Atomic Layer Deposition (ALD) is a promising technique for growing ultrathin, pristine dielectrics on metal substrates, which is essential to many electronic devices. Tunnel junctions are an excellent example which require a leak-free, ultrathin dielectric tunnel barrier of typical thickness around 1 nm between two metal electrodes. A challenge in the development of ultrathin dielectric tunnel barrier using ALD is controlling the nucleation of dielectrics on metals with minimal formation of native oxides at the metal surface for high-quality interfaces between the tunnel barrier and metal electrodes. This poses a critical need for integrating ALD with ultra-high vacuum (UHV) physical vapor deposition. In order to address these challenges, a viscous-flow ALD chamber was designed and interfaced to an UHV magnetron sputtering chamber via a load lock. A sample transportation system was implemented for in situ sample transfer between the ALD, load lock, and sputtering chambers. Using this integrated ALD-UHV sputtering system, superconductor-insulator-superconductor (SIS) Nb/Al/Al2O3/Nb Josephson tunnel junctions were fabricated with tunnel barriers of thickness varied from sub-nm to ~ 1 nm. The suitability of using an Al wetting layer for initiation of the ALD Al2O3 tunnel barrier was investigated with ellipsometry, atomic force microscopy, and electrical transport measurements. With optimized processing conditions, leak-free SIS tunnel junctions were obtained, demonstrating the viability of this integrated ALD-UHV sputtering system for the fabrication of tunnel junctions and devices comprised of metal-dielectric-metal multilayers.
Large-area MoS2 atomic layers are synthesized on SiO2 substrates by chemical vapor deposition using MoO3 and S powders as the reactants. Optical, microscopic and electrical measurements suggest that the synthetic process leads to the growth of MoS2 monolayer. The TEM images verify that the synthesized MoS2 sheets are highly crystalline.
We demonstrated that chemical vapor deposition of graphene on Cu films strongly enhances their thermal diffusivity and thermal conductivity. Deposition of graphene increases the thermal conductivity of 9 micrometer (25 micrometer) thick Cu films by up to 24% (16%) near room temperature. Interestingly, the observed improvement of thermal properties of graphene coated Cu films is primarily due to changes in Cu morphology during graphene deposition and associated with it temperature treatment rather than graphene's action as an additional heat conducting channel. Enhancement of thermal properties…
Transition metal dichalcogenides (TMDs) have recently attracted attention due to their interesting electronic and optical properties. Fabrication of these materials in a reliable and facile method is important for future applications, as are methods to characterize material quality. Here we present the chemical vapor deposition of MoSe2 monolayer and few layer crystals. These results show the practicality of using chemical vapor deposition to reliably fabricate these materials. Low frequency Raman spectra and mapping of shear and layer breathing modes of MoSe2 are presented for the first time.…
We evaluate how a second graphene layer forms and grows on Cu foils during chemical vapor deposition (CVD). Low-energy electron diffraction and microscopy is used to reveal that the second layer nucleates and grows next to the substrate, i.e., under a graphene layer. This underlayer mechanism can facilitate the synthesis of uniform single-layer films but presents challenges for growing uniform bilayer films by CVD. We also show that the buried and overlying layers have the same edge termination.
