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Femtosecond pulsed laser ablation of GaAs
Applied Surface Science 221 (2004) 364–369
Femtosecond pulsed laser ablation of GaAs T.W. Trelenberg*, L.N. Dinh, C.K. Saw, B.C. Stuart, M. Balooch1 Lawrence Livermore National Laboratory, University of California, P.O. Box 808 Livermore, CA 94550, USA Received 25 February 2003; received in revised form 25 February 2003; accepted 20 July 2003
Abstract The properties of femtosecond-pulsed laser deposited GaAs nanoclusters were investigated. Nanoclusters of GaAs were produced by laser ablating a single crystal GaAs target in vacuum or in a buffer gas using a Ti–sapphire laser with a 150 fs minimum pulse length. For in-vacuum deposition, X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) revealed that the average cluster size was approximately 7 nm for laser pulse lengths between 150 fs and 25 ps. The average cluster size dropped to approximately 1.5 nm at a pulse length of 500 ps. It was also observed that film thickness decreased with increasing laser pulse length. A reflective coating, which accumulated on the laser admission window during ablation, reduced the amount of laser energy reaching the target for subsequent laser shots and developed more rapidly at longer pulse lengths. This observation indicates that non-stoichiometric (metallic) ablatants were produced more readily at longer pulse lengths. The angular distribution of ejected material about the target normal was well fitted to a bi-cosine distribution of cos 47 y þ cos 4 y for ablation in vacuum using 150 fs pulses. XPS and AES revealed that the vacuum-deposited films contained excess amorphous Ga or As in addition to the stoichiometric GaAs nanocrystals seen with XRD. However, films containing only the GaAs nanocrystals were produced when ablation was carried out in the presence of a buffer gas with a pressure in excess of 6.67 Pa. At buffer gas pressure on the order of 1 Torr, it was found that the stoichiometry of the ablated target was also preserved. These experiments indicate that both laser pulse length and buffer gas pressure play important roles in the formation of multi-element nanocrystals by laser ablation. The effects of gas pressure on the target’s morphology and the size of the GaAs nanocrystals formed will also be discussed. # 2003 Elsevier B.V. All rights reserved. PACS: 81.07.-b; 61.46.þw; 36.40.-c Keywords: GaAs; Femtosecond; Ablation; Gallium arsenide; Laser
1. Introduction Pulsed laser deposition (PLD) offers a simple, convenient method of producing nano-scaled materi* Corresponding author. Present address: LLNL, 7000 East Ave., L-370, Livermore, CA 94550, USA. E-mail address: [email protected] (T.W. Trelenberg). 1 Present address: Division of Biomaterials and Bioengineering, School of Dentistry, University of California, San Francisco, CA 94143-0858, USA.
als in which quantum confinement effects become significant and whose effects modify the material properties observed [1]. Ablation using very short pulses has shown great promise in facilitating the growth of complex multi-element films with stoichiometries matching those of their parent materials [2]. Gallium arsenide, already important in the electronic and opto-electronic industries, has even greater potential if quantum confinement effects could be used to tune its material properties. PLD of GaAs in the
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0169-4332(03)00937-1
T.W. Trelenberg et al. / Applied Surface Science 221 (2004) 364–369
The experiments were carried out in a vacuum chamber with a high-vacuum base pressure (104 Pa) and provisions for the admission of a buffer gas (SF6 and Ar for this work). The targets were irradiated with 820 nm light from a Ti–sapphire laser operating at 1000 Hz. The minimum pulse length of 150 fs was typically utilized; however, a number of pulse-lengths, up to a 500 ps maximum, were available. Each pulse had an energy of approximately 1.3 mJ and was focused to a spot-size of 300– 400 nm, yielding a fluence of 0.8 J/cm2. Rastering and rotation of the ablation target prevented the target from being ablated in the same area repeatedly without some recovery time. The substrate and target were separated by 11 cm with their surfaces perpendicular to the line between them and the laser striking the target at an angle 458 from the surface normal. The films were deposited onto Si substrates and the ejected material was found to have an angular distribution of cos 47 y þ cos 4 y about the target normal. After removal from the synthesis chamber, the films and targets were transported in air prior to analysis with X-ray diffraction (XRD), Auger electron spectroscopy (AES), X-ray
