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The preparation and characterization of preferred (110) orientation aluminum nitride thin films on Si (100) substrates by pulsed laser deposition
Vacuum 85 (2010) 193e197
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
The preparation and characterization of preferred (110) orientation aluminum nitride thin films on Si (100) substrates by pulsed laser deposition Hongju Chen, Caihong Jia, Xinan Zhang, W.F. Zhang* School of Physics & Electronics, Henan University, Kaifeng 475001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 January 2010 Received in revised form 22 April 2010 Accepted 18 May 2010
The preferred (110) oriented aluminum nitride (AlN) thin films have been prepared by pulsed laser deposition on p-Si (100) substrates. The films were characterized with X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and atomic force microscope (AFM). The results indicate that the AlN thin films are well-crystallized when laser energy is higher than 300 mJ/puls. The AFM images show that the surface roughness of the deposited AlN thin films gradually increases with increasing laser energy, but the surface morphologies are still very smooth. The crystallinity and morphology of the thin films are found to be strongly dependent on the laser energy. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: AlN thin films p-Si (100) Pulsed laser deposition Preferred orientation
1. Introduction Aluminum nitride (AlN) is a wide direct band gap (6.2 eV) semiconductor material with hexagonal wurtzite structure. It has been considered as a promising material for next-generation microelectronic and optoelectronic devices due to its excellent physical and chemical properties, such as high hardness, high thermal conductivity, high surface acoustic velocity and good chemical stability [1e3]. A variety of deposition methods for the AlN films have been reported, e.g. reactive sputtering [4], metal-organic chemical vapor deposition (MOCVD) [5], laser-molecular beam epitaxy [6], and pulsed laser deposition (PLD) [7]. MOCVD is a feasible method for continuous production. However, the high processing temperatures (higher than 1000 C) [8] induce large internal stresses in the films, which restrict the choice of substrate. In addition, previous report also shows that the smooth and stoichiometric AlN films are difficult to obtain by MOCVD [9]. Meanwhile, low-temperature growth of thin films is one of prominent points of PLD technique, which also can control film thickness and use high purity source materials, etc. AlN thin films are usually grown on foreign substrates. The adoption of silicon substrates offers the advantages of availability of cheap and large diameter substrates. In addition, the heterostructure growth of AlN thin films on silicon substrates is very interesting for the monolithic integration of optoelectronic
* Corresponding author. Tel.: þ86 378 3881 940; fax: þ86 378 3880 659. E-mail address: [email protected] (W.F. Zhang). 0042-207X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.05.012
and microwave devices [10]. In a recent report, the p-Si/AlN/Au heterostructured light-emitting diode has been fabricated and investigated [11]. Consequently, it is very meaningful to prepare and characterize of the AlN thin films. The growth of AlN (002) thin films on p-Si (100) substrates by PLD has been reported [12]. In this work, we prepared and characterized the preferred orientation AlN (110) thin films on p-Si (100) substrates, which can be used for light-emitting optoelectronic devices in the green and blue color region due to their large band gaps and low dielectric constants [13].
2. Experiments The deposition of AlN films was conducted in a typical PLD stainless steel vacuum chamber. Using an UV KrF (l ¼ 248 nm, sFWHM ¼ 25 ns) excimer laser (COMPexPro201, Coherent, USA), the laser beam is focused at 45 on the surface of a high purity AlN target (99.99%) with a lens having 300 mm focal length. Before deposition, the Si substrates were cleaned using acetone, alcohol, hydrofluoric acid (2%), and then washed in distilled water. To avoid the target piercing and the films nonuniform, both the target and the substrates were rotated at the speed of 15 rpm and 10 rpm during the ablation, respectively. The target-to-substrate distance was kept at 5 cm. The AlN thin films were deposited on p-Si (100) substrates (10.8e11.8 U cm) at 600 C under 7.5 105 Pa ambient pressure with different laser energies. The laser frequency was 10 Hz. The deposition process lasted 1 h for all samples. After deposition, the samples were in-situ annealed for 10 min to
H. Chen et al. / Vacuum 85 (2010) 193e197
Si (400)
Table 1 Optical phonon modes and their energies for unstrained AlN [18].
SiO2
Intensity (a. u.)
