Papers - KOBORI Hiromi
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Electromotive Force Generation due to Electron and Impurity Cyclotron Resonances in Compound Semiconductors:（共著） Reviewed
T. Okashita, H. Kobori and T. Ohyama
Physica B 302-303 369 - 373 2001
Joint Work
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Effects of Resonant-Polaron and Band-Nonparabolicity on Cyclotron Resonance in （共著） Reviewed
H. Kobori, T. Nomura and T. Ohyama
Physical Review B 69 2315 - 2323 2001
Joint Work
Authorship：Lead author
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Intensive Far-Infrared Optical Study on the Donor Impurity Band in the presence of a Magnetic Field for n-GaAs:（共著） Reviewed
H. Kobori, M. Inoue and T. Ohyama
Physica B 302-303 17 - 22 2001
Joint Work
Authorship：Lead author
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Comparison of for δ-doped Layer and Bulk Crystals of Si:Sb in the Weak Localization Region:（共著） Reviewed
A. Fujimoto, H. Kobori, T. Ohyama and S. Ishida
Physica B 302-303 7 - 11 2001
Joint Work
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Resonant-Photoelectromagnetic Effect in Semiconductors:（共著） Reviewed
T. Okashita, H. Kobori and T. Ohyama
Physical Review B 64 195333 - 195342 2001
Joint Work
Phys. Rev. B64 pp.195333-195342, (2001)
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Comparison of magnetoconductance of the δ-doped layer and bulk crystal of Si : Sb in the weak localization regime Reviewed
Akira Fujimoto, Hiromi Kobori, Tyuji Ohyama, Shuichi Ishida, Kazuo Satoh, Tadaoki Kusaka, Yoshiharu Kakehi
Physica B: Condensed Matter 302-303 7 - 11 2001
Joint Work
The magnetoresistance of a Si : Sb δ-doped layer has been measured in magnetic fields up to 3.4 T. The results have been compared with those for a heavily doped bulk sample. The portion of the occupation of Sb in δ-doped layer is 0.11 monolayer. We have observed the positive magnetoresistance in the perpendicular weak magnetic fields arising from the weak anti-localization due to spin-orbit interaction for the first time in this system. The ratio of spin-orbit scattering time to inelastic scattering time is fairly large compared with that of the metallic thin films which include heavy elements. When the moderate magnetic field is applied perpendicularly to the layer, the negative magnetoresistance appears. On the other hand, the positive one is observed in parallel fields. The comparative analyses are performed between the two kinds of samples within the framework of weak localization theories. © 2001 Elsevier Science B.V.
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Electromotive force generation due to electron and impurity cyclotron resonances in compound semiconductors Reviewed
Tomonori Okashita, Hiromi Kobori, Tyuzi Ohyama
Physica B: Condensed Matter 302-303 369 - 373 2001
Joint Work
We report on the resonant-photoelectromagnetic (R-PEM) effect in semiconductor, that is the electromotive force generation associated with electron cyclotron resonance (ECR) and impurity cyclotron resonance (ICR). This can be explained with a model that the carriers excited by the resonant absorption of the radiation induce the DC voltage through the thermomagnetic process. In this study, we have applied this technique to the n-InGaAs thin film and observed the negative voltage peak in addition to the usual positive ICR one. From the temperature variation and the resonant magnetic field dependence of these signals, we have concluded that the negative signal is induced by the ECR absorption. In addition, we have confirmed that the inversion of polarity for these signals originates in the inhomogeneous distribution of conduction electrons inside the film, and electrons localized at a fraction of the sample mainly cause the resonant absorption. Through the detailed observation and analysis of the generated DC voltage, we can investigate the dynamics of hot carriers inhomogeneously excited in the sample by the ECR as well as the ICR. Moreover, if we pay special attention to the polarity of the generated voltage, we can determine the distribution of electrons and impurities inside the film. © 2001 Elsevier Science B.V.
