Electroreflectance Spectra Research Articles

Pockels Effect at the Interface between Water and Ti Electrode

The Pockels coefficient of interfacial water between bulk water and a Ti electrode was estimated from the electroreflectance spectra (ΔR/R) to be r13≈−150 pm/V as the maximum value of magnitude, which is comparable in magnitude to the largest coefficient for electrode interfacial water, i.e., 200 pm/V for interfacial water on a transparent oxide electrode. This Pockels signal increased by a factor of about ±3 by applying a DC bias voltage of ±1 V. The reflectance (R) of the Ti electrode had a dip structure in the UV region (3.5–4.5 eV) due to the interference of a 14 nm thick surface oxide film, and the ΔR/R spectra in aqueous electrolyte solution showed a large reflectance change in the UV region with a dispersive shape due to the contribution of the TiO2 film. The reproducibility of the electroreflectance experiment was high, suggesting that the surface oxide film contributes to the large Pockels effect of interfacial water and the robustness of the electrode.

Tuning the optical properties of molybdenum oxide using electric field: A theoretical and experimental study

Molybdenum Oxide (MoO3) with van der Waals structure can lead to the exotic properties under the action of external physical perturbation. Here, a combined density functional theory and an experimental study were performed to analyze the response of MoO3 under electric field. The electric field with the strength varying from 0.05 V/Å to 0.23 V/Å was applied across the MoO3 cell which induced the splitting in the valence and conduction band and decreases the bandgap. A critical electric field of 0.23 V/Å resulted a closing of the bandgap of MoO3 and led semiconductor to metal transition. The results of DFT regarding decrease in the bandgap of MoO3 were further supported experimentally. For this, MoO3 films were fabricated using thermal evaporation and their chemical and optical properties were analyzed using X-ray photoelectron spectroscopy and Electroreflectance (ER). To analyze the optical response, an ER analysis with external voltage varying from 10 to 50 volts was performed on Aluminum/MoO3/Aluminum heterostructure. The resultant ER spectra revealed three distinct critical points that correspond to the fundamental as well as defect related optical transitions involved in MoO3. The Third derivative functional form model applied on the ER spectra depicted a monotonical decrease in the critical points of MoO3 at lower voltage. At high voltage of about 80 volts, the existence of high built-in electric field results the delocalization in the electrons and hole wave function and gives rise to the Franz-Keldysh oscillations.

Comparison between polycrystalline Au and single-crystalline Au(1 1 1) electrodes as the substrate of a cationic organic monolayer based on their anion dependent redox activities

The structures of organic monolayers immobilized on a metal electrode and their functions depend on the substrate surface chemistry and morphology. A better defined crystalline flat electrode usually provides a structurally more ordered organic monolayer, although the use of a single-crystalline electrode costs more for mass production. Because the monolayer structures at both molecular and mesoscopic levels are critical to the functions of the modified electrode, in-depth experimental studies to clarify the relationship between the functions and the surface morphology of the electrode are crucial. In this work, we compared polycrystalline Au (poly-Au) and single-crystalline Au(111) electrodes as the substrates for a redox-active, self-assembled monolayer (SAM) of an alkyl-viologen thiol, 1-(10-mercaptodecyl)-1′-methyl-4,4′-bipyridinium dibromide (MeVC10-SH 2Br−). The voltammetric and electroreflectance measurements were performed at a mirror-polished poly-Au electrode and a flame-annealed single-crystalline Au(111) electrode after their surface modification with a MeVC10-SH monolayer. We found significant differences in the anion effects on the voltammetric behavior between the two substrates, especially in the dependence on anion hydrophobicity. The electroreflectance spectra revealed a tendency that a greater abundance of the viologen radical cation dimers at a Au(111) than that at a poly-Au. A considerable Br− adsorption at the SAM-modified Au(111) electrode was observed because of the loose packing of immobilized MeVC10-SH molecules. We discussed the origins of the contrasting behavior resulting from the difference of the surface.

