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Frequency-dependent conductivity in tris(acetylacetonato)manganese(III) thin films on Si(1 0 0) substrates
Materials Chemistry and Physics 96 (2006) 422–426
Frequency-dependent conductivity in tris(acetylacetonato) manganese(III) thin films on Si(1 0 0) substrates A.A. Dakhel Department of Physics, College of Science, University of Bahrain, P.O. Box 32038, Kingdom of Bahrain Received 2 February 2005; received in revised form 11 June 2005; accepted 19 July 2005
Abstract Thin tris(acetylacetonate)manganese(III) films of amorphous structure were prepared by vacuum deposition on glass and Si (1 0 0) substrates. The as-deposited and annealed-in-vacuum films were characterised by X-ray fluorescence, X-ray diffraction and optical absorption spectroscopy. The prepared title-complex amorphous films were investigated as insulators for Al/insulator/Si(P) metal–insulator–semiconductor (MIS) structures, which were characterised by the measurement of their capacitance and AC-conductance as a function of gate voltage. From those measurements, the state density Dit at insulator/semiconductor interface and the density of the fixed charges in the complex insulator were determined. It was found that Dit was in order of 1011 eV−1 cm−2 and the surface charge density in the insulator film was in order of 1011 –1012 cm−2 .The frequency dependence of the electrical conductivity and dielectric properties of MIS structures were studied at room temperature. The results follow the correlated barrier-hopping (CBH) model, from which the fundamental absorption bandgap, the minimum hopping distance and other parameters of the model were determined. This study shows that the tris(acetylacetonate)manganese(III) films grown on Si(1 0 0) is a promising candidate for high-ε dielectric applications. It displays sufficiently high-ε value in the range 30–40. © 2005 Elsevier B.V. All rights reserved. PACS: 77.55.+f; 72.20.−I; 78.70.En Keywords: Insulating films; Tris(acetylacetonate)manganese(III); Dielectric phenomena; CBH model
1. Introduction The metal ions in metal-substituted organic coordination complexes (MOSC) play a role of attraction centres through strong metal–ligand covalent bonds [1], which enhance the stability of the molecules even under sublimation. Among these MOSC is the dark-brown tris(acetylacetonato)manganese(III) or tris(2,4-pentanedionato)manganese(III), which has several CHEMICAL applications like polymerisation of vinyl monomers, combustion catalyst, curing agent for isocyanate resins and deposition of manganese oxide. Its molecular structure can be described as follows: one acetylacetonate anion [abb. acac: (CH3 COCHCOCH3 )− ] serve as a ligand to a metal ion, forming chemical structure in which the ligand is bonded to the metal ion through both oxygen atoms forming a planer six-membered ring (Mn-O2 C3 ) [2]. This binding E-mail address: [email protected]. 0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.07.034
can be thought of as consisting of a covalent bond through one oxygen atom and a dative-covalent bond through the other oxygen and this bonding is delocalised. In the complex of molecular stoichiometry Mn(acac)3 , each Mn ion is bonded to three acac radicals, so that the Mn-O6 array forms an octahedral coordination polyhedron, which suffers from Jahn–Teller distortion in form of either a tetragonal elongation (TE) (two Mn O = 0.212 nm; four Mn O = 0.193 nm) or a moderate tetragonal compression (TC) (two Mn O = 0.195 nm; four Mn O = 0.200 nm) [1–3]. In the solid state, the complex molecules arrange themselves in three different crystal structures (␣,  and ␥ [2,4]) depending on the preparation conditions. The starting material in the present investigation is the familiar -phase of a monoclinic structure (P21 /c) with a = 1.4013 nm, b = 0. 76 nm, c = 1.6373 nm and β = 99.33◦ [2]. In the present investigation, we have prepared the titlecomplex thick films by sublimation in vacuum on Si(1 0 0) substrates and report here on their AC-electrical properties as
A.A. Dakhel / Materials Chemistry and Physics 96 (2006) 422–426
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a function of frequency. This well-known coordination complex has not been studied before by AC-electrical method.
