Infrared Reflectivity Measurements Research Articles

Realization of a graphene gate field effect transistor for electrochemical detection and biosensors

A study of 4H-SiC annealing temperature was performed using a rf-induction furnace in order to determined the optimal conditions to obtain suitable graphene layers. The graphene layer has been identified and confirmed by electrical measurements and physico-chemical analyses using Atomic Force Microscopy, X-Ray Photoelectron Spectroscopy and Infra-Red reflection measurements. GFET (Graphene Field-Effect Transistors) planar structure has been fabricated, with source and drain contacts formed directly on the graphene by lift-off. For the insulating gate, SiO2 layers have been deposited by sputtering. The graphene layers have been investigated by electrochemical detection with Fe3(CN)6, Ru(NH3)6 and Catechol. Graphene electrodes have also been tested as transducers for biosensors, with acetylthiocholine (AChE), allowing this electrode to detect inhibitors of neurotransmitters. This study demonstrates the ability to include epitaxially grown graphene at the surface of SiC as a channel for the development of transistors, and it shows the efficiency of this layer playing the role of electrode in electrochemistry and that of transducer in biosensing. In addition, these results provide a very promising perspective for the elaboration of ISFETs with a similar graphene synthesis.

Optical constants of silver and copper indium ternary sulfides from infrared reflectivity measurements

Infrared reflection spectra are obtained in the frequency range of 50–2000cm−1 for AgIn5S8 and CuIn5S8 single crystals grown by Bridgman method. All four infrared-active modes are detected, which are in full agreement with the prediction of group-theoretical analysis. Spectral dependence of optical parameters; real and imaginary parts of the dielectric function, the function of energy losses, refractive index, absorption index and absorption coefficient were calculated from reflectivity experiments. The frequencies of transverse and longitudinal optical modes and oscillator strengths were also determined. The bands detected in IR spectra of studied crystals were assigned to various vibration types (valence and valence-deformation) on the basis of the symmetrized displacements of atoms obtained employing the Melvin projection operators.

Multipitched Diffraction Gratings for Surface Plasmon Resonance-Enhanced Infrared Reflection Absorption Spectroscopy.

We demonstrate the application of metal-coated diffraction gratings possessing multiple simultaneous pitch values for surface enhanced infrared absorption (SEIRA) spectroscopy. SEIRA increases the magnitude of vibrational signals in infrared measurements by one of several mechanisms, most frequently involving the enhanced electric field associated with surface plasmon resonance (SPR). While the majority of SEIRA applications to date have employed nanoparticle-based plasmonic systems, recent advances have shown how various metals and structures lead to similar signal enhancement. Recently, diffraction grating couplers have been demonstrated as a highly tunable platform for SEIRA. Indeed, gratings are an experimentally advantageous platform due to the inherently tunable nature of surface plasmon excitation at these surfaces since both the grating pitch and incident angle can be used to modify the spectral location of the plasmon resonance. In this work, we use laser interference lithography (LIL) to fabricate gratings possessing multiple pitch values by subjecting photoresist-coated glass slides to repetitive exposures at varying orientations. After metal coating, these gratings produced multiple, simultaneous plasmon peaks associated with the multipitched surface, as identified by infrared reflectance measurements. These plasmon peaks could then be coupled to vibrational modes in thin films to provide localized enhancement of infrared signals. We demonstrate the flexibility and tunability of this platform for signal enhancement. It is anticipated that, with further refinement, this approach might be used as a general platform for broadband enhancement of infrared spectroscopy.

Pressure-induced phase transition and electrical properties of thermoelectric Al-doped Mg2Si

A recent study has shown the thermoelectric performance of Al-doped Mg2Si materials can be significantly enhanced at moderate pressure. To understand the cause of this phenomenon, we have performed in situ angle dispersive X-ray diffraction and infrared reflectivity measurements up to 17 GPa at room temperature. Contrary to previous experiment, using helium as a pressure transmission medium, no structural transformation was observed in pure Mg2Si. In contrast, a phase transition from cubic anti-fluorite (Fm-3m) to orthorhombic anti-cotunnite (Pnma) was observed in the Al-doped sample at 10 GPa. Infrared reflectivity measurements show the electrical conductivity increases with pressure and is further enhanced after the phase transition. The electron density of states at the Fermi level computed form density functional calculations predict a maximum thermoelectric power factor at 1.9 GPa, which is in good agreement with the experimental observation.

