Thermal and mechanical damage of GaAs in picosecond regime

Optics & Laser Technology 33 (2001) 363–369www.elsevier.com/locate/optlastec

Thermal and mechanical damage of GaAs in picosecond regimeAmit Pratap Singha, Avinashi Kapoora ;∗, K.N. Tripathia, G. Rvindra Kumarb

aDepartment of Electronics Science, University of Delhi, South Campus, Benito Juarez Road, New Delhi-21, IndiabDepartment of Nuclear and Atomic Physics, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai-400 005, India

Received 6 February 2001; accepted 25 April 2001

Abstract

An in-depth study of the single pulse and multiple pulse laser (35 ps; 10 Hz and 1064 nm) damage for threshold 4uence and greater4uence of GaAs 〈1 0 0〉 single crystal is presented. Damage which starts at a power 2×1011 W=cm2 in the form of pits occurs due toaccumulation of laser induced microscopic defects. E8ect of multiple pulse at 9rst makes the pits more prominent in the form of Gaemission. Then the topmost layer is removed. If the number of pulses is further increased new pits are formed in the new surface (beneaththe removed surface) and the above process is repeated. The thermal model is su<cient to explain this morphology. However, for larger4uences, a large cracking and fracture and the possibility of both Ga and As emission in di8erent ratios suggest that mechanical damageis a dominant feature for higher 4uences which arises due to generation of shock waves and rapid vaporization of material. Damagethreshold has been calculated with the help of the thermal model given by Meyer et al. which is in good agreement with our experimentalresults. c© 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Gallium arsenide (GaAs); Laser induced surface damage (LIDT); Thermal damage; Mechanical damage

1. Introduction

Laser induced damage has been observed in semiconduc-tor materials since the advent of lasers and has been theobject of numerous studies over many years [1–4]. The studyof laser-induced damage as a function of material propertiesas well as laser parameters allows a complete characteriza-tion of the damage process in terms of the intrinsic or ex-trinsic thermo-physical and metallurgical properties of thematerial.

Laser induced damage studies in GaAs have been reportedover a wide range of pulse durations and laser wavelengths[1–4]. However, only a few laser induced damage stud-ies have been reported on GaAs, either at larger 4uencesor at multiple pulses. Multiple pulse damage of GaAs hasbeen analyzed by Sardar et al. [5] at pulse duration 10 nsand at laser wavelength 1064 nm. They observed that with4uence below the single shot threshold observable damageoccurs for larger number of pulses, which suggests that dam-age occurs through the accumulation of laser-induced micro-scopic defects in this material and this does not depend onthe etching process. Kanasaki et al. also observed the same

∗ Corresponding author. Tel.: +91-11-6111-955, ext. 201.E-mail address: avinashi [email protected] (A. Kapoor).

mechanism [6]. They have measured the particle emissionfrom the GaAs (1 1 0) surface under the repeated irradia-tion with nanosecond laser pulses for photon energy belowas well as above the band gap. For sub-band-gap energiesthe emission of Ga atoms decreases with increasing photonenergies indicating that the emission is initiated by defectsacting as localization sites of the fundamental excitation.Ra8 et al. [7] also measured the particle emission from theGaAs surface under the multiple pulses and under the non-melting conditions with picosecond pulses. They concludedthat below the melting threshold, the amount of atoms sub-limated per laser pulse indicates that nonthermal processesare dominant.

The present study describes results on laser-induceddamage on single crystal GaAs samples irradiated withNd : YAG laser pulses of 35 ps duration. The objective ofthe study is the evaluation of damage morphological fea-tures in picosecond regime at single and multiple pulses forthreshold 4uence and above the threshold.

2. Experimental details

Freshly prepared well-polished single crystal 〈1 0 0〉 ori-ented (undoped) GaAs was irradiated with Nd : YAG laser

0030-3992/01/$ - see front matter c© 2001 Elsevier Science Ltd. All rights reserved.PII: S 0030 -3992(01)00038 -X

364 A. Pratap Singh et al. / Optics & Laser Technology 33 (2001) 363–369

Fig. 1. SEM micrograph showing: (a) surface damage morphology of GaAs sample at Damage threshold 4uence (F th) for single pulse; (b) enlargedview of the periphery of Fig. 1(a), showing the presence of pits; (c) surface damage morphology of GaAs sample at Damage threshold 4uence (F th)for 50 pulse; (d) enlarged view of the portion near the centre of Fig. 1(c), showing the expansion of pits and Ga-rich phase at the boundary of the pits;(e) enlarged view of periphery of Fig. 1(c), showing the varying nature of pits; (f ) surface damage morphology of GaAs sample at Damage threshold4uence (F th) for 100 pulses, showing a surface removal and new growth of pits; (g) enlarged view of pits, shown in Fig. 1(f ); (h) surface damagemorphology of GaAs sample at Damage threshold 4uence (F th) for 500 pulses; (i) enlarged view of Fig. 1(h), showing layered structure of deep pitswith some droplets; ( j) periphery of Fig. 1(h), showing di8erent ratios of Ga and As phase and local surface removal.

