J O U R N A L O F M AT E R I A L S S C I E N C E : M AT E R I A L S I N E L E C T RO N I C S 1 0 ( 1 9 9 9 ) 6 2 7 ± 6 3 1
Characterization of laser and laser/thermal annealed semiconducting iron silicide thin ®lms A. DATTA, S. KAL*, S. BASU Materials Science Centre and *Department of Electronics and ECE, Indian Institute of Technology, Kharagpur 721302, India E-mail:
[email protected] M. NAYAK, A. K. NATH Industrial CO2 Laser Section, Centre for Advanced Technology, Indore 452013, India, b-FeSi2 is an important semiconducting silicide which is being studied extensively. In this paper, we report our results of the effect of laser and laser-thermal annealing on the properties of b-FeSi2 . 5N purity Fe was deposited on Si substrate and was subsequently irradiated by CW and pulsed laser separately followed by thermal annealing to reduce the laser induced damage. The samples were then characterized by sheet resistance, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), optical re¯ectance and absorption studies. Lastly, b-FeSi2 /n-Si heterojunctions were fabricated and the effect of laser treatment on the junction ideality factor was investigated. All these characterizations indicated the formation of good quality b-FeSi2, particularly after pulsed laser followed by thermal treatment.
1. Introduction
Silicides belong to a very important group of compounds, which are particularly useful in VLSI fabrication [1]. While most of the silicides are metallic in nature, some of them are also semiconducting [2]. Among the semiconducting silicides the, b phase of FeSi2 is quite promising owing to its direct band gap (0.83 to 0.89 eV), high resistance to oxidation even at elevated temperature, non-toxicity and low vapor pressure [3]. Thus, it is a potential material for the development of new optoelectronic devices within the well known Si process technology. Usually this silicide is produced by evaporation of a thin metal ®lm on a Si substrate followed by heat treatment at an appropriate temperature and is normally carried out in a furnace in vacuum or ¯ushed with inert gas. But much less work has so far been reported on laser synthesis and modi®cation of iron silicide. In the present study, we have prepared b-FeSi2 using both CW CO2 laser and excimer (XeCl) pulsed laser.
2. Experimental
5N purity Fe was deposited by electron beam evaporation using an Edward E306A vacuum coating unit on both
1 0 0 and
1 1 1 oriented Si wafers and annealed by CW CO2 laser (energy density ranging from 250 to 400 J cm ÿ 2) and pulsed laser
0.2 to 1.8 J cm ÿ 2 respectively. For CW CO2 laser
l 10:6 mm annealing, the ambient was nitrogen. For excimer (XeCl) pulsed laser
l 0:308 mm annealing, irradiation was done both in He and vacuum atmospheres. The samples were again thermally annealed in Ar H2 atmosphere at a suitable temperature for a ®xed time to 0957±4522
# 1999 Kluwer Academic Publishers
passivate the possible laser-induced damage. Some samples were only thermally annealed at suitable temperatures and times in order to compare the effect of laser beam irradiation. XRD studies on all these samples were done using Philips PW1820 model with a Co line
l 0:1789 nm. Both thermally grown and laser grown followed by thermally treated ®lms were studied for optical absorption and re¯ectance using a Shimadzu spectrophotometer. The absorption and re¯ectance spectra of these ®lms were analyzed. Sheet resistance was measured by a four-point probe technique using a Veeco FPP-5000 model equipment. XPS was also carried out for qualitative elemental analysis. Finally a b-FeSi2 /n-Si heterojunction was fabricated by laser annealing and I±V characteristic was studied.
3. Results and discussion
We categorize the different samples as follows, according to the mode of thermal and laser treatment: TH1: only thermally annealed samples TH2: thermal followed by CW laser annealed samples TH3: thermal-CW laser-thermal annealed samples TH4: thermal-pulsed laser-thermal annealed samples CW1: only CW laser annealed samples CW2: CW laser followed by thermal annealed samples PL1: only pulsed laser annealed samples PL2: pulsed laser followed by thermal annealed samples 627
It is to be noted that only the laser energy was varied during annealing and the temperature and time during thermal annealing were kept ®xed at 850 C and 2 h.
