Short-period Superlattice Structure Research Articles

Low-Temperature Migration-Enhanced Epitaxial Growth of High-Quality (InAs)4(GaAs)3/Be-Doped InAlAs Quantum Wells for THz Applications

This investigation explores the structural and electronic properties of low-temperature-grown (InAs)4(GaAs)3/Be-doped InAlAs and InGaAs/Be-doped InAlAs multiple quantum wells (MQWs), utilizing migration-enhanced epitaxy (MEE) and conventional molecular beam epitaxy (MBE) growth mode. Through comprehensive characterization methods including transmission electron microscopy (TEM), Raman spectroscopy, atomic force microscopy (AFM), pump–probe transient reflectivity, and Hall effect measurements, the study reveals significant distinctions between the two types of MQWs. The (InAs)4(GaAs)3/Be-doped InAlAs MQWs grown via the MEE mode exhibit enhanced periodicity and interface quality over the InGaAs/Be-InAlAs MQWs grown through the conventional molecule beam epitaxy (MBE) mode, as evidenced by TEM. The AFM results indicate lower surface roughness for the (InAs)4(GaAs)3/Be-doped InAlAs MQWs by using the MEE mode. Raman spectroscopy reveals weaker disorder-activated modes in the (InAs)4(GaAs)3/Be-doped InAlAs MQWs by using the MEE mode. This originates from utilizing the (InAs)4(GaAs)3 short period superlattices rather than InGaAs, which suppresses the arbitrary distribution of Ga and In atoms during the InGaAs growth. Furthermore, pump–probe transient reflectivity measurements show shorter carrier lifetimes in the (InAs)4(GaAs)3/Be-doped InAlAs MQWs, attributed to a higher density of antisite defects. It is noteworthy that room temperature Hall measurements imply that the mobility of (InAs)4(GaAs)3/Be-doped InAlAs MQWs grown at a low temperature of 250 °C via the MEE mode is superior to that of InGaAs/Be-doped InAlAs MQWs grown in the conventional MBE growth mode, reaching 2230 cm2/V.s. The reason for the higher mobility of (InAs)4(GaAs)3/Be-doped InAlAs MQWs is that this short-period superlattice structure can effectively suppress alloy scattering caused by the arbitrary distribution of In and Ga atoms during the growth process of the InGaAs ternary alloy. These results exhibit the promise of the MEE growth approach for growing high-performance MQWs for advanced optoelectronic applications, notably for high-speed optoelectronic devices like THz photoconductive antennas.

The Band-Gap Studies of Short-Period CdO/MgO Superlattices

Trends in the behavior of band gaps in short-period superlattices (SLs) composed of CdO and MgO layers were analyzed experimentally and theoretically for several thicknesses of CdO sublayers. The optical properties of the SLs were investigated by means of transmittance measurements at room temperature in the wavelength range 200–700 nm. The direct band gap of {CdO/MgO} SLs were tuned from 2.6 to 6 eV by varying the thickness of CdO from 1 to 12 monolayers while maintaining the same MgO layer thickness of 4 monolayers. Obtained values of direct and indirect band gaps are higher than those theoretically calculated by an ab initio method, but follow the same trend. X-ray measurements confirmed the presence of a rock salt structure in the SLs. Two oriented structures (111 and 100) grown on c- and r-oriented sapphire substrates were obtained. The measured lattice parameters increase with CdO layer thickness, and the experimental data are in agreement with the calculated results. This new kind of SL structure may be suitable for use in visible, UV and deep UV optoelectronics, especially because the energy gap can be precisely controlled over a wide range by modulating the sublayer thickness in the superlattices.

Designer Topological Insulator with Enhanced Gap and Suppressed Bulk Conduction in Bi2Se3/Sb2Te3 Ultrashort-Period Superlattices.

A novel approach to suppress bulk conductance in three-dimensional (3D) topological insulators (TIs) using short-period superlattices (SLs) of two TIs is presented. Evidence for superlattice gap enhancement (SGE) was obtained from the reduction of bulk background doping from 1.2 × 1020 cm-3 to 8.5 × 1018 cm-3 as the period of Bi2Se3/Sb2Te3 SLs is decreased from 12 nm to 5 nm. Tight binding calculations show that, in the ultrashort-period regime, a significant SGE can be achieved for the resulting SL. Ultrathin short-period SLs behave as new designer TIs with bulk bandgaps up to 60% larger than the bandgap of the constituent layer of largest bandgap, while retaining topological surface features. Evidence for gap formation was obtained from ellipsometric measurements. Analysis of the weak antilocalization cusp in low-temperature magneto-conductance confirms that the top and bottom surfaces of the SL structure behave as Dirac surfaces. This approach represents a promising platform for building truly insulating TIs.

