S. Kasouit

Electronic transport in microcrystalline silicon controlled by trapping and intra-grain mobility

Abstract Time-resolved microwave conductivity (TRMC) and diffusion-controlled time-resolved microwave conductivity (DTRMC) were used on various microcrystalline silicon samples to investigate the mechanisms which control the mobility in these heterogeneous materials. First, in the diffusion mode, electron μ e and hole μ h mobilities were measured: their similar value excludes control of the transport by the amorphous interstitial tissue, if it exists. Next it is shown that TRMC is insensitive to the grain boundary barrier, while DTRMC is sensitive to grain boundary barrier effects. The once more similar values of the mobility obtained by the two measurements suggest that grain boundary barriers do not control the mobility. Finally, a detailed analysis of the TRMC transients as a function of the carrier density and deposition conditions reveals the contribution of recombination and trapping below and in the 0.1–0.2 eV energy range. We conclude that the grain quality, including low trapping inside or outside, is an essential factor in increasing the microcrystalline silicon mobility.

Microcrystalline silicon: An emerging material for stable thin‐film transistors

Top-gate and bottom-gate microcrystalline-silicon thin-film transistors (TFTs) have been produced at low temperature (150-250°C) by the standard radio-frequency glow-discharge technique using three preparation methods: the hydrogen dilution of silane in hydrogen, the layer-by-layer technique, and the use of SiF 4 -Ar-H 2 feedstock. In all cases, a stable top-gate TFT with mobility values around 1 cm 2 /V-sec have been achieved, making them suitable for basic circuit on glass applications. Moreover, the use of SiF 4 gas combined with specific plasma treatments of the a-SiN:H dielectric produces large columns, even at the interface with the dielectric. This leads to stable bottom-gate TFTs, fully compatible with todays a-Si:H production facilities, reaching mobility values up to 3 cm 2 /V-sec. These devices are an interesting alternative to laser-crystallized polysilicon thin films in a growing number of applications.

Transport mechanism of microcrystalline silicon thin films

Abstract The dark conductivity, σd(T), and the time-resolved microwave conductivity (TRMC) of hydrogenated microcrystalline silicon (μc-Si:H) films prepared by hot-wire chemical vapor deposition (HWCVD) and very-high-frequency plasma-enhanced CVD have been investigated. The σd(T) simulations were carried out based on a simplified energy band model. Fitting the σd(T) data indicates that the electrical transport mechanism in μc-Si:H film depends on the nanocrystallite volume fraction (Xc). Around room temperature, thermionic emission dominates for samples with low Xc, however, for high Xc, tunneling is the major conduction mechanism. In the low temperature range, the behavior of films with low-Xc is fairly explained by variable range hopping conduction. An activation energy of 0.15 eV related to oxygen was found in HWCVD films. TRMC measurements show that the mobility increases with Xc.

Fluorine and hydrogen effects on the growth and transport properties of microcrystalline silicon from SiF4 precursor

Abstract Structural and electrical properties of microcrystalline silicon (μc-Si:H) films grown at 200 °C from SiF4 precursor have been studied. A particular growth mechanism in which the material is completely crystallised from the initial stages of deposition is revealed by in situ ellipsometry measurements. Time resolved microwave conductivity (TRMC) measurements were performed on samples deposited at various conditions and an optimum in the total pressure and the hydrogen dilution which maximise the crystalline fraction and the transport properties was found. A mobility as high as 7 cm 2 / V s was measured for a 0.14 μm thick sample. The nature of the substrate (a-SiN:H and SiO2) and its deposition temperature were found to affect the composition of the μc-Si:H deposited on it.

Effect of deposition conditions and dielectric plasma treatments on the electrical properties of microcrystalline silicon TFTs

We study the growth of microcrystalline silicon films on silicon nitride as a function of the deposition conditions and the dielectric plasma treatment.For thin film transistors processed in the bottom gate configuration, we obtain stable transistors with mobilities of 0.7 cm V s , indicating that we have indeed achieved a microcrystalline channel even in the bottom gate 2 y1 y1 approach.Moreover, we found an opposite correlation between the linear mobility and the crystallization kinetics of the material deduced from in-situ ellipsometry measurements; i.e. the faster the crystallization kinetics, the lower the mobility. This result is discussed in terms of smaller grain size for films with fast crystallization, and is supported by Raman spectroscopy measurements. These results provide a guideline for further improving the mobility of the transistors. 2002 Elsevier Science B.V. All rights reserved.

