Effect of arsenic species on the kinetics of GaAs nanowires growth by molecular beam epitaxy

GaAs nanowires (NWs) are grown on GaAs (1 1 1) B substrates in a molecular beam epitaxy system, by Au-assisted vapor–liquid–solid growth. We compare the characteristics of NWs elaborated with As₂ or As₄ molecules. In a wide range of growth temperatures, As₄ leads to growth rates twice faster than As₂. The shape of the NWs also depends on the arsenic species: with As₄, regular rods can be obtained, while pencil-like shape results from growth with As₂. From the analysis of the incoming fluxes, which contributes to the NWs formation, we conclude that the diffusion length of Ga adatoms along the NW sidewalls is smaller under As₂ flux as compared to that under As₄ flux. It follows that As₂ flux is favourable to the formation of radial heterostructures, whereas As₄ flux is preferable to maintain pure axial growth.     

The structure of vapour-deposited arsenic sulphides prepared using a cooled (78 K) substrate

Amorphous molecular As₄S₄ and non-stoichiometric “As₂S₃” evaporated films have been prepared by condensing onto a substrate at liquid nitrogen temperature in situ in the X-ray diffractometer. The structures of these films are shown to be similar, thus confirming the importance of As₄S₄ (realgar-type) molecules in amorphous films prepared by evaporation of “As₂S₃” glass. The local molecular packing in the as-deposited amorphous As₄S₄ film shows a preference for a packing arrangement similar to that found in the α-crystalline phase. Both as-deposited films contain gross structural defects such as molecular clusters and/or voids which irreversibly reduce in importance as the film structures relax on warming to ambient temperatures. Also, the As₄S₄ film partially devitrifies to the high-temperature β-crystalline phase on warming to ambient, but no devitrification occurs in the non-stoichiometeric film due to the presence of more than one molecular type.     

Dependence of photo-oxidation on Ag(Cu)-content in Ag(Cu)–As2Se3 films

We report the dependence of photo-oxidation (i.e., the formation of As₂O₃ microcrystals) on Ag (or Cu) content in Ag-doped As₂Se₃ and Cu-doped As₂Se₃ films. These chalcogenide films were prepared by thermal evaporation and photodoping, and their film surface was illuminated in air with an Ar laser beam of a wavelength of 0.5145 μm. From viewpoint of applications, we paid attention to the probability and the beginning optical intensity of the As₂O₃ microcrystals formation, and the photodarkening effect as a function of Ag (Cu) content. It has been confirmed that the addition of metals Cu or Ag into As₂Se₃ films is very useful in for suppressing or weakening such a harmful oxidation reaction. It has also been found that there is a distinct difference in these properties between Ag–As₂Se₃ and Cu–As₂Se₃ films, which is attributed to the difference in the coordinate number between Ag and Cu atoms.     

A route to single-crystalline ZnO films with low residual electron concentration

Single-crystalline ZnO films have been grown on a-plane sapphire in plasma assisted molecular beam epitaxy by introducing a high-temperature ZnO buffer layer. The residual electron concentration of the films can be lowered to 1.5×10¹⁶ cm⁻³, comparable with the best value ever reported for ZnO films grown on a rare and costly substrate of ScAlMgO₄. A 3×3 reconstruction has been observed on the films grown in this route, which reveals that the films have very smooth surface. X-ray phi-scan spectrum of the films shows six peaks with 60° intervals, and two-dimensional X-ray diffraction datum indicates the single-crystalline nature of the films. Low temperature photoluminescence spectrum of the films shows a dominant free exciton emission and five phonon replicas, confirming the high quality of the films.     

Atomic structure and short- and medium-range order parameters in amorphous chalcogenide films prepared by different methods

The atomic structure of amorphous As₂Se₃ and As₂S₃ films prepared by thermal evaporation in a vacuum and by RF ion-plasma sputtering has been studied by the methods of X-ray diffraction and Raman spectroscopy. The techniques of film preparation had different conditions of substance vaporization and atom condensation on a substrate. It has been established that films prepared by these methods have significant differences in the dimensions of the medium-range order and in the local atomic structure, which causes considerable differences in their electronic properties.     

