Multiquantum-well Structures Research Articles

Polarization effect on electronic band structure of InGaN/GaN multi‐quantum wells

AbstractWe have characterized the effect of piezoelectric field in strained InGaN/GaN multi‐quantum wells (MQWs) by using the deep level transient spectroscopy (DLTS). The photoluminescence peak of the quantum wells was observed at 2.612 eV roughly matched with DLTS results. And the carrier profile obtained by the relation of capacitance‐voltage showed carrier accumulation peaks representing the quantum wells. The thermal activation energy calculated from DLTS spectra decreased by about 0.12 eV with increasing the reverse bias field. This energy shift can be explained by the partial compensation of the internal piezoelectric field. In this report, we suggested the DLTS technique as a useful method to characterize the quantum confined Stark effect in InGaN/GaN MQWs structure. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Assessment of various LED structure designs for high‐current operation

AbstractUsing modelling, we have examined alternative designs of light‐emitting diode (LED) structures, aimed at lowering the non‐equilibrium carrier concentration and, hence, at suppression of Auger recombination in the active regions of the diodes. Both a multi‐quantum‐well (MQW) heterostructure and that with a thick single active layer are compared. The MQW structure is found to be ineffective for improvement of the LED internal quantum efficiency (IQE). In contrast, LEDs with a thick single active layer turn out to be quite promising for high‐current operation. Detailed simulation provides a guideline for further optimization of the LED heterostructures. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Gain characteristics and lasing of Ga(NAsP) multi‐quantum well structures

AbstractLasing is demonstrated for a Ga(NAsP) multi quantumwell structure grown pseudomorphically on GaP using metal‐organic vapour‐phase epitaxy. For a series of temperatures ranging from 8 K to 290 K, the sample emission characteristics are determined. With increasing pump power, a spectral narrowing of the emitted light along with a blue‐shift is observed as the lasing threshold is approached.Above the threshold, the emission splits up spectrally into a pattern of clearly distinguishable modes. In addition, the potential of this material concerning roomtemperature lasing is investigated by means of a variable stripe‐length experiment, where a modal gain of 10 cm–1 is observed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Reduction in efficiency droop, forward voltage, ideality factor, and wavelength shift in polarization-matched GaInN/GaInN multi-quantum-well light-emitting diodes

Blue light-emitting diodes (LEDs) with polarization-matched GaInN/GaInN multi-quantum-well (MQW) active regions are grown by metal-organic vapor-phase epitaxy. The GaInN/GaInN MQW structure reduces the magnitude of polarization sheet charges at heterointerfaces in the active region. The GaInN/GaInN MQW LEDs are shown to have enhanced light-output power, reduced efficiency droop, a lower forward voltage, a smaller diode ideality factor, and decreased wavelength shift, compared with conventional GaInN/GaN MQW LEDs.

MOVPE growth of InGaAs/GaAsP-MQWs for high-power laser diodes studied by reflectance anisotropy spectroscopy

We have intensively studied the growth and strain behaviour of In x Ga 1− x As multi-quantum wells (MQWs) for application in high-power laser diodes. Their growth in metal-organic vapour-phase epitaxy (MOVPE) was monitored by reflectance anisotropy spectroscopy (RAS). The influence of indium content variation in the InGaAs quantum wells and strain compensation using GaAsP layers on the reflectance anisotropy for single and multi-quantum wells has been studied. A clear dependence of the RAS signal on the indium and phosphorus content has been found. Furthermore, the beginning of strain relaxation during growth can be monitored by a difference in the RAS signals of the GaAs layers below and above the MQW structure. With optimised growth conditions complete laser structures have been grown. Laser diodes with very small far-field angles and different numbers of quantum wells were processed into broad-area lasers, which show output powers up to 60 W in pulse mode.