3. Results and discussion Fig. 1 shows the X-ray diffraction results obtained from films which were created in vacuum using different laser pulse lengths. Diffraction peaks are clearly observed and are located in positions consistent with those expected for GaAs, as indicated by the Joint Committee on Powder Diffraction Standards (JCPDS) [9] plot presented beneath the experimental data. Analysis of the peak widths using the Debye–Scherrer equation indicates that the diffraction comes from ˚ in size for pulse GaAs crystallites of roughly 70 A lengths ranging from 150 fs up to 25 ps. The film grown with 500 ps pulses also shows weak diffraction ˚ crystallites. The low diffraction coming from 15 A quality is in part due to the size of the crystallites and to the low material yield at this pulse length. This is consistent with the results from earlier work which revealed that stoichiometric crystallites are formed with femtosecond pulses [8] but are not seen when using nanosecond pulses [4]. At the intermediate pulse length of 500 ps, near amorphous crystallites consisting of only 2–3 GaAs unit cells are produced, indicating that a 500 ps pulse length is close to the maximum 6000
Crystallite size (Å)
2. Experiment
photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Sputtering to remove oxide layers was performed when necessary.
Intensity (arb units)
nanosecond regime has already produced stoichiometric GaAs nanoparticles that exhibit these finitesize effects, provided the ablations are carried out in the presence of a background gas [3]. However, films deposited in vacuum were found to be non-crystalline and to lack stoichiometric concentrations of gallium and arsenic [4]. The use of femtosecond-pulsed lasers provides the opportunity to study this system in a regime where the driving pulse terminates prior to the onset of electron-phonon coupling and the pulse’s total energy is deposited before the target can react thermally [5–7]. A recent study has shown that these ultra-short pulses produce stoichiometric crystallites in the 2–20 nm range even in the absence of a background gas [8]. This work investigates the effect of femtosecond and low-picosecond laser pulse lengths on the ablation of GaAs, as well as the effects of background gas pressure. The effects these parameters have on the ablation targets themselves will also be discussed.
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2θ(Degrees) Fig. 1. X-ray diffraction results obtained from films created via ablation in vacuum using pulse lengths ranging from 150 fs to 500 ps.
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pulse length that can be used to form crystallites in vacuum. The inset of Fig. 1 shows that there is no systematic variation in crystallite size for pulse lengths shorter than 25 ps. However, the significant change in size observed for the 500 ps sample indicates that there is a change in the laser–target interaction mechanism. This change was also evident when attempts were made to determine deposition rate as a function of pulse length. During these experiments, the ablated material was found to accumulate as a reflective coating on the inside of the admission window. The accumulation was negligible for 150 fs pulses, reducing laser power to the target by only 4% after 90 min of ablation, and was barely discernible to the eye. However, using 500 ps pulses, a metallic film was clearly visible and reduced the laser power reaching the target by 50% within 15 min. Compared to the shorter pulse length trials, metallic phase material was more readily produced when using the longer 500 ps pulses. While this corroborates the change in the dominant laser–target interaction mechanism suggested by X-ray diffraction, it made the determination of deposition rates (relative or absolute) impossible, as the amount of laser energy delivered to the target in a set time period varied with pulse length. The films deposited during short pulse laser ablation in vacuum, while containing stoichiometric GaAs crystallites, were not found to be stoichiometric overall. Fig. 2 consists of several Auger spectra obtained from the laser deposited films, as well as a control scan from a clean GaAs wafer. Scan (d) was taken on a clean GaAs wafer and provides a 1:1 Ga-to-As ratio reference. Scans (a–c) are from films deposited using various pulse lengths (150 fs, 25 ps, and 500 ps, respectively) in vacuum (1 104 Pa). The Ga-to-As ratios are very similar for these three films and do not appear to be affected by changes in pulse length. Comparison with scan (d) shows that these films are not stoichiometric, containing on the order of 20% excess gallium. Film (e) of Fig. 2 was generated via ablation in 66.7 Pa of SF6, and a comparison with scan (d) reveals that the stoichiometric ratio of Ga-to-As has been preserved. Short-pulse laser ablation (less than hundreds of picoseconds) on GaAs intrinsically creates GaAs nanocrystals [8], but simultaneously it also generates a significant quantity of atomic species.