Physical vapor deposition can be used to prepare highly stable organic glass systems where the molecules show orientational and translational ordering at the nanoscale. We have used low-dose four-dimensional scanning transmission electron microscopy (4D STEM), enabled by a fast direct electron detector, to map columnar order in glassy samples of a discotic mesogen using a 2 nm probe. Both vapor deposited and liquid cooled glassy films show domains of similar orientation, but their size varies from tens to hundreds of nanometers, depending on processing. Domain sizes are consistent with surface…
The critical separation of clusters, corresponding to the maximum density of clusters, affects growth characteristics during physical vapor deposition (PVD). In particular, this separation can affect surface smoothness in growing single-crystalline films, grain size distribution in growing polycrystalline films, and diameter in growing nanorods. This Letter reports a theoretical expression of the critical separation as a function of deposition conditions and accompanying verifications using lattice kinetic Monte Carlo (MC) simulations of PVD on Cu{111}. In contrast to existing theories, the th…
CdTe has been a special semiconductor for constructing the lowest-cost solar cells and the CdTe-based Cd1-xZnxTe alloy has been the leading semiconductor for radiation detection applications. The performance currently achieved for the materials, however, is still far below the theoretical expectations. This is because the property-limiting nanoscale defects that are easily formed during the growth of CdTe crystals are difficult to explore in experiments. Here we demonstrate the capability of a bond order potential-based molecular dynamics method for predicting the crystalline growth of CdTe fi…
Single-walled carbon nanotube (SWCNT) films are promising materials for transparent conductive films (TCFs) with potential applications in flexible displays, touch screens, solar cells and solid-state lighting1,2. However, further reductions in resistivity and in cost of SWCNT films are necessary for high quality TCF products3. Here, we report an improved floating catalyst chemical vapor deposition method to directly and continuously produce ultrathin and freestanding SWCNT films at the hundred meter-scale. Both carbon conversion efficiency and SWCNT TCF yield are increased by three orders of …
The deposition of boron-doped amorphous carbon thin films on SiO2 substrate was achieved via a focused ion beam-assisted chemical vapor deposition of triphenyl borane (C18H15B) and triphenyl borate (C18H15BO3). The existence of boron in the deposited film from triphenyl borane, with a precursor temperature of 90 °C, was confirmed by a core level X-ray photoelectron spectroscopy analysis. The film exhibited a semiconducting behavior with a band gap of 285 meV. Although the band gap was decreased to 197 meV after an annealing process, the film was still semiconductor. Additionally, a drastic red…
We investigate key electrical properties of monolayer graphene assembled by chemical-vapor-deposition (CVD) as impacted by supporting substrate material. Graphene field-effect transistors (GFETs) were fabricated with carbon channel placing directly on hexagonal boron nitride (h-BN) and SiO2, respectively. Small-signal transconductance (gm) and effective carrier mobility (μeff) are improved by 8.5 and 4 times on h-BN, respectively, as compared with that on SiO2. Compared with GFET with exfoliated graphene on SiO2, gm and μeff measured from device with CVD graphene on h-BN substrate exhibits com…
By molecular dynamics simulation, the chemical vapor deposition of amorphous carbon onto graphite and diamond surfaces was studied. In particular, we investigated the effect of source H/C ratio, which is the ratio of the number of hydrogen atoms to the number of carbon atoms in a source gas, on the deposition process. In the present simulation, the following two source gas conditions were tested: one was that the source gas was injected as isolated carbon and hydrogen atoms, and the other was that the source gas was injected as hydrocarbon molecules. Under the former condition, we found that a…
The nucleation of carbon caps on small nickel clusters is studied using a tight binding model coupled to grand canonical Monte Carlo simulations. It takes place in a well defined carbon chemical potential range, when a critical concentration of surface carbon atoms is reached. The solubility of carbon in the outermost Ni layers, that depends on the initial, crystalline or disordered, state of the catalyst and on the thermodynamic conditions, is therefore a key quantity to control the nucleation.
Uniform single layer graphene was grown on single-crystal Ir films a few nanometers thick which were prepared by pulsed laser deposition on sapphire wafers. These graphene layers have a single crystallographic orientation and a very low density of defects, as shown by diffraction, scanning tunnelling microscopy, and Raman spectroscopy. Their structural quality is as high as that of graphene produced on Ir bulk single crystals, i.e. much higher than on metal thin films used so far.
