A constituent analysis of semiconductors using intense visible laser radiation

A constituent analysis of semiconductors using intense visible laser radiation E. WlNTNER, ~ J . S C H M I D T It is s h o w n that the constituent surface atoms of several binary s e m i c o n d u c t o r s vaporized by intense visible laser light can be easily identified by their characteristic radiation in the ultra-violet. These lines, between 2 0 0 nm and 4 0 0 nm, originate from the recombination of neutral excited atoms. The measurements were performed at room temperature and at liquid He temperature. This may represent an explanation for the very strongly intensity dependent ultra-violet signals occurring in recent SHG examinations of surface order in GaAs w i t h illumination in the visible. KEYVVORDS: spectroscopy, lasers, semiconductors, ultra-violet

Introduction Electric fields in intense p u l s e d laser r a d i a t i o n o f several M W cm -2 are strong e n o u g h to v a p o r i z e a t o m s from the surface o f any a b s o r b i n g m a t e r i a l 1. The e v a p o r a t e d a t o m s are usually in the excited state a n d hence emit c h a r a c t e r i s t i c r e c o m b i n a t i o n r a d i a t i o n by which they can be identified. In this p a p e r it is shown that the metal constituents o f several b i n a r y s e m i c o n d u c t o r materials, that is, CdS, CdSe, GaAs, InSb emit characteristic ultra-violet lines which can be easily e x p l a i n e d as the characteristic lines o f neutral atoms. T h e result seems to be relevant to e x p l a i n the origin of the ultra-violet intensity in S H G short pulse surface e x a m i n a t i o n s of, for e x a m p l e z, G a A s at intensities at which a p h a s e t r a n s i t i o n from the crystalline structure to a p h a s e of lower o r d e r takes place. Such studies 3,4 play an i m p o r t a n t role in the u n d e r s t a n d i n g o f laser a n n e a l i n g

d i s a p p e a r quickly. F i g 1 shows the typical e m i s s i o n spectrum of C d S at 300 K in the spectral range from 200 - 400 nm. O n e can clearly recognize a n u m b e r of lines. T h e m e a s u r e d linewidth c o r r e s p o n d s to the resolution o f the s p e c t r o m e t e r used. T h e intensity o f the e m i s s i o n o b s e r v e d at ~ = 361.1 nm against the n u m b e r o f the pulse is shown in F i g 2. The signal reaches its m a x i m u m intensity after a b o u t 20 laser pulses a n d r e m a i n s then c o n s t a n t for a n o t h e r 80 pulses. T h e r e a f t e r the intensity decays to very low values after a p p r o x i m a t e l y 350 pulses.

20L

Experiment

.~ 15

The exciting laser source for this e x p e r i m e n t was a N 2 laser p u m p e d dye laser p r o d u c i n g t u n a b l e n a r r o w b a n d (AX ,-~0.02 nm) pulses ,v 3 ns long in the spectral range of 474 n m - 485 rim. The energy per pulse was a b o u t 150 p.J a n d the repetition rate a few Hz. T h e b e a m was focused by a lens w i t h . / = 10 cm onto the s a m p l e at room t e m p e r a t u r e or 2.4 K ( i m m e r s e d in s u p e r f l u i d He) y i e l d i n g laser fluences o f a few J cm -2. This excitation created an emission spectrum i n d e p e n d e n t of the actual wavelength c h o s e n a n d insensitive to incident polarization. D e f o c u s i n g caused the signal to

=

The authors are at the IAEE, Technical University of Vienna, Gusshausstr. 27, A-1040 Wien, Austri& Received 22 March 1985. Revised 13 September 1985.

0030-3992/85/060319-02/$03.00 OPTICS AND LASER TECHNOLOGY. DECEMBER 1985

~, ~ 10

"~ _~

j

m

5

0 225

I 275

I 325

I 375

X (nm) Fig. 1

Ultra-violet emission spectrum of CdS at 300 K

© 1985 Butterworth 8- Co (Publishers) Ltd 319

Table 1

List o f o b s e ~ e d

CdS

r-

I 50

0

I 100

I 150

I 200

i 250

1 300

Number of pulses

[ A m C

4

231.28 257.31 274.86 325.11 340.37 346.62 361.06 361.29

326.0 340.5 346.5 361.0

In*

Sb*

GaAs

Ga*

252.80 259.80

287.6 294.0 404.0 418.0

287.4 294.3 403.2 417.2

271.03 287.70 303.94 325.61

p r o p a g a t i o n in air. The collisions with He a t o m s p r o b a b l y q u e n c h the emission of atomic lines.

