J. Olea

Lifetime recovery in ultrahighly titanium-doped silicon for the implementation of an intermediate band material

The doping of conventional semiconductors with deep level (DL) centers has been proposed to synthesize intermediate band materials. A recent fundamental study of the nonradiative recombination (NRR) mechanisms predicts the suppression of the NRR for ultrahigh DL dilutions as a result of the delocalization of the impurity electron wave functions. Carrier lifetime measurements on Si wafers doped with Ti in the 1020–1021 cm−3 concentration range show an increase in the lifetime, in agreement with the NRR suppression predicted and contrary to the classic understanding of DL action.

Titanium doped silicon layers with very high concentration

Ion implantation of Ti into Si at high doses has been performed. After laser annealing the maximum average of substitutional Ti atoms is about 1018 cm−3. Hall effect measurements show n-type samples with mobility values of about 400 cm2/V s at room temperature. These results clearly indicate that Ti solid solubility limit in Si has been exceeded by far without the formation of a titanium silicide layer. This is a promising result toward obtaining of an intermediate band into Si that allows the design of a new generation of high efficiency solar cell using Ti implanted Si wafers.

High quality Ti-implanted Si layers above the Mott limit

In this paper, we present a detailed characterization of high quality layers of Si implanted with Ti at high doses. These layers are intended to the formation of an intermediate band (IB) solar cell. The main requirement to obtain an IB material is to reach an impurity concentration beyond the Mott limit, which is, in this case, much higher than the solid solubility limit. To overcome this limit we used the combination of ion implantation and pulsed-laser melting as nonequilibrium techniques. Time-of-flight secondary ion mass spectrometry measurements confirm that Ti concentration exceeds the theoretical Mott limit in the implanted layer, and glancing incidence x-ray diffraction and transmission electron microscopy measurements prove that good crystallinity can be achieved. Sheet resistance and Hall effect mobility show uncommon characteristics that can only been explained assuming the IB existence.

Sub-bandgap absorption in Ti implanted Si over the Mott limit

We have analyzed the structural and optical properties of Si implanted with very high Ti doses and subsequently pulsed-laser melted (PLM). After PLM, all samples exhibit an abrupt and roughly uniform, box-shaped Ti profile, with a concentration around 2 × 1020 cm−3, which is well above the Mott limit, within a 150 nm thick layer. Samples PLM-annealed at the highest energy density (1.8 J/cm2) exhibit good lattice reconstruction. Independent of the annealing energy density, in all of the samples we observe strong sub-bandgap absorption, with absorption coefficient values between 4 × 103 and 104 cm−1. These results are explained in terms of the formation of an intermediate band (IB) originated from the Ti deep levels.

Two-layer Hall effect model for intermediate band Ti-implanted silicon

Si samples have been implanted with very high Ti doses (over the theoretical Mott limit) to obtain an intermediate band (IB) in the host semiconductor. The electronic transport properties of this material have been analyzed by temperature-dependent sheet resistance and Hall effect measurements in the 7–400 K range. The experimental results are successfully explained by means of an analytical two-layer model, in which the implanted layer and the substrate behave as an IB/n-Si type junction. We deduce that the IB is located at 0.38 eV below the conduction band, which is around one third of the Si bandgap, i.e., theoretically close to the optimum location for an IB. Finally, we obtain that carriers at the IB behave as holes with a mobility of 0.4–0.6 cm2 V−1 s−1. This extremely low mobility is the one expected for a semifilled, metallic band, being this metallic condition of the IB a requirement for IB solar cells.

Electronic transport properties of Ti-impurity band in Si

In this paper we show that pulsed laser melted high dose implantation of Ti in Si, above the Mott transition, produces an impurity band (IB) in this semiconductor. Using the van der Pauw method and Hall effect measurements we find strong laminated conductivity at the implanted layer and a temperature dependent decoupling between the Ti implanted layer (TIL) and the substrate. The conduction mechanism from the TIL to the substrate shows blocking characteristics that could be well explained through IB theory. Using the ATLAS code we can estimate the energetic position of the IB at 0.36 eV from the conduction band, the density of holes in this band which is closely related to the Ti atomic density and the hole mobility in this band. Band diagrams of the structure at low and high temperatures are also simulated in the ATLAS framework. The simulation obtained is fully coherent with experimental results.

Sub-bandgap spectral photo-response analysis of Ti supersaturated Si

We have analyzed the increase of the sheet conductance (ΔG□) under spectral illumination in high dose Ti implanted Si samples subsequently processed by pulsed-laser melting. Samples with Ti concentration clearly above the insulator-metal transition limit show a remarkably high ΔG□, even higher than that measured in a silicon reference sample. This increase in the ΔG□ magnitude is contrary to the classic understanding of recombination centers action and supports the lifetime recovery predicted for concentrations of deep levels above the insulator-metal transition.

Experimental verification of intermediate band formation on titanium-implanted silicon

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(+)/n junction.

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.

Room-temperature operation of a titanium supersaturated silicon-based infrared photodetector

We report room-temperature operation of 1 × 1 cm2 infrared photoconductive photodetectors based on silicon supersaturated with titanium. We have fabricated these Si-based infrared photodetectors devices by means of ion implantation followed by a pulsed laser melting process. A high sub-band gap responsivity of 34 mV W−1 has been obtained operating at the useful telecommunication applications wavelength of 1.55 μm (0.8 eV). The sub-band gap responsivity shows a cut-off frequency as high as 1.9 kHz. These Si-based devices exhibit a non-previous reported specific detectivity of 1.7 × 104 cm Hz1/2 W−1 at 660 Hz, under a 1.55 μm wavelength light. This work shows the potential of Ti supersaturated Si as a fully CMOS-compatible material for the infrared photodetection technology.