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Electrical characterizations of a controllable field emission triode based on low temperature
synthesized ZnO nanowires
View the table of contents for this issue, or go to the journal homepage for more 2006 Nanotechnology 17 83
(http://iopscience.iop.org/0957-4484/17/1/014)
Nanotechnology 17 (2006) 83–88 doi:10.1088/0957-4484/17/1/014
Electrical characterizations of a
controllable field emission triode based on
low temperature synthesized ZnO
nanowires
Chia Ying Lee
1, Tseung Yuen Tseng
1,2,4, Seu Yi Li
3and Pang Lin
31Department of Electronics Engineering and Institute of Electronics, National Chiao Tung
University, Hsinchu 300, Taiwan
2Department of Materials and Mineral Resources Engineering, National Taipei University of
Technology, Taipei 106, Taiwan
3Institute of Materials Science and Engineering, National Chiao Tung University,
Hsinchu 300, Taiwan E-mail:tseng@cc.nctu.edu.tw Received 26 August 2005 Published 1 December 2005 Online atstacks.iop.org/Nano/17/83 Abstract
Fabrication and field emission properties of a ZnO nanowire (NW) triode were investigated in this study. The ZnO NWs have a single-crystalline wurtzite structure,∼50 nm diameter and 3.4 × 1010cm−2number density. The ZnO NW triode shows good and controllable emission properties with the turn-on anode electric field (at a current density of 1µA cm−2), threshold anode electric field (at a current density of 1 mA cm−2) and field enhancement factor of 1.6, 2.1 V µm−1and 3340, respectively. The ZnO NW triode exhibits transistor characteristics with a gate leakage region, linear region and saturation region. Furthermore, the controllable field emission performance of the ZnO NW triode can be enhanced by illumination and argon ion bombardment. A low temperature Si-based microelectronic compatible fabrication process was provided for successfully making ZnO NW based triodes with good field emission properties.
1. Introduction
Current trends in nanotechnology and nanomaterials play an important role in the potential applications of photonic, electro-optical and electronic devices because of their unique physical and chemical properties [1–4]. In particular, exploration of the materials for flat panel displays has been a hot topic for the last few decades. High aspect ratio one-dimensional (1D) nanostructures, such as nanotubes, nanowires and nanobelts, have been extensively studied with a view to use in vacuum microelectronic devices, including field emission displays (FEDs), electron sources, microwave devices and high power rf amplifiers, because of the advantage of the low turn-on electric field and high electron emission efficiency [3–5].
4 Author to whom any correspondence should be addressed.
The main advantages for the FED are a large area display, high uniformity, high productivity, high brightness, low cost, low power consumption and reliability. From this point of view, triode-type FED devices with low driving voltage, high resolution and controllable electron emission characteristics are candidates for constituting a new generation of FED devices.
ZnO, with its wide band gap (3.4 eV) and large exciton binding energy (60 meV), has attracted much attention since possible applications in phosphors, transparent conducting films for solar cells, ultra-violet (UV) laser devices and flat panel displays were suggested [6–9]. Recently, ZnO NW emitters have been reported to exhibit good emission properties with high stability, low threshold electric field, high emission current density, good emission stability and
C Y Lee et al
durability [9,13]. The ZnO NWs have been synthesized by various procedures [10–12]; however, the main challenge in fabricating the NW based FED devices is the high synthesis temperature which retards the integration processes for FED device structures. A hydrothermal method would offer a superior route for FED fabrication because of the catalyst-free growth, low cost, low reaction temperature, large area and uniform production, environment friendliness and process compatibility with VLSI.
Although field emission triodes based on CNTs and diamond films have been developed [14,15], devices based on ZnO NWs are seldom discussed. In this article, a hydrothermal method is adopted to fabricate ZnO NWs in a field emission triode device at 75◦C, and the controllable field emission characteristics are investigated.
