P Delaye

H J Von Bardeleben

G Roosen

Photorefractive measurements in electron irradiated semi-insulating GaAs

Keywords: 71.55.Eq, 78.50.Ge, 42.70.Nq

We present photorefractive measurements at 1.06µm and 1.3µm performed in electron irradiated GaAs. Irradiation with electrons of kinetic energies ≥ 1MeV introduces intrinsic electrically active defects, which modify the Fermi level position and allow to modify the electron hole competition mechanism of the photorefractive effect. Further, it has been shown that the optical absorption in the 1.3 to 1.5 µm spectral range can be increased, which might allow to enlarge the useful spectral range of GaAs towards optical telecommunication windows. The native and irradiation induced defects have been assessed by electron paramagnetic resonance and optical absorption spectroscopy conducted at T=300K and 77K.

The direct influence of an irradiation induced mid-gap defect on the photorefractive effect is experimentally and theoretically demonstrated .

I. INTRODUCTION

The photorefractive (PR) effect in electrooptic photoconductive semiconductor materials is related to the optically induced charge redistribution between deep defect levels.

In the semi-insulating III-V materials GaAs and InP, one deep defect is responsible for both photocreation and trapping of free carriers : it is the EL2 and the Fe In defects, respectively.

Both defects introduce a deep level (EL2 0/+ and Fe -/0 ), situated around mid-gap, which pins the Fermi level. Different studies have been performed in order to analyze the influence of these defects on the PR effect in these materials [1][2][3][4][5].

In undoped, liquid encapsulated Czochralski grown GaAs, the EL2 defect is present at concentrations [EL2] ≈ (2 ± 1) x 10 16 cm -3 . The occupancy of the EL2 defect is very variable from one sample to another depending on the growth characteristics and on the concentration of the residual contaminants in the sample. The occupancy ratio [EL2 + ]/[EL2 0 ] does vary from 0.1 to 1 depending on the growth conditions. Modern state of the art material is characterized by an occupation ratio < 0.1, which is not optimal for PR applications. The photorefractive properties of GaAs are very dependent on this ratio, especially around 1.3µm [3], because of the electron-hole competition. The occupancy ratio, one would like to have for wavelengths around 1.3µm, is greater than 1. We propose in this paper a new approach, based on the irradiation with high energy electrons, to control the occupation ratio, which is expected to improve the PR response at 1.3 µm. Irradiation creates in the gap a variety of deep defects that change the compensation mechanism in the material, inducing a change in the Fermi level position. We will show in the following, that the irradiation indeed leads to an increase of the [EL2 + ]/[EL2 0 ] ratio.

The outline of the paper is the following. After a presentation of electron irradiation induced defects in GaAs, we present results of electron paramagnetic resonance (EPR) and optical absorption measurements, which are performed in order to monitor the change in the occupation ratio of EL2 and the introduction of the irradiation induced defects. We then present the PR effect measurements realized in the electron irradiated samples. We show that our results can be interpreted in a two defect PR model, which includes, besides the EL2 defect, only one of the irradiation induced defects, H3, which introduces a mid-gap level equally.

II. ELECTRONIC IRRADIATION

A. sample and irradiation characteristics

To analyze the effect of irradiation in undoped semi-insulating GaAs, we use a set of samples issued from the same ingot. These samples are characterized before irradiation by a total EL2 concentration [EL2]=1.3x10 16 cm -3 and a compensation ratio [EL2 + ]/[EL2 0 ] close to 1 [3].

The samples are irradiated with electrons of kinetic energy higher than the displacement threshold of atoms (200keV). We choose an energy of 2MeV, which allows to have defect introduction rates in the order of 10 -2 cm -1 and gives rise to a homogenous defect distribution over the thickness of the sample (≈1.5mm) The irradiation doses are chosen between 1 and 5 x 10 16 electrons per squared centimeters. The irradiation is performed at room temperature.

B. irradiation induced defects

The defects created by electron irradiation in n and p-type samples and particularly their electrical properties have been studied before [6]. The primary defects created by irradiation are Frenkel pairs (V As -As i , V Ga -Ga i ) in the two arsenic and gallium sublattices. Recently it has been demonstrated [7,8] that the formation of antisite defects, As Ga and Ga As also takes place with high introduction rates. All these intrinsic defects are electrically active and their presence will modify the Fermi level and also the lifetime of the free carriers.

The electrical properties of the defects introduced in n-and p-type GaAs have been determined by Deep Level Transient Spectroscopy (DLTS) [6]. These defects have been classified as electron traps called E1 to E5, and holes traps called H0-H3. All these defects are stable at room temperature and may play a role in our experiments.

Table 1 gives some of the parameters of these defects [6]. T 0 is the temperature of the DLTS peak (corresponding to a thermal emission coefficient of 70s -1 ). τ is the introduction rate of the defect and, if Φ is the flux of electrons (in cm -2 ), the concentration of created defects is N T = τΦ. E e is the energy level of the defect, taken from the conduction band for electron traps E and from the valence band for hole traps H. σ a is the apparent capture crosssection (σ na for the capture of electrons of the conduction band by traps E and σ pa for the capture of holes of the valence band by traps H). Thermal emission coefficient ß as a function of temperature T is given by [9,10] :

! n = 2.28 " 10 2 0 T 2 # n a exp $ E e k B T % & ' ( ) * ! p = 1.7 " 10 2 1 T 2 # p a exp $ E e k B T % & ' ( ) *
with E e and σ a given in Table 1. We then calculate the value of ß at T=300K (ß n for electron traps and ß p for hole traps).

The electron and hole traps E and H have been evidenced both as majority and minority carrier traps. Thus they are expected to be present in all samples independently of their conductivity type and in particular in semi-insulating samples, for which no quantitative electrical measurements are possible. All the characteristics of the defects of Table 1, can be used for our semi-insulating samples. The only parameter, which is expected to vary with the conductivity of the sample, is the introduction rate τ given in Table 1 for n-and p-type samples. A RPE measurement has given an introduction rate of arsenic antisite varying from 1cm -1 in n-type samples to 10 -2 cm -1 in semi-insulating samples.

Table 1 gives us some important informations on the defects created by irradiation and on their eventual influence on the PR effect. The introduction rate of the different defects, in our semi-insulating crystals, is expected to be of the order 10 -2 to 1cm -1 ,which means that defects are introduced in concentrations of typically 10 15 cm -3 for the electron dose (10 16 e -.cm 2 ) we have chosen. The next point concerns the thermal emission rate of defects.

Studies of the PR effect have shown that the optical emission rate of carriers must exceed their thermal emission rate [11]. A high thermal emission coefficient will be thus prejudicial to the PR properties of a given defect. The occupancy ratio of the defect is another important parameter for the PR effect (it will appear later in the effective trap density N eff ). We now discuss this point at the light of the results given in Table 1. The occupancy ratio depends on the position E a of the energy level of the defect compared to E f the Fermi level position. For example, for an acceptor level A which can exist under two states of charge : neutral A 0 (concentration N a 0 ) and with a valence band electron captured A -(concentration N a ! ), we have a ratio of concentrations :

N a 0 N a ! = g a exp E a ! E f k B T " # $ % & '
with g a a degeneracy factor which value is in general close to 1. As soon as E a ! E f is greater than 4k B T (k B T=0.025eV at T=300K), N a 0 N a ! becomes smaller than 10 -2 (i.e. a small value of the effective trap density

N eff = N a ! N a 0 N a ! + N a 0 )
. A similar argument can be used for a donor level, leading to the same conclusion.

In our semi-insulating sample, the Fermi level is pinned before irradiation on the 0/+ level of the native EL2 defect, i.e. E f ≈ 0.7eV (from the conduction band). After irradiation the samples stay semi-insulating, and the Fermi level changes only a little (as it will be shown by our EPR and optical absorption results), which means that the Fermi level stays around 0.7eV. We see in Table 1 that all the irradiation defects, except H3 and E4, are at least 0.2eV away from the Fermi level. So they are all in one charge state only (empty of electrons for E traps and full of electrons for H traps). All these defects are thus characterized by a very small effective trap density, which means that they will have a negligible influence on the PR effect.

For this reason we will consider only the two defects H3 and E4 for the analysis of the PR results in the following. The generation and recombination of holes is expected to be determined by the H3 defect .

III. SAMPLE CHARACTERIZATION

Before measuring and analyzing the photorefractive properties of the electron irradiated samples we present in this section the results, obtained by Electron Paramagnetic Resonance (EPR), absorption and electrical conductivity measurements on the same samples.

