Effect Of Electronic Excitation Research Articles

Electric properties of undoped hydrogenated amorphous silicon semiconductors irradiated with self-ions

This paper reports dark conductivity (DC), photoconductivity (PC), and Seebeck coefficient variations of undoped hydrogenated amorphous silicon semiconductors irradiated with protons and Si ions. Both the DC and PC values show nonmonotonic variations with increasing a fluence in the case of proton irradiation, whereas the monotonic decreases are observed in the case of Si ion irradiation. From results of the Seebeck coefficient variation due to proton irradiation, it is shown that the increase in DC and PC in the low fluence regime is caused by donor-center generation. Also, it is shown by analyzing the proton energy dependence and the energy deposition process that the donor-center generation is based on the electronic excitation effect. On the other hand, the decrease in DC and PC in the high fluence regime is attributed to the carrier removal effect and the carrier lifetime decrease due to the accumulation of dangling bonds, respectively. The dangling bond generation due to ion irradiation is mainly caused by the displacement damage effect and therefore it is different from the generation process in the Staebler–Wronski effect.

Modelling the effects of electronic excitations in ionic-covalent materials

High energy radiation events in ionic and covalent materials can lead to highly excited electronic configurations which, over time, relax to the ground state, either radiatively by emitting photons, or non-radiatively. Non-radiative relaxation involves the transfer of energy to the lattice and this can result in lattice heating, defect formation or even phase changes. The effects of the relaxation mechanisms on the atomic configuration are challenging to model accurately by standard methods. The situation is further complicated by interactions between electronic excitations and pre-existing defects, possibly created by other radiation events. In this paper we describe a range of mechanisms by which the electronic energy is transferred to the lattice and the resulting effects on the atomic configuration, along with the different techniques that are used to model these effects.

Role of Electronic Excitation in the Amorphization of Ge-Sb-Te Alloys

First-principles molecular dynamics simulation reveals the effects of electronic excitation in the amorphization of Ge-Sb-Te. The excitation makes the phase change an element-selective process, lowers the critical amorphization temperature considerably, for example, to below 700K at a 9% excitation, and reduces the atomic diffusion coefficient with respect to that of melt by at least 1 order of magnitude. Noticeably, the resulting structure has fewer wrong bonds and significantly increased phase-change reversibility. Our results point to a new direction in manipulating ultrafast phase-change processes with improved controllability.

Structural and optical study of irradiation effect in GaN epilayers induced by 308 MeV Xe ions

Wurtzite GaN epilayers irradiated at room temperature with 308 MeV 129Xe 35+ ions to fluences of 1 × 10 13 and 3 × 10 13 cm −2 have been studied by contact mode atomic force microscopy (AFM), high-resolution X-ray diffraction (HRXRD), micro-Raman scattering and photoluminescence (PL) spectroscopy. The AFM images showed that the surface of GaN films was etched efficiently due to the Xe ion irradiation. The initial step-terrace structure on GaN surface was eliminated completely at a fluence of 3 × 10 13 cm −2. HRXRD and Raman results indicated that the Xe ion irradiation led to a homogenous lattice expansion throughout the entire ∼3 μm-thick GaN films. The lattice expansion as well as the biaxial compressive stress of the films was increasing with the increase of ion fluence. PL measurements showed that a dominant yellow luminescence band in the as-grown GaN films disappeared, but a blue and a green luminescence bands were produced after irradiation. Based on these results, the strong electronic excitation effect of 308 MeV Xe ions in GaN is discussed.

Ion irradiation effects on the exchange bias in IrMn/Co films

The present work reports on the influence of ion irradiation in exchange-coupled bilayers. Magnetron-sputtered IrMn4/Co films were irradiated with 40 keV He+ ions and the dependence of their magnetic properties was studied as function of ion fluence and current used during the irradiations. The effects of ion damage and electronic excitation were also studied through additional irradiations with H+ and Ne+ ions. The results show a clear dependence of the exchange-bias field on the defects caused by the ion bombardment. No correlations with other irradiation effects were observed.

Excitation-Induced Low Temperature Epitaxial Growth of Ge on Si(100)

ABSTRACTThe effect of laser-induced electronic excitations on the self-assembly of Ge quantum dots (QD) on Si(100)-(2x1) grown by pulsed laser deposition is studied. The experiment was conducted in ultrahigh vacuum. A Q-switched Nd:YAG laser (wavelength λ = 1064 nm, 10 Hz repetition rate) was split into two beams; one used to ablate a Ge target while the other to electronically excite the substrate. In situ reflection high-energy electron diffraction (RHEED) and ex situ atomic force microscopy (AFM) were used to study the morphology of the grown QDs. The dependence of the QD morphology on substrate temperature and ablation and excitation laser energy density was studied. Electronic excitation is shown to affect the surface morphology. For Ge coverage of 22 monolayer, it was observed that the excitation laser reduces the epitaxial growth temperature to 250 °C, a temperature at which no epitaxy is possible without excitation. Applying the excitation laser to the substrate during the growth changes the QD morphology and island density and improves the size uniformity of QDs at 390 °C. Surface diffusion measurement calculated from RHEED recovery curves show that the excitation-laser increases the surface diffusion of the Ge atoms. A purely electronic mechanism of enhanced surface diffusion of the Ge adatoms is involved.

