Phonon-assisted Indirect Transitions Research Articles

Photon-Momentum-Enabled Electronic Raman Scattering in Silicon Glass.

The nature of enhanced photoemission in disordered and amorphous solids is an intriguing question. A point in case is light emission in porous and nanostructured silicon, a phenomenon that is still not fully understood. In this work, we study structural photoemission in heterogeneous cross-linked silicon glass, a material that represents an intermediate state between the amorphous and crystalline phases, characterized by a narrow distribution of structure sizes. This model system shows a clear dependence of photoemission on size and disorder across a broad range of energies. While phonon-assisted indirect optical transitions are insufficient to describe observable emissions, our experiments suggest these can be understood through electronic Raman scattering instead. This phenomenon, which is not commonly observed in crystalline semiconductors, is driven by structural disorder. We attribute photoemission in this disordered system to the presence of an excess electron density of states within the forbidden gap (Urbach bridge) where electrons occupy trapped states. Transitions from gap states to the conduction band are facilitated through electron-photon momentum matching, which resembles Compton scattering but is observed for visible light and driven by the enhanced momentum of a photon confined within the nanostructured domains. We interpret the light emission in structured silicon glass as resulting from electronic Raman scattering. These findings emphasize the role of photon momentum in the optical response of solids that display disorder on the nanoscale.

Electron–phonon physics from first principles using the EPW code

EPW is an open-source software for ab initio calculations of electron–phonon interactions and related materials properties. The code combines density functional perturbation theory and maximally localized Wannier functions to efficiently compute electron–phonon coupling matrix elements, and to perform predictive calculations of temperature-dependent properties and phonon-assisted quantum processes in bulk solids and low-dimensional materials. Here, we report on significant developments in the code since 2016, namely: a transport module for the calculation of charge carrier mobility under electric and magnetic fields using the Boltzmann transport equation; a superconductivity module for calculations of phonon-mediated superconductors using the anisotropic multi-band Eliashberg theory; an optics module for calculations of phonon-assisted indirect transitions; a module for the calculation of small and large polarons without supercells; and a module for calculating band structure renormalization and temperature-dependent optical spectra using the special displacement method. For each capability, we outline the methodology and implementation and provide example calculations.

In Operando Quantification of Single and Multiphoton Photocurrents in GaP and InGaN Photodetectors with Phase-Modulated Femtosecond Light Pulses

Single and multiphoton absorptions in wide band gap semiconductors determine the functionality of photodetectors, lasers, and light-emitting diodes (LEDs). Although electronic structure strongly influences the different orders of multiphoton absorptions in semiconductors, it has not been feasible to quantify their contributions on the functionality of devices. A lack of proper measurement techniques has been the hurdle. Here, we present a simple and sensitive method based on the phase modulation of femtosecond pulses to quantify single-, two-, and three-photon absorptions in GaP and InGaN photodetectors. Our results show that only three-photon absorption contributes to the photocurrent in the InGaN device when excited by femtosecond pulses at 1030 nm. On the other hand, single-, two-, and three-photon absorptions have comparable contributions in the GaP detector. The three contributions have different origins: linear photocurrent is attributed to the absorption by the impurities in the doped regions, two-photon photocurrent is from the phonon-assisted indirect transition from the valence band to the conduction band minimum, and three-photon photocurrent is from the direct transition to the conduction band. We also demonstrate that the method can be applied to image the heterogeneity of multiphoton photocurrent in devices. Our work could be adapted for “in operando” characterization of optoelectronic systems.

Band tailing in tellurium excess CdTe crystals grown from vapour phase

Absorption spectra of hexagonally structured, tellurium excess p-type cadmium telluride (CdTe) grown from vapour phase having conductivity 2.529 × 104 (Ω cm)−1 was studied in the range 700–880 nm. The existence of phonon-assisted indirect transition was confirmed. The direct and indirect bandgaps and the involved phonon energy were identified. The analysis revealed a band tailing and the presence of a strong internal electric field responsible for the tunnelling of charge carriers.