A. Electron paramagnetic resonance EPR measurements have been performed on the as-grown and on the electron irradiated samples in order to determine the variation of the Fermi level position via the change in the occupation ratio of the EL2(0/+) level and the introduction of electron irradiation induced paramagnetic defects. It has been shown before, that the electron irradiation does not modify the total EL2 concentration itself [12]. The EPR measurements have been performed with an X-band spectrometer ; the measuring temperature was between 4K and 16K. The utilization of a spin standard sample (Al 2 O 3 :Cr) allows to determine quantitatively the defect (EL2 + ) concentration. The defects were measured under thermal equilibrium conditions and under 1.05µm optical excitation, the wavelength leading to the quenching of the EL2 related As Ga spectrum..

Previous EPR studies have allowed the assignment of microscopic models for three irradiation induced defects [START_REF] Jia | Proc. 21st Int. Conf. Physics of Semiconductors[END_REF]: an Arsenic antisite defect with slightly modified EPR parameters as compared to the EL2 defect and no optically induced metastability, the Gallium antisite defect and the Gallium vacancy defect.

Before irradiation the samples show only one paramagnetic defect, whose Spin

Hamiltonian parameters identify it as the positively charged EL2 defect (Fig. 1) (g-factor g=2.03 and hyperfine interaction constant A=890x10 -4 cm -1 [START_REF] Wagner | [END_REF]). The concentration of this defect is determined to be 6.6x10 15 cm -3 . This defect is completely quenchable.

After a first irradiation with a dose of 1x10 16 e -.cm -2 the concentration of the positively charged As Ga defect is increased by ≈30% to a value of 9.2 x 10 15 spin.cm -3 . Further irradiation up to a final dose of 5x10 16 cm -2 does no longer modify this concentration (Table 2). The fact that the As Ga + concentration is not further modified does not imply, that no further defects are created by the irradiation ; it only demonstrates, that the compensation ratio N D -N A stays constant. In addition to the native As Ga defect (related to EL2) we observe equally the irradiation induced As Ga defect, the EPR spectrum of which is not quenchable (Table 2). The values of Table 2 show that the irradiation changes the EL2 + concentration changes by ≈10% only.

In addition to the EL2 related arsenic antisite defect we observe under optical excitation a different EPR spectrum, which we have assigned previously to the Ga As -defect (Fig. 1).

The intensity of this spectrum increases linearly with the irradiation dose (Table 2). The optical properties of this defect and in particular the photoionization spectra of the 2-/-charge states have not been measured yet. It seems however, that the Ga As 2-/-level can be associated with the H1 level at E V +0.25eV. Consequently low temperature photoexcitation at 1.2eV is expected to photoionize the Ga As 2-defect transferring a free electron in the conduction band.

B. Absorption

The absorption spectra for all our samples are deduced from measured transmission spectra at room temperature using an expression that take into account multiple reflections :

T = 1 ! R ( ) 2 exp !" l ( ) 1 ! R 2 exp !2" l ( )
. Coefficient of reflection R is calculated with the value of the refractive index given by Ref. [15,16]. Results are given in Table 3 for the two wavelengths of interest 1.06µm and 1.32µm. Following the irradiation dose we observe an increase of absorption for the two wavelengths, which is seen over the whole spectrum (Fig. 2), with a smooth bump around 1.3µm. From the absorption spectra of the non irradiated sample (D2) at room temperature (Fig. 2), we deduce the total concentration of the EL2 defect. We use the wavelength of 1.19µm [17] where electron and hole photoionization cross-sections are equals : S n =S p =4.9x10 -17 cm 2 [18]. At λ=1.19µm, we have α=0.65cm -1 which gives for the EL2 total concentration, [EL2]=1.3x10 16 cm -3 . This parameter is important as the electronic irradiation does not affect the total concentration of EL2 but only the occupancy ratio of the defect ([EL2 + ]/[EL2 0 ]). So we take this value of concentration, determined in the non irradiated sample, for all the other irradiated samples.

Complementary absorption measurements are performed at low temperature (T=77K).

They will provide much more information. Several reasons call for these measurements.

Firstly absorption band widths depend on temperature and smaller temperatures will lead to thinner bands and then, more distinct features in the absorption spectra. The second and main reason is that, at this temperature, we can use the quenching properties of EL2 defect [19].

The disappearance of the absorption bands related to the EL2 defect will reveal other absorption bands related to irradiation induced defects. This quenching phenomenon is illustrated in Fig. 3 where we see the absorption spectra of sample D2 (non irradiated) before and after quenching of EL2 defect by illumination of the sample with the light of a Nd:YAG laser emitting at 1.06µm. The nearly total disappearance of absorption confirms that EL2 is the main defect for the absorption of the samples before irradiation. The slightly negative absorption observed in Figure 3 at long wavelength is certainly due to an overestimation of about 2% of the index of refraction (and thus of R), which value was given at 103K in ref. [16] and not at 77K, temperature where our experiment is performed.

In irradiated samples after quenching of EL2 absorption, we see the appearance of an absorption band centered at 1.32µm, as well as an absorption tail near the gap (Fig. 4) which both increase with the irradiation dose. The absorption tail is caused by the multiple defects that are created close to valence and conduction band. At the wavelengths of interest (1.06µm and 1.32µm) this absorption tail has only little influence, so we do not discuss it further. The absorption band centered at 1.3µm covers the range of interest and grows linearly with the irradiation dose (Fig. 5). We attribute this absorption band to the photoemission of holes from the H3 defect, the photoionization cross-section spectrum of which has been determined by Deep Level Optical Spectroscopy experiments in p-type crystals [20]. This spectrum, presented in Figure 6, corresponds exactly to the absorption spectra we measure (Fig. 4). The electron photoionization cross-section spectra for E4 defect is not known. We assume it to be similar to the electron photoionization cross section spectra of EL2 0 [21] (i.e. a strong influence of the higher L and X conduction bands). This means that E4 is partly responsible for the absorption tail near the gap and is of low influence at 1.06µm and at 1.32µm compared to the absorption due to H3. Thus H3 only has to be considered in the spectral region we use.

C. Conductivity

We perform dark conductivity and photoconductivity measurements in the different irradiated and non irradiated samples. The ohmic contacts are realized with silver paint. The characteristics is ohmic at low applied field ( smaller than 300V.cm -1 ) in all the samples.

We observe an increase of the dark conductivity of the samples with the irradiation dose (Table 4). When high fields are applied, we observe a non-ohmic behavior in the sample, which corresponds to the field enhancement of the capture cross-section of electrons on the EL2 defect [22]. Note that this non-ohmic character decreases with increasing dose of irradiation corresponding to a decreasing role of the EL2 defect on the dark conductivity.

Photoconductivity measurements are performed at the two wavelengths of 1.06µm and 1.32µm with an incident illumination on the crystal of 16mW.cm -2 for both wavelengths. We observe a decrease of the photoconductivity with the dose of irradiation (Table 4). The photocurrent is verified to be linear with the illumination.

This decrease of photoconductivity and increase of dark conductivity cannot be explained by the observed variations of the EL2 occupancy ratio. Another reason is that the modification of this ratio goes through an increase of the role of holes as EL2 + increases, that would lead to an increase of photoconductivity at 1.32µm as this photoconductivity is mainly due to holes. Thus, the natural way to explain the observed feature of the conductivity is to introduce a second defect, which can be the H3 and/or E4 defect. For dark conductivity, as electrons are predominant, the effect of the secondary defect is mainly to increase the number of free carriers and then to increase conductivity. For photoconductivity the effect is different, as mainly holes are generated at 1.06µm and 1.32µm as well. Valence band is emptied from these holes by recombination on the H3 defect, when it is created by irradiation, and photoconductivity decreases. In fact the effects are much more complicated than here presented and only numerical simulations can show the precise effect of irradiation on conductivity. Unfortunately, even in a simplified model where we neglect the charge redistribution in the levels (i.e. Fermi level variation) induced by irradiation, the number of unknown parameters is too important for the simulation to be reliable. The important point stays that it is necessary to introduce this secondary defect to explain the photoconductivity results.

D. Conclusion on sample characterization

The results of our combined EPR, absorption, and conductivity measurements lead to the conclusion that electron irradiation shifts the Fermi level position such as to obtain an occupancy ratio for EL2 close to one and creates one mid-gap defect H3, a hole trap, responsible for the absorption band centered at 1.3µm. This defect influences both the dark and photoconductivity of the irradiated samples. Its presence will also be sensitive in PR measurements and will require an extension of the PR model to a two defect model comprising EL2 and H3 as it will be seen in the following.