Ultrafast thermoelectric properties of gold under conditions of strong electron-phonon nonequilibrium

The electronic scattering rates in metals after ultrashort pulsed laser heating can be drastically different than those predicted from free electron theory. The large electron temperature achieved after ultrashort pulsed absorption and subsequent thermalization can lead to excitation of subconduction band thermal excitations of electron orbitals far below the Fermi energy. In the case of noble metals, which all have a characteristic flat d-band several electron volts well below the Fermi energy, the onset of d-band excitations has been shown to increase electron-phonon scattering rates by an order of magnitude. In this paper, we investigate the effects of these large electronic thermal excitations on the ultrafast thermoelectric transport properties of gold, a characteristic noble metal. Under conditions of strong electron-phonon nonequilibrium (relatively high electron temperatures and relatively low lattice temperatures, Te⪢TL), we find that the Wiedemann–Franz law breaks down and the Seebeck coefficient is massively enhanced. Although we perform representative calculations for Au, these results are expected to be similar for the other noble metals (Ag and Cu) due to the characteristic large d-band separation from the Fermi energy.

Ab initio-based diffusion theory and tracer diffusion in Ni–Cr and Ni–Fe alloys

In this paper, ab initio modeling is used to predict diffusion relevant thermodynamic and kinetic information for dilute Ni–Cr and Ni–Fe alloys. The modeling results are then used to determine the phenomenological coefficient matrices and the tracer diffusion coefficients for both vacancy and interstitial mediated diffusion. In addition to predicting diffusion coefficients, this ab initio-based approach provides information typically inaccessible to experiments, including the different contributions to diffusion (e.g., electronic excitation effects), the species dependence of interstitial diffusion, and the deviations from Arrhenius-type relations, which are often used to describe and extrapolate experimental diffusion data. It is found that: (1) Cr is the fastest diffusing species in Ni by both vacancy and interstitial diffusion, followed by Fe and then Ni. The enhanced diffusivity of Cr is primarily due to differences in migration barriers and binding energies, not pre-exponential factors. (2) Fe and Cr solutes in Ni have weak interactions with vacancies but Cr solutes bind strongly to interstitial defects. (3) Cr exhibits non-Arrhenius behavior in both vacancy and interstitial mediated diffusion. (4) Temperature dependent electronic contributions have a significant impact on the diffusion in some cases. (5) The vacancy diffusion mechanism in Ni–Cr changes as a function of temperature resulting in vacancy–solute drag below 460 K.

Clarification of high density electronic excitation effects on the microstructural evolution in UO 2

In order to understand the properties of ion tracks and the microstructural evolution under accumulation of ion tracks in UO 2, 100 MeV Zr 10+ and 210 MeV Xe 14+ ions irradiation examinations have been done at a tandem accelerator facility of JAEA-Tokai, and it has been observed the microstructure by means of a transmission electron microscope (TEM) and a scanning electron microscope (SEM) in CRIEPI. Comparison of the diameter of ion tracks between UO 2 and CeO 2 under irradiation with 100 MeV Zr 10+ and 210 MeV Xe 14+ ions at room temperature clarify that the sensitivity on high density electronic excitation of UO 2 is much less than that of CeO 2. By the cross-sectional observation of UO 2 under irradiation with 210 MeV Xe 14+ ions at 300 °C, elliptical changes of fabricated pores that exist till ∼6 μm depth and the formation of dislocations have been observed in the ion fluence over 5 × 10 14 ions/cm 2. The drastic changes of surface morphology and inner structure in UO 2 indicate that the overlapping of ion tracks will cause the point defects, enhance the diffusion of point defects and dislocations, and form the sub-grains at relatively low temperature.

