Selective Chemical Vapor Deposition Research Articles

Fabrication and properties of lateral Josephson junctions with a RuO2 weak link

Ruthenium dioxide (RuO2) is a metallic rutile oxide with a number of interesting properties. For a long time, it was considered to be a highly conductive normal metal and a Pauli paramagnet. Recently, it was found that the material is antiferromagnetic, with small magnetic moments of the order of 0.05 Bohr magneton and an ordering temperature above 300 K. The presence of magnetic moments should have clear consequences when trying to induce superconductivity in RuO2. We used a selective area chemical vapor deposition method to grow nanostrips of RuO2 on TiO2 substrates. On these nanostrips, superconducting contacts were made of MoGe, and a weak link was fabricated with a Focused Ion Beam. We find that the device behaves as a Josephson junction, including a Fraunhofer-like response to a magnetic field, for distances between the contacts below 70 nm. We estimate the induced singlet coherence length ξ to be about 12 nm, which seems a reasonable number when small magnetic moments are present.

Laser-textured biomimetic copper leaf with structural-wettability dual gradient for efficient fog harvesting

Water collection has gradually become research hot topic owing to its potential for alleviating water shortages in arid areas. To date, numerous artificial materials with hierarchical structure have been fabricated to improve water-collecting efficiency. Here, a multi-bioinspired fog-harvesting artificial Cu-Leaf, deriving from Leaf skeletons, Namib desert beetle, and Cactus spines, is fabricated by ultrafast laser selective ablation, chemical etching and chemical vapor deposition, which achieve efficient water-collecting by combining fog capture, droplet growing and directional transportation together. The four-level wedge-shaped tracks of Cu-Leaf skeleton and its wetting property were finely adjusted and their correlations with the fog-harvesting capability were systematically studied. Force analysis related to fog capture, droplet growing and the droplet directional transportation on the hydrophobic/hydrophilic patterned skeleton surfaces have been highlighted. The optimal biomimetic Cu-Leaf performed superior fog-harvesting property with harvesting rate of 544.03 mg/h cm2, which is 2.5 times that of the original Cu sheet. Moreover, we designed an integrated fog-harvesting device (FHD) that could realize the dynamic fog-harvesting and efficient water storage. Therefore, this work enhances the understanding of the fundamental research and provides valuable strategy for design and preparation of sustainable and efficient fog harvesting systems.

Room Temperature Light Emission from Superatom-like Ge-Core/Si-Shell Quantum Dots.

We have demonstrated the high-density formation of super-atom-like Si quantum dots with Ge-core on ultrathin SiO2 with control of high-selective chemical-vapor deposition and applied them to an active layer of light-emitting diodes (LEDs). Through luminescence measurements, we have reported characteristics carrier confinement and recombination properties in the Ge-core, reflecting the type II energy band discontinuity between the Si-clad and Ge-core. Additionally, under forward bias conditions over a threshold bias for LEDs, electroluminescence becomes observable at room temperature in the near-infrared region and is attributed to radiative recombination between quantized states in the Ge-core with a deep potential well for holes caused by electron/hole simultaneous injection from the gate and substrate, respectively. The results will lead to the development of Si-based light-emitting devices that are highly compatible with Si-ultra-large-scale integration processing, which has been believed to have extreme difficulty in realizing silicon photonics.

Study of Cu-growth feature by selective low-pressure chemical vapor deposition using a CuI precursor

In this study, Cu-growth features are investigated using selective low-pressure CVD of Cu on Ru(001) using a copper(I) iodide precursor. While discrete columnar grains are formed below 320 °C, lateral Cu islands are grown above 380 °C. The CuI dissociation efficiency, which depends on the growth temperature, indicates two activation energies corresponding to I2 desorption from Cu and Ru. The activation energy below 320 °C and above 380 °C are identified as the energies required for the desorption of I2 from Cu(111) and Ru(001), respectively. At 400 °C, nucleation occurs at the initial stage, followed by the lateral growth of Cu-islands. A proposed growth model, involving dissociated Cu-atom contributions toward increasing the Cu-island height and Cu coverage on Ru, can explain the island growth features dependent on the growth time. Further, the growth model reveals an important factor to achieve thinner Cu layers growth entirely covering Ru.

