J.C. Delgado

Correlation between hydrogen thermal evolution and morphology in glow discharge deposited a-Si:H films

Abstract The effects of annealing on sample morphology are investigated for hydrogenated amorphous silicon (a-Si:H) thin films prepared by plasma decomposition of silane in a d.c. low-pressure glow discharge reactor. A group of samples was studied which were annealed at a constant temperature rate, measuring at the same time the flux of the effused hydrogen from the films. All samples were observed using scanning electron microscopy before and after the annealing, and in some cases the morphology of samples was observed at intermediate temperatures. For some of the samples the morphology does not change after annealing and for others the surface is partially or completely destroyed after the thermal process. A close correlation has been found between this surface destruction and the presence of a sharp low-temperature peak in the hydrogen thermal evolution spectra. An interpretation based on the stress changes produced under annealing in this structure and its relaxation after hydrogen release is used to explain the results.

Structural characterization of a-SiC:H by thermal desorption spectroscopy

Abstract The structure of a-Si1−xCx:H thin films prepared from different mixtures of SiH4 and CH4 was studied by thermal desorption spectroscopy. The influence of the gas phase composition and the thickness on the hydrogen evolution spectra is presented and discussed. A third hydrogen evolution was observed in a-Si1−xCx films with higher carbon content. The results show that the surface controlled desorption processes become dominant when increasing the carbon content.

Influence of the technological parameters on the growth of a-Si:H by a low pressure d.c. plasma process

Abstract The influence of technological parameters such as the location in the reactor, the pressure and the silane flux on the deposition of hydrogenated amorphous silicon has been studied. The thin films have been grown in a low pressure d.c. hot cathode glow discharge with electrostatic confinement. The electrical parameters of the plasma have been obtained from Langmuir probe measurements. In this type of reactor a high efficiency of silane usage is achieved, which is attributed to the effect of the electrons coming from the hot filament. The changes of the deposition rate with the technological parameters have been studied. The surface structure of the samples is inappreciable in some of them and granular like in others. This structure is favoured by conditions such as the sample orientation in the reactor facing the cathode, high pressures, for which the ion bombardment is lower, and intermediate silane fluxes. The existence of two phases which form a reticular-like structure has been revealed through a strong chemical attack on some samples. SiH is the almost exclusive bond between silicon and hydrogen atoms.

Hydrogenated amorphous silicon films obtained by a low pressure dc glow discharge

Films of a-Si: H have been deposited by means of a dc hot cathode discharge of SiH4 with electrostatic confinement at a pressure as low as 0.4 Pa. The plasma used is quite quiescent as has been observed by means of reproducible Langmuir probe measurements. Substrates have been placed at different locations in between the electrodes, some of them facing the anode and the others facing the cathode.Films deposited on substrates facing the cathode present a granular, non-columnar, structure, an IR spectrum with only SiH absorption peaks, and a very low photoresponse. Films deposited on substrates facing the anode have a similar IR spectrum but are homogeneous, have lower hydrogen content, and present a high photoresponse. The optical absorption coefficientα shows in all samples theαnE=C(E−E0)x behaviour, but with exponentx=3 and notx=2 as is usually considered in a-Si∶H.

Study of post-deposition contamination in low-temperature deposited polysilicon films

The presence of hydrogen in polysilicon films obtained at low temperatures by hot-wire CVD and the post-deposition oxidation by air-exposure of the films are studied in this paper. The experimental results from several characterization techniques (infrared spectroscopy, X-ray photoelectron spectroscopy, secondary ion mass spectrometry and wavelength dispersive spectroscopy) showed that hydrogen and oxygen are homogeneously distributed at grain boundaries throughout the depth of the films. Hydrogen is introduced during the growth process and its concentration is higher in samples deposited at lower temperatures. Oxygen diffuses along the grain boundaries and binds to silicon atoms, mainly in Si 2 O groups.

Influence of Substrate Temperature on the Properties of A-Si:H P-Layers Obtained from Trimethylboron

a-Si:H p-layers doped by trimethylboron (TMB) were obtained by PECVD in a monochamber reactor with a rotating substrate holder. The influence of the substrate temperature (T s ) on the film properties was systematically studied for two different doping gas concentrations. The incorporation of boron, hydrogen and carbon was studied by Secondary Ion Mass Spectrometry (SIMS). Optical properties were determined by means of Photothermal Deplection Spectroscopy (PDS) and optical transmission. Dark conductivity (aj and activation energy (E act ) were measured electrically. Our results show that Σ d has a marked dependence on substrate temperature, although boron atom concentration depends only slightly on T s . The optical gap for samples obtained at the higher concentration also depends on T s and its dependence is related to the hydrogen content, as boron content does not change. P-i-n diodes were obtained with the p-layer deposited from TMB.

Persistent photoconductivity in undoped a-Si:H/a-SiC:H multilayers

Abstract Persistent photoconductivity (PPC) was observed in a-Si:H/a-Si 1− x C x :H (hydrogenated amorphous silicon and hydrogenated amorphous silicon-carbon alloys) multilayers. The influence of bilayer thickness, band gap, wavelength of the light, duration of illumination and temperature on this phenomenon was studied. PPC and photoconductivity increase when the layer thickness increases, which has been attributed to interface recombination. PPC also increases when the carbon content of a-Si 1− x C x :H increases. PPC for samples with low carbon content exhibits a decrease vs. illumination time attributed to the light-induced created defects. The temperature dependence of PPC presents an activated behaviour.

Fast Degradation with Pulsed Light of a-Si:H P-I-N Photodiodes

Amorphous Silicon p-i-n photodiodes were obtained by PECVD in a reactor with a rotating substrate holder. Reverse currents as low as 5 × 10 −11 A/cm 2 at a bias of − 2V were measured using a guard ring electrode to minimize lateral edge currents. The devices were degraded by a Xenon flash lamp in open circuit conditions. The kinetics of the degradation process was evaluated by studying the long time dark current transient under reverse bias.

Characterization of Amorphous Silicon Films through Infrared Spectroscopy and Hydrogen Thermal Effusion

Samples of pure and alloyed a:Si-H have been characterized by hydrogen thermal effusion conjointly to infrared absorption spectroscopy, The following four points have been studied; (1) total and partial H contents, i.e. subdivided into covalently bonded, IR-active H and not-covalently bonded, IR-inactive H, (2) Detection of weakly bonded H, (3) H preferential attachment in alloyed networks, (4) H thermal stability.