A. Pratap Singh et al. / Optics & Laser Technology 33 (2001) 363–369 365

Fig. 1. (continued )

(Continuum radiation PY61C-10). The laser consists ofa single 4ashlamp pumped oscillator based on the hybridmode locking: passive mode locking by dye and activemode locking by an acousto-optic modulator operated at69:95 MHz. One of the 6–8 pulses circulating in the cavityis damped by means of a Pockel’s cell. The oscillator pro-duces 35 ps pulses at 1064 nm with output energy of 3–4 mJat a repetition rate of 10 Hz. The output beam may be ex-panded and ampli9ed to about 70 mJ by means of a singlepass ampli9er pumped by two 4ashlamps. Using a Gaus-sian beam (TEM00 mode) the samples were irradiated atnormal incidence(1=e2∼ laser spot size (diameter) ∼7 mm)at room temperature under normal atmospheric pressure.

3. Results

After irradiation by laser pulses the damaged spot hasbeen analysed by SEM. The successive aspects of the dam-age morphology at single and multiple pulses for three char-acteristic 4uence regimes—Damage threshold 4uence (Fth);

Two times of Fth (2×Fth) and Three times of Fth (3×Fth)are as follows:

3.1. Damage threshold ;uence (Fth)

Damage was 9rst observed at a power density of2×1011 W=cm2 revealing the spattering in the central por-tion and in the periphery of the damaged spot (Fig. 1(a)).Magnifying the peripheral area (Fig. 1(b)), it is clear thatthis spattering has been formed in the form of pits. Interest-ingly these pits make a long ordered chain, which may arisedue to electrodynamic interference e8ects which furtherdepends on the wavelength and orientation of light [8].

Increasing the number of pulses to 50 pulse the wholefocussed area is damaged (Fig. 1(c)) in the form of pits.The depth and diameter of these pits increase and the gapsbetween the pits are reduced. In the periphery these pitsin their preliminary stage can be seen (Fig. 1(d)). Somedroplets are also present. Increasing the number of pulses to100 pulse the whole focused area is removed. A new growthin the form of pits and splashing is also evident. It should


Fig. 2. SEM micrograph showing: (a) surface damage morphology of GaAs sample at two times the threshold 4uence for 100 pulses; (b) periphery ofFig. 2(a), showing a chain of pits; (c) surface damage morphology of GaAs sample at two times the threshold 4uence for 500 pulses; (d) enlarged viewof periphery of Fig. 2(c) showing the small pits and crack lines.

be that when the complete damaged area took the form ofpits (for 50 pulses), then due to the bright periphery (whitecolor) (Ga rich phase) of the pits, complete area is bright(white colour) but after the Ga rich phase (white colour)explosion (for 100 pulses) the complete area becomes dark(black colour). We presume that this should be an As richphase [9] which arises following the Ga rich phase explosion(Fig. 1(e)). Fig. 1(f) shows the magni9ed view of these pits.When the number of pulses increases further to 500 pulsesthe whole focussed area is once again damaged with similarkinds of pits, which is comparatively smaller in diameter anddepths. Fig. 1(g) shows the damaged spot. Here splashingof some bright droplets of Ga atoms is also present. If wecompare the results for 1 pulse and 500 pulses we 9nd thatthe results are similar in terms of pits, the only di8erenceis that for 500 pulses the topmost layer has been removedand pits are formed in the new layer (beneath the removedlayer). These results suggest that if we further increase thenumber of pulses we should get similar results for the newsurface (beneath the removed surface).

3.2. Two times the threshold ;uence (2Fth)

Figs. 2(a)–(d) shows the damage morphology for thispower 4uence. Here also the damage morphology proceedsin similar manner as for damage threshold 4uence (forma-tion of pits, their expansion and then surface removal) butcrack lines are also introduced in the damaged spot.

3.3. Three times the threshold ;uence (3Fth)

Fig. 3(a)–(c) shows the damage morphology for thepower 4uence three times greater than the threshold 4uence.In the 9gure a large crack and fracture can be seen besidesthe pit formation and surface removal. These results sug-gest that increasing the laser 4uence, damage morphologyis dominated by cracking and fracture as compared to pitsformation and surface removal. Also for threshold 4uencewe have seen either a bright spot (white color) as Ga-richphase (for 50 pulses) or a dark spot (black color) as As-rich


Fig. 3. SEM micrograph showing: (a) surface damage morphology of GaAs sample at three times the threshold 4uence for 100 pulses, showing crackingand surface removal; (b) enlarged view of periphery of Fig. 3(a) showing a chain of pits; (c) surface damage morphology of GaAs sample at threetimes the threshold 4uence for 500 pulses, showing large cracking, fracture and surface removal.

phase (for 100 pulses), but for larger 4uence (2×Fth and3×Fth), the spot has a complex color composition of blackand white colour. It is expected that for larger 4uenceswith the Ga emission the possibility of As emission can-not be ruled out as suggested by Jong et al. [10]. Herethe temperature is su<cient to evaporate both Ga and Asatoms.

