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28 Jul 2008

Volume 93, Issue 4, Articles (04xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 93, 043101 (2008); http://dx.doi.org/10.1063/1.2963352 (3 pages)

Xin Fu, Jun Jiang, Wenzheng Zhang, and Jun Yuan
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Observation of interdot energy transfer between InAs quantum dots

K. Nishibayashi, T. Kawazoe, M. Ohtsu, K. Akahane, and N. Yamamoto

Appl. Phys. Lett. 93, 042101 (2008); http://dx.doi.org/10.1063/1.2945289 (3 pages) | Cited 9 times

Online Publication Date: 28 July 2008

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We observed interdot excitation energy transfer between vertically aligned InAs quantum dots (QDs) separated by a 24-nm-thick spacer layer. This transfer was explained by resonant energy transfer via an optical near-field interaction between the first excited state of small QDs and the second excited states of large QDs. The excitation intensity dependence of the photoluminescence intensities showed that the energy transfer times were 75 ps at 15 K and 4 ns at 77 K. Our experimental results indicated that the III-V compound semiconductor QDs are appropriate for nanophotonic switching devices.
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78.67.Hc Quantum dots
78.55.Cr III-V semiconductors
73.21.La Quantum dots
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials

Role of structural defects in the unipolar resistive switching characteristics of Pt/NiO/Pt structures

Chanwoo Park, Sang Ho Jeon, Seung Chul Chae, Seungwu Han, Bae Ho Park, Sunae Seo, and Dong-Wook Kim

Appl. Phys. Lett. 93, 042102 (2008); http://dx.doi.org/10.1063/1.2963983 (3 pages) | Cited 24 times

Online Publication Date: 28 July 2008

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We investigated the resistive switching characteristics of two types of Pt/NiO/Pt structures with epitaxial and polycrystalline NiO layers. Both of these Pt/NiO/Pt structures exhibited unipolar resistive switching. Pt/epitaxial-NiO/Pt showed unstable switching or no resistance state change after several repeated runs. Pt/polycrystalline-NiO/Pt showed very reproducible switching. The experimental data indicated that microstructural defects (e.g., grain boundaries) played crucial roles in the reliability of the unipolar resistive switching behavior. This was further supported by first-principles calculations.
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73.40.Rw Metal-insulator-metal structures
68.65.Ac Multilayers
68.35.Ct Interface structure and roughness
68.35.Dv Composition, segregation; defects and impurities
61.72.Mm Grain and twin boundaries

Enhancement of thermoelectric properties of sputtered Bi–Sb–Te thin films by electric current stressing

Chien-Neng Liao, Kuen-Ming Liou, and Hsu-Shen Chu

Appl. Phys. Lett. 93, 042103 (2008); http://dx.doi.org/10.1063/1.2965487 (3 pages) | Cited 12 times

Online Publication Date: 28 July 2008

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Both electrical conductivity and Seebeck coefficient of sputtered Bi–Sb–Te films were enhanced by introducing a high density of electric current through the films during thermal annealing. The electrically stressed Bi–Sb–Te films were found to have lower carrier concentration but much higher mobility than the films thermally treated at the same temperatures. A mechanism based on electromigration-induced preferential Sb diffusion is proposed to explain the observed electrical transport properties and precipitation of Sb-rich phase in the electrically stressed Bi–Sb–Te films. The study shall lead to an effective strategy of improving thermoelectric properties of Bi–Sb–Te films by electric current stressing.
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73.61.At Metal and metallic alloys
72.15.Jf Thermoelectric and thermomagnetic effects
73.50.Lw Thermoelectric effects
66.30.Qa Electromigration
81.40.Gh Other heat and thermomechanical treatments

Electron mobility in N-polar GaN/AlGaN/GaN heterostructuress

David F. Brown, Siddharth Rajan, Stacia Keller, Yun-Hao Hsieh, Steven P. DenBaars, and Umesh K. Mishra

Appl. Phys. Lett. 93, 042104 (2008); http://dx.doi.org/10.1063/1.2965483 (3 pages) | Cited 5 times

Online Publication Date: 29 July 2008

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The drift mobility of two-dimensional electron gasses in N-polar GaN/AlGaN/GaN heterostructures was measured with capacitance and resistance measurements using gated transmission line method structures. A decrease in mobility with increasing reverse bias of the gate was observed. A variational wave function was used to calculate the mobility limited by optical phonon, alloy disorder, and ionized impurity scattering as a function of gate bias, and found to match the experimental data well. Three distinct regimes where phonon, alloy, and impurity scattering are dominant were observed.
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73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
72.20.Fr Low-field transport and mobility; piezoresistance
72.10.Fk Scattering by point defects, dislocations, surfaces, and other imperfections (including Kondo effect)

