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Sep 4, 2013 - Electron-Field-Emission Properties of Gallium. Compound by Ammonification of Ga2O3 Nanowires. Han-Ting Hsueh, Wen-Yin Weng, ...
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IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 12, NO. 5, SEPTEMBER 2013

Electron-Field-Emission Properties of Gallium Compound by Ammonification of Ga2O3 Nanowires Han-Ting Hsueh, Wen-Yin Weng, Tsung-Ying Tsai, and Shoou-Jinn Chang

Abstract—The authors report the growth of β-Ga2 O3 nanowires and the conversion of β-Ga2 O3 nanowires to gallium nitride (GaN) nanowires through ammonification, and the fabrication of nanowires-based field emitters. The threshold field of Ga2 O3 nanowires was 5.65 V/μm. After ammonification under 750, 800 and 900 ◦ C, the threshold fields became smaller which were 3.82, 3.03 and 2.12 V/μm, respectively. While the ammonification temperature was increased to 950 ◦ C, the threshold field drastically increased to 13.13 V/μm. Index Terms—Gallium nitride (GaN), Ga2 O3 , nanowires.

I. INTRODUCTION VER the past decades, field-emission properties of various materials have been studied because they are potentially useful for flat panel displays. It is known that performances of field emitters depend strongly on the morphology, density, and material quality [1]. With a good geometric field enhancement factor (β), considerable attention has been focused on the use of one-dimensional (1-D) nanotubes or nanowires (NWs) as field emitters in recent years [2]. For example, field-emission properties of carbon nanotubes have been extensively studied [3]. Field emitters based on metal oxides, such as SnO2 [4], In2 O3 [5], ZnO [6], CuO [7], and MoO3 [8] have also been demonstrated. Among them, monoclinic gallium oxide (β-Ga2 O3 ) with a wide band gap of 4.9 eV is an interesting material that has attracted much attention. It has been demonstrated that β-Ga2 O3 NWs can be used as solar-blind photodetectors [9]. The formation of oxygen vacancies in β-Ga2 O3 dominates its electric characteristic as an n-type semiconductor. Such a semiconductor can then be used for transparent electronic devices [10] and gas sensors [11]. GaN is also a wide band gap material that is promising for high-temperature electronic devices and light emitters. 1-D GaN NWs can be synthesized by various methods such as laser ablation [12], pyrolysis approach [13], and chemical vapor deposition [14]. Recently, Li et al. have demonstrated the direct nitridation of Ga2 O3 NWs and converted it to GaN NWs [15].

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Manuscript received February 18, 2013; revised May 28, 2013; accepted June 11, 2013. Date of publication June 18, 2013; date of current version September 4, 2013. The review of this paper was arranged by Associate Editor E. Tutuc. H.-T. Hsueh is with the National Nano Devices Laboratories, Tainan 741, Taiwan (e-mail: [email protected]). W.-Y. Weng, T.-Y. Tsai, and S.-J. Chang are with the Institute of Microelectronics and Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNANO.2013.2268854

They reported that GaN NWs could be converted to β-Ga2 O3 NWs by annealing in oxygen ambience, and annealing β-Ga2 O3 NWs in NH3 ambience could convert them back to GaN NWs. The application in field emitters of pure β-Ga2 O3 NWs and GaN NWs have also been discussed in the previous literature. Liu et al. grew quasi-aligned GaN NW arrays via thermal evaporation of Ga2 O3 /GaN powders and their threshold field of emission was about 7.0 V/μm [16]. Liu et al. also grew GaN bicrystalline NWs by directly reacting the starting Ga2 O3 powders with ammonia in a horizontal resistance and their threshold field was ∼7.5 V/μm [17]. Yamashita et al. grew the GaN nanorod with native silicon oxides by radio-frequency plasma-enhanced molecular beam epitaxy and they got a lower threshold field of 2.5 V/μm [18]. P-doped GaN NWs were also grown via the thermal evaporation process with Ga2 O3 /GaN/InP powders by Liu et al., and their threshold field of emission was ∼5.1 V/μm [19]. However, to our knowledge, no further details about microstructure characterization and field-emission effects were reported for the dependence of properties on different nitridation of β-Ga2 O3 NWs. In this study, we report the growth of β-Ga2 O3 NWs and then the conversion of β-Ga2 O3 NWs to GaN NWs through ammonification with different temperature and the fabrication of field emitters. Physical, optical, and electrical properties of the fabricated field emitters were also discussed.

