Multiquantum-well Light-emitting Diodes Research Articles

Improved light output power of InGaN/GaN MQW LEDs by lower temperature p-GaN rough surface

Mg-doped p-GaN epitaxial layers grown at different temperatures were prepared and characterized. It was found that we could achieve a higher hole concentration and a rough surface by reducing the growth temperature down to 800 °C. In 0.23Ga 0.77N/GaN multiquantum well (MQW) light-emitting diodes (LEDs) with such a low 800 °C-grown p-GaN cap layer were also fabricated. It was also found that one could lower the LED operation voltage from 3.68 to 3.36 V and enhance the 20 mA LED output power by the 800 °C-grown p-GaN cap layer. However, it was also found the leakage current was larger and the lifetime was shorter for the LEDs with the 800 °C-grown p-GaN cap layer.

Study of AlGaInP multiquantum-well/double heterostructure light-emitting diodes with In-added GaP window layer regrown by antimony-based liquid phase epitaxy

The characteristics of surface-emitting AlGaInP multiquantum-well (MQW) and double heterostructure (DH) light-emitting diodes (LEDs) with a GaP window layer regrown by a GaP source melt with the addition of indium in the liquid phase epitaxy (LPE) system are studied. Using antimony in the source melt instead of gallium eliminates the phenomenon of melt back at the top layer of the LED during the LPE regrowth process. Meanwhile, adding In and Zn to the source melt significantly increases the GaP growth rate and effectively reduces residual melt during the regrowth process. Thus, the AlGaInP LED with a Zn-doped GaP window layer regrown by In being added, Sb-based LPE exhibits a lower turn-on voltage and higher light output power than that without the regrown GaP window layer. However, the emission peak wavelength of the MQW LED shifts a little after the LPE regrowth process. The DH LED does not shift. Thus, the DH structure is more suitable than the MQW structure for the LPE regrowth process of GaP.

Formation of Nonalloyed Low Resistance Ni/Au Ohmic Contacts to p-Type GaN Using Au Nanodots

The effects of the insertion of Au nanodots (4-10 nm in diameter) at the Ni/GaN interface on the electrical properties of Ni/Au ohmic contacts to p-type GaN have been investigated. As-deposited Ni/Au contacts with Au nanodots show better electrical behavior than contacts without Au nanodots. Nanodot contacts produce a specific contact resistance of 8.4 X 10 - 4 Ω cm 2 . The multiquantum-well light-emitting diodes (LEDs) are fabricated with the nanodot Ni/Au contact layers. Nanodot LEDs show a lower operating voltage compared with LEDs made with a conventional Ni/Au contact layer.

InGaN/GaN multi-quantum dot light-emitting diodes

It has been demonstrated that InGaN/GaN blue light-emitting diodes (LEDs) with multiple quantum dot (MQD) were successfully fabricated by metal-organic chemical vapor deposition. We have formed nanoscale InGaN self-assembled QDs in the well layers of the active region with a typical 3-nm height and 10-nm lateral dimension. With a 20-mA DC injection current, the forward voltage was 3.1 and 3.5V for MQD LED and conventional nitride-based multi-quantum well (MQW) LED with the same structure, respectively. It was also found that EL peak position of the MQD LED is more sensitive to the amount of injection current, as compared to the conventional MQW LEDs.

Optical and electrical properties of homoepitaxially grown multiquantum well InGaN/GaN light-emitting diodes

We have studied the electrical and optical properties of homoepitaxially grown InGaN/GaN multiquantum-well light-emitting diodes over a wide range of temperatures and currents. We found thermionic emission rather than tunneling transport to be the most probable mechanism responsible for radiative recombination processes. As we point out, the thermal activation energies of Si donors in the n-type GaN and Mg acceptors in the p-type GaN can explain the temperature dependence of the device series resistance. Furthermore, the presence of strong localization effects in the active InGaN layers can be deduced from the observed considerable blueshifts of the electroluminescence emission peak with increasing temperature and current. We found the values of the internal and external quantum efficiencies of homoepitaxially grown light emitting diodes to be 38% and 1.9%, respectively.

