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Hydrogen dissociation on high-temperature tungsten
Surface Science 600 (2006) 2207–2213 www.elsevier.com/locate/susc
Hydrogen dissociation on high-temperature tungsten Wengang Zheng, Alan Gallagher
*
JILA, University of Colorado and National Institute of Standards and Technology, Boulder, CO 80309-0440, USA Received 13 June 2005; accepted for publication 13 March 2006 Available online 25 April 2006
Abstract H2 dissociation on polycrystalline tungsten is measured from 1700 to 3000 K using the filament temperature (T) change and a normalized H-atom density at the chamber surface. The dissociation probability per H2 filament collision (Pdiss) saturates at 0.40 at high T and has a 2.25 ± 0.05 eV apparent activation energy when Pdiss 1. This activation energy is consistent with prior data and models, but the H2 pressure dependence is not. Pdiss is independent of the H2 pressure for this entire T range and the 1–85 mTorr pressure range studied, contradicting the primary model that has been used to explain H2 dissociation on tungsten and other metals. We show that some apparently contradictory prior measurements are actually consistent with our observations and with each other, once this pressure dependence of Pdiss is recognized. 2006 Elsevier B.V. All rights reserved. Keywords: Polycrystalline surfaces; Tungsten; Hydrogen molecule; Dissociation; Desorption; Catalysis; Models of surface kinetics
1. Introduction The dissociation of H2 on metal surfaces, particularly tungsten (W), has a long history because of its early importance in lighting and a continuing importance in catalysis and H sources. In addition, the present study is motivated by its importance in the production of thin-film silicon devices by ‘‘hot-wire deposition’’ [1]. Measurements of H2 dissociation on W are mostly of three types: (1) reduction of H2 pressure in a closed vessel as H produced on a 1100–1800 K filament reacts with a coating on the chamber surfaces [2–4]; (2) measurement of power removed from a 1700–2800 K filament [5]; and (3) measurement of H produced on a 2100–2900 K filament [6,7]. Other measurements also provide constraints on any model for this dissociation. Examples are: exposure of clean 6300 K tungsten to a known H2 flux, followed by measurement of H2 evolution as the metal temperature (T) is raised [4,8], measurement of surface H concentration [9], surface
*
Corresponding author. Tel.: +1 303 492 7841; fax: +1 303 492 5235. E-mail address: [email protected] (A. Gallagher).
0039-6028/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.03.032
H diffusion [10], H2 beam attenuation due to adsorption [11], and H–D exchange reactions on the surface [12]. A few calculations study dissociative adsorption and H2 desorption on W [13]; in contrast, scores of papers appear every year about these processes on silicon surfaces, e.g., [14] and references therein. (These references are representative, not inclusive.) The present measurements on polycrystalline W fall into categories (2) and (3). Summarizing these studies, it appears that H2 dissociatively adsorbs with a probability S(T) per collision, either without an (adsorbed H2) precursor state or with a weakly-bound precursor state [11]. (S is also a function of the incident H2 energy distribution and the surface fractional coverage, h [11]. The incident energy distribution is essentially constant, and we expect h 1 in the present measurements, so this will be ignored here.) H atoms on the W surface either desorb individually, overcoming a barrier EW–H, or as H2 when a pair of H atoms in close proximity overcomes a smaller barrier (EW–H2 ) [4]. Also, at T > 1200 K of interest here, H only resides on the W surface [15]. Thus, the dissociation probability (Pdiss) per H2–metal collision is S(T) times the fraction of adsorbed H that desorbs as H rather than as H2(fH=H2 ). At high T,
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H atoms are desorbed very rapidly, h 1, most H atoms desorb individually, and fH=H2 ffi 1. At low T, H2 desorption is energetically favorable and fH=H2 1. In this case, h depends on the balance between the H adsorption rate (AH) and the H2 desorption rate (DH2), both as rates per unit area. It is customary to take DH2 ¼ C m nm expðEW–H2 =kT Þ, where n is the H surface density, m is the desorption order and Cm is a constant. Except at high coverage, one expects second-order (m = 2) desorption when the H atoms reside and move independently on the surface. This is generally observed, but on some surfaces (e.g., Si(1 0 0), 2 · 1), there is an energetic advantage to H atoms residing at adjacent sites, yielding first-order (m = 1) desorption except at very low h [14]. An even greater exception occurs on some aluminum surfaces, where H saturation of dominant desorption sites occurs at moderate-to-large coverage, yielding zero-order (m = 0) desorption [16]. Most observations of H2 evolution from W are consistent with second-order desorption [7,8,13]. For conditions where most adsorbed H desorbs as H2, 1=m DH2 ffi AH/2 yields n / AH . Since the H desorption rate (DH) should be proportional to n, and at low coverage AH is proportional to the H2 pressure (p), Pdiss / DH/PH2 / p1+1/m. Using the H2-loss method, Brennan and Fletcher [2] measured Pdiss / p1/2, corresponding to m = 2, for H release from polycrystalline W at T = 1200–1350 K. Our analysis indicates that their reported 1800 K data fits Pdiss / p3/8. Previous H2-loss measurements, that also obtained Pdiss / p1/2 in the 1300 K region, are cited by Brennan and Fletcher. They obtained a dissociation activation energy (Ediss) of 2.30 ± 0.05 eV, and similar values were reported in the earlier measurements. Using a different Hadsorbing cold surface, Stobinski and Dus [3] obtained Ediss ffi 2.25 eV but did not investigate the dependence on p. Umemoto et al. [7] measured H vapor density (nH) in a chamber containing 2–60 mTorr of H2 and a W filament at T = 1250–2200 K. They observed Ediss = 2.48 eV and nH / p1/2. The H flux due to H production on the filament should equal nH times the diffusion rate to chamber surfaces where the H is consumed. This diffusion rate is proportional to p1, so this implies a H flux proportional to p1/2. Since the H flux also equals (AH/S)Pdiss and AH / p is expected, this implies Pdiss / p3/2. This Ediss is close to the previous results, but Pdiss / p3/2 is far from any other measured pressure dependence. (If the chamber were coated with amorphous silicon from other studies, SiH4 would be etched from surfaces by H, perhaps producing such an anomalous p dependence.) Similar values of Ediss have also been observed for H2 dissociation on other metals [17]. The striking fact that 2Ediss ffi EH2, the vapor-phase H2 dissociation energy, for all metals studied has long been recognized. This suggests that Ediss may be independent of the H atom binding energy to the particular metal surface. In Section 3, we will review a model for second-order desorption that yields 2Ediss = EH2 for all metal surfaces. Summarizing the discussion of the previous two paragraphs, it appears that most data on H2 desorption from
W is consistent with second-order desorption. Most measured p and T dependence of H2 dissociation on W fit expectations based on second-order desorption. Thus, we started these H2 dissociation measurements expecting to merely improve knowledge of S(T) and two additional rate constants that appear in Hickmott’s second-order desorption model [4]. However, our data (using polycrystalline W) clearly contradicts second-order desorption and appears to require a major reassessment of all models for H2 dissociation on W and perhaps on many other metals. 2. Measurements The W filament is mounted along one axis of a 6-way stainless-steel cross, with 6.3 cm diameter tubing and copper-seal flanges. The chamber is evacuated by a turbomolecular pump, typically to 108 Torr. The hydrogen pressure is monitored by a capacitance manometer, and a window 30 cm from the filament allows filament observation. A 0.3 mm diameter orifice in one flange leads to a separately turbopumped, threshold-ionization mass spectrometer (TIMS) of our design. Hydrogen and argon enter through metal tubing, leak valves, and a computer-operated valve and exit through a gate valve that is used to adjust flow to the pump. All data is taken with continuous flow of hydrogen, and often also argon, to prevent impurity buildup in the dissociation chamber. Filament 833 nm emission (I) is measured by a silicon photodiode behind an interference filter and used to establish T using I / exp(hm/kT), normalized at 2300 K to T from a calibrated red-wavelength–intensity-match pyrometer with emissivity and window-transmission corrections. This device is checked by comparing measured and calculated filament resistance, using published resistivities and direct current to achieve high accuracy. Data has been taken with a 17 cm long 20 · 760 lm foil to minimize the cold-end length and with 380 and 500 lm diameter wires of 17–25 cm length. We did this to vary the applied voltage and the end corrections and to confirm that typical polycrystalline W is being used. In all cases, the T distribution at the ends were calculated and used to correct the effective hot surface length versus T. (The hot length is typically 2–7% less than the total length.) H2 dissociation on the filament is measured by two methods: changes in T as H2 is removed from the chamber and H atom flux to the chamber surfaces measured by TIMS. In the T-change method, the filament voltage (VF) is held constant as H2 is removed, yielding a T increase from TH2 to Tvac. This causes a decrease DPR = VFDiF in heating power, where iF is the current, and an increase DPrad = DI(dPvac/dI) in the radiated power, where P(I)vac is the measured radiated power (iFVF) versus I in vacuum. The power loss due to the H2 is DP(TH2) =DPR + DPrad. Below 1300 K, R = VF/iF is more accurate than the 833 nm emission (I) for establishing T, and DPrad = DR(dPvac/dR) is used. Both methods agree from 1300 to 2000 K, but thermionic current can contribute to iF at higher T, so I is used to establish T. (Using VF > 0 minimizes this cur-
W. Zheng, A. Gallagher / Surface Science 600 (2006) 2207–2213
Langmuir D(dimensionless)
0.1
α=0.4
0.01
α=0.07
Conduction
Dissociation 1E-3 0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1000K/T
Fig. 1. Measured power change divided by the H2 collision rate with the filament and by EH2 = 4.5 eV plotted versus 1000/T (K). A 380 lm diameter W filament was used for the data shown. The equivalent quantity from PH2 = 15 and 39 mTorr data of Langmuir and Mackay is shown as solid lines. This quantity is identified as Pdiss + aCH2/EH2, where Pdiss is the dissociation probability, aCH2/EH2 is the thermal-conduction contribution, a is the thermal accomodation coefficient, and CH2 is the H2 thermal energy. The dashed lines are aCH2/EH2 for a = 0.07 and 0.4. The solid line labeled Pdiss is normalized average data from Fig. 2, and the (j) points are the data minus this Pdiss line.
