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The synthesis of ZnO nanowires and their subsequent use in high-current field-effect transistors formed by dielectrophoresis alignment
ARTICLE IN PRESS
Physica E 40 (2008) 866–872 www.elsevier.com/locate/physe
The synthesis of ZnO nanowires and their subsequent use in high-current field-effect transistors formed by dielectrophoresis alignment Seung-Yong Leea,1, Ahmad Umara,b,1, Duk-Il Suha, Ji-Eun Parka, Yoon-Bong Hahnb,2, Jeong-Yong Ahnc, Sang-Kwon Leea, a School of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea School of Chemical Engineering and Technology, Nanomaterials Processing Research Centre, Chonbuk National University, Jeonju 561-756, Republic of Korea c Department of Statistical Informatics, Chonbuk National University, Jeonju 561-756, Republic of Korea
b
Received 10 September 2007; received in revised form 5 October 2007; accepted 18 October 2007 Available online 13 November 2007
Abstract The synthesis of zinc oxide (ZnO) nanowires was achieved by thermal evaporation on a steel alloy substrate. Various material characteristics such as X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and Raman scattering analysis indicated that the synthesized ZnO nanowires were single crystalline with a wurtzite hexagonal phase, and were preferentially synthesized in the c-axis direction. In addition, the straightforward and successful alternating current (AC) dielectrophoresis (DEP) method that can be used to align and manipulate ZnO nanowires as well as to fabricate high-performance multiple-channel field-effect transistors (FETs) with a back-gate structure were also investigated. The DEP results indicated that the number of aligned ZnO nanowires increased with the increasing AC voltages. Moreover, we demonstrated that the DEP-prepared multiple ZnO nanowires FETs can manage on-current exceeding 1 mA at a low-bias voltage. Our approach to build up the high-current nano-FETs offers substantial opportunities for further practical electronics and photonics device applications. r 2007 Elsevier B.V. All rights reserved. PACS: 73.63.–b; 73.63.Bd Keywords: Dielectrophoresis; ZnO nanowires; Field-effect transistors (FETs); Electrostatic screening effect
1. Introduction One-dimensional (1D) nanostructure systems such as nanowires, nanotubes, and nanobelts have fascinating distinctive features. They are potentially ideal building blocks for nano-electronic and nano-photonic device applications. Among the wide range of 1D semiconductor Corresponding author at: Department of Semiconductor Science and Technology, Chonbuk National University, 664-14 Dukjin-Dong, Dukjin-Ku, Jeonju, Republic of Korea. Tel.: +82 63 270 3973; fax:+82 63 270 3585. E-mail address: [email protected] (S.-K. Lee). 1 These authors contributed equally to this work. 2 Also for correspondence.
1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.10.094
nanomaterials, the 1D nanostructure of II–VI semiconductor zinc oxide (ZnO) nanowires has acquired a special place because of its diversity of properties: direct wide band gap (3.37 eV), large saturation velocity (3.2 107 cm s1), high breakdown voltage, and large exciton-binding energy (60 meV) at room temperature [1]. Because of these distinct properties these ZnO nanowires have an opportunity to be recognized as one of the most effective materials for the fabrication of efficient nanodevices and nanosystems [2–5]. They are also an attractive material for ultraviolet (UV) optoelectronic devices and can efficiently serve an important role in the application of mechanical devices, sensors, and field emitters. For these purposes, a variety of fabrication approaches have been reported on
ARTICLE IN PRESS S.-Y. Lee et al. / Physica E 40 (2008) 866–872
demonstrations of the synthesis of 1D ZnO nanostructures (nanowire, nanorods, and nanobelts) in the literature. Such approaches are the template methods, the vapor transports and condensation methods, the metal organic chemical vapor deposition (MOCVD) methods, and the solid–vapor process [6–11]. Nevertheless, it is important to realize the transport properties of the 1D ZnO nanowires in utilizing them for the construction of high-performance electronic and photonic devices such as field-effect transistors (FETs), chemical and bio-sensors, and UV photo-sensors. To fabricate nanometer-scale electronic and photonic devices with a bottom-up technique, a traditional approach is to start with a random dispersion from a nanowire suspension, followed by an electrode fabrication at a known nanostructure location on the substrates using conventional e-beam lithography (EBL) and ion-beam lithography (IBL) [1]. Despite these widely used techniques for the fabrication of nanodevices with these conventional lithography techniques, a new fabrication concept to manipulate and align these semiconductor nanostructures, which can offer high performance and low price is required for wafer-based large-scale integration. Among various approaches for the manipulation of nanostructures, dielectrophoresis (DEP) is an attractive technique for the effective, inexpensive, and parallel manipulation of semiconductor nanostructures. DEP has been used to manipulate nanostructures such as carbon nanotubes [12], SnO2 nanobelts [13], CdS nanowires [14], GaN nanowires [15,16], ZnO nanowires [17], and gold nanowires [18]. Recently, we have extensively investigated the characteristics of DEP with a single-crystalline gallium nitride (GaN) nanowire and then proved that these DEPs together with device fabrication were extraordinarily promising for electronic and photonic device applications as well as for large-scale applications [15,16]. Nevertheless, we still require systematic detailed studies on the DEP characterization and subsequent DEP-prepared multiple FETs for practical electronic and photonic device and wafer-based large-scale integration applications. In this paper, we describe both the material and electrical characterizations of single-crystalline ZnO nanowires that were synthesized by the thermal evaporation method. In addition to these characterizations, we present highcurrent-driving multiple ZnO nanowires FETs. These transistors were formed by assembling ZnO nanowires on a SiO2/Si substrate with an optimized DEP condition in a three-probe scheme.
