- Email: [email protected]
Pulsed laser deposition of crystalline LaB6 thin films
Applied Surface Science 247 (2005) 384–389 www.elsevier.com/locate/apsusc
Pulsed laser deposition of crystalline LaB6 thin films V. Craciun a,*, D. Craciun b a
Major Analytical Instrumentation Center, Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA b Laser Department, National Institute for Laser, Plasma, and Radiation Physics, Bucharest, Romania Available online 2 April 2005
Abstract The deposition of LaB6 thin films by the pulsed laser deposition (PLD) technique was investigated. X-ray photoelectron and Auger electron spectroscopy (XPS, AES), X-ray diffraction and reflectivity were used to characterize the properties of the deposited films. It has been found that crystalline films could be grown only by using laser fluences around 10 J/cm2 or higher and substrate temperatures in excess of 800 8C. Cubic LaB6 films (a = 0.4157 nm) exhibiting a strong (1 0 0) texture were deposited under a residual vacuum better than 1 10 6 Torr at 850 8C. These films were smooth, with surface roughness values below 1.4 nm and mass densities around 4.88 g/cm3, very close to the theoretical LaB6 density of 4.71 g/cm3. XPS and AES investigations showed that the outermost 2–3 nm of the surface region contained a significant amount of oxygen and La–O and B–O bonds. Once this surface region was removed by sputtering, the oxygen content decreased to values below 10%. # 2005 Elsevier B.V. All rights reserved. Keywords: Thin films; LaB6; Laser ablation; Refractive coatings; X-ray reflectivity
1. Introduction LaB6 is a refractory compound characterized by a high melting temperature, excellent thermal stability, high hardness and one of the lowest work function for electron emission [1]. It crystallizes in the CsCl type lattice (a = 0.4153 nm, JCPDS #06-0401), where all boron atoms are contained in B6 octahedra, which are arranged as units in a BCC type lattice and surrounded by eight La atoms. This interesting arrangement explains the material low rezistivity and * Corresponding author.
work function. However, the same arrangement might be the cause of difficulties in depositing high quality thin films. Ar implantation within the deposited film during sputtering might cause additional problems [2], whereas better results were obtained by the electron beam evaporation technique [3,4]. Interestingly, there is no article describing the use of the pulsed laser deposition (PLD) technique for the deposition of LaB6 thin films. Since PLD has been shown to posses several advantages with respect to other thin film deposition techniques, we investigated its use to deposit LaB6 films and present the results here.
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.01.071
V. Craciun, D. Craciun / Applied Surface Science 247 (2005) 384–389
2. Experiment The depositions were conducted in a typical PLD system using a KrF excimer laser (l = 248 nm). The laser parameters used were 8–11 J/cm2 fluence and a 5–10 Hz repetition rate. The films were deposited for different times on p++ Si (1 0 0) substrates (supplied by MEMC) that had their native oxide stripped by dipping them in 1% HF solution, at substrate temperatures from 500 up to 850 8C under residual vacuum (9 10 6 to 4 10 7 Torr). The films surface and interfacial roughness, mass density and thickness were investigated by X-ray reflectometry (XRR, Panalytical X’Pert MRD system). The same instrument was used for structural characterization in symmetrical and grazing incidence X-ray diffraction (XRD and GIXD). The chemical composition and bonding of the films were investigated by X-ray photoelectron spectroscopy (XPS) in a Perkin Elmer 5100 system using Mg Ka radiation (1253.6 eV) and by Auger electron spectroscopy (AES, Perkin Elmer PHI 660).
