ISSN 1998-0124 33 CN 11-5974/O4
Nano Res. Res. 2012, 2012, 5(1): 5(1): 33–42 33–42 Nano DOI 10.1007/s12274-011-0182-1 Research Article
Doping-Free Fabrication of Carbon Nanotube Thin-Film Diodes and Their Photovoltaic Characteristics Qingsheng Zeng1, Sheng Wang1 (), Leijing Yang1, Zhenxing Wang1, Zhiyong Zhang1, Lianmao Peng1 (), Weiya Zhou2, and Sishen Xie2 () 1 2
Key Laboratory for the Physics and Chemistry of Nanodevices, Department of Electronics, Peking University, Beijing 100871, China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
Received: 7 September 2011 / Revised: 20 October 2011 / Accepted: 23 October 2011 © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011
ABSTRACT Random networks of single-walled carbon nanotubes (SWCNTs) were have been grown by chemical vapor deposition on silicon wafers and used for fabricating field-effect transistors (FETs) using symmetric Pd contacts and diodes using asymmetrical Pd and Sc contacts. For a short channel FET or diode with a channel length of about 1 μm or less, the device works in the direct transport regime, while for a longer channel device the transport mechanism changes to percolation. Detailed electronic and photovoltaic (PV) characterizations of these carbon nanotube (CNT) thin-film devices was carried out. While as-fabricated FETs exhibited typical p-type transfer characteristics, with a large current ON/OFF ratio of more than 104 when metallic CNTs were removed via a controlled breakdown, it was found that the threshold voltage for the devices was typically very large, of the order of about 10 V. This situation was greatly improved when the device was coated with a passivation layer of 12 nm HfO2, which effectively moved the threshold voltages of both FET and diode back to center around zero or turned these device to their OFF states when no bias was applied on the gate. PV measurements were then made on the short channel diodes under infrared laser illumination. It was shown that under an illumination power density of 1.5 kW/cm2, the device resulted in an open circuit voltage VOC = 0.21 V and a short circuit current ISC = 3.74 nA. Furthermore, we compared PV characteristics of CNT film diodes with different channel lengths, and found that the power transform efficiency decreased significantly when the device changed from the direct transport to the percolation regime.
KEYWORDS Carbon nanotube, photovoltaic, diodes, thin film, doping-free
1. Introduction Carbon nanotubes (CNTs) have attracted immense interest due to their outstanding properties and potential applications in nanoelectronic and optoelectronic devices [1]. Semiconducting single-walled carbon nano-
tubes (SWCNTs) have been extensively investigated as the channel materials in high performance fieldeffect transistors (FETs) and have been shown, using device performance metrics, to outperform those based on Si—due particularly to the extremely high carrier mobility of CNTs [2]. But the performance of CNT
Address correspondence to Sheng Wang,
[email protected]; Lianmao Peng,
[email protected]; Sishen Xie,
[email protected]
34 thin-film-based nanoelectronic devices lags a long way behind that based on individual CNTs [3–6]. Although medium-scale CNT thin-film integrated circuits, composed of about 100 thin-film transistors (TFTs), have been demonstrated on flexible substrates [3], the performance of typical CNT thin-film FETs is degraded due to the admixture of semiconducting and metallic CNTs which usually results in a low current ON/OFF ratio making them less suitable for high performance applications. Nevertheless CNT TFTs have been investigated for their special electric characteristics which are different from those of individual SWCNTs [3–6], and found to have the advantages of mechanical flexibility, high-yield fabrication of integrated circuits and excellent electrical performance, which are difficult to achieve using organic molecule or polymer-based materials. High-performance CNT TFTs and complementary metal-oxide semiconductor (CMOS) logic circuits based on separated carbon nanotubes also