Tunable electronic structure and structural transition of GaAs clusters at high pressure and temperature

Salhi A., Alshaibani S., Alaskar Y., Albadri A., Alyamani A.

The optical quality enhancement of a GaAs template grown on Silicon substrate using InGaAs/GaAs Dislocation Filters (DFs) combined with an annealing step have been assessed optically using Photoluminescence (PL) of embedded InAs/InGaAs QDs within a GaAs matrix. An annealing temperature after the growth of the DFs of 690 °C was shown to be optimum, giving an enhanced PL emission from the embedded InAs QDs in terms of intensity and Full Width at Half Maximum (FWHM). InAs quantum dots capped with GaAsSb grown at different temperatures were grown on the optimized GaAs template on Si. The prepared samples were characterized by PL, Atomic Force Microscopy (AFM), excitation power and dependent PL at 77 K and 300 K. At 77 K, the InAs/GaAsSb grown at 484 °C showed a type II band alignment with an emission wavelength of 1297 nm, which is shorter than the emission obtained from the reference sample, where the GaAsSb-capped dots were grown on GaAs (1357 nm). By reducing the GaAsSb capping layer growth temperatures to 465 °C and 450 °C, the wavelength at 77 K was extended to 1375 nm and 1480 nm respectively, resulting from an increased Sb content in the capping layer. At 300 K, a long wavelength emission of 1623 nm have been achieved.

Schlaepfer F., Lucchini M., Sato S.A., Volkov M., Kasmi L., Hartmann N., Rubio A., Gallmann L., Keller U.

Resolving the fundamental carrier dynamics induced in solids by strong electric fields is essential for future applications, ranging from nanoscale transistors1,2 to high-speed electro-optical switches3. How fast and at what rate can electrons be injected into the conduction band of a solid? Here, we investigate the sub-femtosecond response of GaAs induced by resonant intense near-infrared laser pulses using attosecond transient absorption spectroscopy. In particular, we unravel the distinct role of intra- versus interband transitions. Surprisingly, we found that despite the resonant driving laser, the optical response during the light–matter interaction is dominated by intraband motion. Furthermore, we observed that the coupling between the two mechanisms results in a significant enhancement of the carrier injection from the valence into the conduction band. This is especially unexpected as the intraband mechanism itself can accelerate carriers only within the same band. This physical phenomenon could be used to control ultrafast carrier excitation and boost injection rates in electronic switches in the petahertz regime. Significant enhancement of carrier injection into the conduction band is observed for GaAs subjected to intense resonant near-infrared laser pumping. Attosecond-resolved investigation reveals the interplay between the intra- and interband transitions.

Ono S., Kikegawa T.

The high-pressure behavior of gallium arsenide, GaAs, has been investigated using an in-situ X-ray powder diffraction technique in a diamond anvil cell combined with a resistance heating method, at pressures and temperatures up to 25 GPa and 1000 K respectively. The pressure-induced phase transition from a zincblende to an orthorhombic ( Cmcm ) structure was observed. This transition occurred at 17.3 GPa and at room temperature, where a negative temperature dependence for this transition was confirmed. The transition boundary was determined to be P (GPa) = 18.0 − 0.0025 × T (K).

Li Q., Shen K., Yang R., Zhao Y., Lu S., Wang R., Dong J., Wang D.

Comparative study of GaAs and CdTe solar cell under low-intensity light irradiance was carried out to study the cell device performance in response to the changed light irradiance intensity. For highly efficient GaAs solar cell, the series Rs and the shunt Rp resistance were found to be low/high enough to have almost undetectable negative effect on the cell device performance at low-intensity light irradiance. The robust diode parameters guaranteed good cell performance at low-intensity light irradiance. An ideal logarithmic function was established to describe the variation of cell efficiency with light irradiance intensity for GaAs solar cell. For CdTe polycrystalline solar cell, the relatively large series resistance Rs and small shunt resistance Rp, compared to that of the GaAs solar cell, significantly decreased the cell fill factor. With increased light illumination intensity, both the diode ideality factor A and the reverse saturation current density J0 were increased due to the high density of interface states at the CdS/CdTe junction. The comparative study and conclusions drawn in this work provide device fabrication improvement direction for the CdTe solar cell.

Kurban M., Erkoç Ş.

Al-Ghamdi M.S., Sayari A., Sfaxi L.

