Andreas J. Huber

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.

Pseudoheterodyne detection for background-free near-field spectroscopy

The authors present a detection technique for scattering-type near-field optical microscopy capable of background interference elimination in the entire near-UV to far-IR spectral range. It simultaneously measures near-field optical signal amplitude and phase by interferometric detection of scattered light utilizing a phase-modulated reference wave. They compare its background suppression efficiency to other known methods and experimentally show that it provides a reliable near-field optical material contrast even in the case where both noninterferometric and homodyne interferometric detection methods fail.

Near-field imaging of mid-infrared surface phonon polariton propagation

We demonstrate that mid-infrared surface phonon polariton propagation on a SiC crystal can be imaged by scattering-type near-field optical microscopy. From the infrared images, we measure the wave vector and the propagation length of locally excited surface phonon polaritons. Our method can be also applied to surface plasmon polaritons and allows to study surface polaritons in subwavelength-scale structures.

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 techniques 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.

Nanoscale Infrared Absorption Spectroscopy of Individual Nanoparticles Enabled by Scattering-Type Near-Field Microscopy

Infrared absorption spectroscopy is a powerful and widely used tool for analyzing the chemical composition and structure of materials. Because of the diffraction limit, however, it cannot be applied for studying individual nanostructures. Here we demonstrate that the phase contrast in substrate-enhanced scattering-type scanning near-field optical microscopy (s-SNOM) provides a map of the infrared absorption spectrum of individual nanoparticles with nanometer-scale spatial resolution. We succeeded in the chemical identification of silicon nitride nanoislands with heights well below 10 nm, by infrared near-field fingerprint spectroscopy of the Si-N stretching bond. Employing a novel theoretical model, we show that the near-field phase spectra of small particles correlate well with their far-field absorption spectra. On the other hand, the spectral near-field contrast does not scale with the volume of the particles. We find a nearly linear scaling law, which we can attribute to the near-field coupling between the near-field probe and the substrate. Our results provide fundamental insights into the spectral near-field contrast of nanoparticles and clearly demonstrate the capability of s-SNOM for nanoscale chemical mapping based on local infrared absorption.

Focusing of surface phonon polaritons

Surface phonon polaritons (SPs) on crystal substrates have applications in microscopy, biosensing, and photonics. Here, we demonstrate focusing of SPs on a silicon carbide (SiC) crystal. A simple metal-film element is fabricated on the SiC sample in order to focus the surface waves. Pseudoheterodyne scanning near-field infrared microscopy is used to obtain amplitude and phase maps of the local fields verifying the enhanced amplitude in the focus. Simulations of this system are presented, based on a modified Huygens’ principle, which show good agreement with the experimental results.

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.

Acoustic terahertz graphene plasmons revealed by photocurrent nanoscopy

Terahertz (THz) fields are widely used for sensing, communication and quality control. In future applications, they could be efficiently confined, enhanced and manipulated well below the classical diffraction limit through the excitation of graphene plasmons (GPs). These possibilities emerge from the strongly reduced GP wavelength, λp, compared with the photon wavelength, λ0, which can be controlled by modulating the carrier density of graphene via electrical gating. Recently, GPs in a graphene/insulator/metal configuration have been predicted to exhibit a linear dispersion (thus called acoustic plasmons) and a further reduced wavelength, implying an improved field confinement, analogous to plasmons in two-dimensional electron gases (2DEGs) near conductive substrates. Although infrared GPs have been visualized by scattering-type scanning near-field optical microscopy (s-SNOM), the real-space imaging of strongly confined THz plasmons in graphene and 2DEGs has been elusive so far-only GPs with nearly free-space wavelengths have been observed. Here we demonstrate real-space imaging of acoustic THz plasmons in a graphene photodetector with split-gate architecture. To that end, we introduce nanoscale-resolved THz photocurrent near-field microscopy, where near-field excited GPs are detected thermoelectrically rather than optically. This on-chip detection simplifies GP imaging as sophisticated s-SNOM detection schemes can be avoided. The photocurrent images reveal strongly reduced GP wavelengths (λp ≈ λ0/66), a linear dispersion resulting from the coupling of GPs with the metal gate below the graphene, and that plasmon damping at positive carrier densities is dominated by Coulomb impurity scattering.

High-power femtosecond mid-IR sources for s-SNOM applications

We demonstrate two high-power femtosecond mid-infrared (mid-IR) sources that can be combined with a scattering-type scanning near-field optical microscope (s-SNOM). The first one is based on difference frequency generation (DFG) between the two signal wavelengths of a high-power dual-signal-wavelength periodically poled lithium niobate (PPLN) optical parametric oscillator (OPO) and covers the spectral range from 10.5 μm to 16.5 μm. The second one is an AgGaSe2 OPO pumped by the PPLN OPO. With this mid-IR OPO we obtained up to 113 mW average idler power at 4857 nm with more than 40 cm−1 FWHM spectral width. We demonstrate mid-IR near-field spectra and near-field images that we obtained by combining the broadband femtosecond mid-IR DFG source with an s-SNOM.