Swift heavy ion irradiation of pyrochlore oxides: Electronic energy loss threshold for latent track formation

Pyrochlore pellets with the Gd 2(Ti 2− x Zr x )O 7 stoichiometry ( x = 0, 1 and 2) were irradiated with swift heavy ions in order to investigate the effects of electronic excitation and to determine the electronic stopping power threshold for track formation. XRD results showed that the electronic excitation induced by 870 MeV Xe and 780 MeV Kr ions leads to: (i) a crystalline–amorphous transition for Gd 2Ti 2O 7 and Gd 2TiZrO 7, (ii) a phase transition towards an anion-deficient fluorite structure (order–disorder transition) for Gd 2Zr 2O 7. Thus, zirconate pyrochlores present a better radiation resistance under swift heavy ion irradiation than titanate pyrochlores. Moreover results underline the existence of an electronic stopping power threshold around 13–14 keV/nm, below which phase transformations do not occur.

Color center formation in silica glass induced by high energy Fe and Xe ions

Silica glass samples were implanted with 1.157 GeV 56Fe and 1.755 GeV 136Xe ions to fluences range from 1 × 10 11 to 3.8 × 10 12 ions/cm 2. Virgin and irradiated samples were investigated by ultraviolet (UV) absorption from 3 to 6.4 eV and photoluminescence (PL) spectroscopy. The UV absorption investigation reveals the presence of various color centers (E′ center, non-bridging oxygen hole center (NBOHC) and ODC(II)) appearing in the irradiated samples. It is found that the concentration of all color centers increase with the increase of fluence and tend to saturation at high fluence. Furthermore the concentration of E′ center and that of NBOHC is approximately equal and both scale better with the energy deposition through processes of electronic stopping, indicating that E′ center and NBOHC are mainly produced simultaneously from the scission of strained Si–O–Si bond by electronic excitation effects in heavy ion irradiated silica glass. The PL measurement shows three emissions peaked at about 4.28 eV (α band), 3.2 eV (β band) and 2.67 eV (γ band) when excited at 5 eV. The intensities of α and γ bands increase with the increase of fluence and tend to saturation at high fluence. The intensity of β band is at its maximum in virgin silica glass and it is reduced on increasing the ions fluence. It is further confirmed that nuclear energy loss processes determine the production of α and γ bands and electronic energy loss processes determine the bleaching of β band in heavy ion irradiated silica glass.

Nonthermal laser-induced formation of crystalline Ge quantum dots on Si(100)

The effects of laser-induced electronic excitations on the self-assembly of Ge quantum dots on Si(100)-(2×1) grown by pulsed laser deposition are studied. Electronic excitations due to laser irradiation of the Si substrate and the Ge film during growth are shown to decrease the roughness of films grown at a substrate temperature of ∼120 °C. At this temperature, the grown films are nonepitaxial. Electronic excitation results in the formation of an epitaxial wetting layer and crystalline Ge quantum dots at ∼260 °C, a temperature at which no crystalline quantum dots form without excitation under the same deposition conditions.

Radiation effects in yttria-stabilized zirconia: Comparison between nuclear and electronic processes

Yttria-stabilized zirconia (YSZ) single crystals were irradiated with heavy ions at energies ranging from a few MeV to a few GeV in order to compare the effects of nuclear collisions and electronic excitations. The amount of damage created by irradiation was measured as a function of the ion fluence by Rutherford backscattering and channeling spectrometry (RBS/C). Monte-Carlo simulations of RBS/C data show that both the depth distributions of radiation damage and the damage build-up strongly depend on the type of irradiation. At low energy (4MeV Au ions) the damage exhibits a peak around the ion projected range (∼500nm) and it accumulates with a double-step process; at high energy (940MeV Pb ions) the damage profiles are rather flat up to several micrometers and the damage accumulation is monotonous (one-step). RBS/C and X-ray diffraction results show that the various irradiations never lead to total disordering of irradiated layers (amorphization), even at the highest fluences used in the experiments.

Production and stability of radiation-induced defects in MgAl2O4 under electronic excitation

This paper investigates the formation process of radiation-induced defects in magnesium aluminate spinel and their stability using transmission electron microscopy, with emphasis on the effects of electronic excitation. Small interstitial-type dislocation loops disappeared under electron-induced electronic excitation. The elimination rate of the loops was found to be one order higher than for α-alumina. The disappearance of dislocation loops by a dissociation mechanism into isolated interstitials is discussed through analysis of the growth-and-shrink process of the loops. HARECXS analysis on cross section specimens irradiated with 350MeV Au ions has shown the progress of cation disordering along ion tracks to be a function of electronic stopping power, (dE/dx)e. Cations were found to exchange their sites toward a random configuration. Such disordering appears from (dE/dx)e=10keV/nm, and increases in size with increasing (dE/dx)e to reach nearly 10nm in diameter at 30keV/nm, under an assumption of a fully disordered configuration.

