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Institute
Microscopic and spectroscopic characterization of interfaces and dielectric layers for OFET devices
(2009)
Copper oxide (CuO) is an attractive p-type oxide semiconductor with narrow band gap that has intriguing properties such as non-toxicity, chemical stability, electrocatalytic activity, abundant availability and low production cost. Polymeric materials have attracted great interest owing to their high flexibility, light weight, low cost production and high resistance against corrosion. Poly(vinylidene fluoride) (PVDF) is a commonly used piezoelectric polymer due to its excellent flexibility and piezoelectric properties. In this work, we adopted a one-pot synthesis of CuO nanowires by facile and selective chemical etching process of Cu substrate. XPS, XRD, SEM data ascribed the formation of polycrystalline CuO nanowires over the entire Cu surface. It has been observed that the uniformity, shape and size of the nanowires could be conveniently controlled by etching time and etchant concentration. We have also prepared piezoelectric nanogenerator based on the flexible PVDF/CuO composite film, where the traditional electrical poling treatment was avoided. In addition, dielectric properties have been studied to demonstrate the role of interfacial polarization.
Aluminum nitride films prepared by plasma atomic layer deposition using different plasma sources
(2018)
Aluminum nitride (AlN) thin films are promising for versatile applications in optoelectronics, electronics, piezoelectrics, and acoustics due to their remarkable properties such as wide band gap, high dielectric constant, low electrical conductivity, good piezoelectric coefficient and high ultrasonic velocity. We present a comparative study of AlN films grown by plasma-enhanced atomic layer deposition at 350°C silicon wafers in the SENTECH SI ALD LL system using TMA and NH3 where either a capacitively coupled plasma (CCP) or a direct PTSA (planar triple spiral antenna) source was applied. The films were characterized by ellipsometry, XPS and electrical measurements. The layer properties are discussed concerning the varied ALD process parameters. In general, the process using the direct PTSA source delivered films with higher refractive index and better homogeneity over the wafer achieving also higher growth rates per cycle (GPC) in reduced total cycle durations. Films with refractive index in the range of 2.05 and permittivity around 8 could be realized with a GPC of 1.54 Å/cycle.
We study the electronic structure of ultrathin Al₂O₃, HfO₂, and TiO₂ ALD films by resonant photoelectron spectroscopy. We identify intrinsic defects which are responsible for the active sites in interface reactions, for the incorporation of intrinsic charges, and for the formation of local dipole momenta. All of these features determine the surface potentials and the reactivity of the surface of the atomic layer deposition coated systems. We give examples of charges and dipoles in Al₂O₃, on a study of the surface potentials in HfO₂, and relate the intrinsic defects in TiO₂ to their electrochemical relevance.
A self-poled piezoelectric nanogenerator (NG) based on bismuth oxide (Bi2O3) nanostructures (BNS)-doped poly(vinylidene fluoride) (PVDF) is demonstrated. The in-situ formation of BNS incorporated in the PVDF matrix is realized through a simple solution casting technique that favors the nucleation and stabilization of 99.6% of the electroactive phase in PVDF (i.e., beta/gamma-phases). The enhancement of the output voltage (3.6 V) and current (2.4mA) of the NG based on a nonelectrically poled BNS containing PVDF composite film is achieved and demonstrated by simple repeated human finger imparting. The improvement of the output performance is influenced by the generation of the electroactive polar beta-phase in PVDF, due to the electrostatic interactions between the –CH2–/–CF2– dipoles of PVDF and the surface charge of the BNS.
The NGs are able to charge up capacitors demonstrating that the power generation from such a hybrid device structure can be utilized for powering various portable devices.
In situ real-time ellipsometry (irtE) with a very high time resolution of 24 ms was applied to monitor the inductively coupled plasma enhanced atomic layer deposition (ALD) process of Al₂O₃ thin films to precisely resolve each step of the ALD process and its complete cycle. The influence of plasma power, plasma pulse duration, and deposition temperature on the film growth characteristics was investigated. Ex situ ellipsometry [UV-VIS-NIR-SE (ultraviolet-visible-nearinfrared-spectroscopic ellipsometry) and IR-SE (infrared spectroscopic ellipsometry)] and x-ray photoelectron spectroscopy revealed the bulk properties (thickness, refractive index, chemical composition, and carbon incorporation) of the films, which together with the in situ results are compared to those of the films prepared by thermal ALD (T-ALD). The ICPEALD (inductively coupled plasma enhanced ALD) films were deposited at substrate temperatures between 80 and 250 °C and the role of plasma power (50–300 W) and its pulse duration (1–20 s) was investigated at 250 °C. The reference T-ALD layers were prepared at 200 °C. The ICPEALD process of Al₂O₃ shows an increased growth rate, and the produced films exhibit higher carbon contaminations than the T-ALD Al₂O₃ films. Plasma pulse times of up to 15 s further increase the content of carbon and CH species; at the same time, the refractive index decreases. The optical properties of ICPEALD deposited Al₂O₃ films are comparable with those of the T-ALD films for low plasma power and short plasma pulse durations. For the ICPEALD films, UV absorption is found and it is dependent on the deposition parameters. irtE resolves process effects that correlate with the bulk properties of Al₂O₃, such as impurities and oxygen deficiencies.
The design and development of intrinsically stretchable all-organic self-powered sensors concurrently perceiving temperature and pressure remain a challenge but deliver an exciting platform to realize environmentally friendly wearable electronics. In this approach, a biomimetic all-organic stretchable energy harvester is designed by a xylitol-added poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS/Xyl) film as a compatible overlay electrode with polyaniline-reinforced one-dimensional aligned poly(vinylidene fluoride) hybrid electroactive soft nanowires. The gradient of elastic modulus between the electrode and the active nanowire component enables the all-organic device to manifest excellent power-generating performance under external temperature fluctuation (∼3 μW/m2 under ΔT ∼ 92 K) and mechanical force (∼31 μW/cm2 at 30 N). Importantly, the device renders simultaneous energy scavenging of temperature and pressure changes under pressing and stretching conditions (∼20%). The excellent mechanosensitivity (∼100 mV/N), fast response time (∼1 ms), outstanding mechanical and thermal stability, and good temperature resolution <10 K enable the harvester to act as an epidermal sensor, which simultaneously detects and discriminates both subtle pressure and thermal deviations exposed to an epidermis surface. The real-time recording and wireless transferring of physiological signals to a smartphone indicate an effective way to realize remote healthcare monitoring for early intervention.
The band alignment of two large band gap oxides was studied by a combination of XPS and UPS using consecutive sputter steps to unravel the electronic structure and elemental composition of each layer and the interface region. An Al2O3/ZnO heterointerface (10 nm Al2O3 on 59 nm ZnO) was grown on top of a Si single crystal substrate by consecutive thermal and plasma-assisted atomic layer deposition (ALD) respectively. The valence band maximum of Al2O3 was found to be 1.1 eV below that of ZnO, the conduction band minimum 2.3 eV above, resulting in a type-I staggered heterojunction. A reduction of ZnO to elemental Zn in the interface region was detected by the apparent shoulder of the Zn 2p and 2s core levels and by the Zn LMM Auger. This suggests an ALD interface formation mechanism different from previous models identified for other heterointerfaces.
Indium oxide thin films were deposited on Si (100) by plasma-enhanced atomic layer deposition (PEALD) using trimethylindium (TMIn) and oxygen plasma (O2) in a low-temperature range of 80 to 200 °C. The In2O3 layers were characterized by in-situ spectroscopic ellipsometry (SE), ex-situ X-ray photoelectron spectroscopy (XPS) and electrical measurements. The SE data show a growth rate of 0.56 Å/cycle within the ALD window (100 to 150 °C) with a thickness inhomogeneity of ≤1.2%. In addition, the highest refractive index is 2.07 (at 632.8 nm) for the layer grown at 150 °C, and the films exhibit indirect and direct band gaps of 2.8±0.1 eV and 3.3±0.2 eV, respectively. XPS characterization indicates no carbon incorporation and a temperature-dependent off-stoichiometry of the layers. The chemical analysis of the In 3d and O 1s core levels confirms the formation of In-O bonds and suggests the additional presence of hydroxyl groups and defects. With increasing temperature, the contribution of OH groups and defects decreases whereas that of In-O bonds increases. Notably, higher growth temperatures result in an indium rich phase within the layers.
