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Properties and electronic structure of heavily oxygen-doped GaN crystals
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Chemical Physics Letters 451 (2008) 222–225 www.elsevier.com/locate/cplett
Properties and electronic structure of heavily oxygen-doped GaN crystals Akira Miura a, Shiro Shimada a,*, Masaaki Yokoyama b, Hiroto Tachikawa a, Toshio Kitamura c a
Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan b Horiba Ltd., Tokyo 101-0031, Japan c National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8568, Japan Received 25 May 2007; in final form 4 December 2007 Available online 8 December 2007
Abstract We investigated the properties of heavily oxygen-doped single GaN crystals and proposed their electronic structure based on firstprinciple calculations. The crystals were grown by carbothermal reduction and nitridation of Ga2O3 at 1180 °C for 2–8 h. The crystals were found to have high-crystallinity and high-concentration oxygen (4–9 1020 atoms/cm3). Cathodoluminescence measurements of the 8-h-grown crystals showed a shift of band-edge-related luminescence peak toward higher energy (ca. 120 meV). Density of state of oxygen-doped GaN supercell was calculated using the first-principle to support this upward shift of luminescence energy. Ó 2007 Elsevier B.V. All rights reserved.
Wurtzite-type GaN is a direct semiconductor with a band gap energy of ca. 3.4 eV and high luminescent efficiency, enabling its use in optoelectronic devices such as light emitting diodes from blue to ultraviolet regions [1]. For these devices to be engineered, doping of GaN is an important technology to control optical and electronic properties. Reportedly, low-concentration oxygen and silicon in the GaN substitute for nitrogen and gallium, respectively, generating shallow donor levels by releasing electrons [2]. On the other hand, the high-concentration doping of GaN with these impurities showed different luminescence properties. Some researchers have reported that the luminescence peak energy decreases with increasing doping concentration of silicon [3,4], probably because of the band gap renormalization effect. Others have described that the upward shift of the luminescence energy occurs because of the high electron concentration produced by the high-concentration doping of oxygen [5,6], as explained
*
Corresponding author. Fax: +81 11 706 6576. E-mail address: [email protected] (S. Shimada).
0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.12.007
by Moss–Burstein shift. Therefore, these two phenomena caused by high-concentration doping leave room for investigation of GaN properties with high-concentration impurities, especially the luminescence property and electronic structure. The present study examines properties of heavily oxygen-doped GaN single crystals grown by carbothermal reduction and nitridation (CRN) of Ga2O3. Based on these results, their electronic structure is described using a computational approach. The starting material for the growth of heavily oxygendoped GaN crystals was a mixture of Ga2O3 and graphite powders in the mole ratio of Ga2O3/C = 1/20 [7]. In a horizontal furnace, 7.2 g of the starting material in a silica glass boat 100 mm long was set in a silica glass tube. A silica glass bar connected to a motor pushed the boat at a constant rate of 0.8 cm/h into the heating zone of 970– 1060 °C, where Ga2O vapor was generated by the carbothermal reduction of Ga2O3 in Ar (purity >99.999%) flowing at 100 ml min1. The Ga2O vapor was transported into a silica glass crucible by Ar flow [8] and nitrided at 1180 °C with NH3 (purity >99.99997%) flowing at 300 ml min1 for 2, 4, 6 and 8 h.
A. Miura et al. / Chemical Physics Letters 451 (2008) 222–225
ments of the crystals grown for 4–8 h indicated a FWHM of ca. 30 arcsec in the ð1 0 1 0Þ diffraction peaks, thereby demonstrating a high-crystallinity comparable to that of crystals grown using the Na flux method [12]. Examination by EPMA revealed that the crystals grown for 2–8 h contain almost the same amount of gallium and nitrogen, in addition to 0.5–1.5 at.% oxygen. No region existed in the crystals where gallium, nitrogen and/or oxygen were condensed locally. The SIMS results (Table 1) show that these crystals contain the oxygen as a major impurity (4–9 1020 atoms/cm3) and the silicon and carbon as minor ones. A large difference in the hydrogen concentration of the crystals grown for 2–4 h and 6–8 h is apparent, but it remains unclear why such a large difference occurs in these crystals. It was proposed in our previous Letters [7,13] that the GaN crystals are grown by CRN of Ga2O3, as follows: Ga2 O3 ðsÞ þ 2CðsÞ ! Ga2 OðgÞ þ 2COðgÞ Ga2 OðgÞ þ 2NH3 ðgÞ ! 2GaNðsÞ þ 2H2 ðgÞ þ H2 OðgÞ
Table 1 SIMS results of the four elements (O, H, Si, C) at a depth down to 5 lm from the surface of the crystals grown for 2–8 h Growth time/h
Impurity/atoms/cm3
2 4 6 8
9 1020 5 1020 4 1020 4 1020
Oxygen
Hydrogen 3 1019 4 1019 6 1017 <5 1017
Silicon
Carbon
3 1018 2 1018 2 1018 2 1018
8 1016 3 1016 4 1016 1 1017
3.59eV (c) (b) Χ1
3.55eV
(a) Χ33
1.2 1.6 2 2.4 2.8 3.2 3.6 4 3 Energy/ eV
Fig. 1. Optical micrographs of crystals grown for (A) 4 h and (B) 8 h.
