Structural, Optical and AC Conductivity Studies on Polycrystalline-Si/Nanocrystalline-FeSi2 Composite Thin Films

We investigated an electrical conduction mechanism of Si/FeSi2 composite films by measuring a frequency dependence of AC electrical conductivity. The results were analyzed based upon Jonscher's power law. The hopping conduction obeying the Jonscher’s power law was observed for a-Si single films after annealing as well as Si/FeSi2 composite films annealed at 550 and 900°C. From the analysis of XRD, optical absorption, TEM/EDS and AC conductivity, we conclude that the electrical conduction mechanisms in polycrystalline (poly)-Si/β-FeSi2 annealed at 550°C and poly-Si/αand β-FeSi2 annealed at 900°C are due to electron hopping via the conduction band of β-FeSi2 and α-FeSi2 nanocrystals embedded in poly-Si thin films, respectively.


Introduction
In recent years, a lot of research efforts in developing all-solid-state secondary battery with high safety and high capacity has been devoted and especially thin-film all-solid-state secondary batteries, which consists of an insulating layer sandwiched between two layers of p-type and n-type inorganic semiconductors, have been proposed [1,2]. A. Nakazawa proposed a secondary battery with following structures; TiO2, SnO2 or ZnO as a n-type layer, NiO or CuAlO2 as a p-type layer, and the charging layer filled with an n-type metal oxide semiconductors of fine particles which were coated with insulating materials [1]. Here we focus on Si/β-FeSi2 composite thin films, which contain nanocrystalline β-FeSi2 particles within Si matrix [3,4]. Crystalline Si (c-Si) has an indirect bandgap (Eg ind ) of 1.12 eV [5], while the amorphous Si (a-Si) has Eg ind values of about 1.4-1.8 eV [6,7]. β-FeSi2 also exhibits semiconducting properties with a direct band-gap (Eg dir ) of 0.87 eV and Eg ind values of 0.765 eV [8,9]. Therefore, band structure of Si/β-FeSi2 composite thin films becomes similar to the charging layer of proposed secondary battery [1]. Furthermore, the difference between band gaps of Si and β-FeSi2 can be arranged by the degree of crystallinity on Si. From the above points, we believe that Si/β-FeSi2 composite thin films can be used as a charge layer for all-solidstate secondary battery.
However, the electrical conduction mechanism of Si/β-FeSi2 composite films is still unclear. In this study, structural, optical and alternating current (AC) electrical properties of polycrystalline-Si/β-FeSi2 composite thin films were evaluated and the results were compared with those of a-Si single films and the β-FeSi2 single films. The electrical conduction mechanism was discussed based upon the Jonscher's law [10].
Si thin films doped with Fe atoms were deposited on the quartz substrates by RF magnetron cosputtering (SPF-210H, CANON ANELVA Co., Ltd.) at 0.5 Pa in Ar gas (6N) atmosphere. After the deposition, these samples were annealed in an infrared lamp heating system (Mila-5000, ADVANCE RIKO, Inc.) at 550 or 900°C for 5 hours in a constant Ar gas flow rate of 100 sccm to form Si/β-FeSi2 composite thin films. The a-Si single films and the β-FeSi2 single films were also deposited on the quartz substrates using the Si target as described previously and FeSi2 target (4N, φ101.6×t1 mm, Kojundo Chemical Laboratory Co., Ltd.), respectively and then they were annealed under the same above-mentioned conditions for comparison. The film thickness of all samples was around 300 nm. Finally, two rectangular-shaped Al electrodes (area: 16×6.0 mm 2 , their distance: 4.5 mm) were deposited on these samples by vacuum thermal evaporation for AC conductivity analysis. Then these samples were alloyed at 450°C for 5 min in Ar gas to form an ohmic contact.
