Photoreflectance Spectra Research Articles

Observation of temperature-dependent heavy- and light-hole split direct bandgap and tensile strain from Ge0.985Sn0.015 using photoreflectance spectroscopy

Temperature- (T-) dependent photoreflectance (PR) measurements have been made for the tensile-strained, undoped Ge0.985Sn0.015 film grown on n-Si substrate by ultra-high vacuum chemical vapor deposition method. The PR spectra at room temperature consist of two signals at around 0.739 and 1.022 eV, which are assigned to the direct transitions from conduction Γ valley to valence and spin-orbit split-off bands, respectively. The T-dependent PR measurements show tensile-strain split direct bandgap transitions from the Γ valley to the light-hole (ELH) and heavy-hole (EHH) bands at energies of 0.772 and 0.803 eV at 12 K, respectively, which are not usually observable from the photoluminescence measurements for relatively high Sn content Ge1-ySny samples. The PR signals for both HH and LH bands are blue shifted and their intensities decrease with increasing temperature, but both LH and HH PR signals persist through 240 K and only one HH PR signal is observed at room temperature. It has been observed that the separation energy between the EHH and ELH increases as T decreases, which clearly indicates an increase in tensile strain as T decreases. From the analysis of the T-dependent separation energy between the ELH and EHH, the T-dependent tensile strain in the Ge0.985Sn0.015 film was obtained, which might not be easily measured using the X-ray diffraction method.

Strain study of GaAs/In x Ga 1−x As/GaAs structures grown by MOVPE

Abstract GaAs/InxGa1 − xAs/GaAs strained and partially relaxed structures were grown by metalorganic vapor phase epitaxy and in situ monitored by laser reflectometry (LR). Two structures were formed by a single InxGa1 − xAs layer, and the third comprises three superposed InxGa1 − xAs layers having increasing indium contents. LR plots as function of time were recorded to extract growth rates and thicknesses of active and cap layers. In order to study the strain effect on structural and optical properties of these heterostructures, high resolution X-ray diffraction (HRXRD) and photoreflectance (PR) measurements were performed. HRXRD curves are developed to calculate strain tensor components, indium composition, and thicknesses of strained and partially relaxed layers. Besides, valence-band splitting and band-gap energy shift were measured by best fitting PR spectra at 300 K. Experimental energy values determined as a function of indium composition and relaxation rate were compared to those obtained by the elastic strain theory. For single and superposed InxGa1 − xAs active layers, a good correlation between experimental results and theoretical predictions was obtained.

Nanotextured Si Solar Cells on Microtextured Pyramidal Surfaces by Silver-assisted Chemical Etching Process

We investigated nanotextured Si solar cells using the silver-assisted chemical etching process. The nanotexturing process is very sensitive to the concentration of chemical etching solution. The high concentration process results in a nanowire formation for the nanosurfaces and causes severe surface damage to the top region of the micropyramids. These nanowires show excellent light absorption in photoreflectance spectra and radiative light emission in photoluminescence spectra. However, the low concentration process forms a nano-roughened surface and provides high minority carrier lifetimes. The nano-roughened surfaces of the samples show the improved electrical cell properties of quantum efficiency, conversion efficiency, and cell fill factor due to the reduction in the formation of the over-doped dead layer.

Photoreflectance investigation of band gap renormalization and the Burstein–Moss effect in Si doped GaN grown by MOVPE

This work reports on the photoreflectance (PR) study of Si doped GaN layers (GaN:Si) with different doping levels (2.1×1017–2.8×1019cm−3). GaN:Si samples have been successfully grown on sapphire substrate by metalorganic vapor phase epitaxy (MOVPE). Room temperature PR spectra showed Franz–Keldysh oscillation (FKO) features, reflecting the existence of a built-in electric field in the epilayers. The band gap energy determined using the FKO theory, is observed first to decrease and then to increase with increasing electron concentration. This result is explained by the competing effects of band gap renormalization and Burstein-–Moss band filling. Based on theoretical approach and experimental results, the interplay between these phenomena is systematically studied.

