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Gallium nitride nanowires doped with magnesium
Materials Letters 63 (2009) 978–981
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Gallium nitride nanowires doped with magnesium Dongdong Zhang, Chengshan Xue ⁎, Huizhao Zhuang, Haibo Sun, Yuping Cao, Yinglong Huang, Zouping Wang, Ying Wang, Yongfu Guo Institute of Semiconductors, College of Physics and Electronics, Shandong Normal University, Jinan 250014, China
a r t i c l e
i n f o
Article history: Received 31 August 2008 Accepted 14 January 2009 Available online 22 January 2009 PACS: 71.55.Eq 81.05.Ea 61.72.uj 62.23.Hj 61.46.Km 81.16.-c 81.15.Cd
a b s t r a c t GaN nanowires doped with Mg have been synthesized on Si (111) substrate through ammoniating Ga2O3 films doped with Mg under flowing ammonia atmosphere. The Mg-doped GaN nanowires were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscopy (HRTEM) and photoluminescence (PL). The results demonstrate that the nanowires were single crystalline with hexagonal wurzite structure. The diameters of the nanowires ranged 20–30 nm and the lengths were about hundreds of micrometers. The intense PL peak at 359 nm showed a blueshift from the bulk band gap emission, attributed to Burstein–Moss effect. The growth mechanism of the crystalline GaN nanowires is discussed briefly. © 2009 Elsevier B.V. All rights reserved.
Keywords: Crystal growth Nanomaterials Nanowires Mg-doped
1. Introduction One-dimensional nanostructure materials such as nanowires, nanotubes, and nanocables are of considerable interest due to their application potentials as building blocks of nanodevices such as channels of nanotransistors [1,2], cavities of nanolasers [3], and probe tips of atomic force microscopes [4]. Gallium nitride is an important direct band gap semiconductor, which is intense interest for UV or blue emitters, detectors, and high-temperature electronic devices [5,6]. In order to improve its performances in both electronic and optoelectronic devices, however, appropriate doping is necessary. Up to now, various doping methods and impurities have been tried and reported. Such as, ferromagnetism in Mn-doped GaN nanowires (theories of computational [7] and experimental results [8]), temperature-dependent single-electron tunneling effect in lightly and heavily doped GaN nanowires [9], and observation of hysteretic magnetoresistance in Mn-doped GaN nanowires with the mesoscopic Co and Ti/Au contacts [10], near UV photoluminescence of Hg-doped GaN nanowires [11], etc. In this paper, high-quality Mg-doped GaN nanowires have been synthesized on Si (111) substrate through ammoniating Ga2O3 films doped with Mg. Because ionic radius of Mg
⁎ Corresponding author. Fax: +86 531 86180017. E-mail address: [email protected] (C. Xue). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.01.044
(0.65 Å) is slightly bigger than that of Ga (0.62 Å), the GaN composition can be easily substituted by Mg under certain conditions. Motivated by this approach, possibly, Mg substitutes part of Ga in GaN based on a chemical reaction, which may turn GaN nanoparticles into Mg-doped GaN nanowires by replacement of the small part of GaN. 2. Experimental procedures Mg doped GaN nanowires have been synthesized through ammoniating Ga2O3 films doped with Mg under flowing ammonia atmosphere. In the first step, Ga2O3 films doped with Mg were deposited on Si (111) substrates by sputtering the Mg target with purity of 99.99% and the sintered Ga2O3 target with purity of 99.999% in a JCK-500A radio frequency magnetron sputtering system. The conditions of sputtering process were as follows: the RF sputtering power was 150 W and the frequency was 13.56 MHz; the DC sputtering power was 20 W; the background pressure was 0.9 × 10− 3 Pa; The working gas was pure Ar (≥99.99%) and the working pressure was 2 Pa. There were 30 cycles in this process, the total thickness of the Ga2O3 films was about 600 nm. Fig. 1 shows the growth sketch of the Ga2O3 films. The single sputtering cycle was that: The un-doped Ga2O3 layer was deposited firstly with thickness of about 20 nm, and then the Mg layer was deposited for 2 s According to the experimental sputtering rate of Mg (200 nm/min), the thickness of Mg layer in
D. Zhang et al. / Materials Letters 63 (2009) 978–981
979
Fig. 1. The growth sketch of the Mg doped Ga2O3 films.
every cycle was about 7 nm. In the second step, as-deposited Ga2O3 thin films were ammoniated in a conventional tube furnace at 900 °C for 15 min. After reaction, a light white layer was found on the substrate surface.
