Behavior of Sn atoms in GeSn thin films during thermal annealing: Ex-situ and in-situ observations

Abstract

Thermally induced crystallization processes for amorphous GeSn thin films with Sn concentrations beyond the solubility limit of the bulk crystal Ge-Sn binary system have been examined by X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction, and (scanning) transmission electron microscopy. We paid special attention to the behavior of Sn before and after recrystallization. In the as-deposited specimens, Sn atoms were homogeneously distributed in an amorphous matrix. Prior to crystallization, an amorphous-to-amorphous phase transformation associated with the rearrangement of Sn atoms was observed during heat treatment; this transformation is reversible with respect to temperature. Remarkable recrystallization occurred at temperatures above 400 °C, and Sn atoms were ejected from the crystallized GeSn matrix. The segregation of Sn became more pronounced with increasing annealing temperature, and the ejected Sn existed as a liquid phase. It was found that the molten Sn remains as a supercooled liquid below the eutectic temperature of the Ge-Sn binary system during the cooling process, and finally, β-Sn precipitates were formed at ambient temperature.

Carrier and heat transport properties of poly-crystalline GeSn films for thin-film transistor applications

“…The crystallization temperature of the Ge can be decreased via Ge-Sn alloying, and high-quality GeSn crystals can be attained by low-temperature annealing. [7][8][9][10][11][12][13][14][15][16][17][18] In addition, a high carrier mobility of 320 cm 2 V −1 s −1 was observed in poly-GeSn films fabricated with ∼2% Sn by using solid-phase crystallization at a low temperature (380 °C) owing to their high crystallinity. 12 To further increase the carrier mobility, the Sn concentration in the Ge lattice must be increased because theoretical calculations show that the band structure changes and the carrier mobility increases at Sn concentrations exceeding 6.5%.…”

Atomic Pathways of Solute Segregation in the Vicinity of Nanoscale Defects

“…This allows the atoms to easily form β-Sn clusters, which is the thermodynamically stable phase at room temperature. Surface segregation of Sn was evidenced by several experimental investigations on GeSn [27][28][29][30] . Vacancy-assisted diffusion might play a key role in the diffusion at a smaller length, such as to the surface from the sub-surface layers or to the dislocations from the neighboring matrix.…”

“…This is manifested by a steady increase in the solute concentration and the density of NRAC closer to the dislocation. The Sn atoms have a strong tendency to reach the surface, where they can diffuse freely with a nominal diffusion barrier. − Vacancy-assisted diffusion might play a key role in the diffusion at a smaller length from the neighboring matrix toward the surface or dislocations. On the other hand, atomic transport from deeper layers up to the surface is primarily dislocation-assisted, as highlighted in the current work.…”

The thermal stability of epitaxial GeSn layers

“…In fact, a high density of point defects could evolve in complex vacancy defects/clusters acting as seeds for the formation of non-substitutional segregated β-Sn defects, 12 as reported for a rather extreme case for the annealing of amorphous Ge 0.92 Sn 0.08 layers. 18 To gain more insight on the annealing-driven plastic strain relaxation and segregation processes, in Fig. 4, we plot the change of a 1 (relative to its value at room temperature) as a function of T A for the three partially relaxed samples.…”