Low-temperature scanning tunneling microscopy and near-edge X-ray absorption fine structure investigation of epitaxial growth of F<Subscript>16</Subscript>CuPc thin films on graphite

Low-temperature scanning tunneling microscopy (LT-STM) and near-edge X-ray absorption fine structure (NEXAFS) measurements are used to study the epitaxial growth and molecular orientation of organic thin films of copper hexadecafluorophthalocyanine (F₁₆CuPc) on highly oriented pyrolytic graphite (HOPG). Our results show that F₁₆CuPc molecules lie flat on HOPG up to 5 nm thickness, stabilized by interfacial and interlayer π–π interactions. LT-STM experiments reveal the coexistence of two different in-plane orientations of the F₁₆CuPc monolayer on HOPG. On the second layer of F₁₆CuPc on HOPG, however, all F₁₆CuPc molecules possess the same in-plane orientation.     

Investigation of local electronic transport and surface potential distribution of Cu(In,Ga)Se2 thin-films

Local current mapping and surface potential distributions on polycrystalline Cu(In,Ga)Se₂ (CIGS) films are investigated by conductive atomic force microscopy and Kelvin probe force microscopy. The two kinds of samples fabricated by co-evaporation had extremely different conversion efficiencies of 10% and 0.2% for stoichiometric and Cu- and Se-deficient compositions, respectively. We examined the microscopic reasons for the differences in the local electrical properties. Current mapping and current–voltage behaviors were measured at intragrain regions (IGs) and grain boundaries (GBs). Electronic transport between a Pt scanning probe and the CIGS layer is explained by the Schottky conduction mechanism. The surface potential distribution shows an intriguing relation with topological variation, inferring that a local built-in potential is possibly formed on positively charged GBs. The surface potential is about 100 mV, which shows energy band bending near GBs in the films. Exciton separation near GBs is explained by the bending of the conduction and valence bands, which is sensitive to compositional and structural inhomogeneities.     

Mobility–diffusivity relationship for heavily doped organic semiconductors

The relationship between the diffusivity D _n and the mobility μ _n of chemically doped organic n-type semiconductors exhibiting a disordered band structure is presented. These semiconductors have a Gaussian-type density of states. So, calculations have been performed to elucidate the dependence of D _n /μ _n on the various parameters of this Gaussian density of states. Y. Roichman and N. Tessler (Appl. Phys. Lett. 80:1948, 2002), and subsequently Peng et al. (Appl. Phys. A 86:225, 2007), conducted numerical simulations to study this diffusivity–mobility relationship in organic semiconductors. However, almost all other previous studies of the diffusivity–mobility relationship for inorganic semiconductors are based on Fermi–Dirac integrals. An analytical formulation has therefore been developed for the diffusivity/mobility relationship for organic semiconductors based on Fermi–Dirac integrals. The D _n /μ _n relationship is general enough to be applicable to both non-degenerate and degenerate organic semiconductors. It may be an important tool to study electrical transport in these semiconductors.     

Growth of nanocrystalline CuIn3Se5 (OVC) thin films by ion exchange reactions at room temperature and their characterization as photo-absorbing layers

Nanocrystalline CuIn₃Se₅ thin films have been grown on ITO glass substrates using chemical ion exchange reactions with CdS, in alkaline medium at pH 11. The as-deposited films were annealed in air at 200 °C for 30 min and characterized using X-ray diffraction (XRD), transmission electron microscopy, energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, and scanning electron microscopy to study the structural, compositional and morphological properties. The XRD patterns reveal the nanoparticles size to be of 18-20 nm diameter, while from the SEM images the nanoparticles size is estimated to be 20-30 nm. It is observed that the annealed films contain nanocrystallites connected with each other through grain boundaries, with grain size of about 100-125 nm and have an overall n-type electrical conductivity and higher photoconductivity. The current-voltage (I-V) characteristics (in dark and light) of these films indicated the formation of a Schottky like junction between the n-CuIn₃Se₅ (OVC) and CdS/ITO layers.     

