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XPS study of InTe and GaTe single crystals oxidation
Materials Chemistry and Physics 97 (2006) 98–101
XPS study of InTe and GaTe single crystals oxidation O.A. Balitskii a,∗ , W. Jaegermann b a
b
Department of Electronics, Lviv Ivan Franko National University, Dragomanov Str. 50, 79005 Lviv, Ukraine Department of Materials Science, Darmstadt University of Technology, Petersenstrasse 23, 64287 Darmstadt, Germany
Received 7 June 2005; received in revised form 11 July 2005; accepted 23 July 2005
Abstract Using X-ray photoelectron spectroscopy (XPS) thermal oxidation of indium and gallium tellurides single crystals was studied. It was established, that oxidation produces layers of both metal and tellurium oxides on the surface, which drastically differs from indium and gallium sulphides and selenides own oxides. Possibility of formation of the other tellurium containing phases is discussed. © 2005 Elsevier B.V. All rights reserved. Keywords: X-ray photo-emission spectroscopy (XPS); Oxidation; Indium and gallium tellurides
1. Introduction
2. Experimental
Among III–VI compounds not much attention has been paid to investigations of indium and gallium tellurides. There are some works devoted to structure, electronic and optical properties but only few [1,2]—to the heterostructures (HS), based on those compounds. The main reasons for that are low symmetry groups (monoclinic C2m for GaTe and tetragonal I4/mcm for InTe), responsible for the complicity in theoretical calculations. The simplest method of HS fabrication is thermal oxidation. In contrary to indium and gallium selenides [3–5] and sulphides [6], thermal oxidation of InTe and GaTe has not been studied to date. Third group selenides are characterised by a complicated sequence of intermediate phase, formed during oxidation (selenates and selenides with the higher Se content). In contrary, the corresponding sulphides are more acceptable for HS construction due to much higher volatility of sulphur. It appears that formation of MeTe-own oxide structures (Me–In, Ga) and detailed investigation of obtained films composition should be useful for future applications.
The experimental investigations were conducted on not intentionally doped indium and gallium tellurides, grown by the Bridgmen method. The cleavage of the samples produces mirror like oxygen and carbon-free surfaces, which do not need additional treatment. Thermal oxidation was performed in air at the definite temperatures from interval 420 to 725 ◦ C in previously heated furnace. The oxidation time was chosen as 5 min for every sample, as XPS is a mostly surface sensitive technique and oxidation of bulk is unnecessary. After the oxidation, the samples were mouted to the XPS chamber. Earlier XPS was successfully used for description of GaSe [3] and InSe [4] thermal oxidation processes. XPS measurements were carried out in an ultra high vacuum (10−9 mbar) chamber. Spectra were recorded using a PHI 5700 MultiTechnique spectrometer with monochromatic Al K␣ irradiation (hν = 1486.6 eV) as an excitation source. The spectrometer was calibrated with respect to the binding energy of the 4f level of gold (Eb (Au 4f7/2 ) = 84.00 eV). The oxidation was observed by measuring the binding energies of indium, gallium, oxygen and tellurium core levels. Due to Ref. [3,4] spectral positions of corresponding elements Auger peaks are not very informative for the oxidation process.
∗
Corresponding author. Tel.: +380 322 964200; fax: +380 322 729467. E-mail address: [email protected] (O.A. Balitskii).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.07.055
O.A. Balitskii, W. Jaegermann / Materials Chemistry and Physics 97 (2006) 98–101
99
Fig. 1. Evolution of XPS spectra of Te, O and In core levels during InTe thermal oxidation.
