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We have investigated correlation between luminescence property and particle size of nanocrystalline silicon (nc-Si) fabricated by controlling Si concentration in an amorphous SiOx (a-SiOx) films. The Si concentration in the a-SiOx film was increased with increasing a RF power and lowering a gas pressure. The increase of Si concentration led to expansion of the particle size of nc-Si. The particle size of nc-Si was varied from 1.8 nm up to 3.5 nm for the sample introduced the Si concentration from 0.7 % up to 9 %. The luminescent color from nc-Si grains, which differs in size, showed a red/green/blue lights.
The electrical transport properties of field effect transistor (FET) devices made of silicon nanowires (SiNWs) synthesized by pulsed laser vaporization (PLV) were studied. From as-grown PLV-SiNW FET, we found p-channel FET behavior with low conductance. To improve conductance, spin on glass (SOG) and vapor doping were used to dope phosphorus and indium into SiNW, respectively. From doping after synthesis, we could successfully make both n- and p-channel FET devices.
The threshold voltage shift of a p-channel Ge/Si hetero-nanocrystal floating gate memory device was investigated both numerically and phenomenologically. The numerical investigations, by solving 2-D Poisson-Boltzmann equation, show that the presence of the Ge on Si dot tremendously prolongs the retention time, reflected by the time decay behavior of the threshold voltage shift. The increase of the thickness of either Si or Ge dot will reduce the threshold voltage shift. The shift strongly depends on the dot density. Nevertheless, only a weak relation between the threshold voltage shift and the tunneling oxide thickness was found. A circuit model was then introduced to interpret the behavior of threshold voltage shift, which agrees well with the results of the numerical method.
Hydrogen passivation effect on the enhancement of photoluminescence (PL) of Er ions in SiO2 films contained Si nanocrystallites (nc-Si) has been investigated. Er-doped SiO2 films were fabricated by laser ablation of Er-deposited Si substrate in oxygen gas atmosphere. The PL intensity of Er ions and nc-Si were increased by hydrogen gas treatments, while ESR signal intensity of residual defects located at the interfaces between nc-Si and SiO2 was decreased. These results indicate that hydrogen passivation of residual defects is useful for the enhancement of the Er PL.
Structural modifications in nanocrystalline silicon-silicon dioxide (nc-Si/SiO2) superlattices (SL) under high intensity laser irradiation have been studied experimentally and theoretically. The melting threshold in nc-Si/SiO2 SLs was found in the range of 5–8 kW/cm2 for cw excitation by 514 nm line of Ar+ laser and ∼11–15 mJ/cm2 for pulsed irradiation by 248 nm, π∼20 ns KrF laser. The irradiation of the samples above the melting threshold induces the irreversible modification of nc-Si layers, which is controlled by the thickness of the separating SiO2 layers, and increases the PL intensity increases by more than 300%.
High density boron-doped silicon nanowire arrays were fabricated within the pores of anodized alumina membranes via vapor-liquid-solid (VLS) growth Anodized alumina membranes with a nominal pore diameter of 200 nm served as templates for the sequential electrodeposition of silver, cobalt, and gold which served as the backside electrical contact, ohmic contact metal and catalyst metal for VLS growth, respectively. Boron-doped silicon nanowires were then synthesized within the pores by VLS growth using silane (SiH4) and trimethylboron (TMB) gas sources. Arrays of Al dots were deposited on the top surface of the membrane after nanowire growth. A series of samples was prepared with different SiNW lengths and boron doping levels. Two point probe measurements were used to measure the I-V characteristics of the silicon nanowire arrays before and after annealing. Nanowire resistivity and contact resistance were determined from plots of resistance versus nanowire length. The resistivity of the SiNW was observed to decrease with the addition of TMB during growth.
The formation of self-assembled quantum dots (QD) is of increasing interest for applications in optical, nanoelectronic, biological and quantum computing systems. From the perspective of fabrication technology, there are great advantages if the whole device can be made using a single Si substrate. Furthermore, GeSi is a model semiconductor system for fundamental studies of growth and material properties. In practice, as the MBE growth of heterostructures is inherently a non-equilibrium process, the formation of self-assembled nanostructures is both complex and sensitive to growth and overgrowth conditions. The morphology, structure and composition of QDs can all change during growth. It is therefore crucial to understand their structures at different stages of growth at the atomic scale. Here, the characterization of QD growth using high-resolution high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) imaging is presented. Both the formation of uncapped QDs and the effect of the encapsulation are investigated, and the morphological and compositional evolution of the QDs and wetting layers are observed directly at the atomic scale for the first time. During encapsulation, the Ge content in the centres of the QD remains unchanged, despite significant intermixing, lateral spreading and a laterally inhomogeneous Ge distribution inside the Ge QD. The initial non-uniform wetting layer for the uncapped Ge QD becomes uniform after encapsulation, and a 3-monolayer-thick core with ∼ 60% Ge content is formed in the 2 nm-thick wetting layer with an average Ge content of ∼ 30%. The results were obtained by direct analysis of the Z-contrast STEM imaging without involving complex image simulations.
