Effects of Hydrogen Plasma Treatment on Field-Emission Characteristics of Palladium Nanogap Emitters

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Effects of Hydrogen Plasma Treatment on Field-Emission

Characteristics of Palladium Nanogap Emitters

Chih-Hao Tsai,a Kuan-Jung Chen,aFu-Ming Pan,a,zHsiang-Yu Lo,bYiming Li,b Mei-Chao Chiang,cand Chi-Neng Moc

a

Department Materials Science and Engineering and bDepartment Communication Engineering, National Chiao-Tung University, Hsinchu, Taiwan

c

Chunghwa Picture Tubes, Limited, Taoyuan, Taiwan

Nanogaps were prepared on the Pd line electrode by focused ion beam, and electron field-emission characteristics of the Pd nanogap emitter subject to hydrogen plasma treatment were studied. The as-prepared nanogap had smooth and uniform gap edges, and thus field-emission characteristics of the nanogap emitter were primarily dependent on the gap separation. After the hydrogen plasma treatment, the field-emission property of the Pd nanogap emitter was significantly enhanced. The improvement in the field-emission property was mainly attributed to formation of a ragged morphology on the nanogap emitter during the hydrogen plasma treatment. The ragged morphology provided more emitting sites with a high field enhancement factor. The Fowler– Nordheim plot was used to elucidate the dependence of field-emission characteristics of the Pd nanogap emitter on the plasma-induced ragged morphology.

Manuscript submitted December 7, 2007; revised manuscript received August 31, 2008. Published October 8, 2008.

Electrodes with a nanometer-scale gap have many appealing ap-plications, such as molecular electronics,1,2 biomolecular detection,3,4 and vacuum microelectronics.5-7However, because of the complexity and reliability of nanogap fabrication and integra-tion, practical applications of most nanogap devices are still far beyond realization. Recent demonstration of a prototype surface conduction electron emission display共SED兲 is probably the one na-nogap application having the greatest commercial potential and re-ceiving the most attention in recent years.8The nanogap in a surface conduction electron emission共SCE兲 device of the SED display was fabricated on palladium oxide line electrodes deposited by ink-jet printing. To produce the nanogap, the PdO electrode was subject to a series of electrical forming and activation processes. Carbon-aceous materials were required to be selectively deposited on the Pd electrode to further narrow down the nanogap to a width of 4–6 nm. Because of the complex fabrication processes, production of the SED display is costly and probably unreliable. In a previous study, we successfully fabricated a Pd nanogap field emitter with the gap separation of⬍30 nm by hydrogenation of Pd line electrodes.9The mechanistic principle of the Pd nanogap formation was based on the well-known phenomenon that hydrogen absorption in a Pd film can lead to hydride formation and hydride phase transformation, which will result in a compressive stress in the Pd film.10Presently, we are undertaking various approaches to improve field-emission character-istics of the Pd nanogap SCE device, including optimization of hy-drogenation conditions and hydrogen plasma treatments. Earlier studies have shown that adsorption of hydrogen could enhance field-emission properties of electron field emitters.11,12In the study, we used hydrogen plasma to modify the Pd nanogap electrodes and studied improvements in field-emission properties of the nanogap emitter. The hydrogen plasma treatment could not only create a ragged morphology on the Pd nanogap emitter by ion bombardment but also could cause a work function lowering. Because using fo-cused ion beam 共FIB兲 to fabricate nanostructured patterns is a simple and reliable method, we used FIB in this study to prepare nanogaps with well-defined gap separations on the Pd line electrode instead of using the Pd hydrogenation process, which generally pro-duces nanogaps with irregular edge shape. Field-emission character-istics of the FIB-prepared nanogap emitter could be significantly improved by the hydrogen plasma treatment.

Experimental

A schematic of the Pd thin-film emitter structure is shown in Fig. 1. The p-type共100兲 silicon wafer was used as the substrate. A SiO2

layer 100 nm thick was first thermally grown on the Si substrate. The Pd line electrode was contacted with the underlying Pt/Ti pad electrodes. Before electron beam evaporation共E-beam兲 deposition of the Pt thin-film pad electrode共10 nm thick兲, the Ti layer 共5 nm thick兲 was E-beam deposited on the oxide as an adhesion layer. A photolithographical liftoff process was used to pattern the Pt/Ti pad electrodes. The Pd thin-film strip共30 nm thick and 3 ␮m wide兲 was then E-beam deposited on the Pt/Ti pad electrode, and the strip pattern was also defined by the liftoff method. A FIB system共FEI Company兲 was used to prepare the nanogap on the Pd line electrode using a gallium ion source at a beam energy of 30 keV. The gap separation was controlled by tuning the ion beam current, and the achievable minimum gap separation in the study was⬃25 nm. The surface of the Pd line electrode was modified by hydrogen plasma in a microwave plasma chemical vapor deposition system 共ASTeX PDS-17兲 under the following treatment condition: microwave fre-quency, 2.45 GHz; power density, 124 W/cm2_{; H}

2 flow rate,

100 sccm; working pressure, 30 Torr; and substrate temperature, 300°C. The lattice constant of the Pd thin film before and after the plasma treatment was studied by a grazing incident X-ray diffracto-meter共Bede D1兲. Surface morphology was examined by scanning electron microscope关共SEM兲, JEOL 6500兴 and atomic force micro-scope共Veeco D5000兲. Field-emission properties of the Pd nanogap emitter were studied with a Keithley 237 source measurement unit under a vacuum condition of⬃5 ⫻ 10−6_Torr.

