ZrO₂ Ferroelectric Field-Effect Transistors Enabled by the Switchable Oxygen Vacancy Dipoles

Abstract

This paper investigates the impacts of post-rapid thermal anneal (RTA) and thickness of ZrO₂ on the polarization P and electrical characteristics of TaN/ZrO₂/Ge capacitors and FeFETs, respectively. After the RTA ranging from 350 to 500 °C, TaN/ZrO₂/Ge capacitors with 2.5 and 4 nm-thick amorphous ZrO₂ film exhibit the stable P. It is proposed that the ferroelectric behavior originates from the migration of the voltage-driven dipoles formed by the oxygen vacancies and negative charges. FeFETs with 2.5 nm, 4 nm, and 9 nm ZrO₂ demonstrate the decent memory window (MW) with 100 ns program/erase pulses. A 4-nm-thick ZrO₂ FeFET has significantly improved fatigue and retention characteristics compared to devices with 2.5 nm and 9 nm ZrO₂. The retention performance of the ZrO₂ FeFET can be improved with the increase of the RTA temperature. An MW of ~ 0.46 V is extrapolated to be maintained over 10 years for the device with 4 nm ZrO₂.

Background

Doped poly-HfO₂ ferroelectric field-effect transistors (FeFETs) have attracted considerable interest in the non-volatile memory (NVM) applications due to their CMOS process compatibility [1]. Although the decent electrical performance has been demonstrated in doped HfO₂-based FeFETs [2], some fundamental limitations still plague their practical applications, including the high thermal budget of 500 °C annealing required to form orthorhombic crystal phases and the undesired leakage current along the grain boundaries with the scaling down of ferroelectric thickness. Ferroelectricity has been widely observed in a variety of different materials, e.g., Sb₂S₃ nanowires [3], GaFeO₃ film [4], LaAlO₃-SrTiO₃ film [5], and amorphous Al₂O₃ containing nanocrystals [6, 7]. Recently, we reported the FeFETs with partially crystallized ZrO₂ gate insulator functioning as NVM and analog synapse [8]. Although the ZrO₂ transistors exhibited decent electrical performance with the thinner thickness compared to the reported doped HfO₂, the underlying mechanism for the ferroelectricity in ZrO₂ film remains unclear. It is critical and important to elucidate the origin of the switchable polarization P for evaluating the performance limit of ZrO₂ FeFETs.

In this work, TaN/ZrO₂/Ge FeFETs with 2.5 nm, 4 nm, and 9 nm-thick insulators are fabricated. The switchable P in TaN/ZrO₂/Ge capacitor is proposed to originate from the migration of voltage-driven oxygen vacancies and negative charges. The impacts of ZrO₂ thickness and the post-rapid thermal annealing (RTA) on the P of TaN/ZrO₂/Ge and the memory window (MW), endurance, and retention characteristics of FeFETs are investigated.

Methods

FeFETs with ZrO₂ gate insulator were fabricated on 4-in. n-Ge(001) substrate using a similar process in [8, 9]. After the pre-gate cleaning in the diluted HF (1:50) solution, Ge wafers were loaded into an atomic layer deposition (ALD) chamber. ZrO₂ films with thicknesses of 2.5 nm, 4 nm, and 9 nm were deposited at 250 °C using TDMAZr and H₂O as precursors of Zr and O, respectively. A 100-nm-thick TaN gate electrode was deposited by reactive sputtering. After the gate electrode formation, the source/drain (S/D) regions were implanted by BF₂⁺ at a dose of 1 × 10¹⁵ cm⁻² and an energy of 20 keV. A total of 15 nm nickel (Ni) S/D contacts were formed by a lift-off process. Finally, the RTA at 350, 450, and 500 °C for 30 s was carried out.

Figure 1 a shows the schematic of the fabricated transistor. Figure 1b–d shows the transmission electron microscope (TEM) images of the TaN/ZrO₂/Ge samples with 2.5, 4, and 9 nm-thick ZrO₂, respectively. All the samples underwent an RTA at 500 °C for 30 s. The 2.5 nm ZrO₂ sample remains an insulator film after the annealing. For the 4 nm sample, although some nanocrystals are observed, ZrO₂ maintains to be an amorphous layer. While full crystallization occurs for the 9 nm ZrO₂ film. Notably, an interfacial layer (IL) of GeO_x exists between the ZrO₂ and Ge channel region, although it is too thin to be observed in the TEM images.

Fig. 1.

a Schematic of the fabricated TaN/ZrO₂/Ge FeFET. b, c, and d HRTEM images of the TaN/ZrO₂/Ge stacks with different ZrO₂ thicknesses. The samples underwent an RTA at 500 °C for 30 s

Results and Discussion

Figure 2 shows the P vs. voltage (V) curves for the TaN/ZrO₂/Ge capacitors with different ZrO₂ thicknesses and different annealing temperatures. The solid lines with different colors represent the minor loops with various sweeping voltage range (V_range). The measurement frequency is 1 kHz. The 2.5 nm and 4 nm ZrO₂ devices can exhibit stable ferroelectricity after an RTA at 350 °C. Figure 3 plots the remnant P (P_r) as a function of the sweeping V range curves for the capacitors annealed at various temperatures.

