Ferroelectric-like Behavior Originating from Oxygen Vacancy Dipoles in Amorphous Film for Non-volatile Memory

Traditional ferroelectric devices suffer a lack of scalability. Doped HfO2 thin film is promising to solve the scaling problem but challenged by high leakage current and uniformity concern by the polycrystalline nature. Stable ferroelectric-like behavior is firstly demonstrated in a 3.6-nm-thick amorphous Al2O3 film. The amorphous Al2O3 devices are highly scalable, which enable multi-gate non-volatile field-effect transistor (NVFET) with nanometer-scale fin pitch. It also possesses the advantages of low process temperature, high frequency (~GHz), wide memory window, and long endurance, suggesting great potential in VLSI systems. The switchable polarization (P) induced by the voltage-modulated oxygen vacancy dipoles is proposed.


Background
Ferroelectric random access memory (FeRAM) based on conventional perovskite ferroelectrics (e.g., PZT) has been one of the commercial non-volatile memories (NVMs) [1], although it cannot be scaled and not CMOS-compatible. Ferroelectricity was widely observed in a variety of different materials, such as porcine aortic walls [2], Sb 2 S 3 nanowires [3], GaFeO 3 film [4], doped poly-HfO 2 films [5], nanocrystalline hydroxyapatite films [6], and LaAlO 3 -SrTiO 3 film [7]. Among these materials, doped-HfO 2 films have attracted special interests for the NVM application due to their CMOS process compatibility. But the polycrystalline structure is inevitable to generate ferroelectricity in doped-HfO 2 , which brought obstacles for device application to overcome as follows: 1) it is incompatible with the gate-last processing with regard to the thermal budget of 500°C required to form orthorhombic crystal phases [8]; 2) power consumption is induced from undesired leakage current along the grain boundaries, which increases exponentially along with the scaling down of ferroelectric thickness. Recently, a theoretical study proposed that the additional ferroelectricity in thick poly-HfO 2 (>5 nm) can come from the long-range correlations in the assembly of electric dipoles created by oxygen vacancies [9]. The defect charge trapping/detrapping mechanism was observed to produce the ferroelectric-like behavior in a 5-nm-thick amorphous Al 2 O 3 for a multi-state memory, which, however, suffers from a very low trapping/detrapping frequency (e.g.,~500 Hz) [10].
In this work, stable ferroelectric-like behavior is demonstrated in a 3.6-nm-thick amorphous Al 2 O 3 film, where the switchable polarization (P) is proposed to be induced by the voltage-modulated oxygen vacancy dipoles. The amorphous Al 2 O 3 film possesses the advantages of low process temperature and the operating frequency up to~GHz, which enable multi-gate nonvolatile field-effect transistor (NVFET) with nanometerscale fin pitch. Al 2 O 3 NVFET memory with a 100-ns pulse width program/erase (P/E) voltages and over 10 6 P/E cycles endurance is demonstrated. The effects of electrodes and film thickness on the P in Al 2 O 3 capacitors are also investigated. The amorphous non-volatile devices show a promising future in VLSI memories.

