Szu-Chao Chen

Enhanced Output Power of GaN-Based LEDs With Nano-Patterned Sapphire Substrates

GaN-based light-emitting diodes (LEDs) with an emitting wavelength of 450 nm were grown on nano-patterned sapphire substrates (NPSS) fabricated by nanosphere lithography. The crystalline quality of the epitaxial film could be improved by using the NPSS technique. The output power of LED grown on NPSS was 1.3 and 1.11 times higher than those of LEDs grown on conventional and patterned sapphire substrates at the injection current of 20 mA, respectively. The enhancement in output power could be contributed to the efficiently scattering by NPSS. But some voids formed at the GaN/NPSS interface cause a thermal dissipation problem of NPSS LED operated at high injection current.

Energy levels of GaSb grown by metalorganic chemical vapor deposition

Undoped GaSb epilayers were grown on GaSb and GaAs substrates using the metalorganic chemical vapor deposition (MOCVD) technique, and photoluminescence (PL). Hall effect measurements were used to characterize the undoped epilayers. For undoped GaSb epilayers, four energy levels revealed by PL spectra are shown to be due to acceptors, using KTdp/dEf va Ef‐Ev data obtained from Hall effect results, where p is the hole concentration, Ef and Ev are the Fermi level and the top of valence band, respectively. The acceptor levels lie at 3, 58, 80, and 130 meV above the top of valence band, respectively. The donor level is 15 meV below the bottom of the conduction band. These acceptor levels exist in almost all of the undoped p‐type GaSb epilayers grown at high temperature, while the donor level exists only in the undoped n‐type GaSb epilayers grown by low‐temperature MOCVD. It is found that these levels may be changed using the different growth temperatures.

Critical optical properties of AA-stacked multilayer graphenes

The band structures and optical properties of AA-stacked multilayer graphenes are calculated by the tight-binding model and gradient approximation. For a nL-layer AA-stacked graphene, there are nL peaks at both low and middle frequencies. The threshold energy of odd-layer graphene is much lower than that of even-layer graphene for nL<10. The differences in the electronic structures and optical properties between the odd and even layers are reduced with increasing nL. When nL grows to 30 (200), the spectra of 2D graphene are almost identical to those of 3D graphite at middle (low) frequencies.

Measurement of AlInAsSb/GaInAsSb heterojunction band offset by photoluminescence spectroscopy

We have grown unstrained Al0.66In0.34As0.85Sb0.15/Ga0.64In0.36As0.84Sb0.16 multiple-quantum-well (MQW) structures on InP substrates by metalorganic vapor phase epitaxy. Low-temperature photoluminescence was performed for these MQW structures. By comparing the luminescence peak energies with the theoretical calculations, we estimated the conduction-band offset ratio to be 0.75±0.10 for the Al0.66In0.34As0.85Sb0.15/Ga0.64In0.36As0.84Sb0.16 heterostructure.

Curvature Effects on Magnetoelectronic Properties of Nanographene Ribbons

Magnetoelectronic properties of curved nanographene ribbons are investigated through the Peierls tight-binding model. Those properties are strongly affected by an effectively non-uniform magnetic field and the finite-size effect. The curvature converts quasi-Landau levels into oscillating parabolic energy bands with several band-edge states. Such extra states induce asymmetric prominent peaks in the density of states. The number, height, spacing and frequency of those peaks depend strongly on the field strength and the central angle. Moreover, the spatial symmetry of the Landau wave function is broken. The simple proportionality relationship between the wave function on the two sublattices A and B disappears, which implies that the essential physical properties might be drastically changed. Nanographene ribbons are very different from carbon nanotubes in electronic properties, mainly owing to the boundary conditions.

Normal-incidence intersubband and interband optical transitions in GaSb-InAs superlattices

Two interesting interband and intersubband optical transitions in the range of infrared wavelength were demonstrated using a type II GaSb-InAs superlattice structure. The interband transition results from the coupling between the wave functions of the first conduction and the first heavy-hole subband when Zn-doped GaSb p-type cap and buffer layers are used. On the other hand, the intersubband transition, which is a result of the strong mixing of the heavy-hole band and the light-hole band, was achieved by using InAs n-type cap and buffer layers. The wavelengths of interband and intersubband transitions is in the ranges of 3-5 /spl mu/m and 8-14 /spl mu/m, respectively. Consequently, there is a possibility of fabricating infrared photodetectors with the GaSb-InAs superlattices.<<ETX>>

High quality undoped n‐type GaSb epilayers by low‐temperature metalorganic chemical vapor deposition

We obtain undoped n‐type GaSb epilayers by low‐temperature metalorganic chemical vapor deposition at a low growth temperature of 450u2009°C reproducibly. Different conduction types of GaSb and different energy levels are compared by photoluminescence spectra. For n‐type undoped GaSb, the FWHM of dominant peak and bound exciton are 11 and 1.3 meV, respectively. Because semi‐insulating GaSb substrates cannot be obtained, we cannot use the Hall effect to determine the carrier concentration and mobility of the homoepilayer. In order to identify the conduction types of GaSb, ohmic contact and Schottky barrier are made by Au/Ge/Ni and Au, respectively. The concentrations of undoped n‐type GaSb homoepilayers obtained from I‐V and C‐V measurements are 1.44×1017–3.0×1017 cm−3, respectively. The mobility and concentration of undoped p‐type GaSb heteroepilayers are 758 cm2/Vu2009s and 9.0×1015 cm−3 at 300 K, respectively.

Improvement of AlGaInP light emitting diode by sulfide passivation

The brightness of AlGaInP light emitting diodes (LEDs) has been raised by a factor of 1.12 at 20 mA by sulfide passivation. Meanwhile, the sulfide also can decrease leakage current of AlGaInP LEDs at -2 V to nearly one thousandth of that in the as-fabricated device. The possible causes for the brightness increase of AlGaInP LEDs after sulfide treatment including surface roughness, reduction of Fresnel loss, and effective injection of carriers were demonstrated.

Improvement of AlInP-AlGaInP MQW light-emitting diode by meshed contact layer

The optical output power of the AlInP-AlGaInP light-emitting diodes was pronouncedly enhanced 1.45 times at 20 mA without degrading the electrical characteristics by introducing the meshed GaAs contact layer. Both theoretical simulation and experimental analysis were used to demonstrate the feasibility of the proposed structure.

Optical and structural characterization of InAs/GaSb superlattices

InAs/GaSb superlattices sandwiched between conventional InAs layers were grown by low pressure metal organic chemical vapor deposition. Period and roughness of the superlattices were examined by field emission transmission electron microscopy. Room temperature infrared absorption spectra for InAs/GaSb superlattices were obtained by Fourier-transform infrared spectroscopy. The effects of varying the doping levels and thicknesses of the InAs sandwiching layers on the absorption spectra of InAs/GaSb superlattices were studied. It was found that by choice of suitable doping levels and cap/buffer thicknesses, the resulting fermi level equalization (as in normal homo or heterojunctions) thereby allowed the setting or “pinning” of the superlattice Fermi level to any desired value within the range made available by the original bulk material characteristics in conjunction with the doping conditions. When the thicknesses of the InAs sandwiching layers became less than 1 μm, the sandwiching effect and the intersubb...