Electroreflectance Research Articles

Electroreflectance spectroscopy of few-layer MoS2 : Issues related to A1s exciton subspecies, exciton binding energy, and inter-layer exciton

Optical spectra of few-layer transition metal dichalcogenide semiconductors reveal several transitions whose character and origins continue to be debated. We have studied hBN encapsulated few-layer MoS2 films using electroreflectance (ER) spectroscopy. Two strong features are seen in the reflectance spectrum of trilayer MoS2 around the ground state A–exciton transition. In ER, the corresponding features show opposite phase response to the applied voltage. Evidence from first principles ER line shape simulation and photoluminescence spectroscopy suggests that these two features are likely to be A1s exciton subspecies proposed earlier, whose energy depends on which layer the electron–hole pair is located in. The first excited state A2s exciton transition is also identifiable in ER. Through the two-dimensional hydrogenic exciton model, it enables an approximate estimation of the exciton binding energy Eb. The increase in Eb with decreasing film thickness, which originates from reduced dielectric screening, is phenomenologically analyzed through a film thickness and capping material dependent effective dielectric constant. Extending this idea, we show that the A1s exciton is mostly confined to a single S–Mo–S layer, as predicted by theory. An inter-layer (IL) exciton is expected in bilayer and thicker 2H-MoS2 films. However, we show that there can be bilayer films where the IL exciton is absent, which may be related to increased carrier concentration.

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.

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.

Electroreflectance study of antimony doped ZnO thin films grown by pulsed laser deposition

In this research, antimony doped ZnO (SZO) thin films with various doping content have been grown on a c-Al2O3 substrate by pulsed laser deposition. The effect of the applied electric field on the bandgap of SZO thin films was studied by electroreflectance (ER) spectroscopy using a capacitor-type geometry. Hall effect measurements indicate that the p-type conductivity of SZO is realized for the Sb2O3 weight percentage at 2%. The blue shift of the energy bandgap was observed in thin films after increasing the doping concentration. The Burstein-Moss effect is the crucial mechanism for the blue shift of the SZO bandgap. Furthermore, we found the red shift of bandgap in all samples, which was measured under various electric fields by ER spectroscopy. The changes of the optical transition in the band structure should be the origin of the red shift behaviors of the SZO bandgap under the presence of the electric field. Based on our results, we can design and optimize the bandgap of SZO for optoelectronic devices.

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.

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.

Optical Characterization and Photovoltaic Performance Evaluation of GaAs p-i-n Solar Cells with Various Metal Grid Spacings

GaAs p-i-n solar cells are studied using electroreflectance (ER) spectroscopy, light beam induced current (LBIC) mapping and photovoltaic characterization. Using ER measurements, the electric field across the pn junction of a wafer can be evaluated, showing 167 kV/cm and 275 kV/cm in the built-in condition and at −3 V reverse bias, respectively. In order to understand the effect of the interval between metal grids on the device’s solar performance, we performed LBIC mapping and solar illumination on samples of different grid spacings. We found that the integrated photocurrent intensity of LBIC mapping shows a consistent trend with the solar performance of the devices with various metal grid spacings. For the wafer used in this study, the optimal grid spacing was found to be around 300 μm. Our results clearly show the importance of the metal grid pattern in achieving high-efficiency solar cells.

Resonant optical studies of GaAs/AlGaAs Multiple Quantum Well based Bragg Structures at excited states

ABSTRACTOptical reflectance (OR) and electro-reflectance (ER) spectroscopies were employed to study the resonant optical properties of GaAs/AlGaAs multiple quantum wells based Resonant Bragg Structures (RBS) at excited states. The RBS samples have 60 periods of GaAs/AlGaAs quantum well/barrier grown on semi-insulating GaAs substrates by molecular beam epitaxy with slightly different thicknesses of well/barrier. We observed enhanced OR and ER features when exciton energy coincides with the energy of the Bragg reflection peak, a double resonance condition. Bragg peak can significantly be tuned by changing the angle of incidence of the light. Exciton energies can be tuned by changing the temperature, external electric field and the thickness of the quantum wells. By tuning the Bragg peak for double resonance in the RBS samples of different thicknesses, we observed the electro-reflectance features related to the transitions of x(e2-hh2), x(e2-hh1), x(e2-lh1), x(e2-hh3) and x(e1-hh3) excitons along with the sharp features of x(e1-hh1) and x(e1-lh1) ground state exciton transitions from the ER experiments. The excitonic transitions x(e2-hh1), x(e2-lh1) and x(e2-hh3) which are prohibited at zero electric field, were also observed due to the increased overlap of the electron and hole wave functions caused by the electric field; built-in electric field or applied DC bias.

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).

Absolute technique for measuring internal electric fields in InGaN/GaN light-emitting diodes by electroreflectance applicable to all crystal orientations

The internal electric fields in III-polar (0001), N-polar , and semipolar InGaN/GaN light-emitting diodes were investigated by electroreflectance (ER) spectroscopy. The ER spectra reflected the difference in the direction and strength of internal electric fields. Phase analyses of the ER signal revealed that only III-polar InGaN wells have the opposite direction of the internal electric field at zero bias voltage; this finding is in good agreement with the results of numerical analyses. Quantitative analyses of internal electric fields were conducted by the linewidth analyses of ER spectra. Our experimental results indicate that the absolute value of internal electric fields can be measured from ER spectra.

