Evaluation of structural,electronic, thermoelectric,and optical results of C-doped HfO2 by first-principle's investigation

In this work, we report the ab initio numerical simulation investigation on the crystal lattice, electronic structure, optical, and transport properties of pure and C-doped crystalline hafnium dioxide (c-HfO₂) using FP-LAPW method. Different exchange correlation functionals like generalized gradient approximation (GGA) of PBE-sol and Tran and Blaha's modified Becke-Johnson exchange potential (mBJ) within density functional theory have been used. Two kinds of defects in cubic pure HfO₂ have been investigated: one is substitution of Hf atom by C impurity and other substitution of O atom by C impurity in crystalline HfO₂. The computed results indicate that impurity energy bands as a result of 2p states of C are found to present in the band gap of c-HfO₂. Few of these bands are present at the conduction band minimum, which results to a noteworthy band gap contraction, and hence electrons close to Fermi level get transferred in doped c-HfO₂. We have also analysed the dielectric function, absorption coefficient, optical conductivity, optical function, electron energy loss, and reflectivity for both pure HfO₂ and doped with C. Furthermore, the temperature-dependent transport properties of C-doped HfO₂ are also discussed in terms of Seeback coefficient, thermal conductivities, electronic conductivities, power factor, and figure of merit in the temperature range 0 to 1200 K. The calculated value of PF for pure HfO₂ was found to increase from 0.01 × 10¹² WK⁻²m⁻¹s⁻¹ at 50 K to 1.79 × 10¹² WK⁻²m⁻¹s⁻¹ at 1200 K and for HfO_{2(1 − x)}C_x it was found to increase from 0.06 × 10¹² WK⁻²m⁻¹s⁻¹ at 50 K to 0.25 × 10¹² WK⁻²m⁻¹s⁻¹ at 1200 K.     

Electron properties of n- and p-CuInSe2

Using two-temperature synthesis method with the further directed crystallization under the radial and slight horizontal temperature gradients, facilitating the convection and mixing of the melt, the authors obtained monocrystals of n- and p-CuInSe₂ with controlled deviation from the stoichiometry using the excess In and Se. We have carried out the measurements of the conductivity and Hall coefficient in the temperature interval 70–415 K and investigated Hall mobility as the function of temperature, determining the dominating carrier scattering mechanisms. It was found that the electrical properties of n- and p-CuInSe₂ are caused by the defects of various types depending on the growth conditions and stoichiometry deviations. The energy position of the impurity levels was identified to be 0.055 ± 0.003 eV and 0.022 ± 0.003 eV above the valence band for acceptor levels and 0.010 ± 0.002 eV below the conduction band for the donor level.     

Impurity photovoltaic effect in GaAs solar cell with two deep impurity levels

The impurity photovoltaic (IPV) effect has been extensively investigated for silicon solar cells with indium impurities. A small positive effect on the efficiency was theoretically predicted, provided that efficient light trapping schemes are applied. Since IPV effects are predicted to be more pronounced in wider band gap materials and possibly also by introducing more than one impurity level, we carried out a numerical study on GaAs solar cells with copper impurities. Indeed, copper introduces multiple acceptor type mid-gap levels in GaAs, two of which are investigated here: one at 0.14 eV and another at 0.40 eV above the valence band edge. We used a solar cell device simulator (SCAPS) specially adapted to include the IPV effect. We varied the impurity concentration and the light trapping parameters of the device, and calculated the occupation probability of the impurity levels, the generation and recombination through these levels (thermal and optical) and the solar cell characteristics: quantum efficiency, QE(λ), short-circuit current J_sc, open-circuit voltage V_oc and efficiency η.A significant IPV effect with extended response in the infrared is calculated when efficient light trapping is present. When an internal reflection coefficient R=0.99 can be reached at both the front and the back surface of the solar cell, an increase of the short-circuit current densities 2 mA/cm² can be obtained. The consequences of two levels instead of one are calculated and discussed, leading to a trade-off between J_sc improvement and a V_oc decrease.     

