G. Pucker

High power efficiency in Si-nc/SiO2 multilayer light emitting devices by bipolar direct tunneling

We demonstrate experimentally bipolar (electrons and holes) current injection into silicon nanocrystals in thin nanocrystalline-Si/SiO2 multilayers. These light emitting devices have power efficiency of 0.17% and turn-on voltage of 1.7 V. The high electroluminescence efficiency and low onset voltages are attributed to the radiative recombination of excitons formed by both electron and hole injection into silicon nanocrystals via the direct tunneling mechanism. To confirm the bipolar character, different devices were grown, with and without a thick silicon oxide barrier at the multilayer contact electrodes. A transition from bipolar tunneling to unipolar Fowler–Nordheim tunneling is thus observed.

Propagation losses of silicon nitride waveguides in the near-infrared range

Si3N4∕SiO2 waveguides have been fabricated by low pressure chemical vapor deposition within a complementary metal–oxide–semiconductor fabrication pilot line. Propagation losses for different waveguide geometries (channel and rib loaded) have been measured in the near infrared as a function of polarization, waveguide width, and light wavelength. A maximum thickness of single Si3N4 of 250 nm is allowed by the large stress between Si3N4 and SiO2. This small thickness turns into significant propagation losses at 1544 nm of about 4.5dB∕cm because of the poor optical mode confinement factor. Strain release and control is possible by using multilayer waveguides by alternating Si3N4 and SiO2 layers. In this way, propagation losses of about 1.5dB∕cm have been demonstrated thanks to an improved optical mode confinement factor and the good quality of the interfaces in the waveguide.

Low-voltage onset of electroluminescence in nanocrystalline-Si/SiO2 multilayers

Thin film metal-oxide-semiconductor light emitting devices (LEDs) based on nanocrystalline silicon multilayer structure were grown by plasma-enhanced chemical vapor deposition. Room temperature electroluminescence was studied under direct current and time-resolved pulsed-current injection schemes. Multilayer LEDs operating at voltages below 5 V and electroluminescence turn-on voltage of 1.4–1.7 V are demonstrated. The turn-on voltage is less than 3.2 V which corresponds to the barrier height at the silicon oxide interface for electrons. Electrical injection in the multilayer LED is controlled by direct tunneling of electrons and holes among silicon nanocrystals. This injection regime is different than the Fowler–Nordheim tunneling that controls the electron injection in single thick layer LED operating at high voltages. A comparison of the power efficiency for the multilayer based LED and a similar single thick layer LED shows larger power efficiency for the former than for the second. Our results open ne...

Silicon-based near-infrared tunable filters filled with positive or negative dielectric anisotropic liquid crystals

Complementary metal-oxide-semiconductor-compatible tunable Fabry–Perot microcavities filled with liquid crystals (LCs) were realized and studied in the near-infrared region. The microcavities were produced by chip bonding technique, which allows one to infill LC between two [SiO2/Si]3λ/4 (λ=1.5 μm) dielectric Bragg reflectors separated by 950-nm-thick SiO2 posts. Liquid crystals with positive and negative dielectric anisotropy were used, i.e. MerckE7 (Δe=13.8) and Merck-6608 LC (Δe=−4.2). Mirror-integrated electrodes allow an external bias to induce an electric field and to tune the LC properties and, hence, the microcavity resonance. Electric-field-induced shifts of the second-order cavity modes of 127 and 49 nm were obtained for Merck-E7 and Merck-6608 LC, with driving potentials of 5 and 10 V, respectively.

Graded-size Si quantum dot ensembles for efficient light-emitting diodes

We propose a simple way to engineer the energy band gap of an ensemble of silicon nanocrystal (Si-NC) embedded in SiO2 via thickness/composition profiling of Si-NC multilayers. By means of a complementary metal-oxide-semiconductor compatible process, light emitting diodes (LEDs) which incorporate graded energy gap Si-NC multilayers in the active region have been grown. Electrical and optical properties of these graded Si-NC LEDs demonstrate the ability of the proposed method to tailor the optoelectronic properties of Si-NC devices.

