Resonant Raman spectroscopy is realized on closely spaced nanowire based quantum wells. Phonon qu... more Resonant Raman spectroscopy is realized on closely spaced nanowire based quantum wells. Phonon quantization consistent with 2.4 nm thick quantum wells is observed, in agreement with the cross-section transmission electron microscopy measurements. The creation of a high density plasma within the quantized structures is demonstrated by the observation of coupled plasmonphonon modes. The density of the plasma and thereby the plasmon-phonon interaction is controlled with the excitation power. At large excitation power, an optically induced transparency of ∼85% is obtained due to the saturation in the filling of the quantized states in the quantum wells. This work represents a base for further studies on confined high density charge systems in nanowires. arXiv:1101.4847v1 [cond-mat.mes-hall]
g-factor tuning in self-assembled quantum dots Appl. Phys. Lett. 96, 133102 (2010); 10.1063/1.336... more g-factor tuning in self-assembled quantum dots Appl. Phys. Lett. 96, 133102 (2010); 10.1063/1.3367707 Observation of an electrically tunable exciton g factor in InGaAs/GaAs quantum dots Appl. Phys. Lett. 96, 053113 (2010); 10.1063/1.3309684 Size-dependent radiative decay time of excitons in GaN/AlN self-assembled quantum dots Appl. Phys. Lett. 83, 984 (2003); 10.1063/1.1596382
We report on the impact of the doping concentration design on the performance of silicon microwir... more We report on the impact of the doping concentration design on the performance of silicon microwire arrays as photovoltaic devices. We have fabricated arrays with different p-and ndoping profiles and thicknesses, obtaining mean efficiencies as high as 9.7% under AM 1.5G solar illumination. The results reveal the importance of scaling the microwire diameter with the depletion width resulting from doping concentrations. The doping of the core should be kept low in order to reduce bulk recombination. Furthermore, the thickness of the n-shell should be kept as thin as possible to limit the emitter losses.
Nanowire diameter has a dramatic effect on the absorption cross-section in the optical domain. Th... more Nanowire diameter has a dramatic effect on the absorption cross-section in the optical domain. The maximum absorption is reached for ideal nanowire morphology within a solar cell device. As a consequence, understanding how to tailor the nanowire diameter and density is extremely important for high-efficient nanowire-based solar cells. In this work, we investigate mastering the diameter and density of self-catalyzed GaAs nanowires on Si(111) substrates by growth conditions using the self-assembly of Ga droplets. We introduce a new paradigm of the characteristic nucleation time controlled by group III flux and temperature that determine diameter and length distributions of GaAs nanowires. This insight into the growth mechanism is then used to grow nanowire forests with a completely tailored diameter-density distribution. We also show how the reflectivity of nanowire arrays can be minimized in this way. In general, this work opens new possibilities for the cost-effective and controlled...
Physical properties of surfaces are extremely important for initiation and nucleation of crystal ... more Physical properties of surfaces are extremely important for initiation and nucleation of crystal growth, including nanowires. In recent years, fluctuations in surface characteristics have often been related to unreproducible growth of GaAs nanowires on Si by the Ga-assisted method. We report on a systematic study of the occurrence of GaAs nanowire growth on silicon by the Ga-assisted method for different kinds of silicon oxides: native, thermal and hydrogen silsesquioxane (HSQ). We find that success in achieving nanowires and the growth conditions such as gallium rate and substrate temperature depend mainly on the physical properties of the surface: oxide stoichiometry, oxide thickness and surface roughness. These results constitute a step further towards the integration of GaAs technology on the Si platform.
The influence of the oxide in Ga-assisted growth of GaAs nanowires on Si substrates is investigat... more The influence of the oxide in Ga-assisted growth of GaAs nanowires on Si substrates is investigated. Three different types of oxides with different structure and chemistry are considered. We observe that the critical oxide thicknesses needed for achieving nanowire growth depends on the nature of oxide and how it is processed. Additionally, we find that different growth conditions such as temperature and Ga rate are needed for successful nanowire growth on different oxides. We generalize the results in terms of the characteristics of the oxides such as surface roughness, stoichiometry and thickness. These results constitute a step further towards the integration of GaAs technology on the Si platform.
