Search Results - (Author, Cooperation:E. I. Chen)

2012-08-21

Nature Publishing Group (NPG)

0028-0836

1476-4687

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Animals ; *Cellular Reprogramming ; Chromatin/genetics/metabolism ; DNA Methylation ; DNA-Binding Proteins/*metabolism ; *Epigenesis, Genetic ; Exons/genetics ; Fibroblasts/metabolism ; Homeodomain Proteins/genetics/metabolism ; Humans ; Induced Pluripotent Stem Cells/*cytology/*metabolism ; Introns/genetics ; Mice ; Poly(ADP-ribose) Polymerases/genetics/*metabolism ; Proto-Oncogene Proteins/*metabolism ; Receptors, Estrogen/genetics/metabolism

1089-7550

AIP Digital Archive

Physics

Data are presented on the 300 K (and 77 K) continuous photopumped laser operation of AlyGa1−yAs-GaAs-InxGa1−xAs quantum-well heterostructures (QWHs) that have been oxidized (H2O vapor+N2, 425 °C) selectively along the high-band-gap heterolayers located on either side of a GaAs waveguide (WG) region with an InxGa1−xAs QW in its center. Transmission electron microscope images show that the oxide-embedded GaAs-InxGa1−xAs WG+QW active region remains unoxidized and thus is sandwiched between electrically insulating AlyGa1−yAs native oxide layers. The thresholds for photopumped 300 K continuous laser operation of the as-grown and oxide-embedded QWHs are approximately equal after differences in the Ar+-laser photopumping efficiencies are considered.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Data are presented on the photopumped laser operation of planar AlAs–GaAs superlattice (SL) minidisk lasers. The SL minidisk (70 A(ring) AlAs, 30 A(ring) GaAs; 100 periods; ∼37 μm diameter) is defined by impurity-induced layer disordering (IILD), followed by wet oxidation (N2+H2O vapor, 400 °C which surrounds the minidisk with a low-refractive-index AlGaAs oxide. The planar minidisks exhibit laser operation at λ∼7540 A(ring), with wider mode separation (Δλ∼13 A(ring)) than disks defined by only IILD (a smaller refractive index step) and cleaved sample edges. The mode separation of Δλ∼13 A(ring) corresponds to disk modes that utilize the perimeter of the oxide-defined disks. In the fabrication of the SL minidisks, IILD forms a structural and doping difference beyond the disk perimeter that acts, in effect, as a p–n junction during etching or wet oxidation. Etch profiles are shown demonstrating this behavior. © 1996 American Institute of Physics.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