2. Experiments
A field emission triode based on ZnO NWs was fabricated on a p-type Si(100) substrate. Figure 1(a) illustrates the fabrication flow of a ZnO NW triode field emission array. The process began with plasma-enhanced chemical vapour deposition (PECVD) of SiO2 on the Si substrate, followed
by electron beam evaporator deposition of gate electrodes, made of Al. The thicknesses of the insulating oxide and the gate metal were 500 and 100 nm, respectively. The emitter regions were defined by a standard photolithography and wet etching process. An ultrathin ZnO film (∼70 Å) was deposited on the substrate by rf sputtering (13.56 MHz) under Ar sputtering gas at a base pressure of 20 mTorr; this was used as a seeding layer to prepare well-aligned ZnO NWs by the hydrothermal method. Then, the photoresistance (PR) layer was stripped, and the unwanted seeding above the metal gate layer was lifted off. Last, the substrate was put into an aqueous solution (Milli-Q, 18.2 M cm) of zinc nitrate hexahydrate
(Zn(NO3)2·6H2O, 0.01 M) and diethylenetriamine (HMTA,
C6H12N4, 0.01 M) in a sealed vessel at 75◦C for 30 min.
A more detailed description of the hydrothermal method was given in [1]. When the NWs were grown hydrothermally on the substrate, the fabrication of the triode device was completed.
The crystal structure of the ZnO NWs was examined by means of x-ray diffraction (XRD, MAC Science, MXP18, Japan). The surface morphologies of the NWs were observed by field emission scanning electron microscopy (FE-SEM, Hitachi S-4700I, Japan) and high resolution transmission electron microscopy (HR-TEM, Philips Tecani-20). The test scheme corresponding to the field emission measurement is illustrated in figure1(b). The measurement was carried out in a vacuum chamber (base pressure of 1× 10−6Torr) equipped with a valve through which Ar gas flows in, to adjust the measuring pressure. A Keithley 237 current–voltage analyser was used for measuring the field emission characteristics and a power supply was adopted for the control of the gate bias
(Vg). A copper electrode probe that serves as an anode with
the tip diameter of 500µm was placed at a distance of 500 µm from the tips of the NWs. The electrode distance was adjusted using a precision screwmeter with an accuracy of±0.1 µm.
Figure 1. (a) Schematic description of fabrication processes for the
field emission triode. (b) Test system for the triode mode.
3. Results and discussion
A low magnification SEM image of the fabricated 2 × 2 ZnO based triode array is shown in the inset of figure 2, indicating that the cathode active region is a square opening of 100×100 µm2, and the distance between the two active regions
is 500µm. The ZnO NWs are successfully and selectively grown inside the gate hole with ZnO seeding areas, but no NWs and other impurities are grown on the gate regions. Figure2
is an enlarged image of the triode device, in which there are well-aligned ZnO NWs with an average diameter of 50 nm and a number density of 3.4 × 1010cm−2. These ZnO NWs
are randomly oriented, and uniformly and selectively grown on ZnO seeding layers inside the cathode active regions.
Figure 3(a) shows the XRD pattern of the ZnO NWs of the field emission triode. The peaks at 33.08◦ and 38.5◦ in the XRD patterns are caused by the Si substrate and Al metal gate, respectively. The crystal structure of these ZnO NWs is wurtzite and no other phases appear. The lattice constants calculated from the XRD patterns of hydrothermally 84
Figure 2. Typical FE-SEM micrographs near the gate edge of a
ZnO NW triode; the inset shows the 4× 4 array triode with ZnO NWs grown inside gate holes.
