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The ultrafast reflectivity of silicon, excited and probed with femtosecond laser pulses, is studied for different wavelengths and energy densities. The confinement of carriers in a thin surface layer delimited by a nanoscale Si-layered system buried in a Si heavily-doped wafer reduces the critical density of carriers necessary to create the electron plasma by a factor of ten. We performed two types of reflectivity measurements, using either a single beam or two beams. The plasma strongly depends on the photon energy density because of the intervalley scattering of the electrons revealed by two different mechanisms assisted by the electron–phonon interaction. One mechanism leads to a negative differential reflectivity that can be attributed to an induced absorption in X valleys. The other mechanism occurs, when the carrier population is thermalizing and gives rise to a positive differential reflectivity corresponding to Pauli-blocked intervalley gamma to X scattering. These results are important for improving the efficiency of Si light-to-electricity converters, in which there is a possibility of multiplying carriers by nanostructurization of Si.

A nanoscale layer of amorphized silicon is obtained by implantations with silicon ions through a P-doped FZ-silicon wafer material few nanometers below the wafer surface. After a controlled annealing, the amorphized silicon material is sandwiched between two layers of recrystallized silicon. Defects remain at the interface c-Si/a-Si/c-Si. Photoluminescence at very low temperature is experimented to determine the energy levels generated by this design. TEM pictures show that some nanocrystalline elements are located close to the interface surrounded by a-Si. However, the photoluminescence spectra do not present any signal of luminescence from them. This could be due to random sizes of nanocrystals. Then, a scan from energies below the silicon bandgap has been realized at 8 K. The spectrum is composed of multiple narrow peaks close to the conduction band and a broadband from 0.78 eV to 1.05 eV. In order to determine the origin of these signals, spectra of three distinct peaks were collected at different temperatures from 8 K to 120 K. The broadband collapses more rapidly by increasing the temperature than the narrow lines and theirs maxima of intensity differ.

Si modifications implemented at the nanoscale lead to optoelectronic and photovoltaic effects that can widen applications of conventional Si devices. The investigation exploits charge carrier and photon flux transformations at a so-called carrier collection limit. Comparison of the collection efficiencies of the same sample with and without a buried nanosystem allows a better understanding of the optical (absorbance) and electronic (carrier collection) behaviors. Experimental evidence for enhanced absorbance of a strained nanoscale Si-layered system has been found.

A characteristic photoluminescence of a superficial Si nanolayer realized by ion implantation has been observed. This effect, being totally independent of those shown recently for a nanoscale Si-layered system, is similar to that produced by Si nanocrystals (Si nc). To visualize the nature and give evidence of this effect, we fabricated samples in two different ways: (i) by incorporation of Si nc into thin SiO2 films deposited on Si wafer by the spin-on-glass method and (ii) by a nanoscale superficial crystalline-Si modification using medium-energy ion implantation and thermal treatment. In both cases the UV-to-red light conversion has been observed to be independent of wafer post-implantation damage. To show the UV-to-red conversion contribution, we use the ion modified superficial Si layer with its well-defined potential barrier, the so-called carrier collection limit. Such a modified Si structure gives us a method of deconvoluting several optoelectronic features observed experimentally on modified Si. ...

The experimental collection efficiency of a nanoscale Si-layered system formed around a buried amorphization is investigated. A self-consistent calculation that takes into account the number of photons removed in the front deactivated zone and those transmitted to deeper layers leads to the result that more than one electron per absorbed short-wavelength photon is collected in the active zone lying below a carrier collection limit. This result suggests that it could be possible to make an efficient third generation Si solar cell based on such a design on the condition that photocarriers generated in the surface zone be collected.

