Bruno Teixeira de
Poças Falcão
Estudos espectroscópicos de propriedades físicas
de sistemas de nanopartículas de silício cristalino
Spectroscopic studies of physical properties of
crystalline silicon nanoparticle systems
2020
Bruno Teixeira de
Poças Falcão
Estudos espectroscópicos de propriedades físicas
de sistemas de nanopartículas de silício cristalino
Spectroscopic studies of physical properties of
crystalline silicon nanoparticle systems
Tese apresentada à Universidade de Aveiro para cumprimento dos requisitos necessários à obtenção do grau de Doutor em Engenharia Física, realizada sob a orientação científica do Doutor Rui Nuno Pereira, Investigador Principal do Departamento de Física da Universidade de Aveiro, e do Professor Doutor Joaquim Pratas Leitão, Professor Auxiliar do Departamento de Física da Universidade de Aveiro.
Para os meus pais, por serem a maior inspiração da minha vida
o júri
presidente Prof. Doutor Aníbal Manuel de Oliveira Duarte
Professor Catedrático, Universidade de Aveiro
vogais Prof. Doutor Mikhail Igorevich Vasilevskiy
Professor Catedrático, Universidade do Minho
Prof.ª Doutora Teresa Maria Fernandes Rodrigues Cabral Monteiro
Professora Associada c/ Agregação, Universidade de Aveiro
Prof. Doutor Hugo Manuel Brito Águas
Professor Associado, Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa
Doutora Katharina Lorenz
Investigadora Auxiliar, Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico
Doutor Rui Nuno Marques Pereira
agradecimentos Esta tese resulta da participação e do envolvimento de várias pessoas e instituições que foram fundamentais para a execução do trabalho aqui
apresentado. Em particular, destaco o papel dos meus orientadores, Doutor Rui N. Pereira e Professor Joaquim P. Leitão, aos quais agradeço e reconheço o desafio lançando há vários anos, o apoio, a paciência, e especialmente a sua amizade ao longo de toda esta aventura.
Ao CENIMAT/I3N (Universidade Nova de Lisboa, Portugal), em particular ao grupo dos Professores Hugo Águas e Rodrigo Martins, pela colaboração que foi
estabelecida para a síntese de nanopartículas de silício a partir de plasmas não térmicos induzidos por radiofrequência. Agradeço especialmente à Lídia Ricardo, pela incansável produção de amostras.
Ao grupo do Doutor Hartmunt Wiggers (Universität Duisburg-Essen, Alemanha), pela produção de todas as amostras estudadas de nanopartículas de silício sintetizadas a partir de plasmas não térmicos induzidos por micro-ondas. À Doutora Rosário Soares, por incontáveis medidas de difração de raios-X nas amostras estudadas e pela valiosa ajuda na análise dos dados experimentais. À Professora Rosário Correia e ao Professor Andrés Cantarero (Universidad de Valencia, Espanha), pelo contributo fundamental no trabalho de espectroscopia de Raman dedicado ao estudo da interação electrão-fonão.
Aos Doutores José Coutinho e Daniel Gouveia, pelo envolvimento no trabalho do modelo de confinamento de fonões, nomeadamente pela discussão científica e suporte na implementação de alguns algoritmos.
Ao Professor Luís Rino e aos meus colegas e amigos, Nuno Santos, Alexandre Botas, Derese Desta, Jennifer Teixeira, Nabiha Ben Sedrine, Marta Sousa e Roberto Costa, por apoiarem a execução de trabalho laboratorial e/ou contribuírem diretamente com medidas experimentais.
Ao Departamento de Física e à Universidade de Aveiro, pela oportunidade de dar aulas ao abrigo do regime de contrapartidas. Estendo o agradecimento a todos os docentes que me acompanharam ao longo do meu percurso académico e cujo papel foi determinante para chegar até aqui.
Ao i3N e à unidade de investigação FSCOSD/i3N, por me terem acolhido e proporcionado os recursos materiais e humanos e o suporte financeiro para a realização do trabalho aqui apresentado.
A todos os meus amigos por me aturarem durante este período, em especial aos meus colegas “da salinha”, e do grupo de espectroscopia.
À Joana, por nunca me largar a mão, pelo carinho, pela paciência e por todo o apoio incondicional, especialmente nos momentos de maior caos e alienação. Obrigado meu amor, ainda agora começamos!
Aos meus pais, a quem tudo devo.
Este trabalho teve o suporte financeiro da Fundação para a Ciência e Tecnologia e do Fundo Europeu de Desenvolvimento Regional através dos projectos
UID/CTM/50025/2019, POCI-01-0145-FEDER-007688, UID/CTM/50025/2013, PTDC/CTM-ENE/2514/2012, RECI/FIS-NAN/0183/2012, FCOMP-01-0124-FEDER-027494 e PTDC/FIS/112885/2009. É igualmente reconhecido o suporte financeiro do Danish Strategic Research Council através do projecto 10-0939691DSF e da Polish National Agency for Academic Exchange através do projecto
palavras-chave Nanocristais e nanopartículas de silício, oxidação superficial, tensão, interação electrão-fonão, confinamento de fonões, recombinação de portadores de carga, propriedades optoelectrónicas, dispersão de Raman, espectroscopia de fotoluminescência.
resumo As nanopartículas de silício (Si-NPs) são um nanomaterial ecológico e
promissor para aplicações (opto)eletrónicas e biomédicas devido às suas propriedades físicas e químicas inigualáveis e ajustáveis através do tamanho. A adaptação de Si-NPs para aplicações específicas e a efetiva materialização de tecnologias baseadas em nanosilício só pode ser alcançada através de uma caracterização profunda e de um entendimento fundamental das suas propriedades, bem como de um conhecimento adequado dos fenómenos que governam estes materiais à nanoescala.