Effect of Defects on Strain Relaxation in InGaN/AlGaN Multiple‐Quantum‐Well Near‐Ultraviolet Light‐Emitting Diodes

Three similar‐structure InGaN/AlGaN multiple‐quantum‐well near‐ultraviolet (NUV) light‐emitting diodes (LEDs) are utilized to investigate the microscopic effect of defects on strain relaxation. Consistent correlations among the crystal quality, the piezoelectric field (FPZ), the internal quantum efficiency (IQE), and the bandgap shrinkage of NUV LEDs are obtained by investigating the macroscopic characterizations. The difference in crystal quality (or the defect density) of NUV LEDs is found by the ideality factor, the emission microscope image, the Shockley–Read–Hall coefficient, and the IQE. Electroreflectance spectra are used to calculate FPZ of NUV LEDs. FPZ, the IQE, and the peak‐wavelength shift at driving currents are increased with the samples’ crystal quality compared to the reference sample. Also, FPZ, the IQE, and the peak‐wavelength shift are decreased with the increase in samples’ defect densities. A similar result is found for the bandgap shrinkage. This effect significantly indicates that the strain relaxation is induced by defects. Herein, a model that systematically explains the observed changes in macroscopic properties of NUV LEDs is proposed.

Electroreflectance of Cu2(Cd1−x,Znx)SnS4 thin film solar cells

The effect of Cu off-stoichiometry and Zn alloying on the fundamental absorption region of ${\mathrm{Cu}}_{2}\mathrm{CdSn}{\mathrm{S}}_{4}$ (CCTS) absorbers in complete solar cells has been investigated using electroreflectance (ER) spectroscopy at room temperature. It is found that ER spectra consist of contributions from two different sources, one of which corresponds to band gap transition in the absorber layer and the other to the interference effect in the window layer. ER measurements on CCTS samples reveal a near-constant band gap energy of 1.37--1.38 eV and a relatively small broadening of 60--90 meV in the probed $0.8<\mathrm{Cu}/(\mathrm{Cd}+\mathrm{Sn})<0.89$ compositional range, in contrast to related kesterites ${\mathrm{Cu}}_{2}\mathrm{ZnSn}{(\mathrm{S},\mathrm{Se})}_{4}$. The analysis of the band gap in ${\mathrm{Cu}}_{2}({\mathrm{Cd}}_{1\ensuremath{-}x},{\mathrm{Zn}}_{x})\mathrm{Sn}{\mathrm{S}}_{4}$ alloys yields a quadratic dependence on Zn content with a bowing parameter of 0.4 eV. Finally, the broadening parameters of the band gap transitions as well as their compositional dependence are evaluated and discussed.

Recovering the optical transitions in tin oxide thin films at room temperature using electroreflectance

Observing the optical transitions in Tin Oxide at room temperature is a difficult task because of its forbidden bandgap nature in bulk form. Moreover, it possesses the well know low emission efficiency issue in its reduced dimensional thin film structures . In this work, we use Electroreflectance (ER) method to recover the optical transitions in tin oxide thin films . A metal-oxide-metal geometry, with SnO2 film was sandwiched between two metal electrodes , was illuminated by the light under an external perturbation voltage. This results the well-enhanced critical points in the ER spectra with a suppressed featureless background. The characteristics of observed critical points are found using the third derivative functional form model. The points are identified as defect transition and surface exciton transition originated due to the presence of defect states formed by the oxygen vacancies . In addition to this, the free exciton transition, from deep of the valence band to the conduction band , is explicitly revealed by ER spectra of r-SnO 2 film. A built-in electric field driven by external voltage is supposed to be the main factor for enhancing the critical points associated with optical transitions. • The optical transitions in SnO 2 films were observed at room temperature by using electroreflectance method. • A metal-oxide-metal structure was fabricated to explore the optical transitions in SnO 2. • The electroreflectance method with external perturbation of 10 V was applied across the geometry. • The electroreflectance spectra revealed the defect and exciton transitions within the visible, near UV, and UV region.

