Carbon-free Precursor Research Articles

Online preparation of high-quality BN coatings with atomic diffusion based on carbon-free water-soluble precursor

Online preparation of high-quality BN coatings with atomic diffusion based on carbon-free water-soluble precursor

Photoelectrochemical properties of p-type CuBi2O4 prepared by spray pyrolysis of carbon-free precursor aqueous solution combined with post-annealing treatment

CuBi2O4 photoelectrodes were fabricated by a spray pyrolysis method using a carbon-free precursor solution. The optimized CuBi2O4 generated a photocurrent of −0.94 mA cm−2via H2O2 reduction at 0.6 VRHE under simulated sunlight illumination.

Thermal Atomic Layer Deposition of Silicon Carbonitride: A Density Functional Theory Study

Silicon carbonitride (SiCN) films have received considerable attention due to their lower dielectric constant and better etch selectivity than silicon nitride (SiN) films [1]. Therefore, SiCN is replacing SiN in a variety of applications, such as dielectric barriers for copper interconnects or sidewall spacers for self-aligned contacts. SiCN films have been mainly prepared by plasma-enhanced chemical vapor deposition (PECVD) or plasma-enhanced atomic layer deposition (PEALD). However, plasma-based deposition techniques suffer from poor film quality and lack of conformability on high-aspect-ratio patterns. Thermal atomic layer deposition at high process temperatures can deposit high-quality thin films with excellent conformality. However, no studies have been published on thermal ALD in SiCN, while there are a few reports on PEALD in SiCN. In this work, we studied the thermal ALD process of SiCN by alternating exposures to a carbon-containing silicon precursor and NH3 and compared it with the thermal ALD of SiN processes using carbon-free precursors. Density functional theory (DFT) calculations were used to model and simulate the continuous ligand exchange reactions between the silicon precursor and the substrate surface. The carbon incorporation in the growing film when using carbon-containing precursors was predicted by DFT, which is in good agreement with the experimental observations. Acknowledgments This work was supported by Hansol Chemical Co., Ltd.

Recombination dynamics of indirect excitons in hexagonal BN epilayers containing polytypic segments grown by chemical vapor deposition using carbon-free precursors

Hexagonal (h) BN is a semiconductor that crystallizes in layers of a two-dimensional honeycomb structure. Since hBN exhibits high quantum efficiency (QE) near-band edge emission at around 5.8 eV in spite of the indirect bandgap, hBN has a potential for the use in deep-ultraviolet light emitters. For elucidating the emission dynamics of indirect excitons (iXs) in hBN, spatially and temporally resolved luminescence measurements were carried out on hBN epilayers grown using carbon-free precursors. In addition to major μm-side flat-topped (0001) hBN columnar grains, sub-μm-scale polytypic segments were identified, which were likely formed by certain growth instabilities. The hBN domains exhibited predominant emissions of phonon-assisted fundamental iXs at 5.7–5.9 eV and a less-pronounced 4.0-eV emission band. The photoluminescence lifetime (τPL) for the iX emissions was 54 ps, which most likely represents the midgap recombination lifetime (τMGR) for an iX reservoir. Because τPL did not change while the cathodoluminescence (CL) intensity increased with temperature above 100 K, both the immobile character of iXs and strong exciton–phonon interaction seem significant for procreating the high QE. The CL intensity and τPL of the 5.5 eV band monotonically decreased with temperature, indicating that τPL represents τMGR, most probably a nonradiative lifetime, around the real states. Equally significant emissions at 6.035 eV at 12 K and 6.0–6.1 eV at 300 K were observed from the polytypic segments, most probably graphitic bernal BN, which also exhibited negligible thermal quenching property.

Comprehensive characterization and analysis of hexagonal boron nitride on sapphire

Hexagonal boron nitride (h-BN) is considered as one of the most promising materials for next-generation quantum technologies. In this paper, we report bulk-like multilayer h-BN epitaxially grown using a carbon-free precursor on c-plane sapphire with a strong emphasis on material characterization and analysis. In particular, structural, morphological, and vibrational properties, and chemical bonding of such van der Waals materials are presented. Between as-grown h-BN and c-plane sapphire, a compressive residual strain induced by both lattice mismatch and thermal expansion mismatch is examined by both theoretical and experimental studies. Atomic force microscopy revealed and scanning electron microscopy supported the presence of wrinkles across the entire surface of the film, likely due to biaxial compressive strain further verified by Raman spectroscopy. Stacking orders in h-BN with a clearly layered structure were confirmed by high resolution transmission electron microscopy, showing that the films have crystallographic homogeneity. Chemical analysis of the as-grown films was done by x-ray photoelectron spectroscopy, which confirmed the formation of stoichiometric h-BN films with excellent uniformity. This wafer-scale chemical vapor deposition-grown layered h-BN with 2D morphology facilitates applications in the fields of quantum- and deep ultraviolet-photonics.