Electrical conduction studies of hot wall deposited CdSexTe1−x thin films

CdSe_xTe_1−x thin films of different compositions have been deposited on cleaned glass substrates using the hot wall deposition technique under conditions very close to thermodynamical equilibrium with minimum loss of material. The electrical conductivity of the deposited films has been studied as a function of temperature. All the films showed a transition from phonon-assisted hopping conduction through the impurity band to grain-boundary-limited conduction in the conduction/valence band at temperature around 325 K. The conductivity has been found to vary with composition; it varied from 0.0027 to 0.0198 Ω⁻¹ cm⁻¹ when x changed from 0 to 1. The activation energies of the films of different compositions determined at 225 and 400 K have been observed to lie in the range 0.0031–0.0098 and 0.0285–0.0750 eV, respectively. The Hall-effect studies carried out on the deposited films revealed that the nature of conductivity (p or n-type) was dependent on film composition; films with composition x=0 and 0.15 have been found to be p-type and the ones with composition x=0.4, 0.6, 0.7, 0.85 and 1 have been observed to exhibit n-type conductivity. The carrier concentration has been determined and is of the order of 10¹⁷ cm⁻³. The majority of carrier mobilities of the films have been observed to vary from 0.032 to 0.183 cm² V⁻¹ s⁻¹ depending on film composition. The study of the mobility of the charge carriers with temperature in the range of 300–450 K showed that the mobility increased with power of temperature indicating that the type of scattering mechanism in the studied temperature range is the ionized impurity scattering mechanism.     

Energy band structure parameters and their data, derived from the measurements of minority carrier current density in heavily doped emitters of silicon devices

In the n(p)-type heavily doped emitter region (HDER) of silicon devices, at room temperature, we have investigated the minority-carrier lifetime and the energy band structure parameters such as band-gap narrowing, apparent band-gap narrowing, unperturbed Fermi energy shift, optical gap, and reduced interacting density-of-majority conduction (valence) band states effective mass. As used in our previous paper, the present treatment is also based on the two assumptions for minority-carrier transport parameters. Gaussian impurity density profile, and accurate expression for minority-carrier mobility. Our empirical models for minority-carrier lifetime and band-gap narrowing are proposed and determined such that their curves versus the impurity concentration lie in between 3heir existing experimental data. Then, from a conjunction between electrical and optical phenomena, it is found that our theoretical values of such the energy band structure parameters are in good accordance with our own corresponding data, derived from the measurements of minority-carrier saturation current density in the n(p)-type HDER of silicon devices.     

Segregation phenomena in large-size cast multicrystalline Si ingots

In cast multicrystalline silicon ingots impurity concentrations vary along the ingot height due to segregation phenomena during the directional solidification. It is expected that these concentrations are the highest at the top of the ingot which solidifies last. The bottom of the ingot which solidifies first, and which is longer in contact with the crucible floor is contaminated by solid state diffusion. As a consequence, lifetime (τ_n) and diffusion length (L_n) of minority carriers are the highest in the central part of the ingot and decrease strongly in the top as well as at the bottom. However, the impurity concentration is so high at the extremities of the ingot that additional solid state segregation phenomena occur at extended defects, which extract impurities from the adjacent grains. That is revealed at grain boundaries (GBs) by τ_n and L_n scan maps and also by the variation of the mobility of the majority carriers which cross GBs.     

Interface characterisation of ZnSe/CuGaSe2 heterojunction

The ZnSe/CuGaSe₂ heterojunctions were fabricated by flash evaporation technique of CuGaSe₂ onto the (110) surface of ZnSe crystals. CuGaSe₂ layers had thickness 2–4 μm and showed a hole concentration up to (1.5–18.0)×10¹⁸ cm⁻³ and mobility μ4–24 cm² V⁻¹ s⁻¹ at 300 K. The charge carrier concentration in ZnSe crystals at 300 K was n=5.6×10¹⁶ cm⁻³ and their mobility μ=300 cm² V⁻¹ s⁻¹. The investigated ZnSe/CuGaSe₂ heterojunctions have at the interface an intermediate layer with a thickness of 450–750 Å and a linear graded band gap as well as an i-ZnSe compensated layer with a thickness of 1–2 μm and resistivity ρ10⁸–10⁹ Ω cm. The i-ZnSe layer is highly compensated due to the presence of Cu acceptor impurities. In this layer the Fermi level position E_c−F₀690 meV and a trap level position E_t−F₀17 meV were determined. The total trap concentration in the i-ZnSe layer is N_t5×10¹⁴ cm⁻³. The mean free path of excited charge carriers in the graded band gap region was calculated as λ55 Å. On the basis of experimental data analysis of electrophysical properties of both ZnSe/CuGaSe₂ heterojunctions and constituent materials the energetic band diagram of the investigated heterostructures is proposed. The current transport mechanism through ZnSe/CuGaSe₂ heterojunctions is consequently elucidated.     

