Low Dark Conductivity Research Articles

Optimal H2-dilution playing key role in accomplishing significant nanocrystallinity with both Si and Ge moieties in SiGe nanocomposite thin film network

Accomplishing significant nanocrystallinity in SiGe alloy network during low-temperature growth in RF-PECVD is a challenging task because it involves grossly different decomposition kinetics of source gases GeH4 and SiH4. The major limitation lies in its rapid deterioration of optoelectronic qualities at increased Ge-inclusion, via the overpopulation of dangling bonds, two-phase heterostructure contributing two transport paths, and non-uniform distribution of H-bonding within its two competent counterparts that lead towards complete amorphization of the film network. In the present work, SiGe alloy films are grown in 13.56 MHz RF-PECVD at a low TS ∼220 °C, by varying H2-dilution to the (SiH4 + GeH4) plasma. At very low H2-dilution, Ge-dominated low bandgap SiGe material with a dominant amorphous network and subsequent low dark-conductivity is obtained. At an increased H2-dilution, a highly nanocrystalline SiGe network with significant nc-Ge component is produced; however, a reduced Ge-content with H-bonding in mostly poly-hydride configuration opposes bandgap narrowing. The nc-SiGe film possesses optimally narrow optical bandgap (Eg ∼1.5 eV) by substantial Ge present in the network, and significant dark-conductivity (σD ∼2.38 × 10–3 S.cm−1) arising from considerable crystallinity in both Ge-Ge and Si-Si moieties, prepared at optimum H2-dilution [H2 / (SiH4 + GeH4)] = 25. It appears appropriate for an ideal absorber-layer in the bottom sub-cell of tandem-structured nc-Si solar cells.

Morphological Control of 2D Hybrid Organic-Inorganic Semiconductor AgSePh.

Silver phenylselenolate (AgSePh) is a hybrid organic-inorganic two-dimensional (2D) semiconductor exhibiting narrow blue emission, in-plane anisotropy, and large exciton binding energy. Here, we show that the addition of carefully chosen solvent vapors during the chemical transformation of metallic silver to AgSePh allows for control over the size and orientation of AgSePh crystals. By testing 28 solvent vapors (with different polarities, boiling points, and functional groups), we controlled the resulting crystal size from <200 nm up to a few μm. Furthermore, choice of solvent vapor can substantially improve the orientational homogeneity of 2D crystals with respect to the substrate. In particular, solvents known to form complexes with silver ions, such as dimethyl sulfoxide (DMSO), led to the largest lateral crystal dimensions and parallel crystal orientation. We perform systematic optical and electrical characterizations on DMSO vapor-grown AgSePh films demonstrating improved crystalline quality, lower defect densities, higher photoconductivity, lower dark conductivity, suppression of ionic migration, and reduced midgap photoluminescence at low temperature. Overall, this work provides a strategy for realizing AgSePh films with improved optical properties and reveals the roles of solvent vapors on the chemical transformation of metallic silver.

Effect of thermodynamic parameters on properties of silicon-carbon films prepared by radio-frequency plasma-enhanced chemical vapor deposition for anti-reflective and photo-luminescent coatings

This work reports an experimental study on the synthesis of hydrogenated amorphous silicon-carbon (a-SiC:H) films with improved antireflective and photo-luminescent characteristics. These films were prepared by plasma-enhanced chemical vapor deposition at a radio frequency of 13.56 MHz, varying the thermodynamic parameters of pressure, gas flows, and temperature. Silane (SiH4), methane (CH4), and hydrogen (H2) were the precursor gases. In a first experiment, composition in gas phase was varied and correlated to the composition in solid phase. Absorption spectra, conductivity, refractive index, optical gap, and photoluminescence (PL) were analyzed. Optical gap and fraction of carbon in gas phase showed a linear dependence with the atomic fraction of carbon in solid phase. Results indicated that the Si0.4C0.6 alloy exhibited a high PL as well as an optimal combination of optical gap and refractive index to be applied as antireflective coating. The subsequent optimization of PL was carried out by a fractional experiment, by varying pressure, H2 flow, and temperature. Results revealed that PL can be improved at high pressure, without H2 flow, and low temperature during glow discharge. Enhancement of PL was correlated to the proper concentration of silicon and carbon in the films, low dark conductivity, negative AM 1.5 conductivity, fluctuating current at low voltage, the increment of Si−H2, C−H2, and C=C bonds, along with vibrational energies in the range of 3190–3585 cm−1.