Inherent selective pulsed chemical vapor deposition of aluminum oxide in nm scale

Inherent selective pulsed chemical vapor deposition (CVD) of aluminum oxide on Si and SiO2 in preference to SiCOH has been studied. SiCOH is alkyl (-CxHy) terminated SiO2, which was used as a non-reactive surface. For aluminum oxide deposition, pulsed CVD processes using aluminum tri sec-butoxide (ATSB) and trimethylaluminum (TMA) were tested. ATSB alone can induce a pulsed thermal CVD reaction to selectively deposit aluminum oxide at 300 °C sample temperature; ∼4 nm of aluminum oxide were selectively deposited on Si and SiO2 in preference to SiCOH. By adding a periodical pulse of TMA during ATSB pulses, higher selectivity was achieved with ∼12 nm and ∼14 nm of aluminum oxide on Si and SiO2 with around 0.4–0.5 nm root mean square (RMS) roughness at 330 °C. The high selectivity persisted on the nanoscale: STEM showed that ∼10 nm of aluminum oxide could be selectively deposited on nanoscale patterned features. The selectively deposited AlOx films showed low k (∼4) dielectric performance with low leakage (around 10−6 A/cm2 at ± 2 V for a 15 nm thick film). This selective aluminum oxide pulsed CVD process has the potential to be applicable in nanoscale fabrication, such as low k spacer and etch stop layer.

Core-shell GaN/AlGaN nanowires grown by selective area epitaxy.

GaN/AlGaN core-shell nanowires with various Al compositions have been grown on GaN nanowire array using selective area metal organic chemical vapor deposition technique. Growth of the AlGaN shell using pure N2 carrier gas resulted in a smooth surface for the nonpolar m-plane sidewalls with superior optical properties, whereas, growth using a mixed N2/H2 carrier gas resulted in a striated surface similar to the commonly observed morphology in the growth of nonpolar III-nitrides. The Al compositions in the AlGaN shells are found to be less than the gas phase input ratio. The systematic reduction in efficiency of Al incorporation in the AlGaN shells with increasing the Al molar flow in the gas phase is attributed to geometric loss, strain-limited Al incorporation, and increased gas phase parasitic reactions. Defect-related luminescence has been observed for AlGaN shells with Al content ≥ 30% and the origin of the defect luminescence has been determined as the (VIII-2ON)1- complex. Microstructural analysis of the AlGaN shells revealed that the dominant defects are partial dislocations. Growth of the nonpolar m-plane AlxGa1-xN/AlyGa1-yN quantum wells on the sidewalls of the GaN nanowires produced arrays with excellent morphology and optical emission, which demonstrated the viability of such a growth scheme for large area efficient ultraviolet LEDs as well as for next generation ultraviolet micro-LEDs.

Inherent selective pulsed chemical vapor deposition of amorphous hafnium oxide / titanium oxide nanolaminates

Water-free Inherent selective pulsed chemical vapor deposition (CVD) of HfO2/TiO2 nanolaminates on Si and SiO2 in preference to SiCOH has been studied. SiCOH is highly porous alkylated SiO2, which is used as a nonreactive low-k dielectric. Ti(OiPr)4 [titanium(IV) isopropoxide] and Hf(OtBu)4 [Hafnium tert butoxide] were used in the CVD study. Previous studies showed that metal alkoxide precursors could form oxide films through thermal decomposition. However, single oxide films greater than 2 nm can be rough due to crystallization. To solve this issue, HfO2/TiO2 nanolaminate structures were studied. With sequential dosing of each precursor in a supercycle at 300 °C sample temperature, HfO2/TiO2 nanolaminate films with thin (less than 3 nm) sublayers were selectively deposited. The films were smooth with root mean square (RMS) roughness lower than 0.5 nm and almost amorphous from XRD analysis; this is unexpected since both oxides readily crystalize. Amorphous nanolaminate oxide film deposition with high selectivity was achieved by controlling each sublayer thickness and the Hf:Ti ratio. TEM studies proved that ∼ 20 nm of the nanolaminate film could also be selectively deposited on nanoscale patterned surfaces. This selective amorphous nanolaminate oxide CVD process has a potential to be applicable in the nanoscale patterning in MOSFET fabrication.

