spectroscopic diagnostics of a laser erosion plasma formed from cusbs2 polycrystals

Optics and Spectroscopy, Vol. 97, No. 4, 2004, pp. 633–639. Translated from Optika i Spektroskopiya, Vol. 97, No. 4, 2004, pp. 674–680.Original Russian Text Copyright © 2004 by Shuaibov, Chuchman, Dashchenko.

LASERS AND THEIR APPLICATIONS

Spectroscopic Diagnostics of a Laser Erosion PlasmaFormed from CuSbS2 Polycrystals

A. K. Shuaibov, M. P. Chuchman, and A. I. DashchenkoUzhgorod State University, Uzhgorod, 88000 Ukraine

e-mail: [email protected] March 20, 2003; in final form, January 15, 2004

Abstract—The spectra and dynamics of emission from regions of a laser plasma torch located at different dis-tances from a polycrystalline CuSbS2 target irradiated by a neodymium laser (W = (3–5) × 108 W/cm2, τ =20 ns, f = 12 Hz, λ = 1.06 µm) were investigated. The emission data were used to estimate the average temper-ature (≤0.82 eV) and the electron density ((1.82–1.92) × 1016 cm–3) in the laser torch and the recombinationtimes of ions (tr(S

2+) = 15 ns, tr(Cu+) = 65–85 ns), as well as to analyze the efficiency of filling of excited atomiclevels. A model describing the target destruction and the evolution of the processes accompanying spread of thelaser plasma is proposed. © 2004 MAIK “Nauka/Interperiodica”.

INTRODUCTION

Emission spectroscopy is a contactless method char-acterized by high informativeness and thus is beingmore and more widely used in various fields of scienceand technology. Information obtained by this methodsupplements to a significant degree data of the probeanalysis and mass spectroscopy of a laser plasma.Emission spectroscopy is especially promising for thediagnostics of laser deposition of films of complexcomposition [1, 2], which is important for obtainingmaterials with specified properties [3].

Rapid scientific and technical progress is imposingever-stricter requirements for high-quality materialswith a wide set of properties for quantum electronicsand microelectronics and calling for minimum produc-tion expenditures at the same time. Thus, complex chal-cogenides [4, 5], which exhibit various electrical andphoto-, thermo-, acousto-, piezo-, and ferroelectriceffects, are of much interest. An example of such acompound is polycrystalline CuSbS2. Its multicompo-nent composition and variety of deposition conditionsensure a wide range of possibilities for changing thestructure of this material and, simultaneously, its phys-ical properties. However, this problem calls for adetailed study of the physical and chemical processes ina laser torch [6].

The aim of this study is to obtain insight into the for-mation and spread of the products of laser erosion ofpolycrystalline CuSbS2 by temporally and spatiallyresolved emission spectroscopy.

EXPERIMENTAL

A laser plasma was formed as a result of irradiationof a target made of a polycrystalline blend based onCuSbS2 by Nd laser radiation (W = (3–5) × 108 W/cm2,

0030-400X/04/9704- $26.00 © 20633

τ = 20 ns, f = 12 Hz, λ = 1.06 µm). The experimentalsetup we used made it possible to investigate the emis-sion spectra (along with the oscillograms of spectrallines in the wavelength range 200–600 nm) from theregions located at the distances r = 1 and 7 mm from thetarget surface [7]. The sensitivity of the recording sys-tem was calibrated using hydrogen and tungsten lamps,which allowed us to measure the relative intensities ofspectral lines [8]. The pressure of residual gases in thevacuum chamber where the target was installed did notexceed 7 Pa. The target surface was tilted at an angle of60° with respect to the laser beam and the emissionintensity was measured in the direction perpendicularto the beam. The laser radiation was focused into a spot0.3–0.5 mm in diameter by a lens with focal length f =50 cm, and the torch emission was measured using alens with focal length f = 10 cm. The time resolutionamounted to 2–3 ns and the error of intensity measure-ments was 10%.

The optical characteristics obtained were used toestimate, employing the technique described in [9, 10],the temperature and density of electrons (Te, ne), the ionrecombination time (tr), and the efficiency of filling ofdifferent energy states of atoms (N/g) in the laserplasma torch:

(1)

(2)

(3)

(4)

Ng----

IλAg-------,=

Te

E2 E1–N1g2/N2g1( )ln

------------------------------------,=

tr∆t

∆ I/Imax( )ln----------------------------,=

ne 8.75 1027–

z3trTe

9/2–×( )1/2–

,=

004 MAIK “Nauka/Interperiodica”

634

SHUAIBOV

et al

.