Fig. 3. XRD pattern of Mg doped GaN nanowires with a hexagonal wurtzite structure.
3. Results and discussions 3.1. SEM analysis The typical SEM images of the Mg doped GaN nanowires are shown in Fig. 2. Fig. 2 (a) shows that the light-white layer is composed of high-density GaN nanowires, crossing each other and homogeneously distributing over the whole surface of Si substrate. From Fig. 2 (b), further observation image demonstrates that the nanowires have diameters ranged from 20 nm to 30 nm and lengths typically up to hundreds of micrometers, indicating a wonderful aspect ratio. 3.2. XRD analysis Fig. 3 shows the XRD pattern of the samples. The diffraction peaks in the panel are at 2θ = 32.2°, 34.1°, 36.4° correspond to the (100), (002), (101) planes of GaN, respectively. Compared with un-doped GaN [12], the d-spacing of (100) planes of the Mg-doped GaN increases from 0.273 nm to 0.281 nm. Because the ionic radius of Mg and Ga are 0.65 Å and 0.62 Å, respectively, the doping of Mg induces the increase of the crystal constant of GaN nanowires. Moreover, no other peaks of impurities appears in the spectra, indicating the predominant single wurtzite GaN phase of the deposit and the sharp diffraction peaks reveal that the GaN nanowires possess good crystalline quality. 3.3. HRTEM analysis The TEM image of nanowires shown in Fig. 4(a) reveals that the GaN nanowires with a uniform diameter of about 25 nm. It is worth
noting that no metal particle is observed on the surface of the GaN nanowires. Slightly rough surfaces are also found, indicating that the samples probably have defects. The insert in the upper-right-hand corner is the selected area electron diffraction (SAED) pattern of the nanowire, which can be indexed to the reflection of single crystalline hexagonal wurtzite GaN with the growth direction parallel to the [100] direction of hexagonal unit cell. But there are many other diffraction spots in the diffraction pattern. We think that these may be induced by the doped-Mg. These particular diffraction spots reflect the crystal structure of Mg. Fig. 4(b) shows the HRTEM lattice image of the single nanowire. The clear lattice fringes confirm that the nanowires are high quality hexagonal single-crystalline GaN. The distance between the two fringes is 0.281 nm, which is larger than the plane distance of un-doped GaN (100) plane with 0.273 nm. In Fig. 4(c), the EDX spectrum of single GaN nanowire shows that the nanowire is composed predominantly of Ga ~45 at.% and N ~48.5 at.%, a small amount of Mg ~5 at.% and a trace of O and C, suggesting the GaN nanowires are doped with Mg. 3.4. PL analysis Fig. 5 shows the measurement of PL spectrum at room temperature. The nanowires show four emission peaks at 359 nm, 384 nm, 425 nm, 442 nm, respectively. It has been well known that a bulk-type GaN shows photoluminescence at 370 nm at room temperature [13]. 3 According to the following equation: Ev = 1:24 λ × 10 ev, the peak at 359 nm corresponding to Ev = 3.45ev. An obviously blueshift of the band gap emission occurs from the 3.39 eV of bulk GaN to 3.45 eV of Mg-doped GaN. When GaN is doped with Mg, the excess carriers
Fig. 2. Typical SEM images of the Mg doped GaN nanowires at different magnifications.