We focus here on a study of the growth of polymorphous and protocrystalline silicon materials wit... more We focus here on a study of the growth of polymorphous and protocrystalline silicon materials with respect to the well-established amorphous and microcrystalline silicon. Protocrystalline films correspond to a slow crystallisation process, in which the films grow densely in the first monolayers, but their porosity and roughness increase with thickness, allowing the nucleation of crystallites, and finally the formation of a microcrystalline phase. On the contrary, polymorphous films remain dense, independent of their thickness. The control of the temperature gradient between the RF electrode and the substrate holder allows a switch from microcrystalline to polymorphous silicon growth, which strongly supports our hypothesis of polymorphous films being formed by simultaneous contributions of silicon radicals and clusters to the growth. ᮊ
Surface recombination represents a handicap for high-efficiency solar cells. This is especially i... more Surface recombination represents a handicap for high-efficiency solar cells. This is especially important for nanowire array solar cells, where the surface-to-volume ratio is greatly enhanced. Here, the effect of different passivation materials on the effective recombination and on the device performance is experimentally analyzed. Our solar cells are large area top-down axial n-p junction silicon nanowires fabricated by means of Near-Field Phase-Shift Lithography (NF-PSL). We report an efficiency of 9.9% for the best cell, passivated with a SiO 2 /SiN x stack. The impact of the presence of a surface fixed charge density at the silicon/oxide interface is studied.
Reliable doping is required to realize many devices based on semiconductor nanowires. Group III-V... more Reliable doping is required to realize many devices based on semiconductor nanowires. Group III-V nanowires show great promise as elements of high-speed optoelectronic devices, but for such applications it is important that the electron mobility is not compromised by the inclusion of dopants. Here we show that GaAs nanowires can be n-type doped with negligible loss of electron mobility. Molecular beam epitaxy was used to fabricate modulation-doped GaAs nanowires with Al0.33Ga0.67As shells that contained a layer of Si dopants. We identify the presence of the doped layer from a high-angle annular dark field scanning electron microscopy cross-section image. The doping density, carrier mobility, and charge carrier lifetimes of these n-type nanowires and nominally undoped reference samples were determined using the noncontact method of optical pump terahertz probe spectroscopy. An n-type extrinsic carrier concentration of 1.10 ± 0.06 × 10(16) cm(-3) was extracted, demonstrating the effec...
We review different strategies to achieve a three-dimensional energy bandgap modulation in a nano... more We review different strategies to achieve a three-dimensional energy bandgap modulation in a nanowire (NW) by the introduction of self-assembled 0D, 1D and 2D quantum structures, quantum dots (QDs), quantum wires (QWRs) and quantum wells (QWs). Starting with the well-known axial, radial (coaxial/ prismatic) or polytypic quantum wells in GaN/AlN, GaAs/AlAs or wurtzite/zinc-blende systems, respectively, we move to more sophisticated structures by lowering their dimensionality. New recent approaches developed for the self-assembly of GaN quantum wires and InAs or AlGaAs quantum dots on single nanowire templates are reported and discussed. Aberration corrected scanning transmission electron microcopy is presented as a powerful tool to determine the structure and morphology at the atomic scale allowing for the creation of 3D atomic models that can help us to understand the enhanced optical properties of these advanced quantum structures.
ABSTRACT Magnetic nanotubes (NTs) are interesting for magnetic memory and magnonic applications. ... more ABSTRACT Magnetic nanotubes (NTs) are interesting for magnetic memory and magnonic applications. We report magnetotransport experiments on individual 10 to 20 μm long Ni and CoFeB NTs with outer diameters ranging from 160 to 390 nm and film thicknesses of 20 to 40 nm. The anisotropic magnetoresistance (AMR) effect studied from 2 K to room temperature (RT) amounted to 1.4% and 0.1% for Ni and CoFeB NTs, respectively, at RT. We evaluated magnetometric demagnetization factors of about 0.7 for Ni and CoFeB NTs having considerably different saturation magnetization. The relatively large AMR value of the Ni nanotubes is promising for RT spintronic applications. The large saturation magnetization of CoFeB is useful in different fields such as magnonics and scanning probe microscopy using nanotubes as magnetic tips.
