【作者】 代天军；

【导师】 李言荣；

【作者基本信息】 电子科技大学 ， 微电子学与固体电子学， 2019， 博士

【摘要】 近年来,因其优异的光电子学、电子学、化学和机械学性能,二维（2D）层状结构过渡金属二硫族化合物（TMDs）受到广泛关注,就电子学和光电子学而言,具有半导体特性的TMDs最为重要,虽然,人们已经演示了其在电子学和光电子学中的巨大应用潜力,但要实现TMDs从实验室概念器件演示到实质工业应用,仍有许多基础和技术问题需要解决,其中,晶圆级二维材料的可控制备仍然面临许多挑战。虽然,人们尝试了各种方法制备晶圆级少层及单层TMDs薄膜,但仍然缺乏层数可控的晶圆级外延单晶薄膜制备的有效方法。本论文以MSe₂为研究对象,其中过渡金属M选为Mo,围绕晶圆级TMDs的可控制备这一技术目标,分别采用原子层沉积（ALD）技术和分子束外延（MBE）技术,开展晶圆级均匀连续的2D MoSe₂薄膜制备研究,结合微观结构及电学特性测量,初步探索了所制备MoSe₂薄膜的器件应用,论文的整体研究工作如下:基于ALD沉积的自限制生长模式在薄膜厚度控制方面的优势,论文以Mo（CO）₆作为Mo源,采用氧等离子体作为氧源,先开展了MoO₃薄膜的ALD生长工艺研究,在生长温度为162℃时,实现了原子层沉积模式,生长速率为0.076nm/cycle,通过ALD沉积周期数实现了所沉积MoO₃薄膜厚度的精确控制,在此基础上,对所制备MoO₃薄膜进行了晶化处理,表征了其晶体结构和形貌特征变化,并测量了所沉积MoO₃薄膜的光学性质和介电特性,结果显示:所沉积MoO₃薄膜的禁带宽度约为4 eV,介电常数约为17,在1 V偏压下,薄膜的纵向漏电电流仅为6.43×10^-77 A/cm²,绝缘特性良好。在实现了MoO₃薄膜厚度控制的基础上,论文进一步利用双温区CVD设备,在富Se的气氛下,对所沉积MoO₃薄膜进行硒化处理,以制备MoSe₂薄膜（为了描述简便,本论文称该方法为“两步法”）,对所制备MoSe₂薄膜的结构、组分、形貌、厚度和均匀性进行了表征。表征结果显示:在优化条件下,所制备的MoSe₂薄膜为晶圆级均匀连续的少层二维材料,所制备MoSe₂薄膜的层数与MoO₃薄膜ALD沉积的周期数具有强列的线性关系,因此,通过控制MoO₃薄膜ALD沉积周期数,实现了所制备MoSe₂薄膜层数的准确控制。论文采用光刻工艺,研制了叉指电极结构的光电导器件和背栅结构的场效应晶体管（FET）,表征了“两步法”所制备MoSe₂薄膜的光电特性和电学特性。通过在MoSe₂薄膜表面不同区域所制备叉指电极结构光电导器件的性能统计显示:所制备晶圆级MoSe₂薄膜面内均匀性良好,且少层MoSe₂薄膜具有优异光电响应特性,在638 nm波长的光照下,其光电导器件的光/暗电流比达到690,响应时间约为22 ms,器件的光响应度约100 A/W,外量子效率（EQE）值约为19668%,器件的比探测率D^*值高达2×10¹³ Jones。此外,基于背栅结构的FET性能测试结果显示:采用两步法所制备的2H-MoSe₂为p型导电特性,载流子沟道迁移率为0.08 cm² V^-11 s^-1。虽然,基于ALD技术的“两步法”在层数控制方面具有优势,但在硒化处理阶段,薄膜也同时经历着固相外延的过程,由于MoO₃和MoSe₂的层间van der Waals（VdWs）结合特点,固相外延生长MoSe₂薄膜的成核位置和成核密度难以有效调控,导致所制备MoSe₂薄膜呈现出多晶结构特征。为了提高所制备晶圆级MoSe₂薄膜的晶体质量,本论文进一步采用MBE技术生长了MoSe₂薄膜。为此,论文以高纯MoSe₂颗粒和Se颗粒为源料,分别采用电子束蒸发器和低温束源炉产生MoSe₂和Se分子束流,以Si、SiO₂/Si、Graphene（Gr）/Si和Gr/SiO₂/Si为衬底,研究了MoSe₂薄膜的制备（本论文称该方法为“一步法”）,并采用原子力显微技术（AFM）、透射电子显微镜技术（TEM）、拉曼光谱分析技术等手段,对“一步法”制备的MoSe₂薄膜进行了表征。研究结果显示:所生长材料连续均匀,且为六角晶体结构的2H-MoSe₂薄膜,与“两步法”相比,在确保晶圆级薄膜层数控制的基础上,显著提高了所制备MoSe₂薄膜的晶体质量。在优化的生长温度下,通过控制沉积速率和沉积时间,实现了所生长MoSe₂薄膜原子层数的有效控制,成功制备出3层和7层的MoSe₂薄膜,其中,7层MoSe₂薄膜的禁带宽度约为1.42 eV。在制备出了高质量MoSe₂薄膜的基础上,论文采用聚甲基丙烯酸甲酯（PMMA）辅助转移方法,实现了晶圆级MoSe₂薄膜的完整转移,并采用光刻工艺,研制了叉指电极结构的光电导器件和背栅结构的FET,表征了“一步法”所制备MoSe₂薄膜的光电特性和电学特性。研究结果显示,采用“一步法”所制备MoSe₂薄膜具有优异的光电响应特性,在600 nm波长的光照下,其光电导器件的光响应度约为120A/W,响应时间约为40 ms,比探测率D^*约为3.6×10¹³ Jones。但与“两步法”所制备MoSe₂薄膜的p型导电特性不同,采用“一步法”所制备的MoSe₂薄膜表现为n型导电特性,且其FET呈现出双极型特性,空穴沟道迁移率大约为0.01 cm²V^-11 s^-1。在MoSe₂薄膜晶体质量改善的基础上,论文构造了Gr/MoSe₂ VdWs异质结,并研制了顶栅结构的FET,测量结果显示:器件表现为NMOS特性,开关比高达10⁵,电子迁移率约为410 cm² V^-11 s^-1,有望为二维材料的电子学应用发展出一条可行的技术方案。更多还原

【Abstract】 In recent years,the layered transition metal dichalcogenides（TMDs）,a class of two-dimensional（2D）materials,have attracted much attention due to their excellent optoelectronic,electrionic,chemical,and mechanical properties.The semiconductor property of TMDs is most important for electronics and optoelectronics.Although its great potential in electronics and optoelectronics has been demonstrated,there are still many technical issues that need to be addressed to realize TMDs from conceptual device