Cu₂O Thin-Film Transistor for CMOS Logic Circuit Through the Chemical Reaction During Annealing in N₂ Atmosphere

Metal oxide semiconductors have been emerging as a semiconductor material for next-generation electronic devices such as switching devices in displays, thin film diodes, solar cells, and Dynamic Random-Access Memory (DRAM) due to several advantages such as low cost, various deposition methods, low processing temperature, and processability on large-area glass substrate. However, it is difficult to construct a low power logic circuit based on Complementary Metal-Oxide-Semiconductor (CMOS) with only n-type metal oxide TFTs, so p-type metal oxide TFTs with good electrical performance is required. Cuprous oxide (Cu₂O) is a potentially promising p-type semiconductor due to the intrinsically good electrical properties such as high hole mobility over 100 cm²/Vs and the possibility to fabricate devices using conventional semiconductor processes. Despite an intrinsic high hole mobility, Cu₂O TFT has still shown poor electrical performances. Several studies proposed dissociative Cu defects and CuO impurities as the cause of the poor electrical properties of Cu₂O TFTs. First, dissociative Cu (interstitial Cu) defects create a donor-like sub-gap states in the band gap of p-type Cu₂O, causing high off-current. Second, the other considering point is CuO impurities in the Cu₂O thin film, which induce sub-gap states that serve as a hole trapping site above the VBM of Cu₂O because the bandgap of CuO (1.4 eV) is smaller than that of Cu₂O (2.17 eV). Thus, dissociative Cu (interstitial Cu) defects and CuO impurities play a critically detrimental role in the quality of Cu₂O thin film. Dissociative Cu defects cause high off-current and CuO impurities cause low field effect mobility. Therefore, it is essential to reduce dissociative Cu and CuO impurities in order to improve the electrical performance of Cu₂O TFTs.<br/>Here, we demonstrated Cu₂O TFTs with high field effect mobility and low off-current through reduction of CuO impurities and dissociative Cu defects by simply controlling the fabrication conditions without inducing any additional layers or chemical treatments. In the Radio Frequency (RF) sputtering deposition process, a Cu_xO thin film with a high CuO ratio was fabricated by using a mixed gas of Ar and O₂ as working gas. Then, post annealing was performed while flowing N₂ gas. Oxygen added to the working gas during deposition increases the CuO ratio in the copper oxide thin film and consequently reduces dissociative Cu defects. Also, the N₂ atmosphere in annealing plays an important role in reducing Cu_xO to Cu₂O and prevents oxidation to CuO phase. During post annealing, when CuO - CuO transformed to Cu₂O, oxygen is generated, and the generated oxygen oxidized dissociative Cu defects to CuO. The formed CuO also combined with another CuO and transited to Cu₂O. These chemical reactions occur repeatedly and continuously during the annealing process, creating a chain reaction. By XRD and XPS analysis, we confirmed that the fabrication method of Cu₂O thin film combined with controlled deposition and anneal effectively reduces CuO impurities and dissociative Cu defects, the ratio of dissociative Cu defects decreased from 11.3% to 3.1%, which are responsible for the degradation of the electrical properties of Cu₂O TFTs. The fabricated Cu₂O TFTs were showed the field effect mobility of 1.19 ± 0.06 cm²/Vs, the on/off current ratio of 1.09 ± 0.06 × 10⁵. And the Cu₂O TFT device exhibited a Vth shift of 3.31 V under negative bias stress (Vds = -10V, Vgs = -20V, 4000s). As a practical application, we demonstrated CMOS inverter with n-type IGZO TFT and p-type Cu₂O TFT and this device achieved a gain of 14 at V_DD = 20 V. Our new fabrication route showed that it was possible to simply fabricate a high-quality Cu₂O thin film, effectively improve the electrical properties of Cu₂O TFTs, and make low-power CMOS logic circuits based on metal oxide semiconductors.