show great potential for future nanotube-based thin-film macro-electronics and display electronics [7–9]. A semiconducting CNT thin film has a direct band gap and may in principle be used for photovoltaic (PV) applications. Apart from their superior electrical and thermal conductivity, semiconducting CNTs in a thin film may be tuned to exhibit a wide range of absorption from ultraviolet to infrared to match the solar spectrum [10, 11]. In particular it has been shown that carefully prepared CNT thin films exhibited almost 100% absorption of visible light [12]. Strong carrier multiplication (CM) or multiple exciton generation (MEG) effects have also been observed in CNT systems [13, 14], which may in principle lead to a potentially higher energy conversion efficiencies than that defined by the Shockley–Queisser limit [15]. Moreover, high purity semiconducting SWCNT films obtained with density differentiation [16] are now widely available and will further promote the application of CNT in solar cell or other photoelectric devices. Among other applications, CNT network films have been used in organic solar cells [17] serving as electrodes or active light absorption and charge collection materials simultaneously in CNT–Si heterojunction [18, 19]. However, the unique intrinsic PV properties of CNTs have not been fully exploited in those structures. In a typical PV device, a built-in field is essential
Nano Res. 2012, 5(1): 33–42 for efficient separation of light-generated electrons and holes. Several types of single SWCNT-based diodes have been demonstrated, including those based on chemical doping [20], split gate [21], and asymmetric contacts [22–25]. However, diodes fabricated by chemical doping are usually unstable in the usual ambient environment and are seldom used as PV devices [20]. The p–n and p–i–n junction diodes based on split gate or electrostatic doping exhibit ideal diode characteristics, but this type of device requires more complex fabrication process and multi-voltage controls which are not suitable for general PV applications [21]. Recently, we reported observations of PV effects for a semiconducting SWCNT which was asymmetrically contacted with Pd and Sc (or Y), and named this device a barrier-free bipolar diode (BFBD) [22–24]. In this CNT diode, the conduction band of the CNT is aligned with the Fermi level of the low work function metal Sc or Y [26, 27], while the valence band is aligned with the Fermi level of the large work function metal Pd [28]; an electric field is thus formed without chemical doping the CNT channel. It has been shown that this type of CNT diode may be used as an efficient photodetector [23] and light-emitting diode [24]. But so far this technique has not been used for constructing a CNT network based PV devices, and it is the aim of this paper to investigate the electronic and photoelectronic characteristics of CNT thin film devices constructed using this technique.
2. Experimental Random networks of SWCNTs used in this work were grown by chemical vapor deposition (CVD) [29] on a silicon wafer covered with 500 nm thermally grown SiO2. This method can produce CNT networks with homogeneous density with a large area up to the square centimeter scale. The network density is about 1–2 per μm2 (tube length 2–20 μm) and the randomly aligned SWCNTs have diameters between 1.1–1.7 nm and an average diameter of about 1.5 nm as measured by Raman resonance. As-grown CNT networks were cut into stripes with a width W = 50 μm via UV lithography and O2 plasma etching. All electrodes were patterned by electron beam lithography and deposited by electron beam evaporation. All transport
Nano Res. 2012, 5(1): 33–42 measurements were carried out using Keithley 4200 semiconductor analyzer at room temperature. Photovoltaic measurements were carried out using a laser with λ = 785 nm and the laser spot size was varied to fit the width of the device channel which was typically much larger than the channel length of the device. The power density of the focused laser beam on the device may be varied from 0 to 1.5 kW/cm2.
