Recrystallization of germanium surfaces by femtosecond laser pulses

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Optics & Laser Technology 35 (2003) 87–97

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Recrystallization of germanium surfaces by femtosecond laser pulsesAmit Pratap Singh, Avinashi Kapoor∗, K.N. Tripathi

Department of Electronics Science, Delhi University, South Campus, Benito Juarez Road, New Delhi-21, India

Received 2 January 2002; received in revised form 20 August 2002; accepted 1 October 2002

Abstract

The damage morphology of germanium surfaces using femtosecond laser pulses of various 1uences and number of pulses is reported.The single pulse damage threshold in the present experiment was 9:7 ± 4:0 × 10−13 W=cm2. The experimental threshold value wascompared with theory, considering the damage threshold as the melting threshold. The cooling rate calculated on the basis of presentresults is 2:4× 1015◦C=s. Recrystallization was the common feature of the damage morphology. For 1uences greater than the single pulsedamage-threshold micropits and spherical grains of micron size were formed in the damaged surface. Ablation (surface removal) was alsoobserved at higher 1uences (at two or three times of damage threshold value). The damage morphology, induced by multiple pulses, wasuna8ected for linear and circular polarization.? 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Recrystallization; Germanium surface; Femtosecond laser pulses; Damage morphology; Ripples; Grains; Linear polarization; Circularpolarization

1. Introduction

The advent of ultrashort laser pulses has revolutionizedthe <eld of laser–matter interaction in recent years [1,2]. Themain interest has been focused on ablation studies [2]. Bychoosing the proper laser parameters such as wavelength,intensity, and pulse duration, a great 1exibility is availableto modify laser ablation to suit speci<c task. The distinc-tive advantages achieved by ultrashort (femtosecond) laserpulses [2] have stimulated the interest in the physical un-derstanding of the mechanisms of short pulse laser ablation.But where the question is of surface morphology of semi-conductor materials following the laser interaction, it can besaid that ablation studies to date are not suBcient as theyhave concentrated primarily on surface removal. The un-derstanding of surface morphology after laser treatment isessential in order to develop a better control and better useof surface structure. Previous laser-induced damage studieshave led to an understanding of the structural transformationin semiconductor surfaces [3,4].This paper reports on the damage morphology of ger-

manium surface 〈1 0 0〉 irradiated with femtosecond laserpulses. Germanium is another technologically importantsemiconductor material after silicon and gallium arsenide.

∗ Corresponding author.E-mail address: [email protected] (A. Kapoor).

Unfortunately after the invention of ultrashort pulse lasers,the <eld of laser–semiconductor interaction has concentratedon silicon and gallium arsenide [2]. However, several exper-iments have been performed on germanium using compar-atively longer duration pulses. Elci et al. have used a pumpand probe technique to study the damage morphology ingermanium surfaces irradiated by 5 ps laser pulses [5]. Theemphasis of the work was on transient processes, which takeplace at the subpicosecond level. They showed that afterexcitation electron densities of 2 × 1020 cm−3 are created.These are the density levels at which screening of energyrelaxation takes place both in direct and indirect band gapsemiconductors and the relaxation extends over a time spanof 100 ps. Meyer et al. arrived at a similar conclusion [3].They presented a theory for laser-induced damage in semi-conductors and compared this with experiments which wereperformed using a Ruby laser (pulse duration: 40 ns, wave-length: 0:69 �m). They showed that when the damage occursin a germanium surface at a power 1uence of 7×106 W=cm2(damage threshold values), carrier densities of the order of1020 cm−3 could be attained. At such high densities the re-laxation occurs through Auger processes. Willis et al. usedscanning electron microscope images to explain the dam-age morphology of germanium surfaces irradiated with mi-crosecond laser pulses [4]. They observed regular moltenpatterns in the form of ripples in the damaged surface ofgermanium.

