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Ultrafast Optical Measurements of Defect Creation in Laser Irradiated SiO2
Guillaume Petite, Philippe Daguzan, Stéphane Guizard, Philippe Martin
To cite this version:
Guillaume Petite, Philippe Daguzan, Stéphane Guizard, Philippe Martin. Ultrafast Optical Measure- ments of Defect Creation in Laser Irradiated SiO2. Journal de Physique III, EDP Sciences, 1996, 6 (12), pp.1647-1676. �10.1051/jp3:1996102�. �jpa-00249549�
Ultrafast Optical Measurements of Defect Creation in Laser Irradiated Si02
Guillaume Petite (*), Philippe Daguzan, St6phane Guizard and Philippe Martin
CEA/DSM/DRECAM, Service de Recherche sur les Surfaces et l'Irradiation de la Matibre, CEN Saclay, 91191 Gif-sur-Yvette Cedex, France
(Received 21 December 1996, revised 12 March 1996, accepted 19 March 1996)
PACS.71.38.+I Polarons and electron-phonon interactions
PACS.72.20.Jv Charge carriers: generation, recombination, lifetime and trapping PACS.78.47.+p Time-resolved optical spectroscopies and other ultrafast optical
measurements in condensed matter
PACS.79.20.Ds Laser-light impact phenomena
Abstract. Optical methods using sub-picosecond laser pulses allow to study the kinetic of defect creation in Si02, caused by an intense electronic excitation. A first intense "pump" pulse
is used to create a high density (up to 10~~ cm~~) of e-h pair- A second, weaker pulse is then
used to probe the state of the material after an adjustable delay, with a time resolution of the order of10~~~
s. A first investigation using photoelectron spectroscopy shows that the electrons
can reach kinetic energies in the conduction band in large excess of the photon energy, through three-body electron-photon-phonon transitions (a sequential absorption process). "Transient
Frequential Interferometry" is used to measure the instantaneous refractive index, I-e- the free carrier density (conduction electrons), and to confirm the existence of the absorption by
conduction electrons. Transient absorption can be used to monitor the appearance of point defects following the trapping of the free carriers. We show that, contrary to what is observed in other oxides (A1203 and MgO), the trapping process is extremely fast (150 fs), and occurs at all temperatures in the triplet state of the Self Trapped Exciton (STE). A permanent absorption is shown to appear at room temperature only, resulting from the thermal conversion of STE into colored centres. Finally, we study from a theoretical point of view the transport of conduction
electrons with help of two different methods: Monte-Carlo simulations, which allow to introduce in a convenient way the efTect of the laser field, and solving the time-evolution of the density
matrix equations, a more exact treatment in principle required in Si02 because of the strong electron-phonon coupling, but which does not yet allow to include the effect of a strong laser field.
Introduction
It is no wonder that the behaviour of wide-bandgap dielectrics under strong laser irradiation has been the subject of a constant interest from the community of the laser users. Among the
many practical uses of such materials, a very important one is that of optical material since
they are transparent to visible radiation. However, because of the constant increase of the
(*) Author for correspondence (e-mail: g.petite@cea,fr)
© Les #ditions de Physique 1996
commonly available laser intensities (TW cm~~ are nowadays routinely achieved with table- top laser systems), even the purest wide-bandgap (transparent) material can be coupled to any
visible radiation, through non-linear processes, provided its intensity is large enough, and this will affect its properties in many different ways: non-linear contributions to the refractive index will make the beam propagation (self-focusing), as well as its spectrum (self-phase modulation), intensity dependent and non-linear absorption of energy1&~ill eventually lead to the destruction of the material.
