J. Phys. Chem. B 1998, 102, 5069-5076
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Chemistry of HNO3 on Ge(100) Armen Avoyan, Craig Tindall, and John C. Hemminger* Department of Chemistry and the Institute for Surface and Interface Science, UniVersity of California, IrVine, California 92697 ReceiVed: August 12, 1997; In Final Form: April 20, 1998
We describe here the initial bonding geometry and decomposition mechanism of HNO3 on Ge(100). A combination of high-resolution electron energy loss spectroscopy, Auger electron spectroscopy, and temperature programmed desorption experiments have been used to shed light on the details of this complex but technologically important system. It was found that HNO3 adsorbs molecularly at 190 K in a predominantly bidentate bonding configuration. When the substrate is warmed to 425 K, the molecule decomposes. In a process that involves two adjacent HNO3 adsorbates, H2O and NO desorb while O remains on the surface. Further heating above 600 K causes the O to desorb as GeO.
1. Introduction Silicon is the dominant material used in the microelectronics industry. Consequently, an enormous amount of effort has been devoted to studying the properties of this material.1 Because its sister element germanium does not possess all of the qualities that make silicon such a useful material, its application to microelectronics has been much less extensive.2 Despite this, there are specialty applications for which germanium is the material of choice. One example of this is the blocked impurity band (BIB) infrared detector. When fabricated using germanium, these detectors are sensitive to wavelengths as long as 220 µm.3,4 Fabricating BIB detectors using germanium rather than silicon is challenging because there is much less processing experience with this element. In addition, some important aspects of the chemistry of germanium are less favorable for device processing than those of silicon. Because of this, there is interest in understanding the fundamental chemistry of the germanium surface, which is related to the chemical processing required to fabricate devices such as Ge/Ga BIB infrared detectors. In the case of germanium, one difficult challenge in fabricating devices is caused by the fact that the thermally grown oxide, GeO2, is water-soluble. Hence, it is unsuitable as a material for long-term passivation. However, the nitride and oxynitride compounds have been used for passivation with some success.5,6 In a previous paper, we discussed an HREELS study of the chemistry of the nitrogen hydrides on Ge(100).7 In the present work we report the results of a study of an active oxidizer HNO3. HNO3 is a widely used compound in semiconductor processing. It is used for cleaning wafers and is a component in many semiconductor etching solutions.8 Nonetheless, although several studies of HNO3 chemistry on metal surfaces have been reported,9,10 little fundamental work appears to have been done with this compound on silicon or germanium. Using a variety of techniques, HREELS, AES, and TPD, we show here that HNO3 reacts with Ge(100) under ultrahigh vacuum (UHV) conditions to oxidize the surface. We further propose a reaction mechanism and bonding geometry that is consistent with our data. Our studies show that HNO3 adsorbs molecularly at low temperatures. When the substrate is warmed to 425 K, the
HNO3 molecule dissociates, evolving NO and H2O while simultaneously depositing oxygen on the surface. Further heating of the substrate to 600 K results in the desorption of the oxygen. 2. Experimental Section The experiments were carried out in a stainless steel UHV system with a base pressure of 2 × 10-10 Torr. This base pressure is somewhat higher than that reported in our previous work because HNO3 very readily sticks to the walls of the chamber and is much more difficult to pump away than HN3 that had been used in previous studies. This UHV chamber houses an Auger electron spectrometer (AES) manufactured by Perkin-Elmer (model F10-55 CMA electron optics), a lowenergy electron diffraction (LEED) system (Varian model 9812145), and a high-resolution electron energy loss spectrometer (HREELS) manufactured by LK Technologies (model LK2000). The typical resolution attainable with this instrument from a Pt sample is about 25 cm-1. There was a significant amount of quasi-elastic scattering apparent in the HREELS spectra of our clean Ge surfaces that results from the scattering of electrons in the HREELS beam by the surface plasmon modes. Consequently, the resolution attainable with the clean Ge(100) surface was dependent on temperature. In our experiments this was typically around 124 cm-1 fwhm at a surface temperature of 180 K. The HREELS beam narrowed significantly upon adsorption of the HNO3. This behavior is the same as that seen in the case