Si (100)
AlN (110)
194
600 mJ/puls 500 mJ/puls 400 mJ/puls 300 mJ/puls Si substrate 30
35
40
45
50
55
60
65
70
75
80
2 (deg.) Fig. 1. XRD patterns of the AlN films deposited at 600 C under 7.5 105 Pa with different laser energy of 300, 400, 500, 600 mJ/puls. The inset shows the XRD pattern of the AlN target.
improve the quality of films. The film thickness was estimated to be in the range of 580e770 nm by using the separation of the interference maxima in the optical reflectance spectra of the AlN thin film [14], corresponding to the deposition rate of 9.7e12.8 nm/min. The crystalline quality of the films is characterized by X-ray diffraction (XRD) (DX-2500) with Cu Ka radiation with l ¼ 0.1541 nm. The Raman spectra were measured by a laser Raman Spectrometer (Renishaw RM1000) with 457.5 nm solid-state laser. The chemical bonds present in the films are investigated using FTIR spectroscopy (AVATAR360). X-ray photoelectron spectra (XPS) were recorded on an Axis Ultra system with monochromatic Al Ka X-rays (1486.6 eV) operated at 45 W and 15 kV with a background pressure of approximate 5.0 109 Torr. Surface morphologies of the AlN thin films were characterized with AFM (SPA400). 3. Results and discussion
Mode
E2(low)
A1(TO)
E2(high)
E1(TO)
A1(LO)
E1(LO)
Energy (cm1)
248.6
611.0
657.4
670.8
890.0
912.0
structure, according to the JCPDS No. 65-0820. Fig. 1 displays the XRD patterns of p-Si (100) substrates and AlN films deposited at 600 C under 7.5 105 Pa with different laser energies of 300, 400, 500 and 600 mJ/puls. As can be seen, all the XRD patterns of Fig. 1 have a sharp peak at 2q ¼ 33.04 , which was reported that it was the AlN (100) [15]. The authors also argued and attributed the peak at 2q ¼ 33.04 to the (100) peak of silicon substrates because Si (100) also shows the diffraction at the same site. Another peak at 58.8 is considered as the AlN (110). The sample deposited with 300 mJ/puls hardly shows AlN peak, but other samples exhibit preferred orientated AlN (110) peaks. To obtain the in-plane distribution of the projection of the c-axis of the (110) crystallites, X-ray F scanning was performed to determine the isotropy/anisotropy of the film properties in the phi-plane. Fig. 2 shows the X-ray F scanning of Si (111) substrates and AlN (100) films. There are no peaks observed in Fig. 2a indicating that the AlN film for 600 mJ/puls laser energy is of in-plane isotropy. The inplane isotropy could be due to low substrate temperature. Meanwhile it can be observed that the intensity of the AlN (110) peak increases with increasing the laser energy. For the samples deposited with different laser energies of 400, 500 and 600 mJ/puls, the average particle sizes are about 11.8, 12.2 and 13.5 nm, respectively, estimated using the Scherrer’s equation D ¼ 0.89l/ (bcosq) [16], where l is the wavelength of Cu Ka radiation in 1.541 Å, b is the full width at half maximum in radians obtained using Jade software, and q is the scattering angle. It indicates that the higher the laser energy, the larger the grain size and the better crystallization. Generally, increase of the laser energy can enhance the ablation effect and promote single adatom formation resulting in more island aggregation density [17]. As a result, a certain kinetic energy of the AlN complex particles is beneficial for crystalline growth since it promotes surface diffusion of adatoms. In the present case, the crystalline quality of AlN films gets optimum for the laser energy of 600 mJ/puls.
3.1. XRD patterns analysis The XRD pattern of the AlN target is shown in the inset of Fig. 1. The diffraction intensity conforms to the hexagonal wurtzite
Counts (a. u.)
120
180
240
300
360
Phi (deg.) Si (111)
0
60
120
180
240
300
360
Phi (deg.) Fig. 2. X-ray F scanning of (a) AlN (100) films and (b) Si (111) substrates.
600
670
60
650
600 mJ/puls 500 mJ/puls 400 mJ/puls 300 mJ/puls
700
750
824
620
AlN
0
b
According to the group theory, wurtzite AlN structure belongs to 4 (C6 mc) with two formula units per primitive the space group C6V 3
Intensity (a. u.)
