Tunable photoluminescence and electroluminescence of size-controlled silicon nanocrystals in nanocrystalline-Si/SiO2 superlattices

We present photoluminescence and electroluminescence of silicon nanocrystals deposited by plasma-enhanced chemical vapor deposition (PECVD) using nanocrystalline silicon/silicon dioxide (nc-Si/SiO₂) superlattice approach. This approach allows us to tune the nanocrystal emission wavelength by varying the thickness of the Si layers. We fabricate light emitting devices (LEDs) with transparent indium tin oxide (ITO) contacts using these superlattice materials. The current-voltage characteristics of the LEDs are measured and compared to Frenkel-Poole and Fowler-Nordheim models for conduction. The EL properties of the superlattice material are studied, and tuning, similar to that of the PL spectra, is shown for the EL spectra. Finally, we observe the output power and calculate the quantum efficiency and power conversion efficiency for each of the devices.     

Optical properties of Si/CaF2 superlattices

We present a first-principle theoretical study of the dielectric functions of Si/CaF₂ superlattices. In particular, we investigate how the optical response depends on the thickness of the Si layers. Our results show that for very thin Si slabs (well width less than 20 Å) optical excitation peaks are present in the visible range. These peaks are related to strong transitions between localized states. Moreover, the static dielectric costant is considerably reduced. From the comparison made with recent experimental data on similar systems we conclude that the quantum confinement, a good surface passivation and the presence of localized states are the key ingredients in order to have photoluminescence in confined silicon based systems.     

Ellipsometric spectroscopy study of photoluminescent Si/SiO2 systems obtained by magnetron co-sputtering

Si nanograins embedded in silica matrix were obtained by magnetron cosputtering of both Si and SiO₂ at different substrate temperature (200–700°C) and thermal annealing at 1100°C. The samples were characterized by ellipsometric spectroscopy, high-resolution electron microscopy observations and photoluminescence. The highest excess of Si atoms was found to be incorporated for deposition temperature near 400–500°C, giving rise to a maximum PL and a shift of the peak position towards lower energy. These features might be interpreted in terms of quantum size effects and of density of grains, even though the interface states seem to be involved in the improvement of the photoluminescence efficiency.     

Anneal temperature dependence of Si/SiO2 superlattices photoluminescence

Photoluminescence spectroscopy, Fourier transform infrared spectroscopy, X-ray reflectometry and high resolution electron microscopy have been used to interpret the photoluminescence properties of annealed (3/19 nm) Si/SiO₂ multilayers grown by reactive magnetron sputtering. The multilayers show an emission in the visible and near-infrared range after heat treatment from 900°C which tends to decrease from 1200°C. Three different origins for the photoluminescence activity have been found. An anneal temperature of 1200°C is necessary to optimise the silicon crystallisation within the silicon sublayers.     

Optical properties of Si-rich SiO2 films in relation with embedded Si mesoscopic particles

Optical properties of Si-rich SiO₂ films prepared by an RF cosputtering method are discussed. From the infrared and Raman spectroscopy together with the electron microscopy, it is shown that Si mesoscopic particles embedded in solid matrices with the sizes ranging from ˜ 10 nm (nanocrystals) to less than ˜1 nm (clusters) can be obtained by the cosputtering and post-annealing. The absorption and photoluminescence spectra are presented. For our samples, a red luminescence peak analogous to that of porous Si is observed for films containing Si clusters rather than nanocrystals. Raman spectra which evidence the success in the heavy doping of B atoms into Si nanocrystals are also discussed.     

The origin of photoluminescence in Ge-implanted SiO2 layers

Ge ions were implanted at 100 keV with 3×10¹⁶ cm⁻² into a 300  nm thick SiO₂ layer on Si. Visible photoluminescence (PL) around 2.1 eV from an as-implanted sample is observed, and faded out by subsequent annealing at 900°C for 2 h. However, PL shows up again after annealing above 900°C at the same peak position. Compared with the as-implanted sample, significant increase of Ge–Ge bonds is measured in X-ray photoelectron spectroscopy, and the formation of Ge nanocrystals with a diameter of 5 nm are observed in transmission electron microscopy from the sample annealed at 1100°C. We conclude that the PL peak from the sample annealed above 900°C is caused by the quantum confinement effects from Ge nanocrystals, while the luminescence from the as-implanted sample is due to some radiative defects formed by Ge implantation.     

