Superlattice Band Gap Research Articles

Study on the tunable band gap of LiNaPb2Br6 superlattice and its optical properties

The property changes between APbBr3 (A = Li, Na) and LiPbBr3/NaPbBr3 superlattice (LiNaPb2Br6), as well as the electronic structure and optical properties of LiNaPb2Br6 under various pressures, have been investigated using the GGA-PBE method based on first-principles. The research shows that the band gap (Eg) value of LiNaPb2Br6 is 1.674 eV at 0 GPa, which is between that of LiPbBr3 and NaPbBr3. Meanwhile, the results indicate that the Eg of LiNaPb2Br6 gradually decreases with the increase in pressure, reaching 0.080 eV at 7 GPa, and its valence band top crosses the Fermi level at 8 GPa. Therefore, by combining the construction of superlattices with the application of pressure, the Eg of LiNaPb2Br6 can be continuously regulated in the range of 0–1.674 eV. Since the optical properties of a material primarily depend on its band structure, the optical properties of LiNaPb2Br6 can be widely adjusted by continuously tuning the Eg of this superlattice. The calculated results demonstrate that the optical properties of LiNaPb2Br6 were optimized in comparison to APbBr3 (A = Li, Na). In addition, the increase in pressure improves the absorption capability of LiNaPb2Br6 in both the visible and ultraviolet regions, thereby expanding the potential applications of LiNaPb2Br6 in the optical field.

Toward Red Light Emitters Based on InGaN-Containing Short-Period Superlattices with InGaN Buffers

In order to shift the light emission of nitride quantum structures towards the red color, the technological problem of low In incorporation in InGaN−based heterostructures has to be solved. To overcome this problem, we consider superlattices grown on InGaN buffers with different In content. Based on the comparison of the calculated ab initio superlattice band gaps with the photoluminescence emission energies obtained from the measurements on the specially designed samples grown by metal-organic vapor phase epitaxy, it is shown that by changing the superlattice parameters and the composition of the buffer structures, the light emission can be shifted to lower energies by about 167 nm (0.72 eV) in comparison to the case of a similar type of superlattices grown on GaN substrate. The importance of using superlattices to achieve red emission and the critical role of the InGaN buffer are demonstrated.

Tin diselenide/zirconium disulfide terahertz acoustic multi-layer superlattice for liquid sensing applications of acetonitrile; reconsidering Voigt-Reuss-Hill schemes

Studying the mechanical properties mismatching of SnSe2/ZrS2 2D materials with Voigt-Reuss-Hill (VRH) schemes promise a novel structure for sensing applications. For the first time, we studied the effect of mechanical mismatching properties of SnSe2/ZrS2 multilayers with VRH schemes on the acoustic wave’s propagation through acoustic superlattice (AS). We proposed [(SnSe2/ZrS2)4] superlattice with VRH schemes as a new THz multichannel sensor for acetonitrile sensing. The [(SnSe2/ZrS2)4] with Voigt scheme introduced several resonant peaks for acetonitrile inside the superlattice bandgap compared to Hill and Reuss schemes. The appropriate relation between resonance frequency shift and resonance bandwidth is the key to improving the multichannel sensor's sensitivity to liquid characteristics. Further, we studied the effect of low and high temperatures on our multichannel sensor. Furthermore, the highest sensitivity and resonance frequency were recorded by our AS with the Voigt scheme towards acetonitrile having values of 0.287 (GHz/(kg/m3)) and 0.237 THz at 0 °C. The results show a novel sensitivity of acetonitrile for low-temperature environments.

Uncovering Topological Edge States in Twisted Bilayer Graphene.

Twisted bilayer graphene (t-BLG) has recently been introduced as a rich physical platform displaying flat electronic bands, strongly correlated states, and unconventional superconductivity. Studies have hinted at an unusual Z2 topology of the moiré Dirac bands of t-BLG. However, direct experimental evidence of this moiré band topology and associated edge states is still lacking. Herein, using superconducting quantum interferometry, we reconstructed the spatial supercurrent distribution in t-BLG Josephson junctions and revealed the presence of edge states located in the superlattice band gaps. The absence of edge conduction in high resistance regions just outside the superlattice band gap confirms that the edge transport originates from the filling of electronic states located inside the band gap and further allows us to exclude several other edge conduction mechanisms. These results confirm the unusual moiré band topology of twisted bilayer graphene and will stimulate further research to explore its consequences.

