- Email: [email protected]
Interaction of hydrogen with InN thin films elaborated on InP(1 0 0)
Surface Science 601 (2007) 3722–3725 www.elsevier.com/locate/susc
Interaction of hydrogen with InN thin films elaborated on InP(1 0 0) M. Krawczyk
b
a,*
, A. Bilin´ski a, J.W. Sobczak a, S. Ben Khalifa b, C. Robert-Goumet b, L. Bideux b, B. Gruzza b, G. Monier b
a Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warszawa, Poland LASMEA, UMR 6602 CNRS, Blaise Pascal University, Campus Scientifique des Ce´zeaux, 63177 Aubie`re Cedex, France
Available online 5 April 2007
Abstract III–V semiconductor compound structures are widely applied in technology of advanced microelectronics, optoelectronics, and gas sensors. In this paper, we report on the use of XPS to characterize in situ the interaction of thermally activated hydrogen atoms and hydrogen molecules with InP(1 0 0) surfaces covered by thin InN overlayers. XPS spectra were taken with an ESCALAB-210 spectrometer after repeated hydrogenation cycles at temperatures up to 350 C. The evolution of the In 3d, In 4d, P 2p, N 1s, O 1s and C 1s photoelectron spectra was carefully monitored. The XPS spectra of the hydrogen exposed surface revealed significant differences compared to those from the non-hydrogenated surface. InN films were found to be weakly reactive to hydrogen under experimental conditions explored. The behavior of P atoms at the hydrogenated surface was dependent on the parameters characterizing each hydrogenation (exposure, hydrogen species used, annealing temperature). Moreover, the heavily hydrogenated surface exhibited a phosphorus enrichment. 2007 Elsevier B.V. All rights reserved. Keywords: Indium phosphide; Indium nitride film; Molecule–solid reactions; Atom–solid reactions; Hydrogen molecule; Hydrogen atom; Hydrogenation; X-ray photoelectron spectroscopy
1. Introduction The interaction of hydrogen with different III–V semiconductor surfaces is a key aspect in a number of technological processes (crystal growth, crystal defects and bulk impurities passivation, procedures of surface cleaning and etching) [1]. In addition, hydrogen strongly affects the properties of semiconducting materials. It is always electrically active, and usually counteracts the prevailing conductivity of the semiconductor. In some materials, including indium nitride (InN), hydrogen acts, however, as a source of doping [2]. Therefore, the interaction of hydrogen with InN seems to be a key aspect of this technologically important material. Its stability at high temperature is strongly affected by the presence of H2 in the ambient which, in turn, influ*
Corresponding author. Tel.: +48 22 3433403; fax: +48 22 3433333. E-mail address: [email protected] (M. Krawczyk).
0039-6028/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.04.003
ences the quality of material grown on different semiconductor compound substrates. In particular the nitridation of InP(1 0 0) substrates has been recently studied in detail [3–6]. Through these studies, both the growth of InN thin films on the substrate and its mechanism have been evidenced. However, investigations of hydrogen interaction with InN-covered InP(1 0 0) system are still lacking. In the present work, we report the partial results of systematic XPS studies of the interaction between the hydrogen molecules (atoms) and the InN/InP(1 0 0) system. Here, we focus on the chemical composition of differently prepared InN/InP(1 0 0) surfaces. 2. Experimental Thin layers of InN were grown on S-doped InP(1 0 0) substrates (n = 4.7 · 1016 cm 3) at temperatures up to 250 C. The substrate cleaning treatments and their nitridation process have been already described elsewhere [3–6].
