Undoped μc-Si Research Articles

μc-Si thin film transistors with very thin active layer

N-type microcrystalline silicon (μc-Si) top-gate Thin Film Transistors (TFTs) are fabricated at a maximum temperature of 180°C using different thicknesses of undoped μc-Si active layers. The effect of the thickness on the TFT performance is experimentally studied and then modeled using Silvaco software tools. The experimental high improvement of the subthreshold swing and the limitation of the rear channel effect, when using very thin active layer, are shown to be due to the increase of the lateral electrical field between the source or drain and the active layer. This increase of the lateral field is shown to be much more important for defected active layer as the microcrystalline silicon one compared to single crystalline silicon active layer. The importance of the use of very thin active layer for amorphous or ploy–micro–nano-crystalline silicon based TFTs is then demonstrated.

Electrical and Mechanical Behaviors of Microcrystalline TFTs Deposited on PEN

Coplanar N-type TFTs are fabricated at low temperature T<180{degree sign}C on a plastic PEN (Polyethylene naphthalate) substrate. Then, these TFTs are electrically characterized to extract their parameters. TFTs are bended at different radius of curvature R. Mobility field effect increases when the radius of curvature decreases. TFT present characteristics up to R = 1 cm, beyond it characteristics fall down. Conductivity increases also when bending the undoped µc-Si:H film only.

Hydrogen-induced-intrinsic transport in undoped microcrystalline silicon

It is demonstrated that microcrystalline silicon (μc-Si:H) of intrinsic character can be produced by post-growth atomic hydrogen treatment. Undoped μc-Si:H films with a dark-conductivity activation energy (Ea) of about 0.20 eV were grown by plasma-enhanced chemical vapor deposition, and then subsequently exposed to an atomic hydrogen plasma. The hydrogen treatment is shown to result in a gradual increase in the Ea with increasing treatment time, followed by saturation at about 0.57 eV, a value observed for truly intrinsic μc-Si:H films. In the saturated state, the dark conductivity is on the order of 10−7 S/cm. The dark conductivity prefactor is found to follow the Meyer–Neldel rule. It is proposed that charge transport takes place in amorphous-like tissue surrounding the crystalline grains. The results are attributed to the Fermi level shift due to a change in the gap state distribution.

Model calculation of phototransport properties of minority carriers of fully crystalline undoped µc-Si:H

The steady state photoconductivity (SSPC) as a function of temperature and light intensity was measured on microstructurally well-characterized plasma deposited highly crystalline undoped hydrogenated microcrystalline silicon ( µc-Si:H) films. Numerical modeling of SSPC was carried out using our proposed effective density of states profile. The simulation results of phototransport properties are found to be in good agreement with the experimental findings. The results and information gathered about the majority and minority carriers are compared to reported studies on microstructurally similar µc-Si:H material. We show that simulation of SSPC can yield reliable information about the phototransport properties of both majority and minority carriers.

Stability of Microcrystalline Silicon TFTs

Stability of microcrystalline silicon (µc-Si:H) Top-Gate TFTs processed at T<200oC and using silicon dioxide or silicon nitride as gate insulator is studied. Excellent stability with silicon nitride as gate insulator is shown. The stability of TFTs with silicon dioxide is shown to depend on the thickness and then on the structure of undoped µc-Si:H active layer. TFTs with thick active layer present high mobility but are non stable. When using thin active layer, TFTs become very stable but with lower mobility. This mobility is however much higher than its value for TFTs with silicon nitride. Results show the possibility to increase the present usual low mobility of stable µc-Si:H TFTs, saving in the same time the stability.

Influence of boron doping on roughness microcrystalline silicon

We have studied roughness of boron-doped microcrystalline Si (μc-Si) surfaces with an emphasis on the influence of heavy doping. μc-Si films were prepared using plasma-enhanced chemical vapor deposition (PECVD) with different boron concentrations in gas phase from 0% to 2%. Growth-induced roughening of μc-Si surfaces was monitored ex situ using an atomic force microscope (AFM). With an increase in the deposition time, the surface width (rms roughness), w, of undoped μc-Si surface exhibited usual behaviors; first, (a) w increased, (b) slightly dropped, (c) rose again, and then (d) gradually increased. In the case of B-doped μc-Si, w differently behaved; (a) w increased very soon, (b) slightly dropped, (a′) rose again, (b′) slightly dropped again, (c) rose, and finally (d) gradually increased. The quick increase in w indicates that boron doping promotes the nucleation, and the repeated nucleation is responsible for the behavior (a′)–(b′). Additionally, the nucleation density, that was derived using the lateral correlation length of surface heights, monotonically increased with an increase in the boron concentration. The effects of boron doping are discussed with the catalytic effects and the formation of the surface-covering layer.