980
D. Zhang et al. / Materials Letters 63 (2009) 978–981
Fig. 4. HRTEM and EDX images of the Mg-doped GaN samples.
supplied by the impurities to the conduction band contribute to decrease the electrical conductivity of GaN. And due to a large density of states of GaN near the conduction-band minimum, the conduction band edge is filled by excessive carriers provided by the impurities, leading to a blueshift of optical band-to-band transitions. That is consistent with the Burstein–Moss effect [14]: BM
ΔEg
=
ℏ2 k2F 1 1 + me mh 2
where kF is the Fermi wave vector, me and mh are the effective masses
of the electrons and holes in the conduction band. Additionally the other peak at 384 nm corresponding to 3.26 eV represents the transition form the conduction-band edge to the acceptor level AM (acceptor Mg) which lies about 150 meV above the valence-band maximum. The peak at 425 nm corresponding to 2.95 eV is most likely due to transition from deep donor-like states caused by Mg impurities to the covalence-band states. The peak at 442 nm corresponding to 2.80 eV has been attributed to the contribution of donor–acceptor pair (DAP). These are consistent with the observation of Zolper et al. [15]. The emission energy E(r) is affected by the Coulomb interaction between acceptors and donors according to the following equation: 2 Eðr Þ = Eg −ðEA + ED Þ + eer , where Eg(=3.39 eV) is the band gap, EA (=0.15 eV) and ED are the binding energy of acceptor and donor, r (≈50 nm) is the separation between the donor and acceptor and ε is the dielectric constant. ED is calculated to be 420 meV. The deep level could be assigned to a deep Mg-related complex, such as, Mgi or MgGaVN [16,17]. Of course, convincing explanation on the relation of PL is my further work.
3.5. Discussion about growth process
Fig. 5. The PL spectra of Mg-doped GaN nanowires.
Based on the above analysis, the growth process can be described as follows. In the sputtering process, multilayer structures of the Ga2O3 thin films doped with Mg have been obtained. The thickness of un-doped Ga2O3 layer in every cycle equals with tens of atom layers. The Mg layer with the thickness of 7 nm has about 3 atomic layers. So in the growth process, Mg has more opportunity to substitute the position of Ga. In the ammoniating process, NH3 decomposes into NH2, NH, H2 and N when the ammoniating temperature is above
D. Zhang et al. / Materials Letters 63 (2009) 978–981
800 °C [18]. The Ga2O3 particles are reduced to gaseous Ga2O by H2 and then GaN molecules are synthesized through the reaction between Ga2O and NH3. At that temperature, vaporized Mg was doped into the GaN particles to occupy the position of the vacancy of gallium. All the reactions can be expressed as below: 2NH3 ð g ÞYN2 + 3H2 ð g Þ Ga2 O3 ðsÞ + 2H2 ð gÞYGa2 Oð g Þ + 2H2 Oð g Þ
981
less than 30 nm, and lengths up to hundreds of micrometers. The Mgdoped GaN nanowires show intense emission peaking at 359 nm that exhibits a clear blueshift from the bulk band gap emission (370 nm). Acknowledgements This project was supported by the Key Research Program of the National Natural Science Foundation of China (No. 90201025) and the National Natural Science Foundation of China (No. 90301002).
