New types of periodic structures in laser-induced chemical vapor deposition

New types of periodic structures that occur in laser-induced chemical vapor deposition have been investigated, mainly for polycrystalline Si deposited on glass substrates covered with a thin amorphous Si layer. These periodic structures, which are believed to be a general phenomenon in pyrolytic laser-induced processing, limit the resolution obtainable in direct writing of patterns.On leave from Fudan University, Department of Electronic Engineering, Shanghai, People's Republic of China     

Lateral growth rates in laser CVD of microstructures

Lateral growth rates of Ni spots deposited on absorbing substrates by decomposition of Ni(CO)₄ with visible Kr⁺ laser light have been measured. The experimental data are consistent with the calculated temperature distributions. The mechanism of decomposition is thermal with an apparent chemical activation energy of 22±3 kcal/mole for the temperature range 350 KT500 K.     

CARS study of SiH4−NH3 reaction process in glow discharge plasma

We have investigated the glow discharge plasma of SiH₄–NH₃ mixture by CARS. The decomposition rate of NH₃ is linearly dependent on SiH₄ partial pressure but that of SiH₄ is not affected by the mixing ratio.     

Low-temperature formation of silicon nitride films using pulsed-plasma CVD under near atmospheric pressure

Silicon nitride (SiN_X) film fabrication on polyethylene terephthalate (PET) substrates has been achieved at a low temperature (∼100 °C) by plasma enhanced chemical vapor deposition operated at near atmospheric pressures. A short-pulse based power system was employed to maintain a stable discharge of SiH₄, H₂ and N₂ in near atmospheric pressures without the use of any inert gases such as He. The deposited films were characterized by X-ray photoelectron spectroscopy. Cross sections of the films were observed by scanning electron microscope (SEM). Despite the use of N₂ in place of NH₃, a high deposition rate (290 nm/min) was obtained by this near-atmospheric-pressure plasma.     

Bistable growth in laser chemical vapor deposition

This is the first report on the observation of a bistability in laser-CVD. The effect is demonstrated for silicon deposition from silane by means of Ar⁺-laser radiation.     

Deposition of thin rhodium films by plasma-enhanced chemical vapor deposition

Thin films of rhodium have been prepared starting from dicarbonyl-2.4-pentadionato-rhodium(I), Rh(CO)₂C₅H₇O₂, by plasma enhanced CVD. The dependence of the deposition rate and film properties on substrate temperature, partial pressure of the organometallic and on hydrogen has been studied. Metal contents of 100% and thin-film resistivities as low as 5 times the bulk resistivity of rhodium have been achieved.     

Size effect in optical spectra of microcrystalline diamond powders and CVD films

Received: 24 March 1997/Accepted: 27 May 1997     

Periodic structures in pyrolytic laser-CVD of W from WCl6

In laser-induced direct writing of W stripes from an admixture of WCl₆ and H₂ non-coherent periodic structures have been observed. These structures appear only in the presence of small amounts of O₂. Together with this structure formation, an oscillating change in the surface absorptivity and morphology has been observed. A special setup that permits to determine the absorptivity of deposits on a micrometer scale has been developed.     

Raman diagnostics of CVD systems: Determination of local temperatures

The suitability of local temperature measurements by cw Raman spectroscopy for the CH₄/H₂ CVD system has been established. The temperature profiles in a model reactor were derived from H₂ pure rotational lines and from hot bands of thev ₁ vibrational band of CH₄. Experimental results are presented for substrate temperatures of 773 K and of 1473 K. High accuracy of measurement and excellent agreement with theoretical solutions for the temperature field within the reactor were found.     

