Optics & Laser Technology 35 (2003) 21–24www.elsevier.com/locate/optlastec

Laser-induced damage studies in GaAs

Amit Garg∗, Avinashi Kapoor, K.N. TripathiDepartment of Electronic Science, University of Delhi, South Campus, Benito Juarez Road, New Delhi-110021, India

Received 31 May 2002; received in revised form 15 August 2002; accepted 20 August 2002

Abstract

Laser-induced damage studies have been carried out on monocrystal GaAs at 1:06 �m wavelength as a function of pulse repetition ratein the nanosecond regime. The single shot observed laser damage threshold is 0:9 J=cm2. It has been found that the damage thresholddecreases when the sample is irradiated with large number of pulses. However, the above e3ect is observed only when the repetition rateis higher than 1 Hz. Various laser damage mechanism theories have been discussed to explain the results.? 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Multiple pulse damage; Thermal stress; Accumulation e3ects

1. Introduction

The study of laser irradiation on semiconductors has beena subject of great interest over the years. Laser-induced dam-age studies in solids helps in understanding the interaction ofhigh intensity radiation with matter. The damage thresholdof the material depends on the wavelength of laser, its pulsewidth, focussed spot size, number of pulses, surface andbulk characteristics of the material. The laser-induced dam-age studies in terms of material properties and these laser pa-rameters allows a complete characterization of the damageprocess in terms of the intrinsic or extrinsic thermo-physicaland metallurgical properties of the material.

In general, there are many signs that are responsible fordamage on materials. For example, damage in the form ofsmall pits followed by severe catering by localized heatingdue to surface defects or appearance of sharp crack lines atlow power densities due to thermal shock. In case of mul-tiple pulse damage, the growth of microscopic defects mayprecede the catastrophic damage or it may be due to ma-terial changes prior to damage. Various theories have beensuggested such as thermal melting due to cumulative e3ects,accumulation e3ects, thermoelastic stresses, avalanchemultiplication, multiphoton ionization, growth of micro-scopic defects, etc. [1–5] to explain the multiple pulse laserdamage.

∗ Corresponding author.E-mail address: avinashi kapoor@yahoo.com (A. Garg).

Laser-induced damage studies in GaAs have been re-ported over a wide range of pulse durations and laser wave-lengths [6–12]. However, most of these studies relate tomorphological damage and ion emission. In general, mostof these studies indicate that the laser damage for multi-ple pulses is smaller than that for a single laser pulse. Alsomultiple pulse damage studies has been con@ned to a repeti-tion frequency of 10–20 Hz. To completely understand thedamage mechanisms in GaAs, it is very important to carryout these studies at low repetition frequencies. The presentstudy reports laser-induced damage studies on monocrystalGaAs samples as a function of pulse repetition frequency in0.1–20 Hz range. The results are discussed in the light ofvarious existing theories.

2. Experimental details

Single crystal 〈100〉 oriented GaAs (undoped, mirror pol-ished) samples are investigated in the present work. For amirror—like polished surface, the GaAs samples were @rstground with @nest grades of alumina slurry and then suc-cessively polished with 4, 2 and 1 �m size diamond pasteson Selvyt cloth. Finally super polishing was achieved on thesurface with 0:1 �m size diamond paste. This results in anoptically clean surface. The samples were then thoroughlycleaned in trichloroethylene, acetone and isopropyl alcoholto remove all the contaminants. The samples were @nallyrinsed in deionized water.

0030-3992/02/$ - see front matter ? 2002 Elsevier Science Ltd. All rights reserved.PII: S0030 -3992(02)00118 -4

22 A. Garg et al. / Optics & Laser Technology 35 (2003) 21–24

Fig. 1. Experimental setup for the study of laser-induced damage studies.

