Number Of Superlattice Periods Research Articles

Investigation of infrared photoluminescence spectra of Ge1-x-y Si x Sn y /Si nanostructures

This work investigates the luminescence properties of pseudomorphic nanostructures with Ge1-x-y Si x Sn y /Si superlattices (SL) grown on silicon substrates by molecular beam epitaxy. It was shown that the addition of Sn (y = 0.07) to the alloy layers within the structures results in a significant shift of the photoluminescence (PL) spectra towards longer wavelengths (2.0-3.5 μm) compared to similar Ge0.7Si0.3/Si superlattices. A series of experiments involving etching the structures to different depths have shown that the observed photoluminescence occurs exactly in the Ge1-x-y Si x Sn y /Si SL region, with the sublinear nature of the PL power dependence indicating the probable participation of defects in radiative recombination. A significant increase in low-temperature PL was observed with a decrease in the thickness of narrow-gap SL layers from 6 to 2 nm, as well as with an increase in the number of superlattice periods from 10 to 30. The obtained structures exhibit relatively good temperature stability of luminescence, which is preserved up to 160 K. Thus, the current work demonstrates the possibility of creating mid-wave IR emitting epitaxial structures on silicon substrates.

Topologically protected spin diffusion and spin generator using chalcogenide superlattices

Spintronics is expected to be the basis for future ultra-low-energy nanoelectronic devices. To operate such devices at room temperature, amplifiers, batteries, capacitors, as well as spin current sources are required. Here we report a chalcogenide superlattice composed of GeTe and Sb2Te3 layers that have a topologically protected spin diffusion length exceeding 100 μm at room temperature. A spin generator is demonstrated by combining magnetic injectors (TbFeCo) with this superlattice. The spin current was found to increase exponentially with the number of superlattice periods. We used this effect to demonstrate a 15-fold increase in the spin current. In addition, spin rectification is possible by growing the superlattice layers with atomic-level thickness accuracy. The reported chalcogenide superlattice spin generators and rectifiers open new opportunities to design low-energy spintronic integrated circuits and quantum computers.

Performance enhancement of yellow InGaN-based multiple-quantum-well light-emitting diodes grown on Si substrates by optimizing the InGaN/GaN superlattice interlayer

A specially designed InGaN/GaN superlattice (SL) interlayer was inserted between n-GaN and a multiple quantum well to enhance the performance of yellow light-emitting diodes (LEDs) grown on Si (111). The number of SL periods was determined to be the key to enhancing the external quantum efficiency and reducing forward voltage. Our results show that more SLs could suppress nonradiative recombination by eliminating micron-scale indium-rich clusters and could promote hole injection with increased V-pit size. However, too many SLs reduce the effective luminescence area and lead to many voids formed in the p-type layer. We demonstrate that 32 is the optimum number of SLs for yellow InGaN/GaN LEDs, obtaining a high light output power of 63 mW with a dominant wavelength of 568 nm, and a low forward voltage of 2.38 V at 200 mA (20 A/cm2).

Low-dimensional thermoelectricity in graphene: The case of gated graphene superlattices

Low-dimensional thermoelectricity is a key concept in modern thermoelectricity. This concept refers to the possibility to improve thermoelectric performance through redistribution of the density of states by reducing the dimensionality of thermoelectric devices. Among the most successful low-dimensional structures we can find superlattices of quantum wells, wires and dots. In this work, we show that this concept can be extended to cutting-edge materials like graphene. In specific, we carry out a systematic assessment of the thermoelectric properties of quantum well gated graphene superlattices. In particular, we find giant values for the Seebeck coefficient and the power factor by redistributing the density of states through the modulation of the fundamental parameters of the graphene superlattice. Even more important, these giant values can be further improved by choosing appropriately the angle of incidence of Dirac electrons, the number of superlattice periods, the width of the superlattice unit cell as well as the height of the barriers. We also find that the power factor presents a series of giant peaks, clustered in twin fashion, associated to the oscillating nature of the conductance. Finally, we consider that low-dimensional thermoelectricity in graphene and related 2D materials is promising and constitutes a possible route to push forward this exciting field.

Metamorphic Al0.5Ga0.5N:Si on AlN/sapphire for the growth of UVB LEDs

In this paper we investigate the growth of metamorphic Al0.5Ga0.5N:Si on c-plane AlN/sapphire. The structural properties of the AlGaN:Si pseudo substrates and the electro-optical characteristics of subsequently grown UVB LEDs are being examined. We demonstrate, that superlattices allow the controlled strain relaxation of Al0.5Ga0.5N by rearrangement of threading dislocations, thus preventing the formation of cracks. This study investigates AlN/GaN superlattices with a nominal GaN layer thickness between 1.0nm and 2.5nm at a fixed AlN layer thickness of 2.5nm. The number of superlattice-periods was also varied between 20 and 120. It was found that beyond a GaN layer thickness of 1.5nm three-dimensional structures are formed. Additionally, these three-dimensional structures reduce the local defect density of the subsequently grown Al0.5Ga0.5N layer. Although the Al0.5Ga0.5N layer appears to be almost fully relaxed, the relaxation state of this pseudo substrate, was found to be dependent on the GaN layer thickness in the superlattice. After optimizing the superlattice structure we were able to grow crack free 4μm thick Si-doped Al0.5Ga0.5N layers and on top UVB LEDs with a fully strained active region emitting at 310nm with output powers of more than 18mW at 500mA.