of the nitrogen hydrides.7 In this chamber it is also possible to make temperature programmed desorption measurements using a UTI model 100C quadrupole mass spectrometer and a custom-built power supply that utilizes a power feedback circuit to heat the Ge crystal.11 The experiments were performed on Ge(100) crystals. The sample mounting and heating techniques will be described elsewhere.12 The primary dopant in the samples used was gallium at a level of approximately 6 × 1016 cm-3. AES analysis was performed with a Perkin-Elmer (Physical Electronics) single-pass cylindrical mirror analyzer. Typical electron beam parameters were 3 kV and ∼1 µA. The spectra were recorded in first derivative mode. The LEED system was used to check the surface structure as a diagnostic and also to
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TABLE 1: HNO3 Vibrational Energies and Assignments frequency assignments unidentate structureb bidentate structureb
HREELS modes (cm-1)a 3550 (impact) 1614 (dipole) 1259 (impact) 890 (dipole) 761 (impact)
O-H stretch NO2 asymmetric stretch (1480-1550) NO2 symmetric stretch (1290) N-OH stretch (1000) NO2 out-of-plane bend (740)
O-H stretch N-OH stretch (1630) NO2 asymmetric stretch (1250) NO2 symmetric stretch (985) NO2 symmetric bend (785)
362
Ge-O stretch
Ge-O stretch
infrared gas phase (cm-1)c 3550 (ν1) 1709 (ν2) 1304 (ν4) 879 (ν5) 647 (ν6) 580 (ν7) 763 (ν8) 458 (ν9)
O-H stretch NO2 antisymmetric stretch N-OH bend N-OH stretch NO2 angle deformation N-OH bend NO2 out of plane bend O-H torsion
a From this work. b Frequencies in parentheses are those of Sn(IV)(NO3)2 and Sn(IV)(NO3)4 complexes for the given structure. c Lee, J. T.; Rice, J. E. J. Phys. Chem. 1992, 96, 650 and references therein.
characterize the structure of the adsorbate layer. The TPD spectra were recorded using a temperature ramp rate of 2 K/s. The desorbed species were monitored using the UTI quadrupole mass spectrometer (QMS) which was covered with a shroud to prevent molecules that desorb from the sample mount from entering the ionizer region of the QMS. The HNO3 was obtained from Aldrich (Optima grade, 70% solution in H2O). This was mixed with H2SO4 also obtained from Aldrich (Trace Metal Grade, 96-97% in H2O). H2SO4 has a very low vapor pressure (10-9 Torr) and a high affinity for H2O. Thus, it acts as an effective drying agent.13 This causes the composition of the vapor above the mixture to be essentially anhydrous HNO3. As described in more detail below, the HREELS spectra of HNO3 adsorbed on Ge(100) give no indication of H2O adsorption.14 The HNO3 was introduced into the chamber using an all-glass doser with a 0.004 in. diameter capillary 1.0 cm long at the end to limit the flux.15 With a dosing pressure of 2 Torr behind the capillary, by use of our dosing geometry and sample size, this should result in a dosing rate of 0.9 langmuir (L)/s.
Figure 1. Structure of the isolated nitric acid molecule.
3. Results and Discussion 3.1. Structure and Properties of HNO3(gas). The electronic structure of HNO3 exhibits some similarities to the structure of HN3, which we have studied previously on Ge(100).7 The nitrate ion (NO3-) is planar with all N-O bond distances close to 1.22 Å. The molecular orbitals of the ion can be constructed from the three σ bonds (sp2 hybridization) and the pz orbitals of the nitrogen atom. A total of 24 valence electrons are contained in the bonding and nonbonding orbitals. This is a stable configuration for a tetra-atomic species that is similar to the 16 electron triatomic and 10 electron diatomic ionic structures. For example, N3- is a 16 electron structure.16,17 In nitric acid, there is a hydrogen atom that is covalently bonded to one of the oxygen atoms. Electron diffraction and microwave experiments have shown that in the gas phase, the HNO3 molecule is planar.16 The addition of this H atom results in a substantial weakening (lengthening) of the N-O bond closest to the hydrogen. Again, this is very similar to the behavior of the HN3 molecule. In that case, the N-N bond closest to the H atom was substantially weakened and it was seen that this substantially determined the chemistry of the molecule on Ge(100).7 Although less decisive than in the case of HN3, it will be seen that this structural property of HNO3 will also play a key role in its chemistry with Ge(100). One interesting feature of the HNO3 molecule is the relatively high frequency of the N-OH torsional mode. Neglecting anharmonic effects, Cohn et al.18 have estimated the rotational barrier height to be about 8.4 kcal/mol. More recent and much more sophisticated theoretical calculations have confirmed this.19 The point of this is simply to show that the barrier to rotational
Figure 2. HREELS spectrum following a 60 L dose of HNO3 on Ge(100). The sample was dosed at 190 K.
