F. Evangelisti

Dark J‐V characteristic of p‐i‐n a‐Si:H solar cells

The J‐V characteristics of p‐i‐n solar cells in the dark have been simulated by solving numerically the full set of transport equations. We have analyzed the dependence of dark current on the density of gap states in the intrinsic layer and on the presence of interface states between the p and i layers. The dependence on the thickness of the intrinsic layer has also been investigated and we have found that the agreement with the experimental data is possible only in the presence of interface states. A simplified model which qualitatively explains the results of the complete simulation has been developed. We have also investigated the changes of J‐V characteristics with measurement temperature obtaining results in good agreement with the experimental data without introducing any tunneling mechanism.

Photoemission studies of amorphous semiconductor heterojunctions

Abstract Recent results on amorphous semiconductor heterojunctions are reported. The main focus is on the discussion of valence band discontinuities between amorphous silicon and silicon-based alloys. It is suggested that the compositional disorder is responsible for the negligible discontinuities found.

Influence of hydrogen dilution on the optoelectronic properties of glow discharge amorphous silicon carbon alloys

This work reports the influence of diluting with hydrogen the gaseous mixture of methane and silane on the electrical and optical properties of silicon–carbon alloys deposited by glow discharge. Impressively high secondary photoconductivities, low density of gap defects states and Urbach energy parameter were found in materials prepared in suitable hydrogen dilution conditions. Experiments on temperature dependence of dark and under white light illumination conductivity are also reported and interpreted. Depending on the dilution and substrate temperature, the Fermi level shift toward the conduction band, suggesting a combined effect of low density of defects and sensitization as the causes of the high photoconductivities. IR studies show that appropriate hydrogen dilution and substrate temperature prevent the silicon atoms from forming high order hydrides. Consequently, a more compact structure with better electronic properties is obtained.

EXAFS investigation of amorphous-to-crystal transition in Ge

Abstract The structure of a-Ge as a function of the deposition temperature Ts has been studied by the EXAFS technique for the first time. The results demonstrate the sensitivity of EXAFS to structural differences in the medium range. The data provide strong evidence for a continuous transition from the amorphous to the crystalline phase over a temperature interval form 130°C to 300°C. The analysis of the data is in line with the microcrystal model of amorphous Ge films and allows an estimate of the average grain size for the lower range Ts used.

Gap states in a-Si:H by photoconductivity and absorption

Abstract The spectral dependence of photoconductivity in a series of a-Si:H samples has been analyzed in order to derive the absorption coefficient α. It has been found that the exponent β which relates photoconductivity and generation rate varies with photon energy and chopping frequency. Its critical influence upon the value of α is established. The Urbach tail in the region 1.4–1.8 eV has been studied and found to be independent of extrinsic gap states and hydrogen content. Finally an inverse correlation has been determined between the photoconductivity lifetime and the extrinsic gap-state density, as measured from the corresponding integrated area of the absorption coefficient.

Experimental reflectivity spectra and additional boundary conditions in CdS

Abstract Reflectivity spectra of the A ( n = 1) exciton in CdS samples with various surface conditions are compared with those calculated using three different “additional boundary condition” (a.b.c.). It is found that, contrary to expectations, no conclusion can be drawn about the validity of the different a.b.c., because the surface interacts with the exciton in a complicated manner not accounted for by theories.

Infrared and optical study of a-SiN alloys☆

Abstract a-Si1−xNx:H alloys produced by the GD decomposition of SiH4+NH3+H2 have been studied by infrared and visible spectroscopy. By interpreting the strength of the two IR bands at 840 and 970 cm−1 in the frame of a statistical model, we found that on the average N is bonded to Si in trigonal sites, and that for x>0.20 stoichiometric clusters appear in the lattice. The optical gap parallels this structural modification, showing a linear increase in the region x=0.00–0.20 followed by a steeper increase at larger nitrogen content.

On the intensity dependence of the photoconductivity in a-Si:H

Abstract The recombination kinetics of the steady-state photoconductivity is investigated. It is found that the occurence of kinetics intermediate between monomolecular and bimolecular cannot be explained in terms of a movement of the quasi-Fermi level in an exponential distribution of trapping states. A model is proposed that quantitatively predicts the dependence of the photoconductivity on the gap-state density as well as the experimental values of the exponent β which relates photoconductivity and generation rate.

Infrared analysis of a-Ge1−xNx:H

Abstract Infrared analysis of non-stoichiometric hydrogenated germanium-nitrogen alloys (a-Ge 1−x N x :H) is presented. The films were prepared by rf glow discharge and rf sputtering using various gases. The infrared spectra show different features depending on the deposition method and conditions. In the samples prepared by rf sputtering in an Ar + NH 3 atmosphere, the presence of Nue5f8H 2 bonding groups were observed. The absorption band for the Nue5f8H 2 bending vibration is at 1435 cm −1 . The stretching mode is composed of at least four bands in the 2800 to 3600 cm −1 range. The same samples, in addition, have an absorption band at 545 cm −1 which we attributed to the Geue5f8NH 2 vibrations. The samples prepared by glow discharge at a substrate temperature of 230°C in a GeH 4 + NH 3 atmosphere do not contain Nue5f8H 2 absorption bands.

Atomic and electronic structure of a-Si1−xCx:H alloys

Abstract The atomic and the electronic structure of a-Si1−xCx:H alloys are discussed with the emphasis on the information that can be obtained from structural probes like EXAFS and X-ray scattering and with electronic probes like photoemission and Auger spectroscopies.