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Plasma dynamics studies during deposition of thin film PbTe on a glass substrate
ARTICLE IN PRESS
Vacuum 80 (2006) 841–849 www.elsevier.com/locate/vacuum
Plasma dynamics studies during deposition of thin film PbTe on a glass substrate E. Rodriguez, E. Jimenez, L. Moya, C.L. Cesar, L.P. Cardoso, L.C. Barbosa Departamento de Eletroˆnica Quaˆntica, Instituto de Fı´sica ‘‘Gleb Wataghin’’, Universidade Estadual de Campinas, P.O. Box 6165, CEP 13084-971, SP, Brazil
Abstract PbTe thin films were prepared by pulsed laser deposition using a Nd:YAG laser (532 nm) in an argon atmosphere. Dynamic processes in the gas phase induced by Nd:YAG laser ablation of PbTe are investigated by analyzing the light emitted by the plume. Pressure in the chamber varied from 1.0 105 to 1.0 mbar. Space and time-resolved optical spectroscopy measurements indicate the presence of both neutral, Pb(I) and Te(I) and ionized, Pb(II) and Te(II), species. The velocities of the species remain unchanged for argon pressures up to 101 mbar, which suggests that expansion of the plume occurs without further collision with the foreign gas in this pressure range. It is found that an addition of 5 101 mbar Ar enhances the emission line intensity and leads to a decrease in species velocity observed at large distances from the target surface. Furthermore, a region of maximum emission intensity at distance of 5–10 mm from the target is found when a gas environment is present. r 2005 Elsevier Ltd. All rights reserved.
1. Introduction Lead telluride belongs to the group of AIVBVI semiconductors which crystallize in the FCC NaCl-type structure under ambient conditions. This material exhibits properties, which are unusual, and possibly unique, relative to other semiconductors. For instance, it possesses a high dielectric constant, high carrier mobility and a narrow fundamental band gap E0 0.3 eV whose Corresponding author. Tel.: +55 19 3788 5452;
fax:+55 19 3788 5427. E-mail address: eugenio@ifi.unicam.br (E. Rodriguez).
temperature coefficient (dE0/dT) is positive [1]. This material has also been the subject of many research efforts, due to the technological importance for its use in a variety of optoelectronic device applications like infrared detectors and tunable mid-infrared quantum well diode lasers [2]. More recently, PbTe grown in the form of quantum dots has been studied because of attendant non-linear optical properties [3]. The fact that PbTe exhibits quantum confinement effects in the region used for optical communications [4,5] makes this material an excellent candidate for developing switching devices [6].
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.07.005
ARTICLE IN PRESS E. Rodriguez et al. / Vacuum 80 (2006) 841–849
842
For many years, pulsed laser deposition (PLD), which enables stoichiometric mass transfer from the target to the substrate in a convenient gas atmosphere, has become a powerful technique for fabricating high-quality films of high-Tc superconductor oxides and a number of other materials including piezoelectrics and ferroelectrics. More recently, this technique was used to deposit polycrystalline and epitaxially oriented films of PbTe on KCl(1 0 0) and BaF2(1 1 1) substrates [7,8]. It is believed that the interaction between the ablated particles and the background gas greatly influences the quality of the films; however, details concerning particle kinetics were not entirely understood. This stimulated a large number of studies that were conducted in the past few years to develop a better understanding of the plasma expansion dynamics. The aim of this paper is to study the influence of background gas on the plume dynamics so that suitable conditions may be selected in order to grow high-quality thin films.
2. Experimental arrangement The basic experimental set-up used for PLD is shown in Fig. 1. It consists of a vacuum chamber pumped to a base pressure of 1 107 mbar by a turbo molecular pump. Laser ablation of a PbTe target (99.99%) was performed using the second harmonic of a Q-Switched Quantel Nd:YAG laser (532 nm, 4 ns, 20 Hz). The energy per pulse
Power Supply
Vacuum Chamber
Optical fiber Spectrometer Photomultilier
Digitiz er
PC
Boxcar
Plotter
Fig. 1. Experimental arrangement used for the experiments.
measured at the target surface was 70 mJ, which yielded a fluence of 2.2 J/cm2. The pulsed laser beam is focused at an incidence angle of 451 onto a rotating target, using a 30 cm lens. In order to analyze the influence of argon pressure, the experiments were carried out in vacuum (1.0 105 mbar) and under an argon pressure of 0.1, 0.5 and 1.0 mbar, at fixed energy density. High-purity argon (99.999%) was used for the experiments. The light emitted from a selected spatial region of the laser-induced plasma is collected by a lens (f ¼ 12.5 cm), placed and centered at two times the focal distance. The resulting convergent beam is directed into a silica/silica optical fiber (200 mm high OH silica core) that guides the radiation to the entrance slit of a KRATOS spectrometer (2.0 A˚ resolution). The optical image of the plume is scanned along the target normal to analyze different segments of it, with a spatial resolution of 200 mm. The light emitted is collected by a photomultiplier (15 ns rise time) connected to a boxcar averager for recording the spectra (320–600 nm). For the time-resolved measurements, the spectrometer was tuned to a selected wavelength and the signal from the photomultiplier was then fed into a 1 GHz digitizer (Tektronik TDS 7104). The boxcar analyzer and the oscilloscope were triggered by a reference signal originating from the laser cavity that was coincident with the onset of the laser pulse. Thin films of PbTe were grown using a target–substrate distance of 5 cm and an argon pressure in the chamber of 0.5 mbar. The laser fluence was varied from 2.2 to 20 J/cm2 by keeping the laser energy constant and reducing the laser spot size. The films were deposited onto BK7 Corning Glass substrates maintained at room temperature. Just before use, they were ultrasonically cleaned for 10 min each in (a) distilled water with detergent, (b) hydrochloric acid, (c) distilled water, (d) acetone, and (e) ethanol, and finally dried in flowing pure argon. The crystalline structure of the films was studied using an X-ray diffractometer with FeKa radiation. The surface morphology of the deposited films was analyzed by scanning electron microscopy (SEM) using a Jeol JSM 6330 F microscope.
