Yong Ju Park

Optical properties of silicon nanoparticles by ultrasound-induced solution method

White-light-emitting silicon nanoparticles, whose surfaces were passivated with butyl, were prepared using a focused ultrasonic energy. The white light was achieved by controlling only the size distribution without adding any fluorescent ions. The white-light-emitting silicon nanoparticles had a wide size distribution of 1–5 nm and an average size of 2.7 nm, which were sufficiently small to indicate the quantum confinement effect for silicon. The photoluminescence spectrum covered a wide range of 320 nm–700 nm with a full width at half maximum of approximately 190 nm.

Electrical characterization of InAs/InP self-assembled quantum dots by deep level transient spectroscopy

In this paper, electrical characterization of InAs/InP self-assembled quantum dots is studied. The PL signals shown a peak at 0.67 eV with a full-width at half-maximum of about 50 meV.

Linearized multiple-quantum-well electro-absorption modulator by quantum well intermixing technique

In this paper, we propose a new technique to suppress the non- linearity of multiple quantum well (MQW) electro-absorption (EA) modulator, mainly due to an exponential-like transmission characteristics of EA modulator and non-linearity of quantum confined stark effect (QCSE), by intermixing MQW absorption region. Optical properties and its dependence on applied bias voltages of intermixed InGaAs/InGaAsP MQW absorption region, such as transition energy and gain (or absorption) spectrum have been calculated by solving Luttinger-Kohn Hamiltonian. It has been shown that the transfer function of a MQW-EA modulator can be tailored by introducing differently intermixed regions along the waveguide direction. It has been also shown that proposed technique can suppress IMD2 (2nd order intermodulation distortion) by 39.6 dB and enhance spurious free dynamic range (SFDR) by a 3.6 dB by choosing proper combination of interdiffusion lengths and waveguide lengths.

Microstructures and Growth Characteristics of Self-Assembled InAs/GaAs Quantum Dots Investigated by Transmission Electron Microscopy

The microstructure and strain characteristics of self-assembled InAs/GaAs quantum dots (QDs) were studied by using transmission electron microscopy. Compressive strain was induced to uncapped QDs from GaAs substrate and the misfit strain largely increased after the deposition of GaAs cap layer. Tensile strain outside QD was extended along the vertical growth direction; up to 15 nm above the wetting layer. Vertically nonaligned and aligned stacked QDs were grown by adjusting the thickness of GaAs spacer layers. The QDs with a lens-shaped morphology were formed in the early stage of growth, and their apex was flattened by the out-diffusion of In atoms upon GaAs capping. However, aligned QDs maintained their lens-shaped structure with round apex after capping. It is believed that their apex did not flatten because the chemical potential gradient of In was relatively low due to the adjacent InAs QD layers.

Enhancement of optical properties of self-assembled quantum dots for infrared photodetectors by thermal annealing

Intermixing effects of MOCVD (metal organic chemical vapor deposition) grown InGaAs SAQDs (self-assembled quantum dots) covered with SiO2 and SiNx-SiO2 dielectric capping layers were investigated. The intermixing of SAQDs was isothermally performed at 700°C by varying annealing time under the N2-gas ambient. It was confirmed from the PL measurement after the thermal annealing that, the emission energy of SAQDs was blue-shifted by 190 meV, the FWHM (full width at half maximum) was narrowed from 76 meV to 47 meV and the PL intensity was increased. SiNx-SiO2 double capping layer have been found to induce larger PL intensity after the thermal annealing of SAQDs compared to SiO2 single capping layer. The results can be implemented for increasing quantum efficiency and tuning the detection wavelength in quantum dot infrared photodetector (QDIP).

Role of InxGa1-xAs strain relaxation layers in optical and structural properties of InAs/GaAs quantum dots

Effects of InxGa1-xAs strain relaxation layers on the optical and structural properties of InAs quantum dots (QDs) were studied systemically. 300 K-photoluminescence (PL) shows that PL peak energy of the QDs is blue-shifted in GaAs/InAs QDs/5 nm-thick In0.1Ga0.9As structure compared to GaAs/InAs QDs/GaAs structure. This is attributed to the intermixing of materials between the QDs and the InGaAs layer below the QDs, whereas capping of a 5 nm-thick In0.1Ga0.9As layer leads to red shift due to strain relaxation effect. As thickness of InxGa1-xAs capping layer (TI) increases, 300 K-PL peaks experience red shift below TI < ~7 nm. Unlikely, TI above 7 nm results in blue shift. Considering average height of the QDs is ~ 7 nm, this is attributed to intermixing of material between the QDs and InGaAs capping layers. The blue shift in x = 0.2 over TI > ~7 nm is relatively smaller compared to that in x = 0.1. It is noteworthy that strain difference between the InAs QDs and the InxGa1-xAs is smaller in x = 0.2 rather than in x = 0.1. Finally, InAs QDs are sandwiched by asymmetric thickness (7.5 nm-thick capping InGaAs, 0, 1.2, and 2.5 nm-thick bottom InGaAs) of In0.2Ga0.8As layers. 300 K-PL spectrum shows that 1.2 nm-thick bottom InGaAs leads to the longest wavelength (1306 nm) among this sample set. This is attributed to reduced barrier height and ignorable accumulated strain effect in thin bottom InGaAs layers. In this report, we justify merit of dots in an asymmetric well structure over conventional dots in a symmetric well structure and strain relaxation structure for the control of PL peak energy.

Effects of Growth Sequence on Optical and Structural Properties of InAs/GaAs Quantum Dots Grown by Atomic Layer Molecular Beam Epitaxy

The influence of growth sequence on optical and structural properties of InAs/GaAs quantum dots (QD) grown by atomic layer epitaxy was investigated systematically. It is found that growth interruption (GI) after In is more effective than non-GI after In in reducing the density of coalescent dots, and reducing the dot width distribution of the QDs. Meanwhile, dot densities are approximately doubled by non-GI after In. GI after As reduces dot height distribution compared with non-GI after As. Generally, GI after In plays a more critical role than GI after As does in formation of the QDs. The sample grown with In/GI/As/GI sequence shows the lowest 300 K-photoluminescence (PL) linewidth (∼30 meV), high PL peak separation between ground and 1st excited level (∼80 meV). From the result, it is known that In/GI/As/GI is the favorable growth sequence among the sample sets. Temperature dependence of PL linewidth shows that the In/GI/As/GI sample is insensitive to cryostat temperature and it is attributed to weak wetting effect. Thinner wetting layers shown in a cross-sectional TEM image supports this.

Linearization of quantum well electro-absorption modulator by quantum well intermixing technique for analog optical links

A Quantum Well Intermixing (QWI) technique has been introduced to modify the transfer function of quantum well electroabsorption (EA) modulator for analog optical links. The linearization of transfer function of EA modulator has been achieved by introducing QW absorption regions differently intermixed each other along the waveguide. A 39.6 dB of IMD2 (2nd order intermodulation distortion) suppression and a 3.6 dB of SFDR (Spurious Free Dynamic Range) enhancement have been achieved.