Forming Gas Atmosphere Research Articles

Vertically Well-Aligned ZnO Nanoscintillator Arrays with Improved Photoluminescence and Scintillation Properties.

ZnO nanoarrays were grown via a low-temperature hydrothermal method. Solutions, each with different additive combinations, were prepared and evaluated. The effects of the additives involved in the growth procedure, i.e., ammonium hydroxide and sodium citrate, were studied in terms of the morphological, optical and scintillation properties of the ZnO nanostructures. Measurement of the nanorod (NR) length, corresponding photoluminescence (PL) and scintillation spectra and their dependence on the additives present in the solution are discussed. ZnO NRs grown on a silica substrate, whose UV transmission was found to be better than glass, showed high-quality structural and optical properties. It was found that the addition of sodium citrate significantly reduced defects and correspondingly increased the intrinsic near-band-edge (NBE) UV emission intensity at ~380 nm. To obtain high-quality nanostructures, samples were annealed in a 10% H2 + 90% N2 atmosphere. The anneal in the forming gas atmosphere enhanced the emission of the UV peak by reducing defects in the nanostructure. NRs are highly tapered towards the end of the structure. The tapering process was monitored using time growth studies, and its effect on PL and reflectance spectra are discussed. A good alpha particle response was obtained for the grown ZnO NRs, confirming its potential to be used as an alpha particle scintillator. After optimizing the reaction parameters, it was concluded that when ammonium hydroxide and sodium citrate were used, vertically well-aligned and long ZnO nanoarrays with highly improved optical and scintillation properties were obtained.

Copper-Based Nanomaterials for Fine-Pitch Interconnects in Microelectronics.

ConspectusNanostructured copper-based materials have emerged as a new generation of robust architectures for realizing high-performing and reliable interconnection in modern electronic packaging. As opposed to traditional interconnects, nanostructured materials offer better compliance during the packaging assembly process. Due to the high surface area-to-volume ratio of nanomaterials, they also enable joint formation by sintering through thermal compression at much lower temperatures compared to bulk counterparts. Nanoporous Cu (np-Cu) films have been employed in electronic packaging as materials that facilitate a chip-to-substrate interconnection, realized by a Cu-on-Cu bonding after sintering.In this Account, we discuss the use of self-supported np-Cu films for low-temperature joint formation. The novelty of this work comes from the incorporation of tin (Sn) into the np-Cu structure, thus ensuring lower sintering temperatures with a goal of producing Cu-Sn intermetallic alloy-based joints between two Cu substrates. The incorporation of Sn is done using an all-electrochemical bottom-up approach that involves the conformal coating of fine-structured np-Cu (initially formed by dealloying of Cu-Zn alloys) with a thin layer of Sn.This Account provides insight on existing technologies for using nanostructured films as materials for interconnects as well as the optimization studies for the Sn-coating processes as a new alternative approach. The applicability of the synthesized Cu-Sn nanomaterials for low-temperature joint formation is also discussed. To realize this new approach, the Sn-coating process is administered by a galvanic pulse plating technique, which is optimized to preserve the porosity in the structure with a Cu/Sn atomic ratio that allows for the formation of the Cu6Sn5 intermetallic compound (IMC). Nanomaterials obtained using this approach are subjected to joint formation by sintering at temperatures between 300 and 200 °C under 20 MPa pressure in forming gas atmosphere. Cross-section characterization of the formed joints postsintering reveals densified bonds with minimal porosity that consist predominantly of the Cu3Sn IMC. Furthermore, these joints are less prone to structural inconsistencies compared to existing joints formed using purely np-Cu. The results presented in this Account provide a glimpse into a facile and cost-effective approach for synthesizing nanostructured Cu-Sn films and illustrate their applicability as new interconnect materials.