A comparative study of electroluminescence from Ge／SiO2 and Si／SiO2 films

Ge/SiO_2 and Si/SiO_2 films were deposited using the two-target alternation magnetron sputtering technique. The Au/Ge/SiO_2/p-Si and Au/Si/SiO_2/p-Si structures were fabricated and their electroluminescence (EL) characteristics were comparatively studied. Both Au/Ge/SiO_2/p-Si and Au/Si/SiO_2/p-Si structures have rectifying property. All the EL spectra from the two types of the structure have peak positions around 650－660 nm. The EL mechanisms of the structures are discussed.     

Si/SiO2 multilayers: synthesis by reactive magnetron sputtering and photoluminescence emission

This paper relates a complete study of Si/SiO₂ multilayer (ML) structures. First, we suggest an original way of synthesis based on reactive magnetron sputtering of a pure silica target. The photoluminescence spectra of these MLs consist of two Gaussian bands in the visible-near infrared spectral region. The stronger one (I band) is fixed at about 780 nm and probably due to interface states. The weaker one (Q band) is tuneable with the Si sublayer thickness and originates from a radiative recombination within the nanosized Si layers. For this latter band the peak position is a function of the Si sublayer thickness and shows a discontinuity at 30 Å. This corresponds to an Si phase change. For thicknesses above 30 Å, the sublayers are composed of nanocrystalline silicon whereas below 30 Å the sublayers are made of amorphous silicon. We develop a model based on a quantum well to which we have added an interfacial region between Si and SiO₂. It is characterised by an interfacial potential of 0.3 eV. This model depicts the simultaneous behaviour of Q and I bands for an Si sublayer thickness below 30 Å.     

Silicon nanoparticles formation in annealed SiO/SiO2 multilayers

We present a study on amorphous SiO/SiO₂ superlattice performed by grazing-incidence small-angle X-ray scattering (GISAXS). Amorphous SiO/SiO₂ superlattices were prepared by high-vacuum evaporation of 3 nm thin films of SiO and SiO₂ (10 layers each) onto Si(1 0 0) substrate. After the deposition, samples were annealed at 1100 °C for 1 h in vacuum, yielding to Si nanocrystals formation. Using a Guinier approximation, the shape and the size of the crystals were obtained. The size of the growing nanoparticles in the direction perpendicular to the film surface is well controlled by the bilayer thickness. However, their size varies more significantly in the direction parallel to the film surface.     

Substrate temperature dependence of the photoluminescence efficiency of co-sputtered Si/SiO2 layers

Si particles embedded in an SiO₂ matrix were obtained by co-sputtering of Si and SiO₂ at various deposition temperatures T_d (200–700°C) and annealing at different temperatures T_a (900–1100°C). The systems were characterized by X-ray photoelectron, Raman scattering, infrared absorption and photoluminescence spectroscopy techniques. The results show that the photoluminescence efficiency is strongly dependent on the degree of phase separation between the Si nanocrystals and the SiO₂ matrix. This is likely connected with the Si/SiO₂ interface characteristics, together with the features indicating the involvement of quantum confinement.     

Room temperature visible light emission from Si/SiO2 multilayers: Roles of interface electronic states and silicon phase

We have studied luminescence properties and microstructure of 20 patterns Si/SiO₂ multilayers. The photoluminescence spectra consist of two gaussian bands in the visible-infrared spectral region. It has been demonstrated that the strong PL band is caused by the radiative recombination in the Si/SiO₂ interfaces states, whereas the weaker band originates from radiative recombination in the nanosized Si layers. The peak shift of this latter band shows a discontinuity that corresponds to a crystalline-to-amorphous phase change when the Si layers are thinner than 30 Å. The peak energy as a function of the layer thickness is interpreted using a quantum confinement model in the case of amorphous Si layers.     

