Naoufal Bahlawane

Advances in the deposition chemistry of metal-containing thin films using gas phase processes

Metal thin films are indispensable for the processing of a number of modern devices that benefit from their electronic, magneto-electric and optical properties. Application trends progressively involve integration into functional devices which feature three-dimensional nanostructures with increasingly high aspect ratios. These conditions promote increased interest towards non-line-of-sight deposition processes such as Chemical Vapour Deposition (CVD) and Atomic Layer Deposition (ALD). The deposition of metals using CVD or ALD is, however, less developed than that of the oxide counterparts. Several persisting limitations are directly related to the deposition chemistry. In the present perspective, relevant deposition chemical approaches are discussed along with their corresponding characteristics. Challenging issues regarding the purity and the nucleation kinetics are addressed. Using the intrinsic reactivity of the metals themselves to catalyse their own growth is one of the promising approaches emphasised here. Based on our recent work, the potential of this approach is discussed with respect to the growth of reactive and noble transition metals, pure or as alloys, as thin films or as embedded nanoparticles in functional oxide matrix thin films.

Rational Design of Functional Oxide Thin Films with Embedded Magnetic or Plasmonic Metallic Nanoparticles

Getting into films: semiconductor thin films containing magnetic or plasmonic metal nanoparticles are key materials for the development of high-efficiency solar cells, bright light-emitting diodes, and new magnetoelectric devices. The catalytically driven chemical vapor deposition offers a unique way to combine deposition of the metallic nanoparticles with that of functional oxides to produce such films.

Electrical Switching in Semiconductor-Metal Self-Assembled VO2 Disordered Metamaterial Coatings.

As a strongly correlated metal oxide, VO2 inspires several highly technological applications. The challenging reliable wafer-scale synthesis of high quality polycrystalline VO2 coatings is demonstrated on 4” Si taking advantage of the oxidative sintering of chemically vapor deposited VO2 films. This approach results in films with a semiconductor-metal transition (SMT) quality approaching that of the epitaxial counterpart. SMT occurs with an abrupt electrical resistivity change exceeding three orders of magnitude with a narrow hysteresis width. Spatially resolved infrared and Raman analyses evidence the self-assembly of VO2 disordered metamaterial, compresing monoclinic (M1 and M2) and rutile (R) domains, at the transition temperature region. The M2 mediation of the M1-R transition is spatially confined and related to the localized strain-stabilization of the M2 phase. The presence of the M2 phase is supposed to play a role as a minor semiconducting phase far above the SMT temperature. In terms of application, we show that the VO2 disordered self-assembly of M and R phases is highly stable and can be thermally triggered with high precision using short heating or cooling pulses with adjusted strengths. Such a control enables an accurate and tunable thermal control of the electrical switching.

Noncatalytic thermocouple coatings produced with chemical vapor deposition for flame temperature measurements.

Chemical vapor deposition (CVD) and metal-organic chemical vapor deposition (MOCVD) have been employed to develop alumina thin films in order to protect thermocouples from catalytic overheating in flames and to minimize the intrusion presented to the combustion process. Alumina films obtained with a CVD process using AlCl(3) as the precursor are dense, not contaminated, and crystallize in the corundum structure, while MOCVD using Al(acetyl acetone)(3) allows the growth of corundum alumina with improved growth rates. These films, however, present a porous columnar structure and show some carbon contamination. Therefore, coated thermocouples using AlCl(3)-CVD were judged more suitable for flame temperature measurements and were tested in different fuels over a typical range of stoichiometries. Coated thermocouples exhibit satisfactory measurement reproducibility, no temporal drifts, and do not suffer from catalytic effects. Furthermore, their increased radiative heat loss (observed by infrared spectroscopy) allows temperature measurements over a wider range when compared to uncoated thermocouples. A flame with a well-known temperature profile established with laser-based techniques was used to determine the radiative heat loss correction to account for the difference between the apparent temperature measured by the coated thermocouple and the true flame temperature. The validity of the correction term was confirmed with temperature profile measurements for several flames previously studied in different laboratories with laser-based techniques.

Abnormal behaviors in electrical transport properties of cobalt-doped tin oxide thin films

Electrical transport behaviors of SnO2-based oxides are absolutely essential, either for the understanding of physiochemical properties, or their practical applications. In this paper, an abnormal change in electrical transport is reported upon cobalt doping. A far-from-equilibrium technique—pulsed spray evaporation chemical vapor deposition (PSE-CVD), is investigated for the fabrication of Sn1−xCoxO2−δ (x = 0–0.18) thin films. Upon cobalt doping, the Hall mobility improves gradually and a ten-fold enhancement was noticed for Sn0.82Co0.18O2−δ relative to pure SnO2 films. This unexpected effect induces a dramatic drop in the electrical resistivity. Post-annealing treatment and XPS investigation indicate that the occurrence of surface-stabilized tin interstitials may be the primary reason for the unusual enhancement in conductivity. Cobalt doping not only generates the interstitial tin cations, but also stabilizes to a great extent their presence at the surface. This study may help to illumine new insight for the understanding of doping strategies, and offer a potential route for transport-related applications.