motion of the OH group is much higher than that seen in simple saturated organic molecules such as ethane or butane. There, one typically sees rotational barriers of from 1-4 kcal/mol.18,20 This result suggests that there is a relatively strong intramolecular hydrogen bond between the H atom and O2 (H-O2 distance ) 2.143 Å)19 as shown in Figure 1. 3.2. Adsorption and Bonding Geometry of HNO3 on Ge(100) at 190 K. 3.2.1. HNO3 on Ge(100). Figure 2 shows an HREELS spectrum of HNO3 adsorbed on Ge(100) at 190 K. The sample was dosed with 60 L of HNO3. The proposed bonding geometries and respective HREELS mode assignments, along with HNO3 gas-phase frequencies, are summarized in Table 119 and will be discussed subsequently. The direct comparison of the IR spectrum of a gas-phase molecule with our HREEL spectrum does not yield a straightforward assignment of the experimental modes. Nonetheless, the HNO3 thermal desorption and HREELS warmup data followed by AES surface analysis, each of which will be discussed in more detail in a separate section below, suggest a very strong adsorbate-substrate interaction that the HNO3
Chemistry of HNO3 on Ge(100)
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(a)
(b)
Figure 3. Absolute intensity of HNO3 vibrational modes vs electron detector angle away from the specular direction.
molecule experiences upon adsorption, without falling apart. Thus, one would expect the vibrational modes of the adsorbed molecule to be significantly perturbed from their respective gasphase values. To aid in the assignments of the HNO3(ads) vibrational modes, HREELS measurements were performed as a function of angle away from the specular direction. Dipoleactive vibrational modes are expected to show angular dependencies that are peaked in the specular scattering direction and that follow the angular dependence of the elastically scattered (zero energy loss) electrons.21 Figure 3 shows the behavior of the intensity of the various modes of HNO3 on Ge(100) as the angle of the electron energy analyzer of the HREELS is varied. The 1614 and the 890 cm-1 modes show a significant amount of dipole character. The intensities of the 3550 and 1259 cm-1 modes remain approximately constant or even slightly increase in intensity as the analyzer is rotated from the specular angle to 26° off specular, indicating that they are not dipole-active. The one mode that is of intermediate intensity, the 761 cm-1 mode, peaks in the specular direction but exhibits a much broader angular distribution than the elastically scattered electrons, suggesting that both dipole scattering and shorter range impact scattering mechanisms contribute to the intensity. The intensities of the two strongest modes, the 1614 and 890 cm-1, are seen to behave in a manner similar to that of the elastic peak as a function of analyzer angle, while the modes that are weaker in the lowtemperature HREELS spectrum are seen to be impact-scattered. There are two potentially reasonable bonding geometries for molecular adsorption of HNO3 on Ge(100): a unidentate structure bonded to the Ge through O3 (see Figure 4a), and a bidentate structure bonded to Ge through both O2 and O3 (see Figure 4b). Both of these kinds of structures are known in metal complexes of the nitrate ion.22 Although the vibrational spectra associated with these two structures are expected to exhibit similar peaks, the mode assignments for the peaks are quite different as is shown in Table 1. In Table 1 we list the observed mode frequencies for HNO3 adsorbed on Ge(100) at 190 K along with the mode frequencies obtained from gas-phase IR spectra of HNO3. We also indicate the assignments of the HNO3(ads) modes for the unidentate and bidentate bonding geometries. These assignments are based on comparison with IR assignments of Sn(IV)(NO3)2 and Sn(IV)(NO3)4 complexes.22 Although the frequencies expected for the bidentate structure are slightly better matches for our
Figure 4. (a) Schematic of a unidentate bonding geometry for HNO3 on Ge(100). (b) Schematic of a bidentate bonding geometry for HNO3 on Ge(100), which is supported by the present experiments.