v

2

0 225

275

325

375

x (nm)

Fig. 3

Ultra-violetemissionspectrumof CdS at 2.4 K

M i c r o s c o p i c inspection o f the irradiated spot showed an o b s e r v a b l e a m o u n t o f surface d a m a g e in the form of a small dip. If the crystals are e x a m i n e d at a very low cryogenic temperature, s i m i l a r but less p r o n o u n c e d spectra are o b t a i n e d as shown in F i g 3. The overall intensity is c o n s i d e r a b l y less ('~1/3) a n d the peaks are much less d o m i n a n t over the diffuse background. Despite the cooling, the s a m p l e s showed a s i m i l a r surface erosion as described before. The d a m a g e o f the surface supports the c o n c l u s i o n that atoms were v a p o r i z e d by the intense laser electromagnetic fields. F o r G a A s for example, the t h r e s h o l d l]uence lbr surface melting I is 30 mJ c m 2 a value which wits far exceeded. Detailed e x a m i n a t i o n of the spectra shows that they originate from the neutral excited metal constituent atoms o f the s e m i c o n d u c t o r examined. Table 1 lists all the lines identified in the spectral range of interest It can be obser~,ed that the c o r r e s p o n d e n c e with the t a b u l a t e d data 5 is e x c e l l e n t The fact that only the lines o f the metal a t o m s are detected is due to the choice of the region o f observation: for e x a m p l e s u l p h u r has emission lines at much longer wavelengths. The difference between the spectral emission at the two e x p e r i m e n t a l t e m p e r a t u r e s may be due to the fact that greater energy is required to v a p o r i z e super-cooled a t o m s y i e l d i n g a s m a l l e r emission intensity at liquid helium temperatures. Furthermore, there is p r o b a b l y a strong interaction of the v a p o r i z e d a t o m s with the liquid helium e n v i r o n m e n t c o m p a r e d to the nearly-free

320

Cd*

* Landolt- B6rnstein5

>-

.#

252.9 260.3 271.0 287.6 304.3 325.5

nm)

CdSe

230.0 257.0 275.0 326.6 339.5 347.0 361.1 361.3 InSb

Fig. 2 Intensityof the emissionat X= 361.1 nm for the emission spectrum of CdS againstthe numberof pulses

lines(kin

The b e h a v i o u r of the emitted intensity can be u n d e r s t o o d in the following way: starting with a fresh spot on the sample, only a certain fraction of the focused light energy is absorbed. With the g r a d u a l erosion of the surface this fraction increases and so does the e v a p o r a t i o n rate and therefore the emitted intensity. With the increasing depth o f the crater on the s u r f a c e the focusing geometry c h a n g e s and hence a s m a l l e r energy density is reached y i e l d i n g less evaporation. The collection geometry is also altered g r a d u a l l y so that less light is collected by the detection system.

Conclusion The result could be useful for an investigation of surface compositions. Further, this observation can explain the o c c u r r e n c e o f ultra-violet signals in laser surface e x a m i n a t i o n s 2 by S H G which deviate from the usual q u a d r a t i c d e p e n d e n c e above the t h r e s h o l d lbr surface damage. As the signals show no a n g u l a r d e p e n d e n c e a n d are associated, at laser tluences o f ' ~ 1 J c m 2 with blue sparks at the surface they arc p r o b a b l y emitted by v a p o r i z e d atoms.

Acknowledgement The authors would like to t h a n k Professor H.A, H a u s for his critical r e a d i n g o f the m a n u s c r i p t

References 1 Lietoila, A., Gibbons, J.F, "Laser Electron-Beam Inlcractions with Solids', Edited by Appleton, B.R., (cller. G.K., North Hollan& New York 2 Malvezzi, A.M., Liu, J.M., Bloembergen, N. Appl Phys Lett 415 (1984) 11)19 3 Shank, C.V., Yen, R., Hirliman, C. Phys Rer Lett 50 11983) 454 4 Malvezzi, A.M., Kurz, H., Bloembergen, N. "UItrafast Phenomena IV. Edited by Auston, D.H. and Eisemhal. K. Springer Series Chem. Physics 38 (1984) 118 5 Landoh-Bi}rnstcin, Vol. I, Part I {Edited by Hellwege, R.), Springer-Verlag Berlin ( 1977}

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