We studied the influence of adsorbates on field emission (FE) properties of well-aligned ZnO nanoneedle arrays with varying initial electric field, vacuum gap, and ambient pressure. The FE current exhibits hysteresis upon a loop of applied voltage. On applying an initial electric field of 3.85×106 V/m, the turn-on voltage increases ∼50%, Fowler-Nordheim plots show better linearity, as well as the hysteretic behavior can be annihilated. It is found that the FE current depends on the vacuum gap and the ambient pressure. The FE current increased sensitively with increasing the ambient pressure P when P<∼7×10−5 Pa, and it saturates in the pressure range of 7×10−5–3×10−4 Pa above which the FE current drops. The phenomena can be explained by the adsorbate effects. The adsorbate states modify the effective work function of the samples, and two emission regimes, adsorption-controlled and desorption-controlled processes, were proposed. Our results are useful for practical applications as the I–V characteristics ca...

Effect of adsorbates on field-electron emission from ZnO nanoneedle arrays

From a linear relation between the applied bias and the anode–sample distance, the electric field necessary for field emission (FE) can be obtained reproducibly. For heavily Si-doped AlN and AlxGa1−xN (0.38⩽x<1), the band gap (Al-mole fraction, x), Si-dopant density (NSi), and thickness dependences of the field emission are investigated. After optimizing the sample structure, we obtained a FE current density of 0.22 A/cm2 with a 0.3-mm-diameter rod anode from 0.8-μm-thick Si-doped (NSi:1×1021 cm−3) AlN.

Field-emission characteristics and large current density of heavily Si-doped AlN and AlxGa1−xN (0.38⩽x<1)

Nanoporous alumina membranes represent templates for the synthesis and alignment of filamentous nanostructured materials. Porous aluminium oxide (PAOX) membranes are ideal for high-temperature synthesis. They are transparent, flexible, freestanding and thermally robust. Their pore sizes can be varied over a wide range. The membrane backside seems suited for structuring processes on the nanoscale, for which it may serve as a master. After introducing the formation process of PAOX membranes, synthesis of carbon nanotubes (CNTs) with different morphologies, their electrical field emission properties, and the formation of polymeric filaments and structured polymer surfaces with the aid of PAOX are reported in this microreview.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

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Carbon and Polymer Filaments in Nanoporous Alumina

The field emission current densities j from n-type semiconductors are obtained using a fully exact calculation scheme and are plotted in the Fowler–Nordheim (FN) coordinates. It is found that the FN plot for a semiconductor has the nonlinear slope that originates in the emission mechanism. The slope of the FN plot has three values, each value representing the slope in the three distinct regions of field F. This implies that the tunneling probability makes three distinct contributions to the electron emission according to the value of F. It is also found that the slope is modified with the carrier concentration and the band gap.

Analysis of the slope of the Fowler–Nordheim plot for field emission from n-type semiconductors

ZnO:Ga nanowires with low turn-on field for field-emission lighting

The carrier concentration-dependence of field emission is theoretically investigated calculating the conduction band contributions to emission current for several n-type semiconductors. The calculated field emission current densities j are found to increase slowly with increasing concentration n for n-type Ge, Si, GaN semiconductors. For n-type GaAs, j increases before a certain value of n and decreases after it. Such n-dependence of all semiconductors varies slightly with the applied field. It is to note that the internal voltage drop due to field penetration is a crucial quantity in determining the n-dependence. The band gap shrinkage effect is meaningful only in high carrier concentrations.

Carrier concentration-dependence of field emission from semiconductors

The conductance of a microscopic constriction in a two-dimensional electron gas is obtained as a function of both the constriction width and curvature. When the quantized conductance G at plateaus is given by the channel number N_c times the quantum unit 2e²/h, N_c is found to be a function of not only the width and but also the curvature. At a given W, N_c increases by one whenever the constriction curvature decreases by about a certain value. Until the shape smoothness becomes comparable to the two parallel boundaries, there exist more channels available for conduction in a smaller-curvature constriction than in a larger-curvature one. This result is very interesting because N_c has been considered to depend on the width W only. This reflects that the number of the quantized transverse levels depend on both the constriction width and curvature in a two-dimensional electron gas.

Curvature - dependence of the quantized conductance of a microscopic constriction in a two - dimensional electron gas

A hyperbolic model is used to describe the shape effect of the quantized conductance of a microscopic constriction in a two-dimensional electron gas. The constriction is given by two hyperbolas of β = β 0 and π-β 0 in elliptic coordinates (α, β). The conductance G of the constriction is calculated by using Mathieu's functions which satisfy Schrodinger's equation and the hyperbolic boundary conditions. It is found that the number of channels N C depends not only on the constriction width W but also the slope-related coordinate β 0 . It is also found that tunnelling is the important factor to determine the shape of the quantized G graph and depends on β 0 . Quantization of G is more possible for the large-β 0 constriction (i.e. the long and smooth shape) than for the small-β 0 constriction (i.e. the short and steep shape). For comparison, we calculate G of the hyperbolic constrictions using the adiabatic approximation that the shape is smooth enough for quantization. All adiabatic results for every β 0 have almost the same G curve, which is similar to one obtained by the first scheme for smoothly changing shape, say β 0 = 45°.

Shape effects of the conductance of a quantum ballistic constriction in a two-dimensional electron gas

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Carrier concentration-dependence of field emission from semiconductors

Curvature - dependence of the quantized conductance of a microscopic constriction in a two - dimensional electron gas

Shape effects of the conductance of a quantum ballistic constriction in a two-dimensional electron gas