Min-Kyu Kim,Ik-Jyae Kim,Jang-Sik Lee
Defect Engineering of Hafnia-Based Ferroelectric Materials for High-Endurance Memory Applications
Zirconium-doped hafnium oxide (HfZrOx) is one of the promising ferroelectric materials for next-generation memory applications. To realize high-performance HfZrOx for next-generation memory applications, the formation of defects in HfZrOx, including oxygen vacancies and interstitials, needs to be optimized, as it can affect the polarization and endurance characteristics of HfZrOx. In this study, we investigated the effects of ozone exposure time during the atomic layer deposition (ALD) process on the polarization and endurance characteristics of 16-nm-thick HfZrOx. HfZrOx films showed different polarization and endurance characteristics depending on the ozone exposure time. HfZrOx deposited using the ozone exposure time of 1 s showed small polarization and large defect concentration. The increase of the ozone exposure time to 2.5 s could reduce the defect concentration and improve the polarization characteristics of HfZrOx. When the ozone exposure time further increased to 4 s, a reduction of polarization was observed in HfZrOx due to the formation of oxygen interstitials and non-ferroelectric monoclinic phases. HfZrOx, with an ozone exposure time of 2.5 s, exhibited the most stable endurance characteristics because of the low initial defect concentration in HfZrOx, which was confirmed by the leakage current analysis. This study shows that the ozone exposure time of ALD needs to be controlled to optimize the formation of defects in HfZrOx films for the improvement of polarization and endurance characteristics.
Defect Engineering of Hafnia-Based Ferroelectric Materials for High-Endurance Memory Applications

Zirconium-doped hafnium oxide (HfZrOx) is one of the promising ferroelectric materials for next-generation memory applications. To realize high-performance HfZrOx for next-generation memory applications, the formation of defects in HfZrOx, including oxygen vacancies and interstitials, needs to be optimized, as it can affect the polarization and endurance characteristics of HfZrOx. In this study, we investigated the effects of ozone exposure time during the atomic layer deposition (ALD) process on the polarization and endurance characteristics of 16-nm-thick HfZrOx. HfZrOx films showed different polarization and endurance characteristics depending on the ozone exposure time. HfZrOx deposited using the ozone exposure time of 1 s showed small polarization and large defect concentration. The increase of the ozone exposure time to 2.5 s could reduce the defect concentration and improve the polarization characteristics of HfZrOx. When the ozone exposure time further increased to 4 s, a reduction of polarization was observed in HfZrOx due to the formation of oxygen interstitials and non-ferroelectric monoclinic phases. HfZrOx, with an ozone exposure time of 2.5 s, exhibited the most stable endurance characteristics because of the low initial defect concentration in HfZrOx, which was confirmed by the leakage current analysis. This study shows that the ozone exposure time of ALD needs to be controlled to optimize the formation of defects in HfZrOx films for the improvement of polarization and endurance characteristics.

