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Some electrical properties of diamond-like carbon thin films
ARTICLE IN PRESS
Vacuum 74 (2004) 325–330
Some electrical properties of diamond-like carbon thin films E. Staryga*, G.W. Ba) k, M. D"u’zniewski ! z, Wolcza ! ! ! z, Poland Institute of Physics, Technical University of Łod’ nska 219, 93-005 Łod’ Accepted 22 December 2003
Abstract Diamond-like carbon (DLC) belongs to very interesting materials used for a number of practical applications. It was noted that the electrical properties of DLC films obtained by RF PCVD discharge depend substantially on the deposition conditions. The results and discussion of the electrical properties of DLC films and DLC/Si heterostructures is presented. The electrical conductivity results are explained in terms of hopping mechanism. The relation between charge transport, structure of the energy gap and the deposition conditions is discussed. r 2004 Elsevier Ltd. All rights reserved. Keywords: Diamond-like carbon; DLC; Charge transport; Charge mobility
1. Introduction Carbon atoms can exist in three different chemical states, involved in sp3, sp2 and sp1 chemical coordination. This enables to form a great amount of structures, which may contain also other chemical elements. In general, two main types of diamond-like structures can be identified: hydrogenated (a-C:H) and non-hydrogenated (a-C) ones. The first ones are often referred to as diamond-like hydrocarbons and the second ones are referred to as diamond-like carbon (DLC), though these terms are not quite consistently used. During the second part of the last century a great interest was taken in both diamond and *Corresponding author. Tel.: +4842-6313651; fax: +48426313639. E-mail addresses: [email protected] (E. Staryga), [email protected] (G.W. Ba) k), mdfi[email protected] (M. D"u’zniewski).
DLC structures. For its physical properties DLC thin films are considered as material, which may be applied for some kind of both passive and active electronics elements. The main structural difference between the diamond (natural or synthetic) and the diamondlike systems is the presence and the amount of sp2 carbon phase. DLC films contain a good portion of sp2 bonding, sometimes up to 60%. The relation between the amount of graphitic sp2 and aliphatic sp3 bonding is one of the most important features of DLC, which strongly influences the physical properties of DLC films [1,2]. There are many methods of deposition of DLC thin films [3–5]. Among them, the PCVD methods are the most frequently used. The average amount of hydrogen in the films obtained from methane by the RF PCVD technique decreases from 57 at% for the zero bias voltage down to about 30 at% for 1000 V bias voltage [6]. The amount of sp2 and sp3 sites in DLC films deposited with RF PCVD
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2003.12.150
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technique is also a function of the self-bias voltage and, in general, the fraction of sp2 sites increases with increasing value of the negative self-bias voltage [7,8]. The conclusion resulting from the structural studies is that the concentration of localised states in the forbidden gap may be expected to increase with increasing value of the negative self-bias, because both the increase of sp2 phase and the decrease of hydrogen content should give rise to increase of the localised states in the gap. In our work we show experimental results to confirm the presented hypothesis. The electron drift mobility in DLC thin films obtained by RF PCVD technique was measured previously using the discharge under open circuit conditions [9]. The electron drift mobility in DLC at room temperature turns out to be rather low, of the order of 106 cm2/V s. Such a low mobility value suggests that hopping transport among localised states dominates. The activation energy of mobility is about 0.03 eV. This means that the hopping transport takes place in a narrow band of states. The mobility value in the investigated DLC films obtained by r.f. glow discharge from methane was found to increase a little with increasing negative self-bias voltage applied [10]. This means that the electron drift mobility increases with decreasing hydrogen content and decreasing amount of sp3 bonds.
2. Experimental The deposition of DLC films was carried out in a conventional parallel-plate plasma-chemical reactor powered from the r.f. generator which was supplied through a matching box [11]. The negative self-bias voltage was changed from 20 to 900 V. The films were deposited at the pressure of about 50 Pa from methane. The films were deposited on platinum and steel (AISI 316 L) substrates and also on silicon wafers (the resistivity of the n-type and p-type silicon used was between 3 and 12 O cm) placed on the r.f. powered, negatively self-biased electrode. The powered electrode was maintained at room temperature during deposition by circulating coolant water.
The thicknesses of the investigated layers were from 0.02 to 0.80 mm. The measurements of the I2V characteristic were carried out in the temperature range 77– 400 K in the dark in vacuum of about 0.13 Pa using DN1754 cryostat (Oxford Instruments) connected with the temperature controller ITC-502 (Oxford Instruments). The Keithley 487 electrometer worked under computer control and Test Point software (Capital Equipment Corporation) was used to control the measurement procedure. For the measurements of the recombination lifetime of charge carriers a gold comb electrode was deposited on the surface of DLC films. The light beam (duration of pulse 6 ms) fell upon the surface covered with comb electrodes and generated free electron–hole pairs at the surface of DLC. The current pulse was registered using DSO 5804 digital oscilloscope. The measuring system was under computer control, which enabled to store the experimental data. The dielectric measurements were carried out using Solartron 1260 Impedance Analyser combined with a Chelsea Dielectric Interface.