2. Experimental details The synthesis of crystalline tris(acetylacetonato)manganese(III) powder has been described in detail elsewhere [5]. Thin films of the complex (Mn-acac) were slowly deposited (about 0.1 nm s−1 ) by thermal sublimation using Mo boat in a vacuum system of about 10−3 Pa on Si(1 0 0) substrates held at room temperature. The boron-doped at level of 1.2 × 1022 m−3 Si wafer substrates were thermo-chemically cleaned with 50% potassium hydroxide solution at 65 ◦ C for 15 min. Two sorts of Mn-acac layers were investigated; namely the as-deposited and the annealed in vacuum of about 10−3 Pa at about 100 ◦ C for 10 min. The annealing was done in order to produce microstructural variations to reduce the density of the trapped charges in the deposited layers. The electrical measurements on the title-complex layers as insulators were done on samples prepared in form of metal–insulator–semiconductor MIS structures, which constructed by deposition of aluminum-film electrodes of about 150 nm. The complex layer thickness was measured by Gaertner L117 ellipsometer of λ = 632.8 nm to be about 495 nm with refractive index of about 1.3. The Mn content in the deposited film samples and the constituent powder was probed by the X-ray fluorescence (XRF) method. The exciting Ni-filtered X-ray Cu radiation (λ = 0.15405 nm) was incident on the film surface at 15◦ and the fluorescent yield was collected at 90◦ by using an Amptek XR-100CR, Si detector. The crystal structure was investigated with a Philips PW 1729 X-ray diffractometer using Cu K␣ radiation with a scanning speed of 0.01◦ s−1 . The AC-electrical measurements were performed using a Keithley 3330 LCZ instrument with a signal of 50 mV and the DC-measurements were carried out with a Keithley 614 electrometer.
Fig. 1. XRF spectrum of tris(acetylacetonate)manganese(III) powder and thin film grown on Si substrate. The exciting line was Cu K␣ line of energy 8.047 keV.
as-deposited and the annealed thin films show amorphous structure. The spectral normal transmittance T(λ) of the amorphous Mn-acac film grown on glass substrate in the transparent and absorption regions (200–1100 nm) was measured by UV–VIS-Shimadzu double beam spectrophotometer and the results are shown in Fig. 3. The transmittance data were corrected relative to the optically identical uncoated substrate. The investigated sample has high transparency T > 0.90 in the transparent region and a sharp absorption edge, which refer to the direct transitions. The energy gap was calculated according to the method discussed in Ref. [6] and using Hamberg et al. absorption relation [7], to be about 3.66 eV. 3.2. Capacitance–voltage analysis Prior to AC-electrical conductivity measurements, the constructed MIS structures should be electrically characterised by measuring their high-frequency capacitance as a function of gate voltage. Those measurements determine the effective relative permittivity (RP) of the insulating layer, the
3. Characterisation details 3.1. Mn-acac films characterisation Fig. 1 shows the XRF spectrum of Mn-acac powder and thin film grown on Si substrate. There are two considerable peaks; namely a Si K␣ signal of energy 1.74 KeV and a Mn K␣ signal of energy 5.898 keV. The appearance of the Mn signal radiated from the deposited thin film ensures the stability of Mn-acac molecules during the sublimation at about 150 ◦ C. Fig. 2 shows the X-ray diffraction (XRD) pattern of the prepared Mn(acac)3 powder, which shows a monoclinic structure of parameters a = 1.41 nm, b = 0.77 nm, c = 1.65 nm and β = 99.4◦ that are almost comparable to those given in Ref. [2] for the -phase. The XRD patterns of both the
Fig. 2. X-ray diffraction of tris(acetylacetonate)manganese(III) powder and thin film. The beam was Cu K␣ and the scan speed was 0.01◦ s−1 .
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A.A. Dakhel / Materials Chemistry and Physics 96 (2006) 422–426 Table 1 The values of the flat-band voltage VFB , the density of the charges in the insulator layer Qins , the interfacial density of states Dit and the effective relative permittivity εins at 293 K for tris(acetylacetonate)manganese(III) films of amorphous structures grown on Si substrates Sample As-prepared Annealed
Fig. 3. Normal spectral transmittance T(λ) and absorption coefficient α(E) (inset) of amorphous Mn(acac)3 film grown on glass substrate in transparent and fundamental absorption regions.