Enhancement of deformation of redox-active hydrogel as an actuator by increasing pendant viologens and adding filler or counter-charged polymer

Strategies for enhancement of redox-driven deformation of a hydrogel, viologen pendant poly-L-Lysine based glutaraldehyde cross-linked hydrogel (PLLV-GA-gel), were pursued. To surpass the previously reported fastest reductive contraction, a 93 % volume decrease within 100 s on dithionite reduction, and to accelerate expansion on oxidation, we examined the pendant rate dependence and the effect of the addition of conductive fillers or an anionic polymer, poly(styrene sulfonate) (PSS). In testing deformation of water-floated PLLV-GA-gels, the contraction of a 25 % pendant rate gel was greater and faster than others. The initial rate of reductive contraction of the gels with fillers was in the order of graphene nano-platelet (GNP) > gold nanoparticle > carbon nanotube (CNT) > no-filler. The initial rates of swelling of GNP and CNT gels on oxidation were faster than the no-filler gel. PSS incorporation enhanced the swelling rate and amplitude but slowed down the contraction. All the PLLV-based samples on a Au electrode showed largely diffusion-controlled redox reactions of viologen in voltammograms and radical cation dimer rich light-absorption features in electroreflectance spectra. The guide for the choice of conductive filler and coexistence of anionic polymer described herein should be applicable to enhance the deformation of many redox-driven hydrogel actuators.

Impact of a surface on the electro-reflectance spectra of n-Si(110) and their polarization anisotropy

The electro-reflectance spectra, including their polarization dependencies were analyzed for n-Si(110) in the energy range of 2.9-3.8 eV. Based on the optical anisotropy of electro-optical effect, two contributions originated from a surface, (isotropic part relates to the linear electro-optical effect which inherent for (110) surface) and bulk, (anisotropic part relates to the Franz–Keldysh effect) were identified and separated. The presence of such extreme is explained by the zero value of the electron wave function on the surface and (or) the structure gettering of the free carriers.

Long range self-organisations of small metallic nanocrystals for SERS detection of electrochemical reactions

Gold electrodes were modified by silver and gold nanocrystals (NCs) that self-organize onto the surface. Their optical properties were explored by measuring electroreflectance spectra as a function of electrode potential. Below their oxidation potential, no shift of the reflectance maximum was observed for Ag NCs. This can be explained by a low interfacial capacitance resulting from the impossibility for the electrolyte to penetrate into the hydrophobic layer created by the NCs dodecanethiol ligands. Conversely, a non-monotonous evolution was observed with the electrode potential for oleylamine capped Au NCs. This behavior is suggesting a less dense hydrophobic layer, allowing significant electrolyte penetration. Next, electroactive compounds were adsorbed on the the Au NCs assemblies and characterized by Raman spectroelectrochemistry. In the first system displaying a single electron transfer with no coupled chemical reaction, only the spectrum intensity changed, because oxidation generated a Raman resonant radical cation. The second study considered 4-nitrothiophenol, for which up to 6 electrons and 6 protons may be transferred. In this case, the nitro band disappeared upon reduction, and the spectrum displayed typical features of the formed 4-aminothiophenol.

Electrochemical and spectroelectrochemical probing of the ionic channel in Nafion films using the redox of perfluoroalkyl viologen

