H. Meiling

Device-quality polycrystalline and amorphous silicon films by hot-wire chemical vapour deposition

Abstract We describe how high-quality intrinsic hydrogenated amorphous silicon (a-Si: H), as well as purely intrinsic single-phase hydrogenated polycrystalline silicon (poly-Si: H), can be obtained by hot-wire chemical vapour deposition (HWCVD). The deposition parameter space for these different thin-film materials has been optimized in the same hot-wire deposition chamber. A review of the earlier work shows how such high-quality films at both ends of the amorphous-crystalline scale have evolved. We incorporated both the amorphous and the polycrystalline silicon films in n-i-p solar cells and thin-film transistors (TFTs). The solar cells, with efficiencies in excess of 3%, confirm the material quality of both the a-Si: H and the poly-Si: H i-layer materials, but more work is needed to improve the interfaces with the doped layers. The TFTs made with a-Si: H and poly-Si: H channels show quite similar characteristics, such as a field-effect mobility of 0·5cm2 V−1 s−1, indicating that the channel region has a...

STABLE AMORPHOUS-SILICON THIN-FILM TRANSISTORS

Hydrogenated amorphous silicon, a-Si:H, prepared with the hot-wire chemical vapour deposition technique is incorporated in thin-film transistors (TFTs). High-quality TFTs are fabricated with this type of a-Si:H, which we deposited at a rate of 17 A/s. TFTs with a current switching ratio of 5×105, a threshold voltage of 6.3 V, and an electron field-effect mobility in the saturation regime of 0.6 cm2/V s are obtained. These TFTs do not show any threshold voltage shift upon prolonged gate voltage application, in contrast to conventional a-Si:H TFTs. This has been achieved by optimizing the electronic properties of the hot-wire layer, and by optimizing the interface between the gate dielectric and the hot-wire layer.

Structural properties of a-Si:H related to ion energy distributions in VHF silane deposition plasmas

Abstract We present measurements on typical silane-hydrogen RF/VHF deposition plasmas and the corresponding a-Si:H films deposited from these plasmas. A range of process settings was used, covering both the α and the γ′ regime of the discharge. Mass resolved ion energy distributions were measured at the grounded electrode to determine the ion flux at the growing surface. Although the main precursors are radicals, in the lower pressure α regime the flux of ions towards the surface can account for at least 10% of the observed growth rate. In the γ′ regime this contribution to the growth of the film is less. We measured internal stress, hydrogen concentration, hydrogen bonding configuration, and refractive index to determine the effects of the ion bombardment on the structure of the deposited a-Si:H films. Good structural properties, i.e. a refractive index of about 4.25 at 600 nm and a minimum number of SiH2 bonds, are found above a threshold energy of 5 eV per deposited atom. This observation is explained in terms of knock-on processes of the deposited atoms by ions and an increased mobility of the growth precursors at the surface. Both these processes promote the formation of a dense film.

Low-temperature deposition of polycrystalline silicon thin films by hot-wire CVD

Polycrystalline silicon films have been prepared by hot-wire chemical vapor deposition (HWCVD) at a relatively low substrate temperature of 430°C. The material properties have been optimized for photovoltaic applications by varying the hydrogen dilution of the silane feedstock gas, the gas pressure and the wire temperature. The optimized material has 95% crystalline volume fraction and an average grain size of 70 nm. The grains have a preferential orientation along the (2 2 0) direction. The optical band gap calculated from optical absorption by photothermal deflection spectroscopy (PDS) showed a value of 1.1 eV, equal to crystalline silicon. An activation energy of 0.54 eV for the electrical transport confirmed the intrinsic nature of the films. The material has a low dangling bond-defect density of ∼ 1017 cm3. A photo conductivity of 1.9 × 10−5 Ω−1cm−1 and a photoresponse (σphσd) of 1.4 × 102 were achieved. A high minority-carrier diffusion length of 334 nm as measured by the steady-state photocarrier grating technique (SSPG) and a large majority-carrier mobility-lifetime (μτ) product of 7.1 × 10−7cm2V−1 from steady-state photoconductivity measurement ensure that the poly-Si : H films possess device quality. A single junction nip cell made in the configuration n+-c-Si/i-poly-Si: H/p-μc-Si : H/ITO yielded 3.15% efficiency under 100 mW/cm2 AM 1.5 illumination.

