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Characterization of semiconducting iron silicide films produced by furnace annealing
October 1999
Materials Letters 41 Ž1999. 89–95 www.elsevier.comrlocatermatlet
Characterization of semiconducting iron silicide films produced by furnace annealing A. Datta
a,)
, S. Kal b, S. Basu
a
a
b
Materials Science Center, Indian Institute of Technology, Kharagpur 721302, India Microelectronics Center, Department of Electronics and ECE, Indian Institute of Technology, Kharagpur 721302, India Received 25 February 1999; accepted 28 April 1999
Abstract Semiconducting iron silicide films Žb-FeSi 2 . were prepared by e-beam evaporation of 5N purity iron on Si substrates and subsequent annealing at different temperatures and time under reducing atmosphere. The composition and the semiconducting b-phase were confirmed by X-ray diffraction. Refractive index and thickness were measured using ellipsometry. Atomic Force Microscopy ŽAFM. was used as a tool for quantitative three dimensional surface analysis. Conductivity was studied as a function of temperature down to 13.7 K. Hall coefficient was measured at room temperature and the carrier concentration was determined. Band gap was calculated from the optical absorption data and the Photoluminescence ŽPL. study was conducted to observe possible emission from the silicide films. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Semiconducting silicide; Thin film; Preparation; Characterization
1. Introduction Metal silicides with their higher electrical conductivity find an important place in submicron silicon microelectronics. They are extensively used in integrated circuit applications, for example as schottky barriers, as ohmic contacts and as low resistivity metallization material for gate and interconnect w1x. Although, majority of silicides are metallic in nature, some silicides have been found as semiconducting. Among them, b-iron silicide is the most promising candidate. The growing interest in beta iron silicide ) Corresponding author. Fax. q91-3222-55303; E-mail: [email protected]
is chiefly due to its narrow direct band gap Ž Eg s 0.85–0.89 eV. w2–7x and thus a potential candidate for opto-electronic applications in IR region of electromagnetic spectrum. Moreover, since it can be easily grown epitaxially on top of both ²100: and ²111: silicon crystals at temperature around 6008C w8x, the most favourable application lies in integrated lasers and IR detectors. Although, a number of reports is available till date on thin film deposition and characterization of semiconducting iron silicide, much work is yet to be done to establish the material for practical applications. The present investigation is thus aimed at the preparation of b-FeSi 2 by furnace annealing and characterization. XRD analysis confirmed the formation of this compound. The
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 1 0 9 - 3
90
A. Datta et al.r Materials Letters 41 (1999) 89–95
refractive index ŽR.I.. at two different annealing times and corresponding thickness were measured by ellipsometry. AFM was used as a high resolution
tool to study surface microroughness. Conductivity was studied as a function of temperature down to 13 K. Hall coefficient was measured at room tempera-
Fig. 1. X-ray diffraction pattern for iron silicide film deposited on single crystal Si²100: with different annealing temperature and time Ža. 6508C for 2 h; Žb. 7508C for 2 h; Žc. 8508C for 2 h; Žd. 8508C for 5 h.
A. Datta et al.r Materials Letters 41 (1999) 89–95 Table 1 The refractive index ŽR.I.. and the thickness of silicide layers on p-Si ²100: Sample
Annealing
Refractive
no.
temperature and time
index ŽR.I..
Thickness ˚. ŽA
1 2
8508C, 1 h 8508C, 2 h
2.531 2.435
3366 3591
91
mined by optical absorption method using Shimadzu MPC-3100 spectrophotometer. Photoluminescence ŽPL. was studied by a FTPL setup at 9.5 K.
3. Results and discussion
ture and the carrier concentration was determined. Optical absorption and PL studies were also carried out to determine the energy band gap and sub band gap states.
