V. Filip

Growth of aligned carbon nanotubes by plasma-enhanced chemical vapor deposition: Optimization of growth parameters

Direct-current plasma-enhanced chemical vapor deposition (CVD) with mixtures of acetylene and ammonia was optimized to synthesize aligned carbon nanotubes (CNTs) on Co- or Ni-covered W wires with regard to wire temperature, wire diameter, gas pressure, and sample bias. A phase diagram of CNT growth was established experimentally in this optimization process. It was revealed by transmission electron microscopy that Co-catalyzed CNTs encapsulated a Co carbide nanoparticle at their tip, disagreeing with a previous report that Co particles were located at the base of CNTs CVD grown on Co-covered Si substrates [C. Bower et al., Appl. Phys. Lett. 77, 2767 (2000)]. This leads to the conclusion that the formation mechanism of aligned CNTs depends significantly on the catalyst support material as well as the catalyst material itself. Since the sample bias strongly affected the morphology of CNTs, the selective supply of positive ions to CNT tips was possibly responsible for the alignment of growing CNTs.

Modeling the electron field emission from carbon nanotube films.

A theoretical framework for the electron field emission from carbon nanotubes (CNTs) is discussed. Using the tunneling theory, the influence of the detailed electron energy dispersion is proven to be of little importance for the electron field emission. By means of numerical computations in a simplified model, the influence of the environment on the local field on a CNT is discussed for an aligned CNT film. In a simple triangular model for the potential energy barrier at the tube end, a tunneling probability was obtained. A statistical model was developed for the structural and functional parameters of aligned CNT films. Practical CNT films of excellent alignment, obtained directly on a tungsten wire by plasma-enhanced chemical vapor deposition, were analyzed by this statistical model. Their distribution in the enhancement factors was thus deduced. An indirect method to get the average electrical parameters of the film using only a limited amount of experimental data was thus established.

Modeling of the electron field emission from carbon nanotubes

Using a tunneling approach for the field emission from a single carbon nanotube, expressions for the emission current as a function of the anode voltage and of the emitted electron energy spectrum are obtained. The low dimensionality of the electronic system of a carbon nanotube is taken into account. The extraction field on the nanotube’s tip is evaluated using numerical computations. For nanotubes of practical interest, having large enough diameters, it is demonstrated that the influence of the detailed form of the electron energy dispersion relations is not of major importance. This influence could be generally embedded in a numerical factor entering the expression of the emission current. The influence of the various tube parameters on the characteristics is also identified and analyzed. An approximate formula for use in practical analysis in field emission is deduced and its validity for different nanotube sizes is verified.

Temperature-dependent light-emitting characteristics of InGaN/GaN diodes

Temperature-dependent light-emitting and current–voltage characteristics of multiple-quantum well (MQW) InGaN/GaN blue LEDs were measured for temperature ranging from 100 to 500 K. The measurement results revealed two kinds of defects that have pronounced impact on the electroluminescent (EL) intensity and device reliability of the LEDs. At low-temperature ( 300 K), deep traps due to the structure dislocations at the interfaces significantly reduce the efficiency for radiative recombination though they can enhance both forward and reverse currents significantly. In addition, the significant enhancement of trap-assisted tunneling current causes a large heat dissipation and results in a large redshift of the emission peak at high temperature.

Nitrogen Incorporation into Hafnium Oxide Films by Plasma Immersion Ion Implantation

The physics and effects of nitrogen incorporation into hafnium oxide (HfO2) films were studied in detail. We found that only a trace amount (~5%) of nitrogen can be introduced into the HfO2 films by plasma immersion ion-implantation, regardless of implantation dose. We proposed that the nitrogen incorporation is mainly due to the filling of O vacancies in the as-deposited HfO2 films and the nitridation of silicide bonds at the HfO2/Si interface. Temperature-dependent capacitance–voltage and current–voltage characteristics measurements indicate that both interface and oxide trap densities were greatly reduced as the results of the nitrogen filling of the O-vacancies and the nitridation of interfacial hafnium silicide bonds.

Emission statistics for Si and HfC emitter arrays after residual gas exposure

Field emission arrays (FEAs) comprising 100 Si or HfC coated Si emitters have been fabricated. The FEAs emission properties were measured in ultra high vacuum conditions and after being subject to Ar and O2 residual gases with partial pressures in the range 10−6 to 10−4Pa. The influence of residual gases on the FEAs field emission properties has been comparatively assessed using the model parameter extraction method. The array field emission model uses equations that describe the electron emission current from individual emitters of given radius R and work function ϕ together with nonuniform distribution functions giving the dispersion of these parameters within the array. A simplified relationship for the array emission current depending on a single integral instead of a double one has been derived. A nonlinear extraction algorithm using partial derivatives of the objective function is used, which gives fast and accurate results. Both Si and HfC emitters exhibit (to a different degree) tightening of the ...

Characterization of enhanced field emission from HfC-coated Si emitter arrays through parameter extraction

In this paper a HfC coating layer was deposited on the tips resulting in enhanced field emission properties. The field emission temporal evolution was measured in ultra-high vacuum conditions and also in O/sub 2/ and Ar gases of different concentration.

Field electron emission from carbon nanotubes grown by plasma-enhanced chemical vapor deposition

Densely distributed, aligned carbon nanotubes were grown on Co-covered W wires by dc-plasma-enhanced chemical vapor deposition with a mixture of acetylene and ammonia as precursors. Each nanotube was capped with a Co carbide particle. The diameter of the nanotubes ranged from 20 to 130 nm, and their average length was about 5 μm. Their field emission characteristics, measured in a modified ultra-high vacuum scanning electron microscope, were analyzed by a statistical field emission model developed for aligned carbon nanotube film cathodes. An indirect method was thus established to obtain the average electrical parameters of the film using only a limited amount of experimental data.

Proposal for a new self-focusing configuration involving porous silicon for field emission flat panel displays

Field emission displays (FEDs) work in principle in a similar way with conventional cathode ray tubes (CRTs), namely emitted electrons excite phosphors which in turn emit visible light. However, FEDs obtain the electrons through field emission from a distributed network of sharp emitters, and there is no special deflection system for them. The size of the light spot associated with an island of emitters (a pixel) depends on many factors, such as display geometrical dimensions and operating conditions. In this article, a special self-focusing configuration for FEDs is analyzed using a combined analytical and numerical approach. The proposed configuration involves a distributed network of electron emitting areas which are concave in shape and not plane as usual, one for each phosphor pixel. The cathode concave areas are covered with a porous silicon layer, having sharp fibrils with several nanometers radius of curvature. The concave shape has a built-in electron self-focusing feature but, as a by-product, d...

Model parameter extraction for nonlinear Fowler–Nordheim field emission data

The traditional approach of analyzing field emission (FE) data using the “emission area” and “field enhancement” factors is faulty. Instead, the emission current I should be computed through integration of the tunneling current density over the emitter surface, taking into account the local variation of the electric field. As a consequence, FE data represented as Fowler–Nordheim (FN) plots ln(I/V2) versus 1/V are nonlinear regardless of the model used to derive the electric field and the modified FN plot ln(I/V3) versus 1/V is more suitable for representing the FE data in a linear way. In this article, a general approach for extraction of model parameters using wide-range FE data is proposed. A nonlinear least-square fitting procedure is applied to emission data based on the statistical gated emitter/triode model tailored for the particular emitter configuration. Uniform distributions for the emitter radius R and work function φ in the array are considered, although other distributions may be included in ...