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silicon heterojunctions
Diamond & Related Materials 22 (2012) 37–41
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Temperature-dependent resistive switching of amorphous carbon/silicon heterojunctions Xili Gao a, b, Xiaozhong Zhang a, b,⁎, Caihua Wan a, b, Jimin Wang a, b, Xinyu Tan a, b, Dechang Zeng c a b c
Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing, 100084, People's Republic of China National Center for Electron Microscopy (Beijing), Tsinghua University, Beijing 100084, People's Republic of China School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, People's Republic of China
a r t i c l e
i n f o
Article history: Received 28 April 2011 Received in revised form 6 December 2011 Accepted 13 December 2011 Available online 22 December 2011 Keywords: Amorphous carbon Heterojunction Resistive switching
a b s t r a c t Amorphous graphite-like carbon (a-GLC) films were deposited on n-Si substrates by pulsed laser deposition (PLD) technique to form a-GLC/Si heterojunctions. The a-GLC/Si heterojunctions deposited at 300 K showed nonvolatile and reproducible resistive switching behavior at low temperatures (80–150 K). Applied proper bias voltage, the resistance of the heterojunctions could be switched from one state to the other state. The ratio of the high/low resistances could be greater than 10. This phenomenon might be attributed to the effects of the reverse barrier and the large density of traps in a-GLC film on the carriers. This a-GLC/Si heterojunctions might provide a new approach to fabricate higher density and low-temperature functional memory. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, it is an appealing aim of the memory industry to design memory cells with high density, long retention and strong stability in extreme work conditions. Many attentions have been paid to search high-κ dielectric materials [1,2] and fabricate a memory device with different mechanisms [3,4]. Carbon, as a unique material, shows amazing characteristics in memory device. It can be fabricated to crystal, polycrystalline and amorphous allotropes by different methods [5–7]. The electrical properties of carbon materials are characterized by their microstructure and volume fraction of sp 3 cluster, which can be transformed under proper thermal [8,9] or focused electron beam irradiate treatment [10,11]. In contrast to other switchable memory materials, carbon is of mono-atomic nature, may be scalable to very small feature size (even single bonds) which would increase memory's density significantly [12]. Moreover, carbon is compatible with silicon and this would integrate with silicon-based device easily. These special characteristics make carbon material an outstanding alternative in flash memory. Recently, switching behaviors have been reported intensively in one-dimensional carbon-based materials [13–15]. These devices show large high/low resistances ratio, nonvolatile and reproducible properties. However, it is still a problem to fabricate millions of memory cells based on individual nanosized in parallel, because it is hard to eliminate variation among individual structures and thus hard to reproducibly control properties of these cells [16]. Compared with
⁎ Corresponding author. Tel.: + 86 10 62773999; fax: + 86 10 62771160. E-mail address: [email protected] (X. Zhang). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.12.012
one-dimensional memory element, two-dimensional memory cell (film) exhibited more potential applications. Sinitskii et al. [17] found nonvolatile and reproducible resistive switching behavior in lithographic graphitic films and attributed this behavior to the formation of crack generated by Joule Heat. Besides phase changed switching behavior, carrier captured switching behavior was also reported in amorphous carbon films based on their large density of defect states [18,19]. However, to our best knowledge, most of the carrier captured switching behaviors were observed in doped diamond-like carbon (DLC) films deposited on metal or metallic compound substrates. In this article, we reported the temperature-dependent two-terminal resistive switching of undoped a-GLC/silicon heterojunctions fabricated by pulsed laser deposition (PLD) method. The a-GLC/Si heterojunctions showed reproducible and nonvolatile switching behaviors and the high/low resistance ratio was greater than 10 at low temperature (80 K). However, the switching behavior disappeared gradually with increasing temperatures. This a-GLC/Si heterojunctions showed different operating mechanisms which might provide a new approach to fabricate higher density and low-temperature functional memory. 2. Experiment The a-C films were deposited on n-type (100) silicon substrates by PLD method at various temperatures under 10 Pa of argon. Pure graphite (99.99%) was firstly ball milled for 24 h and then was hot-pressed into cylinders at 1300 °C and 40 MPa for 30 min in Argon atmosphere. The as-fabricated cylinders were used as a target for PLD. The silicon substrates are n-type semiconductors with resistivity of 0.55–0.8 Ω cm. Before deposition, the substrates were ultrasonically cleaned in ethanol, acetone, and then de-ionized water. After etched in diluted HF solution
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X. Gao et al. / Diamond & Related Materials 22 (2012) 37–41