3. Results and discussion The films deposited under residual vacuum levels in the high 10 6 Torr and up to a substrate temperature of 800 8C were found to be amorphous by XRD investigations. Films deposited at 850 8C exhibited crystallinity, the lower the residual vacuum, the higher the intensity of the X-ray diffraction peaks. Although the films contained only La, B, and O atoms, as evidenced by XPS and AES investigations, very often we could not match the acquired diffraction patterns to one of the more than 50 reference patterns corresponding to various chemical compounds in the La–B–O family. A typical example is shown in Fig. 1a, where the XRD patterns acquired both in the symmetrical and grazing incidence from a film deposited under a residual vacuum atmosphere of 9 10 6 Torr and a substrate temperature of 850 8C are displayed. There are several factors that can account for this difficulty. First, the crystalline structure of LaB6 allows for a significant fraction of La atoms to be volatilized without the collapsing of the rigid skeleton. Many impurities present in the residual
385
atmosphere, mainly oxygen and water could be trapped inside the crystalline structure and alter the local chemistry and arrangement of atoms. Simple calculations [5] showed that the oxygen partial pressure required to prevent La2O3 formation at room temperature is of the order of 10 200 atm, therefore surface oxidation is unavoidable. Secondly, there is a large thermal mismatch between the LaB6 film and the Si substrate. The thermal expansion coefficients are 6.0 10 6 K 1 for LaB6 versus 2.6 10 6 K 1 for Si, a difference that can induce thermal stress in such thin films and shift the patterns. Thirdly, films deposited by PLD tend to be highly textured, many of the XRD peaks being either smaller than in the powder form or even absent. However, under a low residual vacuum of 9 10 7 Torr, a high laser fluence of 10 J/cm2 and at a substrate temperature of 850 8C, highly textured LaB6 thin films, with the (1 0 0) planes parallel to the substrate, were deposited as shown in Fig. 1b. The lattice parameter estimated from the (1 0 0) peak position was 0.4157 nm, very close to the tabulated position of 0.4153 nm, and the grain size, estimated from the FWHM of the same line was around 20 nm. The estimated size of the crystalline grains was very similar to the thickness of the films, indicating that the growth started with this orientation from the very early stages of the deposition. Inspecting the region of the critical angle for the XRR spectra acquired from films deposited under various conditions, we observed that as the residual vacuum level decreased, the value of the critical angle shifted towards lower values that correspond to lower film densities. Since both La oxides and La borates exhibit densities higher than that of LaB6, this is an indication that the content of oxygen within the films was reduced and their stoichiometry evolved towards that of LaB6. A complete XRR spectrum acquired from one of our best LaB6 films is displayed in Fig. 2. The properties of this film, estimated by simulating the acquired XRR spectra using the WingixaTM software from Panalytical and a three-layer model (interfacial layer, LaB6, and surface contamination layer), are shown in Table 1. One can note that the surface roughness (rms value) was estimated to be 1.4 nm, a very good value for LaB6 films. There are a range of values in the
386
V. Craciun, D. Craciun / Applied Surface Science 247 (2005) 384–389
Fig. 1. XRD spectra of thin films deposited using a laser fluence of 10 J/cm2 at a substrate temperature of 850 8C under a residual vacuum of the order of 5 10 6 Torr (a) and (b) 9 10 7 Torr; the vertical dotted lines indicate the reference position and intensity for stoichiometric LaB6 powder (JCPDS #06-0401).
literature for the density of LaB6, because of different La to B ratios this compound can exhibit various oxygen contamination levels. The density estimated for our film, 4.88 g/cm3, is very close to the theoretical density of 4.71 corresponding to stoichiometric LaB6 in a cubic lattice with a = 0.4153 nm, a good indication of the quality of these PLD grown films.
XPS investigations revealed the presence of C 1s O 1s lines on the surface of the deposited films as shown in Fig. 3. Neglecting the contribution of the adventitious C, the O, La, and B concentrations on the surface of an as-received film were found to be 36.5%, 10.3%, and 53.2%, respectively. After 4 min of sputtering with 4 keV Ar+ at an emission current of 25 mA, when around 2 nm of the material was
V. Craciun, D. Craciun / Applied Surface Science 247 (2005) 384–389
387
Fig. 2. XRR acquired spectrum and its simulation for a LaB6 film deposited under the lowest residual vacuum. The results of the simulation are displayed in Table 1.
Table 1 The results of the simulation of the XRR spectrum using a three layer model for one of the high quality LaB6 film deposit at 850 8C under 9 10 7 Torr Deposition time [min]
30
Layer’s thickness [nm] IL
LaB6/r [g/cm3]
CL/rms
2.2
22.5/4.88
1.0/1.4
IL, interfacial layer; CL, contamination layer.
removed, the amount of C was negligible while that of O, La, and B were found to be 23.5%, 15.3%, and 61.2%, respectively. One cannot only note the decrease of the O contamination but also a decrease of the B to La ratio, most likely caused by the preferential sputtering of B with respect to La. An
AES depth profile of a 23-nm thick film is shown in Fig. 4. One can note the very low level of oxygen and carbon signals after the outermost 3 nm of the surface region was removed by Ar ion sputtering. To obtain a good quantitative estimation of the oxygen level, around 12 nm of the film was removed and then the bulk composition was investigated. The resulting AES spectra, displayed in Fig. 5 indicated a ratio of B to La of around 2.3 and an oxygen content of less than 10%. Since the XRR and XRD results indicated that the film was very similar to a stoichiometric LaB6 compound we think that the low B to La ratio estimated by AES is caused by the preferential B sputtering during Ar ion bombardment, similar to the XPS result.
