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ABSTRACT: Cyclotron resonance has been observed in ZnO heterostructures with dense two dimensional electron gas at a wide range of the sub-millimeter wave up to 651 GHz. The typical carrier concentration of the sample is found to be higher than 1012 cm–2 from the Shubnikov de Haas oscillation measurement, and the quantum Hall effect has also been observed at high magnetic fields up to 25 T.The electron effective mass in the ZnO two dimensional system has just been determined as m* = 0.32 ± 0.01 m0 from the magnetic field dependence of the cyclotron resonance experiments. The obtained two dimensional mass is enhanced in comparison with the bare effective mass of three dimensional systems. It is partially due to the large electron-phonon interaction in ZnO (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 03/2010; 7(6):1599 - 1601.
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H. Tampo,
H. Shibata,
K. Maejima,
T.-W. Chiu,
H. Itoh,
A. Yamada,
K. Matsubara,
P. Fons,
Y. Chiba,
T. Wakamatsu,
Y. Takeshita,
H. Kanie, S. Niki
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ABSTRACT: The band profiles of ZnMgO/ZnO heterostructures were confirmed through surface potential measurements by Kelvin probe force microscopy. A simple model for the band profile was proposed and the various band parameters were evaluated experimentally and theoretically based on the band model. The band profile was calculated and validated with experimental results using the Schrödinger–Poisson equation. The energy level of the ZnMgO surface donor state, which serves as the source of the two-dimensional electron gas in ZnMgO/ZnO heterostructures, was estimated from the band parameters; nearly identical energy levels around 0.8 eV were obtained for Zn1−xMgxO layers with Mg compositions x ranging from 0.12 to 0.42 and the corresponding charge densities were estimated to be 8×1012 cm−2.
Applied Physics Letters 06/2009; 94(24):242107-242107-3. · 3.84 Impact Factor
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H. Tampo,
H. Shibata,
K. Maejima,
A. Yamada,
K. Matsubara,
P. Fons,
S. Kashiwaya, S. Niki,
Y. Chiba,
T. Wakamatsu,
H. Kanie
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ABSTRACT: Both the formation mechanism and the origin of the two-dimensional electron gas (2DEG) in ZnMgO/ZnO heterostructures have been investigated. The 2DEG in the heterostructures was confirmed to originate from polarization-induced charge and was found to be dominant for transport at low temperatures as well as room temperature (RT) by transport measurements. The origin of 2DEG was concluded to be the surface of the ZnMgO layer based on both capacitance-voltage measurements and the dependence of the carrier concentration on the ZnMgO layer thickness. The largest sheet carrier concentration was 1.1×1013 cm−2 and the highest mobility for the heterostructure was obtained for a Mg composition of 0.61 at RT.
Applied Physics Letters 11/2008; 93(20):202104-202104-3. · 3.84 Impact Factor
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ABSTRACT: A strong excitonic optical transition in a Zn1−xMgxO alloy grown by radical source molecular beam epitaxy was observed using both optical reflectivity measurements and photoluminescence (PL) measurements. Clear and strong reflectance peaks at room temperature (RT) were observed from 3.42 eV (x = 0.05) to 4.62 eV (x = 0.61) from ZnMgO layers at RT. Distinct clear PL spectra at RT were also observed for energies up to 4.06 eV (x = 0.44). The peak intensity of the reflected signal increased for x values up to x ∼ 0.2 simultaneously with an increase in PL intensity; however, a Stokes shift between the reflectance peak and the PL peak was not observed for x values below 0.2. These facts suggest that the oscillator strength of ZnMgO is enhanced by alloying, and the underlying mechanism is discussed. Furthermore, we demonstrate that the strong reflectance properties even at RT provide an easy method to determine the Mg composition of a thin ZnMgO layer in a ZnMgO/ZnO heterostructure.
