Investigation of the laserAl2O3(1120) surface interaction using excitation by pairs ofpicosecond laser pulsesAlex V. Hamza, Robert S. Hughes Jr., L. L. Chase, and H. W. H. Lee Citation: Journal of Vacuum Science & Technology B 10, 228 (1992); doi: 10.1116/1.586306 View online: http://dx.doi.org/10.1116/1.586306 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/10/1?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Raman scattering and xray diffractometry studies of epitaxial TiO2 and VO2 thin films and multilayers on αAl2O3(1120) J. Appl. Phys. 73, 2841 (1993); 10.1063/1.353036 Investigation of lasersurface interactions and optical damage mechanisms using excitation by pairs ofpicosecond laser pulses Appl. Phys. Lett. 57, 443 (1990); 10.1063/1.103660 Desorption kinetics and excimer formation of pyrene on Al2O3(1120) J. Chem. Phys. 91, 5778 (1989); 10.1063/1.457530 Infrared resonant desorption of butane from Al2O3(1120): Evidence for an ordered adlayer from vibrational modeselectivity J. Chem. Phys. 90, 3389 (1989); 10.1063/1.455841 Summary Abstract: Interactions and electronic energy transfer between molecules on dielectric surfaces:Phenanthrene on Al2O3(1120) J. Vac. Sci. Technol. A 6, 852 (1988); 10.1116/1.575091

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Investigation of the laser-Al2 0 3 (1120) surface interaction using excitation by pairs of picosecond laser pulses

Alex V. Hamza, Robert S. Hughes, Jr., L. L. Chase, and H. W. H. Lee University of California. Lawrence Livermore National Laboratory. Livermore. California 94550

(Received 15 April 1991 ; accepted 5 September 1991 )

The lifetime of a laser-induced surface excitation on Al20 3 (1120) that leads to detectable emission of neutral or positively charged particles and further to optical surface damage is measured to be a - 200 ps by varying the delay between pairs of 80 ps pulses at 1064 nm. The dependence of the emission threshold fluence on the time delay between laser pulses suggests a low order absorption process.

I. INTRODUCTION

Recently, the laser-induced desorption of aluminum from Al2 0 3 (1120) by 1064 nm photons has been reported.! The observation of high energy (- 7 eV) aluminum ions due to laser irradiation suggested a nonlinear photon absorption process. If surface modification occurs only above a thresh­old fluence, it is not possible to directly observe the depend­ence of the absorption of laser light on laser intensity or f1uence. For such situations Chase and co-workers2 have developed an experimental method to determine whether the laser-surface interaction is linear or nonlinear in the laser intensity and to measure the lifetime of the surface excitation leading to emission of atoms, molecules, and ions produced by the laser irradiation. The experiment involves compari­son of the single laser pulse emission threshold fluence to the threshold for emission induced by a pair of laser pulses sepa­rated by a variable delay. We report here on the results of such an experiment for the interaction of 1.17 eV photons with a Al20 3 (1120) surface.

II. EXPERIMENTAL

The experiment has been described previously2 and is only briefly discussed here. The AI2 0 3 (1120) samples were purchased from Saphikon, Inc., undoped, 99.999% pure, polished with diamond paste to less than 25 nm roughness, and oriented to within 1° as checked by Laue diffraction. The 12.7 X 12.7 X 1 mm samples were placed in ultrahigh vacu­um (base pressure less than I X 10- 9 Torr) and cleaned by Ar + sputtering for one-half hour (I keY beam energy and 27 flA beam current) and annealed to surface temperatures greater than 1300 K. The surface temperature was moni­tored by a chromel-alumel thermocouple held on the surface by a high temperature ceramic adhesive. We have previously shown that this annealing procedure produces clean and or­dered [( 3 Xl) low energy electron diffraction pattern] sur­faces.! However, no in situ surface characterization was available in the present work. A maximum energy of - 12 m] in a 80 ps pulse from a Nd3 + :Y AG regenerative amplifi­er was focused to a 150 flm spot diameter. The angle of inci· dence of the linearly polarized laser light was along the sur­face normal. A UTI quadrupole mass spectrometer (QMS) detected neutral and positively charged particles emitted fol-

lowing laser excitation. The QMS was set to monitor mass­to-charge ratio equal to 27 through the course of this study; however, the QMS was not able to filter the-particles, possi­bly due to their high energy.! Thus, above the the emission threshold, signal was detected at any rf setting on the quad­rupole rods. In addition particles were detected both with the ionizer on and with the ionizer off, suggesting the major­ity of the particles are ions. The bias on the secondary elec­tron mUltiplier and the flight time of the particles indicated that photon and electron emission was not monitored. The ion reference energy, the potential of the grid surrounding the ionizer of the QMS, of + 15 e V limited the sensitivity of the detector to desorbing ions.