Data are presented demonstrating the effect that cavity length, and thus Q, has upon quantum well heterostructure edge-emitting laser diodes that are reduced to microcavity thickness. The lasers, with reduced mode density and enhanced spontaneous emission, are defined vertically by a Ag top-contact mirror and a closely spaced (∼0.9 μm) high-contrast AlAs native oxide-GaAs distributed Bragg reflecting bottom mirror. For shorter and shorter diode lengths (700→70 μm, and still lesser mode density) the light versus current (L–I) characteristic below threshold is at first steeper and steeper (amplified stimulated emission), until, at a diode length of ∼100 μm, the loss in Q and insufficient gain are manifest as a downward bend in the L–I curve and a shift to higher threshold current where bandfilling to a higher state (shorter wavelength) contributes more gain. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Data are presented demonstrating double injection and negative resistance in stripe-geometry oxide-aperture AlyGa1−yAs–GaAs–InxGa1−xAs quantum well heterostructure lasers. The buried oxide laser structures are defined, in current and cavity, by laterally oxidizing the higher Al composition upper and lower cladding layers from a mesa edge (a ridge), thus, forming a narrow oxide-defined buried aperture (∼2μm). Post fabrication annealing (425 °C in N2) removes the negative resistance, indicating that the crystal growth and oxidation processes introduce products such as H and OH in the active region that compensate the dopants. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Data are presented on the 300 K photopumped (pulsed) laser operation of ultrathin (∼0.15 μm) planar microdisk lasers. The tiny lasers employ the wet oxidation of an AlGaAs layer beneath the disks to form semiconductor cavities that are in solid contact but optically isolated from the GaAs substrate. The resulting microcavity lasers operate at low pump intensities and exhibit spectra characteristic of whispering gallery mode microdisk lasers. © 1996 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Data are presented on the 300 K photopumped (pulsed) laser operation of a visible-spectrum (λ=650 nm) AlAs–AlGaAs/InAlP–InGaP quantum-well heterostructure (QWH) crystal that utilizes high-index-contrast AlAs-native-oxide/Al0.6Ga0.4As distributed Bragg reflector mirrors. The mirrors are formed by the lateral oxidation (H2O+N2, 425 °C) of two sets of four "buried'' AlAs layers that are separated by Al0.6Ga0.4As. These mirrors, which create a high-Q cavity in the vertical direction, "sandwich'' a one-wavelength InAlP–InGaP QW active region, thus forming a compact microcavity that "tunes'' the carrier scattering and recombination into a narrow spectrum (∼25 A(ring)) and supports laser operation in the vertical direction. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Data are presented demonstrating a GaAs-based metal–oxide semiconductor field effect transistor employing in the gate region a laterally formed native oxide of AlAs. The gate oxide, formed by a water vapor process, is similar to that used successfully in recently developed semiconductor laser devices. The transistors described here represent an extension of the "wet'' oxidation Al-based III–V native oxide technology employed successfully in light-emitting and laser devices. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Data are presented on the 300-K continuous and pulsed photopumped laser operation of AlyGa1−yAs-GaAs-InxGa1−xAs quantum-well heterostructure (QWH) crystals that utilize large-index-step high-contrast distributed Bragg reflector mirrors. The mirrors are formed by selective lateral oxidation (H2O+N2, 425 °C) of three lower and three upper AlAs layers in the structure, resulting in enhanced cavity Q in the vertical direction. The laterally oxidized mirrors, a small lower and an upper "stack'' that sandwich a lateral waveguide and double QW active region, are of sufficient quality to permit vertical-cavity laser operation of the QWH crystals.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Data are presented on deep-oxide planar buried-heterostructure AlGaAs–GaAs quantum well heterostructure laser diodes fabricated using a self-aligned process that combines native oxide and impurity-induced layer disordering (IILD) technologies. Silicon IILD intermixes the waveguide layers on either side of an active area stripe and allows "wet'' oxidation to penetrate and create a low-index (n∼1.7) deep-oxide structure for electrical and optical confinement. Continuous-wave (cw) threshold currents of ∼3.4 mA are measured for ∼3.5-μm-wide active regions (L∼250 μm), with maximum cw output powers greater than 29 mW/facet and external differential quantum efficiencies as high as 70% (300 K, uncoated facets). © 1994 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Data are presented showing that a GaAs p-n tunnel diode can be modified, and improved, with the introduction of an InxGa1−xAs layer (Lz∼100 A(ring)) in the barrier region to reduce the energy gap (and carrier mass) and increase the tunneling probability without sacrificing the high injection barrier and voltage of GaAs. Peak tunnel current densities in the range (1–1.5)×103 A/cm2 are obtained, with peak-to-valley current ratios of ∼20:1 and voltage "swings'' from peak tunnel current to equal injection current of (approximately-greater-than)1 V (≤1 V for GaAs). The C-doped GaAs(p+)-InGaAs(n+)-GaAs(n+) diodes are grown by metalorganic chemical vapor deposition and are compared to GaAs tunnel diodes fabricated by the usual alloy process (i.e., local liquid phase epitaxy).

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Data are presented on the growth, by metalorganic chemical vapor deposition, and fabrication of n-p (n-up) AlGaAs-GaAs-InGaAs quantum-well heterostructure lasers using a p+-n+ GaAs-InGaAs reverse-biased tunnel junction to contact the n-type GaAs substrate. The lasers operate continuously at 300 K with a threshold of ∼37 mA for a 10-μm-wide native-oxide-defined gain-guided stripe (cavity length ∼375 μm). Comparison tunnel junctions similar to those used in the diode lasers are also fabricated and exhibit low reverse-biased series resistances (∼2.2 Ω, area ∼4.5×larger).

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Data are presented on the photopumped laser operation of an AlAs-AlyGa1−yAs-GaAs-InxGa1−xAs quantum well heterostructure in which the GaAs-InxGa1−xAs active region is embedded, top and bottom, in native oxide. The upper and lower wider gap confining regions of the laser are selectively converted to oxide, leaving the active region intact. The oxidation (H2O+N2, 425 °C) proceeds laterally (perpendicular to the crystal growth direction) from a chemically etched mesa edge. The photopumped oxide-embedded heterostructure operates as a laser continuously at 77 K and pulsed at 300 K. In comparison with the as-grown crystal, the oxidized sample shows no significant laser threshold degradation.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

