Miguel A Vidal - Academia.edu (original) (raw)
Papers by Miguel A Vidal
Cancer Research, 2009
Prevalence of methylene tetrahydrofolate reductase (MTHFR) gene mutations in South Indian populat... more Prevalence of methylene tetrahydrofolate reductase (MTHFR) gene mutations in South Indian population was investigated from a total of 608 samples, 420 adults and 188 newborns. Detection of mutation was carried out focussing on the two most common mutations of the MTHFR gene (C677T and A1298C) using PCRbased RFLP method. T-allele frequency was almost similar between the newborns and adults (0.0904, 0.1012). However, a higher T-allele frequency was observed in females (0.1538 and 0.12 in adults and newborns respectively) than males (0.0556 and 0.05 in adults and newborns respectively). In the case of the other mutation, C-allele frequency was almost similar with no sex-bias (0.414 and 0.402 in males and females respectively). Biochemical correlation of fasting plasma total homocysteine to MTHFR genotype revealed mild to moderate hyperhomocysteinemia in mutants. Plasma total homocysteine in males was found to be higher than in females in both normal and mutant individuals. TT homozygous women had higher plasma homocysteine. The high T-allele frequency, elevated plasma homocysteine and low folate intake in women could well be a risk factor for birth defects. The gender bias observed in this autosomal recessive trait was an interesting finding and is discussed.
Single wall carbon nanotubes are dispersed in water with the water-soluble polymer poly-vinylpyrr... more Single wall carbon nanotubes are dispersed in water with the water-soluble polymer poly-vinylpyrrolidone and the surfactant sodium dodecylbenzene sulfonate, and then deposited by evaporative deposition onto degeneratively-doped silicon wafer substrates. These deposits were examined by scanning electron microscopy, which revealed highly-ordered arrays of large single wall carbon nanotube bundles. Various solution concentrations were prepared and deposition conditions were varied to determine their affect on the single wall carbon nanotube arrays. These observations were related to existing lyotropic liquid crystal theory and theories explaining the behavior of carbon nanotubes in solution, which allowed for further development and interpretation of the phase diagram which describes the behavior of single wall carbon nanotubes in lyotropic liquid crystal systems, and how competing liquid crystal systems in the same solution directly affect the ordering of the single wall carbon nanotube arrays.
Nanotechnology, 2012
In an effort to combine group III-V semiconductors with carbon nanotubes, a simple solution-based... more In an effort to combine group III-V semiconductors with carbon nanotubes, a simple solution-based technique for gallium functionalization of nitrogen-doped multi-wall carbon nanotubes has been developed. With an aqueous solution of a gallium salt (GaI 3 ), it was possible to form covalent bonds between the Ga 3+ ion and the nitrogen atoms of the doped carbon nanotubes to form a gallium nitride-carbon nanotube hybrid at room temperature. This functionalization was evaluated by x-ray photoelectron spectroscopy, energy dispersive x-ray spectroscopy, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy S Online supplementary data available from stacks.iop.org/Nano/23/325601/mmedia
Journal of Applied Physics, 2010
Infrared optical studies were carried out in a group of cubic InN samples grown by gas source mol... more Infrared optical studies were carried out in a group of cubic InN samples grown by gas source molecular beam epitaxy on MgO ͑001͒ substrates. Room temperature ͑RT͒ reflectance and low-temperature ͑LT͒ transmittance measurements were performed by using fast Fourier transform infrared spectrometry. Reflectance fittings allowed to establish that -InN films have large free-carrier concentrations present ͑Ͼ10 19 cm −3 ͒, a result that is corroborated by Hall effect measurements. Each sample explored exhibited a different optical absorption edge. The Varshni parameters that describe adequately the optical absorption edge responses with temperature are obtained for the set of samples studied. The observed temperatures changes, from LT to RT, are the lowest reported for III-V semiconductor binary compounds. The temperature coefficient of the conduction band depends on the strength of the electron-phonon interaction ͑e-ph-i͒, as well as on the thermal expansion. It has been predicted that cubic InN has one of the smallest e-ph-i of all III-V compounds, which is corroborated by these results. The variation in values of absorption edges is clearly consistent with the Burstein-Moss and band renormalization effects, produced by high free electron concentrations. It is shown that the conduction band in -InN, analogous to wurtzite InN, follows a nonparabolic behavior.