3.1. Sheet resistance
Sheet resistance
Rsh of various samples was measured by the 4-point probe technique using a Veeco FPP-5000 model instrument. The plots of sheet resistance versus laser energy density for different samples are shown in Fig. 1. The trends found and their possible explanations are given below. CW1 and CW2 samples showed an increase in sheet resistance with increase in laser energy density. CW1 samples showed lower Rsh than that of CW2 samples. As laser energy was increased, a better reaction between Fe and Si might have taken place and resulted in an increase in Rsh . In the case of CW1 samples rapid laser irradiation might have caused incomplete solid state reaction between Fe and Si and there might be some metallic silicide phase together with b-FeSi2 and trace amounts of Fe also. Hence, lower resistance was seen in CW1 samples compared to CW2 samples. But after thermal annealing of CW1 samples for 2 h at 850 C, Rsh increased because of enhanced reaction between Fe and Si for the formation of the semiconducting phase of FeSi2 . For pulsed laser samples, we obtained a similar nature of variation of sheet resistance with pulse energy as in the case of CW and TH samples. The measured sheet resistance value with pulsed laser energy is shown in the inset of Fig. 1. On the other hand, Rsh of TH2 samples was found to be always higher than TH1 samples. The reason may be the disorder produced in the crystal lattice because of the sudden laser impact. However, Rsh dropped when TH2 samples were further annealed thermally, thereby supporting the above statement. The typical value of sheet resistance
Rsh for TH1 samples was 3.0±3.5 kO/&. The sample was prepared by thermal annealing at 850 C for 2 h. In the case of CW2 and PL2 samples, we found a smaller decrease in sheet resistance than that reported earlier
Figure 1 Sheet resistance
Rsh of different samples as a function of laser energy.
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[3, 9, 12]. In our case the deposited Fe thickness was 90± 100 nm and the corresponding silicide thickness from a theoretical estimate should be around 350 nm. Hence, the computed resistivity falls in the range 10±120 mO cm whereas the value of this parameter was reported to be 100±1000 mO cm. This is almost an order of magnitude reduction in resistivity in our case, which might have resulted from various factors, e.g. presence of low resistivity phase even after thermal annealing, effect of substrate on electrical transport and uncertainty in measuring exact silicide thickness.
3.2. X-ray photoelectron spectroscopy (XPS)
The XPS study of some of the samples was done using a VG ESCA LAB MKII model equipment. The surface was examined without any etching. At low and moderate power laser irradiation, no Si signal was observed, indicating the absence of any Si compound right on the surface. Hence, the Fe-Si reaction might have started at the interface but the surface layers of Fe still did not react with Si, up to a particular laser energy. We obtained C and O peaks splitting indicating qualitatively the presence of one or more phases of iron oxide on the surface. However, the splitting of the C peak is quite unusual and yet to be explained. However, after thermal annealing, CW2 samples showed the distinct peaks of C, O, Fe and Si. Also, no splitting of O and C peaks was observed in this case. The TH1 samples also showed almost the same behavior as the CW2 samples.
3.3. X-ray diffraction analysis
We have studied almost all types of samples, as was mentioned earlier. All CW1, PL1 samples showed no sharp peaks indicating a poor crystallinity (Fig. 2). One broad peak around 15 and another around 34 are seen. The signal from b-FeSi2 is likely to be in the peak around 34 , as evident from standard powder diffraction data.
Figure 2 XRD spectra of only CW laser annealed (CW1) and only pulsed laser annealed (PL1) samples.
Linear particle size was calculated using Scherrer's equation Phkl
Figure 3 XRD spectra of CW laser followed by thermal annealed (CW2) and pulsed laser followed by thermal annealed (PL2) samples.
CW2 samples showed the characteristics displayed in Fig. 3. The samples showed a sharp peak at an angle (34.3 ), which de®nitely points to a h2 2 0i and h2 0 2i b-FeSi2 plane. Some other planes of b-FeSi2 are also present but diffraction lines parallel to the substrate plane were absent, indicating that a polycrystalline ®lm has been formed with predominant crystallite orientation along h2 2 0i direction. For pulsed laser annealed samples (PL1, PL2) the diffractogram is shown in Figs. 2 and 3. In the case of PL1 samples
2 0 2,
2 2 0 and
2 2 1 planes are present in a broad background. At higher pulse energies (1 to 1.8 J cm ÿ 2) the peaks broadened, indicating a stress developed in the ®lm by high energy laser irradiation. After thermal annealing of the PL1 samples, we obtained the spectrum of PL2 samples. A sharp peak indicated that the
2 0 2)/
2 2 0 plane is the dominating feature of the spectrum with a couple of very small peaks. This is an interesting result which we observed only in our pulsed laser annealed samples. The intensity was highest amongst all cases indicating a grain growth mainly along the
2 0 2/
2 2 0 direction. Hence, it may be that a kind of solid phase epitaxy (SPE) is achieved in our process. The lattice parameters were calculated and are given in Table I. The values are more or less in good agreement with the reported values. However, a general trend was noticed that the values of a and b decreased from reported values, whereas the value of c increased in both CW2 and PL2 samples. Hence, it may possibly be interpreted that a and b, which are in the direction parallel to the interface, are shortened, and c, which is perpendicular to the substrate, is expanded. This is a case of so-called biaxial compressive strain developed in the ®lm.
