Semiconductor Alloys Research Articles

Density Functional Theory Study of the Gas Phase and Surface Reaction Kinetics for the MOVPE Growth of GaAs1-yBiy.

The kinetics of chemical reactions occurring during the metal-organic vapor phase epitaxy (MOVPE) of GaAs1-yBiy have been studied using density functional theory (DFT). GaAs1-yBiy is a metastable semiconductor alloy that has potential applications in high-performance long-wavelength emitters. Its growth is complicated by the low solubility of Bi within the GaAs lattice, which leads to phase segregation under conventional III-V semiconductor growth conditions. In this study, the thermochemical and kinetic parameters of the gas-phase pyrolysis and surface reactions occurring in the MOVPE growth of GaAs1-yBiy from trimethyl bismuth, tertiary butyl arsine, and triethyl gallium are calculated from first-principles electronic structure and vibrational mode calculations. These calculations indicate that the pyrolysis products AsH2 and Bi(CH3)2 are the principle sources for the deposition of their respective metallic elements. The surface-adsorbed methyl species and their interaction with the gas-phase pyrolysis products lead to the self-limiting growth described within this model. The calculated thermochemical and kinetic values provide initial parameters for the development of a microkinetic model of GaAs1-yBiy deposition.

Laser-Driven Phase Segregation and Tailoring of Compositionally Graded Microstructures in Si–Ge Nanoscale Thin Films

The ability to manipulate the composition of semiconductor alloys on demand and at nanometer-scale resolutions is a powerful tool that could be exploited to tune key properties such as the electronic band gap, mobility, and refractive index. However, existing methods to modify the composition involve altering the stoichiometry by temporal or spatial modulation of the process parameters during material growth, limiting the scalability and flexibility for device fabrication. Here, we report a laser processing method for localized tailoring of the composition in amorphous silicon-germanium (a-SiGe) nanoscale thin films on silicon substrates, postdeposition, by controlling phase segregation through the scan speed of the laser-induced molten zone. Laser-driven phase segregation at speeds adjustable from 0.1 to 100 mm s-1 allows access to previously unexplored solidification dynamics. The steady-state spatial distribution of the alloy constituents can be tuned directly by setting the laser scan speed constant to achieve indefinitely long Si1-xGex microstructures, exhibiting the full range of compositions (0 < x < 1). To illustrate the potential, we demonstrate a photodetection application by exploiting the laser-written polycrystalline SiGe microstripes, showing tunability of the optical absorption edge over a wavelength range of 200 nm. Our method can be applied to pseudobinary alloys of ternary semiconductors, metals, ceramics, and organic crystals, which have phase diagrams similar to those of SiGe alloys. This study opens a route for direct laser writing of novel devices made of alloy microstructures with tunable composition profiles, including graded-index waveguides and metasurfaces, multispectral photodetectors, full-spectrum solar cells, and lateral heterostructures.

Growth of dilute quaternary alloy InPNBi and its′ characterization

Abstract InPNBi alloy semiconductor is grown for the first time on InP substrates by liquid phase epitaxy (LPE) technique by adding minute amounts of polycrystalline InN and Bi to the growth melt (upto 3 wt% of InN and 3 wt% of Bi in the melt). Energy dispersive X-ray (EDX) spectroscopy shows the presence of Bi and N in the material. Crystalline quality of the layer and the lattice contraction and dilation with respect to the substrate are demonstrated by high resolution X-Ray diffraction (HRXRD) measurements on layers containing N and Bi in different ratios. This result is qualitatively substantiated from the values of N and Bi contents obtained from the analysis of the X-ray photoelectron spectroscopy (XPS) measurements performed on each layer. XPS further provides details of the N and Bi bonding with In in the lattice. Raman spectroscopy measurements are done on the layers to investigate the different vibrational modes associated with the constituent elements and their bonds. 10 K photoluminescence (PL) indicate band gap reduction in the alloy upto 63 meV due to the incorporation of N and Bi atoms at the P sites of the InP lattice.

