GIPO newsletter

Virtual Channels for Flowing Magnetic Beads

Professor Jui-che Tsai

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We have demonstrated the feasibility of a novel microfluidic system with virtual channels formed by ‘walls’ of magnetic fields, including collecting channels, transporting channels and function channels. The channels are defined by the nickel patterns. With its own ferromagnetism, nickel can be magnetized using an external magnetic field; the nickel structures then generate magnetic fields that can either guide or trap magnetic beads. A glass substrate is sandwiched between the liquid containing magnetic beads and the chip with nickel structures, preventing the liquid from directly contacting the nickel (Fig. 1). Collecting channels with different shapes are compared (Fig. 2). Moreover, we demonstrate I-, S- and Y-shaped transporting channels can steer magnetic beads smoothly. Also, a switchable trapping channel implemented with a bistable mechanism can perform the passing and blocking of a magnetic bead.

Figure 1

Figure 2

S. H. Tang, H. W. Chiang, M. C. Hsieh, Y. D. Chang, P. F. Yeh, W. Y. Shieh, and J. C. Tsai, “An approach to implement virtual channels for flowing magnetic beads,” Journal of Micromechanics and Microengineering, vol. 24, no. 7, 075016, July 2014.

Gate-Bias Stress Stability of P-Type SnO Thin-Film Transistors Fabricated by RF-Sputtering

Professor I-Chun Cheng
Graduate Institute of Photonics and Optoelectronics, National Taiwan University

Inverted-staggered bottom-gate rf-sputtered p-type SnO thin-film transistors (TFTs) with an atomic layer deposited HfO₂ gate dielectric were demonstrated on glass substrates, and the gate-bias stress stability of these TFTs was investigated. The SnO TFT exhibits a threshold voltage of 2.5 V, a field-effect hole mobility of 0.24 cm²V⁻¹s⁻¹, a sub-threshold swing of 2 V/decade, and an ON/OFF current ratio of 10³. Under gate-bias stress, the transfer characteristics shift with the same polarity as the stress voltage, whereas the sub-threshold swing and field-effect mobility remain almost unaltered. The threshold voltage shifts under various gate-bias stress voltages are well fitted by the stretch-exponential equation. The characteristic trapping time of carriers increases from 1.2 × 10⁴ s to 6.8 × 10⁵ s when the gate-bias stress voltage changes from −12.5 V to −5 V and increases from 1.3 × 10³ s to 4.7 × 10⁴ s when the gate-bias stress voltage decreases from 12.5 V to 5 V. This indicates that the dominant mechanism of the threshold voltage shift is the charge trapping at the interface between the active layer and the gate dielectric or at the gate dielectric near the interface. Larger amounts of threshold voltage shifts observed in the positive gate-bias stress may be caused by the bias-induced adsorption of oxygen on the unpassivated backchannel surface in addition to charge trapping.

Figure 1 (a) Transfer and (b) output characteristics of inverted-staggered bottom-gate SnO TFT.

Figure 2 (a) Transfer characteristics of SnO TFTs at various gate-bias stress times. (b) Threshold voltage shift (Vth) as a function of gate-bias stress time under various gate-bias stress voltages at both polarities.

Ref: I-Chung Chiu and I-Chun Cheng, “Gate-bias stress stability of p-type SnO thin-film transistors fabricated by rf-sputtering,” IEEE Electron Device Letters, vol. 35, pp. 90-92, 2014

3D Analysis of the influence of random indium alloy fluctuations in InGaN quantum wells on the device behavior

Professor Yuh-Renn Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

In this research project, our group worked with Prof. James S. Speck in SSLEEC in UCSB. We work on developing 3D analytical simulation tool to be capable to analyze the carrier transport and recombination behavior in random alloy InGaN quantum well. With the experimental result of indium distribution of InGaN quantum well obtained by Atom Probe tomography provided from UCSB. We analyze the influence of the intrinsic random indium fluctuation in the InGaN quantum wells on the carrier transport, efficiency droop, and emission spectrum in GaN-based light emitting diodes (LEDs). Both real and randomly generated indium fluctuations were used in 3D simulations and compared to quantum wells with a uniform indium distribution. We found that without further hypothesis the simulations of electrical and optical properties in LEDs such as carrier transport, radiative and Auger recombination, and efficiency droop are greatly improved by considering natural nanoscale indium fluctuations. The detail result is published in JAP 116, 113104 (2014).