[sub 2x] semiconductor alloys](https://attachments.academia-assets.com/46452754/thumbnails/1.jpg)
](Applied Physics Letters, 2000
Influence of nonstoichiometry on long-range order and its measurement in ( GaAs ) 1−x Ge 2x and r... more Influence of nonstoichiometry on long-range order and its measurement in ( GaAs ) 1−x Ge 2x and related alloys Ge-related faceting and segregation during the growth of metastable ( GaAs ) 1−x ( Ge 2 ) x alloy layers by metal-organic vapor-phase epitaxy
Journal of Applied Physics, 2009
We have measured the infrared reflectance anisotropy of bulk wurzite ͑0001͒ GaN. Reflectance diff... more We have measured the infrared reflectance anisotropy of bulk wurzite ͑0001͒ GaN. Reflectance difference ͑RD͒ spectra have been determined from the experimental reflectance for light polarization along the sample axes ͓0110͔ and ͓1120͔. The spectra show an optical anisotropy around the frequency of the E 1 ͑LO͒ and A 1 ͑LO͒ GaN phonons. The spectra are influenced by the structural quality of the samples, as revealed by measurements of high resolution x-ray diffraction and Raman scattering. The RD may be originated by inhomogeneous anisotropic broadening of the optical phonons induced by crystal size distribution. Based on the sensitivity of the infrared spectra to the crystalline quality, we propose this infrared technique as a useful optical probe for hexagonal GaN characterization.
![Research paper thumbnail of Determination of the optical energy gap of Ge[sub 1−x]Sn[sub x] alloys with 0<x<0.14](https://attachments.academia-assets.com/46452746/thumbnails/1.jpg)
](Applied Physics Letters, 2004
The optical energy gap of Ge 1Ϫx Sn x alloys has been determined from transmittance measurements,... more The optical energy gap of Ge 1Ϫx Sn x alloys has been determined from transmittance measurements, using a fast-Fourier-transform infrared interferometer. Our results show that the change from indirect to direct band gap occurs at a lower critical Sn concentration (x c ) than the value predicted from the virtual crystal approximation, tight binding, and pseudopotential models. However, a close agreement between the experimental results and the predictions with deformation potential theory is observed. The concentration x c , which is theoretically expected to be 0.09, actually it is observed to lie between 0.10Ͻx c Ͻ0.13.
![Research paper thumbnail of Ge[sub 1−x]Sn[sub x] alloys pseudomorphically grown on Ge(001)](https://attachments.academia-assets.com/46452744/thumbnails/1.jpg)
](Applied Physics Letters, 2003
The optical energy gap of Ge 1Ϫx Sn x alloys has been determined from transmittance measurements,... more The optical energy gap of Ge 1Ϫx Sn x alloys has been determined from transmittance measurements, using a fast-Fourier-transform infrared interferometer. Our results show that the change from indirect to direct band gap occurs at a lower critical Sn concentration (x c ) than the value predicted from the virtual crystal approximation, tight binding, and pseudopotential models. However, a close agreement between the experimental results and the predictions with deformation potential theory is observed. The concentration x c , which is theoretically expected to be 0.09, actually it is observed to lie between 0.10Ͻx c Ͻ0.13.
Journal of Applied Physics, 2001
Strained layer superlattices of GaAs/Ge/GaAs and Ge/GaAs/Ge have been grown by magnetron sputteri... more Strained layer superlattices of GaAs/Ge/GaAs and Ge/GaAs/Ge have been grown by magnetron sputtering of different intercalated layer thickness. The samples exhibited good crystallographic quality, pseudomorphic growth on the substrate, as well as superlattice characteristics. Layer periodicity, concentration profile and the thicknesses of the resultant films were examined by high-resolution x-ray diffraction, secondary ion mass spectroscopy, infrared ͑IR͒ optical transmission measurements, and Raman spectroscopy. The heterostructures exhibited IR attenuation peaks in transmission between 0.5 and 1.0 eV, whose energy position was characterized as a function of the thickness of the intercalated thinner layers. The combined results of these techniques reveal that the intended GaAs layers are in fact composed of (GaAs) 1Ϫx (Ge 2 ) x alloys with a few percent Ge content. Experimental and theoretical results have been modeled with the transmittance model, which assumes that light hits the surface normally and takes the alternating layer thicknesses as variable parameters. Both, experimental and theoretical results agree to within 3%.
