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A. GOLTZE~ and C. SCHWAB: Differentiation of Paramagnetic Spectra in GaAs 649 phys. stat. sol. (b) 160, 649 (1990) Subject classification: 71.70 and 76.30; S7.12 Groupe Recherches Physiques et Matkriaux, Centre de Recherches Nuclkaires, IN2P3-C.N.R.S., Universitk Louis Pasteur, Strasbourg ') Qualitative and Quantitative Differentiation of Paramagnetic Anion-Antisite-Related Spectra in GaAs BY A. GOLTZEN~ and C. SCHWAB The electron paramagnetic resonance spectra related to the anion antisite in GaAs may be sorted out into two groups according to their behaviour under photoexcitation and microwave saturation. This qualitative separation is quantitatively supported by different values for the central hyperfine constant and for the component linewidth of the quadruplet spectrum usually modeled by an isolated AS : : defect. These observations lead to reinvestigate the initial extrapolation, based on the isolated P,, centre in Gap, which results in the identification of the former quadruplet. It supports the existence of several antisite-related defects likely differing by their first shell as in Gap. Les spectresde resonance paramagnktique electroniquerelatifs a l'antisite anionique dans GaAs peuvent itre classes en deux groupes selon leur comportement sous excitation optique ou sous saturation microonde. Cette separation qualitative est confirmee quantitativement par l'observation de valeurs diffkrentes des constantes &interaction hyperfine centrale et des largeurs de raies du quadruplet habituellement modelist par un defaut As&, isolk. Ces observations nous amtnent a reexaminer l'extrapolation initiale, basee sur le centre P,, isolk dans Gap, qui a conduit a l'identification du quadruplet prkckdent. Ceci confirme l'existence de plusieurs dkfauts associes a l'antisite differant probablement par leur premitre couche comme dans Gap. 1. Introduction Electron paramagnetic resonance (EPR) is a measure of the interaction of an external applied magnetic field on a local magnetic moment using microwave absorption between electronic levels split by the ZEEMAN effect [l, 21. Hence an EPR analysis of the electronic configuration of a defect basically rests on an atomic structure approach. In a real crystal, the simplest case for such a magnetic moment may be depicted by an unpaired electron or hole bound to a lattice defect, depending on the charge state with respect to the perfect lattice. This unpaired spin is subjected to a predominant central hyperfine interaction; it involves the nuclear spin I of the core of the defect and its strength is described by the central hyperfine coupling constant A,, whose magnitude is essentially a measure of the local electronic density. The unpaired spin further undergoes ligand hyperfine interactions; depending on the nature and number of surrounding nuclei, the latter are represented by an envelope function of overall linewidth AH. In favorable cases, the individual components are resolved allowing one to separate the contributions of the successive shells around the defect. Translating the effects of the neighbourhood, this interaction determines the site ascription. Hence, at least in theory, EPR appears as a suitable tool for identifying the nature and location of intrinsic defects in a compound ') 23, rue du Loess, F-67037 Strasbourg Cedex, France.

Qualitative and Quantitative Differentiation of Paramagnetic Anion-Antisite-Related Spectra in GaAs

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A. G O L T Z E ~ and C. SCHWAB: Differentiation of Paramagnetic Spectra in GaAs 649

phys. stat. sol. (b) 160, 649 (1990)

Subject classification: 71.70 and 76.30; S7.12

Groupe Recherches Physiques et Matkriaux, Centre de Recherches Nuclkaires, IN2P3-C.N.R.S., Universitk Louis Pasteur, Strasbourg ')

Qualitative and Quantitative Differentiation of Paramagnetic Anion-Antisite-Related Spectra in GaAs

BY A. GOLTZEN~ and C. SCHWAB

The electron paramagnetic resonance spectra related to the anion antisite in GaAs may be sorted out into two groups according to their behaviour under photoexcitation and microwave saturation. This qualitative separation is quantitatively supported by different values for the central hyperfine constant and for the component linewidth of the quadruplet spectrum usually modeled by an isolated AS:: defect. These observations lead to reinvestigate the initial extrapolation, based on the isolated P,, centre in Gap, which results in the identification of the former quadruplet. It supports the existence of several antisite-related defects likely differing by their first shell as in Gap.

