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Transition metal impurities and band offsets in wide gap II–VI semiconductors: Zn1−xMnxSe(Ni) compounds
Solid State Communications, Vol. 91, No. 4, pp. 279-282,1994 Elsevier Science Ltd Printed in Great Britain 0038-1098/94 $7.00+.00 0038-1098(94)00346-7
Pergamon
TRANSITION METAL IMPURITIES AND BAND OFFSETS IN WIDE GAP II-VI SEMICONDUCTORS: Znl_xMnxSe(Ni ) COMPOUNDS V.R. Galakhov, T.P. Surkova, V.I. Sokolov, E.Z. KurTnaev Institute of Metal Physics, Russian Academy of Sciences-Ural Division, 620219 Yekaterinburg GSP-170, Russia Chr. Zubr~gel, H. 0nl0, M. Neumann Fachbereich Physik, Universit~it OsnabrOck, 49069 Osnabr0ck, Germany S.A. Permogorov, L.N. Tenishev A.F. Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St.Petersburg, Russia (Received 31 March 1994 by P.H. Dederichs)
accepted for publication 9 May 1994 Znl.xMnxSe(Ni) solid solutions have been analysed by absorption, reflectivity and resonant photoemission measurements. Valence band offsets for the compounds with different Mn concentrations were estimated from the threshold energy h0)th of the photoionization of Ni2+ ions as well as from photoemission spectra. The contribution of the Mn3d derived states to the valence band was determined. 1. Introduction
grown by the Bridgman method. Mn and Ni were added to the melt, producing a Ni concentration of ~ 6 * 1017 cm -3 for all samples and a Mn molar fraction x = 0.0 to 0.37. The Mn concentration was determined by energy dispersive X-ray fluorescence analysis with an accuracy of _+10%. Freshly cleaved or polished platelets were used for the optical measurements. Absorption and reflection spectra were recorded using a DFS-12 monochromator with a photomultiplier tube connected to a photon counting system in the spectral energy range 1.4 - 2.1 eV. The samples were mounted in a helium flow cryostat with controllable temperatures. The resonant photoemission spectra were obtained at the storage ring BESSY in Berlin using the monochromator TGM2 in an energy range from 30 to 70 eV. The photoemitted electrons were analysed with a commercial ADES 400 system. The energy resolution was set to - 0.2 eV. The crystals were cleaved in vacuum and were heated up to 250 ° C in situ in order to clean the surface. The pressure during the experiment was about 2 * 10-10 Torr.
Some years ago it has been supposed that the energy levels of 3d transition metals (TM) in the series of isovalent II-VI and III-V compounds hardly depend on the host band position but rather follow a certain internal host reference, the so-called average dangling bond energy level1,2. It was established also for some compounds that the energy differences between TM energy levels in the series of isovalent semiconductors are equal to the respective band offsets2,3. It appears that this approach can be applied for rare earths also4. All this caused us to undertake systematic and consistent investigations of this interesting phenomenon. For this purpose we have chosen Znl.xMnxSe(Ni) compounds with different contents of Mn. The Mn induced shifts of the energy bands and the Ni formed 3d impurity levels in the energy gap 5,6. The position of the TM(Ni) energy levels in the energy gap and the shift of valence and conduction bands for various compositions was controlled by the observation of the photoionization process Ni2+(3dS)+h0)th --~ Ni+(3dg)+h (h denotes a hole in the valence band) together with measurements of the excitonic reflection and resonant photoemission. The latter measurements allowed us to deduce the Mn3d contributions to the density of states in the valence band.
3. Results and Discussion ' 3.1. Optical Spectra The characteristics of the investigated samples are listed in Table 1. Exciton reflectivity measurements were done by us earlier to establish the band gap energies for the same samplesS,6. The position of the point R~ = (Rmax + Rmin)/2 was taken as a free
2. Experimental The single crystals used in this work were
279
280
WIDE GAP II-Vl SEMICONDUCTORS
Table 1: Composition x, free exciton energies EFE and threshold energies h0)th for the Ni2*--+ Ni* photoionization process in Znl_xMnxSe(Ni) crystals at 5K. For all samples the Ni concentration is ~ 6 * 1017 cm -3.
