Valence Force Field Research Articles

Composition modulation and local structure in strained diluted GaInNAs nitride alloy thin layers on GaAs substrates

ABSTRACTThe theoretical study reported here is devoted to diluted GaInNAs nitride alloys, and focuses on correlation between local chemical atomic neighborhood and atomic distances. The model used is Valence Force Field approximation, and we model a periodic Ga1−xInxNyAs1−y alloy film deposited on a GaAs substrate. The surface is dimerised (for simplicity we consider 2×1 anion rich surfaces). First starting from a random film, first and second nearest neighbor distances are calculated, and the corresponding histogram drawn. Then a pseudo-annealing process is simulated by allowing the N atoms to choose their optimal location. This pseudo-annealing strongly enhances the number of In-N bonds, in agreement to experimental studies. From this statistical study, fine structure of each peak of the histograms is shown to be not directly related to a given surrounding chemical distribution up to 3rd nearest neighbors. The positions (in distance) of the peaks appear not to be modified by alloy composition nor alloy segregation, which only alter relative intensities and peak shapes. Last, we consider stripes of In-rich and In-poor zones: calculated energy variations show a strong tendency for N atoms to completely desert In-poor zones.

Columnar [001]-oriented nitrogen order in Ga(NAs) and (GaIn)(NAs) alloys

By calculations in the framework of the valence force field method, we show that nitrogen atoms in diluted GaAs1−xNx tend to align along the [001] direction. In quaternary alloys Ga1−yInyAs1−xNx this tendency is observed only in “as-grown” samples, while in the annealed samples nitrogen atoms build more energetically favorable bonds with indium. Experimentally observed inhomogeneous strain profiles in these material systems, as well as their dissolution upon annealing, agree qualitatively with results of the calculations.

Influence of chlorine-substitution in pyrimidine ring on proton donor ability in H-bond and parameters of amino group of 2-amino pyrimidine

The stretching and deformational vibrations of amino group in 2-aminopyrimidine (1), 2-amino-4-chloro-6-methylpyrimidine ( 2) and 2-amino-4,6-dichloropyrimidine ( 3) as well as its 1:1 and 1:2 complexes with different proton acceptors: CH 3CN, dioxane, tetrahydrofurane, dimethylformamide, dimethylsulphoxide and hexamethylphosphoramide were studied by IR spectroscopy and quantum chemical calculations. The influence of temperature, within the range 285–330 K, in CCl 4 solutions, on the first spectral moment M (1), effective band half width 2( M (2)) 1/2 and integrated intensity B, of the stretching vibrations of –NH 2 group of the free and 1:1 H-bonded complexes of compounds 1–3 with proton acceptors was studied. Parameters of linear dependency of the spectral moments on temperature were established. The thermodynamic characteristics of the complex formation process were determined. The vibrational and electrooptic problems, in framework of R NH 2 model of valence force field, were solved and the γ(HNH) angle, force constants K(NH) and electrooptic ∂ μ/∂ q parameters for the free and H-bonded molecules of aminopyrimidines 1–3 were calculated. It was found that formation of 1:1 complexes with proton acceptors led to 10–15% decrease of the force constant K 1(NH) of N H bond directly involved into hydrogen bond formation, and only 1% increase of the force constant K 2(NH) for the “free” N H bond. The comparison of K(NH) force constants of H-bonded complexes of different composition showed that H-bond in 1:1 complexes is stronger than in 1:2 complexes. It was shown, that chlorine-substitution in 4,6-positions of 2-aminopyrimidine significantly influences the donor ability of –NH 2 group in H-bond, as well as spectral, force, electrooptic and thermodynamic characteristics of amino group of bonded molecules. Theoretical calculations at DFT-B3LYP/6-31+G(d,p) level were performed and atom charge redistribution upon hydrogen bond formation was determined. Charge transfer process between the proton donor and proton acceptor molecules, in dependency on the character of proton acceptor molecule was discussed. The results of quantum chemical calculations for free and H-bonded aminopyrimidines correspond to the results of the experiment and calculation in model of valence force field.