We explored the electrochemistry of a perfluoroalkyl viologen, N,N′-di-(1H,1H,2H,2H-perfluorobutyl)-4,4′-bipyridinium dichloride (FC4VFC4) in a Nafion film on an Au electrode. The capability of FC4VFC4 as a redox-active probe for micro-environments of the ionic channels in the Nafion film was highlighted by clarifying the difference of its electrochemical behavior from that of its alkyl analog, dibutyl viologen (C4VC4). The time course of cyclic voltammograms (CVs) was tracked after immersion of a Nafion film-coated Au electrode in viologen solutions of various concentrations, Cs. The strongly Cs-dependent time change revealed the formation of an aggregate of FC4VFC4 in the ionic channels in the Nafion film. The aggregate blocks its own redox reaction at Cs ≥ 10 μM. At Cs = 5 μM, FC4VFC4 exhibited a quasi-reversible response. In contrast, C4VC4 showed a quasi-reversible response in the Cs range from 5 μM to 500 μM. We also used the time dependent change of electroreflectance (ER) spectra to track the state of viologens in the close proximity of the electrode|Nafion interface. After the immersion in 50 μM FC4VFC4 solution, the ER signals of the redox of FC4VFC4 molecules, being not in direct contact with the Au surface, rapidly increased in line with the change of CVs. What followed was a steep decrease of ER signal while the CV redox current was still increasing. It was also found that the FC4VFC4 aggregates in the Nafion film block the electrode reactions of [Ru(NH3)6]2+/3+, methylviologen, and C4VC4, which otherwise show reversible or quasi-reversible responses in the absence of FC4VFC4. All the results, especially the sharp contrast of the behavior of FC4VFC4 to that of C4VC4, revealed that either or both intermolecular perfluoro chain-chain interaction and perfuorinated FC4VFC4 side chain-Nafion perfluoroether side chain interaction are the key to determine the chemical micro-environment in the ionic channels.

Calibration of Polarization Fields and Electro-Optical Response of Group-III Nitride Based c-Plane Quantum-Well Heterostructures by Application of Electro-Modulation Techniques

The polarization fields and electro-optical response of PIN-diodes based on nearly lattice-matched InGaN/GaN and InAlN/GaN double heterostructure quantum wells grown on (0001) sapphire substrates by metalorganic vapor phase epitaxy were experimentally quantified. Dependent on the indium content and the applied voltage, an intense near ultra-violet emission was observed from GaN (with fundamental energy gap Eg = 3.4 eV) in the electroluminescence (EL) spectra of the InGaN/GaN and InAlN/GaN PIN-diodes. In addition, in the electroreflectance (ER) spectra of the GaN barrier structure of InAlN/GaN diodes, the three valence-split bands, Γ9, Γ7+, and Γ7−, could selectively be excited by varying the applied AC voltage, which opens new possibilities for the fine adjustment of UV emission components in deep well/shallow barrier DHS. The internal polarization field Epol = 5.4 ± 1.6 MV/cm extracted from the ER spectra of the In0.21Al0.79N/GaN DHS is in excellent agreement with the literature value of capacitance-voltage measurements (CVM) Epol = 5.1 ± 0.8 MV/cm. The strength and direction of the polarization field Epol = −2.3 ± 0.3 MV/cm of the (0001) In0.055Ga0.945N/GaN DHS determined, under flat-barrier conditions, from the Franz-Keldysh oscillations (FKOs) of the electro-optically modulated field are also in agreement with the CVM results Epol = −1.2 ± 0.4 MV/cm. The (absolute) field strength is accordingly significantly higher than the Epol strength quantified in published literature by FKOs on a semipolar ( 11 2 ¯ 2 ) oriented In0.12Ga0.88N quantum well.

Averaged angle-resolved electroreflectance spectroscopy on Cu(In,Ga)Se2 solar cells: Determination of buffer bandgap energy and identification of secondary phase

Currently, the use of Zn(O,S) as buffer material for Cu(In,Ga)Se2 (CIGS) solar cells is intensely studied in order to further boost the performance of these devices. In this context, nondestructive analytical tools are needed that enable the determination of buffer bandgap energies in the complete device. To this end, we developed a spectroscopic approach based on electroreflectance (ER). From a set of measured angle-resolved ER (ARER) spectra, an averaged modulus spectrum is numerically calculated. This method suppresses the commonly observed detrimental line-shape distortions due to interference effects in the layered device structure and thus enables the determination of bandgap energies even for thin buffer layers. To verify the working principle of ARER, we first apply it to CIGS absorber and CdS buffer layers. Then, we utilize it to investigate CIGS solar cells with Zn(O,S) buffers. All ARER results are compared to the results of diffuse ER, a technique previously developed for the suppression of interference fringes. We demonstrate that ARER is the superior ER method for nondestructive bandgap determination of thin buffer layers in complete CIGS solar cells. Moreover, a Cu containing compound was determined as a secondary phase in the Zn(O,S) buffer by combined ARER studies, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy.

