Teimuraz Mchedlidze

Residual stress in Si nanocrystals embedded in a SiO2 matrix

Multiple quantum wells consisting of alternating Si and SiO2 layers were studied by means of Raman scattering. The structures were fabricated by the remote plasma enhanced chemical vapor deposition of amorphous Si and SiO2 layers on quartz substrate. The structures were subjected to a rapid thermal annealing procedure for Si crystallization. The obtained results suggest that the Si layers consist of nanocrystals embedded in an amorphous Si phase. It was found that the silicon nanocrystals inside 2nm thin layers are under high residual compressive stress. Moreover, the metastable Si III phase was detected in these samples supporting the presence of large compressive stresses in the structures. The compressive stress could be relaxed upon local laser annealing.

Influence of Dislocation Loops on the Near-Infrared Light Emission From Silicon Diodes

The infrared light emission of forward-biased silicon diodes is studied. Through ion implantation and anneal, dislocation loops were created near the diode junction. These loops suppress the light emission at the band-to-band peak around 1.1 mum. The so-called D1 line at 1.5 mum is strongly enhanced by these dislocation loops. We report a full study of photoluminescence and electroluminescence of these diodes. The results lead to new insights for the manufacturing approach of practical infrared light sources in integrated circuits.

Light induced solid-phase crystallization of Si nanolayers in Si/SiO2 multiple quantum wells

The process of light-induced crystallization (LIC) of nanometer-thick amorphous silicon (a-Si) layers in Si/SiO2 multiquantum wells (MQW) was investigated using Raman spectroscopy. In the present investigations, a laser was employed as the light source. An analysis of obtained and previously published results suggests strong influence of radiation wavelength on the outcome of the process. Namely, for certain ranges of wavelengths and radiation fluxes the crystallization proceeds through the light-induced solid phase crystallization (LISPC) process. An optimal set of radiation wavelength and flux values allows formation of fully crystallized and almost strain-free layers of nanocrystalline silicon (Si-nc). The difference in the absorption coefficients between a-Si and Si-nc was considered responsible for the obtained results. A mechanism explaining the wavelength and the radiation flux dependence was proposed. Understanding of the mechanism of LISPC in MQW structures would allow improving the LIC processes...

Electroluminescence from p-i-n structure fabricated using crystalline silicon on glass technology

Strong electroluminescence was detected at room temperature from a p-i-n structure fabricated using crystalline silicon on glass technology. The luminescence spectra at small to moderate carrier injection levels contains strong peak with maximum at energy position Eph∼0.8 eV. Additionally, a broad emission band in the range of energies 1 eV<Eph<1.16 eV appears at high injection levels. Obtained results suggest that the low energy peak can be attributed to dislocation related luminescence (DRL), while at least part of the high-energy emission band should be attributed to band-to-band transitions. A shift in the DRL peak position by the electric field present in the structure was observed. The shift is related to strong Stark effect. The relatively high efficiency of room temperature luminescence suggests the possibility for application of the structure for all-silicon light emitter.

Regular Dislocation Networks in Silicon Part I: Structure

Dislocation networks obtained by hydrophobic wafer bonding of Si (100) are investigated. The twist and tilt misorientations induce two interacting dislocation networks. Advanced bonding techniques are applied and optimized allowing to eliminate the tilt and to control the twist misorientation. At very low twist angles the interfaces no longer exhibit regular dislocation networks. Properties of dislocation networks are discussed.

Influence of electric field on spectral positions of dislocation-related luminescence peaks in silicon: Stark effect

Spectral positions of dislocation-related luminescence (DRL) peaks from dislocation loops located close to a p-n junction in silicon were shifted by carrier injection level. We suppose that the excitonic transition energies of DRL were reduced by an effective electric field at dislocation sites due to quadratic Stark effect (QSE). The field results from built-in junction field reduced by carrier injection. A constant of the shift, obtained from fitting of the data with QSE equation, was 0.0186 meV/(kV/cm)2. The effect can explain the diversity of DRL spectra in silicon and may allow tuning and modulation of DRL for future photonic applications.

