Y. Tang

Polarized cathodoluminescence study of selectively grown self-assembled InAs/GaAs quantum dots

We have examined the optical properties of self-assembled InAs quantum dots (QDs) grown on prepatterned GaAs(001) substrates with polarization sensitive and time-resolved cathodoluminescence (CL) imaging and spectroscopy techniques. The InAs QDs were formed using a novel application in self-assembled molecular beam epitaxial growth, which entailed the growth of InAs on preformed [110]-oriented stripe mesas. Interfacet In adatom migration occurred along the stripe side-walls during growth, enabling the selective formation of linear arrays of InAs QDs on the stripe mesas. The total InAs deposition needed to induce the two-dimensional to three-dimensional morphology change on the stripes is less than that required to initiate QD formation on the unpatterned substrates. The QDs formed on the mesa top were found with a luminescence distribution redshifted relative to QDs in the valley region, indicating that QDs with a larger average size were formed on the mesa top. The lower density of QDs in the valley reg...

Linearly polarized and time-resolved cathodoluminescence study of strain-induced laterally ordered (InP)2/(GaP)2 quantum wires

The optical properties of (InP)2/(GaP)2 bilayer superlattice (BSL) structures have been examined with linearly polarized cathodoluminescence (CL), time-resolved CL spectroscopy, and cathodoluminescence wavelength imaging. An In and Ga composition modulation of ∼18% forms during the metalorganic chemical vapor deposition growth of short period (InP)2/(GaP)2 bilayer superlattices. Transmission electron microscopy showed a period of ∼800u2009A along the [110] direction, resulting in coherently strained quantum wires. A strong excitation dependence of the polarization anisotropy and energy of excitonic luminescence from the quantum wires was found. The results are consistent with a phase-space and band filling model that is based on a k⋅p and two dimensional quantum confinement calculation which takes the coherency strain into account. CL images reveal that defects in the BSL originate from the GaAs substrate and/or the initial stages of InGaP growth. The effects of defects on the band filling, carrier relaxation...

Vibration energy harvester with low resonant frequency based on flexible coil and liquid spring

This paper reports an electromagnetic vibration-energy harvester with low resonant frequency based on liquid spring composed of ferrofluid. Cylinder magnet array formed by four disc NdFeB magnets is suspended by ferrofluid in a laser-machined acrylic tube which is wrapped by flexible planar coil fabricated with microfabrication process. The magnet array and coil are aligned automatically by the ferrofluid. Restoring force when the magnet array is deviated from the balance position is proportional to the deviated distance, which makes the ferrofluid work as a liquid spring obeying Hooks law. Experimental results show that the electromagnetic energy harvester occupying 1.8u2009cc and weighing 5 g has a resonant frequency of 16u2009Hz and generates an induced electromotive force of Vrmsu2009=u20092.58u2009mV (delivering 79 nW power into matched load of 21 Ω) from 3u2009g acceleration at 16u2009Hz.

An optical method for studying carrier diffusion in strained (InP)2/(GaP)2 quantum wires

The carrier transport in strain-induced laterally ordered (InP)2/(GaP)2 quantum wire (QWR) samples was examined with a noncontact Haynes–Shockley diffusion measurement which utilized time-resolved scanning cathodoluminescence. An anisotropy in ambipolar diffusion along the [110] and [110] directions (perpendicular and parallel to the QWRs, respectively) was observed. The temperature dependence of this anisotropy was measured, revealing that carrier diffusion along the QWR direction is thermally activated.

Spatial variations in luminescence and carrier relaxation in molecular beam epitaxial grown (InP)2/(GaP)2 quantum wires

Phase separation in III–V semiconductors has led to a unique method for fabricating quantum wires via a strain induced lateral ordering process. Quantum wire (QWR) arrays were formed during the gas source molecular beam epitaxial (MBE) growth of (InP)2/(GaP)2 bilayer superlattices (BSLs) and were studied by time-resolved and linearly polarized cathodoluminescence. Nonlinear optical properties, such as phase-space filling effects, were observed to be indicative of the QWR nature of the samples. Samples prepared by gas source MBE were found to have a greater uniformity, smaller QWRs, and higher optical quality in comparison to those obtained by metal–organic chemical vapor deposition. Misfit dislocations also formed in one of the BSL samples, indicating a partial strain relaxation at the GaAs/InGaP and BSL/InGaP interfaces. The carrier relaxation, transport, and collection in the QWRs were studied with time-resolved cathodoluminescence.

Ambipolar diffusion anisotropy induced by defects in nipi‐doped In0.2Ga0.8As/GaAs multiple quantum wells

The influence of strain‐induced defects on the ambipolar diffusive transport of excess electrons and holes in the δ‐doped InGaAs/GaAs multiple quantum well system has been examined with a new technique called electron‐beam‐induced absorption modulation (EBIA). The excess carrier lifetime and diffusion coefficient are obtained by a one‐dimensional diffusion experiment that utilizes EBIA. An anisotropy in the ambipolar diffusion along both high‐symmetry 〈110〉 directions is found, and this is seen to correlate with the distribution of dark line defects observed in cathodoluminescence.

Electron beam-induced absorption modulation imaging of strained In0.2Ga0.8As/GaAs multiple quantum wells

We have examined the effects of electron‐hole plasma generation on excitonic absorption phenomena in nipi‐doped In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) using a novel technique called electron beam‐induced absorption modulation imaging. The electron‐hole plasma is generated by a high‐energy electron beam in a scanning electron microscope and is used as a probe to study the MQW absorption modulation. The influence of structural defects on the diffusive transport of carriers is imaged with a μm‐scale resolution.

Time‐resolved cathodoluminescence study of carrier relaxation in strained (InP)2/(GaP)2 quantum wires

The carrier relaxation kinetics and nonlinear optical properties of strain‐induced laterally ordered (InP)2/(GaP)2 quantum wire (QWR) samples were examined with time‐resolved cathodoluminescence. A temperature dependence of the QWR luminescence decay time reveals that thermal activation of carriers in the QWR and transfer to and from In0.49Ga0.51P barriers plays an important role in determining the measured lifetimes. The presence of disorder in the QWRs was found to induce inhomogeneous regions which exhibit large variations in carrier capture and band filling.

Absorption modulation induced by electron beam excitation of strained In0.2Ga0.8As/GaAs multiple quantum wells

The effects of excess carrier generation on excitonic absorption phenomena in nipi‐doped In0.2Ga0.8As/GaAs multiple quantum wells (MQWs) was examined using a novel technique called electron beam‐induced absorption modulation imaging. The nipi‐doping induced barrier height is determined by measuring the frequency response of the absorption modulation as a function of temperature and employing a model which is based on thermal excitation of carriers in the limit of Boltzmann statistics. Spatial steps in the absorption modulation which correlate with the positions and orientation of dark line defects imaged in cathodoluminescence are observed. These results indicate the existence of defects in the MQWs which impede the ambipolar diffusive transport of the spatially separated electron‐hole plasma.

Stackable dual-layer coil based on wafer-level transfer technique for electromagnetic energy harvester

This paper reports a stackable dual layer coil based on ZnO sacrificial layer and thermal-release-tape transfer technique for electromagnetic energy harvester. The power density of electromagnetic energy harvester can be improved significantly using multilayer coil with high coil turns. Five of microfabricated dual-layer coil plates are stacked for 450 turns for an electromagnetic vibration-energy harvester, which produces 116 μW from 3.6 g vibration at resonant frequency of 160 Hz.