Kyoung Chan Kim

Gain-dependent linewidth enhancement factor in the quantum dot structures

We measured the linewidth enhancement factor (alpha factor) of InAs quantum dot (QD) laser diodes (LDs) with two different QD structures. One is a normal QD LD with the same energy bandgap for each active QD layer, while the other is chirped with different energy bandgaps. The differential gain of the chirped InAs QD LDs is found to be about five times smaller than that of normal InAs QD LDs, whereas no overall wavelength shift with injection currents is observed in both QD LDs. The alpha factor is approximately five times higher in the chirped InAs QD LDs than in the normal InAs QD LDs. This relatively large alpha factor in the chirped InAs QD LDs is attributed to the asymmetrical, wide inhomogeneous gain profile.

High power single-lateral-mode operation of InAs quantum dot based ridge type laser diodes by utilizing a double bend waveguide structure

We report on the high-power, single-lateral-mode operation of InAs quantum dot (QD) based ridge type laser diodes (LDs) by utilizing a double bend (DB) waveguide structure. The LDs were designed so that only fundamental modes propagate and higher optical modes are suppressed through the bent regions. DB waveguide LDs allow the use of wide ridge widths for fundamental mode operations, which helps to increase their output power via the increase in their net gain. We measured continuous wave single-lateral-mode output power of up to 310 mW from InAs QD DB waveguide LDs manufactured with 10-μm-wide stripes without facet coating.

Optical Characteristics and the Linewidth Enhancement Factor Measured from InAs/GaAs Quantum Dot Laser Diodes

We report the optical characteristics and the linewidth enhancement factor (alpha-factor) of InAs/GaAs quantum dot (QD) laser diodes (LDs) with a 5-mum-wide stripe and 1-mm-long cavity. Continuous wave (CW) operation of the laser yielded a kink-free output power of 160 mW with an external efficiency of 0.35 W/A. The threshold current was 28 mA and the lasing occurred at 1286 nm, solely at the ground state (GS) up to a current of 345 mA, without switching to the excited state (ES). Then, to estimate the alpha-factor of the LDs, we investigated the differential gain and wavelength shift versus the current. The differential gain was 2.37 cm-1/mA at 1286 nm, and the wavelength shift was 0.00046 nm/mA, which resulted in an alpha-factor of 0.057 at 1286 nm. The alpha-factor decreased with increasing wavelength. To the best of our knowledge, this is the lowest value of the alpha-factor that has been reported so far, indicating that our QD-LD has excellent beam quality under high-power operation due to the elimination of the filamentation.

InGaN/GaN White Light-Emitting Diodes Embedded With Europium Silicate Thin Film Phosphor

This article describes the successful fabrication of europium-silicate thin film phosphor and its application to InGaN/GaN white light-emitting diodes (LEDs) in order to improve the photometric properties of the LEDs, including their correlated color temperatures (CCT) and color rendering index (CRI). The europium-silicate compounds are deposited on GaN templates grown on sapphire substrates by RF-sputtering and then annealed at 1000°C in an N2 ambient to form a thin film phosphor that produces yellow or red emissions. The thin film phosphor is then patterned with stripes to grow a GaN buffer layer by epitaxially laterally overgrown GaN (ELOG) techniques, on which LED structures are grown by metal organic chemical vapor deposition. The ELOG sample shows no pits on the surface, and the full widths at half maximum (FWHMs) of its X-ray rocking curve for the (002) and (102) planes are as low as 249 and 416 arcsec, respectively. The optical spectrum from the embedded thin film phosphor is adjusted to have a maximum intensity at 560-600 nm and a FWHM as wide as 90 nm to make up for the low efficiency at these wavelengths of conventional YAG-based yellow phosphor. Finally, we observed a tristimulus coordinate (x, y) = (0.33, 0.39), CCT = 5607 K, and CRI = 77.6 from the white LEDs with thin film phosphor as compared with (x, y) = (0.30, 0.28), CCT = 8467 K, and CRI = 66.52 for the white LEDs without thin film phosphor.

Influence of post-annealing on the electrical properties of metal/oxide/ silicon nitride/oxide/silicon capacitors for flash memories

We report the effect of post-annealing on the electrical properties of metal/oxide/silicon nitride/oxide/silicon (MONOS) capacitors. Four samples, namely as-deposited and annealed at 750, 850 and 950 °C for 30 s in nitrogen ambient by a rapid thermal process, were prepared and characterized for comparison. The best performance with the largest memory window of 4.4 V and the fastest program speed of 10 ms was observed for the sample annealed at 850 °C. In addition, the highest traps density of 6.84 × 1018 cm−3 was observed with ideal trap distributions for the same sample by capacitance–voltage (C–V) measurement. These results indicate that the memory traps in the ONO structure can be engineered by post-annealing to improve the electrical properties of the MONOS device.

Design and Simulation of an 808 nm InAlAs/AlGaAs GRIN-SCH Quantum Dot Laser Diode

Quantum dots were designed within a GRIN-SCH(Graded index - Separate confinement Heterostructure) heterostructure to create a high power InAlAs/AlGaAs laser diode. 808 nm light emission was with a quantum dot composition of In0.665Al0.335As and wetting layer composition of Al0.2Ga0.8As by LASTIP simulation software. Typical characteristics of GRIN structures such as high confinement ratios and Gaussian beam profiles were shown to still apply when quantum dots are used as the active media. With a dot density of 1.0x1011 dots/cm2, two quantum dot layers were found to be good enough for low threshold, high-power laser applications.