3. Results and discussion Fig. 1 presents evolution of tellurium 3d, oxygen 1s and indium 3d levels emission during InTe oxidation. Due to spin-orbital splitting of tellurium and indium 3d core levels, In 3d5/2 and Te 3d5/2 sublevels were analysed for chemical shifts. The complex emission lines were decomposed into Gaussians, centred due to binding energies of elements in corresponding compounds. The cleaved InTe sample is
characterised by the absence of oxygen and binding energies of In 3d5/2 and Te 3d5/2 levels equal to 444.7 and 572.7 eV, respectively. With increased oxidation temperatures the intensity of indium core level emission drops with the simultaneous appearance of the asymmetric shape from the higher binding energy region. The In 3d peaks are, thus, decomposed into two Gaussians. The Gaussian, positioned in the higher binding energies region (Eb (In 3d5/2 ) = 445.0 eV), due to [7,8], should be attributed to the indium oxide. With
100
O.A. Balitskii, W. Jaegermann / Materials Chemistry and Physics 97 (2006) 98–101
further increase of oxidation temperature the intensity of the emission peak at 444.7 eV sharply decreases and the peak disappears at 590 ◦ C. The intensity of component, attributed to In2 O3 grows up to temperature 590 ◦ C. Then the intensity of the indium emission remains stable with some small fluctuations. The position of the tellurium 3d5/2 level shifts at the beginning of oxidation from 572.7 to 576.6 eV. The high sensitivity of this line to oxidation was revealed in [9], where as prepared indium telluride films possess strong XPS emission near 576.6 eV. The binding energy of the tellurium 3d5/2 line well correlates with that one in TeO2 (in TeO3 the binding energy of the tellurium 3d5/2 level is much higher [7]). The intensity of this peak grows until the temperature
reaches 515 ◦ C. Afterwards the intensity of the Te 3d5/2 emission slowly decreases until the temperature approaches 725 ◦ C. The last temperature is very close to the melting point of tellurium dioxide (733 ◦ C), so there should be essential losses of this phase from the surface. The oxygen 1s emission is asymmetrical and can be perfectly fitted by two Gaussians for all oxidation temperatures. The first one at 529.8 eV corresponds to oxygen in indium oxide [4]. The second one, positioned at 532.4 eV, should be attributed to oxygen in TeO2 and possible adsorbed OH− from the ambient. The intensity of the first contribution saturates at 515 ◦ C with following unessential fluctuations; the intensity of the second contribution reaches its maximum earlier (420 ◦ C) and slightly falls
Fig. 2. Evolution of XPS spectra of Te, O and Ga core levels during GaTe thermal oxidation.
O.A. Balitskii, W. Jaegermann / Materials Chemistry and Physics 97 (2006) 98–101
down with the temperature increasing, which correlates well with tellurium line behavior. Fluctuations of all peaks intensities can be explained by the redistribution of indium and tellurium oxides during reactive oxygen diffusion into the crystal. We should discuss the possibility of In2 Te3 formation, as correspondent phase (In2 Se3 ) was observed during InSe oxidation. The indium 3d5/2 line shifts very slightly even for the much higher oxidation state. The tellurium 3d5/2 line for In2 Te3 is measured at about 572.5–572.7 eV [9]; it practically coincides with that one of InTe. Due to high reactivity even as prepared by co-evaporation indium telluride thin films are characterised by a high quantity of tellurium dioxide on the surface [9]. As even low oxidation temperature does not permit to observe emission from In–Te bonds we could not prove or reject the question about In2 Te3 formation. Fig. 2 illustrates the oxidation process for gallium telluride. The cleaved samples are characterised by the absence of oxygen; the binding energies of the Ga 3d and Te 3d5/2 levels are 19.5 and 573.1 eV, respectively. The second one correlates well with the data for GaTe, prepared by MOCVD [10]. The Ga 3d peak as reported for GaTe in Ref. [10] should be situated at 20.5 eV but as assessed from other sources [3,7] only high oxidation states of Ga (mainly gallium oxides) can shift the Ga 3d emission to such high binding energies above 20 eV. Thus the position of the Ga 3d peak in [10] should be explained by the formation of Ga-oxides due to uncontrolled oxygen, maybe from the CVD precursor. During oxidation only at the lowest temperature (420 ◦ C) minor amounts of GaTe can be found as the intensity of peak at 19.5 eV decreased by more than one order. For all other oxidation temperatures the component with the spectral position of 20.5 eV prevails. As was mentioned above, this component can clearly be identified as gallium in Ga2 O3 [3,7]. With the increase of oxidation temperature the intensity of at the Ga2 O3 emission at 20.5 eV increases and saturates at 590 ◦ C. The peak is symmetrical and higher oxidation states of gallium [3] were not found. A shoulder at about 23 eV, observed at oxidation temperatures of 590–725 ◦ C, is probably an emission of the oxygen 2s level [7]. The tellurium 3d5/2 emission at 420 ◦ C shows two contributions with practically similar intensity. The intensity of the high binding energy emission from TeO2 reached its maximal value at 515 ◦ C, afterwards its intensity remains stable. The behavior of low binding energy tellurium contribution is more complicated. The intensity of Te 3d5/2 peak at 573.1 eV decreases when the oxidation temperature increases, but at 650 ◦ C its intensity becomes almost comparable to the tellurium oxide contribution with simultaneous shift to the lower binding energy region (572 eV). The only possible phase with such a binding energy of tellurium is Ga2 Te3 [10]. At the same time the Ga 3d peak position corresponds exactly to gallium oxide. This is because of characteristic changes in the escape depth of the photoelectrons, as also discussed below. Further increasing temperature results in the presence of only a TeO2 contribution in the tellurium spectrum. The behavior of the
101
oxygen 1s level is almost the same as in the case of InTe oxidation. Two components with 530.9 and 532.4 eV binding energies (attributed to Ga2 O3 and TeO2 , respectively) are observed. The intensity of the gallium oxide component is insignificantly affected by increasing the oxidation temperature, while the intensity of the high binding energy emission, corresponding to tellurium oxide decreases when the temperature increases up to 650 ◦ C.
4. Conclusions We have established that the oxidation processes of GaTe and InTe are rather different. Indium telluride has both indium and tellurium in the surface termination layer with dangling bonds. The oxidation reaction, thus, results in simultaneous formation of metal and chalcogen oxides mixtures. Layered GaTe is fully tellurium terminated, which makes the surface more resistant to oxidation. The reason for the disappearance of the bulk component of the Ga 3d line at initial oxidation stage is attributed to the high quantity of oxygen which intercalates between the single layers of the layered compounds [3,4]. The escape depth of Ga 3d electrons (with about 1.4 keV kinetic energies) is approximately 1.7 nm, coinciding with the thickness of a GaTe single layer. Thus XPS detects oxidized gallium atoms on the boundaries between layers. Such unusual oxidation picture, as compared with the other III–VI compounds, is caused by the absence of any essential losses of chalcogen during oxidation.
Acknowledgements One of the authors (O.A. Balitskii) acknowledges partial financial support of DAAD (Grant No. A/04/15854). The authors wish to thank E. Golusda for the assistance in experiments.
References [1] C. Coskun, S. Aydogan, H. Efeoglu, Semicond. Sci. Technol. 19 (2004) 242. [2] V.N. Katerinchuk, M.Z. Kovalyuk, Techn. Phys. Lett. 25 (1999) 54. [3] H. Iwakuro, C. Tatsuyama, S. Ishimura, Jpn. J. Appl. Phys. 21 (1982) 94. [4] I. Miyake, T. Tanpo, C. Tatsuyama, Jpn. J. Appl. Phys. 23 (1984) 172. [5] O.A. Balitskii, N.N. Berchenko, V.P. Savchyn, J.M. Stakhira, Mater. Chem. Phys. 65 (2000) 130. [6] O.A. Balitskii, V.P. Savchyn, P.V. Savchyn, Physica B 355 (2005) 365. [7] J. Chastian, R.C. King Jr. (Eds.), Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics Inc., Minnesota, 1995. [8] O.A. Balitskii, V.P. Savchyn, B. Jaeckel, W. Jaegermann, Physica E 22 (2004) 921. [9] N. Guettari, C. Amory, M. Morsli, J.C. Bernede, A. Khelil, Thin Solid Films 431/432 (2003) 497. [10] E.G. Gillian, A.R. Barron, Chem. Mater. 9 (1997) 3037.
