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8 Sep 2008

Volume 93, Issue 10, Articles (10xxxx)

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Appl. Phys. Lett. 93, 101905 (2008); http://dx.doi.org/10.1063/1.2977760 (3 pages)

Mei Wang, Yinwei Li, Tian Cui, Yanming Ma, and Guangtian Zou
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Strain dependence of indirect band gap for strained silicon on insulator wafers

J. Munguía, G. Bremond, J. M. Bluet, J. M. Hartmann, and M. Mermoux

Appl. Phys. Lett. 93, 102101 (2008); http://dx.doi.org/10.1063/1.2978241 (3 pages) | Cited 5 times

Online Publication Date: 8 September 2008

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We have used low temperature photoluminescence measurements in order to quantify the impact of strain effect on the Si indirect band gap in 9 nm thick tensely strained silicon on insulator layers. A redshift of the transverse optical phonon excitonic recombination in the strained silicon layer was evidenced as the strain in the layer is increased. Band gap shrinkages in the Δ direction equal to 130±3 meV, 184±3 meV, and 239±3 meV were obtained for 0.87±0.03%, 1.22±0.05%, and 1.54±0.06% strain values. These measured indirect transitions are in good agreement with the calculated strained silicon indirect band gap values.
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71.20.Mq Elemental semiconductors
71.35.-y Excitons and related phenomena
63.20.kk Phonon interactions with other quasiparticles
78.66.Db Elemental semiconductors and insulators
78.55.Ap Elemental semiconductors

Ti-based nonalloyed Ohmic contacts for Al0.15Ga0.85N/GaN high electron mobility transistors using regrown n+-GaN by plasma assisted molecular beam epitaxy

Hui-Chan Seo, Patrick Chapman, Hyun-Ick Cho, Jung-Hee Lee, and Kyekyoon (Kevin) Kim

Appl. Phys. Lett. 93, 102102 (2008); http://dx.doi.org/10.1063/1.2979702 (3 pages) | Cited 4 times

Online Publication Date: 8 September 2008

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A technique for regrowing n+-GaN layers has been developed to realize nonalloyed Ohmic contacts using plasma assisted molecular beam epitaxy. The contact resistance and device performance were measured of a recessed-gate with the regrowth and of recessed-source/drain AlGaN/GaN high electron mobility transistors (HEMTs). With the regrown n+-GaN layers and recessed drain/source, a low contact resistance of 0.6 Ω mm was obtained for Ti/Au contacts to AlGaN. The peak drain current (IDS,max) and maximum transconductance (gm,max) of the AlGaN/GaN HEMTs with nonalloyed Ohmic contacts were 573 mA/mm and 60 mS/mm, respectively. These results demonstrate that the regrowth of highly doped GaN layers is crucial in achieving low-resistance nonalloyed Ohmic contacts for the HEMT structures.
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85.30.Tv Field effect devices
85.75.Hh Spin polarized field effect transistors

Ultrafast carrier dynamics and terahertz conductivity of photoexcited GaAs under electric field

Qing-li Zhou, Yulei Shi, Bin Jin, and Cunlin Zhang

Appl. Phys. Lett. 93, 102103 (2008); http://dx.doi.org/10.1063/1.2980026 (3 pages) | Cited 11 times

Online Publication Date: 8 September 2008

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The ultrafast carrier dynamics and terahertz conductivity in semi-insulating GaAs have been investigated under electric field (E) by using optical pump-terahertz probe technique. The measurements indicate that the terahertz transmission change induced by the pump pulses at high E is smaller than that without E. We attribute this phenomenon to carrier scattering into the L valley, which leads to a drop in carrier mobility. The calculated transient photoconductivities fit well with the Drude–Smith model, being consistent with our intervalley scattering model.
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72.20.Ee Mobility edges; hopping transport
72.40.+w Photoconduction and photovoltaic effects

Dynamic carrier lifetime imaging of silicon wafers using an infrared-camera-based approach

