Eric G. Barbagiovanni

Quantum confinement in Si and Ge nanostructures

We apply perturbative effective mass theory as a broadly applicable theoretical model for quantum confinement (QC) in all Si and Genanostructures including quantum wells(QWs), wires (Q-wires), and dots(QDs). Within the limits of strong, medium, and weak QC, valence and conduction band edge energy levels (VBM and CBM) were calculated as a function of QD diameters, QW thicknesses, and Q-wire diameters. Crystalline and amorphous quantum systems were considered separately. Calculated band edge levels with strong, medium, and weak QC models were compared with experimental VBM and CBM reported from X-ray photoemission spectroscopy (XPS), X-ray absorption spectroscopy (XAS), or photoluminescence(PL). Experimentally, the dimensions of the nanostructures were determined directly, by transmission electron microscopy(TEM), or indirectly, by x-ray diffraction (XRD) or by XPS. We found that crystalline materials are best described by a medium confinement model, while amorphous materials exhibit strong confinement regardless of the dimensionality of the system. Our results indicate that spatial delocalization of the hole in amorphous versus crystalline nanostructures is the important parameter determining the magnitude of the band gap expansion, or the strength of the quantum confinement. In addition, the effective masses of the electron and hole are discussed as a function of crystallinity and spatial confinement.

Quantum confinement in Si and Ge nanostructures: Theory and experiment

The role of quantum confinement (QC) in Si and Ge nanostructures (NSs) including quantum dots, quantum wires, and quantum wells is assessed under a wide variety of fabrication methods in terms of both their structural and optical properties. Structural properties include interface states, defect states in a matrix material, and stress, all of which alter the electronic states and hence the measured optical properties. We demonstrate how variations in the fabrication method lead to differences in the NS properties, where the most relevant parameters for each type of fabrication method are highlighted. Si embedded in, or layered between, SiO2, and the role of the sub-oxide interface states embodies much of the discussion. Other matrix materials include Si3N4 and Al2O3. Si NSs exhibit a complicated optical spectrum, because the coupling between the interface states and the confined carriers manifests with varying magnitude depending on the dimension of confinement. Ge NSs do not produce well-defined luminesc...

Role of quantum confinement in luminescence efficiency of group IV nanostructures

Experimental results obtained previously for the photoluminescence efficiency (PLeff) of Ge quantum dots (QDs) are theoretically studied. A log-log plot of PLeff versus QD diameter (D) resulted in an identical slope for each Ge QD sample only when E-G similar to (D-2 + D)(-1). We identified that above D approximate to 6.2 nm: E-G similar to D-1 due to a changing effective mass (EM), while below D approximate to 4.6 nm: E-G similar to D-2 due to electron/hole confinement. We propose that as the QD size is initially reduced, the EM is reduced, which increases the Bohr radius and interface scattering until eventually pure quantum confinement effects dominate at small D

Quantum confinement in nonadditive space with a spatially dependent effective mass for Si and Ge quantum wells

Abstract We calculate the effect of a spatially dependent effective mass (SPDEM) [adapted from Costa Filho et al. (2011)] on an electron and a hole confined in a quantum well (QW). In the work of Costa Filho et al., the translation operator is modified to include an inverse character length scale, γ , which defines the SPDEM. The introduction of γ means that translations are no longer additive. In nonadditive space, we choose a ‘skewed’ Gaussian confinement potential defined by the replacement x → γ − 1 ln ( 1 + γ x ) in the usual Gaussian potential. Within the parabolic approximation γ is inversely related to the QW thickness and we obtain analytic solutions to our confinement Hamiltonian. Our calculation yields a reduced dispersion relation for the gap energy ( E G ) as a function of QW thickness, D : E G ~ D − 1 , compared to the effective mass approximation: E G ~ D − 2 . Additionally, nonadditive space contracts the position space metric thus increasing the occupied momentum space and reducing the effective mass, in agreement with the relation: m o ⁎ − 1 ∝ ∂ 2 E / ∂ k 2 . The change in the effective mass is shown to be a function of the confinement potential via a point canonical transformation. Our calculation agrees with experimental measurements of E G for Si and Ge QWs.