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88 A.P. Singh et al. / Optics & Laser Technology 35 (2003) 87–97

Time resolved experiments on Si, GaAs, InSb providesuBcient evidence that, in the case of ultrashort laser pulses,phase transitions are so fast that these transitions could notbe explained by thermal processes. Recently the damagestudies on Silicon surface induced by femtosecond laserpulses showed that the damage morphology of silicon sur-faces is sensitive to pulse duration [6]. This fact stimulatedthe present work. In the present study, details of the morpho-logical features of damaged germanium surfaces for various1uences and number of pulses are reported. The e8ects ofdi8erent polarization were also studied.

2. Experimental details

The laser used in the present experiments was a tita-nium:sapphire (Ti:S) system (wavelength 806 nm, pulseduration 110 fs, prf 10 Hz) based on the principle ofchirped pulse ampli<cation [7]. It comprises an oscillator,pulse-stretcher, regenerative ampli<er, multipass ampli<er,and a grating pulse-compressor. The oscillator produces76 MHz pulses in a titanium:sapphire (Ti:S) crystal by Kerrlens mode-locking. The average mode-locked output poweris 450 mW at 90 fs with bandwidth of 20 nm. These pulsesare taken to a folded stretcher con<guration, which tempo-rally stretches the pulse to about 200 ps. The stretched pulseis then coupled into the regenerative ampli<er (RGA). Thelatter is pumped by 80 mJ of 532 nm radiation from a 6 nsNd:YAG laser (SURELITE) operating at a repetition rateof 10 Hz. A Pockels cell selects pulses from the oscillatorpulse train at 10 Hz which get ampli<ed in the RGA from afew nJ per pulse to about 10 mJ after 11 passes in the RGA.The ampli<ed output pulse is switched out by the Pockelscell. Further ampli<cation is achieved by a multipass am-pli<er system pumped by 300 mJ from the Nd:YAG laser,which increases the pulse energy to 80 mJ.

He-Ne Laser

Lens

Beam Splitter

Sample

Ti:Saphhire Neutral Density

Laser System Filter

Photodetector

Energymeter

Fig. 1. Experimental setup.

The 200 ps stretched pulse is <nally compressed to about100–110 fs by a grating pulse compressor. The <nal outputobtained from this laser system is 50–55 mJ per pulse, withpulse duration of 100–110 fs (with a Gaussian 1/e width)at a repetition rate of 10 Hz. The variation in pulse duration(100–110 fs) is due to the dispersion in optical elements inthe beam path. The resulting peak output power is 0:5 TW.The laser pulses were temporally, spatially, and spectrallycharacterized at the output and at various intermediatestages. The TEM00 mode astigmatic laser beam was fo-cused by a 75-cm focal length lens. The sample was placedat a distance of 71 cm from the lens. The beam diameter,determined by a slit scanning technique, was 1:5 ± 0:3 cmat 71 cm from lens (is at the sample position) and was in-dependent of the pulse repetition rate. The incident energyintensity was adjusted on the sample both by incorporatingneutral density <lters in the beam path as well as by plac-ing the samples between lens and its focal point. The slitwas scanned across the beam at all the sample locationsfor the purpose of studying the beam shape and estimatingthe beam size. The He–Ne laser beam was used as a probebeam and focused with the help of 75-cm focal length lens,onto the site (75-cm from the lens (Fig. 1)) to be damaged.This beam had to be focused to a spot with dimensionssmaller than the Ti:sapphire spot size to avoid the e8ectof undamaged surface during the re1ectivity measurements(of He–Ne laser beam) [8]. The re1ectivity of He–Ne laserwas measured few seconds after sample irradiation. In thepresent de<nition of damage threshold (reduction of re1ec-tivity of He–Ne laser beam by more than 10%), the inducedchanges were permanent (irreversible changes). Hence thedamage threshold is de<ned as the value of the energy thatresults in scattering centers, suBcient to reduce the re1ec-tivity by 10%. A schematic diagram of the experimentalsetup of the damage studies is shown in Fig. 1. All theexperiments were performed in ambient air. Well-polished

A.P. Singh et al. / Optics & Laser Technology 35 (2003) 87–97 89

germanium single crystals 〈1 1 0〉 were used as the sam-ple to be damaged. The damaged (induced by femtosecondlaser pulses) samples of germanium surfaces were later ob-served under a scanning electron microscope (LEOS-440PC BASED Digital SEM). The SEM is equipped with astandard Polaroid camera that permits the user to view andtake pictures of the samples immediately.