Spectroscopy Letters, 2014
We have examined the effect of high temperature on single-wall carbon nanotubes under air and nit... more We have examined the effect of high temperature on single-wall carbon nanotubes under air and nitrogen ambient by Raman spectroscopy. We observe the temperature dependence of the radial breathing mode and the G-band modes. The thermal expansion coefficient (b) of the bundled nanotubes is obtained experimentally using the estimated volume from Raman scattering. b behaves linearly with temperature from 0.33 Â 10 À5 K À1 to 0.28 Â 10 À5 K À1 in air and from 0.58 Â 10 À5 K À1 to 0.47 Â 10 À5 K À1 in nitrogen ambient, respectively. The temperature dependence of the radial breathing mode Raman frequencies is consistent with a pure temperature effect.
Recibido el 16 de enero de 2012; aceptado el 9 de febrero de 2012
![Research paper thumbnail of Nonlinear behavior of the energy gap in Ge[sub 1−x]Sn[sub x] alloys at 4 K](https://attachments.academia-assets.com/46452745/thumbnails/1.jpg)
](Applied Physics Letters, 2007
The optical energy gap of Ge 1−x Sn x alloys ͑x ഛ 0.14͒ grown on Ge substrates has been determine... more The optical energy gap of Ge 1−x Sn x alloys ͑x ഛ 0.14͒ grown on Ge substrates has been determined by performing transmittance measurements at 4 K using a fast fourier transform infrared interferometer. The direct energy gap transitions in Ge 1−x Sn x alloys behave following a nonlinear dependence on the Sn concentration, expressed by a quadratic equation, with a so called bowing parameter b 0 that describes the deviation from a simple linear dependence. Our observations resulted in b 0 RT = 2.30± 0.10 eV and b 0 4 K = 2.84± 0.15 eV, at room temperature and 4 K, respectively. The validity of our fit is limited for Sn concentrations lower than 15%.
[sub x] alloys epitaxially grown on GaAs](https://attachments.academia-assets.com/46452757/thumbnails/1.jpg)
](Journal of Applied Physics, 2001
͑GaAs͒ 1Ϫx ͑Si 2 ) x metastable alloys grown on ͑001͒, ͑110͒, ͑112͒, and ͑111͒ GaAs substrates, w... more ͑GaAs͒ 1Ϫx ͑Si 2 ) x metastable alloys grown on ͑001͒, ͑110͒, ͑112͒, and ͑111͒ GaAs substrates, with Si fractions in the range 0рxр0.43, were studied by Raman scattering. Two modes near the LO and TO modes of GaAs, besides two local modes associated either with Si-As or Si-Ga and Si-Si bonds, are observed in the Raman spectra. The ratio of integrated intensities of TO-like and LO-like modes (I TO /I LO ) is used to evaluate the short range order. It is observed that the zinc blende to diamond transition reported in the literature for these alloys does not influence the Si fraction dependence of the short range order.
Physical Review B, 2001
Infrared reflection experiments were performed on wurtzite InN films with a range of free-electro... more Infrared reflection experiments were performed on wurtzite InN films with a range of free-electron concentrations grown by molecular-beam epitaxy. Measurements of the plasma edge frequencies were used to determine electron effective masses. The results show a pronounced increase in the electron effective mass with increasing electron concentration, indicating a nonparabolic conduction band in InN. We have also found a large Burstein-Moss shift of the fundamental band gap. The observed effects are quantitatively described by the k•p interaction within the two-band Kane model of narrow-gap semiconductors.
Epitaxial single crystal films of Ge, with thickness from 0.2 to 2.4 µm, were grown on GaAs (001)... more Epitaxial single crystal films of Ge, with thickness from 0.2 to 2.4 µm, were grown on GaAs (001) by rf sputtering. These layers were characterized by High Resolution X-Ray Diffraction (HRXRD). Measured rocking curves show that pseudomorphic samples with good structural quality can be obtained by this growth technique. Asymmetrical reflections (115) and (-1-15) are used to determine the in-plane and in-growth lattice parameters of the grown films. From the behavior of these parameters and Ge diffraction peak broadening, with the layer thickness, an experimental value of 1.8 µm is obtained for the critical thickness of Ge grown on GaAs (001).