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for zinc and oxygen, respectively. First, the substrate was ultrasonically cleaned using acetone and sequentially with isopropyl alcohol (IPA). In a typical reaction process, 3 g of metallic zinc powder was sprayed in a quartz boat and placed in the center of a furnace. We placed the substrate adjacent to the source material. The furnace was evacuated to a pressure of 5–10 Torr, which remained during the entire reaction process prior to the reaction. Then, the furnace was rapidly heated to the temperature in a range of 600–630 1C under a constant flow of nitrogen and oxygen with flow rates of 100 and 250 sccm (standard cubic centimeter per minute), respectively. The reaction lasted for about 30–90 min. After the desired time, the furnace was cooled to room temperature under a constant flow of nitrogen gas. A light gray-colored product resulted on the entire substrate surface, which was then characterized in terms of its structural, optical, and electrical properties. The as-synthesized ZnO nanowires were characterized using field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM) equipped with selected area electron diffraction (SAED), X-ray diffraction (XRD), Raman scattering and photoluminescence (PL) spectroscopy. For the DEP experiments, we prepared the ZnO nanowires suspensions by sonicating a 10 ml IPA solution. The metal electrodes (Ti/Au=50/100 nm) were prepared using a standard photo-lithography process on a 4 in thermally oxidized Si (1 0 0) wafer. One chip (5 5 mm2) consisted of 20 opposing pairs of electrodes. Its shape included a 4 mm gap between two metal electrodes as shown in Fig. 1. Fig. 1 shows a schematic view of the fabrication of ZnO nanowires FETs, where the ZnO nanowires are aligned in the gap over the 20 metal electrodes with an optimized DEP condition. A drop of the ZnO nanowires suspension (3 ml) was placed on the selected gap using a micropipette; then, an alternating current (AC) electrical field was applied across the electrodes. After the suspension
2. Experimental details The synthesis of single-crystalline ZnO nanowires was achieved via a thermal evaporation system that consisted of a horizontal quartz tube furnace with a halogen lamp heating system. A steel alloy (Fe: 72.8%, Cr: 22%, Al: 5%, Y: 0.1%, and Zr: 0.1%) with a size of 1.5 1.5 cm2 was used as the substrate. A metallic zinc powder (99.999%, Aldrich Co.) and oxygen gas were used as source materials
Fig. 1. Schematic view of the DEP-aligned ZnO nanowires in the gap over the 20 metal electrodes.
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completely dried out, the images of the samples were taken by FE-SEM or an UV optical microscope for counting successfully aligned ZnO nanowires between the electrodes. These counts were used as a basis for the calculation of the alignment yield in the electrodes. The yield rate is defined by the percentage of the number of metal electrodes where at least one ZnO nanowire aligns out of the 20 opposing pairs of electrodes. The current–voltage (I–V) measurements were performed on several DEP-prepared FET device structures with a semiconductor parameter analyzer structure at room temperature using a HP 4156A semiconductor parameter analyzer in the range of 20 fA–100 mA. 3. Results and discussion 3.1. Material characteristics The detailed structural properties of the as-synthesized ZnO nanowires were obtained using FE-SEM and TEM observations as shown in Fig. 2. Fig. 2(a) shows FE-SEM images of the synthesized ZnO nanowires, indicating that the as-synthesized nanowires were vertically aligned and synthesized onto the substrate surface with high density. The typical diameters and lengths of these nanowires were determined to be in the range of 120–150 nm and 10–15 mm, respectively. Fig. 2(b) shows the X-ray diffraction patterns, which were measured with Cu-K radiation, of the assynthesized ZnO nanowires onto the steel alloy substrate. The peaks obtained from the spectrum exhibited similar
peaks indexed to the hexagonal structure of the bulk ZnO. These peaks revealed that the synthesized nanowires were single crystalline with a wurtzite hexagonal phase. No trace of zinc or other impurities were detected from the spectrum. Hence, we confirmed that the products were pure hexagonal ZnO nanowires. Additionally, the dominated and higher-intensity peak of ZnO (0 0 0 2) at 34.21 as compared to other peaks in the spectrum also served to corroborate that the synthesized products were highly crystalline with a wurtzite hexagonal phase and were synthesized along the c-axis direction. Fig. 2(c) demonstrates the low-magnification TEM image of a ZnO nanowire which was almost consistent with FE-SEM observations. The TEM image indicated that the nanowires were uniform in diameter, with a diameter of 120 nm, throughout their length. The corresponding SAED pattern, projected along the (2 1 1 0) zone axis, substantiated that the ZnO nanowires were a single crystalline and were synthesized along the (0 0 0 2) direction. The corresponding atomically resolved HR-TEM image is shown in Fig. 2(d), which shows that the distance between the two lattice planes was approximately 0.52 nm which corresponded to the d-spacing of the (0 0 0 1) crystal planes of the wurtzite ZnO. It further confirmed that the synthesized ZnO nanowires were single crystalline wurtzite ZnO, and synthesized along the (0 0 0 1) crystal plane. The corresponding SAED pattern was consistent with the observed HR-TEM data (inset of Fig. 2(d)). The optical properties of the as-synthesized nanowires were determined using Raman scattering and PL measurements at room temperature.
