Types Of Structural Defects Research Articles

Investigation of the growth characteristics of epitaxial GaN layers on sapphire by microcathodoluminescence

A study was made of the growth regimes of undoped epitaxial GaN layers under various substrate nitriding conditions. It was observed that at a nitriding temperature of ∼1000 °C films are formed with typical growth characteristics in the form of hexagonal pyramids separated by a smoothed surface. The cathodoluminescence pattern in the pyramids revealed a fine structure in which a region of donor-acceptor recombination could be identified. The formation of acceptor levels in this region was attributed to intrinsic structural defects in the GaN layers with unsaturated (broken) bonds. The presence of a donor-acceptor recombination line in mirror-smooth epitaxial films may indicate that these films contain this type of structural defect.

Impurity Reaction with Dislocations in Semiconductors

Basic aspects of impurity reaction with dislocations in semiconductors are discussed on the basis of the work of the author's group. Some important insights into the mechanism of impurity reaction with extended structural defects are deduced from a theoretical treatment of impurity distribution around a structural defect in thermal equilibrium, experimental observations on dislocation pinning, and the characteristics in the development of precipitates on various types of structural defects. The morphology of impurity precipitates formed on a dislocation plays an important role in the strength of dislocation pinning. Generally, the impurity–dislocation reaction is dependent on the species and concentration of impurities as well as the core structure of dislocation, and also on the thermal process of the crystal, i.e. the cooling rate from high temperature.

Impurity Reaction with Dislocations in Semiconductors

Basic aspects of impurity reaction with dislocations in semiconductors are discussed on the basis of the work of the author's group. Some important insights into the mechanism of impurity reaction with extended structural defects are deduced from a theoretical treatment of impurity distribution around a structural defect in thermal equilibrium, experimental observations on dislocation pinning, and the characteristics in the development of precipitates on various types of structural defects. The morphology of impurity precipitates formed on a dislocation plays an important role in the strength of dislocation pinning. Generally, the impurity–dislocation reaction is dependent on the species and concentration of impurities as well as the core structure of dislocation, and also on the thermal process of the crystal, i.e. the cooling rate from high temperature.

Minority Carrier Diffusion Length in AlGaN: A Combined Electron Beam Induced Current and Transmission Microscopy Study

Combined electron beam induced current and transmission electron microscopy measurements have been performed on both undoped and Si-doped AlGaN epitaxial films with aluminium contents x ranging from x=0 to x=0.79. TEM analysis shows the distribution and type of structural defects, the content of cubic phase and the decomposition phenomena strongly to depend on the Al content of the layers. The influence of the cubic phase on the electronic properties of the samples is discussed taking into account the high piezoelectric effect present in these alloys. The diffusion length of minority carriers has been determined by EBIC measurements and a lower limit of the lifetime has been estimated.

Influence of radiation on thin ZnS-Tb films

Changes in thin zinc-sulfide films under the action of the γ-radiation of Co60 are studied by investigating electroluminescence spectra of terbium embedded in these films as a luminescent probe. It is shown that changes in the relation of the intensities of bands, a decrease in their halfwidth and the background component, and simplification of the spectrum are observed in a short-wave region of the Tb radiation spectrum that corresponds to5D3→7Fj transitions. The same modification of the radiation spectrum is characteristic of ZnS films whose crystalline structure is ordered in the course of thermal annealing at a temperature of 350°C. Based on the analysis of the data obtained it is inferred that irradiating the ZnS films with small radiation doses of 104–105 rad leads to the ordering of their crystalline structure due to the elimination of one of the types of structural defects.

A high-resolution electron microscopy study of structural defects in YBa 2Cu 4O 8 superconductor

Structural defects in YBa 2Cu 4O 8 have been observed by means of high-resolution electron microscopy, coupled with multi-slice image simulations. The as-synthesized crystals contained several types of structural defects, as follows: (i) Planar defects consisting of a local structure of YBa 2Cu 5O 9 with triple CuO chains resulting from the insertion of one extra CuO chain at the location of the original double CuO chains. (ii) Planar faults consisting of thin 90° rotation twins about the c-axis. The layer thickness of the a– c plane domain in the b– c plane domain was 1 2 unit cell along the c-axis of YBa 2Cu 4O 8. (iii) The coexistence of YBa 2Cu 4O 8 layers and YBa 2Cu 3O 7− y layers within one crystal. (iv) an edge dislocation with Burgers vector of either b = b〈010〉 or a〈100〉 on a single CuO chain layer, and dislocations with a Burgers vector b = 1 2 [110] on double CuO chain layers, which exist at the boundary of the 90° twins.