Growth of Monolayer and Multilayer MoS2 Films by Selection of Growth Mode: Two Pathways via Chemisorption and Physisorption of an Inorganic Molecular Precursor.

We report facile growth methods for high-quality monolayer and multilayer MoS2 films using MoOCl4 as the vapor-phase molecular Mo precursor. Compared to the conventional covalent solid-type Mo precursors, the growth pressure of MoOCl4 can be precisely controlled. This enables the selection of growth mode by adjusting growth pressure, which facilitates the control of the growth behavior as the growth termination at a monolayer or as the continuous growth to a multilayer. In addition, the use of carbon-free precursors eliminates concerns about carbon contamination in the produced MoS2 films. Systematic studies for unveiling the growth mechanism proved two growth modes, which are predominantly the physisorption and chemisorption of MoOCl4. Consequently, the thickness of MoS2 can be controlled by our method as the application demands.

Atomic layer deposition of HfO2 films using carbon-free tetrakis(tetrahydroborato)hafnium and water

Thin hafnium oxide films were prepared by atomic layer deposition using a carbon-free precursor, tetrakis(tetrahydroborato)hafnium [Hf(BH4)4], and H2O. Film growth was studied using an in situ quartz crystal microbalance and Fourier transform infrared spectroscopy measurements. Self-limiting growth was observed between 100 and 175 °C, but the thermal decomposition of the Hf precursor occurred at higher temperatures. The film properties were investigated using x-ray photoelectron spectroscopy, x-ray reflectivity, x-ray diffraction, ellipsometry, time-of-flight secondary ion mass spectrometry, and x-ray absorption spectroscopy. The as-deposited films were found to consist of an amorphous mixture of HfO2 and B2O3, and had a lower density and lower refractive index compared to pure HfO2 thin films. Annealing the films to >750 °C yielded crystalline monoclinic HfO2 with a density of 9 g/cm3 and a refractive index of 2.10.

Preparation of sol-gel ZrO2 films with high laser-induced damage threshold under high temperature.

In this study, three different types of ZrO2 films were prepared with different precursors and additives using the sol-gel method. High-temperature annealing was implemented to investigate the impact of temperature on optical properties, microstructure, surface morphologies and absorption of these films. According to the laser-induced damage threshold (LIDT) tests on films having experienced annealing and those implemented with in-situ high temperature, the ZrO2 film with ZrOCl2·8H2O as the precursor and copolymer of silicone and polyaldoxyl ether as the additive had the highest resistance to laser-induced damage. After annealing at 623 K, its LIDT was 21.4 J/cm2, while that at an in-situ high temperature of 523 K was 23.9 J/cm2. The strong high temperature resistance was likely attributed to the usage of carbon-free precursor and high temperature-resistant additive, which contributed to low carbon contents and less structural damage caused by organic matter evaporation. In this context, there were less high temperature-induced impurity and structural defects, leading to higher LIDT values. This study provided a novel method for preparing high temperature-resistant sol-gel films, which shed light upon wider potential application of sol-gel films at high-temperature conditions.

Pure deep-ultraviolet cathodoluminescence from rocksalt-structured MgZnO grown with carbon-free precursors

Rocksalt-structured MgZnO thin films of optical band gaps as large as about 5.8–6.6 eV were grown on MgO substrates by a mist chemical vapor deposition method using carbon-free precursors. MgZnO thin films with the dislocation density of less than 107 cm−2 were obtained. Pure deep-ultraviolet cathodoluminescence was observed without noticeable emission at the longer wavelengths, being attributed to improved crystallinity as well as reduced carbon impurity. The highest emission energy (the shortest emission wavelength) at the luminescence peak was 6.24 eV (199 nm) at 6 K.

