Self-assembling conditions of 1O4Ca nanoclusters in ZnTe:(Ca,O)

Self-assembling conditions for 1O4Ca tetrahedral nanoclusters in ZnTe:(Ca, O) with Ca content up to 0.02 and with the ultradilute limit of oxygen are represented. The causes of self-assembling are a CaO and ZnTe bonding preferential over a CaTe and ZnO one and decrease of the strain energy after nanocluster formation. Self-assembling conditions were studied in the temperature range from 200 ^°C to 1305 ^°C. The occurrence of nanoclusters depends only on the Ca content and temperature. The temperature of self-assembling completion when all oxygen atoms are in 1O4Ca nanoclusters is determined by the Ca and oxygen contents.     

Self-assembling of C and Sn in Ge

Self-assembling of isoelectronic C and Sn impurities in Ge is predicted. The formation of the 1C4Sn tetrahedral cells is thermodynamically profitable in Ge-rich C_xSn_yGe_1−x−y (4x<y) alloys in the ultra dilute C impurity limit with 1×10^-8x1×10^-3. The concentrations of Sn atoms when all C atoms are surrounded only by Sn atoms are estimated for the lower molecular beam epitaxy, intermediate annealing and higher bulk crystallization temperatures. The origin of this phenomenon is a considerable decrease of the strain energy after self-assembling. The same self-assembling in Si is thermodynamically non-profitable due to the large cohesive energy of Si–C chemical bonds.     

Nanoscale order in ZnS:(Cd,O)

A thermodynamic model of self-assembling of isoelectronic impurities in ZnS:(Cd, O) is presented. Self-assembling results in an occurrence of 1O4Cd tetrahedral clusters. The self-assembling conditions are derived by the minimum condition of the free energy of ZnS:(Cd, O) for Cd and oxygen in the dilute and ultra dilute limits, respectively. The occurrence of 1O4Cd clusters and the completion of self-assembling when all oxygen atoms are in clusters are results of the continuous phase transitions. The temperature of a self-assembling occurrence does not depend on oxygen content and is a function of Cd concentration. The fulfilled estimates show that at temperature of 300 °C all oxygen atoms should be in 1O4Cd clusters if Cd content is insignificantly larger than 1%.     

Nanoscale order in GaAs:(B, Sb)

Nanoscale order caused by self-assembling of 1B4Sb and 4B10Sb clusters in GaAs:(B, Sb) is described. Self-assembling occurs in wide ranges of temperature and impurity concentration. Co-doping with boron and Sb isoelectronic impurities transforms GaAs into GaAs-rich B_xGa_1−xSb_yAs_1−y quaternary alloy. The self-assembling conditions are obtained from 0 to 800 °C with boron and Sb concentrations from x=1×10^–5 to x=2×10^–4 and from y=5×10^–4 to y=0.01, respectively. If Sb content is much larger than that of boron almost all boron atoms are in 1B4Sb clusters up to 800 °C and other boron impurities are isolated. If boron content is nearly equal or larger than that of Sb the formation of 4B10Sb clusters is preferential.     

Interpretation of isomer shifts of substitutional119Sn and129I in group IV semiconductors

The measurements of the isomer shift in Mössbauer effect of substitutionally implanted¹¹⁹Sn and¹²⁹I atoms in group IV semiconductors show that the contact density of 5s electrons at the impurity ńucleus decreases when going from Ge to Si and diamond. A satisfactory interpretation of this dependence can be given based on a simple perturbation approach: Interaction of the host crystal with the bonding valence electrons of the impurity atom causes a charge redistribution of these electrons and results in decrease of occupancy of the 5s level at the impurity atom. Some general aspects of this problem with substitutional isoelectronic and donor impurities are discussed.     

Application of XAFS spectroscopy to studying the microstructure and electronic structure of quantum dots

Variations in the microscopic structural parameters of semiconductor nanoclusters (interatomic distances, coordination numbers, and types of neighboring atoms) under variation of the preparation conditions of Ge/Si, GaN/AlN, and InAs/AlAs heterostructures are determined by the EXAFS-and XANES spectroscopy methods. The effect of preparation conditions on interphase diffusion and structure parameters in semiconductor nanoclusters is revealed. The effect of the temperature of synthesis at different stages, the substrate orientation and pretreatment, the composition of the molecular beam (in particular, the presence of Ge⁺ cations) on the local composition of nanostructures is studied. Relations between the size and shape of nanoparticles and their local spatial characteristics (interatomic distances, stoichiometric composition, and phase boundary characteristics) are examined.     

