Su-hee Chae

Room-temperature GaN vertical-cavity surface-emitting laser operation in an extended cavity scheme

A GaN-based vertical-cavity surface-emitting laser (VCSEL) has been demonstrated in an extended cavity structure. A VCSEL device had a long extended cavity, which consisted of a sapphire substrate as well as a GaN epilayer and had an integrated microlens on one side. High-reflection dielectric mirrors were deposited on both sides of the laser cavity. The laser was optically pumped and operated at room temperature. The VCSEL device lased at a low threshold excitation intensity of 160 kW/cm2. In contrast to a conventional microcavity-VCSEL structure, the VCSEL operated in multiple longitudinal modes with mode spacing consistent with its physical thickness.

Crystal-polarity dependence of Ti/Al contacts to freestanding n-GaN substrate

The effect of crystal polarity on the electrical properties of Ti/Al contacts to n-GaN substrate has been investigated. The Ti/Al contacts prepared on Ga-face n-GaN substrate became ohmic with a contact resistivity of 2×10−5 Ω cm2 after annealing at temperatures higher than 600 °C for 30 s. On the contrary, the contacts on N-face n-GaN substrate exhibited nonlinear current–voltage curve and high Schottky barrier heights over 1 eV were measured at the same annealing conditions. These results could be explained by opposite piezoelectric-field at GaN/AlN heterostructure resulted from different polarity of the GaN substrate.

High rate etching of sapphire wafer using Cl2/BCl3/Ar inductively coupled plasmas

Abstract Sapphire wafers which are generally used for the fabrication of GaN-based optoelectronic devices are found to be very difficult in lapping, polishing, and cutting for packaging due to the chemical stability and hardness of sapphire. To study possibilities of replacing some of these processes by dry etching, (0001) sapphire wafers were etched using inductively coupled plasmas as a function of gas combination of Cl2/BCl3 and Ar/Cl2/BCl3. The increase of BCl3 in Cl2/BCl3 increased the etch rates. Also, the increase of BCl3 in Cl2 improved the etch selectivity over photoresist. With the mixture of 50% Cl2/50% BCl3, sapphire etch rates of 362.7 nm min−1 could be obtained and, by the addition of 20% Ar in this mixture, the etch rates increased further to 377.5 nm min−1. When the sapphire etching with 50% Cl2/50% BCl3 was applied to lapped wafers for polishing, the surface roughness was decreased from 12.95 to 1.43 nm and the smoothness was better than mechanically polished sapphire surface (5.3 8 nm).

Effect of V-Shaped Pit Size on the Reverse Leakage Current of InGaN/GaN Light-Emitting Diodes

We report the effect of V-shaped pit size, inverted hexagonal pits embedded in InGaN multiple quantum well, on the reverse leakage current of InGaN light-emitting diodes (LEDs). It was found that the reverse leakage current of InGaN LEDs with larger V-shaped pits is significantly reduced from 1.80 mA down to 3.84 nA at -30 V by several orders of magnitude. We claim that this improvement is accounted for increased Poole-Frenkel barrier height of carrier trapped at the deep centers via enlarged V-pit formation, consequently resulting in lower reverse current of InGaN LEDs.

Highly efficient InGaN/GaN blue LED on 8-inch Si (111) substrate

We have grown LED structures on top of a robust n-type GaN template on 8-inch diameter silicon substrates achieving both a low dislocation density and a 7 um-thick template without crack even at a sufficient Si doping condition. Such high crystalline quality of n-GaN templates on Si were obtained by optimizing combination of stress compensation layers and dislocation reduction layers. Wafer bowing of LED structures were well controlled and measured below 20 μm and the warpage of LED on Si substrate was found to strongly depend on initial bowing of 8-inch Si substrates. The full-width at half-maximum (FWHM) values of GaN (0002) and (10-12) ω-rocking curves of LED samples grown on 8-inch Si substrates were 220 and 320 arcsec. The difference between minimum and maximum of FWHM GaN (0002) was 40 arcsec. The dislocation densities were measured about 2~3×108/cm2 by atomic force microscopy (AFM) after in-situ SiH4 and NH3 treatment. The measured quasi internal quantum efficiency of 8-inch InGaN/GaN LED was ~ 90 % with excitation power and temperature-dependent photoluminescence method. Under the un-encapsulated measurement condition of vertical InGaN/GaN LED grown on 8-inch Si substrate, the overall output power of the 1.4×1.4 mm2 chips representing a median performance exceeded 484 mW with the forward voltage of 3.2 V at the driving current of 350 mA.

