Ambipolar microcrystalline silicon thin-film transistors

Hydrogenated microcrystalline silicon (µc-Si:H) has recently received significant attention as a promising material for thin-film transistors (TFTs) in large area electronics due to its high electron and hole charge carrier mobilities. We report on ambipolar TFTs based on microcrystalline silicon prepared by plasma-enhanced chemical vapor deposition at temperature of 160 °C with high electron and hole charge carrier mobilities of 40 cm²/Vs and 10 cm²/Vs, respectively. The ambipolar microcrystalline silicon TFTs provide a simple route in realizing large area integrated circuits at low cost. The electrical characteristics of the ambipolar microcrystalline silicon TFTs will be described and the first results on ambipolar inverters will be presented. The influence of the ambipolar TFT characteristics on the performance of the inverter will be also discussed.     

Intrinsic microcrystalline silicon prepared by hot-wire chemical vapour deposition for thin film solar cells

Microcrystalline silicon (μc-Si:H) prepared by hot-wire chemical vapour deposition (HWCVD) at low substrate temperature T_S and low deposition pressure exhibits excellent material quality and performance in solar cells. Prepared at T_S below 250 °C, μc-Si:H has very low spin densities, low optical absorption below the band gap, high photosensitivities, high hydrogen content and a compact structure, as evidenced by the low oxygen content and the weak 2100 cm⁻¹ IR absorption mode. Similar to PECVD material, solar cells prepared with HWCVD i-layers show increasing open circuit voltages V_oc with increasing silane concentration. The best performance is achieved near the transition to amorphous growth, and such solar cells exhibit very high V_oc up to 600 mV. The structural analysis by Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows considerable amorphous volume fractions in the cells with high V_oc. Raman spectra show a continuously increasing amorphous peak with increasing V_oc. Crystalline fractions X_C ranging from 50% for the highest V_oc to 95% for the lowest V_oc were obtained by XRD. XRD-measurements with different incident beam angles, TEM images and electron diffraction patterns indicate a homogeneous distribution of the amorphous material across the i-layer. Nearly no light induced degradation was observed in the cell with the highest X_C, but solar cells with high amorphous volume fractions exhibit up to 10% degradation of the cell efficiency.     

Correlation of structural and optoelectronic properties of thin film silicon prepared at the transition from microcrystalline to amorphous growth

Photovoltaic properties of 4 µm thick microcrystalline silicon p–i–n solar cells have been studied, over a range of crystallinity determined using Raman spectroscopy. Low-crystallinity material (below 10%) appears to absorb disproportionately strongly in the infrared, possibly due to increased light scattering or to relaxation of the crystal momentum selection rule. A minimum in solar cell efficiency is observed under AM1.5 illumination when V_OC ≈ 580 mV, with blue response most strongly affected. This is consistent with a reduction in electron mobility to a value below that of amorphous silicon for low-crystallinity material, in agreement with time-of-flight measurements.     

Formation of microcrystalline-Si thin film transistors by using self-aligned nickel-silicided process

The formation of microcrystalline-Si thin film transistors (μC-Si TFTs) by using self-aligned nickel-silicided process has been studied. The μC-Si TFTs have been generally fabricated as the top-gate staggered-type structure due to the limitation of process temperature up to 200 °C for practical device applications on organic polymer substrates. However, at the processing temperature of 200 °C, this proposed self-aligned nickel-silicided scheme can cause better device characteristics of μC-Si TFTs than the general top-gate staggered structure, without extra photo masking step. As compared to the general top-gate staggered structure, the self-aligned nickel-silicided scheme can lead to larger bending of energy band near the source region, which facilitates causing more carrier tunneling from the source contact electrode. As a result, this proposed self-aligned nickel-silicided scheme can obviously cause a larger on-state current of μC-Si TFTs than the previous top-gate staggered structure.     

Carrier mobilities in microcrystalline silicon films

For a better understanding of electronic transport mechanisms in thin-film silicon solar cell quality films, we have investigated the Hall mobility for electrons in microcrystalline/amorphous silicon over a range of crystallinities and doping concentrations.We find that Hall mobility increases with increasing doping concentration in accordance with earlier measurements. With increasing amorphous fraction, the measured mobility decreases suggesting a negative influence of the additional disorder. The results suggest a differential mobility model in which mobility depends on the energy level of the carriers that contribute to the electrical current.     