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Energy (eV) Fig. 2. AES spectra for substrates created under various conditions. Scans (a)–(c) are from ablations in vacuum at pulse lengths of 150 fs, 25 and 500 ps, respectively, while (e) comes from 150 fs pulses in a background of SF6 at 66.7 Pa. Scan (d) is from a clean GaAs wafer, provides a 1:1 Ga-to-As ratio reference.
Seen with AES, these atoms condense into an amorphous form, and are therefore not seen in X-ray diffraction. These amorphous contaminates destroy the stoichiometry of the vacuum-ablated films and, as discussed earlier, reduce the laser power delivered to the target by their accumulation as a reflective metallic film on the admission window. A sufficiently high background pressure will scatter atomic material that has a mass comparable to that of the background gas [2]. However, the background gas will leave unaffected the much more massive nanoclusters. This produces a pressure range in which the atomic species are scattered but the stoichiometric nanoclusters arrive at the substrate surface unimpeded. For GaAs, this pressure is found to be approximately 6.67 Pa. A buffer gas can also be used to preserve the surface stoichiometry of the ablation target. Fig. 3 shows high resolution XPS spectra of the Ga(2p3=2 ) and As(3d5=2 ) core levels taken from three different targets. Scan (a) is taken from a clean GaAs wafer as a 1:1 Ga/As
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Fig. 3. XPS results for several target surfaces: (a) a clean GaAs wafer; (b) a target ablated in (6:67 104 Pa) of SF6 ; and (c) a target ablated in vacuum (104 Pa). The ablations used 150 fs pulses.
reference. Scan (b) is from an ablation carried out in a 6:67 104 Pa background of SF6 and is found to have a Ga-to-As ratio consistent with the 1:1 ratio seen in (a). Plot (c) was taken from a target ablated in vacuum with identical laser parameters but in comparison with (a) is seen to have a significant amount of excess arsenic. An explanation for this may lie in the cooling rates experienced by the two surfaces. The AFM phase images shown in Fig. 4 were obtained from two target surfaces that were ablated in different background pressures of SF6 : 6666 Pa for the left image and 6:67 104 Pa for the right. The 6666 Pa image shows evidence of large scale melting. The smooth, rounded features and large
Fig. 4. AFM phase images obtained from two target surfaces upon which ablation (PL ¼ 150 fs) was carried out using different SF6 background pressures, 6666 Pa (left) and 6:67 104 Pa (right). The left image shows evidence of large-scale melting (smooth surfaces, spherical features). The right image shows evidence of melting as well, but the final roughness and smaller feature size indicate a more rapid cooling process. The 6666 Pa image is representative of images seen for SF6 pressures of less than 6666 Pa.
spherical droplets are indicative of a surface that was melted and then resolidified. The 6666 Pa image is representative of surfaces ablated in pressures of less than 6666 Pa, as similar images were seen in those cases. The 6:67 104 Pa image, while still showing evidence of a melted surface, has a large number of smaller droplets, which seems to indicate that the surface cooled much more rapidly than the targets ablated in reduced background pressures. It is believed that the cooling afforded by the background gas at near atmospheric pressure allows the target surface to retain its stoichiometry during ablation. At very high temperatures, such as the maximum following the laser pulse, the vapor pressure difference between Ga and As becomes insignificant. However, at lower temperatures, before resolidification, they are quite different and species segregation can occur. Convection from a non-planar surface via a background gas at near atmospheric pressure can become a significant source of thermal losses as surface roughness increases (thereby increasing surface area). It can be shown that heat lost through convection can become as significant as that lost through radiation for slightly roughened surfaces and that convection losses can approach those of conduction for extremely rough surfaces [10]. A large convective cooling component will minimize vapor-pressure driven segregation and will tend to preserve the original stoichiometry of the target surface. The distribution of particles as a function of angle was also investigated for ablation occurring in vacuum and is shown in Fig. 5. The films for these measurements were grown simultaneously on a number of
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T.W. Trelenberg et al. / Applied Surface Science 221 (2004) 364–369
150 fs and 500 ps results in this work appear to indicate that the sub-picosecond processes are responsible for the forward-focused ablation, which has a much greater contribution with 150 fs pulses, while the cos4 term, dominant with 500 ps pulses, is the result of ablation due to more conventional thermal processes. Time-of-flight studies of GaAs ablation with femtosecond pulses have shown that the ablations consist of two discrete distributions (in time): a fast pulse of ionized material followed by a broader plume of neutrals [13], which provides further evidence for the two-component angular distribution discussed here. Fig. 5. Distribution of nanoparticles as a function of angle for ablation in vacuum. A sum of two cosines, cos 47 y þ cos 4 y (shown individually beneath the fit), was required to fit the data (solid circles), implying that there is more than one ablation process responsible for the growth of the film. AFM measurements (insets, 1 mm full scale) show that larger clusters are preferentially scattered to larger angles.