observed spectrum, taking into account the resolution of our HREELS experiments and the possible frequency differences between HNO3 adsorbed on Ge and the comparison complexes, we are not able to decide on the correct geometry from comparison of the frequencies alone. The scattering mechanism information obtained from the off-specular scattering experiments described above does provide additional insight in this case. The modes that are most useful in our analysis are the 1614, 1259, and 890 cm-1 modes. As we will see, the dipole (1614 and 890 cm-1) and impact (1259 cm-1) character of these modes is most consistent with the bidentate bonding geometry. For the bidentate structure the 1614 cm-1 mode would be assigned to the N-OH stretch and the 890 cm-1 mode would be assigned to the NO2 symmetric stretch. In the bidentate structure both of these modes would be expected to have dipole derivatives perpendicular to the surface and thus would be dipole-excited. The 1259 cm-1 mode is assigned to the NO2 asymmetric stretch for the bidentate structure. In this geometry this mode would have a dipole derivative parallel to the surface. Thus, in the bidentate structure the 1259 cm-1 mode would be observed only through an impact scattering mechanism, as we observe experimentally. In contrast, if the unidentate structure was correct, the 1614 cm-1 mode would be assigned to the NO2 asymmetric stretch, which should not be strongly dipole-scattered in contrast to our observation. The 1259 cm-1 mode would be assigned to the NO2 symmetric stretch for a unidentate structure and should show strong dipole excitation character, again in contrast to our observations. Thus, although the frequencies we observe do not let us decide between the bidentate and unidentate structures, the excitation mechanisms for these three modes are definitely more consistent with the bidentate structure. The origin of the ability to experimentally distinguish between the bidentate and the unidentate structures is the difference in the mode assignments. In particular, the bidentate structure is expected to have weaker N-O bonds, leading to lower frequency of NO2 symmetric and asymmetric stretches. To complete the discussion of the HNO3 vibrational modes, it should be mentioned that the modes associated with the
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TABLE 2: Vibrational Energies and Assignments for NO2(gas) NO2(Gas) (cm-1)a ν1 ν2 ν3 a
symm stretch in-plane bend asymm stretch
1323 750 1618
From ref 16, p 375.
hydrogen motion, except for the O-H stretch, which appears to be unaffected by the adsorption, are either effectively screened by the surface or simply not resolved. The presence of the 761 cm-1 symmetric NO2 bending mode is a good indication of the molecularity of the adsorption. The strong mode at 362 cm-1 is most likely to be the Ge-O stretch, which is not too surprising in view of the fact that the NO2 section of the molecule is interacting strongly enough with the Ge(100) surface to cause it to fall apart when the substrate is heated, as will be discussed shortly. At this point there are a couple of side issues that we should discuss. The first is whether any water is coming in with the nitric acid and simultaneously adsorbing onto the surface. As mentioned previously, the HREELS spectrum rules this out. There is a troublesome hydrocarbon contamination problem (C-H modes at ∼2950 cm-1), but no H2O contamination is seen in the spectrum. The differences between the H2O vibrational spectrum and the 190 K HNO3 spectrum are as follows. First, the HREELS spectrum of H2O adsorbed on Ge(100) shows a strong peak at 1944 cm-1 due to the Ge-H stretching mode. In addition there are two peaks, one at 684 and 920 cm-1 due to Ge-OH bending modes.14 The first is definitely not seen at all in the HNO3 spectrum, and the fact that this and the Ge-H mode are not seen provides good evidence that the HNO3 is essentially anhydrous on the surface. The nearest peak to 920 cm-1 in the HNO3 spectrum is 890 cm-1. Thus, for the reasons just stated, it is very unlikely that the 890 cm-1 peak is due to O-H groups bonded directly to the surface. In our studies of H2O adsorption on Ge(100) at this temperature, even very large (2000 L) doses of H2O failed to deposit any molecular water on the surface. The presence of molecular water on the surface would be indicated by peaks at 1600, 880, and 520 cm-1. HNO3 also has a peak very close to 880 cm-1. The 520 cm-1 peak is not observed, and the 1600 cm-1 can be plausibly attributed to molecular HNO3. In addition, even at lower temperatures where molecular H2O has been observed in previous studies,14 adsorption of H2O on Ge(100) is accompanied by some dissociative adsorption that gives rise to strong Ge-H stretching mode at 1944 cm-1. Again, as pointed out previously, this is not observed. Hence, the low-temperature HREELS spectrum demonstrates that our method of mixing the concentrated (70% HNO3 in H2O) with H2SO4 is an effective way of retaining the water in the liquid phase. Since even small doses (