Layered semiconductor structures like delta-dopings and buried amorphizations, where modified optoelectronic features result simultaneously from material composition and from device design, can considerably widen optoelectronic applications of conventional materials. Multi-interface novel devices (MINDs) based on a nanoscale Si-layered system buried within the heavily P-doped Si wafer have an unusual reflection, absorption and internal light propagation, which can be dominated by a dense free-carrier gas confined within a surface potential well. First, a model of optical functions of the heavily doped Si:P using experimental data published previously for extremely heavily P-doped Si using the Transition Matrix Approach (TMA) to simulate the electromagnetic optical response and field propagation has been constructed. The dielectric function combines oscillation functions and a dense free-carrier gas (Lorentz-Drude approach) and can take into account an inhomogeneous P-doping distribution. Next, an optical model of the real multi-interface device, based on electron microscopy data, has been constructed. A simplified sequence of buried, optically active interfaces and corresponding layers (with transformed material and refraction indexes) is possible due to a planar geometry. Finally, we compare our simulated and experimental reflectivity. In this way we could determine particularly difficult-to-measure parameters. The method presented could be useful for device characterization during the fabrication.

A new class of ordered structures that exhibit exceptional properties not readily observed before in nature or in the laboratory is called metamaterials. Their properties arise from qualitatively new response functions that are not observed in the constituent materials and result from the inclusion of artificially fabricated, intrinsic and extrinsic, lowdimensional components. Low-dimensional or nanostructured Si materials as, for example, nanoscale Si-layered systems combined with an active interface with its crystalline defects show new PV behavior never observed before in nature and in engineering. To observe such nanolayers buried within a Si single-crystal one has to conserve a local strain that plays an important role in the metamaterial formation. To do this, one uses techniques based on X-ray spectroscopy or more recently proposed SEM and EDS images of just cleaved edges. The microscopy results of layered structures have been compared with those obtained from reflectivity simulations from our code based on Lorentz-Drude theory and experimental reflectivity measured in integrated hemispheres. An excellent agreement can be observed.

One of the most challenging topics of today's research and development concerns efficiency of light-to-electricity conversion, preferably on Si-derived devices. The best of the possible/imaginable solutions has to allow overcoming the indirect Si bandgap constraints. This aim becomes realizable by transforming the hard photon-matter interaction into a soft photon-electron-electron interaction with additional new low-energy mechanisms to allow a multistage conversion cycle. Such effects have been observed by us within new Si-derived metamaterials obtained by multiple transformations, leading to a nanoscale Si-layered system. In such systems, a giant photoconversion could be observed for the first time due to hot electron interactions with active interfaces and conditioned crystalline defects transformed into Si metamaterial units, called tectons. Today it seems to be the best way to overcome conversion shortages of the bulk, thinfilm or any other Si-based devices. We present in this work a background and three experimental demonstrations of giant photoconversion.

Nanoscale Si-layered systems represent an attractive way to enlarge optical and electrical functions in Si optoelectronic, photonic and PV technology. Physical interactions transform the initial Si material to a new Si-based metamaterial. The device architecture also plays a role in specific nonlinear features. The newly observed behavior requires a better insight into understanding the mechanisms determining the macroscopic performance. We report here some specific electrical properties resulting from the complexity of the electron transport in different test structures designed and manufactured by us. One of the most important parameters concerns state of the device surface. Measurements have been carried out in different conditions of illumination (spectral composition, intensity, with/without optical bias), acquisition mode (duration of acquisition) and device polarization mode (photodiode, photovoltaic). Time-resolved current collection with stabilized voltages, as well as time-resolved voltage variation under stabilized currents, both made under light excitation, allowed observation of extremely long time constants.

Analytic representation of optical function is necessary for the device and structural modeling. Nowadays optoelectronic devices consist of complex materials combined together, therefore accurate representation of each part is even more important and results in prediction of overall structure properties. The complete model for amorphous and crystalline Si:P dielectric function is presented. Range of accuracy, known problems and model parameters are studied and described. New interesting features of Si:P dielectric functions are discussed. The influence of dopants and free-carriers is taken into account and studied separately and their overlap is also analyzed. The influence of Drude damping time on the optical response of heavily doped Si:P is studied. All results are then compared with experimental data.

Several phenomena normally considered separately are taken into account simultaneously to model a new solar‐cell structure. A planar substructure inserted in the cell emitter is provided at its faces with two L‐H interfaces which modify recombination mechanisms inside the accumulation layers. The theoretical modelization and one‐dimensional numerical simulation show its yield to be 35%–40%. In practice, fabrication would necessitate uniting recent technological advances in the same fine substructure to ensure: (i) absorption of low‐energy photons; (ii) optical confinement of the infrared light; and (iii) formation of the L‐H interfaces at its edges.

