Structure characterization of MBE-grown (Zn,Cr)Se layers

Journal of Alloys and Compounds 382 (2004) 92–99

Structure characterization of MBE-grown (Zn,Cr)Se layers M. Jouanne a , J.F. Morhange a , E. Dynowska b,∗ , E. Łusakowska b , W. Szuszkiewicz b , L.W. Molenkamp c , G. Karczewski b,c a

Laboratoire des Milieux Désordonnés et Hétérogènes UMR 7603, Université Pierre et Marie Curie, 140 rue Lourmel, 75015 Paris, France b Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland c Experimental Physics III, University of Würzburg, Am Hubland, Würzburg, Germany Received 13 March 2004; received in revised form 13 April 2004; accepted 14 April 2004

Abstract Cr-based diluted magnetic semiconductors (DMSs) have attracted a lot of attention within the last few years because of their possible applications in the area of spintronics. Modern growth techniques of such materials require efficient methods of characterization of their structure. In this work, the structure of Cr-rich ZnSe layers grown by MBE on (0 0 1) GaAs substrate was investigated by means of the high-resolution X-ray diffraction, atomic force microscopy and Raman scattering. Direct evidence of the mixed Zn1−x Crx Se crystal in the zinc-blende phase is demonstrated by the high resolution X-ray diffraction (a part of Cr is shown to occupy the cationic sites in the crystal lattice). The presence of amorphous or crystalline Se- and Cr-related precipitates in selected samples is demonstrated, their influence on the properties of (Zn,Cr)Se layers is discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Thin films; Nanofabrications; X-ray diffraction; Inelastic light scattering; Diluted magnetic semiconductors; (Zn,Cr)Se; Atomic force microscopy

1. Introduction The diluted magnetic semiconductors (DMSs)—in particular, II–VI-based mixed crystals—have been intensively investigated for more than 30 years and are relatively well-known today. Most of early papers have been devoted to the properties of mixed crystals containing Mn, Fe or Co because the solubility limit for these transition metals in II–VI host lattice is very high. For many years, it was thought that II–VI-based DMSs exhibit only antiferromagnetic properties at low temperatures. A possible ferromagnetic behaviour for some of the above-mentioned mixed crystals was considered more recently. The first suggestion of possible new properties of DMSs doped with other transition metals than Mn, Fe or Co was given in Ref. [1]. In particular, the stability of the ferromagnetic state in Cr-doped, II–VI-type DMSs was predicted nearly 10 years ago using theoretical calculations based on tight binding approximation [2,3]. This property was also considered in some more recent theoretical results, like the ab ∗ Corresponding author. Tel.: +48-22-843-6034; fax: +48-22-843-0926. E-mail address: [email protected] (E. Dynowska).

0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.05.039

initio electronic band structure calculations [4–6] and the analysis of the structural stability of Cr-doped zinc-blende chalcogenides [7]. Just after the first reports suggesting possible ferromagnetic properties of II–VI-type DMSs containing Cr, an experimental evidence of this behaviour was demonstrated for several highly Cr-doped bulk crystals, like ZnSe:Cr [8–10], ZnS:Cr [9,10], ZnTe:Cr [10,11] and CdS:Cr [12,13]. Meanwhile, a progress in the technology also made it possible to grow selected crystals containing up to a few percent of Cr. The bulk Hg1−x Crx Se mixed crystals [14,15] were the first DMS of this type, (Zn,Cr)Te and (Zn,Cr)Se thin layers were the next examples ([16,17,18], respectively). In some cases, the ferromagnetic character of investigated samples was clearly established up to the room temperature, as was reported for (Zn,Cr)Te layers, for which the Cr content corresponds to the Zn1−x Crx Te nominal mixed crystal composition x = 0.2 [19,20]. Today, it is well established that for Cr-based II–VI DMSs the p–d exchange interaction is of the ferromagnetic-type but the expected ferromagnetic character of the d–d superexchange still requires clear experimental confirmation. The ferromagnetic behaviour of selected DMS attracts now a lot of attention because of possible applications in