Structural study of Mn-doped ZnO films by TEM

The structural study of diluted magnetic semiconductors is important for interpreting the ferromagnetic behavior associated with the materials. In the present work, a series of low concentration Mn-doped ZnO thin films synthesized by pulsed laser deposition was studied by electron microscopy. All films show the wurtzite structure with (001) preferred growth orientation on the Si substrate. Electron diffraction experiments indicate the deterioration of the growth orientation in some areas of the films with increasing Mn concentration, and the existence of a secondary phase, of Mn₂O₃-type, in the films with larger Mn concentrations. High-resolution electron microscopy images confirm the existence of the secondary phase in the grain boundary of the Mn-doped ZnO phase. The magnetic properties of Mn-doped ZnO are discussed in relation to the structures of the films.     

Carrier density distribution in silicon nanowires investigated by scanning thermal microscopy and Kelvin probe force microscopy

The use of scanning thermal microscopy (SThM) and Kelvin probe force microscopy (KPFM) to investigate silicon nanowires (SiNWs) is presented. SThM allows imaging of temperature distribution at the nanoscale, while KPFM images the potential distribution with AFM-related ultra-high spatial resolution. Both techniques are therefore suitable for imaging the resistance distribution. We show results of experimental examination of dual channel n-type SiNWs with channel width of 100 nm, while the channel was open and current was flowing through the SiNW. To investigate the carrier distribution in the SiNWs we performed SThM and KPFM scans. The SThM results showed non-symmetrical temperature distribution along the SiNWs with temperature maximum shifted towards the contact of higher potential. These results corresponded to those expressed by the distribution of potential gradient along the SiNWs, obtained using the KPFM method. Consequently, non-uniform distribution of resistance was shown, being a result of non-uniform carrier density distribution in the structure and showing the pinch-off effect. Last but not least, the results were also compared with results of finite-element method modeling.     

Locally enhanced light–matter interaction of MoS2 monolayers at density-controllable nanogrooves of template-stripped Ag films

Transition metal dichalcogenide (TMD) monolayers, such as MoS_2, possess a direct optical bandgap are useful for emerging ultrathin optoelectronics in the visible light range, whereas their thin thickness limits light absorption and emission properties. To address this drawback, one promising approach is to hybridize plasmonic nanostructures with monolayer TMDs to utilize local field enhancement effects owing to localized surface plasmon resonance (LSPR). Herein, we propose a strong enhancement of the local light–matter interaction in MoS₂ monolayers on naturally generated nanoscale grooves. The nanogrooves are formed at grain boundaries (GBs) of template-stripped metal film substrates that are fabricated by mechanically stripping Ag films deposited on an ultra-flat Si substrate, wherein the nanogroove densities are systematically modulated by the Ag film thickness. We observe an effective photoluminescence enhancement factor of 758 and a Raman spectroscopy intensity enhancement of approximately 5 times in MoS₂ on the subwavelength-scale nanogrooves, compared with that on grain planes, which is attributed to a strong local field enhancement of the LSPR effect. Moreover, this plasmonic enhancement effect is elucidated by dark-field scattering spectroscopy and optical simulations. Our results can facilitate the utilization of density-controllable plasmonic nanogrooves synthesized without nanopatterning techniques for plasmonic hybrids on 2D semiconductors.     

Structural and electronic implications for carrier injection into organic semiconductors

We report on the structural and electronic interface formation between ITO (indium-tin-oxide) and prototypical organic small molecular semiconductors, i.e., CuPc (copper phthalocyanine) and α-NPD (N,N′-di(naphtalen-1-yl)-N,N′-diphenyl-benzidine). In particular, the effects of in situ oxygen plasma pretreatment of the ITO surface on interface properties are examined in detail: Organic layer-thickness dependent Kelvin probe measurements revealed a good alignment of the ITO work function and the highest occupied electronic level of the organic material in all samples. In contrast, the electrical properties of hole-only and bipolar organic diodes depend strongly on the treatment of ITO prior to organic deposition. This dependence is more pronounced for diodes made of polycrystalline CuPc than for those of amorphous α-NPD layers. X-ray diffraction and atomic force microscopic (AFM) investigations of CuPc nucleation and growth evidenced a more pronounced texture of the polycrystalline film structure on the ITO substrate that was oxygen plasma treated prior to organic layer deposition. These findings suggest that the anisotropic electrical properties of CuPc crystallites, and their orientation with respect to the substrate, strongly affect the charge carrier injection and transport properties at the anode interface.     