Structural and optical properties of Si/SiO2 multi-quantum wells

Structural and optical properties of Si/SiO₂ multi-quantum wells (MQW) were investigated by means of Raman scattering and photoluminescence (PL) spectroscopy. The MQW structures were fabricated on a quartz substrate by remote plasma enhanced chemical vapour deposition (RPECVD) of alternating amorphous Si and SiO₂ layers. After layer deposition the samples were subjected to heat treatments, i.e. rapid thermal annealing (RTA) and furnace annealing. Distinct PL signatures of confined carriers evidenced formation of Si-nanocrystals (nc-Si) in annealed samples. Analyses of Raman spectra also show presence of nc-Si phase along with amorphous-Si (a-Si) phase in the samples. The strong influence of the annealing parameters on the formation of nc-Si phase suggests broad possibilities in engineering MQW with various optical properties. Interestingly, conversion of the a-Si phase to the nc-Si phase saturates after certain time of furnace annealing. On the other hand, thinner Si layers showed a disproportionately lower crystalline volume fraction. From the obtained results we could assume that an interface strain prevents full crystallization of the Si layers and that the strain is larger for thinner Si layers. The anomalous dependence of nc-Si Raman scattering peak position on deposited layer thickness observed in our experiments also supports the above assumption.     

Quantum confined luminescence in Si/SiO2 superlattices

Amorphous Si/SiO₂ superlattices with periodicities between 2 and 5 nm have now been grown on (1 00) Si wafers by several different techniques: molecular beam epitaxy, magnetron sputtering, and plasma enhanced chemical vapor deposition (PECVD). With the first two methods little or no hydrogen is incorporated during growth and visible photo-luminescence (PL) is obtained at wavelengths from 520 to 800 nm. The shift in the PL peak position with Si layer thickness is consistent with quantum confined band-to-band recombination. Annealing the sputtered superlattices at temperatures up to 1100°C results in a very bright red PL that is similar in intensity to that observed in porous Si samples. For large numbers of periods (e.g., 425) the PL is strongly modulated in intensity owing to optical interference within the superlattice. Similar quantum confined, but defect induced, PL is also observed in the PECVD grown superlattices, where the amorphous Si layers are heavily hydrogenated.     

Investigation on preparation and physical properties of nanocrystalline Si/SiO2 superlattices for Si-based light-emitting devices

Structures containing silicon nanocrystals (nc-Si) are very promising for Si-based light-emitting devices. Using a technology compatible with that of silicon, a broader wavelength range of the emitted photoluminescence (PL) was obtained with nc-Si/SiO₂ multilayer structures. The main characteristic of these structures is that both layers are light emitters. In this study we report results on a series of nc-Si/SiO₂ multilayer periods deposited on 200 nm thermal oxide SiO₂/Si substrate. Each period contains around 10 nm silicon thin films obtained by low-pressure chemical vapour deposition at T=625°C and 100 nmSiO₂ obtained by atmospheric pressure chemical vapour deposition T=400°C. Optical and microstructural properties of the multilayer structures have been studied by spectroscopic ellipsometry (using the Bruggemann effective medium approximation model for multilayer and multicomponent films), FTIR and UV–visible reflectance spectroscopy. IR spectroscopy revealed the presence of SiO_x structural entities in each nc-Si/SiO₂ interface. Investigation of the PL spectra (using continuous wave-CW 325 nm and pulsed 266 nm laser excitation) has shown several peaks at 1.7, 2, 2.3, 2.7, 3.2 and 3.7 eV, associated with the PL centres in SiO₂, nc-Si and Si–SiO₂ interface. Their contribution to the PL spectra depends on the number of layers in the stack.     

Monte Carlo analysis of electron heating in Si/SiO2 superlattices

In this work, we investigate the transport properties of Si/SiO₂ superlattices with a multiband one-particle Monte Carlo simulator. Using the envelope function approximation, we solve the Krönig–Penney potential along the growth direction z using a tight-binding-like analytical form; we also consider parabolic dispersion along the xy plane. The scattering mechanisms introduced in the simulator are confined optical phonons, both polar and nonpolar.Owing to the very flat shape of the bands along the growth direction, very low drift velocities are found for vertical transport. However, the simulation shows that, for oblique fields, the transport properties along the vertical direction are strongly influenced by the in-plane component of the electric field: in this way higher vertical drift velocities can be obtained. The results point out that in-plane carrier heating and multiband properties are responsible for this behavior.     

Electroluminescence from Si／SiO2 films deposited on p—Si substrates

The structure of Au/Si/SiO₂/p-Si has been fabricated using the magnetron sputtering technique. It has a very good rectifying behaviour. Visible electroluminescence (EL) has been observed from the Au/Si/SiO₂/p-Si structure at a forward bias of 5V or larger. A broad band with one peak around 650－660 nm appears in all the EL spectra of the structure. The effects of the thickness of the Si layer in the Si/SiO₂ films and of the input electrical power on EL spectra are studied systematically.     

