Comparison of perturbative and nonperturbative calculations for Urbach's law

1968 ◽  
Vol 46 (18) ◽  
pp. 2087-2089 ◽  
Author(s):  
Dennis Dunn ◽  
I. P. Batra
Keyword(s):  
Electron Energy ◽  
Weak Interaction ◽  
Electronic Absorption ◽  
Optical Phonon ◽  
Energy Gap ◽  
Phonon Energy ◽  
Optical Phonons ◽  
Absorption Coefficients ◽  
Wide Range ◽  
Nonperturbative Method

In many semiconductors and insulators electronic absorption occurs below the direct electron-energy gap, owing (we assume) to the interaction of electrons with optical phonons. In this paper we compare absorption coefficients, over a wide range of interaction strengths, calculated by two different prescriptions. The first method is a perturbative one, which takes into account only one-phonon processes, while the second is a nonperturbative method, which takes into account many-phonon processes.We find that for the (weak) interaction involved in InSb the two methods give essentially the same results for photon energies [Formula: see text] in the interval [Eg – kθ, Eg], where Eg is the energy gap and kθ is the optical phonon energy. For stronger interactions the perturbation results are nowhere applicable.

Electron-Phonon Interaction and Joule Heating in Nanostructures

2008 ◽  
Author(s):  
Eric Pop
Keyword(s):  
Carbon Nanotubes ◽  
Electric Fields ◽  
Joule Heating ◽  
Optical Phonon ◽  
Phonon Transport ◽  
Phonon Energy ◽  
Strained Silicon ◽  
Vibrational Modes ◽  
Optical Phonons ◽  
Ambient Gas

The electron-phonon energy dissipation bottleneck is examined in silicon and carbon nanoscale devices. Monte Carlo simulations of Joule heating are used to investigate the spectrum of phonon emission in bulk and strained silicon. The generated phonon distributions are highly non-uniform in energy and momentum, although they can be approximately grouped into one third acoustic (AC) and two thirds optical phonons (OP) at high electric fields. The phonon dissipation is markedly different in strained silicon at low electric fields, where certain relaxation mechanisms are blocked by scattering selection rules. In very short (∼10 nm) silicon devices, electron and phonon transport is quasi-ballistic, and the heat generation domain is much displaced from the active device region, into the contact electrodes. The electron-phonon bottleneck is more severe in carbon nanotubes, where the optical phonon energy is three times higher than in silicon, and the electron-OP interaction is entirely dominant at high fields. Thus, persistent hot optical phonons are easily generated under Joule heating in single-walled carbon nanotubes suspended between two electrodes, in vacuum. This leads to negative differential conductance at high bias, light emission, and eventual breakdown. Conversely, optical and electrical measurements on such nanotubes can be used to gauge their thermal properties. The hot optical phonon effects appear less pronounced in suspended nanotubes immersed in an ambient gas, suggesting that phonons find relaxation pathways with the vibrational modes of the ambient gas molecules. Finally, hot optical phonons are least pronounced for carbon nanotube devices lying on dielectrics, where the OP modes can couple into the vibrational modes of the substrate. Such measurements and modeling suggest very interesting, non-equilibrium coupling between electrons and phonons in solid-state devices at nanometer length and picoseconds time scales.

scholarly journals The Band-Gap Studies of Short-Period CdO/MgO Superlattices

2021 ◽  
Vol 16 (1) ◽  
Author(s):  
Ewa Przeździecka ◽  
P. Strąk ◽  
A. Wierzbicka ◽  
A. Adhikari ◽  
A. Lysak ◽  
...  
Keyword(s):  
Band Gap ◽  
Layer Thickness ◽  
Energy Gap ◽  
Band Gaps ◽  
Short Period ◽  
Wide Range ◽  
Salt Structure ◽  
Short Period Superlattices ◽  
Direct Band ◽  
Sublayer Thickness

AbstractTrends in the behavior of band gaps in short-period superlattices (SLs) composed of CdO and MgO layers were analyzed experimentally and theoretically for several thicknesses of CdO sublayers. The optical properties of the SLs were investigated by means of transmittance measurements at room temperature in the wavelength range 200–700 nm. The direct band gap of {CdO/MgO} SLs were tuned from 2.6 to 6 eV by varying the thickness of CdO from 1 to 12 monolayers while maintaining the same MgO layer thickness of 4 monolayers. Obtained values of direct and indirect band gaps are higher than those theoretically calculated by an ab initio method, but follow the same trend. X-ray measurements confirmed the presence of a rock salt structure in the SLs. Two oriented structures (111 and 100) grown on c- and r-oriented sapphire substrates were obtained. The measured lattice parameters increase with CdO layer thickness, and the experimental data are in agreement with the calculated results. This new kind of SL structure may be suitable for use in visible, UV and deep UV optoelectronics, especially because the energy gap can be precisely controlled over a wide range by modulating the sublayer thickness in the superlattices.

