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1 Oct 1999

Volume 86, Issue 7, pp. 3497-4057

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Theory of phase stabilities and bonding mechanisms in stoichiometric and substoichiometric molybdenum carbide

Håkan W. Hugosson, Olle Eriksson, Lars Nordström, Ulf Jansson, Lars Fast, Anna Delin, John M. Wills, and Börje Johansson

J. Appl. Phys. 86, 3758 (1999); http://dx.doi.org/10.1063/1.371284 (10 pages) | Cited 21 times

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First principles, total energy methods have been applied to predict the relative stabilities of the four experimentally verified MoC phases: the cubic δ(NaCl) phase and the three hexagonal γ(WC), η and γ′(TiAs) phases. The effect of vacancies on the relative stability of these four phases was investigated using a model structure with ordered vacancies within the carbon sublattice. For stoichiometric MoC, the γ phase was found to be the most stable followed by γ′, δ, and η, but for substoichiometric MoC0.75, the order of relative stability was changed and the substoichiometric δ phase was found to have the lowest energy followed by γ′ and γ. A study of the electronic structure revealed vacancy induced peaks in the density of state and the electron density attached to these peaks was analyzed and found to emanate from unscreened Mo–Mo bonds through the carbon vacancy site. Finally, the oxygen stabilization of the γ′ MoC phase was studied. © 1999 American Institute of Physics.
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71.15.Nc Total energy and cohesive energy calculations
71.15.-m Methods of electronic structure calculations
61.72.J- Point defects and defect clusters
61.50.Lt Crystal binding; cohesive energy
71.20.Ps Other inorganic compounds
64.70.-p Specific phase transitions
81.30.Dz Phase diagrams of other materials
61.66.Bi Elemental solids
61.66.Dk Alloys

Band-gap and k.p. parameters for GaAlN and GaInN alloys

S. K. Pugh, D. J. Dugdale, S. Brand, and R. A. Abram

J. Appl. Phys. 86, 3768 (1999); http://dx.doi.org/10.1063/1.371285 (5 pages) | Cited 17 times

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Using a semi-empirical pseudopotential method, a set of band-structure calculations are performed on a range of GaInN and GaAlN alloys in both the zinc-blende and wurtzite structures. Pseudopotentials for the bulk materials are described by suitable V(q) functions, and these are used to construct the alloy pseudopotentials. The band gap as a function of alloy composition is studied, and it is found that there is no significant bowing in the case of GaAlN. The bowing is larger for GaInN, although heavily dependent on the strain present. A more detailed study of the wurtzite alloys is carried out for low Al and In fractions. Wurtzite k⋅p parameters for several alloys at concentrations commonly used in devices are obtained from the semi-empirical band structure using a Monte Carlo fitting procedure. © 1999 American Institute of Physics.
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71.20.Nr Semiconductor compounds
71.15.-m Methods of electronic structure calculations
71.15.Dx Computational methodology (Brillouin zone sampling, iterative diagonalization, pseudopotential construction)

Rapidly convergent expansion method for calculating the effective conductivity of three-dimensional lattices of symmetric inclusions

Constantin Simovski and Sailing He

J. Appl. Phys. 86, 3773 (1999); http://dx.doi.org/10.1063/1.371286 (7 pages) | Cited 5 times

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An exact method is introduced to determine the electric potential in an infinite rectangular lattice of particles described by curvilinear coordinates in which Laplace's equation separates. The potential is expanded in harmonic functions, and suitable auxiliary functions are used to obtain an infinite system of linear algebraic equations for the expansion coefficients. Special attention is paid to lattices of spheres and prolate spheroids. For these cases, the truncated system converges very rapidly as the number of terms in the truncation series increases. The method works well for calculating the effective conductivity for dense or sparse inclusions, and for highly conducting lattices or lattices of cavities. Numerical results for the effective conductivity are given and compared with data obtained by other methods. © 1999 American Institute of Physics.
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72.10.Bg General formulation of transport theory
61.72.Qq Microscopic defects (voids, inclusions, etc.)