rent.) We plot this data versus 1/TH2 in Fig. 1, where TH2 is T with the H2 present, for several p between 2 and 30 mTorr. It is plotted as DP(TH2)/{(nH2v/4)ASEH2}, where (nH2v/4)AS is the rate of H2 collisions with the filament, nH2(v) is the H2 density (velocity), AS is the filament hot surface area and EH2 is the vapor–H2 dissociation energy at 0 K. We equate DP(TH2) to (nH2v/4)AS(PdissEH2 + EC), so Pdiss + EC/EH2 is plotted in Fig. 1, where EC is the conduction cooling energy per H2–filament collision. There is essentially no pressure dependence to this quantity, as is also true of additional data that is not plotted to avoid excess overlap. The lines and square points in Fig. 1 will be discussed after presenting the H-atom data. The H2–filament collision rate, nH2vAS/4, is proportional 1=2 to T vapor , and we calculate that the incident H2 is at Tvapor ffi 300 K at H2 pressures below 20 mTorr. This calculation provides a correction at higher pressures, as described below. EC = a(T)C(T)H2 is also used, where a(T) is an effective thermal accomodation coefficient, C(T)H2 = [3k(T Tvapor) + E(T)v] is the specific heat of H2, and E(T)v = kTx/{exp(x) 1}, with x = 6330 K/T as the H2 vibrational specific heat. This uses equilibrated H2 release at T as a guide to interpreting EC, although the actual energy distribution of the released H2 is not known. At T where dissociation is significant, an assumed equilibrated H release at T yields almost the same energy, since 4kT ffi 3kT + E(T)v at high T. The H-atom data reported in Fig. 2 is obtained with 20 mTorr of argon also in the chamber. This assures a constant H temperature and angular distribution at the orifice
0
10
[H]/pressure (normalized to Pdiss )
5mT 10mT 2mT 15mT 20mT 30mT 10mT ribbon
2209
-1
10
-2
10
S&F 4mT 1mT 10mT 30mT Hickmott 1mT Hickmott 50mT
-3
10
0.30
0.35
0.40
0.45
0.50
0.55
0.60
1000/T Fig. 2. The H atom signal from the TIMS (points) versus 1000/T (K) for a 380 lm diameter W filament and the H2 pressures indicated. The curved lines are from Eq. (1), using Hickmott vH and vH2 values, ED1 = 2.25 eV, S = 0.4 and PH2 = 1 mTorr (dotted line), 50 mTorr (solid line), and 5 mTorr (line through the data). The open stars are the data of Smith and Fite, at PH2 < 0.2 mTorr.
to the TIMS and hence a sampling efficiency that is independent of p. (As expected, removing the argon for PH2 > 10 mTorr does not alter the H signal.) The H-atom signal is proportional to the H density at the orifice, which is proportional to the total H flux from the filament. Flux/density is also proportional to Prec, the recombination probability per chamber-surface collision. Thus, H-signal/p / Pdiss if Prec is constant as T and p are varied. Varying the chamber surface temperature, surface-cleaning procedure (e.g., electropolish or baking), and prior H exposure have not significantly affected the H signal, supporting the assumption that Prec is constant. A different error that can arise in H-atom data is excess H or H+, due to H2 dissociation and ionization by the thermionic current, perhaps aided by the Hþ 2 þ H2 ! Hþ þ H reaction. This anomalous signal can be distin3 guished by its thermionic-emission T dependence, a VF threshold near 17 V, and a dependence on the sign of VF. It has been eliminated from the data presented here by using positive VF below 17 V. The measured H-signal/p is plotted in Fig. 2 for several H2 pressures between 1 and 30 mTorr. Within experimental uncertainty, seen as 20% variations in the data shown, all observations with foil and different diameter filaments were in agreement with the data shown. The 20% signal variations at high T are attributed to drifts in mass spectrometer sensitivity; the increased noise and occasional oscillations at low T result from small signal/ noise and baseline drifts. This data was typically taken with 30-min up-down scans of VF; any hysteresis is included within the points shown. Since H-signal/p / Pdiss (assuming constant Prec) in Fig. 2, it is normalized at high T to Pdiss, as determined in the next paragraph from Fig. 1 data. The data in Fig. 1 equals Pdiss + a(T)C(T)H2/EH2, where C(T) and EH2 are known properties of H2. At high T, where
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Fig. 1 data is essentially constant, we assume that all adsorbed H2 dissociates, i.e., Pdiss = S(T). It is also necessary that a(T) P S(T), since dissociation is accompanied by H release, presumably with a kinetic temperature of T. We therefore assume that a(T) = Pdiss in this high-T region, yielding a = Pdiss = 0.40 from the data in Fig. 1. Since the H signal in Fig. 2 is proportional to Pdiss, an average H signal versus T is plotted as a line labeled Pdiss in Fig. 1; it is normalized to 0.40 at high T. The difference between the average data and this line represents the conduction contribution; it is plotted as solid-square points. For comparison, the calculated thermal-conduction contribution, a(T)C(T)H2/EH2, is plotted as dashed lines for a(T) = 0.4 and 0.07. As can be seen, the low-T conduction data roughly fits the a = 0.07 line, and the high-T data fits the a = 0.40 line as it was normalized to do. The conduction data makes a smooth transition between these two a values and might be identified as S(T). The p = 15 and 39 mTorr, power-change data of Langmuir and Mackay is plotted as solid lines in Fig. 1. (Their higher-p data requires an unknown correction for the effective temperature of the impinging H2.) This data, from almost 100 years ago, is very similar to the present data. The Smith and Fite data, corresponding to p < 0.2 mTorr, is shown in Fig. 2. It is also close to our data, with a moderate discrepancy that is well within their data uncertainty. In Fig. 3, we show the p dependence of Pdiss + EC/EH2 at 2200 K. The triangles are from Fig. 1 data, and the circles are from a slow up-down p scan, where VF is fixed and the H2 flow is repeatedly switched on and off. This T is chosen to be in the regime where adsorbed H2 is primarily released
1
Dissociation probability at 2200K
Tf = 2200K Brennan & Fletcher extrapolated
Hickmott's model
Langmuir 0.1
S&F corrected by sqrt(T g)
0.01 0.1
1
10
100
Pressure (mT)
Fig. 3. Measured H2 pressure dependence of Pdiss at 2200 K. The (,) symbols are H-atom signals from T scans in Fig. 2 and the (s) are powerchange data from a pressure scan at a fixed filament voltage, set to yield 2200 K at low pressure. The latter points have been corrected to a constant TH2 above 10 mTorr. An approximate further correction for the decreasing H2–filament collision rate is shown as a line from 10 to 85 mTorr. The star points are Langmuir and Mackay data, and the horizontal line is Smith and Fite data.