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Resonant photoelectromagnetic effect in semiconductors Reviewed
Tomonori Okashita, Hiromi Kobori, Tyuzi Ohyama
Physical Review B - Condensed Matter and Materials Physics 64 ( 19 ) 2001
Joint Work
Extensive studies of the electromotive force (EMF) generation induced by the resonant magnetoabsorption with the far-infrared excitation have been performed. As for the resonant photoelectromagnetic (RPEM) effect whose origins had been obscure, we obtained various information on fundamental properties of the generated EMF and gave the reasonable understandings for the mechanism of the generation of the RPEM voltage. In addition, we propose a simple model consistent with our experimental results. All of the experimental results we have obtained here support the thermomagnetic model where the RPEM voltage is generated through thermomagnetic process due to the temperature gradient induced inside the samples. Accordingly, we concluded that the observed RPEM voltage is generated by the optically induced Nernst-Ettingshausen effect. This is a conventional method to investigate some nonequilibrium electronic systems and their relaxation mechanism complementary to the transmission and photoconductivity measurements, and we can apply the results obtained by this study for the investigation in dynamical properties of electronic system. © 2001 The American Physical Society.
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Intensive far-infrared optical study on the donor impurity band in the presence of a magnetic field for n-GaAs Reviewed
H. Kobori, M. Inoue, T. Ohyama
Physica B: Condensed Matter 302-303 17 - 22 2001
Joint Work
We have intensively studied donor impurity bands formed by the ground and excited states in the presence of a magnetic field by means of the far-infrared optical technique. With this view, we have carried out measurements of the impurity cyclotron resonance absorption (ICRA) for hydrogenic donors in n-GaAs. Concerning the optical transition between the 1s and 2p+1 states, the variation of the full-width at half-maximum (FWHM) for the ICRA has been investigated as functions of the donor concentration and the magnetic field. Those dependencies of the FWHM have been discussed in consideration of energy bandwidths of the 1s and 2p+1 states for donors in the presence of a magnetic field. The energy bandwidths have been estimated by use of the tight binding approximation for the linear combinations of atomic orbitals, assuming the simple cubic configuration of donors. The qualitative agreement between experimental results and numerical estimation has been obtained. © 2001 Elsevier Science B.V.
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Weak-localization analysis for the Upper-Hubbard transport in Si:Sb Reviewed
A. Fujimoto, H. Kobori, K. Fujii, T. Ohyama, S. Ishida
Physica Status Solidi (B) Basic Research 218 ( 1 ) 177 - 180 2000.3
Joint Work
Magnetoresistance (MR) in Si:Sb just below the critical concentration of metal-insulator transition has been carried out as stepping foot into the non-ohmic region of the electric current density. In the ohmic region, the resistivity at low temperatures obeys Efros-Shklovskii type variable-range hopping and the positive MR is observed. As the electric current density increases, the positive MR is depressed and then the negative one appears. This behavior has been explained by the transfer of electrons, due to the rise of the electron temperature, from the strongly localized states to the upper Hubbard band whose state is assumed to be the weakly localized one.
DOI： 10.1002/(SICI)1521-3951(200003)218:1<177::AID-PSSB177>3.0.CO;2-G
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The drastic change of magnetoresistance by the rise of electron temperature in Sb doped Si Reviewed
A. Fujimoto, H. Kobori, T. Ohyama, S. Ishida
Physica Status Solidi (B) Basic Research 210 ( 2 ) 263 - 268 1998.12
Joint Work
Temperature, electric current and magnetic field dependences of dc resistivity and Hall coefficient have been measured for the Si:Sb sample whose donor concentration is 0.80Nc, where Nc is the critical concentration of the metal-insulator transition. From the observation of ∈2 conduction the hard Coulomb gap seems to open at low temperatures, and we have found that the resistivity, Hall coefficient and magnetoresistance sensitively depend on the electric current density. These data have been analyzed using a two-band model including the upper and lower Hubbard bands. In particular, below Nc positive magnetoresistance is observed in the low electric current density regime. As the electric current density increases, a crossover to negative magnetoresistance is observed. It is concluded that these phenomena originate from the rise of the electron temperature and the transfer of electrons from strongly localized to weakly localized states in the upper Hubbard band.
