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The impact of ultra thin silicon nitride buffer layer on GaN growth on Si (1 1 1) by RF-MBE
Applied Surface Science 257 (2011) 2107–2110
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The impact of ultra thin silicon nitride buffer layer on GaN growth on Si (1 1 1) by RF-MBE Mahesh Kumar a,b , Mohana K. Rajpalke a , Basanta Roul a,b , Thirumaleshwara N. Bhat a , Neeraj Sinha c , A.T. Kalghatgi b , S.B. Krupanidhi a,∗ a b c
Materials Research Centre, Indian Institute of Science, Bangalore 560012, Karnataka, India Central Research Laboratory, Bharat Electronics, Bangalore 560013, Karnataka, India Office of Principal Scientific Advisor, Government of India, New Delhi 110011, India
a r t i c l e
i n f o
Article history: Received 9 August 2010 Accepted 21 September 2010 Available online 25 September 2010 Keywords: Nitridation MBE Silicon nitride GaN
a b s t r a c t Ultra thin films of pure silicon nitride were grown on a Si (1 1 1) surface by exposing the surface to radiofrequency (RF) nitrogen plasma with a high content of nitrogen atoms. The effect of annealing of silicon nitride surface was investigated with core-level photoelectron spectroscopy. The Si 2p photoelectron spectra reveals a characteristic series of components for the Si species, not only in stoichiometric Si3 N4 (Si4+ ) but also in the intermediate nitridation states with one (Si1+ ) or three (Si3+ ) nitrogen nearest neighbors. The Si 2p core-level shifts for the Si1+ , Si3+ , and Si4+ components are determined to be 0.64, 2.20, and 3.05 eV, respectively. In annealed sample it has been observed that the Si4+ component in the Si 2p spectra is significantly improved, which clearly indicates the crystalline nature of silicon nitride. The high resolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM) and photoluminescence (PL) studies showed a significant improvement of the crystalline qualities and enhancement of the optical properties of GaN grown on the stoichiometric Si3 N4 by molecular beam epitaxy (MBE). © 2010 Elsevier B.V. All rights reserved.
1. Introduction GaN material system is gathering great interest from the viewpoint of its application to optical devices as well as electronic devices because of its wide band gap nature [1]. GaN films used in the mass production of such devices are grown by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) on sapphire and SiC substrates [2–4]. However, they are limited in size and still rather expensive. Attempts to grow GaN on substrates like Si or quartz, which is available at low cost, and large areas have resulted in highly defective films due to high lattice mismatch and thermal expansion coefficient. Although some results have been reported on the successful growth of GaN on Si using AlN [5,6], SiC [7,8] and InGaN [9] buffer layers, but a defect-induced yellow luminescence (YL) was commonly present, which affects the optical properties of GaN. To date, among all the reported buffer layers for GaN on Si heteroepitaxy, the AlN buffer layer approach yields the best results reported in the literature [10,11]. However, the mutual solubility of Al and Si is very high at the buffer-layer growth temperature (∼820 ◦ C versus eutectic temperature 577 ◦ C). Therefore, interdiffusion of Al and Si at the interface is severe,
∗ Corresponding author. E-mail address: [email protected] (S.B. Krupanidhi). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.09.058
resulting in high unintentional doping levels in the epilayers and Si substrates [12,13]. To overcome this serious drawback, it has been demonstrated that a silicon nitride buffer layer can be used for the GaN growth [14–16]. In the present study, the annealing effect on the chemical bonding configurations of the surface and interface of an ultrathin silicon nitride film formed by RF nitrogen plasma on to Si (1 1 1) are investigated using core level photoelectron spectroscopy. The Si 2p core-level spectra reveal the distinct components for the nitrided Si species not only in stoichiometric Si3 N4 but also in the intermediate nitridation states of Si1+ and Si3+ . The crystalline qualities, surface morphologies and optical properties of GaN film grown on silicon nitride by RF-MBE were studied by HRXRD, SEM and PL.