In this work, we would like to understand in detail how is the energy absorbed by the material, by which mechanisms this energy, primarily absorbed by the electrons, is redistributed among the different degrees of liberty of the lattice, and in which way, and on which time scales,
are the material's properties (optical, mechanical, affected. Most of the basic concepts necessary to describe this intricate interaction, and among them all those pertaining to the electron-phonon coupling, have been at hand for some times, but their use in the context of materials whose electronic structure itself is sometimes only roughly known is still a matter of debate. It is fair to say that a lot of the physics developed during the 80's to describe the interaction of laser light with semiconductors has to be seriously re,>isited and that, very often, the models found to be adequate in this latter case fail to represent the reality of the
interaction with wide-bandgap materials.
One of the recent breakthroughs in laser technology consists in the possibility of obtaining high intensity light in pulses 1&~hose duration can be made as short as a few tens of femtoseconds, and this throughout the whole near IR, visible and near UV range. This offers a unique possibility of obtaining an insight into the dynamics of the energy exchanges in the material.
Indeed, the time constant associated with the energy transfer between the electrons and the lattice, governed by the electron-phonon coupling, is of the order of the picosecond (and as
we will see, sometimes less). Using high intensity sub-picosecond pulses, a number of so-called
"pump/probe" experiments can be thought of, where a first intense laser pulse is used to induce
instantaneously a strong electronic perturbation of the material, which is then probed after an
adjustable delay, with a second (weaker) laser pulse. A number of classical optical methods
can be used as a probe, which almost all measure the instantaneous refractive index of the material (real or imaginary part) Another possibility is of course a direct observation of the electrons through photoemission techniques. With such experiments, it is possible to obtain a time resolved information on processes which, when studying the effect of more classical, e-g.
nanosecond, laser pulses are used, have to be integrated over the pulse: the later part of the pulse acts on a material whose properties have evolved as the result of the interaction with the earlier parts of the pulse. Therefore, ultrashort laser pulses can bring about an information which is qualitatively different from that obtained with more conventional ones, in the sense that they give access to the intrinsic behaviour of the material.
Intense sub-picosecond lasers do not only serve the purpose of understanding their own effect
on optical materials, but also allow to simulate the effect of practically any irradiation which
proceeds through deposition of electronic energy, that 18 to say ionizing radiations. Indeed, if such radiations usually initially proceed by excitation of core electrons (rather than valence ones, like lasers), after a time of the order of a few 10~~~ s, due to the interplay of secondary ionizations, Auger relaxation, the electronic state of the irradiated matter resembles very much the one obtained with laser irradiation: a more or less dense electron-hole plasma involving
conduction electrons, mostly at the bottom of the conduction band, and valence holes. The excitation densities which can be achieved with table-top laser systems are in the range of those resulting from even the most intense ionizing radiations (highly stripped heavy ions in the GeV energy range), producing for instance solid state density plasmas 1&~ith temperatures well o,>er
the kev range. Even if the electronic states reached with these different irradiations are not
the same, they share the fact that they are reached in a time short enough that the lattice has not begun to react, that we referred to above as "instantaneously"- It so appears that the informations obtained with lasers concerning for instance the creation of point defects by
electronic excitation will be useful in understanding the general problem of irradiation defects,
and particularly its dynamical aspects. There, lasers possess so far an indisputable advantage
due to the possibility of precisely timing two laser pulses, and to the large variety of optical methods available to study the electronic state of matter.
Three different methods, yielding complementary informations, will be presented in this paper. The first one consists in observing directly the photoexcited electrons, or at least those which have been excited above the vacuum level, leading to photoemission- One can obtain this way informations concerning the energy deposition mechanisms, as well as a picture of the electronic state following immediately the excitation. Optical probing of the perturbed
material can also be achieved with the use of interferometric or absorption measurements. As
we will see, transient interferometry experiments allow to study the free-carrier dynamics, and
more precisely the trapping of electronic excitation. This localization of electronic excitation
can be at the origin of point defect creation, which can be probed using their transient (or permanent) optical absorption. In principle, these three methods offer an almost complete
description of the different stages of the material's evolution: excitation, electronic relaxation, lattice relaxation.
