A high carrier-mobility crystalline silicon film directly grown on polyimide using SiCl4/H-2 microwave plasma for flexible thin film transistors

摘要

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 H-2-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 H-2 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 cm(2) 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 10(6) cm(2) V-1 s(-1), a threshold voltage of 2.5 V, and an on/off current ratio of 1.2 x 10(6). 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.