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3. Results 3.1. Total emission spectrum Plasma emission spectra shown in Fig. 2 were acquired using a 12 ms integration time for two chamber pressures: 1.0 105 mbar; and, in an argon atmosphere (0.5 mbar). In both cases, the spectra were recorded at a distance of 5 mm from the target to avoid the contribution of the continuous emission corresponding to the electrons, which is strong for distances from the target less than 2 mm. Spectra obtained show several discrete peaks superimposed on a continuous background. Most peaks were identified as belonging to lead and tellurium, neutral or singly ionized. New emission lines were not found in the spectrum when recorded in an argon atmosphere in comparison with the spectrum obtained in vacuum. However, an increase in the intensity of the emission lines is observed in the case of spectra obtained in the presence of argon. Spectral lines with wavelength shorter than 415 nm show a slight increase in the emission intensity; however, longer wavelength features show a considerable increase in intensity. As seen in Fig. 2, the emission peaks are noticeably broadened above the natural line width PbI
TeII
6 Intensity (a.u.)
PbII
PbI
9
&
Nd:YAG
TeII
laser PbII
3
(b) 0 4 2
(a)
0 350
400 450 Wavelength (nm)
500
550
Fig. 2. Spectra of the temporally integrated plasma emission recorded at 5 mm from the PbTe target using an incident energy density of 2.2 J/cm2. The pressure in the vacuum chamber was (a) 1.0 105 mbar and (b) 0.5 mbar.
843
(104 A˚), and no pressure dependence on peaks width is observed. Stark broadening is mainly responsible for the peak width yielding linewidths on the order of 2 A˚ [9]. Those changes are smaller than the resolution of the spectroscopic system used, and consequently the main reason they could not be detected. 3.2. Time evolution of species The time evolution of four atomic emission lines has been studied. The lines were identified according to standard references [10,11]. Each line belongs to a different emitting species: Pb(I) (l ¼ 405.8 nm) to neutral atomic lead [12], Pb(II) (l ¼ 504.3 nm) to singly ionized atomic lead [13], Te(I) (l ¼ 875.8 nm) to neutral atomic telluride [14] and Te(II) (l ¼ 424.2 nm) to singly ionized atomic telluride [15]. Table 1 presents other relevant data concerning these atomic lines. Each emission line has been studied in real time at different distances from the target surface. The emission spectra of all measured spectral lines show common features, which are illustrated in Figs. 3 and 4 using the results obtained for Pb(I). The emission spectra were recorded both in vacuum (Fig. 3) and under an argon atmosphere (Fig. 4) at different distances from the target surface. The overall emission intensity is higher in argon than in vacuum and therefore it can be recorded at greater distances from the target surface when ablation takes place in the presence of argon (Fig. 4). Real time emission originated from the surface of the target for the 405.8 nm line, clearly shows that there are two components that contribute to the total emission. The first one appears during the first phase of plasma formation, approximately 10–20 ns after the incident laser pulse. This component is associated with the peak of continuous emission (present throughout the spectrum range studied) and rapidly decreases as the distance from the target increases. The second component, which peaks 80–100 ns after the laser pulse, has a transient and a long tail and corresponds to the emission characteristic of the atomic lead. This peak is slightly displaced for longer times as the distance from the target
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Table 1 Energy levels of four emission lines present in the plume Species
Pb(I)
Pb(II)
Te(I)
Te(II)
Lower level Configuration K (cm1) E (eV)
Upper level Configuration K (cm1) E (eV)
6s26p2 3P0 10650.32 1.32 6s27p 2P01/2 74458.99 9.23 5p36s 5S20 44 253 5.49 5s25p26p 3P5/2 102324.95 12.69
6s26p27s 3P10 35287.22 4.37 6s27d 2D3/2 94284.58 11.69 5p36p 3P2 55667.76 6.90 5s25p26p {6d,7s} 125906.50 15.61
DE (eV)
Wavelength (nm)
Ionization energy (eV)
405.8
7.42
504.3
15.03
875.8
9.01
424.2
18.6
3.05
2.46
1.41
2.92
The transitions involved were quoted from Refs. [10–15].
x10-2
x10-1
x10-1
4
2
x10-1
g
c
b
5
f
2
d
2
h
c
0
g
e
b
0
f
0 d
-2 -2
a
Intensity (a.u.)
Intensity (a.u.)
a
0 e
-5
-2 -10
-4
-4
-15
-4
-6 -6
-20
0.0 (I)
1.0 Time (µs)
0.0 (II)
1.5
3.0
Time(µs)
Fig. 3. Emission transients of the emission line Pb(I) 405.8 nm recorded at 105 mbar for different target-substrate distances. I: (a) surface of the target, (b) 2 mm and (c) 4 mm. II: (d) 6 mm, (e) 8 mm, (f) 10 mm and (g) 12 mm.The signal is in arbitrary units, but the scale is the same in order to enable a comparison between the waveforms.
0
(I)
1
Time (µs)
0
2
(II)
5
10
Time (µs)
Fig. 4. Emission transients of the emission line Pb(I) 405.8 nm recorded at 0.5 mbar for different target–substrate distances. I: (a) target surface, (b) 3 mm, (c) 6 mm and (d) 9 mm. II: (e) 13 mm (f) 17 mm (g) 21 mm and (h) 25 mm.The signal is in arbitrary units, but the scale is the same in order to enable a comparison between the waveforms.