We present a study of the optical properties of InAs quantum dot (QD) solar cells (SC) grown by molecular beam epitaxy on GaAs(11N)A orientation substrates (N = 4, 5). Optical properties of InAs/GaAs (11N)A QD SCs were characterized by spectroscopic ellipsometry (SE), photoluminescence (PL) and photocurrent (PC) measurements. From SE data, optical constants are calculated and the different transition energies for the two SCs are calculated and identified in the energy range 1–6 eV. The surface and volume energy loss functions are calculated and show different dependences on the incident photon energy for the two InAs/GaAs QD SCs in the energy interval 1–6 eV. PL measurements revealed good optical properties of the InAs QD SC grown on GaAs(115)A compared to that grown on GaAs(114)A orientation substrate. Two additional peaks are observed at 1.17 and 1.34 eV in the PL spectrum of the SC grown on the (115)A direction and are attributed to the transitions between excited states in the InAs QDs. Photocurrent measurements show that above GaAs bandgap, higher photocurrent is measured for the sample grown on the (114)A orientation. This was attributed to the multiple exciton generation in InAs QDs since in this sample the built-in field is strong enough to accelerate photocarriers generated in GaAs.

Saghrouni H., Jomni S., cherif A., Belgacem W., Beji L.

This paper describes the electrical and dielectric characteristics for the first time of the high-k Dy 2 O 3 oxide film deposited on the porous GaAs substrate by electron beam deposition under ultra vacuum. Morphological characterization is investigated by atomic force microscopy (AFM). The electrical and dielectric properties of Co/Au/Dy 2 O 3 /n-porous GaAs structure were studied in the temperature range of 80–500 K. The conductance and capacitance measurements were performed as a function of bias voltage and frequency. The dielectric constant (e′), dielectric loss (e″) and dielectric loss tangent (tanδ) of the structure are obtained from capacitance–voltage (C–V) and conductance–voltage (G/ω–V) measurements. These parameters are found to be strong functions of temperature and bias voltage. In the forward bias region, C–V plots show a negative capacitance (NC) behavior, e′–V plots for each temperature value take negative values as well. Such negative values of C correspond to the maximum of the conductance (G/ω). The negative capacitance values appear abnormal when compared to the conventional behavior of ideal Schottky barrier diode (SBD) and metal–oxide–semiconductor (MOS) structures. The following behavior of the C and e′ in the forward bias region has been explained with the minority-carrier injection and relaxation theory. From DC conductance study, electronic conduction is found to be dominated by thermally activated hopping at high temperature. Activation energy is deduced from the variation of conductance with temperature. The Nyquist plots exhibited single semi-circular arcs which were well fitted to an equivalent circuit.

Smida A., Laatar F., Hassen M., Ezzaouia H.

This paper consists to present first results concerning the structure of porous GaAs layer (por-GaAs-L) prepared by using HF/HNO 3 as acidic solution in vapor etching (VE) method. In order to clarify this method, we detail here its principle and explain how por-GaAs-Ls are formed, taking into account the influencing of the exposure time of the GaAs substrate to the acid vapor. The etched GaAs layers have been investigated by UV–visible and PL analysis. One porous layer was performed to be characterised by Atomic Force Microscopy (AFM), FTIR spectroscopy, and X-Ray Diffraction (XRD). The porous structure was constituted by a nanocrystals with an average size about 6 nm. These nanocrystals were calculated from XRD peak using Scherrer׳s formula, AFM imaging, and also by using effective mass approximation model from effective band gap.

Kurban M., Barış Malcıoğlu O., Erkoç Ş.

A molecular dynamics simulations using a recently developed CdZnTe bond order potential is carried out to study structural and thermodynamical properties of the CdZnTe spherical-like ternary nanoparticles with 167–357 atoms in the temperature range 100 K–600 K. The heat capacity calculation is performed as depending the size and the stoichiometry at various temperatures using a non-equilibrated molecular dynamics simulation strategy. Furthermore, the segregation phenomena of Cd, Zn, and Te atoms in the Cd–Zn–Te nanoparticles are investigated by calculating the order parameter R depending on nanoparticle size and temperature. The radial distribution function has also been calculated for the Cd 0.50 Zn 0.50 Te nanoparticle with 357 atoms at 100 K and 600 K.

Syum Z., Woldeghebriel H.

The stable structure of (GaAs)3 and (GaAs)7 has been found by the first principle calculation previously. Here we use ultra-soft pseudo potentials with plane wave methods within the framework of the density functional approach to investigate the partial charge density and bonding mechanism of the pristine and doped clusters. We found that the electronic states at the vicinity of the Fermi level come mainly from p states with very small contributions from s states. Study of the orbital charge density of the doped clusters reveals that conduction is possible through molecular orbitals other than the lowest unoccupied molecular orbital level. The pristine cluster and doped clusters have Fermi level below the midgap, that is near to the HOMO level but slight difference has been observed and there is a band shift in energy levels of the doped clusters towards the higher energy level.

