Temperature Dependence Of Band Gap Energy Research Articles

Temperature-Dependent Absorption of Ternary HfS2−xSex 2D Layered Semiconductors

In this study, we present the investigation of optical properties on a series of HfS2−xSex crystals with different Se compositions x changing from 0 to 2. We used the chemical-vapor transport method to grow these layered ternary compound semiconductors in bulk form. Their lattice constants and crystal properties were characterized by X-ray diffraction, high-resolution transmission electron microscopy, and Raman spectroscopy. We have performed absorption spectroscopies to determine their optical band-gap energies, which started from 2.012 eV with x = 0, and gradually shifts to 1.219 eV for x = 2. Furthermore, we measured the absorption spectroscopies at different temperatures in the range of 20–300 K to identify the temperature dependence of band-gap energies. The band-gap energies of HfS2−xSex were determined from the linear extrapolation method. We have noticed that the band-gap energy may be continuously tuned to the required energy by manipulating the ratio of S and Se. The parameters that describe the temperature influence on the band-gap energy are evaluated and discussed.

Performance comparison of near- and far-field temperature-dependent thermophotovoltaic systems with tungsten emitter and GaxIn1-xAs cell

The near-field effect greatly increases the radiative heat flux. The enhanced radiative heat flux results in increasing heat production in photovoltaic cells. A temperature-dependent model of the near-field thermophotovoltaic cell (TPVC) with a tungsten emitter and a GaxIn1-xAs cell is established and optimized to compare the performances of far- and near-field TPVCs, in which the temperature-dependent band-gap energy and dielectric function as well as finite-time heat transfer between the photovoltaic cell and the environment are all taken into account. The effects of voltage, emitter temperature, heat transfer parameter, and mole fraction of Ga on the performance of TPVCs are analyzed. It is found that the maximum power density of the near-field TPVC is above three times of that of the far-field TPVC, while the cell temperature at the maximum power density in the near-field TPVC has risen by 14.7 K in comparison with that in the far-field TPVC even if the heat transfer parameter is as high as 9.5 W/(cm2 K). The performances of the near-field TPVC are more affected by the heat transfer parameter than those of the far-field TPVC. When the heat transfer parameter is reduced to 1.5 W/(cm2 K), cell temperature in the near-field TPVC is increased by 126 K and the maximum power density is decreased by 5.43 W/cm2, while in the far-field TPVC, the cell temperature is increased by 40 K and the maximum power density is decreased by 0.68 W/cm2. Moreover, it is pointed out that increasing the emitter temperature, decreasing the cell temperature, and selecting the suitable mole fraction of component Ga in GaxIn1-xAs are some effective approaches to boost the power density and efficiency of far- and near-field TPVCs.

Optical and structural characteristics of electrodeposited Cd 1-xZnxS nanostructured thin films

Abstract The structural and optical characteristics of Cd1-xZnxS (CdZnS) thin films grown by the electrodeposition method were investigated in the present paper. The crystalline structure of the grown CdZnS thin film was determined as cubic wurtzite due to observed diffraction peaks associated with (111) and (220) planes. Atomic compositional ratios of the constituent elements were obtained using energy dispersive spectroscopy and doping concentration of the Zn was found as 5% (x ~ 0.05). Scanning electron microscopy image of the studied thin film indicated that grown film is nanostructured. Raman spectra of CdS and CdZnS thin films were measured and it was seen that observed longitudinal optical modes for CdZnS present a blue-shift. Temperature-dependent band gap energy characteristics of the thin films were studied performing transmission experiments in the 10–300 K temperature range. The analyses of the recorded transmittance spectra showed that direct band gap energy of the films decreases from 2.56 eV (10 K) to 2.51 eV (300 K) with the increase of temperature. The band gap energy vs. temperature dependency was studied applying well-known Varshni optical model and various optical parameters of the films were reported according to the results of the applied model.

Thermoelectrically-Cooled InAs/GaSb Type-II Superlattice Detectors as an Alternative to HgCdTe in a Real-Time Mid-Infrared Backscattering Spectroscopy System.