ARTICLE IN PRESS E. Rodriguez et al. / Vacuum 80 (2006) 841–849
temperatures of 12,000 K are calculated for the plasma. This value is in the range of reported plasma temperatures generated by PLD in other experiments [17]. The comparison between the emission of the species in vacuum and in a gas environment allow two regions to be distinguished as a function of distance (d), to target. For 0pdp1 mm, the emission intensity has the same behavior in both environments. At larger distances dX2 mm the emission intensity depends on the pressure of the background gas. When a background gas is introduced into the chamber, the time delay between the laser pulse and the transient emission maximum no longer exhibits a linear dependence on the distance to the
(c)
PbII
(b)
TeII
(a)
TeI
20
10
Distance from target (mm)
increases. The moment when the maximum value of intensity is reached may be associated with the time required for most of the Pb(I) atoms pass through the observation point. At shorter and longer times, there are the particles that travel at higher and lower velocity, respectively. Therefore, the time delay for the maximum emission as a function of the distance from the target enables the average velocities of the species that travel in the plume to be determined. On the target perimeter (0–1 mm), the peak of continuous emission is superimposed on the emission characteristic of a particular species, thus masking or overshadowing it. The temporal resolution of the spectroscopic system used does not allow separation of both peaks. For this reason, the intensity values for distances less than 1 mm will not be taken into consideration for the calculation of the velocities reported in this paper. When experiments are carried out in vacuum, the delay associated with the emission exhibits a linear relationship with the distance from the target within the 2–12 mm distance from the target. Slopes of the straight lines adjusted by linear regression, give the average velocity of the components of the plume during the expansion. The velocities calculated for the different species studied were as follows: (7.670.1) 105 cm/s for Pb(I), (1.470.2) 106 cm/s for Pb(II), (1.307 0.02) 106 cm/s for Te(I), and (1.3570.03) 105 cm/s for Te(II). Velocities of neutral atoms were lower than those of the corresponding ions. According to the model proposed by Singh and Narayan [16], the plasma generated by the interaction of the laser with the target undergoes an adiabatic expansion after the termination of the laser pulse. The thermal energy is rapidly converted into kinetic energy and the plume front attains asymptotic pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiexpansion velocities of vm ¼ 2=ðg 1Þ gkT=M where k is the Boltzman constant, m is the atomic mass of the species and g ¼ Cp/Cv is the specific heat ratio at constant pressure and volume, respectively. Using an approximate value of 1.2 for g that takes into account the degrees of freedom associated with ionization and dissociation and the calculated value of vm ¼ 7.6 105 cm/s and the atomic mass for neutral lead of M ¼ 207.2 u,
845
0
20
10
0
20
10
0 0
1000
2000 3000 Time (ns)
4000
Fig. 5. Time delay between the laser pulse and the maximum emission intensity from (a) Te(I), (b) Te(II) and (c) Pb(II) transitions as a function of the distance from the target. Results are recorded in vacuum (W) and 0.5 mbar argon (J). The solid line are linear fits of the results recorded in vacuum (105 mbar).
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target. This observation is illustrated in Fig. 5, which shows results obtained for three measured emission lines. It was possible to detect the emission of the atomic transitions studied at times on the order of several microseconds after the laser pulse was terminated. For the Pb(I) emission line for example, the transient emission at 25 mm distance from the target was detected about 5 ms after the end of the laser pulse. These times are considerably larger than those of the lifetimes of the respective transitions (usually some tens of nanoseconds), thus suggesting that the plume maintains a high temperature within our observation region (2–25 mm from the target) and that the atoms are re-excited many times during their travel. Part of the plume’s expansions energy is transferred to the background gas as thermal energy in the gas shock front creating more electrons by ionization of the surrounding gas atoms. These electrons through collisions are most likely responsible for the additional excitation of the plume atoms during their travel [18,19].
1.2
Values of total emission intensity obtained from boxcar integration of the signal for each spectral line studied were plotted as a function of the distance from the target for different pressures in the chamber. Fig. 6 shows the results for the line corresponding to Te(II) (l ¼ 424.2 nm). In the case of experiments conducted over the pressure range from 1.0 105 mbar up to 0.1 mbar, the total emission intensity decreases exponentially as a function of the distance from the target. When the pressure in the chamber varies between 0.5–1.0 mbar, the intensity does not exhibit an exponential decrease across the whole interval of target–substrate distances. A region of considerable increase in the emission intensity appears, thus suggesting plasma compression. The distance where the maximum emission appears varies for each species and is displaced to shorter distances as the pressure increases.
1.0 mbar
(c)
0.5 mbar
(b)
10-1 mbar
(a)
10-5 mbar
0.8
0.6
0.4
0.2
0.0
0
3.3. Variation of the total intensity of emission as a function of the distance from the target
(d)
1.0
Intensity (a.u.)
846
5
10
15
20
25
Distance to the target (mm) Fig. 6. Plots of the temporally integrated emission intensities of Te(II) 424.2 nm vs. distance to the target for ablation in (a) 1.0 105 mbar, (b) 0.1 mbar argon; (c) 0.5 mbar argon and (d) 1.0 mbar argon. Lines are a guide for the eye.
3.4. Structural and morphological properties of the films Structural analyses of the samples were carried out by means of X-ray diffraction at room temperature (TE300 K) using y:2y scans in a Philips (PW 11700) diffractometer system with Fe Ka radiation (l ¼ 1.93597 A˚) and using a graphite monochromator for the diffracted beam. The measurements were acquired using an 0.021 step size and 2 s/step. Fig. 7 shows the XRD patterns of the PbTe target and two films deposited at different laser fluences. The target is polycrystalline with intense characteristic peaks (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2) and (4 0 0), corresponding to the NaCl fcc structure of PbTe.
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Spectra for as-deposited films grown at 2.2 J/ cm2 (Fig. 7a) and 20 J/cm2 (Fig. 7b) indicate that they are also polycrystalline. The film grown at lower laser fluence is preferentially oriented in the (1 1 1) direction, however, the film grown at higher fluence is preferentially oriented in the (2 0 0) direction, reproducing the crystal orientation present in the target. The diffractograms were analyzed by means of Warren’s method [20] and the peak corresponding to (0 1 2) reflection of the standard Al2O3 powder which appears at 2yffi32.301 was used for instrumental calibration purposes in determining the crystallite size. Using this method it was possible to determine the
Intensity (a.u.)
(b)
847
crystallite size for both films and target. The results of these calculations indicated average crystallite size on the order of 32.4 and 25.2 nm for the samples grown at lower fluence (Fig. 7a) and higher fluence (Fig. 7b) laser fluence respectively, and 80.5 nm for the target. Fig. 8 shows SEM images of both films presented in Figs. 7a and b. The surface of the film grown at the lower fluence (Fig. 8a) shows micro-particulates in the form of droplets (splashing effect), caused by ejection of molten material from the target surface [21]. The film grown at higher fluence exhibits a very smooth surface and practically no presence of particulates or droplets. Crystallites with dimensions on the order of 25–30 nm are seen on the film surface in good agreement with the crystallite size determined from measured XRD data.
4. Discussions (a)
(200)
20
30
(220) (222) (311)
(111)
40
50 2θ (deg.)
