Analysis of oxygen vacancies in Ru/ZnO Schottky contacts for a MOM selector and its application for 1R1S structure

Abstract

본 연구는 단방향의 metal/ZnO 쇼트키 접촉을 이용하여 궁극적으로 MOM 구조의 양방향 셀렉터를 제조하는 것에 관한 것이다. 기존에 사용되던 metal/ZnO 쇼트키 접촉은 광학소자용으로 주로 사용되었는데, 이 경우 ZnO의 성질은 그 분야에 요구되는 광학적 성질에 의해서 정해졌다. 즉, ZnO의 밴드갭과 산소 결손에 의한 녹색 영역대 파장 방출의 조절이 metal/ZnO 쇼트키 접촉을 제작하는 데 고려되어야 할 요구사항들이다. 그러나 높은 전자 이동도, 넓은 범위를 가지는 전자 농도, 전도대역 밀도 등은 ZnO의 selector로의 적용 가능성을 보여줌에도 불구하고, ZnO의 이러한 분야로의 적용은 연구 진행이 미진하다. 따라서 본 연구에서는 metal/ZnO 쇼트키 접촉을 이용한 셀렉터 제작 시의 ZnO 물성의 지표를 제시하고, 이를 1R1S 구조에 적용하였다.

2 장에서는 60nm, 100nm, 200nm의 서로 다른 두께의 ZnO를 사용하여 Ti/ZnO/Ru/Si MOM 구조를 제조하여 쇼트키 다이오드 기반 선택기에 사용되는 산화물로서 ZnO의 물성을 제시했다. ZnO의 두께를 증가시키면 결함 밀도가 증가하여 쇼트키 접촉 특성이 나빠졌다. 여기서, 결함의 증가로 인한 광학 유전 상수의 증가가 이미지 힘에 의해 유발되는 장벽 저하 효과에 어떻게 영향을 미치는지 확인하기 위해 기존 쇼트키 방정식을 수정했고, 다른 광학적 측정값과의 비교를 통해 수정된 방정식이 옳음을 증명하였다. 즉, 두께의 증가로 인한 광학 유전율의 증가는 장벽 저하 효과를 약화시킨다. 또한, 결손 증가는 전류 스윕 특성을 저하시킨다. 결과적으로 소자의 통합 측면과 성능을 고려할 때 60nm의 ZnO 박막을 사용하는 것이 좋은 셀렉터를 만드는 방법이다.

3 장에서는 실험 장벽 높이가 높은 Ru/ZnO 쇼트키 접촉으로 인한 문제를 연구하였다. Ru/ZnO 쇼트키 접촉은 이론적으로 0.76 eV 정도의 쇼트키 장벽을 갖지만, 본 연구에서는 1.2 eV 수준의 높은 쇼트키 장벽을 가짐을 확인하였다. 이로 인한 가장 큰 문제는 역 바이어스에서 전류가 포화되지 않고 전기장의 제곱근에 비례한다는 것이다. 이러한 현상은 산소 결손으로 인한 트랩에 갇힌 전자가 여기되고 전도에 관여하는 PFE 메커니즘에 의해 설명될 수 있다. 이러한 문제는 단방향 셀렉터에서 치명적인 누설 전류 증가를 야기할 수 있으며, 또한 단방향 메모리 소자의 파괴에 기인할 수 있어 치명적이다. Ru/ZnO 쇼트키 접촉이 이론값보다 높게 형성된 근거가 될 수 있는 주장은 다음의 두 가지이다. 첫째, 장벽 높이가 증가하는 두 가지 이유는 RuO2의 형성과 Zn-극의 형성 때문이다. 두 경우 모두 현재까지는 산소 결손으로 인해 발생되는 문제로 여겨지며, 산소 결손의 수를 줄임으로써 PFE를 예방할 수 있을 것으로 보인다.