Direct growth of flat micrometer-sized bilayer graphene islands in between molybdenum disulfide sheets is achieved by chemical vapor deposition of ethylene at about 800 °C. The temperature assisted decomposition of ethylene takes place mainly at molybdenum disulfide step edges. The carbon atoms intercalate at this high temperature, and during the deposition process, through defects of the molybdenum disulfide surface such as steps and wrinkles. Post growth atomic force microscopy images reveal that circular flat graphene islands have grown at a high yield. They consist of two graphene layers s…
Although chemical vapor deposition (CVD) is one of the preferred routes to synthetic diamond crystals, a full knowledge of the point defects produced during growth is still incomplete. Here we exploit the charge and spin properties of nitrogen-vacancy (NV) centers in type-1b CVD diamond to expose an optically and magnetically dark point defect, so far virtually unnoticed despite an abundance comparable to (if not greater than) that of substitutional nitrogen. Indirectly-detected photo-luminescence spectroscopy indicates a donor state 1.6 eV above the valence band, although the defect's microsc…
We have synthesized a large amount of boron-doped multiwalled carbon nanotubes (MWNTs) by hot-filament chemical vapor deposition. The synthesis was carried out in a flask using a methanol solution of boric acid as a source material. The scanning electron microscopy, transmission electron microscopy, and micro-Raman spectroscopy were performed to evaluate the structural properties of the obtained MWNTs. In order to evaluate the electrical properties, temperature dependence of resistivity was measured in an individual MWNTs with four metal electrodes. The Ramman shifts suggest carrier injection …
GaN/SiO2 core/shell nanowires are grown by cobalt phthalocyanine catalyst assisted vapor-liquid-solid route, in which Si wafer coated with a mixture of gallium and indium is used as the source for Ga and Si and ammonia is used as the precursor for nitrogen and hydrogen. Gallium in the presence of indium and hydrogen, which results from the dissociation of ammonia, forms Si-Ga-In alloy at the growth temperature around 910 degree Celsius. This alloy acts as the source of Si, Ga and In. A detailed study using a variety of characterization tools reveals that these wires, which are several tens of …
This work presents a comparison of the structural, chemical and electronic properties of multi-layer graphene grown on SiC(000-1) by using two different growth approaches: thermal decomposition and chemical vapor deposition (CVD). The topography of the samples was investigated by using atomic force microscopy (AFM), and scanning electron microscopy (SEM) was performed to examine the sample on a large scale. Raman spectroscopy was used to assess the crystallinity and electronic behavior of the multi-layer graphene and to estimate its thickness in a non-invasive way. While the crystallinity of t…
A comparative study of the structural, microstructural and magnetic properties of CrO$_2$ thin films grown onto (110) and (100) TiO$_2$ rutile single crystal substrates by chemical vapor deposition (CVD), using CrO$_3$ as chromium precursor and either oxygen or argon as carrier gas is presented. Our results show that growth under argon carrier gas leads to high quality CrO$_2$ epilayers with structural and magnetic properties similar to those obtained using the more standard oxygen carrier gas. Furthermore, we interpret the larger magnetic coercivity observed for the (110) oriented films in te…
The growth of graphene on copper by atmospheric pressure chemical vapor deposition in a system free of pumping equipment is investigated. The emphasis is put on the necessity of hydrogen presence during graphene synthesis and cooling. In the absence of hydrogen during the growth step or cooling at slow rate, weak carbon coverage, consisting mostly of oxidized and amorphous carbon, is obtained on the copper catalyst. The oxidation originates from the inevitable occurrence of residual oxidizing impurities in the reactor's atmosphere. Graphene with appreciable coverage can be grown within the vac…
45-degree rotated epitaxial diamond nucleation on both (001) and {111} planes of a silicon substrate was observed using chemical vapor deposition with nucleation enhancement by electron emission. The epitaxial relationship between diamond and Si is D(001)//Si(001) and D<100>//Si<110> in both cases. While the case of diamond nucleation on Si(001) may be explained in Verwoerd's model, the other case has no theoretical model as yet. The as-grown diamond was characterized by scanning electron microscopy and Raman spectroscopy. It is speculated that a suitable combination of low temperature and low…
We report one-pot chemical vapor deposition (CVD) growth of large-area Janus SeMoS monolayers, with the asymmetric top (Se) and bottom (S) chalcogen atomic planes with respect to the central transition metal (Mo) atoms. The formation of these two-dimensional semiconductor monolayers takes place upon the thermodynamic equilibrium-driven exchange of the bottom Se atoms of the initially grown MoSe2 single crystals on gold foils with S atoms. The growth process is characterized by complementary experimental techniques including Raman and X-ray photoelectron spectroscopy and the growth mechanisms a…