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the area of spintronics. Practical realisation of such devices as the spin filters for an injection or detection of spin-polarized carriers is of great interest. Single crystals, thin layers or quantum structures containing Ga1−x Mnx As, EuS or Cr-based II-VI semiconducting compounds are thus the object of extensive studies in many research centres. Selected II–VI-based DMSs, for which the ferromagnetism occurs at room temperature, are of course, of particular importance. It should also be mentioned that spintronics is not the only possible application of Cr containing materials. There is also a great interest in the near- and mid-infrared semiconductor tunable lasers. Transition metal-doped chalcogenides with broad wavelength tunability at room temperature and a high optical efficiency could be excellent candidates for this purpose. The chromium as a dopant has also attracted a lot of attention because of an effective Cr2+ photoluminescence at wavelengths near 2.4 ␮m. It is one of the reasons why the wide-gap, Cr-doped II–VI semiconducting compounds like ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, Cd1−x Znx Te, Cd1−x Mgx Te or Cd1−x Mnx Te have been an object of intensive studies during the last decade (for detail see e.g., Refs. [21–23]). Cr-doped ZnSe is a particularly interesting material because of a small lattice mismatch between ZnSe and GaAs, which could be very useful for several optoelectronic devices. For example, possible application of ZnSe:Cr as a nonlinear light up-conversion material (converting red-green light into blue) has been demonstrated very recently [24,25]. Thus, it is not surprising that every improvement of the material properties or a progress in an understanding of the physical phenomena in such materials attracts a lot of attention. In spite of several experimental efforts undertaken within the last few years, there still exists in the literature a discussion on the origin of the ferromagnetic behaviour observed for numerous II–VI semiconducting compounds containing a relatively high amount of Cr. The first possibility is that the Cr ions occupy the substitutional cationic position in the host lattice so that a real mixed crystal is created. In such a case, the ferromagnetic property of material would be an inherent property of this II–VI-based DMS resulting from the superexchange interaction between Cr ions randomly distributed in the crystal lattice. The second explanation, based on the fact that the expected Cr solubility limit in II–VI compounds is very low, lay on the magnetic properties of other phases built of Cr-related precipitates. While most of the possible precipitates would exhibit antiferromagnetic properties some could present a ferromagnetic order in selected temperature range. The low-temperature molecular beam epitaxy (MBE) technique made it possible to grow II–VI-based DMSs with a relatively large Cr concentration. During the last few years, extensive studies of (Zn,Cr)Te layers containing up to 20% of Cr were performed [19,20]. According to recent data, [19,20,26] in the low composition range, a part of Cr ions can be introduced into the zinc-blende

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lattice and occupy Zn sites. The lattice parameter value for Zn1−x Crx Te mixed crystal in the composition range up to x ∼ 0.02 linearly increases with the crystal composition. With an increasing Cr content, new phases (non-identified Cr-related precipitates) are created and the lattice parameter value for grown (Zn,Cr)Te layer does not change or can even decrease in some cases [26]. Recently, (Zn,Cr)Se epilayers with a significant Cr concentration were grown by MBE [27]. According to both the electron microprobe analysis and the magnetization measurements by means of SQUID magnetometer, the Cr content was as high as 2.1% of the total amount of metals. First results of studies (mostly limited to the magnetic properties and to the photoluminescence of grown layers) were shown in this paper. The photoluminescence data demonstrated an important concentration of Cr ions in Cr2+ states in the layers. This charge state of chromium was the only one detected by this method but an indirect evidence of Cr3+ states (deduced from the magnetization per ion Cr) was also given in Ref. [27]. The analysis of the magnetization curves obtained for Cr-rich ZnSe layers showed a strong ferromagnetic coupling between Cr spins, but, at the same time, demonstrated that the Cr ions are not distributed randomly. Those observations lead to the conclusion that the observed magnetic behaviour of the layers is very similar to that expected for a superparamagnetism. The authors suggested the presence of ZnCr2 Se4 precipitations as a possible explanation of this effect (quite similar magnetic behaviour suggesting a superparamagnetism was also recently observed for (Zn,Cr)Te layers containing 8% of Cr [26]). Finally, it should be mentioned that up till now a direct proof of even partial substitution of Zn by Cr in ZnSe does not exist for (Zn,Cr)Se layers. The goal of present investigations is to check if the classical methods of crystal structure characterization (like a standard X-ray diffraction (XRD), a high resolution X-ray diffraction (HRXRD), an atomic force microscopy (AFM) and Raman scattering) are sensitive enough to identify the phase composition of the MBE-grown layers as well as to determine the real composition of these layers.