Multigram preparation of tungsten microfibers via needle-less electrospinning of phosphotungstic acid

Metallic tungsten microfibers were prepared in a multigram yield by electrospinning from the aqueous phosphotungstic acid/polyvinyl alcohol (PVA) solution in three steps. Green composite microfibers of H3PW12O40 /PVA were easily electrospun from precursor solutions on a Nanospider instrument with a cylindrical rotating electrode. Subsequent oxidation of the organic PVA matrix and decomposition of phosphotungstic acid in the air at 600 °C provided ceramic WO3/P2O5 fibers. Finally, prepared oxide fibers were reduced in a forming gas atmosphere at temperatures up to 1000 °C. During the reduction, all phosphorus was removed, and pure metallic tungsten microfibers were produced.

Cathodoluminescence and optical absorption spectroscopy of plasmonic modes in chromium micro-rods

Manipulating light at the sub-wavelength level is a crucial feature of surface plasmon resonance (SPR) properties for a wide range of nanostructures. Noble metals like Au and Ag are most commonly used as SPR materials. Significant attention is being devoted to identify and develop non-noble metal plasmonic materials whose optical properties can be reconfigured for plasmonic response by structural phase changes. Chromium (Cr) which supports plasmon resonance, is a transition metal with shiny finished, highly non-corrosive, and bio-compatible alloys, making it an alternative plasmonic material. We have synthesized Cr micro-rods from a bi-layer of Cr/Au thin films, which evolves from face centered cubic to hexagonal close packed (HCP) phase by thermal activation in a forming gas ambient. We employed optical absorption spectroscopy and cathodoluminescence (CL) imaging spectroscopy to observe the plasmonic modes from the Cr micro-rod. The origin of three emission bands that spread over the UV–Vis-IR energy range is established theoretically by considering the critical points of the second-order derivative of the macroscopic dielectric function obtained from density functional theory (DFT) matches with interband/intraband transition of electrons observed in density of states versus energy graph. The experimentally observed CL emission peaks closely match the s–d and d–d band transition obtained from DFT calculations. Our findings on plasmonic modes in Cr(HCP) phase can expand the range of plasmonic material beyond noble metal with tunable plasmonic emissions for plasmonic-based optical technology.

Annealing temperature controlled crystallization mechanism and properties of gallium oxide film in forming gas atmosphere

AbstractGallium oxide (Ga2O3) films had been fabricated on Al2O3(0001) substrate by employing pulsed laser deposition (PLD) and annealed at different temperatures under forming gas (FG) atmosphere (95% N2 + 5% H2). The influence of annealing temperature on the structural, optical, chemical composition, and surface morphological properties of the Ga2O3 thin films was investigated comprehensively. The annealing processes with hydrogen gas play a crucial role in the characteristics of Ga2O3 thin films. A crystallization mechanism of Ga2O3 films controlled by annealing temperature has been proposed firstly and analyzed systematically, which contains three kinds of competitive mechanism, namely the thermal enhanced crystallization, the enhanced H2 dissociative adsorption on Ga2O3 surfaces, and the high‐temperature decomposition of Ga2O3. Both Ga+ and Ga3+ oxidation valence states were presented in all samples, which indicated lattice oxygen deficiency in Ga2O3 films. The variation of the non‐lattice oxygen proportion of Ga2O3 films related to the crystallization mechanism firstly increased and then decreased with the increase of annealing temperature. The detailed crystallization mechanism of PLD‐Ga2O3 films annealed in FG offers a guideline and references for the further fabrication of high‐quality Ga2O3 films and their applications in high‐performance devices.

