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Adsorption and thermal decomposition of NH3 on NiAl(001) and NiAl(111)
Applied Surface Science 126 Ž1998. 176–184
Adsorption and thermal decomposition of NH 3 on NiAl ž001 / and NiAl ž111 / G. Schmitz ) , P. Gassmann 1, R. Franchy IGV, Forschungszentrum Julich, D-52425 Julich, Germany ¨ ¨ Received 3 November 1997; accepted 19 November 1997
Abstract The adsorption and thermal decomposition of ammonia ŽNH 3 . on NiAlŽ001. and NiAlŽ111. was investigated by means of high-resolution electron energy loss spectroscopy ŽEELS. and temperature programmed desorption ŽTPD.. At 80 K, NH 3 adsorbs molecularly forming NH 3 multilayers on both surfaces. These multilayers desorb by heating the samples to about 100 K. On NiAlŽ001. the further thermal decomposition of the remaining monolayer-NH 3 takes place for considerably lower temperatures than on NiAlŽ111.. For the Ž001. surface, heating to 150 K leads to a partial decomposition of NH 3 to NH. After annealing at 380 K, the NH 3 and NH species are dissociated completely and atomic nitrogen is adsorbed on the NiAlŽ001. surface. In the case of NiAlŽ111., the decomposition of NH 3 to NH occurs at about 260 K. In contrast to NiAlŽ001., evidence is found for a recombinative thermal desorption of N2 , H 2 , and NH 3 at 134, 270, and 275 K, respectively. At 420 K, NiAlŽ111. is covered with NH which decomposes to atomic nitrogen upon annealing at 750 K. We assume the different atomic composition of the outermost layer of NiAlŽ001. and NiAlŽ111. to be responsible for the observed differences. q 1998 Elsevier Science B.V. Keywords: Adsorption; Atomic composition; Annealing; Atomic nitrogen
1. Introduction AlN as well as GaN and InN belongs to the group of large band gap III–V semiconductors which have attracted much interest recently due to their promising properties for the use in optoelectronic devices. Thin, well-ordered AlN films can be prepared on the Ž001. and Ž111. surface of the intermetallic alloy
) 1
Corresponding author. E-mail: [email protected]. Present address: Dresdner Bank, Frankfurt, Germany.
NiAl upon adsorption of NH 3 at 80 K and subsequent annealing at elevated temperatures w1,2x. This causes the decomposition of NH 3 and the reaction of formed N atoms with Al atoms that segregate to the surface. The grown films have a wurtzite crystal ˚ for structure with lattice parameters of 3.14 " 0.04 A ˚ Ž . the NiAl 001 substrate and 3.11 " 0.04 A for NiAlŽ111., respectively w3x. High-resolution EEL spectra of ordered AlN films show a Fuchs–Kliewer phonon mode at 865 cmy1 in good agreement with theoretical spectra on the base of the dielectric theory. In good agreement with reported values for bulk
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 7 . 0 0 6 0 0 - 4
G. Schmitz et al.r Applied Surface Science 126 (1998) 176–184
AlN, the band gap of these films was determined to be 6.1 " 0.1 eV for the NiAlŽ001. substrate and 6.0 " 0.2 eV for NiAlŽ111., respectively. Gap states at 1.1 and 5.1 eV were found for AlNrNiAlŽ001. as well as for AlNrNiAlŽ111.. In a similar way, thin GaN films on the Ž001.-surface of the intermetallic alloy CoGa can be prepared upon adsorption of NH 3 at 80 K and subsequent thermal decomposition w4x. In order to gain insight into the complex growth processes of thin AlN films on NiAl applying the preparation method described above, a detailed study of the adsorption and thermal decomposition of NH 3 is of fundamental importance. In a previous paper, we reported on the elemental steps in the growth of AlN thin films on NiAl upon thermal decomposition of NH 3 w3x. In this article we focus on aspects of the adsorption and decomposition of NH 3 on NiAl in view of the different behavior of the Ž111. and the Ž001. surface studied by high-resolution electron energy loss spectroscopy ŽEELS. and temperature programmed desorption ŽTPD.. Though the structure and the electronic properties of the grown AlN films are very similar for both surface orientations, the thermal decomposition of NH 3 turns out to be different for NiAlŽ001. and NiAlŽ111.. NiAl orders in a CsCl type structure with a lattice ˚ The Ž001. surface has a square constant of 2.89 A. unit cell and is terminated either by Al- or Ni-atoms. Al-termination was determined from low-energy electron diffraction ŽLEED. IrV studies w5x. In a recent study, Blum et al. w6x showed that the termination and structure of the clean NiAlŽ001. surface depend on the cleaning procedure. In contrast to the Ž001. surface, the NiAlŽ111. surface has an open structure consisting of alternating Ni and Al layers. From low-energy ion scattering in the neutral impact collision mode ŽNICISS. and scanning tunneling microscopy ŽSTM. studies, it is known that the clean NiAlŽ111. surface is Ni terminated with Al atoms in the second layer w7x. The paper is organized as follows: Section 2 deals with the experimental methods and the cleaning procedure of the NiAl samples. In Section 3, the EELS and TPD results are presented and discussed for NH 3rNiAlŽ001. ŽA. and NH 3rNiAlŽ111. ŽB.. Section 3.3 finally deals with concluding remarks on the different decomposition behavior for NH 3 adsorbed on the NiAlŽ001. and NiAlŽ111. surface.
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2. Experimental The experiments were performed in a two level stainless-steel ultrahigh vacuum ŽUHV. chamber at a base pressure of about 5 = 10y1 1 mbar. The upper level contains a cylindrical mirror analyzer ŽCMA. for the Auger electron spectroscopy ŽAES., a threegrid low energy electron diffraction ŽLEED. system, an ion gun and a quadruple mass spectrometer ŽQMS. for TPD. For the TPD measurements, the sample was positioned in front of the aperture ŽB s 3 mm. of the enclosure of the QMS. The sample was heated by electron impact and the temperature was measured using a NiCrrNiAl Žtype K. thermocouple that was spot-welded at the top of the sample. We used a constant heating rate of 2.0 Krs which was controlled by a new self-optimizing digital PID temperature controller. This controller can generate temperature ramps in the range of 15–1500 K with an accuracy of about 0.1%, and will be described in detail elsewhere w8x. The lower level contains a computer-controlled EEL spectrometer which is based on the 1278 cylindrical deflector w9x. The adsorption of 14 NH 3 and 15 NH 3 Ž99.9% purity. was performed by backfilling the UHV chamber. The exposures were measured without correction to the ion gauge sensitivity. In order to avoid readsorption during EELS measurements, NH 3 was adsorbed in small doses. The NH 3 covered surface was annealed to the specified temperatures for about 1 min. Before starting the EELS measurement, the sample was allowed to cool down to 80 K again. The NiAl single crystals were cut by spark erosion and polished mechanically. They were oriented with an accuracy of 0.58. After bake-out of the UHV chamber the samples showed carbon, sulfur, and oxygen contaminations. The cleaning procedure for NiAlŽ001. and NiAlŽ111. is similar. Heating the samples in an oxygen atmosphere Ž1 = 10y6 mbar. to 1100 K leads to oxidation and desorption of the carbon and sulfur species. Simultaneously, this procedure causes the formation of Al-oxide films on the NiAl substrate that can be removed by subsequent annealing to 1500 K. The high annealing temperatures cause the segregation of carbon to the surface. By repeated cycles of the cleaning procedure, the carbon contamination could be removed and a carbon depleted zone in the surface region was estab-
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lished. Sharp Ž1 = 1. LEED pattern were found for the clean NiAlŽ001. w10,11x and NiAlŽ111. w12x surfaces.