We report on the development of thermoelectrically cooled (TE-cooled) InAs/GaSb type-II superlattice (T2SL) single element infrared (IR) photodetectors and exemplify their applicability for real-time IR spectroscopy in the mid-infrared in a possible application. As the European Union’s Restriction of Hazardous Substances (RoHS) threatens the usage of the state-of-the-art detector material mercury cadmium telluride (MCT), RoHS-compatible alternatives to MCT have to be established for IR detection. We use bandgap engineered InAs/GaSb T2SLs to tailor the temperature-dependent bandgap energy for detection throughout the required spectral range. Molecular beam epitaxy of superlattice samples is performed on GaAs substrates with a metamorphic GaAsSb buffer layer. Photolithographic processing yields laterally-operated T2SL photodetectors. Integrated in a TE-cooled IR detector module, such T2SL photodetectors can be an alternative to MCT photodetectors for spectroscopy applications. Here, we exemplify this by exchanging a commercially available MCT-based IR detector module with our T2SL-based IR detector module in a real-time mid-infrared backscattering spectroscopy system for substance identification. The key detector requirements imposed by the spectroscopy system are a MHz-bandwidth, a broad spectral response, and a high signal-to-noise ratio, all of which are covered by the reported T2SL-based IR detector module. Hence, in this paper, we demonstrate the versatility of TE-cooled InAs/GaSb T2SL photodetectors and their applicability in an IR spectroscopy system.

Bond-photon-phonon thermal relaxation in the M(X, X2) (M = Mo, Re, Ta, Ge, Sn; X = S, Se, and Te).

We systematically investigated the temperature-dependent bandgap energy and Raman shift on the bond length and bond energy, Debye temperature, and atomic cohesive energy for M(X, X2) via bond relaxation methods. It is revealed that the thermal decay of both bandgap energy and phonon frequency arose from the thermal integration of the specific heat of Debye approximation. The results indicate that (i) the bandgap energy relaxation is due to the thermal excitation-induced weakening of the bond energy, and the phonon frequency was just a function of bond length and bond energy; (ii) the Debye temperature determines the nonlinear range at low temperatures; (iii) the reciprocal of the atomic cohesive energy governs the linear behavior at high temperatures. Thus, the outcomes of this study include fundamental information about photon, phonon, and the thermal properties of layered semiconductors, which are crucial to develop the new generations of thermal and electronic applications of devices based on layered semiconductors.

Electronic and optical properties of layered chalcogenide FeIn2Se4

We investigated the spectral-dependent dielectric function and temperature-dependent bandgap energy of layered chalcogenide FeIn2Se4 crystals. The critical-point energy and Lorentzian broadening were analyzed by fitting second-derivative spectra using the standard critical point model. The temperature effect of the bandgap energy was analyzed based on an analytical model considering both thermal lattice expansion and electron–phonon interactions. We also extensively analyzed the temperature dependence of absorption tails and identified their possible origins. The dielectric functions and absorption coefficient in the photon-energy range of 0.75–4.75 eV were obtained. The results also showed that optical phonon modes associated with the electron–phonon interactions could be closely related to the average phonon energy.

Temperature-dependent studies of exciton binding energy and phase-transition suppression in (Cs,FA,MA)Pb(I,Br)3 perovskites