Ga2 Oð g Þ + 2NH3 ð g ÞY2GaNðsÞ + 2H2 ð gÞ + H2 Oð g Þ The growth of Mg-doped GaN nanowires may attribute to the surface energy distribution of the Si(111) substrate. It may produce some energetically favored sites for the absorption of gas-phase reactants [19]. Namely, the vapor phase of Ga2O react with NH3 yielding GaN molecules, which directly deposition the energetically favored sites of substrate to form the GaN crystalline nuclei without the formation process of liquid phase. Once GaN nuclei are formed, nanostructure materials would then grow from the nuclei. The GaN molecules diffuse to the nuclei from new reaction driven by a lower chemical potential [20]. The GaN continually molecules diffuse and agglomerate into the GaN micrograins. Then the very small GaN micrograins grow up with the progress of the ammoniating and accordingly lay the foundations for the growth of nanostructured GaN. 4. Conclusions GaN nanowires doped with Mg have been synthesized through ammoniating Ga2O3 films doped with Mg at 900 °C for 15 min. The morphology, structure and optical properties of the synthesized GaN nanowires were investigated by SEM, XRD, HRTEM, XPS and PL. Most of the nanowires have a hexagonal wurtzite structure with diameters
References [1] Lauhon LJ, Gudiksen MS, Wang D, Lieber CM. Nature 2002;420:57. [2] Ham M-H, Choi J-H, Hwang W, Park C, Lee W-Y, Myoung J-M. Nanotechnology 2006;17:2203. [3] Huang MH, Mao S, Feick H, Yan H, Wu Y, Kind H, et al. Adv Mater 2001;13:113. [4] Lee W, Jeong MC, Myoung JM. Nanotechnology 2004;15:254. [5] Fasol G. Science 1996;272:1751. [6] Nakamura S. Science 1998;281:956. [7] Kim JR, Kim BK, Lee IJ, Kim JJ. Phys Rev B 2004;69:233303. [8] Han DS, Park J, Rhie KW, Kim S, Chang J. Appl Phys Lett 2005;86:032506. [9] Wang Q, Sun Q, Jena P. Phys Rev Lett 2005;95:167202. [10] Han SE, Oh H, Kim JJ, Seong HK, Choi HJ. Appl Phys Lett 2005;87:062102. [11] Zhou SM. Physica E 2006;33:394–7. [12] Perlin P, Jauberthiecarillon C, Itie JP, et al. Phys Rev B 1992;45. [13] Monemar B. Phys Rev B 1974;10:676. [14] Zhou SM, Zhang XH, Meng X, Lee ST. Nanotechnology 2004;15:1152. [15] Zolper JC, Crawford MH, Howard AJ, Ramer J, Hersee SD. Appl Phys Lett 1996;68:200. [16] Smith M, Chen GD, Lin JY, Jiang HX, Salvador A, Sverdlov BN. Appl Phys Lett 1996;68:1883. [17] Gross M, Henn G, Ziegler J, Allenspacher P, Cychy C, Schröder H. Mater Sci Eng B 1999;59:94. [18] Xue SB, Zhuang HZ, Xue CS, Hu LJ, Li BL, Zhang SY. Chin Phys 2007;16:1405. [19] Ai Y, Xue C, Sun C, Sun L, Zhuang H, Wang F, et al. Mater Lett 2007;61:2833–6. [20] Debnath RK, Meijers R, Richter T, Stoica T, Calarco R, Luth H. Appl Phys Lett 2007;90:123117.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Gallium nitride nanowires doped with magnesium Dongdong Zhang, Chengshan Xue ⁎, Huizhao Zhuang, Haibo Sun, Yuping Cao, Yinglong Huang, Zouping Wang, Ying Wang, Yongfu Guo Institute of Semiconductors, College of Physics and Electronics, Shandong Normal University, Jinan 250014, China
a r t i c l e
i n f o
Article history: Received 31 August 2008 Accepted 14 January 2009 Available online 22 January 2009 PACS: 71.55.Eq 81.05.Ea 61.72.uj 62.23.Hj 61.46.Km 81.16.-c 81.15.Cd
a b s t r a c t GaN nanowires doped with Mg have been synthesized on Si (111) substrate through ammoniating Ga2O3 films doped with Mg under flowing ammonia atmosphere. The Mg-doped GaN nanowires were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscopy (HRTEM) and photoluminescence (PL). The results demonstrate that the nanowires were single crystalline with hexagonal wurzite structure. The diameters of the nanowires ranged 20–30 nm and the lengths were about hundreds of micrometers. The intense PL peak at 359 nm showed a blueshift from the bulk band gap emission, attributed to Burstein–Moss effect. The growth mechanism of the crystalline GaN nanowires is discussed briefly. © 2009 Elsevier B.V. All rights reserved.