ð1Þ ð2Þ
A large amount of the oxygen in these crystals should derive from Ga2O and/or CO. The existence of the hydrogen, carbon and silicon in the crystals is probably attributable to contents of NH3, CO and silica glass, respectively [8]. Fig. 2 shows the CL spectra measured at room temperature and 18 K of the 4-h- and 8-h-grown crystals, together with that of an epitaxial film grown on sapphire substrates using metalorganic chemical vapor deposition (MOCVD). The 4-h-grown crystals shows weak and broad lumines-
Luminescence Intensity (a.u.)
The lattice parameters of the crystals were determined using X-ray powder diffraction (XRD, RINT 2200; Rigaku Corp.). A high-resolution X-ray diffractometer (D8 Discover; Bruker AXS) with a four-crystal Cu Ka1 monochromator was used for the rocking curve measurements to obtain the full-width at half-maximum (FWHM) of the ð1 0 1 0Þ diffraction peaks, with the (0 0 0 1) plane parallel to the scattering plane. Both the spot size of X-ray beam and the width of detector slit were 0.5 mm. Electron probe microanalysis (EPMA, JXA-8900 M; JEOL) was used for determination of the major components in the crystals. Secondary ion mass spectroscopy (SIMS, IMS-4f; Cameca Instrument Japan K.K. Ltd.) analysis was performed using Cs+ ion at an accelerating voltage of 14.5 kV to determine the concentrations of H, C, O, and Si elements in the crystals. Cathodoluminescence (CL) measurements of the crystals were performed at room temperature and 18 K at an accelerating voltage of 5 kV using a scanning electron microscopy (SEM) connected to a CL detection system [9]. Raman spectra of the crystals were obtained using the 457.9 and 514.5 nm laser lines with a liquid-nitrogencooled charge-coupled device (CCD). The density functional theory (DFT) was applied to calculate the electronic structure of undoped and oxygendoped GaN. The crystal wave functions were expanded in terms of a plane wave basis set. The cutoff energies of the plane-wave expansion for wave functions and charge densities were 25.0 and 200 Rydberg, respectively. Generalized gradient approximation (GGA) [10] was performed using the projector augmented wave method. Fig. 1 shows optical micrographs of typical crystals grown for 4 and 8 h. It was found that they grew to pale brown or black millimeter-sizes in hexagonal-prism shapes. XRD analysis indicated that the crystals grown for 2–8 h had the wurtzite-type structure with lattice parameters of a0 = 0.3189 nm and c0 = 0.5184 nm, agreeing with the reported values of colorless GaN single crystals grown using Na flux method [11]. X-ray rocking curve measure-
223
3.2 3.4 3.6 Energy/ eV
3.8
Fig. 2. CL spectra (A) measured at room temperature of (a) 4-h- and (b) 8-h-grown crystals, and (c) epitaxial film grown by MOCVD, and CL spectrum (B) measured at 18 K of 8-h-grown crystal.
