光电所所讯

Broadband optical waveguide couplers with arbitrary coupling ratios designed using a genetic algorithm

Professor Ding-Wei Huang

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

台湾大学光电所黄定洧教授

In this work, we present a generalized design of broadband optical waveguide couplers with arbitrary coupling ratios on the silicon-on-insulator platform. The device is segmented into 34 short sections, where the propagation constant and the coupling coefficient of each section are viewed as variables during the optimization process. The optimal variable combination is determined by a genetic algorithm. We can achieve a performance superior to that of other design methods with fewer degrees of freedom. For 75%/25%, 50%/50%, 25%/75%, and 0%/100% couplers, the device lengths are 34 μm and the ±2% bandwidths are all in excess of 100 nm at the central wavelength of 1580 nm.

Po-Han Fu, Yi-Chou Tu, and Ding-Wei Huang* (2016, Dec.), “Broadband optical waveguide couplers with arbitrary coupling ratios designed using a genetic algorithm,” Optics Express, 24 (26), pp.30547-30561. (SCI)

Fig. 1. Schematic of the broadband optical waveguide coupler.

Fig. 2. Output power versus wavelength for λ = 1500 – 1660 nm calculated by the CMT, BEM and FEM.

Mechanical Bending Effect on the Performance of Flexible Complementary Oxide-TFT-Based Circuits Employing n-ZnO and p-SnO TFTs

Professor I-Chun Cheng

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

台湾大学光电所陈奕君教授

Complementary thin-film transistor (TFT) circuitry consisting of inverters is the building block for realizing integrated logic circuits on large-area and/or flexible substrates. Although numerous oxide-TFT circuits have been reported previously, most of them were based on n-channel MOSFET (NMOS) technology. Here, we report flexible fully oxide-based complementary metal–oxide–semiconductor (CMOS) inverters and ring oscillators by the monolithic integration of flexible n-channel zinc oxide (ZnO) and p-channel tin monoxide (SnO) thin-film transistors (TFTs) on polyimide substrates. The TFTs have an inverted-staggered bottom-gated structure and were fabricated by a low-temperature rf-sputtering technique. The static voltage gain of a flexible oxide-TFT-based CMOS inverter with a geometric aspect ratio of 5 is ~12 at a supplied voltage (V_DD) of 12 V. An oscillation frequency of ~18.4 kHz is obtained from a five-stage flexible oxide-TFT-based CMOS voltage control ring oscillator at V_DD of 12 V. The mechanical bending effect on the electrical performance of the on-foil devices and circuits is then investigated by mechanical bending tests. The results show that the static voltage gain of the CMOS inverter and the peak-to-peak voltage and oscillation frequency of the ring oscillator decrease as the outward bending curvature (tensile strain) increases, while the influence of the inward bending curvature mechanical (compressive strain) is negligible.

Fig. 1. (a) Static voltage transfer characteristics and voltage gain curves at different supplied voltages of the flexible oxide-TFT-based CMOS inverter. (b) Static voltage transfer characteristics of the CMOS inverter operated at V_DD = 12 V under various bending levels. Inset: The static gain value of the CMOS inverter as a function of bending curvature.

Fig. 2. (a) Output waveform of a five-stage flexible CMOS ring oscillator. (b) Oscillation frequency and peak-to-peak voltage value as a function of bending curvature.

Application of localization landscape theory of disorder in semiconductors. Application to carrier localization, absorption, transport and recombination in light emitting diodes

Professor Yuh-Renn Wu's laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

台湾大学光电所吴育任教授

This work was the collaboration work with France team in Ecole polytechnique in Paris and UCSB SSLEC in united states under the support of Taiwan-ANR collaboration project. The goal is to directly apply the localization landscape theory into the simulation program in order to achieve an efficiency way for obtaining the effective quantum potential. In the first PRB paper, we try to prove that by solving the localization landscape u to obtain the effective quantum potential 1/u, we can bypass the time consuming process of solving Schrodinger equation eigen value problems.

Fig. 1. Schematic structure of the self-consistent Poissonlandscape model allowing us to bypass solving the Schrödinger equation. [1]

Fig. 2. (a), (c) The conduction band potential (E_c) and effective quantum confining potential (W_c) for m-plane (top) and c-plane (bottom) double-period SLs. (b), (d) The corresponding integrated density of states (IDOS). [2]

As shown in Fig.2, by solving the localization landscape, we can obtained the effective quantum potential Wc as shown in red line in Fig. 2(a) and 2(c). By applying the distribution of quantum potential to obtain the integrated density of states, it would be close to the result of Schrodinger eigen solver. However, the speed is a few order faster.

