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1 Jul 2003

Volume 94, Issue 1, pp. 1-808

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Spatial and temperature dependence of the spectroscopic profile of a magnetic atom adsorbed on a metal surface—Co/Cu(111)

Yuki Shimada, Hideaki Kasai, Hiroshi Nakanishi, Wilson Agerico Diño, Ayao Okiji, and Yukio Hasegawa

J. Appl. Phys. 94, 334 (2003); http://dx.doi.org/10.1063/1.1572543 (8 pages) | Cited 11 times

Online Publication Date: 20 June 2003

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As a representative Kondo system, consisting of a magnetic atom adsorbed on a metal surface, we consider a Co adatom on a Cu(111) surface. We introduce general expressions for the tunneling current and the differential conductance dI/dV, and discuss the corresponding spatial and temperature dependencies of the dI/dV line shapes that can be observed when the system is probed with a scanning tunneling microscope (STM). Starting with the STM tip sufficiently far from the Co adatom, the corresponding dI/dV line shape initially has an asymmetric structure about the Fermi level, which gradually becomes symmetric with decreasing STM tip–Co adatom distance, due to the competition between first-order (direct electron tunneling from the STM tip to the metal surface) and second-order (via the magnetic adsorbate) tunneling processes. The corresponding line shape also shows a strong temperature dependence, increasing in width with increasing temperature. As an aid to studying and observing these temperature dependencies experimentally, we suggest the utility of considering the temperature dependence of the derivative of dI/dV with respect to the bias voltage V, i.e., the d2I/dV2 line shape. © 2003 American Institute of Physics.
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75.70.Ak Magnetic properties of monolayers and thin films
72.25.Mk Spin transport through interfaces
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Cc Other ferromagnetic metals and alloys
68.43.Fg Adsorbate structure (binding sites, geometry)
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)

Electric-field-induced anisotropy of excitonic optical properties in semiconductor quantum dots

H. Gotoh, H. Kamada, T. Saitoh, H. Ando, and J. Temmyo

J. Appl. Phys. 94, 342 (2003); http://dx.doi.org/10.1063/1.1578524 (6 pages) | Cited 3 times

Online Publication Date: 20 June 2003

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We report the anisotropic excitonic optical properties (polarization dependent photoemission and photoabsorption) induced by applying a lateral electric field in a single semiconductor quantum dot. The excitonic optical polarization characteristics are examined using theoretical calculation and optical measurement. The optical properties are numerically analyzed taking into account the quantum dot potential, electric field, and electron–hole Coulomb interaction. We evaluate the polarization properties from calculated exciton wave functions. The polarization properties depend strongly on the size of the quantum dots and the spatial symmetry of the hole part of the wave function. There is large electric-field-induced anisotropy in thin quantum dots to a larger lateral extent even where the polarization properties are completely isotropic without an electric field. We compare the theoretical results with experimental results for InGaAs quantum dots obtained using the microphotoluminescence technique. Qualitative agreement between the theoretical results and experimental ones is obtained. © 2003 American Institute of Physics.
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78.67.Hc Quantum dots
78.20.Jq Electro-optical effects
71.35.Cc Intrinsic properties of excitons; optical absorption spectra
78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
73.21.La Quantum dots

Schottky barrier height and interfacial state density on oxide-GaAs interface

J. S. Hwang, C. C. Chang, M. F. Chen, C. C. Chen, K. I. Lin, F. C. Tang, M. Hong, and J. Kwo

J. Appl. Phys. 94, 348 (2003); http://dx.doi.org/10.1063/1.1578528 (6 pages) | Cited 4 times

Online Publication Date: 20 June 2003

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Photoreflectance (PR) and Raman spectra were employed to investigate the interfacial characteristics of a series of oxide films on GaAs. The barrier heights across the interfaces and the densities of interfacial states are determined from the PR intensity as a function of the pump power density. The oxide-GaAs structures fabricated by in situ molecular beam epitaxy exhibit low interfacial state densities in the low 1011 cm−2 range. The density of the interface states of the Ga2O3(Gd2O3)–GaAs structure is as low as (1.24±0.14)×1010 cm−2. The Ga2O3(Gd2O3) dielectric film has effectively passivated the GaAs surface. Additionally, Raman spectra were used to characterize the structural properties of the oxide films. © 2003 American Institute of Physics.
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73.20.At Surface states, band structure, electron density of states
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
73.30.+y Surface double layers, Schottky barriers, and work functions
78.30.Hv Other nonmetallic inorganics