Les spectres de resonance paramagnktique electronique relatifs a l'antisite anionique dans GaAs peuvent itre classes en deux groupes selon leur comportement sous excitation optique ou sous saturation microonde. Cette separation qualitative est confirmee quantitativement par l'observation de valeurs diffkrentes des constantes &interaction hyperfine centrale et des largeurs de raies du quadruplet habituellement modelist par un defaut As&, isolk. Ces observations nous amtnent a reexaminer l'extrapolation initiale, basee sur le centre P,, isolk dans Gap, qui a conduit a l'identification du quadruplet prkckdent. Ceci confirme l'existence de plusieurs dkfauts associes a l'antisite differant probablement par leur premitre couche comme dans Gap.

1. Introduction

Electron paramagnetic resonance (EPR) is a measure of the interaction of an external applied magnetic field on a local magnetic moment using microwave absorption between electronic levels split by the ZEEMAN effect [l, 21. Hence an EPR analysis of the electronic configuration of a defect basically rests on an atomic structure approach.

In a real crystal, the simplest case for such a magnetic moment may be depicted by an unpaired electron or hole bound to a lattice defect, depending on the charge state with respect to the perfect lattice. This unpaired spin is subjected to a predominant central hyperfine interaction; it involves the nuclear spin I of the core of the defect and its strength is described by the central hyperfine coupling constant A,, whose magnitude is essentially a measure of the local electronic density. The unpaired spin further undergoes ligand hyperfine interactions; depending on the nature and number of surrounding nuclei, the latter are represented by an envelope function of overall linewidth AH. In favorable cases, the individual components are resolved allowing one to separate the contributions of the successive shells around the defect. Translating the effects of the neighbourhood, this interaction determines the site ascription. Hence, at least in theory, EPR appears as a suitable tool for identifying the nature and location of intrinsic defects in a compound

') 23, rue du Loess, F-67037 Strasbourg Cedex, France.

650 A. GOLTZEN~ and C. SCHWAB

semiconductor as vacancies, interstitials or antistructure defects are likely to escape most common methods of chemical analysis.

However for GaAs, where a proper identification of defects is of paramount importance for the industry, the spectroscopic signature of the ligand hyperfine interaction remains uncomplete even in the most favorable case. This has its origin in the relatively large I values and nuclear magnetic moments of the matrix constituting elements and low ionicity of this binary compound. The consequences of this situation are best illustrated by the interpretation of the quadruplet spectrum first discovered in as-grown materials [3] and attributed to the isolated anion antisite, As:: in ionic notation or As;, in electrical notation (Fig. 1).

In fact the EPR signature consisting of four nearly identical lines only implies that it results from a central hyperfine interaction of an S = 112 electron with an I = 312 nucleus. Discarding extrinsic contaminants on the basis of chemical analysis data, one is then essentially left with the two elements of the matrix, both of which have only isotopes with a nuclear spin I = 312. Gallium has two isotopes, namely 69Ga and 71Ga with 60% and 40% relative natural abundances, respectively; arsenic is 100% abundant with its "As isotope [4]. As the nuclear magnetic moment of 'lGa is about 1.3 times that of 69Ga, the equal strength and symmetrical lineshape of the quadruplet is then in favour of an As ascription of the I = 3/2 nucleus. In principle, the lack of any further resolved structures in each quadruplet component prohibits the precise location of this misplaced As atom in the lattice. A priori, it could be either substitutionnal or interstitial, offering three possibilities in total as in a sphalerite structure there are two tetrahedral interstitial sites depending on the anionic or cationic nature of the atoms of the first surrounding shell, but only one substitional defect site, i.e. the anti-structure site.

