Noise measurements in electron-beam-evaporated amorphous silicon thin films

The low frequency noise of vacuum-evaporated amorphous silicon films, both intrinsic and prepared under a partial pressure of hydrogen, was measured over a range of temperatures. The current noise spectral density shows a predominance of 1/?^α noise at room temperature, where α lies between 0.8 and 1.04, and the conduction proceeds primarily by the hopping of carriers near the Fermi energy level. As the temperature is raised towards the region of extended states conduction the noise spectrum changes to more of a generation-recombination type with α having values from 1.66 to 2.04 at the highest temperatures. These results imply that no extremely large, very narrow band of gap states was formed under our fabrication conditions and they therefore agree with earlier work on the mixed conduction process and with field effect data.     

Electron spin resonance study of amorphous hydrogenated carbon films

Amorphous hydrogenated carbon films of high electrical resistivity were studied using electron spin resonance (ESR) both as prepared and after vacuum heat treatment. The films were produced by deposition from a d.c. magnetron glow discharge containing acetylene. In the as-prepared state, all films gave a composite ESR signal with a broad and a narrow component suggesting the presence of two distinct phases. When the films were annealed, an intense narrow signal was produced which had the same temperature dependence of the signal intensity and the linewidth as that reported in the literature for heated carbonaceous material. By analogy with published work on the heating of carbonaceous material, a model of the structure is proposed based on threefold coordinated graphitic sheets formed by fusion of aromatic rings. Calculations of the average molecular weight of the graphitic sheets are given for two annealing temperatures.     

Effect of thermal coupling on the electronic properties of hydrogenated amorphous silicon thin films deposited by electron cyclotron resonance

In photovoltaic devices, rather thin intrinsic layers of good quality materials are required and high deposition rates are a key point for a cost-effective mass production. In a previous study we have shown that good quality amorphous silicon (a-Si:H) films can be deposited by matrix distributed electron cyclotron resonance (MDECR) plasma CVD at very high deposition rates (∼ 2.5 nm/s). However, only thick films (> 1 μm) exhibited good transport properties. A very poor thermal coupling between the substrate holder and the substrate is the main reason for such a behaviour. We present here experimental data which support this conclusion as well as the improved transport and defect-related properties of new very thin a-Si:H samples (thickness around 0.3 μm) deposited at a higher temperature than the previous ones.     

Electron spin resonance in thin film silicon after low temperature electron irradiation

Paramagnetic defects in amorphous and microcrystalline silicon (a-Si:H and μc-Si:H) with various structure compositions and doping levels were investigated by electron spin resonance (ESR). Samples were prepared by PECVD. The defect density was varied with 2 MeV electron bombardment at 100 K and stepwise annealing in the range of 80 K-433 K. In intrinsic material the spin density of the dominant ESR signal, presumably originating from dangling bonds (db), increases by up to 3 orders of magnitude after irradiation. In doped μc-Si:H material the pronounced conduction electron (CE) resonance disappears after irradiation and is replaced by the db resonance like in the irradiated intrinsic material. Generally the initial spin density and the line shape can be restored upon annealing at 433 K. Additional features at g-values of g ≈ 2.010 and g ≈ 2.000 in the ESR spectra are observed after irradiation together with the strongly enhanced Si db line at about g = 2.004-2.005. These features decrease rapidly on the first annealing steps and cannot be observed after the final annealing stage.     

Aluminum doped silicon carbide thin films prepared by hot-wire CVD: Investigation of defects with electron spin resonance

Al-doped p-type μc-SiC:H is prepared in a wide range of HWCVD preparation parameters like Al-doping ratio, deposition pressure, substrate and filament temperatures. We investigate the structural and electrical properties, and focus on identification of paramagnetic defect states by electron spin resonance (ESR). Nominally undoped μc-SiC:H is of a high n-type conductivity (σ_D = 10^− 6-10^− 1 S/cm) and shows a narrow central ESR line (g ≈ 2.003, peak-to-peak linewidth ?H_pp ≈ 4 G) with two pairs of satellites and a spin density N_S = 10¹⁹ cm^− 3. Al-doping results in the compensation of dark conductivity to as low as σ_D = 10^− 11 S/cm and at higher doping concentrations to effective p-type material. Increase of Al-doping results in reduction of crystallinity (I_C^IR), ESR line shifts to g ≈ 2.01 and becomes as broad as ?H_pp ≈ 30 G, not unlike to the resonance of singly occupied paramagnetic valence band tail states in a-Si:H. ESR spectrum of highly crystalline Al-doped μc-SiC:H however has a g-value very close to undoped μc-SiC:H. Electron spin density in compensated material decreases to 5 × 10¹⁷ cm^− 3 before it increases again for the highly doped material.     