K. Ramspeck, S. Reissenweber, J. Schmidt, K. Bothe, and R. Brendel

Appl. Phys. Lett. 93, 102104 (2008); http://dx.doi.org/10.1063/1.2972122 (3 pages) | Cited 16 times

Online Publication Date: 9 September 2008

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We present a calibration-free dynamic infrared carrier lifetime mapping technique, yielding images of the carrier lifetime of multicrystalline silicon wafers within seconds. Images of the infrared emission of the sample under test are taken directly after switching on a monochromatic illumination source and after steady-state conditions have been established in the sample. Making use of the proportionality between the infrared emission and the free carrier density inside the sample, the carrier lifetime is calculated from the signal ratio of these two images by an analytical method. We achieve an excellent agreement when comparing our results with carrier lifetime mappings obtained by the microwave-detected photoconductance decay technique.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
84.37.+q Measurements in electric variables (including voltage, current, resistance, capacitance, inductance, impedance, and admittance, etc.)

Adsorption-controlled growth of EuO by molecular-beam epitaxy

R. W. Ulbricht, A. Schmehl, T. Heeg, J. Schubert, and D. G. Schlom

Appl. Phys. Lett. 93, 102105 (2008); http://dx.doi.org/10.1063/1.2973180 (3 pages) | Cited 15 times

Online Publication Date: 10 September 2008

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Using molecular-beam epitaxy, we demonstrate the adsorption-controlled growth of epitaxial EuO films on single crystalline (110) YAlO3 substrates. Four-circle x-ray diffraction (XRD) reveals phase-pure, epitaxial, (001)-oriented films with rocking curve full width at half maxima as narrow as 34 arc sec (0.0097°). The critical thickness for the onset of relaxation of (001) EuO on (110) YAlO3 ( ∼ 2% lattice mismatch) was determined from XRD measurements to be 382±25 Å. A saturation magnetization of 6.96±0.07μB/Eu, a value close to the theoretical limit of 7μB/Eu, is observed.
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68.55.at Other materials
68.43.Mn Adsorption kinetics
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Photoresponse of the Schottky junction Au/SrTiO3:Nb in different resistive states

D. S. Shang, J. R. Sun, L. Shi, and B. G. Shen

Appl. Phys. Lett. 93, 102106 (2008); http://dx.doi.org/10.1063/1.2978240 (3 pages) | Cited 17 times

Online Publication Date: 10 September 2008

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A systematic study on photovoltaic effects has been performed for the Schottky junction Au/SrTiO3:0.05 wt %Nb, the resistance of which can be tuned, by applied electric pulses, between ∼ 1 and ∼ 200 MΩ. It is found that, despite the great change in junction resistance, the photocurrent across the junction is constant when the power and wavelength of incident light are fixed. The corresponding Schottky barrier, deduced from the photoresponse data is ∼ 1.5 eV, independent of junction resistance. This result suggests the invariance of the interfacial barrier during resistance switching and the occurrence of filamentary conduction channels.
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73.30.+y Surface double layers, Schottky barriers, and work functions
72.40.+w Photoconduction and photovoltaic effects
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates

Electrochemical fabrication of ultralow noise metallic nanowires with hcp crystalline lattice

Amrita Singh, T. Phanindra Sai, and Arindam Ghosh

Appl. Phys. Lett. 93, 102107 (2008); http://dx.doi.org/10.1063/1.2979709 (3 pages) | Cited 2 times

Online Publication Date: 10 September 2008

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We experimentally demonstrate that low-frequency electrical noise in silver nanowires is heavily suppressed when the crystal structure of the nanowires is hexagonal closed pack (hcp) rather than face centered cubic (fcc). Using a low-potential electrochemical method we have grown single crystalline silver nanowires with hcp crystal structure, in which the noise at room temperature is two to six orders of magnitude lower than that in the conventional fcc nanowires of the same diameter. We suggest that motion of dislocations is probably the primary source of electrical noise in metallic nanowires, which is strongly diminished in hcp crystals.
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81.07.Bc Nanocrystalline materials
82.45.Yz Nanostructured materials in electrochemistry
61.66.-f Structure of specific crystalline solids
87.80.Kc Electrochemical techniques

