eV Activation Energy Research Articles

Interpretation of negative temperature dependence of hydriding reaction in LaNi5-Mg alloys by modified Chou model

A modified Chou model with parameters of temperature (T), initial hydrogen pressure (P0), enthalpy change (ΔH) and entropy change (ΔS) is proposed in consideration of thermodynamic driving force. The meaningful activation energy Ev can be calculated for the negative temperature dependence of hydriding reaction in hydrogen storage materials. This model is verified by the hydriding/dehydriding experimental data of (LaNi5)1−xMgx (x=0, 0.018, 0.041, 0.063). The calculated results indicate that the hydriding kinetics of (LaNi5)1−xMgx alloys are controlled by diffusion of hydrogen atom in hydride. The addition of small amount of magnesium improves the kinetic performance of LaNi5 slightly. Furthermore, the modified Chou model was compared with the previous model that successfully explain the negative temperature depending hydrogenation reaction of Mg-based alloys. The fitting results show that the modified Chou model can describe the hydrogen absorption kinetic mechanism of negative temperature dependent materials more accurately.

Thermal stability of simple tetragonal and hexagonal diamond germanium

Exotic phases of germanium, that form under high pressure but persist under ambient conditions, are of technological interest due to their unique optical and electrical properties. The thermal evolution and stability of two of these exotic Ge phases, the simple tetragonal (st12) and hexagonal diamond (hd) phases, are investigated in detail. These metastable phases, formed by high pressure decompression in either a diamond anvil cell or by nanoindentation, are annealed at temperatures ranging from 280 to 320 °C for st12-Ge and 200 to 550 °C for hd-Ge. In both cases, the exotic phases originated from entirely pure Ge precursor materials. Raman microspectroscopy is used to monitor the phase changes ex situ following annealing. Our results show that hd-Ge synthesized via a pure form of a-Ge first undergoes a subtle change in structure and then an irreversible phase transformation to dc-Ge with an activation energy of (4.3 ± 0.2) eV at higher temperatures. St12-Ge was found to transform to dc-Ge with an activation energy of (1.44 ± 0.08) eV. Taken together with results from previous studies, this study allows for intriguing comparisons with silicon and suggests promising technological applications.

Evolution of on-resistance (RON) and threshold voltage (VTH) in GaN HEMTs during switch-mode operation

The evaluation and investigation of electrical parameters drifts during device operation is one of the mandatory task that has to be performed in order to improve GaN-power devices stability. In this work the authors would like to present a novel characterization method that allows the simultaneous monitoring of both RON and VTH during the switch-mode operation. By applying the proposed measurement method with different operating parameters it has been possible to provide a preliminary interpretation of the physical mechanisms leading to the observed RON increase and VTH positive shift. In particular, the observed RON increase, which is thermally activated with a 0.83eV activation energy, has been ascribed to the presence of Carbon-related traps within the device buffer layer. On the other hand, the positive VTH shift has instead been related to interface defects at the dielectric/III–N interface and/or to bulk traps in the gate dielectric.

Light-induced lifetime degradation in high-performance multicrystalline silicon: Detailed kinetics of the defect activation

We examine the defect activation kinetics in block-cast high-performance multicrystalline silicon (HP mc-Si) under illumination at elevated temperature. Our lifetime analysis shows that the observed light-induced lifetime degradation consists of two separate stages: a fast stage followed by a slow stage. Our experiments reveal that both degradation stages can be fitted using a sum of two exponential decay functions. The resulting degradation rate constants depend both on the temperature and the light intensity applied during degradation. For the fast component, we determine an activation energy of (0.89 ± 0.04)eV from an Arrhenius plot of the degradation rate and for the slow component we determine a value of (0.94 ± 0.06)eV. The activation energies are relatively large, leading to a very pronounced dependence of the degradation rates on temperature. We also observe that both degradation rates show a linear dependence on the applied light intensity during degradation in the examined intensity range between 0.25 and 1.5 suns.

Modifying Surface Chemistry of Metal Oxides for Boosting Dissolution Kinetics in Water by Liquid Cell Electron Microscopy.