200 300 400 500 λ, nm

406.

3 C

u I

465.

7 S

II

327.

4 C

u I

324.

8 C

u I

287.

8 Sb

I259.

8 Sb

I25

2.9

Sb I

504.

7 S

II

527.

9 S

I52

1.8

Cu

I51

5.3

Cu

I51

0.6

Cu

I

Fig. 1. Emission spectrum of a laser erosion plasma of the CuSbS2 compound (r = 7 mm). Wavelengths (in nm) are indicated nearthe spectral lines.

where I and λ are the intensity and wavelength of aspectral line, respectively; A is the transition probabil-ity; g and E are the statistical weight and energy of alevel; ∆t is the variation in time; ∆ln(I/Imax) is the differ-ence of the logarithms of intensities normalized to themaximum value; and z is the ion charge.

The emission spectra of the plasma were identifiedand the populations of energy levels determined usingreference books [11, 12].

OPTICAL CHARACTERISTICSOF THE PLASMA

In the emission spectra of the laser torch formed bylaser irradiation of CuSbS2 polycrystals, lines of copper(λ < 300 nm) and antimony (λ > 300 nm) were domi-nant. Sulfur manifested itself in the form of weak linesS I and S II. The contribution of S ions to the emissionspectrum exceeded that of S atoms. The spectrum ofemission from the region spaced 7 mm from the targetis shown in Fig. 1. The results of the spectrum identifi-cation and the relative intensities of spectral lines (I)(taking into account the sensitivity of the measuringsystem (kλ)) are listed in Table 1.

Comparing the obtained results with similar investi-gations of the emission of laser plasmas formed by irra-diation of elemental Cu and Sb [13, 14], we note that,for the polycrystalline target, the contribution of Cuatoms to the emission spectrum increased at λ ≥300 nm, whereas the number of spectral lines due totransitions from the energy levels filled as a result ofmany-electron excitation of Cu I atoms (shifted levels)and their intensity decreased. For Sb I in a laser torch

from the CuSbS2 compound, the emission intensity andthe number of spectral lines decreased as well.

When the laser power is not very high, the mainproducts of the crystalline target erosion are singlycharged ions of its components and complex ions of thesame chemical composition as the compound [15].Thus, it can be suggested that the main channel of exci-tation is the reactions of dielectronic and dissociativerecombination. In the case of dielectronic recombina-tion, collisions between bound and free electrons leadto the diffusion of bound electrons via levels with sim-ilar energies to a narrow region where radiative transi-tions are dominant [10]. The energy of the narrowregion and the gap between the ionization energy andthe energy of the highest level from which emissionwas detected are listed in Table 2 for pure elements andCuSbS2.

The energy gap between the ionization potential ofan element and the energy corresponding to the narrowregion in the recombination flux of this element islarger in the case of laser erosion of a polycrystallinecompound. The electron temperature is higher for lasertorches formed from pure elements [16, 17]. The rela-tion between the above quantities can be written as Ei –Enr = 3/2(Te) [10]. The inconsistency obtained may berelated to the competitive processes of filling of Cu Iand Sb I levels under conditions of efficient exchangeby electrons with the same energies. It can also be seenfrom Table 1 that in situ diagnostics of laser depositionof CuSbS2 should be performed with the spectral linesof Cu I and Sb I in the visible and ultraviolet ranges,respectively.

OPTICS AND SPECTROSCOPY Vol. 97 No. 4 2004

SPECTROSCOPIC DIAGNOSTICS OF A LASER EROSION PLASMA 635

Investigation of the time characteristics of the emis-sion from the laser torch (Fig. 2) showed that the short-est (~100 ns) times are characteristic of the emission ofS II (λ = 301.6 nm). The energy of the upper state ofS II can be considered to correspond to the narrowregion in the recombination flux of the ion componentof the plasma: E = 20.37 eV.

The oscillograms of the spectral lines in the emis-sion spectra under study differ from the correspondingoscillograms for the emission of pure targets. The rea-sons are the significant contribution of dissociativerecombination to the spectra of CuSbS2 and the differ-ence in the mechanisms of target destruction in bothcases. It can be seen from the dynamics of the emissionof the plasma laser torch from CuSbS2 that the heightsof both maxima at all the wavelengths are approxi-mately the same and the lifetime of the second maxi-mum in the emission spectrum of the plasma torch fromthe compound is shorter than in the emission spectra oftorches from pure elements. In the first stage of evapo-ration, the torch emission dynamics coincides with thelaser pulse dynamics in both cases.

