OBSERVATION OF Se 77 SUPERHYPERFINE STRUCTURE ON THE ELECTRON-PARAMAGNETIC RESONANCE OF Fe3+ (3d S ) IN CUBIC ZnSe

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1 R540 Philips Res. Repts 20, , 1965 OBSERVATION OF Se 77 SUPERHYPERFINE STRUCTURE ON THE ELECTRON-PARAMAGNETIC RESONANCE OF Fe3+ (3d S ) IN CUBIC ZnSe by J. DIELEMAN Abstract The electron-paramagnetic-resonance spectrum of Fe 3 +(3d 5 ) in cubic ZnSe has been studied at 9 3 Gel». The spectrum shows the usual behaviour of a 3d5 configuration with a 6S5/2 ground state in a cubic environment. The relevant parameters in the spin Hamiltonian are an isotropic g = ± 0'0002 and a cubic-field-splitting parameter a = (48'3 ± 0'5).10-4 cm-i at 77 "K. Hyperfine structure due to the Fe5? isotope is resolved at 1 9"K, the value of the hyperfine-splitting constant A = (6 75 ± 0.05).10-4 cm-i. The values of these parameters are compared to those found for Fe3+ in cubic ZnS and cubic ZnTe. Just as has been done by others for the 3d5 configurations of Cr+ and Mn 2 + the variation of the parameters for Fe 3 + in the series ZnS, ZnSe, ZnTe are discussed in terms of increasing covalent bonding in this series. Resolved hyperfine structure has been observed at 1 9 "K on the M = +t... -t transition due to the interaction with the nuclear magnetic moment of Se?? on one of the four equivalent nearestneighbour Se sites surrounding the Fe 3 +. This interaction is axial with the symmetry axes lying in the Fe-Se directions. The hyperfine parameters are Til = (11'5 ± 0.2).10-4 cm-i if H is parallel to the hyperfine axis and Tl. =(6'7 ± 0'2).10-4 crrr-! if H is perpendicular to this axis. 1. Introduetion Although quite a number of papers have appeared in the last two years on the electron-paramagnetic-resonance (e.p.r.) spectra of Cr+, Mn 2 + and Fe 3 + in a number ofii-vi compounds 1,2,3), there are members ofthis series which have not yet been investigated: This paper describes some interesting results obtained on the e.p.r. of Fe3+ in cubic ZnSe. 2. Samples The single crystals of cubic ZnSe were prepared either by Ie transport or from the melt. They contained Fe as a trace impurity in concentrations of per cmê or lower. One batch was doped with iron enriched to 98% in Fe Apparatus. Our e.p.r. data were taken with the aid of a "Varian" spectrometer operating at about 9300 Mc/s. Magnetic-field modulation was used with a frequency of lookc/s at 77 "K and 200 cis at 2 "K. At both temperatures provisions were made to allow optical excitation of the samples in the microwave cavity.

2 THE ELECTRON-PARAMAGNETIC RESONANCE OF FeB+ (3d5) IN CUBIC ZnSe Production of Fe 3 + The e.p.r. pattern of Fe3+ could be produced in several ways, one being optical excitation of the samples with the 436-mfL line of a mercury-discharge tube. In that case holes generated by the optical excitation are captured by divalent iron, leading to partial conversion of Fe 2 + into Fe 3 +. Another way was to refire the samples in high selenium pressures, which also resulted in partial conversion.of Fe 2 + into Fe 3 +, in all probability by the formation of compensating zinc vacancies. The divalent iron was converted quantitatively into Fe3+ by diffusing a slight excess of copper over iron into the crystals. 5. Results and discussion The behaviour of the allowed fine-structure transitions in the magnetic field can be described by the usual spin Hamiltonian Hf for the case of a 6S 5 /2 ground state in a cubic field 4): Hf = gf3h. S + ia [Sz4 +Sy4 + Sz4-iSfCS + 1)(3S 2 + 3S-1)], (1) where g is the spectroscopie splitting factor, f3 is the Bohr magneton, H is the magnetic field, Sis the electron-spin operator,a is the cubic-field-splitting parameter, Sz, Sy and Sz are the components ofthe electron-spin operator along the cubic <100> axes of the crystal, and S = 5/2. Since in our case a«hs the formulas of Kronig and Bouwkamp 5) could be used to obtain the values of the parameters occurring in eq. (1). Their values are given in table I together with the values for Fe 3 + in cubic ZnS and cubic ZnTe. For illustration the e.p.r. pattern obtained at 77 OK for H 11 [001]is shown in fig. 1, whereas the field position versus 8 of the central (---t+-...+d transition for H rotating in a (110) plane is given in fig aa. +- Sa Fig. 1. The derivative e.p.r. spectrum at 77 "K if the magnetic field is parallel to the [OOI] axis with H increasing from the left to the right. The centralline is less broadened by crystalline strains than the others.