The threshold for detectable emission due to a single exci­tation pulse was measured by increasing the laser fluence on a single spot until emission was observed. The emission threshold fluence was defined as the fluence when emission was either constant or increased on successive laser shots. New spots were used for subsequent measurements. The well-defined single pulse emission threshold fluence was 16 ± 4 J/cm2. The error in the absolute fluence is based on the difficulty in measuring the 150 flm 1/ e2 beam diameter with the scanning knife-edge technique. 3 The relative threshold fluence was reproducible to within 10%. The ab­solute threshold fluence for an 80 ps pulse is, within experi­mental error, the same as the value of the visible damage threshold of 12 ± 2 J / cm 2 obtained with an 8 ns pulse.! This is quite a surprising result since the intensity in the 80 ps pulse is two orders of magnitude higher than the 8 ns pulse at the same fluence. This point is discussed further below. The surface temperature rise is estimated to be less than 150 K based on the thermal properties of bulk sapphire, assuming a Gaussian pulse and a surface absorbance of I X 10 - 4 (see Ref. 4 for estimation of surface temperature rise). The value of 1 X 10 - 4 surface absorbance is an upper bound for 1064 nm light based on photothermal deflection measurements of Dreyfus et al. 5 Visible damage was determined by direct observation by a 40 times telescope with the surface illumi­nated by both He-Ne probe light and white light. The visible damage threshold of the laser-exit side of the sample corre­lated with the emission threshold from the laser-incident side of the sample with the 80 ps pulse. The QMS was posi­tioned in line-of-sight of the incident side; therefore, emis-

228 J. Vac. Sci. Technol. B 10 (1), Jan/Feb 1992. 0734-211 X/92/010228-03$01.00 ® 1992 American Vacuum Society 228

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229 Hamza et 81.: Laser-AI20 3(1120) surface Interaction

sion was detectable from the incident side only. Damage on the exit side ofthe sample unfortunately obscured visual ob­servation of the incident side.

The measurement of the detectable emission threshold was repeated using pairs of pulses. The pulses in the pair had nearly equal ftuences and were separated by a variable time delay from 170 ps to 10 ns. The emission threshold was de­termined for the sum of the ftuences in each pulse.

III. RESULTS AND DISCUSSION

The pair of 80 ps pulses gives a threshold for emission as a function of two variables-the ftuence of each individual pulse and the delay between the pulses. In order to interpret the data a simple model is used, based on the following as­sumptions. 2 First, it is assumed that the emission results from an absorption mechanism that is proportional to F n

,

where Fis the laser ftuence to the surface. Such a power law is expected for linear (n = 1) or multiphoton (n> I) ab­sorption. Second, this absorption produces some excitation (electronic, thermal, vibrational, etc.) that decays exponen­tially with a time constant Te. The decay need not be expo­nential, however, e.g., thermal excitations. Any appropriate functional form may be used. An exponential decay is the simplest and is appropriate for low density electronic excita­tions. Third, the instantaneous value of this excitation must exceed a threshold value to cause a detectable emission of particles from the surface. With these assumptions in mind, the threshold value of the excitation for two pulses must be the same as for one pulse. Therefore, the condition at the detectable emission threshold is expressed by setting the sum of the excitation remaining after the first pulse in the pair [(FI )n exp( - The)] plus the excitation from the second pulse in the pair [(F2 )n] equal to the excitation produced with only a single pulse [(l;r],

(FI )n exp( ~e T) + (F2)n = (F)n. (1)

In Eq. I, FI is the ftuence in the first pulse, and F2 is the ftuence in the second pulse at the two-pulse threshold for detectable emission. F is the ftuence necessary to produce a detectable emission in a single pulse, and T is the time delay between pulse pairs. If both pulses have the same ftuence at threshold (FI = F2 ), then we define a ratio, R = 2FI / F, which is equal to

[ ( T)] - lin R ( T) = 2 I + exp ~.. . (2)