Counts (a. u.)
a
3.2. Raman and IR spectra analysis
800
850
-1
Raman shift (cm ) Fig. 3. Raman spectra of the AlN films samples.
900
H. Chen et al. / Vacuum 85 (2010) 193e197
1107
Transmittance (a. u.)
620
600 mJ/puls 500 mJ/puls 400 mJ/puls
300 mJ/puls
AlN (674) 500.0
1.0k
1.5k
2.0k
2.5k
3.0k
3.5k
195
silicon substrates [20]. The Raman peak at 670 cm1 of the AlN thin films enhanced with laser energy, meaning that the crystalline quality improves. As is well known, Raman and FTIR have been considered as an effective technique to study structural and physical properties. Fig. 4 shows the FTIR absorption spectra of all the samples, and the FTIR spectroscopic analysis was scanned from 400 to 3900 cm1. Raw data are presented, as the attempted subtraction of silicon background signals from these spectra was not successful. As shown in Fig. 4, the dominant absorption peak at 674 cm1 in FTIR spectra corresponds to the AlN transverse optical (TO) phonon mode [21,22]. The peaks at 620 and 1107 cm1 are due to silicon substrates [23,24]. Besides the FTIR spectra of the samples prepared at 400, 500 and 600 mJ/puls, the FTIR spectrum of 300 mJ/puls is also distinct, indicating that AleN bond is already formed in the sample. Comparing FTIR to the XRD and Raman spectra, the samples except for 300 mJ/puls are well-crystallized.
-1
Wavenumber (cm ) 3.3. XPS analysis Fig. 4. FTIR patterns of the AlN films samples.
cell. Theory analysis predicts the zone-center optical modes in the representation as G ¼ A1þ2B1þE1þ2E2. The A1, E1, and E2 modes are Raman active, while the B1 modes are inactive. Furthermore, the A1 and E1 modes are each split into the longitudinal optical (LO) and transverse optical (TO) components, thus creating the A1 (LO, TO) and E1 (LO, TO) modes [18]. The feature peaks of AlN in the region from 200 to 500 cm1 are attributed to acoustical phonons and the peaks between 600 and 900 cm1 are assigned to optical phonons [19]. The Raman phonon energies of the AlN films are summarized in Table 1 [18]. The Raman spectra of the samples are shown in Fig. 3. Three peaks were centered around 620, 670, and 824 cm1. The peak at 670 cm1 is corresponding to the E1 (TO) modes of AlN [18]. The peak at 620 cm1 does not coincide with those in Table 1, but support the published result for A1 (TO) 620 cm1 [19] phonon mode. The peak at 824 cm1 is attributable to a phonon mode of the 1000
1000
O1s
300 mJ/puls
900 800
O1s
400 mJ/puls
900 800
700
700
600
N1s
Al2p
400
Al2p
300
C1s
500
Al2s
400
N1s
600
C1s
500
Intensity (a. u.)
The XPS analysis was performed to determine the chemical state and atomic compositions of the AlN thin films. Adjusted spectra based on neutral C 1s photoelectron peak at 284.8 eV [25,26], of the AlN films were shown in Fig. 5. Photoelectron peaks from aluminum, nitrogen, carbon and oxygen can be seen. The single peaks of Al 2p, N 1s and O 1s are located at about 74.0, 397.0, and 532.0 eV, respectively. The peaks of N 1s and Al 2p are corresponding to AlN [25,27], indicating the formation of AlN thin films. The composition ratios of the films can be approximately obtained from the XPS spectrum of each element using the analyzing software of XPS system (see Fig. 6). The content of carbon is not shown. The diagram shows that the oxygen contents approximately account for 28 at.%, which is the highest in the compositions. It could be attributed to the high gettering efficiency of Al metal for oxygen into oxide, when AlN films are exposed to atmospheric oxygen. In this case an incorporation of high contamination of oxygen in the films was observed. Another likely
Al2s
300
200
200
100
100
0
0 0
50 100 150 200 250 300 350 400 450 500 550 600
1000
0
50 100 150 200 250 300 350 400 450 500 550 600
1000
500 mJ/puls
900
O1s C1s
800
O1s
600 mJ/puls
900
N1s
800
700
700
600
C1s
600
N1s
500
Al2s
400 300
Al2s
500 400
Al2p
300
Al2p
200
200
100
100
0
0 0
50 100 150 200 250 300 350 400 450 500 550 600
0
50 100 150 200 250 300 350 400 450 500 550 600
Binding energy (eV) Fig. 5. XPS spectra of the AlN thin films.