In the second paper, we try to apply the landscape theory in the 3D modeling to explain the Urbach tails in semiconductors related to compositional disorder. The Urbach tail observed in InGaN alloy quantum wells of solar cells and LEDs by biased photocurrent spectroscopy is shown to be characteristic of the ternary alloy disorder. The broadening of the absorption edge observed for quantum wells emitting from violet to green (indium content ranging from 0% to 28%) corresponds to a typical Urbach energy of 20 meV. Our 3D model is used for obstaining the effective potential in disorder. This model incorporating compositional disorder accounts well for the experimental broadening of the Urbach tail of the absorption edge. For energies below the Urbach tail of the InGaN quantum wells, type-II well-to-barrier transitions are observed and modeled. This contribution to the below-band-gap absorption is particularly efficient in near-ultraviolet emitting quantum wells. When reverse biasing the device, the well-to-barrier below-band-gap absorption exhibits a red-shift, while the Urbach tail corresponding to the absorption within the quantum wells is blue-shifted, due to the partial compensation of the internal piezoelectric fields by the external bias. The good agreement between the measured Urbach tail and its modeling by the localization theory demonstrates the applicability of the latter to compositional disorder effects in nitride semiconductors.

Fig. 3. Absorption curves for InGaN/GaN structures of different indium composition at 0 V computed using the 3D theoretical model incorporating compositional disorder and QW electric field as a function of different smoothing parameters (dashed line). The responsivity curves measured by BPCS of InGaN/GaN solar cells of different indium composition at 0 V are also shown (continuous lines). [2]

In the third paper, we finally apply this model in studying the carrier transport and recombination. We provide a novel method to account for quantum disorder effects into the classical drift-diffusion model of semiconductor transport through the localization landscape theory. Quantum confinement and quantum tunneling in the disordered system change dramatically the energy barriers acting on the perpendicular transport of heterostructures. In addition, they lead to percolative transport through paths of minimal energy in the two-dimensional (2D) landscape of disordered energies of multiple 2D quantum wells. This model solves the carrier dynamics with quantum effects self-consistently and provides a computationally much faster solver when compared with the Schrödinger equation resolution. The theory also provides a good approximation to the density of states for the disordered system over the full range of energies required to account for transport at room temperature. The current-voltage characteristics modeled by three-dimensional simulation of a full nitride-based light emitting diode (LED) structure with compositional material fluctuations closely match the experimental behavior of high-quality blue LEDs. The model allows also a fine analysis of the quantum effects involved in carrier transport through such complex heterostructures. Finally, details of carrier population and recombination in the different quantum wells are given.

Fig. 4. (a), (b) The perspective views along the z direction of vertical transport of the 1/u_e and z component of current (J_z), respectively. (c), (d) The 1/u_e and J_z value in the midplane (x-y plane) of the third QW. All figures are solved by 1/u-Poisson-DD model, where the LED current density is 20 A.cm^-2.

All these simulation tools are shared at http://yrwu-wk.ee.ntu.edu.tw, where users in academics can download for usages.

References:

1. Marcel Filoche, Marco Piccardo, Yuh-Renn Wu, Chi-Kang Li, Claude Weisbuch, and Svitlana Mayboroda, "Localization landscape theory of disorder in semiconductors I: Theory and modeling", Phys. Rev. B, 95, 144204, April, 2017. DOI: 10.1103/PhysRevB.95.144204

2. Marco Piccardo, Chi-Kang Li, Yuh-Renn Wu, James S. Speck, Bastien Bonef, Bob Farrell, Marcel Filoche, Lucio Martinelli, Jacques Peretti, and Claude Weisbuch, "Localization landscape theory of disorder in semiconductors II: Urbach tails of disordered quantum well layer", Phys. Rev. B, 95, 144205, April, 2017. DOI: 10.1103/PhysRevB.95.144205

3. Chi-Kang Li, Marco Piccardo, Li-Shuo Lu, Svitlana Mayboroda, Lucio Martinelli, Jacques Peretti, James S. Speck, Claude Weisbuch, Marcel Filoche, and Yuh-Renn Wu *, "Localization landscape theory of disorder in semiconductors III: Application to transport and radiative recombination in light emitting diodes.", Phys. Rev. B, 95, 144206, April, 2017. DOI: 10.1103/PhysRevB.95.144206