Period Superlattice Structure Research Articles

Polymorphic Ga2S3 nanowires: phase-controlled growth and crystal structure calculations.

The polymorphism of nanostructures is of paramount importance for many promising applications in high-performance nanodevices. We report the chemical vapor deposition synthesis of Ga2S3 nanowires (NWs) that show the consecutive phase transitions of monoclinic (M) → hexagonal (H) → wurtzite (W) → zinc blende (C) when lowering the growth temperature from 850 to 600 °C. At the highest temperature, single-crystalline NWs were grown in the thermodynamically stable M phase. Two types of H phase exhibited 1.8 nm periodic superlattice structures owing to the distinctively ordered Ga sites. They consisted of three rotational variants of the M phase along the growth direction ([001]M = [0001]H/W) but with different sequences in the variants. The phases shared the same crystallographic axis within the NWs, producing novel core–shell structures to illustrate the phase evolution. The relative stabilities of these phases were predicted using density functional theory calculations, and the results support the successive phase evolution. Photodetector devices based on the p-type M and H phase Ga2S3 NWs showed excellent UV photoresponse performance.

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.

Decoupling the refractive index from the electrical properties of transparent conducting oxides via periodic superlattices.

We demonstrate an alternative approach to tuning the refractive index of materials. Current methodologies for tuning the refractive index of a material often result in undesirable changes to the structural or optoelectronic properties. By artificially layering a transparent conducting oxide with a lower refractive index material the overall film retains a desirable conductivity and mobility while acting optically as an effective medium with a modified refractive index. Calculations indicate that, with our refractive index change of 0.2, a significant reduction of reflective losses could be obtained by the utilisation of these structures in optoelectronic devices. Beyond this, periodic superlattice structures present a solution to decouple physical properties where the underlying electronic interaction is governed by different length scales.

Plasmons in graphene moiré superlattices.

Moiré patterns are periodic superlattice structures that appear when two crystals with a minor lattice mismatch are superimposed. A prominent recent example is that of monolayer graphene placed on a crystal of hexagonal boron nitride. As a result of the moiré pattern superlattice created by this stacking, the electronic band structure of graphene is radically altered, acquiring satellite sub-Dirac cones at the superlattice zone boundaries. To probe the dynamical response of the moiré graphene, we use infrared (IR) nano-imaging to explore propagation of surface plasmons, collective oscillations of electrons coupled to IR light. We show that interband transitions associated with the superlattice mini-bands in concert with free electrons in the Dirac bands produce two additive contributions to composite IR plasmons in graphene moiré superstructures. This novel form of collective modes is likely to be generic to other forms of moiré-forming superlattices, including van der Waals heterostructures.

Retraction Note: Microstructure of Co precipitation strengthened B2-ordered NiAl

A transmission electron microscopy investigation on the phase decomposition of B2-ordered (Ni,Co)Al supersaturated with Ni and Co has revealed the precipitation of (Ni,Co)2Al which has not been expected from the reported equilibrium phase diagram. The (Ni,Co)2Al phase has a hexagonal structure and takes a rodlike shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appears in the hexagonal (Ni,Co)2Al phase. The orientation relationship between the (Ni,Co)2Al precipitates and the B2-(Ni,Co)Al matrix is found to be (0001)p//(111)B2 and [\( \bar 1 \)2\( \bar 1 \)0]p//[\( \bar 1 \)10]B2, where the suffix p and B2 denote the (Ni,Co)2Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardens appreciably by the fine precipitation of the (Ni,Co)2Al phase.

Retraction Note: Microstructures and morphologies of B2-ordered NiAl(Co) and FeAl(Co)

Fine dispersion of disordered phases is obtained in a Ni-Al-Co and Fe-Al-Co ternary system. A transmission electron microscopy investigation has been performed in the present work on the precipitation of supersaturated B2-ordered (Ni,Co)Al and α-Fe in B2-ordered FeAl(Co) with different stoichiometries. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of (Ni,Co)Al and B2-FeAl(Co) increased appreciably by the fine precipitation of (Ni,Co)2Al, α-Fe, and overage softening occurred after prolonged aging. In case of B2-ordered (Ni,Co)Al, the (Ni,Co)2Al phase had a hexagonal structure and took a rod-like shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appeared in the hexagonal (Ni,Co)2Al phase. The orientation relationship between the (Ni,Co)2Al precipitates and the B2-(Ni,Co)Al matrix was (0001)p//(111)B2 and \([\bar 12\bar 10]_p //[\bar 110]_{B2}\), where the suffix p and B2 denote the (Ni,Co)2Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardened appreciably by the fine precipitation of the (Ni,Co)2Al phase. On the other hand, in case of B2-FeAl(Co), the disordered α-Fe phase was present as a precipitate in a B2-FeAl(Co) matrix and had a cubic-cubic orientation with the matrix. At the early aging periods, prismatic dislocation loops formed in the B2-FeAl(Co) matrix. B2-FeAl(Co) matrix was typically hardened by the precipitation of α-Fe.

