FG Angewandte Physik und Halbleiterspektroskopie
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Atomic layer deposition (ALD) is known to produce amorphous and defect-rich films in a layer-by-layer fashion, which can potentially give rise to unexpected material properties. In particular, ultrathin films (few monolayers) will show the highest complexity, as the substrate-material interaction will play a major role during deposition. Therefore, it is crucial to understand the early stages of growth of the ALD process to control and potentially tailor this interfacial interaction. Applying a surface science approach combined with complementary ex-situ characterization, we have studied by in-situ X-ray photoelectron spectroscopy (XPS) the early stages of ceria (CeOx) growth on SiO2 substrates deposited by thermal-ALD using Ce(thd)4/O3. Interestingly, an initial mixture of Ce3+ and Ce4+ was observed, although only Ce4+ may be expected considering the used precursor and oxidant. This fact, together with a deviation from the ideal layer-by-layer growth and a higher growth rate during the first cycles, indicates a significant influence of the substrate of the ALD reaction mechanism as well as a correlation between morphology and ceria oxidation state.
Ni/ceria catalysts exhibit a high activity for methane to methanol conversion, making them very promising for applications within a sustainable economy. Possibly, their activity may be strongly enhanced due to the facile exchange between Ce4+ and Ce3+ states, with the latter likely responsible for activating O-H and C-H bonds. Here, we aim to unravel the complex metal-oxide interactions in the inverse CeOx/Ni(111) system under oxidizing and reducing environments. Using low-energy electron diffraction (LEED) we find that the CeOx(111) grown by reactive molecular beam epitaxy preferentially aligns with the main directions of the Ni(111) substrate or is azimuthally rotated by ±10°. By using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS), we find that less NiO is formed during deposition of CeOx than when it is held without ceria at the same conditions (O2 partial pressure and temperature). Finally, we observe a complex behavior of the cerium and nickel oxidation states when exposing the system to O2 or H2 atmospheres.
Perovskite solar cells (PSCs) excel in achieving high open-circuit voltages (VOC) for narrow bandgaps (∼1.6 eV) but face challenges with wide-bandgap perovskites, like methylammonium lead trichloride (MAPbCl3) with a 3.03 eV bandgap. These materials are transparent in visible absorbing ultraviolet (UV) light. However, achieving uniform film crystallization remains a hurdle. Here, we enhance MAPbCl3 crystallization by manipulating annealing atmospheres (nitrogen, air, and MACl vapor). Excess MACl vapor improves surface coverage, which is crucial for film stability. We demonstrate that the microstructure of the perovskite film, including surface morphology, grain boundaries, and interfaces, can affect the photovoltaic properties. The subsequently obtained VOC of 1.78 V is the highest recorded for single-junction PSCs to the best of our knowledge. Surprisingly, the conventional hole-transport layer spiro-OMeTAD, optimized for narrow bandgaps, sustains such high voltages. Photoluminescence measurements reveal a trap-assisted recombination peak at 1.65 eV, indicating deep traps as significant to voltage loss in MAPbCl3.
β-Ga2O3 (GaO) holds great promise in high-power applications. GaO is predicted to outperform the leading technologies based on SiC and GaN due to a three times higher calculated critical field strength. It has been identified that alloying with Al results in an almost linear increase in the bandgap between Ga2O3 (4.85eV) and Al2O3 (7eV). A critical issue in the bandgap engineering of GaO is maintaining reasonable electron mobility, which is low for the pure material (150cm2/V/S). This project aims to explore GaO by Al alloying, which allows tunable wide bandgaps for engineering material properties and at the same time maintains excellent crystal quality. PXRD and their Rietveld refinement on several AlGaO crystals indicate a preferential ordering of Al and Ga over octahedral and tetrahedral sites. The results of an SXRD study using synchrotron radiation addressing the surface structure of β-AlxGa2−xO3(100) substrates with x=0−0.25. Our results indicate that the GaO(100) surface is nearly bulk terminated and remains smooth up to 800°C and 10−5mbar of molecular oxygen.
The use of cerium oxide for hydrogen sensing is limited by the low electrical conductivity of layers deposited from a ceria target. To increase the electrical conductivity, partially reduced cerium oxide layers were obtained from a metallic cerium target by reactive magnetron sputtering. The proportions of the oxidation states Ce3+, present in reduced species, and Ce4+, present in fully oxidized species, were determined by ex-situ XPS. For electrical characterization, films were deposited on planarized tungsten finger electrodes. IV curves were measured over several days to investigate possible influences of oxygen and humidity on electrical conductivity. The morphological stability of the layers under ambient conditions was investigated by microscopical methods. The XPS results show a significant amount of Ce3+ in the layers. The electrical conductivity of as-grown samples is several orders of magnitude higher than that of samples grown from a ceria target. However, the conductivity decreases over time, indicating an oxidation of the layers. The surface morphology of the samples was found to be changing drastically within days, leading to partial delamination.