EFE(eV)
X 0.0 0.13 0.16 0.18 0.19 0.37
both (eV) 1.838 1.820 1.814 1.812 1.794 1.816
2.802 2.798 2.840 2.900 2.906
exciton energy EFE , where Rrnax and Rmi n denote the maximum and the minimum of the reflection coefficient in the region of the exciton structure. We note that the observed reflectivity features in Znl.xMnxSe(Ni ) are not absolutely identical with those reported for Znl.xMnxSe solid solutions 7,8. This difference can be connected with the presence of Ni ions in our samples. Absorption spectra were obtained for all crystals, in the range of internal Ni2+(3d 8) 3TI(F ) -~ 3TI(P ) and 3TI(F ) --~ 1T2(G ) transitions (1.4 - 1.8 eV) and in the range of charge transfer transitions Ni 2+ (3d 8) + he)th Ni*(3dg)+h. Internal transitions were used only for a control of the incorporation of the Ni ions into the lattice in the doubly charged state. Fig.1 presents absorption spectra for several samples in the energy range of Ni photoionization. The spectra are presented in a O~2/3 VS h(O plot which should give a straight line for the photoionization
Vol.91,No.4
process cutting the energy axis at the impurity ionization energy both 9. The threshold energies hO)th are listed in the Table 1. These values have been obtained by more precise calculations and differ slightly from those given by some of us eadier5,6. The value h~th = 1.838 eV for ZnSe(Ni) (x = 0.0) is consistent with the previous results 9-11. As can be seen from Fig.1 and Table 1 the threshold energy is decreasing by 20 meV for the smallest Mn concentration (x = 0.03). The further decrease of h(Oth with x is less essential, except for x = 0.37. We believe that it should not be taken into account because at x = 0.3 a structural phase transition is found 7. Assuming that the difference between TM (namely Ni) ionization energies is equal to respective band offsets1, 2 we get the values for valence band offsets (VBO's) for Znl_xMnxSe(Ni) solid solutions through the whole range of compositions. These values, changing from 20 meV for x = 0.03 to ~ 40 meV for x = 0.19 are in good agreement with those derived from the magnetic field studies ( < 20 meV) of ZnSe/Zno.77Mno.23Se quantum wells 12 however they
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Fig.2: Energy distribution curves of Zno.s4Mno.16Se(Ni): a) at the Mn3p-3d resonance (he = 50 eV, solid line), b) off the resonance (he) = 48 eV, dotted line), c) energy distribution curve of Zno.97Mno.03Se(Ni) at he) = 50 eV (dotted line). (a) - (b) the difference curve between the spectra taken on and off the resonance for Zno.84MnoJ6Se(Ni). (a) - (c) the difference curve for Zno.84Mno.16Se(Ni) and Zno.97Mno.o3Se(Ni ) at he = 50 eV. At the bottom the Mn ~ X-ray emission speclnJm of 7_.no.9oMno.loSe is shown on the scale of photon energies.
