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27 Dec 1999

Volume 75, Issue 26, pp. 4049-4210

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Nanometer-scale studies of Al–Ga interdiffusion and As precipitate coarsening in nonstoichiometric AlAs/GaAs superlattices

B. Lita, Smita Ghaisas, R. S. Goldman, and M. R. Melloch

Appl. Phys. Lett. 75, 4082 (1999); http://dx.doi.org/10.1063/1.125543 (3 pages) | Cited 5 times

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We have investigated the effects of post-growth annealing on Al–Ga interdiffusion and As precipitate coarsening in AlAs/GaAs superlattices grown by molecular-beam epitaxy at low temperatures. High-resolution x-ray diffraction spectra show a significant decrease in the number and intensity of satellite peaks for the ex situ annealed compared with the as-grown superlattices, a feature which is often attributed to a reduction in interface abruptness. However, our cross-sectional scanning tunneling microscopy images show significant variation in the apparent superlattice period of the ex situ annealed compared with the as-grown superlattices. For the as-grown superlattices, preferential As precipitation on the GaAs side of AlAs/GaAs interfaces is evident. In the ex situ annealed superlattices, a preference for As precipitates at the GaAs on AlAs interface is apparent, although the As precipitates are no longer restricted to the interface region. Thus, the apparent change in superlattice period is likely due to variations in As precipitate density, which may be influenced by AlAs–GaAs alloying at the AlAs/GaAs interfaces. © 1999 American Institute of Physics.
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68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties
68.35.Fx Diffusion; interface formation
64.75.-g Phase equilibria
61.72.Cc Kinetics of defect formation and annealing
68.35.Ct Interface structure and roughness
66.30.Ny Chemical interdiffusion; diffusion barriers

Carbon-related defects in proton-irradiated, n-type epitaxial Si1−xGex

T. P. Leervad Pedersen, A. Nylandsted Larsen, and A. Mesli

Appl. Phys. Lett. 75, 4085 (1999); http://dx.doi.org/10.1063/1.125544 (3 pages) | Cited 12 times

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Ci and CiCs defects, created by proton irradiation of n-type, strain-relaxed, epitaxial Si1−xGex of 0.005 ⩽ x ⩽ 0.5, have been studied using deep-level transient spectroscopy (DLTS). The ionization enthalpies of the two defects relative to the conduction band edge, ΔH, are found to increase linearly with increasing Ge content. It is shown that the corresponding levels are not pinned to any of the band edges. Furthermore, it is shown that, for both defects, the slopes, δΔH/δx, as well as the full width at half maximum (FWHM) of the corresponding DLTS peaks, are similar. These observations are in agreement with conclusions deduced from previous electron-paramagnetic resonance (EPR) measurements in pure silicon, stating that, for both defects, the trapped electron is preferentially located at the Ci atom because of its larger electronegativity as compared to those of silicon and germanium. The anneal temperature of the Ci defect, and correspondingly the in-growth temperature of the CiCs complex, increase with increasing Ge content. This is equivalent to an increasing retardation of the diffusion of Ci in Si1−xGex with increasing Ge content. © 1999 American Institute of Physics.
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71.55.Ht Other nonmetals
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.80.Jh Ion radiation effects
61.82.Fk Semiconductors
61.72.Cc Kinetics of defect formation and annealing

Thermal stability of supersaturated MgxZn1−xO alloy films and MgxZn1−xO/ZnO heterointerfaces

A. Ohtomo, R. Shiroki, I. Ohkubo, H. Koinuma, and M. Kawasaki

Appl. Phys. Lett. 75, 4088 (1999); http://dx.doi.org/10.1063/1.125545 (3 pages) | Cited 51 times

