Keunjoo Kim

Room‐temperature visible photoluminescence from silicon‐rich oxide layers deposited by an electron cyclotron resonance plasma source

Highly split, visible light emissions at room temperature were observed in the range from 335 to 650 nm in silicon‐rich oxide filmsdeposited in the plasma phase of a mixture of silane and oxygen. The mechanism of the light emissions is classified into two categories. The photoluminescence bands at both 365 and 469 nm are related to the intrinsic defects of the E′ center and the neutral oxygen vacancy, respectively. However, the relatively sharp peaks at 403 and 535 nm are correlated with the development of polycrystalline core of Si‐enriched parts.

Temperature‐dependent critical layer thickness for strained‐layer heterostructures

We systematically analyze the stress‐strain‐temperature relationships and include thermal strain contributions to the misfit‐strain only formalism of strained‐layer heterostructures. Application of this theory to the GexSi1−x/Si (100) and InxGa1−xAs/GaAs (100) system demonstrates that the thermal effect lowers the critical layer thickness significantly on both systems, in excellent agreement with experimentally measured values. Empirical formulae of the critical layer thickness in terms of a mole fraction and temperature for these systems are provided.

Growth law of silicon oxides by dry oxidation

A theoretical description of the kinetic mechanism of thermal oxidation in silicon is proposed by complementing the Deal - Grove model. The relationship of the classical linear - parabolic growth law is generalized to the logarithmic growth law which provides a complete description for the whole regime of oxide films. In particular, the enhanced oxidation rate in the thin regime may be attributed to the diffusion length, which is characterized by the difference between the activation energies of the diffusion process and the reaction process. Our fitting of the logarithmic growth law to several experimental results shows excellent agreement and the fitting parameters also provide activation energies of 1.50 and 2.49 eV for the interfacial reaction and diffusion in oxide respectively.

Thermal Oxynitridation of Silicon in N 2 O Ambients

A model for the growth kinetics of a dielectric film obtained by oxidation of silicon in nitrous oxide environment is presented. The complete logarithmic form of oxidation kinetics is introduced for understanding the oxynitridation mechanism which includes information of the role of nitrogen in oxide film. The model fits experimental data excellently, both for short and long growth times. The chemical reaction energies of 1.81 and 1.1 eV are required for the oxynitridation for short and long growth times, respectively. This result suggests that the initial stage of the dielectric growth requires the higher reaction energy to form the SiO x N y layer and the lower energy is needed for the bulk oxide formation from the reaction with the interfacial layer of the SiO x N y . For the N 2 O oxidation, there are two barriers for the chemical reactions for NO and O 2 species dissociated from N 2 O molecules.

Thermal quenching effect of an infrared deep level in Mg-doped p-type GaN films

The thermal quenching of an infrared deep level of 1.2–1.5 eV has been investigated on Mg-doped p-type GaN films, using one- and two-step annealing processes and photocurrent measurements. The deep level appeared in the one-step annealing process at a relatively high temperature of 900 °C, but disappeared in the two-step annealing process with a low-temperature step and a subsequent high-temperature step. The persistent photocurrent was residual in the sample including the deep level, while it was terminated in the sample without the deep level. This indicates that the deep level is a neutral hole center located above a quasi-Fermi level, estimated with an energy of EpF=0.1–0.15 eV above the valence band at a hole carrier concentration of 2.0–2.5×1017/cm3.

Enhanced quantum efficiency of amorphous silicon thin film solar cells with the inclusion of a rear-reflector thin film

We investigated the growth mechanism of amorphous silicon thin films by implementing hot-wire chemical vapor deposition and fabricated thin film solar cell devices. The fabricated cells showed efficiencies of 7.5 and 8.6% for the samples without and with the rear-reflector decomposed by sputtering, respectively. The rear-reflector enhances the quantum efficiency in the infrared spectral region from 550 to 750 nm. The more stable quantum efficiency of the sample with the inclusion of a rear-reflector than the sample without the rear-reflector due to the bias effect is related to the enhancement of the short circuit current.

Reactor design rules for GaN epitaxial layer growths on sapphire in metal-organic chemical vapour deposition

The thermal process of the growth of GaN-based semiconductors was analysed for two home-made horizontal reactors. The reactors were designed to make the ammonia gas flow in the opposite direction to the main gas flow. For two horizontal reactors different in dimension, the low Reynolds numbers of Re = 2.94 and 4.15 were chosen for stable laminar flow and the Rayleigh numbers governing the heat convection were optimized to the values of Ra = 6.0 and 76.2, respectively. The qualities of GaN and InGaN films were characterized by Hall effect measurement, x-ray diffraction and photoluminescence and compared with respect to the reactor dependency.

Nanopyramid Formation by Ag Metal-Assisted Chemical Etching for Nanotextured Si Solar Cells

Copyright ©2015 KIEEME. All rights reserved. This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited. pISSN: 1229-7607 eISSN: 2092-7592 DOI: http://dx.doi.org/10.4313/TEEM.2015.16.4.206 OAK Central: http://central.oak.go.kr

Nanotextured Si Solar Cells on Microtextured Pyramidal Surfaces by Silver-assisted Chemical Etching Process

We investigated nanotextured Si solar cells using the silver-assisted chemical etching process. The nanotexturing process is very sensitive to the concentration of chemical etching solution. The high concentration process results in a nanowire formation for the nanosurfaces and causes severe surface damage to the top region of the micropyramids. These nanowires show excellent light absorption in photoreflectance spectra and radiative light emission in photoluminescence spectra. However, the low concentration process forms a nano-roughened surface and provides high minority carrier lifetimes. The nano-roughened surfaces of the samples show the improved electrical cell properties of quantum efficiency, conversion efficiency, and cell fill factor due to the reduction in the formation of the over-doped dead layer.

Substrate imposed stress-strain effect on photoluminescence in hydrogenated amorphous silicon alloys

Hydrogenated amorphous silicon films were deposited on the unstrained and strained Si substrates by an electron cyclotron resonance plasma source. The photoluminescence spectra show that emission energies are different from each other. The redshift of photoluminescence induced by the biaxial tensile stress is increased with decreasing the temperature in the range of 0.11–0.17 eV. The stress effect also enhances the quantum efficiency significantly. The pseudomorphic growth of Si on a relaxed Si0.75Ge0.25 (100) surface provides the strain energy of about 0.17 eV. These comparable results indicate that the shift of emission energy is attributed to the stress effect perturbing the polysilane structure.