M. Krawczyk et al. / Surface Science 601 (2007) 3722–3725
Briefly, substrate nitridation and InN growth were performed with a glow discharge cell (GDS). In this kind of nitrogen source, continuous plasma was produced, and a majority of N atomic species were successfully created. Through the consumption of metallic In droplets, the nitrogen flux combined with In metal allowed to produce thin layers (e.g. four monolayers) of InN on InP(1 0 0). Following nitridation of two InP(1 0 0) substrates, InN layers were grown at different growth conditions: (1) nitridation process time of 35 min, two atomic monolayers have been created (the sample exposed to molecular hydrogen); (2) to create four atomic monolayers: indium evaporation time of 3 min, nitridation process time of 30 min (the sample exposed to atomic hydrogen). The UHV system used here for hydrogenation treatments consisted of a preparation and an analysis chamber separated by a gate valve. The analysis chamber was equipped with an ESCALAB-210 spectrometer [7,8]. XPS spectra were collected using the Al Ka source (300 W, 15 kV, 20 mA) at a take-off angle of 68 to analyze a thin surface layer. The In 3d, In 4d, P 2p, N 1s, O 1s and C 1s photoelectron spectra were acquired with a pass energy set at 50 eV. Data were analyzed using the AVANTAGE program (Thermo VG Scientific, UK), including satellite subtraction, Shirley background subtraction and fitting procedure. Quantification was performed using the multiline method [9]. Charging effects were corrected using the C 1s photoelectron line from surface contaminants at 285.0 eV. In the preparation chamber, hydrogen exposure at increasing temperature up to 350 C were performed. Molecular hydrogen was admitted to the whole preparative chamber in three successive exposures (in Langmuirs, 1 L = 10 6 Torr s): 4.50 · 109 L (p = 5.0 mbar, t = 20 min, T = 100 C), 5.17 · 109 L (p = 4.6 mbar, t = 25 min, T = 250 C), and 3.37 · 109 L (p = 3.0 mbar, t = 25 min, T = 350 C). In our studies the total H2 exposure was calculated to be 13.04 · 109 L. Atomic hydrogen was generated by passing hydrogen gas through a coiled tungsten filament at 2000 K. The atomic hydrogen doser was kept about 7 cm away from the sample during dosing. In order to perform the H-atom hydrogenation, we used three successive H2 exposures: 1.35 · 109 L (p = 3.0 mbar, t = 10 min, T = 100 C), 0.67 · 109 L (p = 0.5 mbar, t = 30 min, T = 250 C), and 0.81 · 109 L (p = 0.6 mbar, t = 30 min, T = 350 C) in presence of a hot filament. The total exposure of H2 considered was 2.83 · 109 L. It is important to note that the hydrogen dosage reported in Langmuirs by other authors refers to the total amount of gas introduced into the chamber, and not to the H dosage, which (depending on the experimental set-up) can be a very small fraction of the total gas dosage. Only 10% of the hydrogen molecules were estimated to dissociate into atomic hydrogen under the experimental conditions used [10]. The H2 dissociation probability on high temperature tungsten has been found to be 0.4 at 3000 K [10]. Moreover, it has been also found to be independent of the H2 pressure (for
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p < 0.13 mbar, but such independence might continue well at the higher p, according to [10]). After exposure the InN/ InP(1 0 0) samples were transferred under UHV to the XPS chamber for subsequent surface analyses. The samples examined here were sputter cleaned by 3 keV Ar+ irradiation (4 lA/cm2, 3 min) prior to a first hydrogenation. After this treatment, common surface contaminants, carbon and oxygen, were reduced to a minimum. Since the experimental conditions separating the contaminations removal and N depletion have not yet been defined, an optimum surface cleaning process was applied where N depletion could be minimized without InN surface damage could be achieved. 3. Results and discussion Present XPS results provide detailed information about the chemical situation of differently nitridated InP(1 0 0) substrates after their interaction with H2 and/or H at 100 (not involved here), 250 and 350 C. It should be underlined that molecular hydrogen does not stick on III–V semiconductor surfaces and does not dissociate at room temperature. Fig. 1 presents the In 3d (a) and the P 2p (b) photoelectron lines for the non-hydrogenated InN/ InP(1 0 0) surface (signals 1) and after thermally activated hydrogen exposures at 250 and 350 C (signals 2–3). In the same figure (Fig. 1c), the intensity ratio of the P 2p and the In 3d3/2 lines, as a function of hydrogen (molecules/atoms) exposure, is also shown. As can be seen in Fig. 1a and b, both the In 3d and P 2p doublets exhibit a significant difference compared to those recorded prior to the hydrogen treatment. This difference reflects in both the intensity behavior (P 2p) and the energetic