Resonant Raman spectroscopy is realized on closely spaced nanowire based quantum wells. Phonon qu... more Resonant Raman spectroscopy is realized on closely spaced nanowire based quantum wells. Phonon quantization consistent with 2.4 nm thick quantum wells is observed, in agreement with the cross-section transmission electron microscopy measurements. The creation of a high density plasma within the quantized structures is demonstrated by the observation of coupled plasmonphonon modes. The density of the plasma and thereby the plasmon-phonon interaction is controlled with the excitation power. At large excitation power, an optically induced transparency of ∼85% is obtained due to the saturation in the filling of the quantized states in the quantum wells. This work represents a base for further studies on confined high density charge systems in nanowires. arXiv:1101.4847v1 [cond-mat.mes-hall]
g-factor tuning in self-assembled quantum dots Appl. Phys. Lett. 96, 133102 (2010); 10.1063/1.336... more g-factor tuning in self-assembled quantum dots Appl. Phys. Lett. 96, 133102 (2010); 10.1063/1.3367707 Observation of an electrically tunable exciton g factor in InGaAs/GaAs quantum dots Appl. Phys. Lett. 96, 053113 (2010); 10.1063/1.3309684 Size-dependent radiative decay time of excitons in GaN/AlN self-assembled quantum dots Appl. Phys. Lett. 83, 984 (2003); 10.1063/1.1596382
We report on the impact of the doping concentration design on the performance of silicon microwir... more We report on the impact of the doping concentration design on the performance of silicon microwire arrays as photovoltaic devices. We have fabricated arrays with different p-and ndoping profiles and thicknesses, obtaining mean efficiencies as high as 9.7% under AM 1.5G solar illumination. The results reveal the importance of scaling the microwire diameter with the depletion width resulting from doping concentrations. The doping of the core should be kept low in order to reduce bulk recombination. Furthermore, the thickness of the n-shell should be kept as thin as possible to limit the emitter losses.
Nanowire diameter has a dramatic effect on the absorption cross-section in the optical domain. Th... more Nanowire diameter has a dramatic effect on the absorption cross-section in the optical domain. The maximum absorption is reached for ideal nanowire morphology within a solar cell device. As a consequence, understanding how to tailor the nanowire diameter and density is extremely important for high-efficient nanowire-based solar cells. In this work, we investigate mastering the diameter and density of self-catalyzed GaAs nanowires on Si(111) substrates by growth conditions using the self-assembly of Ga droplets. We introduce a new paradigm of the characteristic nucleation time controlled by group III flux and temperature that determine diameter and length distributions of GaAs nanowires. This insight into the growth mechanism is then used to grow nanowire forests with a completely tailored diameter-density distribution. We also show how the reflectivity of nanowire arrays can be minimized in this way. In general, this work opens new possibilities for the cost-effective and controlled...
Physical properties of surfaces are extremely important for initiation and nucleation of crystal ... more Physical properties of surfaces are extremely important for initiation and nucleation of crystal growth, including nanowires. In recent years, fluctuations in surface characteristics have often been related to unreproducible growth of GaAs nanowires on Si by the Ga-assisted method. We report on a systematic study of the occurrence of GaAs nanowire growth on silicon by the Ga-assisted method for different kinds of silicon oxides: native, thermal and hydrogen silsesquioxane (HSQ). We find that success in achieving nanowires and the growth conditions such as gallium rate and substrate temperature depend mainly on the physical properties of the surface: oxide stoichiometry, oxide thickness and surface roughness. These results constitute a step further towards the integration of GaAs technology on the Si platform.
The influence of the oxide in Ga-assisted growth of GaAs nanowires on Si substrates is investigat... more The influence of the oxide in Ga-assisted growth of GaAs nanowires on Si substrates is investigated. Three different types of oxides with different structure and chemistry are considered. We observe that the critical oxide thicknesses needed for achieving nanowire growth depends on the nature of oxide and how it is processed. Additionally, we find that different growth conditions such as temperature and Ga rate are needed for successful nanowire growth on different oxides. We generalize the results in terms of the characteristics of the oxides such as surface roughness, stoichiometry and thickness. These results constitute a step further towards the integration of GaAs technology on the Si platform.