Dissolution of metal oxides is fundamentally important for understanding mineral evolution and micromachining oxide functional materials. In general, dissolution of metal oxides is a slow and inefficient chemical reaction. Here, by introducing oxygen deficiencies to modify the surface chemistry of oxides, we can boost the dissolution kinetics of metal oxides in water, as in situ demonstrated in a liquid environmental transmission electron microscope (LETEM). The dissolution rate constant significantly increases by 16-19 orders of magnitude, equivalent to a reduction of 0.97-1.11 eV in activation energy, as compared with the normal dissolution in acid. It is evidenced from the high-resolution TEM imaging, electron energy loss spectra, and first-principle calculations where the dissolution route of metal oxides is dynamically changed by local interoperability between altered water chemistry and surface oxygen deficiencies via electron radiolysis. This discovery inspires the development of a highly efficient electron lithography method for metal oxide films in ecofriendly water, which offers an advanced technique for nanodevice fabrication.

Static and Dynamic Disorder in Metastable Phases of Tin Oxide

The multistage transformation of Sn3O4 to SnO2 in an oxygen-free atmosphere has been studied using hydrothermally synthesized samples, both pristine and heated, with in situ X-ray diffractometry, Raman spectroscopy, differential scanning calorimetry, dielectric spectroscopy, and ex situ X-ray photoelectron and nuclear magnetic resonance spectroscopies. Experimental evidence consistently shows that, beginning from 250 °C, Sn3O4 converts to Sn2O3 and, from 350 °C, to SnO2, accompanied by the formation of metallic tin. These two stages are associated with frequency-dependent dielectric losses, as revealed by impedance measurements, which also indicate a significant conductivity of 1.78 eV in activation energy. It is speculated that the relaxation-like processes relate to the transition from static to dynamic disorder and then to order in the oxygen sublattice.

Reduced deuterium retention in simultaneously damaged and annealed tungsten

Deuterium (D) retention in polycrystalline tungsten (W) with copper (Cu) ion damage concurrently produced at elevated surface temperature is investigated. An in situ heated stage held W samples at a controlled temperature up to 1243 K, which were subjected to displacement damage produced by 3.4 MeV Cu ions. D retention is subsequently explored by exposure of the W samples held at 383 K to a D2 plasma ion fluence of 1024 D+/m2. Nuclear reaction analysis (NRA), utilizing the D(3He,p)4He nuclear reaction, is used to probe the D concentration in the near surface up to 6 μm. Thermal desorption spectroscopy (TDS) is used to measure outgassed HD and D2 molecules to determine the bulk D concentration. Both NRA and TDS measure a significant reduction in D retention for samples damaged at elevated temperature. TDS quantitatively shows that the lowest energy trap remains largely unaffected while higher energy traps, induced by Cu ions, are annealed and approach intrinsic concentrations as the temperature during ion damage approaches 1243 K. Analysis of TDS data yields an activation energy of (0.10 ± 0.02) eV for recovery of ion-damage induced traps at elevated temperature.

Reduction of interface traps between poly-Si and SiO2 layers through the dielectric recovery effect during delayed pulse bias stress

We investigate the interface trap behavior between tunneling oxide and poly-Si channel layer post erase/write cycling with a delayed pulse by using deep level transient spectroscopy. For comparison of the defect states depending on the stress pulses, a Schottky and a metal–oxide semiconductor device were fabricated. A defect state at about Ec −0.51 eV in the Schottky device was measured before the annealing process. Three-hole trap states with activation energies of Ev +0.28 eV, Ev +0.53 eV, and Ev +0.76 eV appeared after the post-annealing process. The electron trap was about Ec −0.15 eV after erase/write 3000 cycling was applied at ±10 V for 100 ms at 25 °C and 85 °C. These defect states may have an effect on the charge loss behavior of the electrons localized in the charge trap layer at the retention mode of three-dimensional non-volatile memory devices. Dramatically, after the endurance stress was applied with a delayed pulse of 300 cycling at 85 °C for 50.4 h, no interface traps of the deep level transient spectroscopy spectra appeared. Dielectric recovery can decrease the density of the interface trap and improve the retention properties. This may have been caused by the passivation effect on the dangling bond of the interface traps.

Erbium diffusion in titanium dioxide

The diffusivity of erbium in the anatase phase of titanium dioxide (TiO2) has been studied for various temperatures ranging from 800 °C to 1, 000 °C. Samples of TiO2, with a 10 nm thick buried layer containing 0.5 at% erbium, were fabricated by radio-frequency magnetron sputtering and subsequently heat treated. The erbium concentration profiles were measured by secondary ion mass spectrometry, allowing for determination of the temperature-dependent diffusion coefficients. These were found to follow an Arrhenius law with an activation energy of (2.1±0.2) eV. X-ray diffraction revealed that the TiO2 films consisted of polycrystalline grains of size ≈ 100 nm.