We demonstrate the direct dry transfer of large area Chemical Vapor Deposition graphene to several polymers (low density polyethylene, high density polyethylene, polystyrene, polylactide acid and poly(vinylidenefluoride-co-trifluoroethylene) by means of only moderate heat and pressure, and the later mechanical peeling of the original graphene substrate. Simulations of the graphene-polymer interactions, rheological tests and graphene transfer at various experimental conditions show that controlling the graphene-polymer interface is the key to controlling graphene transfer. Raman spectroscopy an…
The direct liquid injection chemical vapor deposition (DLI-CVD) technique has been used for the growth of cobalt ferrite (CFO) films on (100)-oriented MgAl$_2$O$_4$ (MAO) substrates. Smooth and highly epitaxial cobalt ferrite thin films, with the epitaxial relationship $\mathrm{MAO} (100)\:[001] \parallel \mathrm{CFO} (100)\:[001]$, are obtained under optimized deposition conditions. The films exhibit bulk-like structural and magnetic properties with an out-of-plane lattice constant of $8.370\;\mathrmÅ$ and a saturation magnetization of $420\;\mathrm{kA/m}$ at room temperature. The Raman spect…
Graphene has attracted a lot of research interests due to its exotic properties and a wide spectrum of potential applications. Chemical vapor deposition (CVD) from gaseous hydrocarbon sources has shown great promises for large-scale graphene growth. However, high growth temperature, typically 1000°C, is required for such growth. Here we demonstrate a revised CVD route to grow graphene on Cu foils at low temperature, adopting solid and liquid hydrocarbon feedstocks. For solid PMMA and polystyrene precursors, centimeter-scale monolayer graphene films are synthesized at a growth temperature down …
Recently, monolayer SnS, a two-dimensional group IV monochalcogenide, was grown on a mica substrate at the micrometer-size scale by the simple physical vapor deposition (PVD), resulting in the successful demonstration of its in-plane room temperature ferroelectricity. However, the reason behind the monolayer growth remains unclear because it had been considered that the SnS growth inevitably results in a multilayer thickness due to the strong interlayer interaction arising from lone pair electrons. Here, we investigate the PVD growth of monolayer SnS from two different feed powders, highly pur…
In most envisioned applications, full utilization of a graphene-carbon nanotube (CNT) construct requires maintaining the integrity of the graphene layer during the CNT growth step. In this work, we exhibit an approach towards controlled CNT growth atop graphene substrates where the reaction equilibrium between source hydrocarbon decomposition and carbon saturation into and precipitation from the catalyst nanoparticles shifts towards CNT growth rather than graphene consumption. By utilizing C2H4 feedstock, we demonstrate that the low temperature growth permissible with this gas suppresses undes…
The formation of various uncommon shaped voids along with regular triangular and square voids in the epitaxial 3C-SiC films on Si has been investigated by optical microscopy and atomic force microscopy. Heteroepitaxial growth of 3C-SiC films on Si (001) and (111) substrates has been performed using hexamethyldisilane in a resistance-heated chemical vapor deposition reactor. The influence of the orientation of the Si substrate in determining the shape of the voids has clearly been observed. In addition, the growth period and the growth-temperature have been considered as the major parameters to…
We synthesized centimeter-scale single- to few-layer graphene (FLG) films via chemical vapor deposition (CVD) on Ni foils. We demonstrates that the precipitation mechanism may not be the only important mechanism in the formation of graphene by CVD in Ni system, and that controlling the cooling rate in the CVD process may not be the appropriate way to control the thickness of graphene films. In addition, we are the first to demonstrate the transfer of centimeter-scale FLG from Ni foil to transparent flexible polyethylene terephthalate substrates via an efficient roll-to-roll process. Comparing …
We report on a laser annealing procedure which greatly improves the quality of suspended monolayers of chemical vapor deposition (CVD) grown MoSe2. Annealing with a green laser locally heats the suspended flake to approximately 600 K, which both removes contaminants and reduces strain gradients. At 4 K, we observe linewidths as narrow as 3.5 meV (1.6nm) full-width at half-max (FWHM) for both photoluminescence (PL) and reflection. Large peak reflectances up to 47% are also observed. These values are comparable to those of the highest quality hexagonal boron nitride (hBN) encapsulated samples. W…
