M. Balgos - Academia.edu (original) (raw)

Papers by M. Balgos

Research paper thumbnail of Dynamics of Optically-Generated Carriers in Si (100) and Si (111) Substrate-Grown GaAs/AlGaAs Core-Shell Nanowires

Nanoscale research letters, 2015

GaAs/Al0.1Ga0.9As core-shell nanowires (CSNWs), with average lateral size of 125 nm, were grown o... more GaAs/Al0.1Ga0.9As core-shell nanowires (CSNWs), with average lateral size of 125 nm, were grown on gold nanoparticle-activated Si (100) and Si (111) substrates via molecular beam epitaxy. Room temperature-photoluminescence (RT-PL) from the samples showed bulk-like GaAs and Al0.1Ga0.9As bandgap emission peaks at 1.43 and 1.56 eV, respectively. Higher PL emission intensity of the sample on Si (111) compared to that on Si (100) is attributed to uniform Al0.1Ga0.9As shell passivation of surface states on Si (111)-grown CSNWs. Carrier dynamics in two different temporal regimes were studied. In the sub-nanosecond time scale (300-500 ps), time-resolved radiative recombination efficiency of carriers was examined. In the 0-4 ps range, surface field-driven ballistic transport of carriers was probed in terms of the radiated terahertz (THz) waves. Time-resolved PL measurements at 300 K revealed that the carrier recombination lifetime of the GaAs core on Si (100)-grown CSNWs is 333 ps while that...

Research paper thumbnail of Self-catalyzed growth of GaAs nanorods on silicon via MBE

AIP Conference Proceedings, 2011

ABSTRACT Self-Catalyzed GaAs nanorods (NRs) were grown directly on Silicon (100) and (111) substr... more ABSTRACT Self-Catalyzed GaAs nanorods (NRs) were grown directly on Silicon (100) and (111) substrates via Molecular Beam Epitaxy. Scanning Electron Microscopy shows that the nanorod lengths ranging from 1 to 12 um with diameter between 30 to 200 nm can be achieved. It is established that nanorod growth is favored between 660 to 680 °C, time studies reveal that NRs grow initially via a VLS mechanism and that Ga flux influences the tilt angle of NRs.

Research paper thumbnail of Growth of zincblende GaAs-AlGaAs core-shell nanowires on Si (111) by MBE using AAO-patterned Au nanoparticle catalyst

AIP Conference Proceedings, 2011

ABSTRACT Gold (Au) nanoparticle‐catalyzed GaAs‐AlGaAs core‐shell nanowires were grown by molecula... more ABSTRACT Gold (Au) nanoparticle‐catalyzed GaAs‐AlGaAs core‐shell nanowires were grown by molecular beam epitaxy on Si (111) subtrates. The Au nanoparticles were pre‐patterned using an anodized aluminum oxide template. Scanning electron microscopy reveals nanowire diameters of 130–220 nm, lengths of approximately 6.5 μm and density of 2×108 cm−2. Photoluminescence measurement of the core‐shell nanowires reveals that they are of good optical quality.

Research paper thumbnail of Application of external tensile and compressive strain on a single layer InAs/GaAs quantum dot via epitaxial lift-off

physica status solidi (b), 2013

Tensile and compressive strains via epitaxial lift-off (ELO) techniques were applied on single-la... more Tensile and compressive strains via epitaxial lift-off (ELO) techniques were applied on single-layer InAs/GaAs quantum dots (QDs). At low temperatures, due to the difference in thermal expansion coefficients of the ELO film and host substrate, the ELO QDs film bonded to Si and MgO substrates experienced tensile and compressive strain, respectively. At 13 K, we observed that the photoluminescence (PL) spectra of the ELO film bonded to MgO blueshifts by 10 meV while the ELO film bonded to Si redshifts by 8.5 meV with respect to the ground state of the as-grown sample. The estimated tensile and compressive strains at this temperature were determined by monitoring the valence-band splitting of the GaAs PL peak. The film bonded to Si has a light hole (lh) to heavy hole (hh) energy separation of 4.6 meV, resulting to values of strain, " ¼ 6.049 Â 10 À4 and stress, X ¼ 0.746 Â 10 À3 kbar or 74.6 MPa. On the other hand, the film bonded on MgO has an lh-hh energy separation of 3.7 meV, giving " ¼ 4.8 Â 10 À4 and X ¼ 0.24 Â 10 À3 kbar or 24 MPa. Furthermore, we also observed a reversal of the PL intensity peak between the ground and excited-state transition of the film bonded on silicon only. A plateau-like feature between the two peaks also emerged, indicating the presence of another optical transition, which is enhanced due to application of tensile strain. We associated this peak to the 1LO-phonon replica of the PL transition resulting from the excited state. Based on these observations, this reversal is most likely attributed to the reduction of the carrier-relaxation mechanism from excited states to the ground-state transition upon the application of tensile strain. Finally, the result of this study showed the efficacy of the ELO technique as an alternative way of introducing variable tensile and compressive strain in the InAs/GaAs QD's heterostructure.