Journal of Crystal Growth, 2015
In x Ga 1 À x N (001) ternary alloys were grown on GaN/MgO (100) substrates in a plasma assisted ... more In x Ga 1 À x N (001) ternary alloys were grown on GaN/MgO (100) substrates in a plasma assisted molecular beam epitaxy system. We determined the in-plane [001] and in-growth [110] lattice parameters, as well as the bulk lattice parameter of the alloys for different In concentrations by high resolution X-ray diffraction. The In concentration was determined assuming Vegard's law for the alloy lattice parameter. The optical energy gap of In x Ga 1 À x N has been determined by transmittance measurements from absorption edges for several In concentrations. Our results show that the alloys have a direct band gap for all In concentrations and a bowing parameter b ¼ 1.84.
Applied Physics Letters, 2014
![Research paper thumbnail of In-plane and out-of-plane lattice parameters of [11n] epitaxial strained layers](https://attachments.academia-assets.com/46452775/thumbnails/1.jpg)
](Journal of Crystal Growth, 2006
A procedure is proposed to determine the in-plane (a || ) and out-of-plane (a ? ) lattice paramet... more A procedure is proposed to determine the in-plane (a || ) and out-of-plane (a ? ) lattice parameters of ½1 1 n epitaxial cubic strained layers by high resolution X-ray diffraction (HRXRD) rocking curves (RC). The common approach followed to obtain the lattice parameters from asymmetrical diffraction RC of ½0 0 1 grown films, is extended to apply it to ½1 1 n grown layers. Epitaxial pseudomorphic Ge layers were grown on ½0 0 1, ½1 1 0, ½1 1 1, ½1 1 2, ½1 1 3 and ½1 1 4 GaAs substrates to analyze them by HRXRD. Reciprocal space maps (RSM) were also measured to obtain the lattice parameters of these samples. We observe an excellent agreement of the RC and RSM results, which demonstrates the validity of the suggested RC approach. r
Journal of Applied Physics, 2010
InN films were grown on MgO substrates with a -GaN buffer layer using the gas source molecular b... more InN films were grown on MgO substrates with a -GaN buffer layer using the gas source molecular beam epitaxy technique. Initially, at typical growth rates from 0.09 to 0.28 ML/sec and at 500°C substrate temperature, the growth was performed in a layer by layer way as revealed by in situ reflection high-energy electron diffraction ͑RHEED͒. In all samples studied, a critical thickness of ϳ5 ML in InN pseudomorphic layer was measured with a frame by frame analysis of RHEED patterns recorded on video. After reaching critical thickness, the InN films undergo a relaxation process, going from two-dimensional growth to three-dimensional, as evidenced by the transformation of the RHEED patterns that change from streaky to spotty. Depending on the In cell temperature, either nanocolumnar InN or flat cubic final films are grown, as can be corroborated by scanning electron microscopy. The experimental critical thickness ͑h c ͒ value of 5 ML is compared to values calculated from different critical thickness models.
Cancer Research, 2009
Prevalence of methylene tetrahydrofolate reductase (MTHFR) gene mutations in South Indian populat... more Prevalence of methylene tetrahydrofolate reductase (MTHFR) gene mutations in South Indian population was investigated from a total of 608 samples, 420 adults and 188 newborns. Detection of mutation was carried out focussing on the two most common mutations of the MTHFR gene (C677T and A1298C) using PCRbased RFLP method. T-allele frequency was almost similar between the newborns and adults (0.0904, 0.1012). However, a higher T-allele frequency was observed in females (0.1538 and 0.12 in adults and newborns respectively) than males (0.0556 and 0.05 in adults and newborns respectively). In the case of the other mutation, C-allele frequency was almost similar with no sex-bias (0.414 and 0.402 in males and females respectively). Biochemical correlation of fasting plasma total homocysteine to MTHFR genotype revealed mild to moderate hyperhomocysteinemia in mutants. Plasma total homocysteine in males was found to be higher than in females in both normal and mutant individuals. TT homozygous women had higher plasma homocysteine. The high T-allele frequency, elevated plasma homocysteine and low folate intake in women could well be a risk factor for birth defects. The gender bias observed in this autosomal recessive trait was an interesting finding and is discussed.