GaN/AlN short-period superlattices for intersubband optoelectronics: A systematic study of their epitaxial growth, design, and performance

We have studied the effect of growth and design parameters on the performance of Si-doped GaN/AlN multiquantum-well (MQW) structures for intersubband optoelectronics in the near infrared. The samples under study display infrared absorption in the 1.3–1.9 μm wavelength range, originating from the photoexcitation of electrons from the first to the second electronic level in the QWs. A commonly observed feature is the presence of multiple peaks in both intersubband absorption and interband emission spectra, which are attributed to monolayer thickness fluctuations in the quantum wells. These thickness fluctuations are induced by dislocations and eventually by cracks or metal accumulation during growth. The best optical performance is attained in samples synthesized with a moderate Ga excess during the growth of both the GaN QWs and the AlN barriers without growth interruptions. The optical properties are degraded at high growth temperatures (>720 °C) due to the thermal activation of the AlN etching of GaN. From the point of view of strain, GaN/AlN MQWs evolve rapidly to an equilibrium average lattice parameter, which is independent of the substrate. As a result, we do not observe any significant effect of the underlayers on the optical performance of the MQW structure. The average lattice parameter is different from the expected value from elastic energy minimization, which points out the presence of periodic misfit dislocations in the structure. The structural quality of the samples is independent of Si doping up to 1020 cm−3. By contrast, the intersubband absorption spectrum broadens and blueshifts with doping as a result of electron-electron interactions. This behavior is independent of the Si doping location in the structure, either in the QWs or in the barriers. It is found that the magnitude of the intersubband absorption is not directly determined by the Si concentration in the wells. Instead, depending on the Al mole fraction of the cap layer, the internal electric field due to piezoelectric and spontaneous polarization can deplete or induce charge accumulation in the QWs. In fact, this polarization-induced doping can result in a significant and even dominant contribution to the infrared absorption in GaN/AlN MQW structures.

Structural investigation of GaAs1−xBix/GaAs multiquantum wells

GaAs 1 − x Bi x / GaAs multiquantum wells (MQWs) have been grown in the layer-by-layer mode of molecular beam epitaxy. A well-defined multilayered structure of the MQWs has been confirmed by cross-sectional transmission microscopy and high-resolution x-ray diffraction measurements. Photoluminescence has been observed from GaAs1−xBix/GaAs MQW at room temperature. The MQW structures have been confirmed to be thermally stable even after annealing up to 800 °C, although they need to be grown at a low temperature (350–400 °C) for Bi incorporation.

Influence of patterned TiO2/SiO2 dielectric multilayers for back and front mirror facets on GaN-based laser diodes

Ridge InGaN multi-quantum-well-structure (MQW) edge-emitting laser diodes (LDs) were grown on (0001) sapphire substrates by low-pressure metal-organic chemical vapour deposition (MOCVD). The dielectric TiO2/SiO2 front and back facet coatings as cavity mirror facets of the LDs have been deposited with electron-beam evaporation method. The reflectivity of the designed front coating is about 50% and that of the back high reflective coating is as high as 99.9%. Under pulsed current injection at room temperature, the influences of the dielectric facets were discussed. The threshold current of the ridge GaN-based LDs was decreased after the deposition of the back high reflective dielectric mirrors and decreased again after the front facets were deposited. Above the threshold, the slope efficiency of the LDs with both reflective facets was larger than those with only back facets and without any reflective facets. It is important to design the reflectivity of the front facets for improving the performance of GaN-based LDs.

Polarization-Insensitive GaInNAs–GaInAs MQW-SOA With Low Noise Figure and Small Gain Tilt Over 90-nm Bandwidth (1510–1600 nm)

rdquoWe developed a broadband polarization-insensitive multiquantum-well (MQW) semiconductor optical amplifier (SOA) for application as an optical switch in broadcast-and-select-type optical packet switching systems. We adopted a GalnNAs-GalnAs MQW structure active layer to decrease the noise figure (NF) around the gain peak wavelength together with a small gain tilt in the C-band. The device exhibited a smaller NF on the shorter wavelength side of the gain peak compared with a GalnNAs strained-bulk SOA. The reduced NF resulted in a large effective gain bandwidth of up to 90 nm (1510-1600 nm). The device also exhibited the small gain tilt (<1.2 dB) and small polarization-dependent gain (<0.8 dB) in the C-band.