Binding energy (eV)
196
H. Chen et al. / Vacuum 85 (2010) 193e197
1.0
32
Al N O
28
0.9 0.8
Roughness (nm)
Composition (at.%)
30
26 24
0.7 0.6 0.5 0.4
22
0.3 20
0.2 300
400
500
600
0.1 300
350
Laser energy (mJ/puls) Fig. 6. The change of composition ratios of the films with the increasing laser energy (the content of C is not given).
reason for the observed high amount of oxygen could be the residual gas pressure in the vacuum chamber [28]. In addition, along with the increase of laser energy, the ratio of N/Al is from 0.67 to 0.99, close to 1. It indicates the formation of near-stoichiometric AlN in the films for the laser energy approaching 600 mJ/puls. 3.4. AFM characterization The surface morphology and roughness of the AlN thin films deposited with different laser energies of 300, 400, 500 and 600 mJ/puls are studied by AFM (see Fig. 7). The roughness is represented by the root mean square (RMS) value, i.e. the standard deviation of the height Zi of all points within a given scan area and qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PN 2 is given by a formula, Rrms ¼ i ¼ 1 ðZi Zav Þ =N , where Zan is the average vertical height within the scan area. As can be seen from Fig. 8, the roughness of all samples are less than 1 nm, which prove the AlN films grown on p-Si (100) substrates at 600 C under
400
450
500
550
600
Laser energy (mJ/puls) Fig. 8. Absolute rms roughness of the AlN thin films as a function of laser energy.
7.5 105 Pa with different laser energies of 300, 400, 500 and 600 mJ/pulse are quite smooth and uniform. With the laser energy increase, roughness increases gradually from 0.189 nm to 0.931 nm, which shows that the higher the laser energy, the larger roughness. The increase of surface roughness is in good agreement with the increase of average grain size mentioned above. The as-prepared AlN films on p-Si (100) substrates could be useful for applications in different optoelectronic devices, in which the roughness of AlN films is very important. 4. Conclusions In summary, AlN thin films with preferred (110) orientation have been obtained on p-Si (100) substrates by PLD technique. XRD, Raman, FTIR, XPS and AFM are utilized to characterize the AlN films. It is found that the crystalline quality improves with laser energy. The as-grown oriented AlN (110) films may be promising in optoelectronic devices. Acknowledgements This work was supported by the Project of Cultivating Innovative Talents for Colleges and Universities of Henan Province (Grant No. 2002006) and the Natural Science Foundation of Department of Education of Henan Province (2009B48003) and the Key Technologies R & D Program of Henan Province (092102210005). References
Fig. 7. AFM images of surface morphologies of the AlN thin films at different laser energy: (a) 300, (b) 400, (c) 500, and (d) 600 mJ/puls. The scanned areas are 1.0 1.0 mm2.
[1] Yoder MN. IEEE Trans Electron Devices 1996;43:1633. [2] Cherng JS, Chang DS. Vacuum 2010;84:653. [3] Jacquot A, Lenoir B, Dauscher A, Verardi P, Craciun F, Stolzer M, et al. Appl Surf Sci 2002;186:507. [4] Iriarte GF, Engelmark F, Katardjiev IV. J Mater Res 2002;17:1469. [5] Amano H, Sawasaki N, Akasaki I, Toyoda Y. Appl Phys Lett 1986;48:353. [6] Schenk HPD, Kaiser U, Kipshidze GD, Fissel A, Hobert H, Schulze J, et al. Mater Sci Eng B 1999;59:84. [7] Ristoscu C, Ducu C, Socol G, Craciunoiu F, Mihailescu IN. Appl Surf Sci 2005;248:411. [8] Luo WJ, Wang XL, Guo LC, Xiao HL, Wang CM, Ran JX, et al. Microelectron Eng 2008;39:1710. [9] Hu TL, Mao SW, Chao CP, Wu MF, Huang HL, Gan D. J Electron Mater 2007;36:81. [10] Liu B, Gao J, Wu KM, Liu C. Solid State Commun 2009;149:715. [11] Zhao JL, Tan ST, Iwan S, Sun XW, Liu W, Chua SJ. Appl Phys Lett 2009;94:093506. [12] An ZH, Men CL, Yu J, Chu PK, Lin CL. J Appl Phys 2003;94:1934. cı S, Duman S, Tütüncü HM, Srivastava GP. Diamond Relat Mater [13] Bag 2009;18:1057.