The experiment determines FI , F2, and F. In the case where FI and F2 are nearly equal, R is determined from R = (FI + F2 )/F. In thecasewhereFI andF2 are different, R can be determined from the data and Eqs. 1 and 2 for various values of n. In those cases, however, the variation in R on the choice of n (for I <n < 3) was negligible compared with the measured experimental error. Shown in Fig. I is a plot of R versus time delay T between the pulses. The filled squares represent the value of R measured from FI , F2, and F. Fits to the data are shown for n = 1, n = 2, and n = 3. For n = I the best fit yields a value of the exponential decay time for the excitation of 180 ps; for n = 2 the fit yields a decay

J. Vac. Sci. Technol. B, Vol. 10, No.1, Jan/Feb 1992

2.0

1.8 n=3 ~. = sso psec

~ ........ -.- --.... .

t._ ISOpsec

A1,o,(I120)-(3x I)

A.=l064nm puIsc width 80 psec

229

1.0tL.....~-';1-!;;OO:---~~~~~--'--7:tO~OO-~~~~~~-:-':tO~OOO·

Delay, 1:, [psec]

FIG. J. Ratio R of the two pulse detectable emission threshold to the single pulse threshold versus the delay time between the pulses. Filled squares are the measured values. Error bars are 15%, based on statistics of the single and double pulse detectable emission threshold measurements. The line drawn are fitsofEq. (2) to the data. The solid lineis for n = I and T, = 180 pSi the dotted line is for n = 2 and T, = 350 pSi the dashed line is for n = 3 and T, = 550 ps.

time for the excitation of 350 ps; and for n = 3 the fit yields a decay time for the excitation of550 ps. n = 1 gives the best fit to the data; however, the n = 2 fit is within the determined error of the measurements. It should be noted that the fact that the damage thresholdfluences are equal for 80 ps and 8 ns pulses suggests that linear absorption is the dominant in­teraction mechanism.

A number of qualitative observations are made. At long time delays (;;.1 ns) the two incident laser pulses behave as independent pulses, i.e., each pulse must have the required threshold ftuence to induce detectable desorption by itself. Therefore, R = 2 at long time delays. At shorter time delays ( < I ns) the two incident laser pulses are no longer indepen­dent, i.e., the effects of the second laser pulse are affected by the excitations produced by the first laser pulse. Conse­quently, the total ftuence required in the two pulses to induce a detectable desorption signal decreases and R becomes less than 2. As the time delay decreases from I ns to 177 ps the total ftuence, and thus R, continues to decrease. Unfortu­nately, it was not possible to make meaningful double pulse measurements for shorter delay times ( < 177 ps), because the 80 ps laser pulses begin to overlap temporally.

The characteristic decay times of the excitation on the order of a couple hundred picoseconds place limits on the mechanism leading to desorption. The first mechanism to be addressed is simple heating of the surface. The absorption of 1064 nm light by bulk sapphire is negligibly small such that any heating by the incident pulse would involve absorption of the light by an "absorbing or defect" region. Assuming the material around the absorbing region has the same isotropic thermal properties as bulk sapphire, the cooling times of the sapphire around these regions can be calculated as a function of the radius of the absorbing region by using the thermal diffusion equation in spherical coordinates. In order for the cooling time to be the order of 200 ps, the radius of the ab­sorbing region must be less than 300 A. Assuming volume

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230 Hamza tit al: Laser-AI20 3(1120) surface Interaction

heating of the absorbing region and a volume heat capacity of the region similar to bulk sapphire, the absorption coeffi­cient of the region must be - 625 cm - I or greater to produce a temperature rise to 2300 K, the melting point of sapphire. However, the previously observed preferential desorption of aluminum and the desorption energy for the aluminum par­ticles of - 7 eV after excitation with S ns laser pulses I argues strongly against a thermal mechanism, assuming the mecha­nisms for S ns and SO ps pulses are similar.

Excitation lifetimes on the order of a couple hundred pico­seconds are consistent with lifetimes for electronic excita­tions measured in solids. Bokor and co-workers6 have mea­sured the time dependence of the population of the 1T*

surface state and conduction band states on Si ( 111 ) - (2 Xl) with a pump-probe photoemission experiment. Both electronic excitations have lifetimes of a few hundred picoseconds. The ejection of halide atoms due to the decay of self-trapped excitons in alkali halides is perhaps the best studied desorption induced by electronic transitions in insu­lators. Williams and co-workers 7 have used pump-probe ex­periments to measure the time evolution of defects in alkali halides. Based on their pump-probe measurements, esti­mates of the lifetime of self-trapped excitons in alkali halides at room temperature vary from 200 to Sooo ps. Schildbach and Hamza I have suggested that the excitation of an exciton localized on the aluminum in Alz 0 3 may lead to the desorp­tion of Al particles. The measurement of an excitation life­time of a couple hundred picoseconds could be consistent with the decay of the exciton in sapphire.

