Growth and properties of single-crystalline Ge nanowires and germanide/Ge nano-heterostructures

Single-crystalline Ge nanowires have been synthesized on Au-coated Si substrates through a thermal evaporation, condensation method and vapor-liquid-solid mechanism. The [111] growth direction of the Ge nanowires was analyzed using HRTEM and fast Fourier transform diffraction patterns. Global back-gated Ge nanowire field-effect transistors (FETs) on the Si ₃N₄ dielectrics were fabricated and studied, showing p-type behavior and a field effect hole mobility of 44.3 cm² V ^-1 s^-1. The Ge channel length could be well controlled through the annealing process. After a rapid thermal annealing (RTA) process, Ni₂Ge/Ge/Ni₂Ge nano-heterostructures were formed. The electrical transport properties were effectively improved by the heterojunction rather than the metal contact. The epitaxial relationship between Ge and orthorhombic Ni₂Ge was Ge[110]//Ni₂Ge[110] and Ge(-11-1)//Ni₂Ge(1-1-2). From electrical transport properties, the measured resistivity of the Ge nanowires was much lower than intrinsic bulk Ge material. A room temperature photoluminescence spectrum of the Ge nanowires possessed a broad blue emission with a peak at 462 nm in wavelength, which was attributed to the oxide-related defect states. Due to the existence of the defects, a Ge nanowire FET was able to detect visible light and serve as a nanowire photodetector.