Statistical model of ternary group-III nitrides

We have derived a statistical model of zinc-blende ternary group-III nitrides, assuming the crystal lattice distortion due to lattice constant mismatch to make the major contribution to the free energy of mixing of these compounds and using a new methodology based on the numerical calculation of the configuration partition functions of the alloys. As a result, we have found a nonlinear compositional dependence of the interaction parameter for each of the ternary alloys, $\mathrm{AlGaN}$, $\mathrm{InGaN}$, and $\mathrm{AlInN}$, as well as the temperature-dependent contribution to its free energy originated from the configurational dispersion of the mixing energy. The valence-force field simulations made with the Keating potential have provided the model parameters for these compounds. A simple analytical approximation applicable in the whole range of composition variation is suggested for the interaction parameters of the ternary alloys. The theoretical predictions are compared with those obtained by other approaches and with the data on conventional III-V compounds.

Spectral and energy characteristics of dioxybenzenes

In the approximation for the valence force field with quantum-chemical estimation of the intensities of the absorption bands of IR spectra, calculation of the main spectral-structural and energy characteristics of dioxy-benzenes is performed. This made it possible to suggest energetically more favorable conformations and assign the frequencies of normal vibrations to individual types of symmetry.

Simulation of the dynamic properties of Hg2 Hal 2 crystals (Hal=Cl, Br, I)

The dispersion curves for crystal lattices of univalent mercury halides Hg2Hal2 (Hal=Cl, Br, I) are calculated from the experimental frequencies at singular points of the Brillouin zone and the velocities of sound in the framework of the valence-force field model. The results of calculating the dispersion branch of the TA soft mode along the Γ-X direction of the Brillouin zone and the calculated elastic moduli are presented. The calculated values are in good agreement with the experimental data.

Specific structural and compositional properties of (GaIn)(NAs) and their influence on optoelectronic device performance

We have grown (GaIn)(NAs) lattice-matched bulk as well as compressively strained multi-quantum-well structures by metal-organic vapour-phase epitaxy (MOVPE) suitable for either solar cell or laser applications, respectively. By applying a specific novel TEM dark-field technique columnar strain fields, which are possibly caused by chain-like N ordering in the samples, have been detected. Valence force field calculations show that indeed these chains are energetically stable in Ga(NAs). This chain-like ordering can be dissolved in (GaIn)(NAs), however, upon appropriate annealing, as verified experimentally. On the other hand we find that device performance especially of lasers is limited by carbon impurities in the active (GaIn)(NAs) region of the lasers. The strong affinity of N–C results in an enhanced incorporation of C if the N content in the material is increased. The paper also shows the sources of C incorporation in (GaIn)(NAs) MOVPE growth and how its incorporation can possibly be avoided.

X-ray diffraction on laterally modulated (InAs)n∕(AlAs)m short-period superlattices

Lateral composition modulation in InAs∕AlAs short-period superlattices was investigated by x-ray grazing-incidence diffraction and coplanar x-ray diffraction at a “normal” wavelength and at an anomalous wavelength, for which diffraction from the (200) planes does not exhibit a chemical contrast. The experimental data were compared with theoretical simulations assuming that the interfaces consist of a periodic sequence of monoatomic steps. The displacement field in the superlattice was calculated by continuum elasticity and using a valence-force field method. From the fit of the experimental data to the theory, the lengths of individual atomic terraces were determined.

