Using high pressure to unravel the mechanism of visible emission in amorphous Si/SiO${}_x$ nanoparticles

Regarding light absorption/emission efficiency, silicon presents the fundamental drawback of its indirect band gap. It is long known, though, that optical properties are greatly enhanced in materials which comprise different kinds of nanocrystalline Si covered by or embedded in Si oxide layers. Conversely, their amorphous counterparts have received far less attention, such that no general consensus about the emission mechanism prevails. We report here on an efficiently luminescent material based on amorphous Si nanoparticles (a-Si NPs) embedded in a nonstoichiometric Si oxide matrix, which exhibits intense, broadband emission from the a-Si NPs, spectrally separable from the defect luminescence of the suboxide matrix. Apart from the brightness of the emitted light, the nanometer-size a-Si inclusions present the technological advantage of needing very moderate annealing temperatures (450${}^{\circ }$C–700${}^{\circ }$C) for their production. The combined use of high pressure, experimentally as well as theoretically, allowed us to trace back the microscopic origin of the photoluminescence to radiative recombination processes between confined states of the a-Si NPs. The signature of quantum confinement is found in the magnitude and sign of the pressure coefficient of different optical transition energies. The pressure derivatives exhibit a universal dependence on particle size, determined solely by the confinement energy of the discrete electron state involved in the radiative recombination process.

Figure 1

(a) Energy-filtered TEM micrographs of the sample annealed at 550${}^{\circ }$C clearly showing the presence of Si NPs embedded in the suboxide matrix. The Si NPs and the Si substrate appear bright, whereas the dark regions correspond to suboxide material. (b) Idem (a) but for the sample annealed at 700${}^{\circ }$C. (c) Particle-size histogram of the sample annealed at 550${}^{\circ }$C constructed by sampling more than 300 NPs. The resulting size distribution is fairly sharp with a mean diameter of (2.1 $\pm$ 0.2) nm. (d) Idem (c) but for the sample annealed at 700${}^{\circ }$C. The mean diameter of (2.3 $\pm$ 0.3) nm is only slightly larger.

Figure 2

(a) Fourier-transform infrared (FTIR) absorption spectra of the as-grown silicon oxide layer and four different pieces annealed at 450${}^{\circ }$C, 550${}^{\circ }$C, 625${}^{\circ }$C, and 700${}^{\circ }$C in the frequency region of the asymmetric Si-O-Si stretching vibration of oxygen atoms in twofold-coordinated bridging bonding sites. The integrated intensity of this absorption band is used to monitor the changes in composition of the suboxide upon annealing. (b) The oxygen content of the SiO${}_x$ matrix as determined by FTIR for the five studied samples. Thermal annealing triggers phase separation processes in the suboxide matrix, leading to an increasingly stoichiometric matrix material and the further formation of pure Si NPs, as the annealing temperature increases.

Figure 3

(a) The red-shift of the whole PL band with increasing annealing temperature, i.e., for larger Si NP diameters, is a clear indication that quantum confinement effects are at work. The extremely faint emission from the as-grown layer is associated with luminescence from defect centers in the suboxide matrix. (b) An example of the line-shape fitting analysis of the PL spectra performed with four Gaussian peaks labeled $E_1,...,E_4$. The three lowest-energy peaks, which can be assigned to optical transitions between different confined states of the a-Si NPs, exhibit similar red-shifts upon annealing, i.e., with increasing average NP diameter $d$, as displayed in the inset.

Figure 4

(a) In vacuum and under strong illumination with a blue laser (488-nm line with a power density of about 2.5 kW/cm${}^2$), the incipient PL emission of the as-grown sample transforms into a broad band centered at around 2.2 to 2.3 eV, that grows continuously in intensity becoming the dominant peak. (b), (c) Idem (a) for the samples annealed at 550${}^{\circ }$C and 700${}^{\circ }$C, respectively. In all three cases, it is the same emission band that appears upon light irradiation. This emission disappears again by breaking the vacuum letting air (O${}_2$) into the sample chamber.

Figure 5

(a) PL spectra of the sample annealed at 550${}^{\circ }$C measured at different hydrostatic pressures up to 9 GPa. High pressure causes a small but detectable red-shift of the emission band of the Si NCs. (b) Idem (a) for the sample annealed at 700${}^{\circ }$C. (c) The maximum peak position of the three lowest-energy Gaussians as a function of pressure, showing a decreasing linear dependence above 2 to 3 GPa. All data points corresponding to several subsequent pressure upstrokes and downstrokes are shown. The slopes exhibit a striking systematics upon variations in crystal size and depending on the energy of the optical transition.

Figure 6

(a) Valence and conduction band density of states (DOS) calculated for the depicted Si nanocrystal with a diameter of 3 nm (solid curve). Peaks corresponding to discrete states were artificially broadened to simulate inhomogeneous broadening in the ensemble, leading to a smooth DOS with peaklike features like the ones marked with dashed vertical lines for the conduction band. (b) The calculated linear pressure coefficients of conduction band eigenvalues referred to the top of the valence band for six different crystal sizes between 1.5 and 4 nm (open symbols). Closed symbols with error bars represent the measured values of the pressure coefficient for transition energies $E_1,E_2$, and $E_3$. The inset shows how the calculated pressure coefficient of the conduction band ground state becomes more negative with increasing diameter, reaching eventually the value given by the pressure derivative of the lowest indirect band gap in bulk Si.