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13 Sep 2010

Volume 97, Issue 11, Articles (11xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 97, 113701 (2010); http://dx.doi.org/10.1063/1.3487998 (3 pages)

Sarah E. Baker, Michael D. Pocha, Allan S. P. Chang, Donald J. Sirbuly, Stefano Cabrini, Scott D. Dhuey, Tiziana C. Bond, and Sonia E. Létant
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Experimental and simulation study of breakdown voltage enhancement of AlGaN/GaN heterostructures by Si substrate removal

Domenica Visalli, Marleen Van Hove, Puneet Srivastava, Joff Derluyn, Johan Das, Maarten Leys, Stefan Degroote, Kai Cheng, Marianne Germain, and Gustaaf Borghs

Appl. Phys. Lett. 97, 113501 (2010); http://dx.doi.org/10.1063/1.3488024 (3 pages) | Cited 8 times

Online Publication Date: 13 September 2010

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The breakdown mechanism in GaN-based heterostructures (HFETs) grown on silicon substrate is investigated in detail by TCAD simulations and silicon substrate removal technique. High-voltage electrical measurements show that the breakdown voltage saturates for larger gate-drain distances. This failure mechanism is dominated by the avalanche breakdown in the Si substrate. High-voltage TCAD simulations of AlGaN/GaN/Si substrate structures show higher impact ionization factor and electron density at the Si interface indicating a leakage current path where avalanche breakdown occurs. Experimentally, by etching off the Si substrate the breakdown voltage no longer saturates and linearly increases for all gate-drain gaps. We propose the silicon removal technique as a viable way to enhance the breakdown voltage of AlGaN/GaN devices grown on Si substrate.
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73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
77.22.Jp Dielectric breakdown and space-charge effects
72.20.Ht High-field and nonlinear effects
73.20.At Surface states, band structure, electron density of states

Mn-doped nanocrystals in light-emitting diodes: Energy-transfer to obtain electroluminescence from quantum dots

Arup K. Rath, Saikat Bhaumik, and Amlan J. Pal

Appl. Phys. Lett. 97, 113502 (2010); http://dx.doi.org/10.1063/1.3489099 (3 pages) | Cited 11 times

Online Publication Date: 14 September 2010

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We fabricate light-emitting diodes (LEDs) based on Mn-doped ZnS nanocrystals along with hole-transporting N,N′ bis(3-methylphenyl)-N,N′-diphenyl-benzidine (TPD). With Mn-doping, ZnS nanostructures exhibit a strong photoluminescence. The LEDs exhibit electroluminescence (EL) from Mn-doped ZnS quantum dots and TPD. In order to open up channels for energy-transfer from TPD to quantum dots and to achieve EL from only the nanoparticles, we grow core-shell nanoparticles with Mn-doped ZnS in the core and CdS as the shell layer. Excitons formed in TPD can now transfer their energy directly to the shell-layer to yield EL from only the nanoparticles.
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78.60.Fi Electroluminescence
78.55.Et II-VI semiconductors
61.72.uj III-V and II-VI semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.U- Doping and impurity implantation
71.20.Nr Semiconductor compounds
68.65.Hb Quantum dots (patterned in quantum wells)
73.21.La Quantum dots
73.63.Kv Quantum dots
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)

Sonic gradient index lens for aqueous applications

Theodore P. Martin, Michael Nicholas, Gregory J. Orris, Liang-Wu Cai, Daniel Torrent, and José Sánchez-Dehesa

Appl. Phys. Lett. 97, 113503 (2010); http://dx.doi.org/10.1063/1.3489373 (3 pages) | Cited 14 times

Online Publication Date: 14 September 2010

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We study the acoustic scattering properties of a phononic crystal designed to behave as a gradient index lens in water, both experimentally and theoretically. The gradient index lens is designed using a square lattice of stainless-steel cylinders based on a multiple scattering approach in the homogenization limit. We experimentally demonstrate that the lens follows the graded index equations derived for optics by mapping the pressure intensity generated from a spherical source at 20 kHz. We find good agreement between the experimental result and theoretical modeling based on multiple scattering theory.
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43.58.Ls Acoustical lenses and microscopes
43.20.Dk Ray acoustics
43.20.Fn Scattering of acoustic waves

Instability in threshold voltage and subthreshold behavior in Hf–In–Zn–O thin film transistors induced by bias-and light-stress

Khashayar Ghaffarzadeh, Arokia Nathan, John Robertson, Sangwook Kim, Sanghun Jeon, Changjung Kim, U-In Chung, and Je-Hun Lee