Numerical modeling of steady state photoconductivity process in highly crystallized undoped μc-Si:H films

The steady state photoconductivity as a function of temperature and light intensity was measured on plasma-deposited highly crystalline undoped hydrogenated microcrystalline silicon (μc-Si:H) thin films possessing different microstructure and morphology. Different phototransport behaviors were observed in films having dissimilar microstructural attributes and we have shown this to be linked to different features of the density of states (DOS) in the gap. The phototransport properties of μc-Si:H films having small grains, high density of inter-grain and inter-columnar boundary regions containing disordered phase show light intensity exponent ( γ) variation between 0.5 and 1, and thermal quenching effect in temperature-dependent photoconductivity. Based on our mutually corroborative simulation and experimental results, we have proposed the effective DOS maps for the μc-Si:H system, which differ for films having different microstructures.

Improved microcrystalline silicon TFTs

Electron field effect mobility of microcrystalline silicon TFTs, around 40cm2/Vs, is obtained thanks to an optimized process. This mobility is about 10 times the usual value of 1–2cm2/Vs. This jump is due to the present process where the undoped active layer and the doped layer are deposited in the same run, only by switching on the doping gas valve when the thickness of undoped μc-Si film is reached. The process allows to remove the interface between these two layers and to improve the crystalline fraction inside the doped μc-Si film.

Top-gate microcrystalline silicon TFTs processed at low temperature (

N-type as well P-type top-gate microcrystalline silicon thin film transistors (TFTs) are fabricated on glass substrates at a maximum temperature of 200 °C. The active layer is an undoped μc-Si film, 200 nm thick, deposited by Hot-Wire Chemical Vapor. The drain and source regions are highly phosphorus (N-type TFTs) or boron (P-type TFTs)-doped μc-films deposited by HW-CVD. The gate insulator is a silicon dioxide film deposited by RF sputtering. Al–SiO 2–N type c-Si structures using this insulator present low flat-band voltage,−0.2 V, and low density of states at the interface D it=6.4×10 10 eV −1 cm −2. High field effect mobility, 25 cm 2/V s for electrons and 1.1 cm 2/V s for holes, is obtained. These values are very high, particularly the hole mobility that was never reached previously.

Creation of metastable defects in microcrystalline silicon films by keV electron irradiation

The effect of electron irradiation with 40 keV electrons on the sub-gap optical absorption, dark conductivity and photoconductivity of μc-Si:H films has been investigated. The experiments are carried out on undoped μc-Si:H film, prepared on quartz substrate by electron cyclotron resonance chemical vapor deposition (ECRCVD). The absorption coefficient in the sub-gap region ( hν<1.2 eV) as well as the photoconductivity of μc-Si:H films change considerably after electron irradiation. Annealing of the irradiated films for 1 h at 180 °C temperature restored the initial state. The observed parameter changes are attributed to the creation of metastable defects (silicon dangling bonds) in the disordered phase of the material near the surfaces of the microcrystalline columns in μc-Si:H films induced by electron irradiation. The inversely proportional dependence of photoconductivity on the concentration of these defects suggests that the dangling bonds created by electron irradiation are the dominant recombination centers in electron irradiated μc-Si:H films.

Microcrystallisation in Si:H: the effect of gas pressure in Ar-diluted SiH4 plasma

Abstract Using noble gas argon as a diluent of SiH 4 in RF glow discharge, undoped μc-Si:H thin films have been developed at a low power density of 30 mW/cm 2 . It has been found that the gas pressure is a critical factor for the growth of μc-Si:H films. Undoped μc-Si:H films having σ D ∼10 −6 S/cm and Δ E 2 , a more crystalline as well as highly conducting ( σ D ∼10 −4 S/cm) μc-Si:H film has been achieved at a deposition rate of 30 A/min, which is much higher than that attained from H 2 -diluted SiH 4 plasma, by conventional approach. The crystallinity of the films has been identified by the sharp Raman peak at ∼520 cm −1 and a large number of micrograins in the TEM micrographs. The metastable state of Ar, denoted as Ar * , plays the crucial role in inducing microcrystallisation by transferring its de-excitation energy at the surface of the growing film. A mechanism has been proposed to explain the dependence of the formation of μc-Si:H film on the working gas pressure in the plasma.