Growth of GaAs1–xBix/GaAs multi‐quantum wells by molecular beam epitaxy

AbstractGaAs1–xBix/GaAs multi‐quantum well (MQW) structures were grown for the first time by molecular beam epitaxy (MBE). Successful growth of MQWs was demonstrated by observing clear satellite peaks in high‐resolution X‐ray diffraction (HR‐XRD) measurements. Photoluminescence (PL) emission at room temperature (RT) was observed from the MQWs. The peak energy of RT PL emitted from GaAs0.948Bi0.052/GaAs MQWs increased with decreasing thickness of the well layer, which implies quantum size effect. The MQW structures were thermally stable even after annealing up to 800 °C, and the PL emission was observed even after annealing up to 600 °C. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Self‐sustained current oscillations in a multi‐quantum‐well spin polarized structure with normal contacts

AbstractSelf‐sustained current oscillations (SSCO) are found in a nonlinear electron spin dynamics model of a n‐doped dc voltage biased semiconductor II–VI multi‐quantum well structure (MQWS) having one or more of its wells doped with Mn. Provided one well is doped with magnetic impurities, spin polarized current can be obtained even if normal contacts have been attached to this nanostructure. Under certain conditions, the system exhibits static electric field domains and stationary current or moving domains and time‐dependent oscillatory current. We have found SSCO for nanostructures with four or more QWs. The presence of SSCO depends on the spin‐splitting induced by both, the exchange interaction and the external magnetic field. We also calculate the minimal doping density needed to have SSCO, and a bound above which SSCO disappear. This range is crucial to design a device behaving as a spin polarized current oscillator. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The effect of growth temperature on structural quality of GaN/AlN quantum wells by metal-organic chemical vapour deposition

The effect of the growth temperature on the surface and interface quality for the GaN/AlN multiquantum well (MQW) layer grown by metal-organic vapour chemical deposition is investigated. The obtained GaN/AlN MQW structure is almost coherent to the underlying AlGaN layer at improved growth conditions. With a relatively low growth temperature, the GaN/AlN MQW growth rate increases, the surface roughness reduces considerably and no macro steps are observed, resulting in a better periodicity of MQW.

High Optical Response in Forward Biased (In,Ga)N–GaN Multiquantum-Well Diodes Under Barrier Illumination

The authors report on the current-voltage (I-V) characteristic under forward biases obtained in low leakage, small size p-(In,Ga)N-GaN-n multiquantum well diodes. Under barrier illumination, the devices present a high optical response with capabilities to detect optical powers in the pW range without further amplification. This response is attributed to the screening of the internal electric fields. Recombination times of a few seconds are found to be associated to this mechanism. Moreover, a step-like feature is found in the I-V characteristic before the diode turn-on voltage. Our model proposes tunneling current through the multiquantum-well structure as responsible of this feature. Fast modulation of the tunneling effect under barrier illumination is used to evaluate the detection of low photon fluxes.

Metamorphic growth of InAlAs/InGaAs MQW and InAs HEMT structures on GaAs

The large electron mobility at room temperature and the absence of Schottky barrier to metals make InAs two-dimensional electron gas (2DEG) a good candidate for SPIN-FET (FET—field-effect-transistor) applications. So far the growth was done either on the InAlAs epilayers compositionally matched to InP substrates, or on Sb-based compound semiconductors. Here we aim to grow InAs 2DEG on GaAs substrates by using a strain-relaxing buffer layer. We introduce In 0.4Al 0.6As glue layer to the metamorphic structure and investigate the physical properties of InAlAs/InGaAs multi-quantum-well (MQW) structures via X-ray diffraction, transmission electron microscopy, and photoluminescence. We find that the use of As dimer instead of tetramer and the choice of proper growth temperature are essential for successful growth. InAs-inserted-channel InAlAs/InGaAs inverted high-electron-mobility transistor (HEMT) structures show promising results for SPIN-FET application.

Improved Internal Quantum Efficiency of Green Emitting InGaN/GaN Multiple Quantum Wells by In Preflow for InGaN Well Growth

A series of green emitting In1-xGaxN/GaN multi-quantum wells (MQWs) has been grown by using low pressure metal organic chemical vapor deposition. Effects of In flow prior to InGaN well growth for a short duration on structural and optical properties of the MQWs have been studied. In pre-flow does not affect the structural properties such as In composition and interface abruptness of the InGaN/GaN MQW structure. But, it red shifts the photoluminescence (PL) emission peak of MQWs with increase of In pre-flow duration. For specific emission energy, the surface pit density of MQWs grown with In pre-flow is significantly lower than that of MQWs grown without In pre-flow. In pre-flow samples show about 30% enhancement of PL internal quantum efficiency in deep green region (>530 nm) compared to MQWs grown without In pre-flow.

