High-stability pH sensing with a few-layer MoS2 field-effect transistor.

Recently, molybdenum disulfide (MoS2), an emerging 2D material, has become an alternative candidate for ultra-sensitive biosensors due to its semiconducting behavior and the unique layer-by-layer atomic structure. Here, we report on highly stable and repeatable real-time pH sensing with few-layer MoS2 field-effect transistor (FET) biosensors, fabricated with both HfO2 and Al2O3/HfO2 gate dielectrics on top of MoS2 flakes exfoliated from natural crystals onto SiO2/Si samples. Both types of sensors demonstrate a highly linear, stable and repeatable response over a wide pH range with near-ideal pH sensitivity close to the theoretical limit of 59.6 mV pH-1. Ascribing from a different device operation regime in the pH sensing test-subthreshold regime for a sensor with an Al2O3/HfO2 dielectric and linear regime for a sensor with HfO2-a sensor with Al2O3/HfO2 shows significantly higher current sensitivity (∼105-fold) and relatively better linearity than a sensor with HfO2, while the latter shows relatively higher stability and higher repeatability. An Al2O3/HfO2-coated MoS2 FET reveals a high sensitivity or low detection limit of 0.01 pH.

A Comprehensive Study on the Effect of TiN Top and Bottom Electrodes on Atomic Layer Deposited Ferroelectric Hf0.5Zr0.5O2 Thin Films.

The discovery of ferroelectricity in HfO2-based materials in 2011 provided new research directions and opportunities. In particular, for atomic layer deposited Hf0.5Zr0.5O2 (HZO) films, it is possible to obtain homogenous thin films with satisfactory ferroelectric properties at a low thermal budget process. Based on experiment demonstrations over the past 10 years, it is well known that HZO films show excellent ferroelectricity when sandwiched between TiN top and bottom electrodes. This work reports a comprehensive study on the effect of TiN top and bottom electrodes on the ferroelectric properties of HZO thin films (10 nm). Investigations showed that during HZO crystallization, the TiN bottom electrode promoted ferroelectric phase formation (by oxygen scavenging) and the TiN top electrode inhibited non-ferroelectric phase formation (by stress-induced crystallization). In addition, it was confirmed that the TiN top and bottom electrodes acted as a barrier layer to hydrogen diffusion into the HZO thin film during annealing in a hydrogen-containing atmosphere. These features make the TiN electrodes a useful strategy for improving and preserving the ferroelectric properties of HZO thin films for next-generation memory applications.

Stress-Induced Crystallization of Thin Hf1- XZr XO2 Films: The Origin of Enhanced Energy Density with Minimized Energy Loss for Lead-Free Electrostatic Energy Storage Applications.

Increasing interest in the development of alternative energy storage technologies has led to efforts being taken to improve the energy density of dielectric capacitors with high power density. However, dielectric polymer materials still have low energy densities because of their low dielectric constant, whereas Pb-based materials are limited by environmental issues and regulations. Here, the energy storage behaviors of atomic layer-deposited Hf1- XZr XO2 ( X = 0-1) thin films (10 nm) and the phase transformation mechanism associated with an enhancement of their energy density are reported using unipolar pulse measurements. Based on electrical and material characterization, the energy density and energy efficiency are dependent on the Zr content, and stress-induced crystallization by the encapsulating Hf1- XZr XO2 films with TiN top electrodes prior to annealing can enhance the energy density (up to 47 J/cm3 at a small voltage value of 3.5 MV/cm) while minimizing energy loss even at low process temperatures (400 °C). This work will facilitate the realization of Hf1- XZr XO2-based capacitors for lead-free electrostatic energy storage applications.

Probing Interface Defects in Top-Gated MoS2 Transistors with Impedance Spectroscopy.

The electronic properties of the HfO2/MoS2 interface were investigated using multifrequency capacitance-voltage (C-V) and current-voltage characterization of top-gated MoS2 metal-oxide-semiconductor field effect transistors (MOSFETs). The analysis was performed on few layer (5-10) MoS2 MOSFETs fabricated using photolithographic patterning with 13 and 8 nm HfO2 gate oxide layers formed by atomic layer deposition after in-situ UV-O3 surface functionalization. The impedance response of the HfO2/MoS2 gate stack indicates the existence of specific defects at the interface, which exhibited either a frequency-dependent distortion similar to conventional Si MOSFETs with unpassivated silicon dangling bonds or a frequency dispersion over the entire voltage range corresponding to depletion of the HfO2/MoS2 surface, consistent with interface traps distributed over a range of energy levels. The interface defects density (Dit) was extracted from the C-V responses by the high-low frequency and the multiple-frequency extraction methods, where a Dit peak value of 1.2 × 1013 cm-2 eV-1 was extracted for a device (7-layer MoS2 and 13 nm HfO2) exhibiting a behavior approximating to a single trap response. The MoS2 MOSFET with 4-layer MoS2 and 8 nm HfO2 gave Dit values ranging from 2 × 1011 to 2 × 1013 cm-2 eV-1 across the energy range corresponding to depletion near the HfO2/MoS2 interface. The gate current was below 10-7 A/cm2 across the full bias sweep for both samples indicating continuous HfO2 films resulting from the combined UV ozone and HfO2 deposition process. The results demonstrated that impedance spectroscopy applied to relatively simple top-gated transistor test structures provides an approach to investigate electrically active defects at the HfO2/MoS2 interface and should be applicable to alternative TMD materials, surface treatments, and gate oxides as an interface defect metrology tool in the development of TMD-based MOSFETs.