60
target (400) 70
80
Fig. 7. Typical X-ray diffraction patterns of two films grown at room temperature, 0.5 mbar argon and different energy densities: (a) 2.2 J/cm2; (b) 20 J/cm2. For comparison, the target diffraction pattern was also included.
Optical emission spectroscopy was successfully applied to obtain information of the dynamics of the plasma generated by PLD. As a result, it was possible to identify and study the behavior of ionized and neutral species within the plume. Spatial distribution is different for each specie: the ionized species appear first and with slightly greater velocity than the neutral species. This fact may be explained by coulomb interactions within the first moments of plasma expansion.
Fig. 8. SEM image of the film surface grown at different fluences: (a) 2.2 J/cm2, (b) 20 J/cm2.
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Continuous emission observed in the target perimeter (Fig. 3), reaches its maximum value a few nanoseconds after the laser pulse, irrespective of the distance from the target. The peak is very intense throughout the measured spectral range, which suggests the existence of a dense cloud of excited electrons in this region responsible for most of the continuous background and for the excitation of the species ejected during the first phases of plasma expansion. Such a continuous background is fundamentally a result of Bremsstrahlung and recombination emission. As the plume expands into the vacuum, the density of electrons decreases and, consequently, the likelihood of excitation of the ejected species becomes lower, dropping to zero at distances a few millimeters from the target surface, as shown in Figs. 3 and 4 for Pb(I). The presence of a background gas during ablation has two important effects on the plasma expansion process. First, the emission significantly increases for argon pressures greater than or equal to 0.1 mbar (Fig. 4). Secondly, for distances greater than 2 mm, it produces a non-linear dependence between the delay for the maximum emission and the distance from the target (Fig. 5). The ejected particles are slowed down. This slowing down has been explained in terms of elastic scattering of the ablated particles by molecules or atoms of the ambient gas [22]. As indicated above, a region of considerable increase in the emission intensity appears when experiments were carried out under argon at pressures from 0.5–1.0 mbar. The existence of this high-compression region in the plasma should result in an homogenization of the distribution of specie velocities. The particles would reach the substrate together, which may improve the properties of the films. A model to describe this behavior of intensity as a function of the distance from the target does not exist in the literature; however, detailed knowledge of this phenomenon may help in the control of film growth. The (1 1 1) preferential orientation shown in samples grown at lower fluence is probably due to a predominant splashing effect.
5. Conclusion Real time studies of plasma emission generated by PbTe ablation enabled the detection of both neutral and ionized Pb and Te at distances close to the position where the substrate is located. The presence of such species may influence properties of the deposited films. The presence of ambient gas during the process greatly influences the plasma expansion dynamics. The emission intensity considerably increases and the species are slowed down on their path to the substrate. A plasma compression region was found over the pressure range from 0.5 to 1.0 mbar. This region may produce a homogenization in the distribution of the velocities of the ejected species, thus improving the film properties. The increase in laser fluence arising from decreasing the laser spot size has two positive effects on the films: first, it increases the energy of the ionized species in the plume favoring the growth of crystalline films; second, it leads to a considerable reduction of droplet density on the film surface which also improves the film quality. A laser fluence of 20 J/cm2 and a pressure of 0.5 mbar Ar are specified as suitable conditions to grow highly oriented PbTe films showing a relatively smooth surface morphology and practically no presence of particulates or droplets. The fact that we obtained highly oriented PbTe films at room temperature on glass substrates is particularly remarkable and has not been reported in literature. Acknowledgements This work was supported by FAPESP under project 00/12079-0. The authors also kindly acknowledge CEPOF and PRONEX for partial support and Laborato´rio de Microscopia Eletroˆnica from LNLS for the SEM images. References [1] Suzuki N, Adachi S. Jpn J Appl Phys 1994;193:33. [2] Spra¨nger B, Schiessl U, Lambrecht A, Bo¨tter H, Tacke M. Appl Phys Lett 1988;53:2582.
ARTICLE IN PRESS E. Rodriguez et al. / Vacuum 80 (2006) 841–849 [3] Rodriguez E, Jimenez E, Neves AAR, Jacob GJ, Ce´sar CL, Barbosa LC. Physica E 2005;26:361–5. [4] Tudury GE, Marquezini MV, Ferreira LG, Barbosa LC, Cesar CL. Phys Rev B 2000;62:7357. [5] Jacob GJ, Cesar CL, Barbosa LC. Phys Chem of Glasses 2002;43C:250. [6] Rodriguez E, Jimenez E, Neves AAR, Jacob GJ, Cesar CL, Barbosa LC. Appl Phys Lett 2005;86:1. [7] Baleva M, Mateeva E, Momtchilova M. J Phys Condens Matter 1992;4:8997. [8] Jacquot A, Lenoir B, Boffoue´ MO, Dauscher A. Appl Phys A 1999;69:S613. [9] Geohegan DB. In: Chrisey DB, Hubler GK, editors. Pulsed laser deposition of thin films. New York: Wiley; 1994 Chapter 5. [10] Moore CE. Atomic Energy levels as derived from the analyses of optical spectra (Nat. Stand. Ref. Data Ser., Nat. Bur. Stand.), vol 1and 3, Washington DC, 1971. [11] Weast RC. Handbook of Chemistry and physics, 72nd ed. Boca Raton, FL, USA: CRC Press; 1992.
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[12] Wood D, Andrew KL. J Opt Soc Am 1968;58:818. [13] Wood DR, Ross CB, Scholl PS, Hoke M. J Opt Soc Am 1974;64:1159. [14] Morillon C, Verges J. Phys Scr 1975;12:129. [15] Handrup MB, Mack JE. Physica 1964;30:1245. [16] Singh RK, Narayan J. Phys Rev B 1990;41:8843. [17] Dyer PE, Greenough RD, Issa A, Key PH. Appl Phys Lett 1988;53:534. [18] Gu HP, Lou QH, Cheung NH, Chen SC, Wang ZY, Lin PK. Appl Phys B 1994;58:143. [19] Gonzalo J, Vega F, Afonso CN. J Appl Phys 1995;77: 6588. [20] Warren BE. X-ray difraction. London: Adilson-Wesley; 1969. p.13–16. [21] Cheung JT, Sankur H. Growth of thin-films by laserinduced evaporation. CRC Crit Rev in Solid State Mater Sci 1988;15:63. [22] Kools JCS. J Appl Phys 1993;74:10.
