Co-deposited Fe with Ti

The fabrication procedures of patterned CNT emitters were shown schematically in Figures 2-16(a) ~ 2-16(d). As shown in Figure 2-16(a), about 1-μm-thick photoresist was spin-coated on an n-type silicon wafer (100) with low resistance and square cells with lengths of 10 μm, 100 μm, and 1000 μm were defined by photolithography. Then a Ti layer with 50-nm in thickness was deposited by the E-beam evaporation system as a buffer layer and 5-nm-thick catalytic metal, Fe, was co-deposited subsequently with different weight percentages of Ti in the same system, as shown in Figure 2-16(b). Then the patterns were formed after the photoresist was removed by lift-off method as depicted in Figure 2-16(c).

Finally, CNTs were grown selectively by thermal CVD system to investigate the growth phenomenon and the field emission properties of CNTs that was shown in Figure 2-16(d). In the thermal CVD chamber, the samples with different weight percentages of Ti were pre-treated in atmospheric pressure at 700 with ℃ H2 (500 sccm) and CH4 (200 sccm) for 5 min to form catalytic Fe nanoparticles and grown CNTs subsequently in atmospheric pressure at 700 with C℃ 2H4 (5 sccm) for 10 min as depicted in Figure 2-16(e).

Scanning electron microscopy (SEM) was performed to discover the density and the morphology of CNTs. The wall structure and crystallinity of CNTs were determined by high-resolution transmission electron microscopy (HRTEM) and the components of nanoparticles were analyzed by energy dispersive spectroscopy (EDS). A high-vacuum measurement environment with a base pressure of 5x10^-6 Torr was set up to characterize the field emission properties of CNTs (Figure 2-5). Cathode contact was made directly on the wafer. A glass plate coated with indium-tin-oxide (ITO) was positioned 120 μm to 160 μm above the sample surface as an anode. All cables were shielded except for the ground return path to the power source. The emission current densities of CNTs were measured as a function of applied electric field, using Keithley 237 high voltage units as DC source and Keithley 238

high current units as ground source. The measurement instruments were auto-controlled by the computer with IEEE 488 interface. The cathode luminescence could be obtained from the ITO glass with P22 phosphor coating on it.

2.3.3.2 Results and Discussion

Figures 2-17 (a) ~ (e) showed the SEM images of CNTs from the top view with different weight percentages of Ti corresponded to 0 %, 36 %, 53 %, 70 %, and 85 %, respectively.

From the SEM observation, the density of CNTs decreased if the weight percentage of Ti in the co-deposition of Fe and Ti increased. To calculate the quantity conveniently, the pictures were divided into one fourth and then estimated the number of CNTs. Afterwards, the number which was multiplied by four was approximate density value. For the case of weight percentage of Ti was 0 %, the density of CNTs was about 8x10⁹ emitter/cm². With the increase of weight percentage of Ti, the density of CNTs was getting low. When the weight percentage of Ti was increased to 85 %, the density of CNTs had already dropped to 8x10⁶ emitter/cm². The approximate densities of CNTs could be counted and listed in Table 2-3(a).

As a speculation, the percentage of Fe catalytic nanoparticles buried under the Ti film could be increased with the increase of weight percentage of Ti and the Ti film could hinder the growth of CNTs as a diffusion barrier. With thicker Ti film, the probability of carbon radicals to diffuse into the catalyst would be reduced therefore reduced the growth of CNTs. To estimate the length of CNTs, the SEM images were taken from a 90° viewing angle and were shown in Figures 2-18 (a) ~ (e) corresponded to Figures 2-17 (a) ~ (e). With the increase of weight percentage of Ti, on the one hand the turn-on field would be raised resulted from the shortening of length of CNTs, on the other hand the turn-on field would be lowered which might result from the decrease of density of CNTs. So there would be appearances of optimal value between the two mechanisms and they would be verified later.

In order to investigate the microstructure of CNTs, the high resolution transmission

microscope (HRTEM) images were taken in Figure 2-19. Figure 2-19(a) showed the HRTEM image of CNTs with low magnification and a nanoparticle was enclosed in the CNTs. From the HRTEM images with high magnification in Figures 2-19(b), the layer-by-layer structure of graphite could be clearly observed. It could also demonstrate that the CNT in this TEM image was a multi-wall structure. Moreover, the composition of the catalytic nanoparticle was analyzed by energy dispersive spectrum (EDS) via the TEM instrument. The EDS spectrum in Figure 2-20 showed that only Fe, C, and Cu existed in the results of the EDS analysis. The peaks of Cu were from the Cu mesh which was used to hold CNTs in the TEM system. The peak of C was from the CNT and the peaks of Fe were from the catalytic nanoparticle enclosed in the CNT. No signal of Ti was detected which indicated that the Fe would not form the alloy with Ti in this process. From the images of HRTEM and the analysis of EDS, Fe played a critical role in the growth of CNTs and Ti seemed not involve in the formation of CNTs by Ti-Fe alloy.