Figure 3. (a) XRD pattern of the ZnO NW triode. (b) HRTEM
micrograph of ZnO NWs; the inset shows the corresponding SAED of the NWs.
grown ZnO NWs are a = b = 3.25 Å and c = 5.21 Å, which are consistent with what is recorded in ICDD No 80-0074. An HR-TEM image and the corresponding selected area electron diffraction (SAED) pattern of the hydrothermally grown ZnO NWs are shown in figure 3(b), illustrating the growth orientation and crystal structure of the NWs. As shown in the figure, the ZnO NWs grew uniformly along the [002] direction and the distance between parallel [002] lattice fringes of the ZnO NW is 5.21 Å. The SAED pattern indexed in figure3(b) shows that the ZnO nanowire is a single-crystalline structure. The lattice constants calculated from the indexed pattern are a = b = 3.25 Å and c = 5.21 Å, which are consistent with those calculated from the XRD result.
The emission current density(J ) versus gate bias (Vg)
characteristic plots at various applied electric fields(Ea) are
Figure 4. (a) Field emission current density versus gate voltage (J–Vg) curves under the various applied electric fields. (b) Relation
of transconductance versus gate voltage for the field emission triode. (c) Field emission current density versus applied electric field curves under the gate voltages of 0, 10 and 18 V. The inset shows the corresponding Fowler–Nordheim plots.
shown in figure4(a), indicating that the controllable transistor behaviour can be divided into three parts: gate leakage region, linear region and saturation region. After Ea becomes larger
than the threshold electric field, the electrons can emit from ZnO NW emitters without applying Vg. Then, J decreases
with increase in Vgin the gate leakage region. With increase
of Vgfrom 0 V, the electric field gradient near the ZnO NW
emitters will increase due to the short emitter–gate spacing, and consequently some emitted electrons might be trapped by the gate under the gate leakage region, which results in the lowering of J . The emission current density in the gate leakage region decreases as the gate bias increases since there are more electrons trapped by the gate. As Vgis continuously
increased up to 14 V, J abruptly increases in the linear region. The linear intercept on the Vgaxis is defined as the threshold
gate voltage of the linear region, Vgth. It is believed that
C Y Lee et al
enough to accelerate the electrons; consequently, the emitting electrons gain momentum while passing through the gate to the anode without being trapped. Finally, the field emission current density is saturated when Vgis larger than 18 V. The
high emission current of the triode operated in the saturating region may be due to the short gate–tip spacing, small gate aperture and high aspect ratio of the ZnO NWs. J saturation in the saturating region occurs owing to the space charge effect of the semiconductor emitters [16]. Here, the gate leakage region is defined as the field emission off region and the saturation region is the on region. Then, the on/off current density ratio of this field emission triode is about 102under the anode electric field of 2.2 V µm−1.
The field emission characteristics can also be observed in the variation of the small single transconductance (gm).
Figure4(b) depicts the relationships between gm and Vgfor
the ZnO NW based field emission triode. gmis expressed as
follows: gm= d Ie dVg Va . (1)
It is to be noted that gmis nearly zero below Vgthbecause Ie is very small in the gate leakage region. gm increases in
the linear region, goes through a maximum at the point of inflection of the linear region to the saturation region in the
J –Vgcurve, and then decreases in the saturating region. The ZnO NW based triode exhibits a high gmof 2.2 µS under the
applied electric field of 2.2 V µm−1and a low operating gate bias of 17 V, which is the optimized operation voltage of the field emission triode. µ is another parameter used to evaluate the ability of the gate voltage to respond to the emission current density, depending on the anode voltage (Va) and the gate
voltage(Vg), which is expressed as µ =dVa dVg I . (2)
From figure4(a), we find thatµ is about 100 below the current density of 2 mA cm−2. Although this µ value is smaller than that (µ = 250) for a triode based on a plasma-enhanced chemical vapour deposition diamond film [17], the low temperature (75◦C) synthesized ZnO NWs are used here for the first time in the fabrication of a field emission triode device with controllability.