Au-Al intermetallic compounds: A series of more efficient LSPR materials for hot carriers-based applications than noble metal Au

Abstract Hot carriers generated by the non-radiative decay of localized surface plasmon resonance (LSPR) of noble metal nanoparticles are widely investigated in applications ranging from solar energy conversion to photocatalysis, and photodetection. From the perspective of sustainable development of the LSPR based applications, it is important to find metallic materials that are cheap and plentiful and show excellent hot carrier properties. In this context, we investigate the possibility of alloying Au with Al to develop efficient LSPR materials for hot carriers-based applications. Taking thermodynamically stable AuAl2 and AuAl as examples, we study their electronic structures, generation rates, energetic distribution, relaxation times, and mean free paths of hot carriers originating from direct and phonon-assisted indirect electronic transitions, which are two of the main channels for generating high-energy hot carriers from the decay of LSPR, and compare the results with that of Au and Al metals. It is found that both AuAl2 and AuAl show larger hot carrier generation rates than Au and the resulting hot carriers are energetically higher than that in Au. Meanwhile, the transport properties of hot carriers in AuAl2 and AuAl outperform that of Au, which is manifested through comparable or even longer relaxation times and mean free paths. Consequently, it is believed that the Au-Al intermetallic compounds should be a series of efficient LSPR materials for hot carriers-based applications, which is comparable with or even better than the noble metal Au. These results are helpful for the sustainable development of related fields.

Isotope Effect in Bilayer WSe2.

Isotopes of an element have the same electron number but differ in neutron number and atomic mass. However, due to the thickness-dependent properties in MX2 (M = Mo, W; X = S, Se, Te) transition metal dichalcogenides (TMDs), the isotopic effect in atomically thin TMDs still remains unclear especially for phonon-assisted indirect excitonic transitions. Here, we report the first observation of the isotope effect on the electronic and vibrational properties of a TMD material, using naturally abundant NAWNASe2 and isotopically pure 186W80Se2 bilayersingle crystals over a temperature range of 4.4-300 K. We demonstrate a higher optical band gap energy in186W80Se2 than in NAWNASe2 (3.9 ± 0.7 meV from 4.41 to 300 K), which is surprising as isotopes are neutral impurities. Phonon energies decrease in the isotopically pure crystal due to the atomic mass dependence of harmonic oscillations, with correspondingly longer E2g and A21g phonon lifetimes than in the naturally abundant sample. The change in electronic band gap renormalization energy is postulated as being the dominant mechanism responsible for the change in optical emission spectra.

Electrodynamic properties of the semimetallic Dirac materialSrMnBi2: Two-carrier-model analysis

The electrodynamics of free carriers in the semimetallic Dirac material $\mathrm{SrMnB}{\mathrm{i}}_{2}$ was investigated using optical spectroscopy and first-principles calculations. Using a two-carrier-model analysis, the total free-carrier response was successfully decomposed into individual contributions from Dirac fermions and non-Dirac free carriers. Possible roles of chiral pseudospin, spin-orbit interaction (SOI), antiferromagnetism, and electron-phonon $(e\text{\ensuremath{-}}ph)$ coupling in the Dirac fermion transport were also addressed. The Dirac fermions possess a low scattering rate of $\ensuremath{\sim}10\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ at low temperature and thereby experience coherent transport. However, at high temperatures, we observed that the Dirac fermion transport becomes significantly incoherent, possibly due to strong $e\text{\ensuremath{-}}ph$ interactions. The SOI-induced gap and antiferromagnetism play minor roles in the electrodynamics of the free carriers in $\mathrm{SrMnB}{\mathrm{i}}_{2}$. We also observed a seemingly optical-gap-like feature near 120 meV, which emerges at low temperatures but becomes filled in with increasing temperature. This gap-filling phenomenon is ascribed to phonon-assisted indirect transitions promoted at high temperatures.

