Strain-compensated Superlattice Research Articles

Large tunable bandgaps in the InAs/AlAs strain-compensated short-period superlattices grown by molecular beam epitaxy

To explore the bandgap tunability in strain-compensated superlattices, we have grown a series of InAs/AlAs strain-compensated short-period superlattices (SPSs) with different period lengths by molecular beam epitaxy. Although the average indium composition of the InAs/AlAs SPS samples remains the same, the bandgaps of these SPSs measured by photoluminescence show a strong dependence on the period length, ranging from 1.41 to 1.01 eV as the period length varies from 4 ML to 10 ML. This fine control on the period length can extend the cutoff wavelength of this digital-alloy-like InAlAs (lattice matched to InP) material up to 1230 nm at room temperature. Multiple transitions are observed in Fourier transform infrared spectra, which agree well with the calculation and confirm the confinement in this structure. The strain effect in tuning the band structures and the band alignments is demonstrated, showing that longer period length together with smaller conduction band offset has led to the smaller effective bandgap of the InAs/AlAs SPS.

A Study of Spin-Polarized Electron Source with Strain-Compensated Superlattice

A Study of Spin-Polarized Electron Source with Strain-Compensated Superlattice

Material and device characterization of Type-II InAs/GaSb superlattice infrared detectors

This work investigates midwave infrared Type-II InAs/GaSb superlattice (SL) grown by molecular beam epitaxy on GaSb substrate. In order to compensate the natural tensile strain of the InAs layers, two different shutter sequences have been explored during the growth. The first one consists of growing an intentional InSb layer at both interfaces (namely GaSb-on-InAs and InAs-on-GaSb interfaces) by migration enhanced epitaxy while the second uses the antimony-for-arsenic exchange to promote an ‘InSb-like’ interface at the GaSb-on-InAs interface. SLs obtained via both methods are compared in terms of structural, morphological and optical properties by means of high-resolution x-ray diffraction, atomic force microscopy and photoluminescence spectroscopy. By using the second method, we obtained a nearly strain-compensated SL on GaSb with a full width at half maximum of 56 arcsec for the first-order SL satellite peak. Relatively smooth surface has been achieved with a root mean square value of about 0.19 nm on a 2 µm × 2 µm scan area. Finally, a p-i-n device structure having a cut-off wavelength of 5.15 µm at 77 K has been demonstrated with a dark-current level of 8.3 × 10−8 A/cm2 at −50 mV and a residual carrier concentration of 9.7 × 1014 cm−3, comparable to the state-of-the-art.

Strain-Compensated InGaAsP Superlattices for Defect Reduction of InP Grown on Exact-Oriented (001) Patterned Si Substrates by Metal Organic Chemical Vapor Deposition.

We report on the use of InGaAsP strain-compensated superlattices (SC-SLs) as a technique to reduce the defect density of Indium Phosphide (InP) grown on silicon (InP-on-Si) by Metal Organic Chemical Vapor Deposition (MOCVD). Initially, a 2 μm thick gallium arsenide (GaAs) layer was grown with very high uniformity on exact oriented (001) 300 mm Si wafers; which had been patterned in 90 nm V-grooved trenches separated by silicon dioxide (SiO2) stripes and oriented along the [110] direction. Undercut at the Si/SiO2 interface was used to reduce the propagation of defects into the III–V layers. Following wafer dicing; 2.6 μm of indium phosphide (InP) was grown on such GaAs-on-Si templates. InGaAsP SC-SLs and thermal annealing were used to achieve a high-quality and smooth InP pseudo-substrate with a reduced defect density. Both the GaAs-on-Si and the subsequently grown InP layers were characterized using a variety of techniques including X-ray diffraction (XRD); atomic force microscopy (AFM); transmission electron microscopy (TEM); and electron channeling contrast imaging (ECCI); which indicate high-quality of the epitaxial films. The threading dislocation density and RMS surface roughness of the final InP layer were 5 × 108/cm2 and 1.2 nm; respectively and 7.8 × 107/cm2 and 10.8 nm for the GaAs-on-Si layer.