The relationship between J and Eafor the ZnO NW based
triode for different Vg is shown in figure 4(c). The
turn-on electric field (Eon, at a current density of 1.0 µA cm−2)
and threshold electric field (Eth, at a current density of
1.0 mA cm−2) are 1.6 and 2.1 V µm−1under zero gate bias, respectively. As the Vg increases to 10 V, J is depressed to
36µA cm−2under an Eaof 2.2 V µm−1. When Vgincreases
to 18 V, Ethslightly decreases to 2.0 V µm−1but J abruptly
increases to 12 mA cm−2under an Ea of 2.2 V µm−1. The
corresponding F–N plots (ln(J/E2) versus E−1) of the ZnO NW based triode are depicted in the inset of figure 4(c), indicating that the measured field emission characteristics fit the F–N relationship. The F–N relationship is as follows:
J = Aβ 2E2 φ exp −Bφ3/2 β E (3)
Figure 5. (a) Photo-enhanced field emission characteristics of the
ZnO NW triode device operating at a Vgof 20 V. The inset shows
the corresponding F–N plots. (b) Photo-enhanced field emission current density versus gate voltage(J–Vg) curves under various
applied electric fields; the inset shows gmversus Vgunder an Eaof
2.2 V µm−1.
where J is the current density, E the applied field, the work function of the ZnO (5.37 eV),β the field enhancement factor, A = 1.56 × 10−10(AV−2eV) and B = 6.83 × 103
(V eV−3/2µm−1) [9]. Thus, the value ofβ can be calculated from the slope of the F–N plot. As shown in the figure, the F–N plot under a Vgof 10 V deviates from the F–N fitting, and
those under voltages Vgof 0 and 18 V obey the relationship
with the same slope. The calculatedβ value for a ZnO NW based triode under a Vgof 0 V is 3048. It is well known that the β value depends only on the geometry, structure, tip size and
number of emitters on the substrate; thus, theβ value should be constant as gate voltage varies. It is suggested that the observed large deviation from the F–N fit under a Vgof 10 V
is attributable to the gate trapping being the main mechanism in the gate leakage region.
The measurement of field emission properties of a field emission triode based on low temperature synthesized ZnO NWs under 30 W incandescent lamp irradiation with a Vgof
20 V (operating in the gate controlled saturation region) was carried out to investigate the influence of the illumination on the triode. As shown in figure5(a), the triode operating in the saturation region exhibits typical field emission characteristics under illumination. This J –E curve can also be divided into three parts: zero emission (region 1 of figure5(a)), F–N field emission (region 2) and current saturation regions (region 3).
Eonand Ethdecrease to 0.9 and 1.3 V µm−1, respectively, and
the maximum current density increases to 2.5 A cm−2under the Vgof 20 V and Eaof 2.2 V µm−1. Theβ value (3050) of
the illuminated ZnO NW based field emission triode calculated from the slopes of the F–N plot (see the inset in figure5(a)) is 86
Figure 6. (a) The emission current density of the ZnO NW based
field emission triode under various pressures. (b) The 1st and 50th sweeps of J –Eacurves and the corresponding F–N plots.
(This figure is in colour only in the electronic version)
close to that of the dark one. Therefore, it is demonstrated that the carriers in the ZnO NWs are excited during the illumination, leading to increase of the emission current density.
The J –Vgplots with various fields Eafor a triode under
30 W incandescent lamp irradiation are shown in figure5(b). It is indicated that the triode under such illumination keeps the transistor showing controllable behaviour that can be separated into a gate leakage region, linear region and saturation region. There is a large increase in the field emission current density under the optical illumination and the threshold gate bias of the triode operated under the illumination is about 20 V. The average current density in the off region under the field Eaof
2.2 V µm−1is about 0.1 mA cm−2, while that in the on region is about 0.5 A cm−2. Thus, the triode exhibits controllable field emission characteristics under illumination, and the on/off current density ratio of this triode is about 5000 under the anode electric field of 2.2 V µm−1. The inset in figure5(b) shows the relationship between gmand Vgwith an Eaof 1.6 V µm−1
under illumination; this exhibits a high gmof 10µS under the
anode field of 1.6 V µm−1and a gate bias of 20 V, which is the optimized operation voltage for such a triode. Moreover, theµ value is about 200 under 2 mA cm−2. Obviously, not only was the emission current density photoenhanced but also the controllability is enhanced under the optical illumination. Figure 6 shows the field emission characteristics of the triode measured under various pressures, obtained to investigate the influence of the measuring pressure on the characteristics. This ZnO NW based triode was swept from 0 to 2.2 V µm−1 with a Vg of 0 V (avoiding the effects
from the gate) under 1× 10−6Torr in the first 29 operations. In the first 29 tests, the field emission current densities are
Figure 7. FE-SEM images of ZnO NWs after the 50th J –Easweep.
