Si Band Gap Research Articles

Preparation of the BaF<sub>2</sub>: Ce<sup>3+</sup>, Yb<sup>3+</sup> Nanoparticles and near Infrared Luminescent Properties

The BaF2: Ce3+, Yb3+ nanoparticles doped with different concentration were prepared by a hydrothermal method. The structure,morphology and photoluminescence properties were studied. In BaF2: Ce3+, Yb3+ nanoparticles, a strong near infrared emissionat 900 – 1050 nm matching to energy of Si band gap of Si-based solar cells was observed under the excitation of 5d level of Ce3+. As a downconversion luminescent convertor, the kinds of materials have potential to be applied in front of Si-based solar cells to reduce thermalization loss and enhance conversion efficiency of solar cells.

Carrier-transport mechanism of Er-silicide Schottky contacts to strained-silicon-on-insulator and silicon-on-insulator.

The current-voltage characteristics and the carrier-transport mechanism of the Er-silicide (ErSi1.7) Schottky contacts to strained-silicon-on-insulator (sSOI) and silicon-on-insulator (SOI) were investigated. Barrier heights of 0.74 eV and 0.82 eV were obtained for the sSOI and SOI structures, respectively. The barrier height of the sSOI structure was observed to be lower than that of the SoI structure despite the formation of a Schottky contact using the same metal silicide. The sSOI structure exhibited better rectification and higher current level than the SOI structure, which could be associated with a reduction in the band gap of Si caused by strain. The generation-recombination mechanism was found to be dominant in the forward bias for both structures. Carrier generation along with the Poole-Frenkel mechanism dominated the reverse-biased current in the SOI structure. The saturation tendency of the reverse leakage current in the sSOI structure could be attributed to strain-induced defects at the interface in non-lattice-matched structures.

Energy band structures of group IV compound semiconductors for solar cells

The impact of isotropic lattice strain and the composition ratios of mono and binary systems of group IV compounds with diamond structures on energy band structures were analyzed with ab initio calculations by covering wider ranges of strain (−30 to 30%) and compositions (0–100%) than those in actual cases. The bandgap of C crystal increased (decreased) under isotropic compressive (tensile) strain while the bandgap of Si (Ge) crystal reached maximum under tensile strain of 5% (0%). The bandgaps of Ge–Sn (Si–Ge and Si–Sn) at 1:1 stoichiometry reached maximum under compressive strain of 5% (0%). These values slightly differed from the values predicted by Vegard's law. The bandgaps of 1:1 compounds Si–C (Ge–C and Sn–C) reached maximum under tensile strain of 5% (0%). The bandgaps remarkably differed from the values predicted by Vegard's law when the compounds contained C atoms.

Potential-induced degradation in solar cells: Electronic structure and diffusion mechanism of sodium in stacking faults of silicon

Sodium decorated stacking faults (SFs) were recently identified as the primary cause of potential-induced degradation in silicon (Si) solar-cells due to local electrical short-circuiting of the p-n junctions. In the present study, we investigate these defects by first principles calculations based on density functional theory in order to elucidate their structural, thermodynamic, and electronic properties. Our calculations show that the presence of sodium (Na) atoms leads to a substantial elongation of the Si-Si bonds across the SF, and the coverage and continuity of the Na layer strongly affect the diffusion behavior of Na within the SF. An analysis of the electronic structure reveals that the presence of Na in the SF gives rise to partially occupied defect levels within the Si band gap that participate in electrical conduction along the SF.

Phonon-assisted transient electroluminescence in Si

The phonon-replica infrared emission is observed at room temperature from indirect band gap Si light-emitting diode under forward bias. With increasing injection current density, the broadened electroluminescence spectrum and band gap reduction are observed due to joule heating. The spectral-resolved temporal response of electroluminescence reveals the competitiveness between single (TO) and dual (TO + TA) phonon-assisted indirect band gap transitions. As compared to infrared emission with TO phonon-replica, the retarder of radiative recombination at long wavelength region (∼1.2 μm) indicates lower transition probability of dual phonon-replica before thermal equivalent.

Efficient near-infrared down-conversion in KCaGd(PO_4)_2:Ce^3+,Yb^3+

Abstract: A novel near-infrared (NIR) quantum cutting KCaGd(PO4)2:Ce3+,Yb3+ phosphor was successfully developed. Due to the cooperative energy transfer from one Ce3+ to two Yb3+, an intense NIR emission around 1021 nm of Yb3+:2F5/2-2F7/2 transition was obtained under 324 nm excitation. Yb3+ concentration dependent quantum efficiency has been calculated and the theoretical maximum efficiency approaches up to 158.2%. Because the emission of Yb3+ around 1021 nm is matched with the band gap of crystalline Si, the phosphors could be a potential candidate for silicon-based solar cells.