as H2, as indicated by the rapid variation with T, while providing good signals. The pressure-scan data is the most accurate, since other data comes from different times and may be influenced by small drifts in apparent T and in TIMS sensitivity. The pressure-scan data is free of hysteresis but requires a T correction for PH2 > 10 mTorr since VF was fixed and the H2 cools the filament. TH2 decreases by DT as p increases, and the raw data (not shown) has been corrected to 2200 K by adding (dPdiss/dT)DT. As p increases past 20 mTorr, this T-corrected data slowly drops below a constant value of Pdiss. This drop is attributed to a decrease in the rate of H2 collisions with the filament, since the nearby vapor is heated by hot H and H2 from the filament. A calculation of Tvapor one mean-free-path from the filament, using the EC data in Fig. 1 as the energy 1=2 flow to the vapor and nH2 v / T vapor , corrects this data to the line in Fig. 3. (The calculation yields Tvapor = 600 K at 85 mTorr.) While this is only an estimate of this effect, it supports the conclusion that Pdiss is independent of p up to the highest pressures studied (85 mTorr). Summarizing Fig. 3, Pdiss at 2200 K is independent of p from 1 to 85 mTorr. From Fig. 2, one can see that this result also holds over the full range of T shown there (1700– 3000 K). Pdiss at 2200 K from Smith and Fite [6] and from the Langmuir and Mackay low-pressure data [5] is shown in Fig. 3. The latter is independent of p but 40% higher than our data. This could be due to filament T calibration or other factors discussed next. There are a variety of factors that can cause errors in the measured DP(TH2). One is the release of molecules from the wall, directly by filament radiation or by wall heating. These molecules react with and cool the filament, causing an apparent change in Pdiss if their action is altered by adding H2. A related mechanism, famous for rapidly destroying W-filament lights with the ‘‘water cycle’’, is H reduction of tungsten-oxide at the chamber wall, releasing water molecules that oxidize the filament and cause WxOy evaporation. Here we clean the chamber stainless-steel surfaces and expose them to H atoms from the filament (with H2 flow) to reduce W on the walls before taking DP(TH2) and H-atom data. To test for typical vapor impurities in the H2, we verified that Pdiss did not change when the inlet tubing was cooled to 78 K. We also observed hysteresisfree H atom and DP(TH2) data above 1700 K during 30 min, 1000 ! 3000 ! 1000 K temperature scans. However, a(T < 1400 K) was sensitive to the time since high-T cleaning of the filament; it decreased during H2 exposure after high-T operation. This contributes a hysteresis at the lower T in Fig. 1, where the entire up-down temperature-scan data is shown. Apparently the filament surface and a are not unique functions of T below 1400 K. As mentioned above, thermionic currents can cause errors, as they are influenced by H2 vapor at high T and negative V. This causes an apparent contribution to filament resistance and thus T and DP(TH2). This is less severe for positive V and is avoided in the present data by utilizing 833 nm emission to determine T in this region.
W. Zheng, A. Gallagher / Surface Science 600 (2006) 2207–2213
3. Discussion We have compared our data to the Langmuir and Mackay power-change data in Fig. 1. Their vacuum quality was much lower, and establishing T was much harder in 1911. Yet the data is very similar to ours, even including the conduction cooling below 1500 K. Impurities would be most serious at the highest T and might have caused their higher saturation value of Pdiss. The Pdiss data of Smith and Fite, taken at p < 0.2 mTorr and shown in Figs. 2 and 3, is slightly higher than our data, but as it is at very small p, it strongly supports a independent of p. Smith and Fite suggested that an oxygen layer might exist on their W surface at the lower T, but this is not supported by observations reported in Livshits et al. [18]. Most of the difference from our data looks like an 80 K (4%) temperature shift, which is not surprising in view of the difficulty of establishing T at this level of accuracy. Our definition of a, provided above, is different than theirs and yields a(1250 K) ffi 0.20 from their Fig. 5 data. This is about three times the value determined from the DP(TH2) data in Fig. 1. (As noted above, we find that a in this T range is sensitive to the amount of H2 exposure after filament ‘‘cleaning’’ at T = 3000 K.) Brennan and Fletcher [2] measured H2 loss in a closed vessel due to dissociation on the filament followed by H reaction with a molybdenum oxide (MoOx) film on the walls. Earlier experiments of this type, cited by them, obtained a similar dependence on p and T but apparently suffered from contamination of the filament. Brennan and Fletcher observed a very clear (dPH2/dt)/p / p1/2 due to H desorption from the W for T = 1150–1350 K and p = 1–6 mTorr. The above formula defining Pdiss yields (dp/dt)/p / SPdiss, and it is likely that S is independent of PH2 since any dependence normally requires significant h, and h should grow rapidly as T decreases. Thus, observing systematic behavior across this T range implies small h. Consistent with this, the Hickmott parameters (presented below) yield 2% coverage at 1150 K and 6 mTorr. From our analysis of their Fig. 2, we believe Brennan and Fletcher also observe SPdiss / p3/8 for T = 1800 K and p = 0.01– 0.3 mTorr. In Fig. 3, we show their results extrapolated to 2100 K; the p dependence is obviously different than our data. This comparison would look the same at 1800 K, since our H-atom data yields Pdiss / p0 down to 1800 K and has essentially the same T dependence as their data. Thus, this disagreement occurs for data taken at the same T and for essentially the same range of p. (Most of our data is at larger p, but p0 dependence would only be expected at lower pressures than those of [2], according to [4].) This discrepancy cannot be due to H-reaction nonlinearity at their MoOx-coated chamber surface, because this would also influence the T dependence. One possibility is that the H– MoOx reaction releases H2O, which reacts with the W and changes its reactivity. Stobinski and Dus [3] also measured H2 loss in a closed vessel, and they carefully established the linearity of the H-atom reaction with a 78 K gold surface layer (which
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would not produce a volatile product). They report H2 loss on W for T = 1020–1315 K but only at p = 2 mTorr. Their data is consistent with 2.25 eV dissociation activation energy, and they state that their H-atom production agrees with an equation from Langmuir for vapor-phase-equilibrium H2 dissociation. We are uncertain regarding what was actually observed, and dividing their Eq. (2) by the H2-filament collision rate to obtain Pdiss, yields 2–3 times our data in Fig. 2. Hickmott [4] also measured H2 loss due to H reaction with the chamber surface, which was glass that had been baked, then cooled to 77 K. He asserted that the H atoms were well absorbed by this surface prior to reaching a saturation density. By using very low pressures and H2 flows, he thus measured a H2 flow decrease that equaled half the flow of H desorbed from the filament. He also provided a model for the full p and T dependence of H2 dissociation based on second-order H2 desorption. As this nicely demonstrates the relationship between dissociation and H2 desorption order, we will reproduce it here before comparing to his data. The H adsorption rate per filament surface area from H2 vapor is AH = 2S(PH2v/4kTvapor), where v (Tvapor) is the vapor thermal velocity (temperature). We assume that S depends only on T, since h 1 is expected for the conditions of interest here. Since EW–H2 is the activation energy for H2 desorption and EH2 is the vapor–H2 dissociation energy, desorbing to two free H atoms requires EW–H2 þ EH2 and the H-atom binding to W is EW–H ¼ ðEW–H2 þ EH 2 Þ=2. The H-atom desorption rate is kHn, where n is the surface H density and kH = mHexp(EW–H/kT). Assuming independent H atom locations, the H desorption rate as H2 is kH2n2 (from 2kH2(n2/2)), where kH2 = mH2exp([2E W–H EH2]/ kT). This yields, in steady state, AH ¼ k H n þ k H 2 n 2
ð1Þ
from which n ¼ ðk H =2k H2 Þf1 þ ð1 þ CÞ 4AH k H2 =k 2H
1=2
g;
ð2Þ
4AH mH2 fexpðEH2 =kT Þg=m2H .
¼ Two rewith C ¼ leased H represent one dissociation, so this occurs at a rate of kHn/2. Thus, the dissociation probability (Pdiss) per H2–W collision equals (kHn/2)/(AH/S), yielding P diss ¼ 2Sf1 þ ð1 þ CÞ
1=2
g=C:
ð3Þ
The fascinating feature of Eq. (3) is that C, and thus Pdiss, are independent of EW–H. For small C, corresponding to high T and small p, Pdiss ffi S; almost all adsorbed H is desorbed as H and Pdiss is independent of T and PH2. For large C, corresponding to low T and high PH2 with most adsorbed 1=2 H desorbed as H2, P diss ffi 2S=C 1=2 ¼ Sk H =ðAH k H2 Þ / 1=2 2 P H2 expðEH2 =2kT Þ. (Equivalently, AH2 ffi kH2n and Hdesorption = kHn ffi kH(AH/kH2)1/2.) Thus, when Pdiss 1, the dissociation activation energy (Ediss) is 1/2 of the vapor– H2 dissociation energy and is independent of the specific metal as long as EW–H2 > 0. This predicted Pdiss, using Hickmott’s mH and mH2 values, is plotted in Fig. 2 for S = 0.4, with
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W. Zheng, A. Gallagher / Surface Science 600 (2006) 2207–2213
three p chosen to bracket and match our data. EW–H2 ffi 1:4 eV on polycrystalline W [4] and 1.15–1.6 eV on individual planes [8]. The latter has been obtained from H2 desorption studies and an equilibrium-surface-coverage measurement [9]. Using EW–H2 ¼ 1:4 eV yields EW–H = 2.95 eV. As already noted, Ediss ffi EH2/2 = 2.24 eV has been observed in several W studies [2,3,7] as well as in the present measurements, and 5–20% higher values have been observed on tantalum and rhenium [21]. In further agreement with this model, Pdiss / p1/2 in the limit where mostly H2 is released agrees with Brennan and Fletcher’s data and other measurements using MoOx surface layers that are cited there. Thus, a considerable body of data supports Eqs. (1)–(3) and second-order H2 desorption. Hickmott asserts that the above model fits his dissociation data, but we do not agree. His Figs. 7–9 show measured H flux versus p for a range of T between 1170 and 1460 K, and p = 109 106 Torr. The measured H flux fits linear functions of p, with slopes that vary as exp(Ediss/kT) with Ediss = 2.9 eV (his Fig. 10). Since Pdiss / (H flux)/p, this is equivalent to Pdiss independent of p and exponential in 1/T, in agreement with our observations. In contrast, the solution of Hickmott’s Eq. (10) (our Eq. (1)) has one limit (with mostly H desorbed) where DH is linear in p and independent of T, and the other limit (with mostly H2 desorbed) where H flux / p1/2exp(Ediss/ kT). Thus, Hickmott’s measurements, as well as those of Smith and Fite and of Langmuir, agree with our observations and disagree with the form of Eq. (1). We have looked for modifications to the physical model that might explain the dissociation data presented here. A successful model must explain two features that occur in the region where Pdiss 1: (1) Pdiss / exp(Ediss/kT) with Ediss ffi EH2/2 and (2) Pdiss / p0. The Hickmott, secondorder desorption model obtains result (1) but simultaneously 1=2 obtains P diss / AH2 , and as AH should be proportional to p at low h this violates (2). Lifshits et al. [19] provide some strong arguments for Pdiss / p1/2, based on equilibrium relations and some assumptions, and it is useful to consider an equilibrium situation, with the vapor enclosed by the hot W. For T and p that yield small fractional vapor dissociation, the vapor H density is proportional to p1/2, and the H and H2 collision rates with the W are proportional to p1/2 and p. By detailed balance, the H and H2 desorption rates must match their adsorption rates, so if the adsorption probability is large for both specie then H desorption should be proportional to p1/2. Since the H2 collision rate is much larger than for H, this suggests Pdiss / p1/2. An equivalence of surface H resulting from H or H2 adsorption, as well as adsorption probabilities independent of p, are implicit in this argument. Recognizing that some of these assumptions do not hold may be useful. Of course, the experiments also differ in having essentially no H collisions with the W, as well as H2 incident with a temperature far below T. We have not found a convincing explanation that obtains both features but will mention some ideas that have been considered.
