Interface Engineering of Ferroelectric/Metal and Ferroelectric/Semiconductor interface for HfO2-based ferroelectrics

김세현 서울대학교 대학원 2025 국내박사

The amount of data that needs to be processed by computers has increased exponentially due to advancements in modern smart technologies. To handle such large volumes of data, memory devices must be developed in a more efficient and sophisticated manner. Ferroelectric memories, such as ferroelectric random-access memory (FeRAM), ferroelectric field-effect transistors (FeFETs), and ferroelectric tunnel junctions (FTJs), have been considered promising candidates for next-generation memory devices due to their high efficiency. In 2011, a remarkable achievement in downscaling the ferroelectric materials was reported where the ferroelectricity was maintained at 10 nm thickness range using HfO2-based thin films. However, in pursuit of more efficient devices, extensive efforts have been made to scale down the thickness of HfO2-based ferroelectrics to below 5 nm. As the film thickness decreases, the proportion of the interface relative to the entire device increases, thereby amplifying the influence of interfacial non-idealities. These interfacial effects can significantly degrade the ferroelectric properties and adversely impact overall device performance. The interfaces in ferroelectric devices can be broadly categorized into ferroelectric/metal and ferroelectric/semiconductor interfaces. At the ferroelectric/metal interface, a non-ferroelectric interfacial layer is often formed during processing, which degrades the ferroelectric properties. Moreover, oxygen vacancies (Vo) concentrated at the interface can lead to domain wall pinning and the formation of non-ferroelectric phases, resulting in the wake-up effect. At the ferroelectric/semiconductor interface, when a HfO2-based ferroelectric layer is deposited on a Si substrate via atomic layer deposition (ALD), the oxygen precursor can oxidize the Si surface, forming a low-permittivity and defective SiO2 interfacial layer. Due to its low dielectric constant, this SiO2 layer experiences a large voltage drop during device operation, causing dielectric breakdown to occur earlier than in the ferroelectric layer, thus limiting the reliability of the device. In addition, insufficient charge compensation from the Si substrate induces a depolarization field, which can cause back-switching of ferroelectric dipoles. Interface traps at the Si substrate arise from oxidation/reduction reactions and residual impurities during processing, and they act as a major factor deteriorating the subthreshold swing (S.S.) in FeFETs. Therefore, controlling interfacial properties to suppress such non-idealities is essential for maintaining device performance in the context of continued downscaling. To mitigate the wake-up effect caused by Vo at the ferroelectric/metal interface, a process strategy employing Mo electrodes, which can supply oxygen during thermal processing, was implemented. Hf0.5Zr0.5O2 (HZO) films grown on Mo electrodes exhibited a higher fraction of the ferroelectric orthorhombic phase and a lower concentration of Vo compared to those grown on conventional TiN electrodes. As a result, the devices demonstrated enhanced remanent polarization (Pr) values and a suppressed wake-up effect. These findings suggest that employing Mo as an electrode material effectively reduces Vo concentration at the ferroelectric/metal interface, thereby improving both the ferroelectric performance and reliability of the device. To suppress the limited endurance and depolarization field effects induced by the low-permittivity SiO2 interfacial layer formed during processing at the ferroelectric/semiconductor interface, a thin Ti metal layer was deposited on the Si substrate and subsequently oxidized during processing to form a high-permittivity (k) TiO2 interfacial layer. A very thin 5 nm HZO film was employed in the MFIS capacitor incorporating the TiO2 interfacial layer. The TiO2 layer acted as a potential well, enhancing the charge compensation capability. As a result, the depolarization field was effectively reduced, suppressing back-switching of ferroelectric dipoles. This led to a large 2Pr value of 60 μC/cm2 and significantly improved endurance of up to 108 cycles even under reduced operating voltages. To control the interface trap density (Dit) at the Si/SiO2 interface and border traps in HZO layer, various interfacial layers including SiO2, TiO2, and Al2O3 were employed, and the Dit characteristics of MFIS devices were comparatively analyzed. Among these materials, TiO2 exhibits a Gibbs free energy of formation most similar to that of SiO2. As a result, the TiO2 interfacial layer effectively supplies oxygen to the HZO and SiO2 layers. This supply leads to improved interface and border trap characteristics at the interface. 정보 기술의 발전으로 인해 컴퓨터가 처리해야 하는 데이터의 양은 기하급수적으로 증가하였다. 이러한 방대한 데이터를 처리하기 위해서는 메모리 소자의 고효율화 및 고도화가 필수적이다. 강유전체 기반 메모리인 강유전체 랜덤 액세스 메모리(FeRAM), 강유전체 전계효과 트랜지스터(FeFET), 강유전체 터널 접합(FTJ) 등은 높은 효율성으로 인해 차세대 메모리 소자로 주목받고 있다. 2011년, HfO2 기반의 박막을 활용하여 10 nm 수준에서도 강유전성이 유지된다는 결과가 보고되었으며, 이후 더 높은 소자 효율을 달성하기 위해 HfO2 기반 강유전체의 두께를 5 nm 이하로 줄이기 위한 연구가 활발히 진행되었다. 하지만 박막의 두께가 감소함에 따라 전체 소자 내에서 계면의 비중이 상대적으로 증가하고, 이에 따라 계면의 결함들이 소자 특성에 미치는 영향이 더욱 커지게 된다. 이러한 계면 특성은 강유전 특성을 저하시킬 뿐 아니라 전체 소자의 성능에도 부정적인 영향을 미친다. 강유전체 소자에서의 계면은 크게 강유전체/금속 계면과 강유전체/반도체 계면으로 나뉜다. 강유전체/금속 계면에서는 공정 중에 비강유전성 계면층이 형성되는 경우가 있으며, 이는 강유전성을 저하시킨다. 또한 계면에 집중된 산소 공공(Vo)은 도메인 벽의 pinning이나 비강유전 상의 형성을 유도하여 wake-up effect를 유발한다. 강유전체/반도체 계면에서는 원자층증착법 (ALD) 공정을 통해 HfO2 기반 강유전체 층이 Si 기판 위에 증착될 경우 산소 공급원이 Si 표면을 산화시켜 저유전율 및 defect가 존재하는 SiO2 계면층을 형성한다. 이 SiO2 층은 유전율이 낮기 때문에 소자 동작 시 전압 강하가 집중되어 강유전체층보다 먼저 절연 파괴가 발생할 수 있으며, 이는 소자의 endurance를 제한하는 주요 원인이 된다. 또한, Si 기판의 불충분한 전하 보상 능력은 탈분극 전기장을 유도하여 dipole의 back-switching을 초래할 수 있다. 이 외에도, 공정 중 산화-환원 반응이나 불순물 잔류 등에 의해 Si 기판에서 계면 trap이 형성되고, 이는 FeFET에서 subthreshold swing (S.S.) 특성을 저하시킨다. 따라서 이러한 비이상성을 억제하기 위한 계면 특성 제어는 소자 스케일 다운이 진행됨에 따라 더욱 중요해지고 있다. 강유전체/금속 계면에서 Vo에 의해 발생하는 wake-up effect를 억제하기 위해 산소를 공급할 수 있는 Mo 전극을 활용한 공정 전략이 적용되었다. Mo 전극 위에 성장된 Hf0.5Zr0.5O2(HZO) 박막은 기존 TiN 전극 위에 성장된 박막보다 강유전성 orthorhombic 상의 분율이 높고 Vo의 농도가 낮음을 나타내었다. 이로 인해, 잔류 분극(Pr) 값이 증가하고 wake-up effect가 크게 억제되었다. 이는 Mo 전극이 열처리 과정 중 HZO 쪽으로 산소를 공급하여 강유전 특성과 신뢰성을 동시에 향상시킬 수 있음을 보여준다. 강유전체/반도체 계면에서 공정 중 형성된 저유전율 SiO2 계면층으로 인해 발생하는 endurance 제한 및 탈분극 전기장 효과를 억제하기 위해, Si 기판 위에 Ti 금속 박막을 증착한 후 공정 중 산화시켜 고유전율(k) TiO2 계면층을 형성하였다. 이때, 5 nm 두께의 매우 얇은 HZO 박막을 사용하여 MFIS 커패시터를 제작하였다. TiO2 층은 포텐셜 우물로 작용하여 전하 보상 능력을 향상시키고, 이에 따라 탈분극 전기장을 효과적으로 감소시켜 dipole의 back-switching을 억제하였다. 결과적으로 2Pr 값은 60 μC/cm2로 크게 향상되었으며, 구동 전압을 낮춘 조건에서도 108 사이클까지 우수한 endurance 특성을 보였다. 또한 Si/SiO2 계면에서의 interface trap density(Dit)를 제어하기 위해 SiO2, TiO2, Al2O3 등의 다양한 계면층을 적용하여 MFIS 소자의 Dit 특성을 비교 분석하였다. 그 결과, TiO2는 SiO2와 가장 유사한 Gibbs 자유 에너지를 가지고 있어 IL로 사용할 경우 HZO 박막이 SiO2 층에서 산소를 빼앗아오는 oxygen scavenging 현상을 억제할 수 있었다. 이 억제를 통해 계면의 interface와 border trap 특성도 향상됨을 확인하였다.

