Electrostatic charging e ects in fast H interactions with thin Ar

Transcription

1 Nuclear Instruments and Methods in Physics Research B 157 (1999) 116±120 Electrostatic charging e ects in fast H interactions with thin Ar lms D.E. Grosjean a, R.A. Baragiola a, *, W.L. Brown b a Laboratory for Atomic and Surface Physics, Engineering Physics, University of Virginia, Thornton Hall, Charlottesville, VA 22901, USA b Bell Labs, Lucent Technologies, Murray Hill, NJ 07974, USA Abstract We have studied electron emission, luminescence and sputtering from thin Ar lms excited by 2 MeV protons. We varied the voltage, V a, of the anode surrounding the target, which extracts electrons from the lm and results in unbalanced positive charges. The resulting large internal electric elds alter sputtering and luminescence. At the beginning of irradiation of a freshly deposited lm, we observe that a positive anode voltage of a few hundred volts produces a large, catastrophic increase in sputtering which gradually disappears as irradiation progresses. We discuss the results in terms of dielectric breakdown induced by macroscopic charging associated with deep charged defect sites. Ó 1999 Elsevier Science B.V. All rights reserved. PACS: Rf; Dy; Jp 1. Introduction * Corresponding author. Tel.: ; fax: ; raul@virginia.edu Charging of insulators is a common problem in ion interactions with insulators. Charging occurs due to the inability to drain away excess positive charges that originate from implanted ions and emitted secondary electrons. Some outstanding questions of this phenomenon are: what conditions determine the degree of charging, what are the charge-trapping sites and how are they distributed in the insulator. Rare-gas solids are good model systems to study charging due to ion irradiation because they are relatively simple, being made up of a single element and they can often be treated as a dense gas, whose properties under irradiation are well known. Trapped positive charge results in an electrostatic potential in the solid that has in uence on other phenomena. The electric eld may separate electrons and holes generated by ionizing collisions, hindering their recombination, and thus lowering the luminescence and sputtering they produce [1,2]. The electrostatic potential has two origins. A microscopic and transient potential is formed along the ionization track produced by the ion, which decays in a time of the order of 0.5 ns in Ar [3], due to recombination. For insulating lms on a metal substrate, there is an additional, X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S X(99)

2 D.E. Grosjean et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 116± position-dependent, decay time due to the migration of unbalanced positive charge to the substrate, where it is neutralized by electron transfer. Unbalanced charges result from the initial loss of energetic electrons into vacuum (electron emission) and into the substrate. The remaining electrons, degrading in energy due to collisions, distribute around the positive charges partially screening them. The unbalanced charges, together with their images, set up strong electric elds in the lms. For solid Ar, the mobility of a hole is so large (1000 cm 2 s 1 V 1 ), that a hole located 30 nm from the substrate, which experiences an initial image eld of 3 kv/cm, will reach the substrate before it can self-trap into an excimer Ar 2 (1±10 ps) [4]. After self-trapping, the hole-mobility drops by nearly ve orders of magnitude but, for the lm thickness typically used (50±200 nm), the time to reach the substrate and quench (in the absence of recombination) is still shorter than 20 ls, for electric elds >100 V/cm. In contrast to these short times, we observe e ects due to large macroscopic potentials with relaxation times of seconds [1,5,6]. These long times then indicate that there exist in the lm less mobile, or ``trapped'', charges that accumulate with irradiation time. For the case of 33 kev H incident on Ar lms, we found that a charge density builds up to a maximum value: r q/cm 2 for the surface charge, and q q/cm 3 for the bulk charge [6], where q is the elementary positive charge. The persistence of these charged traps means that they are tightly bound, possibly at voids or other defect sites, and at the surface. Here we describe experimental manifestations of electrostatic charging of thin lms of solid argon, during irradiation with 2 MeV protons. They include modi cations of electron emission, luminescence, and sputtering, and irradiation dose effects. We observe evidence of dielectric breakdown for lms much thinner than previously reported [5,6]. The experimental methods have been described in detail before [1,2,4]. They involve the growth of pure Ar lms by vapor deposition in ultrahigh vacuum onto a clean Au substrate cooled to 8 K and irradiation with 2 MeV ions. The geometrical arrangement is that of a vacuum diode; the lm is deposited on a grounded substrate and is surrounded by a cylindrical anode that is biased with a variable positive voltage, V a. Typically we record, as a function of anode voltage, the current to the substrate, bulk luminescence from the lm (excimer Ar 2 2Ar transition at 9.8 ev) with an ultraviolet spectrometer and the increase in Ar partial pressure in the target chamber caused by sputtering, with a mass spectrometer. 2. Results and discussion Fig. 1 shows the variation of electron emission, luminescence and sputtering as a function of V a Fig. 1. Variation of sputtering, photon and electron emission yields with anode voltage. Notice the sharp increase of sputtering near 400 V (``catastrophic'' sputtering) and the fading of the breakdown behavior with irradiation dose (``conditioning''). Labels next to the curves indicate doses in units of ions/ cm 2. The voltage was scanned at 25 V/s, in the direction shown by the arrows.

