Theoretical evaluation of stability and efficiency of two-junction two-terminal amorphous tandem solar cells is performed using an analytical model which takes into account not only optical characteristics and transport properties but also photodegradation effects due to dangling bond formation under light exposure. Requirements for material quality and device design are particularly investigated.

The fabrication and characterization of near infrared photodetectors integrated on silicon substrates are reported on where the active layer is a thermally evaporated polycrystalline germanium. Recent results are presented in the effort to enhance the optoelectronic properties of the poly-Ge film in terms of uniformity for multiple device integration, speed and responsivity. In particular we demonstrate a 16 pixel linear array, a speed of photoresponse of about 650 ps and an enhancement of responsivity by a factor of four. The fabrication process, including substrate cleaning and preparation, requires temperatures lower than 300°C being fully compatible with silicon technology.

We report on a novel solid-state spectrum analyzer in the near infrared. The device is an array of six photodetectors based on polycrystalline germanium film evaporated on a silicon substrate and each element is a wavelength-selective detector. We describe the fabrication and characterization of such device and we demonstrate its capability both as a wavelengthmeter for quasi-monochromatic light beams and as a spectrum analyzer.

Dynamic behavior of photogenerated carriers in diamond-based UV photodetectors is investigated over a wide excitation frequency range, enabling an analysis of the influence of film morphology and impurity content on device response times. Under pulsed light excitation, short time detector photoresponse varies from 2.5 to 10 ns, whereas carrier lifetimes estimated under steady-state illumination lie in the 0.1-1 ns range, exhibiting a small dependence on the film microstructure. Conversely, very long response times, strongly dependent on film characteristics, are detected by decreasing the excitation frequency. Such results are discussed in terms of carrier recombination at defect- and impurity-related centers, trapping at localized states close to the band edges, and dispersive transport. It is suggested that device response times are mainly related to charge trapping either into discrete or continuously distributed energy levels, rather than to recombination of carriers at midgap defect states.

Nanocrystalline silicon formation by CW laser-irradiation of amorphous silicon–oxygen alloys of variable composition has been investigated as a function of both laser power density and alloy composition. Structural changes during the annealing treatment were monitored ‘in situ’ by micro Raman spectroscopy. Careful Raman lineshape analysis in the 450–550 cm−1 range allowed the evaluation of Si nanocrystal size distribution. It is shown that depending on the size of the nanocrystallites, irradiation can yield to a strong photoluminescence (PL) enhancement or quenching.

We report the fabrication of efficient near-infrared photodetectors based on polycrystalline Ge films on Si (100). In view of integration with Si electronics, metal-semiconductor-metal photodiodes have been realized, for the first time to the best of our knowledge, by low-temperature evaporation. The experiments performed on non-optimized devices indicate a substantial sensitivity at both 1.32 and 1.55 μ m, with a responsivity as high as l6mA W−1 at 1.32 μm with 0.2 V bias, and a time response of about 4ns.

Self-supported CVD diamond films having coplanar contacts spaced down to 100 mu m have been characterized in the temperature range 300-600 K by electrical measurements in the dark and under illumination at lambda = 220 nm. Up to 600 K the dark conductivity values are smaller than 10(-9) S cm(-1) and spectrally resolved photocurrent yield measurements in the range 200-800 nm exhibit more than four orders of magnitude discrimination between intrinsic (below 230 nm) and extrinsic (above 230 nm) photoresponses. A thermal activation of the UV photoresponse is also observed, giving an average temperature coefficient for intrinsic photoresponses around 1%/degrees C. (C) 2000 Elsevier Science S.A. All rights reserved.

Transient photoresponse of CVD diamond-based detectors has been studied in the time domain 10−9s–103s by both chopped-light and pulsed laser excitations. A broad distribution of response times over the whole investigated range is observed, related to the quasi-continuous distribution of gap states induced by structural defects and impurities. Very slow components between 1 and 103s also reflect non-diamond phase content and can be modified by surface treatments. Response times in the range 100ns–100ms generally corresponds to trapping processes at relatively deep states, but in very defected material they can be correlated to dispersive transit time contribution, as observed in amorphous materials. Under 7ns pulsed excitation decay times shorter than 3ns are observed in fastest detectors. Such times seem related to trapping processes at shallow states rather than to recombination at midgap states, which involves characteristic times shorter than 1ns.

Stress distribution and diamond phases in polycrystalline diamond film have been investigated by micro-Raman spectroscopy. Intensity, lineshape and peak position of the diamond Raman line largely change on the film surface, particularly if grain center and grain boundary regions are compared. Whereas in the center of the grains the characteristic diamond Raman line is observed, close to grain boundaries an additional feature at 1326 cm(-1) is detected. Such a Raman component may be related to the presence of hexagonal microphases formed by a defect-induced symmetry modification of the diamond lattice. Grain boundaries appear also to be affected by a strongly anisotropic stress, which induces a frequency shift and splitting of the Raman peak. Stress configurations compatible with the experimental observations are proposed and discussed.

We report on Ge Schottky photodiodes epitaxially grown on Si (100) exhibiting a good responsivity in the near infrared up to 1.55 μm. The Ge epitaxial layers, grown by ultrahigh vacuum chemical vapor deposition, were deposited in two steps differing by the substrate temperature. With this procedure it was possible to obtain films thicknesses comparable with light penetration depth in the 1.3–1.6 μm range. The photodiodes have a photocurrent which increases as a function of voltage bias, reaching a maximum responsivity of 0.12 A/W at 1.3 μm under a reverse bias of 4 V. The leakage current density at the saturation voltage is 1 nA/μm2. The results show that the proposed approach is promising for the fabrication of 1.3–1.55 μm near-infrared photodetectors integrated on silicon chips.

