Field-emission properties of nanocrystalline tin oxide films

Sensors and Actuators B 107 (2005) 474–478

Field-emission properties of nanocrystalline tin oxide films Vitor Baranauskas∗ , M´arcio Fontana, Zhao Jing Guo, Helder Jos´e Ceragioli, Alfredo Carlos Peterlevitz Faculdade de Engenharia El´etrica e Computa¸ca˜ o, Universidade Estadual de Campinas, Av. Albert Einstein N. 400, 13083-852 Campinas SP, Brazil Received 8 July 2004; received in revised form 2 November 2004; accepted 3 November 2004 Available online 30 December 2004

Abstract The room temperature field-emission properties of tin oxide films have been studied. The samples were prepared on Si and glass substrates using the spray deposition technique using vapors of pentahydrated stannic chloride and ethanol diluted in hydrogen. Raman and Atomic force microscopy (AFM) analyses have been carried out to study the relation between the surface morphology and chemical composition with the emission properties of the samples. It was found that the deposition temperature has a dramatic influence on the electron field-emission properties. This effect is explained in terms of the temperature dependence of the hydrolysis reaction to form n-type SnO2 grains and the increase of the film roughness with the temperature. © 2004 Elsevier B.V. All rights reserved. Keywords: Field electron emission; Tin oxide; Stannic oxide; SnO2 ; Displays

1. Introduction Tin oxide films possess interesting structural and electronic properties that suggest a number of novel and useful applications for electronic gas sensors [1–4], flat display devices [5,6], computer touch screens, thin-film transistors, transparent electrodes and photovoltaic cells [7,8]. A variety of processes have been used to prepare SnO2 coatings, such as sol–gel [9], chemical precipitation [4,10], sputtering [11,12], chemical vapor deposition (CVD) [13,14], and spray pyrolysis [6,15]. Thin-films of SnO2 may be produced with unique properties, such as low electrical resistivity (n-type degenerate semiconductor), high optical transparency (band-gap energy Eg ∼ 3.5 eV), high or low porosity and high chemical stability. However, the electron field-emission properties of SnO2 have not been investigated in detail, since in most vacuum applications this material is mainly used as anode. ∗

Corresponding author. Tel.: +55 19 3 788746; fax: +32891395. E-mail address: [email protected] (V. Baranauskas).

0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2004.11.004

This work describes the study of the electron fieldemission properties of tin oxide films prepared by the spray deposition technique on Si substrates. The experimental results show a strong influence of the deposition temperature on the emission threshold field. The morphological and chemical structure of the samples, revealed by atomic force microscopy (AFM) and Raman spectroscopy, and their effects on the electron field-emission threshold are discussed.

2. Experimental details The samples were deposited using a spray pyrolysis system described elsewhere [16]. The experimental set-up consists of a reservoir of liquid Sn solution that feeds by gravity a jet gas nozzle. The nozzle is induced by the flow of hydrogen through a small orifice to the atmosphere. Control valves are placed to independently adjust the hydrogen and the Sn solution flow rates. The substrates were mounted horizontally over a steel hot-plate placed below the jet nozzle in order to receive an uniform spray coating. A 0.2 M solution

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of pentahydrated stannic chloride (SnCl4 ·5H2 O) in ethanol (C2 H5 OH) was used in the feed reservoir. Hydrogen flow was kept at 7 l min−1 at 1520 Torr pressure and the solution spray rate was around 1 ml min−1 . Deposition temperatures, measured by a chromel–alumel thermocouple, were controlled via the power supplied to the hot-plate, and were set in the range 523–673 K, corresponding to deposition rates of about 0.4–4.6 nm s−1 , respectively. After establishing the stability of the deposition parameters described, the substrates were introduced in the reactor for 60 s. Soda-lime glass and n-type Si substrates were used. Prior to the deposition, they were ultrasonically degreased in acetone, ethyl alcohol, rinsed in distilled water and dried in hot nitrogen. Surface morphology of the grown layers was examined by Atomic Force Microscopy (AFM) using a Nanoscope II AFM. Micro-Raman spectroscopy were carried out at ambient temperature using a Renishaw microprobe system, employing the output of an Ar+ laser (6 mW power) for excitation at λ = 514.5 nm. The characterization of field-emission properties was performed in a specially designed vacuum system by the control of distance (d) and parallelism between anode–cathode (samples) surfaces using a precisely combined XYZ-angular micrometer stage. The measurement of the Fowler–Nordheim (F–N) current versus bias voltage (I–V) was done for a fixed anode–cathode distance of d = 25 ␮m in parallel plate configuration. A Cu rod of 3 mm diameter was used as anode. The threshold field (Et ) was measured by the slope of the bias versus anode–cathode distance curves for a standard electron current density of 500 nA cm−2 , fitted to straight lines [17].