Fig. 2.

Measured P vs. V characteristics of the TaN/ZrO₂/Ge capacitors with different ZrO₂ thicknesses and various annealing temperatures

Fig. 3.

Comparison of P_max as a function of V_range for the TaN/ZrO₂/Ge capacitors with different ZrO₂ thicknesses and various annealing temperatures

Figure 3 shows the comparison of P_max as a function of V_range for the TaN/ZrO₂/Ge capacitors with the different ZrO₂ thicknesses and the various RTA temperatures. For the 4 nm ZrO₂ devices, as the annealing temperature increases from 350 to 450 °C, a larger V_range is required to obtain a fixed P_max. This is attributed to the fact that the higher annealing temperature produces the thicker interfacial layers (ILs) between at Ge/ZrO₂ and ZrO₂/TaN interfaces, leading to a larger unified capacitance equivalent thickness (CET). For the 2.5 nm ZrO₂ capacitors, the sample with 500 °C annealing has a lower V_range than does the 350 °C annealing sample with the same P_max. Although the ILs get thicker with the increased RTA temperature, some ZrO₂ was consumed by the oxygen scavenging and interdiffusion at the interface. For the very thin ZrO₂ device, the latter is dominant. Compared to the 2.5 nm ZrO₂ capacitor, a much larger V_range is required to achieve a similar P_max. However, the 9 nm ZrO₂ capacitor does not exhibit the higher V_range in comparison with the 4 nm device. This is due to the crystal ZrO₂ that has a much higher κ value than does the amorphous film, which significantly reduces the CET of the 9 nm device.

Figure 4a shows the extracted evolution of the positive and negative P_r, denoted by $P_{\mathrm{r}}^{+}$and $P_{\mathrm{r}}^{-}$, respectively, for the 4 nm-thick ZrO₂ capacitors with RTA at different temperatures over 10⁶ sweeping cycles measured at 1 kHz. Devices annealed at 350 °C and 450 °C exhibit the obvious wake-up effect. No wake up or imprint is observed for the 4 nm ZrO₂ ferroelectric capacitor underwent annealing at 500 °C. Figure 4b compares the P_r as a function of sweeping cycles for the devices with the different ZrO₂ thicknesses. The 4 nm ZrO₂ ferroelectric capacitor achieves improved stability of P_r endurance compared to the 2.5 nm and 9 nm devices during the 10⁶ endurance test.

Fig. 4.

aP_r vs. the number of ms-pulse sweeping cycles for 4 nm ZrO₂ capacitors with different RTA temperatures. bP_r vs. number of ms-pulse sweeping cycles for the ZrO₂ capacitors after annealing at 500 °C

The switching P is observed in amorphous ZrO₂ capacitance, and it is inferred that the mechanism must be different from that of the reported doped poly-HfO₂ ferroelectric films. We propose that the underlying mechanism for ferroelectric behavior is related to the oxygen vacancy dipoles. It is well known that, as TaN metal deposited, the Ta oxygen scavenger layers will increase the oxygen vacancy concentration inside ZrO₂ [10]. Oxygen vacancies also appear at the ZrO₂/Ge interface. Figure 5 shows the schematics of the switchable P in TaN/ZrO₂/Ge originating from the migration of oxygen vacancies and negative charges to form the positive and negative dipoles. It is speculated that the negative charges in ZrO₂ are related to the Zr vacancy [11], which is similar to those in Al₂O₃ film [12]. The migration of the voltage-driven oxygen vacancies has been widely demonstrated in resistive random-access memory devices [13, 14]. Notably, this is the first demonstration of three-terminal non-volatile transistors dominated by the voltage-driven oxygen vacancies.

Fig. 5.

Schematics of the mechanism for switchable P in ZrO₂ capacitors, which is attributed to the migration of voltage-driven oxygen vacancies and negative charges to form dipoles

The P-V hysteresis enables the ZrO₂ FeFETs to obtain a large and stable MW for the embedded NVM (eNVM) applications. Figure 6 shows the measured I_DS-V_GS curves of 2.5, 4, and 9 nm ZrO₂ FeFETs for the two polarization states with 1 μs program/erase (P/E) conditions. The transistors were annealed at 500 °C. Program (erase) operation is achieved by applying positive (negative) voltage pulses to the gate of the ZrO₂ FeFETs, to raise (lower) its threshold voltage (V_TH). V_TH is defined as V_GS at 100 nA·W/L, and MW is defined as the maximum change in V_TH. All the FeFETs with various ZrO₂ thicknesses have the MW above 1 V with 1 μs P/E pulses. To achieve a similar MW, a higher erase voltage is needed for the 9 nm ZrO₂ FeFET compared to the other two transistors. It is seen that a larger magnitude erase V_GS is required to obtain the roughly equal shift of I-V relative to the initial curve compared to the program V_GS. It is speculated that the oxygen vacancies contributing to the P mainly come from the reaction between TaN and ZrO₂, like the initial state of the device in Fig. 5a. As a positive V_GS (program) is applied, the oxygen vacancies diffuse and accumulate in the layer near the ZrO₂/Ge interface (Fig. 5b), where the distribution of the oxygen vacancy dipoles is quite different from the initial state. So it is easy to shift the I-V curve to a higher |V_TH| with a positive V_GS. However, as a negative V_GS (erase) is applied, the back diffusion of oxygen vacancies brings the gate stack back to its original state (Fig. 5c). So the magnitude of the negative erase V_GS has to be increased to achieve the equivalent shift of I-V to the positive program V_GS.

