Temperature Dependence Of Bands Research Articles

A simple parameterization of ozone infrared absorption for atmospheric heating rate calculations

A simple parameterization of ozone absorption in the 9.6‐μm region which is suitable for two‐ and three‐dimensional stratospheric and tropospheric models is presented. The band is divided into two parts, a band center region and a band wing region, grouping together regions for which the temperature dependence of absorption is similar. Each of the two regions is modeled with a function having the form of the Goody random model, with pressure and temperature dependent band parameters chosen by empirically fitting line‐by‐line equivalent widths for pressures between 0.25 and 1000 mbar and ozone absorber amounts between 1.0×10−7 and 1.0 cm atm. The model has been applied to calculations of atmospheric heating rates using an absorber amount weighted mean pressure and temperature along the inhomogeneous paths necessary for flux computations. In the stratosphere, maximum errors in the heating rates relative to line‐by‐line calculations are 0.1 K/d, or 5% of the peak cooling at the stratopause. In the troposphere the errors are at most 0.005 K/d.

Surface photovoltage effects in photoemission from metal/GaP(110) interfaces: Temperature-dependent Fermi level movement

In recent experiments of metal deposition onto cleaved GaP(110) surfaces we have shown that light sources used in photoelectron spectroscopy may induce a surface photovoltage (SPV), which causes a substantial deviation from the ground state potential distribution, and may induce errors in the determination of band bending by photoemission. Here we analyze the temperature-dependent movement of the surface Fermi level in n- and p-type GaP(110) surfaces as a function of indium and silver deposition, taking into account the presence of the SPV. It is found that changes in the substrate temperature not only modify the adlayer morphology and metallicity, but also the surface electron-hole recombination rate. We observe that the temperature-dependent shift of the semiconductor core levels is always accompanied by a similar shift of the metal core level and Fermi edge, suggesting that the reversible temperature-dependent band bending recently reported for metal/III–V semiconductor interfaces is related to the SPV, and does not represent a ground state property of the interfacial electronic structure. Implications of these results on current models concerning Schottky barrier formation are discussed.

Raman spectra of blue bronze Rb 0.3MoO 3

Temperature dependent Raman spectra of quasi-one-dimensional blue bronze Rb 0.3MoO 3 are reported. Similarly to what it was found by Travaglini et al. ( Solid State Commun. 45, 289, 1983) in K 0.3MoO 3, a low temperature-low frequency strongly temperature dependent band is assigned to the amplitude mode of the charge density wave. In addition, the relevant infrared data are reviewed. A Raman weighted one-dimensional phonon density of states found above mid-gap energies is traced to the symmetric stretching-breathing mode. It suggests a zone center branch softening of at least 147 cm −1.

Role of photocurrent in low-temperature photoemission studies of Schottky-barrier formation.

Photoelectron spectroscopy is frequently used to study band bending in semiconductors due to charge stored in surface or interface states. This paper examines how such experimental results are modified by photovoltages generated within the band-bending region not only by ambient light sources, but by the incident x rays themselves. Recent experiments which have suggested dopant-dependent and reversible temperature-dependent band bending in the initial stages of formation of the metal-GaAs(110) interface are used as an example. It is shown here that the reported dependence derives from a photovoltaic effect.

Confined excitons under biaxial compression and tension

The properties of confined excitons have been investigated under both biaxial compression and tension, which result upon cooling from the different thermal expansion coefficients of a thin GaAs/AlGaAs multiple quantum well (MQW) sample and various sample holders. Exciton transitions connected with higher subbands have been observed in optical transmission and identified by their temperature dependence using subband calculations based on an 8×8 k·p Hamiltonian with temperature and stress dependent band edge energies. We identify among others the “forbidden” L1-C2 transition.

Thermally reversible band bending for Bi/GaAs(110): Photoemission and inverse-photoemission investigations.

