Photoreflectance Spectra Research Articles

Characterization of surface electric field in β-FeSi2 by Franz–Keldysh oscillations

The surface electric field (F) of β-FeSi2 has been investigated by Franz–Keldysh oscillations (FKOs) observed in photoreflectance spectra. The FKO signals were observed in an undoped β-FeSi2 epitaxial layer (3 nm) grown on p+-type β-FeSi2 layers (UP+ structure) by molecular beam epitaxy. The surface electric field obtained from the FKO at 11 K was F = 220 kV/cm. The surface electric field was almost independent of the excitation light power and temperature, showing that the surface electric field was not affected by the surface photovoltage effect. In the dependence of F on the thickness of the undoped layer (d), F was nearly constant at d = 2–56 nm. The result showed that the surface electric field of β-FeSi2 was applied to a thin region (<2 nm) at the surface.

Study of the Photo-Response of Doped GaAs with Aging

The aging of semiconductor materials is a topic of current interest. We studied the photo-response of epitaxial samples of GaAs doped with Ge and Sn up to 1 × 1019 atoms cm−3. These samples were stored in a dry and dark environment for 26 years. We realized photoluminescence measurements at different temperatures and photoreflectance spectra at 300 K in three periods: 1995, 2001 and 2021. We found that environmental oxygen formed defects in GaAs, leaving lattice vacancies that provoked changes in the optical photo-response. In addition, we found that the vacancy concentrations could be as large as 5 × 1017 atoms cm−3 over the 26 years. In this work, we demonstrate that the aging of semiconductor materials occurs even when they are not used within a functioning circuit, with the changes being greater when the material is not doped. Knowing about the aging of materials is important for the industry, particularly for the semiconductor industry, because aging-induced deterioration influences prices and guarantees.

Photoreflectance Analysis of InAsPSb/InGaAs Multi-Quantum Well LED Structures with Different Well/Barrier Numbers

InAsPSb is an emerging material used as an efficient barrier in quantum well structures, and the resulting devices can be employed in the mid-infrared region of the electromagnetic spectrum. This study investigates the photoreflectance spectra of two InAsPSb/InGaAs multi-quantum well light-emitting diodes with 6 and 15 quantum well periods. The photoreflectance of the samples was analyzed at various temperatures and excitation powers. By examining the Franz-Keldysh oscillations in the spectra, we explored the influence of the number of well layers on the electric field strength in the junction. The results showed that the number of quantum wells can influence the electric field at the junction, potentially impacting the overall performance of the devices. The simulation of the electric field strength aligns with the results of the photoreflectance analysis. This suggests that the field extracted from Franz-Keldysh oscillations characterizes the field inside the multi-quantum wells, offering potential reasons for the observed effects on the number of multi-quantum wells in the field.

Photoreflectance spectroscopy of BiOCl epitaxial thin films

We have observed a new optical transition in the photoreflectance spectra of indirect-gap BiOCl thin films, which were grown on SrTiO3 substrates. The position of this transition is close in energy to its bulk critical point (CP) energy. Moreover, these are significantly lower than a higher-lying direct-type CP from an energetic point of view. The spectral line shape analysis for our observed signal suggests the presence of an excitonic effect of this compound. We determined its dependence of the optical anomaly on temperature ranging from 80 K to RT. We adopted the Varshni model for this analysis. At last, we compared photonic properties of BiOCl with those of an element and binary semiconductors.

Phase-sensitive analysis of a two-color infrared photodetector using photoreflectance spectroscopy

The phase diagrams of photoreflectance spectra were investigated for an InGaAs two-color infrared photodetector. The diagrams for a high excitation intensity revealed that the spectrum is multi-component. The origin of these components was investigated, and the photoreflectance spectra and phase diagrams were also measured for an angle-polished version at different depths. With the help of the polished sample, the variation of the phase delay angles and the trapping time constants was tracked for different depths. Additionally, the polished version enables us to find a confirmation for the origins of the multi-component nature of the whole phase diagram. It can be concluded that when the phase delays or time constants of various components are very close, more attention should be paid to interfering with the phase-sensitive investigations of layered materials. As a main result, the consistency of the phase delay with interface trap densities was confirmed qualitatively. Using a reciprocal space map of the sample, this result can be a piece of experimental evidence for a correlation between the photoreflectance time constant and trap densities in the junctions. This non-contact method enables the characterization of layered devices, offering a valuable tool for achieving high-performance devices.