Fig. 1. (a) The calculated QW conduction band potential map with indium fluctuations at a forward bias of 2.94V. The dark blue color shows the deepest potential (higher indium composition) in the QW. (b) One of the electron eigen wavefunction. The wavefunction localizes in the corresponding high indium region. (c) The calculated emission spectra when including the indium fluctuation at 20A/cm² current density. The blue line is the average result of 20 random cases, the black line is the ideal uniform QW, and the dashed lines represent five different random fluctuation maps. (d) Change of emission peak energy with current density. Ideal QWs with 50% and 100% polarization values are shown for comparison. The blue circle is the emission peak calculated in the indium fluctuation case.

Fig. 2. (a) The simulated structure for modeling the carrier transport, which includes 6 QWs and one AlGaN electron blocking layer (EBL). (b) The calculated conduction band potential at 3.1 V. (c) The radiative recombination rate at a forward bias of 3.1 V. The unit is (1/cm³ s).

Vertically-Aligned Polymer-Stabilized Liquid Crystal (VA-PSLC) with a Curing Voltage

Professor Wing-Kit Choi

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We have recently been developing a new Liquid Crystal (LC) mode known as “Vertically-Aligned Polymer-Stabilized Liquid Crystal (VA-PSLC) with a curing voltage”. In this LC mode, a curing voltage was found to help reduce the otherwise strong scattering effect of a VA-PSLC. This LC mode was placed inside a Fabry-Perot cavity (coated with dielectric mirrors) to achieve a fast-response wavelength-tunable filter. Wavelength tuning range was found to decrease as curing voltage increased, which is consistent with what we expected since molecules had a larger pretilt angle when the curing voltage was higher. Shortening of response time (> 10X) was found as polymer concentration increased since the polymer effect helped improve the response speed. These filters can have potential in wavelength tuning applications such as Wavelength Division Multiplexing (WDM) in telecommunication systems where high speed is desirable. This new LC mode can also find applications where both fast response time and phase-only modulation are desirable (such as LCoS, beam steering, LC lens etc.)

Fig. 1. (Top) VA-PSLC with pretilt angle generated by a curing voltage. (Bottom) Structure of a VA-PSLC Fabry-Perot filter.

Fig. 2. Much reduced scattering (T↑) by adding a curing voltage.

Fig. 3. Shift of resonance peaks by applying voltage to a Fabry-Perot filter filled with VA-PSLC with a curing voltage.

Fig. 4. Shortening of response time as polymer concentration increases.

Ref: Wing-Kit Choi and Yan-Min Li , “Vertically-Aligned Polymer-Stabilized Liquid Crystals (VA-PSLC) with a Curing Voltage for fast-response wavelength tuning Applications”, accepted and to be published in Molecular Crystals and Liquid Crystals in March/April, 2015

Investigation of Junction Thermal Characteristics of Light-Emitting Transistors

Professor Chao-Hsin Wu

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

We demonstrate the DC temperature-variation measurement including thermal-electric feedback coefficient to extract the junction temperature and thermal resistance of the light-emitting transistor (LET). The schematic of the device top view and cross section are shown (Fig. 1). The quantum-wells (QWs) embedded in the base region of the heterojunciton bipolar transistor (HBT) can enhance radiative recombination and carry out the energy by photons instead of phonons. Therefore, self-heating effect can be mitigated.

The thermal-electric feedback coefficient of the LET in this work is -2.07 mV/K under collector current of 2 mA (Fig. 2) and is larger than that (-1.66 mV/K) of the reference HBT. The thermal resistance of the LET is 993.3 ^oC/W at power consumption of 44.19 mW, while that of the reference HBT is 1027 ^oC/W at power consumption of 21 mW (Fig. 3). The large magnitude of the thermal-electric feedback coefficient of the LET can be explained by carrier dynamics mechanism in QWs such as temperature-dependent thermionic emission. By tuning the QW design of the LET, a more temperature-sensitive device can be achieved for future sensor application.

This work has been accepted by IEEE Trans. Electron Devices.

Fig. 1. (a) Schematic device top view layout and (b) cross section of the LET. The emitter area is 18 μm × 18 μm.

Fig. 2. T_a corresponding to several constant V_BE values of P. All curves are at I_C = 2 mA. The dashed lines are linear fitting to the symbols, extended to cross the P = 0 axis.

Fig. 3. R_th under different power dissipation P. The red circle points are the R_th of the LET and the blue rectangular points are the R_th of the HBT.