We not only perform growth and scanning tunneling microscopy studies of magnetic films on semiconductors using molecular beam epitaxy / scanning tunneling microscopy system in an existing lab, but we also carry out development of a novel molecular beam epitaxy/pulsed laser deposition and superconducting magnet cryogenic spin-polarized scanning tunneling microscopy system in a completely new lab.
We study the growth of iron nitride on gallium nitride using molecular beam epitaxy with Fe e-beam evaporation and radio frequency N-plasma growth. Thin iron nitride layers of thickness about 16 nm are grown and monitored in situ using reflection high energy electron diffraction. The samples following growth are analyzed ex situ using a variety of techniques including X-ray diffraction, Rutherford backscattering, and atomic force microscopy. The crystal phase and orientation with respect to the GaN substrate are deduced by monitoring the structure, morphology, and lattice constant evolution of the iron nitride film. The growth is discussed in terms of a 2-dimensional to 3-dimensional growth mode transition.
The investigation of the initial phase of sub-monolayer iron deposition on GaN(0001) pseudo-1×1-1+1/12 surface is carried out. To begin with, we verify an atomically smooth GaN growth surface with in situ reflection high energy electron diffraction. Scanning tunneling microscopy shows smooth terraces separated by single and double height bilayer atomic steps. About 0.42 ML iron is deposited on a smooth GaN surface, and the subsequent scanning tunneling microscopy images reveal waffle-like 2-dimensional islands with a height of ~ 1.8-2.0 Å, growing in a 2-dimensional mode outward from the GaN step edges of the pseudo-1×1-1 + 1/12 surface. A clear 6×6 structure is observed for the islands. The waffle-like islands also grow in the GaN spiral growth regions.
Studies of iron/Ga-rich N-polar GaN(000-1) reveal the formation of quantum spintronic nanostructures on N-polar GaN(000-1), formed by the deposition of iron onto the gallium-rich surface and investigated using scanning tunneling microscopy in situ. The iron-induced islands are spontaneously formed after Fe deposition in the temperature range of 210-360 °C. Higher deposition temperature leads to larger width islands with uniform quantum thickness. This thickness corresponds to about 3 (or 4) atomic layers. The islands also exhibit an atomically smooth surface with a zig-zag row structure having 4×2 periodicity.
For the increasing need of nitride semiconductors and semiconductor based structures in electronic, optoelectronic and spintronic applications, and due to the powerful capabilities of scanning tunneling microscopy and spin-polarized scanning tunneling microscopy in probing structural, electronic, and even magnetic properties, we carry out the development of a nitrogen plasma assisted molecular beam epitaxy/pulsed laser epitaxy facility integrated with a cryogenic superconducting magnet scanning tunneling microscope system. The custom-designed molecular beam epitaxy growth system supports up to eight sources, including up to seven effusion cells plus a radio frequency plasma source, for growing a variety of complex materials, such as nitride semiconductors and magnetic materials, and incorporates in situ reflection high energy electron diffraction and pulsed laser epitaxy. The custom-designed STM head has a modular design, consisting of an upper body and a lower body. The upper body contains the approach and scanning mechanism, as a key unit, while the lower body accepts molecular beam epitaxy/pulsed laser deposition grown samples using compression springs and sample skis. The design further enables tip exchange without removing the sample holder from the STM. The modular design has the advantage of conveniently adapting the microscope to different applications and chamber systems in the future without changing the upper body design. A sample/tip handling system is designed and optimized for both the molecular beam epitaxy growth system and the scanning tunneling microscope system.