Ammonia photodissociation promoted by Si(100).

Using in situ X-ray photoelectron spectroscopy measurements after reaction, we show that hydrogen-terminated Si(100) perturbs the bonding of physisorbed NH3 enabling a photochemical decomposition pathway at wavelengths different from those characteristic of either the molecule in the gas phase or the semiconductor bandgap. UV illumination only of gas phase NH3 at partial pressures from 0.1 to 100 Torr produced a maximum at 10 Torr in the N surface coverage. This is in good agreement with a model of the radical production rate showing that at this pressure the gas density balances the flux of photons at the surface with energies sufficient to dissociate NH3. UV illumination of both the gas phase and the surface produced a monotonic increase in the N coverage with pressure as well as coverages that were 3-10 times higher than when only the gas phase was illuminated. The amine saturation coverage scaled with the UV fluence at 10 Torr and 75 °C, reaching 6.9 × 10(14) atoms/cm(2) (∼1 N atom per Si surface atom) at 19 mW/cm(2) and 12 × 10(14) atoms/cm(2) (∼1.8 N per Si) at 35 mW/cm(2). Monochromatic illumination showed that the wavelengths driving deposition were not correlated with the Si bandgap, but instead were roughly the same as gas phase photodissociation (λ < 220 nm). The primary driving force to replace the hydrogen termination with amine groups was direct photodissociation of NH3 molecules whose electronic structure was perturbed by interaction with the surface. Amine groups enhanced the surface reaction of water present as a contaminant in the source gas. These results show that molecules in weakly bound surface states can have a dramatic impact on the photochemistry.

Carrier generation in irradiated Si detectors and its impact on the electric field profile

The reverse current of irradiated Si detectors was analyzed in terms of carrier generation via defects with the energy levels in the Si bandgap. The data for detectors irradiated by 23GeV protons and 1MeV neutrons were obtained in the temperature (T) range of 200–300K. Two models of the bulk generation current were developed to allow fitting and simulation of the reverse current vs. T data. These are a model of carrier generation via a single effective energy level for the direct estimation of detector performance, and a model of two generation centers with the effective energy levels originating from radiation-induced deep levels adjacent to the midgap. The influence of the bandgap temperature dependence (Eg(T)) on the parameters of the current generation centers was estimated. The first model gives the effective level energy of Ev+0.65eV and a linear dependence of the effective defect concentration on fluence. The second model involves contribution from deep donors and deep acceptors positioned at Ev+0.48eV and Ec−0.52eV, respectively, and is adapted for simultaneous calculation of the reverse current and the electric field distribution in irradiated detectors. The results of the study show that: (a) both models fit well to the experimental data; (b) contribution of the Eg(T) dependence to Et does not exceed 5%; (c) taking into consideration the bulk generation current gives the correct electric field distribution, which is a key reference for other characteristics of irradiated Si detectors; and (d) the generation lifetime is significantly larger than the carrier trapping time constant pointing to additional levels taking part in trapping of the drifting charge.

Design of a lattice-matched III–V–N/Si photovoltaic tandem cell monolithically integrated on silicon substrate

In this paper, we present a comprehensive study of high efficiencies tandem solar cells monolithically grown on a silicon substrate using GaAsPN absorber layer. InGaAs(N) quantum dots and GaAsPN quantum wells have been grown recently on GaP/Si susbstrate for applications related to light emission. For photovoltaic applications, we consider the GaAsPN diluted nitride alloy as the top junction material due to both its perfect lattice matching with Si and ideal bandgap energy for current generation in association with the Si bottom cell. Numerical simulation of the top cell is performed. The effect of layer thicknesses and doping on the cell efficiency are evidenced. In these structures a tunnel junction (TJ) is needed to interconnect both the top and bottom sub-cells. We compare the simulated performances of different TJ structures and show that the GaP(n+)/Si(p+) TJ is promising to improve performances of the current-voltage characteristic.

Designing III-V multijunction solar cells on silicon

Single junction Si solar cells dominate photovoltaics but are close to their efficiency limits. This paper presents ideal limiting efficiencies for tandem and triple junction multijunction solar cells subject only to the constraint of the Si bandgap and therefore recommending optimum cell structures departing from the single junction ideal. The use of III-V materials is considered, using a novel growth method capable of yielding low defect density III-V layers on Si. In order to evaluate the real potential of these proposed multijunction designs, a quantitative model is presented, the strength of which is the joint modelling of external quantum efficiency and current-voltage characteristics using the same parameters. The method yields a single parameter fit in terms of the Shockley-Read-Hall lifetime. This model is validated by fitting experimental data of external quantum efficiency, dark current, and conversion efficiency of world record tandem and triple junction cells under terrestrial solar spectra without concentration. We apply this quantitative model to the design of tandem and triple junction solar cells, yielding cell designs capable of reaching efficiencies without concentration of 32% for the best tandem cell and 36% for the best triple junction cell. This demonstrates that efficiencies within a few percent of world records are realistically achievable without the use of concentrating optics, with growth methods being developed for multijunction cells combining III-V and Si materials.

