Interdiffusion in Heusler film epitaxy on GaAs(001)

We report the role of interdiffusion in molecular beam epitaxy of the binary Heusler alloy system Fe${}_{1-x}$Si${}_x$/GaAs(001), employing a variety of complementary techniques that, in their combination, provide quantitative insight into the dynamics of the involved processes. The main properties of the investigated Fe${}_{0.84}$Si${}_{0.16}$ and Fe${}_{0.76}$Si${}_{0.24}$ films—growth, epitaxy, crystallographic order, interface quality, saturation magnetization, coercive field, and magnetic anisotropy—are in perfect agreement with the literature. Additionally, our results reveal a strong interdiffusion of Fe and Si into the GaAs substrate as well as of As and Ga into the Fe${}_{1-x}$Si${}_x$ films, creating intermixed layers of 2–3-nm thickness in both film and substrate. Interdiffusion is dominant already at moderate growth temperatures required for crystallographic ordering, thus demanding new concepts including appropriate diffusion barriers for the development of ferromagnet/semiconductor hybrid systems.

Figure 1

Sphere model of the $~$ lattice of Fe$45\%$Si: It consists of a regular array of body-centered cubes where the corners (A, C) are occupied by Fe atoms and the center atom is alternatingly Fe (B) or Si (D).

Figure 2

High-resolution TEM images along [110] of the interface region of Fe$D{0}_3$Si${}_3$/GaAs(001): (a) ${}_{0.84}$C, (b) ${}_{0.16}$C, (c) $T_{\mathrm{G}}=150\hspace{0.5em}{}^{°}$C. Vertical lines illustrate pseudomorphic growth and arrows mark interface. Inset shows a larger section of the $200\hspace{0.5em}{}^{°}$C film.

Figure 3

XRD spectra (intensity versus relative x-ray momentum change in reciprocal lattice units) of Fe$250\hspace{0.5em}{}^{°}$Si$150\hspace{0.5em}{}^{°}$ films on GaAs(001) deposited at 150 ${}_{0.84}$C, 200 ${}_{0.16}$C, and 250 ${}^{°}$C. In addition to the (004) reflection of the Fe${}^{°}$Si${}^{°}$ films, pronounced thickness oscillations are observed at all growth temperatures pointing to a good surface and interface quality.

Figure 4

(a) XRD spectra (intensity versus relative x-ray momentum change in reciprocal lattice units) along specific superlattice reflections of a Fe${}_{0.84}$Si${}_{0.16}$ film deposited at 200${}_{0.76}$ onto GaAs(001). Note that particularly the intensity of the (115) reflection is very low, where even for the GaAs(115) peak less than 10 counts/s are detected (compared to ${}_{0.24}$ counts/s for the GaAs(004) peak). (b) Corresponding HRTEM images along [110] of the interface region of Fe${}^{°}$Si${10}^4$/GaAs(001); dashed line indicates affected region in the GaAs substrate (see text).

Figure 5

Magnetic hysteresis loops (magnetization ${}_{0.76}$ versus magnetic field ${}_{0.24}$) of Fe$M$Si$H\parallel [110]$/GaAs(001) measured at room temperature: (a) Fe${}_{1-x}$Si${}_x$ films deposited at 150 ${}_{0.84}$C, 200 ${}_{0.16}$C, and 250 ${}^{°}$C. (b) Fe${}^{°}$Si${}^{°}$ films deposited at 200 ${}_{0.76}$C in comparison with the respective Fe${}_{0.24}$Si${}^{°}$ film.

Figure 6

Film force (force per unit film width as a function of the mean film thickness) evolving during and after MBE growth of various Fe${}_{0.84}$Si${}_{0.16}$ films deposited onto GaAs(001) at 100 ${}_{0.84}$C, 150 ${}_{0.16}$C, 200 ${}^{°}$C, and 250 ${}^{°}$C. For comparison, the force curve of a pure Fe film deposited at room temperature is included; slope of dashed lines corresponds to indicated film stress. Film force curves in the inset are subsequently displaced by 1N/m to enable a better view on the respective induction periods.

Figure 7

Film force (force per unit film width as a function of the mean film thickness) evolving during and after MBE growth of an Fe${}^{°}$Si${}^{°}$ film deposited onto GaAs(001) at 200 ${}_{0.76}$C. For comparison, also the force curves of a respective Fe${}_{0.24}$Si${}^{°}$ film and a pure Fe film deposited at room temperature are included. Dashed lines indicate the film forces evolving due to the maximum misfit stress calculated by bulk lattice constants.

Figure 8

Auger depth profiles of Fe, Si, As, Ga, and O of the Fe${}_{0.84}$Si${}_{0.14}$ film deposited at 200 ${}_{0.76}$C onto GaAs(001); dashed line marks the lower film end as indicated by the disappearance of the oxygen signal and used for thickness calibration (upper scale), the expected decay of the Fe signal in the case of a sharp interface, and the kink in the Si signal.

Figure 9

Quantified (left) and normalized (right) STEM-EDXS linescans of Fe, Si, Ga, and As in (a) Fe${}_{0.24}$Si${}^{°}$/GaAs(001) and (b) Fe${}_{0.84}$Si${}_{0.16}$/GaAs(001), both deposited at 200 ${}_{0.76}$C; insets show the positions at which the EDX spectra were taken.