(1) Prepairing. As noted in Section 1, on the Si(1 0 0), 2 · 1 reconstructed surface there is an energetic advantage to H atoms residing at adjacent sites, and this yields first-order desorption except at very low h [14]. If such ‘‘prepairing’’ occurred on W, the factor n2/2 in the last term of Eq. (1) would be replaced by n/2. This would yield Pdiss = SkH/(kH + kH2), which is independent of PH2 at all T. At high T where kH2 kH, it yields Pdiss ffi S, as is measured. At lower T, where kH kH2 and Pdiss 1, it yields Pdiss / exp(Ediss/ T) with Ediss ¼ EW–H EW–H2 ¼ ðEH2 EW–H2 Þ=2 ¼ 1:55 eV for EW–H2 ¼ 1:4 eV. This is very far from the measured ED1 = 2.25–2.9 eV, which pretty well rejects this as a possible explanation. (2) Slow diffusion. Another way to obtain Pdiss / p0 is to assume that the H atoms from each H2 adsorption evaporate as H2 before diffusing into a homogeneous distribution on the W surface. The diffusing-pair overlap probability decays as s/t, where s = d2/DH, d is the H–H reaction diameter, and DH is the H surface-diffusion coefficient. If, as is likely, most H are constrained to step edges (on polycrystalline W), the resulting one-dimensional pair overlap decays yet more slowly, as (s/t)1/2. If diffusion is inhibited by a barrier, this might be reasonable, but Daniels and Gomer [10] find diffusion barriers to be 0.15 eV, which is relatively unimportant at T > 1500 K. But the most serious objection to this idea is that it yields the same (incorrect) Ediss as in the previous paragraph; it is just another cause of ‘‘prepairing’’. The discrepancy is even worse if a diffusion barrier is invoked, as this energy subtracts from the Ediss of prepairing. (3) A range of adsorption energies. If EWH is inhomogeneous over the surface, the distribution of H will be as well, and the H–H reaction probability will not vary as n2. If the stronger binding sites are clustered, as might occur at step edges, the h dependence of H2 desorption will be weakened. As a limiting case, H2 release / n0 at small Pdiss occurs on aluminum [16] and is generally attributed to release primarily from small regions where H-saturated surface sites occur. Estimating the effect of various possible binding-site distributions has not been done here. (4) H release during breakup. The direct adsorption of H2 into two adsorbed H atoms is exothermic by ED2 ffi 1.4 eV, while the breakup into one adsorbed and one free H is endothermic by (EH2 ED2)/2 ffi 1.6 eV. Thus, this method of producing a free H does not have the observed activation energy of 2.3 eV. (5) T dependence of S(T). If a significant portion of the T dependence of Pdiss resulted from T dependence of S(T), this would influence the apparent activation energy. We observe S(T > 2000 K) ffi 0.40 and S(1200 K) ffi 0.07, Alnot et al. [19] obtain S = 0.5 from T = 200–1150 K and Butler et al. observe S(300 K) ffi 0.5 [11]. Additional values and limits have been reported, generally with S P 0.1. There are two problems with using S(T) to explain the data: S(T) appears to be a slowly varying function of T, and it would be difficult to obtain the large range of Pdiss values in Fig. 2 or in [7] from only S(T). However, it could alter the above relations between Ediss and the other energies.
W. Zheng, A. Gallagher / Surface Science 600 (2006) 2207–2213
(6) The mathematical indication. One can achieve relations (1) and (2) from Eq. (1) if the last term is multiplied by AH or anything proportional to p. This seems to require a model in which H2 release still requires two adsorbed H atoms in proximity but with an added assist from a diffusing-adsorbed H2 or a hot atom diffusing after a recent H2 adsorption. Since many pairs of adsorbed H reside at close range with insufficient energy to escape as H2, this multibody process might dominate by increasing the available energy for H2 escape. Also, some H–D mixing data has been explained with a ‘‘Eley–Rideal’’ model in which hotatom diffusion on the surface (following H2 adsorption) produces the molecular desorption [20]. This might yield the suggested link between AH and H2 desorption, although in the present case two adsorbed H must be involved. An alternative would be to divide the other two terms of Eq. (1) by p, equivalent to assuming that H adsorption is independent of p and H release is independent of H surface density. Measurements of other observables are needed to support the existence of such unexpected behavior. 4. Conclusions We provide the H2 dissociation probability (Pdiss) on high temperature W for the pressure and temperature region of interest for many applications, particularly ‘‘hot-wire’’ silicon-film deposition. We show that several previous high temperature measurements are consistent with our results, once one recognizes that Pdiss is independent of the H2 pressure. (We show this for H2 pressures (p) below 0.1 Torr but expect it to continue well above this pressure.) Our measured T dependence of Pdiss is also consistent with a 2.25 eV dissociation activation energy on W, in agreement with most prior data. This simplifies calculation of H flux in various applications. However, Pdiss / (p)0 contradicts second-order H2 desorption and the most successful model for this dissociation process. This model makes good physical sense and explains some important features of H2 dissociation on metals, but its predicted
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pressure dependence appears to be a fatal flaw. We discuss several possible modifications to the model but do not find a clearly valid explanation of this unexpected feature of the data. Our results throw major doubt on current understanding of H2 dissociation, accompanied by H release, from metals. Acknowledgements This work was supported in part by the National Renewable Energy Laboratory of the US Department of Energy. We wish to thank A.V. Phelps for many valuable contributions. References [1] Proceedings of the Second International Conference on Cat-CVD (Hot Wire CVD) Process, Thin Solid Films 430 (2003) 1. [2] D. Brennan, P.C. Fletcher, Proc. Roy. Soc. 250 (1959) 389. [3] J. Stobinski, R. Dus, Vacuum 46 (1995) 433. [4] T.W. Hickmott, J. Chem. Phys. 32 (1960) 810. [5] I. Langmuir, G.M.J. Mackay, J. Am. Chem. Soc. 36 (1914) 1708. [6] J.N. Smith Jr., W.L. Fite, J. Chem. Phys. 37 (1962) 898. [7] H. Umemoto, K. Ohara, D. Morita, Y. Nozaki, A. Masuda, H. Matsumura, J. Appl. Phys. 91 (2002) 1650. [8] P.W. Tamm, L.D. Schmidt, J. Chem. Phys. 54 (1971) 4775. [9] T.-U. Nahm, R. Gomer, Surf. Sci. 375 (1997) 281. [10] E.A. Daniels, R. Gomer, Surf. Sci. 336 (1995) 245. [11] D.A. Butler, B.E. Hayden, J.D. Jones, Chem. Phys. Lett. 217 (1994) 423. [12] K. Moritani, M. Okada, M. Nakamura, T. Kasai, Y. Murata, J. Chem. Phys. 115 (2001) 9947 (also references therein). [13] A. Forni, M.C. Desjonqueres, D. Spanjaard, G.F. Tantardini, Surf. Sci. 274 (1992) 161. [14] E.S. Tok, J.R. Engstrom, H. Chuan Kang, J. Chem. Phys. 118 (2003) 3294. [15] R. Frauenfelder, J. Vac. Sci. Technol. 6 (1969) 388. [16] A. Winkler, G. Pozgainer, K.D. Rendulic, Surf. Sci. (251–252) (1991) 886. [17] S.A. Redman, C. Chung, K.N. Rosser, M.N.R. Ashford, Phys. Chem. Chem. Phys. 1 (1999) 1415. [18] A.I. Livshits, F.El. Balghiti, M. Bacal, Plasma Source Sci. Technol. 3 (1994) 465. [19] P. Alnot, A. Cassuto, D.A. King, Surf. Sci. 215 (1989) 29. [20] Th. Kamler, S. Wehner, J. Kuppers, J. Chem. Phys. 109 (1998) 4071.