voltage border of the accumulation region and the density of charges in the insulating layer and in the film/Si substrate interface, from which one can decide the possibility of construction of an MIS device Fig. 4 shows the gate-voltage dependence of the capacitance C(Vg ) and AC-conductance G(Vg ) at 100 kHz using a parallel-circuit mode at room temperature. The correction of the parallel conductance (Gc ) for the series resistance was done according to the relations given in Refs. [8,9]. However, those curves have the ordinary forms of MIS structures. The capacitance of the MIS structure at high frequencies in accumulation mode is controlled basically by the dielectric properties of the bulk insulator [10]. Therefore, it is possible to calculate the effective RP (εins ) of the insulator from the accumulation capacitance per unit area C acc at 100 kHz, and the results are given in Table 1. These values are well
VFB (V)
Qins (charges cm−2 )
Dit (eV−1 cm−2 )
εins
−12.3 −3.3
+5.0 × 1012
7.2 × 1011 5.3 × 1011
39.2 29.1
+7.8 × 1011
above 3.82 of the SiO2 that is usually used as insulator in the MIS devices. But, the structural changes due to annealing of the prepared layer reduce the relative permittivity by about 25%. The flat-band voltage of the C(Vg ) curves are measured and given in Table 1. Then, it is possible to estimate the density per unit area of the charges (Qins ) in the film according to the method given in Refs. [10,11] and the results are presented in Table 1. The density Qins is strongly reduced under annealing, which is expectable as the annealing reduces the density of vacancies, dangling bonds and various types of defects and improving the stoichiometry and homogeneity in the sample [12]. Generally, these charges Qins are considered as trap centres, which control the DC-conduction process. The interface trap density Dit at Si midgap is estimated from the combination of a single high-frequency (ω = 2π × 105 s−1 ) C(Vg ) and G(Vg ) characteristics using Hill’s method [13]: Dit =
Gmax /ω 2 eA [(Gmax /ωCox )2 + (1 − Cm /Cox )2 ]
(1)
where A is the gate area and Gmax is the maximum measured conductance in the G–Vg plot with its corresponding measured capacitance Cm . Therefore, Hill’s method needs to determine Gmax from G(V) curve, capacitance at accumulation and at voltage position of Gmax . The result of calculation of Dit is given in Table 1, which is not high enough to pin the Fermi level of the Si substrate [14]. Consequently, the interface traps and defects cannot prevent the construction of an MIS device.
4. AC conductivity The experimental dependence of AC conductivity of insulators on the frequency is usually expressed according to the following relation [15]: σAC = σDC (0) + Aσ ωs
Fig. 4. Gate-voltage dependence of the capacitance C(Vg ) and the corrected conductance Gc (Vg ) measured at 100 kHz for Al/Mn(acac)3 /Si(P) MIS device at room temperature. The measuring signal was 50 mV.
(2)
where σ DC (0) is the DC conductivity, Aσ is the preexponential and s is the exponent. Therefore, the suitable explanation for the conduction in insulators is the thermally activated hopping of carriers that was formulated theoretically through the correlated barrier hopping (CBH) model, which led to the power law (Aσ ωs ) giving the following
A.A. Dakhel / Materials Chemistry and Physics 96 (2006) 422–426
Fig. 5. Frequency dependence of: (a) the AC conductivity σ AC (ω) and (b) the capacitance C(ω) measured at room temperature under accumulation voltages for films grown on Si substrate.
relation [16–18]: s=1−
6kB T WM + kB T ln(ωτo )
(3)
where WM is the maximum barrier height, which is equal to the bandgap of the material for bipolaron hopping [18] and ω is the average of the used frequency range of fitting. In the CBH model, the current carriers hop between sites over a columbic barrier of height W of magnitude W = Wm − (ncarr e2 /πεεo r), where r is separation between two hopping sites and ncarr is the number of carriers involved in a hop (ncarr = 1 or 2 for single or bipolaron processes, respectively). Fig. 5 presents the frequency dependence of the AC conductivity σ AC and the capacitance C of the constructed MIS structures measured at room temperature in the frequency range of 0.5–100 kHz under the accumulation voltage of −14 V for the as prepared and −8 V for the annealed sample. Those characteristics are determined mainly by the capacitance and the AC conductivity of the insulating layer. Therefore, the measured data can be analysed according to the
