Sn Concentration Effects on Polycrystalline GeSn Thin Film Transistors

Abstract

Thin-film transistor (TFT) applications of GeSn have attracted attention as a means of improving the performance of electronic devices. Based on our advanced solid-phase crystallization and TFT process technologies, we comprehensively studied the relationship between the film properties and TFT characteristics of polycrystalline GeSn. The initial Sn concentration <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${x}_{\mathrm {i}}$ </tex-math></inline-formula> significantly changed the crystal and electrical properties of the GeSn layer. Excess Sn ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${x}_{\mathrm {i}} \ge 4.5$ </tex-math></inline-formula> %) precipitated in GeSn and degraded its properties, whereas the appropriate amount of Sn effectively passivated defects in Ge and reduced the density of defect-induced acceptors and grain boundary traps while maintaining a high Hall hole mobility (>200 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> ). The performance of the accumulation-mode TFTs fabricated under 400 °C also strongly depended on <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\boldsymbol {x}_{\mathrm {i}}$ </tex-math></inline-formula> , achieving both a high field-effect mobility (170 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> ) and on/off ratio (10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${x}_{\mathrm {i}} = 1.6$ </tex-math></inline-formula> %. This performance was shown to be the highest among Ge-based TFTs with grain boundaries in the channel.