Functional ultra-thin oxide films deposited by atomic layer deposition on structured substrates
(2021)
In the last decades, atomic layer deposition (ALD) has gained prominence in the materials and surface science communities owing to its high potential for integration as a scalable process in microelectronics. ALD's largest strengths are its well-controlled layer-by-layer deposition and growth conformity on 3D structures. Yet, the ALD technique is also well known to lead to amorphous and defective, non-stoichiometric thin films, resulting in modified materials properties that may even preferentially be used in certain applications. To study these issues, we have developed an in-situ ALD reactor attached to an X-ray photoelectron spectroscopy (XPS) system, capable of switching between both pump and flow-type operation. This novel tool allows to cover the entire range of compounds and recipes used in ALD, thus clarifying the role of such defects at different deposition stages, growth conditions and film/substrate interfaces. To exemplify these sorts of studies, we show the deposition of Al2O3 5-10 nm films on nanostructured Si, and their use as substrates for functional CeOx ALD deposits.
A super-cycle approach to atomic layer deposition of indium-gallium-zinc oxide at low temperature
(2023)
The continuing development of multifunctional devices needs novel multicomponent oxide layers, demanding a high control of both composition and thickness during their preparation. To this end, single metal oxides exhibiting high structural quality and conformity have successfully been grown by atomic layer deposition (ALD). However, the deposition of more complex compounds with specific optical and electrical properties is still challenging. In this work, we follow a bottom-up approach to design an ALD super-cycle to grow mixed indium-gallium-zinc oxide (IGZO) films with a controllable composition. For the formation of the individual indium, gallium, and zinc oxides, we found the use of plasma-enhanced ALD (PEALD) at 150 °C to be favorable when using the organometallic precursors trimethylindium, trimethylgallium, and diethylzinc together with oxygen plasma. The PEALD approach of IGZO films can particularly overcome a nucleation delay within the ZnO sub-cycle known from thermal ALD, achieving a higher growth per cycle and improving the quality and composition homogeneity of the films as shown by in-situ spectroscopic ellipsometry and ex-situ X-ray photoelectron spectroscopy.
Atomic layer deposition (ALD) exhibits a high potential for integration as a scalable process in microelectronics, allowing well-controlled layer-by-layer deposition and conformal growth on 3D structures. Yet, the ALD technique is also well known to lead to amorphous and defective, non-stoichiometric films, potentially resulting in modified materials properties that, in the case of ultra-thin deposits, can also be affected by film/substrate interaction. Interestingly, initial in situ X-ray photoemission spectroscopy (XPS) measurements of ceria ALD-deposits on Al2O3/Si, sapphire, and SiO2 substrates confirm a Ce3+/Ce4+ mixture dependent on the substrate interaction, deposit thickness, and morphology. Using near-ambient pressure XPS, we have significantly reduced ultrathin (< 10 nm) ceria films grown by ALD by exposing them to different O2/H2 partial pressures at moderate temperatures (< 525K). Notably, the total amount of reduction to Ce3+ is found to depend on the deposit thickness and initial ceria/substrate interaction. Furthermore, the intrinsic defects related to the ALD method seem to play a critical role in the reversible reduction at room temperature.
The photosensitive and rectifying properties of Ga2O3 are described by (n-type) intrinsic (π-) electrons. These polaronic screened multiatomic carriers populate the intrinsic defect states within the ionic gap; their spectroscopic evidence is based on resonant photoemission spectroscopy data that also provide the ionicity factor of Ga2O3 and the size of the ionic gap. The π-electron density depends on the internal potential and its photo- and field-induced dipole contributions, and it describes the observed combined ohmic-exponential carrier densities and current–voltage dependences. The π-electron dynamics is caused by pairing and dissociation dipoles in the bulk of Ga2O3. The material properties of the electrode contribute via external chemical potentials and define the criteria for ohmic and rectifying contacts. This quantitative and predictive concept not only convinces by perfect agreement with published experimental data but also points toward the achievable performance limits of UV absorbers and rectifying devices.
This work presents a new ultra-high vacuum cluster tool to perform systematic studies of the early growth stages of atomic layer deposited (ALD) ultrathin films following a surface science approach. By combining operando (spectroscopic ellipsometry and quadrupole mass spectrometry) and in situ (X-ray photoelectron spectroscopy) characterization techniques, the cluster allows us to follow the evolution of substrate, film, and reaction intermediates as a function of the total number of ALD cycles, as well as perform a constant diagnosis and evaluation of the ALD process, detecting possible malfunctions that could affect the growth, reproducibility, and conclusions derived from data analysis. The homemade ALD reactor allows the use of multiple precursors and oxidants and its operation under pump and flow-type modes. To illustrate our experimental approach, we revisit the well-known thermal ALD growth of Al2O3 using trimethylaluminum and water. We deeply discuss the role of the metallic Ti thin film substrate at room temperature and 200 °C, highlighting the differences between the heterodeposition (<10 cycles) and the homodeposition (>10 cycles) growth regimes at both conditions. This surface science approach will benefit our understanding of the ALD process, paving the way toward more efficient and controllable manufacturing processes.
Atomic layer deposition (ALD) allows preparation of conformal coatings with possibility to control their thickness at the submonolayer level, making it a good tool for depositing active layers on 3D structures. Our group is working on cerium oxide-based materials for hydrogen detection, which is difficult at ambient conditions due to the low sensitivity and long response time of the sensors. The cerium oxide layers prepared by ALD contain a lot of defects and provide an opportunity to overcome these complications. Thickness and morphology of the oxide films play an important role in defining the Ce3+/Ce4+ ratio, as well as the interface with the used substrate. Here, we compare cerium oxide thin films deposited by ALD techniques on SiO2 and Al2O3 substrates. The results reveal that the interface to the substrate can considerably influence the reactivity of the cerium oxide toward hydrogen and oxygen. Preparation of the oxides using two different precursors (commercial Ce(thd)4 and custom Ce(dpdmg)3) has been demonstrated to affect the redox properties of the films, their reactivity, and the reversibility.
It is assumed that plasma-enhanced atomic layer deposition (PEALD) cannot be used to prepare thin films on sensitive organic-inorganic perovskites because the plasma destroys the perovskite film and thus deteriorates its photophysical properties. Here, we prove that using an appropriate geometry of the ALD system (SENTECH SI PEALD system) and suitable process parameters it is possible to coat perovskites with alumina by PEALD. Spectromicroscopy followed by electrical characterisation reveal that as long as the PEALD process is not optimized (too long plasma pulses) one gets degradation of the perovskite as well as dissociation of the created iodine pentoxide (during PEALD) under light that causes a valence band maximum (VBM) shift to the Fermi level and thus significantly decreases the solar cell efficiency. However, once the PEALD process parameters are optimized, no VBM shift is observed. Moreover, the solar cell efficiency depends inversely on process temperature and layer thickness.
Localized defect states and charge trapping in Al₂O₃ films prepared by atomic layer deposition
(2017)
The evaluation of the electronic structure and intrinsic defect mechanisms in Al₂O₃ thin films is essential for their effective use in applications with desired functionality such as surface passivation schemes for solar cells [1]. We present a comparative study of different Al₂O₃ films grown by atomic layer deposition (ALD) [2]. The layers were deposited on different substrates using the same aluminum precursor (TMA, trimethylalumium) and employing different process parameters (thermal-ALD, plasma-enhanced-ALD, substrate temperature). These films were characterized by resonant photoelectron spectroscopy and by electrical measurements (capacitance-voltage). For all films investigated intrinsic defect states within the electronic band gap were observed including excitonic, polaronic, and charge-transfer defect states, where their relative abundance is subject of the choice of ALD parameters and of the used substrate. The spectroscopic assigned in-gap defect states are related with electronic charges as determined in the electrical measurements. [1] G. Dingemans and W.M.M. Kessels, J. Vac. Sci. Technol. A 30, 040802 (2012). [2] K. Henkel, M. Kot, D. Schmeißer, J. Vac. Sci. Technol. A 35, (2017), accepted.