Hydrogen Assisted Magnesiothermic Reduction of Y-Doped, Nanocrystalline TiO2

The direct reduction of TiO2 to low-oxygen titanium metal is achievable via Hydrogen Assisted Magnesiothermic Reduction (HAMR). To investigate and leverage the oxygen-scavenging properties of rare-earth dopant species on the HAMR process, Y-doped and undoped TiO2 powders were synthesized and characterized. HAMR blends incorporating the synthesized TiO2 were reduced under forming gas atmospheres. X-ray powder diffraction (XRD) and scanning electron microscope (SEM) characterization was performed prior to and following reduction. The TiO2 powders were observed to be dense and nanocrystalline. Following reduction, more extensive development of intermediate HAMR phases was observed as a result of Y-doping. The microstructure/phase evolution of the HAMR reduction phases was observed to deviate from the expected for dense TiO2 particles. Rapid restructuring of the TiO2 particle interiors was attributed to increased bulk diffusion rates of nanocrystalline materials. Doped nanocrystalline TiO2 powders were identified as potential alternative feedstocks for HAMR experiments. The byproduct MgO phase was observed to grow as a particle agglomerating network that is dense when formed at 750 °C and porous when formed at 900 °C.

Double Layer Growth of ZnO Nanorods by a Low Temperature Solution Method: Synthesis and Photoluminescence Properties

In the present work, double layer Zinc Oxide (ZnO) nanorods (NRs) were fabricated hydrothermally and their photoluminescence (PL) properties were investigated. Two different recipes and their combination were used to obtain double layer vertically well-aligned ZnO NRs. These recipes include polyethylenimine (PEI) and citrate as additives in the growth solution resulting long NRs with a broad defect emission and relatively short NRs with a near band-edge ultraviolet (UV) emission, respectively. Double layer growth of long-long (LL), long-short (LS) and short-short (SS) ZnO NRs were considered. Grown samples were annealed in a forming gas atmosphere for a better quality NR structure. LL ZnO NRs (long ZnO NRs grown on a long ZnO NR layer) with a 65 µm thickness showed a broad yellow-orange PL emission and no any near band-edge UV emission was observed. LS ZnO NRs representing short ZnO NRs grown on a long ZnO NR layer (LS1= 36 µm, LS2= 48 µm and LS3= 44 µm) showed an enhanced near band-edge UV emission when compared to that of the long ZnO NRs. The UV intensity was found to decrease with the increase in thickness of the NRs in LS samples. Finally, SS ZnO NR sample, (short ZnO NRs grown on a short ZnO NR layer) which has a thickness of 33 µm, displayed a stronger near band-edge UV emission with a negligible broad emission than that of as-grown SS ZnO NRs (UV peak intensity ratio 59). This study should be important for applications where longer NRs with enhanced PL properties are strictly required.

Optoelectronic Properties of Antimony Doped Tin Oxide Thin Films Obtained by Spray Pyrolysis

Antimony doped tin oxide (ATO) thin films are deposited on corning glass substrate using the spray pyrolysis technique. The experimental parameters such as distance between the substrate and source (10-30 cm), substrate temperature (350-450°C) and atmospheres (Nitrogen and Forming gas) are varied to study their effect on the properties of ATO thin films. The ATO thin film annealed at 425°C exhibits the lowest electrical resistivity of 2.23×10-2 Ω-cm. Besides, the film annealed in the nitrogen atmosphere showed a less resistivity value of 9.06×10-3 (Ω-cm) than the forming gas atmosphere. The film doped with 3 at% of Sb revealed the highest figure of merit value of 11.45x10-2 Ω-1. The preferential orientation is observed at the (200) diffraction plane in all the cases from the structural studies. Furthermore, the intensity of the diffraction planes decreases as the temperature increases. The average transmittance of 75% is obtained for ATO thin films.

Thermal annealing ambiance effect on phosphorus passivation and reactivation mechanisms in silicon-based Schottky diodes hydrogenated by MW-ECR plasma

