Silicon nitride deposited by plasma-enhanced chemical vapor deposition (PECVD) from SiH4 and NH3 is not crystalline Si3N4 with a specific stoichiometric ratio, but rather SiNxHy, an Amorphous network in which the N/Si ratio can vary from zero in Amorphous Si (a-Si) to almost two. The rate at which injected electrons are trapped in SiNxHy decreases by over 103 when N/Si is increased from 4/3 to the N saturation point, and the trapping centers are therefore likely to be Si dangling bonds. Trapping is undesirable in semiconductor devices except in memory applications, because it shifts the device's flat-band voltage, although in a-Si thin-film transistors (TFT's) the nitride trapping rate can easily be made Much smaller than the rate of electron trapping in deep states of the a-Si. When deposition plasma chemistry is driven to the N-saturation point, SiNxHy has a trapping rate lower than that of the best PECVDSiO2, although this is still 104 above that of thermal oxide grown on single-crystal Si. N-saturated deposition requires not only a high NH3/'SiH4 feed ratio, but also sufficient plasma power to activate the NH3 so that it will completely consume the SiH4 in formation of the tetra-aminosilane [Si (NH2) 4] precursor molecule. Then, the Si in the Amorphous network becomes entirely bonded to N rather than to both H and N, and NH groups act like O does in SiO2 to lower the coordination of the network and thus avoid dangling bonds.

The ‘time-of-flight’ technique measured electron and hole drift mobilities are shown to be independent of deposition parameters in capacitively coupled, diode type RF-PECVD a-Si:H. However, the carrier mobilities can be varied by orders of magnitude by changing the bias applied to the mesh type controlling electrode in a triode type reactor. The room temperature hole drift mobility increases to 10−1cm2/Vs when an appropriate bias is applied to the mesh providing optimum energy ion stimulation of the film growth surface during deposition. Analysis of the field and temperature dependences of the drift mobilities shows that in the best specimens the conduction (valence) band tail density of states are approximately exponential with a reciprocal slope of lOMeV (25MeV).

We have previously demonstrated selective area deposition of n+ Macrocrystalline silicon at 250°C using time modulated silane flow into a hydrogen plasma, and applied the technique to form high performance top-gate Amorphous silicon TFT's with two mask sets. In this paper, we discuss issues related to process scale-up, including the effect of deposition rate on selectivity loss and non-uniformity. Uniformity can be achieved with higher growth rates by expanding the window for selectivity, and using conditions well within the process limits. We show that lower pressure and higher rf power can enlarge the window by enhancing the hydrogen-Mediated silicon etching.

The drive-current of low-temperature (∼300°C) deposited TFTs has been increased by replacing the a-Si:H channel, and source and drain materials with μc-Si. Lightly B2H6 doped, near-intrinsic μc-Si has been used as the channel layer of the TFTs, and n+ μc-Si was used for the source and drain contacts. The compensation of intrinsic defects in the undoped μc-Si by boron doping increases the dark conductivity activation energy from -0.35 eV to 0.8 eV. TFTs were fabricated in a bottom gate structure, and required an H2 plasma treatment to produce devices with effective channel mobilities of -6.8 cm2/V-s and threshold voltages of -3.7 V in the saturation region.

Effects of intermittent deposition on the defect density in hydrogenated Amorphous silicon (a-Si:H) are investigated at various substrate temperatures by using a mechanical shutter, while maintaining the discharge continuously. The intermittent deposition experiments, where monolayer growth and intermission (waiting time) are repeated in cycles, enable us to study surface dangling bond (DB) recombination and thermal hydrogen desorption separately from other reactions on the growth surface. The defect density in films prepared at lower substrate temperatures decreases with the waiting time, while that deposited at higher substrate temperatures increases with the waiting time.

Polysilicon films with a large mobility can be deposited at tempertures lower than about 400 °C by the catalytic chemical vapor deposition (cat-CVD) Method using a silane and hydrogen gas mixture. The mechanism of polysilicon growth and the structure of polysilicon itself are studied by TEM and X-ray diffraction. It is found that the polysilicon grows from an initial step of the deposition but that the grains start to orient preferentially along a (220) direction as the film grows. It is also found that such preferential axis of the grains is likely to orient toward a heated catalyzer as a sunflower always likes to face the sun.