Hydrogen dissociation on high-temperature tungsten Wengang Zheng, Alan Gallagher
*
JILA, University of Colorado and National Institute of Standards and Technology, Boulder, CO 80309-0440, USA Received 13 June 2005; accepted for publication 13 March 2006 Available online 25 April 2006
Abstract H2 dissociation on polycrystalline tungsten is measured from 1700 to 3000 K using the filament temperature (T) change and a normalized H-atom density at the chamber surface. The dissociation probability per H2 filament collision (Pdiss) saturates at 0.40 at high T and has a 2.25 ± 0.05 eV apparent activation energy when Pdiss 1. This activation energy is consistent with prior data and models, but the H2 pressure dependence is not. Pdiss is independent of the H2 pressure for this entire T range and the 1–85 mTorr pressure range studied, contradicting the primary model that has been used to explain H2 dissociation on tungsten and other metals. We show that some apparently contradictory prior measurements are actually consistent with our observations and with each other, once this pressure dependence of Pdiss is recognized. 2006 Elsevier B.V. All rights reserved. Keywords: Polycrystalline surfaces; Tungsten; Hydrogen molecule; Dissociation; Desorption; Catalysis; Models of surface kinetics
1. Introduction The dissociation of H2 on metal surfaces, particularly tungsten (W), has a long history because of its early importance in lighting and a continuing importance in catalysis and H sources. In addition, the present study is motivated by its importance in the production of thin-film silicon devices by ‘‘hot-wire deposition’’ [1]. Measurements of H2 dissociation on W are mostly of three types: (1) reduction of H2 pressure in a closed vessel as H produced on a 1100–1800 K filament reacts with a coating on the chamber surfaces [2–4]; (2) measurement of power removed from a 1700–2800 K filament [5]; and (3) measurement of H produced on a 2100–2900 K filament [6,7]. Other measurements also provide constraints on any model for this dissociation. Examples are: exposure of clean 6300 K tungsten to a known H2 flux, followed by measurement of H2 evolution as the metal temperature (T) is raised [4,8], measurement of surface H concentration [9], surface
*
Corresponding author. Tel.: +1 303 492 7841; fax: +1 303 492 5235. E-mail address: [email protected] (A. Gallagher).
0039-6028/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2006.03.032
H diffusion [10], H2 beam attenuation due to adsorption [11], and H–D exchange reactions on the surface [12]. A few calculations study dissociative adsorption and H2 desorption on W [13]; in contrast, scores of papers appear every year about these processes on silicon surfaces, e.g., [14] and references therein. (These references are representative, not inclusive.) The present measurements on polycrystalline W fall into categories (2) and (3). Summarizing these studies, it appears that H2 dissociatively adsorbs with a probability S(T) per collision, either without an (adsorbed H2) precursor state or with a weakly-bound precursor state [11]. (S is also a function of the incident H2 energy distribution and the surface fractional coverage, h [11]. The incident energy distribution is essentially constant, and we expect h 1 in the present measurements, so this will be ignored here.) H atoms on the W surface either desorb individually, overcoming a barrier EW–H, or as H2 when a pair of H atoms in close proximity overcomes a smaller barrier (EW–H2 ) [4]. Also, at T > 1200 K of interest here, H only resides on the W surface [15]. Thus, the dissociation probability (Pdiss) per H2–metal collision is S(T) times the fraction of adsorbed H that desorbs as H rather than as H2(fH=H2 ). At high T,
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W. Zheng, A. Gallagher / Surface Science 600 (2006) 2207–2213
H atoms are desorbed very rapidly, h 1, most H atoms desorb individually, and fH=H2 ffi 1. At low T, H2 desorption is energetically favorable and fH=H2 1. In this case, h depends on the balance between the H adsorption rate (AH) and the H2 desorption rate (DH2), both as rates per unit area. It is customary to take DH2 ¼ C m nm expðEW–H2 =kT Þ, where n is the H surface density, m is the desorption order and Cm is a constant. Except at high coverage, one expects second-order (m = 2) desorption when the H atoms reside and move independently on the surface. This is generally observed, but on some surfaces (e.g., Si(1 0 0), 2 · 1), there is an energetic advantage to H atoms residing at adjacent sites, yielding first-order (m = 1) desorption except at very low h [14]. An even greater exception occurs on some aluminum surfaces, where H saturation of dominant desorption sites occurs at moderate-to-large coverage, yielding zero-order (m = 0) desorption [16]. Most observations of H2 evolution from W are consistent with second-order desorption [7,8,13]. For conditions where most adsorbed H desorbs as H2, 1=m DH2 ffi AH/2 yields n / AH . Since the H desorption rate (DH) should be proportional to n, and at low coverage AH is proportional to the H2 pressure (p), Pdiss / DH/PH2 / p1+1/m. Using the H2-loss method, Brennan and Fletcher [2] measured Pdiss / p1/2, corresponding to m = 2, for H release from polycrystalline W at T = 1200–1350 K. Our analysis indicates that their reported 1800 K data fits Pdiss / p3/8. Previous H2-loss measurements, that also obtained Pdiss / p1/2 in the 1300 K region, are cited by Brennan and Fletcher. They obtained a dissociation activation energy (Ediss) of 2.30 ± 0.05 eV, and similar values were reported in the earlier measurements. Using a different Hadsorbing cold surface, Stobinski and Dus [3] obtained Ediss ffi 2.25 eV but did not investigate the dependence on p. Umemoto et al. [7] measured H vapor density (nH) in a chamber containing 2–60 mTorr of H2 and a W filament at T = 1250–2200 K. They observed Ediss = 2.48 eV and nH / p1/2. The H flux due to H production on the filament should equal nH times the diffusion rate to chamber surfaces where the H is consumed. This diffusion rate is proportional to p1, so this implies a H flux proportional to p1/2. Since the H flux also equals (AH/S)Pdiss and AH / p is expected, this implies Pdiss / p3/2. This Ediss is close to the previous results, but Pdiss / p3/2 is far from any other measured pressure dependence. (If the chamber were coated with amorphous silicon from other studies, SiH4 would be etched from surfaces by H, perhaps producing such an anomalous p dependence.) Similar values of Ediss have also been observed for H2 dissociation on other metals [17]. The striking fact that 2Ediss ffi EH2, the vapor-phase H2 dissociation energy, for all metals studied has long been recognized. This suggests that Ediss may be independent of the H atom binding energy to the particular metal surface. In Section 3, we will review a model for second-order desorption that yields 2Ediss = EH2 for all metal surfaces. Summarizing the discussion of the previous two paragraphs, it appears that most data on H2 desorption from
W is consistent with second-order desorption. Most measured p and T dependence of H2 dissociation on W fit expectations based on second-order desorption. Thus, we started these H2 dissociation measurements expecting to merely improve knowledge of S(T) and two additional rate constants that appear in Hickmott’s second-order desorption model [4]. However, our data (using polycrystalline W) clearly contradicts second-order desorption and appears to require a major reassessment of all models for H2 dissociation on W and perhaps on many other metals. 2. Measurements The W filament is mounted along one axis of a 6-way stainless-steel cross, with 6.3 cm diameter tubing and copper-seal flanges. The chamber is evacuated by a turbomolecular pump, typically to 108 Torr. The hydrogen pressure is monitored by a capacitance manometer, and a window 30 cm from the filament allows filament observation. A 0.3 mm diameter orifice in one flange leads to a separately turbopumped, threshold-ionization mass spectrometer (TIMS) of our design. Hydrogen and argon enter through metal tubing, leak valves, and a computer-operated valve and exit through a gate valve that is used to adjust flow to the pump. All data is taken with continuous flow of hydrogen, and often also argon, to prevent impurity buildup in the dissociation chamber. Filament 833 nm emission (I) is measured by a silicon photodiode behind an interference filter and used to establish T using I / exp(hm/kT), normalized at 2300 K to T from a calibrated red-wavelength–intensity-match pyrometer with emissivity and window-transmission corrections. This device is checked by comparing measured and calculated filament resistance, using published resistivities and direct current to achieve high accuracy. Data has been taken with a 17 cm long 20 · 760 lm foil to minimize the cold-end length and with 380 and 500 lm diameter wires of 17–25 cm length. We did this to vary the applied voltage and the end corrections and to confirm that typical polycrystalline W is being used. In all cases, the T distribution at the ends were calculated and used to correct the effective hot surface length versus T. (The hot length is typically 2–7% less than the total length.) H2 dissociation on the filament is measured by two methods: changes in T as H2 is removed from the chamber and H atom flux to the chamber surfaces measured by TIMS. In the T-change method, the filament voltage (VF) is held constant as H2 is removed, yielding a T increase from TH2 to Tvac. This causes a decrease DPR = VFDiF in heating power, where iF is the current, and an increase DPrad = DI(dPvac/dI) in the radiated power, where P(I)vac is the measured radiated power (iFVF) versus I in vacuum. The power loss due to the H2 is DP(TH2) =DPR + DPrad. Below 1300 K, R = VF/iF is more accurate than the 833 nm emission (I) for establishing T, and DPrad = DR(dPvac/dR) is used. Both methods agree from 1300 to 2000 K, but thermionic current can contribute to iF at higher T, so I is used to establish T. (Using VF > 0 minimizes this cur-
W. Zheng, A. Gallagher / Surface Science 600 (2006) 2207–2213
Langmuir D(dimensionless)
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α=0.07
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Dissociation 1E-3 0.3
0.4
0.5
0.6
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Fig. 1. Measured power change divided by the H2 collision rate with the filament and by EH2 = 4.5 eV plotted versus 1000/T (K). A 380 lm diameter W filament was used for the data shown. The equivalent quantity from PH2 = 15 and 39 mTorr data of Langmuir and Mackay is shown as solid lines. This quantity is identified as Pdiss + aCH2/EH2, where Pdiss is the dissociation probability, aCH2/EH2 is the thermal-conduction contribution, a is the thermal accomodation coefficient, and CH2 is the H2 thermal energy. The dashed lines are aCH2/EH2 for a = 0.07 and 0.4. The solid line labeled Pdiss is normalized average data from Fig. 2, and the (j) points are the data minus this Pdiss line.