Stability of hot‐wire deposited amorphous‐silicon thin‐film transistors

For the first time hydrogenated amorphous silicon, a‐Si:H, deposited with the hot‐wire technique is incorporated in thin‐film transistors (TFTs). Amorphous silicon was deposited at a rate of 20 A/s. TFTs with a switching ratio of 105, a threshold voltage of 16.9 V, and a field‐effect mobility μs of 0.001 cm2/V s are obtained. Upon gate voltage stress, virtually no change in any of these TFT parameters is observed. Conventional state‐of‐the‐art TFTs deposited in a 13.56 MHz glow discharge showed a threshold voltage shift of more than +12 V. The interface between the gate dielectric and the hot‐wire a‐Si:H layer needs further optimization. After gate voltage stress, the TFTs containing hot‐wire a‐Si:H have superior quality with respect to the threshold voltage.

Deposition‐rate reduction through improper substrate‐to‐electrode attachment in very‐high‐frequency deposition of a‐Si:H

We have tracked down one of the major causes for nonuniformities in film thickness in large‐area deposition of hydrogenated amorphous silicon, a‐Si:H. To simulate improper substrate‐to‐electrode attachment we deliberately introduced a gap behind the substrate. The rf‐excitation‐frequency dependence of the influence of this gap on the deposition rate is presented. We show that a local small gap behind the glass has a detrimental effect on the local deposition rate, and therefore on the uniformity of the films. For example, at a frequency of 60 MHz typically the reduction of the deposition rate amounts to 25% when a gap of 1 mm is present. To explain the observed effects the plasma‐sheath dynamics are considered. The relations between the dc self‐bias voltage, the amplitude of the applied rf voltage, and the deposition rate are determined experimentally. A theoretical model that explains the reduction of the deposition rate is presented. We conclude from the model that the ion density in the sheath is indep...

Modeling and Scaling of a-Si:H and Poly-Si Thin Film Transistors

We have developed analytic SPICE models for hydrogenated amorphous silicon (a-Si:H) and polysilicon (poly-Si) thin film transistors (TFTs) which accurately model all regimes of operation, are temperature dependent to 150°C, and scale with device dimensions. These models have been presented in [1, 2]. In this work, we compare the current-voltage characteristics predicted by our models with the measured characteristics from TFTs fabricated at different foundries. We compare the extracted device parameters in order to evaluate the robustness of our models and to determine a suitable default parameter set. We also use the models to examine the effects of device scaling for short channel TFTs. The models can be accessed using the circuit simulator AIM-Spice [3], which is available at http://nina.ecse.rpi.edu/aimspice .

Structure and hydrogen content of stable hot-wire-deposited amorphous silicon

Thin-film transistors incorporating a hot-wire chemical-vapor-deposited silicon layer have been shown to exhibit superior electronic stability as compared to glow-discharge-deposited amorphous silicon devices. Hot-wire-deposited silicon films of various thicknesses (37–370 nm) on silicon dioxide were investigated. The films are structurally inhomogeneous. Raman measurements and transmission electron microscopy show that isolated cone-shaped crystals grow within a primarily amorphous layer. The amorphous interface region has a low hydrogen content of 2.0±0.2 at. %, while the films exhibit an enhanced hydrogen concentration in the surface region. The bond-angle distribution in the amorphous phase is comparable to that of device-quality glow-discharge-deposited amorphous silicon.

THE INVERSE MEYER-NELDEL RULE IN THIN-FILM TRANSISTORS WITH INTRINSIC HETEROGENEOUS SILICON

The electron transport in undoped heterogeneous silicon deposited by hot-wire chemical vapor deposition is found to exhibit conventional as well as greatly pronounced inverse Meyer–Neldel behavior, when including this type of silicon in a field-effect device. The heterogeneous nature of the semiconductor in the channel region near to the gate insulator allows the Fermi level to be pushed deeply into the conduction-band tail states of the amorphous constituent of the material, which is the transport-limiting phase. By considering the band alignment at the interface of the crystalline inclusions and the amorphous phase, the occurrence of an activation energy smaller than 0.1 eV (required to observe the inverse Meyer–Neldel behavior) can be expected.

Ion bombardment in silane VHF deposition plasmas

The measurement of mass resolved ion energy distributions at the grounded substrate in an RF glow discharge allows to determine the ion flux and the ion energy flux towards the surface of a growing hydrogenated amorphous silicon (a-Si:H) layer. This provides the means to study the influence of ions on the structural properties of a-Si:H. Here the authors focus on the {alpha}-{gamma}{prime} transition as occurs in silane-hydrogen plasmas at an RF frequency of 50 MHz and a substrate temperature of 250 C. The structural properties of the layers appear to depend on the kinetic energy of the arriving ions. This is supported by measurements of ion fluxes under other deposition conditions and by characterization of the corresponding layers. The influence of ions on the growth is discussed in terms of their flux, and the amount of delivered kinetic and potential energy to the growing film. The measurements suggest that a threshold energy of about 5 eV per deposited atom is needed for the construction of a dense amorphous silicon network.