Annealing effects on structural and electrical properties of Fe/Si interface Asian Journal of Physics
141 Vol. 19, Nos. 2 & 3 (2010) 141-146
Annealing effects on structural and electrical properties of Fe/Si interface Chhagan Lal, Renu Dhunna and I P Jain Centre for Non-conventional Energy Resources, University of Rajasthan, Jaipur, 302004, India
Fe films of 80 nm thickness were deposited onto Si (111) substrate using electron beam evaporation technique at 2 × 10–7 torr vacuum. Samples were annealed in 3 × 10 –5 torr vacuum at 500 and 600 °C temperatures for one hour for the formation of silicide phases. GIXRD results have revealed the formation a stable FeSi2 disilicide at the interface on annealing at 600 °C temperature. The Schottky Barrier Height (SBH) is calculated from I-V curves and Norde method [F(V)-V]. It is concluded that the SBH decreases with increasing the annealing temperature.© Anita Publications. All rights reserved.
1 Introduction Metal-Semiconductor surface and interface investigations have attracted scientist for intensive research due to their potential technological applications in microelectronics [1] since early 1980's. Fe/Si system is studied extensively as it finds applications in the development of magnetic silicon based heterostructures. Fe/Si thin films show abnormal magnetic and transport behaviors. The possibility of growing ferromagnetic layers on silicon is of great interest from a fundamental point of view since these systems provides peculiar opportunities for studying the links between atomic and magnetic properties. The growth of such magnetic layers on semiconducting substrates is also very important for possible applications in microelectronics, since it allows the integration of magnetic devices in silicon or similar technologies and opens up new possibilities in the area of data storage or of sensors. Iron is potentially an interesting metal for epitaxial growth silicon substrate and was the subject of several studies [2-5]. Spontaneous and significant intermixing exists for iron films, even at room temperature and the presence of Si in iron layer strongly affects ferromagnetism even in the epitaxial layers in such a way that magnetism can be quenched when Si concentrations come closer to FeSi stoichiometry [6]. It was shown that the use of appropriate templates considerably improves the structural quality of the layer and limits the interdiffusion between the deposited ferromagnetic elements and the Si substrate [7]. Ferromagnetic iron layers deposited epitaxially onto Si (111) substrate exhibit perpendicular and in-plane magnetism over a broad thickness range [8-11] which lead to complex spin reorientation mechanism involved during magnetization process [8, 12]. The characteristics at the interface are dictated by interdiffusion thickness, uniformity of interdiffusion parallel to interface and the structural properties at the interface. Various previous structural studies are important for the information about the crystalline nature of Fe layer and amorphous Si spacer layer [13-15]. The formation of iron silicide and its crystalline structure in spacer layer region is reported in sputtered Fe/Si multilayer [13, 14 - 16]. The metastable phases observed in the case of iron silicides crystallize in a cubic lattice (CsCl, fluorite). The cubic silicon substrate favors the growth of epitaxial layers with this symmetry (eventually slightly deformed along the substrate normal). The structures present in some cases a significant defect content, so that a broad range of stoichiometries can be observed (e.g. from FeSi(CsCl) to FeSi2(CsCl) [17, 18]. On the other hand, there exist bulk iron silicides crystallizing in cubic structures very similar to the metastable phases for Fe:Si stoichiometries up to Fe3Si. Kanel et al. [18] and Alvarez et al. [19] gave a detailed account on the phases observed and also on their properties. Particularly the Fe/Si interface is of fundamental interest in the development of magnetic, silicon based heterostructures. Recent interest has been focused on the investigation of the electronic structure of Corresponding author : E-mail:
[email protected] (Dr Chhagan Lal)
Chhagan Lal, Renu Dhunna and I P Jain
142
semiconducting Fe silicide films [20-25] due to their possible applications in the development of siliconintegrated optoelectronic devices. Lately another dimension has been added to this field of research due to the observation of antiferromagnetic interlayer coupling in Fe/Si multilayers [26, 27]. Various groups have attempted to understand the electrical and structural properties of Schottky contacts on n-GaN. Kalinina et al. [28], Wang et al. [29], Khanna et al. [30] and Reddy et al. [31] investigated Cr, Au and Ni Schottky contacts on n-GaN and reported their barrier height to be 0.53 and 0.58 eV for Cr, 1.03 eV for Au, 1.15 and 1.11 eV for Ni Schottky contacts using I-V and C-V measurements. Wang et al. [29] fabricated Pt/nGaN Schottky diode and showed a barrier height of 0.82 eV by I-V method for the as-deposited contact. Lal et al [32] also calculated the Schottky Barrier Height (SBH) of Ti/Si contacts by I-V method for asdeposited and annealed samples. In the present work we report on the structural and electrical properties of Fe/Si contacts as a function of annealing temperature. Structural properties have been studied using Grazing Incidence X-ray Diffraction (GIXRD). Electrical properties (I-V curves) of the system have been studied in vacuum and SBH has been calculated by using these I-V curves and Norde method. 2 Experiment The n-type Si (111) substrates of area 1×1 cm2 and of thickness 500 µm were initially chemically cleaned by a conventional procedure (TCE, acetone, methanol) and then dipped into a diluted HF (1:10) solution to remove impurities or any oxide layer present before loading into vacuum chamber. Fe films of 80 nm were deposited on to silicon substrate using electron beam evaporation at room temperature and at 2 × 10–7 torr vacuum. The Fe/Si samples were annealed at 500 and 6000C for one hour in 3 × 10 –5 torr vacuum for the formation of silicide phases. 3 Results and Discussions
F e (2 00 )
F eS i2(22 4)
F e(1 10 ) F eS i(2 10 )
S i(1 11 )
F eS i2(40 2)
GIXRD Study
F e Si(2 22 )
c. at 600 0 C
0
b. at 500 C
a. As-depo sited
20
30
40
50
60
2 Fig 1: GIXRD pattern of Fe/Si contacts
70
80
Annealing effects on structural and electrical properties of Fe/Si interface
143
As deposited and annealed samples were studied using Grazing Incidence X-ray Diffraction (GIXRD) to find the phases present at the interface. GIXRD measurements of pristine and annealed samples with Cu K radiation ( = 1.54 Å) were recorded in the 2 range 20-80 to identify the phases. Figure 1(ac) shows the diffraction pattern of the as deposited bilayer system and annealed samples at different temperatures. GIXRD results have revealed the formation a stable FeSi 2 disilicide at the interface on annealing at 600°C temperature. All the measurements show that intensity of Fe (110) and Fe (200) decreases, with increase in annealing showing some interfacial reaction for the formation of silicide. The x-ray diffraction is sensitive to the crystallite size calculated using the well known Scherrer formula, the average crystallite size (D) is given [33] as:
0.9 D (Cos )
... (1)
where is the X-ray wavelength, is the diffraction angle and is the line width of the diffraction profile resulting from small crystallite size. Using above equation, the crystallite size of FeSi2 phase was calculated and found to be 74.55 Å ± 0.02 Å. Electrical Measurements The contact properties obtained from the I-V characteristics (forward and reverse) of Fe/Si diodes as a function of annealing temperature are shown in Fig 2. It is observed that the characteristics of Fe/Si Schottky diodes are uniform over different diodes. Forward I-V characteristics were fitted using the relation for the thermionic emission over a barrier [34].