Copper oxide appears to be a promising candidate for a hole transport layer (HTL) in emerging perovskite solar cells. Reasons for this are its good optical and electrical properties, cost-effectiveness, and high stability. However, is this really the case? In this study, we demonstrate that copper oxide, synthesized by a spray-coating method, is unstable in contact with formamidinium lead triiodide (FAPI) perovskite, leading to its decomposition. Using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and ultraviolet–visible (UV–vis) spectrophotometry, we find that the entire copper oxide diffuses into and reacts with the FAPI film completely. The reaction products are an inactive yellow δ-FAPI phase, copper iodide (CuI), and an additional new phase of copper formate hydroxide (CH2CuO3) that has not been reported previously in the literature.
In this work, a 500 nm-thick cerium oxide thin film was prepared by electron beam evaporation. It was found that the deposition of 7 nm thick Pd catalyst was required for obtaining a sensor response to hydrogen. The Pd/CeOx sensing structure has a high response of 5000 towards 25 ppm H2 at a working temperature of 200 °C and exhibits a sensor response of 1.3 at temperatures near ambient. Furthermore, the sensing structure exhibited excellent response/recovery kinetics. The results confirm that the CeOx-based materials are a promising material for the fabrication of room-temperature hydrogen sensors.
This work introduces a simplified deposition procedure for multidimensional (2D/3D) perovskite thin films, integrating a phenethylammonium chloride (PEACl)‐treatment into the antisolvent step when forming the 3D perovskite. This simultaneous deposition and passivation strategy reduces the number of synthesis steps while simultaneously stabilizing the halide perovskite film and improving the photovoltaic performance of resulting solar cell devices to 20.8%. Using a combination of multimodal in situ and additional ex situ characterizations, it is demonstrated that the introduction of PEACl during the perovskite film formation slows down the crystal growth process, which leads to a larger average grain size and narrower grain size distribution, thus reducing carrier recombination at grain boundaries and improving the device's performance and stability. The data suggests that during annealing of the wet film, the PEACl diffuses to the surface of the film, forming hydrophobic (quasi‐)2D structures that protect the bulk of the perovskite film from humidity‐induced degradation.
The transition towards a new, renewable energy system based on green energy vectors, such as hydrogen, requires not only direct energy conversion and storage systems, but also the development of auxiliary components, such as highly sensitive hydrogen gas sensors integrated into mass devices that operate at ambient conditions. Despite the recent advances in nanostructured metal oxide thin films in terms of simple fabrication processes and compatibility with integrated circuits, high sensitivity, and short response/recovery times usually require the use of expensive noble metals or elevated tem-peratures (>250 ºC), which results in high power consumption and poor long-term stability. This article presents the first steps of the work on developing a novel resistive hydrogen gas sensor based on ultrathin cerium oxide films, compatible with complementary metal oxide semiconductor technology and capable of operating at room temperature. Here, we show a multidisciplinary bottom-up approach combining different work areas for the sensor development, such as sensor architecture, sensing mechanism and deposition strategy of the active layer, electrical contact design depending on the desired electrical output, and fast testing under controlled environments.
Inverse oxide/metal catalysis allows achieving better catalytic performance than its traditional counterpart. For example, in cerium-based inverse catalyst systems, the Ce3+ states have been shown to be the active sites for methanol synthesis. This suggests that the activity can be enhanced by promoting those through alloying with trivalent, catalytically active rare-earth metals, as, e.g. Sm. We present low-energy and X-ray photoemission electron microscopy (LEEM/XPEEM), investigations that show how epitaxially grown (100)- and (111)-oriented CeO2 islands may be modified and/or alloyed by post-deposited metallic Sm. For the Ce1−xSmxO2−δ/Ru(0001) system, the CeO2 (111)-oriented islands undergo a structural change, concomitant with a partial conversion from Ce4+ to Ce3+. Surprisingly, for Ce1−xSmxO2−δ/Cu(111) the result is found to be face-dependent since only (100)-oriented CeOx islands were reduced whereas the (111)-oriented islands remained unaltered. Both systems have been exposed to reducing (H2) and oxidizing (CO2) conditions, resulting in higher reduction and in a complete recovery of the Ce4+ states, respectively. These unexpected results indicate a complex interaction not only between cerium and the doping element, but also an intricate interplay with the metallic substrate.