강유전체 HfO2의 도펀트 의존성 및 강유전체 터널 접합 메모리 소자 응용에 대한 연구

홍승현 한국산업기술대학교 일반대학원 2020 국내석사

Ferroelectric HfO2 has been steadily studied due to compatibility of its excellent CMOS process and ferroelectricity with a thickness of several nano-meters of thin film. In addition, ferroelectric HfO2 has been attracted attention as a suitable material for application to Ferroelectric Tunnel Junction (FTJ), a ferroelectric resistive memory using tunneling current. Ferroelectric HfO2 thin films have various parameters such as dopants, process conditions, and device structure, which affect the electrical properties of the device. Therefore, further research is needed to investigate the effects and correlation of these various parameters on device characteristics. In this study, to investigate the effect of the dopant, the electrical properties of ferroelectric thin films according to the dopant (Zr, Si) applied to HfO2 were analyzed. In particular, the first-order reversal curve (FORC) was applied to analyze the characteristics of the device in order to understand the domain behavior due to various effects caused by cycling (Wake-up, Fatigue). Based on the results, the optimum process conditions and device structure of FTJ were evaluated to maximize the On/Off ratio. Finally, memory properties and reliability of the device were evaluated for the application of memory devices. 강유전체 HfO2는 우수한 CMOS 공정 호환성과 수 nm 두께의 박막에서도 강유전성을 발현된다는 이점으로 인해 많은 연구가 진행되고 있다. 또한 강유전체 HfO2는 터널링 전류를 이용하는 강유전체 저항성 메모리인 Ferroelectric Tunnel Junction (FTJ)에 적용하기에 적합한 물질로 주목받고 있다. 강유전체 HfO2 박막에는 여러 도펀트와 공정조건 및 소자구조가 적용되고, 다양한 공정 파라미터들이 소자의 전기적 특성에 영향을 미친다. 따라서, 이 다양한 파라미터들이 소자특성에 미치는 영향과 상관관계에 대한 깊은 연구가 필요하다. 본 연구에서는 HfO2에 적용되는 도펀트인 Zr, Si 에 따른 강유전체 박막의 전기적 특성을 분석하였다. 특히 사이클링에 의해 발생하는 여러 효과 (Wake-up, Fatigue) 에 따른 도메인의 거동을 파악하기 위해 First-order Reversal Curve (FORC)를 적용하여 소자의 변화를 분석하였다. 그 결과를 바탕으로, 최적의 공정조건 및 소자구조를 도출하여 On/Off ratio를 극대화 했다. 마지막으로, 메모리 소자 적용을 위해 여러 메모리 특성을 분석하고 소자의 신뢰성을 평가하였다.

Inducing reversible polarization switching in HfO2- ZrO2 thin films for energy and memory applications