Atomic structure and electronic properties of the ∑37(610) and ∑29(520) [001] tilt grain boundaries in Ge

400 kV high resolution electron microscopy (HREM), deep level transient spectroscopy (DLTS) and steady state electrical measurements have been applied to 37(610) and 29(520) [001] tilt grain boundaries (GBs) in germanium bicrystals. The atomic boundary structures were revealed by experimental HREM images taken under different defocus conditions. Later, structure models were refined by means of a trial-and-error method applying alternatively the image simulation and the molecular static calculation of relaxed structures. The structures were shown to be consistent with the modified structural unit model. Although the structures are different for the two GBs studied, DLTS data and steady state measurements were found to be quite similar for both GBs. Thus, the results point to the extrinsic origin of localized deep states at the GBs. The analysis of DLTS spectra indicates the impurity segregation at the boundary, e.g., the formation of vacancy-type oxygen complexes of a donor-like state at E _c-0.21 eV, which results in the fluctuation of the potential barrier. Defects in the GBs—like facets, atomic steps and secondary grain boundary dislocations—which are characteristic of both boundaries can act as nuclei to the impurity segregation.Presented at the Workshop on High-Voltage and High-Resolution Electron Microscopy, February 21–24, 1994, Stuttgart, Germany.     

Preparation and characterization of the LiInSe2 thin films

Summary p-type LiInSe₂ films have been prepared by the rapid evaporation method onn-type Si andn-type GaP. Various characterization techniques such as the X-ray analysis, the Rutherford backscattering (RBS) analysis and the scanning electron microscopy were used to evaluated the quality of the films. The rectifications of the preliminary heterojunctions are demonstrated. Paper presented at the ?V International Conference on Ternary and Multinary Compounds?, held in Cagliari, September 14–16, 1982.     

Microstructures of grain boundaries in (001)-oriented strontium bismuth tantalate thin films grown by pulsed laser deposition

(001)-oriented strontium bismuth tantalate thin films have been grown on Pt/TiO₂/SiO₂/Si (100) substrates by pulsed laser deposition. The room-temperature current–electric field dependence of the films has been investigated, which revealed a space-charge-limited conduction mechanism. The microstructures of grain boundaries and structural defects in these films were also examined by transmission electron microscopy and high-resolution transmission electron microscopy, respectively. The grains of the films deposited at 550 °C exhibited polyhedral morphologies, and the average grain size was about 50 nm in length and 35 nm in width. At a small misorientation angle (8.2°) tilt boundary, a regular array of edge dislocations with about 3-nm periodic distance was observed, and localized strain contrast near the dislocation cores was also observed. The Burgers vector b of the edge dislocation was determined to be [110]. At a high misorientation angle (39.0°) tilt grain boundary lattice strain contrast associated with the distortion of lattice planes was observed, and the mismatching lattice images occurred at about 2 nm along the boundary. The relationship between microstructural defects at grain boundaries and leakage currents of these films is also discussed. Received: 8 September 2000 / Accepted: 18 December 2000 / Published online: 28 February 2001     

Defect studies in small CdTe clusters

We study Cd vacancy formation in prototype stoichiometric and non-stoichiometric CdTe clusters with and without passivation. For certain clusters like Cd₁₃Te₁₆, vacancy leads to severe distortion of the geometry due to propagation of defect. Annealing of the vacancy out of the cluster is observed in all unpassivated clusters. Passivated clusters retain their initial geometry and vacancy induced structural distortions are not seen in these clusters since the defect gets localized. Vacancy also induces intragap states. However, it was observed that the passivation of the dangling bonds created by the vacancy removes the intragap states. In an attempt to have CdTe clusters with extrinsic carriers, we substituted a Cd atom by its adjacent atoms Pd/Ag/In/Sn in these CdTe clusters. Substitutional doping of Cd by metal atoms increases the stability of unpassivated clusters. For certain clusters, metal atom doping leads to a half-metallic character. Pd/Ag-doped clusters are p-type semiconductors whereas In-doped clusters are n-type semiconductors. Sn doping in these clusters does not result in n-type semiconductors.     