Temperature-dependent direct transition energy in Ge0.99Sn0.01 film grown on Si measured by photoreflectance spectroscopy

Optical properties of a p-Ge0.99Sn0.01 film grown on an n-Si substrate have been investigated as a function of temperature and excitation laser intensity using photoreflectance (PR) spectroscopy. The Ge0.99Sn0.01 film was grown by ultra-high vacuum chemical vapor deposition method. Room temperature PR spectrum shows a dominant signal assigned to a direct transition from the conduction Γ valley to valence band at around 0.73 eV. The transition to spin-orbit split-off band is also observed at around 1.0 eV. In addition, Franz–Keldysh oscillations (FKOs) due to the internal electric field are observed above the direct bandgap transition energy. The direct transition energy obtained by an analysis of FKO extremum was 0.728 eV at room temperature. The internal electric fields are reduced as the laser excitation intensity increases due to the photovoltage effect. The temperature dependence of direct transition energy was also investigated.

Photoreflectance analysis of annealed vanadium-doped GaAs thin films grown by metalorganic vapor phase epitaxy

Abstract In this study, we investigate the optical properties of annealed vanadium-doped GaAs films grown on GaAs substrates by metalorganic vapor phase epitaxy. The temperature dependence of the photoreflectance (PR) of as-grown GaAs:V films has been studied. We used the fit with Third-Derivative Functional Form model to evaluate the physical parameters. The temperature dependence of band gap and spin–orbit energies can be described by the Bose–Einstein statistical expression. The PR spectra of the samples are measured after thermal annealing in order to check any improvement in the optical quality of the material. The PR signal amplitude of GaAs:V samples decreased after thermal annealing. Degradation of the PR signal for annealing temperature at about 850 °C is observed revealing a poor quality of the layer surface states and an important density of the recombination centers. The lock-in phase analysis of PR spectra allows to determine the time constant for GaAs:V sample before and after thermal annealing.

H-point exciton transitions in bulk MoS2

Reflectance and photoreflectance spectrum of bulk MoS2 around its direct bandgap energy have been measured at 12 K. Apart from spectral features due to the A and B ground state exciton transitions at the K-point of the Brillouin zone, one observes additional features at nearby energies. Through lineshape analysis the character of two prominent additional features are shown to be quite different from that of A and B. By comparing with reported electronic band structure calculations, these two additional features are identified as ground state exciton transitions at the H-point of the Brillouin zone involving two spin-orbit split valance bands. The excitonic energy gap at the H-point is 1.965 eV with a valance bands splitting of 185 meV. While at the K-point, the corresponding values are 1.920 eV and 205 meV, respectively.

Optical properties and electronic energy‐band structure of Cu2ZnSnS4

AbstractOptical properties of the quaternary Cu2ZnSnS4 semiconductor have been studied by optical absorption, photoluminescence, photoreflectance, and thermoreflectance measurements. Optical absorption measurements suggest that Cu2ZnSnS4 is a direct‐gap semiconductor having the band gap of ∼1.46 eV at 11 K. The photoreflectance and thermoreflectance spectra reveal distinct structures at energies of the critical points in the Brillouin zone. By performing the band‐structure calculation, these critical points are successfully assigned to specific points in the Brillouin zone. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Temperature-dependent modulated reflectance of InAs/InGaAs/GaAs quantum dots-in-a-well infrared photodetectors

We present a photoreflectance (PR) study of multi-layer InAs quantum dot (QD) photodetector structures, incorporating InGaAs overgrown layers and positioned asymmetrically within GaAs/AlAs quantum wells (QWs). The influence of the back-surface reflections on the QD PR spectra is explained and a temperature-dependent photomodulation mechanism is discussed. The optical interband transitions originating from the QD/QW ground- and excited-states are revealed and their temperature behaviour in the range of 3–300 K is established. In particular, we estimated the activation energy (∼320 meV) of exciton thermal escape from QD to QW bound-states at high temperatures. Furthermore, from the obtained Varshni parameters, a strain-driven partial decomposition of the InGaAs cap layer is determined.