3. Results and discussion 3.1. Thermal decomposition of NH3 adsorbed on NiAl(001) at 80 K Fig. 1 shows a series of EEL spectra of 1 Langmuir ŽL. NH 3 adsorbed on NiAlŽ001. as a function of annealing temperature. Spectrum a which was taken after the adsorption at 80 K exhibits six losses at 360, 530, 1095, 1610 and 3390 cmy1 , with a shoulder at about 3260 cmy1 . For comparison, spectrum d in Fig. 1 was taken for an exposure of 10 L NH 3rNiAlŽ001. at 80 K. This spectrum exhibits six losses at 410, 820, 1095, 1640 and 3390 cmy1 , again
Fig. 1. A set of EEL spectra of NH 3 on NiAlŽ001. recorded as a function of annealing temperature. Spectrum a was taken after the adsorption of 1 L NH 3 at 80 K. Spectra b and c show the loss feature after annealing at 150 and 380 K. Spectrum d was taken after the adsorption of 10 L NH 3 at 80 K.
showing a shoulder at about 3260 cmy1 . In order to assign the losses, the observed vibrational frequencies are compared to results found for NH 3 multilayers on AgŽ110. w13x, for a condensed phase of NH 3rFeŽ110. w14x and for solid NH 3 w15x Žsee Table 1.. The broad low-frequency loss structure up to about 380 cmy1 Žwith maximum at 360 cmy1 . in spectrum a is attributed to the frustrated libration n l with a contribution of the hindered translation n t . The mode at 550 cmy1 corresponds to the NiAl– ŽNH 3 . stretching vibration of NH 3 molecules on the surface. The asymmetric high-frequency loss feature consists of the symmetric Ž ns . and antisymmetric Ž na . stretch of NH 3 at 3260 and 3390 cmy1 . The losses at 1095 and 1610 cmy1 are the corresponding symmetric Ž ds . and degenerate Ž dd . deformation modes. Due to the good agreement with the reported frequencies from literature Žsee Table 1., spectrum a can unequivocally be attributed to multilayer NH 3 . In particular, the librational mode at 360 cmy1 w13x and Žas will be shown below. the mode at 1095 cmy1 clearly indicate the formation of a NH 3 multilayer upon disordered condensation of NH 3 . This is in agreement with the observed strong decrease in the amount of elastically scattered electrons for 1 L NH 3rNiAlŽ001. compared to the clean surface. Under the assumption that NH 3 is adsorbed only on high-symmetry sites, a reduced number of dipole active modes would be expected. For example, 0.05 L NH 3rFeŽ110. corresponds to a C 2v symmetry for adsorbed NH 3 molecules with only three dipole active modes w14x. The observed vibrational frequencies for an exposure with 10 L NH 3 can be attributed in a similar way. The frustrated libration n l is the most intense mode observed in the spectrum. In addition, it is shifted to higher frequencies and is now located at 410 cmy1 , in excellent agreement with the condensed phase of NH 3rFeŽ110. w14x. Double excitation of this mode Ž2 n l . leads to the loss at 820 cmy1 . The stretching mode NiAl– ŽNH 3 . is masked by the intense loss structure at 410 cmy1 and can only be observed as a shoulder. The asymmetric high-frequency loss feature consisting of the symmetric Ž ns . and antisymmetric Ž na . stretch of NH 3 at 3260 and 3390 cmy1 is not shifted. The symmetric deformation mode Ž ds . is still located at 1095 cmy1 and the loss intensity is strongly increased. The
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Table 1 Comparison of the vibrational frequencies Žin cmy1 . of NH 3 rNiAlŽ001. at 80 K with values from literature Vibrational mode
NH 3 rNiAlŽ001. 1L
nt nl 2n l n ŽNH 3 –NiAl. ds dd ns na
360 530 1095 1610 3260 3390
10 L shoulder 410 820 shoulder 1095 1640 3260 3390
NH 3 rAgŽ110. w13x Žmultilayer.
NH 3rFeŽ110. w14x Žcondensed phase.
400 800
410 820
1070 1630 3320 3380
1095 1640 3270 3430
degenerate deformation mode Ž dd . is shifted by about 30 cmy1 to higher frequencies and is now found at 1640 cmy1 . Again, this value is in excellent agreement with reported values from literature Žsee Table 1.. Spectrum b shows the situation after annealing the sample at 150 K. It exhibits four losses at 630, 1245, 1530 and 3330 cmy1 . The relative intensity of the losses compared to spectrum a has strongly decreased. The disappearance of the low-frequency loss at 360 cmy1 indicates the desorption of the multilayer. We assume that spectrum b corresponds to NH 3 adsorbed in the first layer Žin the following referred to as monolayer-NH 3 .. The mode at 630 cmy1 is attributed to the NiAl– ŽNH 3 . stretching vibration of NH 3 molecules on the surface. The losses at 1245 and 1530 cmy1 originate in the symmetric Ž ds . and degenerate deformation mode Ž dd . of NH 3 , respectively. Compared to spectrum a the dd mode shows a shift to a lower frequency which is similar to the shift observed for decreasing NH 3 coverages Žcompare spectrum d for 10 L with spectrum a for 1 L in Fig. 1.. The frequency value of the symmetric deformation mode Ž ds . for monolayerNH 3 Ž1245 cmy1 . differs considerably from the multilayer value Ž1095 cmy1 .. Careful examination of the peak at 1095 cmy1 in spectrum a shows that this loss has an asymmetric lineshape with a shoulder at the high-frequency side. We assume that this shoulder is due to a contribution of monolayer-NH 3 in spectrum a. In an isotope exchange experiment with 15 NH 3 on NiAlŽ001. both contributions could be distinguished conclusively: The EEL spectrum for 2.1 L 15 NH 3 adsorbed at 80 K showed two distinct
NH 3 w15x Žsolid. 141 360
1060 1646 3223 3378
losses at 1075 and 1265 cmy1 in the frequency range under investigation. The loss at 1075 cmy1 is close to the value of condensed NH 3 and is therefore attributed to the multilayer, while the mode at 1265 cmy1 originates in the monolayer contribution. A monolayer and multilayer contribution to the ds mode was also observed for NH 3rPtŽ111. w16x. The relatively broad high-frequency loss structure at 3330 cmy1 consists of the stretch vibrations ns and na . However, this high-frequency structure is now more symmetric compared to spectrum a. Since NH has only one vibrational mode in this frequency region, it is probable that in addition to NH 3 molecules NH radicals are coadsorbed. Therefore, we assume that in the temperature range from 80 to 150 K decomposition of NH 3 to NH occurs. Annealing at 380 K leads to a further reduction of the number of loss peaks. Spectrum c exhibits three losses at 245, 465 and 1215 cmy1 . The loss at 245 cmy1 is attributed to the vibration of Al atoms of the NiAl substrate. This loss was also found on clean NiAlŽ111. surfaces w12x. The loss at 465 cmy1 originates in the stretch vibration NiAl–N of atomic nitrogen on the surface. The observed value is close to the frequencies found for several other substrates Že.g., NrFeŽ111.: 465 cmy1 w17x, NrCuŽ110.: 427 cmy1 w18x, NrNiŽ111.: 490 cmy1 w19x.. The very weak loss at 1215 cmy1 is attributed to the vibration of atomic hydrogen on the surface. In an isotope exchange experiment with 15 NH 3 on NiAlŽ001., we found that this mode remains unchanged while the vibration with nitrogen contribution is shifted. This assignment is also supported by the metal–H stretching mode found on low indexed tungsten surfaces
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around 1260 to 1290 cmy1 w20x. Since no highfrequency loss peaks are observed, it is concluded that no NH x species are adsorbed on the surface. Hence, NH 3 is completely decomposed.
