Ultrathin ferroic HfO2-ZrO2 superlattice gate stack for advanced transistors

With the scaling of lateral dimensions in advanced transistors, an increased gate capacitance is desirable both to retain the control of the gate electrode over the channel and to reduce the operating voltage¹. This led to a fundamental change in the gate stack in 2008, the incorporation of high-dielectric-constant HfO₂ (ref. ²), which remains the material of choice to date. Here we report HfO₂-ZrO₂ superlattice heterostructures as a gate stack, stabilized with mixed ferroelectric-antiferroelectric order, directly integrated onto Si transistors, and scaled down to approximately 20 ångströms, the same gate oxide thickness required for high-performance transistors. The overall equivalent oxide thickness in metal-oxide-semiconductor capacitors is equivalent to an effective SiO₂ thickness of approximately 6.5 ångströms. Such a low effective oxide thickness and the resulting large capacitance cannot be achieved in conventional HfO₂-based high-dielectric-constant gate stacks without scavenging the interfacial SiO₂, which has adverse effects on the electron transport and gate leakage current³. Accordingly, our gate stacks, which do not require such scavenging, provide substantially lower leakage current and no mobility degradation. This work demonstrates that ultrathin ferroic HfO₂-ZrO₂ multilayers, stabilized with competing ferroelectric-antiferroelectric order in the two-nanometre-thickness regime, provide a path towards advanced gate oxide stacks in electronic devices beyond conventional HfO₂-based high-dielectric-constant materials.

Accurate Threshold Voltage Reliability Evaluation of Thin Al2O3 Top-Gated Dielectric Black Phosphorous FETs Using Ultrafast Measurement Pulses

Few-layer black phosphorus (BP) has attracted significant interest in recent years due to electrical and photonic properties that are far superior to those of other two-dimensional layered semiconductors. The study of long term electrical stability and reliability of black phosphorus field effect transistors (BP-FETs) with technologically relevant thin, and device-selective, gate dielectrics, stressed under realistic (closer to operation) bias and measured using state-of-the-art ultrafast reliability characterization techniques, is essential for their qualification and use in different applications. In this work, air-stable BP-FETs with a thin top-gated dielectric (15 nm Al₂O₃, SiO₂ equivalent thickness of 5 nm) were fabricated and comprehensively characterized for threshold voltage ( V_th) instability under negative gate bias stress at various measurement delays ( t_m), stress biases ( V_GSTR), temperatures ( T), and stress times ( t_str) for the first time. Thin top-gated oxide enables low V_GSTR that is closer to the operating condition and ultrafast V_th measurements with low delay ( t_m = 10 μs, due to high drain current) that ensure minimal recovery. The resultant time kinetics of V_th degradation (Δ V_th) shows fast saturation at longer stress times and low-temperature activation energy. V_th instability in these top-gated devices is suggested to be dominated by hole trapping, which is modeled using first-order equations at different V_GSTR and T. It is shown that measurements using larger t_m show lower degradation magnitude that do not saturate due to recovery artifacts and give inaccurate estimation of hole trap densities. Conventional, thick, and global back-gated oxide BP-FETs were also fabricated and characterized for varying t_m (1 ms being the lowest due to a low drain current level for thick oxide), V_GSTR, and T to benchmark our top-gated results. Nonsaturating Δ V_th in the back-gated devices is shown to result from recovery artifacts due to the large t_m (1 ms and greater) values. Finally, using a V_GSTR and T-dependent first-order model, we show that the top-gated Al₂O₃ BP-FETs with scaled gate oxide thickness can match state-of-the-art Si reliability specifications at operating voltage and room/elevated temperature.

Energy auditing and carbon footprint under long-term conservation agriculture-based intensive maize systems with diverse inorganic nitrogen management options

A greater energy grant in diesel-fed machinery driven farming substantiate the higher GHGs emission along with improper input (fertilizer, pesticide and irrigation) use and intensive soil management. Practicing conservation tillage, residue retention and diversified crop rotations were advocated because of their multiple benefits. Hence we explored the energy requirement and carbon footprint of conservation agriculture (CA) based maize production systems. Coated N fertilizer [sulphur coated urea (SCU) and neem coated urea (NCU)] were compared with unfertilized and uncoated prilled urea (PU) in the scenario of with and without residue retention on permanent beds (PB) under diversified maize systems [MMuMb, maize-mustard-mungbean and MWMb, maize-wheat-mungbean] in search of a sustainable and energy efficient production system with lesser C-footprint. Results of the 4-year study showed that crops planted on permanent bed with crop residue (PB+R) registered 11.7% increase in system productivity compared to PB without residue (PB-R). N management through Neem coated urea (NCU) recorded 2.3 and 10.9% higher system productivity compared with non-coated prilled urea plot under PB-R and PB+R, respectively. MMuMb was marginally superior than MWMb system in terms of cropping sequence yield, profitability, and energy and carbon use efficiency. Crop residue retention in zero tilled PB increased cost of cultivation by 125 and 147 USD/ha in MMuMb and MWMb systems, respectively. The quantified carbon footprint value was higher in MWMb system. In CA-based practices, crop residues management contributed the highest energy input (61.5-68.4%) followed by fertilizer application (17-20%). Among N management practices, neem coated urea (NCU) significantly improved system productivity and profitability in all the residue applied plots compared to un-fertilized and prilled urea (PU) applied plots. Similarly, higher energy output was also observed in NCU treated plots. However, carbon footprint value was higher in PU (268-285 CO₂-e kg/Mg) plots than NCU (259-264 CO₂-e kg/Mg) treated plots. Thus, the study supports and recommends that the CA-based MMuMb system with efficient N management through NCU is an environmentally safe, clean and energy efficient one, hence can reduce carbon footprint, will ensure food security and will mitigate climate change.