We observed the light emission from the indium ultra-thin films induced by scanning tunneling microscopy (STM) and revealed its process, combining the scanning tunneling spectroscopy (STS).

STM is used for researching surface structures, electronic properties, and reactions, while providing information on electro luminescence as a metal-insulator-metal junction[1]. STM-induced luminescence(STML) studies on bulk metal surfaces were reported and its process is understood by the radiative decay of the tip-surface junction plasmon excited by inelastic tunneling. It is of great importance to investigate the effect of altering the sample metal to a thin film because quantum size effects are expected to affect the electronic properties and thus the nature of the light emission. In this study, we created In thin films on Si(1119 substrate and investigated the luminescence properties on them.

We prepared √7×√3 In/Si(111) superstructure which is known to indium double layer structure[2]. We also prepared three-eight atomic layer films layer by layer, depositing to the double layer structure at low temperature. Each film extended over tens of nanometers and exhibited √3×√3 periodicity in STM image, indicating that atomically flat In(111) film is formed.

STS spectra acquired at each film showed some peaks. Analyzing with phase quantization rules[3], it is revealed that the peaks are originated from quantum well states, which come from the confinement of indium sp electrons in direction to the film thickness.

STML from the films was observed and its peaks presented bias voltage and film thickness dependence peculiar to thin films. STML spectra also exhibited the tip shape dependence of intensity distributions. Combining with the STS results, the light emission is attributed to the radiative decay of the junction plasmon which is excited by inelastic tunneling to the QWSs.

[1] R. Berndt, et al., Phys. Rev. Lett., 67, 3796 (1991)

[2] T. Shirasawa, et al., Phys. Rev. B., 99, 100502(R) (2019)

[3] M. Milun, et al., Rep. Prog. Phys., 65, 99 (2002)