Ion-induced electrostatic charging of ice

Nuclear Instruments and Methods in Physics Research B 268 (2010) 2888–2891

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Ion-induced electrostatic charging of ice J. Shi, M. Famá, B.D. Teolis 1, R.A. Baragiola * University of Virginia, Laboratory for Atomic and Surface Physics, Charlottesville, VA 22904, USA

a r t i c l e

i n f o

Article history: Received 9 October 2009 Received in revised form 30 March 2010 Available online 24 April 2010 Keywords: Electrostatic charging Ion irradiation Ice

a b s t r a c t We studied electrostatic charging on amorphous ice films induced by the impact of 100 keV Ar+ ions at 45° incidence. We derived the positive surface electrostatic potential from the kinetic energy of sputtered molecular ions. Measurements were performed as a function of film thickness, ion flux and accumulated fluence. The main results are (a) films charge up to a saturation value, following an exponential time dependence. (b) The time constant for charging is approximately proportional to the reciprocal of the ion flux. (c) The maximum surface voltage depends on film thickness and ion flux. (d) Charging does not occur for films thinner than the maximum range of projectile. (e) Dielectric breakdown is observed for surface potentials above 100 V. We explain the measurements with a model in which charges can drift into the substrate or be trapped temporarily near the ionization range of the projectiles. A charge can be released from the trap by the electric field produced by a nearby charge injected by subsequent projectiles. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Electrostatic charging of dielectrics by ion impact results from trapping of charges implanted by the projectile or resulting from the emission of secondary electrons and, to a much smaller extent, secondary ions. The dielectric can discharge partially or totally if it is in contact with a conducting surface or by dilution of the charge concentration by diffusion and drift through a thick dielectric. The charging process starts with the creation of electron/hole pairs from the ionization collisions by the projectile and the energetic electrons they produce. Electrons and holes that do not immediately recombine may be trapped at localized defects, contributing to charging, or can transfer to a conducting electrode (substrate), leaving an excess of charges with opposite polarity in the film. A positive potential on the surface may, in turn, inhibit electron emission depending on the presence of external electric fields. The electric field produced by the trapped charges is limited to the value of the dielectric strength. Our motivation is to understand and predict the electrostatic charging of objects in space, in particular icy objects. Water ice is abundant on the surfaces of many objects in the outer solar system, where it is exposed to UV photons, solar wind, cosmic rays and energetic charged particles trapped by the planetary magnetosphere. Radiation effects induced by energetic ions in ice have been recently reviewed [1,2]. Later work includes radiation chemistry (molecular decomposition and synthesis) [3], sputtering [4,5], sec* Corresponding author. Tel.: +1 434 982 2907; fax: +1 434 924 1353. E-mail address: [email protected] (R.A. Baragiola). 1 Present address: Southwest Research Institute, San Antonio, TX 78227, USA. 0168-583X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2010.04.013

ondary ion emission [6,7] amorphization of crystalline ice [8], and compaction of microporous amorphous ice [9,10]. Surface electric fields in icy planetary objects are unknown since no space mission has landed on them. Their importance lies in their ability to deflect and even reflect magnetospheric particles from reaching the surface of the icy satellites. A full study of electrostatic charging of ice must include the hypothesis of a net electrical polarization of ice during condensation [11,12]. However, considering the magnitude of the effects discussed in this paper, should ferroelectricity exist in solar ices, it could be masked by charging effects induced by magnetospheric energetic particles or UV irradiation. In addition to their astronomical importance, water ice and other condensed gases are a convenient group of insulators to study charging effects, since thin films can be easily deposited from the vapor and be removed by moderate heating. Electrostatic charging effects in solidified noble gases caused by ion bombardment have been previously studied in our laboratory [13–15]. Here we report on experimental studies of charging effects on ice induced by 100 keV Ar+ ions. The electrical potential of the surface was derived from measurements of the energies of ejected secondary ions, and was measured as a function of film thickness, ion energy, current flux and integrated fluences.