Temperature dependence of the electron energy gap of highTCsuperconductors studied by work function spectroscopy

1994 ◽  
Vol 64 (13) ◽  
pp. 1726-1728
Author(s):  
S. Westermeyr ◽  
R. Müller ◽  
J. Scholtes ◽  
H. Oechsner
Keyword(s):  
Temperature Dependence ◽  
Work Function ◽  
Electron Energy ◽  
Energy Gap

scholarly journals The Use of Synchrotron Radiation in the Energy Calibration of Astronomical Apparatus

1972 ◽  
Vol 14 ◽  
pp. 642-646
Author(s):  
G. A. Gurzadyan ◽  
J. B. Ohanesyan
Keyword(s):  
Thin Films ◽  
Synchrotron Radiation ◽  
Electron Energy ◽  
Electron Accelerator ◽  
Energy Calibration ◽  
Ideal Solution ◽  
X Rays ◽  
Special Equipment ◽  
Physics Institute ◽  
Wide Range

The problem of energy calibration of astrophysical apparatus is essential for every or almost every space experiment. The utilization of synchrotron radiation from an electronic accelerator should perhaps be taken as an ideal solution of this problem, if of course, such a possibility is available.Special equipment for the extraction of synchrotron radiation has been devised at the circular electron accelerator with a maximum electron energy of 6 GeV, in the Physics Institute of Erevan (Gurzadyan and Ohanesyan, 1972). The equipment is designed primarily for the energy calibration of astronomical apparatus operating in a vacuum and hard ultraviolet and X rays. However, the equipment can also be applied to a wide range of experiments relating to the physics of solids, crystallography, physics of thin films, X rays, etc.

Pump–probe measurement of lifetime engineering in SiGe quantum wells below the optical phonon energy

2005 ◽  
Vol 20 (10) ◽  
pp. L50-L52 ◽  
Author(s):  
C R Pidgeon ◽  
P J Phillips ◽  
D Carder ◽  
B N Murdin ◽  
T Fromherz ◽  
...  
Keyword(s):  
Quantum Wells ◽  
Optical Phonon ◽  
Probe Measurement ◽  
Phonon Energy ◽  
Pump Probe ◽  
Sige Quantum Wells

The role of optical phonons in intermediate layer-mediated thermal transport across solid interfaces

2017 ◽  
Vol 19 (28) ◽  
pp. 18407-18415 ◽  
Author(s):  
Eungkyu Lee ◽  
Tengfei Luo
Keyword(s):  
Molecular Dynamics ◽  
Molecular Dynamics Simulation ◽  
Optical Phonon ◽  
Thermal Transport ◽  
Intermediate Layer ◽  
Dynamics Simulation ◽  
Optical Phonons ◽  
Spectral Matching

A study with molecular dynamics simulation shows that optical phonon vibrational spectral matching by an intermediate layer can significantly impact thermal transport across diatomic solid interfaces.

40 Years Trajectory of Amorphous Semiconductor Research

2000 ◽  
Vol 609 ◽  
Author(s):  
Yoshihiro Hamakawa
Keyword(s):  
Thin Film ◽  
Solar Cells ◽  
Energy Gap ◽  
Early Period ◽  
Basic Research ◽  
Amorphous Semiconductor ◽  
Film Deposition ◽  
Terrestrial Environment ◽  
Heterojunction Solar Cells ◽  
Wide Range

ABSTRACTA review is given on a research trajectory of amorphous and microcrystalline semiconductors and their device applications proceeded since 1970. A brief explanation on the motivation to start amorphous semiconductor research is given to produce a new kind of synthetic semiconductor having continuous energy gap controllability with valency electron controllability through our experience of modulation spectroscopy in semiconductors.The first material we have challenged is Si-As-Te chalcogenide semiconductor which has a very wide vitreous region in Gibb's Triangle. A series of systematic experiments has been carried out in the terrestrial environment since 1971, and also within the TT-500A rocket experiment in 1980, and the Spacelab. J experiments FMPT (First Material Processing Test) project in 1992. The second material is hydrogenated amorphous silicon (a-Si:H) and its alloys started in 1976 just after the Garmisch Partenkirchen ICALS-6. With some basic research on the a-Si:H film deposition technology and film quality improvement, our continuous effort to improve the efficiency bore the tandem type solar cells in 1979, and also new products of a-SiC:H and a-SiGe:H in the early period of 1980s are described. These innovative device structures and materials have bloomed in the middle of 1980s in R & D phase such as a-SiC/a-Si heterojunction solar cells, a-Si/a-SiGe and also a-Si/poly-Si tandem type solar cells, and industrialized in recent few years. New kind of trials on full-color thin film light emitting devices has also been recently initiated with wide range of band gap controllability of a-SiC:H.The third material is microcrystalline silicon (µc-Si) and their alloys which gathers a tremendous R & D effort as a promised candidate for the bottom cell of the a-Si/µc-Si tandem solar cells aimed for the all-round plasma CVD process for the next age thin film photovoltaic devices. In the final part of presentation, a brief discussion will be given on a technological evolution from “bulk crystalline age” to “multilayered thin film age” in the semiconductor optoelectronics toward 21 century.