Strain inhomogeneities in epitaxial BaFe2As2 thin films

Epitaxial BaFe₂As₂ thin films grown by pulsed laser deposition on spinel substrates with a thin iron buffer layer show strain‐induced superconductivity. Using high resolution electron backscatter diffraction in a scanning electron microscope very small orientation changes (≤ 0.2°) of the lattice and strain inhomogeneities on a length scale of few 100 nm or less were measured. Furthermore, partially strain relaxed areas were revealed. It was found, that the quality of the BaFe₂As₂ thin films crucially depends on the iron buffer layer.     

Characteristics of doped oxides and their use in silicon device fabrication

Thin films of binary SiO₂ based mixed oxides are finding increasing use in the fabrication of silicon discrete devices and integrated circuits. The mechanical and chemical properties of these glasses, and thus their use in the fabrication of any specific device structure, is quite dependent upon composition. We have developed an infrared method for determining the composition of glasses in the (a) B₂O₃-SiO₂ system over the range 0–100% B₂O₃, (b) As₂O₃-SiO₂ system over the range 0–20% As₂O₃, and (c) P₂O₅-SiO₂ system over the range 0–20% P₂O₅. This technique is non-destructive and may be used to unambiguously characterize films in the 1000–10000 Å, range such as are commonly used in device manufacture. The use of these glasses as sources for diffusion of dopants into silicon has been extensively studied, and results will be given. In addition, the infrared vibrational spectra for the borosilicate system has been studied in detail allowing insights into the character of the bonding in these vapor-deposited glassy solids.     

Structure of pulsed-laser deposited arsenic-rich As–S amorphous thin films,and effect of light and temperature

Thin amorphous films from As–S system (∼As₅₀S₅₀) were prepared by the pulsed laser deposition technique. Light- and thermally-induced changes of structure of studied films have been investigated using Raman scattering spectroscopy results and interpreted in terms of chemical reactions and/or phase transitions between individual structural units. The irradiation of as-deposited thin films causes light-induced reactions, in which As₄S₃, α- and β-As₄S₄, and pararealgar/χ-As₄S₄ molecules are formed. Thermal-annealing of exposed thin films leads to the formation of β-As₄S₄ molecules.     

Mass spectrometric analysis of arsenic trisulfide

Mass spectrometric analysis of arsenic trisulfide, thermally vaporized from amouphous bulk and thin films, is reported. The results suggest that arsenic trisulfide evaporates as As₄S₆ molecules, although its molecular ion is absent from the mass spectrum because of fragmentation after ionization. Analysis of vaporization from oxidized specimens is also presented which supports a proposed mechanism for light-enhanced vaporization of As₂S₃.     

RbxK1-xTiOPO4 Thin Films Grown on KTiOPO4 Substrates by Liquid Phase Epitaxy

The properties of Rb_xK_1-xTiOPO₄ thin films grown on KTiOPO₄ substrates are reported. High quality films were obtained using the flux liquid phase epitaxy method, and optical waveguiding was observed in these epitaxial films. X-ray analysis shows that the epitaxial films are single crystals films. Their cell constants are different from those of the substrates. The epitaxy growth rate and other film properties were compared for films grown on different faces. Two different surface morphologies were observed for films grown on (100) faces. The morphologies on (201) and (201) faces were also different. The quality of films grown on the (201) face was better. The reason for a structure inversion for films grown on the z⁻ face is discussed.     

Experimental characterization of amorphous As–Se–Sb alloys

Thin films were thermally evaporated from ingot pieces of the As₃₀Se_70−xSb_x (with 2.5 ⩽ x ⩽ 17.5 at.%) glasses under vacuum of ∼10⁻⁵ Torr. Increasing Sb content was found to affect the thermal and optical properties of these films. Non-direct electronic transition was found to be responsible for the photon absorption inside the investigated films. The chemical bond approach has been applied successfully to interpret the decrease of the glass optical gap with increasing Sb content. Decreasing the thermal stability of the As₃₀Se_70−xSb_x specimen by increasing Sb content is responsible for occurring the amorphous–crystalline process at lower temperatures. Binary As₂Se₃ and Sb₂Se₃ phases are the main components of the stoichiometric As₃₀Se₆₀Sb₁₀ composition.     