Effects of doping on the transport properties of CoSb3

H. Anno, K. Matsubara, Y. Notohara, T. Sakakibara, and H. Tashiro

J. Appl. Phys. 86, 3780 (1999); http://dx.doi.org/10.1063/1.371287 (7 pages) | Cited 48 times

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Effects of doping on the transport properties of CoSb3 have been systematically investigated using Ni, Pd, and Pt as donor impurities. It is shown that the Hall mobility, the Seebeck coefficient, and the electrical conductivity depend strongly not only on the carrier concentration but also on these donor impurities. Our theoretical analysis suggests that the electron effective mass and the conduction band deformation potential are significantly affected by both the doping levels and the donor impurities. These doping effects in CoSb3 can be attributed to (1) the changes in the electronic structure with doping and (2) the specific nature of the conduction band structure, in particular, the nonparabolicity of the band which can be explained in terms of a two-band Kane model. The observed changes in the electronic properties with doping are also consistent with the predictions of a recent band structure calculation of CoSb3. On the other hand, the lattice thermal conductivity decreases markedly with increasing carrier concentration, and is almost independent of the donor impurities. Our analysis based on the Debye model indicates that the coupling of the point-defect (alloy) scattering with the electron-phonon scattering plays an important role in reducing the lattice thermal conductivity in heavily doped n-type CoSb3. The effects of doping on the phonon scattering are also discussed on the basis of a model calculation as a function of the electronic properties and the impurity properties (atomic mass and size). As a result, it is found that the strength of the electron-lattice interaction (the electron-phonon coupling), which is closely related to the effective mass and the deformation potential, is an important factor affecting the scattering of phonons as well as charge carriers in heavily doped n-type CoSb3. © 1999 American Institute of Physics.
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72.20.Fr Low-field transport and mobility; piezoresistance
72.20.My Galvanomagnetic and other magnetotransport effects
72.20.Dp General theory, scattering mechanisms
72.20.Pa Thermoelectric and thermomagnetic effects
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
71.55.Ht Other nonmetals
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
72.80.Ga Transition-metal compounds

Quantized current jumps in silicon photoconductors at room temperature

Y. Haddab, P.-A. Besse, and R. S. Popovic

J. Appl. Phys. 86, 3787 (1999); http://dx.doi.org/10.1063/1.371288 (5 pages)

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We report on the observation of quantized jumps due to single-carrier trapping and detrapping at defect states in silicon photoconductors of 103 μm3 in volume. A specifically designed electrical test structure in a low-doped (2×1014 cm−3) silicon crystal was fabricated. It consists in four substrate resistances connected in a Wheatstone bridge. After the exposure to light, the bridge offset voltage recovers its equilibrium value with steps of 5–10 μV, corresponding to the emission or capture of a single carrier. Such structures also display random telegraph signals in the dark, with steps of similar amplitude. This behavior is observed with structures processed on Czochralski-grown substrates and not with those processed on float-zone substrates. Simple calculations based on quantized free carrier concentration variations corroborate the above measurements. © 1999 American Institute of Physics.
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72.80.Cw Elemental semiconductors
72.40.+w Photoconduction and photovoltaic effects
71.55.Cn Elemental semiconductors
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Role of oxygen vacancy defect states in the n-type conduction of β-Ga2O3

Zoltán Hajnal, József Miró, Gábor Kiss, Ferenc Réti, Péter Deák, Roy C. Herndon, and J. Michael Kuperberg

J. Appl. Phys. 86, 3792 (1999); http://dx.doi.org/10.1063/1.371289 (5 pages) | Cited 39 times

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Based on semiempirical quantum-chemical calculations, the electronic band structure of β-Ga2O3 is presented and the formation and properties of oxygen vacancies are analyzed. The equilibrium geometries and formation energies of neutral and doubly ionized vacancies were calculated. Using the calculated donor level positions of the vacancies, the high temperature n-type conduction is explained. The vacancy concentration is obtained by fitting to the experimental resistivity and electron mobility. © 1999 American Institute of Physics.
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71.55.Ht Other nonmetals
72.80.Jc Other crystalline inorganic semiconductors
61.72.J- Point defects and defect clusters
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
71.20.Nr Semiconductor compounds
72.20.Fr Low-field transport and mobility; piezoresistance

