Electronic structure and optical properties of Sn and SnGe quantum dots

Self-assembled quantum dots in a Si–Ge–Sn system attract research attention as possible direct band gap materials, compatible with Si-based technology, with potential applications in optoelectronics. In this work, the electronic structure near the point and interband optical matrix elements of strained Sn and SnGe quantum dots in a Si or Ge matrix are calculated using the eight-band k·p method, and the competing L-valley conduction band states were found by the effective mass method. The strain distribution in the dots was found with the continuum mechanical model. The parameters required for the k·p or effective mass calculation for Sn were extracted by fitting to the energy band structure calculated by the nonlocal empirical pseudopotential method. The calculations show that the self-assembled Sn/Si dots, sized between 4 and 12 nm, have indirect interband transition energies between 0.8 and 0.4 eV and direct interband transitions between 2.5 and 2.0 eV. In particular, the actually grown, approximately cylindrical Sn dots in Si with a diameter and height of about 5 nm are calculated to have an indirect transition (to the L valley) of about 0.7 eV, which agrees very well with experimental results. Similar good agreement with the experiment was also found for SnGe dots grown on Si. However, neither of these is predicted to be direct band gap materials, in contrast to some earlier expectations

The DFT Study of Electronic and Optical Properties of the Surface Functional SiGe, GeSn and GeSn Nanostructures

The electronic and optical properties of Si, Ge, and Sn nanostructures are widely studied for various applications, including drug delivery, cell imaging, biosensing and biomedical. This work considers the effect on electronic and optical properties of SiGe, SiSn and GeSn nanostructures by varying the surface functional and the structure size. The considered structures are about spherical-shaped, with a zinc-blende crystal structure, and H, O+H, OH, and NH2-capped. The optimized structures and their absorption energies are calculated by density functional theory (DFT) and time-dependent density functional theory TD-DFT techniques. In all calculations, the B3LYP and 6-31g basis are used for investigation of electronic and optical properties for SiGe nanostructures, while the LanL2DZ is used for SiSn and GeSn nanostructures. The results show that the optical gap depends not only on the size but also on the terminations on the nanostructure surface. This dependence allows for the possibility of electronic and optical gap engineering

The study of structural, morphological and optical properties of (Al, Ga)-doped ZnO: DFT and experimental approaches

ZnO is a widely studied material for several applications, such as a photocatalyst, a working electrode for dye-sensitized solar cells, and for thermoelectric devices. This work studies the effects of an increase in the number of carriers by doping ZnO with Al and Ga. The 6.25 mol% Al-doped ZnO, 6.25 mol% Ga-doped ZnO, and 12.5 mol% (Al, Ga)-co-doped ZnO nanoparticles were prepared using the combustion method. The prepared samples were then characterized by X-ray diffraction, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and UV–visible spectroscopy techniques. Moreover, the density functional theory (DFT) was also employed for computational study of Al and Ga doped ZnO. Optimized crystal structures, density of states (DOS) and band structure of these systems were calculated using Vienna Ab initio Simulation Package code. From this study, Al and Ga are found to play an important role in both the morphology and optical properties of the ZnO: Al and Ga doping can change the band gap and the Fermi level position in the ZnO. The prepared samples were characterized for their thermoelectric properties, and these were also modelled, using BolzTraP code, for ZnO, Al-doped ZnO, Ga-doped ZnO and (Al, Ga)-co-doped ZnO. The Seebeck coefficient, electrical conductivity, relaxation time, electronic thermal conductivity and power factor were all analysed. The experimental and computational results all point in the same direction, indicating that the thermoelectric properties of ZnO change because the semiconductor ZnO transforms into metallic ZnO when doped with Al and Ga. This leads to ZnO showing different thermoelectric properties, particularly Ga-doped ZnO and (Al, Ga)-co doped ZnO: they provide a high electrical conductivity and power factor. Therefore, it is expected that these favorable properties might promote the ZnO to be a potential candidate for improved efficiency thermoelectric devices