The main objective of this work is to investigate the effect of thermal annealing in forming gas atmosphere on the mechanism of deactivation and reactivation of phosphorus in silicon-based Schottky diodes. Firstly, the microwave plasma power, initial phosphorus concentration in the samples and hydrogen flux were fixed as 650 W, 1015 cm–3, and 30 sccm, respectively, to investigate the behavior of different working parameters of diodes, specifically the duration and temperature of hydrogenation. Secondly, few samples hydrogenated at 400 °C for 1 h were annealed under the forming gas (10% H2 + 90% N2) within the temperature range from 100 to 700 °C for 1 h. The profiles of active phosphorus concentration were monitored by evaluating the change in concentration of phosphorus after hydrogenation or thermal annealing in a forming gas environment through capacitance-voltage measurements. The obtained results depict the temperature and duration of hydrogenation, which ultimately reveals the complex behavior of phosphorous and hydrogen in silicon. However, the phosphorus passivation rate is homogeneous over all the depths measured at 400 °C. The thermal annealing in a forming gas indicates the increase in passivation rate of phosphorus as a function of annealing temperature, till the passivation rate attains saturation in the sample annealed at 400 °C. At higher temperatures, a decrease in the concentration of phosphorous-hydrogen complexes is observed due to the dissociation of these complexes and reactivation of phosphorus under thermal effect.

Pressureless sinter joining of bare Cu substrates under forming gas atmosphere by surface-oxidized submicron Cu particles

Pressureless sinter joining of bare Cu substrates under forming gas atmosphere by surface-oxidized submicron Cu particles

Improving the Characteristics of Niobium-Doped Titanium Dioxide Nanofilm with Moderate Hydrogen Incorporation

20 nm thick anatase niobium-doped titanium dioxide (TNO) nanofilm with resistivity of 2.9 × 10−3 Ω·cm, visible-light transmittance of over 92% and work function of 4.43 eV was obtained by radio-frequency magnetron sputtering method and a diluted H2 atmosphere annealing. A rapid annealing in a forming gas atmosphere of 3%H2 and 97%N2 at 500 °C reduced the resistivity by 3 orders of magnitude. Furthermore, the utilization of a “glass cover” (GC) in the annealing process decreased the resistivity by approximately 3 times and increased the refractive index by approximately 3.6% for the anatase TNO nanofilm. The secondary ion mass spectrometry measurement confirmed that the GC restrained the hydrogen diffusion from the surface into TNO occurred during the annealing. Employed as the antireflection layer in hydrogenated amorphous Si solar cells, the TNO nanofilm annealed with GC enabled an open-circuit voltage of 0.91 V, a gain of 6.5% in short-circuit current density and a reduction of 30.3% in series resistance, resulting in a gain of approximately 12.8% in photoelectric conversion efficiency. These results suggest that moderate H incorporation is beneficial for improving the electrical and optical characteristics of TNO thin films.

Self-aligned-TiSi2 bottom contact with APM cleaning and post-annealing for sputtered-MoS2 film

Electrical contact characteristics between self-aligned titanium silicide (TiSi2) and sputtered-molybdenum disulfide (MoS2) films were newly demonstrated. In contrast with metal contacts, the surface of the TiSi2 bottom-contact was cleaned by using an ammonia and hydrogen peroxide mixture (APM) before MoS2 deposition, because the TiSi2 film has high oxidation resistance. In order to extract the contact resistance, a transmission line model device was fabricated. A two-order reduction in the contact resistance was achieved by a post-annealing at 650 °C in forming gas ambient (3% H2 in N2). This reduction was attributed to selective titanium diffusion from bottom contact into the MoS2 film. The TiSi2 contact is thus speculated as a candidate for a practical contact material in MoS2 devices.

Investigation of the Effects of Forming Gas Annealing on Al2O3/GaN Interface

In this work, the effects of post-deposition annealing in forming gas ambient on the structure of the Al2O3/GaN interface are investigated. Using capacitance–voltage (C–V) and conductance–frequency (G–f) measurements, the quality of Al2O3/GaN metal–insulator–semiconductor (MIS) devices subjected to different annealing conditions was assessed. Annealing at 350°C for 10 and 20 min resulted in the lowest density of trap states (< 1011 cm−2eV−1), while higher-temperature annealing resulted in increased density of interface traps. Scanning transmission electron microscopy, energy-dispersive x-ray spectroscopy (STEM/EDS), and x-ray photoelectron spectroscopy (XPS) were used to determine the impact of the anneal on the structure of the oxide and interface. It was found that after annealing at 600°C for 20 min, the oxide structure was not significantly changed, though the density of interface states was found to increase by nearly an order of magnitude. These results suggest that the post-deposition annealing of Al2O3/GaN in forming gas passivates defects at the interface but does not cause significant structural change away from the interface.