Results and Discussion

The FIB prepared nanogap on the Pd thin-film line electrode had smooth gap edges and a uniform gap separation. The plane-view SEM images of the nanogaps with a gap separation of ⬃30 and ⬃90 nm are shown in Fig. 2a and b, respectively. From the top view, both the nanogaps had very similar morphological features, such as edge smoothness and uniformity. The rough feature shown on the side area of the Pd line electrode was likely a result of the liftoff lithography process. The photoresist pattern might disturb Pd deposition on the region near the edge of the Pd line electrode.

z_{E-mail: [email protected]}

Figure 1. Schematic cross-sectional diagram of the palladium emitter struc-ture.

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Because of the smaller gap separation, the nanogap with a 30 nm separation had a much better field-emission efficiency than the one with the 90 nm separation. Figure 3 shows the field-emission current of the two nanogap emitters as a function of the applied voltage 关current-voltage 共I-V兲 curve兴. The dependence of the emission cur-rent of a field emitter on the applied voltage is described by the Fowler–Nordheim共F-N兲 relation I = A

冉

␣␤ 2 ␾d2

冊

V 2_exp− B␾3/2d ␤V 关1兴

where V is the applied voltage, ␣ is the emitting area, ␾ is the cathode work function, d is the distance between the cathode and anode 共i.e., gate in the SCE device兲, ␤ is the field enhancement factor, and A and B are generally considered as constants under a typical field-emission condition. The field enhancement factor␤ re-lates the applied voltage with the local electric field E_locat the emit-ting site by the relation Eloc=␤共V/d兲 and is strongly dependent on

the geometric shape of the field emitter. The insets in Fig. 3 are the corresponding F–N plots of the I-V curves. The linear feature of the F-N plots suggests that electron emission in the nanogap followed the F–N field-emission mechanism. From the F-N plot, the turn-on voltages共V_t兲 of the 30 and 90 nm nanogaps, which are herein de-fined as the bias voltage at which the F-N plot begins to exhibit a linear rising feature, were⬃50 and ⬃125 V, respectively. Accord-ing to Eq. 1, the slope of the F-N plot共S兲 is given by

S =− B␾ 3/2_d

␤ 关2兴

The F-N slopes of the 30 and 90 nm nanogaps were determined from Fig. 3 to be −245 and −1455, respectively. The measured slope

of the 90 nm nanogap was about 6 times that of the 30 nm nanogap. Because the slope is linearly proportional to the nanogap separation and inversely proportional to the field enhancement factor, the 30 nm nanogap must have a ␤ value twice that of the 90 nm nanogap.

Field-emission characteristics of the FIB-prepared nanogaps were significantly improved when the Pd line electrode was subject to the hydrogen plasma treatment, which could greatly modify the surface morphology and chemical composition of the Pd electrode. Because of the very small mass, hydrogen atoms can easily diffuse into the Pd lattice and occupy octahedral interstitial sites of the face-centered-cubic共fcc兲 lattice.10,13Hydrogen absorption in Pd is known to form two hydride phases,␣ and ␤ phases, depending on the hydrogen concentration and the absorption temperature.13Phase transformation from the solid solution phase共␣兲 to the interstitial compound phase共␤兲 is accompanied by a lattice expansion and thus results in a large film stress. When the Pd thin film was subject to the hydrogen plasma treatment, hydrogen radicals in the plasma could be effectively absorbed in the Pd electrode, thereby forming the Pd hydride phases. Lattice expansion of the Pd electrode due to the hydride phase transformation was studied by X-ray diffraction 共XRD兲 analysis. Figure 4 shows the XRD spectra of a blanket Pd thin film treated by the hydrogen plasma for various treatment times. The XRD spectrum of the as-deposited Pd thin film showed diffrac-tion peaks from the 共111兲, 共200兲, 共220兲, and 共311兲 lattice orienta-tions. The lattice constant, derived from Bragg’s equation, of the as-deposited Pd thin film was 3.871 Å. As the plasma treatment time increased, all the diffraction peaks exhibited shifts toward lower diffraction angles, indicating an increase in the lattice spacing. The dependence of the共111兲 peak shift and the fcc lattice constant on the plasma treatment time is shown in the inset of Fig. 4. After the hydrogen plasma treatment of 5 min, the lattice constant increased from 3.871 to 3.919 Å, revealing that Pd hydride was formed in the Pd thin film.10It is interesting to note that the intensity of the共220兲 peak increased with the plasma treatment time. The increase of the 共220兲 peak implied preferential texturing developed in the Pd elec-trode during the plasma treatment. Work function is a strong func-tion of the crystal orientafunc-tion. It has been reported that single crys-talline Pd has work functions of 5.20, 5.65, and 5.95 eV for the 共110兲, 共100兲, and 共111兲 orientations, respectively.14

