Electric properties of carbon-doped n-type β-FeSi2/p-type Si heterojunction diodes

Motoki Takahara*, Tarek M. Mostafa, Ryuji Baba, Suguru Funasaki, Mahmoud Shaban, Nathaporn Promros, and Tsuyoshi Yoshitake 1Department of Applied Science for Electronics and Materials, Kyushu University, Kasuga, Fukuoka 816-8580, Japan 2Department of Electrical Engineering, Aswan Faculty of Engineering, Aswan University, Aswan 81542, Egypt 3Department of Physics, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang, Chalongkrung Road, Bangkok 10520, Thailand


Introduction
In recent years, development of silicon-based optoelectronic devices has received much attention [1][2][3].Light sources and detectors, which are compatible with the existing silicon technologies and can operate in the telecommunication wavelengths of 1.3 and 1.55 m of the near infrared (NIR) light, are required for the further development of modern silicon-based optoelectronic devices.The orthorhombic semiconducting iron disilicide (β-FeSi2) can be epitaxially grown on Si substrates with small lattice mismatches of 2~5% [4,5], which makes it compatible with Si integration.In addition, it possesses large optical absorption coefficients (greater than 10 5 cm 1 at light energies above 1.2 eV) and a direct optical band gap of approximately 0.85 eV above an indirect bandgap of 0.78 eV, which is relevant to optical fiber telecommunication wavelengths [6].Moreover, its compositional elements (Fe and Si) are nontoxic and abundant in nature [7].It has been reported that the photoluminescence and electroluminescence at wavelength of ~1.5 μm are observable in devices based on β-FeSi2 [8,9].Their photoelectronic properties of β-FeSi2 are immensely interesting for new applications to silicon-based optoelectronic devices.However, there have been few reports on the applications to near infrared (NIR) photodetectors so far.
Previously, n-type β-FeSi2/p-type Si heterojunctions were electrically studied and evaluated as near infrared photodiodes.They showed a rectifying action similarly to conventional p-n heterojunctions [2,10,11].However, its action is accompanied by large leakage dark current.We believe that the main reason for the large leakage current is large residual carrier densities of β-FeSi2 thin films, which might exceed 10 17 cm -3 .
In this study, we suggest C-doping into β-FeSi2 for reducing the carrier density of -FeSi2.The following effects are supposed: (i) C flexibly forms chemical bonds with different elements, so that incorporation of C atoms into β-FeSi2 lattices might compensate imperfections and lattice defects; (ii) C atoms possibly attract free electron in β-FeSi2 due to its larger electronegativity which exceeds that of Si; (iii) the incorporation of C atoms into β-FeSi2 thin films is easily made during the film preparation by sputtering with C-blended FeSi2 targets.We investigated the effects of C incorporation into -FeSi2 thin films on the current-voltage (J-V) characteristics and NIR photodetection properties, at a wavelength of 1.3 m, of n-type β-FeSi2/p-type Si heterojunctions.In addition, the effects of C-doping were structurally investigated by X-ray diffraction and Raman spectroscopic measurements.

Experimental details
Undoped and C-doped n-type β-FeSi2 thin films were deposited on p-type Si(111) substrates, with an electrical resistivities of 10 Ωcm, at a substrate temperature of 560°C by a radio frequency magnetron sputtering (RFMS) method.Prior to the film deposition, the Si substrates were cleaned with a dilute hydrofluoric acid solution (0.5% HF) and rinsed with deionized water to remove their native oxide layers.After that, they were immediately introduced into the sputtering apparatus in which the base pressure was lower than 3 × 10 5 Pa.FeSi2 alloy (undoped, 0.5 at.%, and 1.0 at.%C-doped) targets (purity: 4N) were used as source materials for the film deposition.The sputtering deposition was made at a pressure of 2.66 × 10 1 Pa with introducing Ar gas at flow rates of 15 sccm.After that, front (Al on the β-FeSi2 film) and back (Pd on the Si substrate) ohmic contacts were formed at room temperature.The β-FeSi2 thin films were structurally evaluated by X-ray diffraction (XRD) and Raman scattering spectroscopy (JASCO NRS-2100).The schematic illustration of a n-type β-FeSi2/p-type Si heterojunction diode is shown in Fig. 1.In this device structure, NIR light transmitted through the Si substrate, directly can reach a depletion region in the β-FeSi2 film.The current-voltage (J-V) characteristics were measured in the dark and under illumination with a 6 mW, 1.31 μm laser diode.