position of all contributions considered (In 3d, P 2p). The levels of the hydrogen covered surface were shifted to higher binding energies by 0.8 and 1.2 eV for the In 3d and P 2p1/2, respectively. In analogy with previous studies carried out on InP(1 0 0) [11,12], our XPS In 3d spectra exhibit the presence of different In–P–O surface species for the nonhydrogenated sample. Their evolutions were clearly evidenced during heavy hydrogen treatment at temperatures up to 350 C. Moreover, an exact line shape analysis of these spectra (not shown here) does not confirm the findings of earlier XPS data [13,14], exhibiting the dominant metallic character (the transition from In–In configuration into that of metallic In) of the InP(1 0 0) surface after heavy hydrogen exposure. Beside the higher binding energy shift of P 2p hydrogenated core level spectra, their remarkable line shape and intensity changes are observable in Fig. 1b. The P 2p spectra were fitted using similar parameters applied in [4,6], and decomposed into two doublets corresponding to the P–In and P–N contributions (not shown here). The intensity (i.e. the population of each contribution considered) of the P 2p line drastically increases for the hydrogen exposed surface. This is reflected strongly in the P 2p/In 3d3/2 intensity ratio (Fig. 1c), especially for hydrogen exposures in
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M. Krawczyk et al. / Surface Science 601 (2007) 3722–3725
As follows from Fig. 1c, molecular and thermally activated hydrogen causes substantial changes in the XPS intensity ratio of the P 2p to the In 3d3/2 line. Up to 5 · 109 L H2 (molecules) there is almost no change in the ratio, then there is a drastic rise up to 0.072. In the case of hydrogen atoms exposure (atoms), however, this ratio initially decreases from 0.097 to 0.074. After an exposure of 2 · 109 L, there is a sudden increase in the P 2p/In 3d3/2 intensity ratio. It is important to note that Fig. 1c refers to hydrogen molecules and/or atoms interacted with the two differently prepared samples, as clearly indicated above. Therefore, the initial value of the P2p/In 3d3/2 intensity ratio differs considerably for the non-hydrogenated sample (at hydrogen exposure of 0 L). For both the samples studied here, the observed rise of the ratio is consistent with the formation of phosphorus-rich surface [15]. This result can be related to the presence of a more disordered InP(1 0 0) substrate surface induced by annealing in hydrogen environment, containing weak In–P bonds or P-interstitial atoms which are more reactive toward H atoms, and producing PHx (x = 1–3) species on the surface. Furthermore, phosphorus hydrides are more stable in the advanced than in the initial stages of hydrogenation interaction. It has been known [16] that PH3 thermally decomposes on the surface at T > 300 C, producing mainly P4 and P2H4. Fig. 2 shows the In 4d (valence band) photoelectron spectra for prolonged atomic hydrogen exposure ranging from 0 L (spectrum 1) to 2.83 · 109 L (spectrum 3). Details of the hydrogenation exposure and identification of different components [4,5] are presented in Table 1. With increasing hydrogen exposure, the In 4d surface component peak positions shifts towards larger binding energy maximally by about 0.8 eV. The observed surface core-level shifts (SCLS) are significantly higher than the 0.45 eV shift
Fig. 1. Evolutions of (a) the In 3d; (b) the P 2p and (c) intensity ratio of the P 2p and In 3d3/2 photoelectron spectra from InN/InP(1 0 0) surface exposed to atomic/molecular hydrogen: (1) non-hydrogenated; (2) 2.02 · 109 L, 250 C; (3) 2.83 · 109 L, 350 C. The total exposure is given in Langmuirs of molecular hydrogen in presence of a hot filament.
presence of a hot filament. A more extended report of the XPS results seen in Fig. 1a and b is currently under way and will be presented elsewhere.
Fig. 2. Evolution of the In 4d photoelectron spectra from InN/InP(1 0 0) surface exposed to atomic hydrogen: (1) non-hydrogenated; (2) 2.02 · 109 L, 250 C; (3) 2.83 · 109 L, 350 C. The total exposure is given in Langmuirs of molecular hydrogen in presence of a hot filament.
M. Krawczyk et al. / Surface Science 601 (2007) 3722–3725
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Table 1 Binding energies (in eV), identification and relative proportions for different contributions in the In 4d photoelectron spectra from InN/InP(1 0 0) surface exposed to atomic hydrogen H exposure (109 L)
In–X (X = In, P, N) contribution (%) In–In
0 1.35 2.02 2.83
16.1(7) 15.8(3) 15.8(3) 16.0(3)
In–P 16.8(5) 17.1(2) 17.2(2) 17.3(2)
17.0(34) 17.4(36) 17.4(36) 17.8(37)
In–N 17.7(22) 18.2(24) 18.1(24) 18.4(25)
18.2(19) 18.3(21) 18.3(21) 18.7(19)
18.9(13) 19.3(14) 19.2(14) 19.5(14)
The total exposure is given in Langmuirs of molecular hydrogen in presence of a hot filament.
