Low Charge Carrier Concentration Research Articles

Quantum Hall effect-insulator transition in the InAs/GaAs system with quantum dots

The InAs/GaAs structures consisting of quantum-dot layers with electronic properties typical of two-dimensional systems are investigated. It is found that, at a low concentration of charge carriers, the variable-range-hopping conductivity is observed at low temperatures. The localization length corresponds to characteristic quantum-dot cluster sizes determined using atomic-force microscopy (AFM). The quantum Hall effect-insulator transition induced by a magnetic field occurs in InAs/GaAs quantum-dot layers with metallic conductivity. The resistivities at the transition point exceed the resistivities characteristic of electrons in heterostructures and quantum wells. This can be explained by the large-scale fluctuations of the potential and, hence, the electron density.

EuGa 2± xGe 4∓ x: preparation, crystal chemistry and properties

The title compound was prepared from a mixture of the elements by melting in a glassy carbon crucible (HF furnace, argon atmosphere). EuGa 2± x Ge 4∓ x crystallizes in a new structure type (orthorhombic symmetry, space group Cmcm (no. 63), a=4.1571(3) Å, b=11.268(1) Å, c=13.155(1) Å, V=616.2(2) Å 3, Z=4, Pearson symbol oC28) and is characterized by a 3D framework of 4-fold bonded (4b) E atoms ( E=Ga,Ge) with channels parallel to the short a-axis. The europium atoms lie in the E 18 holes of those channels. The structure can also be described as an intergrowth of segments from BaAl 2Si 2 and from a reconstructed diamond type of structure. The (Ga,Ge) 24 framework of the title compound is equivalent to that of the Si(Al) atoms in the zeolite structure of CsAlSi 5O 12. Magnetic susceptibility measurements show that EuGa 2± x Ge 4∓ x undergoes a transition from a paramagnetic to an antiferromagnetic ordered state at about 9 K. In the paramagnetic region, an oxidation state of 2+ for europium ( μ eff≈8 μ B/Eu) was obtained. The crystal chemical formula of EuGa 2Ge 4 is Eu 2+[(4b)Ga 1−] 2[(4b)Ge 0] 4 according to a charge balanced Zintl phase. This was confirmed by quantum chemical calculations (TB-LMTO, ELF). However, like several other similar compounds, e.g. alkaline earth metal clathrates, EuGa 2± x Ge 4∓ x has a metal-like temperature dependence of the electrical resistivity with a low charge-carrier concentration in agreement with the low value of the calculated density of states.

Drop Size and Velocity Measurements in an Electrostatically Produced Hydrocarbon Spray

Liquid hydrocarbons are difficult to atomize electrostatically at practical flow rates due to their high resistivities and low concentration of charge carriers. However special “charge injection” techniques have been used in this investigation to produce combustible sprays of oils. An experimental investigation of the drop size and velocity distributions within a charged kerosine spray is presented, using a PDA system and photographic methods. Bimodal size distributions are found with a central core of larger drops or ligament formations near the nozzle surrounded by a sheath of smaller drops. Because of the bimodal character the concept of average diameter for the spray is difficult to apply so that there is little practical use defining a relationship between mean drop diameter and mean specific charge without a knowledge of a relationship between charge and size of individual drops. Examination of the velocity component distributions showed the processes which control the two-zone characteristics of the spray. The larger drops have a high inertia and were less deflected by the space charge force within the spray and it is argued that the larger drops possess a smaller specific charge compared with the smaller drops which reinforces the tendency for the large drops to remain along the spray centreline. For the smaller drops the converse is the case, to the extent that at low flow rates their trajectories have a negligible axial velocity component and recirculation toward the earthed injector body is observed.

O KVV Auger emission versus resonant photoemission at the O K edge of high- Tc superconductors

Photoelectron spectroscopy results on single crystals of the superconductors Bi 2Sr 2CaCu 2O 8,Bi 2Sr 2CuO 6, Ba 0.6K 0.4BiO 3 and the semiconductor Ba 0.9K 0.1BiO 3 are reported for the photon energy region around the O K absorption threshold. The development of the O- KVV Auger structure has been carefully monitored as a function of photon energy. A non-monotonic behavior displaying a feature at a constant binding energy of about 14 eV was found for Bi 2Sr 2CaCu 2O 8 and Bi 2Sr 2CuO 6 in a narrow photon energy region of 1 eV at the main edge of the O K absorption spectrum around 530 eV. The corresponding enhancement, connected with the autoionization of O 2 p states, is absent in Ba 1− x K x BiO 3 in contrast to Bi 2Sr 2CaCu 2O 8 and Bi 2Sr 2CuO 6. The resonant enhancement is more pronounced for Bi 2Sr 2CuO 6 as compared to Bi 2Sr 2CaCu 2O 8, which can be explained by a lower charge carrier concentration in the former case, leading to a more localized nature of intermediate O 2 p states. The model parameters Cu d– d and O p– p Coulomb interactions and the charge transfer energy Δ are estimated from the experiments.

Structural, electrical and optical properties of SnO 2-x:F-layers deposited by DC-reactive magnetron-sputtering from a metallic target in Ar–O 2/CF 4 mixtures

F-doped SnO x films have been prepared by DC-reactive magnetron sputtering in Ar-O 2/CF 4 mixtures at a deposition temperature of 180°C. The optical and electrical parameters (specific resistance, charge carrier concentration, mobility) were correlated with the chemical composition and the structural properties (phases, grain size). Small concentrations of F increase both the charge carrier concentration and the mobility. Higher fluorine incorporation into the films does not improve the electrical properties, which is attributed to the formation of tin fluorides. The maximum fluorine concentration in the SnO 2−x:F-films, measured by RBS, is 7×10 21 cm −3 (corresponding to 8 at.%). Comparison of the charge carrier concentrations with the fluorine content shows that the electrical activation of the fluorine is smaller than 3.4%. The low electrical activation of the fluorine is tentatively explained by the formation of tin fluorides, leading to low charge carrier concentrations (N D≈1.4× 10 20cm −3). This limits the specific resistivities to values >7×10 −3 Ωcm.

