Multiple Quantum Wells Active Region Research Articles

Non-equilibrium quantum well populations and active region inhomogeneity in polar and nonpolar III-nitride light emitters

Strong disparity of electron and hole transport in III-nitride materials is commonly accepted as a main reason for inhomogeneous carrier injection in multiple-quantum well (MQW) active regions of light emitters operating in visible spectral range. In this work, we show that two more factors, specifically (i) excessive depth of III-nitride QWs and (ii) strongly non-equilibrium character of electron and hole populations in optically active QW, are responsible for the active region inhomogeneity in GaN-based light emitters. Modeling shows that electron and hole populations of deep III-nitride QWs are highly imbalanced and substantially deviate from thermodynamic equilibrium with corresponding mobile carrier subsystems in the device active region. In turn, large residual QW charges provide strong impact on the active region electrical uniformity and QW injection conditions. We demonstrate that, as a result of non-equilibrium effects in QW population, even nonpolar III-nitride light emitters with deep QWs suffer from inhomogeneous carrier injection, large QW residual charges, and overall electrical non-uniformity of MQW active regions.

Carrier Transport in InGaN MQWs of Aquamarine- and Green-Laser Diodes

We studied experimentally and theoretically the substrate-orientation impact on carrier transport and capture in InGaN multiple quantum well (MQW) laser diodes (LDs) with emission in the aquamarine-green spectral range. A new simulation approach was developed to analyze this behavior of LEDs and LDs emitting at these wavelengths. We show that due to deep carrier confinement, the thermal escape from a QW in such devices is negligible. The carrier distribution among QWs is therefore determined by the carrier transport and capture rates. We also show that the ballistic transport mechanism is dominant in this type of MQW active region. In c-plane structures, this mechanism is tunneling-assisted, and therefore, the transport is much slower than in nonpolar and semipolar structures. Because of this, a strong carrier injection nonuniformity observed in c-plane LDs, causes the threshold current increase when number of QWs is >;2. This effect is not observed in semipolar LDs because the carrier transport rate is faster than the capture rate.

Growth and characteristics of GaInN/GaInN multiple quantum well light-emitting diodes

We demonstrate GaInN multiple quantum well (MQW) light-emitting diodes (LEDs) having ternary GaInN quantum barriers (QBs) instead of conventional binary GaN QBs for a reduced polarization mismatch between QWs and QBs and an additional separate confinement of carriers to the MQW active region. In comparison with GaInN LEDs with conventional GaN QBs, the GaInN/GaInN LEDs show a reduced blueshift of the peak wavelength with increasing injection current and a reduced forward voltage. In addition, we investigate the density of pits emerging on top of the MQW layer that are correlated with V-defects and act as a path for the reverse leakage current. The GaInN/GaInN MQW structure has a lower pit density than the GaInN/GaN MQW structure as well as a lower reverse leakage current. Finally, the GaInN/GaInN MQW LEDs show higher light output power and external quantum efficiency at high injection currents compared to the conventional GaInN/GaN MQW LEDs. We attribute these results to the reduced polarization mismatch and the reduced lattice mismatch in the GaInN/GaInN MQW active region.

Effect of Residual Stress and Sidewall Emission of InGaN-Based LED by Varying Sapphire Substrate Thickness

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The effect of residual stress and the sidewall emission in InGaN–GaN films with different thickness of sapphire substrate were investigated. The peak wavelength of electroluminescence was blue-shifted as thinning the sapphire substrate, because the wafer bowing-induced mechanical stress alters the piezoelectric field in the InGaN–GaN multiple quantum-well active region of the light-emitting diode (LED). A sideview LED with 170-<formula formulatype="inline"><tex Notation="TeX">$\mu\hbox{m}$</tex> </formula>-thick sapphire exhibited the highest output power of 14.04 mW at a forward current of 20 mA, improved by 7% compared to that with 80-<formula formulatype="inline"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula>-thick sapphire. The maximum output power can be obtained by considering both the photon escaping probability from the edges of the sapphire and the photon absorption probability in the sapphire as well as the residual mechanical stress induced by the wafer bowing. </para>