Band Gap Tuning of Twinned GaAsP Ternary Nanowires

GaAs1–xPx ternary alloy nanowires have drawn much interest because their tunable band gaps, which range from the near-infrared to visible region, are promising for advanced and integrated nanoscale optoelectronic devices. In this study, we synthesized compositionally tuned GaAs1–xPx (0 ≤ x ≤ 1) alloy nanowires with two average diameters of 60 and 120 nm by vapor transport method. The nanowires exhibit exclusively twinned superlattice structures, consisting of zinc blende phase twinned octahedral slice segments between wurtzite phase planes. Smaller diameter and higher P content (x) result in shorter periodic superlattice structures. The band gap of the smaller diameter nanowires is larger than that of the larger diameter nanowires by about 90 meV, suggesting that the twinned superlattice structure increases the band gap. The increase in band gap is ascribed to the higher band gap of the wurtzite phase than that of the zinc blende phase.

Drastic improvement of oxide thermoelectric performance using thermal and plasma treatments of the InGaZnO thin films grown by sputtering

Single-crystal InGaO 3(ZnO) m thin films with periodic superlattice structures suitable for transparent thermoelectric applications were fabricated using a commercially available c-plane sapphire substrate, an epitaxial ZnO buffer layer, a thermal treatment at 900 °C, and an Ar plasma treatment. The introduction of the epitaxial ZnO buffer layer led to a significant reduction in the lattice mismatch at the interface with the InGaO 3(ZnO) m films. The sandwich structure of the ZnO/InGaZnO/ZnO resulted in an increase in the ZnO content in the superlattice InGaO 3(ZnO) m thin films. With respect to thermoelectric properties, the formation of a perfect, layered structure induced an increase in the Seebeck coefficient and, at the same time, a decrease in the thermal conductivity. After complete crystallization, the Ar plasma treatment resulted in a considerable decrease in the electrical resistivity without microstructural changes and without a large decrease in the thermal conductivity. As a result, the thermoelectric properties using n-type oxide semiconductors were dramatically improved.

Composition and Phase Tuned InGaAs Alloy Nanowires

InxGa1−xAs (0 ≤ x ≤ 1) alloy nanowires (average diameter = 50 nm) were synthesized at 800 °C with complete composition tuning by the thermal evaporation of GaAs/InAs powders. X-ray diffraction and Raman spectroscopy confirmed the complete composition tuning over the whole range. They exhibit exclusively a superlattice structure composed of zinc blende phase twinned octahedral slice segments having alternating orientations along the axial [111] direction and wurtzite phase twin planes. When the mole fraction (x) approaches 0.5, the period of the twinned superlattice structures becomes shorter, showing a controlled wurtzite−zinc blende polytypism. At x = 0.5, the wurtzite phase is dominant with the shortest superlattice periodicity (∼2 nm). The smaller diameter consistently induces shorter periodic superlattice structures. This unique polytypism shows that the incorporation of In (or Ga) and the smaller diameter promotes the crystallization of the nanowires in the wurtzite phase. These InxGa1−xAs nanowires produce an efficient THz emission, showing minimized carrier mobility at x = 0.5, where the superlattice stacking faults are maximized.