Fig. 2. (a) Low-magnification FE-SEM image of the ZnO nanowires. The inset is a high-magnification FE-SEM image. (b) The X-ray diffraction (XRD) pattern of the ZnO nanowires on a steel alloy substrate. (c) Low-resolution TEM image of an individual ZnO nanowire. The inset is a selective area electron diffraction (SAED) pattern taken along the [0 0 0 1] zone axis. (d) The lattice-resolved TEM image of the ZnO nanowire. The inset is a SAED pattern taken along the [0 0 0 1] zone axis.
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Fig. 3. (a) A typical Raman scattering and (b) photoluminescence (PL) spectrum of the ZnO nanowires. All measurements were performed at room temperature.
Fig. 3(a) shows a typical Raman-scattering spectrum of the as-synthesized ZnO nanowires. With a wurtzite hexagonal phase, ZnO belongs to the space group C46v with two formula unit primitive cells with all of the atoms occupying the C3V sites. At the G point of the Brillouin zone, singlecrystalline ZnO nanowires had eight sets of optical phonons. Among the eight sets of optical modes, the A1, E1, and E2 modes were Raman active. Among these the E2 modes were Raman active only while the A1 and E1 were also infrared active and therefore divided into two components: longitudinal (LO) and transverse (TO) optical components [19]. Fig. 3(a) shows a sharp, strong, and dominated peak at 437 cm1, assigned as a Raman-active
optical phonon E2 mode, a characteristic peak for the wurtzite hexagonal ZnO [19]. A very small peak at 581 cm1 was also observed which was attributed to the E1L mode corresponding to the impurities and structural defects (oxygen vacancies, zinc interstitial, free carriers, etc.) of the deposited nanostructures [20]. In addition to these, two very suppressed peaks at 330 and 378 cm1 were also observed and attributed to the E2H–E2L (multi-phonon process) and A1T modes, respectively [21]. Therefore, the presence of the highest Raman-active E2 mode in the spectrum with a suppressed and very short E1L mode suggested that the as-synthesized ZnO nanowires exhibited a good crystal quality with significantly less structural
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defects and impurities. Fig. 3(b) exhibits the typical roomtemperature PL spectrum for the as-synthesized ZnO nanowires. A strong and sharp UV emission centering at 382 nm was observed. The UV emission was also identified as a near band edge emission (NBE). It originated from the free excitons’ emissions, which were close to the band gap of the ZnO at about 3.37 eV.
force highly depends on the volume of the nanowires, Clausius–Mosotti factor, and the gradient of the electric field. In Fig. 4(d), the images from FE-SEM show that the number of aligned nanowires across the electrodes increased with increase in the applied AC electric field
at 20 MHz 10 Vp-p
3.2. DEP alignment characterization To investigate the distribution of the DEP characterization, we prepared five sets of the samples (25 samples in total). The measurements of the alignment yield were used with five samples at each of the AC voltages (1, 5, 10, 15, 20 Vpp) at a fixed frequency of 20 MHz [15,16]. The typical FE-SEM images of the aligned ZnO nanowires on the Ti/Au metal electrodes using dielectrophoresis with an enlarged image are shown in Figs. 4(a) and (b) where the ZnO nanowires were well-aligned across the metal electrodes. Fig. 4(c) shows the alignment yield of ZnO nanowires in the gap over the 20-electrode arrays for the applied AC electric field in the range of 1–20 Vpp at a fixed frequency of 20 MHz which was the optimum condition for the ZnO nanowire suspensions with IPA according to previous studies [15,16]. This figure indicates that the alignment yield of the ZnO nanowires strongly depended on the AC electric field and increased linearly with the increasing AC voltages. The alignment yield reached up to 60% with an electric field of 20 Vpp, our results can be explained by the well-known DEP force model exerted on a homogeneous cylindrical and long nanowire with an AC electric field. It is given by [3,4]: 2 pr l ~ 2 Þ, F DEP ¼ (1) m KðoÞrðE rms 2 n m KðoÞ ¼ Re . (2) m
Fig. 4. (a) Typical FE-SEM images of the aligned ZnO nanowires in the gap with an enlarged image. (b) The measurements were performed at an AC field of 10 Vpp and a frequency of 20 MHz. (c) Distribution yield rate for the aligned ZnO nanowires on the SiO2/Si substrates as a function of the AC electric field (1, 5, 10, 15, 20 Vpp) at a fixed frequency of 20 MHz. To verify the distribution of the yield rate, five samples were used for each electric field measurement. The solid line denotes the fitting line as a visual guide. (d) FE-SEM images of the aligned ZnO nanowires in the gap for the AC electric field (1, 5, 10, 15, 20 Vpp) at the frequency of 20 MHz in the gap over the 20 electrodes.
