Effects of nitrogen doping on optical and electrical properties of nanocrystalline FeSi2 films prepared by sputtering

Nitrogen-doped nanocrystalline-FeSi2 (NC-FeSi2) thin films were deposited on SiO2 substrates at room temperature by radio frequency magnetron sputtering, and the effects of nitrogen-doping were experimentally studied. X-ray diffraction measurements revealed that the lattice constant of nano-sized grains increases by nitrogen doping and finally the film becomes amorphous. Optical absorption spectral and electrical measurements indicated that the optical bandgap is evidently enlarged and the electrical conductivity is significantly decreased by nitrogen doping, respectively. It was experimentally demonstrated that nitrogen doping drastically modulate NC-FeSi2 optically and electrically.


Introduction
In recent years, semiconducting iron disilicide (-FeSi2) has attracted much attention as a new candidate for silicon-based optoelectronic devices [1,2,3,4,5].-FeSi2 has large optical absorption coefficients, which are greater than 10 5 cm -1 at photon energies above 1.2 eV [6,7].It possesses an indirect optical bandgap of 0.78 eV and a direct bandgap of 0.85 eV [8,9], which is relevant to an optical fiber for telecommunication wavelengths of 1.3 and 1.55 m of the near infrared (NIR) light [10,11].Moreover, it also can be epitaxially grown on Si substrates with small lattice mismatches of 2~5% [12,13].In addition, -FeSi2 is a nontoxic material, and its compositional elements (Fe and Si) are abundant in nature [14].Nanocrystalline iron disilicide (NC-FeSi2) comprising nano-sized -FeSi2 grains exhibits physical properties similar to -FeSi2 [15].It can be grown on any solid substrates at room temperature [16,17].Its optical bandgap value is close to that of -FeSi2.Moreover, it has large optical absorption coefficients of greater than those of -FeSi2 [18].Their optoelectronic properties of -FeSi2 and NC-FeSi2 are immensely interesting for new applications to silicon-based NIR detection devices.Although the application of -FeSi2 to NIR photodetectors has already been reported [19,20], there have ever been few reports on the application for NC-FeSi2 so far [15].
Previously, we have reported that NC-FeSi2 thin films have n-type conduction with a large residual carrier density of probably on the order of 10 19 cm 3 , which is larger than that of -FeSi2 by orders of magnitude [17,21].n-Type NC-FeSi2/p-type Si heterojunction diodes were electrically studied and evaluated as NIR photodiodes.They showed a rectifying action similarly to that of conventional p-n heterojunction diodes.However, its action is accompanied by a large reverse leakage current, which might be attributed to the large carrier density and the existence of grain boundaries that act as centers for leakage in the NC-FeSi2 layer.
In this study, we suggest nitrogen doping into NC-FeSi2 for reducing the carrier density of NC-FeSi2.The incorporation of nitrogen atoms, which is one of representative interstitial elements, is expected to terminate dangling bonds and compensate lattice defects.Additionally, nitrogen atoms possibly attract free electrons in NC-FeSi2 due to its large electronegativity.We investigated the effects of nitrogen-doping on the structural, optical, and electrical properties of NC-FeSi2.

Experimental details
200-nm-thickness undoped and nitrogen-doped NC-FeSi2 thin films were deposited on SiO2 substrates at room temperature by radio frequency magnetron sputtering (RFMS) using an FeSi2 alloy target (purity: 4N).Prior to the film deposition, the SiO2 substrates were rinsed in acetone with an ultrasonic processor.After that, they were immediately introduced into the sputtering apparatus.The base pressure was lower than 310 5 Pa, and the sputtering deposition of the films was carried out at a fixed pressure of 2.66  10 1 Pa.Nitrogen-doped films was deposited in atmospheres comprised of N2 and Ar mixed gases.The nitrogen content was adjusted by changing the N2 and Ar gases inflow ratio.Concretely, the film deposition was made at N2/Ar inflow ratios of 0.1/15 and 1.0/15.Al ohmic contacts were deposited on the surface of the NC-FeSi2 films at room temperature for the measurement of the electrical conductivity.The crystalline structure was examined by X-ray diffraction (XRD).The optical absorption coefficients were measured using a ultraviolet/visible/near-infrared spectrometer equipped with a integration sphere.The electrical conductivity were measured in the temperature range of 200-500 K using the van der Pauw method (HL5500PC; Accent).