(Invited) Colloidal Quantum Dots for Enhancing Performance of Avalanche Amorphous-Selenium Photodetectors

Photodiodes using amorphous-selenium (a-Se) have gained significant attention in medical diagnostic imaging due to their avalanche phenomena. At high electric fields above 70 V/μm, holes in a-Se undergo impact ionization which increases the effective sensitivity of the photodetector. However, at the high electric fields required for avalanche gain, dark current due to charge injection from metallic electrodes can become a significant source of noise, limiting the dynamic range of the device. To maximize gain while maintaining low dark conductivity, and to achieve reliable and repeatable impact ionization without irreversible breakdown, we need a non-insulating n-type hole-blocking/electron-transporting layer with a high dielectric constant. Among previously reported hole blocking layers that include both polymers and crystalline inorganic materials, vacuum deposited bulk cerium dioxide (CeO2) has shown promising results and has been successfully utilized in n-i-p avalanche a-Se sensors. However, bulk CeO2 cannot be directly deposited on a-Se owing to the low crystallization temperature of a-Se. In this talk, I will present our results on colloidal CeO2 quantum dots as potential hole blocking layers for a-Se devices. These CeO2 quantum dots are electronically passivated by surfactants/ligands and spin coated directly on a-Se at room temperature without inducing crystallization. Owing to quantum confinement, the QDs have a larger bandgap than bulk CeO2 thus raising the effective hole energy barrier. Prototype devices based on these QDs as interface layers on a-Se achieved an ultra-low dark current of 30 pA/cm2 at 70 V/μm which is three orders of magnitude lower than any avalanche a-Se device previously reported and may be the lowest dark current ever recorded for an avalanche device operated at room temperature.

Application of a Bio-Nanocomposite Tissue as an NIR Optical Receiver and a Temperature Sensor

The application of a type of bio-nano tissue, consisting of tobacco cells and multiwalled carbon nanotubes, as a sensitive near-infrared region (NIR) bolometer operating at room temperature was investigated. An electrical resistor-type sensor was fabricated by the evaporation of lateral gold contacts. A very low dark conductivity was achieved by a high-voltage treatment of a strongly conducting nanotube network. Although before the treatment the electrical transport was dominated by the percolative transport in the nanotube network, after the voltage stress the electronic transport in the low-conductivity tissue was controlled only by the intrinsic electrical properties of the biological matrix. In this latter case, the conductivity was extremely temperature and humidity dependent. However, on operating the sensor tissue in a small temperature window around room temperature, the change in humidity could be neglected. Under these conditions, the multifunctional device functioned as a very high sensitivity temperature sensor with a temperature coefficient of resistance of more than −20%/K. Sensitive bolometer operation with a very good signal-to-noise ratio was demonstrated by irradiation of the tissue with low-power LEDs in the near-infrared range between 780 and 1720 nm.

Structural and optoelectronic characteristics of nanocrystalline silicon oxide film as absorber layer for thin film solar cells

The mixed phase nanocrystalline silicon oxide (nc-SiOx:H) thin films have been prepared by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) system from SiH4, H2 and CO2 mixture. The effect of oxygen incorporation on the structural and opto-electrical properties of nc-Si:H films was investigated. As the CO2 flow rate increased, the crystalline volume fraction in the films decreased, while the optical band gap increased slowly. During the phase transition, both oxygen and hydrogen content increased drastically with the increasing CO2 flow rate, while the Urbatch energy and microstructure factor showed obvious reduction, which indicated an enhancement of the medium range order of the films and good passivation of the inner voids. Device quality nc-SiOx:H film was fabricated with low microstructure factor of 0.32 and high crystalline volume fraction around 50%, along with wide optical band gap of ∼1.75 eV, low dark-conductivity σd ∼7.6 × 10−6 S/cm and high photosensitivity of about 2 × 102. On flat substrate a solar cell was prepared with high Voc of 0.62 V and Jsc of 14.57 mA/cm2. These results revealed the potential of nc-SiOx:H film as an absorber layer for thin film solar cells.