Structure and low temperature properties of SrB6

A detailed study of the room temperature structure of high quality single crystalline SrB6 has been made. Minute changes in interatomic distances may drastically affect the electronic spectrum of this compound. Electronic transport properties both at zero and non-zero frequencies above 4He temperatures indicate that SrBP6 is close to be a semiconductor. However, at very low temperatures this compound enters a metallic state with a low concentration of itinerant charge carriers. Possible correlation effects are indicated by anomalous temperature dependences of the electrical resistivity and the specific heat below 1 K.

Impedance spectroscopy on [formula omitted] crystals

Crystals of Na +−β″-Al 2O 3 and Na + Ho 3+ −β″-Al 2O 3 , with varying degrees of exchange, have been investigated by impedance spectroscopy using sputtered chromium films as electrodes. For this work a sample holder was developed to enable studies of the in-plane conductivity of even very thin β″-alumina crystals. The ionic conductivities of the investigated Na +−β″-Al 2O 3 crystals were found to be one order of magnitude lower than those reported in the literature, a discrepancy caused by a lower charge carrier concentration. On the other hand, the activation energy and the diffusion coefficients of the sodium ions, as deduced from the Nernst-Einstein equation, agree well with previous studies. In the case of the Na + Ho 3+ −β″-Al 2O 3 crystals, a strong decrease of the ionic conductivity has been observed with increasing degree of exchange. To determine the bulk conductivities an equivalent circuit had to be applied which differed from the one used in case of Na +−β″-Al 2O 3. With an additional RC-element in parallel these spectra could be fitted with satisfying results. This RC-element may be regarded as a second relaxation process of the Na + ions.

Structure-property relationships in poly(o-phenylenediamine) derivatives

Abstract Various poly(o-phenylenediamine) derivatives (e.g. poly(2,3-diaminotoluene) (PDT) and poly(N-n-butyl-o-phenylenediamine) (PBPD)) have been prepared electrochemically and characterized. Cyclic voltammetry measurements in different acidic conditions have revealed a one-step oxidation process in these polymers which implies the formation of deprotonated imines in PDT and of some oxidized (charged) substituted imines in PBPD. However, in-situ conductivity measurements showed that both materials have low electrical conductivities (with a maximum of 10−4−10−3 S cm−1). On the basis of results obtained on model compounds (5,12-dihydro-5,7,12,14-tetraazapentacene and some derivatives), these low electrical conductivities can be explained by a low concentration of charge carriers and poor interchain contacts.

The thermoelectric power in In 2O 3

The n-type semiconductivity of In 2O 3 is confirmed by the sign of the Seebeck coefficient. From a comparison of Hall and Seebeck data a value of 0.14 m o is deduced for the density-of-states effective mass. The incorporation of divalent cations results in a higher absolute value of the Seebeck coefficient, which is reflected in the lower charge carrier concentration, in accordance qualitatively with the defect model for In 2O 3.

Instability of the localized small polaron in high electric fields

A model is proposed for the effect of high electric fields on low conductivity materials where the major mode of conduction is by hopping of localized small polarons. It is predicted that the mobility increases with the field up to a threshold field where delocalization of the small polaron occurs. At this field the mobility increases discontinuously by a few orders of magnitude, and the conduction mode is in a polaronic band. Observation of the instability has been made on polycrystalline spinel Co 1+χCr 2−χO 4 and on single crystal Co 0.994Li 0.006O. The I− V characteristics have been determined by dc and ac at 60 Hz and by repetitive pulsing at low repetition rates. Thick specimens were used (0.01 to 0.10 mm) to assure the bulk nature of the transition. The instability consists of a transition from a non-ohmic low conductivity state at low fields to a high conductivity state when a breakdown field E b is attained. The high conductivity is sustained until the field is lowered below a minimum field E m, at which point the low conductivity state is restored. The model can be applied to amorphous semiconductors as well, under the assumption that the low conductivity mechanism is small polaron hopping, rather than very low concentration of charge carriers in a high mobility band. The intrinsic conduction of these materials can be attributed then to the activation energy of the hopping motion.

Pulsed nuclear magnetic resonance of gamma-irradiated lithium hydride

The growth of wurtzite ZnTe thin films with thickness between 250 and 1000 nm on borosilicate glass substrates by electron beam evaporation is reported. The formation of the wurtzite structure was confirmed using X-ray diffraction. The films showed diffraction peaks originating from the (110), (016) and (116) planes, indicating absence of any preferred orientation. The transmission of all the films was of the order of 80% in the near IR region. The refractive index of the wurtzite ZnTe phase increased with increase in thickness from 3.0 at 250 nm to 4.2 for the 1000 nm thickness film at a wavelength of 1800 nm. The optical band gap of these films increased with thickness showing values of 0.85, 0.9 and 0.98 eV at 250, 400 and 1000 nm thickness, respectively. Chemical composition studies revealed that the films were mildly non-stoichiometric with excess Te. Comparison with the zinc blende structure of ZnTe shows that the wurtzite structure has a higher refractive index, lower band gap and lower charge carrier concentration.