Device quality a-Si:H has been deposited at 3 Å/s using the “hot wire” technique with 1% SiH4 in He as a source gas. To achieve this deposition rate despite the high dilution, the filament was positioned at 1–2 cm from the substrate. This short distance introduces a large non-uniformity across the substrate in the deposition rate as well as in the film properties. This experimental fact was used to analyze which factors in the deposition determine film quality. We find that radiation from the filament is not an important factor. Data taken from samples deposited at various distances, pressures and flows suggest that the film quality is influenced by radical reactions with SiH4. However, this assumption alone predicts too strong a pressure dependence. The influence of deposition rate as an additional factor can explain the results.

A scanning tunneling microscope has been used to study the topology of the surface of device-quality, hydrogenated Amorphous silicon deposited by rf discharge from silane or “hot wire” CVD. The substrates were oxide-free single-crystal silicon or GaAs. Films studied were either grown in our laboratory and observed with no air exposure, or grown at other laboratories producing device-quality photovoltaic cells and viewed after air exposures of less than 30 Minutes. Thin films (10 nm) representing early growth stages appear significantly smoother than the thicker films. The topology of thick films (> 50 nm) has large variations over individual samples. While many regions can be characterized as “rolling hills”, atomically flat areas are sometimes observed nearby in our films. In most regions the observed slopes were 10% or less from the horizontal, but some steep-sided valleys, indicating incipient voids, are seen. Overall surface roughness measured on sub-Micron areas of our films is very inhomogeneous. Uniformity of the films grown off site was much better, although no atomically flat regions were observed, surface roughness can be estimated.

Silicon oxycarbide microcrystalline layers, n- and p-doped, highly conductive and highly transparent have been produced using a Two Consecutive Decomposition and Deposition Chamber (TCDDC) system. The films exhibit suitable properties for optoelectronic applications where wide band gap materials with required conductivity and stability are needed. In this paper we present the role of partial oxygen pressure (po2) in controlling the composition, structure and transport properties (conductivity, σd and optical gap, Eop) of silicon oxycarbide microcrystalline layers.

Low temperature processing is necessary for the fabrication of thin-film transistors for electronics-on-glass applications, including active matrix flat-panel displays. One method to achieve this involves the use of pulsed excimer-laser annealing of an Amorphous silicon layer on top of an SiO2 layer. The intense UV laser is absorbed in the Amorphous silicon region, Maintaining a low average temperature. The thickness of the underlying SiO2 layer affects the solidification velocity and hence the grain size of the annealed layer. Previous work has concluded that the resultant grain size is small (<100 nm) and further work is needed in finding ways of increasing grain size. This paper describes how grain size is affected by varying the thickness of the SiO2 layer. Correlations will be discussed.between the solidification velocities and grain size as affected by the varying thickness of the SiO2 layer. The paper includes a comparison between experimental and theoretical results, using equations based on energy balance considerations.

We present a study of the optoelectronic and structural properties of a-Si:H deposited by Hot-Wire chemical vapor deposition (HW-CVD) from SiH4 and H2 at “Medium” (Tfil ≃ 1500°C) and “high” (Tfil ≃ 1900 °C) filament temperatures. For each tungsten filament temperature regime, the following deposition parameters are varied: (i) pressure (p ∼ 10−2 — 0.5 Torr); (ii) substrate temperature (Tsub ∼ 180 — 270 °C); (iii) silane flow rate (FsiH4 ∼ 1 — 10 ccm) and (iv) hydrogen flow rate (FH2 ∼ 0 — 10 seem). Films deposited at Tfil ∼ 1900 °C in a low pressure regime (p ∼ 2.7 × 10−2Torr) using flows of 5 sccm for both H2 and SiH4 had high deposition rates (rd ∼ 8 Ås−1). These films showed an optical bandgap, E9Tauc ≃ 1.7 eV, a dark conductivity σd ∼ 10−8Scm−1 with an activation energy Eα,σd ≃ 0.8 eV, and photoconductivity σph ≥ 10−5Scm−1 (σph ∼ 10−5). Films deposited at Tju = 1500 °C and p ≃ 0.3 Torr, showed 1.7 < E9Tauc < 2 eV, 10−5 < σd < 10−3Scm−1, 0.2 < Eα,σ d < 0.5 eV and σph/Σd < 102. For the same Tfit and p ∼ 3 × 10−2 — 0.1 Torr, the films show 1.7 < E9Tauc < 2 eV, 10−3 < Σd < 10−1Scm−1 and σph/σd < 1. Films deposited using molybdenum and rhenium filaments at Tfil ≃ 1900 °C show E9Tauc ≃ 1.7 eV and σd ∼ σph ∼ 10−7Scm−1

