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Localization and defects in axial (In,Ga)N/GaN nanowire heterostructures investigated by spatially-resolved luminescence spectroscopy  [PDF]
Jonas L?hnemann,Christian Hauswald,Martin W?lz,Uwe Jahn,Michael Hanke,Lutz Geelhaar,Oliver Brandt
Physics , 2014, DOI: 10.1088/0022-3727/47/39/394010
Abstract: (In,Ga)N insertions embedded in self-assembled GaN nanowires are of current interest for applications in solid state light emitters. Such structures exhibit a notoriously broad emission band. We use cathodoluminescence spectral imaging in a scanning electron microscope and micro-photoluminescence spectroscopy on single nanowires to learn more about the mechanisms underlying this emission. We observe a shift of the emission energy along the stack of six insertions within single nanowires that may be explained by compositional pulling. Our results also corroborate reports that the localization of carriers at potential fluctuations within the insertions plays a crucial role for the luminescence of these nanowire based emitters. Furthermore, we resolve contributions from both structural and point defects in our measurements.
First-principles study of the electronic structure and optical properties of GaN(0001) surface

Du Yu-Jie,Chang Ben-Kang,Zhang Jun-Ju,Li Biao,Wang Xiao-Hui,

物理学报 , 2012,
Abstract: The band structure, the density of states, the surface energy, the work function, and the optical properties of GaN(0001)(2×2) clean surface are calculated systematically by the first-principles plane-wave ultro-soft pseudopotential method based on the density function theory. It is found that the band structure of GaN(0001) surface changes greatly after relaxation, the surface has metallic conductive properties, and there is obvious surface state near the bottom of conduction band. In the effect of dipole moment, the surface charges shift and Ga-terminated surface is positive polar surface. the surface energy and the work function of GaN (0001) surface are obtained to be 2.1 J.m-2 and 4.2 eV, respectively. The optical properties of GaN (0001) surface and bulk phase GaN are analyzed and compared. It is found that there is big difference between them.
X-ray magnetic circular dichroism characterization of GaN/Ga1-xMnxN digital ferromagnetic heterostructure  [PDF]
J. I. Hwang,M. Kobayashi,G. S. Song,A. Fujimori,A. Tanaka,Z. S. Yang,H. J. Lin,D. J. Huang,C. T. Chen,H. C. Jeon,T. W. Kang
Physics , 2007, DOI: 10.1063/1.2769944
Abstract: We have investigated the magnetic properties of a GaN/Ga1-xMnxN (x = 0.1) digital ferromagnetic heterostructure (DFH) showing ferromagnetic behavior using soft x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). The Mn L2,3-edge XAS spectra were similar to those of Ga1-xMnxN random alloy thin films, indicating a substitutional doping of high concentration Mn into GaN. From the XMCD measurements, it was revealed that paramagnetic and ferromagnetic Mn atoms coexisted in the Ga1-xMnxN digital layers. The ferromagnetic moment per Mn atom estimated from XMCD agreed well with that estimated from SQUID measurements. From these results, we conclude that the ferromagnetic behavior of the GaN/Ga1-xMnxN DFH sample arises only from substitutional Mn2+ ions in the Ga1-xMnxN digital layers and not from ferromagnetic precipitates. Subtle differences were also found from the XMCD spectra between the electronic states of the ferromagnetic and paramagnetic Mn2+ ions.
Electronic Structures of Wurtzite GaN with Ga and N Vacancies
PANG Chao,SHI Jun-Jie,ZHANG Yan,K S A Butcher,T L Tansley,J EDownes,SHANG Jia-Xiang,

中国物理快报 , 2007,
Abstract: The electronic band structures of wurtzite GaN with Ga and N vacancy defects are investigated by means of the first-principles total energy calculations in the neutral charge state. Our results show that the band structures can be significantly modified by the Ga and N vacancies in the GaN samples. Generally, the width of the valence band is reduced and the band gap is enlarged. The defect-induced bands can be introduced in the band gap of GMV due to the Ga and N vacancies. Moreover, the GaN with high density of N vacancies becomes an indirect gap semiconductor. Three defect bands due to Ga vacancy defects are created within the band gap and near the top of the valence band. In contrast, the N vacancies introduce four defect bands within the band gap. One is in the vicinity of the top of the valence band, and the others are near the bottom of the conduction band. The physical origin of the defect bands and modification of the band structures due to the Ga and N vacancies are analysed in depth.