2. Experimental 5N purity iron ŽAldrich Chemicals, USA. was deposited on ²100: oriented single crystal silicon by electron beam evaporation. The substrates had resistivity between 5 and 25 V cm. They were first degreased in organic solvents ŽTCE, Acetone and Methanol one after another. and then etched in H 2 SO4 q H 2 O 2 Ž2:1. solution for 20–30 min followed by repeated washing with DI water. Finally Si was dipped into 10% HF solution for 1–2 min before loading into e-beam chamber. The iron coating was done at a vacuum better than 10y5 Torr for about 20–25 min using an Edward vacuum coating unit, model E306 A. A resistive heating furnace up to 10008C was used for the purpose of annealing. The furnace was provided with precision programmable temperature controller to maintain the temperature accurately. Arrangement was also made to maintain a gaseous ambient inside the furnace. A reducing atmosphere of 2% high purity hydrogen in 98% high purity argon was used during annealing. Annealing temperature was varied between 5508C and 8508C and the time between 2 h and 5 h. The formation of iron silicide film was verified by X-ray diffraction method using CuK a radiation. The effect of time of annealing on the film was monitored by refractive index measurements using ellipsometer ŽGaertner, USA.. The resistivity was measured using Van der Pauw configuration which was subsequently applied for Hall effect measurements to find out the carrier concentration. The energy band gap Ž Eg . was deter-
3.1. XRD analysis The X-ray diffraction pattern is given in Fig. 1. By annealing up to 7508C for 2 h a mixture of different stoichiometry, e.g., Fe 2 Si, FeSi, b-FeSi 2 was obtained, but by increasing the annealing temperature to 8508C only b-FeSi 2 peaks were obtained. In every case Si peaks were also detected due to the fact that the normal angle XRD was used in the present investigation. The substrate peaks could be avoided by grazing angle XRD which was not available to the authors. Similar results were also presented by Bost and Mahan w3x. 3.2. Ellipsometry Refractive Index ŽR.I.. and thickness of b-FeSi 2 were measured by ellipsometry using He–Ne laser ˚ . at an angle of incidence 708. The Ž l s 6328 A results are given in Table 1. It was seen that R.I. decreased as time of annealing increased but thickness increased. With our average Fe layer thickness ˚ it was found that 1 A˚ of Fe results in of 972 A, ˚ of silicide which is in good agreement 3.46–3.69 A with published data w5,10x. 3.3. Atomic force microscopy (AFM) AFM is one of the most powerful tools to study the surface topography of amorphous and polycrys-
Table 2 AFM results on iron silicide thin films on ²100: p-Si Sample no.
Annealing temperature and time
Average roughness, R a Žnm.
RMS roughness, R rms Žnm.
ar75 ar11 ar12 ar14
7508C, 2 h 8508C, 1 h 8508C, 2 h 8508C, 4 h
44.51 69.90 72.04 69.44
53.68 85.07 85.28 87.53
92
A. Datta et al.r Materials Letters 41 (1999) 89–95
talline thin films w11,12x. Available optical techniques are mostly related to one dimensional quantitative analysis of surface. AFM, basically a stylus based technique, is capable of acquiring highly accu-
rate three dimensional topograph over an extended scale with a high spatial resolution. AFM measurements were conducted with a Burleigh Aris 3300 model equipment which can scan a region of 3.5
Fig. 2. Three-dimensional AFM images of iron silicide film annealed at Ža. 7508C for 2 h Žb. 8508C for 1 h Žc. 8508C for 2 h Žd. 8508C for 4 h.