Impact of the substrate lattice constant on the emission properties of InGaN/GaN short-period superlattices grown by plasma assisted MBE

Abstract In this work we investigate InGaN/GaN short period superlattices (SPSLs) grown on (0001) GaN and on relaxed In0.2Ga0.8N buffers with varying degree of plastic relaxation. The SPSLs were fabricated by plasma assisted molecular beam epitaxy (PAMBE). Despite the nominal SPSL structure consisted of InN monolayers (MLs) embedded in GaN barriers, we observed single monolayer InGaN layers instead of InN. We study the photoluminescence (PL) of such SPSLs as a function of the substrates lattice constant. Increase of the substrate lattice constant from a = 3.183 A (for GaN grown on sapphire) to a = 3.216 A resulted in a peak emission shift from 379 nm to 419 nm. We attribute the PL red-shift to the increased In incorporation in the ML-thick InGaN on relaxed buffers as a consequence of the reduced lattice mismatch between the substrate and the film. Our experiment confirms that coherent growth of InN is prohibited on GaN and points out the important role of substrate lattice constant engineering for realization of InN/GaN SPSLs.

Room temperature yellow InGaAlP quantum dot laser

We report simulation of the conduction band alignment in tensile–strained GaP–enriched barrier structures and experimental results on injection lasing in the green–orange spectral range (558–605 nm) in (AlxGa1–x)0.5In0.5P–GaAs diodes containing such barriers. The wafers were grown by metal–organic vapor phase epitaxy side–by–side on (8 1 1)A, (2 1 1)A and (3 2 2)A GaAs substrates, which surface orientations were strongly tilted towards the [1 1 1]A direction with respect to the (1 0 0) plane. Four sheets of GaP–rich quantum barrier insertions were applied to suppress the leakage of non–equilibrium electrons from the gain medium. Two types of the gain medium were applied. In one case 4–fold stacked tensile–strained (In,Ga)P insertions were used. Experimental data shows that self–organized vertically–correlated quantum dots (QDs) are formed on (2 1 1)A– and (3 2 2)A–oriented substrates, while corrugated quantum wires are formed on the (8 1 1)A surface. In the other case a short–period superlattice (SPSL) composed of 16–fold stacked quasi–lattice–matched 1.4 nm–thick In0.5Ga0.5P layers separated by 4 nm–thick (Al0.6Ga0.4)0.5In0.5P layers was applied. Laser diodes with 4–fold stacked QDs having a threshold current densities of ∼7–10 kA/cm2 at room temperature were realized for both (2 1 1)A and (3 2 2)A surface orientations at cavity lengths of ∼1 mm. Emission wavelength at room temperature was ∼599–603 nm. Threshold current density for the stimulated emission was as low as ∼1 kA/cm2. For (8 1 1)A–grown structures no room temperature lasing was observed. SPSL structures demonstrated lasing only at low temperatures <200 K. The shortest wavelength (558 nm, 90 K) in combination with the highest operation temperature (150 K) was realized for (3 2 2)A–oriented substrates in agreement with theoretical predictions.

Investigation of digital alloyed AlInSb metamorphic buffers

Al1-xInxSb metamorphic step-graded buffers with Al0.6In0.4Sb terminal layers, designed to serve as a virtual substrate to support integrated InAs0.5Sb0.5 long-wave infrared absorber layers, were grown on GaSb wafers via molecular beam epitaxy. Two different structural profiles were used to define the effective composition of each buffer step: one based on digital alloys (1 nm period, ∼1.6 unit cells) and the other based on short period superlattices (10 nm period, ∼16 unit cells). Characterization via optical Nomarski microscopy, x-ray diffraction reciprocal space mapping, and transmission electron microscopy indicates that the digital alloy based structure behaves similar to that expected for a conventional bulk ternary alloy based structure, while the short period superlattice structure exhibits significantly hindered relaxation within the buffer layers.