WIDE GAP II-VI SEMICONDUCTORS
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F'~j.3: Constant initial state (CIS) spectra of Zno.84Mno.16Se(Ni) for initial state energies in the valence band region. are almost one order of magnitude smaller than those reported elsewhere 13. In the latter paper the VBO for Zno.81Mno.19Se crystals has been determined from photoionization spectra as a difference between the binding energies of the Zn3d core levels for the ternary and its parent binary compounds. To clarify the situation resonant photoemission investigations were performed on our set of Znl_xMnxSe crystals. 3.2. Resonant Photoemission Fig.2 shows the photoelectron spectra of Zn0.84Mn0J6Se(Ni ) obtained on (hco = 50 eV) and off (ho~ = 48 eV) the Mn 3p-3d resonance and those of Zno.97Mno.o3Se(Ni) obtained at h0~ = 50 eV. The intensities of spectra have been normalized to the photon flux. The energy scale is aligned at the top of the valence band of the compounds and was controlled by the inspection of the energetic position of the Zn3d maxima (the strong peaks at -9.4 eV). Within the limits of the experimental error (0.05 eV) the position of the top of the valence band of the compounds with different x did not change in respect to the Zn3d maxima. This is in an agreement with our optical data which gave a value for the VBO not more than 20 meV for these crystals. For a photon energy of 50 eV the cross sections for Se 4p are significantly smaller than those for Mn3d. Taking into account the concentrations of Mn and Se one can deduce the corresponding Se4p and Mn3d contributions to a common photoemission spectrum. The curve (a) - (b) in Fig.2 shows the dif-
281
ference spectrum from photoemission curves taken on and off resonance for the Zn0.84Mn0.1eSe(Ni) sample. On the other hand the difference curve ( a ) - (c) was deduced by substracting the photoemission spectrum of Zno.g7Mno.o3Se(Ni) from that of Zno.s4Mn0.16Se(Ni) (at hco=50 eV). Both difference curves have a similar shape at the same binding energy. There are strong maxima at 3.6 - 3.7 eV below the top of the valence band, shoulders in the range 0 - 2.8 eV and satellites at 6 - 8 eV. In the region 8 - 10 eV some artificial features show up originating from the very intensive Zn3d lines. At the low part of Fig. 2 the Mn Lc, X-ray emission spectrum (2p-3d4s transition) taken from our previous work 14 is shown. It gives information about the Mn3d4s density of states for Zn0.9oMno.loSe. The spectrum is brought into coincidence with the maxima of the difference curves (a) - (b) and (a)- (c). For the Mn L~ spectrum we included the photon energy scale in the figure. The large width of the Mn Lc, Xray emission spectrum is due to the short life time of the Mn2p core hole. A hump around 639 eV corresponds to the shoulder at -2 eV in the difference photemission spectra and a shoulder at 632 - 635 eV corresponds to the satellite at 6 - 8 eV. The resonance behaviour of the photoemission spectra is more clearly seen in the constant-initialstate (CIS) spectra shown in Fig.3 where photoemission intensities at various binding energies are plotted as a function of photon energy. There one can see a tendency that the resonance peak in the CIS spectra is more pronounced for the satellite and less for the shoulder in the range of 0 - 3 eV. The resonance behaviour in the photoemission process originates from an interference between direct transitions 3d n --~ 3d n-1 + e- and core excitations 3p63d n --> 3pS3dn*l followed by a super Coster-Kronig transition to 3p63d n-1 + e-. Since the predominant component in the ground state configuration is d 5 with a small amount of dSL (L denotes a hole in the ligand p-level) it is expected that d4 like final states are enhanced by the resonance process rather than dSL like final states. Thus