On the dual role of halogen in magnesium oxyhalide bismuth borate glasses: Insight through optical absorption

The present research communication deals with some significant outcomes related to the compositional dependence of optical absorption in magnesium oxyhalide bismuth borate glasses. These vitreous materials have been synthesized over very wide composition range of xMgX2·(30–x)MgO·20Bi2O3·50B2O3 (X=Cl, F, Br and x=0, 2, 5, 7, 10mol%) via convenient melt-quench route. The analysis of fundamental absorption edge (from UV–Vis spectroscopy data) has been carried out in order to obtain optical band gap (Eopt) and band tailing parameter (B) corresponding to indirect allowed and forbidden Mott’s transitions. The non-linear compositional variations of Eopt have predicted a dual role (network former and modifier) of halogen ions in the magnesium bismuth borate matrix. Moreover, evaluation of Urbach’s energy has been done and order of its values indicates the presence of phonon-assisted indirect transitions in each synthesized composition.

Phonon-assisted indirect transitions in angle-resolved photoemission spectra of graphite and graphene

Indirect transitions of electrons in graphene and graphite are investigated by means of angle-resolved photoemission spectroscopy (ARPES) with several different incident photon energies and light polarizations. The theoretical calculations of the indirect transition for graphene and graphite are compared with the experimental measurements for highly-oriented pyrolytic graphite and a single-crystal of graphite. The dispersion relations for the transverse optical (TO) and the out-of-plane longitudinal acoustic (ZA) phonon modes of graphite and the TO phonon mode of graphene can be extracted from the inelastic ARPES intensity. We find that the TO phonon mode for $k$ points along the $\Gamma$-$K$ and $K$-$M$-$K'$ directions in the Brillouin zone can be observed in the ARPES spectra of graphite and graphene by using a photon energy of about 11.1 eV. The relevant mechanism in the ARPES process for this case is the resonant indirect transition. On the other hand, the ZA phonon mode of graphite can be observed by using a photon energy of about 6.3 eV through a nonresonant indirect transition, while the ZA phonon mode of graphene within the same mechanism should not be observed.

Temperature dependence of optical absorption coefficient of 4H- and 6H-SiC from room temperature to 300 °C

Optical absorption coefficients of 4H- and 6H-SiC were measured at cutoff wavelengths from a value of 0 to 400–500 cm−1 at various temperatures that is from room temperature to 300 °C. The redshift of the absorption edge with increasing temperature was observed. It is caused by a decrease in bandgap energy and a change in phonon occupation with increasing temperature. By considering TA-, LA-, and TO-mode phonon-assisted indirect transitions, the measured data are well fitted using a theoretical model. The obtained parameters are reported.

Non-linear optical and electrical properties of germanate glasses

Glasses in the system Na2O–GeO2 were prepared via melt quenching technique. The optical absorption spectra were measured in the wavelength range from 190 to 1000nm. The fundamental absorption edge for all the glasses was analyzed in terms of the theory proposed by Davis and Mott. The position of the absorption edge and hence the value of the optical band gap were found to show anomalous behavior depending on the glass composition. The absorption in these glasses is believed to be associated with indirect transitions. The origin of the Urbach energy is associated with phonon-assisted indirect transitions. The temperature-dependence of DC conductivity for these glasses has been examined in the temperature range 463–623K. It reveals that the obtained activation energy (0.04–0.54eV) of these glasses was glass composition dependent.

Influence of SiO 2 on dispersive conductivity and absorption edge of calcium bismuthate glasses

The dynamics of Ca 2+ ions and the optical properties of calcium bismuth silicate glasses were investigated using relaxation spectroscopy and optical absorption spectra. These glasses were prepared by normal melt quench technique. Density, molar volume and glass transition temperature (T g) have been measured. A large difference in the values of T g and crystalline temperature (T C) suggests the thermal stability of these glasses. The ac and dc conductivities, activation energy for dc conduction (E dc) and for relaxation frequency (E τ) were extracted from the impedance spectra. The compositional variation in conductivity has been attributed to the presence of mixed glass former effect in these glasses. Both electric modulus and the conductivity formalism have been employed to study the relaxation dynamics of charge carriers in these glasses. A single ‘master curve’ for normalized plots of all the modulus isotherms observed for a given composition indicates that the dynamic processes for ions in these glasses are temperature independent. Similar values of E dc and E τ indicate that the ions have to overcome the energy barrier of same height in their conduction as well as relaxation. The observed conductivity spectra follows power law with exponent ‘s’ which increases regularly with frequency and approaches unity at higher frequencies. It has been observed that the position of the absorption edge and cutoff wavelength shift towards blue as Bi 2O 3/SiO 2 ratio decreases. The compositional variation in optical band gap (E g) has been attributed to the influence of the CaO and SiO 2 on conduction band. The values of Urbach energy confirm the existence of phonon-assisted indirect transitions in the present glass system. The average electronic polarizability of oxide ion (α O 2−) and the optical basicity (Λ th) have been estimated from the calculated values of the optical band gap and were found to be dependent directly on Bi 2O 3/SiO 2 ratio.