Electronic and Structural Properties of N-Type Microcrystalline Silicon Oxide (Mc-Siox:H) Films for Applications in Thin Film Silicon Solar Cells

The growth of n-type μc-SiOx:H layers in the thickness range of up to 700nm is investigated. We show that the electrical and structural properties of μc-SiOx:H are thickness dependent but to a weaker extend in comparison with n-type μc-Si:H. μc-SiOx:H layers with low dark conductivity in the order of 10-11 S/cm, measured in the planar direction, can be successfully used as n-layers in amorphous silicon solar cells, resulting in an improved cell performance due to reduced parasitic absorption in the n-type layer. The results suggest anisotropic electrical transport in μc-SiOx:H materials, supported by an observation of elongated crystalline regions in high resolution TEM microscopy.

Improving and Characterizing (Cd,Zn)Te Crystals for Detecting Gamma-Ray Radiation

${\rm Cd}_{0.9}{\rm Zn}_{0.1}{\rm Te}$ ingots were synthesized from pure components (6N purity Cd, Zn, and Te with In as the dopant) and subsequently grown from the melt under an argon overpressure. Graphite crucibles (with and without an inner coating of pyrolytic BN) were used. The temperature gradient in the solidification zone was 7–30 K/cm, and the growth rate was 0.6-1.0 mm/hour. We investigated the chemical composition, structure, and electrical properties of the as-grown crystals, and established relationships between the crystal properties and the growth conditions. The bottom, middle, and top of the ingots had n-type conductivity, but slightly different properties. Resistivity reached a maximum in the middle of the ingots ( ${(2.5 - 5.0)} \times {10^{10}}~\hbox{Ohm-cm}$ ), and was less at the edges $\sim 0.8 \times {10^{10}}~\hbox{Ohm-cm}$ . The value of the bandgap was minimal in the middle of the ingots ( $\sim 1.5~\hbox{eV}$ ), and 1.53-1.55 eV at the edges. The compensation degree ( ${{\rm N}_{\rm d}}/{{\rm N}_{\rm a}}$ ) of the energy level responsible for the low dark conductivity showed a maximum value at the bottom of the ingots ( $\sim 60 - 90\% $ ), and a minimum in the middle part (1-2%). The crystals were then used to fabricate Cd(Zn)Te detectors for gamma-ray radiation.

Electronic transport in NbSe₂ two-dimensional nanostructures: semiconducting characteristics and photoconductivity.

The electronic transport properties of two-dimensional (2D) niobium diselenide (NbSe2) layer materials with two-hexagonal single-crystalline structures grown by chemical vapor transport were investigated. Those NbSe2 nanostructures isolated simply using mechanical exfoliation were found to exhibit lower conductivity and semiconducting properties, compared with their bulk metallic counterparts. Benefiting from lower dark conductivity, NbSe2 nanoflakes exhibit a remarkable photoresponse under different wavelengths and intensity excitations. The photocurrent responsivity and photoconductive gain can reach 3.8 A W(-1) and 300, respectively; these values are higher than those of graphene and MoS2 monolayers and are comparable with those of GaS and GaSe nanosheets. The presence of electron trap states at the surface was proposed as an explanation for the reduced dark conductivity and enhanced photoconductivity in the 2D NbSe2 nanostructures. This work identifies another possibility for the application of a metallic layer material as an optoelectronic component in addition to an ultrathin transparent conducting material.

High rate deposition of microcrystalline silicon films using jet-type inductively coupled plasma chemical vapor deposition

Microcrystalline silicon (μc-Si) films were deposited at a high rate and low temperature using jet-type inductively coupled plasma chemical vapor deposition (jet-ICPCVD), and the deposition rate, microstructure and electrical properties of the deposited films were investigated. It was demonstrated that a high deposition rate of over 20 nm/s can be achieved while maintaining high crystallinity and low dark conductivity. The deposition rate is well controlled by regulating the generation rate and transport of growth precursors. High crystallinity of the films results principally from hydrogen-induced chemical annealing. Furthermore, the excellent electrical properties benefit from the low oxygen content and/or low deposition temperature.