H. Chen et al. / Vacuum 85 (2010) 193e197 [14] Rosenberger LW. Dissertation of Doctor. Modification and Characterization of aluminum nitride surfaces for an acoustic Wave Biosensor; 2007. pp. 23e24. [15] Lv L, Li QS, Li L, Zhang LC, Wang CF, Qi HX. Chin Phys Lett 2007;24:553. [16] Chen GY, Somesfalean G, Liu Y, Zhang ZG, Sun Q, Wang FP. Phys Rev B 2007;75:195204. [17] Zhang DM, Guan L, Li ZH, Pan GJ, Tan XY, Li L. Appl Surf Sci 2006;253:874. [18] Davydov VY, Kitaev YE, Goncharuk IN, Smirnov AN. Phys Rev B 1998;58:12899. [19] Hobert H, Dunken HH, Meinschien J, Stafast H. Vib Spectrosc 1999;19:205. [20] Ribeiro CTM, Alvarez F, Zanatta AR. Appl Phys Lett 2002;81:1005. [21] Shukla Gaurav, Khare Alika. Appl Surf Sci 2008;255:2057. [22] Mahmood A, Machorro R, Muhl S, Heiras J, Castillon FF, Farias MH, et al. Diamond Relat Mater 2003;12:1315.
197
[23] Dong FQ, Wan P, Feng QM, Song GB, Peng TJ, Li P, et al. J Wuhan Univ Technology-Mater Sci Ed 2004;19:94. [24] Niu JJ, Yang DR, Sha J, Wang JN, Li M. Mater Lett 2007;61:894. [25] Moulder JF, Stickle WF, Sobol PE, Bomben KD. Handbook of X-ray photoelectron spectroscopy. Eden Prarie MN 55344: Perkin-Elmer Corp., Physical Electronics Division; 1992. [26] Moulder JF, Stickle WF, Sobol PE, Bomben KD. Handbook of X-ray photoelectron spectroscopy. Eden Prairie, Minnesota: Physical Electronics Inc; 1995. [27] Song ZR, Yu YH, Shen DS, Zou SC, Zheng ZH, Luo EZ, et al. Mater Lett 2003;57:4643. [28] Dimitrova V, Manova D, Djulgerova R. Surf Coat Technol 2000;123:12.
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Vacuum journal homepage: www.elsevier.com/locate/vacuum
The preparation and characterization of preferred (110) orientation aluminum nitride thin films on Si (100) substrates by pulsed laser deposition Hongju Chen, Caihong Jia, Xinan Zhang, W.F. Zhang* School of Physics & Electronics, Henan University, Kaifeng 475001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 January 2010 Received in revised form 22 April 2010 Accepted 18 May 2010
The preferred (110) oriented aluminum nitride (AlN) thin films have been prepared by pulsed laser deposition on p-Si (100) substrates. The films were characterized with X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and atomic force microscope (AFM). The results indicate that the AlN thin films are well-crystallized when laser energy is higher than 300 mJ/puls. The AFM images show that the surface roughness of the deposited AlN thin films gradually increases with increasing laser energy, but the surface morphologies are still very smooth. The crystallinity and morphology of the thin films are found to be strongly dependent on the laser energy. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: AlN thin films p-Si (100) Pulsed laser deposition Preferred orientation
1. Introduction Aluminum nitride (AlN) is a wide direct band gap (6.2 eV) semiconductor material with hexagonal wurtzite structure. It has been considered as a promising material for next-generation microelectronic and optoelectronic devices due to its excellent physical and chemical properties, such as high hardness, high thermal conductivity, high surface acoustic velocity and good chemical stability [1e3]. A variety of deposition methods for the AlN films have been reported, e.g. reactive sputtering [4], metal-organic chemical vapor deposition (MOCVD) [5], laser-molecular beam epitaxy [6], and pulsed laser deposition (PLD) [7]. MOCVD is a feasible method for continuous production. However, the high processing temperatures (higher than 1000 C) [8] induce large internal stresses in the films, which restrict the choice of substrate. In addition, previous report also shows that the smooth and stoichiometric AlN films are difficult to obtain by MOCVD [9]. Meanwhile, low-temperature growth of thin films is one of prominent points of PLD technique, which also can control film thickness and use high purity source materials, etc. AlN thin films are usually grown on foreign substrates. The adoption of silicon substrates offers the advantages of availability of cheap and large diameter substrates. In addition, the heterostructure growth of AlN thin films on silicon substrates is very interesting for the monolithic integration of optoelectronic