High-performance n-channel organic thin-film transistors based on the dual effects of heterojunction and surface modification

This paper presents two n-channel organic heterojunction transistors with modified insulator by using hexadecafluorophthalocyaninatocopper (F₁₆CuPc)/copper phthalocyanine (CuPc) and F₁₆CuPc/pentacene as the active layers. Compared with a single-layer device, it reports that an improved field-effect mobility and a 6-fold higher drain current are observed. The highest mobility of 0.081~cm²/(V.s) was obtained from F₁₆CuPc/CuPc heterojunction devices. This result is attributed to the dual effects of the organic heterojunction and interface modification. Furthermore, for two heterojunction devices, the performance of the F₁₆CuPc/CuPc-based transistor is better than that of F₁₆CuPc/pentacene. This is attributed to the morphologic match of two organic components.     

Structure and morphology of CuPc and F16CuPc pn heterojunction

This article reports structure and morphology of copper phthalocyanine (CuPc) and fluorinated copper phthalocyanine (F₁₆CuPc) pn heterojunction. Highly ordered CuPc and F₁₆CuPc polycrystalline thin films with the 2 0 0 plane spacing s of 1.30 and 1.56 nm, respectively, could be continuously grown via an intermediate-phase layer. Compared with CuPc, the intermediate-phase layer is much thinner when F₁₆CuPc is used as the first layer. The rougher the first layer is, the thicker the intermediate-phase layer is. Similarly, the 2 0 0 plane spacings of the intermediate-phase layer are dependent on morphology of the first layer. Furthermore, morphology of the heterostructure is mainly dominated by that of CuPc films. Due to the thicker intermediate-phase layer in the CuPc/F₁₆CuPc heterostructure, the thin film transistors (TFT) performance is obviously inferior to that of the F₁₆CuPc/CuPc device.     

Thermoelectric properties of manganese silicide films

Manganese silicide films (MnSi and MnSi_1.73) were grown on silicon and glass substrates. The films were characterized by X-ray diffraction, Auger electron spectroscopy, and thermoelectric measurements. The films were n-type when they were deposited on n-type (100) silicon substrates with resistivity of 3–5 ohm-cm. P-type films were obtained when the films were deposited on glass or n-type (100) silicon substrates with resistivity of greater than 6000 ohm-cm. An analysis of the resistivity data in the intrinsic regime suggests that MnSi_1.7 has an energy gap of 0.984 eV. PACS 73.50.Lw; 72.20.-i; 72.80.Ga; 73.61.-r; 73.61.Le     

Saturation magnetization distribution in the vicinity of a grain boundary in a plate of a triaxial silicon-iron-type ferromagnet

The one-dimensional saturation magnetization distribution M _S(x) in the vicinity of grain boundaries (GBs) is studied in $(01\bar 1)$ plates of soft magnetic alloys of the Fe-3% Si type in the framework of the theory of micromagnetism. The mechanisms of the emergence of spatial inhomogeneities of the saturation magnetization near planar GBs are discussed. Calculations show that macroscopic regions Δx≈10 mm in extent arise near GBs because of the μ* effect. This result is supported by the experimental data.     