rent.) We plot this data versus 1/TH2 in Fig. 1, where TH2 is T with the H2 present, for several p between 2 and 30 mTorr. It is plotted as DP(TH2)/{(nH2v/4)ASEH2}, where (nH2v/4)AS is the rate of H2 collisions with the filament, nH2(v) is the H2 density (velocity), AS is the filament hot surface area and EH2 is the vapor–H2 dissociation energy at 0 K. We equate DP(TH2) to (nH2v/4)AS(PdissEH2 + EC), so Pdiss + EC/EH2 is plotted in Fig. 1, where EC is the conduction cooling energy per H2–filament collision. There is essentially no pressure dependence to this quantity, as is also true of additional data that is not plotted to avoid excess overlap. The lines and square points in Fig. 1 will be discussed after presenting the H-atom data. The H2–filament collision rate, nH2vAS/4, is proportional 1=2 to T vapor , and we calculate that the incident H2 is at Tvapor ffi 300 K at H2 pressures below 20 mTorr. This calculation provides a correction at higher pressures, as described below. EC = a(T)C(T)H2 is also used, where a(T) is an effective thermal accomodation coefficient, C(T)H2 = [3k(T Tvapor) + E(T)v] is the specific heat of H2, and E(T)v = kTx/{exp(x) 1}, with x = 6330 K/T as the H2 vibrational specific heat. This uses equilibrated H2 release at T as a guide to interpreting EC, although the actual energy distribution of the released H2 is not known. At T where dissociation is significant, an assumed equilibrated H release at T yields almost the same energy, since 4kT ffi 3kT + E(T)v at high T. The H-atom data reported in Fig. 2 is obtained with 20 mTorr of argon also in the chamber. This assures a constant H temperature and angular distribution at the orifice
0
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[H]/pressure (normalized to Pdiss )
5mT 10mT 2mT 15mT 20mT 30mT 10mT ribbon
2209
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1000/T Fig. 2. The H atom signal from the TIMS (points) versus 1000/T (K) for a 380 lm diameter W filament and the H2 pressures indicated. The curved lines are from Eq. (1), using Hickmott vH and vH2 values, ED1 = 2.25 eV, S = 0.4 and PH2 = 1 mTorr (dotted line), 50 mTorr (solid line), and 5 mTorr (line through the data). The open stars are the data of Smith and Fite, at PH2 < 0.2 mTorr.
to the TIMS and hence a sampling efficiency that is independent of p. (As expected, removing the argon for PH2 > 10 mTorr does not alter the H signal.) The H-atom signal is proportional to the H density at the orifice, which is proportional to the total H flux from the filament. Flux/density is also proportional to Prec, the recombination probability per chamber-surface collision. Thus, H-signal/p / Pdiss if Prec is constant as T and p are varied. Varying the chamber surface temperature, surface-cleaning procedure (e.g., electropolish or baking), and prior H exposure have not significantly affected the H signal, supporting the assumption that Prec is constant. A different error that can arise in H-atom data is excess H or H+, due to H2 dissociation and ionization by the thermionic current, perhaps aided by the Hþ 2 þ H2 ! Hþ þ H reaction. This anomalous signal can be distin3 guished by its thermionic-emission T dependence, a VF threshold near 17 V, and a dependence on the sign of VF. It has been eliminated from the data presented here by using positive VF below 17 V. The measured H-signal/p is plotted in Fig. 2 for several H2 pressures between 1 and 30 mTorr. Within experimental uncertainty, seen as 20% variations in the data shown, all observations with foil and different diameter filaments were in agreement with the data shown. The 20% signal variations at high T are attributed to drifts in mass spectrometer sensitivity; the increased noise and occasional oscillations at low T result from small signal/ noise and baseline drifts. This data was typically taken with 30-min up-down scans of VF; any hysteresis is included within the points shown. Since H-signal/p / Pdiss (assuming constant Prec) in Fig. 2, it is normalized at high T to Pdiss, as determined in the next paragraph from Fig. 1 data. The data in Fig. 1 equals Pdiss + a(T)C(T)H2/EH2, where C(T) and EH2 are known properties of H2. At high T, where
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Fig. 1 data is essentially constant, we assume that all adsorbed H2 dissociates, i.e., Pdiss = S(T). It is also necessary that a(T) P S(T), since dissociation is accompanied by H release, presumably with a kinetic temperature of T. We therefore assume that a(T) = Pdiss in this high-T region, yielding a = Pdiss = 0.40 from the data in Fig. 1. Since the H signal in Fig. 2 is proportional to Pdiss, an average H signal versus T is plotted as a line labeled Pdiss in Fig. 1; it is normalized to 0.40 at high T. The difference between the average data and this line represents the conduction contribution; it is plotted as solid-square points. For comparison, the calculated thermal-conduction contribution, a(T)C(T)H2/EH2, is plotted as dashed lines for a(T) = 0.4 and 0.07. As can be seen, the low-T conduction data roughly fits the a = 0.07 line, and the high-T data fits the a = 0.40 line as it was normalized to do. The conduction data makes a smooth transition between these two a values and might be identified as S(T). The p = 15 and 39 mTorr, power-change data of Langmuir and Mackay is plotted as solid lines in Fig. 1. (Their higher-p data requires an unknown correction for the effective temperature of the impinging H2.) This data, from almost 100 years ago, is very similar to the present data. The Smith and Fite data, corresponding to p < 0.2 mTorr, is shown in Fig. 2. It is also close to our data, with a moderate discrepancy that is well within their data uncertainty. In Fig. 3, we show the p dependence of Pdiss + EC/EH2 at 2200 K. The triangles are from Fig. 1 data, and the circles are from a slow up-down p scan, where VF is fixed and the H2 flow is repeatedly switched on and off. This T is chosen to be in the regime where adsorbed H2 is primarily released
1
Dissociation probability at 2200K
Tf = 2200K Brennan & Fletcher extrapolated
Hickmott's model
Langmuir 0.1
S&F corrected by sqrt(T g)
0.01 0.1
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Pressure (mT)
Fig. 3. Measured H2 pressure dependence of Pdiss at 2200 K. The (,) symbols are H-atom signals from T scans in Fig. 2 and the (s) are powerchange data from a pressure scan at a fixed filament voltage, set to yield 2200 K at low pressure. The latter points have been corrected to a constant TH2 above 10 mTorr. An approximate further correction for the decreasing H2–filament collision rate is shown as a line from 10 to 85 mTorr. The star points are Langmuir and Mackay data, and the horizontal line is Smith and Fite data.
as H2, as indicated by the rapid variation with T, while providing good signals. The pressure-scan data is the most accurate, since other data comes from different times and may be influenced by small drifts in apparent T and in TIMS sensitivity. The pressure-scan data is free of hysteresis but requires a T correction for PH2 > 10 mTorr since VF was fixed and the H2 cools the filament. TH2 decreases by DT as p increases, and the raw data (not shown) has been corrected to 2200 K by adding (dPdiss/dT)DT. As p increases past 20 mTorr, this T-corrected data slowly drops below a constant value of Pdiss. This drop is attributed to a decrease in the rate of H2 collisions with the filament, since the nearby vapor is heated by hot H and H2 from the filament. A calculation of Tvapor one mean-free-path from the filament, using the EC data in Fig. 1 as the energy 1=2 flow to the vapor and nH2 v / T vapor , corrects this data to the line in Fig. 3. (The calculation yields Tvapor = 600 K at 85 mTorr.) While this is only an estimate of this effect, it supports the conclusion that Pdiss is independent of p up to the highest pressures studied (85 mTorr). Summarizing Fig. 3, Pdiss at 2200 K is independent of p from 1 to 85 mTorr. From Fig. 2, one can see that this result also holds over the full range of T shown there (1700– 3000 K). Pdiss at 2200 K from Smith and Fite [6] and from the Langmuir and Mackay low-pressure data [5] is shown in Fig. 3. The latter is independent of p but 40% higher than our data. This could be due to filament T calibration or other factors discussed next. There are a variety of factors that can cause errors in the measured DP(TH2). One is the release of molecules from the wall, directly by filament radiation or by wall heating. These molecules react with and cool the filament, causing an apparent change in Pdiss if their action is altered by adding H2. A related mechanism, famous for rapidly destroying W-filament lights with the ‘‘water cycle’’, is H reduction of tungsten-oxide at the chamber wall, releasing water molecules that oxidize the filament and cause WxOy evaporation. Here we clean the chamber stainless-steel surfaces and expose them to H atoms from the filament (with H2 flow) to reduce W on the walls before taking DP(TH2) and H-atom data. To test for typical vapor impurities in the H2, we verified that Pdiss did not change when the inlet tubing was cooled to 78 K. We also observed hysteresisfree H atom and DP(TH2) data above 1700 K during 30 min, 1000 ! 3000 ! 1000 K temperature scans. However, a(T < 1400 K) was sensitive to the time since high-T cleaning of the filament; it decreased during H2 exposure after high-T operation. This contributes a hysteresis at the lower T in Fig. 1, where the entire up-down temperature-scan data is shown. Apparently the filament surface and a are not unique functions of T below 1400 K. As mentioned above, thermionic currents can cause errors, as they are influenced by H2 vapor at high T and negative V. This causes an apparent contribution to filament resistance and thus T and DP(TH2). This is less severe for positive V and is avoided in the present data by utilizing 833 nm emission to determine T in this region.
