Changes In Dark Conductivity Research Articles

Reversible and irreversible effects after oxygen exposure in thick (>1 μm) silicon films deposited by VHF-PECVD on glass substrates investigated by dual beam photoconductivity

Metastability and instability effects due to oxygen exposure in thick intrinsic hydrogenated microcrystalline silicon films deposited by very high frequency plasma enhanced chemical vapour deposition on smooth glass substrates were investigated using temperature-dependent dark conductivity, steady state photoconductivity, and sub-bandgap absorption measurements obtained using the dual beam photoconductivity (DBP) method. No significant changes in dark conductivity and photoconductivity were detected even after long-term air exposure of samples in room ambient as well as after oxygen exposure when samples were characterized in oxygen ambient. However, characterization of the oxygen-exposed state in high vacuum caused an increase in dark conductivity and photoconductivity as well as a significant decrease in the sub-bandgap absorption coefficient spectra in the low energy region in samples with [Formula: see text]. These changes are partially irreversible for samples [Formula: see text] and mostly reversible for compact materials with significant amorphous fraction. No detectable metastable changes occurred in microcrystalline silicon samples with [Formula: see text] as well as in pure amorphous silicon.

Post-growth annealing effects in compensated μc-Si:H samples

In this work we study the effect of low temperature annealing on the transport properties of μc-Si:H. We performed post-growth annealing on a series of micro-doped μc-Si:H samples having different degrees of compensation, extending from n-type to p-type character. The samples were isothermally annealed at 423 K, 10 K below the deposition temperature. An increase of up to four orders of magnitude was observed in the dark conductivity ( σ dk) for the low temperature region. Simultaneously, the activation energy ( E a) of the dark conductivity changes depending on the doping level and the annealing time. Absorption coefficient and density of states measurements have been done for different annealing stages by using the constant photocurrent method and the modulated photocurrent technique, respectively. The density of defects in the midgap region increases during the annealing while the band tails remain unaffected. The σ dk and E a behavior can be interpreted in terms of a shift of the Fermi level towards the valence band. A model based in doping activation is proposed in order to explain this behavior.

Transient and modulated photoconductivity in microcrystalline silicon

ABSTRACTWe report on transient and modulated photoconductivity experiments with undoped michrystalline silicon in which access to density-of-states information is limited because the Fermi level results in occupancy of localised states in the energy range which is scanned. Simulation results show that a defect peak will be masked if most of the distribution is occupied because of the Fermi level position and the density-of-states determined from the experimental data is not an image of the true distribution. Another difficulty with obtaining reliable density-of-states distributions in microcrystalline silicon is the metastability of samples with respect ot adsorption of gases. If dark-conductivity changes are large upon heat treatment in vacuum, the modulated and transient photocurrent response are also affected to a large degree and the density-of-states profiles apart form being influenced by the Fermi level position thus also depend on the thermal history of the sample.

Pentacene thin-films obtained by thermal evaporation in high vacuum

Pentacene thin-films were obtained by thermal evaporation in high vacuum of a pure (98%) commercially available source. All the samples were grown at room temperature (25 °C) and high deposition rates (>20 Å/s) on Corning glass substrates. The microstructure of the pentacene thin-films evidences the coexistence of thin-film and bulk triclinic crystalline phases. The optical absorption edge close to 1.7 eV, together with the high optical absorption in the visible range, make pentacene a promising candidate for low cost solar cells. Doping series of samples were obtained by dipping pieces cut from the same pentacene sample into a solution of iodine in acetonitrile for different times (<1 h). Thereby, room temperature dark conductivity changes from 3×10 −7 to 4×10 −5 Ω −1 cm −1. Finally, glass/SnO/pentacene/Al and glass/ZnO:Al/pentacene/Au structures were fabricated showing rectifying characteristics with on/off ratios of approximately 3 orders of magnitude.