2. Experimental details The experiments were conducted in a cryopumped, ultra-high vacuum chamber with a base pressure of 1010 Torr. Using a capillary array doser, ice films of pure degassed water were vapor

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1.0

Normalized Vs

0.8

200

150

For films of thicknesses from 1000 to 2900 ML, the film voltage increased monotonically with ion fluence and saturated for F above 1.8  1013 Ar+ cm2, to a value that increases with film thickness, as shown in Fig. 1 (bottom). In the top panel of this figure, we show the charging curves normalized to their respective saturation values. The fit shown in this figure corresponds to the exponential function:



V s ðtÞ t ¼ 1  exp  V s ð1Þ s



1

100

200

300 400 Time (s)

600

700

1000

100

10

1 0.1

1

10 10

ð2Þ

500

Fig. 1. Bottom panel: charging curves for ice films thicknesses between 1000 ML and 3000 ML, grown and irradiated at 80 K with 100 keV Ar ions at an ion flux j = 3.1  1010 cm2 s1. Top panel: normalized charging curves fitted with Eq. (1).

100 +

1000

2

Flux (x10 Ar /cm s)

ð1Þ

where Vs(1) is the film voltage at saturation fluence, and s a time constant, which describes the charging process. In this case, for a flux of 3.1  1010 ions cm2 s1, the average s is 150 s with a spread of ±30 s for different film thickness. For films thinner than 600 ML, the secondary ions energy remains constant at 5 eV even at high fluences, indicating that these films do not charge. We tested the dependence of the charging time constant s with the ion flux j in similar conditions as in the experiments shown in Fig. 1. The dependence, as shown in Fig. 2, follows a behavior which we approximate here as:

sr

1030 ML 1230 ML 1440 ML 1640 ML 2050 ML 2460 ML 2870 ML

100

0 0

Charging Time Constant (s)

3.1. Charging

¼ rj þ

0.4

50

The mass spectrum of positive secondary ions ejected by 100 keV Ar+ impact on a 1250 ML thick ice film at 80 K shows peaks for the ionized water molecule H2O+, the dissociation prodþ þ þ + + ucts H+, Hþ 2 , O , OH , new species such as H3 , O2 , and H2 O2 , and + the protonated water cluster ions (H2O)nH . Energy scans were done for H3O+ ions which, though not as intense as H+, have less kinetic energy. We propose that the energy of the sputtered ions Ep results from an intrinsic sputtered energy E0 plus the energy gained by acceleration from the surface at a potential, Vs to the spectrometer at ground, plus a work function correction, which is neglected here. Thus, a measurement of Ep gives the surface potential Vs = (Ep  E0)/e. In the following, we report our results of the evolution of Vs with irradiation time, ion flux, ion fluence and film thickness.

s

0.6

0.0

3. Results

1

1030 ML 1230 ML 1440 ML 1640 ML 2050 ML 2460 ML 2870 ML

0.2

Vs (volt)

deposited at 80 K onto a cooled, gold-coated, quartz crystal resonator microbalance. By measuring the resonant frequency of the crystal, which is proportional to the deposited mass per unit area, we determined the column density g of the films with a sensitivity of 0.04 ML [16] (we define 1 ML = 1015 molecules cm2 or approximately a surface monolayer). The values of g ranged from 100 to 20,000 ML. Using thin film optical interference spectroscopy (250–700 nm) we precisely determined the film thickness z and the density (g/z). Unless otherwise indicated, all the experiments reported here were done with the samples grown and irradiated at 80 K. Ion beams from a mass analyzed ion accelerator were scanned uniformly and entered the sample at a 45° incident angle. We chose Ar+ ions as projectiles because they produced a relatively large secondary ion signal that allowed fast measurements of ion energy distributions. We measured the ion beam flux, j with a FarR aday cup, and derived the fluence, F ¼ jdt. Secondary ions emitted normal from the surface were detected with a Hiden EQS 300 sector field electrostatic energy analyzer and quadrupole mass spectrometer located 3.8 cm from the surface of the microbalance. The Hiden EQS 300 is capable of being operated at a mass resolution of 0.01 amu and an energy resolution of 0.05 eV.