3. Results and discussion 3.1
Properties of CNT thin film FET
We first consider the structure of a CNT network FET as shown in Fig. 1(a). The Pd electrodes had a thickness of 50 nm and the CNT channel length between adjacent electrodes was typically 1 μm. Field-effect measurements were carried out by using the n+ Si as the backgate, and typical results are shown in Fig. 1(b). The as-made thin film FET showed hardly any modulation
35 with the gate voltage. This is because statistically speaking about 1/3 of the CNTs in the as-grown network are metallic. Since the typical length of the CNTs in the network is longer than the channel length of the devices used here (about 1 μm), some metallic CNTs may provide direct paths for carriers to transport between electrodes which are hardly modulated by the back-gate. The direct paths provided by the metallic CNTs were removed by the electrical breakdown procedure developed by the IBM group [30], and the remaining CNTs then result in a typical p-type fieldeffect characteristic (Fig. 1(b)). Before electrical breakdown, the current ION/IOFF ratio of the CNT thin film FET was typically less than ten, and this is largely due to the presence of metallic CNTs in the channel which directly bridge source to drain electrodes. The ION/IOFF ratio can be significantly increased up to about 104–105 after electrical breakdown (Fig. 1(b)), and the breakdown can effectively remove the current contribution from metallic CNTs
Figure 1 Structure and electronic characteristics of CNT thin-film FETs. (a) Schematic diagram showing a Pd-contacted ( p-type) short channel CNT thin-film FET. (b) Transfer characteristics of a typical p-type CNT thin-film FET (with W = 50 μm and L = 1 μm) before and after electrical breakdown, and corresponding (c) output characteristics and (d) gate voltage-dependent transconductance (gm) of the FET for Vds = 0.1 V after electrical breakdown
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by applying a large sweeping voltage between drain/ source electrodes to burn the metallic CNTs, while applying a large positive bias voltage on the back-gate to turn off and preserve the semiconducting CNTs. The output characteristics of the p-type CNT thin film FET are shown in Fig. 1(c) for different gate voltages from –20 V to 20 V with a step of 20 V. The I–V curves appear to be very linear at low bias, demonstrating that Pd forms high quality ohmic contact to the CNT network even after the electrical breakdown. The largest source–drain current is more than 200 μA for Vds = 2 V in this device and the ON-current density (> 4 μA/μm) is comparable to that of the best reported value [7]. Effective removal of metallic CNTs in the channel of the device makes the device perform well as a unipolar p-type device and this is important for obtaining good rectifying characteristics from the diode that is based on the FET. Figure 2(d) shows the gate voltage Vgs-dependent transconductance of the FET at Vds = 0.1 V, demonstrating excellent transconductance of more than 0.3 μS over a large bias range with a maximum at 0.56 μS. To a good approximation, the carrier mobility may be estimated from the transfer characteristics of the device (Fig. 1(b)) using the equation μdevice
L 1 dI ds L gm WC ox Vds dVgs WCox Vds
(1)
where L and W are the length and width of the CNT channel, respectively, and Cox is the gate oxide capacitance per unit area. For a CNT array, taking into consideration the electrostatic coupling between CNTs, the oxide capacitance is given by [8] 1
1 sin h(2 πtox / 0 ) 1 Cox CQ1 ln 0 0 (2) 2 π π R 0 ox in which the quantum capacitance of the nanotube is assumed to be CQ = 4pF/cm [2], the dielectric constant 0 ox = 3.9 × 8.85 × 10–12 F/m for the SiO2 substrate, 1/0 stands for the density of CNTs (here the value is about 1.5 tubes/μm, R = 0.75 nm is the average radius of the CNTs), and tox = 500 nm is the thickness of the dielectric layer. It should be noted that, at most, one third of the CNTs survive the breakdown process as
estimated from the ON/OFF current change before and after breakdown (Fig. 1(b)). This is because not only are metallic CNTs cut off, but also some of the adjacent semiconducting CNTs are also affected during the breakdown process. Taking this effect into consideration, we have calculated that for a typical device with L = 1 μm, 1/0 = 0.5 tubes/μm, and Cox = 1.34 × 10–9 F/cm2. Substituting Cox, the device geometry and transconductance into Eq. (1), the device mobility μdevice is calculated to be 84 cm2/Vs. For the device with L = 3 μm, the transconductance gm = 0.54 μS (not shown here), and device μ is estimated to be 242 cm2/Vs. In previous work, the reported mobilities of CNT TFTs have always been below 100 cm2/Vs even when the channel length was much longer than 3 μm as used here [3, 7]. The mobility of our device is also larger than those TFTs based on separated CNTs and this is due to the fact that no process for transferring CNTs was used, and thus fewer defects were introduced into the CNTs during device fabrication processes. 3.2
Properties of CNT thin film diode
We now consider the fabrication and electrical characteristics of the CNT thin film based diode. Figures 2(a) and 2(b) show the fabrication processes of such a diode based on the as-made p-type FET we just discussed, and Fig. 2(c) is a scanning electron microscopy (SEM) image of a real device. For the FET with a channel length L = 1 μm, after a 50 nm Sc extension was deposited as n-type contact, the device was turned into an asymmetrically contacted diode. The as-made device exhibits a typical p-type transfer characteristic (Fig. 2(d)) and rectifying I–V characteristics (Fig. 2(e)). The point to note is that the CNT channel length is reduced to about 0.6 μm after the extension of the Sc contact as shown in Fig. 2(c). In such a short channel, most of the CNTs are contacted directly by a pair of Pd and Sc electrodes forming direct paths for electronic transport, with only few CNT–CNT junctions between the Pd and Sc electrodes. These junctions are known to increase the series resistance of the diode significantly. There exist not only Schottky Barriers between metallic CNTs (m-CNT) and semiconducting CNTs (s-CNT), but also large contact resistance between m-CNTs or s-CNTs [31].