Microstructures of microcrystalline silicon thin films prepared by hot wire chemical vapor deposition

Undoped hydrogenated microcrystalline silicon (μc-Si:H) thin films were prepared at low temperature by hot wire chemical vapor deposition (HWCVD). Microstructures of the μc-Si:H films with different H₂/SiH₄ ratios and deposition pressures have been characterized by infrared spectroscopy X-ray diffraction (XRD), Raman scattering, Fourier transform (FTIR), cross-sectional transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). The crystallization of silicon thin film was enhanced by hydrogen dilution and deposition pressure. The TEM result shows the columnar growth of μc-Si:H thin films. An initial microcrystalline Si layer on the glass substrate, instead of the amorphous layer commonly observed in plasma enhanced chemical vapor deposition (PECVD), was observed from TEM and backside incident Raman spectra. The SAXS data indicate an enhancement of the mass density of μc-Si:H films by hydrogen dilution. Finally, combining the FTIR data with the SAXS experiment suggests that the Si---H bonds in μc-Si:H and in polycrystalline Si thin films are located at the grain boundaries.     

Ion implantation of microcrystalline silicon for low process temperature top gate thin film transistors

Ion implantation of phosphorus was used to dope amorphous and microcrystalline silicon with the aim of achieving a low-temperature, self-aligned process for forming n⁺ contacts to top-gate thin-film transistors. Amorphous and microcrystalline films made with both RF glow discharge and hot-wire chemical vapor deposition were implanted. The effect of the dose, energy and implantation temperature and subsequent annealing at increasing temperatures on the dark conductivity, activation energy and photoconductivity were studied. Lowering the energy (15 keV) while increasing the dose (10¹⁵ cm⁻²) and the implantation temperature (300°C) resulted in the highest after anneal (300°C) dark conductivity for both hot-wire (0.3 Ω⁻¹ cm⁻¹) and RF (0.2 Ω⁻¹ cm⁻¹) microcrystalline films.     

Development of microcrystalline silicon carbide window layers by hot-wire CVD and their applications in microcrystalline silicon thin film solar cells

Microcrystalline silicon carbide (μc-SiC:H) thin films in stoichiometric form were deposited from the gas mixture of monomethylsilane (MMS) and hydrogen by Hot-Wire Chemical Vapor Deposition (HWCVD). These films are highly conductive n-type. The optical gap E₀₄ is about 3.0-3.2 eV. Such μc-SiC:H window layers were successfully applied in n-side illuminated n-i-p microcrystalline silicon thin film solar cells. By increasing the absorber layer thickness from 1 to 2.5 μm, the short circuit current density (j_SC) increases from 23 to 26 mA/cm² with Ag back contacts. By applying highly reflective ZnO/Ag back contacts, j_SC = 29.6 mA/cm² and η = 9.6% were achieved in a cell with a 2-μm-thick absorber layer.     

Fundamental aspects of low-temperature growth of microcrystalline silicon

The fundamental aspects of the growth of microcrystalline silicon are discussed in terms of the gas-phase reactions, as well as surface reactions of Si-related radicals and atomic hydrogen. The nucleation of crystallites is related to the Si–H complex, accompanied by the compressive stress caused by breaking the Si---Si bond due to atomic hydrogen. The nucleation is followed by epitaxial-like growth. The low-temperature epitaxy was studied on Si(001) surfaces between 100 and 500 °C. The dependence on the hydrogen dilution and deposition temperature of the epitaxial thickness reveals crystal growth facilitated on a homogeneous hydrogen-covered surface. Finally, requirements for high crystallinity and low defect density under high deposition-rate conditions are discussed. Powder formation and its suppression using the hollow-mesh method are also described.     