Single wall carbon nanotubes are dispersed in water with the water-soluble polymer poly-vinylpyrr... more Single wall carbon nanotubes are dispersed in water with the water-soluble polymer poly-vinylpyrrolidone and the surfactant sodium dodecylbenzene sulfonate, and then deposited by evaporative deposition onto degeneratively-doped silicon wafer substrates. These deposits were examined by scanning electron microscopy, which revealed highly-ordered arrays of large single wall carbon nanotube bundles. Various solution concentrations were prepared and deposition conditions were varied to determine their affect on the single wall carbon nanotube arrays. These observations were related to existing lyotropic liquid crystal theory and theories explaining the behavior of carbon nanotubes in solution, which allowed for further development and interpretation of the phase diagram which describes the behavior of single wall carbon nanotubes in lyotropic liquid crystal systems, and how competing liquid crystal systems in the same solution directly affect the ordering of the single wall carbon nanotube arrays.
Nanotechnology, 2012
In an effort to combine group III-V semiconductors with carbon nanotubes, a simple solution-based... more In an effort to combine group III-V semiconductors with carbon nanotubes, a simple solution-based technique for gallium functionalization of nitrogen-doped multi-wall carbon nanotubes has been developed. With an aqueous solution of a gallium salt (GaI 3 ), it was possible to form covalent bonds between the Ga 3+ ion and the nitrogen atoms of the doped carbon nanotubes to form a gallium nitride-carbon nanotube hybrid at room temperature. This functionalization was evaluated by x-ray photoelectron spectroscopy, energy dispersive x-ray spectroscopy, Raman spectroscopy, scanning electron microscopy and transmission electron microscopy S Online supplementary data available from stacks.iop.org/Nano/23/325601/mmedia
Journal of Applied Physics, 2010
Infrared optical studies were carried out in a group of cubic InN samples grown by gas source mol... more Infrared optical studies were carried out in a group of cubic InN samples grown by gas source molecular beam epitaxy on MgO ͑001͒ substrates. Room temperature ͑RT͒ reflectance and low-temperature ͑LT͒ transmittance measurements were performed by using fast Fourier transform infrared spectrometry. Reflectance fittings allowed to establish that -InN films have large free-carrier concentrations present ͑Ͼ10 19 cm −3 ͒, a result that is corroborated by Hall effect measurements. Each sample explored exhibited a different optical absorption edge. The Varshni parameters that describe adequately the optical absorption edge responses with temperature are obtained for the set of samples studied. The observed temperatures changes, from LT to RT, are the lowest reported for III-V semiconductor binary compounds. The temperature coefficient of the conduction band depends on the strength of the electron-phonon interaction ͑e-ph-i͒, as well as on the thermal expansion. It has been predicted that cubic InN has one of the smallest e-ph-i of all III-V compounds, which is corroborated by these results. The variation in values of absorption edges is clearly consistent with the Burstein-Moss and band renormalization effects, produced by high free electron concentrations. It is shown that the conduction band in -InN, analogous to wurtzite InN, follows a nonparabolic behavior.
Applied Physics Letters, 2000
Influence of nonstoichiometry on long-range order and its measurement in ( GaAs ) 1−x Ge 2x and r... more Influence of nonstoichiometry on long-range order and its measurement in ( GaAs ) 1−x Ge 2x and related alloys Ge-related faceting and segregation during the growth of metastable ( GaAs ) 1−x ( Ge 2 ) x alloy layers by metal-organic vapor-phase epitaxy
Journal of Applied Physics, 2009
We have measured the infrared reflectance anisotropy of bulk wurzite ͑0001͒ GaN. Reflectance diff... more We have measured the infrared reflectance anisotropy of bulk wurzite ͑0001͒ GaN. Reflectance difference ͑RD͒ spectra have been determined from the experimental reflectance for light polarization along the sample axes ͓0110͔ and ͓1120͔. The spectra show an optical anisotropy around the frequency of the E 1 ͑LO͒ and A 1 ͑LO͒ GaN phonons. The spectra are influenced by the structural quality of the samples, as revealed by measurements of high resolution x-ray diffraction and Raman scattering. The RD may be originated by inhomogeneous anisotropic broadening of the optical phonons induced by crystal size distribution. Based on the sensitivity of the infrared spectra to the crystalline quality, we propose this infrared technique as a useful optical probe for hexagonal GaN characterization.
Applied Physics Letters, 2004
The optical energy gap of Ge 1Ϫx Sn x alloys has been determined from transmittance measurements,... more The optical energy gap of Ge 1Ϫx Sn x alloys has been determined from transmittance measurements, using a fast-Fourier-transform infrared interferometer. Our results show that the change from indirect to direct band gap occurs at a lower critical Sn concentration (x c ) than the value predicted from the virtual crystal approximation, tight binding, and pseudopotential models. However, a close agreement between the experimental results and the predictions with deformation potential theory is observed. The concentration x c , which is theoretically expected to be 0.09, actually it is observed to lie between 0.10Ͻx c Ͻ0.13.
