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15 Apr 2010

Volume 107, Issue 8, Articles (08xxxx)

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J. Appl. Phys. 107, 081101 (2010); http://dx.doi.org/10.1063/1.3340792 (15 pages)

Anne-Marie Kietzig, Savvas G. Hatzikiriakos, and Peter Englezos
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back to top Electronic Structure and Transport

Electroluminescence behavior of ZnO/Si heterojunctions: Energy band alignment and interfacial microstructure

J. B. You, X. W. Zhang, S. G. Zhang, H. R. Tan, J. Ying, Z. G. Yin, Q. S. Zhu, and Paul K. Chu

J. Appl. Phys. 107, 083701 (2010); http://dx.doi.org/10.1063/1.3385384 (5 pages) | Cited 7 times

Online Publication Date: 19 April 2010

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n-ZnO/p-Si heterojunction light-emitting diodes (LEDs) show weak defect-related electroluminescence (EL). In order to analyze the origin of the weak EL, the energy band alignment and interfacial microstructure of ZnO/Si heterojunction are investigated by x-ray photoelectron spectroscopy. The valence band offset (VBO) is determined to be 3.15±0.15 eV and conduction band offset is −0.90±0.15 eV, showing a type-II band alignment. The higher VBO means a high potential barrier for holes injected from Si into ZnO, and hence, charge carrier recombination takes place mainly on the Si side rather than the ZnO layer. It is also found that a 2.1 nm thick SiOx interfacial layer is formed at the ZnO/Si interface. The unavoidable SiOx interfacial layer provides to a large number of nonradiative centers at the ZnO/Si interface and gives rise to poor crystallinity in the ZnO films. The weak EL from the n-ZnO/p-Si LEDs can be ascribed to the high ZnO/Si VBO and existence of the SiOx interfacial layer.
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85.60.Jb Light-emitting devices

Semiconductor-to-insulator transition of undoped-BaTiO3 in quenched state

Hyung-Soon Kwon, Han-Ill Yoo, Chang-Hoon Kim, and Kang-Heon Hur

J. Appl. Phys. 107, 083702 (2010); http://dx.doi.org/10.1063/1.3385426 (5 pages) | Cited 2 times

Online Publication Date: 19 April 2010

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It has been believed that the defect structure of “undoped” BaTi1−ηO3−δ be governed by background acceptor impurities, most likely AlTi′, in its near stoichiometry regime (δ ≈ 0). Its electrical conductivity versus oxygen activity should, thus, be expected to be similar to that of Al-doped BaTiO3 not only in equilibrium state at elevated temperatures, but also in quenched state at low temperatures. In quenched state, however, the “undoped” BaTiO3 from various sources totally betray this expectation: A landmark of the quenched-state defect structure, semiconductor-to-insulator transition falls at an oxygen activity >10 orders lower than that of the Al-doped and it is rather similar to that of variable-valent Mn-doped BaTiO3. The transition is found to systematically depend on the nonmolecularity (η), suggesting the defect structure being governed by the multiply-charged cation vacancies. The hole-trapping energies of VBa″ and VTi⁗ are estimated and compared with those for the fixed-valent AlTi′ and variable-valent MnTi″.
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71.30.+h Metal-insulator transitions and other electronic transitions
61.72.up Other materials
61.66.Bi Elemental solids
61.66.Dk Alloys
72.80.Sk Insulators
81.40.Gh Other heat and thermomechanical treatments

