Band Gap Of Bulk Si Research Articles

Two-Photon Excitation Spectroscopy of Silicon Quantum Dots and Ramifications for Bio-Imaging.

Two-photon excitation in the near-infrared (NIR) of colloidal nanocrystalline silicon quantum dots (nc-SiQDs) with photoluminescence also in the NIR has potential opportunities in the field of deep biological imaging. Spectra of the degenerate two-photon absorption (2PA) cross section of colloidal nc-SiQDs are measured using two-photon excitation over a spectral range 1.46 < ℏω < 1.91 eV (wavelength 850 > λ > 650 nm) above the two-photon band gap Eg(QD)/2, and at a representative photon energy ℏω = 0.99 eV (λ = 1250 nm) below this gap. Two-photon excited photoluminescence (2PE-PL) spectra of nc-SiQDs with diameters d = 1.8 ± 0.2 nm and d = 2.3 ± 0.3 nm, each passivated with 1-dodecene and dispersed in toluene, are calibrated in strength against 2PE-PL from a known concentration of Rhodamine B dye in methanol. The 2PA cross section is observed to be smaller for the smaller diameter nanocrystals, and the onset of 2PA is observed to be blue shifted from the two-photon indirect band gap of bulk Si, as expected for quantum confinement of excitons. The efficiencies of nc-SiQDs for bioimaging using 2PE-PL are simulated in various biological tissues and compared to efficiencies of other quantum dots and molecular fluorophores and found to be comparable or superior at greater depths.

Nonthermal Plasma-Synthesized Phosphorus–Boron co-Doped Si Nanocrystals: A New Approach to Nontoxic NIR-Emitters

We report on the successful creation of nonthermal plasma-synthesized phorphorus and boron co-doped Si nanocrystals (PB:Si NCs) with diameters (DNC) ranging from 2.9 to 7.3 nm. Peak photoluminescence (PL) emission energies for all PB:Si NC diameters are ca. 400–500 meV lower than the excitonic emission values in intrinsic Si NCs, which can be attributed to prevalent donor–acceptor (D–A) transitions within the co-doped system. This D–A transition model is further evidenced by PL lifetimes within the range of 40–80 μs, faster than what is observed for intrinsic Si NCs. By reducing the level of confinement within PB:Si NCs (i.e., DNC > 4 nm), we are able to red-shift the near-infrared (NIR)-emitting D–A transitions to below the band gap of bulk Si (1.12 eV). We quantify the PL quantum yield (PLQY) for a range of DNC and show that the plasma method can achieve reasonably high PLQY values (12% for DNC = 2.9 nm), even without any optimization of the synthesis or surface chemistry. We posit that perfect charge c...

Electronic properties of a new structured Sin/O superlattice

Silicon is a material which dominants the semiconductor industry and has a well-established processing technology based on it. However, silicon has an indirect-bandgap and is not efficient in light emitting. This limits its applications in optoelectronics. In this paper, we proposed a new structural model for the silicon-based superlattice, i.e., the Sin/O one. The model consists of alternating films of n-layers of Si and a monolayer of oxygen along z-direction, together with a surface cell of Si(001) (2×1) reconstruction in the x-y plane. The importance of employing such a Si(001) (2×1) reconstruction is that all the electrons at interface can be strongly bonded. Our results showed interesting electronic properties, e.g., the band folding and large band gap of bulk Si, when the thickness of the silicon layers was increased (but still thin). Our structure might also offer other interesting properties.

Electronic and geometric investigations of the Ca/Si(1 1 1)-( [formula omitted]) surface

The atomic and electronic structures of the Ca/Si(1 1 1)-( 5 × 2 ) surface have been theoretically investigated by using the pseudopotential method and the local density approximation (LDA) of the density functional theory. The geometrical model is based on the deposition of three Ca lines on the top of a combination of a honeycomb chain channel (HCC) and the Seiwatz (SZ) chain formed by Si-adatoms. The Ca adatoms lie on both H 3 and T 4 symmetry sites. This structural model produces a semiconducting surface band structure with a clear LDA energy gap of 0.56 eV. Ca-derived band gap states have been identified by comparing the band structures for the clean Si(1 1 1)-( 5 × 2 ) and Ca/Si(1 1 1)-( 5 × 2 ) surfaces. Significant charge transfer from the Ca adatoms to neighbouring Si atoms has been concluded by analysing electronic charge density and STM simulations. A discussion of the role of Ca adatoms in improving the chemical passivation of the Si(1 1 1) surface has been attempted by examining the orbital nature of states in the fundamental band gap of bulk Si.

Control of photoluminescence properties of Si nanocrystals by simultaneously doping n- and p-type impurities

The effects of B and P codoping on photoluminescence (PL) properties of Si nanocrystals (nc-Si) are studied systematically. It is shown that the PL intensity of codoped nc-Si is always higher than that of either P- or B-doped nc-Si. The intensity is sometimes even higher than that of pure nc-Si at relatively low P and B concentrations and low annealing temperatures. By doping P and B simultaneously to very high concentrations, the PL peak shifts below the band gap of bulk Si.

