PLD Growth And Properties Of Epitaxial SnO Thin Films

As one of the typical oxide semiconductor materials,SnO is a native p-type material and shows great protential for production of transparent conductive layer and p-type oxide semiconductors-based thin-film transistors(TFTs).However,SnO is less well characterized,like its p-type conductive mechanism and relevant physical and electrical property still need further study.The main reason is that there seldom single-crystals available that could facilitate more detailed studies of SnO.Therefore,high quality SnO films lays foundation of its further reseach and prossesses academic merits and practical utilities.In the present work,we use metallic Sn target and O2 as reactive gas but PLD technique to grow epitaxial SnO films.We systematically studied the influced of oxygen pressure and substrate temperature on the growth of SnO epitaxil thin films,and the thickness dependent properties of epitaxial SnO thin films were studied as well.The main contents and results are as follows:Firstly,we investigated the infuluced of oxygen pressure on the growth of SnO epitaxil thin films.Both the epitaxil SnO and SnO2 thin films were achieved by tuning the O2 pressure.As the substrate temperature was fixed at 575℃,the films deposited at O2 pressure of 0.3 Pa showed the best quality.The films showed an in-plane epitaxial relationship with the substrate of SnO(001)//Al2O3(1012)and SnO(110)//Al2O3(1120).Furthermore,the EF was very close to the top of the VBM of SnO while for the SnO2 film,the energy band gap was observed below EF,which supported electronic structure of the p-type in the SnO film and the n-type one in the SnO2 film.For the Raman result of pure eptaxil SnO films,only A1g mode presented.Secondly,the infuluced of substrate temperature on the growth of SnO epitaxil thin films was investigated.The results showed that the films deposited at substrate temperature of 575℃ showed the best quality with the O2 pressure fixed at 0.3 Pa.It also indicated that the films deposited above 400℃ showed a crystalline SnO structure,while the films deposited at 700℃ showed SnO2 and Sn mixed structure.Further more,we found the film deposited at substrate temperature of 600℃ showed the tin oxide and intermediated mixed phase.And we defined the intermediate phase as Sn3O4,combining the result from XRD and in situ X-ray Raman.The surface morphlogy showed that the surface of the films deposited at high substrate temperature showed smooth and small root mean square roughness(RMS).The optical bandgap were also investegated,we found that the substrate temperature had little influence on the optical band gap of SnO thin films.Finally,SnO thin films were epitaxially grown on Al2O3(1012)substrates with the thickness systematically varied from 3.54 to 60.88 nm.The thickness dependent properties have been investigated.Consequently,we conclude the critical coherent length of epitaxial SnO thin films is 19 nm along with the c-axis.When the film thickness below critical value,the in-plane lattice constant a shrinked and the out-of-plane constant c expanded initially,comparing with the values of the bulk state.As the thickness further increasing above critical value,the lattice constant a expanded while the c-axis lengths shrinked gragually.And both of the lattice constant a and c were constant to the value of 3.807A and 4.867A eventually,which were close to the lattice parameters of bulk SnO.We also found that the bandgap Eg showed a blueshift which was believed to be due mainly to quantum size effect.When changing the film thickness decrasing from 25.61 nm to 3.54 nm,the bandgap of SnO films were tuned from 2.64 eV to2.89 eV.Based on the fitting result,Eg(R→∞)value of 2.66 eV,me*=0.133 me and the exciton diameter of 63.4 A were obtained as well.These physical parameters were all obtained based on the experiment for the first time which is of great significance to the further study of SnO.