Controlled synthesis of Co3O4 spinel with Co(acac)3 as precursor

Cobalt(III) acetylacetonate (Co(acac)3) was used as a precursor to grow pure Co3O4 with pulsed-spray evaporation chemical vapor deposition (PSE-CVD). The effect of solvent and substrate temperature on the growth kinetics and morphology of the films was investigated. The obtained spinel exhibited good catalytic performance.

CVD of Conducting Ultrathin Copper Films

Miniaturization of electronic devices imposes challenges in terms of materials and production methods, and advances in the chemical vapor deposition (CVD) of metals are a key prerequisite toward reliable interconnects that are essential for their functionality. Electrically conducting ultrathin films of pure copper were grown on glass and silicon substrates starting at a temperature of 195 degrees C. The growth kinetics does not exhibit any measurable nucleation time enabling early stage coalescence and high electrical conductivity. In situ monitoring of the CVD process using synchrotron-based mass spectrometry shows that the enhanced dehydrogenation of alcohols by copper(II) acetylacetonate precursor drives the Cu-0 deposition, which is kinetically favorable already at low temperature

Mass-Spectrometric Monitoring of the Thermally Induced Decomposition of Trimethylgallium, Tris(tert-Butyl)Gallium, and Triethylantimony at Low Pressure Conditions

The thermal decomposition of trimethylgallium (GaMe3), tris(tert-butyl)gallium (GatBu3) and triethylantimony (SbEt3) was investigated in a tubular hot-wall reactor coupled with a molecular-beam sampling mass spectrometer, and decomposition mechanisms were proposed. The obtained results confirm the predominance of the surface reactions and reveal that the radical decomposition path of GatBu3 and SbEt3, responsible for the formation of butane and ethane respectively, is restricted to a narrow temperature range in contrast to the molecular route that is responsible for the formation of the corresponding alkenes. GaMe3 decomposes above 480°C, forming essentially methane and also ethane to a lesser extent, whereas GatBu3 decomposes starting 260°C to form predominantly i-butane and i-butene as major species. The decomposition of SbEt3 starts at 400°C and forms n-butane, ethane, and ethene. The selectivity to n-butane increases with the thermolysis temperature. The resulting activation energies of the relevant decomposition paths show good agreement with those among them that have been measured before by temperature-programmed desorption techniques.

Low-temperature thermolysis behavior of tetramethyl- and tetraethyldistibines.

The thermolysis behavior of tetramethyl- and tetraethyldistibine (Sb2Me4 and Sb2Et4) was investigated using a mass spectrometer coupled to a tubular flow reactor under near-chemical vapor deposition (CVD) conditions. Sb2Me4 undergoes a gas-phase disproportionation with an estimated activation energy of 163 kJ/mol. This reaction leads to the formation of methylstibinidine, SbMe, that reacts on the surface to produce antimony film and SbMe3. Unfortunately, this clean decomposition pathway is limited to a narrow temperature range of 300–350°C. At temperatures exceeding 400°C, SbMe3 decomposes following a radical route with a consequent risk of carbon contamination. In contrast, Sb2Et4 disproportionates at the hot wall of the reactor. According to mass-spectrometric data, this reaction is significant starting at a temperature of 100°C, with an apparent activation energy of 104 kJ/mol. Within the temperature range of 100–250°C, the precursor decomposition leads to the formation of antimony films and SbEt3, whereas different molecular reaction pathways are significantly activated above 250°C. The use of Sb2Et4 lowers the risk of carbon contamination compared to Sb2Me4 at high temperature. Therefore, Sb2Et4 is a promising CVD precursor for the growth of antimony films in the absence of hydrogen atmosphere in a wide temperature range.

Prussian Blue Analogs for Rechargeable Batteries

Summary Non-lithium energy storage devices, especially sodium ion batteries, are drawing attention due to insufficient and uneven distribution of lithium resources. Prussian blue and its analogs (Prussian blue analogs [PBAs]), or hexacyanoferrates, are well-known since the 18th century and have been used for hydrogen storage, cancer therapy, biosensing, seawater desalination, and sewage treatment. Owing to their unique features, PBAs are receiving increasing interest in the field of energy storage, such as their high theoretical specific capacity, ease of synthesis, as well as low cost. In this review, a general summary and evaluation of the applications of PBAs for rechargeable batteries are given. After a brief review of the history of PBAs, their crystal structure, nomenclature, synthesis, and working principle in rechargeable batteries are discussed. Then, previous works classified based on the combination of insertion cations and transition metals are analyzed comprehensively. The review includes an outlook toward the further development of PBAs in electrochemical energy storage.