Thickness‐Dependent Asymmetric Potential Landscape and Polarization Relaxation in Ferroelectric Hf_xZr_1−xO₂ Thin Films through Interfacial Bound Charges

Abstract

As complementary metal-oxide-semiconductor (CMOS) scaling down to sub-10 nm node, emerging technology to reduce power consumption enforced by shortchannel effects is actively pursued. [1] At single transistor level, the most effective way is to reduce operating voltage (V DD ) and subthreshold slope (SS). [2] However, SS in metal-oxide-semiconductor fieldeffect transistors (MOSFETs) is limited to 60 mV dec −1 at room temperature due to thermionic emission of carriers in the Boltzmann tail. To this end, novel concepts are proposed to design steep-slope transistors, including tunnel FET (TFET), [3] impact ionization MOS (IMOS), [4] and nanoelectromechanical FET (NEM-FET). [5] While achieving steep slope, these new device concepts sacrifice other aspects. TFETs utilize the band-to-band tunneling current and suffer from low on-state current. The IMOS needs very large bias voltage to drive impact ionization and has stability issues. For NEM-FET, speed is limited to MHz due to the moving of heavy mass. Recently, negative capacitance (NC) effect in an FE/dielectric gate stack is proposed as a possible solution to break the 60 mV dec −1 limitation. [6][7][8][9][10][11][12] HfO 2 -based FE materials are particularly attractive because of: 1) excellent CMOS-compatibility by atomic layer deposition (ALD), [13,14] 2) scalability to ultrathin thickness, [15] 3) no sacrifice of I on , [8] and 4) potential GHz operation. [16] In the original Landau-Ginzburg-Devonshire (LGD) theory, a ferroelectric below its Curie temperature is described by a doublewell free energy landscape. The two degenerate energy minima define two stable spontaneous polarization states, which can be programmed by the application of electric field. The minima of energy states are separated by a metastable region where the second-order differential of energy versus polarization is negative, defining a region of negative capacitance. [6] It has been proposed to stabilize the metastable negative capacitance region by serially adding a dielectric (DE) with parabolic energy landscape, the so-called capacitance matching. [17,18] The Landau-Khalatnikov (L-K) model shows that such quasistatic NC can theoretically give rise to sub-60 mV dec −1 switching without hysteresis in logic devices. [19] However, so far, the experimental observation of negative capacitance still cannot be fully explained by L-K model. The negative capacitance (NC) effect in ferroelectric materials provides a possible solution to break the Boltzmann tyranny and realize steep-slope field-effect transistors (FETs) with sub-60 mV dec −1 subthreshold slope (SS). HfO 2 -based ferroelectrics (FE) have attracted great attention as dielectric candidates for NCFETs due to their excellent scalability and compatibility with complementary metal-oxide-semiconductor technology. However, understanding of the ferroelectric properties of HfO 2 -based FEs, especially at reduced thickness, is still at an early stage. The quasistatic polarization relaxation behavior of ferroelectric Hf x Zr 1−x O 2 (HZO) thin fil...

Intrinsic Nature of Negative Capacitance in Multidomain Hf0.5Zr0.5O2‐Based Ferroelectric/Dielectric Heterostructures

“…We also know that σ IF ≈ P s −7 µC cm −2 , which means that σ IF is even more negative than −7 µC cm −2 , because P s < 0. This is consistent with previous findings that suggest large negative charge densities of this order of magnitude at similar ferroelectric/dielectric interfaces, [ 26,34–37 ] which likely stem from trapped electrons.…”

“…For ferroelectric/dielectric heterostructures, both symmetric (Figure 2c) and asymmetric (Figure 2d) structures with identical Al 2 O 3 thicknesses were fabricated to investigate the effect of trapped and/or fixed charges at the HZO/Al 2 O 3 interface, which seem to play a key role in such capacitors. [ 25,26,33–35 ] In the as fabricated symmetric Al 2 O 3 /HZO/Al 2 O 3 capacitors, the electrostatic effect of similar charge densities at both interfaces should cancel out on average, which is not true for the asymmetric samples. The thickness of the individual layers was set by the number of ALD cycles and was determined from X‐ray reflectivity experiments.…”

Study on the phase transition dynamics of HfO2-based ferroelectric films under ultrafast electric pulse

“…For the origin of HfO 2 ferroelectricity, researchers generally accept that orthorhombic ferroelectric phase (Pca2 1 ) is derived from tetragonal phase (P4 2 /nmc), which may also transform into non-ferroelectric monoclinic phase (P2 1 /c) [22]. Thus, the ferroelectric orthorhombic phase and non-ferroelectric monoclinic phase generally coexist in the thin film [23][24][25]. Therefore, we set the tetragonal phase as the parent phase, the orthorhombic ferroelectric phase and the monoclinic phase as the sub-phases.…”

Polarization Switching and Correlated Phase Transitions in Fluorite‐Structure ZrO2 Nanocrystals

“…[5] Surface relaxation is a crucial issue in fluorite-based ultrathin film devices. [24] Figure 4a,b shows the polarization mapping of ZrO 2 nanocrystals at (001) and (100) surfaces at grain boundaries. The polarization is represented by the length of arrows, while the cyan and yellow arrows represent the polarization higher and lower than 35 μC cm −2 , respectively.…”