Supachok Thainoi

Self-assembled quantum-dot molecules by molecular-beam epitaxy

Self-assembled InAs quantum-dot (QD) molecules having high dot density and aligned dot set structure, which is defined by nanotemplates, were realized by thin capping and regrowth technique in a molecular-beam epitaxy process. Thin capping of GaAs on InAs QDs leads to the creation of nanoholes having a camel-like nanostructure due to anisotropic strain fields along the [11¯0] crystallographic direction and anisotropic surface diffusion accompanying the QD collapse. Regrowth of InAs QDs on the nanohole templates initially results in the formation of QDs with good size uniformity in the middle of features with the shape of propeller blades. This takes place at the regrowth thickness of 0.6 monolayer (ML). The strain at propellers’ edge starts to play its role, creating sets of quantum dots surrounding the initial and centered dots at the regrowth thickness of 1.2 ML. The elongated configuration of propellers’ blades defines the pattern of QD sets having five to six dots on each side. The dot density of the ...

Thin-capping-and-regrowth molecular beam epitaxial technique for quantum dots and quantum-dot molecules

A thin-capping-and-regrowth molecular beam epitaxial technique is proposed and demonstrated to be a suitable approach for the growth of lateral quantum-dot molecules (QDMs). By regrowing on top of nanoholes, previously formed from as-grown quantum dots (QDs) via a thin-capping process, nanopropeller QDs are formed. By repeating the thin-capping-and-regrowth process for several cycles at the regrown thickness of 0.6 ML, nanopropeller QDs are linked along the [11¯0] crystallographic direction, leading to the alignment of QDs. The thin-capping-and-regrowth process is repeated for 1, 3, 5, 7, and 10cycles on different samples for comparison purposes. It is found from ex situ atomic force microscopy that at 7cycles of thin capping and regrowth of QDs, the best alignment of QDs is achieved. This is due to the strain having an optimum condition. The samples that undergo three and five thin-capping-and-regrowth cycles show some randomness of QD formation. When the process is repeated for 10cycles, QDs become rand...

Improvement of PV Performance by Using Multi-Stacked High Density InAs Quantum Dot Molecules

InAs quantum dot molecules (QDMs) are prepared by thin-capping-and-regrowth MBE process. The dot density can be varied between 1010 cm-2, for as-grown quantum dots (QDs), to 1012 cm-2, for multi-stack QDMs. Photocurrent measurements on 1-and 5-stack high-density QDM layers show that these InAs QDMs when embedded inside a GaAs bulk structure extend photon absorption beyond the 850-nm bandedge limited by GaAs. The results also indicate that the higher the number of stacks the higher the resulting current. The presence of high-density QDMs in solar cells thus extends the absorption region and at the same time increase the output current. Electrical characterisations on homojunction (p-n) solar cells with 1-and 5-stack high-density QDMs embedded between the junction show that the 5-stack sample provides a higher short-circuit current density of Jsc=14.4 mA/cm2 compared to 9.6 mA/cm 2 provided by the 1-stack sample. The increase is due entirely to the difference in absorptive dot volume accounted for by the difference in the number of stacks of high-density InAs QDMs. The efficiency of homo-structure 5-stack high-density QDM solar cell is 5.1%

Multi-stacked quantum dots with graded dot sizes for photovoltaic applications

Multiple stacks of quantum dots are proposed as an active layer in a novel solar cells structure. A simple growth technique reported here proves to be an important enabling technology, making uniform, controllable stacks of InAs dots. Schottky solar cells fabricated from the multi-stacked graded dots wafer show current-voltage characteristics which indicate a significant improvement in short-circuit current when compared to the same devices fabricated from a non-graded dots wafer. Spectral response at 1.0-1.4 /spl mu/m region is attributed to the quantum dot layers.

Improved quantum confinement of self-assembled high-density InAs quantum dot molecules in AlGaAs∕GaAs quantum well structures by molecular beam epitaxy

Self-assembled, multistack InAs quantum dot molecules (QDMs) were grown by a modified molecular beam epitaxial (MBE) technique, which involves multiple stacking and multiple cycling of the thin-capping-and-regrowth process, so as to obtain a large volume density of quantum dots on the sample. Furthermore, the high-density InAs QDMs were also grown sandwiched either between a double heterostructure (DHS) or between a quantum-well (QW) structure. It was found from microphotoluminescence (μ-PL) measurements that the QDMs sandwiched between these structures give broader PL spectra than those of the as-grown QDMs. The broadening of the PL spectra is associated with the poorer dot size uniformity, which arises from the long and complicated MBE growth processes. However, comparing between the QDMs in the DHS and in the QW structure, the latter give narrower PL spectra. The narrower PL spectra for the QDM-in-QW structure is attributed to the improved quantum confinement effect arising from the use of the QW.

