Stark effect, polarizability and electroabsorption in silicon nanocrystals

Demonstrating the quantum-confined Stark effect (QCSE) in silicon nanocrystals (NCs) embedded in oxide has been rather elusive, unlike the other materials. Here, the recent experimental data from ion-implanted Si NCs is unambiguously explained within the context of QCSE using an atomistic pseudopotential theory. This further reveals that the majority of the Stark shift comes from the valence states which undergo a level crossing that leads to a nonmonotonic radiative recombination behavior with respect to the applied field. The polarizability of embedded Si NCs including the excitonic effects is extracted over a diameter range of 2.5--6.5 nm, which displays a cubic scaling, $\alpha=c D^3$, with $c=2.436\times 10^{-11}$ C/(Vm), where $D$ is the NC diameter. Finally, based on intraband electroabsorption analysis, it is predicted that p-doped Si NCs will show substantial voltage tunability, whereas n-doped samples should be almost insensitive. Given the fact that bulk silicon lacks the linear electro-optic effect as being a centrosymmetric crystal, this may offer a viable alternative for electrical modulation using p-doped Si NCs.Comment: Published version, 7 pages, 7 figure

Improvement of light emission from Tb-doped Si-based MOS-LED using excess Si in the oxide layer

The fabrication of efficient silicon-based Light Emitting Devices (LEDs) is extremely important for the integration of photonic and electronic components on the same Si platform. In this paper, we report on the room temperature electroluminescence properties of Tb-doped MOS-LED devices with an active layer of SiO2 and Si-rich SiOx produced using the magnetron co-sputtering technique. The electroluminescence properties of both types of devices were studied as a function of processing conditions and material properties. A clear Tb3+ electroluminescence signal from the D-5(4)-> F-7(j) transitions has been observed without any parasitic defect emissions from the active layer hosting the Tb3+ ions. We have shown that the incorporation of excess Si into the active layer significantly enhances the electroluminescence signal, which lowers the turn on voltage below 10 V and is crucially important for meeting the low-power requirements for integrated circuit applications. We also addressed some of the fundamental questions concerning the light generation mechanisms in the Tb-doped system

Improvement of light emission from Tb-doped Si-based MOS-LED using excess Si in the oxide layer

The fabrication of efficient silicon-based Light Emitting Devices (LEDs) is extremely important for the integration of photonic and electronic components on the same Si platform. In this paper, we report on the room temperature electroluminescence properties of Tb-doped MOS-LED devices with an active layer of SiO2 and Si-rich SiOx produced using the magnetron co-sputtering technique. The electroluminescence properties of both types of devices were studied as a function of processing conditions and material properties. A clear Tb3+ electroluminescence signal from the D-5(4)-> F-7(j) transitions has been observed without any parasitic defect emissions from the active layer hosting the Tb3+ ions. We have shown that the incorporation of excess Si into the active layer significantly enhances the electroluminescence signal, which lowers the turn on voltage below 10 V and is crucially important for meeting the low-power requirements for integrated circuit applications. We also addressed some of the fundamental questions concerning the light generation mechanisms in the Tb-doped system

Investigation of silver-induced crystallization of germanium thin films fabricated on different substrates

Silver-induced crystallizations of amorphous germanium (alpha-Ge) thin films were fabricated through electron beam evaporation on crystalline silicon (c-Si) (100), aluminum-doped zinc oxide (AZO), and glass substrates at room temperature. The solid-phase crystallization (SPC) of alpha-Ge films was investigated for various post-annealing temperatures between 300 and 500 degrees C for 60 min. Two crystallization approaches were compared: SPC and metal-induced crystallization (MIC). The structural properties of the Ge thin films fabricated by both methods were studied with Raman and X-ray diffraction (XRD) measurements. The Raman and XRD results indicated that the metal-induced crystallization of the Ge thin films yielded crystallization at temperatures considerably lower than those used in the SPC technique. As expected, the amount of crystallization and the quality of the films were improved with increased annealing temperatures. It was also demonstrated that the same material properties could be obtained using different substrates without any significant variation

An Alternative Metal-Assisted Etching Route for Texturing Silicon Wafers for Solar Cell Applications