Thermoelectric properties of phase separated Ti substituted Zr0.75Hf0.25NiSn0.985Sb0.015 half-Heuslers

Sb is a very effective dopant for ZrNiSn based half-Heusler alloys. The effect of Ti substitution on Zr0.75Hf0.25NiSn0.985Sb0.015 half-Heusler (HH) semiconductor alloys has been investigated to explore the structural modifications and composition variation. TixHf0.25 Zr0.75-xNiSn0.985Sb0.015 (x = 0, 0.15, 0.30, 0.45) alloys were synthesized by induction melting. A set of samples was also annealed for comparative studies. The samples were then sintered using plasma activated sintering (PAS) technique. XRD results confirmed the existence of ZrNiSn type HH compounds. Backscattered electron (BSE) images showed phase separations in the samples. Ti substitution improved the carrier concentration and electrical conductivity of the alloys. Moreover, thermal conductivity was also significantly reduced due to the enhanced phonon scattering. Consequently, a ZT value of 1.11 at 873 K was obtained for 30% Ti substituted (annealed) sample.

Band gap bowings of ternary REN (RE = Sc, Y, La, and Lu) alloys

Structural and electronic properties of ternary alloys of ScN, YN, LaN, and LuN semiconductors have been investigated within the density functional theory. Fully relativistic band structures were obtained with the modified Becke-Johnson approach. Although the lattice parameters of solid solutions of rare-earth elements nitrides exhibit a linear Vegard’s law behavior, the dependences of band gaps on alloys compositions are strongly nonlinear, i.e., substantial band gap bowings are predicted for systems studied. Such strong band gap bowings were not found in any group III nitrides. The Sc/Y/Lu-based materials are indirect band gap (Γ–X) semiconductors. The ternary REN alloys containing La exhibit mainly direct type of band gaps (X–X), which may be completely closed in some Sc1−xLaxN systems. The findings presented in this work suggest that strong band gap bowings in ternary solid solutions of REN semiconductors may enable obtaining band gaps lower than 0.6 eV being characteristic for LaN, which are unavailable in group III nitride materials.

Effects of ligand and chemical affinity of S and Se precursors on the shape, structure and optical properties of ternary CdS1−xSex alloy nanocrystals

Colloidal ternary CdS1−xSex (0 ≤ x ≤ 1) alloy semiconductor nanocrystals (NCs) with spherical and tetrapodal shapes were synthesized successfully by wet chemical method using cadmium oxide (CdO), selenium (Se), and sulfur (S) as precursors and oleic acid (OA), tri-n-octylphosphine (TOP) as ligands in 1-octadecene (ODE) solvent. For the first time, by using TOP and changing Se content, the shape of ternary CdS1−xSex alloy NCs could be tuned from spheres to tetrapods and the crystal structure could be transformed from zincblende (ZB) to wurtzite (WZ). The photoluminescence (PL) spectra of CdS1−xSex alloy NCs synthesized with and without TOP were also completely different, corresponding to one and two excitonic emission peaks. The causes of these interesting results have been discussed.

Highly Mismatched Semiconductor Alloys: From Atoms to Devices

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Space Materials Science in China: I. Experiment Studies under Microgravity

The virtual absence of gravity-dependent phenomena in microgravity allows an in-depth understanding of fundamental events that are normally obscured and therefore are difficult to study quantitatively on Earth. Of particular interest is that the low-gravity environment aboard space provides a unique platform to synthesize alloys of semiconductors with homogeneous composition distributions, on both the macroscopic and microscopic scales, due to the much reduced buoyancy-driven convection. On the other hand, the easy realization of detached solidification in microgravity suppresses the formation of defects such as dislocations and twins, and thereby the crystallographic perfection is greatly increased. Moreover, the microgravity condition offers the possibilities to elucidate the liquid/solid interfacial structures, as well as clarify the microstructure evolution path of the metal alloys (or composites) during the solidification process. Motivated by these facts, growths of compound semiconductors and metal alloys were carried out under microgravity by using the drop tube, or on the scientific platforms of Tiangong-2 and SJ-10. The following illustrates the main results.

Optical Structural and Morphological Studies of Cu0.5Zn0.5Se thin film deposited by chemical bath deposition method

Abstract The Copper Zinc chalcogenides thin films have been considered as an important material for use in optoelectronic devices. These compoundshowever, possess limitations on the ultimate efficiency and stability of devices due to weak nature ZnSebond. Themajorconsequences faced are lattice instability, poor mechanical, optical and electrical proprties.In view of this ,we report, a simple, aqueous chemical route to grow a single range of Cu0.5Zn0.5Se (x= 0.5) alloy semiconductor system on glass substrate at room temperature with special emphasis on their structural , surface morphological , optical , and electrical properties. The XRD study revealed the polycrystalline nature for all composition in a single cubic phase. The film was uniform, adherent, with color varying from faint red for ZnSe to dark red for CuSe .The high transmittance of the film in certain range of the solar spectrum and large band gap make them good material for selective coating and absorber layer for solar cell, respectively.Analysis of absorption spectra gave direct band gap is 1.9 eV.The electrical conductivity and activation energy was found to be a function of composition (x=0.5). Chemical deposition system is useful in depositing Cu0.5Zn0.5Se thin film of desired properties by careful selection of compositional parameter (x = 0.5)