Electroreflectance study on optical anisotropy in β-Ga2O3

Electroreflectance (ER) spectra of β-Ga2O3 were measured using Schottky barrier diodes. The ER spectra were well fitted to a third derivative line shape function by assuming three excitonic transitions. The transition energies exhibited moderate blueshifts with an external reverse bias by reflecting the contribution of transitions at M0 type critical points. Optical anisotropy was selectively observed in the polarized ER spectra. The fittings for the third derivative line shape function gave relatively large broadening parameters (0.11–0.89 eV) due mainly to the large exciton-longitudinal-optical-phonon interaction in β-Ga2O3. The ER spectra exhibited a third derivative line shape with a Franz-Keldysh oscillation superimposed on it. The results demonstrate that ER measurements are useful for investigating optical anisotropy in β-Ga2O3 and related alloys.

Measurement of the Piezoelectric Field in InGaN/AlGaN Multiple-Quantum-Well Near-Ultraviolet Light-Emitting Diodes by Electroreflectance Spectroscopy

The piezoelectric field in InGaN/AlGaN-based multiple-quantum-well (MQW) near-ultraviolet light-emitting diodes (LEDs) was measured by electroreflectance and photocurrent spectroscopies. At forward and reverse biases, both electroreflectance and photocurrent spectra were studied by utilizing similar-structure samples with indium contents of ~4.5%, ~5.5%, ~6.5%, and ~7.5%. The influence of MQW and superlattice structures on the electroreflectance spectra was interactively identified. The relation between the defects and the diffusion of Mg acceptors through the defects into the MQWs was also observed and systematically confirmed through the capacitance-voltage characteristics. The effects of diffused Mg acceptors on electroreflectance spectra, depletion width, and the piezoelectric field were found. The strain relaxation caused by the defects was also systematically investigated. The calculated piezoelectric fields of these samples were in good agreement with the theoretically calculated values.

Investigation into the Distribution of Built-in Electric Fields in LED Heterostructures with Multiple GaN/InGaN Quantum Wells by Electroreflectance Spectroscopy

Electric-field nonuniformity in the active region of a LED (light-emitting diode) heterostructure based on five identical GaN/InGaN quantum wells is investigated by the electroreflectance method. The energies of band-to-band transitions in the quantum wells and barriers are determined from the analysis of electroreflectance spectra using the Kramers–Kronig transforms. The procedure for estimating the electric-field strength in separate quantum wells of the active region by the position of spectral lines is proposed. It is found that the energies of the main transition in quantum wells of the active region with zero bias of the p–n junction differ by a magnitude on the order of 140 meV, which corresponds to a difference in the electric-field strengths of 0.78 MV/cm. It is shown that the nonuniformity of the electric field in the active region depends on the p–n junction bias.

Исследование распределения встроенных электрических полей в светодиодных гетероструктурах c множественными квантовыми ямами GaN/InGaN методом электроотражения

AbstractElectric-field nonuniformity in the active region of a LED (light-emitting diode) heterostructure based on five identical GaN/InGaN quantum wells is investigated by the electroreflectance method. The energies of band-to-band transitions in the quantum wells and barriers are determined from the analysis of electroreflectance spectra using the Kramers–Kronig transforms. The procedure for estimating the electric-field strength in separate quantum wells of the active region by the position of spectral lines is proposed. It is found that the energies of the main transition in quantum wells of the active region with zero bias of the p – n junction differ by a magnitude on the order of 140 meV, which corresponds to a difference in the electric-field strengths of 0.78 MV/cm. It is shown that the nonuniformity of the electric field in the active region depends on the p – n junction bias.

