Angelo Mascarenhas - Academia.edu (original) (raw)
Papers by Angelo Mascarenhas
Journal of Crystal Growth, 2012
Boron is potentially useful for strain balancing compressively strained materials such as InGaAs ... more Boron is potentially useful for strain balancing compressively strained materials such as InGaAs and GaAsBi that are being developed for use in optical and electronic devices. Understanding and improving the incorporation of boron in GaAs is an important first step toward the realization of these strainbalanced systems. Here, we show that the apparent boron incorporation in GaAs, determined from Xray diffraction measurements, decreases as the substrate temperature is increased, although measurements of the metallurgical concentration of boron remain constant. This implies that boron is incorporating preferentially on non-substitutional sites as growth temperature is increased. The addition of a bismuth surfactant flux not only makes the epilayers smoother, but within a narrow range of substrate temperatures, restores the incorporation of substitutional boron.
Physical Review B, 2012
Electrons in semiconductor alloys have generally been described in terms of Bloch states that evo... more Electrons in semiconductor alloys have generally been described in terms of Bloch states that evolve from constructive interference of electron waves scattering from perfectly periodic potentials, despite the loss of structural periodicity that occurs on alloying. Using the semiconductor alloy GaAs 1-x N x as a prototype, we demonstrate a localized to delocalized transition of the electronic states at a percolation threshold, the emergence of a mobility edge, and the onset of an abrupt perturbation to the host GaAs electronic structure, shedding new light on the evolution of electronic structure in these abnormal alloys.
Japanese Journal of Applied Physics, 2016
Dilute bismide semiconductor alloys are a promising material platform for optoelectronic devices ... more Dilute bismide semiconductor alloys are a promising material platform for optoelectronic devices due to drastic impacts of bismuth on the electronic structure of the alloy. At the same time, the details of bismuth incorporation in the lattice are not fully understood. In this work, we conduct Raman scattering spectroscopy on GaP 1%x Bi x epilayers grown by molecular beam epitaxy (MBE) and identify several bismuth-related Raman features including gap vibration modes at 296, 303, and 314 cm %1. This study paves the way for more detailed analysis of the local symmetry at bismuth incorporation sites in the dilute bismide alloy regime.
Physical Review B, 1993
In this paper, polarized photoluminescence is used to study the I-point valence-band structure of... more In this paper, polarized photoluminescence is used to study the I-point valence-band structure of single-variant, spontaneously ordered CsalnP2. A laser beam is focused onto the [110I cleaved edges of
Physical Review B, 2005
The process of spontaneous lateral composition modulation in short-period InAs/AlAs superlattices... more The process of spontaneous lateral composition modulation in short-period InAs/AlAs superlattices has been investigated by grazing-incidence x-ray diffraction. We have developed a theoretical description of x-ray scattering from laterally modulated structures that makes it possible to determine the lateral composition modulation directly without assuming any structure model. From experimental intensity distributions in reciprocal space we have determined the amplitudes of
Nominally lattice-matched GaInAs layers grown by metal organic vapor phase epitaxy on InP substra... more Nominally lattice-matched GaInAs layers grown by metal organic vapor phase epitaxy on InP substrates have been studied using high-resolution x-ray diffraction (HRXRD) to determine the growth conditions under which ordering is introduced. HRXRD provides an independent means to quantify the order parameter of semiconductor heterostructures as well as the ordering on different {111} planes, i.e., double variant ordering. This independent means to determine ordering provides for a better understanding of the effects of ordering on the electronic and optical properties. Double variant ordering was observed for epitaxial layers grown on exact (001) InP substrates, with an order parameter of about 0.1 in both variants. For substrates that were miscut by 6 degrees, single variant ordering was detected. In these cases, an order parameter as high as 0.66 was measured for certain growth conditions. The layers grown on vicinal substrates are all of high crystalline quality, those on (001) substrates exhibit some mosaic spread.
