Temperature Dependence Of Band Gap Research Articles

Thermal characteristics of InGaN-based green micro-LEDs

We have investigated the thermal characteristics of InGaN-based green micro-light-emitting diodes (micro-LEDs) without the passivation layer in a wide junction temperature range of 298–453 K. The decreased temperature coefficient (dVf/dT) of the device with a smaller device size is attributed to the increased series resistances for the smaller devices, largely affected by the defects due to sidewall damage of the active layer. The ideality factor of 2.02 at 298 K suggests that the charge transport mechanism could be defect-assisted tunneling. In addition, it is observed that the ideality factor decreases with increasing temperature. The results of the C–V measurements suggest similar electron and hole concentrations in the depletion region, leading to a balanced electron–hole recombination in the active layer. It was also found that the temperature-dependent bandgaps of ternary In0.3Ga0.7N obtained from electroluminescence spectra of micro-LEDs agree with the calculated values by using the semi-empirical Varshni relationship.

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.

Fang–Howard wave function modelling of electron mobility in AlInGaN/AlN/InGaN/GaN double heterostructures* *Project supported by the Xi’an Science and Technology Program, China (Grant No. 2019217814GXRC014CG015-GXYD14.3) and the Open Project of Key Laboratory of Wide Band-gap Semiconductor Materials, Ministry of Education, China (Grant No. Kdxkf2019-01).

To study the electron transport properties in InGaN channel-based heterostructures, a revised Fang-Howard wave function is proposed by combining the effect of GaN back barrier. Various scattering mechanisms, such as dislocation impurity (DIS) scattering, polar optical phonon (POP) scattering, piezoelectric field (PE) scattering, interface roughness (IFR) scattering, deformation potential (DP) scattering, alloy disorder (ADO) scattering from InGaN channel layer, and temperature-dependent energy bandgaps are considered in the calculation model. A contrast of AlInGaN/AlN/InGaN/GaN double heterostructure (DH) to the theoretical AlInGaN/AlN/InGaN single heterostructure (SH) is made and analyzed with a full range of barrier alloy composition. The effect of channel alloy composition on InGaN channel-based DH with technologically important Al(In,Ga)N barrier is estimated and optimal indium mole fraction is 0.04 for higher mobility in DH with Al0.4In0.07Ga0.53N barrier. Finally, the temperature-dependent two-dimensional electron gas (2DEG) density and mobility in InGaN channel-based DH with Al0.83In0.13Ga0.04N and Al0.4In0.07Ga0.53N barrier are investigated. Our results are expected to conduce to the practical application of InGaN channel-based heterostructures.

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.

Temperature-tuned bandgap characteristics of Bi12TiO20 sillenite single crystals

Bi12MO20 (M: Si, Ge, Ti, etc.) compounds are known as sillenites having fascinating photorefractive characteristics. The present paper reports the structural and optical characteristics of one of the members of this family, Bi12TiO20 single crystals, grown by Czochralski method. X-ray diffraction pattern of the crystal presented sharp and intensive peaks associated with planes of cubic crystalline structure with lattice constant of a = 1.0142 nm. The optical properties were studied by means of room temperature Raman and temperature-dependent transmission experiments at various temperatures between 10 and 300 K. Raman spectrum indicated peaks around 127, 162, 191, 219, 261, 289, 321, 497 and 537 cm−1. The analyses of transmittance spectra indicated the increase of direct bandgap energy from 2.30 to 2.56 eV as temperature was decreased from room temperature to 10 K. The temperature-dependent bandgap characteristics of Bi12TiO20 were analyzed by means of Varshni and O’Donnell-Chen models. The analyses under the light of these models resulted in absolute zero bandgap energy of Eg(0) = 2.56(4) eV, rate of change of bandgap energy of γ = – 1.11 × 10−3 eV/K and average phonon energy of 〈Eph〉 = 8.6 meV.

Temperature effects on optical characteristics of CdSe thin films

CdSe is one of the significant members of II-VI type semiconducting family and it has a wide range of technological applications in which optoelectronic devices take a special position. The present paper reports the structural and optical characteristics of thermally evaporated CdSe thin films. XRD pattern exhibited preferential orientation along (111) plane while atomic composition analyses resulted in the ratio of Cd/Se as closer to 1.0. Temperature-dependent band gap characteristics of CdSe thin films were investigated for the first time by carrying out transmission experiments in the 10–300 K range. The analyses showed that direct band gap energy of the compound decreases from 1.750 (at 10 K) to 1.705 eV (at 300 K). Varshni model was successfully applied to the temperature-band gap energy dependency and various optical constants were determined. Raman spectrum of CdSe thin films was also presented to understand the vibrational characteristics of the compound. The present paper would provide worthwhile data to researchers especially studying on optoelectronic device applications of CdSe thin films.