Growth kinetics and structural perfection of (InN)1/(GaN)1–20 short-period superlattices on +c-GaN template in dynamic atomic layer epitaxy

The growth kinetics and structural perfection of (InN)1/(GaN)1–20 short-period superlattices (SPSs) were investigated with their application to ordered alloys in mind. The SPSs were grown on +c-GaN template at 650 °C by dynamic atomic layer epitaxy in conventional plasma-assisted molecular beam epitaxy. It was found that coherent structured InN/GaN SPSs could be fabricated when the thickness of the GaN barrier was 4 ML or above. Below 3 ML, the formation of SPSs was quite difficult owing to the increased strain in the SPS structure caused by the use of GaN as a template. The effective or average In composition of the (InN)1/(GaN)4 SPSs was around 10%, and the corresponding InN coverage in the ∼1 ML-thick InN wells was 50%. It was found that the effective InN coverage in ∼1 ML-thick InN wells could be varied with the growth conditions. In fact, the effective In composition could be increased up to 13.5%, i.e., the corresponding effective InN coverage was about 68%, by improving the capping/freezing speed by increasing the growth rate of the GaN barrier layer.

Low operation voltage of GaN-based LEDs with Al-doped ZnO upper contact directly on p-type GaN without insert layer

Al-doped ZnO (AZO) film was evaporated on double-side polished sapphire, p-GaN layers, n+-InGaN–GaN short-period superlattice (SPS) structures, and GaN-based light-emitting diodes (LEDs) by e-beam. The AZO film on the p-GaN layer after thermal annealing exhibited an extremely high transparency (98% at 450nm) and a small specific contact resistance of 2.19×10−2Ωcm2, which was almost the same as that of as-deposited AZO on n+-SPS structure. With 20mA injection current, the forward voltages were 3.30 and 3.27V, whereas the output powers were 4.32 and 4.07mW for the LED with AZO on insert n+-SPS upper contact and the LED with AZO on p-GaN upper contact (without insert layer), respectively. The small specific contact resistance and low operation voltage of LED with AZO on p-GaN upper contact was achieved by rapid thermal annealing (RTA) process.

Effect of Growth Temperature on the Luminescence Properties of InP/GaP Short-Period Superlattice Structures

The optical properties of InP/GaP short-period superlattice (SPS) structures grown at various temperatures from <TEX>$400^{\circ}C$</TEX> to <TEX>$490^{\circ}C$</TEX> have been investigated by using temperature-dependent photoluminescence (PL) and emission wavelength-dependent time-resolved PL measurements. The PL peak energy for SPS samples decreases as the growth temperature increases. The decreased PL energy of ~10 meV for the sample grown at <TEX>$425^{\circ}C$</TEX> compared to that for <TEX>$400^{\circ}C$</TEX>-grown sample is due to the CuPt-B type ordering, while the SPS samples grown at <TEX>$460^{\circ}C$</TEX> and <TEX>$490^{\circ}C$</TEX> exhibit the significant reduction of the PL peak energies due to the combined effects of the formation of lateral composition modulation (LCM) and CuPt-B type ordering. The SPS samples with LCM structure show the enhanced carrier lifetime due to the spatial separation of carriers. This study represents that the bandgap energy of InP/GaP SPS structures can be controlled by varying growth temperature, leading to LCM formation and CuPt-B type ordering.

Prediction of giant magnetoelectric effect in LaMnO3/BaTiO3/SrMnO3 superlattice: The role of n-type SrMnO3/LaMnO3 interface

We study the magnetoelectric coupling for the [001]-oriented (LaMnO3)2/(BaTiO3)5/(SrMnO3)2 superlattice, by means of the density functional theory. An interesting transition between ferromagnetic ordering and antiferromagnetic ordering is demonstrated by switching ferroelectric polarization in short-period superlattice structure. The predicted ferroelectrically induced magnetic reconstruction is less sensitive to the choice of Coulomb-correction U within GGA + U scheme. A possible explanation is given in terms of the favorable effect of n-type SrMnO3/LaMnO3 interface. Our results suggest that a sizable magnetoelectric effect may be achieved in the short-period LaMnO3/BaTiO3/SrMnO3 superlattice, hence promising application in electrically controlled magnetic data storage.