Moussa R., Abdiche A., Abbar B., Guemou M., Riane R., Murtaza G., Omran S.B., Khenata R., Soyalp F.

The structural, electronic and optical properties of the GaAs1−x P x ternary alloys together with their binary GaP and GaAs compounds were investigated in the zinc-blende (ZB) phase using the density functional theory. The lattice constant of the GaAs compound decreases while its bulk modulus increases when the doping concentration of the P dopant is increased. In addition, both parameters (lattice constant and bulk modulus) show small deviations from the linear concentration dependence. The energy band gap of the GaAs compound is of the direct nature, which increases with the increase in the P dopant concentration, whereas at higher P dopant concentration, the band gap shifts from direct to indirect character. On the other hand, the hydrostatic pressure has a significant effect on the band structure of the investigated compounds where the binary GaAs compound changes from a direct band gap semiconductor to an indirect band gap semiconductor at P ≥ 5 GPa. Furthermore, the pressure-dependence of the optical properties of the GaAs, GaP and GaAs0.75P0.25 alloy were also investigated, where the calculated zero frequency refractive index and the dielectric function are also compared with the experimental results as well as with different empirical models.

Pluengphon P., Bovornratanaraks T., Vannarat S., Pinsook U.

Ab initio calculations were performed for investigating the high pressure phases of GaAs up to 200 GPa. By comparing the minimum free energies of structures, we found the thermodynamically stable phases of GaAs under pressure beyond GaAs-III (Imm2) with space groups Pmma and P4/nmm at the pressure range of 88–146 GPa and 146–200 GPa, respectively. For discussing the difference results of GaAs IV and V in previous studies, we found that Pmma and P4/nmm are the lower symmetric phases of P6/mmm and CsCl-like, respectively. For analyzing the Pmma→P4/nmm phase transition, we observed the approximated path and found that the barrier of transformation from Pmma to P4/nmm in direction [110] is 0.035 eV. The graph of density of states shows no energy gap in stable phases at 130 and 160 GPa, indicating that Pmma and P4/nmm are the metallic phases. The contour plots of the electron density difference show some valence electron sharing in Pmma which is higher than in P4/nmm. Moreover, the results of elastic parameters and modulus ratio suggested that the Pmma phase is a ductile material, while the P4/nmm phase is a brittle due to the increasing of shear modulus.

Graf A., Sonnenberg D., Paulava V., Schliwa A., Heyn C., Hansen W.

Sallen G., Kunz S., Amand T., Bouet L., Kuroda T., Mano T., Paget D., Krebs O., Marie X., Sakoda K., Urbaszek B.

Optical and electrical control of the nuclear spin system allows enhancing the sensitivity of NMR applications and spin-based information storage and processing. Dynamic nuclear polarization in semiconductors is commonly achieved in the presence of a stabilizing external magnetic field. Here we report efficient optical pumping of nuclear spins at zero magnetic field in strain-free GaAs quantum dots. The strong interaction of a single, optically injected electron spin with the nuclear spins acts as a stabilizing, effective magnetic field (Knight field) on the nuclei. We optically tune the Knight field amplitude and direction. In combination with a small transverse magnetic field, we are able to control the longitudinal and transverse components of the nuclear spin polarization in the absence of lattice strain—that is, in dots with strongly reduced static nuclear quadrupole effects, as reproduced by our model calculations. Optical control of nuclear spin polarization in semiconductor quantum dots is promising for applications in NMR imaging. Sallen et al.report efficient dynamic nuclear polarization at zero magnetic field in strain-free gallium arsenide quantum dots with Knight fields dominating the nuclear quadrupole effects.

Othman M., Kasap E., Korozlu N.

The structural, electronic and optical properties of In x Ga 1 −x As, GaAs 1− y P y ternary and In x Ga 1 −x As 1− y P y quaternary semiconductor alloys are investigated using first-principles plane-wave pseudo-potential method within the LDA approximations. For these alloys lattice parameters, bulk modulus, band gap energy and density of states are calculated. Besides, we have calculated the optical parameters (dielectric functions, energy loss function, reflectivity, absorption and refractive index) of these semiconductor alloys. Our results agree well with the available theoretical and experimental data in the literature.

Bhowmick M., Sellan D., Johnson K., Mikolaichik K., Zhou X., Das A., Ramkumar C., Magill B.A., Smith N.W., Khodaparast G.A.