2. Experimental The growth system used in this study was a RF-MBE system (OMICRON) equipped with a radio frequency (RF) plasma source. The base pressure in the system was below 1 × 10−10 mbar. The undoped Si (1 1 1) substrates were ultrasonically degreased in isopropyl alcohol (IPA) for 10 min and boiled in trichloroethylene, acetone and methanol at 70 ◦ C for 5 min, respectively, followed by dipping in 5% HF to remove the surface oxide. The substrates were thermally cleaned at 900 ◦ C for 1 h in ultra-high vacuum. For sample (a) the nitridation of the substrate was carried out at 530 ◦ C
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Fig. 2. N 1s core-level spectra of silicon nitride film grown on Si (1 1 1) for sample (a) and (b) and the corresponding decompositions N1, N2 and N3. Fig. 1. Si 2p core-level spectra of silicon nitride film grown on Si (1 1 1) for sample (a) and (b). Only intensity attenuation is detected. No energy shift and no new features arise.
for 60 min and for sample (b) the nitridation of the substrate was carried out at 530 ◦ C for 30 min followed by annealing at 900 ◦ C for 30 min in ultra high vacuum and again nitridation at 700 ◦ C for 30 min. Nitrogen flow rate and plasma power were kept at 0.5 sccm and 350 W. The chemical bonding states of silicon nitride surface were measured by core-level photoelectron spectroscopy for both samples. For the both samples (a) and (b), low temperature GaN buffer layer of thickness of 20 nm were grown on silicon nitride at 500 ◦ C followed by 400 nm thick GaN epilayers at 700 ◦ C. The Gallium beam equivalent pressure was kept 5.6 × 10−7 mbar. Nitrogen flow rate and plasma power were kept at 1.0 sccm and 350 W, for buffer layer and GaN growth. The structural characterization and surface morphologies of the samples were carried out by HRXRD and SEM, respectively. The PL spectra were recorded at room temperature using He–Cd laser of 325 nm excitation wavelength with a maximum input power of 30 mW.
Fig. 3. HRXRD rocking curves of both samples obtained from the GaN film grown on Si (1 1 1) substrate using silicon nitride intermediate layer.
3. Results and discussion 3.1. Si 2p core-levels of silicon nitride layer The core-levels photoelectron spectroscopy was carried out to determine the chemical bonding states of silicon nitride surface using Al K␣ radiation (h = 1486.6 eV). In Fig. 1 Si 2p core-level spectra (CLS) were recorded for sample (a) and (b) and it shows how the annealing affects the Si 2p surface sensitive core level spectra. The CLS have been numerically fitted using Lorentzian convoluted with a Gaussian function. The background has been taken into account using a linear profile. In stoichiometric silicon nitride a Si atom is bonded to 4 N atoms (Si4+ ) and an N atom is bonded to 3 Si atoms [17], thus only the component with Si4+ is expected. At the interface two more coordinations are required (Si3+ and Si1+ ) for an ideal match between the silicon nitride and Si (1 1 1) lattices [18]. Fig. 1 also shows the compositions: one bulk and three other components assigned to silicon in Si1+ , Si3+ and Si4+ coordination are needed. The corresponding binding energies are at +0.66, +2.20, +3.05 eV,
respectively (with respect to the bulk position). We summarized the differences in the energy positions in Table 1. For the Si1+ , Si3+ and Si4+ components, the measured ratio are 0.328:0.324:0.348 and 0.133:0.084:0.783 for sample (a) and (b), respectively. From this we can observe that the intensity of the stoichiometric Si3 N4 (Si4+ ) component is almost doubled in annealed sample compare to as grown sample. The absence of Si2+ species is identified in the specTable 1 Binding energy shifts (relatively to the bulk) of the Si 2p core-level for the silicon nitride on Si (1 1 1). Author reference
Si1+ (eV)
Si3+ (eV)
Si4+ (eV)
Theory [22,23] Present work Flammini et al. [19] Dufour et al. [20] Stesmans and Gorp [21] Kim and Yeom [18]
0.7 0.64 0.62 0.60 – 0.64
2.1 2.20 2.13 – – 2.21
2.8 3.05 2.79 2.59 3.15 2.74
M. Kumar et al. / Applied Surface Science 257 (2011) 2107–2110
