Hooyoung Song

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

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.

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

Zero-internal fields in nonpolar InGaN/GaN multi-quantum wells grown by the multi-buffer layer technique

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.

Field dependence of barrier heights and luminescence properties in polar and nonpolar InGaN/GaN single quantum wells

The external field dependence of barrier heights and the internal field dependence of luminescence properties in InxGa1−xN/GaN single quantum wells (SQWs) with polar (x=0.13) and nonpolar (x=0.15) orientations were investigated. The conduction band offset of a SQW was characterized by using deep level transient spectroscopy. At a reverse bias of −3 V, the barrier height of the nonpolar SQW was estimated to be 0.42 eV, which is smaller than the 0.60 eV seen in the polar SQW due to the absence of internal fields along the nonpolar direction. Both samples showed a redshift of barrier heights with increasing reverse bias. The carrier recombination affected by carrier localization, quantum-confined Stark effect, and Varshni’s shift was analyzed through temperature-dependent photoluminescence. Numerical simulations of the barrier heights and internal fields showed good agreement with experimental results.

Carrier Dynamics of Deep-Level States in InGaN/GaN Multiquantum Wells

InGaN/GaN (3 nm/10 nm) multiquantum wells (MQWs) were grown on (0001) sapphire substrates by metal–organic chemical vapor deposition. To characterize the effect of internal electric fields on deep-level states in InGaN MQWs, deep-level transient spectroscopy (DLTS) and bias-dependent photoluminescence (PL) measurements were performed. The thermal activation energy of MQWs obtained from DLTS spectra decreased to 0.15 eV with an increase in reverse measure bias from 2 to 10 V, which indicates partially cancelled internal fields. The blue shift of PL peak energy was about 0.05 eV when the reverse measure bias was changed from 0 to 5 V. This is explained well by carrier dynamics in the conduction band of InGaN/GaN MQWs, which affects carrier recombination energies corresponding to the band edge to edge transitions with changing reverse measure bias. # 2009 The Japan Society of Applied Physics

Effects of enhanced lateral transport on InGaN/GaN light emitting diodes via n-type AlGaN/GaN superlattices

We demonstrate c-plane InGaN/GaN light emitting diodes (LEDs) using polarization engineered n-type AlGaN/GaN superlattices (SLs). Aluminum composition variation and Si-delta doping concepts were incorporated in the SLs design as a means to improve vertical and lateral carrier transport in SLs. Compared to a reference n-type GaN layer having lateral conductivity of 197 Ω cm, a SLs structure exhibited significantly improved lateral conductivity, as high as 569 Ω cm, without any vertical transport degradation. Optimized AlGaN/GaN SLs structure embedded in LED improved current spreading and resulted in 13.7% and 6.7% enhancement of output power and forward voltage at 60 A/cm2, respectively.

Electrical Properties and Defect States in ZnO Substrates Irradiated by MeV Electron-beam

The electrical properties and defect states in ZnO substrates were studied during high-energy electron beam irradiations. 1 MeV and 2 MeV electron-beam with dose of electrons/ were irradiated on Zn-surface of the sample. In the sample irradiated by 1 MeV, the leakage current was increased by electron-beam induced surface defects, while the enhancement of on/off property and the decrease of leakage current appeared in the 2 MeV irradiated sample. From the deep level transient spectroscopy measurements for these samples, it showed that the defect states with the activation energies of -0.33 eV and +0.8 eV are generated during the high energy electron-beam irradiation. Especially, it considered that the -0.33 eV state related with O-vacancy affects to their electrical properties.

Structural, electrical, and optical characterizations of a-plane InGaN/GaN quantum well structures

Sung-Ho Lee, Jae Bum Kim, Ji-Su Son, and Sung-Min Hwang Green Energy Research Center, Korea Electronics Technology Institute, Gyeonggi-do 463–816, Korea GaN and related ternary compounds have been widely used for fabrication of light emitting diodes (LEDs) and laser diodes (LDs). Especially, the low-dimensional systems such as quantum wells (QWs), quantum wires, and quantum dots have been investigated as an effective structure for improving the efficiency of light-emitting devices such as light emitting diodes and laser diodes. Generally, the quantum well active regions in III-nitride optoelectronic devices grown on conventional templates along the polar orientation have critical problems given by the quantum confined Stark effect (QCSE) due to the effects of strong piezoelectric and spontaneous polarizations [1]. However, the QWs grown on nonpolar templates along a- or m-directions are free from the QCSE since the polar-axis lies within the growth plane of the template [2]. In this study, we achieved high quality a-plane GaN films on sapphire substrates and characterized structural, electrical and optical properties in the a-plane InGaN/GaN QW structures. High quality of a-plane GaN templates was confirmed by using selected area diffraction (SAD) patterns and high resolution x-ray diffraction (HRXRD) results. To investigate the electrical properties of aplane GaN QWs structures, the temperature-dependent carrier depth profiles which can determine the carrier confinement with nanoscale spatial resolution were studied. And the redshift of photoluminescence (PL) peaks with increasing temperature will be intensively discussed.

Optical properties of green light-emitting diodes grown on r-plane sapphire substrates

Nonpolar a-plane light emitting diodes (LEDs) in InGaN/GaN multiple quantum well structures were successfully fabricated by metal organic chemical vapour deposition (MOCVD) on r-plane sapphire substrates. An optical output power of 0.26 mW was obtained at a drive current of 20 mA and 1.27 mW at 100 mA, with peak emission wavelengths of 504.5 nm and 503.9 nm, respectively. The peak emission wavelength shift for the green LED was 4.4 nm when the injection current was changed from 5 to 100 mA.

Influence of Growth Parameters on Structural Anisotropy of Epitaxial a‐plane GaN Films

We investigated the structural anisotropy of a‐plane GaN films grown by using multi‐buffer layer technique on (1‐102) r‐plane sapphire substrates. For high quality a‐plane GaN films, multi‐buffer layers with various growth conditions were grown by metal‐organic chemical vapor deposition, and analyzed by using several measurement systems such as optical microscopy, scanning electron microscopy, high resolution x‐ray diffraction. The experimental results showed that the nucleation‐layer thickness and the growth temperature of three‐dimensional (3D) growth layer affect significantly the crystal quality of subsequently grown a‐plane GaN films. When the nucleation‐layer thickness was 150 nm, nuclei were fully coalesced. From the x‐ray diffraction results, it appeared that the growth temperature during 3D islands growth affects the full‐width at half maximum (FWHM) values of x‐ray rocking curves along c‐ or m‐directions. At optimized growth conditions, the omega FWHM values of (11–20) x‐ray rocking curve along ...