Bandgap engineering through the control of strain can be accomplished through many means, such as applying strain epitaxially that typically requires complicated growth and/or material processing. An alternative is applying a one-time shock wave to GaAs, transforming the material to a precisely controllable, permanently pressurized structure. In this study, laser-driven flyer plate experiments were conducted in crystalline GaAs to probe permanent structural changes induced by shock compression. Detailed characterizations after the shock were conducted at the center of the compression through photoluminescence (PL), time-resolved PL, x-ray diffraction (XRD), and Raman spectroscopy. While PL peak positions shifted nonlinearly with increasing pressure, the peak linewidths increased linearly. PL lifetimes shortened with the application of pressure, showing evidence of more defects. The XRD displayed peak shifts and peak broadening with increasing pressure, with a possible systematic decrease in crystallite sizes. The observed transverse optical (TO) and longitudinal optical Raman modes showed nonlinear peak shifts, while the TO mode exhibited linewidth broadening. The results indicate the presence of strong disorders and a high degree of inhomogeneous strain in the shocked material. The compressed GaAs appear to have highly localized regions with smaller grains.

Huang J., Liu Y., Huang T., Liu S., Wu A.

Abstract: Two polymer particles have been prepared by the reaction of N, N, N', N'-tetrakis(4- aminophenyl)-1,4-benzenediamine, 4, 4-biphenyldialdehyde and isophthalaldehyde, and characterized by SEM, FTIR and XRD. Based on methylene blue as the model pollutant, the adsorption properties of two polymer particles have been observed by using different adsorbent dosages, adsorption times and adsorption temperatures. Experimental data show that the removal rates of methylene blue wastewater are 74 % for polymer A and 68 % for polymer B, and the removal rates are up to 84% for polymer A and 74 % for polymer B after photo-catalytic treatment. All these suggest that the adsorption performance of polymer A is more excellent than that of polymer B. In addition, the methylene blue adsorption of the two Schiff base products conforms to the Freundlich adsorption isothermal model.

Peng J., Tikhonov E.

Han Y., Zhang S., Wang Z., Ji X., Cheng J.

The structural evolution, stabilities, charges transfer, and bonding nature of neutral RuSin(n = 3–13) clusters are calculated by using CALYPSO structure prediction combined with B3LYP method. Optimized geometries for RuSin clusters displayed that the Ru atom tends to adhere to the surface of the silicon clusters for n = 3–9, while Ru atom falls into the interior of Sin cage when n = 10. The discussions of stabilities in depth indicated that the RuSi12 is the superatom with good chemical stability, which could be utilized as building blocks for novel Si-based semiconductor materials. The analysis of chemical bonding shows that the Si–Si and Ru-Si σ-bonds have clearly effect the stability of D6h RuSi12 cluster.

Lu W., Hao K., Liu S., Lv J., Zhou M., Gao P.

Abstract Polynitrogen compounds have been intensively studied for potential applications as high energy density materials, especially in energy and military fields. Here, using the swarm intelligence algorithm in combination with first-principles calculations, we systematically explored the variable stoichiometries of yttrium–nitrogen compounds on the nitrogen-rich regime at high pressure, where a new stable phase of YN10 adopting I4/m symmetry was discovered at the pressure of 35 GPa and showed metallic character from the analysis of electronic properties. In YN10, all the nitrogen atoms were sp 2-hybridized in the form of N5 ring. Furthermore, the gravimetric and volumetric energy densities were estimated to be 3.05 kJ g−1 and 9.27 kJ cm−1 respectively. Particularly, the calculated detonation velocity and pressure of YN10 (12.0 km s−1, 82.7 GPa) was higher than that of TNT (6.9 km s−1, 19.0 GPa) and HMX (9.1 km s−1, 39.3 GPa), making it a potential candidate as a high-energy-density material.

Kurban H., Alaei S., Kurban M.

In this work, we perform a theoretical analysis of structural, electronic, and optical properties of pure and Mg-doped amorphous ZnO nanoparticles (a-ZnO NPs) using DFTB method. Our results show that Zn atoms are more preferential for Mg atoms than for O atoms because the number of Mg Zn bonds is greater than that of Mg O. The rise in the content of Mg in a-ZnO NPs leads to an increase of Mg–Zn and Mg O interactions. Mg atoms prefer to locate near the center of a-ZnO NP, but Zn and O atoms nearly preserve their positions which is compatible with radial distribution function peaks. The orbital energies display a decrease in the energy gap from 3.592 to 3.546 eV while increasing Mg content. The LUMO level is also significantly shifted to higher energies. The results also reveal that the performance of pure a-ZnO NP can be enhanced with a subsequent increase in Mg content.