Anisotropy in the electronic structure of V2GeC investigated by soft x-ray emission spectroscopy and first-principles theory  [PDF]
Martin Magnuson,Ola Wilhelmsson,Maurizio Mattesini,Sa Li,Rajeev Ahuja,Olle Eriksson,Hans H?gberg,Lars Hultman,Ulf Jansson
Physics , 2011, DOI: 10.1103/PhysRevB.78.035117
Abstract: The anisotropy of the electronic structure of ternary nanolaminate V2GeC is investigated by bulk-sensitive soft x-ray emission spectroscopy. The measured polarization-dependent emission spectra of V L2,3, C K, Ge M1 and Ge M2,3 in V2GeC are compared to those from monocarbide VC and pure Ge. The experimental emission spectra are interpreted with calculated spectra using ab initio density-functional theory including dipole transition matrix elements. Different types of covalent chemical bond regions are revealed; V 3d - C 2p bonding at -3.8 eV, Ge 4p - C 2p bonding at -6 eV and Ge 4p - C 2s interaction mediated via the V 3d orbitals at -11 eV below the Fermi level. We find that the anisotropic effects are high for the 4p valence states and the shallow 3d core levels of Ge, while relatively small anisotropy is detected for the V 3d states. The macroscopic properties of the V2GeC nanolaminate result from the chemical bonds with the anisotropic pattern as shown in this work.
Electronic structure of Ga$_{1-x}$Cr$_{x}$N and Si-doping effects studied by photoemission and X-ray absorption spectroscopy  [PDF]
G. S. Song,M. Kobayashi,J. I. Hwang,T. Kataoka,M. Takizawa,A. Fujimori,T. Ohkouchi,Y. Takeda,T. Okane,Y. Saitoh,H. Yamagami,F. -H. Chang,L. Lee,H. -J. Lin,D. J. Huang,C. T. Chen,S. Kimura,M. Funakoshi,S. Hasegawa,H. Asahi
Physics , 2008, DOI: 10.1103/PhysRevB.78.033304
Abstract: The electronic structure of the magnetic semiconductor Ga$_{1-x}$Cr$_{x}$N and the effect of Si doping on it have been investigated by photoemission and soft x-ray absorption spectroscopy. We have confirmed that Cr in GaN is predominantly trivalent substituting for Ga, and that Cr 3$d$ states appear within the band gap of GaN just above the N 2$p$-derived valence-band maximum. As a result of Si doping, downward shifts of the core levels (except for Cr 2$p$) and the formation of new states near the Fermi level were observed, which we attribute to the upward chemical potential shift and the formation of a small amount of Cr$^{2+}$ species caused by the electron doping. Possibility of Cr-rich cluster growth by Si doping are discussed based on the spectroscopic and magnetization data.
High-energy spectroscopic study of the III-V nitride-based diluted magnetic semiconductor Ga$_{1-x}$Mn$_{x}$N  [PDF]
J. I. Hwang,Y. Ishida,M. Kobayashi,H. Hirata,K. Takubo,T. Mizokawa,A. Fujimori,J. Okamoto,K. Mamiya,Y. Saito,Y. Muramatsu,H. Ott,A. Tanaka,T. Kondo,H. Munekata
Physics , 2005, DOI: 10.1103/PhysRevB.72.085216
Abstract: We have studied the electronic structure of the diluted magnetic semiconductor Ga$_{1-x}$Mn$_{x}$N ($x$ = 0.0, 0.02 and 0.042) grown on Sn-doped $n$-type GaN using photoemission and soft x-ray absorption spectroscopy. Mn $L$-edge x-ray absorption have indicated that the Mn ions are in the tetrahedral crystal field and that their valence is divalent. Upon Mn doping into GaN, new state were found to form within the band gap of GaN, and the Fermi level was shifted downward. Satellite structures in the Mn 2$p$ core level and the Mn 3$d$ partial density of states were analyzed using configuration-interaction calculation on a MnN$_{4}$ cluster model. The deduced electronic structure parameters reveal that the $p$-$d$ exchange coupling in Ga$_{1-x}$Mn$_{x}$N is stronger than that in Ga$_{1-x}$Mn$_{x}$As.
