Bi nanocrystals embedded in an amorphous Ge matrix grown by pulsed laser deposition

Received: 1 September 1997/Accepted 8 September 1997     

Raman analysis of amorphous silicon ruthenium thin films embedded with nanocrystals

We report the Raman analysis of both as‐deposited and annealed amorphous silicon ruthenium thin films embedded with nanocrystals. In the Raman spectra of as‐deposited films, variations of TO peak indicate a short‐range disorder of a‐Si network with an increase of Ru concentration. The substitutional Ru atoms lower the concentration of Si―Si bonds and suppress the intensity of TO peak, but have less effect on TA, LA and LO peaks. In the Raman spectra of annealed films, characteristic parameters confirm the upgrade of a‐Si network at a low annealing temperature and the emergence of both ruthenium silicide and silicon nanocrystals at 700 °C. Although ruthenium silicide nanocrystals present no Raman peaks in the Raman spectra of as‐deposited samples, the non‐linear variations of intensity ratios I_{LA + LO}/I_TO and I_TA/I_TO still suggest their existence, and these nanocrystals are subsequently verified by high‐resolution transmission electron microscopy. Copyright © 2015 John Wiley & Sons, Ltd.     

Structural and photoluminescence properties of silicon nanocrystals embedded in SiC matrix prepared by magnetron sputtering

Si nanocrystals (NCs) embedded in an SiC matrix were prepared by the deposition of Si-rich Si_1?xC_x/SiC nanomultilayer films using magnetron sputtering, subsequently followed by thermal annealing in the range of 800～1200 °C. As the annealing temperature increases to 1000 °C, Si NCs begin to form and SiC NCs also start to emerge at the annealing temperature of 1200 °C. With the increase of annealing temperature, two photoluminescence (PL) peaks have an obvious redshift. The intensity of the low-energy PL peak around 669～742 nm gradually lowers, however the intensity of high-energy PL peak around 601～632 nm enhances. The low-energy PL peak might attribute to dangling bonds in amorphous Si (a-Si) sublayers, and the redshift of this peak might be related to the passivation of Si dangling bonds. Whereas the origin of the high-energy PL peak may be the emergence of Si NCs, the redshift of this peak correlates with the change in the size of Si NCs.     

Superluminescence of Er3+ in an amorphous silicon matrix

The photoluminescence of Er³⁺ ions in thin films of amorphous silicon has been studied under conditions of intense pumping. A superlinear increase in the amplitude and a shortening of the photoluminescence relaxation times are revealed at a pumping intensity above 200 kW/cm². The data obtained are explained by the threshold “switching-on” of the superluminescence mechanism.     

Memory effect in MIS structures with amorphous silicon nanoparticles embedded in ultra thin SiOx matrix

Metal/${\mathrm{SiO}}_2$/a-Si-${\mathrm{SiO}}_x$/c-Si structures containing amorphous silicon nanoparticles (a-Si NPs) embedded in ultra thin ${\mathrm{SiO}}_x$ matrix are fabricated by thermal evaporation of ${\mathrm{SiO}}_x$ and sputtering of ${\mathrm{SiO}}_2$ layers followed by thermal annealing at $700{}^{\circ }\mathrm{C}$. A memory effect, due to charging of a-Si NPs in ${\mathrm{SiO}}_x$, is observed. The processes of NP charging and discharging are accomplished by applying pulses with alternative polarities. The observed shift of the flat band voltage of the high-frequency C–V curve caused by a voltage pulse of $-15\mathrm{V}$ having duration of 1 s is more than 3 V. In addition, the structures show good retention characteristics which make them promising for application in non-volatile memory devices.     

Anomalous two-phonon absorption in diamond nanocrystals embedded in amorphous carbon

Observation of anomalously strong IR absorption are reported in the region of two-phonon diamond frequencies in amorphous hydrogenated carbon films grown by cosputtering graphite and copper in a plasma. Up to five structural elements of bands having frequencies close to or coinciding with the figures quoted in the literature for two-phonon absorption bands in bulk diamond were observed. This suggests the presence of diamond nanocrystals in the grown layers. Observation of two-phonon absorption in thin films is in itself remarkable because of the smallness of its coefficient in bulk diamond. An estimate of the absorption coefficient in the dominant band at 2140 cm^?1 yields about 200 cm^?1, which exceeds by more than an order of magnitude that for bulk diamond. This can be assigned to an anomalous enhancement of the two-phonon absorption coefficient due to phonon confinement in the small diamond particles nucleated in the amorphous carbon matrix. An estimate of these inclusions from the band width yields about 20 Å. The reasons for the catalytic activity of copper in diamond nucleation are analyzed.     