Research paper thumbnail of Deep level traps and the temperature behavior of the photoluminescence in GaAs/AlGaAs multiple quantum wells grown on off-axis and on-axis substrates

Journal of Luminescence, 2013

Research paper thumbnail of Observation of picosecond carrier lifetimes in GaAs/AlGaAs single quantum wells grown at 630°C

Journal of Luminescence, 2013

ABSTRACT We report short carrier lifetimes for GaAs/AlGaAs isolated single quantum wells (35, 50,... more ABSTRACT We report short carrier lifetimes for GaAs/AlGaAs isolated single quantum wells (35, 50, and 90 angstrom) grown via molecular beam epitaxy at 630 degrees C. Photoluminescence (PL) measured at 14-150 K show defect transitions that significantly thermalize at similar to 77 K. Carrier lifetimes of the quantum wells, measured via time-resolved PL (TRPL), are similar to 21 ps (35 angstrom), similar to 39 ps (50 angstrom) and similar to 48 ps (90 angstrom). Carrier lifetimes and fluence-dependence measurements suggest that at 77 K where the shallow levels have thermalized, deeper level defects are still present and act as effective carrier traps. The defect density appears to vary inversely with well-width, and this observed behavior is explained on the basis of interface roughness.

Research paper thumbnail of Terahertz emission from GaAs-AlGaAs core-shell nanowires on Si (100) substrate: Effects of applied magnetic field and excitation wavelength

Applied Physics Letters, 2013

ABSTRACT Terahertz (THz) emission from GaAs-AlGaAs core-shell nanowires (CSNW) on silicon (100) s... more ABSTRACT Terahertz (THz) emission from GaAs-AlGaAs core-shell nanowires (CSNW) on silicon (100) substrates was investigated using THz time-domain spectroscopy. The applied magnetic field polarity dependence strongly suggests that THz emission originated from photo-carriers in the CSNWs. Optical excitation of the GaAs-AlGaAs core-shell yielded a wider THz emission bandwidth compared with that of just the GaAs core material. This result is currently attributed to faster carrier lifetimes in the AlGaAs shell. The THz emission spectral data are supported by time-resolved photoluminescence studies.

Research paper thumbnail of Photocarrier Transport and Carrier Recombination Efficiency in Vertically Aligned Si Nanowire Arrays Synthesized Via Metal-Assisted Chemical Etching

Applied Physics Express, 2013

Research paper thumbnail of Shell to core carrier-transfer in MBE-grown GaAs/AlGaAs core–shell nanowires on Si(100) substrates

Journal of Luminescence, 2014

ABSTRACT We report on the shell-to-core carrier-transfer in GaAs/Al0.1Ga0.9As core-shell nanowire... more ABSTRACT We report on the shell-to-core carrier-transfer in GaAs/Al0.1Ga0.9As core-shell nanowires grown on Si(1 0 0) substrates via molecular beam epitaxy. The nanowires are dominantly zincblende and are tilted with respect to the substrate surface. Photoluminescence (PL) excitation spectrosocopy at 77 K revealed an abrupt increase in the GaAs PL intensity at excitation above the Al0.1Ga0.9As shell bandgap which is attributed to shell to core carrier-transfer. More carriers from the Al0.1Ga0.9As transfer to the GaAs at T>90 K, as observed in the time-resolved PL and temperature dependence of the relative PL intensities of GaAs and Al0.1Ga0.9As due to the ionization of the traps within the Al0.1Ga0.9As. Using a coupled rate equation model that takes into account shell to core carrier-transfer, the average recombination time constants of Al0.1Ga0.9As shell τrec,s=400 ps (580 ps) and GaAs core τrec,c=600 ps (970 ps) were obtained from the time-resolved PL at 300 K (77 K). Carrier-transfer time constants τCT=50 ps (55 ps) at 300 K (77 K) were also obtained.