IV. PHOTOREFRACTIVE EFFECT IN IRRADIATED GaAs

The photorefractive effect in undoped semi-insulating GaAs is based on optical and electrical properties of the native defect EL2. At 1.32µm the performances of such a sample are strongly dependent on the occupancy ratio of this defect [3]. The irradiation will change the position of the Fermi level, which will become closer to the valence band. It will lead to an increase of the role of the holes in the PR effect and then an improvement of this mechanism in these materials at 1.32µm [START_REF] Ph | Digest of Topical Meeting on Photorefractive Materials, Effects and Devices IV[END_REF].

A. Set-up and experimental results

We study the photorefractive effect by a two-beam coupling experiment. We measure the energy transfer gain ! 0 = I s (with Pump beam) I s (without Pump beam) from a strong pump beam toward a weak probe beam I s from which we deduce photorefractive gain Γ with expression ! 0 = e " #d (d thickness of the sample). The measurement is performed with two diode pumped Nd:YAG lasers emitting at 1.06µm and 1.32µm. These lasers emit vertically polarized beams that are splitted and then recombined on the crystal in order to form an interference pattern with a grating spacing Λ (or grating wave number k=2π/Λ). The grating vector is oriented along the [001] crystal axis and beams are polarized along the [110] crystal direction.

The theoretical variation of photorefractive gain Γ with k is given by an expression developed with the model taking into account bipolar conduction [START_REF] Kukhtarev | [END_REF]25] (hole-electron competition) in the case of grating spacings smaller than the diffusion length of the carriers :

! = 2" n o 3 r 4 1 # cos$ k B T e % 0 k 1+ k 2 / k o 2 with : ! 0 = S n EL2 o [ ] " S p EL2 + [ ] S n EL2 o [ ] + S p EL2 + [ ] and k o 2 = e 2 ! k B T EL2 o [ ] EL2 + [ ] EL2 o [ ] + EL2 + [ ]
In these expressions n 0 is the refractive index of the material (n 0 = 3.44 at 1.06µm, and n 0 = 3.38 at 1.32µm [15,16]); r 41 is the electrooptic coefficient (r 41 =1.72pm.V -1 at 1.06µm and r 41 =1.54pm.V -1 at 1.32µm [26]), θ is the half-angle between the beams inside the crystal (we have for all grating spacings cosθ ≈1), ε is the static dielectric constant (ε=1.13x10 -10 F.m -1 [16]).

ξ 0 is the hole-electron competition coefficient and k o 2 is the inverse squared of the Debye screening length. These parameters depend on the photoionization cross-sections of carriers (S n for electrons and S p for holes) from the deep level EL2 (wavelength dependent parameter) [18] and on the concentrations of the two states of charge of the EL2 defect (wavelength independent parameter).

Experimental results are presented in Table 5. For each sample and each wavelength we verify that the gain saturates with respect to the incident irradiation. We also eliminate the absorption grating components which are present in highly irradiated samples by using properties of symmetry of the PR effect [27]. Γ changes its sign when turning round the crystal of 180° around the [110] direction, whereas absorption components stay identical which allows the extraction of the photorefractive component for all the measurements as presented in Table 5.

We experimentally observe a decrease of the gain at 1.06µm with the irradiation dose leading to a complete cancellation of the gain for the highest dose. Nearly no change is observed at 1.32µm.

For the non irradiated sample D2, we estimate from the measurements at 1.06µm an hole-electron competition coefficient ξ 0 ≈0.7, which gives, taking into account the absorption of this sample α=1cm -1 , α n =S n [EL2 0 ]≈0.8cm -1 and α p = S p [EL2 + ]≈0.2cm -1 . We know from our absorption and EPR measurements that [EL2 + ] = 6x10 15 cm -3 and [EL2 0 ] = 7x10 15 cm -3 .

With the photoionization cross-sections taken in the literature [18] : S n =10 -16 cm 2 and S p =4x10 -17 cm 2 , we can then calculate α n =0.7cm -1 and α p =0.25cm -1 , which corresponds to the experimental estimation using PR measurements. After irradiation with a dose of 5x10 16 e -.cm -2 (sample D8) we have ξ 0 ≈0, which would lead to α n ≈α p . For this sample we have [EL2 + ]=7x10 15 cm -3 (given by EPR measurements (Table 2)) and then [EL2 0 ]=6x10 15 cm -3 , from which we calculate α n =0.6cm -1 and α p =0.3cm -1 and ξ 0 ≈0.3, values very different from what is observed experimentally.

If we would consider that the change of the photorefractive gain at 1.06µm was due to a change of the [EL2 + ]/[EL2 0 ] ratio, we should have observed an increase of a factor 2 to 3 of the gain at 1.32µm, which, obviously, does not exist. Moreover, as seen in EPR experiments, the ratio changes a little only with the irradiation dose and not enough to explain the change of photorefractive gain at 1.06µm.

So, the one defect hole-electron competition model which explains the PR effect in non irradiated GaAs in the 1-1.5µm range [3] and sample D2, fails to explain the photorefractive response of electron irradiated GaAs. We have to take into account the secondary defect created by irradiation that we saw in absorption experiments. We will now describe this model and then show how it applies to the case of electron irradiated GaAs.

B. Photorefractive effect modeling with a secondary trap and hole-electron competition

Recently an extension of the photorefractive model which takes into account a main defect coupled with both conduction and valence band and a secondary defect coupled with either the conduction band or the valence band (Fig. 7) was developed [28]. The analytical expression of the photorefractive gain with a secondary hole trap (coupled with the valence band) is :

! = 2" n o 3 r 4 1 # cos$ k B T e k 1+ k 2 k o 2 + k' o 2 k o 2 %(k) k o 2 + k' o 2 & k' o 2 k' o 2 +k o 2 1 A p ' & %(k) & 1 A p 1 + k 2 ' p 2 ( ) * * * + , - - - ( )
* * * + , - - - with ! (k) = " n # " p ( ) k 2 + " n $ p 2 # " p $ n 2 ( ) A n " n + A p " p ( ) k 2 + A n " n $ p 2 + A p " p $ n 2 ( ) k 0 2 = e 2 ! k B T N 0 N T " N 0 ( ) N T and k' 0 2 = e 2 ! k B T N' 0 N' T " N' 0 ( ) N T '
are linked to the Debye screening length of the two defects and we have

! n = S n N T " N 0 ( ) , ! p = S p N 0 , A n = ! n + S n I 0 S n I 0 , A p = ! p + S p I 0 S p I 0 , A' p = ! ' p +S' p I 0 S' p I 0 , ! p 2 = e k B T " p N T # N 0 ( ) µ p , ! n 2 = e k B T " n N 0 µ n .
In these expressions S represents the photoionization cross-section, ß the thermal emission coefficient, γ the recombination coefficient and µ the mobility. N T -N 0 is the concentration of neutral traps and N 0 the concentration of ionized traps. The prime corresponds to the secondary defect, n and p indices deal with electrons and holes respectively.

We see on this expression that the influence of the secondary defect occurs through the two terms k' 0 2 and A' p only. We can then understand in what conditions a secondary defect effectively influence the PR effect. The first point deals with term A' p which is linked to the thermal emission coefficient ß' n of the secondary defect. For example, for most of the defects of Table 1 this coefficient is greater than S n I 0 (which is of the order of magnitude of 10s -1 for typical illumination I 0 of some hundreds of mW.cm -2 and value of S n around 10 -16 cm 2 ), which means that in most of the cases we have

C. Discussion of results

In the case of semiconductors the previous expression of the gain can be simplified by doing the usual approximation of grating spacing smaller than the diffusion length (i.e.

k 2 >> ! n 2 ,! p 2 ).
In that case we have

1 1 + k 2 ! p 2 <<1 and ! (k) becomes ! (k) = " n # " p ( ) A n " n + A p " p ( ) .
We also suppose that the illumination is sufficient to assure that the gain does not vary with illumination, i.e. that A n = A p = A' p = 1. We then have an expression of the photorefractive gain that reduces to :

! = 2" n o 3 r 4 1 # cos$ k B T e k 1+ k 2 k o 2 + k' o 2 k o 2 % 0 & k' o 2 k o 2 + k' o 2 ' ( ) * + , with ! 0 = " n # " p ( ) " n + " p ( )
Before irradiation there is only one defect in the sample, the EL2 defect and we have k' 0 2 =0. The expression reduces to the expression of the one defect electron-hole competition model [25]. When increasing irradiation dose we create two defects H3 and E4 close to midgap. Considering the results of absorption measurements we can consider that H3 has the greatest influence on the absorption spectra at the two wavelengths we use (i.e. 1.06µm and 1.32µm). The H3 defect will then act as a secondary defect coupled with valence band and introduced with a concentration proportional to the dose of irradiation (Fig. 5). We neglect the redistribution of charges inside the main defect EL2 and then suppose that ξ 0 does not change with increasing irradiation. Then the only effect of irradiation is an increase of k' 0 2 starting from a zero value.

