Laser thermal annealing effects on single crystal gallium phosphide

We have studied the laser thermal annealing (LTA) effects on single crystal GaP. The samples have been analyzed by means of Raman spectroscopy, glancing incidence x-ray diffraction (GIRXD), and transmission electron microscopy (TEM) measurements. After LTA process, the Raman spectra of samples annealed with the highest energy density show a forbidden TO vibrational mode of GaP. This result suggests the formation of crystalline domains with a different orientation in the annealed region regarding the GaP unannealed wafer. This behavior has been corroborated by GIXRD measurements. TEM images show that the LTA produces a defective layer with disoriented crystalline domains in the surface. The depth of this defective layer increases with the energy density of LTA. The lack of crystallinity after LTA processes could be related with the high bond energy value of GaP

Pulsed Laser Melting Effects on Single Crystal Gallium Phosphide

We have investigated the pulse laser melting (PLM) effects on single crystal Gal?. The samples have been studied by means of Raman spectroscopy, glancing incidence X-ray diffraction (GIRXD), van der Pauw and Hall effect measurements. After PLM process, the Raman spectra of samples annealed with the highest energy density show a forbidden TO vibrational mode of Gal?. This suggests the formation of crystalline domains with a different orientation in the Gal? PLM region regarding to the GaP unannealed region. This behavior has been corroborated by glancing incidence x-ray diffraction measurements. A slightly increase in the sheet resistivity and a suppression of the mobility in PLM samples have been observed in all the measured temperature range. Such annealing effects are a cause of great concern for intermediate band (IB) materials formation where PLM processes are required first, to recovery the lattice crystallinity after high dose ion implantation processes and second, to avoid impurities outdiffusion when the solid solubility limit is exceeded

Insulator-to-metal transition in vanadium supersaturated silicon: variable-range hopping and Kondo effect signatures

We 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

Double ion implantation and pulsed laser melting processes for third generation solar cells

In the framework of the third generation of photovoltaic devices, the intermediate band solar cell is one of the possible candidates to reach higher efficiencies with a lower processing cost. In this work, we introduce a novel processing method based on a double ion implantation and, subsequently, a pulsed laser melting (PLM) process to obtain thicker layers of Ti supersaturated Si. We perform ab initio theoretical calculations of Si impurified with Ti showing that Ti in Si is a good candidate to theoretically form an intermediate band material in the Ti supersaturated Si. From time-of-flight secondary ion mass spectroscopy measurements, we confirm that we have obtained a Ti implanted and PLM thicker layer of 135 nm. Transmission electron microscopy reveals a single crystalline structure whilst the electrical characterization confirms the transport properties of an intermediate band material/Si substrate junction. High subbandgap absorption has been measured, obtaining an approximate value of 10 4 cm(-1) in the photons energy range from 1.1 to 0.6 eV

Electrical characterization of amorphous silicon MIS-based structures for HIT solar cell applications

A complete electrical characterization of hydrogenated amorphous silicon layers (a-Si:H) deposited on crystalline silicon (c-Si) substrates by electron cyclotron resonance chemical vapor deposition (ECR-CVD) was carried out. These structures are of interest for photovoltaic applications. Different growth temperatures between 30 and 200 °C were used. A rapid thermal annealing in forming gas atmosphere at 200 °C during 10 min was applied after the metallization process. The evolution of interfacial state density with the deposition temperature indicates a better interface passivation at higher growth temperatures. However, in these cases, an important contribution of slow states is detected as well. Thus, using intermediate growth temperatures (100–150 °C) might be the best choice

Electrical decoupling effect on intermediate band Ti-implanted silicon layers

We investigated the electrical transport properties of ultraheavily Ti-implanted silicon layers subsequently pulsed laser melted (PLM). After PLM, the samples exhibit anomalous electrical behaviour in sheet resistance and Hall mobility measurements, which is associated with the formation of an intermediate band (IB) in the implanted layer. An analytical model that assumes IB formation and a current limitation effect between the implanted layer and the substrate was developed to analyse this anomalous behaviour. This model also describes the behaviour of the function V/Delta V and the electrical function F that can be extracted from the electrical measurements in the bilayer. After chemical etching of the implanted layer, the anomalous electrical behaviour observed in sheet resistance and Hall mobility measurements vanishes, recovering the unimplanted Si behaviour, in agreement with the analytical model. The behaviour of V/Delta V and the electrical function F can also be successfully described in terms of the analytical model in the bilayer structure with the implanted layer entirely stripped

Far infrared photoconductivity in a silicon based material: vanadium supersaturated silicon

We have analyzed the spectral sub-bandgap photoresponse of silicon (Si) samples implanted with vanadium (V) at different doses and subsequently processed by pulsed-laser melting. Samples with V concentration clearly above the insulator-metal transition limit show an important increase of the photoresponse with respect to a Si reference sample. Their photoresponse extends into the far infrared region and presents a sharp photoconductivity edge that moves towards lower photon energies as the temperature decreases. The increase of the value of the photoresponse is contrary to the classic understanding of recombination centers action and supports the predictions of the insulator-metal transition theory

High quality Al0.37In0.63N layers grown at low temperature (< 300 degrees C) by radio-frequency sputtering

High-quality Al0.37In0.63N layers have been grown by reactive radio-frequency (RF) sputtering on sapphire, glass and Si (111) at low substrate temperature (from room temperature to 300 degrees C). Their structural, chemical and optical properties are investigated as a function of the growth temperature and type of substrate. X-ray diffraction measurements reveal that all samples have a wurtzite crystallographic structure oriented with the c-axis perpendicular to the substrate surface, without parasitic orientations. The layers preserve their Al content at 37% for the whole range of studied growth temperature. The samples grown at low temperatures (RT and 100 degrees C) are almost fully relaxed, showing a closely-packed columnar-like morphology with an RMS surface roughness below 3 nm. The optical band gap energy estimated for layers grown at RT and 100 degrees C on sapphire and glass substrates is of similar to 2.4 eV while it red shifts to similar to 2.03 eV at 300 degrees C. The feasibility of growing high crystalline quality AlInN at low growth temperature even on amorphous substrates open new application fields for this material like surface plasmon resonance sensors developed directly on optical fibers and other applications where temperature is a handicap and the material cannot be heated

Room temperature photo-response of titanium supersaturated silicon at energies over the bandgap

Silicon samples were implanted with high Ti doses and subsequently processed with the pulsed-laser melting technique. The electronic transport properties in the 15–300 K range and the room temperature spectral photoresponse at energies over the bandgap were measured. Samples with Ti concentration below the insulator-metal (I-M) transition limit showed a progressive reduction of the carrier lifetime in the implanted layer as Ti dose is increased. However, when the Ti concentration exceeded this limit, an extraordinary recovery of the photoresponse was measured. This result supports the theory of intermediate band materials and is of utmost relevance for photovoltaic cells and Si-based detectors

Energy levels distribution in supersaturated silicon with titanium for photovoltaic applications

In 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