Thermal annealing effects on Graphene/n-Si Schottky junction Solar cell: Removal of PMMA residues

Thermal annealing is one of most effective way to improve the efficiency of graphene/n-Si Schottky junction solar cell. Here, its underlying mechanism has been investigated by comparative studies in terms of the removal of polymethyl methacrylate (PMMA) residues, using the J-V characteristics, the transient photocurrent and photovoltage measurements. Experimental results have revealed that there are trap states which are originated from the PMMA residues and cause the large photocurrent leakage as the intensity of the incident light increases. It is also found that the PMMA residues accelerate deterioration and rapidly invalidate hole doping effects. Such undesirable PMMA residues were effectively removed by the thermal annealing treatments, serving to reduce the photocurrent leakage and to increase the stability.Comment: 18 pages, 7 figures, 5 table

Capacitance characterization of Graphene/n-Si Schottky junction solar cell with MOS capacitor

We have demonstrated a simple and accurate method for characterizing the capacitance of Graphene/n-Si Schottky junction solar cells (GSSCs) which embed the metal-oxide-semiconductor (MOS) capacitor. We measured two types of GSSCs, one with thermal annealing treatments (w-a) and one without (wo-a). It was found that the wo-a GSSC exhibits a two-step feature in the phase versus forward bias voltage relationship, which may be attributed to the presence of polymethyl methacrylate residues. By considering the capacitance of the MOS capacitor (Cmos) and its standard deviation, we successfully obtained the capacitance of the Schottky junction (CSch), and evaluated meaningful built-in potentials (Schottky barrier heights) which are 0.51V (0.78eV) and 0.47V (0.75eV) for the w-a and wo-a GSSCs, respectively, by the Mott-Schottky analysis. We also briefly discuss the relationship between CSch and the Nyquist and Bode plots, finding that the RC time constant decreases due to the subtraction of Cmos.Comment: 16 pages, 8 figure

Iron-Based Heavy Quasiparticles in SrFe4_{4}Sb12_{12}: An Infrared Spectroscopic Study

Temperature-dependent infrared reflectivity spectra of SrFe$_{4}$Sb$_{12}$ has been measured. A renormalized Drude peak with a heavy effective mass and a pronounced pseudogap of 10 meV develops in the optical conductivity spectra at low temperatures. As the temperature decreases below 100 K, the effective mass ($m^{*}$) rapidly increases, and the scattering rate ($1/\tau$) is quenched. The temperature dependence of $m^{*}$ and $1/\tau$ indicates that the hybridization between the Fe 3d spins and the charge carriers plays an important role in determining the physical properties of SrFe$_{4}$Sb$_{12}$ at low temperatures. This result is the clear evidence of the iron-based heavy quasiparticles.Comment: 5 pages, 5 figure

Magnetic Field-Induced Superconductor-Insulator-Metal Transition in an Organic Conductor: An Infrared Magneto-Optical Imaging Spectroscopy

The magnetic field-induced superconductor-insulator-metal transition (SIMT) in partially deuterated $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br, which is just on the Mott boundary, has been observed using the infrared magneto-optical imaging spectroscopy. The infrared reflectivity image on the sample surface revealed that the metallic (or superconducting) and insulating phases coexist and they have different magnetic field dependences. One of the magnetic field dependence is SIMT that appeared on part of the sample surface. The SIMT was concluded to originate from the balance of the inhomogenity in the sample itself and the disorder of the ethylene end groups resulting from fast cooling.Comment: 5 pages, 5 figures, to appear in Phys. Rev.

Capacitance characterization of graphene/n-Si Schottky junction solar cell with MOS capacitor

We have demonstrated a simple and accurate method for characterizing the capacitance of Graphene/n-Si Schottky junction solar cells (GSSCs) which embed the metal-oxide-semiconductor (MOS) capacitor. We measured two types of GSSCs, one with thermal annealing treatments (w-a) and one without (wo-a). It was found that the wo-a GSSC exhibits a two-step feature in the phase versus forward bias voltage relationship, which may be attributed to the presence of polymethyl methacrylate residues. By considering the capacitance of the MOS capacitor (C _mos ) and its standard deviation, we successfully obtained the capacitance of the Schottky junction (C _Sch ), and evaluated meaningful built-in potentials (Schottky barrier heights) which are 0.51 V (0.78 eV) and 0.47 V (0.75 eV) for the w-a and wo-a GSSCs, respectively, by the Mott–Schottky analysis. We also briefly discuss the relationship between C _Sch and the Nyquist and Bode plots, finding that the RC time constant decreases due to the subtraction of C _mos