224
A. Miura et al. / Chemical Physics Letters 451 (2008) 222–225
400
shift of E2 mode imply that the high-concentration oxygen changed the bonding strength between gallium and nitrogen. The density of states (DOSs) of undoped and oxygendoped wurtzite-type GaN were obtained using first-principle calculations to explain the effect of the oxygen content in GaN on its electronic structure. Fig. 4A shows the Ga54N54 and Ga54N53O1 supercells of undoped and substitutionally oxygen-doped GaN, respectively, using the three-fold translation of the primitive unit cell of GaN with the measured lattice parameters of the obtained crystals. The oxygen concentration in the oxygen-doped GaN supercell (Ga54N53O1) is taken as ca. 8 1020 atoms/cm3, being almost equal to that of the grown crystals. Fig. 4B shows the total DOSs of (a) undoped and (b) oxygendoped GaN, and (c) the decomposed DOS for oxygen of oxygen-doped GaN, together with their expanded Fig. 4B(a0 –c0 ); the zero of energy was defined as the valence-band edge, and the Fermi level (Ef) as the middle between the highest energy state with valence electrons and the lowest state without the electrons in the ground state. The substitutional oxygen raises the Fermi level and forms the two states (Y and Z) in oxygen-doped GaN above and below the conduction-band edge (X) of undoped GaN, as shown in Fig. 4B(a0 and b0 ). These two states (Y and Z) in decomposed DOS for oxygen of oxygen-doped GaN (Fig. 4B(c0 )) overlap considerably with those in the total DOS (Fig. 4B(b0 )) to be hybrid states. Because the DOS of the Ga54N53O1 supercell with the substitutional oxygen should reflect an electronic structure of heavily oxygen-doped GaN crystals, the upward energy
(c) (b) (a) 564 566 568 570 572 -1 Raman Shift (cm-1)
A1(LO) E1 (LO)
(b) A1 (TO) E1(TO) E2
Raman Intensity (a.u.)
cence peaks at ca. 3.4 and ca. 1.8 eV (Fig. 2A(a)), whereas the 8-h-grown crystals show an intense luminescence peak at 3.54 eV. The CL spectra obtained for the 2 and 6 h were similar to those for the 4 and 8 h, respectively. Importantly, the luminescence peak energy of the 8-h-grown crystals (3.54 eV) is 120 meV higher than that of the MOCVD film (3.42 eV) (Fig. 2A(b) and (c)). The luminescence spectrum measured at 18 K of the 8-h-grown crystal gives overlapping intense luminescence peaks at 3.55 and 3.59 eV with tailing at the lower-energy side (Fig. 2B). It has been known that Raman spectra of wurtzite-type GaN show A1(TO), E1(TO), E2, A1(LO) and E1(LO) modes [14–18], depending on the crystallographic orientation [15–17], electron concentration [15,16,18], strain [15,17] and bonding strength. Raman spectra obtained for the ð1 0 1 0Þ plane of the 4-h- and 8-h-grown crystals exhibit the A1(TO), E1(TO) and E2 modes, but not the A1(LO) and E1(LO) modes (Fig. 3a and b). It has been reported that the absence of A1(LO) and E1(LO) modes is due to the crystallographic orientation [15–17] and/or high electron concentration >1019 cm3 [15,16,18]. Since the spectra of the GaN powders produced by grinding the grown crystals also exhibited the absence of A1(LO) and E1(LO) modes, the nonexistence of these modes is attributed to not the crystallographic orientation but the high electron concentration, which is probably derived from the high-concentration substitutional oxygen for nitrogen in GaN. The high-resolution Raman spectra of the E2 mode of these crystals and a strain-free MOCVD film are shown in the inset in Fig. 3; a strain-free state of the film is recognized by the frequency of E2 mode (567.7 cm1) [14,17]. The FWHM value of the E2 mode of the crystals is ca. 3.5 cm1, which is broader than that for the MOCVD film (ca. 2.0 cm1). The frequency of E2 mode of the crystals are 567.5 and 567.3 cm1, respectively, indicating slightly downward frequency shifts from that of the MOCVD film. The broadening and slight frequency
500 600 700 800 Raman Shift (cm-1)
(a)
900
Fig. 3. Raman spectra taken on the ð1 0 1 0Þ plane of (a) 4-h- and (b) 8-hgrown crystals. Positions of Raman modes in wurtzite-type GaN (Ref. [14]) are marked by lines. Inset spectra with a spectral resolution of 0.2 cm1 show E2 mode of (a0 ) 4-h- and (b0 ) 8-h-grown crystals, and (c0 ) stress-free epitaxial film grown by MOCVD.
Fig. 4. (A) Supercell structures of undoped and oxygen-doped GaN crystals. (B) Total DOSs of (a) undoped and (b) oxygen-doped GaN, and (c) decomposed DOS for oxygen of oxygen-doped GaN. The figures on the left side show an energy region of 6.5 to +6.5 eV; the figures on the right side depict an energy region of 0.5 to +4.8 eV. Ef indicates Fermi level.
