Chiao Chang

Superradiant modes in Fibonacci quantum wells under resonant conditions

It is first presented that superradiant modes exist in Fibonacci quantum wells within the exact regions that are obtained using the gap map diagram, rather than the traditional resonant Bragg condition. The results show that three limited regions are derived from the diagram, which correspond to bandgaps with widths that differ from each other. The regions in which the superradiant modes do not occur are also defined clearly. Moreover, the proposed method can be used to determine whether superradiant modes occur in multiple quantum wells that have non-periodical arrangements, including quasiperiodic sequences and correlated disorder sequences.

Temperature-dependent electroluminescence from GeSn heterojunction light-emitting diode on Si substrate

The electroluminescence from a Ge/GeSn/Ge p–i–n light-emitting diode on Si was investigated under different temperatures ranging from 25 to 150 K. The diode was operated at a low injection current density of 13 A/cm2. We obtained no-phonon- and phonon-assisted replicas in emission spectra. Also, the relationship between indirect bandgap energy and temperature was investigated. The temperature-dependent bandgap energy followed Varshnis empirical expression with α = 4.884 × 10−4 eV/K and β = 130 K.

Room-temperature 2-μm GeSn P-I-N homojunction light-emitting diode for inplane coupling to group-IV waveguides

We report the electroluminescence of a planar p-i-n diode based on an undoped GeSn layer where the p- and n-type electrodes are fabricated by using the CMOS process of ion implantation. The measurement shows a broad spectrum at a peak energy located below the bulk bandgap of Ge associated with indirect optical transition analyzed by taking into account composition- and strain-dependent modeling. This work provides an alternative approach to the fabrication of GeSn-based p-i-n light-emitting diodes as well as moving towards the integration with waveguided on-chip group IV photonic devices.

Quasi-Bragg conditions in Thue-Morse dielectric multilayers.

The existence of quasi-Bragg conditions (QBCs) in Thue-Morse dielectric multilayers (TMDMs), based on high reflectance and large forbidden gap regions in its transmission spectra, is presented. In contrast to a single traditional Bragg condition (TBC) in periodic bilayers, there are three QBCs in TMDMs. The formula for each of these QBCs is derived. The results show that one of these QBCs just overlaps the TBC. However, a singular point with perfect transmission exists in this QBC at the quarter-wave stack, at which high reflection exists in the periodic bilayers.

Strongly localized modes in one-dimensional defect-free magnonic quasicrystals

Signal storage in magnonic quasicrystals using a slow spin-wave mode, rather than the quasinormal mode of traditional periodic magnonic crystals, is proposed, which is analogous to the slow light mode in the field of optics. Compared to traditional materials, richer and more wavelength-selective sharp resonances are achieved using the quasicrystals with a fewer number of layers, because of the peculiar fractal transmission spectra of quasicrystals. The number of sharp resonance and the quality factor for the sharp resonances in the transmission spectra also increases as the generation order of the magnonic quasicrystal increases. This generic nature allows the storage of signals using spin wave, for a wide range of quasiperiodic systems.

Comparative investigation of Schottky barrier height of Ni/n-type Ge and Ni/n-type GeSn

We report an investigation of the Schottky barrier height (SBH) of Ni/n-type Ge and Ni/n-type GeSn films that is annealed at a wide range of temperatures. Both voltage- and temperature-dependent current–voltage (I–V) measurements are performed. From the analysis of these nonlinear I–V traces, the SBH is found and the results shows that the SBH of Ni/n-type GeSn (a) is smaller than that of Ni/n-type Ge and (b) decreases with the Sn content of the surface GeSn layer associated with the thermal annealing. By modeling the composition- and strain-dependent energy bandgap (Eg), the relationship between the SBH and Eg is established and it is found that SBH/Eg ∼0.8. These results suggest that the GeSn film could serve as an interfacial layer for the reduction of the SBH in Ge-based electronic devices that are desirable for applications.