Somsak Panyakeow

Amorphous Visible-Light Thin Film Light-Emitting Diode Having a-SiN:H as a Luminescent Layer

Hydrogenated amorphous silicon nitride (a-SiN:H) has been applied for the first time as a luminescent active layer (i-layer) in an amorphous visible-light thin film light-emitting diode (TFLED). The TFLED has the structure of glass substrate/ITO/p a-SiC:H/i a-SiN:H/n a-SiC:H/Al. Visible red and yellow emissions can be observed at room temperature from the TFLEDs in which the optical energy gap of i-a-SiN:H is larger than 2.4 eV. The brightness of the red TFLED was 0.5 cd/m2, with a forward injection current density of 2000 mA/cm2 for the 0.033 cm2 cell area.

Effect of arsenic species on the formation of (Ga)InAs nanostructures after partial capping and regrowth

Surface morphologies of self-assembled (Ga)InAs nanostructures grown by partial-capping-and-regrowth technique using gas-source molecular beam epitaxy (GSMBE) and solid-source molecular beam epitaxy (SSMBE) are compared. With SSMBE under an As4 ambient, as-grown quantum dots (QDs) change to a camel-like nanostructure after being partially overgrown with GaAs. When additional InAs is deposited, quantum-dot molecules are created. In comparison, with GSMBE under As2 overpressure, as-grown QDs are transformed into quantum rings after partial capping with GaAs and then, after regrowth, become double QDs. At higher regrowth temperature, QD rings are formed.

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 ...

Self-assembled nanoholes and lateral QD bi-molecules by molecular beam epitaxy and atomically precise in situ etching

Abstract We present a systematic study of nanoholes formation on top of capped InAs quantum dots (QDs) by using AsBr 3 in situ etching. The etching process can be divided into two regimes depending on the nominal etching depth and the thickness of the GaAs cap layer. In the first regime, 6-nm deep and 50-nm wide nanoholes are formed, which we ascribe to a strain selectivity of the etchant. Further supply of the etching gas causes the hole diameter to increase, while the depth stays approximately constant. Photoluminescence (PL) was used to confirm the removal of the buried InAs QDs during etching. The holes can be overgrown with InAs such that an atomically flat surface is recovered. Further InAs deposited on the filled-hole layer forms into pairs of self-assembled QDs—so-called lateral QD bi-molecules. These QD bi-molecules show narrow and intense PL signal at room temperature.

InAs/GaAs self-organized quantum dots on (4 1 1)A GaAs by molecular beam epitaxy

Self-organized InAs QDs formation on (4 1 1)A GaAs and on controlled (1 0 0) GaAs substrates were studied by insitu RHEED observation, AFM measurement and PL spectroscopy. The transition from two-dimensional to threedimensional growth on (4 1 1)A was observed by RHEED pattern transformation to indicate the critical layer thickness of InAs QDs formation. The result is almost the same as that on (1 0 0). Improvement of QDs alignment having dot size uniformity on (4 1 1)A was observed by AFM measurement. Enhanced optical properties of QDs on (4 1 1)A was shown by low-temperature PL spectroscopy. Strong PL peak at 966 nm having FWHM of 35 nm was obtained from the 3stacked QDs on (4 1 1)A. The narrower PL FWHM of QDs peak on (4 1 1)A comes from the improved uniformity of QDs on (411)A. # 2001 Elsevier Science B.V. All rights reserved.

Effect of carrier gas on response of oxide semiconductor gas sensor

Abstract It is known that response mechanism of an oxide semiconductor gas sensor based on a redox process occurs at the surface of the sensor. The present investigation used an alcohol gas sensor made of tin oxide as an example to demonstrate the role of oxygen molecules in carrier gases on the response of the sensor. It is found that high concentration of oxygen in the carrier gases reduced the sensor recovery time and background conductance, and enhanced its dynamic range and linearity of measurement. These results can be explained by a reaction model of oxidation by oxygen and reduction by reducing gas at the surface of the sensor.

Visible thin film light emitting diode using a-SiN:H/a-SiC:H heterojunctions

Abstract A visible a-SiN:H/a-SiC:H heterojunction thin film light emitting diode (TFLED) has been developed. The TFLED has a structure of glass/ITO/p a-SiC:H (2.0 eV)/i a-SiN:H (2.4–4.0 eV)/n a-SiC:H (2.0 eV)/A1. The thickness of the p-i-n layers are 150 A, 200–1000 A and 300 A, respectively. The results have shown that the optimized thickness of the i-layer is approximately 500 A, and the emitting color can be changed from red, yellow, green to white-blue as the optical energy gap of the i-layer is increased. The brightness of the TFLEDS is mostly constant under the pulse-mode operation in the frequency range of dc-1 MHz.

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...

Monitoring and data analysis of a PV system connected to a grid for home applications

Abstract Monitoring of a PV system connected to a grid was conducted to collect the system performance and compared to a PV stand-alone system. Daily solar inputs and load outputs for home applications of the two systems were recorded. Balance and surplus of energy in the systems were observed during dry and summer seasons when high solar radiation was recorded. During the raining season, with thunderstorms, solar radiation was low and grid cut-off occasionally occurred. Consequently, energy deficiencies and grid back-up of the systems were observed. It was found that the battery size of the PV system connected to the grid was reduced by a factor of 0.5–0.7 compared to the PV stand-alone counterpart for similar load behaviors. The merits of the PV system connected to the grid during grid cut-off was confirmed; the system proved to be appropriate for tropical countries where unstable electricity supply from the grid can occur during monsoon season.

The effects of relaxed InGaAs virtual substrates on the formation of self-assembled InAs quantum dots

Self-assembled InAs quantum dots (QDs) are grown on InGaAs/GaAs virtual substrates (VS) by molecular beam epitaxy. By growing InAs QDs on partially relaxed InGaAs layers with different thicknesses, hence with varying degrees of relaxation, we obtain InAs QDs which are aligned or grouped into distinct units. It was found that growths of InAs QDs on these VS result in preferential alignment along [1 1 0] and [1 −1 0] directions, and the thicker the InGaAs relaxed layer the higher the chance that QDs coalesce into distinct ensembles of QDs. We attribute the results to the difference in the underlying surface undulation due to different degrees of VS relaxation and to strain asymmetries, as confirmed by atomic force microscopy and high-resolution x-ray diffraction. A schematic description of InAs QD growth on partially relaxed InGaAs layers is given.