Eun-Chel Cho

Silicon Quantum Dots in a Dielectric Matrix for All-Silicon Tandem Solar Cells

We report work progress on the growth of Si quantum dots in different matrices for future photovoltaic applications. The work reported here seeks to engineer a wide-bandgap silicon-based thin-film material by using quantum confinement in silicon quantum dots and to utilize this in complete thin-film silicon-based tandem cell, without the constraints of lattice matching, but which nonetheless gives an enhanced efficiency through the increased spectral collection efficiency. Coherent-sized quantum dots, dispersed in a matrix of silicon carbide, nitride, or oxide, were fabricated by precipitation of Si-rich material deposited by reactive sputtering or PECVD. Bandgap opening of Si QDs in nitride is more blue-shifted than that of Si QD in oxide, while clear evidence of quantum confinement in Si quantum dots in carbide was hard to obtain, probably due to many surface and defect states. The PL decay shows that the lifetimes vary from 10 to 70 microseconds for diameter of 3.4 nm dot with increasing detection wavelength.

Structural characterization of annealed Si1−xCx/SiC multilayers targeting formation of Si nanocrystals in a SiC matrix

Amorphous Si1−xCx/SiC multilayer films were prepared by alternating deposition of Si-rich Si1−xCx and near-stoichiometric SiC layers by using magnetron sputtering. The as-deposited films were annealed at different temperatures (Ta) from 800 to 1100 °C. The influence of Ta and Si content in the Si-rich layer on the layered structural stability and on the formation of Si and/or SiC nanocrystals (NCs) is investigated by a variety of analytical techniques, including x-ray reflectivity (XRR), x-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, and Fourier transform infrared spectrometry (FTIR). XRR showed that Si1−xCx/SiC multilayers annealed at temperatures of up to 800 °C retain their layered structure. XRD revealed that Si NCs were formed in samples with a high Si content in the Si-rich layer for Ta≥800 °C. At annealing temperatures of 900 °C or greater, the formation of Si NCs was accompanied by the formation of β-SiC NCs. Additionally, the formation of Si and SiC NCs was c...

Clear quantum-confined luminescence from crystalline silicon/SiO2 single quantum wells

Crystalline silicon single quantum wells (QWs) were fabricated by high-temperature thermal oxidation of ELTRAN® (Epitaxial Layer TRANsfer) silicon-on-insulator (SOI) wafers. The Si layer thicknesses enclosed by thermal SiO2 range from 0.8 to 5 nm. Luminescence energies from such QWs vary from 1.77 to 1.35 eV depending on the Si layer thickness, without evidence for interface-mediated transition seen in earlier work. The ability to detect quantum-confined luminescence seems to arise from the use of ELTRAN SOI wafers, from suppressed interface state luminescence by high-temperature oxidation and, possibly, from interface matching by crystalline silicon oxide.

Fabrication and characterization of Si nanocrystals in SiC matrix produced by magnetron cosputtering

Si-rich amorphous silicon carbide thin films were prepared by magnetron cosputtering and were subsequently annealed to form Si nanocrystals embedded in a SiC matrix. A sputter target consisted of a patterned Si wafer on top of a carbon target. The ratio of carbon to silicon in deposited films was adjusted by means of a different silicon wafer open area. X-ray photoelectron spectroscopy spectra show that various compositions were obtained by changing the sputtered area ratio of carbon to silicon target. Analysis of atomic force microscopy shows that surface roughness increases significantly after annealing. Transmission electron microscopy reveals that Si nanocrystals do not form at temperatures less than 800°C, while they are clearly established, with sizes ranging from 3to7nm, as the temperature is at 1100°C. IR spectra show that increase in annealing temperature for the Si-rich Si1−xCx (x<0.5) films favors the formation of Si–C bonds and increase of the short-range order. Optical studies show a blueshif...