Applied Physics Letters, 2007
Light scattering measurements in the dilute isoelectronically doped alloy GaAs1-xBix reveal a lar... more Light scattering measurements in the dilute isoelectronically doped alloy GaAs1-xBix reveal a large free electron population photogenerated by continuous-wave laser excitation at low temperature. Low-temperature time-resolved photoluminescence of the bismuth related near-band-gap states show carrier lifetimes of several nanoseconds. The authors attribute this to trapping of photoexcited holes at bismuth pair or cluster states located near the valence band maximum.
Applied Physics Letters, 2002
Applied Physics Letters, 1998
The nature of spontaneous lateral composition modulation and its relationship to surface morpholo... more The nature of spontaneous lateral composition modulation and its relationship to surface morphology during the growth of (AlAs)m(InAs)n short-period superlattices by molecular beam epitaxy are investigated as a function of the global strain between the short-period superlattice and ...
ABSTRACT The optical transitions in GaAs_1-xBix (x<3.6%) were studied by electromodulated ... more ABSTRACT The optical transitions in GaAs_1-xBix (x<3.6%) were studied by electromodulated and photoluminescence spectroscopy. We find a rapid decrease of the band gap energy as a function of Bi concentration, indicative of a considerable bowing coefficient. The valence band splitting increases faster than that of GaAs under similar compressive strain whereas the temperature dependence of the observed GaAsBi transitions is similar to that of GaAs. An analysis of photoluminescence results will be presented and a discussion of the characteristics of GaAsBi with respect to other isoelectronic alloys will be given.
Applied Physics Letters, 2014
Direct imaging of minority electron transport via the spatially resolved recombination luminescen... more Direct imaging of minority electron transport via the spatially resolved recombination luminescence signature has been used to determine carrier diffusion lengths in GaInP as a function of doping. Minority electron mobility values are determined by performing time resolved photoluminescence measurements of carrier lifetime on the same samples. Values at 300 K vary from ~ 2000 to 400 cm 2 /Vs and decrease with increasing doping. Anisotropic diffusion lengths and strongly polarized photoluminescence are observed, resulting from lateral composition modulation along the [110] direction. We report anisotropic mobility values associated with carrier transport parallel and perpendicular to the modulation direction.
Bulletin of the American Physical Society, 2013
Submitted for the MAR14 Meeting of The American Physical Society Imaging Long-Range Carrier Diffu... more Submitted for the MAR14 Meeting of The American Physical Society Imaging Long-Range Carrier Diffusion Across Grains in Polycrystalline CdTe1 KIRSTIN ALBERI, BRIAN FLUEGEL, HELIO MOUTINHO, RAMESH DHERE, JIAN LI, ANGELOMASCARENHAS, National Renewable Energy Laboratory — The use of polycrystalline semiconductors in electronic devices enables low cost fabrication on large area substrates. Understanding the extent to which structural defects and impurities influence carrier transport in these materials is increasingly important as device performance is maximized, but most conventional characterization techniques often cannot directly probe their effects. We have applied a novel photoluminescence imaging technique to directly observe carrier diffusion in the presence of grain boundaries and impurities in poly-CdTe films. Our results show that the grain boundaries in this material are relatively transparent to free carrier and exciton diffusion as compared to poly-GaAs. Furthermore, a netw...
Fundamental electronic and optical properties of the GaxIn1-xP alloy system are investigated for ... more Fundamental electronic and optical properties of the GaxIn1-xP alloy system are investigated for applications requiring high bandgaps in the range of ˜1.9--2.2 eV, corresponding to the composition range 0.51
Handbook of Luminescent Semiconductor Materials, 2011
Superlattices and Microstructures, 2012
Optical pumping of a type-I/type-II coupled asymmetric quantum well pair induces a spatially sepa... more Optical pumping of a type-I/type-II coupled asymmetric quantum well pair induces a spatially separated two dimensional charge carriers plasma in the well&amp;#39;s wide and narrow parts. Treating the two coupled wells as a single system we find that the eigenstate probability distribution localizes exclusively either in the wide or the narrow parts of the well pair. The energy of the
Two-dimensional tilings of an optically birefringent material are proposed as an orientational su... more Two-dimensional tilings of an optically birefringent material are proposed as an orientational superlattice for light. The light modes that result from loss-free refraction are analyzed. It is shown that the behavior for light propagation in such lattices leads to totally bound as well as propagating states. The behavior of light propagation in several such tilings is compared.