Multiple-cation mixed-halide (Cs,FA,MA)Pb(I,Br)3 perovskites containing cesium, formamidinium (FA), and methylammonium (MA) possess excellent properties for a wide range of optoelectronic applications such as thin-film photovoltaics or lasers. We investigate the role of excitons and the exciton binding energy EB, relevant for the effectiveness of charge separation in solar cells, as well as the temperature-dependent bandgap energy Eg which is used as an indicator for crystal phase transitions. Generalized Elliott fits of absorption spectra offer the possibility to determine both EB and Eg. However, since excitonic effects are non-negligible even at room temperature, a careful and detailed analysis of the spectra is crucial for a correct interpretation. Therefore, an additional evaluation based on a so-called f-sum rule is applied to achieve an improved reliability of the results at higher temperatures. The obtained EB values of 20–24 meV for Cs-containing mixed perovskite compounds are below the ones of 24–32 meV and 36–41 meV for pure methylammonium lead iodide (MAPbI3) and bromide (MAPbBr3), respectively, and, thus, facilitate charge-carrier separation in photovoltaic applications. Furthermore, temperature-dependent (T = 5–300 K) studies of Eg in (Cs,FA,MA)Pb(I,Br)3 indicate a suppressed crystal phase transition by the absence of any phase-transition related signatures such as the well-known jump of about 100 meV in MAPbI3. We verify these results using temperature-dependent electroreflectance spectroscopy, which is a very reliable technique for the direct and non-destructive determination of optical resonances of the absorber layer in complete solar cells. Additionally, we confirm the suppression of the phase transition in Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 by temperature-dependent X-ray diffraction.

A COMPARATIVE ANALYSIS OF EFFECT OF TEMPERATURE ON BAND-GAP ENERGY OF GALLIUM NITRIDE AND ITS STABILITY BEYOND ROOM TEMPERATURE USING BOSE–EINSTEIN MODEL AND VARSHNI’S MODEL


 High temperature stability of band-gap energy of active layer material of a semiconductor device is one of the major challenges in the field of semiconductor optoelectronic device design. It is essential to ensure the stability in different band-gap energy dependent characteristics of the semiconductor material used to fabricate these devices either directly or indirectly. Different models have been widely used to analyze the band-gap energy dependent characteristics at different temperatures. The most commonly used methods to analyze the temperature dependence of band-gap energy of semiconductor materials are: Passler model, Bose–Einstein model and Varshni’s model. This paper is going to report the limitation of the Bose–Einstein model through a comparative analysis between Bose–Einstein model and Varshni’s model. The numerical analysis is carried out considering GaN as it is one of the most widely used semiconductor materials all over the world. From the numerical results it is ascertained that below the temperature of 95o K both the models show almost same characteristics. However beyond 95o K Varshni’s model shows weaker temperature dependence than that of Bose–Einstein model. Varshni’s model shows that the band-gap energy of GaN at 300o K is found to be 3.43eV, which establishes a good agreement with the theoretically calculated band-gap energy of GaN for operating at room temperature.

Temperature-dependent electroluminescence from GeSn heterojunction light-emitting diode on Si substrate

The electroluminescence from a Ge/GeSn/Ge p–i–n light-emitting diode on Si was investigated under different temperatures ranging from 25 to 150 K. The diode was operated at a low injection current density of 13 A/cm2. We obtained no-phonon- and phonon-assisted replicas in emission spectra. Also, the relationship between indirect bandgap energy and temperature was investigated. The temperature-dependent bandgap energy followed Varshni’s empirical expression with α = 4.884 × 10−4 eV/K and β = 130 K.

Influence of interparticle electronic coupling on the temperature and size dependent optical properties of lead sulfide quantum dot thin films

We report on the quantum dot (QD) size, temperature, and inter-dot coupling dependence on the optical absorption and emission for PbS QD thin films. Inter-dot coupling is induced by ligand exchange from oleic acid to 1,2-ethanedithiol, and the expected band gap red-shift observed for coupled QD thin films is accompanied by a modification to the temperature-dependence of the band gap energy. The amplitude and temperature dependence of the photoluminescence (PL) Stokes shift support recombination via a mid-gap state and also indicate that the application of band gap-specific models to fit the temperature dependence PL peak energy is inadequate. Electronically coupled QD thin films show PL quenching with decreasing temperature, following a Boltzmann model which is consistent with thermally activated carrier transport. Enhancing the inter-dot coupling results in the dynamic PL decay signal changing from single- to bi-exponential behavior, reveals a size-dependent transport activation energy, and yields a negative temperature dependent band gap energy for the smallest QD diameters.

