Optics & Laser Technology 34 (2002) 23–26www.elsevier.com/locate/optlastec

E!ect of polarization on the surface damagemorphology of GaAs singlecrystal during irradiationwith picosecond laser pulses

Amit Pratap Singha, Avinashi Kapoora ; ∗, K.N. Tripathia, G. Ravindra KumarbaDepartment of Electronics Science, University of Delhi, South Campus, Benito Juarez Road, New Delhi 21, India

bDepartment of Nuclear and Atomic Physics, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India

Received 21 May 2001; accepted 11 September 2001

Abstract

A comparative study of damage morphology in GaAs induced by s- , p- and linearly polarized laser light (1:064 �m; 35 ps) is presented.For linearly polarized light damage initiates in the form of pits. This material damage occurs below the surface. For s- or p-polarizedlight material damage involves only the surface layer. For larger 5uences or number of pulses the di!erences are less marked and thedamage morphology occurs in a similar manner either for linearly polarized or s- or p-polarized light. Ripples are formed when multipleirradiation is used due to interference between the front and back faces of the test sample. The spacing of these ripples is 3 �m, which isin good accordance with the reported work of Guosheng et al. (Phys. Rev. B 26 (1982) 5366). c© 2002 Elsevier Science Ltd. All rightsreserved.

Keywords: s-Polarization; p-Polarization; Pits; Damage threshold; Ripples

1. Introduction

Laser-induced damage studies in GaAs have been madeusing a large number of variables. For example, Bertolotti etal. [1] performed their experiments on 〈1 1 1〉 oriented GaAs,polished with >ne abrasive powder using a ruby laser witha pulse duration of 500 �s. They showed that at the damagethreshold 5uence (20 kW cm−2) crack lines were formed.However, craters were formed at higher power densities.Tsu et al. [2] reported the recrystallization of ion-implantedamorphous GaAs surfaces during their damage studies. Theyused a 10 ns pulsed Nd:YAG laser. Meyer et al. [3] theo-retically analyzed the laser-induced damage thresholds forGaAs at 0:69 and 1:06 �m wavelengths. According to theirtheory, in GaAs the free career life time at 300 K is dom-inated by radiation recombination and at higher tempera-tures by Auger recombination. Jong et al. [4] studied dam-age morphology on 〈1 0 0〉 oriented Cr-doped GaAs using aruby laser with 25 ns pulse duration and energy densities of2 J=cm2. They showed the material degradation by the loss

∗ Corresponding author. Tel.: +91-11-688-9750x201.E-mail address: avinashi kapoor@yahoo.com (A. Kapoor).

of large amounts of Ga and As from GaAs single crystal.Sardar et al. [5] presented both single and multiple pulselaser (10 ns; 10 Hz, and 1064 nm) damage of 〈1 0 0〉 sin-gle crystal GaAs surfaces. According to their analysis thedamage shows a defect dominated morphology and accu-mulation behavior, both for heavy etched and light etchedGaAs and for controlled oxide growth material. Kuanr et al.[6] showed the role of surface e!ects in their experiments,which involved 〈1 0 0〉 oriented Si-doped GaAs single crys-tal irradiated by a Nd : glass laser (wavelength 1:06 �m andpulse duration 300 �s). They showed that the laser-induceddamage threshold of lapped GaAs samples were lower thanthat of mirror-polished samples.In earlier work in damage studies of GaAs 〈1 0 0〉 induced

by linearly polarized laser light, in the picosecond regime[7], it was observed that damage was initiated in the formof distinctive pits [7] (see Figs. 1 and 2). Increasing theintensity or number of pulses, resulted in these pits becomingmore prominent and then large surface removal occurred.This type of removal includes the layer beneath the surfaceof GaAs. The present paper continues that line of investiga-tion by considering the e!ect of polarization. We have usedthe same experimental setup in order to obtain good com-parison with the results of that paper.