The low order absorption of 1.17 e V photons must also be reconciled with the high energy of the desorbing aluminum ions (-7 eV).1 In an electronic mechanism, linear or sec­ond order photon absorption requires an initially occupied electronic state 1.17 or 2.33 eV below the conduction band minimum (CBm) for an interband excitation leading to de­sorption. If only excitation of the aluminum exciton is re­quired for desorption then the initially occupied electronic state could lie 2.17 or 3.33 eV below the CBm, since the exciton lies 1.0 eV below the CBm. Indeed, Schildbach and Hamza I have observed electronic states in the band gap of Alz 0 3 (1120) - (12 X 4) with electron energy loss spec­troscopy (ELS). The loss energy of these surface or impurity states is centered at 3.5 eV with approximately 2 eV full width at half-maximum. The loss energies for the surface states are shifted to slightly lower loss energy, centered at 2.7 e V, on the (3 Xl) surface. 8 If these states are occupied and the loss energy is due to transitions to the CBm, then these states lie 2.7 ± 1.0 eV below the CBm. A low order photon absorption and an electronic desorption mechanism sug­gests the initial states for absorption lie high in the band gap (within - 3 e V of the CBm). Filled electronic states high in the band gap would result from a space charge layer, possi­bly due to aluminum interstitials. The surface may contain aluminum adatoms to form the reconstructed surface (see Ref. 1).

A second possibility to reconcile the low order absorption with a high energy excitation is that an intermediate state in

J. Vac. ScI. Technol. B, Vol. 10, No.1, Jan/Feb 1992

230

a resonant enhanced absorption process will become satu­rated. If initially empty surface states 1.17 or 2.33 eV above the valence band maximum (VBM) become saturated, the absorption process can appear to be first or second order in the laser ftuence. Such a scenario to explain the low order absorption also does not require the build up of a space charge layer in order to have filled surface states high in the band gap.

Another possible interpretation of the - 200 ps decay time may be the cooling of a laser-induced plasma. The in­ability to mass select the desorbing moieties in the present study make it uncertain that the photon absorption mecha­nism is the same for both S ns and 80 ps pulses. The first pulse in the two pulse experiment may produce a plasma (elec­trons and ions) at the sample surface. The second pulse can subsequently heat the plasma such that returning particles can damage the surface and leaving particles have sufficient energy to overcome the ion reference energy (see Sec. II)

and be detected by the mass spectrometer. Laser-induced plasma formation has been proposed as a mechanism re­sponsible for visible surface damage. 9 Thus, the threshold for detectable emission could be the result of plasma forma­tion and subsequent plasma heating by the laser pulse. The decay time reported here may be a measure of the plasma decay time.

IV. CONCLUSIONS

Using pairs of picosecond pulses, the interaction of 1064 nm light with an Alz 0 3 (1120) surface was investigated. The order of the absorption process was shown to be low, n = 1 or n = 2. In addition, the lifetime of the excitation leading to detectable emission was - 150-400 ps.

ACKNOWLEDGMENTS

The authors would like to thank the referees for helpful suggestions. This work was performed under the auspices of the Division of Material Sciences, Office of Basic Energy Sciences, U.S. Department of Energy, by Lawrence Liver­more National Laboratory under Contract No. W-7405-ENG-4S.

1 M. A. Schildbach and A. V. Hamza, Phys. Rev. B (submitted). 2 L. L. Chase, H. W. H. Lee, and R. S. Hughes, Jr., App!. Phys. Lett. 57, 443

(1990). J Y. Suzaki and A. Tachibana, Applied Optics 14, 2809 (1975). 4 J. L. Brand and S. M. George, Surf. Sci. 167, 341 (1986). 'R. W. Dreyfus, F. A. McDonald, and R. J. von Gutfeld, J. Vac. Sci. Tech-no!. B 5, 1521 (1987).

6 J. Bokor, Science 246, 1130 and references therein (1989). 7R. T. Williams, Opt. Engin. 28,1024 and reference therein (1989). 8 M. A. Schildbach (unpublished). 9 R. F. Haglund, Jr., N. H. Tolk, and G. W. York, in Laser-Induced Damage

in Optical Materials 1985 (U.S. GPO, Washington, DC, 1985), NBS Spe­cial Publication 746, p. 497.

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