Plasma-deposited germanium nitride gate insulators for indium phosphide metal-insulator-semiconductor field-effect transistors

Germanium nitride (Ge3N4) films were deposited on indium phosphide (InP) compound semiconductor substrates using plasma-enhanced chemical vapor deposition. The depositions were performed in a capacitively coupled parallel-plate reactor using two different processes. In the first process, germane (GeH4), nitrogen (N2), and ammonia (NH3) reactant gases were used. In the second process, germane and pure nitrogen were used as reactant gases. The films were deposited using 13.56 MHz rf excitation to a typical thickness of 800 Å with a refractive index of 2.11–2.16. The breakdown field strength of the films was greater than 106 V/cm. Auger electron spectroscopy did not indicate significant chemical composition differences between the two deposition processes. In order to study the feasibility of plasma-deposited germanium nitride as a gate insulator, metal-insulator-semiconductor field-effect transistors (MISFETs) with 2-μm channel lengths were fabricated on InP. The device transconductance and threshold voltage for the GeH4/N2/NH3 process were 17 mS/mm and −3.6 V, respectively. The drain-source breakdown voltages were greater than 10 V. The GeH4/N2/NH3 deposition process was preferred over the GeH4/N2 process because of enhanced thickness uniformity, increased deposition rate, increased resistivity, reduced interface state density, and comparable MISFET electrical characteristics.