and rinsed in de-ionized water, Si substrates were quickly placed into the deposition chamber. Ablation of a rotating composite target has been performed using a 248 nm UV excimer laser (Lambda Physik LPX 205) delivering a laser fluence of ~8 J/cm2 at a repetition rate of 10 Hz. The substrate holder was rotated at a constant speed of ~30 rpm and the target to substrate distance was fixed at about 5 cm. Indium (In) electrodes of 2 × 2 mm 2 were pressed on the carbon film and the experimental results show that the In electrodes have Ohmic contact to the carbon films. The current–voltage (I–V) curves of the a-GLC/Si heterojunctions were measured at various temperatures by using a two-probe method with a Keithley 2400 sourcemeter. The ratio of sp 2 and sp 3 cluster was investigated by Raman spectrum (Renishaw RM2000). Double–Gaussian peak model was used to fit peaks in Raman spectra. 3. Results and discussion 3.1. The Raman spectra of a-C films Fig. 1(a) shows the Raman spectra of the a-C films deposited at T = 300, 573, 873, 973 K. It is well known that the Raman spectrum of carbon materials can be fitted to the D band at 1350 cm− 1 and the G band at 1580 cm− 1. The ratio of the intensities between D and G peaks provides information on sp2/sp3 and the sp2 cluster size in the films. The full width at half maximum of G peak (FWHMG) shows the disorder degree of the a-C film. From these spectra and the fitted results (Fig. 1(b)) two main variations were clearly observed: with increasing deposited temperatures, (1) FWHMG decreased from 116 to 90 cm− 1 and (2) the ratio of ID/IG increased from 1.8 to 3.2 at first, and then decreased to 1.4 when the deposited temperatures increase
from 573 to 973 K. The results indicated these a-C films were GLC which had low sp 3 contents (b20%). Meanwhile the sp 3 contents and the disorder degree of a-GLC films decreased with increasing deposited temperatures [20]. 3.2. The current–voltage characteristics of a-GLC/Si heterojunctions deposited at 300 K Fig. 2(a) is the current–voltage (I–V) characteristics of the a-GLC/ Si heterojunctions deposited at 300 K measured at 80 K. As the voltage was swept in a sequence of −5 V → 0 V → 5 V → 0 V → −5 V, the I–V curves of a-GLC/Si heterojunction showed an obvious resistive switching behavior (low and high resistance states). It is noteworthy that, instead of sweeping, voltage pulses could be also used to achieve the switching behavior at low bias voltages. A pulse voltage of 5 V (or − 5 V) for 1 ms was used to write (or erase). After each write or erase operation, the device resistance was consecutively read at 0.5 V for 10 times, and the resistances of low and high state were about 10 7 and 10 8 Ω, respectively. After at least 20 circles of writeread (10 times)–erase-read (10 times) operations, the memory device showed no degradation (Fig. 2(c)). To investigate the retention properties, the device was consecutively read at 0.5 V for 3000 s after a write or erase operation, and the result showed (Fig. 2(b)) that this voltage pulse generated switching behavior was nonvolatile. The switching behavior observed in our work was different from those reported switching behaviors of carbon films [17–19]. In our case, our sample was pure a-GLC film rather than N-doped [18] or B-doped [19] a-DLC film. Moreover, the switching behavior observed in our sample only appeared at low temperatures (80–150 K) rather than at high temperatures (>250 K) as observed in N-doped a-DLC film/alumina substrate structure [18]. To investigate the origin of the switching behavior, a-GLC/glass structure was also fabricated. The result showed the contact between indium electrodes and aGLC film was Ohmic and there was no switching behavior observed, indicating barrier might play an important role in this kind of resistive switching effect. Considering that the a-GLC film contained a large number of defects, trapping effect on carriers should be also taken into account in the resistive switching behavior of a-GLC/Si heterojunctions. We proposed a plausible mechanism to understand the abnormal temperature-dependent resistive switching behavior of a-GLC/Si heterojunctions. When an a-GLC film was deposited on the n-Si substrates, a heterojunction was then formed. As both indium electrodes were placed on the a-GLC film, the structure of a-GLC/Si can be modeled as two heterojunctions connected serially (inset of Fig. 2(a)). Considering that the resistance of reverse biased heterojunction is much larger than that of forward biased heterojunction, the I–V characteristic of the a-GLC/Si heterojunction could be simply depicted by the reverse biased heterojunction. Thus, the current density could be expressed as: [21] J ¼ J s ½expðqV=kT Þ−1 J s ¼ qDp pn0 =Lp þ qDn np0 =Ln
ð1Þ
where J, Js are current density and the saturation current density, respectively. q, V, k and T are elementary charge, applied bias voltage, the Boltzmann constant and temperature, respectively. pn0 and np0 are the equilibrium minority carrier density in the n-side and p-side, respectively. Lp and Ln are the diffusion length of holes and electrons, respectively. Dp and Dn are the diffusivity of the holes and electrons, respectively. For the reverse biased voltage, V b 0 and |V|> > kT/q (7.0 mV, when T = 80 K). Eq. (1) could be rewritten as Fig. 1. (a) Raman spectrum of a-C films deposited on Si substrate at various temperatures and (b) the full width at half maximum of G peaks and the ratio of the intensity of D and G peaks (ID/IG).
