Cheng-Ta Kuo

Quasiregular quantum-dot-like structure formation with postgrowth thermal annealing of InGaN'GaN quantum wells

Postgrowth thermal annealing of an InGaN/GaN quantum-well sample with a medium level of nominal indium content (19%) was conducted. From the analyses of high-resolution transmission electron microscopy and energy filter transmission electron microscopy, it was found that thermal annealing at 900 °C led to a quasiregular quantum-dot-like structure. However, such a structure was destroyed when the annealing temperature was raised to 950 °C. Temperature-dependent photoluminescence (PL) measurements showed quite consistent results. Blueshift of the PL peak position and narrowing of the PL spectral width after thermal annealing were observed.

Quantum-well-width dependencies of postgrowth thermal annealing effects of InGaN/GaN quantum wells

Optical measurements of temperature-dependent photoluminescence (PL) spectral peak, integrated PL intensity and PL decay time, and microstructure analyses with high-resolution transmission electron microscopy showed the strong dependencies of thermal annealing effects on quantum well (QW) width in InGaN/GaN QW structures. With different QW widths, different levels of strain energy were built. Upon thermal annealing, energy relaxation resulted in the reshaping of quantum dots and hence the changes of optical properties. Thermal annealing at 800 °C of a narrow QW width (2 nm) structure led to regularly distributed quantum dots (QDs) and improved optical quality. However, thermal annealing at the same temperature of a sample of larger QW width (4 nm) did not show QD formation. In this situation, even higher local strains around QWs were speculated. Also, degraded optical quality was observed.

Low Operation Voltage of Nitride-Based LEDs with Al-Doped ZnO Transparent Contact Layer

We have developed nitride-based multiquantum well light-emitting diodes (LEDs) with E-beam evaporated Al-doped ZnO (AZO) transparent contact layers (TCLs). With 20 mA injection current, it was found that forward voltages were 3.32, 3.33, and 4.91 V, while output powers were 10.1, 11.8, and 6.0 mW for the indium-tin-oxide LED, E-beam evaporated AZO LED, and sputter-evaporated AZO LED, respectively. The low operation voltage of the E-beam evaporated AZO LED is attributed to the deposition of low-resistivity TCLs, and the elimination of plasma damage in the p-GaN layer is attributed to use of the E-beam evaporated AZO. The high output power of the E-beam evaporated AZO LED is due to the enhancement of light extraction resulting from the high refractive index of AZO TCL.

Optical Simulation and Fabrication of Nitride-Based LEDs With the Inverted Pyramid Sidewalls

In this study, the numerical and experimental demonstrations for the enhancement of light-extraction efficiency in nitride-based LEDs with randomly inverted pyramid sidewalls (IPSs) by chemical etching of the chip edge are presented. With 20 mA injection current, it was found that forward voltages were 3.69 and 3.75 V while output powers were 7.07 and 8.95 mW for the conventional LED and inverted pyramid sidewall LED, respectively. The larger LED output power is attributed to the increased light-extraction efficiency by IPSs.

Dislocation reduction in GaN with multiple MgxNy∕GaN buffer layers by metal organic chemical vapor deposition

Unintentionally doped GaN epitaxial layers with a conventional single low temperature (LT) GaN buffer layer and with multiple MgxNy∕GaN buffer layers were grown on sapphire substrates by metal organic chemical vapor deposition. The multiple MgxNy∕GaN buffer layers exhibit a low nuclei density, increasing the volume of defect-free regions and reducing the dislocations associated with the grain boundaries. Therefore, the GaN with multiple MgxNy∕GaN buffer layers reveals an asymmetrical reflection (102) with a small full width at half maximum, and a higher mobility, lower background concentration, and lower etching pit density than the GaN with the LT GaN buffer layer.

Diffusion mechanism and photoluminescence of erbium in GaN

Abstract Erbium has been diffused into GaN for the first time. A weak spontaneous emission is observed in the photoluminescence spectra after the diffusion process during 168 h at 800 °C under N2 atmosphere. The diffusion coefficient of erbium in GaN is obtained in Arrenhius expression to be D=1.8±1.3×10 −12 exp (−1±0.4 eV /kT) cm2/s. The result shows that the Er diffusion mechanism might be an interstitial-assisted process. The luminescence characteristics of the Er-diffused GaN is compared with the Er-implanted GaN. The methods to enhance the emission intensity of the Er-diffused GaN are discussed.

Er diffusion into gallium nitride

Abstract In this study, we report the diffusion mechanism of Er in GaN and the optical properties of Er-diffused GaN. The diffusion coefficient has been measured by Rutherford backscattering spectroscopy and secondary ion mass spectrometry. Erbium is also implanted into GaN to compare the photoluminescence (PL) characteristics. The PL emission of Er-diffused and implanted GaN can be observed in infrared region. The emission intensity efficiency related to the Er concentration distribution after the Er diffusion is discussed.

Optical characteristics of InGaN/GaN quantum well structures with embedded quantum dots

We compared the results of optical characterization between five InGaN/GaN quantum well samples of different well widths. Temperature dependencies of photoluminescence (PL) spectral positions, integrated PL intensities, and PL intensity decay times at PL peaks of all the five samples showed three temperature ranges of different variation trends. The radiative efficiencies of the samples in the high temperature range had the same decay slope, which is supposed to be determined by the defect structures outside clusters. The radiative efficiencies in the medium temperature range varied among samples, indicating different defect structures in the regions between coupled clusters in different samples. Consistent results of temperature dependent variations between the integrated PL intensity and PL decay time among these samples provided clues for reasonable interpretations. Also, we showed the strong dependencies of thermal annealing effects on quantum well (QW) width in InGaN/GaN QW structures. Thermal annealing at 800 °C of a narrow QW width (2 nm) structure led to improved optical quality. However, thermal annealing at the same temperature of a sample of larger QW width (4 nm) resulted in degraded optical quality.

Array of GaN-based transverse junction blue light emitting diodes with regrown n-type regimes

In this research, we demonstrate array of transverse-junction (TJ) blue light-emitting-diodes (LEDs), which are specified as a horizontal carrier flow instead of side-by-side injection, with a consequence of InxGa1-xN/GaN multiple-quantumwells (MQWs) as the active region. The demonstrated devices were carried out by the re-growth of n-type GaN on the sidewall of p-type GaN. Regarding the transverse carrier flow of injected carriers, these TJ-LEDs, as compared to the control related to traditional vertical junction structure, can effectively spread injected currents more uniformly, minimize the problem of nonuniform carrier-distribution and current crowding effect, and achieve 35% improvement of power performance.

Femtosecond pump-probe studies on carrier dynamics in InGaN/GaN quantum wells with indium aggregated quantum dot structures

Temperature-dependent pump-probe measurements were conducted for observing the process of carrier relaxation into localized states of quantum dots, which were formed through indium aggregation in InGaN/GaN quantum well structures of various parameters.