Spatial correlations and optical properties in three-dimensional deterministic aperiodic structures.

Photonic systems have strongly varying optical properties depending on the spatial correlations present in a given realization. In photonic crystals the correlations are spatially periodic forming Bravais lattices whereas the building blocks of an amorphous medium are randomly distributed without any long-range order. In this manuscript we study the optical properties of so-called deterministic aperiodic structures which fill the gap between the aforementioned two limiting cases. Within this group we vary the spectrum of the spatial correlations from being pure-point over singularly-continuous to absolutely-continuous. The desired correlations are created in direct-laser written three-dimensional polymer structures using one construction principle which allows us to attribute the optical behaviour solely to the encoded spectrum. Infrared reflection measurements reveal the characteristic response of each spectral type verifying the successful fabrication of large deterministic aperiodic structures. To prove the presence of the correlations in all directions we perform transmission experiments parallel to the substrate by means of micro-optical mirrors placed next to the structures. Transport measurements reveal a strong dependence of the effective beam width at the output facet on the encoded lattice type. Finally, we reproduce the lattice type dependent transport behavior in numerical calculations ruling out extrinsic experimental reasons for these findings.

Optical spectroscopy study of the three-dimensional Dirac semimetalZrTe5

Three dimensional (3D) topological Dirac materials are under intensive study recently. The layered compound ZrTe$_5$ has been suggested to be one of them by transport and ARPES experiments. Here, we perform infrared reflectivity measurement to investigate the underlying physics of this material. The derived optical conductivity exhibits linear increasing with frequency below normal interband transitions, which provides the first optical spectroscopic proof of a 3D Dirac semimetal. Apart from that, the plasma edge shifts dramatically to lower energy upon temperature cooling, which might be associated with the consequence of lattice parameter shrinking. In addition, an extremely sharp peak shows up in the frequency dependent optical conductivity, indicating the presence of a Van Hove singularity in the joint density of state.

An investigation of sol–gel spin coating growth of wurtzite GaN thin film on 6H–SiC substrate

In this study, wurtzite gallium nitride (GaN) thin film was directly grown on hexagonal silicon carbide (6H–SiC) substrate without buffer layer using sol–gel spin coating method followed by annealing and nitridation process. The entire growth process was investigated in-depth. The results revealed that the conversion of GaN thin film proceeds through an intermediate of amorphous gallium(I) sub-oxide (Ga2O). In this case, the amorphous Ga2O was converted into GaN thin film after being nitridated at 950°C under ammonia ambient. The intermediate of amorphous Ga2O can only be identified through infrared reflectance measurements.

Synthesis of hydrogen-doped zinc oxide transparent conductive films by RF magnetron sputtering

Hydrogen-doped ZnO (ZnO:H) thin films were synthesized on glass substrates by using an RF magnetron sputtering system and adding H2 to Ar sputtering gas. The doping effects of hydrogen on ZnO thin films were systematically investigated under various deposition conditions. Resistivity decreased with increasing H2 fraction from 0 to 5%. The Hall effect and infrared reflectance measurements showed that the increase in H2 fraction induced increases in hydrogen incorporation amount and carrier density. These results can be understood by considering that hydrogen acts as a donor, as was previously reported. Effective hydrogen doping in ZnO during synthesis was realized by the suppression of the increase in substrate temperature not only in the bulk but also at the top surface. The lowest resistivity of 1.5 × 10−3 Ω·cm and the highest carrier density of 2.7 × 1020 cm−3 were obtained under optimized conditions without metal dope.

Pressure dependence of the refractive index in wurtzite and rocksalt indium nitride

We have performed high-pressure Fourier transform infrared reflectance measurements on a freestanding InN thin film to determine the refractive index of wurtzite InN and its high-pressure rocksalt phase as a function of hydrostatic pressure. From a fit to the experimental refractive-index curves including the effect of the high-energy optical gaps, phonons, free carriers, and the direct (fundamental) band-gap in the case of wurtzite InN, we obtain pressure coefficients for the low-frequency (electronic) dielectric constant ε∞. Negative pressure coefficients of −8.8 × 10−2 GPa−1 and −14.8 × 10−2 GPa−1 are obtained for the wurtzite and rocksalt phases, respectively. The results are discussed in terms of the electronic band structure and the compressibility of both phases.

