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1. General Classification for Presenting the Experimental Results

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1. General Classification for Presenting the Experimental Results CHAPTER V X-RAY ABSOHPTION SPECTRA OF S01V1E IN'l'hRME'l'ALLIC COMPOUr\DS 1. General classification For presenting the experimental results obtained on the selenides, the present chapter has been divided into three parts for the convenience of discussion. The first part describes the results on some group I selenides (Ag2Se, CuSe and CU3Se2) and the second describes the work on group II selenides (ZnSe, CdSe and HgSe). In the last part of the chapter is described the work on germanium selenides (GeSe and GeSe2). GROUP I SELENIDES 2. Results Fig. 17 shows the diffraction patterns of the samples of Ag2Se, CuSe and CU3Se2 prepared in the laboratory follow­ ing the general method outlined in Chapter II, along with those of silver, selenium and copper. Cu Ka radiation was 55 56 used to record the diffraction patterns which were taken on a Philips powder camera of diameter 114.6 mm. The radiation was filtered by a nickel foil in order to render the beam monochromatic. Exposures ranging between 4-6 hours were gi vena The diffraction patterns, which are consistent with the crystal structures52 ,85,86 of the three compounds indi­ cate the formation of the respective lattices and therefore of the compounds. In Table 6 below are given the wavelengths, v/R values and the corresponding energies of the Se K discontinuity in metallic selenium and in the compounds Ag2Se, Cu:Se, and TABLE 6 DA TA ON Se K ABJOHPTION DISCON TINUI frf Absorber A v/R Energy(E) L\E X.U eV eV Metallic selenium 977.82 931.94 12651.1 .:to.07 .:t 0.5 Ag 2Se 977.62 932.13 12653.7 2.6 +0.08 .:t 0.5 CuSe 977.59 932.16 12654.1 3.0 +0.06 .:!: 0.5 CU3Se2 977.45 932.29 12655.9 4.8 .:to•07 + 0.5 It is seen from the last column of Table 6 that the Se K V. B . UJ t ..J <;( .\ ~... r'::#:/ ; .. "?': u . < '.- en o ~ ---.. 1-•' .. -~-.' 'r ~ . '- 1 - I ~o z I .' K.. -..,......;--~ . --.I.!_" - K . ..... · .. -" - '. ...'. .... ~ . 0.. .. .' . ~ · .". - ~",' .' , .' ~ .. : · -", t. .--: - ';~:~ ' :~~ FJ G : 1'8:- 0;- Se K~{e~; s · ORPt: ION I~ SELENIUM .. ".. -::".'- - ::~ b- .SQ .. I< A BSQRPTION INc; c ~QUP t . SEL EN I DES 57 discontinuity in all t~ese compounds shifts towards high el)ergy side with respect to that in metallic selenium. The shifts are well above the estimated experimental error. 3. Discus~:;ion It has been shown in Chapter IV that the absorption discontinuity in a semiconductor represents the transition of the inner elect Ton to the top of the valEnce band. Thus the shift in the discontinuity would arise due to the altered position of the top of the valence band, which can be ex- plained as follows : Detailed crystal structure analyses of these compounds (except for CuSe)52 do not appear to have been done. Pre- liminary structure data show that these compounds are not formed by directional bonds such as sp3, pJ, sp2 etc. Hence bonds of ionic nature may be expected in these compounds to gi ve the la.ttice structures accc·rding to Lave 1 s geometrical prlnclp. I es. 87 lb·t can e lmaglne, . d t h at some 0 f t h e 5 s I electrons of silver or 4s1 electrons of copper, which are more or less free, are transferred to the valence band of selenium, thus giving a nearly filled subshell on the anion. The resulting nearly filled valence band of the compound would thus be centred on the anion (selenium atoID) , which eventually gives semiconducting properties to the compounds. The shifts of the Se K discontinuity in these compounds bring direct evidence for the electron transfer. The X-ray absorption process is indicated in Figs. 18a and b. N N :s :s 01 (b 16 It might be mentioned here that Bally and Milller have reported a shift of 8.1 eV in the Se K edge in CU2Se. Our shifts for euSe and Cu)Se2 along with that of Bally and ~tiller for CU2Se in increasing order show that the transfer Of electrons to the anion is proportional to the cation­ richness of the compound. However, it is not possible from these studies a~ this stage to say exactly how many electrons take part in the transfer. The study of the X-ray absorption discontinuities of silver and copper in these compounds would have been very helpful in confirming the transfer of electrons to the anion as envisaged above. However, as we have employed the photo­ graphic method to record the spectra, the absorption discon­ tinuity of silver could not be studied. It was also not