Journal of Applied Physics, 2009
GaxIn1−xP alloys grown by metalorganic vapor phase epitaxy (MOVPE) are known to exhibit spontaneo... more GaxIn1−xP alloys grown by metalorganic vapor phase epitaxy (MOVPE) are known to exhibit spontaneous long-range ordering that results in a modification of the alloy electronic band structure. Using time resolved and time integrated photoluminescence studies at 9 K, we demonstrate that the change in alloy ordering in GaxIn1−xP alloys can transform the conduction to valence band optical transition from direct to indirect for a given Ga concentration. This finding may enable sequential growth of alternate layers of high bandgap direct and indirect semiconductor alloys with similar lattice constants, opening various possibilities for device applications.
International Journal of Nanotechnology, 2008
In this review we describe the growth and properties of the dilute bismide semiconductor alloy Ga... more In this review we describe the growth and properties of the dilute bismide semiconductor alloy GaAs 1-x Bi x and show how its properties are in certain respects complementary to the dilute nitride alloy, GaNyAs1-y. Like the dilute nitrides the dilute bismides show a giant band gap bowing effect in which a small concentration of the alloying element has a disproportionate effect on the band gap, however in the case of the bismide the band gap reduction is associated with an increase in the energy of the valence band maximum (VBM) rather than a reduction in the energy of the conduction band minimum (CBM). Under standard GaAs growth conditions Bi acts as a surfactant with associated improvements in surface quality. In order to incorporate Bi, growth temperatures below 400 o C are used with As2/Ga flux ratios close to unity. The electron mobility of GaAs is only weakly affected by Bi alloying, in contrast to the dilute nitrides where the electron mobility decreases rapidly with N alloying. Bi alloying also produces a giant bowing effect in the spin orbit splitting in the valence band. Strong room temperature photoluminescence is observed. Prospects for future device applications of this new compound semiconductor alloy are discussed.
Advanced Materials, 2008
A number of nanometer-scale photovoltaic (PV) concepts based on semiconductor nanowires have been... more A number of nanometer-scale photovoltaic (PV) concepts based on semiconductor nanowires have been developed or proposed in recent years, with either inorganic/organic hybrid [1-4] or all-inorganic approaches. [5-9] The quasi-one-dimensional (quasi-1D) structure is perhaps the optimized choice for optoelectronic devices such as solar cells and photodetectors, because it allows for maximal advantage to be taken of reduced dimensionality whilst retaining the last and only needed conduction channel. Besides the possibility of exploring quantum effects at the nanoscopic scale, [7,8] the quasi-1D system could be superior to the bulk material even at the mesoscopic scale, where the lateral size falls below the carrier diffusion length, for instance, by reducing the nonradiative recombination and carrier scattering loss [10,11] through elimination of the unnecessary lateral transport and the resulting recombination loss. Additionally, a nanowire array constitutes a natural architecture, such as a photonic crystal, [12] for light trapping. The charge separation of the electron and hole is a key step in the generation of solar power in a PV device. In a conventional solar cell, it is typically achieved by a planar p-n homojunction along the path of the current flow or longitudinally. In nanometer-architecture PV devices, however, the charge separation is often facilitated by a type II or staggered energy alignment of a heterojunction, constructed from two materials for which both the valance and conduction bands of one component lie lower in energy than the corresponding bands of the other component. Such heterojunctions have been intensively investigated for solar cell applications, including dyesensitized solar cells (DSSCs), [13] quantum-dot-sensitized solar
Journal of Crystal Growth, 2012
Boron is potentially useful for strain balancing compressively strained materials such as InGaAs ... more Boron is potentially useful for strain balancing compressively strained materials such as InGaAs and GaAsBi that are being developed for use in optical and electronic devices. Understanding and improving the incorporation of boron in GaAs is an important first step toward the realization of these strainbalanced systems. Here, we show that the apparent boron incorporation in GaAs, determined from Xray diffraction measurements, decreases as the substrate temperature is increased, although measurements of the metallurgical concentration of boron remain constant. This implies that boron is incorporating preferentially on non-substitutional sites as growth temperature is increased. The addition of a bismuth surfactant flux not only makes the epilayers smoother, but within a narrow range of substrate temperatures, restores the incorporation of substitutional boron.
