Oxygen annealing effect on field-emission characteristics of hydrothermally synthesized Al-doped ZnO nanowires

Oxygen annealing effect on

field-emission characteristics of hydrothermally

synthesized Al-doped ZnO nanowires

Jyh-Liang Wang

⁎

, Tsang-Yen Hsieh

, Po-Yu Yang

, Chuan-Chou Hwang

, Der-Chi Shye

, I-Che Lee

a

Department of Electronics Engineering, Ming Chi University of Technology, Taipei 24301, Taiwan

b_{Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, Hsinchu 30010, Taiwan}

a b s t r a c t

a r t i c l e i n f o

Available online 18 January 2012

Keywords:

Al-doped zinc oxide (AZO) Hydrothermal growth Nanowire

Field emission Oxygen annealing

Al-doped ZnO (AZO) nanowires (NWs) were synthesized using low-temperature hydrothermal growth to in-vestigatefield emission (FE) characteristics. The intensity ratio of NBE peak to DLE peak (R=INBE/IDLE)

in-creases and the half-maximum of NBE peaks (FWHMNBE) decreases with the Al contents. Experimental

results reveal the FE characteristics of AZO NWs are functions of Al content. Moreover, the larger R value and the smaller FWHMNBEare found as the oxygen annealing temperature increases. The hydrothermal

AZO NWs annealed in oxygen ambience with an appropriate temperature (i.e. 300–500 °C) can demonstrate the further improved FE properties (i.e. the turn-onfield of 1.70 V/μm (at 1 μA/cm2_{), thresholdfield of 2.92 V/μm}

(at 1 mA/cm2_{), andβ of 4547).}

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Various nanomaterials with high aspect ratio have been intensively investigated forfield emission (FE) characteristics in the last decade [1–3]. Among those studies, ZnO nanostructures are suffered much at-tention for the prolonging device lifetimes of thefield emitters, which can be attributed to the properties of a direct energy wide-bandgap (i.e. ~3.37 eV) at room temperature, a large exciton binding energy (i.e. ~60 meV), thermal stability, chemical stability, and high mechani-cal strength [4,5]. Lately, the ZnO nanostructures are doped with group III metal elements (i.e. Al)[6,7], resulting in the reduced electrical resistance[9]. The Al-doped ZnO (AZO) nanostructures have been pre-sented for the high conductance andfine crystal quality[7,8], which can be expected to have a low turn-on_{field and a high emission current.} ZnO nanostructures have been prepared by several methods, such as metal-organic vapor phase epitaxy (MOVPE)[10], metal-organic chemi-cal vapor deposition (MOCVD)[11], thermal evaporation method[12], and vapor–liquid–solid (VLS) methods [13]. However, the reported methods usually demand high process temperature (>600 °C) and re-sult in some concerns during the fabrications offield emission display (FED) devices, i.e. the limitation of substrate materials and process inte-gration issues [10–13]. Conversely, a solution-based hydrothermal method exhibits the benefits of low reaction temperature, low facility cost, catalyst-free growth, uniform production, large area, and com-patibility with plastic electronics, which will be appropriate for the FED fabrication[14]. Nevertheless, only finite report presents the

low-temperature hydrothermal fabrications of AZO nanostructures and related FE characteristics[8]. Furthermore, the structural defects of metal oxide (i.e. oxygen vacancies, oxygen interstitials, zinc vacan-cies, and zinc interstitials) produce potential wells that can trap and af-fect the movement of carriers, which degrade the device performance and may be vanished during oxygen ambience annealing[15]. Thus, a post-annealing processed in oxygen ambience at a moderate tempera-ture (i.e. 300–500 °C) shall be considered to compensate or reduce the structural defects of ZnO nanostructures. In this study, not only the FE characteristics of the low-temperature hydrothermal AZO nanostruc-tures are further investigated, but also the influence of post-annealing in oxygen ambience on the FE characteristics of hydrothermal AZO nanostructures will be explored.