possible to study the Cu K absorption discontinuity since the W L~2 line emitted by the X-ray tube falls very near to it. As has been already stated in Chapter III, the molecular orbital description which applies essentially to covalent compounds, cannot be employed to draw schematic band struc­ tures for these compounds which are of the class (2) type (iono-covalent; Chapter III - Section 7). GROUP II SELENIDES 4. Res u 1 t s In Fig. 19 are shown the powder diffraction patterns of ZnSe, HgSe and CdSe along with those of zinc, selenium and cadmium. These patterns are consistent with the crystal 59 structures of the compounds reported in literature,53 indi­ cating the formation of the compounds. Data on the Se K discontinuity and in the compounds ZnSe, CdSe, HgSe (in metallic selenium) are presented in Table 7. It is observed that the 5e K discontinuity shifts towards lower energies in all these compounds with respect to the discontinuity in metallic selenium. TABLE 7 DA TA ON Se K ABSOH.PTION DISCONTINUITY Absorber A viR Energy(E) ~E X. U. eV eV Metallic selenium 977.82 931.94 12651.1 .:to•07 -+ 0.5 ZnSe 978.08 931.69 12647. ? .:to•O? + 0.5 3·4 CdSe 977.99 931.78 12648.9 .:to•05 + 0.4 2.2 HgSe 978.11 931.6b 126h7.3 .:to.07 + 0.5 3.8 5. Discussion ZnSe and HgSe have zinc blende (ZnS) structure53 and CdSe has zincite (ZnO) structure. 53 In these structures every atom is tetrahedrally surrounded by four other atoms. Predominantly covalent bonds are formed by hybridized sp3 orbitals on selenium and metal atoms. Schematic band struc- 60 tures for these compounds are shown in Figs. 11 and 12. It is interesting to compare the band structure of pure selenium (Fig. 10) with that of selenium in these compounds (Fig. 11). In the band structure for the compounds the non-bonding band disappears. Also the valence band of selenium in the com­ pounds contains only four electrons as against six in the s and p bonding and non-bonding orbitals of pure selenium. This results in lowering of the top of the valence band of selenium in the compounds, which would give the shift of the ~bsorption discontinuity towards low energy side. Alter- natively, it may be possible to attribute the shift of the Se K discontinuity in the compounds to the large exchange integral (Chapter III - Section B) in covalent compounds. It might be mentioned here that the shifts towards high energy side of the LI, LII and LIII absorption discontinuities of both cadmium and tellurium in CdTe obtained by Noreland, Ekstig and Auleytner15 do not agree with our results. It is rather difficult to comment on the res~ults of these authors at this stage. We shall only mention here that the LII, LIll absorptions starting from p levels are not very suit­ able for this study in which the valence and conduction bands have p symmetry. The Ll edge, although starts from s BS state, is very much b roa d ene d d ue to Auger effect. GElliVlliNIUM SELENIDES 6. Res u 1 t s The powder diffraction patterns of germanium selenide JC:·~I.:I 3 +.--------- 1'--' q I co (J') 000( ~ ~~ ~~~ ;J\ r- rr r'Ar 7\ +~ -0( VI;( I')-< .:< :>iJ'1 » » cr 0- CT til (J) VI ." ~ IV o 61 (GeSe) and germanium diselenide are shown in Fig. 20, along with those of selenium and germanium. They are consistent 54,55 with the lattices of these compounds reported in literature, which indicates the formation of s~able lattices in stoi- chiometric proportions. In Fig. 21a is shown a spectrogram with magnification four indicating the Se K and Ge K absorption discontinuities in GeSe and the reference lines of tungsten. A microphoto- meter record taken with ma~nification 4 of the same is also attached (Fig. 21b) along with the spectrogram. Table 8 gives the data on the Se K discontinuity in metallic selenium and in germanium selenides. In germanium selenide the discontinuity is found to shift towards higher energies by about 2.7 eV, while in germanium diselenide it is observed to shift towards lower energies by about 2.6 eV with respect to the edge in metallic selenium. TABLE 8 DATA ON Se K ABSOliPTION DISCONTINUITY Absorber A viR Energy(E) .6~ x. U. eV eV Metallic selenium 977.82 931.94 12651.1 .:t 0.07 + 0.5 GeSe 977.60 932.14 12653.8 .:t. 0.C)7 + 0.5 2.7 2.6 f}eSe2 978.02 931.75, 12648.5 + 0.07 -+ 0.5 62 Data on the Ge K absorption discontinuity in GeSe, GeSe2 and ~ermanium are given in Table 9. It is observed that the Ge K discontinuity in GeSe shifts towards low energy side by about 1.5 eV with respect to that in germanium while in GeSe2 it remains almost unaffected.
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