Person: Montero Álvarez, Daniel
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Daniel
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Montero Álvarez
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Universidad Complutense de Madrid
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Item Energy levels distribution in supersaturated silicon with titanium for photovoltaic applications(Applied physics letters, 2015) Pérez, E.; Castán, H.; García, H.; Dueñas, S.; Bailón, L.; Montero Álvarez, Daniel; García-Hernansanz, R.; García Hemme, Eric; Olea Ariza, Javier; González Díaz, GermánIn the attempt to form an intermediate band in the bandgap of silicon substrates to give it the capability to absorb infrared radiation, we studied the deep levels in supersaturated silicon with titanium. The technique used to characterize the energy levels was the thermal admittance spectroscopy. Our experimental results showed that in samples with titanium concentration just under Mott limit there was a relationship among the activation energy value and the capture cross section value. This relationship obeys to the well known Meyer-Neldel rule, which typically appears in processes involving multiple excitations, like carrier capture/emission in deep levels, and it is generally observed in disordered systems. The obtained characteristic Meyer-Neldel parameters were Tmn = 176 K and kTmn = 15 meV. The energy value could be associated to the typical energy of the phonons in the substrate. The almost perfect adjust of all experimental data to the same straight line provides further evidence of the validity of the Meyer Neldel rule, and may contribute to obtain a deeper insight on the ultimate meaning of this phenomenon. (C) 2015 AIP Publishing LLC.Item Estimation of the melting threshold of Ti supersaturated Si using time resolved reflectometry and haze measurements(Semiconductor Science and Technology, 2023) Montero Álvarez, Daniel; Caudevilla Gutiérrez, Daniel; Algaidy, S.; García Hernansanz, Rodrigo; Suler, A; Acosta-Alba, P.; Kerdiles, S.; Pastor Pastor, David; García Hemme, Eric; Roy, F.; Olea Ariza, JavierHyperdoped or supersaturated semiconductors are gathering the attention of industry and research institutions due to their sub-bandgap photon absorption properties. In this study, two fast and non-invasive techniques, time-resolved reflectometry (TRR) and Haze Measurements, are applied to infer the melt and solidification regimes of Ti supersaturated 300 mm silicon wafers, aiming to ease the characterization process towards high volume manufacturing of supersaturated materials. Ti supersaturation is attained by using an ion implantation process with a dose 3 x 10(15) cm(-2), which amorphizes the surface. Crystalline quality is then recovered by means of a XeCl UV nanosecond laser annealing process. TRR technique is used to determine two different melting and solidification processes of the laser annealed implanted surface. A first brief, low temperature peak (alpha peak) is associated with the melting process of the amorphized surface, followed by a longer peak/plateau (beta (1) peak/plateau), linked to the melting process of the crystalline phase below the amorphized layer, at sufficiently high laser fluences. Haze technique is used to indirectly measure the crystalline quality after the solidification process of the laser-annealed surface. Atomic force microscopy measurements are used to obtain the surface roughness value and cross-section high resolution transmission electron microscopy micrographs to check crystalline quality.Item Deposition of Intrinsic a-Si:H by ECR-CVD to Passivate the Crystalline Silicon Heterointerface in HIT Solar Cells(IEEE journal of photovoltaics, 2016) García Hernansanz, Rodrigo; García Hemme, Eric; Montero Álvarez, Daniel; Prado Millán, Álvaro Del; Olea Ariza, Javier; San Andrés Serrano, Enrique; Martil De La Plaza, Ignacio; González Díaz, GermánWe have deposited intrinsic amorphous silicon (a-Si:H) using the electron cyclotron resonance (ECR) chemical vapor deposition technique in order to analyze the a-Si:H/c-Si heterointerface and assess the possible application in heterojunction with intrinsic thin layer (HIT) solar cells. Physical characterization of the deposited films shows that the hydrogen content is in the 15-30% range, depending on deposition temperature. The optical bandgap value is always comprised within the range 1.9- 2.2 eV. Minority carrier lifetime measurements performed on the heterostructures reach high values up to 1.3 