Prolonged spin relaxation time in Zn-doped GaAs/GaAsP strain-compensated superlattice

A GaAs/GaAsP strain-compensated superlattice (SL) is a highly promising spin-polarized electron source. To realize higher quantum efficiency, it is necessary to consider spin relaxation mechanisms. We have investigated the electron spin relaxation time in a Zn-doped GaAs/GaAsP strain-compensated SL by time-resolved spin-dependent pump and probe reflection measurements. The long spin relaxation time of 104 ps was observed at room temperature (RT), which is about three times longer than that of conventional undoped GaAs multiple quantum wells. Even when the excitation power increases from 30 to 110 mW, the change in the spin relaxation time at RT was small. This relationship implies that the intensity of the electron beam can be increased without affecting the spin relaxation time. These results indicate that a Zn-doped GaAs/GaAsP strain-compensated SL has the great advantage for use as a spin-polarized electron source.

Analysis of quantum efficiency improvement in spin-polarized photocathode

GaAs/GaAsP strain-compensated superlattices (SLs) were developed for spin-polarized photocathode applications. High crystal quality was maintained with SL thicknesses up to 720 nm (90-pairs); however, the quantum efficiency (QE) did not increase linearly with the SL thickness but became saturated starting from an SL thickness of 192 nm (24-pairs). Time-resolved photoluminescence measurements revealed that the carrier lifetime in the GaAs/GaAsP strain-compensated SL was as short as 20.5 ps at room temperature, which causes the elimination of photoexcited electrons before emission. A simulation based on a diffusion model was implemented to quantitatively evaluate the effect of the carrier lifetime on the QE. The simulation results were in good agreement with the experimental results and demonstrate that a carrier lifetime of over 120 ps is required for a two-fold improvement of the QE.

GaNAs/InGaAs Superlattice Solar Cells with High N Content in the Barrier Grown by All Solid-State Molecular Beam Epitaxy**Supported by the National Natural Science Foundation of China under Grant No 61274134, the University of Science and Technology Beijing Talents Start-up Program under Grant No 06105033, and the International Cooperation Projects of Suzhou City under Grant No SH201215.

We demonstrate nearly 1 eV GaN0.03As0.97/In0.09Ga0.91As strain-compensated short-period superlattice solar cells by all solid-state molecular beam epitaxy. The optimal period thickness for the superlattice growth is achieved to realize high structural quality. Meanwhile, the annealing conditions are optimized to realize a photoluminescence (PL) at a low temperature. However, no PL signal is detected at room temperature, which could be reflected by a lower open-circuit voltage of the fabricated devices. The GaN0.03As0.97/In0.09Ga0.91As superlattice solar cells show a reasonably-high short-circuit current density (Jsc) of over 10 mA/cm2. Furthermore, a concentration behavior is measured, which shows a linear relationship between Jsc and concentration ratios. The extrapolated ideality factor and saturated current density by the concentration action are in good agreement with that extracted by the dark case of the p-i-n diodes.

Effect of crystal quality on performance of spin-polarized photocathode

GaAs/GaAsP strain-compensated superlattices (SLs) with thickness up to 90-pair were fabricated. Transmission electron microscopy revealed the SLs are of high crystal quality and the introduced strain in SLs layers are fixed in the whole SL layers. With increasing SL pair number, the strain-compensated SLs show a less depolarization than the conventional strained SLs. In spite of the high crystal quality, the strain-compensated SLs also remain slightly depolarized with increasing SL pairs and the decrease in spin-polarization contributes to the spin relaxation time. 24-pair of GaAs/GaAsP strain-compensated SL demonstrates a maximum spin-polarization of 92% with a high quantum efficiency of 1.6%.