Esta tese é dedicada ao estudo de propriedades físicas de Si-NPs sintetizadas em plasmas não térmicos, com tamanhos na escala nanométrica e diferentes terminações na superfície. Estes materiais são investigados teórica e
experimentalmente, nomeadamente através de estudos espectroscópicos, com vista a compreender as interações electrão-fonão sob iluminação, a correlacionar a evolução da oxidação com o aparecimento e a natureza de tensão, e a resolver várias questões em aberto relacionadas com a
recombinação de portadores de carga e a origem da emissão de luz. Através de experiências de dispersão de Raman em filmes finos contendo Si-NPs com uma superfície oxidada, e, comparativamente, em silício volúmico, verifica-se que o reduzido transporte elétrico e a baixa condutividade térmica nas Si-NPs conduzem a uma população não térmica de fonões óticos em condições de excitação ótica moderada/elevada. A importância da anisotropia e do desdobramento das curvas dos fonões óticos na estimativa do tamanho das Si-NPs, a partir das dependências da posição e largura do espectro de Raman com o tamanho, é demonstrada através do aperfeiçoamento do modelo de confinamento de fonões (PCM) tendo em conta estes efeitos. A evolução da oxidação da superfície de Si-NPs terminadas com H é avaliada através de medidas de dispersão de Raman e espectroscopia de infravermelho em função do tempo de exposição ao ar. As variações da posição e largura de Raman obtidas experimentalmente são comparadas com as previsões teóricas dadas pelo PCM, a partir das quais se conclui que não existe tensão
significativa nas Si-NPs terminadas com H e que o crescimento do óxido à superfície conduz ao aparecimento de stress compressivo (até 1.2 GPa). Com base em vários estudos de fotoluminescência, verifica-se que o crescimento do óxido em Si-NPs com tamanhos de ~ 3 nm leva à alteração da origem da luminescência, deixando de envolver apenas estados relacionados com o núcleo e passando a envolver estados relacionados com o núcleo e com a superfície. Também se mostra que nas Si-NPs oxidadas a importância relativa destes dois canais radiativos depende intimamente do seu tamanho. Para tamanhos superiores a ~ 3.2 nm, a luminescência é dominada por transições com origem no núcleo, enquanto que para tamanhos inferiores a ~ 2.8 nm a luminescência passa a ser rapidamente dominada por transições envolvendo o óxido à medida que o tamanho diminui. Estabelece-se ainda que a
recombinação (não)radiativa de portadores de carga foto-gerados é dominada por mecanismos monomoleculares envolvendo estados induzidos por defeitos quando excitados com potências relativamente baixas, e por mecanismos bimoleculares envolvendo estados dentro do hiato energético sob potências de excitação elevadas. Além disso, verifica-se que as Si-NPs terminadas com H são particularmente sensíveis à foto-excitação, mesmo sob exposição residual a moléculas de oxigénio e água, devido à catalisação da oxidação pela luz, o que deverá ter um impacto significativo em propriedades físicas como a condutividade térmica e elétrica. Este trabalho destaca a importância das técnicas de espectroscopia na investigação de propriedades físicas de Si-NPs.
keywords Silicon nanocrystals and nanoparticles, surface oxidation, strain, electron-phonon interactions, phonon confinement, charge carrier recombination, optoeletronic properties, Raman scattering, photoluminescence spectroscopy.
abstract Crystalline silicon nanoparticles (Si-NPs) are a promising environmentally friendly nanomaterial for (opto)electronic and biomedical applications, owing to their unparalleled and size-tuneable physical and chemical properties. The controlled adaptation of Si-NPs for specific applications and the effective materialization of nanosilicon-based technologies can only be achieved through a deep
characterization and a fundamental understanding of the properties of Si-NPs, including unveiling the phenomena ruling these materials at the nanoscale. This thesis focuses on the study of physical properties of Si-NPs synthesized in nonthermal plasmas, with sizes in the nanometre range and with different forms of surface termination. These materials are investigated theoretically and
experimentally, with emphasis on spectroscopic studies, in view of understanding electron-phonon interactions under illumination, of linking the evolution of the oxidation phenomena with the appearance and the nature of strain, and of solving various open questions related to the recombination of charge carriers and origin of light emission. Through several Raman experiments conducted on surface-oxidized Si-NP thin films and, comparatively, on bulk crystalline silicon (c-Si), it is found that under moderate/high optical excitation the low electrical transport and reduced thermal conductivity of the Si-NPs leads to a nonthermal population of optical phonons. In general, this phenomenon may invalidate a reliable estimation of the temperature of nanomaterials from Raman spectroscopy. The phonon confinement model (PCM) is improved by considering the effects of anisotropy and splitting of the optical phonon dispersion curves, through which it is demonstrated the
importance of these effects in the accurate estimation of the Si-NPs size based on the size-dependent shift and broadening of the Raman spectrum. The evolution of the natural surface oxidation process of H-terminated Si-NPs is monitored by Raman spectroscopy and infrared measurements as a function of air exposure time. The obtained experimental variations of the Raman shift and linewidth are compared with theoretical predictions from the PCM, from which it is concluded that in H-terminated Si-NPs the strain is negligible and that the continuous growth of an oxide shell leads to the appearance of an increasing compressive stress (up to 1.2 GPa). Based on several photoluminescence studies, it is found that, for Si-NPs with average size of ~ 3 nm, the origin of the luminescence changes from involving solely core-related states to core- plus surface-related states upon development of a surface oxide. In addition, it is shown that, for surface-oxidized Si-NPs, the relative importance of these two radiative recombination channels has a clear dependence on the nanoparticle size. For Si-NPs with sizes larger than ~ 3.2 nm, the luminescence is dominated by optical transitions originating within the
nanoparticle core, whereas for sizes smaller than ~ 2.8 nm the luminescence becomes quickly dominated by transitions involving the oxide shell as the size decreases. Moreover, it is established that, independently of the surface
passivation, the (non)radiative recombination of photo-generated charge-carriers is dominated by monomolecular processes involving defect-induced states at
relatively low excitation powers and by bimolecular processes involving sub-band gap states at high excitation powers. Further, it is found that H-terminated Si-NPs are quite sensitive to photo-excitation even under residual oxygen and water molecule exposure due to surface oxidation catalysed by illumination, which should have a significant impact on inter-NP connectivity and in physical properties like thermal and electrical conductivities. This work highlights the importance of spectroscopy techniques in the investigation of the physical properties of Si-NPs.