Fig. 3. XPS survey spectrum of an as-received LaB6 film.
388
V. Craciun, D. Craciun / Applied Surface Science 247 (2005) 384–389
Fig. 4. AES depth profile of a 23-nm thick LaB6 film deposited on Si.
Fig. 5. AES survey spectrum acquired after in situ removal of 12 nm by Ar+ sputtering.
4. Conclusions LaB6 thin films were deposited on Si substrates by the pulsed laser deposition technique. A combination of high laser fluence, high substrate temperature, and low residual vacuum was required to obtain crystalline films. Under these conditions, the films grew with a strong (1 0 0) texture.
The films density of 4.88 g/cm3 was around the tabulated value of 4.71 g/cm3 while the surface morphology was very smooth, with a roughness value (rms) of 1.4 nm. The surface layer contained a significant amount of oxygen, which decreased to values below 10% after the first 2–3 nm of the outermost surface region was removed by Ar ion sputtering. Further experiments to measure the
V. Craciun, D. Craciun / Applied Surface Science 247 (2005) 384–389
electron emission properties of these films are in progress.
Acknowledgement This work was funded by the Office of Naval Research under grant no. N00014-03-1-0605. We thank Eric Lambers from MAIC for help with AES investigations
389
References [1] Technical Publications, CERAC incorporate 7(2), 1997. [2] S.J. Mroczkowski, J. Vac. Sci. Technol. A9 (1991) 586. [3] Y. Okamoto, T. Aida, S. Shinada, Jpn. J. Appl. Phys. 26 (1987) 1722. [4] A. Yutani, A. Kobayashi, A. Kinbara, Appl. Surf. Sci. 70/71 (1993) 737. [5] R.T. Dehoff, Thermodynamics in Materials Science, McGrawHill Science/Engineering/Math, 1993.
Pulsed laser deposition of crystalline LaB6 thin films V. Craciun a,*, D. Craciun b a
Major Analytical Instrumentation Center, Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA b Laser Department, National Institute for Laser, Plasma, and Radiation Physics, Bucharest, Romania Available online 2 April 2005
Abstract The deposition of LaB6 thin films by the pulsed laser deposition (PLD) technique was investigated. X-ray photoelectron and Auger electron spectroscopy (XPS, AES), X-ray diffraction and reflectivity were used to characterize the properties of the deposited films. It has been found that crystalline films could be grown only by using laser fluences around 10 J/cm2 or higher and substrate temperatures in excess of 800 8C. Cubic LaB6 films (a = 0.4157 nm) exhibiting a strong (1 0 0) texture were deposited under a residual vacuum better than 1 10 6 Torr at 850 8C. These films were smooth, with surface roughness values below 1.4 nm and mass densities around 4.88 g/cm3, very close to the theoretical LaB6 density of 4.71 g/cm3. XPS and AES investigations showed that the outermost 2–3 nm of the surface region contained a significant amount of oxygen and La–O and B–O bonds. Once this surface region was removed by sputtering, the oxygen content decreased to values below 10%. # 2005 Elsevier B.V. All rights reserved. Keywords: Thin films; LaB6; Laser ablation; Refractive coatings; X-ray reflectivity
1. Introduction LaB6 is a refractory compound characterized by a high melting temperature, excellent thermal stability, high hardness and one of the lowest work function for electron emission [1]. It crystallizes in the CsCl type lattice (a = 0.4153 nm, JCPDS #06-0401), where all boron atoms are contained in B6 octahedra, which are arranged as units in a BCC type lattice and surrounded by eight La atoms. This interesting arrangement explains the material low rezistivity and * Corresponding author.
work function. However, the same arrangement might be the cause of difficulties in depositing high quality thin films. Ar implantation within the deposited film during sputtering might cause additional problems [2], whereas better results were obtained by the electron beam evaporation technique [3,4]. Interestingly, there is no article describing the use of the pulsed laser deposition (PLD) technique for the deposition of LaB6 thin films. Since PLD has been shown to posses several advantages with respect to other thin film deposition techniques, we investigated its use to deposit LaB6 films and present the results here.
