Phosphorus Doping Effect In Si Nanocrystals/SiO₂ Multilayers

As the development of the nano-semiconductor technology,the size of silicon-based devices is decreasing rapidly,and it is now in the scale of nanometers.The silicon nanocrystals(Si NCs)have been attracting more and more interests due to their novel optical and electronic properties which is distinguished from their bulk counterpart,and they have been widely used in the nanoelectronic and optoelectronic devices,such as the next-generation solar cells,non-volatile memories and Si-based monolithic optoelectronic integrations.To improve the performance of the devices of Si NCs,it is important to realize the impurity doping and study the influence of doping on the electronic structures and optoelectronic properties of Si NCs.In bulk Si,the mechanism and the influence of doped impurities(such as boron and phosphorus)are relatively clear.However,for the impurities in Si NCs,it is still an open question of the impurities distribution,activation efficiency,relative location of the impurity energy level and influence on the optical/electronic characteristics.Thus,it is quite necessary to make deep and systematic investigations of the doping behavior in Si NCs,which is also an attractive and interesting topic in the worldwide.In this thesis,we mainly study the doping behaviors and influence on the Si NCs'electronic structures and optoelectronic properties of the impurities,especially phosphorus(P).With the plasma enhanced chemical vapor deposition(PECVD)system,the amorphous Si(a-Si)/SiO2 multilayers are obtained by alternatively repeating the a-Si deposition/oxidation processes,and the Si NCs/SiO2 multilayers are prepared after the subsequent high temperature annealing.By adding doping gas(like PH3)during the a-Si layer deposition process,the doped samples can be obtained.With the electron spin resonance(ESR)and x-ray photoelectron spectroscopy(XPS)measurements,we determine the location of P atoms in the multilayers and study the dependence of doping behaviors on the Si NC size,doping concentrations and annealing temperatures.We demonstrate experimentally that P atoms can be partially doped into Si NCs.Besides,we make further studies of the mechanism and light emitting behavior of the subband near-infrared(NIR)light(<1.leV).We can find that the NIR light emission can only be observed in the ultra-small sized(-2nm)P doped Si NCs,and it is gone for Si NCs with larger size or other impurities,like boron.Considering the analysis of the electronic structure and the study of the light emitting characteristics,we discuss the subband light emitting mechanism.What's more,the localized surface plasma resonance(LSPR)effect is observed in the heavily P doped Si NCs/SiO2 multilayers,and we study the signal dependence on the doping concentration and Si NCs size.The primary results and innovations are listed below:1.We fabricated the P doped a-Si/SiO2 multilayers with controllable thickness in the PECVD system and obtained the Si NCs embedded in SiO2 after high temperature annealing.P doping was realized by adding PH3 during the a-Si deposition process.With the Raman scattering and cross-sectional transmission electron microscopy(TEM)measurements,we characterized the size of Si NCs,the crystalline ratio and the structure of multilayers,and it was found that the average size of Si NCs is in the range of 2-8nm which was consistent with the thickness of corresponding Si layers.By using the XPS measurement,we found that P atoms are mainly distributed in the Si layer or the interface of Si/SiO2,and they don't tend to move into the SiO2 even after high temperature annealing.Both the experimental and theoretical studies reported previously were in good agreement with our results.2.With the temperature-dependent ESR technique,we systematically studied the ESR spectra of P doped Si NCs with various doping concentrations and annealing temperatures.We found that the signal due to the Si dangling bonds(DBs)at surface of Si NCs existed in the room-temperature ESR spectra.After P doping,this signal disappeared,which indicates that the DB defects are passivated by P atoms.Meanwhile,the low-temperature ESR spectra revealed that P atoms are partially doped into Si NCs and they result in the appearance of the conduction electron(CE)signal.Compared to the CE in bulk Si,it was broadened in Si NCs due to the quantum confinement.Interestingly,the hyperfine structure(HFS)was observed in the ESR spectra when the doping concentration was low,which indicates only one P atom exists in a single Si NC.With the doping concentration and annealing temperature increasing,the number of incorporated P atoms was increasing.As a result,the HFS disappeared and the CE signal was enhanced.When the concentration and annealing temperature were both high enough,the Si crystalline structure was partially destroyed by P atoms,and a new kind of defects were introduced.By analyzing the temperature dependence of the CE signal's integrated intensity,we found that the electrons mainly obey the"Curie paramagnetism" at a low doping concentration and mainly obey the"Pauli paramagnetism" at a high doping concentration.3.Based on the subband NIR light emission from the P doped Si NCs which was observed previously,we made further studies in the properties and mechanism of this PL signal.It was found that the subband NIR emission can only be observed in the P doped Si NCs with an ultra-small size(?2nm),and it cannot be seen in Si NCs with larger size or other impurities(like B).Then we systematically studied the PL spectra of the 2nm P doped Si NCs/SiO2 multilayers and observed two PL peaks:one located at?890nm which is corresponding to the band-to-band transition or the radiative recombination center at the interface,and another located at?1250nm which is due to the P induced radiative deep level.We found that with the doping concentration increasing,the number of deep levels in Si NCs is increasing.So the 890nm peak is reduced and the 1250nm peak is enhanced.When the doping concentration is further increased,both peaks are reduced.This can be assigned to the P induced defects under the high doping concentration.We also measure the lifetime of the two PL peaks and we found that the lifetime of 1250nm peak is much shorter than that of 890nm peak,which indicates a much higher recombination rate.The temperature dependent PL measurements of the 1250nm peak revealed that the peak blue shifts under low temperature due to the broadened bandgap.The integrated intensity of the PL peak increase and then decreases with the temperature increasing,which is a typical characteristic of light emission from Si NCs.4.Based on the P doped Si NCs/SiO2 multilayers with various doping concentrations,we observed the LSPR effect by the using the FTIR measurements.The LSPR originates from the free carriers generated by the dopants.We found that when the doping concentration is over 6%and the size of Si NCs is over 4nm,the LSPR can be observed.With the concentration increasing,the LSPR peak will blue shift.By the ESR and EDX measurements,we demonstrated that the LSPR related free electrons are provided by P atoms incorporated in the Si NCs.With the energy of the LSPR peak,we estimated the density of free electrons and calculated the effective doping concentration.Besides,we also found that the LSPR peak of B-P codped Si NCs is blue shifted in comparison with the P doped sample.It is possible that the B and P atom pairs are formed and they enhance the doping efficiency of P atoms.