Ar Argon (element)
a-Si Amorphous silicon
a-Si:H Hydrogenated amorphous silicon
BET Brunauer–Emmett–Teller
BSE Backscattered electron(s)
c-Si Bulk crystalline silicon
BX Bound exciton
BZ Brillouin zone
CB Conduction band
CCD Charge-coupled device
CVD Chemical vapour deposition
cw Continuous wave
DAP Donor-acceptor pair
DOS Density of states
DPSS Diode-pumped solid-state (laser)
EDS Energy dispersive spectroscopy
EPR Electron paramagnetic resonance
ESR Electron spin resonance
FFT Fast Fourier transform (algorithm)
FTIR Fourier transform infrared (spectroscopy)
FWHM Full width at half maximum
HRTEM High resolution transmission electron microscopy/microscope ICDD International centre for diffraction data
KVFF Keating valence force field (model)
LA Longitudinal acoustic (phonon mode)
LHe Liquid helium
LN Liquid nitrogen
LO Longitudinal optical (phonon mode)
MCT Mercury cadmium telluride (detector)
NA Numerical aperture
NC(s) Nanocrystal(s)
NMR Nuclear magnetic resonance
NP(s) Nanoparticle(s)
NTP Nonthermal plasma
OML Orbital motion limited (theory)
PCM Phonon confinement model
PECVD Plasma enhanced chemical vapour deposition
PL Photoluminescence
PMS Particle mass spectrometer
PMT Photomultiplier tube
p-Si Porous silicon
poly-Si Polycrystalline silicon
QCM Quantum confinement model
QD(s) Quantum dot(s)
RCF Richter, Campbell and Fauchet (phonon confinement model)
RF Radio frequency
RPM Rotation per minute
RS Raman spectroscopy
RWL Richter, Wang and Lei (phonon confinement model)
SAD Selected-area diffraction (pattern)
SE Secondary electron(s)
SEM Scanning electron microscopy/microscope
Si-NC(s) Silicon nanocrystal(s) Si-NP(s) Silicon nanoparticle(s)
TA Transverse acoustic (phonon mode)
TEM Transmission electron microscopy/microscope
TO Transverse optical (phonon mode)
ULF Ultra low frequency (filter)
UV Ultraviolet
VB Valence band
VIS Visible
a Lattice parameter
A Anharmonic constant; Gaussian component (Raman and PL spectra)
A− Ionized acceptor
A0 Neutral acceptor
An Transition probability of the monomolecular recombination At Transition probability of the bimolecular recombination
B Anharmonic constant; Gaussian component (Raman and PL spectra)
C Fourier coefficient; anharmonic constant
dox Oxide shell thickness
D Diffusion coefficient; anharmonic constant
D− Ionized donor
D0 Neutral donor
e Electron charge
E Energy
~
E Electric field vector
EA Acceptor binding energy
Ebind Exciton binding energy
ED Donor binding energy
Eg Band gap energy
EPL PL photon energy
f Ratio between populations of vibrational states
F Coefficient
G Gaussian component (Raman spectrum); carrier generation rate
h Planck constant
I Intensity
IAS anti-Stokes intensity IS Stokes intensity
kB Boltzmann constant
K Constant; high symmetry point (Brillouin zone); shape factor for spherical particles (Scherrer equation)
Kα X-ray emission line
L Particle diameter/size; atomic chain length; diffusion length; high symmetry point (Brillouin zone); Lorentzian component (Raman spectrum)
n Refraction index; energy level (number); electron density
nm Reaction rate (Elovich equation)
Nr Density of recombination centres
p Hole density
pn Population of the vibrational state n P / Pexc Excitation power
~
P Polarization vector
~
q Phonon wavevector
~
Q Plane wave displacement vector
R Ratio (intensity)
R Raman tensor
tm Characteristic time (Elovich equation) tox / tair Oxidation/air exposure time
T Temperature
W Weighting function
X High symmetry point (Brillouin zone)
X2 Goodness of fit (figure of merit in the Williamson-Hall profile analysis)
α Absorption coefficient; thermal expansion coefficient; rate; constant
β Line broadening (XRD); fitting coefficient (energy shift)
γ Grüneisen parameter (isothermal)
Γ Raman peak linewidth; high symmetry point (centre of Brillouin zone)
η Intensity ratio of second- to first-order Raman scattering
θ Bragg angle
λ Wavelength
µ Charge transport mobility
ρ Depolarization ratio; Raman scattering efficiency
σ Raman cross section; standard deviation
τ Thermalization time
τr Radiative (bimolecular) recombination rate
Φ Bloch wave function; size distribution
χ Electric susceptibility
Ψ Phonon wave function
ω Frequency
ω(q) Phonon dispersion curve
1.1 Number of articles published since 1995 containing the keys “silicon
nanoparti-cles”, “silicon quantum dots”, or “silicon nanocrystals” . . . 3
1.2 Photoluminescence of Si-NPs spanning from the UV to the NIR . . . 4
1.3 In vivo fluorescence imaging of tumours in mice using biodegradable lumines-cence porous silicon nanoparticles . . . 6
1.4 Si-NPs based field effect transistor . . . 7
2.1 Schematic illustration of a (100) Si wafer. Theoretical and experimental Raman intensity from (100) backscattering . . . 20
2.2 Temperature dependence of the frequency shift of the optical phonon mode in bulk c-Si . . . . 24
2.3 Diagrams representing three- and four-phonon anharmonic decay processes . 25 2.4 Phonon weighting functions . . . 27
2.5 Comparison of optical phonon dispersion functions in Si along [100] . . . 29
2.6 Simulated phonon dispersion curves of Si . . . 31
2.7 Schematic diagram illustrating radiative and nonradiative transitions . . . 33
2.8 Charge carrier density as function of generation rate and the corresponding transition between monomolecular and bimolecular recombination processes . 38 2.9 Diagram of the oxidation steps of Si in air at room temperature . . . 40
2.10 Collisionality regimes considered in the particle charging model . . . 43
2.11 Energy flux between a particle and the surrounding plasma environment . . . 44
3.1 Schematic representation of a MW flow-through nonthermal plasma reactor . 62 3.2 XRD patterns from the Si-NPs synthesized by microwave nonthermal plasmas 63 3.3 HRTEM characterization of a sample containing Si-NPs with diameter of 34 nm 63 3.4 Schematic representation of a RF flow-through nonthermal plasma reactor . . 64