Deep-Ultraviolet Luminescence of Rocksalt-Structured MgxZn1−xO (x > 0.5) Films on MgO Substrates

Rocksalt-structured MgxZn1−xO films with Mg composition x of 0.47, 0.57, and 0.64 were grown on (100)-oriented MgO substrates using mist chemical vapor deposition. Cathodoluminescence measurements showed deep ultraviolet (DUV) emission peaking at 4.88 eV (254 nm), 5.15 eV (241 nm), and 5.21 eV (238 nm), respectively, at 12 K. The peak energies were lower than the band gap energies by ca. 1 eV, suggesting that the deep ultraviolet (DUV) emission may be recognized as near band edge luminescence but is associated with impurities, defects, or band fluctuations. The use of carbon-free precursors in the growth is suggested to eliminate carbon impurities and to improve the optical properties of MgxZn1−xO.

Functionalization of SiO2 Surfaces for Si Monolayer Doping with Minimal Carbon Contamination.

Monolayer doping (MLD) involves the functionalization of semiconductor surfaces followed by an annealing step to diffuse the dopant into the substrate. We report an alternative doping method, oxide-MLD, where ultrathin SiO2 overlayers are functionalized with phosphonic acids for doping Si. Similar peak carrier concentrations were achieved when compared with hydrosilylated surfaces (∼2 × 1020 atoms/cm3). Oxide-MLD offers several advantages over conventional MLD, such as ease of sample processing, superior ambient stability, and minimal carbon contamination. The incorporation of an oxide layer minimizes carbon contamination by facilitating attachment of carbon-free precursors or by impeding carbon diffusion. The oxide-MLD strategy allows selection of many inexpensive precursors and therefore allows application to both p- and n-doping. The phosphonic acid-functionalized SiO2 surfaces were investigated using X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy, whereas doping was assessed using electrochemical capacitance voltage and Hall measurements.

Enhanced Conductivity in the Metastable Intermediate in Li x FePO4 Electrode

Olivine LiFePO4 is a major commercial cathode material for lithium ion batteries due to its high specific theoretical capacity, low cost, nontoxicity, and superb thermal/chemical stability. In addition to the above properties, olivine LiFePO4 shows an exceptional high-rate capability despite its poor intrinsic electronic conductivity and sluggish two-phase reaction process. Last year, a non-equilibrium solid solution reaction process was experimentally demonstrated by Grey et al[1]. Independent research was conducted by several groups and confirmed these results[2] , [3]. Recently, we have determined the crystal structure of intermediate Li2/3FePO4 [4]. However, until now there has been no investigation on the transport properties of the metastable intermediate phase formed at the phase boundary interface during the non-equilibrium process due to the following extreme technical difficulties. First, the solid solution phase Li x FePO4 is metastable during electrochemical process and disappears within a few seconds after relaxation[1,3]. We overcame this limitation by quenching Li x FePO4 (x = 0.6) at 350 °C to room temperature. This quenched phase remained stable for a couple of weeks, which enabled sufficient time to measure the intrinsic conductivity. Second, conductive carbon may form during sintering at high temperature from possible carbon sources such as polyolefin worn from jars, organic solvent used in milling and carbon-containing precursors (e.g., oxalate FeC2O4·2H2O). Finally, impurities such as Li3PO4, Li4P2O7 and Fe x P resulting from off-stoichiometric mixing or the carbon reduction effect above 800 °C will largely increase the apparent conductivity of LiFePO4 by several orders of magnitude[5]. The scattering of the LiFePO4 conductivity data resulting from the above extrinsic effects has troubled and confused scientists for a long time. In our present study, pure carbon-free LiFePO4 and FePO4 were prepared using carbon-free precursors and controlling sintering parameters, and the intrinsic conductivity of quenched single phase Li x FePO4 (x = 0.6) was measured. The following figure shows representative Nyquist plots of carbon-free LiFePO4, carbon-free FePO4, and quenched Li0.6FePO4 samples measured at 340 K. Of particular interest is that the electronic resistance of the quenched single-phase Li0.6FePO4 was approximately 2 orders of magnitude smaller than those of the two end-members (LiFePO4 and FePO4) at 340 K. The temperature-dependent conductivity was also measured. Across the entire temperature range of 30 to 350 °C, the single-phase Li0.6FePO4 shows approximately 2 orders of magnitude superior electronic conductivity. The activation energy of Li0.6FePO4 obtained from the slope of the temperature-dependent conductivity was almost identical to the two end members (LiFePO4 and FePO4), which means that the enhanced electronic conductivity originates from the increased charge carrier density. In summary, a metastable solid solution phase Li x FePO4 (x = 0.6) was isolated by the optimized quenching protocol and found to have approximately 2 orders of magnitude enhanced electronic conductivity over the two end members of LiFePO4 and FePO4. Our research reveals that the single-phase transformation mechanism not only relaxes the interface strain energy but also enhances the intrinsic charge transport, enabling the high-rate capability of olivine LiFePO4.

