Amorphous oxide semiconductors thin-film transistors (AOs TFTs) have gained interest over the years because of their advantages over a-Si and poly-Si transistors. In particular, tin oxide is used as a semiconductor material because of its abundance, low toxicity, and high intrinsic mobility. Performance-wise, tin oxide TFTs need improvement because the reported average mobility remains < 10 cm²/Vs.¹ Previous research used silicon doping to reduce tin oxide’s oxygen vacancies and improve its performance as a semiconductor channel. Currently, Si_xSn_yO TFTs are fabricated using vacuum process, but solution processing is important to realize large-scale and high-throughput electronics. Sol-gel method is one of the conventional routes to produce solution-processed metal oxides. However, it requires high temperature (≤500°C) to facilitate film densification which is not compatible with flexible substrates having low degradation temperatures. Among the approaches for low-temperature TFT fabrication, solution combustion synthesis (SCS) has garnered significant attention because of its simplicity and versatility.² SCS uses the energy released by combustion to help the formation of more M-O-M networks in the film. Thus, there is no need to use high temperature to condense and densify the film. This study attempts to produce Si_xSn_yO TFTs through solution processing for the first time. We investigated the effect of solution combustion synthesis to the quality of Si_xSn_yO films. Next, we optimized the aging time and oxidizer to fuel ratio (ammonium nitrate to urea) to determine the films with better quality. For aging time optimization, Si_xSn_yO precursors were stirred at RT for 24-h, 48-h, and 72-h. Lastly, we compared the TFT performance in terms transfer characteristics of Si_xSn_yO TFTs fabricated by sol-gel and SCS annealed at the same temperature.

Secondary ion mass spectrometry (SIMS) analysis was used to compare the carbon impurities present in both Si_xSn_yO films. The sol-gel film contained higher amounts of carbon impurities, about an order of magnitude higher as shown in Figure 1. This proves that SCS provides extra energy to help degrade the carbon-containing compounds in the film. Carbon atoms are considered impurities that can introduce defects in the Si_xSn_yO film. These defects are detrimental to the performance characteristics of the fabricated TFT device. In addition, the thickness of the films was determined using X-ray reflectivity. The SCS film is 15.70 nm-thick while the sol-gel film is 18.88-nm thick. Furthermore, optimization of the Si_xSn_yO films via SCS revealed that oxidizer to fuel ratio affects the roughness and amount of M-O bonds formed in the film. The 3:1 ratio failed to produce a film while other ratios were successful in producing films. Although the film with 1:1 ratio has root mean square roughness (R_q) value of 0.380 nm, it contains depressions and voids. On the other hand, the film using a 1:3 ratio showed a uniform and even surface with R_q value of 0.377 nm. In addition, characterization of the two films through X-ray photoelectron spectroscopy (XPS) showed higher percentage of M-O bonds in the O1s spectrum (530.5 eV) for the film with 1:3 ratio. The values are 45.9% and 59.3% for 1:1 and 1:3 ratios, respectively. The amount of oxygen vacancy and H- containing compounds also decreased in the 1:3 film.

Si_xSn_yO TFTs were fabricated from both SCS and sol-gel methods. Both TFTs showed high off-current and highly negative V_th, as shown in Figure 1. Therefore, both TFTs still contain several defects which suggests that 3% Si doping may be insufficient and must be increased to produce a significant effect. The sol-gel fabricated TFT also showed lower drain current. Therefore, the low annealing temperature at 300 ˚C is insufficient to densify the film and reduce impurities. On the other hand, SCS-assisted fabrication showed higher mobility at 3.14 cm²/Vs – confirming that SCS provided additional energy to degrade impurities and improve the M-O network in the Si_xSn_yO channel. In conclusion, we were able to produce Si_xSn_yO TFTs with improved mobility using solution combustion synthesis. However, the high V_th and high off-current issues need to be addressed by increasing silicon dopant addition and exploring other fuel options. But overall, we can confirm that SCS is beneficial in improving the quality of Si_xSn_yO films annealed at 300 ˚C. In the future, this low temperature fabrication is important to realize flexible Si_xSn_yO TFT devices.

Acknowledgements

This research was supported by NAIST foundation grant.

References

1. Chang, H., et al. ACS Applied Electronic Materials, 3(11), 4943–4949.
2. Carlos, E., et al. Chemistry - A European Journal, 26(42), 9099–9125.