Controlling potential barrier height by changing V-shaped pit size and the effect on optical and electrical properties for InGaN/GaN based light-emitting diodes

The internal quantum efficiency (IQE) of InGaN/GaN multiple quantum wells (MQWs) with blue light emission was improved by inserting an InGaN/GaN superlattice (SL) beneath the MQWs. While the SL technique is useful for improving the light-emitting diode (LED) performance, its effectiveness from a multilateral point of view requires investigation. V-shaped pits (V-pits), which generate a potential barrier and screen the effect of the threading dislocation, are one of the candidates for increasing the light emission efficiency of LEDs exceptionally. In this research, we investigated the relationship between the V-pit and SL and revealed that the V-pit diameter is strongly correlated with the IQE by changing the number of SL periods. Using scanning near-field optical microscopy and photoluminescence measurements, we demonstrated the distinct presence of the potential barrier formed by the V-pits around the dislocations. The relationship between the V-pit and the number of SL periods resulted in changing the potential barrier height, which is related to the V-pit diameter determined by the number of SL periods. In addition, we made an attempt to insert pit expansion layers (PELs) composed of combination of SL and middle temperature grown GaN layer instead of only SL structure. As a result of the evaluation of LEDs using SL or PEL, the EL intensity was strongly related to pit diameter regardless of the structures to form the V-pits. In addition, it was clear that larger V-pits reduce the efficiency droop, which is considered to be suppression of the carrier loss at high injection current.

Electrophysical characteristics and structural parameters of metamorphic HEMT nanoheterostructures In0.7Al0.3As/In0.7Ga0.3As/In0.7Al0.3As containing superlattices with different numbers of periods in the metamorphic buffer

The results of studying the electrophysical characteristics and structural parameters of metamorphic In0.7Al0.3As/In0.7Ga0.3As/In0.7Al0.3As HEMT nanoheterostructures epitaxially grown on GaAs (100) substrates have been presented. A linear metamorphic buffer with inserted unbalanced superlattices characterized by different numbers of periods is used. Transmission electron microscopy has shown that an increase in the number of superlattice periods from 5 to 30 promotes the improvement of the crystal structure. In this case, the electrophysical parameters of metamorphic HEMT nanoheterostructures are also significantly improved.

Spontaneous lateral modulation in short-period superlattices investigated by grazing-incidence x-ray diffraction

The process of spontaneous lateral composition modulation in short-period InAs-AlAs superlattices has been investigated by grazing-incidence x-ray diffraction. We have developed a theoretical description of x-ray scattering from laterally modulated structures that makes it possible to determine the lateral composition modulation directly without assuming any structure model. From experimental intensity distributions in reciprocal space we have determined the amplitudes of the modulation and its degree of periodicity and their dependence on the number of superlattice periods. From the data it follows that the modulation process cannot be explained by bunching of monolayer steps and most likely, it is caused by stress-driven morphological instabilities of the growing surface.

Ordered quantum dot molecules and single quantum dots formed by self-organized anisotropic strain engineering

An ordered lattice of lateral InAs quantum dot (QD) molecules is created by self-organized anisotropic strain engineering of an (In,Ga)As∕GaAs superlattice (SL) template on GaAs(311)B by molecular-beam epitaxy, constituting a Turing pattern in solid state. The SL template and InAs QD growth conditions, such as the number of SL periods, growth temperatures, amount and composition of deposited (In,Ga)As, and insertion of Al-containing layers, are studied in detail for an optimized QD ordering within and among the InAs QD molecules on the SL template nodes, which is evaluated by atomic force microscopy. The average number of InAs QDs within the molecules is controlled by the thickness of the upper GaAs separation layer on the SL template and the (In,Ga)As growth temperature in the SL. The strain-correlated growth in SL template formation and QD ordering is directly confirmed by high-resolution x-ray diffraction. Ordered arrays of single InAs QDs on the SL template nodes are realized for elevated SL template and InAs QD growth temperatures together with the insertion of a second InAs QD layer. The InAs QD molecules exhibit strong photoluminescence (PL) emission up to room temperature. Temperature-dependent PL measurements exhibit an unusual behavior of the full width at half maximum, indicating carrier redistribution solely within the QD molecules.

