Growth of Fe on Fe(001) and Correlation with RHEED

Molecular beam epitaxy (MBE) is used to produce artificial structures with abrupt interfaces at the single atomic layer level. The nonequilibrium nature of MBE, in most applications, leads to a supersaturation of adsorbed species, which must undergo complicated processes such as dissociation, diffusion, nucleation, and film growth. The most widely used technique to monitor MBE growth is reflection-high-energy-electron-diffraction (RHEED), in which a high energy (typically 10 keV to 30 keV) electron beam strikes a surface at grazing incidence and the resulting diffraction pattern is monitored. Through detailed modeling of RHEED intensity oscillations there have been attempts to extract information on the surface morphology and processes such as diffusion and nucleation, but real space measurements have been lacking.

We have used scanning tunneling microscopy (STM) to obtain real space images of the homoepitaxial growth of Fe on Fe(001) which we correlate with the corresponding RHEED patterns and RHEED intensity oscillations. Such measurements are illustrated in Fig. 1 which shows RHEED intensity measurements of the (0,0) diffracted beam during growth and the RHEED patterns and the STM images of the surface morphology after growth was stopped at five oscillations. Diffusion kinetics which are very temperature dependent strongly influence the growth as it progresses from island growth to thin film growth. At the lower temperature of 20° C, the higher propensity for nucleation and the barriers to step edge diffusion result in a rough film with five exposed layers as seen in Fig. 1(a). The islands have a mean center-to-center spacing of approximately 5 nm. This structure gives rise to a splitting of the diffraction beams in the RHEED pattern. The observed splitting is in agreement with simulations calculated from the surface correlation function obtained from the real space STM image in Fig. 1(a). The decay of the RHEED intensity oscillations is correlated with the surface roughness which detailed measurements of the surface morphology at this temperature show increases monotonically with film thickness. An increased diffusion rate at higher temperatures results in a larger terrace structure as seen in Fig. 1(b) for the 180° C growth temperature. Interestingly, the surface roughness is comparable to the growth at 20° C, and there are still five layers exposed. This is an indication that the barrier to step diffusion has not yet been overcome at this temperature. The RHEED intensity shows stronger oscillations but still with a significantly damped intensity envelope. Nearly layer-by-layer growth is observed at a growth temperature of 250° C shown in Fig. 1(c). The surface morphology is seen to consist of three layers with one layer predominant. Layer-by-layer growth is characterized by the RHEED intensity oscillations returning to nearly their initial intensity value with very little damping. Further measurements at this temperature support a kinematic description of the RHEED intensity oscillations observed for this system.

Figure 1: STM image, diffraction patterns, and RHEED (0,0) beam intensity measurements of Fe on Fe(001) growth.

Homoepitaxial Growth of Iron and a Real Space View of Reflection-High-Energy-Electron-Diffraction
Scaling of Diffusion-Mediated Island Growth in Iron on Iron Homoepitaxy
The Growth of Iron on Iron Whiskers


Joseph A. Stroscio
Daniel T. Pierce
Robert J. Celotta


Robert Dragoset - NIST

Supported in part by the Office of Naval Research

---

Online: May 1996
Last Updated: February 2008

epgwebmaster@nist.gov