scholarly journals Characteristics of Coherent Optical Phonons in a Hexagonal YMnO3 Thin Film

2019 ◽  
Vol 9 (4) ◽  
pp. 704 ◽  
Author(s):  
Takayuki Hasegawa
Keyword(s):  
Thin Film ◽  
Optical Phonon ◽  
Laser Pulses ◽  
Generation Process ◽  
Neel Temperature ◽  
Optical Phonons ◽  
Néel Temperature ◽  
Temperature Range ◽  
Lower Temperature Range ◽  
Lower Temperature

This paper reviews our recent study on a coherent optical phonon in a hexagonal YMnO3 thin film together with related optical studies in hexagonal RMnO3 (R = Y, Lu, Ho) compounds. Coherent phonons have been observed in RMnO3 compounds by pump-probe spectroscopy with subpicosecond laser pulses, whereas the observation of coherent optical phonons was reported only in LuMnO3. Recently, we succeeded in the observation of the coherent optical phonon in a YMnO3 thin film. The generation process of the coherent optical phonon is assigned to a displacive mechanism, which is identical to that in LuMnO3. The coherent optical phonon is observed in the temperature range from 10 K to room temperature, while the oscillation intensity strongly decreases as the temperature increases to the Néel temperature of ~70 K from a lower temperature range. It is interesting that the temperature dependence is largely different from that in LuMnO3. We describe that the result can be qualitatively explained by the property of an isostructural transition around the Néel temperature in RMnO3 compounds. In addition, we briefly discuss ultrafast incoherent responses of excited electronic states from the viewpoint of the excitation photon energy of laser pulses.

Solvent Effects upon the Electronic Absorption and Fluorescence Spectra of Pteridines and Riboflavin and Their Ground and First Excited Singlet State Dipole Moments

Pteridines ◽  
1991 ◽  
Vol 3 (3) ◽  
pp. 153-163 ◽  
Author(s):  
J.-J. Aaron ◽  
M. D. Gaye ◽  
C. Párkányi ◽  
C. Boniface ◽  
T. W. N Bieze ◽  
...  
Keyword(s):  
Electronic Absorption ◽  
Singlet State ◽  
Dipole Moments ◽  
Stokes Shift ◽  
Fluorescence Emission ◽  
Excited Singlet State ◽  
Theoretical Ground ◽  
Excited Singlet ◽  
Solvatochromic Shift ◽  
Wide Range

The electronic absorption. fluorescence excitation, and fluorescence emission spectra of a series of pteridines (lumazine, xanthopterin, isoxanthopterin, biopterin) and riboflavin (vitamin B2) were measured at room temperature (298 K) in a number of solvents covering a wide range of polarities (dioxane, ethyl ether, chloroform, ethyl acetate, 1-butanol. 2-propanol, ethanol, methanol, dimethylformamide, acetonitrile, and dimethyl sulfoxide). The effects of the solvent upon the spectral properties are discussed. Experimental groundstate dipole moments were measured for selected compounds and were used in combination with the spectral data to evaluate their first excited singlet-state dipole moments by means of the solvatochromic shift method (Bakhshiev's and Kawski-Chamma-Viallet's equations based on the variation of the Stokes shift with the solvent dielectric constant-refractive index term). The theoretical ground and excited singlet-state dipole moments for all pteridines and riboflavin were calculated as a vector sum of the π-component (obtained by the PPP method) and the a-component (obtained from a-bond moments). A second set of theoretical values was obtained by using the CNDO/2method. A good agreement was observed between the experimental and the theoretical values. Excited singlet-state dipole moments are higher than the ground state values by 1 to 6 Debye units for all the pteridines under study with the exception of xanthopterin.

Sign in / Sign up

Export Citation Format

Share Document