Quantum confinement in Si and Ge nanostructures: effect of crystallinity

We look at the relationship between the preparation method of Si and Ge nanostructures (NSs) and the structural, electronic, and optical properties in terms of quantum confinement (QC). QC in NSs causes a blue shift of the gap energy with decreasing NS dimension. Directly measuring the effect of QC is complicated by additional parameters, such as stress, interface and defect states. In addition, differences in NS preparation lead to differences in the relevant parameter set. A relatively simple model of QC, using a ‘particle-in-a-box’-type perturbation to the effective mass theory, was applied to Si and Ge quantum wells, wires and dots across a variety of preparation methods. The choice of the model was made in order to distinguish contributions that are solely due to the effects of QC, where the only varied experimental parameter was the crystallinity. It was found that the hole becomes de-localized in the case of amorphous materials, which leads to stronger confinement effects. The origin of this result was partly attributed to differences in the effective mass between the amorphous and crystalline NS as well as between the electron and hole. Corrections to our QC model take into account a position dependent effective mass. This term includes an inverse length scale dependent on the displacement from the origin. Thus, when the deBroglie wavelength or the Bohr radius of the carriers is on the order of the dimension of the NS the carriers ‘feel’ the confinement potential altering their effective mass. Furthermore, it was found that certain interface states (Si-O-Si) act to pin the hole state, thus reducing the oscillator strength.

Photoluminescence efficiency of self-assembled germanium dots

Under the proviso that the existing tight-binding (TB) and effective mass (EM) theoretical models provide a good description of the Ge dot energy gap versus dot diameter, this work investigates the effect of nanoparticle size and the size distribution on the near infrared PL spectrum obtained from self-assembled Ge dots grown on a thin layer of TiO2 or SiO2 on Si. For the as-grown samples, the dot PL emission occupies a wide near-infrared band between 0.8 and 1 eV. The PL efficiency versus dot size for four samples was obtained in three steps. Firstly, the PL spectrum was converted to an intensity plot versus dot diameter rather than energy by taking the PL emission from each dot to occur at the dot bandgap calculated using the TB or EM model. Secondly, a numerical form for the physical size distribution of that sample was obtained by performing a least-squares fit of a Gaussian to the dot size distribution measured by atomic force microscopy or transmission electron microscopy. Finally, the PL efficiency versus dot size was calculated using the fitted Gaussian dot size distribution to normalize the PL intensity distribution obtained in the first step. Although the absolute intensities of the PL from the samples vary, the calculated curves are all well-fitted by straight lines on a log-log plot with essentially the same slope for all samples, which indicates that under weak confinement there is a universal power-law increase in PL efficiency with decreasing dot size.

Diffusion In Nano‐Scale Metal‐Oxide/Si And Oxide/SiGe/Si Structures

MOSFETs incorporating SiGe and InxGa1−xAs channels are an attractive option to further increase complementary MOS logic performance since these alloys offer electron mobilities significantly higher than Si. The poor electrical quality and low chemical stability of native oxides, leading to Fermi level pinning and high defect densities, often prevents the fabrication of competitive devices. Oxygen transport in model 2–10 nm thick systems, including Si, SiGe ultra‐thin alloys, and hafnium oxide films grown by atomic layer deposition was studied by high‐resolution ion scattering (MEIS) and conventional Rutherford Backscattering Spectroscopy (RBS) in combination with isotope tracing. We found much lower activation barrier value in the case of HfO2/Si which is attributed to distinctly different oxygen incorporation mechanisms. The growth rate is two orders of magnitude faster for SiGe ultra‐thin alloys and it cannot be described by the Deal‐Grove model and its modifications, nor the reactive layer model. Poten...

Optical Properties of Si Quantum Dots in Silica via an Implantation Mask

We studied photoluminescent properties and luminescent decay dynamics in Si quantum dots (QDs) produced by Si implantation in SiO 2, and their modification by the application of an implantation mask. Silicon quantum dots were prepared by ion implantation, followed by high temperature annealing leading to nanocrystal nucleation and growth. The mask was prepared by spin-coating silica microspheres to achieve laterally-selective implantation, to control QD size and separation. Transmission electron microscopy (TEM) images were obtained to verify the diameter of the quantum dots. We observe a noticeable peak shift and narrowing in the photoluminescence spectra with the application of the implantation mask. Observed maxima in the photoluminescence spectra are compared with a quantum field theoretical model using an infinite confining 1D potential for Si quantum dots. We comment on the role of excitation transfer by observing a change in the dispersion exponent of the luminescent decay dynamics due to the mask.