DOI： 10.1002/(SICI)1521-3951(199812)210:2<263::AID-PSSB263>3.0.CO;2-X
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Resonant-Photoelectromagnetic Effects in Far-Infrared Regions for Compound Semiconductors Reviewed
Tomonori Okashita, Hiromi Kobori, Tyuzi Ohyama
Journal of the Physical Society of Japan 67 ( 2 ) 675 - 678 1998.2
Joint Work
The dc voltage generation associated with cyclotron resonance (CR) and impurity cyclotron resonance (ICR) in n-InGaAs and n-InSb has been studied. In both materials, it is found that the observed voltage decreases rapidly with increasing temperature and the line shape has similar structure to the absorption coefficient. Using a thermomagnetic model where a heat current induces a voltage through the Nernst-Ettingshausen effect, we have explained the experimental results. This technique allows us to investigate resonant absorption for a sample in which transmission measurements are difficult to perform, as well as to study the mechanism of phonon transport through the sample.
DOI： 10.1143/JPSJ.67.675
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Electromotive force generation induced by cyclotron resonance in semiconductors Reviewed
T. Okashita, H. Kobori, T. Ohyama
ICMMT 1998 - 1998 International Conference on Microwave and Millimeter Wave Technology, Proceedings 1998-August 698 - 701 1998
Joint Work
© 1998 IEEE. We report on the electromotive force generation associated with electron cyclotron resonance (ECR) as well as impurity cyclotron resonance (ICR) in n-InGaAs. The observed voltage traces with variation in the magnetic field show similar structure to that of the absorption, and decrease rapidly with increasing temperature. An effect characteristic of the thin films is also observed. Comparing with results for the bulk samples and using thermomagnetic model we demonstrate the mechanism of voltage generation in films, and show the usefulness of this method to study the phonon transportation inside the sample and/or at interfaces between thin film and its substrate, and, what is more, to study resonant absorption.
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Bound to unbound state transition of hydrogenic donors in semiconductors with a static screened Coulomb potential - A trial on variational calculation Reviewed
H. Kobori, T. Ohyama
Journal of Physics and Chemistry of Solids 58 ( 12 ) 2065 - 2068 1997.12
Joint Work
The ground state energy of a hydrogenic donor in the static screened Coulomb potential for single-valley semiconductors is calculated as a function of the screening wavenumber qs by use of the variational principle. We employ a more realistic variational wavefunction rather than a hydrogenic one in consideration of the gentle extinction of the true wavefunction in the radial direction due to the strong screening. The effective Bohr radius aD of a screened hydrogenic donor and the average distance 〈r〉 of the bound electron from the positive donor atom are also obtained as a function of qs. We obtained the result that the critical value of qsaB for the unbound state was 1.13, compared with 1 for a hydrogenic type, with an effective Bohr radius aB of an isolated hydrogenic donor. It is found that the Mott concentration is given by N1/3c = 0.325 at qsaB = 1.13 for our calculation (N1/3c aB = 0.254 at qsaB = 1 for the hydrogenic type) if we assume Thomas-Fermi screening and the equal concentration of free electrons and donors at the nonmetal-metal transition (Mott Transition) point. © 1997 Elsevier Science Ltd. All rights reserved.
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Variational calculations of energy levels associated with 1s, 2p and 3d states of hydrogenic donors for semiconductors in arbitrary magnetic field Reviewed
H. Kobori, T. Ohyama
Journal of Physics and Chemistry of Solids 58 ( 12 ) 2057 - 2064 1997.12
Joint Work
The energies of 1s, 2p-1, 2p0, 2p+1, 3d-2, 3d-1, 3d+1 and 3d+2 states for an isolated hydrogenic donor in single-valley semiconductors are calculated as functions of magnetic field by use of the variational principle. We employ the variational wave functions which are applicable to an arbitrary magnetic field strength. At the low and high magnetic field limits, these variational wave functions become equal to the hydrogen and harmonic-oscillator types. Analytic expressions for the expectation values of the energies are presented according to the variational wave functions which we employ. The binding energy is also calculated as a function of magnetic field. It is found that our present calculations show reliable results in an arbitrary magnetic field. © 1997 Elsevier Science Ltd. All rights reserved.