Low-dose radiation effects in extrinsic photoconductors

M. Patrashin, N. Hiromoto, B. Fouks, I. A. Maslov, and V. M. Ledenev

J. Appl. Phys. 86, 3797 (1999); http://dx.doi.org/10.1063/1.371290 (7 pages) | Cited 1 time

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Experimental results and a model describing long-term radiation-induced changes in extrinsic photoconductors are presented. Experiments, carried out at low temperatures and low photon backgrounds, simulated typical operating conditions of extrinsic infrared detectors used in space- based platforms and instruments. The observed changes in the carrier lifetime and current–voltage characteristics at a low electric field, as well as a long-term relaxation of the effects after removal of the irradiation are related to a recharging of the impurities in the bulk during the irradiation. The proposed model agrees well with experimental data and despite its simplicity, provides relevant information on how to deal with these effects in practical applications. © 1999 American Institute of Physics.
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61.82.Fk Semiconductors
72.40.+w Photoconduction and photovoltaic effects
73.50.Pz Photoconduction and photovoltaic effects
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
85.60.Gz Photodetectors (including infrared and CCD detectors)
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths

Study of direct tunneling through ultrathin gate oxide of field effect transistors using Monte Carlo simulation

E. Cassan, S. Galdin, P. Dollfus, and P. Hesto

J. Appl. Phys. 86, 3804 (1999); http://dx.doi.org/10.1063/1.371291 (8 pages) | Cited 13 times

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Direct tunneling gate currents of ultrathin gate oxide thickness metal oxide semiconductor field effect transistors (MOSFETs) are modeled in a two-step calculation procedure based on the treatment of physical microscopic data acquired during Monte Carlo device simulation. Gate currents are obtained by weighting the carrier perpendicular energy distribution at the Si/SiO2 and N+-poly–Si/SiO2 interfaces by the electron transmission probability, which is calculated by the one-dimensional Schrödinger equation resolution with the transfer-matrix method. The procedure is applied to a 0.07 μm gate length and 1.5 nm gate oxide thickness transistor, for which the gate and drain voltage influences on gate currents are studied by assuming at first a uniform gate oxide layer. It is shown that the maximum gate current is obtained for one of the two static points of complementary metal oxide semiconductor inverters: VGS = VDD and VDS = 0, which raises a severe problem of standby power consumption. The contribution of hot carriers to the tunnel current is evaluated and is found to be small in case of such ultrathin oxide n-MOSFETs: contrary to thick (>5 nm) gate oxide transistors, the maximum gate current is not linked to the carrier energy peak in the channel but is located near the source well where the electron concentration is the largest. Oxide thickness fluctuations are then considered by meshing the oxide surface area and assuming a Gaussian law for the local oxide thickness deviation to the mean value. It is shown that a correct agreement is achieved with experimental published data when the oxide film nonuniformity is included in the calculation. Gate currents mapping for different bias conditions are given and analyzed, which show that very high current densities run through the oxide layer in the vicinity of weak points. An estimate of the surface through which flows the major part of the current is made, and a link between the highly nonuniform current leakage and the soft-breakdown mechanism of the oxide layer is proposed. © 1999 American Institute of Physics.  
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85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling
02.70.Rr General statistical methods
73.40.Gk Tunneling
02.50.Ng Distribution theory and Monte Carlo studies
85.40.-e Microelectronics: LSI, VLSI, ULSI; integrated circuit fabrication technology

Amorphous and microcrystalline silicon films grown at low temperatures by radio-frequency and hot-wire chemical vapor deposition