3. Results 3.1. Electrical conductivity of DLC films The results obtained previously show that the electrical conductivity at room temperature increases by nearly 8 orders of magnitude with increasing self-bias voltage [12], while the activation energy decreases from 0.8 eV down to about 0.1 eV. The measurements show that in principle the conductivity obeys relation s ¼ s0 expðDEa =kTÞ over the temperature range 223–400 K. However, the previous data relates to a comparatively narrow temperature range. In order to examine the electrical conductivity more exactly, the measurements in a wider temperature range (163–400 K) were carried out. The results are shown in Fig. 1 in log s vs. 1/T scale. All the curves shown in Fig. 1 consist of two straight-line parts. The low-temperature ones are described by relatively low activation energies (from 0.05 eV up to 0.07 eV for decreasing value
ARTICLE IN PRESS E. Staryga et al. / Vacuum 74 (2004) 325–330
3.2. Electrical conductivity of DLC/Si heterostructures
-4 0.15eV
log(σ[S/m])
-5
0.22eV
-6
∆Ea [eV] 0.050eV
3 0.055eV
-7
0.28eV
2
-8 0.070eV
1
-9 -10 2
3
4 5 1000/T[1/K]
6
327
7
Fig. 1. Conductivity vs. reciprocal of temperature in DLC films prepared at various self-bias voltage: 1FVe ¼ 150 V; 2FVe ¼ 250 V; 3FVe ¼ 400 V.
of the negative self-bias voltage). The activation energies of the high-temperature parts are a few times greater (from 0.15 to 0.28 eV). The changes of the activation energies take place at about 260 K, which may suggest that we have to do with some change of conduction mechanism at this temperature. The conductivity in the two temperature ranges can be well described with the expression s ¼ s0 expðEa =kTÞ; where s0LT ¼ ð1:2 107 2 1:8 105 Þ S/m for the low temperature range and s0HT ¼ ð1:5 103 25:1 103 Þ S/m for the high temperature one. Both the low mobility value (m ¼ 1:6 106 28:0 106 cm2/V s [9]) and the low values of the pre-exponential factors s0 suggest that the hopping transport among localised states dominates in DLC thin films. This suggestion is in agreement with the earlier results [13–17]. In case of the low-temperature conduction it may be supposed that we have to do with charge transport in a narrow band of states due to its very low activation energy (0.05–0.07 eV). However, in the case of high-temperature range the activation of charge carriers between various bands of localised states within the gap may be supposed to take place. Let us note that in case of hopping transport among localised states the electrical conductivity may be expected to increase with increasing concentration of the localised states. This is in agreement with the results presented above.
The best-known heterostructure containing DLC thin films is metal/DLC/Si system [18–21], though some data concerning DLC/diamond junction are also available [22]. However, there is no theory describing the electrical properties of heterostructures containing poorly conducting amorphous thin films, in which the charge transport is dominated with hopping of charge carriers among localised states, as it has been found in DLC thin films [12–15]. In this situation a simplified equation of p–n junction can be used for approximate description of conducting properties of the investigated DLC/Si heterostructures: eU I ¼ Id0 exp ; ð1Þ ZkT where e is the electron charge, U is the voltage, Z is the ideality factor, k is the Boltzmann constant and T is the temperature. Id0 is the current corresponding to the voltage approaching zero. The ideality factor Z describes the mathematical similarity of I2V characteristics to the characteristics of classical p–n junction. In case of classical p–n junctions the value of the ideality factor changes between 1 and 2. In general, two different cases of rectifying DLC/Si structures may exist. In the first one the DLC film does not limit the current flow through the system. In this case the system behaves like a typical rectifying p–n junction. In the second case the behaviour of DLC/Si structure provided with metal electrode strongly depends on the voltage applied. At the lower voltages the conduction is usually limited by DLC/Si junction but at the higher voltages the conduction becomes limited by the bulk resistance of DLC film and the system resembles a typical MIS structure. The rectification ratio of undoped DLC/Si heterostructures rather does not exceed 102 which is not an overwhelming result [20,21]. Figs. 2 and 3 show current–voltage characteristics of Au/DLC/p-Si/Ag and Au/DLC/n-Si/Au heterostructures at various temperatures. A remarkable difference between the conduction of DLC/n-Si and DLC/p-Si heterostructures has
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328
been detected. The conduction of both junctions is asymmetric and the forward direction occurs when a negative potential is applied at the silicon substrate regardless of the type of silicon substrate.
I [A]
1.5E-06 1.0E-06 5.0E-07 0.0E+00
-2
-1
0
1
2
3
-5.0E-07 -1.0E-06 -1.5E-06 -2.0E-06
4 U [V]
77 K 120 K 180 K 240 K 300 K 325 K 340 K
-2.5E-06
Fig. 2. Current–voltage characteristics of Au/DLC/p-Si/Ag heterostructure at various temperatures.