Research paper thumbnail of Dynamics of Optically-Generated Carriers in Si (100) and Si (111) Substrate-Grown GaAs/AlGaAs Core-Shell Nanowires

Nanoscale research letters, 2015

GaAs/Al0.1Ga0.9As core-shell nanowires (CSNWs), with average lateral size of 125 nm, were grown o... more GaAs/Al0.1Ga0.9As core-shell nanowires (CSNWs), with average lateral size of 125 nm, were grown on gold nanoparticle-activated Si (100) and Si (111) substrates via molecular beam epitaxy. Room temperature-photoluminescence (RT-PL) from the samples showed bulk-like GaAs and Al0.1Ga0.9As bandgap emission peaks at 1.43 and 1.56 eV, respectively. Higher PL emission intensity of the sample on Si (111) compared to that on Si (100) is attributed to uniform Al0.1Ga0.9As shell passivation of surface states on Si (111)-grown CSNWs. Carrier dynamics in two different temporal regimes were studied. In the sub-nanosecond time scale (300-500 ps), time-resolved radiative recombination efficiency of carriers was examined. In the 0-4 ps range, surface field-driven ballistic transport of carriers was probed in terms of the radiated terahertz (THz) waves. Time-resolved PL measurements at 300 K revealed that the carrier recombination lifetime of the GaAs core on Si (100)-grown CSNWs is 333 ps while that...

Research paper thumbnail of Self-catalyzed growth of GaAs nanorods on silicon via MBE

AIP Conference Proceedings, 2011

ABSTRACT Self-Catalyzed GaAs nanorods (NRs) were grown directly on Silicon (100) and (111) substr... more ABSTRACT Self-Catalyzed GaAs nanorods (NRs) were grown directly on Silicon (100) and (111) substrates via Molecular Beam Epitaxy. Scanning Electron Microscopy shows that the nanorod lengths ranging from 1 to 12 um with diameter between 30 to 200 nm can be achieved. It is established that nanorod growth is favored between 660 to 680 °C, time studies reveal that NRs grow initially via a VLS mechanism and that Ga flux influences the tilt angle of NRs.

Research paper thumbnail of Growth of zincblende GaAs-AlGaAs core-shell nanowires on Si (111) by MBE using AAO-patterned Au nanoparticle catalyst

AIP Conference Proceedings, 2011

ABSTRACT Gold (Au) nanoparticle‐catalyzed GaAs‐AlGaAs core‐shell nanowires were grown by molecula... more ABSTRACT Gold (Au) nanoparticle‐catalyzed GaAs‐AlGaAs core‐shell nanowires were grown by molecular beam epitaxy on Si (111) subtrates. The Au nanoparticles were pre‐patterned using an anodized aluminum oxide template. Scanning electron microscopy reveals nanowire diameters of 130–220 nm, lengths of approximately 6.5 μm and density of 2×108 cm−2. Photoluminescence measurement of the core‐shell nanowires reveals that they are of good optical quality.

Research paper thumbnail of Application of external tensile and compressive strain on a single layer InAs/GaAs quantum dot via epitaxial lift-off

physica status solidi (b), 2013

Tensile and compressive strains via epitaxial lift-off (ELO) techniques were applied on single-la... more Tensile and compressive strains via epitaxial lift-off (ELO) techniques were applied on single-layer InAs/GaAs quantum dots (QDs). At low temperatures, due to the difference in thermal expansion coefficients of the ELO film and host substrate, the ELO QDs film bonded to Si and MgO substrates experienced tensile and compressive strain, respectively. At 13 K, we observed that the photoluminescence (PL) spectra of the ELO film bonded to MgO blueshifts by 10 meV while the ELO film bonded to Si redshifts by 8.5 meV with respect to the ground state of the as-grown sample. The estimated tensile and compressive strains at this temperature were determined by monitoring the valence-band splitting of the GaAs PL peak. The film bonded to Si has a light hole (lh) to heavy hole (hh) energy separation of 4.6 meV, resulting to values of strain, " ¼ 6.049 Â 10 À4 and stress, X ¼ 0.746 Â 10 À3 kbar or 74.6 MPa. On the other hand, the film bonded on MgO has an lh-hh energy separation of 3.7 meV, giving " ¼ 4.8 Â 10 À4 and X ¼ 0.24 Â 10 À3 kbar or 24 MPa. Furthermore, we also observed a reversal of the PL intensity peak between the ground and excited-state transition of the film bonded on silicon only. A plateau-like feature between the two peaks also emerged, indicating the presence of another optical transition, which is enhanced due to application of tensile strain. We associated this peak to the 1LO-phonon replica of the PL transition resulting from the excited state. Based on these observations, this reversal is most likely attributed to the reduction of the carrier-relaxation mechanism from excited states to the ground-state transition upon the application of tensile strain. Finally, the result of this study showed the efficacy of the ELO technique as an alternative way of introducing variable tensile and compressive strain in the InAs/GaAs QD's heterostructure.