Gaussian and Faddeeva broadening of the Franz-Keldysh oscillations at three-dimensional critical-points

Faddeeva broadening (FB) couples the physical characteristics of Gaussian and Lorentzian broadening into a single profile shaping the spectral response of dielectric functions. The electric field induces Franz-Keldysh oscillations (FKOs) as deviations in the dielectric function of semiconductors. We investigated the impact of Gaussian and FB on FKOs at the isotropic M0 of three-dimensional critical points. The twenty-pole Martin–Donoso–Zamudio quadrature was applied to compute the FB of FKOs in differential reflectivity based on the electro-optic function, H(z), with the complex Airy function, Ai(z). H(z) was simplified into a compact product of to reduce the truncation error and determine its asymptotics. We derived a general asymptotic expansion of order N, accounting for Stokes’ phenomenon as the arg(z) crosses The computation of H(z) is detailed for twelve digits of accuracy. The effects of higher Gaussian and FB widths on the damping of FKOs are assessed using FKO peak-to-peak attenuations. For the low-field regime, this work H(z) smoothly evolves into Aspnes’ third derivative functional form (TDFF). Between low- to intermediate-field regimes, we investigated the boundaries for the relevant use of the TDFF by assessing parameters extracted from simulated lineshapes, and the average root mean-squared error. We fitted of the Faddeeva broadened single Airy model to published experimental photoreflectance (PR) lineshapes of InGaAsP. To model degenerate light-and heavy-holes transitions, we fitted of the Faddeeva broadened double Airy model to published experimental contactless electroreflectance (CER) lineshapes of GaAs. In addition to legacy mean-squared error () and coefficient of determination (), the goodness of fit was evaluated using the unbiased Akaike information criteria. The effectiveness of Faddeeva versus Lorentzian broadening in sample PR and CER spectra analysis was assessed using Airy function FKO models. Concludingly, the FB of the differential reflectivity model outperformed the Lorentzian broadening even with the empirical energy-dependent width.

Fourier-transform infrared photoreflectance spectroscopy of the InSb/InAs/In(Ga,Al)As/GaAs metamorphic heterostructures with a superlattice waveguide

Fourier-transform infrared photoreflectance (PR) spectroscopy was used to study the energy spectrum of InSb/InAs/In(Ga,Al)As/GaAs metamorphic heterostructures with a superlattice waveguide at room temperature (RT). Theoretical calculations in the framework of the eight-band Kane model were performed to obtain a reliable knowledge of the actual energies of the most probable optical transitions. The experimental results were analyzed to determine the influence of the design features and stress balance on the energy spectra of the structures. Photoluminescence studies performed at 11 K and RT, as well as the determination of the internal quantum efficiency of luminescence, enabled us to characterize the emission characteristics of the structures, regardless of their waveguide efficiency. The structure with a 5-nm-thick GaAs insertion within the metamorphic buffer layer exhibited the highest probability of the main optical transition observed in the PR spectra as well as the highest luminescence intensity and quantum efficiency.

Influence of interface states on built-in electric field and diamagnetic-Landau energy shifts in asymmetric modulation-doped InGaAs/GaAs QWs

The impact of interface defect states on the recombination and transport properties of charges in asymmetric modulation-doped InGaAs/GaAs quantum wells (QWs) is investigated. Three sets of high-mobility InGaAs QW structures are systematically designed and grown by the metal-organic vapor phase epitaxy technique to probe the effect of carrier localization on the electro-optical processes. In these structures, a built-in electric field drifts electrons and holes towards the opposite hetero-junctions of the QW, where their capture/recapture processes are assessed by temperature-dependent photoreflectance, photoluminescence, and photoconductivity measurements. The strength of the electric field in the structures is estimated from the Franz Keldysh oscillations observed in the photoreflectance spectra. The effects of the charge carrier localization at the interfaces lead to a reduction of the net electric field at a low temperature. Given this, the magnetic field is used to re-distribute the charge carriers and help in suppressing the effect of interface defect states, which results in a simultaneous increase in luminescence and photoconductivity signals. The in-plane confinement of charge carriers in QW by the applied magnetic field is therefore used to compensate the localization effects caused due to the built-in electric field. Subsequently, it is proposed that under the presence of large interface defect states, a magnetic field-driven diamagnetic-Landau shift can be used to estimate the fundamental parameters of charge carriers from the magneto-photoconductivity spectra instead of magneto-photoluminescence spectra. The present investigation would be beneficial for the development of high mobility optoelectronic and spin photonic devices in the field of nano-technology.