The materials concerned in this paper will be mainly the wide-bandgap oxides, and more
precisely Si02, A1203, MgO- This is of course because of their practical importance, but
also because they are a representative sample of the iono-covalent character shared by such materials. However, we will also mention other work performed on alkali halides, an other class of wide-bandgap dielectrics whose properties are in many respect similar to those of oxides, who are subject to the same type of processes, and whose electronic structure (and particularly
that of their point defects) is better known. We will essentially consider the case of pure bulk material, which will reveal itself complicated enough not to address the more difficult issues of the surface of interface processes.
Because of its particular importance in many technologies, there has been a rather large
number of studies on irradiation defects in Si02 (1-19j- The E( centre is probably the most
important of these defects, and it has been extensively studied both experimentally and the-
oretically. It involves one electron located at an oxygen vacancy, and is characterized by an optical absorption band at 5-8 eV and an anisotropic Electron Paramagnetic Resonance (EPR)
spectrum iii. An atomic configuration was proposed, based on a linear combination of molec- ular orbitals method [2], in which the electron is localized predominantly in an sp~ orbital centered at one of the silicon atoms neighbouring the vacanc», the other silicon undergoing a highly asymmetric relaxation bringing it in a planar configuration with its backbonded oxygens- It was later on shown that the "pluckered" configuration (where the silicon relaxes through the
oxygen plane) was in fact more stable [3]. Quite comprehensive reviews of the paramagnetic defects in crystalline quartz can be found in references [4, 5]. E( centres have been shown to result not only from high energy electron (> 200 kev) irradiation as in iii, but also from U-V- laser irradiation [6,7], or X-ray irradiation [8]. More recently, transient defects resulting from the same type of irradiation were observed [9-11]. In addition to a transient luminescence at 2-8 eV observed in [9], it was shown that this defect, known as the Self-Trapped-Exciton (STE) was associated with a transient volume change as well as a transient absorption line at 5-2 eV [10j. Luminescence, volume change and absorption have a lifetime of1 ms at low temperatures, and are rapidly quenched for temperatures above 150 K- Excitation spectra of the 2-8 eV luminescence using VUV radiation were measured in [11], showing that it is di-
rectly associated with the excitation of electron-hole pairs, eventually in a bound exciton state.
Several models have been proposed concerning the atomic structure of the STE in Si02. Let
us just mention the most recent one, widely accepted now, which is due to Fischer et al. [12].
Using an ab-initio Hartree-Fock cluster calculation, they show that trapping of a hole on an oxygen and of an electron on a neighbouring silicon produces a displacement of the oxygen atom almost along the hexagonal axis- A calculation of the electronic structure corresponding
to such a lattice relaxation was performed by Schluger [13]. In particular, it concludes to the existence of a metastable triplet state, with a minimum of energy corresponding to an oxygen
displacement of 0-3 I, to which corresponds a forbidden radiative recombination line at an energy of 2.6 eV. Such a prediction is in very good agreement with the experimental findings
of Itoh et al. [10j We have limited ourselves here to the evidences concerning intrinsic defects of Si02, but many other varieties of the above centres associated with impurities (essentially, Al, Ge and OH) have been reported.