2. Experimental details From 1- to 3-␮m thick, (0 0 1)-oriented ZnSe layers were grown by MBE on (0 0 1) GaAs substrates in standard Se-rich growth conditions. During the growth, some layers were doped with Cr. Because of the high Cr melting temperature (resulting in a very low flux of chromium ions when heating the Cr effusion cell) a special effusion cell operating at temperatures up to TCr ∼ 1240 ◦ C was used for Cr incorporation. For the comparison, some layers nominally containing Cr were grown when the effusion cell was kept at significantly lower temperature (starting from TCr = 600 ◦ C). Finally, in some cases two-step growth process was applied. The first step consisted of the deposition of a thick pure ZnSe buffer layer while during the second one,

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the main layer containing Cr was grown. In every case, the substrate temperature was kept at 300 ◦ C, which, according to the literature data, corresponds to the optimal condition for the growth of ZnSe layer on a GaAs substrate. The growth rate was equal to 0.7 ␮m/h. The presence of Cr in ZnSe layers and its concentration was investigated previously on selected twin samples by electron probe microanalysis, a photoluminescence and magnetization measurements with the use of a SQUID magnetometer. Details of these studies and the numerical values were recently published in Ref. [27]. Systematic studies of the surface morphology of samples both pure and nominally doped with Cr were performed using AFM, which provides a powerful technique for microscopic surface roughness measurements. AFM data were collected with a Digital Instruments Multi Mode Scanning Probe Microscope using a tapping mode option. The typical sample surface area analyzed by AFM was 100 ␮m2 and for each sample a few different areas were explored. The phase analysis of the layers was performed by a conventional X-ray diffraction method. The high-resolution X-ray diffraction measurements (MRD Philips diffractometer in double- and triple-crystal configurations) were used for both an estimation of the crystal quality and the precise measurements of the lattice parameters of these layers. The Raman spectra were obtained in a quasi-backscattering geometry on a Jobin–Yvon U1000 spectrometer equipped with holographic gratings and a S20 photomultiplier. The 514.5 nm Ar+ laser line served for the excitation. Spectra were collected within the frequency range from 0 to 350 cm−1 , with a spectral resolution better than 2 cm−1 . The investigated samples were glued on the cold finger of a continuous-flow helium cryostat. All Raman spectra were

taken at room and at low temperature (T ∼ 15 K). Both the Stokes and anti-Stokes part of Raman spectrum were collected and analyzed. The exact frequency positions and the width (FWHM) of all structures observed in Raman spectra were fitted assuming the Lorentzian form of phonon lines.

3. Results and discussion The crystal quality of all grown layers (both pure and containing Cr) was estimated on the basis of FWHM values of the 0 0 4 Bragg reflections (rocking curves), obtained as a result of ω-scan when applying the double-crystal configuration of the diffractometer. Some examples of the diffraction data are presented in Fig. 1. The typical FWHM value of 0 0 4 rocking curve for pure MBE-grown ZnSe on GaAs substrate is about 110 arcsec, while in the case of the second sample containing Cr-rich ZnSe layer this value is equal to 262 arcsec. The (Zn,Cr)Se layer was grown on a ZnSe buffer layer, so one can suggest that, in this case, two rocking curves corresponding to two separate epitaxial layers are superimposed. The difference between the lattice parameters az (perpendicular to the surface) of these two layers must be very small so the corresponding peaks are not well separated. Indeed, further measurements performed in 2θ-ω mode with a crystal-analyzer (triple-crystal geometry) show only a single peak for pure ZnSe layer (Fig. 2), but confirmed a double structure of the 0 0 4 diffraction peak in the case of (Zn,Cr)Se/ZnSe heterostructure (Fig. 3). One should stress that in our case an increase of the lattice parameter value because of possible Cr in the interstitial sites (analogous to the well-known case of Mn in Ga1−x Mnx As) can be excluded. The reason is that transition metal impurity in-

Fig. 1. 0 0 4 rocking curves for two investigated samples. Left side: single layer grown when TCr = 600 ◦ C (no Cr detected). Right side: layer grown on ZnSe buffer layer, TCr = 1240 ◦ C (presence of Cr confirmed).