First-Principles Study of n*AlN/n*ScN Superlattices with High Dielectric Capacity for Energy Storage.

As a paradigm of exploiting electronic-structure engineering on semiconductor superlattices to develop advanced dielectric film materials with high electrical energy storage, the n*AlN/n*ScN superlattices are systematically investigated by first-principles calculations of structural stability, band structure and dielectric polarizability. Electrical energy storage density is evaluated by dielectric permittivity under a high electric field approaching the uppermost critical value determined by a superlattice band gap, which hinges on the constituent layer thickness and crystallographic orientation of superlattices. It is demonstrated that the constituent layer thickness as indicated by larger n and superlattice orientations as in (111) crystallographic plane can be effectively exploited to modify dielectric permittivity and band gap, respectively, and thus promote energy density of electric capacitors. Simultaneously increasing the thicknesses of individual constituent layers maintains adequate band gaps while slightly reducing dielectric polarizability from electronic localization of valence band-edge in ScN constituent layers. The AlN/ScN superlattices oriented in the wurtzite (111) plane acquire higher dielectric energy density due to the significant improvement in electronic band gaps. The present study renders a framework for modifying the band gap and dielectric properties to acquire high energy storage in semiconductor superlattices.

Modeling of (GaP)m/(AlP)n superlattices using empirical tight-binding method

In this paper, we investigated behavior of energy gap and refractive index of (GaP) m /(AlP) n superlattices structure (SLS) grown on GaP (100) substrate in terms of GaP and AlP thicknesses. For this purpose, we used the empirical tight-binding method (ETBM) with sp 3 s* approximation and considering first nearest neighbor and spin-orbit interactions. Since in this method, predicting 15 parameters in individual balk Hamiltonian matrix is very important, we used genetic algorithm to obtain a new set of parameters that can accurately predict the superlattice structures energy gaps with about 1.2% deviation from the experimental data points. We used these parameters and also considered stress, strain and energy band lineups for this SLS to simulate the energy gap and refractive index for a range of GaP and AlP thicknesses. • Introduction of new orbitals interaction parameters for GaP/AlP by using genetic algorithm, which help to have better description in bulk and superlattices. • We compared the result of simulation by experimental values and results with other method and other orbitals interaction parameters. • Prediction of GaP/AlP superlattice bandgap energy and refractive index by varying the thicknesses of both AlP and GaP and extracting a range of bandgap variations.

Strong mid-infrared photoresponse in small-twist-angle bilayer graphene

Recently the small-twist-angle (< 2{\deg}) bilayer graphene has received extraordinary attentions due to its exciting physical properties. Compared with monolayer graphene, the Brillouin zone folding in twisted bilayer graphene (TBG) leads to the formation of superlattice bandgap and significant modification of density of states. However, these emerging properties have rarely been leveraged for the realization of new optoelectronic devices. Here we demonstrate the strong, gate-tunable photoresponse in mid-infrared wavelength range of 5 to 12 um. A maximum extrinsic photoresponsivity of 26 mA/W has been achieved at 12 um when the Fermi level in a 1.81{\deg} TBG is tuned to its superlattice bandgap. Moreover, the strong photoresponse critically depends on the formation of superlattice bandgap, and it vanishes in the gapless case with ultrasmall twist angle (< 0.5{\deg}). Our demonstration reveals the promising optical properties of TBG and provides an alternative material platform for tunable mid-infrared optoelectronics.

Tunable photoelectronic properties of hydrogenated-silicene/halogenated-silicene superlattices for water splitting

We propose a novel two-dimensional (2D) lateral superlattice based on silicene alternately saturated by hydrogen and halogen atoms, named as hydrogenated-silicene/halogenated-silicene superlattices (SHSXSLs, X = F, Cl, Br, and I). Employing ab initio electronic calculations, we systematically investigate the structural, electronic, and optical properties of SHSXSLs. Our results show that these superlattices are much stable than silicene as saturating the dangling bonds and the bandgap and carrier effective mass of superlattices can be modulated by the width of components due to the confinement and interfacial effects. By calculating the band alignment, it is found that all superlattices exhibit the type I alignment. Specifically, SHSCl,BrSLs have very small valence band offset and large conduction band offset, which can separate electrons and holes in these systems. Furthermore, the band edge positions of SHSCl,BrSLs satisfy the redox potential of the water splitting and SHSCl,BrSLs exhibit strong absorption in the visible region, suggesting that these superlattices are promising materials for photovoltaics and photocatalytics. This new type of superlattice structure can be applied to other 2D materials with strong activity to enhance the stability and modulate the properties effectively.