J ¼ −J s ¼ − qDp pn0 =Lp þ qDn np0 =Ln :
ð2Þ
X. Gao et al. / Diamond & Related Materials 22 (2012) 37–41
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Fig. 2. Resistive switching behavior of a-GLC/Si heterojunction measured at 80 K. (a) I–V characteristics of a-GLC/Si junction (Insert is the schematic illustration of the electrical measurement). (b) The retention time of a-GLC/Si heterojunction . (c) Cyclic endurance of the same device: applying pulse of 5 V and − 5 V for 1 ms turn the device to low and high resistance state, respectively. After each write or erase operation, the device resistance was read at 0.5 V 10 times.
The a-GLC film was highly disordered and contained a large density of traps which would remarkably affect the carrier density or mobility [22]. In this article, we would mainly focus on the effect of traps on the carrier density. To understand the switching behavior more clearly, extended and localized states distribution in a-C and band structure of a-GLC/Si heterojunctions are schematically shown in Fig. 3 according to Robertson's results [23]. EC, EV and Ef are the bottom of conduction band, the top of valence band and Fermi level, respectively. Si and C represent those quantities in silicon substrates and carbon matrix, respectively. El and Ed are and localized states and defect states, respectively. When reverse bias voltages were applied on the a-GLC/Si heterojunction, EfC tended to shift up in the neutral area and bent downward in the depletion area. However, considering the pinning effect of defect states on Fermi level of a-GLC, it was very difficult to further increase the applied voltage until the defects were fully filled by injected carriers. Phenomenologically, the slope of log(I)–log(V) was 13 (Fig. 4), much larger than 1(Ohmic transport region) and 2 (space charge limited region) in this case [24–26]. On the other hand, when the injected electrons began to fill the defects which assist hopping of carriers, the density of majority carrier in a-GLC would decrease in a large amount. Even though the applied voltage was removed, these excess electrons occupying the defects would still stay there because these defect states were within the range of mobility edges and thus those injected carriers were impossible to escape from these defects. This also meant, at the same time, the minority density was increased after the applied voltage was removed. According to formula (2), the reversal current density (− Js) of these heterojunctions was mainly determined by and, concretely speaking, proportional to minority density (np0) of a-GLC which is
reversal to majority density. Therefore, the increase of minority density in a-GLC due to historical reversal bias voltages would lead to a larger reversal current density. Phenomenologically, the heterojunctions were
Fig. 3. Schematic of (a) extended and localized state distribution in a-C and (b) band structure of a-GLC/Si heterojunction.
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Fig. 4. Relation between log(I) and log(V) of the a-GLC/Si heterojunction at 80 K. Fig. 5. The high and low resistance states of a-GLC/Si heterojunctions as a function of temperature.
