Direct measurement method for lattice spacing on regular crystalline surface using scanning tunneling microscope and laser interferometry

This article describes the instrument used for direct measurement of the lattice spacing on regular crystalline surfaces using a scanning tunneling microscope (STM) as a detector and a phase modulation homodyne interferometer as the reference scale. It consists of a STM head with a YZ-axes tip scanner, a precise X-axis sample stage with flexure springs and interferometer optics. The three-dimensional atomic STM image of the crystalline surface can be obtained by combining the movements of the YZ-axes tip scanner and the X-axis sample stage. The displacement of the X-axis sample stage can be measured directly with the phase modulation homodyne interferometer, which can determine the optical path difference of a wavelength times integer with picometer resolution. To reduce the thermal deformation effects, the STM head and the sample stage were fabricated with low linear thermal expansion cast iron, the X-axis was selected as fast scanning and the measurement axis, and a thermostabilized cell with 0.05 K temperature fluctuation was employed. The lattice spacing on the crystalline surface can be determined from the number of lattice spacings along the X-axis in the atomic STM image and the measured displacement of the sample stage by the interferometer.