I [A]
2.0E-06 0.0E+00
-2
-1
0
1
2
3
4 U [V]
-2.0E-06 77 K 120 K 180 K 240 K 300 K 325 K 340 K
-4.0E-06 -6.0E-06 -8.0E-06 -1.0E-05 -1.2E-05
Fig. 3. Current–voltage characteristics of Au/DLC/n-Si/Ag heterostructure at various temperatures.
However, the conductivity of the DLC/n-Si heterostructures in the forward direction is about 3–7 times higher than that of its DLC/p-Si counterpart. The values of the ideality factor describing the I2V characteristics of the investigated DLC/Si junctions changes in the range between 21.2 down to 3.25 (see Table 1), decreasing with increasing temperatures. Giebel and Pavesi [23] obtained similar results for heterostructures containing porous silicon. The values of Z found by Giebel and Pavesi were in the range between 45 and 2.3 and were also decreasing with increasing temperature. The authors suggested that the comparatively high values of Z are characteristic of hopping transport of charge in amorphous porous silicon films. The independence of the diode tendency on the type of silicon substrate can be understood when the band structure of both systems is taken into account. When a negative potential is applied at the silicon, it gives rise to an increased concentration of electrons at the DLC/silicon junction regardless of the type of silicon used. Depending on the conductivity of DLC film the increased concentration of electrons need not lead neither to accumulation nor inversion like in typical MIS systems. In both cases of n-Si and p-Si substrates the electrons from silicon can be injected into the complex band of localised electron states which has been shown to exist in the forbidden gap of DLC [3,24]. Assuming that the electron transport is dominating in case of charge transport via the localised stated in the forbidden gap [10], it becomes understandable that the directive tendency of DLC/Si systems is independent of the type of silicon. It becomes also
Table 1 The ideality factor Z of I2V characteristics of the investigated heterostructures Sample
Au/DLC/n-Si/Ag Au/DLC/p-Si/Ag Au/DLC/n-Si/Au Au/DLC/n-Si/Al
Temperature 77 K
120 K
180 K
240 K
300 K
325 K
340 K
16.2 21.2 9.02 14.9
9.83 14.5 6.80 8.14
6.67 8.72 5.29 6.99
5.66 6.98 4.29 4.08
5.68 6.29 3.68 —
5.34 5.87 3.42 3.34
5.37 5.54 3.25 3.47
ARTICLE IN PRESS E. Staryga et al. / Vacuum 74 (2004) 325–330 EA EA EA
0.9 eV
1.2 eV
1.6 eV
EA 1.9 eV
clear that the forward currents in DLC/n-Si structure is higher than that in DLC/p-Si ones. This is due to the higher concentration of electrons at the junction in the case of DLC/n-Si junction, which leads to injection of greater number of electrons into DLC film in this type of heterostructures.
329
EF
3.3. Electronic structure of the energy gap EB 3
2
The s bonds of the carbon sp and sp sites give rise to bonding (s) and antibonding (s ) band states, separated by a wide band gap (close to 5–6 eV, similar to that of diamond). The p bonds of the carbon sp2 sites form weaker bonds, which introduce bonding (p) and antibonding (p ) states within the s2s gap. These states form the band edges and control the width of the optical gap. The edges form tails of localised states, which is typical for many amorphous covalent structures. It is experimentally confirmed that there exist numerous localised states in the energetic gap in DLC thin films. The states may be due to either various clusters of sp2 sites (including graphitic phase) and from ‘‘free’’ (i.e. not saturated with hydrogen atoms) sp3 sites. This means, that the concentration of the localised states in the gap should increase with increasing amount of sp2 sites and with decreasing amount of hydrogen atoms. Taking into account that the amount of sp2 sites increases with increasing negative self-bias voltage [7,8] and the amount of hydrogen decreases with increasing negative self-bias voltage [6] one can expect that the concentration of localised states in the gap should increase with increasing negative self-bias voltage. This has been confirmed by the measurements of the optical gap [12]. Robertson [3] has shown that significant reduction of the gap can be achieved by distortions of p-bonded pairs. It has also been shown that the optical band gap can change in a wide range (from about 0.4 to 4.0 eV) due to changing amount of graphitic phase [25,26]. Measurements of thermally stimulated current (TSC) have shown the existence of deep charge traps (0.57–0.76 eV) in the forbidden gap of DLC
EB EB EB (a)
(b)
(c)
(d)
Fig. 4. The model of the structure of energy gap illustrating the change within it for DLC prepared at various values of the selfbias voltage: (a) Ve ¼ 20 V, amount of phase sp2 40%; (b) Ve ¼ 150 V, amount of phase sp2 45%; (c) Ve ¼ 400 V, amount of phase sp2 55%; (d) Ve ¼ 600 V, amount of phase sp2 62%.