To further elucidate the mechanisms of NH 3 decomposition upon annealing, TPD measurements on the system 15 NH 3 were performed. The main desorption products are H 2 , 15 NH 3 and 15 N2 . Fig. 2 shows
Fig. 2. The TPD spectra for the mass 2 ŽH 2 . signal Ža., the mass 18 Ž15 NH 3 . signal Žb., and the mass 30 Ž 15 N2 . signal Žc. for increasing exposures of 15 NH 3 adsorbed on NiAlŽ001..
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thermal desorption spectra for the mass 2 ŽH 2 . signal Ža., the mass 18 Ž 15 NH 3 . signal Žb. and the mass 30 Ž 15 N2 . signal Žc. as a function of 15 NH 3 exposure. The H 2 signal exhibits two desorption peaks at 95 Ž b . and 212 K Žg .. The b peak is attributed in part to the decomposition of NH 3 to NH q H 2 and in part to hydrogen as a cracking pattern of the simultaneously desorbing NH 3 molecules from the multilayer. The g peak indicates the decomposition of the residual NH 3 monolayer and of adsorbed NH molecules to N atoms. This interpretation is in agreement with the EELS results. The mass 18 signal Žb. shows only one peak at 95 K which is due to the NH 3 multilayer desorption. Also the mass 30 signal Žb. mainly shows a peak at 95 K which can be explained as the cracking pattern of the 15 NH 3 signal. Therefore, the TPD measurements confirm the assumption that NH 3 adsorbed on NiAlŽ001. at 80 K thermally decomposes in two steps. In addition, NH 3 multilayer desorption is observed at 95 K. As was reported in a previous paper w3x, further annealing of the sample at 1250 K leads to the formation of AlN on the surface. 3.2. Thermal decomposition of NH3 adsorbed on NiAl(111) at 80 K The adsorption and thermal decomposition of NH 3 was also studied on the NiAlŽ111. surface. It turns out that mechanisms of NH 3 decomposition on NiAlŽ111. are different from those found for the NiAlŽ001. surface. Fig. 3 shows a series of EEL spectra of 1 L NH 3 adsorbed on NiAlŽ111. as a function of annealing temperature. Spectrum a was taken after the adsorption at 80 K. It exhibits seven losses at 340, 625, 1105, 1260, 1625 and 3365 cmy1 with a shoulder at 3220 cmy1 . As in the case of 1 L NH 3rNiAlŽ001. at 80 K Žspectrum a of Fig. 1. the observed losses can be attributed to NH 3 adsorbed in multilayers. The low-frequency losses at 340 and 625 cmy1 are the frustrated librational mode n l and the stretching vibration NiAl– ŽNH 3 . of NH 3 molecules on the surface, respectively. The high-frequency losses at 3220 and 3365 cmy1 originate in the symmetric Ž ns . and antisymmetric Ž na . stretching vibrations of NH 3 . The corresponding degenerate deformation mode Ž dd . is located at 1625 cmy1 . The symmetric deformation
Fig. 3. A set of EEL spectra of NH 3 on NiAlŽ111. as a function of annealing temperature. Spectrum a was taken after the adsorption of 1 L NH 3 at 80 K. Spectra b–e were obtained after heating the sample to the specified temperatures.
mode Ž ds . consists of a multilayer contribution at 1105 cmy1 and a monolayer contribution at 1260 cmy1 . Spectrum b was recorded after annealing the sample to 150 K. It exhibits seven losses at 460, 635, 1230, 1540, 1900 and 3350 cmy1 with a weak shoulder at 3200 cmy1 . The losses at 1900 and 460 cmy1 are attributed to the CO stretch and the n ŽNiAl–CO. vibration of CO molecules that are coadsorbed on the surface from the residual gas. The observed frequency values are in accordance with those found for 1 L COrNiAlŽ111. w21x. The remaining five losses can be explained by monolayerNH 3 . Again, the disappearance of the mode at 340 indicates the desorption of the multilayer. The loss at 635 cmy1 refers to the stretching vibration NiAl– ŽNH 3 .. The modes at 3200 and 3350 are attributed to the symmetric Ž ns . and antisymmetric Ž na . stretching vibrations. The corresponding symmetric Ž ds . and degenerate deformation modes Ž dd . are located at
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1230 and 1540 cmy1 . A similar shift of the dd mode to lower frequencies Ž1625 cmy1 in spectrum a to 1540 cmy1 in spectrum b. was also observed for the system NH 3rNiAlŽ001. Žcf. Fig. 1.. After heating to 420 K, spectrum c in Fig. 3 shows only two modes at 620 and 3300 cmy1 . It exhibits no deformation modes and the highfrequency loss is now narrow and symmetric. This spectrum can be clearly attributed to adsorbed NH, since NH has only one high-frequency loss which is assigned to the NH stretch. The loss at 620 cmy1 originates in the stretching vibration NiAl– ŽNH. of NH molecules on the surface. The observed frequencies are close to those found for NHrNiŽ111. Ž n ŽNi–NH. s 620 cmy1 , n ŽNH. s 3340 cmy1 . w22x and NHrNiŽ110. Ž n ŽNH. s 3240 cmy1 . w19x. For NHrNiŽ111. an additional weak loss at 1270 cmy1 is observed which is attributed to the deformation mode d ŽNH.. Since the deformation mode d ŽNH. is not dipole active for NH molecules adsorbed in an upright geometry, we assume this binding configuration for NHrNiAlŽ111.. Further annealing to 550 K leads to spectrum d. It exhibits four losses at 310, 465, 830 and 3300 cmy1 . The very weak loss at 3300 cmy1 is attributed to a small amount of NH radicals adsorbed on the surface. The mode at 465 cmy1 originates in the stretching vibration NiAl–N of atomic nitrogen on the surface. The same mode was observed for NrNiAlŽ001. Žcf. Section 3.1.. The loss at 310 cmy1 can be attributed to the vibration of Al atoms of the NiAl substrate. On the clean NiAlŽ111. surface this loss was found at 240 cmy1 w12x. Probably due to the presence of atomic nitrogen on the surface, the frequency is shifted to a higher value. The loss at 830 cmy1 indicates the formation of AlN. AlN is characterized by a mode at 865 cmy1 w3x. Due to the overlapping with the intense mode at 465 cmy1 , this mode seems to be shifted to lower frequencies. After annealing to 750 K, spectrum e still shows the vibration of Al atoms at 310 cmy1 , the stretching vibration NiAl–N of atomic nitrogen at 465 cmy1 and the mode at 865 cmy1 indicating the formation of AlN. The additional shoulder at 600 cmy1 is attributed to an intermediate state of AlN w3x. The disappearance of the high-frequency losses indicates the complete decomposition of all NH x species.
TPD measurements were also performed for NH 3rNiAlŽ111.. The main desorption products have been monitored. Fig. 4 shows thermal desorption spectra for mass 2 ŽH 2 . signal Žspectrum a., mass 17 ŽNH 3 . signal Žb., mass 28 ŽN2 or CO. signal Žc. and mass 14 ŽN. signal Žd. for an exposure of 1 L 14 NH 3 . Spectrum a exhibits three distinct H 2 desorption peaks at about 260 Ž b ., 466 Žg 1 . and 596 K Žg 2 .. The broad b peak is attributed in part to the decomposition of NH 3 to NH q H 2 and in part to a hydrogen desorption of adsorbed H atoms. For the system HrNiAlŽ110. a similar hydrogen desorption was found at about 270 K w23x. Spectrum c indicates the desorption of N2 at 134 K. The symmetric lineshape suggests a second order desorption. In a recent paper, a similar TPD spectrum for a recombinative desorption of atomic nitrogen on NiAlŽ111. is found w24x. Therefore, it can be concluded that NH 3 is partly dissociated at this temperature. While the N atoms formed upon NH 3 dissociation desorb at 134
Fig. 4. A set of TPD spectra of 1 L NH 3 adsorbed on NiAlŽ111. showing the mass 2 ŽH 2 . signal Ža., the mass 17 ŽNH 3 . signal Žb., the mass 28 ŽN2 and CO. signal Žc., and the mass 14 ŽN. signal Žd..
