Electrical characteristics and reliability problems of ferroelectric thin films

Abstract

For decades, ferroelectric has been studied a lot as non-volatile memory devices because of its bi-stable properties. It was recently highlighted again with the discovery of HfO2, which shows ferroelectricity even in few nanometers thin film. The reliability issue in the application of ferroelectric memory devices is always important and it still need to be further studied. We focused our research on the reliability issues, especially retention, fatigue and wake-up effect. In chapter 2, we investigated the effects of Pb content in Pb(Zr0.52Ti0.48)O3 (PZT) and electrode materials on the ferroelectric properties of metal/PZT/metal thin film capacitors for non-volatile memory device applications. The increase of Pb content resulted in a change from the non-ferroelectric pyrochlore phase to the ferroelectric perovskite phase, and consequently, the enhancement of polarization-voltage hysteresis loops. Non-noble metal/PZT/noble metal asymmetric electrode structures were fabricated by using non-noble metals and a noble metal with different work functions. The asymmetric electrode and the resultant internal field, not only made a shift in the initial hysteresis loops, but also caused continuous degradation of the reliability characteristics of ferroelectric capacitors. In chapter 3, we investigated the effect of repetitive switching of polarization on the ferroelectric Pt/ PZT/Pt thin film capacitor by using impedance spectroscopy. From the Cole-Cole plot, the equivalent circuit is described as a combination of the bulk part (a capacitor) and the interface part (the constant phase element (CPE), and a parallelly-connected resistor). The circuit parameters were analyzed at various stages of switching. An early increase and a subsequent decrease of the bulk capacitance may represent the wake-up and fatigue phenomena, respectively. The change in the interface part was characterized by an increase in resistance and the growth of n, the exponent of CPE, which may have come from a reduction of defects and the diminished inhomogeneity in the interfacial layer, respectively. The change in the resistance and the coefficient of the CPE in the interface part collectively resulted in an increase in the interfacial impedance. The coercive voltage, which may have intrinsically increased due to the repetitive switching, was even larger as a result of the increased interfacial impedance. In chapter 4, we investigated the effect of wake-up on the polarization switching dynamics of Si doped HfO2 thin films with an imprint. From the polarization-voltage hysteresis loops, wake-up and imprint effects were observed. We measured the polarization switched by the voltage pulse at various durations and voltages. The switching behaviors could be explained by the nucleation-limited-switching model with a Lorentzian distribution of the characteristic switching time. Due to the wake-up phenomena, the center of the distribution increased, and the width became narrower, which could be related to improvement of the uniformity of the film. The polarization switching data from the positive and negative bias pulses was analyzed by using the same empirical equation with the effective electric-field corrected for the imprint effect. The increase of the characteristic time due to wake-up was carried out by an increase in the field-independent coefficient, τ_1, without a change in the activation field, α.|강유전체는 bi-stable한 특성 때문에 오랜기간 비휘발성 메모리소자로 많이 연구되었다. 최근 얇은 두께에서도 강유전성을 보이는 HfO2의 발견으로 다시 한번 집중을 받았다. 강유전성 메모리소자로 응용하는데 있어서 분극의 신뢰성 문제는 언제나 중요한 이슈이고 이직도 연구되어야 할 부분이 많이 남아있다. 우리는 신뢰성 문제 중에 특히 retention, fatigue, wake-up에 집중하여 연구하였다. 2장에서는 Pb 함량과 전극 물질의 종류가 PZT 박막의 전기적인 특성과 신뢰성 문제에 미치는 영향을 연구하였다. Pb 함량이 적을 경우, 강유전성이 아닌 Pyrochlore 상이 만들어져서 박막의 잔여분극값이 적게 나왔다. 하부전극이 noble meter이고 상부전극이 non-noble meter일 경우, 계면에서 전극이 PZT의 산소를 뺏어와 박막에 내부 전기자을 형성하였다. 