W. Zheng, A. Gallagher / Surface Science 600 (2006) 2207–2213
3. Discussion We have compared our data to the Langmuir and Mackay power-change data in Fig. 1. Their vacuum quality was much lower, and establishing T was much harder in 1911. Yet the data is very similar to ours, even including the conduction cooling below 1500 K. Impurities would be most serious at the highest T and might have caused their higher saturation value of Pdiss. The Pdiss data of Smith and Fite, taken at p < 0.2 mTorr and shown in Figs. 2 and 3, is slightly higher than our data, but as it is at very small p, it strongly supports a independent of p. Smith and Fite suggested that an oxygen layer might exist on their W surface at the lower T, but this is not supported by observations reported in Livshits et al. [18]. Most of the difference from our data looks like an 80 K (4%) temperature shift, which is not surprising in view of the difficulty of establishing T at this level of accuracy. Our definition of a, provided above, is different than theirs and yields a(1250 K) ffi 0.20 from their Fig. 5 data. This is about three times the value determined from the DP(TH2) data in Fig. 1. (As noted above, we find that a in this T range is sensitive to the amount of H2 exposure after filament ‘‘cleaning’’ at T = 3000 K.) Brennan and Fletcher [2] measured H2 loss in a closed vessel due to dissociation on the filament followed by H reaction with a molybdenum oxide (MoOx) film on the walls. Earlier experiments of this type, cited by them, obtained a similar dependence on p and T but apparently suffered from contamination of the filament. Brennan and Fletcher observed a very clear (dPH2/dt)/p / p1/2 due to H desorption from the W for T = 1150–1350 K and p = 1–6 mTorr. The above formula defining Pdiss yields (dp/dt)/p / SPdiss, and it is likely that S is independent of PH2 since any dependence normally requires significant h, and h should grow rapidly as T decreases. Thus, observing systematic behavior across this T range implies small h. Consistent with this, the Hickmott parameters (presented below) yield 2% coverage at 1150 K and 6 mTorr. From our analysis of their Fig. 2, we believe Brennan and Fletcher also observe SPdiss / p3/8 for T = 1800 K and p = 0.01– 0.3 mTorr. In Fig. 3, we show their results extrapolated to 2100 K; the p dependence is obviously different than our data. This comparison would look the same at 1800 K, since our H-atom data yields Pdiss / p0 down to 1800 K and has essentially the same T dependence as their data. Thus, this disagreement occurs for data taken at the same T and for essentially the same range of p. (Most of our data is at larger p, but p0 dependence would only be expected at lower pressures than those of [2], according to [4].) This discrepancy cannot be due to H-reaction nonlinearity at their MoOx-coated chamber surface, because this would also influence the T dependence. One possibility is that the H– MoOx reaction releases H2O, which reacts with the W and changes its reactivity. Stobinski and Dus [3] also measured H2 loss in a closed vessel, and they carefully established the linearity of the H-atom reaction with a 78 K gold surface layer (which
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would not produce a volatile product). They report H2 loss on W for T = 1020–1315 K but only at p = 2 mTorr. Their data is consistent with 2.25 eV dissociation activation energy, and they state that their H-atom production agrees with an equation from Langmuir for vapor-phase-equilibrium H2 dissociation. We are uncertain regarding what was actually observed, and dividing their Eq. (2) by the H2-filament collision rate to obtain Pdiss, yields 2–3 times our data in Fig. 2. Hickmott [4] also measured H2 loss due to H reaction with the chamber surface, which was glass that had been baked, then cooled to 77 K. He asserted that the H atoms were well absorbed by this surface prior to reaching a saturation density. By using very low pressures and H2 flows, he thus measured a H2 flow decrease that equaled half the flow of H desorbed from the filament. He also provided a model for the full p and T dependence of H2 dissociation based on second-order H2 desorption. As this nicely demonstrates the relationship between dissociation and H2 desorption order, we will reproduce it here before comparing to his data. The H adsorption rate per filament surface area from H2 vapor is AH = 2S(PH2v/4kTvapor), where v (Tvapor) is the vapor thermal velocity (temperature). We assume that S depends only on T, since h 1 is expected for the conditions of interest here. Since EW–H2 is the activation energy for H2 desorption and EH2 is the vapor–H2 dissociation energy, desorbing to two free H atoms requires EW–H2 þ EH2 and the H-atom binding to W is EW–H ¼ ðEW–H2 þ EH 2 Þ=2. The H-atom desorption rate is kHn, where n is the surface H density and kH = mHexp(EW–H/kT). Assuming independent H atom locations, the H desorption rate as H2 is kH2n2 (from 2kH2(n2/2)), where kH2 = mH2exp([2E W–H EH2]/ kT). This yields, in steady state, AH ¼ k H n þ k H 2 n 2
ð1Þ
from which n ¼ ðk H =2k H2 Þf1 þ ð1 þ CÞ 4AH k H2 =k 2H
1=2
g;
ð2Þ
4AH mH2 fexpðEH2 =kT Þg=m2H .
¼ Two rewith C ¼ leased H represent one dissociation, so this occurs at a rate of kHn/2. Thus, the dissociation probability (Pdiss) per H2–W collision equals (kHn/2)/(AH/S), yielding P diss ¼ 2Sf1 þ ð1 þ CÞ
1=2
g=C:
ð3Þ
The fascinating feature of Eq. (3) is that C, and thus Pdiss, are independent of EW–H. For small C, corresponding to high T and small p, Pdiss ffi S; almost all adsorbed H is desorbed as H and Pdiss is independent of T and PH2. For large C, corresponding to low T and high PH2 with most adsorbed 1=2 H desorbed as H2, P diss ffi 2S=C 1=2 ¼ Sk H =ðAH k H2 Þ / 1=2 2 P H2 expðEH2 =2kT Þ. (Equivalently, AH2 ffi kH2n and Hdesorption = kHn ffi kH(AH/kH2)1/2.) Thus, when Pdiss 1, the dissociation activation energy (Ediss) is 1/2 of the vapor– H2 dissociation energy and is independent of the specific metal as long as EW–H2 > 0. This predicted Pdiss, using Hickmott’s mH and mH2 values, is plotted in Fig. 2 for S = 0.4, with
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W. Zheng, A. Gallagher / Surface Science 600 (2006) 2207–2213
three p chosen to bracket and match our data. EW–H2 ffi 1:4 eV on polycrystalline W [4] and 1.15–1.6 eV on individual planes [8]. The latter has been obtained from H2 desorption studies and an equilibrium-surface-coverage measurement [9]. Using EW–H2 ¼ 1:4 eV yields EW–H = 2.95 eV. As already noted, Ediss ffi EH2/2 = 2.24 eV has been observed in several W studies [2,3,7] as well as in the present measurements, and 5–20% higher values have been observed on tantalum and rhenium [21]. In further agreement with this model, Pdiss / p1/2 in the limit where mostly H2 is released agrees with Brennan and Fletcher’s data and other measurements using MoOx surface layers that are cited there. Thus, a considerable body of data supports Eqs. (1)–(3) and second-order H2 desorption. Hickmott asserts that the above model fits his dissociation data, but we do not agree. His Figs. 7–9 show measured H flux versus p for a range of T between 1170 and 1460 K, and p = 109 106 Torr. The measured H flux fits linear functions of p, with slopes that vary as exp(Ediss/kT) with Ediss = 2.9 eV (his Fig. 10). Since Pdiss / (H flux)/p, this is equivalent to Pdiss independent of p and exponential in 1/T, in agreement with our observations. In contrast, the solution of Hickmott’s Eq. (10) (our Eq. (1)) has one limit (with mostly H desorbed) where DH is linear in p and independent of T, and the other limit (with mostly H2 desorbed) where H flux / p1/2exp(Ediss/ kT). Thus, Hickmott’s measurements, as well as those of Smith and Fite and of Langmuir, agree with our observations and disagree with the form of Eq. (1). We have looked for modifications to the physical model that might explain the dissociation data presented here. A successful model must explain two features that occur in the region where Pdiss 1: (1) Pdiss / exp(Ediss/kT) with Ediss ffi EH2/2 and (2) Pdiss / p0. The Hickmott, secondorder desorption model obtains result (1) but simultaneously 1=2 obtains P diss / AH2 , and as AH should be proportional to p at low h this violates (2). Lifshits et al. [19] provide some strong arguments for Pdiss / p1/2, based on equilibrium relations and some assumptions, and it is useful to consider an equilibrium situation, with the vapor enclosed by the hot W. For T and p that yield small fractional vapor dissociation, the vapor H density is proportional to p1/2, and the H and H2 collision rates with the W are proportional to p1/2 and p. By detailed balance, the H and H2 desorption rates must match their adsorption rates, so if the adsorption probability is large for both specie then H desorption should be proportional to p1/2. Since the H2 collision rate is much larger than for H, this suggests Pdiss / p1/2. An equivalence of surface H resulting from H or H2 adsorption, as well as adsorption probabilities independent of p, are implicit in this argument. Recognizing that some of these assumptions do not hold may be useful. Of course, the experiments also differ in having essentially no H collisions with the W, as well as H2 incident with a temperature far below T. We have not found a convincing explanation that obtains both features but will mention some ideas that have been considered.