G. Schmitz et al.r Applied Surface Science 126 (1998) 176–184
K, the H atoms are desorbed first at about 270 K. It arises the question, why atomic hydrogen was not observed in EEL spectrum b after annealing a 150 K. Atomic hydrogen is expected to have a stretching vibration NiAl–H close to the ds mode at 1230 cmy1 Žcf. Section 3.1. and is probably masked by this peak. The g 1 and g 2 peaks in spectrum a are attributed to the decomposition of NH to adsorbed nitrogen and desorbing hydrogen. Since there are two H 2 desorption peaks, we assume that NH occupies two adsorption sites with different dissociation energies. Spectrum b ŽNH 3 . shows a single peak at 275 K which is due to a recombinative NH 3 desorption of adsorbed atomic nitrogen and hydrogen Žcf. Section 3.3.. For higher NH 3 exposures an additional multilayer desorption peak at about 100 K is observed in the NH 3 signal. This is in good agreement with the EELS results which show the multilayer desorption in the temperature range of 80–150 K. The peak at 408 K in the mass 28 spectrum c is attributed to desorbing CO, because the mass 14 signal Žcracking pattern of N2 . in spectrum d shows no corresponding peak. On the other hand, the peak at 134 K in spectrum c can unequivocally be identified as desorbing N2 . The observed CO desorption at 408 K is in good agreement with the EELS measurements. The spectrum at 150 K clearly indicated adsorbed CO, whereas at 420 K the corresponding loss features disappeared. As was reported in a previous paper w3x, further annealing of the sample at 1150 K leads to the
reaction of the nitrogen atoms with Al under the formation of AlN on the surface. Simultaneously, an intermediate state during the AlN film growth is found for this temperature. After heating to 1250 K, the AlN growth is completed which is indicated by one distinct loss at 865 cmy1 w3x. 3.3. Concluding remarks The different mechanisms of the thermal decomposition of NH 3 on NiAlŽ001. and NiAlŽ111. are summarized in Table 2. On NiAlŽ001. the thermal decomposition of NH 3 takes place for considerably lower temperatures than on NiAlŽ111.. For example, while NH 3rNiAlŽ001. is completely decomposed to atomic nitrogen at 380 K, NH radicals still are observed on the NiAlŽ111. surface at 550 K. The decomposition process on NiAlŽ111. is completed upon annealing at about 600 K. We assume that the reason for this different behavior may be the different atomic composition of the outermost layer of the substrate. As mentioned in Section 1, NiAlŽ111. has an open structure with Ni atoms in the outermost layer as could be shown conclusively w7x. The second layer consists of Al atoms. For NiAlŽ001., Blum et al. w6x showed that the termination and structure of the clean surface depend on the cleaning procedure. In a recent study w24x, we found evidence that our cleaning procedure, which is described in Section 2, leads to an Al terminated surface. The observed NH 3 signal of the TPD measurements Žspectrum b. in
Table 2 Comparison of the different mechanisms of NH 3 decomposition on NiAlŽ001. and NiAlŽ111. Temperature
NiAlŽ001.
NiAlŽ111.
80 K Up to 150 K
NH 3 multilayer Desorption of the multilayer at 95 K Partially decomposition of NH 3 to NH ad q H 2 Monolayer-NH 3 q NH ad decomposition of NH 3 and NH to Nad at 212 K Ždecomposition complete.
NH 3 multilayer Desorption of the multilayer at 100 K Partially decomposition of NH 3 to Nad q H ad Monolayer-NH 3 ŽqCO. Decomposition of the monolayer-NH 3 to NH ad at 260 K Recombinative desorption of N2 and H 2 at 134 and about 270 K Recombinative desorption of NH 3 at 275 K
150 K Up to 380 K
380 K 420 K Up to 550 K
Nad
550 K Up to 750 K
Nad w3x The formation of AlN starts w3x
183
NH ad Decomposition of NH in the g 1 state to Nad at 466 K Formation of AlN NH ad q Nad q AlN Decomposition of NH in the g 2 state to Nad at 596 K Ždecomposition complete.
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Figs. 2 and 4. can serve as an example for the effect of the Ni atoms in the first layer of NiAlŽ111.. The observed recombinative NH 3 desorption at 275 K on the NiAlŽ111. surface can be attributed to the hydrogenation capabilities of Ni. On the other hand, on the Al terminated NiAlŽ001. surface only the multilayer desorption at 95 K is observed. Similar differences were found for the decomposition of CH 3 OH on NiAlŽ001. ŽAl-termination. and NiAlŽ110. Ž50% Ni atoms and 50% Al atoms in the outermost layer. w25x. On NiAlŽ110. the hydrogenation capabilities of Ni atoms were able to remove hydrocarbon fragments from the alloy as methane, whereas this reaction channel appears to be very limited on the Al terminated Ž001. surface w25x. For the growth of the AlN films, the formation of atomic nitrogen is the important step. Further annealing leads to the reaction of nitrogen atoms with Al atoms that segregate from the bulk to the surface. Since the EEL spectra of both surfaces are very similar for temperatures higher than about 750 K w3x, it can be concluded that if atomic nitrogen is present on the surface, the subsequent AlN growth is independent of surface termination. Acknowledgements G.S. gratefully acknowledges the financial support by the Deutsche Forschungsgemeinschaft Žproject Fr 1137r2-1.. References w1x P. Gassmann, F. Bartolucci, R. Franchy, J. Appl. Phys. 77 Ž1995. 5718. w2x P. Gassmann, J. Boysen, G. Schmitz, F. Bartolucci, R. Franchy, Solid State Commun. 97 Ž1996. 1.
