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Impurity doping is a key factor that needs to be addressed to control the optical properties of silicon quantum dots (Si QDs) for their applications in optoelectronic devices. Although there have been several studies to understand the effect on n-type and p-type dopant on the luminescence properties of Si QDs, the exact influence of the dopant impurities on the optical properties and the underlying mechanism are yet to be understood. For this purpose, we investigate the luminescence properties of Si QDs/SiO 2 multilayer structure doped with B and P. Using rf-magnetron sputtering and ion implantation techniques, we prepared three kinds of samples: undoped, B-doped and P -doped. Results from photoluminescence measurements indicate that the significant luminescence suppression in the doped samples can be mainly attributed to the Auger non-radiative process occurring between photoexcited excitons and free carriers. Our interpretation should serve to resolve the controversy around the effect of impurity doping on the luminescence properties of Si QDs.

We investigated nano-floating gate memory having a charge trap layer (CTL) composed of cobalt germanide nanostructure (ns-CoGe). A tunneling oxide layer; a CTL containing Co, Ge, and Si; and a blocking oxide layer were sequentially deposited on a p-type silicon substrate by RF magnetron sputtering and low-pressure chemical vapor deposition. We optimized the CTL formation conditions by rapid thermal annealing at a somewhat low temperature (about 830 °C) by considering the differences in Gibbs free energy and chemical enthalpy among the components. To characterize the charge storage properties, capacitance–voltage (C–V) measurements were performed. Further, we used X-ray photoelectron spectroscopy for chemical analysis of the CTL. In this work, we not only report that the C–V measurement shows a remarkable opening of the memory window for the ns-CoGe compared with those of nanostructures composed of Co or Ge alone, but also clarify that the improvement in the memory characteristics originates in the nanostructure formation, which consists mainly of Co-Ge bonds. We expect ns-CoGe to be a strong candidate for fabrication of next-generation memory devices.

Transparent BaSnO3 thin films have been proposed as an alternative transparent conducting oxide (TCO). Although bulk synthesis and high-quality fabrication of epitaxial films are well established, there are still unsolved aspects about their electronic structure, such as the direct or indirect nature and the size of the band gap. We investigated the electronic structure of epitaxial BaSnO3 thin films using in situ angle-resolved photoemission spectroscopy. We directly measured an indirect band gap of 3.7 eV, a value compatible with those of previous reports, but we also identified additional in-gap states at −1.6 eV below the conduction band minimum that we attribute to intrinsic defects, mainly oxygen vacancies.

Controlling the emission wavelength by the quantum confinement or the size effect of silicon quantum dots (Si QDs) has a limitation in that it can be only approximately 600 nm or above. To overcome this limitation, in the recent decade, numerous studies have been conducted for reproducing the entire visible range from blue to red from Si QDs by their surface functionalization. In this study, we fabricated alkyl- and amine-passivated Si QDs and investigated their emission mechanism following surface functionalization. We interpreted the emission mechanism of the surface-functionalized Si QDs by combined experimental analysis of the decay time dynamics and density functional theory-based model calculation of the highest occupied molecular orbital-lowest unoccupied molecular orbital electronic structures. Our comprehensive studies could provide insight into the debated problems of the luminescence mechanism of surface-functionalized Si QDs.

We propose a chemical wet etching technique that does not make use of toxic Hydrofluoric solution to produce cladding-etched optical fiber intended for use as a platform for a fiberized saturable absorber. The wet etching technique is based on the use of a mixed solution of NH4F and (NH4)2SO4, which is not toxic, and a small, specially-devised etching cradle to achieve precise control of the etched fiber length. A fiberized saturable absorber was implemented by depositing the graphene oxide particles on top of the residual cladding for our cladding-etched fiber by using the drop & dry method. The saturable absorber was incorporated into an erbium-doped fiber ring cavity to test its suitability as a mode-locker with a modulation depth larger than 4.3%. Femtosecond pulses with a temporal width of ∼615 fs were readily obtained at a repetition rate of ∼17.09 MHz. The saturable absorption and laser performance of our saturable absorber were compared to those of an absorber on a core-etched fiber platform prepared through the formation of microdroplets of HF and surface tension-driven flow, as in [57] .