Photoluminescence studies of In0.7Al0.3As/In0.75Ga0.25As/In0.7Al0.3As metamorphic heterostructures on GaAs substrates

The influence of the design of the metamorphic buffer of In0.7Al0.3As/In0.75Ga0.25As metamorphic nanoheterostructures for high-electron-mobility transistors (HEMTs) on their electrical parameters and photoluminescence properties is studied experimentally. The heterostructures are grown by molecular-beam epitaxy on GaAs (100) substrates with linear or step-graded InxAl1 − xAs metamorphic buffers. For the samples with a linear metamorphic buffer, strain-compensated superlattices or inverse steps are incorporated into the buffer. At photon energies ħω in the range 0.6–0.8 eV, the photoluminescence spectra of all of the samples are identical and correspond to transitions from the first and second electron subbands to the heavy-hole band in the In0.75Ga0.25As/In0.7Al0.3As quantum well. It is found that the full width at half-maximum of the corresponding peak is proportional to the two-dimensional electron concentration and the luminescence intensity increases with increasing Hall mobility in the heterostructures. At photon energies ħω in the range 0.8–1.3 eV corresponding to the recombination of charge carriers in the InAlAs barrier region, some features are observed in the photoluminescence spectra. These features are due to the difference between the indium profiles in the smoothing and lower barrier layers of the samples. In turn, the difference arises from the different designs of the metamorphic buffer.

Optimized interfacial management for InGaAs/GaAsP strain-compensated superlattice structure

In this paper, the interfacial management for InGaAs/GaAsP strain-compensated superlattice (SL) structures with thin wells and barriers and a large number of hetero-interfaces has been investigated. By means of X-ray diffraction (XRD) measurement and high-accuracy in-situ curvature measurement, we found that the hetero-interfaces with substantial lattice mismatch tend to generate interfacial defects, which can be mitigated by the insertion of ultrathin GaAs interlayers. However, an inadequate gas-switching sequence induces unintended atomic content near the hetero-interfaces and in the GaAs insertion layers, which influences the average strain of the structure. It was found effective to extend the stabilization time for the arsenic and phosphorus mixture before GaAsP barrier growth to 3s and to insert an 1s hydrogen purge after InGaAs well growth. Static photoluminescence (PL) and time-resolved photoluminescence (TRPL) results were also used to evaluate the crystal quality of grown structures.

InGaAs/AlInAs strain-compensated Superlattices grown on metamorphic buffer layers for low-strain, 3.6 μm-emitting quantum-cascade-laser active regions

Short-wavelength (λ∼3.6μm) quantum-cascade-laser (QCL) designs, employing a metamorphic buffer layer (MBL) on a GaAs substrate, have been developed for strong carrier confinement to the active regions, as a result of implementing the deep well and tapered active-region concepts. The strain·thickness product values for the quantum wells and barriers comprising the QCL active regions (ARs) are kept basically the same as those employed for longer wavelength (λ∼4.8μm) QCL AR structures grown on InP substrates. Strain-compensated superlattice (SL) structures, representative of the QCL AR, are grown by metalorganic vapor phase epitaxy (MOVPE) on an AlInGaAs compositionally step-graded MBL. Structural characterization of the SL structures underscores the importance of reducing the top-surface roughness of the underlying MBL. Intersubband absorption has been observed for doped SL structures grown on hydride-VPE-grown MBLs.