Transmission electron microscopy and cathodoluminescence of tensile-strained GaxIn1−xP/InP heterostructures. I. Spatial variations of the tensile stress relaxation

We have investigated the optical and structural properties of tensile-strained GaxIn1−xP/InP heterojunctions by cathodoluminescence (CL) in the scanning electron microscope and by transmission electron microscopy (TEM). The lattice mismatch of the samples is ranging from 0.4% (x=5.5%) to 0.84% (x=11.8%). We show, in agreement with previous studies, that the relaxation of tensile-strained epilayers occurs by the emission of partial and perfect dislocations. The numerous twins and stacking faults which are found in the epilayers act as efficient recombination centers for electron-hole pairs and appear as dark line defects (DLDs) in CL images. ‘‘Ladderlike’’ configurations of these defects are found both by TEM and CL in samples with a lattice mismatch larger than 0.5%. We also demonstrate that DLDs are contaminated by impurities. Areas with networks of perfect dislocations are found between the DLDs. The analysis of the dislocation types allows us to suggest that the growth of low-mismatched samples is two dimensional, and that it is three dimensional in highly mismatched samples. Finally, the spatial variations of the strain relaxation throughout the samples are studied by 77-K CL spectroscopic measurements and it is shown that these variations can be correlated with the various types of structural defects.

Cluster quantum-chemical study of the chemisorption of methane on a lithium-promoted magnesium oxide doped by zinc oxide

The reaction of a methane molecule with a lithium-doped magnesium oxide catalyst ( Li MgO ) containing small amounts of Zn 2+ cations (Zn/Li/MgO) was theoretically studied using a modified MINDO/3 method and applying a supermolecular approach. The surface of magnesium oxide (MgO) was modelled by a Mg 32O 32 four-layer molecular cluster containing all types of structural defects i.e., low-coordinated magnesium and oxygen ions (Mg 2+ LC and O 2− LC) of various faces, edges, corners etc. Molecular clusters of lithium-promoted magnesia ( Li MgO ) were simulated by isomorphic substitution of Mg 2+ LC by Li + LC; the excess negative charge of the cluster was compensated by a proton connected to an O 2− 3C site. For Zn-doped Li MgO or MgO an isomorphic substitution of Mg 2+ LC by Zn 2+ LC was assumed. The calculations indicate that for Zn/Li/MgO or Zn MgO the substitution of a threefold coordinated magnesium cation by zinc is more favourable by energetics than for four- and five-fold coordinated Mg cations. The computational results are used to interpret the experimentally observed increase of C 2+ hydrocarbons selectivity in the oxidative coupling of methane when doping a NaOH-promoted CaO catalyst with minor amounts of Zn 2+ cations.

Microstructure and electrical properties of La 2− xSr xCuO 4-YBa 2Cu 3O 7−δ multilayers deposited by laser ablation

Superconducting films consisting of alternating layers of La 2− x Sr x CuO 4 (22 nm thick) and YBa 2Cu 3O 7−δ (26 nm thick) were deposited on (100) SrTiO 3 substrates by excimer laser ablation. The films, five bilayers thick, exhibited good superconducting properties. X-ray diffraction and transmission electron microscopy investigations showed that they were heteroepitaxially grown with the c-axis of both materials normal to the substrate surface. Particular attention was paid to the microstructure of the layers as well as to the different interfaces. Cross-sectional TEM investigations showed that all the interfaces were quite sharp but not equally rough. The YBCO over LSCO interface was wavy while the LSCO over YBCO interface was nearly atomically flat. The interpretation of these different shapes and their influence on the next deposited layer will be reported, as well as the presence of different types of structural defects in the layers.

