Subnanometer-resolved measurement of the tunneling effective mass using bulk plasmons

Abstract

Superlattices are periodic structures where the constituents alternate between low-and high-bandgap materials; the resulting quantum confinement tailors the resulting device properties and increases their operating speed. Amorphous carbon is an excellent candidate for both the well and barrier layers of the superlattices, leading to a fast and reliable device manufacturing process. We show theoretically and experimentally that, using low energy-loss spatially resolved spectroscopy, we can characterize the component layers of a superlattice. We measure quantum confinement of the electron wave function in the superlattice's wells and calculate the effective tunneling mass for amorphous carbon superlattices as m * = 0.067m e . This effective mass makes diamondlike carbon films as feasible candidate for electronic devices. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2188593͔ Diamond-like carbon films have optical band gaps from 1.2-4.0 eV controllable through the deposition parameters, such as the plasma power. 1 They are an attractive option for the semiconductor industry due to the inexpensive, fast and reliable manufacturing method. When the deposition parameter is varied cyclically during deposition, we obtain a bandgap modulation in an essentially homogenous material system, 2,3 leading to higher device speeds and controllable electronic properties. The confining potential of a band-gapmodulated artificial structure on the electron wave function leads to the quantization of the particle momentum and energy, controlled by the well's width and depth and the number of alternating barrier and well layers. 4,5 This can lead to the tailoring of devices for specific applications, such as frequency generators for the mobile phone industry. The design and testing of manufacturing processes for superlattices require not only the ability to characterize the morphology of the individual layers, but also their individual electronic properties. This places electron energy loss spectroscopy ͑EELS͒ in a transmission electron microscope ͑TEM͒ in the unique position of providing all this information. Here, we use an alternative method of acquiring spectral information on a subnanometer scale, and then set the theoretical basis for interpreting the measured collective excitation on the subnanometer scale. We then extract the tunneling effective mass by modeling the changes measured in the collective excitations using the "particle-in-a-box" quantum confinement model.In an electron microscope, it is possible to image the barrier and well layers constituent of a superlattice, even when their respective physical properties are very similar, at large defoci ͓Fig. 1͑a͔͒. 6 Energy loss spectroscopic profiling provides spatially resolved energy-loss spectra across linear features, under parallel illumination. 7 This method utilizes the manner in which a Gatan imaging filter forms images and energy-loss spectra, by collecting two-dimensional data sets with one axis, the energy loss and the other axis the spatial dimension norma...

Negative differential conductance observed in electron field emission from band gap modulated amorphous-carbon nanolayers

“…The E 2 − E 1 is calculated to be 0.45 eV ͑E 1 = 0.2 eV and E 2 = 0.65 eV͒ for the effective mass m * ϳ 0.06, r ϳ 4, and w = 3 nm which is further confirmed by modeling. 8,15 Hence, the average value of the band bending in the sp 2 layer attributed to the space potential is ϳ0.2 eV and is comparable to E 1 . When E c of the Si substrate is aligned with the first confined energy E 1 by this band bending, resonant tunneling can occur and hence the emission current will increase.…”

“…The fully depleted sp 2 a-C layer could create electric fields much greater than the macroscopic field external to it. [12][13][14] Similar to the space charge induced band bending model for amorphous materials in which the Si substrate acts as the electron source, [12][13][14][15] Semet et al 5 and Wang et al 7 proposed an identical hypothesis for GaN based double layered structures. We postulate electron injection from Si into the sp 2 "well layer" through the 3 nm thick sp 3 layer by field penetration.…”

Optical excitations in electron microscopy

“…8(a) as a solid curve. Spatially-resolved bulk-plasmon spectroscopy has been used to identify the presence of metals in a specimen [e.g., Na in either Na 2 O or silicate glasses (Jiang et al, 2008)] and to demonstrate the effect of quantum confinement in carbon nanostructures by tracing local variations in the plasmon energy (Stolojan et al, 2006), which are related to changes in the conduction-electron density through Eq. (A2).…”

Coherent quantum transport features in carbon superlattice structures

“…DLC superlattice devices would facilitate a new class of devices with robust operating conditions. Quantum confinement has been firmly established in these disordered superlattice structures 13 which showed evidence of negative differential resistance (NDR) 14 15 16 . DLC double barrier structures showed a greater peak to valley ratio as well as demonstrating high frequency performance capabilities up to 110 GHz and bias-related hysteresis in the current-voltage characteristics 12 .…”