3 208 I.DIELEMAN As can be seen from table I the g-value of Fe 3 + increases regularly on going. from ZnS to ZnTe. This behaviour is similar to that of Cr+ 1) and Mn2+ 2).. Now, Fidone and Stevens 6) have shown that covalent bonding affects the Fig. 2. The field position of the central (+t... -t) line of Fe3+ in cubic ZnSe at 77 "K as a function of (J, where (J is the angle between the [001] axis and the magnetic field H, with H rotating in a (110) plane. The solid curve corresponds to the field positions calculated with the aid of the formulas of Kronig and Bouwkamp 5). Circles are experimental points. Ho TABLEI E.p.r. parameters for Fe 3 + in cubic ZnS, ZnSe and ZnTe compound ZnS ZnSe ZnTe cation-anion separation (A) temperature of 0bservation COK) g ± ± ± a (10-4 crrr+) ± O S 48 3 ± ± 20 A (Fe 57 ) (10-4 cnr") 7 70 ± ± ± 0 2 T (Se77) (10-4 cm- 1 ) - I ± 0 2*) j_ 6 7 ± 0 2 reference 3b I this paper 3c *) Observed at 2 "K; 11 or j_ means parallel or perpendicular to the Fe-Se direction. -

4 THE ELECTRON.PA~MAGNETIC RESONANCE OF Fe 3 + (3d5) IN CUBIC ZnSe 209 g-value. Furthermore, as will be discussed later on in this paper, the regular decrease in hyperfine splitting for all three ions on going from ZnS to ZnTe. proves that covalent bonding between the impurity and its neighbours increases monotonically in this series 7). Qualitatively the effect is seen to be the same for all three ions: an increase in covalent bonding gives rise to an increasing positive constribution to the g- value. Further regularities are that the higher the charge on the impurity (Cr+, Mn 2 +, Fe 3 +) the larger is the g-value and the _more pronounced is the increase in the g-value on going from ZnS tó ZnTe. The behaviour of a in this series of compounds is peculiar. Gabriel, Johnston and Powell 8) showed that for ionic crystals a should decrease with increasing interionic distance. Title 1, 2) found that for Cr+ and Mn2+ the effect on the a- value of increasing interatomie distances (see table I) in the series ZnS to ZnTe was more than compensated by the increase in covalent bonding, i.e. a remained positive and increased regularly in the series from ZnS to ZnTe. However, Fe3+ behaves quite differently. The parameter a decreases from ZnS to a smaller positive value in ZnSe and changes sign from ZnSe to ZnTe. For Fe 57 (natural abundance 2,17%, I = t) 8) the spin Hamiltonian (1) has to be augmented by a hyperfine-structure term Hws: = A I.S, (2) where A is the hyperfine-structure constant and I is the nuclear-spin operator. The value of A was determined at 2 "K on a ZnSe crystal doped intentionally with Fe enriched in the Fe 57 isotope; see fig. 3 for illustration ofthe Fe57 hyperfine splitting on the central fine-structure line. The values of A, shown in table Fig. 3. The Fes? h.f.s, on the central Ms = +t+--+-t transition at 2 "K, observed on a ZnSe crystal doped with Fe 5? (the two most intense lines); H was parallel to the [111] axis.