Purity and resistivity improvements for electron-beam-induced deposition of Pt

Electron-beam-induced deposition (EBID) of platinum is used by many researchers. Its main application is the formation of a protective layer and the “welding material” for making a TEM lamella with a focused ion beam thinning process. For this application, the actual composition of the deposition is less relevant, and in practice, both the mechanical strength and the conductivity are sufficient. Another important application is the creation of an electrical connection to nanoscale structures such as nano-wires and graphene. To serve as an electrical contact, the resistivity of the Pt deposited structure has to be sufficiently low. Using the commonly used precursor MeCpPtMe3 for deposition, the resistivity as created by the basic process is 10+5–10+6 higher than the value for bulk Pt, which is 10.6 µΩ cm. The reason for this is the high abundance of carbon in the deposition. To improve the deposition process, much attention has been given by the research community to parameter optimization, to ex situ or in situ removal of carbon by anneal steps, to prevention of carbon deposition by use of a carbon-free precursor, to electron beam irradiation under a high flux of oxygen and to the combination with other techniques such as atomic layer deposition (ALD). In the latter technique, the EBID structures are used as a 1-nm-thick seed layer only, while the ALD is used to selectively add pure Pt. These techniques have resulted in a low resistivity, today approaching the 10–150 µΩ cm, while the size and shape of the structure are preserved. Therefore, now, the technique is ready for application in the field of contacting nano-wires.

On the potential use of carbon-free mesoporous precursors of LiFePO4 for lithium-ion batteries electrode

Abstract This paper describes the preparation, characterization and electrochemical study of various iron and lithium iron phosphates. The pristine iron phosphate samples obtained were amorphous materials with various textural properties, in terms of average pore size, pore order and specific surface area. Some solids were mesoporous, featuring high surface area, narrow pore size distribution and high pore volume. Lithium iron phosphates were obtained by chemical lithiation of these iron phosphates, using LiI dissolved in acetonitrile. These materials were amorphous and electrochemically inactive against lithium. Following heating at 550 °C, the materials crystallized and showed disparate electrochemical performance in lithium half-cells. This can be ascribed not only to the presence in the samples of impurities of different nature (and inherent to the synthesis method), but also to different textural and morphological properties. In fact, the best electrochemical performance was that of crystallized LiFePO 4 obtained from the mesoporous iron phosphate and consisting of rounded nanoparticles with high surface area and adequate porosity. As a result, a better electrolyte impregnation in the porous structures is highly expected. Furthermore, enhancement of the lithium ion diffusion inside the network is observed. This justifies the potential use of carbon-free mesoporous precursors of LiFePO 4 . However, introduction of a conductive carbon coating onto the phosphate particles and/or removal of the impurities prove necessary for good capacity retention upon cycling.

High‐Performance ZnO Transistors Processed Via an Aqueous Carbon‐Free Metal Oxide Precursor Route at Temperatures Between 80–180 °C

An aqueous and carbon-free metal-oxide precursor route is used in combination with a UV irradiation-assisted low-temperature conversion method to fabricate low-voltage ZnO transistors with electron mobilities exceeding 10 cm(2) /Vs at temperatures <180 °C. Because of its low temperature requirements the method allows processing of high-performance transistors onto temperature sensitive substrates such as plastic.

Electrical properties of platinum interconnects deposited by electron beam induced deposition of the carbon-free precursor, Pt(PF3)4

Comprehensive analysis of the electrical properties, structure and composition of Pt interconnects, developed via mask-less, electron beam induced deposition of the carbon-free Pt precursor, Pt(PF3)4, is presented. The results demonstrate significantly improved electrical performance in comparison with that generated from the standard organometallic precursor, (CH3)3Pt(CpCH3). In particular, the Pt interconnects exhibited perfect ohmic behavior and resistivity that can be diminished to 0.24 × 10−3 Ω cm, which is only one order of magnitude higher than bulk Pt, in comparison to 0.2 Ω cm for the standard carbon-containing interconnects. A maximum current density of 1.87 × 107 A cm−2 was achieved for the carbon-free Pt, compared to 9.44 × 105 A cm−2 for the standard Pt precursor. The enhanced electrical properties of the as-deposited materials can be explained by the absence of large amounts of carbon impurities, and their further improvement by postdeposition annealing in N2. In-situ TEM heating experiments confirmed that the annealing step induces sintering of the Pt nanocrystals and improved crystallinity, which contributes to the enhanced electrical performance. Alternative annealing under reducing conditions resulted in improved performance of the standard Pt interconnects, while the carbon-free deposit suffered electrical and structural breakage due to formation of larger Pt islands.