2. Experimental procedures

The AZO nanostructures were hydrothermally grown on the glass substrates[8]. In experiments, a 200 nm-thick AZOfilm was sput-tered on glass substrates to serve as a seed layer during the hydro-thermal growth. The precursor solution containing 2.5 mmol zinc nitrate hexahydrate (Zn (NO3)26H2O), 2.5 mmol

hexamethylenetet-ramine (HMTA) and distilled water. Aluminum nitrate nonahydrate (Al (NO3)39H2O) powders were used as the Al content source to

add in the precursor solution. The dosages of Al dopant (Al/(Al+ Zn)) were designed as 0 at.% (undoped ZnO), 1 at.%, 2 at.% and 3 at.% in the precursor solutions, accordingly. Next, the samples were placed in the mixed solution at 85 °C for 1 h on a hot plate. Then, the samples were washed in DI water in order to eliminate the residual salts and dried with blowing nitrogen gas. Subsequent to the hydrothermal growth, some samples were picked up and annealed in oxygen ambience to

⁎ Corresponding author at: 84 Gungjuan Rd., Taishan, Taipei 24301, Taiwan. Tel.: +886 2 29089899x4861; fax: +886 2 29085247.

E-mail address:[email protected](J.-L. Wang).

0257-8972/$– see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2012.01.019

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Surface & Coatings Technology

evaluate the effect of oxygen ambient annealing, which will be explained by the technologies of material analyses and electrical mea-surement later. The oxygen ambient annealing was executed in a tube furnace with the parameters as oxygen gasflow rate of 60 sccm, process pressure of ~1 atm, annealing temperature of 300–500 °C, and 1 h dura-tion. The morphology of the grown AZO nanostructures was observed by field-emission scanning electron microscopy (FE-SEM, Hitachi S-4700I). The photoluminescence (PL) spectra were executed with the excitation wavelength at 325 nm of a He–Cd laser. Electron FE characteristics of samples were measured in a vacuum chamber under 1.0× 10−7Torr at room temperature using high-voltage units (Keithley 237) with an IEEE 488 interface controlled by a personal computer.

3. Results and discussion

In the published study[8], the AZO nanostructures with various Al contents were synthesized successfully on AZO/glass substrate by hy-drothermal growth at 85 °C.Fig. 1shows the FE-SEM images for the growth of AZO nanostructures on the AZO/glass substrate, while the Al contents were designed in the range of 0 at.%–3 at.%. The mor-phologies of AZO nanostructures can be identified as nanowires (NWs). The AZO NWs are well ordered and vertically aligned on the substrate with controllable length (~1μm). The vertical nanostructure employed as an emission cathode usually profit for the field-emission [11]. The insets present the top-view FE-SEM images of AZO NWs with various Al contents. The aligned AZO NWs outline can be seen as well-defined hexagons associated with the wurtzite structure of ZnO single crystal. The variation of NWs dimensions with various Al con-tents is not obvious. The diameter of AZO NWs is observed as ~100 nm by naked eyes from the images. Generally, no distinction in morphology is found among the AZO NWs with different Al contents. In addition, the Al compositions of AZO NWs were experimentally ana-lyzed by XPS X-ray photoelectron spectroscopy (Physical Electronics PHI-1600) as 0 at.%, 0.31 at.%, 0.87 at.%, and 1.98 at.%, respectively to

the designed Al contents of 0 at.%, 1 at.%, 2 at.%, and 3 at.%. In the subse-quent discussion, the actual Al contents of samples are remarked ac-cordingly to the XPS analysis.

Fig. 2(a) indicates the room-temperature PL spectra of AZO NWs, which can be distinguished into two components: one is the UV emis-sion owing to the near band-edge emisemis-sion (NBE)[16], and the other is the deep level emission (DLE) in the visible region due to the pres-ence of structural defects[10]. When Al element is doped in ZnO, the Al ions can be presented as Al3 +and compete with Zn ions to con-sume the residual O ions in ZnO matrix, which decreases the concen-tration of oxygen interstitials in the AZO NWs[17]. Compare to the undoped ZnO NWs, the weaker DLE of AZO NWs reflects the reduced structural defects andfine crystal quality[8,10]. Moreover, the com-parative intensity ratio of NBE peak to DLE peak (R = INBE/IDLE) and

full-width at half-maximum of NBE peaks (FWHMNBE) can be

associ-ated with the relative amount of structural defects in ZnO[21,22], and used to quantify the structural quality of AZO nanostructures. Fig. 2(b) performs the analysis of PL emission spectra on the R values and FWHMNBE. The R values increase as the Al contents increase. On

the contrary, the FWHMNBE decreases with the Al contents. AZO

NWs with 1.98 at.% Al content exhibits the larger R and smaller FWHM (owing to the strong intensity of NBE peak, weak intensity of DLE peak, and sharp profile of NBE peak). This result suggests that hydrothermal AZO NWs with well-designed Al content can pro-duce the fewer structural defects and superior crystallinity, which result in the increases of carrier concentration, carrier lifetime and mobility [20], and may benefit for conductivity and FE characteristics[8]. Addi-tionally, the inset reveals that the NBE peaks shift from 380 nm to 372 nm as the increasing of Al content, linked to Burstein–Moss effect [10].

