Yong Gon Seo

Demonstration of nonpolar a-plane InGaN/GaN light emitting diode on r-plane sapphire substrate

High crystalline a-plane (112¯0) GaN epitaxial layers with smooth surface morphology were grown on r-plane (11¯02) sapphire substrate by metalorganic chemical vapor deposition. The full width at half maximum of x-ray rocking curve was measured as 407 arc sec along c-axis direction, and the root mean square roughness was 1.23 nm. Nonpolar a-plane InGaN/GaN light emitting diodes were subsequently grown on a-plane GaN template, and the optical output power of 0.72 mW was obtained at drive current of 20 mA (3.36 V) and 2.84 mW at 100 mA (4.62 V) with the peak emission wavelength of 477 nm.

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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The extraction efficiency of nonpolar a-plane (11-20) GaN LEDs on sapphire substrates has been enhanced by selectively etching the mesa sidewall faces and the n-type GaN surfaces with photoenhanced chemical wet etching. Submicron-sized trigonal prisms having prismatic planes of {1-100} were clearly displayed on the n-type GaN surfaces as well as the sidewall face after 5 min etching at 60 degrees C. The radiation patterns have shown that more light is extracted in all directions and the output powers of surface textured a-plane GaN LEDs have increased by 25% compared with control samples. PEC wet etching produced unique feature of etching morphology on the mesa sidewall faces and the n-type GaN surface.

) GaN Light-Emitting Diodes on Sapphire Substrate

The properties of Ti/Al/Ni/Au Ohmic contacts on n-type a-plane GaN epitaxial layers directly grown on r-plane sapphire substrates are reported. The minimum specific contact resistance of ~10?5???cm2 was achieved after annealing at 650?700??C. Ohmic contact properties were measured using transmission line method patterns oriented in both the m- and c-axis directions of a-plane GaN. The sheet resistance of a-plane GaN along the c-axis was two times higher than that along the m-axis, which shows significant electric anisotropy in the two orientations.

Enhanced light extraction of nonpolar a-plane (11-20) GaN light emitting diodes on sapphire substrates by photo-enhanced chemical wet etching.

We report on orange a-plane light-emitting diodes (LEDs) with InGaN single quantum well (SQW) grown on r-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). The peak wavelength and the full-width at half maximum (FWHM) at a drive current of 20mA were 612.2 nm and 72 nm, respectively. The device demonstrated a blue shift in emission wavelength from 614.6 nm at 10 mA to 607.5 nm at 100 mA, representing a net shift of 7.1 nm over a 90 mA range, which is the longest wavelength compared with reported values in nonpolar LEDs. The polarization ratio values obtained from the orange LED varied between 0.36 and 0.44 from 10 to 100mA and a weak dependence of the polarization ratio on the injection current was observed.

Ohmic contact properties of non-polar a-plane GaN films on r-plane sapphire substrates

We demonstrate highly stable blue-emission on the semipolar (11-22) InGaN/GaN multi-quantum well light-emitting diode (LED) that has been prepared by the direct lateral-overgrowth method. The LED clearly displays bright blue-emission (λ ∼ 441 nm) with the reduced quantum-confined Stark effect. The variation of the emission-wavelength takes place only at the small injection-current range (<10 mA), and the net variation is less than 1.3 nm for wide driving-current ranges (5–100 mA). After the stabilization of light emission, the optical power is linearly increased with increasing the driving-current (1–1.8 mW). Additionally, the LED exhibits the stable light-polarization characteristics.

Orange a-plane InGaN/GaN light-emitting diodes grown on r-plane sapphire substrates

We report on the new fabrication method of a-plane InGaN light emitting diodes (LEDs) using the epitaxy on patterned insulator on sapphire substrate (EPISS). Cathodoluminescence spectrum of the fully coalesced a-plane GaN template showed that band edge emission intensity of the wing region was four times higher than that of the window region. Threading dislocations and basal stacking faults densities in wing region were ~1×10⁷ cm⁻² and ~5☓10⁴ cm⁻¹, respectively. Blue-emitting (443.4 nm) a-plane InGaN LED employing EPISS showed the optical power of 3.1 mW and the EL FWHM of 25.2 nm at the injection current of 20 mA.

Highly stable blue-emission in semipolar (11-22) InGaN/GaN multi-quantum well light-emitting diode

The effects of different SiNx interlayers on the structural and electrical properties of nonpolar Si-doped a-plane (11–20) GaN films grown on r-plane (1–102) sapphire were investigated. The surface roughness depends strongly on the SiNx coverage, deposition temperature and number of SiNx layers. The in-plane anisotropy of on-axis x-ray rocking curves (XRCs) (full width at half-maximum) was significantly decreased by the introduction of multiple SiNx-treated GaN interlayers, indicating coherently scattering domains of uniform size. Off-axis XRC measurements were also employed to investigate the effects on the mosaic twist corresponding to edge dislocation and the I1-type basal-plane stacking fault (BSF) density. Hall effect measurement showed that the electrical conductivity was the highest when multiple SiNx/GaN interlayers were employed. The measured sheet resistances (Rsh) along the c-axis were higher than those along the m-axis. These anisotropic conductivities could be explained by BSFs acting as carrier scattering centers. The ratios of Rsh along the two in-plane orientations also correlated well with the BSF densities.

Enhanced electroluminescence of a-plane InGaN light emitting diodes grown on oxide-patterned r-plane sapphire substrates.

The potential of nonpolar a-plane InGaN/GaN multi-quantum wells (MQWs), which are free from a strong piezoelectric field, was demonstrated. An a-GaN template grown on an r-plane sapphire substrate by the multi-buffer layer technique showed high structural quality with an omega full width at half maximum value along the c-axis of 418 arcsec obtained from high-resolution x-ray diffraction analysis. From barrier analysis by deep level transient spectroscopy, it appeared that a-plane InGaN/GaN MQWs can solve the efficiency droop problem as they have a lower electron capture barrier than the c-plane sample. The peak shift of the temperature-dependent photoluminescence signal for the nonpolar InGaN/GaN MQWs was well fitted by Varshnis empirical equation with zero-internal fields. A high photoluminescence efficiency of 0.27 from this sample also showed that nonpolar MQWs can be the key factor to solve the efficiency limitation in conventional c-plane GaN based light emitting diodes.

Structural and electrical anisotropies of Si-doped a-plane (11?20) GaN films with different SiNx interlayers

We report a type of hybrid optical fiber created by filling the central hole of a silica hollow optical fiber (HOF) with an organic polymer to serve as the core. After suitable curing of the polymer filling of the HOF, a self-assembled one-dimensional polymer-air periodic structure was created without the need for an amplitude mask. This acts as a long-period fiber grating device with an axial refractive index modulation. Details of the fabrication method for the hybrid fiber grating and its transmission spectra analysis are reported.