demonstration to practical industrial application.For example,there are still many challenges for wafer-scale 2D materials synthesis with thickness controllability.Various methods have been tried to produce few-layer/single layer TMDs with wafer-scale,but there is a lack of an effective method that can realize the epitaxy of wafer-level single crystal thin films with layer controllability.In this dissertation,MSe₂（M=Mo）was chosed for exploring a controllable preparation technology that can produce wafer-scale TMDs.Both atomic layer deposition（ALD）and molecular beam epitaxial（MBE）techniques were employed to prepare 2D MoSe₂ thin films with wafer-scale uniformity and continuity.Based on the measurement of microstucture and electrical properties,the device applications of the prepared MoSe₂ thin films were preliminarily explored.The overall research content and results are as follows:The self-limiting growth characteristic renders ALD a great advantage in film thickness control.Thus,this work introduced a“two-step”method to grow wafer-scale MoSe₂ by selenizing molybdenum oxide（MoO₃）thin films in a Se rich atmosphere,where the MoO₃ thin films were deposited by ALD.Firstly,the growth process characteristics of the ALD MoO₃ thin films were investigated,using molybdenum hexacarbonyl（Mo（CO）₆）and oxygen plasma as the starting materials.Self-limiting growth was verified at a deposition temperature of 162℃,and the growth rate was determined to be 0.76?/cycle.The thickness of the MoO₃ films can be precisely controlled by adjusting the number of ALD cycles.Afterward,the prepared MoO₃ films were crystallized,and their crystal structure and morphology were characterized.The optical properties and dielectric properties of the deposited MoO₃ films were measured.The results show that,the bandgap of the as-grown MoO₃ is 4 eV and its dielectric constant was found to be about 17.Moreover,a very low leakage current of 6.43×10^-7A/cm² was obtained at 1 V gate bias,indicating the good insulating properties of the films.The structure,composition,morphology,thickness and uniformity of the produced MoSe₂ films were characterized.The results show that,the few-layer MoSe₂ films with wafer-scale uniformity and continuity can be achieved,and the thicknesses of the synthesized MoSe₂ films can be precisely controlled by adjusting the number of MoO₃ALD cycles.In order to investigate the photoelectric and electrical properties of the MoSe₂ films synthesized by the two-step method,the photodetectors（PDs）with an interdigital geometry and field effect transistors（FETs）with back gate structure were developed.The performance statistics of PDs displayed that the device behaviors are reproducible in a large number of devices owing to the uniformity of large area MoSe₂films.Impressively,the few-layer MoSe₂ exhibited excellent optoelectronics characteristics