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18 Oct 2004

Volume 85, Issue 16, pp. 3343-3639

Issue Cover Spotlight Figure

Appl. Phys. Lett. 85, 3570 (2004); http://dx.doi.org/10.1063/1.1807953 (2 pages)

X. N. Zhang, C. R. Li, Z. Zhang, and Z. X. Cao
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Spatial variation of luminescence from AlGaN grown by facet controlled epitaxial lateral overgrowth

A. Bell, R. Liu, U. K. Parasuraman, F. A. Ponce, S. Kamiyama, H. Amano, and I. Akasaki

Appl. Phys. Lett. 85, 3417 (2004); http://dx.doi.org/10.1063/1.1807950 (3 pages) | Cited 6 times

Online Publication Date: 22 October 2004

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Interesting phenomena have been observed in the epitaxial lateral overgrowth of AlxGa1−xN alloys using facet control on serrated GaN templates. A complex microstructure is observed that involves misfit dislocation arrays that are closely related to regions with significantly large variations in composition. The dislocations are on inclined planar boundaries and result from basal-plane slip, which is allowed in this inclined facet geometry. The spatial variation of the aluminum composition in the overgrowth region is determined by cathodoluminescence spectroscopy and ranges from x=0.06 to 0.27, for constant growth conditions that after planarization result in a uniform composition at x=0.16. These results indicate that aluminum incorporation depends significantly on the growth direction with marked preference for facets parallel to the basal plane.
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78.66.Fd III-V semiconductors
78.60.Hk Cathodoluminescence, ionoluminescence
68.55.-a Thin film structure and morphology
68.55.A- Nucleation and growth
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)

Photon tunneling via surface plasmon coupling

A. Passian, A. Wig, A. L. Lereu, F. Meriaudeau, T. Thundat, and T. L. Ferrell

Appl. Phys. Lett. 85, 3420 (2004); http://dx.doi.org/10.1063/1.1793351 (3 pages) | Cited 7 times

Online Publication Date: 22 October 2004

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The measurement of a photonic signal via plasmon-plasmon coupling in curved thin metal films is presented. In domains of subwavelength dimension, we calculate the resonant dispersion relations by modeling the curved thin film as a single sheeted hyperboloid of revolution. We show that several such surface modes are accessible optically at frequencies below the plasma frequency of the metal.
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68.55.-a Thin film structure and morphology
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Optical properties of single InAs quantum dots in close proximity to surfaces

C. F. Wang, A. Badolato, I. Wilson-Rae, P. M. Petroff, E. Hu, J. Urayama, and A. Imamoğlu

Appl. Phys. Lett. 85, 3423 (2004); http://dx.doi.org/10.1063/1.1806251 (3 pages) | Cited 33 times

Online Publication Date: 22 October 2004

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The optical properties of single InAs∕GaAs quantum dots (QDs) were studied as a function of their distance from the air∕GaAs interface. A short-period superlattice structure allows us to controllably shorten the distance between the QDs and the surface in 6-nm steps. The QD luminescence intensity and lifetime measurements show that quantum tunneling effect results in a sharp reduction in fluorescence efficiency and lifetime when the wetting-layer–surface distance is within 9 nm. For distances between 15 and 40 nm, broadening of the photoluminescence linewidths of single QDs was observed. Since exciton recombination time and efficiency are in this case unchanged with respect to bulk QDs, the observed line broadening is most likely due to dephasing or spectral diffusion processes.
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73.21.La Quantum dots
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.40.Gk Tunneling
78.67.Hc Quantum dots
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
78.55.Cr III-V semiconductors
78.35.+c Brillouin and Rayleigh scattering; other light scattering

Epitaxial growth of Eu2O3 thin films on LaAlO3 substrates by polymer-assisted deposition

Y. Lin, H. Wang, M. E. Hawley, S. R. Foltyn, Q. X. Jia, G. E. Collis, A. K. Burrell, and T. M. McCleskey

Appl. Phys. Lett. 85, 3426 (2004); http://dx.doi.org/10.1063/1.1806265 (3 pages) | Cited 8 times