Interface of anodic sulfide-oxide on n-type InSb

A novel anodic sulfidization process for forming native sulfide-oxide films on n-type InSb is described. The results of Auger electron spectroscopy analysis indicate that native sulfide-oxide films are formed from aqueous sulfide solutions. The measured capacitance-voltage characteristics of metal-insulator-semiconductor devices indicate that the films have a low fixed surface charge density of the order of 3×10¹⁰ cm^–2 and small hysteresis. The native sulfide-oxide films leave the surface of n-type InSb practically at flatband and in this respect are superior to the passivation layers of anodic oxide and direct plasma SiN _x . The interface between InSb and its native sulfide-oxide in combination with plasma CVD SiN _x has excellent electrical properties.     

The grain size distribution in nanocomposite films

The grain size distributions and related mechanisms in nanocomposite films with nanostructures comprising a nanocrystalline (nc) phase surrounded by an amorphous (a) matrix under different amorphous phase amounts (V _a) have been analyzed by using a Monte Carlo grain growth model. The results show that with the V _a value increasing to a critical value of ～28%, the grain size distribution approaches lognormality, and it becomes off-lognormal when the V _a value is larger or smaller than ～28%. The simulated results are in a good agreement with the experiment. It is shown that the homogenous or inhomogeneous grain growth mode, determined by the energy exerted on the grain boundary, originates in lognormal or off-lognormal grain size distributions in nanocomposite films. Also, in a system with lognormal grain size distribution, the amorphous phase just covers all grain boundaries (GBs) and the length obtained by summing the boundary circumference of all nanograins is the longest. It is expected that this microstructure can result in exceptional properties of nanocomposite films.     

Σ3 CSL boundary distributions in an austenitic stainless steel subjected to multidirectional forging followed by annealing

The effect of processing and annealing temperatures on the grain boundary characters in the ultrafine-grained structure of a 304-type austenitic stainless steel was studied. An S304H steel was subjected to multidirectional forging (MDF) at 500–800°C to total strains of ~4, followed by annealing at 800–1,000°C for 30 min. The MDF resulted in the formation of ultrafine-grained microstructures with mean grain sizes of 0.28–0.85 μm depending on the processing temperature. The annealing behaviour of the ultrafine-grained steel was characterized by the development of continuous post-dynamic recrystallization including a rapid recovery followed by a gradual grain growth. The post-dynamically recrystallized grain size depended on both the deformation temperature and the annealing temperature. The recrystallization kinetics was reduced with an increase in the temperature of the preceding deformation. The grain growth during post-dynamic recrystallization was accompanied by an increase in the fraction of Σ3ⁿ CSL boundaries, which was defined by a relative change in the grain size, i.e. a ratio of the annealed grain size to that evolved by preceding warm working (D/D₀). The fraction of Σ3ⁿ CSL boundaries sharply rose to approximately 0.5 in the range of D/D₀ from 1 to 5, which can be considered as early stage of continuous post-dynamic recrystallization. Then, the rate of increase in the fraction of Σ3ⁿ CSL boundaries slowed down significantly in the range of D/D₀ > 5. A fivefold increase in the grain size by annealing is a necessary condition to obtain approximately 50% Σ3ⁿ CSL boundaries in the recrystallized microstructure.     

Grain Boundaries in Barium Strontium Titanate Thin Films: Structure, Chemistry and Influence on Electronic Properties

In this paper, we investigate the role of grain boundaries in polycrystalline (Ba _x Sr_1–x )Ti_1+y O_3+z films, grown by metal organic vapor deposition, in the accommodation of nonstoichiometry, as well as their role in the strong composition dependence of the electric and dielectric behavior observed in these films. High-spatial resolution electron energy-loss spectroscopy is used for the analysis of composition and structural changes at grain boundaries, as a function of film composition. The existence of amorphous, titanium rich, TiO₂-like phases at the grain boundaries of films with large amounts of excess Ti (y 0.08) may explain the non-monotonic resistance degradation behavior of the films as a function of Ti content. However, we show that a grain boundary phase model fails to explain the strong composition dependence of the dielectric behavior. Electron energy-loss spectra indicate a distortion of the Ti–O octahedra in the grain interiors in samples with increasing Ti excess. The decrease of the dielectric constant with increasing amounts of excess Ti is therefore more likely due to Ti accommodation in the grain interiors.