XPS study of InTe and GaTe single crystals oxidation O.A. Balitskii a,∗ , W. Jaegermann b a
b
Department of Electronics, Lviv Ivan Franko National University, Dragomanov Str. 50, 79005 Lviv, Ukraine Department of Materials Science, Darmstadt University of Technology, Petersenstrasse 23, 64287 Darmstadt, Germany
Received 7 June 2005; received in revised form 11 July 2005; accepted 23 July 2005
Abstract Using X-ray photoelectron spectroscopy (XPS) thermal oxidation of indium and gallium tellurides single crystals was studied. It was established, that oxidation produces layers of both metal and tellurium oxides on the surface, which drastically differs from indium and gallium sulphides and selenides own oxides. Possibility of formation of the other tellurium containing phases is discussed. © 2005 Elsevier B.V. All rights reserved. Keywords: X-ray photo-emission spectroscopy (XPS); Oxidation; Indium and gallium tellurides
1. Introduction
2. Experimental
Among III–VI compounds not much attention has been paid to investigations of indium and gallium tellurides. There are some works devoted to structure, electronic and optical properties but only few [1,2]—to the heterostructures (HS), based on those compounds. The main reasons for that are low symmetry groups (monoclinic C2m for GaTe and tetragonal I4/mcm for InTe), responsible for the complicity in theoretical calculations. The simplest method of HS fabrication is thermal oxidation. In contrary to indium and gallium selenides [3–5] and sulphides [6], thermal oxidation of InTe and GaTe has not been studied to date. Third group selenides are characterised by a complicated sequence of intermediate phase, formed during oxidation (selenates and selenides with the higher Se content). In contrary, the corresponding sulphides are more acceptable for HS construction due to much higher volatility of sulphur. It appears that formation of MeTe-own oxide structures (Me–In, Ga) and detailed investigation of obtained films composition should be useful for future applications.
The experimental investigations were conducted on not intentionally doped indium and gallium tellurides, grown by the Bridgmen method. The cleavage of the samples produces mirror like oxygen and carbon-free surfaces, which do not need additional treatment. Thermal oxidation was performed in air at the definite temperatures from interval 420 to 725 ◦ C in previously heated furnace. The oxidation time was chosen as 5 min for every sample, as XPS is a mostly surface sensitive technique and oxidation of bulk is unnecessary. After the oxidation, the samples were mouted to the XPS chamber. Earlier XPS was successfully used for description of GaSe [3] and InSe [4] thermal oxidation processes. XPS measurements were carried out in an ultra high vacuum (10−9 mbar) chamber. Spectra were recorded using a PHI 5700 MultiTechnique spectrometer with monochromatic Al K␣ irradiation (hν = 1486.6 eV) as an excitation source. The spectrometer was calibrated with respect to the binding energy of the 4f level of gold (Eb (Au 4f7/2 ) = 84.00 eV). The oxidation was observed by measuring the binding energies of indium, gallium, oxygen and tellurium core levels. Due to Ref. [3,4] spectral positions of corresponding elements Auger peaks are not very informative for the oxidation process.
∗
Corresponding author. Tel.: +380 322 964200; fax: +380 322 729467. E-mail address: [email protected] (O.A. Balitskii).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.07.055
O.A. Balitskii, W. Jaegermann / Materials Chemistry and Physics 97 (2006) 98–101
99
Fig. 1. Evolution of XPS spectra of Te, O and In core levels during InTe thermal oxidation.
