The Preparation Of Wafer-Scale Two-dimensional MSe₂ Thin Films For Electronics And Optoelectronics

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.