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Efficiency droop is generally observed in electroluminescence under high current injection. Optical characterization on efficiency droop in InGaN/GaN multiple-quantum-well structures has been conducted at 12 K. Clear droop behaviors were observed for the sample excited by above-bandgap excitation of GaN with pulse laser. The results show that dislocation is not the crucial factor to droop under high carrier density injection, and Auger recombination just slightly affects the efficiency. The radiative recombination may be mainly affected by a multi-carrier-related process (diffusion and drift with a factor of n3.5 and n5.5) at the interface between GaN barrier and InGaN well.

We studied the growth technique for the dislocation reduction in a-plane GaN grown by metal organic chemical vapour deposition (MOCVD) using AlInN buffer layer, high temperature atomic layer epitaxy, and trenched r-sapphire technique. By using these techniques, the crystal quality was much improved. We also fabricated blue light emitting diodes (LEDs) on a-plane GaN film over r-sapphire substrate and on a-plane bulk GaN substrate. The electroluminescence (EL) characteristics of the LED samples were examined, and we found that the EL near field pattern from homo-epitaxially grown a-GaN based LED was spatially uniform, although the pattern from the LED on r-sapphire substrate was not uniform. The output power at the wavelength of 430nm was 0.72mW at the 20mA injection current for the sample on a-plane bulk GaN. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In the growth of InGaN/GaN multi-quantum well (MQW) heterostructures by metal organic chemical vapor deposition, V-defects attached to threading dislocations have been observed and investigated. Energy-dispersive X-ray analysis and conventional transmission electron microscopy studies were carried out in order to determine the In composition and investigate the behavior of the dislocations. The V-defects are limited by {101¯1} lattice planes, they are attached to threading dislocations and may start at the third quantum well. The associated dislocation runs up into the overgrown GaN layer. Some (a+c) dislocations were shown to decompose inside the multi-quantum well, giving rise to a misfit segment in the c-plane and a V-shape defect.

We investigated the structural and optical properties of AlInGaN/AlGaN quantum wells (QWs) and the quaternary AlInGaN epilayers as a function of V/III flux ratio (flow rate of NH3). From the photoluminescence and cathodoluminescence (CL) measurements, the emission wavelengths of the AlInGaN/AlGaN QWs linearly became blueshifted and the intensities decreased with increasing the flow rate of NH3. In the X-ray rocking curve, a pattern of the sample grown at lower NH3 flow rate gives an indication of the good crystalline quality of the films. The monochromated scanning CL image of the AlInGaN epilayers measured at around 80 K showed that more densely and uniformly distributed bright spot densities, indicating In segregation regions, increased at lower NH3 flow rate. Therefore, these results suggest that the relatively decrease of NH3 flow rate results in the increase of In composition fluctuations into the AlInGaN alloys, which generate exciton localized states, and thereby enhances the emission. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Structural and optical properties of InGaN/GaN triangular-shaped multiple quantum well (QW) structures were investigated under various conditions of growth parameters such as growth temperature, flow rate of Ga and/ or In composition, and well and barrier widths. The optical properties affected by the growth parameters were well correlated with an In band gap, which is determined by the potential depth and the In composition in the well region. The emission peak energy was almost independent of the barrier width due to the relaxation of the piezoelectric fields in the triangular-shaped QWs. Photoluminescence spectra of the InGaN/GaN multiple QW structures showed a parabolic curve centered at 2.66 eV. The optical property of the triangular-shaped multiple QWs was substantially improved due to formation of quantum dot-like In composition fluctuations.

Structural and optical properties of InGaN/GaN multiple triangular quantum well (QW) structures with different threading dislocation (TD) densities of 1.5×108 (sample A) and 4.5×108 cm-2 (sample B) have been studied. High resolution transmission electron microscopy and x-ray diffraction analysis showed more fluctuation of local In composition in the sample B, which was attributed to the stress field created by the dislocations as it provides a driving force for the migration of In atoms towards dislocations. Severe degradation of photoluminescence intensity of the sample B was also observed at < 50 K. The optical and structural properties of the InGaN/GaN triangular QW structures are overall substantially affected by the TD density.