Current percolation in ultrathin channel nanocrystalline silicon transistors

X. Guo, S. R. P. Silva, and T. Ishii

Appl. Phys. Lett. 93, 042105 (2008); http://dx.doi.org/10.1063/1.2965807 (3 pages)

Online Publication Date: 29 July 2008

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The ultrathin channel nanocrystalline silicon transistor shows greatly improved switching performance and has demonstrated its candidacy for low power applications. In this work, by careful observation of the current-voltage and threshold voltage characteristics, we find that current percolation occurs when the channel is thinner than 3.0 nm due to strong quantum confinement induced large potential variations over the channel. We show that the device channel width must be at least 0.3 μm to avoid percolative “pinch off” for 0.5 μm channel length devices. Theoretical analysis performed on the devices agrees well with the experimental data and provides important guidelines to model and optimize the devices for circuit design.
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85.30.Tv Field effect devices
85.30.De Semiconductor-device characterization, design, and modeling
85.35.-p Nanoelectronic devices
73.50.Dn Low-field transport and mobility; piezoresistance
73.61.Cw Elemental semiconductors

Direct observation of oxygen movement during resistance switching in NiO/Pt film

Chikako Yoshida, Kentaro Kinoshita, Takahiro Yamasaki, and Yoshihiro Sugiyama

Appl. Phys. Lett. 93, 042106 (2008); http://dx.doi.org/10.1063/1.2966141 (3 pages) | Cited 58 times

Online Publication Date: 29 July 2008

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We demonstrate that both a low resistance state and a high resistance state can be written by bipolar voltage application in a local region of NiO/Pt films by using conducting atomic force microscopy. To investigate how oxygen played a role in the resistance switching phenomenon, a local writing process in 18O tracer gas atmosphere was carried out and the composition change was examined by time-of-flight secondary ion mass spectroscopy. As a result, it was revealed that oxygen moves to the anode side, and the composition of the NiO surface might change thereby causing the change in resistance.
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73.61.Ng Insulators
78.66.Nk Insulators
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces

Seebeck coefficients of n-type (Ga,In)(N,As), (B,Ga,In)As, and GaAs

G. Homm, P. J. Klar, J. Teubert, and W. Heimbrodt

Appl. Phys. Lett. 93, 042107 (2008); http://dx.doi.org/10.1063/1.2959079 (3 pages) | Cited 6 times

Online Publication Date: 30 July 2008

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We measured the Seebeck coefficients of n-type (Ga,In)(N,As), (B,Ga,In)As, and GaAs epitaxial layers with doping concentrations ranging from 1017 to 1019 cm−3 in the temperature range between 50 and 290 K. Despite the significant differences in electronic structure between the nonamalgamation type quaternary alloys and the binary GaAs, the temperature dependence of the Seebeck coefficient for samples of similar doping concentration is almost the same for all three semiconductor systems. The finding can be explained by the similarity of the dispersions of the extended phonon states of the three semiconductor systems in conjunction with a dominant phonon drag contribution to the Seebeck coefficient in the temperature range studied.
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73.50.Lw Thermoelectric effects
73.61.Ey III-V semiconductors
71.20.Nr Semiconductor compounds
63.20.dd Measurements
61.72.uj III-V and II-VI semiconductors
61.72.sd Impurity concentration

The great improvement effect of pores on ZT in Co1−xNixSb3 system

Qinyu He, Shejun Hu, Xingui Tang, Yucheng Lan, Jian Yang, Xiaowei Wang, Zhifeng Ren, Qing Hao, and Gang Chen

Appl. Phys. Lett. 93, 042108 (2008); http://dx.doi.org/10.1063/1.2963476 (3 pages) | Cited 12 times

Online Publication Date: 30 July 2008

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In this paper, the Co1−xNixSb3+y system (x = 0.1,0.2; y = 0, 0.05) has been prepared by direct current induced hot press and annealing. Pores were made by annealing pressed Co1−xNixSb3.05 pellets into Co1−xNixSb3 pellets during annealing. It was found that from room temperature to 500 °C, the pores with diameter larger than 1 μm has significantly increased the Seebeck coefficient, considerably decreased the thermal conductivity, almost maintained the electrical conductivity constant, and therefore markedly improved the dimensionless figure of merit (ZT) of Co1−xNixSb3 system. By comparing the samples, we conclude that inducing pores into skutterudites is an effective route to greatly improve the ZT.
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72.15.Jf Thermoelectric and thermomagnetic effects
81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
81.40.Gh Other heat and thermomechanical treatments
72.15.Eb Electrical and thermal conduction in crystalline metals and alloys