A. Datta et al.r Materials Letters 41 (1999) 89–95
93
Fig. 2 Žcontinued..
mm = 3.5 mm. Typical measures of surface roughness are the average roughness R a and RMS roughness R rms of a specified area of interest on the surface w13x. A summary of the measurement results on our iron silicide samples processed in different conditions is given in Table 2. We observed an increase in roughness with increase in annealing temperature with fixed cooling rate for all films Ž3–58Crmin.. Charalabos et al. w14x did a systematic study on surface roughness of iron silicide film and obtained similar observation. The surface roughness value Žboth R a and R rms . in case of sample ar12 is found considerably higher than that of ar75. However, roughness values did not change appreciably Žrather remained almost same. in case of sample ar14 and ar11 which were annealed at 8508C for 4 h and 1 h, respectively. There must be a stress in the silicide film mainly due to the lattice mismatch between the substrate and the film. This stress should be properly relieved to get a relatively uniform and smooth film. The parameters on which this stress mainly depends are grain size of the silicide and formation of structural defects at the growth temperature. So when cooling rate was kept fixed and growth temperature was raised, silicides with larger grain size resulted. Larger grain size led to stress of higher magnitude. For a fixed cooling rate, this stress developed could not be relieved similarly as in case of the sample grown at lower temperature and as a result a higher surface roughness was created. It is to be noted that increasing annealing time did not significantly affect the film roughness which essentially indicates that with a fixed annealing temperature,
time has not that much effect on grain size. The Fig. 2a–d shows the AFM images of the samples in three dimensional form. To the best knowledge of the authors no systematic study of iron silicide surface by AFM has so far been reported. Detailed AFM study of the b-FeSi 2 films is in progress. 3.4. ConductiÕity Van der Pauw technique was used to measure the conductivity of our b-FeSi 2 sample grown at 8508C for 2 h. The conductivity as a function of reciprocal temperature is shown in Fig. 3a. Measurements were done from 300.3 K to 13.7 K using a liquid He closed cycle cryogenerator. The measurement and data aquisation were fully computer controlled. It was observed that conductivity increases with increasing temperature as expected for a semiconductor. It was also found that conductivity decreases too rapidly at very low temperature zone Žfrom 70.7 K to 13.7 K.. Conductivity Ž s . as a function of inverse temperature ŽTy1 . was plotted and a straight line was obtained in the temperature range 300.3 K to 74.7 K Žsee Fig. 3b.. The activation energy of 16 meV was calculated from the slope. The sign of Hall coefficient indicates that the majority carriers are holes. The carrier concentration calculated at room temperature was 8.11 E17 cmy3 . 3.5. Optical absorption Optical absorption was carried out using a Shimadzu MPC 3100 spectrophotometer. The direct
94
A. Datta et al.r Materials Letters 41 (1999) 89–95
Fig. 3. Conductivity as a function of temperature Ža. from 300.3 K to 13.7 K Žb. from 300.3 K to 74.7 K.
A. Datta et al.r Materials Letters 41 (1999) 89–95
95
density of iron silicide and that might create enough non-radiative recombination centers. 4. Conclusion
Fig. 4. Optical Absorption Spectrum of iron silicide film on p-Si ²100:.