Quantum confinement in amorphous silicon quantum dots embedded in silicon nitride

Amorphous silicon quantum dots (a-Si QDs) were grown in a silicon nitride film by plasma enhanced chemical vapor deposition. Transmission electron micrographs clearly demonstrated that a-Si QDs were formed in the silicon nitride. Photoluminescence and optical absorption energy measurement of a-Si QDs with various sizes revealed that tuning of the photoluminescence emission from 2.0 to 2.76 eV is possible by controlling the size of the a-Si QD. Analysis also showed that the photoluminescence peak energy E was related to the size of the a-Si QD, a (nm) by E(eV) = 1.56+2.40/a(2), which is a clear evidence for the quantum confinement effect in a-Si QDs.     

Structural investigations of Ge nanoparticles embedded in an amorphous SiO2 matrix

Transmission electron microscopy and X-ray photoelectron spectroscopy analyses are performed to investigate Ge nanoparticles embedded in an amorphous SiO₂ matrix. GeSiO thin films are prepared by two methods, sol?Cgel and radio frequency magnetron sputtering. After the deposition, the sol?Cgel films are annealed in either N₂ (at 1 atm and 800 °C) or H₂ (at 2 atm and 500 °C), and the sputtered films in H₂ (at 2 atm and 500 °C), to allow Ge segregation. Amorphous Ge-rich nanoparticles (3?C7 nm size) are observed in sol?Cgel films. Crystalline Ge nanoparticles in the high pressure tetragonal phase (10?C50 nm size) are identified in the sputtered films. The size of the nanoparticles increases with Ge concentration in the volume of the film. At the film surface, the Ge concentration is much larger that in the volume for both sol?Cgel and sputtered films. At the same time, at the film surface, only oxidized Ge is observed.     

Excitonic photoluminescence characteristics of amorphous silicon nanoparticles embedded in silicon nitride film

We have investigated the photoluminescence (PL) properties of amorphous silicon nanoparticles (a-Si NPs) embedded in silicon nitride film (Si-in-SiN_x) grown by helicon wave plasma-enhanced chemical vapor deposition (HWP-CVD) technique. The PL spectrum of the film exhibits a broad band constituted of two Gaussian components. From photoluminescence excitation (PLE) measurements, it is elucidated that the two PL bands are associated with the a-Si NPs and the silicon nitride matrix surrounding a-Si NPs, respectively. The existence of Stokes shift between PL and absorption edge indicates that radiative recombination of carriers occurs in the states at the surface of the Si NPs, whereas their generation takes place in the a-Si NPs cores and the silicon nitride matrix, respectively. The visible PL of the film originates from the radiative recombination of excitons trapped in the surface states. At decreasing excitation energy (E_ex), the PL peak energy was found to be redshifted, accompanied by a narrowing of the bandwidth. These results are explained by surface exciton recombination model taking into account there existing a size distribution of a-Si NPs in the silicon nitride matrix.     

Relative enhancement of photoluminescence intensity of passivated silicon nanocrystals in silicon dioxide matrix

Photoluminescence (PL) intensity of passivated silicon nanocrystals (Si NCs) embeded in an SiO₂ matrix is compared with that of unpassivated ones. We investigate the relative enhancement of PL intensity (I_R) as a function of annealing temperature and implanted Si ion dose. The I_R increases simultaneously with the annealing temperature. This demonstrates an increase in the number of dangling bonds (DBs) with the degree of Si crystallization via varying the annealing temperature. The increase in I_R with implanted Si ion dose is also observed. We believe that the near-field interaction between DBs and neighboring Si NCs is an additional factor that reduces the PL efficiency of unpassivated Si NCs.     

Effect of excimer laser annealing on the silicon nanocrystals embedded in silicon-rich silicon nitride film

The investigations of silicon-rich silicon nitride film grown by plasma-enhanced chemical vapor deposition and then annealed by excimer laser have been carried out systematically. The surface roughness and the crystallization of the films have significantly been improved after the excimer laser annealing. The samples demonstrate visible photoluminescence emission under optical excitation at room temperature. It is found that the emission peak energy as well as emission intensity changes with laser annealing conditions, and the relevant mechanism is discussed in detail. Our investigation exhibits the size controllability of silicon nanocrystals embedded in the silicon nitride film, which implies promising applications in optoelectronic devices such as light-emitting diodes and solar cells.     

Temperature dependence of photoluminescence from CdSe nanocrystals embedded in silica matrix

In this work, CdSe nanocrystals (NCs) embedded in SiO₂ matrix were grown by radio frequency (RF)-sputtering technique. X-ray technique was used to characterise the structural properties of the system. The NC's size was estimated to be around 4±1 nm in diameter. The temperature dependence of the photoluminescence from the CdSe/SiO₂ system showed carriers thermal exchange between the NCs and deep defects in the matrix. The evolution of the excitonic energy emission with temperature is about 10 meV in the temperature range 15-295 K. This weak shift was explained by thermal mismatch between the matrix and the NCs.     

