[show abstract][hide abstract] ABSTRACT: We report on the local stress distribution in a GaN-based vertical light-emitting diode (V-LED) fabricated using two types of separation methods: a chemical lift-off (CLO) procedure and a laser lift-off (LLO) technique. The CLO LED exhibits a stronger donor-bound exciton (D0X) emission than the LLO LED, owing to its textured surface morphology and lower amount of damage to the structure. On the basis of the photoluminescence and Raman spectroscopy results, we determine that the CLO GaN LED has an 82 MPa lower residual stress than the LLO GaN LED. Therefore, the CLO technique can be considered as a more effective method to fabricate stress-relieved high-brightness LEDs.
Journal of Physics D Applied Physics 03/2013; 46(15):155104. · 2.53 Impact Factor
[show abstract][hide abstract] ABSTRACT: Four types of GaN-based light-emitting diodes (LEDs) with V-pits formed in different regions were grown by metal–organic chemical vapor deposition. The position of the V-pits embedded in the layers of the LED structures was controlled by varying the growth temperature. We achieved the highest output power and lowest leakage current values with the LED structures comprising V-pits embedded in active regions and the p-GaN textured surface. The V-pit formation enhances the light output power and reverse voltage values by 1.3 times the values of the conventional LED owing to the enhancement of the light scattering probability and the effective filtering of threading dislocations.
[show abstract][hide abstract] ABSTRACT: The diameter of ZnO nanowires grown by chemical vapour deposition was controlled by employing CrN buffer structures on the c-Al2O3 substrate. The nanosized CrN islands with different morphologies were prepared by nitridation of thickness-controlled Cr film in NH3 atmosphere. The ZnO nanowires grew normal to the surface of the CrN/c-Al2O3 templates due to reduction in the lattice mismatch between ZnO and c-Al2O3 by the CrN buffer layer. Investigation of the interface between CrN and ZnO by high resolution transmission electron microscopy revealed the presence of reactive layers such as ZnCr2O4 and Cr2O3. The diameter of nanowires significantly affected their stimulated emission characteristics. At room temperature, the threshold intensity for stimulated emission increased from 35 to above 500 kW cm−2 as the diameter of ZnO nanowires decreases from 223 to 77 nm. This dependence of threshold intensity for stimulated emission from nanowires is caused by an increase in the surface recombination and/or enhanced leakage of optical field in narrower nanowires.
Journal of Physics D Applied Physics 10/2009; 42(22):225403. · 2.53 Impact Factor
[show abstract][hide abstract] ABSTRACT: We report the electrical characteristics of vertical and lateral type light emitting diodes (LEDs) grown with CrN buffer layer. The LED with CrN buffer showed lower reverse leakage current than the reference sample grown with conventional low-temperature GaN buffer. It was also observed that the density of open core screw dislocation was smaller by one order of magnitude, which was thought to relate to the leakage current of devices. The vertical type LED fabricated by chemical etching of CrN buffer showed lower series resistance, lower turn-on voltage, and larger light output power than those of the conventional LEDs.
[show abstract][hide abstract] ABSTRACT: Vertically-aligned zinc oxide (ZnO) nano-needles have been selectively grown on the Si (100) substrates using chemical vapor transport and condensation method without metal catalyst. The selective nucleation of nano-needles was achieved by the controlled treatment of substrate surface using zinc acetate aqueous solution. The nano-needles were selectively grown on the zinc acetate treated area, while the nano-tetrapod structures were formed on the non-treated area. The nano-needles have uniform tip-diameter and length, about 10 nm and 2-3 microm, respectively. The angle of the ZnO nano-needles from the substrate was 90 +/- 0.2 degrees. The structural and optical properties of nano-needles and nanotetrapod structures were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL). The results showed that ZnO nano-needles grow along the c-axis of the crystal plane due to the c-oriented ZnO nanoseeds formed by zinc acetate treatment. The nano-needles have strong ultraviolet emission peak of 3.29 eV with green emission of 2.3 eV at room temperature. This selective growth technique of vertical nano-needles using aqueous solution method has potential applications in the field emission devices or optoelectronic devices hybridized with silicon based electronic devices.
Journal of Nanoscience and Nanotechnology 12/2006; 6(11):3351-4. · 1.15 Impact Factor
[show abstract][hide abstract] ABSTRACT: We propose a rocksalt (RS) structured chromium nitride (CrN) layer as a novel buffer for the growth of gallium nitride (GaN) films on c-sapphire substrate. A RS-CrN buffer has good advantages for growing GaN films on c-sapphire from the viewpoints of lattice mismatch and thermal expansion coefficient. The RS-CrN layer has mid value of both the lattice constant and the thermal expansion coefficient of GaN and c-sapphire. It is verified that the lattice mismatch between CrN and c-sapphire of 6.2-6.7%, which is experimentally estimated from reflection high-energy electron diffraction (RHEED) characterization, about 6.2-6.7%, agree well with the calculated value of 6.6%. By these advantages, N-polarity GaN epilayers grown on RS-CrN buffer layer show remarkably improved crystal quality when compared with those of grown on conventional low temperature (LT) GaN buffer using molecular beam epitaxy (MBE). For a further improvement of GaN crystallinity, we performed the surface treatment of Ga-exposure on CrN buffer layer. The Ga-exposure treatment on the RS-CrN buffer layer drastically improved the crystal quality of upper GaN epilayer, which was identified by measuring structural and optical properties using X-ray diffraction (XRD) and Photoluminescence (PL), respectively. The improved crystallinity is resulted not from the polarity conversion but from the stress relaxation effect by a metallic Ga interlayer.
Physica Status Solidi (c). 01/2006; 3(6):1388-1391.