Applied Physics Letters 12/2007; 91(26):261907-261907-3. · 3.84 Impact Factor
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ABSTRACT: The authors report that high-quality Zn1−xMgxO alloys are very brilliant light emitters, even more brilliant than ZnO, particularly in the high-temperature region; both the emission bandwidth and the oscillator strength of the photoluminescence from Zn1−xMgxO alloys increase remarkably with increasing Mg composition ratio x. The authors have revealed that the increase in the oscillator strength is mainly due to the increase in the activation energy required for the nonradiative recombination processes. Therefore, it is suggested that the localization of excitons, because of the compositional fluctuation, takes place in Zn1−xMgxO alloys and that the degree of the localization increases with increasing x.
Applied Physics Letters 03/2007; 90(12):124104-124104-3. · 3.84 Impact Factor
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ABSTRACT: We have investigated the initial stages of growth and relaxation of ZnO grown on {110} (a-sapphire) films ranging from 2.5 to 100 nm in thickness. A significant feature of this process is the change in symmetry in going from the twofold symmetry of the a-sapphire surface to the six-fold symmetry of ZnO. Asgrown films were investigated by atomic force microscopy (AFM), high-resolution x-ray diffraction, and polarized XAFS. AFM measurements indicated planar growth even from the earliest stages of epitaxy reflective of the long surface diffusion length of Zn. High-resolution x-ray diffraction measurements demonstrated resolution-limited (0002) rocking curves for films up to 30 nm thick (early growth) upon which the onset of diffuse scattering was observed, eventually coming to completely dominate for films thicker than 100 nm (late-growth). Polarized XANES and EXAFS measurements were performed on the Zn edge to elucidate structural changes from early to late-growth. Analysis of EXAFS data indicated an anisotropic accomodation of misfit with large in-plane bond rotations and with only a slight outward relaxation of the ZnO lattice along the [0001] direction. Electrical measurements indicate that the electron concentration in this initial layer is degenerate.
Physica Scripta 11/2006; 2005(T115):523. · 1.20 Impact Factor
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ABSTRACT: A two-dimensional electron gas was observed in Zn polar ZnMgO/ZnO (ZnMgO on ZnO) heterostructures grown by radical source molecular beam epitaxy. The electron mobility of the ZnMgO/ZnO heterostructures dramatically increased with increasing Mg composition and the electron mobility (μ ∼ 250 cm2/V s) at RT reached a value more than twice that of an undoped ZnO layer (μ ∼ 100 cm2/V s). The carrier concentration in turn reached values as high as ∼ 1×1013 cm−2 and remained nearly constant regardless of Mg composition. Strong confinement of electrons at the ZnMgO/ZnO interface was confirmed by C-V measurements with a concentration of over 4×1019 cm−3. Temperature-dependent Hall measurements of ZnMgO/ZnO heterostructures also exhibited properties associated with well defined heterostructures. The Hall mobility increased monotonically with decreasing temperature, reaching a value of 2750 cm2/V s at 4 K. Zn polar “ZnMgO on ZnO” structures are easy to adapt to a top-gate device. These results open new possibilities for high electron mobility transistors based upon ZnO-based materials.