depending on the deposition conditions [24]. For DLC films produced at the self-bias voltage Ve ¼ 20 V the energy gap shows two trap levels (0.68 and 0.76 eV deep), while only one level (0.57 eV) is observable in films deposited at selfbias voltage Ve ¼ 50 V. An increase in the Ve parameter during deposition gives rise to a decrease in the trap level depths in the forbidden gap. The results of both optical and electrical measurements enable to present the scheme of the forbidden gap of DLC structures obtained at various values of the self-bias voltage Ve (see Fig. 4). For the lowest values of Ve the gap is 1.9 eV wide, the concentration of the localised states at the edges is comparatively low. There exist some trapping levels within the gap registered by TSC measurements. The concentration of the states increases with the increasing self-bias voltage which results in narrowing the forbidden gap. The edge of conduction band approaches the trapping level which is registered in TSC experiment as shallowing the traps. In general the concentration of localised states increases and the structure of the forbidden gap becomes more complex with increasing value of the self-bias voltage.
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4. Summary and conclusions
References
The following conclusions may be put forward from the results presented above:
[1] Davis CA, Veerasamy VS, Amaratunga GAJ, Milne WL, McKenzie DR. Philos Mag B 1994;69:1121. [2] Rengan A, Narayan J, Jahnke C, Bedge S, Park JL, Li M. Mater Sci Eng 1992;B15:15. [3] Robertson J. Mater Sci Eng 2002;R37:129. [4] Bachmann PK, Van Enckevort W. Diamond Relat Mater 1992;1:1021. [5] Szmidt J. Diamond-like layers produced using plasmaenhanced deposition methods for applications in microelectronics. Warsaw: Sci Bull. Electronic 108, Publish. House Warsaw University of Technology; 1995. [6] Tamor MA, Vassell WC, Carduner KR. Appl Phys Lett 1991;58:592. [7] Schafer J, Ristein J, Ley L. Diamond Relat Mater 1994;3:861. [8] Koldl P, Wild C, Dischler B, Wagner J, Ramsteiner M. Mater Sci Forum 1990;52:41. ! [9] Mycielski W, Staryga E, Lipinski A. Thin Solid Films 1993;235:13. ! [10] Mycielski W, Staryga E, Lipinski A, Mitura S, Soko"owska A. Diamond Relat Mater 1994;3:858. [11] Ha!s Z, Mitura S, Wendler B. Pro Int Ion Eng Congres ISIAT-IPAT’83, Kyoto, 1983. p. 1143. ! [12] Staryga E, Lipinski A, Mitura S, Ha!s Z. Thin Solid Films 1986;145:17. [13] Meyerson B, Smith F. J. Non Cryst Solids 1980;35/36: 435. [14] Jones DI, Stewart AD. Philos Mag 1982;B46:423. [15] Dasgupta D, Demichelis F, Tagliaferro A. Philos Mag B 1991;63:1255. [16] Silva SRP, Carey JD. Diamond Relat Mater 2003;12:151. [17] Godet C. Diamond Relat Mater 2003;12:159. [18] Amaratunga GAJ, Segal DE, McKenzie DR. Appl Phys Lett 1991;59:69. [19] Mandel Th, Frischoltz M, Helbig R, Birkle S, Hammersmidt A. Appl Surf Sci 1993;65/66:795. [20] D"u’zniewski M, Staryga E, Stanishewski V, Tochitski EI, Mitura S, Ba) k GW. Adv Mater Opt Electron 1996;6:409. [21] D"u’zniewski M, Staryga E, Ba) k GW. IEEE Trans Dielectr Electr Insul 2001;8:418. [22] Chan KK, Silva SRP, Amaratunga GAJ. Thin Solid Films 1992;212:232. [23] Giebel G, Pavesi L. Phys Status Sol (A) 1995;151:355. [24] Staryga E. Diamond Relat Mater 1994;3:865. [25] Rusli JR, Amaratunga GAJ. J Appl Phys 1996;80:2998. [26] Chen CW, Robertson J. J Non Cryst Solids 1998;227/ 230:602.
1. The electrical conductivity of DLC films obtained by RF PCVD technique strongly depends on the self-bias voltage and changes in a broad range (approximately 1012–105 S/m). For the temperatures below 260 K the activation energy equals a few tenths of electronovolt and increases remarkably for the higher temperatures which probably results from thermal activation of charge carriers between the bands of localised states in the forbidden gap. Both the increase in mobility and conductivity suggest that the concentration of localised states in the forbidden gap increases with increasing value of the self-bias voltage. 2. Both DLC/p-Si and DLC/n-Si junctions show rather weak rectifying properties. The current– voltage characteristics of the junctions verify the domination of hopping transport in DLC films. The conduction in the forward direction of DLC/n-Si junction is a few times higher than that of DLC/p-Si heterostructure and this may be interpreted as a result of electron transport domination in undoped DLC films. 3. The presented results of both electrical and optical investigations suggest that the forbidden gap is reduced with increasing value of the negative self-bias voltage. The reduction results from the increasing concentration of localised states of the band edges. As a result, the measured effective depth of trapping levels is also reduced. The increasing concentration of localised states may result in formation of subbands of localised states within the gap. The thermal activation of charge carriers between the sub-bands may be responsible for electrical conduction in the higher temperatures (i.e. above approximately 260 K).