We investigated the structural and optical properties of InGaN/GaN triangular-shaped multiple quantum well (MQW) structures as a function of emission wavelengths. As the peak wavelength became longer, the average In composition increased linearly and the peak linewidth in the PL measurements became broad. Also, the PL intensity showed the highest value at 23% In and the interface showed the best abruptness, but the lowest intensity at 30% In. A decrease in emission intensity is ascribed to the crystalline imperfection such as defects and/or impurities caused by increased misfit strain in the QW regions with relatively higher In composition.

Graded-In-content InGaN/GaN quantum wells (QWs) with high brightness blue and green emission were grown on sapphire by metalorganic chemical vapor deposition. The optical properties were studied by temperature-dependent photoluminescence (PL), PL excitation (PLE), and time-resolved PL. Phase separations are revealed by PL measurement and verified by PLE with different absorption edges. From temperature-dependent PL, the integrated intensity of the main InGaN blue and green emission (∼462 nm for blue and ∼509 nm for green) decreased by about a factor of 13.2 and 7.4, only about one order of magnitude, with increasing temperature from 10 to 300 K, indicating strong carrier localization and high quantum efficiency in graded-In-content multiple QWs. Time-resolved PL shows a clear increase of decay time with temperature from 14 to 90 K for blue emission samples and to 50 K for green emission samples, then decrease with more higher temperature. From the time-resolved PL results, a clear carrier transfer is observed from weakly to strongly localized states, which plays an important role to enhance brightness and quantum efficiency in graded-In-content InGaN QWs. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Substantial improvement of electrical and optical properties of InGaN/GaN multiple quantum wells (MQWs) was obtained with a triangular band structure. Transmission electron microscopy (TEM) images from the–50 nm. The light-emitting diodes (LEDs) with the triangular QWs showed a lower operation voltage, a higher light output power, and higher intensities and narrower line widths of electroluminescence spectra than those with the rectangular QWs. Very bright and uniform light emission from the triangular MQW LEDs was also observed at a low injection current, but spatially non-uniform emission from the rectangular ones.

In-doped GaN thin films were grown by metalorganic chemical vapor deposition (MOCVD) with different trimethylindium (TMIn) flow rates and those structural and electrical characteristics were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), Hall effect measurement and deep level transient spectroscopy (DLTS). The full width at half maximum (FWHM) of X-ray rocking curves and the pit density, which is estimated by AFM images, decrease with increasing TMIn flow rate. It seems that In doping has definite effects of reducing dislocation density in In-doped GaN films. Hall mobility increases with increasing TMIn flow rate. As an explanation, from an analysis of temperature-dependent Hall effect measurements based on a two-layer mixed conduction model, the mobility contribution of the epilayer compared to that of the interface layer between GaN and sapphire is increased due to reduction of dislocation density in In-doped GaN thin films. It is also observed that the reduction of deep level E2 is related to In-doping.

Effects of growth interruption on the optical and the structural properties of InGaN/GaN quantum wells were investigated by using photoluminescence, transmission electron microscopy, optical microscopy, and high resolution x-ray diffraction. The InxGa1−xN/GaN (x>0.2) quantum wells used in this study were grown on c-plane sapphire by using metalorganic chemical vapor deposition. The interruption was carried out by closing the group-III metalorganic sources before and after the growths of the InGaN quantum well layers. The transmission electron microscopy images show that with increasing interruption time, the quantum-dot-like regions and well thickness decreased due to indium reevaporation or the thermal etching effect. As a result the photoluminescence peak position was blueshifted and the intensity was reduced. Temperature- and excitation-power-dependent photoluminescence spectra support the results of transmission electron microscopy measurements. The sizes and the number of V defects did not differ with the interruption time. The interruption time is not directly related to the formation of defects. The V defect originates at threading dislocations and inversion domain boundaries due to higher misfit strain.