P. Alpuim, V. Chu, and J. P. Conde

J. Appl. Phys. 86, 3812 (1999); http://dx.doi.org/10.1063/1.371292 (10 pages) | Cited 58 times

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The effect of hydrogen dilution on the optical, transport, and structural properties of amorphous and microcrystalline silicon thin films deposited by hot-wire (HW) chemical vapor deposition and radio-frequency (rf) plasma-enhanced chemical vapor deposition using substrate temperatures (Tsub) of 100 and 25 °C is reported. Microcrystalline silicon (μc-Si:H) is obtained using HW with a large crystalline fraction and a crystallite size of ∼30 nm for hydrogen dilutions above 85% independently of Tsub. The deposition of μc-Si:H by rf, with a crystallite size of ∼8 nm, requires increasing the hydrogen dilution and shows decreasing crystalline fraction as Tsub is decreased. The photoconductivity, defect density, and structure factor of the amorphous silicon films (a-Si:H) are strongly improved by the use of hydrogen dilution in the Tsub range studied. a-Si:H films with a photoconductivity-to-dark conductivity ratio above 105, a deep defect density below 1017 cm−3, an Urbach energy below 60 meV and a structure factor below 0.1 were obtained for rf films down to 25 °C (at growth rates ∼0.1–0.4 Å/s) and for HW films down to 100 °C (at growth rates ∼10 Å/s), using the appropriate hydrogen dilution. In the low Tsub range studied, the growth mechanism, film properties, and the amorphous to microcrystalline silicon transition depend on the flux of atomic hydrogen available. The properties of the films are compared to those of samples produced at 175 and 250 °C in the same reactors. © 1999 American Institute of Physics.
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73.61.Cw Elemental semiconductors
78.66.Db Elemental semiconductors and insulators
68.55.-a Thin film structure and morphology
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
71.20.Mq Elemental semiconductors
81.05.Cy Elemental semiconductors
73.61.Jc Amorphous semiconductors; glasses
78.66.Jg Amorphous semiconductors; glasses
81.05.Gc Amorphous semiconductors
52.77.Bn Etching and cleaning
52.77.Dq Plasma-based ion implantation and deposition
72.40.+w Photoconduction and photovoltaic effects

Linear electroabsorption in semi-insulating GaAs/AlGaAs asymmetric double quantum wells

María Aguilar, M. Carrascosa, F. Agulló-López, F. Agulló-Rueda, M. R. Melloch, and D. D. Nolte

J. Appl. Phys. 86, 3822 (1999); http://dx.doi.org/10.1063/1.371293 (4 pages) | Cited 5 times

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Electroabsorption has been investigated in semi-insulating asymmetric GaAs/AlGaAs double quantum wells presenting high linear Stark responses, adequate for photorefractive applications. We have used the envelope function approximation to calculate the linear Stark shifts of the energy levels and select a suitable structure for the experimental study. The experimental data indicate that the response to the applied field critically depends on a complicated interplay of effects that compete or cooperate to suppress or enhance the electroabsorption. For positive field polarity, the competing contributions of the overlapping e1–hh1 and e1–hh2 transitions partially cancel the electroabsorption despite large linear Stark shifts. On the other hand, small negative fields induce large electroabsorption because the Stark shifts of the two transitions have opposite signs. © 1999 American Institute of Physics.
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78.66.Fd III-V semiconductors
73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
78.20.Jq Electro-optical effects
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Microstructural investigation of oxidized Ni/Au ohmic contact to p-type GaN

Li-Chien Chen, Fu-Rong Chen, Ji-Jung Kai, Li Chang, Jin-Kuo Ho, Charng-Shyang Jong, Chien C. Chiu, Chao-Nien Huang, Chin-Yuen Chen, and Kwang-Kuo Shih