In situ ellipsometry has been applied for monitoring silicon-thin-film growth on a glass substrate in RF-discharge plasma of SiF4+SiH4 diluted with H2 (80%). Polycrystalline silicon films were obtained at substrate temperature of 300°C and RF power density of 500MW/cm2. Volumetric fraction of c-Si increased from 50 to 80% by increasing the SiF4/ (SiF4+SiH4) flow ratio from 0 to 0.5. The fraction of c-Si also increased to 63% by sequential repetition of deposition with SiH4/H2 and etching with SiF4. The etching rate of a-Si was less than that of c-Si. These results suggest that semi-preferential etching of a-Si enhances the increase of c-Si fraction. A gas-phase reaction simulation has suggested that the dominant deposition precursor is SiH3, and the etchant is F in the present condition.

Coaxial helical resonators are used in the field of high frequency engineering for narrow band selection of signals from about 10 MHz to 500 MHz (VHF and low UHF range) [1, 2]. Their favorable geometric construction suggests their utilization as plasma source for etching and deposition of a-Si:H at frequencies between 30 and 200 MHz - an additional, but still unusual application.

The resonator consists of a cylindric outer conductor, which is closed on one side by a bottom plate, and a rotationally symmetrical inner conductor of spiral form (helix) Measuring γ/4. One end of the helix is grounded to the bottom plate (short circuit), the other end is open (open circuit). Power supply is feasible either galvanically by connecting to the helix or inductively by means of a coupling coil. Applying a suitable coupling factor allows operation at resonance without matchbox. The plasma is generated within the resonator and above its open end, which can be adjusted to special conditions of application with the aid of several grids. In contrast to the source commonly used, the coaxial helix plasma source has proved advantageous with regard to simple design, easy mounting, Minimal outer electrical equipment, no dependence of generated plasma on substrate measures, good breakdown and discharge behavior, low supply voltage (some tens of volts), high power density, and, due to a discharge occurring without filament, also to life durability and no thermally generated particle emission. The application of very high frequencies leads to a low ion bombardment intensity of a-Si:H, and thus additional magnetic fields are not necessary.

In situ ellipsometry reveals that particle formation influences the growth of glow discharge a-Si:H. This particle formation is observed even under discharge conditions leading to the deposition of device quality Material. It is found that less dense material is deposited during the particle-induced initial transient stage of the discharge which influences the properties of the subsequently growing “bulk” film. The effect of special power gradient ignition procedures is discussed. A significant increase of solar cell efficency is achieved by choosing “soft” start conditions for the i-layer deposition.

Hydrogenated Amorphous silicon films (a-Si:H) were prepared by mercury sensitized photo-CVD using SiH4 and SiH2Cl2 gas mixture. The effects of chlorine (Cl) on the electrical, optical and structural properties of the a-Si:H films were investigated. It was found that a small addition of SiH2Cl2 to the deposition system increases the photo-sensitivity, decreases the defect density, and reduces the light induced degradation of a-Si:H films. At a large addition of SiH2Cl2, however, the CI incorporation into the films was observed, resulting in the deterioration of the film properties. The electrical and optical properties were successfully improved by the H2 dilution when the films were deposited with a large addition of SiH2Cl2.