A study is presented on minority carrier transport parameters, in particular mobility–lifetime products, in natural and CVD diamond materials with different morphology. μτ products are obtained by photocurrent versus field measurements and their values range between 10−8–10−5 cm2V−1 depending on grain size and orientation. Observed variations are mainly related to mobility changes, while minority carrier lifetimes are almost constant around 10−10–10−9 s. The origin of such a behaviour is analysed in terms of gap state distribution arising from structural defects, impurities and non-diamond microphases.

We report on the fabrication of a detector array for the near infrared on silicon substrate. Thermally evaporated polycrystalline germanium is used as the active layer in the device which consists of 16 pixel with dot-pitch of about 100 micron; the single pixel has a metal-semiconductor-metal structure. We demonstrate a responsivity of 16mA/W at 1.3 micron and extending down to 1.55 micron. At the same wavelength an operation speed in the nanoseconds range is demonstrated. The overall fabrication process, including substrate cleaning and preparation, requires temperatures lower than 350 degrees C being fully compatible with silicon electronics.

Transient photoresponse of CVD diamond-based detectors has been studied in the time domain 10(-9) s-10(3) s by both chopped-light and pulsed laser excitations. A broad distribution of response times over the whole investigated range is observed, related to the quasi-continuous distribution of gap states induced by structural defects and impurities. Very slow components between 1 and 10(3) s also reflect non-diamond phase content and can be modified by surface treatments. Response times in the range 100 ns-100 ms generally correspond to trapping processes at relatively deep states, but in very defected material they can be correlated to dispersive transit time contribution, as observed in amorphous materials. Under 7 ns pulsed excitation decay times shorter than 3 ns are observed in fastest detectors. Such times seem related to trapping processes at shallow states rather than to recombination at midgap states, which involves characteristic times shorter than 1 ns. (C) 1999 Elsevier Science S.A. All rights reserved.

Photoluminescence data are presented for diamond films grown by different techniques (hot-filament CVD, microwave PECVD, DC arc-jet) and excited in the range 450–650 nm by different laser sources. In large-grain textured samples, sharp vibronic, structures, related to nitrogen and silicon impurity centers, are clearly observed, whereas in small-grain, randomly oriented films spectra are dominated by broad luminescence bands, whose maxima move with excitation energy. In the former case a careful deconvolution of emission spectra allows us to obtain vibronic parameters of the centers and their fine structure details, while excitation spectra give the energy location of higher electronic excited states. On the other hand, the line-shape and the excitation-dependent shift of broad luminescence bands are related to a continuous distribution of gap states, able to trap photogenerated electron-hole pairs.

Electrical and optoelectronic characterization of p–i–n devices based on a-SiOx:H thin film alloys is presented. It is shown that near infrared electroluminescence can be obtained for applied fields higher than a typical threshold depending on the p/i and n/i interface structure. Electroluminescence (EL) intensity under AC excitation is independent of frequency up to 20kHz and exhibits rise and decay times of about 10μs. Optical properties of a-SiOx:H materials allow the integration of LED devices and optical waveguides, which can be coupled to planar silicon detectors for obtaining Si-based optoelectronic circuits.

A novel one-dimensional array of photodetectors for the near infrared up to 1.55 /spl mu/m is reported. The device is based on polycrystalline Ge thermally evaporated on silicon, and consists of 16 pixels at a 100 /spl mu/m pitch. A responsivity of 16 mA/W was measured at 1.3 /spl mu/m with nanosecond response time.

Due to the ever growing use of optical fibers in communications, for both long distances and local area networks, there is an increasing effort toward the realization of high speed and high efficiency optical detectors, operating in the low-absorption regions (1.3-1.55µm) of silica fibers. Although III-V semiconductors provide high detection efficiency in the range of interest, incorporating them in the well-established Si-based very large scale integration technology is difficult and expensive[ 1 ]. Ge-on-Si structures represents a viable alternative solution for near infrared photodetection, owing to their narrow band gap and full compatibility with silicon technologies. Pure germanium, indeed, represents the best candidate for absorbing in 1.3-1.55 µm photodetectors, due to its direct bandgap of 0.85eV. However, owing to the high lattice mismatch with Si, it is not easy to obtain Ge films with characteristics suitable for integrated electronics: thickness (efficiency of the device), flatness (realization of submicron lithography), defect-free structure (high-speed).

Carrier injection mechanism, electroluminescence properties and stability of p-i-n thin film light emitting diodes (TFLED) based on amorphous silicon-oxide are investigated for various intrinsic layer thickness and oxygen percentage. It is suggested that multistep tunneling across n-i and p-i barriers through localized tail states is the main injection mechanism in TFLED, occurring at different bias values for each type of carrier. Such a mechanism can explain the existence of a threshold voltage for the appearence of electroluminescence and the dependence of EL intensity on current. Results on TFLED long term stability are also presented and discussed in the frame of a simple degradation model.

The role of localized defect centers and continuous gap states distribution in photoluminescence, photoconductivity, and photoresponse time dependence of diamond films have been analyzed for different film morphology and grain orientations. Sharp spectroscopic features related to impurity centers or broad components associated with continuous distribution of gap states prevail in the spectra depending on film microstructure and deposition technique. It is shown that defects, either localized or continuously distributed, reduce carrier lifetimes and slow down carrier transport by trapping effects. The observation of metastability effects after UV illumination giving an increase of subgap photoresponse is also reported.