Insight into influence of thermodynamic coefficients on transient negative capacitance in Zr-doped HfO2 ferroelectric capacitors

We study the influence of the thermodynamic coefficients on transient negative capacitance for the Zr-doped HfO₂ (HZO) ferroelectric capacitors by the theoretical simulation based on the Landau-Khalatnikov (L-K) theory and experimental measurement of electrical properties in the resistor-ferroelectric capacitor (R-FEC) circuit. Our results show that the thermodynamic coefficients α, β and γ also play a key role for the transient NC effect besides the viscosity coefficient and series resistor. Moreover, the smaller coefficients α and β, the more significant the transient NC effect. In addition, we also find that the thermodynamic process of transient NC does not obey the generally accepted viewpoint of Gibbs free energy minimization.     

Wake-up effect in Hf0.4Zr0.6O2 ferroelectric thin-film capacitors under a cycling electric field

We examined the wake-up effect in a TiN/Hf_0.4Zr_0.6O₂/TiN structure. The increased polarization was affected by the cumulative duration of a switched electric field and the single application time of the field during each switching cycle. The space-charge-limited current was stable, indicating that the trap density did not change during the wake-up. The effective charge density in the space-charge region was extracted from capacitance-voltage curves, which demonstrated an increase in free charges at the interface. Based on changing characteristics in these properties, the wake-up effect can be attributed to the redistribution of oxygen vacancies under the electric field.     

Improved performance of MoS2 FET by in situ NH3 doping in ALD Al2O3 dielectric

Since defects such as traps and oxygen vacancies exist in dielectrics, it is difficult to fabricate a high-performance MoS$_{2}$ field-effect transistor (FET) using atomic layer deposition (ALD) Al$_{2}$O$_{3}$ as the gate dielectric layer. In this paper, NH$_{3}$ in situ doping, a process treatment approach during ALD growth of Al$_{2}$O$_{3}$, is used to decrease these defects for better device characteristics. MoS$_{2}$ FET has been well fabricated with this technique and the effect of different NH$_{3}$ in situ doping sequences in the growth cycle has been investigated in detail. Compared with counterparts, those devices with NH$_{3}$ in situ doping demonstrate obvious performance enhancements: $I_{\rm on}/I_{\rm off}$ is improved by one order of magnitude, from $1.33\times 10^{5}$ to $3.56\times 10^{6}$, the threshold voltage shifts from $-0.74 $ V to $-0.12$ V and a small subthreshold swing of 105 mV/dec is achieved. The improved MoS$_{2}$ FET performance is attributed to nitrogen doping by the introduction of NH$_{3}$ during the Al$_{2}$O$_{3}$ ALD growth process, which leads to a reduction in the surface roughness of the dielectric layer and the repair of oxygen vacancies in the Al$_{2}$O$_{3}$ layer. Furthermore, the MoS$_{2}$ FET processed by in situ NH$_{3}$ doping after the Al and O precursor filling cycles demonstrates the best performance; this may be because the final NH$_{3}$ doping after film growth restores more oxygen vacancies to screen more charge scattering in the MoS$_{2}$ channel. The reported method provides a promising way to reduce charge scattering in carrier transport for high-performance MoS$_{2 }$ devices.     

Crystallization behaviors of ultrathin Al-doped HfO_2 amorphous films grown by atomic layer deposition

In this work, ultrathin pure HfO_2 and Al-doped HfO_2films(about 4-nm thick) are prepared by atomic layer deposition and the crystallinities of these films before and after annealing at temperatures ranging from 550℃ to 750℃ are analyzed by grazing incidence x-ray diffraction. The as-deposited pure HfO_2 and Al-doped HfO_2 films are both amorphous. After550-℃ annealing, a multiphase consisting of a few orthorhombic, monoclinic and tetragonal phases can be observed in the pure HfO_2 film while the Al-doped HfO_2 film remains amorphous. After annealing at 650℃ and above, a great number of HfO_2 tetragonal phases, a high-temperature phase with higher dielectric constant, can be stabilized in the Al-doped HfO_2 film. As a result, the dielectric constant is enhanced up to about 35. The physical mechanism of the phase transition behavior is discussed from the viewpoint of thermodynamics and kinetics.     

Growth mechanism of atomic-layer-deposited Ti Al C metal gate based on TiCl_4 and TMA precursors

TiAlC metal gate for the metal-oxide-semiconductor field-effect-transistor(MOSFET) is grown by the atomic layer deposition method using TiCl_4 and Al(CH_3)_3(TMA) as precursors. It is found that the major product of the TiCl_4 and TMA reaction is TiAlC, and the components of C and Al are found to increase with higher growth temperature. The reaction mechanism is investigated by using x-ray photoemission spectroscopy(XPS), Fourier transform infrared spectroscopy(FTIR), and scanning electron microscope(SEM). The reaction mechanism is as follows. Ti is generated through the reduction of TiCl_4 by TMA. The reductive behavior of TMA involves the formation of ethane. The Ti from the reduction of TiCl_4 by TMA reacts with ethane easily forming heterogenetic TiCH_2, TiCH=CH_2 and TiC fragments. In addition,TMA thermally decomposes, driving Al into the Ti C film and leading to TiAlC formation. With the growth temperature increasing, TMA decomposes more severely, resulting in more C and Al in the TiAlC film. Thus, the film composition can be controlled by the growth temperature to a certain extent.