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We have examined the thermal stability of wurtzite-phase MgxZn1−xO alloy films and ZnO/MgxZn1−xO bilayer films with x exceeding the reported solubility limit of 0.04. When a Mg0.23Zn0.78O film was annealed, the segregation of MgO started at 850 °C and the band gap was reduced to the value of that for an x = 0.15 film after annealing at 1000 °C. Mg0.15Zn0.85O films showed no change of the band gap even after annealing at 1000 °C. Therefore, we conclude that the thermodynamic solubility limit of MgO in MgxZn1−xO epitaxial film is about x = 0.15. The thermal diffusion of Mg across the MgxZn1−xO/ZnO interface was observed only after annealing above 700 °C. Unlike other II–VI semiconductors, ZnO-based alloy films and heterointerfaces are stable enough for the fabrication of high-crystallinity heterostructures. © 1999 American Institute of Physics.
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68.60.Dv Thermal stability; thermal effects
68.35.Md Surface thermodynamics, surface energies
64.75.-g Phase equilibria
61.72.Cc Kinetics of defect formation and annealing
78.40.Fy Semiconductors
78.66.Hf II-VI semiconductors
71.20.Nr Semiconductor compounds
68.55.-a Thin film structure and morphology
66.30.Ny Chemical interdiffusion; diffusion barriers

The role of particle energy and pulsed particle flux in physical vapor deposition and pulsed–laser deposition

Stefan G. Mayr, Michael Moske, Konrad Samwer, Maggie E. Taylor, and Harry A. Atwater

Appl. Phys. Lett. 75, 4091 (1999); http://dx.doi.org/10.1063/1.125546 (3 pages) | Cited 10 times

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Surface morphology evolution of thin films generated by physical and pulsed-laser deposition depending on the incident particle energy and the pulse rate is investigated using a continuum growth model. The model includes curvature-induced surface diffusion, the Schwoebel barrier and surface atom displacement as main surface processes. The numerical solution of the model is in very good agreement with the results of kinetic Monte Carlo simulations, which also serve to estimate the continuum growth parameters, and with experimental results on thin Si films. The increase of the incident particle energy, starting from thermal energy, fundamentally influences the surface topography, changing from self-affine to self-organized morphology. © 1999 American Institute of Physics.
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81.15.Fg Pulsed laser ablation deposition
68.55.-a Thin film structure and morphology
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.35.Fx Diffusion; interface formation

Strong photoluminescence from monosubstituted polyacetylenes containing biphenylyl chromophores

Yuan Ming Huang, Weikun Ge, Jacky W. Y. Lam, and Ben Zhong Tang

Appl. Phys. Lett. 75, 4094 (1999); http://dx.doi.org/10.1063/1.125547 (3 pages) | Cited 17 times

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Monosubstituted polyacetylenes are generally regarded as unlikely candidates for photoluminescent materials. We have, however, observed intense deep-blue emission in a series of photoexcited poly(1–alkynes) (PAs) containing biphenylyl pendants (–{HC=C[(CH2)m–OCO–biphenyl–OC7H15]}n–where m = 2, 3, 4, and 9). The photoluminescence (PL) is readily observable by naked eyes under normal room illumination conditions, whose integrated intensity is threefold higher than that of poly(1–phenyl–1–butyne), a well-known highly luminescent disubstituted polyacetylene. A red PL band has also been detected in the PAs. Using the extended-Hückel-tight-binding method, we have calculated the density of states of the PAs, and it is found that the functional pendants have dramatically modified the band states of polyacetylene. The strong PL of the PAs is satisfactorily explained by the engineered electronic structures. © 1999 American Institute of Physics.
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42.70.Jk Polymers and organics
78.55.Mb Porous materials
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
71.20.Rv Polymers and organic compounds

Local structures of free-standing AlxGa1−xN thin films studied by extended x-ray absorption fine structure

Kin Man Yu, W. Shan, C. J. Glover, M. C. Ridgway, William S. Wong, and W. Yang

Appl. Phys. Lett. 75, 4097 (1999); http://dx.doi.org/10.1063/1.125548 (3 pages) | Cited 5 times

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Local structural information for the first two atomic shells surrounding Ga atoms in free standing AlxGa1−xN alloy films has been obtained by extended x-ray absorption fine structure spectroscopy. For an AlN mole fraction ranging from 0 to 0.6, we found that the first shell Ga–N bond length had only a weak composition dependence, roughly one quarter of that predicted by Vegard’s Law. In the second shell, the Ga–Ga bond length was significantly longer than that of Ga–Al (Δ ∼ 0.04–0.065 Å). A bond-type specific composition dependence was observed for the second shell cation–cation distances. While the composition dependence of the Ga–Ga bond length is ∼70% of that predicted by Vegard’s Law, the Ga–Al bond length was essentially composition independent. These results suggested that local strain in AlxGa1−xN was also accommodated by lattice distortion in the Al cation sublattice. © 1999 American Institute of Physics.
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68.55.-a Thin film structure and morphology
61.05.cj X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc.
78.70.Dm X-ray absorption spectra