Spectral Phonon Transport Engineering Using Stacked Superlattice Structures

Materials with ultra-low thermal conductivity have important applications in the areas of thermal insulation and energy harvesting. Thermal conductivity of materials can be greatly tuned by the scattering of phonons with miro-/nanostructures. Here, we show a new method to spectrally engineer the phonon transport by using stacked superlattice structures, which is proved by the studies of one-dimensional atomic chain model. The spectral phonon transmission is obtained using atomistic Green’s function approach. The results show that single superlattice can effectively block phonons due to the formed superlattice phonon bandgaps. When multiple superlattices are stacked in series, phonons can be blocked in a much wider spectral range due to the stacked phonon bandgaps. By design, the thermal conductance can be reduced by ten times across a stacked multiple superlattice structure, and the phonon transmission result shows that most phonons can be effectively scattered except for the ones with very low frequency.

ZnO/(Zn)MgO polar and nonpolar superlattices

The bandgaps of short period ZnO/(Zn)MgO superlattices deposited on c-, m-, and a-ZnO substrates were examined both theoretically and experimentally. Ab initio calculations showed that the bandgaps of c-oriented polar superlattices are smaller than those of nonpolar ones; however, this is mainly due to different geometric configurations, because the influence of internal electric fields existing in polar superlattices is not very significant. The calculations revealed that for 5–6 MgO monolayers in the barriers, the bandgap values become independent of the barrier thickness, which suggests that such superlattices can be treated as sets of isolated ZnO wells. In the experimental part of this work, it is demonstrated that short period ZnO/MgO and ZnO/ZnMgO superlattices can be grown successfully on differently oriented crystalline bulk ZnO substrates using molecular beam epitaxy. The bandgaps of the superlattices were determined from low temperature photoluminescence measurements. It is shown that they agree well with the theoretical results.

Structural Engineering of Zinc‐Blend/Wurtzite BN Superlattices

AbstractBy constructing Zinc‐Blende (ZB)/Wurtzite (WZ) boron nitride (BN) (ZB1‐x/WZx‐BN) superlattices (SLs), their electronic structure and mechanical properties were investigated by first‐principles techniques based on density functional theory. It is found that the band structure is significantly modulated by the ratio of WZ phase in ZB/WZ‐BN SLs. With the increase of WZ ratio, the band gap of SLs decreases and then increases dramatically. More importantly, transformation from type‐I to type‐II band alignment between ZB and WZ can be tuned by adjusting the ratio of WZ to be &gt;0.6. In addition, a universal expression of the band gap is proposed to reveal a nonlinear dependence of the band gap within the structure of SLs as induced by from the interface effect achieved via structural transformation. The elastic constants results revealed that the change of ZB/WZ ratio poses no significant effect on the mechanical properties. The effect of structure modulation on band structure and mechanical properties is significantly important for future engineering of novel semiconductor SLs.

Characterization of Ge Quantum Dot Optical Waveguides for High Speed Optical Modulators

Quantum Dot (QD) Optical Modulators can provide high speed modulation in low cost indirect bandgap materials. Si based optical modulators can be realized with the inclusion of self-assembled Ge QDs to provide low cost, high speed CMOS compatible optical devices. In this paper, we present the optical characterization of a novel Ge-QD Si-SiO2 based waveguide for use in as an optical modulator. Optical performance figures of merit are investigated including insertion loss (IL) measurements, and Wavelength dependent loss (WDL). We present a multimode waveguide fabricated with conventional CMOS processing. The waveguide provides 4.43dB/cm loss and individual discrete absorption regimes corresponding to the unique minibands produced by superlattice properties of the self-assembled Ge QDs in the IR regime. Absorption properties of the Ge QDs are demonstrated and verified against the QD superlattice bandgap model. Analysis and simulation is presented to qualitatively compare the QD bandgap energies with the reported optical properties. The QD functionalized structure demonstrates the fundamental optical principles of a QD waveguide, setting the foundation for a active modulation testing of this QD based optical modulator.

