Silicon Nanocrystals
2.3 Photoluminescence Properties
thesis work [95, 96] using a combination of scanning probe measurements and reflection high energy electron diffraction (rheed). His experiments were conducted using the same sample set we use in the optoelectronic experiments described in this thesis, allowing us to make direct use of his results. These samples were fabricated during our collaboration with Intel on 300 mm substrates by implantation with 5 keV Si+ ions to a total fluence of 1.27 × 1016 cm−2 and were annealed in an RTA furnace at 1080 ◦C for 5 minutes in an atmosphere containing 2% oxygen. His measurements show that the nanocrystals fab- ricated by this procedure are crystalline and that they are distributed in an approximate monolayer within the oxide layer. He estimates the areal density of nanocrystals in the samples to be ∼4 ×1012 cm−2. Figure 2.3 is reproduced from Feng’s thesis, and shows a measurement of the distribution of silicon nanocrystal diameters that suggests a mean di- ameter of∼2.5 nm. This size is somewhat too small to correspond to the photoluminescence spectra we measure, and probably implies a decrease in diameter during the etching proce- dure used to separate the nanocrystals from the oxide matrix prior to the vacuum atomic force microscopy measurement. A distribution of nanocrystal diameters centered at 2.5 nm would imply that ∼26% of the implanted silicon contributes to nanocrystal formation. If the etching procedure has reduced the average diameter from∼4 nm, as may be more con- sistent with the range of observed photoluminescence wavelengths, ∼82% of the implanted silicon contributes to nanocrystal formation. The remainder of the implanted silicon could be incorporated into the substrate during the nanocrystal formation process, adding∼1 nm of silicon to the interface. Alternatively, the implanted silicon could be incorporated in the oxide or present in agglomerates that are too small to observe using vacuum AFM.
minescence spectrum of a sample fabricated at Intel during our collaboration is shown in figure 2.4. This spectrum is entirely consistent with previously reported silicon nanocrystal photoluminescence results. It is now well established that the characteristic near-infrared photoluminescence, typically observed in the range from ∼650 nm to ∼950 nm, originates from the band-to-band recombination of quantum confined excitons [52]. Interface states involving oxygen bonds are thought to play an important role in smaller nanocrystals, which emit photons at lower energies than predicted by theory [112]. Reports of silicon nanocrystal photoluminescence at green or blue wavelengths in oxide matrices tend to be met with skep- ticism, and are commonly assumed to be misinterpretations of defect luminescence within the oxide matrix [24]. Within the near-infrared emission band, the emission wavelength can be tuned by controlling the diameter of the silicon nanocrystals [104]. While the strength of band-to-band radiative recombination increases with decreasing nanocrystal size over this spectral range, a transition to a direct gap band structure has not been observed. It has been proposed that the oxygen bond related interface states dominate the recombination for small nanocrystals that might otherwise show direct gap behavior [52]. For this reason, silicon nanocrystals embedded in nonoxide matrices such as Si3N4 have recently attracted attention [57]. However, the radiative quantum efficiency of silicon nanocrystals embedded in silicon nitride may be lower than the quantum efficiency of nanocrystals in SiO2.
The absorption characteristics of silicon nanocrystals can be measured directly by trans- mission measurements, but these experiments may be effected by absorption that is es- sentially unrelated to the excitation of quantum confined excitons in silicon nanocrystals.
A preferred approach is to perform photoluminescence excitation (PLE) experiments, in which the emission spectrum is monitored as the excitation wavelength is changed. These measurements primarily show that the average absorption in silicon nanocrystal ensembles closely resembles absorption in bulk silicon (i.e., it is essentially featureless and generally increases into the UV) [80]. This is perhaps unsurprising as typical∼3–4 nm diameter sili- con nanocrystals contain order 103 silicon atoms and likely have a continuum of conduction band states available to absorb light at UV wavelengths. Electron energy loss spectroscopy (eels) measurements also suggest that the conduction band is bulk-like in silicon nanocrys- tals. [113]
Despite being bright in relation to bulk silicon, single nanocrystals are still feeble light emitters (<300 fW) and so the vast majority of reported experiments have been studies of
Figure 2.4. A photoluminescence spectrum for silicon nanocrystals fabricated during our collaboration with Intel Corporation. The nanocrystals are excited by ∼50 W/cm2 pump illumination at 457.9 nm.
large ensembles of nanocrystals. Ensembles of nanocrystals are complicated optical systems that can exhibit different photoluminescence properties than isolated nanocrystals. For example, ensemble photoluminescence spectra are usually quite broad as a consequence of nanocrystal size inhomogeneity while single nanocrystal measurements made at low tem- perature show narrow (∼2–20 meV) homogeneous spectra [114]. The single nanocrystal measurements tend to vary widely from nanocrystal to nanocrystal, suggesting that photo- luminescence is sensitive to small changes in nanocrystal composition or environment that are averaged out in ensemble measurements. These experiments also provide evidence for the excitation of complicated phonon modes that suggest that the shape of a nanocrystal may be important in addition to the size. Single nanocrystal photoluminescence measure- ments made at room temperature show that the homogeneous spectra broaden considerably to ∼120–150 meV [115]. To our knowledge, no PLE or recombination scale time-resolved photoluminescence experiments on single nanocrystals have yet been reported.
Other known photoluminescence properties of interest for silicon nanocrystals in silicon
oxide include the optical excitation cross section (∼6 ×1016 cm2 at 488 nm) [80] and the single-triplet exchange energy gap for excitons (∼10 meV) [116]. The exchange splitting explains the temperature dependence of silicon nanocrystal photoluminescence intensity in terms of a slow recombination rate for the parity forbidden triplet-to-ground state tran- sitions (∼0.1–0.3% of the singlet-to-ground transition rate) and the probability that the exciton will be found in the more optically active singlet state in thermal equilibrium.