set to low resistance state. In the erase process, forward bias voltages were applied on the a-GLC/Si heterojunction. The injected electrons filled in the defect states would be removed by the electric field and the density of majority carrier would recover. This meant the a-GLC/Si device was reset to the high resistance state again. Fig. 4 shows the relation between log(I) and log(V) of the a-GLC/Si heterojunction at 80 K. The slope of log(I)–log(V) could be used to monitor the current injection in solids, especially for the insulators with traps in which the trap filling progress is often misunderstood as breakdown. Considering the traps and thermal carriers, the log(I)–log(V) of insulators contain three limiting curves: Ohm's law, Child's law and a trap-filled-limit curve which has a voltage threshold and enormously steep current rise [24–26]. Normally, the carriers start to fill the traps when the applied voltage is larger than the threshold value. In this progress, the slope of log(I)–log(V) would increase steeply. As long as the traps were fully filled, the slope of log(I)–log(V) would decrease and fit Child's law again. For upsweep process, the log(I)–log(V) curve thus showed three different sections. Under low applied voltages, the slope of log(I)–log(V) was about 3.5 and it steeply increased to 13.8 when the applied voltage was larger than a threshold value, which meant injected carriers began to fill the traps [24–26]. When the applied voltage further increased, the slope of log(I)–log(V) decreased to 3.3, indicating that most traps were filled by injected carriers. For downsweep process, the slope of the log(I)– log(V) was about 3.2 and it was not changed with decreasing of applied voltages, which meant the trapped carriers had not escaped from the trapping states. This result was in accordance with our proposed mechanism of switching behavior in a-GLC/Si heterojunction. The resistive switching behavior observed in our a-GLC/Si heterojunctions was also strongly temperature-dependent (Fig. 5). The resistive switching behavior occurred only in low temperature range of 80–150 K, and disappeared above 150 K. At 80 K, after write (5 V) or erase (− 5 V), the resistance read at 0.5 V is about 10 7 (ON) and 10 8 Ω(OFF), respectively. However, the difference between ON and OFF decreased gradually as temperature increases and it became zero when temperature was above 150 K. For higher temperatures (>200 K), the a-GLC/Si heterojunction showed very different conduct mechanism from that at low temperatures, which might result from the disappearance of the switching behavior. Fig. 6 is the relation between log(I) and log(V) of the aGLC/Si heterojunction at 200 K, which shows that the slope of log(I)–log(V) was about 1.2 at 200 K and decreased gradually to 1 with increasing temperatures (in the both upsweep and downsweep process). This indicated that the dominant mechanism in higher temperature (> 200 K) was Ohmic rather than space charge limit current dominated at low temperatures [27] (80–150 K). When the Ohmic transport mechanism dominated, no net carriers were injected into the memory cells and thus no switching behavior was observed at high temperatures.
3.3. The relation of resistive switching behavior and deposited temperatures Fig. 7 shows the I–V characteristics of the a-GLC/Si heterojunctions measured at 80 K. It could be seen that the I–V characteristic of the aGLC/Si heterojunction deposited at 300 K (black line) showed an obvious resistive switching behavior (low and high resistance states) as the voltage was swept in a sequence of −5 V → 0 V → 5 V → 0 V → −5 V. However, the resistive switching behavior weakened with increasing deposited temperature and totally disappeared when the deposited temperature was higher than 873 K. Considering the Raman results, it could be concluded that higher disorder degree and higher sp3/sp2 ratio resulting from a low depositing temperature led to a higher trap density which was crucial for this resistive switching phenomenon. Compared to flash memory device (three-terminal), a-GLC/Si heterojunctions are two-terminal which do not require such complex fabrication process or geometries as three-terminal device, providing a new approach to fabricate higher density memory. Moreover, this abnormal switching behavior might be used in some special environment where conventional memories could not function normally. 4. Conclusions a-GLC/Si heterojunctions were fabricated by PLD technique at various temperatures. The results showed the higher disorder degree and higher sp3/sp2 ratio lead to a higher trap density which was crucial for this resistive switching phenomenon. The heterojunctions deposited at 300 K showed reproducible and nonvolatile resistive switching behavior at low temperatures. By applying a proper bias voltage, resistive switching from one state to another state could be achieved. The
Fig. 6. Relation between log(I) and log(V) of the a-GLC/Si heterojunction at 200 K. Inset shows relation between log(I) and log(V) under vary temperatures.
X. Gao et al. / Diamond & Related Materials 22 (2012) 37–41
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Fig. 7. I–V characteristics of a-GLC/Si heterojunctions deposited at various temperatures measured at 80 K.
ratio of high/low resistance was greater than 10 at low temperature (80 K). This resistive switching behavior might be attributed to the combination effect of the reverse barrier and the large density of traps in a-GLC film. The temperature-dependent property might be attributed to the different conduction mechanisms in different temperature ranges. Acknowledgements We would like to acknowledge the financial support by The National Science Foundation of China (Grant nos. U0734001 and 11074141) and The Ministry of Science and Technology of China (Grant no. 2009CB929202).