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Magnetic-field-induced narrowing of far-infrared magneto-optical absorption of neutral donors in GaAs Reviewed
Hiromi Kobori, Masutaka Inoue, Tyuzi Ohyama
Solid State Communications 93 ( 5 ) 363 - 366 1995.2
Joint Work
We have studied the magnetic-field-induced narrowing of far-infrared (FIR) magneto optical absorption line for is-2p+1 transition of hydrogenic shallow donors in n-GaAs, of which line-width is dominated by the concentration broadening. The concentration range of donor is 1.5 × 1015 cm-3 to 6.0 × 1016cm-3, which extends over the concentration for occurrence of metal-insulator transition (Molt transition), i.e., 1.6 × 1016cm-3. The linewidth narrowing due to magnetic field is considered to arise from the strong shrinkage of 2p+1 state. © 1995.
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Direct evidence for a charge-controlled optical quenching of el2 centers in semi-insulating gaas Reviewed
Tyuzi Ohyama, Tetsuo Shimizu, Hiromi Kobori, Eizo Otsuka
Japanese Journal of Applied Physics 32 ( 5 R ) 1889 - 1897 1993.5
Joint Work
We present results of microwave photoconductivity as well as far-infrared laser magnetooptical absorption measurements obtained for LEC (liquid encapsulated Czochralski)-grown semi-insulating GaAs crystals. At low temperature we have observed a “photo-quenching effect” both for signals of microwave photoconductivity and for the shallow donor Zeeman absorption measured using a far-infrared laser, under the condition of BBG (below- band-gap) photoexcitation. Through these experimental observations, we conclude that the shallow donor is associated with the metastable state of the EL2 center in LEC-grown semi-insulating GaAs. From the temperature dependence of microwave photoconductivity decay with BBG pulsed photoexcitation, we can estimate the characteristic activation energy, AEk = 2AQ meV, which originates from a barrier in the conduction band related to the Franck-Con- don shift of the EL2 metastable state. © 1993 The Japan Society of Applied Physics.
DOI： 10.1143/JJAP.32.1889
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High magnetic field effects on electron-bound hole recombination in InSb Reviewed
H. Kobori, T. Ohyama, E. Otsuka
Physica B: Physics of Condensed Matter 184 ( 1-4 ) 128 - 131 1993.2
Joint Work
The recombination rate between conduction electrons and holes bound to acceptors has been measured in InSb as a function of magnetic field (up to 9 T) at liquid-helium temperature. The abrupt and slow decreases of the recombination rate are observed in low and high magnetic fields, respectively. © 1993.
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Magnetic field dependence of polaron mass in CdTe Reviewed
Hiromi Kobori, Tyuzi Ohyama, Eizo Otsuka
Solid State Communications 84 ( 4 ) 383 - 388 1992.10
Joint Work
Polaron cyclotron resonance experiments are performed for high-quality CdTe samples at 4.2 K by use of far-infrared lasers. To study the polaron effect in a magnetic field precisely, various far-infrared wavelengths (513, 419, 394, 295, 220 and 172 microm) are employed on the conditions of photo- and electric field excitations. The polaron mass is obtained as a function of magnetic field for the n=0→n=1 ( n: Landau quantum number) and n=1→n=2 cyclotron resonance transitions. Regarding the n=0→n=1 transition, the experimental results are in good agreement with the theoretical prediction of F. M. Peeters and J.T. Devreese. © 1992.
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Diffusion and recombination of photocarriers in Ge and GaAs
H. Nakata, H. Kobori, A. Uddin, T. Ohyama, E. Otsuka
Semiconductor Science and Technology 7 ( 3 B ) 1992
Joint Work
The dynamical behaviour of carriers in highly photo-excited Ge and GaAs is investigated by far-infrared magneto-absorption and photoconductivity measurement. The authors observe far-infrared absorption due to hot holes in electron-hole gas in Ge and extension of carrier lifetime by impact-ionization of impurities in GaAs.