Performance Of Thin-film Devices Research Articles

Energy-Level Interpretation of Carbazole Derivatives in Self-Assembling Monolayer.

Energy-level alignment is a crucial factor in the performance of thin-film devices, such as organic light-emitting diodes and photovoltaics. One way to adjust these energy levels is through chemical modification of the molecules involved. However, this approach may lead to unintended changes in the optical and/or electrical properties of the compound. An alternative method for energy-level adjustment at the interface is the use of self-assembling monolayers (SAMs). Initially, SAMs with passive spacers were employed, creating a surface dipole moment that altered the work function (WF) of the electrode. However, recent advancements have led to the synthesis of SAM molecules with active spacers. This development necessitates considering not only the modification of the electrode's WF but also the ionization energy (IE) of the molecule itself. To measure both the IE of SAM molecules and their impact on the electrode's WF, a relatively simple method is photo-electric emission spectroscopy. Solar cell performance parameters have a higher correlation coefficient with the ionization energy of SAM molecules with carbazole derivatives as spacers (up to 0.97) than the work function of the modified electrode (up to 0.88). Consequently, SAMs consisting of molecules with active spacers can be viewed as hole transport layers rather than interface layers.

Self-Assembled Monolayers for Interfacial Engineering in Solution-Processed Thin-Film Electronic Devices: Design, Fabrication, and Applications.

Interfacial engineering has long been a vital means of improving thin-film device performance, especially for organic electronics, perovskites, and hybrid devices. It greatly facilitates the fabrication and performance of solution-processed thin-film devices, including organic field effect transistors (OFETs), organic solar cells (OSCs), perovskite solar cells (PVSCs), and organic light-emitting diodes (OLEDs). However, due to the limitation of traditional interfacial materials, further progress of these thin-film devices is hampered particularly in terms of stability, flexibility, and sensitivity. The deadlock has gradually been broken through the development of self-assembled monolayers (SAMs), which possess distinct benefits in transparency, diversity, stability, sensitivity, selectivity, and surface passivation ability. In this review, we first showed the evolution of SAMs, elucidating their working mechanisms and structure-property relationships by assessing a wide range of SAM materials reported to date. A comprehensive comparison of various SAM growth, fabrication, and characterization methods was presented to help readers interested in applying SAM to their works. Moreover, the recent progress of the SAM design and applications in mainstream thin-film electronic devices, including OFETs, OSCs, PVSCs and OLEDs, was summarized. Finally, an outlook and prospects section summarizes the major challenges for the further development of SAMs used in thin-film devices.

Accurate determination of band tail properties in amorphous semiconductor thin film with Kelvin probe force microscopy

The disordered microscopic structure of amorphous semiconductors causes the formation of band tails in the density of states (DOS) that strongly affect charge transport properties. Such band tail properties are crucial for understanding and optimizing thin-film device performance with immense relevance for large area electronics. Among the available techniques to measure the DOS, Kelvin Probe Force Microscopy (KPFM) is exceptional as it enables precise local electronic investigations combined with microscopic imaging. However, a model to interpret KPFM spectroscopy data on amorphous semiconductors of finite thickness is lacking. To address this issue, we provide an analytical solution to the Poisson equation for a metal–insulator–semiconductor junction interacting with the atomic force microscope tip. The solution enables us to fit experimental data for semiconductors with finite thickness and to obtain DOS parameters, such as band tail width, doping density, and flat band potential. To demonstrate our method, we perform KPFM experiments on Indium–Gallium–Zinc Oxide (IGZO) thin-film transistors (IGZO-TFTs). DOS parameters compare well with values obtained with photocurrent spectroscopy. We demonstrate the relevance of the developed method by investigating the impact of ionizing radiation on DOS parameters and TFT performance. Our results provide clear evidence that the observed shift in threshold voltage is caused by static charge in the gate dielectric, leading to a shift in flat band potential. Band-tails and doping density are not affected by the radiation. The developed methodology can be easily translated to different semiconductor materials and paves the way for quantitative microscopic mapping of local DOS parameters in thin-film devices.

Controlling supramolecular structures in iron tetrasulfonated phthalocyanine films through pH variation: implications for thin-film device performance.