But referring to Gap, where a centre with a full 31PGa-31P4 signature has been evidenced and ascribed to an isolated phosphorus antisite P,,[5], the EPR quadruplet has been similarly attributed to an isolated AsGa-As4 defect [3]. Though convincing the likelyhood of the physico-chemical arguments, mainly based on a scaling of the A. parameter, may appear,

I ' I I I I

Fig. 1. (a) Experimental spectrum ascribed to As;: in GaAs [8]. (b) Least mean-square fit of (a) to a quadruplet constrained to the isolated As:: model. (A, is given here in (g- 'pe-') reduced units)

Differentiation of Paramagnetic Anion-Antisite-Related Spectra in GaAs 65 1

it must be reminded here that even for GaP the initial choice between the two possible locations of the misplaced P atom, i.e. between a substitutional anti-structure site (PGa-P4) or an interstitial site (Pi-P,) is not supported by a complete EPR signature as only the ligand hyperfine structure due to the first shell has been resolved so far.

Of course, several theoretical arguments supporting the anti-structure configurations have been put forward for both GaP and GaAs. We deliberately have not reported them here since, primarily dealing with GaAs, the experimental data should be firmly established prior reference to any theoretical modeling.

2. Experimental Facts

We shall show that there is experimental evidence that the EPR quadruplet spectra observed in bulk as-grown [3], particle-irradiated [6 to 81, and plastically deformed GaAs [9 to 111, though all constrainable to the same As% model of an isolated centre, are actually differing in several aspects.

First, they may be qualitatively separated into two groups according to whether they are sensitive or not to a 1 pm illumination at low temperatures (T 5 140 K). A photo- quenching under these conditions is usually considered as a prerequisite for an eventual link between Ash: -related centres and the ubiquitous “electrical” deep donor EL2 controlling the FERMI level position in the band gap of bulk semi-insulating GaAs. It implicitely implies the following processes:

hv hv As&, + e- P As:, or As;, P As:, + h+ ,

hv As:, + As%,

where e- and h+ are electrons and holes, respectively. As$, designates the metastable state of the corresponding AsGa-related species.

Henceforth reactions (1) and (2) describe a transient neutralisation of the As&, centre during illumination, whereas reaction (2) translates the irreversible conversion of the neutral As:, into its metastable configuration, thus creating a sink for the whole process as EPR only detects the ionized centres.

Initially EL2 has been defined by photocapacitance measurements in conducting materials [12, 131; it was later recognized by optical absorption in semi-insulating materials [14], the common feature of both experiments being the existence of a metastable state optically accessed. For EPR measurements, the practical difficulty is, however, distinguishing a true metastability, characteristic of EL2, from a mere charge transfer involving a low detrapping rate of photocarriers, for instance.

At 4.2 K, grown-in or deformation-induced Ashi-related defects are both photo- quenchable [9,15 to 171, whereas the particle (fast neutrons or high energy electrons) generated ones are not [7]. Recent experiments have determined a potential origin for some controversies concerning the photo-behaviour of neutron-induced Ask: centres [18]. Indeed systematic investigations showed that in a given sample the photoquenchability of the latter could only be apparent as it depends on the relative ratio of grown-in and irradiation- generated antisites. This suggests that the conflicting experimental reports may stem from different initial concentrations ratios of the two sorts of defects, a point having so far been ignored. On the other hand, similar systematics for the deformed materials could be hindered as the initial stoichiometry deviation of the samples seems to limit the maximum

652 A. GOLTZEN~ and C. SCHWAB

concentration of the As&:-related defects to some 1OI6 cm-3 on account of the usual growth conditions [ 191. As a result, the comparison between deformation-revealed and deformation- generated antisites would remain beyond experimental reach due to an insufficient concentration range achievable by plastic deformation.

