Carrier and heat transport properties of polycrystalline GeSn films on SiO2

Abstract

We evaluated the potential of polycrystalline (poly-) GeSn as channel material for the fabrication of thin film transistors (TFTs) at a low thermal budget (<600 C). Poly-GeSn films with a grain size of $50 nm showed a carrier mobility of $30 cm 2 V À1 s À1 after low-temperature annealing at 475-500 C. Not only carrier mobility but also thermal conductivity of the films is important in assessing the self-heating effect of the poly-GeSn channel TFT. The thermal conductivity of the poly-GeSn films is 5-9 W m À1 K À1 , which is significantly lower compared with 30-60 W m À1 K À1 of bulk Ge; this difference results from phonon scattering at grain boundaries and Sn interstitials. The poly-GeSn films have higher carrier mobility and thermal conductivity than poly-Ge films annealed at 600 C, because of the improved crystal quality and coarsened grain size resulting from Sn-induced crystallization. Therefore, the poly-GeSn film is well-suited as channel material for TFTs, fabricated with a low thermal budget. V

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

“…The distortion decreases with increasing annealing temperature. The lattice contrast estimated from the 111 peak location is as follows: 0.577 nm for specimens annealed at 400 C, 0.574 nm for 450 C, 0.572 nm for 475 C, and 0.568 nm for 500 C. 10 These values are larger than that of Ge (0.5658 nm), suggesting the incorporation of Sn into the Ge matrix. The reduction of the lattice constant with increasing annealing temperature suggests the ejection of Sn atoms from Ge.…”

“…We have recently examined the electrical and thermal properties of polycrystalline GeSn thin films grown on SiO 2 and demonstrated their potential capabilities as a high-performance TFT channel material. 10 We also found that the carrier mobility and density suddenly change between 460 C and 480 C. This alteration is strongly correlated with Sn segregation from the matrix; therefore, understanding the Sn behavior during thermal annealing is technologically important for controlling the physical properties of the materials. However, previous studies were performed using ex-situ techniques, and direct observations of crystallization processes have never been performed.…”

“…10 ) In addition to the reflections associated with GeSn with a diamond structure, some spots resulting from segregated Sn (marked with arrows) are observed in the diffraction pattern of Fig. 4(c).…”

“…It should be noted that the Sn crystallites were not detected at 475 C in our previous cross-sectional TEM observations. 10 The diffraction patterns were taken from a wider region in plane-view specimens than in cross-sectional ones; therefore, a small amount of Sn crystallites is successfully detected in the present study. The bright-field images above 475 C contain Sn grains.…”

“…The sample preparation techniques are described elsewhere. 10 The as-deposited and annealed specimens were characterized by X-ray photoelectron spectroscopy (XPS), grazing incidence X-ray diffraction (GIXRD), transmission electron microscopy (TEM), and scanning transmission electron microscopy (STEM). XPS analyses were performed using a Kratos AXIS Ultra DLD.…”