425
models specified for insulators. The present results can be explained in the framework of the power law or the CBH model. The fitting value of “s” was 0.953 for frequencies f > 5 kHz for both sample sorts. This means that the conductivity data follow the CBH model conditioned by s < 1 as given in Eq. (3), which provides the value of Wm to be about 3.66 eV. This value is equal to the fundamental absorption band, which means that the conduction is realised by a bipolaron process. The application of the CBH model, according to the analysis given elsewhere [19], gives the concentration of hopping sites (Nhs ) to be of order 1022 cm−3 for bipolaron process. The obtained value of Nhs is one order of magnitude higher than the concentration of manganese ions (2.3 × 1021 cm−3 ) in the investigated material. This may refer to the influence of imperfections like dangling bonds and fluctuation of density and chemical compositions as sites participating in the conduction process. Accordingly, the annealing process, which reduces the imperfection density, reduces also the concentration of conduction sites by about five times in the present investigation. The values of r were calculated to be r ≥ 0.08 nm for the as-deposited and r ≥ 0.12 nm for the annealed sample, which might refer to realization of some molecular arrangement in the annealed film. The value of the parameter σ DC (0) was extracted to be in the range of 10−4 to 10−5 S m−1 for both samples. This parameter would be measured if CBH model can be applied for the low frequency range (say f < 1 kHz). But, the charges Qins , which can response to the measuring signal at low frequencies interfere the experimental data to comply with the CBH model. However, it has been found that the extracted value of σ DC (0) was comparable to the real DC-conductivity measured by the usual DC method using Ohm’s law under the accumulation voltage for each sample. The capacitance decreases with increasing signal frequency, which can be attributed to the effect of charge redistribution by carrier hopping on centres [20]. According to Kramers–Kronig (KK) relations [21–23], the theoretical consequence of the application of the power-law guides to the following relation: C(ω) ∝ ωs −1 for the frequency dependence of the capacitance at accumulation voltage [24]. But, the applicability of the KK relation in the present work with exponent s = 0.85 as shown in Fig. 4b is limited especially for the as-deposited sample due to higher value of Qins .
5. Conclusion In the present work, amorphous tris(acetylacetonate)manganese(III) films were prepared on Si (P) substrates by sublimation in a vacuum chamber. The films were characterised by XRF, XRD and spectral optical absorption. The constructed Al/insulator/Si MIS devices were characterised by measuring their capacitance and AC-conductance as a function of gate voltage. The frequency dependence of the AC conductivity obeys the power law. It appears that the hop
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of current carriers between defects is realised over potential barriers (W) of values depend on the separation of those sites according to the CBH model. This study shows that the tris(acetylacetonate)manganese(III) films grown on Si(1 0 0) is a promising candidate for high-ε dielectric applications, it displays sufficiently high-ε value around 40. Furthermore, it is possible to obtain an acceptable level of Dit and Qins , which is necessary condition for the high-k gate dielectric material applications according to the road map stated in Ref. [25].
Acknowledgement The Author is grateful to Dr. Ahmed Y. Ali-Mohamed from Dept. of Chemistry, Coll. of science, University of Bahrain, Kingdom of Bahrain for his many useful and interesting discussions about the metal-substituted organic materials. References [1] J.C. Bailar Jr., H.J. Emeleus, R. Nyholm, A.F. Trotman-Dickenson (Eds.), Comprehensive Inorganic Chemistry, Pergamon Press, Oxford, 1975, p. 872. [2] J.P. Fackler, A. Avdeef, Inorg. Chem. 13 (1974) 1864. [3] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, fifth ed., Willey-Interscience Publication, NY, 1988, p. 705. [4] B.R. Stults, R.S. Marianelli, V.W. Day, Inorg. Chem. 18 (1979) 1853.