found for InP(1 1 0) under hydrogen exposure up to 104 L range [17]. This SCLS result for the In 4d line agree quite well with a simple model of charge-transfer induced chemical shifts by a charge redistribution among the surface atoms, giving rise to increased ionicity at the surface [18]. In addition, this change may be also connected with relaxation and reconstruction processes at the surface. One of the effects of atomic hydrogen is also an increase in the spin–orbit splitting value of the In–In contribution in the In 4d XPS spectra in the 0.7–1.4 eV range, as shown in Table 1. Similarly, the Au 5d spin–orbit splitting has been found to increase from 1.5 to about 2.1 for the Au/ InP(1 1 0) interface [19]. In both the cases, the change may be indicative of a relaxation (or screening effect) mentioned above. It is also important to note that hydrogen insertion into the In–In dimer bonds produces electrondeficient bridging In–H–In hydrides [20]. The XPS data provide clear evidence that nitride layers seem to be quite stable during the hydrogen treatment since no significant change of In–N bonds is observed. The shift to higher energies of the N 1s peak (not shown here, BE = 398.9 eV for N–H, BE = 400.4 eV for NH3) is consistent with the formation of N–H bonds on the InN surface. Thus, a thin surface outmost layer rich in In–H and N–H is formed upon exposure to atomic hydrogen of the surface studied. In the future, the combination of XPS and in situ TDS will be of high relevance for more detailed information about both the formation and dissociation of In, N and P hydrides. 4. Conclusions Here we reported and discussed new XPS data on the hydrogenation of a InN-covered InP(1 0 0) surface at increasing temperature up to 350 C. The InN layers have been grown on the substrate at different growth conditions. It was found that the interaction of InN thin layers with hydrogen depends on the reactant state. Furthermore, hydrogen atoms were found to be more effective in this relatively weak interaction under experimental conditions examined.
InN did not form a protective layer of the InP(1 0 0) substrate. High hydrogen exposures produced probably its surface disruption through the alteration of surface stoichiometry. XPS results led to the conclusion of a final surface rich in phosphorus saturated with hydrogen through P–H bonds. References [1] S. Nannarone, M. Pedio, Surf. Sci. Rep. 51 (2003) 1, and references therein. [2] Ch. G. Van den Walle, Physica B 376–377 (2006) 1. [3] M. Petit, Y. Ould-Metidji, C. Robert, L. Bideux, B. Gruzza, V. Matolin, Appl. Surf. Sci. 212–213 (2003) 601. [4] L. Bideux, Y. Ould-Metidji, B. Gruzza, V. Matolin, Surf. Interface Anal. 34 (2002) 712. [5] M. Petit, C. Robert-Goumet, L. Bideux, B. Gruzza, V. Matolin, S. Arabasz, B. Adamowicz, D. Wawer, M. Bugajski, Surf. Interface Anal. 37 (2005) 615. [6] M. Petit, D. Baca, S. Arabasz, L. Bideux, N. Tsud, S. Fabik, B. Gruzza, V. Chab, V. Matolin, K.C. Prince, Surf. Sci. 583 (2005) 205. [7] M. Krawczyk, L. Zommer, J.W. Sobczak, A. Jablonski, M. Petit, C. Robert-Goumet, B. Gruzza, Surf. Sci. 566–568 (2004) 856. [8] M. Krawczyk, L. Zommer, A. Kosin´ski, J.W. Sobczak, A. Jablonski, Surf. Interface Anal. 38 (2006) 644. [9] A. Jablonski, B. Lesiak, L. Zomer, M.F. Ebel, H. Ebel, Y. Fukuda, Y. Suzuki, S. Tougaard, Surf. Interface Anal. 21 (1994) 724. [10] W. Zheng, A. Gallagher, Surf. Sci. 600 (2006) 2207. [11] J.F. Wager, W.H. Makky, C.W. Wilmsen, L.G. Meiners, Thin Solid Films 95 (1982) 343. [12] G. Hollinger, E. Bergignat, J. Joseph, Y. Robach, J. Vac. Sci. Technol. A 3 (1985) 2082. [13] O. Mhamedi, F. Proix, J. P Lacharme, C.A. Sebenne, Surf. Sci. 199 (1988) 121. [14] J. Woll, Th. Allinger, V. Polyakov, J.A. Schaefer, A. Goldmann, W. Erfurth, Surf. Sci. 315 (1994) 293. [15] H. Ninomiya, T. Sugino, K. Matsuda, J. Shirafuji, Jpn. J. Appl. Phys. 32 (1993) L12. [16] G. Bruno, M. Losurdo, P. Capezzuto, J. Vac. Sci. Technol. A 13 (1995) 349. [17] F. Proix, C.A. Se´benne, B. El Hafsi, K. Hricovini, R. Pinchaux, J.E. Bonnet, Phys. Rev. B 43 (1991) 14581. [18] V. Hinkel, L. Sorba, K. Horn, Surf. Sci. 194 (1988) 597. [19] I.A. Babalola, W.G. Petro, T. Kendelewicz, I. Lindau, W.E. Spicer, J. Vac. Sci.Technol. A 1 (1983) 762. [20] K. Raghavachari, Q. Fu, G. Chen, L. Li, C.H. Li, D.C. Law, R.F. Hicks, J. Am. Chem. Soc. 124 (2002) 15119.