Keywords: Crystal growth Nanomaterials Nanowires Mg-doped
1. Introduction One-dimensional nanostructure materials such as nanowires, nanotubes, and nanocables are of considerable interest due to their application potentials as building blocks of nanodevices such as channels of nanotransistors [1,2], cavities of nanolasers [3], and probe tips of atomic force microscopes [4]. Gallium nitride is an important direct band gap semiconductor, which is intense interest for UV or blue emitters, detectors, and high-temperature electronic devices [5,6]. In order to improve its performances in both electronic and optoelectronic devices, however, appropriate doping is necessary. Up to now, various doping methods and impurities have been tried and reported. Such as, ferromagnetism in Mn-doped GaN nanowires (theories of computational [7] and experimental results [8]), temperature-dependent single-electron tunneling effect in lightly and heavily doped GaN nanowires [9], and observation of hysteretic magnetoresistance in Mn-doped GaN nanowires with the mesoscopic Co and Ti/Au contacts [10], near UV photoluminescence of Hg-doped GaN nanowires [11], etc. In this paper, high-quality Mg-doped GaN nanowires have been synthesized on Si (111) substrate through ammoniating Ga2O3 films doped with Mg. Because ionic radius of Mg
⁎ Corresponding author. Fax: +86 531 86180017. E-mail address: [email protected] (C. Xue). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.01.044
(0.65 Å) is slightly bigger than that of Ga (0.62 Å), the GaN composition can be easily substituted by Mg under certain conditions. Motivated by this approach, possibly, Mg substitutes part of Ga in GaN based on a chemical reaction, which may turn GaN nanoparticles into Mg-doped GaN nanowires by replacement of the small part of GaN. 2. Experimental procedures Mg doped GaN nanowires have been synthesized through ammoniating Ga2O3 films doped with Mg under flowing ammonia atmosphere. In the first step, Ga2O3 films doped with Mg were deposited on Si (111) substrates by sputtering the Mg target with purity of 99.99% and the sintered Ga2O3 target with purity of 99.999% in a JCK-500A radio frequency magnetron sputtering system. The conditions of sputtering process were as follows: the RF sputtering power was 150 W and the frequency was 13.56 MHz; the DC sputtering power was 20 W; the background pressure was 0.9 × 10− 3 Pa; The working gas was pure Ar (≥99.99%) and the working pressure was 2 Pa. There were 30 cycles in this process, the total thickness of the Ga2O3 films was about 600 nm. Fig. 1 shows the growth sketch of the Ga2O3 films. The single sputtering cycle was that: The un-doped Ga2O3 layer was deposited firstly with thickness of about 20 nm, and then the Mg layer was deposited for 2 s According to the experimental sputtering rate of Mg (200 nm/min), the thickness of Mg layer in
D. Zhang et al. / Materials Letters 63 (2009) 978–981
979
Fig. 1. The growth sketch of the Mg doped Ga2O3 films.
every cycle was about 7 nm. In the second step, as-deposited Ga2O3 thin films were ammoniated in a conventional tube furnace at 900 °C for 15 min. After reaction, a light white layer was found on the substrate surface.
Fig. 3. XRD pattern of Mg doped GaN nanowires with a hexagonal wurtzite structure.
3. Results and discussions 3.1. SEM analysis The typical SEM images of the Mg doped GaN nanowires are shown in Fig. 2. Fig. 2 (a) shows that the light-white layer is composed of high-density GaN nanowires, crossing each other and homogeneously distributing over the whole surface of Si substrate. From Fig. 2 (b), further observation image demonstrates that the nanowires have diameters ranged from 20 nm to 30 nm and lengths typically up to hundreds of micrometers, indicating a wonderful aspect ratio. 3.2. XRD analysis Fig. 3 shows the XRD pattern of the samples. The diffraction peaks in the panel are at 2θ = 32.2°, 34.1°, 36.4° correspond to the (100), (002), (101) planes of GaN, respectively. Compared with un-doped GaN [12], the d-spacing of (100) planes of the Mg-doped GaN increases from 0.273 nm to 0.281 nm. Because the ionic radius of Mg and Ga are 0.65 Å and 0.62 Å, respectively, the doping of Mg induces the increase of the crystal constant of GaN nanowires. Moreover, no other peaks of impurities appears in the spectra, indicating the predominant single wurtzite GaN phase of the deposit and the sharp diffraction peaks reveal that the GaN nanowires possess good crystalline quality. 3.3. HRTEM analysis The TEM image of nanowires shown in Fig. 4(a) reveals that the GaN nanowires with a uniform diameter of about 25 nm. It is worth
