C. Godet

VARIABLE RANGE HOPPING REVISITED: THE CASE OF AN EXPONENTIAL DISTRIBUTION OF LOCALIZED STATES

Abstract Numerical modeling of single-phonon hopping transport within distributions of localized electronic states shows that many transport data for amorphous semiconductors and polymers are consistent with exponential tail state distributions rather than with Motts hypothesis of an energy-independent distribution near the Fermi level. Although both cases lead to a T −1/4 temperature dependence of the 3D hopping conductivity, they can be discriminated by their different correlations between the slope and the conductivity prefactor. This numerical approach allows a determination of the localization parameter N ( E F ) γ −3 , where (1/ γ ) is the decay length of the electronic wave function, and the assessment of the validity conditions for the single-phonon approximation.

Structural and electronic properties of electron cyclotron resonance plasma deposited hydrogenated amorphous carbon and carbon nitride films

Hydrogenated amorphous carbon and carbon nitride films (a-C1−xNx:H) have been synthesized from methane, acetylene, or acetylene–nitrogen precursor gases using a high density electron cyclotron resonance plasma. The deposition and etching rates, along with the film stoichiometry, density, Raman signature of the sp2 phase, and optical and transport properties, have been studied as a function of plasma parameters (microwave power and negative bias of the substrate). While low-density H-rich carbon films have been grown from methane for ion energies up to 200 eV, films grown using acetylene have been obtained at high deposition rate (1.1 nm s−1) with H content below 25 H at. % and density of 2.0 g cm−3, which makes them interesting as electronic materials. For dense carbon nitride alloys, the maximum (N/N+C)=0.35 is limited by the vanishing growth rate, which results from ion-assisted chemical etching mechanisms. A larger N2 plasma etching rate related with lower film density is observed for (N/N+C) values ab...

The origin of visible photoluminescence from silicon oxide thin films prepared by dual-plasma chemical vapor deposition

In order to understand the radiative recombination mechanisms in silicon oxides, photoluminescence properties (PL) of H-rich amorphous silicon oxide thin films grown in a dual-plasma chemical vapor deposition reactor have been related to a number of stoichiometry and structure characterizations (x-ray photoelectron spectroscopy, vibrational spectroscopy, and gas evolution studies). The visible photoluminescence at room temperature from a-SiOx:H matrixes with different compositions, including different bonding environments for H atoms, has been studied in the as-deposited and annealed states up to 900 °C. Three commonly reported PL bands centered around 1.7, 2.1, and 2.9 eV have been detected from the same type of a-SiOx:H material, only by varying the oxygen content (x = 1.35, 1.65, and 2). Temperature quenching experiments are crucial to distinguish the 1.7 eV band, fully consistent with bandtail-to-bandtail recombination, from the radiative defect luminescence mechanisms attributed either to defects rel...

Hydrogen content estimation of hydrogenated amorphous carbon by visible Raman spectroscopy

In the present study, we report the hydrogen content estimation of the hydrogenated amorphous carbon (a‐C:H) films using visible Raman spectroscopy in a fast and nondestructive way. Hydrogenated diamondlike carbon films were deposited by the plasma enhanced chemical vapor deposition, plasma beam source, and integrated distributed electron cyclotron resonance techniques. Methane and acetylene were used as source gases resulting in different hydrogen content and sp2∕sp3 fraction. Ultraviolet-visible (UV-Vis) spectroscopic ellipsometry (1.5–5eV) as well as UV-Vis spectroscopy were provided with the optical band gap (Tauc gap). The sp2∕sp3 fraction and the hydrogen content were independently estimated by electron energy loss spectroscopy and elastic recoil detection analysis-Rutherford back scattering, respectively. The Raman spectra that were acquired in the visible region using the 488nm line shows the superposition of Raman features on a photoluminescence (PL) background. The direct relationship of the sp2...

Role of mobile hydrogen in the amorphous silicon recrystallization

The plasma deposition of nanocrystalline silicon thin films is usually performed under a high flux of atomic hydrogen and hydrogenated chemical species. The growth mechanisms are investigated using the layer‐by‐layer deposition of dense nanocrystalline silicon, obtained at 250 °C by alternating SiH4 and H2 plasmas. In the steady state, a minimum exposure time to the hydrogen plasma is necessary to recrystallize the amorphous top layer (10–85 A). It is shown that this critical time is determined by the diffusion time of some mobile H through the top a‐Si:H layer. The recrystallization is discussed in relation to the diffusion of hydrogen leading to the nanovoid and broken bond formation processes.

Physics of bandtail hopping in disordered carbons

Electrical transport data in disordered forms of carbon are reviewed and compared with the bandtail hopping characteristics obtained from a modeling of phonon-assisted tunneling within an exponential density of states distribution. In the weak electric field limit, the dependence of the effective carrier mobility and the ohmic conductivity on the localization parameter N(EF)γ−3 and the disorder parameter E0 are computed using the filling rate method and the Kubo formula. Most of the data for amorphous carbon-based alloys and ion-implanted diamond match the bandtail hopping predictions and the changes in the localization parameter can thus be investigated as a function of the synthesis conditions. For nanotube mats or cluster beam deposited films, the situation is more complex and transport data probably reflect the heterogeneity of the microstructure.

