In this work we investigate operation in the Geiger mode of the new single photon avalanche photo diode (SPAD) SAP500 manufactured by Laser Components. This SPAD is sensitive in the range 400-1000 nm ...and has a conventional reach-through structure which ensures good quantum efficiency at the long end of the spectrum. By use of passive and active quenching schemes we investigate detection efficiency, timing jitter, dark counts, afterpulsing, gain and other important parameters and compare them to the "standard" low noise SPAD C30902SH from Perkin Elmer. We conclude that SAP500 offers better combination of detection efficiency, low noise and timing precision.
We prepared the anatase nanotubes (NT) and other nanostructured titania as electron accepting/transmitting layers in solar cells, using titanium anodization. Upon gaining control over the ...anodization, the parameters were held constant in order to observe the role of the deposited layers (by electron beam evaporation and magnetron sputtering) on the NT yield. The structural and microstructural parameters were investigated using FIB-FEG-SEM, XRD, Raman and GIXRD. Differences in the titanium layers play a significant role on the type of titania nanostructures achieved. Only dense homogeneous titanium layer surface, uniform in thickness and without cracks at macroscale, with uniformly seized isotropic nanoparticles, will enable uniform electrochemical etching and thus favourable, reproducible formation of the titania NT, and upon thermal treatment the anatase NT. Transparent conductive oxide (TCO) layers, which are not in direct contact to the NT, can also exhibit influence on the morphology of the titania NT charge transfer layer. In order to reach large uniform areas of nanotubes, which is required for solar cell application, the preparation of titanium layer has to be tailored.
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•Nanostructured anatase was considered for buffer layers in solar cell devices.•Observed different Ti morphologies also show different susceptibility to anodization.•Role of Ti thin film deposition on yield of ordered anatase nanotubes was shown.•TCO layer can also exhibit some influence on charge transfer layer morphology.•Use of most favourable assembly should enable improved DSSC or PSC.
► Amorphous-nanocrystalline silicon thin films were prepared by PECVD. ► Silicon nanocrystalls embeded in amorphous silicon matrix. ► The in-depth distribution of hydrogen atoms was estimated by ...TOF-ERDA. ► Non uniform distribution of hydrogen across the depth with maximum close to a-Si:H/a-nc-Si:H interface.
The in-depth distribution of hydrogen atoms in 100nm-thick, amorphous-nanocrystalline, silicon films (a-nc-Si:H) was estimated by time-of-flight elastic recoil detection analysis (TOF-ERDA) using a previously described set-up. The layer with nanocrystals was deposited on a 50nm amorphous layer by plasma-enhanced chemical vapor deposition (PECVD), using silane gas that was diluted with hydrogen. High-resolution transmission electron microscopy (HRTEM) showed that the films contained nanocrystals of silicon embedded in an amorphous Si:H matrix. The size of the nanocrystals and the crystal-to-amorphous ratio increased in the direction from the substrate toward the surface of the film. The amorphous matrix appeared uniform, except for the area close to the a-Si:H/a-nc-Si:H interface, where spots that were brighter than average appeared. These areas can be attributed to the presence of less-dense material, presumably voids. It is assumed that the surface of the voids is “decorated” with hydrogen that saturates the silicon “dangling bonds”. This is why the distribution of hydrogen should indicate a density fluctuation in the material. Consistent with this assumption, the TOF-ERDA showed a non-uniform distribution of hydrogen across the depth, with a maximum value close to the a-Si:H/a-nc-Si:H interface that coincides with the less-dense material seen by the HRTEM. This supports the idea about the important influence of voids on crystal formation, particularly in the nucleation phase. After a heat treatment at 400°C, the distribution of hydrogen remained the same, while the total hydrogen concentration decreased. This indicated that the type of hydrogen bonding was the same across the amorphous network and assumed that the areas of less-dense material are agglomerates of smaller voids.
A two-step procedure for the formation of silver nanoparticles embedded in a glass matrix is studied. The procedure consists of: (i) the inclusion of silver ions in the glass matrix by electric field ...assisted dissolution of Ag film deposited on the glass and (ii) the aggregation of silver by thermal annealing. The optical properties of the sample, dominated by the surface plasmon resonance of metal nanoparticles in the visible spectral range, are studied by optical spectroscopy. The structural characterization is carried out by grazing-incidence small-angle X-ray scattering measurements performed at the Synchrotron Elettra (Italy) and silver depth profiles are determined using Rutherford backscattering. The results suggest that the depth profile of Ag might be tailored by modification of the parameters of metal film dissolution (electric field and temperature). Variation of thermal annealing parameters (temperature and time) allows control of the nanoparticles size. Thus, the surface plasmon absorption intensity and line shape are changed, enabling tuning of the optical properties of the sample.
Titanium dioxide thin films were deposited by filtered cathodic arc evaporation (FCAE) from a Ti target in an oxygen atmosphere onto (a) fluorine-doped tin oxide substrates SnO
2
:F (FTO) and (b) ...glass microscope slides. The growth rate calculated from film thickness profilometry measurements was found to be approximately 0.8 nm/s. The films were highly transparent to visible light. x-Ray photoemission spectroscopy analysis of the Ti 2p electron binding- energy shift confirmed the presence of a TiO
2
stoichiometric compound. The results for the root-mean-square (RMS) surface roughness of the films deposited onto FTO substrates evaluated by atomic force microscopy suggested nanostructured film surfaces. When exposed to hydrogen plasma, TiO
2
films revealed insignificant changes in the optical spectra. The initial sheet resistance of the SnO
2
:F layer was 14 Ω/sq. The deposition of the top TiO
2
layer (45 nm thick) over the FTO electrode resulted in an increase of the sheet resistance of 2 Ω/sq. In addition, the sheet resistance of the double-layer FTO/TiO
2
transparent conductive oxide (TCO) electrode increased by 1 Ω/sq as a result of H
+
plasma exposure. Regardless of the TiO
2
film’s low conductivity, a thin protective layer could be coated onto FTO films (presumably 15 nm thick) due to their high transparency, offering high resistance to aggressive H
+
plasma conditions. In this paper we show that ∼50-nm-thick TiO
2
coating on FTO films provides sufficient protection against deterioration of transparency and conductivity due to hydrogen radical exposure.