Outlooks for development of silicon nanoparticle memory cells

Phase change memory is based on changes in the optical, electrical or other properties of materials during phase transitions, e.g. an amorphous to crystalline transition. Currently existing and potential applications of this memory are primarily based on multicomponent alloys of metals and semiconductors. However single-component nanoparticles including Si ones are also of interest as promising nanosized memory cells. The potential for developing this type of memory cells is confirmed by the fact that the optical absorption index of bulk amorphous silicon is of the same order of magnitude as that of crystalline silicon. Certainly this phenomenon can hardly be implemented with a single nanoparticle the size of which is within light wavelength. Using molecular dynamics and the Stillinger-Weber potential we have studied the regularities of melting and the conditions of crystallization of silicon nanoparticles containing within 105 atoms. We have shown that cooling of nanosized silicon drops at a 0.2 TK/s rate or higher rates causes their amorphous transition whereas single-component nanosized metallic drops crystallize in molecular dynamics experiments even at a 1 TK/s rate. Further heating of amorphous silicon nanoparticles containing above 5 ∙ 104 atoms causes their crystallization in a specific temperature range from 1300 to 1400 K. We have concluded that there is a possibility of developing phase change memory cells on the basis of the above phase transitions. An amorphous transition of a nanoparticle can be achieved by its melting and further cooling to room temperature at a 0.2 TK/s rate whereas a crystalline transition, by its heating to 1300–1400 K at a 0.2 TK/s rate followed by cooling. Results of molecular dynamics experiments suggest there is a minimum silicon nanoparticle size for which the development of phase change memory cells becomes theoretically impossible at a given temperature change rate. For a 0.2 TK/s temperature change rate this minimum size is 12.4 nm (number of atoms approx. 5 ∙ 104).

Clustering in ZnBVI-rich SrxZn1−xOyBVI1−y (BVI = S, Se, Te) highly mismatched alloys

Abstract The clustering properties of dilute highly mismatched semiconductor alloys attract considerable interest recently. Non-homogeneity of clusters and strong internal strains are the main disadvantages of such alloys. 4O10Sr clustering in ZnBVI-rich SrxZn1−xOyBVI1−y (BVI = S, Se, Te) up to 800 °C are presented. Reduction of the internal strains is a cause of the cluster formation. The cohesive energies, stiffness coefficients, bond stretching and bond bending elastic constants of zinc blende SrS and SrSe are estimated. All or almost all oxygen atoms should be in 4O10Sr clusters up to 800 °C in SrxZn1−xOyS(Se)1−y if alloy content is y > 1 × 10−3 (5 × 10−4) and x > 4y. SrxZn1−xOyTe1−y are completely 4O10Sr cluster ordered with contents from the ultra dilute limit (x > 1 × 10−4, y = 1 × 10−4) up to 800 °C. The densities of 4O10Sr clusters were calculated by minimization of the free energy. The binding energy of the electron bound to the oxygen tetrahedron in ZnTe:O at RT is estimated. The obtained results demonstrate that SrxZn1−xOyBVI1−y are promising semiconductors with the reduced internal strains and identical polyatomic clusters containing oxygen tetrahedrons.

DFT Calculation on the Electronic Structure and Optical Properties of InxGa1-xN Alloy Semiconductors

Using the density functional theory (DFT) of the first principle and Generalized gradient approximation method, the electronic structures and optical properties of the InxGa1-xN crystals with different x (x = 0.25, 0.5, 0.75, 1) have been calculated in this paper. The influence of the electronic structure on the properties has been analyzed. Then the influence of doping quantity on the characteristics has been summarized, which also indicates the trend of complex dielectric function, absorption spectrum and transitivity. With the increase of x, the computational result shows that the optical band gap (i.e.Eg) of the InxGa1-xN crystal tends to be narrow, then the absorption spectrum shifts to the low-energy direction. And the Fermi energy slightly moves to the bottom of conduction band which would cause the growth of conductivity by increasing x. In a word, the InxGa1-xN compound can be achieved theoretically the adjustable Eg and photoelectric performance with x, which will be used in making various optoelectronic devices including solar cell and sensors.