Ordering of quantum dot molecules by self-organization

Ordered groups of InAs quantum dots (QDs), lateral QD molecules, are created by self-organized anisotropic strain engineering of a (In,Ga)As/GaAs superlattice (SL) template on GaAs (311)B by molecular beam epitaxy. During stacking the SL template self-organizes into a highly ordered two-dimensional (In,Ga)As and, thus, strain field modulation on a mesoscopic length scale, constituting a Turing pattern in solid state. InAs QDs preferentially grow on top of the SL template nodes due to local strain recognition, forming a lattice of separated groups of closely spaced ordered QDs. The SL template and InAs QD growth conditions like the number of SL periods, growth temperatures, amount and composition of deposited (In,Ga)As, and insertion of Al-containing layers are studied in detail for optimized QD ordering within and among the InAs QD molecules on the SL template nodes, which is evaluated by atomic force microscopy. The average number of InAs QDs within the molecules is controlled by the thickness of the upp...

Formation of InAs quantum dot arrays on GaAs (100) by self-organized anisotropic strain engineering of a (In,Ga)As superlattice template

We demonstrate the formation of well-defined InAs quantum dot (QD) arrays by self-organized engineering of anisotropic strain in a (In,Ga)As/GaAs superlattice (SL). Due to the accumulation and improvement of the uniformity of the strain-field modulation along [011], formation of InAs QD arrays along [0-11] with 140 nm lateral periodicity is clearly observed on the SL template when the number of SL periods is larger than ten. By enhancing the In adatom surface migration length at low growth rates, clear arrays of single InAs QDs are obtained. The QD arrays exhibit strong photoluminescence efficiency that is not reduced compared to that from InAs QD layers on GaAs. Hence, ordering by self-organized anisotropic strain engineering maintains the high structural quality of InAs QDs.

Localized destructive interference in X-ray specular reflectivity

Localized destructive interference can be obtained for specific values of the angle of incidence when studying semiconductor superlattices with X-ray reflectivity at fixed wavelength. Kinematical calculations show that the difference between the real parts of the refractive index of the layers must be small, and that the number of superlattice periods must be optimized to enhance the destructive interference. An experimental example, involving a CdTe/CdZnTe superlattice, and optical matrix simulations for SiGe/Si superlattices are presented to illustrate this effect. Finally, it is shown that such structures can be tailored to act as energy filters at fixed angles of incidence.

Spikes in the Current Self-Oscillations of Doped GaAs/AlAs Superlattices

In doped, weakly coupled superlattices, current self-oscillations occur due to a recycling motion of a charge monopole, which forms the boundary between two electric field domains. In addition to these fundamental oscillations, the current also contains a high-frequency modulation in form of spikes, which are caused by the tunneling process of the charge monopole over a single superlattice period. The number of spikes within one fundamental oscillation can therefore serve as a measure for the number of superlattice periods, which are involved in the recycling motion. Spiking may reflect non-stationary tunneling effects for sufficiently large applied electric fields.

Morphological and compositional variations in strain-compensated InGaAsP/InGaP superlattices

We have investigated the properties of strain-compensated InGaAsP/InGaP superlattices, grown by metalorganic vapor phase epitaxy, with and without InP interlayers inserted in the InGaP barrier. Using cross-sectional scanning tunneling microscopy and spectroscopy, we observe lateral variations in layer thickness and electronic properties. When the number of superlattice periods is increased from 8 to 16, the growth front develops large undulations in the top two to four superlattice periods. For structures with InP layers inserted in the InGaP barrier, only slight undulations of the top superlattice periods occur. We discuss the origins of the growth front undulations in terms of the elastic relaxation of strain arising from thickness and/or composition variations in the superlattice layers. Finally, we observe a fourfold periodicity of the [001] atomic spacing, presumably arising from atomic ordering in the alloys.

New Relaxation Mechanism in Short Period Si/Ge Strained-Layer Superlattices

AbstractHigh quality Si/Ge strained-layer superlattices composed of a sequence of alternating 3 monolayers pure Si and 9 monolayers pure Ge have been grown by molecular beam epitaxy at 310°C on Ge(001) substrates. In order to investigate the transition from coherent to incoherent growth in these tensily strained structures a set of samples with varying number of superlattice periods has been studied by transmission electron microscopy. It is found that superlattices as thick as 33 nm at least show perfect and defect-free layer growth whereas for thicker superlattices strain accommodation occurs. For this strained heteroepitaxial system we observed, to our knowledge, for the first time the formation of microtwins as the only relaxation mechanism. High-resolution lattice imaging reveals that the twin lamellae result from successive glide of 90° (a/6)<112> Shockley partial dislocations on adjacent {111} planes from the surface towards the bulk. The activation barrier which has to be overcome in the case of 90° partial dislocations is compared with the energies required for the nucleation of 60° perfect and 30° partial misfit dislocation half-loops.