Effects of impurities on the degradation and long-term stability for solid oxide fuel cells

The effect of impurities on the degradation of performances was investigated for the flatten tube type SOFC stack. The durability tests of 20-cells stack were conducted at 750 °C (1023 K) with dry H₂ for more than 5000 h under a constant current density of 0.3 A cm⁻². The voltage loss showed a linear relationship between voltage loss and operation time (about 1.5%/1000 h). The ohmic resistance increased with operation time while the polarization resistance showed constant values. After the long-term operation test, the concentration levels of impurities were measured at cathode and interlayer by secondary ion mass spectrometry (SIMS). The concentrations of several elements were successfully determined in ppm levels. The concentrations of several elements increased with operation time (Na, Al, Si, and Cr), which suggested the transports and depositions on the cell component surface. The increase of resistance and impurity concentration at the interlayer were estimated from the literature data and SIMS impurity analysis.     

Temperature dependence of the conductivity in large-grained boron-doped ZnO films

The temperature dependence of the conductivity is investigated as a function of boron doping in large-grained, degenerate polycrystalline ZnO films prepared by low-pressure chemical vapor deposition. Carrier transport in undoped and lightly doped films is mainly controlled by the grain boundary; field emission through grain boundaries limits the conductivity below 90 K, while thermally activated thermoionic-field emission leads to an increase in the conductivity with the temperature near room temperature. In contrast, carrier transport in highly doped films is mainly governed by intra-grain scattering, which does not depend on the temperature for degenerate electron gases, limits the mobility below 120 K, whereas a metallic behavior (decrease in conductivity with increasing temperature) is observed at room temperature, which is linked to the ionized impurity scattering. The transition between the “semiconductor”-like and metallic-like behavior at room temperature takes place for a film with carrier concentration between 6×10¹⁹ and 9×10¹⁹ cm⁻³.     

Spectral Remote Sensing for Furnaces and Flames

The spectral remote sensing (SRS) technique to measure the gas temperature profile in a combustion system is introduced from fundamental concept to lab-size bench test. As the bases of the technique, computationally working models of narrow band emission in the CO₂ 4.3μ -wide band are tested, and inversion algorithms to convert the spectral intensity data into a temperature profile are proposed. Actual applications to three lab-size test benches are made to confirm the practical applicability of the technique. With the latest inversion methods and instruments, the SRS technique can retrieve the temperature profile within a 20–30 K error for peak temperature of 1500 K within seconds. A scheme called BCIM incorporating base function approach and nodal temperature approach is recommended. It is also shown that a pure concentration problem cannot be handled by SRS. However, SRS can be applied to combustion flames thanks to a strong temperature/concentration correlation.     

Influence of Cr on structural and optical properties of TiO2:Cr nanopowders prepared by flame spray synthesis