band gap of 0.86 eV was obtained for FeSi 2 film on ²100: p-Si. There was a noticeable sub-band absorption Žsee Fig. 4. which was supposed to be originated from higher density of defects in the film. 3.6. Photoluminescence (PL) PL was carried out in FT mode using a 488 nm Ar-ion laser and LN2 cooled Ge detector at 9.5 K. Two samples were taken for study — one annealed at 8508C for 2 h and another at 8508C for 4 h. Spectra were taken from 1.77 mm to 1.03 mm. For the first sample emissions around 1.14 eV and 1.09 eV were observed while in case of second sample emission only at 1.14 eV was observed. These emissions are attributed to Si, at 1.14 eV due to band edge and at 1.09 eV due to defect which has been eliminated after 4 h annealing. No peak due to silicide layer, however, was observed. Oostra et al. w9x also observed no emission from their silicide layer whereas Dimitriadis et al. w5x observed broad band edge emission from the silicide film they prepared. The result we got may be due to high defect
The electron beam evaporation followed by thermal annealing at 8508C for 2 h. could produce semiconducting b-FeSi 2 film. Optimization of annealing temperature and time could improve the quality of the deposited film as was verified from XRD. AFM results quantitatively indicated a more rough surface at higher annealing temperature. Conductivity of the film was studied as a function of temperature. A carrier activation energy was calculated from a linear fit. Optical absorption spectrum, PL and carrier concentration indicate that the prepared b-FeSi 2 films have large number of defects, somewhat intrinsic in nature. Acknowledgements The authors thankfully acknowledge the help of A. Pramanik, L.K. Bera, Dr. D. Pal and Dr. A. Dhar of IIT Kharagpur. References w1x S.P. Murarka, Silicides for VLSI Applications, Academic Orlando, 1983. w2x M.C. Bost, J.E. Mahan, J. Appl. Phys. 64 Ž1988. 2034. w3x M.C. Bost, J.E. Mahan, J. Appl. Phys. 58 Ž1985. 2696. w4x M.C. Bost, J.E. Mahan, J. Vac. Sci. Technol. B5 Ž1986. 1336. w5x C.A. Dimitriadis et al., J. Appl. Phys. 68 Ž1990. 1726. w6x K. Lefki et al., J. Appl. Phys. 69 Ž1991. 352. w7x S. Basu, V.L.N. Avadhani, Ind. J. Phys. 63 A Ž5. Ž1989. 434. w8x M.G. Grimaldi et al., Appl. Surf. Sci. 74 Ž1994. 19. w9x D.J. Oostra et al., J. Appl. Phys. 74 Ž7. Ž1993. 4347. w10x M. Ozvold et al., Thin Solid Films 263 Ž1995. 92. ˇ w11x G. Binnig, C.F. Quate, Phys. Rev. Lett. 56 Ž1986. 930. w12x K.L. Westra, D.J. Thomson, Thin Solid Films 257 Ž1995. 15. w13x A.C. Diebold, B. Doris, Surface and Interface Analysis 20 Ž1993. 127. w14x A. Charalabos, J. Appl. Phys. 68 Ž1990. 93.
Materials Letters 41 Ž1999. 89–95 www.elsevier.comrlocatermatlet
Characterization of semiconducting iron silicide films produced by furnace annealing A. Datta
a,)
, S. Kal b, S. Basu
a
a
b
Materials Science Center, Indian Institute of Technology, Kharagpur 721302, India Microelectronics Center, Department of Electronics and ECE, Indian Institute of Technology, Kharagpur 721302, India Received 25 February 1999; accepted 28 April 1999
Abstract Semiconducting iron silicide films Žb-FeSi 2 . were prepared by e-beam evaporation of 5N purity iron on Si substrates and subsequent annealing at different temperatures and time under reducing atmosphere. The composition and the semiconducting b-phase were confirmed by X-ray diffraction. Refractive index and thickness were measured using ellipsometry. Atomic Force Microscopy ŽAFM. was used as a tool for quantitative three dimensional surface analysis. Conductivity was studied as a function of temperature down to 13.7 K. Hall coefficient was measured at room temperature and the carrier concentration was determined. Band gap was calculated from the optical absorption data and the Photoluminescence ŽPL. study was conducted to observe possible emission from the silicide films. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Semiconducting silicide; Thin film; Preparation; Characterization
1. Introduction Metal silicides with their higher electrical conductivity find an important place in submicron silicon microelectronics. They are extensively used in integrated circuit applications, for example as schottky barriers, as ohmic contacts and as low resistivity metallization material for gate and interconnect w1x. Although, majority of silicides are metallic in nature, some silicides have been found as semiconducting. Among them, b-iron silicide is the most promising candidate. The growing interest in beta iron silicide ) Corresponding author. Fax. q91-3222-55303; E-mail: [email protected]
is chiefly due to its narrow direct band gap Ž Eg s 0.85–0.89 eV. w2–7x and thus a potential candidate for opto-electronic applications in IR region of electromagnetic spectrum. Moreover, since it can be easily grown epitaxially on top of both ²100: and ²111: silicon crystals at temperature around 6008C w8x, the most favourable application lies in integrated lasers and IR detectors. Although, a number of reports is available till date on thin film deposition and characterization of semiconducting iron silicide, much work is yet to be done to establish the material for practical applications. The present investigation is thus aimed at the preparation of b-FeSi 2 by furnace annealing and characterization. XRD analysis confirmed the formation of this compound. The
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 9 . 0 0 1 0 9 - 3
90
A. Datta et al.r Materials Letters 41 (1999) 89–95
refractive index ŽR.I.. at two different annealing times and corresponding thickness were measured by ellipsometry. AFM was used as a high resolution
tool to study surface microroughness. Conductivity was studied as a function of temperature down to 13 K. Hall coefficient was measured at room tempera-
Fig. 1. X-ray diffraction pattern for iron silicide film deposited on single crystal Si²100: with different annealing temperature and time Ža. 6508C for 2 h; Žb. 7508C for 2 h; Žc. 8508C for 2 h; Žd. 8508C for 5 h.