* Corresponding author. Tel.: þ86 378 3881 940; fax: þ86 378 3880 659. E-mail address: [email protected] (W.F. Zhang). 0042-207X/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2010.05.012
and microwave devices [10]. In a recent report, the p-Si/AlN/Au heterostructured light-emitting diode has been fabricated and investigated [11]. Consequently, it is very meaningful to prepare and characterize of the AlN thin films. The growth of AlN (002) thin films on p-Si (100) substrates by PLD has been reported [12]. In this work, we prepared and characterized the preferred orientation AlN (110) thin films on p-Si (100) substrates, which can be used for light-emitting optoelectronic devices in the green and blue color region due to their large band gaps and low dielectric constants [13].
2. Experiments The deposition of AlN films was conducted in a typical PLD stainless steel vacuum chamber. Using an UV KrF (l ¼ 248 nm, sFWHM ¼ 25 ns) excimer laser (COMPexPro201, Coherent, USA), the laser beam is focused at 45 on the surface of a high purity AlN target (99.99%) with a lens having 300 mm focal length. Before deposition, the Si substrates were cleaned using acetone, alcohol, hydrofluoric acid (2%), and then washed in distilled water. To avoid the target piercing and the films nonuniform, both the target and the substrates were rotated at the speed of 15 rpm and 10 rpm during the ablation, respectively. The target-to-substrate distance was kept at 5 cm. The AlN thin films were deposited on p-Si (100) substrates (10.8e11.8 U cm) at 600 C under 7.5 105 Pa ambient pressure with different laser energies. The laser frequency was 10 Hz. The deposition process lasted 1 h for all samples. After deposition, the samples were in-situ annealed for 10 min to
H. Chen et al. / Vacuum 85 (2010) 193e197
Si (400)
Table 1 Optical phonon modes and their energies for unstrained AlN [18].
SiO2
Intensity (a. u.)
Si (100)
AlN (110)
194
600 mJ/puls 500 mJ/puls 400 mJ/puls 300 mJ/puls Si substrate 30
35
40
45
50
55
60
65
70
75
80
2 (deg.) Fig. 1. XRD patterns of the AlN films deposited at 600 C under 7.5 105 Pa with different laser energy of 300, 400, 500, 600 mJ/puls. The inset shows the XRD pattern of the AlN target.
improve the quality of films. The film thickness was estimated to be in the range of 580e770 nm by using the separation of the interference maxima in the optical reflectance spectra of the AlN thin film [14], corresponding to the deposition rate of 9.7e12.8 nm/min. The crystalline quality of the films is characterized by X-ray diffraction (XRD) (DX-2500) with Cu Ka radiation with l ¼ 0.1541 nm. The Raman spectra were measured by a laser Raman Spectrometer (Renishaw RM1000) with 457.5 nm solid-state laser. The chemical bonds present in the films are investigated using FTIR spectroscopy (AVATAR360). X-ray photoelectron spectra (XPS) were recorded on an Axis Ultra system with monochromatic Al Ka X-rays (1486.6 eV) operated at 45 W and 15 kV with a background pressure of approximate 5.0 109 Torr. Surface morphologies of the AlN thin films were characterized with AFM (SPA400). 3. Results and discussion
Mode
E2(low)
A1(TO)
E2(high)
E1(TO)
A1(LO)
E1(LO)
Energy (cm1)
248.6
611.0
657.4
670.8
890.0
912.0