Photoluminescence study of ZnO films prepared by thermal oxidation of Zn metallic films in air

Y. G. Wang, S. P. Lau, H. W. Lee, S. F. Yu, B. K. Tay, X. H. Zhang, and H. H. Hng

J. Appl. Phys. 94, 354 (2003); http://dx.doi.org/10.1063/1.1577819 (5 pages) | Cited 124 times

Online Publication Date: 20 June 2003

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Zinc oxide (ZnO) films were synthesized by thermal oxidation of metallic zinc films in air. The influence of annealing temperatures ranging from 320 to 1000 °C on the structural and optical properties of ZnO films is investigated systematically using x-ray diffraction and room temperature photoluminescence (PL). The films show a polycrystalline hexagonal wurtzite structure without preferred orientation. Room temperature PL spectra of the ZnO films display two emission bands, predominant excitonic ultraviolet (UV) emission and weak deep level visible emission. It is observed that the ZnO film annealed at 410 °C exhibits the strongest UV emission intensity and narrowest full width at half maximum (81 meV) among the temperature ranges studied. The excellent UV emission from the film annealed at 410 °C is attributed to the good crystalline quality of the ZnO film and the low rate of formation of intrinsic defects at such low temperature. The visible emission consists of two components in the green and yellow range, and they show different temperature dependent behavior from UV emission. Their possible origins are discussed. © 2003 American Institute of Physics.
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78.66.Hf II-VI semiconductors
78.55.Et II-VI semiconductors
81.05.Dz II-VI semiconductors
81.65.Mq Oxidation
61.72.Cc Kinetics of defect formation and annealing

Controlled p doping of the hole-transport molecular material N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine with tetrafluorotetracyanoquinodimethane

Weiying Gao and Antoine Kahn

J. Appl. Phys. 94, 359 (2003); http://dx.doi.org/10.1063/1.1577400 (8 pages) | Cited 13 times

Online Publication Date: 20 June 2003

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We investigate p-type doping of the hole-transport organic molecular material N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) with tetrafluorotetracyanoquinodimethane (F4-TCNQ) using direct and inverse photoemission spectroscopy, contact potential difference measurements, and in situ current–voltage (I–V) measurements. The close match between the ionization energy of α-NPD and the electron affinity of F4-TCNQ leads to an efficient charge transfer between highest occupied molecular orbital of the host and lowest occupied molecular orbital of the dopant. The Fermi level moves down towards the valence states by 0.62 eV in the 0.5% doped film with respect to the undoped film, and a narrow space charge layer (∼60 Å) forms at the interface with Au. Hole injection in the doped devices increases by several orders of magnitude due to tunneling through the depletion region. The large relaxation energy of the ionized α-NPD molecule limits the movement of the Fermi level and, ultimately, the hole injection. © 2003 American Institute of Physics.
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73.40.Ns Metal-nonmetal contacts
61.72.up Other materials
79.60.Fr Polymers; organic compounds
73.20.At Surface states, band structure, electron density of states
71.20.Rv Polymers and organic compounds

Study of excitonic ground state energies in coupled three-quantum dot systems for far-infrared laser applications

Z. Y. Lai and W. Z. Shen

J. Appl. Phys. 94, 367 (2003); http://dx.doi.org/10.1063/1.1577408 (8 pages) | Cited 4 times

Online Publication Date: 20 June 2003

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Effective mass theory and variation method are used to calculate the ground state energies of excitons in coupled three-quantum dot (3-QD) systems. To calculate multicenter integrals involving two particle wave functions, a series of spheres are used to cover the coordinate space and further approximation has been made when the radii of the integral spheres are suitably selected around the three spherical QDs in near equal radius. The results obtained from the calculation of In0.5Ga0.5As/GaAs and GaAs/Al0.2Ga0.8As coupled 3-QD systems are basically in agreement with the experiments. Based on the energy level schemes in these systems, we make suggestions for the application of far-infrared and/or terahertz sources. © 2003 American Institute of Physics.
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73.21.La Quantum dots
71.35.-y Excitons and related phenomena
78.67.Hc Quantum dots
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
42.55.Px Semiconductor lasers; laser diodes