Applied Physics Letters 09/2006; 89(13):132113-132113-3. · 3.84 Impact Factor
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ABSTRACT: The effects of water vapor introduction during the growth of Cu(In1–xGax)Se2, specifically CuInSe2 (CISe), Cu(In,Ga)Se2 (CIGSe), and CuGaSe2 (CGSe) thin films were studied. We have developed thus far a novel technique to improve CIGSe (x ∼ 0.5) cell performance by means of water vapor introduction during CIGSe deposition. In this study, we have examined the effectiveness of water vapor introduction for other x -compositions (CISe and CGSe). Variations in the electrical properties observed in CIGSe (x ∼ 0.5), that is, increasing hole density and conductivity with water vapor introduction, were also observed in CISe and CGSe. Water vapor introduction affected solar cell performance as well; open circuit voltages, short circuit current densities, and efficiencies were improved. The improvements in cell performance are thought to be related to annihilation of donor defects arising from Se-vacancies by incorporation of oxygen from the water vapor. In addition to this, the sodium content in the CIGSe layers was found to depend on the partial pressure of water vapor during deposition. This result suggests that the improvement mechanism is also related with the so-called ‘Na-effects'. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physica Status Solidi (A) Applications and Materials 08/2006; 203(11):2609 - 2614. · 1.46 Impact Factor
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ABSTRACT: Mo films deposited on glass substrates by rf sputtering, and Cu(InGa)Se2 (CIGS) films deposited on the Mo films by the three-stage method and CuGaSe2 (CGS) films deposited directly on glass substrates by co-evaporation of elemental source materials were observed by X-ray diffraction (XRD) pole figure measurements. The texture analysis of the Mo films revealed a wide variety of solid-spatial orientations, being dependent on the sputtering device configuration and deposition conditions. The in-plane orientation of the CIGS or CGS films was affected by the Mo film texture, however, the predominant crystallographic lattice plane along the substrate surface was found to be close to (115) in any case. It must be noticed that the CIGS film is apt to grow in (115) orientation because of the lattice plane nature although this orientation cannot be detected by the XRD 2θ –θ scan due to its zero structural factor. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physica Status Solidi (A) Applications and Materials 08/2006; 203(11):2639 - 2643. · 1.46 Impact Factor
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ABSTRACT: We have developed a novel technique to improve Cu(In<sub>1-x</sub>Ga<sub>x</sub>)Se<sub>2</sub> (CIGSe) cell performance by means of water vapor introduction during CIGSe deposition. We have examined thus far the effectiveness of water vapor introduction on CIGSe with particular emphasis on a Ga/(In+Ga) ratio x of around 0.5. In this study, the effects of water vapor introduction on thin film properties and solar cell performance for other x-compositions, specifically CuInSe<sub>2</sub> (CISe) and CGSe<sub>2 </sub> (CGSe) were studied. Variations in the electrical properties observed in CIGSe (x~0.5), that is, increasing hole density and conductivity with water vapor introduction, were also observed in CISe and CGSe. Water vapor introduction affected solar cell performance as well. The observed improvements in cell performance are thought to be related to the annihilation of donor defects arising from Se vacancies by incorporation of oxygen from the water vapor. In addition, the sodium content in the CIGSe layers was found to increase with increasing oxygen content with water vapor introduction. This result suggests that the mechanism behind the improvement is also related with an enhancement of so-called Na effects
Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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ABSTRACT: The built-in potential, carrier collection and eventually the conversion efficiency of the Cu(In,Ga)Se<sub>2</sub>-based solar cell were calculated for the best performance as a function of doping concentrations coupled with the arrangement of the junction location. The results were illustrated as contour maps on donor-acceptor concentration coordinates. It was shown that the conversion efficiency map becomes a saddle shape because a reduced space charge region of high doping concentration limits carrier collection due to small minority carrier diffusion length in CIGS, indicating that a high doping concentration does not necessarily results in high performance. Besides it can be said that buried junction formation fortunately makes device design condition less heavy
Photovoltaic Energy Conversion, Conference Record of the 2006 IEEE 4th World Conference on; 06/2006
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ABSTRACT: The crystallographic polarity of ZnO bulk crystals and epilayers were determined by X-ray diffraction (XRD) using anomalous dispersion near the Zn K-edge. The method is not destructive and is straightforward to carry out using a typical XRD measurement system. The polarity difference between the Zn (0001) and O (000) surfaces could be easily determined using a {0002} diffraction peak and the Bremstrahlung radiation from a Cu rotating anode source. By using the normalized pre and post-Zn K-edge diffraction intensity ratios of the {0002} diffraction peak, Zn polar and O polar ZnO layers could always be distinguished but, the absolute value of the ratio was found to change with layer thickness. Acid etching results confirmed the veracity of the polarity determination of the XRD measurement. To test the technique, Zn and O polar ZnO layers were grown by radical source molecular beam epitaxy on MgO buffer layers on c-sapphire substrate and O polar ZnO layers were grown on a-plane substrates and measured using the X-ray technique with excellent agreement. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 03/2006; 3(4):1018 - 1021.