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3.2. N 1s core-levels of silicon nitride layer Fig. 2 shows N 1s core-level spectra and their decompositions for the sample (a) and (b). From the raw data, one can find a dominant peak at the center, and the obvious shoulders at both sides. These spectra are straightforwardly decomposed into three components: the main peak (N2) is located at 397.65 eV and the others (N1 and N3) shifted relatively by −0.74 ± 0.01 and +0.76 ± 0.01 eV. Among the three N components, the major species N2 is naturally ascribed to N atoms in the stoichiometric Si3 N4 layer. From this we can observe that the intensity of the N2 peak is almost doubled in annealed sample (b), compare to as grown sample (a) this assignment is consistent with the observation of a dominant Si4+ component in the Si 2p spectrum of Fig. 1 and is corroborated by the binding energy of N2, which is in good agreement with that reported for stoichiometric Si3 N4 layer [18]. 3.3. HRXRD and SEM of GaN film The crystalline quality of GaN epilayers grown on ultra thin silicon nitride film on Si (1 1 1) substrate was determined by full width half maximum (FWHM) of HRXRD rocking curves of (0 0 0 2) symmetry planes of GaN epilayers, as shown in Fig. 3. The XRD peaks of GaN film grown on stoichiometric Si3 N4 layer (sample (b)) shows lower FWHM of ∼10 arcmin, indicating improved crystalline quality compared to as grown sample (a) which has FWHM of ∼18 arcmin. Fig. 4 shows the SEM of GaN film grown on ultra thin silicon nitride film on Si (1 1 1) substrate. It can be seen that the surface of the GaN film grown on stoichiometric Si3 N4 layer (sample (b)) is smooth and crack-free. Fig. 4. SEM images of both samples of the GaN film grown on Si (1 1 1) substrate using silicon nitride intermediate layer.
trum, which is expected to be at around +1.5 eV [24]; the maximum allowed intensity of the Si2+ component, if put intentionally, was less than 1% of the total intensity of the interfacial components of Si1+ –Si3+ . This behavior clearly shows the absence of oxidation of Si (1 1 1).
3.4. Photoluminescence measurements of GaN film The optical properties of both samples were further characterized by PL measurements using a He–Cd laser (30 mW, 325 nm) as the excitation source at room temperature and shown in Fig. 5. The GaN film shows a strong band-edge emission peak at 3.433 eV. From figure it can be seen that FWHM of PL spectra of GaN film grown on stoichiometric Si3 N4 layer (sample (b)) shows lower values, indicating excellent optical properties compared to as grown sample (a). Room temperature PL measurements also illustrate the absence of YL in both samples, which is due to the presence of gallium vacancies and deep level impurities. Generally, the absence of yellow emission may also be attributes to higher doping, causing carrier contribution is the range of 1019 cm−3 . However, the measured values in the present case were almost two orders of magnitude less. Such situation ruled out the contributions for the absence of yellow emission from heavy unintentional doping. 4. Conclusion
Fig. 5. Room temperature PL spectra of both samples.
In conclusion, a thin silicon nitride layer was grown on Si (1 1 1) surface by exposing the surface to radio-frequency (RF) nitrogen plasma. The clear Si 2p emission features for the interfacial Si1+ and Si3+ states are observed with core-level shifts of 0.64 and 2.20 eV, respectively, and those for the stoichiometric Si3 N4 layer are observed with a core-level shift of 3.05 eV. These values are in good agreement with the recent theoretical and experimental values. No Si2+ species is observed in the Si 2p spectra indicating an abrupt and defect-free interface due possibly to the nearly perfect lattice matching between crystalline Si3 N4 and Si (1 1 1). In annealed sample it has been observed that the Si4+ component in the Si 2p spectra is significantly improved, which clearly indicates the crystalline nature of silicon nitride. The GaN film grown on stoichiometric Si3 N4 layer (annealed sample) shows good crystalline qualities, smooth
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surface and excellent optical properties compared to as grown sample. References [1] S. Nakamura, S. Pearton, G. Fasol, The Blue Laser Diode, Springer, Berlin, 2000. [2] S. Tripathy, S.J. Chua, P. Chen, Z.L. Miao, J. Appl. Phys. 92 (2002) 3503. [3] V.A. Joshkin, C.B. Parker, S.M. Bedair, L.Y. Krasnobaev, J.J. Cuomo, R.F. Davis, A. Suvkha, Appl. Phys. Lett. 72 (1998) 2838. [4] H.J. Lee, H. Ryu, C. Lee, K. Kim, J. Crystal Growth 19 (1998) 621. [5] X. Ni, L. Zhu, Z. Ye, Z. Zhao, H. Tang, W. Hong, B. Zhao, Surf. Coat. Technol. 198 (2005) 350. [6] M.H. Kim, C. Sone, J.H. Yi, E. Yoon, Appl. Phys. Lett. 71 (1997) 1228. [7] L. Sugiura, K. Itaya, J. Nishio, H. Fujimoto, Y. Kokubun, J. Appl. Phys. 82 (1997) 4877. [8] W.J. Schaff, H. Wu, J.P. Choudhury, M. Murphy, T. Eustis, B. Foutz, O. Ambacher, L.F. Eastman, Solid-State Electron. 44 (2000) 259. [9] R.A. Oliver, M.J. Kappers, C.J. Humphreys, G.A. Briggs, J. Appl. Phys. 97 (2005) 013707. [10] J.W. Yang, A. Lunev, G. Simin, A. Chitnis, M. Shatalov, M. Asif Khan, J.E. Van Nostrand, R. Gaska, Appl. Phys. Lett. 76 (2000) 273.