A. Datta et al.r Materials Letters 41 (1999) 89–95 Table 1 The refractive index ŽR.I.. and the thickness of silicide layers on p-Si ²100: Sample
Annealing
Refractive
no.
temperature and time
index ŽR.I..
Thickness ˚. ŽA
1 2
8508C, 1 h 8508C, 2 h
2.531 2.435
3366 3591
91
mined by optical absorption method using Shimadzu MPC-3100 spectrophotometer. Photoluminescence ŽPL. was studied by a FTPL setup at 9.5 K.
3. Results and discussion
ture and the carrier concentration was determined. Optical absorption and PL studies were also carried out to determine the energy band gap and sub band gap states.
2. Experimental 5N purity iron ŽAldrich Chemicals, USA. was deposited on ²100: oriented single crystal silicon by electron beam evaporation. The substrates had resistivity between 5 and 25 V cm. They were first degreased in organic solvents ŽTCE, Acetone and Methanol one after another. and then etched in H 2 SO4 q H 2 O 2 Ž2:1. solution for 20–30 min followed by repeated washing with DI water. Finally Si was dipped into 10% HF solution for 1–2 min before loading into e-beam chamber. The iron coating was done at a vacuum better than 10y5 Torr for about 20–25 min using an Edward vacuum coating unit, model E306 A. A resistive heating furnace up to 10008C was used for the purpose of annealing. The furnace was provided with precision programmable temperature controller to maintain the temperature accurately. Arrangement was also made to maintain a gaseous ambient inside the furnace. A reducing atmosphere of 2% high purity hydrogen in 98% high purity argon was used during annealing. Annealing temperature was varied between 5508C and 8508C and the time between 2 h and 5 h. The formation of iron silicide film was verified by X-ray diffraction method using CuK a radiation. The effect of time of annealing on the film was monitored by refractive index measurements using ellipsometer ŽGaertner, USA.. The resistivity was measured using Van der Pauw configuration which was subsequently applied for Hall effect measurements to find out the carrier concentration. The energy band gap Ž Eg . was deter-
3.1. XRD analysis The X-ray diffraction pattern is given in Fig. 1. By annealing up to 7508C for 2 h a mixture of different stoichiometry, e.g., Fe 2 Si, FeSi, b-FeSi 2 was obtained, but by increasing the annealing temperature to 8508C only b-FeSi 2 peaks were obtained. In every case Si peaks were also detected due to the fact that the normal angle XRD was used in the present investigation. The substrate peaks could be avoided by grazing angle XRD which was not available to the authors. Similar results were also presented by Bost and Mahan w3x. 3.2. Ellipsometry Refractive Index ŽR.I.. and thickness of b-FeSi 2 were measured by ellipsometry using He–Ne laser ˚ . at an angle of incidence 708. The Ž l s 6328 A results are given in Table 1. It was seen that R.I. decreased as time of annealing increased but thickness increased. With our average Fe layer thickness ˚ it was found that 1 A˚ of Fe results in of 972 A, ˚ of silicide which is in good agreement 3.46–3.69 A with published data w5,10x. 3.3. Atomic force microscopy (AFM) AFM is one of the most powerful tools to study the surface topography of amorphous and polycrys-
Table 2 AFM results on iron silicide thin films on ²100: p-Si Sample no.