Appl. Phys. Lett. 97, 113504 (2010); http://dx.doi.org/10.1063/1.3480547 (3 pages) | Cited 17 times

Online Publication Date: 14 September 2010

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Electrical bias and light stressing followed by natural recovery of amorphous hafnium-indium-zinc-oxide (HIZO) thin film transistors with a silicon oxide/nitride dielectric stack reveals defect density changes, charge trapping and persistent photoconductivity (PPC). In the absence of light, the polarity of bias stress controls the magnitude and direction of the threshold voltage shift (ΔVT), while under light stress, VT consistently shifts negatively. In all cases, there was no significant change in field-effect mobility. Light stress gives rise to a PPC with wavelength-dependent recovery on time scale of days. We observe that the PPC becomes more pronounced at shorter wavelengths.
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85.30.Tv Field effect devices
72.40.+w Photoconduction and photovoltaic effects
77.22.-d Dielectric properties of solids and liquids

Determination of the interface δ-hole density in a blue-emitting organic semiconductor diode by electromodulated absorption spectroscopy

Mi Zhou, Rui-Qi Png, Sankaran Sivaramakrishnan, Perq-Jon Chia, Chaw-Keong Yong, Lay-Lay Chua, and Peter K. H. Ho

Appl. Phys. Lett. 97, 113505 (2010); http://dx.doi.org/10.1063/1.3467469 (3 pages) | Cited 3 times

Online Publication Date: 16 September 2010

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The hole density at the interface of a poly(fluorene-alt-triarylamine) (TFB) with p-doped poly(3,4-ethylenedioxythiophene) (PEDT) anodes has been determined from electromodulation of the TFB subgap polaron band. At 295 K, this δ-hole density σOSC is approximately 1×1012 cm−2, for which the current–voltage characteristics indicate an Ohmic contact. However at 30 K, σOSC falls to 2×1011 cm−2, and the contact characteristics approach the injecting-blocking boundary. The PEDT/TFB vacuum offset inside the device is inferred to be 0.1 eV, and so the Fermi level is not as deeply pinned in the TFB gap as suggested by ultraviolet photoemission measurements.
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85.60.Jb Light-emitting devices

Hot carrier solar cells: Achievable efficiency accounting for heat losses in the absorber and through contacts

A. Le Bris and J.-F. Guillemoles

Appl. Phys. Lett. 97, 113506 (2010); http://dx.doi.org/10.1063/1.3489405 (3 pages) | Cited 10 times

Online Publication Date: 16 September 2010

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The hot carrier solar cell enables the photovoltaic efficiency limit to be approached by tapping into what are normally heat losses. Previous models neglected thermalization in the absorber and assumed ideally energy selective contacts that allow minimum heat losses upon carrier extraction. The proposed improved model includes both realistic contacts and thermalization rates. The heat flux due to carrier extraction is computed. Results show that spectrally broad semiselective contacts are compatible with an efficiency exceeding the single junction limit, which would considerably facilitate the realization of the device.
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88.40.H- Solar cells (photovoltaics)
88.40.J- Types of solar cells

Structural characterization of amorphous Ta2O5 and SiO2–Ta2O5 used as solid electrolyte for nonvolatile switches

Naoki Banno, Toshitsugu Sakamoto, Noriyuki Iguchi, Masashi Matsumoto, Hideto Imai, Toshinari Ichihashi, Shinji Fujieda, Kazuhiko Tanaka, Satoshi Watanabe, Shu Yamaguchi, Tsuyoshi Hasegawa, and Masakazu Aono

Appl. Phys. Lett. 97, 113507 (2010); http://dx.doi.org/10.1063/1.3488830 (3 pages)

Online Publication Date: 17 September 2010

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Diffusivity of Cu in amorphous (a-) Ta2O5 is increased by low temperature annealing above 350 °C but the increase is suppressed by adding SiO2 to Ta2O5. To clarify the reasons, we investigated the structural difference between a-Ta2O5 and a-SiO2–Ta2O5. The results show that the low temperature annealing does not cause polycrystallization of Ta2O5 but purges weakly bonded oxygen atoms from the bulk and decreases the film density. Adding SiO2 to Ta2O5 is shown to increase the coordination number of Ta–O, which results in the improved thermal stability.
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61.43.Er Other amorphous solids
81.40.Gh Other heat and thermomechanical treatments
82.45.Gj Electrolytes
64.70.dg Crystallization of specific substances
84.32.Dd Connectors, relays, and switches
66.30.H- Self-diffusion and ionic conduction in nonmetals
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