4 장에서는 Ru/ZnO/Ru MOM 구조 대신 질소가 도핑된 ZnO를 사용한 Ru/ZnO:N/Ru MOM 구조를 제조하여 양방향 선택기로 제시했다. 밴드갭은 비슷한 채로, 광학 유전 상수는 감소하였다. 이를 통해 산소 결손이 감소되었다고 가정할 수 있으며, 이러한 결손 감소는 Ru/ZnO 쇼트키 접촉의 특성 개선에 큰 영향을 끼친다. 향상된 쇼트키 셀렉터 특성은 쇼트키 장벽 높이를 감소시키고, 제로-바이어스 편이 현상을 감소시키고, 기존에 발생하던 불균일한 장벽 높이 현상이 제거된 것으로 보인다. 결과적으로, 보다 우수한 Ru/ZnO:N/Ru 셀렉터를 제조하여 1R1S 구조에 적용하였고, 장치가 잘 작동함을 확인 하였다.

결론적으로, Ru/ZnO 쇼트키 접촉의 기본적인 물성과 문제를 단방향 셀렉터 소자를 제작하여 확인하였고, 이를 해결하기 위해 Ru/ZnO:N/Ru 양방향 소자를 제작하여 셀렉터 특성이 향상되었음을 증명하였다. 또한 1R1S 구조 제작 시 소자가 적절하게 동작하는 것을 확인하였다. 따라서 본 연구가 향후 cross array 구조로의 ZnO의 셀렉터 적용 시 가이드라인으로서 역할을 수행할 수 있을 것으로 기대된다.

|Basically, this thesis relates to the fabrication of a bidirectional selector of MOM structure using unidirectional metal/ZnO Schottky contacts. Conventional metal/ZnO Schottky contacts were mainly used for optical devices, in which case the properties of ZnO were determined by the optical properties required in a related field. In other words, the control of green band-to-wavelength emission by ZnO bandgap and oxygen vacancies are requirements to be considered in fabricating metal/ZnO Schottky contacts. However, although high electron mobility, a wide range of electron concentrations, and conduction band densities show the possibility of applying ZnO as a selector, the application of ZnO to this field is not well studied. Therefore, in this study, ZnO properties as a selector using metal/ZnO Schottky contact were presented and applied to the 1R1S structure.

In Chapter 2, Ti/ZnO/Ru/Si MOM structures were fabricated using ZnO with different thicknesses of 60 nm, 100 nm, and 200 nm, and the physical properties of ZnO were presented as oxides used in Schottky diode-based selectors. Increasing the thickness of ZnO increased the defect density, resulting in poor Schottky contact properties. Here, we modified the existing Schottky equation to see how the increase in optical dielectric constant due to the increase in defects affects the barrier lowering effect caused by the image force. the modified equation is well-matched with other optical measurements. That is, the increase in optical permittivity due to the increase in thickness weakens the barrier lowering effect. In addition, increased defects degrade the current sweep characteristics. As a result, considering the performance of the device, using a 60 nm ZnO thin film is an excellent way to make a useful selector.

In Chapter 3, limits caused by Ru/ZnO Schottky contacts with high experimental barrier heights were studied. The Ru/ZnO Schottky contacts theoretically have a Schottky barrier of about 0.76 eV, but in this study, the barrier height shows a high value of 1.2 eV. The biggest problem is that at reverse bias the current is not saturated and is proportional to the square root of the electric field. This phenomenon can be explained by the PFE mechanism in which electrons trapped by the oxygen deficiency are excited and involved in conduction. This problem can cause fatal leakage current increase in the unidirectional selector and the destruction of the unidirectional memory device. There are two possible arguments that the Ru/ZnO Schottky contact can be formed higher than the theoretical value. The first one is the increased barrier height by the formation of RuO2, and second is the formation of the Zn-polar interface. In both cases, it is considered to be a problem caused by oxygen deficiency, and it is possible to prevent PFE by reducing the number of oxygen deficiencies.

In Chapter 4, a Ru/ZnO:N/Ru MOM structure using nitrogen-doped ZnO instead of the Ru/ZnO/Ru MOM structure was prepared and presented as a bidirectional selector. The optical dielectric constant decreased, while the bandgap was similar. This may be a reduction of oxygen deficiency, which has a significant impact on improving the properties of the Ru/ZnO Schottky contacts. Improved selector characteristics with Schottky contact seem to reduce Schottky barrier height, reduce zero-bias shift, and eliminate the existing non-uniform barrier height. As a result, a better Ru/ZnO:N/Ru selector was prepared and applied to the 1R1S structure, it was confirmed that the device works well.