Results of synchrotron-radiation photoemission, inverse-photoemission, and low-energy electron diffraction studies of the Bi/GaAs(110) interface over the temperature range 20 to 300 K are presented. At 300 K, the first Bi monolayer grows in zigzag chains along the [11\ifmmode\bar\else\textasciimacron\fi{}0] direction to form an ordered (1\ifmmode\times\else\texttimes\fi{}1) overlayer. This overlayer is semiconducting with a gap of 0.7 eV. Continued deposition results in Bi island growth atop the initial monolayer (ML) and conversion from semiconducting to semimetallic character. Bi deposition at 50 K results in layer-by-layer growth with no long-range order. Temperature- and coverage-dependent studies of band bending show symmetric behavior for n- and p-type GaAs(110) with matched bulk dopant concentrations. For 300-K deposition, the Fermi level ${\mathit{E}}_{\mathit{F}}$ moves rapidly toward final positions 0.74 and 0.98 eV below the conduction-band minimum for n- and p-type GaAs doped at ${10}^{17}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. For 50-K deposition, the bands remain nearly flat to coverages of 1--2 ML where ${\mathit{E}}_{\mathit{F}}$ moves rapidly toward midgap. Significantly, both techniques also show that Fermi-level movement is thermally reversible, such that the position of ${\mathit{E}}_{\mathit{F}}$ in the surface band gap depends on temperature. A small part of the ${\mathit{E}}_{\mathit{F}}$ movement is due to nonequilibrium processes involving the creation of electron-hole pairs by the incident photon or electron beam followed by the transport of the minority carriers to the surface region by the electric field in the depletion region. This surface voltage is most important at low temperature, and studies which vary the incident photon flux by over 3 orders of magnitude establish that the photovoltage has a small (\ensuremath{\le}100 meV), but non-negligible, effect on the band bending measured under ``normal'' experimental conditions. Hence, equilibrium processes control coupling of substrate and adsorbate-induced states in the GaAs band gap through the semiconductor depletion region and they determine the temperature-dependent interface band bending.

Infrared spectroscopy of CO on NaCl(100). II. Vibrational dephasing and band shapes

Temperature dependence of the fundamental vibrational transition of CO on NaCl(100) single crystal surfaces has been measured between 4 and 55 K using Fourier-transform infrared spectroscopy. The absorption profiles at various temperatures are very nearly symmetric. At 4 K, the transition is centered at 2154.93 cm−1, and the full-width at half-maximum (FWHM) is 0.17 cm−1; at 55 K, the transition shifts to 2155.14 cm−1, and the FWHM broadens to 0.39 cm−1. The temperature-dependent frequency shift and broadening of the fundamental transition are interpreted by a vibrational phase relaxation process, in which the coupling between the stretching mode of CO and a low frequency CO-surface mode causes the dephasing of the excited state vibrational motion. The Boltzmann factor associated with the spectroscopic temperature dependence allows us to determine that the surface mode has a frequency of 40 cm−1 . We associate this mode with the frustrated translational motion of CO over the NaCl(100) surface. The observed temperature-dependent absorption band undoubtedly is a consequence of collective motions among the CO stretching vibrations within the monolayer. However, the current model, that assumes this collective motion is coupled by the CO transition dipoles, cannot explain the data. Further theoretical work will be required to understand the infrared spectroscopy of CO on NaCl(100). Below 10 K, the CO fundamental absorption becomes temperature independent. However, the limiting bandwidth and bandshape depend on the NaCl(100) crystal surface preparation. For one set of crystals, the 4 K bandshape was Lorentzian with a FWHM of 0.17 cm−1 . For another set, the 4 K bandshape was Gaussian with a FWHM of 0.11 cm−1. We believe these bandwidths, narrower than those reported previously for any adsorbate system, are limited by heterogeneities of the monolayer and/or substrate. The 0.11 cm−1 bandwidth provides a lower limit of 45 ps for the lifetime of vibrationally excited CO on NaCl(100).