Clustering in Bismide Semiconductor Alloys

Clustering in Bismide Semiconductor Alloys

Structural and electronic properties of dilute-selenide gallium oxide

First-principles density functional theory is applied to investigate the electronic and structural properties of dilute-Se β-Ga2(O1−xSex)3 alloys with the Se-content ranging from 0% to 16.67%. The findings showed that the addition of Se has significant effect on the β-Ga2O3 alloy properties. The equilibrium volume and lattice parameters of β-Ga2(O1−xSex)3 alloys are presented, showing a general expansion with increasing Se-content. Further analysis indicates the expansion rate in the c (001) direction is much larger than that in the a and b directions, in which the information provides important guidance for the manufacturing of the β-Ga2(O1−xSex)3/Ga2O3-based material. From our analysis of the band structures, the β-Ga2(O1−xSex)3 alloys exhibit indirect bandgap property with the bandgap energy decreasing dramatically from 4.868 eV to 2.759 eV. The wavelength derived from the direct bandgap energy covers a regime from 255 nm to 475 nm, implying the potential of β-Ga2(O1−xSex)3 alloys in an ultraviolet photodetector and visible light applications. In addition, electron effective masses are calculated and presented for the β-Ga2(O1−xSex)3 alloys, in which the electron effective mass reduces as the Se-content increases. As a part of a highly mismatched alloy semiconductor class, dilute-Se Ga2(O1−xSex)3 is discussed for the first time with no prior literature in our work, and our findings indicate the potential implementation of GaOSe alloys for electronic and optoelectronic device applications.

Electronic structure theory of unusually matched BAs1-xPx alloys using high-throughput ab-initio computation

BAsP alloys symbolize an appealing class of highly matched ternary III-V semiconductors alloys; do to their unique weak unlikeness in electronegativity and atomic size mismatch between Arsenic (As) and Phosphorus (P). Here using evolutionary algorithm and density functional theory, we explore local atomic structure, thermodynamic, mechanical, dynamical, electronic and band gap variation of BAs-BP system. From out of structure search, tetragonal B2AsP phase (space groupP4‾m2) has been unveiled. The calculated very weak mixing enthalpy for the ordered B2AsP phase, suggest its thermodynamic borderline instability. Analysis of computed elastic constants and phonon spectrum reveal that B2AsP compound is mechanically and dynamically stable. Moreover it is found that the first neighbor cation-anion distances B–As and B–P remain very close to that in the pure binary compounds and corresponds to only ~25% of the change predicted in the virtual crystal approximation. The phonon band structure of B2AsP evinces multiple atypical features such as over bending of the upmost optical phonon branch and a missing LO-TO splitting at zone-centre. Furthermore the Born effective charge of B, As, and P reveals a rather weak dependence with composition. For the electronic properties, we find that the valence band maximum of the ordered B2AsP phase evidence unusual strong coupling between B–As and B–P p-states which reflects strong anomalous B–As and B–P covalent bonds. Finally a nearly vanishing band-gap bowing (b~0.1eV) has been found which results from the small size and chemical differences between As and P atoms.

Chemical Vapor Deposition of Boron‐Incorporated Graphitic Carbon Nitride Film for Carbon‐Based Wide Bandgap Semiconductor Materials

This article discusses the growth of B atoms incorporated into the graphitic carbon nitride (g‐C3N4) thin films on c‐plane sapphire substrates at various growth temperatures by thermal chemical vapor deposition (CVD) using melamine and ammonia borane as precursors. The B incorporation is achieved at a growth temperature of 618 °C, which is slightly higher than the optimal growth temperature of g‐C3N4 thin films. The signal peak for the B 1s core level attributed to B—N bonds is observed by X‐ray photoelectron spectroscopy, indicating the realization of B incorporation into g‐C3N4 films. With an increase in the growth temperature up to 650 °C, a monotonically increasing B composition and a decreasing C composition are observed, implying that B atoms are incorporated into g‐C3N4 by the substitution into C sites. A shift of photoluminescene (PL) peak energy is observed. The PL peak shifts from 2.75 eV for unintentionally doped thin film to 3.60 eV for B‐incorporated thin films with a composition of 8.0%. The B composition dependence of PL peak energies is in good agreement with the quadratic function, suggesting that the bandgap bowing occurs due to B incorporation into the g‐C3N4 thin films as in conventional compound semiconductor alloys.

