【作者】 张杰；

【导师】 叶志镇； 吕建国；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2014， 博士

【摘要】 近年来,基于氧化物半导体的薄膜晶体管(TFTs)因在未来大尺寸、高帧率和高分辨率的平板显示器中的潜在应用而备受关注。现今的有源矩阵液晶显示器(AMLCD)和有源矩阵有机发光二极管显示(AMOLED)普遍采用的是非晶硅的TFT技术。和非晶硅TFT技术相比,氧化物半导体TFT技术存在诸多优点,包括高迁移率、高光学透过率和低工艺温度等。这些优点使得氧化物TFT技术更适合运用于透明显示和柔性显示。在本文中,我们研究了三种不同的氧化物半导体,分别是氧化锌(ZnO)、铟镓锌氧(InGaZnO, IGZO)和铟铝锌氧(InAlZnO, IAZO)。本文的主要研究内容和结果可以概括如下：1.我们制备了以ITO和金属Cr作为源漏电极的ZnO TFTs,研究了源漏电极与ZnO沟道层的接触对TFT性能的影响。ITO电极能与沟道层形成了更好的接触,其器件性能也明显优于Cr电极的TFT。2.我们采用了退火控制ZnO薄膜载流子的方法来实现ZnO TFTs的器件特性。发现对室温制备的ZnO TFTs,在N2气氛下采用低温退火比较合适,既能增加ZnO沟道层的迁移率和降低缺陷态密度,又能保证低的关态电流。低温退火的缺点是不能消除薄膜中高温退火才能消除的缺陷态。3.我们利用TFT的结构研究了ZnO薄膜中持续光电导(PPC)现象,发现栅极电压能够控制光电流衰退的快慢。我们提出一个可能的模型解释这种栅压可控PPC现象的原因。在栅极电压的作用下,电子与电性相反的氧空位Vo2+和空穴的分离,导致复合速率的下降。4.我们制IGZO TFTs并研究了其稳定性,发现了IGZO TFTs的两种不稳定机制。室温制备的IGZO TFTs具有栅压连续扫描不稳定性,表现为第二次扫描的转移特性曲线出现大的偏移,这是由沟道中浅陷阱缺陷导致的不稳定性。这些浅陷阱缺陷是由氧空位与薄膜中的空隙共同引起的,通过氧气气氛高温退火能够消除这种缺陷。退火后的IGZO TFTs具有栅极偏压工作不稳定性,表现为转移特性曲线平行右移而其亚阈值摆幅S几乎不变。栅极偏压不稳定性归因于绝缘层与沟道层界面的陷阱机制。5.我们在室温条件下沉积了非晶IAZO薄膜,研究了退火温度对薄膜结构,形貌,光学和电学性能的影响。薄膜的非晶态结构即使在高温退火下也能稳定存在。退火处理会影响薄膜的原子重新排列,但对表面形貌影响不大。退火后薄膜的电阻率不断下降,迁移率也有所提高。随着退火温度的升高,薄膜的吸收边发生红移,其光学禁带宽度不断下降。6.我们制备了IAZO TFTs,发现其具有与IGZO TFTs相当的电学性能。低温退火的IAZO TFTs具有跟室温制备的IGZO TFTs一样的连续扫描不稳定性。XPS的结果显示退火温度升高,氧空位含量减少,表明引起不稳定的浅陷阱缺陷与氧空位相关。高温退火后这种不稳定性消失,只存在界面陷阱机制引起的偏压工作不稳定性。7.我们研究了环境因素对IAZO TFTs稳定性的影响,验证了空气成分如何影响TFT性能。干燥的N2和02几乎不影响TFT性能。湿度对TFT的性能有很大影响,一方面增加了沟道层的表面电导,另一方面导致阈值电压的偏移。进一步的偏压稳定性测试显示,氧气的湿度越大,阈值电压变化越大,稳定性越差。我们提出水辅助氧吸附的模型解释了湿氧对氧化物TFT稳定性的影响。更多还原