Investigation of valence-band splitting in InN by low-temperature photoreflectance spectroscopy

Temperature-dependent photoluminescence (PL) and photoreflectance (PR) spectroscopy and room-temperature Raman spectroscopy and X-ray diffraction have been utilized to investigate the optical properties, electron concentration, crystalline quality, and electronic band structures, especially valence-band splittings, of InN films grown by plasma-assisted molecular beam epitaxy (PAMBE) and metal–organic chemical vapor deposition (MOCVD). The smaller thermal activation energies imply the PAMBE-grown InN film exhibits low-density localized states from band tail states. PR signals of the InN film are detectable when the temperature is below about 100 K due to the cooling down of free electrons to trap states. For the MOCVD-grown InN film, no PR signal is observed even at 15 K due to the higher free electron concentration. To analyze the energetic positions of the features in the PR spectra without ambiguity, the moduli of individual PR resonances are considered. Based on the PR results and appropriate Hamiltonian, the values of the crystal-field splitting and the spin–orbit splitting in InN are experimentally determined as 26.8 and 14.5 meV, respectively. Theoretical and experimental reports are compared and discussed to verify this result.

Photoreflectance and photoluminescence study of localization effects in GaAsBi alloys

Photoreflectance (PR) and photoluminescence (PL) spectra of GaAs1−xBix alloys grown by metalorganic vapor phase epitaxy, for x up to 4.8%, were measured at temperatures ranging from 12 to 300K. The PR signal shifts due to the temperature change decreases with increasing Bi content of GaAsBi alloys. For temperature below 100K, a dominant peak in PL spectra of GaAsBi was observed. This peak is attributed to carrier localization resulting from Bi-related localized states in GaAsBi. A decrease in PR signal has been also found when the temperature was lowered. This behavior is attributed to a weakening of modulation efficiency, which is induced by carrier localization that has been evidenced in low temperature PL. The localized state emission partly contributes to the decrease in the band gap energy shift. In addition, at high temperatures the small PR signal shift is due to the reduction in the temperature dependence of the band gap energy. The analysis of the band gap energy evolution with temperature using the Bose–Einstein statistical expression shows that the average phonon energy is much larger than that expected from the linear interpolation between GaAs and GaBi. This fact is related to the interaction between electrons and phonons localized at Bi atoms playing an important role in the reduction of the temperature dependence of the band gap energy of GaAsBi alloys.

Experimental and theoretical studies of band gap alignment in GaAs1−xBix/GaAs quantum wells

Band gap alignment in GaAs1−xBix/GaAs quantum wells (QWs) was studied experimentally by photoreflectance (PR) and theoretically, ab initio, within the density functional theory in which the supercell based calculations are combined with the alchemical mixing approximation applied to a single atom in a supercell. In PR spectra, the optical transitions related to the excited states in the QW (i.e., the transition between the second heavy-hole and the second electron subband) were clearly observed in addition to the ground state QW transition and the GaAs barrier transition. This observation is clear experimental evidence that this is a type I QW with a deep quantum confinement in the conduction and valence bands. From the comparison of PR data with calculations of optical transitions in GaAs1−xBix/GaAs QW performed for various band gap alignments, the best agreement between experimental data and theoretical calculations has been found for the valence band offset of 52 ± 5%. A very similar valence band offset was obtained from ab initio calculations. These calculations show that the incorporation of Bi atoms into GaAs host modifies both the conduction and the valence band. For GaAs1−xBix with 0 < x < 0.074, the conduction band shifts lineary at a rate of ∼33 meV per % Bi, which only slightly decreases with Bi concentration. Whereas the valance band shift is clearly non-linear. Reducing initially at a rate of ∼51 meV per % Bi for low concentrations of Bi and then at a significantly reduced rate of ∼20 meV per % Bi near the end of the studied composition range. The overall reduction rate of the band gap is parabolic and the reduction rates change from ∼84 to ∼53 meV per % Bi for lower and higher Bi concentrations, respectively. The calculated shifts of valence and conduction bands give the variation of valence (conduction) band offset between GaAs1−xBix and GaAs in the range of ∼60%–40% (∼40%–60%), which is in good agreement with our conclusion derived from PR measurements.