(1) Prepairing. As noted in Section 1, on the Si(1 0 0), 2 · 1 reconstructed surface there is an energetic advantage to H atoms residing at adjacent sites, and this yields first-order desorption except at very low h [14]. If such ‘‘prepairing’’ occurred on W, the factor n2/2 in the last term of Eq. (1) would be replaced by n/2. This would yield Pdiss = SkH/(kH + kH2), which is independent of PH2 at all T. At high T where kH2 kH, it yields Pdiss ffi S, as is measured. At lower T, where kH kH2 and Pdiss 1, it yields Pdiss / exp(Ediss/ T) with Ediss ¼ EW–H EW–H2 ¼ ðEH2 EW–H2 Þ=2 ¼ 1:55 eV for EW–H2 ¼ 1:4 eV. This is very far from the measured ED1 = 2.25–2.9 eV, which pretty well rejects this as a possible explanation. (2) Slow diffusion. Another way to obtain Pdiss / p0 is to assume that the H atoms from each H2 adsorption evaporate as H2 before diffusing into a homogeneous distribution on the W surface. The diffusing-pair overlap probability decays as s/t, where s = d2/DH, d is the H–H reaction diameter, and DH is the H surface-diffusion coefficient. If, as is likely, most H are constrained to step edges (on polycrystalline W), the resulting one-dimensional pair overlap decays yet more slowly, as (s/t)1/2. If diffusion is inhibited by a barrier, this might be reasonable, but Daniels and Gomer [10] find diffusion barriers to be 0.15 eV, which is relatively unimportant at T > 1500 K. But the most serious objection to this idea is that it yields the same (incorrect) Ediss as in the previous paragraph; it is just another cause of ‘‘prepairing’’. The discrepancy is even worse if a diffusion barrier is invoked, as this energy subtracts from the Ediss of prepairing. (3) A range of adsorption energies. If EWH is inhomogeneous over the surface, the distribution of H will be as well, and the H–H reaction probability will not vary as n2. If the stronger binding sites are clustered, as might occur at step edges, the h dependence of H2 desorption will be weakened. As a limiting case, H2 release / n0 at small Pdiss occurs on aluminum [16] and is generally attributed to release primarily from small regions where H-saturated surface sites occur. Estimating the effect of various possible binding-site distributions has not been done here. (4) H release during breakup. The direct adsorption of H2 into two adsorbed H atoms is exothermic by ED2 ffi 1.4 eV, while the breakup into one adsorbed and one free H is endothermic by (EH2 ED2)/2 ffi 1.6 eV. Thus, this method of producing a free H does not have the observed activation energy of 2.3 eV. (5) T dependence of S(T). If a significant portion of the T dependence of Pdiss resulted from T dependence of S(T), this would influence the apparent activation energy. We observe S(T > 2000 K) ffi 0.40 and S(1200 K) ffi 0.07, Alnot et al. [19] obtain S = 0.5 from T = 200–1150 K and Butler et al. observe S(300 K) ffi 0.5 [11]. Additional values and limits have been reported, generally with S P 0.1. There are two problems with using S(T) to explain the data: S(T) appears to be a slowly varying function of T, and it would be difficult to obtain the large range of Pdiss values in Fig. 2 or in [7] from only S(T). However, it could alter the above relations between Ediss and the other energies.
W. Zheng, A. Gallagher / Surface Science 600 (2006) 2207–2213
(6) The mathematical indication. One can achieve relations (1) and (2) from Eq. (1) if the last term is multiplied by AH or anything proportional to p. This seems to require a model in which H2 release still requires two adsorbed H atoms in proximity but with an added assist from a diffusing-adsorbed H2 or a hot atom diffusing after a recent H2 adsorption. Since many pairs of adsorbed H reside at close range with insufficient energy to escape as H2, this multibody process might dominate by increasing the available energy for H2 escape. Also, some H–D mixing data has been explained with a ‘‘Eley–Rideal’’ model in which hotatom diffusion on the surface (following H2 adsorption) produces the molecular desorption [20]. This might yield the suggested link between AH and H2 desorption, although in the present case two adsorbed H must be involved. An alternative would be to divide the other two terms of Eq. (1) by p, equivalent to assuming that H adsorption is independent of p and H release is independent of H surface density. Measurements of other observables are needed to support the existence of such unexpected behavior. 4. Conclusions We provide the H2 dissociation probability (Pdiss) on high temperature W for the pressure and temperature region of interest for many applications, particularly ‘‘hot-wire’’ silicon-film deposition. We show that several previous high temperature measurements are consistent with our results, once one recognizes that Pdiss is independent of the H2 pressure. (We show this for H2 pressures (p) below 0.1 Torr but expect it to continue well above this pressure.) Our measured T dependence of Pdiss is also consistent with a 2.25 eV dissociation activation energy on W, in agreement with most prior data. This simplifies calculation of H flux in various applications. However, Pdiss / (p)0 contradicts second-order H2 desorption and the most successful model for this dissociation process. This model makes good physical sense and explains some important features of H2 dissociation on metals, but its predicted
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pressure dependence appears to be a fatal flaw. We discuss several possible modifications to the model but do not find a clearly valid explanation of this unexpected feature of the data. Our results throw major doubt on current understanding of H2 dissociation, accompanied by H release, from metals. Acknowledgements This work was supported in part by the National Renewable Energy Laboratory of the US Department of Energy. We wish to thank A.V. Phelps for many valuable contributions. References [1] Proceedings of the Second International Conference on Cat-CVD (Hot Wire CVD) Process, Thin Solid Films 430 (2003) 1. [2] D. Brennan, P.C. Fletcher, Proc. Roy. Soc. 250 (1959) 389. [3] J. Stobinski, R. Dus, Vacuum 46 (1995) 433. [4] T.W. Hickmott, J. Chem. Phys. 32 (1960) 810. [5] I. Langmuir, G.M.J. Mackay, J. Am. Chem. Soc. 36 (1914) 1708. [6] J.N. Smith Jr., W.L. Fite, J. Chem. Phys. 37 (1962) 898. [7] H. Umemoto, K. Ohara, D. Morita, Y. Nozaki, A. Masuda, H. Matsumura, J. Appl. Phys. 91 (2002) 1650. [8] P.W. Tamm, L.D. Schmidt, J. Chem. Phys. 54 (1971) 4775. [9] T.-U. Nahm, R. Gomer, Surf. Sci. 375 (1997) 281. [10] E.A. Daniels, R. Gomer, Surf. Sci. 336 (1995) 245. [11] D.A. Butler, B.E. Hayden, J.D. Jones, Chem. Phys. Lett. 217 (1994) 423. [12] K. Moritani, M. Okada, M. Nakamura, T. Kasai, Y. Murata, J. Chem. Phys. 115 (2001) 9947 (also references therein). [13] A. Forni, M.C. Desjonqueres, D. Spanjaard, G.F. Tantardini, Surf. Sci. 274 (1992) 161. [14] E.S. Tok, J.R. Engstrom, H. Chuan Kang, J. Chem. Phys. 118 (2003) 3294. [15] R. Frauenfelder, J. Vac. Sci. Technol. 6 (1969) 388. [16] A. Winkler, G. Pozgainer, K.D. Rendulic, Surf. Sci. (251–252) (1991) 886. [17] S.A. Redman, C. Chung, K.N. Rosser, M.N.R. Ashford, Phys. Chem. Chem. Phys. 1 (1999) 1415. [18] A.I. Livshits, F.El. Balghiti, M. Bacal, Plasma Source Sci. Technol. 3 (1994) 465. [19] P. Alnot, A. Cassuto, D.A. King, Surf. Sci. 215 (1989) 29. [20] Th. Kamler, S. Wehner, J. Kuppers, J. Chem. Phys. 109 (1998) 4071.