x 5.0 k 5 m
80 70 Frequency (20 MHz)
60 Alignment Yield (%)
Here, K(o) is the real part of the Clausius–Mosotti factor, r is the radius of the nanowires, l is the length of the nanowires, n is the dielectric constant of the ZnO nanowires (3.9), and m is the dielectric constant of the liquid medium (in our case IPA, em=18.3e0) [15]. In Eq. (2), the asterisk denotes that the dielectric constant is a complex quantity which is related to the conductivity (s) and the angular frequency (o) through the standard formula, ¼ iðs=oÞ. As shown in Eq. (1), the DEP
x 1.0 k 20 m
50 40 30 20 10 0 0
5
10 15 AC voltage,Vp-p, (V)
20
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6.0u
1 Vp-p AC 20 MHz
1 23 4
4.0u
5 Vp-p
ID Drain Current (A)
5 2.0u
0.0
-2.0u
-4.0u
10 Vp-p -6.0u -4
-2
0
2
4
VDS Source-Drain Voltage (V) 1.4u
15 Vp-p
1
20 Vp-p
ID Drain Current (A)
2 3 4
1.2u
5 6
1.0u
Fig. 4. (Continued)
strength. These results agreed well with previous results for a GaN-nanowires suspension with IPA [16]. 3.3. Electrical characterization (FET-measurements) First, we have characterized the transistor performance for ZnO nanowires FETs, which were prepared by an optimized DEP technique. The electrical characterization was performed directly with DEP-prepared ZnO nanowires FETs without any further process on the ZnO nanowire FETs as shown in Figs. 1 and 4(a),(c). Fig. 5(a) shows the current–voltage (ID–VDS) characteristics of an AC DEPaligned ZnO nanowires FET at different gate voltage (VG). As shown in Fig. 5(a), the ZnO nanowire conductance increases for a VG greater than zero and decreases for a VG less than zero. This behavior indicated that these nanowires had n-type of dopants. From the previous studies [2], this n-type behavior of the as-synthesized un-doped ZnO nanowires was due to the contribution of oxygen vacancies
-40
-20
0 20 VG Gate Voltage (V)
40
Fig. 5. (a) Typical drain current (ID) as a function of the source-drain voltage (VDS) with different gate voltages, VG (40 to40 V, 20 V step) for the DEP-prepared ZnO nanowires FETs. Curves 1–5 correspond to gate voltage of 40, 20, 0, 20, and 40 V, respectively. (b) ID–VG curves of the DEP-prepared ZnO nanowires FETs at different VDS (1.00–0.90, 0.02 V step). Curves 1–6 correspond to the bias voltages of 1.0, 0.98, 0.96, 0.94, 0.92, and 0.90 V, respectively. All measurements were performed at room temperature.
and/or zinc interstitials. As indicated in Fig. 5(a), the ID does not completely deplete in the ID–VDS transfer characteristics. Fig. 5(b) shows ID–VG curves under several VDS values in the range of 1.0–0.9 V, indicating that the on–off current ratio (Ion/Ioff) is extremely low compared to the previous results for a single ZnO nanowire [2,22,23]. This low value could be explained by the electrostatic screening effect of the DEP-aligned ZnO nanowires. As shown in Fig. 4(a), (b), and (d), the aligned ZnO nanowires are crossing and stacking among themselves.
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This infrequent crossing and stacking of the nanowires could prevent the current transport in the FETs. On the other hand, we found that the DEP-prepared ZnO nanowires FETs managed the current in the order of 41 mA, indicating that these multiple ZnO nanowires could be useful to apply to practical electronic circuits. The estimated carrier mobility from the gate-modulation characteristics [dI/dVG ¼ m(C/L2)VDS] was on the order of 15.9 cm2 V1 s1. Moreover, we estimated the carrier concentration of 3 1018 cm3 using the relation I ¼ nqmEA; where n is the carrier concentration, E is the electric field, m is the mobility (15.9 cm2 V1 s1), and A is the area (7.85 1011 cm2), even though our DEP-prepared ZnO nanowires FETs did not completely deplete as shown in Fig. 5(a). It could be caused by the poor ohmic contacts on the ZnO nanowires. Detailed studies on the ohmic contacts to unintentionally doped ZnO nanowires are required to improve the performance of the DEPprepared nanowires-based FETs. 4. Conclusion In summary, single crystalline aligned ZnO nanowires were successfully synthesized by catalyst-free thermal evaporation on a steel alloy substrate. XRD, TEM, SAED, and Raman scattering analysis demonstrated that the synthesized ZnO nanorods were single crystalline with a wurtzite hexagonal phase and preferentially synthesized along the c-axis direction. Sharp and strong NBEs emissions with no green emission in the room temperature PL spectra indicated that our synthesized ZnO nanowires had a good crystal quality with excellent optical properties. Moreover, we also successfully demonstrated a novel and expedient approach, which was a powerful technique for extracting the electrical properties from many semiconductor nanowires, using AC DEP. In addition, our DEPprepared multiple ZnO nanowires FETs exhibited a high drain current of 1 mA. We believe that these nanodevices are especially promising for practical use in a highintegrated circuit. They can be useful for a nano-LED in a lab-on-a-chip. Acknowledgments This work was supported by Grant no. R01-2006-00011306-0 from the Basic Research Program of the Korea
Science & Engineering Foundation. Seung-Yong Lee, Ahmad Umar, Duk-Il Suh, and Ji-Eun Park are thankful to the Korea Research Foundation’s graduate research fellowship program (Post BK21) for financial support.