The luminescence emission arising from SiGe layers oxidized in dry or wet atmospheres has been studied and the results obtained in both cases have been compared. Additional characterization of the samples by Raman and FTIR spectroscopies, which give information on the remaining SiGe layer and on the composition of the growing oxides respectively, have allowed the luminescence and the structural features of the samples at each stage of the oxidation processes to be correlated. SiGe layers of two different thickness have been used in order to clearly establish the origin of the different emissions, eliminating the contribution of the oxide and linking them to the presence of nanoparticles.
Rib-loaded silica waveguides containing Si nanocrystals were grown by quadruple implantation of Si ions into a 2 μm-thick thermally-grown SiO2 layer. The thickness of the resulting flat-profile active region was about 300 nm, with a 9.5% Si excess (determined by X-ray photoelectron spectroscopy). Complete phase separation and nanocrystal formation was assured by annealing at 1100 °C, and studied by means of optical tools such as Raman and luminescence. The rib-loaded structure of the waveguides was fabricated by photolithographic and reactive ion etching processes, with patterned rib widths ranging from 1 to 8 μm. Efficient light propagation was observed when end-fire coupling a probe signal both at 633 nm and 780 nm into the waveguides, with attenuation losses as low as 11 dB/cm. Signal amplification experiments, with pulsed and continuous wave (CW) top pumping, have shown increased signal absorption when the pump power is raised. This couples with the lack of any fast component in the time decay of the amplified spontaneous emissions as measured by ns pulsed pumping Variable Stripe Length (VSL) experiments. These two phenomena are interpreted as due to the lack of stimulated emission in these nanocrystalline systems.
Using a waveguide spectrometer chip as an example, we describe how high index contrast waveguides systems such as silicon-on-insulator can be combined with microphotonic design rules to extend the performance of waveguide devices. The challenges arising in the implementation of silicon microphotonic technology are discussed, and recent work addressing the issues of waveguide coupling, polarization sensitivity, waveguide loss and massively parallel data acquisition is reviewed.
Optical properties of birefringent porous-silicon layers are studied within the density functional theory. Starting from a (110)-oriented supercell of 32 silicon atoms, columns of atoms in directions  and  are removed and the dangling bonds are saturated with hydrogen atoms. The results show an in-plane anisotropy in the dielectric function and in the refractive index (n). The difference Δn defined as n -n is compared with experimental data and a good agreement is observed. Also, the possibility in determining the morphology of pores by using polarized lights is analyzed.
We have characterized the mechanism of energy transfer from Si nanoparticles to Er3+ ions in different silicate glasses, namely soda-lime and aluminium silicates, and made the comparison with pure silica. By means of a multi-implantation scheme we have formed a 350 nm thick glass layer with a uniform Si excess (5% or 15% atomic excess) and an Er distribution. Several Er doses were chosen so that the resulting Er peak concentration could vary from 2 × 1019 up to 6 × 1020 cm−3. Fused silica wafers coimplanted in the same conditions were used as a reference material in order to compare the different efficiency and mechanisms of Er emission as a function of the host silicate composition. Thermal treatment at low temperature has been performed to enhance the photoluminescence around 1540 nm. Large PL emission, compared to structures doped only with Er, has been successfully detected in all co-implanted glasses, with similar intensity. Moreover, we have measured PL lifetimes from 2.5 to 12 ms (depending on the Er dose and Si excess), and estimated an Er effective excitation cross-section of the order of 1 × 10−17 cm2.
We have measured the growth rate of silicon nanowires (SiNWs) in the diameter range of 3 to 40 nm (8.4 nm on average), which were grown by chemical vapor deposition (CVD) at temperatures between 365 °C and 495 °C. It is found that SiNWs with smaller diameters grow slower than those with larger ones, and a critical diameter at which growth stops completely exists. The growth rate of the thinner SiNWs stronger depends on growth temperature than that of thicker ones in previous studies. We discuss the dependence by thermodynamics theory.
We show ultra-fast Raman gain dynamics in submicron-size silicon-on-insulator strip waveguide. Using high confinement structures and pico-second pump pulses, we show 6-dB net nonlinear gain with 20.7-W peak pump power in a 7-mm long waveguide.
In this paper, we report Raman Scattering (RS) and photoluminescence (PL) measurements of Ge nanowires (NWs) grown via vapor-liquid-solid (VLS) using chemical vapor deposition silicon substrates consisting of (100) and (111) crystallographic orientations. Ge NWs grown are ∼40 nm in diameter, approximately a micrometer in length, and a sharp narrow Raman peak at ∼300 cm−1 indicates single crystal quality. An absence of SiGe peak in the Raman spectra indicates that SiGe interdiffusion is insignificant for the NW volume. Low temperature PL-intensity-dependence spectra indicate that the observed emission originates at the Ge NW – Si substrate interface, where SiGe intermixing has been detected. This interface is formed differently for (111) and (100) oriented Si substrates due to the <111> preferential growth direction of Ge NWs.