Both groups of As&: -related centres behave also differently under microwave saturation. At 4.2 K, grown-in and deformation-induced antisites are readily saturable at a microwave power level of about 100 mW. To the opposite, the neutron-generated centres are insensitive to power level, but a slight saturation effect is observed with the electron-induced ones [20].

The decay temperatures are also different with the cause of the defects. However, due to the different sensitivity levels involved for such experiments, such a comparison may not be straightforward. Therefore, we shall not insist on this aspect for the moment.

Of particular note is the observation that the first group is akin to growth-induced As&: defects, which may be related to stoichiometric composition and dislocation density. Indeed plastic deformation simulates the local stresses and strains that may occur during pulling or cooling of the crystal. The second group instead is a bulk simulation of the intrinsic defects associated with the lattice damage present in the shallow penetration region of ions subsequent to implantation doping of the GaAs wafers. As a result, these two groups of defects may also differ with respect to the physico-chemical thermodynamic equilibrium.

The inspection of the spin Hamiltonian parameters A, and AH brings further support to the separation of the As&:-related centers into at least two groups, which are the same as above. The former parameters are actually derived from a fit of the spin Hamiltonian:

H = g p , H . S + A,Z. S (3)

yielding a stick diagram, which is then convoluted by a Gaussian of linewidth AH for comparison with the isotropic quadruplet spectra. Table 1 lists the mean values of A, and AH thus determined for the different types of samples. As-grown and plastically deformed samples display larger A, and lower AH values by comparison with the particle-irradiated samples (Fig. 2).

Independent of the corresponding decays of the intensities of the As&-related signals, the variations of A, and AH with annealing temperature are too of particular interest as neutron- and deformation-induced As$: -related centres behave quite differently. Fig. 3 shows that the values of A, and AH for neutron-irradiated GaAs gradually change with annealing temperature becoming close to the values of the first group in the 450 to 600 "C temperature range [21]; in contrast these parameters remain practically unaffected by the

Table 1 EPR parameters of As::-related defects in GaAs

generation method recording temperature central hypefine linewidth ref. T(K) constant A, [FWHM]

(w4 cm-') A H ( ~ o - ~ T)

as-grown 4.2 plastic deformation 4.2 n-irradiation 4.2 e --irradiation 4.2

907 f 2 420 f 30 [411 [111

890 k 2 510 f 23 181 892 f 2 490 f 25 [*I

912 f 4 429 k 16

(Values determined at 0.1 mW microwave power level) * Data obtained by the fits of [20]

Differentiation of Paramagnetic Anion-Antisite-Related Spectra in GaAs 653

I I I I

Fig.2. Variation of the hyperfine constant and linewidth of the As:: quadruplet vs. generation parameters for neutron-irradiated (squares) and plastically deformed (triangles) GaAs. For the lowest neutron fluence, the parameters are strongly dected by an important contribution of the grown-in AsGa centre

Fig. 3. Variation of the hyperfine constant and linewidth of the As&: quadruplet generated by neutron irradiation (@ = 1.5 x 10l6 cm-') or plastic deformation ( E = 10%) vs. annealing temperature. 0 neutron irradiation, A plastic deformation

thermal treatment for the grown-in and deformation-induced antisites of the second group. These different behaviours may be accounted for by the fact that particle-generated defects are not in the same thermodynamic equilibrium with the lattice as those induced by a plastic deformation at 400 "C as evoked before.

Summing up, the different behaviours of the two groups of EPR-revealed As&:-related centres with respect to photosensitivity and microwave saturation suggest an ascription to different defects. Assuming that anion antisites are effectively involved in both cases, it means at least different sorts of As,,-related complexes. The analysis of the spin Hamiltonian parameters of the quadruplet A, and AH confirms the existence of the two groups, but also hints at the possibility of a partial thermal conversion for the particle-generated complexes during their decay. Their parameters become then close to those of grown-in and deformation-induced defects.