Investigation of van der Waals crystals of GaSe and GaS-=SUB=-x-=/SUB=-Se-=SUB=-1-x-=/SUB=- by photoreflectance method

Photoreflectance spectra of layered undoped GaSe and GaSxSe1-x crystals present Franz--Keldysh oscillations indicating the near-surface built-in electric field, that can participate in the separation of photoinduced charge carriers in ultrahigh-sensitive photodetectors based on these materials. The measured value of the field strength in GaSxSe1-x turned out to be almost 1.5 times less than in GaSe, that may indicate a smaller number of free charge carriers in the solid solution. The broadening parameter of GaSxSe1-x spectral lines is also significantly lower than in the case of GaSe. This is due to the fact that isovalent atoms, being added into the GaSe, fill Ga vacancies, reducing the number of defects and the concentration of intrinsic charge carriers. The high-field modulation mode observed in the photoreflectance spectrum of GaSxSe1-x doped with an Al donor impurity indicates a relatively small thickness of the depletion region due to the presence of a large number of free electrons. Keywords: photoreflectance, Franz--Keldysh oscillations, GaSe, gallium monoselenide, van der Waals crystals, layered crystals.

Исследование ван-дер-ваальсовых кристаллов GaSe и GaS-=SUB=-x-=/SUB=-Se-=SUB=-1-x-=/SUB=- методом фотоотражения

Photoreflectance spectra of layered undoped GaSe and GaSxSe1-x crystals present Franz — Keldysh oscillations indicating the near-surface built-in electric field, that can participate in the separation of photoinduced charge carriers in ultrahigh-sensitive photodetectors based on these materials. The measured value of the field strength in GaSxSe1-x turned out to be almost 1.5 times less than in GaSe, that may indicate a smaller number of free charge carriers in the solid solution. The broadening parameter of GaSxSe1-x spectral lines is also significantly lower than in the case of GaSe. This is due to the fact that isovalent atoms, being added into the GaSe, fill Ga vacancies, reducing the number of defects and the concentration of intrinsic charge carriers. The high-field modulation mode observed in the photoreflectance spectrum of GaSxSe1-x doped with an Al donor impurity indicates a relatively small thickness of the depletion region due to the presence of a large number of free electrons.

Systematic optical study of high-x InxGa1-xAs/InP structures for infrared photodetector applications

Optical and structural properties in high-x InxGa1-xAs (x > 0.65) samples with varying indium concentration grown on InP (100) substrate are reported. By increasing the indium fraction, it was found by the high-resolution X-ray diffraction (HR-XRD) study that the dislocation density in the InxGa1-xAs epitaxial layer significantly increased, and the surface quality deteriorated remarkably. Photoreflectance (PR) spectra show the presence of Franz-Keldysh Oscillations (FKOs) features above the InxGa1-xAs energy bandgap. The strain-induced electric field is then estimated directly from the FKOs periods. Temperature-dependent photoluminescence (TDPL) measurements from 10 K to 300 K showed carrier locations (S-shape). This abnormal behavior is due to the dislocation density associated with fluctuations in the indium concentration. A quasi-stationary rate equation model for the temperature-dependent luminescence spectra of the localized state material system is proposed to interpret the band gap emission process quantitatively. Low-temperature (10 K) time-resolved PL measurements show the increase of lifetime with increasing the indium concentration. Yet, the addition of only 1.7% of indium concentration results in a strong enhancement of PL lifetime by ∼ 80%.All these results reveal a more precise picture of the localization and recombination mechanisms of photogenerated carriers in the InGaAs layer, which could be the crucial factors in controlling the performance of high indium content InGaAs SWIR detector.

Characterization of In(Ga,Al)As/GaAs metamorphic heterostructures for mid-IR emitters by FTIR photoreflectance spectroscopy

Infrared photoreflectance (PR) spectra of In(Ga,Al)As/GaAs metamorphic heterostructures have been obtained using a novel photomodulation FTIR spectroscopy technique. An analysis of the PR spectra features allowed us to estimate the critical point energies corresponding to the direct interband transitions in various regions of the In(Ga,Al)As heterostructures, and distinguish the PR signals originating from Fabry-Perot interference. Observation of Franz-Keldysh oscillations originating from the InAlAs virtual substrate and an InGaAs waveguide layer has enabled determination of the built-in electric field intensities within the heterostructures. The obtained results open up possibilities for contactless control of free carrier concentration in In(Ga,Al)As/GaAs metamorphic heterostructures developed for growth of emitters of mid-IR spectral range.