noting that no metal particle is observed on the surface of the GaN nanowires. Slightly rough surfaces are also found, indicating that the samples probably have defects. The insert in the upper-right-hand corner is the selected area electron diffraction (SAED) pattern of the nanowire, which can be indexed to the reflection of single crystalline hexagonal wurtzite GaN with the growth direction parallel to the [100] direction of hexagonal unit cell. But there are many other diffraction spots in the diffraction pattern. We think that these may be induced by the doped-Mg. These particular diffraction spots reflect the crystal structure of Mg. Fig. 4(b) shows the HRTEM lattice image of the single nanowire. The clear lattice fringes confirm that the nanowires are high quality hexagonal single-crystalline GaN. The distance between the two fringes is 0.281 nm, which is larger than the plane distance of un-doped GaN (100) plane with 0.273 nm. In Fig. 4(c), the EDX spectrum of single GaN nanowire shows that the nanowire is composed predominantly of Ga ~45 at.% and N ~48.5 at.%, a small amount of Mg ~5 at.% and a trace of O and C, suggesting the GaN nanowires are doped with Mg. 3.4. PL analysis Fig. 5 shows the measurement of PL spectrum at room temperature. The nanowires show four emission peaks at 359 nm, 384 nm, 425 nm, 442 nm, respectively. It has been well known that a bulk-type GaN shows photoluminescence at 370 nm at room temperature [13]. 3 According to the following equation: Ev = 1:24 λ × 10 ev, the peak at 359 nm corresponding to Ev = 3.45ev. An obviously blueshift of the band gap emission occurs from the 3.39 eV of bulk GaN to 3.45 eV of Mg-doped GaN. When GaN is doped with Mg, the excess carriers
Fig. 2. Typical SEM images of the Mg doped GaN nanowires at different magnifications.
980
D. Zhang et al. / Materials Letters 63 (2009) 978–981
Fig. 4. HRTEM and EDX images of the Mg-doped GaN samples.
supplied by the impurities to the conduction band contribute to decrease the electrical conductivity of GaN. And due to a large density of states of GaN near the conduction-band minimum, the conduction band edge is filled by excessive carriers provided by the impurities, leading to a blueshift of optical band-to-band transitions. That is consistent with the Burstein–Moss effect [14]: BM
ΔEg
=
ℏ2 k2F 1 1 + me mh 2
where kF is the Fermi wave vector, me and mh are the effective masses
of the electrons and holes in the conduction band. Additionally the other peak at 384 nm corresponding to 3.26 eV represents the transition form the conduction-band edge to the acceptor level AM (acceptor Mg) which lies about 150 meV above the valence-band maximum. The peak at 425 nm corresponding to 2.95 eV is most likely due to transition from deep donor-like states caused by Mg impurities to the covalence-band states. The peak at 442 nm corresponding to 2.80 eV has been attributed to the contribution of donor–acceptor pair (DAP). These are consistent with the observation of Zolper et al. [15]. The emission energy E(r) is affected by the Coulomb interaction between acceptors and donors according to the following equation: 2 Eðr Þ = Eg −ðEA + ED Þ + eer , where Eg(=3.39 eV) is the band gap, EA (=0.15 eV) and ED are the binding energy of acceptor and donor, r (≈50 nm) is the separation between the donor and acceptor and ε is the dielectric constant. ED is calculated to be 420 meV. The deep level could be assigned to a deep Mg-related complex, such as, Mgi or MgGaVN [16,17]. Of course, convincing explanation on the relation of PL is my further work.
3.5. Discussion about growth process
Fig. 5. The PL spectra of Mg-doped GaN nanowires.
Based on the above analysis, the growth process can be described as follows. In the sputtering process, multilayer structures of the Ga2O3 thin films doped with Mg have been obtained. The thickness of un-doped Ga2O3 layer in every cycle equals with tens of atom layers. The Mg layer with the thickness of 7 nm has about 3 atomic layers. So in the growth process, Mg has more opportunity to substitute the position of Ga. In the ammoniating process, NH3 decomposes into NH2, NH, H2 and N when the ammoniating temperature is above
D. Zhang et al. / Materials Letters 63 (2009) 978–981
800 °C [18]. The Ga2O3 particles are reduced to gaseous Ga2O by H2 and then GaN molecules are synthesized through the reaction between Ga2O and NH3. At that temperature, vaporized Mg was doped into the GaN particles to occupy the position of the vacancy of gallium. All the reactions can be expressed as below: 2NH3 ð g ÞYN2 + 3H2 ð g Þ Ga2 O3 ðsÞ + 2H2 ð gÞYGa2 Oð g Þ + 2H2 Oð g Þ
981
less than 30 nm, and lengths up to hundreds of micrometers. The Mgdoped GaN nanowires show intense emission peaking at 359 nm that exhibits a clear blueshift from the bulk band gap emission (370 nm). Acknowledgements This project was supported by the Key Research Program of the National Natural Science Foundation of China (No. 90201025) and the National Natural Science Foundation of China (No. 90301002).