The improvement of atomic layer deposited SiO2/4H-SiC interfaces via a high temperature forming gas anneal

This letter reports on the improvement of a SiO2 layer formed by atomic layer deposition on 4H-SiC, using a post-deposition anneal in forming gas ambient. Capacitance–voltage measurements revealed good electrical properties, compared to a thermal oxide which was grown in N2O, with flatband voltage values averaging at -0.29 V and a low positive mobile ion charge density in the order of 1010 cm−2. XPS analysis revealed the FG annealed sample to have the most Si rich interface comparatively to other PDAs, with a C:Si ratio of 0.72, allowing more Si bonds to be terminated. SIMS analysis identified an increase in hydrogen near the interface of the FG-annealed sample with a peak concentration of 2.12 × 1021 cm−3. It is concluded that the improvement in electrical performance is due to the hydrogen passivating trap states at the SiO2/4H-SiC interface.

Effects of Post-Deposition Annealing Time in Forming Gas Ambient on Y2O3 Films Deposited on Silicon Substrate

Effects of post-deposition annealing (PDA) time (15, 30, and 45 min) at 800°C in forming gas (95% N2-5% H2) ambient was systematically studied for RF-magnetron sputtered Y2O3 films on n-type Si(100) substrate. X-ray diffraction characterization has revealed the detection of Y2O3 phase oriented in (400), (440), (541), and (543) planes for all of the investigated samples. In addition, capacitance-voltage characteristics of the investigated Al/Y2O3/Si-based metal-oxide-semiconductor capacitors were also presented in this work.

Effect of Forming Gas Furnace Annealing on the Ferroelectricity and Wake-Up Effect of Hf0.5Zr0.5O2 Thin Films

The effect of furnace annealing on the ferroelectricity, leakage current, and wake-up effect in Hf0.5Zr0.5O2 (HZO) ultrathin film is studied as a function of furnace annealing temperature and gas environment after crystallizing the films with rapid thermal annealing in the presence of a TiN capping electrode. HZO films are deposited using atomic layer deposition in a Ge-HZO-TiN stack with Pt as the top contact electrode. The increment in the remanent polarization (Pr) is higher when the films are furnace annealed in the forming gas ambient. Forming gas furnace annealed films show an order of magnitude less leakage current as compared to the nitrogen annealed films. Dynamic hysteresis current measurements of the HZO ultrathin films show a faster merging of the four switching peaks after cycling the virgin forming gas furnace annealed films. H-incorporation during forming gas furnace annealing does not degrade the ferroelectric properties of HZO ultrathin films unlike conventional ferroelectrics, such as PZT or SBT. Higher Pr, lower leakage current, and an improved wake-up effect of the HZO ultrathin films show its resistance to degradation by forming gas furnace annealing, which makes ferroelectric HfO2 an ideal material for next-generation ferroelectric memory devices.

Investigation of phases and chemical states of tin titanate films grown by atomic layer deposition

Although perovskite tin titanate (SnTiO3, STO) has been predicted to have ferroelectricity with a large spontaneous polarization, the implementation of metastable perovskite STO remains a great challenge. In this study, the atomic layer deposition of STO films was attempted using bis(1-dimethylamino-2-methyl-2-propoxy)tin(II) as the Sn source, titanium-tetrakis-isopropoxide as the Ti source, and water as the oxygen source. It was found that during the process, adsorption of both the precursors was enhanced on the heterogeneous reaction surface because of the correlation growth behavior. STO films remained amorphous below 250 °C. Furthermore, although the film was crystallized at 270 °C, the perovskite phase was not identified. Despite the use of the Sn precursor with Sn2+, Sn ions in all the films transformed into Sn4+. Although the postdeposition annealing process in a forming gas atmosphere was conducted for the perovskite conversion, the STO film transformed into Ti5Sn3, an intermetallic compound, at 500 °C.

