Seogwoo Lee

Effects of Basal Stacking Faults on Electrical Anisotropy of Nonpolar a-Plane (

We report on the effects of basal stacking faults (BSFs) on the electrical anisotropy and the device characteristics of nonpolar a-plane GaN (1120) light-emitting diodes (LEDs) on r-plane (1102 ) sapphire substrates. The sheet resistance in the direction parallel to the c-axis [0001] is 18%-70% higher than the one in the direction parallel to the m-axis [1100 ]. The anisotropic conductivity of faulted a-plane GaN films can be explained by carrier scatterings from BSFs. It is also shown that the output power of nonpolar a-plane GaN LEDs are significantly influenced by the presence of BSFs, which laterally hampers the carrier transport in the n-GaN layer, especially in the direction parallel to the c-axis in faulted nonpolar nitride films.

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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.

) GaN Light-Emitting Diodes on Sapphire Substrate

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.

Leakage current improvement of nitride-based light emitting diodes using CrN buffer layer and its vertical type application by chemical lift-off process

Raman and emission properties of a nonpolar a-plane InGaN/GaN blue-green light emitting diode (LED) on an r-sapphire substrate are investigated and compared with a conventional c-plane blue-green LED. The output power of the a-plane LED was 1.4 mW at 20 mA. The c-plane LED has higher EQE, but it reaches the maximum at a lower forward current and the droop is faster than the a-plane counterpart. As the reverse bias increased, a blueshift in the PL spectra was not observed in the a-plane structure, which is indicative of an absence of quantum confined Stark effects. However, a strong blueshift in the electroluminescence spectra was still present, which means the In localization effects are relevant in nonpolar InGaN/GaN quantum wells. In the Raman spectra, a strong anisotropy of E2(high) phonon modes was observed. By comparing the frequency of the E2(high) modes, we demonstrate that the residual compressive strain in an a-plane LED is significantly smaller than in the polar counterpart.

Improvement of Light Extraction Efficiency and Reduction of Leakage Current in GaN-Based LED Via V-Pit Formation

We experimentally clarify the effects of barrier dopings on the polarization induced electric fields and the band structure in InGaN/GaN blue light emitting diodes. Both effects were independently verified by using electric field modulated reflectance and capacitance-voltage measurement. It is shown that the Si barrier doping does reduce the polarization induced electric field in the quantum wells. But the benefit of Si-doping is nullified by modification of the band structure and depletion process. With increased number of doped barriers, smaller number of quantum wells remains in the depletion region at the onset of the diffusion process, which can reduce the effective active volume and enhance the electron overflow.

Raman and emission characteristics of a-plane InGaN/GaN blue-green light emitting diodes on r-sapphire substrates

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.

Experimental verification of effects of barrier dopings on the internal electric fields and the band structure in InGaN/GaN light emitting diodes

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.

Structural and stimulated emission characteristics of diameter-controlled ZnO nanowires using buffer structure

The authors have studied the photoluminescence (PL) intensities of a-plane (11-20) GaN films (a-GaN) as a function of x-ray rocking-curve (XRC) linewidth values measured in both c- and m-axis directions. PL intensity of well-known luminescence lines such as 3.47 eV (bound exciton emission), 3.41 eV (basal-plane stacking fault related emission), and 3.29 eV (defect induced emission) are discussed in terms of XRC linewidth values. PL intensities reveal a close relationship with XRC linewidth measured in the c-axis direction, while an unusual relationship was observed between PL intensity and XRC linewidth in the m-axis direction. Inhomogeneous strain along the m-axis direction of a-GaN film is discussed as a cause of XRC linewidth broadening, rather than the formation of structural defects.

Local stress distribution in GaN vertical light-emitting diodes fabricated using CLO and LLO methods

We have applied surface treatment with ammonia to laterally grown GaN. The technique of sublimation was used for the bulk growth of GaN at a temperature of 1080°C, with a reacting gas consisting of NH/sub 3/ and N/sub 2/, on a substrate of metalorganic chemical vapor deposited GaN film/sapphire. With the surface treatment, the laterally grown GaN appears to have an improved surface and structure: cracks are conspicuously reduced as observed with a UV-microscope and crystal uniformity is increased as shown by X-ray diffraction.