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Fig. 2. The 2θ-ω scan (0 0 4 reflection) taken for the sample grown when TCr = 600 ◦ C.

troduced to III–V semiconducting compound is electrically active (Mn replacing Ga in Ga1−x Mnx As acts as an acceptor). As a result of the self-compensation mechanism with an increasing Mn content, a part of Mn ions, is forced to occupy the interstitial sites in the crystal lattice (such ions act as double donors). This mechanism does not exist for transition metal doped II–VI compounds and Cr in the interstitial site should be an exception in our case. The splitting of the 0 0 4 peak proves that at least a part of Cr atoms really occupy cationic sites in the ZnSe matrix, which results in the zinc-blende Zn1−x Crx Se mixed crystal. From the angular position of the 0 0 4 peak measured in the 2θ-ω mode, the lattice parameters in the growth di-

rection (az ) can be calculated. In the case of pure ZnSe (Fig. 2) we obtained az = 0.56677 ± 0.00001 nm, which means that the layer is under residual thermal tensile strain (the lattice parameter for bulk ZnSe is equal to a = 0.56688 nm). Such behaviour is well-known—the reason of it is the difference between thermal expansion coefficients of the layer and the substrate materials. The az values, obtained for (Zn,Cr)Se/ZnSe heterostructure (Fig. 3), are: az1 = 0.56650 nm and az2 = 0.56683 nm. A problem arises as to which component of the split peak can be attributed to the Cr-rich ZnSe layer (which is expected to be the Zn1−x Crx Se mixed crystal) and which one to pure ZnSe. For a proper interpretation, one has to take into account that ZnSe buffer

Fig. 3. The 2θ-ω scan (0 0 4 reflection) taken for the sample grown when TCr = 1240 ◦ C.

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is capped by (Zn,Cr)Se layer. As we know from our earlier studies [28], the cap layer causes strain of the covered layer so, we can attribute az1 to the ZnSe buffer. The (Zn,Cr)Se/ZnSe structure is pseudomorphic (due to very small lattice mismatch between these two layers, both of them more than 1 ␮m thick) so we can estimate the relaxed lattice parameter of Zn1−x Crx Se according to the formula: arel =

az + 2(c12 /c11 )axy 1 + 2(c12 /c11 )

(1)

where az is the lattice parameter of Zn1−x Crx Se layer in the growth direction; axy is in-plane lattice parameter of ZnSe calculated from the above formula with arel = 0.56688 nm; c12 and c11 are elastic constants of ZnSe [29]. As a result, we obtained arel = 0.56705 ± 0.00002 nm for Zn1−x Crx Se layer. This result differs noticeably from the lattice parameter value of pure ZnSe. This finding can be considered as the first direct evidence of a Zn1−x Crx Se mixed crystal phase of zinc-blende structure in (Zn,Cr)Se layer (one should remind previous indirect indication in favour of this possibility, resulting from the spectrum of photoluminescence, characteristic for the Cr2+ state [27]). Moreover, on the basis of the observed difference between lattice parameters one can try to estimate the composition of this mixed crystal. To our knowledge, the only experimental data relating an increase of the lattice parameter value for MBE-grown (Zn,Cr)Se layer to an increase of Cr content are presented in Ref. [18]. According to the authors, the X-ray diffraction and the reflection high-energy electron diffraction (RHEED) indicated that high structural layer quality was maintained for Cr incorporation up to ∼1019 atoms cm−3 (corresponding to the Zn1−x Crx Se mixed crystal composition x ∼ 0.001). Samples with Cr concentration close to ∼4 × 1020 atoms cm−3 had the lattice parameter exceeding that for ZnSe by 0.0015–0.0020 nm. This is a huge effect unknown for any II–VI-based, DMS mixed crystals studied till now, so we strongly believe that it should have a different origin. Surprisingly, for Cr concentration smaller by more than one order of magnitude, the lattice parameter value reported in the same paper, decreases only slightly. The precision of XRD measurements is not as good as that for HRXRD so we will not use the numerical values given in Ref. [18] for our estimations. Recently, in two theoretical papers the expected lattice parameter value of a hypothetical CrSe of zinc-blende structure was calculated. Almost identical values, 0.580 and 0.5833 nm, are given in Ref. [30,31], respectively. Both papers also predicted the lattice parameter value for a hypothetical zinc-blende CrTe in a reasonable agreement with the value extrapolated from existing data for Zn1−x Crx Te mixed crystal [17]. This agreement is a strong argument in favour of the quality of these calculations. However, it should also be mentioned that recent data taken for Zn1−x Crx Te mixed crystal [26] suggest noticeably faster increase of the lattice parameter when increasing Cr content.