Other effects of laser irradiation on Si02 have been studied and in particular photoemission [14j, which produced one of the first evidences that multiphoton interband transitions of a
relatively high order (9) where possible under such conditions. Shen et al- [lsj also showed that laser irradiation of Si02 (as well as Nacl) in the visible increased the near IR absorption of the samples, which they explained by the hypothesis of the laser heating of the'photoinjected
conduction electrons, a process that will be studied in detail hereafter. Finally, because of the importance of the use of Si02 in electronic devices, the electron transport was extensively studied, essentially with use of Monte-Carlo Simulations [16-18j, eventually including the effect of a strong laser field [19j
This paper is organised as follows: in Section I, we will recall the main processes that contribute to the laser-material interaction, and which will be separately studied later. In Section 2, we will briefly present some general aspects of the pump/probe techniques used in our experiments. Section 3 will describe the photoemission experiments, and discuss the electronic energy deposition mechanisms and their dependence on the different laser parameters (mainly wavelength and intensity, but also pulse duration)- In Section 4, the evolution of the free electron density resulting from the excitation will be studied using transient interferometry
experiments. In Section 5, we will show that the evolution of the free carrier density can be, in
Si02, correlated with the appearance of specific point defects, which can be observed through their optical absorption. Before concluding this paper, Section 6 will be devoted to a discussion of the models used so far to account for the free-electron relaxation in the conduction band,
and to the specific aspects of our materials which are characterized by a high electron-phonon coupling.
1. Fundamental Mechanisms of Energy Absorption and Relaxation
Figure I presents a summary of the different mechanisms whereby electrons of the material
exchange energy with the laser field on one hand (absorption) and with the lattice on the other hand (relaxation). The band structure of the material has been reduced to its simplest expression, that is to the fact that there is a gap much larger than the photon energy. This is of
course not enough to interpret e-g- the result of a photoelectron spectroscopy experiment, but
will be sufficient in the framework of this section which deals only with the basic mechanisms.
It is now well accepted that the initial absorption mechanism -labelled (I) in Figure I- has a
multiphoton character [20j- The low photon energy (compared to the gap) is compensated by
the possibility, owing to the high intensity of the lasers used here, to absorb simultaneously
several photons. Such multiphoton transitions have been extensively studied in the gas phase and exist also in the solid state. They are the quantum counterpart of non-linear optics. A
C-B- (3)
(4)
o (6>
p-s-
(i) ~7)
D-s-
Fig- 1- Schematized electronic structure of a wide-bandgap insulator (V.B-: Valence Band, C-B-:
Conduction Band, D-S-: Defect State, P-S-: "polaron" trap state, 0: vacuum level) and elementary elec- tronic processes following the injection of a free-carrier density into the C-B- by multiphoton excitation
(1): electron-phonon relaxation (2), photoemission (3), conduction electron laser heating (4), trapping (or self-trapping) into deep state (5), trapping into polaron state (6), radiative recombination (7)-
few remarks have to be made for the reader who is not familiar with such processes:
(I) There is no need for intermediate states in the bandgap to make these transitions possible.
The state reached by an electron from the top of the valence band by one photon absorption
does not conserve energy (it is often called a "virtual" state)- This only means that its lifetime
is limited by the Heisenberg principle to something of the order of its energy mismatch to the
nearest allowed state, that is to say the inverse of the photon energy (the laser period). If
the intensity is high enough, the mean time interval between photons may be less than this
lifetime, so that absorption of one more photon can take place, and so on. This sets an order of magnitude of the minimum time to be considered when we say "simultaneous" it means in
a time shorter than the field period. Obviously, if there is an energy conserving intermediate state, this will dramatically boost the transition probability simply because the lifetime of such
states is many orders of magnitude higher than that of virtual states.
(ii) The more photons need to be absorbed, the less probable is the multiphoton transition.
In the intensity range considered here, these probabilities are essentially determined using lowest (non vanishing) order perturbation theory, and the corresponding transition rates can be written under the form:
Wn = anf~ (I-I)
where Wn the n~~ order transition rates in s~~, F the photon flux in cm~~s~~, and an a
generalized cross section in cm~~ s~~~ An approximate scaling law for the order of magnitude
of an is:
an = 10~~~ x 10~~~(~~~~ (1-2)
Obviously, if there are occupied defect states in the bandgap which can be ionized by absorp-
tion of less photons than needed for an interband transition, they can easily dominate the
photoinjection process, and this even in very low concentrations. However, a way of promoting the higher order processes is to work at high photon fluxes. Typically, by comparison of (1-1)