Interaction of hydrogen with InN thin films elaborated on InP(1 0 0) M. Krawczyk
b
a,*
, A. Bilin´ski a, J.W. Sobczak a, S. Ben Khalifa b, C. Robert-Goumet b, L. Bideux b, B. Gruzza b, G. Monier b
a Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warszawa, Poland LASMEA, UMR 6602 CNRS, Blaise Pascal University, Campus Scientifique des Ce´zeaux, 63177 Aubie`re Cedex, France
Available online 5 April 2007
Abstract III–V semiconductor compound structures are widely applied in technology of advanced microelectronics, optoelectronics, and gas sensors. In this paper, we report on the use of XPS to characterize in situ the interaction of thermally activated hydrogen atoms and hydrogen molecules with InP(1 0 0) surfaces covered by thin InN overlayers. XPS spectra were taken with an ESCALAB-210 spectrometer after repeated hydrogenation cycles at temperatures up to 350 C. The evolution of the In 3d, In 4d, P 2p, N 1s, O 1s and C 1s photoelectron spectra was carefully monitored. The XPS spectra of the hydrogen exposed surface revealed significant differences compared to those from the non-hydrogenated surface. InN films were found to be weakly reactive to hydrogen under experimental conditions explored. The behavior of P atoms at the hydrogenated surface was dependent on the parameters characterizing each hydrogenation (exposure, hydrogen species used, annealing temperature). Moreover, the heavily hydrogenated surface exhibited a phosphorus enrichment. 2007 Elsevier B.V. All rights reserved. Keywords: Indium phosphide; Indium nitride film; Molecule–solid reactions; Atom–solid reactions; Hydrogen molecule; Hydrogen atom; Hydrogenation; X-ray photoelectron spectroscopy
1. Introduction The interaction of hydrogen with different III–V semiconductor surfaces is a key aspect in a number of technological processes (crystal growth, crystal defects and bulk impurities passivation, procedures of surface cleaning and etching) [1]. In addition, hydrogen strongly affects the properties of semiconducting materials. It is always electrically active, and usually counteracts the prevailing conductivity of the semiconductor. In some materials, including indium nitride (InN), hydrogen acts, however, as a source of doping [2]. Therefore, the interaction of hydrogen with InN seems to be a key aspect of this technologically important material. Its stability at high temperature is strongly affected by the presence of H2 in the ambient which, in turn, influ*
Corresponding author. Tel.: +48 22 3433403; fax: +48 22 3433333. E-mail address: [email protected] (M. Krawczyk).
0039-6028/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2007.04.003
ences the quality of material grown on different semiconductor compound substrates. In particular the nitridation of InP(1 0 0) substrates has been recently studied in detail [3–6]. Through these studies, both the growth of InN thin films on the substrate and its mechanism have been evidenced. However, investigations of hydrogen interaction with InN-covered InP(1 0 0) system are still lacking. In the present work, we report the partial results of systematic XPS studies of the interaction between the hydrogen molecules (atoms) and the InN/InP(1 0 0) system. Here, we focus on the chemical composition of differently prepared InN/InP(1 0 0) surfaces. 2. Experimental Thin layers of InN were grown on S-doped InP(1 0 0) substrates (n = 4.7 · 1016 cm 3) at temperatures up to 250 C. The substrate cleaning treatments and their nitridation process have been already described elsewhere [3–6].
