Mapping Free-Carriers in Multijunction Silicon Nanowires Using Infrared Near-Field Optical Microscopy

Abstract

We report the use of infrared (IR) scattering-type scanning near-field optical microscopy (s-SNOM) as a nondestructive method to map free-carriers in axially modulation-doped silicon nanowires (SiNWs) with nanoscale spatial resolution. Using this technique, we can detect local changes in the electrically active doping concentration based on the infrared free-carrier response in SiNWs grown using the vapor-liquid-solid (VLS) method. We demonstrate that IR s-SNOM is sensitive to both p-type and n-type free-carriers for carrier densities above ∼1 × 10 cm. We also resolve subtle changes in local conductivity properties, which can be correlated with growth conditions and surface effects. The use of s-SNOM is especially valuable in low mobility materials such as boron-doped p-type SiNWs, where optimization of growth has been difficult to achieve due to the lack of information on dopant distribution and junction properties. s-SNOM can be widely employed for the nondestructive characterization of nanostructured material synthesis and local electronic properties without the need for contacts or inert atmosphere.

Probes for Ultrasensitive THz Nanoscopy

“…Scattering-type scanning near-field optical microscopy (s-SNOM) 1 is an emerging scanning probe technique, which extends the power of optical techniques deep into the subwavelength regime for nanoscale imaging and spectroscopy from visible to terahertz frequencies. Applications include nanoscale chemical materials identification via molecular vibrational spectroscopy, [2][3][4][5] mobile carrier mapping in semiconductor nanostructures, [6][7][8][9] polariton mapping in 2D materials, [10][11][12][13] and studies of metal-insulator transitions and strongly correlated quantum materials. [14][15][16][17] In s-SNOM, a metallic, cantilevered atomic force microscopy (AFM) tip is illuminated with focused laser radiation and the backscattering from the tip is detected by a far-field detector.…”

Photoassisted Electron–Ion Synergic Doping Induced Phase Transition of n-VO2/p-GaN Thin-Film Heterojunction

“…After 30 min of photoassisted hydrogen-doping treatment, the resistance of the pristine VO 2 gap dropped by 2 orders of magnitude (as seen in Figure d), and it was converted to a low-resistivity metallic state (Figure S10). It was known that s-SNOM could directly detect the near-field optical signal near the tip of the AFM probe and accurately reflect the differences in the dielectric properties of the sample surface, , with the highest resolution up to 10 nm, with the configuration in Figure e. Because the phase transition of the 50 nm VO 2 gap was accompanied by a change of the dielectric parameters, the s-SNOM image should be able to directly explore the phase transition upon the photoassisted hydrogenation in the VO 2 nanogap.…”

Rapid Infrared Spectroscopic Nanoimaging with nano-FTIR Holography

“…Scattering-type scanning near-field optical microscopy (s-SNOM) is an emerging tool for infrared (IR) nanoimaging that extends the analytical power of infrared light to the deep subwavelength scale. It has thus found successful application in a variety of fields, including the nanoscale identification of molecular vibration signatures, − the probing of polaritonic excitations in novel nanophotonic structures, − the mapping of the free carrier concentration in semiconductors, − and the study of metal–insulator transitions and strongly correlated quantum materials. − In s-SNOM, the metal-coated tip of an atomic force microscope (AFM) is illuminated with monochromatic IR light and produces a tightly confined and strongly enhanced IR hot spot at the tip apex. When brought in proximity to a sample surface, the tip interacts with the sample via near-field coupling.…”