A. Miura et al. / Chemical Physics Letters 451 (2008) 222–225
shift of the CL peak observed in the 6–8-h-grown crystals can be explained by both the formation of high-energy hybrid state (Z) and the upward shift of Fermi level. It has been reported that the large band gap of heavily oxygen-doped InN film measured using optical absorption might result from the formation of an InOxNy alloy phase (alloying effect) and Moss–Burstein shift [19]. Consequently, the formation of the high-energy hybrid state and the upward shift of Fermi level could be explained by the formation of a GaOxNy alloy phase and Moss–Burstein shift, respectively. The observed tailing peak might reflect the state (Y) (Fig. 2A(b)). Although the role of high-concentration oxygen in GaN on its electronic structure, shown by the DOSs, can support the CL energy shift, it is not clear why the CL shift was not observed in the 2–4-h-grown crystals (Fig. 2A(a)). Since only the 2–4-h-grown crystals contained both the highconcentration hydrogen >1019 atoms/cm3 and oxygen >1020 atoms/cm3 (Table 1), it is possible that the high-concentration hydrogen in the heavily oxygen-doped GaN crystals could prevent the CL shift. In summary, we investigated the properties of heavily oxygen-doped GaN single crystals grown by the CRN of Ga2O3 at 1180 °C for 2–8 h. The crystals had the wurtzite-type structure with a0 = 0.3189 nm and c0 = 0.5184 nm and showed their high-crystallinity. The 4-hand 8-h-grown crystals exhibited broadening and slight frequency shift of the Raman E2 mode allowed by the wurtzite-type GaN, but not the LO modes. These crystals contained the high oxygen concentration (4–5 1020 atoms/cm3), but the hydrogen concentration was much lower in the 8-h-grown crystals than in the 4-h-grown ones. The CL spectra of the 8-h-grown crystals exhibited the blue shift of the band-edge-related peak (120 meV) with the tailing at lower-energies, which was explained by both the formation of the high-energy hybrid state and the upward shift of the Fermi level, based on the first-principle calculation of DOS of Ga54N53O1 supercell.
225
Acknowledgements We would like to express our sincere appreciation to Prof. T. Sekiguchi (National Institute for Materials Science) for CL measurements and helpful suggestions, and to Prof. S. Nakashima (National Institute of Advanced Industrial Science and Technology) for Raman measurements and valuable advice. We would also like to express our hearty thanks to Dr. M. Jo (The Open Facility of Hokkaido University Sousei Hall) for the rocking curve measurements, to K. Akutsu (MST) for the first-principle calculations. This study was partly supported by the Government Science Research Fund of the Ministry of Education of Japan (No. 18655082) for S.S. References [1] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Jpn. J. Appl. Phys. 34 (1995) L979. [2] H. Wang, A.-B. Chen, J. Appl. Phys. 87 (2000) 7859. [3] M. Yoshikawa et al., J. Appl. Phys. 86 (1999) 4400. [4] W. Li, Aizhen Li, Thin Sol. Film. 401 (2001) 279. [5] D. Meister et al., J. Appl. Phys. 88 (2000) 1811. [6] P. Prystawko et al., Phys. State. Sol. (b) 210 (1998) 437. [7] A. Miura, S. Shimada, T. Sekiguchi, J. Cryst. Growth 299 (2007) 22. [8] A. Miura, S. Shimada, T. Sekiguchi, M. Yokoyama, B. Mizobuchi, J. Cryst. Growth (in press). [9] T. Sekiguchi, H.S. Leipner, Appl. Phys. Lett. 67 (1995) 3777. [10] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [11] M. Aoki, H. Yamane, M. Shimada, T. Kajiwara, S. Sarayama, F.J. DiSalvo, Cryst. Growth Design 2 (2002) 55. [12] T. Yamada, H. Yamane, H. Iwata, S. Sarayama, J. Cryst. Growth 281 (2005) 242. [13] S. Shimada, R. Taniguchi, J. Cryst. Growth 263 (2004) 1. [14] V.Y. Davydov et al., Phys. Rev. B 58 (1998) 899. [15] H. Harima, J. Phys.: Condens. Matter 14 (2002) R967. [16] T. Kozawa, T. Kachi, H. Kano, Y. Taga, M. Hashimoto, N. Koide, K. Manabe, J. Appl. Phys. 75 (1994) 1098. [17] L.I. Hwan et al., J. Appl. Phys. 83 (1998) 5787. [18] H. Harima, H. Sakashita, T. Inoue, S. Nakashima, J. Cryst. Growth 189–190 (1998) 672. [19] K.S.A. Butcher, T.L. Tansley, Superlattice Microst 38 (2005) 1.