김금도 서울대학교 대학원 2019 국내박사

도핑한 HfO2 및 HfO2-ZrO2 고용체 박막에서 나타나는 강유전성과 전계 유도 강유전성은 2011년 처음 보고된 이후 집중적으로 연구 되어왔다. 이러한 박막들은 기존 페로브스카이트 기반의 강유전체 박막과 비교할 때 훨씬 큰 밴드 갭 에너지 (~5.5 eV) 및 Si-compatibility 등을 가져 미래 반도체 소자로 사용되기에 뚜렷한 이점들을 나타내었다. 반면, 기존의 연구들은 주로 전극-강유전체-전극 또는 전극-강유전체-(상유전체)-반도체 전극 구조 상에서 비가역적 분극 스위칭에 기반한 강유전체 물성과 이와 관련된 비휘발성 메모리 소자 측면에 주목하였으며, 특정 조건 하에서 HfO2-ZrO2 박막 상에 유도될 수 있는 가역적 분극 스위칭 현상에 대해서는 체계적으로 연구가 되지 않았다. 가역적 분극 스위칭은 인가한 전계가 제거되어도 스위칭된 상태가 유지되는 일반적인 강유전체 커패시터와 반대로, 인가한 전계가 제거될 때 스위칭된 자발 분극이 초기 분극 상태로 되돌아오는 특성을 의미한다. 이러한 분극 스위칭 특성을 보이는 강유전체 박막은 외부 전압이 없는 상태에서 어떤 메모리 상태도 유지할 수 없기 때문에 비휘발성 메모리 소자로써는 사용되기 힘든 반면, 자발 분극 스위칭에 의해 유도되는 큰 커패시턴스 변화를 통해 에너지 및 음의 커패시턴스 (negative capacitance; NC) 소자로써 유망한 특성들을 나타낼 수 있다. 따라서 본 연구는 HfO2-ZrO2 박막 상에서 챕터 3과 4에서 제시되는 두 가지 다른 방법을 이용하여 가역적 분극 스위칭을 유도하고, 이를 각각 에너지 및 NC 메모리 소자로써 활용한 내용에 대해 다루었다. 챕터 3에서는 무극성의 정방(tetragonal) 상을 안정화하는 방법을 통해 가역적 분극 스위칭을 유도하고, 이를 통해 MIM 커패시터의 정전기적 에너지 저장 소자 응용을 연구하였다. HfO2-ZrO2 박막 내에서는 정방 상이 안정화될 때 인가한 전계에 의해 가역적으로 극성의 사방정계(orthorhombic) 상으로 상전이가 가능하다. 이는 P-E 커브 상에서 큰 Pmax와 작은 Pr을 갖는 이중-히스테리시스를 나타나게 하는데, 이는 정전기적 슈퍼커패시터 소자에 적합한 특성이다. 그러나 현재까지 HfO2-ZrO2 박막에서 나타나는 큰 에너지 저장 특성은 ~10 nm 이상의 두꺼운 두께에서 낮은 유전율의 규모를 증가시키는 데 단사정(monoclinic) 상의 생성으로 인한 스케일업의 장애를 가져왔다. 단사정 상의 생성이 원자층 증착법(atomic layer deposition; ALD) 과정 중 부분적인 insitu-결정화 현상과 큰 관련이 있음을 고려하여 낮은 ALD 온도에서의 Hf0.5Zr0.5O2 박막 증착과 이를 이용한 에너지 저장 특성을 체계적으로 연구하였다. 낮은 증착 온도에서 탄소와 질소 도펀트 및 그레인 크기의 변화에 의해 Hf0.5Zr0.5O2 박막의 결정상은 극성의 사방정 상에서 정방 상으로 변화하였고, 이를 통해 가역적 분극 스위칭을 통한 전계유도 강유전성을 나타낼 수 있었다. 210 oC에서 증착한 Hf0.5Zr0.5O2 박막은 기존의 280 oC에서 증착한 Hf0.3Zr0.7O2 박막에 비해 ~40 nm 두께까지 더 작은 단사정 상 생성에 의한 열화를 보였고, 이를 통해 개선된 에너지 저장 특성을 나타내었다. 챕터 4에서는 강유전체 층과 전극 층 사이에 유전층 삽입을 통해 탈분극 전계를 유도하고 이를 이용하여 가역적 분극 스위칭을 유도한 연구를 진행하였다. 강유전체 물질의 NC 현상에 의해 가역적 분극 스위칭 중에 나타나는 강유전체/상유전체 이중층의 커패시턴스는 이중층 중에 존재하는 상유전체 층의 단일 커패시턴스보다 커질 수 있다. 이 현상은 (순간적인) NC 현상에 의한 커패시턴스 증폭으로 불리며, 전계 효과 트랜지스터 (field-effect transistor)의 저전력 동작 및 DRAM (dynamic random access memory) 커패시터 소자를 개발하기 위한 목적으로 많은 관심을 끌고 있다. 그러나 현재까지 강유전체/상유전체 이중층 커패시터에서 관찰한 NC 효과는 Pb(Zr,Ti)O3 및 BaTiO3와 같은 페로브스카이트 결정 구조에 기초한 에피텍셜 (epitaxial) 강유전체 박막에서만 보고가 되었고, 이러한 박막들은 현재 상보형 금속 산화 반도체 (complementary metal oxide semiconductor; CMOS) 공정에 사용되기 어려운 한계를 지닌다. 따라서 본 챕터에서는 ALD를 통해 증착한 비정질-Al2O3/다결정-Hf0.3Zr0.7O2 이중층 구조에서 가역적 분극 스위칭을 유도하고, 이 때 나타나는 순간적인 NC 현상을 연구하였다. Zr 조성 및 결정화 조건은 순간적인 NC 현상을 뚜렷하게 관찰하기 위해 세밀하게 조정되었다. 5-10 nm 두께의 Al2O3/Hf0.3Zr0.7O2 이중층 박막의 넓은 전압 범위에서 커패시턴스 증폭 현상을 관찰하였다. NC 현상에 의해 추가적으로 유도 가능한 전하 밀도는 적층된 상유전체 층의 커패시턴스에 의해 제한됨을 확인하였다. 상유전체 층을 통한 누설 전류를 억제하는 것이 이중층 커패시터 상에서 히스테리시스 없는 안정적인 충/방전 거동에 필수적인 조건임을 확인하였다. Ferroelectricity and field-induced ferroelectricity in doped HfO2 and HfO2-ZrO2 solid solution films have been studied since the ferroelectricity in HfO2 thin film was first reported in 2011. The significant merits for the utilization in future semiconductor devices could be found in such thin films, such as large electrical band gap (~5.5eV) and Si-compatibility, compared to the conventional perovskite-based ferroelectric thin films. However, the previous reports mainly focused on the ferroelectric properties based on irreversible polarization switching at the metal-ferroelectric-metal (MIM) or metal-ferroelectric-(insulator)-semiconductor structures and the related nonvolatile memory aspects, but the reversible polarization switching properties induced in the HfO2–ZrO2 thin film at the optimized processes were not studied in detail. Contrary to the irreversible polarization switching operation observed in conventional MFM capacitor, the reversible polarization switching specifically means the switching behavior where the switched polarization by the action of external electric field shows the reversible back-switching when the electric field is removed. The ferroelectric layer which shows this type of polarization switching can be hardly used as nonvolatile memory device because it cannot retain any memory state in absence of external voltage, but the large capacitance change induced by the Ps reversal facilitates the utilization in energy storage and negative capacitance devices in combination with the promising electrical and physical properties of HfO2-ZrO2 thin film. Therefore, this work presents the reversible polarization switching in HfO2-ZrO2 thin films induced by two different methods in chapter 3 and 4. The first method presented in chapter 3 uses the stabilization of nonpolar tetragonal phase, where the study was carried out for the potential application of the MFM capacitor in electrostatic energy storage device. When the tetragonal phase is stabilized in HfO2-ZrO2 thin films, the electric field-induced reversible phase transition to polar orthorhombic phase can be possible due to the structural similarity between them. It facilitates the double-hysteresis curve in P-E diagram with large Pmax and low Pr, which is beneficial characteristic for electrostatic supercapacitor device. However, to date, the high energy storage performances observed in the field-induced ferroelectric HfO2- or ZrO2-based films have had an obstacle to scale-up due to the involvement of low-k monoclinic phase at the large thickness (> ~10 nm). Considering that the monoclinic phase formation is closely related with the in-situ (partial) crystallization during the atomic layer deposition (ALD) process, in this chapter, the ALD temperature of Hf0.5Zr0.5O2 thin films was lowered, and its influence on the energy storage performances was systematically examined. Carbon and nitrogen dopants incorporated at a low deposition temperature in combination with grain size decrease change the polymorphism of Hf0.5Zr0.5O2 thin film from the genuine ferroelectric to field induced (incipient) ferroelectric crystal structure. The Hf0.5Zr0.5O2 thin film deposited at 210 °C shows improved resistance to degradation by monoclinic phase involvement up to ~ 40 nm compared to the previously-reported Hf0.3Zr0.7O2 thin films. The second method presented in chapter 4 uses the induction of depolarization field by inserting dielectric layer between ferroelectric and electrode layers. Due to the negative capacitance effect of ferroelectric material, the ferroelectric/dielectric bilayer capacitance density observed during the reversible polarization switching can be larger than the capacitance density of the constituent dielectric layer in the bilayer structure. This phenomenon is called capacitance boosting by (transient) negative capacitance effect, and now attracting a great deal of attention in work towards low-power operation of field effect transistors and extremely large capacitance density in dynamic random access memory. However, to date, observation of the NC effect in dielectric/ferroelectric bilayer capacitors has been limited to the use of epitaxial ferroelectric thin films based on perovskite crystal structures, such as Pb(Zr,Ti)O3 and BaTiO3, which is not compatible with current complementary metal oxide semiconductor technology. This chapter, therefore, reports on the transient NC effect in amorphous-Al2O3/polycrystalline-Hf0.3Zr0.7O2 bilayer systems prepared using atomic layer deposition. The thin film processing conditions are carefully tuned to achieve the appropriate ferroelectric performances that are a prerequisite for the examination of the transient NC effect. Capacitance enhancement is observed in a wide voltage range in 5–10 nm thick Al2O3/Hf0.3Zr0.7O2 bilayer thin films. It is found that the capacitance of the dielectric layer plays critical role in the determination of additional charge density induced by the NC effect. In addition, inhibition of the leakage current is important for stabilization of nonhysteretic charge–discharge behavior of the bilayers. The mean-field approximation combined with classical Landau formalism precisely reproduces the experimental results.