[11] A. Dadgar, M. Poschenrieder, J. Blasing, K. Fehse, A. Diez, A. Krost, Appl. Phys. Lett. 80 (2002) 3670. [12] C.L. Wu, J.C. Wang, M.H. Chan, T.T. Chen, S. Gwo, Appl. Phys. Lett. 83 (2003) 4530. [13] E. Calleja, M.A. Sanchez-Garcıa, D. Basak, F.J. Saˇınchez, F. Calle, P. Youinou, E. Munoz, J.J. Serrano, J.M. Blanco, C. Villar, T. Laine, J. Oila, K. Saarinen, C.H. Hautojarvi, D.J. Molloy, I. Somerford, Harrison, Phys. Rev. B 58 (1998) 1550. [14] H.M. Lee, C.T. Kuo, H.W. Shiu, C.H. Chen, S. Gwo, Appl. Phys. Lett. 95 (2009) 222104. [15] W.Y. Uen, Z.Y. Li, S.M. Lan, S.M. Li, J. Crystal Growth 280 (2005) 335. [16] S. Sakai, T. Wang, Y. Morishima, Y. Naoi, J. Crystal Growth 221 (2003) 334. [17] G.L. Zhao, M. Bachlechner, Phys. Rev. B 58 (1998) 1887. [18] J. Kim, H. Yeom, Phys. Rev. B 67 (2003) 035304. [19] R. Flammini, R. Belkhou, F. Wiame, S. Iacobucci, A.T. Ibrahimi, Surf. Sci. 579 (2005) 188. [20] G. Dufour, F. Rochet, H. Roulet, F. Sirotti, Surf. Sci. 304 (1994) 33. [21] A. Stesmans, G.V. Gorp, Phys. Rev. B 52 (1995) 8904. [22] G.M. Rignanese, A. Pasquarello, Appl. Phys. Lett. 76 (2000) 553. [23] G.M. Rignanese, A. Pasquarello, Phys. Rev. B 63 (2001) 075307. [24] Y.D. Chung, J.W. Kim, C.N. Whang, H.W. Yeom, Phys. Rev. B 65 (2002) 155310.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The impact of ultra thin silicon nitride buffer layer on GaN growth on Si (1 1 1) by RF-MBE Mahesh Kumar a,b , Mohana K. Rajpalke a , Basanta Roul a,b , Thirumaleshwara N. Bhat a , Neeraj Sinha c , A.T. Kalghatgi b , S.B. Krupanidhi a,∗ a b c
Materials Research Centre, Indian Institute of Science, Bangalore 560012, Karnataka, India Central Research Laboratory, Bharat Electronics, Bangalore 560013, Karnataka, India Office of Principal Scientific Advisor, Government of India, New Delhi 110011, India
a r t i c l e
i n f o
Article history: Received 9 August 2010 Accepted 21 September 2010 Available online 25 September 2010 Keywords: Nitridation MBE Silicon nitride GaN
a b s t r a c t Ultra thin films of pure silicon nitride were grown on a Si (1 1 1) surface by exposing the surface to radiofrequency (RF) nitrogen plasma with a high content of nitrogen atoms. The effect of annealing of silicon nitride surface was investigated with core-level photoelectron spectroscopy. The Si 2p photoelectron spectra reveals a characteristic series of components for the Si species, not only in stoichiometric Si3 N4 (Si4+ ) but also in the intermediate nitridation states with one (Si1+ ) or three (Si3+ ) nitrogen nearest neighbors. The Si 2p core-level shifts for the Si1+ , Si3+ , and Si4+ components are determined to be 0.64, 2.20, and 3.05 eV, respectively. In annealed sample it has been observed that the Si4+ component in the Si 2p spectra is significantly improved, which clearly indicates the crystalline nature of silicon nitride. The high resolution X-ray diffraction (HRXRD), scanning electron microscopy (SEM) and photoluminescence (PL) studies showed a significant improvement of the crystalline qualities and enhancement of the optical properties of GaN grown on the stoichiometric Si3 N4 by molecular beam epitaxy (MBE). © 2010 Elsevier B.V. All rights reserved.