Finally, the zinc-blende CrSe lattice parameter value could also be estimated from the extrapolation of X-ray diffraction data taken for Hg1−x Crx Se mixed crystals (where x < 0.05) [32]. Such extrapolation is subjected to some errors but, nevertheless, it suggests a value of about 0.60 nm, close to the theoretically predicted value. Under these circumstances, we believe that it is possible to use the expected zinc-blende CrSe lattice parameter value 0.5833 nm in order to estimate the real composition of Zn1−x Crx Se mixed crystal with a limited accuracy. As a result of such calculations, a mixed crystal composition x = 0.009 ± 0.002 is determined, which gives (considering the total amount of Cr atoms introduced during the growth) no more than 50% of Cr in the substitutional cationic position. As it was mentioned before, there is a very low probability that remaining Cr atoms occupy the interstitial sites. These atoms must precipitate forming other phases. One can speculate that the Cr solubility limit in ZnSe probably cannot be higher than 1%, in agreement with all existing data for ZnSe:Cr grown by various techniques until now. It also means that a correct interpretation of the magnetic properties of (Zn,Cr)Se layers with a high Cr content requires at least two contributions: one due to zinc-blende Zn1−x Crx Se mixed crystal (the superexchange between Cr2+ ions) and the second one due to precipitates containing Cr3+ ions. In order to identify possible precipitates, Raman scattering studies were performed on ZnSe layers grown for a different TCr . All the structures observed in Raman spectra resulted from light scattering by phonons. The measured spectra were compared to spectra taken on pure ZnSe bulk crystals and to previously published data. Fig. 4 shows the evolution of the Raman spectra when increasing TCr (only the Stokes parts of spectra are presented). The upper spectrum corresponds to the pure bulk ZnSe crystal. Apart from large first-order Raman scattering from TO and LO phonon, some small structures, which can be attributed to second order scattering from two-phonon combination (mostly additive), were also observed. The spectrum presented in the middle was recorded on the sample grown when TCr = 600 ◦ C. As one can see, only the first-order Raman scattering was observed in this case (the two-phonon combinations were not observed for MBE-grown thin layers because of the low efficiency of such processes). The small layer thickness made it possible the observation of the LO- and TO-phonons of the GaAs substrate. For the zinc-blende structure, the Raman scattering in backscattering geometry on a (0 0 1) plane is forbidden for the TO-phonon. However, we observe peaks situated at the TO phonons frequencies of both the ZnSe layer and the GaAs substrate. The first effect can be justified by misalignments in the experiment geometry, the second one is due to high GaAs doping. A similar spectrum (not shown in Fig. 4) was obtained for the layer grown when TCr = 1050 ◦ C. A new situation appears when TCr ∼ 1240 ◦ C, as shown in the bottom spectrum in Fig. 4. New weak and very broad Raman structures, covering nearly all the spectral range under investigations, are observed. Least-squares