w3x P. Gassmann, G. Schmitz, J. Boysen, F. Bartolucci, R. Franchy, J. Vac. Sci. Technol. A 14 Ž3. Ž1996. 813. w4x P. Gassmann, G. Schmitz, R. Franchy, Surf. Sci. 380 Ž1997. L459. w5x H.L. Davis, J.R. Noonan, Mater. Res. Soc. Symp. Proc. 83 Ž1987. 3. w6x R.-P. Blum, D. Ahlbehrendt, H. Niehus, Surf. Sci. 366 Ž1996. 107. w7x H. Niehus, W. Raunau, K. Besocke, R. Spitzel, G. Comsa, Surf. Sci. 225 Ž1990. L8. w8x G. Schmitz, F. Bartolucci, P. Gassmann, R. Franchy, to be published. w9x H. Ibach, Electron Energy Loss Spectrometers, The Technology of High Performance, Springer Series in Optical Science, Springer, Berlin, 1991. w10x P. Gassmann, R. Franchy, H. Ibach, Surf. Sci. 319 Ž1994. 95. w11x P. Gassmann, R. Franchy, H. Ibach, J. Electron Spectrosc. Rel. Phen. 64r65 Ž1993. 315. w12x K. Kitamura, T. Nagao, T. Osaka, R. Franchy, C. Oshima, Jpn. J. Appl. Phys. 32 Ž1993. 3253. w13x J.L. Gland, B.A. Sexton, G.E. Mitchel, Surf. Sci. 115 Ž1982. 623. w14x W. Erley, H. Ibach, J. Electron Spectrosc. Rel. Phen. 31 Ž1983. 161. w15x O.S. Binbrek, A. Anderson, Chem. Phys. Lett. 15 Ž1972. 421. w16x B.A. Sexton, G.E. Mitchel, Surf. Sci. 99 Ž1980. 523. w17x M.-C. Tsai, U. Seip, I.C. Bassignana, J. Kuppers, G. Ertl, ¨ Surf. Sci. 155 Ž1985. 387. w18x D. Heskett, A. Baddorf, E.W. Plummer, Surf. Sci. 195 Ž1988. 94. w19x J.L. Gland, G.B. Fisher, G.E. Mitchell, Chem. Phys. Lett. A 119 Ž1985. 89. w20x C. Backx, B. Feuerbach, B. Fitton, R.F. Willis, Phys. Lett. A 60 Ž1977. 145. w21x R. Franchy, M. Wuttig, H. Ibach, Surf. Sci. 189r190 Ž1987. 438. w22x I.C. Bassignana, K. Wagemann, J. Kuppers, G. Ertl, Surf. ¨ Sci. 175 Ž1986. 22. w23x A.T. Hanbicki, A.P. Baddorf, E.W. Plummer, B. Hammer, M. Scheffler, Surf. Sci. 331–333 Ž1995. 811. w24x G. Schmitz, F. Bartolucci, P. Gassmann, J. Masuch, R. Franchy, submitted to J. Chem. Phys. w25x B.-R. Sheu, D.R. Strongin, J. Catal. 154 Ž1995. 379.
Adsorption and thermal decomposition of NH 3 on NiAl ž001 / and NiAl ž111 / G. Schmitz ) , P. Gassmann 1, R. Franchy IGV, Forschungszentrum Julich, D-52425 Julich, Germany ¨ ¨ Received 3 November 1997; accepted 19 November 1997
Abstract The adsorption and thermal decomposition of ammonia ŽNH 3 . on NiAlŽ001. and NiAlŽ111. was investigated by means of high-resolution electron energy loss spectroscopy ŽEELS. and temperature programmed desorption ŽTPD.. At 80 K, NH 3 adsorbs molecularly forming NH 3 multilayers on both surfaces. These multilayers desorb by heating the samples to about 100 K. On NiAlŽ001. the further thermal decomposition of the remaining monolayer-NH 3 takes place for considerably lower temperatures than on NiAlŽ111.. For the Ž001. surface, heating to 150 K leads to a partial decomposition of NH 3 to NH. After annealing at 380 K, the NH 3 and NH species are dissociated completely and atomic nitrogen is adsorbed on the NiAlŽ001. surface. In the case of NiAlŽ111., the decomposition of NH 3 to NH occurs at about 260 K. In contrast to NiAlŽ001., evidence is found for a recombinative thermal desorption of N2 , H 2 , and NH 3 at 134, 270, and 275 K, respectively. At 420 K, NiAlŽ111. is covered with NH which decomposes to atomic nitrogen upon annealing at 750 K. We assume the different atomic composition of the outermost layer of NiAlŽ001. and NiAlŽ111. to be responsible for the observed differences. q 1998 Elsevier Science B.V. Keywords: Adsorption; Atomic composition; Annealing; Atomic nitrogen
1. Introduction AlN as well as GaN and InN belongs to the group of large band gap III–V semiconductors which have attracted much interest recently due to their promising properties for the use in optoelectronic devices. Thin, well-ordered AlN films can be prepared on the Ž001. and Ž111. surface of the intermetallic alloy
) 1
Corresponding author. E-mail: [email protected]. Present address: Dresdner Bank, Frankfurt, Germany.
NiAl upon adsorption of NH 3 at 80 K and subsequent annealing at elevated temperatures w1,2x. This causes the decomposition of NH 3 and the reaction of formed N atoms with Al atoms that segregate to the surface. The grown films have a wurtzite crystal ˚ for structure with lattice parameters of 3.14 " 0.04 A ˚ Ž . the NiAl 001 substrate and 3.11 " 0.04 A for NiAlŽ111., respectively w3x. High-resolution EEL spectra of ordered AlN films show a Fuchs–Kliewer phonon mode at 865 cmy1 in good agreement with theoretical spectra on the base of the dielectric theory. In good agreement with reported values for bulk
0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 Ž 9 7 . 0 0 6 0 0 - 4
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AlN, the band gap of these films was determined to be 6.1 " 0.1 eV for the NiAlŽ001. substrate and 6.0 " 0.2 eV for NiAlŽ111., respectively. Gap states at 1.1 and 5.1 eV were found for AlNrNiAlŽ001. as well as for AlNrNiAlŽ111.. In a similar way, thin GaN films on the Ž001.-surface of the intermetallic alloy CoGa can be prepared upon adsorption of NH 3 at 80 K and subsequent thermal decomposition w4x. In order to gain insight into the complex growth processes of thin AlN films on NiAl applying the preparation method described above, a detailed study of the adsorption and thermal decomposition of NH 3 is of fundamental importance. In a previous paper, we reported on the elemental steps in the growth of AlN thin films on NiAl upon thermal decomposition of NH 3 w3x. In this article we focus on aspects of the adsorption and decomposition of NH 3 on NiAl in view of the different behavior of the Ž111. and the Ž001. surface studied by high-resolution electron energy loss spectroscopy ŽEELS. and temperature programmed desorption ŽTPD.. Though the structure and the electronic properties of the grown AlN films are very similar for both surface orientations, the thermal decomposition of NH 3 turns out to be different for NiAlŽ001. and NiAlŽ111.. NiAl orders in a CsCl type structure with a lattice ˚ The Ž001. surface has a square constant of 2.89 A. unit cell and is terminated either by Al- or Ni-atoms. Al-termination was determined from low-energy electron diffraction ŽLEED. IrV studies w5x. In a recent study, Blum et al. w6x showed that the termination and structure of the clean NiAlŽ001. surface depend on the cleaning procedure. In contrast to the Ž001. surface, the NiAlŽ111. surface has an open structure consisting of alternating Ni and Al layers. From low-energy ion scattering in the neutral impact collision mode ŽNICISS. and scanning tunneling microscopy ŽSTM. studies, it is known that the clean NiAlŽ111. surface is Ni terminated with Al atoms in the second layer w7x. The paper is organized as follows: Section 2 deals with the experimental methods and the cleaning procedure of the NiAl samples. In Section 3, the EELS and TPD results are presented and discussed for NH 3rNiAlŽ001. ŽA. and NH 3rNiAlŽ111. ŽB.. Section 3.3 finally deals with concluding remarks on the different decomposition behavior for NH 3 adsorbed on the NiAlŽ001. and NiAlŽ111. surface.