Ferroelectric materials based on hafnium oxide have the potential to overcome the limitations of perovskite-based ferroelectric materials because of their several advantages, including a low process temperature, high scalability, and complementary metal-oxide-semiconductor compatibility. Due to these advantages, zirconium-doped hafnium oxide (HfZrOx) has been used as a ferroelectric material for negative capacitance transistors, next-generation memory devices, and neuromorphic devices. However, HfZrOx has shown a limitation in endurance characteristics compared with perovskite-based ferroelectric materials. The degradation of HfZrOx under repeated electrical stress is due to the generation and accumulation of defects. Therefore, to improve the endurance characteristics, optimization of the defects in HfZrOx is required. The formation of defects in HfZrOx can be affected by the types of electrodes, annealing temperature, and ozone exposure time of the atomic layer deposition (ALD) process. In particular, the ozone exposure time may be a critical parameter to the formation of oxygen defects in HfZrOx, such as oxygen vacancy and interstitial defects because ozone reacts with the Hf and Zr precursors to form the HfZrOx layer during the ALD process. These defects can affect the leakage current and breakdown of HfZrOx during operation. Previously, it was shown that the formation of the non-ferroelectric monoclinic phase could be suppressed and the growth of the ferroelectric orthorhombic phase could be promoted by optimizing the ozone exposure time. Also, due to different phase compositions, the ferroelectric characteristics such as remnant polarization and coercive electric field could be affected by the ozone exposure time. However, most of the previous studies analyzed the effect of ozone exposure time on the defect and electrical properties of thin HfZrOx films with a thickness of <10 nm. The impact of the ozone exposure time may be different in thicker HfZrOx films due to different in-plane stress induced in the films. Also, as thicker HfZrOx films are beneficial for memory applications due to their large memory window, the effect of ozone exposure on the defect and electrical properties of HfZrOx should be further analyzed. In this study, we investigate the polarization and endurance characteristics of a 16-nm-thick HfZrOx according to the ozone exposure time. When the ozone exposure time increases from 1 to 2.5 s, a wake-up effect is reduced and the polarization characteristics of HfZrOx are improved. A longer ozone exposure time of 4 s induces the reduction of polarization in HfZrOx. The effect of the ozone exposure time on the endurance characteristics of HfZrOx is investigated. The optimized ozone exposure time (2.5 s) improves the endurance characteristics of HfZrOx by reducing the defects in HfZrOx. These results show that optimization of the ozone exposure time is essential to improve the ferroelectric and endurance characteristics of HfZrOx.