Metal-assisted etching (MAE) can be used to form antireflective and light-trapping structures on crystalline silicon solar cells. This method has been widely used to form nanowires and nanoholes on their surfaces. In this study, the MAE technique with additional hole-injection mechanisms has been investigated to form surface nanostructures with various shapes. The effect of each chemical's percentage, as well as the etching time, has been studied on the surface geometry and optical performance. The average reflection from the surface was reduced to less than 3% over the solar spectrum. The light-trapping properties of the structures were investigated through absorption measurements on thin wafers, and a nearly 90% average absorption was obtained for samples with a 50-mu m thickness. The smoothing of the surface and lifetime of the chemical solutions has been systematically studied. As a result, control over the surface geometry and optical properties have been obtained with this new single-step etching approach. Passivation of the surface with SiO2 is also investigated to realize device implementation in the next step

Structural and electrical analysis of poly-Ge films fabricated by e-beam evaporation for optoelectronic applications

We have investigated the relationship between structural and electrical properties of Ge thin films deposited on single crystal silicon (100) substrates by electron beam evaporation at room temperature. Post-thermal annealing was applied to obtain poly-crystalline Ge thin films. The structural effects of the annealing temperature and annealing time on the crystallization of Ge films were analyzed using Raman and X-ray diffraction measurements. Raman and X-ray diffraction spectra revealed a structural evolution from amorphous to crystalline phase with increasing annealing temperature and annealing time. It was found that high quality poly-crystalline Ge films were obtained with crystallization ratio of 90% at an annealing temperarure of 500 degrees C following the crystallization threshold of 450 degrees C. Effects of structural ordering on the electrical properties were investigated through current-voltage characteristics of fabricated heterostructure devices (Ge/p-Si). Smooth cathode-anode interchange in the diode behavior has been clearly observed following the structural ordering as a function of annealing temperature in a systematic way. These outcomes could be exploited for engineering of low-cost Ge based novel electronic and opto-electronic devices

Mechanisms of light emission from terbium ions (Tb3+) embedded in a Si rich silicon oxide matrix

Mechanisms of light emission in Tb doped Si rich SiOx matrix prepared by magnetron sputtering are studied by photoluminescence spectroscopy (PL). Characteristic PL peaks of Tb3+ ions and Si nanocrystals are simultaneously observed with an inverse relationship between their intensity. With a prolonged heat treatment at high temperatures, light emission from Tb3+ ions enhances at the expense of total quenching of the PL signal from the nanocrystals. It is suggested from the annealing studies as a function of process conditions and structural characterization that the light emission from Tb ions is mediated by trap states formed in the band gap of the oxide matrix by TbxSiyOz complexes or excess Si states

The quantum confined Stark effect in silicon nanocrystals

The quantum confined Stark effect (QCSE) in Si nanocrystals embedded in a SiO(2) matrix is demonstrated by photoluminescence (PL) spectroscopy at room and cryogenic temperatures. It is shown that the PL peak position shifts to higher wavelengths with increasing applied electric field, which is expected from carrier polarization within the quantum dots. It is observed that the effect is more pronounced at lower temperatures due to the improved carrier localization at the lowest energy states of the quantum dots. Experimental results are shown to be in good agreement with phenomenological model developed for the QCSE model

Metal-assisted nano-textured solar cells with SiO2/Si3N4 passivation

We demonstrate the fabrication of nano-sized surface textured crystalline silicon by a metal-assisted electroless etching method with nitric acid added as the hole injection agent. This method generates randomly shaped cone-like structures that offer a clear advantage over nanowires by enabling straightforward passivation with standard techniques. Average reflection values as low as 3% have been achieved. Optimizing the thickness of anti-reflective coatings, the doping depth and the screen-printed metal firing process increases the short circuit current of the cell by 0.82 mA/cm(2) over the reference cells, which had a pyramidal texture without nano texturing

Effect of Au on the crystallization of germanium thin films by electron-beam evaporation

Metal induced crystallization is a widely used method to form crystalline/polycrystalline structures at low temperatures. In this work, Au was applied to enhance the crystallization of amorphous Ge films. Ge films with thicknesses of similar to 1.5 mu m were fabricated by electron beam evaporation on c-Si substrate with and without very thin Au layer. Crystallization properties of Ge films were analyzed for different growth and post annealing temperatures varied between 270 degrees C and 730 degrees C. The structures of polycrystalline Ge films were investigated by employing X-ray diffraction (XRD), Raman spectra and scanning electron microscopy (SEM)