Specific features of photoelectric properties of layered films of amorphous hydrogenated silicon

Temperature dependences of photoconductivity of layered and conventional undoped films of amorphous hydrogenated silicon have been studied within a wide range of temperatures (130–420 K) and illumination intensities (0.1–60 mW cm−2). It is established that a higher photosensitivity of layered films compared with conventional films is governed by a low dark conductivity of layered films as a consequence of a deeper position of the equilibrium Fermi level in the band gap and the absence of temperature quenching of photoconductivity in these films. It is shown that these specific features of electrical and photoelectric properties of layered films can be attributed to a low concentration of silicon dangling bonds in comparison with the concentration of oxygen-related acceptor centers, which feature a larger capture coefficient for holes.

Preparation, Structures, and Physical Properties of Tetrakis(alkylthio)tetraselenafulvalene (TTCn-TSeF, n = 1–15)

Abstract A series of tetrakis(alkylthio)tetraselenafulvalene compounds (TTCn-TSeF, n = 1–15) were prepared by a one-step reaction between dialkyl disulfide and tetralithiated TSeF. Molecular properties (redox potentials and optical absorptions in solution) and solid-state properties (thermal behaviors, electric conductivities, and molecular and crystal structures for n = 1 and 10) were studied. TTCn-TSeF compounds are weak electron donor molecules and characterized by small on-site Coulomb repulsion. TTC1-TSeF has a high-dimensional conduction network owing to the presence of high-dimensional heteroatomic contacts, “Atomic-Wire Effect.” The π-moieties of TTC10-TSeF were fastened by the alkyl chains (“Fastener Effect”) to form π-columns and there are a variety of short heteroatomic contacts resulting in two-dimensional electronic structure. Electrical conductivity exhibited peculiar enhancement for n = 1 and 7 ≤ n ≤ 14 owing to the presence of high-dimensional conduction paths. These compounds may manifest high carrier mobility, and are good candidates for the field-effect transistor channel based on the advantageous features: low dark conductivity, low donor ability, on-site Coulomb repulsion energy, high-dimensional π-electron structure, and high solubility in organic solvents.

Electrical properties of a-Si:H thin films as a function of bonding configuration

Hydrogenated amorphous silicon (a-Si:H) thin films were fabricated by Radio Frequency (RF) magnetron sputtering. For solar-cell applications, a-Si:H layers are required to show low dark conductivity and high photoconductivity and, thus, high photosensitivity. Hydrogen flow ratio and working pressure were mainly adjusted to control bonding configurations and hydrogen concentration in the films. At a high working pressure of 12 mTorr, all of the prepared amorphous and microcrystalline silicon films showed a dominant IR absorption peak at 2100 cm −1, which indicates a Si–H 2 stretching mode, grain boundaries and microvoids. When the working pressure was decreased to as low as 3 mTorr with a hydrogen flow ratio of 0.1, the bonding configuration of the films was mainly Si–H as determined by the dominant IR absorption peak at 2000 cm −1, and the photosensitivity of the films was maximized to 760.

Fullerene complexes with divalent metal dithiocarbamates: structures, magnetic properties, and photoconductivity

A series of complexes of fullerenes C60 and C70 with metal dithiocarbamates {MII(R2dtc)2}·Cm (m = 60 or 70) and metal dithiocarbamates coordinated to nitrogen-containing ligands (L), {MII(R2dtc)2)x·L}·C60 (x = 1 or 2), where M = Cu, Zn, Cd, Hg, Mn, or Fe, R = Me, Et, Prn, Pri, or Bun, L is 1,4-diazabicyclo[2.2.2]octane (DABCO), N,N′-dimethylpiperazine, or hexamethylenetetramine, were synthesized. The shape of dithiocarbamate molecules is sterically compatible with the spherical shape of C60, resulting in an efficient interaction between their π systems. The resulting compounds are characterized by a layered or three-dimensional packing of the fullerene molecules. In the C60 complexes, iron(II) and manganese(II) dithiocarbamates exist in the high-spin states (S = 2 and 5/2). The magnetic susceptibility of {MII(Et2dtc)2}2·Cm (M = Fe or Mn, m = 60 or 70) in the temperature range of 200–300 K is described by the Curie-Weiss law with Θ = −250 and −96 K and with maxima at 110 and 46 K, respectively, which is indicative of a strong antiferromagnetic spin coupling between MII. The Weiss constants for the [{MII(Et2dtc)2}2·DABCO]·C60·(DABCO)2 complexes (M = Fe or Mn) are 1.7 and 0.3 K, respectively. The magnetic moments of the complexes containing Fe and Mn dithiocarbamates slightly increase at temperatures below 50 and 35 K, respectively, which is evidence of the ferromagnetic spin coupling between MII in {MII(Et2dtc)2}2·DABCO. Single crystals of the complexes exhibit low dark conductivity (10−10–10−11 S cm−1). The visible light irradiation of these crystals leads to an increase in the photocurrent by two–three orders of magnitude. The photogeneration of free charge carriers in the complexes occurs both due to the photoexcitation of metal dithiocarbamate (CuII(Et2dtc)2) and through the charge transfer from metal dithiocarbamate (MII(Et2dtc)2, M = Zn or Cd) to C60.

