Epitaxial Growth of n-Type-FeSi 2 Thin Films on p-Type Si ( 111 ) Substrates by Radio-Frequency Magnetron Sputtering and Rectifying Action of Heterojunctions

Tarek M. Mostafa, Motoki Takahara, Ryuji Baba, Suguru Funasaki, Mahmoud Shaban*, Nathaporn Promros**, Aki Tominaga, Maiko Nishibori, 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, Bangkok 10520, Thailand 4Research Center for Synchrotron Light Applications, Kyushu University, Kasuga, Fukuoka 816-8580, Japan


Introduction
In the last few years, the orthorhombic semiconducting phase of iron disilicide (β-FeSi2) has received much attention owing to its potential application to optoelectronics such as near infrared detectors and photovoltaics [1,2].β-FeSi2 has a large absorption coefficient, which is 200-fold larger than that of crystalline silicon at 1.5 eV, and possesses indirect and direct optical band gaps of 0.78 eV and 0.85 eV, respectively, which is relevant to an optical fiber for telecommunication wavelengths (1.3 and 1.5 μm) [3][4][5].It is also compatible with silicon technology due to small lattice mismatches of 2 -5% [6,7].From the ecological point of view, β-FeSi2 is a nontoxic material, and its elements (Fe and Si) are abundant in nature [8].-FeSi2 is a potential semiconductor applicable to silicon-based optoelectronic devices.However, there have been a few reports on heterojunction photodiodes comprising -FeSi2 and Si thus far because it is difficult to fabricate diodes that exhibit good rectifying action.
In order to form β-FeSi2/Si heterojunctions, several techniques such as ion beam synthesis [9,10], molecular beam epitaxy [11], chemical vapor deposition [12], and reactive deposition epitaxy [11] have been employed.However, the heterojunctions prepared by these techniques seems not to exhibit good rectifying action as photodiodes.The most probable reason might be the diffusion of Fe atoms into Si substrates.It results in the generation of deep trap levels in the Si layer, which should act as leakage centers in the rectifying action and trap centers for photogenerated carriers [14].The Fe diffusion is enhanced with increasing temperature.Although post-annealing at temperatures higher than 800 C is generally applied for growing -FeSi2 and for enhancing the crystalline quality of -FeSi2, it accelerates the diffusion of Fe atoms into Si substrates.In order to avoid such a situation, we have ever employed facing target direct current sputtering for as-growing -FeSi2 thin films on Si(111) epitaxially at substrate temperatures as low as possible [15].On the other hand, although a conventional magnetron sputtering, which enable us to deposit large-area films and is suitable for industrial applications, has ever been employed for the deposition of -FeSi2 thin films, there has been a few reports on the demonstration of -FeSi2/Si heterojunctions as photodiodes [16].
In this work, we employed a conventional radio-frequency magnetron sputtering (RFMS) for fabricating -FeSi2/Si heterojunctions, wherein -FeSi2 thin films were deposited on Si(111) substrates without post-annealing and studied the formation of the -FeSi2/Si heterojunctions.It was found that the epitaxial growth of -FeSi2 is an important key factor for producing rectifying action of the heterojunctions.