under 638 nm laser illumination,including a high light/dark current ratio of 690,a fast response time of 22 ms,an ultrahigh photoresponsivity approaching 100A/W,a high external quantum efficiency（EQE）of¹9668%and a high specific detectivity of up to 2×10¹³ jones.Furthermore,the FET exhibited a field-effect mobility of 0.08 cm² V^-11 s^-1,and revealed that the 2H-MoSe₂ prepared by the two-step method is p-type conductive.Although the“two-step”method based on ALD shows great advantages in layer number control,there is a process of solid phase epitaxy during the conversion from MoO₃ to MoSe₂ in selenization,which makes it difficult to regulate the nucleation sites and nucleation density of MoSe₂ films effectively,resulting in the polycrystalline nature of MoSe₂.Thus,in this work,molecular beam epitaxy（MBE）was employed to produce MoSe₂ films for the purpose of improving the crystal quality of the wafer-level MoSe₂film（“one-step”method）.2D semiconducting MoSe₂ films were successfully synthesized on Si,SiO₂/Si,Graphene（Gr）/Si and Gr/SiO₂/Si substrates,respectively,using MoSe₂ and Se particles as the starting materials.The as-synthesized 2H-MoSe₂ films were characterized by atomic force microscopy（AFM）and transmission electron microscopy（TEM）,Raman spectrum,and so on,revealing six polycrystalline layers of hexagonal structure and with wafer-scale uniformity and continuity.Compared with the“two-step”method,the crystal quality of the prepared MoSe₂ film was significantly improved.3-layer and 7-layer MoSe₂ thin films were successfully prepared at the optimized growth conditions,demonstrating that the atomic layers of MoSe₂ films can be effectively tuned.The bandgap of 7-layer MoSe₂ was determined to be 1.42 eV.The as-grown wafer-scale MoSe₂ film on SiO₂/Si substrate can be easily transferred by the PMMA-assisted transfer method.PDs based on MoSe₂ thin layers synthesized by the one-step method in an interdigital geometry demonstrate excellent photoelectric performance under the illumination of 600 nm incident light,including a high photo-responsivity of 120 A/W,a rapid response time of 40 ms,and a ultrahigh detectivity D^*of 3.6×10¹³ jones.FET fabricated with MoSe₂ film shows ambipolar behavior,and the the field-effect holes mobility of the device is about 0.01 cm² V^-11 s^-1.Unlike the p-type of the MoSe₂ film prepared by the two-step method,the MoSe₂ film synthesized by the one-step method exhibits n-type conductivity characteristics.On the basis of the improved crystal quality of MoSe₂ thin film,a FET based on a Gr/MoSe₂ van der Waals（vdWs）heterostructure with a top-gate geometry was constructed,which demonstrated a NMOS digital behavior,a current ON/OFF ratio of up to 10⁵ and a field-effect mobility of 410 cm² V^-11 s^-1.It is expected to develop a feasible technical scheme for the electronic application of 2D materials.更多还原