Temperature-dependent optical constants of monolayer {text {MoS}}_2, {text {MoSe}}_2, {text {WS}}_2, and {text {WSe}}_2: spectroscopic ellipsometry and first-principles calculations

The temperature-dependent (T = 4.5 , hbox {-} , 500 , hbox {K}) optical constants of monolayer {text {MoS}}_2, {text {MoSe}}_2, {text {WS}}_2, and {text {WSe}}_2 were investigated through spectroscopic ellipsometry over the spectral range of 0.73–6.42 eV. At room temperature, the spectra of refractive index exhibited several anomalous dispersion features below 800 nm and approached a constant value of 3.5–4.0 in the near-infrared frequency range. With a decrease in temperature, the refractive indices decreased monotonically in the near-infrared region due to the temperature-dependent optical band gap. The thermo-optic coefficients at room temperature had values from 6.1 times 10^{-5} to 2.6 times 10^{-4} , hbox {K}^{-1} for monolayer transition metal dichalcogenides at a wavelength of 1200 nm below the optical band gap. The optical band gap increased with a decrease in temperature due to the suppression of electron–phonon interactions. On the basis of first-principles calculations, the observed optical excitations at 4.5 K were appropriately assigned. These results provide basic information for the technological development of monolayer transition metal dichalcogenides-based photonic devices at various temperatures.

Efficient thick film non-fullerene organic solar cells enabled by using a strong temperature-dependent aggregative wide bandgap polymer

In this work, a temperature-dependent aggregative polymer (PBDT-DTBTO) has been successfully explored to realize efficient thick film organic solar cells (OSCs). PBDT-DTBTO possesses a deep HOMO energy level of −5.35 eV and a wide bandgap of 1.76 eV, which is complementary to the near-infrared absorption of the paired non-fullerene acceptor, INPIC-4F. We reveal the efficacy of the temperature-dependent aggregative PBDT-DTBTO/INPIC-4F bulk-heterojunction (BHJ) blend for thick film OSCs through tuning the processing solvent and thermal annealing temperature. The results show that the device using chlorobenzene as the processing solvent only yields a mediocre power conversion efficiency (PCE) of 8.69%. Whereas, a superior PCE of 13.1% can be achieved by using chloroform (CF) as the processing solvent and proper thermal annealing. The morphological analysis shows that the bicontinuous network of the PBDT-DTBTO/INPIC-4F BHJ blend possesses a preferential face-on orientation when using CF as the processing solvent and thermal annealing, resulting in suppressed charge recombination and superior charge transport in the derived device. Moreover, the PBDT-DTBTO/INPIC-4F-based OSC demonstrates a high thickness toleration of the BHJ layer, which can still deliver a high PCE of 12.6% while the thickness of the BHJ layer is increased to ~320 nm. More encouragingly, a promising PCE of over 12% can also be achieved for the device fabricated by blade-coating, revealing its great potential for the high-throughput solution-coating process.

Temperature-dependent band gaps in several semiconductors: from the role of electron–phonon renormalization

Temperature dependence of band gap is one of the most fundamental properties for semiconductors, and has strong influences on many applications. The renormalization of the band gap at finite temperatures is due to the lattice expansion and the phonon-induced atomic vibrations. In this work, we apply the recently-developed electron–phonon renormalization (EPR) method to study the temperature-dependent band gap in some classical covalent (diamond, Si, and SiC) and ionic semiconductors (MgO and NaCl). The contributions from both the lattice expansion and the phonon-induced atomic vibrations at finite temperatures are considered. The results show that the band gaps Eg all decrease as temperature T increases, consistent with the experiments and other theoretical studies (e.g., from 0 K to 1500 K, the reductions are ∼0.451 eV for diamond and ∼1.148 eV for MgO, respectively). The covalent compounds investigated show weaker temperature dependences of Egs than the ionic compounds, due to the much weaker lattice expansions and therefore low contributions from these. The zero-point motion effect has greater influence on the band gap in semiconductors with light atoms, such as diamond (reduction ∼0.437 eV), due to larger atomic displacements. By decomposing the EPR effect into respective phonon modes, it is found that the high-frequency optical phonon vibrations dominate the temperature-dependent band gap in both covalent and ionic compounds. Our work provides the fundamental understandings on the temperature-dependent band gaps caused by lattice dynamics.