Superlinear photovoltaic effect in Si nanocrystals based metal-insulator-semiconductor devices

Superlinear-variation in short circuit photocurrent with increasing incident optical power has been observed in metal-insulator-semiconductor structures having a silicon rich oxinitride active layer containing silicon nanocrystals. A model has been elaborated where an internal gain mechanism explains the superlinear photovoltaic effect. The internal gain mechanism is due to secondary carrier generation (SCG) from sub-bandgap levels in the nanocrystal. SCG is caused by impact excitation from the photogenerated conduction band electrons. The sub-bandgap levels are associated to traps formed at the dielectric/Si-nanocrystals interface.

Optical response of one-dimensional (Si/SiO2)m photonic crystals

One-dimensional photonic crystals made of (Si/SiO2)m multilayers with m=2,…8 have been grown on SiO2 4-in. wafers by repeated polysilicon low-pressure chemical vapor deposition, oxidation, and wet etching steps. The poly-Si and SiO2 layers were about 220 and 660 nm thick, respectively, thus realizing λ/4 distributed Bragg reflectors. Spectroscopic ellipsometry in the 1.4–5 eV range was used to determine the dielectric function of poly-Si and the actual layer thicknesses, as well as to check the structural and compositional homogeneity of the structures. In order to measure the photonic crystal properties, specular reflectance and transmittance measurements were performed from 0.2 to 6 eV at different angles of incidence θ⩽50° and for transverse electric and transverse magnetic polarizations. The stop-bands characteristic of Bragg reflector multilayers appear up to the fifth order and become more pronounced with increasing m, reaching almost complete rejection for m=4 periods. The experimental spectra were...

Room temperature luminescence from (Si/SiO2)n (n=1,2,3) multilayers grown in an industrial low-pressure chemical vapor deposition reactor

A simple complementary metal–oxide–semiconductor compatible process for the preparation of very thin (1–5 nm thick) poly-Si layers embedded in SiO2 is presented. The process consists of repeated cycles of poly-Si deposition, oxidation, and wet etching steps. Periodic structures with up to three Si/SiO2 layers were grown using this process. Transmission electron microscopy analyses show that the layered structure can be conserved down to a Si layer thickness of 2 nm. For thinner layers the resulting structure is more granular like. Samples with a Si-layer thickness lower than 3 nm show room temperature photoluminescence at about 1.55 eV that shifts to higher energies when the thickness is further reduced. The maximum shift obtained with respect to the c-Si band gap is 0.55 eV. Intensity of the photoluminescence as a function of temperature shows a behavior similar to the one observed for 0 and one-dimensional Si structures. On the basis of the thickness dependence, the temperature dependence and the satura...

Oscillatory vertical coupling between a whispering-gallery resonator and a bus waveguide.

We report on a theoretical and experimental study of the optical coupling between a whispering-gallery type resonator and a waveguide lying on different planes. In contrast to the usual in-plane geometry, the present vertical one is characterized by an oscillatory behavior of the effective coupling as a function of the vertical gap. This behavior manifests itself as oscillations in both the resonance peak waveguide transmission and the mode quality factor. An analytical description based on coupled-mode theory and a two-port beam-splitter model of the waveguide-resonator vertical coupling is developed for arbitrary phase-matching conditions and is successfully used to interpret the experimental observations.

Thermo-optical bistability with Si nanocrystals in a whispering gallery mode resonator

We report on the observation of optical bistability in an integrated planar microresonator with embedded silicon nanocrystals (Si-ncs). The phenomenon originates from the thermo-optical modulation of the silica-embedded Si-ncs refractive index, which in turn alters the spectral position of the resonator mode. The estimated thermo-optical coefficient of the Si nanocrystalline material, dn/dT≈2.92×10(-5)>  K(-1), is an order of magnitude lower than that of bulk silicon. Both time-resolved pump-and-probe experiments and numerical simulations confirm that the silica host is responsible for the heat dissipation from the resonator. Moreover, a negligible Q-factor degradation at pump powers as high as 100 mW, along with the absence of a fast component in time-resolved measurements, confirm the minute contribution from excited carriers effects. These observations, combined with the already published large third-order nonlinearities of Si-ncs (an order of magnitude larger than in bulk Si), make this system an outstanding candidate for low-power on-chip nonlinear comb generation.