【Abstract】 Recently, thin-film transistors (TFTs) based on oxide semiconductors have attracted considerable attention because of their potential application in the next generation flat panel displays, which are characterized by large screen, high frame rate and high resolution. Compared to amorphous silicon TFTs that is currently used in active-matrix liquid crystal display (AMLCD) and active-matrix organic light-emitting-diode display (AMOLED), oxide semiconductors TFTs have numerous advantages including high mobility, high optical transparency and low processing temperature, making them suitable to be used in transparent and flexible display.In this thesis, our work focused on three different oxide semiconductors:zinc oxide (ZnO), indium gallium zinc oxide (InGaZnO, IGZO) and indium aluminum zinc oxide (InAlZnO, IAZO). The major contents of this thesis are summarized as follows:1. ZnO TFTs with ITO or Cr as Drain/Source electrodes were fabricated. The effect of contact with different electrodes on the performance was investigated. ITO electrodes can form a better contact with the ZnO channel, thus TFTs with ITO electrodes showed superior performance than TFTs with Cr electrodes.2. Annealing treatment was used to control the carrier concentration in ZnO films and achieve the operation of ZnO TFTs. For as-deposited ZnO TFTs, low-temperature annealing under N2is preferable. Because the low-temperature can not only increase the mobility and reduce the defects density, but also keep a low off-current. The disadvantage of low-temperature annealing is its disability to remove the defects that only high-temperature annealing can do.3. We employed a TFT device to investigate the persistent photoconductivity (PPC) in ZnO films. The phenomenon that the rate of photocurrent decay can be controlled by the gate bias was observed. We proposed a plausible model to explain the PPC controlled by gate-bias. Under the gate-bias, the electric field will drive electrons away from the electropositive oxygen vacancies (VO2+) and holes, leading to the reduction of recombination rate between electrons and VO2+or holes.4. The IGZO TFTs were fabricated and their stability was investigated. Two different instability mechanisms were observed in IGZO TFTs. The first one is shallow trap mechanism that happens to as-deposited IGZO TFTs when the transfer characteristic curves were continuously swept. These shallow traps are formed when an oxygen vacancy site is adjacent to a large open space in the film. And high-temperature annealing with O2can annihilate these shallow traps. After high-temperature annealing, IGZO TFTs only suffer from gate-bias stress instability, characterized by parallel positive threshold voltage shift with no change of sub-threshold swing. The interface trapping mechanism is responsible for gate-bias stress instability.5. Amorphous IAZO films were deposited at room temperature and annealed at different temperatures. The effects of annealing temperature on the structural, morphology, optical and electrical performance of the films were investigated. The amorphous structure is stable even at high-temperature annealing. The annealing treatment would lead to redistribute of atom in the film while show no significant influence on the morphology. The annealing treatment also increased the mobility and reduced the resistivity of the film. Red shift was observed for the absorption edge for the film and the optical band gap narrowed as the annealing temperature increase.6. IAZO TFTs were fabricated, showing comparable electrical performance to IGZO TFTs. IAZO TFTs annealed at low temperature had the positive shift of transfer characteristic curve under continuous sweeping, as same as as-deposited IGZO TFTs. XPS results showed the number of oxygen vacancies reduced as the annealing temperature increased, indicating the shallow traps are related to oxygen vacancies. This instability disappear after high-temperature annealing, only left the gate-bias stress instability cause by interface trapping site.7. The impact of ambient on stability of IAZO TFTs was investigated and how the ingredients of ambient affect the TFT performances was confirmed. It was observed that dry O2and N2had little influence on the TFT performances, while the relative humidity of O2had significant impact on the TFT performances. Firstly, it induced the back surface conductivity of the channel layer. Secondly, it led to positive shift of threshold voltage. Further study on the gate-bias stressing showed the larger of humidity of O2, the large shift of threshold voltage and the less stable for the TFT devices. A water-assisted oxygen absorption model was proposed to account for the impact of wet O2on stability of oxide TFT.更多还原