With the growing use of optoelectronics in information technology, manipulating light is almost as important as manipulating electrons. Unfortunately silicon, workhorse of modern microelectronics, is next to useless in optical applications. There has been a massive effort to overcome silicon's inadequacies, and ways of coaxing silicon to handle light are under development but a key component — the laser — has been problematic. Last year a silicon laser was produced, but it involved metres of optical fibre. Now workers in Intel's research labs have come up with an all-silicon laser on a single chip. The device is compact and readily integrated with other silicon components. The possibility of light generation and/or amplification in silicon has attracted a great deal of attention1 for silicon-based optoelectronic applications owing to the potential for forming inexpensive, monolithic integrated optical components. Because of its indirect bandgap, bulk silicon shows very inefficient band-to-band radiative electron–hole recombination. Light emission in silicon has thus focused on the use of silicon engineered materials such as nanocrystals2,3,4,5, Si/SiO2 superlattices6, erbium-doped silicon-rich oxides7,8,9,10, surface-textured bulk silicon11 and Si/SiGe quantum cascade structures12. Stimulated Raman scattering (SRS) has recently been demonstrated as a mechanism to generate optical gain in planar silicon waveguide structures13,14,15,16,17,18,19,20,21. In fact, net optical gain in the range 2–11 dB due to SRS has been reported in centimetre-sized silicon waveguides using pulsed pumping18,19,20,21. Recently, a lasing experiment involving silicon as the gain medium by way of SRS was reported, where the ring laser cavity was formed by an 8-m-long optical fibre22. Here we report the experimental demonstration of Raman lasing in a compact, all-silicon, waveguide cavity on a single silicon chip. This demonstration represents an important step towards producing practical continuous-wave optical amplifiers and lasers that could be integrated with other optoelectronic components onto CMOS-compatible silicon chips.

An all-silicon Raman laser

We analyze theoretically the optical properties of ideal semiconductor crystallites so small that they show quantum confinement in all three dimensions [quantum dots (QD's)]. In the limit of a QD much smaller than the bulk exciton size, the linear spectrum will be a series of lines, and we consider the phonon broadening of these lines. The lowest interband transition will saturate like a two-level system, without exchange and Coulomb screening. Depending on the broadening, the absorption and the changes in absorption and refractive index resulting from saturation can become very large, and the local-field effects can become so strong as to give optical bistability without external feedback. The small QD limit is more readily achieved with narrow-band-gap semiconductors.

Theory of the linear and nonlinear optical properties of semiconductor microcrystallites

Size dependence of nanostructures: Impact of bond order deficiency

The quantum-confined Stark effect in single cadmium selenide (CdSe) nanocrystallite quantum dots was studied. The electric field dependence of the single-dot spectrum is characterized by a highly polarizable excited state (∼10 5 cubic angstroms, compared to typical molecular values of order 10 to 100 cubic angstroms), in the presence of randomly oriented local electric fields that change over time. These local fields result in spontaneous spectral diffusion and contribute to ensemble inhomogeneous broadening. Stark shifts of the lowest excited state more than two orders of magnitude larger than the linewidth were observed, suggesting the potential use of these dots in electro-optic modulation devices.

Quantum Confined Stark Effect in Single CdSe Nanocrystallite Quantum Dots

The scaling of complementary metal oxide semiconductor (CMOS) transistors has led to the silicon dioxide layer used as a gate dielectric becoming so thin that the gate leakage current becomes too large. This led to the replacement of SiO2 by a physically thicker layer of a higher dielectric constant or ‘high-K’ oxide such as hafnium oxide. Intensive research was carried out to develop these oxides into high quality electronic materials. In addition, the incorporation of Ge in the CMOS transistor structure has been employed to enable higher carrier mobility and performance. This review covers both scientific and technological issues related to the high-K gate stack – the choice of oxides, their deposition, their structural and metallurgical behaviour, atomic diffusion, interface structure, their electronic structure, band offsets, electronic defects, charge trapping and conduction mechanisms, reliability, mobility degradation and oxygen scavenging to achieve the thinnest oxide thicknesses. The high K oxides were implemented in conjunction with a replacement of polycrystalline Si gate electrodes with metal gates. The strong metallurgical interactions between the gate electrodes and the HfO2 which resulted an unstable gate threshold voltage resulted in the use of the lower temperature ‘gate last’ process flow, in addition to the standard ‘gate first’ approach. Work function control by metal gate electrodes and by oxide dipole layers is discussed. The problems associated with high K oxides on Ge channels are also discussed.

