Thin Film Diffusion Research Articles

Spin‐on‐dopant phosphorus diffusion in germanium thin films for near‐infrared detectors

AbstractThreading dislocations in germanium thin films on silicon introduce acceptor states in the germanium bandgap close to the valence band. Thus, highly defected germanium thin films spontaneously exhibit a p‐type behavior. Here we report on spin‐on‐dopant diffusion of phosphorus in thermally evaporated, highly defected germanium thin films. We demonstrate effective compensation of the acceptor states associated to dislocations by means of post‐growth doping.We discuss phosphorus diffusion in these highly defected films and pinpoint the benefits of spin‐on‐doping by realizing and testing near‐infrared photodiodes in evaporated Ge on Si, achieving high responsivities which compare well with those of state‐of‐the‐art Ge p‐i‐n photodiodes. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Monte Carlo Simulation of Carrier Diffusion in Organic Thin Films with Morphological Inhomogeneity

Monte Carlo simulation was carried out to understand the influence of morphological inhomogeneity on carrier diffusion in organic thin films. The morphological inhomogeneity was considered in the simulation by incorporating the regions of low energetic disorder in a host lattice of high energetic disorder which decreases the overall energetic disorder of the system. For the homogeneous films, the carrier diffusion was found to decrease upon decreasing the energetic disorder. In contrast to this, in the case of inhomogeneous films the carrier diffusion enhanced upon decreasing the overall energetic disorder, up to an optimum value and beyond which the carrier diffusion decreased. Through our simulation, we observed that the behavior of carrier diffusion in the inhomogeneous case is due to the morphology dependent carrier spreading, which acts in addition to the thermal and non-thermal field assisted diffusion mechanisms. This morphological dependence of carrier spreading arises due to the generation of packets of carriers with different jump rates, which is after effect of slow relaxation of the carriers generated in the less disordered regions of inhomogeneous system. Our simulation of morphology dependent carrier spreading and its influence on the basic diffusion process provide deeper insight into the charge transport mechanisms in organic thin films.

Enhanced diffusion caused by surface reactions in thin films of Sn–Cu–Mn

This paper describes an investigation of low-temperature diffusion in thin films of Sn/Cu, Cu/Mn and Sn/Cu/Mn. The combination of Rutherford backscattering spectroscopy, grazing incidence X-ray diffraction and Auger electron spectroscopy has been used to provide information about the concentration distribution of components and the depth of the various phases and interfaces. The results show that processes that occur on a free surface of thin films can stimulate diffusion of elements in the bulk. It is shown that the element with higher affinity to oxygen tends to move to the surface in order to oxidize and that this oxidation process provides a driving force for further diffusion. Introduction of a third layer to a bilayer system may lead to changes in the surface topography caused by the reactive diffusion between upper layers and it causes the development of fast, ‘short-circuit’ diffusion paths and so increases the diffusion of elements from the bottom to the top. Reactive diffusion between Sn and Cu in the top layers leads to significant changes at the surface with features of the size of 10 to 20nm appearing. These changes provide shortcuts for the diffusion of Mn atoms from the bottom layer to the free surface. As a consequence of these processes, the sequence of layers of different elements in thin-film structures alters diffusion and results in different sample properties.

CdZnS thin films sublimated by closed space using mechanical mixing: A new approach

Cadmium sulfide (CdS) is a prominent material for its tunable band gap used as a window layer in II–VI semiconductor solar cells. The light trapping capability of window layer is one of the powerful tools to enhance the efficiency of the cell. CdS and zinc (Zn) powders were mixed mechanically with different weight percents to make CdZnS (CZS) powder. CZS was deposited onto an ultrasonically cleaned glass substrate using close spaced sublimation (CSS) technique. CZS as-deposited thin films were characterized for structural, surface morphology with energy dispersive X-rays (EDX) and optical properties for the use of window layer in CdS/CdTe based solar cells. The different Zn concentrations in CZS played a vital role on crystallite size in structural analysis and optical properties e.g. transmission, absorption coefficient and energy band gap, etc. The crystallite size of as-deposited CZS thin films were increased as Zn concentration was increased up to certain value. The energy band gap varies from 2.42eV to 2.57eV for as-deposited CZS thin films with increasing Zn concentrations and surface morphology changes also. These changes were occurred due to zinc diffusion in CdS thin films. An angle resolved transmission data was taken to check the behavior of CdS and CZS thin film at different angles. A comparative study was carried out between CdS thin films and CZS thin films for the use of good window layer material.

