Surface Science 128 (1983) L243-L248 L243 North-Holland Publishing Company

SURFACE SCIENCE LETTERS

EJECTION OF ATOMS AND MOLECULES FROM HIGHLY EXCITED CdS

A. NAMIKI, K. WATABE, H. FUKANO, S. NISHIGAKI and T. NODA

Department of Electrical Engineering and Electronics, Toyohashi University of Technology, Tempaku, Toyohashi 440, Japan

Received 28 December 1982

The main products, sulfur dimers ($2), sulfur atoms (S) and cadmium atoms (Cd), ejected from highly excited CdS, show Maxwellian velocity distributions characterized by an unusually low kinetic temperature, indicating that the laser induced damage of CdS in high-density electron-hole plasma does not undergo a thermal melting, but a direct solid-gas phase transition driven by dimerization of adjacent sulfur atoms.

Investigation of particle ejection from surfaces irradiated by an intense laser is becoming an active research topic in the field of semiconductor physics and chemistry. In the case of intense laser irradiation of band gaps of semiconduc- tors. there exists a fundamental problem about the mechanism for the motive force to cause ejection of particles, which results in lattice rearrangement and surface damage. Two mechanisms have been presented: one is thermal motion of atoms obeying classical mechanics in the molten phase [1,2], and the other quantum mechanical dynamics of atoms in a high-density electron-holeplasma [3]. The time-of-flight (TOF) spectra of ejected particles bring us much information about the atom dynamics on surfaces and in bulk as well. Assuming a Maxwellian velocity distribution, Stritzker et al. observed that the systematic increase of the temperature of evaporating silicon atoms reached 3000 K in TOF spectra with an increase of the laser power up "to 2 J / c m 2, suggesting that the laser annealing of Si was due to thermal melting [2]. The observed kinetic temperature seems to exhibit a true lattice temperature according to a recent Raman experiment [4]. For compound semiconductors, however, Nakayama et al. stated that the evidences of the non-thermal process were based on the experimental facts of non-Maxwellian velocity distributions of ejected particles from ZnO[5] and GAP[6], and the existence of an extraor- dinary low threshold for the dye laser fluence to cause the ejection of phosphor in the wavelength region of the indirect band gap of GAP[5]. They stressed the role of the electron-hole plasma in which through Anderson's negative-U interaction [7] two holes localize on one atom which then desorbs from the

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L244 A. Namiki et al. / Ejection of atoms and molecules from CdS

surface [8]. The apparently different manner of the neutral particle ejection between Si and compound semiconductors seems to need a view point of the nature of the chemical bonds and the optical properties of each material for full understanding of the high-density electronic excitation effects. Under these circumstances, CdS is one of the most interesting semiconductors to study the laser effects, because precise experiments have been done with respect to the optical properties for excitons, excitonic molecules, and electron-hole plasma [9], while no efforts have been made for the lattice dynamics in the high-den- sity electron-hole plasma. In this Letter, we show that the emitting atoms and molecules from CdS irradiated at 300 K with a nitrogen laser have a Maxwel- lian velocity distribution characterized by a very low kinetic temperature and that the high-density electron-hole plasma undergoes the incoherent solid-gas phase transition driven by the dimerization force of adjacent sulfur atoms.

TOF spectra of emitted particles from a CdS single crystal (Teikoku Tsushin Company) were measured with a quadrupole mass spectrometer (QMS) (ULVAC, MSQ-150A). The surface parallel to the c axis of the CdS crystal was etched in concentrated chloric acid for a few seconds. In order to avoid an excessive charge-up due to the emission of charged particles or to keep the Fermi level fixed, the rear surface was coated with indium to obtain Ohmic contact and then connected to the sample holder grounded together with the detection circuits. A home made nitrogen laser with 5 ns pulse width was defocussed onto the sample normal to the surface at a repetition rate of 0.2 Hz. The beam spot size was measured directly from the burned area of the sample to be 7.5 x 10 -3 cm 2. The atoms and molecules ejected 40 ° from the surface normal were detected with the QMS under the base pressure of 2 x 10 -7 Tort. The signal from the electron multiplier was terminated with a suitable load resister and then fed into an oscilloscope (Tektronix, 466) directly. The flight path from the sample to the exit of the ionization chamber was 7.0 cm long. The transit time through the quadrupoles to the exit of the electron multiplier was measured for various residual gases by pulse control of the focusing lens voltage for ions. From these results the relevant transit time for each atom or molecule, which must be subtracted to obtain the correct flight time, was estimated. The resolution of the QMS was set rather lowered (M/AM ~ 1 M) intentionally to obtain strong and clear signals. The laser power was monitored for each shot by measuring the luminescence from an emissive filter (Toshiba, L-42) irradiated with the laser splitted with a Quartz plate. The absolute intensity had been calibrated with a power meter (Gentek, ED-100) beforehand. The experiments were done at room temperature, around 300 K.