A parametric study of high density plasma etching of SrBi2Ta2O9 (SBT) thin films was carried out in a planar type inductively coupled plasma (ICP) etcher with different chemistries of Cl2/Ar, Cl2/NF3/Ar, and Cl2/NF3/O2/Ar. The etch rate was a strong function of gas concentration, ICP source power, and rf chuck power. Both Cl2/NF3/Ar and Cl2/NF3/O2/Ar plasmas showed maximum etch rates of ∼1600 A/min at 5 mTorr, 700 W ICP power, and 150 W rf chuck power. The addition of NF3 and O2 into the Cl2/Ar mixture played an important role in not only enhancing the etch rate, but also smoothing the etched surface by reducing the kinetic energy of ions. Electrical properties of the SBT films were quite dependent of plasma chemistries: Cl2/NF3/O2/Ar showed the least damage in the films and resulted in overall the best polarization-electric field hysteresis loop compared to other chemistries.

Dielectric SrTiO3 thin films were deposited on Pt/Si and Ir/Si substrates by plasma enhanced metalorganic chemical vapor deposition using high purity Ti(O-i-C3H7)4, Sr(tmhd)2, and oxygen. The deposition rates were quite dependent on bubbler temperature, substrate temperature, and rf source power. The dissociation of Ti(O-i-C3H7)4 was substantially enhanced by increasing the rf source power, while that of Sr(tmhd)2 increased with the substrate temperature. Surface morphology showed a smooth surface and good adhesion at the interface. Based on the growth characteristics of films, it was found that the deposition of SrTiO3 films was controlled by the decomposition rate of Sr(tmhd)2 on the substrate surface.

Optical properties and carrier dynamics of high-efficiency green-light-emitting InGaN/GaN multiple quantum wells (MQWs) with graded-In-content were studied by photoluminescence (PL), PL excitation, and time-resolved PL techniques. Two separated InGaN-related peaks were clearly found in PL spectra due to strong phase separation in the well of the graded-In-content InGaN MQWs. The integrated intensity of the main InGaN green emission (∼510 nm) decreased by only about a factor of 7 with increasing temperature from 10 to 300 K, indicating strong carrier localization and high quantum efficiency. Strong carrier transfer from low-In-content region with weak carrier localization to high-In-content part with strong carrier localization was observed by time-resolved PL. Therefore, we conclude that the effective carrier transfer from weakly to strongly localized states plays an important role to enhance brightness and quantum efficiency in the green-light-emitting InGaN MQWs with graded-In content.

InGaN/GaN triangular shaped multiple quantum wells (QWs) grown by grading In composition with time were adopted as an active layer of blue light-emitting diodes (LEDs). Compared to the LEDs with conventional rectangular QW structures, the triangular QW LEDs showed a higher intensity and a narrower linewidth of electrical luminescence (EL), a lower operation voltage, and a stronger light-output power. EL spectra of the triangular-QW-based LEDs also showed that the peak energy is nearly independent of the injection current and temperature, indicating that the triangular QW LED is more efficient and stable than the rectangular one.

Structural and optical properties of various shapes of quantum wells (QWs), including rectangular, triangular, trapezoidal, and polygonal ones are investigated. Photoluminescence (PL) measurements show that the highest light emission efficiency and the best reproducibility in the intensity and wavelength are obtained from trapezoidal QWs. The temperature dependence of PL spectra indicates the more localized nature of excitons in the trapezoidal QWs. A plan-view transmission electron microscopy shows that quantum dots (QDs) are formed inside the dislocation loop in trapezoidal QWs. The distribution of QDs in size and composition becomes more uniform with trapezoidal QWs than with rectangular QWs, leading to superior light-emission characteristics. It is suggested that QD engineering and dislocation control are possible, to some extent, by the modulation of the QW shape in InGaN/GaN-based light-emitting devices.