3.5 Si-NP size distributions estimated from TEM data for samples NP37 and NP49, and TEM image from sample NP49 . . . 66
3.6 Image of the MBraun glovebox . . . 67
3.7 SEM images of Si-NP thin-films . . . 68
4.1 Raman spectra of Si-NPs with various crystallite sizes, bulk c-Si and a-Si . . 76
4.2 Temperature dependence of the frequency of the optical phonon mode observed in thermal excitation experiments of a Si-NPs (34 nm) film and of bulk c-Si . 78
4.3 Anti-Stokes and Stokes Raman spectra measured at different excitation power densities . . . 80
4.4 Temperature dependence of the Raman peak position obtained in optical
excitation experiments for different Si-NP samples . . . . 81
4.5 Raman peak position Ω versus intensity ratio R . . . . 83
4.6 Schemes representing the allowed and hindered diffusion of photoexcited elec-trons away from the Raman measurement (excitation) spot in a bulk material and in a nanoparticulate system . . . 85
4.7 Estimated values of the coefficient F . . . . 86
5.1 Phonon dispersion curves of Si simulated from “our model” . . . 94
5.2 Raman spectra of a 3 nm Si-NC resulting from “our model” along the high symmetry directions . . . 96
5.3 Size distribution and Raman spectrum of Si-NCs synthesized at 3 Torr . . . . 98
5.4 Comparison between theoretical and experimental Raman spectrum . . . 99
5.5 Raman shift and linewidth as a function of NC diameter . . . 100
6.1 Size distribution, XRD pattern, and Raman spectrum from as-grown Si-NPs 109
6.2 EPR spectrum from as-grown Si-NPs . . . 112
6.3 FTIR spectra in the range of stretching modes of Si–O–Si and Si–H bonds recorded for as-grown Si-NPs and after long-term exposure to air . . . 113
6.4 Dependence of the intensity of FTIR bands from the surface species Si–O–Si and Si4−x–Si–Hx as a function of the oxidation time . . . 114
6.5 Raman intensity at 1100 cm−1 as a function of oxidation time . . . 115
6.6 Relation between the Raman peak position and linewidth as a function of oxidation time . . . 116
6.7 Evolution of size distribution parameters as a function of the oxide shell thickness117
6.8 Schematic illustration of the evolution of oxidation in a small Si-NP . . . 120
7.1 Room temperature PL spectra of H-terminated Si-NPs . . . 130
7.2 Dependence of the peak energy and of the integrated PL intensity on the normalized excitation power of H-terminated Si-NPs . . . 131
7.3 Dependence of the PL intensity ratio on the normalized excitation power of the H-terminated Si-NPs . . . 132
7.4 Evolution of room temperature PL spectra of Si-NPs during oxidation under ambient conditions . . . 135
7.5 Peak energy and PL intensity ratio dependencies on the time of exposure to air of Si-NPs . . . 136
7.6 Dependence of the peak energy and of the integrated PL intensity on the normalized excitation power of long-term surface-oxidized Si-NPs . . . 137
7.7 Diagram of the oxidation steps of a Si-NP under ambient (dark) conditions and illumination . . . 139
8.1 Structural and morphological properties of Si-NPs synthesized at 12 Torr . . 151
8.2 Dependence of the NP size of as-grown Si-NPs on the discharge pressure . . . 152
8.3 Raman spectra of as-grown Si-NPs synthesized at 12 and 2 Torr . . . 153
8.4 FTIR spectra and integrated intensity ratio between the siloxanes and surface hydrides bands as a function of NP size in surface-oxidized Si-NPs . . . 155
8.5 PL spectra as a function of discharge pressure (NP size) of surface-oxidized Si-NPs . . . 156
8.6 Fitting model used in the deconvolution of PL spectra . . . 157
8.7 Dependence of the PL integrated intensity ratio on the NP size . . . 158
8.8 Dependence of the PL peak energy on the NP size . . . 160
8.9 Dependence of the Gaussian curves parameters on the NP size . . . 163
2.1 Raman selection rules of the optical phonon modes in Si for backscattering from a (001) surface . . . 20
3.1 List of Si-NP batches produced by MW plasma . . . 62
3.2 List of Si-NP batches produced by RF plasma . . . 65
4.1 Parameters obtained in the fitting of Equation (4.1) to the data of the tem-perature dependence of Ω of the optical phonon mode for the Si-NPs and bulk
c-Si, obtained in the thermal excitation experiments . . . . 79
5.1 Fitting coefficients Fn (cm−1) describing the TO and LO dispersion curves
along the high-symmetry directions . . . 95
6.1 Raman peaks observed in the spectrum of the as-grown Si-NPs . . . 110
7.1 Values of β estimated from fittings to the experimental data of H-terminated and surface-oxidized Si-NPs . . . 136
List of Abbreviations i
List of Symbols iii
List of Figures v
List of Tables xi
Contents xiii
1 Introduction 1
1.1 Silicon: (nano) science and technology . . . 1
1.2 Thesis organization, main topics, and other remarks . . . 7
1.3 References . . . 8
2 Theoretical background 17
2.1 Raman scattering . . . 17
2.1.1 Fundamentals of Raman scattering . . . 17
2.1.2 Selection rules . . . 19
2.1.3 Stokes and anti-Stokes intensity ratio . . . 21
2.1.4 Anharmonic effects . . . 22
2.1.5 Phonon confinement (model) . . . 25
2.2 Radiative and nonradiative transitions in photoluminescence . . . 32
2.2.1 Interband, intraband, and excitonic recombination . . . 32
2.2.2 Band tails and dangling bond defects . . . 35
2.2.3 Kinetic description of luminescence processes . . . 36
2.3 Natural oxidation of silicon . . . 38
2.3.1 The Cabrera-Mott model . . . 39
2.4 Synthesis of silicon nanoparticles in nonthermal plasmas . . . 40
2.4.1 Nanoparticle nucleation . . . 41
2.4.2 Cluster agglomeration (nanoparticle charging) . . . 42
2.4.3 Nanoparticle surface growth (nanoparticle heating) . . . 43
3 Experimental techniques and samples 53
3.1 Experimental techniques . . . 53
3.1.1 Raman spectroscopy . . . 53
3.1.2 Photoluminescence spectroscopy . . . 55
3.1.3 Infrared (absorption) spectroscopy . . . 56
3.1.4 Electron paramagnetic resonance . . . 56
3.1.5 X-ray diffraction . . . 57