DTRMC, a probe of transverse transport in microcrystalline silicon

Diffusion-induced time resolved microwave conductivity is a method for transverse transport studies in semiconductors deposited on crystalline silicon. In this paper, by measuring layers with different thickness, we show that the transit time of carriers is controlled by diffusion. Moreover, measurements as a function of flux allow us to deduce the diffusion coefficients of electrons and holes and to separate surface and bulk recombination, depending on transport being in unipolar or bipolar regime. We finally compare the diffusion coefficient deduced from this method with TRMC mobility.

34.4: Invited Paper: Microcrystalline Silicon: An emerging Material for Stable Thin Film Transistors

Top gate and bottom gate microcrystalline silicon thin film transistors (TFTs) have been produced by the radio frequency glow discharge technique using three preparation methods: the standard hydrogen dilution of silane in hydrogen, the use of the layer-by-layer technique, and the use of SiF4-Ar-H2 feedstock. In all cases, stable top gate TFT with mobility values around 1 cm2/V.s have been achieved, making them suitable for basic circuit on glass applications. Moreover, the use of SiF4 gas combined with specific treatments of the a-SiN:H dielectric in bottom gate TFTs, fully compatible with todays a-Si:H production facilities, lead to an enhancement of the mobility which reaches stable values around 3 cm2/V.s.

Noninvasive electrical characterization of semiconductor at bulk and interfaces

The Spectroscopic Ellipsometry and the Time Resolved Microwave Conductivity (TRMC) are efficient tools for in-situ non invasive characterizations during the growth of semiconductors and interfaces. From ellipsometry, one estimates the optical absorption, structural composition of the material in the bulk and near the interface. The TRMC measures the transient microwave reflectivity induced by carriers photogenerated by a pulsed laser. From TRMC, one may estimate the mobility of the carriers in a thin film or in bulk materials, the carrier lifetime in the bulk or near the surface. Particularly, we characterize microcrystalline silicon: electron and hole mobility, electron mobility inside the grain, trapping. We also analyze the semiconductor/dielectric interface, particularly for c-Si/SiO2. Using various UV laser fluxes, we can characterize the surface recombination, estimate the interface field and compare with the density of states obtained from capacitance measurement. The results are compared with simulation.

Non-invasive electrical characterization of semiconductor interfaces

Abstract The semiconductor–insulator interface is very critical for the operation of various devices. The time-resolved microwave conductivity (TRMC) is an efficient tool for non-invasive characterizations during the growth of semiconductors and interfaces. TRMC measures the transient microwave reflectivity induced by UV laser-photogenerated carriers. The analysis of the signal (amplitude and shape) as a function of carrier density enables to separate between bulk and interface recombination, and to estimate surface state density, surface recombination velocity, and the effect of interface electric field. The experiment is numerically modeled. The measurements are achieved for interfaces such as c-Si/SiO 2 , c-Si/Si 3 N 4 , and μc-Si/Si 3 N 4 , and are correlated with capacitance measurements as well as with model simulation.

Noninvasive electrical characterization of semiconductor and interface

The Spectroscopic Ellipsometry and the Time Resolved Microwave Conductivity (TRMC) are efficient tools for in-situ non invasive characterizations during the growth of semiconductors and interfaces. From ellipsometry, one estimates the optical absorption, structural composition of the material in the bulk and near the interface. The TRMC measures the transient microwave reflectivity induced by carriers photogenerated by a pulsed laser. From TRMC, one may estimate the mobility of the carriers in a thin film or in bulk materials, the carrier lifetime in the bulk or near the surface. Particularly, we characterize microcrystalline silicon : electron and hole mobility, electron mobility inside the grain, trapping.