177
2. Experimental The experiments were performed in a two level stainless-steel ultrahigh vacuum ŽUHV. chamber at a base pressure of about 5 = 10y1 1 mbar. The upper level contains a cylindrical mirror analyzer ŽCMA. for the Auger electron spectroscopy ŽAES., a threegrid low energy electron diffraction ŽLEED. system, an ion gun and a quadruple mass spectrometer ŽQMS. for TPD. For the TPD measurements, the sample was positioned in front of the aperture ŽB s 3 mm. of the enclosure of the QMS. The sample was heated by electron impact and the temperature was measured using a NiCrrNiAl Žtype K. thermocouple that was spot-welded at the top of the sample. We used a constant heating rate of 2.0 Krs which was controlled by a new self-optimizing digital PID temperature controller. This controller can generate temperature ramps in the range of 15–1500 K with an accuracy of about 0.1%, and will be described in detail elsewhere w8x. The lower level contains a computer-controlled EEL spectrometer which is based on the 1278 cylindrical deflector w9x. The adsorption of 14 NH 3 and 15 NH 3 Ž99.9% purity. was performed by backfilling the UHV chamber. The exposures were measured without correction to the ion gauge sensitivity. In order to avoid readsorption during EELS measurements, NH 3 was adsorbed in small doses. The NH 3 covered surface was annealed to the specified temperatures for about 1 min. Before starting the EELS measurement, the sample was allowed to cool down to 80 K again. The NiAl single crystals were cut by spark erosion and polished mechanically. They were oriented with an accuracy of 0.58. After bake-out of the UHV chamber the samples showed carbon, sulfur, and oxygen contaminations. The cleaning procedure for NiAlŽ001. and NiAlŽ111. is similar. Heating the samples in an oxygen atmosphere Ž1 = 10y6 mbar. to 1100 K leads to oxidation and desorption of the carbon and sulfur species. Simultaneously, this procedure causes the formation of Al-oxide films on the NiAl substrate that can be removed by subsequent annealing to 1500 K. The high annealing temperatures cause the segregation of carbon to the surface. By repeated cycles of the cleaning procedure, the carbon contamination could be removed and a carbon depleted zone in the surface region was estab-
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lished. Sharp Ž1 = 1. LEED pattern were found for the clean NiAlŽ001. w10,11x and NiAlŽ111. w12x surfaces.
3. Results and discussion 3.1. Thermal decomposition of NH3 adsorbed on NiAl(001) at 80 K Fig. 1 shows a series of EEL spectra of 1 Langmuir ŽL. NH 3 adsorbed on NiAlŽ001. as a function of annealing temperature. Spectrum a which was taken after the adsorption at 80 K exhibits six losses at 360, 530, 1095, 1610 and 3390 cmy1 , with a shoulder at about 3260 cmy1 . For comparison, spectrum d in Fig. 1 was taken for an exposure of 10 L NH 3rNiAlŽ001. at 80 K. This spectrum exhibits six losses at 410, 820, 1095, 1640 and 3390 cmy1 , again
Fig. 1. A set of EEL spectra of NH 3 on NiAlŽ001. recorded as a function of annealing temperature. Spectrum a was taken after the adsorption of 1 L NH 3 at 80 K. Spectra b and c show the loss feature after annealing at 150 and 380 K. Spectrum d was taken after the adsorption of 10 L NH 3 at 80 K.
showing a shoulder at about 3260 cmy1 . In order to assign the losses, the observed vibrational frequencies are compared to results found for NH 3 multilayers on AgŽ110. w13x, for a condensed phase of NH 3rFeŽ110. w14x and for solid NH 3 w15x Žsee Table 1.. The broad low-frequency loss structure up to about 380 cmy1 Žwith maximum at 360 cmy1 . in spectrum a is attributed to the frustrated libration n l with a contribution of the hindered translation n t . The mode at 550 cmy1 corresponds to the NiAl– ŽNH 3 . stretching vibration of NH 3 molecules on the surface. The asymmetric high-frequency loss feature consists of the symmetric Ž ns . and antisymmetric Ž na . stretch of NH 3 at 3260 and 3390 cmy1 . The losses at 1095 and 1610 cmy1 are the corresponding symmetric Ž ds . and degenerate Ž dd . deformation modes. Due to the good agreement with the reported frequencies from literature Žsee Table 1., spectrum a can unequivocally be attributed to multilayer NH 3 . In particular, the librational mode at 360 cmy1 w13x and Žas will be shown below. the mode at 1095 cmy1 clearly indicate the formation of a NH 3 multilayer upon disordered condensation of NH 3 . This is in agreement with the observed strong decrease in the amount of elastically scattered electrons for 1 L NH 3rNiAlŽ001. compared to the clean surface. Under the assumption that NH 3 is adsorbed only on high-symmetry sites, a reduced number of dipole active modes would be expected. For example, 0.05 L NH 3rFeŽ110. corresponds to a C 2v symmetry for adsorbed NH 3 molecules with only three dipole active modes w14x. The observed vibrational frequencies for an exposure with 10 L NH 3 can be attributed in a similar way. The frustrated libration n l is the most intense mode observed in the spectrum. In addition, it is shifted to higher frequencies and is now located at 410 cmy1 , in excellent agreement with the condensed phase of NH 3rFeŽ110. w14x. Double excitation of this mode Ž2 n l . leads to the loss at 820 cmy1 . The stretching mode NiAl– ŽNH 3 . is masked by the intense loss structure at 410 cmy1 and can only be observed as a shoulder. The asymmetric high-frequency loss feature consisting of the symmetric Ž ns . and antisymmetric Ž na . stretch of NH 3 at 3260 and 3390 cmy1 is not shifted. The symmetric deformation mode Ž ds . is still located at 1095 cmy1 and the loss intensity is strongly increased. The
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Table 1 Comparison of the vibrational frequencies Žin cmy1 . of NH 3 rNiAlŽ001. at 80 K with values from literature Vibrational mode
NH 3 rNiAlŽ001. 1L
nt nl 2n l n ŽNH 3 –NiAl. ds dd ns na
360 530 1095 1610 3260 3390
10 L shoulder 410 820 shoulder 1095 1640 3260 3390
NH 3 rAgŽ110. w13x Žmultilayer.
NH 3rFeŽ110. w14x Žcondensed phase.
400 800
410 820
1070 1630 3320 3380
1095 1640 3270 3430
degenerate deformation mode Ž dd . is shifted by about 30 cmy1 to higher frequencies and is now found at 1640 cmy1 . Again, this value is in excellent agreement with reported values from literature Žsee Table 1.. Spectrum b shows the situation after annealing the sample at 150 K. It exhibits four losses at 630, 1245, 1530 and 3330 cmy1 . The relative intensity of the losses compared to spectrum a has strongly decreased. The disappearance of the low-frequency loss at 360 cmy1 indicates the desorption of the multilayer. We assume that spectrum b corresponds to NH 3 adsorbed in the first layer Žin the following referred to as monolayer-NH 3 .. The mode at 630 cmy1 is attributed to the NiAl– ŽNH 3 . stretching vibration of NH 3 molecules on the surface. The losses at 1245 and 1530 cmy1 originate in the symmetric Ž ds . and degenerate deformation mode Ž dd . of NH 3 , respectively. Compared to spectrum a the dd mode shows a shift to a lower frequency which is similar to the shift observed for decreasing NH 3 coverages Žcompare spectrum d for 10 L with spectrum a for 1 L in Fig. 1.. The frequency value of the symmetric deformation mode Ž ds . for monolayerNH 3 Ž1245 cmy1 . differs considerably from the multilayer value Ž1095 cmy1 .. Careful examination of the peak at 1095 cmy1 in spectrum a shows that this loss has an asymmetric lineshape with a shoulder at the high-frequency side. We assume that this shoulder is due to a contribution of monolayer-NH 3 in spectrum a. In an isotope exchange experiment with 15 NH 3 on NiAlŽ001. both contributions could be distinguished conclusively: The EEL spectrum for 2.1 L 15 NH 3 adsorbed at 80 K showed two distinct
NH 3 w15x Žsolid. 141 360
1060 1646 3223 3378
losses at 1075 and 1265 cmy1 in the frequency range under investigation. The loss at 1075 cmy1 is close to the value of condensed NH 3 and is therefore attributed to the multilayer, while the mode at 1265 cmy1 originates in the monolayer contribution. A monolayer and multilayer contribution to the ds mode was also observed for NH 3rPtŽ111. w16x. The relatively broad high-frequency loss structure at 3330 cmy1 consists of the stretch vibrations ns and na . However, this high-frequency structure is now more symmetric compared to spectrum a. Since NH has only one vibrational mode in this frequency region, it is probable that in addition to NH 3 molecules NH radicals are coadsorbed. Therefore, we assume that in the temperature range from 80 to 150 K decomposition of NH 3 to NH occurs. Annealing at 380 K leads to a further reduction of the number of loss peaks. Spectrum c exhibits three losses at 245, 465 and 1215 cmy1 . The loss at 245 cmy1 is attributed to the vibration of Al atoms of the NiAl substrate. This loss was also found on clean NiAlŽ111. surfaces w12x. The loss at 465 cmy1 originates in the stretch vibration NiAl–N of atomic nitrogen on the surface. The observed value is close to the frequencies found for several other substrates Že.g., NrFeŽ111.: 465 cmy1 w17x, NrCuŽ110.: 427 cmy1 w18x, NrNiŽ111.: 490 cmy1 w19x.. The very weak loss at 1215 cmy1 is attributed to the vibration of atomic hydrogen on the surface. In an isotope exchange experiment with 15 NH 3 on NiAlŽ001., we found that this mode remains unchanged while the vibration with nitrogen contribution is shifted. This assignment is also supported by the metal–H stretching mode found on low indexed tungsten surfaces
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around 1260 to 1290 cmy1 w20x. Since no highfrequency loss peaks are observed, it is concluded that no NH x species are adsorbed on the surface. Hence, NH 3 is completely decomposed.