3.1.6 Electron microscopy . . . 59
3.1.7 Profilometry . . . 60
3.2 Samples . . . 61
3.2.1 Silicon nanoparticles synthesized with microwave plasmas . . . 61
3.2.2 Silicon nanoparticles synthesized with radiofrequency plasmas . . . 64
3.2.3 Samples preparation . . . 66
3.3 References . . . 69
4 Light-induced nonthermal population of optical phonons 73
4.1 Introduction . . . 73
4.2 Experimental details . . . 75
4.3 Room temperature Raman spectra . . . 75
4.4 Thermal excitation experiments . . . 77
4.5 Optical excitation experiments . . . 79
4.6 Nonthermal population of optical phonons . . . 82
4.7 Conclusions . . . 85
4.8 Supplemental material: experimental determination of the coefficient F . . . 86
4.9 References . . . 87
5 Raman spectrum of nanocrystals: phonon dispersion splitting and anisotropy 91
5.1 Introduction . . . 91
5.2 Improved phonon confinement model . . . 93
5.3 Experimental details . . . 97
5.4 Theory versus experiment . . . 97
5.5 Conclusions . . . 101
5.6 References . . . 101
6 Oxidation and strain in free-standing silicon nanoparticles 105
6.1 Introduction . . . 105
6.2 Experimental details . . . 107
6.3 Characterization of as-grown Si-NPs . . . 108
6.4 Ambient-oxidation of Si-NPs . . . 112
6.5 Strain in free-standing Si-NPs . . . 117
6.6 Conclusions . . . 120
7 Photoluminescence and recombination mechanisms in H-terminated and
(photo-)oxidized silicon nanoparticles 127
7.1 Introduction . . . 128
7.2 Experimental details . . . 129
7.3 Photoluminescence of H-terminated Si-NPs . . . 129
7.4 Photoluminescence of surface-oxidized Si-NPs . . . 134
7.5 Oxidation kinetics under illumination and ambient (dark) conditions . . . 138
7.6 Nature of optoelectronic properties and recombination channels . . . 140
7.7 Conclusions . . . 143
7.8 References . . . 143
8 Size-dependent critical transition in the origin of the photoluminescence of
surface-oxidized silicon nano-particles 147
8.1 Introduction . . . 148
8.2 Experimental details . . . 149
8.3 Dependence of nanocrystal size on discharge pressure . . . 151
8.4 Size-dependent photoluminescence of surface-oxidized Si-NPs . . . 155
8.5 Conclusions . . . 162
8.6 Supplemental material: maximum of PL as a function of nanoparticle size . . 163
8.7 References . . . 165
9 Summary and outlook 171
1
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h
a
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Introduction
1.1
Silicon: (nano) science and technology
At the heart of virtually every electronic and optoelectronic technology is silicon (Si), an indirect band gap∗ semiconductor material belonging to group IV of the periodic table [1,2]. There are numerous arguments behind the unparalleled significance of Si in technology, among which, but not exclusively, the fact that it is the second most abundant element on Earth’s crust (making it cheap), it can be processed/grown with very low impurity concentration as a single crystal and practically defect-free, its device integration is easily scalable, exhibits good electrical characteristics, and it has the ability to form a stable oxide (SiO2) with desirable dieletric, mechanical, and protective properties [3]. In the past 60 years, Si has been the dominant material in the (micro)electronics industry, especially in transistors, mostly due to the profound technological know-how and the huge investments done in the manufacturing and infrastructure to scale down billions of components into a single chip [4]. Despite the expected change in the upcoming years to a promising more-than-Moore technology based on sequential monolithic integration of high capacity memory into 3D nanosystems, with new oxide films and heterostructures, Si should continue being a basilar material [4–6]. In the field of terrestrial photovoltaics, crystalline Si (c-Si) technologies (“first-generation”) dominate the global market with a share of about 95%, whereas all thin-film technologies (“second-” and “third-generation”) account for the remaining 5% of the total annual production (as of 2017) [7]. This is largely due to the high efficiency output (almost 27%) of Si wafer-based solar cells [8, 9], their stablitity, and because they rely on established process technologies with an enormous background, as in the semiconductor industry in general. Si-based materials,
∗
Amongst many other properties, this implies that the eletronic transitions usually involve the absorption or emission of phonons to satisfy the conservation of energy and crystal momentum, which, from the technological point of view, may be rather undesirable.
namely c-Si, amorphous Si (a-Si), and polycrystalline Si (poly-Si), are also the most common materials currently used in the commercial production of micro/nano-electro-mechanical systems (MEMS/NEMS), e.g., sensors, actuators, and passive structures, which often exploit mechanical properties rather than electrical properties [10, 11]. The overwhelming dominance of Si in many technologies is undeniable and is only natural to envision its usage in the field of optoelectronics [12]. However, despite the demonstration of some photonic applications [13–16] and efficient avalanche photodetectors [17], the generalized optical exploitation of Si-based optoelectronic devices is one of the most challenging problems of materials research due to the indirect energy band gap of Si. Unsurprisingly, the majority of optoelectronic devices, such as light-emitting diodes (LEDs), laser diodes, photodetectors, optical amplifiers, and optical modulators, are made from direct band gap III-V compound semiconductors such as GaAs, InP, InN, AlN, GaN, and GaSb, and their alloys [17]. In this regard, reducing the size of the Si crystal to the nanoscale, where confinement and surface effects play crucial roles [18,19], and the advent of nanotechnology may provide ingenious solutions to overcome the intrinsic limitations of Si.