Low-temperature growth of gallium oxide thin films by plasma-enhanced atomic layer deposition
(2020)
Gallium oxide (Ga2O3) thin films were deposited by plasma-enhanced atomic layer deposition (PEALD) applying a capacitively coupled plasma source where trimethylgallium (TMGa) as the gallium precursor and oxygen (O2) plasma were used in a substrate temperature (Ts)in the range of 80–200 °C. TMGa exhibits high vapor pressure and therefore facilitates deposition at lower substrate temperatures. The Ga2O3 films were characterized by spectroscopic ellipsometry (SE), x-ray photoelectron spectroscopy (XPS), and capacitance-voltage (C-V) measurements. The SE data show linear thickness evolution with a growth rate of ∼0.66 Å per cycle and inhomogeneity of ≤2% for all samples. The refractive index of the Ga2O3 thin films is 1.86 ± 0.01 (at 632.8 nm) and independent of temperature, whereas the bandgap slightly decreases from 4.68 eV at Ts of 80 °C to 4.57 eV at 200 °C. XPS analysis revealed ideal stoichiometric gallium to oxygen ratios of 2:3 for the Ga2O3 layers with the lowest carbon contribution of ∼10% for the sample prepared at 150 °C. The permittivity of the layers is 9.7 ± 0.2 (at 10 kHz). In addition, fixed and mobile oxide charge densities of 2–4E12 and 1–2E12 cm−2, respectively, were observed in the C-V characteristics. Moreover, the Ga2O3 films show breakdown fields in the range of 2.2–2.7 MV/cm. Excellent optical and electrical material properties are maintained even at low substrate temperatures as low as 80 °C. Hence, the TMGa/O2 PEALD process is suitable for electronic and optoelectronic applications where low-temperature growth is required.
Flexible and wearable e-skin sensors are attracting a great interest for their smart sensing applications in next-generation electronics. However, implant ability, sensitivity, and biosignal detection capability in a self-powered manner are the prime concerns in embedded devices. In particular, electrode compatibility and imperishability have become challenging issues in wearable sensors due to the poor compatibility and fragileness of metal electrodes. In this context, we report on a skin-interactive metal-free spongy electrode in a piezoelectric sensor where highly aligned poly(vinylidenefluoride) (PVDF) nanofibers (NFs) arrays are introduced as the piezoelectric active component and conducting polyaniline- (PANI-) coated PVDF (PANI−PVDF) NFs mats served as flexible electrodes. Notably, a 99% yield of piezoelectric phases of the aligned PVDF arrays is the key factor to exhibit promising mechano-sensitivity (0.8 V/kPa) performance that in turn helps in human-health monitoring. The sensor shows excellent mechanical to electrical energy conversion that enable to sense human finger touch (10 V under 10 kPa) with energy conversion efficiency of 53%. Most importantly, due to the compatible electrodes excellent mechanical stability has been found showing negligible degradation over 12,000 periodic cycles. Furthermore, under mechanical stimuli, it is also possible to charge up a capacitor (1μF) to 4 V within 60 s confirming the possibility to use the device as a self-powered piezo-organic-e-skin sensor (POESS). This type of structural design enables to trace elusive movement of muscles and the operation in several conditions such as bending, compression and stretching. We demonstrated various human gestures monitoring, such as wrist bending, neck stretching, and arm compressions, throat movements during drinking water, coughing actions, and swallowing. In addition, diverse specific phonation recognition, heart-pulse measurement and its respective short-time Fourier transform (STFT) analysis indicate an efficient and convenient way of monitoring human-health status particularly in hospital-free mode.
A comparative study of thin aluminum nitride (AlN) films deposited by plasma-enhanced atomic layer deposition in the SENTECH SI ALD LL system applying either a direct inductively coupled plasma (ICP) or an indirect capacitively coupled plasma (CCP) source is presented. The films prepared with the ICP source (based on a planar triple spiral antenna) exhibit improved properties concerning the growth rate per cycle, total cycle duration, homogeneity, refractive index, fixed and mobile electrical charges, and residual oxygen content compared to the CCP source, where the comparison is based on the applied plasma power of 200 W. The increase of the plasma power to 600 W in the ICP process significantly reduces the residual oxygen content and enhances the electrical breakdown field. The AlN layers grown under these conditions, with a growth rate per cycle of 1.54 Å/cycle, contain residual oxygen and carbon concentrations of about 10% and 4%, respectively, and possess a refractive index of 2.07 (at 632.8 nm).
AlN has remarkable properties (wide band gap, low electrical and thermal conductivity, high dielectric constant, piezoelectricity) and is attractive for (opto)electronic and sensor applications. However, high oxygen content within nitride films is always a critical issue due to the thermodynamically favorable oxidation against nitridation resulting in deteriorated materials properties. In order to clarify whether the oxidation is a surface-limited or a bulk process elemental depth profiling is essential. In this work XPS in combination with Ar+ sputtering is applied to carry out depth profiling of AlN films prepared by plasma-enhanced atomic layer deposition using different parameters (plasma source, power and pulse duration). Particularly, the Al2p core levels are analyzed where the signals are decomposed into four components, representing weaker contributions of pure AlN and aluminum oxide phases as well as stronger signals of mixed oxygen-rich and nitrogen-rich phases. After sputtering (providing access to the deeper part of the film) the pure AlN phase content increases while the pure aluminum oxide content stays relatively constant. These issues are discussed with regard to the preparation parameters employed and accompanying XRD and electrical measurements.
This paper reports a self-powered,flexible, piezo- and pyro-electric hybrid nanogenerator (NG) device that can be fixed on different locations of human skin for detecting static and dynamic pressure variations and can also monitor temperature fluctuations during the respiration process. An efficient and cost-effective fabrication strategy has been developed to create electrospun poly(vinylidene fluoride) (PVDF)/graphene oxide (GO) nanofibers, which are used to create a highly sensitive wearable pressure sensor and pyroelectric breathing sensor. The sensor can accurately and rapidly
detect pressures as low as 10 Pa with a high sensitivity (4.3 V/kPa), a key performance indicator for wearable sensors. Importantly, the sensor exhibits a high sensitivity to bending and stretching by finger, wrist, and elbow. The pressure sensor is also highly sensitive to vocal vibrations when attached to the human throat. The device can generate a maximum output power density of∼6.2 mW/m2 when subjected to a compressive stress, which enhances its range of applications. Moreover, it is demonstrated that doping with GO improves the pyroelectric energy harvesting and sensing performance of the device under repeated temperature fluctuations. The PVDF/GO-based nanogenerator has a maximum pyroelectric output power density of∼1.2 nW/m2 and can sense temperature changes during respiration, which makes it promising as a pyroelectric breathing sensor. It is demonstrated that processing of the PVDF-GO self-powered multifunctional pressure and pyroelectric breathing sensor can be up-scaled for fabricating compact and high-performance electronic skins for application in health monitoring, motion detection, and portable electronics.
Al2O3-Atomic Layer Deposited Films on CH3NH3PbI3 : Intrinsic Defects and Passivation Mechanisms
(2019)
The initial interaction of atomic layer deposited films of Al2O3 at room temperature on CH3NH3PbI3 (MAPI) films is studied. Synchrotron radiation–based photoelectron spectroscopy is applied to analyze the initial changes in the Al-derived features by comparing samples with different Al2O3 film thicknesses.
It is found that polarons and excitons, both intrinsic defects of Al2O3, play a key role in the interface formation. The polaronic states uptake a charge from the MAPI substrate. This charge is transferred to and stabilized in the excitonic state of Al2O3 which is assigned to predominately tetrahedral coordinated Al sites. This charge transfer is initiated by vacancies present in the MAPI substrate and stabilizes a covalent bonding at the Al2O3–MAPI interface but also causes a roughening of the interface which may lead to the formation of grain boundaries.
On top of the rough interface, 2D Al2O3 clusters with an increasing number of octahedrally coordinated Al—O bonds grow, and with increasing Al2O3 coverage, they introduce self-healing of the structural defects.
We report on the interaction of CH₃NH₃PbI₃ substrates with AlOx films prepared by atomic layer deposition at room temperature. We use synchrotron radiation-based photoemission spectroscopy and study the Pb 4f, I 3d, and Al 2p core levels as well as the corresponding valence band states (Pb 5d and O 2p). A Cooper minimum is observed for both the Pb 5d and O 2p states, and it indicates that the bonding at the interface must be covalent. We focus on the electronic properties of the substrate and its interface and identify a delicate charge balance between polaronic and excitonic states in MAPI and AlOₓ. The perovskite vacancy sites, identified by changes in the core-level intensities, mediate the charge balance and covalent interaction.