To further elucidate the mechanisms of NH 3 decomposition upon annealing, TPD measurements on the system 15 NH 3 were performed. The main desorption products are H 2 , 15 NH 3 and 15 N2 . Fig. 2 shows
Fig. 2. The TPD spectra for the mass 2 ŽH 2 . signal Ža., the mass 18 Ž15 NH 3 . signal Žb., and the mass 30 Ž 15 N2 . signal Žc. for increasing exposures of 15 NH 3 adsorbed on NiAlŽ001..
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181
thermal desorption spectra for the mass 2 ŽH 2 . signal Ža., the mass 18 Ž 15 NH 3 . signal Žb. and the mass 30 Ž 15 N2 . signal Žc. as a function of 15 NH 3 exposure. The H 2 signal exhibits two desorption peaks at 95 Ž b . and 212 K Žg .. The b peak is attributed in part to the decomposition of NH 3 to NH q H 2 and in part to hydrogen as a cracking pattern of the simultaneously desorbing NH 3 molecules from the multilayer. The g peak indicates the decomposition of the residual NH 3 monolayer and of adsorbed NH molecules to N atoms. This interpretation is in agreement with the EELS results. The mass 18 signal Žb. shows only one peak at 95 K which is due to the NH 3 multilayer desorption. Also the mass 30 signal Žb. mainly shows a peak at 95 K which can be explained as the cracking pattern of the 15 NH 3 signal. Therefore, the TPD measurements confirm the assumption that NH 3 adsorbed on NiAlŽ001. at 80 K thermally decomposes in two steps. In addition, NH 3 multilayer desorption is observed at 95 K. As was reported in a previous paper w3x, further annealing of the sample at 1250 K leads to the formation of AlN on the surface. 3.2. Thermal decomposition of NH3 adsorbed on NiAl(111) at 80 K The adsorption and thermal decomposition of NH 3 was also studied on the NiAlŽ111. surface. It turns out that mechanisms of NH 3 decomposition on NiAlŽ111. are different from those found for the NiAlŽ001. surface. Fig. 3 shows a series of EEL spectra of 1 L NH 3 adsorbed on NiAlŽ111. as a function of annealing temperature. Spectrum a was taken after the adsorption at 80 K. It exhibits seven losses at 340, 625, 1105, 1260, 1625 and 3365 cmy1 with a shoulder at 3220 cmy1 . As in the case of 1 L NH 3rNiAlŽ001. at 80 K Žspectrum a of Fig. 1. the observed losses can be attributed to NH 3 adsorbed in multilayers. The low-frequency losses at 340 and 625 cmy1 are the frustrated librational mode n l and the stretching vibration NiAl– ŽNH 3 . of NH 3 molecules on the surface, respectively. The high-frequency losses at 3220 and 3365 cmy1 originate in the symmetric Ž ns . and antisymmetric Ž na . stretching vibrations of NH 3 . The corresponding degenerate deformation mode Ž dd . is located at 1625 cmy1 . The symmetric deformation
Fig. 3. A set of EEL spectra of NH 3 on NiAlŽ111. as a function of annealing temperature. Spectrum a was taken after the adsorption of 1 L NH 3 at 80 K. Spectra b–e were obtained after heating the sample to the specified temperatures.
mode Ž ds . consists of a multilayer contribution at 1105 cmy1 and a monolayer contribution at 1260 cmy1 . Spectrum b was recorded after annealing the sample to 150 K. It exhibits seven losses at 460, 635, 1230, 1540, 1900 and 3350 cmy1 with a weak shoulder at 3200 cmy1 . The losses at 1900 and 460 cmy1 are attributed to the CO stretch and the n ŽNiAl–CO. vibration of CO molecules that are coadsorbed on the surface from the residual gas. The observed frequency values are in accordance with those found for 1 L COrNiAlŽ111. w21x. The remaining five losses can be explained by monolayerNH 3 . Again, the disappearance of the mode at 340 indicates the desorption of the multilayer. The loss at 635 cmy1 refers to the stretching vibration NiAl– ŽNH 3 .. The modes at 3200 and 3350 are attributed to the symmetric Ž ns . and antisymmetric Ž na . stretching vibrations. The corresponding symmetric Ž ds . and degenerate deformation modes Ž dd . are located at
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1230 and 1540 cmy1 . A similar shift of the dd mode to lower frequencies Ž1625 cmy1 in spectrum a to 1540 cmy1 in spectrum b. was also observed for the system NH 3rNiAlŽ001. Žcf. Fig. 1.. After heating to 420 K, spectrum c in Fig. 3 shows only two modes at 620 and 3300 cmy1 . It exhibits no deformation modes and the highfrequency loss is now narrow and symmetric. This spectrum can be clearly attributed to adsorbed NH, since NH has only one high-frequency loss which is assigned to the NH stretch. The loss at 620 cmy1 originates in the stretching vibration NiAl– ŽNH. of NH molecules on the surface. The observed frequencies are close to those found for NHrNiŽ111. Ž n ŽNi–NH. s 620 cmy1 , n ŽNH. s 3340 cmy1 . w22x and NHrNiŽ110. Ž n ŽNH. s 3240 cmy1 . w19x. For NHrNiŽ111. an additional weak loss at 1270 cmy1 is observed which is attributed to the deformation mode d ŽNH.. Since the deformation mode d ŽNH. is not dipole active for NH molecules adsorbed in an upright geometry, we assume this binding configuration for NHrNiAlŽ111.. Further annealing to 550 K leads to spectrum d. It exhibits four losses at 310, 465, 830 and 3300 cmy1 . The very weak loss at 3300 cmy1 is attributed to a small amount of NH radicals adsorbed on the surface. The mode at 465 cmy1 originates in the stretching vibration NiAl–N of atomic nitrogen on the surface. The same mode was observed for NrNiAlŽ001. Žcf. Section 3.1.. The loss at 310 cmy1 can be attributed to the vibration of Al atoms of the NiAl substrate. On the clean NiAlŽ111. surface this loss was found at 240 cmy1 w12x. Probably due to the presence of atomic nitrogen on the surface, the frequency is shifted to a higher value. The loss at 830 cmy1 indicates the formation of AlN. AlN is characterized by a mode at 865 cmy1 w3x. Due to the overlapping with the intense mode at 465 cmy1 , this mode seems to be shifted to lower frequencies. After annealing to 750 K, spectrum e still shows the vibration of Al atoms at 310 cmy1 , the stretching vibration NiAl–N of atomic nitrogen at 465 cmy1 and the mode at 865 cmy1 indicating the formation of AlN. The additional shoulder at 600 cmy1 is attributed to an intermediate state of AlN w3x. The disappearance of the high-frequency losses indicates the complete decomposition of all NH x species.