Nano Res. 2012, 5(1): 33–42 In order to obtain stable and high-performance CNT thin film diodes, a transparent coating material HfO2 was introduced in order to protect the device. The diode exhibits a p-type transfer characteristic when the channel is exposed to air (see the blue line in Fig. 2(d)). This unipolar p-type behavior is due to the adsorption of oxygen on the CNTs, which shifts the threshold of the device far away from 0 V [32, 33]. The influence of O2 adsorption can be avoided by covering the channel
37 with 12 nm HfO2 via atomic layer deposition (ALD) at 90 °C. After depositing HfO2 as a passivation layer, the CNTs in the channel are protected from O2 adsorption and become more intrinsic. On the other hand, the oxygen vacancies in the HfO2 layer bring positive charges near the CNTs, modulating (pushing down) the energy band of the CNT similar to a floating gate and shifting the threshold of the device toward negative gate voltage. The CNT thin film diode thus
Figure 2 Fabrication, structure, and performance of CNT thin-film diodes. Schematic diagrams showing the structure of (a) an as-fabricated CNT thin-film diode being asymmetrically contacted by Pd and Sc and (b) with the device being coated by an additional HfO2 layer. (c) SEM image showing a real CNT thin-film diode with W = 50 μm and L = 0.6 μm. (d) Transfer characteristics and I–V characteristics of the diode (e) before and (f) after being covered with a 12 nm HfO2 layer
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exhibits an ambipolar characteristic (the red line in Fig. 2(d)), with a threshold voltage slightly less than zero. The current of the diode under bias Vds = 1 V increases significantly at large positive gate voltage Vg while decreasing slightly at negative Vg after HfO2 covering, so that the ION in the p-region and that in the n-region have essentially the same order of magnitude. 3.3
Photovoltaic properties of CNT thin film diodes
The total diode current results from a balance between the dark current Idark and light-generated current ISC, i.e. I = Idark – ISC. While the light-generated current ISC is hardly dependent on gate voltage, Idark depends strongly on Vgs as shown in Fig. 2(d). In order to get the highest PV efficiency for the CNT diode, the diode is best biased at its OFF state in which Idark is minimum. For the as-made CNT diode (the blue curve in Fig. 2(d)), a large gate voltage of more than 10 V is needed to turn the device into its OFF state. This requirement for using a large bias is highly undesirable in real PV applications. Fortunately, after the coating of HfO2 was applied the CNT diode device was found to be largely in its OFF state for zero gate voltage (see Fig. 2(d)), suggesting that the diode may be used with high power transform efficiency without extra gate control. The coating of HfO2 not only increases the device stability but also improve the power conversion efficiency by providing a suitable threshold shift to move the OFF state to a zero gate (Vg = 0 V) condition. On the other hand, regardless of whether the device channel is covered with HfO2 or not, the CNT device always exhibits excellent rectifying characteristic just like an ordinary diode device (see Figs. 2(e) and 2(f)). A typical PV response of the thin film CNT diode to infrared light illumination is shown in Fig. 3(a). In all PV measurements, the Sc electrode is grounded and the bias is applied on the Pd electrode. Since the CNT is asymmetrically contacted, a built-in electric field exists along the CNT (Fig. 3(b)). Under light illumination, the devices result in both open circuit voltage (VOC) and short circuit photocurrent (ISC) as shown in Fig. 3(a). The operating principle of the CNT diode is illustrated using the energy band diagrams depicted in Fig. 3(b). The top diagram shows the energy bands of a short semiconducting CNT being ohmically contacted with Pd and Sc as drain and source
respectively under zero bias. The valence band of the CNT is aligned with the Fermi level of Pd and the conductance band of the CNT is aligned with the Fermi level of Sc. Because the channel length of the device is of the order of the gate oxide thickness (~500 nm), the band bending of the CNTs is extended all the way through the device due to the weak coupling of the back-gate with the channel. Therefore, a built-in electric field is formed all over the channel, providing the required field to separate light-generated electron–hole pairs and to collect them in the electrodes (Fig. 3(b)). It should be noted that the direction of the light-generated current as shown in Fig. 3(b) is opposite