Photoreflectance studies of temperature and hydrostatic pressure dependencies of direct optical transitions in BGaAs alloys grown on GaP

BGaAs layers with boron concentrations of 4.1%, 7.4%, and 12.1% are grown by molecular beam epitaxy on a GaP substrate and studied by optical absorption and photoreflectance (PR) spectroscopy with both temperature and hydrostatic pressure dependence. The direct optical transitions from the bands composing the valence band—namely heavy-hole, light-hole, and spin–orbit split-off—to the conduction band are clearly observed in the PR spectra. For the abovementioned optical transitions, their temperature dependencies are obtained in the range from 20 K to 300 K, and analyzed by Varshni and Bose–Einstein relations. Furthermore, the BGaAs alloys are also studied with hydrostatic pressure up to ∼18 kbar, revealing pressure coefficients of direct optical transitions. The obtained results are discussed within the framework of the band anticrossing model and chemical trends.

Optical characterization by photoreflectance of GaN after its partial thermal decomposition

In the current study, we investigated the partial decomposition of GaN layers grown on SiN-treated sapphire substrate in an atmospheric pressure metal organic vapor phase epitaxy (AP-MOVPE) vertical reactor under N2 ambient at 1200 °C. Under these experimental conditions, we studied the decomposition of different coalescence degrees GaN layers and the early thermal decomposition stages of GaN films. By the help of the scanning electron microscope (SEM), the surface morphology of the decomposed GaN epilayers was examined. We demonstrated that the thermal decomposition leads to an irregular etching process, which was carried out step by step. The optical properties of the decomposed films were then studied by using the photoreflectance (PR) technique at room temperature and at 11 K. We determined a logarithmic relation between the built-in electric field in the surface of GaN layers and the stress value. A model is also proposed to extract the stress value from PR spectra. It should be noted also that by using the thermal decomposition process, the stress inside GaN layers can be reduced by a factor of 6.

Simple and direct estimation method for split-off hole effective mass from Franz–Keldysh oscillations appearing in photoreflectance spectra: a feasibility study using GaAs epitaxial layer structures at room temperature

We investigated the feasibility of directly estimating split-off hole effective mass m so from Franz–Keldysh oscillations using non-destructive and non-invasive photoreflectance spectroscopy. We used an undoped/n-type GaAs epitaxial structure and two GaAs p–i–n diode structures. We observed the phenomenon that Franz–Keldysh oscillations, which originate both the E 0 fundamental transition and from the E 0 + Δso transition energy, appear in the photoreflectance spectra at room temperature. Initially, from the electro-optic constant ℏΘ HH of the Franz–Keldysh oscillations originating from the E 0 fundamental transition, we deduced the built-in electric field F in each sample with the use of the relation ℏΘ HH = (e 2ℏ2 F 2/2μ HH)1/3, where the quantity μ HH is the reduced effective mass of the electron and heavy hole. In the next step, we calculated the reduced effective mass μ so of the electron and split-off hole using the relation ℏΘ so = (e 2ℏ2 F 2/2μ so)1/3, where the quantity ℏΘ so is the electro-optic constant of the Franz–Kelysh oscilations from the E 0 + Δso transition. Finally, we estimated the split-off hole effective mass m so from the reduced effective mass μ so. The estimated split-off hole effective mass agrees with the reported value obtained experimentally. Thus, we conclude that the split-off hole effective mass is estimatable from analyzing the Franz–Keldysh oscillations in the photoreflectance spectra. We also compare the present hole effective masses to those theoretically known and discuss the appropriateness of the electronic-band calculations.

Impact of Built‐In Electric Field on the Emission Characteristics of InAs/GaAs Quantum Dot Laser Structure

The impact of the built‐in electric field on the emission characteristics of an InAs/GaAs quantum dot (QD) laser structure is investigated by performing systematic spectroscopic measurements. Photoluminescence (PL) features related to the ground state (GS) and excited state (ES) transitions of a QD ensemble are clearly observed in the temperature range of 8–300 K. The cross‐sectional transmission electron microscopy image and a systematic analysis of temperature‐dependent PL data confirm the excellent crystalline and optical quality of the QD laser structure. It is found that the values of the activation energy associated with GS and ES transitions of the QD sample reasonably match with the energy splitting of PL features. This successfully explains the trends observed in temperature‐dependent PL spectra, where thermal transfer of carriers to the wetting/barrier layer via QD excited states is proposed to be a major decay channel. An interesting observation is made where the integrated intensity of PL features increases up to 75 K and falls thereafter during the heating cycle. Such a peak‐like behavior of the integrated intensity is explained by invoking the temperature dependence of the built‐in electric field, which is estimated from the analysis of Franz–Keldysh oscillations observed in photoreflectance spectra and is important in the design of QD lasers.