Enhancement of Seebeck coefficient in graphene superlattices by electron filtering technique

We show theoretically that the Seebeck coefficient and the thermoelectric figure of merit can be increased by using electron filtering technique in graphene superlattice based thermoelectric devices. The average Seebeck coefficient for graphene-based thermoelectric devices is proportional to the integral of the distribution of Seebeck coefficient versus energy of electrons. The low energy electrons in the distribution curve are found to reduce the average Seebeck coefficient as their contribution is negative. We show that, with electron energy filtering technique using multiple graphene superlattice heterostructures, the low energy electrons can be filtered out and the Seebeck coefficient can be increased. The multiple graphene superlattice heterostructures can be formed by graphene superlattices with different periodic electric potentials applied above the superlattice. The overall electronic band gap of the multiple heterostructures is dependent upon the individual band gap of the graphene superlattices and can be tuned by varying the periodic electric potentials. The overall electronic band gap of the multiple heterostructures has to be properly chosen such that, the low energy electrons which cause negative Seebeck distribution in single graphene superlattice thermoelectric devices fall within the overall band gap formed by the multiple heterostructures. Although the electrical conductance is decreased in this technique reducing the thermoelectric figure of merit, the overall figure of merit is increased due to huge increase in Seebeck coefficient and its square dependency upon the Seebeck coefficient. This is an easy technique to make graphene superlattice based thermoelectric devices more efficient and has the potential to significantly improve the technology of energy harvesting and sensors.

InAs/GaSb Type-II superlattice photodiode array inter-pixel region blue-shift by femtosecond (fs) laser anneal

A post-growth blue-shift in the band gap of an undoped InAs/GaSb Type-II superlattice (5.5 μm cutoff wavelength), as a result of 775 nm, 150 fs laser annealing, is presented. A band gap blue-shift of ∼72 meV in the - and p-layer etched inter-pixel region, laser annealed superlattice is achieved. Using an inter-diffusion model, the dominant group-III and group-V diffusion coefficients are found to be and respectively. Confirmation of the unaltered condition of the superlattice in a Ti/Au masked pixel area establishes this blue-shifted superlattice band gap to be the reason behind the improved inter-pixel isolation resulting from the fs laser annealing technique.

Tunable energy gaps and mid-infrared optical properties in InAs/GaSb type-II superlattices

We investigate on the infrared (IR) optoelectronic properties in short-period InAs/GaSb type-II superlattices (SLs) by a modified eight-band [Formula: see text] model. The electronic mini-band structures for such SLs are evaluated by the modified eight-band [Formula: see text] model, incorporating the microscopic interface effect. We find that with varying the values around 20/25 Å for the InAs/GaSb layer widths, the tunable mid-IR bandgaps can be achieved effectively. The SL bandgap from 275 to 346 meV can be achieved by decreasing the InAs layer thickness from 23 to 17 Å at a fixed GaSb layer thickness of 24 Å, or from 254 to 313 meV by increasing the GaSb layer thickness from 18 to 27 Å at a fixed InAs layer thickness of 21 Å. Correspondingly the optical absorptions in such systems can be tuned evidently. Our theoretical results are in good agreement with experimental data over a series of SL samples. This study confirms further that short-period InAs/GaSb type-II SLs are of great importance for IR applications.

A designing principle for low dark-current strained layer superlattices

A systematic study of native point defect states is carried out to understand the origin of gap states in strained layer superlattices. Using a combination of first principles Hamiltonian, tight-binding Hamiltonian, and Green's function approach, we calculate the native point defect states in bulk InAs, bulk GaSb, and ten InAs-GaSb strained layer superlattices. We observe that the defects in strained layer superlattices located away from the interface create fewer mid gap states if the constituent bulk materials do not have defect states in that energy interval. This can be used as a design principle to shift the superlattice band gap and reduce or eliminate the mid-gap states.