ms, indicating a well-passivated a-Si:H/c-Si heterointerface for deposition temperatures as low as 100°C. In addition, we prove that the metal-oxide- semiconductor conductance method to obtain interface trap distribution can be applied to the a-Si:H/c-Si heterointerface, since the intrinsic a-Si:H layer behaves as an insulator at low or negative bias. Values for the minimum of D_it as low as 8 × 10^10 cm^2 · eV^-1 were obtained for our samples, pointing to good surface passivation properties of ECR-deposited a-Si:H for HIT solar cell applications.Item Insulator-to-metal transition in vanadium supersaturated silicon: variable-range hopping and Kondo effect signatures(Journal of physics D: applied physics, 2016) Martil De La Plaza, Ignacio; García Hemme, Eric; Montero Álvarez, Daniel; García Hernansanz, Rodrigo; Olea Ariza, Javier; González Díaz, GermánWe report the observation of the insulator-to-metal transition in crystalline silicon samples supersaturated with vanadium. Ion implantation followed by pulsed laser melting and rapid resolidification produce high quality single-crystalline silicon samples with vanadium concentrations that exceed equilibrium values in more than 5 orders of magnitude. Temperature-dependent analysis of the conductivity and Hall mobility values for temperatures from 10K to 300K indicate that a transition from an insulating to a metallic phase is obtained at a vanadium concentration between 1.1 × 10^(20) and 1.3 × 10^(21) cm^(−3) . Samples in the insulating phase present a variable-range hopping transport mechanism with a Coulomb gap at the Fermi energy level. Electron wave function localization length increases from 61 to 82 nm as the vanadium concentration increases in the films, supporting the theory of impurity band merging from delocalization of levels states. On the metallic phase, electronic transport present a dispersion mechanism related with the Kondo effect, suggesting the presence of local magnetic moments in the vanadium supersaturated silicon material.Item Limitations of high pressure sputtering for amorphous silicon deposition(Materials research express, 2016) García Hernansanz, Rodrigo; García Hemme, Eric; Montero Álvarez, Daniel; Olea Ariza, Javier; San Andrés Serrano, Enrique; Prado Millán, Álvaro Del; Ferrer, F. J.; Martil De La Plaza, Ignacio; González Díaz, GermánAmorphous silicon thin films were deposited using the high pressure sputtering (HPS) technique to study the influence of deposition parameters on film composition, presence of impurities, atomic bonding characteristics and optical properties. An optical emission spectroscopy (OES) system has been used to identify the different species present in the plasma in order to obtain appropriate conditions to deposit high purity films. Composition measurements in agreement with the OES information showed impurities which critically depend on the deposition rate and on the gas pressure. We prove that films deposited at the highest RF power and 3.4 × 10^−2 mbar, exhibit properties as good as the ones of the films deposited by other more standard techniques.Item Transport mechanisms in silicon heterojunction solar cells with molybdenum oxide as a hole transport layer(Solar energy materials and solar cells, 2018) García Hernansanz, Rodrigo; García Hemme, Eric; Montero Álvarez, Daniel; Olea Ariza, Javier; Prado Millán, Álvaro Del; Martil De La Plaza, Ignacio; Voz Sánchez, Cristobal; Gerling, Luis; Puigdollers, Joaquin; Alcubilla, R.Heterojunction solar cells based on molybdenum sub-oxide (MoOx) deposited on n-type crystalline silicon have been fabricated. The hole selective character of MoOx is explained by its high workfunction, which causes a strong band bending in the Si substrate. This bending pushes the surface into inversion. In addition, the sub-stoichiometry of the evaporated MoOx layers leads to a high density of states within the bandgap. This is crucial for charge transport. The J-V electrical characteristics at several temperatures were analysed to elucidate the dominant charge transport mechanisms of this heterojunction structure. We have identified two different transport mechanisms. At low bias voltage, transport is dominated by hole tunnelling through the MoOx gap states. At higher voltage the behaviour is similar to a Schottky junction with a high barrier value, due to the high MoOx work function. These results provide a better understanding of the hole selective character of MoOx/n-type silicon heterocontacts, which is key to further improve this new kind of solar cells.