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Physica E 40 (2008) 866–872 www.elsevier.com/locate/physe
The synthesis of ZnO nanowires and their subsequent use in high-current field-effect transistors formed by dielectrophoresis alignment Seung-Yong Leea,1, Ahmad Umara,b,1, Duk-Il Suha, Ji-Eun Parka, Yoon-Bong Hahnb,2, Jeong-Yong Ahnc, Sang-Kwon Leea, a School of Semiconductor and Chemical Engineering, Chonbuk National University, Jeonju 561-756, Republic of Korea School of Chemical Engineering and Technology, Nanomaterials Processing Research Centre, Chonbuk National University, Jeonju 561-756, Republic of Korea c Department of Statistical Informatics, Chonbuk National University, Jeonju 561-756, Republic of Korea
b
Received 10 September 2007; received in revised form 5 October 2007; accepted 18 October 2007 Available online 13 November 2007
Abstract The synthesis of zinc oxide (ZnO) nanowires was achieved by thermal evaporation on a steel alloy substrate. Various material characteristics such as X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and Raman scattering analysis indicated that the synthesized ZnO nanowires were single crystalline with a wurtzite hexagonal phase, and were preferentially synthesized in the c-axis direction. In addition, the straightforward and successful alternating current (AC) dielectrophoresis (DEP) method that can be used to align and manipulate ZnO nanowires as well as to fabricate high-performance multiple-channel field-effect transistors (FETs) with a back-gate structure were also investigated. The DEP results indicated that the number of aligned ZnO nanowires increased with the increasing AC voltages. Moreover, we demonstrated that the DEP-prepared multiple ZnO nanowires FETs can manage on-current exceeding 1 mA at a low-bias voltage. Our approach to build up the high-current nano-FETs offers substantial opportunities for further practical electronics and photonics device applications. r 2007 Elsevier B.V. All rights reserved. PACS: 73.63.–b; 73.63.Bd Keywords: Dielectrophoresis; ZnO nanowires; Field-effect transistors (FETs); Electrostatic screening effect
1. Introduction One-dimensional (1D) nanostructure systems such as nanowires, nanotubes, and nanobelts have fascinating distinctive features. They are potentially ideal building blocks for nano-electronic and nano-photonic device applications. Among the wide range of 1D semiconductor Corresponding author at: Department of Semiconductor Science and Technology, Chonbuk National University, 664-14 Dukjin-Dong, Dukjin-Ku, Jeonju, Republic of Korea. Tel.: +82 63 270 3973; fax:+82 63 270 3585. E-mail address: [email protected] (S.-K. Lee). 1 These authors contributed equally to this work. 2 Also for correspondence.
1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.10.094
nanomaterials, the 1D nanostructure of II–VI semiconductor zinc oxide (ZnO) nanowires has acquired a special place because of its diversity of properties: direct wide band gap (3.37 eV), large saturation velocity (3.2 107 cm s1), high breakdown voltage, and large exciton-binding energy (60 meV) at room temperature [1]. Because of these distinct properties these ZnO nanowires have an opportunity to be recognized as one of the most effective materials for the fabrication of efficient nanodevices and nanosystems [2–5]. They are also an attractive material for ultraviolet (UV) optoelectronic devices and can efficiently serve an important role in the application of mechanical devices, sensors, and field emitters. For these purposes, a variety of fabrication approaches have been reported on
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demonstrations of the synthesis of 1D ZnO nanostructures (nanowire, nanorods, and nanobelts) in the literature. Such approaches are the template methods, the vapor transports and condensation methods, the metal organic chemical vapor deposition (MOCVD) methods, and the solid–vapor process [6–11]. Nevertheless, it is important to realize the transport properties of the 1D ZnO nanowires in utilizing them for the construction of high-performance electronic and photonic devices such as field-effect transistors (FETs), chemical and bio-sensors, and UV photo-sensors. To fabricate nanometer-scale electronic and photonic devices with a bottom-up technique, a traditional approach is to start with a random dispersion from a nanowire suspension, followed by an electrode fabrication at a known nanostructure location on the substrates using conventional e-beam lithography (EBL) and ion-beam lithography (IBL) [1]. Despite these widely used techniques for the fabrication of nanodevices with these conventional lithography techniques, a new fabrication concept to manipulate and align these semiconductor nanostructures, which can offer high performance and low price is required for wafer-based large-scale integration. Among various approaches for the manipulation of nanostructures, dielectrophoresis (DEP) is an attractive technique for the effective, inexpensive, and parallel manipulation of semiconductor nanostructures. DEP has been used to manipulate nanostructures such as carbon nanotubes [12], SnO2 nanobelts [13], CdS nanowires [14], GaN nanowires [15,16], ZnO nanowires [17], and gold nanowires [18]. Recently, we have extensively investigated the characteristics of DEP with a single-crystalline gallium nitride (GaN) nanowire and then proved that these DEPs together with device fabrication were extraordinarily promising for electronic and photonic device applications as well as for large-scale applications [15,16]. Nevertheless, we still require systematic detailed studies on the DEP characterization and subsequent DEP-prepared multiple FETs for practical electronic and photonic device and wafer-based large-scale integration applications. In this paper, we describe both the material and electrical characterizations of single-crystalline ZnO nanowires that were synthesized by the thermal evaporation method. In addition to these characterizations, we present highcurrent-driving multiple ZnO nanowires FETs. These transistors were formed by assembling ZnO nanowires on a SiO2/Si substrate with an optimized DEP condition in a three-probe scheme.
