Recrystallization of germanium surfaces by femtosecond laser pulses

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Optics & Laser Technology 35 (2003) 87 – 97 www.elsevier.com/locate/optlastec

Recrystallization of germanium surfaces by femtosecond laser pulses Amit 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 was compared with theory, considering the damage threshold as the melting threshold. The cooling rate calculated on the basis of present results is 2:4 × 1015◦ C=s. Recrystallization was the common feature of the damage morphology. For 1uences greater than the single pulse damage-threshold micropits and spherical grains of micron size were formed in the damaged surface. Ablation (surface removal) was also observed at higher 1uences (at two or three times of damage threshold value). The damage morphology, induced by multiple pulses, was una8ected 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; Circular polarization

1. Introduction The advent of ultrashort laser pulses has revolutionized the
Corresponding author. E-mail address: [email protected] (A. Kapoor).

Unfortunately after the invention of ultrashort pulse lasers, the
0030-3992/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 0 - 3 9 9 2 ( 0 2 ) 0 0 1 4 6 - 9

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

Time resolved experiments on Si, GaAs, InSb provide suBcient evidence that, in the case of ultrashort laser pulses, phase transitions are so fast that these transitions could not be explained by thermal processes. Recently the damage studies on Silicon surface induced by femtosecond laser pulses showed that the damage morphology of silicon surfaces is sensitive to pulse duration [6]. This fact stimulated the present work. In the present study, details of the morphological features of damaged germanium surfaces for various 1uences and number of pulses are reported. The e8ects of di8erent polarization were also studied.

The 200 ps stretched pulse is
2. Experimental details The laser used in the present experiments was a titanium:sapphire (Ti:S) system (wavelength 806 nm, pulse duration 110 fs, prf 10 Hz) based on the principle of chirped pulse ampli
He-Ne Laser

Lens Beam Splitter Sample

Ti:Saphhire Laser System

Neutral Density Filter Photodetector Energymeter

Fig. 1. Experimental setup.

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

germanium single crystals
1 1 0 were used as the sample to be damaged. The damaged (induced by femtosecond laser pulses) samples of germanium surfaces were later observed under a scanning electron microscope (LEOS-440 PC BASED Digital SEM). The SEM is equipped with a standard Polaroid camera that permits the user to view and take pictures of the samples immediately. 3. Results 3.1. Damage threshold 7uence The damage in the germanium surfaces were
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single pulse and in the form of micropits, ripples and recrystallization for multiple pulses (100 pulse), is followed. The only di8erence was found in the area covered by the above said patterns. Figs. 5 and 6 show the damaged spot and the periphery of the damaged spot, respectively. This damage was produced using 100 pulses of linearly polarized light. To make a suitable comparison in the present study the circularly polarized light was also used. The damage, produced by the circularly polarized light using the 100 pulses at the threshold 1uence is given in Fig. 7. It is clear from Figs. 5–7 that the state of polarization does not a8ect the damage morphology at this level of 1uence. 3.3. Three times the damage threshold 7uence Increasing further the value of 1uence to three times the threshold 1uence, the morphology for a single pulse is the same as for threshold 1uence in the form of recrystallization (Figs. 8–14). Now, at this 1uence the complete periphery turns into the recrystallized form (Figs. 8 and 9) for linearly polarized light and Figs. 10 and 11 for circularly polarized light). But for 100 pulses the morphology changes considerably in comparison to the threshold 1uence or twice the threshold 1uence (Figs. 12–14). Spherical grains of few micron size are formed in the damaged surface of germanium (Fig. 13). Fig. 13, shows the central portion of the damaged spot (Fig. 12) irradiated with 100 pulses using circularly polarized light. Fig. 14 shows the periphery of the damaged spot (Fig. 12) showing the redeposited particles, which were ablated (surface removal). Under similar

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

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

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

Fig. 4. The magni
conditions the complete damaged spot for linearly polarized light is shown in Fig. 15. Ablation (surface removal) was a common feature for multiple pulses at higher 1uences (two times the damage threshold 1uence and three times the damage threshold 1uence) (Figs. 6, 9 and 14).

4. Discussion In the present observation the damage morphologies of germanium surfaces were studied. Recrystallization was the common feature in the present damage morphology as it

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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 and at three times the damage threshold 1uence. In the present observation the damage threshold was indeed the melting threshold and not ablation threshold as the surface is recrys-

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

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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 by hot carrier di8usion and the melting is determined only by the 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 only one photon absorption process, excluding the two photon absorption processes, i.e. assuming that heating occurs primarily over the linear absorption depth [9]. Further assumptions were made that parameters such as thermal

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

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Fig. 9. The magni
Fig. 10. The completely damaged spot at three times the threshold 1uence (of single pulse) for 1 pulse using circularly polarized light.

conductivity, speci
Two limiting cases may be distinguished to study the temperature pro
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A.P. Singh et al. / Optics & Laser Technology 35 (2003) 87 – 97

Fig. 11. The magni
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
where k is the thermal conductivity, cv the speci
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Fig. 13. The magni
Fig. 14. The periphery of the damaged spot shown in Fig. 12.