We deposited a-Si:H thin-films by PECVD of dichlorosilane (SiH2Cl2) diluted with H2 in a DC-triode glow-discharge reactor. The dilution ratio, given as the ratio of H2 to SiH2Cl2 flows, was varied from 0 to 130 and the pressure was varied from 0.3 to 0.75 Torr as the hydrogen fraction was increased. We report on the properties of the deposition plasma and the resulting films; namely the plasma pressure-voltage characteristics and the films' chemical composition, structure, and optoelectronic properties. For hydrogen dilution ratios greater than 2:1, dense, rigid films are deposited, with optical bandgaps from 1.75 to 1.9 eV. With increasing hydrogen dilution a growth rate of 5 Å/s is maintained while photoconductivities increase and defect densities decrease. The best films are obtained at a dilution ratios of 16:1 to 32:1, where we have obtained a dark conductivity of 10−13 S/cm, a photoconductivity of 2×10−7 S/cm (at G=1020 cm-3 s−1) and a defect density of 1.5×1016 cm−3.

Two versions of a deposition method referred to as CYCLIC-CVD are presented. Both are based on the repetition of cycles involving two steps: a) a thin a-Si:H layer deposition and b) a hydrogen plasma treatment. In the first version the cycles consist of a thermal CVD deposition step at 500–550 °C followed by a hydrogen plasma step. Device quality a-Si:H films of less than 3 at.% hydrogen content have been prepared. In the second version, which we call Closed Chamber CVD (CC-CVD) the deposition step is done by conventional PE CVD. However, the hydrogenation step is performed with closed reactor chamber. The latter step provides conservation of silicon mass at an equilibrium between H-etching and redeposition. As a result, μc-Si films of high crystallinity are produced with a high overall deposition rate.

A new dual-plasma (surface wave-coupled microwave and capacitively-coupled radiofrequency) PECVD reactor for high growth rate of Amorphous insulating alloys is being developped. A high flexibility for thin film materials synthesis is expected, because the energy of the ion bombardment can be monitored independently from the microwave plasma chemistry. In situ diagnostics (Optical EMission Spectroscopy and Spectroscopie Ellipsometry) are used for the optimization of the dual-Mode plasma deposition of hydrogenated Amorphous silicon a-Si:H and silicon oxides a-SiOx:H (with 0 ≤ × ≤ 2). The growth of stoichiometric oxide at 3.3 nm / s has been achieved.

We performed plasma impedance measurements at room temperature for a hydrogen plasma using an impedance analyser. The plasma excitation frequency range spans from 40 to 70 MHz. Both the real and imaginary part of the impedance decrease monotonously with increasing frequency. These measurements are in agreement with prior experimental observations [1], that the required peak-to-peak voltage between the electrodes is reduced at higher excitation frequencies. Using a simple equivalent circuit for the plasma this effect can be mainly attributed to the increased sheath capacitance. Furthermore, by modelling the sheath with a simple parallel plate capacitor, its thickness could be estimated: it decreases from 2 MM at 40 MHz to about 1.4 MM at 70 MHz plasma excitation frequency. Finally, a possible link between the decreasing sheath thickness on the increase of deposition rate is discussed.

PECVD hydrogenated Amorphous silicon (a-Si:H) layers have been deposited at high rates using helium dilution of the reacting precursors. This allows the fabrication of very thick layers (a few tens of microns) within reasonable processing times. This technique can be applied to the fabrication of thick p-i-n detector devices to be used for direct X-ray detection. Several p-i-n devices have been processed at deposition rates as high as 15 A/sec without significant changes in the optoelectronic properties of the a-Si:H films. Capacitance-Voltage measurements have been performed in order to measure the residual space charge density in the intrinsic region of the devices. The results demonstrate a moderate increase of the N*DoS concentration with the deposition rate. DC bias photoconductivity measurements have also been performed. The experimental data have been fitted with a multiple trapping transport model for holes. The model parameters have been studied and related to the electronic properties of the films.