The observed neutral species ejected after the laser irradiation are S, S 2, S 3, S 4 and Cd. Typical examples of TOF spectra from the QMS are shown in fig. 1. The upper signal is a trace of emitted S 2 molecules as a function of time. The first sharp band is ascribed to positive ions and the later broad band to $2

A. Namiki et al. / Ejection of atoms and molecules from CdS L245

neutrals. Assuming the ionization efficiency of S 2 molecules to be about 10 -4 in the ionization chamber, the intensity ratio of cations to neutrals is on the order of 10-5. Therefore, in this work we focus our attention to neutral species. The complicated behaviour of cations will be reported elsewhere. The signal from neutrals is composed of two parts: one has a clear peak at around 200 Fs, and the other no peak but a long tail extending to a few tens of milliseconds. When the centre mass number ( M / z ) was changed slightly, only the latter part remained while the former disappeared (lower trace). The' decay speed of the latter was clearly correlated to the speed of evacuation. When the spot of the irradiation was set in off-axis of the QMS and another aperture was put in front of the ionization chamber to restrict a much smaller solid angle, a signal similar to the latter part was observed. Taking into account interference effects on the mass resolution, due to the velocity components perpendicular to the QMS axis, the latter part may be tentatively assigned as the background signal resUlting from random scattering of the ejected particles with the instrumental constituents. We thus regard here only the first part as the real component of the particles emitted directly from the irradiated position. This subtraction of the background component was relatively important for Cd compared to S 2, whereas such a subtraction was not needed for S 3 and S 4.

z o ~

o

rO ~ Z

0.2V/div

0.1Vldiv

I00 /~s/div

Fig. 1. Oscilloscope traces of the output signals of $2: upper trace, M / z ~ 63.3; lower trace, M / z = 64.5. The laser power is 117 nO/crn 2.

L246 A. Namiki et al. / Ejection of atoms and mol.ecules from CdS

If the t rans la t iona l mot ion of the emi t ted par t ic les obeys the Maxwel l ian veloci ty d i s t r ibu t ion law, the observed T O F spect ra should fit to a t ime d i s t r ibu t ion funct ion f ( t ) with a pa r ame te r t o as [10]

f ( t ) = const × 1 / t 4 × e x p ( - t 2 / t 2 ) , (1)

and the effective t empera tu re Tel f is re la ted to t o as

Tel r = m12/2kt2o, (2)

where m is the mass of the emi t ted molecules, l the flight d is tance and k the Bol tzmann constant . Fig. 2 shows the observed t ime d i s t r ibu t ion of the ejected

par t ic les together with the one s imula ted by eq. (1). The curves of S, S 2, S 3 and S 4 can be f i t ted to the s imula ted one with a lmost s imilar values of the p a r a m e t e r t 0, ind ica t ing that these species have a s imilar velocity d i s t r ibu t ion charac te r ized by the mean veloci ty ( v ) ( = 2 1 / ~ / ~ t o ) = 3.3 x 104 c m / s . The exper imenta l curve of C d can also be f i t ted to eq. (1) with a value of t o s imilar to that of sulfur molecules, a l though it is s l ightly large. Because of the s imilar veloci ty d is t r ibut ions , the relat ive difference among the T~fe's for each species arises appa ren t ly f rom the difference in mass as recognized f rom eq. (2). The fact that each molecule has its own specific t empera tu re means that no equi l ib r ium exists be tween the ejected par t ic les and the kinetic t empera tu res do not necessar i ly indicate the lat t ice tempera ture . The T~ff's were measured under three di f ferent laser power condi t ions , being summar ized in table 1. A l though the error in Tel r is somewhat large because of the ins t rumenta l reso lu t ion l imited by the ambigu i ty of the t ransi t t ime for the ioniza t ion c h a m b e r to the e lec t ron mult ipl ier , the Tcef's of S, S 2 and Cd are found to be ra ther cons tan t within the exper imenta l uncer ta in ty for the present laser power. Both facts of ex t remely low T~ff for the .ma in p roduc t s S 2, S and Cd. and independence of T~ff on laser power may indicate that the laser induced eject ion f rom CdS does not undergo a thermal melt ing, since the mel t ing poin t

Table 1 Effective/emperature Tet ¢, relative yield Y, and power index n

Species T~fr (K) ya) n c~

71 mJ/cm 2 133 mJ/cm 2 147 mJ/cm ~

S 180+ 30 180_+ 30 200_+40 31 2.7 S 2 320 + 60 290 _+ 50 310 5:60 100 2.7 S 3 470-+ 80 510-+90 1.5 3.2 $4 590_+90 0.3 3.2 Cd 330 + 40 320 + 50 66 b ) 3.0

a) Corrected for the total detection efficiency of the QMS. b) Corrected for the relative isotopic abundance, 24% at M / z = 112. ) The slope obtained from fig. 3.