Tin – an unlikely ally for silicon field effect transistors?

AbstractWe explore the effectiveness of tin (Sn), by alloying it with silicon, to use SiSn as a channel material to extend the performance of silicon based complementary metal oxide semiconductors. Our density functional theory based simulation shows that incorporation of tin reduces the band gap of Si(Sn). We fabricated our device with SiSn channel material using a low cost and scalable thermal diffusion process of tin into silicon. Our high‐κ/metal gate based multi‐gate‐field‐effect‐transistors using SiSn as channel material show performance enhancement, which is in accordance with the theoretical analysis. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Origin of visible and near-infrared photoluminescence from chemically etched Si nanowires decorated with arbitrarily shaped Si nanocrystals

Arrays of vertically aligned single crystalline Si nanowires (NWs) decorated with arbitrarily shaped Si nanocrystals (NCs) have been fabricated by a silver assisted wet chemical etching method. Scanning electron microscopy and transmission electron microscopy are performed to measure the dimensions of the Si NWs as well as the Si NCs. A strong broad band and tunable visible (2.2 eV) to near-infrared (1.5 eV) photoluminescence (PL) is observed from these Si NWs at room temperature (RT). Our studies reveal that the Si NCs are primarily responsible for the 1.5–2.2 eV emission depending on the cross-sectional area of the Si NCs, while the large diameter Si/SiOx NWs yield distinct NIR PL consisting of peaks at 1.07, 1.10 and 1.12 eV. The latter NIR peaks are attributed to TO/LO phonon assisted radiative recombination of free carriers condensed in the electron–hole plasma in etched Si NWs observed at RT for the first time. Since the shape of the Si NCs is arbitrary, an analytical model is proposed to correlate the measured PL peak position with the cross-sectional area (A) of the Si NCs, and the bandgap (Eg) of nanostructured Si varies as Eg = Eg (bulk) + 3.58 A−0.52. Low temperature PL studies reveal the contribution of non-radiative defects in the evolution of PL spectra at different temperatures. The enhancement of PL intensity and red-shift of the PL peak at low temperatures are explained based on the interplay of radiative and non-radiative recombinations at the Si NCs and Si/SiOx interface. Time resolved PL studies reveal bi-exponential decay with size correlated lifetimes in the range of a few microseconds. Our results help to resolve a long standing debate on the origin of visible–NIR PL from Si NWs and allow quantitative analysis of PL from arbitrarily shaped Si NCs.

Preparation and Photoluminescent Properties of Ce&lt;sup&gt;3+&lt;/sup&gt;, Yb&lt;sup&gt;3+&lt;/sup&gt; Co-Doped YF&lt;sub&gt;3&lt;/sub&gt; Nanophosphors for Solar Cells

An efficient near-infrared (NIR) quantum cutting (QC) nanophospors with Ce3+, Yb3+ co-doped in YF3 had been prepared by hydrothermal method and characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), photoluminescence spectra and decay dynamics. The obtained nanorods were uniform and monodisperse. Under the excitation of 5d level of Ce3+, an intense NIR emission at 900 1100 nm was observed which match to the energy of Si band gap of Si - based solar cells. In the Ce3+, Yb3+ codoped YF3, the lifetime of Ce3+ decreases and the quantum efficiency (QE) increases with increasing Yb3+ concentration.

Magnetostriction‐strain‐induced enhancement and modulation of photovoltaic performance in Si‐p–n/TbxDy1−xFe2 composite

High photovoltaic efficiency is a key index in the application of silicon (Si) solar cells. In this study, a composite of a photovoltaic Si p–n junction solar cell and a magnetostrictive TbxDy1−xFe2 alloy was fabricated. By utilizing the magnetostrictive strain to modulate the energy bandgap of Si, the open‐circuit voltage and the maximum photovoltaic output power of the Si p–n junction solar cell could be enhanced by ∼12% and 9.1% under a dc magnetic field of ∼250 mT, respectively. The significantly enhanced photovoltaic performance and the simple fabrication process make the Si‐p–n/TbxDy1−xFe2 composite a promising material for high‐efficiency solar cell devices. The structure of the proposed Si‐p–n/TbxDy1−xFe2 laminated composite.

