Ferroelectric thin films have been widely investigated for application in high density data storage. The probe storage based on the ferroelectric thin films has been extensively studied by atomic force microscopy (AFM). The probe storage has potential memory device that overcomes the storage density limit of the data storage based on the magnetic materials. It was well known that ferroelectric film has the thinner and the smaller domain, which could form stably. As the film thickness is decreased, the ferroelectric/electrode interface might effects more strongly on the domain switching property. In this study, we investigated the effect of interface between ferroelectric film and bottom electrode on the domain switching and retention properties of ferroelectric thin film using AFM. And we fabricated the $PbTiO_3$ (PTO) ferroelectric thin film which had improved domain switching and retention properties by changing the fabrication method of PTO thin film.
We fabricated the PTO ferroelectric thin films by gas phase reaction sputtering. Using conventional gas phase reaction sputtering method, the lower part of seed layer could un-completely react with following deposited material. Therefore defects and un-reacted seed layer could remain at the ferroelectric/electrode interface. And these affect to the domain switching and retention properties negatively. Therefore we fabricated PTO thin films with various deposition conditions to obtain the completely reacted PTO thin film.
We fabricated three types of PTO thin films as following.
ⅰ) PTO film fabricated by continuous method with 90 W PbO reaction power
ⅱ) PTO film fabricated by layer by layer method with 90 W PbO reaction power
ⅲ) PTO film fabricated by layer by layer method with 50 W PbO reaction power
We investigated the domain switching and retention properties of PTO samples by AFM. Firstly, we performed hysteresis measurements of each samples by applying voltage to the ferroelectric thin films with various maximum/minimum voltages. Secondly, we investigated domain switching properties of each samples by area poling experiments. Thirdly, we observed the retention properties of the written domains using piezoelectric force microscopy (PFM). Finally, we performed the current mapping experiment for investigating leakage properties.
We found that domain switching behaviors are responsible in the existence of the defect and un-reacted $TiO_2$ at the ferroelectric/electrode interface. And we found that PTO film which was fabricated by layer by layer method with 50 W PbO reaction power showed much more improved domain switching and retention properties than other PTO films.
In summary, we have investigated the domain switching and retention properties of $PbTiO_3$ ferroelectric thin films using AFM. It was found that interface between ferroelectric thin film and bottom electrode affects on the domain switching property. The space charge at interface between ferroelectric film and bottom electrode is responsible in the domain switching property of ferroelectric thin film. And we could obtain the PTO film which had improved domain switching and retention properties by changing the deposition conditions.
현재 강유전체 박막을 이용한 저장매체의 개발이 활발히 이루어 지고 있다. 특히 강유전체 박막과 원자력현미경을 이용한 탐침 정보저장장치의 경우 자성물질을 이용한 저장장치에 비해서 훨씬 높은 정보저장밀도를 구현할수 있는 가능성이있다. 탐침 정보저장장치에서는 강유전체 박막의 두께가 얇을 수록 더욱 작은 도메인을 형성할 수있다는 결과가 보고 되고 있다. 그러나 박막의 두께가 얇아질수록 강유전체 박막과 하부 전극사이의 계면이 강유전체 박막의 도메인 거동에 미치는 영향은 커진다. 그리하여 본 연구에서는 매우 얇은 강유전체 $PbTiO_3$ 박막을 제조하여, 제조된 강유전체 박막의 도메인 특성과 하부전극과 강유전체 박막 사이의 계면이 도메인의 거동에 미치는 영향을 원자력 현미경을 이용하여 분석해 보았다. 본 실험을 위해서 가스상 반응 스퍼터링법을 이용하여 매우 얇은 $PbTiO_3$ 박막을 제조하였으며, 원자력 현미경을 이용하여 박막의 hysteresis loop 측정, area poling 실험, retention 실험 , leakage 특성 측정 등을 시행하였으며 그 결과 hysteresis loop shift, 높은 area poling voltage, poor 한 retention 특성, 높은 leakage current등 매우 나쁜 도메인 특성이 관찰되었다. 그 이유는 강유전체 박막과 하부전극 사이의 계면에 존재하는 defect 혹은 미처 반응하지 못한 $TiO_2$ 물질이 상부 강유전체 도메인에 나쁜 영향을 미쳤기 때문으로 생각되며 제조 방식을 지속적 방법에서 단계적 방법으로 그리고 PbO reaction power를 낮추는 실험을 순차적으로 수행하여 새로운 박막을 제조하여 동일한 실험을 수행 한 결과 더욱 향상된 도메인 거동을 관찰할 수 있었다.
