Polycrystalline Si thin-film transistors fabricated at ≤800 °C: Effects of grain size and {110} fiber texture

Abstract

The grain size and {110} fiber texture of low‐pressure chemical‐vapor‐deposited polycrystalline Si films were first modified using the ‘‘seed selection through ion channeling’’ process. These films were self‐implanted at 200 keV, at normal incidence, to various doses (5–20×10¹⁴ cm⁻²), and were subsequently recrystallized at 600 °C. The as‐deposited film was characterized by the smallest grain size, 0.08 μm, among all films and a weak {110} texture. The film processed with a dose of 11×10¹⁴ cm⁻² had an intermediate average grain size of 1.0 μm but was characterized by the strongest {110} texture among all films. The film processed with a dose of 20×10¹⁴ cm⁻², on the other hand, was characterized by the largest average grain size of 2.0 μm among all films but had no crystallographic texture. Metal‐oxide‐semiconductor thin‐film transistors were then fabricated on these three films to examine the effects of grain size and fiber texture on device performance. Both n‐ and p‐channel transistors were fabricated, using a self‐aligned process with a maximum processing temperature of 800 °C. For both types of transistors, the as‐deposited film offered the worst device performance, exhibiting the highest threshold voltages (4 and −4 V, respectively) and the lowest channel mobilities (0.4 and 0.5 cm²/V s, respectively). The 11×10¹⁴ cm⁻² film offered the best device performance, exhibiting the lowest threshold voltages (1.2 and −3.0 V, respectively) and the highest channel mobilities (42 and 34 cm²/V s, respectively). The 20×10¹⁴ cm⁻² film, on the other hand, exhibited equal or slightly worse threshold voltages (1.6 and −3.0 V, respectively) and channel mobilities (35 and 24 cm²/V s, respectively, than the 11×10¹⁴ cm⁻² film. These results demonstrate that both the grain size and grain orientation can be important factors in determining polycrystalline transistor performance. For grain sizes in the order of 1–2 μm, a stronger {110} texture can, in fact, lead to a better transistor material than a larger average grain size.