Atomic-Scale Investigation of Nano-Domain Structure in HfO2-Based Ferroelectric Thin Films by Advanced (S)TEM Analysis

박건우 서울대학교 대학원 2024 국내박사

Atomic-Scale Investigation of Nano-Domain Structure in HfO2-Based Ferroelectric Thin Films by Advanced (S)TEM Analysis Kunwoo Park School of Chemical and Biological Engineering The Graduate School Seoul National University This thesis delves into the fundamental understanding of ferroelectric properties in nanocrystalline HfO2 thin films, which are gaining significant attention in both academic and semiconductor industry circles due to their novel ferroelectric characteristics at sub-10nm thicknesses. This research not only explores the inherent properties of HfO2 films used in CMOS technology but also unravels the complex mechanisms underpinning the ferroelectric phase in these materials. Key findings include the stabilization of the ferroelectric phase through silicon doping, which leverages silicon's tendency to form strong covalent bonds with oxygen, thereby altering the phase stability in favor of the ferroelectric orthorhombic phase. Scanning Transmission Electron Microscopy (STEM) and Density Functional Theory (DFT) calculations confirm that silicon doping introduces monoclinic structural components into the orthorhombic phase, facilitating ferroelectric transition. Additionally, the study investigates the effects of oxygen deficiency on the ferroelectric switching properties of Si-doped HfO2 films. Controlled annealing experiments demonstrate that high-temperature processes enhance oxygen vacancies, thereby reducing the energy barrier for polarization flipping, as evidenced by STEM, Electron Energy-Loss Spectroscopy (EELS), and first-principles calculations. Furthermore, the research reveals that <111>-oriented grains in yttrium-doped HfO2 (Y:HfO2) films induce a high-symmetry mixed-(R3m, Pnm21) phase, which enhances polarization under in-plane compressive strain. This phenomenon, along with the cooperative effect of nanoscale orthorhombic grains and high-symmetry phase grain boundaries, is crucial in determining the overall ferroelectricity of the Y:HfO2 film. In-depth analysis of ferroelectric domain networks and oxygen position mapping elucidates that polarization is suppressed at vertical domain walls, whereas horizontal domain alignment with zero-width walls shows active polarization. This thesis provides significant insights into the complex nature of ferroelectric HfO2 thin films, offering valuable implications for the future design and application of these materials in advanced semiconductor technologies. Keywords: Ferroelectric HfO2 thin films, nanocrystalline structure analysis, doping effects, high-temperature annealing effects, electron microscopy and spectroscopy, ferroelectric domain network Student Number: 2018-23882 본 논문은 나노 두께의 HfO2 박막이 갖는 강유전 특성에 대한 심층적인 이해를 제공한다. 특히 10nm 이하의 두께에서 나타나는 이 박막의 독특한 강 유전 특성은 학계와 반도체 산업에 큰 관심을 불러일으키고 있다. 본 연구는 CMOS 기술에서 활용되는 HfO2 필름의 기본적인 특성 뿐만 아니라, 이 재료들의 강유전 상태를 형성하는 복잡한 메커니즘을 규명한다. 이 논문의 핵심 발견 중 하나는 실리콘 도핑을 통해 강유전 상을 안정화시키는 기작을 밝혔다는 것이다. 실리콘은 산소와 강한 공유 결합을 형성하는 경 향이 있으며, 이는 강유전 상의 안정성을 변화시키는 데 기여한다. 주사 투과 전자 현미경(STEM)과 밀도 범함수(DFT) 계산을 통해 실리콘 도핑이 강유전상 내부의 단사정 구조 요소를 도입하여 강유전 상태로의 전환을 용이하게 함을 확인했다. 또한 본 연구는 산소 결핍이 실리콘 도핑된 HfO2 필름의 강유전 스위칭 특성에 미치는 영향을 분석한다. 고온 열처리 과정을 통해 조절된 산소 결핍은 분극 반전의 에너지 장벽을 낮추는 것으로 나타났으며, 이는 STEM, 전자 에너지 손실 분광법(EELS), 및 제일원리 계산으로 입증되었다. 이 연구는 또한 이트륨 도핑된 HfO2 (Y:HfO2) 필름에서 <111>-방향 결정 립이 혼합형 고대칭 상(R3m, Pnm21)을 유도하며, 이는 입면 압축 변형 하에서 분극이 강화됨을 밝혀냈다. 나노 수준의 강유전 상 결정립의 상호작용이 Y:HfO2 필름의 전체적인 강유전성을 결정하는 중요한 요소로 나타났다. 강유전 영역 네트워크와 산소 위치 매핑에 대한 정밀 분석은 수직 영역 도메인 벽에서는 분극이 억제되나, 너비가 없는 도메인 벽을 가진 수평으로 정렬된 영역에서는 활성 분극이 관찰됨을 밝혔다. 결론적으로, 이 논문은 강유전 HfO2 박막의 복잡한 특성에 대한 깊은 통찰력을 제공하며, 이를 통해 고급 반도체 기술에서 이 재료들의 미래 설계 및 응용에 대한 중요한 가이드라인을 제시한다.