Conductance fluctuations in VHF-PECVD grown hydrogenated microcrystalline silicon thin films

Coplanar conductance fluctuations or excess noise of undoped hydrogenated microcrystalline silicon (μc-Si : H) thin films grown by VHF-PECVD from silane–hydrogen mixtures with silane concentrations from 2% to 6% have been studied between room temperature and 470 K. We report that undoped μc-Si : H thin films show similar noise-power spectra to those of undoped a-Si : H films in a coplanar sample geometry. At lower temperatures, the noise with the slope α=0.60±0.07 and at higher temperatures, the noise with the slope α close to unity dominate the spectrum. The noise magnitude decreases with decreasing silane concentration and becomes strongly temperature dependent with increased crystallinity.

A comparison of microcrystalline silicon prepared by plasma-enhanced chemical vapor deposition and hot-wire chemical vapor deposition: electronic and device properties

The optoelectronic properties of undoped μc-Si : H have been investigated, with emphasis on the states close to the edges of the band gap. The usefulness of the constant photocurrent method (CPM) for the determination of the absorption coefficient, α(E), is critically described. Combined with carefully evaluated photothermal deflection spectroscopy data, CPM spectra yield valuable information on the transport and dynamics of photo-generated carriers. By comparing photoluminescence and Raman spectra on high-quality samples prepared by plasma-enhanced and hot-wire chemical vapor deposition, with different silane concentrations in the gas stream, a correlation between the microstructure and photoluminescence energy is obtained. It is proposed that the density of band tail states is reduced with increasing silane concentration, leading to an increase in the photoluminescence energy and in the open-circuit voltages of solar cells.

Promotion of microcrystallization by argon in moderately hydrogen diluted silane plasma

Using argon as a diluent of SiH 4 , undoped hydrogenated microcrystalline silicon (μc-Si:H) films, having σ D ∼10 −5 S cm −1 , were prepared at a very high deposition rate of 36 Å/min. Micrograins were identified with several well-defined crystallographic orientations. The effect of variation of Ar-dilution on the electrical and structural properties of Si:H films were studied systematically. Addition of H 2 to the Ar-diluted SiH 4 plasma improved the network structure by eliminating defects, introducing structural reorientation and grain growth, although, reducing the deposition rate. Accordingly, highly conducting ( σ D ∼10 −3 S cm −1 ) undoped μc-Si:H film was achieved utilizing energy released by de-excitation of metastable state of Ar (denoted as Ar*), in association with network modulation by atomic hydrogen in the plasma.

Role of hydrogen in controlling the growth of μc-Si:H films from argon diluted SiH4 plasma

Hydrogenated microcrystalline silicon thin films have been prepared by the rf glow discharge method using argon as a diluent of SiH4 to achieve a high growth rate. μc-Si:H film having conductivity ∼10−5 S cm−1 was achieved at a deposition rate of 36 Å/min at a moderate power density of 90 mW/cm2, without hydrogen dilution. Micrograins were identified with several well defined crystallographic orientations. Inhomogeneity and porosity at the grain boundary zone have a significant effect on the electrical properties of the films due to adsorption when exposed to atmosphere. However, by adding hydrogen to the Ar-diluted SiH4 plasma, a homogeneous and improved network structure without having any effect of adsorption was obtained at a reduced deposition rate. Highly conducting (σD∼10−3 S cm−1) undoped μc-Si:H film was prepared at a deposition rate of 15 Å/min having 90% crystalline volume fraction. The energy released by the de-excitation of Ar* in the plasma initiates rapid nucleation in the Si network and atomic hydrogen in the plasma helps in the defect elimination, structural reorientation, and grain growth.

Influence of electron and proton irradiation on the electronic properties of microcrystalline silicon

We report and detail changes in the electronic properties of microcrystalline silicon (μc-Si) films after 1-MeV electron and 100-MeV proton irradiation. We studied these irradiation effects on undoped μc-Si samples from plasma-enhanced chemical vapour deposition (PECVD) and hot-wire chemical vapour deposition (HWCVD). The samples under investigation also cover a doping series of HWCVD μc-Si samples with a room-temperature dark conductivity range between 10 −7 S/cm and a few S/cm for both n-type and p-type doping in the as-deposited state. Experimental methods applied before and after irradiation include dark conductivity, photoconductivity for the majority carriers and steady-state photocarrier grating for the minority carriers. Application of modulated photocurrent and constant photocurrent methods provide information on changes in the density of states. Annealing effects on the electronic properties are reported for sample storage at room temperature.