Perhaps the most notable moment that glimpsed the materialization of all-Si based opto-electronic devices occurred in the early 1990s when Leigh Canham observed photoluminescence (PL) in the visible range from porous Si (p-Si) at room-temperature [20,21]. The behaviour was readily attributed to spacial confinement of charge carriers resulting from the decrease of the crystal size down to the nanoscale and the consequent overlapping of the electron and hole wave functions that allowed quasi-direct radiative transitions to occur in such indirect band gap semiconductor. In the case of Si, the electronic quantum confinement effect occurs when at least one spatial dimension is smaller than the Bohr’s radius of the exciton (an electrostatically bounded electron-hole pair), which is ∼ 4.9 nm [22]. Importantly, these findings made clear that the indirect band gap of bulk c-Si and the poor optical properties responsible for Si being particularly unsuited for optoelectronic applications, such as the low absorption coefficient in the visible spectral region, the low radiative recombination process rates (about 102 s−1), and the long relative radiative lifetimes (in the milisecond range) [19], could be surpassed by “defeating the tyranny” of the energy and momentum conservation laws. In spite of the revolutionary breakthrough, drawbacks like the physical stability, reproducibility, and the inefficiency of the electrochemical etching technique to produce p-Si quickly paved the way for the investigation of other morphologies, namely of more isolated and spherically-shaped Si nanostructures [23,24]. Ever since, nanostructured Si in the form of nanocrystals (NCs), nanoparticles (NPs), and quantum dots (QDs),† has received extensive interest given their potential application not only for optoelectronics but also for electronic, biomedical, and sensing applications [25], as witnessed by the increasing number of publications in the last
†
The terms related to nanostructured Si can be distinguished as follows: (i) Si-NCs - mono- or poly-crystals of Si with a (near-)spherical shape; (ii) Si-NPs - amorphous or crystalline Si-NPs that may present a coreshelllike structure as, for instance, when SiNCs have a capping layer such as a surface oxide shell; (iii) SiQDs -nanostructures of Si within the quantum size regime where a discretization of the energy levels occurs inside the material, as in sufficiently small Si-NCs and NPs, islands, and other low-dimensional structures. Without any loss of generality, throughout this thesis the term Si-NPs is preferentially adopted to refer to nanoparticles with a crystalline Si core.
1 9 9 5 2 0 0 0 2 0 0 5 2 0 1 0 2 0 1 5 2 0 2 0 0 2 4 6 8 1 0 1 2 1 4 N u m b er o f ar ti cl es ( × 1 0 3 ) Y e a r s i l i c o n n a n o p a r t i c l e s s i l i c o n q u a n t u m d o t s s i l i c o n n a n o c r y s t a l s
Figure 1.1: Number of articles published since 1995 containing the keys “silicon nanoparticles”, “silicon quantum dots”, and “silicon nanocrystals”, as obtained in ScienceDirect in a search performed
on October 30, 2019.
decades (Figure1.1).
The numerous technological prospects have driven research towards the development of several deposition and synthesis methods specially capable of controllably produce high quality Si-NPs with pronounced confinement effects and tunable physical properties. In general, these techniques can be divided in two main categories according to their outputs, the ones that lead to the direct nucleation of Si-NCs inside dielectric thin films and the ones that result in the nucleation and growth of free-standing Si-NPs as solid powders or dispersed in solutions [24]. The controlled synthesis of Si-NCs embedded within dielectric matrices is additive in nature and can be achieved through several deposition techniques usually found in the semiconductor industry (e.g., chemical vapour deposition routes, thermal evaporation, or sputtering) [18,24,26–43]. Moreover, this approach enables the dispersion of the Si-NCs in wide band gap dielectric matrices, such as Si dioxide (SiO2), Si nitride (Si3N4), or Si carbide (SiC), which not only ensures the electronic confinement but also provides an additional degree of freedom for band gap engineering in devices based on Si-NPs [44–47]. For instance, the idea of organizing QDs into a 3D periodic superlattice to form minibands [48,49], taking advantage of the QD size, interdot distance, and the composition of the dielectric layer, may enable the fabrication of highly efficient all-Si “third-generation” tandem photovoltaic cells, with increased absorption throughout the whole visible spectrum and enhanced charge carrier mobilities due to minibands [50–53]. Successful demonstrations of luminescent devices (e.g., lasers and LEDs) exploiting quantum confinement effects and the nature of the surrounding ambient have also been accomplished with Si-NCs embedded within dielectric environments [54–56]. In spite of some advances in the synthesis of matrix-embedded Si-NCs [57,58], the current techniques usually lack a complete control over the size and density of the NCs. Also, the surface passivation, funcionalization, and intentional doping have proven to be physically
UV/blue Visible Red/NIR
(a) (b) (c) (d)
Figure 1.2: Photoluminescence of Si-NPs spanning from the UV to the NIR. (a) As-grown ligand-terminated (surface-covered with hydrophilic amino groups) Si-NPs synthesized through a wet chemistry route [100]. (b) Solution-processed Si-NPs capped with 1-heptene [101]. (c) Surface oxidized and hydrogen-terminated (right, red-emitting) Si-NPs produced by an electrochemical method [88]. (d) Si-NPs dispersed in methanol and presumably surface oxidized, synthesized in a low-pressure nonthermal plasma [102].
challenging [59, 60]. Moreover, the high temperatures required in the post-deposition thermal annealing (∼ 1 000 − 1 200 ◦C), to enable the phase separation between Si and the dieletric matrix and the subsequent NC formation, may introduce compatibility problems in terms of technological integration [1, 24, 25, 41–43]. Alternatively, free-standing Si-NPs often constitute a better platform for the development of applications based on nanosilicon and are also more suitable for the investigation of fundamental properties. Furthermore, there are several gas-, liquid-, and solid-phase synthesis routes [24, 27, 61–74] that allow, in general, better size and surface engineering of the nanosilicon material. However, liquid-based chemical synthesis often have poor size control, are time consuming, have small yields, and may suffer from contaminations that can degrade the optoelectronic properties of the Si-NPs [75,76]. Conversely, the gas-phase methods based on silane (SiH4) decomposition in nonthermal plasmas [72–74] are particularly appealing due to their cost effectiveness, scalability, high production rates, and excellent precursor utilization, and because they provide high quality Si-NPs in the sub-10 nm size range, with significantly narrow size distributions and desirable optoelectronic properties [77]. Moreover, gas-phase plasma approaches have been particularly successful in the intentional n- and p-type electronic doping of Si-NPs [73,78–83].