The field emission properties of CNTs with different weight percentages of Ti were shown in Figure 2-21(a) which showed the relationship between emission current density and electric field (J-E). The corresponding Fowler-Nordheim plots for CNTs were depicted in Figure 2-21(b) and the linearity slope of the plots confirmed the field emission phenomena.

From Table 2-3, the turn-on field defined at 10 μA/cm² of CNTs decreased from 3.679 V/μm to 3 V/μm with the weight percentage of Ti in the co-deposition of Fe and Ti increased from 0

% to 53 %. It might result from the suppression of the screening effects with the density of CNTs dropped from 8x10⁹ emitter/cm² to 5x10⁸ emitter/cm². However, the turn-on field did not always decrease with the increase in the weight percentage of Ti. When the weight percentage of Ti was added to 70 %, the turn-on field would rise to 4.594 V/μm. It might due to the decrease in the length of CNTs which would result in the reduction of aspect ratio and cause the decline in the field enhancement factor (β). When the weight percentage of Ti was added to 85 %, only little CNTs could be observed via the SEM images due to the majority of

Fe catalytic nanoparticles buried under the Ti film and the Ti film could hinder the growth of CNTs as a diffusion barrier. With thicker Ti film, the probability of carbon radicals to diffuse into the catalyst would be reduced therefore reduced the growth of CNTs. The threshold field defined at 10 mA/cm² of CNTs field emission diode decreased from 5.75 V/μm to 4.706 V/μm with the weight percentage of Ti increased from 0 % to 36 %. The density of CNTs was decreased from 8x10⁹ emitter/cm² to 2x10⁹ emitter/cm². Here, the rise of the threshold field when the weight percentage of Ti was 53 % might due to the reduction of field emission sites.

The field emission current density was improved from 8 mA/cm² to 26.96 mA/cm² at electric field of 5.5 V/μm when the weight percentage of Ti was increased from 0 % to 36 %, respectively. When the weight percentage of Ti was 53 %, the field emission current density dropped to 8.24 mA/cm² at 5.5 V/μm because of the reduction of field emission sites.

Moreover, if the weight percentage of Ti was over 36 %, the field emission current density reduced with the increase of weight percentage of Ti and the field emission current density was almost zero when the weight percentage of Ti was 85 %. Table 2-3(b) showed the summarized field emission properties of CNTs grown with different weight percentages of Ti.

Figure 2-22 showed the luminescent images on the ITO anode with phosphor coating on it. The distance of spacer was 100 μm and these samples were square cells with length of 1 cm. Samples (a), (b), and (c) showed the Fe/Ti (5 nm/50 nm) film at applied electric field 5, 6, and 7 V/μm, respectively. With the increase of electric field, the brightness of luminescent image would be increased and these three images were all revealed poor uniformity from Figures 2-22 (a), (b), and (c) observation. Samples (d), (e), and (f) showed the Fe/Ti (5 nm/ 50 nm) film and a 2-nm-thick Ti capping layer was deposited after pre-treatment at applied electric field 5, 6, and 7 V/μm, respectively. In Figures 2-22 (d), (e), and (f), the field emission current density would be larger and the luminescent images would also be more bright corresponded to Figures 2-22 (a), (b), and (c). However, the uniformity had not been improved by these methods. Samples (g), (h), and (i) showed the weight percentage of Ti was

36 % in the co-deposition of Fe and Ti were deposited on the 50-nm-thick Ti buffer layer at applied electric field 5, 6, and 7 V/μm, respectively. From Figures 2-22 (g), (h), and (i) observation, the uniformity would be improved obviously and the brightness of luminescent image was sufficient for commercial application at applied electric field 7 V/μm.

2.3.3.3 Summary

The density of CNTs could be controlled with different weight percentages of Ti and reduced the turn-on field and improved the field emission current density due to the suppression of screening effects. Low density could reduce the turn-on field, however, the number of the emitting sites would also be reduced hence cause the reduction in field emission current density. So it needed an optimized weight percentage of Ti to gain a reasonable field emission current density. In this experiment, the optimum weight percentage of Ti was found out to be 36 %. The field emission current density was improved from 8 mA/cm² to 26.96 mA/cm² at the electric field of 5.5 V/μm and the turn-on field was decreased from 3.679 V/μm to 3.059 V/μm with the weight percentage of Ti increased from 0 % to 36 %.

From the luminescent images observation, the uniformity would be improved obviously by the co-deposition of Fe and Ti and the brightness on the ITO anode was sufficient for commercial application at applied electric field 7 V/μm.