TPD measurements were also performed for NH 3rNiAlŽ111.. The main desorption products have been monitored. Fig. 4 shows thermal desorption spectra for mass 2 ŽH 2 . signal Žspectrum a., mass 17 ŽNH 3 . signal Žb., mass 28 ŽN2 or CO. signal Žc. and mass 14 ŽN. signal Žd. for an exposure of 1 L 14 NH 3 . Spectrum a exhibits three distinct H 2 desorption peaks at about 260 Ž b ., 466 Žg 1 . and 596 K Žg 2 .. The broad b peak is attributed in part to the decomposition of NH 3 to NH q H 2 and in part to a hydrogen desorption of adsorbed H atoms. For the system HrNiAlŽ110. a similar hydrogen desorption was found at about 270 K w23x. Spectrum c indicates the desorption of N2 at 134 K. The symmetric lineshape suggests a second order desorption. In a recent paper, a similar TPD spectrum for a recombinative desorption of atomic nitrogen on NiAlŽ111. is found w24x. Therefore, it can be concluded that NH 3 is partly dissociated at this temperature. While the N atoms formed upon NH 3 dissociation desorb at 134
Fig. 4. A set of TPD spectra of 1 L NH 3 adsorbed on NiAlŽ111. showing the mass 2 ŽH 2 . signal Ža., the mass 17 ŽNH 3 . signal Žb., the mass 28 ŽN2 and CO. signal Žc., and the mass 14 ŽN. signal Žd..
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K, the H atoms are desorbed first at about 270 K. It arises the question, why atomic hydrogen was not observed in EEL spectrum b after annealing a 150 K. Atomic hydrogen is expected to have a stretching vibration NiAl–H close to the ds mode at 1230 cmy1 Žcf. Section 3.1. and is probably masked by this peak. The g 1 and g 2 peaks in spectrum a are attributed to the decomposition of NH to adsorbed nitrogen and desorbing hydrogen. Since there are two H 2 desorption peaks, we assume that NH occupies two adsorption sites with different dissociation energies. Spectrum b ŽNH 3 . shows a single peak at 275 K which is due to a recombinative NH 3 desorption of adsorbed atomic nitrogen and hydrogen Žcf. Section 3.3.. For higher NH 3 exposures an additional multilayer desorption peak at about 100 K is observed in the NH 3 signal. This is in good agreement with the EELS results which show the multilayer desorption in the temperature range of 80–150 K. The peak at 408 K in the mass 28 spectrum c is attributed to desorbing CO, because the mass 14 signal Žcracking pattern of N2 . in spectrum d shows no corresponding peak. On the other hand, the peak at 134 K in spectrum c can unequivocally be identified as desorbing N2 . The observed CO desorption at 408 K is in good agreement with the EELS measurements. The spectrum at 150 K clearly indicated adsorbed CO, whereas at 420 K the corresponding loss features disappeared. As was reported in a previous paper w3x, further annealing of the sample at 1150 K leads to the
reaction of the nitrogen atoms with Al under the formation of AlN on the surface. Simultaneously, an intermediate state during the AlN film growth is found for this temperature. After heating to 1250 K, the AlN growth is completed which is indicated by one distinct loss at 865 cmy1 w3x. 3.3. Concluding remarks The different mechanisms of the thermal decomposition of NH 3 on NiAlŽ001. and NiAlŽ111. are summarized in Table 2. On NiAlŽ001. the thermal decomposition of NH 3 takes place for considerably lower temperatures than on NiAlŽ111.. For example, while NH 3rNiAlŽ001. is completely decomposed to atomic nitrogen at 380 K, NH radicals still are observed on the NiAlŽ111. surface at 550 K. The decomposition process on NiAlŽ111. is completed upon annealing at about 600 K. We assume that the reason for this different behavior may be the different atomic composition of the outermost layer of the substrate. As mentioned in Section 1, NiAlŽ111. has an open structure with Ni atoms in the outermost layer as could be shown conclusively w7x. The second layer consists of Al atoms. For NiAlŽ001., Blum et al. w6x showed that the termination and structure of the clean surface depend on the cleaning procedure. In a recent study w24x, we found evidence that our cleaning procedure, which is described in Section 2, leads to an Al terminated surface. The observed NH 3 signal of the TPD measurements Žspectrum b. in
Table 2 Comparison of the different mechanisms of NH 3 decomposition on NiAlŽ001. and NiAlŽ111. Temperature
NiAlŽ001.
NiAlŽ111.
80 K Up to 150 K
NH 3 multilayer Desorption of the multilayer at 95 K Partially decomposition of NH 3 to NH ad q H 2 Monolayer-NH 3 q NH ad decomposition of NH 3 and NH to Nad at 212 K Ždecomposition complete.
NH 3 multilayer Desorption of the multilayer at 100 K Partially decomposition of NH 3 to Nad q H ad Monolayer-NH 3 ŽqCO. Decomposition of the monolayer-NH 3 to NH ad at 260 K Recombinative desorption of N2 and H 2 at 134 and about 270 K Recombinative desorption of NH 3 at 275 K
150 K Up to 380 K
380 K 420 K Up to 550 K
Nad
550 K Up to 750 K
Nad w3x The formation of AlN starts w3x
183
NH ad Decomposition of NH in the g 1 state to Nad at 466 K Formation of AlN NH ad q Nad q AlN Decomposition of NH in the g 2 state to Nad at 596 K Ždecomposition complete.
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Figs. 2 and 4. can serve as an example for the effect of the Ni atoms in the first layer of NiAlŽ111.. The observed recombinative NH 3 desorption at 275 K on the NiAlŽ111. surface can be attributed to the hydrogenation capabilities of Ni. On the other hand, on the Al terminated NiAlŽ001. surface only the multilayer desorption at 95 K is observed. Similar differences were found for the decomposition of CH 3 OH on NiAlŽ001. ŽAl-termination. and NiAlŽ110. Ž50% Ni atoms and 50% Al atoms in the outermost layer. w25x. On NiAlŽ110. the hydrogenation capabilities of Ni atoms were able to remove hydrocarbon fragments from the alloy as methane, whereas this reaction channel appears to be very limited on the Al terminated Ž001. surface w25x. For the growth of the AlN films, the formation of atomic nitrogen is the important step. Further annealing leads to the reaction of nitrogen atoms with Al atoms that segregate from the bulk to the surface. Since the EEL spectra of both surfaces are very similar for temperatures higher than about 750 K w3x, it can be concluded that if atomic nitrogen is present on the surface, the subsequent AlN growth is independent of surface termination. Acknowledgements G.S. gratefully acknowledges the financial support by the Deutsche Forschungsgemeinschaft Žproject Fr 1137r2-1.. References w1x P. Gassmann, F. Bartolucci, R. Franchy, J. Appl. Phys. 77 Ž1995. 5718. w2x P. Gassmann, J. Boysen, G. Schmitz, F. Bartolucci, R. Franchy, Solid State Commun. 97 Ž1996. 1.
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