structure, according to the JCPDS No. 65-0820. Fig. 1 displays the XRD patterns of p-Si (100) substrates and AlN films deposited at 600 C under 7.5 105 Pa with different laser energies of 300, 400, 500 and 600 mJ/puls. As can be seen, all the XRD patterns of Fig. 1 have a sharp peak at 2q ¼ 33.04 , which was reported that it was the AlN (100) [15]. The authors also argued and attributed the peak at 2q ¼ 33.04 to the (100) peak of silicon substrates because Si (100) also shows the diffraction at the same site. Another peak at 58.8 is considered as the AlN (110). The sample deposited with 300 mJ/puls hardly shows AlN peak, but other samples exhibit preferred orientated AlN (110) peaks. To obtain the in-plane distribution of the projection of the c-axis of the (110) crystallites, X-ray F scanning was performed to determine the isotropy/anisotropy of the film properties in the phi-plane. Fig. 2 shows the X-ray F scanning of Si (111) substrates and AlN (100) films. There are no peaks observed in Fig. 2a indicating that the AlN film for 600 mJ/puls laser energy is of in-plane isotropy. The inplane isotropy could be due to low substrate temperature. Meanwhile it can be observed that the intensity of the AlN (110) peak increases with increasing the laser energy. For the samples deposited with different laser energies of 400, 500 and 600 mJ/puls, the average particle sizes are about 11.8, 12.2 and 13.5 nm, respectively, estimated using the Scherrer’s equation D ¼ 0.89l/ (bcosq) [16], where l is the wavelength of Cu Ka radiation in 1.541 Å, b is the full width at half maximum in radians obtained using Jade software, and q is the scattering angle. It indicates that the higher the laser energy, the larger the grain size and the better crystallization. Generally, increase of the laser energy can enhance the ablation effect and promote single adatom formation resulting in more island aggregation density [17]. As a result, a certain kinetic energy of the AlN complex particles is beneficial for crystalline growth since it promotes surface diffusion of adatoms. In the present case, the crystalline quality of AlN films gets optimum for the laser energy of 600 mJ/puls.
3.1. XRD patterns analysis The XRD pattern of the AlN target is shown in the inset of Fig. 1. The diffraction intensity conforms to the hexagonal wurtzite
Counts (a. u.)
120
180
240
300
360
Phi (deg.) Si (111)
0
60
120
180
240
300
360
Phi (deg.) Fig. 2. X-ray F scanning of (a) AlN (100) films and (b) Si (111) substrates.
600
670
60
650
600 mJ/puls 500 mJ/puls 400 mJ/puls 300 mJ/puls
700
750
824
620
AlN
0
b
According to the group theory, wurtzite AlN structure belongs to 4 (C6 mc) with two formula units per primitive the space group C6V 3
Intensity (a. u.)
Counts (a. u.)
a
3.2. Raman and IR spectra analysis
800
850
-1
Raman shift (cm ) Fig. 3. Raman spectra of the AlN films samples.
900
H. Chen et al. / Vacuum 85 (2010) 193e197
1107
Transmittance (a. u.)
620
600 mJ/puls 500 mJ/puls 400 mJ/puls
300 mJ/puls
AlN (674) 500.0
1.0k
1.5k
2.0k
2.5k
3.0k
3.5k
195
silicon substrates [20]. The Raman peak at 670 cm1 of the AlN thin films enhanced with laser energy, meaning that the crystalline quality improves. As is well known, Raman and FTIR have been considered as an effective technique to study structural and physical properties. Fig. 4 shows the FTIR absorption spectra of all the samples, and the FTIR spectroscopic analysis was scanned from 400 to 3900 cm1. Raw data are presented, as the attempted subtraction of silicon background signals from these spectra was not successful. As shown in Fig. 4, the dominant absorption peak at 674 cm1 in FTIR spectra corresponds to the AlN transverse optical (TO) phonon mode [21,22]. The peaks at 620 and 1107 cm1 are due to silicon substrates [23,24]. Besides the FTIR spectra of the samples prepared at 400, 500 and 600 mJ/puls, the FTIR spectrum of 300 mJ/puls is also distinct, indicating that AleN bond is already formed in the sample. Comparing FTIR to the XRD and Raman spectra, the samples except for 300 mJ/puls are well-crystallized.
