Two-band Conduction Model Research Articles

Limits of Ohm’s law in ultra-pure metals with almost ideal crystal lattices

By electric transport measurements on high-purity bulk molybdenum single crystals at helium-temperature, it is shown that when approaching the ideal metallic crystal structure, there are deviations from Ohm’s law that can exceed 30% even at low current densities ≅3A/mm2. This is due to the intrinsic circular magnetic field of the measuring current in the range of B≅1mT, which generates a magnetoresistance according to the two-band conduction model. With increasing lattice vibrations and/or crystal defects, this inherent uncertainty finally reduces to immeasurably small values. Finally, Ohm’s law for a nearly ideal lattice structure can be strictly exact only in the theoretical limiting case of an infinite lattice plane, because then the magnetic fields of all electrons of the current compensate to zero.

High conductivity and low activation energy in p-type AlGaN

Record-low p-type resistivities of 9.7 and 37 Ω cm were achieved in Al0.7Ga0.3N and Al0.8Ga0.2N films, respectively, grown on single-crystal AlN substrate by metalorganic chemical vapor deposition. A two-band conduction model was introduced to explain the anomalous thermal behavior of resistivity and the Hall coefficient. Relatively heavy Mg doping (5 × 1019 cm−3), in conjunction with compensation control, enabled the formation of an impurity band exhibiting a shallow activation energy of ∼30 meV for a wide temperature range. Valence band conduction associated with a large Mg ionization energy was dominant above 500 K. The apparently anomalous results deviating from the classical semiconductor physics were attributed to fundamentally different Hall scattering factors for impurity and valence band conduction. This work demonstrates the utility of impurity band conduction to achieve technologically relevant p-type conductivity in Al-rich AlGaN.

Experimental evidence of impurity-band transport in p-type polycrystalline germanium film

Herein, we report on the Hall transport properties of a p-type polycrystalline germanium film, which is grown by the DC magnetron sputtering method. The average grain size of 85 nm is calculated. The resistivity and Hall coefficient of the germanium film were measured at temperatures between 100 K and 350 K. At room temperature (T = 300 K), the resistivity of 1.06 Ω cm, the hole concentration of 7.6 × 1017 cm−3 and mobility of 7.76 cm2V−1s−1 are obtained. The temperature dependence of Hall coefficient presents a steep maximum at a specific temperature (T☆ ≈ 215 K). At the same time, the slope of the log resistivity versus 1/T curve is changed. The unusually results can be well explained by a two-band conduction model, which includes the valence band with a high mobility, and the acceptor impurity band with a much lower mobility. This work gives an evidence for impurity-band conduction in the polycrystalline germanium film.

Relaxation of the electric current in Si3N4: Experiment and numerical simulation

The relaxation of the electric current in a metal–nitride–oxide–semiconductor structure has been measured experimentally. The experiment has been compared with the calculation based on the two-band conduction model and the multiphonon mechanism of the ionization of traps. The upper estimate obtained for the recombination cross section from the comparison of the experiment with the calculation is found to be 5 × 10–13 cm2.

Electrical properties in free-standing GaN substrates fabricated by hydride vapor-phase epitaxy and self-separation technique

The authors report on the electrical properties of free-standing GaN substrates, using one typical set of low crystalline quality (type I) and high crystalline quality (type II), fabricated by hydride vapor-phase epitaxy and self-separation technique. Type I has a large electron concentration of ~1018 cm−3 and a small electron mobility of ~85 cm2/Vsec while type II has a small electron concentration of ~5 × 1015 cm−3 and a large electron mobility of ~400 cm2/Vsec. In type I, the electron concentration decreases with the decrease of temperature until a critical temperature and then increases at lower temperatures while in type II, the electron concentration continuously decreases with the decrease of temperature. These different electrical properties are ascribed to the difference in the structural defect densities of the two samples. These indicate that the electron-transport mechanism of the defective free-standing GaN substrates is dominated by the two-band conduction model. These results are consistent with the temperature dependence of the electron mobility and with the structural properties observed by using high-resolution X-ray diffraction and atomic-force microscopy.

Thickness dependence of electrical properties of polycrystalline GaSbAs thin films grown on glass substrates: Analysis on the basis of a two-band conduction model using a differential Hall-effect method

Polycrystalline films of undoped p-type GaSb0.8As0.2 with different thicknesses ranging from 0.1 to 1μm were grown on glass substrates at a substrate temperature of 300°C by molecular-beam deposition. Hall-effect measurements in the temperature range of 10–400K revealed the coexisting valence-band and impurity-band conduction. A differential Hall-effect method was applied for the purpose of obtaining concentration and mobility of holes in the surface-side region of the film eliminating the effects of the underlying substrate-side region. Furthermore, a two-band conduction model was applied for the purpose of obtaining concentration and mobility of holes in the valence band eliminating the effects of the impurity-band conduction. The concentrations of a shallow acceptor level, a deep acceptor level, and a compensating donor level are deduced through fitting the experimental temperature dependence of valence-band hole concentration by the solution of the charge-balance equation. The mean value and the standard deviation of the grain-boundary barrier height as well as the mean free path are deduced through fitting the experimental temperature dependence of mobility by assuming the thermionic emission of holes over grain-boundary potential barriers with fluctuated heights. The estimated results of the above parameters are presented as a function of the distance from the substrate interface. Especially, it is shown that the mean free path of valence-band holes is almost unchanged while the height of grain-boundary potential barriers decreases with the distance from the substrate. The obtained results suggest the necessity of improving the electrical properties of GaSbAs layers near at the beginning of their deposition by some method, e.g., by inserting buffer layers, in the practical application for thin film transistors.