substrates placed at a fixed radius from the point of laser incidence on the target. The thickness of the deposited material as a function of angle fit well to a sum of two cosines, cos 47 y þ cos 4 y. The presence of the two distributions suggests that there is more than one ablation process at work. As the AFM insets show, the broad (cos4) term was responsible for the production of clusters that were 150% larger than those generated in the tightly forward-focused (cos47) portion of the ablation plume. These films were found to contain GaAs nanocrystals, but as before, the films themselves lacked stoichiometry. However, unlike the films from Fig. 2, these films were rich in arsenic. Using 500 ps pulses, a bi-cosine angular distribution was also produced, consisting of a forwardfocused component and the broad cos 4 y component. However, for the longer 500 ps pulse length, it was the cos 4 y contribution that was the dominant component. Short-pulse target interaction studies have shown that metallic and semiconducting ablation targets become liquid in timescales that are too short to be accounted for by conventional thermal processes [11,12]. However, following the phase change, these surfaces are found to behave as though they were generated via conventional thermal processes [6]. The
4. Summary Pulsed laser deposited GaAs nanocluster films and ablation targets produced using 500 ps laser pulses were investigated. The two-component angular distribution of the ejected ablation products observed in these experiments confirms the existence of a thermal process (wide angular distribution) and a faster laser– material interaction (sharp forward distribution). The experimental results also suggest that the preservation of the irradiated target’s stoichiometry and the formation of stoichiometric GaAs nanocrystalline films require that ablation occur in the presence of a background gas and use laser pulses that are 25 ps or shorter. Background pressures on the order of 6.67 Pa were found to be sufficient to scatter excess Ga and As atoms in the ablation plume and produce stoichiometric GaAs nanocrystalline films. Pressures on the order of 6:67 104 Pa were found to provide rapid convection cooling of the target surfaces following a laser pulse, allowing the stoichiometry of the target to be preserved.
Acknowledgements The authors would like to acknowledge the technical support of Octavio Cervantes and Cheryl Evans. This work was performed under the auspices of the US Department of Energy by the University of California, Lawrence Livermore National Laboratory, under contract No. W-7405-ENG-48.
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References [1] X. Duan, J. Wang, C.M. Lieber, Appl. Phys. Lett. 76 (9) (2000) 1116–1118. [2] D.B. Chrisey, G.K. Hubler (Eds.), Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. [3] J. Perrie`re, E. Millon, M. Chamarro, M. Morcrette, C. Andreazza, Appl. Phys. Lett. 78 (19) (2001) 2949–2951. [4] L.N. Dinh, S. Hayes, C.K. Saw, W.M. II, M. Balooch, Appl. Phys. Lett. 75 (15) (1999) 2208–2210. [5] D. von der Linde, K. Sokolowski-Tinten, J. Bialkowski, Appl. Surf. Sci. 109/110 (1997) 1–10. [6] D. von der Linde, K. Sokolowski-Tinten, Appl. Surf. Sci. 154–155 (1999) 1–10. [7] P. Saeta, J.K. Wang, Y. Siegal, N. Bloembergen, E. Mazur, Phys. Rev. Lett. 67 (8) (1991) 1023–1026.
