Ho-Sang Kwack

Room-temperature operation of InP-based InAs quantum dot laser

A ridge waveguide quantum dot (QD) laser with a stripe width of 15 /spl mu/m was fabricated by using the seven-stacked InAs QD layers based on the InAlGaAs-InAlAs material system on InP [001] substrate. Room-temperature lasing operation was observed at 1.501 /spl mu/m, which is the first observation from the InAs QDs with the InAlGaAs-InAlAs structure. The characteristic temperature of the InAs QD laser calculated from the temperature dependence of threshold current density was 135 K in the temperature range from 200 K to room temperature.

Characteristics of ZnO Thin Films by Means of Plasma-Enhanced Atomic Layer Deposition

Zinc oxide films were grown in a temperature range of 75-150°C by plasma-enhanced ALD for the application of flexible transparent thin-film transistor (TFT). We have investigated the effect of precursor pulsing time, temperature, and radio frequency power on the film growth. The X-ray diffraction patterns of polycrystalline ZnO films showed rather preferred (002) orientation even at a low temperature of 150°C. We could obtain highly resistant ZnO film, being suitable for the active layer of TFT, at low temperature with the reactive oxygen plasma to result in decreased carrier density.

Manipulation of the structural and optical properties of InAs quantum dots by using various InGaAs structures

The structural and optical properties of self-assembled InAs quantum dots (QDs) with various InGaAs structures were investigated by transmission electron microscopy (TEM) and photoluminescence (PL). The emission peak position of InAs QDs covered by a 6 nm In0.15Ga0.85As layer was 1.26 μm with PL linewidth of 31 meV, which is narrower than that of QDs in a GaAs matrix. By inserting a 1 nm In0.15Ga0.85As layer below the InAs QD layer with a 6 nm In0.15Ga0.85As overgrowth layer, the emission peak position was redshifted with larger energy-level spacing between the ground states and the first excited states compared to that of QDs with an In0.15Ga0.85As overgrowth layer only. By covering the InAs QDs on a 1 nm In0.15Ga0.85As layer with an 8 nm InxGa1−xAs layer having graded In composition, the emission peak position was 1.32 μm with relatively larger energy-level spacing and narrower PL linewidth compared to QDs covered by an In0.15Ga0.85As layer. The longer emission wavelength with relatively larger energy-l...

InAs-InAlGaAs quantum dot DFB lasers based on InP [001]

The InAs-InAlGaAs quantum dot (QD) lasers with the InAlGaAs-InAlAs material system were fabricated on distributed feedback (DFB) grating structures on InP (001). The single-mode operation of InAs-InAlGaAs QD DFB lasers in continuous-wave mode was successfully achieved at the emission wavelength of 1.564 mum at room temperature. This is the first observation on the InP-based QD lasers operating around the emission wavelength window of 1.55 mum. The threshold current density of the InAs-InAlGaAs QD DFB laser with a cavity length of 1 mm and a ridge width of 3 mum, in which one of the cleaved facets was coated with 95% high-reflection, was 1.23 kA/cm2 (176 A/cm2 for single QD layer). The sidemode suppression ratio value of the QD DFB laser was as high as 42 dB at the driving current of 100 mA

Long-wavelength laser based on self-assembled InAs quantum dots in InAlGaAs on InP (001)

Seven stacks of self-assembled InAs quantum dots (QDs) separated by 28nm thick InAlGaAs barriers were grown on InP (001) substrate by a solid-source molecular-beam epitaxy and were investigated by cross-sectional transmission electron microscopy and photoluminescence spectroscopy. Gain guided broad-area lasers with a stripe width of 75μm were fabricated by using the seven-stacked InAs QD layers with the InAlGaAs–InAlAs material system on InP (001). The lasing operation from InAs QDs was observed up to 260K and the characteristic temperature of the uncoated QD laser calculated from the temperature dependence of threshold current density was 377K for temperatures up to 200K, and 138K above 200K. The drastic decrease in the characteristic temperature above 200K was mainly related to the thermal behavior of carriers in QDs, and possibly the thermal coupling of the QDs to the wetting layer and the waveguide region.