Vacuum 80 (2006) 841–849 www.elsevier.com/locate/vacuum
Plasma dynamics studies during deposition of thin film PbTe on a glass substrate E. Rodriguez, E. Jimenez, L. Moya, C.L. Cesar, L.P. Cardoso, L.C. Barbosa Departamento de Eletroˆnica Quaˆntica, Instituto de Fı´sica ‘‘Gleb Wataghin’’, Universidade Estadual de Campinas, P.O. Box 6165, CEP 13084-971, SP, Brazil
Abstract PbTe thin films were prepared by pulsed laser deposition using a Nd:YAG laser (532 nm) in an argon atmosphere. Dynamic processes in the gas phase induced by Nd:YAG laser ablation of PbTe are investigated by analyzing the light emitted by the plume. Pressure in the chamber varied from 1.0 105 to 1.0 mbar. Space and time-resolved optical spectroscopy measurements indicate the presence of both neutral, Pb(I) and Te(I) and ionized, Pb(II) and Te(II), species. The velocities of the species remain unchanged for argon pressures up to 101 mbar, which suggests that expansion of the plume occurs without further collision with the foreign gas in this pressure range. It is found that an addition of 5 101 mbar Ar enhances the emission line intensity and leads to a decrease in species velocity observed at large distances from the target surface. Furthermore, a region of maximum emission intensity at distance of 5–10 mm from the target is found when a gas environment is present. r 2005 Elsevier Ltd. All rights reserved.
1. Introduction Lead telluride belongs to the group of AIVBVI semiconductors which crystallize in the FCC NaCl-type structure under ambient conditions. This material exhibits properties, which are unusual, and possibly unique, relative to other semiconductors. For instance, it possesses a high dielectric constant, high carrier mobility and a narrow fundamental band gap E0 0.3 eV whose Corresponding author. Tel.: +55 19 3788 5452;
fax:+55 19 3788 5427. E-mail address: eugenio@ifi.unicam.br (E. Rodriguez).
temperature coefficient (dE0/dT) is positive [1]. This material has also been the subject of many research efforts, due to the technological importance for its use in a variety of optoelectronic device applications like infrared detectors and tunable mid-infrared quantum well diode lasers [2]. More recently, PbTe grown in the form of quantum dots has been studied because of attendant non-linear optical properties [3]. The fact that PbTe exhibits quantum confinement effects in the region used for optical communications [4,5] makes this material an excellent candidate for developing switching devices [6].
0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.07.005
ARTICLE IN PRESS E. Rodriguez et al. / Vacuum 80 (2006) 841–849
842
For many years, pulsed laser deposition (PLD), which enables stoichiometric mass transfer from the target to the substrate in a convenient gas atmosphere, has become a powerful technique for fabricating high-quality films of high-Tc superconductor oxides and a number of other materials including piezoelectrics and ferroelectrics. More recently, this technique was used to deposit polycrystalline and epitaxially oriented films of PbTe on KCl(1 0 0) and BaF2(1 1 1) substrates [7,8]. It is believed that the interaction between the ablated particles and the background gas greatly influences the quality of the films; however, details concerning particle kinetics were not entirely understood. This stimulated a large number of studies that were conducted in the past few years to develop a better understanding of the plasma expansion dynamics. The aim of this paper is to study the influence of background gas on the plume dynamics so that suitable conditions may be selected in order to grow high-quality thin films.
2. Experimental arrangement The basic experimental set-up used for PLD is shown in Fig. 1. It consists of a vacuum chamber pumped to a base pressure of 1 107 mbar by a turbo molecular pump. Laser ablation of a PbTe target (99.99%) was performed using the second harmonic of a Q-Switched Quantel Nd:YAG laser (532 nm, 4 ns, 20 Hz). The energy per pulse
Power Supply
Vacuum Chamber
Optical fiber Spectrometer Photomultilier
Digitiz er
PC
Boxcar
Plotter
Fig. 1. Experimental arrangement used for the experiments.
measured at the target surface was 70 mJ, which yielded a fluence of 2.2 J/cm2. The pulsed laser beam is focused at an incidence angle of 451 onto a rotating target, using a 30 cm lens. In order to analyze the influence of argon pressure, the experiments were carried out in vacuum (1.0 105 mbar) and under an argon pressure of 0.1, 0.5 and 1.0 mbar, at fixed energy density. High-purity argon (99.999%) was used for the experiments. The light emitted from a selected spatial region of the laser-induced plasma is collected by a lens (f ¼ 12.5 cm), placed and centered at two times the focal distance. The resulting convergent beam is directed into a silica/silica optical fiber (200 mm high OH silica core) that guides the radiation to the entrance slit of a KRATOS spectrometer (2.0 A˚ resolution). The optical image of the plume is scanned along the target normal to analyze different segments of it, with a spatial resolution of 200 mm. The light emitted is collected by a photomultiplier (15 ns rise time) connected to a boxcar averager for recording the spectra (320–600 nm). For the time-resolved measurements, the spectrometer was tuned to a selected wavelength and the signal from the photomultiplier was then fed into a 1 GHz digitizer (Tektronik TDS 7104). The boxcar analyzer and the oscilloscope were triggered by a reference signal originating from the laser cavity that was coincident with the onset of the laser pulse. Thin films of PbTe were grown using a target–substrate distance of 5 cm and an argon pressure in the chamber of 0.5 mbar. The laser fluence was varied from 2.2 to 20 J/cm2 by keeping the laser energy constant and reducing the laser spot size. The films were deposited onto BK7 Corning Glass substrates maintained at room temperature. Just before use, they were ultrasonically cleaned for 10 min each in (a) distilled water with detergent, (b) hydrochloric acid, (c) distilled water, (d) acetone, and (e) ethanol, and finally dried in flowing pure argon. The crystalline structure of the films was studied using an X-ray diffractometer with FeKa radiation. The surface morphology of the deposited films was analyzed by scanning electron microscopy (SEM) using a Jeol JSM 6330 F microscope.
