The Preparation of Wafer-Scale Two-dimensional MSe2 Thin Films for Electronics and Optoelectronics

Author:Dai Tian Jun

Supervisor:li yan rong


Degree Year:2019





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.