To study the mechanism of formation of excitedstates in more detail, we plotted the time dependence ofthe logarithm of intensity normalized to the maximum(Fig. 3). Taking into account that the lifetimes ofexcited states are much shorter than the time of obser-vation of emission, we assume that the formation ofexcited states is primarily determined by the evapora-tion conditions and processes in the plasma. The depen-dences in Fig. 3, approximated by straight-line por-tions, show that, in the time ranges 100–175 and 175–250 ns, quasi-stationary excitation conditions are real-ized (judging by the time of their appearance), whichare determined by the recombination processes or col-lisions. The corresponding recombination times areindicated in Fig. 3. It is of interest that S ions emit mostintensively at t = 10–25 ns, while upper excited statesof Cu atoms are filled most efficiently at t ≈ 90 ns aswell. The most intense radiative transitions from thelower states of Cu I correspond to the midpoint of thistime interval.

We explain this effect as follows. The formation ofmost excited particles due to the target evaporationoccurs in two regimes [10], to which the two maximaon oscillograms correspond. The first maximum is theresult of the action of laser radiation. The energy ofphotons is absorbed by the electronic shells of mole-cules located on the target surface. The probability ofabsorbing the energy of a photon is highest for valenceelectrons of the atoms forming a molecule, which arelocated in the conduction band and are responsible forthe formation of chemical bonds. The outer electrons ofa molecule, acquiring a sufficient energy, either emergeinto the vacuum or depart into the target bulk. Theescape of weakly bound outer electrons from the inter-action region (as well as the acquisition of energy byinner electrons of a molecule) leads to heating of the


target and breaking of chemical bonds [18, 19]. Thus,in the first stage of evaporation, intense formation oftarget atoms and their ionization occur.

This mechanism of energy absorption accounts forthe appearance of emission from the shifted levels ofCu I, as well as the formation of S II ions, despite thesignificant ionization potential. Coulomb interactionbetween ions also favors evaporation. Since CuSbS2 is

Table 1. Intensity distribution in the spectrum of a lasertorch from CuSbS2

λ, nm(atom, ion) Upper level Eexc, eV I/kλ, rel. units

527.9 S I 5p3P2, 1, 0 9.21 0.10

521.8 Cu I 4d2D5/2 6.19 1.00

515.3 Cu I 4d2D3/2 6.19 0.50

510.6 Cu I 4p2 3.82 0.40

504.7 S II 4p2 16.52 0.10

465.7 S II 4p4 16.52 0.05

455.3 S II 4p12 17.81 <0.05

427.3 Cu I 5s14D7/2 7.74 <0.05

406.3 Cu I 5d2D3/2, 5/2 6.87 <0.05

368.5 Cu I 4d14F5/2 9.09 <0.05

327.4 Cu I 4p2 3.79 0.25

324.8 Cu I 4p2 3.82 0.20

301.6 S II 4d12G7/2 20.37 0.05

287.8 Sb I 5p2(3P0)6s4P1/2 5.36 0.10

277.0 Sb I 5p2(3P1)6s4P3/2 5.69 <0.05

267.1 Sb I 5p2(3P1)6s2P5/2 5.69 <0.05

259.8 Sb I 5p2(3P1)6s2P1/2 5.82 0.25

252.9 Sb I 5p2(3P2)6s2P3/2 6.12 0.10

231.1 Sb I 5p2(3P0)6s4P1/2 5.36 <0.05

P3/2°

P3/2°

S3/2°

P3/2°

P1/2°

P3/2°

Table 2. Location of the narrow region on the energy scale, Enr,and the energy gap between the ionization energy Ei and Enr

Element *Cu I Sb ICuSbS2

*Cu I Sb I S I

Enr, eV 9.12 7.51 9.09 6.12 9.21

Ei – Enr, eV 0.7 1.1 0.99 2.5 2.37

* The energy gap is counted from the highest shifted energy term.