Effect of Bi 2O 3 content on physical and optical properties of 15Li 2O–15K 2O– xBi 2O 3–(65− x) B 2O 3: 5V 2O 5 glass system

Multi-component bismuth borate glasses doped with vanadium ions 15Li 2O–15K 2O– xBi 2O 3–(65− x) B 2O 3: 5V 2O 5, ( x=3, 5, 7, 10, 12 and 15) have been prepared using conventional melt quench technique. Characterization of the prepared glasses has been done using X-ray diffraction, differential scanning calorimetry and density measurements. The effect of Bi 2O 3 content on the optical properties of the present glass system is studied from the optical absorption spectra recorded in the wavelength range 200–800 nm. The fundamental absorption edge has been identified from the optical absorption spectra. The values of optical band gap for indirect allowed transitions have been determined using available theories. The origin of the Urbach energy is associated with the phonon-assisted indirect transitions. The density and molar volume studies indicate that Bi 2O 3 in these glasses is acting partly as network modifier and partly as network former. The variations in the optical band gap energies, density and molar volume with Bi 2O 3 content have been discussed in terms of changes in the glass structure. Values of the theoretical optical basicity, average crosslink density and the average electronic polarizability are also reported.

Thermal and optical properties of tellurite glasses doped erbium

Er3+-doped tellurite glasses with molar compositions of 75TeO2–20ZnO–(5 − x) Na2O–xEr2O3 (x = 0, 0.5, 1, 2, 3, and 4 mol%) have been elaborated from the melt-quenching method. The effects of Er2O3 concentration on the thermal stability and optical properties of tellurite glasses have been discussed. From the differential scanning calorimetry (DSC) profile, the glass transition temperature T g, and crystallization onset temperature T x are estimated. The thermal stability factor, defined as ∆T = T x − T g, was higher than 100 °C. It suggests that tellurite glass exhibits a good thermal stability and consequently is suitable to be a potential candidate for fiber drawing. Furthermore, the stability factor increases with Er2O3 concentration up to 2 mol% then presents a continue decrease suggesting of beginning of crystallization of highly doped tellurite glasses. The refractive index and extinction coefficient data were obtained by analyzing the experimental spectra of tanΨ and cos∆ measured by spectroscopic ellipsometry (SE). The complex dielectric functions (e = e1 + ie2) of the samples were estimated from regression analysis. The fundamental absorption edge has been identified from the optical absorption spectra and was analyzed in terms of the theory proposed by Davis and Mott. The values of optical band gap for direct and indirect allowed transitions have been determined. An important decrease of the optical band gap was found after Er doping. It was assigned to structural changes induced from the formation of non-bridging oxygen. The absorption coefficient just below the absorption edge varies exponentially with photon energy indicating the presence of Urbach’s tail. The origin of the Urbach energy is associated with the phonon-assisted indirect transitions.

Electroluminescence in Metal-Oxide-Semiconductor Tunneling Diodes with Ultra Thin Silicon

Electroluminescence characteristics of a metal-oxide-semiconductor tunneling diode on a silicon-on-insulator wafer have been studied. The spectrum from the diode is peaked at 1050 nm (1.18 eV) and at 1145 nm (1.08 eV). The spectrum from the diode on a silicon wafer is peaked at 1145 nm. The peak at 1145 nm can be assigned as phonon-assisted indirect transitions. It is indicated that the peak at 1050 nm is due to the quantum confinement in an ultra thin silicon layer.