Phonons in Quantum-Dot Quantum Well**The project supported by the Special Funds for the State Key Basic Research Program of China under Grant No. G2000068305 and National Natural Science Foundation of China under Grant No. 60176007

Phonon modes of AlAs/GaAs/AlAs and GaAs/AlAs/metal Pb quantum-dot quantum wells (QDQW's) with the whole scale up to 90 Å are calculated by using valence force field model (VFFM) based on group theory. Their optical frequency spectra are divided into two nonoverlapping bands, the AlAs-like band and the GaAs-like band, originated from and having frequency interval inside the bulk AlAs optical band and bulk GaAs optical band, respectively. The GaAs-LO -like modes of QDQW's that have maximum bulk GaAs-LO parentages in all modes covering the whole frequency region and all symmetries have always A1 symmetry. Its frequency is controllable by adjusting the structure parameters. In AlAs/GaAs/AlAs, it may be controlled to meet any designed frequency in GaAs-like band. The results on GaAs/AlAs/metal Pb QDQW's show the same effect of reducing in interface optical phonons by using the metal/semiconductor interface revealed ever by macroscopic model. The frequency spectra in both GaAs-like and AlAs-like optical phonon bands are independent of the thickness of Pb shell as long as the thickness of Pb shell is no less than 5 Å. Defects at metal/AlAs interface have significant influence to AlAs-like optical modes but have only minor influence to GaAs-like optical modes. All these results are important for the studying of the e-ph interaction in QD structures.

Nearest-neighbor distances in strained thin films of random pseudobinary semiconductor alloys: A calculational methodology

A method, based on macroscopic elastic theory, is presented, which predicts the nearest-neighbor distances in strained pseudobinary semiconductor thin films. The method applies to films grown on the (001) face of substrates with the zincblende or diamond structure. Based on crystallographic and elastic parameters, the bond lengths are determined through minimization of the elastic energy calculated in the framework of the valence force field method. Good agreement with experimental data is obtained by considering only bond stretching terms. The effectiveness of this method is shown in a number of cases taken from literature.

Molecular simulation study of miscibility of ternary and quaternary InGaAlN alloys

Molecular simulations are conducted to determine the limits of miscibility of a valence force field model for zinc-blende-structured In1−x−yGaxAlyN semiconductor alloys. The transition matrix Monte Carlo method is used to calculate the free energy of the model alloys as a function of temperature and alloy composition (considering both x and y ranging from zero to unity). Analysis of the free-energy surface provides values for the upper critical solution temperature of the ternary alloys: InGaN (1550 K), InAlN (2700 K), and GaAlN (195 K). The miscibility envelope of the quaternary alloy is determined at 773 K and 1273 K. The excess properties of the mixtures are calculated, and it is found that the excess entropy is negligible, and the excess enthalpy is nearly independent of temperature. Consequently, regular-solution theory provides a good description of the thermodynamic properties of the alloys, and comparison of the simulation results with the phase behavior previously reported using regular-solution theory finds good agreement. Structural properties of the ternary compounds are examined in terms of the local compositions. For InGaN it is found (surprisingly) that there is a slight preference for In atoms to have Ga atoms rather than other In atoms as neighbors, in comparison to a random mixture. The two other ternary compounds exhibit the expected behavior, in which the (small) deviations from random mixing tend to favor segregation of like atoms. Among the ternaries, GaAlN is found to show the greatest deviations from random mixing.

Effects of Sb N and period on the electronic properties of GaAs/GaInNAsSb superlattices

We have calculated the bond distributions and atom positions of GaAs/GalnNAsSb superlattices using Keating's semiempirical valence force field (VFF) model and Monte Carlo simulation. The electronic structures of the superlattices are calculated using folded spectrum method (FSM) combined with an empirical pseudopotential (EP) proposed by Williamson et al.. The effects of N and Sb on superlattice energy levels are discussed. We find that the deterioration of the optical properties induced by N can be explained by the localization of the conduction-band states around the N atom. The electron and hole effective masses of the superlattices are calculated and compared with the effective masses of the bulk GaAs and GaInAs.