Chemical Physics Letters 451 (2008) 222–225 www.elsevier.com/locate/cplett
Properties and electronic structure of heavily oxygen-doped GaN crystals Akira Miura a, Shiro Shimada a,*, Masaaki Yokoyama b, Hiroto Tachikawa a, Toshio Kitamura c a
Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan b Horiba Ltd., Tokyo 101-0031, Japan c National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8568, Japan Received 25 May 2007; in final form 4 December 2007 Available online 8 December 2007
Abstract We investigated the properties of heavily oxygen-doped single GaN crystals and proposed their electronic structure based on firstprinciple calculations. The crystals were grown by carbothermal reduction and nitridation of Ga2O3 at 1180 °C for 2–8 h. The crystals were found to have high-crystallinity and high-concentration oxygen (4–9 1020 atoms/cm3). Cathodoluminescence measurements of the 8-h-grown crystals showed a shift of band-edge-related luminescence peak toward higher energy (ca. 120 meV). Density of state of oxygen-doped GaN supercell was calculated using the first-principle to support this upward shift of luminescence energy. Ó 2007 Elsevier B.V. All rights reserved.
Wurtzite-type GaN is a direct semiconductor with a band gap energy of ca. 3.4 eV and high luminescent efficiency, enabling its use in optoelectronic devices such as light emitting diodes from blue to ultraviolet regions [1]. For these devices to be engineered, doping of GaN is an important technology to control optical and electronic properties. Reportedly, low-concentration oxygen and silicon in the GaN substitute for nitrogen and gallium, respectively, generating shallow donor levels by releasing electrons [2]. On the other hand, the high-concentration doping of GaN with these impurities showed different luminescence properties. Some researchers have reported that the luminescence peak energy decreases with increasing doping concentration of silicon [3,4], probably because of the band gap renormalization effect. Others have described that the upward shift of the luminescence energy occurs because of the high electron concentration produced by the high-concentration doping of oxygen [5,6], as explained
*
Corresponding author. Fax: +81 11 706 6576. E-mail address: [email protected] (S. Shimada).
0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.12.007
by Moss–Burstein shift. Therefore, these two phenomena caused by high-concentration doping leave room for investigation of GaN properties with high-concentration impurities, especially the luminescence property and electronic structure. The present study examines properties of heavily oxygen-doped GaN single crystals grown by carbothermal reduction and nitridation (CRN) of Ga2O3. Based on these results, their electronic structure is described using a computational approach. The starting material for the growth of heavily oxygendoped GaN crystals was a mixture of Ga2O3 and graphite powders in the mole ratio of Ga2O3/C = 1/20 [7]. In a horizontal furnace, 7.2 g of the starting material in a silica glass boat 100 mm long was set in a silica glass tube. A silica glass bar connected to a motor pushed the boat at a constant rate of 0.8 cm/h into the heating zone of 970– 1060 °C, where Ga2O vapor was generated by the carbothermal reduction of Ga2O3 in Ar (purity >99.999%) flowing at 100 ml min1. The Ga2O vapor was transported into a silica glass crucible by Ar flow [8] and nitrided at 1180 °C with NH3 (purity >99.99997%) flowing at 300 ml min1 for 2, 4, 6 and 8 h.
A. Miura et al. / Chemical Physics Letters 451 (2008) 222–225
ments of the crystals grown for 4–8 h indicated a FWHM of ca. 30 arcsec in the ð1 0 1 0Þ diffraction peaks, thereby demonstrating a high-crystallinity comparable to that of crystals grown using the Na flux method [12]. Examination by EPMA revealed that the crystals grown for 2–8 h contain almost the same amount of gallium and nitrogen, in addition to 0.5–1.5 at.% oxygen. No region existed in the crystals where gallium, nitrogen and/or oxygen were condensed locally. The SIMS results (Table 1) show that these crystals contain the oxygen as a major impurity (4–9 1020 atoms/cm3) and the silicon and carbon as minor ones. A large difference in the hydrogen concentration of the crystals grown for 2–4 h and 6–8 h is apparent, but it remains unclear why such a large difference occurs in these crystals. It was proposed in our previous Letters [7,13] that the GaN crystals are grown by CRN of Ga2O3, as follows: Ga2 O3 ðsÞ þ 2CðsÞ ! Ga2 OðgÞ þ 2COðgÞ Ga2 OðgÞ þ 2NH3 ðgÞ ! 2GaNðsÞ þ 2H2 ðgÞ þ H2 OðgÞ
Table 1 SIMS results of the four elements (O, H, Si, C) at a depth down to 5 lm from the surface of the crystals grown for 2–8 h Growth time/h
Impurity/atoms/cm3
2 4 6 8
9 1020 5 1020 4 1020 4 1020
Oxygen
Hydrogen 3 1019 4 1019 6 1017 <5 1017
Silicon
Carbon
3 1018 2 1018 2 1018 2 1018
8 1016 3 1016 4 1016 1 1017
3.59eV (c) (b) Χ1
3.55eV
(a) Χ33
1.2 1.6 2 2.4 2.8 3.2 3.6 4 3 Energy/ eV
Fig. 1. Optical micrographs of crystals grown for (A) 4 h and (B) 8 h.