Online Publication Date: 22 October 2004

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Eu2O3 thin films were epitaxially grown on (001)LaAlO3 (LAO) substrates using a polymer-assisted deposition technique. Microstructural studies by x-ray diffraction and transmission electron microscopy show that the films are epitaxial with an orientational relationship of (001)Eu2O3‖(001)LAO and [1math0]Eu2O3‖[100]LAO. We have found a systematic change in the out-of-plane lattice parameter of Eu2O3 (a⊥) with the annealing temperature, which is believed to be from different strain effects.
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68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
68.37.Lp Transmission electron microscopy (TEM)
81.40.Gh Other heat and thermomechanical treatments

Strong affinity of hydrogen for the GaN(000-1) surface: Implications for molecular beam epitaxy and metalorganic chemical vapor deposition

J. E. Northrup and J. Neugebauer

Appl. Phys. Lett. 85, 3429 (2004); http://dx.doi.org/10.1063/1.1808227 (3 pages) | Cited 18 times

Online Publication Date: 22 October 2004

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The stabilities of clean and hydrogen covered GaN(000-1) surfaces are determined using density functional theory together with a finite temperature thermodynamics approach. Hydrogen has an extremely high affinity for the N-face surface: Even under ultrahigh vacuum conditions as realized in molecular beam epitaxial growth, with a residual hydrogen pressure of 10−12 atm, the hydrogen terminated surface is, for very N-rich conditions, more stable than any clean surface. A transition to a surface covered by a Ga adlayer is predicted to occur as the Ga chemical potential increases. In typical metalorganic chemical vapor deposition conditions the (000-1) surface is predicted to be covered by 0.75 monolayers of hydrogen. The slower growth rate on the (000-1) surface in comparison to the (0001) surface is attributed to low adsorption of N on the H-covered (000-1) surface.
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81.05.Ea III-V semiconductors
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.47.Fg Semiconductor surfaces
68.35.Rh Phase transitions and critical phenomena
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.43.Mn Adsorption kinetics
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces

Upconversion luminescence of Al2O3–SiO2:Ce3+ glass by femtosecond laser irradiation

Hongpeng You, Tomokatsu Hayakawa, and Masayuki Nogami

Appl. Phys. Lett. 85, 3432 (2004); http://dx.doi.org/10.1063/1.1808234 (3 pages) | Cited 13 times

Online Publication Date: 22 October 2004

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Near-infrared to visible upconversion luminescence was observed in a sol-gel processed Al2O3–SiO2 glass containing Ce3+ ions under femtosecond laser irradiation. The optical properties of the glasses reveal that the upconversion luminescence comes from the 5d→4f transition of the Ce3+ ions. The relationship between the intensity of the Ce3+ emission and the pump power reveals that a three-photon absorption predominates in the conversion process from the near-infrared into the blue luminescence. The analysis of the upconversion mechanism suggests that the upconversion luminescence may come from a three-photon simultaneous absorption that leads to a population of the 5d level in which the characteristic luminescence occurs.
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78.55.Qr Amorphous materials; glasses and other disordered solids
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
78.66.Jg Amorphous semiconductors; glasses
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Stretchability of thin metal films on elastomer substrates

Teng Li, Zhenyu Huang, Z. Suo, Stéphanie P. Lacour, and Sigurd Wagner

Appl. Phys. Lett. 85, 3435 (2004); http://dx.doi.org/10.1063/1.1806275 (3 pages) | Cited 70 times