Multiplex H_2 CARS thermometry in a microwave assisted diamond CVD plasma

Rapid temperature measurements in a low pressure, microwave assisted, diamond CVD plasma are reported. By using a “modeless” laser as the Stokes source for H CARS, accurate single-shot and averaged temperatures were obtained which agreed with values obtained from laser induced fluorescence measurements. The speed of data acquisition afforded by multiplex CARS allowed variations of temperature to be monitored with changing plasma conditions induced by variations of pressure, gas composition and microwave power. The application of the technique for “on-line” monitoring of plasma processes is briefly discussed. Received: 13 February 1996 / Accepted: 12 June 1996     

Laser-induced thermoelectric voltage in normal state MgB2 thin films

Laser-induced voltage has been observed in c-axis oriented MgB₂ thin film at room temperature. The amplitude of the signal is approximately proportional to the film thickness. For the film with the thickness of 150 nm, a very fast response has been detected when the film was irradiated by a 308 nm pulsed laser of 20 ns duration. The rise time and full width at half-maximum of the signal are about 3 and 25 ns, respectively. The physical origin of the laser-induced voltage can be attributed to a transverse thermoelectricity due to the anisotropic thermopower in MgB₂.     

Direct writing of gold lines by laser-induced chemical vapor deposition

Stripes of gold metal were deposited by focussing an Ar⁺ laser (514nm) onto glass substrates in a heated vacuum cell containing the evaporator and the precursor. MeAuPMe₃, Me₃AuPR₃ (R = Me,Et) were used as precursors. Using MeAuPMe₃ or Me₃AuPEt₃, deposits of high quality were obtained above 40° C and 60° C evaporator temperature, respectively. With Me₃AuPMe₃ the same deposits of gold stripes were possible near room temperature. The stripes were characterized by scanning profilometry, electrical resistivity, SEM and SAM measurements. In general, the stripe resistivity was between 1.5 and 7 times of the bulk metal.     

ArF laser CVD of hydrogenated amorphous silicon: The role of buffer gases

ArF laser-induced CVD has been employed to generate hydrogenated amorphous silicon (a-Si:H) from Si₂H₆ gas dilute with He, Ar, or H₂. The formation of amorphous films or powder is found to depend critically on the kind of buffer gas, the stationary total and partial gas pressures, and the substrate temperature. These dependences have been investigated in the 1–5 Torr pressure and 100–400 °C temperature ranges. They are semiquantitatively discussed in terms of ArF laser photolysis of disilane, gas heating by heat flow from the substrate and laser irradiation, diffusion, and gas phase polymerization. Furthermore, photo ionization has been observed but found irrelevant for the a-Si:H layer properties. The photo and dark conductivities ( _ph, _d) of the semiconductor layers are determined by the substrate temperature. The _ph values range between 10^–7 and 10^{–4 –1} cm^–1 and the _d values between 10^–11 and 10^{–8 –1} cm^–1. The maximum ratio _ph/ _d amounts to 4×10⁴. The layers are further characterized by their optical band gap and activation energy. The layer properties are compared to literature values of amorphous films prepared by various photo, HOMO, and plasma CVD methods.     

Synthesis of carbon nanotubes by CVD: Effect of acetylene pressure on nanotubes characteristics

The effect of acetylene partial pressure on the structural and morphological properties of multi-walled carbon nanotubes (MWCNTs) synthesized by CVD on iron nanoparticles dispersed in a SiO₂ matrix as catalyst was investigated. The general growing conditions were: 110 cm³/min flow rate, 690 °C synthesis temperature, 180 Torr over pressure and two gas compositions: 2.5% and 10% C₂H₂/N₂. The catalyst and nanotubes were characterized by HR-TEM, SEM and DRX. TGA and DTA were also carried out to study degradation stages of synthesized CNTs. MWCNTs synthesized with low acetylene concentration are more regular and with a lower amount of amorphous carbon than those synthesized with a high concentration. During the synthesis of CNTs, amorphous carbon nanoparticles nucleate on the external wall of the nanotubes. At high acetylene concentration carbon nanoparticles grow, covering all CNTs’ surface, forming a compact coating. The combination of CNTs with this coating of amorphous carbon nanoparticles lead to a material with high decomposition temperature.     