0030-3992/02/$ - see front matter c© 2002 Elsevier Science Ltd. All rights reserved.PII: S 0030 -3992(01)00087 -1

24 A. Pratap Singh et al. / Optics & Laser Technology 34 (2002) 23–26

Fig. 1. The damaged spot induced by linearly polarized light (single pulseand at damage threshold 5uence).

Fig. 2. The periphery of damaged spot of Fig. 1. Chain of deep pits isvisible in Figure.

2. Experimental details

A damage experiment on well-polished GaAs 〈1 0 0〉single crystal was performed using a frequency dou-bled Q-switched Nd:YAG laser (Continuum PY61C-10,35 ps; 10 Hz and 1064 nm). The sample was mounted ona piezo table. The spot size of the beam was determined bythe knife edge method. The energy incident on the samplecould be adjusted using neutral density >lters. A He–Nelaser beam was used as a probe beam to detect the damage.The probe beam, which was smaller than the spot size, wasfocussed onto the site to be damaged and after re5ectionfrom the sample fell on to the photodetector. The incidenceof damage was determined by the change in re5ectivity ofthe He–Ne laser beam.

Fig. 3. The damaged spot induced by s-polarized light (single pulse andat damage threshold 5uence).

Fig. 4. The magni>ed view of damaged spot of Fig. 3. The bright areais re5ection from the residual metallic Ga.

3. Results

After laser irradiation, the damaged spot was analyzed us-ing a scanning electron microscope. Fig. 3 shows the dam-aged spot induced by a single laser pulse at the threshold5uence (2× 1011 W=cm2) using s-polarized light. The spotwas segmented into bright and dark regions. To unravel themechanism, the central portion of damaged spot was en-larged (Fig. 4). Fig. 4 clari>es that bright and dark regionswere formed shallowly as compared to deep pits, which wereformed using linearly polarized pulses. The bright region isa Ga-rich phase, which was formed due to the preferableevaporation of arsenic [1]; as arsenic has a lower vaporiza-tion temperature in comparison to gallium. It is clear fromFig. 4 that there was not any deep pit formation at the mi-cron level. In the case of linearly polarized light damage

A. Pratap Singh et al. / Optics & Laser Technology 34 (2002) 23–26 25

Fig. 5. The damaged spot induced by p-polarized light (single pulse andat damage threshold 5uence).

Fig. 6. The magni>ed view of damaged spot of Fig. 5.

occurs below the surface. For p-polarized light (Figs. 5 and6) the morphology is very similar as for s-polarized light inthe form of segmented bright and dark regions. Figs. 5 and6 show the damaged spot and a magni>ed view of the cen-tral portion of the spot, respectively, induced by single pulsep-polarized laser light at the threshold 5uence. The damagemorphology for linearly polarized light is shown in Figs. 1and 2. This damage was induced at the threshold 5uence(2 × 1011 W=cm2) using single pulse. Figs. 1 and 2 weretaken from Ref. [7] which show the complete damaged spotand the periphery of the damaged spot, respectively. Fig. 1shows spattering at the central and peripheral portions of thedamaged spot. These spattering are indeed a chain of pitsthat is clearer in Fig. 2. These pits were in a molten form.The formation of pits suggest that the defect initiated dam-age morphology was similar to the results that were shownpreviously using micro [6] or nanosecond laser pulses [5].For larger number of pulses and large power density,

the di!erences were less marked and damage proceeds

Fig. 7. The damaged spot induced by s-polarized light (100 pulse and 2times the threshold 5uence). This feature is very similar to that whichwas reported earlier [7].