one can assign the satellite to a d4 final state and one can suggest substantial overlap between d 4 and dSL final states for the low-binding energy shoulder. For a similar compound Cdl_xMnxTe configuration-interaction calculations on the model cluster [MnTe4] 6- were performed Is. A main peak at 3.5 - 3.7 eV is attributed mainly to 5E final-states however with some contributions of ST2 final states. These states are predominantly of dSL final state character originating from L ~ d charge transfer screening of d hole (d 4) states. The satellite region (5 - 9 eV) is more d 4 like and consists of 5E and 5T2 states. The region near the top of the valence band is due to dSL states. When we compare our experimental results for
282
WIDE GAP II-VI SEMICONDUCTORS
Znl.xMnxSe with calculationsis it is obvious that our semiconductor shows charge transfer type behaviour rather than Mott-Hubbard behaviour. The low binding energy states are due to dSL charge transfer states. For the Mn I_= X-ray emission spectrum one can suggest a two-step process18 2p.d5 --->2p.dSl_ --> d5L. For a charge-transfer insulator the Mn I_= X-ray emission spectrum reveals a final state of 3dnL rather than 3d n-l. The shape of the Mn L= spectrum is similar to the difference curve (a) - (b) in resonance. So, the Mn L,~ maximum represents the 3dSL final-states with a small contribution of the poorly screened (atom like) 3d4 states. 4. Concluding Remarks We have investigated the Ni photoionization process and the photoemission spectra of Znl_xMnxSe(Ni ) compounds which allowed us to conclude that VBO's are likely to be very small for compounds with different Mn compositions and the band gap change is mostly caused by the shift of the conduction band. The reported value of the VBO for Zn0.81Mno.19Se13 which is almost one order of magnitude higher than that obtained in our experi-
Vol.9 l,No.4
ment can mean that the ascription of the shifts of the Zn3d core levels between the ternary alloys and their parent binary compound to a shift of the valence band edges as proposed by the authors needs further confirmation. We would like to remark that Znl_xMnxSe(Ni) compounds which we have studied in the present work are not perfectly suited for the solution of this problem. The presence of Mn ions with a half filled 3d shell leads to the appearance of Mn3d levels in the valence band and in the energy gap with a high degree of hybridization with anion pstates and large exchange interaction even in the absence of a magnetic field. Therefore we believe that the investigation of solid solutions without magnetic components (for example Znl_xCdxSe(Ni) or ZnSxSel_x(Ni ) would be favourable. - We are very grateful to Drs. N.N. Kolesnikov and Yu.N. Ivanov for growing the crystals and to Dr. W. Dobrowolski for measurements of the crystal composition. This work was supported by the Deutsche Forschungsgemeinschaft (V. Galakhov) and the German Bundesministerium for Science and Technology (BMFT, Project 05 5MPAAB 7) and partly by the International Science Foundation (Soros Foundation). Acknowledgement
References:
1. 2. 3. 4. 5.
6. 7. 8.
J.M. Langer, H. Heinreich, Phys. Rev. Lett. 5S (1985) 1414 J.M. I_anger, C. Delerue, M. Lannoo, H Heinreich, Phys. Rev. B 38 (1988) 7723 J. Tersoff, W. Harrison, Phys. Rev. Lett. 58 (1987) 2367 C. Delerue, M. Lannoo, Phys..Rev. Lett. 67 (1991) 3006 S.A. Permogorov, LN. Tenishev, V.I. Sokolov, T.P. Surkova, Defects and Diffusion Forum (ed. by N.T. Bagraev) Trans. Tech. Publication Ltd., Switzerland, Vol. 103-106, p.193 (1993) S.A. Permogorov, L.N. Tenishev, V.I. Sokolov, T.P. Surkova, Acta Physica Polonica A82 (1992) 705 R.B. Bylsma, W.M. Becker, J. Kossut, U. Debska, Phys. Rev. B33 (1986)8207 A. Twardowski, T. Dietl, M. Demlaniuk, Solid State Commun. 48 (1983) 845
9. 10. 11. 12. 13. 14.
15. 16.