Comparative full-band Monte Carlo study of Si and Ge with screened pseudopotential-based phonon scattering rates

Phuong Hoa Nguyen, Karl R. Hofmann, and Gernot Paasch

J. Appl. Phys. 94, 375 (2003); http://dx.doi.org/10.1063/1.1579860 (12 pages) | Cited 24 times

Online Publication Date: 20 June 2003

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In a previous article [J. Appl. Phys. 92, 5359 (2002)], we presented a combination of a full-band Monte Carlo method using an advanced band structure and a variable Brillouin zone discretization, with phonon scattering rates based on the screened pseudopotential considering the positions of the atoms in the elementary cell. To make the method suitable for sufficiently fast applications, such as device simulations, the simplest wave number dependent approximation was introduced. It contains an average of the cell structure factor, and only two fit parameters: The acoustic and the optical deformation potentials. As the pseudopotential, the Ashcroft model potential is chosen, and screening is taken into account using the Lindhard dielectric function. In the present article, based on the study of the influence of the two deformation potentials on the electron and hole drift velocities in Si and Ge, we show how to select the deformation potentials. Depending on the targeted agreement with experimental results, the pairs of deformation potentials for electrons and holes can be used uniformly for a wide temperature range or separately for different temperatures. For Ge, we achieve remarkable quantitative agreement with the temperature, field, and orientation dependencies of experimental electron and hole drift velocities in the wide temperature range from 77 to 300 K with a single set of the two deformations potentials for each carrier type. A detailed comparative simulation of the transport properties in Ge and Si at different temperatures is presented which is comprised of the steady-state dependence of the drift velocity on the electric field, the low-field mobility, and transient transport. Peculiarities of the drift velocity-field dependencies, such as the anisotropy, and a negative differential mobility are discussed in terms of the different band structures in connection with the field dependence of the simulated distribution functions. For doped materials, ionized impurity scattering is included. The resulting dependencies on the doping level are consistent with experimental values. © 2003 American Institute of Physics.
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72.20.Dp General theory, scattering mechanisms
72.80.Cw Elemental semiconductors
02.70.Uu Applications of Monte Carlo methods
63.20.K- Phonon interactions
71.15.Dx Computational methodology (Brillouin zone sampling, iterative diagonalization, pseudopotential construction)
71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations

Characterization of low-frequency noise in molecular beam epitaxy-grown GaN epilayers deposited on double buffer layers

W. K. Fong, S. W. Ng, B. H. Leung, and Charles Surya

J. Appl. Phys. 94, 387 (2003); http://dx.doi.org/10.1063/1.1579843 (5 pages) | Cited 1 time

Online Publication Date: 20 June 2003

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We report the growth of high-mobility Si-doped GaN epilayers utilizing unique double buffer layer (DBL) structures, which consist of a thin buffer layer and a thick GaN intermediate-temperature buffer layer (ITBL). In this study, three types of DBL were investigated: (i) thin GaN low-temperature buffer layer /GaN ITBL (type I); (ii) nitridated Ga metal film/GaN ITBL (type II); and (iii) thin AlN high-temperature buffer layer /GaN ITBL (type III). Systematic measurements were conducted on the electron mobilities and the low-frequency noise over a wide range of temperatures. It is found that the electron mobilities of the GaN films are substantially improved with the use of DBLs, with the sample using type III DBL which exhibits the highest low-temperature mobility. Furthermore, the same sample also demonstrates the elimination of deep levels at 91 and 255 meV below the conduction band. This is believed to result from the relaxation of tensile stress during growth with the use of type III DBLs. © 2003 American Institute of Physics.
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73.61.Ey III-V semiconductors
73.50.Dn Low-field transport and mobility; piezoresistance
72.70.+m Noise processes and phenomena
68.60.Bs Mechanical and acoustical properties
72.20.Fr Low-field transport and mobility; piezoresistance
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors

Scattering of electrons in silicon inversion layers by remote surface roughness

F. Gámiz and J. B. Roldán

J. Appl. Phys. 94, 392 (2003); http://dx.doi.org/10.1063/1.1577227 (8 pages) | Cited 10 times

Online Publication Date: 20 June 2003

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A model to study the effect of the roughness at the poly-Si/SiO2 interface in silicon inversion layers on the electron mobility is obtained. Screening of the resulting perturbation potential by the channel carriers is taken into account, considering Green’s functions for metal–oxide–semiconductor geometry, i.e., taking into account the finite thickness of the gate oxide. Mobility of electrons is evaluated at room temperature by the Monte Carlo method, taking into account the simultaneous contribution of phonon scattering, SiO2/Si interface roughness scattering, Coulomb scattering, and remote surface roughness scattering. The contribution of excited subbands is considered. The resulting remote surface roughness scattering is shown to be strongly dependent on the oxide thickness, and degrades mobility curves at low inversion charge concentrations. The results obtained show that the effect of this scattering mechanism cannot be ignored when the oxide thickness is below 5 nm, (as in actual devices), even when (as is usual) very high doping concentrations are used. © 2003 American Institute of Physics.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
68.35.Ct Interface structure and roughness

Hydrogen plasma treatment effects on electrical and optical properties of n-ZnO

A. Y. Polyakov, N. B. Smirnov, A. V. Govorkov, K. Ip, M. E. Overberg, Y. W. Heo, D. P. Norton, S. J. Pearton, B. Luo, F. Ren, and J. M. Zavada

J. Appl. Phys. 94, 400 (2003); http://dx.doi.org/10.1063/1.1579114 (7 pages) | Cited 28 times

Online Publication Date: 20 June 2003

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The effects of hydrogen plasma treatment on high-quality bulk n-ZnO crystals were studied. It is shown that after plasma exposure at 200 °C for 0.5 h the hydrogen penetrates into the material down to about 20 μm and shows concentrations close to 1017 cm−3 in that region. The incorporation of this hydrogen coincides with an increase in the shallow donor concentration to about the same level as the concentration of hydrogen. In contrast to that in most other semiconductor materials, hydrogen plasma treatment of ZnO is shown to increase the concentration of the already existing electron and hole traps and to introduce electron traps near 0.55 eV, earlier observed in proton irradiated samples. The effect is at least partially due to the surface damage caused by plasma exposure. Despite this increase in the density of deep traps, the luminescence intensity in the near band-edge region is shown to increase down to the depth corresponding to the hydrogen penetration depth in the material. © 2003 American Institute of Physics.
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71.55.Gs II-VI semiconductors
72.80.Ey III-V and II-VI semiconductors
78.60.Hk Cathodoluminescence, ionoluminescence
52.77.-j Plasma applications
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Effects of piezoelectricity and spontaneous polarization on localized excitons in self-formed InGaN quantum dots

Jun-jie Shi and Zi-zhao Gan

J. Appl. Phys. 94, 407 (2003); http://dx.doi.org/10.1063/1.1576490 (9 pages) | Cited 23 times

Online Publication Date: 20 June 2003

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Exciton states confined in wurtzite InxGa1−xN/GaN strained quantum dots (QDs) are investigated within the framework of effective-mass approximation and variational approach, including three-dimensional confinement of the electrons and holes in QDs and a strong built-in electric field effect due to the piezoelectricity and spontaneous polarization. The relationship between exciton states and structural parameters of QDs is studied in detail. Our results show that the In-rich QDs-like are formed spontaneously due to In compositional fluctuations in the InxGa1−xN layer. The strong built-in electric field in InxGa1−xN/GaN strained QDs gives rise to a marked reduction of the effective band gap of QDs and leads to a remarkable electron–hole spatial separation. This effect has a strong influence on exciton states and optical properties of QDs especially for the QDs with large height (⩾5 nm) along the grown direction of the heterostructures. A good agreement has been obtained between the calculated and measured emission wavelengths for different InxGa1−xN/GaN strained QDs. © 2003 American Institute of Physics.
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73.21.La Quantum dots
71.35.Cc Intrinsic properties of excitons; optical absorption spectra
77.65.Ly Strain-induced piezoelectric fields
78.67.Hc Quantum dots
68.65.Hb Quantum dots (patterned in quantum wells)
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
77.22.Ej Polarization and depolarization