Influence of chromium incorporation on structural and optical properties of titanium dioxide nanopowders obtained by flame spray synthesis, FSS is studied by means of: X-ray diffraction, XRD; Raman spectroscopy; transmission electron spectroscopy, TEM; photoelectron spectroscopy, XPS and optical spectrophotometry over the ultraviolet, UV and visible range of the light spectrum from 250 nm to 2200 nm. The specific surface area, SSA, of the powders has been adjusted from 48 m²/g for TiO₂ + 0.1at.% Cr to 177 m²/g for TiO₂ + 15 at.% Cr which is accompanied by a decrease in the anatase grain size from 21 nm to 5 nm. The anatase-to-rutile ratio changes with Cr³⁺ concentration but there is no evidence of precipitation of chromium oxides or chromium titanates. Incorporation of Cr³⁺ into TiO₂ lattice, as proved by XPS, is found to affect the electronic structure of TiO₂, as indicated by the optical spectrophotometry. The impurity band is formed within the forbidden band gap of titanium dioxide which results in the additional absorption within the visible range of the light spectrum. The general aim of this work is to improve the visible light absorption and hence the efficiency of photocatalytic decomposition of organic contaminants.     

Spectrophotometric study of the corrosion behaviour of chromium in molten alkali carbonates

The corrosion of chromium species in molten (Li/K)₂CO₃ and (Li/Na/K)₂CO₃ are investigated by spectrophotometry. A strong charge-transfer absorption band is observed in the ultraviolet region with λ_max=372 nm, and the molar absorptivity of CrO₄²⁻ in molten (Li/K)₂CO₃ is calculated. The corrosion rates are investigated by means of increasing the concentration of CrO₄²⁻ in the melts. Under an air atmosphere, the corrosion rate is slower in molten (Li/K)₂CO₃ than in molten (Li/Na/K)₂CO₃ and Cr₂O₃ corrodes more easily in molten (Li/K)₂CO₃ than Cr. With an atmosphere of CO₂=1 atm, there is no increase in the CrO₄²⁻ concentration in the solution phase during a period of 5 h.     

Effects of low level boron doping of the i-layer on the performance of SiC p-i-n devices

a-Si: H devices have been prepared by plasma decomposition of a silane mixture containing ppm levels of diborane. The level of background impurities was controlled by outgassing, monitored by residual gas analysis and finally measured in the solid by SIMS analysis. Two groups of samples were prepared which differed by an order of magnitude in their impurity concentration. The effect of boron addition to the i-layer differed strongly depending on the background impurity level. At high impurity levels ([O] = 1 × 10²⁰ atoms/cm³) a well defined maximum in device performance occurred at [B₂H₆] = 3 ppm, while at lower background levels boron addition served only to lower the device performance. Quantum efficiency measurements indicate that the changes in device performance are due to corresponding changes in the collection length and average μτ product. The ambipolar diffusion length determined by the surface photovoltage yielded similar results.     

Potential of Mn doped In1−xGaxN for implementing intermediate band solar cells

In this paper we propose that the incorporation of Mn into In_1−xGa_xN can produce material suitable for intermediate band (IB) solar cells. For x≈0.22 we predicted that the resulting material would have a total bandgap of 1.11 eV, with the IB located at about 0.74 eV from the valence band (VB). The resulting limiting efficiency is 53.4% (maximum light concentration and assuming that the sun is a black body at 6000 K and the cell operates at 300 K). The use of Mn offers an additional advantage of high solubility in the semiconductor host. The concentration of Mn can exceed the critical value of about 6×10¹⁹ cm^-3, which is considered to be the threshold to inhibit non-radiative recombination and create a true intermediate band.     

Improvement of solar cell efficiencies by impurity transitions

Transitions via impurities, in addition to transitions from the valence band to the conduction band, increase the generation rate of electron–hole pairs. By the presence of impurities, however, the recombination rate is also increased. In this study, the recombination process is assumed to be entirely radiative. The probability for photon absorption in generation and for photon emission in recombination depend on the occupation probability of the impurity states. The variation of the occupation resulting in a variation of the optical properties along a current–voltage characteristic is taken into account. With a single impurity state a maximal efficiency of 46% for the AM0 spectrum is found for a band gap of 2.3 eV and an impurity level at 0.88 eV. No improvement is found for band gaps less than 1.2 eV. For two different impurity levels, transitions between these levels must be excluded, otherwise the lower level is mostly occupied and the higher level is mostly empty leading to poor optical absorption for some of the possible impurity transitions.     