Photoemission spectroscopy and velocity analysis of sputtered carbon atoms, ions, and clusters Cm0,± (m ⩽ 4)

Abdul Qayyum, M. Naeem Akhtar, Tasneem Riffat, and Shoaib Ahmad

Appl. Phys. Lett. 75, 4100 (1999); http://dx.doi.org/10.1063/1.125549 (3 pages) | Cited 4 times

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Diagnostics of the carbon atoms, ions, and clusters Cm0,± (m ⩽ 4) sputtered from a graphite surface under 10 keV Xe+ bombardment reveals the presence of neutral Cm0 and positively charged Cm+ species in the photoemission spectra. The complete absence of Cm+ in the mass spectra by a velocity analyzer is supplemented by the presence of the negatively charged Cm−. The velocity spectra are dominated by C and C2− with peaks due to C3− and C4−. These results may help us to understand the contribution of neutral and charged species in the heavy-ion sputtering of graphite and the energetics of regenerative soot. © 1999 American Institute of Physics.
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79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
36.40.Mr Spectroscopy and geometrical structure of clusters

Size-dependent second-harmonic generation by nanocrystals prepared by plasma-enhanced chemical-vapor deposition

D. Milovzorov and T. Suzuki

Appl. Phys. Lett. 75, 4103 (1999); http://dx.doi.org/10.1063/1.125550 (3 pages) | Cited 8 times

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The second-harmonic generation (SHG) from silicon nanocrystallites in polycrystalline films prepared by low-temperature plasma-enhanced chemical-vapor deposition has been studied. We observed an increase in the SHG signal due to the quantum-size effect and shape distortion by decrease in the sizes of crystallites. It was shown that the resonant SHG spectra have a size-dependent fine structure. Also, the power dependence of the SHG response from polycrystalline silicon films was investigated. © 1999 American Institute of Physics.
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42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
78.66.Vs Fine-particle systems
78.66.Db Elemental semiconductors and insulators
78.30.Am Elemental semiconductors and insulators
61.46.-w Structure of nanoscale materials

Strain relief and its effect on the properties of GaN using isoelectronic In doping grown by metalorganic vapor phase epitaxy

Shigeo Yamaguchi, Michihiko Kariya, Shugo Nitta, Hiroshi Amano, and Isamu Akasaki

Appl. Phys. Lett. 75, 4106 (1999); http://dx.doi.org/10.1063/1.125551 (3 pages) | Cited 26 times

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We studied the effect of isoelectronic In doping on the crystalline and optical properties of GaN grown on sapphire with H2 or N2 carrier gas by metalorganic vapor phase epitaxy. The relationship between lattice constants c and a obtained by x-ray diffraction analysis showed that with increasing trimethylindium (TMI) flow during growth, the strain in GaN decreased, and accordingly, the tilting and the twisting components of crystalline mosaicity also decreased. In addition, the Raman shift, the excitonic photoluminescence peak energy, and the its linewidth shifted in accordance with the magnitude and the sign of the strain in GaN, regardless of the carrier gas used. These results revealed that for a smaller TMI flow region, In was incorporated so that the crystallinity of GaN improved, and for a larger TMI flow region, In substituted for Ga so that alloying formation might have occurred. © 1999 American Institute of Physics.
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81.05.Ea III-V semiconductors
68.60.Bs Mechanical and acoustical properties
78.55.Cr III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
68.55.-a Thin film structure and morphology
78.66.Fd III-V semiconductors
61.72.uj III-V and II-VI semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
78.30.Fs III-V and II-VI semiconductors
71.35.-y Excitons and related phenomena

Crystal tilting in GaN grown by pendoepitaxy method on sapphire substrate

Ig-Hyeon Kim, C. Sone, Ok-Hyun Nam, Yong-Jo Park, and Taeil Kim

Appl. Phys. Lett. 75, 4109 (1999); http://dx.doi.org/10.1063/1.125552 (3 pages) | Cited 28 times