Therefore, field-emission characteristics of the Pd nanogap can vary with the change in the work function of the electron-emitting surface due to evolu-tion of the preferential texturing in the electrode.

The surface morphology of the Pd electrode was significantly Figure 2. Plane-view SEM images of the FIB prepared Pd nanogaps with a

gap separation of:共a兲 30 and 共b兲 90 nm.

Figure 3. 共Color online兲 Field-emission I-V curves and the corresponding F-N plots共inset兲 of the FIB prepared nanogap emitters with a gap separation of:共a兲 30 and 共b兲 90 nm.

Figure 4. 共Color online兲 XRD spectra of the Pd thin film as a function of hydrogen treatment time:共a兲 as-deposited, 共b兲 1, 共c兲 3, and 共d兲 5 min. The fcc lattice constant of the Pd thin films as a function of the plasma treatment time is shown in the inset.

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modified by the hydrogen plasma treatment. Figure 5 shows atomic force microscopy共AFM兲 images of a blanket Pd thin film before and after the hydrogen plasma treatment. The surface of the as-deposited Pd thin film was smooth as shown in Fig. 5a with a root-mean-square 共rms兲 roughness of 0.66 nm. After the hydrogen plasma treatment, the film surface became very rough and the rms roughness increased with the plasma treatment time. The Pd film subject to the plasma treatment of 5 min had an rms roughness of ⬃7.30 nm 共Fig. 5b兲. The SEM images of one of the gap edges of the 90 nm Pd nanogap before and after the plasma treatment of 5 min are shown in Fig. 6a and b, respectively. The SEM images show only one edge of the nanogap because the other side of the nanogap electrode was etched away by a FIB so that the sidewall edge of the nanogap could be clearly examined. Before the plasma treatment, the FIB-prepared nanogap had smooth gap edges. However, the edge of the 90 nm nanogap became jagged after the plasma treat-ment, and voids were even produced on the Pd electrode as shown in the plane-view SEM image of Fig. 6c. The development of the rugged morphology on the Pd electrode was likely due to hydrogen ion bombardment. Displacement of surface atoms and formation of vacancies could take place during hydrogen ion bombardment. Con-current atom and vacancy migrations under the energetic plasma condition could lead to agglomeration of atoms and void formation on the Pd electrode, resulting in a surface of rugged microtopogra-phy. In addition, as described above, formation of the hydride phase during hydrogen plasma treatment could result in a compressive stress in the Pd electrode. If the stress was not distributed uniformly in the Pd line electrode because of defects and grain boundaries,

then it could enhance atom migration toward a less stressed area during the plasma treatment and, thus might play a non-neglegible role in assisting surface roughening.15

Figure 7 shows field-emission I-V curves and the corresponding F-N plots of the 90 nm nanogap emitter subject to the hydrogen plasma treatment of various times. After the plasma treatment, the nanogap emitter showed a great improvement in field-emission properties. For the nanogap with the plasma treatment of 5 min, the turn-on voltage was⬃40 V, which was even smaller than that of the as-prepared 30 nm nanogap, and a field-emission current as high as 2 mA was obtained at a bias of 65 V. The field-emission current increased significantly with the plasma treatment time. The F-N slopes of the nanogap emitters treated with the hydrogen plasma for 1, 3, and 5 min were −231, −118, and −90, respectively. Because the nanogaps exhibited a trivial change in the average gap separation after the plasma treatment, the plasma-treated nanogap emitters must have a larger field enhancement factor or a smaller work func-tion as revealed by Eq. 2. Formafunc-tion of Pd hydrides on a smooth Pd thin film by hydrogenation may result in either a small decrease or increase in the work function, depending on the hydrogen absorption temperature.16,17It was also reported that increase in film roughness could significantly reduce the work function of metallic thin films.16,18A work function reduction as large as 2 eV after hydrogen absorption in Pd at 78 K has been observed and mainly ascribed to the severe change in the Pd film morphology.16Therefore, it is likely that the larger rms roughness of the Pd nanogap emitters receiving a longer plasma treatment could induce a larger work function lower-ing and thus lead to a shallower F-N slope. We used a UV photo-electron spectroscope共RKI model AC-2兲 to measure work functions of the Pd thin film at 298 K. The work functions of the Pd thin film before and after the hydrogen plasma treatment for 5 min were 5.08 and 4.65 eV, respectively. According to Eq. 2, the slope of the F-N plot varies with the work function by a power of 3/2. If only the measured change in the work functions of the Pd thin film before and after the plasma treatment were taken into account, then the corresponding F-N slope should be decreased by just a factor of 1.14. Thus, the 16-fold decrease in the F-N slope suggested that the ␤ value was increased by a factor of ⬃14 after the plasma treatment of 5 min, assuming no significant change in d, the nanogap separa-tion. Therefore, the large increase in the␤ value was the predomi-nant cause leading to the improvement in field-emission character-istics of the nanogap emitter after the plasma treatment.

