Han-Su Cho

Nitrogen-polar GaN growth evolution on c-plane sapphire

This letter presents a study on N-polar GaN growth evolution on sapphire using a low-temperature GaN buffer, which is distinctly different from the two-step growth of Ga-polar GaN according to both in situ reflectance and ex situ microscopy. Annealed N-polar GaN buffer exhibits densely packed tiny grains, serving as a template for the subsequent high-temperature GaN growth, which starts in a quasi-two-dimensional mode without any roughening-recovery process. Atomically smooth N-polar GaN has been achieved with no stacking fault or inversion domain observed. The mosaic microstructure, electrical, and optical properties of N-polar GaN are compared with those of Ga-polar GaN.

Free-Standing GaN Layer by Combination of Electrochemical and Photo-Electrochemical Etching

A free-standing GaN layer was produced by combining electrochemical (EC) etching from the front surface, photo-electrochemical (PEC) etching from the back surface, and subsequent regrowth of GaN on the porous template thus produced. The EC etching resulted in the formation of etch channels on the surface portion of the starting film, whereas the back-side PEC etching gave rise to a columnar structure supporting the entire film. When the n-GaN layer was regrown on such template, the underlying columnar structure provided weak places for easy separation and transfer of the film by mechanical bonding.

Microcathodoluminescence spectra evolution for planar and nanopillar multiquantum-well GaN-based structures as a function of electron irradiation dose

Effects of low energy electron beam irradiation (LEEBI) of planar and nanopillar InGaN/GaN multiquantum well light emitting diode structures are discussed. The bands observed in microcathodoluminescence (MCL) spectra were attributed to recombination involving two types of InGaN quantum dots with lower (2.92 eV MCL band) and higher (2.75 eV) indium concentration. During the LEEBI treatment, the intensity of both MCL lines first decreased, presumably due to the introduction of radiation defects, then, after the dose of 0.2 C/cm2 increased, reached a maximum and then again decreased. At the same time, the peak energy showed a red shift at low irradiation doses and a blue shift at high doses. The results are explained by an interplay between the increasing density of nonradiative recombination defects and quantum dots during irradiation. The difference between the nanopillar and planar structures is attributed to a stronger impact of surface defects in nanopillars.

Enhanced optical properties of nanopillar light-emitting diodes by coupling localized surface plasmon of Ag/SiO2 nanoparticles

We fabricated nanopillar light-emitting diodes (LEDs) embedded with Ag/SiO2 nanoparticles, and investigated the energy coupling processes between the localized surface plasmons of nanoparticles and the active quantum well regions of nanopillar LEDs. These nanoparticle-embedded nanopillar LEDs showed considerable increases in photoluminescence and electroluminescence intensities, compared with reference nanopillar LEDs. The observed optical enhancement was explained by the increased spontaneous emission rate caused by energy coupling from excitons in the InGaN/GaN quantum well active region of the LEDs to the localized surface plasmon modes of the Ag/SiO2 nanoparticles. A strongly enhanced characteristic photoluminescence decay also confirmed such an explanation.

Facile Fabrication of Free-Standing Light Emitting Diode by Combination of Wet Chemical Etchings

Free-standing GaN light-emitting diode (LED) structure with high crystalline quality was fabricated by combining electrochemical and photoelectrochemical etching followed by regrowth of LED structure and subsequent mechanical detachment from a substrate. The structural quality and composition of the regrown LED film thus produced was similar to standard LED, but the photoluminescence and electroluminescence intensity of the LED structures on the etched template were several times higher than for standard LED. The performance enhancement was attributable to additional light scattering and improved crystalline quality as a result of the combined etching scheme.

Electrical Characteristics and Deep Traps Spectra of Undoped GaN Films Grown on Si Using Different Strain-Relieving Buffer Types

Electrical properties of GaN films grown on Si by molecular beam epitaxy using various types of strain-relieving layers have been studied by means of Hall/van der Pauw measurements, capacitance-voltage profiling, admittance spectroscopy, and deep levels transient spectroscopy with electrical and optical injection. The electrical properties of all grown films were determined by relatively deep electron traps N1, N2, and N3 with aggregate concentration of ~1017 cm-3. Freezing out of these traps led to the films freezing out down to the depth corresponding to the nearest underlying heterointerface where a strong band bending caused a sharp nonuniformity of charge carriers concentration. For AlN or Al-rich AlGaN underlying films, this band bending could cause formation of hole sheet charge leading to apparent conductivity to appear p-type in Hall. Other deep traps detected in the grown films were N4 and N5 acceptors with levels near Ec - 0.6 eV, and hole traps H1 and H2 with levels near Ev + 0.9 eV. Possible consequences of the observed phenomena for designing the thick GaN stand-off films in power transistors are briefly discussed.