Influence of rapid-thermal-annealing temperature on properties of rf-sputtered SnOx thin films

doi:10.1016/j.apsusc.2014.11.115

Applied Surface Science

Volume 327, 1 February 2015, Pages 358-363

https://doi.org/10.1016/j.apsusc.2014.11.115 Get rights and content

Highlights

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Sputtered SnO_x are annealed by infrared rapid thermal annealing (RTA) technique.
•
RTA (225, 245, 265 °C) processes improve the transparency and increase the bandgaps.
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All measurable films exhibit p-type conductivity after RTA-processing.
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Lower metallic Sn reduces carrier scattering and compensating electron carriers.
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The best hole mobility achieved is 0.78 cm² V⁻¹ s⁻¹ for films annealed at 265 °C.

Abstract

We investigated rf-sputtered SnO_x thin films that were processed by the infrared rapid thermal annealing (RTA) technique. The films were RTA-processed at 225, 245, and 265 °C for 2.5 min in ambient air. X-ray diffraction analyses indicate the existence of metallic Sn and SnO phases in the films. After RTA processing, metallic Sn decreases and the total amount of SnO increases. The oxidation of metallic Sn in the films becomes more significant as the temperature increases from 225 °C to 265 °C. X-ray photoelectron spectroscopy reveals that the SnO phase is the dominant phases after RTA processing. The transmittance in the visible light wavelength region improves after RTA processing and increases with the annealing temperature. The Tauc bandgap is calculated as 1.8 eV for as-deposited and increases to ∼2.8 eV after RTA processing. p-Type conductivity is confirmed for all measurable RTA-processed films by Hall measurement and Seebeck coefficient measurement. The best hole mobility achieved is 0.78 cm² V⁻¹ s⁻¹ for films annealed at 265 °C and the corresponding hole carrier concentration is 4.28 × 10¹⁷ cm⁻³.

Introduction

Transition metal oxides are potential base materials for next generation large-area electronics owing to their higher mobility compared to hydrogenated amorphous silicon (a-Si:H), the current industrial standard material. The development of n-type oxide electronic materials is more mature, and many n-type semiconductor oxides such as InGaZnO [1], [2] have already been used in the electronics industry. On the other hand, the development of p-type semiconductors lags behind; nevertheless, high quality p-type oxides are strongly desired because they are essential for the development of CMOS technology [3], [4]. Several p-type materials have been developed thus far such as p-type doped ZnO [5], [6], [7], [8], NiO [9], [10], [11], [12], and Cu₂O-based oxides [13], [14], [15], [16], [17]; the last has been alloyed with Al₂O₃ to form (Cu,Al)-based oxides [18], [19], [20], [21], [22], [23], [24], which have higher transparency in the visible wavelength region. The p-type conductivity of SnO arises from its tin vacancy which forms shallow acceptor states [25]. For p-type oxides in general, the hole mobilities are low because the hole transport paths (i.e. valence band maxima) are made mainly from rather localized O 2p orbitals that are directional and have high electronegativity. Holes are thus localized in deep 2p levels and transported via hopping mechanism [26]. Compared with other p-type oxides, SnO is possibly a better native p-type oxide. Sn 5s energy level is similar to that of O 2p, orbitals may hybridize such that holes are delocalized to possess higher hole mobility [27]. Therefore, SnO has been applied to make thin-film transistors, which are building blocks in large-area electronics [26], [28], [29], [30]. From the viewpoint of the application of SnO thin films to large-area electronics, sputtering is the most popular deposition technology [28], [30], [31], [32], [33]. However, there remain some challenges concerning the narrow processing window of SnO technology—the film quality of films are highly sensitive to the sputtering pressure, oxygen content during sputtering, annealing temperature, and meta-stability of the disproportionation reaction of SnO → Sn + SnO₂ at ∼600 K [26], [29], [30], [32], [33], [34], [35], [36].