Crystallization is a key for ferroelectricity which is a collective behavior of microscopic electric dipoles. On the other hand, uncontrolled crystallization leads to uneven morphology and random crystal orientations, which undermines the application potential of ferroelectric thin films. In this work, we introduce a film fabrication method of low-temperature physical vapor deposition followed by restrained crystallization, with electrical properties monitored in real-time by in situ measurements. This method was adopted to fabricate films of 2-methylbenzimidazole (MBI), whose molecule crystal…
In this work, we report on the addition of GeH4 gas during homoepitaxial growth of 4H-SiC by chemical vapour deposition. Ge introduction does not affect dramatically the surface morphology and defect density though it is accompanied with Ge droplets accumulation at the surface. The Ge incorporation level inside the 4H-SiC matrix, ranging from few 1017 to few 1018 at.cm-3, was found to be mainly affected by the growth temperature and GeH4 flux. Other growth parameters like C/Si ratio, polarity, or off-orientation did not show any significant influence. On the other hand, adding GeH4 led to the …
Two-dimensional (2D) transition metal dichalcogenides (TMDs), especially MoS2 and WS2 recently attract extensive attentions due to their rich physics and great potential applications. Superior to graphene, MS2 (M = Mo/W) monolayers have a native direct energy gap in visible frequency range. This promises great future of MS2 for optoelectronics. To exploit properties and further develop more applications, producing large-scale single crystals of MS2 by a facile method is highly demanded. Here, we report the synthesis of large-scale triangular single crystals of WS2 monolayer from a chemical vap…
Tuning the surface energy of fluorinated diamond-like carbon coatings via plasma immersion ion implantation plasma-enhanced chemical vapor deposition with 1,1,1,2-tetrafluoroethane
This paper reports on the growth and characterization of hexagonal boron nitride (hBN) and its use for solid-state thermal neutron detection. The hBN epilayers were grown by metalorganic chemical vapor deposition on sapphire substrates at a temperature of 1350 C. X-ray diffraction peak from the (002) hBN plane at a 2theta angle of 26.7 deg exhibited the c-lattice constant of 6.66 Å for these films. A strong peak corresponding to the high frequency Raman active mode of hBN was found for the films at 1370.5/cm. hBN-based solid-state neutron detectors were fabricated and tested with a metal-semic…
The effect of Fe and Ni catalysts on the synthesis of carbon nanotubes (CNTs) using atmospheric pressure chemical vapor deposition (APCVD) was investigated. Field emission scanning electron microscopy (FESEM) analysis suggests that the samples grow through a tip growth mechanism. High-resolution transmission electron microscopy (HRTEM) measurements show multiwalled carbon nanotubes (MWCNTs) with bamboo structure for Ni catalyst while iron filled straight tubes were obtained with the Fe catalyst. The X-ray diffraction (XRD) pattern indicates that nanotubes are graphitic in nature and there is n…
We present a transfer-free preparation method for graphene on hexagonal boron nitride (h-BN) crystals by chemical vapor deposition of graphene via a catalytic proximity effect, i.e. activated by a Cu catalyst close-by . We demonstrate the full coverage by monolayer graphene of half-millimeter-sized hexagonal boron nitride crystals exfoliated on a copper foil prior to growth. We demonstrate that the proximity of the copper catalyst ensures high yield with the growth rate estimated between of 2μm/min to 5μm/min . Optical and electron microscopies together with confocal micro-Raman mapping confir…
We control the anisotropic molecular packing of vapor-deposited glasses of ABH113, a deuterated anthracene derivative with promise for future OLED materials, by changing the deposition rate and substrate temperature at which they are prepared. We find that, at substrate temperatures from 0.65Tg to 0.92Tg, deposition rate significantly modifies the orientational order in the vapor-deposited glasses as characterized by X-ray scattering and birefringence. Both measures of anisotropic order can be described by a single deposition rate-substrate temperature superposition (RTS). This supports the ap…
We characterize nanostructures of Bi2Se3 that are grown via metalorganic chemical vapor deposition using the precursors diethyl selenium and trimethyl bismuth. By adjusting growth parameters, we obtain either single-crystalline ribbons up to 10 microns long or thin micron-sized platelets. Four-terminal resistance measurements yield a sample resistivity of 4 mOhm-cm. We observe weak anti-localization and extract a phase coherence length l_phi = 178 nm and spin-orbit length l_so = 93 nm at T = 0.29 K. Our results are consistent with previous measurements on exfoliated samples and samples grown v…