3 118 D.E. Grosjean et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 116±120 for a 75 nm lm irradiated by 2 MeV protons. The saturation of electron emission requires V a larger than 400 V, rather than the few volts needed in the case of the bare metal substrate. The reason for the need of high V a is that a positive potential develops at the beam spot (5 mm dia.) very close to the much larger grounded substrate. This creates, in front of the beam spot on the surface, a potential barrier for electrons that only disappears at large V a [6]. Electrons that cannot overcome the barrier are returned to the sample. By analogy to our studies on a similar apparatus [6], the voltage at which the I±V a curves saturate indicate surface potentials of 60 V. In contrast, experiments with 55 kev proton irradiation of 60 nm Ar lms showed saturation at V a 20 V and beam spot potential less than 1 V [6]. The seemingly contradictory results may result from a number of di erences between the experiments. Low energy protons deposit their charge close to the surface, where they capture an electron. They then travel through the solid mostly as neutral H. This is another scource of excess positive charge that that may be trapped at defect sites at the outer regions of the lm. However, we believe that the most likely reason for the di erence between the experiments is the dissimilar lm growth temperatures. In the experiments reported here, we used 8 K, and therefore expect our lms to contain a signi cantly higher density of defect sites, including pores, compared with those of the kev experiments, where the lms were grown at 20 K. Large electric elds may induce dielectric breakdown in the lm. Evidence for breakdown is the sharp increase in the Ar partial pressure in the chamber when large anode potentials are applied. Fast uctuations or spikes accompany the pressure increase. A possible reason for this increased sputtering, which we term `catastrophic' is the heat spike generated by dielectric breakdown, which may also account for the enhanced sputtering of solid D 2 with high density of trapped charges [7]. For thicker lms, breakdown e ects are stronger and the increase in the sputtering yield can be more than an order of magnitude. We notice that the breakdown behavior is not very reproducible. This observation strengthens the view that defects are important in charging and breakdown, since the quantity and type of defects are likely quite variable for di erent thin lm depositions. Remarkably, the breakdown behavior fades upon prolonged irradiation and eventually disappears (Fig. 1). We call this phenomenon `conditioningõ. The irradiation doses required for conditioning increase with lm thickness and decrease for high de/dx projectiles, such as 2 MeV Ne. The uctuations, or spikes, that accompany the pressure bursts decrease with irradiation dose. We note that the doses required for conditioning only remove of the order of monolayers, thus lm thinning by sputtering is not important. The disappearance of the breakdown behavior suggests a decrease in the density of trapped charges. Therefore, we propose that conditioning is to due to radiation-induced annealing of regions of the lms with high concentration of defects. The hysteresis seen in Fig. 1, and previously reported is related to the time required for the buildup of charged traps, a few seconds at our current densities. The characteristic time in the hysteresis curve is inversely related to the beam current density [1]. As positive charges build up, they increase the electric eld they generate which in turn moves them faster to the substrate, a selflimiting process. The electric eld also redirects electrons that might otherwise reach the substrate to the surface, where they can be ejected even if they have thermal energies, since Ar has a negative electron a nity. The internal electric eld also forces the positive holes to the substrate, where they are neutralized, thereby decreasing the potential of each ionization track. Thus, charging has two opposite e ects: it enhances electron emission by moving electrons inside the lm towards the surface and decreasing the positive potential, and it decreases collection of the emitted electrons by building up a potential barrier in front of the beam spot, for insu ciently large V a. Hysteresis is also seen in luminescence and sputtering because these phenomena depend partially on the availability of electrons. When electrons are extracted, they cannot participate in recombination with holes and thus decrease that part of luminescence and sputtering which is started with a recombination into the Ar 2 excimer [1].

