The Transparency Of Cd(0001) Films Grown On Si(111)-7×7: Imaging The Interface At Atomic Scale

As we all know,understanding and controlling the growth of epitaxial films is always an important goal for people to pursue for a long time.Precisely controlling the epitaxial films at atomic scale is always a long cherished goal.To understand the growth law of epitaxial films,we must determine the interface structure buried below the films during the growth of films firstly.Therefore,The study on the interface structure of thin films becomes very necessary.It not only can help to control the growth of epitaxial films,but also reveal the microscopic mechanism and growth law of epitaxial thin films.The interfacial structure plays a crucial role in controlling the growth of epitaxial film.However,it is still a formidable task to observe the interface structure at the atomic scale.The incident wave is usually used as the detection signal to detect the interfacial structure in the experiment.The detected signals from the interface become very weak due to the strong attenuation and decoherence when the incident wave passes through the films,so it is very difficult to obtain the information of the interface structure.With the advent of scanning tunneling microscopy(STM),significant progress has been made in observing the interface structure of thin films.Earlier studies reported that scanning tunneling microscopy has been used to observe the interfacial structure and morphology of metal or Semiconductor substrate buried under the metal films.Altfeder et al.made important contributions in this area of study.They have observed the step of Si(111)-7×7 below the Pb islands by low temperature scanning tunneling microscope and revealed the phenomenon of electron interference fringes is originated from the step of Si in 1997.In the following year,they have demonstrated the typical Si(111)-7×7 superstructure below the Pb islands can be directly imaged by LT-STM.Although the 7×7 superstructures are visible,the 12 Si adatoms in the unit cells can not be resolved.Hence,imaging the 7×7 superstructures below thin films at the atomic scale has not been achieved.In this work,using molecular beam epitaxy we have grown the Cd epitaxial films with highly flat on Si(111)-7×7 substrate,and realized the imaging of interface structure of Cd(0001)at atomic scale by using low temperature scanning tunneling microscopy.Under high bias voltage,we only observe the Si(111)-7×7 reconstruction,but under relative low bias voltage,we can clearly observe the atomic image of Si(111)-7×7.And that means during the growth process of Cd films,the unique structure of Si(111)-7×7 substrate is not damaged and remains intact.We attribute the transparency and high resolution interface imaging of Cd(0001)films to the anisotropic effective mass of the electrons in Cd films.In addition,the quantization of longitudinal electron lead to the transparency of Cd films also.Moreover,imaging capability of interface depends not only on the tip bias but also on the thickness of Cd.For a given island,the lateral resolution of films changes when the tip bias is changed.For a given bias,imaging capability of interface also changes when the number of Cd layers is changed by one.It is cause by quantum size effect.Furthermore,the surfaceroughness of the Cd islands with even or odd monolayers have similar properties.The surfaceroughness of the Cd islands is originated from the electron of disordered clusters from the buried interface,rather than the Cd films.