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Contents lists available at SciVerse ScienceDirect
Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond
Temperature-dependent resistive switching of amorphous carbon/silicon heterojunctions Xili Gao a, b, Xiaozhong Zhang a, b,⁎, Caihua Wan a, b, Jimin Wang a, b, Xinyu Tan a, b, Dechang Zeng c a b c
Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing, 100084, People's Republic of China National Center for Electron Microscopy (Beijing), Tsinghua University, Beijing 100084, People's Republic of China School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, People's Republic of China
a r t i c l e
i n f o
Article history: Received 28 April 2011 Received in revised form 6 December 2011 Accepted 13 December 2011 Available online 22 December 2011 Keywords: Amorphous carbon Heterojunction Resistive switching
a b s t r a c t Amorphous graphite-like carbon (a-GLC) films were deposited on n-Si substrates by pulsed laser deposition (PLD) technique to form a-GLC/Si heterojunctions. The a-GLC/Si heterojunctions deposited at 300 K showed nonvolatile and reproducible resistive switching behavior at low temperatures (80–150 K). Applied proper bias voltage, the resistance of the heterojunctions could be switched from one state to the other state. The ratio of the high/low resistances could be greater than 10. This phenomenon might be attributed to the effects of the reverse barrier and the large density of traps in a-GLC film on the carriers. This a-GLC/Si heterojunctions might provide a new approach to fabricate higher density and low-temperature functional memory. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nowadays, it is an appealing aim of the memory industry to design memory cells with high density, long retention and strong stability in extreme work conditions. Many attentions have been paid to search high-κ dielectric materials [1,2] and fabricate a memory device with different mechanisms [3,4]. Carbon, as a unique material, shows amazing characteristics in memory device. It can be fabricated to crystal, polycrystalline and amorphous allotropes by different methods [5–7]. The electrical properties of carbon materials are characterized by their microstructure and volume fraction of sp 3 cluster, which can be transformed under proper thermal [8,9] or focused electron beam irradiate treatment [10,11]. In contrast to other switchable memory materials, carbon is of mono-atomic nature, may be scalable to very small feature size (even single bonds) which would increase memory's density significantly [12]. Moreover, carbon is compatible with silicon and this would integrate with silicon-based device easily. These special characteristics make carbon material an outstanding alternative in flash memory. Recently, switching behaviors have been reported intensively in one-dimensional carbon-based materials [13–15]. These devices show large high/low resistances ratio, nonvolatile and reproducible properties. However, it is still a problem to fabricate millions of memory cells based on individual nanosized in parallel, because it is hard to eliminate variation among individual structures and thus hard to reproducibly control properties of these cells [16]. Compared with
⁎ Corresponding author. Tel.: + 86 10 62773999; fax: + 86 10 62771160. E-mail address: [email protected] (X. Zhang). 0925-9635/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2011.12.012
one-dimensional memory element, two-dimensional memory cell (film) exhibited more potential applications. Sinitskii et al. [17] found nonvolatile and reproducible resistive switching behavior in lithographic graphitic films and attributed this behavior to the formation of crack generated by Joule Heat. Besides phase changed switching behavior, carrier captured switching behavior was also reported in amorphous carbon films based on their large density of defect states [18,19]. However, to our best knowledge, most of the carrier captured switching behaviors were observed in doped diamond-like carbon (DLC) films deposited on metal or metallic compound substrates. In this article, we reported the temperature-dependent two-terminal resistive switching of undoped a-GLC/silicon heterojunctions fabricated by pulsed laser deposition (PLD) method. The a-GLC/Si heterojunctions showed reproducible and nonvolatile switching behaviors and the high/low resistance ratio was greater than 10 at low temperature (80 K). However, the switching behavior disappeared gradually with increasing temperatures. This a-GLC/Si heterojunctions showed different operating mechanisms which might provide a new approach to fabricate higher density and low-temperature functional memory. 2. Experiment The a-C films were deposited on n-type (100) silicon substrates by PLD method at various temperatures under 10 Pa of argon. Pure graphite (99.99%) was firstly ball milled for 24 h and then was hot-pressed into cylinders at 1300 °C and 40 MPa for 30 min in Argon atmosphere. The as-fabricated cylinders were used as a target for PLD. The silicon substrates are n-type semiconductors with resistivity of 0.55–0.8 Ω cm. Before deposition, the substrates were ultrasonically cleaned in ethanol, acetone, and then de-ionized water. After etched in diluted HF solution
38
X. Gao et al. / Diamond & Related Materials 22 (2012) 37–41
