Multiple Quantum Well Region Research Articles

On the efficiency droop in InGaN multiple quantum well blue light emitting diodes and its reduction with p-doped quantum well barriers

Multiple quantum well (MQW) InGaN light emitting diodes with and without electron blocking layers, with relatively small and large barriers, with and without p-type doping in the MQW region emitting at ∼420nm were used to determine the genesis of efficiency droop observed at injection levels of approximately ⩾50A∕cm2. Pulsed electroluminescence measurements, to avoid heating effects, revealed that the efficiency peak occurs at ∼900A∕cm2 current density for the Mg-doped barrier, near 550A∕cm2 for the lightly doped n-GaN injection layer, meant to bring the electron injection level closer to that of holes, and below 220A∕cm2 for the undoped InGaN barrier cases. For samples with GaN barriers (larger band discontinuity) or without p-AlGaN electron blocking layers the droop occurred at much lower current densities (⩽110A∕cm2). In contrast, photoluminescence measurements revealed no efficiency droop for optical carrier generation rates corresponding to the maximum current density employed in pulsed injection measurements. All the data are consistent with heavy effective mass of holes, low hole injection efficiency (due to relatively lower p-doping) leading to severe electron leakage being responsible for efficiency droop.

III–Nitride-Based Microarray Light-Emitting Diodes with Enhanced Light Extraction Efficiency

In this paper, the enhancement of light extraction efficiency in III–nitride-based light-emitting diodes (LEDs) with an array of microstructures is demonstrated numerically and experimentally at the near-ultraviolet (n-UV) spectral region. Two different microstructures are adopted to study the mechanism, including the shallow etching of microstructures on the indium–tin-oxide (ITO) p-contact and the deep etching of microholes through the InGaN/GaN multiple-quantum-well (MQW) active structure and down to the n-type GaN layer. Multiple point sources with TE and TM polarizations in the MQW region are utilized in the finite-difference time domain (FDTD) analysis to model the behaviour of LED. The simulated beam patterns of both LEDs reveal that the microstructures act as diffraction gratings to extract guided light in the n-type GaN layer. The measured results of beam patterns also verify the prediction in the numerical analysis.

Growth of periodic micropits InGaN‐based LED structure on wet‐etch patterned sapphire substrate

AbstractGrowth of periodic micropits InGaN‐based light emitting diode (LED) structure on wet‐etch patterned sapphire substrate (WPSS) was introduced. Two‐step growth of GaN on the triangle shape of WPSS promotes the ability to control the density/size of honeycomb shape GaN template and resulting in the threading dislocation density of 8‐9×106 cm‐2. Moreover, this template also was acting as the seeds to conduct the periodic distribution of micropits InGaN/GaN multiple quantum wells (MQWs). Spatially and spectrally resolved luminescence from periodic micropits LED is obtained using near‐field scanning optical microscope (NSOM). The results revealed that the excited carrier has facilitated the confinement at the horizontal MQW region rather than in the inclined facet MQW. Consequently, intensity of the light emitting from the horizontal region is much higher than that of the inclined facets region. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Impurity bound-to-unbound terahertz sensors based on beryllium and silicon δ-doped GaAs∕AlAs multiple quantum wells

Beryllium and silicon δ-doped GaAs∕AlAs multiple quantum wells (MQWs) were designed and fabricated for selective sensing of terahertz radiation. Optical characterization of the structures by photoreflectance spectroscopy has indicated a presence of built-in electric fields–from 18 up to 49kV∕cm depending on the structure design–located mainly in the cap and buffer layers, but do not penetrating into the MQWs region. Terahertz sensing under normal incidence via impurity bound-to-unbound transitions is demonstrated in photocurrent experiments within 0.6–4.2THz in silicon-doped MQWs and 3.5–7.3THz range in beryllium-doped MQWs at low temperatures.

Origin of efficiency droop in GaN-based light-emitting diodes

The efficiency droop in GaInN∕GaN multiple-quantum well (MQW) light-emitting diodes is investigated. Measurements show that the efficiency droop, occurring under high injection conditions, is unrelated to junction temperature. Furthermore, the photoluminescence output as a function of excitation power shows no droop, indicating that the droop is not related to MQW efficiency but rather to the recombination of carriers outside the MQW region. Simulations show that polarization fields in the MQW and electron blocking layer enable the escape of electrons from the MQW region and thus are the physical origin of the droop. It is shown that through the use of proper quaternary AlGaInN compositions, polarization effects are reduced, thereby minimizing droop and improving efficiency.

