P 44 ISSRNS 2012: Abstracts / Synchrotron Radiation in Natural Science Vol. 11, No 1 – 2 (2012)
PHOTOEMISSION BINDING ENERGY LOCAL CHANGE CAUSED BY CRYSTALLINE LOCAL STRUCTURE
B.A. Orlowski1∗, A. Szczerbakow1, P. Dziawa1, K. Gas1, A. Reszka1, S. Thiess2, and W. Drube2
1Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02–668 Warsaw, Poland
2Institut f¨ur Experimentalphysik, Universit¨at Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany Keywords: crystalline structure, electronic structure, synchrotron radiation, photoemission spectroscopy
∗e-mail : orbro@ifpan.edu.pl
The paper is concern to the problem of crys- talline local structure appearing in the ternary crystal and corresponding to it local electronic structure. In Pb0.94Cd0.06Te [1, 2] crystal local crys- talline structure is created due to introduction of Cd ion on a sit of Pb ion. In the case created ternary crystal still survive in the rock salt structure like it is for PbTe. As the ionic radius of Cd ion is smaller than Pb ion the lattice collapse appears around the introduced Cd ion. These crystalline local structure creates the electronic local structure.
The paper presents comparison of spectra of two kind of semiconductor samples. The one is the ternary crystal of Pb0.94Cd0.06Te with the frozen rock salt structure. In these crystal the crys- talline local structure is created due to differences of the radiuses size of cations Pb2+ and Cd2+ and due to frozen technology process of the crystal.
The second one sample is the layer nanostructure CdTe/Pb0.95Eu0.05Te/CdTe grown in thermal equi- librium by MBE method [3].
The bulk single crystals of Pb1−xCdxTe were grown by self-selecting vapor growth (SSVG) method which is marked by near equilibrium growth conditions [1, 2]. The metastable Pb1−xCdxTe crys- tals of about 1 ccm volume with (001) natural facets were obtained with FWHM rocking curve values of below 2 arcmin and almost absolutely uniform composition in respect to the CdTe molar fraction x. The study of X-ray diffraction of Pb1−xCdxTe ternary crystal performed in room temperature con- firm a single-phase rock-salt solution. It means, that small ionic radius Cd2+ion is taking a crystal region corresponding to the ion Pb2+ of big ionic radius.
The Tunable High Energy X-ray Photoemission Spectroscopy (THE-XPS) with the application of the synchrotron radiation was used to investigate electronic structure of the samples. The experiment was performed using the (THE-XPS) spectrometer at wiggler beam line station BW2 of the Doris III storage Ring, HASYLAB, DESY, Hamburg. Dou- ble crystal monochromator Si(111) covering an en- ergy range from 2.4 to 10 keV with a monochromatic photon flux of about 5 × 1012 photons/s and with total energy resolution power of 0.5 eV for radiation energy around 3000 eV was used. The photoemis- sion studies were performed at room temperature.
To simplify further consideration lets take one dimensional chains of the PbTe, PbCdTe and
CdTe presented in Fig. 1. First row of the figure presents PbTe clean crystal chain while the third row presents CdTe clean crystal chain. The distance between Pb – Te atoms are bigger than it is for dis- tance between Cd – Te atoms. The middle row cor- responds to the case when the Cd ion is introduced on the seat of Pb ion and the structure of PbTe chain changes locally and creates local structure around Cd ion. The Te ions nearest to introduced Cd ion will change their position to move closer to the Cd ion to approach the Cd – Te distance corresponding to CdTe chain (row 3). These movement of Te ions in direction to Cd ion will lead to the increase of the distance between these Te ions and their neigh- bors Pb ions. As a result we can obtain new local structure around the Cd ion. In the local structure region we can expect that the distance between Cd – Te and Pb – e ions will increase in comparison to these distances obtained for CdTe and PbTe chains.
Figure 1 : One dimensional structure of PbTe, PbCdTe, and CdTe. The Cd ion is inserted on the place of Pb ion. Two Te ligands ions of Cd ion shift closer to the inserted Cd2+ ion. The distance between Cd – Te is now smaller than previous Pb – Te and bigger than in CdTe. The next Pb ion moves closer to Cd ion and its distance Pb – Te is bigger in comparison to the previous Pb – Te distance in PbTe crystal. Let’s assume that the next distances of Pb – Te remains as previous in PbTe.
Local change of binding energy EB:
- for Cd2+ and Pb2+ electrons increases, conduction band is expected to shift down
- for Te2− electrons EB decreases, valence band is ex- pected to shift up.
Local crystalline structure leads to the creation of local electronic structure. Local increase of the distance between cation and anion leads to the in- crease of binding energy of electrons of cation and to the decrease of binding energy of electrons of anion.
The results of the measured binding energies are presented at Table I. In the local structure region the distance between Pb — Te increases and it leads to the increase of binding energies of Pb2+ ion
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ISSRNS 2012: Abstracts / Synchrotron Radiation in Natural Science Vol. 11, No 1 – 2 (2012) P 44
Table 1 : Photoemission binding energies and their dif- ferences for frozen crystal Pb0.94Cd0.06Te and layer CdTe/Pb0.95Eu0.05Te/CdTe grown in thermal equilib- rium.
Core levels binding energy Eb(eV)
Samples
Cation Anion
Pb5d5/2 Pb4f7/2 Te4d5/2 Te3d5/2
Pb5d3/2 Pb4f5/2 Te4d3/2 Te3d3/2
Pb0.94Cd0.06Te 18.67 137.18 39.86 572.02 21.33 142.10 41.34 582.57 CdTe/Pb0.95... 18.45 137.32 39.87 571.94
...Eu0.05Te/CdTe 21.11 142.24 41.36 582.48 EbFrCrystal, EbLayer +0.22 +0.14 -0.01 -0.08 +0.22 +0.14 -0.02 -0.09
electrons and to the decrease of ion Te2− electrons binding energies (table I row 3).
The results consistently confirm the consider- ation of the presented abstract and leads to the conclusions: in crystal appears local structure, dis- tance of Pb — Te increases in the region of local structure; binding energies (table I row 1 and 2) of electrons of Pb2+ ions are higher for frozen crystal than for layer grown in thermal equilibrium (Table 1, row 3), while for electrons of Te2−ion the binding energies are smaller for frozen crystal (Table 1, row
1 and 2) than for layer grown in thermal equilibrium (Table 1, row 3).
Acknowledgments: The authors acknowledge support by MSHE of Poland research Projects DESY/68/2007 and by the European Community via the Research In- frastructure Action under the FP6 “Structuring the Eu- ropean Research Area” Programme (through the In- tegrated Infrastructure Initiative “Integrating Activity on Synchrotron and Free Electron Laser Science”) at DESY. Partially supported by European Union within the European Regional Development Fund, through an Innovative Economy grant (POIG.01.01.02-00-108/09) and (POIG.01.01.02-00-008/08).
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