ALD로 증착한 Hf1-xZrxO2박막에서의 O3 dose 및 적층 방식에 따른 polarization 특성 연구

소남우 연세대학교 대학원 2020 국내석사

Since the ferroelectric properties of Si doped HfO2 were confirmed in 2011, many studies of HfO2-based ferroelectrics have been conducted. Compared to the existing perovskite structure ferroelectric, the fluorite-based HfO2-based ferroelectric exhibits ferroelectric characteristics even at a thin thickness (~ 10nm), which has an advantage in application to DRAM capacitors and 3D NAND Flash. Ferroelectric properties can be obtained by adding various dopants (Si, Zr, Y, Al, Gd, Sr, La, etc.) to the HfO2 film and depositing a capping metal film, and then performing a heat treatment process. Among them, Zr doped HfO2 (HZO) shows the ferroelectric properties within a wide composition range (~ 50%), as well as the antiferroelectric properties when the Zr content is increased. So HZO has been actively researched. Oxidant is used as an oxygen source to form an oxide film during the ALD process. Ozone (O3) and H2O are typical oxidants. Among them, O3 is used as an oxidant because it has a higher oxidizing power than H2O, an excellent ability to remove impurities in thin films, and a short purging time. In this paper, we studied the polarization characteristics according to the O3 dose and HfO2 / ZrO2 stacking sequences in the Hf1-xZrxO2 thin film deposited by ALD. First, the polarization characteristic changes in relation to the change of O3 dose in HfO2 were studied. As a result of the measurement of the polarization voltage, it shows the paraelectric characteristics, and there was no change according to the amount of O3 dose. In the other literature, there was a change in the amount of O3 dose. The reason for the difference between the experimental result and the literature result is expected to be caused by the difference in the process temperature and the O3 dose range. Second, the polarization characteristic changes in relation to the change of O3 dose in ZrO2 were studied. As a result of polarization voltage measurement, it shows antiferroelectric properties, and when the O3 dose increases, the anti-ferroelectric properties increase. The predicted cause derived through physical property analysis and literature investigation is the degree of oxidation of the underlying TiN formed at the beginning of the process when the O3 dose is changed, and the oxygen vacancy in the bulk film decreases after the beginning, causing a change in the oxygen content of the final film and affecting the crystallinity of the tetragonal phase. It is presumed that the antiferroelectric properties change. Third, the change in polarization characteristics of O3 dose change in Hf1-xZrxO2 (HZO) thin film was studied. Hf0.38Zr0.62O2 showed ferroelectric properties, the maximum value of Remnant Polarization (Pr) was confirmed, and Hf0.25Zr0.75O2 showed antiferroelectric-like properties. In each case, the Pr value tends to increase when the O3 dose increases, and it is thought that the expected cause of the polarization change is the same as that estimated in ZrO2. Fourth, the effect of polarization on the HfO2 / ZrO2 stacking sequences in the HZO thin film was studied. Polarization change occurred in the case of Zr-rich Hf0.25Zr0.75O2, and it was confirmed that the Pr value was increased in the ZrO2 starting group (ZH, ZZ) compared to the HfO2 starting group (HH, HZ). As a result of examining the cause of the polarization change through literature review, when the thickness of the ZrO2 starting layer increases, it acts as a seed layer and affects the crystallinity of the HZO thin film, thus increasing the orthorhombic phase in the film, which is estimated to increase polarization. 2011년 Si doped HfO2에서 강유전체 (Ferroelectric) 특성이 확인된 이후로 HfO2 기반의 강유전체 연구가 많이 진행되고 있다. 기존 Perovskite 구조의 Ferroelectric 대비 Fluorite 구조의 HfO2 기반의 Ferroelectric 경우 얇은 두께 (~10nm)에서도 Ferroelectric 특성을 보임으로써 DRAM Capacitor와 3D NAND Flash 등의 Application 적용에 유리한 장점을 가지고 있다. HfO2 막에 다양한 Dopant(Si, Zr, Y, Al, Gd, Sr, La 등)를 첨가하여 Capping Metal막을 증착 한 다음 열처리 공정을 진행하면 Ferroelectric 특성을 얻을 수 있는데, 그 중 Zr doped HfO2(HZO)인 경우 넓은 조성 범위내(~50%) Ferroelectric 특성을 보이고 뿐만 아니라 Zr 함량 증가 시 반강유전체 (Antiferroelectric) 특성을 보여 해당 물질에 대한 많은 연구가 활발히 진행되고 있다. ALD 공정 진행 시 산화 막을 형성하기 위하여 산소 공급원으로 산화제 (Oxidant)를 사용하는데, 대표적인 Oxidant로 Ozone (O3), H2O가 있다. 이 중 O3은 H2O대비 높은 산화력과 박막내 불순물 제거 능력이 뛰어나고, Purging Time이 짧은 이점이 있어 Oxidant로 많이 사용되고 있다. 본 논문에서는 ALD로 증착 한 Hf1-xZrxO2 박막에서의 O3 Dose 및 HfO2/ZrO2 적층 방식에 따른 Polarization 특성에 대한 연구를 진행하였다. 첫번째로, HfO2, 단일 박막에서 O3 Dose변화에 대한Polarization 특성 변화를 연구하였다. Polarization Voltage측정결과 Paraelectric 특성을 보이며, O3 Dose양에 따른 변화가 없었다. 다른 문헌에서는 O3 Dose양에 대한 변화가 있었는데, 실험 결과와 문헌 결과가 차이나는 이유는 공정 온도와 O3 Dose 범위 차이에 의해 발생한 것으로 예상된다. 두번째로, ZrO2, 단일 박막에서 O3 Dose변화에 대한 Polarization 특성 변화를 연구하였다. Polarization Voltage 측정결과 Antiferroelectric 특성을 보이며, O3 Dose 증가 시 Anti-ferroelectric 특성이 증가하였다. 물성 분석 및 문헌 조사를 통해 도출한 예상 원인은 O3 Dose 변화 시 공정 초반에 형성되는 하부 TiN 산화정도와 초반 이후 Bulk막내의 oxygen vacancy감소로 최종 막의 oxygen양의 변화를 일으키게 되고 Tetragonal 상의 결정화도에 영향을 끼쳐 Antiferroelectric 특성이 변화되는 것으로 추정된다. 세번째로, Hf1-xZrxO2 (HZO) 박막에서 O3 Dose변화에 대한 Polarization 특성 변화를 연구하였다. Hf0.38Zr0.62O2에서는 Ferroelectric특성을 보이며, Remnant Polarization(Pr)의 최대값을 확인하였고 Hf0.25Zr0.75O2에서는 Antiferroelectric-like 특성을 보였다. 각각의 경우 O3 Dose 증가 시 Pr값이 증가하는 경향을 보이며, Polarization 변화되는 예상 원인은 ZrO2에서 추정했던 원인과 동일하다고 생각된다. 네번째로, HZO박막에서 HfO2/ZrO2 적층 방식에 대한 Polarization 영향성을 연구하였다. Polarization 변화는 Zr-rich인 Hf0.25Zr0.75O2경우에 발생하며, HfO2 Starting group (HH, HZ)대비 ZrO2 Starting group (ZH, ZZ)에서 Pr값이 커짐을 확인하였다. Polarization 변화되는 예상 원인을 문헌 조사를 통해 파악해 본 결과 ZrO2 Starting Layer의 두께가 증가한 경우 seed layer 로 작용하고 이로 인해 HZO박막의 결정성에 영향을 주어 막 내의 Orthorhombic 상을 증가시켜 Polarization이 커진다고 추정된다