Selective Chemical Vapor Deposition Growth of WS2/MoS2 Vertical and Lateral Heterostructures on Gold Foils.

Vertical and lateral heterostructures consisting of atomically layered two-dimensional (2D) materials exhibit intriguing properties, such as efficient charge/energy transfer, high photoresponsivity, and enhanced photocatalytic activities. However, the controlled fabrication of vertical or lateral heterojunctions on metal substrates remains challenging. Herein, we report a facile and controllable method for selective growth of WS2/MoS2 vertical or lateral heterojunctions on polycrystalline gold (Au) foil by tuning the gas flow rate of hydrogen (H2). We find that lateral growth is favored without H2, whereas vertical growth mode can be switched on by introducing 8–10 sccm H2. In addition, the areal coverage of the WS2/MoS2 vertical heterostructures is tunable in the range of 12–25%. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) results demonstrate the quality and absence of cross-contamination of the as-grown heterostructures. Furthermore, we investigate the effects of the H2 flow rate on the morphology of the heterostructures. These pave the way to develop unprecedented 2D heterostructures towards applications in (opto)electronic devices.

Selective chemical vapor deposition of HfB2 on Al2O3 over SiO2 and the acceleration of nucleation on SiO2 by pretreatment with Hf[N(CH3)2

We show that growth of the metallic ceramic HfB2 by CVD from Hf(BH4)4 at 220 °C is inherently selective on Al2O3 over SiO2: a 10.4-nm film grows on Al2O3 in 16 min, whereas only 0.07 nm of HfB2 grows on SiO2 in 18 min. Nucleation occurs on both SiO2 and Al2O3; however, the Al2O3 surface has a much higher density of nuclei such that HfB2 islands quickly coalesce to form continuous films, followed by steady-state growth of HfB2. On SiO2, nucleation is sparse and coalescence of the islands takes much longer; as a result, the overall growth rate is slower. Sparse nucleation on SiO2 also leads to a rough layer with a broad height distribution function: for a deposit containing 1.6 × 1015 Hf atoms/cm2 (equivalent to a bulk thickness of 0.5 nm for HfB2), the rms roughness is 3.8 nm on SiO2 but only 1.3 nm on Al2O3. The difference in the formation rate of nuclei (and thus the area density of nuclei) is attributed to the different acid-base character of hydroxyl groups on these oxide surfaces. We also found that, when growth on SiO2 is desired, the surface can be modified by exposure to tetrakis(dimethylamido)hafnium, which adsorbs to saturation at ∼1 monolayer. Subsequent exposure of this pretreated surface leads to an increased density of HfB2 nuclei, a reduced coalescence time, and a smaller roughness of the resulting surface from 3.8 to 1.7 nm. By contrast, a similar pretreatment on Al2O3 has little effect on the roughness of subsequently grown HfB2 films, which are already relatively smooth when grown on untreated alumina surfaces.