High resolution measurement of the thermal expansion coefficient of semiconductor multilayer lateral nanostructures

We measured the thermal expansion coefficient (TEC) of a vertically stacked multi-quantum-well (MQW) structure buried under a thick GaAs top layer before and after lateral patterning of the GaAs top layer. After patterning the TEC of the whole MQW structure differs from that of the planar structure by about 20%. Based on calculations in terms of methods of finite elements the effect is explained by the influence of the strain field originating from the bottom edges of the etched nanostructure. Due to the long range nature of this strain field the strain release within the individual quantum wells changes as a function from the distance from the valley. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence Properties Affected by Carrier Transport between X and Different First Excited States Originating from Interface Imperfection in GaAs/AlAs Multi-Quantum Wells

The authors investigated the electric-field dependence of photoluminescence (PL) spectra in a GaAs/AlAs multi-quantum well (MQW) structure, in which QWs with different thicknesses coexist. PL properties in this sample are chiefly determined by the coexistence of 21-monolayer (ML) and 23-ML QWs, because the islands of 22-ML QWs are too small to have an effect on radiative processes. Two PL signals are found in the shorter-wavelength region, caused by X1–Γ2 resonances, because the different thicknesses of GaAs QWs lead to different resonant voltages and wavelengths. We also estimated the carrier densities of excited states from the observed two PL intensities and by calculations of overlap integrals of wave functions between electron and heavy-hole states. This result clearly indicates that the carrier density in the Γ2 in the 21-ML QW becomes larger than that in the 23-ML QW when the X1–Γ2 resonance of the 21-ML QW occurs. This demonstrates that interface imperfection creates novel carrier transport paths and affects carrier densities as well as optical properties in excited states in MQW structures.

Characterization of GaSb/AlGaSb Multi-Quantum-Well Structures Grown on Si(001) Substrates

We prepared GaSb/AlGaSb multi-quantum-well (MQW) structures on n-type silicon (001) substrates by molecular beam epitaxy (MBE). To minimize dislocations in MQW layers and decrease the total thickness of the epitaxial layer, we employed not only an AlSb initiation layer but also a superlattice buffer layer (SL-BL) in a moderately thick GaSb buffer layer. For comparison we also fabricated other MQW structures on a considerably thick GaSb buffer layer without SL-BL. The obtained atomic force microscopy (AFM), transmission electron microscopy (TEM) images and high-resolution X-ray diffraction (XRD) patterns indicated that the definite MQW structures for both the samples and the quality of MQW layers prepared using SL-BL were generally better than those of the reference sample. The PL emission of these samples at about 1.30–1.55 µm was observed at room and low temperatures. The dependence of PL emission energy on GaSb well width was well explained by the finite square well potential model.

增加微腔激光器的自发辐射因子

The microcavity and the influence of nonradiative recombination can control spontaneous emission. An analytic resolution of rate equation is studied for microcavity lasers. The relationship between output properties and structural parameters of multi-quantum wells (MQWs) is obtained. One of the most important consequences of the increased spontaneous emission factor is the reduction of laser threshold. It is found that the characteristic curve of a thresholdless laser is strongly nonradiative depopulation-dependent. The light output is increased by the enhanced well number and the reduced width. In particular, there is an optimal well number corresponding to the lowest threshold current density for MQW structure in the microcavity lasers.

The effect of coupling barrier thickness on structural and optical properties in asymmetrically coupled GaN/Al0.5Ga0.5N/GaN multiquantum wells

The structural and optical properties of coupled, multiquantum-well structures, consisting of GaN(10 Å)/Al0.5Ga0.5N(12 or22 Å)/GaN(20 Å) bounded by Al0.5Ga0.5N (100 Å) barriers, were investigated by high resolution x-ray diffraction, transmission electron microscopy, and photoluminescence measurements. We studied the carrier dynamics in asymmetric double quantum wells by analyzing the temperature-dependent and time-resolved photoluminescence spectra. Carrier tunneling between the coupled quantum wells affected the optical properties. In our analysis of the effect of the carrier tunneling in terms of free-carrier screening, the effect was much weaker for the sample with Tib=12 Å than for that with Tib=22 Å.