Porous silicon (PS), well known as a visible luminescent material, has been revealed to be an attractive candidate for an ultrasonic emitter recently. Since the ultrasonic emission is based on a thermal process it is important to evaluate thermal properties of PS. We then evaluated the thermal conductivity of PS from the phase delay in a photoacoustic (PA) signal. We report the dependence of thermal conductivity on anodization time in forming PS.
Er doped Si-rich SiO2 films were deposited through reactive RF magnetron co-sputtering and subjected to a single annealing step to simultaneously form silicon nanocrystals (Si-nc's) and activate the Er emission. Reference Er in stoichiometric SiO2 (Er:SiO2) films were deposited for comparison and the Er emission in the presence of Si-nc's was optimized with respect to the annealing temperature. The Er emission from Er in SiO2 containing Si-nc's (Er:SiO2+Si-nc) films is maximized at annealing temperatures between 600 °C and 800 °C, where the 1.54 μm emission is enhanced by more than two orders of magnitude relative to Er:SiO2 samples. Efficient energy coupling between Si-nc's and Er ions was demonstrated through excitation cross section measurements and non-resonant Er excitation experiments for samples annealed at temperatures as low as 600 °C. Since strong emission can be achieved from Er:SiO2+Si-nc films deposited through a standard CMOS process and annealed at temperatures below 700 °C, they can be used to fabricate CMOS compatible light
Recent interest in biological imaging, remote sensing, and biochemical spectroscopy has made nanoscale Terahertz (THz) emitters based on semiconductors become extremely attractive. THz lasers with good performance (emitting over 1 milliWatt CW at 10 K) have been fabricated from the group III-V compounds by molecular beam epitaxy, but such devices are complex with over 500 quantum wells and barriers and are incompatible with low cost Si processing. Similar devices from the SiGe system have had difficulties because the strain limits the total number of active layers, and have produced relatively poor performance (peak powers near 5 nW), even using strain symmetric active regions on virtual substrates of relaxed SiGe buffers. We report here on the characteristics of a new type of THz emitting device with higher powers (above 0.1 milliWatt) based on radiative impurity transitions in doped silicon, which can be simply fabricated without Ge alloying.
The THz emitters were fabricated from both p-type and n-type doped silicon wafers with typical resistivities of 5 Ω-cm, using conventional photolithography and metal contact lift off. Samples were electrically pulsed and the electroluminescence was measured by Fourier Transform Infrared spectrometry. At 4.2 K, the emission spectra showed several peaks centered around 8.1 THz for the boron doped device, 7 to 13 THz for the gallium doped device, and 6.6 THz for the phosphorus doped device. The electroluminescence was attributed to radiative transitions from excited p-like to s-like hydrogenic dopant states, with emission energies in remarkable agreement with the known dopant absorption levels. Current-voltage measurement suggested an excitation mechanism based on impact ionization of neutral dopants by hot carriers. The net quantum efficiency (emitted photons per injected electron) was estimated to be up to 2 × 10-3. The temperature dependence will be reported with operation above 30 K.
Si nanoclusters (Si-nc) embedded in SiO2 present outstanding luminescent emission in the visible and are the material of choice for the realization of efficient light sources integrated with Si technology. PECVD is an attractive preparation route but there is still the need to understand how Si excess and matrix composition affect the precipitation of Si-nc and their photoluminescence (PL) efficiency. The SiOx PECVD layers studied here have a Si excess up to 50% and a thickness between 50 and 100 nm. The phase separation, precipitation and growth of the Si-nc have been achieved by annealing at 1250 °C. For reference, the same study has been performed in Si-nc/SiO2 materials synthesized by ion implantation and annealing. Refractive index and thickness measured by ellipsometry show a densification of the layers after the H release during annealing. A detailed composition profile has been determined by XPS and FTIR analyses and shows almost complete phase separation except for the interfaces, where a depletion of Si-nc is found. EFTEM demonstrates that isolated Si-nc are formed for Si excess up to 25% while for higher Si excess a continuous Si phase is observed. The PL efficiency in PECVD samples is maximized for a Si excess of 17% which is the same Si excess than that for the most emitting implanted samples. No dependence of PL efficiency has been found on the presence of Nitrogen in the matrix (up to the 10%).
Er3+, and Nd3+ doped Si/Al/SiO2 and thin films have been prepared by rf co-sputtering. Some of these films were annealed to 700°C. Erbium doped Si/Al/SiO2 films were prepared with two different sputtering configurations: one configuration with a large quantity of Al and a second configuration with a smaller quantity of Al. The configuration with large quantity of Al shows a diminished luminescence at 1.53 μm, but this emission is increased by substrate heating. The configuration with smaller quantity of Al shows emission at 1.525 μm similar in intensity to the Er-doped Si/SiO2. The spectral shape for the 4I13/2→4I15/2 emission is broader than for an analogous Er3+ doped Si/SiO2. The smaller quantity of Al configuration increases the solubility of Nd3+ (and luminescence for high Nd3+ concentration) in Si/SiO2 films and changes the spectral shape of the 4F3/2 emission with respect to the Nd3+ doped Si/SiO2 films.