The study of temperature dependent strain in Ge epilayer with SiGe/Ge buffer layer on Si substrate with different thickness

Temperature dependent near band edge transitions in Ge epilayer on Si substrate with different thickness were studied by using photoreflectance (PR) and piezoreflectance (PzR) measurements. The identification of conduction to light-hole (LH) band and conduction to heavy-hole (HH) band transitions originated from the strain induced valence band splitting results from tensile strain have been achieved by comparing the relative intensities of PR and PzR spectra. The strain induced energy splitting between LH and HH can be reduced with a thinner substrate and can be accounted by the mismatch of thermal expansion coefficients between the Si substrate and Ge epilayer. We also studied temperature dependent energy splitting between LH and HH through theoretical calculations. The comparison of energy splitting between experiments and theoretical calculations were made and discussed.

Optical properties study of InxGa1−xAs/GaAs structures using spectral reflectance, photoreflectance and near-infrared photoluminescence

Optical properties of InxGa1−xAs films grown on GaAs substrates by metalorganic vapor phase epitaxy were investigated. Spectral reflectance (SR) and photoreflectance (PR) at room temperature and near-infrared photoluminescence (PL) at 10K were performed. SR signals in the range of 200–1700nm provided the x-dependence of the critical point energies E1, E1+Δ1 and E2. Furthermore, band-gap and spin-orbit splitting energies, as well as their broadening parameters were determined from PR spectra and studied as function of In composition ranging from 0 to 0.37. On the other hand, the origins of luminescence bands observed in PL spectra were revealed. A redshift of 16meV/%In in the band-to-band transition was obtained. All results issued from different characterizations tools are correlated and compared to the literature.

Electronic and structural characterisation of Cu3BiS3 thin films for the absorber layer of sustainable photovoltaics

Thin films of p-type Cu3BiS3 with an orthorhombic wittichenite structure, a semiconductor with high potential for thin film solar cell absorber layers, were synthesised by thermal annealing of Cu and Bi precursors, magnetron sputtered on Mo/glass substrate, with a layer of thermo-evaporated S. The elemental composition, structural and electronic properties are studied. The Raman spectrum shows four modes with the dominant peak at 292cm−1. Photoreflectance spectra demonstrate two band gaps EgX and EgY, associated with the X and Y valence sub-bands, and their evolution with temperature. Fitting the temperature dependencies of the band-gaps gives values of 1.24 and 1.53eV for EgX and EgY at 0K as well as the average phonon energy. Photoluminescence spectra at 5K reveal two bright and broad emission bands at 0.84 and 0.99eV, which quench with an activation energy of 40meV. The photocurrent excitation measurements demonstrate a photoresponse and suggest a direct allowed nature of the band gap.

Temperature dependence study of near-band-edge transitions of compressively strained quaternary GaAsPSb layer by photoreflectance and piezoreflectance spectroscopy

The optical properties of a strained GaAsPSb layer grown on GaAs substrate by metal–organic vapor-phase epitaxy were characterized by photoreflectance (PR) and piezoreflectance (PzR) techniques. The strain induced splitting of the valence band was observed. Identification of conduction to heavy-hole band and conduction to light-hole band transitions were achieved by comparing the relative intensities of PR and PzR spectra. The results revealed that the GaAsPSb film is under compressive strain. The temperature dependences of near-band-edge transition energies were analyzed using Varshni and Bose–Einstein expressions in the temperature range from 77 to 400 K. The parameters that describe the temperature variations of the near-band-edge transition energies and broadening parameters were evaluated and discussed.