Electronic Structure of Heavy Fermion Ce Compounds Studied by Photoemission Spectroscopy

Heavy fermions, one of the strongly correlated electrons systems (SCES), have attractedmuch attention due to the wide ground states from magnetism to non-magnetism through aquantum critical point (QCP). This phase variation shows the similar tendency with theothers of SCES, i･e･ the High-Tc cuprate superconductors and organic conductors, despite ofa lack evidence of connection among them. Therefore, the explicit understanding of thephase variation in heavy fermion will provide the clues to resolve the problems of SCES. Inheavy fermion system, the ground state varies as a function of the cf-hybridization strength, responsible for Kondo effects, between the local 4f electrons and the itinerant conductionelectrons in the Doniach phase diagram. In spite of the numerous research for the heavyfermions, the role of the cf-hybridization through the QCP remains poorly understood in theview of the electronic structure, because of the difficulty of a systematic experiments andthe direct observation of the cf-hybridization band. In his thesis, the two heavy fermion systems, CeNi1-xCoxGe2 and CeCoGe1.75Si1.25, aremainly studied by several kinds of photoemission (PE) spectroscopies. First, in order to investigate Ce 4f characters across the QCP, he has performed the resonant, hard X-ray and high-resolution PE on the isostructural heavy fermion CeNi1-xCoxGe2 system, where the ground state changes from an antiferromagnetic (0 ≦ x ≦ 0.2) to a non-magneticregime (0.4 ≦ x ≦ 1) through the QCP(x = 0.3). In the resonant PE, the bulk properties of Ce4f electrons are obtained from the quantitative analysts of the non-crossing approximation(NCA) with considering the surface sensitivity of Ce 4d-4f and 3d-4f resonant PE spectra, respectively. The obtained bulk properties reveals the detail electronic structure, e.g. crystalline electric field (CEF) effects, and firstly shows that the Ce 4f electronic structurecontinuously develops across the QCP with increasing cf-hybridization intensity. In the hard X-ray PE, Ce 3d core-level was measured at photon energy, hv = 7941.5 eV, where the obtained spectra almost reflect the bulk properties due to the large escape depth, λ－200 Å Analysis of Ce 3d5/2 peaks also shows the continuity of Ce 4f electronic structureacross the QCP in agreement with the results of the resonant PE. Multiplet structures areclearly observed in Ce 3d3/2 peaks. And it is first found in the X-ray PE experiment thatthese multiplet structures are strongly related to CEF effects. Moreover, the quasi-Particle peak due to cf-hybridization and the detail electronic structuredue to CEF effects is directly observed in the high-resolution PE, showing the consistencywith the results of both resonant and hard X-ray PE. Secondly, the angle resolved photoemission (ARPES) study on a heavy fermionCeCoGe1.75Si1.25 has been performed to explicitly understand cf-hybridization band in amomentum space. Even though the single impurity Anderson model (SIAM) well explainsthe spectra of the angle-integrated photoemission, Ce 4f electrons are actually affected bythe periodic potential in the solids. It surely makes the cf-hybridization bands. ARPES datashow that CeCoGe1.75Si1.25system is the quasi two-dimensional system: there is smalldispersion along the c-axis. Here, he presents the first observation of both the renormalizedCe 4f band and conduction band due to cf-hybridization by uslng Ce 4d-4f resonant ARPES. The quasi-particle peak, the so-called tail of the Kondo resonance, is strongly enhancedacross the intersected place of Ce 4f band and Co 3d conduction band in contrast to theSIAM expectation. This may be a reason why SIAM model well explains theangle-integrated PE. Furthermore, the cf-hybridization band is scaled by using the simpleperiodic Anderson model (PAM). Surprisingly, the cf-hybridization strength exhibits theanisotropy in the momentum space, which can not be expected in SIAM and simple PAMwhere the cf-hybridization strength is isotropic. This reveals that in heavy fermion system, cf-hybridization takes place in a lattice and the anisotropy of Kondo coupling should beconsidered