The samples were placed on a ground plate and installed in a high-vacuum chamber system for FE characteristic measurement. A glass plate coated with indium-tin-oxide (ITO) and phosphor was oppo-sitely positioned 100μm above the sample surface to act as an

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Fig. 1. (a) FE-SEM images of AZO nanostructures on the AZO/glass substrate. The Al contents were designed in the range of (a) 0 at.%, (b) 0.31 at.%, (c) 0.87 at.%, and (d) 1.98 at.%. The insets give the related top-view images.

anode.Fig. 3(a) gives the FE characteristics of samples grown on AZO/ glass substrates. The turn-on fields (defined as the electric field extracted at a current density of 1μA/cm2_{) were reduced with Al}

con-tents. Similarly, the thresholdfields (estimated as the electric field with at a current density of 1 mA/cm2_{) were also decreased}

accord-ingly to the Al contents. The AZO NWs demonstrate the higher cur-rent density, lower turn-on field and lower threshold field than those of undoped ZnO NWs. Furthermore, the FE curves of and AZO NWs can be modeled by the Fowler–Nordheim (F–N) equation expressed as

J¼ Aβ2 E2=ϕ

 

exp− Bϕ 3=2=βE; ð1Þ

where J denotes the current density (A/m2_{), V denotes the applied}

voltage, E (V/d) is the applied electricfield, d denotes the distance be-tween anode electrode and top of NWs, A = 1.56 × 10− 10(A eV/V2_),

B = 6.83 × 109_{(V/m eV}3/2_{)[18],ϕ denotes the work function of ZnO}

NWs (eV), andβ denotes the field enhancement factor, respectively. Here, the work function of ZnO can be assumed as 5.4 eV[19].β is known to depend on several factors, including the geometry crystal structure, the material conductivity, density of the nanostructures and the work function.β can be extracted from the slope of ln (J/E2₎

versus 1/E in F–N plots[23]. The inset ofFig. 3(a) demonstrates the corresponding F–N plots of the emission current. The values of β for AZO NWs emitters were apparently promoted correspondingly to the Al contents of 0–1.98 at.%. Furthermore,Fig. 3(b) carries out the

statistical distributions of turn-onfields and β. As the Al content in-creases, the smaller turn-onfield, and the larger β value with fewer fluc-tuations can be aware. The experimental FE characteristics are consistent with the addressed analysis of PL spectra. As a result, the bet-ter FE characbet-teristics (i.e. the smaller turn-onfield, and the larger β value with fewer _{fluctuations) are obtained for AZO NWs with} 1.98 at.% Al content, which were considered for oxygen ambient annealing.

Fig. 4displays the FE-SEM images of the as-grown AZO NWs and samples annealed in oxygen ambience at 300 °C–500 °C, while the Al content wasfixed as 1.98 at.%. No evident change on the morphology of AZO NWs is observed after oxygen annealing at various tempera-tures.Fig. 5(a) shows the PL emission spectra of the samples before and after oxygen annealing at 300–500 °C. The DLE intensities de-creased as the annealing temperature in oxygen ambience. The anneal-ing at higher temperature does reduce the number of zinc interstitials, and the oxygen ambience during annealing also inhibits the oxygen va-cancies[15]. The inset ofFig. 5(a) reveals an increased intensity of NBE peak (ΔINBE) for the 500 °C-annealed sample, which may be attributed

to the compensated structural defects during oxygen annealing. Like-wise, the analysis of PL emission spectra on the related R values and FWHMNBEare applied to quantify the structural quality of AZO NWs

after oxygen annealing.Fig. 5(b) discloses the larger R value and the smaller FWHMNBEas the annealing temperature increases. It

recom-mends that the more compensated structural defects with an improve-ment on crystal structure can be existed for hydrothermal AZO NWs after the oxygen annealing at higher temperature (i.e. 500 °C).

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b

Intensity (Arb. Unit)

Wavelength (nm)

1.98 at.% Al 350 360 370 380 390 400 410

Intensity (Arb. Unit)

Wavelength (nm)

Fig. 2. (a) Room-temperature PL emission spectra for the AZO NWs samples with different Al contents. (b) The analysis of PL emission spectra on the R values (R = INBE/IDLE) and

FWHMNBE.