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ABSTRACT: The crystallographic polarity of ZnO epilayers was determined by x-ray diffraction (XRD) using anomalous dispersion near the Zn K edge. The method is not destructive and is straightforward to carry out using a typical XRD measurement system. The polarity difference between the Zn (0001) and O (000) surfaces could be easily determined using a {0002} diffraction peak and the Bremstrahlung radiation from a Cu rotating anode source. By using the normalized pre- and post-Zn K-edge diffraction intensity ratios of the (0002) diffraction peak, Zn polar and O polar ZnO layers could always be distinguished but, the absolute value of the ratio was found to change with layer thickness. The absolute value of the ratio with layer thickness was found to have a linear dependence on layer thickness allowing determination of the polarity of (0001) ZnO epilayers with a single x-ray measurement and the known layer thickness in conjunction with standard data. Acid etching results confirmed the veracity of the polarity determination of the XRD measurement. To test the technique, Zn and O polar ZnO layers were grown by radical source molecular beam epitaxy (RS MBE) on MgO buffer layers on c-sapphire substrate and O polar ZnO layers were grown on a-plane substrates and measured using the x-ray technique with excellent agreement.
Applied Physics Letters 09/2005; 87(14):141904-141904-3. · 3.84 Impact Factor
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M. Watanabe,
M. Sakai,
H. Shibata,
H. Tampo,
P. Fons,
K. Iwata,
A. Yamada,
K. Matsubara,
K. Sakurai,
S. Ishizuka, S. Niki,
K. Nakahara,
H. Takasu
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ABSTRACT: Photoluminescence properties of nominally undoped ZnO thin films grown by radical-source molecular-beam epitaxy have been investigated as a function of (i) sample growth conditions, (ii) excitation laser power density, and (iii) measurement temperatures. Altogether four excitonic emission peaks were observed at photon energy of 3.3646, 3.3606, 3.3572, and 3.3331 eV, which are tentatively denoted as emission peaks A, D, F, and G, respectively. We have classified the defect types responsible for the emission peaks into the following two groups; (i) D and F, whose responsible defect types are suggested to be residual impurities such as aluminum and indium, respectively, and (ii) A and G, whose responsible defect types are suggested to be intrinsic defects such as oxygen vacancies, interstitial zinc, and extended structural defects particularly for G.
Applied Physics Letters 05/2005; 86(22):221907-221907-3. · 3.84 Impact Factor
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K. Tamura,
K. Nakahara,
M. Sakai,
D. Nakagawa,
N. Ito,
M. Sonobe,
H. Takasu,
H. Tampo,
P. Fons,
K. Matsubara,
K. Iwata,
A. Yamada, S. Niki
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ABSTRACT: Ga-doped ZnO (ZnO:Ga) was fabricated by molecular-beam epitaxy to employ as highly transparent contact electrode on p-GaN layer. ZnO:Ga films have higher transparency in near uv and blue wavelength range than the conventional Ni/Au oxidized alloy p-electrode. The as-deposited ZnO:Ga p-electrodes worked as ohmic contacts for the p-GaN layers without any kind of post annealing processes. The optical power of InGaN light-emitting diode (LED) fabricated with ZnO:Ga p-contact was doubled compared to that with Ni/Au p-electrode. The LED chips with ZnO:Ga were operated at forward current of 20 mA in the environment of temperature of 121 °C, humidity of 85%, and pressure of 1.6 atm. The tested LED has lifetime of 80 and more hours in this environment. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (a) 09/2004; 201(12):2704 - 2707. · 1.21 Impact Factor
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ABSTRACT: Al-doped Zn1−xMgxO films have been deposited on glass substrates at a substrate temperature of 200 °C by a pulsed laser deposition system. A resistivity of 3×10−4 Ω cm was obtained at x=0.06. Film resistivity was found to increase with further increases in Mg composition. The maximum band gap of films with a resistivity ρ⩽1×10−3 Ω cm was found to be 3.97 eV, demonstrating band-gap engineering possibilities in the range of Eg=3.5–3.97 eV with a resistivity ρ⩽1×10−3 Ω cm. The average transmittance of the films was higher than 90% in the wavelength region λ=400–800 nm, a range suitable for transparent conducting film applications.
