Decrease In Hole Concentration Research Articles

Suppression of compensation from nitrogen and carbon related defects for p-type N-doped ZnO

In this letter, the authors performed a comprehensive study on suppression mechanism of compensation from nitrogen and carbon related complex defects in N-doped ZnO grown by metal-organic chemical vapor deposition. The chemical bonding information of donorlike substitutional complex defects, (NN)O and (NC)O, were restrained with low N/O ratio, leading to the conduction type conversion. High epitaxial temperature has more suppressing effect on the formation of desired acceptor NO than that of (NC)O, as evident by the decreasing hole concentration. Upon utilization of such suppression effect, this study provides a promising route to realize p-type ZnO.

Moisture sensitivity of p-ZnO/n-Si heterostructure

A p-ZnO/n-Si thin film heterojunction is fabricated on an n-type Si substrate by pulsed laser deposition (PLD). The crystallinity of the junction materials and surface morphology are examined by an X-ray diffractometer (XRD), scanning electron microscope (SEM) and cross-sectional transmission electron microscope (TEM). The current–voltage ( I– V) characteristics of the p–n heterostructure show nonlinear diode like behavior. Zinc oxide doped with urea as nitrogen source shows p-type conductivity, which is further confirmed by its moisture sensing ability. The moisture sensitivity of the heterostructure shows the increase of resistance due to decrease of hole concentration and thus reveals the p-type conductivity of nitrogen doped ZnO film. Nitrogen doped p-ZnO/n-Si thin film heterojunction shows almost linear variation of resistance with relative humidity (RH) in the range 30–97% with a response and recovery time of 12 s and 36 s respectively at normal atmospheric temperature and pressure.

Mechanisms of Decrease in Hole Concentration in Al-Doped 4H-SiC by Irradiation of 200 keV Electrons

From the temperature dependence of the hole concentration p(T) in a lightly Al-doped 4H-SiC epilayer irradiated with several fluences of 200 keV electrons, the density of Al acceptors with 0.2 V E + eV decreases significantly with increasing fluence, whereas the density of unknown defects with 0.37 V E + eV increases with fluence and then decreases slightly. Although only C vacancies increase with fluence because 200 keV electrons can displace only C atoms, only the increase in the density of C monovacancies cannot explain the changes of p(T) by 200 keV electron irradiation. It may be necessary to consider the relationship between C vacancies and Al acceptors.

Distinct doping dependences of the pseudogap and superconducting gap ofLa2−xSrxCuO4cuprate superconductors

We have performed a temperature-dependent angle-integrated photoemission study of ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Cu}{\mathrm{O}}_{4}$ covering from lightly doped to heavily overdoped regions and oxygen-doped ${\mathrm{La}}_{2}\mathrm{Cu}{\mathrm{O}}_{4.10}$. The superconducting gap energy ${\ensuremath{\Delta}}_{\mathrm{sc}}$ was found to remain small for decreasing hole concentration while the pseudogap energy ${\ensuremath{\Delta}}^{*}$ and temperature ${T}^{*}$ increase. The different behaviors of the superconducting gap and the pseudogap can be explained if the superconducting gap opens only on the Fermi arc around the nodal $(0,0)\text{\ensuremath{-}}(\ensuremath{\pi},\ensuremath{\pi})$ direction while the pseudogap opens around $\ensuremath{\sim}(\ensuremath{\pi},0)$. The results suggest that the pseudogap and the superconducting gap have different microscopic origins.

Relationship between the superconducting gap and the pseudogap: Temperature-dependent photoemission study of La 2− xSr xCuO 4 and La 2CuO 4.10

We have performed a detailed temperature-dependent angle-integrated photoemission study of lightly-doped to heavily-overdoped La 2− x Sr x CuO 4 and oxygen-doped La 2CuO 4.10. We found that, while the magnitude of the (small) pseudogap Δ ∗ increased with decreasing hole concentration, that of the superconducting gap Δ sc did not increase.

Unexpected behavior of diamagnetic signal in polycrystalline Bi 2Sr 2Ca 1− xY xCu 2O 8+ δ

The effect of superconductivity and inhomogeneity on the phase coherence between superconducting grains for Bi 2Sr 2CaCu 2O 8+ δ (Bi-2212) is discussed. We find that in the UD regime, inhomogeneity plays a central role in establishing the phase coherence between superconducting grains. By decreasing hole concentration, p, the phase stiffness for the thermal fluctuations is suppressed, whereas the phase coherence between the superconducting grains is enhanced. In particular, the phase coherence between the superconducting grains is established almost immediately as the superconductivity for Bi-2212 with p ≃ 0.075 appears.