Here we have to consider two cases depending on the wavelength of study. At 1.06µm

we have, before irradiation, ξ 0 >0 ( α n >α p ) which means that factor k o 2 ! 0 " k' o 2 will decrease with increasing k' 0 2 due to a competition between the two terms. We observe then a decrease of the photorefractive gain down to a total cancellation of the gain. At 1.32µm the two defects have the same acceptor nature with ξ 0 <0 (α n <α p ) for the EL2 main defect. Term

! k o 2 " 0 + k' o 2 (
) is always negative and never changes its sign when we increase k' 0 2 . The

reduction factor k o 2 ! 0 " k' o 2 k o 2 + k' o 2
changes only slightly and then the irradiation has little influence on the photorefractive gain at 1.3µm as observed experimentally.

V. CONCLUSION

The effect of electron irradiation on the photorefractive properties of semi-insulating GaAs crystals has been studied. A characterization of the irradiated samples by EPR, absorption and conductivity measurements shows that the EL2 occupation ratio can be increased by irradiation up to a value of ≈1. They also reveal that the H3 hole trap, which

gives rise to a level at E v +0.71eV, is responsible for the absorption band at 1.3µm. This H3 defect modifies the PR effect in the irradiated samples. The disappearance of the PR gain at 1.06µm and the absence of variation of the gain at 1.32µm with increasing irradiation dose are due to a competition between the EL2 defect and the H3 defect. The measurements and model presented here show that the influence of a second defect in the PR effect is only possible if this defect is very close to the Fermi level at mid-gap.

Figure 1 :

 1 Figure 1 : EPR spectrum of the arsenic antisite (top curve) and of the gallium antisite (bottom curve).

Figure 2 :

 2 Figure 2 : Absorption spectra at room temperature (T=300K) of non irradiated sample (D2) and of D8 (irradiation dose of 5x 10 16 e -.cm -2 ).

Figure 3 :

 3 Figure 3 : Absorption spectra at T=77K of non irradiated sample (D2), before (1) and after (2) quenching of EL2.

Figure 4 :

 4 Figure 4 : Absorption spectra at T=77K of non irradiated sample (D2), of sample D4(irradiation dose of 2x10 16 e -.cm -2 ) and of sample D8 (irradiation dose of 5x10 16 e -.cm -2 ) after quenching of EL2.

Figure 5 :

 5 Figure 5 : Absorption at λ=1.32µm at T=77K after quenching of EL2 as a function of the irradiation dose.

Figure 6 :

 6 Figure 6 :Hole photoionization cross-section spectra from defect H3[20].

Figure 7 :

 7 Figure 7 :Energy-band diagram of crystal with a main hole-electron trap (with a total concentration N T ) and a secondary hole trap (with a total concentration N' T ).

Figure 1 Figure 2

 12 Figure 1

Figure 3

 3 Figure 3

Figure 4

 4 Figure 4

Figure 5

 5 Figure 5

Figure 6

 6 Figure 6

Figure 7

 7 Figure 7

Table 1

 1 are at more than 0.2eV from the Fermi level except H4 and E3, so these defects will be characterized by very low effective trap densities and we will have for all these defects k' 0 2 << k 0 2 . In conclusion, in semiconductors around room temperature and with moderate illumination (some hundreds of mW.cm -2 ) it will be necessary to take into account a secondary defect only if it is sufficiently close to the Fermi level to be under two states of charge and have a moderate thermal emission coefficient or if it is present in high concentration in order to compensate for its distance from the Fermi level.

	1 A' p	<< 1. The second point to discuss is the

Table Captions Table 1 :

 Captions1 Traps created by room temperature electron irradiation[6] (see text for symbol definitions).

Table 2 :

 2 Concentrations of defects measured in electron irradiated samples by electron paramagnetic resonance.

Table 3 :

 3 Room temperature absorption of samples at λ=1.06µm and at λ=1.32µm as a function of the irradiation dose.

Table 4 :

 4 Dark conductivity σ d and photoconductivity σ ph at λ=1.06µm and at λ=1.32µm as a function of the irradiation dose.

Table 5 :

 5 Photorefractive gain Γ, at λ=1.06µm and at λ=1.32µm, as a function of the

	irradiation dose.

Table 2

 2 SamplesIrradiation dose (x 10 16 e -.cm -2 )

	α (1.06 µm)	α (1.32 µm)
	(cm -1 )	(cm -1 )

Table 3

 3 Sampleσ d (Ω -1 cm -1 ) σ ph (1.06 µm)(Ω -1 cm -1 )

	σ ph (1.32 µm)
	(Ω -1 cm -1 )

Table 4 Γ

 4 (cm -1 )

	Wavelength	λ = 1.06 µm	λ = 1.32 µm
	Grating spacing	0.87 µm	3 µm	1.07 µm	2.1 µm
	D2	0.32	0.11	-0.12	-0.085
	D5	0.44	0.15	-0.14	-0.10
	D4	0.28	0.10	-0.14	-0.09
	D6	0.25	0.08	-0.13	-0.11
	D7	0.19	0.07	-0.12	-0.095
	D8	≈ 0	≈ 0	-0.14	-0.095

Table 5

 5

Acknowledgments : P. Delaye thanks Dr. B. Briat for his help during the low temperature absorption experiments conducted in his laboratory (ESPCI). This work was supported by Direction des Recherches Etudes et Techniques (DRET).

Trap

T o (K) τ (cm -1 ) E e (eV) σ a (cm


Yves Belaud

Philippe Delaye

Jean-Claude Launay

Gérald Roosen

PHOTOREFRACTIVE RESPONSE OF CdTe:V UNDER AC ELECTRIC FIELD FROM 1 TO 1.5µm

Keywords: Classification : 42-65, 72-80E, 78-20J

The AC field technique leading to the enhancement of the photorefractive effect is applied to Vanadium-doped Cadmium Telluride. Net photorefractive gain is obtained for the three wavelengths studied : 1.06µm, 1.32µm and 1.55µm. We observed a change in the sign of the photorefractive gain between 1.06µm and 1.55µm, which represents the first evidence of an electron-hole competition in this material. Theoretical interpretation is performed using an extension of the AC field theoretical model with bipolar conduction.

Introduction :

The development of optical communication networks and metrology systems at the eye-safe wavelength of 1.5µm, increases the interest for the extension of the photorefractive effect towards the wavelength region of 1.3-1.5µm. Cadmium Telluride (CdTe) appears to be a good candidate for such applications, as its sensitivity extends beyond 1.5µm and a photorefractive effect can be obtained with the low excitation energies delivered by lasers diodes [1][2][START_REF] Ziari | 3rd Topical Meeting on Photorefractive Materials[END_REF]. In order to obtain the strong photorefractive gain necessary to exceed the threshold of the double phase conjugate mirror [START_REF] Weiss | [END_REF] (or other self pumped phase conjugate mirrors), the use of an externally applied electric field will be required. We perform a characterization of our sample performances using the two wave mixing technique and applying a square AC electric field. This AC field technique permits to keep the π/2 phase-shift which maximizes the two beam coupling photorefractive gain [5,6].

We here present the results of the study conducted at the three wavelengths 1.06µm, 1.32µm and 1.55µm. Net photorefractive gain is obtained at any of these wavelengths with applied field of 10kV.cm -1 and field frequency around 100Hz. We obtain, for the first time to our knowledge in this kind of material, a change of the photorefractive majority carrier between the different wavelengths. We compare our experimental results to the theory, with a model of photorefractive effect with square AC field taking into account bipolar conduction (hole-electron competition).