Effects of strain-compensated AlGaN/InGaN superlattice barriers on the optical properties of InGaN light-emitting diodes

In this article, metalorganic chemical vapor deposition (MOCVD)-grown InGaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with Al0.03Ga0.97N and Al0.03Ga0.97N/In0.01Ga0.99N superlattices-barrier layers on c-plane sapphire were studied for the influence of the strain-compensated barrier on the optical properties of the LEDs. High-resolution X-ray diffraction (HRXRD) analysis shows that the LEDs with a strain-compensated superlattice barrier (SC-SLB) have better interface quality than those using AlGaN. This difference in quality may result from the alleviation of strain relaxation in superlattice layers to improve the crystalline perfection of the epitaxial structures. It was also found that the degree of the exciton localization effect rises considerably as InGaN grows directly on the AlGaN barrier layers. However, the increase in the strength of the polarization fields within the MQWs (as evaluated from bias-dependent photoluminescence (PL) measurement) could reduce the radiative efficiency of the LEDs and shift their PL peaks toward long wavelengths. With suitable control of crystalline quality and the reduced quantum-confined Stark effect in the MQWs, the SC-SLB LEDs operating at 150-mA-current show a 22.3% increase in light output power as compared to their conventional counterparts.

MBE growth and characterization of 5-μm quantum-cascade lasers

Quantum-cascade lasers, which are obtained by molecular-beam epitaxy, emit at wavelengths of 5.0 μm at 77 K and 5.2 μm at 300 K and are based on a design with four quantum wells in the active region with vertical transitions and strain-compensated superlattices with high-efficiency injection and a short lifetime of the ground state are fabricated. The typical thresholds for lasing at 300 K were in the range 4–10 kA/cm2. The maximum emission power was as high as ∼1 W, the maximum lasing temperature was ∼450 K, and the maximum characteristic temperature T 0 ≈ 200 K. The use of a modified process of postgrowth treatment made it possible to reproducibly obtain high-quality devices.

MBE growth of 5 μm quantum cascade lasers

We have demonstrated quantum cascade lasers (QCL) emitting at ∼5μm based on efficient injection of the four quantum wells active region design with vertical transitions and strain-compensated superlattice with high injection efficiency and short ground state lifetime. Using a processing sequence adjusted for QCL state of the art devices were reproducibly obtained.

Enhanced Luminescence of InGaN Light Emitting Diodes with Strain-Compensated Superlattice Barriers

In this paper, we propose to use the strain-compensated superlattices as the barrier layers within InGaN light emitting diodes (LEDs). High resolution X-ray diffraction measurements indicate good control of the interface quality for the LEDs incorporated with a strain-compensated superlattice barrier (SC-SLB). Furthermore, the thermal activation energy of the LEDs with an SC-SLB in the temperature-dependent photoluminescence (PL) measurements is estimated as about , which is comparable to that of the normal LEDs. This result implies that the amount of crystalline defects, such as threading dislocation, is not significant enough to cause the deterioration of device characteristics. Because of the decrease in the strength of the piezoelectric fields within the multiple quantum wells (MQWs), the LEDs with an SC-SLB exhibit a blueshift and an increased PL intensity as compared to the normal LEDs. The lower quantum-confined Stark effect in the MQWs means that the light output power of the modified LEDs is superior to that of the normal LEDs over the entire operating range. The SC-SLB-containing LEDs also have an improved efficiency drop because the SC-SLB could provide a higher barrier height to suppress the carrier leakage.

Characteristics of mid-IR-emitting deep-well quantum cascade lasers grown by MOCVD

Varying composition, deep-well quantum cascade (QC) laser structures are grown using metal-organic chemical vapor deposition (MOCVD). The compositions of the quantum wells and barriers in the active regions differ from those in the extractor/injector regions. Since the highly strained layers (i.e., In 0.68Ga 0.32As and Al 0.75In 0.25As) are not grown throughout the structure, the net cumulative strain can be lower than that in conventional QC lasers. Those layers’ thicknesses and compositions were calibrated using X-ray diffraction (XRD) spectra of strain-compensated superlattice (SL) structures. The XRD spectra from 30-stage, deep-well QC structures confirm that extremely accurate layer thicknesses and compositions can be achieved via MOCVD. Fabricated ridge-guide devices, lasing at ∼4.8 μm, exhibit ultra-low temperature sensitivity of their electro-optical characteristics by comparison to those of conventional QC lasers emitting in the 4.5–5.5 μm wavelength range.