A representative set of amorphous–nano-crystalline Si thin films was deposited by radio-frequency plasma enhanced chemical vapor deposition using silane highly diluted by hydrogen. By Raman ...spectroscopy it was found that the variation of silane to hydrogen ratio resulted in films with crystal fraction between 0 and 55vol.% and individual crystal sizes between 2 and 20nm with bi-modal, broad size distribution. High resolution transmission microscopy, done on certain number of samples, confirmed the nano-meter size of crystallites and bi-modal size distribution. The optical properties measured by Fourier transform photocurrent spectroscopy and photo thermal deflection spectroscopy correspond to the material with structure between amorphous and crystalline. The spectral distribution of relative quantum efficiency of photovoltaic solar cell made from this material shows ‘blue shift’ with increase of crystal to amorphous fraction. This result is discussed as a possible consequence of quantum effects accompanied with actual size and size distribution of crystals.
Thin film structures consisting of nano-crystalline and amorphous silicon layers deposited on glass by plasma enhanced chemical vapour deposition have been studied by optical spectroscopy methods ...(transmittance, photo-thermal deflection spectroscopy and photo-current spectroscopy) while structure was examined by Raman spectroscopy. The nano-crystalline layers were grown on the same amorphous layers, using different radio-frequency (RF) discharge powers, leading to different structural and optical properties. The energy dependence of the absorption coefficient above the band gap agrees well to the bimodal size distribution of crystals and crystal fraction estimated by Raman spectroscopy. For energies below the band gap, the comparison of the absorption of the bi-layer systems with respect to single amorphous layer reveals that the samples produced at higher RF discharge present a higher disorder degree (Urbach edge increases) and higher number of structural defects (absorption related to the defects increases).
Thin Si films, with thickness between 100 and 300 nm, were deposited by PECVD (Plasma Enhanced Chemical Vapour Deposition) in silane gas (SiH
4) highly diluted by hydrogen. The degree of dilution and ...the discharge power were varied in order to obtain different crystalline to amorphous fractions in the films. Two types of substrates were used. The first one was amorphous and relatively flat while the second one was polycrystalline with a roughness of a few tens of nanometers. The crystal fraction in the deposited samples, as estimated by Raman spectroscopy, varied between 0 and 40%, and the individual crystal size was between 2 and 8 nm. The larger individual crystals are usually present in those samples with the highest crystal fraction. The sample density, estimated upon the spectral distribution of the dielectric function in the infra red, was 15–25% less than the density of crystalline silicon. The GISAXS pattern of all of the examined samples indicated the presence of not-spherical-like “particles” in the “bulk” of the thin films, with an average “particle” size between 1.5 and 4 nm. These “particles” are most probably voids and their shape indicates columnar growth. By applying the GISAXS technique on samples deposited on different substrates, the borderline deposition conditions between “transport limited growth” and “growth dominantly influenced by plasma surface reactions” was estimated.
Due to lack of long-range ordering, the density of amorphous silicon is lower than in its crystalline form. By hydrogenation of material, the density difference becomes larger. In order to study the ...effect quantitatively, a series of thin amorphous hydrogenated silicon films, with variation in hydrogen to silicon ratio from 6 to 31 at%, deposited by magnetron sputtering, was examined by UV–visible–IR spectroscopy and nuclear methods.
The film density of deposited films was estimated using three different approaches. In the first one, the density was estimated by using effective medium approximation (EMA) analysis of a long wavelength dielectric function. In particular, the basic and modified Maxwell–Garnett models were tested. The second set of values for density was obtained by analysis of stretching vibrations of Si–H bonds in the IR part of the spectrum, by using an earlier published method. The results of all methods applied are compared and their plausibility is discussed.
Amorphous hydrogenated silicon (a-Si:H) with high hydrogen content (10–40
at.%), and non-stehiometric silicon–carbon (Si
1−
x
C
x
) thin films with a variation of the carbon to silicon ratio up to ...0.3, were deposited by using a magnetron sputtering source. The Si
1−
x
C
x
thin films were partially crystallised after deposition by thermal annealing up to 1050
°C.
The GISAXS (Grazing Incidence Small Angle X-ray Scattering) spectra of all of the prepared specimens indicate the presence of “particles” in the “bulk” of the films. For the a-Si:H samples, “particles” are most probably voids agglomerates with a variation in size between 3 and 6
nm. The mean value of the size distribution of the “particles” increases while its width slightly decreases with the hydrogen content in the film. This indicates a better structural ordering which is consistent with the results of Raman spectroscopy that show a decrease of the ratio between intensities of transversal acoustic (TA) and transversal optic (TO) phonon peaks,
I
TA/
I
TO, and a narrowing of the TO peak with increasing hydrogen content. These results are discussed as a consequence of the beneficial influence of hydrogen bombardment during the film growth.
For Si
1−
x
C
x
thin films, the “particles” are assumed to be SiC nano-crystals with a size between 2 and 14
nm and they are larger in films with a higher carbon concentration. Inside each of the films, the crystals are larger closer to surface and they grow faster in the direction parallel to the surface than in that which is perpendicular to it.
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