Atomic layer deposition (ALD) is well known to lead to amorphous and defective, non-stoichiometric films, potentially resulting in modified material properties that can also be affected by film/substrate interaction in the case of ultra-thin growths. For example, the formation, diffusion, and recovery of oxygen vacancies can be favored in disordered, reducible metal oxides compared to more ordered deposits, whereas interdiffusion processes can critically affect the film/substrate interface region. These effects have extensively been studied for thin thermal-ALD ceria films (below 15 nm) by combining in-situ and ex-situ characterization techniques in our lab and at synchrotron radiation facilities. While using alumina or silica substrates modifies the initial growth rate, Ce3+/Ce4+ ratio, and ceria morphology, the formation of different species at the interface affects its reactivity. Interestingly, the experiments have shown high reducibility of ALD-ceria ultrathin films on silica for very low hydrogen concentrations, even at room temperature, whereas for alumina substrates the formation of aluminates at the interface prevents further oxidation. Moreover, the comparison with more ordered films indicates a key role of the defective structure of ALD films in Ce3+/Ce4+ conversion.
Atomic layer deposition (ALD) has been used extensively to grow homogeneous films with excellent coverage and atomic-scale thickness control for a variety of applications. However, remaining challenges include the investigation of novel precursor-oxidant combinations for low-temperature deposition as well as unraveling the complex interplay between substrate and coating for ultrathin films. In this work, we present a detailed investigation of ultrathin cerium oxide films grown using the novel Ce(dpdmg)3 precursor with H2O and O2. Following a surface science-based approach, we have combined operando spectroscopic ellipsometry and in-situ X-ray photoelectron spectroscopy to allow rapid process optimization and determination of the complex relation between oxide stoichiometry, film thickness and ALD growth parameters, revealing a distinct dependence of inital Ce3+ content on the film thickness and choice of oxidant. This offers the possibility of adjusting the oxide properties to application requirements e.g. in gas sensing by choosing a suitable precursor-oxidant combination.
The need to store and use hydrogen safely as part of green economy based on renewable energy evokes a necessity to reliably detect it at ambient conditions. The majority of currently used sensors are working at elevated temperatures (200-500 °C). In this work, we demonstrate that ceria films deposited on a commercial electrode by atomic layer deposition (ALD) and electron beam evaporation (EBE) electrically respond to hydrogen (from 20 to 500 ppm) at much lower temperatures (50-200 °C). The results reveal that <1.5 nm thin Pd adlayer increases the electrical response by several orders of magnitude for both ceria films. The NAP-XPS study under changing oxidative/reductive atmospheres sheds light on the mechanism of Pd-CeOx thermal activation and the role of the deposition technique in the reactivity of the oxide.
Understanding the interactions in hybrid systems based on graphene and functional oxides is crucial to the applicability of graphene in real devices. Here, we present a study of the structural defects occurring on graphene during the early stages of the growth of CoO, tailored by the electronic coupling between graphene and the substrate in which it is supported: as received pristine graphene on polycrystalline copper (coupled), cleaned in ultra-high vacuum conditions to remove oxygen contamination, and graphene transferred to SiO2/Si substrates (decoupled). The CoO growth was performed at room temperature by thermal evaporation of metallic Co under a molecular oxygen atmosphere, and the early stages of the growth were investigated. On the decoupled G/SiO2/Si samples, with an initial low crystalline quality of graphene, the formation of a CoO wetting layer is observed, identifying the Stranski-Krastanov growth mode. In contrast, on coupled G/Cu samples, the Volmer-Weber growth mechanism is observed. In both sets of samples, the oxidation of graphene is low during the early stages of growth, increasing for the larger coverages. Furthermore, structural defects are developed in the graphene lattice on both substrates during the growth of CoO, which is significantly higher on decoupled G/SiO2/Si samples mainly for higher CoO coverages. When approaching the full coverage on both substrates, the CoO islands coalesce to form a continuous CoO layer with strip-like structures with diameters ranging between 70 and 150 nm.
The high performance of platinum–tin catalysts for oxidation reactions has been linked to the formation of tin oxides at the metal surface, but little is known about the structure of these oxides or the chemical behavior that determines their catalytic properties. We show here how surface oxides on Pt3Sn(111) incorporate oxygen at the metal interface, which may be subsequently removed by reaction with CO. The storage mechanism, where oxygen uptake occurs without loss of interfacial Pt–Sn bonds, is enabled by the peculiar asymmetrical coordination state of Sn2+. O atoms are bound at pocket sites in the 2D oxide sheet between these outward-buckled Sn atoms and metallic Sn in the alloy surface below.