Structural properties of these thin films were evaluated by XRD (SmartLab, Rigaku CO., Ltd.) and TEM/EDS (H-90000NAR, Hitachi High-Technologies Co., Ltd.) analysis and their optical properties were evaluated by optical transmittance and reflectance (V-670 spectrophoto-meter, Jasco CO., Ltd.). Electrical characteristics of these thin films were evaluated by AC conductivity analysis (4284A LCR meter, Agilent Technologies, Inc.) at a DC bias voltage of 0 V and an AC bias voltage of either 0.05 or 1 V as a function of frequency between 100 Hz and 1 MHz. Figure 1 shows XRD spectra of annealed samples: (a) FeSi2 single films subjected to annealing at 550°C, (b) Si single films subjected to annealing at 550°C, (c) Si/FeSi2 composite films subjected to annealing at 550°C, and (d) Si/FeSi2 composite films subjected to annealing at 900°C. It was found from Fig. 1(a) that FeSi2 single films subjected to annealing at 550°C shows a polycrystalline β-FeSi2 structure. XRD spectrum of (b) Si single films subjected to annealing at 550°C exhibit an amorphous Si structure, while that of (c) Si/FeSi2 composite films subjected to annealing at 550°C consists of polycrystalline Si and polycrystalline β-FeSi2. The cause of formation of polycrystalline Si at low temperature of 550°C is considered to be due to the metal induced crystallization (MIC) [11,12]. It should be noted that (d) Si/FeSi2 composite films subjected to annealing at 900°C consist of metallic α-FeSi2, semiconducting β-FeSi2 and poly-Si. We estimated the crystallite sizes, D of poly-Si for samples (c) and (d) by Scherrer's equation (k=0.9) [13] using the F.W.H.M. values of Si (311) peaks in Figs. 1 (c) and 1 (d), and they were determined to be 3.5 and 7.5 nm, respectively. show TEM images of the poly-Si/β-FeSi2 and poly-Si/αand β-FeSi2 composite thin films formed by annealing at 550 and 900℃, respectively. From the TEM/EDS analysis, the black dots and grey area were identified as FeSi2 and Si, respectively. The particle sizes of FeSi2 dot crystals shown in Figs. 2 (a) and 2(b) were estimated using ImageJ (an open source image processing program) to be 6.8 and 18.8 nm, respectively. Similarly, the distances between FeSi2 particles shown in Figs. 2 (a) and 2(b) were estimated to be 8.95 and 19.5 nm, respectively. Figure 3 shows optical absorption spectra of annealed samples, which were derived from optical transmittance and reflectance measurements. It is reasonable that the optical absorption coefficient of (d) poly-Si/α-and β-FeSi2 composite films is much smaller than that of (c) poly-Si/β-FeSi2 composite thin films because the reflectance increases with increasing the amount of metallic α-FeSi2. Optical indirect and direct band-gap values (Eg ind and Eg dir ) for β-FeSi2 and Si were determined from the optical absorption spectra using Tauc relation [14]. They were Eg dir =0.74 eV for β-FeSi2 single films, Eg ind =1.51 eV for a-Si single films, Eg ind /Eg dir =1.24/0.88 eV for poly-Si/β-FeSi2 composite films, and Eg ind /Eg dir =1.08/0.96 eV for poly-Si/α-and β-FeSi2 composite films. It is known that the bandgap of Si decreases with increasing the degree of crystallinity of Si [15]. As mentioned above, crystallite sizes of poly-Si for poly-Si/β-FeSi2 composite films annealed at 550°C and poly-Si/α-and β-FeSi2 composite films annealed at 900°C were determined to be 3.5 and 7.5 nm, respectively. The difference of the degree of crystallinity is one of the possible origin of the difference of Eg ind for poly-Si. On the other hand, Eg dir for β-FeSi2 depends on chemical composition of Fe/Si as well as the degree of crystallinity of β-FeSi2. The reason for difference in Eg dir for β-FeSi2 is currently unclear. Further investigation on chemical composition of Fe/Si for β-FeSi2 nanocrystals is needed. The summary of analysis results from TEM images and optical absorption spectra is listed in Table Ⅰ.   Figure 4(a) shows the angular frequency dependence of the electrical AC conductivity for various films after annealing. It is known that the AC electrical conductivity of a substance exhibiting hopping conduction obeys Jonscher's power law and it is expressed as Eq. (1) [10,16,17],

Results and Discussion
where A is a constant dependent on temperature, ω is the angular frequency, f is frequency and s is the frequency exponent (0 < s ≤ 1). Hopping conduction was observed for three types of films; a-Si single films after annealing at 550℃, poly-Si/β-FeSi2 composite films formed by annealing at 550℃ and poly-Si/α-and β-FeSi2 composite films formed by annealing at 900℃. It has been suggested that the electric conduction mechanism of amorphous materials is mainly due to hopping conduction [18,19]. Therefore, it is natural that a-Si single films after annealing show hopping conduction in a wide range of angular frequency. On the other hand, poly-Si/β-FeSi2 and poly-Si/αand β-FeSi2 composite films show hopping conduction only at higher angular frequencies and in a wide range of angular frequency, respectively. The discussion of these electrical conduction mechanisms will be described later.