636 SHUAIBOV et al.

a p-type semiconductor with a band gap width of0.84 eV [20], while the photon energy insignificantlyexceeds 1 eV, photoelectrons arise, which, apparently,compensate the positive charge and are accumulatedunder the heated surface layer of the target. The pres-

0 50 100 150 200 t, ns

327.4 Cu I

521.8 Cu I

515.3 Cu I

510.6 Cu I

324.8 Cu I

301.2 S III, rel. units

Fig. 2. Oscillograms of spectral emission lines from thecentral region of the erosion torch (r = 1 mm). Wavelengths(in nm) are indicated near the lines.

0 100

–3

–2

200 t, ns

–1

0ln(I/Imax)

24

15

54110

7462

85

115

307

146108 1492 6

5

43

1

Fig. 3. Time dependences of the logarithm of spectral lineintensity normalized to the maximum value (r = 1 mm):(1) λ = 301.2 nm, S II; (2) λ = 324.8 nm, Cu I; (3) λ =510.6 nm, Cu I; (4) λ = 515.3 nm, Cu I; (5) λ = 521.8 nm,Cu I; (6) λ = 324.7 nm, Cu I. The recombination times(in ns) are indicated near the curves.

ence of maxima of the electron temperature and densitybetween the two emission maxima favors this hypothe-sis [17].

The aforementioned high electron temperature maycause an intense excitation of lower energy states ofCu I (Fig. 3). Since it is mainly the increase in pressurethat stops evaporation [10], this increase also facilitatesthe diffusion of copper, which is characteristic of thecompound under study [21].

During the first stage of evaporation, recombinationoccurs and excited states are intensively filled; the effi-ciency of plasma emission may be as high as 50–80%of the absorbed energy [22]. Thus, the action of hotvapor particles, high electron temperature, and intenseplasma emission lead to the evaporation of internal lay-ers of the target material. In this case, electrons shouldplay a key role, which is confirmed by the substantialsimilarity between the torches from solid targetsformed by laser radiation and by electron fluxes [23].

The mechanisms of energy transfer change in thecourse of time and the amount of energy transferred tothe surface decreases. Thus, the deeper the layers of thetarget material that are evaporated, the higher the prob-ability of conserving the chemical structure. Therefore,the dissociative recombination is more pronounced bythe end of evaporation; it also contributes to the delayof radiative transitions from lower levels and to the timeof recombination to these levels.

PARAMETERS OF THE TORCH PLASMA

Before determining the parameters of the torchplasma, it is pertinent to review the structure of CuSbS2and the features of its phase transitions. This compoundis characterized by melting at a constant temperature.The dominance of one of the components of this com-pound during its preparation leads to several thermaleffects manifesting themselves in thermograms: differ-ent phase states of the alloy components can exist inde-pendently. Upon melting, the CuSbS2 compound maydecompose into simpler molecules, which exist in bothliquid and vapor phases. The boundaries of the temper-ature range of existence of the multiphase substancewere not found [24].

In the case under consideration, the characteristictime of energy transfer is of the same order of magni-tude as the time of ionic reactions [6]; therefore, duringa time interval of about 10 ns, when the thermal diffu-sion can be neglected, the short-range order and thebond structure [25, 26] (Fig. 4) can change significantlyonly in the vapor phase. The structure of covalent bondsin the CuSbS2 molecule suggests that its stability isretained up to the degree of ionization z = 5. In CuSbS2crystals, sulfur serves as a donor of conduction elec-trons, providing a free carrier concentration of about1017 cm–3 [27].



During collisions, recombination, and irradiation ofmultiply charged molecules of the evaporated sub-stance, the effect of bond redistribution and the chargeexchange [28, 29] initiate the formation of ionized frac-tals and atoms. In the case under consideration, themost likely pairs are Cu–S, Sb–S, and Cu–Sb, whosedissociation and recombination lead to the formation ofionized and excited atoms.

The recombination times of Cu+ and S2+ ions wereused to find the electron density: 1.82 × 1016 and 1.92 ×1016 cm–3, respectively. The difference can be attributedeither to the error of indirect measurements (for all thecalculations, it does not exceed 25%) or, what is morelikely, to the fact that the recombination times were cal-culated in the time intervals 90–100 and 125–175 ns forthe S and Cu ions, respectively. The temperature enter-ing formula (4) was calculated from the Boltzmann dis-tribution: 0.71 eV for copper and 0.81 eV for sulfur ata distance of 1 mm from the target.