The experimental arrangement is as shown schematicallyin Fig. 1. A Q-switched Nd:YAG laser with pulse durationof 20 ns (FWHM) at a fundamental frequency of 1:064 �mwas used. The output of the laser was Gaussian. The TEM00

mode laser beam was focussed by a lens of 45 cm focallength. The beam spot size determined by the knife edgescanning technique was of the order of 500 �m at the focalspot. The incident energy on the samples was adjusted bothby incorporating neutral density @lters in the beam path aswell as by placing the sample between lens and its focalpoint. The knife edge was scanned across the beam at allthe sample locations for the purpose of studying the beamshape and estimating the beam size. A frequency stabilizedHe–Ne laser was used as a probe beam and focussed ontothe site to be damaged. The beam was focussed to a spotwith dimensions smaller than Nd:YAG spot size. The re-Jectivity was measured both before and after the irradiationand the induced damage was observed to be permanent. Theexperiment was performed at room temperature in air undernormal atmospheric pressure.

3. Results and discussion

Several sites were irradiated over a range of energies neardamage threshold. The maximum variation in the thresholdvalue, from site to site, was smaller than 8%. The damagethreshold is de@ned as that value of energy that results inscattering centers that are suLcient to reduce reJectivityby 10%. Fig. 2 shows a graph between relative reJectivityvs. laser pulse energy for a single shot. It can be seen thatdamage started to occur at 0:9 J=cm2. The value matcheswell with the results earlier reported on GaAs [13].

A comparison between the experimental and theoreticalvalues can be made by calculating the surface temperaturerise using the heat conduction equation [14].

�c@T@t

= x@T 2

@x2 + (1 − r)I0h e−x

Fig. 2. Relative reJectivity vs. laser pulse energy.

for the following boundary conditions

T (x)t=0 = T;

(@T@x

)x=0

= 0;

Tx→(x) = 0:

The solution of heat conduction equation is

T = I0h (1 − r)[[

[exp(D2t) − 1]D2 e−x

+1D2

{2( t�

)1=2exp

(−x2

4Dt

)−(x2

D

)

× erfc

[12

(x2

Dt

)1=2]}

− 1(D2)1=2

exp(D2t)2(D2)1=2

×{

e−xerfc

[12

(x2

Dt

)1=2

− (D2t)1=2

]

− e−xerfc

[12

(x2

Dt

)1=2

+ (D2t)1=2

]}]]

erfcx = 1 − 2√�

∫ x

0exp(−u2) du;

where is the absorption coeLcient, D = K=�c is the ther-mal di3usion coeLcient, c is the speci@c heat, � is the den-sity of the material, K is the thermal conductivity, r is thereJection coeLcient, t is the time measured from the begin-ning of a pulse, T0 is the temperature of the crystal beforeirradiation and LD = (Dt0)1=2 is the thermal di3usion lengthcorresponding to pulse duration t0. In the present case, thelaser spot size is greater than LD (LD=0:707 �m), i.e. thereis negligible lateral heat di3usion; the problem can thus beregarded as one dimensional. At the sample surface corre-sponding to x= 0 and at a moment t = t0 which correspond

A. Garg et al. / Optics & Laser Technology 35 (2003) 21–24 23

Fig. 3. ReJectivity as a function of number of pulses for laser Juence of0:7 J=cm2: �; 0:1 Hz and 1 Hz (overlapping); 4; 2 Hz; ×; 20 Hz.

to maximum heating, the surface temperature is given by

T (x = 0) =2I0h

√t(1 − r)

c�(�D)1=2

(1 +

√�

2[exp(D2t0) − 1]

(D2t0)1=2

−√�

2exp(D2t0)(D2t0)1=2 erf(D2t0)1=2

)+ T0;

where

erfx =2√�

∫ x

0exp(−u2) du:

If the damage threshold is assumed to correlate with sur-face melting then it would occur when the peak surfacetemperature reaches 1238◦C. This calculated LIDT is0:93 J=cm2. This agrees well with an observed LIDT of0:9 ± 0:072 J=cm2.