Online Publication Date: 22 October 2004

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Many flexible electronic surfaces comprise inorganic films on organic substrates. Mechanical failure of such integrated structures of stiff and compliant materials poses a significant challenge. This letter studies the stretchability of metal films on elastomer substrates. Our experiment shows that, when stretched, elastomer-supported metal films rupture at strains larger than those reported for freestanding films. We use a finite element code to simulate the rupture process of metal films. A freestanding metal film ruptures by forming a single neck. By contrast, a metal film on an elastomer substrate may develop an array of necks before rupture. While the pre-rupture necks do not change the electrical conductance appreciably, they elongate the metal film, leading to a large overall rupture strain.
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68.60.Bs Mechanical and acoustical properties
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
62.20.M- Structural failure of materials
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Textured nano-agglomerates embedded in the thin film of La0.8Sr0.2MnO3

Y. L. Zhu, B. Wu, X. L. Ma, D. X. Li, H. B. Lu, Z. H. Chen, and G. Z. Yang

Appl. Phys. Lett. 85, 3438 (2004); http://dx.doi.org/10.1063/1.1808236 (3 pages)

Online Publication Date: 22 October 2004

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We have observed nano-scale chemical composition separation in the thin film of La0.8Sr0.2MnO3 by means of high-angle-angular-dark-field imaging, elemental mapping, and composition line-scanning in a transmission electron microscope. It is found that highly dense and dimensionally uniform nano-agglomerates are embedded in the film. The ratio of Mn∕La in the nano-agglomerates is fluctuated and generally much higher than that in their surrounding medium. Such compositional inhomogeneities result in a textured microstructure, which is one of the most important parameters for tunning electronic properties in the colossal magnetoresistance oxides.
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61.46.-w Structure of nanoscale materials
68.55.-a Thin film structure and morphology
68.37.Lp Transmission electron microscopy (TEM)
64.75.-g Phase equilibria
61.66.Bi Elemental solids
61.66.Dk Alloys

Evolution of stress in GaN heteroepitaxy on AlN∕Si(111): From hydrostatic compressive to biaxial tensile

A. Krost, A. Dadgar, J. Bläsing, A. Diez, T. Hempel, S. Petzold, J. Christen, and R. Clos

Appl. Phys. Lett. 85, 3441 (2004); http://dx.doi.org/10.1063/1.1808237 (3 pages) | Cited 16 times

Online Publication Date: 22 October 2004

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The initial steps of GaN growth on an AlN buffer layer on Si(111) substrates by metalorganic vapor phase epitaxy were investigated using field emission scanning electron microscopy, micro-photoluminescence, as well as by conventional and grazing incidence x-ray diffraction. A series of GaN layers was grown for various times ranging from 7.5 s to several minutes, doubling the growth time for each step. The AlN buffer layer is noncontinuous and consists of (0001)-oriented AlN islands with a mean diameter of about 50 nm. On top of these nucleation centers three-dimensional growth of GaN was observed. With increasing growth times up to 30 s these islands further expanded and their distribution became more homogeneous. At 60 s coalescence started with homogeneously distributed islands, and after 120 s the layer was fully coalesced. The layers grown for 7.5 and 15 s are under a high compressive hydrostatic pressure, which might be enhanced by the lattice mismatch between AlN and GaN. For longer growth times a biaxial tensile stress is observed. The occurrence of the biaxial tensile stress correlates with the onset of island coalescence. The x-ray results are in agreement with low-temperature optical measurements showing a consistent energy shift of the near band gap luminescence and longitudinal optical Raman modes with respect to relaxed GaN.
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68.55.A- Nucleation and growth
81.05.Ea III-V semiconductors
78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
68.37.Vj Field emission and field-ion microscopy
68.55.-a Thin film structure and morphology

Ultrafast optical switching at the telecommunication wavelength by single electron transfer between fluorene-containing redox polymer and its counter anion

Toshihiko Nagamura, Yasuyuki Nagai, Akihiro Furube, and Shigeo Murata

Appl. Phys. Lett. 85, 3444 (2004); http://dx.doi.org/10.1063/1.1808238 (3 pages) | Cited 5 times

