Investigation of carrier transport in semiconductor nanowires by scanning probe techniques

In this work, electron beam induced current (EBIC) and scanning photocurrent microscopy (SPCM) were used to quantitatively investigate the electronic properties of silicon nanowire devices. For the first time, it was shown that minority carrier diffusion lengths in phosphorous-doped silicon nanowires are significantly reduced from their bulk values because of nonradiative recombination at the nanowire surface. Diffusion lengths were measured quantitatively by EBIC analysis of nanowire Schottky diodes.;SPCM analysis of two-terminal ohmic n-type silicon nanowire devices revealed a nonuniform electric field along the channel length suggesting an increased resistivity toward the nanowire tip. Etching of the nanowire surface eliminated the gradient indicating that the origin was a surface doping profile. Using four-terminal device geometries, it was shown that quantitative one-dimensional potential profiles and effective carrier concentrations may be obtained. The surface etching process was also used to fabricate high-performance n-Si nanowire FETs that operate based on the n+-n junctions introduced by the etch. SPCM confirms that, in the subthreshold regime, the dominant resistance in the device becomes the n+-n junctions at the edges of the etched region while in the on-state modulation of the carrier concentration in the etched channel determines the device transfer characteristics.;The effect of nanowire nonuniformity on FET performance was investigated using tapered boron-doped silicon nanowires. The transistor threshold voltages and subthreshold slopes were shown to change monotonically along the length of the nanowire with improved transistor characteristics at the tip. SPCM analysis indicated the relative contribution from contact resistance increased toward the nanowire tip where the carrier concentration is reduced in the absence of significant surface doping.;A combination of current-voltage analysis and SPCM was used to investigate silicon nanowire Schottky diodes. Schottky barrier properties obtained by the two methods show good agreement and were used to estimate the internal electric field at the metal-semiconductor junction as well as the effective carrier concentration of the semiconductor.