[5] G. Pass, H. Sutcliffe, Practical Inorganic Chemistry: Preparations, Reactions and Instrumental Methods, second ed., Chapman and Hall Ltd., London, 1979. [6] E.G. Birgin, I. Chambouleyron, J.M. Martinez, J. Comp. Phys. 151 (1999) 862. [7] I. Hamberg, C.G. Granqvist, K.-F. Berggren, B.E. Sernelius, L. Engstrom, Phys. Rev. B 30 (1984) 3240. [8] S. Pal, D.N. Bose, Appl. Surf. Sci. 181 (2001) 179. [9] E.H. Nicollian, J.R. Brews, MOS Physics and Technology, Wiley, New York, 1982. [10] S.M. Sze, Physics of Semiconductor Devices, second ed., John Wiley & Sons Inc., 1981, p. 397. [11] D.A. Neamen, Semiconductor Physics and Devices-Basic principles, second ed., Irwin-McGraw-Hill Comp USA Inc, 1997, p. 434. [12] J.P. de Neufville, S.C. Moss, S.R. Ovshinsky, J. Non-Cryst. Solids 13 (1973) 191. [13] W.H. Hill, Solid State Electron. 23 (1980) 987. [14] N. Konofaos, Microelectron. J. 35 (2004) 421. [15] R.M. Hill, A.K. Jonscher, J. Non-Cryst. Solids 32 (1979) 53. [16] S.R. Elliott, Adv. Phys. 36 (1987) 135. [17] A.R. Long, Adv. Phys. 31 (1982) 553. [18] S.R. Elliott, Philos. Mag. 36 (1977) 1291. [19] A.A. Dakhel, Chem. Phys. Lett. 393 (2004) 528. [20] A. Vasudevan, S. Carin, M.A. Melloch, S. Harmon, Appl. Phys. Lett. 73 (1998) 671. [21] A. Mansingh, R.P. Tandon, J.K. Vaid, J. Phys. Chem. Solids 36 (1975) 1267. [22] K.K. Srivastava, A. Kumar, O.S. Panwar, L.N. Lakshminarayan, J. Non-Cryst. Solids 33 (1979) 205. [23] M.A.M. Seyam, Appl. Surf. Sci. 181 (2001) 128. [24] A. Ghosh, Phys. Rev. B 41 (1990) 1479. [25] K. Cho, Comput. Mater. Sci. 23 (2002) 43.
Frequency-dependent conductivity in tris(acetylacetonato) manganese(III) thin films on Si(1 0 0) substrates A.A. Dakhel Department of Physics, College of Science, University of Bahrain, P.O. Box 32038, Kingdom of Bahrain Received 2 February 2005; received in revised form 11 June 2005; accepted 19 July 2005
Abstract Thin tris(acetylacetonate)manganese(III) films of amorphous structure were prepared by vacuum deposition on glass and Si (1 0 0) substrates. The as-deposited and annealed-in-vacuum films were characterised by X-ray fluorescence, X-ray diffraction and optical absorption spectroscopy. The prepared title-complex amorphous films were investigated as insulators for Al/insulator/Si(P) metal–insulator–semiconductor (MIS) structures, which were characterised by the measurement of their capacitance and AC-conductance as a function of gate voltage. From those measurements, the state density Dit at insulator/semiconductor interface and the density of the fixed charges in the complex insulator were determined. It was found that Dit was in order of 1011 eV−1 cm−2 and the surface charge density in the insulator film was in order of 1011 –1012 cm−2 .The frequency dependence of the electrical conductivity and dielectric properties of MIS structures were studied at room temperature. The results follow the correlated barrier-hopping (CBH) model, from which the fundamental absorption bandgap, the minimum hopping distance and other parameters of the model were determined. This study shows that the tris(acetylacetonate)manganese(III) films grown on Si(1 0 0) is a promising candidate for high-ε dielectric applications. It displays sufficiently high-ε value in the range 30–40. © 2005 Elsevier B.V. All rights reserved. PACS: 77.55.+f; 72.20.−I; 78.70.En Keywords: Insulating films; Tris(acetylacetonate)manganese(III); Dielectric phenomena; CBH model
1. Introduction The metal ions in metal-substituted organic coordination complexes (MOSC) play a role of attraction centres through strong metal–ligand covalent bonds [1], which enhance the stability of the molecules even under sublimation. Among these MOSC is the dark-brown tris(acetylacetonato)manganese(III) or tris(2,4-pentanedionato)manganese(III), which has several CHEMICAL applications like polymerisation of vinyl monomers, combustion catalyst, curing agent for isocyanate resins and deposition of manganese oxide. Its molecular structure can be described as follows: one acetylacetonate anion [abb. acac: (CH3 COCHCOCH3 )− ] serve as a ligand to a metal ion, forming chemical structure in which the ligand is bonded to the metal ion through both oxygen atoms forming a planer six-membered ring (Mn-O2 C3 ) [2]. This binding E-mail address: [email protected]. 0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.07.034
can be thought of as consisting of a covalent bond through one oxygen atom and a dative-covalent bond through the other oxygen and this bonding is delocalised. In the complex of molecular stoichiometry Mn(acac)3 , each Mn ion is bonded to three acac radicals, so that the Mn-O6 array forms an octahedral coordination polyhedron, which suffers from Jahn–Teller distortion in form of either a tetragonal elongation (TE) (two Mn O = 0.212 nm; four Mn O = 0.193 nm) or a moderate tetragonal compression (TC) (two Mn O = 0.195 nm; four Mn O = 0.200 nm) [1–3]. In the solid state, the complex molecules arrange themselves in three different crystal structures (␣,  and ␥ [2,4]) depending on the preparation conditions. The starting material in the present investigation is the familiar -phase of a monoclinic structure (P21 /c) with a = 1.4013 nm, b = 0. 76 nm, c = 1.6373 nm and β = 99.33◦ [2]. In the present investigation, we have prepared the titlecomplex thick films by sublimation in vacuum on Si(1 0 0) substrates and report here on their AC-electrical properties as
A.A. Dakhel / Materials Chemistry and Physics 96 (2006) 422–426
423
a function of frequency. This well-known coordination complex has not been studied before by AC-electrical method.