ð1Þ ð2Þ
A large amount of the oxygen in these crystals should derive from Ga2O and/or CO. The existence of the hydrogen, carbon and silicon in the crystals is probably attributable to contents of NH3, CO and silica glass, respectively [8]. Fig. 2 shows the CL spectra measured at room temperature and 18 K of the 4-h- and 8-h-grown crystals, together with that of an epitaxial film grown on sapphire substrates using metalorganic chemical vapor deposition (MOCVD). The 4-h-grown crystals shows weak and broad lumines-
Luminescence Intensity (a.u.)
The lattice parameters of the crystals were determined using X-ray powder diffraction (XRD, RINT 2200; Rigaku Corp.). A high-resolution X-ray diffractometer (D8 Discover; Bruker AXS) with a four-crystal Cu Ka1 monochromator was used for the rocking curve measurements to obtain the full-width at half-maximum (FWHM) of the ð1 0 1 0Þ diffraction peaks, with the (0 0 0 1) plane parallel to the scattering plane. Both the spot size of X-ray beam and the width of detector slit were 0.5 mm. Electron probe microanalysis (EPMA, JXA-8900 M; JEOL) was used for determination of the major components in the crystals. Secondary ion mass spectroscopy (SIMS, IMS-4f; Cameca Instrument Japan K.K. Ltd.) analysis was performed using Cs+ ion at an accelerating voltage of 14.5 kV to determine the concentrations of H, C, O, and Si elements in the crystals. Cathodoluminescence (CL) measurements of the crystals were performed at room temperature and 18 K at an accelerating voltage of 5 kV using a scanning electron microscopy (SEM) connected to a CL detection system [9]. Raman spectra of the crystals were obtained using the 457.9 and 514.5 nm laser lines with a liquid-nitrogencooled charge-coupled device (CCD). The density functional theory (DFT) was applied to calculate the electronic structure of undoped and oxygendoped GaN. The crystal wave functions were expanded in terms of a plane wave basis set. The cutoff energies of the plane-wave expansion for wave functions and charge densities were 25.0 and 200 Rydberg, respectively. Generalized gradient approximation (GGA) [10] was performed using the projector augmented wave method. Fig. 1 shows optical micrographs of typical crystals grown for 4 and 8 h. It was found that they grew to pale brown or black millimeter-sizes in hexagonal-prism shapes. XRD analysis indicated that the crystals grown for 2–8 h had the wurtzite-type structure with lattice parameters of a0 = 0.3189 nm and c0 = 0.5184 nm, agreeing with the reported values of colorless GaN single crystals grown using Na flux method [11]. X-ray rocking curve measure-
223
3.2 3.4 3.6 Energy/ eV
3.8
Fig. 2. CL spectra (A) measured at room temperature of (a) 4-h- and (b) 8-h-grown crystals, and (c) epitaxial film grown by MOCVD, and CL spectrum (B) measured at 18 K of 8-h-grown crystal.