1. Introduction GaN material system is gathering great interest from the viewpoint of its application to optical devices as well as electronic devices because of its wide band gap nature [1]. GaN films used in the mass production of such devices are grown by metalorganic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE) on sapphire and SiC substrates [2–4]. However, they are limited in size and still rather expensive. Attempts to grow GaN on substrates like Si or quartz, which is available at low cost, and large areas have resulted in highly defective films due to high lattice mismatch and thermal expansion coefficient. Although some results have been reported on the successful growth of GaN on Si using AlN [5,6], SiC [7,8] and InGaN [9] buffer layers, but a defect-induced yellow luminescence (YL) was commonly present, which affects the optical properties of GaN. To date, among all the reported buffer layers for GaN on Si heteroepitaxy, the AlN buffer layer approach yields the best results reported in the literature [10,11]. However, the mutual solubility of Al and Si is very high at the buffer-layer growth temperature (∼820 ◦ C versus eutectic temperature 577 ◦ C). Therefore, interdiffusion of Al and Si at the interface is severe,
∗ Corresponding author. E-mail address: [email protected] (S.B. Krupanidhi). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.09.058
resulting in high unintentional doping levels in the epilayers and Si substrates [12,13]. To overcome this serious drawback, it has been demonstrated that a silicon nitride buffer layer can be used for the GaN growth [14–16]. In the present study, the annealing effect on the chemical bonding configurations of the surface and interface of an ultrathin silicon nitride film formed by RF nitrogen plasma on to Si (1 1 1) are investigated using core level photoelectron spectroscopy. The Si 2p core-level spectra reveal the distinct components for the nitrided Si species not only in stoichiometric Si3 N4 but also in the intermediate nitridation states of Si1+ and Si3+ . The crystalline qualities, surface morphologies and optical properties of GaN film grown on silicon nitride by RF-MBE were studied by HRXRD, SEM and PL.
2. Experimental The growth system used in this study was a RF-MBE system (OMICRON) equipped with a radio frequency (RF) plasma source. The base pressure in the system was below 1 × 10−10 mbar. The undoped Si (1 1 1) substrates were ultrasonically degreased in isopropyl alcohol (IPA) for 10 min and boiled in trichloroethylene, acetone and methanol at 70 ◦ C for 5 min, respectively, followed by dipping in 5% HF to remove the surface oxide. The substrates were thermally cleaned at 900 ◦ C for 1 h in ultra-high vacuum. For sample (a) the nitridation of the substrate was carried out at 530 ◦ C
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Fig. 2. N 1s core-level spectra of silicon nitride film grown on Si (1 1 1) for sample (a) and (b) and the corresponding decompositions N1, N2 and N3. Fig. 1. Si 2p core-level spectra of silicon nitride film grown on Si (1 1 1) for sample (a) and (b). Only intensity attenuation is detected. No energy shift and no new features arise.
for 60 min and for sample (b) the nitridation of the substrate was carried out at 530 ◦ C for 30 min followed by annealing at 900 ◦ C for 30 min in ultra high vacuum and again nitridation at 700 ◦ C for 30 min. Nitrogen flow rate and plasma power were kept at 0.5 sccm and 350 W. The chemical bonding states of silicon nitride surface were measured by core-level photoelectron spectroscopy for both samples. For the both samples (a) and (b), low temperature GaN buffer layer of thickness of 20 nm were grown on silicon nitride at 500 ◦ C followed by 400 nm thick GaN epilayers at 700 ◦ C. The Gallium beam equivalent pressure was kept 5.6 × 10−7 mbar. Nitrogen flow rate and plasma power were kept at 1.0 sccm and 350 W, for buffer layer and GaN growth. The structural characterization and surface morphologies of the samples were carried out by HRXRD and SEM, respectively. The PL spectra were recorded at room temperature using He–Cd laser of 325 nm excitation wavelength with a maximum input power of 30 mW.