Sample and experimental set-up presentations :

The CdTe sample we study was grown using modified Bridgman technique and Vanadium-doped with 1.5x10 19 at.cm -3 . The dark conductivity is 6x10 -11 Ω -1 cm -1 . The absorption spectrum (Fig. 1) clearly shows two absorption bands centered at 1µm and 1.5µm, the last one extending up to 1.7µm

The photorefractive effect is analysed in a two beam coupling geometry. The sources are diode-pumped Nd:YAG lasers emitting at 1.06µm and 1.32µm and a laser diode emitting at 1.55µm. Two s-polarized beams, a strong pump I p and a small probe I s , interfere in the crystal ; the grating vector is oriented along (001) axis. The resulting space-charge field creates energy transfer from the pump to the probe. With a Ge photodiode, we measure the transmission of the probe beam with and without the presence of the pump and deduce the photorefractive gain Γ [7]. The interaction length of the crystal is l = 0.866cm. For this crystal, we previously reported on the variation of the photorefractive two beam coupling gain versus grating spacing without applied electric field [8]. This permitted to determine an effective trap density N eff = (1.2 ± 0.3)x10 15 cm -3 and an electron-hole competition coefficient ξ 0 = 0.51±0.06 at 1.06µm. Using the method described in ref. [START_REF] Ph | Etude des non-linéarités photoréfractives dans les composés semiisolants III-V et II-VI : Influence d'une irradiation électronique[END_REF], we determined that electrons were the majority carriers at this wavelength. The photorefractive gain at 1.32µm was found close to zero indicating a nearly perfect hole-electron compensation at this wavelength. Conducting now the same measurements at 1.55µm, we obtain a gain Γ= -0.09 cm -1 for a grating period of Λ=1.8 µm. From this measurement we deduce an electron-hole competition coefficient ξ 0 ≈ -0.3. This change in sign of the photorefractive gain gives an evidence of an electron-hole competition in this crystal with holes becoming a majority carriers at 1.55µm while electrons were majority carriers at 1.06µm.

For AC field experiments, the electric field is applied along (001) through silver-painted electrodes. The field is given by a high voltage amplifier characterized by a slew-rate of 100V.µs -1 . The frequency of the square field is varied from 1Hz to 10kHz. The temporal shape of the field inside the crystal was checked by looking at the modulation of the transmission of the crystal placed between crossed polarizers (Pockels effect). At high frequency, the field is trapezoidal with a constant slope corresponding to the slew-rate value. At low frequency the field is perfectly square. We also checked that the field applied uniformly in the crystal with no high field regions accross it.

Experimental results under AC field :

The photorefractive gain is influenced by different experimental parameters : the amplitude E 0 and the frequency F of the applied electric field, the pump to probe beam ratio ß, the total incident illumination I 0 , the grating spacing Λ. We study the photorefractive gain variation versus these parameters.

Firstly the evolution of the photorefractive response with pump to probe beam ratio ß = I P / I s is conducted while maintaining a constant incident illumination. At 1.06µm and 1.55µm the gain continuously increases with this ratio (Fig. 2) showing a saturation that occurs for ß larger than 10 4 . This point will be discussed later. At 1.32µm due to the smaller value of the gain, the influence of ß should be seen for smaller value of this ratio. Indeed we do not see any variation of the gain with ß for the range of ß of interest. For all the measurements we will present in the following, the pump to probe beam ratio is chosen high enough to be close to the saturation of the photorefractive gain.

We look at the variation of the gain with the applied electric field amplitude E 0 , for the different wavelengths (Fig. 3). In all cases, the higher the field, the higher the photorefractive gain. The change of the sign of the photorefractive gain between 1.06µm and 1.55µm, as well as the small gain at 1.32µm confirm the experiments performed without applied field in the same crystal. For all these measurements the frequency was optimized, i.e. the frequency was chosen high enough, compared to the inverse of the photorefractive time constant, to reach the stationary regime [6]. Of course, this cutoff frequency will depend on both the applied electric field amplitude and the incident illumination. As predicted [6], we note that the cutoff frequency decreases with the amplitude of the applied electric field (fig. 4). However for fields above 4 kV.cm -1 , we observe a non expected reduction of the plateau (Fig. 4): the photorefractive gain decreases when we increase the frequency. This point will be discussed later. In the same way, as the response time of the photorefractive effect is inversely proportional to the incident illumination [7], the cutoff frequency will vary proportionally to this illumination [6]. We effectively observe such a variation (Fig. 5) which gives a cutoff frequency of 30Hz for an incident illumination of 5mW.cm -2 at 1.06µm for a field of 10 kV.cm -1 . At 1.55µm, the cutoff frequency is 5Hz for an incident illumination of 3mW.cm -2 and a field of 10 kV.cm -1 (see Fig. 4).

At very low incident illumination the influence of the dark conductivity will decrease the gain. We can define the equivalent dark irradiance Id as the irradiance at which the gain is reduced by a factor of 2. In our crystal, we measure Id ≈ 50 µW.cm -2 at 1.32µm. At 1.06µm and 1.55µm, we could not reach a low enough illumination to observe a decrease of the gain. This value is equivalent to what was found in other crystals [1][2][START_REF] Ziari | 3rd Topical Meeting on Photorefractive Materials[END_REF] and is coherent with the very low value of the dark conductivity we previously measured in this crystal [8].

Discussion :

In order to compare our experimental results with the theory, we use the model developed in [6] with the differential equations governing the space-charge field given by the electron-hole competition model [START_REF] Strohkendl | [END_REF]. When the frequency of the electric field overpass the cutoff frequency and the space-charge field is in the stationary regime, the expression of the photorefractive gain Γ reduces to :

! = 2" n 0 3 r 4 1 # cos$ k B T e k 0 2 k % & ' ( ) * n + * p ( ) + * n , n 2 I n + * p , p 2 I p ( ) * n + * p ( ) + * n I n k 0 2 + , n 2 ( ) + * p I p k 0 2 + , p 2 ( )
with :

I n = k 2 + ! n 2 k 2 + ! n 2 ( ) 2 + k 2 V 2 and I n = k 2 + ! p 2 k 2 + ! p 2 ( ) 2 + k 2 V 2 . V = E 0 e k B T , ! n 2 = e k B T 1 µ n " n and ! p 2 = e k B T 1 µ p " p
µ n(p) and τ n(p) are the mobility and recombination time of electrons (holes). α n and α p are the parts of absorption that give electrons and holes respectively. They are deduced from the value of ξ 0 and α according to the relations : ! = ! n + ! p and ! 0 = " n # " p " n + " p . Taking a single carrier model, the photorefractive gain expression reduces to the one found in ref. [5].

All the material parameters that influence the photorefractive gain through the theoretical expression are known except the recombination times τ n and τ p . The mobilities for electrons and holes are µ n =520 cm 2 V -1 s -1 and µ p =80 cm 2 V -1 s -1 [11]. The product n 0 3 r 41 is taken equal to 120 for the three wavelengths [1,2]. We thus adjust the theoretical curve to the experimental points for the three wavelengths by varying the values of τ n and τ p . At 1.06µm due to the electron dominated characteristic of the gain, the adjustment is more sensitive to τ n , whereas at 1.55µm it is τ p which is important as holes dominate. At 1.32µm where we have almost perfect electron-hole competition, theoretical gain is mainly sensitive to the ratio between τ n and τ p (fig. 3). We determine from the adjustment : τ n =0.1ns and τ p =0.5ns,. The ξ 0 values used are : ξ 0 = 0.51 at 1.06µm as determined in previous experiments [8], ξ 0 = 0.03 at 1.32µm and ξ 0 = -0.35 at 1.55µm which correspond to the value we estimate from measurements at higher grating spacing. Using higher values for τ n and τ p will quickly give theoretical gains twice higher than experimental ones. For example, at 1.06µm the theoretical gain matches the experimental one (say 2.5 cm -1 at 5 kV.cm -1 ) with the above values of τ n . Increasing τ n by a factor of 2 leads to a photorefractive gain of 4.2 cm -1 , whereas a decrease by 2 gives a photorefractive gain of 1 cm -1 . This example indicates how sensitive to the values of τ n and τ p the adjustments are. The µτ products we deduce are: µ n τ n =5x10 -8 cm 2 V -1 and µ p τ p =4x10 -8 cm 2 V -1 ; they are smaller than the one determined in other samples [1,2,12] but remain not too far from them.