The main question is now translating the different EPR signatures of the As&:-related centres into different defect configurations. Therefore, we shall revisit the experimental data concerning the P,,-related spectra in GaP and refer to some recent theoretical evaluations of A,, and AH for the isolated AsGa centre and some of its complexes with As and Ga vacancies.

42 physica (b) 160/2

654 A. GOLTZEN~ and C. SCHWAB

3. Gallium Phosphide Revisited

It is well known that not only the isolated antisite P&: -P4, but also its complexes labeled P,,-P,X and P,,-plx,, where X stands for unknown defects with zero nuclear spin, have been identified in GaP [5,22 to 241. Typical examples for the latter could be a phosphorus vacancy, V,, as an example for an intrinsic defect or a carbon acceptor, Cp, for an extrinsic defect; the latter is indeed an ubiquitous contaminant of both GaP and GaAs.

Table 2 lists the corresponding EPR parameters values as derived from the following spin Hamiltonian:

H = g p , H . S + A,Z.S+ ~ A , , [ S . Z [ (4)

where A, , , is the ligand hyperfine interaction constant of the I-th neighbour of nuclear spin I,. For identical tetrahedrally coordinated first neighbours, the A1,[ values are equal for a magnetic field H , 11 [Ool], and

Neglecting the slight variation of the central hyperfine constant with temperature, we observe that both A , and A , are essentially independent of the means of generation of the P,,-P4 centre. They, however, considerably differ with the actual defect configuration P,,-p,x or PX,.

In view of the experimental situation involving now likely different configurations for the As,,-related defects, the use of the sole isolated P,, defect, i.e. P,,-P,, for an extrapolation toward GaAs may now be questioned. In order to ease the comparison between antisite defects in both materials, we report the AH(GaAs) linewidths as extrapolated from the AH(GaP) linewidths, calculating the envelope of the hyperfine structure due to the first shell according to the general expression [25]:

A , , [S . Z, = A , S . Z.

where N , is the number of nuclei in the I-th shell and ti, the relative abundance of isotope i of nuclear spin Z with its hyperfine energy W,,,. The isotropic contribution of the latter is given by

Table 2 EPR parameters of PG,-related defects in GaP

defect generation recording central ligand ref. configuration method temperature hypefine hyperfine

constant A, constant A, T(K) cm-’) (lo-, cm-’)

pGapp4 as-grown 20 966 & 13 81.5 f 4 [51 n-irradiation 4.2 989 f 10 80 f 4 1401 e--irradiation 10 lo00 84 1231

1231

pGa-px3 e--irradiation 77 700 225 1241

77 925 -

pGa-p3x e--irradiation 17 704 117 f 8 1221

Differentiation of Paramagnetic Anion-Antisite-Related Spectra in GaAs 655

where pe and pN are the electronic and nuclear magnetic moments, respectively. [y(O)l2) is the electronic density at the nucleus. Since P and As have only one isotope each, the former relation simplifies considerably; thus we may define a scaling factor F by setting

according to relation (5). In order to avoid any a priori hypothesis on the actual structure of the antisite

configuration in GaAs, we estimate the W,,(GaAs)/W,,(GaP) ratio from the ratio of the atomic isotropic hyperfine coupling constants do [4] '):

finally yielding F NN 2.11. Changing material involves a matrix-dependent factor k as we have shown in the case of InP [26]. In the present case, this further correction is not relevant as we will compare the parameters for a set of defects in a given compound, GaP or GaAs.

The AH(GaP) values are then calculated by replacing W',(GaP) by the corresponding values of A , as reported in Table 2 in the simplified formula (5) resulting from the existence of a single isotope for phosphorus, 31P with I = 1/2:

[8 lg (2)]'/' ( ;), I2 AH(GaP) = N A I - = 1.25A,N1"

gepe (9)

where AH and A , are expressed in T and cm-', respectively. The calculated values, corresponding to H, 11 [Ool], are listed on Table 3.