However, this Juence was suLcient to induce laser dam-age to the samples if they were irradiated with a number ofpulses at high enough prf.

To study the e3ect of multiple pulse induced damage,we kept the laser energy at 0:7 J=cm2 which is below thesingle pulse laser damage threshold. But at the same time,the Juence is suLcient to cause the laser damage to thesamples if irradiated with N number of pulses in one mode.Fig. 3 shows the e3ect on reJectivity as a function of numberof pulses for di3erent pulse repetition rate. It can be seenthat there is no change in the reJectivity when the pulserepetition rate is 1 Hz or less but the reJectivity decreaseswhen the pulse repetition rate is higher than 1 Hz.

Fig. 4 shows the variation of damage threshold as a func-tion of number of pulses for a pulse repetition rate of 20 Hz.It can be seen that the laser damage threshold decreases sig-ni@cantly from 0.9 to 0:36 J=cm2 when the number of pulsesis increased to 40.

It is quite evident from the above observations that pulserepetition rate and number of pulses play a signi@cant rolein determining the damage threshold of GaAs. The aboveobservations are interpreted in light of undermentioned pos-sibilities.

One of the most probable damage mechanisms to explainthe above observations is the Cumulative E3ect i.e. cumu-

Fig. 4. Damage threshold as a function of laser pulses for pulse repetitionfrequency of 20 Hz.

lation of the temperature when irradiated with number ofpulses. To have an idea of rise in surface temperature withtime, we calculated the rise in temperature of GaAs after40 pulses at the pulse repetition rate of 20 Hz using therelation [15]:

T (z; t)t ¿ �=2I0K

(kt

)1=2

ierfc(

z2(kt)1=2

)

− 2I0K

(k

(t − �))1=2

ierfc(

z2(k(t − �))1=2

);

where k and K are the thermal di3usivity and thermal con-ductivity and � is the pulse width. The calculated valuesshow that the temperature rise after 1 �s is 248◦C drop-ping to 292◦C at the end of 1 ms. The cumulative tempera-ture rise at the end of a train of 40 pulses at 20 Hz is nearthe room temperature. Therefore, even 40 pulses cannotcause that appreciable rise in temperature for the mate-rial to get damaged. Therefore, thermal melting is clearlyruled out as the possible damage mechanism for multiplepulses.

Another possibility that has been put forward by manyresearchers [16–18] is accumulation of laser-induced micro-scopic defects in this material. The accumulation mecha-nism is likely to involve the growth of small structural defectand/or the aggregation and clustering of point defects. Thesedefects, which are basically clusters of impurities pinned atcrystal defects produced during the multiple-pulse laser ir-radiation, can act as extended absorption centers by trappingimpurity ions and thus lead to macrodamage. In our studies,the multiple pulse damage is noticeable only when the pulserepetition rate is higher than 1 Hz but not at lower rates.These observations indicate that the accumulation e3ect in-creases at higher pulse repetition rates while it occurs veryslowly at lower rates.

Laser-induced cyclic thermal stress [5,19,20] is anotherpossibility to explain the multiple pulse damage in our case.

24 A. Garg et al. / Optics & Laser Technology 35 (2003) 21–24

The thermal-stress amplitude induced by the laser beam isproportional to the surface temperature. Under repeated irra-diation, the amplitude of thermal stress increases and whenit becomes larger than the plastic yield point, plastic strainis produced and manifested as plastic slip deformation. Thisplastic slip deformation which accumulates on the surfaceincrease the surface roughness and is followed by increaseabsorbed laser energy. In our case, the multiple pulse dam-age was observed only at higher pulse repetition rates be-cause the relaxation times of these thermally generated stressrange from a millisecond to second. If the material is irradi-ated with another pulse during this period, the stress cumu-lates, otherwise it is relaxed and the e3ect of the @rst pulsebecomes insigni@cant. However, in addition to the abovemechanism of stress generation, another possibility of gen-eration of localized stress can be by localized absorption dueto migration and pinning of impurities.