RETRACTED ARTICLE: A study on the microstructure of precipitation strengthened B2-ordered NiAl

Microstructural control to produce a multiphase structure and there by improve the high temperature strength as well as low temperature ductility of intermetallics has received much attention. A transmission electron microscopy investigation has been performed in the present work on the precipitation of supersaturated B2-ordered (Ni,Co)Al and α-Cr in B2-ordered β-NiAl with different stoichiometry. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of (Ni,Co)Al and β-NiAl increases appreciably by the fine precipitation of (Ni,Co)2Al and α-Cr, and overage softening occurs after prolonged aging. In the case of B2-ordered (Ni,Co)Al, the (Ni,Co)2Al phase has a hexagonal structure and takes a rod-like shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appears in the hexagonal (Ni,Co)2Al phase. The orientation relationship between the (Ni,Co)2Al precipitates and the B2-(Ni,Co)Al matrix is found to be (0001)p//(111)B2 and [ $\bar 1$ 2 $\bar 1$ 0]p//[ $\bar 1$ 10]B2, where the suffixes p and B2 denote the (Ni,Co)2Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardens appreciably by fine precipitation of the (Ni,Co)2Al phase. On the other hand, in the case of B2-NiAl, perfect lattice coherency is retained at the interfaces between the α-Cr particles and the matrix during the initial stage of aging. After prolonged aging, a loss of coherency occurs by the attraction of matrix dislocations to the particle/matrix interface followed by climbing around the particles.

Microstructure and Strength of B2-ordered (Ni,Co)Al

A transmission electron microscopy (TEM) investigation of the morphologies and crystal structures of precipitates in supersaturated B2-ordered (Ni,Co)Al has revealed that rod-like precipitates of the (Ni,Co)2Al phase with a hexagonal structure form parallel to the direction of the B2 matrix. By aging at temperatures below 973 K, a long period superlattice structure of hexagonal (Ni,Co)2Al was formed. The (Ni,Co)Al hardens appreciably by the precipitation of these phases. An energy dispersive spectroscopy (EDS) was used to analyze the compositions of each phase formed in the B2-(Ni,Co)Al. The effects of the dispersion of the (Ni,Co)2Al phase on the temperature dependence of the strength of the B2-(Ni,Co)Al have been investigated over a temperature range from 298 K to 1173 K.

Occurrence of short-range-ordered incommensurate states in γ-TiAl

This paper examines a lesser-known form of quasiperiodicity in alloys, namely, quasiperiodicity in superlattice structures. Unlike quasicrystals, a quasiperiodic superlattice (QPSL) structure has an underlying periodic lattice and it is the arrangement of the two atomic species on this lattice that makes the superlattice quasiperiodic. Central to the discussion will be a QPSL structure, comprising three different building blocks (tiles), whose approximants are the Al5Ti3 and Al11Ti7 structures in Al-rich γ-TiAl. The three types of tiles represent unimolecular clusters of three different superlattice structures derived from γ-TiAl. Two different types of antiphase boundaries (APBs) occur in Al5Ti3. Their energies have been changed continuously in a Monte Carlo simulation to study transformations, either to the QPSL or to known periodic superlattice structures in Al–Ti. Some of the intermediate stages of transformation are short-range ordered states, which exhibit intensity maxima at incommensurate positions in the reciprocal space and different geometrical patterns of diffuse intensity. Very good agreement has been found in the simulation results, the results of a 4D to 2D projection scheme and TEM observations.

Imaging the evolution of lateral composition modulation in strained alloy superlattices.

Scanning tunneling microscopy is used to investigate the morphological evolution of GaAs/InAs short period superlattice structures. The layers of the superlattice, either grown in compression or tension, exhibit an island or trench morphology. With increasing film thickness, the islands or trenches grow in size and develop a characteristic spacing along [110] of approximately 150 A. This is the first experimental evidence to suggest that lateral composition modulation arises from both thickness variations of the layers and compositional nonuniformities within the atomic plane.

GaN-Based Light Emitting Diodes with Si-Doped In0.23Ga0.77N/GaN Short Period Superlattice Current Spreading Layer

The electrical properties of the Si-doped n+-In0.23Ga0.77N/GaN short period superlattice (SPS) structure were investigated and compared with those of a conventional Mg-doped GaN contact layer. The secondary ion mass spectroscopy (SIMS) data clearly shows a simultaneous presence of Si and In in the surface region. Temperature dependent Hall measurement showed that such a SPS structure exhibits a high sheet electron concentration. It was found that we could reduce the 20 mA LED forward voltage from 3.78 V to 2.94 V and also reduce the series resistance of the LED from 41 Ω to 10 Ω by introducing such an n+-InGaN/GaN SPS top contact. It was also found that we could improve the output power and LED lifetime by employing such a SPS structure.