Annealing temperature and time
Average roughness, R a Žnm.
RMS roughness, R rms Žnm.
ar75 ar11 ar12 ar14
7508C, 2 h 8508C, 1 h 8508C, 2 h 8508C, 4 h
44.51 69.90 72.04 69.44
53.68 85.07 85.28 87.53
92
A. Datta et al.r Materials Letters 41 (1999) 89–95
talline thin films w11,12x. Available optical techniques are mostly related to one dimensional quantitative analysis of surface. AFM, basically a stylus based technique, is capable of acquiring highly accu-
rate three dimensional topograph over an extended scale with a high spatial resolution. AFM measurements were conducted with a Burleigh Aris 3300 model equipment which can scan a region of 3.5
Fig. 2. Three-dimensional AFM images of iron silicide film annealed at Ža. 7508C for 2 h Žb. 8508C for 1 h Žc. 8508C for 2 h Žd. 8508C for 4 h.
A. Datta et al.r Materials Letters 41 (1999) 89–95
93
Fig. 2 Žcontinued..
mm = 3.5 mm. Typical measures of surface roughness are the average roughness R a and RMS roughness R rms of a specified area of interest on the surface w13x. A summary of the measurement results on our iron silicide samples processed in different conditions is given in Table 2. We observed an increase in roughness with increase in annealing temperature with fixed cooling rate for all films Ž3–58Crmin.. Charalabos et al. w14x did a systematic study on surface roughness of iron silicide film and obtained similar observation. The surface roughness value Žboth R a and R rms . in case of sample ar12 is found considerably higher than that of ar75. However, roughness values did not change appreciably Žrather remained almost same. in case of sample ar14 and ar11 which were annealed at 8508C for 4 h and 1 h, respectively. There must be a stress in the silicide film mainly due to the lattice mismatch between the substrate and the film. This stress should be properly relieved to get a relatively uniform and smooth film. The parameters on which this stress mainly depends are grain size of the silicide and formation of structural defects at the growth temperature. So when cooling rate was kept fixed and growth temperature was raised, silicides with larger grain size resulted. Larger grain size led to stress of higher magnitude. For a fixed cooling rate, this stress developed could not be relieved similarly as in case of the sample grown at lower temperature and as a result a higher surface roughness was created. It is to be noted that increasing annealing time did not significantly affect the film roughness which essentially indicates that with a fixed annealing temperature,
time has not that much effect on grain size. The Fig. 2a–d shows the AFM images of the samples in three dimensional form. To the best knowledge of the authors no systematic study of iron silicide surface by AFM has so far been reported. Detailed AFM study of the b-FeSi 2 films is in progress. 3.4. ConductiÕity Van der Pauw technique was used to measure the conductivity of our b-FeSi 2 sample grown at 8508C for 2 h. The conductivity as a function of reciprocal temperature is shown in Fig. 3a. Measurements were done from 300.3 K to 13.7 K using a liquid He closed cycle cryogenerator. The measurement and data aquisation were fully computer controlled. It was observed that conductivity increases with increasing temperature as expected for a semiconductor. It was also found that conductivity decreases too rapidly at very low temperature zone Žfrom 70.7 K to 13.7 K.. Conductivity Ž s . as a function of inverse temperature ŽTy1 . was plotted and a straight line was obtained in the temperature range 300.3 K to 74.7 K Žsee Fig. 3b.. The activation energy of 16 meV was calculated from the slope. The sign of Hall coefficient indicates that the majority carriers are holes. The carrier concentration calculated at room temperature was 8.11 E17 cmy3 . 3.5. Optical absorption Optical absorption was carried out using a Shimadzu MPC 3100 spectrophotometer. The direct
94
A. Datta et al.r Materials Letters 41 (1999) 89–95
Fig. 3. Conductivity as a function of temperature Ža. from 300.3 K to 13.7 K Žb. from 300.3 K to 74.7 K.