Modulation of charge transport in diamond-based layers

A. Serra, D. Manno, T. Siciliano, G. Micocci, A. Tepore, M. Rossi, M. L. Terranova, V. Sessa, S. Piccirillo, and S. Orlanducci

J. Appl. Phys. 94, 416 (2003); http://dx.doi.org/10.1063/1.1579542 (7 pages) | Cited 2 times

Online Publication Date: 20 June 2003

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Doping of diamond by substitutional insertion of metallic species or production of diamond/metals nanocomposite layers has been obtained by a hybrid chemical vapor deposition based technique. The potential of such an approach makes it possible to obtain a wide class of purposely designed diamond-based structures characterized by specific properties of charge transport. Reflection high-energy electron diffraction, scanning electron microscopy and x-ray dispersive spectrometry have been used to study the structural and compositional characteristics of some Nd-, W- and Ti-containing diamond films. The peculiar electrical properties conferred to the host diamond layers by the insertion of various metals have been investigated in the range of 25–500 K by performing Hall effect and conductivity measurements. The mechanism of charge transport and the electrical properties of these materials are found to be mainly governed by organization of the metallic species, which can be in different forms, such as dispersion at the atomic scale and the distribution of isolated clusters or aggregates localized at grain boundaries. Depending on the microstructure, the resulting materials can behave as p-type semiconductors, characterized by resistivity values as low as 3.3×10−3 Ω cm and high values of Hall mobility, or show metal-like conduction, with resistivity as low as 2.2×10−1 Ω cm. The insertion of metallic species does not perturb the crystalline quality of the host diamond matrix and, consequently, the layers produced combine the outstanding properties of diamond with electrical behavior that can be modulated for specific applications. © 2003 American Institute of Physics.
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73.61.Cw Elemental semiconductors
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
68.55.-a Thin film structure and morphology
68.55.Nq Composition and phase identification
61.72.Mm Grain and twin boundaries
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
72.20.My Galvanomagnetic and other magnetotransport effects

Temperature dependence of the impact ionization coefficients in GaAs, cubic SiC, and zinc-blende GaN

Louis Tirino, Michael Weber, Kevin F. Brennan, Enrico Bellotti, and Michele Goano

J. Appl. Phys. 94, 423 (2003); http://dx.doi.org/10.1063/1.1579129 (8 pages)

Online Publication Date: 20 June 2003

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In this article we discuss the effect of temperature on the impact ionization coefficients in wide band-gap semiconductors and compare it to that of bulk GaAs. The impact ionization coefficients as a function of temperature are examined for three semiconductors: gallium arsenide, cubic phase silicon carbide, and zinc-blende phase gallium nitride. It is found that the magnitude of the phonon energy is principally responsible for changes in the impact ionization coefficients as a result of temperature change. While the energy band gap of a material does have a temperature dependence that directly affects the impact ionization transition rate, that change is much smaller than the relative change in the magnitude of the phonon-scattering rates in all of the materials studied here. The phonon energies are found to play a vital role in the magnitude of the change in the scattering rates as a function of temperature. Materials with relatively small phonon energies have phonon scattering rates that change considerably with temperature, and therefore have impact ionization coefficients that also change considerably with changes in temperature. Conversely, the phonon scattering rate in materials with a large phonon energy is less affected and thus these materials have impact ionization coefficients that are relatively insensitive to changes in the temperature. © 2003 American Institute of Physics.
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72.20.Ht High-field and nonlinear effects
81.05.Ea III-V semiconductors
81.05.Hd Other semiconductors
71.38.-k Polarons and electron-phonon interactions
72.20.Fr Low-field transport and mobility; piezoresistance
72.20.Dp General theory, scattering mechanisms
72.80.Ey III-V and II-VI semiconductors
63.20.K- Phonon interactions
72.80.Jc Other crystalline inorganic semiconductors