A DFT study of C, SiC, Si, and SiGe colloidal quantum dots for bioimaging

Photoluminescence of colloidal nanocrystals or quantum dots has great potential in bioanalysis and diagnostic applications, as well as in optoelectronics. In this work C, SiC, Si, and SiGe colloidal quantum dots are formed based on the diamond structure or zinc blende structure with various diameters. Then, an energy-optimized structure was developed, and the electronic structure was investigated using density functional theory (DFT). The absorption coefficient of the energy spectrum of these dots is studied by employing a time-dependent density functional theory (TD-DFT) method. The calculated geometries indicated that these dots are nearly spherical. The electronic structure reveals that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of energy level can be tuned by changing the quantum dot size, i.e., the energy gaps are reduced when the diameter of these dots is increased. The studied absorption energy reveals that the absorption peak is in the UV-vis range. Moreover, the absorption peak can be engineered, i.e., the absorption wavelength position is blueshifted when the size of the quantum dot is increased, both in the same materials, but in different forms and in the same form of different materials

Improved Thermoelectric Properties of SrTiO3 via (La, Dy and N) Co-Doping: DFT Approach

This work considers the enhancement of the thermoelectric figure of merit, ZT, of SrTiO3 (STO) semiconductors by (La, Dy and N) co-doping. We have focused on SrTiO3 because it is a semiconductor with a high Seebeck coefficient compared to that of metals. It is expected that SrTiO3 can provide a high power factor, because the capability of converting heat into electricity is proportional to the Seebeck coefficient squared. This research aims to improve the thermoelectric performance of SrTiO3 by replacing host atoms by La, Dy and N atoms based on a theoretical approach performed with the Vienna Ab Initio Simulation Package (VASP) code. Here, undoped SrTiO3 , Sr0.875La0.125TiO3 , Sr0.875Dy0.125TiO3 , SrTiO2.958N0.042, Sr0.750La0.125Dy0.125TiO3 and Sr0.875La0.125TiO2.958N0.042 are studied to investigate the influence of La, Dy and N doping on the thermoelectric properties of the SrTiO3 semiconductor. The undoped and La-, Dy- and N-doped STO structures are optimized. Next, the density of states (DOS), band structures, Seebeck coefficient, electrical conductivity per relaxation time, thermal conductivity per relaxation time and figure of merit (ZT) of all the doped systems are studied. From first-principles calculations, STO exhibits a high Seebeck coefficient and high figure of merit. However, metal and nonmetal doping, i.e., (La, N) co-doping, can generate a figure of merit higher than that of undoped STO. Interestingly, La, Dy and N doping can significantly shift the Fermi level and change the DOS of SrTiO3 around the Fermi level, leading to very different thermoelectric properties than those of undoped SrTiO3 . All doped systems considered here show greater electrical conductivity per relaxation time than undoped STO. In particular, (La, N) co-doped STO exhibits the highest ZT of 0.79 at 300 K, and still a high value of 0.77 at 1000 K, as well as high electrical conductivity per relaxation time. This renders it a viable candidate for high-temperature applications

Finite-difference calculation of the electronic structure of artificial graphene, the 2D hexagonal AlwGa1-wAs/GaAs structure with tunable interactions

The energy dispersion relation of two dimensional hexagonal lattice of GaAs quantum wires embedded in AlwGa1-wAs matrix, called artificial graphene, was calculated by the finite difference method with periodic boundary conditions. The validity of the finite difference based code was checked by comparing the bound state energies of various two dimensional systems with appropriate boundary conditions with analytic solutions or the results obtained by COMSOL software, which uses the finite element method, and a very good agreement was found. The energy dispersion relation calculated for artificial graphene structure shows massless Dirac particles, characteristic for real graphene. Therefore, artificial graphene-like structures have properties similar to those of real graphene, and are tailorable by appropriate structure engineering

SiGeSn Ternaries for Efficient Group IV Heterostructure Light Emitters.