We report on the electronic structure of gallium oxide (Ga₂O₃) single crystals as studied by resonant photoelectron spectroscopy (resPES). We identify intrinsic electronic defects that are formed by mixed-atomic valence states. We differentiate three coexisting defect states that differ in their electronic correlation energy and their spatial localization lengths. Their relative abundance is described by a fractional ionicity with covalent and ionic bonding contributions. For Ga₂O₃, our analyses of the resPES data enable us to derive two main aspects: first, experimental access is given to determine the ionicity based on the original concepts of Pauling and Phillips. Second, we report on multi-atomic energy loss processes in the Ga2p core level and X-ray absorption data. The two experimental findings can be explained consistently in the same context of mixed-atomic valence states and intrinsic electronic defects.
The surface oxidation of a titanium oxynitride (TiOxNy) film after long-time storage of 25 month in ambient conditions is investigated. The TiOxNy film is prepared by plasma-enhanced atomic layer deposition using etrakis(dimethylamino)titanium and nitrogen plasma, and the film is characterized by Ar+
ion sputtering of the film surface in combination with x-ray photoelectron spectroscopy (XPS) as well as by angle-resolved XPS. The total thickness of an oxygen-enriched layer at the surface of the TiOxNy films is found to be about 0.7 nm and it consists of a sequence of a 0.4 nm thick TiON/TiO2 enriched layer followed by a 0.3 nm thick TiO2 enriched layer underneath compared to the bulk composition of the film which shows constant values of 29% TiN, 29% TiO2, and 42% TiON. The results suggest that the TiON enrichment takes place initially at the surface followed by a surface and subsurface oxidation.
In this work we report on the preparation of poly(vinylidene fluoride) (PVDF) films with various mass fractions (wt%) of hydrated rare earth metal salt [(NH4)4Ce(SO4)4.2H2O] (CAS) and its electroactive phase (β- and γ-phase) formation. It was found that CAS can yield high amount of electroactive phases in PVDF. Compared to the neat PVDF film, a significantly increment of dielectric constant (ε ∼ 41) and a low loss factor (tan δ ∼ 0.8) were observed in the PVDF-CAS composite films. Thus the PVDF-CAS composite films with high electroactive phase and superior dielectric properties may meet the requirements of next generation electronic components.
Oxide surface coatings are of importance in tailoring interface properties with respect to surface passivation, adjustment of surface potentials, or providing active centers for surface reactions. In this contribution, we report about surface coatings prepared by the atomic layer deposition (ALD) method. ALD is known for its conformal growth of ultrathin, dense films which exhibit a low concentration of pinholes.
Natural piezoelectric materials are of increasing interest, particularly for applications in biocompatible, implantable, and flexible electronic devices. In this paper, we introduce a cost-effective, easily available natural piezoelectric material, that is, sugar in the field of wearable piezoelectric nanogenerators (PNGs) where low electrical output, biocompatibility, and performance durability are still critical issues. We report on a high-performance piezoorganic nanogenerator (PONG) based on the hybridization of sugar-encapsulated polyvinylidenefluoride (PVDF) nanofiber webs (SGNFW). We explore the crucial role of single-crystal sugar having a fascinating structure along with the synergistic enhancement of piezoelectricity during nanoconfinement of sugar-interfaced macromolecular PVDF chains. As a consequence, the SGNFW-based PONG exhibits outstanding electricity generation capability (e.g.,∼100 V under 10 kPa human finger impact and maximum power density of 33 mW/m2) in combination with sensitivity to abundantly available different mechanical sources (such as wind flow, vibration, personal electronics, and acoustic vibration). Consequently, it opens up suitability in multifunctional self-powered wearable sensor designs for realistic implementation. In addition, commercially available capacitors are charged up effectively by the PONG because of its rapid energy storage capability. The high performance of the PONG not only offers“battery-free”energy generation (several portable units of light-emitting diodes and a liquid crystal display screen are powered up without using external storage) but also promises its use in wireless signal transmitting systems, which widens the potential in personal health care monitoring. Furthermore, owing to the geometrical stress confinement effect, the PONG is proven to be a highly durable power-generating device validated by stability test over 10 weeks. Therefore, the organic nanogenerator would be a convenient solution for portable personal electronic devices that are expected to operate in a self-powered manner.
The electronic structure of ZnO is dominated by coexisting electron and ionic states/bonds caused by charge polarization and self-trapping phenomena of the valence charges. Their interplay manifests itself in intrinsic electronic defects which have different degree of spatial localization and electronic correlation. The relative abundance of intrinsic defects can be determined by the ionicity factor fi and its value can be derived from three independent experimental procedures in a consistent way. This approach also explains the complex satellite features in the XPS and XAS data. Ionicity and satellite formation are two experimental findings which can be explained consistently in the same context of mixed-atomic valence states and intrinsic electronic defects.
This chapter reports about tin oxide (SnO₂), a material which belongs to the transparent conducting oxide family and is best characterized by its high conductivity, high carrier mobility, and the ability to form p-type conductivity. We correlate these properties in terms of localized intrinsic electronic defect states which are resolved by resonant photoelectron spectroscopy measurements. We describe SnO₂ in terms of an inhomogeneous, mixed-ionic-covalent semiconductor in which these intrinsic electronic defects state are stabilized.
In this study, the authors compared different Al₂O₃ films grown by atomic layer deposition (ALD) with the same aluminum precursor but on different substrates. The authors employed different process parameters such as thermal-ALD and plasma-enhanced-ALD using different substrate temperatures ranging from 280 °C down to room temperature. They characterized these films by resonant photoelectron spectroscopy and by electrical measurements. They established that generally the ALD-Al₂O₃ films show characteristic features of bulk Al₂O₃. For all films investigated, the authors found intrinsic defect states within the electronic band gap and identified excitonic, polaronic, and charge-transfer defect states. The authors gave an atomistic model to explain these intrinsic defects and found that their relative abundance is subject of the choice of ALD parameters and of the substrate used. They were able to relate the spectroscopic assigned in-gap defect states with the electronic charges as determined in our electrical measurements.
Valence band (VB) spectra of titanium dioxide (TiO2) and oxynitride (TiOxNy) films prepared by different atomic layer deposition (ALD) processes are compared and related to electrical characterization [current–voltage (JV) and capacitance–voltage (CV)] results. By increasing the nitrogen amount in the TiO2 film, band-gap narrowing is observed. The band-gap decrease is related to the contribution of the nitrogen density of states, which induces defects within the band-gap and thus reduces its optical band-gap. In-gap states are found in the VB spectra at 1 eV below the Fermi energy in all investigated ALD samples, i.e., in TiO2 as well as in TiOxNy films. An exponential correlation between leakage current density and in-gap state intensity is derived
by the combination of JV measurements and VB spectra, whereas the in-gap states seem to have no influence on hysteresis and fixed oxide charges found in the CV data. It is argued that the in-gap states in TiO2 and TiOxNy have an excitonic or polaronic origin. Both, band-gap narrowing and in-gap state intensity can be tuned by the ALD process selection and the variation of its parameters.
A comparative study of thin titanium oxynitride (TiOxNy) films prepared by plasma enhanced atomic layer deposition using tetrakis(dimethylamino)titanium (TDMAT) and N2 plasma as well as titanium(IV)isopropoxide and NH3 plasma is reported. The comparison is based on the combination of Ti2p core level and valence band spectroscopy and current-voltage measurements. The TDMAT/N2 process delivers generally higher fractions of TiN and TiON within the Ti2p spectra of the films and stronger photoemissions within the bandgap as resolved in detail by high energy resolution synchrotron-based spectroscopy. In particular, it is shown that higher TiN contributions and in-gap emission intensities correlate strongly with increased leakage currents within the films and might be modified by the process parameters and precursor selection.
Platinum, being the most efficient and stable catalyst, is used in photoelectrochemical (PEC) devices. However, a minimal amount of Pt with maximum catalytic activity is required to be used to minimize the cost of production. In this work, we use an environmentally friendly, ost-effective, and less Pt-consuming method to prepare PEC devices for the hydrogen evolution reaction (HER). The Pt monolayer catalyst is selectively deposited on a Au-nanoisland-supported boron-doped p-type Si (100) photocathode. The PEC device based on the Si photocathode with an ultralow loading of the Pt catalyst exhibits a comparable performance for the HER to that of devices with a thick Pt layer. In addition, we demonstrate that by using a thin TiO2 layer deposited by atomic layer deposition photo-oxidation of the Si photocathode can be blocked resulting in a stable PEC performance.