The temperature dependence of intersubband absorption in a barrier-doped GaAs/Al 0.3Ga 0.7As quantum well structure

We compare the results of an envelope function (EFA) calculation of absorption spectra as a function of temperature of a barrier-doped GaAs/Al 0.3Ga 0.7As multiple quantum well (MQW) with experimental results. The linewidth, total integrated area, and peak positions of the intersubband transition (IT) in the MQW were measured as a function of temperature with the infrared absorption technique. We infer the density of electrons σ in the well from the total integrated intensity of the IT. Using the temperature dependence of σ we find that the Fermi energy in our MQW is also temperature dependent. The peak position of the IT is found to exhibit a nonlinear blue shift as the temperature is lowered. We calculate a one-electron energy band structure of the MQW versus the in-plane wave vector as a function of temperature using Ekenberg's nonparabolic-anisotropic EFA formalism. In our calculation we employ temperature dependent effective masses in both the well and barrier materials and temperature dependent conduction band offset and nonparabolicity parameter. Using the calculated bands and Fermi distribution functions to reflect the occupancies of both the ground and excited states with temperature dependent Fermi levels, we also obtain the optical absorption spectra as a function of temperature. These “raw” calculated spectra are then convoluted with a Lorentzian broadening function using the broadening parameters obtained from the measured lineshapes. The peak of the broadened spectra shows hardly any shift as a function of temperature. We conclude that the exchange interaction for the ground state electrons, which is temperature dependent owing to the temperature dependence of σ, is responsible for the observed peak positions as well as for their blue shift.

Cathodoluminescence of cadmium diphosphide crystals under uniaxial deformation

Cathodoluminescence spectra of CdP2 crystals are studied at 6 K. The spectra are excited by a pulsed beam of electrons under uniaxial deformation of the samples. It is established that the creation of tension in the a and c directions leads to a shift of all band maxima of the spectral short-wavelength range to the low-energy and high-energy region. This is explained by a proportional change in the forbidden bandwidth induced by deformation. Variations in the intensity of certain sites of the radiation spectrum under crystal deformation as well as the investigation of temperature dependences of bands make it possible to explain their nature. The 2.142 eV line is associated with recombination of free excitons and the 2.132 and 2.110 eV bands are induced by recombination of excitons which are localized on isolated and nitrogen pair centres, respectively. Excitons which are localized on neutral acceptors and produced by intrinsic defects are responsible for the radiation with maximum of 2.097 eV. [Russian Text Ignored].

Temperature-dependent Al/GaAs(110) interface formation and adatom energy references.

Synchrotron-radiation photoemission studies of the formation of Al/GaAs(110) interfaces have been performed as a function of substrate temperature for 60\ensuremath{\le}T\ensuremath{\le}300 K for n- and p-type doped samples. The results show temperature-dependent changes in surface Fermi-level position, surface morphology, and the distribution of released substrate atoms in the overlayer. Detailed examination shows a separation in energy of \ensuremath{\sim}1.0 eV for the Al 2p binding energy for n- and p-type GaAs at low coverage. This equals the difference in band bending for the two substrates and demonstrates that the adatom energy reference is an intrinsic level of the semiconductor, not the Fermi level. Substrate band bending approaches its final value when ${E}_{F}$ becomes the energy reference for the overlayer, and this occurs at the onset of metallic overlayer behavior. Temperature-dependent band bending observed below \ensuremath{\sim}1 monolayer can be understood in terms of the coupling of an adsorbate energy level to the semiconductor via steady-state tunneling and thermionic emission. The high-coverage results at all temperatures are consistent with metallicity.

The effect of the temperature shift of the energy bands on the thermopower of quasi-free electrons in semiconductors

Abstract The thermopower of non-degenerate semiconductors is calculated for temperature-dependent energy bands using the quantum statistical formulae for the linear response. The effective, temperature-dependent, electronic and phononic energies are determined through a canonical transformation of the electron—phonon Hamiltonian and a mean-field approximation in the second-order correction. For the renormalized electron-phonon system, the phonon-drag effects are neglected.