Interlayer and excited-state exciton transitions in bulk 2H−MoS2

Photoreflectance (PR) spectrum of bulk $2H\text{\ensuremath{-}}{\mathrm{MoS}}_{2}$ at energies around its direct band gap is shown to have at least three distinct spectral features over a certain temperature range, although only two are seen in absorption and reflectance spectra. These observations are compared with results from many-body perturbation theory band structure calculations that include exciton formation through the Bethe-Salpeter equation. Apart from the ground state $A\phantom{\rule{4pt}{0ex}}(n=1)$ exciton transition around 1.92 eV at 25 K, the next spectral feature around 1.96 eV, whose origin has been much debated, is identified here with a transition $I$ with strong interlayer contributions. Its greater sensitivity to electric fields is established through electroreflectance measurements. The third one around 1.99 eV, which is prominent only in PR, is shown to arise from the first excited state $A\phantom{\rule{4pt}{0ex}}(n=2)$ exciton transition. We discuss the previously unexplained observation as to why the $A\phantom{\rule{4pt}{0ex}}(n=2)$ feature dominates the PR spectrum at high temperatures. The $A$ exciton is shown to have a quasi-two-dimensional (2D) nature with binding energy of $77\ifmmode\pm\else\textpm\fi{}3$ meV under a 2D hydrogenic exciton model.

Investigation of energy transitions in MoS2 by photoreflectance spectroscopy method

Photoreflectance spectroscopy was used to study the energy spectra of natural molybdenum disulfide samples at various temperatures. Measurements using Fourier transform infrared spectrometer make it possible to observe a fine structure in the photoreflectance spectra which appears due to transitions involving excited excitonic states. The K-exciton binding energy obtained from our experiments is about 90 meV. In addition, an optical transition involving the excited state from the H-point of the Brillouin zone was observed, and the corresponding H-exciton binding energy was found to be 77 meV.

Reflectivity and Photoreflectivity Spectra of Structures with Quantum Wells Based on ZnO

Reflectivity and modulated photoreflectivity spectra of ZnO and Zn1 – xMgxO epitaxial layers as well as ZnO/Zn1 – xMgxO quantum wells have been studied at normal and oblique light incidence. Comparison of experimental and calculated spectra allowed refinement of certain parameters of excitons in these structures and the order of energy bands in ZnO.

Enhancement of L-band optical absorption in strained epitaxial Ge on Si-on-quartz wafer: Toward extended Ge photodetectors

Enhanced optical absorption in the L band (1.565–1.625 μm) of optical communication is reported for a Ge epitaxial layer grown on a Si-on-quartz (SOQ) wafer toward an extended range Ge photodetector (PD) in Si photonics. Ge epitaxial layers are grown using ultrahigh vacuum chemical vapor deposition at 600 °C on three different wafers of bonded SOQ, bonded Si-on-sapphire (SOS), and ordinary bonded Si-on-insulator (SOI). In the Ge layer, depending on thermal expansion mismatch between the Ge layer and base substrate, different amounts of in-plane biaxial lattice strain are generated. X-ray diffraction shows that an enhanced tensile strain of 0.32 ± 0.02% is generated in Ge on SOQ, which is approximately two times higher than 0.17 ± 0.02% in Ge on SOI, whereas the strain in Ge on SOS shows a slightly compressive value of −0.06 ± 0.02%. Photoreflectance spectra for these Ge layers show a change in the direct bandgap energy in accordance with the strain. In particular, the direct bandgap energy reduces from 0.775 ± 0.003 eV for Ge on SOI to 0.747 ± 0.003 eV for strain-enhanced Ge on SOQ, being comparable to 0.74 eV for the standard III–V PD material of In0.53Ga0.47As on InP. Furthermore, enhanced optical absorption in the L band is realized for Ge on SOQ, as measured using free-space pin PDs. These results indicate that the operating wavelength range of Ge PD on SOQ extends from C (1.530–1.565 μm) to the L band.