Abrupt Change in Dark Conductivity on C60 Polycrystalline Films at the Rotational Order/Disorder Transition

The temperature dependence of dark conductivity of C60 fullerites in the form of polycrystalline films sandwiched between metal electrodes is systematically studied. At the rotational order/disorder phase transition temperature Tc of about 256 K, an abrupt change in dark conductivity of 4–5 orders of magnitude within a narrow temperature range of less than 2 K is observed. The conductivity for this sample decreases as the temperature crosses Tc from fcc phase to sc phase. From the Arrhenius plot of dark conductivity, the activation energies are estimated to be 53±4 meV for the fcc phase and 111±6 meV for the sc phase. To explain these experimental findings, the electric transport through parallel, extremely pure, and homogeneous C60 monocrystals between two metal contacts is postulated and described, and a model based on the local lattice potential induced by rotational order of C60 molecules is proposed.

Transition from amorphous to microcrystalline Si:H: effects of substrate temperature and hydrogen dilution

Structural changes in silicon films with variations of hydrogen dilution and substrate temperature have been studied. At a critical value of hydrogen dilution in silane ( φ) a sharp transition from amorphous to microcrystalline phase has been observed. An increase of substrate temperature from 170 to 340 °C shifts the transition point from φ=95.5% to 93.5%. The change in dark conductivity and absorption corroborate with the change in crystalline volume fraction in the films. The grain size varies from 45 to 360 Å depending on deposition conditions. Optical absorption and hydrogen content in the film decreases drastically with formation of microcrystalline structure. Silicon films developed at 340 °C show moderate photosensitivity together with low light induced degradation at low crystallinity, which might be suitable properties for solar cell application.

Increasing The Dark Conductivity Activation Energy in Undoped Microcrystalline Silicon by Post-Growth Anneals

ABSTRACTUndoped µc-Si:H film of the strong n-type character with the dark conductivity activation energy of about 0.28 eV was annealed. Annealing was carried out by slowly increasing the temperature from 25 °C to 450 °C at a constant rate of 12 °C/min (one annealing cycle). Annealing effects were monitored by measuring the changes in dark conductivity, oxygen and hydrogen concentrations, and photoluminescence (PL). Dark conductivity activation energy gradually increases with increasing the number of annealing cycles to a saturation value of about 0.6 eV. There is little or no change in the oxygen concentration, but the hydrogen concentration decreases with increasing the number of annealing cycles. The PL band near 1.2 eV disappears with annealing, while the low energy PL band near 0.85 eV dominates rather as the number of annealing cycles increases. A possible explanation will be discussed.

Electrical Characterization of Chemically Deposited CdS Thin Films under Magnetic Field Application

CdS films were grown onto glass substrates by the chemical-bath deposition (CBD) technique using cadmium chloride and thiourea as Cd and S ion suppliers, respectively. The growth process was carried out with and without constant magnetic field applications. The field was applied perpendicular to the substrate at two values of the magnetic induction: 0.040 and 0.077 T. As result of the application of the magnetic field during the growth process, changes in dark conductivity and carrier concentration were observed, added to the evolution of native defect energy levels, evidenced from van der Pauw and Hall effects versus temperature data, and thermally stimulated current measurements, respectively. In particular, the electrical properties of the CBD-CdS films become enhanced by field application.

Spin-dependent photoconductivity in CVD- and MBE-grown silicon-on-sapphire

Spin-dependent photoconductivity is observed in (100) silicon films grown on sapphire by CVD and MBE. The CVD films are either in their as-grown state or have undergone single or double solid-phase epitaxial regrowth. For all samples a resonant decrease in photoconductivity is observed at a field of about 0.34 T for a microwave frequency of about 9.6 GHz and at about 3.3 mT when the frequency is about 92 MHz. The fractional change in photoconductivity at resonance is measured as a function of the magnetic field strength, microwave or radiofrequency power, temperature, light intensity and sample voltage. The results are interpreted in terms of a quantum mechanical treatment of the pair model of Kaplan, Solomon and Mott and values are extracted for the spin relaxation time, pair dissociation rate and singlet recombination rate. In some samples a resonant change in dark conductivity is also observed and interpreted.

Anomalous conductivity phenomenon and quantum size effects in a new microstructure-modulated superlattice

Quantum size effects such as the modulation of activation energy have been observed and investigated in hydrogenated microcrystalline silicon (μc-Si:H)/hydrogenated amorphous silicon (a-Si:H) microstructure-modulated superlattices. The longitudinal dark conductivity changes anomalously around a temperature in the temperature dependence of the longitudinal dark conductivity for these superlattices. Studies indicate that these superlattices exhibit negative resistance above room temperature. We owe the anomalous conductivity phenomenon to the multibarrier resonant tunneling through the μc-Si:H/a-Si:H multiple-quantum-well structure. By considering the resonant tunneling effects, the anomalous conductivity behavior can be successfully explained.