Heteroborospherene nanoclusters as high-performance nonlinear optical materials

Today, the design of new compounds with giant nonlinear optical responses is attracted to many researchers. Inspired by an interesting finding of a new class of heteroborospherenes which were formed by doping four carbon atoms in the B364- nanocluster (C4B32), we suggest the alkali metal-doped C4B32 (M@C4B32, M=Li, Na, and K) nanoclusters as high-performance nonlinear optical materials. Our results show that the alkali metal atoms have a considerable effect on the structural and electronic properties of the C4B32 nanocluster. We found that the doping alkali metal can remarkably decrease the HOMO-LUMO gap and significantly increases the first hyperpolarizability of the C4B32 nanocluster. Also, our results reveal that the first hyperpolarizability of the M@C4B32 nanoclusters can be progressively enhanced by increasing the atomic number of alkali metals. The effect of external electric fields on the nonlinear optical responses of the M@C4B32 has been systematically explored. We found that the first hyperpolarizability of the M@C4B32 compounds can be gradually increased by increasing the imposed external electric field from zero to the critical external electric field along the charge transfer direction (M→C4B32). Accordingly, this work presents an efficient strategy to improve the nonlinear optical responses of the heteroborospherenes.     

Size effects of semiempirical large unit cell method in comparison with nanoclusters properties of diamond-structured covalent semiconductors

Self-consistent Hartree–Fock method within the framework of large unit cell (LUC) formalism using complete neglect of differential overlap (CNDO) is used to simulate size effects on nanoclusters of covalent crystalline diamond-structured semiconductor C, Si, Ge, and Sn using k=0 approximation. Three sizes are investigated namely 8, 64, and 216 atom LUCs. Cohesive energy, energy gap, valence band width, and hybridization orbitals are obtained from electronic structure calculations. Charge distribution, density of states, and orbital wave functions are also reported. Sensitivity analysis of the dependence of some of the calculated properties on model parameters is performed. Results revealed that electronic properties converge to some limit as the size of the LUC increases and that the 216 atoms LUC is very near to the bulk of these materials. Increasing LUC size or atomic number of covalent semiconductor removed some electronic cloud from bonding region to the spherical region around the atom. The same is true in going to higher energy levels. Increasing the size of LUC also resulted in a decrease of energy gap, increasing valance band width, and generally slightly increasing the cohesive energy. The model predicts energy gap reduction in going from ultra-small nanoclusters to the bulk of the four elements to be around 2 eV. The approximations imposed on the present model calculations leading to resemblance with nanoclusters properties in addition to the evidences of such resemblance such as orbital shapes and trends of data are discussed. Small nanoclusters are expected to have stronger directional bonds than in their bulk structure. The smallest cells (8 atoms) are found to be of a slightly longer lattice constant of the order 0.01, 0.1, 0.04, and 0.02 a.u. for diamond, silicon, germanium, and tin, respectively. X-ray form factors show a slight decrease of low-angle lines when we go to larger cells that was also found experimentally. To the best of my knowledge, this is the first systematic determination of covalent nanoclusters lattice constant and X-ray form factors variation with ultra-small nanocluster size.     

The Si,Ge, Sn,Pb doped (6,3) Chiral single-walled carbon nanotubes: A computational study

Electronic structure properties including bond lengths, bond angles, dipole moments (μ), energies, band gaps, NMR parameters of the isotropic and anisotropic chemical shielding parameters for the sites of various atoms were calculated using the density functional theory for Si, Ge, Sn, Pb doped (6,3) Chiral single-walled carbon nanotubes (SWCNTs). The calculations indicated that average bond lengths were as: Pb3C>Sn3C>Ge3C>Si3C>C3C. The dipole moments for Si, Ge, Sn, Pb doped (6,3) Chiral single-walled carbon nanotubes structures show fairly large changes with respect to the pristine model.     

