Large Electron Concentration Research Articles

Growth and Properties of Bulk Single Crystals of GaN

ABSTRACTIn this paper we review recent developments in the growth of bulk GaN crystals by a high-pressure, high-temperature method. We also provide information on various physical properties of bulk GaN material. Then, some preliminary results on the homoepitaxial growth of GaN are given. In the second part of this paper we discuss the following problems: the possible origin of the large free electron concentration in undoped GaN material, the parasitic effect of yellow luminescence and the nature of Zn- and Mg-acceptors.

Self-consistent quantum-mechanical calculations in ultrathin silicon-on-insulator structures

Schrödinger’s and Poisson’s equations have been self-consistently solved in ultrathin silicon-on-insulator (SOI) structures. The interaction between the front and back inversion layers has been studied as a function of the silicon film thickness, electron concentration, and temperature. The splitting of the subband energy levels, due to the possibility of quantum-mechanical interaction between the two inversion layers, strongly depends on the silicon film thickness tSi. The total subband structure can qualitatively vary with the applied gate voltages. For the thinnest devices (tSi<15 nm), volume inversion may occur even for rather large electron concentration and low temperature. For intermediate SOI thicknesses (tSi≂15–50 nm), many more subbands are populated in SOI structures than in their bulk silicon counterparts.

An envelope function description of the quantum well formed in strained layer SiGe/Si modulation doped field effect transistors

Schrödinger and Poisson equations are solved self-consistently to calculate the quantum mechanical properties of the quantum well (QW) formed in SiGe/Si/SiGe strained layer modulation doped field effect transistors. The strain produces an electron quantum well in Si. The two-dimensional electron gas distribution is calculated to show excellent confinement at low temperature. The confinement is poor at very low electron concentration at room temperature. However, it improves with increasing number of channel electrons. Moreover, a large electron concentration in a wide QW may result in the loss of confinement.

Magneto-transport in InAs/AlSb quantum wells with large electron concentration modulation

We report measurements of magnetocapacitance, Hall resistance, and magnetoresistance of gated InAs/AlSb quantum wells, By varying the voltage between the front gate and the two-dimensional electron gas, we were able to make magneto-transport measurements at 4.2 K for electron concentrations covering a range from 3 × 10 11 to 3 × 10 12 cm −2. A strongly modified magnetocapacitance signal and a drop in the mobility were observed as the second subband became populated. The relation of gate voltage to density of mobile carriers obtained from these measurements was in agreement with the simple capacitor model, indicating the absence of Fermi level pinning in the quantum well.

Boron and carbon in nickel, iron, and Ni3Al

Small amounts of boron and carbon in nickel, iron, and Ni3Al behave similarly except that carbon, when segregated into grain boundaries in Ni3Al and other nickel rich alloys, promotes rather than suppresses intergranular embrittlement. It is suggested that this stems from the absence of a stable carbide in the nickel system, as contrasted with the readiness to form carbides and borides in the Fe–C, Ni–B, and Fe–B systems. Excess carbon in nickel rich grain boundaries is thus expected instead to form graphite-like structures, so weakening the boundaries. This behaviour is attributed to a large electron concentration, leading to some occupation of antibonding spd hybrid states and a positive heat of solution.MST/1449

Electron-electron collision broadening of the conduction band edge state in heavily doped n-type indium oxide

Thin films of heavily doped In2O3 have been reported to show high infra-red reflectance combined with good transmittance of visible radiation and have found use in transparent heat reflectors. The introduction through doping, of a large free electron concentration, produces changes in the electron-electron interactions and a lifetime broadening for some band states. This energy broadening is evaluated for the band edge state over the range of electron concentration 1019−1022 cm−3. The model used incorporates exchange through the use of an antisymmetric wave function in the squared matrix element of the electron-electron Coulombic interaction. For a dopant concentration of 5×1020cm−3 the band edge broadening is found to be some 47 meV. The effect on the calculated broadening of the inclusion of exchange is shown to be small for all dopant concentrations used in solar collector films.