Selective pulsed chemical vapor deposition of water-free HfOx on Si in preference to SiCOH and passivated SiO2

HfOx was selectively deposited on Si in preference to SiCOH using an oxygen-containing precursor, Hf(OtBu)4, via a pulsed chemical vapor deposition. The water-free process avoids nucleation by water on passivated oxide surfaces. The uniform HfOx film was deposited on Si by the thermal decomposition of the precursor, and the inherent selectivity of HfOx was retained over SiCOH until ~ 2 nm of HfOx was deposited on Si at 250 °C. Additional selectivity enhancement was demonstrated by employing molecular surface passivation and reversible adsorption/desorption of the precursor via control of the purge time. 1,1,3,3-tetramethyldisilazane (TMDS) and Bis(dimethylamino)dimethylsilane (DMADMS) were employed to passivate reactive –OH sites on SiO2 and SiCOH to increase the selectivity to ~ 7 nm HfOx deposition on Si before any significant nucleation occurred on TMDS passivated SiCOH. It is expected that the selective water-free HfOx deposition can be used for patterning nanoscale structures in MOSFET devices.

Phase selective CVD growth and photoinduced 1T → 1H phase transition in a WS2 monolayer

We report the direct chemical vapour deposition (CVD) growth of the metastable 1T phase of a WS2 monolayer and the in situ phase transition characteristics with the aid of Raman, photoluminescence and fluorescence microscopy.

Superconductivity in nano- and micro-patterned high quality single crystalline boron-doped diamond films

We demonstrate nano- and micro-size patterning processes for high quality single crystal superconducting boron-doped diamond films with low damage using selective microwave plasma chemical vapour deposition and selective oxygen plasma etching. The offset critical temperature Tc(offset) of the micro strip is 10.2 K even if the strip width is 1 μm. However, we show that the critical temperature at zero resistivity Tc(zero) of several narrow strips is decreased owing to the unexpected defects induced by polishing damage and natural nanoscale scratch injury. These defects will induce a two-step superconducting transition. The probability of this tendency increases with decreasing the strip width. We also reveal that the critical current density Jc increases with decreasing the strip width, especially below 30 μm. By using a transport measurement, the maximum Jc is estimated to be 917,000 A/cm2 at 2.0 K with a strip width of 2 μm, which is the highest value reported for a superconducting diamond film. The upper critical field is estimated to be 11.5 T by the WHH approximation and 9.3 T by BCS fitting. Such high values mean superconducting diamond wiring can be used under a high magnetic field. We also attempt to fabricate nano-patterning and reveal that the transition temperature suddenly decreases below 400 nm, and superconductivity is not observed below 300 nm. This work contributes to the future development of superconducting nano- and micro-electro mechanical systems by exploiting the excellent properties of robustness, processability, high transition temperature, critical current density, and critical field associated with diamond.

Toward the use of single crystal diamond based detector for ion-beam therapy microdosimetry

In this work, the fabrication and characterization of a microdosimeter based on a synthetic single crystal diamond is reported. The microdosimeter is realized by means of both standard photolithography and selective chemical vapor deposition techniques to accurately define its micrometric sensitive volume.Experimental measurements were carried out at the Ruđer Bošković Institute microbeam facility using different particles such as proton, helium, lithium, carbon and oxygen. Ion beam induced charge (IBIC) technique was performed to characterize the microdosimeter response in terms of its charge collection properties. The experimental data were also analyzed by means of Geant4 Monte Carlo simulations.Diamond based microdosimeter shows a well-defined active volume. Homogeneity of the response was estimated at about 7% and linked to structural defects of the diamond surface as deduced by AFM inspection. The detector response shows a good linear behavior for ions of different kind and energy, indicating that the detector is suitable for measuring a wide range of particles and LET i.e. 100 ÷ 3000 keV/μm. Finally, microdosimetric capabilities of the diamond based microdosimeter were preliminarily tested in low LET radiation fields (i.e. protons beam).