The role of hydrogen dilution of silane and phosphorus doping on hydrogenated microcrystalline silicon (μc-Si:H) films prepared by hot-wire chemical vapor deposition (HW-CVD) technique

The electrical, structural and optical properties of undoped and phosphorus doped μc-Si:H films prepared by a HW-CVD technique have been studied. The hydrogen (H2) dilution of silane has been varied carefully to produce undoped μc-Si:H films. The amorphous-to-microcrystalline transition was observed for a hydrogen dilution ratio >0.75. The phosphorus doped μc-Si:H films were deposited by varying the phosphine (PH3) gas flow rate. The structural properties of these films have been investigated by Raman spectroscopy, low angle X-ray diffraction spectroscopy and Fourier transform infrared vibrational spectroscopy. Electrical characterization has been carried out by dark conductivity and charge carrier activation energy measurements. The phosphorus doped μc-Si:H films showed that the addition of PH3 to the source gases promotes the growth of crystallinity. The increase in crystallite size and crystalline volume fraction with the addition of PH3 to the source gases indicates that it enhances the crystallization of the μc-Si:H film. Low angle XRD studies shows that the PH3 doped μc-Si:H does not show any preferential orientation crystallites. For optimized deposition conditions PH3 doped μc-Si:H films with high dark conductivity (0.4 S/cm), low activation energy (0.03 eV) and high band gap (1.82 eV) were obtained with a high deposition rate (13 Å/s). However, for these optimized conditions, the hydrogen content was relatively large (8.3 at.%).

Passivation of oxygen-related donors in microcrystalline silicon by low temperature deposition

Low-temperature processing for high-performance solar cells based on hydrogenated microcrystalline silicon (μc-Si:H) has been developed using a conventional rf plasma-enhanced chemical vapor deposition (PECVD) technique at an excitation frequency of 13.56 MHz under a high deposition pressure condition. Among pin type solar cells, it is found that deposition temperature of i-layer at 140 °C is effective particularly for improving open circuit voltage (Voc), surprisingly without deteriorating short circuit current or fill factor. Carrier density of undoped μc-Si abruptly decreases for deposition temperatures lower than 180 °C, and the improvement of Voc is ascribed to a decrease of shunt leakage current arising from the oxygen-related donors. This implies that oxygen-related donors can be passivated at low deposition temperatures and that hydrogen plays an important role for the passivation. We propose a simple model for the hydrogen passivation of oxygen related donors. We apply this passivation technique to solar cells, and consequently a conversion efficiency of 8.9% (Voc=0.51 V, Jsc=25 mA/cm−1, FF=0.70) has been obtained in spite of an oxygen concentration of 2×1019 cm−3 in combination with device optimization such as a p-layer. Effect of deposition temperature of i-layer upon other solar cell parameter, short circuit current, and fill factor is also discussed.

Heterogeneity in microcrystalline-transition state: Origin of Si-nucleation and microcrystallization at higher rf power from Ar-diluted SiH4 plasma

Using very high Ar-dilution to the SiH4 plasma, good quality amorphous Si:H films could be obtained at very low rf power. The a-Si:H film, prepared at a very low deposition rate of ∼10 Å/min, exhibited a σPh∼1×10−4 S cm−1, σPh/σD∼105, a notably wide optical gap of 2.10 eV and a very good stability against thermal annealing effects with reasonable light induced degradation. At higher rf power undoped μc-Si:H films were prepared with a high σD∼1×10−4 S cm−1, at a deposition rate of 30 Å/min from &amp;lt;1 sccm of SiH4. Micrograins were identified with several well-defined crystallographic orientations. However, porosity in the grain boundary zone contributed a significant amount of adsorbed effects on the electrical properties. At very high powers, the growth of a columnar network structure was demonstrated. Long-range structural relaxation permitted by the non-rigid and heterogeneous network structure associated with the physical vapor deposition-like growth at the microcrystalline-transition state, has been identified as the origin of nucleation to the Si network and microcrystallization at higher power. It is proposed that Ar* in the Ar-diluted plasma provides the energy required for nucleation and grain growth during microcrystallization, and plays an analogous role as atomic H does during chemical annealing in H2-diluted plasma.

Detection of bottom depletion layer and its influence on surface photovoltage measurement in μc-Si:H

The surface photovoltage (SPV) measurement is a very suitable method to assess the minority carrier diffusion length in microcrystalline silicon (μc-Si:H). Surface states create space charge region, needed for the SPV technique and diffusion length evaluation, but additionally another space charge region, the bottom one, can appear. We present results of monitoring experimentally the bottom space charge region by SPV technique in thick layers of undoped μc-Si:H grown on different substrates. In layers grown on p+ crystalline silicon substrate and a-Si:H sublayers the surface photovoltage as a function of the absorption coefficient is characterised by a distinct peak at low absorption. Mathematical modelling prescribes the experimental data to a photovoltage signal originating in the bottom interface μc-Si:H/substrate.