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], Sb2S3 nanowires [3], GaFeO3 film [4], doped poly-HfO2 films [5], nanocrystalline hydroxyapatite films [6], and LaAlO3-SrTiO3 film [7]. Among these materials, doped-HfO2 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-HfO2, 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-HfO2 (>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 Al2O3 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 Al2O3 film, where the switchable polarization (P) is proposed to be induced by the voltage-modulated oxygen vacancy dipoles. The amorphous Al2O3 film possesses the advantages of low process temperature and the operating frequency up to ~GHz, which enable multi-gate non-volatile field-effect transistor (NVFET) with nanometer-scale fin pitch. Al2O3 NVFET memory with a 100-ns pulse width program/erase (P/E) voltages and over 106 P/E cycles endurance is demonstrated. The effects of electrodes and film thickness on the P in Al2O3 capacitors are also investigated. The amorphous non-volatile devices show a promising future in VLSI memories.

Methods

Amorphous Al2O3 films were grown on Si(001), Ge(001), and TaN/Si substrates by atomic layer deposition (ALD). TMA and H2O 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 Al2O3 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/Al2O3 gate stack were fabricated on Ge(001). After gate formation, source/drain (S/D) regions were implanted by BF2+ with a dose of 1 × 1015 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 Al2O3 capacitor and the p-channel NVFET. There is an interfacial layer (IL) between the electrode and the Al2O3 film. Figure 1c and d show the high-resolution transmission electron microscope (HRTEM) images of the TaN/Al2O3/Ge stacks with different amorphous Al2O3 thicknesses (tAlO) after an RTA at 350 °C.

Fig. 1

Schematics of the fabricated a Al2O3 capacitors with various electrodes and b Al2O3 NVFET. c and d HRTEM images of the fabricated TaN/Al2O3/Ge stacks with different tAlO, showing the amorphous Al2O3 films after an RTA at 350 °C

Results and Discussion

Figure 2 shows the measured P vs. voltage V characteristics for the amorphous Al2O3 capacitors with different tAlO 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 tAlO, TaN/Al2O3/Ge capacitor achieves a higher saturation P (Psat) 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 IL between TaN and Al2O3 produces more oxygen vacancies, contributing to a stronger switching P, compared to the TiAlO and WAlO ILs. P-V curves in Fig. 2d indicate that TaN/Al2O3/TaN capacitor has a much higher Psat in comparison with TaN/Al2O3/Ge, which is attributed to the fact that dual TaAlO ILs provide higher oxygen vacancy concentration. While Psat is significantly lower from that with Si bottom electrode (Fig. 2e), compared with the Ge electrode. This result indicates that Al2O3/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/Al2O3(6 nm)/Ge capacitor, exhibiting a higher Vc and an almost identical Psat as compared to that from the device with 3.6 nm of Al2O3 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].

Fig. 2

Measured P vs. V characteristics of the Al2O3 capacitors with different electrodes. a, b, and c showing the  P-V curves of TaN/Ti/Al2O3/Ge, TaN/Al2O3/Ge, and W/Al2O3/Ge, respectively, with a 3.6-nm tAlO. d and e showing that the Psat is enhanced(reduced) by using TaN(Si) as the bottom electrode instead of Ge. f TaN/Al2O3(6 nm)/Ge capacitor has a higher Vc and a similar Psat compared to the 3.6-nm-thick device in b. g and h Endurance measurements showing no degradation of Pr and Vc observed after 104 sweeping cycles for a TaN/Al2O3(3.6 nm)/Ge capacitor

Figure 2g and h show the extracted evolution of the positive and negative remnant P (Pr) and coercive V (Vc) values, respectively, over 104 sweeping cycles for a TaN/Al2O3/Ge capacitor. No wake-up, imprint, or fatigue effect is observed. Vc of the device is ~1.8 V, indicating that the E in the Al2O3 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]. Psat of the devices ranges from 1 to 5 μC/cm2, corresponding to a reasonable oxygen vacancy concentration in the range 3~15×1012 cm-2 assuming they have charge of plus two.

The underlying mechanism for ferroelectric-like behavior associated with oxygen vacancies in Al2O3 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/Al2O3/Ge, which originates from the segregation of voltage-modulated 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 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 Al2O3/Ge and (Ti, TaN, and W)/Al2O3/Ge samples are measured and shown in Fig. 4). For all the metal/Al2O3/Ge samples, there is a metal oxide IL formed between metal and Al2O3, which are proposed to be the reservoir of oxygen ions and vacancies, which is consistent with Ref. [17].

Fig. 3

Schematics of the mechanism for ferroelectric-like behavior in Al2O3 capacitors. Switchable P is due to the migration of oxygen vacancies and negative charges to form dipoles

Fig. 4

Core level XPS spectra of a Al2O3/Ge, b TaN/Al2O3/Ge, c Ti/Al2O3/Ge, and d W/Al2O3/Ge samples

To characterize the electrical performance of Al2O3 NVFET as NVM, program (erase) operation is achieved by applying positive (negative) voltage pulses to the gate, to raise (lower) its threshold voltage (VTH). Figure 5a shows how the linear-region transfer characteristics of the Al2O3 NVFET shift relative to the initial IDS-VGS curve measured with ±4 V program (erase) voltages with 100 ns pulse width. Here, VTH is defined as a VGS at 100 nA⋅W/L, and MW is defined as the maximum change in VTH. The Al2O3 NVFET obtains an MW of 0.44 V, though amorphous Al2O3 film has smaller Pr than the reported doped HfO2 films [5, 8]. It is noted that the high operating frequency up to 10 MHz of Al2O3 NVFET memory, which is indicative of that switchable P in Al2O3 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 Al2O3 devices to further study the device endurance. Figure 5b shows the plots of VTH vs. P/E cycle number, suggesting a stable MW can be maintained without a significant degradation over 106 P/E cycles for a 3.6-nm-thick Al2O3 NVFET.

Fig. 5

a Measured IDS-VGS curves of a 3.6-nm-thick Al2O3 NVFET for the initial and two polarization states. An MW of 0.44 V is obtained. b Endurance measurement demonstrates that no MW degradation is observed after 106 P/E cycles

Notably, the ferroelectric-like behavior observed in the amorphous Al2O3 devices can be extended to the universal amorphous oxides, e.g., hafnium oxide (HfO2) and zirconium oxide (ZrO2).

Conclusions

Stable ferroelectric-like behavior is first realized in capacitors with a thin amorphous Al2O3 insulator. Switchable P in amorphous Al2O3 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 Al2O3 NVFET achieves an MW of 0.44 V and over 106 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.