Methods
Amorphous Al 2 O 3 films were grown on Si(001), Ge(001), and TaN/Si substrates by atomic layer deposition (ALD). TMA and H 2 O vapor were used as the precursors of Al and O, respectively. During the deposition, the substrate temperature was maintained at 300°C. Different top metal electrodes, including TaN/Ti, TaN, and W, were deposited on Al 2 O 3 surfaces by reactive sputtering. Capacitors with different electrodes were fabricated by lithography patterning and dry etching. Rapid thermal annealing (RTA) at 350°C for 30 s was performed. NVFETs with TaN/Al 2 O 3 gate stack were fabricated on Ge(001). After gate formation, source/drain (S/ D) regions were implanted by BF 2 + with a dose of 1 × 10 15 cm -2 and an energy of 20 keV, and 20 nm-thick nickel S/D metal electrodes were then formed by lift-off process. Figure 1a and b shows the schematics of the fabricated Al 2 O 3 capacitor and the p-channel NVFET. There is an interfacial layer (IL) between the electrode and the Al 2 O 3 film. Figure 1c and d show the highresolution transmission electron microscope (HRTEM) images of the TaN/Al 2 O 3 /Ge stacks with different amorphous Al 2 O 3 thicknesses (t AlO ) after an RTA at 350°C . Figure 2 shows the measured P vs. voltage V characteristics for the amorphous Al 2 O 3 capacitors with different t AlO and various top and bottom electrodes. The measurement frequency is 1 kHz. As shown in Fig. 2a-c, with a fixed 3.6 nm of t AlO , TaN/Al 2 O 3 /Ge capacitor achieves a higher saturation P (P sat ) compared to the devices with TaN/Ti and W top electrodes. The ferroelectric-like behavior is strongly correlated with interfaces, and it is proposed that the formation of TaAlO x IL between TaN and Al 2 O 3 produces more oxygen vacancies, contributing to a stronger switching P, compared to the TiAlO x and WAlO x ILs. P-V curves in Fig. 2d indicate that TaN/Al 2 O 3 /TaN capacitor has a much higher P sat in comparison with TaN/ Al 2 O 3 /Ge, which is attributed to the fact that dual TaAlO x ILs provide higher oxygen vacancy concentration. While P sat is significantly lower from that with Si bottom electrode (Fig. 2e), compared with the Ge electrode. This result indicates that Al 2 O 3 /Si interface quality is better, i.e., fewer oxygen vacancies, compared to that from the device based on Ge substrate. Figure 2f shows the P-V curves of a TaN/Al 2 O 3 (6 nm)/Ge capacitor, exhibiting a higher V c and an almost identical P sat as compared to that from the device with 3.6 nm of Al 2 O 3 film in Fig. 2b. It is noted that the reason for the unclosed P-V loops is because a leakage indeed exists. It was reported that the large offset at an electric field of zero always occurred with a large field, and it always disappeared gradually with the smaller sweeping range of V [11,12]. Figure 2g and h show the extracted evolution of the positive and negative remnant P (P r ) and coercive V (V c ) values, respectively, over 10 4 sweeping cycles for a TaN/ Al 2 O 3 /Ge capacitor. No wake-up, imprint, or fatigue effect is observed. V c of the device is~1.8 V, indicating that the E in the Al 2 O 3 film is 4~6 MV/cm and in the ILs can exceed 8 MV/cm, which is high enough to drive the oxygen vacancies [13,14]. P sat of the devices ranges from 1 to 5 μC/cm 2 , corresponding to a reasonable oxygen vacancy concentration in the range 3~15×10 12 cm -2 assuming they have charge of plus two. The underlying mechanism for ferroelectric-like behavior associated with oxygen vacancies in Al 2 O 3 devices is discussed. The migration of the voltage-driven oxygen vacancies has been widely demonstrated in resistive random-access memory devices [15,16]. Figure 3 shows the schematics of the switchable P in TaN/Al 2 O 3 /Ge, which originates from the segregation of voltagemodulated oxygen vacancies and negative charges to form the electrical dipoles. It is reasonable to infer that the movable oxygen vacancies mainly arise from the formation of TaAlO x IL and are located in the vicinity of the top interface at the initial state (Fig. 3a). Figure 3b and c indicate how the positive and negative P are formed, respectively, with the modulation of the oxygen vacancy and negative charge dipoles under the applied voltage. X-ray photoelectron spectra (XPS) of Al 2 O 3 /Ge and (Ti, TaN, and W)/Al 2 O 3 /Ge samples are measured and shown in Fig. 4). For all the metal/Al 2 O 3 /Ge samples, there is a metal oxide IL formed between metal and Al 2 O 3 , which are proposed to be the reservoir of oxygen ions and vacancies, which is consistent with Ref. [17].

Results and Discussion
To characterize the electrical performance of Al 2 O 3 NVFET as NVM, program (erase) operation is achieved by applying positive (negative) voltage pulses to the gate, to raise (lower) its threshold voltage (V TH ). Figure 5a shows how the linear-region transfer characteristics of the Al 2 O 3 NVFET shift relative to the initial I DS -V GS curve measured with ±4 V program (erase) voltages with 100 ns pulse width. Here, V TH is defined as a V GS at 100 nA⋅W/L, and MW is defined as the maximum change in   V TH . The Al 2 O 3 NVFET obtains an MW of 0.44 V, though amorphous Al 2 O 3 film has smaller P r than the reported doped HfO 2 films [5,8]. It is noted that the high operating frequency up to 10 MHz of Al 2 O 3 NVFET memory, which is indicative of that switchable P in Al 2 O 3 originates from the migration of voltage-driven oxygen vacancy to form dipoles, not from defects charge trapping/detrapping. Alternating program and erase pulses were applied to the Al 2 O 3 devices to further study the device endurance. Figure 5b shows the plots of V TH vs. P/E cycle number, suggesting a stable MW can be maintained without a significant degradation over 10 6 P/ E cycles for a 3.6-nm-thick Al 2 O 3 NVFET. Notably, the ferroelectric-like behavior observed in the amorphous Al 2 O 3 devices can be extended to the universal amorphous oxides, e.g., hafnium oxide (HfO 2 ) and zirconium oxide (ZrO 2 ).

Conclusions
Stable ferroelectric-like behavior is first realized in capacitors with a thin amorphous Al 2 O 3 insulator. Switchable P in amorphous Al 2 O 3 capacitors is demonstrated by P-V loops and NVFET test. The ferroelectric-like behavior is proposed to be originating from the interface oxygen vacancies and ions dipoles. The 3.6-nm-thick Al 2 O 3 NVFET achieves an MW of 0.44 V and over 10 6 cycle endurance under ±4 V at 100 ns P/E condition. All in all, this work opened a new world for amorphous oxide ferroelectric devices, which are promising for multi-gate (fin-shaped, nanowire, or nanosheet) NVFETs with potentially nano-scaled fin pitch in VLSI systems. Authors' Contributions YP carried out the experiments and drafted the manuscript. GQH, YP, and WWX designed the experiments. FNL, NZ, and CGD helped to measure the device. GQH, YL, ZF, and HD helped to revise the manuscript. YH supported the study. The authors read and approved the final manuscript.

Funding
The authors acknowledge the support from the National Key Research and Development Project (Grant Nos. 2018YFB2202800 and 2018YFB2200500) and the National Natural Science Foundation of China (Grant Nos. 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.