Role of Individual Bands in the Unusual Temperature-Dependent Band Gap of Methylammonium Lead Iodide

The optical band gap of methylammonium lead iodide (CH3NH3PbI3) is known to increase with temperature; such a dependence is referred to as an unusual behavior when compared to conventional semiconductors. In addition, the band gap exhibits a sharp decrease in a narrow temperature range during the orthorhombic-to-tetragonal phase-transition of the material. In this work, we investigate the conduction and valence band energies to deliberate on the role of individual bands in yielding the unusual temperature dependence of the transport gap and also its sharp decrease at the transition temperature. Scanning tunneling spectroscopy (STS) of CH3NH3PbI3 films performed at different temperatures allowed us to derive the band energies across the two structural phases and at the transition temperature as well. The temperature dependence of band energies has been substantiated by the changes in the lattice parameters (from synchrotron radiation X-ray diffraction studies at different temperatures) vis-à-vis the orbital overlaps responsible in forming the conduction and valence bands. The STS studies have provided useful information in gaining deeper insights into the unusual temperature dependence of hybrid halide perovskite solar cells.

Two-dimensional d-π conjugated metal-organic framework based on hexahydroxytrinaphthylene

The development of new two-dimensional (2D) d-π conjugated metal-organic frameworks (MOFs) holds great promise for the construction of a new generation of porous and semiconductive materials. This paper describes the synthesis, structural characterization, and electronic properties of a new d-π conjugated 2D MOF based on the use of a new ligand 2,3,8,9,14,15-hexahydroxytrinaphthylene. The reticular self-assembly of this large π-conjugated organic building block with Cu(II) ions in a mixed solvent system of 1,3-dimethyl-2-imidazolidinone (DMI) and H2O with the addition of ammonia water or ethylenediamine leads to a highly crystalline MOF Cu3(HHTN)2, which possesses pore aperture of 2.5 nm. Cu3(HHTN)2 MOF shows moderate electrical conductivity of 9.01 × 10−8 S·cm−1 at 385 K and temperature-dependent band gap ranging from 0.75 to 1.65 eV. After chemical oxidation by I2, the conductivity of Cu3(HHTN)2 can be increased by 360 times. This access to HHTN based MOF adds an important member to previously reported MOF systems with hexagonal lattice, paving the way towards systematic studies of structure-property relationships of semiconductive MOFs.

Temperature dependence of the topological phase transition of BiTeI from first principles

A topological phase transition from a trivial insulator to a $\mathbb{Z}_2$ topological insulator requires the bulk band gap to vanish. In the case of noncentrosymmetric materials, these phases are separated by a gapless Weyl semimetal phase. However, at finite temperature, the gap is affected by atomic motion, through electron-phonon interaction, and by thermal expansion of the lattice. As a consequence, the phase space of topologically nontrivial phases is affected by temperature. In this paper, the pressure and temperature dependence of the indirect band gap of BiTeI is investigated from first principles. We evaluate the contribution from both electron-phonon interaction and thermal expansion, and show that their combined effect drives the topological phase transition towards higher pressures with increasing temperature. Notably, we find that the sensitivity of both band extrema to pressure and topology for electron-phonon interaction differs significantly according to their leading orbital character. Our results indicate that the Weyl semimetal phase width is increased by temperature, having almost doubled by 100 K when compared to the static lattice results. Our findings thus provide a guideline for experimental detection of the nontrivial phases of BiTeI and illustrate how the phase space of the Weyl semimetal phase in noncentrosymmetric materials can be significantly affected by temperature.

Electron-phonon interactions using the projector augmented-wave method and Wannier functions

We present an ab-initio density-functional-theory approach for calculating electron-phonon interactions within the projector augmented-wave method. The required electron-phonon matrix elements are defined as the second derivative of the one-electron energies in the PAW method. As the PAW method leads to a generalized eigenvalue problem, the resulting electron-phonon matrix elements lack some symmetries that are usually present for simple eigenvalue problems and all-electron formulations. We discuss the relation between our definition of the electron-phonon matrix element and other formulations. To allow for efficient evaluation of physical properties, we introduce a Wannier-interpolation scheme, again adapted to generalized eigenvalue problems. To explore the method's numerical characteristics, the temperature-dependent band-gap renormalization of diamond is calculated and compared with previous publications. Furthermore, we apply the method to selected binary compounds and show that the obtained zero-point renormalizations agree well with other values found in literature and experiments.

Temperature dependent linewidth rebroadening in quantum dot semiconductor lasers

We experimentally and analytically investigate the influence of temperature on the linewidth of an InP quantum dot (QD) laser. The full width half maximum of the peak in the optical spectrum strongly depends on the pump current and rebroadens at high injection levels. We show that with increasing temperature these effects are amplified. Applying a QD laser model including the excited and ground state with detailed balance scattering rates, we are capable of reproducing the experimentally observed data qualitatively and thus show that a relatively simple QD-laser model is capable of capturing this complex behavior. Additionally, we include a temperature dependent energy band gap reduction needed to fit the data and show that this effect enhances the rebroadening effect for higher temperatures.