Improving the performance of thin film-based devices is a crucial factor for their successful application, mainly for organic electronic semiconductors. The adjustment of supramolecular structuring of thin films plays a role in the optical and electrical properties. In this sense, we investigated how various pH values, such as 2.5, 6.0, and 9.0, of the solutions influenced the growth of iron tetrasulfonated phthalocyanine (FeTsPc) Layer-by-Layer (LbL) films and their respective supramolecular structures as well as their electrochemical properties. The supramolecular structures were evaluated via UV-vis absorption spectroscopy, quartz crystal microbalance (QCM), micro-Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and cyclic voltammetry. The different pH values of the solution induce different degrees of molecular aggregation for FeTsPc (monomer, dimer, and aggregate formation). For instance, the higher the pH, the higher the aggregation. Films produced at pH 2.5 were organized preferentially with the molecules perpendicular to the substrate, while films at pH 6.0 and 9.0 were organized preferentially with the molecules parallel to the substrate. Besides, the film produced at pH 2.5 results in higher film thickness, higher stability, and better electrocatalytic behavior for the electrochemical detection of catechol. The results presented here enhance the understanding of nanostructured films, helping to harness supramolecular organization to improve the performance of thin-film devices.

Spatially Resolved Spectroscopic Characterization of Nanostructured Films by Hyperspectral Dark-Field Microscopy.

Nanostructured films have been widely used for preparing various advanced thin-film devices because of their unique electrical, optical, and plasmonic characteristics associated with the nano-size effect. In situ, nondestructive and high-resolution characterization of nanostructured films is essential for optimizing thin-film device performance. In this work, such thin-film characterization was achieved using a hyperspectral dark-field microscope (HSDFM) that was constructed in our laboratory by integrating a hyperspectral imager with a commercial microscope. The HSDFM allows for high-resolution (Δλ = 0.4 nm) spectral analysis of nanostructured samples in the visible-near-infrared region with a spatial resolution as high as 45 nm × 45 nm (corresponding to a single pixel). Four typical samples were investigated with the HSDFM, including the gold nanoplate array, the self-assembled gold nanoparticle (GNP) sub-monolayer, the sol-gel nanoporous titanium dioxide (TiO2) film, and the layer-stacked molybdenum disulfide (MoS2) sheet. According to the experimental results, the plasmon resonance scattering bands for nanoplate clusters are identical with those for individual gold nanoplates, indicating that the gap between adjacent nanoplates is too large to allow plasmonic coupling between them. A different case was observed with the self-assembled GNP sub-monolayer in which the aggregated clusters with the internal plasmonic interaction show a considerable red-shift of the plasmon resonance band relative to the isolated single GNP. In addition, the protein adsorption on the nanoporous TiO2 film was observed to be inhomogeneous on the microscale, and the stepped boundaries of the MoS2 sheet were clearly observed. A quasi-linear dependence of the single-pixel light intensity on the step height was obtained by combining the HSDFM with atomic force microscopy. The minimum thickness detectable by the present HSDFM is 6.5 nm, corresponding to the 10-layer MoS2 film. The work demonstrated the outstanding applicability of the HSDFM for nanostructured film characterization.

Unravelling atomically resolved structure of a high-k dielectric oxide-semiconductor interface: Exit wave reconstruction and ab-initio calculation insights

Heterostructure interfaces play a major role in defining the performance of thin-film devices. High-k dielectric oxide-semiconductor heterostructures are being extensively investigated as promising candidates for future integrated circuits, thus it becomes important to precisely probe the interfaces at the atomic scale for technological advancements. In this work, a high-k dielectric oxide (Gd2O3)-semiconductor (Ge) interface was characterized at the atomic scale using complex-valued exit wave reconstructed from a set of focal series high-resolution transmission electron microscopy (HRTEM) images acquired without objective lens spherical aberration correction. The complexity of this characterization lies in removing image artefacts produced by amorphous layer deposited on the imaged region during ion milling which was successfully solved using an algorithm to remove amorphous background developed recently. The final result reveals that the interface of the present study is atomically sharp and flat. The thickness of the imaged region along viewing direction was estimated from channelling map. Comparing reconstructed amplitude of experimental data with that of simulated one generated using Density Functional Theory (DFT) optimized interface structure, it was found that the Gd2O3 layers were terminated at the Gd atoms in the interface.

