Low temperature fabrication of amorphous silicon thin film transistors by dc reactive magnetron sputtering

Abstract

We deposit hydrogenated amorphous silicon ($a$ -Si:H) and hydrogenated amorphous silicon nitride $\mathrm{(a-}{\mathrm{SiN}}_x:\mathrm{H})$ films using dc reactive magnetron sputtering at 125 and 230 °C substrate temperatures. We characterize the structural properties using infrared absorption, thermal hydrogen evolution, and refractive index measurements, and evaluate the electrical quality using capacitance–voltage and leakage current measurements. Nitride layers deposited at both temperatures have dominant N–H bonding with hydrogen concentration approximately ${10}^{22}/{\mathrm{cm}}^{\mathrm{3}}.$ Metal-insulator-semiconductor devices have been fabricated on $c$ -Si and show electrical leakage of ${\text{<5×10}}^{\text{−8}}\hspace{0.17em}{\mathrm{A/cm}}^{\mathrm{2}}$ at 3 MV/cm field, flat band voltage magnitude $\text{<1\hspace{0.17em}}\mathrm{V},$ and hysteresis $\text{<2\hspace{0.17em}}\mathrm{V}.$ Low temperature $a$ -Si:H films show monohydride dominant bonding. The photo- to dark conductivity ratio is ${\text{5×10}}^5$ for films deposited at 125 °C. Inverted staggered thin film transistors have been fabricated with the optimized layers. Thin film transistors deposited at 230 °C have a field effect mobility of $\text{0.8\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{2}}\mathrm{/V\hspace{0.17em}s},$ an $I_{\mathrm{on}}{\mathrm{/I}}_{\mathrm{off}}$ ratio of ${\text{5×10}}^5,$ a subthreshold slope of 1.2 V/decade, and a threshold voltage of 4 V; those deposited at 125 °C have a field effect mobility of $\text{0.3\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{2}}\mathrm{/V\hspace{0.17em}s},$ an $I_{\mathrm{on}}{\mathrm{/I}}_{\mathrm{off}}$ ratio of ${\text{5×10}}^5,$ and a threshold voltage of 3 V.