Measurement of phonon damping by nanostructures

The understanding of phonon lifetime and scattering rates is attracting an increasing interest due to the major role of phonon in thermal and electrical conductivity which are key properties for technological applications. The infrared complex dielectric function of a crystal is determined by the harmonic characteristics of the phonon together with the intrinsic and extrinsic phonon scattering rates. In order to investigate the interplay between the phonon intrinsic scattering and the scattering of the phonon by a nanostructured surface, infrared reflectivity measurements from SiC nano-pyramids on SiC substrate have been analysed using a Kramers-Kronig conversion technique to deduce the infrared complex dielectric function. Then, the real and imaginary parts of the dielectric function were fitted simultaneously by using a theoretical model for the dielectric constant that considers frequency-dependent phonon damping at the center of the Brillouin zone. It has been found that surface nanostructuring strongly enhances the overall scattering rate of the phonon at the Brillouin zone center.

Anisotropic dielectric response of lead zirconate crystals in the terahertz and infrared range at low temperature

Polarized terahertz transmittance and infrared reflectivity measurements were performed on selected lead zirconate crystals at T = 10 K. The fit of the spectra enabled us to obtain all three principal components of its anisotropic dielectric function and the corresponding parameters of the polar phonon modes.

Infrared-active phonon modes in monoclinic multiferroicMnWO4

We report on polarized infrared reflectivity measurements of multiferroic, monoclinic ${\mathrm{MnWO}}_{4}$ between 10 and 295 K. All five nonvanishing components of the dielectric tensor have been determined in the frequency range of the phonons. All infrared-active phonon modes (7 ${A}_{u}$ modes and 8 ${B}_{u}$ modes) are unambiguously identified. In particular, the strongest ${B}_{u}$ modes have been overlooked in previous studies, in which the monoclinic symmetry was neglected in the analysis. The combined analysis of reflectance data measured in different experimental geometries (${R}_{ac}$ and ${R}_{p}$) is particularly helpful for a proper identification of the ${B}_{u}$ modes. Using a generalized Drude-Lorentz model, we determine the temperature dependence of the phonon parameters, including the orientation of the ${B}_{u}$ modes within the $ac$ plane. The phonon parameters and their temperature dependence were discussed controversially in previous studies, which did not include a full polarization analysis. Our data do not confirm any of the anomalies reported above 20 K. However, in the paramagnetic phase we find a drastic reduction of the spectral weights of the weakest ${A}_{u}$ mode and of the weakest ${B}_{u}$ mode with increasing temperature. Below 20 K, the parameters of the ${A}_{u}$ phonon modes for $E\phantom{\rule{0.16em}{0ex}}\ensuremath{\parallel}\phantom{\rule{0.16em}{0ex}}b$ show only subtle changes, which demonstrate a finite but weak coupling between lattice dynamics and magnetism in ${\mathrm{MnWO}}_{4}$. A quantitative comparison of our infrared data with the quasistatic dielectric constant ${\ensuremath{\varepsilon}}_{b}$ yields a rough estimate for the oscillator strength $\ensuremath{\Delta}{\ensuremath{\varepsilon}}_{\mathrm{em}}⪅0.02$ of a possible electromagnon for $E\phantom{\rule{0.16em}{0ex}}\ensuremath{\parallel}\phantom{\rule{0.16em}{0ex}}b$. Furthermore, we report on a Kramers-Kronig-consistent model which is able to describe non-Lorentzian line shapes in compounds with monoclinic symmetry.

Reversible Optical Switching of Infrared Antenna Resonances with Ultrathin Phase-Change Layers Using Femtosecond Laser Pulses

Recently, phase-change materials (PCMs) have gained a lot of interest in the field of active metamaterials and plasmonics due to their switchable optical properties. In the infrared spectral range the huge contrast in the refractive index between an amorphous and a crystalline phase can be employed for nonvolatile tuning of nanoantenna or metasurface resonances. To make use of such concepts in devices, the reversible switching of the active material has to be realized. Here we demonstrate such reversible cycling by applying femtosecond pulses from a Ti:sapphire laser. These optical pulses trigger the phase transitions of the PCM thin film, which is covering infrared nanoantennas. Ge3Sb2Te6 is chosen as the PCM, since it offers very low losses in the infrared spectral range. The layer geometry presented is exceptionally thin (∼1/50 of the operating wavelength) and the design intentionally avoids lossy capping layers. Infrared reflectivity measurements verify the laser-induced resonance shifts of the plasmo...