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for zinc and oxygen, respectively. First, the substrate was ultrasonically cleaned using acetone and sequentially with isopropyl alcohol (IPA). In a typical reaction process, 3 g of metallic zinc powder was sprayed in a quartz boat and placed in the center of a furnace. We placed the substrate adjacent to the source material. The furnace was evacuated to a pressure of 5–10 Torr, which remained during the entire reaction process prior to the reaction. Then, the furnace was rapidly heated to the temperature in a range of 600–630 1C under a constant flow of nitrogen and oxygen with flow rates of 100 and 250 sccm (standard cubic centimeter per minute), respectively. The reaction lasted for about 30–90 min. After the desired time, the furnace was cooled to room temperature under a constant flow of nitrogen gas. A light gray-colored product resulted on the entire substrate surface, which was then characterized in terms of its structural, optical, and electrical properties. The as-synthesized ZnO nanowires were characterized using field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM) equipped with selected area electron diffraction (SAED), X-ray diffraction (XRD), Raman scattering and photoluminescence (PL) spectroscopy. For the DEP experiments, we prepared the ZnO nanowires suspensions by sonicating a 10 ml IPA solution. The metal electrodes (Ti/Au=50/100 nm) were prepared using a standard photo-lithography process on a 4 in thermally oxidized Si (1 0 0) wafer. One chip (5 5 mm2) consisted of 20 opposing pairs of electrodes. Its shape included a 4 mm gap between two metal electrodes as shown in Fig. 1. Fig. 1 shows a schematic view of the fabrication of ZnO nanowires FETs, where the ZnO nanowires are aligned in the gap over the 20 metal electrodes with an optimized DEP condition. A drop of the ZnO nanowires suspension (3 ml) was placed on the selected gap using a micropipette; then, an alternating current (AC) electrical field was applied across the electrodes. After the suspension
2. Experimental details The synthesis of single-crystalline ZnO nanowires was achieved via a thermal evaporation system that consisted of a horizontal quartz tube furnace with a halogen lamp heating system. A steel alloy (Fe: 72.8%, Cr: 22%, Al: 5%, Y: 0.1%, and Zr: 0.1%) with a size of 1.5 1.5 cm2 was used as the substrate. A metallic zinc powder (99.999%, Aldrich Co.) and oxygen gas were used as source materials
Fig. 1. Schematic view of the DEP-aligned ZnO nanowires in the gap over the 20 metal electrodes.
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completely dried out, the images of the samples were taken by FE-SEM or an UV optical microscope for counting successfully aligned ZnO nanowires between the electrodes. These counts were used as a basis for the calculation of the alignment yield in the electrodes. The yield rate is defined by the percentage of the number of metal electrodes where at least one ZnO nanowire aligns out of the 20 opposing pairs of electrodes. The current–voltage (I–V) measurements were performed on several DEP-prepared FET device structures with a semiconductor parameter analyzer structure at room temperature using a HP 4156A semiconductor parameter analyzer in the range of 20 fA–100 mA. 3. Results and discussion 3.1. Material characteristics The detailed structural properties of the as-synthesized ZnO nanowires were obtained using FE-SEM and TEM observations as shown in Fig. 2. Fig. 2(a) shows FE-SEM images of the synthesized ZnO nanowires, indicating that the as-synthesized nanowires were vertically aligned and synthesized onto the substrate surface with high density. The typical diameters and lengths of these nanowires were determined to be in the range of 120–150 nm and 10–15 mm, respectively. Fig. 2(b) shows the X-ray diffraction patterns, which were measured with Cu-K radiation, of the assynthesized ZnO nanowires onto the steel alloy substrate. The peaks obtained from the spectrum exhibited similar
peaks indexed to the hexagonal structure of the bulk ZnO. These peaks revealed that the synthesized nanowires were single crystalline with a wurtzite hexagonal phase. No trace of zinc or other impurities were detected from the spectrum. Hence, we confirmed that the products were pure hexagonal ZnO nanowires. Additionally, the dominated and higher-intensity peak of ZnO (0 0 0 2) at 34.21 as compared to other peaks in the spectrum also served to corroborate that the synthesized products were highly crystalline with a wurtzite hexagonal phase and were synthesized along the c-axis direction. Fig. 2(c) demonstrates the low-magnification TEM image of a ZnO nanowire which was almost consistent with FE-SEM observations. The TEM image indicated that the nanowires were uniform in diameter, with a diameter of 120 nm, throughout their length. The corresponding SAED pattern, projected along the (2 1 1 0) zone axis, substantiated that the ZnO nanowires were a single crystalline and were synthesized along the (0 0 0 2) direction. The corresponding atomically resolved HR-TEM image is shown in Fig. 2(d), which shows that the distance between the two lattice planes was approximately 0.52 nm which corresponded to the d-spacing of the (0 0 0 1) crystal planes of the wurtzite ZnO. It further confirmed that the synthesized ZnO nanowires were single crystalline wurtzite ZnO, and synthesized along the (0 0 0 1) crystal plane. The corresponding SAED pattern was consistent with the observed HR-TEM data (inset of Fig. 2(d)). The optical properties of the as-synthesized nanowires were determined using Raman scattering and PL measurements at room temperature.
