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3 Dec 2007

Volume 91, Issue 23, Articles (23xxxx)

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Appl. Phys. Lett. 91, 232101 (2007); http://dx.doi.org/10.1063/1.2818712 (3 pages)

T. Kita, D. Chiba, Y. Ohno, and H. Ohno
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A few-electron vertical In0.56Ga0.44As quantum dot with an insulating gate

T. Kita, D. Chiba, Y. Ohno, and H. Ohno

Appl. Phys. Lett. 91, 232101 (2007); http://dx.doi.org/10.1063/1.2818712 (3 pages)

Online Publication Date: 3 December 2007

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Using an Al2O3 gate insulator by atomic layer deposition and air-bridge drain electrode, we fabricated a quantum dot with few electrons based on an In0.56Ga0.44As resonant tunneling diode structure. Artificial atomic properties manifested themselves in magnetotransport, enabling the determination of effective electron g factors. Results show that the insulating gate structure used here is effective for realizing quantum dots made of narrow-gap semiconductors for studying spin-related phenomena.
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85.30.Kk Junction diodes
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
85.75.-d Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields

Kinetics of Ni3Si2 formation in the Ni2Si–NiSi thin film reaction from in situ measurements

J. A. Kittl, M. A. Pawlak, C. Torregiani, A. Lauwers, C. Demeurisse, C. Vrancken, P. P. Absil, S. Biesemans, C. Detavernier, J. Jordan-Sweet, and C. Lavoie

Appl. Phys. Lett. 91, 232102 (2007); http://dx.doi.org/10.1063/1.2822411 (3 pages) | Cited 3 times

Online Publication Date: 3 December 2007

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The kinetics of Ni3Si2 formation in the Ni2Si–NiSi thin film reaction were determined from simultaneous in situ x-ray diffraction (XRD) measurements, performed using a synchrotron source, and sheet resistance measurements. Samples consisted of 90 nm Ni/100 nm polycrystalline-Si/SiO2 stacks, of interest for fully silicided gate applications, on (100) Si. After initial formation of a Ni2Si/NiSi bilayer, these films reacted to form Ni3Si2. The evolution of sheet resistance and of the intensity of XRD peaks were used to extract the fraction of Ni3Si2 formed during ramp and isothermal annealings. A Kissinger analysis was performed for ramp annealing with ramp rates of 1, 3, 5, 9, and 27 °C/s, obtaining the activation energy of Ni3Si2 formation, Ea = 1.92±0.15 eV. A Kolmogorov-Johnson-Mehl-Avrami analysis was performed for isothermal anneals, finding an Avrami exponent of 2.1±0.2, suggesting two-dimensional growth. This is consistent with a nucleation controlled process for Ni3Si2 formation, with nucleation sites at different positions in the thin film, and subsequent lateral two-dimensional propagation of the transformation front parallel to the film surface. Implications for Ni fully silicided gate applications are discussed.
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81.40.Gh Other heat and thermomechanical treatments
73.61.At Metal and metallic alloys

Low nonalloyed Ohmic contact resistance to nitride high electron mobility transistors using N-face growth

Man Hoi Wong, Yi Pei, Tomás Palacios, Likun Shen, Arpan Chakraborty, Lee S. McCarthy, Stacia Keller, Steven P. DenBaars, James S. Speck, and Umesh K. Mishra

Appl. Phys. Lett. 91, 232103 (2007); http://dx.doi.org/10.1063/1.2820381 (3 pages) | Cited 14 times

Online Publication Date: 3 December 2007

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Nonalloyed Ohmic contacts on Ga-face n+-GaN/AlGaN/GaN high electron mobility transistor (HEMT) structures typically have significant contact resistance to the two-dimensional electron gas (2DEG) due to the AlGaN barrier. By growing the HEMT structure inverted on the N-face, electrons from the contacts were able to access the 2DEG without going through an AlGaN layer. A low contact resistance of 0.16 Ω mm and specific contact resistivity of 5.5×10−7 Ω cm2 were achieved without contact annealing on the inverted HEMT structure.
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85.30.Tv Field effect devices
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