Low-energy electronic structure of thehigh-Tc cuprates La2−xSrxCuO4 studied by angle-resolved photoemission spectroscopy

We have performed a systematic angle-resolved photoemission spectroscopy (ARPES) study of thehigh-Tc cuprates La2−xSrxCuO4, ranging from the underdoped insulator to the superconductor to the overdoped metal. Wehave revealed a systematic doping evolution of the band dispersions and (underlying)Fermi surfaces, pseudogap and quasi-particle features under the influence of strongelectron–electron interaction and electron–phonon interaction. The unusual transport andthermodynamic properties are explained by taking into account the pseudogap opening andthe Fermi arc formation, due to which the carrier number decreases as the doped holeconcentration decreases.

Structural and superconducting properties of neodymium added (Bi,Pb) 2Sr 2CaCu 2O y

The effects of neodymium (Nd) addition on the phase evolution, structural and superconducting properties of (Bi,Pb) 2Sr 2CaCu 2O y [(Bi,Pb)-2212] prepared by solid state synthesis in bulk polycrystalline form were studied. The Nd content was varied from x = 0 to 0.5 on a general stoichiometry of Bi 1.7Pb 0.4Sr 2.0Ca 1.1Cu 2.1Nd x O y . The samples were characterized by differential thermal analysis (DTA), powder X-ray diffraction, scanning electron microscopy (SEM) equipped with energy dispersive X-ray analysis (EDX), resistance–temperature ( R– T) measurements and superconductivity measurements at 64 K. It was found that the melting temperature of (Bi,Pb)-2212 slightly increases and the endotherm broadens due to the Nd-addition. The c-lattice parameter initially decreases and then increases with Nd addition. The critical temperature ( T C) and the critical current density ( J C) of the added samples are highly enhanced. The added sample shows a maximum onset critical temperature ( T C-onset) of 95.56 K ( x = 0.3) and a maximum critical current density of 719 A/cm 2 at 64 K ( x = 0.2) against 76.7 K and 100 A/cm 2, respectively, for the pure sample. The results show that the enhancement in superconducting properties are not due to any improvement in microstructure or grain growth, but due to a decrease in hole concentration as a result of Nd doping, which changes the system from ‘over-doped condition’ to ‘optimally doped condition’.

P-GaN Cathode Degradation Correlated with Hydrogen Passivation during Water Photolysis under UV Light

We investigated the mechanism of p-GaN cathode degradation in Na2SO4 solution under UV light irradiation. During water photolysis, photocurrent and hydrogen production decreased with time at a zero bias. Samples degraded by UV light irradiation showed an increase in photoluminescence (PL) intensity and a decrease in hole concentration compared with those before UV light irradiation. Annealing the degraded samples at 850 °C in N2 ambient resulted in a decrease in PL intensity and an increase in hole concentration. These changes suggest that hydrogen passivation occurs as the mechanism of p-GaN cathode degradation during water photolysis.

Effect of site selective doping of Sm on the superconducting and flux pinning properties of(Bi, Pb)-2212

The effect of site selective doping of Sm on the superconducting and flux pinning propertiesof (Bi, Pb)-2212 superconductor has been investigated. Pure, Sm substituted at differentcationic sites such as Bi, Sr and Ca, and Sm added (Bi, Pb)-2212 superconductingsamples are prepared by solid state synthesis. Remarkable improvements in theTC,JC and flux pinning properties have been observed both in the Sm substituted and Sm addedsamples and the properties are found to be highly site dependent. The bulk flux pinning force(FP) calculated fromthe field dependent JC values shows that the irreversibility lines of Sm doped samples are shifted towardshigher fields to different extents depending on the site in which the substitution oraddition is made. These changes are due to a decrease in hole concentration and thecreation of point-like defects because of the replacement of cations by rare earthions and also may be due to nanoscale secondary phases present in the samples.

Hole concentration dependence of penetration depth and upper critical field inBi2Sr2(Ca,Y)Cu2O8+δextracted from reversible magnetization

We have measured the reversible magnetization of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi-2212) with different hole concentrations as a function of temperature and magnetic field. By employing the vortex fluctuation model, the values of the penetration depth and the upper critical field are obtained. As the hole concentration decreases, fluctuation effects become more remarkable due to the decrease in superfluid density. The upper critical field has a maximum value when the hole concentration is around 0.19.

Anomalous transport and ferromagnetism in the diluted magnetic semiconductor Sb2−xCrxTe3

We grew single crystals of the new dilute magnetic semiconductor p-Sb2−xCrxTe3 (x=0,0.0115,0.0215) and investigated their transport, magneto-transport and magnetic properties in the temperature interval 1.7–300K. In high fields we clearly observed Shubnikov–de Haas oscillations, that show a decrease in hole concentration for Cr-doped samples. A negative magnetoresistance and anomalous Hall effect are observed in Cr-doped samples. Ferromagnetic order is found below the Curie temperature Tc≈5.8K (x=0.0215) and Tc≈2.0K (x=0.0115). In the ferromagnetic phase we observe large magnetic anisotropy with the easy-axis parallel to the C3 crystallographic axis.