TEM characterization of high superconductive multilayered thin films

Abstract Superconductive multilayered thin films have been deposited by excimer laser ablation and d.c. magnetron sputtering. The laser deposited films were grown on (100) SrTiO 3 substrates and consisted of alternating layers of La 2− x Sr x CuO 4 (LSCO) and YBa 2 Cu 3 O 7−δ (YBCO) while the sputtered films, deposited on (100) MgO substrates, consist of a 3 nm MgO layer sandwiched in 100 nm YBCO layers. Cross-sectional transmission electron microscopy (TEM) specimens have been prepared so as to investigate the different interfaces and the local structure of the films. The results show that the laser deposited films are epitaxially grown with the c- axis normal to the surface. The interfaces are rather sharp but some roughness of the YBCO over LSCO interface, possibly related to the presence of shear defects in the LSCO layers, is evidenced. The observations of the sputtered films reveal the presence of some a- axis oriented YBCO grains and the partial crystallization of the amorphous deposited MgO layer. Some a- axis oriented grains are also observed in the laser deposited films but only in the upper layers. The different types of structural defects as well as the different interfaces, characterized by high resolution electron microscopy (HREM), will be reported.

Elastic and dissipative properties of high-Tc superconductors

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HCN: Hydrogen‐Bonded Versus Conjugated Polymer

AbstractResults of a density‐functional study of linear HCN and CNH monomers, the corresponding hydrogen‐bonded polymers, and a conjugated polymer (CHN)x with C and N atoms alternating along a zigzag chain are reported. Minimizing the total energy as a function of structure results in realistic structures for the monomers and the conjugated polymer; but the local‐density approximation may be responsible for the too short hydrogen bonds found for the hydrogen‐bonded polymers. For the optimized structures we present electronic energies, densities of states, photoelectron spectra, momentum densities, and reciprocal form factors. Comparing these clearly demonstrates both similarities and differences for the various systems. It is further demonstrated that the conjugated polymer can lower the total energy upon a C–N bond‐length alternation. A model is then constructed with which solitons and polarons in the conjugated polymer are examined. Both types of structural defects are stable for the charged chains and induce two gap‐states. Finally, polarons in the hydrogen‐bonded chains are discussed.

Ge xS 1− x glasses. Defects, density and structure

The density, X-ray scattering and ESR ofGe x S 1− x glasses within0.10 ⩽ x ⩽ 0.44 have been measured. The region of compositions in question is subdivided into three intervals: 0.10 ⩽ x ⩽ 0.325, 0.325 ⩽ x ⩽ 0.30and0.30 ⩽ 0.44 according to the types of paramagnetic centers (PC) and the specific features of the concentration dependences of the experimental quantities under study. It is found that the density, the PC concentration and the features of the radial distribution functions are correlated in each interval. The concentration dependence of the relative amount of two types of PC is the first interval is obtained. The influence of γ-irradiation at 300 K upon the structure and concentration of PC is studied. It is found that under γ-irradiation the amount of PC either increases (first and second interval) or decreases (third interval) and the unpaired spin density is redistributed among different types of structural defects. New types of PC do not arise under γ-irradiation. In the framework of the structural chemical analysis the models of structural units and their transformation with glass composition are suggested. Based on the proposed model for the glass network, the experimental data are analyzed.

P-Doping of epitaxial ZnSe using group I elements

Our work on MBE growth of ZnSe doped with elemental lithium and sodium sources is presented. Lithium doping has resulted in low resistivity p-ZnSe with uncompensated acceptor concentrations in excess of 5 × 10 16 cm −3 (resistivity as low as 2.9 Ω cm). Variations of the film properties with growth conditions, as well as some potential problems involving the formation of electrical contacts, are discussed. As yet we have not been able to achieve p-type conversion of the ZnSe films using a sodium doping source. Photoluminescence measurements suggest the possible association of Na with some type of structural defect. Possible reasons for the observed differences between the Li and Na dopants, as well as the prospects for future success in doping of ZnSe using Group I elements, are discussed.