Fabrication and electrical characteristics of Si nanocrystal/c-Si heterojunctions

Heterojunctions (HJs) were fabricated from p-type Si nanocrystals (Si NCs) embedded in a SiC matrix on an n-type crystalline Si substrate. Transmission electron microscopy revealed that Si NCs are clearly established, with sizes in the range of 3–5nm. The HJ diodes showed a good rectification ratio of 1.0×104 at ±1.0V at 298K. The ideality factor, junction built-in potential, and open-circuit voltage are ∼1.24, 0.72V, and 0.48V, respectively. Measurement of temperature-dependent I-V curves in forward conduction suggests that, in the medium voltage range, junction interface recombination can be described as the dominant current transport mechanism.

Resonant tunneling through defects in an insulator: Modeling and solar cell applications

In this paper, a model for electron tunneling through defects in an insulator is presented. The three-dimensional results for the electron transmission coefficient can be obtained by characterizing the tunneling process in terms of a defect density and capture cross section. Fitting the model parameters by comparison with the results of a full three-dimensional tunneling-through-defect simulation, this model can be used to calculate and predict the electron transmission for various spatial distributions of defects without performing the complex three-dimensional calculations. Energy selective contacts using the resonant tunneling for carrier extraction have been proposed as a means to achieve a higher efficiency in future generations of photovoltaic devices. Resonant tunneling through defects in an insulator, where the defects may be atoms or quantum dots, may provide a possible implementation for such energy selective contacts. With the present model, the influences of the tunneling effective mass, insul...

Time-resolved and time-integrated photoluminescence analysis of state filling and quantum confinement of silicon quantum dots

In this paper we report studies of the optical properties of silicon quantum dot structures. From time-resolved and time-integrated photoluminescence measurements we investigate the state-filling effect and carrier lifetime, and discuss the parabolic confinement of quantum dot structures and the large energy splitting between quantum dot levels. The photoluminescence intensities for different quantum dot levels decay with a stretched exponential function and the decay times are in the range 2–100μs depending on the observation wavelength and the dot size.

Excitation dependence of photoluminescence in silicon quantum dots

We have studied the optical properties of silicon quantum dots (QDs) embedded in a silicon oxide matrix using photoluminescence (PL) and time-resolved PL. A broad luminescence band is observed in the red region, in which the time evolution exhibits a stretched exponential decay. With increasing excitation intensity a significant saturation effect is observed. Direct electron?hole recombination is the dominant effect in the red band. A relatively narrow peak appears around 1.5?eV, which is attributed to the interface states overlapping with transition from the ground state of the silicon QDs. The saturation factor increases slowly with detection photon energy between 1.5 and 1.8?eV, which is attributed to the emission from zero-phonon electron?hole recombination. At higher photon energies the significantly increased saturation factor suggests a different emission mechanism, most likely the defect states from silicon, silicon oxide or silicon rich oxide.

Ultrafast carrier dynamics of Si quantum dots embedded in SiN matrix

Femtosecond spectrally resolved two-color three-pulse nonlinear spectroscopy is used to study the dynamics and coherence properties of excited carriers in Si quantum dot structures embedded in silicon nitride. A very short dephasing time of <180fs at room temperature is observed. Ultrashort population relaxation times of ∼400fs and 6–10ps are measured and discussed in the context of the different contributions from transverse optical and transverse acoustic phonon-assisted transitions.

Fabrication and characterization of tin-based nanocrystals

Sn-based nanocrystals were prepared by depositing Sn-rich SiO2 films using a cosputtering process and a subsequent vacuum annealing. Transmission electron microscopy (TEM) and x-ray diffraction showed formation of Sn nanocrystals evenly distributed in SiO2 matrix at relatively low annealing temperature of 400°C. The size of Sn nanocrystals increased with increasing annealing temperature. X-ray photoelectron spectroscopy revealed that Sn was partially oxidized during the cosputtering process forming Sn oxide nanoclusters of 3.4±0.6nm in diameter after annealing, as observed by TEM. The Sn-based nanocrystal films exhibited wide optical bandgap around 4.2–4.4eV and a slightly high-energy shift with increasing annealing temperature. This result is in close agreement with the absorption in the Sn oxide nanoclusters as well as Sn-related oxygen defects in the matrix.