0:89 l b1=2 cos yhkl
where l wavelength of radiation used, b1=2 half peak width in radians and yhkl angle of diffraction Using the spectra in Figs. 2 and 3 the particle size calculated along the
2 0 2/
2 2 0 direction for different types of samples are given in Table II. From the table, it is clear that the linear particle size increased from the only laser annealed samples to the laser followed by thermally annealed samples in the case of both CW and pulsed laser. This indicates that the very small sized particles grown immediately after laser irradiation showed grain growth after thermal annealing for suf®cient time at an appropriate temperature. This effect appears to be prominent in the case of the pulsed laser annealed sample where appreciably larger grains were obtained compared to CW laser annealed samples. It is believed that in the very short time involved during pulsed laser annealing, the surface temperature remains below the melting point, which enhances solid phase epitaxial regrowth resulting in larger grains.
3.4. Optical absorption and re¯ectance
The optical absorption and re¯ectance data were recorded for a series of samples using a Shimadzu spectrophotometer. The absorption data were taken with reference to a bare Si substrate. The light was incident on the silicide layer. Both CW2, PL2 and TH1 samples were examined for absorption and re¯ectance study. The re¯ectance spectra of both CW2 and PL2 samples are almost the same showing distinct ®ne structures. In Fig. 4, three peaks around 0.93, 0.89 and 0.74 eV in PL2 sample and three peaks around 0.93, 0.90 and 0.74 eV in the CW2 sample were observed. The transitions at 0.89 and 0.90 eV are due to the band edge. The transition at 0.93 eV is present in both the spectra and is not reported elsewhere. However, Bost and Mahan [4] mentioned two direct transitions at 0.89 and 1.01 eV by absorption measurements. The transition at 0.74 eV was also found in two spectra and may originate from any sub-band gap defect state. It is to be noted that the estimation of optical parameters is highly dependent on the growth process and in the present case the values can change because of the laser treatment [5]. Two characteristic re¯ectance spectra are shown in Fig. 4. In contrast, the only thermally grown samples (TH1) showed no ®ne structure; instead a uniformly increasing spectral feature from 0.85 to 1.75 mm was obtained. The optical absorption measurement for determining band gap was also performed. We know that the nature of the band gap transition can be evaluated by measuring
T A B L E I Lattice parameters calculated for laser-thermal annealed samples from XRD spectra Lattice parameters
CW laser annealed samples
Pulsed laser annealed samples
[13]
[14]
a(nm) b(nm) c(nm)
0.9694 0.7764 0.7852
0.9855 0.7789 0.7882
0.9863 0.7791 0.7833
0.9879 0.7799 0.7839
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T A B L E I I Particle size calculated from XRD spectra Type of samples
Calculated particle size
P2 0 2=2 2 0 (nm)
CW1 CW2 PL1 PL2
11.2 95.4 19.0 155.9
the absorption coef®cient a as a function of photon energy hn. For a direct transition a is given by [6] a
hn A
hn7Edg 1=2 for an indirect transition by 2 a
hn A
hn7Eind g 7Eph
Edg
Egind
where and are the magnitudes of the direct and indirect gap, respectively, A is a constant for an allowed direct transition, Eph is the phonon energy and A contains the probability of phonon emission. The value of the direct transition, 0.87 eV, at RT (Fig. 5) is in good agreement with earlier reports [7] and also with our own re¯ectance measurements. Also, a tail below the band edge was observed, supposedly due to the absorption of defect states in the gap. While trying to ®t our data to the equation stated above for an indirect gap, no appreciable linear region was found from which the band gap could be calculated. Hence, no indirect transition was detected by us unlike some other reports [2, 8]. Optical parameters, namely the real part of the refractive index
n, the absorption constant
k, the undispersed refractive index
no , and the real and imaginary parts of relative permittivity, were calculated from the re¯ectance and absorption spectra. The value of the undispersed refractive index
no was found to be 5.5 which is in good agreement with the earlier published value for thermally annealed samples [4]. The details of the estimation of these optical parameters will be reported elsewhere.