Ultrathin epitaxial InAs layer relaxation on cross-hatch stress fields

Thin, highly-strained InAs layers epitaxially grown on GaAs/InGaAs cross-hatch surfaces undergo postgrowth transformations that yield several morphologies ranging from aligned quantum wires to quantum dots and micron-scale pyramids. The shape varieties result from the multiple pathways created from the combined/competitive effects of asymmetric adatom diffusions, subsurface stress fields and misfit energy minimization. These morphologies reveal the multiple outcomes of metastable states between the two- and the three-dimensional transition that if properly captured and engineered may open up new windows of opportunities both in devices such as sensors and in fundamental quantum studies.

Self-assembled composite quantum dots for photovoltaic applications

InAs and InGaAs quantum dots were prepared by MBE onto GaAs substrates. Quantum dots size was found to be 40-50 nm in diameter and 4-7 nm in height by AFM. PL peak of InAs quantum dots is at 980 nm, and at a shorter wavelength for InGaAs dots. Composite InAs/InGaAs quantum dots should give a wider spectral response in solar cell applications. Stabilized performance of quantum dots structure at high temperatures is an attractive feature. A possibility to store photo-generated carriers in the composite structure is another important prospect.

Selective growth of InAs/GaAs self-organized quantum dots by shadow mask technique

The 10-mm stripes of 3-stacked self-organized InAs QDs layers embedded in GaAs matrix were selectively grown on GaAs substrates by MBE with shadow mask technique. The shadow mask structure composed of a 1-mm GaAs mask layer and a 1.5-m mA l 0.5Ga0.5As spacer layer was prepared by LPE and MBE. The window stripes were lithographically defined and were opened by conventional chemical etching. The self-organized QDs formation was observed by monitoringthe 2D to 3D transition on an unpatterned GaAs substrate. PL spectrum from shadow mask sample confirms the QDs formation and reveals the different optical properties from QDs on the unpatterned substrate. The blue-shift of PL spectrum of QDs grown through shadow mask is due to the different dot sizes. # 2001 Elsevier Science

Study on GaAs/GaAlAs MQW structure for photovoltaic applications

Abstract GaAs/GaAlAs MQW structures with varying well widths having narrowest wells on the top layers and gradually wider wells for the inner layers were studied and confirmed by Auger spectroscopy and photoluminescence measurements. Multiplication of quantum wells gives stronger photoluminescence due to higher density of quantized states in the structure. This MQW structure was experimented in the photoconductivity and photocurrent measurements at room temperature. It is found that at low multiplication of quantum wells in MQW structure, the photoconductivity effect was mainly controlled by bulk material as shown in the spectral response. Photocurrent at low bias voltage shows relatively better spectral sensitivity at shorter wavelength. The photoconductivity and photocurrent measurements indicate that appropriate MQW structures acting as broader absorbers due to graded characters of quantized energy states are needed for photovoltaic application. Integration of MQW structure to solar cell structure was also investigated.

Raman and photoluminescence properties of type II GaSb/GaAs quantum dots on (001) Ge substrate

We investigate structural Raman and photoluminescence properties of type II GaSb/GaAs quantum dots (QDs) grown on (001) Ge substrate by molecular beam epitaxy. Array of self-assembled GaSb QDs having an areal density of ∼1.66 × 1010 dots/cm2 is obtained by a growth at relatively low substrate temperature (450 °C) on a GaAs surface segmented into anti-phase domains (APDs). Most of QDs form in one APD area. However, a few QDs can be observed at the APD boundaries. Raman spectroscopy is used to probe the strain in GaAs layer. Slight redshift of both LO and TO GaAs peaks are observed when GaSb QDs are buried into GaAs matrix. Optical properties of capped QDs are characterized by photoluminescence measurement at low temperatures (20 K and 30 K). Emission peaks of GaSb/GaAs QDs are found in the range of 1.0-1.3 eV at both temperatures. Slight redshift is observed when the laser excitation power is increased at 20 K while blueshift of QD peak is observed at 30 K. We attribute this abnormal behavior to the contribution of overlapped GaSb wetting layer peak in the PL emission as well as the feature of type II band structure.