A high carrier-mobility crystalline silicon film directly grown on polyimide using SiCl4/H2 microwave plasma for flexible thin film transistors

Abstract

An approach for the direct synthesis of crystalline Si films with high carrier mobility on a flexible polyimide (PI) substrate is reported. Microwave plasma enhanced chemical vapor deposition was applied using H₂-diluted silicon tetrachloride (SiCl₄) as a precursor. Dense crystalline Si films with a columnar structure were directly obtained on an unheated substrate without the extra recrystallization technique and the transfer process under an optimal H₂ flow rate of 100 sccm. The films show a crystalline volume fraction of >98% and a high growth rate of 4.1 nm s⁻¹. Detailed TEM studies show that crystalline Si films can be directly grown on a substrate without an amorphous incubation layer even at the initial stage. Moreover, a thin layer of a single-crystalline (111) grain exists on the topmost surface. Such a microstructure feature is responsible for the measured high Hall carrier mobility of 170 cm² V⁻¹ s⁻¹. The adhesion between the Si film and the PI substrate achieves the highest rank, 5B. Mechanical static bending tests reveal the critical radius of curvature (R_c) at 8.0 mm. The decrease in electrical conductivity is only 23% after dynamic bending tests at R_c for 1000 cycles. Top-gate flexible Si thin film transistors employing an optimized 100 nm crystalline Si channel layer exhibit a field-effect carrier mobility as high as 106 cm² V⁻¹ s⁻¹, a threshold voltage of 2.5 V, and an on/off current ratio of 1.2 × 10⁶. No obvious deterioration of transfer characteristics was observed when the devices were subjected to bending at R_c. These results thus represent important steps toward a low-cost approach to high-performance flexible crystalline Si film based electronics.