Optical emission spectroscopy as a process control tool during plasma enhanced chemical vapor deposition of microcrystalline silicon thin films

The decisive criterion associated with the species emission intensity ratio (Hα/SiH*) which characterizes the crystallinity of microcrystalline silicon (μc-Si) film was found to display an unstable behavior resulting from species concentration variation during μc-Si film growth with optical emission spectroscopy (OES) tool. In this study, a real-time process control system i.e. closed-loop system was developed. It aims to control the species intensity ratio with OES device in a very high frequency (VHF) plasma enhanced chemical vapor deposition reactor, via modulating the VHF power and silane dilution to improve μc-Si film growth for high efficiency a-Si/μc-Si tandem solar cell. The experiment results show that the closed-loop system stabilized the Hα/SiH* intensity ratio within a variation of 5% during the μc-Si film deposition process. Higher growth rate of μc-Si film with the same crystallinity was obtained in the closed loop system which consumed less power and SiH4 gas than in the open loop system, i.e. without process control.     

Effects of dilution ratio and seed layer on the crystallinity of microcrystalline silicon thin films deposited by hot-wire chemical vapor deposition

We deposited microcrystalline silicon (μc-Si) by hot-wire chemical vapor deposition (HWCVD) at different thickness and dilution ratio, with and without seed layer. As the dilution ratio increased, we observed an increase in the amount of microcrystalline phase in the film, a change in the structure of the grains and a loss of the (220) preferential orientation. The films deposited over a seed layer had a larger fraction of crystalline phase than films deposited with the same parameters but without a seed layer. For high dilution ratios (R=100), most of the film grows epitaxially at the interface with the Si substrate, but a microcrystalline film slowly replaces the single-crystal phase. For low dilution ratios (R=14), the film starts growing mostly amorphously, but the amount of crystalline phase increases with thickness.     

Shutterless deposition of phosphorous doped microcrystalline silicon by Cat-CVD

In this paper we present results on phosphorous-doped μc-Si:H by catalytic chemical vapour deposition in a reactor with an internal arrangement that does not include a shutter. An incubation phase of around 20 nm seems to be the result of the uncontrolled conditions that take place during the first stages of deposition. The optimal deposition conditions found lead to a material with a dark conductivity of 12.8 S/cm, an activation energy of 0.026 eV and a crystalline fraction of 0.86. These values make the layers suitable to be implemented in solar cells.     

Hot-wire deposition of amorphous and microcrystalline silicon using different gas excitations by a coiled filament

Microcrystalline silicon (μc-Si:H) and amorphous silicon (a-Si:H) films were deposited using a hot-wire CVD (HWCVD) system that employs a coiled filament. Process gasses, H₂ and Si₂H₆, could be directed into the deposition chamber via different gas inlets, either through a coiled filament for efficient dissociation or into the chamber away from the filament, but near the substrates. We found that at low deposition pressure (e.g. 20 mTorr) the structure of the films depends on the way gases are introduced into the hot-wire chamber. However, at higher pressure (e.g. 50 mTorr), Raman measurement shows similar results for films deposited with different gas inlets.     

Influence of hot wire on the formation of microcrystalline silicon in MWECR-CVD system

The high-crystallinity and low-defect-density microcrystalline silicon films (μc-Si:H) was prepared by using a new hot-wire-assisted microwave electron cyclotron resonance-chemical vapor deposition (HWAMWECR-CVD) system. In this system the hot wire plays an important role in suppressing the growth of a-Si:H in favor of μc-Si:H, thus improving the physical properties. The experimental results show that the μc-Si:H film prepared by using this new system, the crystalline volume fraction is increased from 16.4% to 63.2%, the photoconductivity is increased by two orders of magnitude, the optical band gap is decreased to 1.59 eV, and the light-induced degradation keeps almost constant compared to that prepared by conventional system.     

Microstructure characterization of microcrystalline silicon thin films deposited by very high frequency plasma-enhanced chemical vapor deposition by spectroscopic ellipsometry

We applied ex situ spectroscopic ellipsometry (SE) on silicon thin films across the a-Si:H/μc-Si:H transition deposited using different hydrogen dilutions at a high pressure by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD). The optical models were based on effective medium approximation (EMA) and effective to estimate the thickness of the amorphous incubation layer and the volume fractions of amorphous, microcrystalline phase and void in μc-Si:H thin films. We obtained an acceptable data fit and the SE results were consistent with that from Raman spectroscopy and atomic force microscopy (AFM). We found a thick incubation layer in μc-Si:H thin films deposited at a high rate of ~ 5 Å/s and this microstructure strongly affected their conductivity.     