Driving Quick and Large Amplitude Contraction of Viologen-Incorporated Poly-l-Lysine-Based Hydrogel by Reduction.

A glutaraldehyde-cross-linked poly-l-lysine-based hydrogel with pendant viologens was synthesized with various [viologen unit]/[Lys unit] ratios. The hydrogel with the ratio of 25% was extensively characterized. Characterization of the hydrogel revealed that (i) water content reaches 95.8%, (ii) unbound viologen unit is absent, and (iii) lyophilized gel, showing porous structure, is rewettable. The hydrogel in contact with a gold, glassy carbon, pyrolytic graphite, or ITO electrode was electroactive, showing voltammetric responses involving diffusion process. Occurrence of electron transfer between viologen sites was verified by ESR measurements. The electroreflectance spectrum demonstrated significant dimerization of one-electron-reduced forms, viologen radical cations. Most remarkably, reduction of the hydrogel by dithionite showed a 93% volume decrease, being near to the water content, at 100 s and finally 97% contraction at 380 s. The initial rate of contraction was 0.02 s-1, and the swelling weight ratio was over 17. The contraction rate was much faster than that observed when adding of redox-inactive salts in the outer medium. Recovery by reoxidation was relatively sluggish even when O2-saturated water was used. The quick and large-amplitude contraction with removal of water from the gel body should be the consequence of permeability of reductant, interchain π-stacking of one-electron-reduced forms (viologen monoradical monocation), desolvation of viologen upon reduction, electron hopping between viologen units and its acceleration with contraction, and effective osmotic pressure difference.

Evaluation of the Built-in Voltage of a Au/n-GaAs Schottky Barrier Diode by Using Electroreflectance Spectroscopy

The built-in voltage (V bi ) of a Au/n-GaAs Schottky barrier diode was examined by using electroreflectance (ER) spectroscopy. The ER spectra were measured at various modulation voltages (v ac ) and dc bias voltages (V bias ). The interface electric fields (E i ) were obtained from the observed Franz-Keldysh oscillations (FKO) in the ER spectra. When v ac was increased, the amplitudes of the ER spectra varied linearly, but the line shapes of the ER spectra did not change. E i decreased from 19.3 × 104 V/cm to 4.39 × 104 V/cm with increasing V bias from −0.5 V to 0.6 V. From the relationship between V bias and E i 2 , the V bi and the carrier concentration (n) were found to be 0.70 V and 2.4 × 1016 cm −3, respectively.

Interactive Study of Electroreflectance and Photocurrent Spectra in InGaN/GaN-Based Blue LEDs

We experimentally and theoretically investigate the relationship between the electroreflectance (ER) and photocurrent (PC) spectra, and how they can be utilized to estimate the flat-band voltage and the bandgap energy of the InGaN/GaN-based quantum-well (QW) structure in blue light-emitting diodes. With theoretical modeling of ER and PC spectra, we calculate the changes in both the refractive index ( $\Delta \text{n}$ ) and optical absorption ( $\Delta \alpha )$ spectra from experimental ER and PC data by using the Kramers–Kronig relation. Then, we compare $\Delta \text{n}$ (or $\Delta \alpha )$ spectra obtained differently from the ER and PC data and try to comprehend their physical meanings interactively. From these combined studies, we propose an exact method of determining the flat-band voltage, the piezoelectric field, the emission energy, the effective bandgap energy, and the Stokes shift of a QW structure under the quantum-confined Stark effect (QCSE).

Blue shift in the optical transitions of ZnO thin film due to an external electric field

An electric field can act as an efficient external perturbation to induce optical changes in a material. This paper describes the influence of an external electric field on the excitonic and band transitions in zinc oxide thin film investigated through electroreflectance. The electroreflectance spectra were identified as excitonic and band transitions. Both types of transitions were found to shift toward high energy as the magnitude of the electric field increased. Such a blue shift was accompanied by an increase in the exciton binding energy of the system.