Multiple Quantum Wells Light-emitting Diodes Research Articles

Improvement of near-ultraviolet InGaN-GaN light-emitting diodes through higher pressure grown underlying GaN layers

The 410-nm near-ultraviolet (near-UV) InGaN-GaN multiple quantum-wells light-emitting diodes (LEDs) with low-pressure-grown (200 mbar) and high-pressure-grown (400 mbar) Si-doped GaN underlying layers were grown on c-face sapphire substrates by metal-organic vapor phase epitaxy. Increasing the growth pressure during the initial growth of the underlying n-type GaN epilayers of the near-UV InGaN-GaN LEDs was found to reduce the amount of threading dislocations that originated from the GaN-sapphire interfaces. The electroluminescence intensity of LEDs with underlying GaN layers grown at a higher pressure was nearly five times larger than that of LED with layers grown at lower pressure. Additionally, two-order reduction of leakage current was also induced for the LEDs grown at a higher pressure.

Nitride-based near-ultraviolet multiple-quantum well light-emitting diodes with AlGaN barrier layers

The In0.05Ga0.95N/GaN, In0.05Ga0.95N/Al0.1Ga0.9N, and In0.05Ga0.95N/Al0.18Ga0.82N multiple-quantum well (MQW) light-emitting diodes (LEDs) were prepared by metal-organic chemical-vapor deposition. (MOCVD). It was found that the 20-mA electroluminescence (EL) intensity of the InGaN/Al0.1Ga0.9N 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 a larger injection current suggest that Al0.1Ga0.9N-barrier layers can provide a better carrier confinement and effectively reduce leakage current. In contrast, the EL intensity of the InGaN/Al0.18Ga0.82N MQW LED was smaller because of the relaxation that occurred in the MQW active region of the sample.

Improvement in light-output efficiency of InGaN/GaN multiple-quantum well light-emitting diodes by current blocking layer

The fabrication and characterization of an InGaN/GaN multiple-quantum well (MQW) light-emitting diode (LED) with a SiO2 current blocking layer inserted beneath the p-pad electrode is described. The light-output power and external quantum efficiency for the InGaN/GaN MQW LED chip with a current blocking layer were significantly increased compared to those for the conventional InGaN/GaN MQW LED chip. The increase in the light-output power can be attributed to the injection of additional current into the light-emitting quantum well layer of the LED by the SiO2 current blocking layer and a reduction in parasitic optical absorption in the p-pad electrode.

High-density plasma-induced etch damage of InGaN/GaN multiple quantum well light-emitting diodes

Plasma-induced damage of InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) has been studied in terms of forward turn-on and reverse breakdown voltages, together with etch rate and surface morphology. The physical degradation of sidewall along with rough surface morphology of n–GaN caused by increased ion scattering induced the deterioration of the forward and reverse voltages. The forward turn-on voltage was relatively independent of the pressure up to 20 mTorr. The reverse breakdown voltage showed the worst degradation at 75% Cl2 mainly because of a sidewall contamination. It was found that the turn-on voltage is sensitive to the surface roughness of the etched n–GaN and the breakdown voltage is strongly affected by the sidewall contamination. Annealing under nitrogen after the mesa etching improved the electrical properties of the InGaN/GaN MQW LEDs.

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.

Optical Study of AlGaN/GaN Multiple Quantum Well Structures Grown on Laterally Overgrown GaN Templates

We report on optical studies of Al0.1Ga0.9N/GaN structures with five 30 Å thick GaN quantum wells (QWs) grown by metal organic vapor phase epitaxy using the lateral overgrowth technique. Overgrown regions demonstrate better structural and optical properties. The low temperature photoluminescence (PL) is dominated by the multiple quantum well (MQW) emission at 3.53 eV with the linewidth of ∼50 meV. The PL decay time for this line was measured to be ∼600 ps. Comparison with an AlGaN/GaN MQW light-emitting diode (LED) structure is done. The LED structure was grown with a p-type doped AlGaN top layer and a p-GaN contact layer. The PL spectrum of the LED structure shows besides the donor–acceptor pair recombination from the top layer an additional 60 meV wide line at 3.64 eV. The presence of two MQW peaks may be related to the potential gradient present across the MQW structure.