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.01.071
V. Craciun, D. Craciun / Applied Surface Science 247 (2005) 384–389
2. Experiment The depositions were conducted in a typical PLD system using a KrF excimer laser (l = 248 nm). The laser parameters used were 8–11 J/cm2 fluence and a 5–10 Hz repetition rate. The films were deposited for different times on p++ Si (1 0 0) substrates (supplied by MEMC) that had their native oxide stripped by dipping them in 1% HF solution, at substrate temperatures from 500 up to 850 8C under residual vacuum (9 10 6 to 4 10 7 Torr). The films surface and interfacial roughness, mass density and thickness were investigated by X-ray reflectometry (XRR, Panalytical X’Pert MRD system). The same instrument was used for structural characterization in symmetrical and grazing incidence X-ray diffraction (XRD and GIXD). The chemical composition and bonding of the films were investigated by X-ray photoelectron spectroscopy (XPS) in a Perkin Elmer 5100 system using Mg Ka radiation (1253.6 eV) and by Auger electron spectroscopy (AES, Perkin Elmer PHI 660).
3. Results and discussion The films deposited under residual vacuum levels in the high 10 6 Torr and up to a substrate temperature of 800 8C were found to be amorphous by XRD investigations. Films deposited at 850 8C exhibited crystallinity, the lower the residual vacuum, the higher the intensity of the X-ray diffraction peaks. Although the films contained only La, B, and O atoms, as evidenced by XPS and AES investigations, very often we could not match the acquired diffraction patterns to one of the more than 50 reference patterns corresponding to various chemical compounds in the La–B–O family. A typical example is shown in Fig. 1a, where the XRD patterns acquired both in the symmetrical and grazing incidence from a film deposited under a residual vacuum atmosphere of 9 10 6 Torr and a substrate temperature of 850 8C are displayed. There are several factors that can account for this difficulty. First, the crystalline structure of LaB6 allows for a significant fraction of La atoms to be volatilized without the collapsing of the rigid skeleton. Many impurities present in the residual
385
atmosphere, mainly oxygen and water could be trapped inside the crystalline structure and alter the local chemistry and arrangement of atoms. Simple calculations [5] showed that the oxygen partial pressure required to prevent La2O3 formation at room temperature is of the order of 10 200 atm, therefore surface oxidation is unavoidable. Secondly, there is a large thermal mismatch between the LaB6 film and the Si substrate. The thermal expansion coefficients are 6.0 10 6 K 1 for LaB6 versus 2.6 10 6 K 1 for Si, a difference that can induce thermal stress in such thin films and shift the patterns. Thirdly, films deposited by PLD tend to be highly textured, many of the XRD peaks being either smaller than in the powder form or even absent. However, under a low residual vacuum of 9 10 7 Torr, a high laser fluence of 10 J/cm2 and at a substrate temperature of 850 8C, highly textured LaB6 thin films, with the (1 0 0) planes parallel to the substrate, were deposited as shown in Fig. 1b. The lattice parameter estimated from the (1 0 0) peak position was 0.4157 nm, very close to the tabulated position of 0.4153 nm, and the grain size, estimated from the FWHM of the same line was around 20 nm. The estimated size of the crystalline grains was very similar to the thickness of the films, indicating that the growth started with this orientation from the very early stages of the deposition. Inspecting the region of the critical angle for the XRR spectra acquired from films deposited under various conditions, we observed that as the residual vacuum level decreased, the value of the critical angle shifted towards lower values that correspond to lower film densities. Since both La oxides and La borates exhibit densities higher than that of LaB6, this is an indication that the content of oxygen within the films was reduced and their stoichiometry evolved towards that of LaB6. A complete XRR spectrum acquired from one of our best LaB6 films is displayed in Fig. 2. The properties of this film, estimated by simulating the acquired XRR spectra using the WingixaTM software from Panalytical and a three-layer model (interfacial layer, LaB6, and surface contamination layer), are shown in Table 1. One can note that the surface roughness (rms value) was estimated to be 1.4 nm, a very good value for LaB6 films. There are a range of values in the
386
V. Craciun, D. Craciun / Applied Surface Science 247 (2005) 384–389
Fig. 1. XRD spectra of thin films deposited using a laser fluence of 10 J/cm2 at a substrate temperature of 850 8C under a residual vacuum of the order of 5 10 6 Torr (a) and (b) 9 10 7 Torr; the vertical dotted lines indicate the reference position and intensity for stoichiometric LaB6 powder (JCPDS #06-0401).
literature for the density of LaB6, because of different La to B ratios this compound can exhibit various oxygen contamination levels. The density estimated for our film, 4.88 g/cm3, is very close to the theoretical density of 4.71 corresponding to stoichiometric LaB6 in a cubic lattice with a = 0.4153 nm, a good indication of the quality of these PLD grown films.
