Joo-sung Kim

Electrically Driven Quantum Dot/Wire/Well Hybrid Light‐Emitting Diodes

Electrically driven quantum dot, wire, and well hybrid light-emitting diodes are demonstrated by using nanometer-sized pyramid structures of GaN. InGaN quantum dots, wires, and wells are formed at the tops, edges, and sidewalls of pyramids, respectively. The hybrid light-emitting diodes containing low-dimensional quantum structures are good candidates for broad-band highly efficient visible lighting sources.

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 yellow photoluminescence from {11–22} InGaN multiquantum-well grown on nanoscale pyramid structure

InGaN/GaN multiquantum wells (MQWs) with a peak wavelength of 570 nm are grown on nanosize GaN hexagonal pyramid structures. Temperature dependent photoluminescence (PL) measurements from 10 to 300 K show a high integrated intensity ratio of 0.45. The emission energy of the MQW monotonically decreases with temperature increase, showing the absence of localized potential. Power dependent PL shows no noticeable blueshift caused by piezoelectric field screening effect. Comparative study of the PL results with those of the InGaN MQW on microsize pyramid show that nanosize pyramids play an important role in suppressing piezoelectric field in addition to the semipolar growth direction. We attribute the high luminescence efficiency of the MQW on nanosize pyramid structures to effectively suppressed piezoelectric field and potential localization.

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.

OPTICAL AND STRUCTURAL PROPERTIES OF ZNCDSE/ZNSSE/ZNMGSSE SEPARATE CONFINEMENT HETEROSTRUCTURES

We have grown ZnCdSe/ZnSSe/ZnMgSSe separate confinement heterostructures by molecular‐beam epitaxy. Strain on the ZnSSe layer is calculated from x‐ray and photoluminescence data. The temperature dependence of band‐gap energy and the photoluminescence intensity in the Cl‐doped ZnCdSe active layers is compared with that of undoped ones.

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.