Investigation of some light-matter interactions in Multi-Interface Novel Devices (MIND) containing a nanoscale Si-layered system have led to a method for predicting free-carrier density dependent nonlinear optical properties as a function of doping, light excitation intensity and carrier injection. The approach is based on the well-known t-matrix approximation. A simplified a few-layer optical model has been constructed that will reproduce the main features/parameters of real systems. The degree of model and simulation self-consistency is discussed using basic physical functions and published experimental data. Near perfect agreement between the simulated model and the corresponding experimental results has been obtained. In this way, the simulation allowed us to determine the main origins/components of the strong optical nonlinearity characteristic of one of the most specific MIND behaviours.

A complete model for the heavily doped and/or highly excited Si:P dielectric function is presented in the limit of the free-carrier gas approach. New interesting features of Si:P dielectric functions are presented and discussed. The influence of dopants and free-carriers is taken into account independently and their common features usually assumed in the literature, are analysed. The influence of Drude damping time on the optical response of heavily doped Si:P is studied. All results are compared with experimental data.

A new approach to the description of the accumulated space-charge in the stationary-state and in the steady-state is demonstrated for multilayer (n + -n-n + ) homostructures. The steady-state evolution of accumulated populations of the free-carrier fractions (having anisotropic thermal oscillations) has been considered on the basis of a quasi-equilibrium approximation as a function of the increasing bias. This analytical description takes into account the general solution for Poisson's equation developed for the stationary-state and recently used for the computation of the steady-state low-field potential barrier-height reduction. Excellent agreement is found between our characteristics and previously published experimental and numerical results

The structural changes produced in the near-surface region of single-crystal Si upon fabrication of delta back-surface-field devices were studied by x-ray diffraction in an offset-asymmetric geometry. The results demonstrate that implantation of 180-keV P+ ions into Si with a dose on the order of 1015 cm-2, followed by thermal annealing, allows buried amorphous Si layers to be produced. The structures thus prepared suffer a significant lattice strain perpendicular to the interface.

Our previous modeling and simulation demonstrated that a considerable increase in Si single‐crystal back surface field (BSF) solar cell performance is possible by modifications to give a so‐called delta‐BSF device. In our first measurements a widened absorption (up to λ≤3200 nm) and infrared photoconductivity (where the upper wavelength limit λ≤1900 nm was imposed by the emitting lamp) have been observed.

In an experimental investigation carried out on multilayer silicon homostructures (of the n + -n-n + type) an electric interaction between the two homointerfaces through majority carriers as a function of the geometrical factor has been shown. The two model homointerfaces were formed in the same monocrystal (CVD epitaxy) by only an abrupt change in the impurity doping level. Samples differing in the spacing of their parallel interfaces were characterized in the stationary and steady states (static and quasi-static regimes) to reveal modifications in their macroscopic transport. It has been demonstrated that the thermionic emission current, normally taken into account in the barrier transport (with large potential barriers) dominates in only two of five characteristic bias intervals. The two other phenomena, also related to the geometrical factor, i.e. the diffusion-drift and tunneling currents, dominate the conduction over practically the whole dc-bias range. These results allow the study of short and long range electrical interactions as well as the free carrier micromovement of simple and complex semiconductor interfaces of multilayer devices

The detailed balance method is used to study the thermodynamic efficiency of an intermediate band photovoltaic cell with low threshold Auger generation. Hot electrons generated by high-energy photons pump electrons from the intermediate band to the conduction band. The intermediate band is filled up after absorption of low-energy photons. A thermodynamic efficiency of about 70% is obtained, which is higher than the maximal efficiency of 63.2% for an intermediate band solar cell without Auger generation. The optimum band gap is shifted towards the silicon band gap. This result gives silicon its place in the new generation of solar cell materials using generation mechanism absent in single-band-gap solar cells.

The anisotropy of the photocurrent as a function of photon energy has been measured in the range of the fundamental gap of GaSe. A high, temperature-sensitive, anisotropy of the photocurrent is observed for light energies slightly higher than the gap. The results give independent evidence for the disorder-induced localisation of the electronic states along the crystallographic c axis and confirm the interpretation of the DC and drift conductivity properties of this material.