exponential temperature pro
In the present case it can easily be seen that condition (2Dth tp )1=2 1 is indeed satis
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A.P. Singh et al. / Optics & Laser Technology 35 (2003) 87 – 97

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

Physical parameters

Value

T (K)

References

Thermal conductivity (K) Density (g=cm3 ) Speci
0.60 (T/300) W=cm K 5.32 (0:303 + 0:0184(300=T )) 1214◦ C 0.35

200 ¡ T ¡ 1214 K

[3] [3] [3] [3] [3]

faster than 1014◦ C=s have never been explored with the help of picosecond or longer duration pulses. The structure of the surface layer heat treatment is determined by the crystal-growth speed and cooling rate. The maximum crystallization speed in Ge is about 104 cm=s [12]. Further, if the melted layer is supercooled in a short time compared to a characteristic time (scaling as the heated depth divided by crystallization speed) it is possible that the irradiated spot might revert to the crystalline phase. In the present case for a heating depth of 1 m [10] the condition is ful
T ¿ 300 K

sample was assumed so that the problem could be treated as one-dimensional. The heating depth (linear) was taken as 10−4 cm [9]. Using these
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recombination where an extra half-plane above the slip plane and an extra half-plane below the slip plane can annihilate each other. In either case the level of disorder within a crystal should be reduced. At threshold 1uence, using 100 laser pulses, the pits are formed in the Ge surface. Indeed, the use of multiple pulses creates isothermal conditions and defects are able to move resulting in the formation of pits, but due to ultrashort nature of the pulse, localization of heat occurs and pits remain at a comparatively small (few micron) size. The localization of heats also occurred at twice the threshold 1uence (using 100 pulses) in the form of micron sized (3–7 m (approx.)) spherical grain. For multiple pulses either for two or three times the threshold 1uence, surface removal (Figs. 5, 11 and 14) was observed, though surface removal was absent even at threshold 1uence in the case of single pulses. Mass removal is possible when the material undergoes a change of the fundamental state of aggregation and transforms into a volatile phase, e.g., a gas or plasma. In the case of a single pulse, the material does not exceed the melting point even for three times the damage threshold 1uence as only restructuring in the form of recrystallization occurs and no surface removal is observed. Ablation (surface removal) is only possible when the temperature of liquid (melt) approaches the critical (vaporization) temperature. Since ablation involves the displacement of heavy particles, the ablation times are generally rather long in comparison to the energy relaxation time required to reach the thermal energy distribution, (i.e. a Fermi–Dirac and a Bose–Einstein distribution of electron and phonons, respectively); therefore, ablation is expected to be a thermal process. However, in the case of ultrashort laser pulse as in the present case, thermodynamically metastable states such as superheated liquids and supersaturated vapors may be formed under highly transient conditions [2]. To make the results more generalized, the light with different polarizations (linearly and circularly) was used. It can be said on the basis of the present results that polarization does not a8ect the damage morphology for any 1uence using multiple pulses. However, a detailed separate study of polarization dependence, which is beyond the scope of the present paper, requires particular attention in the case of single pulse. 5. Conclusion In the present investigation the damage morphology of germanium surface shows recrystallization through ultrafast cooling. The recrystallized portion is further nucleated to form the spherical grain of micron size for multiple pulses.

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The formation of micron sized pits for higher than threshold 1uence suggests a defect-induced damage morphology. It has been proposed that defects would be annihilated through recrystallization 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 Organization, New Delhi, India) for their cooperation in the preliminary stage of the work. We thank Dr. R.K. Saxena from National Physical Laboratory, New Delhi, India, for providing the SEM facility. Thanks are also due to Dr. G. Ravindra Kumar from Tata Institute of Fundamental Research for providing the necessary experimental setup. References [1] Zewail A. Femtosecond chemistry, vols. 1 and 2. Heidelberg: Verlag Chemie, 1995. [2] Sokolowski-Tinten K, Bialkowski J, Cavalleri van der Linde D, Oparin A, Meyer-ter Vehn J, Anisimov SI. Transient states of matter during short pulse laser ablation. Phys Rev Lett 1998;81(1):224–7. [3] Meyer JR, Kruer MP, Bartoli FJ. Optical heating in semiconductors: Laser damage in Ge, Si, InSb and GaAs. J Appl Phys 1980;51:5513 –22. [4] Willis LJ, Emmony DC. Laser damage in germanium. Opt Laser Technol 1975;222–8. [5] Elci, Ahmet, Scully O, Marlan, Smirl L, Arthur, Matter C, John, Ultrafast transient response of solid-state plasmas. I. Germanium, theory and experiment. Phys Rev B 1977;16(1):191–221. [6] Singh, Pratap A, Kapoor, Avinashi, Tripathi KN, Kumar, Ravindra G. Laser damage studies of silicon surfaces using ultra-short laser pulse. Opt Laser Technol 2002;34/1:37– 43. [7] Stricklend, Donna, Mourou, Gerard. Compression of ampli