Photoluminescence study of InGaN/GaN multiple-quantum-well with Si-doped InGaN electron-emitting Layer

InGaN/GaN multiple-quantum-well (MQW) structure with Si-doped InGaN electron-emitting layer (EEL) was grown by metal–organic chemical vapor deposition and their characteristics were evaluated by photoluminescence (PL) measurements. In a typical structure, a low indium composition and wide potential well was used to be an EEL, and a six-fold MQW was used to be an active layer where the injected carriers recombine. By comparing the PL spectral characteristics of the MQW samples, the PL intensity of MQW with EEL is about 10 times higher than that of typical MQW. Experimental results indicate that the high electron capture rate of the MQW active region can be achieved by employing EEL.

Barrier effect on hole transport and carrier distribution in InGaN∕GaN multiple quantum well visible light-emitting diodes

Carrier distributions governed by hole transport in InGaN∕GaN multiple quantum well (MQW) visible light-emitting diodes (LEDs) were investigated using conventional blue LEDs and dual-wavelength blue-green LEDs. It was found that holes were dominantly distributed in the QW close to the p-GaN layer in LEDs with conventional MQW active regions at a current of 20mA. A decrease in the thickness or the height of the quantum-well potential barrier enhanced hole injection into the MQWs located near the n-GaN layer. Reducing the thickness of a GaN quantum-well barrier between the blue QW and green QW did not degrade the electroluminescence (EL) intensity of the LED. In contrast, reducing the potential height of the barrier with material of possibly compromised quality resulted in a degradation of the EL intensity of the LED.

Combined analytical-finite difference time-domain full wave simulation of mode-locked vertical-extended-cavity semiconductor lasers

A time-domain approach based on the finite difference time-domain (FDTD) method is described for vertical-extended-cavity surface-emitting lasers (VECSELs). To permit the simulation of realistic devices with large extended cavities a combined analytical-FDTD model is developed. This approach uses plane wave propagation in the linear extended cavity to couple the active mirror and the semiconductor saturable absorber mirror (SESAM) in which the standard FDTD method is applied. This allows for a significant computational time reduction. The material response in the active quantum wells (QWs) and the absorber is incorporated by an infinite impulse response digital filter. The model is validated by the simulation of a passively mode-locked VECSEL with a multiple QW active region and a single QW SESAM.

InGaN ∕ GaN Multiple-Quantum-Well LEDs with Si-Doped Barriers

multiple-quantum-well (MQW) light-emitting diodes (LEDs) with undoped and Si-doped barriers were fabricated. It was found that we could enhance crystal quality of the MQW structure by introducing Si doping into the barrier layers. It was also found that carriers can be injected into the MQW active region more easily for the LED with Si-doped barriers. With a injection current, it was found that forward voltages were 3.29 and for the LEDs with undoped barriers and Si-doped barriers, respectively. The output power and external quantum efficiency of the LED with Si-doped barriers were also found to be larger.

High-Confinement Strained MQW for Highly Polarized High-Power Broadband Light Source

This letter presents highly polarized edge light-emitting diodes with high-confinement, strained, multiple quantum-well active regions. We demonstrate +40 dB of polarization extinction along with 16 dBm of output power from an 800-mum-long centered quantum-well device. By characterizing the polarization extinction and gain of devices with different lengths and optical confinement, we show that the polarization extinction is dominated by the polarization sensitivity of the gain

Midinfrared InGaAsSb quantum well lasers with digitally grown tensile-strained AlGaAsSb barriers