and rinsed in de-ionized water, Si substrates were quickly placed into the deposition chamber. Ablation of a rotating composite target has been performed using a 248 nm UV excimer laser (Lambda Physik LPX 205) delivering a laser fluence of ~8 J/cm2 at a repetition rate of 10 Hz. The substrate holder was rotated at a constant speed of ~30 rpm and the target to substrate distance was fixed at about 5 cm. Indium (In) electrodes of 2 × 2 mm 2 were pressed on the carbon film and the experimental results show that the In electrodes have Ohmic contact to the carbon films. The current–voltage (I–V) curves of the a-GLC/Si heterojunctions were measured at various temperatures by using a two-probe method with a Keithley 2400 sourcemeter. The ratio of sp 2 and sp 3 cluster was investigated by Raman spectrum (Renishaw RM2000). Double–Gaussian peak model was used to fit peaks in Raman spectra. 3. Results and discussion 3.1. The Raman spectra of a-C films Fig. 1(a) shows the Raman spectra of the a-C films deposited at T = 300, 573, 873, 973 K. It is well known that the Raman spectrum of carbon materials can be fitted to the D band at 1350 cm− 1 and the G band at 1580 cm− 1. The ratio of the intensities between D and G peaks provides information on sp2/sp3 and the sp2 cluster size in the films. The full width at half maximum of G peak (FWHMG) shows the disorder degree of the a-C film. From these spectra and the fitted results (Fig. 1(b)) two main variations were clearly observed: with increasing deposited temperatures, (1) FWHMG decreased from 116 to 90 cm− 1 and (2) the ratio of ID/IG increased from 1.8 to 3.2 at first, and then decreased to 1.4 when the deposited temperatures increase
from 573 to 973 K. The results indicated these a-C films were GLC which had low sp 3 contents (b20%). Meanwhile the sp 3 contents and the disorder degree of a-GLC films decreased with increasing deposited temperatures [20]. 3.2. The current–voltage characteristics of a-GLC/Si heterojunctions deposited at 300 K Fig. 2(a) is the current–voltage (I–V) characteristics of the a-GLC/ Si heterojunctions deposited at 300 K measured at 80 K. As the voltage was swept in a sequence of −5 V → 0 V → 5 V → 0 V → −5 V, the I–V curves of a-GLC/Si heterojunction showed an obvious resistive switching behavior (low and high resistance states). It is noteworthy that, instead of sweeping, voltage pulses could be also used to achieve the switching behavior at low bias voltages. A pulse voltage of 5 V (or − 5 V) for 1 ms was used to write (or erase). After each write or erase operation, the device resistance was consecutively read at 0.5 V for 10 times, and the resistances of low and high state were about 10 7 and 10 8 Ω, respectively. After at least 20 circles of writeread (10 times)–erase-read (10 times) operations, the memory device showed no degradation (Fig. 2(c)). To investigate the retention properties, the device was consecutively read at 0.5 V for 3000 s after a write or erase operation, and the result showed (Fig. 2(b)) that this voltage pulse generated switching behavior was nonvolatile. The switching behavior observed in our work was different from those reported switching behaviors of carbon films [17–19]. In our case, our sample was pure a-GLC film rather than N-doped [18] or B-doped [19] a-DLC film. Moreover, the switching behavior observed in our sample only appeared at low temperatures (80–150 K) rather than at high temperatures (>250 K) as observed in N-doped a-DLC film/alumina substrate structure [18]. To investigate the origin of the switching behavior, a-GLC/glass structure was also fabricated. The result showed the contact between indium electrodes and aGLC film was Ohmic and there was no switching behavior observed, indicating barrier might play an important role in this kind of resistive switching effect. Considering that the a-GLC film contained a large number of defects, trapping effect on carriers should be also taken into account in the resistive switching behavior of a-GLC/Si heterojunctions. We proposed a plausible mechanism to understand the abnormal temperature-dependent resistive switching behavior of a-GLC/Si heterojunctions. When an a-GLC film was deposited on the n-Si substrates, a heterojunction was then formed. As both indium electrodes were placed on the a-GLC film, the structure of a-GLC/Si can be modeled as two heterojunctions connected serially (inset of Fig. 2(a)). Considering that the resistance of reverse biased heterojunction is much larger than that of forward biased heterojunction, the I–V characteristic of the a-GLC/Si heterojunction could be simply depicted by the reverse biased heterojunction. Thus, the current density could be expressed as: [21] J ¼ J s ½expðqV=kT Þ−1 J s ¼ qDp pn0 =Lp þ qDn np0 =Ln
ð1Þ
where J, Js are current density and the saturation current density, respectively. q, V, k and T are elementary charge, applied bias voltage, the Boltzmann constant and temperature, respectively. pn0 and np0 are the equilibrium minority carrier density in the n-side and p-side, respectively. Lp and Ln are the diffusion length of holes and electrons, respectively. Dp and Dn are the diffusivity of the holes and electrons, respectively. For the reverse biased voltage, V b 0 and |V|> > kT/q (7.0 mV, when T = 80 K). Eq. (1) could be rewritten as Fig. 1. (a) Raman spectrum of a-C films deposited on Si substrate at various temperatures and (b) the full width at half maximum of G peaks and the ratio of the intensity of D and G peaks (ID/IG).