3. Results

3.1. Damage threshold 7uence

The damage in the germanium surfaces were <rst ob-served at the 1uence of 9:7±4:0×10−13 W=cm2. The samesingle pulse damage threshold value was observed for linearand circular polarization. Fig. 2 shows the complete dam-aged spot of a single pulse at the threshold 1uence usinglinearly polarized light. A thin sharp ring surrounds the dam-aged spot. This ring is made of amorphous germanium [2].In the periphery of the damaged spot the recrystallization ofthe germanium surface is also evident which is clearer forlarger 1uences. Increasing the number of pulses to 100 atthe same 1uence, the central portion of the damaged spot is<lled by micropits and ripples (Figs. 3 and 4). Fig. 4 is amagni<ed view of the central portion of the damaged spotshown in Fig. 3.

3.2. Two times the damage threshold 7uence

Increasing the power 1uence to twice the threshold1uence the same trend in the form of recrystallization for

Fig. 2. The completely damaged spot at threshold 1uence (of single pulse) for 1 pulse using linearly polarized light.

single pulse and in the form of micropits, ripples and re-crystallization for multiple pulses (100 pulse), is followed.The only di8erence was found in the area covered by theabove said patterns. Figs. 5 and 6 show the damaged spotand the periphery of the damaged spot, respectively. Thisdamage was produced using 100 pulses of linearly polar-ized light. To make a suitable comparison in the presentstudy the circularly polarized light was also used. Thedamage, produced by the circularly polarized light usingthe 100 pulses at the threshold 1uence is given in Fig.7. It is clear from Figs. 5–7 that the state of polarizationdoes not a8ect the damage morphology at this level of1uence.

3.3. Three times the damage threshold 7uence

Increasing further the value of 1uence to three times thethreshold 1uence, the morphology for a single pulse is thesame as for threshold 1uence in the form of recrystallization(Figs. 8–14). Now, at this 1uence the complete peripheryturns into the recrystallized form (Figs. 8 and 9) for lin-early polarized light and Figs. 10 and 11 for circularly po-larized light). But for 100 pulses the morphology changesconsiderably in comparison to the threshold 1uence or twicethe threshold 1uence (Figs. 12–14). Spherical grains of fewmicron size are formed in the damaged surface of germa-nium (Fig. 13). Fig. 13, shows the central portion of thedamaged spot (Fig. 12) irradiated with 100 pulses usingcircularly polarized light. Fig. 14 shows the periphery ofthe damaged spot (Fig. 12) showing the redeposited parti-cles, which were ablated (surface removal). Under similar


Fig. 3. The completely damaged spot at threshold 1uence (of single pulse) for 100 pulses using linearly polarized light.

Fig. 4. The magni<ed view of central portion of damaged spot shown in Fig. 3.

conditions the complete damaged spot for linearly polarizedlight is shown in Fig. 15. Ablation (surface removal) wasa common feature for multiple pulses at higher 1uences(two times the damage threshold 1uence and three times thedamage threshold 1uence) (Figs. 6, 9 and 14).

4. Discussion

In the present observation the damage morphologies ofgermanium surfaces were studied. Recrystallization was thecommon feature in the present damage morphology as it


Fig. 5. The completely damaged spot at two times the threshold 1uence (of single pulse) for 100 pulses using linearly polarized light.

Fig. 6. The periphery of the damaged spot shown in Fig. 5.

was observed at threshold 1uence, twice the threshold andat three times the damage threshold 1uence. In the presentobservation the damage threshold was indeed the meltingthreshold and not ablation threshold as the surface is recrys-

tallized and no surface removal has occurred at the thresh-old 1uence. Recent ultrafast X-ray experiment showed thatprocesses that take place in germanium surfaces, near themelting threshold of femtosecond pulses, are thermalized


Fig. 7. The completely damaged spot at two times the threshold 1uence (of single pulse) for 100 pulses using circularly polarized light.