Heavy-ion irradiation of pyrochlore oxides: Comparison between low and high energy regimes

Pyrochlore pellets with Gd2(Ti2−xZrx)O7 stoichiometry were irradiated with heavy ions at energies ranging from a few MeV to a few GeV in order to compare the effects of nuclear collisions and electronic excitations. The damage created by irradiation was characterized as a function of the ion fluence by X-ray diffraction. The structural modifications induced by irradiation were shown to depend on both the sample composition and the type of irradiation. At low energy (4MeV Au ions), the susceptibility to radiation-induced amorphization exhibits a systematic decrease with increasing Zr content. At high energy (1.5GeV Xe or 2.6GeV U ions), similar structural changes are observed at much lower fluences. The lattice parameter increases for low energy irradiation, particularly in the case of amorphizable pyrochlores (Gd2Ti2O7 and Gd2TiZrO7), whereas it decreases for high energy irradiation.

Ab initioinvestigation of boron diffusion paths in germanium

Experimental data indicate that boron diffuses very differently in Ge than in Si. To examine the kinetics of boron diffusion, density functional calculations were performed on a variety of boron diffusion mechanisms, including interstitial and vacancy-mediated paths, as well as a correlated exchange mechanism. It was found that although vacancy and correlated exchange mechanisms possess high diffusion barriers comparable with experiment, the barrier for interstitial-mediated diffusion lies around $3.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and is similar to those found for boron diffusion in Si. This estimate is well below the experimental activation energy. The difference is attributed to the failure of the theory to include the effect of electronic excitations.

Swift heavy ion-induced swelling and damage in yttria-stabilized zirconia

Yttria-stabilized zirconia single crystals were irradiated with a variety of different swift heavy ions (from 145MeV C to 2.6GeV U) in a broad fluence range (between 1011 and 1015cm−2). Surface profilometry measurements show that the out-of-plane expansion of the samples increases versus ion fluence up to an asymptotic value of about 0.2%; the larger the ion mass, the smaller the fluence needed to reach saturation. Rutherford backscattering spectrometry data on irradiated crystals under channeling conditions give clear evidence of lattice damage creation for ions heavier than Br corresponding to a threshold electronic stopping power about 20keVnm−1. In contrast, no threshold is found for the swelling data which scale fairly well with the number of displacements per atom (except for the irradiation with U ions). In all cases we find a linear increase of the out-of-plane expansion versus the concentration of F+-type centers (singly ionized oxygen vacancies) measured by electron paramagnetic resonance spectroscopy in as-received and thermochemically reduced samples, regardless of crystal orientation. The nature of the lattice disorder and the respective effects of elastic collisions and electronic excitations on the ion-induced volume expansion and damage are discussed.

Effect of high electronic excitation in swift heavy ion irradiated semiconductors

The damage evolution due to high electronic energy deposition during 375MeV Xe and 593MeV Au ion irradiation was studied in the semiconductors InP, GaP, GaAs, AlAs and Ge using RBS and TEM. In InP the high electronic energy deposition leads to the formation of amorphous tracks. In the other materials no effect of the electronic energy deposition on damage formation was found. The TEM investigations show that in InP each impinging ion produces a track. With the use of the extended thermal spike model lattice temperatures and maximum radii of the molten zones were calculated for InP and Ge. Whereas in InP the lattice temperature exceeds the melting temperature confirming the observed formation of amorphous tracks, this is not the case for Ge. The calculated radii of the molten zones in InP are in good agreement with the track radii determined from the TEM images.

Study on Effects of Heavy Ion Irradiation on CeO2 by Using Synchrotron Radiation X-Ray Absorption Spectroscopy -as a Simulation Study for Radiation Damage in High-Burnup Light Water Reactor Fuels-

AbstractCeO2 thin films were irradiated with 200MeV Xe ions. Effects of the irradiation were studied by using Extended X-ray Absorption Fine Structure (EXAFS) measurement at SPring8 synchrotron radiation facility. EXAFS spectra for the irradiated thin films near the Ce K-edge show that the coordination number for oxygen atoms around Ce atom decreases and that the Ce-O Debye-Waller factor increases by the irradiation. The atomic distance between oxygen atom and Ce atom does not vary within the accuracy of EXAFS measurement. The effect of high density electronic excitation on the structure of CeO2 is discussed.

Ferromagnetism induced by heavy-ion irradiation in fullerene films

Magnetic measurements using a superconducting quantum interference device and magnetic force microscopy were performed on fullerene films irradiated with $250\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ Ar and $92\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ Si ions, to compare the effects of electronic excitation and collisional cascade on the magnetization. A ferromagnetic behavior increasing with ion fluence is observed. The magnetization is attributed to (i) the formation of an amorphous carbon network and (ii) the incorporation of oxygen in the irradiated films.