Structural and optical properties of shape-engineered InAs quantum dots

Shape-engineered InAs quantum dots (QDs) were grown by using thin In0.15Ga0.85As and a monolayer (ML) of InAs with different periods on 3 ML InAs QDs and their structural and optical properties were investigated by transmission electron microscopy (TEM), photoluminescence (PL), and photoreflectance (PR) spectroscopy. Cross-sectional TEM images of the QD samples showed that the shape, particularly the height of the QD, could be effectively controlled without any significant degradation in QD quality, such as the generation of dislocations in the QD, thus changing the optical properties. PL and PR spectra indicated that all the layers required for the formation of shaped-engineered QD were grown without any degradation in QD properties. PL spectra of shape-engineered QDs that had been subjected to rapid thermal annealing showed an abnormal behavior compared to those of conventionally grown InAs QDs.

Effects of a thin InGaAs layer on InAs quantum dots embedded in InAl(Ga)As

Influences of a thin In0.32Ga0.68As layer on the structural and optical properties of self-assembled InAs quantum dots (QDs) embedded in an InAl(Ga)As matrix, which was lattice-matched to an InP substrate, were investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) spectroscopy. AFM and TEM images showed that the size of QDs grown on a thin In0.32Ga0.68As layer was increased, with a reduction in size fluctuation compared to that of QDs on an InAl(Ga)As layer. The shape of the QD was also more isotropic, indicating that the QD would be closer to an ideal zero-dimensional system. The PL peak position of the InAs QDs grown on a 1.5 nm In0.32Ga0.68As layer was 1.55 μm, with linewidth broadening of 64 meV that was somewhat narrower than those of the QD samples without the In0.32Ga0.68As layer, which agreed well with the AFM and TEM results.

Vertical stacks of shape-engineered InAs∕InAlGaAs quantum dot and its influences on the lasing characteristics

The formation characteristics on the vertical stacks of shape-engineered InAs∕InAlGaAs quantum dots (QDs), which were formed by the alternate growth method (AGQDs), were studied in terms of the modulation in strain field and phase separation. The threshold current of the broad-area laser diodes (LDs) with five stacks of the AGQDs (AGQD-LDs) was 4.5 times smaller than that of the LDs with seven stacks of the conventionally grown QDs (CQD-LDs). The slope efficiency for the AGQD-LDs was 0.16W∕A, which was higher than that of the CQD-LDs of 0.9W∕A. These results can be attributed to better confinement of the electron wave function in QDs.

Well-defined excited states of self-assembled InAs∕InAlGaAs quantum dots on InP (001)

Self-assembled InAs∕InAlGaAs quantum dots (QDs) in an InAlGaAs matrix on InP (001) substrates were grown by the alternate growth method (AGQD), where an InAs layer with a thickness of 1 monolayer (ML) and an InAlGaAs layer with a thickness of 1 ML were alternately deposited. Cross-sectional transmission electron microscopy images indicated that the aspect ratio (height/width) for the AGQDs was ∼0.25, which was higher than ∼0.10 of conventionally grown InAs QDs. The photoluminescence (PL) peak position for the ground states of the AGQDs was 1.485μm with a linewidth broadening of 42meV at room temperature, while the PL linewidth for the conventionally grown QDs was 85meV. And the peaks for the excited-state transitions were also clearly observed from the excitation-power dependent PL. This is the first observation on the well-defined excited-state transitions from the InP-based InAs QDs, even though there were several reports on the features of the excited states.

1.55

For the fabrication of quantum dot (QD) distributed feedback (DFB) lasers, self-assembled InAs/InAlGaAs QDs were grown on the InP/InGaAs grating structures by a molecular beam epitaxy. Ridge-waveguide QD DFB lasers with a stripe width of 3 mum were fabricated. Single-mode lasing operation around the wavelength of 1.56 mum was successfully achieved under the pulsed and continuous-wave (CW) modes at room temperature. The lasing operation was also observed up to 70 degC under pulsed mode, which is the first observation on the single-mode lasing around 1.55 mum. The characteristic temperatures calculated from the temperature dependence of the threshold current density were 121.8 K from room temperature to 45 degC, which was reduced to 74.2 K from 45 degC to 70 degC.