Light-emitting silicon pn diodes

We report on the electrical and optical characteristics of silicon light-emitting pn diodes. The diodes are prepared by ion implantation of boron at high doses and subsequent high-temperature annealing. Under forward bias, the diodes emit infrared electroluminescence closely below the band gap of bulk Si. We present a rate-equation model for bound excitons, free excitons and free carriers which successfully describes the electrical and optical behaviour of the diodes at low temperatures. Especially, an electrical bistability observed below 50 K is shown to be based on the interplay of bound excitons, free excitons and free carriers in the active area of the diodes. The ionisation of bound excitons is the origin of an improved electroluminescence from the diodes at higher lattice temperatures.

Luminescence of porous silicon in a weak confinement regime

We report on luminescence properties of porous silicon emitting efficient light only a few tens of meV above the band gap of bulk Si. This emission band has a well-defined low-energy limit coincident with the position of the lowest possible luminescing exciton state of bulk silicon. The resonant photoluminescence spectrum exhibits all possible combinations of zero-phonon and momentum-conserving TA- and TO-phonon assisted absorption and exciton emission processes known for bulk silicon. We found that TO-phonon assisted processes give a major contribution to the light emission. We discuss the implications of these studies for the understanding of the origin of porous silicon photoluminescence.

Oxide Formation on Si(100)-2×1 Surfaces Studied by Scanning Tunneling Microscopy/Scanning Tunneling Spectroscopy

The oxide formation on Si(100)-2×1 surfaces at room temperature has been studied by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). The change in electronic structures due to oxidation has been discussed based on the spatially resolved STS spectra. It was found in the initial stage of oxidation that the local density-of-states (LDOS) of back bond states and dangling bond sites decreases dramatically at the oxidation-induced bright protrusions. The STS spectra of the 2×1 region with an oxygen exposure of 4.5 L suggest that oxygen atoms adsorb on the back bond sites. At the average SiO2 thickness of 0.4 nm, the surface states of Si(100)-2×1 vanish completely and the energy width of the gap near the Fermi level becomes very close to the band gap of bulk Si. In addition, the observed LDOS is spatially uniform in spite of the presence of bumps in the STM image. The STS spectra indicate the formation of the valence band and the conduction band of SiO2 at the oxide thickness of 0.4 nm.

Schottky barriers at epitaxial silicide/Si interfaces

We studied the electronic structure of the YSi 2/Si(111) and NiSi 2/Si(001) interfaces using the linear muffin in orbitals in the atomic sphere approximation (LMTO-ASA) based on the local density approximation (LDA). Together with the previous results on the CoSi 2/Si(111) and NiSi 2/Si(111) interfaces, we showed that LMTO-ASA calculations with a large supercell give an adequate Schottky barrier height (SBH: E F −: E V) for real silicide/Si interfaces although the LDA depresses the band gap of bulk Si to almost half of the experimental value. An eightfold NiSi 2/Si(001) interface showed almost the same SBH as the type A NiSi 2/Si(111) interface. From the relation between the interface structure and the calculated SBH, we speculate that the bond angle at the interface affects the SBH

Characterization of Porous Silicon: Structural, Optical and Electrical Properties

ABSTRACTThe aim of this paper is to provide a better understanding of photoluminescent porous silicon (PS) microstructure in relation to their electronic properties: absorption band edge shift1 and quantum confinement hypothesis1,2, dielectric constant evolution and electroluminescence characteristics.Results concerning the p type PS microstructure characterization by X ray diffraction and electron microscopy are presented showing a noticeable decrease in crystallite size and surface area with decreasing substrate doping and increasing porosity.The optical transmission of homogeneous free-standing PS layers of different porosities and substrate dopings is studied, showing no evidence of a direct energy gap in PS. On the contrary, a large blue shift of the optical absorption edge, taking into account the total Si mass content in the PS film, is demonstrated. This shift is well correlated with the crystallite size variations with porosity and substrate doping and is attributed to a quantum confinement of electronic wavefunctions in the nanocrystallites.On the other hand, ellipsometry measurements show the PS absorption to be little affected by the microcrystalline structure of the material in the 3.5–5 eV range, i.e. above the direct band gap of bulk Si. This indicates that, if confirmed, the quantum confinement strongly affects the PS joint density of states in the vicinity of the Si band edge and, as could be expected, to a much lesser degree near the edge of the confining potential.Capacitance voltage measurements of thin PS layers allow the determination of the dielectric constant which is shown to decrease with increasing porosity. This behavior is in reasonable agreement with theε values deduced from the transmission experiments in the near infrared. Furthermore, it is shown that this dependence on porosity is well accounted for by the Bruggeman effective medium approximation.Finally, recent results concerning visible light emission from solidstate porous silicon devices will be presented: I-V characteristics, electroluminescence intensity and dynamic, quantum efficiency and device ageing.

Bonding at silicon/insulator interfaces

Various techniques for the spectroscopy of interfaces are discussed such as core level spectroscopy, near-edge absorption, and optical second harmonic generation. The CaF2/Si(111) interface serves as a model for determining electronic states at an interface. A pair of interface states is found, one occupied, the other empty. They represent the bonding (antibonding) combinations of the orbitals that hold the interface together. The interface layer exhibits new properties, e.g., more than twice the band gap of bulk Si. The bonding at complex interfaces, such as SiO2/Si, can also be analyzed using the simple relation between core level shift and oxidation state. Only models with an extended interface are compatible with the observed distribution of intermediate oxidation states.