Online Publication Date: 22 October 2004

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Ultrafast and large absorption change in the optical telecommunication wavelength region was achieved in solid films by making transitory expansion of a π-electronic system with photoinduced electron transfer and reverse reactions between redox-active ion pairs. Femtosecond dynamics of absorption changes at about 500–1550 nm due to single electron transfer was studied for a poly(tetramethyleneoxide) film containing 2,7-bis(4-pyridiniumyl)fluorene unit as part of the main chain. It showed ON response in less than 100 fs and fastest OFF response in 240 fs at the telecommunication wavelength. This polymer film may be applied to tetrahertz all optical information processing in the telecommunication system.
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42.70.Jk Polymers and organics
42.65.Pc Optical bistability, multistability, and switching, including local field effects
42.79.Sz Optical communication systems, multiplexers, and demultiplexers
78.40.Me Organic compounds and polymers
78.30.Jw Organic compounds, polymers
73.50.Pz Photoconduction and photovoltaic effects
42.65.Re Ultrafast processes; optical pulse generation and pulse compression

A hybrid epitaxy method for InAs on GaP

A. Chen, A. Yulius, J. M. Woodall, and C. C. Broadbridge

Appl. Phys. Lett. 85, 3447 (2004); http://dx.doi.org/10.1063/1.1808241 (3 pages) | Cited 3 times

Online Publication Date: 22 October 2004

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The interface formation mechanism during the molecular-beam epitaxy (MBE) of InAs∕GaP has been studied with the aid of the In–Ga–P phase diagram. It is discovered that an initial dissolution and crystallization process similar to liquid phase epitaxy (LPE) may happen at sufficiently high temperature, resulting in a graded composition at the interface. Consequently, “parasitic LPE∕MBE” is the name for this hybrid form of MBE. High-resolution TEM images confirm the existence of the interfacial layer in the sample grown at high temperature. The graded interface smears out the band offset and leads to a nonrectifying heterojunction. Low-temperature (LT) MBE growth can turn off the LPE component, enabling the growth of an abrupt interface. Based on this “LPE∕MBE” model, a LT MBE technique is developed to grow an abrupt InAs∕InGaP interface for heterojunction power Schottky rectifiers. The LT InAs∕InGaP heterojunction demonstrates nearly ideal Schottky rectifier characteristics, while the sample grown at high temperature shows resistive ohmic characteristics. The LT InAs∕InGaP Schottky diode also demonstrates good stability with respect to anneal temperature, similar to the InAs∕GaP heterojunctions.
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68.55.A- Nucleation and growth
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
68.37.Lp Transmission electron microscopy (TEM)
73.30.+y Surface double layers, Schottky barriers, and work functions
81.40.Gh Other heat and thermomechanical treatments
81.30.Dz Phase diagrams of other materials
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions

Highly flexible transparent electrodes for organic light-emitting diode-based displays

Jay Lewis, Sonia Grego, Babu Chalamala, Erik Vick, and Dorota Temple

Appl. Phys. Lett. 85, 3450 (2004); http://dx.doi.org/10.1063/1.1806559 (3 pages) | Cited 77 times

Online Publication Date: 22 October 2004

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Multilayer indium-tin-oxide (ITO)–Ag–ITO stacks were evaluated as transparent conductors for flexible organic light-emitting diode (OLED) displays. The ITO–metal–ITO (IMI) samples exhibited significantly reduced sheet resistance over ITO and greater than 80% optical transmission. The IMI films deposited on plastic substrates showed dramatically improved mechanical properties when subjected to bending both as a function of radius of curvature as well as number of cycles to a fixed radius. OLEDs were fabricated on both ITO and IMI anodes, and the devices with IMI anodes showed improved performance at current densities greater than 1 mA∕cm2 due to the improved conductivity of the anode.
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85.60.Jb Light-emitting devices
85.60.Pg Display systems
73.40.Ns Metal-nonmetal contacts
62.20.F- Deformation and plasticity
81.40.Lm Deformation, plasticity, and creep
62.20.D- Elasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations
78.60.Fi Electroluminescence
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials

Compressibility and pressure-induced phase transformation of Ti3GeC2

Zhongwu Wang, C. S. Zha, and M. W. Barsoum

Appl. Phys. Lett. 85, 3453 (2004); http://dx.doi.org/10.1063/1.1808491 (3 pages) | Cited 21 times

Online Publication Date: 22 October 2004

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In order to explore the large shear-strain-induced polymorph, α‐Ti3GeC2 polycrystals were investigated by using a synchrotron radiation source to 64 GPa under nonhydrostatic state. Upon compression to 26.6 GPa, α‐Ti3GeC2 starts to transform to β‐Ti3GeC2. As compared to α‐Ti3GeC2, the cell parameter a of β‐Ti3GeC2 is slightly shorter, and both c and c∕a larger. These two polymorphs exhibit similar compressibility. The bulk modulus is calculated to be 179 (±10) GPa at a fixed K′=4.0. This is lower than that of Ti3SiC2, and close to that of Ti3Si0.5Ge0.5C2. The compressibilities of these two Ti3GeC2 polymorphs do not display an apparent anisotropy, and differ from the large anisotropies observed from Ti3SiC2 and Ti3Si0.5Ge0.5C2.
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81.40.Lm Deformation, plasticity, and creep
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
62.50.-p High-pressure effects in solids and liquids
64.70.K- Solid-solid transitions
62.20.D- Elasticity
62.20.F- Deformation and plasticity

Simultaneous short-range smoothening and global roughening during growth of hydrogenated amorphous silicon films

Brent A. Sperling and John R. Abelson

Appl. Phys. Lett. 85, 3456 (2004); http://dx.doi.org/10.1063/1.1777414 (3 pages) | Cited 13 times

Online Publication Date: 22 October 2004

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We investigate the evolution of surface morphology during hot-wire chemical vapor deposition of amorphous silicon films onto rough substrates. Using in situ spectroscopic ellipsometry, we find that the surface smoothens as the film grows. However, postdeposition atomic force microscopy reveals that the roughness is actually increasing linearly. We resolve this discrepancy by examining the power spectrum densities of the atomic force images, which indicate that the growth surface experiences both short-range smoothening and global roughening. The ellipsometry data are consistent with the short-range atomic force microscopy data, but they exclude information about the long-range components of roughness. The slope of the power spectrum density indicates surface diffusion is the dominant smoothening mechanism; the linear increase in roughness is consistent with columnar growth caused by self-shadowing.
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81.05.Cy Elemental semiconductors
81.05.Gc Amorphous semiconductors
68.55.-a Thin film structure and morphology
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.47.Fg Semiconductor surfaces
68.55.A- Nucleation and growth
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.35.Fx Diffusion; interface formation

Mechanical aspects of epitaxial ferroelectric Pb(Zr0.5Ti0.5)O3 films investigated by nanoindentation methods and piezoresponse force microscopy

D. Liu and K. W. White

Appl. Phys. Lett. 85, 3459 (2004); http://dx.doi.org/10.1063/1.1806563 (3 pages) | Cited 4 times

Online Publication Date: 22 October 2004

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Mechanical behaviors of a lead zirconium titanate (PZT) thin film are investigated using a unique combination of nanoindentation methods and piezoresponse force microscopy (PFM) in various domain areas, following local domain pattern writing. The interaction between the ferroelectric domain character and the mechanical behavior is mapped using PFM for subsequent nanoindentation. Significant anisotropic behaviors between (100) and (001) domains are observed. An anomalous increase of indentation elastic modulus in the region near a domain wall is observed.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.55.-g Dielectric thin films
68.60.Bs Mechanical and acoustical properties
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.Qp Friction, tribology, and hardness
62.20.D- Elasticity
62.20.M- Structural failure of materials
77.80.Dj Domain structure; hysteresis
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.37.Ps Atomic force microscopy (AFM)

Suppressed surface morphology instabilities in amorphous hydrogenated silicon deposition

George T. Dalakos, Joel L. Plawsky, and Peter D. Persans

Appl. Phys. Lett. 85, 3462 (2004); http://dx.doi.org/10.1063/1.1808495 (3 pages) | Cited 7 times