Analysis of forward tunneling current in InGaN/GaN multiple quantum well light-emitting diodes grown on Si (111) substrate

We report the characteristics of forward tunneling current in InGaN/GaN multiple quantum well light-emitting diodes (LEDs) grown on Si (111) substrate. Temperature-variable current-voltage (I–V) measurement from 80 K to 400 K reveals that forward current regimes can be distinguished by corresponding slopes in semi-logarithmic plot, which are associated with different forward conduction mechanisms of InGaN LED. Temperature-insensitive tunneling behavior appears to be dominant at low current injection regime for InGaN LEDs on Si. Conductive atomic force microscopy analysis indicates that V-pits associated with threading dislocations could be main leakage path of forward tunneling current of InGaN LED on Si.

Single-mode blue-violet laser diodes with low beam divergence and high COD level

We demonstrate GaN-based high-power single transverse-mode laser diodes (LDs) emitting at 405 nm. LD structures are designed to exhibit a high level of catastrophic optical damage and small beam divergence angle. By the control of refractive index profiles, we achieved a vertical beam divergence angle of as low as 17.5/spl deg/ and maximum output power of as high as 470 mW under continuous-wave operation condition. In addition, nearly fundamental transverse-mode operation is demonstrated up to 500-mW pulsed output power by far-field investigation.

Recent achievements of AlInGaN based laser diodes in blue and green wavelength

AlInGaN based blue and blue-green LDs were investigated with regard to the characteristics of GaN semiconductor laser diodes. High power, single mode blue LDs with high COD level (~334mW under CW operation at 25°C, kink-free at 150mW) and long lifetime (~10000 hours under CW operation, 50mW 25°C) were achieved. No significant characteristic differences between blue LDs on LEO-GaN/sapphire and GaN substrate were observed. The blue-green LD which has the wavelength of 485 nm was successfully fabricated and demonstrated under CW operation 25°C, while it showed poor performances of LD characteristics compared to those of blue LDs. We believe that the poor performance of blue-green LDs were caused by the piezo-electric effect by lattice mismatch along C-axis of GaN, In fluctuation by lattice mismatch and In solubility limit in InGaN QWs and thermal annealing which was performed during the p-layer growth.

High power AlInGaN-based blue-violet laser diodes

High power and high efficiency AlInGaN-based laser diodes with 405 nm were fabricated for the post-DVD applications. Magnesium doped AlGaN/GaN multiple quantum barrier (MQB) layers were introduced into the laser diode structure, which resulted in considerable improvement in lasing performances such as threshold current and slope efficiency. Asymmetric waveguide structure was used in order to improve the characteristics of laser diodes. Aluminum content in the n-cladding layer was varied in connection with the vertical beam divergence angle and COD level. By decreasing Al content in the n-cladding layer, the vertical divergence angle was reduced to 17 degree and the COD level was enhanced to over 300mW. The maximum output power reached as high as 470 mW, the highest value ever reported for the narrow-stripe GaN LDs. In addition, the fundamental transverse-mode operation was clearly demonstrated up to 500 mW-pulsed output power.

Magnetically enhanced inductively coupled plasma etching of 6H-SiC

In this study, 6H-SiC wafers were etched using a magnetically enhanced inductively coupled SF/sub 6/-based plasmas (MEICP) and their etch characteristics were investigated. The etch characteristics of SiC and the etch selectivities over metal thin films such as Cu and Ni were investigated as a function of inductive power, operating pressure, additive gas percentage, etc. To understand the etch mechanism, the etched SiC and Cu surfaces were examined by X-ray photoelectron spectroscopy (XPS) and the radical and ion densities in the plasmas were measured by optical emission spectroscopy (OES) and a Langmuir probe, respectively. The obtained highest etch rate was about 1.9 /spl mu/m/min with 90%SF/sub 6//10%O/sub 2/. By XPS analysis, it could be confirmed that the addition of small oxygen percentage assisted in forming volatile SiFx by the reaction with carbon on the SiC surface. In our experimental conditions, the increase of thickness by the formation of a reaction product instead of etching was observed on the Cu mask layer, therefore, the calculated selectivity of SiC to Cu was infinite. Using the Cu mask, 80-100 /spl mu/m thick SiC substrates could be fully etched with vertical etch profiles and smooth etch sidewalls.