高性能增强型Si基ZnO单晶薄膜晶体管的制备与研究

ZnO thin film transistor (TFT), with the advantages of high field mobility, insensitivity with the visible light and controllable electrical properties, is considered as a core cell in the fast developing transparent electronic systems, which could be applied in a variety of areas. So far, most of the research on ZnO TFT is mainly focusing on poly-crystal ZnO with a low field mobility due to the carriers scattering from the grain boundaries. Besides, the concentration of the background carriers in poly-crystal ZnO is usually very high, resulting in a depletion TFT mode where a gate voltage is necessary to cut off the circuit. Moreover, although IGZO-TFT can improve the device performance, the reserve of In and Ga on the earth is very limited, making it urgent to find out an appropriate new material to replace IGZO. Therefore, in this project, high-quality single-crystal ZnO thin film with high carrier mobility is proposed to substitute IGZO as an active layer in the TFT device structure. In order to combine with the mature Si micro-electronic industry, single-crystal ZnO thin film will be deposited on the Si substrate by the unique interfacial engineered molecular beam epitaxial technique. The influence of the growth temperature and film thickness on the carriers will be investigated in order to find out a reproducible way to fabricate the channel layer. High-quality single-crystal oxide insulator films such as MgO, BeO and MgZnO, which are compatible with the interfacial engineering, will be inserted between the Si substrate and ZnO active layer, and their distinctive dielectric properties will also be explored. The properties of the interface between ZnO active layer and the above-mentioned insulator layers will be specially researched with the purpose to decrease the density of interfacial defects. At last, after the improvement of the device structure and ohmic contact, enhancement-mode ZnO-TFT with a high filed mobility and on/off ratio will be realized, which should be a great progress in the field of high-performance ZnO-TFT.

作为迅猛发展的透明电子学器件系统的一个核心元器件，ZnO基薄膜晶体管（TFT）具有迁移率高、对可见光透明、电学性能可调范围大、光敏退化性小等优点，具有广阔的应用前景。目前ZnO基TFT的研究主要存在以下问题：沟道层通常为多晶，迁移率无法进一步提升，工作模式为耗尽型，功耗高；非晶InGaZnO的In和Ga储量十分有限、价格非常昂贵，只能作为过渡阶段产品使用。针对这些现状，本项目拟利用独创的界面控制技术，通过分子束外延法在硅衬底上制备高质量的ZnO单晶薄膜作为n型沟道，研究生长温度及薄膜厚度对载流子浓度的影响，获得沟道层的可控制备工艺；研究MgO、BeO及MgZnO等外延薄膜的介电性能，重点研究ZnO沟道和上述绝缘层的界面特性，获得降低界面缺陷密度的方法。进一步通过优化TFT器件结构以及改善欧姆接触，大幅提高器件迁移率、开关比，从而在低能耗增强型ZnO基TFT的研制中获得重大进展。

作为迅猛发展的透明电子学系统中的一个核心元件，ZnO基薄膜晶体管(thin film transistor, TFT)具有迁移率高、电学可调范围大、对可见光透明、光敏退化性小等优点，具有广阔的应用前景。目前ZnO基TFT的研究主要存在以下问题：沟道层通常为多晶，迁移率难以进一步提升，工作模式多为耗尽型，功耗高；非晶InGaZnO的In和Ga储量十分有限，价格昂贵，只能作为过渡阶段产品使用。针对以上问题，本项目通过独创的界面控制技术，利用分子束外延法在Si衬底上制备了高质量的ZnO单晶薄膜做为n型沟道层，研究了基于MgO、BeO、MgxZn1-xO等不同介电层材料的底栅型单晶ZnO-TFT器件性能，设计了基于MgZnO/ZnO 异质结场效应晶体管、MgZnO/ZnO/MgZnO双异质结场效应晶体管以及双源极ZnO-TFT等新型器件，发现：1). 基于MgO介电层的ZnO-TFT漏电流较大，未表现出TFT开关特性，需进一步优化；2). 基于BeO介电层的ZnO-TFT开关比>107，亚阈值摆幅为0.8V/decade，开启电压为1V，为典型的低功耗增强型工作模式。然而由于BeO/ZnO晶格失配引入的界面缺陷，器件场致迁移率很低，仅为2.13cm2/V/s；3). MgxZn1-xO/ZnO晶格失配很小，可极大降低界面缺陷。当Mg组分增加至50%时，获得的晶体管开关比为105，工作在低功耗的增强型模式下，场致迁移率为~20cm2/V/s，表现出巨大的发展潜力。进一步，设计了基于MgZnO/ZnO/MgZnO双异质结新型场效应晶体管，顶层MgZnO可在ZnO沟道层与Ti/Au电极之间引入肖特基势垒，通过栅压与漏压对势垒高度的共同调控作用，获得了新颖的I-V特性，并应用至紫外探测领域， 可实现“写入”、 “存储”及“清零”等不同功能；4). 为进一步提高器件稳定性，设计了新型双源极器件结构，增加一个中间电极，通过调控中间电极，改变沟道内的电场分布，从而加强对沟道载流子的调控，稳定阈值偏压；5). 设计了源栅短接的器件结构，在柔性全透明衬底上获得了基于ZnO材料的场效应二极管，器件整流比高达5×108，高于普通结型二极管3-4个数量级。以上研究结果填补了单晶ZnO-TFT的研究空白，显示了ZnO-TFT在未来柔性全透明显示技术中的巨大应用前景。