High-Indium-Content InGaN/GaN Multiple-Quantum-Well Light-Emitting Diodes

High-indium-content InGaN/GaN multi-quantum-well (MQW) light-emitting diode (LED) structures were epitaxially grown by metalorganic vapor phase epitaxy (MOVPE). With 70% indium in the InGaN well layers, it was found that the photoluminescence (PL) full-width at half maximum (FWHM) is stronger than that in the case of low-indium-content InGaN/GaN MQW LED structures. It was also found that the peak position of electroluminescence (EL) fabricated In0.7Ga0.3N/GaN LED depends strongly on injection current. As injection current increased from 1 mA to 150 mA, it was found that the output color of the In0.7Ga0.3N/GaN LED changed from orange to yellow, to yellowish green, and finally to yellowish white.

Si and Zn co-doped InGaN-GaN white light-emitting diodes

InGaN-GaN double heterostructure (DH) and multiquantum-well (MQW) light-emitting diodes (LEDs) with Si and Zn co-doped active well layers were prepared by metalorganic chemical vapor deposition (MOCVD). It was found that we could observe a broad long-wavelength donor-acceptor (D-A) pair-related emission at 500/spl sim/560 nm. White light can thus be achieved by the combination of such a long-wavelength D-A pair emission with the InGaN band-edge-related blue emission. By increasing the DMZn mole flow rate to 360 nmole/min, we could achieve a Si and Zn co-doped In/sub 0.3/Ga/sub 0.7/N-GaN MQW LED with color temperature of 4100 K, color rendering index of 70, and color coordinates x=0.383, y=0.405. It was also found that the 20-mA forward voltage and the breakdown voltage of such Si and Zn co-doped In/sub 0.3/Ga/sub 0.7/N-GaN MQW LEDs were both smaller than those of the conventional phosphor-converted white LEDs.

Effect of thickness variation in high-efficiency InGaN/GaN light-emitting diodes

In x Ga (1−x) N/GaN multiquantum-well light-emitting diodes (LEDs) having periodic thickness variations (TVs) in InxGa(1−x)N active layers exhibit substantially higher optical efficiency than LEDs with uniform InxGa(1−x)N layers. In these nanostructured LEDs, the thickness variation of the active layers is found to be more important than the In composition fluctuation in quantum confinement of excitons (carriers). Detailed scanning transmission electron microscopy-atomic number Z contrast analysis, where image contrast is proportional to Z2 (Z being the atomic number), was carried out to investigate the variation in thickness as well as the spatial distribution of In. In the nanostructured LEDs, there are short-range thickness variations (SR-TVs) (3–4 nm) and long-range thickness variations (LR-TVs) (50–100 nm) in InxGa(1−x)N layers. It is envisaged that LR-TV is key to quantum confinement of the carriers and enhancement of the optical efficiency. We propose that the LR-TV is caused by two-dimensional strain in the InxGa(1−x)N layer below its critical thickness. The SR-TV may be caused by In composition fluctuation.

400-nm InGaN-GaN and InGaN-AlGaN multiquantum well light-emitting diodes

The 400-nm In/sub 0.05/Ga/sub 0.95/N-GaN MQW light-emitting diode (LED) structure and In/sub 0.05/Ga/sub 0.95/N-Al/sub 0.1/Ga/sub 0.9/N LED structure were both prepared by organometallic vapor phase epitaxy. It was found that the use of Al/sub 0.1/Ga/sub 0.9/N as the material for barrier layers would not degrade crystal quality of the epitaxial layers. It was also found that the 20-mA electroluminescence intensity of InGaN-AlGaN multiquantum well (MQW) LED was two times larger than that of the InGaN-GaN MQW LED. The larger maximum output intensity and the fact that maximum output intensity occurred at larger injection current suggest that AlGaN barrier layers can provide a better carrier confinement and effectively reduce leakage current.

Influence of Si-doping on the characteristics of InGaN-GaN multiple quantum-well blue light emitting diodes

A detailed study on the effects of Si-doping in the GaN barrier layers of InGaN-GaN multiquantum well (MQW) light-emitting diodes (LEDs) has been performed. Compared with unintentionally doped samples, X-ray diffraction results indicate that Si-doping in barrier layers can improve the crystal and interfacial qualities of the InGaN-GaN MQW LEDs. It was also found that the forward voltage is 3.5 and 4.52 V, the 20-mA luminous intensity is 36.1 and 25.1 mcd for LEDs with a Si-doped barrier and an unintentionally doped barrier, respectively. These results suggests that one can significantly improve the performance of InGaN-GaN MQW LEDs by introducing Si doping in the GaN barrier layers.