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[8] L.N. Dinh, S.E. Hayes, A.E. Wynne, M.A. Wall, C.K. Saw, B.C. Stuart, M. Balooch, A.K. Paravastu, J.A. Reimer, J. Materials Sci. 37 (2002) 3953–3958. [9] The Joint Committee on Powder Diffraction Standards (JCPDS) by the International Centre for Diffraction Data (ICDD), Swarthmore, PA, USA. [10] H.S. Carslaw, J. Jaeger, Conduction of Heat in Solids, second ed., Oxford, 1959. [11] K. Sokolowski-Tinten, J. Bialkowski, D. von der Linde, Phys. Rev. B 51 (20) (1995) 14186–14198. [12] H.W.K. Tom, G.D. Aumiller, C.H. Brito-Cruz, Phys. Rev. Lett. 60 (14) (1988) 1438–1441. [13] A. Cavalleri, K. Sokolowski-Tinten, J. Bialkowski, D. von der Linde, Appl. Phys. Lett. 72 (19) (1998) 2385– 2387.
Femtosecond pulsed laser ablation of GaAs T.W. Trelenberg*, L.N. Dinh, C.K. Saw, B.C. Stuart, M. Balooch1 Lawrence Livermore National Laboratory, University of California, P.O. Box 808 Livermore, CA 94550, USA Received 25 February 2003; received in revised form 25 February 2003; accepted 20 July 2003
Abstract The properties of femtosecond-pulsed laser deposited GaAs nanoclusters were investigated. Nanoclusters of GaAs were produced by laser ablating a single crystal GaAs target in vacuum or in a buffer gas using a Ti–sapphire laser with a 150 fs minimum pulse length. For in-vacuum deposition, X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) revealed that the average cluster size was approximately 7 nm for laser pulse lengths between 150 fs and 25 ps. The average cluster size dropped to approximately 1.5 nm at a pulse length of 500 ps. It was also observed that film thickness decreased with increasing laser pulse length. A reflective coating, which accumulated on the laser admission window during ablation, reduced the amount of laser energy reaching the target for subsequent laser shots and developed more rapidly at longer pulse lengths. This observation indicates that non-stoichiometric (metallic) ablatants were produced more readily at longer pulse lengths. The angular distribution of ejected material about the target normal was well fitted to a bi-cosine distribution of cos 47 y þ cos 4 y for ablation in vacuum using 150 fs pulses. XPS and AES revealed that the vacuum-deposited films contained excess amorphous Ga or As in addition to the stoichiometric GaAs nanocrystals seen with XRD. However, films containing only the GaAs nanocrystals were produced when ablation was carried out in the presence of a buffer gas with a pressure in excess of 6.67 Pa. At buffer gas pressure on the order of 1 Torr, it was found that the stoichiometry of the ablated target was also preserved. These experiments indicate that both laser pulse length and buffer gas pressure play important roles in the formation of multi-element nanocrystals by laser ablation. The effects of gas pressure on the target’s morphology and the size of the GaAs nanocrystals formed will also be discussed. # 2003 Elsevier B.V. All rights reserved. PACS: 81.07.-b; 61.46.þw; 36.40.-c Keywords: GaAs; Femtosecond; Ablation; Gallium arsenide; Laser
1. Introduction Pulsed laser deposition (PLD) offers a simple, convenient method of producing nano-scaled materi* Corresponding author. Present address: LLNL, 7000 East Ave., L-370, Livermore, CA 94550, USA. E-mail address: [email protected] (T.W. Trelenberg). 1 Present address: Division of Biomaterials and Bioengineering, School of Dentistry, University of California, San Francisco, CA 94143-0858, USA.