Tungsten nanocrystal memory devices improved by supercritical fluid treatment

C. H. Chen, T. C. Chang, I. H. Liao, P. B. Xi, C. T. Tsai, P. Y. Yang, Joe Hsieh, Jason Chen, U. S. Chen, and J. R. Chen

Appl. Phys. Lett. 91, 232104 (2007); http://dx.doi.org/10.1063/1.2803937 (3 pages) | Cited 3 times

Online Publication Date: 4 December 2007

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A supercritical CO2 (SCCO2) fluid technique is proposed to improve electrical characteristics for W nanocrystal nonvolatile memory devices, since the thickness and quality of tunnel oxide are critical issues for the fabrication of nonvolatile memory devices. After SCCO2 treatments, C-V curves are restored to normal, as well as the leakage current of W nanocrystal memory devices are reduced significantly. It reveals that W nanocrystal memory devices could be formed with shorter oxidation time, moreover, dangling bonds and trapping states initially created within an incomplete oxidized film will be efficiently repaired after SCCO2 treatment.
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85.35.-p Nanoelectronic devices

Kondo effect in a semiconductor quantum dot coupled to ferromagnetic electrodes

K. Hamaya, M. Kitabatake, K. Shibata, M. Jung, M. Kawamura, K. Hirakawa, T. Machida, T. Taniyama, S. Ishida, and Y. Arakawa

Appl. Phys. Lett. 91, 232105 (2007); http://dx.doi.org/10.1063/1.2820445 (3 pages) | Cited 27 times

Online Publication Date: 4 December 2007

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Using a laterally fabricated quantum-dot (QD) spin-valve device, we experimentally study the Kondo effect in the electron transport through a semiconductor QD with an odd number of electrons (N). In a parallel magnetic configuration of the ferromagnetic electrodes, the Kondo resonance at N = 3 splits clearly without external magnetic fields. With applying magnetic fields (B), the splitting is gradually reduced, and then the Kondo effect is almost restored at B = 1.2 T. This means that, in the Kondo regime, an inverse effective magnetic field of B ∼ 1.2 T can be applied to the QD in the parallel magnetic configuration of the ferromagnetic electrodes.
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73.63.Kv Quantum dots
73.40.Sx Metal-semiconductor-metal structures
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
72.10.Fk Scattering by point defects, dislocations, surfaces, and other imperfections (including Kondo effect)

Metallic conductivity at the CaHfO3/SrTiO3 interface

Keisuke Shibuya, Tsuyoshi Ohnishi, Mikk Lippmaa, and Masaharu Oshima

Appl. Phys. Lett. 91, 232106 (2007); http://dx.doi.org/10.1063/1.2816907 (3 pages) | Cited 9 times

Online Publication Date: 5 December 2007

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The transport and photoluminescence properties of interfaces between amorphous CaHfO3 films and SrTiO3 single crystal substrates were investigated. Although both materials are band-gap insulators, the interfaces were metallic. The sheet carrier density was found to depend strongly on the laser fluence used during the CaHfO3 layer fabrication by pulsed laser deposition, indicating the presence of oxygen vacancies at the interface. The Hall mobility of the heterostructures saturated at 2000 cm2/V s at low temperature. Photoluminescence spectra showed an oxygen vacancy-related emission band at around 420 nm.
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73.40.-c Electronic transport in interface structures
78.55.Hx Other solid inorganic materials
78.66.Nk Insulators
73.61.Ng Insulators
72.20.My Galvanomagnetic and other magnetotransport effects
81.15.Fg Pulsed laser ablation deposition

Substrate engineering for high-performance surface-channel III-V metal-oxide-semiconductor field-effect transistors

Yi Xuan, Peide D. Ye, and Tian Shen

Appl. Phys. Lett. 91, 232107 (2007); http://dx.doi.org/10.1063/1.2822892 (3 pages) | Cited 28 times