Deposition of ultraviolet photoconductive films of amorphous hydrogenated carbon

The structural, optical, and transport properties of hydrogenated amorphous carbon films deposited on the grounded electrode under different dilutions (pure CH4, CH4+H2, and CH4+Ar) in radio frequency powered plasma enhanced chemical vapor deposition process have been studied. Though all the samples deposited were observed to be polymer-like carbon, they have different optical and transport properties. While some of the samples were highly conductive under dark (∼10−1Scm−1 at room temperature), others exhibited very low (&amp;lt;10−12Scm−1) dark conductivity and were photoconductive (∼10−9–10−10Scm−1) only under ultraviolet (UV) light (wavelength λ&amp;lt;250nm). There appears to be a close link between the transport properties and optical absorption. Based on optical absorption, we propose two possible models for the distribution of the density of states involving the π and σ states. The evolution of these states and of the deep defect absorption has a profound impact on the transport properties and particularly on the UV photoconductivity. Comparing the two models, it seems more probable that the UV photoconductivity (for photon energy &amp;gt;4.96eV) is rather linked to transitions involving the σ-σ* states.

Postdeposition thermal annealing and material stability of 75 °C hydrogenated nanocrystalline silicon plasma-enhanced chemical vapor deposition films

This paper examines the material stability and effects of postdeposition thermal annealing of highly crystalline undoped hydrogenated nanocrystalline silicon (nc-Si:H) films prepared by conventional 13.56-MHz plasma-enhanced chemical vapor deposition at 75 °C. Electrical, structural, and chemical composition properties of the nc-Si:H films at ambient atmosphere were studied before and after the thermal annealing. The as-grown nc-Si:H films showed not only a low dark conductivity (σdark) of the order of 10−7–10−6S∕cm but also good stability after exposure to ambient atmosphere due to low oxygen concentration (CO∼2×1018atoms∕cm3) and stable hydrogen passivation of grain boundaries. On the other hand, the σdark and photoconductivity (σphoto) of the nc-Si:H films showed instability during annealing at temperatures higher than the deposition temperature and subsequent exposure to ambient atmosphere. Here, both the σdark and σphoto increased after annealing and then decreased with exposure to ambient atmosphere. In particular, the σdark decreased below the as-grown value proportional to the annealing temperature. This was not observed in nc-Si:H films which have a hydrogenated amorphous silicon nitride (a-SiN:H) passivation layer on the same time scale. There was no significant change in the microstructure of the films. However, the hydrogen concentration (CH) decreased slightly after annealing, while the CO increased. The conductivity instability is therefore strongly related to oxygen desorption and absorption due to thermal annealing and subsequent exposure to ambient atmosphere, respectively. Based on the experimental results, we propose a simple model in terms of hydrogen effusion-assisted oxygen absorption.

Charge carrier mobility in regioregular poly(3-hexylthiophene) probed by transient conductivity techniques: A comparative study