Online Publication Date: 22 October 2004

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The surface chemistry associated with a‐Si:H growth by plasma-enhanced chemical vapor deposition is unique in that the hydrogen-passivated surface results in a low threshold energy for surface diffusion relative to the cohesive energies of the material. We show that helium ion bombardment enhances the hopping rate of loosely bound film precursors without substantially affecting the condensed a‐Si:H material. Our investigative approach consists of examination of the temporal evolution of the surface topography under different substrate bias conditions. Without biasing the substrate, the surface morphology becomes unstable, producing mounded-type structures, consistent with shadowing growth instability. Biasing the substrate suppresses these instabilities and allows an initially rough a‐Si:H surface to be smoothed during subsequent deposition.
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81.05.Cy Elemental semiconductors
81.05.Gc Amorphous semiconductors
68.47.Fg Semiconductor surfaces
68.35.B- Structure of clean surfaces (and surface reconstruction)
81.65.Rv Passivation
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
68.35.Fx Diffusion; interface formation
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Hydrogen site energetics in LaNi5Hn and LaCo5Hn: Toward predicting hydrides

J. F. Herbst and L. G. Hector

Appl. Phys. Lett. 85, 3465 (2004); http://dx.doi.org/10.1063/1.1808503 (3 pages) | Cited 2 times

Online Publication Date: 22 October 2004

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We have investigated the energetics of hydrogen site occupation in LaNi5Hn (hexagonal P63mc structure) and LaCo5Hn (orthorhombic Cmmm structure) via calculation of the site-dependent enthalpies of hydride formation ΔH. For each structure ΔH was determined for a broad variety of hydrogen configurations. In LaNi5Hn (LaCo5Hn) we find that the minimum ΔH occurs for hydrogen filling of the 2b6c16c2 (4e4h) sites, precisely those identified by neutron diffraction. Hydrogen-richer hydrides are predicted for both structures, in qualitative agreement with experiments performed at higher pressures.
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65.40.G- Other thermodynamical quantities
61.66.Fn Inorganic compounds

Thermal degradation of electrical properties and morphology of bulk single-crystal ZnO surfaces

Rohit Khanna, K. Ip, Y. W. Heo, D. P. Norton, S. J. Pearton, and F. Ren

Appl. Phys. Lett. 85, 3468 (2004); http://dx.doi.org/10.1063/1.1801674 (3 pages) | Cited 17 times

Online Publication Date: 22 October 2004

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Bulk ZnO substrates were rapid thermal annealed in either air or N2 at temperatures up to 500°C. The root-mean-square roughness of the surface as measured by atomic force microscopy begins to increase even after 200°C anneals in N2 or 300°C in air. The Schottky barrier height, ΦB, obtained from Pt∕Au diodes fabricated on these surfaces shows a decrease in effective barrier height upon annealing from 0.35 eV in control samples to ∼0.30 eV in those annealed at 400°C and an increase in the saturation current density (from 4 A cm−2 in the control samples to 17–37 A cm−2 in the annealed samples). These results show that the ZnO surface is degraded by quite low annealing temperatures and care must be exercised in designing the thermal budget for processing of ZnO devices. At much higher annealing temperatures (1000°C), the surface completely decomposes to leave metallic Zn droplets.
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85.30.Hi Surface barrier, boundary, and point contact devices
73.40.Ns Metal-nonmetal contacts
61.72.Cc Kinetics of defect formation and annealing
73.30.+y Surface double layers, Schottky barriers, and work functions
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.37.Ps Atomic force microscopy (AFM)
64.75.-g Phase equilibria
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Sulfur passivation for shallow Pd∕W∕Au ohmic contacts to p-InGaSb

S. H. Wang, S. E. Mohney, J. A. Robinson, and B. R. Bennett

Appl. Phys. Lett. 85, 3471 (2004); http://dx.doi.org/10.1063/1.1807013 (3 pages) | Cited 8 times

