1. Introduction

Compared to the conventional cold cathode, carbon nanotubes (CNTs) have many advantages, including low emission threshold fields, large emission current density, eximious emission stability, and long lifetime^[1-5]. Therefore, CNTs have been considered as a proper candidate for field emission cathode arrays^[6-8], the electron resource of X-ray tubes^{[9, 10]}, flat lamps^{[11, 12]} and backlight units (BLUs)^{[13, 14]} for liquid crystal displays (LCDs) because of their excellent field emission characteristics. As is known, the efficiency of cold cathode fluorescent lamp (CCFLs) or recent light emitting diode (LEDs) is so high that they have been widely used. But they have some disadvantages such as use of inverter and Hg vapor and non-uniformity in large area because of line-light source and point-light source^[15-17]. To overcome these disadvantages, field emission arrays (FEAs) with CNTs emitters^[17-19], as an area-light source, have attracted much attention for BLUs in LCD although its luminous efficiency is much lower than those of CCFLs and LEDs. Compared with the backlight units of conventional CCFLs or LEDs, carbon nanotube-based BLUs (CNT-BLUs) for LCD are highlighted as a device to improve video images for future LCD television, especially in terms of contrast ratio and moving picture response time (MPRT).

So far, the triode structures with CNT field emitters mainly include normal-gate^[20], under-gate^{[14, 21]} and planar-gate types^{[17, 22, 23]}, which have been paid much attention due to their low driving voltage, excellent field emission characteristics, and long lifetime. Planar-gate structures have been developed for cost-effective CNT-BLUs in which the cathode and gate are interdigitated and paralleled on the same plane. By contrast, the cathode and gate electrodes in normal-gate or under-gate structures must be isolated by appropriate dielectric layers, accompanied by unavoidable extra deposition and lithography processes. Furthermore, the planar-gate triode has advantages of high efficient deposition of emitters as well as low power consumption in operation^[17].

The planar-gate electron source with CNT field emitters plays a key role in the field emission because of its simple structure, fabrication process, and good field emission characteristics, as described in a previous report^{[17, 22, 23]}. However, there are few available details of the field emission characteristics in a planar-gate electron source with patterned CNT emitters for BLUs in LCD. In this paper, we present the fabrication, field emission characteristics, supporting results of numerical simulation, and direct observation of lighting patterns on phosphor screen of CNT-BLUs.

2. Experimental

The whole preparation process of the novel planar-gate electron source with patterned CNT field emitters is shown in Fig. 1. As shown in Fig. 1(a), Cr/Cu/Cr multilayer films with a thickness of 50 nm were successively deposited on the glass substrate (10 cm $\times$ 10 cm) by DC magnetron sputtering. The multilayer films were coated with a positive photoresist (RZJ-304) by spin coating, and then photoresist patterns were formed by using a developer after UV exposure. To fabricate the cathode and gate electrodes of the planar-gate triode, etching of the Cr layer was carried out in a solution with a 6 : 3 : 100 weight ratio of concentrated KMnO$_{4}$, NaOH and H$_{2}$O and etching of Cu layer was completed in the 4 wt% FeCl$_{3}$ solution. After Cr/Cu/Cr multilayer layer etching, the photoresist pattern was removed by acetone and cleaned by deionized (DI) water, and then interdigitated electrodes were obtained, as illustrated Fig. 1(b). As shown in Fig. 1(c), Al$_{2}$O$_{3}$ bars were formed between the matrix of the planar-gate triode by photolithography, magnetron sputtering, and the lift-off method in order to deposit the patterned CNT emitters on the cathode. Finally, the processed substrate was immersed in the CNT suspension, and then the CNTs were deposited on the cathode by electrophoretic deposition. Prior to electrophoresis, the CNTs, whose diameter was lower than 10 nm and length in the range of 5-10 $\mu$m, were pretreated, including acid-treating with the mixture of H$_{2}$SO$_{4}$ and HNO$_{3}$, sensitizing treatment in aqueous solution of SnCl$_{2}$-HCl, and activating treatment in an aqueous solution of PdCl$_{2}$-HCl. After each pretreatment, the CNTs were rinsed with DI water and then dried in air at 110 ℃ for 24 h. The pretreated CNTs were dispersed in an isopropyl alcohol solution containing a little dissolved Mg(NO$_{3})$$_{2}$$\cdot$6H$_{2}$O, ethyl cellulose, and so on, followed by electrophoretic deposition on the cathode of the planar-gate triode. During the process of electrophoretic deposition, the gate electrodes of the planar-gate triode were used as the anode plate and the cathode of this triode was used as the cathode plate. The DC voltage was applied between the cathode and gate electrodes for 5 min. CNT field emitters were selectively deposited on the cathode of the planar-gate triode. As shown in Fig. 1(d), the novel planar-gate electron source with patterned CNT field emitters was fabricated.