similar to those obtained with the average current density of 2 mA cm−2. Then, the following two sweeps were carried out under 1× 10−3Torr with Ar gas flowing in, and the current density is abruptly increased to 27 mA cm−2. Finally, the pressure was decreased to 1× 10−6Torr again, for the last 19 operations and the field emission current density remains at the average value of 27 mA cm−2, which shows a significant increase in comparison with that in the first 29 tests under the same measuring pressure. The field emission characteristics of the 1st and 50th sweeps of the triode are depicted in figure6(b), indicating that the Ethof the 1st sweep is 2.1 V µm−1while
that of the 50th sweep 1.6 V µm−1. The calculatedβ value of the 50th sweep of this triode device is 5203. Thus, such a triode exhibits better emission properties, including low turn-on and threshold electric fields, high emissiturn-on current density and a highβ value after the measurement at the high pressure of 1× 10−3Torr.
The field emission ability andβ value strongly depend upon the morphology of the ZnO NWs. Figure7shows the FE-SEM image of the ZnO NWs after measuring in high pressure and sweeping 50 times, indicating that these ZnO NWs have smaller tips than the original ones (figure2). It is suggested that these ZnO NWs measured under high pressure were bombarded with argon ions leading to the formation of smaller tips at the front of the NWs. Therefore, the observed improved emission properties of the triode are mainly due to such smaller tips of ZnO NWs. Moreover, these ZnO NWs exhibit the better field emission ability and higher β values than ZnO NWs synthesized by the hydrothermal method
(β ∼ 550) [18] and ZnO nanoneedles formed by Ar ion bombardment (β ∼ 1134) [19]. This result also provides a possible simple method for enhancing the field emission properties of ZnO NW based triodes.
On the basis of the previous studies by our group [9], we can say that vapour–liquid–solid (VLS) synthesized ZnO NWs with a higher aspect ratio and smaller tip diameter exhibit better field emission characteristics (low threshold electric field, high current density and high β value (β ∼ 7180)). However, the ZnO NWs used in field emission devices are restricted to ones having high reaction temperatures. In this paper, the hydrothermal method provides a low temperature process for fabricating ZnO NWs, which would be not only compatible with the Si based microelectronic fabrication process but also possible for use in polymer based flexible electro-optical applications.
C Y Lee et al
4. Conclusions
In this report, a field emission triode based on low temperature hydrothermally grown ZnO NWs was fabricated and characterized. The ZnO NW based triode emitter was designed with a 100× 100 µm2cathode active region, which
exhibits gate controllable behaviour and emits electrons at a threshold gate bias of 14 V with the saturation current density of 12 mA cm−2 and gm of 2.2 µS, at a low operating Ea
of 2.2 V µm−1 and an on–off ratio of up to 102. These
controllable field emission properties of the triode can be enhanced by illumination. Moreover, the ZnO NW based triode exhibited better field emission properties when it was measured under high pressure, leading to the formation of smaller tips of the ZnO NWs. Our field emission triode with controllable transistor characteristics is expected to be appropriate for field emission display applications.
Acknowledgment
This work was supported by the National Science Council of Republic of China. Contract No NSC 94–2216–E–009–007.
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