Langmuir-Blodgett films in amorphous silicon MIS structures

In this paper we describe the properties of MIS structures based on hydrogenated amorphous silicon (a-Si:H) and organic films deposited using the Langmuir-Blodgett technique. Results are reported for undoped a-Si:H passivated with an insulating film of cadmium stearate 80 nm thick. The deposition of the monolayers was found to be critically dependent on the surface condition of the semiconductor.The capacitance data display well-defined accumulation and depletion regions and suggest that inversion is obtained when the device is reverse biased. The conductance data are similar in shape to those observed for conventional MOS structures on crystalline silicon. However, hysteresis and frequency dispersion effects complicate their interpretation in terms of surface state densities.From this preliminary investigation we conclude that useful MOS devices incorporating both thick and thin insulating layers can be based on the a-Si:H/Langmuir-Blodgett film system.     

Conduction electron spin resonance in gold

Conduction electron spin resonance has been observed in transmission through pure gold slabs of 100, 260, and 490 µm thickness covered on each side by a thin ferromagnetic film. This resonance corresponds tog=2.11±0.01 and broadens rapidly above 10 K, in agreement with spin orbit relaxation by phonons.Laboratoire associé au CNRS.     

Depth profile measurements of aluminium film on phosphorus-doped hydrogenated amorphous silicon layers by Auger electron spectroscopy

Depth profile measurements are reported using Auger electron spectroscopy in aluminium films on phosphorus-doped hydrogenated amorphous silicon (a-Si:H) films prepared by r.f. glow discharge deposition. The data show that silicon is present in the aluminium film after heat treatment at a temperature as low as 200 °C. Thermal annealing before the aluminium deposition decreased the silicon signal strength in the aluminium film. The silicon signal was not detected in the aluminium film in contact with the a-Si:H through holes in an SiN film but the aluminium signal was detected in the a-Si:H film, probably at the uncovered margin of the SiN contact holes.     

Optical absorption in glow discharge amorphous silicon films

Both undoped films and films doped with phosphine were deposited by the r.f. glow discharge decomposition of silane onto 7059 glass and single-crystal silicon wafers. The absorption edge in the near infrared was examined and the refractive index was calculated from transmittance interference fringes. The various silicon- hydrogen infrared absorption bands were examined to see whether hydrogen incorporation varied with the growth conditions. Our films invariably showed the presence of secondary and tertiary bonded hydrogen.     

Cracking behavior of evaporated amorphous silicon films

The residual stress in amorphous silicon films deposited by evaporation is investigated with different substrate temperatures. The stress measured from all the films studied in this paper is tensile. The level of stress decreases from 580 MPa to 120 MPa with increasing substrate temperature from 60 °C to 350 °C. When the film becomes thicker, strain increases and cracks are formed for stress relaxation. 10 µm thick amorphous Si films are deposited at 350 °C without cracks. This cracking behavior is theoretically studied and confirmed by experiment.     

The influence of unstable signals for electron spin resonance dosimetry with synthetic A-type carbonated apatite

Synthetic A-type carbonated apatite prepared in controlled conditions was irradiated at room temperature with 60Co gamma rays. The ESR spectrum was associated to axial CO2- and orthorhombic CO3- species. Radicals used as dose markers in biological apatites are long-lived paramagnetic species. The stability of the post-irradiation signal of A-type apatite was investigated for almost 2 years. Measurements showed variations in the spectra attributed to unstable CO3- species, which can be eliminated by thermal treatments at 100 degrees C for 24 h. Results indicated the potential use of an A-type carbonated apatite as a dosemeter.     

Effect of NH3 flow rate on growth, structure and luminescence of amorphous silicon nitride films by electron cyclotron resonance plasma

In this paper, hydrogenated amorphous silicon nitride (a-SiN_x:H) films have been deposited using an electron cyclotron resonance chemical vapor deposition system. The effect of NH₃ flow rate R on the deposition rate, structure and luminescence were studied using various techniques such as optical emission spectroscopy, Fourier Transform Infrared absorption (FTIR), X-ray photoelectron spectroscopy (XPS) and fluoro-spectroscopy, respectively. Optical emission behavior of SiH₄ + NH₃ plasma shows that atomic Si radical concentration determines the film deposition rate. Structural transition of a-SiN_x film from Si-rich one to near-stoichiometric/N-rich one with R was revealed by FTIR and the two phase separation of a-Si and a-Si₃N₄ was also convinced in Si-rich SiN_x films by XPS. Either photo- or electroluminescence for all the SiN_x films with R > 3 sccm shows a strong light emission in visible light wavelength range. As R < 6 sccm, recombination of electrons and holes in a-Si quantum dots is the main mechanism of photo/electroluminescence for Si-rich SiN_x films, however, for photoluminescence, gap states' luminescence is also in competition; as R > 6 sccm, light emission of the SiN_x film originates from defect states in its band gap.     

Control of the structure in zone-melted silicon films on amorphous substrates

In situ observations of a crystallization front (solid-liquid interface) during zone-melting recrystallization of thin silicon films on amorphous substrates by laser beams are described. Cellular structure of the front is investigated, and a transition from faceted to rounded front depending on thickness of the film is noted.