Temperature and CO2 concentration measurements in the exhaust stream of a liquid-fueled combustor using dual-pump coherent anti-Stokes Raman scattering (CARS) spectroscopy

Single-shot, dual-pump coherent anti-Stokes Raman scattering (CARS) measurements of N₂ and CO₂ were performed in the exhaust stream of a swirl-stabilized JP-8-fueled combustor under sooting conditions. The combustor is designed to study particulate formation and particle-size distributions for different flame conditions and therefore is operated at near-stoichiometric overall fuel-air ratios. Various jet fuels and additive concentrations were studied. These conditions pose a significant challenge for temperature measurements using standard N₂ CARS due to strong flame emission and absorption of the CARS signal by the C₂ Swan band. With the dual-pump CARS technique employed in this study, the N₂ CARS signal is generated at a wavelength (496 nm) that is not absorbed by C₂, and concentration measurements of CO₂ can be performed. The standard deviations of the single-shot temperature measurements were approximately 3-4% of the mean values for equivalence ratios ranging from 0.4 to 1.1, whereas those of the single-shot CO₂ concentration measurements were between 9 and 20% of the mean values. Previous single-shot temperature and CO₂ concentration measurements using dual-pump CARS in this liquid-fueled combustor were limited to an equivalence ratio of 0.45, with standard deviations in temperature of about 5-6% of the mean value of 1143 K (Lucht et al., AIAA J. 41 (4) (2003) 679-686). The current study demonstrates a significant improvement in the applicability of single-shot CARS temperature and CO₂ concentration measurements to practical, swirl-stabilized combustors under sooting conditions.     

Electrical and optical characterisation of CuInS2 crystals and polycrystalline coevaporated thin films

Single crystals CuInS₂ were grown by iodine vapour transport method, whereas polycrystalline thin films were obtained by coevaporation technique from three sources. The temperature dependence of the hole mobility in valence band is analysed by taking into account contributions from several scattering mechanisms of the charge carriers. To account for the temperature dependant conductivity of polycrystalline CuInS₂ thin films, grainboundary conduction process was suggested. In the low temperature region, we interpret the data in terms of the Mott law and the analysis is very consistent with the variable range hopping. However, thermionic emission is predominant at high temperatures. Photoluminescence measurements have been performed on CuInS₂ crystals and the analysis has revealed that the emission is mainly due to free-to-bound and donor–acceptor pair transitions. The band gap of that compound is derived from the excitonic emission line at 1.53 eV.     

X-ray absorption spectroscopic study of photovoltaic solar cell materials of the type AIBIIIC2VI with special reference to CuInSe2

The K absorption discontinuity of copper has been recorded for compounds of the type A^IB^IIIC₂^VI using a Cauchois-type bent crystal X-ray spectrograph. The chemical shift in the positions of the discontinuity in the compounds relative to that in pure copper metal is found to be governed by the effective charge on the copper ion. A linear relationship between the chemical shifts and the band gaps has also been observed. This dependence is useful in determining the values of the band gaps in materials for which they are not known.It is shown that an approximate picture of the band structure of CuInSe₂ can be obtained from X-ray absorption near-edge structure studies.     

Development of a hydrogen impurity analyzer based on mobile-gas chromatography for online hydrogen fuel monitoring

Hydrogen is an excellent alternative energy source, particularly for vehicles. Despite the expansion of a considerable number of infrastructures, such as hydrogen refueling stations, there is a lack of efficient inspection methods for monitoring the hydrogen fuel quality. In this study, a hydrogen impurity analyzer (HIA) based on mobile gas chromatography with a thermal conductivity detector is developed and evaluated for the quality assurance of hydrogen fuel. Accordingly, O₂, N₂, and Ar which help in monitoring air leaks at hydrogen refueling stations, and CH₄, which can also be detected by HIA, are selected as target impurities. The HIA reached limits of detection of 2.93, 0.72, 0.84, and 1.54 μmol/mol for O₂, Ar, N₂, and CH₄, respectively. Moreover, the ISO 14687 requirements are satisfied with respective HIA expanded uncertainties of 2.6, 8.7, 8.2, and 9.4% (coverage factor k = 2). The developed system is ISO-compliant and offers enhanced mobility for online inspections.