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how uncertain as the A2 phonon is normally Raman inactive (it is infra-red active) [34]. Fig. 6 summarizes the results and shows the evolution of the phonon frequencies for increasing temperatures of the Cr effusion cell. The points at TCr = 0 ◦ C are experimental data taken for pure bulk ZnSe crystal. The expected phonon frequencies for the most probable Cr-related precipitate (ZnCr2 Se4 ) [35,36] and those corresponding to the trigonal Se are also shown in this figure. Considering these results, it seems difficult to identify by Raman spectroscopy a presence of ZnCr2 Se4 in the deposited layers. If this compound is present in the form of precipitates, the size of grains should be very small. The influence of the Cr on Raman scattering spectra has been investigated for bulk ZnSe crystal containing up to about 0.6% of Cr [37]. The authors of Ref. [37] observed two modes at about 200 cm−1 and 230 cm−1 , which they attribute to modes localized on Cr atom in a substitutional position at Zn site. The suggestion of lower mode frequencies when substituting Cr to Zn seems somehow unlikely. Unfortunately, in our samples, the spectral range of interest is obscured by the Raman signal of Se precipitates so we cannot confirm this observation.

Fig. 4. Comparison of Raman scattering spectra taken at room temperature for a few investigated samples (see text for details).

fit of these structures (Fig. 5) show that they are centred close to the frequency positions of the phonons in Se [33] and are probably due to amorphous Se (in the form of linear chains). Apart from those broad Raman structures, weak peaks near 107 and 140 cm−1 are also observed. The mode near 140 cm−1 can be attributed to the E1 mode of trigonal crystalline Se, the mode near 107 cm−1 could be the A2 mode of the same crystal but this attribution is some-

Fig. 5. Raman scattering spectrum taken at room temperature for the sample analyzed in Figs. 1 and 3 (TCr = 1240 ◦ C). The solid line corresponds to experimental data, thin solid curves are results of a fit assuming a Lorentzian lineshape of the phonon structures.

Fig. 6. Comparison of the phonon mode frequencies observed for layers grown at different conditions with those corresponding to pure ZnSe, amorphous and trigonal Se, and ZnCr2 Se4 . All experimental data (full circles) were taken at room temperature. Diamonds and open circles correspond to the frequency values for trigonal Se and ZnCr2 Se4 at room temperature, respectively, small full circles with bars represent the maximum positions and the width of structures attributed to amorphous Se, respectively. For the clarity, data obtained for one from two samples grown at the highest temperature of the Cr effusion cell (TCr ∼ 1240 ◦ C) are slightly shifted.

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Fig. 7. Details of the sample surface for selected layers observed by AFM. Temperature of the Cr effusion cell: (a) TCr = 600 ◦ C and (b) TCr ∼ 1240 ◦ C.

AFM made it possible to investigate the surface morphology for samples grown at different conditions. Fig. 7 shows the comparison of the surface for two samples grown when TCr = 600 ◦ C and TCr ∼ 1240 ◦ C, respectively. None of these surfaces is really flat as it could be expected for pure ZnSe layer grown at optimal conditions. The first layer (TCr = 600 ◦ C) did not contain Cr, as it was demonstrated by the electron microprobe and was deposited directly on GaAs substrate (Fig. 7a). The surface of this sample is more flat, the root mean square roughness (RMS) is equal to 11.08 nm. Outside the zone of big structures seen in Fig. 7a one can also observe areas of the order of 1 ␮m2 , for which the RMS is in the range from 0.64 to 0.75 nm only. The second layer containing Cr was grown on ZnSe buffer layer (Fig. 7b). The surface of the second sample is not as smooth as that corresponding to the previous one (RMS = 22.47 nm). This increase of the roughness of the surface could result from some amount of Se- and Cr-related precipitates in the second sample. It is well-known that when increasing the thickness of a layer, some polycrystalline islands develop even on the surface of pure ZnSe layers [38]. It is also known that small growth hillocks, resulting from 3D growth, can nucleate on imperfections on the interface, such as defects or impurities for several heterostructures (see, e.g., Ref. [39]). As we believe, really small grains of Se- or Cr-related precipitates (like clusters or nanocrystals) can play an active role in the creation and growth of such hillocks on the layer surface. The change from quasi 2D growth (the case of ZnSe) to a 3D growth mode, observed for (Zn,Cr)Se layer, can be considered as an indirect evidence of such precipitates. Finally, it should be mentioned that the direct observation of Se precipitates by AFM has been reported in bulk ZnSe crystals grown by physical vapour transport with a slight excess of Se [40]. An upper limit of the diameter of the Se inclusions was estimated to be about 20 nm in that case. It should also be mentioned that the formation of nanometer size Se clusters on the surface of ZnSe epilayers grown by metal organic chemical vapour deposition (MOCVD) has been recently reported [41]. According to this paper, Se clusters preferentially emerged from areas of the epilayer where there was a