to the dark current, i.e. the current generated by photons overcomes the potential barrier under forward-bias. When the bias V is equal to open circuit voltage VOC, these two currents cancel each other and result in a zero net current. At zero gate bias with Vg = 0 V, the CNT device is basically in its OFF state as shown in Fig. 2(d) when the device is passivated with HfO2, i.e. the dark current is a minimum, yielding maximum light-generated current and photovoltage. When the illumination power density increases from 375 W/cm2 to 1.5 kW/cm2, ISC and VOC increase from 0.87 nA, 0.15 V to 3.74 nA and 0.21 V respectively as shown in Fig. 3(a). It should be noted that although the power intensity used here is much larger than that of the sun (100 mW/cm2), the illumination density is still about an order of magnitude lower than that required in a single nanotube-based diode to obtain a similar photovoltage and photocurrent [23]. Under the same illumination density, the multiple CNTs in the thin film device thus produce more photogenerated carriers accumulating at the electrodes than that for a device based on a single nanotube. Figure 3(c) shows the dependence of both ISC and VOC on the incident power densities. For a power density below 1.5 kW/cm2, ISC varies linearly with the power density, and VOC increases logarithmically with the power density and tends to saturate gradually. The key performance merit for a PV device is conversion efficiency, which is defined by η = (IMVM)/Pin = (FFISCVOC)/Pin
(3)
where IM and VM describe the bias point where the power generation (IMVM) is at a maximum, ISC is the
Nano Res. 2012, 5(1): 33–42 short-circuit current, Pin is the incident power and FF = (IMVM)/(ISCVOC) is the fill factor (see Fig. 3(d)). For our CNT thin film devices, FF ranges from 0.27 to 0.33. The incident power Pin is estimated by considering the total area of the CNT sections perpendicular to the incident beam as the actual effective area, and the area is in turn estimated using the diameter and channel length of each CNT in the device measured with atomic force microscopy (AFM) and SEM. The actual value of η is estimated to be more than 5% following the method used in earlier reports [28]. We now consider how the PV efficiency of our CNT thin film diode is affected by the CNT–CNT junctions in the channel (Fig. 4(a)). A long channel device with length L = 2.5 μm and W = 50 μm (see the red curve in Fig. 4(b)) was fabricated via the same process as that used for short channel device described above. Both short and long channel devices show good rectifying I–V curves in the dark as shown in Fig. 4(b). However,
39 the transport characteristics of the device of a long channel device with L = 2.5 μm is very different from that of a short channel device with L = 0.6 μm, and this difference is largely due to the increasing resistance at network junctions (see Fig. 4(b)) [31, 34]. While a long channel device works in the percolation regime, the transport in a short channel device is dominated by direct transport, i.e. CNTs connecting directly the Pd and Sc electrodes such as that shown in Fig. 2(c). The current at forward bias for the short channel device with L = 0.6 μm is much larger than that of the long channel device with L = 2.5 μm, while the reverse or leakage current is also larger due to the much larger tunneling current for a short channel device under reverse bias condition. Figure 4(a) shows that there exist many more CNT–CNT junctions in a long channel device than in a short channel device as evidenced in Fig. 2(c), resulting in more complex transport mechanisms [5, 31, 34]. The total resistance of a device
Figure 3 Photovoltaic and electronic characteristics of a typical CNT thin-film diode. (a) I–V characteristics of a short channel diode device with L = 0.6 μm measured in the dark and under illumination. (b) Depicted energy band diagrams of an asymmetrically contacted semiconducting CNT in the dark (top), being illuminated and electron–hole pairs being produced (middle) and light-generated electron–hole pairs being separated and collected while being illuminated (bottom). (c) Experimental data and fit results for open circuit voltage and short circuit current as a function of illumination power density. (d) The fourth quadrant I–V curve and power generation under an illumination power density of 750 W/cm2
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Figure 4 Structure and electronic characteristics of CNT thin-film diodes. (a) SEM image showing a long channel device with L = 2.5 μm and the presence of many CNT–CNT junction between the Sc and Pd electrodes. (b) I–V characteristics of a typical short channel device with L = 0.6 μm and a long channel device with L = 2.5 μm measured in the dark
