Infrared spectroscopic near-field mapping of single nanotransistors.

We demonstrate the application of scattering-type scanning near-field optical microscopy (s-SNOM) for infrared (IR) spectroscopic material recognition in state-of-the-art semiconductor devices. In particular, we employ s-SNOM for imaging of industrial CMOS transistors with a resolution better than 20 nm, which allows for the first time IR spectroscopic recognition of amorphous SiO(2) and Si(3)N(4) components in a single transistor device. The experimentally recorded near-field spectral signature of amorphous SiO(2) shows excellent agreement with model calculations based on literature dielectric values, verifying that the characteristic near-field contrasts of SiO(2) stem from a phonon-polariton resonant near-field interaction between the probing tip and the SiO(2) nanostructures. Local material recognition by s-SNOM in combination with its capabilities of contact-free and non-invasive conductivity- and strain-mapping makes IR near-field microscopy a versatile metrology technique for nanoscale material characterization and semiconductor device analysis with application potential in research and development, failure analysis and reverse engineering.

Nanoscale free-carrier profiling of individual semiconductor nanowires by infrared near-field nanoscopy.

We report quantitative, noninvasive and nanoscale-resolved mapping of the free-carrier distribution in InP nanowires with doping modulation along the axial and radial directions, by employing infrared near-field nanoscopy. Owing to the technique's capability of subsurface probing, we provide direct experimental evidence that dopants in interior nanowire shells effectively contribute to the local free-carrier concentration. The high sensitivity of s-SNOM also allows us to directly visualize nanoscale variations in the free-carrier concentration of wires as thin as 20 nm, which we attribute to local growth defects. Our results open interesting avenues for studying local conductivity in complex nanowire heterostructures, which could be further enhanced by near-field infrared nanotomography.

Infrared nanoscopy of strained semiconductors.

Knowledge about strain at the nanometre scale is essential for tailoring the mechanical and electronic properties of materials. Flaws, cracks and their local strain fields can be detrimental to the structural integrity of many solids. Conversely, the controlled straining of silicon can be used to improve the performance of electronic devices. Here, we demonstrate that infrared near-field microscopy allows direct, non-invasive mapping and a semiquantitative analysis of residual strain fields in polar semiconductor crystals with nanometre-scale resolution. Our experiments with silicon carbide crystals yield optical images of nanoindents showing strain features as small as 50 nm and the evolution of nanocracks. In addition, by imaging nanoindents in doped silicon, we provide experimental evidence for plasmon-assisted near-field imaging of free-carrier properties in nanoscale strain fields. Near-field infrared strain mapping provides possibilities for nanoscale material and device characterization, and could become a tool for nanoscale mapping of the local free-carrier mobility in strain-engineered semiconductors.

Terahertz near-field nanoscopy of mobile carriers in single semiconductor nanodevices.

We introduce ultraresolving terahertz (THz) near-field microscopy based on THz scattering at atomic force microscope tips. Nanoscale resolution is achieved by THz field confinement at the very tip apex to within 30 nm, which is in good agreement with full electro-dynamic calculations. Imaging semiconductor transistors, we provide first evidence of 40 nm (lambda/3000) spatial resolution at 2.54 THz (wavelength lambda=118 microm) and demonstrate the simultaneous THz recognition of materials and mobile carriers in a single nanodevice. Fundamentally important, we find that the mobile carrier contrast can be directly related to near-field excitation of THz-plasmons in the doped semiconductor regions. This opens the door to quantitative studies of local carrier concentration and mobility at the nanometer scale. The THz near-field response is extraordinary sensitive, providing contrast from less than 100 mobile electrons in the probed volume. Future improvements could allow for THz characterization of even single electrons or biomolecules.

Local excitation and interference of surface phonon polaritons studied by near-field infrared microscopy.

We demonstrate that mid-infrared surface phonon polariton excitation, propagation and interference can be studied by scattering-type near-field optical microscopy (s-SNOM). In our experiments we image surface phonon polaritons (SPPs) propagating on flat SiC crystals. They are excited by weakly focused illumination of single or closely spaced metal disks we fabricated on the SiC surface by conventional photolithography. SPP imaging is performed by pseudo-heterodyne interferometric detection of infrared light scattered by the metal tip of our s-SNOM. The pseudo-heterodyne technique simultaneously yields optical amplitude and phase images which allows us to measure the SPP wave vector--including its sign--and the propagation length and further to study SPP interference. High resolution imaging of SPPs could be applied to investigate for example SPP focusing or heat transfer by SPPs in low dimensional nanostructures.