In this study, we investigated room temperature rf-sputtered SnO_x thin films post-processed by the infrared rapid thermal annealing (RTA) technique. Compared with the conventional furnace annealing processes, RTA offers several advantages such as the ability of isothermal and short heating cycles, avoiding temperature gradients within the sample chamber, and having accurate control over the heating cycles [37]. The temperature-time curve can be adjusted by varying the parameters of the PID controller. Infrared RTA processing on SnO_x thin films with a temperature ranged between 300 and 500 °C has been investigated by Liang et al. [38]. In this study, the RTA processing temperature is maintained below the disproportionation temperature (327 °C). For all samples, the heating durations were fixed at 2.5 min. The temperatures were varied as 225, 245, and 265 °C. The SnO_x thin films were then characterized and the best achieved hole mobility is 0.78 cm² V⁻¹ s⁻¹ with a corresponding hole carrier concentration of 4.28 × 10¹⁷ cm⁻³ for films annealed at 265 °C.

Section snippets

Experimental methods

One hundred twenty-nanometer-thick SnO_x thin films were rf-sputter-deposited on glass substrates (Corning Eagle XG) from a pure Sn target (99.995%) in O₂/Ar mixed atmosphere without any intentional heating of the substrate. The rf power, deposition pressure, O₂/Ar flow rate ratio were 50 W, 0.533 Pa, and 3.9%, respectively. The samples were then post-annealed at 225, 245, and 265 °C for 2.5 min (ramping time = 30 s) in air using an infrared RTA system. An X-ray diffractometer (XRD, PANalytical, X’pert

Results and discussion

Fig. 1 shows the XRD patterns of SnO thin films annealed at different temperature using RTA. No sharp peaks were identified in the XRD pattern of the as-deposited SnO film, indicating the film's amorphous nature. After RTA processing at 225, 245, and 265 °C for 2.5 min, the films crystallized with diffraction peaks located at 2θ = 29.88°, 33.33°, 47.85°, and 57.44°, which corresponds to SnO (1 0 1), SnO (1 1 0), SnO (2 0 0), and SnO (2 1 1), respectively, (JCPDS no. 06-0395); peaks located at 2θ = 30.63° and

Conclusion

Room-temperature rf-sputtered-deposited SnO_x thin films were processed using an infrared RTA technique. XRD reveals the amorphous nature of the as-deposited films that crystallized after RTA processing. The direct Tauc bandgap is calculated as 1.8 eV for the as-deposited film and increases to ∼2.8 eV after RTA processing. The XPS results indicated that the SnO phase is the dominant phase after RTA processing. The resistivity of SnO_x RTA-processed at 225 °C is comparatively higher than that of

Acknowledgments

The authors gratefully acknowledge funding support from the Ministry of Science and Technology of Taiwan under grant nos. MOST 103-2221-E-002-057 (JZC), MOST 100-2221-E-002-151-MY3 (ICC) and MOST 101-2628-E-002-020-MY3 (ICC). The authors would like to express gratitude to Mr. Chung-Yuan Kao for his assistance with the EPMA operation. SEM was conducted at the Instrumentation Center of the National Taiwan University. JZC and ICC are currently visiting research scholars at Princeton University,

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Influence of rapid-thermal-annealing temperature on properties of rf-sputtered SnOx thin films

Highlights

Abstract

Introduction

Section snippets

Experimental methods

Results and discussion

Conclusion

Acknowledgments

J. Alloys Compd.

J. Alloys Compd.

Appl. Surf. Sci.

J. Eur. Ceram. Soc.

Appl. Surf. Sci.

Appl. Surf. Sci.

J. Alloys Compd.

Thin Solid Films

J. Eur. Ceram. Soc.

Appl. Surf. Sci.

Solid State Ionics

Solid State Ionics

Inorg. Chim., A—Art. Lett.

Thin Solid Films

Int. J. Therm. Sci.

Thin Solid Films

Zinc concentration dependence study of solution processed amorphous indium gallium zinc oxide thin film transistors using high-k dielectric

Appl. Phys. Lett.

Diffusion–limited a-IGZO/Pt Schottky junction fabricated at 200 degrees C on a flexible substrate

IEEE Electron Device Lett.

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Appl. Phys. Lett.

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Adv. Mater.

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J. Phys. D: Appl. Phys.

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Influence of rapid-thermal-annealing temperature on properties of rf-sputtered SnO_x thin films