als in which quantum confinement effects become significant and whose effects modify the material properties observed [1]. Ablation using very short pulses has shown great promise in facilitating the growth of complex multi-element films with stoichiometries matching those of their parent materials [2]. Gallium arsenide, already important in the electronic and opto-electronic industries, has even greater potential if quantum confinement effects could be used to tune its material properties. PLD of GaAs in the
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0169-4332(03)00937-1
T.W. Trelenberg et al. / Applied Surface Science 221 (2004) 364–369
The experiments were carried out in a vacuum chamber with a high-vacuum base pressure (104 Pa) and provisions for the admission of a buffer gas (SF6 and Ar for this work). The targets were irradiated with 820 nm light from a Ti–sapphire laser operating at 1000 Hz. The minimum pulse length of 150 fs was typically utilized; however, a number of pulse-lengths, up to a 500 ps maximum, were available. Each pulse had an energy of approximately 1.3 mJ and was focused to a spot-size of 300– 400 nm, yielding a fluence of 0.8 J/cm2. Rastering and rotation of the ablation target prevented the target from being ablated in the same area repeatedly without some recovery time. The substrate and target were separated by 11 cm with their surfaces perpendicular to the line between them and the laser striking the target at an angle 458 from the surface normal. The films were deposited onto Si substrates and the ejected material was found to have an angular distribution of cos 47 y þ cos 4 y about the target normal. After removal from the synthesis chamber, the films and targets were transported in air prior to analysis with X-ray diffraction (XRD), Auger electron spectroscopy (AES), X-ray
3. Results and discussion Fig. 1 shows the X-ray diffraction results obtained from films which were created in vacuum using different laser pulse lengths. Diffraction peaks are clearly observed and are located in positions consistent with those expected for GaAs, as indicated by the Joint Committee on Powder Diffraction Standards (JCPDS) [9] plot presented beneath the experimental data. Analysis of the peak widths using the Debye–Scherrer equation indicates that the diffraction comes from ˚ in size for pulse GaAs crystallites of roughly 70 A lengths ranging from 150 fs up to 25 ps. The film grown with 500 ps pulses also shows weak diffraction ˚ crystallites. The low diffraction coming from 15 A quality is in part due to the size of the crystallites and to the low material yield at this pulse length. This is consistent with the results from earlier work which revealed that stoichiometric crystallites are formed with femtosecond pulses [8] but are not seen when using nanosecond pulses [4]. At the intermediate pulse length of 500 ps, near amorphous crystallites consisting of only 2–3 GaAs unit cells are produced, indicating that a 500 ps pulse length is close to the maximum 6000
Crystallite size (Å)
2. Experiment
photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). Sputtering to remove oxide layers was performed when necessary.
Intensity (arb units)
nanosecond regime has already produced stoichiometric GaAs nanoparticles that exhibit these finitesize effects, provided the ablations are carried out in the presence of a background gas [3]. However, films deposited in vacuum were found to be non-crystalline and to lack stoichiometric concentrations of gallium and arsenic [4]. The use of femtosecond-pulsed lasers provides the opportunity to study this system in a regime where the driving pulse terminates prior to the onset of electron-phonon coupling and the pulse’s total energy is deposited before the target can react thermally [5–7]. A recent study has shown that these ultra-short pulses produce stoichiometric crystallites in the 2–20 nm range even in the absence of a background gas [8]. This work investigates the effect of femtosecond and low-picosecond laser pulse lengths on the ablation of GaAs, as well as the effects of background gas pressure. The effects these parameters have on the ablation targets themselves will also be discussed.