For all these calculations we consider that the electric field is perfectly applied, i.e. E 0 =V 0 /d with V 0 the applied voltage and d the interelectrode spacing. The effective applied field can possibly be smaller than V 0 /d. Leaving E 0 as a variable parameter adds a degree of freedom for the adjustment of the experimental data. Taking a reduced value of the field leads to an increase of the possible values of τ n and τ p . However, as we do not know if this reduction of the internal field exists and what is its eventual strength, we decide to neglect this effect, as long as we don't have any evidence of its existence.

The experimentally observed decrease of the gain with increasing the field frequency is not predicted by the model developed in ref. [6] for a perfectly square field. A possible cause of this decrease can be the influence of the shape of the field, i.e. an influence of the slew-rate of the amplifier. To precise this influence we use the model developed in ref. [13] with the peculiar shape of our applied field. According to our calculation the photorefractive gain must decrease because of the slew-rate but this decrease should be noticeable only for frequencies higher than 500Hz. Our decrease occurs at about 100Hz at 1.06µm and around 30Hz at 1.55µm which are much lower values than what we expect theoretically. The origin of this decrease of the gain is still unknown and is currently under study.

The variation of the photorefractive gain with the pump to probe beam ratio shows the evidence of the so called "large signal effect" [12,14]. Indeed our experimental results are not described by the classical law for the effective gain [7] :

! 0 =
1 + " 1+ " e #$l (Fig. 2). In the model for large signal effect, the transmission of the probe beam, in the case of undepleted pump, is given by [14] :

d I S d z = ! f (m) m I S with the modulation m ! 2" # 1
2 and with the function

f (m) = 1 a 1 ! exp(!am) ( ).
A good adjustment of our experimental results is obtained with this function (fig. 2). The value we find for the coefficient a (a=2.7) is coherent with the data obtained in other kinds of materials like BGO [15], whose photorefractive parameters (N eff , µτ, ...) have similar values. All measurements shown on the other curves are conducted with a high pump to probe beam ratio in order to limit this large signal effect.

Conclusion :

We have presented here the first results of the photorefractive experiments we conducted on CdTe:V under applied AC field in the 1-1.5 µm range. The photorefractive gain we measure ( 5.5cm -1 at 1.06µm for a 10kV.cm -1 ) already permits to overpass the threshold of double phase conjugate mirrors ((Γ -α)l > 4) [START_REF] Weiss | [END_REF] with this sample. The results presented here are promising because of the low level of the illumination required (below 1mW.cm -2 ) and the low frequency of the applied electric field (below 100Hz). We developed a model of photorefractive effect under applied AC field with two type of carriers that permits to explain most of our experimental results. We obtained the first evidence of an electron-hole competition in CdTe:V with a change of majority carrier between 1.06µm and 1.55µm. Considering these first results a monopolar conductivity should be obtainable by varying growing or doping conditions. For example, doping of crystals with shalow acceptors would increase the role of holes, leading to optimum photorefractive properties at 1.55 µm. Such an optimization work is currently under progress. 2 : Photorefractive amplification γ 0 (+, ×) as a function of the pump to probe beam ratio ß at 1.06µm and 1.55µm. The thin line is the theoretical variation of γ 0 versus ß according to the law ! 0 = 1 + " 1+ " e #$l . The broad line correspond to the large signal effect curve(see text) with a coefficient a=2.7 at 1.06µm and a=2.2 at 1.55µm. -at 1.06µm :Λ=10µm, ß=10 4 , I inc =7mW.cm -2 .

-at 1.32µm :Λ=21µm, ß=270, I inc =0.7mW.cm -2 .

-at 1.55µm :Λ=11µm, ß=780, I inc =3.4mW.cm -2 .

Figure 4 : Photorefractive gain as a function of the frequency F for different applied field at 1.06µm and 1.55µm : E 0 =2kV.cm -1 (+), E 0 =4kV.cm -1 (∆), E 0 =6kV.cm -1 (×), E 0 =10kV.cm -1 (•).

Figure caption : Figure 1 :

 :1 Figure caption :

Figure

  Figure 2 : Photorefractive amplification γ 0 (+, ×) as a function of the pump to probe beam ratio ß at 1.06µm and 1.55µm. The thin line is the theoretical variation of γ 0

Figure 3 :

 3 Figure 3 : Photorefractive gain Γ as a function of the applied AC field amplitude E 0 for the different wavelength 1.06µm (+), 1.32µm (×) and 1.55µm (o) The line represents the theoretical adjustment (see text). The frequency of the field is optimized for each value of E 0 , the experimental conditions are:

Figure 5 :

 5 Figure5: Variation of the optimal frequency as a function of the incident illumination I inc at 1.06µm (ß=10 4 , E 0 =10kV.cm -1 , Λ=10µm).

Figure 1

 1 Figure 1

Figure

  Figure 2

2

 2 Figure 2

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 10

Figure 3

 3 Figure 3

Figure

  Figure 4

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 4 Figure 4

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 5 Figure 5


Natalie D Snoeren
email: n.snoeren@psych.york.ac.uk

Juan Seguí

Pierre A Hallé

On the role of regular phonological variation in lexical access: Evidence from voice assimilation in French

The present study investigated whether lexical access is affected by a regular phonological variation in connected speech: voice assimilation in French. Two associative priming experiments were conducted to determine whether strongly assimilated, potentially ambiguous word forms activate the conceptual representation of the underlying word. Would the ambiguous word form [sud] (either assimilated soute 'hold' or soude 'soda') facilitate "bagage" 'luggage', which is semantically related to soute but not to soude? In Experiment 1, words in either canonical or strongly assimilated form were presented as primes. Both forms primed their related target to the same extent. Potential lexical ambiguity did not modulate priming effects. In Experiment 2, the primes such as assimilated soute pronounced [sud] used in Experiment 1 were replaced with primes such as soude canonically pronounced [sud]. No semantic priming effect was obtained with these primes. Therefore, the effect observed for assimilated forms in Experiment 1 cannot be due to overall phonological proximity between canonical and assimilated forms. We propose that listeners must recover the intended words behind the assimilated forms through the exploitation of the remaining traces of the underlying form, however subtle these traces may be.

One of the fundamental issues in the auditory word recognition domain pertains to the observation that listeners can recognize words in spite of numerous variations that are present in the speech signal. The perceptual processing of regular surface variations such as phonological assimilation has received a lot of attention in the literature [START_REF] Lahiri | The mental representation of lexical form: a phonological approach to the recognition lexicon[END_REF][START_REF] Gaskell | Phonological variation and inference in lexical access[END_REF], 1998[START_REF] Gow | Assimilation and anticipation in continuous spoken word recognition[END_REF][START_REF] Gow | Assimilation and anticipation in continuous spoken word recognition[END_REF][START_REF] Gow | Does English coronal place assimilation create lexical ambiguity?[END_REF][START_REF] Gow | Feature parsing: Feature cue mapping in spoken word recognition[END_REF][START_REF] Gow | A cross-linguistic examination of assimilation context effects[END_REF][START_REF] Coenen | Variation and assimilation in German: Consequences for lexical access and representation[END_REF][START_REF] Gumnior | Assimilation in existing and novel compounds[END_REF][START_REF] Mitterer | Coping with phonological assimilation in speech perception: Evidence for early compensation[END_REF]Snoeren, Seguí, & Hallé, 2008). Assimilation refers to the process by which a speech segment becomes more like an adjacent segment in terms of one or more phonetic features. Since the explicit conditioning environment (i.e. the adjacent segment) licenses the phonological change, some authors have argued that listeners use phonological context to infer a particular surface form (e.g., [START_REF] Gaskell | Phonological variation and inference in lexical access[END_REF], 1998). This view was recently challenged by some empirical studies presenting evidence that the assimilated speech signal generally contains traces of the underlying segment [START_REF] Nolan | The descriptive role of segments: Evidence from assimilation[END_REF][START_REF] Gow | Assimilation and anticipation in continuous spoken word recognition[END_REF][START_REF] Gow | Does English coronal place assimilation create lexical ambiguity?[END_REF]. In a previous paper, we provided an acoustic-phonetic description of voice assimilation in French, which may occur when two consonants of different voicing are adjacent (Snoeren, Hallé, & Seguí, 2006). For instance, the word-final /p/ in jupe ("dress") tends to become voiced when followed by a voiced consonant (e.g., [yb] in jupe grise). Our acoustic measurements showed that voice assimilation in French can be graded and preserve fine-grained acoustic traces of the underlying form. Importantly, it has been shown that the listener can exploit these traces to access the underlying word form [START_REF] Gow | Does English coronal place assimilation create lexical ambiguity?[END_REF][START_REF] Gow | Feature parsing: Feature cue mapping in spoken word recognition[END_REF].