Physical Review B, 2012
Electrons in semiconductor alloys have generally been described in terms of Bloch states that evo... more Electrons in semiconductor alloys have generally been described in terms of Bloch states that evolve from constructive interference of electron waves scattering from perfectly periodic potentials, despite the loss of structural periodicity that occurs on alloying. Using the semiconductor alloy GaAs 1-x N x as a prototype, we demonstrate a localized to delocalized transition of the electronic states at a percolation threshold, the emergence of a mobility edge, and the onset of an abrupt perturbation to the host GaAs electronic structure, shedding new light on the evolution of electronic structure in these abnormal alloys.
Japanese Journal of Applied Physics, 2016
Dilute bismide semiconductor alloys are a promising material platform for optoelectronic devices ... more Dilute bismide semiconductor alloys are a promising material platform for optoelectronic devices due to drastic impacts of bismuth on the electronic structure of the alloy. At the same time, the details of bismuth incorporation in the lattice are not fully understood. In this work, we conduct Raman scattering spectroscopy on GaP 1%x Bi x epilayers grown by molecular beam epitaxy (MBE) and identify several bismuth-related Raman features including gap vibration modes at 296, 303, and 314 cm %1. This study paves the way for more detailed analysis of the local symmetry at bismuth incorporation sites in the dilute bismide alloy regime.
Physical Review B, 1993
In this paper, polarized photoluminescence is used to study the I-point valence-band structure of... more In this paper, polarized photoluminescence is used to study the I-point valence-band structure of single-variant, spontaneously ordered CsalnP2. A laser beam is focused onto the [110I cleaved edges of
Physical Review B, 2005
The process of spontaneous lateral composition modulation in short-period InAs/AlAs superlattices... more The process of spontaneous lateral composition modulation in short-period InAs/AlAs superlattices has been investigated by grazing-incidence x-ray diffraction. We have developed a theoretical description of x-ray scattering from laterally modulated structures that makes it possible to determine the lateral composition modulation directly without assuming any structure model. From experimental intensity distributions in reciprocal space we have determined the amplitudes of
Nominally lattice-matched GaInAs layers grown by metal organic vapor phase epitaxy on InP substra... more Nominally lattice-matched GaInAs layers grown by metal organic vapor phase epitaxy on InP substrates have been studied using high-resolution x-ray diffraction (HRXRD) to determine the growth conditions under which ordering is introduced. HRXRD provides an independent means to quantify the order parameter of semiconductor heterostructures as well as the ordering on different {111} planes, i.e., double variant ordering. This independent means to determine ordering provides for a better understanding of the effects of ordering on the electronic and optical properties. Double variant ordering was observed for epitaxial layers grown on exact (001) InP substrates, with an order parameter of about 0.1 in both variants. For substrates that were miscut by 6 degrees, single variant ordering was detected. In these cases, an order parameter as high as 0.66 was measured for certain growth conditions. The layers grown on vicinal substrates are all of high crystalline quality, those on (001) substrates exhibit some mosaic spread.