2. Experimental details The synthesis of crystalline tris(acetylacetonato)manganese(III) powder has been described in detail elsewhere [5]. Thin films of the complex (Mn-acac) were slowly deposited (about 0.1 nm s−1 ) by thermal sublimation using Mo boat in a vacuum system of about 10−3 Pa on Si(1 0 0) substrates held at room temperature. The boron-doped at level of 1.2 × 1022 m−3 Si wafer substrates were thermo-chemically cleaned with 50% potassium hydroxide solution at 65 ◦ C for 15 min. Two sorts of Mn-acac layers were investigated; namely the as-deposited and the annealed in vacuum of about 10−3 Pa at about 100 ◦ C for 10 min. The annealing was done in order to produce microstructural variations to reduce the density of the trapped charges in the deposited layers. The electrical measurements on the title-complex layers as insulators were done on samples prepared in form of metal–insulator–semiconductor MIS structures, which constructed by deposition of aluminum-film electrodes of about 150 nm. The complex layer thickness was measured by Gaertner L117 ellipsometer of λ = 632.8 nm to be about 495 nm with refractive index of about 1.3. The Mn content in the deposited film samples and the constituent powder was probed by the X-ray fluorescence (XRF) method. The exciting Ni-filtered X-ray Cu radiation (λ = 0.15405 nm) was incident on the film surface at 15◦ and the fluorescent yield was collected at 90◦ by using an Amptek XR-100CR, Si detector. The crystal structure was investigated with a Philips PW 1729 X-ray diffractometer using Cu K␣ radiation with a scanning speed of 0.01◦ s−1 . The AC-electrical measurements were performed using a Keithley 3330 LCZ instrument with a signal of 50 mV and the DC-measurements were carried out with a Keithley 614 electrometer.
Fig. 1. XRF spectrum of tris(acetylacetonate)manganese(III) powder and thin film grown on Si substrate. The exciting line was Cu K␣ line of energy 8.047 keV.
as-deposited and the annealed thin films show amorphous structure. The spectral normal transmittance T(λ) of the amorphous Mn-acac film grown on glass substrate in the transparent and absorption regions (200–1100 nm) was measured by UV–VIS-Shimadzu double beam spectrophotometer and the results are shown in Fig. 3. The transmittance data were corrected relative to the optically identical uncoated substrate. The investigated sample has high transparency T > 0.90 in the transparent region and a sharp absorption edge, which refer to the direct transitions. The energy gap was calculated according to the method discussed in Ref. [6] and using Hamberg et al. absorption relation [7], to be about 3.66 eV. 3.2. Capacitance–voltage analysis Prior to AC-electrical conductivity measurements, the constructed MIS structures should be electrically characterised by measuring their high-frequency capacitance as a function of gate voltage. Those measurements determine the effective relative permittivity (RP) of the insulating layer, the
3. Characterisation details 3.1. Mn-acac films characterisation Fig. 1 shows the XRF spectrum of Mn-acac powder and thin film grown on Si substrate. There are two considerable peaks; namely a Si K␣ signal of energy 1.74 KeV and a Mn K␣ signal of energy 5.898 keV. The appearance of the Mn signal radiated from the deposited thin film ensures the stability of Mn-acac molecules during the sublimation at about 150 ◦ C. Fig. 2 shows the X-ray diffraction (XRD) pattern of the prepared Mn(acac)3 powder, which shows a monoclinic structure of parameters a = 1.41 nm, b = 0.77 nm, c = 1.65 nm and β = 99.4◦ that are almost comparable to those given in Ref. [2] for the -phase. The XRD patterns of both the
Fig. 2. X-ray diffraction of tris(acetylacetonate)manganese(III) powder and thin film. The beam was Cu K␣ and the scan speed was 0.01◦ s−1 .
