Search

We report the emission properties of various InGaN/GaN quantum wells. The photoluminescence of In[sub x]Ga[sub 1-x]N/GaN (x > 20%) quantum wells under varying external bias voltages has been investigated. A redshift of the photoluminescence peak position and decrease of photoluminescence peak intensity with an increase in voltage were observed regardless of the direction of external bias applied. For the sample in which the quantum dot-like region is abundant in quantum well layers, the peak position and intensity were not influenced much by the externally applied bias voltage. The results show that the origin of strong emission from InGaN/GaN quantum wells can be attributed to exciton localization in the quantum dot-like region and they are consistent with those of high-resolution transmission electron microscopy. [ABSTRACT FROM AUTHOR]

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 In[sub x]Ga[sub 1-x]N/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. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines the temperature induced blue shift of the intersubband absorption peak in GaAs/Al0.3Ga0.7As multiple quantum wells. Calculation as a function of well thickness; Domination of the blue shift by the barrier height; Importance of exchange interactions for well thickness; Reduction of the photocurrent spectrum.

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 n and n) at the interface between GaN barrier and InGaN well. [ABSTRACT FROM AUTHOR]

A significant stress-relaxation was observed in GaN epilayers by integrating a heavily Si-doped GaN (n+-GaN) sacrificial layer in the undoped GaN templates grown on sapphire substrates by metal-organic chemical vapor deposition. Selective GaN growth and electrochemical etching were exploited to achieve embedded air-gaps. Stress-relaxation and its local variations were probed by Raman mapping of high-frequency transverse-optical E2 (high) phonon mode of GaN. Enhanced In incorporation and improved light emission were observed in InGaN/GaN multi-quantum well visible light emitting diode structures fabricated on stress-relaxed GaN-epilayers with embedded air-gaps. Relevant sources for stress reduction and improved optical emission have been discussed. [ABSTRACT FROM AUTHOR]

InGaN/GaN multiple quantum well (MQW) light-emitting diode (LED) structures possessing current spreading layers of various carrier concentrations were grown by a metal organic chemical vapour deposition method, and LEDs were fabricated with various etching depths for the n-contact. The current–voltage and light output power characteristics of these LEDs were investigated. Device properties of LEDs possessing a current spreading layer showed a lower operation voltage and a higher light output power than those without, although the reverse breakdown voltage decreased when the carrier concentration in the current spreading layer was too high. The best LED characteristics are shown in a LED with a carrier concentration of 6 × 1018 cm-3 in the current spreading layer. Operating voltages of LEDs at 20 mA show the best value at an etching depth for the n-electrode of 800 nm from the top layer. The depth of etching for the n-contact in a LED also affects the light output power. On the basis of our results, it is suggested that the etching depth for the n-contact and the carrier concentration of the spreading layer in InGaN/GaN LED should be controlled properly to obtain the best performance from LEDs. [ABSTRACT FROM AUTHOR]

We have reported the effects of growth interruption time on the optical and structural properties of high indium content InxGa1−xN/GaN (x>0.2) multilayer quantum wells (QWs). The InGaN/GaN QWs were grown on c-plane sapphire by metal organic chemical vapor deposition. The interruption was carried out by closing the group-III metal organic sources before and after the growth of the InGaN QW layers. The transmission electron microscopy (TEM) images show that with increasing interruption time, the quantum-dot-like region and well thickness decreases due to indium reevaporation or the thermal etching effect. As a result the photoluminescence (PL) peak position was blue-shifted and the intensity was reduced. The sizes and 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. Temperature dependent PL spectra support the results of TEM measurements. Also, the electroluminescence spectra of light-emitting diode show that dominant mechanism in InGaN/GaN QWs is a localized effect in the quantum-dot-like regions. [Copyright &y& Elsevier]

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 459 nm above the main peaks at 430, 463, and 509 nm with an effective carrier transfer from the higher to main peak sides is characteristic of these structures with various In contents of x<<0.2, 0.20.3, 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 [ABSTRACT FROM AUTHOR]

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. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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.© 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Spatially and spectrally resolved photoluminescence of InGaN/GaN quantum wells grown by metalorganic chemical vapor deposition is studied with near-field scanning optical microscopy (NSOM) and transmission electron microscopy (TEM). High-spatial-resolution NSOM images show bright blue quantum well emission around V defects and yellow emission inside the defects. TEM data suggest that the spatial distribution of blue luminescence is partly due to dislocation gettering by V defects. The yellow emission is attributed to the Ga vacancy-impurity complexes trapped inside V defects. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Spatially and spectrally resolved photoluminescence (PL) from InGaN/GaN quantum wells is obtained using near-field scanning optical microscopy (NSOM). Samples displaying high macroscopic PL intensity revealed nonuniform intensity and linewidth but nearly uniform peak position. It suggests that the contrast in the NSOM image reflects nonuniform distribution of dislocations or defects which act as nonradiative recombination centers. The formation of quantum dots with size of 30±25 nm and their size-dependent interaction with dislocations were observed in plan-view transmission electron microscopy. It is likely that the high luminescence efficiency is due to the efficient localization of excitons in high-density quantum dots located in regions with fewer dislocations. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: Surface morphology of green InGaN/GaN multi-quantum wells (MQWs) grown by using metal organic chemical vapor deposition has been analyzed by using atomic force microscopy, scanning electron microscopy, and cathodoluminescence (CL). Effects of barrier growth temperature, indium composition, and background threading dislocation (TD) density on the evolution of the MQW surface morphology have been studied. Low temperature GaN barrier growth generates a high density of V-pits and inclusions embedded within V-pits. Using trimethylindium flow prior to InGaN well growth, In segregation on GaN barrier is shown to be the prime cause for the formation of some inclusions which appear as bright spots in CL mapping while some inclusions are related to low temperature barrier growth. High temperature GaN barrier growth at 910°C completely suppresses these inclusion defects. In high indium containing InGaN/GaN MQWs, the large lattice mismatch induced strain in the MQW plays the key role in producing V-pits as well as inclusion defects rather than the background TD density of GaN templates. [Copyright &y& Elsevier]