The extraordinary ability to tune efficient luminescence over a broad spectral range (see Figure 1.2) makes Si-NPs highly attractive for light emission applications. The most common strategies to tailor the light emission of Si-NPs consist of reducing their size to shift the emission wavelength from the infrared towards the visible region due to the increasing role of quantum confinement [21,84–86], exploring chemical routes via a wide variety of capping species to passivate, protect, and functionalize their surface [87–92], and by intentionally (co-)doping the NCs core or its surface [81,82, 93–96]. These strategies can either be used independently or in synergistic combination because there is an intimate interplay of the influence of each strategy on the physical properties of the Si-NPs [76,97,98]. For instance, theoretical studies indicate that due to their large surface-to-volume ratio, the electronic band structure of small Si-NPs is extremely sensitive to the surface reconstruction and surrounding environment and can be strongly affected by the atoms located at or in close proximity to the surface [99].
superficial Si atoms, and is the most common form of surface termination in Si-NPs synthesized in nonthermal plasmas [24, 74, 76, 103]. Unfortunately, Si-NPs with a surface passivated with hydrogen are (photo)chemically unstable and thus prone to oxidation when exposed to air [93, 104–106]. In fact, the mere storage of H-terminated Si-NPs under ambient conditions leads to the formation of a native oxide shell with a self-limited thickness of the order of 10% of the NC diameter prior to oxidation [105,107–110]. Surface oxidation is expected to have a profound and complex influence on the optical and electrical properties of Si-NPs due to the variety of types of bonds that oxygen may form on the NP surface, which should induce numerous interface- and oxide-related localized defect states [87, 92, 93, 97, 111–
119]. Effects such as a reduction of the spectral tunability of the light emission and a decrease of the optical band gap [111, 120–125], due to the formation of oxygen-related states within the band gap [111, 114–118, 121, 126], and the observation of self-trapped excitons [127] have all been attributed to the presence of a superficial oxide layer on Si-NPs. Nevertheless, the complexity of O-termination has led to intense debates concerning the light emission properties of surface-oxidized Si-NPs. For instance, some studies reveal that the light emission red-shifts upon surface oxidation [117, 126] while other reports mention an emission blue-shift [71, 85, 93, 112, 116, 117, 119, 128–132]. It has also been argued whether the recombination mechanisms involve just quantum-confined states within the core of the NPs [133], solely oxide-related (highly localized) defect states [134–137], or an interplay of both [119, 132, 133, 138, 139]. Furthermore, the presence of a surface oxide has also been pointed out to promote strain within the NPs core [107, 109, 140] and that the light emission of Si-NPs can be strain-engineered through the modification of the electronic band structure [141–145]. Although most of these works consider that strain increases with the thickening of the oxide shell, they report seemingly contradicting results about strain being strictly compressive, tensile, or a mixture of both depending on the particle size and on the extent of the surface oxidation. The surface capping with organic ligands yields a more (photo-)chemically stable bond with Si atoms, provides good protection against surface oxidation, and prevents aggregation of the Si-NPs in solution [146–148]. From the light emission point of view, organic capping of Si-NPs has a weak impact on the optical band gap when compared with oxide-terminated Si-NPs [149,150], and offers a very broad spectral tunability [89,91] spanning from the from UV/blue [100,101, 151,152], through blue/green [98,153,154] and yellow [155] to red/NIR [156]. Perhaps the most interesting features of Si-NPs capped with organics are the tremendous enhancement of the PL quantum yield [98,155–159], attributed in some works to the elimination and/or passivation of nonradiative surface defects [156–158,160], and the observation of very fast radiative recombination rates, several orders of magnitude higher than those observed for oxide-terminated and even for H-terminated Si-NPs [98]. On the other hand, organic ligands can also be interesting for the bio-functionalization of surface-oxidized Si-NPs [161,162]. Other exotic terminations with the potential to compete with commercial dyes and typical QDs have been reported, as is the case of nitrogen-passivated Si-NPs with PL quantum yield up to 90% [163].
It is not surprising that the exciting properties of free-standing Si-NPs and the prospects envisioned through size engineering and surface chemistry have led to the development of
× 10 6 (p s ¬1 cm ¬2 sr ¬1 ) D-LPSiN P LP SiN P Pre 1 h 3 h 8 h 24 h 6 .0 5 .5 5 .0 D-LPSiN P LPSiN P 8 h8 h 8.6 8.2 7.8 7.4 × 10 6 (p s ¬1 cm ¬2 sr ¬1 ) (a) (b)
Figure 1.3: In vivo fluorescence imaging of tumours in mice using biodegradable luminescence p-Si NPs (LPSiNPs) and LPSiNPS coated with the biopolymer dextran (D-LPSiNPs) to enhance the size and zeta potential of the NPs. (a) Imaging of LPSiNPs and D-LPSiNPs at multiple times. Arrowheads and arrows with solid lines indicate liver and bladder, respectively. (b) Lateral image of the same mice shown in (a), 8 h after LPSiNP or D-LPSiNP injection. Adapted from [177].