Fig. 2. Dependence of the charging time constant s with ion flux. The dashed line is a fit with Eq. (2).

with r = 1.6 ± 0.2  1013 cm2 and sr = (900 ± 300) s. A description of these constants is given in the Section 4. 3.2. Thickness dependence We measured the thickness dependence of the voltage for saturation fluence using films deposited at different thicknesses, and for a 2450 ML film being thinned by sputtering with the ion beam. Fig. 3 shows that for films thinner than 600 ML, the kinetic energy of the emitted ions is approximately constant, 5 ± 1 eV, which we interpret as the intrinsic energy of sputtered H3O+. For thicker

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J. Shi et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2888–2891

40

10

Peak Energy (eV)

ionization region

180

20 150 120

10 90 1800

100

Vs (V)

30

+

Peak H3O Energy (eV)

100

1000 Thickness (ML)

2000 2200 Thickness (ML)

2400

10000

Fig. 3. Peak energy of H3O+ secondary ions vs. film thickness measured at saturation fluence. A thickness of 1000 ML corresponds to 318 nm using a density of 0.94 g/cm3 for compact ice. The symbols corresponds to ice films grown at 80 K to different thicknesses and irradiated at 80 K with a flux of 7.8  1011 cm2 s1 100 keV Ar+. The continuous line and the detail in the inset show measurements on a 2450 ML film as it is eroded by the ion beam due to sputtering. The abrupt drops in the H3O+ energy are interpreted as evidence of dielectric breakdown. The dashed line indicates a linear thickness dependence.

films, Ep increases linearly with thickness and saturates at thicknesses larger than 3000 ML when it reaches 200 V. Above 1800 ML, Ep shows an erratic behavior at high fluences (Fig. 3, inset), with transient discharging (drops in Ep) that we interpret as dielectric breakdown. At the start of breakdown, the ratio of voltage to thickness corresponds to an internal electric field of at least 1.9 MV/cm, the precise value depending on the depth distribution of the trapped charges. We also indicate in Fig. 3 the maximum ionization depth of the projectiles obtained with a TRIM Monte Carlo simulation [17]. 3.3. Initial surface potential We examined the charging curves of 2500 ML amorphous ice films grown between 15 K and 80 K, using an ion flux of 3.1  1010 cm2 s1. Strikingly, the signal of secondary ions did not appear initially but required an incubation time. However, in another experiment where the substrate was biased at +15 V, the secondary ions appeared as soon as the ion beam hit the surface. For a grounded substrate, extrapolation of Vs to zero fluence gives an averaged negative surface potential of 4 ± 1 V, at 80 K. This is of the order of the value of approximately 3 V reported for unirradiated ice films of similar thickness, and attributed to ferroelectricity [11]. We assume that such behavior disappears during irradiation as the orientation of the water dipoles is scrambled, but this will be the subject of further studies. 3.4. Flux dependence After observing a dependence of the results on the ion beam current, we studied the flux dependence of the saturation voltage for 1250 ML films. The results are displayed in Fig. 4 and will be discussed below. 3.5. Summary of findings

 At a fixed flux. The charging rate is independent of film thickness.  The charging time constant is approximately proportional to the reciprocal of the ion flux.

0

0

25

50

75 10

100 +

125

150

2

Flux (10 Ar /cm s) Fig. 4. Surface potential vs. flux of 100 keV Ar+ for an 1250 ML ice film grown and irradiated at 80 K. The line is a fit to Eq. 7.

 Films thinner than the range of projectiles do not charge.  The maximum voltage (at saturation fluences) is proportional to film thickness.  The maximum voltage depends on the ion flux.  Dielectric breakdown appears at surface voltages larger than about 100 V. 4. Discussion Ideally, one would desire to develop a physical model that simultaneously explains all these observations. A simple, preliminary model is presented below. The physical aspects of the model are as follows. At low fluences, charges trap and accumulate in the film due to the implanted ions and ejected secondary electrons. When the charge build up produces a surface potential of several volts, most of emitted secondary electrons return to the surface. Then, the growth rate of the trapped charges is determined by the ion flux j minus the leakage of charges to the substrate where they recombine with electrons tunneling from the valence band of the metal. For projectiles with the energies used in this work, electron capture cross sections are large, 1015 cm2, and therefore the ion charge is initially deposited within a few Å of the surface. With multiply charged projectiles, or at higher velocities, multiple capture processes need to be considered. The ion produced by electron capture joins the thousands of other ions and electrons in the ionization track (the mean energy to create an electron–ion pair is 27 eV). The evolution of the charges and their images induced on the substrate is a very complex and yet unsolved problem but occurs much faster than our measurement times. It is important to note that, before neutralization, which takes several ls [18], the electron cloud in the track will adopt a spatial distribution that minimizes the total energy; this means that the extra positive charge will be at the end of the ionization range rather than close to the surface, where it was deposited. After the track neutralizes, the extra positive charge may be trapped at defects or may drift to the substrate. The amount of charge per unit area, q, determines an electric field (given by Gauss law) E = q/e, which is limited by the dielectric strength of the material EM. Therefore the maximum charge per unit area that can be injected is limited to qM = eEM. It is important to point out that a negligible amount of charge is trapped at the surface, evidenced by the fact that films thinner than 600 ML do not charge (Fig. 3). Trapped charges will be released with a characteristic time constant by thermal fluctuations assisted by the average electric field