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3. Results 3.1. Total emission spectrum Plasma emission spectra shown in Fig. 2 were acquired using a 12 ms integration time for two chamber pressures: 1.0 105 mbar; and, in an argon atmosphere (0.5 mbar). In both cases, the spectra were recorded at a distance of 5 mm from the target to avoid the contribution of the continuous emission corresponding to the electrons, which is strong for distances from the target less than 2 mm. Spectra obtained show several discrete peaks superimposed on a continuous background. Most peaks were identified as belonging to lead and tellurium, neutral or singly ionized. New emission lines were not found in the spectrum when recorded in an argon atmosphere in comparison with the spectrum obtained in vacuum. However, an increase in the intensity of the emission lines is observed in the case of spectra obtained in the presence of argon. Spectral lines with wavelength shorter than 415 nm show a slight increase in the emission intensity; however, longer wavelength features show a considerable increase in intensity. As seen in Fig. 2, the emission peaks are noticeably broadened above the natural line width PbI
TeII
6 Intensity (a.u.)
PbII
PbI
9
&
Nd:YAG
TeII
laser PbII
3
(b) 0 4 2
(a)
0 350
400 450 Wavelength (nm)
500
550
Fig. 2. Spectra of the temporally integrated plasma emission recorded at 5 mm from the PbTe target using an incident energy density of 2.2 J/cm2. The pressure in the vacuum chamber was (a) 1.0 105 mbar and (b) 0.5 mbar.
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(104 A˚), and no pressure dependence on peaks width is observed. Stark broadening is mainly responsible for the peak width yielding linewidths on the order of 2 A˚ [9]. Those changes are smaller than the resolution of the spectroscopic system used, and consequently the main reason they could not be detected. 3.2. Time evolution of species The time evolution of four atomic emission lines has been studied. The lines were identified according to standard references [10,11]. Each line belongs to a different emitting species: Pb(I) (l ¼ 405.8 nm) to neutral atomic lead [12], Pb(II) (l ¼ 504.3 nm) to singly ionized atomic lead [13], Te(I) (l ¼ 875.8 nm) to neutral atomic telluride [14] and Te(II) (l ¼ 424.2 nm) to singly ionized atomic telluride [15]. Table 1 presents other relevant data concerning these atomic lines. Each emission line has been studied in real time at different distances from the target surface. The emission spectra of all measured spectral lines show common features, which are illustrated in Figs. 3 and 4 using the results obtained for Pb(I). The emission spectra were recorded both in vacuum (Fig. 3) and under an argon atmosphere (Fig. 4) at different distances from the target surface. The overall emission intensity is higher in argon than in vacuum and therefore it can be recorded at greater distances from the target surface when ablation takes place in the presence of argon (Fig. 4). Real time emission originated from the surface of the target for the 405.8 nm line, clearly shows that there are two components that contribute to the total emission. The first one appears during the first phase of plasma formation, approximately 10–20 ns after the incident laser pulse. This component is associated with the peak of continuous emission (present throughout the spectrum range studied) and rapidly decreases as the distance from the target increases. The second component, which peaks 80–100 ns after the laser pulse, has a transient and a long tail and corresponds to the emission characteristic of the atomic lead. This peak is slightly displaced for longer times as the distance from the target
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Table 1 Energy levels of four emission lines present in the plume Species
Pb(I)
Pb(II)
Te(I)
Te(II)
Lower level Configuration K (cm1) E (eV)
Upper level Configuration K (cm1) E (eV)
6s26p2 3P0 10650.32 1.32 6s27p 2P01/2 74458.99 9.23 5p36s 5S20 44 253 5.49 5s25p26p 3P5/2 102324.95 12.69
6s26p27s 3P10 35287.22 4.37 6s27d 2D3/2 94284.58 11.69 5p36p 3P2 55667.76 6.90 5s25p26p {6d,7s} 125906.50 15.61
DE (eV)
Wavelength (nm)
Ionization energy (eV)
405.8
7.42
504.3
15.03
875.8
9.01
424.2
18.6
3.05
2.46
1.41
2.92
The transitions involved were quoted from Refs. [10–15].
x10-2
x10-1
x10-1
4
2
x10-1
g
c
b
5
f
2
d
2
h
c
0
g
e
b
0
f
0 d
-2 -2
a
Intensity (a.u.)
Intensity (a.u.)
a
0 e
-5
-2 -10
-4
-4
-15
-4
-6 -6
-20
0.0 (I)
1.0 Time (µs)
0.0 (II)
1.5
3.0
Time(µs)
Fig. 3. Emission transients of the emission line Pb(I) 405.8 nm recorded at 105 mbar for different target-substrate distances. I: (a) surface of the target, (b) 2 mm and (c) 4 mm. II: (d) 6 mm, (e) 8 mm, (f) 10 mm and (g) 12 mm.The signal is in arbitrary units, but the scale is the same in order to enable a comparison between the waveforms.
0
(I)
1
Time (µs)
0
2
(II)
5
10
Time (µs)
Fig. 4. Emission transients of the emission line Pb(I) 405.8 nm recorded at 0.5 mbar for different target–substrate distances. I: (a) target surface, (b) 3 mm, (c) 6 mm and (d) 9 mm. II: (e) 13 mm (f) 17 mm (g) 21 mm and (h) 25 mm.The signal is in arbitrary units, but the scale is the same in order to enable a comparison between the waveforms.