Fig. 2. (a) Low-magnification FE-SEM image of the ZnO nanowires. The inset is a high-magnification FE-SEM image. (b) The X-ray diffraction (XRD) pattern of the ZnO nanowires on a steel alloy substrate. (c) Low-resolution TEM image of an individual ZnO nanowire. The inset is a selective area electron diffraction (SAED) pattern taken along the [0 0 0 1] zone axis. (d) The lattice-resolved TEM image of the ZnO nanowire. The inset is a SAED pattern taken along the [0 0 0 1] zone axis.
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Fig. 3. (a) A typical Raman scattering and (b) photoluminescence (PL) spectrum of the ZnO nanowires. All measurements were performed at room temperature.
Fig. 3(a) shows a typical Raman-scattering spectrum of the as-synthesized ZnO nanowires. With a wurtzite hexagonal phase, ZnO belongs to the space group C46v with two formula unit primitive cells with all of the atoms occupying the C3V sites. At the G point of the Brillouin zone, singlecrystalline ZnO nanowires had eight sets of optical phonons. Among the eight sets of optical modes, the A1, E1, and E2 modes were Raman active. Among these the E2 modes were Raman active only while the A1 and E1 were also infrared active and therefore divided into two components: longitudinal (LO) and transverse (TO) optical components [19]. Fig. 3(a) shows a sharp, strong, and dominated peak at 437 cm1, assigned as a Raman-active
optical phonon E2 mode, a characteristic peak for the wurtzite hexagonal ZnO [19]. A very small peak at 581 cm1 was also observed which was attributed to the E1L mode corresponding to the impurities and structural defects (oxygen vacancies, zinc interstitial, free carriers, etc.) of the deposited nanostructures [20]. In addition to these, two very suppressed peaks at 330 and 378 cm1 were also observed and attributed to the E2H–E2L (multi-phonon process) and A1T modes, respectively [21]. Therefore, the presence of the highest Raman-active E2 mode in the spectrum with a suppressed and very short E1L mode suggested that the as-synthesized ZnO nanowires exhibited a good crystal quality with significantly less structural
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defects and impurities. Fig. 3(b) exhibits the typical roomtemperature PL spectrum for the as-synthesized ZnO nanowires. A strong and sharp UV emission centering at 382 nm was observed. The UV emission was also identified as a near band edge emission (NBE). It originated from the free excitons’ emissions, which were close to the band gap of the ZnO at about 3.37 eV.
force highly depends on the volume of the nanowires, Clausius–Mosotti factor, and the gradient of the electric field. In Fig. 4(d), the images from FE-SEM show that the number of aligned nanowires across the electrodes increased with increase in the applied AC electric field
at 20 MHz 10 Vp-p
3.2. DEP alignment characterization To investigate the distribution of the DEP characterization, we prepared five sets of the samples (25 samples in total). The measurements of the alignment yield were used with five samples at each of the AC voltages (1, 5, 10, 15, 20 Vpp) at a fixed frequency of 20 MHz [15,16]. The typical FE-SEM images of the aligned ZnO nanowires on the Ti/Au metal electrodes using dielectrophoresis with an enlarged image are shown in Figs. 4(a) and (b) where the ZnO nanowires were well-aligned across the metal electrodes. Fig. 4(c) shows the alignment yield of ZnO nanowires in the gap over the 20-electrode arrays for the applied AC electric field in the range of 1–20 Vpp at a fixed frequency of 20 MHz which was the optimum condition for the ZnO nanowire suspensions with IPA according to previous studies [15,16]. This figure indicates that the alignment yield of the ZnO nanowires strongly depended on the AC electric field and increased linearly with the increasing AC voltages. The alignment yield reached up to 60% with an electric field of 20 Vpp, our results can be explained by the well-known DEP force model exerted on a homogeneous cylindrical and long nanowire with an AC electric field. It is given by [3,4]: 2 pr l ~ 2 Þ, F DEP ¼ (1) m KðoÞrðE rms 2 n m KðoÞ ¼ Re . (2) m
Fig. 4. (a) Typical FE-SEM images of the aligned ZnO nanowires in the gap with an enlarged image. (b) The measurements were performed at an AC field of 10 Vpp and a frequency of 20 MHz. (c) Distribution yield rate for the aligned ZnO nanowires on the SiO2/Si substrates as a function of the AC electric field (1, 5, 10, 15, 20 Vpp) at a fixed frequency of 20 MHz. To verify the distribution of the yield rate, five samples were used for each electric field measurement. The solid line denotes the fitting line as a visual guide. (d) FE-SEM images of the aligned ZnO nanowires in the gap for the AC electric field (1, 5, 10, 15, 20 Vpp) at the frequency of 20 MHz in the gap over the 20 electrodes.