An ultrathin Al₂O₃ film deposited on methylammonium lead triiodide (CH₃NH₃PbI₃) perovskite has the capability to suppress the carrier recombination process and improve the perovskite solar cells efficiency
and stability. However, annealing at temperatures higher than 85°C degrades the CH₃NH₃PbI₃ perovskite film. The X-ray photoelectron spectroscopy study performed in this work indicates that it is possible to grow Al₂O₃ by atomic layer deposition on the perovskite at room temperature, however, besides pure Al₂O₃ some OH groups are found and the creation of lead and iodine oxides at the Al₂O₃/CH₃NH₃PbI₃ interface takes place.
The electroactive β phase of poly(vinylidenefluoride) (PVDF) is induced due to the aging time of PVDF solutions. The feasibility of the combination of the three crystalline polymorphs (α, β and γ) is demonstrated where
their relative proportion within the PVDF film can be tailored by the simple monitoring of the preparation conditions. To identify all these phases, Fourier transform infrared (FT-IR) spectroscopy is carried out and it is spotlighted that the vibrational bands at 510 and 841 cm-1 are not sufficient to state the formation of the β phase. The main aim of this work is devoted to develop a better understanding on the thermal stability of these several phases of PVDF, which has a longstanding ambiguity
persisting in this area. It has been found that the in situ thermal FT-IR spectroscopy is one of the best alternatives to understand this important
issue. It is ascertained that the β phase is the least thermally stable phase
among α, β and γ phases, whereas the γ phase is the most thermally stable phase.
The process parameters’ impact of the plasma-enhanced atomic layer deposition (PE-ALD) method on the oxygen to nitrogen (O/N) ratio in titanium oxynitride (TiOxNy) films was studied. Titanium(IV)isopropoxide in combination with NH3 plasma and tetrakis(dimethylamino) titanium by applying N2 plasma processes were investigated. Samples were characterized by the in situ spectroscopic ellipsometry, x-ray photoelectron spectroscopy, and electrical characterization (current–voltage: I-V and capacitance–voltage: C-V) methods. The O/N ratio in the TiOxNy
films is found to be very sensitive for their electric properties such as conductivity, dielectric breakdown, and permittivity. Our results indicate that these PE-ALD film properties can be tuned, via the O/N ratio, by the selection of the process parameters and precursor/coreactant
combination.
Exploring efficient and inexpensive electrocatalysts for the oxidation of water is of great importance for various electrochemical energy
storage and conversion technologies. In the present study, a new water-soluble [Ce
III(DMF) (HSO4)3] complex was synthesized and characterized
by UV−vis, photoluminescence, and high-resolution X-ray photoelectron spectroscopy techniques. Owing to classic 5d →4f transitions, an intense
photoluminescence in the UV region was observed from the water-soluble [CeIII(DMF)(HSO4)3] complex. A stacking electrode was designed where
self-assembled L-cysteine monolayer modified gold was immobilized with the synthesized cerium complex and was characterized by scanning electron microscopy, electrochemical impedance spectroscopy, and cyclic voltam-metry. The resulting electrode, i.e., [CeIII(DMF)(HSO4)3]−L-cysteine-Au
stacks shows high electrocatalytic water oxidation behavior at an
overpotential of η ≈0.34 V under neutral pH conditions. We also demonstrated a way where the overpotential is possible to decrease upon irradiation of UV light.
Titanium oxynitride films are prepared by plasma enhanced atomic layer deposition method using two different precursors and nitrogen sources. Synchrotron radiation-based X-ray photoelectron spectroscopy and X-ray absorption spectroscopy are used to characterize the nitrogen species incorporated
within these films depending on the deposition parameters.
It is found that nitrogen atoms in these films are differently bonded. In particular, it can be distinguished between Ti-ON and Ti-N bonding configurations and molecular nitrogen species caused by precursor fragments.
Atomic Layer Deposition (ALD) is an excellent deposition technique to grow thin film with high homogeneity coverage on ideal surfaces. Here we are interested in the growth properties on non-ideal (stepped, inert) surfaces. Using STM we studied the growth of Al2O3 by ALD on stepped surface of HOPG and silver covered HOPG (Ag-HOPG). The HOPG sample was cleaved to and then cleaned by N2 in vacuum. We obtain regular steps with terrace widths of 50nm - 500nm. Ag was deposited by thermal deposition on the HOPG. The Al2O3 growth at RT was studies after the first, third, and fifth cycle. Silver evaporation
leads to step decoration with island of 1nm-7nm. On the terraces we find the formation of 2D domain of Ag chains. The features caused by the individual ALD cycles of Al2O3 depend on the terrace widths of the HOPG substrate. For small terraces (<100nm) we obtain a growth of 2D domains ( 20nm-25nm) on the edge steps and in the middle of the terrace. For large terraces (>100nm) we find agglomeration of the precursors on individual nucleation sites. Here 3D islands are formed with a height of up to 5nm in the fifth ALD cycle.
Engineering of Sub-Nanometer SiOₓ Thickness in Si Photocathodes for Optimized Open Circuit Potential
(2016)
Silicon is one of the most promising materials to be used for tandem-cell water-splitting devices. However, the electrochemical instability of bare Si makes it difficult to be used for stable devices. Besides that, the photovoltage loss in Si, caused by several factors (e.g., metal oxide protection layer and/or SiO₂/Si or catalyst/Si interface), limits its use in these devices. In this work, we present that an optimized open circuit potential (OCP) of Si can be obtained by controlling the SiOₓ thickness in sub-nanometer range. It can be done by means of a simple and cost-effective way using the combination of a wet chemical etching and the low temperature atomic layer deposition (ALD) of TiO₂. We have found that a certain thickness of the native SiOₓ is necessary to prevent further oxidation of the Si photocathode during the ALD growth of TiO₂. Moreover, covering the Si photocathode with an ALD TiO₂ layer enhances its stability.
Ytterbium (Yb3+) assisted porous poly(vinylidene fluoride) (PVDF) composite film comprising flexible ferroelectretic nanogenerator (FTNG) is highlighted where traditional poling treatment is completely avoided. The piezoelectric output of FTNG is realized by the co-operative activity of self-polarized -CH2/-CF2 dipoles with porous electret-like structure in the composite film. Owing to extraordinary ferroelectric and dielectric properties, FTNG is acting as a highly efficient mechanical energy harvester. It is capable to capture several forms of abundant mechanical energy arising from humanfinger movements, machine vibrations and sound waves. As a proof of concept, under compressive deformation, FTNG is enable to instantly powers up several consumer electronics and thus provides a promising strategy for achieving self-powered electronic devices.
In this work, solar cells with a freshly made CH₃NH₃PbI₃ perovskite film showed a power conversion efficiency (PCE) of 15.4 % whereas the one with 50 days aged perovskite film only 6.1 %. However, when the aged perovskite was covered with a layer of Al₂O₃ deposited by atomic layer deposition (ALD) at room temperature (RT), the PCE value was clearly enhanced. X-ray photoelectron spectroscopy study showed that the ALD precursors are chemically active only at the perovskite surface and passivate it. Moreover, the RT-ALD-Al2O3-covered perovskite films showed enhanced ambient air stability.
Cerium(III)-,N-dimethylformamide-bisulfate
[Ce(DMF)(HSO4)3] complex is doped into poly(vinylidene
fluoride) (PVDF) to induce a higher yield (99%) of the
electroactive phases (β- and γ-phases) of PVDF. A remarkable enhancement of the output voltage (∼32 V) of a nano-generator (NG) based on a nonelectrically poled cerium(III) complex containing PVDF composite film is achieved by
simple repeated human finger imparting, whereas neat PVDF
does not show this kind of behavior. This high electrical output resembles the generation of self-poled electroactiveβ-phase in
PVDF due to the electrostatic interactions between the fluoride of PVDF and the surface-active positive charge cloud of the
cerium complex via H-bonding and/or bipolar interaction among the opposite poles of cerium complex and PVDF, respectively.
The capacitor charging capability of the flexible NG promises its applicability as piezoelectric-based energy harvester. The cerium(III) complex doped PVDF composite film exhibit an intense photoluminescence in the UV region, which might be due to
a participation of electron cloud from negative pole of bipolarized PVDF. This fact may open a new area for prospective development of high-performance energy-saving flexible solid-state UV light emitters.