ALD로 증착한 (HfO2)n(ZrO2)n 박막의 적층 방식에 따른 결정성 및 polarization 특성 연구

정주영 연세대학교 대학원 2017 국내석사

비휘발성 메모리인 Ferroelectric Random Access Memory(FRAM)은 Dynamic Random Access Memory(Dram)과 비슷한 구조와 동작원리를 가지고 있다. 그러므로 데이터를 읽고 쓰는 속도가 빠르며 전력 사용이 적고, 데이터 보유 특성이 우수하여 Dram을 대처하는 차세대 메모리로 주목 받고 있다. 하지만 기존 FRAM에 적용하던 물질인 PZT와 SBT 의 경우 두께를 200nm 이상 일 때 Ferroelectric 특성이 나타난다고 보고 되어 메모리로 적용 시킬 때 단위공정 진행의 문제가 발생하였다. 하지만 HfO2 기반의 FRAM은 10nm 에서도 Ferroelectric 특성을 형성 하기 때문에 새로운 FRAM 물질로 주목 받고 있다. 이 논문의 목적은 Atomic layer deposition (ALD)를 이용하여 HfO2 기반 박막의 Ferroelectric 특성에 대한 연구를 진행 하였다. ALD를 이용하여 증착 된 박막은 Grazing Incidence X-ray Diffraction (GI-XRD), High Resolution Transmission Electron Microscopy(HR-TEM)을 통해 결정성을 분석을 하였고, 전기적 분석을 진행 하기 위해 capacitor-Voltage measurement(C-V)와 polarization-Voltage measurement(P-V)를 측정 하여 비교 하였다. ZrO¬2 조성에 따른 결정성 변화를 확인 하기 위해 HfO2과 ZrO2 의 cycle ratio를 달리하여 박막을 증착하고 전기적 변화에 대해 연구를 진행 하였다. 그리고 박막의 온도와 시간의 따른 의존성을 확인 하기 위해 Rapid thermal processing(RTP)를 이용하여 후속 열공정을 시행하여 결정성의 변화를 확인 하고, 그에 따른 분극 특성을 확인하였다. 마지막으로 (HfO2)n(ZrO2)n 적층 방식에 다르게 증착 하였을 때 미세 구조의 변화와 결정성변화가 Ferroecletric특성 변화에 대해 확인하였다. n=2~6 일 때 까지는 HfO2와 ZrO2이 alloy-structure를 형성 하고 있다. n=10 일 때는 TEM 이미지 상으로는 alloy-structure 를 형성 하지만 STEM 이미지에서는 두 물질일 단일 layer로 증착 되는 것을 확인 할 수 있었다. 이로 인하여 lamination 구조를 형성 했을 때는 두 물질이 계면에 만나는 영역이 작아지므로 orthorhombic phase가 감소 하여 잔류 분극이 감소 한다고 판단 할 수 있다. Ferroelectric Random Access Memory has attracted considerable interest for replace Random Access Memory DRAM due to its similar structure and operation principle. Therefore it has fast speed of read and write date speed, low power voltage and date retention for next generation memory. But it is known that previous FRAM material of PZT and SBT is satisfied with thickness over 200 nm to Ferroelectric characteristic. it happen problem a unit process for memory process The objective of this paper is to provide a possibility to use HfO2 Film by using Atomic layer deposition, which can be application for Ferroelectric memory. Its microstructure, crystallinity and electrical have been analyzed using Grazing Incidence X-ray Diffraction (GI-XRD), High Resolution Transmission Electron Microscopy (HR-TEM), capacitor-Voltage measurement and polarization-Voltage measurement We deposited the thin film varying the cycle ratio of HfO2 and ZrO2 and studied on the electrical change to determine the crystalline change in ZrO2 Concentration. To determine the dependence by Annealing temperature and time of the thin film, I proceed the subsequent thermal processes using the Rapid thermal processing (RTP). so I confirmed the crystallinity change and the polarization characteristics Finally, the ferroelectric characteristics of (HfO2)n(ZrO2)n films deposited by atomic layer deposition were investigated. (HfO2)n(ZrO2)n stacks with thickness of 9 nm were deposited on TiN films by the alternating growth of HfO2 and ZrO2 with various cycles of n ranging from 2 to 20. TEM showed a uniformly mixture structure of HfO2 and ZrO2 when each layer was thinner than 10 Å and showed clearly laminated structure when an individual layer was thicker than 10 Å. The remanent polarization of (HfO2)n(ZrO2)n films was measured and maintained to be about 14 μC/cm2 for the n up to 10 and was reduced to 8.8 μC/cm2 for the laminated structure of n = 20. The reduced remanent polarization of the laminated structure is considered to be attributed to reduced amount of orthorhombic phase with reduced interfaces between HfO2 and ZrO2 layers in the laminated (HfO2)n(ZrO2)n, resulting in the reduction of non-centrosymmetric orthorhombic phase.

Analysis of electrical characteristics and application in doped hafnium oxide thin films

Woo, Jongseok Sungkyunkwan university 2021 국내석사

반도체 기술은 반도체 집적회로의 성능이 24개월 마다 2배씩 증가한다는 무어의 법칙에 따라 발전되어왔다. 하지만 소형화 고집적화가 진행되면서 물리적인 한계에 도달했고 이에 누설전류, 문턱전압의 변화 등의 문제를 가지게 된다. 이를 해결하기위해 새로운 물질, 새로운 구조의 소자들이 연구되고 있으며 HfO2 박막은 이러한 문제를 해결하기 위한 물질로서 연구가 되고있다. HfO2 박막은 고유전율을 가지는 물질로 기존의 Si/SiO2 게이트 구조를 대체할 수 있는 물질이다. 고유전율 게이트 유전체를 사용하면 산화막의 두께를 두껍게하면서도 EOT를 유지할 수 있어 게이트 누설전류를 줄일 수 있다. HfO2 박막은 기존의 MOSFET공정과의 호환성, 높은 밴드갭(~5.8eV), 그리고 고유전율(~30)의 장점을 가지고 있어 새로운 게이트 물질로서 많은 관심을 받고 있다. HfO2 물질은 도핑을 통해 유전체에서 강유전체로 변할 수 있다. 이 결과는 2011년에 독일의 NaMLab에서 Si을 도핑한 HfO2이 강유전성을 가지는 것을 처음 확인되었다. 기존에 연구되던 PZT와 BTO등의 Perovskite 구조를 갖는 강유전체는 두께가 ~100nm 정도로 두꺼울 때 강유전성이 나타나는데 반해, HfO2를 기반으로 하는 커패시터에서는 10nm 이하일 때에도 강유전성을 보인다. 따라서 강유전성을 이용한 차세대 반도체소자에 적합한 물질로 주목받고 있다. 이 논문에서는 Atomic Layer Deposition(ALD)를 이용하여 Al과 Zr이 각각 도핑된 HfO2 박막을 제작하여 전기적 특성을 분석하고 각 물질의 특성에 따른 어플리케이션을 만들어 특성을 확인한다. Al이 도핑된 HfO2는 RRAM의 전도층으로 사용되었고, Rapid Thermal Annealing(RTA) 온도에 따른 소자의 특성을 분석하였다. 그 결과 850°C 30초에서 ~450회의 지구성과 ~103의 on/off 전류 비율을 갖는 것을 확인했다. Zr이 도핑된 HfO2에서는 강유전특성을 구현하고 공정의 연속성 유무에 따른 차이를 비교했다. 또한, Fully-Depleted Silicon On Insulator(FDSOI)와 2N7000에 각각 직렬로 연결하여 소비전력을 줄이는 구조의 소자를 제작하여 전류를 제어했고 동작전류를 ~10-8으로 줄여 저전력 소자로의 발전 가능성을 확인했다. Semiconductor technology has been developed in accordance with Moore's law that the performance of semiconductor integrated circuits doubles every 24 months. However, as the miniaturization and highly compacted, the physical limit has been reached, resulting in problems such as leakage current and changes in threshold voltage. To solve this problem, new materials, new structure elements are being studied, and HfO2 thin film is being studied as a material to solve this problem. HfO2 thin film witch is a material with high-k can replace the existing Si/SiO2 gate structure. The use of high-k gate dielectric can reduce gate leakage current by maintaining EOT while thickening oxide film. The HfO2 thin film has the advantages of compatibility with existing MOSFET process, high band gap(~5.8eV), and high dielectric constant(~30), so it is receiving much attention as a new gate material. The HfO2 material can be changed from dielectric to ferroelectric through doping process. The results were first confirmed in 2011 that HfO2, which doped Si in NaMLab, Germany, had ferroelectrics. While the previously studied perovskite structure of PZT and BTO is thick enough to be ~100 nm, the capacitor based on HfO2 shows ferroelectricity even when it is less than 10 nm. Therefore, it is attracting attention as a material suitable for next-generation devices using ferroelectric materials In this paper, using Atomic Layer Deposition(ALD), Al or Zr doped HfO2 thin film, analyze electrical properties, and make an application according to the characteristics of each material to verify the properties. HfO2 in which Al was doped was used as a conductive layer of RRAM, and the characteristics of the device according to the Rapid Thermal Annealing(RTA) temperature were analyzed. The results confirmed that at 850°C 30 seconds to have ~450 endurance and ~103 on/off current ratio. The ferroelectric properties were obtained in Zr doped HfO2 and the differences between the presence and absence of continuity of the process were compared. In addition, ferroelectric HZO capacitor were connected in series to Full-Depleted Silicon On Insulator (FDSOI) and 2N7000 ,respectively, to reduce power consumption, controlling current and reducing the operational current to ~10-8 to confirm the potential for development into low-power devices.