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pulse length that can be used to form crystallites in vacuum. The inset of Fig. 1 shows that there is no systematic variation in crystallite size for pulse lengths shorter than 25 ps. However, the significant change in size observed for the 500 ps sample indicates that there is a change in the laser–target interaction mechanism. This change was also evident when attempts were made to determine deposition rate as a function of pulse length. During these experiments, the ablated material was found to accumulate as a reflective coating on the inside of the admission window. The accumulation was negligible for 150 fs pulses, reducing laser power to the target by only 4% after 90 min of ablation, and was barely discernible to the eye. However, using 500 ps pulses, a metallic film was clearly visible and reduced the laser power reaching the target by 50% within 15 min. Compared to the shorter pulse length trials, metallic phase material was more readily produced when using the longer 500 ps pulses. While this corroborates the change in the dominant laser–target interaction mechanism suggested by X-ray diffraction, it made the determination of deposition rates (relative or absolute) impossible, as the amount of laser energy delivered to the target in a set time period varied with pulse length. The films deposited during short pulse laser ablation in vacuum, while containing stoichiometric GaAs crystallites, were not found to be stoichiometric overall. Fig. 2 consists of several Auger spectra obtained from the laser deposited films, as well as a control scan from a clean GaAs wafer. Scan (d) was taken on a clean GaAs wafer and provides a 1:1 Ga-to-As ratio reference. Scans (a–c) are from films deposited using various pulse lengths (150 fs, 25 ps, and 500 ps, respectively) in vacuum (1 104 Pa). The Ga-to-As ratios are very similar for these three films and do not appear to be affected by changes in pulse length. Comparison with scan (d) shows that these films are not stoichiometric, containing on the order of 20% excess gallium. Film (e) of Fig. 2 was generated via ablation in 66.7 Pa of SF6, and a comparison with scan (d) reveals that the stoichiometric ratio of Ga-to-As has been preserved. Short-pulse laser ablation (less than hundreds of picoseconds) on GaAs intrinsically creates GaAs nanocrystals [8], but simultaneously it also generates a significant quantity of atomic species.
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Energy (eV) Fig. 2. AES spectra for substrates created under various conditions. Scans (a)–(c) are from ablations in vacuum at pulse lengths of 150 fs, 25 and 500 ps, respectively, while (e) comes from 150 fs pulses in a background of SF6 at 66.7 Pa. Scan (d) is from a clean GaAs wafer, provides a 1:1 Ga-to-As ratio reference.
Seen with AES, these atoms condense into an amorphous form, and are therefore not seen in X-ray diffraction. These amorphous contaminates destroy the stoichiometry of the vacuum-ablated films and, as discussed earlier, reduce the laser power delivered to the target by their accumulation as a reflective metallic film on the admission window. A sufficiently high background pressure will scatter atomic material that has a mass comparable to that of the background gas [2]. However, the background gas will leave unaffected the much more massive nanoclusters. This produces a pressure range in which the atomic species are scattered but the stoichiometric nanoclusters arrive at the substrate surface unimpeded. For GaAs, this pressure is found to be approximately 6.67 Pa. A buffer gas can also be used to preserve the surface stoichiometry of the ablation target. Fig. 3 shows high resolution XPS spectra of the Ga(2p3=2 ) and As(3d5=2 ) core levels taken from three different targets. Scan (a) is taken from a clean GaAs wafer as a 1:1 Ga/As
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Fig. 3. XPS results for several target surfaces: (a) a clean GaAs wafer; (b) a target ablated in (6:67 104 Pa) of SF6 ; and (c) a target ablated in vacuum (104 Pa). The ablations used 150 fs pulses.
reference. Scan (b) is from an ablation carried out in a 6:67 104 Pa background of SF6 and is found to have a Ga-to-As ratio consistent with the 1:1 ratio seen in (a). Plot (c) was taken from a target ablated in vacuum with identical laser parameters but in comparison with (a) is seen to have a significant amount of excess arsenic. An explanation for this may lie in the cooling rates experienced by the two surfaces. The AFM phase images shown in Fig. 4 were obtained from two target surfaces that were ablated in different background pressures of SF6 : 6666 Pa for the left image and 6:67 104 Pa for the right. The 6666 Pa image shows evidence of large scale melting. The smooth, rounded features and large
Fig. 4. AFM phase images obtained from two target surfaces upon which ablation (PL ¼ 150 fs) was carried out using different SF6 background pressures, 6666 Pa (left) and 6:67 104 Pa (right). The left image shows evidence of large-scale melting (smooth surfaces, spherical features). The right image shows evidence of melting as well, but the final roughness and smaller feature size indicate a more rapid cooling process. The 6666 Pa image is representative of images seen for SF6 pressures of less than 6666 Pa.