Electronic structure and structural phase transition in Pb1−xGexTe:Cr under pressure

We investigate galvanomagnetic effects near the insulator-metal transition induced by pressure in Pb1−xGexTe (x=0.10, 0.13) alloys doped with chromium (4.2≤T≤300 K, B≤8 T). We observe nonlinearity in the magnetic field dependencies of Hall voltage and peaks of anomalous scattering in the temperature dependencies of resistivity. These are due to the conductivity via chromium impurity band and to the structural phase transition from the cubic NaCl phase to the rhombohedral GeTe phase. Magnetic field dependencies of the Hall coefficient RH(B) are calculated in the framework of two-band conduction model. The main parameters of charge carriers are determined. Pressure dependencies of the critical phase transition temperature are obtained.

Ferromagnetism in Diluted Magnetic Semiconductors Pb<sub>1-x-y</sub>Ge<sub>x</sub>Cr<sub>y</sub>Te

Magnetic and galvanomagnetic properties of Pb1-x-yGexCryTe (x=0.02-0.20, y=0.01-0.08) solid solutions have been investigated. It was found that the magnetic susceptibility of these alloys contains two contributions: a paramagnetic Curie-Weiss share (T<50 K) due to the paramagnetism of Cr3+ ions and a high temperature ferromagnetic share (T<300 K). Dependence of the concentration of paramagnetic centers on the composition of the matrix was obtained. The magnetic field dependences of the Hall coefficient in the vicinity of the metal-insulator transition were measured and the main parameters of charge carriers in terms of the two-band conduction model were estimated. The experimental results are discussed in the framework of the electronic structure model, assuming varying electrical and magnetic activities of Cr ions as function of germanium content in the alloys.

Theoretical Comparison of Electronic and Lattice Thermal Conductivities in n-type (Bi1-xSbx)2Te3 Solid Solutions

We have theoretically investigated the electronic and lattice thermal conductivities for various carrier concentrations in the n-type (Bi1-xSbx)2Te3 solid solutions using the two-band conduction model with a mixed scattering mechanism. For operation of thermoelectric cooling at low temperatures, the carrier concentration must be controlled so that the electronic thermal conductivity makes the total thermal conductivity have the minimum at an applicable temperature. Under this condition, the electronic thermal conductivity is over 25% of the lattice thermal conductivity and results in the gradual increase of the thermal conductivity. These effects are almost independent of the composition x under the same carrier concentration.

Electronic transport properties in the liquid 3d transitionmetal series

The resistivities and thermopowers of the liquid 3d transition-metal series have been investigated within Ziman's formula based on a spin-polarized approach. This implies the existence of atomic magnetic moments and the use of a two-band conduction model for the resistivity. The calculated resistivities are the lowest obtained until now within the Ziman scheme and better agreement with experiment has been found than with the classical non-polarized calculations. In spite of the improvement with respect to the non-magnetic results, the thermoelectric power values do not attain the degree of agreement observed for the resistivities.

Electronic transport properties in the liquid 3d transition-metal series

Abstract The resistivities and thermopowers of the liquid 3d transition-metal series have been investigated within Ziman's formula based on a spin-polarized approach. This implies the existence of atomic magnetic moments and the use of a two-band conduction model for the resistivity. The calculated resistivities are the lowest obtained until now within the Ziman scheme and better agreement with experiment has been found than with the classical non-polarized calculations. In spite of the improvement with respect to the non-magnetic results, the thermoelectric power values do not attain the degree of agreement observed for the resistivities.