The novel planar-gate electron source with CNT field emitters on the cathodes was observed using optical microscopy. The morphology and wall structure of the CNTs were characterized by field emission scanning electron microscopy (FESEM) and by using a high resolution transmission electron microscope (HRTEM), respectively. Field emission characteristics of the novel planar-gate electron source with patterned CNT field emitters were also investigated.

3. Results and discussion

To optimize the planar-gate triode with proper field emission characteristics for BLU application, we simulated the electric field distributions and electron trajectories by using ANSYS commercial software. During the simulation process, we selected the adjacent cathode and gate electrode and the central location of the cathode was used as an origin of coordinates and the location of the cathode was used as the $x$ axis, which was a coordinate with the linear width of the cathode. The two-dimensional model was obtained on the basis of the plane symmetry of the cathode. In the two-dimensional model, the number of elements was approximately 12136, the minimal element size located near the cathode was about 300 nm, and the maximal element size located near the anode electrode was around 30 $\mu$m, respectively. In addition, the anode voltage and gate voltage was set to 2000 V and 100 V, respectively. The distance between the anode and the cathode plate was 1000 $\mu$m. The simulation results of the electric field distribution and electron trajectories near the cathode are shown in Fig. 2.

Figure 2(a) shows that the electric field strength on the surface of cathode clearly increases with the change of the linear width of the cathode from 50 to 200 $\mu$m. As shown in Fig. 2(b), the electric field strength does not obviously change with the changes of the linear width of the gate electrodes from 50 to 1600 $\mu$m. As can be seen from Fig. 3(c), the electric field strength does not obviously depend on the change of cathode-gate gap (C-G gap) in the range of 10-50 $\mu$m. However, the electric field strength becomes smaller when the C-G gap is greater than 50 $\mu$m.

In addition, we also simulated the electron trajectories in these triode structures. The anode-cathode gap (A-C gap) and the C-G gap were set to be 1000 $\mu$m and 50 $\mu$m, respectively. Cathode and gate line width was 100 $\mu$m, respectively. In simulation, the anode voltage was set to be a constant 2000 V. And the gate voltage was fixed to be 0 V, 50 V, and 100 V. The simulation results are shown in Figs. 2(d)-2(f). The radius of electron-beam spreading changed from 120 to 680 $\mu$m by increasing the gate voltage from 0 to 100 V, which makes electrons easily and fully cover the phosphor of the anode plate. Therefore, we optimize and fabricate the planar-gate triode based on the simulation results. The fabricated planar-gate triode is presented by the optical microscopy in Fig. 3(a).

As shown in Fig. 3(a), there are two groups of interdigitated electrode in this structure, one of them was used as the cathode stripe, and the other was used as the gate stripe. Cathode and gate electrodes were paralleled on the same plane in the form of the so-called planar-gate triode structure. The linear cathode and gate electrodes with the matrix arrays, with width and of 180 $\mu$m and length of 450 $\mu$m, are interdigitated and paralleled on the same plane. The width of the cathode is about 100 $\mu$m and the C-G gap is approximately 50 $\mu$m. The width of the Al$_{2}$O$_{3}$ bars between the Cr/Cu/Cr matrix arrays on the gate electrodes is around 430 $\mu$m. As shown in Fig. 3(b), the patterned CNT field emitters are selectively deposited on the area of cathode covered without the Al$_{2}$O$_{3}$ pattern rather than gate electrodes and the gap between the cathode and gate electrodes of the planar-gate triode. In the process of assembling the CNTs, the gate electrodes of the planar-gate triode are used as the anode plate and cathodes of this triode are used as the cathode plate. When the DC voltage is applied between gate and cathode plate, a uniform electric field is formed. The CNTs migrate towards the cathodes instead of the gate electrodes and the gap under the direction of electric field. At the same time, the positive voltage can prevent CNTs from adsorbing on the gate electrodes. Therefore, CNT field emitters with the same packing density are selectively deposited on the surface of cathodes rather than the gate electrodes and the gap. Inset 1 in Fig. 3(b) is the FESEM image of the CNT field emitters. It clearly shows the morphology of CNTs. Some outermost walls of the CNTs are destroyed and some closed nanotubes can be opened by acid purification, as shown in inset 2 of Fig. 3(b), which can reduce the surface potential barrier of CNTs and improve field emission^{[4, 24]}.

To investigate the field emission characteristics of the novel planar-gate electron source with patterned CNT field emitters, ITO glass coated green phosphor was used as an anode plate and the novel planar-gate field emission electron source with patterned CNT emitters as a cathode. The cathode and anode plate were kept apart by spacers with thickness of 1000 $\mu$m. In addition, the sample was moved into a vacuum chamber with a base pressure of 2 $\times$ 10$^{-5}$ Pa at room temperature. The illustration of field emission investigation is shown in Fig. 4, where $V_{\rm a}$ was the anode voltage, $I_{\rm a}$ the anode emission current, $V_{\rm g}$ the voltage applied to the gate electrodes, and $I_{\rm g}$ the gate current.