The InGaAsSb lasers emitting in the midinfrared with strain-compensated multiple-quantum-well (MQW) active region have been grown by solid-source molecular-beam epitaxy (MBE). Digitally grown tensile-strained AlGaAsSb was employed as barriers to increase the valence-band offset for hole confinement. The laser structures were characterized by reflection high-energy electron diffraction, x-ray diffraction, and photoluminescence. The digital-growth approach for the tensile-strained AlGaAsSb barriers improved the reliability and controllability of MBE growth for the MQW active region. The optical and structural qualities of InGaAsSb MQW were improved significantly as compared to those with random-alloy barriers due to the removal of growth interruption at the barrier/well interfaces in digital growth. As a result, high-performance devices were achieved in the InGaAsSb lasers with digital AlGaAsSb barriers. A low threshold current density of 163A∕cm2 at room temperature was achieved for 1000-μm-long lasers emi...

Control of the color contrast of a polychromatic light-emitting device with CdSe-ZnS nano-crystals on an InGaN-GaN quantum-well structure

Blue-red polychromatic light-emitting devices are fabricated by attaching red-emitting CdSe-ZnS nano-crystals on a blue-emitting InGaN-GaN multiple-quantum-well (MQW) structure. To improve the red/blue intensity contrast, holes of different diameters are fabricated for increasing the direct contact area between the MQW active regions and CdSe-ZnS nano-crystals. By comparing the devices of 10-, 50-, 60-, and 70-mum hole diameters, and a reference device of no hole, it is found that the hole diameter of 60 mum represents an optimized condition from the viewpoint of maintaining high quantum efficiency. However, the device of 10-mum holes has the highest red/blue intensity ratio, which corresponds to a 36% increase. This result is attributed to its largest side-wall area in the holes among various samples

Strain relaxation and surface migration effects in InGaAlAs and InGaAsP selective-area-grown ridge waveguides

Surface migration of the group-III precursors and strain relaxation at the ridge sidewalls are compared for 2.5μm wide waveguides based on InGaAsP and InGaAlAs multiple-quantum-well (MQW) structures. The cross-sectional thickness and strain variations have been measured using synchrotron radiation-based x-ray diffraction with an angular resolution of 2arcs and a beam size of (0.24×0.35)μm2. Indium-rich overgrowth has been observed for the InGaAsP-based waveguides, while InGaAlAs-based waveguides demonstrate thickness uniformity of the MQW active region with a strain relief of 0.4%∕μm at the sidewalls.

Nitride-based LEDs with MQW active regions grown by different temperature profiles

Nitride-based light-emitting diodes (LEDs) with multiple quantum-well active regions were separately prepared by metal-organic vapor phase epitaxy in different temperature profiles. Compared with conventional samples, the reduced reverse leakage current and improved electrostatic discharge characteristics of the LEDs can both be achieved using temperature ramping and temperature cycling methods. However, using the temperature ramping may degrade the optical properties of devices due to desorption of In atoms and/or impurity incorporation. With an emission wavelength of 465 nm, the 20-mA output powers measured were 5.5, 6.0, and 7.9 mW for temperature ramping LED, conventional LED, and temperature cycling LED, respectively.

Effect of strain relaxation and exciton localization on performance of 350-nm AlInGaN quaternary light-emitting diodes

The optical and structural properties of AlInGaN quaternary single and multiple quantum-well structures have been investigated by means of photoluminescence and x-ray diffraction. This comparative study of single quantum-well (SQW) and multiple quantum-well (MQW) structures was carried out in terms of the exciton localization effect and the strain relaxation. A detailed analysis indicated that 13% strain relaxation occurs in the MQW compared to the SQW, which is assumed to be fully strained. Furthermore, the AlInGaN SQW structure showed a stronger localization effect than the MQW. Both these effects result in enhanced emission efficiency for the SQW structure, indicating that it is better suited as the active region for ultraviolet light-emitting diodes (UV-LEDs). Finally, the UV-LEDs with an emission wavelength of about 350nm based on such SQW and MQW active regions were grown. The output power of the SQW UV-LEDs is around 2.3 times higher than that of MQW UV-LEDs.

