Interfaces and junctions in bottom-up nanoscale semiconductor devices.

摘要

A semiconductor device is a system composed of multiple materials, and its functionality depends on the junctions and interfaces between these materials. This dissertation documents a study of junctions and interfaces in one-dimensional nanoscale semiconductor materials. Examined are the insulator interface and the dopant profile in vapor-liquid-solid (VLS)-grown silicon nanowires, the electronic properties of the native surface of InAs nanowires grown using bottom-up methods, and metal-carbon nanotube (CNT) Schottky contacts. The capacitance-voltage (C-V) measurement is refined to examine these junctions and interfaces. For a Si nanowire, the C-V measurement shows that the density of trap states on its interface with Al2O3 insulator ranges from ∼1011/cm2·eV in the midgap to ∼1013/cm2·eV closer to the valence band edge. The boron profile in Si nanowires is found to agree well with predictions from interstitial and vacancy-assisted diffusion model, as in bulk Si. For an InAs nanowire, the C-V technique is used extract the trap density of its native surface, which is ∼3.8x1011/cm2·eV in the mid-gap and ∼1013/cm2·eV near the conduction band edge. The trap lifetime in these InAs nanowires is extracted using the C-V method as well. Accurate measurement of the gate capacitance in back-gated InAs nanowires is found to be necessary to determine accurately the electron mobility. The impact of metal-CNT Schottky contacts on the transistor performance and leakage is examined as well. It is found that both the on-state current and off-state leakage depend strongly on the Schottky Barrier Height (SBH) at the contacts. The scaling of the SBH with the CNT diameter shows that the length of the electrical junction is about 25nm. The metal-CNT Schottky junction is also studied using a new instrument capable of measuring rapidly attofarad (10-18 F)-level capacitances. This study confirms the unpinned nature of the metal-CNT Schottky contact, and shows a way to directly determine the height of that energy barrier.