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Pendeoepitaxy of GaN on sapphire substrate with SiO2 mask is demonstrated and characterized by transmission electron microscopy and double crystal x-ray diffraction. A continuous layer of GaN with low dislocation density was achieved by this method. Parts of the GaN layer are tilted symmetrically toward [11-20] direction and have two kinds of coalesce and tilt boundaries. Each boundary was formed by a vertical array of piled up dislocations with the Burger’s vector of [11-20]. The tilting mechanism in pendeo-epitaxy is discussed in terms of surface interaction between the SiO2 mask and ELO-GaN. © 1999 American Institute of Physics.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.05.Ea III-V semiconductors
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
61.72.Mm Grain and twin boundaries
68.55.-a Thin film structure and morphology
81.15.Kk Vapor phase epitaxy; growth from vapor phase

Highly nonlinear photoluminescence threshold in porous silicon

M. Nayfeh, O. Akcakir, J. Therrien, Z. Yamani, N. Barry, W. Yu, and E. Gratton

Appl. Phys. Lett. 75, 4112 (1999); http://dx.doi.org/10.1063/1.125553 (3 pages) | Cited 37 times

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Porous silicon is excited using near-infrared femtosecond pulsed and continuous wave radiation at an average intensity of ∼ 106 W/cm2 (8×1010 W/cm2 peak intensity in pulsed mode). Our results demonstrate the presence of micron-size regions for which the intensity of the photoluminescence has a highly nonlinear threshold, rising by several orders of magnitude near this incident intensity for both the pulsed and continuous wave cases. These results are discussed in terms of stimulated emission from quantum confinement engineered intrinsic Si–Si radiative traps in ultrasmall nanocrystallites, populated following two-photon absorption. © 1999 American Institute of Physics.
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78.55.Ap Elemental semiconductors
78.45.+h Stimulated emission
78.47.-p Spectroscopy of solid state dynamics
42.65.-k Nonlinear optics

Transmission electron microscopy study of the nitridation of the (0001) sapphire surface

P. Vennéguès and B. Beaumont

Appl. Phys. Lett. 75, 4115 (1999); http://dx.doi.org/10.1063/1.125554 (3 pages) | Cited 16 times

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The purpose of this letter is to study, using transmission electron microscopy, the nitridation of the (0001) sapphire surface which is a key step for the fabrication of high-quality GaN materials. A nitridation conducted at 1080 °C during 7 min at atmospheric pressure with a 2 slm NH3 flow, results in the formation of a fully crystalline 10-atomic-planes-thick AlN film by the chemical transformation of the Al2O3 surface. From measurements of interplanar distances in high-resolution images, we show that this chemical transformation is incomplete, i.e., that a few Al vacancies and/or O atoms remain in the AlN structure. © 1999 American Institute of Physics.
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81.65.Lp Surface hardening: nitridation, carburization, carbonitridation
81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
68.35.Dv Composition, segregation; defects and impurities
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
68.35.B- Structure of clean surfaces (and surface reconstruction)
61.72.J- Point defects and defect clusters

GeSi films with reduced dislocation density grown by molecular-beam epitaxy on compliant substrates based on porous silicon

S. I. Romanov, V. I. Mashanov, L. V. Sokolov, A. Gutakovskii, and O. P. Pchelyakov

Appl. Phys. Lett. 75, 4118 (1999); http://dx.doi.org/10.1063/1.125555 (3 pages) | Cited 15 times

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To grow high-quality heteroepitaxial layers, we propose a compliant silicon substrate consisting of a thin epitaxial silicon film on a high-density porous layer as a membrane and an expansive low-density porous layer as a mechanical damper which shields the overlying layers from the massive wafer. GeSi films over the critical thickness have been grown by molecular-beam epitaxy on these substrates. Transmission electron microscopy analysis shows that Ge0.2Si0.8 films have no dislocations owing to just elastic strain relaxation whereas plastic flow in the pseudomorphic films that are being grown on conventional Si substrates occurs with generation of dislocations in a regular manner. The experimental data on porous silicon structure are presented in some detail and are briefly discussed in connection with substrate compliance. © 1999 American Institute of Physics.
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81.05.Hd Other semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
62.20.F- Deformation and plasticity
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