Compressive intrinsic stress originates in the grain boundaries of dense refractory polycrystalline thin films

Intrinsic stresses in vapor deposited thin films have been a topic of considerable scientific and technological interest owing to their importance for functionality and performance of thin film devices. The origin of compressive stresses typically observed during deposition of polycrystalline metal films at conditions that result in high atomic mobility has been under debate in the literature in the course of the past decades. In this study, we contribute towards resolving this debate by investigating the grain size dependence of compressive stress magnitude in dense polycrystalline Mo films grown by magnetron sputtering. Although Mo is a refractory metal and hence exhibits an intrinsically low mobility, low energy ion bombardment is used during growth to enhance atomic mobility and densify the grain boundaries. Concurrently, the lateral grain size is controlled by using appropriate seed layers on which Mo films are grown epitaxially. The combination of in situ stress monitoring with ex situ microstructural characterization reveals a strong, seemingly linear, increase of the compressive stress magnitude on the inverse grain size and thus provides evidence that compressive stress is generated in the grain boundaries of the film. These results are consistent with models suggesting that compressive stresses in metallic films deposited at high homologous temperatures are generated by atom incorporation into and densification of grain boundaries. However, the underlying mechanisms for grain boundary densification might be different from those in the present study where atomic mobility is intrinsically low.

Charge-Transfer Dynamics in Poly(3-hexylthiophene):Perylenediimide-C60 Blend Films Studied by Ultrafast Transient Absorption

Electron-transfer characteristics of 1,7-dipyrrolidinyl-substituted perylenediimide-fullerene (PyPDI-C60) dyad were studied in solid films by an ultrafast transient absorption technique. The research was further expanded to study a poly(3-hexylthiophene) (P3HT):PyPDI-C60 blend to reveal the charge-transfer processes that take place in bulk heterojunction films, which are potentially suitable for photovoltaic applications. This knowledge is of paramount importance when designing organic photovoltaics since the performance of thin film devices is affected by mechanisms taking place at the molecular level upon photoexcitation. It was demonstrated that the charge- separated state, formed after an electron transfer from the PyPDI to the fullerene moiety of the dyad, has a lifetime of tens of picoseconds. A charge-separated state in which the hole is localized in the P3HT and the electron is in the fullerene moiety with a lifetime of <1 ps was shown for the blend film. The intermolecular charge-separated state ...

Understanding the chemical origin of improved thin-film device performance from photodoped ZnO nanoparticles

The chemical origin of the UV-enhanced conductivity of ZnO nanoparticle thin-films is studied. Improved electrical properties are shown for organic photovoltaics, thin-film transistors and diodes utilizing UV-illuminated nanoparticle ZnO films. Using impedance spectroscopy we find that as prepared ZnO nanoparticle films exhibit clear signatures of grain boundary conduction, which is eliminated following UV illumination in nitrogen atmosphere. Exposure to ambient air is found to increase the grain boundary resistance to its original state after oxygen, then water, adsorb to the surface. Gas chromatography mass spectrometry confirms that, as with bulk ZnO, the UV illumination process results in desorption of oxygen as well as oxidation of carbonaceous species at the surface. Loss of photo-generated holes through this oxidative process results in excess electrons in the ZnO, which in turn produces the observed electronic changes.

Sintering Characterizations of Ag-Nano Film on Silicon Substrate

Nowadays, thin films have many applications in every field. So, in order to improve the performance of thin film devices, it is necessary to characterize their mechanical as well as electrical properties. In this research work we focus on the development of a model for the analysis of the mechanical and electrical properties of silver nanoparticles deposited on silicon substrates. The model consists of inter-particle diffusion modeling and finite element analysis. In this study, through the simulation of the sintering process, it is shown that how the geometry, density, and electrical resistance of the thin film layer are changed with sintering conditions. The model is also used to approximate the values of the film Youngs modulus. Comparing results with experimental results shows the high accuracy of this approach.

Electrical and Reliability Characteristics of Barrierless Cu-Based Contact for High Dielectric Constant Oxide Thin-Film Device Integration

We report a stable Cu-based contact stack to overcome Cu diffusion and oxidation problems encountered during the high dielectric constant oxide thin-film device integration. Our contact stack is also beneficial from the barrierless scheme for improving the thin-film device performance. Interfacial investigation reveals no extensive interaction between and barrierless Cu contact stack after annealing up to . on the low resistivity Cu-based contact stack behaves fairly similar to that on the Pt counterpart as the symmetry of the leakage current is obtained using different work function contacts (Cu and Pt). As Cu is compatible with integrated circuit processing, our barrierless contact stack is readily applicable for integration with Si-based devices.