Topological insulator (TI) materials such as Bi2Te3 and Bi2Se3 have attracted strong recent interests. Large scale, high quality TI thin films are important for developing TI-based device applications. In this work, structural and electronic properties of Bi2Te3 thin films deposited by metal organic chemical vapor deposition (MOCVD) on GaAs (001) substrates were characterized via X-ray diffraction (XRD), Raman spectroscopy, angle-resolved photoemission spectroscopy (ARPES), and electronic transport measurements. The characteristic topological surface states (SS) with a single Dirac cone have b…
Micron-scale single-crystal nanowires of metallic TaSe3, a material that forms -Ta-Se3-Ta-Se3- stacks separated from one another by a tubular van der Waals (vdW) gap, have been synthesized using chemical vapor deposition (CVD) on a SiO2/Si substrate, in a process compatible with semiconductor industry requirements. Their electrical resistivity was found unaffected by downscaling from the bulk to as little as 7 nm in width and height, in striking contrast to the resistivity of copper for the same dimensions. While the bulk resistivity of TaSe3 is substantially higher than that of bulk copper, a…
Development of physical cryptographic primitives for generating strong security keys is central to combating security threats such as counterfeiting and unauthorized access to electronic devices. We introduce a new class of physical cryptographic primitives from layered molybdenum disulfide (MoS2) which leverages the unique properties of this material system. Using chemical vapor deposition, we synthesize a MoS2 monolayer film covered with speckles of multilayer islands, where the growth process is engineered for an optimal speckle density. The physical cryptographic primitive is an array of 2…
Diamond nucleation under very low pressure (0.1-1.0 torr) was obtained at very high nucleation densities and very rapid rates using hot-filament chemical vapor deposition (HFCVD). The density on mirror-polished silicon was as high as 10^10 - 10^11 cm^{-2}, equivalent to the highest density in a microwave-plasma CVD system. That on scratched silicon substrates was up to 10^9 cm^{-2}, 1-2 orders of magnitude higher than that obtained under conventionally low pressure (tens of torr, 10^7 - 10^8 cm^{-2}). Also, the density on scratched titanium substrates was as high as 10^10 cm^{-2}. The samples …
Semiconducting transition metal dichalcogenides (TMDCs) have attracted a lot of attention recently, because of their interesting electronic, optical, and mechanical properties. Among large numbers of TMDCs, monolayer of tungsten diselenides (WSe2) is of particular interest since it possesses a direct band gap and tunable charge transport behaviors, which make it suitable for a variety of electronic and optoelectronic applications. Direct synthesis of large domains of monolayer WSe2 and their growth mechanism studies are important steps toward applications of WSe2. Here, we report systematical …
Combining MoS$_2$ monolayers to form multilayers allows to access new functionalities. In this work, we examine the correlation between the stacking order and the interlayer coupling of valence states in MoS$_2$ homobilayer samples grown by chemical vapor deposition (CVD) and artificially stacked bilayers from CVD monolayers. We show that hole delocalization over the bilayer is allowed in 2H stacking and results in strong interlayer exciton absorption and also in a larger A-B exciton separation as compared to 3R bilayers, where both holes and electrons are confined to the individual layers. Co…
Films of (111)-textured Cu, Ni, and Cu$_x$Ni$_y$ were evaluated as substrates for chemical vapor deposition of graphene. A metal thickness of 400 nm to 700 nm was sputtered onto a substrate of $α-$Al$_2$O$_3$(0001) at temperatures of 250 C to 650 C. The films were then annealed at 1000 C in a tube furnace. X-ray and electron backscatter diffraction measurements showed all films have (111) texture but have grains with in-plane orientations differing by $60^{\circ}$. The in-plane epitaxial relationship for all films was $[110]_{metal}$||$[10\bar{1}0]_{{Al}_{2}{O}_{3}}$. Reactive sputtering of Al…
The growth of amorphous TiO2 and anisotropic-polycrystalline ZnO thin-films is studied by means of coarse-grained three-dimensional kinetic Montecarlo simulations under conditions typically encountered in Plasma Enhanced Chemical Vapor Deposition experiments. The approach developed considers fluctuations in the activation energy for surface diffusion of the coarse particles that are calculated on-the-fly and depends on the mesoscale local morphological/structural landscape. The basis of this approach --which is known to work well to simulate the growth of amorphous materials using a much simpl…
The physical processes occurring in the presence of disorder: point defects, grain boundaries, etc. may have detrimental effects on the electronic properties of graphene. Here we present an approach to reveal the grain structure of graphene by the selective oxidation of defects and subsequent atomic force microscopy analysis. This technique offers a quick and easy alternative to different electron microscopy and diffraction methods and may be used to give quick feedback on the quality of graphene samples grown by chemical vapor deposition.