XPS investigations revealed the presence of C 1s O 1s lines on the surface of the deposited films as shown in Fig. 3. Neglecting the contribution of the adventitious C, the O, La, and B concentrations on the surface of an as-received film were found to be 36.5%, 10.3%, and 53.2%, respectively. After 4 min of sputtering with 4 keV Ar+ at an emission current of 25 mA, when around 2 nm of the material was
V. Craciun, D. Craciun / Applied Surface Science 247 (2005) 384–389
387
Fig. 2. XRR acquired spectrum and its simulation for a LaB6 film deposited under the lowest residual vacuum. The results of the simulation are displayed in Table 1.
Table 1 The results of the simulation of the XRR spectrum using a three layer model for one of the high quality LaB6 film deposit at 850 8C under 9 10 7 Torr Deposition time [min]
30
Layer’s thickness [nm] IL
LaB6/r [g/cm3]
CL/rms
2.2
22.5/4.88
1.0/1.4
IL, interfacial layer; CL, contamination layer.
removed, the amount of C was negligible while that of O, La, and B were found to be 23.5%, 15.3%, and 61.2%, respectively. One cannot only note the decrease of the O contamination but also a decrease of the B to La ratio, most likely caused by the preferential sputtering of B with respect to La. An
AES depth profile of a 23-nm thick film is shown in Fig. 4. One can note the very low level of oxygen and carbon signals after the outermost 3 nm of the surface region was removed by Ar ion sputtering. To obtain a good quantitative estimation of the oxygen level, around 12 nm of the film was removed and then the bulk composition was investigated. The resulting AES spectra, displayed in Fig. 5 indicated a ratio of B to La of around 2.3 and an oxygen content of less than 10%. Since the XRR and XRD results indicated that the film was very similar to a stoichiometric LaB6 compound we think that the low B to La ratio estimated by AES is caused by the preferential B sputtering during Ar ion bombardment, similar to the XPS result.
Fig. 3. XPS survey spectrum of an as-received LaB6 film.
388
V. Craciun, D. Craciun / Applied Surface Science 247 (2005) 384–389
Fig. 4. AES depth profile of a 23-nm thick LaB6 film deposited on Si.
Fig. 5. AES survey spectrum acquired after in situ removal of 12 nm by Ar+ sputtering.
4. Conclusions LaB6 thin films were deposited on Si substrates by the pulsed laser deposition technique. A combination of high laser fluence, high substrate temperature, and low residual vacuum was required to obtain crystalline films. Under these conditions, the films grew with a strong (1 0 0) texture.
The films density of 4.88 g/cm3 was around the tabulated value of 4.71 g/cm3 while the surface morphology was very smooth, with a roughness value (rms) of 1.4 nm. The surface layer contained a significant amount of oxygen, which decreased to values below 10% after the first 2–3 nm of the outermost surface region was removed by Ar ion sputtering. Further experiments to measure the
V. Craciun, D. Craciun / Applied Surface Science 247 (2005) 384–389
electron emission properties of these films are in progress.
Acknowledgement This work was funded by the Office of Naval Research under grant no. N00014-03-1-0605. We thank Eric Lambers from MAIC for help with AES investigations
389
References [1] Technical Publications, CERAC incorporate 7(2), 1997. [2] S.J. Mroczkowski, J. Vac. Sci. Technol. A9 (1991) 586. [3] Y. Okamoto, T. Aida, S. Shinada, Jpn. J. Appl. Phys. 26 (1987) 1722. [4] A. Yutani, A. Kobayashi, A. Kinbara, Appl. Surf. Sci. 70/71 (1993) 737. [5] R.T. Dehoff, Thermodynamics in Materials Science, McGrawHill Science/Engineering/Math, 1993.