Electric field effect on the carrier capture of deep traps in p-type InP

R. Darwich and M. K. Sabra

J. Appl. Phys. 107, 083703 (2010); http://dx.doi.org/10.1063/1.3392798 (7 pages)

Online Publication Date: 21 April 2010

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The electric field effect on the carrier capture cross section of deep traps has been studied. The experimental results on the H4F and H5 hole traps in p-type InP show an enhancement of the capture cross section with the increase in the applied electric field. This enhancement depends on the nature of the deep traps and its peak temperature. Increasing the electric field from 4.1×106 to 2.4×107 V/m leads to an increase in the H4F capture cross section by a factor of 3 to 20. While in the case of H5 it increases by a factor of 2 to 5 by increasing the applied electric field from 8.0×106 to 2.4×107 V/m. A theoretical model has been suggested to explain the electric field effect on the capture cross section. This model deals with the cascade and multiphonon processes semiclassically. Applying this model to the above deep traps, we have found that H4F is negatively charged complex and H5 is positively charged complex.
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81.05.Ea III-V semiconductors
71.55.Eq III-V semiconductors
61.82.Fk Semiconductors
61.80.Fe Electron and positron radiation effects
73.61.Ey III-V semiconductors
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Donor-donor binding in In2O3: Engineering shallow donor levels

Li-Ming Tang, Ling-Ling Wang, Dan Wang, Jian-Zhe Liu, and Ke-Qiu Chen

J. Appl. Phys. 107, 083704 (2010); http://dx.doi.org/10.1063/1.3374644 (5 pages) | Cited 3 times

Online Publication Date: 21 April 2010

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Using first-principles band structure methods, we investigate the interactions between different donors in In2O3. Through the formation energy and transition energy level calculations, we find that an oxygen-vacancy creates a deep donor level, while an indium-interstitial or a tin-dopant induces a shallow donor level. The coupling between these donor levels gives rise to even shallower donor levels and leads to a significant reduction in their formation energies. Based on the analysis of the PBE0-corrected band structure and the molecular-orbital bonding diagram, we demonstrate these effects of donor–donor binding. In addition, total energy calculations show that these defect pairs tend to be more stable with respect to the isolated defects due to their negative binding energies. Thus, we may design shallow donor levels to enhance the electrical conductivity via the donor–donor binding.
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71.55.Ht Other nonmetals
71.15.Nc Total energy and cohesive energy calculations
61.72.jj Interstitials
61.72.jd Vacancies
71.20.Nr Semiconductor compounds
72.80.Jc Other crystalline inorganic semiconductors

Ferromagnetic signal and unconventional Kondo-like effect in the superconducting Y1−xPrxBa2Cu3O7−δ systems

Guixin Cao, Yuanyuan Li, Jincang Zhang, Shixun Cao, Chuanbing Cai, and Xuechu Shen

J. Appl. Phys. 107, 083705 (2010); http://dx.doi.org/10.1063/1.3393941 (5 pages)

Online Publication Date: 21 April 2010

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Electrical and magnetic properties were studied for the Y1−xPrxBa2Cu3O7−δ superconductors with x = 0.44–0.48, which was located near the antiferromagnetic (AFM) and superconducting (SC) phase boundaries. An unconventional Kondo-like effect is observed below Tc for x = 0.48, where the value of the resistivity minimum ρmin increases with increasing magnetic field H. The transition temperature Tmin first increases and then decreases with increasing field strength. Magnetic measurements show that a small fraction of FM phase appears and coexists with the SC and AFM phases. We discuss these findings from the viewpoints of Kondo scattering and electron-electron interaction, and found that the resistivity minimum below Tc is mainly due to the Kondo scattering. The Kondo scattering is identified as SC phase coexisting with AFM and FM clusters, which is as the Kondo singlets resulting from Pr substitution. This phase-separated state is unstable against the magnetic field which suppresses the SC phase and restores the single Kondo peak at low temperatures.
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74.25.Ha Magnetic properties including vortex structures and related phenomena
74.25.fc Electric and thermal conductivity
75.50.Dd Nonmetallic ferromagnetic materials
74.72.-h Cuprate superconductors
75.50.Ee Antiferromagnetics
75.30.Mb Valence fluctuation, Kondo lattice, and heavy-fermion phenomena

Bias dependent two-channel conduction in InAlN/AlN/GaN structures

J. H. Leach, X. Ni, X. Li, M. Wu, Ü. Özgür, H. Morkoç, L. Zhou, D. A. Cullen, D. J. Smith, H. Cheng, Ç. Kurdak, J. R. Meyer, and I. Vurgaftman