Fig. 8. The completely damaged spot at three times the threshold 1uence (of single pulse) for 1 pulse using linearly polarized light.

processes [9,10]. The pulsed energy is rapidly distributed byhot carrier di8usion and the melting is determined only bythe total absorbed energy.The heating and cooling dynamics were calculated to

determine the structure of the surface layer after the laser

treatment. The calculations were performed including onlyone photon absorption process, excluding the two photonabsorption processes, i.e. assuming that heating occursprimarily over the linear absorption depth [9]. Furtherassumptions were made that parameters such as thermal


Fig. 9. The magni<ed view of periphery of damaged spot shown in Fig. 8.

Fig. 10. The completely damaged spot at three times the threshold 1uence (of single pulse) for 1 pulse using circularly polarized light.

conductivity, speci<c heat per volume, the heat di8usivityare all independent of the temperature [11]. Within the un-certainties of the assumed parameters used in the presentcalculation, the results provide a satisfactory prediction ofthe present SEM results.

Two limiting cases may be distinguished to study thetemperature pro<les (i.e. heating and cooling dynamics)following laser pulse interactions. If the optical absorptiondepth �−1 is small compared to the thermal di8usion length(2Dthtp)1=2, the cooling time is of the same order as tp


Fig. 11. The magni<ed view of the periphery of damaged spot shown in Fig. 10.

Fig. 12. The completely damaged spot at three times the threshold 1uence (of single pulse) for 100 pulses using circularly polarized light.

(laser pulse duration). With Dth the thermal di8usivity is de-<ned as

Dth = k=cv�;

where k is the thermal conductivity, cv the speci<c heat perunit mass, � the mass density.If the optical absorption depth �−1 is large compared to

the thermal di8usion length, the light absorption creates an


Fig. 13. The magni<ed view of central portion of damaged spot shown in Fig. 12.

Fig. 14. The periphery of the damaged spot shown in Fig. 12.

exponential temperature pro<le. The cooling rate may bede<ned as [11]

dT=dt = (1− R)�3 (2IDthtp=cv�);where the R denotes the re1ectivity of germanium surfaceand I is laser power in W=cm2.

In the present case it can easily be seen that condition�(2Dthtp)1=2�1 is indeed satis<ed. The cooling time inthis condition may be estimated to be �−2=2Dth [11]. Thevalues used in the present calculation are arranged inTable 1. The cooling rate was calculated to be 2:4×1015◦C=swhile the cooling time was 1:4 × 10−10 s. Cooling rates


Fig. 15. The completely damaged spot at three times the threshold 1uence (of single pulse) for 100 pulses using linearly polarized light.

Table 1Physical parameters of germanium

Physical parameters Value T (K) References

Thermal conductivity (K) 0.60 (T/300) W=cm K 200¡T ¡ 1214 K [3]Density (g=cm3) 5.32 [3]Speci<c heat c (T ) (J=g K) (0:303 + 0:0184(300=T )) T ¿ 300 K [3]Melting point 1214◦C [3]Thermal di8usion coeBcient (D) 0.35 [3]

faster than 1014◦C=s have never been explored with thehelp of picosecond or longer duration pulses. The structureof the surface layer heat treatment is determined by thecrystal-growth speed and cooling rate. The maximum crys-tallization speed in Ge is about 104 cm=s [12]. Further, ifthe melted layer is supercooled in a short time compared toa characteristic time (scaling as the heated depth divided bycrystallization speed) it is possible that the irradiated spotmight revert to the crystalline phase. In the present casefor a heating depth of 1 �m [10] the condition is ful<lled,matching with the observed recrystallization in Ge surface(Fig. 2).Based on the present assumptions, the laser damage

threshold power required to heat the surface from an initialtemperature T0 to the <nal temperature Tf was calculatedas [3]