Vacuum 74 (2004) 325–330
Some electrical properties of diamond-like carbon thin films E. Staryga*, G.W. Ba) k, M. D"u’zniewski ! z, Wolcza ! ! ! z, Poland Institute of Physics, Technical University of Łod’ nska 219, 93-005 Łod’ Accepted 22 December 2003
Abstract Diamond-like carbon (DLC) belongs to very interesting materials used for a number of practical applications. It was noted that the electrical properties of DLC films obtained by RF PCVD discharge depend substantially on the deposition conditions. The results and discussion of the electrical properties of DLC films and DLC/Si heterostructures is presented. The electrical conductivity results are explained in terms of hopping mechanism. The relation between charge transport, structure of the energy gap and the deposition conditions is discussed. r 2004 Elsevier Ltd. All rights reserved. Keywords: Diamond-like carbon; DLC; Charge transport; Charge mobility
1. Introduction Carbon atoms can exist in three different chemical states, involved in sp3, sp2 and sp1 chemical coordination. This enables to form a great amount of structures, which may contain also other chemical elements. In general, two main types of diamond-like structures can be identified: hydrogenated (a-C:H) and non-hydrogenated (a-C) ones. The first ones are often referred to as diamond-like hydrocarbons and the second ones are referred to as diamond-like carbon (DLC), though these terms are not quite consistently used. During the second part of the last century a great interest was taken in both diamond and *Corresponding author. Tel.: +4842-6313651; fax: +48426313639. E-mail addresses: [email protected] (E. Staryga), [email protected] (G.W. Ba) k), mdfi[email protected] (M. D"u’zniewski).
DLC structures. For its physical properties DLC thin films are considered as material, which may be applied for some kind of both passive and active electronics elements. The main structural difference between the diamond (natural or synthetic) and the diamondlike systems is the presence and the amount of sp2 carbon phase. DLC films contain a good portion of sp2 bonding, sometimes up to 60%. The relation between the amount of graphitic sp2 and aliphatic sp3 bonding is one of the most important features of DLC, which strongly influences the physical properties of DLC films [1,2]. There are many methods of deposition of DLC thin films [3–5]. Among them, the PCVD methods are the most frequently used. The average amount of hydrogen in the films obtained from methane by the RF PCVD technique decreases from 57 at% for the zero bias voltage down to about 30 at% for 1000 V bias voltage [6]. The amount of sp2 and sp3 sites in DLC films deposited with RF PCVD
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2003.12.150
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technique is also a function of the self-bias voltage and, in general, the fraction of sp2 sites increases with increasing value of the negative self-bias voltage [7,8]. The conclusion resulting from the structural studies is that the concentration of localised states in the forbidden gap may be expected to increase with increasing value of the negative self-bias, because both the increase of sp2 phase and the decrease of hydrogen content should give rise to increase of the localised states in the gap. In our work we show experimental results to confirm the presented hypothesis. The electron drift mobility in DLC thin films obtained by RF PCVD technique was measured previously using the discharge under open circuit conditions [9]. The electron drift mobility in DLC at room temperature turns out to be rather low, of the order of 106 cm2/V s. Such a low mobility value suggests that hopping transport among localised states dominates. The activation energy of mobility is about 0.03 eV. This means that the hopping transport takes place in a narrow band of states. The mobility value in the investigated DLC films obtained by r.f. glow discharge from methane was found to increase a little with increasing negative self-bias voltage applied [10]. This means that the electron drift mobility increases with decreasing hydrogen content and decreasing amount of sp3 bonds.
2. Experimental The deposition of DLC films was carried out in a conventional parallel-plate plasma-chemical reactor powered from the r.f. generator which was supplied through a matching box [11]. The negative self-bias voltage was changed from 20 to 900 V. The films were deposited at the pressure of about 50 Pa from methane. The films were deposited on platinum and steel (AISI 316 L) substrates and also on silicon wafers (the resistivity of the n-type and p-type silicon used was between 3 and 12 O cm) placed on the r.f. powered, negatively self-biased electrode. The powered electrode was maintained at room temperature during deposition by circulating coolant water.
The thicknesses of the investigated layers were from 0.02 to 0.80 mm. The measurements of the I2V characteristic were carried out in the temperature range 77– 400 K in the dark in vacuum of about 0.13 Pa using DN1754 cryostat (Oxford Instruments) connected with the temperature controller ITC-502 (Oxford Instruments). The Keithley 487 electrometer worked under computer control and Test Point software (Capital Equipment Corporation) was used to control the measurement procedure. For the measurements of the recombination lifetime of charge carriers a gold comb electrode was deposited on the surface of DLC films. The light beam (duration of pulse 6 ms) fell upon the surface covered with comb electrodes and generated free electron–hole pairs at the surface of DLC. The current pulse was registered using DSO 5804 digital oscilloscope. The measuring system was under computer control, which enabled to store the experimental data. The dielectric measurements were carried out using Solartron 1260 Impedance Analyser combined with a Chelsea Dielectric Interface.