Results and Discussion
Figure 2 shows the J-V characteristic curves of undoped and C-doped n-type β-FeSi2/p-type Si heterojunction diodes measured in the dark.The undoped-β-FeSi2/Si heterojunctions shows a large dark reverse current, which was approximately 5 × 10 3 A/cm 2 measured at bias voltage of 3 V. On the other hand, the C-doped-β-FeSi2/Si heterojunctions exhibited approximately 9 × 10 4 A/cm 2 at 3 V, which is owing to a significant reduction in the leakage current.This should be attributed to a reduction in the carrier density of β-FeSi2 by C-doping.
Figure 3 shows the J-V characteristics curves of the diodes comprising undoped, 0.5 at.%C-doped, and 1.0 at.%C-doped β-FeSi2, measured in the dark and illumination with a 6 mW, 1.31 μm laser diode.The reverse current under illumination exceeds that in the dark for these devices.The difference between the illumination and dark currents increases with increasing carbon content, which evidently indicates that C-doping improves the photodetection performances of β-FeSi2/Si heterojunction diodes.This is probably mainly because a depletion region in the -FeSi2 thin films is expanded due to a reduction in the carrier density by C-doping.In order to investigate C-doping effects on the structures of -FeSi2 thin films, XRD and Raman scattering spectroscopic measurements were examined.2- XRD patterns of the -FeSi2 thin films exhibits intense -202/220 peaks and weak -404/440 peaks neighbor to Si-111 and Si-222 peaks, as shown in Fig. 4(a) and 4(b), respectively.The shift of the peaks is undetectable among the patterns.Figure 5 shows the pole figure patterns of the β-FeSi2 thin films.They indicate the existence of three types of epitaxial variant that are rotated at an angle 120 with respect to each other [12].Both undoped and C-doped films were epitaxial grown not only in a direction perpendicular to but also in parallel to Si(111) substrate.There is no significant differences in the pole figure pattern among the films.C-doping has no effects on the crystallinity of β-FeSi2 thin films.In other words, C-doping hardly affects the crystalline and epitaxial growth of β-FeSi2.Figure 6 shows the Raman spectra of the undoped and C-doped β-FeSi2 thin films.Four sharp peaks at 176, 193, 247 and 337 cm -1 related to β-FeSi2 are clearly observed.Figure 6(b)-6(d) show the magnification of the main peaks.The 193, 247, and 337 cm -1 cm peaks are known to be due to total vibrations [13][14][15].They do not exhibit peak shifts at all, which implies that the replacement by doped C atoms does not occur at Fe sites.In the case of Co and Mn doping, these peaks exhibit remarkable shifts due to the replacement of Fe atoms by Co and Mn atoms in -FeSi2 lattices [15].Peaks at 520 cm -1 is due to scattering of the first-order optical phonon of silicon.They exhibit no significant change for C-doping.For the other peaks, similarly, significant differences are not observed.
It was found that crystalline structure of -FeSi2 thin films is not affected by C-doping.As a reason for the enhanced diode and photodetection performance by C-doping, it is supposed that C atoms, which are incorporated into -FeSi2 thin films, terminate dangling bonds and compensate defects in the β-FeSi2 thin films, not spoiling the crystalline structure and epitaxial growth of -FeSi2 thin films.

Conclusions
We fabricated C-doped β-FeSi2/Si heterojunction diodes by RFMS, and C-doping effects on the electrical properties of -FeSi2 were studied.The J-V characteristics of the heterojunction diodes, measured in the dark and under NIR light illumination, exhibited improved rectifying actions and photodetection performances as compared with diodes comprising undoped-β-FeSi2 thin films.This improvement might predominantly be attributed to a reduction in the β-FeSi2 film carrier density, somehow accompanied by C-doping.The structure of the deposited β-FeSi2 thin films were characterized by XRD and Raman scattering spectroscopies.Their characterizations confirmed that C-doping has no significant effects on the crystalline structures including the epitaxial growth of β-FeSi2 thin films on Si(111) substrates.Any significant structural change could not be detected by their spectroscopic measurements.C-doping should contribute to the electrical properties of -FeSi2 in some process that is hardly evidently detectable by XRD and Raman scattering spectroscopies.For instance, we supposes that C atoms terminate dangling bonds and compensate defects in the β-FeSi2 thin films, which results in a reduction in the carrier density.Further studies in detail in order to find the origin of the improved electrical properties of -FeSi2 by C-doping are required.