Quantum-well Light-emitting Diodes Research Articles

High Reliability of AlGaInP Light-Emitting Diodes With Tensile Strain Barrier-Reducing Layer

A high-reliability multiple quantum-well AlGaInP light-emitting diode is grown by inserting a novel tensile strain barrier-reducing (TSBR) structure between the window and cladding layers. The TSBR (/spl sim/150 /spl Aring/ of Ga/sub 0.65/In/sub 0.35/P) film is of lattice size with valence band energy and is located between window and cladding layers, thus, significantly reducing band offset and device forward bias, and accordingly achieves improvement in dynamic resistance and junction heating. Reducing junction heating significantly enhances device reliability and lifetime. The TSBR layer also acts as a buffer layer, reducing band, and junction discontinuity between the misfit layers, an effect that can be attributed to reduced nonradiative recombination and defect density. Accordingly, TSBR is a successful design for improving global life behavior.

Characterization of Minority-Carrier Hole Transport in Nitride-Based Light-Emitting Diodes with Optical and Electrical Time-Resolved Techniques

ABSTRACTForward-to-reverse bias step-recovery measurements were performed on In.07Ga.93N/GaN and Al.36Ga.64N/Al.46Ga.54N quantum-well (QW) light-emitting diodes grown on sapphire. With the QW sampling the minority-carrier hole density at a single position, distinctive two-phase optical decay curves were observed. Using diffusion equation solutions to self-consistently model both the electrical and optical responses, hole transport parameters τp = 758 ± 44 ns, Lp = 588 ± 45 nm, and μp = 0.18 ± 0.02 cm2/Vs were obtained for GaN. The mobility was thermally activated with an activation energy of 52 meV, suggesting trap-modulated transport. Optical measurements of sub-bandgap peaks exhibited slow responses approaching the bulk lifetime. For Al.46Ga.54N, a longer lifetime of τp = 3.0 μs was observed, and the diffusion length was shorter, Lp ≈ 280 nm. Mobility was an order of magnitude smaller than in GaN, μp ≈ 10−2 cm2/Vs, and was insensitive to temperature, suggesting hole transport through a network of defects.

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.

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.

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.

High-Power III-Nitride Emitters for Solid-State Lighting

High-power, large-area InGaN/GaN quantum-well heterostructure light-emitting diodes based on an inverted, or flip-chip, configuration are described. These devices are mounted in specially designed high-power (1-5 W) packages and exhibit high extraction efficiency and low operating voltage. In the blue wavelength regime, output powers greater than 250 mW (1 x 1 mm 2 device) and 1 W (2 x 2 mm 2 device) are delivered at standard operating current densities (50 A/cm 2 ), corresponding to wall-plug efficiencies of 22%-23%. Employing phosphors for the generation of white light, these same devices achieve luminous efficiencies greater than 30 lm/W.

Transient electroluminescence of organic quantum-well light-emitting diodes

Time-resolved electroluminescence (EL) is used for to investigate the dynamic distribution of charge carriers in organic quantum-well light-emitting diodes (QWLEDs). In organic QWLEDs, N,N′-bis-(1-naphthl)-diphenyl-1,1′-biphenyl-4,4′-diamine and 2,9-dimethyl-4,7-diphenyl-1,10phenanthroline (BCP) act as a hole-transporting layer and a potential barrier layer, respectively. Tris(8-quinolinolato) aluminum is used as a potential well, a light-emitting layer, and an electron-transporting layer. Both traditional bilayer LED and QWLEDs with one, two, and four wells are investigated. For the QWLED with one well, the transient EL spectra are similar to those of traditional LEDs. In the transient EL spectra of QWLEDs with two and four wells, two distinct peaks are observed. This is due to the fact that holes first distribute in two different wells next to the NPB layer and recombine with electrons that subsequently move in when a voltage pulse is applied. This accounts for the time-resolved light emission in different recombination zones. The interval between the cutoffs of the two EL peaks is considered to be the transit time for electrons to cross the BCP layer, corresponding to electron mobility of 6×10−7 cm2 V−1 S−1 at 7×105 V/cm.

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.