Online Publication Date: 5 December 2007

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High-performance inversion-type enhancement-mode n-channel In0.65Ga0.35As metal-oxide-semiconductor field-effect transistors (MOSFETs) with atomic-layer-deposited Al2O3 as gate dielectric are demonstrated. A 0.5 μm gate-length MOSFET with an Al2O3 gate oxide thickness of 10 nm shows a gate leakage current less than 5×10−6 A/cm2 at 4 V gate bias, a threshold voltage of 0.40 V, a maximum drain current of 670 mA/mm, and transconductance of 230 mS/mm at drain voltage of 2 V. More importantly, a model is proposed to ascribe this 80% improvement of device performance from In0.53Ga0.47As MOSFETs mainly to lowering the energy level difference between the charge neutrality level and conduction band minimum for In0.65Ga0.35As. The right substrate or channel engineering is the main reason for the high performance of the devices besides the high-quality oxide-semiconductor interface.
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85.30.Tv Field effect devices

Enhanced photoresponse in ZnO nanowires decorated with CdTe quantum dot

R. S. Aga, Jr., D. Jowhar, A. Ueda, Z. Pan, W. E. Collins, R. Mu, K. D. Singer, and J. Shen

Appl. Phys. Lett. 91, 232108 (2007); http://dx.doi.org/10.1063/1.2822896 (3 pages) | Cited 18 times

Online Publication Date: 5 December 2007

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The photoresponse of ZnO nanowires irradiated with photons having energies below the band gap of ZnO (3.4 eV) was studied before and after deposition of CdTe quantum dots via a pulsed electron-beam technique. The small amount of deposited CdTe did not increase the dark current of the samples. However, a substantial increase in the steady state photocurrent was observed after CdTe deposition suggesting a clear photosensitization effect. Results revealed that CdTe influences the photoconductivity transients of ZnO by minimizing its interaction with oxygen in air as well as providing additional traps that serve to increase the photocurrent time constant.
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81.07.Vb Quantum wires
81.07.Ta Quantum dots
81.05.Dz II-VI semiconductors
78.67.Hc Quantum dots
78.67.Lt Quantum wires
78.55.Et II-VI semiconductors

Weak ferromagnetism in Cu-doped GaN

A. L. Rosa and R. Ahuja

Appl. Phys. Lett. 91, 232109 (2007); http://dx.doi.org/10.1063/1.2819602 (3 pages) | Cited 19 times

Online Publication Date: 6 December 2007

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We employ density-functional theory within the generalized-gradient approximation to study the structural and electronic properties of Cu-doped GaN. We find that spin polarization on the Cu atoms in the GaN lattice is very small, leading to rather weak ferromagnetic behavior. We therefore suggest that Cu-doped GaN is unsuitable for spintronics applications. Our results refute previous calculations, which reported robust ferromagnetism in Cu-doped GaN.
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72.25.Dc Spin polarized transport in semiconductors
71.20.Nr Semiconductor compounds
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
61.66.Fn Inorganic compounds
75.50.Pp Magnetic semiconductors

Meyer-Neldel rule in ZnO

Heidemarie Schmidt, Maria Wiebe, Beatrice Dittes, and Marius Grundmann

Appl. Phys. Lett. 91, 232110 (2007); http://dx.doi.org/10.1063/1.2819603 (3 pages) | Cited 4 times

Online Publication Date: 6 December 2007

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Seventy years ago Meyer and Neldel investigated four polycrystalline n-type conducting ZnO rods [ W. Meyer and H. Neldel, Z. Tech. Phys. (Leipzig) 12, 588 (1937) ]. The specific conductivity increased exponentially with temperature. A linear relationship between the thermal activation energy for the specific conductivity and the logarithm of the prefactor was observed. Since then thermally activated processes revealing this behavior are said to follow the Meyer-Neldel (MN) rule. We show that the emission of charge carriers from deep electron traps in ZnO follows the MN rule with the isokinetic temperature amounting to 226±4 K.
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71.55.Gs II-VI semiconductors
72.80.Ey III-V and II-VI semiconductors
81.05.Dz II-VI semiconductors
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.20.Fr Low-field transport and mobility; piezoresistance

