Jae-Young Leem

Visible photoluminescence from self-assembled InAs quantum dots embedded in AlAs cladding layers

Photoluminescence (PL) from InAs self-assembled quantum dots (QD) embedded in the AlAs matrix was strong and clean around 700 nm. PL efficiency remained quite high at room temperature compared to other QD systems embedded in GaAs cladding layers. Transmission electron microscope pictures from the structure showed a clear formation of relatively small and coherently strained InAs QD. The observed blueshift with accompanying broadening of PL spectra with the increase of excitation power is interpreted in terms of local carrier tunneling in a dense QD system. The PL peak redshift with the increase of temperature was very large, as much as 228 meV. The anomalous shift is interpreted as due to activation-energy differences between dots of different sizes.

Nanoscale InGaAs concave disks fabricated by heterogeneous droplet epitaxy

The detailed cross-sectional structure of InGaAs quantum dots fabricated by a heterogeneous droplet epitaxy method was investigated by means of cross-sectional transmission electron microscopy observation. It was confirmed that concave disks without any dislocations or wetting layer were formed at the upper part of the flat surface. This result was consistent with the change of photoluminescence intensity and peak position. The sizes of the disks were estimated to be 30 and 12 nm in lateral and vertical directions, respectively. From this estimation, the occurrence of a phase-separation effect is suggested.

Structural and blue emission properties of Al-doped ZnO nanorod array thin films grown by hydrothermal method

ZnO seed layers were deposited on a quartz substrate using the sol-gel method, and Al-doped ZnO (AZO) nanorod array thin films were grown on the ZnO seed layers by the hydrothermal method with different Al concentrations ranging from 0 to 2.0 at. %. The structural and blue emission properties of the ZnO and AZO nanorod array thin films were investigated using scanning electron microscopy (SEM), x-ray diffraction, Ultraviolet-visible spectroscopy, and photoluminescence (PL). Al doping greatly affects the morphology of AZO nanorod array thin films. For an Al concentration of 2.0 at. %, it can be clearly seen from the SEM image that the hexagonal shape has changed into a prism-like shape. In the PL spectra, it is clear that the intensity ratio of the near-band-edge emission to the deep-level emission (DLE) increases as the Al concentration increases up to 2.0 at. %. The DLE peak (about 2.80 eV) in the blue emission region is found for the AZO nanorod array thin films. The transmittance spectra show that as compared to the ZnO nanorod array thin films, the AZO nanorod array thin films exhibited significantly improved transmittance in the visible region and a blue shift of the absorption edge.

Effects of high potential barrier on InAs quantum dots and wetting layer

Effects of a thin AlAs layer (1 nm) with different position on InAs quantum dots (QDs) and wetting layer have been investigated by transmission electron microscopy (TEM), photoluminescence (PL), and photoreflectance (PR). The PL peak position of InAs QDs directly grown on the thin AlAs is blueshifted from that of InAs QDs grown on the GaAs layer by 171 meV mainly due to the high potential barrier and reduced dot size shown in the TEM image. As the additional GaAs layer (1 and 2 nm) is inserted on top of the AlAs layer, the PL peak position is systematically shifted toward longer wavelength with increase in the thickness. Temperature dependent PL of QD samples shows that a thin AlAs layer significantly influences the thermal activation energy. The wetting layer related peak in PR spectra is changed to lower energy with increase in the thickness of an additional GaAs layer, which is mainly caused by the reduction in the effects of the AlAs layer.

Height-controlled InAs quantum dots by using a thin InGaAs layer

The structural and optical properties of height-controlled InAs quantum dots (QDs) have been investigated by transmission electron microscopy (TEM) and photoluminescence (PL). By depositing 1.4 nm In0.15Ga0.85As and a 1 monolayer (ML) InAs layer with different periods on 3 ML InAs QDs, the height of InAs QDs was systematically controlled with similar lateral size. In TEM images, the indication of dislocations due to the large strain, which can be easily seen in large QDs, is not observed even for the QD sample with the highest aspect ratio (height/width). The PL peak position is shifted toward the longer wavelength with an increase in the aspect ratio of QDs. As the aspect ratio is increased, the full width at half maximum in PL measured at 10 K is decreased from 71 to 34 meV indicating that the inhomogeneous broadening caused by the fluctuation in QD size, especially the height, is significantly reduced.