P-type GaN layers were grown on sapphire by metalorganic chemical-vapor deposition and then rapid thermal annealing (RTA) was performed to electrically activate Mg impurities. Varied acceptor densities were obtained by RTA temperature and Mg concentration. Temperature-dependent Hall effects show that the thermal activation energy of the acceptor (EA) is strongly dependent on the acceptor density (NA), approximated by EA(0)=372−1.16×10−18 NA meV at 0 K. A strong temperature dependence of EA was also obtained in this study.

Optical and structural properties of InGaN/GaN quantum wells having growth interruption were investigated using high-resolution x-ray diffraction, photoluminescence and transmission microscopy. InxGa1−xN/GaN (x>0.25) six pair quantum wells used in this study were grown on c- plane sapphire by metalorganic chemical vapor deposition. The growth interruption was carried out by closing the group-III metal organic sources before and after growth of InGaN quantum well layers. With increasing the interruption time, the quantum dot-like region and well thickness decreases due to indium re-evaporation or thermal etching effect. As a result, PL peak position is blue-shifted and intensity is reduced. The size and number of V-defect did not vary with interruption time. The interruption time is not directly related with formation of the defect. The V-defect in quantum wells originates at threading dislocations and inversion domain boundaries due to higher misfit strain.

We have investigated the properties of two types of InGaN/GaN quantum-well (QW) structures. Photoluminescence (PL) measurements were carried out by varying the external bias voltage. The magnitude of the variation in PL peak position and intensity of trapezoid QWs (TQWs) is much smaller than that of rectangular QWs (RQWs). According to transmission electron microscopy measurements, quantum dots are more densely and uniformly distributed in TQWs than in RQWs. The electroluminescence image of a light-emitting diode fabricated using TQWs as active layers (TQW-LED) is more uniform than that of a light-emitting diode fabricated using RQWs as active layers (RQW-LED). Optical output power of a TQW-LED is larger than that of a RQW-LED. These results show that the origin of strong emission from InGaN/GaN QWs is attributed to exciton localization quantum dots, and InGaN/GaN TQWs are considered as active materials in order to increase performance in optoelectronic device.

Optical characteristics of high-efficiency violet-blue, blue, and green light emissions in InxGa1−xN quantum well (QW) structures with graded In content are investigated. Appearance of additional higher energy peaks at 410, 429, and 459nm above the main peaks at 430, 463, and 509nm with an effective carrier transfer from the higher to main peak sides is characteristic of these structures with various In contents of x 0.3, respectively. Robust carrier localization by uniform, small-size, and high-density phase segregation plays an important role in maintaining high efficiencies over a wide range of In contents in graded-In-content InGaN QW structures.

Investigation is carried out by high-resolution electron microscopy on threading dislocations using data treatments with procedures that allow the extraction of the most likely atomic configurations. We also report In composition fluctuations inside InGaN/GaN quantum wells by coupling HRTEM, image simulation and Finite Element Modelling (FEM) of the thin foil relaxation. The results show that the indium content may be close to x = 1.0 in the clusters and this is much higher that was previously suggested by 2D FEM modelling.

Mg-doped p-type GaN epilayers grown by metalorganic chemical vapor deposition exhibit two different types of photoluminescence (PL) characteristics depending on Mg doping rate; the existence of critical Mg concentration is found where both electrical and optical characteristics show an abrupt change in their behavior. In samples with relatively low Mg concentration, a band edge emission, a peak associated with shallow donor-acceptor pair recombination, and a 2.8 eV blue band emission appear in the PL spectrum. Intensities of all three PL peaks decrease with increasing annealing temperature; the blue band quenches most rapidly. In contrast, in samples grown with high ratio of Mg/Ga flow rate, only a strong blue emission band is observed and the intensity of the blue band increases with the annealing temperature suggesting a different origin from the blue band of lightly doped samples. From the annealing temperature dependence of the blue band intensity, we speculate that the emission mechanism of the blue ...