The temperature and electric-field dependence of charge carrier mobility has been studied by a conventional time-of-flight technique in chemically purified, low dark conductivity samples of regioregular poly(3-hexylthiophene). Subsequently, the mobility of doping-induced charge carriers has been determined using the technique of charge carrier extraction by linearly increasing voltage in the same samples exposed to air. The charge carrier mobility determined by both experimental techniques correspond well to each other at temperatures above $130\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, indicating that these experimental techniques are mutually consistent. The study clearly shows that the typical $\mathrm{log}\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\ensuremath{\propto}\ensuremath{\beta}{E}^{1∕2}$, $\ensuremath{\beta}g0$ Poole--Frenkel-like electric-field dependence of the charge carrier mobility diminishes at temperatures around $250--270\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and $\ensuremath{\beta}$ becomes negative at higher temperatures. Such negative electric-field dependence of mobility observed by both experimental techniques is attributed to positional disorder in a random-organic dielectric and analyzed in the framework of the disorder formalism. Finally, the overall agreement indicates that the mode of charge generation has negligible effect on the temperature- and electric-field dependence of mobility except at the lowest temperatures $(l110\phantom{\rule{0.3em}{0ex}}\mathrm{K})$, where transit time dispersion of the photogenerated charge carriers probed by the ToF technique is more pronounced.

Low Substrate Temperature Deposition of Crystalline SiC using HWCVD

AbstractMicrocrystalline silicon carbide (μc-SiC) was prepared at substrate temperatures between 300°C and 450°C using Hot Wire Chemical Vapour Deposition (HWCVD). The SiC films were deposited from monomethylsilane (MMS) diluted in hydrogen on glass and crystalline silicon substrates. The influence of the hydrogen dilution on the deposition rate and the structural and the optoelectronic properties was investigated. Infrared and Raman spectroscopy and transmission electron microscopy (TEM) were applied to study the structural properties. Highly crystalline material with large columnar grains was obtained at high hydrogen dilutions. The optical absorption below the band gap is high and the dark conductivities are far above the values expected for intrinsic SiC. At lower hydrogen dilution, less crystalline or amorphous Si1-xCx is growing, showing broader IR- and Raman peaks, lower dark conductivity and higher absorption above the band gap energy. An extended nucleation zone with large structural disorder was observed even for highly crystalline material.

Synthesis, crystal structure and photoconductivity of the first [60]fullerene complex with metal diethyldithiocarbamate: {CuII(dedtc)2}2·C60

The first molecular complex of fullerene C60 with metal dithiocarbamate, namely, {CuII(dedtc)2}2.C60(dedtc: diethyldithiocarbamate) (1) was obtained as single crystals. Butterfly-shaped CuII(dedtc)2 molecules efficiently co-crystallized with spherical fullerene molecules to form a layered structure, in which closely packed hexagonal C60 layers alternate with the layers composed of CuII(dedtc)2 dimers. The formation of the complex with C60 changes geometry and the EPR spectrum of starting CuII(dedtc)2. Magnetic susceptibility of 1 follows the Curie-Weiss law in the 300-1.9 K range with the negative Weiss constant of -2.5 K showing a weak antiferromagnetic interaction between CuII centers in the dimers. The crystals of 1 have low dark conductivity of 10(-11) S cm-1, which is consistent with a neutral ground state of the complex. Illumination of the crystals by white light increases the photocurrent by 20-50 times. The photoconductivity spectrum of 1 has a maximum at 470 nm showing that both intermolecular charge transfer between neighboring C60 molecules and photoexcitation of CuII(dedtc)2 can contribute to photogeneration of free charge carriers. The effect of a weak magnetic field with Bo<0.5 T on the photoconductivity of 1 has been found.

Fabrication of high growth rate solar-cell-quality c-Si:H thin films by VHF-PECVD

Several series of Si:H films were fabricated by the very high frequency plasma enhanced chemical vapour deposition (VHF-PECVD) at different substrate temperatures (Ts) and silane concentration (SC=[SiH4]/[SiH4+H2]%). The results of Raman spectroscopy showed structural evolution of the Si:H films with the variation of Ts and SC. The results of x-ray diffraction (XRD) measurements indicated that Ts also influences the crystal orientation of the Si:H films. The modulation effect of Ts on crystalline volume fraction (Xc) is more evident for the high SC, which shows different trend compared to low SC. In addition, the growth rate of the films also showed a regular change with the variation of SC andTs. Different samples in the series showed a similar increase in dark conductivity and a decrease in photosensitivity with increasing Ts and decreasing SC. Device-quality microcrystalline silicon materials were deposited at a high growth rate, characterized by relatively low dark conductivity and relatively high photosensitivity in a certain crystalline range. The microcrystalline silicon solar cell with a conversion efficiency of 4.55% has been prepared by VHF-PECVD.