Enhanced thermoelectric property originating from additional carrier pocket in skutterudite compounds

Wei-Shu Liu, Li-Dong Zhao, Bo-Ping Zhang, Hai-Long Zhang, and Jing-Feng Li

Appl. Phys. Lett. 93, 042109 (2008); http://dx.doi.org/10.1063/1.2965123 (3 pages) | Cited 5 times

Online Publication Date: 30 July 2008

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The effect of additional carrier pocket (ACP) on the thermoelectric property is investigated based on the multiband model, indicating that an ACP indeed enhances the power factor, and hence the thermoelectric figure of merit. The established model can be used to well explain the experimental observations in skutterudite compounds that a small addition of Ni or Pd significantly enhances power factor. However, the ACP effect works only when the edge energy difference EΔ between major carrier pocket and ACP is less than 5kBT.
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72.15.Jf Thermoelectric and thermomagnetic effects
61.66.Dk Alloys
61.72.up Other materials

Electronic level alignment at the deeply buried absorber/Mo interface in chalcopyrite-based thin film solar cells

M. Bär, S. Nishiwaki, L. Weinhardt, S. Pookpanratana, W. N. Shafarman, and C. Heske

Appl. Phys. Lett. 93, 042110 (2008); http://dx.doi.org/10.1063/1.2955532 (3 pages) | Cited 7 times

Online Publication Date: 31 July 2008

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We have investigated the electronic structure of the absorber/back contact interface for S-free [Cu(In,Ga)Se2 (“CIGSe”)] and S-containing [Cu(In,Ga)(S,Se)2 (“CIGSSe”)] chalcopyrites with direct and inverse photoemission. Comparison of the electronic levels of the cleavage planes reveals a pronounced cliff in the conduction band at the CIG(S)Se/Mo interface. For the valence band, we find a flat alignment and a small spike for the CIGSe- and CIGSSe-based structures, respectively.
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84.60.Jt Photoelectric conversion

A direct measurement of the local resistances in a ZnO tetrapod by means of impedance spectroscopy: The role of the junction in the overall resistance

Junghwan Huh, Gyu-Tae Kim, Jong Soo Lee, and Sangtae Kim

Appl. Phys. Lett. 93, 042111 (2008); http://dx.doi.org/10.1063/1.2963483 (3 pages) | Cited 6 times

Online Publication Date: 31 July 2008

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We have measured the local resistances in the arms and at the junction in a ZnO tetrapod separately by means of ac impedance spectroscopy. The resistance at the junction is found to be even greater than that in the arms although a volume fraction of the junction is negligibly small compared to that of the arms. This result clearly demonstrates that the junction plays a decisive role in the overall electrical property of the ZnO-tetrapod and thus determines the functionality of the ZnO tetrapod electronic devices.
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72.20.Fr Low-field transport and mobility; piezoresistance
72.80.Ey III-V and II-VI semiconductors

Correlation of tellurium inclusions and carrier lifetime in detector grade cadmium zinc telluride

Ezzat S. Elshazly and Gary Tepper

Appl. Phys. Lett. 93, 042112 (2008); http://dx.doi.org/10.1063/1.2967726 (3 pages) | Cited 5 times

Online Publication Date: 31 July 2008

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Carrier lifetimes and tellurium inclusion densities in detector grade cadmium zinc telluride crystals grown by the high pressure Bridgman method were optically measured using pulsed laser microwave cavity perturbation and infrared microscopy. Excess carriers were produced in the material using a pulsed laser with a wavelength of 1064 nm and pulse width of 7 ns, and the electronic decay was measured at room temperature. Spatial mapping of lifetimes and defect densities in cadmium zinc telluride was performed to determine the relationship between tellurium defect density and trapping. A strong correlation was found between the volume fraction of tellurium inclusions and the carrier trapping time.
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29.40.Wk Solid-state detectors
81.10.Fq Growth from melts; zone melting and refining
61.72.Qq Microscopic defects (voids, inclusions, etc.)
72.80.Ey III-V and II-VI semiconductors
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
71.55.Gs II-VI semiconductors

Magnetoresistance near the ferromagnetic-paramagnetic phase transition in magnetic semiconductors

B. Brodowska, I. Kuryliszyn-Kudelska, T. Wojtowicz, M. Arciszewska, W. Dobrowolski, E. I. Slynko, V. E. Slynko, X. Liu, and J. K. Furdyna

