Thin crystalline germanium (c-Ge) and silicon (c-Si) films are widely used in current microelectronic and nanoelectronic devices, such as thin film transistors and highly efficient solar cells. However, Si or Ge films grown from the vapor phase are usually amorphous. Crystallization of such films requires annealing temperatures above 500°C, which are often incompatible with the fabrication processes used in industry. Yet, when a semiconductor film of amorphous germanium (a-Ge) or amorphous silicon (a-Si) is in contact with a metal that forms an eutectic phase diagram (e.g, Au, Ag, Al, Bi, Pd), the crystallization temperature of the amorphous film is reduced significantly. This effect is commonly known as “metal-mediated” or “metal-induced crystallization” (MIC). Despite numerous studies of the MIC effect, the reaction mechanism is still unclear, although the interaction at the interface between metal and semiconductor seems to play a key role in activating the process. In this context, the main motivation of the current work is to understand the mechanism of metal-induced crystallization of semiconductor films in eutectic binaries by studying the interfacial interactions in these systems. Layered films formed by the sequential condensation of components in vacuum are a convenient model system, since the thickness of the film or the size of particles is comparable with the width of the interface boundary in a binary system. By heating such a system in a transmission electron microscope,  the interaction between the metal and the semiconductor film can be observed directly in real time.

We have selected gold-germanium (Au-Ge) films as a model system. Layered Au/Ge films are prepared by the condensation of an amorphous continuous film of Ge of 5 nm, followed by the deposition of a 0.2-0.3 nm Au film on top of the Ge film. KCl crystals covered by an arc-deposited 5 nm carbon layer prior to the deposition of Ge and Au are used as a substrate. The structure and morphology of the films were studied in situ in a FEI TECNAI G2 F20 X-TWIN transmission electron microscope within the 20-400°C range with a Gatan 652 Double Tilt Heating Holder. It has been shown that the process of solid state de-wetting of a 0.2-0.3 nm Au films occurs in the 150-200°C temperature range and results in an a-Ge film uniformly covered with Au nanoparticles with an equivalent diameter of 4.4 nm. The eutectic melting of these Au nanoparticles occurs at temperatures 200-250°C and is accompanied by both an abrupt change of the film morphology and crystallization of the a-Ge film. Long-term annealing of the film at a temperature below the eutectic point does not lead to a change in the mean particle size or to the appearance of c-Ge. Instead, the capillary motion of liquid eutectic particles on Ge surface has been observed (Fig. 1). At higher temperatures, this process intensifies and the motion of the particles becomes visible directly by TEM. This effect leads to the crystallization of a whole surface of amorphous germanium film and causes a drastic change in the Au film morphology. In addition, the formation of metastable fcc Ge phase is found and is shown to drive the capillary motion of eutectic particles. Fig. 2 shows TEM images of the same area of the film taken at 370°C with an interval of 10 seconds between each image. From Fig 2a to Fig 2b, it is evident that the eutectic nanoparticles move, leaving c-Ge behind. However, a more detailed examination of the HRTEM images of the crystalline Ge film shown in Figure 2 exhibit an unexpected feature. In particular, the Fast Fourier Transform (FFT) of region I in Figure 2a, which corresponds to a beam direction of B=[110], shows forbidden reflections for the diamond structure, namely {200} type planes (d=0.283 nm). The appearance of such reflections seems to indicate the crystallization of germanium into a metastable fcc phase, which is then converted into the diamond structure.

Figures:

Fig. 1. HAADF STEM images of the Au (0.25)/Ge (5nm) film at T=300°C. Images a) to d) were taken with 1 minute interval between each image. The arrows show some of the coalescence events observed during the experiment.

Fig. 2. a) and b) HRTEM images of the same area of Au/Ge film at T = 370°C with 10 seconds interval between each image. FFT inset corresponds of the crystallized area of the Ge [110] film (region I).

« Back to The 16th European Microscopy Congress 2016

EMC Abstracts - https://emc-proceedings.com/abstract/on-the-mechanism-of-metal-induced-crystallization-an-in-situ-tem-study-of-nanosized-auge-films/