224
A. Miura et al. / Chemical Physics Letters 451 (2008) 222–225
400
shift of E2 mode imply that the high-concentration oxygen changed the bonding strength between gallium and nitrogen. The density of states (DOSs) of undoped and oxygendoped wurtzite-type GaN were obtained using first-principle calculations to explain the effect of the oxygen content in GaN on its electronic structure. Fig. 4A shows the Ga54N54 and Ga54N53O1 supercells of undoped and substitutionally oxygen-doped GaN, respectively, using the three-fold translation of the primitive unit cell of GaN with the measured lattice parameters of the obtained crystals. The oxygen concentration in the oxygen-doped GaN supercell (Ga54N53O1) is taken as ca. 8 1020 atoms/cm3, being almost equal to that of the grown crystals. Fig. 4B shows the total DOSs of (a) undoped and (b) oxygendoped GaN, and (c) the decomposed DOS for oxygen of oxygen-doped GaN, together with their expanded Fig. 4B(a0 –c0 ); the zero of energy was defined as the valence-band edge, and the Fermi level (Ef) as the middle between the highest energy state with valence electrons and the lowest state without the electrons in the ground state. The substitutional oxygen raises the Fermi level and forms the two states (Y and Z) in oxygen-doped GaN above and below the conduction-band edge (X) of undoped GaN, as shown in Fig. 4B(a0 and b0 ). These two states (Y and Z) in decomposed DOS for oxygen of oxygen-doped GaN (Fig. 4B(c0 )) overlap considerably with those in the total DOS (Fig. 4B(b0 )) to be hybrid states. Because the DOS of the Ga54N53O1 supercell with the substitutional oxygen should reflect an electronic structure of heavily oxygen-doped GaN crystals, the upward energy
(c) (b) (a) 564 566 568 570 572 -1 Raman Shift (cm-1)
A1(LO) E1 (LO)
(b) A1 (TO) E1(TO) E2
Raman Intensity (a.u.)
cence peaks at ca. 3.4 and ca. 1.8 eV (Fig. 2A(a)), whereas the 8-h-grown crystals show an intense luminescence peak at 3.54 eV. The CL spectra obtained for the 2 and 6 h were similar to those for the 4 and 8 h, respectively. Importantly, the luminescence peak energy of the 8-h-grown crystals (3.54 eV) is 120 meV higher than that of the MOCVD film (3.42 eV) (Fig. 2A(b) and (c)). The luminescence spectrum measured at 18 K of the 8-h-grown crystal gives overlapping intense luminescence peaks at 3.55 and 3.59 eV with tailing at the lower-energy side (Fig. 2B). It has been known that Raman spectra of wurtzite-type GaN show A1(TO), E1(TO), E2, A1(LO) and E1(LO) modes [14–18], depending on the crystallographic orientation [15–17], electron concentration [15,16,18], strain [15,17] and bonding strength. Raman spectra obtained for the ð1 0 1 0Þ plane of the 4-h- and 8-h-grown crystals exhibit the A1(TO), E1(TO) and E2 modes, but not the A1(LO) and E1(LO) modes (Fig. 3a and b). It has been reported that the absence of A1(LO) and E1(LO) modes is due to the crystallographic orientation [15–17] and/or high electron concentration >1019 cm3 [15,16,18]. Since the spectra of the GaN powders produced by grinding the grown crystals also exhibited the absence of A1(LO) and E1(LO) modes, the nonexistence of these modes is attributed to not the crystallographic orientation but the high electron concentration, which is probably derived from the high-concentration substitutional oxygen for nitrogen in GaN. The high-resolution Raman spectra of the E2 mode of these crystals and a strain-free MOCVD film are shown in the inset in Fig. 3; a strain-free state of the film is recognized by the frequency of E2 mode (567.7 cm1) [14,17]. The FWHM value of the E2 mode of the crystals is ca. 3.5 cm1, which is broader than that for the MOCVD film (ca. 2.0 cm1). The frequency of E2 mode of the crystals are 567.5 and 567.3 cm1, respectively, indicating slightly downward frequency shifts from that of the MOCVD film. The broadening and slight frequency
500 600 700 800 Raman Shift (cm-1)
(a)
900
Fig. 3. Raman spectra taken on the ð1 0 1 0Þ plane of (a) 4-h- and (b) 8-hgrown crystals. Positions of Raman modes in wurtzite-type GaN (Ref. [14]) are marked by lines. Inset spectra with a spectral resolution of 0.2 cm1 show E2 mode of (a0 ) 4-h- and (b0 ) 8-h-grown crystals, and (c0 ) stress-free epitaxial film grown by MOCVD.
Fig. 4. (A) Supercell structures of undoped and oxygen-doped GaN crystals. (B) Total DOSs of (a) undoped and (b) oxygen-doped GaN, and (c) decomposed DOS for oxygen of oxygen-doped GaN. The figures on the left side show an energy region of 6.5 to +6.5 eV; the figures on the right side depict an energy region of 0.5 to +4.8 eV. Ef indicates Fermi level.