We investigated impact of excimer laser annealing on the AlGaN/GaN heterojunction structures, where the 2-dimensional electron gas (2DEG) is formed. When applying homogeneous laser pulses at various laser densities, the electrical resistance of the 2DEG shows sudden increase along with sharp decrease of the carrier mobility and slight decrease of the carrier density. Especially, when applying laser density of 300 mJ/cm 2 , we could obtain such jump of electrical resistance even after 2 min, i.e. 2400 pulses at 20 Hz. Low temperature photoluminescence and x-ray photoemission spectroscopy measurements show that the excimer laser annealing suppresses the coherent charge carriers and oxides the sample surface to form Ga 2 O. We estimate the activation energy of suppressing 2DEG to be 0.89 eV for the ELA process. We suggest that the excimer laser annealing has potential for gate oxide fabrication, surface passivation, and lateral pattering of 2DEG structures.

Cu2ZnSnS4 (CZTS) films were grown by electrochemical deposition, and we measured the work function of the as-grown and of the KCN-etched CZTS surfaces by using Kelvin probe force microscopy (KPFM) and micro-Raman scattering spectroscopy with incident laser wavelengths of 488.0 and 632.8 nm, respectively, and the results indicate that a secondary phase formed at different depths. The KPFM measurements can discriminate phase uniformity at the nano-scale. Secondary phases, such as Cu2−xS (0

Si nanocrystal (NC)/SiO2 multilayers containing interfacial nitrogens are formed by radiofrequency magnetron-sputtering and post-annealing. By analyzing the photoluminescence (PL) of the multilayer structures after proton irradiation (PI), we found that the peak at ∼740 nm was shifted toward shorter wavelengths and that its intensity was considerably suppressed. Furthermore, a new peak simultaneously appeared at ∼500 nm. We interpret that PI not only modifies the shapes of the Si NCs and generate defects at the NC/SiO2 interface, but also induces simultaneously hydrogen passivation (HP) of interfacial nitrogens due to the attenuated protons. To investigate the relationship between the HP of N at the Si NC/SiO2 interface and the observed band gap reduction of Si NC structures within the SiO2 matrix, we modeled the atomic configurations at the interface and applied first-principles calculations. The results clearly indicate that the HP of composite structures containing Si–N bonds at the Si NC/SiO2 interface induces the blue-shift in PL. We expect that this investigation helps to pave the way for developing more efficient Si-based light emitting devices or solar cells.

SiO

For GaN/AlGaN/GaN heterostuructures grown on Si substrates, we investigated the changes in the chemical compositions during photochemical oxidation. We utilized synchtrotron-based high-resolution X-ray photoemission spectroscopy to precisely analyze the stoichiometry change. Epitaxially-grown GaN/AlGaN/GaN multilayer films showed a single-crystalline structure and the dominant bonding character of Ga-N near the surface. After photoelectrochemical oxidation with deionized water, the Ga-N bonding transformed into the Ga-O bonding, in which the stoichiometry was confirmed to be Ga2O3. Thus, photoelectrochemical oxidation with water may be a good candidate for fabricating the gate dielectric layer, Ga2O3, for GaN-based power transistors.

SiO2 is a commonly used insulation layer for QCLs but has high absorption peak around 8 to 10 µm. Instead of SiO2, we used Y2O3 as an insulation layer for DC-QCL and successfully demonstrated lasing operation at the wavelength around 8.1 µm. We also showed 2D numerical analysis on the absorption coefficient of our DC-QCL structure with various parameters such as insulating materials, waveguide width, and mesa angle.