Photoresponse-behavior analysis of ZnAl2Se4 layers by using photocurrent spectroscopy

The photocurrent (PC) variation of photoconductive ZnAl2Se4 layers was investigated as a function of temperature. Three PC peaks corresponding to band-to-band transitions were observed. From the relation between the temperature and the PC-peak position, the temperature-dependent band-gap energy E g was well described by E g (T) = E g (0) − (1.94 × 10−3)T 2/(468 + T). Also, E g (0) was estimated to be 3.5268, 3.6148, and 3.7178 eV at the valence band states of Γ4(z), Γ5(x), and Γ5(y), respectively. A characteristic splitting of the valence band governed by the selection rule was first observed through the method of PC spectroscopy. We noted that the crystal-field splitting and spin-orbit splitting were 0.088 eV and 0.103 eV, respectively. With decreasing temperature, however, the PC intensities decreased, contrary to our expectation. From the log J ph vs. 1/T plot, a dominant level of 37.8 meV was observed in the high-temperature region. Thereby, we suggest that this trapping center due to native defects and impurities limits the PC response with decreasing temperature.

Fundamental temperature-dependent properties of the Si nanocrystal band gap

Ever since the first reports about low-temperature photoluminescence (PL) measurements of Si nanocrystals Si (NCs), a severe deviation from the equations commonly used to describe the temperature dependence of the band-gap energy for bulk materials was reported. Our analysis reveals that the formerly observed deviation is solely attributed to too high excitation power densities that cause a preferential emission enhancement of the smaller NCs within a size distribution, due to the temperature dependence of the radiative exciton lifetime. We report on the successful fit of the temperature-dependent band-gap energy of quantum-confined silicon. By means of four size-controlled Si NC samples (1.5 to 4.5 nm), we are able to prove the validity of these equations down to liquid He temperatures, if sufficiently low excitation power densities (500 $\ensuremath{\mu}$W/cm${}^{2}$) are used. Thereby, it is shown that the characteristics of the band-gap widening of quantum-confined nanocrystalline Si obeys to the same physical law as the bulk Si crystal. In addition, the previously observed decrease of the PL intensity for decreasing temperatures is demonstrated to have its origin in the same measurement artifact caused by excitation saturation.

A study of energy band gap versus temperature for Cu 2ZnSnS 4 thin films

The temperature dependent band gap energy of Cu 2ZnSnS 4 thin film was studied in the temperature range of 77–410 K. Various relevant parameters, which explain the temperature variation of the fundamental band gap, have been calculated using empirical and semi-empirical models. Amongst the models evaluated, the Varshni and Pässler models show the best agreement with experimental data in the middle temperature range. However, the Bose–Einstein model fits reasonably well over the entire temperature range evaluated. The calculated fitting parameters are in good agreement with the estimated value of the Debye temperature calculated using the Madelung–Einstein approximation and the Hailing method.

Temperature-dependent photoresponse characteristics of CuAlSe2 layers by photocurrent spectroscopy

Temperature-dependent photoresponse variation on the CuAlSe2 layers was investigated by photocurrent (PC) spectroscopy as a function of temperature. Three peaks of A, B, and C caused by the band-to-band transition were observed. From these PC peaks, the temperature-dependent band-gap energy was well described by Eg(T) = 2.8382 − (8.68 × 10−4)T2/(155 + T). Thus, the parameter of the split valence band on CuAlSe2 was directly extracted out by means of the PC spectroscopy. The crystal field splitting and the spin-orbit splitting were estimated to be 0.1641 and 0.1778 eV at 10 K, respectively. However, the PC intensities decreased with decreasing temperature. It is against our expectation that the PC intensities increase with decreasing temperature. From the relation of log Jph versus 1/T, where Jph is the PC density, two dominant levels were observed, one at high temperatures and the other at low temperatures. We found out that the activation energy of 48.8 meV at high temperatures is related to the binding energy of the free exciton, whereas the PC-intensity variation at a low temperature below 100 K remains constant. This suggests that trapping centers due to native defects and impurities in CuAlSe2 limit the photoresponse with decreasing temperature.