Emission characteristics improvement in structures with InAs/GaAsN/InGaAsN superlattices

The influence of some parameters of nitrogen-containing heterostructures InAs/GaAsN/InGaAsN with strain-compensated superlattices (SCSL) on their emission characteristics has been studied. It is established that the net strain in the structure affects the photoluminescence (PL) linewidth, internal quantum efficiency, intensity, and wavelength. The maximum PL intensity and minimum full width at half maximum (FWHM) of the PL line were achieved with small strains (0–0.2%), whereas the maximum wavelengths (∼1.76 μm) observed for large strain (about +1%). By adding multilayer InAs inserts in the active InGaAsN quantum well in combination with using strain-compensated GaAsN/InGaAsN superlattices, it is possible to control the room-temperature emission wavelength in the range of 1.45–1.76 μm without significantly deteriorating the emissiion characteristics.

Growth of InGaAs/GaNAs strain-compensated quantum dot superlattice on GaAs (3 1 1)B by molecular beam epitaxy

We have studied the structural and optical properties of 10 stacked layers of self-organized In 0.4Ga 0.6As quantum dots (QDs) grown on GaAs (3 1 1)B substrates by atomic hydrogen-assisted radio frequency (RF)-molecular beam epitaxy. A 40 nm-thick GaN 0.007As 0.993 dilute nitride, which is used to cover each QD layer acts as a strain-compensation layer (SCL). The density of strain-compensated In 0.4Ga 0.6As QDs on GaAs (3 1 1)B can be controlled between 2×10 10 and 1×10 11 cm −2 by simply changing the growth temperature. Closely spaced In 0.4Ga 0.6As QDs on GaAs (3 1 1)B shows an ordered structure, in which we observe clear peaks in the two-dimensional fast Fourier transformation image. The temperature dependence of photoluminescence (PL) spectra shows a narrower linewidth over the whole temperature range 30−300 K for strain-compensated QDs owing to better uniformity in the QD size.

Methods of controlling the emission wavelength in InAs/GaAsN/InGaAsN heterostructures on GaAs substrates

Studies of the properties of InGaAsN compounds and methods of controlling the emission wavelength in InAs/GaAsN/InGaAsN heterostructures grown by molecular beam epitaxy on GaAs substrates are reviewed. The results for different types of heterostructures with quantum-size InGaAsN layers are presented. Among those are (1) traditional InGaAsN quantum wells in a GaAs matrix, (2) InAs quantum dots embedded in an (In)GaAsN layer, and (3) strain-compensated superlattices InAs/GaAsN/InGaAsN with quantum wells and quantum dots. The methods used in the study allow controllable variations in the emission wavelength over the telecommunication range from 1.3 to 1.76 μm at room temperature.

Interface analysis of InAs/GaSb superlattice grown by MBE

We report on the structural characterization of short period type-II InAs/GaSb superlattices (SLs) adapted for mid-infrared detection. These structures, grown by molecular beam epitaxy (MBE) on n-type GaSb substrates, are made up of 10 InAs monolayers (MLs) and 10 GaSb MLs and a strain balanced condition is obtained by inserting an InSb ML at the interface between InAs and GaSb in each superlattice's period. From cross-sectional transmission electron microscopy (TEM) measurements, the interface structure is analysed in detail. Very homogeneous and smooth InAs and GaSb layers all over a 50 periods SL structure are observed and high-resolution images bring out the InSb ML inserted between InAs and GaSb. Room temperature absorbance spectroscopy measurements have been performed on strain-compensated SLs including 50, 100, 300 and 400 periods, for a total absorption zone thickness of 0.32, 0.64, 1.92 and 2.56 μm, respectively. The spectra display reproducible and well-defined features with an absorption coefficient varying between 2×10 3 and 4×10 3 cm −1 in the 3–5 μm mid-infrared domain, a sign of high-quality samples suitable for detection.