Studies of Iron Diselenide Thin Films Deposited by Thermal Evaporation
A. Kassaa1, N. Benslim2
1Research Center in Industrial Technologies CRTI, P.O. Box 64, Cheraga 1604, Algeriers, Algeria.
a.kasaa@crti.dz
2LESIMS Laboratory, Department of Physics, Badji Mokhtar University, Annaba. Algeria.
Abstract— Iron diselenide (FeSe2) composite thin films have been formed onto ultrasonically and chemical cleaned glass substrates by thermal evaporation technique from powder. This latter was prepared by mechanical alloying. The structural evolution of a binary alloy with nominal composition FeSe2
prepared by ball milling was investigated as a function of milling time. The structural properties of the powders and the films were ascertained by x-ray diffraction method. The XRD patterns showed that for milling times up to 1 h, the FeSe2 phase is formed. The band gap Eg estimated from optical absorption data was between 0.8–1.01 eV, depending on preparation conditions such as substrate temperature. High optical absorption coefficients (> 104 cm− 1) were found.
Keywords— FeSe2; Solar energy materials; Mechanical alloying; Thermal evaporation; Thin films.
I. INTRODUCTION
In recent decades, transition metal chalcogenides have motivated increasingly active research because of their outstanding electronic and optical properties [1]. The FeSe2 is a p-type semiconductor with a direct band gap of 1.0 eV [2,3]
and considered as a suitable material for photovoltaic applications[4]. FeSe2 crystallizes both in marcasite orthorhombic structure and pyrite cubic structure and exhibit semi conducting behavior[5]. Under ambient conditions, FeSe2 adopt marcasite structure while high temperature/high pressure synthesis leads to pyrite type structure[6]. In earlier works, iron diselenide thin films were prepared by variety of methods such as: molecular beam epitaxy[7], milling pure elemental powders of iron and selenium [8], and selenization of amorphous iron oxide thin films predeposited by spray pyrolysis[9,10], hydrothermal[11], electrodeposition[12]. In this paper, we report on preparation of the nanocrystalline FeSe2 powder by mechanical alloying (MA) process and investigation of structural and optical properties of the vacuum-evaporated FeSe2 thin films.
II. EXPEREMENTAL
The FeSe2 powder used for evaporation of thin films was obtained by mechanical alloying process. The experiment was done in planetary micro mill pulvérisette7 at room temperature where high purity elemental materials of iron and selenium
with stoichiometric composition were sealed in cylindrical steel balls under argon atmosphere. The powder/balls weight ratio was 1/8. The rotational speed (disc rotational velocity) was selected at 300 tr/min. FeSe2 thin films were grown by flash evaporation of the powder of FeSe2 from a tungsten boat under a vacuum of 2.10−5 Torr onto glass cleaned substrates, in a Balzers evaporation unit. Tungsten boat heated by Joule effect was used to evaporate FeSe2 powder. The substrates were kept at a distance of 08 cm from the tungsten boat. The samples were deposited at room temperature and temperature substrates Ts =350 °C. The structural characterization was carried out by means of X-ray diffraction using a Philips X’Pert diffractometer instrument with copper CuKα radiation (λ=1.54 Å). The samples have been scanned from10ᴼ to 90ᴼ 2θ with a step size of 0.04ᴼ. Peak diffraction angles in the XDR patterns were converted to interplanar d-spacing and thus, Phase identification has been carried out by comparison of the observed d-spacing with those reported by the Joint Committee on Powder Diffraction Standards. A Perkin-Elmer λ9(UV–VIS-NIR) spectrophotometer was used for optical studies in the wavelength range [300–1500 nm] at room temperature. The thicknesses of the FeSe2 films were obtained by means of a Dektak III Profilometer. The absorption coefficient and the band gap Eg of the evaporated FeSe2 thin films determined from transmittance (T) measurements.
III. RESULTS AND DISCUSSIONS A. Structural properties
XRD patterns of the films are shown in fig.1 (a), (b) for films grown on glass substrates at room temperature by thermal evaporation from powders milled during 1h and 3h, respectively. Those films show peaks in different orientations such as (110), (011), (210), (311) exhibit the polycrystalline growth. All these diffracted peaks are in good agreement with the JCPDS (02-0290) file of FeSe2 marcasite orthorhombic structure. From Fig. 1(a), the existence of highly preferred orientation peak at 2θ=23.83º confirms a marcasite orthorhombic structure of the deposited FeSe2 film with preferential orientation along the (110) plane.