Influence of physisorbed water on the conductivity of hydrogen terminated silicon-on-insulator surfaces

G. Dubey, G. P. Lopinski, and F. Rosei

Appl. Phys. Lett. 91, 232111 (2007); http://dx.doi.org/10.1063/1.2822417 (3 pages) | Cited 9 times

Online Publication Date: 6 December 2007

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The sheet resistance of hydrogen terminated silicon-on-insulator substrates increases significantly with time in air due to depletion of free carriers, attributed to the growth of electrically active defects as the surface oxidizes. Surprisingly, physisorbed water (via adsorption from ambient or controlled exposure in vacuum) causes an increase in the conductivity. This effect is largely reversible when the water layer is displaced by inert gas purging, heating, or pumping. The observed conductivity changes are correlated with Hall voltage changes, indicating that the adsorbed water layer induces accumulation of majority carriers on n-doped substrates.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
68.43.-h Chemisorption/physisorption: adsorbates on surfaces
81.65.Mq Oxidation

Selective etching of Pt with respect to PtSi using a sacrificial low temperature germanidation process

N. Breil, A. Halimaoui, T. Skotnicki, E. Dubois, G. Larrieu, A. Łaszcz, J. Ratajczak, G. Rolland, and A. Pouydebasque

Appl. Phys. Lett. 91, 232112 (2007); http://dx.doi.org/10.1063/1.2821143 (3 pages) | Cited 2 times

Online Publication Date: 6 December 2007

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A soft and scalable etching procedure that selectively eliminates Pt without altering PtSi is proposed. The selective etch is based on the low temperature transformation of the excess Pt into a more reactive PtxGey phase that is easily etched in a sulfuric peroxide mixture. The mechanism of PtxGey alloying is detailed based on x-ray diffraction analysis. The innocuousness of the germanidation-based selective etch on the integrity of the PtSi/Si junction is consolidated by Schottky barrier measurements. This process is expected to facilitate the integration and the scalability of PtSi on ultrathin silicon layers.
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81.65.Cf Surface cleaning, etching, patterning
73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Ns Metal-nonmetal contacts

Hydrodynamic model for relaxation of optically injected currents in quantum wells

R. M. Abrarov, E. Ya. Sherman, and J. E. Sipe

Appl. Phys. Lett. 91, 232113 (2007); http://dx.doi.org/10.1063/1.2805385 (3 pages) | Cited 4 times

Online Publication Date: 6 December 2007

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We use a hydrodynamic model to describe the relaxation of optically injected currents in quantum wells on a picosecond time scale, numerically solving the continuity and velocity evolution equations with the Hermite-Gaussian functions employed as a basis. The interplay of the long-range Coulomb forces and nonlinearity in the equations of motion leads to rather complex patterns of the calculated charge and current densities. We find that the time dependence of even the first moment of the electron density is sensitive to this complex evolution.
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73.63.Hs Quantum wells
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Modulation of high current gain (β>49) light-emitting InGaN/GaN heterojunction bipolar transistors

B. F. Chu-Kung, C. H. Wu, G. Walter, M. Feng, N. Holonyak, Jr., T. Chung, J.-H. Ryou, and R. D. Dupuis

Appl. Phys. Lett. 91, 232114 (2007); http://dx.doi.org/10.1063/1.2821380 (3 pages) | Cited 4 times

Online Publication Date: 6 December 2007

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The electrical and optical characteristics of high-gain, small-area InGaN/GaN heterojunction bipolar transistors (HBTs) grown by metal-organic chemical vapor deposition on sapphire substrate are reported. The common-emitter current-voltage characteristics of a 3×10 μm2 emitter device demonstrates a current gain β = ΔIC/ΔIB = 49 at 3 mA and breakdown voltage, BVCEO>70 V. The radiative recombination spectrum of a large area 100×100 μm2 emitter HBT is measured, showing a peak at 387 nm and a full width at half maximum of 47 nm. A 1 kHz modulation input is applied to the HBT and both the optical and electrical outputs of a large area device is demonstrated.
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85.30.Pq Bipolar transistors
85.60.Jb Light-emitting devices