SiGeSn ternaries are grown on Ge-buffered Si wafers incorporating Si or Sn contents of up to 15 at%. The ternaries exhibit layer thicknesses up to 600 nm, while maintaining a high crystalline quality. Tuning of stoichiometry and strain, as shown by means of absorption measurements, allows bandgap engineering in the short-wave infrared range of up to about 2.6 µm. Temperature-dependent photoluminescence experiments indicate ternaries near the indirect-to-direct bandgap transition, proving their potential for ternary-based light emitters in the aforementioned optical range. The ternaries' layer relaxation is also monitored to explore their use as strain-relaxed buffers, since they are of interest not only for light emitting diodes investigated in this paper but also for many other optoelectronic and electronic applications. In particular, the authors have epitaxially grown a GeSn/SiGeSn multiquantum well heterostructure, which employs SiGeSn as barrier material to efficiently confine carriers in GeSn wells. Strong room temperature light emission from fabricated light emitting diodes proves the high potential of this heterostructure approach

The quantum confined Stark effect in N-doped ZnO/ZnO/N-doped ZnO nanostructures for infrared and terahertz applications

The terahertz (THz) frequency range is very important in various practical applications, such as terahertz imaging, chemical sensing, biological sensing, high-speed telecommunications, security, and medical applications. Based on the density functional theory (DFT), this work presents electronic and optical properties of N-doped ZnO/ZnO/N-doped ZnO quantum well and quantum wire nanostructures. The density of states (DOS), the band structures, effective masses, and the band offsets of ZnO and N-doped ZnO were calculated as the input parameters for the subsequent modeling of the ZnO/N-doped ZnO heterojunctions. The results show that the energy gaps of the component materials are different, and the conduction and valence band offsets at the ZnO/N-doped ZnO heterojunction give type-II alignment. Furthermore, the optical characteristics of N-doped ZnO/ZnO/N-doped ZnO quantum well were studied by calculating the absorption coefficient from transitions between the confined states in the conduction band under the applied electric field (Stark effect). The results indicate that N-doped ZnO/ZnO/ N-doped ZnO quantum wells, quantum wires, and quantum cascade structures could offer the absorption spectrum tunable in the THz range by varying the electric field and the quantum system size. Therefore, our work indicates the possibility of using ZnO as a promising candidate for infrared and terahertz applications

Non-equilibrium induction of tin in germanium: towards direct bandgap Ge1−xSnx nanowires

The development of non-equilibrium group IV nanoscale alloys is critical to achieving new functionalities, such as the formation of a direct bandgap in a conventional indirect bandgap elemental semiconductor. Here, we describe the fabrication of uniform diameter, direct bandgap Ge1−xSnx alloy nanowires, with a Sn incorporation up to 9.2 at.%, far in excess of the equilibrium solubility of Sn in bulk Ge, through a conventional catalytic bottom-up growth paradigm using noble metal and metal alloy catalysts. Metal alloy catalysts permitted a greater inclusion of Sn in Ge nanowires compared with conventional Au catalysts, when used during vapour–liquid–solid growth. The addition of an annealing step close to the Ge-Sn eutectic temperature (230 °C) during cool-down, further facilitated the excessive dissolution of Sn in the nanowires. Sn was distributed throughout the Ge nanowire lattice with no metallic Sn segregation or precipitation at the surface or within the bulk of the nanowires. The non-equilibrium incorporation of Sn into the Ge nanowires can be understood in terms of a kinetic trapping model for impurity incorporation at the triple-phase boundary during growth

Investigation of carrier confinement in direct bandgap GeSn/SiGeSn 2D and 0D heterostructures

Since the first demonstration of lasing in direct bandgap GeSn semiconductors, the research efforts for the realization of electrically pumped group IV lasers monolithically integrated on Si have significantly intensified. This led to epitaxial studies of GeSn/SiGeSn hetero- and nanostructures, where charge carrier confinement strongly improves the radiative emission properties. Based on recent experimental literature data, in this report we discuss the advantages of GeSn/SiGeSn multi quantum well and quantum dot structures, aiming to propose a roadmap for group IV epitaxy. Calculations based on 8-band k∙p and effective mass method have been performed to determine band discontinuities, the energy difference between Γ- and L-valley conduction band edges, and optical properties such as material gain and optical cross section. The effects of these parameters are systematically analyzed for an experimentally achievable range of Sn (10 to 20 at.%) and Si (1 to 10 at.%) contents, as well as strain values (−1 to 1%). We show that charge carriers can be efficiently confined in the active region of optical devices for experimentally acceptable Sn contents in both multi quantum well and quantum dot configurations