Electrical characterization of a graphite-diamond-graphite junction fabricated by MeV carbon implantation

The Deep Ion Beam Lithography technique has been extensively adopted in recent years for the fabrication of graphitic electrodes in bulk diamond with a wide range of technological applications. Particularly, it has been recently shown that a high current can be driven in devices consisting of micrometer-spaced sub-superficial graphitic electrodes. This effect has been exploited to stimulate electroluminescence from color centers placed in the active region of the device.A deep understanding of the conduction mechanisms governing charge transport in micro-regions of defective diamond comprised between graphitic electrodes is necessary in order to fully exploit the functionality of these opto-electronic devices, as well as to assess the ion-beam-micromachining of diamond as a convenient technique for the fabrication of solid-state micro-devices.In this work, a temperature-dependent characterization of the electrical properties of a sub-superficial graphite-diamond-graphite junction is presented and discussed. The ohmic behavior observed at low bias voltages is ascribed to a donor level with an activation energy of (0.217±0.002) eV, a value compatible with previous reports on nitrogen-related defects. A transition to a high-current regime above a critical voltage VC was also observed, and interpreted in terms of the Space-Charge-Limited Current model. The temperature-dependent measurements allowed to investigate the role of charge trapping in the charge injection mechanism of the junction. By fitting the temperature dependence in the high-current regime it was possible to determine the relevant trap level of the associated Poole-Frenkel mechanism, leading to a value of (0.278±0.001) eV from the conduction band. The Poole-Frenkel conduction model in high-current regime enabled also a preliminary investigation in the effects of ion implantation on the modification of the dc dielectric constant of diamond.

Structural, electrical, band alignment and charge trapping analysis of nitrogen-annealed Pt/HfO2/p-Si (100) MIS devices

Low leakage current density and high relative permittivity (dielectric constant) are the key factor in order to replace the SiO2 from Si-based technology toward its further downscaling. HfO2 thin films received significant attention due to its excellent optoelectronic properties. In this work, ultra-thin (17 nm) HfO2 films on Si substrate are fabricated by RF sputtering. As deposited films are amorphous in nature and in order to get the reasonable high dielectric constant, the films are annealed (700 degrees C, 30 min) in nitrogen environment. A high refractive index (2.08) and small grain size (similar to 10) nm were extracted from ellipsometry and XRD, respectively. The AFM study revealed a small RMS surface roughness 9 angstrom. For electrical characterization, films are integrated in metal-insulator-semiconductor capacitors structure. The oxide capacitance (C-ox), flat band capacitance (C-FB), flat band voltage (V-FB), and oxide-trapped charges (Q(ot)) calculated from high-frequency (1 MHz) C-V curve are 490, 241 pF, 1.21 V and 1.8 x 10(12) cm(-2), respectively. The dielectric constant calculated from accumulation capacitance is 17. The films show a low leakage current density 6.8 x 10(-9) A/cm(2) at +1 V, and this is due to the reduction in oxygen vacancies concentration as we performed annealing in N-2 environment. The band gap of the films is estimated from O 1s loss spectra and found 5.7 eV. The electron affinity (chi) and HfO2/Si barrier height (conduction band offset) extracted from UPS spectra are 1.88 and 2.17 eV, respectively. A trap state with 0.99 eV activation energy below the conduction band edge is found and assigned to the fourfold coordinated oxygen vacancy in m-HfO

On the solid phase crystallization of In2O3:H transparent conductive oxide films prepared by atomic layer deposition