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 multiple-quantum-well green light-emitting diodes on Si grown by metalorganic chemical vapor deposition

We report the characteristics of InGaN multiple-quantum-well (MQW) green light-emitting diodes (LEDs) on Si (111) substrates. The MQW LEDs were grown on Si by metalorganic chemical vapor deposition using Al0.27Ga0.73N/AlN intermediate layers. The LED on Si showed an operating voltage of 7 V, a series resistance of 100 Ω, an optical output power of 20 μW, and a peak emission wavelength of 505 nm with a full width at half maximum of 33 nm at 20 mA drive current. The optical output power was half as compared to that of green LED on sapphire. The LED also exhibited a stable operation over 500 h under automatic current control (20 mA) condition at 27 °C.

Enhanced output power in an InGaN-GaN multiquantum-well light-emitting diode with an InGaN current-spreading layer

InGaN-GaN multiple quantum-well (MQW) light-emitting diodes (LEDs) with InGaN current-spreading layer were grown by metal-organic vapor-phase epitaxy (MOVPE) and their characteristics were evaluated by current-voltage (I-V), as well as output power measurements. Experimental results indicate that the LEDs exhibited a higher output power and a lower operation voltage than that of conventional LEDs. The external quantum efficiency of InGaN-GaN MQW LEDs for bare chips operated at injection current of 20 mA with InGaN current spreading layer near 5%. This is two times higher than that of conventional LEDs. This could be tentatively attributed to the better current-spreading effect resulting from Si-doped In/sub 0.18/Ga/sub 0.82/N wide potential well in which electron states are not quantized.

Effects of sulfur treatment on electrical and optical performance of InGaN/GaN multiple-quantum-well blue light-emitting diodes

The results of the sulfur treatment of multiple-quantum-well (MQW) light-emitting diodes (LEDs) with (NH4)2S and (NH4)2S+t-C4H9OH solutions prior to the deposition of a light-transmitting p-electrode metal are presented. The room-temperature I–V curves showed that the forward voltages of MQW LEDs treated with the two sulfur solutions decrease by 0.12 and 0.35 V at 20 mA, respectively, compared to the untreated MQW LED, as the result of an improvement in p-Ohmic contact characteristics. The relative light-output power and external quantum efficiency of MQW LEDs increased by a factor of 1.28 for the (NH4)2S treated sample and 2.23 for the (NH4)2S+t-C4H9OH treated sample compared to the untreated sample. In addition, the reverse leakage current characteristic of MQW LEDs was reduced as a result of sulfur treatment. This can be attributed to the passivation of surface and sidewall damages formed after the dry-etching process for a reliable pattern transfer. The present results indicate that the sulfur treatment greatly improves the electrical and optical performance of MQW LEDs.

Band-edge luminescence in quaternary AlInGaN light-emitting diodes

Operation of InGaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with quaternary AlInGaN barriers at room and elevated temperatures is reported. The devices outperform conventional GaN/InGaN MQW LEDs, especially at high pump currents. From the measurements of quantum efficiency and total emitted power under dc and pulsed pumping, we show the emission mechanism for quaternary barrier MQWs to be predominantly linked to band-to-band transitions. This is in contrast to localized state emission observed for conventional InGaN/InGaN and GaN/InGaN LEDs. The band-to-band recombination with an increased quantum-well depth improves the high-current performance of the quaternary barrier MQW LEDs, making them attractive for high-power solid-state lighting applications.

Optical properties in InGaN/GaN multiple quantum wells and blue LEDs

InGaN/GaN multiple quantum wells (MQW) and blue MQW light-emitting diodes were grown by metalorganic vapor phase epitaxy (MOVPE). Band gap narrowing of the PL spectra for the InGaN/GaN MQW LEDs can be observed at room temperature. In addition, the emission wavelength of EL and PL spectra for the MQW blue LEDs exhibits a blue-shift phenomenon when increasing the injection current and laser power, respectively. This luminescence behavior can tentatively be understood as a competition between a spectral red-shift mechanism of piezoelectricity-induced quantum-confined Start effect (PQCSE) and a blue-shift mechanism of band-filling and charge screening effects. Blue MQW LEDs emitting at 465 nm have been successfully fabricated with a narrow FWHM of about 30 nm at 20 mA. The output power of bare chips is better than 1 mA at 20 mA and the forward voltages are less than 3.8 V.

