Young-jo Tak

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.

A Novel Growth Method of Freestanding GaN Using In situ Removal of Si Substrate in Hydride Vapor Phase Epitaxy

We demonstrate the freestanding GaN of 2 in. diameter and 400 µm thickness grown from Si substrate by hydride vapor phase epitaxy. To prevent the formation of cracks in the GaN layer during cooling, the Si substrate was removed at high temperature, which successfully suppressed the tensile stress evolution in GaN. The freestanding GaN exhibited high quality with FWHM of 65 arcsec in (0002) X-ray rocking curve and etch pit density of less than 1×106/cm2. It may be possible to fabricate high-quality freestanding GaN substrates of over 8 in. diameter using this method.

Dependence of reverse bias leakage on depletion width and V-pit size in InGaN/GaN light-emitting diodes grown on silicon

The reverse bias leakage characteristics of InGaN/GaN light emitting diodes (LEDs) grown on Si (111) were investigated as a function of two factors: (1) bulk depletion width and (2) V-pit size. The reverse leakage current showed a decreasing trend with an increase in V-pit size, given a fixed depletion width. Atomic probe tomography was used to verify that a reduction in electric field near the vicinity of threading dislocations suppresses field-assisted carrier emission, reducing reverse leakage. Calculations using the appropriate theory show a reasonable agreement with the experimental results. These findings further elucidate the role of V-pits as passivation for reverse leakage paths and may be useful for not only LEDs but GaN-based power devices as well.