Electrochemistry of Nitrogen-Incorporated Hydrogenated Amorphous Carbon Films

Nitrogen-doped hydrogenated amorphous carbon, films, were synthesized by microwave electron cyclotron resonance plasma-enhanced chemical vapor deposition, using as precursors. Such films, with thickness in the range of a few tens of nanometers, very smooth, and pinhole free, appear as electrode materials with reactivity and effective window potential similar to those of boron-doped polycrystalline diamond or nitrogen-doped amorphous diamond. These films, grown on Si or Ti substrates, were investigated with regard to their electrochemical reactivity toward outer sphere redox reactions, such as ferri/ferrocyanide, by cyclic voltammetry and electrochemical impedance spectroscopy (EIS). It has been established that the electrical conductivity and the electrochemical reactivity vary in opposite directions, the optimum found for the former being around a nitrogen atomic fraction of 0.3, and for the latter around 0.05. EIS showed that for a fast redox reaction, the behavior, though looking similar to a simple electron transfer on a metallic substrate with a partial diffusion control, has another contribution, which could be explained by the presence of a layer at the interface with the solution, in which electron transport would occur by hopping between many localized states. Ti substrate improves the electrical contact with the carbon film with respect to the Si substrate, which decreases the apparent potential difference between the anodic and cathodic redox peaks by about 150 mV. The minimum value of found for the charge-transfer rate constant of lies in the range so far obtained for other modified carbon materials, such as boron-doped polycrystalline diamond or nitrogen-doped amorphous diamond thin films.

Silicon-based, protective transparent multilayer coatings deposited at high rate on optical polymers by dual-mode MW/r.f. PECVD

Protective transparent coatings have been deposited on polycarbonate and CR 39 by dual mode MW/r.f. plasma-enhanced chemical vapour deposition from silane. The layer sequence consists of appropriate adhesion-promoting pretreatment, an intermediate columnar silica layer and a hard, densified nitride layer with adjusted intrinsic comprensive stress by means of a particular coupling of the MW and r.f. plasmas. The deposition rate was up to 250 nm min−1 for silica. Optical filters fabricated by similar processes can be included in the coating structure. Superior abrasion resistance and thermal variation endurance are obtained without the need for any wet process step.

Growth and composition of dual-plasma polymer-like amorphous carbon films

Polymer-like hydrogenated amorphous carbon (a-C:H) films have been grown in a dual-plasma [radio frequency (rf)–microwave (MW)] reactor using butane as a carbon precursor and different mixtures (H2–Ar or He–Ar) in the MW plasma in order to vary the atomic hydrogen flux impinging on the growing film. Decreasing the rf power (i.e., the energy of ion bombardment on a-C:H) or increasing the H atom flux both result in a wide band gap H-rich polymer-like a-C:H network. Nuclear techniques have been combined with UV-visible ellipsometry to determine the stoichiometry, density and growth rate of a-C:H films as a function of the plasma parameters. Parametrization of UV-visible optical properties allows to monitor the changes in the optical parameters (optical gap and density of π states) attributed to the formation of structural units containing C=C double bonds. C–H bonds observed by in situ infrared ellipsometry have been used to investigate the role of ions in the growth processes (densification, cross linking) and to interpret the observed changes in optical parameters in terms of a two-phase microscopic description of polymer-like a-C:H. In order to understand the kinetic results, a phenomenological growth model is proposed including the respective roles of ion bombardment and H atom flux in the activation and deactivation of surface sites available for CxHy radical incorporation (chemisorption) to the growth zone. The activation mechanism corresponds to a chemical modification at the growth zone sites. This adlayer model includes the physisorption of both H atoms and CxHy radicals and explains why the temperature dependence of the deposition rate is found to be opposite for the limiting cases of low and high H atom fluxes. Some consequences of the model on the film stoichiometry (H/C ratio) and microstructure (sp2 C/sp3 C ratio) have also been evaluated.Polymer-like hydrogenated amorphous carbon (a-C:H) films have been grown in a dual-plasma [radio frequency (rf)–microwave (MW)] reactor using butane as a carbon precursor and different mixtures (H2–Ar or He–Ar) in the MW plasma in order to vary the atomic hydrogen flux impinging on the growing film. Decreasing the rf power (i.e., the energy of ion bombardment on a-C:H) or increasing the H atom flux both result in a wide band gap H-rich polymer-like a-C:H network. Nuclear techniques have been combined with UV-visible ellipsometry to determine the stoichiometry, density and growth rate of a-C:H films as a function of the plasma parameters. Parametrization of UV-visible optical properties allows to monitor the changes in the optical parameters (optical gap and density of π states) attributed to the formation of structural units containing C=C double bonds. C–H bonds observed by in situ infrared ellipsometry have been used to investigate the role of ions in the growth processes (densification, cross linking) ...

Field-enhanced electrical transport mechanisms in amorphous carbon films

In order to investigate the localized electronic states in hydrogenated amorphous carbon (a-C : H) films, the temperature and electric field dependences of the current density have been measured in low-field coplanar (Al/a-C : H/Al) and in high-field transverse (TiW/a-C : H/TiW) geometries. The very-low-field conductivity σ = σ00 exp[−(T 0/T)1/4] reveals a band-tail hopping transport mechanism in an Ohmic regime (at least for films above 20 nm thickness) while, for electric field values F > 4 × 103 V cm−1, a different behaviour is evidenced of the form with a transition in the exponent value from n = 2 at low fields (F<3 × 104 V cm−1) towards n ⩾̸ 1/2 at high fields (F > 3 × 105 V cm−1). This field enhancement of electrical transport can be interpreted as arising from either a three-dimensional Poole–Frenkel effect for charged empty defects, or field-assisted hopping out of neutral empty defects (the Apsley–Hughes hopping model). Although a clear discrimination between both models would require very high electric fields (F > 8γkT/e, where γ −1 is the localized wavefunction radius), the Apsley–Hughes model describes accurately the experimental temperature-dependent and field-enhanced transport within localized states (scaling as eF/2γkT with γ −1 = 2.8 ± 0.4 nm) and is also consistent with the variable-range hopping mechanism observed in the Ohmic regime.