Simulation of InAsSb/InGaAs Quantum Dots for Optical Device Applications

ABSTRACTSelf-assembled InAsSb/InGaAs quantum dots are candidates for optical detectors and emitters in the 2–5 micron band with a wide range of applications for atmospheric chemistry studies. It is known that while the energy band gap of unstrained bulk InAs1−xSbx is smallest for x=0.62, the biaxial strain for bulk InAs1−xSbx grown on In0.53Ga0.47As shifts the energy gap to higher energies and the smallest band gap is reached for x=0.51. The aim of the present study is to examine how the electronic confinement in the quantum dots modifies these simple considerations. We have calculated the electronic structure of lens shaped InAs1−xSbx quantum dots with diameter 37 nm and height 4 nm embedded in a In0.53Ga0.47As matrix of thickness 7 nm and lattice matched to an InP buffer. The relaxed atomic positions were determined by minimizing the elastic energy obtained from a valence force field description of the inter-atomic interaction. The electronic structure was calculated with an empirical tight binding approach. For Sb concentrations larger than x=0.5, it is found that the InSb/ In0.53Ga0.47As heterostructure becomes type II leading to no electron confined in the dot. It is also found that the energy gap decreases with increasing Sb content in contradiction with previous experimental results. A possible explanation is a significant variation is quantum dot size with Sb content.

Correct Evaluation of the Effect of Transverse Effective Charges on Phonons in AlAs Quantum Dots**The project supported by the Special Funds for Major State Basic Research Project of China under Grant No. G20000683 and National Natural Science Foundation of China under Grant No. 60176007

An improved valence force field model (VFFM) is suggested to calculate the phonon modes in both bulk specimens and quantum dots (QDs) of AlAs taking account of the effect of transverse effective charges (TCs) correctly. The resultant dispersions of AlAs bulk phonons are in accord better with the results carefully fitted to the experimental data by using 11-parameters rigid-ion model, than those got by ordinary VFFM, especially in the region of near Γ point. For AlAs QDs, TCs are evaluated bond by bond for each phonon mode of QD and its effect on the change of the force on atoms is taken into account to modify further the phonon spectrum. The frequency spectra and densities of phonon states of different irreducible representations calculated by using improved VFFM are compared with the results of ordinary VFFM. The correct evaluation of the TCs is not only important in calculating the phonon spectrum of both bulk and QD specimens accurately, but is also in the further discussion of the electron-phonon (e-ph) interaction, which can be directly related to TCs of ions in QD.

Tight-binding study of the influence of the strain on the electronic properties of InAs/GaAs quantum dots

We present an atomistic investigation of the influence of strain on the electronic properties of quantum dots (QD's) within the empirical $s p^{3} s^{*}$ tight-binding (ETB) model with interactions up to 2nd nearest neighbors and spin-orbit coupling. Results for the model system of capped pyramid-shaped InAs QD's in GaAs, with supercells containing $10^{5}$ atoms are presented and compared with previous empirical pseudopotential results. The good agreement shows that ETB is a reliable alternative for an atomistic treatment. The strain is incorporated through the atomistic valence force field model. The ETB treatment allows for the effects of bond length and bond angle deviations from the ideal InAs and GaAs zincblende structure to be selectively removed from the electronic-structure calculation, giving quantitative information on the importance of strain effects on the bound state energies and on the physical origin of the spatial elongation of the wave functions. Effects of dot-dot coupling have also been examined to determine the relative weight of both strain field and wave function overlap.