3. Results and discussion Fig. 1 shows the three-dimensional morphology of the sample deposited at 573 K, which represents the typical morphologies of the samples revealed by AFM. All the samples exhibit a morphology with uniform particle sizes and good surface coverage. A higher deposition temperature fa-

475

Fig. 2. Typical particle size vs. the deposition temperature of the samples, obtained by AFM measurements.

vored the increase in film roughness and also increased the lateral particle size. The heights of the particles are nearly equal to their diameters. Fig. 2 shows the typical particle size versus the deposition temperature obtained from AFM measurements. The particle size increased from typical values of 10 nm (at 523 K) to around 160 nm (at 673 K). The typical room temperature Raman spectra of the samples synthesized at 523 K, 623 K and 723 K are shown in Fig. 3. Only broad peaks have been observed, which is indicative of a large density of defects and the confinement of the usual crystalline phonon modes in nanocrystals. Spectra of the samples deposited at high (spectrum (c)) and mid (spectrum (b)) temperatures show three broad peaks centered around 480–490 cm−1 , 630–636 cm−1 , and 776–785 cm−1 , respectively. These peaks are close to the frequency of three active Raman vibrational modes (Eg , A1g and B2g ) of crystalline and microcrystalline SnO2 tetragonal rutile structure, which have been experimentally observed at 474 cm−1 , 632 cm−1 and 774 cm−1 [18]. Spectrum (b) also shows a small broad lobe around 530–570 cm−1 which is only observed in nanometer SnO2 grains and is attributed to the effect of Raman surface modes that dominates in the nanoscale [19,20].

Fig. 1. AFM image of the sample deposited at 573 K, which represents the typical morphologies of the samples revealed by AFM.

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Fig. 4. Typical current-voltage data plotted in Fowler–Nordheim (F–N) diagram for samples deposited at (a) 573 K and (b) 623 K, measured for a fixed cathode–anode distance (d) of 25 ␮m.

Fig. 3. Typical Raman spectra of the samples deposited at (a) 523 K, (b) 623 K and (c) 723 K.

The ratio between the intensity of this peak to that of the peak at the A1g (632 cm−1 ) position is used in the literature to estimate the size [20] of the nanocrystals. A rough estimate gives a nanocrystalline size of around 10 nm. This grain size is significant smaller than the morphology measured by the AFM, which suggests that the particle features in the AFM images may be interpreted as agglomerates of nanocrystalline SnO2 grains. Another characteristic of spectrum (b) and spectrum (c) is that the intensity of the broad peaks centered around 480–490 cm−1 are higher than the intensity of the broads peaks centered around 630–636 cm−1 . This behaviour is not consistent with the single crystalline phase, since for crystalline, microcrystalline or nanocrystalline SnO2 grains the intensity of the Eg mode at 474 cm−1 is much smaller than the intensity of the A1g mode at 632 cm−1 . In addition, we may also note the absence of peaks around A1g mode 632 cm−1 the spectrum (a) corresponding to the sample synthesized at low temperature (523 K). The spectrum (a) is consistent with the Raman spectra of either pure SnO or pure Sn(OH)2 compounds in which only peaks around 470 cm−1 and 752–760 cm−1 are observed [21]. Therefore, according to the Raman results, the samples synthesized at 623 K and 723 K consists of SnO2 grains mixed with SnO and/or Sn(OH)2 , but the samples synthesized at 525 K contain only grains of tin monoxide and/or tin hydroxide. The presence of SnO2 , SnO and/or Sn(OH)2 in the samples may be explained as follows. The forced spray flow of pentahydrated stannic chloride (SnCl4 ·5H2 O) and ethanol (C2 H5 OH) to the hot-substrate surface induces hydrolysis reactions with generation of radicals of tin methoxide (Sn( OCH3 )x ), tin