J ¼ −J s ¼ − qDp pn0 =Lp þ qDn np0 =Ln :
ð2Þ
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Fig. 2. Resistive switching behavior of a-GLC/Si heterojunction measured at 80 K. (a) I–V characteristics of a-GLC/Si junction (Insert is the schematic illustration of the electrical measurement). (b) The retention time of a-GLC/Si heterojunction . (c) Cyclic endurance of the same device: applying pulse of 5 V and − 5 V for 1 ms turn the device to low and high resistance state, respectively. After each write or erase operation, the device resistance was read at 0.5 V 10 times.
The a-GLC film was highly disordered and contained a large density of traps which would remarkably affect the carrier density or mobility [22]. In this article, we would mainly focus on the effect of traps on the carrier density. To understand the switching behavior more clearly, extended and localized states distribution in a-C and band structure of a-GLC/Si heterojunctions are schematically shown in Fig. 3 according to Robertson's results [23]. EC, EV and Ef are the bottom of conduction band, the top of valence band and Fermi level, respectively. Si and C represent those quantities in silicon substrates and carbon matrix, respectively. El and Ed are and localized states and defect states, respectively. When reverse bias voltages were applied on the a-GLC/Si heterojunction, EfC tended to shift up in the neutral area and bent downward in the depletion area. However, considering the pinning effect of defect states on Fermi level of a-GLC, it was very difficult to further increase the applied voltage until the defects were fully filled by injected carriers. Phenomenologically, the slope of log(I)–log(V) was 13 (Fig. 4), much larger than 1(Ohmic transport region) and 2 (space charge limited region) in this case [24–26]. On the other hand, when the injected electrons began to fill the defects which assist hopping of carriers, the density of majority carrier in a-GLC would decrease in a large amount. Even though the applied voltage was removed, these excess electrons occupying the defects would still stay there because these defect states were within the range of mobility edges and thus those injected carriers were impossible to escape from these defects. This also meant, at the same time, the minority density was increased after the applied voltage was removed. According to formula (2), the reversal current density (− Js) of these heterojunctions was mainly determined by and, concretely speaking, proportional to minority density (np0) of a-GLC which is
reversal to majority density. Therefore, the increase of minority density in a-GLC due to historical reversal bias voltages would lead to a larger reversal current density. Phenomenologically, the heterojunctions were
Fig. 3. Schematic of (a) extended and localized state distribution in a-C and (b) band structure of a-GLC/Si heterojunction.
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Fig. 4. Relation between log(I) and log(V) of the a-GLC/Si heterojunction at 80 K. Fig. 5. The high and low resistance states of a-GLC/Si heterojunctions as a function of temperature.
