Young-Woo Ok

Growth and characterization of hypothetical zinc-blende ZnO films on GaAs(001) substrates with ZnS buffer layers

A stable wurtzite phase of ZnO is commonly observed. In this letter, we report the growth and characterization of zinc-blende ZnO on GaAs(001) substrates. The ZnO films grown on GaAs(001) substrates using microwave-plasma-assisted metalorganic molecular-beam epitaxy were characterized by reflection high-energy electron diffraction, x-ray diffraction, transmission electron microscope, and atomic force microscope measurements. The use of a ZnS buffer layer was found to lead to the growth of the zinc-blende ZnO films. Although the zinc-blende ZnO films were polycrystalline with columnar structures, they showed bright band-edge luminescence at room temperature.

Temperature dependence of photoluminescence of InGaN films containing In-rich quantum dots

The temperature dependence of the photoluminescence (PL) of InGaN films, grown by metalorganic chemical vapor deposition, has been investigated. A strained InGaN thin film which contains composition-fluctuated regions shows the so-called S-shaped temperature dependence of the dominant PL peak energy. However, an InGaN thick film which contains quantum dot-like In-rich regions shows a sigmoidal temperature dependence of the dominant PL peak energy, as the result of a transfer of carriers from the band-edge related luminescent centers to quantum dot-like In-rich regions. It is also found that the activation energy for the thermal quenching of PL intensity in the InGaN thick film which contains quantum dot-like In-rich regions is larger than that in the strained InGaN thin film which contains composition-fluctuated regions.

Chemical etching of boron-rich layer and its impact on high efficiency n-type silicon solar cells

This paper reports on an effective chemical etching treatment to remove a boron-rich layer which has a significant negative impact on n-type silicon (Si) solar cells with boron emitter. A nitric acid-grown oxide/silicon nitride stack passivation on the boron-rich layer-etched boron emitter markedly decreases the emitter saturation current density J0e from 430 to 100 fA/cm2. This led to 1.6% increase in absolute cell efficiency including 22 mV increase in open-circuit voltage Voc and 1.9 mA/cm2 increase in short-circuit current density Jsc. This resulted in screen-printed large area (239 cm2) n-type Si solar cells with efficiency of 19.0%.

Single-crystalline rocksalt CdO layers grown on GaAs (001) substrates by metalorganic molecular-beam epitaxy

In this letter, we report the growth of single-crystalline rocksalt CdO layers on (001) GaAs substrates using ZnS buffer layers. The growth processes of CdO layers were studied by reflection high-energy electron diffraction (RHEED), and the grown CdO layers were evaluated with atomic force microscopy (AFM), and x-ray diffraction (XRD) measurements. After an initial growth delay, the formation of polycrystalline CdO was observed in RHEED measurements during the initial growth of very thin CdO layers. With the increase of the CdO layer thicknesses, streaky RHEED patterns were observed, which indicate the formation of single-crystalline cubic-phase CdO layers. Surface morphology of the CdO layers observed by AFM was atomically flat with root-mean-square roughness of ∼1 nm. The crystalline structures were elucidated from XRD measurements by the determination of the lattice constant to be 4.686±0.001 A, indicating the single-phase rocksalt CdO structure.

Microstructures and electrochemical properties of Pt-doped amorphous V2O5 cathode films for thin film microbatteries

We have investigated the effects of Pt doping on the structural and electrochemical properties of amorphous (a) V2O5 films grown by radio frequency magnetron sputtering. It is shown that the cycling performance of the a-V2O5 films is dependent upon the Pt content. Glancing angle x-ray diffraction and transmission electron microscopy examination shows that the undoped film is amorphous with some short-range order. However, doping of the films at a rf power of 10 W leads to a complete a-V2O5 film which shows the best cycling behavior among the samples. It is shown that the short-range ordered structure reappears when the films are doped by cosputtering at rf powers of 30 and 50 W. It is further shown that doping at a rf power of 50 W results in the formation of amorphous and nanocrystalline PtO5 phases embedded in the a-V2O5 matrix.

Structural and optical properties of InGaN/GaN multiple quantum wells: The effect of the number of InGaN/GaN pairs

The effect of the number of InGaN/GaN quantum well (QW) pairs on the interfacial structural and optical properties of InGaN/GaN multiple quantum wells (MQWs), as grown by low-pressure metalorganic vapor-phase epitaxy was examined. As the number of QW pairs increased, In-rich InGaN precipitates were more readily detected in the InGaN/GaN MQWs by cross-sectional transmission electron microscope. The intensity of the photoluminescence (PL) peak was decreased and the PL peak was red-shifted with an increase in the number of QW pairs. X-ray diffraction measurements revealed that the interfacial structure between InGaN and GaN were also deteriorated with the increasing number of QW pairs. These results can be attributed to the relaxation of an accumulated strain through the dislocations induced by an increase in the total thickness of the MQWs with an increase in the number of QW pairs. These results suggest that the defects such as dislocations facilitate the formation of In-rich phases in the InGaN layers in the MQWs.

CdO epitaxial layers grown on (0 0 1) GaAs surfaces by metalorganic molecular-beam epitaxy

Atomically flat CdO thin layers were grown on GaAs(00 1) substrates with ZnS buffer layers. The grown layers were evaluated by reflection high-energy election diffraction (RHEED). atomic force microscopy (AFM). X-ray rocking curve (XRC), and transmission electron diffraction (TED) measurements. The RHEED observations of CdO thin layers showed two-dimensional growth mode revealed by streaky patterns with excellent homogeneity. through [110] and [I I 0] directions. This streaky pattern is the reflection of the flat surface morphology in atomic level assessed by AFM measurements with the surface root mean square roughness of 1 nm. The crystalline structure as well as quality of the thin CdO layers was investigated by X-ray diffraction and XRC measurements which indicated the single-crystalline rocksalt CdO thin layers with minimum full-width at half maximum under the optimal growth condition. The TED observation showed the epitaxial relationship between the rocksalt CdO and zincblende GaAs substrate as [1 1 0] CdO ∥[1 1 0] GaAs , [1 1 0] Cao ∥[1 1 0] GaAs , and [0 0 1] CaO ∥[001] GaAs .