Of course a similar comparison for the A, values would be desirable, too. Up to now, it is hindered by lack of suitable experimental data. On one side, given the reported strong thermal variations of A , in both GaP [23] and GaAs [27,28], a valuable comparison would request to make it at identical temperatures if not at the lowest of 4.2 K. On the other side, even the recent rediscovery of an anomalous thermal variation of A, for electron-induced AS&: centres [29] after our initial report for neutron-generated centres [27] leaves these determinations incomplete for GaAs as both belong to the same group of particle-generated antisites; to our knowledge, there are still no data corresponding to the other group of grown-in and deformation-induced antisites.

From an inspection of Table 3, one may infer it is unlikely that all As,,-related centres have a complete first shell with four As atoms in spite of the lack of any report of an experimental anisotropy of the Ash: quadruplet spectra in GaAs3). In similarity to Gap, one is then tempted to assign those with narrower components to isolated AsG=-As~ defects and those with broader ones to complexes of the AsGa-As4-,,X,, type. The nature of X could well be similar in both GaP and GaAs, but it has still to be determined. One should, however, be cautious that our extrapolation requires identical electronic densities on the

') Table 2 in [2]. 3, While in agreement with our trends regarding the variations of the parameter AH, the data of

[41] also reveal its anisotropy, thus leading to a further possibility for discriminating among different As;: -related centres.

42*

656 A. GOLTZEN~ and C. SCHWAB

Table 3 Ligand hyperfine envelope of anion-antisite-related defects in GaP and GaAs

defect A1 AH defect AH (10-4 cm-1) (10-4 T) (10-4 T)

PGa-P, 82 f 2 206 f 10 AsG,-As, 435 * 20 P G a - P J 117 f 8 254 & 17 AsG,-As,X 536 f 36 P,,-P1X 225 f ? 283 f ? AsG,-AslX 597 f ?

first neighbours; this is doubtful as the bond relaxation will not be the same when switching from a 111-V to a V-V bond [35]. Therefore we favour a conservative use of the GaP data in ascribing the difference in AH for AsGa to a difference in the coordination of the misplaced As atom.

The estimate of AH that can be derived from recent ENDOR experiments on GaAs yields a value of A, = + 2A:,anisotropic)1/Z = 184.5 MHz for a centre assigned to AsGa-Asi [31]. This corresponds to AH = 347 x T, which is much less than any reported experimental value for an EPR component linewidth, including the spectrum acribed to AsGa-Asi [32]. It must, however, be kept in mind that ENDOR requires experimental conditions different from EPR, in particular with respect to microwave saturation; this could imply a selection of distinct centres, according to their characteristic time constants.

4. The Extended Huckel Molecular Model

Theoretical calculations, especially for the EPR parameters derivable from experiment, have been surprisingly scarce up to now in spite of the generally admitted suitability of the method. A semi-empirical approach by Zou and Wang [33], based on a cluster calculation, has recently brought some answers assuming that the defects in GaAs were &,-related intrinsic complexes, binary or ternary ones involving VGa andfor V,, vacancies. In effect, this theory well fits with the experimental values of A, and AH if the spectra with broader quadruplet components are ascribed to the simpler defects and those with the narrower components to the ternary complexes, the linewidth reduction being particularly sensitive to the proximity of the V,, defect (see Table4). This theory indicates also the ternary complexes as being the most stable configurations. A similar conclusion has already been reached by van Vechten several years ago on the basis of a thermodynamical analysis [34,35].