Kurban H., Dalkilic M., Temiz S., Kurban M.

In this study, we perform a theoretical investigation using the density functional tight-binding (DFTB) approach for the structural analysis and electronic structure of anatase, brookite and rutile phase TiO2 nanoparticles (NPs). Our results show that the number of Ti-O bonds is greater than that of O-O, while the number of Ti-Ti bonds is fewer. Thus, large amounts of O atoms prefer to connect to Ti atoms. The increase in the temperature of the NPs contributes to an increase in the interaction of Ti–O bonding, but a decrease in the O-O bonding. The segregation of Ti and O atoms shows that Ti atoms tend to co-locate at the center, while O atoms tend to reside on the surface. Increasing temperature causes a decrease of the bandgap from 3.59 to 2.62 eV for the brookite phase, which is much more energetically favorable compared to the bulk, while it could increase the bandgap from 3.15 to 3.61 eV for anatase phase. For three-phase TiO2 NPs, LUMO and Fermi levels decrease. The HOMO level of anatase phase NP decreases, but it increases for brookite and rutile phase TiO2 nanoparticles. An increase in the temperature contributes to the stabilization of anatase phase TiO2 NP due to a decrease in the HOMO energies.

Coşkun B.

In this study, undoped and Ag-doped ZnO thin films were grown on p-type Si substrates using Sol-Gel spin coating technique at room temperature. Optical properties, such as refractive index (n) and extinction coefficient (k) were determined using the optical data in terms of wavelength and photon energy, respectively. Depending on Ag doping concentration, some changes in optical properties thin films were observed. Also, the frequency and voltage dependence of some dielectric properties of thin films have been evaluated using the admittance spectroscopy method in the wide frequency range of 10 kHz to 1 MHz at room temperature. Experimental results show that the real ( e ' ) and imaginary parts ( e ' ' ) of dielectric constants, loss tangent (tanδ), real ( M ' ) and imaginary ( M ' ' ) parts of electric modulus were strong functions of frequency and applied bias voltage in the depletion region. The loss tangent versus applied bias voltage curves shows an increase with increasing frequencies and shifts to applied forward bias region giving a peak at about 3 V. M ' and M ' ' values decreased with increasing frequency and shifts to reverse bias region. In addition, it was determined that a.c. electrical conductivity (σ∗) values of thin films increased with increasing voltage as a function of dielectric loss.

Kurban H., Kurban M., Dalkılıç M.

We perform a theoretical investigation using the density functional tight-binding (DFTB) approach for the structural analysis and electronic structure of copper hydride (CuH) metallic nanoparticles (NPs) of different size (from 0.7 to 1.6 nm). By increasing the size of CuH NPs, the number of bonds, segregation phenomena and radial distribution function (RDF) of binary Cu-Cu, Cu-H and H H interactions are analyzed using new implementations in R code. The results reveal that the number of Cu-Cu bonds is more than that of Cu-H while the number of H H bonds are the less. Thus, a large amount of H atoms prefers to connect to Cu atoms. The increase in the size of the NPs contributes to their stabilization because of the increase in the interaction of H H bonding. The segregation of Cu and H atoms shows that Cu atoms tend to co-locate at the center, while H atoms tend to reside on the surface. From the density of state (DOS) analysis, CuH NPs shows a metallic character which is compatible with experimental data. HOMO and Fermi levels decrease from -3.555 to -3.443 eV and from -3.510 to -3.441 eV. Herein, an increase in the size contributes to the stabilization of CuH NP due to decrease in the HOMO energies.

Niehaus T.A., Melissen S.T., Aradi B., Vaez Allaei S.M.

We calculate the phonon-dispersion relations of several two-dimensional materials and diamond using the density-functional based tight-binding approach (DFTB). Our goal is to verify if this numerically efficient method provides sufficiently accurate phonon frequencies and group velocities to compute reliable thermoelectric properties. To this end, the results are compared to available DFT results and experimental data. To quantify the accuracy for a given band, a descriptor is introduced that summarizes contributions to the lattice conductivity that are available already in the harmonic approximation. We find that the DFTB predictions depend strongly on the employed repulsive pair-potentials, which are an important prerequisite of this method. For carbon-based materials, accurate pair-potentials are identified and lead to errors of the descriptor that are of the same order as differences between different local and semi-local DFT approaches.