A rotating-compensator based reflectance difference spectrometer for fast spectroscopic measurements

In-Plane Optical and Electrical Anisotropy of 2D Black Arsenic

Low-symmetry two-dimensional (2D) semiconductors have attracted great attention because of their rich in-plane anisotropic optical, electrical, and thermoelectric properties and potential applications in multifunctional nanoelectronic and optoelectronic devices. However, anisotropic 2D semiconductors with high performance are still very limited. Here, we report the systematic study of in-plane anisotropic properties in few-layered b-As that is a narrow-gap semiconductor, based on the experimental and theoretical investigations. According to experimental results, we have come up with a simple method for identifying the orientation of b-As crystals. Meanwhile, we show that the maximum mobility of electrons and holes was measured in the in-plane armchair (AC) direction. The measured maximum electron mobility ratio is about 2.68, and the hole mobility ratio is about 1.79.

Reflectance difference microscopy for nanometre thickness microstructure measurements

The discontinuity of medium at the boundary produces optically anisotropic response which makes reflectance difference microscopy (RDM) a potential method for nanometre-thickness microstructure measurements. Here, we present the methodology of RDM for the edge measurement of ultrathin microstructure. The RD signal of microstructure's boundary is mathematically deduced according to boundary condition and polarization optics theory. A normal-incidence RDM setup was built simply with one linear polarizer, one liquid crystal variable retarder and one 5 × objective. Then, the performance of the developed setup was identified using homogenous reflection mirror and high quality linear polarizer. For demonstration, microstructures array with 100 nm step height was measured. The results show that the RD signal is sensitive to the edge and its sign reflects the change direction of the edge. Furthermore, a height sensitivity of better than 10 nm and a spatial resolution of ∼3 μm offer this technique a good candidate for characterizing ultrathin microstructures.

Normal-incidence reflectance difference spectroscopy based on a liquid crystal variable retarder

We propose liquid crystal variable retarder-based reflectance difference spectroscopy for normal-incidence measurements. Principles, instrumentation, data collection and reduction, and calibration procedures are provided. The signal noise is better than 10^-3, and the spectral range is from 1.6 to 2.4 eV with 346 photon energy channels. As a demonstration, reflectance difference signals of a multilayer pentacene film on poly (ethylene terephthalate) (PET) film are presented with different polarization azimuths. The characteristic peaks at 1.8 and 1.97 eV, corresponding to the Davydov splitting of pentacene crystal, are observed, which indicate well-ordered in-plane anisotropic structure of pentacene crystal film on PET. Thanks to normal incidence, this design is immune to adjusting the optical structure for the measurements with different working distances, and the objective lens is easily integrated to realize microarea measurements.

Accurate alignment of optical axes of a biplate using a spectroscopic Mueller matrix ellipsometer

The biplate that consists of two single wave plates made from birefringent materials with their fast axes oriented perpendicular to each other is one of the most commonly used retarders in many optical systems. The internal alignment of the optical axes of the two single wave plates is a key procedure in the fabrication and application of a biplate to reduce the spurious artifacts of oscillations in polarization properties due to the misalignment error and to improve the accuracy and precision of the systems using such biplates. In this paper, we propose a method to accurately align the axes of an arbitrary biplate by minimizing the oscillations in the characteristic parameter spectra of the biplate detected by a spectroscopic Mueller matrix ellipsometer (MME). We derived analytical relations between the characteristic parameters and the misalignment error in the biplate, which helps us to analyze the sensitivity of the characteristic parameters to the misalignment error and to evaluate the alignment accuracy quantitatively. Experimental results performed on a house-developed MME demonstrate that the alignment accuracy of the proposed method is better than 0.01° in aligning the optical axes of a quartz biplate.

Mechanical and Electrical Anisotropy of Few-Layer Black Phosphorus

We combined reflection difference microscopy, electron transport measurements, and atomic force microscopy to characterize the mechanical and electrical anisotropy of few-layer black phosphorus. We were able to identify the lattice orientations of the two-dimensional material and construct suspended structures aligned with specific crystal axes. The approach allowed us to probe the anisotropic mechanical and electrical properties along each lattice axis in separate measurements. We measured the Young's modulus of few-layer black phosphorus to be 58.6 ± 11.7 and 27.2 ± 4.1 GPa in zigzag and armchair directions. The breaking stress scaled almost linearly with the Young's modulus and was measured to be 4.79 ± 1.43 and 2.31 ± 0.71 GPa in the two directions. We have also observed highly anisotropic transport behavior in black phosphorus and derived the conductance anisotropy to be 63.7%. The test results agreed well with theoretical predictions. Our work provided very valuable experimental data and suggested an effective characterization means for future studies on black phosphorus and anisotropic two-dimensional nanomaterials in general.

Optimization for liquid crystal variable retarder-based spectroscopic polarization measurements

We present an approach for improving liquid crystal variable retarder (LCVR)-based spectroscopic polarization measurements. As deduced mathematically, the transfer coefficients from the random intensity noise to the signal noise are functions of modulation parameters of the LCVR, i.e., modulation range (MR) and initial retardation. Simulations allow more details about the roles of two parameters. A broad MR reduces effectively the values of the coefficients and leads to a better signal quality. However, as the MR narrows, initial retardation begins to influence the signal quality. To obtain a high-quality spectrum, a recommended solution is to settle the MR more than π at each wavelength. This treatment has two advantages: non-sinusoidal modulation becomes possible and the modulations do not average to zero. Moreover, it weakens the interference of non-uniform intensity distribution in wavelengths of the signal spectrum. These conclusions are proven in experiments. Further, this approach is valid for other polarimeters and ellipsometers based on LCVRs.

A rapid reflectance-difference spectrometer for real-time semiconductor growth monitoring with sub-second time resolution

We report on a rapid, 32-channel reflectance-difference (RD) spectrometer with sub-second spectra acquisition times and ΔR/R sensitivity in the upper 10(-4) range. The spectrometer is based on a 50 kHz photo-elastic modulator for light polarization modulation and on a lock-in amplifier for signal harmonic analysis. Multichannel operation is allowed by multiplexing the 32 outputs of the spectrometer into the input of the lock-in amplifier. The spectrometer spans a wavelength range of 230 nm that can be tuned to cover E(1) and E(1) + Δ(1) transitions for a number of III-V semiconductors at epitaxial growth temperatures, including GaAs, InAs, AlAs, and their alloys. We present two examples of real-time measurements to demonstrate the performance of the RD spectrometer, namely, the evolution of the RD spectrum of GaAs (001) annealed at 500 °C and the time-dependent RD spectrum during the first stages of the epitaxial growth of In(0.3)Ga(0.7)As on GaAs (001) substrates.