Appl. Phys. Lett. 93, 042113 (2008); http://dx.doi.org/10.1063/1.2966148 (3 pages) | Cited 1 time

Online Publication Date: 1 August 2008

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Magnetic and transport properties of ferromagnetic semiconductors (FMSs) near the ferromagnetic-paramagnetic phase transition were studied in two classes of FMSs: in the III–V-based In1−xMnxSb and Ga1−xMnxAs epitaxial films and in IV–VI-based Sn1−xMnxTe and Sn1−x−yMnxEryTe alloys in bulk crystal form. Magnetoresistance data for both FMS classes are in excellent agreement with the theory of Majumdar and Littlewood, indicating that near the Curie temperature, magnetic fluctuations play a key role as scattering centers. These results also show that the Majumdar–Littlewood model can be applied to magnetically very dilute systems.
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72.20.My Galvanomagnetic and other magnetotransport effects
75.50.Pp Magnetic semiconductors
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)

Enzyme-based lactic acid detection using AlGaN/GaN high electron mobility transistors with ZnO nanorods grown on the gate region

B. H. Chu, B. S. Kang, F. Ren, C. Y. Chang, Y. L. Wang, S. J. Pearton, A. V. Glushakov, D. M. Dennis, J. W. Johnson, P. Rajagopal, J. C. Roberts, E. L. Piner, and K. J. Linthicum

Appl. Phys. Lett. 93, 042114 (2008); http://dx.doi.org/10.1063/1.2966158 (3 pages) | Cited 14 times

Online Publication Date: 1 August 2008

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The detection of lactic acid with ZnO nanorod-gated AlGaN/GaN high electron mobility transistors (HEMTs) was demonstrated. The array of ZnO nanorods provided a large effective surface area with a high surface-to-volume ratio and a favorable environment for the immobilization of lactate oxidase. The HEMT drain-source current showed a rapid response when various concentrations of lactic acid solutions were introduced to the gate area of the HEMT sensor. The HEMT could detect lactic acid concentrations from 167 nM to 139 μM. Our results show that portable, fast response, and wireless-based lactic acid detectors can be realized with AlGaN/GaN HEMT based sensors.
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87.85.-d Biomedical engineering
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
82.80.-d Chemical analysis and related physical methods of analysis
85.30.Tv Field effect devices
87.14.ej Enzymes

Effects of metal electrodes on the resistive memory switching property of NiO thin films

C. B. Lee, B. S. Kang, A. Benayad, M. J. Lee, S.-E. Ahn, K. H. Kim, G. Stefanovich, Y. Park, and I. K. Yoo

Appl. Phys. Lett. 93, 042115 (2008); http://dx.doi.org/10.1063/1.2967194 (3 pages) | Cited 46 times

Online Publication Date: 1 August 2008

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The effects of various metal electrodes on the resistive switching of NiO thin films were investigated. Contrary to the belief that Pt is used for its high work function, which enables Ohmic contact to p-type NiO, resistive switching was observed in films with Ta or Al electrodes with a low work function in the as-deposited state. The resistive switching of films with a Ag or Cu top electrode with a low work function and high free energy of oxidation shows the importance of the formation of an oxide layer at the metal/NiO interface.
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73.40.Ns Metal-nonmetal contacts
73.40.Cg Contact resistance, contact potential
84.30.Sk Pulse and digital circuits

Femtosecond acceleration of electrons under high electric fields in bulk GaAs investigated by time-domain terahertz spectroscopy

Y. M. Zhu, T. Unuma, K. Shibata, and K. Hirakawa

Appl. Phys. Lett. 93, 042116 (2008); http://dx.doi.org/10.1063/1.2967857 (3 pages) | Cited 6 times

Online Publication Date: 1 August 2008

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We have investigated terahertz emission from undoped bulk GaAs photoexcited by femtosecond laser pulses under strong bias electric fields, F, up to 300 kV/cm. The initial positive signal in the terahertz waveforms, which is usually interpreted as electron acceleration in the Γ-valley, is found to increase with increasing F for F<50 kV/cm, but to start decreasing gradually above 50 kV/cm. The observed behavior suggests that the effective acceleration mass of electrons significantly increases with increasing F. The mass enhancement is due most likely to band mixing between the Γ- and the higher satellite valleys under high electric fields.
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78.70.Gq Microwave and radio-frequency interactions
78.47.D- Time resolved spectroscopy (>1 psec)
72.80.Ey III-V and II-VI semiconductors
72.20.Fr Low-field transport and mobility; piezoresistance
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