As described above, the Pd nanogap emitter had a jagged edge and a rough surface after the plasma treatment. Under a negative Figure 5. 共Color online兲 AFM images of Pd thin films 共a兲 before and 共b兲

after the hydrogen plasma treatment of 5 min. The scanning area was 4 ⫻ 4 ␮m, and the height scale was 30 nm.

Figure 6. SEM images of the nanogap edge of the 90 nm nanogap emitter: 共a兲 as-prepared and 共b兲 after the hydrogen plasma treatment of 5 min, and 共c兲 the plan-view SEM image of the Pd 90 nm nanogap emitter after the plasma treatment of 5 min. The SEM images of共a兲 and 共b兲 show only one edge of the nanogap as marked by the arrows. A FIB was used to cut out a square crater on the substrate area, where the other side of the nanogap electrode was located, and therefore, only one sidewall edge can be observed.

Figure 7. 共Color online兲 Field-emission I-V curves of the 90 nm nanogap emitter after the hydrogen plasma treatment of various treatment times:共a兲 as-prepared,共b兲 1, 共c兲 3, and 共d兲 5 min. The corresponding F-N plots are shown in the inset.

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bias, the local electric field at the emitting site with a rugged shape can be greatly enhanced, and thus field emission characteristics of the emitter are effectively improved. Figure 8 shows the dependence of the turn-on voltage and the rms roughness on the plasma treat-ment time. The turn-on voltage decreased with increasing the plasma treatment time. The most dramatic V_tdrop from⬃125 to ⬃ 60 V occurred to the nanogap treated by the hydrogen plasma of 1 min. Although further plasma treatment could reduce more the V_t, the nanogap exhibited a gradual Vtdrop after the first minute of the

plasma treatment. The rms roughness increased with the plasma treatment time and also had the greatest change for the first minute of the plasma treatment, but in a moderate way. According to the F-N plots shown in Fig. 7, the F-N slope showed a similar reduction behavior as well共i.e., the slope decreased most markedly in the first minute of the plasma treatment兲. The dramatic Vt and F-N slope

reduction during the first minute of the plasma treatment seemed to suggest that most effective emission sites with a high field enhance-ment factor were created in the nanogap within the first minute of the plasma treatment, leading to a significant improvement on field-emission properties. Aside from creating field-emission sites, to prolong the plasma treatment could further modify the emitter surface, re-sulting in an increase in the emitting area and a decrease in the work function.

Conclusion

A FIB was used to prepared nanogaps on Pd line electrodes, and effects of the hydrogen plasma treatment on field-emission charac-teristics of the nanogap emitter were studied. The FIB-prepared na-nogap was smooth and uniform on the gap edges, and therefore, the gap separation was the main fabrication parameter determining field-emission characteristics of the nanogap emitter. For the

nan-ogap emitter with a gap separation of 30 nm, a turn-on voltage of 50 V was obtained. After the hydrogen plasma treatment, Pd hy-drides were formed in the Pd nanogap emitter according to XRD analysis. The plasma-treated emitter had jagged nanogap edges and a very rough surface. Field-emission performance of the nanogap emitter was significantly improved after the plasma treatment. The turn-on voltage of the emitter decreased with increasing the plasma treatment time. The 90 nm nanogap emitter treated by hydrogen plasma for 5 min exhibited a lower turn-on voltage and a much higher field-emission current as compared to the 30 nm nanogap emitter without the plasma treatment. The improvement in the field-emission property was attributed to the ragged morphology of the nanogap emitter. The ragged morphology created more emitting sites and enhanced the local electric field. In addition, work function lowering was observed for the Pd thin film after the hydrogen plasma treatment, and the field-emission property of the nanogap emitter might thus be further improved.

Acknowledgments

This work was partly supported by the National Science Council of Taiwan, under contract no. NSC94-2120-M009-008 and Chung-hwa Picture Tubes, Ltd. Technical supports from the National Nano Device Laboratories共NDL兲 is gratefully acknowledged.

National Chiao Tung University assisted in meeting the publication costs of this article.

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