J.M. Nor'as, J.W. Allan, J. Phys. C : Solid State Physics 13 (1980) 3511 S.A. Kazanskii, A.I. Ryskin, Optica I Spectro. skopiya (Russian journal) 31 (1971) 325 V.I. Sokolov, T.P. Surkova, V.V. Chernyaev, Phys. stat. sol. (b) 114 (1982) K185 V. Hefetz, J. Nakahara, A.V. Nurmikko, L.A. Kolodzievski, R.L. Gunshor, S. Datta, Appl. Phys.Lett. 47 (1985) 989 R. Wiedamann, H.-E. Gumlich, M. Kupsch, H.-U. Middelmann, U. Backer, Phys.Rev.B45 (1992) 1172 V.I. Sokolov, A.N. Mamedov, V.V. Chemyaev, E.Z. Kurmaev, V.R. Galakhov, S.N. Namnonov, M.P. Kulakov, A.V. Fadeev, Fizika tverdogo tela (Russian Joumal) 27 (1985) 2118 L. Ley, M. Taniguchi, J. Ghijsen, R.L. Johnsson, A. Fujimod, Phys. Rev.B3S (1.967) 2839 G. DrSger, W. Czoble, J.A. Leiro, Phys. Rev. B45 (1992) 8283
Pergamon
TRANSITION METAL IMPURITIES AND BAND OFFSETS IN WIDE GAP II-VI SEMICONDUCTORS: Znl_xMnxSe(Ni ) COMPOUNDS V.R. Galakhov, T.P. Surkova, V.I. Sokolov, E.Z. KurTnaev Institute of Metal Physics, Russian Academy of Sciences-Ural Division, 620219 Yekaterinburg GSP-170, Russia Chr. Zubr~gel, H. 0nl0, M. Neumann Fachbereich Physik, Universit~it OsnabrOck, 49069 Osnabr0ck, Germany S.A. Permogorov, L.N. Tenishev A.F. Ioffe Physical-Technical Institute, Russian Academy of Sciences, 194021 St.Petersburg, Russia (Received 31 March 1994 by P.H. Dederichs)
accepted for publication 9 May 1994 Znl.xMnxSe(Ni) solid solutions have been analysed by absorption, reflectivity and resonant photoemission measurements. Valence band offsets for the compounds with different Mn concentrations were estimated from the threshold energy h0)th of the photoionization of Ni2+ ions as well as from photoemission spectra. The contribution of the Mn3d derived states to the valence band was determined. 1. Introduction
grown by the Bridgman method. Mn and Ni were added to the melt, producing a Ni concentration of ~ 6 * 1017 cm -3 for all samples and a Mn molar fraction x = 0.0 to 0.37. The Mn concentration was determined by energy dispersive X-ray fluorescence analysis with an accuracy of _+10%. Freshly cleaved or polished platelets were used for the optical measurements. Absorption and reflection spectra were recorded using a DFS-12 monochromator with a photomultiplier tube connected to a photon counting system in the spectral energy range 1.4 - 2.1 eV. The samples were mounted in a helium flow cryostat with controllable temperatures. The resonant photoemission spectra were obtained at the storage ring BESSY in Berlin using the monochromator TGM2 in an energy range from 30 to 70 eV. The photoemitted electrons were analysed with a commercial ADES 400 system. The energy resolution was set to - 0.2 eV. The crystals were cleaved in vacuum and were heated up to 250 ° C in situ in order to clean the surface. The pressure during the experiment was about 2 * 10-10 Torr.
Some years ago it has been supposed that the energy levels of 3d transition metals (TM) in the series of isovalent II-VI and III-V compounds hardly depend on the host band position but rather follow a certain internal host reference, the so-called average dangling bond energy level1,2. It was established also for some compounds that the energy differences between TM energy levels in the series of isovalent semiconductors are equal to the respective band offsets2,3. It appears that this approach can be applied for rare earths also4. All this caused us to undertake systematic and consistent investigations of this interesting phenomenon. For this purpose we have chosen Znl.xMnxSe(Ni) compounds with different contents of Mn. The Mn induced shifts of the energy bands and the Ni formed 3d impurity levels in the energy gap 5,6. The position of the TM(Ni) energy levels in the energy gap and the shift of valence and conduction bands for various compositions was controlled by the observation of the photoionization process Ni2+(3dS)+h0)th --~ Ni+(3dg)+h (h denotes a hole in the valence band) together with measurements of the excitonic reflection and resonant photoemission. The latter measurements allowed us to deduce the Mn3d contributions to the density of states in the valence band.