Changes induced in electrical properties and deep level spectra of p‐AlGaN films by treatment in hydrogen plasma and by proton implantation

The influence of hydrogen plasma treatment (250 °C, 0.5 h) and of proton implantation (proton energy of 200 keV, doses between 1012 and 1015 cm–2) on electrical properties of p-AlGaN films were studied. Hydrogen plasma treatment leads to substantial (more than an order of magnitude) decrease of the hole concentration in the surface layer. The dominant deep traps observed in current deep level transient spectroscopy were the Mg-related hole traps with activation energy of 0.17 eV and 0.19 eV. The concentration of both traps was strongly decreased after the hydrogen plasma treatment. At the same time the MCL intensity in the bandedge wavelength region was greatly increased. The results are explained by partial hydrogen passivation of Mg acceptors and of deep centers responsible for non-radiative recombination. Proton implantation was found to lead to a strong decrease of hole concentration, to a decrease in the intensity of the bandedge MCL signal and to a strong suppression of the signal from the hole traps with activation energy of 0.17 eV–0.19 eV. However, no new prominent deep centers were detected in deep level spectra after the implantation. It is suggested that the main centers responsible for the increased compensation of the layers are located in the upper part of the bandgap. Possible interplay between the hydrogen passivation of deep defects and introduction of new defects due to radiation and possibly due to hydrogen interaction with primary radiation defects will be discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In situ angle-resolved photoemission study on La 1 − xSr xMnO 3 thin films grown by laser MBE

We have performed an in situ angle-resolved photoemission study of atomically flat surfaces of La 1 − x Sr x MnO 3 (LSMO) single-crystal thin films grown on SrTiO 3 (0 0 1) substrates by laser molecular beam epitaxy, and investigated the changes in the band structure induced by hole-doping. The band structures of LSMO x = 0.4 thin films consist of several highly dispersive O 2p-derived bands at the binding energies of 2.5–7 eV, almost dispersionless Mn 3d t 2 g bands at ∼2 eV, and a dispersive Mn 3d e g -derived majority band at ∼0.5 eV. We find that the energy positions of these bands monotonically shift toward higher binding energy with decreasing hole concentration in a rigid-band manner, whereas the lower binding energy part of the e g -derived emission within 1.5 eV of the Fermi level ( E F) gradually disappears. These results suggest that the simple rigid-band model does not describe the electronic structure near E F of LSMO.

Thermoelectric properties of p-type (Bi 2Te 3) x(Sb 2Te 3) 1−x crystals prepared via zone melting

In the present study, p-type (Bi 2Te 3) x (Sb 2Te 3) 1− x crystals with various chemical compositions ( x = 0 , 0.05, 0.10, 0.16, 0.20, 0.24 and 0.26) were fabricated through the zone melting method. Thermoelectric properties, including Seebeck coefficient ( α ), electrical conductivity ( σ ) and thermal conductivity ( κ ), were measured in the temperature range of 300–500 K. The influence of the variations of Bi 2Te 3 content ( x ) on the thermoelectric properties was studied. The increase of Bi 2Te 3 content ( x ) caused a decrease in hole concentration and thus a decrease of σ and an increase of α . The maximum figure of merit ( ZT = α 2 σ T / κ ) of 1.14 was obtained at about 350 K for the composition of 24% Bi 2Te 3–76% Sb 2Te 3 with 3 wt% excess Te.

Metal-to-insulator crossover and pseudogap in single-layerBi2+xSr2−xCu1+yO6+δsingle crystals in high magnetic fields

The in-plane ${\ensuremath{\rho}}_{ab}(H)$ and the out-of-plane ${\ensuremath{\rho}}_{c}(H)$ magneto-transport in magnetic fields up to $28\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ has been investigated in a series of high quality, single crystal, hole-doped La-free Bi2201 cuprates for a wide doping range and over a wide range of temperatures down to $40\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. With decreasing hole concentration going from the overdoped $(p=0.2)$ to the underdoped $(p=0.12)$ regimes, a crossover from a metallic to an insulating behavior of ${\ensuremath{\rho}}_{ab}(T)$ is observed in the low temperature normal state, resulting in a disorder induced metal insulator transition. In the zero temperature limit, the normal state ratio ${\ensuremath{\rho}}_{c}(H)∕{\ensuremath{\rho}}_{ab}(H)$ of the heavily underdoped samples in pure Bi2201 shows an anisotropic 3-D behavior, in striking contrast with that observed in La-doped Bi2201 and LSCO systems. Our data strongly support that the negative out-of-plane magnetoresistance is largely governed by interlayer conduction of quasiparticles in the superconducting state, accompanied by a small contribution of normal state transport associated with the field dependent pseudogap. Both in the optimal and overdoped regimes, the semiconducting behavior of ${\ensuremath{\rho}}_{c}(H)$ persists even for magnetic fields above the pseudogap closing field ${H}_{\mathit{pg}}$. The method suggested by [Shibauchi et al. Phys. Rev. Lett. 86, 5763 (2001)] for evaluating ${H}_{\mathit{pg}}$ is unsuccessful for both under- and overdoped Bi2201 samples. Our findings suggest that the normal state pseudogap is not always a precursor of superconductivity.

