Negative capacitance MgZnO thin-film transistor on glass
Creators: name: Yu, Fangzhou
Publisher: name: No Publisher Supplied
Year: 2021

description: Tradeoff between switching performance and power dissipation is a major challenge in the evolution of integrated circuits (IC). In order to reduce the power consumption of modern electronics, lower operating voltage is needed. However, the Boltzmann limit sets a thermionic bottleneck of the subthreshold swing (SS) value as 60 mV/dec at 300 K in the conventional field-effect transistor (FET); therefore, prevents further lowering of the operating voltage and overall power consumption. Negative capacitance field-effect transistor (NC-FET) has been emerging as a promising device for reducing the SS value below 60 mV/dec. The negative capacitance (NC) effect was observed with ferroelectric layer inserted into the regular gate dielectric stack of a FET. The internal voltage at the ferroelectric-dielectric interface could exceed the applied gate voltage, which accelerates the turn-on behavior of the transistor and hence reduces SS value even below the room temperature Boltzmann limit. This dissertation focuses on the feasibility study of the oxide based negative capacitance thin-film transistor (NC-TFT) on glass substrates. Currently, NC transistors have used ferroelectric (FE) materials including P(VDF-TrFE), AlInN/AlN, Pb(Zr,Ti)O3, and doped HfO2. These devices using the FE materials were built on single crystal substrates; therefore, they are not suitable for low-cost and large-area electronic systems. Furthermore, the previously reported NC-FET devices were opaque due to the materials used, including semiconductor channel, metal contacts, or substrate that would limit the possibly for its application in transparent electronics.The oxide thin-film transistor (TFT) on glass technology focuses on application of large-area and cost-effective electronics, which also desire for low power consumption with steep switching characteristics. ZnO and its compounds, including MgxZn1-xO (MZO) are promising for transparent electronic devices due to its high mobility and transparency in the visible light spectrum, and multifunctional properties through doping. It can also be grown at relatively low temperature on various substrates including glass and plastics. However, TFT made up of the pure ZnO suffers from instability issue due to high defect density. We have used the Mg doped ZnO (MZO) as TFT channel to improve device performances, particularly the thermal, threshold, and negative biasing stability.Ferroelectricity can be induced by doping Mg into ZnO (FE-MZO), where local dipole moments are resulted from the structural distortion. Ni dopants are also introduced into MZO to compensate the residual carriers, therefore, improves ferroelectricity. The resulted Ni0.02Mg0.15Zn0.83O (NMZO) film possesses the preferred c-axis orientation, evidenced by x-ray diffraction (XRD). The comprehensive characterizations were conducted to verify the room temperature ferroelectricity of NMZO, including polarization-electric field (P-E) relationship, effects of temperature and frequency variation on dielectric constant, and NC effect voltage amplification.Ferroelectric NMZO layer was then integrated into the gate dielectric stack (SiO2/NMZO) to form the MZO NC-TFT on glass. The NC effect was observed, and SS value was significantly reduced to 52 mV/dec while keeping the normally-off operation and the high on/off current ratio over 109. The device fabrication process was further improved by decreasing the channel deposition temperature from 400 °C to 125 °C using RF sputtering technology.The feasibility of negative capacitance transparent thin-film transistor (NC-TTFT) was also demonstrated by replacing the source and drain metal contact schemes with Al-doped ZnO (AZO), which is the transparent conductive oxide (TCO) electrode. The NC-TTFT on glass is highly transparent (91%) in the visible range. Using pulsed I-V characterization, an independent test result was obtained, and the minimum SS value was measured as 17 mV/dec.The oxide NC-TFT and NC-TTFT on glass use ZnO-based material system with different functions: a semiconductor MZO as channel, a ferroelectric NMZO in the gate dielectric stack, and an AZO as transparent conductive oxide electrodes for source/drain contact. It simplifies the fabrication process and minimizes contamination; thus, reduces cost. The NC-TTFT on glass technology shows promising potential for large-area and transparent electronics applications, such as emerging augmented reality (AR) smart glasses.