More recently (Snoeren et al., 2008), we investigated the role of phonological context in the perception of assimilated speech in French, using a cross-modal identity priming paradigm.

In Experiment 1, in which primes were presented without the assimilating context, a significantly larger priming effect was obtained for the unassimilated (canonical) than assimilated form of the prime words, suggesting that these priming effects reflected different degrees of activation of phonological representation. Norris, Cutler, McQueen, and Butterfield (2006: 149) noted that, "Identity priming does not necessarily imply that conceptual representations have been activated…" At least, it is conceivable that assimilated word forms mainly activate phonological rather than more abstract lexical representations.

In this study, we directly tested for the possibility that even strongly assimilated forms activate lexical-conceptual representations, by using a cross-modal associative priming paradigm. Importantly, we used naturally (as opposed to deliberately) assimilated words as auditory primes. Ambiguous assimilated forms (such as [su t d ], from either soute or soude) might be less efficient than unambiguous forms (such as [y p b ], jupe being the sole possible underlying word) in priming an associated target, conceivably due to the competition of the non-intended word. Nevertheless, the acoustic traces of the intended word in the speech signal might be sufficient to recover that word, and even ambiguous assimilated forms might prime a target associated to the intended word. In Experiment 1, we test for the associative priming effects for ambiguous and unambiguous assimilated forms.

Experiment 1

Method

Participants

Seventy students at the Psychology Department of Paris-Descartes University, native speakers of French, participated in Experiment 1. Thirty-six students (34 women and two men, mean age 24 years) were assigned to the canonical condition, and 34 participants (30 women and four men, mean age 21 years) to the assimilated condition (see below).

Design and Materials

Twenty-four French monosyllabic nouns ending with a voiceless stop were selected as experimental items: twelve words had a minimal pair for final stop voicing (e.g., soute) and 12 words had none (e.g., jupe). Words with a voiceless rather than voiced final stop were used, because previous research showed that the former are more prone to voice assimilation than the latter. Other lexical characteristics are summarized in Table 1.

The experimental noun items were inserted in noun phrases (determinant + noun + adjective). To obtain the canonical and assimilated versions of each noun, its right context either licensed voice assimilation (e.g., une soute bondée), or not (e.g., une soute pleine). The assimilating and the assimilated consonants always had a different place of articulation in order to avoid gemination. A female non-naïve native speaker of Parisian French was asked to produce the noun phrases in a fluid fashion, without pause between words (to increase the likelihood of voice assimilation). Each noun was excised from its noun-phrase context (care was taken to avoid auditory clicks in the excised stimuli). Peak acoustic intensity was equalized across all stimuli. The excised words served as auditory primes. A native French speaker, expert in the field of auditory speech perception, was asked to categorize the word-final consonant of the stimuli. For all the assimilated words, final consonants were categorized as voiced (e.g., the final stop of voice-assimilated soute was perceived as /d/, not /t/). Acoustically, the assimilated stops all had a fully voiced closure: the "voicing ratio" (Snoeren et al., 2006) was 100%.

A free association procedure was used to select targets semantically associated to primes.

Fifty-eight participants were asked to report the first word that came to their mind upon written presentation of each of the experimental items. For each item, the most frequent response across participants (39% in average) was retained as the associated word to be used as target. The same primes were used in the related and unrelated prime-target pairs by rotating the prime-target associations. For instance, the prime gratte was associated to GUITARE in the related condition and to BAGAGE in the unrelated condition.

Forty-eight filler word-word prime-target pairs were constructed. For 24 of them, prime and target were semantically associated (e.g., crabe -MER); they were not for the 24 others (e.g., larve -LOISIR). The semantic associates were obtained from [START_REF] Ferrand | Normes d'association verbales pour 366 d'objets concrets[END_REF] association tables (mean association rate 27%). Finally, 72 filler word-nonword prime-target pairs were added. For 24 of those, the nonword target was derived from a word semantically related to the prime (e.g., ciel -ESLACE: the nonword eslace is derived from espace 'space', related to ciel 'sky'); these targets are, as it were, "pseudo-related" to their prime. These 24 "pseudo-related" prime-target pairs were inserted to avoid a bias toward positive response for pairs in which a semantic relationship might be detected. The primes in the filler prime-target pairs were matched for frequency with the primes in the experimental pairs. Like the experimental primes, the filler primes were recorded within spoken noun-phrases, then excised.

To summarize, the materials consisted of 24 experimental prime-target pairs (12 with potentially ambiguous primes, 12 with unambiguous primes), 48 filler word-word pairs (half of which with semantically related prime and target), and 72 filler word-nonword pairs (24 of which pseudo-related).

Given the limited number of French words in the potentially ambiguous category, all of them were presented to each subject in either their canonical or assimilated form. That is, Form of realization (canonical vs. assimilated) was a between-subject factor: all the experimental primes were presented in their canonical form (e.g., [sut] for soute) to one group of participants, and in their assimilated form (e.g., [sud] for soute) to the other group. Semantic relation (related vs. unrelated) was counterbalanced across subjects within each group so that each subject saw any given target only once.

Procedure

Prime words were presented auditorily through Sennheiser headphones. At the offset of the prime, the target was displayed on a screen for three seconds. The participant was required to press the "yes" button if the target was a French word, and the "no" button if it was not.

Reaction times were measured from the offset of the prime. The DMDX software [START_REF] Forster | DMDX: A Windows display program with millisecond accuracy[END_REF] was used to run the experiment. In addition to the experimental and filler trials, 10 practice trials were presented in a training phase. The test phase begun with two warm-up trials. The total duration of the experiment was about 15 minutes.

Results and Discussion

The data of five participants in the canonical condition and three in the assimilated condition were not retained, due to technical problems. The overall error rate was 4.2%.

Response times under 300 ms or above 1200 ms (less than 1.5%) were excluded from the RT analyses. The results of Experiment 1 showed a classic semantic priming effect. Targets following a semantically related prime (soute -BAGAGE) were responded to faster than those following an unrelated prime (gratte -BAGAGE), regardless of Word type or Form of realization. The size of the priming effect was relatively small (21 and 26 ms in the canonical and assimilated condition, respectively), as is usually found in associative cross-modal priming experiments (e.g., [START_REF] Moss | Accessing different types of lexical semantic information: Evidence from priming[END_REF].

The results suggest that ambiguous, strongly assimilated forms still activate the lexicalconceptual representation of the intended underlying word. However, before we can attribute the priming effect to the presence of acoustic traces in the assimilated form, we need to rule out an alternative explanation: phonological proximity of canonical and assimilated forms. Previous studies have shown that spoken word recognition is relatively tolerant for deviations from canonical forms (e.g., [START_REF] Bölte | Domato primes paprika: Mismatching pseudowords activate semantic and phonological representations[END_REF][START_REF] Connine | Do the beginnings of spoken words have a special status in auditory word recognition[END_REF]. If an assimilated form (e.g., [sud] for soute) and a similar though canonically realized form (e.g.,

[sud] for soude) are processed in the same fashion by the perceptual system due to their phonological proximity, then soude (canonical form [sud]) should prime BAGAGE as much as the assimilated form prime of soute [sud]. After all, the critical consonant in the voiceassimilated form of soute was perceived as /d/ by a French native speaker. Alternatively, if the canonical form prime of soude does not prime BAGAGE as efficiently as the assimilated form of soute, this would suggest that the perceptual system distinguishes between two versions that are almost identical in their surface form. In the latter case, the semantic priming effect that was obtained in Experiment 1 could only be attributed to the presence of fine-grained acoustic traces of voicelessness in the assimilated form [sud] of soute. Of course, these considerations only concern the words that have a lexical counterpart in terms of final-stop voicing. A second experiment was conducted to test for the possibility that canonical, unassimilated soude primes BAGAGE.

Experiment 2

Method

Participants

Thirty-five native speakers of French (30 female and 5 male students, mean age 22 years), students at Paris-Descartes University, participated in Experiment 2.

Design and Materials

Twelve experimental words such as soude, the voiced counterparts of the words such as soute used in Experiment 1 served as auditory primes. They were recorded in a non-assimilatory context (e.g., une soude brute). Each of them was associated to the same target as that used for its voiceless counterpart in Experiment 1 (e.g., the target for soude was BAGAGE). Words were excised from context using the same procedure as in Experiment 1. The remaining materials were the same as in the canonical condition of Experiment 1. Only the unassimilated forms of the prime words were used. Hence, there was no "Form of realization" factor in this experiment.