In this study, a 16-nm-thick HfZrOx is used, which can provide a sufficient memory window for memory device applications. As the memory window of the ferroelectric transistor is directly related to the coercive field and thickness of the ferroelectric layer, a thicker HZO is used to achieve a larger memory window. The morphology of HfZrOx films with different ozone exposure times was investigated (Figure 1). The root-mean-square (RMS) roughness of HfZrOx films with ozone exposure times of 1, 2.5, and 4 s were 0.297, 0.412, and 0.428 nm, respectively, which confirmed the uniform and conformal growth of HfZrOx films. The RMS roughness increased when the ozone exposure time increased, which could be originated from different phase compositions. The polarization–voltage and switching current–voltage curves of ferroelectric capacitors with the TiN/HfZrOx/TiN structure based on different ozone exposure times were measured. When analyzing HfZrOx with the ozone exposure time of 1 s, antiferroelectric characteristics were observed at the pristine state. The ferroelectric polarization curve was observed after 105 bipolar rectangular pulse cycles using a voltage pulse with an amplitude of ±4 V, width of 5 μs, and rise/fall time of 100 ns. In the polarization curve with a 4-V sweep, the positive- (+Pr) and negative-remnant polarization (−Pr) were 12.6 and −14.4 μC/cm2, respectively (Figure 2a). With an ozone exposure time of 2.5 s, HfZrOx showed ferroelectric polarization characteristics at the pristine state. After 105 bipolar pulse cycles, +Pr and −Pr were 17.4 and −17.2 μC/cm2, respectively, which are larger than those of HfZrOx with an ozone exposure time of 1 s (Figure 2b). When the ozone exposure time increased to 4 s, the ferroelectric polarization characteristics were still observed at the pristine states. However, +Pr and −Pr obtained after 105 bipolar pulse cycles decreased to 15.1 and −15.3 μC/cm2, respectively (Figure 2c). These results indicated that the ozone exposure time could affect the phase composition of HfZrOx. To confirm the effects of ozone exposure time on the phases of HfZrOx, we analyzed the grazing incidence X-ray diffraction (GIXRD) patterns of HfZrOx films with the ozone exposure times of 1, 2.5, and 4 s (Figure 2d). In GIXRD patterns of all HfZrOx films, the peaks from the (111), (200), and (220) planes of the orthorhombic phases were observed, which were the origin of the ferroelectricity of HfZrOx. These peaks could be a mixture of the orthorhombic and tetragonal phases of (101), (110), and (200) planes. For HfZrOx films with ozone exposure times of 1 and 2.5 s, the peak from the monoclinic phase was not observed. However, when the long ozone exposure time (4 s) was used, the peak from the (111) plane of the monoclinic phase was observed (Figure S1). When the oxygen source (i.e. ozone) is overexposed, the monoclinic phase can be induced in HfZrOx films. The formation of the monoclinic phase can reduce the polarization of HfZrOx films. Therefore, the ozone exposure time needs to be optimized to stabilize the ferroelectric orthorhombic phase. The endurance characteristics of HfZrOx with different ozone exposure times were investigated. The devices underwent bipolar rectangular pulse cycles with a width of 5 μs, rise/fall time of 100 ns, and amplitudes of ±3, ±4, and ±5 V (±1.875, ±2.5, and ±3.125 MV/cm). The properties of the devices were measured before and after applying the bipolar pulse cycles. The HfZrOx deposited using the ozone exposure time of 2.5 s exhibited the most stable endurance characteristics. With the ozone exposure time of 1 s, HfZrOx showed a large increment of polarization during the repeated switching operation, which was called as the wake-up effect (Figure 3a). With a higher voltage, polarization increased, but the endurance characteristics were degraded. When the ozone exposure time increased to 2.5 s, a large increment of polarization was not observed during cycles, and longer endurance cycles were observed (Figure 3b). When bipolar pulses with an amplitude of ±3 V were used, HfZrOx could achieve stable polarization characteristics for 109 cycles. The breakdown of the HfZrOx layer was observed after 5 × 109 cycles. With the longer ozone exposure time of 4 s, a wake-up effect similar to HfZrOx with an ozone exposure time of 2.5 s was observed, which was smaller than HfZrOx with an ozone exposure time of 1 s. However, the endurance characteristics were degraded, and the breakdown of the HfZrOx layer was observed after 108 cycles (Figure 3c). These differences may originate from an increase in the defects, such as oxygen vacancies or interstitials in HfZrOx. To investigate the defect concentration and generation rate of HfZrOx, leakage currents of HfZrOx films were analyzed depending on the voltage stress. After applying the voltage stress, leakage currents of HfZrOx films deposited using different ozone exposure times were measured (Figure 4a). The leakage current of HfZrOx films decreased with an increase in the voltage stress time. This behavior may originate from charge trapping by the newly generated oxygen defects. The change of Ileakage is defined as ΔI = Iinitial – I. Here, Iinitial and I are the leakage currents at the initial state and after applying the voltage stress pulse, respectively. To fit ΔI depending on the stress time, ΔI = ΔI0tγ was used. Here, ΔI0 is proportional to the initial defect concentration (i.e. defect concentration before the voltage stress) and γ represents the defect generation rate. Ozone exposure time did not affect the defect generation rates of HfZrOx. However, HfZrOx films showed different initial defect concentrations depending on the ozone exposure times (Figure 4b). HfZrOx with an ozone exposure time of 2.5 s showed a smaller initial defect concentration than HfZrOx with ozone exposure times of 1 and 4 s. HfZrOx films with short ozone exposure time (1 s) showed the largest initial defect concentration in analysis based on leakage current. When the ozone exposure time increased to 2.5 s, the initial defect concentration decreased, which may indicate a decrease in oxygen vacancy concentration. However, when the ozone exposure time was further increased to 4 s, the HfZrOx film showed a larger initial defect concentration than the HfZrOx film using the ozone exposure time of 2.5 s (Figure 4c). With overexposure to the oxygen source, defects, such as oxygen interstitials, can be seen in HfZrOx films. Oxygen interstitials have been suggested as the origin of stabilizing the monoclinic phase in HfZrOx films. HfZrOx film using the ozone exposure time of 4 s showed the peak of the monoclinic phase in GIXRD measurement and the increased initial defect concentration compared with HfZrOx film using the ozone exposure time of 2.5 s. The degraded endurance characteristics of HfZrOx, with an increase in the ozone exposure time from 2.5 to 4 s, seem to be originated from the formation of oxygen interstitials. Therefore, the reason for the improved endurance characteristics of HfZrOx, with an ozone exposure time of 2.5 s, seems to be the low initial defect concentration, including oxygen vacancies and interstitials. The low initial defect concentration can prevent the breakdown of the HfZrOx layer. As the ALD is based on thermally activated chemical reactions, the defect concentration can also be affected by the deposition temperature, type of precursors, and reactants. However, in this work, only the ozone exposure time during the deposition is changed, and all other parameters are fixed. Thus, the difference between HZO films is thought to be originated from the difference in the ozone exposure time, which can change the concentration of oxygen-related vacancies. In this work, we correlated the leakage current of the hafnia-based ferroelectric transistor with the defect inside the HfZrOx film. Although other sources, such as surface roughness, structural defect, and tunneling, could affect the leakage current, we excluded other sources due to the following reasons: First, if the surface roughness was the main contributor to the leakage current difference, the leakage current of the ferroelectric capacitor with a 4-s ozone exposure time should show the highest leakage current, but the ferroelectric capacitor with a 1-s ozone exposure time showed the highest leakage current. Also, structural defects, such as pin holes or pores, were excluded because the thin film was conformally deposited using ALD (Figure 1). Finally, as the thickness of the ferroelectric thin film used in this work was 16 nm, the direct tunneling through the ferroelectric thin film could be suppressed compared with ferroelectric films with a thinner thickness. Thus, we believe that the main contributor to the leakage current is the oxygen-vacancy-related defects that originated from different ozone exposure times. These results indicate that the optimization of the ozone exposure time can reduce the initial defect concentration and lead to the improvement of endurance characteristics of HfZrOx.