4 D.E. Grosjean et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 116± Fig. 2 shows transient breakdown phenomena induced by switching V a for a partially conditioned 77 nm Ar lm. It can be seen that breakdown does not occur instantly after the anode is switched positive, but after several seconds, consistent with the idea that it takes time to build up the charges and hence the internal electric eld. We note that when the anode voltage is switched to zero, there is a small tail in the electron emission current that decays slowly. In a previous study with low energy ions, we found that electron emission current under breakdown condition will remain a few seconds after the ion beam is interrupted [5]. We attributed this e ect to eld emission from the substrate into (and through) the lm that persists until a su cient fraction of the excess charge in the lm is neutralized by part of the eld emission current. Fig. 3 shows similar data for a 75 nm Ar lm that has been fully conditioned. Charging phenomena are still noticed in the transient in luminescence and, on a smaller scale, in sputtering. However, this charging does not result in breakdown phenomena. We notice that, unlike the case of Fig. 2, when V a is switched to zero (in 0.1 s), the electron current returns abruptly to zero and electrons are able to neutralize the trapped charges very fast, as judged by the fast increase in luminescence. A 10% spike is seen in the sputtering yield but not in bulk luminescence and may be related to surface recombination processes. When V a is then switched from 0 to +500 V (in 0.1 s), the electron current saturates immediately indicating that the elds needed to saturate electron redirection inside the lm are achieved rapidly. Luminescence and sputtering decrease rapidly, Fig. 2. Transients induced by switching the anode voltage, for 2 MeV H on a 77 nm Ar lm that shows breakdown behavior. Vertical scales for the sputtering yield (S), luminescence (L) and electron emission yield (c), are in arbitrary units. Fig. 3. Same as Fig. 2, but for a ``conditioned'' 75 nm Ar lm. The arrows direct to transient behavior or its absence, when switching anode voltage.

5 120 D.E. Grosjean et al. / Nucl. Instr. and Meth. in Phys. Res. B 157 (1999) 116±120 since removal of electrons in the lm means less recombination into Ar 2, a precursor state for these emissions. As electrons leave, deep charged traps accumulate slowly, creating a macroscopic electric eld inside the lm. The increase of both luminescence and sputtering with time in each cycle, with a steady electron emission current, suggests that electrons are replenished in the lm, most likely by eld emission from the substrate. In summary, we have observed new breakdown phenomena and transient e ects in the irradiation of thin Ar lms with 2 MeV ions, which are manifestations of electrostatic charging. The high voltages needed for saturation of electron currents and the hysteresis in the I±V a curves are indicative of macroscopic charging, which we attribute to the population of deep traps at defect sites. In some cases, especially for thick lms, charging can be so severe that dielectric breakdown ensues. This is accompanied by a large increase of the sputtering yield by more than an order of magnitude. The defect sites may be annealed by prolonged irradiation under breakdown conditions, which cause a gradual disappearance of the breakdown behavior. Further accounts of our studies will be presented in a future publication. Acknowledgements We acknowledge support from NSF, Division of Materials Research. References [1] D.E. Grosjean, R.A. Baragiola, W.L. Brown, Phys. Rev. Lett. 74 (1995) [2] D.E. Grosjean, R.A. Baragiola, C. Vidal, W.L. Brown, Phys. Rev. B 56 (1997) [3] S. Kubota, M. Hishida, M. Suzuki, J. Ruan, Phys. Rev. B 20 (1979) [4] D.E. Grosjean, Ph.D. Thesis, University of Virginia, [5] D.E. Grosjean, R.A. Baragiola, in: R.A. Baragiola (Ed.), Ionization of Solids by Heavy Particles, Plenum Press, New York, 1992, p [6] R.A. Baragiola, M. Shi, R.A. Vidal, C.A. Dukes, Phys. Rev. B 58 (1998) [7] B. Thestrup, W. Svendsen, J. Schou, O. Ellegaard, Phys. Rev. Lett. 73 (1994) 1444.