M. Krawczyk et al. / Surface Science 601 (2007) 3722–3725
Briefly, substrate nitridation and InN growth were performed with a glow discharge cell (GDS). In this kind of nitrogen source, continuous plasma was produced, and a majority of N atomic species were successfully created. Through the consumption of metallic In droplets, the nitrogen flux combined with In metal allowed to produce thin layers (e.g. four monolayers) of InN on InP(1 0 0). Following nitridation of two InP(1 0 0) substrates, InN layers were grown at different growth conditions: (1) nitridation process time of 35 min, two atomic monolayers have been created (the sample exposed to molecular hydrogen); (2) to create four atomic monolayers: indium evaporation time of 3 min, nitridation process time of 30 min (the sample exposed to atomic hydrogen). The UHV system used here for hydrogenation treatments consisted of a preparation and an analysis chamber separated by a gate valve. The analysis chamber was equipped with an ESCALAB-210 spectrometer [7,8]. XPS spectra were collected using the Al Ka source (300 W, 15 kV, 20 mA) at a take-off angle of 68 to analyze a thin surface layer. The In 3d, In 4d, P 2p, N 1s, O 1s and C 1s photoelectron spectra were acquired with a pass energy set at 50 eV. Data were analyzed using the AVANTAGE program (Thermo VG Scientific, UK), including satellite subtraction, Shirley background subtraction and fitting procedure. Quantification was performed using the multiline method [9]. Charging effects were corrected using the C 1s photoelectron line from surface contaminants at 285.0 eV. In the preparation chamber, hydrogen exposure at increasing temperature up to 350 C were performed. Molecular hydrogen was admitted to the whole preparative chamber in three successive exposures (in Langmuirs, 1 L = 10 6 Torr s): 4.50 · 109 L (p = 5.0 mbar, t = 20 min, T = 100 C), 5.17 · 109 L (p = 4.6 mbar, t = 25 min, T = 250 C), and 3.37 · 109 L (p = 3.0 mbar, t = 25 min, T = 350 C). In our studies the total H2 exposure was calculated to be 13.04 · 109 L. Atomic hydrogen was generated by passing hydrogen gas through a coiled tungsten filament at 2000 K. The atomic hydrogen doser was kept about 7 cm away from the sample during dosing. In order to perform the H-atom hydrogenation, we used three successive H2 exposures: 1.35 · 109 L (p = 3.0 mbar, t = 10 min, T = 100 C), 0.67 · 109 L (p = 0.5 mbar, t = 30 min, T = 250 C), and 0.81 · 109 L (p = 0.6 mbar, t = 30 min, T = 350 C) in presence of a hot filament. The total exposure of H2 considered was 2.83 · 109 L. It is important to note that the hydrogen dosage reported in Langmuirs by other authors refers to the total amount of gas introduced into the chamber, and not to the H dosage, which (depending on the experimental set-up) can be a very small fraction of the total gas dosage. Only 10% of the hydrogen molecules were estimated to dissociate into atomic hydrogen under the experimental conditions used [10]. The H2 dissociation probability on high temperature tungsten has been found to be 0.4 at 3000 K [10]. Moreover, it has been also found to be independent of the H2 pressure (for
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p < 0.13 mbar, but such independence might continue well at the higher p, according to [10]). After exposure the InN/ InP(1 0 0) samples were transferred under UHV to the XPS chamber for subsequent surface analyses. The samples examined here were sputter cleaned by 3 keV Ar+ irradiation (4 lA/cm2, 3 min) prior to a first hydrogenation. After this treatment, common surface contaminants, carbon and oxygen, were reduced to a minimum. Since the experimental conditions separating the contaminations removal and N depletion have not yet been defined, an optimum surface cleaning process was applied where N depletion could be minimized without InN surface damage could be achieved. 3. Results and discussion Present XPS results provide detailed information about the chemical situation of differently nitridated InP(1 0 0) substrates after their interaction with H2 and/or H at 100 (not involved here), 250 and 350 C. It should be underlined that molecular hydrogen does not stick on III–V semiconductor surfaces and does not dissociate at room temperature. Fig. 1 presents the In 3d (a) and the P 2p (b) photoelectron lines for the non-hydrogenated InN/ InP(1 0 0) surface (signals 1) and after thermally activated hydrogen exposures at 250 and 350 C (signals 2–3). In the same figure (Fig. 1c), the intensity ratio of the P 2p and the In 3d3/2 lines, as a function of hydrogen (molecules/atoms) exposure, is also shown. As can be seen in Fig. 1a and b, both the In 3d and P 2p doublets exhibit a significant difference compared to those recorded prior to the hydrogen treatment. This difference reflects in both the intensity behavior (P 2p) and the energetic