4. Discussion

In the present study, damage in GaAs surface is 9rst ob-served at a power density of 2×1011 W=cm2 in the form ofpits. These pits become more prominent for multiple pulses.Increasing the number of pulses, the diameter and depth ofthese pits increase, which is further limited by complete sur-face removal. These results suggest the defect-dominatedmorphology as seen by Kanasaki et al. [6] and Sardar et al.[5] for nanosecond regime. These defects may arise due tononlocalized heating and carrier generation.

Many authors have discussed the pits formation in GaAssurface and Ga emission from the surface in detail. Kanasakiet al. [6] reported that emission of Ga atoms is caused byelectronic excitations of the surface defect. They suggestedtwo possible mechanisms for this emission—excitation bypulse produces two dimensional e–h pairs which are fur-ther trapped by defect and a metastable state is generated.The resulting metastable state is further excited by incom-ing photons. In this case the photon energy dependence isindicative of the number of 2 dimensional e–h pairs. An al-ternative possibility is that metastable state interacts withexcited e–h pairs leading to formation of antibonding states.Hattori et al. [11] explained that cascade excitation of thedefects is the main reason behind these emissions. These re-sults suggest that damage morphology of GaAs surface inthe form of pits is purely of a thermal nature [5].

Meyer et al. [3] have explained theoretically the laserinduced damaged morphology in GaAs. According to theMayer et al. [3] theoretical model, the laser power densityP0 required to heat the surface of semiin9nite semiconductor


Table 1Physical parameters of GaAs

Physical parameters Value Temperature References

Speci9c heat, c (0:303 + 0:015x1) J=g=K T ¿ 300 K 13Thermal conductivity, k 0:425×1−1:1 W=cm=K T ¿ 300 K 13Absorption coe<cient 10 exp[149 eV−1(E′ − 1:38 eV)] cm−1 E′¡ 1:426 eV 13�i (h�; T ) 2:91×104 exp[3:22 eV−1(E′ − 1:79 eV)] cm−1 1:426¡E′¡ 1:83 eV

3:48×104 exp[1:71 eV−1(E′ − 1:83 eV)] cm−1 1:83¡E′¡ 2:5 eVMelting point Tm 1500 K

from an initial temperature T0 to a 9nal temperature Tf isgiven by

P0 =�c(T0)RTLh

[1 − R(T0)]tp; (1)

where � is the mass density of the material, c is the speci9cheat, RT is Tf − T0; R is the re4ectivity and tp is the laserpulse duration. Lh is a heating depth [3]. To avoid error,temperature dependent parameters have been suitably cal-culated according to the appendix in the above paper. Allthe parameters have been given in Table 1. In GaAs on laserexcitation, high concentration of excited electrons and holesa8ects many material properties such as carrier recombina-tion, carrier di8usion and optical absorption. At 300 K theradiation recombination is a dominant process and surfacerecombination velocities fall in the range 105–3×106 cm=s.For higher temperature Auger recombination is the dominantprocess as in other materials [3]. Assuming a nonradiativesurface recombination velocity SNR of 106 cm=s; we havecalculated the value of Lh as 1:09×10−3. Putting all the val-ues in Eq. (1), we 9nd that the value for laser power densityP0 required to heat the surface of GaAs is 1:98×1011 W=cm2,which is very close to our experimental results and showsthat extrinsic mechanism is responsible for heating.

Increasing the power density, the damage morphology isdominated by large cracking and fracture as compared to pitsformation and surface removal. Also the colour compositionof black and white in the damaged spot makes the possibilityof As emission with Ga emission. These facts are not easilyunderstandable by a purely thermal model [3]. A possiblemechanism is mechanical damage. When the laser pulse ismore intense and short, laser induced shock waves may begenerated either by the recoil pressure of rapid vaporizationof the material or because of the interaction of the laserwith plasma. These shock waves can impinge on the targetand travel through the material causing fracture if there isenough shock energy.

5. Conclusion

Laser induced damage in GaAs starts in the form of pits.Increasing the number of pulses, the gap between these pits

is reduced and 9nally the complete surface is removed. Onfurther increasing the number of pulses the same process isrepeated. This demonstrates that layer by layer removal ispossible through multiple pulse laser damage in picosecondregime. Multilayer removal (layer by layer) can be achievedby alternatively roughening the surface using etching (orbombardment by energetic electrons or atoms) and thensmoothing by laser induced multiple pulse damage. Alsosuitable adjustment of the initial pit seeding and photon en-ergy will provide surface patterning and material removal.Laser induced damage initiated by defects in semiconduc-tor surfaces can be utilized for eliminating adatom and kinktype defects from surfaces [6,11].

Increasing the power density (2×Fth and 3×Fth) the dam-age morphology is dominated by a large cracking and frac-ture as compared to pits formation and surface removal.This type of morphology has been explained by mechanicaldamage.

Acknowledgements

We would like to thank 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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Thermal and mechanical damage of GaAs in picosecond regime