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shift of the CL peak observed in the 6–8-h-grown crystals can be explained by both the formation of high-energy hybrid state (Z) and the upward shift of Fermi level. It has been reported that the large band gap of heavily oxygen-doped InN film measured using optical absorption might result from the formation of an InOxNy alloy phase (alloying effect) and Moss–Burstein shift [19]. Consequently, the formation of the high-energy hybrid state and the upward shift of Fermi level could be explained by the formation of a GaOxNy alloy phase and Moss–Burstein shift, respectively. The observed tailing peak might reflect the state (Y) (Fig. 2A(b)). Although the role of high-concentration oxygen in GaN on its electronic structure, shown by the DOSs, can support the CL energy shift, it is not clear why the CL shift was not observed in the 2–4-h-grown crystals (Fig. 2A(a)). Since only the 2–4-h-grown crystals contained both the highconcentration hydrogen >1019 atoms/cm3 and oxygen >1020 atoms/cm3 (Table 1), it is possible that the high-concentration hydrogen in the heavily oxygen-doped GaN crystals could prevent the CL shift. In summary, we investigated the properties of heavily oxygen-doped GaN single crystals grown by the CRN of Ga2O3 at 1180 °C for 2–8 h. The crystals had the wurtzite-type structure with a0 = 0.3189 nm and c0 = 0.5184 nm and showed their high-crystallinity. The 4-hand 8-h-grown crystals exhibited broadening and slight frequency shift of the Raman E2 mode allowed by the wurtzite-type GaN, but not the LO modes. These crystals contained the high oxygen concentration (4–5 1020 atoms/cm3), but the hydrogen concentration was much lower in the 8-h-grown crystals than in the 4-h-grown ones. The CL spectra of the 8-h-grown crystals exhibited the blue shift of the band-edge-related peak (120 meV) with the tailing at lower-energies, which was explained by both the formation of the high-energy hybrid state and the upward shift of the Fermi level, based on the first-principle calculation of DOS of Ga54N53O1 supercell.
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Acknowledgements We would like to express our sincere appreciation to Prof. T. Sekiguchi (National Institute for Materials Science) for CL measurements and helpful suggestions, and to Prof. S. Nakashima (National Institute of Advanced Industrial Science and Technology) for Raman measurements and valuable advice. We would also like to express our hearty thanks to Dr. M. Jo (The Open Facility of Hokkaido University Sousei Hall) for the rocking curve measurements, to K. Akutsu (MST) for the first-principle calculations. This study was partly supported by the Government Science Research Fund of the Ministry of Education of Japan (No. 18655082) for S.S. References [1] S. Nakamura, M. Senoh, N. Iwasa, S. Nagahama, Jpn. J. Appl. Phys. 34 (1995) L979. [2] H. Wang, A.-B. Chen, J. Appl. Phys. 87 (2000) 7859. [3] M. Yoshikawa et al., J. Appl. Phys. 86 (1999) 4400. [4] W. Li, Aizhen Li, Thin Sol. Film. 401 (2001) 279. [5] D. Meister et al., J. Appl. Phys. 88 (2000) 1811. [6] P. Prystawko et al., Phys. State. Sol. (b) 210 (1998) 437. [7] A. Miura, S. Shimada, T. Sekiguchi, J. Cryst. Growth 299 (2007) 22. [8] A. Miura, S. Shimada, T. Sekiguchi, M. Yokoyama, B. Mizobuchi, J. Cryst. Growth (in press). [9] T. Sekiguchi, H.S. Leipner, Appl. Phys. Lett. 67 (1995) 3777. [10] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [11] M. Aoki, H. Yamane, M. Shimada, T. Kajiwara, S. Sarayama, F.J. DiSalvo, Cryst. Growth Design 2 (2002) 55. [12] T. Yamada, H. Yamane, H. Iwata, S. Sarayama, J. Cryst. Growth 281 (2005) 242. [13] S. Shimada, R. Taniguchi, J. Cryst. Growth 263 (2004) 1. [14] V.Y. Davydov et al., Phys. Rev. B 58 (1998) 899. [15] H. Harima, J. Phys.: Condens. Matter 14 (2002) R967. [16] T. Kozawa, T. Kachi, H. Kano, Y. Taga, M. Hashimoto, N. Koide, K. Manabe, J. Appl. Phys. 75 (1994) 1098. [17] L.I. Hwan et al., J. Appl. Phys. 83 (1998) 5787. [18] H. Harima, H. Sakashita, T. Inoue, S. Nakashima, J. Cryst. Growth 189–190 (1998) 672. [19] K.S.A. Butcher, T.L. Tansley, Superlattice Microst 38 (2005) 1.