Influences of Multiquantum Barriers on Carrier Recombination in InGaN/GaN Multiple-Quantum-Well Light-Emitting Diodes

The influences of multiquantum barriers (MQBs) on the carrier confinement and carrier recombination in blue InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) have been investigated in depth over a broad range of temperatures from 20 to 300 K. Time-resolved photoluminescence (TRPL) temporal decay was measured to examine the dynamics of the carriers in both devices. The exciton recombination times of the blue emission are 2.81 and 4.11 ns for the QWs with MQB and GaN barriers, respectively. The former is a reasonable value for the radiative recombination in the structure with MQBs, and results from the enhancement of the exciton confinement. It was found that a device with an MQB structure exhibited higher emission intensity as well as lower temperature sensitivity than the conventional MQW LEDs. The improvement of the quantum efficiency for the MQB device was attributed to the fact that the enhancement of the excitons was confined in the MQW region and inhibited the carrier overflow into the GaN region.

Excitonic ultraviolet lasing in ZnO-based light emitting devices

The authors have fabricated ultraviolet (UV) laser diodes based on ZnO∕BeZnO films. The devices have p-n heterojunction structures with a multiple quantum well (MQW) active layer sandwiched between guide-confinement layers. The MQW active layer comprises undoped ZnO and BeZnO, while the two guide-confinement layers were As-doped p-type ZnO∕BeZnO and Ga-doped n-type BeZnO∕ZnO films, respectively. The exciton binding energy in the MQW region is exceptionally large (263meV). Exciton-related lasing was observed by optically pumping the MQWs. ZnO∕BeZnO-based diodes showed laser action by current injection at room temperature. The lasing mechanism is inelastic exciton-exciton collision.

Optimization of Barrier Structure for Strain-Compensated Multiple-Quantum-Well AlGaInP Laser Diodes

A theoretical analysis of a strain-compensated multiple-quantum-well (MQW) AlGaInP laser diode (LD) has been conducted to minimize the operation current and enhance the operation temperature. The effect of the barrier height on the tensile-strain quantum barriers was studied under the same optical confinement and emission wavelength. The simulation results suggest that a more uniform and higher carrier distribution inside the MQW region can be obtained for a lower barrier height owing to the improvement in carrier injection. The performance of the AlGaInP LD is thus improved. However, when the barrier height is too small, higher spontaneous rates in the quantum barrier region deteriorate the LD performance instead. Theoretical analysis shows that an Al composition of 0.1 for the 0.5%-tensile-strain AlxGayIn1-x-yP barrier is the optimal value for strain-compensated MQW AlGaInP LDs.

Optical property of In 0.2Ga 0.8As/GaAs strained multiple quantum-wells grown by using MOCVD

The photocurrent and the photoluminescence were studied for the In 0.2Ga 0.8As/GaAs strained multiple quantum-wells (MQW) structure grown by using metalorganic chemical vapor deposition. The structural properties of the In 0.2Ga 0.8As/GaAs strained MQW were investigated using high-resolution X-ray diffraction. For In 0.2Ga 0.8As/GaAs strained MQW, the peaks observed in the photocurrent and the photoluminescence spectra were preliminarily assigned to electron–heavy hole (e 1–hh) and electron–light hole (e 1–lh) fundamental excitionic transitions. Two additional transitions related to the MQW region were observed in the photocurrent spectra other than transitions involving the ground state.

Temperature-dependent electroluminescence in InGaN/GaN multiple-quantum-well light-emitting diodes

We present a comparative study on temperature dependence of electroluminescence (EL) of InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with identical structure but different indium contents in the active region. For the ultraviolet (UV) and blue LEDs, the EL intensity decreases dramatically with decreasing temperature after reaching a maximum at 150 K. The peak energy exhibits a large redshift in the range of 20–50 meV with a decrease of temperature from 200 K to 70 K, accompanying the appearance of longitudinal-optical (LO) phonon replicas broadening the low energy side of the EL spectra. This redshift is explained by carrier relaxation into lower energy states, leading to dominant radiative recombination at localized states. In contrast, the peak energy of the green LED exhibits a minimal temperature-induced shift, and the emission intensity increases monotonically with decreasing temperature down to 5 K. We attribute the different temperature dependences of the EL to different degrees of the localization effects in the MQW regions of the LEDs.

MOCVD‐Grown InGaN/GaN MQW LEDs on Si(111)

InGaN/GaN multiple-quantum-well (MQW) light emitting diodes (LEDs) were deposited by metal–organic chemical vapour deposition on Si(111) substrate. In this paper, we present a study on the X-ray diffraction (XRD), electroluminescence (EL), photoluminescence (PL), and current-voltage (I–V) characteristics of the LEDs, fabricated at varied quantum well growth temperature TQW. PL intensity as well as XRD quantum well interface quality show moderate effects of structural defects within the MQW region on the internal quantum efficiency. A comparison between PL and EL intensity hints at severe losses of injected carriers within the low-TQW LEDs, being responsible for drastically decreased EL efficiencies. These could result from imperfection of the p-AlGaN blocking layer on top of the MQW region, as deduced from additional information from the I–V characteristics.

