Silicon films were deposited at moderate substrate temperatures (280-500 o C) from pure silane and a silane-hydrogen mixture (10% SiH 4 , 90% H 2 ) in a hot wire CVD reactor. The morphology, structure and composition of the samples were studied with Scanning Electron Microscopy, Transmission Electron Microscopy, Transmission Electron Diffraction, X-ray diffraction, Raman-spectroscopy and Secondary Ion Mass Spectrometry. The sample deposited at 500 o C with pure silane has an amorphous structure, whereas the samples obtained from silane diluted in hydrogen have a polycrystalline structure even that grown at the lowest temperature

Structural and optical characterization of copper phthalocyanine thin film thermally deposited at different substrate temperatures was the aim of this work. The morphology of the films shows strong dependence on temperature, as can be observed by atomic force microscopy and x-ray diffraction spectroscopy, specifically in the grain size and features of the grains. The increase in the crystal phase with substrate temperature is shown by x-ray diffractometry. Optical absorption coefficient measured by photothermal deflection spectroscopy and optical transmittance reveal a weak dependence on the substrate temperature. Besides, the electro-optical response measured by the external quantum efficiency of Schottky ITO/CuPc/Al diodes shows an optimized response for samples deposited at a substrate temperature of 60ºC, in correspondence to the I -V diode characteristics.

In this paper we present new results on doped c-Si:H thin films deposited by Hot-Wire Chemical Vapour Deposition (HWCVD) in the very low temperature range (125-275 ºC). The doped layers were obtained by the addition of diborane or phosphine in the gas phase during deposition. The incorporation of boron and phosphorus in the films and their influence on the crystalline fraction are studied by Secondary Ion Mass Spectrometry and Raman Spectroscopy respectively. Good electrical transport properties were obtained in this deposition regime, with best dark conductivities of 2.6 S/cm and 9.8 S/cm for the p-and n-doped films respectively. The effect of the hydrogen dilution and the layer thickness on the electrical properties are also studied. Some technological conclusions referred to cross contamination could be deduced from the nominally undoped samples obtained in the same chamber after p-and n-type heavily doped layers.

The possible use of poly(ethylene naphthalate) as substrate for thin silicon solar cells has been studied in this paper. The transparency of this polymer makes it a candidate to be used in both substrate and superstrate configurations. ZnO:Al has been deposited at room temperature on top of PEN. The resulting structure PEN/ZnO:Al presented good optical and electrical properties. PEN has been successfully textured (nanometer and micrometer random roughness) using Hot-Embossing Lithography. Reflector structures have been built depositing Ag and ZnO:Al on top of the stamped polymer. The deposition of these layers did not affect the final roughness of the whole. The reflector structure has been morphologically and optically analysed to verify its suitability to be used in solar cells.

Spectroscopic ellipsometry and high resolution transmission electron microscopy have been used to characterize microcrystalline silicon films. We obtain an excellent agreement between the multilayer model used in the analysis of the optical data and the microscopy measurements. Moreover, thanks to the high resolution achieved in the microscopy measurements and to the improved optical models, two new features of the layer-by-layer deposition of microcrystalline silicon have been detected: ͑i͒ the microcrystalline films present large crystals extending from the a-Si:H substrate to the film surface, despite the sequential process in the layer-by-layer deposition; and ͑ii͒ a porous layer exists between the amorphous silicon substrate and the microcrystalline silicon film. © 1996 American Institute of Physics. ͓S0003-6951͑96͒02730-1͔In situ ellipsometry is a powerful nondestructive technique providing detailed information on the growth mechanisms and optical properties of thin films.1 In order to get information on the composition of a film, the real ͗⑀ 1 ͘ and imaginary ͗⑀ 2 ͘ parts of the pseudodielectric function of the system film plus substrate, deduced from the ellipsometric angles ⌬ and ⌿, are compared to those of an optical model based on Bruggeman's effective-medium theory. 2 The differences between the measured and calculated data are minimized by a linear regression analysis. In recent years we have applied this technique to study the layer-by-layer deposition of microcrystalline silicon films.3 In this method, microcrystalline silicon films are obtained by alternating the deposition of hydrogenated amorphous silicon ͑a-Si:H͒ during a time T Si with its exposure to a hydrogen plasma during a time T H . The major conclusions deduced from these studies are: ͑i͒ the nucleation of a crystalline phase within the a-Si:H network takes place once the initially dense a-Si:H film has been converted into porous a-Si:H by the hydrogen plasma exposure, 4 ͑ii͒ the crystallization of the a-Si:H film deposited during the time T Si is related to the diffusion of hydrogen, leading to nanovoid and broken bond formation processes, 5 and ͑iii͒ there is a substrate dependence of the long term evolution of the properties of the already deposited films. 6 Because most of our previous results were based on in situ ellipsometry and because of the increasing complexity of optical models used to fit the experimental data, we have performed an independent validation of the optical models by high resolution transmission electron microscopy ͑HRTEM͒ measurements. An excellent agreement between the results of the optical models and the HRTEM measurements has been achieved, in agreement with previous reports.7 Moreover, the HRTEM measurements have allowed us to improve the optical models and reveal new features of the layer-by-layer deposition of c-Si.Microcrystalline silicon films were codeposited on different substrates by the layer-by-layer technique. Figure 1 shows the experimental and calculated real and imaginary parts of the pseudo...