Effects of Mg accumulation on chemical and electronic properties of Mg-doped p-type GaN surface

Tamotsu Hashizume

J. Appl. Phys. 94, 431 (2003); http://dx.doi.org/10.1063/1.1580195 (6 pages) | Cited 7 times

Online Publication Date: 20 June 2003

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Chemical and electronic properties of Mg-doped p-GaN surfaces were systematically investigated by x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). The doping density of Mg ranged from 3×1019 to 9×1019 cm−3. The XPS and AES analyses revealed the accumulation of Mg for all the air-exposed and chemically treated p-GaN surfaces. The apparent density of Mg calculated from the XPS integrated intensity and the AES intensity was more than one order higher than the value in bulk determined by secondary ion mass spectroscopy. Mg accumulation as well as large amounts of oxides made up the disordered region on the p-GaN:Mg surfaces. Large surface band bending of 1.2–1.6 eV was found at the p-GaN surfaces even after treatment in KOH and NH4OH solutions, due to the existence of high-density surface states. It was found that electron cyclotron resonance assisted N2 -plasma treatment at 300 °C for 1 min is very effective in removing such surface disordered regions and reducing surface band bending. © 2003 American Institute of Physics.
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61.72.S- Impurities in crystals
68.47.Fg Semiconductor surfaces
81.05.Ea III-V semiconductors
68.35.Dv Composition, segregation; defects and impurities
71.55.Eq III-V semiconductors
79.60.Bm Clean metal, semiconductor, and insulator surfaces
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
79.20.Fv Electron impact: Auger emission
73.20.Hb Impurity and defect levels; energy states of adsorbed species
73.20.At Surface states, band structure, electron density of states

Effects of confined longitudinal optical phonons on the exciton binding energy in a cubic quantum dot

B. El Amrani, M. Fliyou, L. Bensaid, T. Lamcharfi, K. Rahmani, and M. Bouayad

J. Appl. Phys. 94, 437 (2003); http://dx.doi.org/10.1063/1.1580190 (6 pages) | Cited 2 times

Online Publication Date: 20 June 2003

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The binding energy of an exciton in a cubic quantum dot has been calculated with and without including the electron (hole)-confined longitudinal optical phonon interactions, using a variational approach and within the effective mass approximation. The quantum confinement effect is described by an infinitely deep well in the envelope-function approximation. The charge carrier phonon coupling is treated within the adiabatic approximation. As the dot size increases, the results show that the contribution of the confined longitudinal optical phonon on the binding energy decreases and the correction to the excitonic state increases. © 2003 American Institute of Physics.
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73.21.La Quantum dots
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
73.22.Lp Collective excitations
71.35.Lk Collective effects (Bose effects, phase space filling, and excitonic phase transitions)
68.65.Hb Quantum dots (patterned in quantum wells)

Low temperature nanoscopic kinetics of hydrogen plasma-enhanced crystallization of a-Si:H films

