Investigation of the underlying mechanism of the Ag-induced layer exchange by in situ studies in SEM and TEM

Abstract text (incl. figure legends and references)

Thin film semiconductors are indispensable in modern technology and a key element for future applications. In particular, highly crystalline thin film semiconductors are of great interest due to their outstanding performance by reducing the detrimental influence of grain boundaries [1]. Preparing such thin film crystalline semiconductors on foreign (amorphous) substrates can even improve their performance and enable future applications like flexible devices. However, the fabrication on amorphous and/or heat-sensitive substrates are quiet challenging and expensive. A smart approach is the crystallization of the semiconductor directly on the needed substrate. Here, the metal-induced layer exchange (MILE) process offers a direct crystallization route by contacting the amorphous semiconductor with a metal layer, which decreases dramatically the necessary crystallization temperature [2]. Applications of the MILE process are widespread due to the great variety of semiconductor – metal systems. Although this process has been known for a long time, the mechanism behind it is still under debate [3].

In our work, we investigated the layer exchange (LE) process within the silver-silicon system (AgILE). Especially, the early stages before the initial LE starts are studied by in situ microscopy. The sample composition comprises a poly-crystalline Ag layer on the substrate (SiN), an oxide layer and an amorphous Si top layer. During heating, both layers change their position and the Si crystalizes (illustrated in Figure 1a).

Pre-characterization and analysis of the process dynamics are performed by in situ light microscopy (see Figure 1b) and ex situ electron microscopy (see Figure 1d). In situ annealing experiments in SEM are performed by our custom-built chip-based heating stage (illustrated in Figure 1c) [4]. Especially, STEM High Angle Annular Dark-Field (HAADF) imaging facilitates the discrimination between Ag and Si due to the Z-dependency of the signal. To study the occurring reactions in more detail, additional heating experiments in the TEM are executed.

Both heating experiments (700 °C) revealed similar reactions before the initial LE occurs. Figure 2a-d displays the observed reactions in the SEM. After a short initial annealing time, Ag push-ups appear followed by a second reaction, where a new Si-rich phase grows within the appeared Ag push-ups. Afterwards, the LE starts exactly at one previously occurred push-up and grows subsequently through the whole sample. The final state consist of pure Si-crystals (black), reacted area (dark gray) and some unreacted regions (bright).

To gain insights into the inner sample structure, lift-out analysis and non-destructively electron tomography are performed. The results of the lift-out samples revealed a crystalline Si phase in the bottom layer, whereas the top layer consist of initial amorphous Si and pushed-up Ag grains (see Figure 2e). Figure 2g indicates the Ag is pushed up by the lateral growing c-Si. Interrupted in situ heating experiments in the TEM were performed to acquire a tilt series of the present sample state. The resulting 3D tomograms verify the distribution of Ag and Si in both layers (see Figure 2f).

Accordingly, we propose a refined model of the AgILE process due to our detailed microscopic study and observed reactions [5].

[1] 10.1063/1.91832

[2] 10.1088/1361-6463/ab91ec

[3] 10.1002/adem.200800340

[4] 10.1016/j.ultramic.2020.112956

[5] Financial support by DFG via GRK