Dimensions of luminescent oxidized and porous silicon structures

Abstract

X-ray absorption measurements from H-passivated porous Si and from oxidized Si nanocrystals, combined with electron microscopy, ir absorption, a recoil, and luminescence emission data, provide a consistent structural picture of the species responsible for the visible luminescence observed in these samples. The mass-weighted average structures in por-Si are particles, not wires, with dimensions significantly smaller than previously reported or proposed. PACS numbers: 78.70.Dm, 61.10.Lx, 61.46.+w, 78.55. -m The novel properties and possible utility of visible room-temperature luminescence from anodically grown porous silicon (por-Si) have generated intense study [1], from which a growing consensus has emerged for explaining the luminescence with quantum-confined structures [2,3]. (Luminescence from por-Si passivated with 0 rather than H [4] has ruled out a SiH"species, while the absence of Si-0 bonding in x-ray absorption data from por-Si [5] has ruled out siloxenes. ) There remains, however, a basic lack of knowledge regarding the dimensions and shape of the species actually responsible for the optical activity, largely because the material is inhomogeneous, but also because there is little direct structural information from the region lying within the penetration depth of the photoexciting radiation. Even for 80% porosity samples excited by -350-400-nm light (where the luminescence is a maximum [5,6]), this depth is & 5000 A [7] and thus not well suited for standard microscopy or diffraction techniques from as-prepared material. Moreover, Si structures & 20 A, should they be significant, are beyond practical detection with these methods. Added to these experimental limitations are theoretical uncertainties in correlating band gap with Si size, where nanoscale particle or wire dimensions for a given gap vary by more than 100% [1,8]. %e report on x-ray absorption measurements from a series of oxidized Si nanocrystals, whose shapes and sizes are known, and from a variety of anodically grown (Hpassivated) por-Si samples. The data, combined with luminescence emission measurements from each of the systems, establish new and unexpectedly smaller values for the average size of Si structures contained within optically relevant depths of por-Si. In addition, the general importance of extended wire shapes is ruled out. Our results have significant implications for describing the origin of visible photoluminescence from por-Si. The Si E-edge absorption measurements were performed at the National Synchrotron Light Source using the AT&T X158 beam line [9] and InSb(111) monochromating crystals. Samples were kept at 77 K to minimize thermal disorder effects [10]. All data were obtained with total electron yield detection, whose effective sampling depth in 80%%ue porosity Si is & 2500 A [11]. A variety of por-Si samples prepared under very different conditions [12] were studied with transmission electron microscopy (TEM), x-ray and ir absorption, tt recoil, and luminescence excitation and emission spectroscopies. Air exposure of ...

Quantum confinement in Si and Ge nanostructures

“…In this case, they are called pseudopor-Si-QDs or in the case they behave like interconnected dots, spherites. [37] Anodically grown por-Si samples were prepared by Schuppler et al [38] X-ray absorption measurements determined the structures to be closer to c-Si than to a-Si. TEM was used to determine the size and PL measurements were performed at room temperature.…”

Number series of atoms, interatomic bonds and interface bonds defining zinc-blende nanocrystals as function of size, shape and surface orientation: Analytic tools to interpret solid state spectroscopy data

“…In fact, the slightly bigger l edge values derived by NEXAFS spectroscopy for N coord NC = 3.52 could be manifested by expansive stress exerted onto the {001}-faceted Si cubes by their 0.7 nm thick SiO 2 shell. 32 While SiO 2 layers of thickness d SiO 2 ≤ 10 nm thickness on a Si wafer experience compressive stress 5 due to Si wafer thicknesses exceeding d SiO 2 by a factor of ≥ 10 4 , the situation here can be reversed since the volume ratio between oxide embedding and NC favors the SiO 2 shell.…”

Bringing Silicon to Light: Luminescence in Silicon Nanostructures

“…As for -Si, however, the emitted light energy generally falls below that expected from calculations of the energy gap for Si spheres (17). Also, the confinement effect is seen (69)(70)(71) or not seen (72) in emission depending on sample preparation. Interpretation of the nanocrystal PL spectra suffers from the same ambiguities as -Si, i.e., nanocrystal size distribution effects and surface chemistry effects (68).…”