White-light emission from InGaN-GaN multiquantum-well light-emitting diodes with Si and Zn codoped active well layer

Si and Zn codoped In/sub x/Ga/sub 1-x/N-GaN multiple-quantum-well (MQW) light-emitting diode (LED) structures were grown by metal-organic vapor phase epitaxy (MOVPE). It was found that we can observe a broad long-wavelength donor-acceptor (D-A) pair related emission at 500 nm/spl sim/560 nm. White light can thus be achieved by the combination of such a long-wavelength D-A pair emission with the InGaN bandedge related blue emission. It was also found that the electroluminescence (EL) spectra of such Si and Zn codoped InGaN-GaN MQW LEDs are very similar to those measured from phosphor-converted white LEDs. That is, we can achieve white light emission without the use of phosphor by properly adjusting the indium composition and the concentrations of the codoped Si and Zn atoms in the active well layers and the amount of injection current.

Spatial inhomogeneity investigation of QW emission in InGaN MQW LEDs

The luminescence properties of InGaN multiquantum well (MQW) light emitting diodes (LEDs) with various emission wavelengths were investigated using electroluminescence (EL) microscopy (ELM) and micro-EL (μ-EL) spectroscopy. Spatial inhomogeneity of the QW emission was observed as a function of the emission wavelength in spectrally resolved ELM images. The results show that the emission from diodes with shorter emission wavelengths exhibit less spatial inhomogeneity, while bright features were observed on the surface of samples with longer emission wavelengths. μ-EL spectra obtained from 5×5 μm 2 regions show an increase in the full width at half maximum (FWHM) of the spectra with increasing injection current and emission wavelength. With increasing emission wavelength, there is a shift in the main emission area from the p-contact area to the n-contact. The μ-EL spectra obtained from different bright spots in the ELM images demonstrate the variation of main emission peak position and FWHM of the EL spectra. The results suggest that the band-tailing effect due to small self-organised In-rich regions play a key role in the emission of the InGaN MQWs.

InGaN-GaN multiquantum-well blue and green light-emitting diodes

InGaN-GaN multiquantum-well (MQW) blue and green light-emitting diodes (LEDs) were prepared by organometallic vapor phase epitaxy, and the properties of these LEDs were evaluated by photoluminescence (PL), double crystal X-ray diffraction, and electroluminescence (EL) measurements. It was found that there were only small shifts observed in PL and EL peak positions of the blue MQW LEDs when the number of quantum well (QW) increased. However, significant shifts in PL and EL peak positions were observed in green MQW LEDs when the number of QW increased. It was also found that there was a large blue shift in EL peak position under high current injection in blue MQW LEDs. However, the blue shift in green MQW LEDs was negligibly small when the injection current was large. These observations could all be attributed to the rapid relaxation in green MQW LEDs since the In composition ratio in the InGaN well was high for the green MQW LEDs. The forward voltage V/sub f/ of green MQW LEDs was also found to be larger than that of blue MQW LEDs due to the same reason.

Development of high efficiency GaN-based multiquantum-well light-emitting diodes and their applications

Highly efficient GaInN-GaN multiple quantum-well (MQW) light-emitting diodes (LEDs) were successfully developed by the low-temperature AlN buffer layer method for metal-organic vapor phase epitaxy (MOVPE). The light-emitting layer of the GaInN-GaN MQW drastically enhances the performance of GaN-based LEDs in terms of the efficiency and spectrums. Flip-chip (FC) type MQW LEDs have been newly developed to increase efficiency in extracting light from the GaN-based crystal to the outside. The luminous intensities of FC type blue and green LEDs are typically 6 and 14 cd, respectively, at 20 mA. The output power of the FC-type LEDs was 14 mW at 20 mA, which was approximately two times higher than that of the conventional face-up type blue LEDs. The external quantum efficiency of blue FC-type LEDs was as high as 20% at 20 mA. New multicolor package was developed using these high performance nitride-based LEDs and commercial AlGaInP-based red LEDs, the color range of which is the largest among other flat panel display devices.

InGaN-AlInGaN multiquantum-well LEDs

InGaN-GaN and InGaN-AlInGaN multiquantum-well (MQW) light-emitting diodes (LEDs) were both fabricated and their optical properties were evaluated by photoluminescence (PL) as well as electroluminescence (EL). We found that the PL peak position of the InGaN-AlInGaN MQW occurs at a much lower wavelength than that of the InGaN-GaN MQW. The PL intensity of the InGaN-AlInGaN MQW was also found to be larger. The EL intensity of the InGaN-AlInGaN MQW LED was also found to be larger than that of the InGaN-GaN MQW LED under the same amount of injection current. Furthermore, it was found that EL spectrum of the InGaN-AlInGaN MQW LED is less sensitive to the injection current. These observations all suggest that we can improve the properties of nitride-based LEDs by using AlInGaN as the barrier layer.