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temperatures of 12,000 K are calculated for the plasma. This value is in the range of reported plasma temperatures generated by PLD in other experiments [17]. The comparison between the emission of the species in vacuum and in a gas environment allow two regions to be distinguished as a function of distance (d), to target. For 0pdp1 mm, the emission intensity has the same behavior in both environments. At larger distances dX2 mm the emission intensity depends on the pressure of the background gas. When a background gas is introduced into the chamber, the time delay between the laser pulse and the transient emission maximum no longer exhibits a linear dependence on the distance to the
(c)
PbII
(b)
TeII
(a)
TeI
20
10
Distance from target (mm)
increases. The moment when the maximum value of intensity is reached may be associated with the time required for most of the Pb(I) atoms pass through the observation point. At shorter and longer times, there are the particles that travel at higher and lower velocity, respectively. Therefore, the time delay for the maximum emission as a function of the distance from the target enables the average velocities of the species that travel in the plume to be determined. On the target perimeter (0–1 mm), the peak of continuous emission is superimposed on the emission characteristic of a particular species, thus masking or overshadowing it. The temporal resolution of the spectroscopic system used does not allow separation of both peaks. For this reason, the intensity values for distances less than 1 mm will not be taken into consideration for the calculation of the velocities reported in this paper. When experiments are carried out in vacuum, the delay associated with the emission exhibits a linear relationship with the distance from the target within the 2–12 mm distance from the target. Slopes of the straight lines adjusted by linear regression, give the average velocity of the components of the plume during the expansion. The velocities calculated for the different species studied were as follows: (7.670.1) 105 cm/s for Pb(I), (1.470.2) 106 cm/s for Pb(II), (1.307 0.02) 106 cm/s for Te(I), and (1.3570.03) 105 cm/s for Te(II). Velocities of neutral atoms were lower than those of the corresponding ions. According to the model proposed by Singh and Narayan [16], the plasma generated by the interaction of the laser with the target undergoes an adiabatic expansion after the termination of the laser pulse. The thermal energy is rapidly converted into kinetic energy and the plume front attains asymptotic pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiexpansion velocities of vm ¼ 2=ðg 1Þ gkT=M where k is the Boltzman constant, m is the atomic mass of the species and g ¼ Cp/Cv is the specific heat ratio at constant pressure and volume, respectively. Using an approximate value of 1.2 for g that takes into account the degrees of freedom associated with ionization and dissociation and the calculated value of vm ¼ 7.6 105 cm/s and the atomic mass for neutral lead of M ¼ 207.2 u,
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0
20
10
0
20
10
0 0
1000
2000 3000 Time (ns)
4000
Fig. 5. Time delay between the laser pulse and the maximum emission intensity from (a) Te(I), (b) Te(II) and (c) Pb(II) transitions as a function of the distance from the target. Results are recorded in vacuum (W) and 0.5 mbar argon (J). The solid line are linear fits of the results recorded in vacuum (105 mbar).
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target. This observation is illustrated in Fig. 5, which shows results obtained for three measured emission lines. It was possible to detect the emission of the atomic transitions studied at times on the order of several microseconds after the laser pulse was terminated. For the Pb(I) emission line for example, the transient emission at 25 mm distance from the target was detected about 5 ms after the end of the laser pulse. These times are considerably larger than those of the lifetimes of the respective transitions (usually some tens of nanoseconds), thus suggesting that the plume maintains a high temperature within our observation region (2–25 mm from the target) and that the atoms are re-excited many times during their travel. Part of the plume’s expansions energy is transferred to the background gas as thermal energy in the gas shock front creating more electrons by ionization of the surrounding gas atoms. These electrons through collisions are most likely responsible for the additional excitation of the plume atoms during their travel [18,19].
1.2
Values of total emission intensity obtained from boxcar integration of the signal for each spectral line studied were plotted as a function of the distance from the target for different pressures in the chamber. Fig. 6 shows the results for the line corresponding to Te(II) (l ¼ 424.2 nm). In the case of experiments conducted over the pressure range from 1.0 105 mbar up to 0.1 mbar, the total emission intensity decreases exponentially as a function of the distance from the target. When the pressure in the chamber varies between 0.5–1.0 mbar, the intensity does not exhibit an exponential decrease across the whole interval of target–substrate distances. A region of considerable increase in the emission intensity appears, thus suggesting plasma compression. The distance where the maximum emission appears varies for each species and is displaced to shorter distances as the pressure increases.
1.0 mbar
(c)
0.5 mbar
(b)
10-1 mbar
(a)
10-5 mbar
0.8
0.6
0.4
0.2
0.0
0
3.3. Variation of the total intensity of emission as a function of the distance from the target
(d)
1.0
Intensity (a.u.)
846
5
10
15
20
25
Distance to the target (mm) Fig. 6. Plots of the temporally integrated emission intensities of Te(II) 424.2 nm vs. distance to the target for ablation in (a) 1.0 105 mbar, (b) 0.1 mbar argon; (c) 0.5 mbar argon and (d) 1.0 mbar argon. Lines are a guide for the eye.
3.4. Structural and morphological properties of the films Structural analyses of the samples were carried out by means of X-ray diffraction at room temperature (TE300 K) using y:2y scans in a Philips (PW 11700) diffractometer system with Fe Ka radiation (l ¼ 1.93597 A˚) and using a graphite monochromator for the diffracted beam. The measurements were acquired using an 0.021 step size and 2 s/step. Fig. 7 shows the XRD patterns of the PbTe target and two films deposited at different laser fluences. The target is polycrystalline with intense characteristic peaks (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2) and (4 0 0), corresponding to the NaCl fcc structure of PbTe.
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Spectra for as-deposited films grown at 2.2 J/ cm2 (Fig. 7a) and 20 J/cm2 (Fig. 7b) indicate that they are also polycrystalline. The film grown at lower laser fluence is preferentially oriented in the (1 1 1) direction, however, the film grown at higher fluence is preferentially oriented in the (2 0 0) direction, reproducing the crystal orientation present in the target. The diffractograms were analyzed by means of Warren’s method [20] and the peak corresponding to (0 1 2) reflection of the standard Al2O3 powder which appears at 2yffi32.301 was used for instrumental calibration purposes in determining the crystallite size. Using this method it was possible to determine the
Intensity (a.u.)
(b)
847
crystallite size for both films and target. The results of these calculations indicated average crystallite size on the order of 32.4 and 25.2 nm for the samples grown at lower fluence (Fig. 7a) and higher fluence (Fig. 7b) laser fluence respectively, and 80.5 nm for the target. Fig. 8 shows SEM images of both films presented in Figs. 7a and b. The surface of the film grown at the lower fluence (Fig. 8a) shows micro-particulates in the form of droplets (splashing effect), caused by ejection of molten material from the target surface [21]. The film grown at higher fluence exhibits a very smooth surface and practically no presence of particulates or droplets. Crystallites with dimensions on the order of 25–30 nm are seen on the film surface in good agreement with the crystallite size determined from measured XRD data.
4. Discussions (a)
(200)
20
30
(220) (222) (311)
(111)
40
50 2θ (deg.)
60
target (400) 70
80
Fig. 7. Typical X-ray diffraction patterns of two films grown at room temperature, 0.5 mbar argon and different energy densities: (a) 2.2 J/cm2; (b) 20 J/cm2. For comparison, the target diffraction pattern was also included.