The dependence of the logarithm of population ofexcited states on the level energy at a distance of 7 mmis shown in Fig. 5. It can be seen that, with an increasein distance, the temperature becomes more uniform:0.5, 0.49, and 0.33 eV for Cu, S, and Sb, respectively.The low temperature (as shown by the energy distribu-tion of the excited states of Sb) and the lower excitationefficiency can be related to the significant chemicalactivity of antimony [30], which is present in the gasphase in the form of polyatomic molecules [31]. A lowpopulation is also observed for lower excited levels ofcopper as a result of self-absorption at resonance tran-sitions or, judging from the positions of the points, dueto the specific features of breaking of chemical bondsbetween Cu and Sb atoms. It should be noted that theBoltzmann distribution at distances of 1 and 7 mm indi-cates a significant increase in the number of excitedparticles in the plasma torch with an increase in the dis-tance (at r = 7 mm, the population is higher by approx-imately a factor of 1.5).

This behavior of the plasma can be clarified to alarge extent by comparison of the electron temperaturewith the dissociation energies of the most stable molec-ular compounds based on Cu, Sb, and S [5, 32, 33](Table 3). The data listed in Table 3 confirm our sugges-tions and exclude collisions from the possible reasonsof dissociation of CuSbS2 ions. The processes of photo-and recombination-induced dissociation are dominant,respectively, at the beginning and the end of the evapo-ration process. Taking into account that the intensity ofa spectrum is primarily determined by the exposure(due to the specific features of the measuring system),the intensity of an averaged spectrum is related to alarger extent to the recombination mechanism (Fig. 2).Analysis of the temperature dependence of the disso-

ciative recombination coefficient yields α ~ [34].The exponent of the electron temperature in this depen-dence decreases with a decrease in the number of atoms

Te1/2–


in a molecule. Thus, the increase in the emission effi-ciency is in good agreement with the increase in theefficiency of dissociative recombination due to the tem-perature drop. It is likely that the dissociation itselfoccurs with a dominance of certain reactions in bothtime and space.

S

S

S

Sb

Cu

Cu

Sb

CuS

SSb

SCu

S

SbS

S–2

Sb+3

Cu+1

S–2

Fig. 4. Unit cell of a CuSbS2 crystal and the schematic dia-gram of bond formation in a CuSbS2 molecule.

4–2

0

8 12 16 20E, eV

2

4

6

8

ln(N/g)

0.490.50

0.33

Fig. 5. Dependences of the logarithm of population ofexcited states on the level energy: (j) S II, (d) Cu I, and(m) Sb I. The values of Te (in eV) are indicated near thecurves.

638 SHUAIBOV et al.

CONCLUSIONS

Destruction of polycrystalline CuSbS2 under theaction of pulsed laser infrared radiation of moderatepower can be described as sublimation dissociationwith a step-by-step spatial and temporal dynamics(characteristic of laser erosion), which manifests itselfvia emission. The target structure plays an importantrole in the formation of excited states. Chemical reac-tions in the vapor phase, leading to the competitive fill-ing of excited states of the atoms forming the target,affect the plasma parameters substantially.

The average value of the electron density in the laserplasma is consistent with the concentration of freecharge carriers in the solid target and amounts to (1.82–1.92) × 1016 cm–3. With an increase in the distance fromthe target, the electron temperature decreases (0.71 and0.50 eV at r = 1 and 7 mm, respectively) and becomesmore uniform. The recombination times of S III andCu II are 15 and 65–82 ns, respectively. In later stagesof evaporation (when particular processes terminateabruptly, giving way to others), the recombination isquasi-stationary.

The short duration of the emission from the levels inthe vicinity of the narrow region suggests the domi-nance of photostimulated generation of ionized atoms,while the emission dynamics indicates the selectivity ofthe mechanisms of occupation of excited states. It isproposed to perform in situ diagnostics of laser deposi-tion in the ultraviolet and visible spectral regions by thelines of Sb and Cu atoms, respectively.

ACKNOWLEDGMENTS

We are grateful to I.É. Kacher for his helpful partic-ipation in discussions of the mechanism of destructionof a polycrystalline target under the action of pulsedlaser radiation.

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Table 3. Dissociation energies of the most stable compoundsof Cu, Sb, and S

Molecule Ediss, eV

Cu3Sb 0.11

Cu2S 0.82

CuS 0.55

Sb2S3 1.63

CuSbS2 1.2

Cu2 2.05

Sb2 3.35

S2 4.38

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Translated by Yu. Sin’kov

spectroscopic diagnostics of a laser erosion plasma formed from cusbs2 polycrystals

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