Research paper thumbnail of Deep level traps and the temperature behavior of the photoluminescence in GaAs/AlGaAs multiple quantum wells grown on off-axis and on-axis substrates

Journal of Luminescence, 2013

Research paper thumbnail of Observation of picosecond carrier lifetimes in GaAs/AlGaAs single quantum wells grown at 630°C

Journal of Luminescence, 2013

ABSTRACT We report short carrier lifetimes for GaAs/AlGaAs isolated single quantum wells (35, 50,... more ABSTRACT We report short carrier lifetimes for GaAs/AlGaAs isolated single quantum wells (35, 50, and 90 angstrom) grown via molecular beam epitaxy at 630 degrees C. Photoluminescence (PL) measured at 14-150 K show defect transitions that significantly thermalize at similar to 77 K. Carrier lifetimes of the quantum wells, measured via time-resolved PL (TRPL), are similar to 21 ps (35 angstrom), similar to 39 ps (50 angstrom) and similar to 48 ps (90 angstrom). Carrier lifetimes and fluence-dependence measurements suggest that at 77 K where the shallow levels have thermalized, deeper level defects are still present and act as effective carrier traps. The defect density appears to vary inversely with well-width, and this observed behavior is explained on the basis of interface roughness.

Research paper thumbnail of Terahertz emission from GaAs-AlGaAs core-shell nanowires on Si (100) substrate: Effects of applied magnetic field and excitation wavelength

Applied Physics Letters, 2013

ABSTRACT Terahertz (THz) emission from GaAs-AlGaAs core-shell nanowires (CSNW) on silicon (100) s... more ABSTRACT Terahertz (THz) emission from GaAs-AlGaAs core-shell nanowires (CSNW) on silicon (100) substrates was investigated using THz time-domain spectroscopy. The applied magnetic field polarity dependence strongly suggests that THz emission originated from photo-carriers in the CSNWs. Optical excitation of the GaAs-AlGaAs core-shell yielded a wider THz emission bandwidth compared with that of just the GaAs core material. This result is currently attributed to faster carrier lifetimes in the AlGaAs shell. The THz emission spectral data are supported by time-resolved photoluminescence studies.

Research paper thumbnail of Photocarrier Transport and Carrier Recombination Efficiency in Vertically Aligned Si Nanowire Arrays Synthesized Via Metal-Assisted Chemical Etching

Applied Physics Express, 2013

Research paper thumbnail of Shell to core carrier-transfer in MBE-grown GaAs/AlGaAs core–shell nanowires on Si(100) substrates

Journal of Luminescence, 2014

ABSTRACT We report on the shell-to-core carrier-transfer in GaAs/Al0.1Ga0.9As core-shell nanowire... more ABSTRACT We report on the shell-to-core carrier-transfer in GaAs/Al0.1Ga0.9As core-shell nanowires grown on Si(1 0 0) substrates via molecular beam epitaxy. The nanowires are dominantly zincblende and are tilted with respect to the substrate surface. Photoluminescence (PL) excitation spectrosocopy at 77 K revealed an abrupt increase in the GaAs PL intensity at excitation above the Al0.1Ga0.9As shell bandgap which is attributed to shell to core carrier-transfer. More carriers from the Al0.1Ga0.9As transfer to the GaAs at T>90 K, as observed in the time-resolved PL and temperature dependence of the relative PL intensities of GaAs and Al0.1Ga0.9As due to the ionization of the traps within the Al0.1Ga0.9As. Using a coupled rate equation model that takes into account shell to core carrier-transfer, the average recombination time constants of Al0.1Ga0.9As shell τrec,s=400 ps (580 ps) and GaAs core τrec,c=600 ps (970 ps) were obtained from the time-resolved PL at 300 K (77 K). Carrier-transfer time constants τCT=50 ps (55 ps) at 300 K (77 K) were also obtained.