Cw Pitt

The origin of photoluminescence from thin films of silicon‐rich silica

We have carried out a study of the photoluminescence properties of silicon‐rich silica. A series of films grown using plasma enhanced chemical vapor deposition over a range of growth conditions were annealed under argon at selected temperatures. Photoluminescence spectra were measured for each film at room temperature and for selected films at cryogenic temperatures. The photoluminescence spectra exhibit two bands. Fourier transform infrared and electron spin resonance spectroscopies were used to investigate bonding and defect states within the films. The data obtained strongly suggest the presence of two luminescence mechanisms which exhibit different dependencies on film growth conditions and postprocessing. We make assignments of the two mechanisms as (1) defect luminescence associated with oxygen vacancies and (2) radiative recombination of electron‐hole pairs confined within nanometer‐size silicon clusters (‘‘quantum confinement’’).

Optical properties of PECVD erbium-doped silicon-rich silica: evidence for energy transfer between silicon microclusters and erbium ions

We report the fabrication by PECVD of silicon-rich erbium-doped silica films that exhibit both 1535 nm fluorescence and visible photoluminescence. Fluorescence spectra are presented along with absorption spectra that display a strong band edge in the blue, which we ascribe to the presence of Si microclusters. We are unable to observe characteristic Er3+ absorption bands and propose that excitation of the rare earth is via an energy transfer process from Si microclusters.

MODELING THE CONTRIBUTION OF QUANTUM CONFINEMENT TO LUMINESCENCE FROM SILICON NANOCLUSTERS

We present a model for the luminescence spectrum of silicon nanoclusters. We propose that the major contribution to luminescence is from radiative recombination of confined excitons (quantum confinement). Utilizing the effective mass approximation we consider the variation in oscillator strength with cluster size and the associated change in the number of available free carriers. By varying both the mean cluster size and size distribution of silicon nanoclusters, the luminescence spectra are modeled to a good fit. We compare our model with experimental photoluminescence and electroluminescence data from this group and from others.

Luminescence from erbium-doped silicon nanocrystals in silica: Excitation mechanisms

We develop a model for the excitation of erbium ions in erbium-doped silicon nanocrystals via coupling from confined excitons generated within the silicon nanoclusters. The model provides a phenomenological picture of the exchange mechanism and allows us to evaluate an effective absorption cross section for erbium of up to 7.3×10−17 cm2: four orders of magnitude higher than in stoichiometric silica. We address the origin of the 1.6 eV emission band associated with the silicon nanoclusters and determine absorption cross sections and excitonic lifetimes for nanoclusters in silica which are of the order of 1.02×10−16 cm2 and 20–100 μs, respectively.

Evidence of energy coupling between Si nanocrystals and Er3+ in ion-implanted silica thin films

Silica thin films containing Si nanocrystals and Er3+ were prepared by ion implantation. Excess Si concentrations ranged from 5% to 15%; Er3+ concentration for all samples was 0.5%. Samples exhibited photoluminescence at 742 nm (attributed to Si nanocrystals), 654 nm (defects due to Er3+ implantation), and at 1.53 μm (intra-4f transitions). Photoluminescence intensity at 1.53 μm increased ten times by incorporating Si nanocrystals. Strong, broad photoluminescence at 1.53 μm was observed for λPump away from Er3+ absorption peaks, implying energy transfer from Si nanocrystals. Erbium fluorescence lifetime decreased from 4 ms to 1 ms when excess Si increased from 5% to 15%, suggesting that at high Si content Er3+ ions are primarily situated inside Si nanocrystals.

Luminescence efficiency measurements of silicon nanoclusters

We present the results of what we believe to be the first study of the power efficiency of room temperature photoluminescence from thin films of silica containing silicon nanoclusters. Films were prepared by plasma enhanced chemical vapor deposition from silane and nitrous oxide precursors. Luminescence was excited using the 476 nm line of an argon-ion laser. We have measured power efficiencies for samples that exhibit luminescence solely due to radiative recombination of quantum confined excitons. Efficiencies around 0.04% are reported.

Growth of thin‐film niobium and niobium oxide layers by molecular‐beam epitaxy

The ultrahigh vacuum technique of molecular‐beam epitaxy (MBE) has been successfully employed in growing highly oriented polycrystal and single‐crystal niobium oxide layers on z‐cut LiNbO3 substrates. A new variant of the monoxide has been grown and characterized, and the potential advantages and limitations of MBE for growing the higher oxides, such as Nb2O5, are put forward. Niobium metal layers have also been grown on z‐cut LiNbO3. The deposition of excellent quality niobium oxide on α‐alumina (z‐cut sapphire) has been demonstrated, providing an optical waveguiding structure of large differential refractive index. Crystallinity and compositional data are presented for all the above layers. The post‐deposition oxidation of oxygen‐deficient films to the pentoxide has been studied in detail, and system requirements for the in situ growth of Nb2O5 proposed.

The origin of the 0.78 eV luminescence band in dislocated silicon

We report the results of a study into the influence of implanted impurities on luminescence in the region of the well-known D1 luminescence band that is associated with dislocations in silicon. A photoluminescence band at around 0.78 eV, which is sometimes seen in silicon containing a high density of dislocations, has been attributed to the presence of oxygen complexes. In this study we have deposited layers of Si0.9Ge0.1 onto single-crystal Si substrates by MBE in order to induce dislocations in the silicon substrate. The samples have subsequently been implanted with iron, erbium or oxygen in order to study the effect of implanted impurities on D-band photoluminescence at around 800 meV. Following implantation with oxygen, two luminescence bands appear at around 0.85 and 0.78 eV, respectively. These bands are not present in either the unimplanted sample or those subject to Er or Fe implantation. The correlation between oxygen doping and the appearance of these bands supports the conjecture that they are associated with oxygen complexes.

Investigation of energy exchange between silicon nanocrystals and Er3+ in silica

We present a photoluminescence study of 1.53 μm emission from erbium in silicon-rich silica that demonstrates efficient energy exchange between silicon nanocrystals and optically active erbium ions. We develop a rate-equation model of the interaction. Using this, we determine the effective erbium absorption cross-section, which appears to be several orders of magnitude higher than that in stoichiometric silica. This cross-section enhancement suggests the possibility of producing very efficient silicon-based broadband pumped optical devices for integrated optoelectronics, which exploit the nanocrystal, rare-earth coupling mechanism.

Indirect excitation of 1.5 μm emission from Er3+ in silicon-rich silica

We report the observation of near-IR emission from erbium in silicon-rich silica, excited using a filtered white-light source. The characteristic 4I13/2–4I15/2 intra-4f transition at 1535 nm is observed even when excitation wavelengths corresponding to the principal erbium optical absorption bands are removed using selective filtering. We ascribe this effect to an efficient transfer mechanism between silicon nanoclusters present in the silicon-rich silica films and the rare-earth ions. This is in good agreement with our previous work in this area and suggests the possibility of obtaining flashlamp-pumped erbium optoelectronic devices.