Hwan-Kuk Yuh

Experimental determination of current spill-over and its effect on the efficiency droop in InGaN/GaN blue-light-emitting-diodes

We report the experimental determination of current spill-over in InGaN/GaN blue light emitting diodes by measuring the change in the forward current generated by a resonant excitation. To quantify accurately, the absorption of the laser as a function of the forward current was also determined. Two samples that have clearly different behavior of efficiency droop were compared to clarify the relationship between the current spill-over and the efficiency droop. We conclude that the carrier spill-over does occur and can be a significant cause for the efficiency droop but cannot single-handedly account for the efficiency droop.

Less strained and more efficient GaN light-emitting diodes with embedded silica hollow nanospheres

Light-emitting diodes (LEDs) become an attractive alternative to conventional light sources due to high efficiency and long lifetime. However, different material properties between GaN and sapphire cause several problems such as high defect density in GaN, serious wafer bowing, particularly in large-area wafers, and poor light extraction of GaN-based LEDs. Here, we suggest a new growth strategy for high efficiency LEDs by incorporating silica hollow nanospheres (S-HNS). In this strategy, S-HNSs were introduced as a monolayer on a sapphire substrate and the subsequent growth of GaN by metalorganic chemical vapor deposition results in improved crystal quality due to nano-scale lateral epitaxial overgrowth. Moreover, well-defined voids embedded at the GaN/sapphire interface help scatter lights effectively for improved light extraction, and reduce wafer bowing due to partial alleviation of compressive stress in GaN. The incorporation of S-HNS into LEDs is thus quite advantageous in achieving high efficiency LEDs for solid-state lighting.

Well-to-well non-uniformity in InGaN/GaN multiple quantum wells characterized by capacitance-voltage measurement with additional laser illumination

We experimentally investigated well-to-well non-uniformity in InGaN/GaN multiple quantum well (MQW) structures by using capacitance-voltage measurements with additional laser illumination. By varying the illuminating power of the resonant excitation, well-to-well non-uniformity through the MQWs was clearly revealed. The quantum wells (QWs) close to the n-GaN side show higher carrier accumulations and larger position shift as the excitation power is increased, relative to the p-side QWs. Both results were attributed to the existence of stronger piezoelectric fields in the n-side QWs induced by subsequent partial relaxation of strain through the MQWs.

Role of photovoltaic effects on characterizing emission properties of InGaN/GaN light emitting diodes

Strong photovoltaic effects on photoluminescence (PL) spectra in InGaN/GaN blue light emitting diodes were investigated. Due to severe carrier escape from quantum wells, significant photovoltaic effects occur in PL measurement in open-circuit condition, which strongly affect the PL peak position and intensity. We reveal that proper correlation between electroluminescence and PL peak positions cannot be obtained without proper consideration of the photovoltaic effects. By changing sample temperature and the PL excitation power, the generated photovoltage varies in the range of 2.0 to 2.6 V. We show that in the open-circuit condition, which is the usual case, the determination of radiative efficiency by measuring the PL intensity ratio of low-and high-temperature cannot be accurate, and the excitation intensity dependent PL cannot be solely intrinsic either. Both the absorption of incident laser and the carrier escape from the quantum well are bias-sensitive. By a simple and straightforward method, we deter...

Effect of the number of wells on optical and structural properties in InGaN quantum well structures grown by metalorganic chemical vapor deposition

High-quality InGaN quantum well (QW) structures with one, two, three, five, and seven wells were grown by metalorganic chemical vapor deposition. The effect of the number of InGaN QWs on the structural and optical properties was studied by high-resolution x-ray diffraction (HRXRD), atomic force microscopy, low excitation density photoluminescence (PL), high excitation density pulsed PL, and PL excitation (PLE). The 10 K PLE band edge of all the samples is almost same, but the 10 K PL peaks of the InGaN QWs initially blueshifts, and then redshifts as the number of wells increases. HRXRD reciprocal space mapping and high excitation pulsed PL show that this anomalous peak shift is due mainly to potential fluctuations, rather than the piezoelectric field. The degree of potential fluctuations varies with dislocation density, which could be affected by growth interruption, the deposition of strained layers, and the accumulated strain energy in InGaN QW structures.

Effects of Pb/Pt Top Electrode on Hydrogen-Induced Degradation in Pb(Zr,Ti)O3.

Hydrogen-induced ferroelectric degradation of Pb(Zr,Ti)O3 (PZT) films was studied by measuring the hysteresis curves after hydrogen annealing at various temperatures and times. It was found that hydrogen annealing at temperatures below 100°C resulted in an increase of remanent polarization due to the accumulation of hydrogen ions at the Pt/PZT interface. After hydrogen annealing at 400°C, the ferroelectric characteristics of the Pt/PZT/Pt capacitor deteriorated very rapidly. However, when a thin (50 A) layer of Pb was deposited on top of the Pt electrode, it was found that the hydrogen-induced ferroelectric degradation was significantly retarded at 400°C. It is presumed that the kinetic path for producing highly reducing hydrogen ions by dissociative adsorption of hydrogen on Pt electrode surfaces was effectively blocked by the poisoning effect of Pb.

Effects of prestrained layers on piezoelectric field and indium incorporation in InGaN/GaN quantum wells

We investigated the effects of prestrained layers on piezoelectric fields and indium incorporation in blue-emitting InGaN/GaN quantum wells by using reverse-biased electroreflectance spectroscopy. We compared two sets of samples, which have identical structures except an insertion of an additional UV quantum well at the bottom of the blue-quantum well. Indium composition of quantum wells with the additional layer is larger than in the wells without the added layer. In contrast, the magnitude of the piezoelectric fields in the quantum wells with the added well is smaller even with higher In composition. The result shows that the strain in the quantum well is partially released and can be controlled by the insertion of the prestrained layer underneath.

Low-temperature Si epitaxial growth on oxide patterned wafers by ultrahigh vacuum electron cyclotron resonance chemical vapor deposition

Low-temperature electron cyclotron resonance hydrogen plasma cleaning was developed for low-temperature epitaxial growth of Si by ultrahigh vacuum electron cyclotron resonance chemical vapor deposition on oxide-patterned wafers. Defect-free undoped Si epitaxial layers could be obtained by optimizing the hydrogen ion flux and cleaning time, however, in the case of boron-doped Si epitaxial growth, Si epilayers had defect zones away from the bird’s beak along the window edges and a defect-free zone at the center of the window. Cross section transmission electron microscopy and energy dispersive spectroscopy results suggest that the defect zone formation is closely related with local oxygen contamination. Possible origins of the local oxygen contamination are discussed.