Modeling of a GaN-based light-emitting diode for uniform current spreading

The characteristics of the GaN/InGaN multiquantum-well light-emitting diode (LED) have been examined from the view point of uniform current spreading. By means of simple modeling, it was found that the current density and the length of the lateral current path through the transparent layer represent dominant parameters in determining uniform current spreading. In this regard, we studied the effect of current density on the reliability characteristics of the LED. We were able to significantly improve the electrical, optical, and reliability characteristics of the LED in terms of reducing the length of the lateral current path through the transparent layer.

Room-temperature operation at 333 nm of Al0.03Ga0.97N/Al0.25Ga0.75N quantum-well light-emitting diodes with Mg-doped superlattice layers

We demonstrate room-temperature deep ultraviolet (UV) current injection emission from Al0.03Ga0.97N/Al0.25Ga0.75N multiquantum-well (MQW) light-emitting diodes (LEDs) fabricated by metalorganic vapor phase epitaxy. The electroluminescence (EL) peaked at 333.0 nm under pulsed current injection. To our knowledge, this is the shortest wavelength ever reported for nitride QW LEDs. A Mg-doped GaN/AlGaN superlattice (SL) hole conductive layer was used as a p-type layer in order to enable current injection into such deep-level AlGaN QWs. We observed single-peak near band-edge emission from the QWs. The output intensity did not saturate up to current densities of 0.33 kA/cm2. We obtained a reasonable well width dependence on the EL peak wavelength of Al0.01Ga0.99N MQW LEDs, which confirms that the main emission peak originates from the QW regions. These results revealed that the Mg-doped SL hole conductive layers are highly suitable for application to GaN-based UV light-emitting devices.

Current Injection UV-Emission from InAlGaN Multi-Quantum-Well Light-Emitting Diodes

ABSTRACTInAlGaN quaternary material is very attractive for realizing ultraviolet (UV) emitting devices working at 300 – 350 nm wavelength range. We demonstrate current injection into 340 nm-band InAlGaN based UV light emitting diodes (LEDs), for the first time, fabricated by metal organic vapor phase epitaxy (MOVPE). We performed current injection into AlGaN/AlGaN multi quantum well (MQW), bulk InAlGaN quaternary and InAlGaN/InAlGaN MQW LEDs through Mg-doped AlGaN/GaN superlattice hole conductive layers. The injected current density was ranging 0 – 0.5 kA/cm2 under pulsed or CW operation. The intensity of both photoluminescence (PL) and electroluminescence for InAlGaN quaternary-based LED was much higher than that for AlGaN based LEDs at room temperature. From these results InAlGaN quaternary-based QWs are expected to realize high intensity UV LEDs and LDs.

AlGaInP-GaInP compressively strained multiquantum-well light-emitting diodes for polymer fiber application

We have studied the optical properties of the AlGaInP-GaInP light-emitting diodes (LED's), with a compressively strained multiquantum-well (CSMQW) active layer, emitting at 650 nm. It was found that by introducing a compressive strain into the MQW active layer, we can achieve a much faster operation speed and a higher output power. It was also found that a +0.33%, compressive strain can reduce the 10%-90% rise time and/or fall time from 50 to 15 ns. Furthermore, the +0.33% compressive strain can also increase the output power of the MQW AlGaInP-GaInP LED by more than 40%. Such a high operation speed and high output power LED is potentially useful in polymer-based optical fiber communication.

Remarkable improvement in emission efficiency of ZnCdSe/Zn(S)Se LEDs by thermal annealing

Thermal annealing effects on optical and electrical characteristics for p-type and n-type II–VI compound layers (ZnSe, ZnSSe, and MgZnSSe) and on the emission efficiency of ZnCdSe/Zn(S)Se 6 quantum well (QW) light emitting diodes (LEDs) grown by molecular beam epitaxy were investigated. It was clarified that serious degradation of optical and electrical characteristics was not observed up to an annealing temperature of 400°C. In the case of p-MgZnSSe, the maximum permitted annealing temperature was lower than that of Zn(S)Se. The light output of the ZnCdSe/Zn(S)Se multi QW LEDs was enhanced by a factor of three at optimum thermal annealing conditions. The study suggests that this thermal effect for LEDs was produced by the improved crystal quality of ZnCdSe QWs by thermal annealing.