5 210 J. DIELEMAN, I, are seen to decrease on going from ZnS to Zn'Te, which is consistent with increasing covalent bonding in this series. The values are 66 2 %,58,0 % and 36 1 %, respectively (see table 11) of the values calculated by Watson and Freeman 10) for a 100 % ionic lattice. Another feature of the Fe3+ resonance is the absence of appreciable superhyperfine interaction with Zn 67 (natural abundance 4 11 %, I= 5/2) 9) on the twelve nearest-neighbour Zn sites. The values of this interaction reduce in ZnS from cm-1 for Cr+ to cm- 1 for Mn2+ and to less than 0, crrr! for Fe3+ 1), whereas the covalent bonding changes much less rapidly (see table 11). Unfortunately, the natural abundance of S33 is too small (0'75 %, I.:_3/2) 9) to see whether the superhyperfine interaction with the four nearest-neighbour chalcogen sites increases in this series. In this respect ZnSe is much more convenient because of the larger natural abundance of the Se 77 isotope (7 58 %, I= t) 9). Again, no superhyperfine structure (s.h.f.s.) due to Zn67 was observed on the Fe3+ resonance, although table 11 shows that the covalent bonding has increased. However, a quite large superhyperfine interaction with Se77 on one of the four equivalent nearest-neighbour Se sites was resolved at 2 "K. This interaction could be described by adding a superhyperfine term Hs,h.f of the following form to (1): 4 Hs.h.f. = 1: S. Tk.rs, k=l (3) where Tk is the tensor describing the hyperfine interaction with the k-th Se 77 nucleus with nuclear magnetic moment Ik. The interaction was found to be axial along the 'Fe-Se directions (four. equivalent <111> directions of the crystal). For arbitrary directions of the magnetic field T2 = 1112 cosê 8 + Tl.2 sin2 8, where Til and Tl. refer to the orientation of the magnetic field along TABLEII Value of Ä of Cr+, Mn2+, Fe3+ in the compounds ZnS, ZnSe, ZnTe, expressed as percentage of the theoretical value for a pure-ionic lattice. See Title's article ref. 3b, for a more detailed account of this subject ~ e Cr+' Mn2+.Fe3+ ZnS ZnSe ZnTe

6 THE ELECTRON-PARAMAGNETIC RESONANCE OF Fe3+ (3d5) IN CUBIC ZnSe 211 Fig.4. The central (+t t) line of Fe3+ in cubic ZnSe observed at 2 "K. Figure 4a was taken with the magnetic field H 11 [001] and figure 4b with H" [111]. The Fe57 hyperfine structure (indicated by arrows) is just visible as an inflection on the Se77 lines. or perpendicular to the hyperfine axes. For illustration the patterns obtained for the central (-t..-..+t) fine-structure line when HII [001] and HII [111] are shown in figs 4a and 4b. The intensity of each ofthe two Se 77 hyperfine lines, observed with H II [001], was found to be (16 3 ± 0 2)% ofthe main line, which is veryclose to the expected [4.(0 9242) /(0 9242)4].t.100 % = 16 4 %. The values of 111 and Tl. are given in table 1. The Se 77 interaction contains an isotropic part consistent with covalent bonding between iron and selenium. Its observation is also additional evidence for the Fe 3 + being at a substitutional Zn site of the lattice. Its magnitude reflects the hole-trap character of Fe 3 + in ZnSe,just as the large Zn 67 interaction in the case of Cr+ in ZnSe 1) reflects the electron-trap character of Cr+. The changing over of appreciable interaction with nearest-neighbour chalcogen sites and the absence of interaction with nearest-neighbour Zn sites in the case of Fe 3 + to the reverse in the case of Cr+ is in all probability directly connected to the different behaviour ofthe parameter a for these ions in the series ZnS to ZnTe. Acknowledgement The author thanks Mr J. W. de Jong for supplying a number of beautiful ZnSe single crystals and Mr G. Koch for preparing the powder material. Eindhoven, January 1965

7 , 212 J.DIELEMAN REFERENCES 1) Cr+: (review) R. S. Title, Phys. Rev. 133A, 1613, ) Mn2+: (review) R. S. Title, Phys, Rev.131, 2503, ) Fe 3 +: a) CdS: J. Lambe, J. Baker and C. Kikuchi, Phys. Rev. Letters 3, 270,1959. b) ZnS: A. Räuber and J. Schneider, Z. Naturforsch. 17a, 266,1962; or R. S. Title, Phys. Rev. 131, 623, c) ZnTe: J. C. Hensel, Bull. Am. phys. Soc. 9, 244, ) A. Abragam and M. H. L. Pryce, Proc. roy. Soc. London A20S, 135, ) R. De L. Kronig and C. J. Bouwkamp, Physica 6,290, ) 1. Fidone and K. W. H. Stevens, Proc. phys. Soc. London 73, 116, ) J. S. van Wieringen, Discussions Faraday Soc. 19, 118, ) J. R. Ga briel, D. F. J ohnston and M. J. D. Powell, Proc. roy, Soc. LondonA264, 503, D) D. Strominger, J. M. Hollander and G. T. Se ab or g, Rev. mod. Phys.30, 585, ) R. E. Watson and A. J. Freeman, Phys, Rev. 123,2027,1961.