The growth and fabrication of high-quality vertical distributed Bragg reflectors (DBRs) utilizing layers of InAlP and the AlAs native oxide are reported. The III–V epitaxial structures employed in this work consist of alternating layers of InAlP and AlAs grown on GaAs substrates by low-pressure metalorganic chemical vapor deposition (MOCVD). The DBR mirrors are formed by selective lateral oxidation of the AlAs layers (H2O vapor + N2, 450 °C) resulting in a layered structure of single-crystal InAlP and amorphous AlxOy. The oxidized vertical DBR mirrors having only 4.5 pairs exhibit high reflectivity in the 96%–99% range over a wide spectral region (Δλ∼200 nm). The structural and optical properties of these DBR mirrors have been measured and show that the reflectors are of high quality. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Data (300 and 77 K) are presented on the photopumped laser operation of AlxGa1−xAs–GaAs vertical cavity quantum well heterostructure crystals that exhibit enhanced hot-carrier recombination. The mirrors defining the vertical cavity are formed by selective lateral oxidation (H2O+N2, 425 °C, 30 min) of quadruple AlAs layers separated by lower composition AlxGa1−xAs "stop'' layers in order to create upper and lower high-index-step oxide-semiconductor distributed Bragg reflector mirrors. The Q of the compact vertical-cavity (a microcavity) enhances the spontaneous and stimulated recombination of hot carriers, making possible single mode laser operation at an energy corresponding to the second state of the quantum well. The laser operation can be shifted to the first state by cooling to 77 K and shifting the energy gap towards the vertical cavity resonance. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Data are presented on the reduction of layer intermixing (disordering) in AlGaAs–GaAs quantum well heterostructures (QWH) during high-temperature anneals by an initial low-temperature "blocking'' Zn diffusion. Room-temperature photoluminescence measurements of the increase in the lowest electron-to-heavy-hole transition energy in the QW are used to characterize the extent of layer intermixing. Doped (C and Si) samples annealed (850 °C, 12 h) after a low-temperature blocking Zn diffusion (480 °C) exhibit reductions in energy shift from ∼177 meV to as little as ∼18 meV. Similar effects are also observed, but to a lesser extent, for undoped samples. The improved thermal stability is attributed to a Zn-diffusion induced reduction in the number of column-III vacancies in the active layers and is confirmed by secondary-ion mass spectroscopy measurements. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Data are presented on planar S-bend waveguides fabricated in an AlxGa1−xAs–GaAs p-n quantum well heterostructure (QWH) crystal by a self-aligned process combining Si impurity-induced layer disordering and "wet'' native oxidation. In the process, a deep, low-index (n∼1.7) oxide cladding structure is formed, creating a buried channel in the QWH core layers and defining the routing properties of ∼2.5-μm-wide guides. Deep-oxide S-bend waveguides with 100 μm offsets exhibit low excess bend losses with a 3 dB transition distance less than 140 μm for transverse electric polarization. These bend losses are significantly lower than those measured for oxide-only and disordered-only guides. Excellent optical isolation of the guided signal is observed, indicating very little crosstalk occurs between adjacent guides. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Data are presented demonstrating edge-emitting laser diode operation of AlyGa1−yAs– GaAs–InxGa1−xAs quantum well heterostructures modified by the formation of a buried native-oxide distributed Bragg reflecting (DBR) mirror adding vertical confinement to the longitudinal laser cavity. The bottom DBR mirror, combined with the highly reflective top p-contact metallization (Ag), forms a thin broadband vertical cavity. The auxiliary vertical mirrors are tuned to improve the coupling of the spontaneous emission to the longitudinal lasing mode, resulting in reduced threshold currents and modified emission characteristics below threshold. © 1995 American Institute of Physics.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

Data are presented demonstrating improved laser operation of AlxGa1−xAs-GaAs-InyGa1−yAs quantum well heterostructures modified with buried native oxide current-blocking windows. The windows are formed by low temperature (425 °C) anisotropic "wet'' oxidation of an Al0.9Ga0.1As layer exposed at the facets of metallized laser bars. These window devices operate continuously to powers as high as 248 mW/facet (uncoated, ∼10.5 μm aperture), a ∼25% improvement over nonwindow devices.

Electronic Resource

1077-3118

AIP Digital Archive

Physics

A new form of AlyGa1−yAs-GaAs-InxGa1−xAs quantum well heterostructure (QWH) laser that is confined above and below the active region by an insulating low refractive index native oxide is demonstrated. The laser diodes are defined from a mesa edge by the selective lateral oxidation and anisotropic oxidation of high Al composition AlyGa1−yAs layers (y=0.85, 0.87) located above and below the QW and waveguide active region. This structure provides excellent current and optical confinement, resulting in continuous wave threshold currents of ∼8 mA and maximum output powers (uncoated laser) of 35 mW/ facet for a∼2.5 μm aperture.

Electronic Resource