Improved contact for thermal and electrical transport in carbon nanofiber interconnects

Tsutomu Saito, Toshishige Yamada, Drazen Fabris, Hirohiko Kitsuki, Patrick Wilhite, Makoto Suzuki, and Cary Y. Yang

Appl. Phys. Lett. 93, 102108 (2008); http://dx.doi.org/10.1063/1.2979710 (3 pages) | Cited 8 times

Online Publication Date: 10 September 2008

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We study the performance and reliability of carbon nanofiber (CNF) interconnects under high-current stress by examining CNF breakdown for four test configurations, suspended/supported with/without tungsten deposition. The use of W is to improve the CNF-electrode contact. The supported cases show a larger current density just before breakdown than the suspended ones, suggesting an effective heat dissipation to the substrate. The W-deposited contacts reduce the initial total resistance from megaohm range without W to kilo-ohms. High-current stress does not change the total resistance of the test structures with W unlike those without W deposition.
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85.40.Ls Metallization, contacts, interconnects; device isolation

High figures of merit and natural nanostructures in Mg2Si0.4Sn0.6 based thermoelectric materials

Q. Zhang, J. He, T. J. Zhu, S. N. Zhang, X. B. Zhao, and T. M. Tritt

Appl. Phys. Lett. 93, 102109 (2008); http://dx.doi.org/10.1063/1.2981516 (3 pages) | Cited 39 times

Online Publication Date: 10 September 2008

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Mg2(Si,Sn) compounds have shown great promise for thermoelectric applications due to good thermoelectric properties, nontoxicity, and abundantly available constituent elements. Herein we report on the thermoelectric properties and microstructure of high performance Mg2Si0.4−xSn0.6Sbx (0 ⩽ x ⩽ 0.015) alloys. The state-of-the-art ZT value of ∼ 1.1 has been attained in the samples with x = 0.0075  due to the relatively low thermal conductivity. In light of the simple cubic structure and mostly light constituent elements, the reduction in lattice thermal conductivity has been discussed in connection with a fairly large amount of in situ formed nanostructures in these samples.
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72.15.Jf Thermoelectric and thermomagnetic effects
73.63.-b Electronic transport in nanoscale materials and structures
72.15.Eb Electrical and thermal conduction in crystalline metals and alloys

Small scale lateral superlattices in two-dimensional electron gases prepared by diblock copolymer masks

S. Hugger, T. Heinzel, and T. Thurn-Albrecht

Appl. Phys. Lett. 93, 102110 (2008); http://dx.doi.org/10.1063/1.2981567 (3 pages) | Cited 1 time

Online Publication Date: 10 September 2008

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A poly(styrene-block-methylmethacrylate) diblock copolymer in the hexagonal cylindrical phase has been used as a mask for preparing a periodic gate on top of a GaAs/AlxGa1−xAs heterostructure. A superlattice period of 43 nm could be imposed on the two-dimensional electron gas. Transport measurements show a characteristic positive magnetoresistance around zero magnetic field, which we interpret as a signature of electron motion guided by the superlattice potential.
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73.21.Cd Superlattices
73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
72.20.My Galvanomagnetic and other magnetotransport effects

Polar face dependence of the ultrafast UV reflectivity of ZnO single crystal

J. R. Schneck, E. Bellotti, P. Lamarre, and L. D. Ziegler

Appl. Phys. Lett. 93, 102111 (2008); http://dx.doi.org/10.1063/1.2968717 (3 pages) | Cited 1 time