Effects of nitridation durations on the synthesis of wurtzite GaN thin films by spin coating method

In this paper, the effects of nitridation durations on the structural, surface morphologies and lattice vibrational properties of gallium nitride (GaN) thin films grown via spin coating method were reported. X-ray diffraction (XRD) results revealed that all the deposited GaN thin films have wurtzite structure and with GaN(002) preferred orientation. In addition, the crystallinity of the GaN thin films increases with increasing of nitridation durations from 15 to 75 min while it degrades at 105 min. Field-emission scanning electron microscopy observations showed that GaN thin films with hexagon grains were formed at nitridation duration of 45 and 75 min, whereas rose-like microstructures were formed at nitridation duration of 105 min. P-polarized infrared reflectance measurements demonstrated that the reststrahlen feature of GaN thin films gradually increases from 15 to 75 min and diminished at 105 min, which is consistent with the XRD results. Finally, all the results revealed that the duration of 75 min was the most efficient time for the nitridation process.

(Invited) Thermochromic Phase Transitions in VO2-Based Thin Films for Energy-Saving Applications

Materials that contribute to more sustainable technologies are in high demand, and the highest impact can be made in the structures that are most ubiquitous. Widely implemented, thermochromic ‘smart’ building windows have the potential to save energy by reducing the heating and cooling loads on HVAC systems.1,2 Exploiting a crystalline phase transition in VO2, these window coatings shift from infrared transparent on cold days when solar heating is desirable to infrared reflective as the ambient temperature increases. In pure VO2, this transition occurs at 68°C, but the incorporation of 1-5% cation impurities can depress the transition temperature to Earth ambient temperatures.In this talk we describe our results on infrared thermochromism in V1-xMxO2thin films, for M = Ta, Nb, W, Hf, and others. To efficiently screen the vast compositional space of transitional metal impurities in VO2, we employ a combinatorial materials technique for both material synthesis and characterization. Thin film libraries of continuously varying composition are prepared by pulsed laser deposition (PLD) from two or more unary metal oxide targets, e.g. V2O5 and Ta2O5. Taking advantage of the spatial non-uniformity of deposition rates from each target, we produce library samples of composition V1-xMxO2where x can vary from <0.01 to 0.3 across a single 3” diameter wafer. A special high-throughput spectroscopic reflectance measurement apparatus has been constructed for screening these libraries for thermochromic transitions. A thermoelectric heater/chiller with a 3” diameter plate maintains library temperature at a setpoint between 5°C and 85°C, while near infrared (NIR) reflectance measurements are made at many library locations, and hence many unique compositions. In Figure 1, for each composition represented on the V1-xTaxO2 sample library, the NIR reflectance switches from a low reflectance state to a high reflectance state, a consequence of the crystalline phase transition that occurs. The transition is reversible with a <10°C hysteresis, and the material returns to its low reflectance state as the material is cooled back to 20°C. As the Ta impurity content increases, the transition temperature decreases.Figure 2 shows depression of transition temperatures in VO2 due to the incorporation of Nb or Ta. Both are found to effect a temperature depression that is linear with concentration at -4 °C/atomic percent. Additionally, the higher valence state impurities, such as W, Nb, and Ta, depress the VO2transition temperature while Hf incorporation has little to no effect.‘Smart’ window coatings enabled by the metal-insulator phase transition in lightly-substituted VO2have significant energy-saving potential in terms of decreased heating and cooling needs for commercial and residential buildings. Critically, the NIR transmission/reflection switch occurs due to ambient temperature, and so the technology is passive, requiring no external controls or power systems.

Transparent conducting zinc nitride films

Highly conducting Zn3N2 thin films were reactively sputtered on unheated glass. Well-crystallized polycrystalline films were obtained even though the films were deposited at a low temperature. Nitrogen-deficient Zn3N2 films exhibited n-type conductivity and resistivities of the order of 10−3 Ω cm. All the obtained Zn3N2 films had sufficiently high electron mobilities in the range of 30 to 66 cm2 V−1 s−1. Such high mobilities probably originate from the small electron effective mass of 0.27m0 (where m0 denotes free electron mass), which was confirmed by infrared reflectance measurements. Furthermore, we found that intentional oxygen doping reduced the resistivities of Zn3N2 thin films to 8.9 × 10−4 Ω cm. Optical measurements revealed that the direct optical band gap of the Zn3N2 films was in the range of 2.9–3.1 eV. These results suggest that impurity-doped Zn3N2 is a candidate for a new nitride-based transparent conductor.