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A.A. Dakhel / Materials Chemistry and Physics 96 (2006) 422–426 Table 1 The values of the flat-band voltage VFB , the density of the charges in the insulator layer Qins , the interfacial density of states Dit and the effective relative permittivity εins at 293 K for tris(acetylacetonate)manganese(III) films of amorphous structures grown on Si substrates Sample As-prepared Annealed
Fig. 3. Normal spectral transmittance T(λ) and absorption coefficient α(E) (inset) of amorphous Mn(acac)3 film grown on glass substrate in transparent and fundamental absorption regions.
voltage border of the accumulation region and the density of charges in the insulating layer and in the film/Si substrate interface, from which one can decide the possibility of construction of an MIS device Fig. 4 shows the gate-voltage dependence of the capacitance C(Vg ) and AC-conductance G(Vg ) at 100 kHz using a parallel-circuit mode at room temperature. The correction of the parallel conductance (Gc ) for the series resistance was done according to the relations given in Refs. [8,9]. However, those curves have the ordinary forms of MIS structures. The capacitance of the MIS structure at high frequencies in accumulation mode is controlled basically by the dielectric properties of the bulk insulator [10]. Therefore, it is possible to calculate the effective RP (εins ) of the insulator from the accumulation capacitance per unit area C acc at 100 kHz, and the results are given in Table 1. These values are well
VFB (V)
Qins (charges cm−2 )
Dit (eV−1 cm−2 )
εins
−12.3 −3.3
+5.0 × 1012
7.2 × 1011 5.3 × 1011
39.2 29.1
+7.8 × 1011
above 3.82 of the SiO2 that is usually used as insulator in the MIS devices. But, the structural changes due to annealing of the prepared layer reduce the relative permittivity by about 25%. The flat-band voltage of the C(Vg ) curves are measured and given in Table 1. Then, it is possible to estimate the density per unit area of the charges (Qins ) in the film according to the method given in Refs. [10,11] and the results are presented in Table 1. The density Qins is strongly reduced under annealing, which is expectable as the annealing reduces the density of vacancies, dangling bonds and various types of defects and improving the stoichiometry and homogeneity in the sample [12]. Generally, these charges Qins are considered as trap centres, which control the DC-conduction process. The interface trap density Dit at Si midgap is estimated from the combination of a single high-frequency (ω = 2π × 105 s−1 ) C(Vg ) and G(Vg ) characteristics using Hill’s method [13]: Dit =
Gmax /ω 2 eA [(Gmax /ωCox )2 + (1 − Cm /Cox )2 ]
(1)
where A is the gate area and Gmax is the maximum measured conductance in the G–Vg plot with its corresponding measured capacitance Cm . Therefore, Hill’s method needs to determine Gmax from G(V) curve, capacitance at accumulation and at voltage position of Gmax . The result of calculation of Dit is given in Table 1, which is not high enough to pin the Fermi level of the Si substrate [14]. Consequently, the interface traps and defects cannot prevent the construction of an MIS device.
4. AC conductivity The experimental dependence of AC conductivity of insulators on the frequency is usually expressed according to the following relation [15]: σAC = σDC (0) + Aσ ωs
Fig. 4. Gate-voltage dependence of the capacitance C(Vg ) and the corrected conductance Gc (Vg ) measured at 100 kHz for Al/Mn(acac)3 /Si(P) MIS device at room temperature. The measuring signal was 50 mV.
(2)
where σ DC (0) is the DC conductivity, Aσ is the preexponential and s is the exponent. Therefore, the suitable explanation for the conduction in insulators is the thermally activated hopping of carriers that was formulated theoretically through the correlated barrier hopping (CBH) model, which led to the power law (Aσ ωs ) giving the following
A.A. Dakhel / Materials Chemistry and Physics 96 (2006) 422–426
Fig. 5. Frequency dependence of: (a) the AC conductivity σ AC (ω) and (b) the capacitance C(ω) measured at room temperature under accumulation voltages for films grown on Si substrate.