A temperature-dependent regular infrared band model

A temperature-dependent infrared band model for the absorption or emission of spectral lines that are simultaneously broadened by Doppler and collisional effects is developed. The result is given in the form of an infinite series which converges repidly and is appropriate for efficient computation. It reproduces accurately the spectral line-intensities and profiles for symmetric molecules. The model has the apparent limitation of producing lines of equal spacing. However, easily applied techniques are demonstrated for overcoming this limitation. The model has been generalized to include the Voigt-profile equivalent of the Clough line shape, which is important in the microwave region. Applications are suggested, such as improvements to the LOWTRAN atmospheric transmittance code and to emission and absorption modeling in general.

Structural phase transitions in ethylenediammonium dinitrate

Abstract (H3NCH2CH2NH3)(NO3)2 belongs to the triclinic space group PT with Z = 1 at 300 K, with one molecule in the unit cell. Differential scanning calorimetric studies indicate phase transitions at 404 and 412 K. Raman and infrared spectra (10–5000 cm−1) of (H3NCH2CH2NH3XNO3)2 and of its N-deuterated derivative are investigated in the temperature range 90–430 K. Assignments of most of the bands observed are proposed. Temperature-dependent frequencies and band widths reveal the existence of three phases. Analysis indicates rearrangement of the cation and disorder of the anion in the high-temperature phases.

Structure, vibrational and electronic spectra, and bonding in trans-diaquabis(oxalato)vanadate(III) complex salts, A[V(ox)2(H2O)2]·xH2O (A = Cs, K, or NH3Me), and the X-ray crystal structure of the potassium salt (x= 3)

The X-ray crystal structure of potassium diaquabis(oxalato)vanadate(III), K[V(ox)2(H2O)2]·3H2O, has been determined [monoclinic, space group P2/c, with unit-cell parameters a= 7.971(4), b= 5.691(2), c= 14.167(5)A, β= 108.97(3)°, and Z= 2]. The structure has been refined to an R value of 0.032 using 560 independent reflections. In comparison with the corresponding Cs+ and NH3Me+ salts, the K+ salt exhibits a significantly longer bond distance for axially co-ordinated water and this is manifested in band shifts of 500–1 000 cm–1 in the visible region. For all three salts vibrational spectra and single-crystal polarised electronic spectra are reported together with assignments. Analysis of the ligand-field spectra is presented in terms of the angular overlap model. It is shown that the band shifts occurring in the electronic spectrum of the K+ salt can be reproduced by a reduction in the σ-bonding capacity of the co-ordinated water together with a slight increase in the σ bonding of the oxalate ligand, both of which are consistent with structural differences existing between the K+ and the other two salts. Significant anisotropy exists in the metal–oxalate π interaction with the in-plane contribution relatively small. In addition, anomalous temperature-dependent bands resulting from vibronic coupling with internal O–H stretching vibrations are present in the electronic spectra of both the Cs+ and NH3Me+ salts.

Dynamical corrections to density-functional theory for quasiparticles in ferromagnetic 4f systems. II. Finite-temperature results for EuO.

We present a theory for the temperature dependence of the electronic quasiparticle spectrum of ferromagnetic 4f systems. A concrete evaluation for the semiconductor EuO is performed and discussed. The study is based on a d-f exchange model, which has been exactly solved for T=0 in part I of this series. The exact solution allows us to fix the ``free'' Bloch energies ${\ensuremath{\varepsilon}}_{m}$(k) of the model in a highly realistic manner by application of a self-consistent spin-polarized band-structure calculation based on density-functional theory. For finite temperatures the d-f model is approximately solved by a many-body procedure, which takes special care for a proper treatment of spin-exchange processes between localized 4f moments and itinerant conduction electrons. The method reproduces the exact T=0 limiting case. The resulting f-spin correlation functions are calculated by a moment method. We discuss in detail the one-electron spectral density for k vectors along the \ensuremath{\Gamma}L direction, real and imaginary parts of the electronic self-energy, and quasiparticle densities of states. The prominent peaks of the spectral density are used to construct a temperature-dependent quasiparticle band structure. The temperature dependence is to first order due to the magnetization, and to second order due to a transverse correlation function of the localized 4f spins.