position of all contributions considered (In 3d, P 2p). The levels of the hydrogen covered surface were shifted to higher binding energies by 0.8 and 1.2 eV for the In 3d and P 2p1/2, respectively. In analogy with previous studies carried out on InP(1 0 0) [11,12], our XPS In 3d spectra exhibit the presence of different In–P–O surface species for the nonhydrogenated sample. Their evolutions were clearly evidenced during heavy hydrogen treatment at temperatures up to 350 C. Moreover, an exact line shape analysis of these spectra (not shown here) does not confirm the findings of earlier XPS data [13,14], exhibiting the dominant metallic character (the transition from In–In configuration into that of metallic In) of the InP(1 0 0) surface after heavy hydrogen exposure. Beside the higher binding energy shift of P 2p hydrogenated core level spectra, their remarkable line shape and intensity changes are observable in Fig. 1b. The P 2p spectra were fitted using similar parameters applied in [4,6], and decomposed into two doublets corresponding to the P–In and P–N contributions (not shown here). The intensity (i.e. the population of each contribution considered) of the P 2p line drastically increases for the hydrogen exposed surface. This is reflected strongly in the P 2p/In 3d3/2 intensity ratio (Fig. 1c), especially for hydrogen exposures in
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M. Krawczyk et al. / Surface Science 601 (2007) 3722–3725
As follows from Fig. 1c, molecular and thermally activated hydrogen causes substantial changes in the XPS intensity ratio of the P 2p to the In 3d3/2 line. Up to 5 · 109 L H2 (molecules) there is almost no change in the ratio, then there is a drastic rise up to 0.072. In the case of hydrogen atoms exposure (atoms), however, this ratio initially decreases from 0.097 to 0.074. After an exposure of 2 · 109 L, there is a sudden increase in the P 2p/In 3d3/2 intensity ratio. It is important to note that Fig. 1c refers to hydrogen molecules and/or atoms interacted with the two differently prepared samples, as clearly indicated above. Therefore, the initial value of the P2p/In 3d3/2 intensity ratio differs considerably for the non-hydrogenated sample (at hydrogen exposure of 0 L). For both the samples studied here, the observed rise of the ratio is consistent with the formation of phosphorus-rich surface [15]. This result can be related to the presence of a more disordered InP(1 0 0) substrate surface induced by annealing in hydrogen environment, containing weak In–P bonds or P-interstitial atoms which are more reactive toward H atoms, and producing PHx (x = 1–3) species on the surface. Furthermore, phosphorus hydrides are more stable in the advanced than in the initial stages of hydrogenation interaction. It has been known [16] that PH3 thermally decomposes on the surface at T > 300 C, producing mainly P4 and P2H4. Fig. 2 shows the In 4d (valence band) photoelectron spectra for prolonged atomic hydrogen exposure ranging from 0 L (spectrum 1) to 2.83 · 109 L (spectrum 3). Details of the hydrogenation exposure and identification of different components [4,5] are presented in Table 1. With increasing hydrogen exposure, the In 4d surface component peak positions shifts towards larger binding energy maximally by about 0.8 eV. The observed surface core-level shifts (SCLS) are significantly higher than the 0.45 eV shift
Fig. 1. Evolutions of (a) the In 3d; (b) the P 2p and (c) intensity ratio of the P 2p and In 3d3/2 photoelectron spectra from InN/InP(1 0 0) surface exposed to atomic/molecular hydrogen: (1) non-hydrogenated; (2) 2.02 · 109 L, 250 C; (3) 2.83 · 109 L, 350 C. The total exposure is given in Langmuirs of molecular hydrogen in presence of a hot filament.
presence of a hot filament. A more extended report of the XPS results seen in Fig. 1a and b is currently under way and will be presented elsewhere.
Fig. 2. Evolution of the In 4d photoelectron spectra from InN/InP(1 0 0) surface exposed to atomic hydrogen: (1) non-hydrogenated; (2) 2.02 · 109 L, 250 C; (3) 2.83 · 109 L, 350 C. The total exposure is given in Langmuirs of molecular hydrogen in presence of a hot filament.
M. Krawczyk et al. / Surface Science 601 (2007) 3722–3725
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Table 1 Binding energies (in eV), identification and relative proportions for different contributions in the In 4d photoelectron spectra from InN/InP(1 0 0) surface exposed to atomic hydrogen H exposure (109 L)
In–X (X = In, P, N) contribution (%) In–In
0 1.35 2.02 2.83
16.1(7) 15.8(3) 15.8(3) 16.0(3)
In–P 16.8(5) 17.1(2) 17.2(2) 17.3(2)
17.0(34) 17.4(36) 17.4(36) 17.8(37)
In–N 17.7(22) 18.2(24) 18.1(24) 18.4(25)
18.2(19) 18.3(21) 18.3(21) 18.7(19)
18.9(13) 19.3(14) 19.2(14) 19.5(14)
The total exposure is given in Langmuirs of molecular hydrogen in presence of a hot filament.