Optical emission spectroscopy was successfully applied to obtain information of the dynamics of the plasma generated by PLD. As a result, it was possible to identify and study the behavior of ionized and neutral species within the plume. Spatial distribution is different for each specie: the ionized species appear first and with slightly greater velocity than the neutral species. This fact may be explained by coulomb interactions within the first moments of plasma expansion.
Fig. 8. SEM image of the film surface grown at different fluences: (a) 2.2 J/cm2, (b) 20 J/cm2.
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Continuous emission observed in the target perimeter (Fig. 3), reaches its maximum value a few nanoseconds after the laser pulse, irrespective of the distance from the target. The peak is very intense throughout the measured spectral range, which suggests the existence of a dense cloud of excited electrons in this region responsible for most of the continuous background and for the excitation of the species ejected during the first phases of plasma expansion. Such a continuous background is fundamentally a result of Bremsstrahlung and recombination emission. As the plume expands into the vacuum, the density of electrons decreases and, consequently, the likelihood of excitation of the ejected species becomes lower, dropping to zero at distances a few millimeters from the target surface, as shown in Figs. 3 and 4 for Pb(I). The presence of a background gas during ablation has two important effects on the plasma expansion process. First, the emission significantly increases for argon pressures greater than or equal to 0.1 mbar (Fig. 4). Secondly, for distances greater than 2 mm, it produces a non-linear dependence between the delay for the maximum emission and the distance from the target (Fig. 5). The ejected particles are slowed down. This slowing down has been explained in terms of elastic scattering of the ablated particles by molecules or atoms of the ambient gas [22]. As indicated above, a region of considerable increase in the emission intensity appears when experiments were carried out under argon at pressures from 0.5–1.0 mbar. The existence of this high-compression region in the plasma should result in an homogenization of the distribution of specie velocities. The particles would reach the substrate together, which may improve the properties of the films. A model to describe this behavior of intensity as a function of the distance from the target does not exist in the literature; however, detailed knowledge of this phenomenon may help in the control of film growth. The (1 1 1) preferential orientation shown in samples grown at lower fluence is probably due to a predominant splashing effect.
5. Conclusion Real time studies of plasma emission generated by PbTe ablation enabled the detection of both neutral and ionized Pb and Te at distances close to the position where the substrate is located. The presence of such species may influence properties of the deposited films. The presence of ambient gas during the process greatly influences the plasma expansion dynamics. The emission intensity considerably increases and the species are slowed down on their path to the substrate. A plasma compression region was found over the pressure range from 0.5 to 1.0 mbar. This region may produce a homogenization in the distribution of the velocities of the ejected species, thus improving the film properties. The increase in laser fluence arising from decreasing the laser spot size has two positive effects on the films: first, it increases the energy of the ionized species in the plume favoring the growth of crystalline films; second, it leads to a considerable reduction of droplet density on the film surface which also improves the film quality. A laser fluence of 20 J/cm2 and a pressure of 0.5 mbar Ar are specified as suitable conditions to grow highly oriented PbTe films showing a relatively smooth surface morphology and practically no presence of particulates or droplets. The fact that we obtained highly oriented PbTe films at room temperature on glass substrates is particularly remarkable and has not been reported in literature. Acknowledgements This work was supported by FAPESP under project 00/12079-0. The authors also kindly acknowledge CEPOF and PRONEX for partial support and Laborato´rio de Microscopia Eletroˆnica from LNLS for the SEM images. References [1] Suzuki N, Adachi S. Jpn J Appl Phys 1994;193:33. [2] Spra¨nger B, Schiessl U, Lambrecht A, Bo¨tter H, Tacke M. Appl Phys Lett 1988;53:2582.
ARTICLE IN PRESS E. Rodriguez et al. / Vacuum 80 (2006) 841–849 [3] Rodriguez E, Jimenez E, Neves AAR, Jacob GJ, Ce´sar CL, Barbosa LC. Physica E 2005;26:361–5. [4] Tudury GE, Marquezini MV, Ferreira LG, Barbosa LC, Cesar CL. Phys Rev B 2000;62:7357. [5] Jacob GJ, Cesar CL, Barbosa LC. Phys Chem of Glasses 2002;43C:250. [6] Rodriguez E, Jimenez E, Neves AAR, Jacob GJ, Cesar CL, Barbosa LC. Appl Phys Lett 2005;86:1. [7] Baleva M, Mateeva E, Momtchilova M. J Phys Condens Matter 1992;4:8997. [8] Jacquot A, Lenoir B, Boffoue´ MO, Dauscher A. Appl Phys A 1999;69:S613. [9] Geohegan DB. In: Chrisey DB, Hubler GK, editors. Pulsed laser deposition of thin films. New York: Wiley; 1994 Chapter 5. [10] Moore CE. Atomic Energy levels as derived from the analyses of optical spectra (Nat. Stand. Ref. Data Ser., Nat. Bur. Stand.), vol 1and 3, Washington DC, 1971. [11] Weast RC. Handbook of Chemistry and physics, 72nd ed. Boca Raton, FL, USA: CRC Press; 1992.
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[12] Wood D, Andrew KL. J Opt Soc Am 1968;58:818. [13] Wood DR, Ross CB, Scholl PS, Hoke M. J Opt Soc Am 1974;64:1159. [14] Morillon C, Verges J. Phys Scr 1975;12:129. [15] Handrup MB, Mack JE. Physica 1964;30:1245. [16] Singh RK, Narayan J. Phys Rev B 1990;41:8843. [17] Dyer PE, Greenough RD, Issa A, Key PH. Appl Phys Lett 1988;53:534. [18] Gu HP, Lou QH, Cheung NH, Chen SC, Wang ZY, Lin PK. Appl Phys B 1994;58:143. [19] Gonzalo J, Vega F, Afonso CN. J Appl Phys 1995;77: 6588. [20] Warren BE. X-ray difraction. London: Adilson-Wesley; 1969. p.13–16. [21] Cheung JT, Sankur H. Growth of thin-films by laserinduced evaporation. CRC Crit Rev in Solid State Mater Sci 1988;15:63. [22] Kools JCS. J Appl Phys 1993;74:10.