Backilluminated ultraviolet photodetector based on GaN/AlGaN multiple quantum wells

The operation of backilluminated ultraviolet (UV) photodetector based on GaN/Al0.27Ga0.73N multiple quantum wells (MQWs) is reported. The MQW structure was deposited on a 1-μm-thick Al0.35Ga0.65N buffer layer which was epitaxied on a sapphire substrate. Coplanar Ohmic contacts were made on the top side of the MQW structure. By illuminating the Ohmic contact positions from the backside of the detector, a flat and narrow band spectral response is achieved in the UV wavelength range from 297 nm to 352 nm. The electron-heavy hole absorption in the MQW region produces the sharp long-wavelength cutoff at 352 nm and the band-to-band absorption of the Al0.35Ga0.65N buffer layer introduces the sharp short-wavelength cutoff at 297 nm. The polarization-induced electric fields result in a redshift of the long-wavelength cutoff. The response time is measured to be RC limited and determined to be 5 μs at a 50 Ω load.

Interactions between inversion domains and InGaN/GaN multiple quantum wells investigated by transmission electron microscopy

The propagation properties of inversion domains (IDs) in InGaN/GaN multiple quantum well (MQW) structures grown by metalorganic chemical vapor deposition have been investigated by transmission electron microscopy (TEM). The majority of the IDs, originating from the sapphire and/or buffer layer, propagate through the MQWs with normal wurtzite structure retaining their original structural features. Some of IDs could induce V-shaped pits in the MQW structures proposing a new formation mechanism for the so-called V-shaped defects. Detailed measurements show that a few IDs are found to be stopped in abnormal MQW regions, where In droplets appear due to phase separation. We presented direct evidence of pure In-phase droplets by means of high-resolution TEM. The above results provide new information on the structural defects in InGaN/GaN-based materials.

Photoluminescence and Electroluminescence Characterization of In<sub>x</sub>Ga<sub>1-x</sub>N/In<sub>y</sub>Ga<sub>1-y</sub>N Multiple Quantum Well Light Emitting Diodes

We report on a study of radiative recombination in In0.11Ga0.89N/In.0.01Ga0.99N multiple quantum wells (MQWs). The QWs were nominally undoped, while the InGaN barriers were Si doped. The MQW part is situated in the depletion field of a pn-junction structure with electrical contacts, so that both photoluminescence (PL) and electroluminescence (EL) can be studied as a function of bias. The PL and EL spectra are distinctly different, in particular at low temperatures. The spectral properties and related differences in PL decay times reflect different recombination conditions in the MQW region for the individual QWs.

Photoluminescence of Excitons in InxGa1?xN/InyGa1?yN Multiple Quantum Wells

We report on a detailed study of excitons in InxGa1-xN/InyGa1-yN multiple quantum wells (MQWs) with an In composition x in the QWs of about 0.1, and a small In composition y in the barrier of 0.01-0.02. The MOVPE growth procedure was optimized to allow growth without In segregation. The InyGa1-yN barriers had a Si doping of about 5 x 10(18) cm(-3) . The low temperature photoluminescence spectra show two sets of exciton-like spectra with quite different properties. The lower energy emission has a small thermal activation energy (about 5 meV), and thus disappears at elevated temperatures, it is not observed at room temperature. The higher energy exciton state has a decay time of about 5 ns, while the lower energy process is much slower. We have also done preliminary studies on samples where the MQW region is situated in a p-n junction field, with semi-transparent contacts, to study the effects of varying the bias across the MQW structure. The combination of optical data can e interpreted in terms of a substantial potential gradient across the MQW region for both samples. The conclusion is that probably only one QW is emitting at low T (and no bias), and the second lower energy PL peak originates from a shallow notch in the conduction band at the interface between the thick GaN buffer layer and the first Ga(In)N barrier.

Photoluminescence of Excitons in InxGa1—xN/InyGa1—yN Multiple Quantum Wells

We report on a detailed study of excitons in InxGa1—xN/InyGa1—yN multiple quantum wells (MQWs) with an In composition x in the QWs of about 0.1, and a small In composition y in the barrier of 0.01–0.02. The MOVPE growth procedure was optimized to allow growth without In segregation. The InyGa1—yN barriers had a Si doping of about 5 × 1018 cm—3. The low temperature photoluminescence spectra show two sets of exciton-like spectra with quite different properties. The lower energy emission has a small thermal activation energy (about 5 meV), and thus disappears at elevated temperatures, it is not observed at room temperature. The higher energy exciton state has a decay time of about 5 ns, while the lower energy process is much slower. We have also done preliminary studies on samples where the MQW region is situated in a p–n junction field, with semi-transparent contacts, to study the effects of varying the bias across the MQW structure. The combination of optical data can be interpreted in terms of a substantial potential gradient across the MQW region for both samples. The conclusion is that probably only one QW is emitting at low T (and no bias), and the second lower energy PL peak originates from a shallow notch in the conduction band at the interface between the thick GaN buffer layer and the first Ga(In)N barrier.