Fig. 6.

Measured I_DS-V_GS curves of the 2.5, 4, and 9 nm-thick ZrO₂ FeFETs for the initial and two polarization states with 1 μs P/E pulses

As the P/E pulse width is reduced to 100 ns, the ZrO₂ FeFETs still demonstrate the decent MW, as shown in Fig. 7a. Especially, the transistor with 2.5 nm ZrO₂ annealed at 350 °C achieves an MW of 0.28 V. Figure 7b plots MW vs. cycle number for the FeFETs with various ZrO₂ thicknesses with 100 ns P/E pulse condition. The 4 nm ZrO₂ device achieves a significantly improved endurance performance compared to the 2.5 nm and 9 nm ZrO₂ FeFETs, which exhibit the obvious wake-up effect and fatigue within 10³ cycles.

Fig. 7.

aI_DS-V_GS curves of the 2.5, 4, and 9 nm-thick ZrO₂ FeFETs for the two polarization states with 100 ns P/E pulses. The devices underwent an RTA at 500 °C. b FeFET with 4 nm ZrO₂ has an improved endurance compared to the 2.5 and 9 nm ZrO₂ transistors

Finally, the retention testing of the ZrO₂ FeFETs is characterized and shown in Figs. 8 and 9. Figure 8 a shows the evolution of I_DS-V_GS curves for the two polarization states of the 4 nm ZrO₂ FeFETs underwent RTA at 350, 450, and 500 °C. The charge trapping leads to the reduction of the devices with the time. As shown in Fig. 8b, the retention performance of the devices can be improved with the increase of the RTA temperature. An MW of ~ 0.46 V is extrapolated to be maintained over 10 years. Figure 9 compares the retention characteristics of the FeFETs with different ZrO₂ thicknesses. The 4 nm ZrO₂ device has an improved retention performance compared to the transistors with 2.5 and 9 nm-thick ZrO₂.

Fig. 8.

a The evolution of I_DS-V_GS curves for the two polarization states of the 4 nm ZrO₂ FeFETs with different RTA temperatures. b The 4 nm ZrO₂ device annealed at 500 °C has a much better retention performance compared to the transistors with RTA at the lower temperatures

Fig. 9.

a The evolution of I_DS-V_GS curves for the two polarization states for the 2.5, 4, and 9 nm-thick ZrO₂ FeFETs underwent a RTA at 500 °C. b The 4 nm ZrO₂ device has an improved retention performance compared to the transistors with 2.5 and 9 nm-thick ZrO₂

Conclusions

In summary, amorphous ZrO₂ ferroelectric capacitors are experimentally demonstrated, and the ferroelectricity is speculated to be due to the migration of the voltage-driven dipoles formed by the oxygen vacancies and negative charges. FeFETs with 2.5 nm, 4 nm, and 9 nm ZrO₂ have the MW above 1 V with 1 μs P/E pulses. The improved fatigue and retention characteristics are obtained in the 4 nm-thick ZrO₂ FeFET in comparison with the devices with 2.5 nm and 9 nm ZrO₂. The retention test indicates that the 4 nm ZrO₂ transistor keeps an extrapolated 10-year MW of ~ 0.46 V.

Abbreviations

RTA

Rapid thermal anneal

IL

Interfacial layer

TaN

Tantalum nitride

FeFET

Ferroelectric field-effect transistors

TDMAZr

Tetrakis (dimethylamido) zirconium

Ge

Germanium

ZrO₂

Zirconium dioxide

ALD

Atomic layer deposition

HF

Hydrofluoric acid

BF₂⁺

Boron fluoride ion

MW

Memory window

NVM

Non-volatile memory

P_r

Remnant polarization

TEM

Transmission electron microscope

Ni

Nickel

P_max

Maximum polarization

RTA

Repaid thermal annealing

V_range

Sweeping voltage range

Authors’ Contributions

HL and YP carried out the experiments and drafted the manuscript. GQH and YL supported the study and helped to revise the manuscript. NZ and CGD helped to do the polarization measurement. YH provided constructive advice in the drafting. All the authors read and approved the final manuscript.

Funding

The authors acknowledge the support from the National Key Research and Development Project (Grant No. 2018YFB2202800 and 2018YFB2200500) and the National Natural Science Foundation of China (Grant No. 91964202, 61534004, and 61874081).

Availability of Data and Materials

The datasets supporting the conclusions of this article are included in the article.

Competing Interests

The authors declare that they have no competing interests.