3. Results and Discussion ' 3.1. Optical Spectra The characteristics of the investigated samples are listed in Table 1. Exciton reflectivity measurements were done by us earlier to establish the band gap energies for the same samplesS,6. The position of the point R~ = (Rmax + Rmin)/2 was taken as a free
2. Experimental The single crystals used in this work were
279
280
WIDE GAP II-Vl SEMICONDUCTORS
Table 1: Composition x, free exciton energies EFE and threshold energies h0)th for the Ni2*--+ Ni* photoionization process in Znl_xMnxSe(Ni) crystals at 5K. For all samples the Ni concentration is ~ 6 * 1017 cm -3.
EFE(eV)
X 0.0 0.13 0.16 0.18 0.19 0.37
both (eV) 1.838 1.820 1.814 1.812 1.794 1.816
2.802 2.798 2.840 2.900 2.906
exciton energy EFE , where Rrnax and Rmi n denote the maximum and the minimum of the reflection coefficient in the region of the exciton structure. We note that the observed reflectivity features in Znl.xMnxSe(Ni ) are not absolutely identical with those reported for Znl.xMnxSe solid solutions 7,8. This difference can be connected with the presence of Ni ions in our samples. Absorption spectra were obtained for all crystals, in the range of internal Ni2+(3d 8) 3TI(F ) -~ 3TI(P ) and 3TI(F ) --~ 1T2(G ) transitions (1.4 - 1.8 eV) and in the range of charge transfer transitions Ni 2+ (3d 8) + he)th Ni*(3dg)+h. Internal transitions were used only for a control of the incorporation of the Ni ions into the lattice in the doubly charged state. Fig.1 presents absorption spectra for several samples in the energy range of Ni photoionization. The spectra are presented in a O~2/3 VS h(O plot which should give a straight line for the photoionization
Vol.91,No.4
process cutting the energy axis at the impurity ionization energy both 9. The threshold energies hO)th are listed in the Table 1. These values have been obtained by more precise calculations and differ slightly from those given by some of us eadier5,6. The value h~th = 1.838 eV for ZnSe(Ni) (x = 0.0) is consistent with the previous results 9-11. As can be seen from Fig.1 and Table 1 the threshold energy is decreasing by 20 meV for the smallest Mn concentration (x = 0.03). The further decrease of h(Oth with x is less essential, except for x = 0.37. We believe that it should not be taken into account because at x = 0.3 a structural phase transition is found 7. Assuming that the difference between TM (namely Ni) ionization energies is equal to respective band offsets1, 2 we get the values for valence band offsets (VBO's) for Znl_xMnxSe(Ni) solid solutions through the whole range of compositions. These values, changing from 20 meV for x = 0.03 to ~ 40 meV for x = 0.19 are in good agreement with those derived from the magnetic field studies ( < 20 meV) of ZnSe/Zno.77Mno.23Se quantum wells 12 however they
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WIDE GAP II-VI SEMICONDUCTORS
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F'~j.3: Constant initial state (CIS) spectra of Zno.84Mno.16Se(Ni) for initial state energies in the valence band region. are almost one order of magnitude smaller than those reported elsewhere 13. In the latter paper the VBO for Zno.81Mno.19Se crystals has been determined from photoionization spectra as a difference between the binding energies of the Zn3d core levels for the ternary and its parent binary compounds. To clarify the situation resonant photoemission investigations were performed on our set of Znl_xMnxSe crystals. 