Ga2 Oð g Þ + 2NH3 ð g ÞY2GaNðsÞ + 2H2 ð gÞ + H2 Oð g Þ The growth of Mg-doped GaN nanowires may attribute to the surface energy distribution of the Si(111) substrate. It may produce some energetically favored sites for the absorption of gas-phase reactants [19]. Namely, the vapor phase of Ga2O react with NH3 yielding GaN molecules, which directly deposition the energetically favored sites of substrate to form the GaN crystalline nuclei without the formation process of liquid phase. Once GaN nuclei are formed, nanostructure materials would then grow from the nuclei. The GaN molecules diffuse to the nuclei from new reaction driven by a lower chemical potential [20]. The GaN continually molecules diffuse and agglomerate into the GaN micrograins. Then the very small GaN micrograins grow up with the progress of the ammoniating and accordingly lay the foundations for the growth of nanostructured GaN. 4. Conclusions GaN nanowires doped with Mg have been synthesized through ammoniating Ga2O3 films doped with Mg at 900 °C for 15 min. The morphology, structure and optical properties of the synthesized GaN nanowires were investigated by SEM, XRD, HRTEM, XPS and PL. Most of the nanowires have a hexagonal wurtzite structure with diameters
References [1] Lauhon LJ, Gudiksen MS, Wang D, Lieber CM. Nature 2002;420:57. [2] Ham M-H, Choi J-H, Hwang W, Park C, Lee W-Y, Myoung J-M. Nanotechnology 2006;17:2203. [3] Huang MH, Mao S, Feick H, Yan H, Wu Y, Kind H, et al. Adv Mater 2001;13:113. [4] Lee W, Jeong MC, Myoung JM. Nanotechnology 2004;15:254. [5] Fasol G. Science 1996;272:1751. [6] Nakamura S. Science 1998;281:956. [7] Kim JR, Kim BK, Lee IJ, Kim JJ. Phys Rev B 2004;69:233303. [8] Han DS, Park J, Rhie KW, Kim S, Chang J. Appl Phys Lett 2005;86:032506. [9] Wang Q, Sun Q, Jena P. Phys Rev Lett 2005;95:167202. [10] Han SE, Oh H, Kim JJ, Seong HK, Choi HJ. Appl Phys Lett 2005;87:062102. [11] Zhou SM. Physica E 2006;33:394–7. [12] Perlin P, Jauberthiecarillon C, Itie JP, et al. Phys Rev B 1992;45. [13] Monemar B. Phys Rev B 1974;10:676. [14] Zhou SM, Zhang XH, Meng X, Lee ST. Nanotechnology 2004;15:1152. [15] Zolper JC, Crawford MH, Howard AJ, Ramer J, Hersee SD. Appl Phys Lett 1996;68:200. [16] Smith M, Chen GD, Lin JY, Jiang HX, Salvador A, Sverdlov BN. Appl Phys Lett 1996;68:1883. [17] Gross M, Henn G, Ziegler J, Allenspacher P, Cychy C, Schröder H. Mater Sci Eng B 1999;59:94. [18] Xue SB, Zhuang HZ, Xue CS, Hu LJ, Li BL, Zhang SY. Chin Phys 2007;16:1405. [19] Ai Y, Xue C, Sun C, Sun L, Zhuang H, Wang F, et al. Mater Lett 2007;61:2833–6. [20] Debnath RK, Meijers R, Richter T, Stoica T, Calarco R, Luth H. Appl Phys Lett 2007;90:123117.