3. Results and discussion Fig. 1 presents evolution of tellurium 3d, oxygen 1s and indium 3d levels emission during InTe oxidation. Due to spin-orbital splitting of tellurium and indium 3d core levels, In 3d5/2 and Te 3d5/2 sublevels were analysed for chemical shifts. The complex emission lines were decomposed into Gaussians, centred due to binding energies of elements in corresponding compounds. The cleaved InTe sample is
characterised by the absence of oxygen and binding energies of In 3d5/2 and Te 3d5/2 levels equal to 444.7 and 572.7 eV, respectively. With increased oxidation temperatures the intensity of indium core level emission drops with the simultaneous appearance of the asymmetric shape from the higher binding energy region. The In 3d peaks are, thus, decomposed into two Gaussians. The Gaussian, positioned in the higher binding energies region (Eb (In 3d5/2 ) = 445.0 eV), due to [7,8], should be attributed to the indium oxide. With
100
O.A. Balitskii, W. Jaegermann / Materials Chemistry and Physics 97 (2006) 98–101
further increase of oxidation temperature the intensity of the emission peak at 444.7 eV sharply decreases and the peak disappears at 590 ◦ C. The intensity of component, attributed to In2 O3 grows up to temperature 590 ◦ C. Then the intensity of the indium emission remains stable with some small fluctuations. The position of the tellurium 3d5/2 level shifts at the beginning of oxidation from 572.7 to 576.6 eV. The high sensitivity of this line to oxidation was revealed in [9], where as prepared indium telluride films possess strong XPS emission near 576.6 eV. The binding energy of the tellurium 3d5/2 line well correlates with that one in TeO2 (in TeO3 the binding energy of the tellurium 3d5/2 level is much higher [7]). The intensity of this peak grows until the temperature
reaches 515 ◦ C. Afterwards the intensity of the Te 3d5/2 emission slowly decreases until the temperature approaches 725 ◦ C. The last temperature is very close to the melting point of tellurium dioxide (733 ◦ C), so there should be essential losses of this phase from the surface. The oxygen 1s emission is asymmetrical and can be perfectly fitted by two Gaussians for all oxidation temperatures. The first one at 529.8 eV corresponds to oxygen in indium oxide [4]. The second one, positioned at 532.4 eV, should be attributed to oxygen in TeO2 and possible adsorbed OH− from the ambient. The intensity of the first contribution saturates at 515 ◦ C with following unessential fluctuations; the intensity of the second contribution reaches its maximum earlier (420 ◦ C) and slightly falls
Fig. 2. Evolution of XPS spectra of Te, O and Ga core levels during GaTe thermal oxidation.
O.A. Balitskii, W. Jaegermann / Materials Chemistry and Physics 97 (2006) 98–101
down with the temperature increasing, which correlates well with tellurium line behavior. Fluctuations of all peaks intensities can be explained by the redistribution of indium and tellurium oxides during reactive oxygen diffusion into the crystal. We should discuss the possibility of In2 Te3 formation, as correspondent phase (In2 Se3 ) was observed during InSe oxidation. The indium 3d5/2 line shifts very slightly even for the much higher oxidation state. The tellurium 3d5/2 line for In2 Te3 is measured at about 572.5–572.7 eV [9]; it practically coincides with that one of InTe. Due to high reactivity even as prepared by co-evaporation indium telluride thin films are characterised by a high quantity of tellurium dioxide on the surface [9]. As even low oxidation temperature does not permit to observe emission from In–Te bonds we could not prove or reject the question about In2 Te3 formation. Fig. 2 illustrates the oxidation process for gallium telluride. The cleaved samples are characterised by the absence of oxygen; the binding energies of the Ga 3d and Te 3d5/2 levels are 19.5 and 573.1 eV, respectively. The second one correlates well with the data for GaTe, prepared by MOCVD [10]. The Ga 3d peak as reported for GaTe in Ref. [10] should be situated at 20.5 eV but as assessed from other sources [3,7] only high oxidation states of Ga (mainly gallium oxides) can shift the Ga 3d emission to such high binding energies above 20 eV. Thus the position of the Ga 3d peak in [10] should be explained by the formation of Ga-oxides due to uncontrolled oxygen, maybe from the CVD precursor. During oxidation only at the lowest temperature (420 ◦ C) minor amounts of GaTe can be found as the intensity of peak at 19.5 eV decreased by more than one order. For all other oxidation temperatures the component with the spectral position of 20.5 eV prevails. As was mentioned above, this component can clearly be identified as gallium in Ga2 O3 [3,7]. With the increase of oxidation temperature the intensity of at the Ga2 O3 emission at 20.5 eV increases and saturates at 590 ◦ C. The peak is symmetrical and higher oxidation states of gallium [3] were not found. A shoulder at about 23 eV, observed at oxidation temperatures of 590–725 ◦ C, is probably an emission of the oxygen 2s level [7]. The tellurium 3d5/2 emission at 420 ◦ C shows two contributions with practically similar intensity. The intensity of the high binding energy emission from TeO2 reached its maximal value at 515 ◦ C, afterwards its intensity remains stable. The behavior of low binding energy tellurium contribution is more complicated. The intensity of Te 3d5/2 peak at 573.1 eV decreases when the oxidation temperature increases, but at 650 ◦ C its intensity becomes almost comparable to the tellurium oxide contribution with simultaneous shift to the lower binding energy region (572 eV). The only possible phase with such a binding energy of tellurium is Ga2 Te3 [10]. At the same time the Ga 3d peak position corresponds exactly to gallium oxide. This is because of characteristic changes in the escape depth of the photoelectrons, as also discussed below. Further increasing temperature results in the presence of only a TeO2 contribution in the tellurium spectrum. The behavior of the
101
oxygen 1s level is almost the same as in the case of InTe oxidation. Two components with 530.9 and 532.4 eV binding energies (attributed to Ga2 O3 and TeO2 , respectively) are observed. The intensity of the gallium oxide component is insignificantly affected by increasing the oxidation temperature, while the intensity of the high binding energy emission, corresponding to tellurium oxide decreases when the temperature increases up to 650 ◦ C.
4. Conclusions We have established that the oxidation processes of GaTe and InTe are rather different. Indium telluride has both indium and tellurium in the surface termination layer with dangling bonds. The oxidation reaction, thus, results in simultaneous formation of metal and chalcogen oxides mixtures. Layered GaTe is fully tellurium terminated, which makes the surface more resistant to oxidation. The reason for the disappearance of the bulk component of the Ga 3d line at initial oxidation stage is attributed to the high quantity of oxygen which intercalates between the single layers of the layered compounds [3,4]. The escape depth of Ga 3d electrons (with about 1.4 keV kinetic energies) is approximately 1.7 nm, coinciding with the thickness of a GaTe single layer. Thus XPS detects oxidized gallium atoms on the boundaries between layers. Such unusual oxidation picture, as compared with the other III–VI compounds, is caused by the absence of any essential losses of chalcogen during oxidation.
Acknowledgements One of the authors (O.A. Balitskii) acknowledges partial financial support of DAAD (Grant No. A/04/15854). The authors wish to thank E. Golusda for the assistance in experiments.
References [1] C. Coskun, S. Aydogan, H. Efeoglu, Semicond. Sci. Technol. 19 (2004) 242. [2] V.N. Katerinchuk, M.Z. Kovalyuk, Techn. Phys. Lett. 25 (1999) 54. [3] H. Iwakuro, C. Tatsuyama, S. Ishimura, Jpn. J. Appl. Phys. 21 (1982) 94. [4] I. Miyake, T. Tanpo, C. Tatsuyama, Jpn. J. Appl. Phys. 23 (1984) 172. [5] O.A. Balitskii, N.N. Berchenko, V.P. Savchyn, J.M. Stakhira, Mater. Chem. Phys. 65 (2000) 130. [6] O.A. Balitskii, V.P. Savchyn, P.V. Savchyn, Physica B 355 (2005) 365. [7] J. Chastian, R.C. King Jr. (Eds.), Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics Inc., Minnesota, 1995. [8] O.A. Balitskii, V.P. Savchyn, B. Jaeckel, W. Jaegermann, Physica E 22 (2004) 921. [9] N. Guettari, C. Amory, M. Morsli, J.C. Bernede, A. Khelil, Thin Solid Films 431/432 (2003) 497. [10] E.G. Gillian, A.R. Barron, Chem. Mater. 9 (1997) 3037.