Carrier-tunneling-induced photovoltaic effect of InAs/GaAs quantum-dot solar cells

This study reports the observation of the carrier-tunneling-induced photovoltaic (PV) effect in an InAs/GaAs quantum-dot solar cell (QDSC). The illuminated current-voltage (J-V) characteristics and the applied-bias-dependent electroreflectance (ER) were measured at 12 K by using an excitation laser with a wavelength of 975 nm (1.27 eV), which excites only the quantum-dot (QD) states below the GaAs band gap. The J-V results showed a peculiar current curve in the reverse bias region caused by carrier tunneling. The ER results showed that the junction electric field (F) decreased with increasing intensity of the excitation laser (Iex) at different applied-bias-voltages (Va) due to the tunneling-induced PV effect. The PV effect was enhanced by improved tunneling with increasing reverse bias voltage. We also evaluated the tunneling carrier density (σpv) as a function of Va in the QDSC.

Influences of the p-GaN Growth Temperature on the Optoelectronic Performances of GaN-Based Blue Light-Emitting Diodes

We investigate the influence of p-(Al)GaN growth temperature ( $T_{g}$ ) on the optoelectronic performances of InGaN/GaN multiple-quantum-well (MQW) blue light-emitting diodes (LEDs). We systematically combine various characterization techniques, such as current–voltage, current-light output power, capacitance–voltage ( $C$ – $V$ ) under forward and reverse biases, photocurrent and electroreflectance (ER) spectroscopies, temperature-dependent electroluminescence, and the internal quantum efficiency (IQE). From the experimental analyses, it is shown that increasing $T_{g}$ induces: 1) the reduced optical loss by the improved crystal quality of the p-GaN layer (improved light extraction efficiency) and 2) the decreased IQE due to the Mg diffusion from the p-(Al)GaN layer to the MQW region. This paper demonstrates that the fine control of the Mg diffusion from the p-GaN layer to the InGaN/GaN MQW region is the key factor for achieving highly efficient blue LEDs. Moreover, a method of estimating the Mg diffusion length is proposed for the first time by analyzing both the $C$ – $V$ curves and the ER spectra under reverse biases.

Large Stokes-like shift in N-polar InGaN/GaN multiple-quantum-well light-emitting diodes

N-polar (−c-plane) InGaN/GaN light-emitting diodes (LEDs) were grown by metalorganic vapor phase epitaxy, and their optoelectronic properties were evaluated by electroreflectance (ER) and electroluminescence (EL) measurements. In −c-plane LEDs, the emission energy was much lower than that in c-plane LEDs. By comparing EL and ER results, we found that the emission energy was also much lower than the transition energy. The transition energy is in good agreement with X-ray diffraction analysis results. These results indicate that −c-plane LEDs exhibit a larger Stokes-like shift than do c-plane LEDs. This Stokes-like shift is due to the strong potential fluctuation, which is possibly caused by the specific growth patterns of −c-plane III–nitrides. The dominant emission centers of the −c-plane LEDs were suggested to be the localized states of InGaN islands.

Piezoelectric field in highly stressed GaN-based LED on Si (1 1 1) substrate

Stress states in GaN epilayers grown on Si (111) and c-plane sapphire, and their effects on built-in piezoelectric field induced by compressive stress in InGaN/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) were investigated using the electroreflectance (ER) spectroscopic technique. Relatively large tensile stress is observed in GaN epilayers grown on Si (111), while a small compressive stress appears in the film grown on c-plane sapphire. The InGaN/GaN MQWs of LED on c-plane sapphire substrate has a higher piezoelectric field than the MQWs of LEDs on Si (111) substrate by about 1.04MV/cm. The large tensile stress due to lattice mismatch with Si (111) substrate is regarded as external stress. The external tensile stress from the Si substrate effectively compensates for the compressive stress developed in the active region of the InGaN/GaN MQWs, thus reducing the quantum-confined Stark effect (QCSE) by attenuating the piezoelectric polarization from the InGaN layer.

A Study of Piezoelectric Field Related Strain Difference in GaN-Based Blue Light-Emitting Diodes Grown on Silicon(111) and Sapphire Substrates.

We investigate the strain difference in InGaN/GaN multiple quantum wells of blue light-emitting diode (LED) structures grown on silicon(1 11) and c-plane sapphire substrates by comparing the strength of piezo-electric fields in MQWs. The piezo-electric fields for two LED samples grown on silicon and sapphire substrates are measured by using the reverse-bias electro-reflectance (ER) spectroscopy. The flat-band voltage is obtained by measuring the applied reverse bias voltage that induces a phase inversion in the ER spectra, which is used to calculate the strength of piezo-electric fields. The piezo-electric field is determined to be 1.36 MV/cm for the LED on silicon substrate and 1.83 MV/cm for the LED on sapphire substrate. The ER measurement results indicate that the strain-induced piezo-electric field is greatly reduced in the LED grown on silicon substrates consistent with previous strain measurement results by micro-Raman spectroscopy and high-resolution transmission electron microscopy.

Diffuse electroreflectance of thin-film solar cells: Suppression of interference-related lineshape distortions

Electroreflectance (ER) is a standard method to determine the band gap of semiconductor materials that has also been applied to thin-film solar cells (TFSCs). However, the lineshapes in typical ER spectra of TFSCs are significantly distorted compared to the model lineshapes, which are used for spectrum evaluation. These distortions are mainly due to thin-film interferences in the stratified system. In this letter, we demonstrate that these distortions are significantly suppressed in diffuse ER (D-ER) where the diffuse instead of the specular reflection of TFSCs is evaluated. The existence of an ER signal in the diffuse reflectance is shown by two-dimensional finite-difference time-domain simulations. Experimentally, the suppression of interference-related lineshape distortions is demonstrated on a series of Cu2ZnSnSe4 solar cells with different layer thicknesses and therefore different optical path lengths for interference. The same working principle is demonstrated for a Cu(In,Ga)(S,Se)2 solar cell as well. The resulting lineshapes in D-ER can then be interpreted using standard analysis methods such as Aspnes' Third-Derivative Functional Form.