Applied Physics Letters, 2007
Light scattering measurements in the dilute isoelectronically doped alloy GaAs1-xBix reveal a lar... more Light scattering measurements in the dilute isoelectronically doped alloy GaAs1-xBix reveal a large free electron population photogenerated by continuous-wave laser excitation at low temperature. Low-temperature time-resolved photoluminescence of the bismuth related near-band-gap states show carrier lifetimes of several nanoseconds. The authors attribute this to trapping of photoexcited holes at bismuth pair or cluster states located near the valence band maximum.
Applied Physics Letters, 2002
Applied Physics Letters, 1998
The nature of spontaneous lateral composition modulation and its relationship to surface morpholo... more The nature of spontaneous lateral composition modulation and its relationship to surface morphology during the growth of (AlAs)m(InAs)n short-period superlattices by molecular beam epitaxy are investigated as a function of the global strain between the short-period superlattice and ...
ABSTRACT The optical transitions in GaAs_1-xBix (x<3.6%) were studied by electromodulated ... more ABSTRACT The optical transitions in GaAs_1-xBix (x<3.6%) were studied by electromodulated and photoluminescence spectroscopy. We find a rapid decrease of the band gap energy as a function of Bi concentration, indicative of a considerable bowing coefficient. The valence band splitting increases faster than that of GaAs under similar compressive strain whereas the temperature dependence of the observed GaAsBi transitions is similar to that of GaAs. An analysis of photoluminescence results will be presented and a discussion of the characteristics of GaAsBi with respect to other isoelectronic alloys will be given.
Applied Physics Letters, 2014
Direct imaging of minority electron transport via the spatially resolved recombination luminescen... more Direct imaging of minority electron transport via the spatially resolved recombination luminescence signature has been used to determine carrier diffusion lengths in GaInP as a function of doping. Minority electron mobility values are determined by performing time resolved photoluminescence measurements of carrier lifetime on the same samples. Values at 300 K vary from ~ 2000 to 400 cm 2 /Vs and decrease with increasing doping. Anisotropic diffusion lengths and strongly polarized photoluminescence are observed, resulting from lateral composition modulation along the [110] direction. We report anisotropic mobility values associated with carrier transport parallel and perpendicular to the modulation direction.
Bulletin of the American Physical Society, 2013
Submitted for the MAR14 Meeting of The American Physical Society Imaging Long-Range Carrier Diffu... more Submitted for the MAR14 Meeting of The American Physical Society Imaging Long-Range Carrier Diffusion Across Grains in Polycrystalline CdTe1 KIRSTIN ALBERI, BRIAN FLUEGEL, HELIO MOUTINHO, RAMESH DHERE, JIAN LI, ANGELOMASCARENHAS, National Renewable Energy Laboratory — The use of polycrystalline semiconductors in electronic devices enables low cost fabrication on large area substrates. Understanding the extent to which structural defects and impurities influence carrier transport in these materials is increasingly important as device performance is maximized, but most conventional characterization techniques often cannot directly probe their effects. We have applied a novel photoluminescence imaging technique to directly observe carrier diffusion in the presence of grain boundaries and impurities in poly-CdTe films. Our results show that the grain boundaries in this material are relatively transparent to free carrier and exciton diffusion as compared to poly-GaAs. Furthermore, a netw...
Fundamental electronic and optical properties of the GaxIn1-xP alloy system are investigated for ... more Fundamental electronic and optical properties of the GaxIn1-xP alloy system are investigated for applications requiring high bandgaps in the range of ˜1.9--2.2 eV, corresponding to the composition range 0.51
Handbook of Luminescent Semiconductor Materials, 2011
Superlattices and Microstructures, 2012
Optical pumping of a type-I/type-II coupled asymmetric quantum well pair induces a spatially sepa... more Optical pumping of a type-I/type-II coupled asymmetric quantum well pair induces a spatially separated two dimensional charge carriers plasma in the well&amp;#39;s wide and narrow parts. Treating the two coupled wells as a single system we find that the eigenstate probability distribution localizes exclusively either in the wide or the narrow parts of the well pair. The energy of the
Two-dimensional tilings of an optically birefringent material are proposed as an orientational su... more Two-dimensional tilings of an optically birefringent material are proposed as an orientational superlattice for light. The light modes that result from loss-free refraction are analyzed. It is shown that the behavior for light propagation in such lattices leads to totally bound as well as propagating states. The behavior of light propagation in several such tilings is compared.