Effect of Mn concentration and growth temperature on nanostructures and magnetic properties of Ge1−xMnx grown on Si

The nanostructures and magnetic properties of Ge_1−xMn_x thin films grown on Si substrates by molecular beam epitaxy, with different nominal Mn concentrations (1−4%) and different growth temperatures, have been systematically investigated by transmission electron microscopy and superconducting quantum interference device. It was discovered that when Ge_1−xMn_x thin films were grown at 70 °C, with increase in Mn concentration, Mn-rich tadpole shaped clusters started to nucleate at 1% Mn and become dominate in the entire film at 4% Mn. While for the thin films grown at 150 °C, tadpoles was firstly seen in the film with 1% Mn and subsequently Mn-rich secondary precipitates became dominant. The magnetic properties show specific features, which are mainly related to the nature and amount of Mn-rich clusters/precipitates within these thin films.     

Low-Temperature epitaxial growth of InGaAs films on InP(100) and InP(411)<Emphasis Type="Italic">A</Emphasis> substrates

The structural and electrical characteristics of In_0.53Ga_0.47As epitaxial films, grown in the low-temperature mode on InP substrates with (100) and (411)A crystallographic orientations at flow ratios of As4 molecules and In and Ga atoms of γ = 29 and 90, have been comprehensively studied. The use of InP(411)A substrates is shown to increase the probability of forming two-dimensional defects (twins, stacking faults, dislocations, and grain boundaries), thus reducing the mobility of free electrons, and As_Ga point defects, which act as donors and increase the free-electron concentration. An increase in γ from 29 to 90 leads to transformation of single-crystal InGaAs films grown on (100) and (411)A substrates into polycrystalline ones.     

Differential anomalous X-ray scattering studies of amorphous Cd59As41 and Cd26As74

The local structure of amorphous cadmium arsenide semiconducting films has been studied by differential anomalous X-ray scattering. Intensity measurements were carried out on two samples, containing 41 and 74 at.% As, in the vicinity of the absorption K edges of both constituents using synchrotron radiation. The computational procedure, similar to that proposed by Warren for an amorphous sample with more than one kind of atom, was applied to obtain the structural parameters from the experimental data. It has been found that atoms in the amorphous CdAs films remain almost tetrahedrally coordinated and that the investigated films are chemically ordered. The structural changes going from cadmium- to arsenic-rich composition have been revealed. The differential anomalous X-ray scattering technique proved to be effective, providing the evidence for the CdCd and AsAs near neighbour correlations in Cd₅₉As₄₁ and Cd₂₆As₇₄, respectively. The simulations of the differential radial distribution functions have shown that for the amorphous film containing 41 at.% As the distorted tetrahedral structure, intermediate between the CdAs and Si III type structures, is adequate to account for the experimental data. At 74 at.% As, the atomic arrangement can be described satisfactorily by the structural model based on the tetragonal CdAs₂ structure. The structural parameters obtained from the present study and those previously derived using the extended X-ray absorption fine structure and conventional large angle X-ray scattering techniques are compared.     

Single crystal growth and physical properties of superconducting ferro-pnictides Ba(Fe,Co)2As2 grown using self-flux and Bridgman techniques

We have employed the high temperature solutions growth technique and the Bridgman technique to grow cm-size high-quality single crystals of pristine BaFe₂As₂ (Ba122) and its Co-doped superconducting variants. In the first approach, self-flux (Fe, Co)As was used to achieve a homogeneous melt of composition Ba(Fe, Co)_3.1As_3.1 at T=1463 K. The melt was then cooled slowly under a temperature gradient in a double-wall crucible assembly to obtain large and flux-free single crystals of Ba(Fe, Co)₂As₂. In the second approach, single crystals were grown directly from a stoichiometric Ba(Fe, Co)₂As₂ melt at T=1723 K employing the Bridgman technique associated with a vertical tube furnace. Using both techniques single crystals with lateral dimensions up to 25×10 mm² and thickness up to 1 mm were obtained. Details of the two methods are given and a comparative study of the magnetic and transport properties of the single crystals obtained using the two methods is presented.     

Doping behavior of silicon in vapor-grown III–V epitaxial films

An expanded silicon-doping model, applicable for unintentionally doped vapor-grown III–V epitaxial materials, is presented based on additional experimental results of doping-level influences of AsH₃, PH₃, and HCl. This model retains the role of HCl as the silicon generation agent, as suggested in a recently reported silicon-doping model for GaAs. It also includes a surface kinetic factor for the silicon-incorporation efficiency in III–V materials as influenced by the relative vapor-phase concentration of As₂, As₄ and P₂, P₄ species. The present studies indicate a different degree of influence of the HCl and AsH₃ partial pressures on the suppression of the GaAs background doping level. The additive effect of both partial pressures is very similar to that found in the chloride system. Qualitatively, the same sensitivity of GaP doping levels to variations in HCl and PH₃ has been observed; however, the magnitudes of the doping-level changes are different. In all instances, the impurity levels in the epitaxial films are reduced by increases in the partial pressures of the various reactants, with the AsH₃ and PH₃ having a greater influence than the HCl.     