3.2. Resonant Photoemission Fig.2 shows the photoelectron spectra of Zn0.84Mn0J6Se(Ni ) obtained on (hco = 50 eV) and off (ho~ = 48 eV) the Mn 3p-3d resonance and those of Zno.97Mno.o3Se(Ni) obtained at h0~ = 50 eV. The intensities of spectra have been normalized to the photon flux. The energy scale is aligned at the top of the valence band of the compounds and was controlled by the inspection of the energetic position of the Zn3d maxima (the strong peaks at -9.4 eV). Within the limits of the experimental error (0.05 eV) the position of the top of the valence band of the compounds with different x did not change in respect to the Zn3d maxima. This is in an agreement with our optical data which gave a value for the VBO not more than 20 meV for these crystals. For a photon energy of 50 eV the cross sections for Se 4p are significantly smaller than those for Mn3d. Taking into account the concentrations of Mn and Se one can deduce the corresponding Se4p and Mn3d contributions to a common photoemission spectrum. The curve (a) - (b) in Fig.2 shows the dif-
281
ference spectrum from photoemission curves taken on and off resonance for the Zn0.84Mn0.1eSe(Ni) sample. On the other hand the difference curve ( a ) - (c) was deduced by substracting the photoemission spectrum of Zno.g7Mno.o3Se(Ni) from that of Zno.s4Mn0.16Se(Ni) (at hco=50 eV). Both difference curves have a similar shape at the same binding energy. There are strong maxima at 3.6 - 3.7 eV below the top of the valence band, shoulders in the range 0 - 2.8 eV and satellites at 6 - 8 eV. In the region 8 - 10 eV some artificial features show up originating from the very intensive Zn3d lines. At the low part of Fig. 2 the Mn Lc, X-ray emission spectrum (2p-3d4s transition) taken from our previous work 14 is shown. It gives information about the Mn3d4s density of states for Zn0.9oMno.loSe. The spectrum is brought into coincidence with the maxima of the difference curves (a) - (b) and (a)- (c). For the Mn L~ spectrum we included the photon energy scale in the figure. The large width of the Mn Lc, Xray emission spectrum is due to the short life time of the Mn2p core hole. A hump around 639 eV corresponds to the shoulder at -2 eV in the difference photemission spectra and a shoulder at 632 - 635 eV corresponds to the satellite at 6 - 8 eV. The resonance behaviour of the photoemission spectra is more clearly seen in the constant-initialstate (CIS) spectra shown in Fig.3 where photoemission intensities at various binding energies are plotted as a function of photon energy. There one can see a tendency that the resonance peak in the CIS spectra is more pronounced for the satellite and less for the shoulder in the range of 0 - 3 eV. The resonance behaviour in the photoemission process originates from an interference between direct transitions 3d n --~ 3d n-1 + e- and core excitations 3p63d n --> 3pS3dn*l followed by a super Coster-Kronig transition to 3p63d n-1 + e-. Since the predominant component in the ground state configuration is d 5 with a small amount of dSL (L denotes a hole in the ligand p-level) it is expected that d4 like final states are enhanced by the resonance process rather than dSL like final states. Thus one can assign the satellite to a d4 final state and one can suggest substantial overlap between d 4 and dSL final states for the low-binding energy shoulder. For a similar compound Cdl_xMnxTe configuration-interaction calculations on the model cluster [MnTe4] 6- were performed Is. A main peak at 3.5 - 3.7 eV is attributed mainly to 5E final-states however with some contributions of ST2 final states. These states are predominantly of dSL final state character originating from L ~ d charge transfer screening of d hole (d 4) states. The satellite region (5 - 9 eV) is more d 4 like and consists of 5E and 5T2 states. The region near the top of the valence band is due to dSL states. When we compare our experimental results for
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WIDE GAP II-VI SEMICONDUCTORS