Applied Physics Letters, 2003
The optical energy gap of Ge 1Ϫx Sn x alloys has been determined from transmittance measurements,... more The optical energy gap of Ge 1Ϫx Sn x alloys has been determined from transmittance measurements, using a fast-Fourier-transform infrared interferometer. Our results show that the change from indirect to direct band gap occurs at a lower critical Sn concentration (x c ) than the value predicted from the virtual crystal approximation, tight binding, and pseudopotential models. However, a close agreement between the experimental results and the predictions with deformation potential theory is observed. The concentration x c , which is theoretically expected to be 0.09, actually it is observed to lie between 0.10Ͻx c Ͻ0.13.
Journal of Applied Physics, 2001
Strained layer superlattices of GaAs/Ge/GaAs and Ge/GaAs/Ge have been grown by magnetron sputteri... more Strained layer superlattices of GaAs/Ge/GaAs and Ge/GaAs/Ge have been grown by magnetron sputtering of different intercalated layer thickness. The samples exhibited good crystallographic quality, pseudomorphic growth on the substrate, as well as superlattice characteristics. Layer periodicity, concentration profile and the thicknesses of the resultant films were examined by high-resolution x-ray diffraction, secondary ion mass spectroscopy, infrared ͑IR͒ optical transmission measurements, and Raman spectroscopy. The heterostructures exhibited IR attenuation peaks in transmission between 0.5 and 1.0 eV, whose energy position was characterized as a function of the thickness of the intercalated thinner layers. The combined results of these techniques reveal that the intended GaAs layers are in fact composed of (GaAs) 1Ϫx (Ge 2 ) x alloys with a few percent Ge content. Experimental and theoretical results have been modeled with the transmittance model, which assumes that light hits the surface normally and takes the alternating layer thicknesses as variable parameters. Both, experimental and theoretical results agree to within 3%.
Spectroscopy Letters, 2014
We have examined the effect of high temperature on single-wall carbon nanotubes under air and nit... more We have examined the effect of high temperature on single-wall carbon nanotubes under air and nitrogen ambient by Raman spectroscopy. We observe the temperature dependence of the radial breathing mode and the G-band modes. The thermal expansion coefficient (b) of the bundled nanotubes is obtained experimentally using the estimated volume from Raman scattering. b behaves linearly with temperature from 0.33 Â 10 À5 K À1 to 0.28 Â 10 À5 K À1 in air and from 0.58 Â 10 À5 K À1 to 0.47 Â 10 À5 K À1 in nitrogen ambient, respectively. The temperature dependence of the radial breathing mode Raman frequencies is consistent with a pure temperature effect.
Recibido el 16 de enero de 2012; aceptado el 9 de febrero de 2012
Applied Physics Letters, 2007
The optical energy gap of Ge 1−x Sn x alloys ͑x ഛ 0.14͒ grown on Ge substrates has been determine... more The optical energy gap of Ge 1−x Sn x alloys ͑x ഛ 0.14͒ grown on Ge substrates has been determined by performing transmittance measurements at 4 K using a fast fourier transform infrared interferometer. The direct energy gap transitions in Ge 1−x Sn x alloys behave following a nonlinear dependence on the Sn concentration, expressed by a quadratic equation, with a so called bowing parameter b 0 that describes the deviation from a simple linear dependence. Our observations resulted in b 0 RT = 2.30± 0.10 eV and b 0 4 K = 2.84± 0.15 eV, at room temperature and 4 K, respectively. The validity of our fit is limited for Sn concentrations lower than 15%.
Journal of Applied Physics, 2001
͑GaAs͒ 1Ϫx ͑Si 2 ) x metastable alloys grown on ͑001͒, ͑110͒, ͑112͒, and ͑111͒ GaAs substrates, w... more ͑GaAs͒ 1Ϫx ͑Si 2 ) x metastable alloys grown on ͑001͒, ͑110͒, ͑112͒, and ͑111͒ GaAs substrates, with Si fractions in the range 0рxр0.43, were studied by Raman scattering. Two modes near the LO and TO modes of GaAs, besides two local modes associated either with Si-As or Si-Ga and Si-Si bonds, are observed in the Raman spectra. The ratio of integrated intensities of TO-like and LO-like modes (I TO /I LO ) is used to evaluate the short range order. It is observed that the zinc blende to diamond transition reported in the literature for these alloys does not influence the Si fraction dependence of the short range order.
