Shu-Ting Chou

Annealing effect on the Fe/Pt multilayers grown on Al2O3 (0001) substrates

We studied the structure and magnetic properties of molecular-beam epitaxy grown 300 A thick Fe/Pt multilayers with different bilayer thickness and annealing temperature. The Fe/Pt multilayers were deposited on 100 A thick Pt buffer layers at 100 °C on Al2O3 (0001) substrates. The structure of as-deposited Fe/Pt films was fcc(111). While the postannealing temperature ⩾400 °C, an additional FePt(100) orientation was observed. A large coercivity range, namely, 200–16 000 Oe can be tuned by varying the bilayer thickness and annealing temperature.

Influence of doping density on the normal incident absorption of quantum-dot infrared photodetectors

The influences of doping densities at the quantum-dot (QD) region for 30-period InAs∕GaAs quantum-dot infrared photodetectors (QDIPs) are investigated. The InAs∕GaAs QDIPs with a lower doping density can operate at high responsivity and high background-limited-performance temperature. Also observed is the decreasing photocurrent ratio of s∕p-polarized lights for the QDIPs with increasing QD doping density. Compared to the similar photocurrent ratio of s∕p-polarized lights for the GaAs∕AlGaAs quantum-well infrared photodetectors at different applied voltages, the observed voltage-dependent response ratio for QDIPs is attributed to the strong scattering characteristics of QDs occupied with photoexcited electrons for electron transport through the QD region.

InGaAs quantum wire infrared photodetector

We report a 20-layer InxGa1−xAs∕In0.52Al0.24Ga0.24As quantum wire infrared photodetector grown on (001)-axis InP substrate by molecular beam epitaxy. High density InGaAs quantum wires were formed, utilizing the strained-induced lateral-layer ordering process by growing a strain-balanced (GaAs)1.80∕(InAs)2.35 short-period superlattice. This device shows a unique polarized photoresponse which favors the normal-incident infrared radiation polarizing perpendicular to the wire orientation. The photoresponse at 6.3μm exhibited a peak detectivity of 3.13×109cmHz1∕2∕W at 10K.

Effects of Ag pinning layers on the magnetic properties of Fe/Pt multilayer films

Tetragonal L10 ordered Fe∕Pt multilayers were prepared by molecular-beam epitaxy on Pt-buffered MgO(001) substrates at 500°C. An ultrathin Ag (0.5nm) layer was symmetrically inserted into [Fe∕Pt] bilayers with different periods to introduce defects or pinning sites at the interfaces of Ag and Fe∕Pt bilayers. The effects of Ag pinning layers in the Fe∕Pt multilayers have been studied. The insertion of Ag pinning layers effectively reduced the size of magnetic domains. The distribution of angular dependent coercivity of Fe∕Pt multilayers with inserted Ag pinning layers between zero and ten periods, indicates a tendency of the domain-wall motion behavior to be weakened but nucleation mode enhanced.

Quantum-dot infrared photodetectors with p-type-doped GaAs barrier Layers

In this letter, we investigate the effects of p-type-doping concentration in the GaAs barrier layer on the multistack InAs-GaAs quantum-dot infrared photodetectors (QDIPs). The dark current decreases with the p-type-doping density in the GaAs barrier layer. The QDIP with five stacks and a p-type-doping concentration of 1/spl times/10/sup 16/ cm/sup -3/ in the GaAs barrier layer and show an observable spectral response with a peak responsivity of 0.5 A/W at 6 /spl mu/m. With increasing the p-type-doping density up to 1/spl times/10/sup 17/ cm/sup -3/, the QDIP structure shows a reduced photocurrent and a nondetectable response. The QDIPs with p-type-doped GaAs barrier layers and different stack numbers exhibit a saturation photocurrent, where the saturation voltage is proportional to the stack number.