High-speed, dual-function vertical cavity multiple quantum well modulators and photodetectors for optical interconnects

A vertical cavity multiple quantum well (MQW) modulator, which offers high contrast ratio, low insertion loss, low power consumption, low jitter and chirp, and easy two-dimensional integration with silicon electronics, is one of the essential components for dense optical interconnects. The devices we have investigated consist of quantum wells in a Fabry-Perot cavity configured as p-i-n diode. The absorptive characteristics of the MQW region can be modified through field-induced absorption change known as quantum-confined Stark effect (QCSE), which translate to the modulation in the device reflectivity. Modulators can also perform the dual function of a high-efficiency photodetector at the receiver end by using a different electronic control circuit. This has the advantage of doubling the optical I/O of a photonic layer and reducing power losses. In this paper, the high-speed performance of modulators and photodetectors will be presented. We use 26 pairs of 75-A GaAs/AlGaAs quantum wells. The modulators have a high contrast ratio of 11 dB, small driving voltage of 3.5 V, and f3dB bandwidth greater than 18 GHz. If the device is used as a high-speed photodetector, it has a quantum efficiency of 95% and f3dB bandwidth greater than 10 GHz. Results of devices of different sizes, with different bias voltages and termination resistors, are presented.

Analytical modeling of the resolution of photorefractive multiple quantum well spatial light modulators

An analytical model that includes lateral drift of photocarriers is developed to explain the experimental resolution of photorefractive multiple quantum well (MQW) devices. The model is in excellent agreement with a phenomenological fit proposed earlier and it follows a small intensity modulation model closely. Charge distributions with and without lateral drift of carriers at the MQW interfaces are assumed in order to calculate the peak diffraction efficiency as a function of grating period. An effective mobility-lifetime product is used to account for the effect of lateral drift in the MQW region. It is shown that good agreement with experiment is obtained when lateral drift is taken into account. Moreover, the model is applied to the study of design tradeoff between resolution, sensitivity, and diffraction efficiency which are all of crucial importance in the performance of practical devices.

Thermoexcited and photoexcited carrier transports in a GaAs/AlGaAs quantum well infrared photodetector

A study of the carrier transport performance of our quantum well infrared photodetector (QWIP) is carried out in this work. AlGaAs/GaAs QWIP devices were fabricated and characterized. Carrier transport between adjacent quantum wells are modeled by both the tunneling and thermoexcitation effects. The strengths of the two transport effects are quite small (tunneling is small due to thick barrier and thermoexcitation is small due to the low device working temperature). However the thermoexcitation effect dominates in our QWIP devices and the thermally activated carriers transport across the optical active multiple quantum well (MQW) region in the three-dimensional drift-diffusion manner. At zero bias the energy band structure of our sample is almost at a condition of flat band. Due to the thick multiple quantum well region and the high doping levels in the contacts, the external bias largely applies linearly across the MQW region. Current increases with increasing bias and it becomes saturated when the drift velocity becomes saturated. The calculated dark current density agrees well with our experimental measurement. The photoexcited carrier density is almost independent of the external bias. The photocurrent increases with increasing bias because of the increasing drift velocity. It reaches its maximal when the drift velocity saturates and the current then decreases as the external bias further increases due to current continuity and charge conservation.

Characterization of InGaAsP/InP p-i-n solar cell structures using modulation spectroscopy and secondary ion mass spectrometry

We have investigated two In1−xGaxAsyP1−y/InP p-i-n multiple quantum well (MQW) solar cell structures (with and without gold contacts) with intrinsic (i)-InP spacers and two InP p-i-n control samples (with and without gold contacts) using electroreflectance (ER), piezoreflectance (PZR) and secondary ion mass spectrometry (SIMS). From a comparison of the rich ER/PZR spectra from the MQW regions (including the quantum confined Stark effect produced by an external bias) with an envelope function calculation (strain and electric field), we have been able to completely characterize the composition (x,y), strain and well width of the MQWs. From the electric fields originating in the i-InP region of the two InP p-i-n control samples, as determined from the observed Franz–Keldysh oscillations (FKOs), we have evaluated the amount of p-dopant interdiffusion, in agreement with the SIMS data. The FKOs from the i-InP region of the MQW samples reveal an unusual effect, i.e., a modulation-doping field (superimposed on the p-i-n field) due to a transfer of charge from the n-InP layer to the nearest InGaAsP quantum well.