high concentration of nonstoichiometric defects. In our case this additional phase could be an indirect consequence of the Cr effusion cell heating to the highest available temperature and/or could be caused by a presence of Cr-related precipitates. Our data confirm previous results showing that Cr is not introduced into ZnSe at relatively low temperatures of the Cr effusion cell [27]. The magnetization data [27] show the ferromagnetic character of the investigated layers containing Cr grown when TCr = 1240 ◦ C but the question arises whether is it due to Cr in the cationic sites in zinc-blende Zn1−x Crx Se mixed crystal or it is simply due to introduced Cr-related precipitates. There exist a number of candidates for such precipitates, including metallic Cr, chromium selenides (like, e.g., CrSe, CrSe2 , Cr2 Se3 , Cr3 Se4 , Cr5 Se8 , or Cr7 Se8 ) and ZnCr2 Se4 . These phases are mostly antiferromagnetic, but for some of them the magnetic properties are not well-known for a wide temperature range below the room temperature and for a few a ferromagnetic behaviour was demonstrated at low temperatures. ZnCr2 Se4 is a paramagnetic semiconductor at room temperature but becomes ferromagnetic below the Curie temperature TC = 115 K. It remains ferromagnetic up to the next phase transition temperature taking place at the Néel temperature TN = 21 K where it becomes antiferromagnetic with a helical spin structure (see Ref. [42] and references). According to Ref. [27], possible precipitates of this compound could explain their data as a result of superparamagnetism (assuming in addition the lack of helimagnetic order at temperatures below T ∼ 20 K because of a finite size effect). Cr3+x Se4 is an antiferromagnet for x = 0 but it can be a metamagnet or a ferromagnet for x < 0 [43] (the required non-stoichiometry could result from an excess of Se during the MBE growth). Finally, Cr5 Se8 is also an antiferromagnetic crystal but a weak parasitic ferromagnetism was reported for this compound at temperatures below 100 K [44]. Unfortunately, none of ferromagnetic crystals listed above was directly observed by our standard X-ray diffraction measurements within the accuracy of this characterization method and it is likely that such precipitates because of their small size could not be de-

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tected by this technique. Further detailed studies for greater set of samples are required before the final identification of precipitates can be done.

4. Conclusions MBE-grown (Zn,Cr)Se layers containing up to about 2.1% of Cr were investigated by XRD, HRXRD, AFM and Raman scattering. Existence of the zinc-blende Zn1−x Crx Se mixed crystal with the composition x ∼ 0.009 was directly demonstrated by HRXRD for the layer with the highest Cr content, at least a half of transition metal introduced into this layer should create Cr-related precipitates. The Raman spectra taken for layers grown when heating the Cr effusion cell to the temperature TCr ∼ 1240 ◦ C showed a presence of an amorphous and a trigonal (crystalline) Se. The results of AFM measurements demonstrated an influence of TCr on the surface morphology and suggested a presence of some precipitates in (Zn,Cr)Se layers. However, no direct evidence of such precipitates (in particular, of previously suggested in the literature ZnCr2 Se4 spinels) was found within the experimental accuracy by the characterization methods listed above.

Acknowledgements This work was supported in part within European Community programs: ICA1-CT-2000-70018 (Centre of Excellence CELDIS), G1MA-CT-2002-04017 (Centre of Excellence CEPHEUS) and by SPINOSA project (IST-2001-33334). The partial financial support by the French program ACI NR0095 “NANODYNE” is also acknowledged.

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