In summary, we investigated the effects of ozone exposure time in the ALD process on the ferroelectric and endurance characteristics of HfZrOx. With the short ozone exposure time of 1 s, HfZrOx showed wake-up effects and large initial defect concentrations. When the ozone exposure time increased from 1 to 2.5 s, the wake-up effects and initial defect concentration of HfZrOx were reduced. The further increase of ozone exposure time could induce the formation of oxygen interstitials, which might cause the reduction of polarization. Among the ozone exposure condition (1–4 s), an ozone exposure time of 2.5 s showed the largest polarization and most stable endurance characteristics. These improved characteristics were achieved by the optimization of defects and phase composition in HfZrOx by controlling the ozone exposure time. This study provides a feasible strategy to improve the endurance and ferroelectric characteristics of HfZrOx by optimizing the ozone exposure time for promoting the application of HfZrOx to next-generation memory devices.

Hf[N(C2H5)CH3]4 [tetrakis(ethylmethylamido)hafnium (TEMAH)] and Zr[N(C2H5)CH3]4 [tetrakis(ethylmethylamido)zirconium (TEMAZ)] were purchased from UP Chemical, Korea. Si wafers with 100-nm-thick thermally grown SiO2 were used as substrates.

Ferroelectric capacitors with a TiN/HfZrOx/TiN structure were fabricated on the SiO2/Si substrate. First, a 100-nm-thick TiN layer was deposited on the SiO2/Si substrate using direct current sputtering. Then, the 16-nm-thick HfZrOx films were deposited via ALD using the HfO2/ZrO2 ALD cycle ratio of 1:1 at 280 °C. Hf[N(C2H5)CH3]4 (TEMAH), Zr[N(C2H5)CH3]4 (TEMAZ), and ozone with a concentration of 180 g/m3 were used as Hf precursor, Zr precursor, and oxygen source, respectively. The ozone was inserted into the chamber after Hf and Zr precursors were inserted. During the deposition of HfZrOx, ozone exposure times of 1, 2.5, and 4 s were used. The 100-nm-thick TiN electrode formed on the HfZrOx. Then, the ferroelectric characteristics of HfZrOx were induced by thermal annealing under an N2 environment for 1 min at 500 °C. The area of the TiN top electrode was 50 μm × 50 μm.

All characteristics were measured at room temperature under ambient conditions. The polarization–voltage curves and endurance characteristics of HfZrOx with different ozone exposure times were measured using a pulse measurement unit (PMU) (4225-PMU, KEITHLEY Instruments). The leakage current characteristics of HfZrOx with different ozone exposure times were measured using a semiconductor parameter analyzer (4200A-SCS, KEITHLEY Instruments). The surface roughnesses of HfZrOx layers were measured by atomic force microscopy (Park Systems, NX10). The crystal structures of the HfZrOx films were analyzed using the grazing incidence X-ray diffraction (GIXRD) (D/MAX-2500, Rigaku).