Direct Writing of Sub-5 nm Hafnium Diboride Metallic Nanostructures

Sub-5 nm metallic hafnium diboride (HfB(2)) nanostructures were directly written onto Si(100)-2 × 1:H surfaces using ultrahigh vacuum scanning tunneling microscope (UHV-STM) electron beam induced deposition (EBID) of a carbon-free precursor molecule, tetrakis(tetrahydroborato)hafnium, Hf(BH(4))(4). Scanning tunneling spectroscopy data confirm the metallic nature of the HfB(2) nanostructures, which have been written down to lateral dimensions of ∼2.5 nm. To our knowledge, this is the first demonstration of sub-5 nm metallic nanostructures in an STM-EBID experiment.

Improving the conductivity of platinum-containing nano-structures created by electron-beam-induced deposition

Electron-beam-induced deposition (EBID) allows nano-scale three-dimensional rapid prototyping and is a promising nano-lithography alternative. One application is to write conductive lines at a position chosen by the scanning electron microscope (SEM) operator, for example to create contact electrodes to nano-devices. Therefore the deposited material’s resistivity should be as close as possible to the bulk metal resistivity namely 10.62μΩcm in the case of platinum, but incorporation of carbon in the deposit resulting from the use of the traditional organometallic precursors negatively impacts conductivity. Firstly, for deposition from the standard organometallic platinum precursor MeCpPtMe3, we show that for optimal platinum content the beam power density at the substrate surface needs to exceed 10μW/μm2. Secondly, we present a novel post-treatment method for improving the conductivity of these nano-deposits ex-situ: exposure to atomic hydrogen flow. On structures deposited from MeCpPtMe3, the carbon content decreases from (81±2) to (65±2)at.% and the resistivity is decreased to (2.2±0.7)×104μΩcm. Thirdly, we present our investigations with a carbon-free platinum precursor, Pt(PF3)4, where resistivities of (690±30)μΩcm were achieved, and furthermore report the dependence of conductivity with substrate temperature during deposition.

One-step growth of HfSiON films

Hafnium silicate (HfSiO) has been identified as a promising candidate to replace silicon oxide/oxynitride as a high- κ material for gate dielectric applications. Nitrided hafnium silicate has been found to have a number of advantages in film performance. However, two-step processes have been commonly used, i.e. the first step is the deposition of HfSiO film by CVD, ALD or other techniques, and the second step is film nitridation. In this research, HfSiON films (Hafnium silicon oxynitride, or nitrided HfSiO) were directly deposited on Si substrate by Chem. Vap. Depos. using trisilylamine (TSA) and tetrakis(diethylamido)hafnium(IV) (TDEAH) precursors. TSA, a highly volatile and carbon-free precursor, was used as the Si source and was delivered in pure vapor phase without heating. TDEAH was used as the Hf source and delivered by direct liquid injection (vaporizer). HfSiON films were deposited in a single step with no need of a post treatment process for nitrogen incorporation. The HfSiON films can be tuned in wide compositional (Hf, Si, O, N) ranges and high growth rates were achieved. The addition of NH 3 to the reactant gas stream was found to be able to deposit films with high and controllable N content. It was also found that NH 3 had significant impact on film growth rate and composition.

Direct chemical vapor deposition growth of tunable HfSiON films by a new precursor combination

Hafnium silicon oxynitride (HfSiON) films were deposited on 200-mm silicon substrates by low-pressure chemical vapor deposition (LPCVD) from a combination of trisilylamine (TSA) and tetrakis(diethylamido)hafnium(IV) (TDEAH) in the temperature range 450 to 575 °C. A highly volatile and carbon-free silicon precursor TSA was used to deposit HfSiON films for the first time. HfSiON films were deposited in a single step with no need of a post-treatment process for nitrogen incorporation. The film composition was tuned in a wide compositional range, and high growth rates were achieved. NH3 was found to have profound effects on film growth rate, metal ratio (Si% or Hf%), nitrogen incorporation, and carbon residue in the films.