depends both on the density and length of the CNTs. It should be noted that the CNT films used in this work are sparsely filled CNT networks with a density of about 1–2 per μm2. The most important factor determining network diode series resistance is believed to be the resistance of CNT–CNT junctions. Both the density of junctions and individual CNT length in the channel increases with channel length, and so does the total resistance. The I–V characteristics of the CNT diode can be fitted with the diode equation, and series resistance can be retrieved from the slope at high forward bias voltage. For the characteristics shown in Fig. 4(b), the retrieved series resistance Rs is about 508 kΩ for the long channel diode with L = 2.5 μm, while it is only about 25 kΩ for the short channel diode with L = 0.6 μm (see also Table 1). Because all pure metallic paths in the CNT network diode are removed during the breakdown treatment, the large series resistance is attributed to the presence of a Schottky barrier between the metallic tubes and semiconducting tubes. In fact, the voltage used for metallic pathway breakdown in a long channel device is much larger than that in a short channel device. This is because of significant voltage drop at the CNT–CNT junctions. The resistance of the junction between CNTs caused by a Schottky barrier can be as large as few hundred kΩ [31, 34], which increases series resistance Rs and decreases the PV efficiency of the diode device. Under the same illumination power, the CNT thin film diode with a long channel length has a similar VOC as that with a short channel length, but has a much
smaller ISC (see Table 1), yielding smaller values of FF and η, namely ~0.24 and 0.0018% respectively for the long channel diode shown in Fig. 4(b). In a typical CNT network PV device, the CNT–CNT junction is an important factor that decreases the efficiency of the device. Another reason for the reduced efficiency of the long channel device is that the built-in electric field is weakened all over the channel due to the presence of the Schottky barrier at CNT junctions, making the band bending less effective in separating electron–hole pairs generated by photons. Table 1 Comparison of device parameters between two CNT thin-film diodes with different channel lengths under the same illumination conditions with a power density of 750 W/cm2 Device
Rs (kΩ)
FF
L = 0.6 μm
25
0.33
1.87
0.18
0.066
L = 2.5 μm
508
0.24
0.43
0.13
0.0018
ISC (nA) VOC (V)
η (%)
4. Conclusions We have fabricated high performance thin-film FETs and barrier-free bipolar diodes based on SWCNT networks grown via CVD, and studied the photovoltaic effects on the thin-film diodes. The current ION/IOFF ratios (104–105) and ON-current density (> 4 μA/μm) of the FETs are large compared to previous reports of thin film FETs based on SWCNT networks. The mobility (84 cm2/Vs) of the thin-film FET with L = 1 μm is also of the same order as the best reported values, and the value becomes significantly larger for a long
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Nano Res. 2012, 5(1): 33–42 channel device with L = 3 μm (242 cm2/Vs). When coated with a 12 nm HfO2 layer, the threshold voltage of the CNT thin film device can be controlled to center around zero gate voltage and be suitable for efficient PV applications. In particular a diode device with L = 0.6 μm and W = 50 μm shows good rectifying characteristics, an open circuit voltage VOC = 0.21 V, and a short circuit current ISC = 3.74 nA under 1.5 kW/cm2 illumination. We have also demonstrated that the PV efficiency of the diode device is strongly dependent on the CNT–CNT junctions. The power transform efficiency decreases rapidly with increasing diode channel length, when transport mechanism changes from direct transport for short channels to percolation for longer channels. We conclude that CNT network diodes that work in the percolation regime are not suitable for PV applications, since large series resistances or CNT–CNT junctions exist in the channel.
Acknowledgements This work was supported by the Ministry of Science and Technology (Grant Nos. 2011CB933002, 2011CB933001, and 2012CB932302), the Fundamental Research Funds for the Central Universities, the National Science Foundation of China (Grant Nos. 61071013, 61001016, 51072006, 60971003, 90921012, and 51172271), and Beijing Municipal Education Commission (Grant No. YB20108000101).
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