이 전기장으로 인해 Imprint 효과가 발생하였고 이것은 한쪽 분극 방향으로의 신뢰성문제에 큰 악영향을 미쳤다. 3장에서는 Impedance spectroscopy를 이용하여 반복적인 스위칭에 대해 박막의 변화를 조사하였다. Cole-Cole plot으로부터 Equivalent circuit을 박막과 계면으로 나누어 디자인하였다. 박막의 경우, C값이 wake-up과 fatigue단계의 분극값과 같은 경향으로 변화하였다. 반면 wake-up과 fatigue와 상관없이 Vc값은 꾸준히 증가하였으며 이것의 이유는 스위칭이 반복될수록 계면의 Impedance값이 증가하여 인가하는 전압의 많은 부분을 계면이 차지하기 때문이었다. 4장에서는 Si이 도핑된 HfO2 박막의 wake-up 상태에 따라 switching dynamics를 조사하였다. 다결정박막의 switching dynamics model인 NLS을 사용하였다. Wake-up 될수록 시간상수가 커졌다. 그 이유는 반복적인 스위칭으로 인해 화학적으로 또는 구조적으로 박막의 질이 좋아졌기 때문이라고 생각한다. 시간상수의 증가가 activation field α의 증가일것이라고 생각하였으나, 전기장에 관계없는 계수인 τ_1때문이라는 것을 알아내었다.; For decades, ferroelectric has been studied a lot as non-volatile memory devices because of its bi-stable properties. It was recently highlighted again with the discovery of HfO2, which shows ferroelectricity even in few nanometers thin film. The reliability issue in the application of ferroelectric memory devices is always important and it still need to be further studied. We focused our research on the reliability issues, especially retention, fatigue and wake-up effect. In chapter 2, we investigated the effects of Pb content in Pb(Zr0.52Ti0.48)O3 (PZT) and electrode materials on the ferroelectric properties of metal/PZT/metal thin film capacitors for non-volatile memory device applications. The increase of Pb content resulted in a change from the non-ferroelectric pyrochlore phase to the ferroelectric perovskite phase, and consequently, the enhancement of polarization-voltage hysteresis loops. Non-noble metal/PZT/noble metal asymmetric electrode structures were fabricated by using non-noble metals and a noble metal with different work functions. The asymmetric electrode and the resultant internal field, not only made a shift in the initial hysteresis loops, but also caused continuous degradation of the reliability characteristics of ferroelectric capacitors. In chapter 3, we investigated the effect of repetitive switching of polarization on the ferroelectric Pt/ PZT/Pt thin film capacitor by using impedance spectroscopy. From the Cole-Cole plot, the equivalent circuit is described as a combination of the bulk part (a capacitor) and the interface part (the constant phase element (CPE), and a parallelly-connected resistor). The circuit parameters were analyzed at various stages of switching. An early increase and a subsequent decrease of the bulk capacitance may represent the wake-up and fatigue phenomena, respectively. The change in the interface part was characterized by an increase in resistance and the growth of n, the exponent of CPE, which may have come from a reduction of defects and the diminished inhomogeneity in the interfacial layer, respectively. The change in the resistance and the coefficient of the CPE in the interface part collectively resulted in an increase in the interfacial impedance. The coercive voltage, which may have intrinsically increased due to the repetitive switching, was even larger as a result of the increased interfacial impedance. In chapter 4, we investigated the effect of wake-up on the polarization switching dynamics of Si doped HfO2 thin films with an imprint. From the polarization-voltage hysteresis loops, wake-up and imprint effects were observed. We measured the polarization switched by the voltage pulse at various durations and voltages. The switching behaviors could be explained by the nucleation-limited-switching model with a Lorentzian distribution of the characteristic switching time. Due to the wake-up phenomena, the center of the distribution increased, and the width became narrower, which could be related to improvement of the uniformity of the film. The polarization switching data from the positive and negative bias pulses was analyzed by using the same empirical equation with the effective electric-field corrected for the imprint effect. The increase of the characteristic time due to wake-up was carried out by an increase in the field-independent coefficient, τ_1, without a change in the activation field, α.