II. EXPERIMENTS The β-Ga2 O3 NWs used in this experiment were grown with a vapor–liquid–solid (VLS) mechanism by heating a GaN/sapphire template. The GaN/sapphire templates were prepared by depositing a 30-nm-thick GaN low-temperature nucleation layer and a 2-μm-thick GaN epitaxial layer on a sapphire substrate by metal-organic chemical vapor deposition. Prior to grow NWs, the GaN/sapphire templates were first dipped in a diluted hydrochloric acid water solution for 5 min to remove the native oxide. A 3-μm-thick Au film was then deposited onto the GaN surface by sputtering. The templates were subsequently placed on an alumina boat and inserted into a quartz tube furnace purged with 30 sccm Ar gas. The temperature in the furnace was then ramped up at 30 ◦ C/min to 500 ◦ C, and kept at 500 ◦ C for 20 min to form Au nanoparticles. After the formation of Au nanoparticles, O2 gas was introduced into the system at a flow rate of 0.8 sccm and the furnace temperature was raised to 1050 ◦ C. After the growth of β-Ga2 O3 NWs, the furnace was naturally cooled down to room temperature. To convert the β-Ga2 O3 NWs to GaN NWs, the samples were placed into the quartz tube furnace again and then heated with mixed gases of NH3 50 sccm and N2 50 sccm for 1 h. Samples with different

1536-125X © 2013 IEEE

HSUEH et al.: ELECTRON-FIELD-EMISSION PROPERTIES OF GALLIUM COMPOUND BY AMMONIFICATION OF GA2 O3 NANOWIRES

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Fig. 2. XRD spectra of the as-grown (a) Ga2 O3 NWs and (b) GaN NWs which were ammonified under 950 ◦ C.

Fig. 1. Magnified SEM images of (a) Ga2 O3 NWs and ammonification of Ga2 O3 NWs under (b) 750 ◦ C, (c) 800 ◦ C, (d) 900 ◦ C, and (e) 950 ◦ C. (f) Cross-sectional image of (e).

nitridation temperatures of 750, 800, 900 and 950 ◦ C were tested in this experiment. A JEOL JSM-7000 F field-emission scanning electron microscope (SEM) was then used to characterize structural properties of β-Ga2 O3 NWs and GaN NWs. A MAC MXP18 X-ray diffractometer (XRD) was used to characterize crystallographic properties of these NWs. Photoluminescence (PL) properties of the as-grown NWs were also measured by using a He–Cd laser as the excitation source during PL measurements that emitted at 325 nm. During the field-emission measurement, the sample was placed in a vacuum chamber with a base pressure of 4 × 10−6 torr. A tungsten head with a diameter of 1.8 mm was then used as the anode. A Keithley 237 high-voltage source was then used to provide the sweeping electric field and to monitor the emission current. III. RESULTS AND DISCUSSION Fig. 1 shows the top-view SEM structural morphology of the samples before and after ammonification with different temperatures. In Fig. 1(a), it could be found that high-density, randomly oriented 1-D Ga2 O3 NWs were successfully grown across the GaN/sapphire template. The average diameter of these NWs varied from 70 to 150 nm. The round-shaped globule found at the tip of the NW is typically observed during the VLS growth of semiconductor NWs which indicates that our Ga2 O3 NWs grew through the VLS mechanism from a Ga-rich Au–Ga alloyed droplet or a Ga droplet. Fig. 2(a) shows the XRD spectrum measured from the as-grown Ga2 O3 NWs. The sharp XRD peaks observed in the spectrum can be indexed to (−1 0 4), (−2 0 2), (1 1 1), (−1 1 3), (−3 1 3), (3 1 1), and (−2 1 7)

Fig. 3.