Journal of Applied Physics, 2009
GaxIn1−xP alloys grown by metalorganic vapor phase epitaxy (MOVPE) are known to exhibit spontaneo... more GaxIn1−xP alloys grown by metalorganic vapor phase epitaxy (MOVPE) are known to exhibit spontaneous long-range ordering that results in a modification of the alloy electronic band structure. Using time resolved and time integrated photoluminescence studies at 9 K, we demonstrate that the change in alloy ordering in GaxIn1−xP alloys can transform the conduction to valence band optical transition from direct to indirect for a given Ga concentration. This finding may enable sequential growth of alternate layers of high bandgap direct and indirect semiconductor alloys with similar lattice constants, opening various possibilities for device applications.
International Journal of Nanotechnology, 2008
In this review we describe the growth and properties of the dilute bismide semiconductor alloy Ga... more In this review we describe the growth and properties of the dilute bismide semiconductor alloy GaAs 1-x Bi x and show how its properties are in certain respects complementary to the dilute nitride alloy, GaNyAs1-y. Like the dilute nitrides the dilute bismides show a giant band gap bowing effect in which a small concentration of the alloying element has a disproportionate effect on the band gap, however in the case of the bismide the band gap reduction is associated with an increase in the energy of the valence band maximum (VBM) rather than a reduction in the energy of the conduction band minimum (CBM). Under standard GaAs growth conditions Bi acts as a surfactant with associated improvements in surface quality. In order to incorporate Bi, growth temperatures below 400 o C are used with As2/Ga flux ratios close to unity. The electron mobility of GaAs is only weakly affected by Bi alloying, in contrast to the dilute nitrides where the electron mobility decreases rapidly with N alloying. Bi alloying also produces a giant bowing effect in the spin orbit splitting in the valence band. Strong room temperature photoluminescence is observed. Prospects for future device applications of this new compound semiconductor alloy are discussed.
Advanced Materials, 2008
A number of nanometer-scale photovoltaic (PV) concepts based on semiconductor nanowires have been... more A number of nanometer-scale photovoltaic (PV) concepts based on semiconductor nanowires have been developed or proposed in recent years, with either inorganic/organic hybrid [1-4] or all-inorganic approaches. [5-9] The quasi-one-dimensional (quasi-1D) structure is perhaps the optimized choice for optoelectronic devices such as solar cells and photodetectors, because it allows for maximal advantage to be taken of reduced dimensionality whilst retaining the last and only needed conduction channel. Besides the possibility of exploring quantum effects at the nanoscopic scale, [7,8] the quasi-1D system could be superior to the bulk material even at the mesoscopic scale, where the lateral size falls below the carrier diffusion length, for instance, by reducing the nonradiative recombination and carrier scattering loss [10,11] through elimination of the unnecessary lateral transport and the resulting recombination loss. Additionally, a nanowire array constitutes a natural architecture, such as a photonic crystal, [12] for light trapping. The charge separation of the electron and hole is a key step in the generation of solar power in a PV device. In a conventional solar cell, it is typically achieved by a planar p-n homojunction along the path of the current flow or longitudinally. In nanometer-architecture PV devices, however, the charge separation is often facilitated by a type II or staggered energy alignment of a heterojunction, constructed from two materials for which both the valance and conduction bands of one component lie lower in energy than the corresponding bands of the other component. Such heterojunctions have been intensively investigated for solar cell applications, including dyesensitized solar cells (DSSCs), [13] quantum-dot-sensitized solar