J. Appl. Phys. 86, 3826 (1999); http://dx.doi.org/10.1063/1.371294 (7 pages) | Cited 51 times

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The microstructure of oxidized Ni/Au films on p-GaN was examined to elucidate the formation of a low resistance ohmic contact to p-GaN with a field-emission gun transmission electron microscope in conjunction with composition analyses. The p-GaN/Ni/Au samples were heat treated at 500 °C in air mainly composed of a mixture of crystalline NiO, Au, and amorphous Ni–Ga–O phases. Small voids adjacent to the p-GaN film were also observed. The as-deposited Au film converted into discontinuous islands containing small amounts of Ni that connect with p-GaN. NiO formed a continuous film at the surface that covers the Au islands and the amorphous Ni–Ga–O phases. Moreover, NiO partially contacts p-GaN as well as Au islands and the amorphous Ni–Ga–O phase. The orientation relationship of the crystalline NiO, Au-rich islands, and p-GaN film was identified as NiO(111)//Au(11math)//GaN(0002) and NiO[1math0]//Au[1math0]//GaN[11math0]. The results suggested that Ni atoms diffuse through the Au layer onto the surface and react with oxygen to form NiO, whereas Au atoms diffuse towards the inside to form a Au–Ni alloy. The microstructural examination indicated that the crystalline NiO and/or the amorphous Ni–Ga–O phases may significantly affect the low resistance ohmic contact to p-GaN. © 1999 American Institute of Physics.
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73.40.Ns Metal-nonmetal contacts
68.35.Ct Interface structure and roughness

Simulation studies of persistent photoconductivity and filamentary conduction in opposed contact semi-insulating GaAs high power switches

R. P. Joshi, P. Kayasit, N. Islam, E. Schamiloglu, C. B. Fleddermann, and J. Schoenberg

J. Appl. Phys. 86, 3833 (1999); http://dx.doi.org/10.1063/1.371295 (11 pages) | Cited 6 times

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A self-consistent, two-dimensional, time-dependent, drift-diffusion model is developed to simulate the response of high power photoconductive switches. Effects of spatial inhomogeneities associated with the contact barrier potential are incorporated and shown to foster filamentation. Results of the dark current match the available experiment data. Persistent photoconductivity is shown to arise at a high bias even under the conditions of spatial uniformity. Filamentary currents require an inherent spatial inhomogeneity, and are more likely to occur for low optical excitation. Under strong uniform illumination, the spatial nonuniformities were quenched as a result of a polarization-induced collapse in the internal fields. However, strong electric fields resulting at the contacts create a bipolar plasma, and hence, a virtual “double injection.” © 1999 American Institute of Physics.
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73.61.Ey III-V semiconductors
73.50.Pz Photoconduction and photovoltaic effects
85.30.Hi Surface barrier, boundary, and point contact devices
85.30.De Semiconductor-device characterization, design, and modeling

Coulomb blockade thermometry using a two-dimensional array of tunnel junctions

Tobias Bergsten, Tord Claeson, and Per Delsing

J. Appl. Phys. 86, 3844 (1999); http://dx.doi.org/10.1063/1.371296 (4 pages) | Cited 8 times

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We have measured current–voltage characteristics of two-dimensional (2D) arrays of small tunnel junctions at temperatures from 1.5 to 4.2 K. This corresponds to thermal energies larger than the charging energy. We show that 2D arrays can be used as primary thermometers in the same way as one-dimensional (1D) arrays, and even have some advantages over 1D arrays. We have carried out Monte Carlo simulations, which agree with our experimental results. © 1999 American Institute of Physics.
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07.20.Dt Thermometers
73.23.Hk Coulomb blockade; single-electron tunneling
02.50.Ng Distribution theory and Monte Carlo studies
02.70.Rr General statistical methods

Interfacial electronic density of states in phthalocyanine derivative Langmuir–Blodgett films determined by surface potential measurement

Haruo Kokubo, Yutaka Oyama, Yutaka Majima, and Mitsumasa Iwamoto

J. Appl. Phys. 86, 3848 (1999); http://dx.doi.org/10.1063/1.371297 (5 pages) | Cited 4 times

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Surface potentials of phthalocyanine derivative Langmuir–Blodgett (LB) films on metal electrodes were studied. Two kinds of phthalocyanine derivatives (C6PcCu and CuttbPc) were examined as a function of the number of deposited layers. The distributions of spatial excess charge density and electronic density of states at the metal/film interface were determined. The difference in the electrostatic phenomena observed in these two phthalocyanine LB films was attributed to the difference in the density of surface states of these two films. © 1999 American Institute of Physics.
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73.20.At Surface states, band structure, electron density of states
68.18.-g Langmuir-Blodgett films on liquids
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