3. Results 3.1. Electrical conductivity of DLC films The results obtained previously show that the electrical conductivity at room temperature increases by nearly 8 orders of magnitude with increasing self-bias voltage [12], while the activation energy decreases from 0.8 eV down to about 0.1 eV. The measurements show that in principle the conductivity obeys relation s ¼ s0 expðDEa =kTÞ over the temperature range 223–400 K. However, the previous data relates to a comparatively narrow temperature range. In order to examine the electrical conductivity more exactly, the measurements in a wider temperature range (163–400 K) were carried out. The results are shown in Fig. 1 in log s vs. 1/T scale. All the curves shown in Fig. 1 consist of two straight-line parts. The low-temperature ones are described by relatively low activation energies (from 0.05 eV up to 0.07 eV for decreasing value
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3.2. Electrical conductivity of DLC/Si heterostructures
-4 0.15eV
log(σ[S/m])
-5
0.22eV
-6
∆Ea [eV] 0.050eV
3 0.055eV
-7
0.28eV
2
-8 0.070eV
1
-9 -10 2
3
4 5 1000/T[1/K]
6
327
7
Fig. 1. Conductivity vs. reciprocal of temperature in DLC films prepared at various self-bias voltage: 1FVe ¼ 150 V; 2FVe ¼ 250 V; 3FVe ¼ 400 V.
of the negative self-bias voltage). The activation energies of the high-temperature parts are a few times greater (from 0.15 to 0.28 eV). The changes of the activation energies take place at about 260 K, which may suggest that we have to do with some change of conduction mechanism at this temperature. The conductivity in the two temperature ranges can be well described with the expression s ¼ s0 expðEa =kTÞ; where s0LT ¼ ð1:2 107 2 1:8 105 Þ S/m for the low temperature range and s0HT ¼ ð1:5 103 25:1 103 Þ S/m for the high temperature one. Both the low mobility value (m ¼ 1:6 106 28:0 106 cm2/V s [9]) and the low values of the pre-exponential factors s0 suggest that the hopping transport among localised states dominates in DLC thin films. This suggestion is in agreement with the earlier results [13–17]. In case of the low-temperature conduction it may be supposed that we have to do with charge transport in a narrow band of states due to its very low activation energy (0.05–0.07 eV). However, in the case of high-temperature range the activation of charge carriers between various bands of localised states within the gap may be supposed to take place. Let us note that in case of hopping transport among localised states the electrical conductivity may be expected to increase with increasing concentration of the localised states. This is in agreement with the results presented above.
The best-known heterostructure containing DLC thin films is metal/DLC/Si system [18–21], though some data concerning DLC/diamond junction are also available [22]. However, there is no theory describing the electrical properties of heterostructures containing poorly conducting amorphous thin films, in which the charge transport is dominated with hopping of charge carriers among localised states, as it has been found in DLC thin films [12–15]. In this situation a simplified equation of p–n junction can be used for approximate description of conducting properties of the investigated DLC/Si heterostructures: eU I ¼ Id0 exp ; ð1Þ ZkT where e is the electron charge, U is the voltage, Z is the ideality factor, k is the Boltzmann constant and T is the temperature. Id0 is the current corresponding to the voltage approaching zero. The ideality factor Z describes the mathematical similarity of I2V characteristics to the characteristics of classical p–n junction. In case of classical p–n junctions the value of the ideality factor changes between 1 and 2. In general, two different cases of rectifying DLC/Si structures may exist. In the first one the DLC film does not limit the current flow through the system. In this case the system behaves like a typical rectifying p–n junction. In the second case the behaviour of DLC/Si structure provided with metal electrode strongly depends on the voltage applied. At the lower voltages the conduction is usually limited by DLC/Si junction but at the higher voltages the conduction becomes limited by the bulk resistance of DLC film and the system resembles a typical MIS structure. The rectification ratio of undoped DLC/Si heterostructures rather does not exceed 102 which is not an overwhelming result [20,21]. Figs. 2 and 3 show current–voltage characteristics of Au/DLC/p-Si/Ag and Au/DLC/n-Si/Au heterostructures at various temperatures. A remarkable difference between the conduction of DLC/n-Si and DLC/p-Si heterostructures has
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been detected. The conduction of both junctions is asymmetric and the forward direction occurs when a negative potential is applied at the silicon substrate regardless of the type of silicon substrate.
I [A]
1.5E-06 1.0E-06 5.0E-07 0.0E+00
-2
-1
0
1
2
3
-5.0E-07 -1.0E-06 -1.5E-06 -2.0E-06
4 U [V]
77 K 120 K 180 K 240 K 300 K 325 K 340 K
-2.5E-06
Fig. 2. Current–voltage characteristics of Au/DLC/p-Si/Ag heterostructure at various temperatures.