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High-K materials and metal gates for CMOS applications

Two sources of room temperature visible luminescence are identified from SiO2 films containing ion beam synthesized Si nanocrystals. From a comparison of luminescence spectra and photoluminescence decay lifetime measurements between Xe + -implanted SiO2 films and SiO2 films containing Si nanocrystals, a luminescence feature attributable to defects in the SiO2 matrix is unambiguously identified. Hydrogen passivation of the films selectively quenches the matrix defect luminescence, after which luminescence attributable to Si nanocrystals is evident, with a lifetime on the order of milliseconds. The peak energy of the remaining luminescence attributable to Si nanocrystals ``redshifts'' as a function of different processing parameters that might lead to increased nanocrystal size and the intensity is directly correlated to the formation of Si nanocrystals. Upon further annealing hydrogen-passivated samples at low temperatures (< 500 °C), the intensity of nanocrystal luminescence increases by more than a factor of 10.

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Defect‐related versus excitonic visible light emission from ion beam synthesized Si nanocrystals in SiO2

Si nanocrystals (diameter 2–5 nm) were formed by 35 keV Si + implantation at a fluence of 6 × 1016 Si/cm2 into a 100 nm thick thermally grown SiO2 film on Si (100), followed by thermal annealing at 1100 °C for 10 min. The nanocrystals show a broad photoluminescence spectrum, peaking at 880 nm, attributed to the recombination of quantum confined excitons. Rutherford backscattering spectrometry and transmission electron microscopy show that annealing these samples in flowing O2 at 1000 °C for times up to 30 min results in oxidation of the Si nanocrystals, first close to the SiO2 film surface and later at greater depths. Upon oxidation for 30 min the photoluminescence peak wavelength blueshifts by more than 200 nm. This blueshift is attributed to a quantum size effect in which a reduction of the average nanocrystal size leads to emission at shorter wavelengths. The room temperature luminescence lifetime measured at 700 nm increases from 12 µs for the unoxidized film to 43 µs for the film that was oxidized for 29 min.

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Tuning the emission wavelength of Si nanocrystals in SiO2 by oxidation

Synthesis of Ge nanocrystals in SiO2 films is carried out by precipitation from a supersaturated solid solution of Ge in SiO2 made by Ge ion implantation. The films exhibit strong room-temperature visible photoluminescence. The measured photoluminescence peak energy and lifetimes show poor correlations with nanocrystal size compared to calculations involving radiative recombination of quantum-confined excitons in Ge quantum dots. In addition, the photoluminescence spectra and lifetime measurements show only a weak temperature dependence. These observations strongly suggest that the observed visible luminescence in our samples is not due to the radiative recombination of quantum-confined excitons in Ge nanocrystals. Instead, observations of similar luminescence in Xe+ -implanted samples and reversible PL quenching by hydrogen or deuterium suggest that radiative defect centers in the SiO2 matrix are responsible for the observed luminescence.

/pdf/the-role-of-quantum-confined-excitons-vs-defects-in-the-1faheapnac.pdf

The role of quantum-confined excitons vs defects in the visible luminescence of sio2 films containing ge nanocrystals

Silicon nanocrystals with diameters ranging from [approximate]2 to 5.5 nm were formed by Si ion implantation into SiO2 followed by annealing. After passivation with deuterium, the photoluminescence (PL) spectrum at 12 K peaks at 1.60 eV and has a full width at half maximum of 0.28 eV. The emission is attributed to the recombination of quantum-confined excitons in the nanocrystals. The temperature dependence of the PL intensity and decay rate at several energies between 1.4 and 1.9 eV was determined between 12 and 300 K. The temperature dependence of the radiative decay rate was determined, and is in good agreement with a model that takes into account the energy splitting between the excitonic singlet and triplet levels due to the electron-hole exchange interaction. The exchange energy splitting increases from 8.4 meV for large nanocrystals ([approximate]5.5 nm) to 16.5 meV for small nanocrystals ([approximate]2 nm). For all nanocrystal sizes, the radiative rate from the singlet state is 300–800 times larger than the radiative rate from the triplet state.

/pdf/size-dependent-electron-hole-exchange-interaction-in-si-sqp16crdkt.pdf

Size-dependent electron-hole exchange interaction in Si nanocrystals

Frenkel-Poole (FP) trap energies of atomic layer deposited Al2O3 and HfxAlyO thin films with various Hf∕Al compositions have been extracted. Using a method based on the field and temperature dependence of FP conduction, intrinsic trap energies under zero electric field can be extrapolated. Results indicate that FP trap energies increase from 0.56to1.48eV when adding more and more Al to HfO2. The trap energy seems to be inversely proportional to the square of the dielectric constant of the film, suggesting that traps may originate from the same type of defect, whose energy level is mediated by the dielectric constant.

K. S. Min

Papers

Defect‐related versus excitonic visible light emission from ion beam synthesized Si nanocrystals in SiO2

Tuning the emission wavelength of Si nanocrystals in SiO2 by oxidation

The role of quantum-confined excitons vs defects in the visible luminescence of sio2 films containing ge nanocrystals

Size-dependent electron-hole exchange interaction in Si nanocrystals

Frenkel-Poole trap energy extraction of atomic layer deposited Al2O3 and HfxAlyO thin films