Kinetic pathways of diffusion and solid-state reactions in nanostructured thin films

Mass transport and solid-state reactions in nanocrystalline thin films are reviewed. It is illustrated that diffusion along different grain boundaries (GBs) can have important effects on the overall intermixing process between two pure films. These processes can be well characterized by a bimodal GB network, with different (fast and slow) diffusivities. First the atoms migrate along fast GBs and accumulate at the film surface. These accumulated atoms form a secondary diffusion source for back diffusion along slow boundaries. Thus the different GBs of the thin films can be gradually filled up with the diffusing atoms and composition depth profiles reflect the result of these processes. Similar processes can be observed in binary systems with intermetallic layers: instead of nucleation and growth of the reaction layer at the initial interface, the reaction takes place in the GBs and the amount of the product phase grows by the motion of its interfaces perpendicular to the GBs. Thus, the entire layer of the pure parent films can be consumed by this GB diffusion-induced solid-state reaction (GBDIREAC), and a fully homogeneous product layer can be obtained.

Thermochromic properties of VO2 thin film on SiNx buffered glass substrate

VO2 thin films were deposited on soda lime glass substrates with silicon nitride sodium-diffusion barrier layer as diffusion barrier, in order to investigate the effect of sodium ion diffusion on the formation of VO2. SiNx layers with thicknesses over 30nm were found to successfully prevent sodium ion diffusion in VO2 thin film and also contribute to the formation of VO2 thin film, which was confirmed by XRD spectra and XPS measurements. The change of infrared transmittance at 2500nm wavelength with temperature change from room temperature to 80°C was increased significantly, and the optical hysteresis width of the sample decreased by almost 6K as well. The results suggest that applying diffusion barrier can improve the thermochromic properties of the VO2 films for energy-saving smart coatings, and silicon nitride can be one of the effective materials to prevent sodium ion diffusion.

X-Ray Diffraction Study of Plasma Exposed and Annealed AlSb Bilayer Thin Film

Aluminum antimony seems to be a promising semiconducting material for high temperature applications, especially for transistors and P-N junction diodes. Additionally, it is a highly efficient solar material. This paper discusses the plasma induced bilayer diffusion of AlSb bilayer thin films using X-ray diffractogram. AlSb bilayer thin films were prepared on a glass substrate by vacuum evaporation technique. The effect of plasma exposure time and annealing temperature on the micro-structural parameters were investigated. X-ray diffraction studies show that the cubic crystals of Al orient along the (111) plane and the hexagonal crystals of Sb orient along the (003) plane. Newly formed cubic crystals of AlSb are oriented along the (200) plane and they are formed due to the simultaneous growth of Al and Sb crystals during plasma exposure.

Comparison of Cu thin films deposited on Si substrates with different surfaces and temperatures

Deposition and growth of Cu thin films on Si(100), Si(110) and Si(111) substrates are studied using molecular dynamics method. The Cu/Si interface diffusion, surface roughness, crystalline structure and growth orientation of Cu thin films are investigated in detail. The effects of substrate surface and temperature are analyzed. Our simulation results show that the number of Cu atoms getting across the substrate surface for Si(111) substrate is the largest, and the number for Si(110) substrate is the smallest. This is caused by the difference of the linear atomic densities and planar atomic densities of Si crystal in different directions and planes. The growth of Cu thin films deposited on Si(100) substrate is 〈100〉 oriented at low temperature, and gradually changes to be 〈111〉 oriented as the increasing of substrate temperature. On the other side, the growth of Cu thin films deposited on Si(110) and Si(111) substrates is always 〈111〉 oriented. Increasing substrate temperature could effectively reduce surface roughness, increase the number of Cu atoms with face-centered cubic (fcc) structure, but meanwhile increase the Cu/Si interface diffusion. Under the same substrate temperature condition, the number of Cu atoms with fcc structure in thin films deposited on Si(110) substrate is larger than that deposited on Si(111) substrate.

Evaluation of PPC Based Nanocomposite for Biomedical and Food Packaging Applications

Environmental friendly and biodegradable nanocomposite (NC) has been developed by efficient intercalation of poly propylene carbonate (PPC) into the interlayer space of cloisite 20B (C-20B) for biomedical and food packaging applications. The physicochemical and structural characterization was studied by X-ray diffraction (XRD) and Fourier transforms infrared spectroscopy. Thermal stability, mechanical property and anti-water sorption capacity of PPC and nanocomposite were scrutinized by thermal gravimetric analysis (TGA), differential scanning calorimeter (DSC), nanoindentation analyzer and thin film diffusion analyzer, respectively. Nanocomposite showed very high thermal decomposition temperature (Td50%) and 10 °C higher glass transition temperature when compared to pure PPC. In addition the nanocomposite exposes high elastic modulus, hardness and anti-water sorption property as compared to PPC. Therefore, this nanocomposite can be used in biomedical and food packaging applications.