Applied Physics Letters 08/2004; 85(8):1374-1376. · 3.84 Impact Factor
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ABSTRACT: ZnO films were grown on low-temperature (LT) buffer layers on sapphire a-plane (11–20) substrates by radical source molecular-beam epitaxy. The LT buffer layers were found to effect the electrical properties of subsequently grown undoped ZnO films, and their presence was found to be indispensable for the growth of films with low carrier concentrations and high mobilities. Temperature-dependent Hall measurements showed the existence of a degenerate region related to the LT buffer layers. It was found that the effects of degenerate layers could be reduced by using annealing treatments and nitrogen doping of the LT buffer layers. The dominant residual donor energy of 110 meV was found to be different than previously reported. The carrier concentration of a ZnO film fabricated using a nitrogen-doped buffer layer was 7.5×1016 cm−3 with a mobility of 132 cm2/V s at room temperature. © 2004 American Institute of Physics.
Applied Physics Letters 05/2004; 84(22):4412-4414. · 3.84 Impact Factor
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ABSTRACT: ZnO films were grown on sapphire substrates by radical source molecular beam epitaxy (RS-MBE). ZnO low temperature buffer layers were subjected to various treatments. High quality ZnO films were obtained by vacuum annealing plus nitrogen doping of the buffer layer. The carrier concentration of the ZnO film fabricated using this buffer layer was 7.5 × 1016 cm−3 with a mobility of 132 cm2/V sec at RT. Temperature dependent Hall measurements showed implied the existence of degenerate (untreated) buffer layers. Using a nitrogen-doped buffer layer to reduce the influence of the degenerate layer, a donor energy of 110 meV was estimated from temperature dependent Hall measurements. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 02/2004; 1(4):888 - 891.
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ABSTRACT: We have applied in situ diffuse spectroscopic light scattering (SLS) to the three-stage deposition process of polycrystalline Cu(In/sub 1-x/Ga/sub x/)Se/sub 2/ (CIGSe) thin films. The process has been studied by systematic variation of the major process parameters of CIGSe, such as (1) substrate temperature and (2) Ga concentration. SLS signals in the wavelength range of 400/spl sim/800 nm has been used in order to derive the development of surface roughness during growth. SLS was found to be more sensitive to near-surface compositions than the conventional temperature monitoring techniques. Emergence of a Cu-rich phase at the surface before the stoichiometry point was observed by SLS. Optimum conditions for smoother ClGSe surfaces has been proposed for each process parameter.
Photovoltaic Energy Conversion, 2003. Proceedings of 3rd World Conference on; 06/2003
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ABSTRACT: In electron probe beam micro-analysis (EPMA), the electron accelerating voltage decides the penetration depth of the probe beam therefore the voltage dependence of the characteristic X-ray emission intensities reflects the composition depth distribution. This effect can be utilized to determine an accurate composition value for the matrix and to identify other phases. As the measured emission intensity is a convolution of the entire probed depth, a parameter fitting procedures using a computer was developed to deconvolute measured spectra of accelerating voltage dependent data. Using this method, a surface phase segregated on CuGaSe epi-layers grown by molecular-beam epitaxy under Cu-excess conditions was detected and found to be Cu Se . 2 3 2 Oxidation of CuInSe or CuGaSe film surfaces was also detected by EPMA. The thickness of the natural oxide layers evaluated 2 2 by this technique was several nanometers. The weakest point in EPMA analysis is its sensitivity to specimen surface roughness. A theoretical estimation of surface roughness effects matched actual data well. This result suggests possible corrections for measured values on rough surfaces. 2003 Elsevier Science B.V. All rights reserved.
Thin Solid Films 01/2003; 431(432):277-283. · 1.89 Impact Factor