Ab initio modeling of the electronic and spin properties of the [NV]<Superscript>−</Superscript> centers in diamond nanocrystals

The electronic and spin properties of different nanocrystals of carbon are studied. The properties of these cluster systems are modeled in terms of the ab initio (Hartree-Fock) and semiempirical (PM3, AM1) quantum-chemical methods. The calculations are performed for different carbon nanocluster systems: defect-free and with [NV]^? centers, hydrogen passivated (C₃₈H₄₂, C₇₁H₈₄, C₈₆H₇₈), and with a free (unpassivated) surface (C₃₈, C₇₁, C₈₆). The spin properties of unhydrated nanoclusters were studied for the first time. The structure of all the clusters under study was optimized using the total energy minimization principle. It is shown that, in the case of hydrated carbon nanocrystals passivated by hydrogen atoms, diamond-like clusters are formed. The atomic structure of an unpassivated nanocrystal depends on the number of atoms in the cluster, as well as on its initial geometrical parameters. In some cases, clusters with a fullerene-like surface are formed. In hydrogenpassivated diamond nanocrystals with [NV]^? centers, the spin density is localized at the nuclei of C atoms nearest to the center vacancies. For the unpassivated counterparts, the spin density is localized at the nuclei of C atoms forming the surface of the corresponding nanocrystal.     

On the melting of gold nanoclusters formed by pulsed laser deposition on different substrates

The experimental results are presented for the backscattering of 500-eV electrons on Au nanoclusters formed on the surface of highly oriented pyrolytic graphite HOPG(0001) and amorphous SiO₂. It has been found that the measured intensity of the elastically backscattered electrons nonmonotonically depends on the size of nanoclusters. It has been shown that the observed features can be explained by an increase in the rms deviation of the atoms of the Au nanocluster with a decrease in its size. The difference in the dependence of the rms deviation of atoms on the size of the nanoclusters formed on the surfaces of HOPG(0001) and amorphous SiO₂ is qualitatively explained by an increase in the roughness of the nanocluster surface accompanying their formation under the strongly nonequilibrium conditions of pulsed laser deposition.     

Effect of adsorbed Sn on Ge diffusivity on Si(111) surface

The effect of adsorbed Sn as a surfactant on Ge diffusion on a Si(111) surface has been studied by Low Energy Electron Diffraction and Auger Electron Spectroscopy. The experimental dependence of Ge diffusion coefficients on the Si(111) surface versus temperature in the presence of adsorbed Sn atoms has been measured in the range from 300 to 650°C. It has been shown that at a Sn coverage of about 1 monolayer the mobility of Ge atoms increases by several orders of magnitude.      

Initial stages of Ge epitaxy on Si(111) under quasi-equilibrium growth conditions

The results of the structural and morphological studies of Ge growth on a Si(111) surface at the initial stages of epitaxy by means of scanning tunneling microscopy and high-resolution transmission electron microscopy are presented. Epitaxy of Ge has been performed in the temperature range of 300 to 550°C under the quasi-equilibrium growth conditions and low deposition rates of 0.001–0.01 bilayers per minute. The stages of the formation and decay of the nanoclusters as a result of the redistribution of the Ge atoms into two-dimensional pseudomorphic Ge islands before the formation of the continuous wetting layer have been experimentally detected. The positions of the preferable nucleation of three-dimensional Ge islands on the wetting layer formed after the coalescence of the two-dimensional islands have been analyzed. The c2 × 8 → 7 × 7 → c2 × 8 phase transitions due to the lateral growth of the islands and the plastic relaxation of the misfit strains occur on the surface of the three-dimensional Ge islands when their strain state changes. The misfit dislocations gather at the interface and two types of steps lower than one bilayer are formed on the surface of the three-dimensional islands during the relaxation process.     

Modeling structural changes in metals upon irradiation with nanoclusters by means of molecular dynamics in combination with the thermal spike model

A model of structural changes in a copper target when irradiated with Cu₍₁₄₇₎ nanoclusters is studied by means of molecular dynamics, supplemented by the thermal spike model. The results from modeling structural changes include the density and depth of penetration of the nanocluster atoms into the bombarded target, depending on the energy of the nanoclusters. The shapes of the sources in the thermal spike model for describing the energy losses of nanoclusters in a target are determined.     