Evaluation of the junction’s electric field and the ideality factor of GaAs p-n junction solar cells by using photoreflectance spectroscopy

We investigated the optical and the electrical properties of GaAs solar cells (SCs) by using photoreflectance (PR) spectroscopy and current-voltage (J-V) measurements. The electric fields (F j ) in p-n junctions were evaluated through an analysis of the Franz-Keldysh oscillations (FKOs) of the PR spectra. From the excitation light intensity (I ex ) dependence of the PR, we found that the photovoltaic effect resulted in a reduction of the F j . The F j was gradually reduced by up to 37% from the dark built-in electric field when the I ex was increased (I ex = 2.3 ∼ 181 mW/cm2). We evaluated the ideality factor (n) of the SC via PR spectroscopy, and the results matched the results of the J-V measurements well. From these results, we demonstrate that the F j and the n of the SCs can be evaluated by using PR spectroscopy.

Photoreflectance study of the temperature dependence of excitonic transitions in dilute GaAsN alloys

We have studied the temperature dependence of optical transitions in dilute GaAsN alloys using photoreflectance (PR). A delocalized‐state transition is clearly observed in PR spectra while several localized‐state emission lines due to isoelectronic centers appear in photoluminescence spectra. The energy of optical transitions observed in PR spectra is in agreement with the excitonic transition energy in photoluminescence excitation spectra, clearly showing that the optical transitions in PR spectra are excitonic transitions. We analyzed the temperature dependence of the excitonic transition energies using the Bose–Einstein statistical expression, and found that even dilute alloys follow the same trend as GaAsN alloys with higher nitrogen concentrations. We discuss the coexistence of localized and delocalized states in dilute GaAsN alloys in terms of the random distribution of N atoms.

Spin photocurrent spectra induced by Rashba- and Dresselhaus-type circular photogalvanic effect at inter-band excitation in InGaAs/GaAs/AlGaAs step quantum wells

Spin photocurrent spectra induced by Rashba- and Dresselhaus-type circular photogalvanic effect (CPGE) at inter-band excitation have been experimentally investigated in InGaAs/GaAs/AlGaAs step quantum wells (QWs) at room temperature. The Rashba- and Dresselhaus-induced CPGE spectra are quite similar with each other during the spectral region corresponding to the transition of the excitonic state 1H1E (the first valence subband of heavy hole to the first conduction subband of electrons). The ratio of Rashba- and Dresselhaus-induced CPGE current for the transition 1H1E is estimated to be 8.8±0.1, much larger than that obtained in symmetric QWs (4.95). Compared to symmetric QWs, the reduced well width enhances the Dresselhaus-type spin splitting, but the Rashba-type spin splitting increases more rapidly in the step QWs. Since the degree of the segregation effect of indium atoms and the intensity of build-in field in the step QWs are comparable to those in symmetric QWs, as proved by reflectance difference and photoreflectance spectra, respectively, the larger Rashba-type spin splitting is mainly induced by the additional interface introduced by step structures.

Correlation of fundamental photoreflectance spectra with surface quality of bulk ZnTe semiconductor grown from Te solution

Large size ZnTe single crystals were grown from Te‐solution with the vertical Bridgman method. The fundamental photoreflectance (PR) spectra of bulk ZnTe crystal subjected to a successive sequence of surface treatments, i.e., mechanical and chemical polishing, and passivation, have been measured over the wavelength range 200–600 nm at room temperature (RT) for the first time. The shape, relative intensity and evolution of the distinct E0, E1, E1+∆1 and E2 transitions during each stage of wafer processing were investigated. It was found that, those distinct transitions vary with varying crystal surface quality. The intensity of the reflectance peaks is large with sharp peak profile when the surface has good crystalline quality. However, E0 peak can be easily eliminated by a small amount of structural damages on the wafer surface. The dispersion tendency of the E1 and E1+∆1 peaks suggesting the wafer surface is widespread covered by structural damages. The reflection peaks’ intensity were all reduced after passivated with H2O2 solution. E2 peak intensity is more easily to be reduced during passivation.