Titanium oxide (TiO2) deposited by atomic layer deposition (ALD) is used as a protective layer in
photocatalytic water splitting system as well as a dielectric in resistive memory switching. The way
ALD is performed (thermally or plasma-assisted) may change the growth rate as well as the electronic properties of the deposited films. In the present work, the authors verify the influence of the ALD mode on functional parameters, by comparing the growth rate and electronic properties of
TiO2 films deposited by thermal (T-) and plasma-enhanced (PE-) ALD. The authors complete the
study with the electrical characterization of selected samples by means of capacitance–voltage and
current–voltage measurements. In all samples, the authors found a significant presence of Ti3+ states, with the lowest content in the PE-ALD grown TiO2 films. The observation of Ti3+ states
was accompanied by the presence of in-gap states above the valence band maximum. For films thinner than 10 nm, the authors found also a strong leakage current. Also in this case, the PE-ALD films showed the weakest leakage currents, showing a correlation between the presence of Ti3+ states and leakage current density.
A flexible nanogenerator (NG) is fabricated with a poly(vinylidene fluoride) (PVDF)film, where deoxyribonu-cleic acid (DNA) is the agent for the electroactive β-phase
nucleation. Denatured DNA is co-operating to align the
molecular−CH2/−CF2 dipoles of PVDF causing piezo- electricity without electrical poling. The NG is capable of harvesting energy from a variety of easily accessible mechanical
stress such as human touch, machine vibration, football
juggling, and walking. The NG exhibits high piezoelectric
energy conversion efficiency facilitating the instant turn-on of several green or blue light-emitting diodes. The generated energy can be used to charge capacitors providing a wide scope
for the design of self-powered portable devices.
Charakterisierung von TiO2-ALD-Schichten und deren Nutzung für die Passivierung von Si-Photokathoden
(2015)
Wir berichten zunächst über den Einfluss der Methode der Atomlagenabscheidung auf die TiO2-Schichteigenschaften. Insbesondere wird der Zusammenhang zwischen Ti3+- sowie Interbandzuständen mit elektrischen Kennlinien und deren Einfluss auf funktionale Schichteigenschaften diskutiert. Anschließend werden Mikrostrukturierung, Grenzflächen-Engineering und konforme Abscheidung einer TiO2-ALD-Schutzschicht effektiv zur Performance-Steigerung von p-Si-Photokathoden angewendet.
In this work, plasma enhanced atomic layer deposited (PE-ALD) samples were prepared at substrate temperatures in the range between room temperature (RT) and 200 °C and investigated by capacitance–voltage and conductance–voltage recordings. The measurements are compared to standard thermal atomic layer deposition (T-ALD) at 200 °C. Very low interface state density (D it) ~1011 eV−1 cm−2 could be achieved for the PE-ALD process at 200 °C substrate temperature after postdeposition anneal (PDA) in forming gas at 450 °C. The PDA works very effectively for both the PE-ALD and T-ALD at 200 °C substrate temperature delivering also similar values of negative fixed charge density (N fix) around −2.5 × 1012 cm−2. At the substrate temperature of 150 °C, highest N fix (−2.9 × 1012 cm−2) and moderate D it (2.7 × 1011 eV−1 cm−2) values were observed. The as deposited PE-ALD layer at RT shows both low D it in the range of (1 to 3) × 1011 eV−1 cm−2 and low N fix (−4.4 × 1011 cm−2) at the same time. The dependencies of N fix, D it, and relative permittivity on the substrate temperatures and its adjustability are discussed.
A polymer nanogenerator based on silver nanoparticles doped electrospun P(VDF-HFP) nanofibers
(2015)
The rising energy demand within the increasing mobility of human society has driven the development of new alternative power sources for portable devices with ultra-low electric power consumption.
Based on its piezoelectric properties poly-vinylidene fluoride (PVDF) is a good candidate for mechanical energy harvesting for such devices. Moreover the copolymer poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] offers better film flexibility and cost-effectiveness than pure PVDF.
We report on a polymer nanogenerator (PNG) based on electrospun P(VDF-HFP) nanofibers doped with silver nanoparticles (Ag-NPs). The electrospun fibers were characterized by FTIR, XPS, SEM and pressure imparting probe. It has been found that the yield of the piezoelectric phase is increased by the addition of Ag-NPs due to an interaction between surface charges of Ag-NPs and the molecular dipoles. Furthermore, defects in the P(VDF-HFP) electrospun fibers are removed. Accordingly, a significant enhancement in the output power of the PNG was found. Similar trends were also observed with Pd-NPs.
Dünne Al2O3-Schichten wurden bei Substrattemperaturen zwischen Raumtemperatur und 200°C mittels PE-ALD auf Si-Substraten abgeschieden und ellipsometrisch, spektroskopisch und elektrisch charakterisiert. Ihr Potenzial für die chemische und Feldeffektpassivierung der Oberflächenrekombination wird bewertet.
We report on the electrospinning of poly(vinylidene fluoride–hexafluoropropylene) [P(VDF–HFP)] nanofibers doped with silver nanoparticles for the preparation of a polymer based nanogenerator (PNG). It has been found that the yield of the piezoelectric phase is increased by the addition of silver nanoparticles. Furthermore, defects in the P(VDF–HFP) electrospun fibers are removed resulting in a significant enhancement in the output power of the PNG. A maximum generated PNG output voltage of 3 V with a current density of 0.9 μA cm−2 is achieved.
Thermal Atomic layer deposition (T-ALD) of oxides is obtained by the pulsed alternation of a metal precursor and an oxygen source, typically H2O or O3, and the reactions leading to ALD are thermally activated. With plasma enhanced ALD (PE-ALD), instead, the oxygen source is represented by an oxygen-containing plasma. The higher reactivity of the plasma-generated species extend the capabilities of ALD: improved film quality and increased flexibility in process conditions, such as growth at low temperature, are typical advantages of PE-ALD over T-ALD. We report on results on the preparation of thin (<100 nm) aluminum oxide (Al2O3) films on silicon substrates using T-ALD and PE-ALD in the SENTECH SI ALD LL system. Films were deposited in the temperature range between room temperature (RT) and 200∘C. We characterized the films with spectroscopic ellipsometry (thickness, refractive index, growth rate) over 4" wafers and with X-ray photoelectron spectroscopy. All films resulted in a high degree of homogeneity, independent of the deposition temperature. Investigations with capacitance-voltage and conductance-voltage measurements showed a very low interface states density for the PE-ALD films.
We report on results on the preparation of thin (<100 nm) aluminum oxide (Al2O3) films on silicon substrates using thermal atomic layer deposition (T-ALD) and plasma enhanced atomic layer deposition (PE-ALD) in the SENTECH SI ALD LL system. The
T-ALD Al2O3layers were deposited at 200 °C, for the PE-ALD films we varied the substrate temperature range between room temperature (rt) and 200 °C. We show data from spectroscopic ellipsometry (thickness, refractive index, growth rate) over 4” wafers and correlatethem to X-ray photoelectron spectroscopy (XPS) results. The 200°C T-ALD and PE-ALD processes yield films with similar refractive indices and with oxygen to aluminum elemental ratios very close to the stoichiometric value of 1.5.
However, in both also fragments of the precursor are integrated into the film. The PE-ALD films show an increased growth rate and
lower carbon contaminations. Reducing the deposition temperature down to rt leads to a higher content of carbon and CH-species. We also find a decrease of the refractive index and of the oxygen to aluminum elemental ratio as well as an increase of the growth rate whereas the homogeneity of the film growth is not influenced significantly. Initial state energy shifts in all PE-ALD samples are observed which we attribute to a net negative charge within the films.
The electroactive Beta-phase formation in Poly(vinylidene fluoride) by gold nanoparticles doping
(2012)
Polyvinylidene fluoride (PVDF) is a typical fluoro-polymer with complicated polymorphisms due to the semi-crystalline nature. Generally, four crystalline phases, α, β, γ and δ may exist in PVDF crystals. The nonpolar α- and δ-phase has a monoclinic lattice with trans-gauche conformation (TGTG⁄), which is thermodynamically most stable for PVDF. The polar β phase with all-trans (TTTT) planar zigzag conformation is attractive for its piezo-, pyro-, and ferroelectric properties. The polar β-Phase and also semi-polar γ-phase (TTTGTTTG⁄) have obtained wide device applications such as acoustic transducers, piezoelectric sensors, energy harvesting generators, pyroelectric sensors, non-volatile memories and energy storage capacitors. In this work, we explore the possibility to nucleate β-phase in PVDF hydrated salt composite films by addition of the hydrated Rare earth metal salt, (NH4)4 Ce (SO4)4, 2H2O) for the piezo- ferro and high dielectric based device applications. It has been found that the water molecules in (NH4)4 Ce (SO4)4, 2H2O could form hydrogen bonds with PVDF during the crystallization process and the hydrogen bonds promoted the electro-active crystalline β- phase in PVDF with all -trans conformations. In addition, the DMF (solvent used for films preparation) can also assist the rare-earth based nanoparticle preparation, as a result the electrostatic interaction of the surface charge of the nanoparticles and molecular dipoles in PVDF can resist the electroactive β- phase in PVDF.