P = [�c(T0)RTLh]=[1− R(T0)]tp;where RT is Tf − T0. The quantity Lh is the depthof material heated. Uniform irradiation of semiin<nite

sample was assumed so that the problem could be treated asone-dimensional. The heating depth (linear) was taken as10−4 cm [9]. Using these <gures the theoretical thresholdpower was calculated to be 2:4× 10−12 W=cm2, comparedto the actual value of 9:7± 4:0× 10−13 W=cm2.The di8erences between the theoretical and experimental

power values could be reduced further if the temperaturedependence of the parameters was considered.In the present observation the recrystallization occurred in

the germanium surface at the single pulse threshold 1uencein the form of slices of micron depth with layered structure.In the recrystallized portion the planes were formed oneover the other. This morphology suggests the presence ofdefects in the germanium surface [13]. As the cooling time,in the present observation, is high in comparison to the pulseduration, there should be suBcient time for the atoms toreadjust during and after heat di8usion. It is possible thatduring restructuring of the surface, defects could “climb”out of the slip plane. It is also possible that defects couldbe removed during the heating and cooling processes by


recombination where an extra half-plane above the slip planeand an extra half-plane below the slip plane can annihilateeach other. In either case the level of disorder within a crystalshould be reduced.At threshold 1uence, using 100 laser pulses, the pits are

formed in the Ge surface. Indeed, the use of multiple pulsescreates isothermal conditions and defects are able to moveresulting in the formation of pits, but due to ultrashort natureof the pulse, localization of heat occurs and pits remain ata comparatively small (few micron) size. The localizationof heats also occurred at twice the threshold 1uence (using100 pulses) in the form of micron sized (3–7 �m (approx.))spherical grain.For multiple pulses either for two or three times the thresh-

old 1uence, surface removal (Figs. 5, 11 and 14) was ob-served, though surface removal was absent even at threshold1uence in the case of single pulses. Mass removal is possiblewhen the material undergoes a change of the fundamentalstate of aggregation and transforms into a volatile phase, e.g.,a gas or plasma. In the case of a single pulse, the materialdoes not exceed the melting point even for three times thedamage threshold 1uence as only restructuring in the form ofrecrystallization occurs and no surface removal is observed.Ablation (surface removal) is only possible when the tem-perature of liquid (melt) approaches the critical (vaporiza-tion) temperature. Since ablation involves the displacementof heavy particles, the ablation times are generally ratherlong in comparison to the energy relaxation time requiredto reach the thermal energy distribution, (i.e. a Fermi–Diracand a Bose–Einstein distribution of electron and phonons,respectively); therefore, ablation is expected to be a ther-mal process. However, in the case of ultrashort laser pulseas in the present case, thermodynamically metastable statessuch as superheated liquids and supersaturated vapors maybe formed under highly transient conditions [2].To make the results more generalized, the light with dif-

ferent polarizations (linearly and circularly) was used. Itcan be said on the basis of the present results that polariza-tion does not a8ect the damage morphology for any 1uenceusing multiple pulses. However, a detailed separate studyof polarization dependence, which is beyond the scope ofthe present paper, requires particular attention in the case ofsingle pulse.

5. Conclusion

In the present investigation the damage morphology ofgermanium surface shows recrystallization through ultrafastcooling. The recrystallized portion is further nucleated toform the spherical grain of micron size for multiple pulses.

The formation of micron sized pits for higher than threshold1uence suggests a defect-induced damage morphology. Ithas been proposed that defects would be annihilated throughrecrystallization especially in the case of multiple pulses.

Acknowledgements

We are grateful to Dr. R.B. Singh, Dr. B.S. Patel,Dr. R.K. Jain, Dr. R.K. Bagai, and Dr. S.K. Aggrawal(all Scientists from Defense Research Development Or-ganization, New Delhi, India) for their cooperation in thepreliminary stage of the work. We thank Dr. R.K. Saxenafrom National Physical Laboratory, New Delhi, India, forproviding the SEM facility. Thanks are also due to Dr.G. Ravindra Kumar from Tata Institute of FundamentalResearch for providing the necessary experimental setup.

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Recrystallization of germanium surfaces by femtosecond laser pulses