In conclusion, the basic physical properties and problems of the Ru/ZnO Schottky contacts were confirmed by the fabrication of a unidirectional selector. To solve this problem, we fabricated Ru/ZnO:N/Ru MOM bidirectional selector. It was also confirmed that the device works properly when fabricating the 1R1S structure. Therefore, this study is expected to play a role as a guideline when applying a ZnO selector to a cross array structure.; Basically, this thesis relates to the fabrication of a bidirectional selector of MOM structure using unidirectional metal/ZnO Schottky contacts. Conventional metal/ZnO Schottky contacts were mainly used for optical devices, in which case the properties of ZnO were determined by the optical properties required in a related field. In other words, the control of green band-to-wavelength emission by ZnO bandgap and oxygen vacancies are requirements to be considered in fabricating metal/ZnO Schottky contacts. However, although high electron mobility, a wide range of electron concentrations, and conduction band densities show the possibility of applying ZnO as a selector, the application of ZnO to this field is not well studied. Therefore, in this study, ZnO properties as a selector using metal/ZnO Schottky contact were presented and applied to the 1R1S structure.

In Chapter 2, Ti/ZnO/Ru/Si MOM structures were fabricated using ZnO with different thicknesses of 60 nm, 100 nm, and 200 nm, and the physical properties of ZnO were presented as oxides used in Schottky diode-based selectors. Increasing the thickness of ZnO increased the defect density, resulting in poor Schottky contact properties. Here, we modified the existing Schottky equation to see how the increase in optical dielectric constant due to the increase in defects affects the barrier lowering effect caused by the image force. the modified equation is well-matched with other optical measurements. That is, the increase in optical permittivity due to the increase in thickness weakens the barrier lowering effect. In addition, increased defects degrade the current sweep characteristics. As a result, considering the performance of the device, using a 60 nm ZnO thin film is an excellent way to make a useful selector.

In Chapter 3, limits caused by Ru/ZnO Schottky contacts with high experimental barrier heights were studied. The Ru/ZnO Schottky contacts theoretically have a Schottky barrier of about 0.76 eV, but in this study, the barrier height shows a high value of 1.2 eV. The biggest problem is that at reverse bias the current is not saturated and is proportional to the square root of the electric field. This phenomenon can be explained by the PFE mechanism in which electrons trapped by the oxygen deficiency are excited and involved in conduction. This problem can cause fatal leakage current increase in the unidirectional selector and the destruction of the unidirectional memory device. There are two possible arguments that the Ru/ZnO Schottky contact can be formed higher than the theoretical value. The first one is the increased barrier height by the formation of RuO2, and second is the formation of the Zn-polar interface. In both cases, it is considered to be a problem caused by oxygen deficiency, and it is possible to prevent PFE by reducing the number of oxygen deficiencies.

In Chapter 4, a Ru/ZnO:N/Ru MOM structure using nitrogen-doped ZnO instead of the Ru/ZnO/Ru MOM structure was prepared and presented as a bidirectional selector. The optical dielectric constant decreased, while the bandgap was similar. This may be a reduction of oxygen deficiency, which has a significant impact on improving the properties of the Ru/ZnO Schottky contacts. Improved selector characteristics with Schottky contact seem to reduce Schottky barrier height, reduce zero-bias shift, and eliminate the existing non-uniform barrier height. As a result, a better Ru/ZnO:N/Ru selector was prepared and applied to the 1R1S structure, it was confirmed that the device works well.

In conclusion, the basic physical properties and problems of the Ru/ZnO Schottky contacts were confirmed by the fabrication of a unidirectional selector. To solve this problem, we fabricated Ru/ZnO:N/Ru MOM bidirectional selector. It was also confirmed that the device works properly when fabricating the 1R1S structure. Therefore, this study is expected to play a role as a guideline when applying a ZnO selector to a cross array structure.