Yu. L. Khait, R. Weil, R. Beserman, F. Edelman, W. Beyer, and B. Rech

J. Appl. Phys. 94, 443 (2003); http://dx.doi.org/10.1063/1.1574598 (11 pages) | Cited 2 times

Online Publication Date: 20 June 2003

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A nanoscopic kinetic model of controlled plasma-assisted microcrystallite formation (PAμCF) of Si in pre deposited a-Si:H films at low temperatures is proposed. The model suggests mechanisms for enhancement of the Si crystallization in a-Si:H films at low temperatures by treatment of the films in plasma. The model reveals certain kinetic advantages of hydrogen plasmas for the formation of Si crystalline nuclei in a-Si:H compared to other plasmas (Ar plasma, etc.). These advantages make the hydrogen plasma substantially more efficient in the PAμCF of Si in a-Si:H films. The proposed mechanism for PAμCF of Si is associated with the formation on the surface of the a-Si:H film and in the adjacent nanometer material layer of nanoscale (picosecond) short-lived hot spots of high energy density (or effective temperature). The hot spots are generated in the material by energetic plasma ions of energy εis=20–100 eV accelerated by the electrical field in the thin plasma layer near the solid surface. The hot spots promote Si crystallization in a-Si:H. It is shown how the plasma composition, energy, mass, and fluxes of the plasma ions impinging on the surface of the a-Si:H film determine the Si nucleation rate and density of Si microcrystallization. © 2003 American Institute of Physics.
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61.43.Dq Amorphous semiconductors, metals, and alloys
68.55.A- Nucleation and growth
52.77.-j Plasma applications
68.47.Fg Semiconductor surfaces
61.80.Jh Ion radiation effects
61.82.Fk Semiconductors

Physical mechanism for saturation of persistent photoconductivity in a GaAs–AlAs/GaAs single heterojunction

S. Prasad

J. Appl. Phys. 94, 454 (2003); http://dx.doi.org/10.1063/1.1581343 (11 pages) | Cited 1 time

Online Publication Date: 20 June 2003

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The saturation in persistent photoconductivity (PPC) in a Si-doped GaAs–AlAs/GaAs heterostructure is investigated using magnetotransport technique at 4.2 K. Against the donor density of 1.2×1013 cm−2, the maximum electron concentration including the contribution from the two-dimensional channel after 1.41 eV illumination is found to be 7.9×1011 cm−2. This result is anomalous as the DX states are expected to respond to 1.41 eV photons so as to bring the electron density close to the donor density. In order to investigate this issue a high electron-mobility transistor has been fabricated. The channel of this device contains a Hall-bridge pattern. The capacitance–voltage in conjunction with quantum transport measurements reveals that at saturation of PPC a second channel of electron gas is present in GaAs–AlAs superlattice. Furthermore, this channel is a nonconducting one as the electrons in it do not participate in the transport process. The reason behind it is the presence of potential barriers that separate the electron gas in the superlattice (EGS) from the alloyed regions meant for ohmic contacts to two-dimensional electron gas (2DEG). The formation of these potential barriers is linked to the lateral diffusion of In atoms used to make ohmic contacts to 2DEG. At saturation of PPC the electron density of EGS and 2DEG together is found to be 2.2×1012 cm−2. The remaining donors are believed to form electronically inactive complexes. At saturation of PPC the two-dimensional electron density (n2D) is determined by the tunneling probability through the potential barrier adjacent to the alloyed region. During the course of this investigation n2D at 4.2 K measured in the dark was found to depend on the cooldown rate. This phenomenon is attributed to the formation of a nonconducting channel. © 2003 American Institute of Physics.
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72.40.+w Photoconduction and photovoltaic effects
81.05.Ea III-V semiconductors
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.63.-b Electronic transport in nanoscale materials and structures
73.21.Cd Superlattices
85.30.Tv Field effect devices
85.60.-q Optoelectronic devices
72.20.My Galvanomagnetic and other magnetotransport effects
75.47.Pq Other materials
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Theory of electron-mobility degradation caused by roughness with long correlation length in strained-silicon devices

Isao Kitagawa, Takuya Maruizumi, and Nobuyuki Sugii

J. Appl. Phys. 94, 465 (2003); http://dx.doi.org/10.1063/1.1582231 (6 pages) | Cited 3 times

Online Publication Date: 20 June 2003

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To study the influence of surface roughness of a strained-Si/SiGe heterostructure on electron mobility, electron-transport properties were investigated with the ensemble Monte Carlo method. This investigation assumed an electron-motion-deviation model, in which the direction of momentum of electrons is deflected according to the roughness in the channel. It was found that (i) degradation of electron mobility appears even if the correlation length of the roughness is larger than 100 nm and (ii) significant degradation of drift velocity occurs when the direction of momentum of electrons is changed by increasing roughness amplitude. It is, therefore, concluded that decreasing the amplitude of the roughness with long correlation length is the appropriate way to increase electron mobility. © 2003 American Institute of Physics.
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85.30.Tv Field effect devices
73.50.Dn Low-field transport and mobility; piezoresistance
85.30.De Semiconductor-device characterization, design, and modeling
73.25.+i Surface conductivity and carrier phenomena
02.70.Uu Applications of Monte Carlo methods