Thin Film Structure of Tetraceno[2,3-b]thiophene Characterized by Grazing Incidence X-ray Scattering and Near-Edge X-ray Absorption Fine Structure Analysis

Understanding the structure-property relationship for organic semiconductors is crucial in rational molecular design and organic thin film process control. Charge carrier transport in organic field-effect transistors predominantly occurs in a few semiconductor layers close to the interface in contact with the dielectric layer, and the transport properties depend sensitively on the precise molecular packing. Therefore, a better understanding of the impact of molecular packing and thin film morphology in the first few monolayers above the dielectric layer on charge transport is needed to improve the transistor performance. In this Article, we show that the detailed molecular packing in thin organic semiconductor films can be solved through a combination of grazing incidence X-ray diffraction (GIXD), near-edge X-ray absorption spectra fine structure (NEXAFS) spectroscopy, energy minimization packing calculations, and structure refinement of the diffraction data. We solve the thin film structure for 2 and 20 nm thick films of tetraceno[2,3-b]thiophene and detect only a single phase for these thicknesses. The GIXD yields accurate unit cell dimensions, while the precise molecular arrangement in the unit cell was found from the energy minimization and structure refinement; the NEXAFS yields a consistent molecular tilt. For the 20 nm film, the unit cell is triclinic with a = 5.96 A, b = 7.71 A, c = 15.16 A, alpha = 97.30 degrees, beta = 95.63 degrees, gamma = 90 degrees; there are two molecules per unit cell with herringbone packing (49-59 degree angle) and tilted about 7 degrees from the substrate normal. The thin film structure is significantly different from the bulk single-crystal structure, indicating the importance of characterizing thin film to correlate with thin film device performance. The results are compared to the corresponding data for the chemically similar and widely used pentacene. Possible effects of the observed thin film structure and morphology on charge carrier mobility are discussed.

Characteristics of surface and interface for ITO/PTCDA/p-Si thin film device

Interface characteristics possess very important influence on the performance of thin film devices. ITO/PTCDA/p-Si thin film device was set up with vacuum evaporation and sputter deposition method. The surface and interface electron states of ITO/PTCDA/p-Si were investigated by X-ray photoelectron spectroscopy (XPS) and argon ion beam etch techniques. Results indicate that at the interface of ITO/PTCDA/p-Si, not only ITO/PTCDA-Si but also PDCDA-Si can produce diffusion. Moreover, the XPS spectra of each atom appear chemical shifts, and the chemical shifts of Ols and Ols are more remarkable.

Smoothing of ZnO films by gas cluster ion beam

Planarization of wide-band-gap semiconductor ZnO surface is crucial for thin-film device performance. In this study, the rough initial surfaces of ZnO films deposited by r.f. magnetron sputtering on Si substrates were smoothed by gas cluster ion beams. AFM measurements show that the average surface roughness (Ra) of the ZnO films could be reduced considerably from 16.1nm to 0.9nm. Raman spectroscopy was used to monitor the structure of both the as-grown and the smoothed ZnO films. Rutherford back-scattering in combination with channeling effect was used to study the damage production induced by the cluster bombardment.

Deposition of polycrystalline thin films with controlled grain size

Difficulties in controlling the grain size and size distribution in polycrystalline thin films are a major obstacle in achieving efficient performance of thin film devices. In this paper we describe a sputtering technology that allows the control of the grain size and size distribution in sputtered films without the use of seed layers, substrate heating or additives. This is demonstrated for three different materials (Cr, NiFe and FeMn) via transmission electron microscopy imaging and grain size analysis performed using the cumulative percentage method. The mean grain size was controlled only via the sputtering rate. We show that higher sputtering rates promote the growth of larger grains. Similar trends were obtained in the standard deviation, which showed a clear reduction with the sputtering rate.

Comparison of memory switching operation in a number of amorphous chalcogenide systems

A number of chalcogenide glasses were investigated for thin film switching applications. Two ranges of threshold voltage were of interest, 15 and 30 V. The switching performance of thin film devices was evaluated and rated on a simple numerical scale. The memory glasses based on the Ge-Te eutectic gave generally satisfactory performance. Selenium-based glasses exhibited high threshold voltage in thin film form, but had limited lifetime. Threshold voltages of about 30 V were obtained from Bi-As-Se glasses; these proved difficult to lock “ON” and possible reasons for this are discussed.

Heat Flow as a Limiting Factor in Thin-Film Devices

The performance of thin-film devices can be limited by thermal failure. A model for thermal power dissipation of a thin-film device on a glass substrate has provided the basis for the solution of the heat flow equation. Solutions for the maximum temperature as a function of power input and rectangular device and substrate dimensions have been obtained by digital computations. These results have been applied to the TFT and to the Mead device, and it is concluded that the Mead device will suffer thermal destruction before it can achieve a figure of merit greater than 10 Mc/sec whereas the TFT is not limited by thermal considerations until a figure of merit of several hundreds of Mc/sec has been achieved.