Modulated Structure of Misfit Layered Cobalt Oxide [(Ca0.90Bi0.10)2(Co0.95Bi0.05)O3]pCoO2

We have determined the crystal structure of Bi-substituted and Bi-free misfit layered cobalt oxides [Ca2CoO3]0.62CoO2, by a (3+1)-dimensional superspace group approach. Structural parameters have been refined with a superspace group of C2/m(1p0)s0 using powder neutron diffraction data. Bismuth atoms are found to substitute for both Ca and Co atoms in the rock salt-type [Ca2CoO3] subsystem. The resulting structural formula is expressed as [(Ca0.90Bi0.10)2(Co0.95Bi0.05)O3]pCoO2 with a refined b-axis ratio of p=0.6183. By Bi substitution, the modulation of the Co–O distances in the [Ca2CoO3] subsystem is markedly decreased relative to the Bi-free counterpart, whereas such a modulation in the [CoO2] subsystem is slightly increased. The observed increase in the Seebeck coefficient and electrical resistivity of the Bi-substituted phase can be explained in terms of the decrease in hole concentration in the CoO2 sheets.

Deep defect states in diluted magnetic semiconductors Pb1-xSnxTe:Yb

The galvanomagnetic effects in new diluted magnetic semiconductors Pb $_{1-x}$ Sn x Te:Yb were studied to build the energy level diagram under variation of the alloy composition and establish the connection between the parameters of electronic structure and magnetic properties. It was found that the Hall coefficient increases almost by order of magnitude with increasing the temperature. As tin content decreases, while the ytterbium concentration grows, the hole concentration decreases by several times. The results are explained by assuming a formation of an ytterbium-induced deep defect level in the energy spectrum of the alloys, which moves up to the top of the valence band with increasing the ytterbium concentration and pins the Fermi level within the valence band at sufficiently high impurity content. The hole concentration vs. ytterbium content dependence was used to calculate the energy position of the Fermi level in the frame of two-band dispersion law and to determine the position of Yb level in the alloys. The diagram of the charge carrier energy spectrum under varying the alloy composition was built.

Electronic structure of the diluted magnetic semiconductors Pb1−xSnxTe:Yb

AbstractThe galvanomagnetic effects in the new diluted magnetic semiconductors Pb1−xSnxTe:Yb were studied to determine the parameters of the electronic structure and to elucidate its influence on the magnetic properties. It was found that the temperature dependencies of the resistivity ρ and the Hall coefficient RH have a “metallic” character, however the RH changes in anomalous manner: its value increases more than by order of magnitude and then passes through maximum with increasing the temperature. Upon an increase of the ytterbium concentration the hole concentration decreases by more than order of magnitude. The results were explained assuming a formation of deep ytterbium-induced defect level in the valence band of the alloys, which moves up to its top with increasing the ytterbium concentration and pins the Fermi level within the valence band. The energy position of the Fermi level was calculated in the frame of two-band dispersion law and used to determine the position of Yb level in the alloys. The diagram of the charge carrier energy spectrum under varying the alloy composition was built.

Effects of carbon tetrabromide flux, substrate temperature and growth rate on carbon-doped GaAs grown by molecular beam epitaxy

Carbon-doped GaAs (GaAs:C) samples were grown using carbon tetrabromide (CBr 4) as p-type dopant in a solid source molecular beam epitaxy (SSMBE) system. The effects of CBr 4 flux, substrate temperature and growth rate were systematically studied. High CBr 4 flux exceeding 2.6×10 −7 Torr and substrate temperature exceeding 620°C were found to induce the formation of dicarbon defects, resulting in decrease in hole concentration and increase in lattice parameter. Surface morphology images taken using atomic force microscopy (AFM) suggest signs of possible relationship between high surface roughness and dicarbon defects. The optimum growth conditions deduced from our experiment provide a useful baseline for application of GaAs:C as base layer in heterojunction bipolar transistor (HBT).