numerous technological applications. For light emitting applications, several groups have demonstrated LEDs based on quantum-confined Si-NPs operating in the visible and NIR spectral regions, either fabricated from surface-oxidized Si-NPs [164] or from organic-capped Si-NPs [165–171]. In the latter case, external quantum efficiencies up to 8.6% have been reached in organic/inorganic (hybrid) LEDs emitting at 850 nm based on 5 nm Si-NPs [166]. Luminescent Si-NPs have also been proposed as excellent alternatives to the traditional II–VI semiconductor QDs and fluorescent organic dyes for bio-applications owing to their low cytotoxicity, good photostability, and water solubility [172–174]. Besides, Si has the clear advantage that it can be readily degraded to silicic acid and excreted in the urine [175]. To date, several reports have demonstrated the huge benefits of Si-NPs for bio-applications, namely for bio-targeting and drug delivery [176,177] (see Figure1.3), bio-sensing [178–181], bio-imaging [152,174,182], and magnetic resonance imaging [183–186]. In the field of photovoltaics, in addition to the strategies mentioned above for the development of “third-generation” photovoltaic devices comprising Si-NCs embedded in dieletric matrices, the use of Si-NPs with tunable surface terminations and optical band gaps have shown to improve the power conversion efficiency in hybrid solar cells by assisting in light absorption and by increasing the interfacial area for exciton dissociation [187–189]. Other interesting examples of the beneficial integration of Si-NPs in photovoltaics are highly efficient luminescent solar concentrators based on colloidal Si-NPs embedded in polymers [190], back-reflector architectures with buried light-scattering microstructures made of Si-NPs [191], surface coatings for poly-Si solar cells based on Si-NPs luminescent inks [75,192], amongst many others [193]. Si-NPs are also promising candidates to the development of transistors (see Figure 1.4) for (nano)electronics devices [81,194–199], in particular for memory applications, given their potential, e.g., to overcome the limitations of current photolithographic techniques to accomplish very small structures [24]. Indeed, efficient charge storage using Si-NPs has been successfully demonstrated with several metal-oxide-semicondutor architectures [200–204], and have also been integrated into functional flash memory devices [205,206]. Many other applications have been envisioned for Si-NPs over the years, including chemical [178] and physical [207] sensors, batteries [208], and
(a)
(b)
(c)
Figure 1.4: Si-NPs based field effect transistor. (a) Scheme and (b) cross-sectional SEM image of the device structure. (c) HR-TEM image of the Si-NPs with diameters in the range 8 − 15 nm. Adapted from [81].
thermoelectrics [209,210], and is only natural to expect that Si-NPs will continue fostering interest by the scientific community.
On the verge of the thirty anniversary of the first observation of light emission from nanostructured Si, the development of nanotechnologies based on Si-NPs has seen numerous advances and successful demonstrations of a broad spectrum of applications, owing in great extent to tremendous improvements in the synthesis, processing, and in our understanding of the fundamental properties of Si at the nanoscale. Despite the progress in many of these fields, a clear understanding of several aspects is still lacking and further efforts will be required to improve device functionality. In this regard, spectroscopic techniques such as Raman scattering and photoluminescence can be valuable tools to address paramount questions in Si-NPs. In this thesis, we use these techniques to investigate size-dependent confinement effects, surface oxidation, strain, electron-phonon interactions, charge carrier transport, or recombination mechanisms, and their intimate role on the structural, optical, and electrical properties of Si-NPs.
1.2
Thesis organization, main topics, and other remarks
This thesis comprises nine chapters through which the motivation (Chapter 1), fundamen-tals (Chapter2), experimental details (Chapter 3), results (Chapters 4to 8), and conclusions (Chapter 9) are presented. The main experimental results are presented chronologically,
and follow the form of the scientific articles in which they have been or will be published (Chapters4-8), or being prepared for submission (Chapter7). Although, some changes with respect to the corresponding articles have been made to enhance the reader’s experience, such as cross-referencing and inclusion of supplemental materials. As a result of such organization, there can be repetitions of some of the contents presented in the different chapters, namely of the experimental details.
In present chapter (Chapter 1), an introduction is given to contextualize nowadays technology based on Si and the current trends and future perspectives of nanoscience and nanotechnology of Si-NPs, in particular for the development of optoelectronic applications. Chapter2is dedicated to the theoretical background required for the discussion of the results, focusing on the fundamentals of Raman scattering, photoluminescence spectroscopy and
the kinetic description of electron-hole recombination mechanisms, oxidation of Si under ambient conditions, and ends reviewing the basic theory behind the formation of Si-NPs in nonthermal plasmas. Chapter 3 provides the experimental details and a short explanation concerning the basic theoretical concepts of complementary experimental techniques through which the Si-NP samples were further investigated. A detailed description of the investigated Si-NP samples and of their preparation is also given in this chapter. Chapter 4 is the first chapter reporting main results obtained in this thesis. Here, a thorough Raman scattering study on surface-oxidized Si-NPs is presented through which we investigate the electron-phonon interactions under high optical excitation conditions and evaluate the limits of Raman thermometry in these low-dimensional nanoparticulate systems. Chapter 5 describes the theoretical framework of the phonon confinement model (PCM) and how it can be improved by considering the effects of anisotropy and splitting of the optical phonon dispersions to retrieve more accurate information about relevant physical properties of Si-NPs from Raman spectroscopy. Based on these findings, Chapter 6discusses the surface-induced lattice-strain phenomenon occurring during the formation of a native oxide shell when H-terminated Si-NPs are exposed to ambient conditions. The obtained results not only enable the characterization of the nature of strain and of the quantification of stress imposed by the surface oxide, but also demonstrate a simple route for the deconvolution of phonon confinement and strain effects in low-dimensional structures using Raman spectroscopy. Chapters 7 and 8 are dedicated to photoluminescence studies in H-terminated and surface-oxidized Si-NPs. In the former, the recombination mechanisms of photo-generated charge carrier and the origin of luminescence is investigated prior, during, and after the surface oxidation of Si-NPs. In the latter chapter, the origin of the PL in surface-oxidized Si-NPs and its size-dependence are evaluated, from which the apparently conflicting assignments for seemingly similar Si-NPs often reported in the literature are clarified. Finally, in Chapter 9, a summary of the main achievements of the present work is presented and some prospects for future research lines are suggested.
1.3
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