Experimental Processes
300-nm β-FeSi2 thin films were grown on a p-type Si (111) substrates with an electrical resistivity of 10 Ωcm and thickness of 260 µm at substrate temperatures range of 500 -600 ºC by RFMS with an FeSi2 alloy targets (purity: 4N), with an atomic ratio of Fe:Si = 1:2.The deposition rate was approximately 0.5 nm/min at an applied radio-frequency power of 20 W. In order to remove their native oxide layers, the Si substrates were initially immersed in diluted hydrofluoric acid (HF) solution (concentration: 1%) and then rinsed in deionized water.After that, they were immediately mounted in a film preparation chamber evacuated down to a base pressure of less than 10 -4 Pa.During the sputtering process, a pressure inside the chamber was fixed to be 2.66 × 10 -1 Pa by introducing Ar gas (purity: 6N) at a flow rate of 15 sccm.The applied RF power was 20 W. After that, they were transferred to another RFMS apparatus in order to deposit the electrodes.As shown in Fig. 1, Pd (purity, 4N) was deposited on the top of Si surface in finger-shaped pattern, while Al (purity, 4N) was deposited on the entire β-FeSi2 back surface.These depositions were carried out at room temperature.In this device structure, NIR light, which is transmitted through the front-side Si substrate, directly reaches a depletion region in the β-FeSi2 thin film.
The crystalline structure was characterized by X-ray diffraction (XRD) (Rigaku, RINT 2000/PC) using Cu-Kα radiation.The current-voltage (J-V) characteristics of the heterojunctions were measured using a source meter (Keithley 2400) in the dark and under illumination with a 6 mW, 1.31 μm laser diode (Neoark, TC20).To confirm the in-plane orientation of -FeSi2, pole figure measurements were examined.The pole figure patterns concerning -440/404 peak are shown in Fig. 3. Whereas the films deposited at substrate temperatures of higher than 560 C clear exhibit the existence of three types of epitaxial variant that are rotated at an angle 120  with respect to each other [16,17].From these results, the β-FeSi2 thin films deposited at substrate temperatures of higher than 560 C are epitaxially grown on Si(111) with the typical epitaxial relationships with three variants although they contain polycrystalline grains of -FeSi2 in the films.On the other hand, at lower than 540 C, the pole figure patterns do not have diffraction spots due to -FeSi2 clearly, which indicates that the -FeSi2 thin films are hardly epitaxially grown on Si(111).It was found that the epitaxial growth of -FeSi2 on Si(111) initiates from approximately 560 C.J-V characteristics in the dark and under illumination with a 1.31 m light, of the heterojunctions are shown in Fig. 4. The epitaxially-grown films, which were deposited at higher than 560 C, exhibit rectifying action.In addition, they slightly exhibit photocurrents for the illumination.On the other hands, the non-epitaxial films, which were deposited at lower than 540 C, hardly exhibit reifying action, and the behavior of the junctions is nearly ohmic.Of course, they rarely indicate photodetection.While the heterojunctions that employ p-type Si substrates exhibit the rectifying action as diodes, heterojunctions comprising n-type Si substrates and -FeSi2 thin films exhibited ohmic behaviors, which evidently indicates that the -FeSi2 thin films grown by RFMS in this work have n-type conduction, similarly to those prepared by facing targets direct-current sputtering (FTDCS) in our previous work.It was demonstrated that the epitaxial growth of -FeSi2 is necessary for producing rectifying action in the heterojunctions.The carrier concentration of the non-epitaxial -FeSi2 thin films, wherein the crystalline growth is insufficient due to the low substrate temperatures, should be too large to generate a depletion region in the heterojunctions, and a huge number of grain boundaries might act as leakage centers.

Results and discussion
Among the epitaxial films, the 560 C film exhibits the best rectifying behavior.The rectification ratio is gradually degraded with increasing substrate temperature.This implies that the 011102-4 diffusion of Fe atoms into the Si substrates might be enhanced by increasing substrate temperature, which results in the degraded diode performances.Whereas an increase in the substrate temperature enhances the crystalline growth of -FeSi2 and epitaxial growth of -FeSi2, it degrades the junction quality, particularly around the interfaces.
In the film deposition by RFMS, films receive plasma damage since substrates are located in plasma.A reason for the heterojunctions prepared by conventional RFMS being inferior to those prepared by facing targets direct-current sputtering in our previous study [17], wherein substrates are located away from plasma distinctly differently from a situation in conventional RFMS and films receive less damage from plasma, might be a difference in the plasma damage.The film quality of -FeSi2 should be degraded by plasma damage.In addition, the existence of films in plasma makes the effective surface temperature of films be increased, which might enhance the diffusion of Fe atoms into the Si substrates.

Conclusion
Heterojunction diodes, wherein n-type β-FeSi2 thin films were epitaxially grown on p-type Si(111) substrates by RFMS, were fabricated, and their rectifying action and photodetection were investigated.The epitaxial growth of n-type β-FeSi2 thin films on p-type Si(111) substrates was realized at substrate temperatures of 560 -600 ºC, and heterojunctions comprising their epitaxially-grown -FeSi2 thin films exhibit rectifying action.It was experimentally demonstrated that the epitaxial growth of -FeSi2 thin films is indispensable for the formation of the heterojunction diodes.In the substrate temperate range of 560-600 C, the rectification ratio was gradually degraded with increasing substrate temperature.It is considered that an enhancement in the diffusion of Fe atoms into Si substrates with increasing substrate temperature might degrade the junction quality.Both rectifying action and photodetection were inferior to those of similar heterojunctions prepared by facing targets direct-current sputtering in our previous study.Since an obvious difference between RFMS and FTDCS is whether substrates are located in plasma or away from plasma, -FeSi2 thin films prepared by RFMS might be damaged by plasma.

Fig. 2 Fig. 2 .
Fig.2displays the XRD patterns of β-FeSi2 thin films deposited on Si(111) substrates at substrate temperatures between 500 and 600 ºC, measured in (a) 2θ-θ scan and (b) grazing incidence (2θ scan) with an incidence angle of 4 °.The films deposited at 560, 580, and 600 C exhibit 202/220 peaks due to β-FeSi2 in the 2θ-θ patterns, and they are strengthened with increasing substrate temperature.At substrate temperatures lower than 540 C, the -202/220 peaks are not clearly observed.The 2θ patterns exhibit several peaks due to -FeSi2, which indicates the existence of polycrystalline grains of β-FeSi2 in the films.