Efficient near-infrared quantum cutting in Ce3+–Yb3+ codoped glass for solar photovoltaic

We report that Ce3+ can be an efficient sensitizer for Yb3+ and a broadband absorber for blue solar spectra. The broadband excitation for near-infrared (NIR) emission is ranged from 350nm to 500nm. The allowed 4f→5d transition and the adjustable lowest 5d level make the energy transfer from Ce3+ to Yb3+ efficiently. Therefore the quantum efficiency (QE) is also very high up to 154.3%. Furthermore, the energy of Yb3+2F7/2→2F5/2 transition matches well with the band gap of a single-junction Si. Given the broad excitation band, high absorption coefficient and excellent mechanical, thermal, and chemical stability, this system can be used as down-conversion (DC) layer for solar cells.

Polymer-embedded colloidal lead-sulfide nanocrystals integrated to vertically slotted silicon-based ring resonators for telecom applications

The main drawback of the rapidly evolving field of silicon photonics lies in the absence of efficient monolithically integrated radiation sources as a consequence of the indirect bandgap of Si and Ge. Relevant alternatives based on the hybrid combination of Si with optically active materials have to be technologically simple, temporally stable, and provide efficient coupling to the Si waveguides. Lead-sulfide nanocrystals (NCs) were blended into a polymer resist suitable for deep-UV- and electron-beam lithography and integrated into Si-based vertically slotted waveguides and ring resonators. The polymer both stabilizes the NC’s photoluminescence emission against degradation under ambient conditions and allows lithographic patterning of this compound material. After integration into the optoelectronic structures and upon optical pumping, intense photoluminescence emission from ring resonators was recorded at the output of bus-waveguides. The resonator quality factors were investigated for polymer-NC compounds with NC concentrations in the range between 0.1 and 8 vol%. The spontaneous emission rate enhancement for vertically slotted resonators was estimated to be a factor of two higher as compared to unslotted ones. The stable integration of colloidal NCs as well as the improved light coupling to silicon circuits is an important step in the development of silicon-based hybrid photonics.

Sub-Bandgap External Quantum Efficiency in Ti Implanted Si Heterojunction with Intrinsic Thin Layer Cells

In this work we present the manufacturing processes and results obtained from the characterization of heterojunction with intrinsic thin layer solar cells that include a heavily Ti ion implanted Si absorbing layer. The cells exhibit external circuit photocurrent at photon energies well below the Si bandgap. We discuss the origin of this below-bandgap photocurrent and the modifications in the hydrogenated amorphous intrinsic Si layer thickness to increase the open-circuit voltage.

Transition metals (Ti and Co) in silicon and their complexes with hydrogen: A Laplace DLTS study

The electrical properties of transition metals (TM) in Si were often determined by means of the conventional deep level transient spectroscopy (DLTS) technique. In the present study we demonstrate that the poor resolution of the conventional DLTS technique could lead to the wrong interpretation of the experimental data. On the example of well-known lifetime killers such as titanium and cobalt we show that the dominant defects appeared in samples doped with these impurities were previously wrongly attributed to different charged states of interstitial Ti and substitutional Co. Applying the high-resolution Laplace DLTS technique we re-examine the origin of these defects. Some of these defects will be shown to react with H and shallow acceptors leading to the appearance of new complex defects in the band gap of Si. Their nature will also be discussed in the present paper.

Defects Related to Sb-Mediated Ge Quantum Dots

We report on a specific defect, which may form during the growth of Stranski-Krastanov surfactant-mediated Ge/Si (100) islands. Transmission electron microscopy reveals that these loop-like defects are local and could be represented by a missing plane of Ge atoms inside some of Ge islands. This specific defect may generate an electrically active trap within the Si band gap at about 0.3 eV above the Si valence band edge. Deep level transient spectroscopy reveals that at least 1 % of Ge islands may include such defects.

The structure and properties of (aluminum, oxygen) defect complexes in silicon

The atomic structure and electronic properties of aluminum (Al)-related defect complexes in silicon (Si) are investigated using first-principles calculations. Individual substitutional Al (AlSi), interstitial Al (Ali) and their possible complex configurations with oxygen (O) atoms are studied. We find a unique stable complex configuration consisting of an Ali and an oxygen dimer, Ali-2Oi, which introduces deep levels in the band gap of Si. The formation energies of the Ali-2Oi complexes could be lower than that of individual Ali atoms under oxygen-rich conditions. The formation of Ali-2Oi complexes may explain the experimental observation that the coexistence of Al and O results in reduced carrier lifetime in Si wafers.