Room temperature PL spectra of the samples.

of β-Ga2 O3 (JCPDS file no.11-0370). The observation of these peaks indicates that the as-grown NWs were β-Ga2 O3 with the ˚ b = 3.04 A, ˚ and c = 12.23 A. ˚ lattice constants a = 5.80 A, At atmospheric pressure, it is known that GaN decomposes at 800 ◦ C [20]. During the growth of NWs, the GaN epitaxial layer will decompose into metallic Ga and N2 gas since the growth temperature was 1050 ◦ C. The metallic Ga and Au nanoparticles should then form Au–Ga alloy. Thus, with the introduced O2 gas, β-Ga2 O3 NWs can be grown by the VLS process. Figs. 1(b)–(e) show the SEM images after nitridation with NH3 gas. After 750 ◦ C ammonification, the structural morphology did not have much difference, and the surface was as smooth as β-Ga2 O3 NWs. While the temperature increased to 800 ◦ C, the surface started to become rough (Fig. 1(c)). The structure of the NWs showed that severe defects appeared such as crack of the main body after ammonification at 950 ◦ C. Fig. 2 (b) shows the XRD spectrum measured from the as-converted NWs at 950 ◦ C. The sharp XRD peaks observed in the spectrum can be indexed to (1 1 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2), and (2 0 1) of GaN (JCPDS file no.89-8624). The observation of these peaks again indicated that the as-grown β-Ga2 O3 NWs were successfully converted to GaN NWs with the lattice con˚ and c = 5.185 A ˚ at 950 ◦ C. It is known that stants a = 3.189 A NH3 can be decomposed into NH, NH2 , and H2 above 800 ◦ C. In the presence of NH, NH2 , and H2 , it is possible to reduce Ga2 O3 to Ga2 O [21]. The reduced Ga2 O3 , thus, can react with

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IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 12, NO. 5, SEPTEMBER 2013

Fig. 4.

(a) Field-emission characteristics measured from the Ga2 O3 NWs and the ammonified NWs. (b) F-N plots of ln(I/V 2 ) against (1/V ).

NH3 gas through the following equations: Ga2 O(s) + 2NH3 ↔ 2GaN(s) + H2 O(g) + 2H2 (g) ΔGr = −33−0 kJmol−1

(1)

Ga2 O(g) + 2NH3 (g) ↔ 2GaN(s) + H2 O(g) + 2H2 (g) ΔGr = −196−96 kJmol−1

(2)

where ΔGr is the Gibbs free energy. Fig. 1(f) shows the crosssectional SEM of the GaN NWs which was ammonified at 950 ◦ C. The average length of these GaN NWs was around 15– 20 μm and the density of these NWs was around 6 × 1011 cm−3 . These 1-D, randomly oriented, and high-density NWs can, thus, be applied to field emitters. To further investigate the nitridation at different temperatures, Fig. 3 shows the PL spectra of these samples. It was found that the emission located at around 360 nm was obviously observed while the ammonification temperature was higher than 900 ◦ C. Thus, from the above analysis [see Fig. 2(b)], it was known that the complete nitridation could be achieved if the ammonification temperature was 950 ◦ C. Fig. 4 shows the field-emission characteristics measured from these samples. For pure β-Ga2 O3 NWs, as shown in Fig. 4(a), the field emission current increased slowly when the applied electric field was small. The threshold field of β-Ga2 O3 NWs was 5.65 V/μm. At the threshold, it was found that the emission current was only around 3.7 × 10−7 A/cm2 . As we further increased the applied bias, it was found that the measured emission current increased exponentially. With 7.6 V/μm applied electric field, the emission current increased to 4.2 × 10−6 A/cm2 . After ammonifying these samples, it was found that the threshold field tended to lower values. The threshold field of the samples with 750, 800, and 900 ◦ C ammonification temperatures was measured as 3.82, 3.03, and 2.12 V/μm, respectively. As the ammonification temperature was increased to 950 ◦ C, however, the threshold field drastically increased to 13.13 V/μm. In order to understand the lower threshold field of the NWs, the measured I– V curves were analyzed using the Fowler–Nordheim (FN) equation J = (Aβ 2 E2 /Φ) exp (−BΦ3/2 /βE), where J is the current density (A/cm2 ), V is the applied voltage (V), β is the field enhancement factor, A = 1.56 × 10−6 (AeV/V2 ), B = 6.83 × 107

Fig. 5.

Schematic band diagram of NWs in a strong external field.