Fig. 8. The periphery of the spot of Fig. 7. Ripples of spacing 3 �m(approx.).

approximately in a similar manner as for linearly polarizedlight or s- or p-polarized light (Figs. 7 and 8). Figs. 7 and8 shows the damaged spot and the periphery of the spot,respectively, induced by s-polarized light with 100 pulseswith energy twice the threshold 5uence. Fig. 7 clearlyshows the surface removal. This surface removal and com-plex color (black and white) composition at the centralportion of the damaged spot, is very similar to the resultswhich were found in the case of linearly polarized light for100 pulses at twice the threshold 5uence [7]. Fig. 8 showsthe periphery of the spot shown in Fig. 7. Ripples, whichare in fact melted patterns, were formed in the periphery ofthe damaged spot. A detailed study has been presented byGuosheng et al. [8] to understand the nature of these meltpatterns. The spacing of these ripples is 3 �m (approx.),which is in good accordance with the results of Guoshenget al. [8].

26 A. Pratap Singh et al. / Optics & Laser Technology 34 (2002) 23–26

4. Discussion

Using linearly polarized light [7] damage initiates in theform of distinctive deep pits that are molten. It suggests thatafter laser irradiation, defects move through the crystal lat-tice and settle at grain boundaries. Pit formation shows thatdamage occurs below the surface. There is not any pit forma-tion of micron size in the damaged spot if s- or p-polarizedlight is used. The segmentation of the spot into brighter anddimmer regions that are shallow in comparison to deep pits,suggests that defects were not involved so much initially asonly the Ga-rich phase, due to As evaporation, was formed.It can be said that initially at threshold 5uence for singlepulses, in the case of linearly polarized light, damage alsooccurs below the surface through pit formation. In the caseof s- or p-polarized light, the shallow evaporation of arsenicshows that damage involves only the surface layer.However, for larger 5uences or numbers of pulses, it has

been shown that both thermal and thermo-mechanical mech-anisms are in competition both for linearly polarized lightand s- or p-polarized light. At the periphery the formationof ripples suggests melting while at the central portion thesurface removal and complex color composition (bright anddark) suggests that thermo-mechanical forces dominate. Infact, at >rst the surface melts. The central melts boils o!.The edge melt (which is cooler because of conduction outof the beam) builds up and the center then shows the under-lying damage due to cracking and the resulting loss of heatdi!usion.The formation of ripples has been studied in detail by

Guosheng et al. [8]. The formation of ripples (melt patterns)is a universal phenomenon. According to the Guoshenget al. theoretical model, ripples induced by p-polarized lightshould have two types of spacing given by �=1± sin �, where� is the incident wavelength and � is the angle of incidencemeasured from the normal to the surface. In our experiment� was 45◦ and the spacing was �=1−sin �=3 �m (approx.).The spacing for s-polarized light was the order of wave-length of incident light.On the basis of the above discussion we may conclude that

initially for a single pulse at threshold 5uence the damagemorphology is very sensitive to polarization as for linearlypolarized light the material damage occurs below the surfacedue to the formation of deep pits but for s- or p-polarizedlight, shallow evaporation of arsenic shows that only thesurface layer is damaged. However, on increasing the 5uenceor number of pulses both thermal and thermo-mechanicalmechanisms are in competition. Initial sensitivity of laserlight to polarization may be crucial for many applications.

For example, material degradation is a well-known problemduring the thermal annealing of compound semiconductorslike GaAs. Therefore if we use linearly polarized light, morematerial will degrade in the form of pits in comparison to s-or p-polarized light where only the surface layer degradationis seen.

5. Conclusion

It has been shown using laser pulses of picosecond dura-tion that initially the surface of a GaAs single crystal is sen-sitive to di!erent polarizations. For linearly polarized lightdamage occurs below the surface while for s- or p-polarizedlight damage involves only the surface layer. For higher5uences (here the results for 2 times the threshold 5uencewere compared) the di!erences in the damage morphol-ogy was less marked for various polarization’s. It has beenshown that thermal and thermo-mechanical processes are incompetition.

Acknowledgements

We are grateful to Dr. R.B. Singh, Dr. B.S. Patel,Dr. R.K. Jain, Dr. R.K. Bagai, Dr. S.K. Aggrawal for theircooperation in the primary stage of the work.

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