A first-principles study of carbon-related energy levels in GaN. Part I - complexes formed by substitutional/interstitial carbons and gallium/nitrogen vacancies  [PDF]
Masahiko Matsubara,Enrico Bellotti
Physics , 2015,
Abstract: Various forms of carbon based complexes in GaN are studied with first-principles calculations employing Heyd-Scuseria-Ernzerhof hybrid functional within the framework of density functional theory. We consider carbon complexes made of the combinations of single impurities, i.e. $\mathrm{C_N-C_{Ga}}$, $\mathrm{C_I-C_N}$ and $\mathrm{C_I-C_{Ga}}$, where $\mathrm{C_N}$, $\mathrm{C_{Ga}}$ and $\mathrm{C_I}$ denote C substituting nitrogen, C substituting gallium and interstitial C, respectively, and of neighboring gallium/nitrogen vacancies ($\mathrm{V_{Ga}}$/$\mathrm{V_N}$), i.e. $\mathrm{C_N-V_{Ga}}$ and $\mathrm{C_{Ga}-V_N}$. Formation energies are computed for all these configurations with different charge states after full geometry optimizations. From our calculated formation energies, thermodynamic transition levels are evaluated, which are related to the thermal activation energies observed in experimental techniques such as deep level transient spectroscopy. Furthermore, the lattice relaxation energies (Franck-Condon shift) are computed to obtain optical activation energies, which are observed in experimental techniques such as deep level optical spectroscopy. We compare our calculated values of activation energies with the energies of experimentally observed C-related trap levels and identify the physical origins of these traps, which are unknown before.
Challenges in first-principles NPT molecular dynamics of soft porous crystals: A case study on MIL-53(Ga)  [PDF]
Volker Haigis,Yacine Belkhodja,Fran?ois-Xavier Coudert,Rodolphe Vuilleumier,Anne Boutin
Physics , 2014, DOI: 10.1063/1.4891578
Abstract: Soft porous crystals present a challenge to molecular dynamics simulations with flexible size and shape of the simulation cell (i.e., in the NPT ensemble), since their framework responds very sensitively to small external stimuli. Hence, all interactions have to be described very accurately in order to obtain correct equilibrium structures. Here, we report a methodological study on the nanoporous metal-organic framework MIL-53(Ga), which undergoes a large-amplitude transition between a narrow- and a large-pore phase upon a change in temperature. Since this system has not been investigated by density functional theory (DFT)-based NPT simulations so far, we carefully check the convergence of the stress tensor with respect to computational parameters. Furthermore, we demonstrate the importance of dispersion interactions and test two different ways of incorporating them into the DFT framework. As a result, we propose two computational schemes which describe accurately the narrow- and the large-pore phase of the material, respectively. These schemes can be used in future work on the delicate interplay between adsorption in the nanopores and structural flexibility of the host material.
Demonstration of GaN/InGaN Light Emitting Diodes on (100) β-Ga2O3 Substrates by Metalorganic Chemical Vapour Deposition
中国物理快报 , 2008,
Abstract: The growth and fabrication of GaN/InGaN multiple quantum well (MQW) light emitting diodes (LEDs) on (100) β-Ga2O3 single crystal substrates by metal-organic chemical vapour deposition (MOCVD) technique are reported. x-ray diffraction (XRD) θ-2θ scan spectroscopy is carried out on the GaN buffer layer grown on a (100) β-Ga2O3 substrate. The spectrum presents several sharppeaks corresponding to the (100) β-Ga2O3 and (004) GaN. High-quality (0002) GaN material is obtained. The emission characteristics of the GaN/InGaN MQW LED are measurement. The first green LED on β-Ga2O3 with vertical current injection is demonstrated.
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