Interdependence of Structure, Morphology, and Phase Transitions in CVD Grown VO2 and V2O3 Nanostructures

Phase selective chemical vapor deposition of nanostructured vanadium dioxide (VO2) and sesquioxide (V2O3) was achieved by deploying [V(OR)4]n (R = tBu, n = 1 (1), R = Et, n = 3 (2), R = Me, n = 4 (3)). Use of [V(OtBu)4] (1) produced thin films of monoclinic VO2 (M1) at 700 and 800 °C consisting of anisotropic nanostructures with high crystallinity and small hysteresis in the metal-to-semiconductor transition (MST). Film morphologies manifested strong dependence on growth temperatures and exhibited pronounced texturing effects at high temperatures (>700 °C). The microstructure of the films was found to significantly affect the MST behavior of VO2 films. DTA measurements of VO2 films showed MST at 63 °C (700 °C) and 65 °C (800 °C), much lower than the transition temperature observed in single crystal material (68 °C). Precursors were characterized in the solid state (XRD) and solution state (temperature dependent EPR, NMR) to reveal an association–dissociation equilibrium in solution (complexes 2 and 3), in...

Laser-induced reduction and in-situ optical spectroscopy of individual plasmonic copper nanoparticles for catalytic reactions

Copper (Cu) can provide an alternative to gold (Au) for the development of efficient, low-cost and low-loss plasmonic nanoparticles (NPs), as well as selective nanocatalysts. Unlike Au, the surface oxidation of Cu NPs can be an issue restricting their applicability. Here, we selectively reduce the Cu NPs by low power laser illumination in vacuum and use dark-field scattering to reveal in real time the optical signatures of the reduction process and its influence on the Cu NP plasmonic resonance. We then study reactive processes at the single particle level, using individual Cu catalyst nanoparticles for the selective laser-induced chemical vapour deposition of germanium nanostructures.

Photochemical CVD of Ru on functionalized self-assembled monolayers from organometallic precursors

Chemical vapor deposition (CVD) is an attractive technique for the metallization of organic thin films because it is selective and the thickness of the deposited film can easily be controlled. However, thermal CVD processes often require high temperatures which are generally incompatible with organic films. In this paper, we perform proof-of-concept studies of photochemical CVD to metallize organic thin films. In this method, a precursor undergoes photolytic decomposition to generate thermally labile intermediates prior to adsorption on the sample. Three readily available Ru precursors, CpRu(CO)2Me, (η3-allyl)Ru(CO)3Br, and (COT)Ru(CO)3, were employed to investigate the role of precursor quantum yield, ligand chemistry, and the Ru oxidation state on the deposition. To investigate the role of the substrate chemistry on deposition, carboxylic acid-, hydroxyl-, and methyl-terminated self-assembled monolayers were used. The data indicate that moderate quantum yields for ligand loss (φ ≥ 0.4) are required for ruthenium deposition, and the deposition is wavelength dependent. Second, anionic polyhapto ligands such as cyclopentadienyl and allyl are more difficult to remove than carbonyls, halides, and alkyls. Third, in contrast to the atomic layer deposition, acid-base reactions between the precursor and the substrate are more effective for deposition than nucleophilic reactions. Finally, the data suggest that selective deposition can be achieved on organic thin films by judicious choice of precursor and functional groups present on the substrate. These studies thus provide guidelines for the rational design of new precursors specifically for selective photochemical CVD on organic substrates.

Substitutional Carbon Loss in Si:C Stressor Layers Probed by Deep-Level Transient Spectroscopy