hydroxide (Sn( OH)x ) and other intermediate complexes, which may result in the formation of stoichiometric tin hydroxide molecules, such as Sn(OH)4 and Sn(OH)2 . The thermal decomposition at the substrate surface produces adherent SnO (insulator) and/or SnO2 (semiconductor) films on the surface and liberates water vapor according to the 

following reactions: Sn(OH)4 −→ SnO2 + 2H2 O and/or 

Sn(OH)2 −→ SnO + H2 O. Higher substrate temperatures favor the chemical process to increase the concentration of SnO2 . Fig. 4 shows typical Fowler–Nordheim plots for samples deposited at 573 K (curve (a)) and 623 K (curve (b)), measured for a fixed cathode–anode distance (d) of 25 ␮m. The linearity in the ln(I/V2 ) versus 1/V plots suggests that the anode current density is due to the field emission. For a defined turn-on (or threshold) current density it is possible to measure the threshold voltage (Vt ) versus distance (d) using the micrometer vacuum probe station. Fig. 5 shows the threshold voltage (Vt ) versus distance (d) necessary to produce a standard threshold current density of about 500 nA cm−2 for the sample deposited at 623 K. A linear fit to this data provides the corresponding typical threshold field (Eth ) for each sample, which are plotted in Fig. 6 as a function of the deposition temperature. One may see that the threshold field (Eth ) decreased from 18.6 V ␮m−1 to 9.2 V ␮m−1 as the deposition temperature increased from 525 K to 673 K, respectively. The electron field-emission process involves extraction of electrons from the tin oxide film by tunneling through the surface potential barrier to the vacuum. The emitted current depends on (i) the material composition, (ii) the resistivity of the back contact and on (iii) the local field existing at the emitting surface. Small variations in one of these factors have a strong impact on the emitted current. Raman results show that the films deposited at mid and high temperatures are predominantly of stoichiometric crystalline SnO2 embedded with Sn(OH)2 and/or SnO grains, but the presence of SnO2

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4. Conclusions We have examined the field electron emission properties of tin oxide films prepared by the spray deposition technique. Our experiments indicate that the deposition temperature has a dramatic influence on the threshold field (Eth ) for electron emission. A relatively low threshold field of 9.2 V ␮m−1 has been observed for the sample deposited at the highest temperature (673 K). We suggest that this effect is related to the temperature needed for efficient thermal decomposition of the hydroxides to form n-type SnO2 nanocrystals; to increase the electron conduction at grain boundaries; and also to increase the geometrical field enhancement factor by enhancing the grain size (height). The electron emission results from these materials are potentially attractive for both science and technological applications. Fig. 5. Typical threshold voltage (Vt ) vs. distance (d) necessary to produce a standard threshold current density of about 500 nA cm−2 for the sample deposited at 623 K.

Acknowledgments was no detected in the low temperature deposited samples. SnO2 is a n-type semiconductor, and therefore, is a good conductive material for electrons. Previous work also shows that the conductivity of tin oxide films increases with the increase of the deposition temperature due to the increase in the granular coalescence and thermal decomposition of tin hydroxides [22]. Finally, the increase of the grain-size with the temperature, revealed by AFM, leads to variation of the height and spacing between the grains (roughness), and therefore, to an increase in the geometrical field enhancement factor. So, although it is difficult to identify the main process, the observed changes in material composition (i), conductivity (ii) and local field enhancement (iii) may explain in integrated form the large variation of the threshold field (Eth ) observed as a function of the sample deposition temperature.

Fig. 6. Typical threshold field (Eth ) for electron emission of the samples as a function of the deposition temperature.

We thank Professor F.C. Marques from IFGW/UNICAMP for his advice and use of his tin oxide spray system, and Dr. Kanad Mallik for his technical assistance. We also gratefully acknowledge INPE for use of their Raman spectrometer and the Brazilian agencies FAPESP, CAPES and CNPq for partial financial support.

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