Table 4 Variations of ligand hyperfine envelope as a function of anion-antisite complex configuration in GaAs [33]

defect configuration AH variation (%I

As,, 0

AsGa-VA, (VAs on 3'd shell)

AsGa-VAs (VAs on 1"' shell) AsGa-VGa (VGa on 2nd shell)

AsGa-VAs-VGa (V on 1"' and 2"d shells) AsGa-VGa-VAs (V on 20d and 3rd shells)

-25.3 - 8.1 - 4.1 - 29.3 - 11.9

Differentiation of Paramagnetic Anion-Antisite-Related Spectra in GaAs 657

The main difference between the model advocated by Zou and Wang [33,36] and that of van Vechten and Wager [37] rests on the nature of the vacancy in the first shell around the AsGa nucleus. The former authors favour a model with VAs in the first shell, i.e. the AsGaVAsVGa configuration, whereas the second ones support the AsGaVGaVAs configuration with the VAs rejected to the third shell. In both cases, the VGa vacancy remains in the second shell. Intuitively the second model seems in better agreement with the isotropy of the As;:-related spectra, but the molecular model accounts for the lack of anisotropy by a predominant S-character of the electronic orbitals even in the presence of surrounding vacancies.

Admitting that particle-irradiation generates isolated AsGa centres or AsGa-VGa pairs, then the ternary complexes best fit with the grown-in or deformation-induced defects as the latter have the smaller AH values. The linewidth reduction in neutron-irradiated GaAs observed during thermal anneal would then be explained by a thermally activated ternary complex formation. Obviously the latter complexes can already be present in the strained material as the deformation is achieved at 400 "C. Still some differences are expected between the mechanisms in particle-irradiated and deformed materials for the complex formation mechanism that also explain further behaviours. The dislocation climb and glide mechanisms leave large amounts of both types of vacancies [38] so that the AsGaVAsVGa or AsGaVGaVAs ternary complexes formation can be immediate, while in irradiated GaAs, it would be slowed down by the limited availability of VG, centres whose concentration is only comparable to that of AsGa defects.

5. Conclusion

We have shown that both qualitative and quantitative arguments allow to separate the AsGa-related spectra into two groups on the basis of the available experimental EPR data. However, some further experiments may be suggested in order to complete the experimental picture prior to a definite model ascription. The anomalous temperature dependence of the hyperfine coupling constant for particle-irradiated GaAs should be compared to a similar experiment on plastically deformed materials. Furthermore, as the latter involves some specific glide and climb mechanisms for the dislocations, it would be interesting to compare the results to those obtained on materials deformed at room temperature. There one would expect some simple defects as in particle-irradiated GaAs since the thermally assisted complex formation would be hindered. Preliminary experiments have already revealed the enhancement of the Ash: -related centres under such experimental conditions.

Also the existence of two groups of Ash: -related centers undermines the generally accepted proof that the generation of similar antisites in the 10" cm-3 range after irradiation supports the idea that the antisites observed at concentrations limited up to some 10'6cm-3 in as-grown or strained GaAs are altogether of intrinsic nature. In other words, can an involvement of intrinsic atoms still be discarded for the photoquenchable AsGa-related defects, a species whose concentration seems still in the range of major impurities? As this concerns specifically the group which may possibly be related to EL2, the argument is also valid for the corresponding electronic level.

Independent of these reserves, the model of Zou and Wang ascribing the ternary complex AsGaVAsVGa to EL2 remarkably fits with the EPR data. Moreover this model allows a straightforward interpretation of the photoquenching of Ask: centres in so far as the singlet

658 A. GOLTZENE and C. SCHWAB

spectra appearing after illumination could eventually be linked to vacancies generated by the following reaction [39]:

Finally in view of the present complexity concerning GaAs, a deeper insight into the EPR of anion antisites in GaP may be well justified, too.

Acknowledgments

The authors gratefully acknowledge late Prof. Zou Yuanxi and Dr. Wang Guangyu from the Shanghai Institute of Metallurgy for the early communication of their work and stimulating letter exchanges. We express our thanks to Drs. S. Benakki and E. Christoffel for the data compilation.

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(Received December 6, 1988; in revised form March 12, 1990)

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