Growth, characterization and electrical properties of epitaxial erbium silicide

Molecular beam epitaxy (MBE) growth of erbium silicide (ERS) thin films on (111) silicon surface and their characterization are discussed. We demonstrated that continuous and monocrystalline films can be prepared by co-deposition of both erbium and silicon in a 1:2 ratio, on either high or room temperature (RT) silicon substrates, followed by solid phase epitaxy (SPE). Using X-ray diffraction (XRD), transmission electron microscopy (TEM), transmitted electron diffraction (TED) and Rutherford backscattering spectrometry (RBS) we identified the silicide as ErSi 2− x , i.e. erbium disilicide with a large concentration of vacancies. The films were shown to be monocrystalline with possible vacancy ordering. The Si/ErSi 2 /Si structures were subsequently prepared and shown to have a minimum RBS channelling yield χ min value of about 5% for erbium in ErSi 2− x and of 7% for the overgrown silicon. The resistivity of epitaxial ErSi 2− x films was found to be about 35 micro ohm cm at RT. The Hall coefficient was found to change sign (at about 150 K) which was explained in terms of a two-band conduction model. At about 4.5 K both resistivity and Hall coefficient exhibit anomalies associeted with magnetic ordering of erbium atoms demonstrated by divergence of the magnetic susceptibility. The Schottky diodes made on p-silicon show excellent rectifying properties, with a barrier height of 0.74±0.01 eV. The diodes on n-silicon were ohmic at RT and rectifying at low temperatures with a barrier height of 0.29±0.01 eV. Possible electronic applications of ERS are also discussed.

Electronic transport properties of epitaxial erbium silicide/silicon heterostructures

We studied electrical parallel and perpendicular transport in thin epitaxial erbium silicide films obtained by solid phase reaction and by codeposition of Er and Si on (111) Si. Resistivity measurements show that the silicide is metallic with a room-temperature resistivity of 34 μΩ cm; the dependence of the Hall coefficient on temperature can be explained by a two-band conduction model. Magnetic effects are shown to affect the low-temperature resistivity and the Hall coefficient. Perpendicular transport properties are studied by electrical [current-voltage I(V) and capacitance-voltage C(V) characteristics] and internal photoemission methods on erbium silicide/n- or p-type Si diodes. The p-type diodes have a perfect rectifying behavior with a Schottky barrier height of about 0.74 eV measured by I(V) and photoemission methods. The n-type junction is ohmic at room temperature and rectifying at low temperatures; C(V) and optical measurements yield a Schottky barrier height of about 0.28 eV. Some potential applications of erbium silicide/Si heterostructures are presented.

Effect of interactions between sp and 3d impurities on the resistivities of ternary iron-based alloys in relation to the problem of steel temper embrittlement

Abstract The existence of interactions between 8p and 3d elements in an iron matrix is investigated by means of residual resistivity measurements. The experimental results are analysed on a two-band conduction model which takes into account the short-range order. An important Ni-Sb interaction term, which accounts for other physical and metallurgical properties, is pointed out.

Microwave properties of n-type InSb in a magnetic field between 4 and 300 °K

A two-band conduction model is used to determine the properties of shallow-type impurity semiconductors in the presence of microwave and dc magnetic fields as a function of temperature. Measurements using cavity perturbation techniques are employed to determine the properties of n-type InSb and theoretical and experimental results between 4 and 300 °K are compared. The hot-electron effect was found to be insignificant between 77 and 300 °K, and the scattering mechanisms are dominated by acoustic and polar modes over the same temperature range.

Saturation of Impurity Cyclotron Resonance and Effect of 337 µm Radiation on Carrier Numbers and Mobilities in n-InSb

Effect of intense far-infrared radiation on magneto-absorption, photoconduction and photo-Hall effect on n -InSb containing 2.5×10 13 cm -3 electrons were investigated at liquid helium temperatures by using an HCN laser. Saturation of impurity cyclotron resonance (ICR) and enhancement of conduction-electron cyclotron resonance (CCR) with the radiation intensity were found. This can be explained by the effect of relaxation processes where the electrons excited to impurity state (010) drop to Landau state N =0 and accumulate there. The relaxation time for the ICR saturation was obtained as 7×10 -9 sec at 1.7 K. The photoconductivity and the photo-Hall effect were analysed on the basis of the two-band conduction model. The result demonstrates that with an increase of the radiation intensity the number of conduction electrons increases consistently with the proposed relaxation processes, and in addition the impurity-band mobility also increases.

Thermoelectric power and electronic structure of dilute alloys of nickel and cobalt with d transition elements

We present new measurements of thermoelectric power for nickel and cobalt dilute alloys with d transition elements in two temperature ranges: from 1.3 to 4.2 K and from 77 to 300 K. We analyse the measurements at room temperature in a two-band conduction model and deduce the thermopowers associated with the matrix (Smσ) and with the impurities (Soσ) in each sub-band σ. The results obtained for the impurity thermoelectric power confirm the model of electronic structure suggested for this alloy from magnetization and resistivity measurements; this model is characterized by the occurrence of a virtual bound state supward arrowdupward arrow at the Fermi level when the charge difference between the matrix and the impurity is large enough. From the values of impurity thermoelectric power in the supward arrow sub-band, we deduce the position and the width of the virtual bound state.

Pouvoirs thermoelectriques et structure electronique d'alliages a base de cobalt

Measurements of thermoelectric powers of cobalt and its dilute alloys with V, Mo, Ru, Os, Re have been carried out between 77 and 300°K, and analyzed, at room temperature, in a two-band conduction model. The structure of the virtual bound state, associated with impurities is deduced.