spherical droplets are indicative of a surface that was melted and then resolidified. The 6666 Pa image is representative of surfaces ablated in pressures of less than 6666 Pa, as similar images were seen in those cases. The 6:67 104 Pa image, while still showing evidence of a melted surface, has a large number of smaller droplets, which seems to indicate that the surface cooled much more rapidly than the targets ablated in reduced background pressures. It is believed that the cooling afforded by the background gas at near atmospheric pressure allows the target surface to retain its stoichiometry during ablation. At very high temperatures, such as the maximum following the laser pulse, the vapor pressure difference between Ga and As becomes insignificant. However, at lower temperatures, before resolidification, they are quite different and species segregation can occur. Convection from a non-planar surface via a background gas at near atmospheric pressure can become a significant source of thermal losses as surface roughness increases (thereby increasing surface area). It can be shown that heat lost through convection can become as significant as that lost through radiation for slightly roughened surfaces and that convection losses can approach those of conduction for extremely rough surfaces [10]. A large convective cooling component will minimize vapor-pressure driven segregation and will tend to preserve the original stoichiometry of the target surface. The distribution of particles as a function of angle was also investigated for ablation occurring in vacuum and is shown in Fig. 5. The films for these measurements were grown simultaneously on a number of
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150 fs and 500 ps results in this work appear to indicate that the sub-picosecond processes are responsible for the forward-focused ablation, which has a much greater contribution with 150 fs pulses, while the cos4 term, dominant with 500 ps pulses, is the result of ablation due to more conventional thermal processes. Time-of-flight studies of GaAs ablation with femtosecond pulses have shown that the ablations consist of two discrete distributions (in time): a fast pulse of ionized material followed by a broader plume of neutrals [13], which provides further evidence for the two-component angular distribution discussed here. Fig. 5. Distribution of nanoparticles as a function of angle for ablation in vacuum. A sum of two cosines, cos 47 y þ cos 4 y (shown individually beneath the fit), was required to fit the data (solid circles), implying that there is more than one ablation process responsible for the growth of the film. AFM measurements (insets, 1 mm full scale) show that larger clusters are preferentially scattered to larger angles.
substrates placed at a fixed radius from the point of laser incidence on the target. The thickness of the deposited material as a function of angle fit well to a sum of two cosines, cos 47 y þ cos 4 y. The presence of the two distributions suggests that there is more than one ablation process at work. As the AFM insets show, the broad (cos4) term was responsible for the production of clusters that were 150% larger than those generated in the tightly forward-focused (cos47) portion of the ablation plume. These films were found to contain GaAs nanocrystals, but as before, the films themselves lacked stoichiometry. However, unlike the films from Fig. 2, these films were rich in arsenic. Using 500 ps pulses, a bi-cosine angular distribution was also produced, consisting of a forwardfocused component and the broad cos 4 y component. However, for the longer 500 ps pulse length, it was the cos 4 y contribution that was the dominant component. Short-pulse target interaction studies have shown that metallic and semiconducting ablation targets become liquid in timescales that are too short to be accounted for by conventional thermal processes [11,12]. However, following the phase change, these surfaces are found to behave as though they were generated via conventional thermal processes [6]. The
4. Summary Pulsed laser deposited GaAs nanocluster films and ablation targets produced using 500 ps laser pulses were investigated. The two-component angular distribution of the ejected ablation products observed in these experiments confirms the existence of a thermal process (wide angular distribution) and a faster laser– material interaction (sharp forward distribution). The experimental results also suggest that the preservation of the irradiated target’s stoichiometry and the formation of stoichiometric GaAs nanocrystalline films require that ablation occur in the presence of a background gas and use laser pulses that are 25 ps or shorter. Background pressures on the order of 6.67 Pa were found to be sufficient to scatter excess Ga and As atoms in the ablation plume and produce stoichiometric GaAs nanocrystalline films. Pressures on the order of 6:67 104 Pa were found to provide rapid convection cooling of the target surfaces following a laser pulse, allowing the stoichiometry of the target to be preserved.
Acknowledgements The authors would like to acknowledge the technical support of Octavio Cervantes and Cheryl Evans. This work was performed under the auspices of the US Department of Energy by the University of California, Lawrence Livermore National Laboratory, under contract No. W-7405-ENG-48.
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