Near-infrared diode laser spectroscopy of the HCSi radical

The A ̃ 2 Σ +(0 1 0)– X ̃ 2 Π(0 0 0) , A ̃ 2 Σ +(0 1 0)– X ̃ 2 Π(0 1 0) , and A ̃ 2 Σ +(0 2 0)– X ̃ 2 Π(0 1 0) band spectra of HCSi radical were investigated by means of near-infrared diode laser spectroscopy to determine precise molecular constants for the X ̃ 2 Π(0 1 0) and A ̃ 2 Σ +(0 1 0) states. The detailed analysis of the rotationally resolved A ̃ 2 Σ +(0 1 0)– X ̃ 2 Π(0 1 0) band spectra, studied for the first time in the present investigation, leads to the precise determination of molecular constants for the X ̃ 2 Π(0 1 0) state associated with the Renner–Teller interaction. We obtained −0.15126663(53) and 495.00698(30) cm −1 as the Renner–Teller parameter ε and the bending vibrational frequency ω 2, respectively. Based on the molecular constants for the X ̃ 2 Π(0 0 0) and X ̃ 2 Π(0 1 0) states, the rotational levels of the A ̃ 2 Σ +(0 1 0) state were analyzed to obtain molecular constants and information on upper state perturbations. Using the available spectroscopic data, valence force fields for both the X ̃ 2 Π and A ̃ 2 Σ + states were estimated to aid in understanding the vibrational energy levels of the HCSi radical.

Evidence of electron-hole symmetry breaking in poly(p-phenylene vinylene)

We analyze the structural distortions associated with positively and negatively charged states produced upon doping poly (p-phenylene vinylene) (PPV). Vibrational frequencies associated with excitations in these charged states are analyzed on the basis of the valence force field and model-compound approaches. In situ electron paramagnetic resonance measurements confirm the coexistence of paramagnetic and diamagnetic species in segments with different lengths. Moreover, carbon-carbon bond-length deviations calculated for oppositely charged segments with four rings and three vinyl groups $({\mathrm{PV}}_{3}^{2\ensuremath{-}}$ and ${\mathrm{PV}}_{3}^{2+})$ clearly differ. Our results show unambiguously that electron-hole symmetry is not maintained in doped PPV and are able to explain the difference between electron and hole intrachain mobility in PPV.

Vibrational spectra, normal coordinate treatment and simulation of the vibrational spectra of piperazine glyoxime and its Co(III) complex

Newly synthesized Co(III) complexes of piperazine glyoxime (PGO) are examined from the vibrational spectroscopy point of view. A complete interpretation of the vibrational spectra of both the ligand and the complex has been carried out on the basis of normal coordinate analysis. A valence force field has been developed for both of the compounds. The vibrational spectra of the compounds are simulated by a visual basic program prepared to run on an MS Excel data sheet.

Spinodal decomposition range of AlxGa1−xNyAs1−y alloys

We have described a spinodal decomposition range of the low N-content AlxGa1−xNyAs1−y alloys grown on GaAs(001). In our analysis we take into account transformations of the bonds and strain, coherency strain, and elastic energies. The alloys are considered in the simple solution approximation. The strain energy is presented by the interaction parameters between the compounds estimated in the valence-force field model. The spinodal decomposition range of AlxGa1−xNyAs1−y alloys with compositions 0⩽x⩽0.4, y=0.01, 0.02, 0.03, 0.04, and 0.05 are demonstrated up to 1000 °C. It is shown that Al and N increase dramatically the temperature of the phase separation.

FTIR and FT Raman spectra and analysis of poly(4-methyl-1-pentene)

Poly(4-methyl-1-pentene) is the most widely used polymer in industry and medical products. Fourier transform infrared and Raman Spectra of poly(4-methyl-1-pentene) have been recorded in the range of 4000–400 and 4000–100 cm −1, respectively. In the present investigation a detailed assignments of the observed fundamental bands of poly(4-methyl-1-pentene) has been analyzed in terms of peak positions and relative intensities. With the hope of providing more and effective information on the fundamental vibrations, a normal coordinate analysis has also been performed on poly(4-methyl-1-pentene) by assuming Cs symmetry. The simple general valence force field (SGVFF) method has been employed in normal coordinate analysis and the potential energy distribution (PED) has been calculated for each fundamental vibration. The PED contribution corresponding to each of the observed frequencies shows the reliability and accuracy of spectral analysis. The validity of the SGVFF method as a practical tool for complete analysis of vibrational spectra, for poly(4-methyl-1-pentene) is confirmed in the present work.