A. Namiki et al. / Ejection of atoms and molecules from CdS L247

~ 10 3

z ~ 10 2

_= z -~ l~/~__j" ~ ~ I01 ~D Z

"i ~ , , , I 10 -1 0 5OO IOO0

MICRO SECONDS

7

20 I I I I I I I

40 100 200 m J / c m 2

L A S E R POWER

Fig. 2. Fittings of the time distribution of the emitted particles. (e) Experimental points after subtraction of the background signal. ( ) Line simulated by eq. (1) with the most reasonable parameter to: 216 #s for S, 242/,ts for $2, and 308 #s for Cd.

Fig. 3. Log-log plots for signal intensities versus laser power.

is as high as 2000 K, but a direct dissociation of the crystal bond in high-density electron-hole plasma. Another mechanism such as stimulated Brillouin scattering [11] may be ruled out because the present laser energy is much larger than the band gap.

In table 1 the relative yields for the ejected species are also summarized. S 2 is the main product among the ejected sulfur compounds. The yields of S 3 and S 4 are very small, while molecular CdS was never observed even with the strongest laser power. Therefore, for laser ejection in a pulsed damaging process of CdS, the formation of S 2 may be a key process. The yield of each emitted particle versus incident laser power is plotted in fig. 3. For these measurements the laser power density was increased monotonically at the same irradiated position. Reproducibility of signal intensity was checked by revers- ing the experimental process at the same position. The laser power density was rather limited to avoid serious damage of the sample. From fig. 3 the existence of the definite threshold of the laser fluence is unclear. If we adopt a simple power law between the yield and the laser power for this restricted power range, the power indices around 3.0 are obtained as summarized in table 1. This implies that the atoms or molecules are ejected by gathering nearly three photons.

The digged damage, the intense signal of QMS, and the troop motion of

L248 A. Namiki et aL / Ejection of atoms and molecules from CdS

ejected par t ic les with the s imilar veloci ty indicate that eject ion of par t ic les f rom highly exci ted CdS does not arise from an ideal ized surface mono laye r bu t from mult i layers . Therefore , it is na tura l to cons ider that the par t ic le e ject ion is o r ig ina ted ra ther in bulk involving the surface. In the light of the p resen t exper imenta l results, the eject ion process of par t ic les f rom highly exci ted CdS may be hypothes ized by the fol lowing scheme: (1) The intense laser i r rad ia t ion of the band gap makes a h igh-dens i ty e l e c t r o n - h o l e plasma. (2) The ins tabi l i ty of the e l e c t r o n - h o l e p l a sma is caused by d imer iza t ion of ad jacen t sulfur a toms gather ing three holes to neutral ize their ionicity. (3) A sufficient amoun t of S 2 molecules in the e l e c t r o n - h o l e p l a sma leads to the phase t rans i t ion f rom solid to gas incoheren t ly in space and time. (4) I nhomo- genei ty of local b o n d rup ture and local f luctuat ions of the e l e c t r o n - h o l e p l a s m a dens i ty result in a Gauss i an d i s t r ibu t ion of the kinet ic energy or an a p p a r e n t Maxwel l i an veloci ty d i s t r ibu t ion for the ejected part icles.

In a cer ta in sense this hole local iza t ion in the h igh-densi ty e l e c t r o n - h o l e p l a sma may be a real iza t ion of Ande r son ' s negat ive-U interact ion [7,8] associ- a ted with d imer iza t ion of sulfur atoms. Therefore, laser induced vapor iza t ion of CdS is closely cor re la ted to the charge t ransfer con t ro l led process in pho ton acce lera ted vapor iza t ion of CdS surface at e levated t empera tu re [12].

The authors express their sincere thanks to Professor T. Takaish i for his k ind suggest ions and also to Professor M. Kosak i for his k ind help in equ ipmenta t ion . This work was suppor t ed by a G r a n t - i n - A i d for Science Research f rom the Minis t ry of Educa t ion of Japan.

References

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[10] C. Peugnet, J. Appl. Phys. 48 (1977) 3206. [11] D.A. Kramer and R.E. Honig, Appl. Phys. Letters 13 (1968) 115. [12] (a) G.A. Somorjai, Surface Sci. 2 (1964) 218;

(b) G.A. Somorjai and .I.E. Lester, Progr. Solid State Chem. 4 (1967) 1.