qVd 1 exp kT ... (1) Where Vd is the voltage across the diode (Vd = V?IRs), Rs the series resistance, n the ideality factor and I0 is the saturation current given by qV I d I 0 exp d nkT
q b I 0 AA **T 2 exp kT
Fig 2: I-V curves for Fe/Si contacts
... (2)
Chhagan Lal, Renu Dhunna and I P Jain
144
Where A** is the effective Richardson constant, A the diode area and b is the Schottky barrier height. The value of b can be deduced directly from the I-V curves if the effective Richardson constant, A** is known. Its theoretical value is 31.2 A cm–2 K–2 based on the effective mass (m*=0.26m0) of Si and was used here to deduce b. The ln(I) vs V Chhagan Lal characteristics are linear at low forward bias voltages. The linearity of this curve is due to the effect of series resistance. The series resistance influences downward curvatures of forward bias I-V characteristics, but the two parameters are significant in both the linear and nonlinear regions of I-V characteristics. Using Eq. (1), a plot of ln[Id /{1 – exp(–qV/kT)}] versus Vd (Fig. 3(a)) yields I0 as the intercept. Once I0 is determined, the barrier height is estimated from it. The series resistance Rs of the diode can be deduced from I-V measurements in the range Rs , for the as-deposited and annealed Fe/Si Schottky contacts. Measurements showed that the Schottky barrier height of as-deposited Fe/Si contact is 0.59 eV. The estimated barrier heights are 0.54 eV for 500 C and 0.49 eV for 600C annealed contacts, respectively. It is clear from the results that the SBH decreases with increase in the annealing temperature. According to Fermi level pinning model of Bardeen [35], when the interface state density increases from Schottky limit to Bardeen limit, the SBH decreases. One thereby concludes that annealing causes an increase in interface state density, leading to decrease in SBH with the formation of stable iron disilicides (FeSi 2). The forward current-voltage characteristics in each case showed the ideality factor was greater than one, suggesting transport mechanisms other than thermionic emission, such as recombination. 0.01 1E-3 1E-4 1E-5
As-dep 0 500C 0 600C
0.85
a
0.80
1E-6
As-dep 0 500C 0 600C
b
0.75
1E-7 0.70
1E-8
0.65
1E-10
F(V)
Id/[1-exp(-qV/kT)][A]
1E-9 1E-11
0.60
1E-12 1E-13
0.55
1E-14
0.50
1E-15 0.45
1E-16 1E-17 -0.8
-0.6
-0.4 Voltage(V)
-0.2
0.0
0.40 0.0
0.2
0.4 0.6 Voltage(V)
0.8
1.0
Fig 3: (a). Plot of Id/[1- exp(-qV/kT)] vs. V, & (b). Plot of F(V) vs. V for the Fe/Si contacts annealed at different temperatures.
The Norde method was also employed [36] to compare the Schottky barrier height of contacts because high series resistance can hinder an accurate evaluation of barrier height form the standard ln(I)-V plot. In this method a function F(V) is plotted against th®e V. F(V) is given by the following equation
Annealing effects on structural and electrical properties of Fe/Si interface
F (V )
145
V I (V ) ln 2 AA **T 2
... (3)
The effective SBH is given by
b F (Vmin )
Vmin kT 2 q
... (4)
where F(Vmin) is the minimum value of F(V) and Vmin is the corresponding voltage. A plot of F(V) versus V for the samples annealed at different temperatures is shown in Fig. 3(b). Calculation showed that the SBH are 0.588 eV for the as-deposited, 0.545 eV at 500°C and 0.484 eV at 600°C, respectively. 4 Conclusion GIXRD results have revealed a stable disilicide FeSi2 at the interface at higher annealing temperature 600°C. The SBH is calculated by I-V curves (direct method) and Norde method. It is concluded that the SBH is decreases with increasing the annealing temperature. Table 1: Calculated Schottky Barrier Height (SBH) by direct and Norde method SBH
As-deposited (eV)
Annealed 500°C (eV)
Annealed 600°C (eV)
By I-V curve
0.59
0.54
0.49
By Norde method
0.588
0.545
0.484
5 Acknowledgments The authors are thankful to MNRE, New Delhi for financial support for this work. The authors are also thankful to UDC, Indore for providing the GIXRD facility. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.