Band gap renormalization, carrier mobilities, and the electron-phonon self-energy in crystalline naphthalene

Organic molecular crystals are expected to feature appreciable electron-phonon interactions that influence their electronic properties at zero and finite temperature. In this work, we report first-principles calculations and an analysis of the electron-phonon self-energy in naphthalene crystals. We compute the zero-point renormalization and temperature dependence of the fundamental band gap, and the resulting scattering lifetimes of electronic states near the valence- and conduction-band edges employing density functional theory. Further, our calculated phonon renormalization of the $GW$-corrected quasiparticle band structure predicts a fundamental band gap of 5 eV for naphthalene at room temperature, in good agreement with experiments. From our calculated phonon-induced electron lifetimes, we obtain the temperature-dependent mobilities of electrons and holes in good agreement with experimental measurements at room temperatures. Finally, we show that an approximate energy self-consistent computational scheme for the electron-phonon self-energy leads to the prediction of strong satellite bands in the electronic band structure. We find that a single calculation of the self-energy can reproduce the self-consistent results of the band gap renormalization and electrical mobilities for naphthalene, provided that the on-the-mass-shell approximation is used, i.e., if the self-energy is evaluated at the bare eigenvalues.

Photovoltaic Properties of MnGaInS4 Single Crystals

The spectral distribution of photoconductivity and the temperature dependence of photocurrent in MnGaInS4 single crystals have been investigated. Photoconductivity spectra demonstrate both intrinsic and impurity conductivity. In the case of manganese deficiency in crystals, an acceptor-type defect is seen in the wavelength range of 0.64–0.76 μm. The temperature dependence of the MnGaInS4 single crystal bandgap is due to exciton–phonon interaction. In the temperature interval studied, photocurrent rises because of thermal depletion of trapping levels. The activation energy of trapping levels has been calculated.

Anomalous boron isotope effects on electronic structure and lattice dynamics of CuB2O4.

Copper metaborate had a unique crystal structure and exhibited noteworthy magnetic phase transitions at 21 and 10 K. The electronic structure and lattice dynamics of copper metaborate Cu11B2O4 single crystals were investigated and compared with the optical properties of CuB2O4, to assess the boron isotope effect. The optical absorption spectrum at room temperature revealed two charge-transfer bands at approximately 4.30 and 5.21 eV with an extrapolated direct optical band gap of 3.16 ± 0.07 eV. Compared with the data on CuB2O4, the electronic transitions were shifted to lower energies upon the replacement of a heavier boron isotope. The band gap was also determined to be lower in Cu11B2O4. Anomalies in the temperature dependence of the optical band gap were observed below 21 K. Furthermore, 38 Raman-active phonon modes were identified in the room-temperature Raman scattering spectrum of Cu11B2O4, which were also observed in CuB2O4 with a shift to lower frequencies. No broadening caused by isotopic changes was observed. As the temperature decreased, phonon frequencies shifted to higher wavenumbers and the linewidth decreased. Anomalous softening in the Raman peaks below 21 K was also revealed.

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.

Temperature-dependent band gap characteristics of Bi12SiO20 single crystals

Bi12SiO20 single crystals have attracted interest due to their remarkable photorefractive characteristics. Since bandgap and refractive index are related theoretically to each other, it takes much attention to investigate temperature dependency of bandgap energy to understand the behavior of photorefractive crystals. The present study aims at investigating structural and optical characteristics of photorefractive Bi12SiO20 single crystals grown by the Czochralski method. The structural characterization methods indicated that atomic composition ratios of constituent elements were well-matched with the chemical compound Bi12SiO20, and grown crystals have a cubic crystalline structure. Optical properties of crystals were investigated by room temperature Raman spectroscopy and temperature-dependent transmission measurements between 10 and 300 K. The analyses of transmittance spectra by absorption coefficient and derivative spectrophotometry techniques resulted in energy bandgaps decreasing from 2.61 to 2.48 eV and 2.64 to 2.53 eV as temperature was increased from 10 to 300 K. The Varshni model was applied to analyze temperature-bandgap energy dependency.

Temperature-tuned band gap characteristics of InSe layered semiconductor single crystals

Layered structured InSe has attracted remarkable attention due to its effective characteristics utilized especially in optoelectronic device technology. This point directs researchers to investigate optical properties of InSe in great detail. The temperature dependent band gap characteristics of InSe and analyses performed on this dependency have been rarely studied in literature. Here, temperature-dependent transmission and room temperature reflection experiments were performed on InSe layered single crystals. The band gap energy was found around 1.22 eV at room temperature and 1.32 eV at 10 K. The temperature-gap energy dependency was analyzed using Varshni and O'Donnell-Chen models to reveal various optical parameters of the crystal. The structural characteristics; crystalline parameters like lattice constants, lattice strain, dislocation density and atomic compositions of InSe were also determined from the analyses of XRD and EDS measurements.