Online Publication Date: 22 October 2004

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The relationship between the specific contact resistance of nonalloyed Pd∕W∕Au ohmic contacts to p-In0.25Ga0.75Sb and premetallization surface preparations is examined. The resistance of ohmic contacts is minimized when p-InGaSb is exposed briefly to a dilute (NH4)2S solution. This treatment minimizes the thickness of any residual oxide or sulfide layer on the semiconductor and avoids excessive etching of the semiconductor, which would make the contact less shallow. A specific contact resistance of 5.9×10−7 Ω cm2 is achieved for a Pd∕W∕Au (2∕50∕145 nm) contact that consumes no more than 5 nm of InGaSb.
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81.65.Rv Passivation
81.65.Cf Surface cleaning, etching, patterning
73.40.Cg Contact resistance, contact potential
81.05.Ea III-V semiconductors
73.40.Ns Metal-nonmetal contacts

Time-of-flight analysis of charge mobility in a Cu-phthalocyanine-based discotic liquid crystal semiconductor

Hideo Fujikake, Takeshi Murashige, Makiko Sugibayashi, and Kazuchika Ohta

Appl. Phys. Lett. 85, 3474 (2004); http://dx.doi.org/10.1063/1.1805178 (3 pages) | Cited 21 times

Online Publication Date: 22 October 2004

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We used a time-of-flight method to study the charge carrier mobility properties of a molecular-aligned discotic liquid crystal semiconductor based on Cu-phthalocyanine. The heated isotropic-phase semiconductor material was sandwiched between transparent electrodes coated onto glass substrates without conventional alignment layers. This was then cooled, and a discotic liquid crystal semiconductor cell was obtained, which we used to make mobility measurements. The material had a fixed molecular alignment due to the supercooling of the hexagonal columnar mesophase. It was clarified that the carrier mobility for electrons was as high as it was for holes at room temperature. The maximum value of negative charge mobility reached 2.60×10−3 cm2∕V s, although negative carrier mobility is often much lower than positive carrier mobility in other organic semiconductors, including conventional Cu-phthalocyanine vacuum-deposited films.
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73.61.Ph Polymers; organic compounds
61.30.-v Liquid crystals
73.50.Pz Photoconduction and photovoltaic effects
73.50.Dn Low-field transport and mobility; piezoresistance

Integrated optical waveguides with liquid cores

D. Yin, D. W. Deamer, H. Schmidt, J. P. Barber, and A. R. Hawkins

Appl. Phys. Lett. 85, 3477 (2004); http://dx.doi.org/10.1063/1.1807966 (3 pages) | Cited 34 times

Online Publication Date: 22 October 2004

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We report the design, fabrication, and demonstration of single-mode integrated optical waveguides with liquid cores. The principle of the device is based on antiresonant reflecting optical (ARROW) waveguides with hollow cores. We describe design principles for waveguide loss optimization down to 0.1∕cm. Using a fabrication process based on conventional silicon microfabrication and sacrificial core layers, waveguides of varying widths and lengths with volumes covering the pico- to nanoliter range were fabricated. We observe confined mode propagation, measure waveguide losses of 2.4∕cm, and demonstrate that the waveguides possess tailorable wavelength selectivity. The potential for highly integrated, sensitive devices based on these properties of the ARROW waveguides is discussed.
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42.82.Et Waveguides, couplers, and arrays
42.79.Gn Optical waveguides and couplers
42.25.Bs Wave propagation, transmission and absorption

Effects of exciton–biexciton coherent coupling on exciton absorption in quantum dots

Hideki Gotoh, Hidehiko Kamada, Tadashi Saitoh, Hiroaki Ando, and Jiro Temmyo

Appl. Phys. Lett. 85, 3480 (2004); http://dx.doi.org/10.1063/1.1807949 (3 pages) | Cited 1 time