Multiple Nanostructures on Full Surface of GZO/GaN-Based LED to Enhance Light-Extraction Efficiency Using a Solution-Based Method

This paper reports a solution-based method for the application of multiple nanostructures on full surface of GZO/GaN-based LEDs to enhance light-extraction efficiency. Ga-doped ZnO (GZO) was deposited to a thickness of 1μm and an n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -InGaN/GaN short-period superlattice structure was grown to improve the electrical characteristics of the LEDs, including series resistance and operating voltage. A solution-based method was used to control the density of ZnO nanoparticles deposited on the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layer for use as self-assembled etching nanomasks. Multiple nanostructures were simultaneously formed on the surfaces of GZO, p-GaN, and n-GaN by dry etching. The proposed LEDs increase light output power by 10%-27% (at 20 mA) over that of regular GaN-based LEDs. The difference in light output power can be attributed to differences in the shape, thickness, and density of GZO and GaN nanostructures, resulting in a reduction in Fresnel reflection provided by the roughened surface of the GaN-based LEDs.

High Efficiency InGaN Blue Light-Emitting Diode With ${&gt;}{\rm 4}\hbox{-}{\rm W}$ Output Power at 3 A

This letter reports high-power and high-efficiency characteristics of the InGaN-based blue light-emitting diode (LED) operating at > 10-W electrical input power in a single-chip package. The LED chip is fabricated as a vertical-injection structure with chip dimensions of 1.8 mm × 1.8 mm. InGaN/GaN short-period superlattice (SL) structures are employed below multiple-quantum-well active region as current spreading layers. It is found, by simulation, that SL layers are quite effective in improving current spreading and uniformity in carrier distribution. When the characteristics of the fabricated LED package are measured under pulsed operation conditions, efficiency droop is found to be greatly reduced in the LED structure with SL layers. A record high light output power of 4.18 W and external quantum efficiency of 51% are demonstrated at 3-A injection current.

Optimization of InGaN/GaN superlattice structures for high-efficiency vertical blue light-emitting diodes

We investigate carrier distribution and efficiency characteristics of GaN-based vertical blue light-emitting diodes (LEDs) having InGaN/GaN short-period superlattice (SL) structures by using numerical simulations. The SL structures exist between multiple-quantum-well active layers and an n-GaN layer. The In composition and doping concentration of SL layers are found to have strong influence on the electron concentration distribution in the plane of QWs, internal quantum efficiency, and forward voltage of vertical LEDs. The electron distribution becomes homogenous as the In composition increases or doping concentration decreases, which results in the reduction of efficiency droop and the increase of forward voltage. Based on the simulation results, optimum SL structures for obtaining the maximum wall-plug efficiency are found.

The design of 1 eV band-gap of GaNAs/InGaAs short-period super-lattice solar cell

The GaNAs/InGaAs short-period super-lattice(SPSL) with a feature of space separation in In and N constituent and equivalent 1 eV band-gap is one of important structures as an active region to achieve high efficiency of GaInNAs- based solar cell in the future. To experimentally realize the required band-gap for high conversion efficiency in multi- junction solar cells on Ge substrate, we demonstrate a propagation matrix method to calculate some dependences of GaNAs/InGaAs SPSLs on their structural parameters. The results show that the GaNAs/InGaAs SPSL structures can be flexible to obtain the 1 eV energy gap by a reasonable choice of the period number, barrier thickness as well as concentrations of In and N. Additionally, the calculation illustrates a relationship of respective modulation from those super-lattice structures for the SPSL active region which emits or absorbs light at 1 eV in energy.

Investigation of GaN-based light-emitting diodes using a p-GaN/i-InGaN short-period superlattice structure as last quantum barrier

In this work, GaN-based light-emitting diodes (LEDs) with a p-GaN/i-InGaN short-period superlattice (SPSL) structure, p-GaN and undoped GaN last quantum barrier (LQB) have been numerically investigated by using the APSYS simulation software. It has been found that the efficiency droop is significantly improved when the undoped GaN LQB in a typical blue LED is replaced by a p-GaN/i-InGaN SPSL structure. According to the simulation analysis, using the p-GaN/i-InGaN SPSL structure as LQB is beneficial to increasing the hole injection efficiency and decreasing the electron current leakage. Therefore, the radiative recombination and optical power are enhanced.