Investigation of defect states in heavily dislocated thin silicon films

Deep level transient spectroscopy (DLTS) and photoluminescence (PL) were applied for investigation of defect states in thin crystalline silicon (Si) films deposited on glass. The films were fabricated by solid phase crystallization of amorphous Si layers and subsequently were subjected either to rapid thermal annealing or/and to hydrogenation. The study revealed presence of carrier traps and radiative recombination centers characteristic for dislocations in Si. Density of the traps strongly varied depending on the fabrication processes applied to the film. This allowed to link formation of the defects with applied fabrication processes and suggested origins for the traps. Passivation of the dislocation-related defect states by hydrogen was observed and appearance of hydrogen-related traps for the dislocated structures was detected. An increase in intensity of dislocation-related luminescence well correlated with the decrease in density of deep dislocation-related traps.

Regular Dislocation Networks in Si. Part II: Luminescence

Regular dislocation networks formed as a result of the direct bonding of Cz-Si wafers with oxide remnants on the pre-bonding surfaces were investigated. Besides the dislocation network, oxide precipitates were detected at the bonding interface. The precipitate density across the network was ~5×1010 cm-2, except small irregularly distributed circular areas, several mm in diameter, where the density was remarkably lower (<5×108 cm-2). The dislocation network structure was not affected by the change in the precipitate density. Photoluminescence spectroscopy (PL) and light beam induced current (LBIC) mapping were applied for characterization of such dislocation networks. For the locations with high precipitate density, PL signal from dislocations and that from the band-to-band transitions were enhanced. On the other hand, the LBIC results indicated that oxide precipitates are active recombination centers and thus should suppress the observed radiative transitions. The controversy can be explained in the assumption that the D-band PL signal increases due to scattering of excitation light by the precipitates and due to related expansion of the excitation area of the dislocation network. The light reflection from the precipitate layer also enhances the detected band-to-band PL signal. The shape of PL spectra from the samples in the range of photon energies 0.75 – 1.15 eV was not influenced by the oxide precipitates.

Electronic States of Oxygen-Free Dislocation Networks Produced by Direct Bonding of Silicon Wafers

The results of experimental investigations of the dislocation-related DLTS-peaks originated from the dislocation networks (DN) are presented. Samples with DNs were produced by direct bonding of p-type silicon wafers and no enhancement of oxygen concentration was detected near the DN plane. Origins of the DLTS peaks were proposed and a correlation with the dislocation-related photoluminescence data was established based on known dislocation structure of the samples. Two types of shallow DLTS peaks exhibited Pool-Frenkel effect, which could be linked to the dislocation deformation potential. One of the shallow DLTS peaks was related to straight parts of screw dislocations and another - to the intersections of the dislocations.

Determination of the Origin of Dislocation Related Luminescence from Silicon Using Regular Dislocation Networks

The investigation of regular dislocation networks (DN) formed by direct wafer bonding suggests that the D1 and D2 peaks of dislocation-related luminescence (DRL) in silicon is linked to screw dislocations, whereas edge dislocations are responsible for D3 and D4 DRL peaks. Non-radiative recombination activity in DN could be attributed to edge dislocations and could be related to enhanced ability of these dislocations to getter impurity atoms. Obtained relation of DRL intensity with the density of screw dislocations suggests existence of the optimum twist angle for the wafer-bonding geometry for which the DRL intensity has a maximum. The dependence of DRL intensity on the spacing between screw dislocations has the maximum at about 7 nm. Reported radiative and non-radiative recombination properties of DN present substantial interest not only for possible LED applications in all-Si photonics but also for photovoltaics, since DNs represent a model system for grain boundaries controlling carrier lifetime in microcrystalline-Si material.