A. Datta et al.r Materials Letters 41 (1999) 89–95
95
density of iron silicide and that might create enough non-radiative recombination centers. 4. Conclusion
Fig. 4. Optical Absorption Spectrum of iron silicide film on p-Si ²100:.
band gap of 0.86 eV was obtained for FeSi 2 film on ²100: p-Si. There was a noticeable sub-band absorption Žsee Fig. 4. which was supposed to be originated from higher density of defects in the film. 3.6. Photoluminescence (PL) PL was carried out in FT mode using a 488 nm Ar-ion laser and LN2 cooled Ge detector at 9.5 K. Two samples were taken for study — one annealed at 8508C for 2 h and another at 8508C for 4 h. Spectra were taken from 1.77 mm to 1.03 mm. For the first sample emissions around 1.14 eV and 1.09 eV were observed while in case of second sample emission only at 1.14 eV was observed. These emissions are attributed to Si, at 1.14 eV due to band edge and at 1.09 eV due to defect which has been eliminated after 4 h annealing. No peak due to silicide layer, however, was observed. Oostra et al. w9x also observed no emission from their silicide layer whereas Dimitriadis et al. w5x observed broad band edge emission from the silicide film they prepared. The result we got may be due to high defect
The electron beam evaporation followed by thermal annealing at 8508C for 2 h. could produce semiconducting b-FeSi 2 film. Optimization of annealing temperature and time could improve the quality of the deposited film as was verified from XRD. AFM results quantitatively indicated a more rough surface at higher annealing temperature. Conductivity of the film was studied as a function of temperature. A carrier activation energy was calculated from a linear fit. Optical absorption spectrum, PL and carrier concentration indicate that the prepared b-FeSi 2 films have large number of defects, somewhat intrinsic in nature. Acknowledgements The authors thankfully acknowledge the help of A. Pramanik, L.K. Bera, Dr. D. Pal and Dr. A. Dhar of IIT Kharagpur. References w1x S.P. Murarka, Silicides for VLSI Applications, Academic Orlando, 1983. w2x M.C. Bost, J.E. Mahan, J. Appl. Phys. 64 Ž1988. 2034. w3x M.C. Bost, J.E. Mahan, J. Appl. Phys. 58 Ž1985. 2696. w4x M.C. Bost, J.E. Mahan, J. Vac. Sci. Technol. B5 Ž1986. 1336. w5x C.A. Dimitriadis et al., J. Appl. Phys. 68 Ž1990. 1726. w6x K. Lefki et al., J. Appl. Phys. 69 Ž1991. 352. w7x S. Basu, V.L.N. Avadhani, Ind. J. Phys. 63 A Ž5. Ž1989. 434. w8x M.G. Grimaldi et al., Appl. Surf. Sci. 74 Ž1994. 19. w9x D.J. Oostra et al., J. Appl. Phys. 74 Ž7. Ž1993. 4347. w10x M. Ozvold et al., Thin Solid Films 263 Ž1995. 92. ˇ w11x G. Binnig, C.F. Quate, Phys. Rev. Lett. 56 Ž1986. 930. w12x K.L. Westra, D.J. Thomson, Thin Solid Films 257 Ž1995. 15. w13x A.C. Diebold, B. Doris, Surface and Interface Analysis 20 Ž1993. 127. w14x A. Charalabos, J. Appl. Phys. 68 Ž1990. 93.