Optical phonon modes in Al1−xScxN

Optical phonons are measured to probe the origins of the reported anomalously high piezoelectric response in aluminum scandium nitride (Al1−xScxN). Epitaxial layers with 0 ≤ x ≤ 0.16 deposited on sapphire(0001) exhibit a refractive index below the band gap, which increases from 2.03 for x = 0 to 2.16 for x = 0.16, corresponding to a dielectric constant ε∞ = 4.15 + 3.2x. Raman scattering shows that zone-center E2(H) and A1(TO) phonon modes shift to lower frequencies with increasing x, following linear relationships: ω(E2(H)) = 658–233x (cm−1) and ω(A1(TO)) = 612–159x (cm−1). Similarly, zone-center E1(TO) and A1(LO) phonon mode frequencies obtained from specular polarized infrared reflectance measurements red-shift to ω(E1(TO)) = 681–209x (cm−1) and ω(A1(LO)) = 868–306x (cm−1). The measured bond angle decreases linearly from 108.2° to 106.0°, while the length of the two metal-nitrogen bonds increase by 3.2% and 2.6%, as x increases from 0 to 0.16. This is associated with a 3%–8% increase in the Born effective charge and a simultaneous 6% decrease in the covalent metal-N bond strength, as determined from the measured vibrational frequencies described with a Valence-Coulomb-Force-Field model. The overall results indicate that bonding in Al-rich Al1−xScxN qualitatively follows the trends expected from mixing wurtzite AlN with metastable hexagonal ScN. However, extrapolation suggests non-linear composition dependencies in bond angle, length, and character for x ≥ 0.2, leading to a structural instability that may be responsible for the reported steep increase in the piezoelectric response.

Superconducting energy gap versus pseudogap in hole-doped cuprates as revealed by infrared spectroscopy

We present in-plane infrared reflectance measurement on two superconducting cuprates with relatively low ${T}_{c}$: nearly optimally doped Bi${}_{2}$Sr${}_{1.6}$La${}_{0.4}$CuO${}_{6+\ensuremath{\delta}}$ with ${T}_{c}=33$ K and underdoped La${}_{1.89}$Sr${}_{0.11}$CuO${}_{4}$ with ${T}_{c}=30$ K. The measurement clearly reveals that the superconducting energy gap is distinct from the pseudogap. They have different energy scales and appear at different temperatures. The results suggest that the pseudogap is not a precursor to the superconducting state. The data also challenge the longstanding and predominant viewpoint that the superconductivity within the $ab$ plane is in the clean limit and the superconducting pairing energy gap could not be detected by in-plane infrared spectroscopy.

Temperature Dependence of the AlN E1(To) Phonon Decay, Thermal Expansion and Strain Effect in AlN/Sapphire by Infrared Reflection

Infrared reflectivity measurement is carried out for AlN films on sapphire substrates. The frequencies of the symmetry optical phonon E1(TO) in the temperature range from 77 K to 500 K are reported by fitting the experimental reflectivity with the classical multi-oscillators model. Taking the lattice thermal expansion and Klemens process of the phonon decay into account, along with the strain effect introduced by thermal mismatch between the film and the substrate, the temperature effect on the frequency of the optical phonon E1(TO) is revealed. It is shown that the shift of frequency is mainly attributed to the decay process while the strain effect induced by thermal mismatch plays a non-negligible role in the outcomes of the strength and damping parameters.

Measurement of the effect of plasmon gas oscillation on the dielectric properties of p- and n-doped AlxGa1−xN films using infrared spectroscopy

We report on the application of infrared (IR) spectroscopy as an approach to nondestructive optical method for quantitative measurement of relevant optoelectronic properties in complex multilayer systems. We developed a numerical technique to analyze quantitatively the dielectric properties and plasmon gas characteristics from infrared reflectivity measurements. The developed technique is based on the combination of Kramers-Kronig theorem with the classical theory of electromagnetic wave propagation in a system of thin films. We applied the approach to deduce the dielectric properties and plasmon gas characteristics in p- and n-doped AlGaN alloys of various compositions, deposited on AlN(100 nm)/GaN(30 nm)/Al2O3. The results agreed with the electrical measurements, and the back calculation reproduced satisfactory the reflectivity measurements, demonstrating the accuracy of the developed technique.