Nanostructuring Si surface and Si/SiO2 interface using porous-alumina-on-Si template technology. Electrical characterization of Si/SiO2 interface

In this work, anodic porous alumina thin films with pores in the nanometer range are grown on silicon by electrochemistry and are used as masking material for the nanopatterning of the silicon substrate. The pore diameter and density are controlled by the electrochemical process. Through the pores of the alumina film chemical oxidation of the silicon substrate is performed, leading to the formation of regular arrays of well-separated stoichiometric silicon dioxide nanodots on silicon, with a density following the alumina pores density and a diameter adjustable by adjusting the chemical oxidation time. The alumina film is dissolved chemically after the SiO₂ nanodots growth, revealing the arrays of silicon dioxide dots on silicon. In a next step, the nanodots are also removed, leaving a nanopatterned bare silicon surface with regular arrays of nanopits at the footprint of each nanodot. This silicon surface structuring finds interesting applications in nanoelectronics. One such application is in silicon nanocrystals memories, where the structuring of the oxidized silicon surface leads to the growth of discrete silicon nanocrystals of uniform size. In this work, we examine the electrical quality of the Si/SiO₂ interface of a nanostructured oxidized silicon surface fabricated as above and we find that it is appropriate for electronic applications (an interface trap density below 1–3×10¹⁰ eV⁻¹ cm⁻² is obtained, indicative of the high quality of the thermal silicon oxide).     

Characterization of Si nanocrystals into SiO2 matrix

Silicon nanocrystals (nc-Si) have gained great interest due to their excellent optical and electronic properties and their applications in optoelectronics. The aim of this work is the study of growth mechanism of nc-Si into a-SiO₂ matrix from SiO/SiO₂ multilayer annealing, using non-destructive and destructive techniques. The multilayer were grown by e-beam evaporation from SiO and SiO₂ materials and annealing at temperatures up to 1100 °C in N₂ atmosphere. X-rays reflectivity (XRR) and high resolution transmission electron microscopy (HRTEM) were used for the structural characterization and spectroscopic ellipsometry in IR (FTIRSE) energy region for the study of the bonding structure. The ellipsometric results gave a clear evidence of the formation of an a-SiO₂ matrix after the annealing process. The XRR data showed that the density is being increased in the range from 25 to 1100 °C. Finally, the HRTEM characterization proved the formation of nc-Si. Using the above results, we describe the growth mechanism of nc-Si into SiO₂ matrix under N₂ atmosphere.     

Visible photoluminescence from a-Si : H/SiO2 superlattices fabricated by UHV evaporation

Hydrogenated amorphous-Si/SiO₂ (a-Si:H/SiO₂) superlattices with different a-Si : H thickness in the range of a few nanometers have been fabricated by ultra high vacuum evaporator (UHV evaporator). The photoluminescence (PL) of our superlattices is observed in the visible spectral region and the peak energy shifts to higher energy as the a-Si : H layer thickness decreases. The temperature dependence of the PL spectra reveals four sub-bands by fitting. Bands at 2.2, 1.9, 1.65 and 1.45 eV are detected and are attributed to E′δ centers, nonbridging-oxygen–hole centers (NBOHC), Si/SiO₂ interface and a-Si : H layer, respectively. We explain the overall blueshift of the PL spectra by the modification of the contribution of these sub-bands.     

Photoluminescence and electroluminescence investigations at Ge-rich SiO2 layers

Strong blue and violet photo (PL) and electroluminescence (EL) at room temperature was obtained from SiO₂-films grown on crystalline Si, which were either single (SI) or double implanted (DI) with Ge ions and annealed at different temperatures. The PL spectra of Ge-rich layers reach a maximum after annealing at 500–700°C for DI layers or 900–1000°C for SI layers, respectively. Both, PL and EL of 500 nm thick Ge-rich layers are easily visible by the naked eye at ambient light due to their high intensity. Based on excitation spectra we tentatively interpret the blue PL as due to the oxygen vacancy in silicon dioxide.

The EL spectrum of the Ge-implanted oxide correlates very well with the PL one and shows a linear dependence on the injected current over three orders of magnitude. For DI layers much higher injection currents than for SI layers can be achieved. An EL efficiency in the order of 10⁻⁴ for Ge⁺-implanted silicon dioxide was determined.