GaInSb/InAs/AlGaAsSb “W” quantum-well light-emitting diodes

We report on the characterization of molecular-beam epitaxy-grown Ga0.75In0.25Sb/InAs/Al0.15Ga0.85As0.05Sb0.95 “W” quantum-well light-emitting diodes (LED). The devices demonstrated room-temperature emission with a peak wavelength of 3.3 μm. A pulsed current output power of 55 μW with a slope efficiency of 98 μW/A was measured and the 300 K internal efficiency was estimated to be 2.5%. Comparative power analysis suggests that LED and laser diode efficiencies are limited by the same thermally activated nonradiative mechanism, probably Auger recombination.

Correlation of proton radiation damage in InGaAs-GaAs quantum-well light-emitting diodes

The effect of proton irradiation of InGaAs/GaAs quantum-well (QW) light-emitting diodes (LEDs) has been studied at energies ranging from 1 to 500 MeV in order to determine device damage mechanisms. The data are analyzed in terms of the theory of Rose and Barnes (1982), and complete correlation of the data over the entire proton energy range was achieved. This degradation data, along with data from other GaAs-based optoelectronic devices, are discussed in terms of the nonionizing energy loss (NIEL). The energy dependences of the various damage coefficients for proton energies greater than about 10 MeV are bounded by the total NIEL and the elastic NIEL.

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.

Integration of n-type and p-type quantum-well infrared photodetectors for sequential multicolor operation

A multicolor infrared photodetector based on the epitaxial integration of an n-type with a p-type GaAs/AlGaAs quantum-well stack is experimentally demonstrated. Additionally, a quantum-well GaAs light-emitting diode is inserted between the stacks to achieve up-conversion of mid-infrared radiation to near-infrared signal. This device shows a remarkable selectivity on wavelength: depending on the bias voltage the peak wavelength detection can be switched on and off between 9.1 and 4.85 μm.

Large-signal-modulation of high-efficiency light-emitting diodes for optical communication

The dynamic behavior of high-efficiency light-emitting diodes (LEDs) is investigated theoretically and experimentally. A detailed theoretical description of the switch-on and switch-off transients of LEDs is derived. In the limit of small-signal modulation, the well-established exponential behavior is obtained. However, in the case of high injection, which is easily reached for thin active layer LEDs, the small-signal time constant is found to be up to a factor of two faster than the radiative recombination lifetime. Using such quantum-well LEDs, we have demonstrated optical data transfer with wide open eye diagrams at bit rates up to 2 Gbit/s. In addition, we have combined the use of thin active layers with the concept of surface-textured thin-film LEDs, which allow a significant improvement in the light extraction efficiency. With LEDs operating at 0.5 Gbit/s and 1 Gbit/s, we have achieved external quantum efficiencies of 36% and 29%, respectively.

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.

Recent Developments in InGaN-Based Blue Leds and Lds

UV/blue/green/amber InGaN quantum-well structure light-emitting diodes with an external quantum efficiency of 7.5%, 11.2%, 11.6%, and 3.3% were developed. The localization in the InGaN well layer induced by the In composition fluctuations seems to be a key role of the high efficiency of those InGaN-based light-emitting diodes. When the electrons and holes are injected into the InGaN active layer of the light-emitting diodes, these carriers are captured by the localized energy states before they are cap- tured by the nonradiative recombination centers caused by the large number of threading dislocations. InGaN multi-quantum-well structure laser diodes ' with modulation doped strained-layer superlattice cladding layers grown on the epitaxially lateral overgrown GaN substrate were demonstrated to have an estimated lifetime of more than 10000 hours under-room temperature con- tinuous-wave operation. When the laser diode was formed on the GaN layer above the SiO2 mask region without any threading dislocations, the thresh- old current density was as low as 2.7 kA cm -2 . When the laser diode was formed on the window region with the high threading dislocation density, the threshold current density was as high as 4.5 to 9 kA cm -2 . A leakage current

The roles of structural imperfections in InGaN-based blue light-emitting diodes and laser diodes

REVIEW High-efficiency light-emitting diodes emitting amber, green, blue, and ultraviolet light have been obtained through the use of an InGaN active layer instead of a GaN active layer. The localized energy states caused by In composition fluctuation in the InGaN active layer are related to the high efficiency of the InGaN-based emitting devices. The blue and green InGaN quantum-well structure light-emitting diodes with luminous efficiencies of 5 and 30 lumens per watt, respectively, can be made despite the large number of threading dislocations (1 x 10(8) to 1 x 10(12) cm-2). Epitaxially laterally overgrown GaN on sapphire reduces the number of threading dislocations originating from the interface of the GaN epilayer with the sapphire substrate. InGaN multi-quantum-well structure laser diodes formed on the GaN layer above the SiO2 mask area can have a lifetime of more than 10,000 hours. Dislocations increase the threshold current density of the laser diodes.