Online Publication Date: 11 September 2008

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The transient reflectivity of an ∼ 40 fs, 340 nm pulse incident on each face of a ZnO single crystal is reported as a function of carrier density. Three exponentially damped carrier relaxation components can be identified in these responses. Only the Zn face reflectivity reveals a subpicosecond decay (200–300 fs) due to electron scattering. In contrast, the O face reflectivity exhibits an ∼ 2 ps absorptive decay attributed to the trapping of carriers by O-surface defects at the earliest times. The other decay components are consistent with electron cooling via acoustic phonons and exciton recombination for both faces.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.47.J- Ultrafast spectroscopy (<1 psec)
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
63.20.-e Phonons in crystal lattices
71.35.-y Excitons and related phenomena

Enhanced electroluminescence intensity of InGaN/GaN multi-quantum-wells based on Mg-doped GaN annealed in O2

Ping Ma, Yanqin Gai, Junxi Wang, Fuhua Yang, Yiping Zeng, Jinmin Li, and Jingbo Li

Appl. Phys. Lett. 93, 102112 (2008); http://dx.doi.org/10.1063/1.2980032 (3 pages) | Cited 4 times

Online Publication Date: 11 September 2008

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InGaN/GaN multi-quantum-well blue (461±4 nm) light emitting diodes with higher electroluminescence intensity are obtained by postgrowth thermal annealing at 720 °C in O2-ambient. Based on our first-principle total-energy calculations, we conclude that besides dissociating the Mg–H complex by forming H2O, annealing in O2 has another positive effect on the activation of acceptor Mg in GaN. Mg can be further activated by the formation of an impurity band above the valence band maximum of host GaN from the passivated MgGa–ON complex. Our calculated ionization energy for acceptor Mg in the passivated system is about 30 meV shallower than that in pure GaN, in good agreement with previous experimental measurement. Our model can explain that the enhanced electroluminescence intensity of InGaN/GaN MQWs based on Mg-doped p-type GaN is due to a decrease in the ionization energy of Mg acceptor with the presence of oxygen.
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78.60.Fi Electroluminescence
78.67.De Quantum wells
61.72.Cc Kinetics of defect formation and annealing

Strain engineered Si/SiGe resonant interband tunneling diodes with outside barriers grown on Si0.8Ge0.2 virtual substrates

R. Anisha, Niu Jin, Sung-Yong Chung, Ronghua Yu, Paul R. Berger, and Phillip E. Thompson

Appl. Phys. Lett. 93, 102113 (2008); http://dx.doi.org/10.1063/1.2981211 (3 pages)

Online Publication Date: 11 September 2008

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Si-based resonant interband tunneling diodes (RITDs) grown on commercially available Si0.8Ge0.2 virtual substrates were studied. Peak-to-valley current ratios (PVCRs) were improved by utilizing strain induced band offsets to 3.5 with a peak current density (Jp) of 161 A/cm2. More specifically, a tensilely strained Si layer on the p-side and a compressively strained Si0.5Ge0.5 layer on the n-side were added to the design to form enhanced potential barriers away from the tunneling junction. The outside barriers deepen the respective hole and electron quantum wells and also block nonresonant tunneling current, which improved the PVCR significantly. However, due to the large surface roughness of the SiGe virtual substrates used in this study, the RITDs grown on Si0.8Ge0.2 substrates exhibit a smaller PVCR overall than RITDs optimized on standard Si substrates. Better performance is expected by using higher quality SiGe substrates with smaller surface roughness.
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85.30.Kk Junction diodes
85.30.Mn Junction breakdown and tunneling devices (including resonance tunneling devices)
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Electrical spectroscopy of high resistivity ion-implanted layers by current-voltage measurements

Tiziana Cesca, Andrea Gasparotto, and Beatrice Fraboni

Appl. Phys. Lett. 93, 102114 (2008); http://dx.doi.org/10.1063/1.2975372 (3 pages) | Cited 3 times