Relative enhancement of photoluminescence intensity of passivated silicon nanocrystals in a silicon dioxide matrix

Photoluminescence (PL) intensity of passivated silicon nanocrystals (Si NCs) embedded in a SiO 2 matrix is compared with that of unpassivated Si NCs. We investigate the relative enhancement of PL intensity (I R ) as a function of annealing temperature and implanted Si ion dose. The I R increases simultaneously with the annealing temperature. This demonstrates an increase in the number of dangling bonds (DBs) with the degree of Si crystallization varying via the annealing temperature. The increase in I R with implanted Si ion dose is also observed. We believe that the near-field interaction between DBs and neighboring Si NCs is an additional factor that reduces the PL efficiency of unpassivated Si NCs.     

Peculiarities of radiation-induced defect formation in nanocrystals embedded in a solid matrix

The modification of basic radiation-physics effects (elastic defect formation and amorphization) in nanoclusters placed in a solid matrix compared to the same processes in single crystals is considered theoretically. We introduce a diffusion model of defect formation and study the influence of the nanocluster-matrix interface on the displacement energy. Based on the percolation treatment, we show a significant change in the critical nanocluster amorphization dose compared to the case of a crystal. The influence of various types of irradiation on the amorphization processes is considered. Judgments about the variety of radiation-physics effects in nanoobjects are expressed.     

Photoluminescence studies from silicon nanocrystals embedded in spin on glass thin films

Photoluminescence (PL) from silicon nanocrystals (Si-nc) prepared from pulverised porous silicon and embedded in undoped (SOG) or phosphorus-doped spin-on-glass (SOD) solutions was studied. Effects of rapid thermal annealing on the PL was also investigated. A strong room temperature PL signal was observed at 710 nm due to the recombination of electron-hole pairs in Si-nc and the PL maximum shifts to the blue region as the phosphorus concentration in the spin on glass increases. However, the rapid thermal annealing process (30 s, 900°C) quenches the PL response. These results suggest that for Si-nc/SOG (SOD) the surface termination is efficient but high phosphorus doping of Si-nc is detrimental to the PL.     

Controlled growth of silicon nanocrystals in a plasma reactor

Using a powerful multilevel simulation approach, we "visualize" the complete growth dynamics of hydrogenated silicon nanostructures under realistic experimental conditions of a plasma reactor. For the early stages of the synthesis, we demonstrate for the first time how precise control of atomic hydrogen not only permits one to choose between the production of amorphous and crystalline nanoparticles, but also to "steer" the growth toward the formation of elementary "building blocks" for the synthesis of hexagonal silicon nanowires.     

Temperature dependent optical properties of Si nanocrystals embedded in SiO2 matrix

The optical properties of Si nanocrystals (nc-Si) with different sizes (2～5 nm) embedded in a SiO₂ matrix synthesized by the SiO _x /SiO₂ superlattice approach were studied in a temperature range from room temperature to 600 K by spectroscopic ellipsometry. The Maxwell–Garnett effective medium approximation and Lorentz oscillator model were employed to extract the dielectric function of nc-Si. The results show that the temperature dependence of optical properties of nc-Si is sensitive to its size. An empirical expression of Varshni approximation was obtained to characterize the band-gap energy change of nc-Si at different temperatures.     

Effect of surface Si-Si dimers on photoluminescence of silicon nanocrystals in the silicon dioxide matrix

The effect of surface states of silicon nanocrystals embedded in silicon dioxide on the photoluminescent properties of the nanocrystals is reported. We have investigated the time-resolved and stationary photoluminescence of silicon nanocrystals in the matrix of silicon dioxide in the visible and infrared spectral ranges at 77 and 300 K. The structures containing silicon nanocrystals were prepared by the high-temperature annealing of multilayer SiO _x /SiO₂ films. The understanding of the experimental results on photoluminescence is underlain by a model of autolocalized states arising on surface Si-Si dimers. The emission of autocatalized excitons is found for the first time, and the energy level of the autolocalized states is determined. The effect of these states on the mechanism of the excitation and the photoluminescence properties of nanocrystals is discussed for a wide range of their dimensions. It is reliably shown that the cause of the known blue boundary of photoluminescence of silicon nanocrystals in the silicon dioxide matrix is the capture of free excitons on autolocalized surface states.     

Electrical and memory properties of silicon nitride structures with embedded Si nanocrystals

In this work, the electrical and memory behaviour of metal-silicon nitride-silicon structures with an embedded nanocrystalline silicon layer, which either consists of separated silicon nanocrystals, or is a continuous nanocrystalline layer, are presented. The structures were prepared by low-pressure chemical vapour deposition (LPCVD). The effect of the duration of deposition and the structure of the nanocrystalline layer were studied. The writing/erasing behaviour was similar for all the structures, but the retention properties were much worse in the structure with a continuous nanocrystalline layer, than in the structures with separated Si nanocrystals. This indicates that Si nanocrystals play role in charge storage in the studied structures.