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Current Density (mA/cm

2) Electric Field (V/ m) 0.16 0.20 0.24 0.28 0.32 0.36 -12 -11 -10 -9 -8 -7 -6 1/E ( m/V) ln (J/E 2)

Fig. 3. (a) J–E field emission plots of AZO NWs samples with different Al contents. The inset demonstrates the corresponding F–N plots of the emission current. The symbols pre-sent the measured data and solid lines indicate the dependence of F–N theory. (b) The sta-tistical distributions of turn-onfields and β versus the Al contents.

Fig. 6(a) illustrates the characteristics of J–E for the as-grown and annealed samples in oxygen ambience. The annealed sample expresses the larger anode current. The values of thresholdfield are calculated as 3.43 V/μm, 3.41 V/μm, 3.08 V/μm, and 2.92 V/μm, accordingly to the conditions of as-grown and annealing at 300–500 °C. The inset also points out the according F–N plots of the emission current.Fig. 6(b) ex-plores the evolution of turn-onfields and β. The turn-on fields are con-sidered as 2.17 V/μm, 2.14 V/μm, 1.79 V/μm, and 1.70 V/μm, correspondingly for the samples of as-grown and annealed at 300– 500 °C. The values ofβ are 3131, 3272, 4166, and 4547, relatively. The smaller turn-onfield and larger β value can be watched with the higher annealing temperature. Therefore, the enhanced FE properties (i.e. the small turn-onfield and large β) of AZO NWs can be achieved because of the compensated structural defects and an improved crystal structure after the oxygen annealing. In summary, the low-temperature hydro-thermal AZO NWs annealed in oxygen ambience with an appropriate temperature (i.e. 300–500 °C) can demonstrate the excellent field emis-sion characteristics, which are comparable with those fabricated at high temperatures (i.e. 600–990 °C)[11,12]. Consequently, this study indi-cates the potentials on applications of high brightness electron sources and vacuum device.

4. Conclusions

The hydrothermal ZnO and Al-doped ZnO (AZO) nanowires (NWs) were vertically grown on the AZO/glass substrates with the controllable length (~ 1μm) and a diameter of ~100 nm. The R value (R = INBE/IDLE) increases and the half-maximum of NBE peaks

(FWHMNBE) decreases with the Al contents. Thus, the AZO NWs (i.e.

1.98 at.% Al content) show the higher current density, lower turn-on field, lower threshold field, and larger β value than those of undoped ZnO NWs. Moreover, a post-annealing processed in oxygen ambience was conducted to improve the FE properties of AZO NWs. The larger R value and the smaller FWHMNBEare found as the oxygen annealing

temperature increases. The more compensated structural defects with an improvement on crystal structure can be obtained for

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Fig. 4. FE-SEM images of (a) as-grown AZO NWs and samples annealed in oxygen ambience at (b) 300 °C, (c) 400 °C, and (d) 500 °C, respectively.

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Fig. 5. (a) Room-temperature PL emission spectra of as-grown AZO NWs and samples annealed in oxygen ambience at 300–500 °C. (b) The related R values and FWHMNBE

hydrothermal AZO NWs after the oxygen annealing at higher temper-ature (i.e. 500 °C). 500 °C-annealed AZO NWs explore the turn-on field, threshold field, and β as 1.70 V/μm (at 1 μA/cm2_{), 2.92 V/μm}

(at 1 mA/cm2_{), and 4547, separately. Consequently, the enhanced}

FE properties (i.e. the small turn-on field and large β) of low-temperature hydrothermal AZO NWs can be achieved because of the compensated structural defects and improved crystal structure after oxygen annealing.

Acknowledgments

Thanks are also due to the Center for Thin Film Technologies and Applications (CTFTA) in Ming Chi University of Technology, the Nano Facillity Center (NFC) in National Chiao Tung University, and the National Nano Device Laboratory (NDL) for the technical supports.

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a

b

Current Density (mA/cm

2) Annealed at 500 o C Annealed at 400 o C Annealed at 300 oC As-grown 0.20 0.24 0.28 0.32 0.36 -12 -11 -10 -9 -8 -7 -6 1/E (m/V) ln (J/E 2) Annealed at 300o_C Annealed_{at 400}o_C Annealed_{at 500}o_C

Fig. 6. (a) J–E field emission plots for as-grown and annealed samples. The symbols demonstrate the measured data and solid lines indicate the dependence of F–N theory. The inset is the according F–N plots of the emission current. The symbols present the measured data and solid lines indicate the dependence of F–N theory. (b) The evolu-tions of turn-onfields and β to the oxygen annealing temperatures.