Thin yttrium and rare earth oxide films produced by plasma enhanced CVD of novel organometallic π-complexes

Novel volatile cyclooctatetraenyl-pentamethylcyclopentadienyl sandwich complexes have been used as precursors to deposit thin yttrium and rare earth oxide films by means of PECVD. These compounds form pure oxide films in plasmas of argon/oxygen or argon/water-vapour, in nitrous oxide, and carbon dioxide at substrate temperatures of 350–400° C and power densities of 1.0–1.5 W/cm².The films were characterized by metal analysis, carbon analysis, XPS, CTEM electron diffraction, SEM micrographs, and FTIR spectra.     

Hot filament effects on CVD of carbon nanotubes

We grew vertically aligned CNTs via HFCVD using mixtures of methane and hydrogen as feedstock, and investigated the dependence of CNT growth on feedstock composition, filament temperature, and filament types. At the filament temperature of 2050 °C tungsten filaments were more efficient for CNT growth than tantalum ones, and higher CNT growth rates were observed when tungsten filaments were operated at 1900 °C. Regardless of filament temperatures and types, monotonic increase in growth rate of vertically aligned CNTs was observed as we increased the methane concentration in the feedstock. In‐situ investigation of feedstock dissociation revealed the generation of various radical species, and, moreover, a strong correlation between CNT growth rates and relative mole fractions of single‐carbon radicals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Nitrogen incorporation in homoepitaxial ZnO CVD epilayers

ZnO:N thin films have been deposited on oxygen and zinc terminated polar surfaces of ZnO. The nitrogen incorporation in the epilayers, using NH₃ as doping source, was investigated as a function of the growth temperature in the range between 380 °C and 580 °C. We used Raman spectroscopy and low temperature photoluminescence to investigate the doping properties. It turned out that the nitrogen incorporation strongly depends on both, the surface polarity of the epitaxial films and the applied growth temperatures. In our CVD process low growth temperatures and Zn‐terminated substrate surfaces clearly favour the nitrogen incorporation in the ZnO thin films. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A comparative study of field-emission from different one dimensional carbon nanostructures synthesized via thermal CVD system

Different one dimensional (1D) carbon nanostructures, such as carbon nanonoodles (CNNs), carbon nanospikes (CNSs) and carbon nanotubes (CNTs) have been synthesized via thermal chemical vapour deposition (TCVD) technique. The different 1D morphologies were synthesized by varying the substrate material and the deposition conditions. The as-prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). FESEM and TEM images showed that the diameters of the CNNs and CNTs were ∼40 nm while the diameters of the CNSs were around 100 nm. Field emission studies of the as-prepared samples showed that CNSs to be a better field emitter than CNNs, whereas CNTs are the best among the three producing large emission current. The variation of field emission properties with inter-electrode distance has been studied in detail. Also the time dependent field emission studies of all the nanostructures have been carried out.     

Deposition of copper oxide (Cu2O,CuO) thin films at high temperatures by plasma-enhanced CVD

Copper-oxide films are deposited by plasma-enhanced CVD using copper acetylacetonate as a precursor. The influence of various experimental parameters on deposition rate, film composition and resistivity have been studied. The substrate temperature and the bias are the parameters which affect these properties the most. An increase of the substrate temperature changes the phases of the deposit from Cu₂O-CuO over Cu₂O to Cu. At temperatures 500° C the deposition rates are high but the films consist mainly of metallic Cu. A negative bias enhances the deposition rate only slightly but has a strong effect on the film composition and can completely balance the oxygen deficiency. At a bias of –120 V the films consist of pure CuO even at temperatures 500° C.     

Influence of CH3COO on the room temperature photoluminescence of ZnO films prepared by CVD

ZnO films with strong c-axis-preferred orientation have been prepared by a single source chemical vapor deposition technique using zinc acetate as source material at the growth temperature of 230 °C. The strong UV and blue emissions were observed in the photoluminescence spectra of as-grown films. A small quantity of residual zinc acetate was reserved on the surface of as-grown ZnO films and the emission mechanism of blue luminescence was nearly related to the CH₃COO^- of unidentate type. The blue emission disappeared and the green emission appeared after annealing treatment. The green emission is related to the singly ionized oxygen vacancies.