I [A]
2.0E-06 0.0E+00
-2
-1
0
1
2
3
4 U [V]
-2.0E-06 77 K 120 K 180 K 240 K 300 K 325 K 340 K
-4.0E-06 -6.0E-06 -8.0E-06 -1.0E-05 -1.2E-05
Fig. 3. Current–voltage characteristics of Au/DLC/n-Si/Ag heterostructure at various temperatures.
However, the conductivity of the DLC/n-Si heterostructures in the forward direction is about 3–7 times higher than that of its DLC/p-Si counterpart. The values of the ideality factor describing the I2V characteristics of the investigated DLC/Si junctions changes in the range between 21.2 down to 3.25 (see Table 1), decreasing with increasing temperatures. Giebel and Pavesi [23] obtained similar results for heterostructures containing porous silicon. The values of Z found by Giebel and Pavesi were in the range between 45 and 2.3 and were also decreasing with increasing temperature. The authors suggested that the comparatively high values of Z are characteristic of hopping transport of charge in amorphous porous silicon films. The independence of the diode tendency on the type of silicon substrate can be understood when the band structure of both systems is taken into account. When a negative potential is applied at the silicon, it gives rise to an increased concentration of electrons at the DLC/silicon junction regardless of the type of silicon used. Depending on the conductivity of DLC film the increased concentration of electrons need not lead neither to accumulation nor inversion like in typical MIS systems. In both cases of n-Si and p-Si substrates the electrons from silicon can be injected into the complex band of localised electron states which has been shown to exist in the forbidden gap of DLC [3,24]. Assuming that the electron transport is dominating in case of charge transport via the localised stated in the forbidden gap [10], it becomes understandable that the directive tendency of DLC/Si systems is independent of the type of silicon. It becomes also
Table 1 The ideality factor Z of I2V characteristics of the investigated heterostructures Sample
Au/DLC/n-Si/Ag Au/DLC/p-Si/Ag Au/DLC/n-Si/Au Au/DLC/n-Si/Al
Temperature 77 K
120 K
180 K
240 K
300 K
325 K
340 K
16.2 21.2 9.02 14.9
9.83 14.5 6.80 8.14
6.67 8.72 5.29 6.99
5.66 6.98 4.29 4.08
5.68 6.29 3.68 —
5.34 5.87 3.42 3.34
5.37 5.54 3.25 3.47
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0.9 eV
1.2 eV
1.6 eV
EA 1.9 eV
clear that the forward currents in DLC/n-Si structure is higher than that in DLC/p-Si ones. This is due to the higher concentration of electrons at the junction in the case of DLC/n-Si junction, which leads to injection of greater number of electrons into DLC film in this type of heterostructures.
329
EF
3.3. Electronic structure of the energy gap EB 3
2
The s bonds of the carbon sp and sp sites give rise to bonding (s) and antibonding (s ) band states, separated by a wide band gap (close to 5–6 eV, similar to that of diamond). The p bonds of the carbon sp2 sites form weaker bonds, which introduce bonding (p) and antibonding (p ) states within the s2s gap. These states form the band edges and control the width of the optical gap. The edges form tails of localised states, which is typical for many amorphous covalent structures. It is experimentally confirmed that there exist numerous localised states in the energetic gap in DLC thin films. The states may be due to either various clusters of sp2 sites (including graphitic phase) and from ‘‘free’’ (i.e. not saturated with hydrogen atoms) sp3 sites. This means, that the concentration of the localised states in the gap should increase with increasing amount of sp2 sites and with decreasing amount of hydrogen atoms. Taking into account that the amount of sp2 sites increases with increasing negative self-bias voltage [7,8] and the amount of hydrogen decreases with increasing negative self-bias voltage [6] one can expect that the concentration of localised states in the gap should increase with increasing negative self-bias voltage. This has been confirmed by the measurements of the optical gap [12]. Robertson [3] has shown that significant reduction of the gap can be achieved by distortions of p-bonded pairs. It has also been shown that the optical band gap can change in a wide range (from about 0.4 to 4.0 eV) due to changing amount of graphitic phase [25,26]. Measurements of thermally stimulated current (TSC) have shown the existence of deep charge traps (0.57–0.76 eV) in the forbidden gap of DLC
EB EB EB (a)
(b)
(c)
(d)
Fig. 4. The model of the structure of energy gap illustrating the change within it for DLC prepared at various values of the selfbias voltage: (a) Ve ¼ 20 V, amount of phase sp2 40%; (b) Ve ¼ 150 V, amount of phase sp2 45%; (c) Ve ¼ 400 V, amount of phase sp2 55%; (d) Ve ¼ 600 V, amount of phase sp2 62%.
depending on the deposition conditions [24]. For DLC films produced at the self-bias voltage Ve ¼ 20 V the energy gap shows two trap levels (0.68 and 0.76 eV deep), while only one level (0.57 eV) is observable in films deposited at selfbias voltage Ve ¼ 50 V. An increase in the Ve parameter during deposition gives rise to a decrease in the trap level depths in the forbidden gap. The results of both optical and electrical measurements enable to present the scheme of the forbidden gap of DLC structures obtained at various values of the self-bias voltage Ve (see Fig. 4). For the lowest values of Ve the gap is 1.9 eV wide, the concentration of the localised states at the edges is comparatively low. There exist some trapping levels within the gap registered by TSC measurements. The concentration of the states increases with the increasing self-bias voltage which results in narrowing the forbidden gap. The edge of conduction band approaches the trapping level which is registered in TSC experiment as shallowing the traps. In general the concentration of localised states increases and the structure of the forbidden gap becomes more complex with increasing value of the self-bias voltage.