Growth mechanisms and defect structures of B12As2 epilayers grown on 4 H-SiC substrates

Epitaxial growth of icosahedral B₁₂As₂ on c-plane 4 H-SiC substrates has been analyzed. On on-axis c-plane 4 H-SiC substrates, Synchrotron white beam x-ray topography (SWBXT) revealed the presence of a homogenous solid solution of twin and matrix B₁₂As₂ epilayer domains. High resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) both revealed the presence of an ～20 nm thick, disordered transition layer at the interface. (0003) twin boundaries are shown to possess fault vectors such as 1/3[1–100]_B12As2, which originate from the mutual shift between the nucleation sites. On the contrary, B₁₂As₂ epilayers grown on c-plane 4 H-SiC substrates intentionally misoriented from (0001) towards [1–100] is shown to be free of rotational twinning. SWBXT, HRTEM and STEM all confirmed the single crystalline nature and much higher quality of the films. In addition, no intermediate layer between the epilayer and the substrate was observed. It is proposed that the vicinal steps formed by hydrogen etching on the off-axis 4 H-SiC substrate surface before deposition cause the film to adopt a single orientation during nucleation process. This work also demonstrates that c-plane 4 H-SiC with offcut toward [1–100] is potentially a good substrate choice for the growth of high-quality, single crystalline B₁₂As₂ epilayers for future device applications.     

Thin film deposition of Ge33As12Se55 by pulsed laser deposition and thermal evaporation: Comparison of properties

Thin films of Ge₃₃As₁₂Se₅₅ were produced using two deposition techniques, ultra-fast pulsed laser deposition (UFPLD) and thermal evaporation (TE), and their properties have been investigated. The chemical composition of the UFPLD films was virtually identical to the composition of the chalcogenide glass targets, whereas the composition of films deposited by TE was significantly different from the target material. Heating of the substrate with a temperature gradient during deposition produced a gradient in composition in both UFPLD and TE films.     

Growth of Zn3As2 on GaAs by liquid phase epitaxy and their characterization

Zn₃As₂ epitaxial layers were grown on GaAs (1 0 0) substrates by liquid phase epitaxy (LPE) using Ga as the solvent. Zinc mole fraction in the growth melt was varied from 1.07×10^?2 to 6×10^?2. X-ray diffraction spectrum exhibits a sharp peak at 43.3° characteristic of Zn₃As₂ crystalline layer. The peak intensity increases with increase in zinc mole fraction in the growth melt. The compositions of the as-grown Zn₃As₂ layers were confirmed by energy dispersive X-ray (EDX) analysis. Surface morphology was studied using scanning electron microscopy (SEM) and the thickness of the epilayers was also determined. The Hall measurements at 300 K indicate that Zn₃As₂ epilayers are unintentionally p-doped. With an increase of zinc mole fraction in the growth melt, carrier concentration increases and carrier mobility decreases. Infrared optical absorption spectroscopy showed a sharp absorption edge at 1.0 eV corresponding to the reported band gap of Zn₃As₂.     

Electron diffraction RDF analysis of amorphous As2Se3,AAs2Se2Te,As2SeTe2 and As2Te3 films

The local order in amorphous films of As₂Se₃, As₂Se₂Te, As₂SeTe₂, and As₂Te₃ has been examined by scanning electron diffraction with direct recording of the intensity of the elastically scattered electrons. The radial distribution functions indicate that there is a systematic increase in mean nearest neighbor distance as the Te concentration is increased, butthe mean coordination number increases slightly around 2.4. Pair function calculation of models shows that the 3-aand 2-fold coordinations of arsenic and chalcogens are retained in these glasses and the interatomic distances are close to those predicted from the Pauling covalent atomic radii of the constituent atomic species. The short range order appears to be similar in amorphous and crystalline As₂Se₃, but different in the case of As₂Te₃ as found by previous workers on bulk materials.