10 20 30 40 50 60 70 80 90 0
1000 2000 3000 4000 5000 6000 7000 8000
(b) (a)
(200)
(011)
FeSe FeSe2
(122)
(201)
(121)(210)(200)
(110)
Intensity (u.a)
2 (°)
Fig.1 Typical diffractogram of a FeSe2 thin film grown from powders milled during (a) 1h, (b) 3h.
B. Optical properties
(UV–VIS-NIR) spectrophotometer is used to see the optical properties such as transmission, absorption and optical band gap of the prepared films. Fig. 2 show the optical transmission spectra for thin films deposited at room temperature and annealed in air at Tr = 200 °C and Tr = 350 °C for 30min. In the region of long wavelength [900–1500 nm], the transmittance spectra exhibit well-defined peaks due to optical interference resulting from multiple reflections in the films. While in the region [500–800], an abrupt fall of the transmittance curves is observed. The increase in the optical transmission for the annealed material is a consequence of the more homogenized thin films.
400 600 800 1000 1200 1400 1600
0 10 20 30 40 50
60 Annealed at Tr = 200 °C Annealed at Tr = 350 °C
Transmittance (%)
Wavelength (nm)
fig. 2 Optical transmission spectra for thin films at different annealing temperatures.
The absorption coefficient was calculated from optical transmittance data using Lambert's principle [13]:
1 (%)
ln 100 T
d A
(1) Where d is the films thickness; T the transmission and A the parameter depending on the refraction indexes of the sample.
The optical band gap (Eg) of the films can be estimated using the relation:
(
h ) B h(
Eg)m (2) where hν, α, Eg, D and m are incident photon energy, absorption coefficient, the optical band gap, the proportionality constant and exponent, respectively. The m values differ and describe a kind of electronic transition.These values are 1/2, 3/2, 2 and 3 that represent direct allowed, direct forbidden, indirect allowed and indirect forbidden transitions [14]. It is observed that the best straight line is observed for m= 1/2, which is expected for direct allowed transition. The value of Eg was estimated by extrapolating the straight line portion of (αhν)2 versus hν.
The absorption spectra were also calculated from the transmission spectra, as shown in fig. 3. It is found that the absorbance decreases significantly in the region of long wavelength. It is found that all the absorption coefficients approach 104 cm−1 in the region of high photon energies, which is consistent with the results reported by [15]. This value is suitable for photovoltaic solar cells fabrication
1,6 1,8 2,0 2,2 2,4 2,6 2,8 3,0
1 2 3 4 5 6
.104 (cm)-1
h (eV)
Room temperature Ts = 350°C
fig. 3 Absorption spectra for FeSe2 thin films for deposition at room temperature and at substrate temperature
The graphs of (αhν)2 versus hν are plotted in Fig.4 enabling the energy band gaps of the films to be determined. The band gaps found for room temperature and substrate temperature Ts = 350 °C are 0.80 and 1.01eV respectively. From the band gap analysis, we can say that heating substrate has some effect on band gap.
0,0 0,5 1,0 1,5 2,0 0
2 4 6 8 10
Room temperature Ts = 350 °C
((h))2 108 (cm-1eV)2
h(eV)
Fig. 4 Variation of the band gap with photon energy of FeSe2 films at room temperature and at substrate temperature.
IV. CONCLUSION
Mechanical alloying (MA) has been successfully used to synthesize FeSe2 powder. Thin films were prepared from FeSe2 powder by single source thermal evaporation under vacuum onto glass substrates. X-ray diffraction analysis revealed that the prepared films possess polycrystalline in nature with orthorhombic structure with preferential orientation along (1 1 0) plane. Optical studies showed that the band gap of FeSe2 thin films is 0.8eV at room temperature and it increase to 1.01 eV at substrate temperature Ts = 350 °C . All films have absorption coefficients higher than 104cm-1.
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