Znl.xMnxSe with calculationsis it is obvious that our semiconductor shows charge transfer type behaviour rather than Mott-Hubbard behaviour. The low binding energy states are due to dSL charge transfer states. For the Mn I_= X-ray emission spectrum one can suggest a two-step process18 2p.d5 --->2p.dSl_ --> d5L. For a charge-transfer insulator the Mn I_= X-ray emission spectrum reveals a final state of 3dnL rather than 3d n-l. The shape of the Mn L= spectrum is similar to the difference curve (a) - (b) in resonance. So, the Mn L,~ maximum represents the 3dSL final-states with a small contribution of the poorly screened (atom like) 3d4 states. 4. Concluding Remarks We have investigated the Ni photoionization process and the photoemission spectra of Znl_xMnxSe(Ni ) compounds which allowed us to conclude that VBO's are likely to be very small for compounds with different Mn compositions and the band gap change is mostly caused by the shift of the conduction band. The reported value of the VBO for Zn0.81Mno.19Se13 which is almost one order of magnitude higher than that obtained in our experi-
Vol.9 l,No.4
ment can mean that the ascription of the shifts of the Zn3d core levels between the ternary alloys and their parent binary compound to a shift of the valence band edges as proposed by the authors needs further confirmation. We would like to remark that Znl_xMnxSe(Ni) compounds which we have studied in the present work are not perfectly suited for the solution of this problem. The presence of Mn ions with a half filled 3d shell leads to the appearance of Mn3d levels in the valence band and in the energy gap with a high degree of hybridization with anion pstates and large exchange interaction even in the absence of a magnetic field. Therefore we believe that the investigation of solid solutions without magnetic components (for example Znl_xCdxSe(Ni) or ZnSxSel_x(Ni ) would be favourable. - We are very grateful to Drs. N.N. Kolesnikov and Yu.N. Ivanov for growing the crystals and to Dr. W. Dobrowolski for measurements of the crystal composition. This work was supported by the Deutsche Forschungsgemeinschaft (V. Galakhov) and the German Bundesministerium for Science and Technology (BMFT, Project 05 5MPAAB 7) and partly by the International Science Foundation (Soros Foundation). Acknowledgement
References:
1. 2. 3. 4. 5.
6. 7. 8.
J.M. Langer, H. Heinreich, Phys. Rev. Lett. 5S (1985) 1414 J.M. I_anger, C. Delerue, M. Lannoo, H Heinreich, Phys. Rev. B 38 (1988) 7723 J. Tersoff, W. Harrison, Phys. Rev. Lett. 58 (1987) 2367 C. Delerue, M. Lannoo, Phys..Rev. Lett. 67 (1991) 3006 S.A. Permogorov, LN. Tenishev, V.I. Sokolov, T.P. Surkova, Defects and Diffusion Forum (ed. by N.T. Bagraev) Trans. Tech. Publication Ltd., Switzerland, Vol. 103-106, p.193 (1993) S.A. Permogorov, L.N. Tenishev, V.I. Sokolov, T.P. Surkova, Acta Physica Polonica A82 (1992) 705 R.B. Bylsma, W.M. Becker, J. Kossut, U. Debska, Phys. Rev. B33 (1986)8207 A. Twardowski, T. Dietl, M. Demlaniuk, Solid State Commun. 48 (1983) 845
9. 10. 11. 12. 13. 14.
15. 16.
J.M. Nor'as, J.W. Allan, J. Phys. C : Solid State Physics 13 (1980) 3511 S.A. Kazanskii, A.I. Ryskin, Optica I Spectro. skopiya (Russian journal) 31 (1971) 325 V.I. Sokolov, T.P. Surkova, V.V. Chernyaev, Phys. stat. sol. (b) 114 (1982) K185 V. Hefetz, J. Nakahara, A.V. Nurmikko, L.A. Kolodzievski, R.L. Gunshor, S. Datta, Appl. Phys.Lett. 47 (1985) 989 R. Wiedamann, H.-E. Gumlich, M. Kupsch, H.-U. Middelmann, U. Backer, Phys.Rev.B45 (1992) 1172 V.I. Sokolov, A.N. Mamedov, V.V. Chemyaev, E.Z. Kurmaev, V.R. Galakhov, S.N. Namnonov, M.P. Kulakov, A.V. Fadeev, Fizika tverdogo tela (Russian Joumal) 27 (1985) 2118 L. Ley, M. Taniguchi, J. Ghijsen, R.L. Johnsson, A. Fujimod, Phys. Rev.B3S (1.967) 2839 G. DrSger, W. Czoble, J.A. Leiro, Phys. Rev. B45 (1992) 8283