Hydrogen-doped indium oxide (In2O3:H) has emerged as a highly transparent and conductive oxide, finding its application in a multitude of optoelectronic devices. Recently, we have reported on an atomic layer deposition (ALD) process to prepare high quality In2O3:H. This process consists of ALD of In2O3:H films at 100 °C, followed by a solid phase crystallization step at 150–200 °C. In this work, we report on a detailed electron microscopy study of this crystallization process which reveals new insights into the crucial aspects for achieving the large grain size and associated excellent properties of the material. The key finding is that the best optoelectronic properties are obtained by preparing the films at the lowest possible temperature prior to post-deposition annealing. Electron microscopy imaging shows that such films are mostly amorphous, but feature a very low density of embedded crystallites. Upon post-deposition annealing, crystallization proceeds merely from isotropic crystal grain growth of these embedded crystallites rather than by the formation of additional crystallites. The relatively high hydrogen content of 4.2 at. % in these films is thought to cause the absence of additional nucleation, thereby rendering the final grain size and optoelectronic properties solely dependent on the density of embedded crystallites. The temperature-dependent grain growth rate has been determined, from which an activation energy of (1.39 ± 0.04) eV has been extracted. Finally, on the basis of the observed crystallization mechanism, a simple model to fully describe the crystallization process has been developed. This model has been validated with a numerical implementation thereof, which accurately predicts the observed temperature-dependent crystallization behaviour.

Effect of depth of traps in ZnO polycrystalline thin films on ZnO-TFTs performance

ZnO thin films were processed by radio frequency magnetron sputtering at room temperature on p-Si/SiO2 substrates under pure argon (Ar:O2=100:0vol.%) and argon–oxygen mixture (Ar:O2=99:1vol.%) gas environment. Morphological, optical and electrical characteristics of the ZnO films are reported, and they show a clear relationship with the gas mixture employed for the sputtering process. Scanning Electron Microscopy revealed the formation of grains of 15.3 and 19.9nm average sizes and thicknesses of 59nm and 82nm for films growth in pure argon and argon–oxygen, respectively. Photoluminescence measurements at room temperature showed the violet emission band (centered at 3eV) which was only detected in the ZnO film grown under pure argon. From thermally stimulated conductivity measurements two traps with 0.27 and 0.14eV activation energies were identified for films grown in pure argon and argon–oxygen mixture, respectively. The trap at 0.27eV is associated with a level located below the conduction band edge and it is supported by the PL band centered at 3eV. Both types of ZnO films were used as the active channel layer in thin film transistors with thermal SiO2 as gate dielectric. Field effect mobility, threshold voltage and current ratio were improved in the devices with ZnO channel deposited with the argon–oxygen mixture (99% Ar/1% O2 vol.). Threshold voltage decreased from 25V to 15V, field effect mobility and current ratio increased from 0.8 to 2.4cm2/Vs and from 102 to 106, in that order.

Low frequency damping behavior associated with sintering process in Al powder compact

The internal friction behavior of Al green power compact duxing the sintering process was studied as a function of temperature. The internal friction measurements were performed from room temperature to 600 °C. Two typical internal friction peaks were detected corresponding to heating and cooling processes, respectively. The heating peak corresponds to a recrystallization process of deformed Al particles, which is influenced by many extrinsic parameters, such as measuring frequency, strain amplitude, heating rate, power particle size and compacting pressure. However, the intrinsic nature of the peak is originated from the micro-sliding of the weak-bonding interfaces between Al particles and increased dislocation density induced in compressing. The cooling peak with the activation energy of (1.64±0.06) eV is associated with the grain boundary relaxation, which can be interpreted as the viscous sliding of grain boundaries. The similar phenomena are also found in the Mg green powder compact.

Synthesis, characterization and interpretation of screen-printed nanocrystalline CdO thick film for optoelectronic applications

The transparent conductive oxide CdO thick films are prepared by screen printing method followed by sintering route. The structural, optical and electrical properties of as-grown films are characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared (FTIR), ultraviolet–visible (UV–VIS) and DC conductivity measurement techniques. XRD, SEM and AFM studies reveal that the film deposited is polycrystalline, single phase and granular in nature. The crystallite size from XRD pattern’s most strong (111) peak calculated using Debye–Scherrer’s formula is 30 nm. IR transmission spectrum exhibits Cd–O stretching and bending mode peaks at 831 and 672 cm−1, respectively. Electrical properties were characterized by two-probe measurement in the temperature range between 300 and 400 K. Optical transmission spectroscopy and DC conductivity measurements have revealed the semiconducting nature of this film with direct band gap energy 2.53 and 0.29 eV activation energy and dc conductivity 4.5 × 10−7 (’Ωcm) −1.

Self-Diffusion in Amorphous Silicon.