J. Appl. Phys. 107, 083706 (2010); http://dx.doi.org/10.1063/1.3330627 (4 pages) | Cited 3 times

Online Publication Date: 22 April 2010

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Due to growth temperature differences during deposition of GaN heterostructures utilizing InAlN barriers, an inadvertent parasitic GaN layer can form in the InAlN barrier layer. In structures utilizing AlN spacer layers, this parasitic layer acts as a second conduction channel with a carrier density dependent upon polarization charges and lattice strain as well as the surface potential. The effect of an additional GaN spacer layer in InAlN/AlN/GaN structures is assessed using simulations, electron-microscopy observations, magnetoconductivity measurements with gated Hall bar samples, and with quantitative mobility spectrum analysis. We propose a possible formation mechanism for the parasitic layer, and note that although the additional unintended layer may have beneficial aspects, we discuss a strategy to prevent its occurrence.
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85.30.Tv Field effect devices
72.20.My Galvanomagnetic and other magnetotransport effects
81.05.Ea III-V semiconductors

Rigorous calculation of the Seebeck coefficient and mobility of thermoelectric materials

Ashok T. Ramu, Laura E. Cassels, Nathan H. Hackman, Hong Lu, Joshua M. O. Zide, and John E. Bowers

J. Appl. Phys. 107, 083707 (2010); http://dx.doi.org/10.1063/1.3366712 (10 pages) | Cited 12 times

Online Publication Date: 26 April 2010

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The Seebeck coefficient of a typical thermoelectric material is calculated without recourse to the relaxation time approximation (RTA). To that end, the Boltzmann transport equation is solved in one spatial and two k-space coordinates by a generalization of the iterative technique first described by Rode. Successive guesses for the chemical potential profile are generated until current continuity and charge-neutrality in the bulk of the device are simultaneously satisfied. Both the mobility and Seebeck coefficient are calculated as functions of the temperature and the agreement to experimentally obtained values is found to be satisfactory. Comparison is made with the less accurate RTA result, which has the sole advantage of giving closed form expressions for the transport coefficients.
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72.20.Pa Thermoelectric and thermomagnetic effects
72.20.Fr Low-field transport and mobility; piezoresistance

Quadrimolecular recombination kinetics of photogenerated charge carriers in the composites of regioregular polythiophene derivatives and soluble fullerene

Hisaaki Tanaka, Yuki Yokoi, Naoki Hasegawa, Shin-ichi Kuroda, Takayuki Iijima, Takao Sato, and Takakazu Yamamoto