Realization of p-type conduction in undoped MgxZn1−xO thin films by controlling Mg content

Y. F. Li, B. Yao, Y. M. Lu, Z. P. Wei, Y. Q. Gai, C. J. Zheng, Z. Z. Zhang, B. H. Li, D. Z. Shen, X. W. Fan, and Z. K. Tang

Appl. Phys. Lett. 91, 232115 (2007); http://dx.doi.org/10.1063/1.2816914 (3 pages) | Cited 22 times

Online Publication Date: 7 December 2007

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Undoped MgxZn1−xO thin films with Mg content of 0 ⩽ x ⩽ 0.20 were grown on c-sapphire substrate by plasma-assisted molecular beam epitaxy. The MgxZn1−xO shows n-type conduction in Mg content of x ⩽ 0.05, and the carrier concentration decreases slowly from 1018 to 1017 cm−3 with increasing Mg content. However, as x ≥ 0.10, the MgxZn1−xO begins to show p-type conduction, and the carrier concentration goes down sharply to 1015 cm−3 firstly and then increases slowly with increasing Mg content from 1015 to 1016 cm−3. The mechanism of transformation from n to p type and change of the carrier concentrations with Mg content were investigated by photoluminescence and absorption measurements as well as first-principle calculation.
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73.61.Ga II-VI semiconductors
81.05.Dz II-VI semiconductors
78.55.Et II-VI semiconductors
78.66.Hf II-VI semiconductors

Deep traps responsible for hysteresis in capacitance-voltage characteristics of AlGaN/GaN heterostructure transistors

A. Y. Polyakov, N. B. Smirnov, A. V. Govorkov, A. V. Markov, A. M. Dabiran, A. M. Wowchak, A. V. Osinsky, B. Cui, P. P. Chow, and S. J. Pearton

Appl. Phys. Lett. 91, 232116 (2007); http://dx.doi.org/10.1063/1.2823607 (3 pages) | Cited 12 times

Online Publication Date: 7 December 2007

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The origin of hysteresis in capacitance-voltage (C-V) characteristics was studied for Schottky diodes prepared on AlGaN/GaN transistor structures with GaN (Fe) buffers. The application of reverse bias leads to a shift of C-V curves toward higher positive voltages. The magnitude of the effect is shown to increase for lower temperatures. The phenomenon is attributed to tunneling of electrons from the Schottky gate to localized states in the structure. A technique labeled “reverse” deep level transient spectroscopy was used to show that the deep traps responsible for the hysteresis have activation energies of 0.25, 0.6, and 0.9 eV. Comparison with deep trap spectra of GaN buffers and Si doped n-GaN films prepared on GaN buffers suggests that the traps in question are located in the buffer layer.
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85.30.Kk Junction diodes
85.30.Tv Field effect devices

Conduction band offset at the InN/GaN heterojunction

Kejia (Albert) Wang, Chuanxin Lian, Ning Su, Debdeep Jena, and John Timler

Appl. Phys. Lett. 91, 232117 (2007); http://dx.doi.org/10.1063/1.2821378 (3 pages) | Cited 22 times

Online Publication Date: 7 December 2007

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The conduction-band offset between GaN and InN is experimentally determined. InN/n-type GaN isotype heterojunctions grown by molecular beam epitaxy are observed to exhibit Schottky-junction like behavior based on rectifying vertical current flow. From capacitance-voltage measurements on the heterojunction, the Schottky barrier height is found to be ∼ 0.94 eV. The photocurrent spectroscopy measurement by backside illumination reveals an energy barrier height of 0.95 eV across the heterojunction, consistent with the capacitance measurement. By combining electrical transport, capacitance-voltage, and photocurrent spectroscopy measurement results, the conduction band offset between InN and GaN is estimated to be ΔEC = 1.68±0.1 eV.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
73.30.+y Surface double layers, Schottky barriers, and work functions
72.80.Ey III-V and II-VI semiconductors
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