Tung-Po Hsieh

Single photon emission from an InGaAs quantum dot precisely positioned on a nanoplane

This work demonstrates single photon emissions from a site-controlled quantum dot (QD) grown on a self-constructed nanoplane. The size of the nanoplane on the micron-sized multifacet structure is accurately controlled by a low surface reducing rate (∼16nm∕min). Single QD spectral lines were resolved and identified. The antibunching behavior reveals that single photons are emitted from the positioned QD.

Electroreflectance studies of InAs quantum dots with InxGa1−xAs capping layer grown by metalorganic chemical vapor deposition

Electroreflectance spectroscopy was used to study the effect of InxGa1−xAs capping layer on InAs quantum dots grown by metalorganic chemical vapor deposition. The optical transitions of the quantum dots and the InxGa1−xAs capping layer were well resolved. The energy shifts in the InxGa1−xAs capping layer show a different trend as compared to a series of referent InxGa1−xAs quantum wells. These results support the concept of strain-driven alloy decomposition during the InxGa1−xAs layer overgrowth.

Toward Omnidirectional Light Absorption by Plasmonic Effect for High-Efficiency Flexible Nonvacuum Cu(In,Ga)Se2 Thin Film Solar Cells

We have successfully demonstrated a great advantage of plasmonic Au nanoparticles for efficient enhancement of Cu(In,Ga)Se2(CIGS) flexible photovoltaic devices. The incorporation of Au NPs can eliminate obstacles in the way of developing ink-printing CIGS flexible thin film photovoltaics (TFPV), such as poor absorption at wavelengths in the high intensity region of solar spectrum, and that occurs significantly at large incident angle of solar irradiation. The enhancement of external quantum efficiency and photocurrent have been systematically analyzed via the calculated electromagnetic field distribution. Finally, the major benefits of the localized surface plasmon resonances (LSPR) in visible wavelength have been investigated by ultrabroadband pump-probe spectroscopy, providing a solid evidence on the strong absorption and reduction of surface recombination that increases electron-hole generation and improves the carrier transportation in the vicinity of pn-juction.

1.55μm emission from InAs quantum dots grown on GaAs

We report a comparative study on the growth of InAs quantum dots (QDs) on GaAs by metalorganic chemical vapor deposition using triethylgallium (TEGa) and trimethylgallium (TMGa) for the GaAs cap layer. QDs exhibit 1.3μm photoluminescence (PL) at room temperature, as the GaAs cap layer is directly grown on the QDs. The PL emission can be extended to 1.49μm when an In0.25Ga0.75As overgrown layer is inserted between the cap layer and the InAs QDs. The use of TMGa or TEGa for the growth of the GaAs cap layer is essential for a further increase in the emission wavelength of the InAs QDs. Strong PL emission at 1.55μm with a linewidth of 28meV can be obtained as the GaAs cap layer is grown by TEGa, while the optical properties degrade severely when using TMGa.

Ultrafast carrier dynamics in Cu(In,Ga)Se 2 thin films probed by femtosecond pump-probe spectroscopy

Ultrafast carrier dynamics in Cu(In,Ga)Se₂ films are investigated using femtosecond pump-probe spectroscopy. Samples prepared by direct sputtering and co-evaporation processes, which exhibited remarkably different crystalline structures and free carrier densities, were found to result in substantially different carrier relaxation and recombination mechanisms. For the sputtered CIGS films, electron-electron scattering and Auger recombination was observed, whereas for the co-evaporated CIGS films, bandgap renormalization accompanied by band filling effect and hot phonon relaxation was observed. The lifetime of defect-related recombination in the co-evaporated CIGS films is much longer than that in the direct-sputtered CIGS films, reflecting a better quality with higher energy conversion efficiency of the former.

Enhanced light emission from InAs quantum dots in single-defect photonic crystal microcavities at room temperature

The optical properties of InAs quantum dots with photonic crystal microcavity emitting near 1.3μm were investigated at room temperature. The photoluminescence (PL) intensity for quantum dots in cavity was enhanced by two orders of magnitude. The large PL enhancement was attributed to the effects combining the improved extraction efficiency and the enhanced spontaneous emission rate due to the Purcell effect. A threefold Purcell enhancement is observed at room temperature, which is predominantly achieved by the very small mode volume of the photonic crystal microcavity.

Growth of low density InGaAs quantum dots for single photon sources by metal?organic chemical vapour deposition

We report the preparation of low density self-assembled InGaAs on GaAs grown by metal–organic chemical vapour deposition for single photon sources. Through using a set of optimized growth parameters, including the arsine partial pressure, total coverage of quantum dots, and growth temperature, high optical quality quantum dots with density as low as 5 × 106 cm−2 have been obtained. Using local optical excitation through a sub-micron aperture of a single quantum dot, its spectral lines associated with the exciton, biexciton, multi-exciton, and charged exciton have been resolved and identified. Photon correlation measurements show that the single quantum dot can successfully emit antibunched photons.

Selective growth of InAs quantum dots on patterned GaAs

We report selective growth of InAs self-assembled quantum dots (QDs) on nano-ridges (30–50nm) formed by wet chemical etching and epitaxial growth processes. The QDs formed on the ridges exhibit distinctive characteristics from those on the planar region between the ridges in terms of physical shape as well as optical property. The QDs, which align one by one on the top of the ridges, are of an asymmetric shape along the [011] and [011] directions. By defining the width of ridge and the monolayers of QDs, we are able to control the height and the width of these QDs. Compared to the QDs on the planar region between the ridges, the luminescence from QDs on the ridges exhibits a shorter wavelength, which is attributed to higher compressive strain in these QDs.

High extractive single-photon emissions from InGaAs quantum dots on a GaAs pyramid-like multifaceted structure.

This work investigates the single-photon emissions from self-assembled InGaAs quantum dots that are grown on an apex plane of a GaAs pyramid-like multifaceted structure. The number of QDs on a multifaceted structure is estimated by scanning electron microscopy. Single-exciton emissions from individual quantum dots are examined by micro-photoluminescence and by making photon correlation measurements. This experiment demonstrates the improvement of the single-photon extraction efficiency as quantum dots are grown on a reduced apex plane of a multifaceted structure.

Enhancing luminescence efficiency of InAs quantum dots at 1.5μm using a carrier blocking layer

The authors report an effective way to enhance the optical efficiency of InAs quantum dots (QDs) on GaAs emitting at the wavelength of 1.5μm. It is found that the loss of holes from QDs to their proximity via the high indium composition InGaAs overgrown layer, which is necessary for achieving long wavelength emission, is the origin of photoluminescence intensity degradation at high temperature. Inserting a 4nm thick Al0.45Ga0.55As layer, acting as a carrier blocking layer, into the GaAs capping matrix can improve the room temperature photoluminescence peak intensity by five and two times for the ground and first excited states, respectively.