Hot electron distribution in quantum cascade and single stage GaAs/AlGaAs periodic superlattice structures

Abstract We report the experimental determination of the steady-state electronic distribution in the excited miniband of GaAs/Al 0.33 Ga 0.67 As quantum cascade (QC) devices with periodic superlattice (SL) active regions. The populations of the excited subbands in the second miniband were extracted from the analysis of the high-energy tail of interminiband electroluminescence spectra. Single- and multiple-stage structures were investigated. We found that the excited electrons can be described by a thermal distribution, in analogy with the case of InGaAs/InAlAs/InP structures. At powers above the injection threshold a non-equilibrium distribution with a temperature higher than the lattice one is established.

Compositionally Modulated Structures Studied by in Situ Scanning Tunneling Microscopy

AbstractTwo different short period superlattice (SPS) structures with nominally equivalent lattice mismatch, InAs/AlAs and InAs/GaAs are examined using in situ scanning tunneling microscopy (STM). Depending upon the growth conditions, the composition of the InAs/AlAs SPS structure can be either homogeneous or modulated in the lateral direction. Distinct periodic structures are clearly visible in images of the modulated SPS while no periodicity is observed in the homogeneous SPS. For a 30 period SPS consisting of 2 monolayers of AlAs and 2 monolayers of InAs we observe structures 20 nm in size with an average spacing of ∼25 nm in orthogonal directions, which is approximately the same length scale as the composition modulation. Despite the nominally equivalent lattice mismatch, the InAs/GaAs SPS structures are quite different. Some degree of modulation is always observed in these structures. Homogeneous structures are not observed. For the modulated SPS structures, scanning tunneling spectroscopy can be used to characterize the chemical profile.

Effects of electric field on the quasi-bound electronic surface states confined to the boundary of periodic superlattice structures

The effect of an applied electric field on the electronic surface states confined to the boundary of a novel type of quantum well structure composed of a semi-infinite, periodic superlattice bounded by a homogeneous barrier has been studied. It has been found that the surface states are very much sensitive to the applied electric field. Possible applications of such structure as opto-electronic devices are also discussed.

Thermal stability of GaAs (C)/InAs superlattices grown by metalorganic molecular beam epitaxy

The thermal stability of GaAs(C)/InAs superlattices grown by metalorganic molecular beam epitaxy on InP substrates has been examined by Hall measurements, transmission electron microscopy, and high resolution x-ray diffraction. These structures provide an ordered counterpart to a random In0.53Ga0.47As alloy, in which high concentration carbon doping is generally difficult to achieve. In a 43 period (23 Å GaAs/26 Å InAs) superlattice in which the GaAs was C-doped and the InAs undoped an average hole concentration of 7×1019 cm−3 and hole mobility of 20 cm2 V−1 s−1 was achieved. Such structures are stable against rapid thermal annealing (10 s) up to 750 °C. An 850 °C/10 s anneal reduced the hole concentration to 1.5×1019 cm−3, accompanied by the onset of intermixing of the superlattice. The surface morphology of all but very thick (36 Å GaAs/40 Å InAs) period superlattice structures remained specular, even after 850 °C, 10 s annealing. These superlattices show properties suitable for use in a range of electronic and photonic devices, particularly InP-based lasers and heterojunction bipolar transistors.

New deep level luminescence bands observed from both a SiGe alloy layer and Si/Ge superlattice structures

Photoluminescence measurements have been made on Si/Ge short period superlattice structures grown on a SiGe alloy buffer layer and on a similar SiGe alloy layer without a superlattice. Using an InSb detector with a low energy cutoff at ∼450 meV, the major luminescence features observed were two broad bands with maximum intensities at ∼530 and ∼720 meV. The luminescence intensity was found to vary with the superlattice composition but was substantially stronger for the SiGe alloy layer without a superlattice. We ascribe these luminescence features to defects in the SiGe alloy layers. This is supported by the observation that the introduction of deliberate copper contamination at 600 °C dramatically increases the photoluminescence signal.

Gas source molecular beam epitaxy growth of short period GaP/AlP(001) superlattices

Short period GaP/AlP superlattices are grown on GaP and GaAs substrates at 600 °C by gas source molecular beam epitaxy with growth interruption. Alternating monolayer growth of GaP and AlP is confirmed by the observation of the reflection high-energy electron diffraction intensity oscillations during growth. The formation of short period superlattice structures and the zone-folded LO phonons are observed in the x-ray diffraction rocking curves and Raman spectra, respectively.