Figure 5 A typical optical absorption plot of the pulsed laser-thermal (PL2) sample.
3.5. Current±voltage behavior of b-FeSi2 /n-Si heterojunction
The as-grown b-FeSi2 is found to be a heavily doped ptype semiconductor without intentional doping [9]. The growth of b-FeSi2 ®lms on n-type Si substrate results in an anisotype heterojunction which might be of considerable interest. The purpose of the I±V study is to examine the current transport through this anisotype polycrystalline b-FeSi2 /Si heterojunction. Thin ®lms of b-FeSi2 were prepared in the form of circular dots on n/n Si h1 0 0i with r 0:02 O cm for heavily doped
n backside and r 0:48±0:72 O cm for the n-epilayer. Thermal annealing at 850 C for 2 h was carried out to form b-FeSi2 thin ®lm. Then, a pulsed laser was used to irradiate some silicide dots, and thermal annealing was again performed on this structure. In each case, the I±V characteristic was noted. The ohmic contact on the backside was formed by depositing Al by a thermal evaporation technique. An HP 4061A model semiconductor component/test system was used for the purpose. I±V measurements on b-FeSi2 /n-Si using a press contact probe showed an exponential behavior nearly up to 0.1 V and deviated at higher voltages due to a series resistance effect. The voltage range up to 0.2 V is ampli®ed graphically in Fig. 6 for the b-FeSi2 /n-Si junction. The ideality factors calculated for this heterojunction are mentioned in the ®gure itself for a ®xed diode area 0.0188 cm2. It was seen that the n value for the thermally annealed sample increased when irradiated by the pulsed laser and then decreased to the minimum value after further thermal annealing. The dominant current transport mechanism across the b-FeSi2 /n-Si junction is tunneling, as studied by Erlesand and Ostling [10] and Dimitriadis [11] from the temperature dependence of the ideality factor.
4. Conclusion
Figure 4 Optical re¯ectance spectra of CW2 and PL2 samples.
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XRD analysis of only laser annealed samples (CW1, PL1) showed almost the same type of spectra both for
radiation caused damage to the ®lm which could be annealed out by further thermal treatment. Also, a laser treatment alone is not suf®cient for homogeneous reaction between Fe and Si. ESCA results again qualitatively indicated that some Fe oxide and also free Fe might be present just on the surface of the only laser annealed samples. Optical absorption and re¯ection measurements of different samples were performed. Re¯ectance spectra showed two more transitions other than the band-to-band one. The absorption coef®cient
a was estimated and the nature of absorption was found to be direct in our case. Finally, b-FeSi2 /n-Si heterostructures were fabricated and I±V characteristics were studied. It was noticed that the ideality factor
n for the thermally annealed sample increased when irradiated by a pulsed laser and then decreased to the minimum value after further thermal annealing, possibly due to reduction of damage from laser impact. Figure 6 ln J±V plots for different samples and corresponding ideality factors calculated from initial linear portion of the curve for b-FeSi2 /n-Si heterojunction.
CW CO2 laser and XeCl pulsed laser. Peaks are a little sharper in the case of PL1 compared to CW1. The shape of the spectra is broad indicating strain in the ®lm, which might be due to rapid laser irradiation and subsequent cooling. To passivate this laser-induced strain in the ®lms, the samples were reannealed thermally at 850 C for 2 h and XRD of the samples was again performed. These XRD spectra showed sharp peaks. The peak intensity was the highest in PL2 samples. The lattice parameters were calculated and linear particle size was determined for the samples using Scherrer's equation. The particle size was found to increase from CW1 to CW2 as well as from PL1 to PL2 samples the size being the highest for the PL2 samples. This indicates the superiority of the pulsed laser treatment. Sheet resistance
Rsh was measured by a four point probe technique. It was found that Rsh increased from CW1 to CW2 samples and also from TH1 to TH2 samples but decreased in TH3 samples. This, coupled with XRD, proves that the laser
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Received 16 March 1999 and accepted 6 July 1999
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