set to low resistance state. In the erase process, forward bias voltages were applied on the a-GLC/Si heterojunction. The injected electrons filled in the defect states would be removed by the electric field and the density of majority carrier would recover. This meant the a-GLC/Si device was reset to the high resistance state again. Fig. 4 shows the relation between log(I) and log(V) of the a-GLC/Si heterojunction at 80 K. The slope of log(I)–log(V) could be used to monitor the current injection in solids, especially for the insulators with traps in which the trap filling progress is often misunderstood as breakdown. Considering the traps and thermal carriers, the log(I)–log(V) of insulators contain three limiting curves: Ohm's law, Child's law and a trap-filled-limit curve which has a voltage threshold and enormously steep current rise [24–26]. Normally, the carriers start to fill the traps when the applied voltage is larger than the threshold value. In this progress, the slope of log(I)–log(V) would increase steeply. As long as the traps were fully filled, the slope of log(I)–log(V) would decrease and fit Child's law again. For upsweep process, the log(I)–log(V) curve thus showed three different sections. Under low applied voltages, the slope of log(I)–log(V) was about 3.5 and it steeply increased to 13.8 when the applied voltage was larger than a threshold value, which meant injected carriers began to fill the traps [24–26]. When the applied voltage further increased, the slope of log(I)–log(V) decreased to 3.3, indicating that most traps were filled by injected carriers. For downsweep process, the slope of the log(I)– log(V) was about 3.2 and it was not changed with decreasing of applied voltages, which meant the trapped carriers had not escaped from the trapping states. This result was in accordance with our proposed mechanism of switching behavior in a-GLC/Si heterojunction. The resistive switching behavior observed in our a-GLC/Si heterojunctions was also strongly temperature-dependent (Fig. 5). The resistive switching behavior occurred only in low temperature range of 80–150 K, and disappeared above 150 K. At 80 K, after write (5 V) or erase (− 5 V), the resistance read at 0.5 V is about 10 7 (ON) and 10 8 Ω(OFF), respectively. However, the difference between ON and OFF decreased gradually as temperature increases and it became zero when temperature was above 150 K. For higher temperatures (>200 K), the a-GLC/Si heterojunction showed very different conduct mechanism from that at low temperatures, which might result from the disappearance of the switching behavior. Fig. 6 is the relation between log(I) and log(V) of the aGLC/Si heterojunction at 200 K, which shows that the slope of log(I)–log(V) was about 1.2 at 200 K and decreased gradually to 1 with increasing temperatures (in the both upsweep and downsweep process). This indicated that the dominant mechanism in higher temperature (> 200 K) was Ohmic rather than space charge limit current dominated at low temperatures [27] (80–150 K). When the Ohmic transport mechanism dominated, no net carriers were injected into the memory cells and thus no switching behavior was observed at high temperatures.
3.3. The relation of resistive switching behavior and deposited temperatures Fig. 7 shows the I–V characteristics of the a-GLC/Si heterojunctions measured at 80 K. It could be seen that the I–V characteristic of the aGLC/Si heterojunction deposited at 300 K (black line) showed an obvious resistive switching behavior (low and high resistance states) as the voltage was swept in a sequence of −5 V → 0 V → 5 V → 0 V → −5 V. However, the resistive switching behavior weakened with increasing deposited temperature and totally disappeared when the deposited temperature was higher than 873 K. Considering the Raman results, it could be concluded that higher disorder degree and higher sp3/sp2 ratio resulting from a low depositing temperature led to a higher trap density which was crucial for this resistive switching phenomenon. Compared to flash memory device (three-terminal), a-GLC/Si heterojunctions are two-terminal which do not require such complex fabrication process or geometries as three-terminal device, providing a new approach to fabricate higher density memory. Moreover, this abnormal switching behavior might be used in some special environment where conventional memories could not function normally. 4. Conclusions a-GLC/Si heterojunctions were fabricated by PLD technique at various temperatures. The results showed the higher disorder degree and higher sp3/sp2 ratio lead to a higher trap density which was crucial for this resistive switching phenomenon. The heterojunctions deposited at 300 K showed reproducible and nonvolatile resistive switching behavior at low temperatures. By applying a proper bias voltage, resistive switching from one state to another state could be achieved. The
Fig. 6. Relation between log(I) and log(V) of the a-GLC/Si heterojunction at 200 K. Inset shows relation between log(I) and log(V) under vary temperatures.
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References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Fig. 7. I–V characteristics of a-GLC/Si heterojunctions deposited at various temperatures measured at 80 K.
ratio of high/low resistance was greater than 10 at low temperature (80 K). This resistive switching behavior might be attributed to the combination effect of the reverse barrier and the large density of traps in a-GLC film. The temperature-dependent property might be attributed to the different conduction mechanisms in different temperature ranges. Acknowledgements We would like to acknowledge the financial support by The National Science Foundation of China (Grant nos. U0734001 and 11074141) and The Ministry of Science and Technology of China (Grant no. 2009CB929202).
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