The polaron binding energy, Wm and the lower bound cut off to hopping distance, Rmin were calculated using Eqs. (2) [16] and (3) [17], respectively, where k is the Boltzmann constant, T is the temperature, n=1, 2 for single and bi-polarons, e is the elementary charge, and ε and ε0 is the relative dielectric constant of a material and dielectric constant of a vacuum, respectively. Here, the value of ε was calculated from the capacitance value of each sample, which was measured by AC conductivity analysis as a function of frequency. The values of Rmin were obtained assuming n=2 and they are shown in Fig. 4(b) as a function of angular frequency. Curve fitting results using Eqs. (1)-(3) are summarized in Table Ⅱ.
The difference between bottom of conduction bands of crystalline-Si (electron affinity, =4.05 eV) [20] and β-FeSi2 ( =4.71 eV) [20] can be estimated to be 0.66 eV, which almost agrees well with Wm value (0.64 eV) of poly-Si/β-FeSi2 composite films. In addition, as shown in Table I, the distance between FeSi2 particles for poly-Si/β-FeSi2 composite films was 8.9 nm, which is of the same order of magnitude as Rmin value (3.3 nm) of poly-Si/β-FeSi2 composite films. For the above reasons, it is suggested that the electrical conduction in poly-Si/β-FeSi2 composite films is due to electron hopping via the conduction band of β-FeSi2. Similarly, the difference between bottom of conduction bands of crystalline-Si ( =4.05 eV) [20] and α-FeSi2 (work function=5.3-5.5 eV) [21] can be estimated to be 1.43-1.45 eV, which is roughly close to Wm value (1.88 eV) of poly-Si/α-and β-FeSi2 composite films. Work function of 5.3-5.5 eV on α-FeSi2 is larger than electron affinity of 4.71 eV on β-FeSi2. Therefore, it is suggested that the electrical conduction in poly-Si/α-and β-FeSi2 composite films is due to electron hopping via the conduction band of α-FeSi2. The distance between FeSi2 particles for poly-Si/α-and β-FeSi2 composite films was 19.5 nm, which is of the different order of magnitude as Rmin value (2.3 nm) of poly-Si/α-and β-FeSi2 composite films. This reason is currently unclear because poly-Si/α-and β-FeSi2 composite films consist of mixed phase of α-and β-FeSi2. We need to form and evaluate poly-Si/α-FeSi2 single phase composite films to clarify it.

Conclusion
XRD and optical absorption spectra of β-FeSi2 single films showed a polycrystalline β-FeSi2 structure with the Eg dir value of 0.74 eV, while those of a-Si single films exhibited an amorphous Si structure with the Eg ind value of 1.51 eV. We found that the poly-Si/β-FeSi2 composite films annealed at 550℃ consist of polycrystalline Si (Eg ind = 1.24 eV) and nanocrystalline β-FeSi2 (Eg dir = 0.87 eV, the particle size is 6.8 nm). Similarly, we found that the poly-Si/α-and β-FeSi2 annealed at 900℃ composite films consist of nanocrystalline α-FeSi2 and β-FeSi2 (Eg dir = 0.96 eV, the particle size is 18.8 nm) and polycrystalline Si (Eg ind = 1.08 eV).
We investigated an electrical conduction mechanism of Si/FeSi2 composite films by measuring a frequency dependence of AC electrical conductivity. The results were analyzed based upon Jonscher's power law and the three parameters, s, Wm and Rmin were determined by fitting the calculated values to the experimental data. The hopping conduction obeying the Jonscher's power law was observed for a-Si single films after annealing as well as Si/FeSi2 composite films annealed at 550 and 900 ℃. The hopping conduction in a-Si films has been reported so far. From the result of comparison between Wm values and the band diagram of Si/FeSi2 as well as between Rmin values and the distance between FeSi2 particles, we conclude that the electrical conduction mechanisms in poly- Si/β-FeSi2 and poly-Si/α-and β-FeSi2 composite films are due to electron hopping via the conduction band of β-FeSi2 and α-FeSi2 nanocrystals embedded in poly-Si films, respectively.