J. Appl. Phys. 107, 083708 (2010); http://dx.doi.org/10.1063/1.3380839 (9 pages) | Cited 5 times

Online Publication Date: 26 April 2010

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Light-induced electron spin resonance (LESR) measurements have been performed on the composites of regioregular polythiophene derivatives and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in order to study the recombination kinetics of photogenerated charge carriers. We adopt two regioregular polymers with different side chains; head-to-tail poly(3-hexylthiophene) (RR-P3HT) and head-to-head poly(3-dodecynylthiophene-2,5-diyl) [HH-P3(C ≡ CDec)Th]. In both systems, two LESR signals due to positive polarons on the polymer (g ∼ 2.002) and fullerene radical anions (g ∼ 2.000) have been observed. Quadrimolecular recombination (QR) kinetics, previously reported for RR-P3HT/C60 composites, where two positive polarons and two radical anions recombine simultaneously, has been confirmed in both systems by the observation of Iex0.25 dependence of the LESR intensity on the excitation light intensity (Iex) and the decay curve of the LESR intensity. This process implies the formation of doubly-charged states such as bipolarons or polaron pairs on the polymer to attract two radical anions. Temperature dependence of the QR rate constant, γ, in both systems has exhibited a crossover of the transport mechanism from low temperature tunneling to high temperature hopping process, as in the case of RR-P3HT/C60 composites. In the RR-P3HT/PCBM composites, γ has exhibited marked dependencies on the PCBM concentration or annealing, which may be related to the change of the crystallinity of the phase-separated polymer and fullerene domains as well as their interface structures, affecting the carrier mobilities or the trap states at the interface. Associated change of the molecular orientation of RR-P3HT crystalline domains with the lamellar structure has been further confirmed from the anisotropic LESR signals of the cast films on the substrates, exhibiting a qualitative agreement with the reported x-ray or optical analyses. In the HH-P3(C ≡ CDec)Th/PCBM composite, γ has been smaller than those in the RR-P3HT/PCBM composites, reflecting the difference of local structures due to the different molecular structure. Furthermore, the hyperfine-determined LESR linewidth of the positive polaron has exhibited a smaller value than those in the RR-P3HT composite, implying the larger extension of the polaron wave function on the polymer chain, which is consistent with the highly coplanar structure of this polymer.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.61.-r Electrical properties of specific thin films
76.30.Lh Other ions and impurities
71.38.Mx Bipolarons
81.40.Gh Other heat and thermomechanical treatments

Measurement of optical constants of Si and SiO2 from reflection electron energy loss spectra using factor analysis method

H. Jin, H. Shinotsuka, H. Yoshikawa, H. Iwai, S. Tanuma, and S. Tougaard

J. Appl. Phys. 107, 083709 (2010); http://dx.doi.org/10.1063/1.3346345 (11 pages) | Cited 3 times

Online Publication Date: 27 April 2010

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The energy loss functions (ELFs) and optical constants of Si and SiO2 were obtained from quantitative analysis of reflection electron energy loss spectroscopy (REELS) by a new approach. In order to obtain the ELF, which is directly related to the optical constants, we measured series of angular and energy dependent REELS spectra for Si and SiO2. The λ(E)K(ΔE) spectra, which are the product of the inelastic mean free path (IMFP) and the differential inverse IMFP, were obtained from the measured REELS spectra. We used the factor analysis (FA) method to analyze series of λ(E)K(ΔE) spectra for various emission angles at fixed primary beam energy to separate the surface-loss and bulk-loss components. The extracted bulk-loss components enable to obtain the ELFs of Si and SiO2, which are checked by oscillator strength-sum and perfect-screening-sum rules. The real part of the reciprocal of the complex dielectric function was determined by Kramers–Kronig analysis of the ELFs. Subsequently, the optical constants of Si and SiO2 were calculated. The resulting optical constants in terms of the refractive index and the extinction coefficient for Si and SiO2 are in good agreement with Palik’s reference data. The results demonstrate the general applicability of FA as an efficient method to obtain the bulk ELF and to determine the optical properties from REELS measurements.
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79.20.Uv Electron energy loss spectroscopy
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
77.22.Ch Permittivity (dielectric function)

The effect of state disproportion on Na0.5CoO2 and other NaxCoO2 compounds

Y. S. Zhang, K. L. Yao, and Z. L. Liu

J. Appl. Phys. 107, 083710 (2010); http://dx.doi.org/10.1063/1.3386441 (5 pages)

Online Publication Date: 27 April 2010

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The electronic structures of Na0.5CoO2 and other NaxCoO2 compounds (with x = 1, 1/3, and 2/3) have been calculated by first-principle calculations based on density-functional theory. The result shows that state disproportion between Co3+ (S ∼ 0) and Co4+ (S ∼ 1/2) in them is determined by the neighboring environments of cobalt ions and the doping level x. Further analysis reveals that state disproportion plays an important role in the physical features of NaxCoO2 compounds, such as metal-insulator transition or magnetic behavior. From these, we can get a further under-standing on the phase diagram of NaxCoO2 compounds
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71.30.+h Metal-insulator transitions and other electronic transitions
75.50.Ee Antiferromagnetics
61.72.up Other materials
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.20.Ps Other inorganic compounds

