R. J. Choi

Efficient blue light-emitting diodes with InGaN/GaN triangular shaped multiple quantum wells

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.

Carrier dynamics of high-efficiency green light emission in graded-indium-content InGaN/GaN quantum wells: An important role of effective carrier transfer

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 (∼510u2009nm) 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.

Influence of the quantum-well shape on the light emission characteristics of InGaN/GaN quantum-well structures and light-emitting diodes

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.

Strong acceptor density and temperature dependences of thermal activation energy of acceptors in a Mg-doped GaN epilayer grown by metalorganic chemical-vapor deposition

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 NAu2009meV at 0 K. A strong temperature dependence of EA was also obtained in this study.

High quantum efficiency of violet-blue to green light emission in InGaN quantum well structures grown by graded-indium-content profiling method

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.

Bias effect on the luminescent properties of rectangular and trapezoidal quantum-well structures

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.

Phase separations in graded-indium content InGaN/GaN multiple quantum wells and its function to high quantum efficiency

Phase separations have been studied for graded-indium content InxGa1x N/GaN multiple quantum wells (MQWs) with different indium contents by means of photoluminescence (PL), cathodeluminescence (CL) and time-resolved PL (TRPL) techniques. Besides the main emission peaks, all samples show another 2 peaks at the high and low energy parts of the main peaks in PL when excited at 10 K. CL images show a clear contrast for 3 samples, which indicates an increasing phase separation with increasing indium content. TRPL spectra at 15 K of the main emissions show an increasing delay of rising time with indium content, which means a carrier transferring from low indium content structures to high indium content structures.