Luminescence of an InGaN/GaN multiple quantum well light-emitting diode

In GaN/GaN multiple quantum wells (MQW) and blue MQW light-emitting diodes (LEDs) were grown by metal-organic vapor phase epitaxy (MOVPE). Band-gap narrowing of the PL spectra for the InGaN/GaN MQW LEDs can be observed at room temperature. In addition, the emission wavelength of EL and PL spectra for the MQW blue LEDs exhibit a blue-shift phenomenon while increasing the injection current and laser power, respectively. This luminescence behavior can tentatively be understood as a competition between a spectral red-shift mechanism of piezoelectricity-induced quantum-confined Stark effect (PQCSE) and a blue-shift mechanism of band-filling and charge screening effects.

Fabrication of thin-film InGaN light-emitting diode membranes by laser lift-off

Indium–gallium nitride (InGaN) multiple-quantum-well (MQW) light-emitting diode (LED) membranes, prefabricated on sapphire growth substrates, were created using pulsed-excimer laser processing. The thin-film InGaN MQW LED structures, grown on sapphire substrates, were first bonded onto a Si support substrate with an ethyl cyanoacrylate-based adhesive. A single 600 mJ/cm2, 38 ns KrF (248 nm) excimer laser pulse was directed through the transparent sapphire, followed by a low-temperature heat treatment to remove the substrate. Free-standing InGaN LED membranes were then fabricated by immersing the InGaN LED/adhesive/Si structure in acetone to release the device from the supporting Si substrate. The current–voltage characteristics and room-temperature emission spectrum of the LEDs before and after laser lift-off were unchanged.

Phase separation in InGaN/GaN multiple quantum wells and its relation to brightness of blue and green LEDs

InGaN/GaN multiple quantum wells (MQW) and high-brightness MQW light-emitting diodes were grown by production scale metal-organic chemical vapor deposition (MOCVD). We have found that the extent of InGaN phase separation in InGaN MQWs depends strongly on growth conditions. Multiple peaks in photoluminescence (PL) of InGaN/GaN MQWs and electroluminescence (EL) of MQW LEDs can be observed at room temperature. In the presence of InGaN phase separation, photoluminescence of MQW is red-shifted with respect to the expected wavelength calculated for the apparent indium composition determined by X-ray diffraction (XRD). We have determined that InGaN phase separation is necessary for high brightness electroluminescence in LEDs. Under optimal growth conditions, MQWs with very well-defined XRD satellite peaks and PL in the wavelength range of (450–520 nm) can be achieved. High-brightness LEDs emitting at 480 nm have been successfully fabricated with an output power well better than 2 mW at 20 mA and with a forward voltage less than 4 V.

High-efficiency InGaN MQW blue and green LEDs

High-efficiency blue and green light-emitting diodes (LEDs) with multiple quantum well (MQW) InGaN active regions have been grown by low-pressure MOCVD on sapphire substrates. The devices have active regions composed of 12 InGaN wells clad by GaN barrier layers. A high density of crystallographically-oriented pits form during growth of the MQW layers and planarize during the growth of subsequent layers. Blue (∼470nm) LEDs exhibit external quantum efficiencies of 10.6% at 20mA and 8.7% at 100mA. Green LEDs (522nm) have external quantum efficiencies of 4.5% at 20mA. Operating voltages as low as 2.96V at 20mA have been achieved. These are the highest efficiencies published to date for nitride-based MQW LEDs. Blue and blue-green device efficiencies are comparable to the best reported SQW devices at 20mA drive current and exceed the SQW LEDs at higher currents.

Light-Emitting Devices Based on Gallium Nitride and Related Compound Semiconductors

ABSTRACTAsymmetric double heterostructures (ADH) of AlGaN/GaInN/GaN blue light emitting diodes (LEDs) and GaInN/GaN multiple quantum well (MQW) LEDs were fabricated by metalorganic vapor phase epitaxy (MOVPE). The ADH LEDs had spectral emissions peaking at 450 nm and the luminous intensify was 2.5 cd at 20 mA The output power was 3.6 mW at 20mA and the external quantum efficiency was as high as 5.1 % at 20 mA. The GaInN/GaN MQW structure was grown successfully by MOVPE. Fine multi-layer structures 7 – 9 nm thick were detected by secondary ion mass spectroscopy and transmission electron microscopy (TEM). The dislocation density in the MQW was as high as 0.5–2×109 cm−2 by TEM The optical efficiency of the MQW layer was higher than that of a bulk GalnN layer. The intensity of UV emission from MQW LEDs was greater than that of blue light from ADH blue LEDs. The UV emission increased as a super-linear function of injection current at -100°C.