electron mobility, as compared to the uncapped
InAs nanowires. In addition, for the radial heterostructure, the conduction and the valence
band alignments favor a type-I heterojunction, while for the axial heterostructure
a transition from a type-I to a type-II heterojunction could occur at this range of diameters.
From (ii), we observed that for nanowire heterostrutures of similar diameters,
the variation of their structural and electronic properties with the composition possesses
significant deviations from the linear behavior, which are dependent of the nanostructure
diameter. The conduction band offset is approximately zero and the valence band offset
decrease regardless of diameter and composition of the heterostructure. / Neste trabalho realizamos um estudo teórico, baseado na teoria do funcional da
densidade, em nanofios de InAs e InP e em heteroestruturas de nanofios InAs/InP. Inicialmente
estudamos a variação das propriedades estruturais, eletrônicas e mecânicas com
o diâmetro em nanofios de InAs e InP, e as possíveis alterações nas propriedades eletrônicas
destes sistemas sob a influência de uma tensão mecânica externa. Nossos resultados
mostram que todas as propriedades analisadas são alteradas com o aumento do confinamento
quântico. Além disso, a aplicação de uma tensão externa ao longo do eixo de
crescimento dos fios leva a uma transição de gap direto para indireto nos nanofios de
menores diâmetros.
A seguir, avaliamos os efeitos do confinamento quântico na massa efetiva dos portadores
de carga em nanofios de InAs crescidos em diferentes direções cristalográficas.
Encontramos que as massas efetivas dos elétrons e dos buracos aumentam com a redução
do diâmetro, independentemente da direção de crescimento dos nanofios. Contudo, no
intervalo de diâmetro estudado, a massa efetiva dos buracos nos nanofios é significativamente
menor do que a massa efetiva dos buracos no cristal.
Do estudo da estabilidade e das propriedades eletrônicas de nanofios de InAs dopados
substitucionalmente com cádmio e zinco observamos que, independentemente do
diâmetro dessas nanoestruturas, as impurezas de Cd são mais estáveis quando estão no
centro do nanofio, enquanto que as impurezas de Zn se distribuem quase que uniformemente
ao longo do diâmetro do fio. Do ponto de vista eletrônico, observamos que
estas impurezas introduzem níveis aceitadores rasos no gap de energia desses materiais
possibitando um comportamento tipo-p desses nanofios.
Por fim, determinamos: (i) as propriedades estruturais, eletrônicas e mecânicas de
heteroestruturas axiais e radiais de nanofios InAs/InP para um determinado diâmetro; e
(ii) as propriedades estruturais e eletrônicas de heteroestruturas radiais InAs/InP como
uma função do diâmetro e da composição. Em (i), nossos resultados mostram que as propriedades
analisadas possuem valores intermediários entre aqueles dos nanofios de InAs e
InP de mesmo diâmetro. Em particular, observamos que a presença de uma camada de
InP sobre nanofios de InAs aumenta significativamente sua mobilidade eletrônica quando
comparada com a de um nanofio de InAs puro. Além disso, na heteroestrutura radial, o alinhamento
das bandas de condução e das bandas de valência favorece uma heteroestrutura
do tipo I, enquanto que na heteroestrutura axial, uma transição de uma heteroestrutura
do tipo I para uma heteroestrutura do tipo II poderá ocorrer neste intervalo de diâmetros.
Em (ii), para as heteroestruturas com diâmetros similares, observamos que a variação de
suas propriedades estruturais e eletrônicas com a composição possui desvios significativos
do comportamento linear, sendo estes dependentes do diâmetro dessas nanoestruturas.
O descasamento da banda de condução é aproximadamente nulo enquanto que o descasamento
da banda de valência diminui independente do diâmetro e da composição da
heteroestrutura.
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Novel Methods for Controlled Self-Catalyzed Growth of GaAs Nanowires and GaAs/AlxGa1-xAs Axial Nanowire Heterostructures on Si Substrates by Molecular Beam EpitaxyTauchnitz, Tina 12 March 2020 (has links)
GaAs-based nanowires are attractive building blocks for the development of future (opto)electronic devices owing to their excellent intrinsic material properties, such as the direct band gap and high electron mobility. A pre-requisite for the implementation of novel functionalities on a single Si chip is the monolithic integration of the nanowires on the well-established Si complementary-metal-oxide-semiconductor (CMOS) platform with precise control of the nanowire growth process.
The self-catalyzed (Ga-assisted) growth of GaAs nanowires on Si(111) substrates using molecular beam epitaxy has offered the possibility to obtain vertical nanowires with predominant zinc blende structure, while potential contamination by external catalysts like Au is eliminated. Although the growth mechanism is fairly well understood, control of the nucleation stage, the nanowire number density and the crystal structure has been proven rather challenging. Moreover, conventional growth processes are typically performed at relatively high substrate temperatures in the range of 560-630 °C, which limit their application to the industrial Si platform.
This thesis provides two original methods in order to tackle the aforementioned challenges in the conventional growth processes. In the first part of this thesis, a simple surface modification procedure (SMP) for the in situ preparation of native-SiOx/Si(111) substrates has been developed. Using a pre-growth treatment of the substrates with Ga droplets and two annealing cycles, the SMP enables highly synchronized nucleation of all nanowires on their substrate and thus, the growth of exceptionally uniform GaAs nanowire ensembles with sub-Poissonian length distributions. Moreover, the nanowire number density can be tuned within three orders of magnitude and independent of the nanowire dimensions without prior ex situ patterning of the substrate. This work delivers a fundamental understanding of the nucleation kinetics of Ga droplets on native-SiOx and their interaction with SiOx, and confirms theoretical predictions about the so-called nucleation antibunching, the temporal anti-correlation of consecutive nucleation events.