Characterizing Local O2 Diffusive Losses in GDLs of PEFCs Using Simplified Flow Field Patterns (“2D”,“1D”,“0D”)

By means of advanced diagnostic tools – neutron imaging, pulsed gas method (helox or O2 pulses during air operation) and electrochemical impedance spectroscopy (EIS) – three different flow fields are characterized in a differential (small-size cell operated at high stoichiometry) Polymer Electrolyte Fuel Cell (PEFC) to emulate the local behavior of a full-size cell. The channel-rib structures are designed so as to obtain 2D, 1D and 0D gradients of O2 concentration across the gas diffusion layer (GDL). The important bulk-diffusion losses (evaluated with pulsed helox) of the “2D” cell compared to the “1D” cell indicate the effect of lateral oxygen diffusion in the GDL. Non-bulk diffusion losses (Knudsen and/or thin film diffusion, evaluated with pulsed O2) also play a non-negligible role on the overall diffusive losses. In particular, their increase in dry conditions for the “1D” and the “0D” cells might be a consequence of drying effects in the electrode.EIS spectra can be interpreted consistently with the results of the pulsed gas method: the increase of arc diameter can be correlated with the overall mass transport losses including both bulk and non-bulk diffusion losses.

Thermal Diffusivities of Bilayer Films Deposited on Substrates Characterized Simultaneously by Transient Reflecting Grating Technique

The transient reflecting grating (TRG) technique has been applied to characterize the thermal diffusivities of thin films in trilayered structures. Based on the TRG behavior in the trilayered structures of opaque film/transparent film/substrate, a two-dimensional theoretical model describing the thermal diffusion processes in the trilayed structures is presented and calculated by the finite-element method. The normalized TRG signals decayed with time can be expressed as a triple-exponential function with different time-dependent coefficients. The influences of the thermal properties of the first and second layer films on the TRG signals are calculated and discussed separately. By fitting the theoretical decay curves to the experimental measured data of a series of trilayered structures (Au/ZnO/Si and Al/ZnO/Si with different film thicknesses), the thermal diffusivities of both films in the trilayered structures can be evaluated simultaneously. The results show that the thermal diffusivities of the thin films are strongly dependent on the thickness of the thin films and significantly lower than the corresponding bulk materials, and decay with the decreasing thickness of the films.

Oxygen vacancy filament formation in TiO2: A kinetic Monte Carlo study

We report a kinetic Monte Carlo (kMC) investigation of an atomistic model for 3-dimensional structural configurations of TiO2 memristor, focusing on the oxygen vacancy migration and interaction under an external voltage bias. kMC allows the access of experimental time scales so that the formation of well defined vacancy filaments in thin TiO2 films can be simulated. The results show that the electric field drives vacancy migration; and vacancy hopping-induced localized electric field plays a key role for the filament evolution. Using the kMC structure of the filaments at different stages of the formation process, electronic density of states (DOS) are calculated by density functional theory. Filament induced gap states are found which gives rise to a transition from insulating behavior to conducting behavior during the filament formation process. By varying kMC simulations parameters, relations between vacancy diffusion, filament formation, and DOS in the TiO2 thin film are elucidated.

Nanoconfinement Effects of Water on Hydrophilic and Hydrophobic Substrates at Cryogenic Temperatures

To gain insights into properties of nanoconfined water, adsorption structure, wettability, surface diffusion, and desorption kinetics of thin films have been investigated by using time-of-flight secondary ion mass spectrometry and temperature programmed desorption. The interfacial hydrogen bond plays a decisive role in both mobility and detachment of the water molecules, as revealed from comparison between hydroxyl- and perfluoroalkyl-terminated Si substrates. The interaction of water with graphite is stronger than that with ordinary hydrophobic substrates, as manifested by higher dewetting temperature and a lower desorption rate. The water molecules on graphite are reoriented upon heating; free OH groups tend to direct toward the substrate. Such an adsorption structure is thought to be responsible for the attractive force leading to higher wettability on graphite, in accordance with predictions of first principle calculations. At least three peaks are identified in temperature programmed desorption of water from graphite, which can be explained as a change in relative contributions between the water–graphite attractive interaction and the intermolecular hydrogen bonds with increasing coverage.

Role of pore morphology in positronium diffusion in mesoporous silica thin films and in positronium emission from the surfaces

Diffusion of ortho-positronium ($o$-Ps) in mesoporous silica films with different pore morphologies and its emission from the surfaces were studied using the Ps time-of-flight method and positron annihilation $\ensuremath{\gamma}$-ray energy spectroscopy. Probabilities of $o$-Ps formation are almost the same in the studied films, however, the emission intensity of $o$-Ps from the film with much larger cagelike pores is significantly less than that from the film with much smaller tubelike pores, where Ps diffusion between cages is feasible by tunneling. A simple strategy is suggested to use mesoporous silica film for the generation of dense enough Ps for many-positronium experiments.