Atomic layer deposition of Ta-doped SnO2 films with enhanced dopant distribution for thermally stable capacitor electrode applications

Atomic layer deposition (ALD) on Ta-doped SnO2 thin films is proposed as a methodology for the fabrication of capacitor electrodes, for application in dynamic random-access memories (DRAMs). In ALD of doped materials, dopant concentration gradients inevitably occur due to the cyclic process-based characteristics of ALD. In this study, the dopant distribution improves drastically by the decrease in the growth per cycle of TaOx ALD. The modified ALD recipe exploits the low reactivity between Ta and Sn precursors to facilitate a reduction in the growth per cycle. The Ta-doped SnO2 films grown based on this modified ALD recipe exhibit improved crystallinity and conductivity. The ALD process yields excellent conformality of the Ta-doped SnO2 film over a hole structure with a high aspect ratio of ~10, both in terms of physical thickness and composition. Additionally, the Ta-doped SnO2 films serve as a template for the overgrowing dielectric TiO2 film, which induces the formation of a high-temperature phase with a high dielectric constant, rutile TiO2, and exhibit excellent thermal stability even after annealing at 400 °C in forming gas atmosphere. These findings demonstrate that the proposed methodology for the growth of Ta-doped SnO2 can facilitate the fabrication of capacitor electrodes for application in DRAMs.

The comparative study of electrical, optical and catalytic properties of Co3O4 thin nanocrystalline films prepared by reactive high-power impulse and radio frequency magnetron sputtering

This paper deals with the investigation of optical and electrical properties of Co3O4. Their impact on catalytic activity in ethanol dehydrogenation is observed. The change in conductivity is provided by intentionally heating the samples in-situ, variation of oxygen flow rate, introduction of the forming gas (10% H2 in N2) and the application of different deposition regimes. Pure Co3O4, as well as deposited catalysts under a forming gas atmosphere are studied. The deposition of the samples on glass and stainless steel substrates is carried out by means of magnetron sputtering. Both radio-frequency and high power impulse magnetron sputtering deposition technologies are applied. By means of these deposition regimes we managed to obtain films with different conductivity and crystalline structure. The morphology of the films is studied by scanning electron microscopy. The crystalline structure and the surface chemical composition of Co3O4 are investigated using X-ray diffraction and X-ray photoelectron spectroscopy. Electrical and optical properties of catalysts are studied by resistance and optical spectroscopy measurements. The catalytic activity of the films is tested on the reaction of ethanol dehydrogenation by means of selected ion flow-drift tube mass spectrometry.

Heterojunction Solar Cell with Carrier Selective Contact Using MoOx Deposited by Atomic Layer Deposition

Hole carrier selective MoOx film is obtained by atomic layer deposition(ALD) using molybdenum hexacarbonyl[Mo(CO)6] as precursor and ozone(O3) oxidant. The growth rate is about 0.036 nm/cycle at 200 g/Nm of ozone concentration and the thickness of interfacial oxide is about 2 nm. The measured band gap and work function of the MoOx film grown by ALD are 3.25 eV and 8 eV, respectively. X-ray photoelectron spectroscopy(XPS) result shows that the Mo6+ state is dominant in the MoOx thin film. In the case of ALD-MoOx grown on Si wafer, the ozone concentration does not affect the passivation performance in the as-deposited state. But, the implied open-circuit voltage increases from 576 °C to 620 °C at 250 g/Nm after post-deposition annealing at 350 °C in a forming gas ambient. Instead of using a p-type amorphous silicon layer, high work function MoOx films as hole selective contact are applied for heterojunction silicon solar cells and the best efficiency yet recorded (21 %) is obtained.