-1
Wavenumber (cm ) 3.3. XPS analysis Fig. 4. FTIR patterns of the AlN films samples.
cell. Theory analysis predicts the zone-center optical modes in the representation as G ¼ A1þ2B1þE1þ2E2. The A1, E1, and E2 modes are Raman active, while the B1 modes are inactive. Furthermore, the A1 and E1 modes are each split into the longitudinal optical (LO) and transverse optical (TO) components, thus creating the A1 (LO, TO) and E1 (LO, TO) modes [18]. The feature peaks of AlN in the region from 200 to 500 cm1 are attributed to acoustical phonons and the peaks between 600 and 900 cm1 are assigned to optical phonons [19]. The Raman phonon energies of the AlN films are summarized in Table 1 [18]. The Raman spectra of the samples are shown in Fig. 3. Three peaks were centered around 620, 670, and 824 cm1. The peak at 670 cm1 is corresponding to the E1 (TO) modes of AlN [18]. The peak at 620 cm1 does not coincide with those in Table 1, but support the published result for A1 (TO) 620 cm1 [19] phonon mode. The peak at 824 cm1 is attributable to a phonon mode of the 1000
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The XPS analysis was performed to determine the chemical state and atomic compositions of the AlN thin films. Adjusted spectra based on neutral C 1s photoelectron peak at 284.8 eV [25,26], of the AlN films were shown in Fig. 5. Photoelectron peaks from aluminum, nitrogen, carbon and oxygen can be seen. The single peaks of Al 2p, N 1s and O 1s are located at about 74.0, 397.0, and 532.0 eV, respectively. The peaks of N 1s and Al 2p are corresponding to AlN [25,27], indicating the formation of AlN thin films. The composition ratios of the films can be approximately obtained from the XPS spectrum of each element using the analyzing software of XPS system (see Fig. 6). The content of carbon is not shown. The diagram shows that the oxygen contents approximately account for 28 at.%, which is the highest in the compositions. It could be attributed to the high gettering efficiency of Al metal for oxygen into oxide, when AlN films are exposed to atmospheric oxygen. In this case an incorporation of high contamination of oxygen in the films was observed. Another likely
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reason for the observed high amount of oxygen could be the residual gas pressure in the vacuum chamber [28]. In addition, along with the increase of laser energy, the ratio of N/Al is from 0.67 to 0.99, close to 1. It indicates the formation of near-stoichiometric AlN in the films for the laser energy approaching 600 mJ/puls. 3.4. AFM characterization The surface morphology and roughness of the AlN thin films deposited with different laser energies of 300, 400, 500 and 600 mJ/puls are studied by AFM (see Fig. 7). The roughness is represented by the root mean square (RMS) value, i.e. the standard deviation of the height Zi of all points within a given scan area and qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi PN 2 is given by a formula, Rrms ¼ i ¼ 1 ðZi Zav Þ =N , where Zan is the average vertical height within the scan area. As can be seen from Fig. 8, the roughness of all samples are less than 1 nm, which prove the AlN films grown on p-Si (100) substrates at 600 C under
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Laser energy (mJ/puls) Fig. 8. Absolute rms roughness of the AlN thin films as a function of laser energy.
7.5 105 Pa with different laser energies of 300, 400, 500 and 600 mJ/pulse are quite smooth and uniform. With the laser energy increase, roughness increases gradually from 0.189 nm to 0.931 nm, which shows that the higher the laser energy, the larger roughness. The increase of surface roughness is in good agreement with the increase of average grain size mentioned above. The as-prepared AlN films on p-Si (100) substrates could be useful for applications in different optoelectronic devices, in which the roughness of AlN films is very important. 4. Conclusions In summary, AlN thin films with preferred (110) orientation have been obtained on p-Si (100) substrates by PLD technique. XRD, Raman, FTIR, XPS and AFM are utilized to characterize the AlN films. It is found that the crystalline quality improves with laser energy. The as-grown oriented AlN (110) films may be promising in optoelectronic devices. Acknowledgements This work was supported by the Project of Cultivating Innovative Talents for Colleges and Universities of Henan Province (Grant No. 2002006) and the Natural Science Foundation of Department of Education of Henan Province (2009B48003) and the Key Technologies R & D Program of Henan Province (092102210005). References
Fig. 7. AFM images of surface morphologies of the AlN thin films at different laser energy: (a) 300, (b) 400, (c) 500, and (d) 600 mJ/puls. The scanned areas are 1.0 1.0 mm2.
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