In the second part of this thesis, an alternative method called droplet-confined alternate-pulsed epitaxy (DCAPE) for the self-catalyzed growth of GaAs nanowires and GaAs/AlxGa1-xAs axial nanowire heterostructures has been developed. DCAPE enables nanowire growth at unconventional, low temperatures in the range of 450-550 °C and is compatible with the standard Si-CMOS platform. The novel growth approach allows one to precisely control the crystal structure of the nanowires and, thus, to produce defect-free pure zinc blende GaAs-based nanowires. The strength of DCAPE is further highlighted by the controlled growth of GaAs/AlxGa1-xAs axial quantum well nanowires with abrupt interfaces and tunable thickness and Al-content of the AlxGa1-xAs sections. The GaAs/AlxGa1-xAs axial nanowire heterostructures are interesting for applications as single photon emitters with tunable emission wavelength, when they are overgrown with thick lattice-mismatched InxAl1-xAs layers in a core-shell fashion. All results presented in this thesis contribute to paving the way for a successful monolithic integration of
highly uniform GaAs-based nanowires with controlled number density, dimensions and crystal structure on the mature Si platform. / GaAs-basierte Nanodrähte sind attraktive Bausteine für die Entwicklung von zukünftigen (opto)elektronischen Bauelementen dank ihrer exzellenten intrinsischen Materialeigenschaften wie zum Beispiel die direkte Bandlücke und die hohe Elektronenbeweglichkeit. Eine Voraussetzung für die Realisierung neuer Funktionalitäten auf einem einzelnen Si Chip ist die monolithische Integration
der Nanodrähte auf der etablierten Si-Metall-Oxid-Halbleiter-Plattform (CMOS) mit
präziser Kontrolle des Wachstumsprozesses der Nanodrähte.
Das selbstkatalytische (Ga-unterstützte) Wachstum von GaAs Nanodrähten auf Si(111)-Substrat mittels Molekularstrahlepitaxie bietet die Möglichkeit vertikale Nanodrähte mit vorwiegend Zinkblende-Struktur herzustellen, während die potentielle Verunreinigung der Nanodrähte und des Substrats durch externe Katalysatoren wie Au vermieden wird. Obwohl der Wachstumsmechanismus gut verstanden ist, erweist sich die Kontrolle der Nukleationsphase, Anzahldichte und Kristallstruktur der Nanodrähte als sehr schwierig. Darüber hinaus sind relativ hohe Temperaturen im Bereich von 560-630 °C in konventionellen Wachstumsprozessen notwendig, die deren Anwendung auf der industriellen Si Plattform begrenzen.
Die vorliegende Arbeit liefert zwei originelle Methoden um die bestehenden Herausforderungen in konventionellen Wachstumsprozessen zu bewältigen. Im ersten Teil dieser Arbeit wurde eine einfache Prozedur, bezeichnet als surface modification procedure (SMP), für die in situ Vorbehandlung von nativem-SiOx/Si(111)-Substrat entwickelt. Die Substratvorbehandlung mit Ga-Tröpfchen und zwei Hochtemperaturschritten vor dem Wachstumsprozess ermöglicht eine synchronisierte Nukleation aller Nanodrähte auf ihrem Substrat und folglich das Wachstum von sehr gleichförmigen GaAs Nanodraht-Ensembles mit einer sub-Poisson Verteilung der Nanodrahtlängen. Des Weiteren kann die Anzahldichte der Nanodrähte unabhängig von deren Abmessungen und ohne ex situ Vorstrukturierung des Substrats über drei Größenordnungen eingestellt werden. Diese Arbeit liefert außerdem ein grundlegendes Verständnis zur Nukleationskinetik von Ga-Tröpfchen auf nativem-SiOx und deren Wechselwirkung mit SiOx und bestätigt theoretische Voraussagen zum sogenannten Nukleations-Antibunching, dem Auftreten einer zeitlichen Anti-Korrelation aufeinanderfolgender Nukleationsereignisse.
Im zweiten Teil dieser Arbeit wurde eine alternative Methode, bezeichnet als droplet-confined alternate-pulsed epitaxy (DCAPE), für das selbstkatalytische Wachstum von GaAs Nanodrähten und GaAs/AlxGa1-xAs axialen Nanodraht-Heterostrukturen entwickelt. DCAPE ermöglicht das Nanodrahtwachstum bei unkonventionell geringeren Temperaturen im Bereich von 450-550 °C und ist vollständig kompatibel mit der Standard-Si-CMOS-Plattform. Der neue Wachstumsansatz erlaubt eine präzise Kontrolle der Kristallstruktur der Nanodrähte und folglich das Wachstum
von defektfreien Nanodrähten mit phasenreiner Zinkblende-Struktur. Die Stärke der DCAPE Methode wird des Weiteren durch das kontrollierte Wachstum von GaAs/AlxGa1-xAs axialen Quantentopf-Nanodrähten mit abrupten Grenzflächen und einstellbarer Dicke und Al-Anteil der AlxGa1-xAs-Segmente aufgezeigt. Die GaAs/AlxGa1-xAs axialen Nanodraht-Heterostrukturen sind interessant für den Einsatz als Einzelphotonen-Emitter mit einstellbarer Emissionswellenlänge, wenn diese mit gitterfehlangepassten InxAl1-xAs-Schichten in einer Kern-Hülle-Konfiguration überwachsen werden. Alle Ergebnisse dieser Arbeit tragen dazu bei, den Weg für eine erfolgreiche monolithische Integration von sehr gleichförmigen GaAs-basierten Nanodrähten mit kontrollierbarer Anzahldichte, Abmessungen und Kristallstruktur auf der industriell etablierten Si-Plattform zu ebnen.
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