Strong dependency of lithium diffusion on mechanical constraints in high-capacity Li-ion battery electrodes

The effect of external constraints on Li diffusion in high-capacity Li-ion battery electrodes is investigated using a coupled finite deformation theory. It is found that thin-film electrodes on rigid substrates experience much slower diffusion rates compared with free-standing films with the same material properties and geometric dimensions. More importantly, the study reveals that mechanical driving forces tend to retard diffusion in highly-constrained thin films when lithiation-induced softening is considered, in contrast to the fact that mechanical driving forces always enhance diffusion when deformation is fully elastic. The results provide further proof that nano-particles are a better design option for nextgeneration alloy-based electrodes compared with thin films.

Model of lateral diffusion in ultrathin layered films

We consider the diffusion of markers in a layered medium, with the lateral diffusion coefficient being the function of hight. We show that the probability density of the lateral displacements follows a one-dimensional Batchelor’s equation with time-dependent diffusion coefficient governed by the particles’ redistribution in height. For the film of a finite thickness the resulting mean squared displacement exhibits superdiffusion at short times and crosses over to normal diffusion at long times. The approach is used for a description of experimental results on inhomogeneous molecular diffusion in thin liquid films deposited on solid surfaces.

Theoretical investigation of the influence of reaction and diffusion kinetics upon thin-film reactive diffusion

Atomistic kinetic Monte Carlo simulations, considering the thermodynamic model of the nonregular solid solution on a rigid fcc lattice and using the first-neighbor atomic pair interaction Ising model, were performed to investigate the influence of reaction and diffusion kinetics on thin-film reactive diffusion with a semi-infinite substrate. The simulations show that compound bulk energies have no effect on the phase sequence of appearance during sequential phase formation. However, a diffusion asymmetry between the elements composing the solid solution has a strong effect on sequential phase formation. Diffusion asymmetry increases the critical thickness of the simultaneous-to-sequential phase formation transition, modifies the phase formation sequence, and controls which phase is the first to form. Asymmetrical diffusion can promote transient phase formation and can prevent some phases of the bulk phase diagram from forming. In addition, it influences the thicknesses of phases and their lifetimes during thin-film reactive diffusion. Consequently, the control of atom diffusion during reactive diffusion, for example, thanks to the controlled introduction of impurities, should allow the phase formation sequence of a given binary system to be modified.

Impurity-controlled Mo films as diffusion barriers for Cu metallization

Effects of vacuum conditions on the oxygen content and microstructure of Mo layers used with Cu gate lines as thin-film transistor-liquid crystal display diffusion barriers were investigated. Mo was deposited using ion-beam sputtering at 1.0 × 10−5 and 7.0 × 10−7 Torr. The Mo layer oxygen content and the microstructure and changes in chemical composition of the Cu/Mo/SiO2/Si layer during annealing were examined. The Mo layer microstructure was influenced by oxygen; increasing concentration increased the energy required for secondary grain growth. Growth was suppressed at high oxygen levels. Therefore, diffusion barrier performance is enhanced by finer Mo layer grain sizes.

Bandlike Transport in Strongly Coupled and Doped Quantum Dot Solids: A Route to High-Performance Thin-Film Electronics

We report bandlike transport in solution-deposited, CdSe QD thin-films with room temperature field-effect mobilities for electrons of 27 cm(2)/(V s). A concomitant shift and broadening in the QD solid optical absorption compared to that of dispersed samples is consistent with electron delocalization and measured electron mobilities. Annealing indium contacts allows for thermal diffusion and doping of the QD thin-films, shifting the Fermi energy, filling traps, and providing access to the bands. Temperature-dependent measurements show bandlike transport to 220 K on a SiO(2) gate insulator that is extended to 140 K by reducing the interface trap density using an Al(2)O(3)/SiO(2) gate insulator. The use of compact ligands and doping provides a pathway to high performance, solution-deposited QD electronics and optoelectronics.

Control of Grain Boundary Microstructures in Sputtered Gold Thin Films by Surface Energy-Driven Grain Growth

The evolution of grain boundary microstructures in gold thin films during annealing was investigated in order to find a clue to the development of high performance thin films by grain boundary engineering. The {111} oriented grains with the lowest surface energy were preferentially grown by surface energy-driven grain growth during annealing. The sharp {111} texture was developed by annealing at the temperature more than 873K. The remarkably high fraction of low-Σ coincidence site lattice (CSL) boundaries occurred when the area fraction of {111} texture increased to more than 95%. In particular, the fraction of some low-Σ CSL boundaries (Σ1,Σ3,Σ7) for the most sharply {111} textured specimen was found to be one order higher than those predicted for a random polycrystal. The utility of grain boundary engineering is discussed for controlling the performance degradation caused by the percolation phenomena of grain boundary diffusion in gold thin films.