Low temperature electric transport properties in hydrogenated microcrystalline silicon films

The dark conductivity of microcrystalline silicon (μc-Si:H) films, deposited in a RF-PECVD system varying the RF power in the 15-100 W range, has been investigated as a function of the temperature. Under low electric field condition (10³ V/cm), the conductivity of the samples as a function of the exponential of T^− 1/4 presents a linear behaviour in the measured temperature range. The density of states near the Fermi level, the range of hopping and the activation energy for hopping have been evaluated using the diffusion model. A correlation between the hopping parameters and the crystallinity degree has been found.     

Interpretation of the hydrogen evolution during deposition of microcrystalline silicon by chemical transport

Hydrogen diffusion is a crucial step in film growth by chemical vapor deposition of both hydrogenated amorphous silicon (a-Si:H) and hydrogenated microcrystalline silicon (µ-Si:H) materials. To gain an insight into the correlation between hydrogen diffusion and the amorphous to microcrystalline transition, we have exposed freshly deposited intrinsic, boron- and phosphorus-doped a-Si:H thin films to hydrogen (or deuterium) plasma in conditions of µc-Si:H deposition by chemical transport. Using both in-situ and ex-situ characterizations techniques, we examined the kinetics of hydrogen excess evolution during the plasma exposure. Solution of the partial differential equation for the diffusion of mobile H atoms with a specific boundary condition that accounts for the reduction of atomic H flux with the growth of the µc-Si:H layer supports the theory that the out-diffusion is a consequence of the growth of the µc-Si:H layer.     

Role of high-k gate insulators for oxide thin film transistors

In conventional TFTs, SiO₂ or SiN_x have been used as gate insulators. But they could not induce the high on-current due to their low-capacitance. Since they have low-capacitance that originated from low dielectric constant, on-current of TFTs with low-k insulated are limited by low-capacitance. We have investigated high-k materials, such as HfO₂, ZrO₂ and modified structures for the use of gate insulators in oxide thin film transistors. ZrO₂ and HfO₂ are the most attractive materials with their superior properties, such as high breakdown field intensity (~ 15 MV/cm), high dielectric constant (~ 25), and the capability of room-temperature process. Since they have high-capacitance due to high dielectric constant, it can be easily expected to result in high on-current. In this work, we demonstrated the comparison of oxide thin film transistors with HfO₂, ZrO₂ and SiO₂ and the roles of gate insulators are analyzed. In the result, oxide thin film transistors with SiO₂, HfO₂ and ZrO₂ have on-currents of ~100 μA, ~500 μA, and ~3 mA, respectively. Especially oxide thin film transistor with ZrO₂ has larger on-current than oxide thin film transistor with HfO₂. The result means that ZrO₂ is more suitable than HfO₂ for the gate-dielectric material which can be fabricated at room temperature.     

Microcrystalline silicon carbide thin films grown by HWCVD at different filament temperatures and their application in n-i-p microcrystalline silicon solar cells

To optimize the performance of microcrystalline silicon carbide (µc-SiC:H) window layers in n-i-p type microcrystalline silicon (µc-Si:H) solar cells, the influence of the rhenium filament temperature in the hot wire chemical vapor deposition process on the properties of µc-SiC:H films and corresponding solar cells were studied. The filament temperature T_F has a strong effect on the structure and optical properties of µc-SiC:H films. Using these µc-SiC:H films prepared in the range of T_F = 1800-2000 °C as window layers in n-side illuminated µc-Si:H solar cells, cell efficiencies of above 8.0% were achieved with 1 µm thick µc-Si:H absorber layer and Ag back reflector.     

Improved transport properties of microcrystalline silicon films grown by HWCVD with a variable hydrogen dilution process

The structure and the transport properties of microcrystalline silicon films prepared by hot-wire/catalytic chemical vapor deposition (HWCVD/Cat-CVD), using different dilution ratios of silane in hydrogen, were investigated. Spectroscopic ellipsometry analysis revealed an increase in the thickness of amorphous incubation layer formed before nucleation and a reduction of the void volume fraction when hydrogen dilution decreases. Thus, a specific microcrystalline silicon film growth process was proposed, based on a variable dilution of silane in hydrogen. For films prepared in such conditions, the formation of the incubation layer was inhibited, which led to a drastic improvement in carrier transport along the growth direction as proved by the diffusion-induced time-resolved microwave conductivity data.