Adsorption-Induced Electrical Conductivity of Some Ferrocene Derivatives: Rates of Adsorption and Desorption of Vapors

Abstract The change in dark conductivity of some ferrocene derivatives on adsorption of some organic solvent has been studied in sandwich cell configurations at different cell temperatures. Appreciable enhancements in the dark conductivity are observed. Such enhancement depends on the amount of adsorbed vapor, cell temperature as well as the chemical nature of the adsorbate. The sensitivity towards a vapor is also found to vary depending on the nature of the substituent groups in the ferrocene ring. Both the adsorption and desorption rates follow the modified Roginsky–Zeldovich equation. The temperature dependence and the nature of the solid-vapor interaction have been deduced from the rate data analysis.

New results on low level phosphorous doping in a-Si:H

We report on new results of the effects of ppm level phosphine doping in hydrogenated amorphous silicon. The results show that the dark conductivity changes from 5 × 10 −11 to 1 × 10 −3 Scm −1 with ppm phosphine doping. The results can be explained by the low defect state density near mid-gap in a-Si:H and the shift of the Fermi level by doping. These new results mean that the electronic properties of a-Si:H can be tuned by very fine doping without introducing large density of defect states. The application of these results in the photoconductive type image sensors is discussed.

An interesting unconventional photoinduced change in the dark conductivity of a-Si:F:H

The results of light exposure on dark conductivity, its activation energy, and the photoconductivity of a-Si:F:H samples are presented. A light-induced decrease in photoconductivity by a factor of about 3 is observed. The changes in dark conductivity are rather unusual. The direction of the change is found to depend on the position of the Fermi level. At a particular activation energy (about 0.85 ev) no light-induced change in the dark conductivity occurs. This behaviour has been explained in terms of a model. The temperature dependence of the defect creation and annealing has also been reported.

R.F. power dependence of the Staebler-Wronski effect

The Staebler-Wronski effect has been investigated in glow-discharge-deposited hydrogenated amorphous silicon films (about 1 μm thick) grown at different r.f. powers. The samples, exposed to visible light of intensity 350 mW cm −2, showed a change, in both the activation energies for conduction and the pre-exponential factors. The activation energies changed from 0.83–1.05 eV in as-deposited annealed films to 1.03–1.15 eV for exposed films whereas the changes in the pre-exponential factors were from 5.0 × 10 4−4.3 × 10 7 Ω −1 cm −1 to 9.3 × 10 5−2.3 × 10 7 Ω −1 cm −1. Both of the constants for Meyer-Neldel rule also showed a change from 3.46 × 10 −7 Ω −1 cm −1 and 30.91 eV −1 to 1.23 × 10 −10 Ω −1 c −1 and 36.62 eV −1. The pre-exponential factors and the activation energies showed a peak for films deposited at 50 W and the rate of change of the density of dangling bonds with illumination time showed a minimum for the same film. The changes in dark conductivity and photoconductivities from the annealed to the exposed state were found to be at a maximum for samples deposited at 5 W.

A-Si:H prepared by ion bombardment activated hydrogen incorporation during Si evaporation

a-Si:H is prepared by vacuum evaporation of Si from a crucible kept at high positive potential relative to the substrate. The concomitant bombardment of the growing film surface with ionized vapor atoms (Si+) is found to activate the incorporation of hydrogen atoms cracked off from residual water molecules (pressure=2×10−7 Torr); a-Si:H containing up to 25 at. % of H may thus be obtained. The dark conductivity changes from activated behavior (low conductivity) to hopping (high conductivity) behavior by screening off this bombardment, while the dangling bond content (2×1017&amp;lt;Ns &amp;lt;1018 cm−3), as determined by electron-spin resonance, does not change within an order of magnitude. The hydrogen incorporation into the Si structure is studied by infrared absorption, revealing most of the incorporated hydrogen to prevail near the void surfaces in the Si–H2 configuration. The results point at the existence of an inhomogeneous film structure (microstructure), the specific features of which are controlled by the ion bombardment.