In-situ growth characterization of 2D heterostructures: MoSe2 on intercalated graphene/Ru(0001)
(2023)
Despite the great fundamental interest in 2D heterostructures, most of the investigated 2D heterostructures were realized by mechanical exfoliation or chemical vapor deposition in the millibar range, preventing true in-situ characterization of the growth process. Here, we have investigated the growth of MoSe2 on single-layer graphene on Ru(0001) via real-time in-situ low-energy electron microscopy and micro-diffraction. After preparation of the graphene by standard procedures from an ethylene precursor, MoSe2 has been prepared via co-deposition of Mo and Se. Prior Se intercalation of the graphene appears to enhance the subsequent growth of MoSe2 on the graphene. At elevated temperatures, rotational ordering of the MoSe2 is facilitated by the strongly enhanced mobility of single-domain MoSe2 islands that align with the high symmetry orientations of the underlying graphene, indicating a non-negligible interaction between the two van-der-Waals materials. Micro-spot angle-resolved photoemission proves the monolayer nature of the as-grown MoSe2 as well as the free-standing character of the Se-intercalated graphene underneath.
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.
Epitaxial graphene growth has often been studied on close-packed transition metal substrates, e. g., the Ru(0001) surface, which is a well-studied model system for strong graphene-support interaction. Here, we focus on a Ru surface with different symmetry, i. e., the Ru(10-10) surface, to investigate the influence of the presumably modified graphene-substrate interaction on the growth of epitaxial monolayer and bilayer graphene (MLG, BLG) islands. The structural and chemical differences of the graphene on the two different surfaces are investigated by photoemission electron microscopy (PEEM), delivering information on both morphology and electronic structure. In-situ PEEM observation of graphene growth on the Ru(10-10) substrate by ethylene decomposition reveals the growth characteristics of MLG and BLG, the latter showing second layer nucleation via surface segregation of carbon. Furthermore, depending on growth temperature and relative orientation of the growing islands and surface steps, different growth characteristics are observed, in contrast to previous studies of the graphene/Ru(0001) system whereas similar electronic properties seem to prevail. Yet, when the MLG is decoupled from the Ru(10-10) substrate via intercalation of oxygen a distinct shift in work function is identified, slightly different from the resulting shift on Ru(0001).
Hydrogen is an efficient source of clean and environmentally friendly energy. However, because it is explosive at concentrations higher than 4%, safety issues are a great concern. As its applications are extended, the need for the production of reliable monitoring systems is urgent. In this work, mixed copper–titanium oxide ((CuTi)Ox) thin films with various copper concentrations (0–100 at.%), deposited by magnetron sputtering and annealed at 473 K, were investigated as a prospective hydrogen gas sensing material. Scanning electron microscopy was applied to determine the morphology of the thin films. Their structure and chemical composition were investigated by X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The prepared films were nanocrystalline mixtures of metallic copper, cuprous oxide, and titanium anatase in the bulk, whereas at the surface only cupric oxide was found. In comparison to the literature, the (CuTi)Ox thin films already showed a sensor response to hydrogen at a relatively low operating temperature of 473 K without using any extra catalyst. The best sensor response and sensitivity to hydrogen gas were found in the mixed copper–titanium oxides containing similar atomic concentrations of both metals, i.e., 41/59 and 56/44 of Cu/Ti. Most probably, this effect is related to their similar morphology and to the simultaneous presence of Cu and Cu2O crystals in these mixed oxide films. In particular, the studies of surface oxidation state revealed that it was the same for all annealed films and consisted only of CuO. However, in view of their crystalline structure, they consisted of Cu and Cu2O nanocrystals in the thin film volume.
La-doped SrTiO3 thin films with high structural quality were homoepitaxially grown by the metal–organic vapor phase epitaxy (MOVPE) technique. Thermogravimetric characterization of the metal–organic precursors determines suitable flash evaporator temperatures for transferring the liquid source materials in the gas phase of the reactor chamber. An adjustment of the charge carrier concentration in the films, which is necessary for optimizing the thermoelectric power factor, was performed by introducing a defined amount of the metal–organic compound La(tmhd)3 and tetraglyme to the liquid precursor solution. X-ray diffraction and atomic force microscopy verified the occurrence of the pure perovskite phase exhibiting a high structural quality for all La concentrations. The electrical conductivity of the films obtained from Hall-effect measurements increases linearly with the La concentration in the gas phase, which is attributed to the incorporation of La3+ ions on the Sr2+ perovskite sites by substitution inferred from photoemission spectroscopy. The resulting structural defects were discussed concerning the formation of occasional Ruddlesden–Popper-like defects. The thermoelectric properties determined by Seebeck measurements demonstrate the high potential of SrTiO3 thin films grown by MOVPE for thermoelectric applications.