found for InP(1 1 0) under hydrogen exposure up to 104 L range [17]. This SCLS result for the In 4d line agree quite well with a simple model of charge-transfer induced chemical shifts by a charge redistribution among the surface atoms, giving rise to increased ionicity at the surface [18]. In addition, this change may be also connected with relaxation and reconstruction processes at the surface. One of the effects of atomic hydrogen is also an increase in the spin–orbit splitting value of the In–In contribution in the In 4d XPS spectra in the 0.7–1.4 eV range, as shown in Table 1. Similarly, the Au 5d spin–orbit splitting has been found to increase from 1.5 to about 2.1 for the Au/ InP(1 1 0) interface [19]. In both the cases, the change may be indicative of a relaxation (or screening effect) mentioned above. It is also important to note that hydrogen insertion into the In–In dimer bonds produces electrondeficient bridging In–H–In hydrides [20]. The XPS data provide clear evidence that nitride layers seem to be quite stable during the hydrogen treatment since no significant change of In–N bonds is observed. The shift to higher energies of the N 1s peak (not shown here, BE = 398.9 eV for N–H, BE = 400.4 eV for NH3) is consistent with the formation of N–H bonds on the InN surface. Thus, a thin surface outmost layer rich in In–H and N–H is formed upon exposure to atomic hydrogen of the surface studied. In the future, the combination of XPS and in situ TDS will be of high relevance for more detailed information about both the formation and dissociation of In, N and P hydrides. 4. Conclusions Here we reported and discussed new XPS data on the hydrogenation of a InN-covered InP(1 0 0) surface at increasing temperature up to 350 C. The InN layers have been grown on the substrate at different growth conditions. It was found that the interaction of InN thin layers with hydrogen depends on the reactant state. Furthermore, hydrogen atoms were found to be more effective in this relatively weak interaction under experimental conditions examined.
InN did not form a protective layer of the InP(1 0 0) substrate. High hydrogen exposures produced probably its surface disruption through the alteration of surface stoichiometry. XPS results led to the conclusion of a final surface rich in phosphorus saturated with hydrogen through P–H bonds. References [1] S. Nannarone, M. Pedio, Surf. Sci. Rep. 51 (2003) 1, and references therein. [2] Ch. G. Van den Walle, Physica B 376–377 (2006) 1. [3] M. Petit, Y. Ould-Metidji, C. Robert, L. Bideux, B. Gruzza, V. Matolin, Appl. Surf. Sci. 212–213 (2003) 601. [4] L. Bideux, Y. Ould-Metidji, B. Gruzza, V. Matolin, Surf. Interface Anal. 34 (2002) 712. [5] M. Petit, C. Robert-Goumet, L. Bideux, B. Gruzza, V. Matolin, S. Arabasz, B. Adamowicz, D. Wawer, M. Bugajski, Surf. Interface Anal. 37 (2005) 615. [6] M. Petit, D. Baca, S. Arabasz, L. Bideux, N. Tsud, S. Fabik, B. Gruzza, V. Chab, V. Matolin, K.C. Prince, Surf. Sci. 583 (2005) 205. [7] M. Krawczyk, L. Zommer, J.W. Sobczak, A. Jablonski, M. Petit, C. Robert-Goumet, B. Gruzza, Surf. Sci. 566–568 (2004) 856. [8] M. Krawczyk, L. Zommer, A. Kosin´ski, J.W. Sobczak, A. Jablonski, Surf. Interface Anal. 38 (2006) 644. [9] A. Jablonski, B. Lesiak, L. Zomer, M.F. Ebel, H. Ebel, Y. Fukuda, Y. Suzuki, S. Tougaard, Surf. Interface Anal. 21 (1994) 724. [10] W. Zheng, A. Gallagher, Surf. Sci. 600 (2006) 2207. [11] J.F. Wager, W.H. Makky, C.W. Wilmsen, L.G. Meiners, Thin Solid Films 95 (1982) 343. [12] G. Hollinger, E. Bergignat, J. Joseph, Y. Robach, J. Vac. Sci. Technol. A 3 (1985) 2082. [13] O. Mhamedi, F. Proix, J. P Lacharme, C.A. Sebenne, Surf. Sci. 199 (1988) 121. [14] J. Woll, Th. Allinger, V. Polyakov, J.A. Schaefer, A. Goldmann, W. Erfurth, Surf. Sci. 315 (1994) 293. [15] H. Ninomiya, T. Sugino, K. Matsuda, J. Shirafuji, Jpn. J. Appl. Phys. 32 (1993) L12. [16] G. Bruno, M. Losurdo, P. Capezzuto, J. Vac. Sci. Technol. A 13 (1995) 349. [17] F. Proix, C.A. Se´benne, B. El Hafsi, K. Hricovini, R. Pinchaux, J.E. Bonnet, Phys. Rev. B 43 (1991) 14581. [18] V. Hinkel, L. Sorba, K. Horn, Surf. Sci. 194 (1988) 597. [19] I.A. Babalola, W.G. Petro, T. Kendelewicz, I. Lindau, W.E. Spicer, J. Vac. Sci.Technol. A 1 (1983) 762. [20] K. Raghavachari, Q. Fu, G. Chen, L. Li, C.H. Li, D.C. Law, R.F. Hicks, J. Am. Chem. Soc. 124 (2002) 15119.