Photoluminescence Study of AgInS2 by Using Confocal Microscopy System

The photoluminescence (PL) spectra of AgInS2 crystals grown by a hot-press method have been investigated by confocal microspectroscopy. The two-dimensional image of PL intensity, which shows an area of the chalcopyrite and orthorhombic structures, with a high resolution of micrometer order was obtained. The free-exciton and biexciton emissions on the chalcopyrite structure were clearly separated. Furthermore, the temperature dependence of band-gap energy was analyzed.

Photoreflectance study of AgGaTe 2 single crystals

The optical properties of ternary chalcopyrite AgGaTe 2 were studied by photoreflectance spectorscopy (PR). Due to the optimal direct energy gap and high absorption coefficient AgGaTe 2 is a promising material for solar energy conversion. Single crystals used in this work were grown by the vertical Bridgman technique. The PR temperature dependent spectra were measured in the range of 25–300 K. At room temperature two energy gaps in AgGaTe 2 were detected: E g A = 1 . 32 0 eV and E g B = 1 . 425 eV , with temperature coefficients d E g A / dT=−2.1×10 −4 eV/K and d E g B / dT=−3.4×10 −4 eV/K. At low temperature ( T=25 K) these bandgap energies were E g A = 1 . 355 eV and E g B = 1 . 466 eV . Temperature dependence of bandgap energies is maximum at about T=90 K.

The temperature-dependent optical properties of ZnO film deposited on sapphire fiber-ending

In this paper, ZnO thin film was first grown on the sapphire fiber-ending by electron-beam evaporation, which has good morphology and high crystallization quality. The transmittance spectra showed that the sharp optical absorption edge has a red shift with increasing temperature, and the obtained temperature-dependent band gap energy for ZnO follows to the linear relation Eg（T）=340-491×１０-4T. This formula presents the key to fabricate new broad\|measurement range fiber-optic temperature sensors based on semiconductor ZnO by utilizing the optical absorption spectra at different temperatures.

Temperature dependence of absorption band edge of CdTe nanocrystals in glass

CdTe nanocrystals were grown in borosilicate glass by two-step heat treatment process. Their average radii between 2.98±0.07 and 6.15±0.32 nm are determined by using effective mass approximation in the strong confinement limit. The changes in the band gap of CdTe nanocrystals with temperature in the range from 300 to 775 K are found to be between −5.43×10−4 and −3.78×10−4 eV K−1 by using Varshni's empirical temperature-dependent band gap energy change relation. The difference between temperature dependence of band gap energy for CdTe nanocrystals and that of bulk CdTe crystal is proposed to be due to electron–phonon coupling which is stronger in nanocrystals. We also report a method to estimate glass transition temperature on the basis of analysis of temperature dependence of absorption band edge of CdTe nanocrystals in glass.

Experimental investigation of the CMN matrix element in the band anticrossing model for GaAsN and GaInAsN layers

The temperature dependence of bandgap energy of GaAs 0.98N 0.02 and Ga 0.95In 0.05As 0.98N 0.02 compounds has been analysed in order to determine the C MN matrix element of the band anticrossing model. We have found that the element equals 2.48 and 2.60 eV for GaAs 0.98N 0.02 and Ga 0.95In 0.05As 0.98N 0.02 layers, respectively. When the same value has been assumed for annealed layers, an increase in the energy of the resonant nitrogen level E N has been obtained. Two possible mechanisms leading to the increase in this energy are discussed in this work.

Role of nitrogen in the reduced temperature dependence of band-gap energy in GaNAs

We have observed a significant reduction in the temperature dependence of the absorption-edge energy in GaNxAs1−x alloys with x<0.04. The effect has been analyzed in terms of the recently introduced band anticrossing model that considers a coupling of the temperature-independent localized states of substitutional nitrogen atoms and the temperature-dependent extended states of GaAs. The model explains very well the alloy composition and the temperature dependence of the absorption-edge energy. We also compare the parameters that determine the temperature dependence of the band-gap energies in GaNAs and GaInNAs alloys.