The materials were recorded by the same speaker as in Experiment 1.

Procedure

The experimental procedure was identical to that in Experiment 1.

Results and Discussion

The data of one participant were not retained due to technical problems. The overall error rate was 2.33%. As in Experiment 1, RTs outside the [300, 1200 ms] range (0.86 %) were excluded from analyses.

The experimental primes (e.g., soude) did not significantly facilitate their target (e.g., "bagage"). For the experimental prime-target pairs, RTs to target were 530 and 525 ms for related (soude -BAGAGE) and unrelated (grade -BAGAGE) pairs, respectively. This difference did not reach significance, Fs < 1. The absence of priming for pairs such as soude -BAGAGE could not be attributed to a general insensitivity to semantic relationship in this experiment. Indeed, RTs were shorter for the related than unrelated word-word filler pairs (522 vs. 550 ms), t(33) =5.26, p < .001, showing a clear semantic priming effect.

In a separate analysis, we directly compared the outcomes of Experiments 1 and 2 to assess priming effects to the same targets (e.g., BAGAGE) preceded by assimilated primes (soute pronounced [sud]) and by canonical primes (soude pronounced [sud]). A two-way ANOVA was conducted with Form of realization (assimilated -Expt.1 vs. canonical -Expt. 2)

and Semantic relation (related vs. unrelated) as main factors. No significant effect was observed for the two main factors, but their interaction was significant, F 1 (1,63) = 4.09, p < .05, F 2 (1,11) = 6.17, p < .05. This interaction shows that primes whose surface forms are very close may be processed differently, depending on the speaker's intention (e.g., soude vs. soute, both realized

[sud]). The surface form [sud] facilitates BAGAGE only when the speaker's intended word is soute. But how different is this form from the canonical form of soude [sud]? Detailed acoustic analyses revealed that all differences pointed to stronger cues to voicedness in canonical soude than in assimilated soute. In particular, preceding vowel duration, a classic cue to obstruent voicing (Peterson & Lehiste, 1960), was longer for canonical soude (82 vs. 73 ms, one-tailed ttest, t(11) = 2.10, p < .05). The assimilated stops tended to be "less voiced" than the plain voiced stops on other dimensions as well: longer closure duration (57 vs. 54 ms), higher F0 at preceding vowel offset (231 vs. 224 Hz), and lower intensity of the occlusion (67.4 vs. 69.1 dB, one-tailed t-test, t(11) = 2.05, p < .05). These acoustic analyses suggest that, at least for strongly assimilated words, the speaker's intention is reflected by fine-grained secondary acoustic cues that are not captured by the voicing ratio index (see Snoeren et al., 2006).

General Discussion

The purpose of the present study was to investigate whether the assimilated form of a word presented in isolation activates its lexical-conceptual representation. We thus compared the semantic priming effects induced by canonical and (strongly) assimilated word forms. Half of the assimilated primes were potentially ambiguous (e.g., [sud] is a realization of either soute or soude), the other half were not ([yb] can only be a realization of jupe). In Experiment 1, semantic priming effects were observed, regardless of form of realization, and potential ambiguity. In Experiment 2, we used as primes the voiced counterparts of the words such as soute used in Experiment 1 (hence, soude for soute), associated with the same targets. No priming effects obtained for these primes. That is, soude did not prime BAGAGE related to soute.

That assimilated word forms induce semantic priming effects suggests a lexicalconceptual locus for these effects. This finding is in line with previous studies showing that lexical access is not necessarily impaired by regular surface variations in the speech signal, [START_REF] Deelman | Missing information in spoken word recognition: nonreleased stop consonants[END_REF]Sumner & Samuel, 2005). The semantic priming effects found in Experiment 1 also held for ambiguous assimilated forms, compatible with two words differing in the voicing of their final stop. In this case, lexical competition does not block the activation of the lexical-conceptual representation of the intended, underlying word.

In the present study, the priming effects for canonical and assimilated word forms had comparable magnitudes (21 vs. 26 ms). These results contrast with those obtained in our previous study using identity priming (Snoeren et al., 2008). In that study, assimilated word forms induced significantly less priming than canonical forms (Experiment 1: 32 vs. 95 ms), suggesting that identity priming effects mainly reflected the activation of phonological representations. In the present study, using semantic priming, comparable effects were found for canonical and assimilated forms, suggesting a lexical-conceptual locus of these effects. Taken together, the different outcomes for cross-modal identity priming and associative priming indicate that the two paradigms are differentially sensitive to phonological and lexical levels of representation.

As we pointed out earlier, the assimilated word forms used as primes in Experiment 1 were strongly assimilated, as judged by an expert speech scientist and as shown by acoustic measurements, and therefore clearly differed from the canonical forms. It is thus surprising that they gave rise to semantic priming effects of very similar magnitude. A possible account in terms of general tolerance for phonological mismatch (cf. [START_REF] Connine | Do the beginnings of spoken words have a special status in auditory word recognition[END_REF], tested in Experiment 2, does not explain the outcome of Experiment 2: soude [sud] did not prime BAGAGE as soute realized [sud] did. The explanation we propose is that the speech signal retains traces of the underlying voicing of intended words [START_REF] Goldrick | Cascading activation from phonological planning to articulatory processes: Evidence from tongue twisters[END_REF].

Listeners would exploit these traces to access abstract lexical representations, however subtle they are. It is therefore quite remarkable that the speech comprehension system is successful to access the speaker's intentions on the basis of such fine-grained acoustic detail.

  Table2shows the RT and error data in each condition. ANOVAs were conducted on the RT data with Form of realization (canonical vs. assimilated) as a between-subject factor, and Word type (with vs. without a voicing counterpart) and Semantic relation (related vs. unrelated) as within-subject factors.

Only Semantic relation had a significant effect, F 1 (1,60) = 17.09, p < .001 (by-subject), F 2 (1,22) = 7.47, p < .05 (by-item); it did not interact with the other factors. RTs were shorter in the related than unrelated condition (496 vs. 519 ms), and this held for both the canonical and assimilated conditions (canonical: t(30) = 2.41, p < .05; assimilated: t(30) = 3.61, p < .01).

Table 1 .

 1 Lexical properties of the experimental primes used in Experiment 1. "Amb" stands for words potentially ambiguous after assimilation (e.g., soute may be confounded with soude if realized [sud]) and "Unamb" for unambiguous words (e.g., jupe has no voiced counterpart in French); log frequency; number of phonological neighbors (nphon); number of phonological neighbors by substitution of the word-final consonant (FC-phon); number of more frequent word-final consonant phonological neighbors (HF-phon); mean association rate for the associated prime-target pairs (AR); target log frequency (tgt-freq).

	Type	log freq	nphon	FC-phon	HF-phon	AR	tgt-freq
	Amb	0.59	24.18	9.67	4.63	41.3%	1.31
	Unamb	1.42	16.75	6.17	0.92	36.6%	1.7

Table 2 .

 2 Mean RTs in ms (SD between parentheses) and error rates in % to target in Experiment 1 for the canonical and assimilated conditions according to Semantic relation (related or unrelated) and Word type (potentially ambiguous or not).

	"Canonical condition"	"Assimilated condition"
	Related	Unrelated	Related	Unrelated
	[sut]-BAGAGE	[rat]-BAGAGE	[sud]-BAGAGE	[rad]-BAGAGE
	501 (61)	521 (85)	484 (56)	513 (85)
	1.6	3.8	4.3	3.2
	[yp]-ROBE	[kup]-ROBE	[yb]-ROBE	[kub]-ROBE
	509 (73)	530 (89)	490 (57)	511 (72)
	3.8	5.4	4.8	7

Acoustical Society of

 America, 32, 693-703. Snoeren, N.D., Hallé, P.A. & Seguí, J. (2006). A voice for the voiceless: Production and perception of assimilated speech in French. Journal of Phonetics, 34(2), 241-268. Snoeren, N.D., Seguí, J. & Hallé, P.A. (2008)

Author Note

This research was supported by a Ministère de l'Education Nationale, de la Recherche, et de la Technologie (MENRT) doctoral fellowship to the first author. We thank Gareth Gaskell for the stimulating discussions about the content of this paper, as well as three anonymous reviewers who provided insightful comments on an earlier version of the manuscript.