J. Shi et al. / Nuclear Instruments and Methods in Physics Research B 268 (2010) 2888–2891

(Poole–Frenkel mechanism). Detrapping by the average electric field appears unimportant in our experiments, since the charging time constant does not depend on fluence. Detrapping can occur by the local electric field produced by another charge introduced by the projectile. This mechanism can be described by a probability that an incident ion produces a charge within a critical radius rc of the trapped charge, and hence an electric field eðer 2c Þ1 larger than a critical detrapping field that is of the order of the average dielectric strength (e is the elementary + charge). Calling r ¼ pr2c the detrapping cross section, the collisional lifetime is then given by:

sc ¼

1

ð3Þ

rj

Charges that are not trapped are conducted through the insulator with the characteristic dielectric relaxation that was given above as sr = 900 ± 300 s. A previous estimate gives a wide range, 100–1000 s for low-density amorphous ice at 130 K [19]. We now put this simple picture in a quantitative framework. The concentration of trapped charges per unit area q can be described by a differential equation with two terms, the injection of charges is given by the flux of projectiles j and the loss of charges by leakage to the substrate with a time constant s:

dq q ¼j dt s

ð4Þ

As described by Eq. (2), we take s to be given by s1 ¼ rj þ s1 r , which has the correct asymptotic solution, s = sr for j ? 0 and s = (rj)1 in the high flux limit. Replacing s in Eq. (4) and given that the fluence F = jt, we solve for q with the initial condition q(0) = 0 which yields:

qðFÞ ¼

e

r þ ðsr jÞ

1

½1  expðFðr þ ðsr jÞ1 Þ

eLe1

r þ ðsr jÞ1

½1  expðFðr þ ðsr jÞ1 Þ

The saturation voltage is then given by:

V s ð1Þ ¼

eLe1

r þ ðsr jÞ1

ð7Þ

which fits the experimental flux dependence very well, as shown in Fig. 4. We note the limits Vs (F; j ? 0) = 0, since the charges move to the substrate too to accumulate and V s ðF; j ! 1Þ ¼ reLe where the trapped charges achieve the maximum value allowed by the dielectric strength, before they can detrap and drift into the substrate. 5. Conclusions We have presented different experiments done on ion-induced electrostatic charging of amorphous ice films grown on a metal surface, and advanced a simple model that accounts for most of the observations. Further work is needed to account for the flux dependence of the time constant and of the saturation voltage and, in general, the dependence of the results on ion type and energy and the type of substrate. The application of the results to icy surfaces in the solar system need to consider multiple aspects, such as the near simultaneous bombardment with energetic electrons (which have a deeper range), the energy distribution of incoming particles, and the nature and location of other materials. Acknowledgment This research was supported by grant AST0807830 from the National Science Foundation. References

ð5Þ

By Gauss theorem, the charges create an electric field q/e and a potential:

V s ðFÞ ¼

2891

ð6Þ

L is the average distance between the trapped charge (ionization range) and the substrate, and e is the elementary charge. Eq. (6) fits very well the experimental results for charging (Fig. 1) since it is analogous to Eq. (1). It also agrees with some of the observations listed above such as the linear relation between saturation voltage and film thickness for thicknesses larger than the penetration depth of Ar ions ice but smaller than those that would induce dielectric breakdown due to intense internal electric field created.

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

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