The crystallization enthalpy of pure amorphous silicon (a-Si) and hydrogenated a-Si was measured by differential scanning calorimetry (DSC) for a large set of materials deposited from the vapour phase by different techniques. Although the values cover a wide range (200 -480 J/g), the minimum value is common to all the deposition techniques used and close to the predicted minimum strain energy of relaxed aSi (240 ± 25 J/g). This result gives a reliable value for the configurational energy gap between a-Si and crystalline silicon. An excess of enthalpy above this minimum value can be ascribed to coordination defects.In contrast to glasses, whose lower energy states can be accessed by either cooling the liquid at lower rates or by thermal annealing [1], the energy of amorphous tetrahedral semiconductors must be lowered by thermal annealing [2]. Among them, amorphous silicon (a-Si) has been extensively studied due to its technological relevance and because it is usually taken as a model material for covalent amorphous networks.We have recently shown [3] that after thermal annealing, the rms deviation from the tetrahedral angle, Δθ, is approximately 9° for a broad range of pure and hydrogenated a-Si materials. This "relaxed state" of pure a-Si (no H-atoms remain after annealing) gives support to the theoretical prediction [4] that it is not possible to build stable amorphous models below minimum bond-angle dispersion. In other words, there is a 'configurational gap' between a-Si and c-Si that ensures higher entropy in the amorphous state. This discontinuity makes it impossible for the material to evolve from one state to the other by continuously varying its short-range order.The bond-angle dispersion entails a built-in strain energy that is released during crystallization [5] and currently detected by differential scanning calorimetry (DSC) as an exothermic peak [6]. By measuring the crystallization enthalpy, ΔH cryst , we can, thus, determine the value of the 'configurational energy gap' between a-Si and c-Si. To date, the only systematic measurements of ΔH cryst have been taken from a-Si obtained by ion implantation of c-Si [6] (measurements on hydrogenated thin films being very scarce [7]). Since the experimental values felt within a narrow range (see the right side of Fig. 1), it is generally accepted that they correspond to the minimum energy of a-Si.However, since this method of synthesis is very far from equilibrium, there is good reason to suspect that, even after thermal annealing, the material has not reached its lowest energy level. In fact, we suggested [8] that one half of ΔH cryst of this material was due to high concentration of coordination defects and, consequently, predicted that the crystallization enthalpy could be reduced by this factor in defect-free materials.Recent studies of organic glasses [9] have shown that, thanks to a higher molecular mobility at the film free surface [10], deposition from the vapour phase is a way to achieve low-energy amorphous states. This behaviour could also apply ...

In this work, we have studied the texturization process of (100) c-Si wafers using a low concentration potassium hydroxide solution in order to obtain good quality textured

This work studies the use of polymeric layers of polyethylenimine (PEI) as an interface modification of electron-selective contacts. A clearly enhanced electrical transport with lower contact resistance and significant surface passivation (about 3 ms) can be achieved with PEI modification. As for other conjugated polyelectrolytes, protonated groups of the polymer with their respective counter anions from the solvent create an intense dipole. In this work, part of the amine groups in PEI are protonated by ethanol that behaves as a weak Brønsted acid during the process. A comprehensive characterization including high-resolution compositional analysis confirms the formation of a dipolar interlayer. The PEI modification is able to eliminate completely Fermi-level pinning at metal/semiconductor junctions and shifts the work function of the metallic electrode by more than 1 eV. Induced charge transport between the metal and the semiconductor allows the formation of an electron accumulation region. Consequently, electron-selective contacts are clearly improved with a significant reduction of the specific contact resistance (less than 100 mΩ·cm2). Proof-of-concept dopant-free solar cells on silicon were fabricated to demonstrate the beneficial effect of PEI dipolar interlayers. Full dopant-free solar cells with conversion efficiencies of about 14% could be fabricated on flat wafers. The PEI modification also improved the performance of classical high-efficiency heterojunction solar cells.