Preparation and Properties of Hydrogenated Amorphous Silicon Produced by Plasma-Enhanced Chemical Vapor Decomposition of Silane

AbstractWe have investigated the properties of several hydrogenated amorphous silicon (a-Si:H) films prepared by the plasma-enhanced chemical vapor decomposition (PECVD) of silane. This reactor is singular because it operates at pressures as low as 10 mTorr and uses infrared lamps to heat the growth environment. Structural characterization showed that all the samples were amorphous. Most films had hydrogen concentrations about 5%, optical gaps near 1.6 eV, and neutral dangling-bond concentrations of the order of 5 × 1016 Cm−3. Electrical activation energies were typically in the range 0.5 – 0.6 eV and the photoconductivity under ∼100 mW/cm2 white light was 10−4 (Ω-cm)−1. The dark conductivity was near 10−8(Ω-cm)−1. No significant change in dark conductivity or photoconductivity occurred after several hours of exposure to light.

Long-term, light-induced changes in dark conductivity of Kapton

The light-induced increase in dark conductivity of Kapton in space and simulated space environments has been investigated. It was found that a long-term increase in conductivity is caused by an accumulation of electrically active centers generated by solar radiation in the bulk of the material if maintained in vacuum. An analysis of the increase in conductivity as a function of light exposure time indicates that the dark conductivity of Kapton can be raised to a level at which the film can become sufficiently conductive to be antistatic simply by exposing it to solar radiation in orbit.

Amorphous-Si 1−xSn x:H by sputter assisted plasma CVD

Amorphous Si 1−xSn x:H films were deposited by Sputter Assisted Plasma CVD (SAP-CVD) which used glow discharge to decompose silane gas and to sputter a solid tin target simultaneously. Resulting films contain tin as 0<x<0.5 analysed by Auger spectroscopy. Dark conductivity changes from 5×10 −10 (ohm-cm) −1 to 1.0 (ohm-cm) −1 as 0<x<0.5. Optical band gap decreases monotonically from 1.8 eV to 0.2 eV, as x increases from 0 to 0.5.

Some Properties of Intrinsic and Phosphorus Doped Amorphous Silicon Thin Films

The dark conductivity σD and the photoconductivity σPh of intrinsic and n-type (phosphorus doped) a-Si: H films prepared by rf glow discharge of silane gas has been studied. The samples have been prepared under different experimental conditions which include change of rf power, substrate temperature and dopant concentration. The most significant result has been the change in dark conductivity and photoconductivity with the change of rf power within a limited range of 2 to 30 Watts. As shown by infrared spectroscopy the chances in σD and σPh with rf power are mainly due to the change of hydrogen content. The appearance of a kink in σD vs 1/T plot (at T' with higher slope at T>T') has been explained as due to the effect of statistical shift of the Fermi level EF.

Electronic properties of hydrogenated amorphous silicon-germanium alloys

The electronic properties of some binary hydrogenated amorphous silicon-germanium alloys a-SixGe1-x: H in the silicon rich region (x > 0.6) are investigated. Experimental evidence is presented of photo-induced effects similar to those described in a-Si:H (Staebler-Wronski effect). The electronic properties are then studied from the dual point of view of the germanium content dependence and of the photo and thermal histories of the films. The dark conductivity changes between the annealed state and the light-soaked state are interpreted in terms of the variation of the temperature coefficient of the Fermi level. The photoconductivity efficiency is shown to remain close to that of a-Si:H for 1 > x ≽ 0.9 and to strongly decrease when the germanium content is further increased : the photoresponse of the Si0.62Ge 0.38 alloy is 104 times smaller than that of a-Si:H. This deterioration of the photoconductive properties is explained in terms of the increase of the density of gap states following Ge substitution. This conclusion is based on the study of the width of the exponential absorption edge and on the results of photoconductivity time response studies. The latter data are interpreted by means of the model of Rose of trapping and recombination kinetics and it is found that for x ≃ 0.6 the density of states at 0.4-0.5 eV below the mobility edge is 7 x 10 17 eV-1 cm-3 as compared to 2.4 x 1016 eV-1 cm-3 for x = 0.97.