Experimental verification of intermediate band formation on titanium-implanted silicon

Abstract

Intermediate band formation on silicon layers for solar cell applications was achieved by titanium implantation and laser annealing. A two-layer heterogeneous system, formed by the implanted layer and by the un-implanted substrate, was formed. In this work, we present for the first time electrical characterization results which show that recombination is suppressed when the Ti concentration is high enough to overcome the Mott limit, in agreement with the intermediate band theory. Clear differences have been observed between samples implanted with doses under or over the Mott limit. Samples implanted under the Mott limit have capacitance values much lower than the un-implanted ones as corresponds to a highly doped semiconductor Schottky junction. However, when the Mott limit is surpassed, the samples have much higher capacitance, revealing that the intermediate band is formed. The capacitance increasing is due to the big amount of charge trapped at the intermediate band, even at low temperatures. Ti deep levels have been measured by admittance spectroscopy. These deep levels are located at energies which vary from 0.20 to 0.28 eV below the conduction band for implantation doses in the range 10 13 -10 14 at./cm 2 . For doses over the Mott limit, the implanted atoms become nonrecombinant. Capacitance voltage transient technique measurements prove that the fabricated devices consist of two-layers, in which the implanted layer and the substrate behave as an n þ /n junction. V C 2013 American Institute of Physics. [http://dx

Energy levels distribution in supersaturated silicon with titanium for photovoltaic applications

“…16 Other works report values of energy under 10 meV: Antonov et al found a MN energy of 6.9 meV in the diffusion of nanoclusters and islands on solid Xe. 17 The same group 18 studied nanoclusters and islands on CO 2 , obtaining a MN energy of approximately 9 meV, in agreement with the most active phonon band for solid CO 2 .…”

“…The main goal of these studies is the formation of an intermediate band (IB) in the midgap of the semiconductor to induce infrared absorption in this material. 1,2 This approach would enable electrons to be pumped from the valence band (VB) into conduction band (CB) via two-photon absorption with lower energy than the semiconductor band gap. 3 There are two main reasons why obtaining this target is so interesting.…”

Application of Ion Implantation for Intermediate Energy Levels Formation in the Silicon-Based Structures Dedicated for Photovoltaic Purposes

“…As it was assumed, occurrence of this new band could allow the photons of energy lower than E g to be absorbed, generating additional photocurrent and consequently, increasing the PV cells efficiency. The possibility of IB formation was confirmed experimentally for different impurities such as titanium [12], sulphur [13], selenium and tellurium [14]. Moreover, this may be be possible for other elements such as chalcogens or transition metals.…”

“…As it was pointed out, some of the recorded plots did not reveal symptoms of the existence of any deep levels. According to what was suggested in [12], such tendency coincides with the IB theory which assumes that recombination is suppressed when the IB band is formed. However, in order to confirm whether the intermediate band was actually formed, it is necessary to perform further analyses.…”

“…In this context, it is reasonable to investigate if it is possible to induce a shift in the value of E g using appropriately configured ion implantation and whether such operation will influence the performance of PV cells. Since the ion implantation and post-implantation treatment are the processes that require predetermining of a number of correlated variables, the aspect of consideration was a matter of many experimental and theoretical studies [11][12][13][14][15].…”