Faraday effect and λ-modulation absorption spectra of GaP

There are presented the absorption optical spectra of GaP measured by λ-modulation method at room temperature in the spectral region from 505 nm to 700 nm. It is not possible even by λ-modulation to be registered at room temperature the wave bands due to the exciton-phonon interaction. The absorption spectra of GaP carried out by a λ-modulation can be separated exactly in the spectral parts as follows: the transmittance region where the absorption is too slightly expressed; the region determined by the phonon-assisted indirect transitions; the region of the interband absorption. The purpose of Faraday rotation measurements is to establish the influence of the exciton-phonon interaction on the magneto-optical effect. The magneto-optical effect has been investigated by a ϕ-modulation. The spectral dependence of dn/dλ in the transmittance region is determined by the ϕ-modulated spectra.

Thermal effects in photoemission from Bi(1 1 1) films on Si(1 1 1)-(7 × 7)

An angle-resolved photoemission study of epitaxied Bi(1 1 1) film on Si(1 1 1)-(7 × 7) surface has been carried out at a temperature from 10 K to 210 K. The results show that there exists strong temperature dependence in the valence band of Bi(1 1 1) film. The variations in the spectral intensity and shape indicate the importance of phonon-assisted indirect transition in the photoemission process. The thermal sensitivity coefficients and effective Debye temperatures for main spectral peaks are determined from the experimental data.

Structural, optical, physical and electrical properties of V 2O 5·SrO·B 2O 3 glasses

The present work aims to study the structure and variation of optical band gap, density and dc electrical conductivity in vanadium strontium borate glasses. The glass systems xV 2O 5·(40 − x)SrO·60B 2O 3 and xV 2O 5·(60 − x)B 2O 3·40SrO with x varying from 0 to 20 mol% were prepared by normal melt quench technique. Structural studies were made by recording IR transmission spectra. The fundamental absorption edge for all the glasses was analyzed in terms of the theory proposed by Davis and Mott. The position of absorption edge and hence the value of the optical band gap was found to depend on the semiconducting glass composition. The absorption in these glasses is believed to be associated with indirect transitions. The origin of Urbach energy is associated with the phonon-assisted indirect transitions. The change in both density and molar volume was discussed in terms of the structural modifications that take place in the glass matrix on addition of V 2O 5. dc conductivity of the glass systems is also reported. The change of conductivity and activation energy with composition indicates that the conduction process varies from ionic to polaronic one.

The role of V 2O 5 in the modification of structural, optical and electrical properties of vanadium barium borate glasses

Vanadium barium borate glasses were prepared by a normal melt quench technique. The infrared spectra of these V 2O 5·BaO·B 2O 3 glasses were recorded over a continuous spectral range (400–4000 cm −1) in an attempt to study their structure systematically. The conversion from three- to four-fold coordinated boron took place. The fundamental absorption edge for all the glasses was analyzed in terms of the theory proposed by Davis and Mott. The position of the absorption edge and hence the value of the optical band gap was found to depend on the semiconducting glass composition. The absorption in these glasses is believed to be associated with indirect transitions. The origin of the Urbach energy is associated with phonon-assisted indirect transitions. The theoretical optical basicity has been calculated and is correlated with a change in the optical band gap. The variation in density and molar volume with composition has been investigated in terms of the structural modifications that take place in the glass matrix on addition of V 2O 5. The DC electrical conductivity as function of the V 2O 5:BaO and the V 2O 5:B 2O 3 ratio has been measured. The change in conductivity and activation energy with composition indicates that the conduction process varies from ionic to polaronic.

Photoacoustic Spectroscopic Study of Optical Band Gap of Zn1-xBexSe Semiconductors

Using the photoacoustic spectroscopic (PAS) technique for the first time, the composition-dependent optical absorption coefficient and band gap of Zn1-xBexSe semiconductors (with x = 0.0–0.25) have been measured at room temperature. The band gap 0estimated from the PAS spectra varies nonlinearly with Be concentration. The exchange interaction of electrons in conduction and valence bands, effects of polytypes, microstructures, and the mixed crystallization (zinc-blend and wurtzite structures) effect are considered for the analysis of the data. The observed exponential edge (Urbach’s edge) can be considered as an internal Franz–Keldish effect arising from the charged impurity generated and “frozen-in” optical phonon-generated fields. The phonon-assisted indirect transition at the band tail regions for some samples is also observed in the present studies.