Murarka S P, Silicides for VLSI Applications, New York, 1985. Klasgas R, Carbone C, Eberhardt W, Pampuch C, Rader O, Kachel J, Gutat W, Phys Rev, B56(1997)10801. Alvarez J, Hinajeros J J, Michel E G, Castro G R, Miranda R, Phys Rev, 45(1997)14042. Ruhrnshopf K, Borgmann D, Wedler G, Thin Solid Films, 280(1996)171. Gallego J M, Garcia J M, Alvarez J, Miranda R, Phys Rev, B46(1992)13369. Berling D, Gewinner G, Hanf M C, Hricovini K, Hong S, Loegel B, Mehdaoui A, Pirri C, Tuilier M H, Wetzel P, J Magn Magn Mater, 191(1999)331. Bertoncini P, Wetzel P, Berling D, Gewinner G, Ulhaq-Bouillet C, Pierron Bohnes V, Phys Rev, B60(1999) 11123. Qiu Z Q, Pearson J, Bader S D, Phys Rev Lett, 70(1993)1006. Nadir Z H, Lo C K, Hardiman M, J Magn Magn Mater, 156(1996)435. Hong S, Pirri C, Wetzel P, Gewinner G, Boukari S, Beaurepaire E, J Magn Magn Mater, 171(1997)280. Foss S, Merton C, Proksch R, Skidmore G, Schmidt J, Dahlberg E D, Pokhil T, Cheng Y T, J Magn Magn Mater, 190(1998)60. Ding H F, Putter S, Oepen H P, Kirschner J, J Magn Magn Mater, 212(2000)L5. Chaiken A, Michel R P, Wall M A, Phys Rev, B53(1996)5518. Gosele U, Tu K N, J Appl Phys, 53(1982)3252. d’ Heurla F M, J Mater Res, 3(1988)167. Von Kanel H, Mader K A, Muller F, Onda N, Sirringhans H, Phys Rev, B45(1992)13807. Bost M C, Mahan J, J Appl Phys, 58(1985)2896.
146
Chhagan Lal, Renu Dhunna and I P Jain
18. von K~inel H, Onda N, Sirringhaus K, MUller-Gubler E, Gonsalves-Conto S, Schwarz C, Appl Surf Sci. 70/71 (1993)559.t 19. Alvarez J, Hinarejos J J, Michel E G, Castro G R, Miranda R, Phys Rev, B45(1992)14 042. 20. Kobayashi N et al, Thin Solid Films, 270(1995)406. Give the names of all authors. 21. Eisebitt S et al., Phys Rev, B 50(1994)18 330. Give the names of all authors. 22. Lefki K, Muret P, J Appl Phys, 74(1993)1138. 23. Lal C, Dhunna R, Dhaka R S, Barman S R, Jain I P, J Optoelectronics and Advance Materials, 12(2010)177. 24. Lal C, Dhunna R, Jain I P, Material Science in Semiconducting Process, 1(2008)1. 25. Lal C, Jain R K, Jain I P, Bulletin of Material Sciences, 30(2007)153. 26. Toscano S et al., J Magn Magn Mater, 114(1992)L6. 27. Fullerton E E, Mattson J E, J Magn Magn Mater, 117(1992)L301. 28. Kalinina E V et al., J Electron. Mater, 25(1996)831. 29. Wang J, Zhao D G, J Phys D:Appl Phys, 36(2003)1018. 30. Khanna R et al., Appl Surf Sci, 252(2006)622. 31. Reddy V R et al., Mat Sci & Engg, B137(2007)200. 32. Lal C, Dhunna R, Jain I P, Vacuum, 83(2009)931. 33. Danilchenko S N, Kukharenko O G, Moseke C, Protsenko I Yu, Sukhodub L F, Sulkio-Cleff B, Cryst Res Technol, 37(2002)1234. 34. Rhoderick E H, Williams T H, Metal-Semiconductor Contacts, Oxford Science, Oxford, 1988. 35. Bardeen J, Phys Rev, 71(1947)717. 36. Norde H, J Appl Phys, 50(1979)5052. [Received: ; accepted: ] Author: Kindly check your references and put them in exact style of AJP ;Vol No(Year)Pagenumber. Pl check the corrections made by us.Use standard of abbreviations of Journals. Give the names of all the authors.