Therefore, we conclude that thermally induced stress andaccumulation e3ects are the most probable mechanisms oflaser damage in GaAs. However, the primary diLculty inidentifying the damage mechanism is the lack of direct evi-dence of changing material properties on the pulse precedingcatastrophic damage. Direct observations of precatastrophicdamage changes will be useful to identify the exact causemore clearly.

References

[1] Bailey RT, Cruickshank FR, Kerkoc P, Pugh D, SherwoodJN. Surface damage of (-)2-(-methylbenzylamino)-5-nitropyridinesingle crystals induced by pulsed laser radiation. Appl Opt1995;34:1239–44.

[2] Chmel A, Eronko SB. Laser induced generation of structural defectsin vitreous silica and in silicate glass. J Non-Cryst Solids 1985;70:45–52.

[3] Wood RM, Taylor RM, Rouse RL. Laser damage in optical materialsat 1:06 �m. Opt Laser Technol 1975;6:105–11.

[4] Nathan V. Repetitively pulsed laser damage in thin @lms. Proc SPIE1992;1848:583–93.

[5] Jee Y, Becker MF, Walser RM. Laser-induced damage onsingle-crystal metal surfaces. J Opt Soc Amer B 1988;5648–59.

[6] Singh AP, Kapoor A, Tripathi KN, Rvindra Kumar G. Thermaland mechanical damage of GaAs in picosecond regime. Opt LaserTechnol 2001;33:363–9.

[7] Bertolatti M, Sette D, Stagni L, Vitali G. Electron microscopeobservation of laser damage on GaAs, GaSb and InSb. Rad E31972;16:197–202.

[8] 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.

[9] Tsu R, Baglin JE, Lasher GJ, Tang JC. Laser induced recrystallizationand damage in GaAs. Appl Phys Lett 1979;34:153–5.

[10] Wood RM. Laser damage in optical materials. Bristol: Adam HilgerPress, 1986. p. 32.

[11] Wood RM, Sharma SK, Waite P. Single shot, cumulative and prfdependent laser induced damage thresholds. vol. 688, NBS specialpublication, 1983. p. 174.

[12] Wood RM, Sharma SK, Waite P. Variation of laser-induced damagethreshold with laser pulse repetition frequency, vol. 669, NBS specialpublication, 1984. p. 44.

[13] Huang AL, Becker MF, Walser RM. Laser induced damage and ionemission of GaAs at 1:06 �m. Appl Opt 1986;25:3864–70.

[14] Grinberg AA, Mekhitev RF, Ryvkin SM, Salmanov VM,Yaroshetskii ID. Absorption of laser radiation and damage insemiconductors. Sov Phys Solid-State 1967;9:1085–90.

[15] Matsuoka Y. Laser-induced damage to semiconductors. J Phys D:Appl Phys 1976;9:215–24.

[16] Belyaev NN, Bredichin VI, Rubakiha VI, Greidman GI. Agingof -LiIO3 laser irradiated single crystals. Sov Phys JETP 1982;83:1065–8.

[17] Kytina IG, Kinber BE. Fatigue fracture of glasses under laserirradiation conditions. Sov J Quant Electron 1980;7:2427–31.

[18] Wu ST, Bass M. Laser induced irreversible absorption changes inalkali halides at 10:6 �m. Appl Phys Lett 1981;39:948–50.

[19] Koumvakalis N, Lee CS, Bass M. Single and multiple pulsecatastrophic damage in diamond turned Cu and Ag mirrors at 10.6,1.06 and 0:532 �m. Opt Eng 1983;22:419–23.

[20] Porteus JO, Decker DL, Jernigan JL, Faith WN, Bass M. Evaluationof metal mirrors for high power applications by multithresholddamage analysis. IEEE J Quant Electron 1978;QE-14:776–82.