Exchange of oxygen isotopes between carbon dioxide and ion-exchanged zeolites A

The reactivity of the framework of zeolite A has been studied through exchange of oxygen isotopes between carbon dioxide and (Na, Ca)-A and (K, Ca)-A zeolites. Special care was taken to exclude effects of residual water by using a clean vacuum system. All zeolites investigated contained a very reactive part, amounting to 2%, on which the reaction rate was too fast to be followed. This part was assigned to amorphous impurities which could not be detected by X-ray diffraction techniques. The Al—O—Si linkage in zeolite A was not so easily broken as in X- and Y-type zeolites. About a half of the framework oxygen atoms were very difficult to exchange, and were assigned to OIII in crystallographic notation. The exchange rate depended sensitively upon the composition of the zeolite. A model for the activated complex, in the exchange reaction, was derived from this composition dependence. This activated complex was a carbonate ion bridged to the framework containing a pair of exchangeable cations supplied from the four- and eight-membered ring sites. It was concluded that OI is more reactive than OII, since the former has a larger electron concentration than the latter, and more easily forms a carbonate ion. The rate constant showed an unusual temperature dependence; there was a maximum in Arrhenius plots with zeolite K1.0Ca5.5-A, but not with Na12-A. The reason for this is qualitatively discussed.

Electron mobility in heavily doped and compensated gallium arsenide due to scattering by potential fluctuations

The electron mobility at 77K of heavily doped and compensated gallium arsenide is calculate assuming the predominant scattering comes from a smooth random potential due to a large number of charged impurities with correlated impurity distribution. The nonparabolicity of a conduction band is taken into account at large electron concentrations. A set of samples with given compensation ratios and electron concentrations has been used for experimental verification of the model which explains well the experimental mobilities.

Avalanche-initiated space-charge oscillations

Physical mechanisms are proposed for avalanche oscillations in n+-n-n+semiconductor structures. A linearized analysis is proposed for these mechanisms, the results of which agree well with the results of a large-signal computer simulation. The oscillation mechanism is dependent upon a large excess electron concentration that is present at high current levels in the n region of an n+-n-n+structure. This electron concentration causes a net negative space charge in the n region, which in turn causes the electric field to be nonuniform, peaking at the anode n+contact. At sufficiently high current densities, an avalanche zone will form at the anode contact. The resultant carrier generation in this zone creates a hole domain of density sufficient to quench the avalanche. This hole domain then travels across the n region under the influence of the field. The positive space charge of the hole domain depresses the field sufficiently to prevent avalanche from recurring at the anode until the domain has extracted at the cathode. The field variation during this cycle causes transit-time terminal voltage oscillations. It is shown how, under proper conditions, a steady-state plasma region may be established over a substantial portion of the device length. This plasma region will cause the device to exhibit a negative differential resistance, and will also support relaxation oscillations at a frequency comparable to the reciprocal of its extraction time.

Abnormally large electron concentration in the ionosphericF2 region at summer-night in middle latitudes

Abnormal ƒoF2 diurnal variations in summer with low noon and high midnight values, occurring most predominantly around Yakutsk in the Asian zone and Port Lockroy in the Antarctic, are analyzed, and the cause is investigated. The anomaly is thought to be due to the enhancement of electron concentration at summer-night. Under the assumption that this enhancement of electron concentration is caused by the horizontal movement of electrons due to electromagnetic and gravitational forces, the continuity equation in a nonsteady state for a two-dimensional (horizontal) F2 region model is solved numerically. Calculated electron concentration variations with time in the area concerned agree approximately with the observed ones. On this basis, the reason for the occurrence of the anomalous ƒoF2 diurnal variation in restricted areas is inferred. Effectiveness of the two-dimensional treatment is also discussed.

Effect of Doping on the Electron Spin Resonance in Phosphorus Doped Silicon

Electron spin resonance experiments were carried out at room temperature on n-types silicon doped with various amounts of phosphorus. A single absorption line was observed in samples of fairly large electron concentration. The line width of electron spin resonance absorption was found to increase, while the g -factor to decrease with the increase of electron concentration in the sample. The experimental results were found to agree qualitatively with the existing theories of the electron spin resonance of conduction electrons.