x 5.0 k 5 m
80 70 Frequency (20 MHz)
60 Alignment Yield (%)
Here, K(o) is the real part of the Clausius–Mosotti factor, r is the radius of the nanowires, l is the length of the nanowires, n is the dielectric constant of the ZnO nanowires (3.9), and m is the dielectric constant of the liquid medium (in our case IPA, em=18.3e0) [15]. In Eq. (2), the asterisk denotes that the dielectric constant is a complex quantity which is related to the conductivity (s) and the angular frequency (o) through the standard formula, ¼ iðs=oÞ. As shown in Eq. (1), the DEP
x 1.0 k 20 m
50 40 30 20 10 0 0
5
10 15 AC voltage,Vp-p, (V)
20
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6.0u
1 Vp-p AC 20 MHz
1 23 4
4.0u
5 Vp-p
ID Drain Current (A)
5 2.0u
0.0
-2.0u
-4.0u
10 Vp-p -6.0u -4
-2
0
2
4
VDS Source-Drain Voltage (V) 1.4u
15 Vp-p
1
20 Vp-p
ID Drain Current (A)
2 3 4
1.2u
5 6
1.0u
Fig. 4. (Continued)
strength. These results agreed well with previous results for a GaN-nanowires suspension with IPA [16]. 3.3. Electrical characterization (FET-measurements) First, we have characterized the transistor performance for ZnO nanowires FETs, which were prepared by an optimized DEP technique. The electrical characterization was performed directly with DEP-prepared ZnO nanowires FETs without any further process on the ZnO nanowire FETs as shown in Figs. 1 and 4(a),(c). Fig. 5(a) shows the current–voltage (ID–VDS) characteristics of an AC DEPaligned ZnO nanowires FET at different gate voltage (VG). As shown in Fig. 5(a), the ZnO nanowire conductance increases for a VG greater than zero and decreases for a VG less than zero. This behavior indicated that these nanowires had n-type of dopants. From the previous studies [2], this n-type behavior of the as-synthesized un-doped ZnO nanowires was due to the contribution of oxygen vacancies
-40
-20
0 20 VG Gate Voltage (V)
40
Fig. 5. (a) Typical drain current (ID) as a function of the source-drain voltage (VDS) with different gate voltages, VG (40 to40 V, 20 V step) for the DEP-prepared ZnO nanowires FETs. Curves 1–5 correspond to gate voltage of 40, 20, 0, 20, and 40 V, respectively. (b) ID–VG curves of the DEP-prepared ZnO nanowires FETs at different VDS (1.00–0.90, 0.02 V step). Curves 1–6 correspond to the bias voltages of 1.0, 0.98, 0.96, 0.94, 0.92, and 0.90 V, respectively. All measurements were performed at room temperature.
and/or zinc interstitials. As indicated in Fig. 5(a), the ID does not completely deplete in the ID–VDS transfer characteristics. Fig. 5(b) shows ID–VG curves under several VDS values in the range of 1.0–0.9 V, indicating that the on–off current ratio (Ion/Ioff) is extremely low compared to the previous results for a single ZnO nanowire [2,22,23]. This low value could be explained by the electrostatic screening effect of the DEP-aligned ZnO nanowires. As shown in Fig. 4(a), (b), and (d), the aligned ZnO nanowires are crossing and stacking among themselves.
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This infrequent crossing and stacking of the nanowires could prevent the current transport in the FETs. On the other hand, we found that the DEP-prepared ZnO nanowires FETs managed the current in the order of 41 mA, indicating that these multiple ZnO nanowires could be useful to apply to practical electronic circuits. The estimated carrier mobility from the gate-modulation characteristics [dI/dVG ¼ m(C/L2)VDS] was on the order of 15.9 cm2 V1 s1. Moreover, we estimated the carrier concentration of 3 1018 cm3 using the relation I ¼ nqmEA; where n is the carrier concentration, E is the electric field, m is the mobility (15.9 cm2 V1 s1), and A is the area (7.85 1011 cm2), even though our DEP-prepared ZnO nanowires FETs did not completely deplete as shown in Fig. 5(a). It could be caused by the poor ohmic contacts on the ZnO nanowires. Detailed studies on the ohmic contacts to unintentionally doped ZnO nanowires are required to improve the performance of the DEPprepared nanowires-based FETs. 4. Conclusion In summary, single crystalline aligned ZnO nanowires were successfully synthesized by catalyst-free thermal evaporation on a steel alloy substrate. XRD, TEM, SAED, and Raman scattering analysis demonstrated that the synthesized ZnO nanorods were single crystalline with a wurtzite hexagonal phase and preferentially synthesized along the c-axis direction. Sharp and strong NBEs emissions with no green emission in the room temperature PL spectra indicated that our synthesized ZnO nanowires had a good crystal quality with excellent optical properties. Moreover, we also successfully demonstrated a novel and expedient approach, which was a powerful technique for extracting the electrical properties from many semiconductor nanowires, using AC DEP. In addition, our DEPprepared multiple ZnO nanowires FETs exhibited a high drain current of 1 mA. We believe that these nanodevices are especially promising for practical use in a highintegrated circuit. They can be useful for a nano-LED in a lab-on-a-chip. Acknowledgments This work was supported by Grant no. R01-2006-00011306-0 from the Basic Research Program of the Korea
Science & Engineering Foundation. Seung-Yong Lee, Ahmad Umar, Duk-Il Suh, and Ji-Eun Park are thankful to the Korea Research Foundation’s graduate research fellowship program (Post BK21) for financial support.
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