Electronic structure and optical properties of (BeTe)n/(ZnSe)m superlattices

Abstract The structural, electronic and optical properties of (BeTe)n/(ZnSe)m superlattices have been computationally evaluated for different configurations with m = n and m≠n using the full-potential linear muffin-tin method. The exchange and correlation potentials are treated by the local density approximation (LDA). The ground state properties of (BeTe)n/(ZnSe)m binary compounds are determined and compared with the available data. It is found that the superlattice band gaps vary depending on the layers used. The optical constants, including the dielectric function ε(ω), the refractive index n(ω) and the refractivity R(ω), are calculated for radiation energies up to 35 eV.

Strain-modulated Ge superlattices

We present a numerical study of the electronic and optical properties of a model single-element superlattice made of a periodic sequence of relaxed and strained regions of a germanium crystal, realized by means of an externally applied strain. We adopt the tight-binding model to evaluate the strain-driven modifications of the band structure and the optical properties. Superlattice band gaps, spatial confinement of near-gap valence and conduction states, and analysis of their symmetry character, have been obtained for different superlattice periodicities and strain intensities. Our results indicate that, for suitable choices of spatial periodicity and strain values, type-I and direct-gap superlattices, with strong dipole matrix elements, can be realized. Conceptually, we demonstrate that Ge single-element strained superlattices could be active materials for novel Si-compatible optical devices.

Effects of Position and Shape of Atomic Defects on the Band Gap of Graphene Nano Ribbon Superlattices

In this work, we study the behavior of introducing atomic size vacancy in a graphene nanoribbon superlattice. Our investigations are based on the density functional theory (DFT) with the Local Density Approximation in Atomistix Toolkit (ATK). We show that, in addition to its shape, the position of vacancy has a major impact on the electrical properties of a graphene nanoribbon superlattice. We show that the band gap of an armchair graphene nanoribbon may be tuned by introducing an appropriate periodic pattern of vacancies. The band gap changes in a zig-zag manner similar to the variation of band gap of a graphene nanoribbon by changing its width. Keywords—Antidot, Atomistix ToolKit, Superlattice, Vacancy.

Influences of shear deformation on electronic structure and optical properties of B, N doped carbon nanotube superlattices

Carbon nanotubes, one of the most advanced nanoscale materials, have attracted much research attention since they exhibited semiconductor, metal or insulator properties depending on their geometric structures. Carbon nanotubes have great potential in various applications in electronic and optical device. Dopants to the carbon nanotubes intentionally could offer a possible route to change and tune their electronic, optical properties. Another important and effective method is to deform the carbon nanotubes structure. Superlattice structures can offer extra degrees of freedom in designing electronic, optical devices. To understand the involved mechanism, in this paper, the geometry structures, electronic structures and optical properties of the armchair carbon nanotube superlattices doped cyclic alternately with B and N under different shear deformations are investigated by the first-principles method through using the CASTEP code in MS 6.0. It is found that the structures of carbon nanotube superlattices can be dramatically changed by shear deformation. When the shear deformation is less than 9%, the optimization geometry structures of carbon nanotube superlattices are still similar to tubular structures, when the shear deformation is greater than 12%, the geometry structures of these systems have large distortions. The results about the binding energy show that the shear deformation changes the stability of the armchair doped carbon nanotube superlattice. The larger the shear deformation, the lower the stability of the doped carbon nanotube superlattices is. The analysis of charge population show that the covalent bond and ionic bond coexist in the armchair carbon nanotube superlattices doped cyclically alternately with B and N. The band gap of the carbon nanotube superlattice is affected by N, B dopants, as a result, the carbon nanotube superlattice changes from a metal to a semiconductor. Compared with the (5, 5) nanotube superlattices, the band gaps of the (7, 7), (9, 9) doped carbon nanotube superlattices increase. With increasing the shear deformation, the band gap of the doped carbon nanotube superlattices decreases gradually, when the shear deformation is greater than 12%, the band gap changes into 0 eV, the carbon nanotube superlattice changes back into a metal from a semiconductor. The analysis of density of states obtains the same conclusions as the energy band analysis. In optical properties, compared with the armchair carbon nanotube superlattices doped cyclically alternately with B and N without shear deformation, those systems under shear deformation have the peaks of the absorption coefficient and the reflectivity that are all reduced, and are all red-shifted.