Physical Review B, 2001
Infrared reflection experiments were performed on wurtzite InN films with a range of free-electro... more Infrared reflection experiments were performed on wurtzite InN films with a range of free-electron concentrations grown by molecular-beam epitaxy. Measurements of the plasma edge frequencies were used to determine electron effective masses. The results show a pronounced increase in the electron effective mass with increasing electron concentration, indicating a nonparabolic conduction band in InN. We have also found a large Burstein-Moss shift of the fundamental band gap. The observed effects are quantitatively described by the k•p interaction within the two-band Kane model of narrow-gap semiconductors.
Epitaxial single crystal films of Ge, with thickness from 0.2 to 2.4 µm, were grown on GaAs (001)... more Epitaxial single crystal films of Ge, with thickness from 0.2 to 2.4 µm, were grown on GaAs (001) by rf sputtering. These layers were characterized by High Resolution X-Ray Diffraction (HRXRD). Measured rocking curves show that pseudomorphic samples with good structural quality can be obtained by this growth technique. Asymmetrical reflections (115) and (-1-15) are used to determine the in-plane and in-growth lattice parameters of the grown films. From the behavior of these parameters and Ge diffraction peak broadening, with the layer thickness, an experimental value of 1.8 µm is obtained for the critical thickness of Ge grown on GaAs (001).
Journal of Crystal Growth, 2015
In x Ga 1 À x N (001) ternary alloys were grown on GaN/MgO (100) substrates in a plasma assisted ... more In x Ga 1 À x N (001) ternary alloys were grown on GaN/MgO (100) substrates in a plasma assisted molecular beam epitaxy system. We determined the in-plane [001] and in-growth [110] lattice parameters, as well as the bulk lattice parameter of the alloys for different In concentrations by high resolution X-ray diffraction. The In concentration was determined assuming Vegard's law for the alloy lattice parameter. The optical energy gap of In x Ga 1 À x N has been determined by transmittance measurements from absorption edges for several In concentrations. Our results show that the alloys have a direct band gap for all In concentrations and a bowing parameter b ¼ 1.84.
Applied Physics Letters, 2014
Journal of Crystal Growth, 2006
A procedure is proposed to determine the in-plane (a || ) and out-of-plane (a ? ) lattice paramet... more A procedure is proposed to determine the in-plane (a || ) and out-of-plane (a ? ) lattice parameters of ½1 1 n epitaxial cubic strained layers by high resolution X-ray diffraction (HRXRD) rocking curves (RC). The common approach followed to obtain the lattice parameters from asymmetrical diffraction RC of ½0 0 1 grown films, is extended to apply it to ½1 1 n grown layers. Epitaxial pseudomorphic Ge layers were grown on ½0 0 1, ½1 1 0, ½1 1 1, ½1 1 2, ½1 1 3 and ½1 1 4 GaAs substrates to analyze them by HRXRD. Reciprocal space maps (RSM) were also measured to obtain the lattice parameters of these samples. We observe an excellent agreement of the RC and RSM results, which demonstrates the validity of the suggested RC approach. r
Journal of Applied Physics, 2010
InN films were grown on MgO substrates with a -GaN buffer layer using the gas source molecular b... more InN films were grown on MgO substrates with a -GaN buffer layer using the gas source molecular beam epitaxy technique. Initially, at typical growth rates from 0.09 to 0.28 ML/sec and at 500°C substrate temperature, the growth was performed in a layer by layer way as revealed by in situ reflection high-energy electron diffraction ͑RHEED͒. In all samples studied, a critical thickness of ϳ5 ML in InN pseudomorphic layer was measured with a frame by frame analysis of RHEED patterns recorded on video. After reaching critical thickness, the InN films undergo a relaxation process, going from two-dimensional growth to three-dimensional, as evidenced by the transformation of the RHEED patterns that change from streaky to spotty. Depending on the In cell temperature, either nanocolumnar InN or flat cubic final films are grown, as can be corroborated by scanning electron microscopy. The experimental critical thickness ͑h c ͒ value of 5 ML is compared to values calculated from different critical thickness models.