Exciton localization in InGaN quantum well devices

Emission mechanisms of a device-quality quantum well (QW) structure and bulk three dimensional (3D) InGaN materials grown on sapphire substrates without any epitaxial lateral overgrown GaN base layers were investigated. The InxGa1−xN layers showed various degrees of in-plane spatial potential (band gap) inhomogeneity, which is due to a compositional fluctuation or a few monolayers thickness fluctuation. The degree of fluctuation changed remarkably around a nominal InN molar fraction x=0.2, which changes to nearly 0.08–0.1 for the strained InxGa1−xN. This potential fluctuation induces localized energy states both in the QW and 3D InGaN, showing a large Stokes-like shift. The spontaneous emission from undoped InGaN single QW light-emitting diodes (LEDs), undoped 3D LEDs, and multiple QW (MQW) laser diode (LD) wafers was assigned as being due to the recombination of excitons localized at the potential minima, whose lateral size was determined by cathodoluminescence mapping to vary from less than 60 to 300 nm in QWs. Those structures are referred to as quantum disks (Q disks) or segmented QWs depending on the lateral size. Blueshift of the emission peak by an increase of the driving current was explained to be combined effects of band filling of the localized states by excitons and Coulomb screening of the quantum confined Stark effect induced by the piezoelectric field. The lasing mechanisms of the continuous wave In0.15Ga0.85N MQW LDs having small potential fluctuations can be described by the well-known electron-hole-plasma (EHP) picture. However, the inhomogeneous MQW LDs are considered to lase by EHP in segmented QWs or Q disks. It is desirable to use entire QW planes with small potential inhomogeneity as gain media for higher performance LD operation.

Portable, Low-Cost, Solid-State Luminescence-Based O2 Sensor

A novel sensor for quantifying molecular O2 based entirely on solid-state electronics is presented. The sensor is based on the luminescence quenching of tris(4,7-diphenyl-1, 10-phenanthroline)ruthenium(II) ([Ru(dpp)3]2+) by molecular O2. The sensor involves immobilizing the ruthenium complex within a porous sol-gel-processed glass film and casting this film directly onto the surface of a blue quantum-well light-emitting diode (LED). The ruthenium complex is excited by the LED, the [Ru(dpp)3]2+ emission is filtered from the excitation with a low-cost acrylic color filter, and the emission is detected with an inexpensive silicon photodiode. The sensor response to gaseous O2 and dissolved O2 in water is presented. The sensor exhibits fast response times and good reversibility, and detection limits are 0.5%, 0.02%, and 110 ppb, respectively, for O2 in the gaseous (linear Stern–Vobner and multi-site Stern–Volmer analysis) and aqueous phase. This sensor provides a cost-effective alternative to traditional electrochemical-based O2 sensing and also provides a platform for other optically based sensors.

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.

5.5-MeV proton irradiation of a strained quantum-well laser diode and a multiple quantum-well broadband LED

The degradation under 5.5-MeV proton irradiation of two classes of quantum-well-based fiber-optic light sources was evaluated for satellite applications. The first was an InGaAs/GaAs strained-layer quantum-well (QW) laser, and the second was a broadband light-emitting diode (LED) based on dual asymmetric quantum wells in the InGaAs/GaAs/AlGaAs system. The QW LEDs were more tolerant of proton irradiation (-3 dB power at approximately 3*10/sup 13/ protons/cm/sup 2/) than the QW lasers (-3 dB power at approximately 3*10/sup 12/ protons/cm/sup 2/). The LEDs were operated far into gain saturation with a high-loss cavity structure, while the lasers were operated in a region where gain was more sensitive to current density. Therefore, atomic displacement-related recombination sites had a greater detrimental effect upon the lasers than the LEDs. The lasers held constant slope efficiency, and current thresholds increased linearly with proton fluence, while both LED power and slope efficiency decreased with proton fluence. The determined damage factors for both devices were found to fall within values predicted from a universal damage relation previously reported by G.P. Summers et al. (1988).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>