relation [16–18]: s=1−
6kB T WM + kB T ln(ωτo )
(3)
where WM is the maximum barrier height, which is equal to the bandgap of the material for bipolaron hopping [18] and ω is the average of the used frequency range of fitting. In the CBH model, the current carriers hop between sites over a columbic barrier of height W of magnitude W = Wm − (ncarr e2 /πεεo r), where r is separation between two hopping sites and ncarr is the number of carriers involved in a hop (ncarr = 1 or 2 for single or bipolaron processes, respectively). Fig. 5 presents the frequency dependence of the AC conductivity σ AC and the capacitance C of the constructed MIS structures measured at room temperature in the frequency range of 0.5–100 kHz under the accumulation voltage of −14 V for the as prepared and −8 V for the annealed sample. Those characteristics are determined mainly by the capacitance and the AC conductivity of the insulating layer. Therefore, the measured data can be analysed according to the
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models specified for insulators. The present results can be explained in the framework of the power law or the CBH model. The fitting value of “s” was 0.953 for frequencies f > 5 kHz for both sample sorts. This means that the conductivity data follow the CBH model conditioned by s < 1 as given in Eq. (3), which provides the value of Wm to be about 3.66 eV. This value is equal to the fundamental absorption band, which means that the conduction is realised by a bipolaron process. The application of the CBH model, according to the analysis given elsewhere [19], gives the concentration of hopping sites (Nhs ) to be of order 1022 cm−3 for bipolaron process. The obtained value of Nhs is one order of magnitude higher than the concentration of manganese ions (2.3 × 1021 cm−3 ) in the investigated material. This may refer to the influence of imperfections like dangling bonds and fluctuation of density and chemical compositions as sites participating in the conduction process. Accordingly, the annealing process, which reduces the imperfection density, reduces also the concentration of conduction sites by about five times in the present investigation. The values of r were calculated to be r ≥ 0.08 nm for the as-deposited and r ≥ 0.12 nm for the annealed sample, which might refer to realization of some molecular arrangement in the annealed film. The value of the parameter σ DC (0) was extracted to be in the range of 10−4 to 10−5 S m−1 for both samples. This parameter would be measured if CBH model can be applied for the low frequency range (say f < 1 kHz). But, the charges Qins , which can response to the measuring signal at low frequencies interfere the experimental data to comply with the CBH model. However, it has been found that the extracted value of σ DC (0) was comparable to the real DC-conductivity measured by the usual DC method using Ohm’s law under the accumulation voltage for each sample. The capacitance decreases with increasing signal frequency, which can be attributed to the effect of charge redistribution by carrier hopping on centres [20]. According to Kramers–Kronig (KK) relations [21–23], the theoretical consequence of the application of the power-law guides to the following relation: C(ω) ∝ ωs −1 for the frequency dependence of the capacitance at accumulation voltage [24]. But, the applicability of the KK relation in the present work with exponent s = 0.85 as shown in Fig. 4b is limited especially for the as-deposited sample due to higher value of Qins .
5. Conclusion In the present work, amorphous tris(acetylacetonate)manganese(III) films were prepared on Si (P) substrates by sublimation in a vacuum chamber. The films were characterised by XRF, XRD and spectral optical absorption. The constructed Al/insulator/Si MIS devices were characterised by measuring their capacitance and AC-conductance as a function of gate voltage. The frequency dependence of the AC conductivity obeys the power law. It appears that the hop
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of current carriers between defects is realised over potential barriers (W) of values depend on the separation of those sites according to the CBH model. This study shows that the tris(acetylacetonate)manganese(III) films grown on Si(1 0 0) is a promising candidate for high-ε dielectric applications, it displays sufficiently high-ε value around 40. Furthermore, it is possible to obtain an acceptable level of Dit and Qins , which is necessary condition for the high-k gate dielectric material applications according to the road map stated in Ref. [25].
Acknowledgement The Author is grateful to Dr. Ahmed Y. Ali-Mohamed from Dept. of Chemistry, Coll. of science, University of Bahrain, Kingdom of Bahrain for his many useful and interesting discussions about the metal-substituted organic materials. References [1] J.C. Bailar Jr., H.J. Emeleus, R. Nyholm, A.F. Trotman-Dickenson (Eds.), Comprehensive Inorganic Chemistry, Pergamon Press, Oxford, 1975, p. 872. [2] J.P. Fackler, A. Avdeef, Inorg. Chem. 13 (1974) 1864. [3] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, fifth ed., Willey-Interscience Publication, NY, 1988, p. 705. [4] B.R. Stults, R.S. Marianelli, V.W. Day, Inorg. Chem. 18 (1979) 1853.
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