To observe the effect of the C-G gap and the gate voltage on the emission current, $V_{\rm a}$ was fixed to be 2000 V, and $V_{\rm g}$ was swept from 0 to 150 V. The measurement results are shown in Fig. 5. The anode emission current ($I_{\rm a})$ is controlled effectively by gate voltage and the C-G gap. Figure 5 reveals that the anode emission current increases with the increases of gate voltage. To analyze the trend of curve in Fig. 5, the effect of the different $V_{\rm g}$ on the electric field was simulated and the simulation results are presented in Fig. 6. As $V_{\rm g}$ increases from 0, 50, 100 to 150 V, the electric field near the cathode gradually becomes much stronger, which makes electrons easily emit from the patterned CNT field emitters.

The light-emission area of the fabricated electron emission source is about 6 cm $\times$ 8 cm. Therefore, we can estimate that the electron-emission area is approximately 48 cm$^{2}$ according to this triode structure. Here, the voltage is usually defined as turn-on voltage as the voltage required to produce a current density of 1 $\mu$A/cm$^{2}$. As shown in Fig. 5, the turn-on voltage at the emission current of 48 $\mu$A increases from 38 to 112 V with increasing C-G gap from 20 to 60 $\mu$m. This is because the electric field near the cathode also becomes much stronger with the decrease of the C-G gap, as shown in Fig. 2(c), and electrons emitted easily from patterned CNT field emitters are continually accelerated to the anode plate under the modulation of the gate voltage

Representatively, we chose an $I_{\rm a}$-$V$ curve of $d_\text{C-G }$ $=$ 50 $\mu$m to analyze the FN plot. The experimental result is shown in the inset of Fig. 5. The values of ln($I/V^{2})$ and $1/V$ show approximate linearity, indicating that the emitting electrons are mainly resulted from barrier tunneling electrons extracted by the electric field. The field emission characteristic of metal emitters can be described by using the Fowler-Nordheim (F-N) equation^[25]:

$\begin{equation} \ln (I/V^2)\propto \frac{-6.53\times 10^7\varphi ^{3/2}}{\beta }\frac{1}{V}, \\ \end{equation}$

(1)

where $\varphi$ and $\beta$ are the work function and field enhancement factor of the CNT field emitters, respectively. The geometrical field enhancement factor $\beta$ of CNT field emitters is estimated to be about 1.2 $\times$ 10$^{6}$ cm$^{-1}$ from the slope of -621 of the F-N plot under the assumption of a work function of 5 eV for CNT field emitters, which is good enough for various application of field emission. Such a high $\beta$ value in our device can be attributed to the patterned CNT field emitters.

Figure 7 presents the luminescence image on the anode from a planar-gate electron source with patterned CNT field emitters, where the anode and gate voltage are fixed to be 2000 V and 130 V, respectively. As shown in Fig. 7, the emission image shows much uniformity and the luminance reaches 5500 cd/m$^{2}$. Here, the luminous efficiency, $\eta$ (lm/W) is defined as $(\pi A_{\rm e} L)/(V_{\rm a} I_{\rm a})$, where $A_{\rm e}$ and $L$ are the nominal area (m$^{2})$, i.e., the phosphor coated area, and measured luminance (cd/m$^{2})$, respectively^[17]. We estimated $\eta$ $=$ 49.1 lm/W at $V_{\rm a}$ $=$ 2 kV, $I_{\rm a}$ $=$ 845 $\mu$A, $A_{\rm e}$ $=$ 48 cm$^{2}$, and $L$ $=$ 5500 cd/m$^{2}$ in our device.

4. Summary and conclusion

In summary, a novel planar-gate electron source with patterned CNT field emitters has been successfully fabricated by magnetron sputtering, photolithography, the lift-off method, and electrophoretic deposition according to the simulation results. Field emission properties were also investigated. The experiment results show that the turn-on voltage of this triode at current density of 1 $\mu$A/cm$^{2}$ changes from 38 to 112 V with increasing C-G gap from 20 $\mu$m to 60 $\mu$m at an anode bias of 2000 V and the A-C gap of 1000 $\mu$m. Field emission characteristics have been controlled effectively by gate voltage and the C-G gap. The whole surface emission on the fluorescent screen is relatively homogeneous. The luminous efficiency and luminance of the fabricated device reaches 49.1 lm/W and 5500 cd/m$^{2}$, respectively. The planar-gate triode with a patterned CNT emitter can be used as the field emission electron source for flat lamp BLUs in LCD due to its simple structure, fabrication process, and good field emission characteristics.