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4. Summary and conclusions
References
The following conclusions may be put forward from the results presented above:
[1] Davis CA, Veerasamy VS, Amaratunga GAJ, Milne WL, McKenzie DR. Philos Mag B 1994;69:1121. [2] Rengan A, Narayan J, Jahnke C, Bedge S, Park JL, Li M. Mater Sci Eng 1992;B15:15. [3] Robertson J. Mater Sci Eng 2002;R37:129. [4] Bachmann PK, Van Enckevort W. Diamond Relat Mater 1992;1:1021. [5] Szmidt J. Diamond-like layers produced using plasmaenhanced deposition methods for applications in microelectronics. Warsaw: Sci Bull. Electronic 108, Publish. House Warsaw University of Technology; 1995. [6] Tamor MA, Vassell WC, Carduner KR. Appl Phys Lett 1991;58:592. [7] Schafer J, Ristein J, Ley L. Diamond Relat Mater 1994;3:861. [8] Koldl P, Wild C, Dischler B, Wagner J, Ramsteiner M. Mater Sci Forum 1990;52:41. ! [9] Mycielski W, Staryga E, Lipinski A. Thin Solid Films 1993;235:13. ! [10] Mycielski W, Staryga E, Lipinski A, Mitura S, Soko"owska A. Diamond Relat Mater 1994;3:858. [11] Ha!s Z, Mitura S, Wendler B. Pro Int Ion Eng Congres ISIAT-IPAT’83, Kyoto, 1983. p. 1143. ! [12] Staryga E, Lipinski A, Mitura S, Ha!s Z. Thin Solid Films 1986;145:17. [13] Meyerson B, Smith F. J. Non Cryst Solids 1980;35/36: 435. [14] Jones DI, Stewart AD. Philos Mag 1982;B46:423. [15] Dasgupta D, Demichelis F, Tagliaferro A. Philos Mag B 1991;63:1255. [16] Silva SRP, Carey JD. Diamond Relat Mater 2003;12:151. [17] Godet C. Diamond Relat Mater 2003;12:159. [18] Amaratunga GAJ, Segal DE, McKenzie DR. Appl Phys Lett 1991;59:69. [19] Mandel Th, Frischoltz M, Helbig R, Birkle S, Hammersmidt A. Appl Surf Sci 1993;65/66:795. [20] D"u’zniewski M, Staryga E, Stanishewski V, Tochitski EI, Mitura S, Ba) k GW. Adv Mater Opt Electron 1996;6:409. [21] D"u’zniewski M, Staryga E, Ba) k GW. IEEE Trans Dielectr Electr Insul 2001;8:418. [22] Chan KK, Silva SRP, Amaratunga GAJ. Thin Solid Films 1992;212:232. [23] Giebel G, Pavesi L. Phys Status Sol (A) 1995;151:355. [24] Staryga E. Diamond Relat Mater 1994;3:865. [25] Rusli JR, Amaratunga GAJ. J Appl Phys 1996;80:2998. [26] Chen CW, Robertson J. J Non Cryst Solids 1998;227/ 230:602.
1. The electrical conductivity of DLC films obtained by RF PCVD technique strongly depends on the self-bias voltage and changes in a broad range (approximately 1012–105 S/m). For the temperatures below 260 K the activation energy equals a few tenths of electronovolt and increases remarkably for the higher temperatures which probably results from thermal activation of charge carriers between the bands of localised states in the forbidden gap. Both the increase in mobility and conductivity suggest that the concentration of localised states in the forbidden gap increases with increasing value of the self-bias voltage. 2. Both DLC/p-Si and DLC/n-Si junctions show rather weak rectifying properties. The current– voltage characteristics of the junctions verify the domination of hopping transport in DLC films. The conduction in the forward direction of DLC/n-Si junction is a few times higher than that of DLC/p-Si heterostructure and this may be interpreted as a result of electron transport domination in undoped DLC films. 3. The presented results of both electrical and optical investigations suggest that the forbidden gap is reduced with increasing value of the negative self-bias voltage. The reduction results from the increasing concentration of localised states of the band edges. As a result, the measured effective depth of trapping levels is also reduced. The increasing concentration of localised states may result in formation of subbands of localised states within the gap. The thermal activation of charge carriers between the sub-bands may be responsible for electrical conduction in the higher temperatures (i.e. above approximately 260 K).