Fig. 3. HRXRD rocking curves of both samples obtained from the GaN film grown on Si (1 1 1) substrate using silicon nitride intermediate layer.
3. Results and discussion 3.1. Si 2p core-levels of silicon nitride layer The core-levels photoelectron spectroscopy was carried out to determine the chemical bonding states of silicon nitride surface using Al K␣ radiation (h = 1486.6 eV). In Fig. 1 Si 2p core-level spectra (CLS) were recorded for sample (a) and (b) and it shows how the annealing affects the Si 2p surface sensitive core level spectra. The CLS have been numerically fitted using Lorentzian convoluted with a Gaussian function. The background has been taken into account using a linear profile. In stoichiometric silicon nitride a Si atom is bonded to 4 N atoms (Si4+ ) and an N atom is bonded to 3 Si atoms [17], thus only the component with Si4+ is expected. At the interface two more coordinations are required (Si3+ and Si1+ ) for an ideal match between the silicon nitride and Si (1 1 1) lattices [18]. Fig. 1 also shows the compositions: one bulk and three other components assigned to silicon in Si1+ , Si3+ and Si4+ coordination are needed. The corresponding binding energies are at +0.66, +2.20, +3.05 eV,
respectively (with respect to the bulk position). We summarized the differences in the energy positions in Table 1. For the Si1+ , Si3+ and Si4+ components, the measured ratio are 0.328:0.324:0.348 and 0.133:0.084:0.783 for sample (a) and (b), respectively. From this we can observe that the intensity of the stoichiometric Si3 N4 (Si4+ ) component is almost doubled in annealed sample compare to as grown sample. The absence of Si2+ species is identified in the specTable 1 Binding energy shifts (relatively to the bulk) of the Si 2p core-level for the silicon nitride on Si (1 1 1). Author reference
Si1+ (eV)
Si3+ (eV)
Si4+ (eV)
Theory [22,23] Present work Flammini et al. [19] Dufour et al. [20] Stesmans and Gorp [21] Kim and Yeom [18]
0.7 0.64 0.62 0.60 – 0.64
2.1 2.20 2.13 – – 2.21
2.8 3.05 2.79 2.59 3.15 2.74
M. Kumar et al. / Applied Surface Science 257 (2011) 2107–2110
2109
3.2. N 1s core-levels of silicon nitride layer Fig. 2 shows N 1s core-level spectra and their decompositions for the sample (a) and (b). From the raw data, one can find a dominant peak at the center, and the obvious shoulders at both sides. These spectra are straightforwardly decomposed into three components: the main peak (N2) is located at 397.65 eV and the others (N1 and N3) shifted relatively by −0.74 ± 0.01 and +0.76 ± 0.01 eV. Among the three N components, the major species N2 is naturally ascribed to N atoms in the stoichiometric Si3 N4 layer. From this we can observe that the intensity of the N2 peak is almost doubled in annealed sample (b), compare to as grown sample (a) this assignment is consistent with the observation of a dominant Si4+ component in the Si 2p spectrum of Fig. 1 and is corroborated by the binding energy of N2, which is in good agreement with that reported for stoichiometric Si3 N4 layer [18]. 3.3. HRXRD and SEM of GaN film The crystalline quality of GaN epilayers grown on ultra thin silicon nitride film on Si (1 1 1) substrate was determined by full width half maximum (FWHM) of HRXRD rocking curves of (0 0 0 2) symmetry planes of GaN epilayers, as shown in Fig. 3. The XRD peaks of GaN film grown on stoichiometric Si3 N4 layer (sample (b)) shows lower FWHM of ∼10 arcmin, indicating improved crystalline quality compared to as grown sample (a) which has FWHM of ∼18 arcmin. Fig. 4 shows the SEM of GaN film grown on ultra thin silicon nitride film on Si (1 1 1) substrate. It can be seen that the surface of the GaN film grown on stoichiometric Si3 N4 layer (sample (b)) is smooth and crack-free. Fig. 4. SEM images of both samples of the GaN film grown on Si (1 1 1) substrate using silicon nitride intermediate layer.