Energy level control for self-assembled InAs quantum dots utilizing a thin AlAs layer

Ground-state energy of InAs quantum dots (QDs) in the GaAs matrix can be changed significantly by introducing a thin AlAs layer (1 nm). The photoluminescence (PL) peak position of the QDs grown directly on the thin AlAs layer is blueshifted by 171 meV from that of the QDs grown without the AlAs layer. QDs grown on an additional GaAs thin layer on top of the AlAs layer have PL peaks systematically redshifted to lower energy as the GaAs layer becomes thicker. Time-resolved PL shows that the QDs have similar lifetimes, attesting to the fact that all the QDs grown in this way are of high quality, although the energy level change is large and a thin AlAs layer is introduced.

The energy level spacing from InAs/GAas quantum dots : Its relation to the emission wavelength, carrier lifetime, and zero dimensionality

We have found that the level spacing between the ground and first excited states of InAs∕GaAs quantum dots (QDs) increases as the photoluminescence peak energy decreases, that is, as the QD increases in size. By means of simple numerical calculations, we confirm that this seemingly unusual level-spacing behavior originates from the low aspect ratio of typical QDs with a finite potential barrier. Carrier lifetime measurements show that QDs with a lower photoluminescence peak energy tend to have a shorter decay time, which can be attributed to better confinement of the electron wave function and the resultant increase in electron-hole wave function overlap.

Growth of Si-doped InAs quantum dots and annealing effects on size distribution

We investigated the Si-doped InAs quantum dots (QDs) grown by molecular beam epitaxy and the annealing effects on the QD size distribution through photoluminescence (PL) spectroscopy. A double-peak feature in PL was observed from as-grown InAs QDs with Si-doping, and excitation intensity dependence of PL indicated that the double-peak feature is related to the ground-state emission from InAs QDs with bimodal size distribution. The PL spectrum from Si-doped InAs QDs subjected to annealing treatment at 800°C in nitrogen ambient showed three additional PL peaks and blue-shift of the double-peak feature observed from as-grown sample. The excitation-intensity-dependent PL and consideration of thermal stability of carriers through temperature-dependent PL measurement demonstrated that three additional peaks come from the InAs QDs with three new branches of QD occurring during annealing process.

High-quality GaN epilayer grown by newly designed horizontal counter-flow MOCVD reactor

We have fabricated a newly designed horizontal counter-flow reactor for growing high-quality III-V nitrides and characterized the GaN/sapphire(0 0 0 1) grown in it. The surface morphology of the film was featureless and smooth without any defects such as hillocks or truncated hexagonals. The measured background concentration and carrier mobility of the film 1.5 m thick are 4 × 1017/cm3 and 180 cm2/V s, respectively. The defect density measured by TEM is about 1 × 109/cm2 and the FWHM of DCX-ray curving is 336 arcsec, respectively. This crystallinity is similar to what was commonly obtained for GaN on sapphire until recently. The FWHM of the band-edge emission peak measured by PL at room temperature is typically around 14 and 4 meV for the main extonic peak(DBE) at 10 K. Except DBE at 3.490 eV, two minor structures are detected on the high-energy and low-energy shoulder of DBE at 3.498 eV(FE) and 3.483(ABE).

Characteristics of Mg-doped GaN epilayers grown with the variation of Mg incorporation

Abstract We have studied the growth characteristics of Mg-doped GaN epilayers grown by MOCVD with the variation of Cp 2 Mg flow rate. To optimize the p-type conductivity. We investigated the dependence of acceptor concentration on the dopant source (Cp 2 Mg) flow rate. The van der Pauw technique, double-crystal X-ray diffractometry (DCXRD) and photoluminescence (PL) were used to characterize their crystallographic, electrical and optical properties. As the incorporation of Mg in GaN epitaxy increases, the surface morphology and crystallinity of the layers become rough and worse because of the increase of lattice distortion due to the large difference of the atomic size between Ga and Mg. With the increase of Mg incorporation, the resistivity of the epilayers increases abruptly without discontinuity because of the increase of much Mg–H complex not cracked. So, it is possible to know that only the partial amount of Mg–H complex in the layers are unbound by annealing at a certain condition. In spite of the continuous increase of Mg incorporation, the hole concentration of the epilayers first increases and then decreases from a certain Mg flow rate. As well as the results of hole concentration, the blue emission intensity of the layers in PL spectra at room temperature first increases and then decreases from a certain Mg flow rate. Therefore, it can be concluded that there is a limitation in Mg activation and the hole concentration decreases from this limit though the incorporation of Mg increases.