High-Performance of InGaZnO TFTs With an Ultrathin 5-nm Al₂O₃ Gate Dielectric Enabled by a Novel Atomic Layer Deposition Method

Abstract

Al2O3, as one of the gate dielectric materials for thin film transistors (TFTs), has been extensively investigated because of its large bandgap, high breakdown field, and good thermal stability. However, the further development of Al2O3 thin films is limited by the presence of defects such as oxygen vacancies, self-interstitial atoms, or impurity elements. To overcome this obstacle, we have developed a novel method for fabricating Al2O3 thin films by using the atomic layer deposition (ALD) technique. This method replaces the conventional Trimethylaluminium (TMA)/H2O cycles with TMA/TMA/H2O/H2O cycles (referred to as ‘double cycles’), to deposit the Al2O3. The 5-nm ultrathin Al2O3 film showed a high areal capacitance of 660 nF/cm2 at 20 Hz, and a relatively low current density of 10−8 A/cm2 at 1 MV/cm. InGaZnO (IGZO) TFTs with ultrathin Al2O3 gate dielectric grown by double cycles exhibited outstanding performances, such as a near theoretical limit subthreshold swing (SS) of 70 mV/decade, a higher on/off current ratio $(I_{\mathrm{ on}} / I_{\mathrm{ off}}$ ) of 106, an increased field-effect mobility ( $\mu $ ) of 6.5 cm2/Vs, a lower threshold voltage $(V_{\mathrm{ th}})$ of 0.2 V, and a low operating voltage of 3 V. These results are superior to the IGZO TFTs with Al2O3 dielectrics deposited using the single TMA/H2O cycle. Therefore, the implementation of ‘double cycles’ in the fabrication of dielectrics through ALD demonstrates considerable potential for future application in low-power electronic devices.

Keywords

• Ultrathin Al₂O₃
• atomic layer deposition
• double cycles
• thin film transistor