The present Letter reports on self-diffusion in amorphous silicon. Experiments were done on ^{29}Si/^{nat}Si heterostructures using neutron reflectometry and secondary ion mass spectrometry. The diffusivities follow the Arrhenius law in the temperature range between 550 and 700 °C with an activation energy of (4.4±0.3) eV. In comparison with single crystalline silicon the diffusivities are tremendously higher by 5 orders of magnitude at about 700 °C, which can be interpreted as the consequence of a high diffusion entropy.

Defect distributions in Cu(In,Ga)(S,Se)2 solar cells with three different buffer layers

Defect levels and interface states in ZnO:B/buffer/Cu(In,Ga)(S,Se)2/Mo/glass solar cells with a Zn(S,O)/CdS, CdS, or Zn(S,O) buffer layer were investigated by capacitance–voltage (C–V) and admittance spectroscopy. Two acceptor levels, A1 (with Ea,A1 = 0.09±0.01 eV activation energy) and A2 (Ea,A2 = 0.26±0.02 eV) were observed from the temperature dependence of the C–V profiles. Spatial distributions of these acceptors were determined by the bias dependence of the profiles. In addition, two kinds of defect signals, S1 (Ea,S1=0.06 eV and 0.07 eV) and S2 (Ea,S2=0.25 eV), were detected from admittance spectroscopy. Each solar cell exhibited only one signal: S1 in Zn(S,O)/CdS- or Zn(S,O)-buffered solar cells, and S2 in solar cell with a CdS buffer layer. These results demonstrate that buffer layers influence the spatial distributions of intrinsic defect states in the region of the Cu(In,Ga) (S,Se)2 absorber close to the hetero-interface.

Phase transition of nanocrystalline–amorphous Sm10.9Co66.5Fe22.6 alloy: Nucleation, growth kinetics and atomic process

The nucleation, growth kinetics and atomic process of the preexisting α-(Fe,Co) and newly-formed SmCo7 nanocrystals in the annealed nanocrystalline-amorphous Sm10.9Co66.5Fe22.6 alloy have been investigated. The preexisting α-(Fe,Co) phase possesses a growth activation energy of Eg = 0.6 eV, smaller than the nucleation activation energy En = 0.8 eV and growth activation energy Eg = 1.4 eV for the newly-formed SmCo7 phase. This demonstrates an easy growth of the α-(Fe,Co) nanocrystals and a relatively difficult formation of the SmCo7 phase, which leads to a large grain size for the α-(Fe,Co) soft phase in the α-(Fe,Co)/SmCo7 nanocomposite magnets. The activation volumes ΔVg∗ = 0.67 Ω and ΔVg∗ = 0.99Ω (Ω = average atomic volume) for the growth of α-(Fe,Co) and SmCo7 nanocrystals were obtained, indicating that the atom diffusion during the growth of both the α-(Fe,Co) and the SmCo7 are assisted by thermal vacancies. These findings provide a fundamental understanding to the crystallization of the SmCo based nanocomposite magnets.

Charge compensating defects study of YbF‐doped BaF crystals using dielectric loss

AbstractYbF‐doped BaF crystals with various concentrations of YbF (0.05, 0.1, 0.17, 0.2 mol) have been grown using the conventional Bridgman method. Dielectric relaxation is used to study the charge compensating defects in BaF crystals with low YbF concentration. The optical absorption spectra reveal the existence of both Yb (in UV domain) and Yb (in IR domain) ions. In the investigated temperature range (150–320 K) we have observed only one type of dielectric relaxation. This relaxation with activation energy eV is associated with trigonal‐type () centers. The charge compensating defects are discussed while taking also into account the optical absorption spectra and the calculated number of NNN dipoles whose relaxation is observed.

Radiotracer diffusion of 114mIn in Cu(In,Ga)Se2 thin films

Diffusion of 114mIn in polycrystalline Cu(In0.7,Ga0.3)Se2 thin films has been investigated by measuring radiotracer activity depth profiles by means of ion sputtering. The measurements were carried out in the temperature range between 523K and 723K. The obtained diffusivities vary between 7.3×10−17cm2s−1 at 523K and 1.4×10−12cm2s−1 at 723K with an activation energy of (1.42±0.09) eV. The grain boundary diffusivities in this temperature range vary between 1.0×10−12cm2s−1 at 523K and 2.9×10−8cm2s−1 at 723K and a first order-of-magnitude estimation of the activation energy for the grain boundary diffusion yields a value of (1.04±0.18) eV.