Kwang Moo Kim

Electrical and optical characterizations of self-assembled quantum dots formed by the atomic layer epitaxy technique

We investigated the electrical and optical properties of InGaAs self-assembled quantum dots grown using the atomic layer epitaxy (ALE) technique. Dots–in–a–well structures were grown by alternately supplying InAs and GaAs sources on an InGaAs layer and covering with another InGaAs layer. Three samples produced with different numbers of cycles of alternate InAs/GaAs supply were characterized by capacitance-voltage and photoluminescence (PL) measurements. For the ten cycle dots–in–a–well structure, a strong zero-dimensional electron confinement was observed even at room temperature. On the other hand, for the five-cycle structure, the PL results indicate that the InGaAs quantum well structure coexists unstably with premature quantum dots. By comparing the results for samples with different numbers of cycles, we suggest that an ALE dots–in–a–well structure can be formed by the aggregation of In and Ga atoms incorporated into the InGaAs quantum well layer when the number of cycles exceeds the critical number ...

Effects of Doping Profile on Characteristics of InAs Quantum Dots

Capacitance-voltage measurements were carried out to investigate the effects of the doping profile on characteristics of self-assembled InAs quantum dots. Using this technique, we observed features of zero-dimensional electron confinement indicating the presence of quantum dots. However, the number of confined states differed depending on the doping profile, and this fact was confirmed by photoluminescence measurements. The equation in the depletion approximation led us to calculate the distribution of carriers as a function of depth from the sample surface, and the results are in agreement with the depth of the quantum dot layer.

Alignment of InAs quantum dots on a controllable strain-relaxed substrate using an InAs/GaAs superlattice

We fabricated InAs self-assembled quantum dots on a strained layer using molecular beam epitaxy. The controllable strained layer consisted of an InAs/GaAs superlattice and a GaAs spacer layer on a GaAs (001) substrate. We formed two-dimensional arrays of quantum dots along the 〈110〉 directions on the partially strain-relaxed layer that is formed using the superlattice system. The increase in the thickness of the partially strain-relaxed layer resulted in stronger alignment of the quantum dots. The aligned quantum dots are applicable to quantum devices, because they confine carriers well, in spite of the existence of dislocation networks. Strongly aligned quantum dots have a lower carrier transition energy because of their larger size and increased relaxation.

Electrical and optical characteristics of various PPV derivatives (MEH-PPV, CzEH-PPV, OxdEH-PPV)

Abstract To investigate the electrical characteristics of polymer based light emitting diode (LED) devices, we fabricated the hole transport device (HTD) and the electron transport device (ETD). The ITO and Au with high work function were used as electrodes for the HTD, and the Al and Li:Al with low work function were used for the ETD. The active layer materials were poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV), poly[2-( N -carbazolyl)-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (CzEH-PPV), and poly[2-(4-tert-butylphenyl)-5-phenyl-1,3,4-oxadiazole-5(2-ethylhexoxy)-1,4-phenylene vinylene] (OxdEH-PPV). We measured the current density–applied field ( J – E ) characteristics of the HTD and ETD with various thickness at different temperatures. The results of the J – E curves were analyzed by using tunneling model, space charge limited conduction (SCLC) model, etc. In the SCLC model, the mobility of the hole and the electron of MEH-PPV is ∼10 −6 and ∼ 10 −8 cm 2 / V s , respectively. For CzEH-PPV and OxdEH-PPV, the hole mobility is similar to the value of the electron mobility with ∼ 10 −10 cm 2 / V s . The luminescent efficiency of CzEH-PPV or OxdEH-PPV is higher than that of MEH-PPV. The results of photoconductivity of the systems qualitatively agrees with the result of the electrical measurement. We analyze that the balance of the electron and the hole mobility plays an important role for the efficiency of the LEDs.

Interdiffusion and structural change in an InGaAs dots-in-a-well structure by rapid thermal annealing

Post-growth rapid thermal annealing (RTA) has been used to investigate an interdiffusion and the structural change in an InGaAs dots-in-a-well (DWELL) structure grown by molecular beam epitaxy using an alternately supplying InAs and GaAs sources. In the case of the as-grown sample, which has a metastable quantum structure due to an intentional deficit of source materials, it is found that an InGaAs quantum well (QW) coexists with the premature quantum dots (QDs), and an intermediate layer exists between the QW and the QDs. Through the RTA process at 600 and 800°C for 30s, metastable structure changes into a normal DWELL structure composed of QDs and QW as a result of the intermixing of premature QDs and the intermediate layer.

Fabrication and characterization of metal-semiconductor field-effect-transistor-type quantum devices

Quantum dot transistors and nanowire transistors are fabricated from a metal-semiconductor field-effect-transistor-type wafer and are characterized at low temperatures. Clear single-electron tunneling and various quantum effects, such as transport through excited states and negative differential resistance, are observed in our wire device. Our data suggest that the potential fluctuation of the heavily doped GaAs layer has a much larger characteristic length than interimpurity spacing, and that this is due to the low ionization rate (approximately 10%) of the dopant atoms at 4.2 K.Quantum dot transistors and nanowire transistors are fabricated from a metal-semiconductor field-effect-transistor-type wafer and are characterized at low temperatures. Clear single-electron tunneling and various quantum effects, such as transport through excited states and negative differential resistance, are observed in our wire device. Our data suggest that the potential fluctuation of the heavily doped GaAs layer has a much larger characteristic length than interimpurity spacing, and that this is due to the low ionization rate (approximately 10%) of the dopant atoms at 4.2 K.

Transient behavior of self-assembled quantum dots formed by atomic layer epitaxy technique

We investigated the effects of carrier dynamics on the temperature dependence of the photoluminescence (PL) of an InGaAs dots-in-a-well (DWELL) structure. The quantum dots (QDs) were formed by the atomic layer epitaxy (ALE) technique alternately supplying InAs and GaAs sources. It was found from the PL measurements at various temperatures that the DWELL structure was accomplished through the generation process of the intermediate layer between the quantum well (QW) and the QDs during the formation of the QDs inside a QW. The thermal quenching equations on the basis of the rate equation model can be explained by the carrier dynamics, which included in the radiative recombination, the carrier thermal escape and the carrier capture process occurring in these three layers, i.e. QW, QD and the intermediate layer.

Effects of doping methods on characteristics of InAs quantum dots

We compared the effects of two different doping methods on electronic states of self-assembled InAs quantum dots using photoluminescence and C-V spectroscopy. Analytical interpretation will be presented in conjunction with the experimental data.

Alignment of InAs quantum dots on gaas using the manipulation of strain fields

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