Device and Process Design Based on Functional Oxide Thin Films for Advanced Memory Applications

고상한 경희대학교 대학원 2025 국내석사

In this work, we present an integrated approach for implementing high-performance and high- reliability memory technology by precisely controlling the material-specific electrical properties of functional oxide thin films and applying them to the structural design of memory devices. In particular, representative oxide materials such as HfO2, Al2O3, and InGaZnO (IGZO) were applied to various roles such as ferroelectric layers, insulating layers, and channel layers, respectively, and the memory performance parameters such as charge storage characteristics, bias stability, and data retention characteristics of the devices were experimentally verified by controlling the process conditions and the stacked structure, and the possibility of applying them to memory devices was verified. First, the crystallization behavior and ferroelectric characteristic changes were analyzed by introducing an Al2O3 capping layer on an undoped HfO2 thin film. The crystal phase transition and polarization switching characteristics according to the temperature conditions of the HfO2 atomic layer deposition (ALD) process were evaluated, and it was confirmed that the polarization characteristics, leakage current, and endurance of the ferroelectric capacitor with the metal- insulator-ferroelectric-metal (MIFM) structure were significantly improved through the stabilization of the ferroelectric phase, which is the orthorhombic phase (o-phase). This suggests an effective methodology that can implement excellent ferroelectric characteristics only through interface control without separate doping. Second, by designing the IGZO thin film as a double-layer (DL) structure and depositing it with different oxygen partial pressures (PO2) during the sputtering process, we attempted to improve the performance in a 2-transistor-0-capacitor (2T0C) DRAM cell. The heterogeneous interface formed between the upper and lower IGZO layers served as an additional conduction path, and the storage efficiency and retention time were increased by advantageously utilizing the parasitic capacitance through geometric optimization of the active area. Through this, we propose a performance improvement method of 2T0C DRAM cells by combining structural design and material control. Finally, we designed a HfO2/Al2O3 nanolaminate gate insulator (GI) structure to secure electrical reliability while utilizing high-K dielectrics in the thin film transistor (TFT) constituting the 2T0C DRAM cell. By periodically laminating HfO2 and Al2O3 through a 150 °C ALD process, we simultaneously achieved crystallization suppression and interface defect reduction, which led to low-voltage operation and improved bias stress stability. When applied to an actual 2T0C DRAM cell, it was successfully verified by showing excellent long-term retention behavior. These results provide guidelines for the design of insulators for low-temperature oxide transistor- based memories.

Study on the internal structure of ferroelectric Hf1-xZrxO2 thin film systems

김한준 서울대학교 대학원 2018 국내박사

Ferroelectric (FE) property of HfO2 thin films was first reported in 2011 NaMLab in Dresden, Germany, which had a fluorite structure, doped with a few amounts of Si. It was a very intriguing issue on the FE community because the fluorite-type film has only ~ 10 nm thickness, whereas the conventional perovskite type ferroelectrics have > 100 nm thickness. It has merit for fabrication of 3-dimensional structure due to its small thickness. Also, the band gap of the HfO2 thin film is 5.5 eV which is high enough to prevent leakage currents flowing through devices. Having titanium nitride as a metal electrode, combined with an excellent compatibility with Si, HfO2 as a thin film could be the representative industrial-friendly materials for the adoption of memory production technology. It has been widely accepted that the emergence of unexpected ferroelectricity in HfO2 thin films is due to the formation of non-centrosymmetric orthorhombic Pca21 phase. However, it still lacks researches on the emergence of ferroelectricity in this material systems. Therefore, this dissertation aims to resolve the ambiguity of the origin of the emergence of ferroelectricity in thin films through researches on the internal structure of the FE HfO2 thin films. For its robust ferroelectricity, many dopants were induced. (Si, Zr, Y, Al, Gd, Sr, La, etc.) Among these dopants, Zr doped HfO2 has its wide composition range for emerging various electrical characteristics and lower processing temperature for crystallization of films. Therefore, Hf1-xZrxO2 thin films are up-and-coming FE materials for analyzing the mechanism of emerging ferroelectricity. As the first step, the degradation of the FE properties of atomic layer deposited Hf0.5Zr0.5O2 films with increasing thickness was examined. When the thickness of the film increases over 20 nm, the FE properties of the films start to degrade whereas the 10 nm-thick film shows robust FE properties in previous reports. The origin of the degradation was elucidated by phase transition of non-FE monoclinic phase. According to general thin film growth theory, meanwhile, the grain size of the film increases with increasing film thickness. The grain size is the critical factor to get FE properties of Hf0.5Zr0.5O2 films because the surface energy and volumetric energy are affected by the grain size. Therefore, control of the grain size of films is key point to interrupt degradation of FE properties despite of increasing film thickness. In this dissertation, the grain size is successfully controlled by inserting 1nm-thick Al2O3 interlayer at the middle position of the thickness of the FE film. The Al2O3 interlayer could hinder the continual growth of Hf0.5Zr0.5O2 films, and the resulting decrease of grain size prevented the formation of the non-FE monoclinic phase. The Al2O3 interlayer also principally decreased the leakage current of the Hf0.5Zr0.5O2 films. As the next step, a “wake-up effect” on the FE Hf0.5Zr0.5O2 films was examined which refers to the increase in remanent polarization with increasing electric field cycling number before the occurrence of fatigue effect. In this work, the wakeup effect from the Hf0.5Zr0.5O2 was carefully examined by the pulse-switching experiment. At the pristine state, the Hf0.5Zr0.5O2 film mostly showed the FE-like behavior with a small contribution of antiferroelectric(AFE)-like distortion, which could be ascribed to the involvement of AFE phase. The field cycling of only 100 cycles almost wholly transformed the AFE phase into FE phase by depinning the pinned domains. The influence of field cycling on the interfacial layer was also examined through the pulse-switching experiments. In addition to that, the broken FE hysteresis loops achieved from a Hf0.4Zr0.6O2 film was interpreted based on the first order phase transition theory. The two-step polarization switching, which was expected from the theory, could be observed by dynamic pulse switching measurement. The variations in the interfacial capacitance values along with switching time and number of switching cycles could also be estimated from the pulse switching test. Being different from the one-step polarization switching in other FE films, two-step polarization switching produced two slanted plateau regions where the estimated interfacial capacitance values were different from each other. This could be understood based on the quantitative model of the two-step polarization switching with the involvement of an intermediate nonpolar phase. The Hf0.4Zr0.6O2 film changed from AFE-like to FE-like with increasing number of electric field cycling, which could be induced by the field driven phase change. Finally, this thesis presents a new strategy for extending conventional scaling trend in dynamic random access memory (DRAM) by utilizing newly found morphotropic phase boundary (MPB) of solid solution of the HfO2-ZrO2 system. For the purpose, the schematic phase diagram of HZO films with various thickness and Hf:Zr ratio was presented based on the previous works, and the MPB of tetragonal and orthorhombic phase for an abnormal increase in dielectric constant (r) was found. From the C-V characterizations, the extraordinary r values could be observed in the MPB, and it could be confirmed that the composition of films changes with changing film thickness. The Zr contents for MPB decreased with decreasing film thickness owing to the relative decrease of the free energy of o-phase compared to that of t-phase. The minimum tox of 0.59nm could be achieved for 8.1 nm-thick Hf0.5Zr0.5O2 films.

