Intrinsic band-edge photoluminescence from silicon clusters at room temperature

Abstract

We report silicon band-edge photoluminescence ͑PL͒ with photon energy of 1.1 eV and external quantum efficiency ͑EQE͒ better than 10 Ϫ3 in samples prepared by high-temperature oxidation of porous silicon. The integrated PL intensity is insensitive to temperature. The EQE strongly depends on the annealing conditions: temperature, time, and ambient. A model is proposed in which the PL originates from silicon clusters within a nonstoichiometric silicon-rich silicon oxide matrix. ͓S0163-1829͑96͒52336-2͔A number of groups have investigated different types of photoluminescence ͑PL͒ in crystalline silicon (c-Si͒ during the last decades. Recent interest has focused on the visible PL that is observed in Si nanoclusters and in porous Si ͑PSi͒, 1 the infrared PL in silicon-germanium superlattices, 2 and the subgap PL due to impurities in c-Si. 3 Band-edge PL in bulk Si is inefficient and usually observed at low temperatures only because c-Si has an indirect band gap. 4 However, there are a few reports in the literature of room-temperature band-edge PL in Si. 5 The typical external quantum efficiency ͑EQE͒ of this PL is understandably low (р10 Ϫ5 ) but can be increased by better surface passivation. 6 In this work we report a significant increase of the Si band-edge PL EQE to greater than 0.1% and its unexpected weak dependence on temperature. During sample preparation, a large number of parameters may be varied opening additional room for further improvements.The samples were prepared by anodically etching borondoped p ϩ c-Si wafers with a resistivity Ϸ0.05 ⍀ cm in an HF-ethanol solution ͑1:1͒ under a current density JϷ20 mA/cm 2 . After anodization the samples were annealed in dilute oxygen ͑10% O 2 in N 2 ) from 10 min up to 3 h, at temperatures ranging from 800 to 990°C. Fouriertransform infrared ͑FTIR͒ spectra of the oxidized samples show the presence of silicon oxide with an absorption peak near 1080 cm Ϫ1 and the absence of Si-H bonds. The PL measurements were performed using the experimental setup in Ref. 5. Figure 1 compares the PL spectra measured over a temperature range from 12 to 300 K for a sample annealed at 950°C for 30 min. The inset shows a detailed PL spectrum at 12 K with a weak no-phonon ͑NP͒ line at 1.158 eV, a TA-phonon line near 1.14 eV, and a TO-phonon line at 1.1 eV with a replica at 1.04 eV. These PL spectral lines are well known in c-Si and tabulated in Ref. 3. The PL spectrum peak is shifted and becomes broader as the temperature is raised. In the temperature region from 12 to 50 K the PL peak undergoes a small blueshift ͑to shorter wavelength͒ and at temperatures greater than 70 K the PL peak is redshifted ͑to longer wavelength͒. We do not observe any blueshift in the PL spectra attributable to quantum confinement effects.Increasing the temperature broadens the PL spectra. Figure 2 shows that the intensity of the high-energy tail of the PL spectrum is linear on a semilogarithmic scale. The slope increases linearly with temperature, as expected when the carriers follow a Boltzmann distribution. 7...

Si/SiGe Heterointerfaces in One-, Two-, and Three-Dimensional Nanostructures: Their Impact on SiGe Light Emission

“…No significant PL at photon energies close to the bulk c-Si transverse optical (TO) phonon-replica PL peak at ~1.1 eV is found. The spectral positions and relative intensities of these two PL peaks are found to be temperature dependent (see Figure 8B): both PL peaks change their positions significantly with temperature and exhibit the lowest photon energy at ~60 K. The PL1 peak is detectable even at room temperature and exhibits an asymmetric broadening, which [in agreement with Tsybeskov et al (1996)] can be fitted by employing Boltzmann thermal broadening on the high energy side of this feature.…”

Structural and optical properties of axial silicon-germanium nanowire heterojunctions

“…In bulk c-Si, the PL peak energy position and line-shape temperature dependencies are explained by taking into account the exciton binding energy (E exc % 10 meV), the phonon-assisted nature of the band-toband recombination involving mostly TO phonons (E ph % 60 meV) and the temperature dependence of population of energy states in the energy bands. 32 Thus, the bandto-band recombination related PL peak photon energy is E PL ¼ E G À (E ph þ E exc ), and in bulk c-Si at low temperature it is close to 1.1 eV. The temperature dependencies of E G (T) and E PL (T) in c-Si are shown in Fig.…”

Photoluminescence and Raman scattering in axial Si/Ge nanowire heterojunctions

“…The ϳ1.08 eV PL peak exhibits an asymmetric broadening at higher temperatures ͑T Ն 60 K͒, which can be well fitted using Boltzmann thermal broadening on the high photon energy side of the PL spectrum. 19 The PL peaks at ϳ1.08 eV and 0.96 eV do not change their positions significantly with temperature, while the PL peak at ϳ1.0 eV shifts nonmonotonically and exhibits the lowest photon energy between 60 and 80 K.…”