Characterization of epitaxial silicon layers made by reduced pressure/temperature CVD

Abstract A rapid thermal processing (RTP) has been used to fabricate homoepitaxial layers of silicon at reduced pressure (1 to 10 Torr) and reduced temperature (800–900°C). Prior to deposition (the samples being chemically cleaned or used as-received) an “in situ” cleaning step is used at temperatures higher than the deposition temperature or at the same temperature as when using UV radiation. Results from both techniques are compared and discussed concerning residual doping concentration, defect density into the grown layers through Schottky diodes and DLTS measurements. Carrier concentration peaks are observed at the substrate/epitaxial layer interface and when the layers are grown by steps. These peak intensities are correlated with the “in situ” surface cleaning step and growth temperature. Spreading resistance measurements show the autodoping level which is very low even with highly doped p-type substrates. Mobility values are compared with bulk material values. TEM observations are also correlated with the electrical results. We observe that the type of defects generated at 800°C has a smalle r activation energy (and density) than that produced at 900°C. The former one is interpreted as corresponding to shallow impurities and the latter is considered as some type of structural defects. As a conclusion we show the feasibility of an “in situ” low temperature surface cleaning prior to deposition giving rise to results of comparable quality to those obtained using a high temperature cleaning.

Exploratory study of the polycrystalline n-silicon photoelectrode by the scanning laser spot technique

An apparatus was developed which allows scanning of semiconductor/electrolyte interfaces with a small laser spot and measuring the local photocurrent. This apparatus was used for studying the polycrystalline n-Si electrode in methanolic ferrocene solutions. Measurements at different electrode potentials reveal the presence of two different types of structural defects, observable by a local photocurrent decrease, i.e. grain boundaries and surface imperfections.

Influence of Rapid Thermal Annealing on Shallow BF2 Implantation into Pre-Amorphized Silicon

ABSTRACTLow energy, shallow BF2+ implants were carried out at room or liquid nitrogen temperature into deep pre-amorphized (100) Si for better control of the dopant profile and post-annealing structural defects. Cross sectional and angle polished plan view transmission electron microscopy were used to study the structural quality of the implanted layer, while SIMS provided a chemical profile. Four types of structural defects were observed in BF2+ implanted, pre-amorphized samples following rapid thermal annealing with a halogen lamp. An in-situ ion beam annealing and the presence of F in the Si lattice were related to the creation of the defects. Good correlations between F gettering and TEM observed defects were found to exist. Implantation of B+ into a pre-amorphized Si surface and subsequent rapid thermal annealing was found to produce a wide defect-free surface layer.

Thermal degradation of some model compounds for polyvinylchloride

The thermal stability of some model compounds for polyvinylchloride with different types of structural defects was studied. The determinations were carried out in thermal dynamical conditions using the conductometric detection of the evolved hydrogen chloride and thermogravimetrical methods. The kinetic parameters corresponding to the first stages of dehydrochlorination reaction were discussed comparatively. The following thermal stability order was obtained: macromodels withHT > macromodels with ClT > macromodels with ClA.

Vibrational dynamics and structure of doped polyacetylene as a one-dimensional disordered crystal

Vibrational infrared and Raman spectra of doped trans polyacetylene are interpreted on the basis of dynamical calculations which consider the system as a one-dimensional crystal containing structural disorder on the molecular level. Several types of structural defects have been studied. The spectroscopic evidence of topological solitons is not unique and an alternative model can be proposed.

Umwandlungs- und reduktionsprodukte von metastabilem Nb 2O 5(Ox2BI), elektronenoptische untersuchung

A metastable Nb 2O 5 modification, obtained by fast and complete oxidation of the monoclinic form of Nb 12O 29, is transformed into the stable HNb 2O 5, modification by heating in air above 700° C whereas the reduction of the metastable Nb 2O 5 modification at comparable temperatures leads to the compound NbO 2.44 which is also metastable. The electron optical investigation shows that the reaction products have “block structures” which are heavily disordered. Typical structural defects recognizable in the products of both types of reactions are a “zipper-like” sequence of blocks, connections with R-tyue tunnels and characteristic structural elements of the initial structure of the metastable Nb 2O 5 modification. An irregular sequence of structural elements of Nb 12O 29 and Nb 25O 62 is characteristic of NbO 2.44.