The effect of X-ray photoelectron spectroscopy measurement on P(VDF-TrFE) copolymer thin films
(2012)
The impact of prolonged X-ray irradiation during X-ray photoelectron spectroscopy (XPS) measurement was investigated on poly(vinylidene-trifluoroethylene) (P(VDF-TrFE)) thin films. It was observed that
prolonged X-ray irradiation can accelerate the crosslinking of P(VDF-TrFE) and diminish the ferroelectric phase. Fourier transform infrared spectroscopy (FT-IR) data indicate that the ferroelectric phase diminishes completely after 360 kJ of X-ray irradiation dose and it induces the paraelectric phase. In this work,
the main emphasis was given to the optimization of the X-ray irradiation dose during XPS measurements
to maintain the ferroelectric phase within the copolymer films.
Experimental data on the reaction kinetics of catalytic processes are prerequisite for modeling of such reactions and their output. We investigate the CO oxidation on
some noble metal catalysts, like Iridium(111) and Palladium(111) which is a bistable surface reaction. In a specific range of CO content in the feed gas islands of one or the other adsorbate nucleate and grow, which is recorded with photo electron emission microscopy (PEEM). During these transients the coverage on the surface
changes from predominantly oxygen covered to predominantly CO covered resp. vice versa. But the resolution of PEEM and especially its unknown and nonlinear dependence of coverage of the two adsorbates (oxygen and CO) and image intensity make it impossible to extract the changes in coverage. On the other hand the reaction diffusion model based on the Langmuir-Hinshelwood-mechanism delivers
the coverage of oxygen and carbon monoxide. The shapeand wall velocity of those islands has been connected perfectly to each other [CPL 423 (2006) 39], but not the
absolute coverage and their variation during the reaction. Since the domain wall velocity is small on Iridium(111), this has opened the possibility for a detailed
analysis of the changing coverage inside this domain wallutilizing synchrotron radiation and photo electron spectroscopy at BESSY II. The coverage profile of
oxygen and CO inside a moving domain wall was measured for both cases using XPS spectra at O1s, C1s and the valence band during thereaction. Comparison with the reaction-diffusion-model and other catalytic surfaces, like Palladium(111) are done.
Control of β- and γ-Phase formation in electroactive P(VDF-HFP) films by silver nano-particle doping
(2012)
Increasing green house emissions amplify global warming concerns worldwide. Fuel cells, based on polymer electrolyte membranes (PEM) are part of the solution to this issue. We aim at developing and testing new catalysts for fuel cells. Platinum catalysts perform best in PEM fuel cells to date but they are very expensive. For fuel cells to be perfectly applicable and economically affordable, an alternative catalyst with similar or even greater efficiency should be developed.
Metal phthalocyanines are known to be good oxygen reduction catalysts and are associated with high electron transfer properties. We investigated Cobalt Phthalocyanine in its pure state mixed with Carbon Black (CoPc:Carbon Black=1:9) untreated and also after a treatment in N2 at several temperatures, up to 1000°C for 30 minutes.
Results of XPS and EDX analyses reveal that the treatment at higher temperatures causes a rearrangement in the chemical states of Co, N and C atoms. Furthermore, results of cycling voltammetry (CV) analysis reveal that CoPc treated in N2 at 800°C exhibited a higher current density, in conclusion, a higher catalytic activity than the untreated CoPc.
Spectroscopic and microscopic characterization of potential catalysts for the hydrogenation of CO2
(2011)
The reduction of CO2 emissions is a general demand of the society in order to avoid irreversible climate changes. We are investigating potential catalysts for the hydrogenation of CO2 in particular for its application in the so called oxyfuel combustion process.
In order to understand the catalytic behavior of the investigated materials we use the techniques X-Ray Photoemission (XPS) and -absorption (XAS) Spectroscopy as well as X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The prospective of our work is to figure out the electronic and structural properties of the material and to optimize proper catalysts or design new ones for best catalytic activity. Beside the spectro-microscopic investigations the catalysts are tested under Sabatier reaction conditions in terms of activity, stability and sensitivity against poisoning. In our preliminary work we constructed a Sabatier reactor system with analytical parts based on infrared absorption and mass spectroscopy modules. First spectroscopic and catalytic results achieved on a commercially available Ru/Al2O3 catalyst will be presented.
In this thesis electrical characterizations on metal-insulator-semiconductor (MIS) and metal-ferroelectric-insulator-semiconductor (MFIS) stacks are presented. The results are discussed in terms of the increase of the performance
of these stacks in order to optimize the characteristics of devices like the field effect transistor (FET) for logic applications and the ferroelectric field effect transistor (FeFET) for non-volatile and non-destructive readout memory
applications. Therefor, two material classes are investigated.
Firstly, this work focuses on praseodymium oxide (PrxOy) films as a high-kmaterial on silicon (Si) and silicon carbide in MISsamples. In particular the improvement of the performance by the introduction of an aluminum oxynitride buffer layer, which acts as an interdiffusion barrier and reduces the leakage
current, the interface state density and the equivalent oxide thickness, is demonstrated. Secondly, in this work the very attractive ferroelectric material poly[vinylidene
fluoride trifluoroethylene] (P[VDF/TrFE]) is introduced in MFIS stacks on Si. The way of successive improvements of the MFIS stack is extensively discussed. Here, the optimization of the preparation of the samples as well as of the layer parameters in the sequence of the MFIS stacks is depicted. An improved long time stability of the memory functionof the MFIS stack with a
partially optimized layer sequence is exhibited. The electrical results are correlated to spectroscopic findings on both material
systems. Strong interfacial reactions between the PrxOy and Si as well as between the aluminum electrode and the P[VDF/TrFE] layer are observed. For both systems the importance of a buffer layer is discussed and its optimization is addressed, too. On both systems a very nice, as consistent, chain of improvements is shown caused by the spectroscopic controlled preparation combined with electrical measurements in order to obtain the functionality of a given material system.
The influence of an annealing step on the parameters of bulk heterojunction organic solar cells is investigated. In order to fabricate the solar cells we use glass coated with ITO (Indiumtinoxide) as a substrate on which the active layer consisting of P3HT and PCBM is spincoated. Al-electrodes are evaporated on top of the active layer. We use PEDOT:PSS as buffer layer. Each sample is annealed at different temperatures for a short time. Between every temperature step the I-V characteristic of the cell is measured. The following parameters are derived afterwards: FF, Isc (density), Voc. Also the efficiency is estimated. The results show a maximum cell efficiency for drying at 100°C for 20sec. A further important step for preparation is the drying procedure of the PEDOT:PSS layer. Here an improvement of about 50% in cell efficiency is measured after drying at 50°C for 5 days under inert gas atmosphere.
Microscopic and Spectroscopic Characterization of Interfaces and Dielectric Layers for OFET Devices
(2008)
Ferroelectric Field Effect Transistors (FeFETs) are considered as a candidate for future non volatile and non destructive readout memory cells. A possible low cost solution is the use of poly[vinylidene fluoride trifluoroethylene (P[VDF/TrFE]). Using P[VDF/TrFE] we focus on metal ferroelectric insulator semiconductor (MFIS) capacitor structures. In this contribution we will summarize our investigations related to capacitance measurements in dependence of applied bias (C(V)) and time (C(t)). The thickness of the ferroelectric layer as well as the thickness and the material of the buffer layer have been investigated for optimization of the write voltage. CV measurements at higher temperatures deliver decreased flatband voltage shifts within one CV loop resulting in lower memory windows. At temperatures around 100∘C the hysteresis totally vanished, pointing out the ferroelectric behavior of the system. C(t) measurements at a constant bias have been performed for retention analysis of the MFIS stack. After applying a writing pulse we could distinguish between a higher and a lower capacitance state for more than 5 days. This work is supported by Deutsche Forschungsgemeinschaft within priority program 1157 (SCHM 745/11-2).