Synthesis and properties of Si and Ge nanoclusters produced by pulsed laser ablation

We show that conventional pulsed laser ablation (PLA) of Si and Ge targets in inert buffer gases is an efficient method of nanocluster synthesis. From a photoluminescence study of Si and Ge nanoclusters produced by PLA we have demonstrated the possibility of tuning the luminescence band from the near infrared to the near ultraviolet regions. The stabilization of the properties of Si nanoclusters by reactive (H₂ gas) PLA synthesis was proved by photoluminescence measurements. Finally, we report a photoluminescence study of gas-suspended Ge nanoclusters during their preparation. They exhibit a broad luminescence spectrum extended from UV to the blue-green region and modulated by a molecule-like structure. We propose an interpretation of the vibronic structure involving Ge-O-Ge vibrations at the surface of photo-excited clusters. To the best of our knowledge, we report here the first observation of vibrational effects from gas-suspended Ge nanoclusters.     

Local structure around Zn and Ga in solution‐processed In–Ga–Zn–O and implications for electronic properties

We study by X‐ray absorption spectroscopy the local structure around Zn and Ga in solution‐processed In–Ga–Zn–O thin films as a function of thermal annealing. Zn and Ga environments are amorphous up to 450 °C. At 200 °C and 450 °C, the Ga atoms are in a β‐Ga₂O₃ like structure, mostly tetrahedral gallium oxide phase. Above 300 °C, the Zn atoms are in a tetrahedral ZnO phase for atoms inside the nanoclusters. The observed formation of the inorganic structure above 300 °C may be correlated to the rise of the mobility for IGZO TFTs. The Zn atoms localized at the nanocluster boundary are undercoordinated with O. Such ZnO cluster boundary could be responsible for electronic defect levels. Such defect levels were put in evidence in the upper half of the band gap. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Off-centering of Pb and Sn impurities in GeTe induced by strong local stress

Noncentrality of large impurity atoms—Pb and Sn atoms—substituting for Ge atoms in a GeTe lattice has been discovered by means of EXAFS investigations in Ge_0.9Pb_0.1Te and Ge_0.85Sn_0.15Te samples. The transition of impurity atoms into a noncentral position under conditions of a strong local stress is explained by the participation of an unshared electron pair from the impurity atoms in the formation of the chemical bond. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 600–603 (25 April 1996)     

The Peierls phase transition in <Emphasis Type="Italic">n</Emphasis>-Ge caused by impurity interaction

The study of the electron paramagnetic resonance in Ge:As has revealed that the insulating state in uncompensated semiconductors is preserved near the insulator-metal phase transition because of the appearance of lattice distortions. The latter are caused by the interaction of the spins localized on impurity atoms due to the spin-Peierls transition. In Ge:As, this effect manifests itself in the concentration range n = n = 3 × 10¹⁷–3.7 × 10¹⁷ cm⁻³.     

Covalency and compression effects on the isomer shift of substitutional119Sn impurities in group IV semiconductors

Since substitutional tin is an isovalent impurity in group IV semiconductors, it represents an almost ideal probe for the study of the variation of the chemical bond in these materials using the isomer shift measurements. To obtain a realistic formula for the isomer shifts in this case, the impurity problem has been formulated in such a way as to include both the band structure and compression effects on the¹¹⁹Sn impurity. The increase of the isomer shift values measured on the impurity¹¹⁹Sn when going from diamond to silicon and germanium host crystals, is interpreted in terms of increasing dehybridization of the homopolar bond in these materials. The lattice relaxation around the tin impurity, due to its large size, seems to be relatively small, if any, in silicon and germanium, while a shift of the neighbouring carbon atoms outwards by 18% of the nearest neighbour distance has to be assumed to explain the experimental value of the isomer shift of substitutional tin in diamond.     

Interpretation of the isomer shift of interstitially implanted119Sn impurities in group IV semiconductors

Recent Mössbauer measurements of the isomer shift of interstitially implanted¹¹⁹Sn impurities in group IV semiconductors are interpreted in terms of the electron contact density of compressed Sn atoms and ions, respectively. The finite space allowed to the impurity atom in the host crystal is approximated by a Wigner-Seitz sphere. Using a calibration procedure, the dependence of the isomer shift on the Wigner-Seitz radiusR _A has been calculated for several electron configurations of the tin atom (ion). The isomer shift values for¹¹⁹Sn interstitials in diamond, silicon, germanium, and -tin are found to correspond to compressed, neutral tin atoms; furthermore, the relaxation of the host lattice about the impurity Sn atom is discussed.