Novel material and device for ferroelectric memory : thin Hf1-xZrxO2 film and tri-states memory

박민혁 서울대학교 대학원 2014 국내박사

Ferroelectric random access memory (FeRAM) has been considered as one of the best candidates for universal memory. On the other hand, difficult scaling of the memory cell size has hindered the realization of high density FeRAM. Given that size scaling is inherently limited by the complicated crystal structure and difficult processing of ferroelectric materials, exploring new material or new device might be the best solution for that. In this dissertation, Hf1-xZrxO2 thin films and tri-state memory is studied as new material and device for next-generation high-density ferroelectric memory. As the first step, the origin of the ferroelectricity in Hf1-xZrxO2 thin films is examined based on their crystallographic structure, micro-structures and resulting in-plane stress. Although it seems evident that the formation of the non-centrosymmetric Pbc21 orthorhombic phase causes ferroelectricity in the doped and alloyed HfO2-based films, the origin of such evolution has not yet been elucidated. From the electrical and physical characterization on Hf1-xZrxO films with various film thickness (tf) and composition, ferroelectric orthorhombic phase is formed with the composition of ~0.4 - 0.6 and tf of < 25 nm. Even though these conditions are appropriate for the formation of tetragonal phase due to the size effect, the unexpected ferroelectric orthorhombic phase is formed. To elucidate the origin of the phenomenon, the film stress calculated from the change of substrate curvature and that of interplanar distance calculated from the change of diffraction peaks of X-ray diffraction spectra were analyzed, and it could be noticed that the unexpected orthorhombic phase is formed with the tensile strain of >1.5% and the grain size of < 25 nm. The large tensile strain is formed due to the huge tensile stress during the stage of island coalescence of Volmer-Weber type growth. In addition to that, the Hf0.5Zr0.5O2 (HZO) films on Pt bottom electrode (BE) were deposited with tetragonal phase with (111)-prefered orientation, and they hardly showed ferroelectric behavior. On the other hand, the in-plane strain of HZO films on Pt BE was almost equivalent with that on TiN BE, which is large enough for the formation of orthorhombic phase. However, the stress along a-, b-, and c-axis are almost equivalent for the case of (111)-oriented films, so it does not meet the condition of asymmetric stress for orthorhombic phase formation. In addition to that, the effects of annealing temperature (Tanneal) and film tf on the crystal structure and ferroelectric properties of HZO films were examined. The HZO films consist of tetragonal, orthorhombic, and monoclinic phases. The orthorhombic phase content, which is responsible for the ferroelectricity in this material, is almost independent of Tanneal, but decreases with increasing tf. In contrast, increasing Tanneal and tf monotonically increases (decreases) the amount of monoclinic (tetragonal) phase, which coincides with the variations in the dielectric constant. The remanant polarization was determined by the content of orthorhombic phase as well as the spatial distribution of other phases. As the next step, the effects of forming gas annealing (FGA) on the ferroelectric properties of HZO films were examined. Although the H-incorporation during FGA degrades the ferroelectric properties of HZO films, the degree of degradation was much lower compared with other ferroelectrics, such as Pb(Zr,Ti)O3. Pt worked as a catalyst for H-incorporation, and maximum 2Pr loss of ~40% occurred. However, the insertion of a ~20-nm-thick TiN layer between Pt and HZO decreased the degradation to ~12%. HZO is more resistant to degradation by FGA compared with the conventional ferroelectrics, which is a highly promising result for next-generation ferroelectric memory. In addition to that, the effect of the top electrode (TE) on the ferroelectric properties and switching endurance of thin HZO films was examined. The TiN/HZO/TiN capacitor can endure up to 109 times the electric cycling, which is promising for the next-generation memory. RuO2 TE was reduced during annealing due to the reactive TiN BE, resulting in the degradation of the ferroelectric properties and endurance. In addition, the endurance of the TiN/HZO/TiN capacitors was optimized by changing the film thickness and the post-annealing temperature. Finally, this thesis presents a feasible structure and actual operation of a tri-state memory function for high density FeRAM using stacked ferroelectric Pb(Zr,Ti)O3/insulating Al2O3/semiconducting ZnO layers with Pt top and BEs. The complicated electrical responses of the stacked structure to external stimuli were well understood based on the separated trapping of the compensating charges at the Pb(Zr,Ti)O3/Al2O3 and Al2O3/ZnO interfaces and the discrete dissipation of the trapped charges during polarization switching in one direction. This unique function of the structure induced three discrete charge states that can be used to increase the memory density by 50% compared to conventional FeRAM at a given cell size. It is believed that this thesis presents new pathways for the next-generation ferroelectric memory by exploring the new material system of Hf1-xZrxO2 films and by suggesting the new structure and operation of ferroelectric capacitor with novel ferroelectric-insulator-semiconductor heterojunction. Even though the ferroelectric memory is still far from the adoption as a universal memory, the results in this thesis could shed light on this field by suggesting new pathways different from the conventional approach.