X-ray photoemission spectroscopy of Aluminium Oxynitride on Si(001) and the rise as buffer layers
(2008)
Praseodymium oxide (PrxOy) is one of the candidate as high-k transistor gate dielectrics, but aluminium oxynitride (AlxONy) as buffer layer is needed to prevent diffusions from the silicon into PrxOy and to hinder charge injection from the semiconductor into the insulator. The thermal stability and thickness dependence of aluminum oxynitride (AlxONy) has been investigated by synchrotron radiation photoemission spectroscopy (SR-PES). AlxONy layers were prepared by low energy ion-beam assisted deposition (LE-IBAD) at room temperature on a silicon substrate cleaned with HF-acid. Aluminium oxynitride is stable till 800∘C. From photoemission spectra we can assess that silicon oxynitride is built as first and then the growth of AlxONy follows. Resonant inelastic X-ray scattering measurements with synchrotron radiation have been performed. The combination of X-ray absorption spectroscopy (XAS) and RIXS make possible to determinate the band gap of AlxONy.
Carbide was grown on Si (001) - substrates with supply of C2H2. Investigation of the interface Si/SiC (buffer layer) is essential for optimized preparation conditions, leading to a high quality of the growing SiC crystal. In addition to the lattice mismatch of 20 per cent between Si and SiC, which causes defects like dislocations or stacking faults, the chemical homogeneity of this layer is important. XPS was used to characterize the first state of SiC crystal growth on the Si substrate. Carbonizing with C2H2 leads to surface reactions which build up the buffer layer. By using electron spectroscopy, it was possible to characterize the SiC/Si (001) buffer layer, depending on the following reaction parameters: Tsubstrate , PC2H2 , PH2 , the ratio of pressures and the reaction time. The measurements were performed in terms of the content of carbide, oxide and roughness/etching-efficiency (Profilometer) for the reaction parameters above. We found, that the most critical parameter is the temperature of the substrate (Tsubstrate).
The ferroelectric copolymer poly(vinylidene fluoride trifluoroethylene) (P(VDF-TrFE)) is a possible material for low cost applications as organic nonvolatile memory element. We present an XPS interface investigation of the copolymer/electrode interface. Below 100nm film thickness, interface phenomena becomes important. Possible interface reactions could lead to a reduction of the ferroelectric functionality.We present a comparative study of the two interfaces Al/P(VDF-TrFE) and PEDOT:PSS/P(VDF-TrFE). The result is a clear indication of a surface reaction in between the Aluminum-electrodes. In contrast, for PEDOT:PSS the measurements shows a layer by layer formation of the interface PEDOT:PSS/PVDF without any reaction. Second, we present a study of remanent polarization of ultrathin spincoated films of the copolymer, in a thickness range from 5 to 100mn, with organic electrodes like PEDOT:PSS. The measurements are carried out by using the flatband shift of a capacitance-voltage (CV) characteristic. This shift of flatband voltage is used as value for the electric polarization of the polymer.
This work reports on the growth and characterization of cubic silicon carbide thin films. A cold wall low pressure chemical vapour deposition (LPCVD) system has been used to grow the cubic silicon carbide. The two step method, carbonization and subsequent growth, has been used to grow good quality 3C-SiC thin films. The composition and structure of deposited thin films have been analysed by Fourier Transform Infrared Spectroscopy (FTIR), XPS and X-Ray Diffractometry (XRD). The structural quality of the films has been optimized as a function of substrate temperature during carbonization and growth. X-rays diffraction measurements shows that the grown layers are highly oriented crystalline cubic silicon carbide.
P(VDF/TrFE) as a ferroelectric dielectric for Organic Field effect Transistors in MFIS structure
(2005)
MFIS structures consisting of metal, ferroelectric poly[vinylidene fluoride trifluoroethylene](P[VDF/TrFE]), insulator, semiconductor (Si) are fabricated and the influence of the polarization of the P(VDF/TrFE) on the CV characteristics of these structures is investigated. We find a characteristic ferroelectric hysteresis in the CV curves upon increasing the voltage scan window (e.g.-U...U,U=6...30V, |UP(VDF/TrFE)|≤15V, dP(VDF/TrFE)=300 nm). A writing process with adequate electric fields causes large shifts in the flatband voltage ( Δ UFB>4V for the quoted example before) which are stable over days. We report on our investigations which focus on the switching behavior by varying the parameters like polymer thickness, writing field strength and time, long time stability, reversing voltage and insulating material (SiO2, PMMA (Polymethylmetacrylate)). This material combination is favorable for transfer into our full organic concept of field effect transistors for application as a nondestructive readout memory (NDRO) cell.
Organic field effect transistors consisting of the copolymer poly(vinylidene fluoridetrifluoroethylene) (P(VDF-TrFE)) and P3HT (poly(3-hexlythiophene)) as active layer have been fabricated. The influence of P(VDF-TrFE) on the transconductance of organic field effect transistors was investigated and enhanced gate effects are observed compared to similar transistors with PMMA (Polymethylmetacrylate) as gate dielectric. Due to the high dielectric constant of P(VDF-TrFE) (DK=12) an operation voltage smaller than for a conventional organic dielectric like PMMA (DK=3.3) was observed. The thickness of the spincoated dielectric layers is in the range of 2microns and in consequence, no significant hysteresis was found, a prerequisite for transistor operation.
Ferroelectric field effect transistors are considered as future non volatile and non destructive readout memory cells. Conventional Perowskite-type ferroelectric materials are shown to work but they need expensive high temperature and high partial pressure processing steps inducing also unintentional and undefined interfacial layers.A possible low cost solution is the use of poly[vinylidene fluoride trifluoroethylene (P[VDF/TrFE]). This is a ferroelectric polymer which can be spin coated at room temperature onto silicon suited with well defined buffer layers. Using P[VDF/TrFE] we focus on metal ferroelectric insulator semi-conductor (MFIS) structures. In MFIS structure a part of the programming voltage drops over the buffering insulator. We report on attempts to minimize this fraction by optimizing the voltage divider
over the buffer and the ferroelectric layers. CV measurements for different thickness of the buffer (10-235nm) and the ferroelectric layer(100nm-1μm) and for different permittivity values of the buffer layer(SiO2, Al2O3, HfO2) will be presented. This work is supported by Deutsche Forschungsgemeinschaft within priority program 1157 (DSCH 745/11-1).
We study the chemical stability and electrical properties of Pr-oxides-SiC MIS stacks. In MISFET devices for high power applications the electric field scaling at the interface between semiconductor and insulator is determined by the ratio of their permittivity values. A high-k material can be used to optimize the performance of such devices. In previous studies we had to understand that the chemical reactivity of the PrOx/SiC interface causes a destructive interaction yielding silicate and graphite formation as well as poor electrical performance after direct deposition of PrOx onto SiC. Therefore we introduced an additional chemically inert layer and in this contribution we focus on PrOx/AlON as a suitable insulator stack. In our spectroscopic investigations we recognized a stable AlON/3C-SiC interface even for annealing steps up to 900∘C. First electrical characterizations are performed on Si substrates and we find a strong improvement in the leakage current by several orders of magnitude down to values of 10−7 A/cm2 at an EOT of 4nm and interface state densities of mean values of 5*1011/eVcm2. We also report on our ongoing electrical characterization of such stacks on SiC substrates. This work is supported by Deutsche Forschungsgemeinschaft within priority program 1157 (DSCH 745/9-2).
Due to potential use in non volatile memory application, Poly(vinylidene fluoride/trifluoroethylene) (P(VDF-TrFE)) is a great deal of interest, especially to produce organic field effect transistors (OFET). Right now it was found that Aluminum is one of the cost-effective inorganic electrode candidate. On the other hand, poly(3,4 ethylenedioxythiophene):poly(4-styrenesulfonate)(PEDOT:PSS) shows promising behavior for the application as organic electrode. In part of our investigations we have systematically studied surface interactions of organic ferroelectric thin films (P(VDF-TrFE))with inorganic (Al) and organic layers (PEDOT:PSS). By X-ray photoelectron spectroscopy (XPS) is has been found that there is a interfacial reaction with Al and the copolymer. In contrast, for electrodes, made of PEDOT: PSS, we found not such kind of behavior. In this case, we found no evidence for any surface reaction. Furthermore, we present our results concerning the orientation of permanent dipoles in the co-polymer (formed between hydrogen and fluorine atom). Here, we use Near-edge X-ray absorption fine structure (NEXAFS) analysis at the U 49/2 - PGM2 beam line of the BESSY-II Synchrotron. This work is supported by Deutsche Forschungsgemeinschaft (DFG) within priority program 1157 (DSCH 745/11-1).