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.

Simulation of static and dynamic properties of edge-emitting multiple-quantum-well lasers

This paper demonstrates simulation tools for edge-emitting multiple-quantum-well (MQW) lasers. Properties of the strained MQW active region are simulated by eight-band kp calculations. Then, a two-dimensional simulation along the transverse cross section of the device is performed based on a drift-diffusion model, which is self-consistently coupled to equations for the optical field. Furthermore, a method is described that allows for an efficient quasi-three-dimensional simulation of dynamic properties of multisection edge-emitting lasers.

Direct measurement of lateral carrier leakage in 1.3-/spl mu/m InGaAsP multiple-quantum-well capped mesa buried heterostructure lasers

1.3-/spl mu/m InGaAsP multiple-quantum-well (MQW) capped mesa buried heterostructure (CMBH) lasers with mesa widths ranging from 1 /spl mu/m (standard single mode) to 100 /spl mu/m (wide area) were fabricated on one wafer with identical current blocking layers at the sides. The single-mode devices had three times larger nominal threshold current density than the wide-area devices. Measurement of gain and loss in the devices showed the single-mode devices to have 6-8 cm/sup -1/ higher loss and 40% lower optical confinement than the wide-area devices. Beyond these differences, the measurements show that up to 30% of the threshold current in the single-mode CMBH lasers does not contribute to the pumping of the MQW active region. Injection efficiency is measured to be close to unity for both single-mode and wide mesa devices. Scenarios to explain this parasitic current are discussed, including the potential role for nonradiative recombination centers at the regrown epitaxial interface, which can be consistent with all of the experimental results.

ZnCdSeTe-based orange light-emitting diode

We report the fabrication of orange light-emitting diodes (LEDs) by introducing Te into the well layers of ZnCdSe-ZnSSe multiple quantum-well active region. It was found that a 15% Te can result in a 91 nm photoluminescence (PL) red-shift at 16 K and a 81 nm PL red-shift at room temperature. Such a ZnSe-based orange LED can be integrated easily with conventional ZnSe-based blue-green LED on the same GaAs substrate. Thus, such a ZnSe-based orange LED is potentially useful in realizing a new one-chip type white LED without the use of phosphor.

Two-section InGaN multiple-quantum-well laser diode with integrated electroabsorption modulator

Q -switching is demonstrated in a two-section InGaN multiple-quantum-well (MQW) laser diode consisting of an electroabsorption modulator and amplifier (gain) section. The modulator and gain sections are optically coupled and share the same InGaN MQW active region, but they are electrically separated by a narrow dry-etched trench. Applying a reverse bias voltage to the modulator section controls the absorption in the modulator portion of the device by compensating the piezoelectric field in the InGaN quantum wells. Changes in the absorption coefficient as large as 5000 cm−1 were realized with a moderate reverse bias of 7.2 V. By forward biasing, the amplifier section at a constant current of 225 mA and by controlling the reverse bias modulator voltage, the output power of the two-section laser diode could be switched between &amp;lt;0.5 mW (off state) and more than 3 mW (on state) with a laser emission wavelength near 401 nm.

Design and performance of asymmetric waveguide nitride laser diodes

We describe the design and performance characteristics of an asymmetric waveguide nitride laser diode structure, in which the p-cladding layer is placed immediately over the multiple-quantum-well (MQW) active region. Its close proximity to the active region enables it to serve not only as a cladding layer, but also as a potential barrier that confines injected electrons. This structure represents a departure from conventional nitride laser diode structures, where electron confinement is provided by a separate high-aluminum-content AlGaN tunnel barrier layer placed over the MQW active region. The optical confinement factor (/spl Gamma/) remains: comparable to that of the conventional structure, in spite of the QW's displacement from the center of the waveguide. Room-temperature CW operation was achieved with this structure.