(V/cm eV3/2 ), and Φ is the work function. The work functions of Ga2 O3 and GaN were 4.8 and 4.1 eV, respectively [22], [23]. To further investigate the field-emission behavior, we replotted ln (J/E 2 ) as a function of 1/V , as shown in Fig. 4(b). From the F-N plot, the enhancement factor of Ga2 O3 NWs was around 1700. Fig. 4(b) also shows the results of NWs after ammonification. Wang et al. had reported that the nitridation occurred when the temperature was higher than 850 ◦ C, and their XRD results also showed that the sample was partially nitridated at 850 ◦ C. Fig. 3 also corresponded with their results that the PL intensity of 900 ◦ C was much lower than 950 ◦ C which was nitridated completely. For this reason, the sample that was ammonified at 900 ◦ C was partially nitridated, and the work function should not exactly be 4.1 eV when calculating the enhancement factor. However, to simplify the calculation for comparison, the work function of both the samples (900 and 950 ◦ C) was assumed to be the same (4.1 eV). Then, the enhancement factor of the samples which were ammonified at 900 and 950 ◦ C were 493 and 169, respectively. While measuring the field-emission properties, a strong electric field was established between the NW tips and the vacuum. The energy band of NWs was distorted due to the external field which resulted in a thin tunneling barrier and the electron accumulation of NW’s tip. The field-emission phenomenon then happened by the accumulation of electrons which tunneled through the barrier. Fig. 5 schematically depicts the band diagram of NWs, where δ denotes the difference between the Fermi energy and the conduction band. The δ of Ga2 O3 and GaN were calculated to be 1.67 and 0.8 eV, respectively. Thus, under strong external field, GaN NWs which had smaller δ exhibited more distortion and accumulation that made

HSUEH et al.: ELECTRON-FIELD-EMISSION PROPERTIES OF GALLIUM COMPOUND BY AMMONIFICATION OF GA2 O3 NANOWIRES

electron emission more easier. Fig. 4(a) also proved these phenomena that the threshold field of the samples became smaller while the ammonification temperature increased. However, the threshold field of the sample which ammonified under 950 ◦ C increased drastically. This could be explained by the severe defects such as cracks of NWs after ammonified under 950 ◦ C. The NWs have round globules to start with and attain sharper tips with higher annealing temperatures presumably due to the evaporation of Ga if it was a Ga droplet or diffusion of Au–Ga if it was an Au–Ga droplet; the threshold field was expected to become lower due to field enhancement at the tip of sharp NWs. At 950 ◦ C, many defects exist near the tip of the NW so that field enhancement at a single point is diminished and the threshold field is increased. These factors could also contribute to the threshold field values in addition to the composition of the NW. IV. CONCLUSION In summary, we report the growth of Ga2 O3 NWs by the VLS process and GaN NWs by ammonification of Ga2 O3 NWs. Complete ammonification was achieved only when the temperature was higher than 950 ◦ C. The measured turn-on field of emission properties decreased while the ammonification temperature was increased. The results also show that the emission property of pure GaN NWs was poor which was grown by ammonification of Ga2 O3 NWs. REFERENCES [1] Y. H. Yang, C. X. Wang, B. Wang, N. S. Xu, and G. W. Yang, “ZnO nanowire and amorphous diamond nanocomposites and field emission enhancement,” Chem. Phys. Lett., vol. 403, no. 4–6, pp. 248–251, 2005. [2] H. T. Hsueh, T. J. Hsueh, S. J. Chang, F. Y. Hung, C. L. Hsu, W. Y. Weng, C. W. Liu, Y. H. Lee, and B. T. Dai, “Selective growth of silicon nanowires on glass substrate with an ultrathin a-Si:H layer,” Electrochem. Solid-State Lett., vol. 13, no. 4, pp. K29–K31, 2010. [3] Y. K. Ko, J. Geng, S. G. Jang, S. M. Yang, T. W. Jeong, Y. W. Jin, J. M. Kim, and H. T. Jung, “Enhanced field emission of an electric field assisted single-walled carbon nanotube assembly in colloid interstices,” Carbon, vol. 47, no. 6, pp. 1555–1560, 2009. [4] Z. R. Dai, Z. W. Pan, and Z. L. Wang, “Novel nanostructures of functional oxides synthesized by thermal evaporation,” Adv. Funct. Mater., vol. 13, no. 1, pp. 9–24, 2003. [5] S. Kar, S. Chakrabarti, and S. Chaudhuri, “Morphology dependent field emission from In2 O3 nanostructures,” Nanotechnol., vol. 17, pp. 3058– 3062, 2006. [6] C. L. Hsu, S. J. Chang, H. C. Hung, Y. R. Lin, C. J. Huang, Y. K. Tseng, and I. C. Chen, “Vertical single-crystal ZnO nanowires grown on ZnO:Ga/glass templates,” IEEE Trans. Nanotechnol., vol. 4, no. 6, pp. 649–654, Nov. 2005.

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