Online Publication Date: 12 September 2008

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We report on a spectroscopic characterization of electrically compensated high resistivity Fe-implanted InP and GaInP by room temperature space-charge-limited-current measurements. This method results to be a reliable and powerful tool to obtain a quantification of the degree of compensation and the free carrier concentration in the samples, together with the activation energy and density of states distribution of the dominant majority carrier traps. Moreover, by correlating these results with temperature dependent electrical spectroscopy analyses, it is possible to obtain information on material parameters, such as the carrier mobility, not always easily accessible by direct measurements.
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73.61.Ey III-V semiconductors
73.50.Fq High-field and nonlinear effects
73.50.Dn Low-field transport and mobility; piezoresistance
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
68.55.-a Thin film structure and morphology

Beneficial effect of La on band offsets in Ge/high-κ insulator structures with GeO2 and La2O3 interlayers

V. V. Afanas’ev, A. Stesmans, G. Mavrou, and A. Dimoulas

Appl. Phys. Lett. 93, 102115 (2008); http://dx.doi.org/10.1063/1.2972123 (3 pages) | Cited 10 times

Online Publication Date: 12 September 2008

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The electron energy band alignment between (100)Ge and high-κ oxide insulators (ZrO2,Gd2O3,La2Hf2O7) grown by the atomic beam deposition method is analyzed using photoemission of electrons from the Ge valence band. Formation of a thin GeO2 passivation layer before the high-κ deposition is found to significantly reduce the barrier for electrons. However, when La2O3 is deposited as an interlayer, it strongly reacts with the Ge substrate to form a La–Ge–O germanate at the interface, which is found to retain a high barrier for the carriers in Ge.
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79.60.Jv Interfaces; heterostructures; nanostructures
77.55.-g Dielectric thin films
71.20.-b Electron density of states and band structure of crystalline solids

In-plane band gap modulation investigated by secondary electron imaging of GaAsN/GaAsN:H heterostructures

L. Felisari, V. Grillo, F. Martelli, R. Trotta, A. Polimeni, M. Capizzi, F. Jabeen, and L. Mariucci

Appl. Phys. Lett. 93, 102116 (2008); http://dx.doi.org/10.1063/1.2981689 (3 pages) | Cited 7 times

Online Publication Date: 12 September 2008

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GaAsN/GaAsN:H in-plane heterostructures have been investigated by secondary electron (SE) imaging in a field-emission scanning electron microscope. Adjacent GaAsN and GaAsN:H regions show a quite different SE image brightness due to the band gap energy difference between H-free and H-irradiated GaAsN. These findings provide a useful means to characterize the lateral diffusion of H and well support secondary ion mass spectroscopy results regarding the importance of a low hydrogenation temperature in order to obtain sharply defined in-plane heterostructures.
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73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
68.37.Hk Scanning electron microscopy (SEM) (including EBIC)
66.30.H- Self-diffusion and ionic conduction in nonmetals
71.20.Nr Semiconductor compounds
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)

Two dimensionally patterned GaNxAs1−x/GaAs nanostructures using N+ implantation followed by pulsed laser melting

Taeseok Kim, Michael J. Aziz, and Venkatesh Narayanamurti

Appl. Phys. Lett. 93, 102117 (2008); http://dx.doi.org/10.1063/1.2982424 (3 pages) | Cited 3 times

Online Publication Date: 12 September 2008

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We present measurements on two dimensionally patterned GaNxAs1−x dots fabricated in a GaAs matrix using ion implantation followed by pulsed laser melting and rapid thermal annealing. The lithographically patterned GaNxAs1−x regions are imaged by ballistic electron emission microscopy (BEEM). By analyzing the BEEM spectra of the locally confined dots, we observe the decrease in the Schottky barrier height with nitrogen incorporation. The second derivatives of BEEM currents from unpatterned GaNxAs1−x films exhibit a decrease in Γ-like thresholds as the nitrogen concentration increases. The composition dependence of the thresholds agrees well with that of previously studied low temperature molecular beam epitaxy grown alloys.
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61.80.Jh Ion radiation effects
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
81.15.Fg Pulsed laser ablation deposition
81.16.Mk Laser-assisted deposition
81.16.Nd Micro- and nanolithography
81.16.Rf Micro- and nanoscale pattern formation
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