Crystallinity in chemically crosslinked low density polyethylenes: 3. Morphology of the XLPE-2 system

Studies of the morphology of low density branched polyethylene crosslinked using 2% dicumyl peroxide have been carried out using transmission electron microscopy of replicas of surfaces etched with permanganic reagent. Under optimal etching conditions detailed information regarding the morphologies of bulk crystallized samples of low crystallinity can be obtained. The morphology of crosslinked polyethylene is compared with that of the original branched polyethylene as a function of crystallization temperature. Branched polyethylene crystallizes in the form of banded spherulites from 88°C to 103°C with a temperature dependent band periodicity. Introduction of a small amount of crosslinking produces profound effects with banded sheaf or bundle-like morphologies resulting. The absence of lamellar banding in the sheaves of the gel fraction of the crosslinked polyethylene is attributed to pre-existent tie-molecules which are an inherent feature of network junctions in a crosslinked system. In the case of a typical unextracted crosslinked polyethylene, the morphology is found to vary with crystallization temperature. D.s.c. studies reveal that the thermal behaviour of XLPE-2 is not too different from that of OPE. The cause of multiple melting behaviour in this class of material is discussed and it is suggested that in addition to other processes of reorganization, hindrance to solid state thickening may be important. A method of monitoring different stages of lamellar melting for semicrystalline polymers was developed involving annealing at elevated temperatures in the presence of chlorosulphonic acid.

Infrared properties and band gaps of HgTe/CdTe superlattices

Measurement of the optical transmission spectra of four HgTe/CdTe superlattices grown by molecular beam epitaxy has been carried out at 300 and 30 K to determine the absorption coefficients, refractive indexes, and band-gap energies. In all cases the optical absorption coefficients reached high values (near 104 cm−1) at energies above the band gap. The absorption coefficients had an energy dependence of α≊(E−Eg)1/2 which confirms theoretical predictions of a superlattice band structure with a direct gap. Three of the superlattices measured showed little or no temperature dependence for their band gaps while the third, which may be anomalous, showed a temperature-dependent band gap.

Theory of the temperature dependence of the threshold current of an InGaAsP laser

The threshold current of an InGaAsP laser is calculated, where the radiative emission, reflection and absorption losses, and Auger recombination are considered. Moreover, the enhancement of the threshold carrier density at high temperatures is an important point. A mechanism for this enhancement is discussed. Then we obtain an excellent agreement with the measured temperature dependence of the threshold current, in particular the T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</inf> -values for T≷T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</inf> and the break point T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</inf> . The reason for this break point is that the radiative recombination dominates for T < T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</inf> , whereas the strongly temperature dependent valence band Auger process becomes more and more important for T > T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</inf> . It is this process which causes the strong increase of the threshold current in the room temperature range.

Band narrowing and susceptibility enhancement by a quadratic electron-phonon interaction

It is shown that a quadratic electron-phonon interaction leads, under certain conditions, to temperature-dependent band narrowing and an inverse lifetime quadratic in the phonon occupation numbers. These explain the prominent metallic properties of organic metals of the TTF-TCNQ family, in particular the enhancement of the magnetic susceptibility.

Polycrystalline zinc selenide films deposited by a low-temperature close-spaced-vapor-transport method

Polycrystalline films of ZnSe, pure and doped with Cu and Ga, have been deposited by a modified close-spaced-vapor-transport method which requires low temperatures (500 °C) and no subsequent annealing. The resulting grains are of the order of 1 μm. The films were characterized by their photoluminescence emission in the temperature range 10–290 K. The undoped and Cu-doped films display the usual Cu-green and Cu-red and near edge emission bands at low temperatures. In the Ga-doped films only a mildly temperature dependent band centered at ∼620 nm appears at all temperatures. The films have very similar characteristics to those prepared by more sophisticated techniques.