Improved performance of GaN-based light-emitting diodes by using short-period superlattice structures

AbstractAn InGaN/GaN multiple-quantum-well (MQW) light-emitting diode (LED) with a ten-period i (undoped) -InGaN/p (Mg doped) -GaN (2.5 nm/5.0 nm) superlattice (SL) structure, was fabricated. This SL structure that can be regarded as a confinement layer of holes to enhance the hole injection efficiency is inserted between MQW and p-GaN layers. The studied LED device exhibits better current spreading performance and an improved quality, compared with a conventional one without SL structure. Due to the reduced contact resistance as well as more uniformity of carriers injection, the operation voltage at 20 mA is decreased from 3.32 to 3.14 V. A remarkably reduced reverse-biased leakage current (10−7–10−9 A) and higher endurance of the reverse current pulse are found. The measured output power and external quantum efficiency (EQE) of the studied LED are 13.6 mW and 24.8%, respectively. In addition, significant enhancement of 25.4% in output power as well as increment of 5% in EQE for the studied devices is observed, as the studied devices show superior current spreading ability and reduction in dislocations offered by the SL structure.

Modelling of an InAs/GaSb/InSb short-period superlattice laser diode for mid-infrared emission by the k.p method

The electronic band structure and optical gain of an InAs/GaSb/InSb short-period superlattice laser diode on a GaSb substrate are numerically investigated with an accurate 8 × 8 k.p model. Using a realistic graded and asymmetric interface profile, we obtain a reasonable agreement on band gap energy with our experimental data extracted from laser emissions performed on the laser diode. The optical performance in terms of optical gain is then calculated for the laser structure and we demonstrate the utility of interface design to model short-period superlattice structures.

Strain-induced nitrogen incorporation in atomic layer epitaxy growth of InAsN/GaAs quantum wells using metal organic chemical vapor deposition

We report on the growth of high-quality high-indium-content (Ga)InAsN/GaAs quantum wells grown using low-pressure metal organic chemical vapor deposition. The growth was performed employing a strain-controlled atomic layer epitaxy technique. We verified experimentally that the strain enables the incorporation of nitrogen atoms during the atomic layer epitaxy growth of InAsN monolayers on GaAs. Photoluminescence and secondary ion mass spectroscopy measurements indicate that about 2.5% of the nitrogen was incorporated in the grown layers. Utilizing this strain-controlled atomic layer epitaxy technique, we designed and demonstrated highly strained InAsN/GaAs short-period superlattice structure suitable for applications in optical communication.

Influence of a p-InGaN/GaN short-period superlattice on the performance of GaN-based light-emitting diodes

We have investigated the performance of GaN-based blue light-emitting diodes (LEDs) with a p-InGaN/GaN short-period superlattice (SPS) contact layer, which were grown by metal organic chemical vapor deposition. It was found that dot-like features appeared on the surface when a p-InGaN/GaN SPS structure was grown. The LED operating voltage decreased from 3.52 V to 3.15 V and the electrostatic discharge properties of LEDs were improved by using such a SPS structure. The output powers of LEDs were also increased by adjusting the In mole fraction in the SPS. However, the lifetime of LEDs became shorter when such a SPS structure was used.

Raman scattering studies of (GaP)n/(InP)n (n = 1, 1.7, 2) short‐period superlattice structures

AbstractWe report Raman scattering from (GaP)n/(InP)n (n = 1, 1.7, 2) short‐period superlattice (SPS) structures to study the effect of lateral composition modulation (LCM) on the behavior of optical phonons. Cross‐sectional transmission electron microscope images revealed that LCM was formed with complex pattern in the n = 1.7 and n = 2 samples grown at 490 °C. Interestingly, both the InP‐ and the GaP‐like longitudinal optical (LO) phonon energies increased systematically as the number of monolayers was increased from n = 1 to n = 2. A significant broadening of the phonon line shapes was also observed for the n = 1.7 and n = 2 samples. In contrast, for samples grown at 425 °C, both the increase of the LO phonon energies and the broadening of the phonon line shapes were observed only when n = 1.7. Our results demonstrate that the optical phonons in the (GaP)n/(InP)n SPS structures are significantly affected in the occurrence of LCM related to the growth temperature and the number of monolayers.Copyright © 2009 John Wiley &amp; Sons, Ltd.