trum, which is expected to be at around +1.5 eV [24]; the maximum allowed intensity of the Si2+ component, if put intentionally, was less than 1% of the total intensity of the interfacial components of Si1+ –Si3+ . This behavior clearly shows the absence of oxidation of Si (1 1 1).
3.4. Photoluminescence measurements of GaN film The optical properties of both samples were further characterized by PL measurements using a He–Cd laser (30 mW, 325 nm) as the excitation source at room temperature and shown in Fig. 5. The GaN film shows a strong band-edge emission peak at 3.433 eV. From figure it can be seen that FWHM of PL spectra of GaN film grown on stoichiometric Si3 N4 layer (sample (b)) shows lower values, indicating excellent optical properties compared to as grown sample (a). Room temperature PL measurements also illustrate the absence of YL in both samples, which is due to the presence of gallium vacancies and deep level impurities. Generally, the absence of yellow emission may also be attributes to higher doping, causing carrier contribution is the range of 1019 cm−3 . However, the measured values in the present case were almost two orders of magnitude less. Such situation ruled out the contributions for the absence of yellow emission from heavy unintentional doping. 4. Conclusion
Fig. 5. Room temperature PL spectra of both samples.
In conclusion, a thin silicon nitride layer was grown on Si (1 1 1) surface by exposing the surface to radio-frequency (RF) nitrogen plasma. The clear Si 2p emission features for the interfacial Si1+ and Si3+ states are observed with core-level shifts of 0.64 and 2.20 eV, respectively, and those for the stoichiometric Si3 N4 layer are observed with a core-level shift of 3.05 eV. These values are in good agreement with the recent theoretical and experimental values. No Si2+ species is observed in the Si 2p spectra indicating an abrupt and defect-free interface due possibly to the nearly perfect lattice matching between crystalline Si3 N4 and Si (1 1 1). In annealed sample it has been observed that the Si4+ component in the Si 2p spectra is significantly improved, which clearly indicates the crystalline nature of silicon nitride. The GaN film grown on stoichiometric Si3 N4 layer (annealed sample) shows good crystalline qualities, smooth
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surface and excellent optical properties compared to as grown sample. References [1] S. Nakamura, S. Pearton, G. Fasol, The Blue Laser Diode, Springer, Berlin, 2000. [2] S. Tripathy, S.J. Chua, P. Chen, Z.L. Miao, J. Appl. Phys. 92 (2002) 3503. [3] V.A. Joshkin, C.B. Parker, S.M. Bedair, L.Y. Krasnobaev, J.J. Cuomo, R.F. Davis, A. Suvkha, Appl. Phys. Lett. 72 (1998) 2838. [4] H.J. Lee, H. Ryu, C. Lee, K. Kim, J. Crystal Growth 19 (1998) 621. [5] X. Ni, L. Zhu, Z. Ye, Z. Zhao, H. Tang, W. Hong, B. Zhao, Surf. Coat. Technol. 198 (2005) 350. [6] M.H. Kim, C. Sone, J.H. Yi, E. Yoon, Appl. Phys. Lett. 71 (1997) 1228. [7] L. Sugiura, K. Itaya, J. Nishio, H. Fujimoto, Y. Kokubun, J. Appl. Phys. 82 (1997) 4877. [8] W.J. Schaff, H. Wu, J.P. Choudhury, M. Murphy, T. Eustis, B. Foutz, O. Ambacher, L.F. Eastman, Solid-State Electron. 44 (2000) 259. [9] R.A. Oliver, M.J. Kappers, C.J. Humphreys, G.A. Briggs, J. Appl. Phys. 97 (2005) 013707. [10] J.W. Yang, A. Lunev, G. Simin, A. Chitnis, M. Shatalov, M. Asif Khan, J.E. Van Nostrand, R. Gaska, Appl. Phys. Lett. 76 (2000) 273.
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