Structure evaluation
Figure 1 shows the X-ray diffraction patterns of undoped and nitrogen-doped NC-FeSi2 films, measured in grazing incidence (2θ scan) with a fixed incidence angle of 4 °.The pattern of undoped film exhibits a broad diffraction peak in the 2 angle range from 40 to 50 °, which is due to the nanocrystalline structure of NC-FeSi2.In our previous studies, NC-FeSi2 films were examined by transmission electron microscopy (TEM) and XRD, and it was found that the films comprise 3-5 nm grains from the dark-field TEM images and a broad diffraction peak similarly to that of this work is observed in the XRD pattern [15,17,18].Thus, the films in this work should comprises nano-sized grains.The broad peak may result from the overlapping of diffractions from several crystalline planes such as 313, 331, 004, 040, 041, 114, 511, 422, and 133 of -FeSi2.For the nitrogen-doped film deposited at N2/Ar = 0.1/15, the broad peak due to NC-FeSi2 is weakened and shifted to lower angles.This might be because the grain size decreases and the lattices of the grains are expanded by the incorporation of nitrogen atoms.The film deposited at N2/Ar = 1.0/15 exhibits no diffraction peaks, which indicates that the film becomes amorphous-like due to further reduction in the grain size.From the XRD measurements, it was found that the grain growth is suppressed by nitrogen doping and an excessive nitrogen-doping make the film be amorphous-like.deposited at N2/Ar = 1.0/15.A optical absorption coefficient () spectrum derived from the transmittance and reflectance spectra are shown in Fig. 3(a).From the spectral profile, the band gap is estimated to be 1.5 eV, which is evidently larger than that (0.85-0.9 eV) of the undoped NC-FeSi2.Figure 3(b) shows the plot of the absorption coefficient in (h) 1/2 and (h) 2 against the photon energy.Linear parts in the (hν) 2 and (hν) 1/2 plots imply absorptions due to direct and indirect transitions, respectively.The band gap of 1.5 eV is due to the band edge of an indirect transition.

Electrical properties
Figure 4(a) shows the temperature dependence of the electrical conductivity of undoped and nitrogen-doped films.The electrical conductivity clearly decreases with increasing nitrogen content.
The conductivity of the doped film deposited at N2/Ar = 1.0/15 is 164 mS/cm at 300 K, which is approximately three orders of magnitude lower than that of the undoped film.For the origin of the conductivity reduction, the followings are considered: (1) nitrogen atoms, which are expected to attract free electrons, reduce the density of major carriers (electrons), ; and (2) nitrogen atoms might terminate dangling bonds at grain boundaries that probably are a source of free carriers.The activation energy was estimated from the temperature dependence of electrical conductivity using the following Arrhenius law.
Here, Ea, 0, and kB are the activation energy, conductivity of a pre-exponential factor that can be extrapolated from experimental data, and Boltzmann constant, respectively.The gradient of the plot gradually changes with T 1 , which implies that the activation energy varies with temperature.The gradient of the plot in Fig. 4(a), that is to say, the actual activation energy Eact, was calculated using the following equation derived from the transform of Eq. ( 1): (2) Figure 4(b) shows the calculated activation energy with temperature.The activation energy gradually vary with temperature, which implies that the transport of carriers follows a variable hopping theory.In addition, the activation energy increases with increasing nitrogen content.
The temperature dependence of the electrical conductivity in hopping conduction is expressed by the following equation:  Here, Tm is a material dependent constant.Figure 5 shows plots of the electrical conductivity as a function of T 1/m .The value of m depends on the carrier transport mechanism as follows: m = 1 indicates nearest-neighbor hopping, m = 3 and 4 indicate two-and three-dimensional variable range hopping (VRH), and m = 2 indicates Efros-Shklovoskii VRH.As shown in Fig. 5, the m = 2 plots are close to linear for all films, and in particular, the m = 2 plot of the highly nitrogen-doped film deposited at N2/Ar = 1.0/15 is perfectly linear.Carrier transport can be explained by the Efros-Shklovoskii model.Its model supposes long-range electron-electron interactions with a soft Coulomb gap [22].Such a kind of interactions might be caused by the incorporation of nitrogen atoms into the NC-FeSi2 films.

Conclusions
Nitrogen-doped NC-FeSi2 thin films were prepared by RFMS, and nitrogen-doping effects on the optical absorption and electrical properties of the films were experimentally studied.By nitrogen doping, the optical bandgap is enlarged and the electrical conductivity is drastically enhanced, as compared with those of the undoped NC-FeSi2 films.Since the increment in the optical band gap is so large, nitrogen doping should drastically vary the band structure of NC-FeSi2.We consider that the reduction in the electrical conductivity by nitrogen doping is because nitrogen atoms, which attract free electrons owing to a large electronegativity, reduce the carrier density and/or because nitrogen atoms might terminate dangling bonds at grain boundaries that probably are a source of free carriers.On the other hand, it is not ruling out the possibility that a large number of defects induced by the incorporation of nitrogen atoms act as trap centers for carriers, which is necessary to be clarified by further studies from this.