Ultrafine hollow needle formation on silicon

Z. Sanaee and S. Mohajerzadeh

J. Appl. Phys. 107, 083711 (2010); http://dx.doi.org/10.1063/1.3391707 (6 pages) | Cited 1 time

Online Publication Date: 27 April 2010

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A method for the formation of ultrafine hollow needles, microcylindrical and nano-wall structures on silicon substrates is reported. The fabrication of these ultrafine structures is possible through a combination of high-precision high aspect ratio vertical etching of silicon with a small angle vacuum deposition technique where nanometric walls are feasible. These structures can be used as the media to transfer gas and liquid through their tiny holes. In addition, the structure has been used to realize cavity-based capacitance inclination sensor suitable for small angle detection. A capacitance variation of 0.6 pF/degree of inclination has been obtained.
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87.85.J- Biomaterials
87.85.Rs Nanotechnologies-applications
81.65.Cf Surface cleaning, etching, patterning
52.77.Bn Etching and cleaning
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
73.61.Cw Elemental semiconductors

Tuning of electronic properties of nanographene ribbons by a spatially modulated electric field

S. C. Chen, C. P. Chang, C. H. Lee, and M. F. Lin

J. Appl. Phys. 107, 083712 (2010); http://dx.doi.org/10.1063/1.3372761 (7 pages) | Cited 1 time

Online Publication Date: 28 April 2010

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The electronic properties of a nanographene ribbon can be significantly tuned by a spatially modulated electric field. The modulated electric potential results in the changes of the electronic properties, i.e., modified energy dispersions, creation of extra band-edge states, alteration of the energy gap, and induction of semiconductor-metal transition. The number of the free carrier increases with the increment of the field strength. Through further classification of the carbon atoms, the features of the wave functions are clearly presented, and the carrier distribution is drastically modulated under the influence of the electric field. The periodic length and the phase shift of the modulated electric field induce a change in the y-axis symmetry of the ribbon and have a significant influence on the energy of the partial flat bands, the energy gap and the carrier distribution. The characteristics of the band structure are directly revealed in the density of states (DOS). The number, heights, positions, and spacings of the peaks in DOS are significantly changed. At the Fermi level, DOS is considerably enhanced; that is, more free carriers are created. The predicted results can be verified by optical and transport experiments.
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73.22.Pr Electronic structure of graphene
72.80.Vp Electronic transport in graphene
73.20.At Surface states, band structure, electron density of states
71.30.+h Metal-insulator transitions and other electronic transitions
72.20.Fr Low-field transport and mobility; piezoresistance

Anisotropy of the electrical transport properties in a Ni2MnGa single crystal: Experiment and theory

Min Zeng, Meng-Qiu Cai, Siu Wing Or, and Helen Lai Wa Chan

J. Appl. Phys. 107, 083713 (2010); http://dx.doi.org/10.1063/1.3354105 (5 pages) | Cited 1 time

Online Publication Date: 29 April 2010

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Electrical transport properties in ferromagnetic shape memory Ni–Mn–Ga single crystal have been investigated both in experiment and theory by analyzing electrical resistivity along different crystallographic directions during heating. The experimental results show a clear first-order martensitic transformation and a large anisotropic resistivity (AR) of 23.7% at the tetragonal martensitic phase. The theoretical conductivity (σ = 1/ρ), estimated using first-principles calculations combined with classical Boltzman transport theory, proves essential crystallographic anisotropic resistivity (AR = 31%) in the martensitic phase and agrees well with experimental results. The AR in the martensitic phase is reveled to mainly originate from the splitting of the minority-spin Ni 3d and Ga 4p states near the Fermi level and hence reconstruction of the minority-spin Fermi surface upon martensitic transformation.
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72.80.Sk Insulators
64.70.K- Solid-solid transitions
81.30.Kf Martensitic transformations
75.50.Cc Other ferromagnetic metals and alloys
71.15.-m Methods of electronic structure calculations
71.20.Gj Other metals and alloys
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