Light-induced bias stress reversal in polyfluorene thin-film transistors

A. Salleo and R. A. Street

J. Appl. Phys. 94, 471 (2003); http://dx.doi.org/10.1063/1.1581352 (9 pages) | Cited 111 times

Online Publication Date: 20 June 2003

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Gate bias-stress effects in the high-performance semiconducting polymer poly-9,9′ dioctyl-fluorene-co-bithiophene (F8T2) were studied. The bias stress in F8T2 was characterized in devices having various gate dielectric materials—different types of SiO2 and a polymer—and a variety of chemically modified dielectric/semiconductor interfaces. A bias-stress effect was reversed by illuminating the transistor structure with band gap radiation. The recovery rate was directly related to the absorption characteristics of F8T2. We conclude that bias stress in F8T2 is due to hole charge trapping inside the polymer, close to the dielectric interface and not to a structural change in the polymer, or to charge in the dielectric. © 2003 American Institute of Physics.
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85.30.Tv Field effect devices
73.61.Ph Polymers; organic compounds

Electronic structure analysis of Zr silicate and Hf silicate films by using spatially resolved valence electron energy-loss spectroscopy

Nobuyuki Ikarashi and Kenzo Manabe

J. Appl. Phys. 94, 480 (2003); http://dx.doi.org/10.1063/1.1580642 (7 pages) | Cited 25 times

Online Publication Date: 20 June 2003

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Electronic structures near the band gaps of Zr silicate and Hf silicate thin films were investigated experimentally and theoretically. We show that the electronic structure of Zr silicate can be reproduced by a superposition of the electronic structures of ZrO2 and SiO2. Similarly, the electronic structure of Hf silicate can be reproduced by a superposition of the electronic structures of HfO2 and SiO2. This indicates that, in these silicates, the lowest conduction band states are composed mostly of d states of Zr or Hf, and the valence band states mostly of O 2p states. The similarity of the electronic structures of these silicates can be attributed to the similarity of the chemical natures of Zr and Hf atoms. Consequently, when these silicate films are used as gate dielectrics in metal–oxide–semiconductor transistors, the gate leakage current could be strongly affected by d states of Zr or Hf. © 2003 American Institute of Physics.
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71.20.Ps Other inorganic compounds
79.20.Uv Electron energy loss spectroscopy
77.55.-g Dielectric thin films
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
68.49.Jk Electron scattering from surfaces

Enhanced electron emission from carbon nanotubes through density control using in situ plasma treatment of catalyst metal

Jong Hyung Choi, Sun Hong Choi, Jae-Hee Han, Ji-Beom Yoo, Chong-Yun Park, Taewon Jung, SeGi Yu, In-Taek Han, and J. M. Kim

J. Appl. Phys. 94, 487 (2003); http://dx.doi.org/10.1063/1.1581377 (4 pages) | Cited 33 times

Online Publication Date: 20 June 2003

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We controlled the density of carbon nanotubes (CNTs) through in situ NH3 plasma pretreatment and investigated field emission properties with the density variation. Ni catalytic layer was transformed into small nanoparticles with NH3 plasma pretreatment time and power. As NH3 plasma pretreatment time was increased, the growth rate of grown CNTs was gradually decreased. Also, the density of CNTs reduced from 2×109 to 8×106/cm2 with an increase in NH3 plasma pretreatment time from 10 to 30 min for the Ni layer of 10 Å. With a decrease in the density of CNTs, the emission current density was increased and turn on electric field was decreased. We obtained large and uniform emission current (about 9 mA/emission area of 0.49 cm2) from CNTs film with the density of 8×106/cm2. © 2003 American Institute of Physics.
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79.70.+q Field emission, ionization, evaporation, and desorption
85.45.Db Field emitters and arrays, cold electron emitters
81.07.De Nanotubes
81.16.Hc Catalytic methods
52.77.Dq Plasma-based ion implantation and deposition
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
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