Ellipsometric analysis of concentration gradients induced in semiconductor crystals by pulsed laser induced epitaxy

The authors report a novel approach to analyze the behavior of pulsed-laser-induced epitaxy on group-IV semiconductors by spectroscopic ellipsometry measurements and finite-element modeling of the thermo- and hydrodynamic behavior. Gradient-composition epitaxial crystals are obtained from previously atomically sharp heteroepitaxial samples through optically monitored processing with UV excimer laser pulses. Spectroscopic ellipsometry is employed to determine the composition gradient of the semiconductor alloy, and the results are correlated with energy-dispersive x-ray measurements. A finite-element modeling approach is developed based on the experimentally monitored melting dynamics and composition gradient results for understanding and prediction of the dynamics.

Photoluminescence properties of Cu-doped CdTeSe alloyed quantum dots versus laser excitation power and temperature

Ternary alloy semiconductors have been proven to be suitable host lattices, which could be doped with transition metals (TM) to control the bandgap energy (Eg) and optoelectronic properties. Through many works performed on TM-doped ternary alloys, their photoluminescence (PL) properties versus the laser excitation power (Pex) and temperature (T) have not been investigated in detail. This work presents a careful study on the effect of Pex and T on the PL properties of zinc-blend Cd1-xCuxTe0.5Se0.5 alloyed quantum dots (QDs) with x = 0, 0.005 and 0.05, which were synthesized by a wet chemical method. At T = 300 K and Pex = 0.1 mW, we observed the PL spectra of x = 0 and 0.005 exhibiting two emissions peaked at about 644 and 820 nm associated with near-band-edge excitons (Ehigh) and lattice defects and Cu2+ ions (Elow), respectively. Meanwhile, the PL spectrum of x = 0.05 shows only one emission (Elow) associated with Cu2+ ions. Interestingly, at 300 K, as increasing Pex from 0.1 to 100 mW, we observed the redshift of Ehigh and the blueshift of Elow. If fixing Pex at 0.1 mW, the study of PL spectra in the range of T = 15–300 K indicated that the emission-peak positions of Ehigh(T) and Elow(T) could be described by Varshni's empirical law while their intensity could be described by the Arrhenius law. Physical mechanisms related to these phenomena were also discussed.

Growth of III-V semiconductor crystals under microgravity

The low-gravity environment aboard the space provides a unique platform for both understanding the crystal-growth-related phenomena that are masked by gravity on the earth and exploring new crystal growth techniques. III-V semiconductor crystal growths were carried out under microgravity and the main results include: device-grade semi-insulating GaAs single crystal with improved stoichiometry was grown under microgravity condition, and low noise field-effect transistors and analog switch integrated circuits were fabricated and the performances were better than their terrestrial counterparts; detached Bridgman growth was realized in two model systems of GaSb and InSb by suppressing the hydrostatic pressure of melt, and largely reduced dislocation densities in the materials were observed; the contributions of buoyancy-driven convection, Marangoni convection and rotation magnetic field forced convection on the microscopic segregation were carefully studied; the vertical gradient freezing method was employed to grow semiconductor alloys and chemically homogeneous GaInSb crystal was obtained. In this review, the main progresses in these aspects are summarized and the future challenges are discussed.

Computational prediction of nanostructured alloys with enhanced thermoelectric properties

The Materials Genome Initiative calls for a dramatic increase in the rate of materials discovery and development. High-throughput (HT) calculations can advance this goal by efficiently screening a large search space for candidate materials to study in more depth. Thermoelectric materials (TEs) are prime candidates for such HT calculations: The properties required to achieve good performance are known, but systematic ways of improving these properties are scarce. Furthermore, known HT methods for TEs only address bulk crystals---screening realistic multicomponent alloys for their TE properties has yet to be accomplished. In this paper, we use a density functional theory driven HT screening-and-sorting procedure to search for new multicomponent bulk-nanostructured thermoelectric materials. We make maximum use of minimal calculations to obtain eight descriptors of the thermodynamics and TE performance of five-element semiconductor alloy systems from combinations of ternary additions in binary compounds. We use these descriptors to reduce a search space of 29 700 five-element systems to a set of 130 candidates. We screen these candidates using TE descriptors to identify several existing high-performance thermoelectrics as well as promising new material systems awaiting further experimental verification.