Enhanced Normal-Incident Absorption of Quantum-Dot Infrared Photodetectors With Smaller Quantum Dots

Ten-period InAs-GaAs quantum-dot (QD) infrared photodetectors grown under different In adatom supply procedures are investigated. Two In adatom supply procedures of In shutter 1) always opened and 2) periodically opened/closed are adopted in this letter. Larger QD sizes in both height and diameter and more uniform size distribution are observed for samples grown under an In shutter periodically opened/closed condition. The device with QDs grown under the In shutter always opened condition has revealed shorter detection wavelengths and enhanced normal incident absorption. The phenomenon shows that beside the increase of energy difference between confinement states, smaller QD sizes would also enhance the normal incident absorption predicted for the theoretically zero-dimensional QD structures.

Effects of an ultrathin MgO inserted layer on the magnetic properties of FePt films

Face-centered-tetragonal L10 Fe∕Pt multilayers have been prepared by molecular-beam epitaxy on MgO (001) substrates at 500°C. An ultrathin MgO layer, 1, 3, and 5nm in thickness, respectively, was inserted into the middle of Fe∕Pt multilayers by using an e-beam evaporation without introducing any oxygen source. The insertion of MgO layer effectively reduced the average grain size and the size of magnetic domains. Out-of-plane coercivity of the FePt films decreased with increasing thickness of the MgO insert layer. It was found through the studies of angular dependent coercivity that upon MgO insertion the decrease in coercivity is originated from the weakening of domain-wall motion, while domain rotation is enhanced depicting a decoupling of intergrain interaction.

Temperature dependence of carrier dynamics for InAs∕GaAs quantum dot infrared photodetectors

Temperature-dependent micro-photoluminescence (μ-PL) spectra and the spectral response for the 30-period undoped InAs∕GaAs quantum-dot infrared photodetectors (QDIPs) are investigated. The competition of different transition levels within the spectra is observed. Also observed is the influence of dot size and density on the μ-PL characteristics. Consistently, the mid-infrared spectral response for the fabricated QDIPs exhibit the same energy position as the shifted PL spectra relative to the energy of wetting layer, which indicates the multi-transition mechanisms responsible for the QDIP spectral response.

Formation and Characterization of 1.5-Monolayer Self-Assembled InAs/GaAs Quantum Dots Using Postgrowth Annealing

In this paper, we report that low-density InAs/GaAs quantum dots (QDs) can be formed by postgrowth annealing the samples with 1.5-monolayer (ML) InAs coverage, which is thinner than the critical layer thickness for the Stranski-Krastanov growth. The annealing procedure was performed immediately after the deposition of the InAs layer. The effects of annealing time and annealing temperature on the dot density, dot size, and optical characteristics of the QDs were investigated. The optimum annealing conditions to obtain low-density QDs are longer than 60 s and higher than 500degC . Meanwhile, no luminescence can be observed for the wetting-layer, which may suggest that the postgrowth annealing will make the wetting layer thinner and thus reduce the effects of wetting layer on carrier relaxation and recombination. On the other hand, we observe that a decrease of the PL intensity at the annealing conditions of 60 s and 515degC , which is possibly due to the increasing surface dislocations resulted from the In adatom desorption at higher annealing temperature.

Transport mechanisms and the effects of organic layer thickness on the performance of organic Schottky diodes

Experimental results of static and dynamic characteristics for single-layer hole-only devices based on copper phthalcyanine (CuPc) and pentacene are observed in this article. The contribution to injection currents from electrode has been investigated by varying the thickness of the organic film. From the observation of current density versus bias voltage (J-V) characteristics, it is concluded that the space-charge-limited conductivity is the dominant transport mechanism for the organic Schottky diodes. Accordingly, an increase of the organic layer thickness will increase the trapping energy level. However, even with the thin CuPc film down to 50nm, the dynamic cut-off frequency of the device is still limited to 150Hz. Low hole mobility and large active area of the device are responsible for the phenomenon. Dramatic enhancement of cut-off frequency up to 11kHz can be obtained for the pentacene-based Schottky diodes.