We focus here on a study of the growth of polymorphous and protocrystalline silicon materials wit... more We focus here on a study of the growth of polymorphous and protocrystalline silicon materials with respect to the well-established amorphous and microcrystalline silicon. Protocrystalline films correspond to a slow crystallisation process, in which the films grow densely in the first monolayers, but their porosity and roughness increase with thickness, allowing the nucleation of crystallites, and finally the formation of a microcrystalline phase. On the contrary, polymorphous films remain dense, independent of their thickness. The control of the temperature gradient between the RF electrode and the substrate holder allows a switch from microcrystalline to polymorphous silicon growth, which strongly supports our hypothesis of polymorphous films being formed by simultaneous contributions of silicon radicals and clusters to the growth. ᮊ
Surface recombination represents a handicap for high-efficiency solar cells. This is especially i... more Surface recombination represents a handicap for high-efficiency solar cells. This is especially important for nanowire array solar cells, where the surface-to-volume ratio is greatly enhanced. Here, the effect of different passivation materials on the effective recombination and on the device performance is experimentally analyzed. Our solar cells are large area top-down axial n-p junction silicon nanowires fabricated by means of Near-Field Phase-Shift Lithography (NF-PSL). We report an efficiency of 9.9% for the best cell, passivated with a SiO 2 /SiN x stack. The impact of the presence of a surface fixed charge density at the silicon/oxide interface is studied.
Reliable doping is required to realize many devices based on semiconductor nanowires. Group III-V... more Reliable doping is required to realize many devices based on semiconductor nanowires. Group III-V nanowires show great promise as elements of high-speed optoelectronic devices, but for such applications it is important that the electron mobility is not compromised by the inclusion of dopants. Here we show that GaAs nanowires can be n-type doped with negligible loss of electron mobility. Molecular beam epitaxy was used to fabricate modulation-doped GaAs nanowires with Al0.33Ga0.67As shells that contained a layer of Si dopants. We identify the presence of the doped layer from a high-angle annular dark field scanning electron microscopy cross-section image. The doping density, carrier mobility, and charge carrier lifetimes of these n-type nanowires and nominally undoped reference samples were determined using the noncontact method of optical pump terahertz probe spectroscopy. An n-type extrinsic carrier concentration of 1.10 ± 0.06 × 10(16) cm(-3) was extracted, demonstrating the effec...
We review different strategies to achieve a three-dimensional energy bandgap modulation in a nano... more We review different strategies to achieve a three-dimensional energy bandgap modulation in a nanowire (NW) by the introduction of self-assembled 0D, 1D and 2D quantum structures, quantum dots (QDs), quantum wires (QWRs) and quantum wells (QWs). Starting with the well-known axial, radial (coaxial/ prismatic) or polytypic quantum wells in GaN/AlN, GaAs/AlAs or wurtzite/zinc-blende systems, respectively, we move to more sophisticated structures by lowering their dimensionality. New recent approaches developed for the self-assembly of GaN quantum wires and InAs or AlGaAs quantum dots on single nanowire templates are reported and discussed. Aberration corrected scanning transmission electron microcopy is presented as a powerful tool to determine the structure and morphology at the atomic scale allowing for the creation of 3D atomic models that can help us to understand the enhanced optical properties of these advanced quantum structures.
ABSTRACT Magnetic nanotubes (NTs) are interesting for magnetic memory and magnonic applications. ... more ABSTRACT Magnetic nanotubes (NTs) are interesting for magnetic memory and magnonic applications. We report magnetotransport experiments on individual 10 to 20 μm long Ni and CoFeB NTs with outer diameters ranging from 160 to 390 nm and film thicknesses of 20 to 40 nm. The anisotropic magnetoresistance (AMR) effect studied from 2 K to room temperature (RT) amounted to 1.4% and 0.1% for Ni and CoFeB NTs, respectively, at RT. We evaluated magnetometric demagnetization factors of about 0.7 for Ni and CoFeB NTs having considerably different saturation magnetization. The relatively large AMR value of the Ni nanotubes is promising for RT spintronic applications. The large saturation magnetization of CoFeB is useful in different fields such as magnonics and scanning probe microscopy using nanotubes as magnetic tips.
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Papers by Anna Fontcuberta I Morral