Further scaling of CMOS transistors goes hand in hand with continued straining of the channel, in order to boost the device performance. However, in a bulk FinFET architecture several of the standard techniques lose their effectiveness, so that stress engineering becomes more and more difficult [1]. It has been shown that for n-channel bulk FinFETs, Si:C source/drain stressors are very effective [1], explaining recent research interest [2]-[4]. Si:C hetero-epitaxial layers with about 1% C and highly P-doped can be deposited by a selective Chemical Vapor Deposition (CVD) epitaxial growth based on a cyclic deposition and etching process [5]. One important issue, however, is to maintain throughout further processing as high as possible substitutional carbon in the stressors in combination with an as high as possible active P concentration, to reduce the contact resistance [3],[6]. It has been observed that application of a post-deposition laser anneal (LA) increases the dopant activation level [7] but at the same time results in lower layer stress [6], suggesting the loss of substitutional carbon. In this paper, a Deep-Level Transient Spectroscopy (DLTS) analysis is performed to address this issue. Undoped Si:C layers have been deposited on p-type CZ silicon substrates by a cyclic CVD process as described previously [7]. Al Schottky barriers with different diameters have been thermally evaporated. Typical current-voltage (I-V) and 1 MHz capacitance-voltage (C-V) curves are represented in Fig. 1. Fourier-transform (FT) DLTS has been performed from 75 K to room temperature using different bias pulses from VR to VP, as indicated schematically in Fig. 2, showing the depth probed by the measurements. The analysis was performed on as-grown layers and after an 850 oC Rapid Thermal Annealing in N2 for 5 min. As shown in Fig. 3, a hole trap around 180 K is clearly observed in the -0.5 V-->0 V spectrum after RTA, which is absent (or much smaller) in the as-grown sample. Closer to the surface, some hole trap appears as well in the as-grown layer (Fig. 4). Focusing on the RTA sample in Fig. 5, it is clearly shown that in the silicon substrate, a hole trap with activation energy of 0.4 eV is present, which, according to the comparison in Fig. 6 most likely corresponds with the CiOi level. It indicates that during RTA, interstitial carbon (Ci) is formed, next diffuses in the silicon substrate where it becomes trapped by interstitial oxygen (Oi). Similar Cidiffusion has been reported before even during laser annealing [8]. Closer to the epi layer, a broad band of hole trap states is present in Fig. 5, which could correspond with some active carbon cluster type of defects.

Nanoscale arrays of antimony telluride single crystals by selective chemical vapor deposition.

Arrays of individual single nanocrystals of Sb2Te3 have been formed using selective chemical vapor deposition (CVD) from a single source precursor. Crystals are self-assembled reproducibly in confined spaces of 100 nm diameter with pitch down to 500 nm. The distribution of crystallite sizes across the arrays is very narrow (standard deviation of 15%) and is affected by both the hole diameter and the array pitch. The preferred growth of the crystals in the <1 1 0> orientation along the diagonal of the square holes strongly indicates that the diffusion of adatoms results in a near thermodynamic equilibrium growth mechanism of the nuclei. A clear relationship between electrical resistivity and selectivity is established across a range of metal selenides and tellurides, showing that conductive materials result in more selective growth and suggesting that electron donation is of critical importance for selective deposition.

Area-selective chemical vapor deposition of Co for Cu capping layer

Selective chemical vapor deposition (CVD) of Co on Cu interconnect lines is an advantageous method because it can simplify unit process and suppress side effects resulting from the metal etching process. Density functional theory calculations show that CoCp(CO)2 has a relatively higher kinetic barrier (36.2 kcal/mol) on SiO2 than that of Co2(CO)8 (22.9 kcal/mol). This indicates that CoCp(CO)2 could be a promising candidate as a Co precursor for selective CVD. Selective CVD of Co films is demonstrated experimentally by deposition on SiO2 and Cu substrates at 200 and 300 °C. To reduce C contents in the deposited film, a NH3 plasma process is adopted into conventional CVD in a cyclic CVD process. This process enables deposition of Co films with lower C content while maintaining selective deposition.

Controlling Nanowire Growth by Light.

Individual Au catalyst nanoparticles are used for selective laser-induced chemical vapor deposition of single germanium nanowires. Dark-field scattering reveals in real time the optical signatures of all key constituent growth processes. Growth is initially triggered by plasmonic absorption in the Au catalyst, while once nucleated the growing Ge nanowire supports magnetic and electric resonances that then dominate the laser interactions. This spectroscopic understanding allows real-time laser feedback that is crucial toward realizing the full potential of controlling nanomaterial growth by light.