Online Publication Date: 22 October 2004

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We report unusual excitonic absorption spectra with dip-shaped structures caused by exciton–biexciton coherent interactions in quantum dots. The exciton absorption spectrum is measured with the micro-photoluminescence excitation technique in a single InGaAs quantum dot. The spectrum changes from Lorenztian-shaped to dip-shaped with increasing excitation intensity. A theoretical analysis with the density matrix method reveals that exciton–biexciton coherent interactions create dressed states in the exciton–biexciton system and lead to the unusual absorption spectra.
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73.21.La Quantum dots
71.35.Cc Intrinsic properties of excitons; optical absorption spectra
78.55.Cr III-V semiconductors
78.67.Hc Quantum dots
71.20.Nr Semiconductor compounds

Ultra-low-frequency self-oscillation of photocurrent in InxGa1−xAs∕Al0.15Ga0.85As multiple-quantum-well p–i–n diodes

A. Satake, K. Tanigawa, T. Kimura, K. Fujiwara, and N. Sano

Appl. Phys. Lett. 85, 3483 (2004); http://dx.doi.org/10.1063/1.1806268 (3 pages)

Online Publication Date: 22 October 2004

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We report an observation of ultra-low-frequency self-oscillation of photocurrent in InxGa1−xAs∕Al0.15Ga0.85As multiple-quantum-well p–i–n diodes. The photocurrent intensity shows self-oscillations with a characteristic frequency of ∼0.1 Hz at low temperatures under reverse bias voltages. The photocurrent self-oscillation depends on applied bias voltage, temperature, illumination power, and indium content of quantum-well layers. These dependences indicate that the photocurrent self-oscillation is attributed to photogenerated carriers trapped in localized centers within InxGa1−xAs quantum-well regions.
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85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
85.30.Kk Junction diodes
72.40.+w Photoconduction and photovoltaic effects
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
79.60.Bm Clean metal, semiconductor, and insulator surfaces

A potential low-temperature oxide-ion conductor: La2−xBaxMo2O9

S. Basu, P. Sujatha Devi, and H. S. Maiti

Appl. Phys. Lett. 85, 3486 (2004); http://dx.doi.org/10.1063/1.1808505 (3 pages) | Cited 25 times

Online Publication Date: 22 October 2004

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An oxide ion conducting material, La1.94Ba0.06Mo2O9, with ionic conductivity of the order of 0.084 S∕cm at 800 °C in air was prepared by a citrate-nitrate auto-ignition process. A 3% Ba doping has suppressed the resistive transition of unsubstituted La2Mo2O9, which in turn stabilized the high-temperature cubic phase at room temperature as confirmed from x-ray diffraction, differential thermal analysis, and dilatometric studies. Impedance measurements on sintered La1.94Ba0.06Mo2O9 further lend strong support that a small amount of Ba doping has increased the overall conductivity of the parent compound La2Mo2O9 to a notable extent both at low and high temperatures.
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81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)
66.30.H- Self-diffusion and ionic conduction in nonmetals
65.40.De Thermal expansion; thermomechanical effects
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
64.70.K- Solid-solid transitions
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation

Optical properties of AlN and GaN in elevated temperatures

K. B. Nam, J. Li, J. Y. Lin, and H. X. Jiang

Appl. Phys. Lett. 85, 3489 (2004); http://dx.doi.org/10.1063/1.1806545 (3 pages) | Cited 14 times

Online Publication Date: 22 October 2004

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Deep ultraviolet photoluminescence spectroscopy has been employed to study the optical transitions in AlN and GaN epilayers at temperatures from 10 to 800 K, from which the parameters that describe the temperature variation of the energy band gap (α and β or aB and θ) and linewidth broadening have been obtained. These parameters are compared with the previously reported values in AlN and GaN obtained by different methods in narrower temperature ranges. Our experimental results demonstrate that the broader temperature range of measurements is necessary to obtain accurate values of these parameters, particularly for AlN. These results, together with other well-known physical properties of AlN, may expand future prospects for the application of III-nitride materials.
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78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
81.40.Gh Other heat and thermomechanical treatments
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