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Investigation On Mechanism And Transient Dynamics Of Higher Than 60% Internal Quantum Efficiency Photoluminescence In A-SiN_x:O Films

Posted on:2017-02-24Degree:DoctorType:Dissertation
Country:ChinaCandidate:P Z ZhangFull Text:PDF
GTID:1108330485971079Subject:Microelectronics and Solid State Electronics
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Under the investigation on Silicon (Si) based monolithic integrated circuit devices, the most difficult work is the high efficient light source because its indirect band gap structure limits the light emission efficiency. Therefore, the study on how to improve the photo luminescence quantum efficiency (PL QE) and what is the mechanism and dynamics of the light emission is the arduous task for the researchers in this field in more than past two decades. Previous works were mostly focused on Si nanostructured materials, such as porous Si, well passivated colloidal Si nanoparticles, and Si nanocrystals embedded thin films etc. On the other hand, only a few papers have been reported the study of PL QE and PL decay processes from Si-based compounds. In 2006, we found that when we introduced oxygen into the a-SiN_x films, then the PL intensity greatly increased in the a-SiN_x:O films. Furthermore, electroluminescence (EL) of our a-SiN_x:O films has been demonstrated. Moreover, the characteristic of optical gain related to this kind of O-Si-N bonding states has been also reported. Based on the previous research of a-SiN_x:O films in our group, in the present work, we investigated the light induced degradation and further analyzed the PL stability of a-SiN_x:O films. We also intensively investigated the role of N-Si-O bonding in the tunable PL characteristics, and confirmed that the incorporation of O atoms into SiNx networks created a new N-Si-O related luminescent defect states (Nx) in the band gap, which were dominantly contribute to the PL of a-SiN_x:O samples. Then we firstly reported the study on PL IQE and PL EQE from our a-SiN_x:O films, and the PL IQE as high as 60% has been achieved. Furthermore, we understood the dynamics of high PL IQE fromN-Si-O bonding states in a-SiN_x:O films as well as the recombination processes under various measurement temperatures. The innovative results are listed as follows:1. We have firstly demonstrated unusual high PL EQE and IQE of a-SiN_x:O films fabricated by PECVD and followed by in situ plasma oxidation. Previous works were mostly focused on Si nanostructured materials, and so far we didn’t find the reports on the results of PL QE from Si-based compound materials. We employed the direct PL absolute quantum yield (AQY, or called PL EQE) measurements within a calibrated integration sphere, which quantitatively accounts for absolute absorbed excitation photons and emission photons, to obtain the PL EQE valued about 6.6%. In order to obtain the PL IQE of our a-SiN_x:O thin solid films, according to the principle of planar geometry optics, we calculated the light extraction factor N* to estimate the PL IQE from the PL EQE. On the other hand, we can calculate the PL IQE by combing the direct measured optical parameters which are from the integration sphere method, and optical parameters of reflectivity, and transmittance of the a-SiN_x:O thin films. We also derived the PL IQE through investigating the characteristic of the temperature dependent PL (TD-PL). These results show that the PL IQE as high as 60% has been achieved at peak wavelength of about 470 nm, which is the first report on the results of the PL IQE for the a-SiN_x:O films. This PL IQE is much higher than that of Si nanocrystal embedded thin films. The relevant results above are published online in Appl. Phys. Lett.105,011113 (2014).2. We intensively studied on the effect of oxygen incorporation in Si-N network disorder structures, by using a combination of optical characterizations, Fourier transform infrared (FTIR) spectra, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) measurements. The results of FTIR, XPS, ESR and PL excited measurements (PLE) indicated that the incorporation of oxygen atoms into silicon nitride (a-SiN_x) networks not only reduced the band tail structure disorder (Urbach tail width EU), but also created new defect states N-Si-O (Nx) in the band gap. Then we studied the mechanisms for tunable PL in the 1.90-2.95 eV range from our a-SiN_x:O films, and we have discovered the distinctive PL characteristics from a-SiN_x:O films with various NH3/SiH4 ratios. The PL peak energy (EPL) is independent of the excitation energy (Eexc) and the PL intensity (IPL) is regardless of the optical band gap (Eopt) but is proportional to the N* defects concentration, both of which are completely different from the PL characteristics by band tail states recombination mechanism, in which the EPL is proportional to Eexc (when Eexc≤ Eopt) and the IPL is dependent on the relative position of Eexc and Eopt. It is interesting to find that the EPL Stokes Shift showed a near constant value of about 0.75 eV, which were independent on the Eopt of the samples. Based on the N-Si-O bonding configurations and the distinctive PL characteristics, the radiative recombination mechanism through the N-Si-O defect states has been proposed. The relevant results above are published online in Appl. Phys. Lett.106,231103 (2015).3. We explored the dynamics of high PL IQE from luminescent N-Si-O bonding states in a-SiN_x:O films by using a combination of time resolved photo luminescence (TR PL) and TD-PL measurements. Both of the spectral profile and EPL from TD-SSPL and TD-TIPL measurements are independent with the variation of temperature, indicating the observed PL is origin from defect states. The TRPL measurements which include microsecond range PL (μs-TRPL) and nanosecond range PL (ns-TRPL) measurement modes, indicate the a-SiN_x:O films exhibit ns PL decay dynamics. We discovered that the ns-PL decay dynamics is independent of pumping fluence (WPF) and uniform across the PL spectrum, which is different from the PL decay behavior in a-SiN_x films and indicates Auger processes made almost no contribution to radiative recombination in the ns-PL of a-SiN_x:O films. Particularly, we precisely monitored the temporal evolution of the PL spectra profile from sub-nanoseconds to nanoseconds. The time-independent PL peak energy and PL spectral profile in a-SiN_x:O films are also different from the reported band tail related PL dynamics in a-SiN_x films that the EPL varies with time after the excitation. Therefore, we can conclude again that the observed ultrafast ns blue PL of a-SiN_x:O films is attributed to the radiative recombination at the luminescent N-Si-0 bonding states. Notice that, we directly calculated the radiative lifetimes at room temperature by using the measured PL lifetimes and PLIQE value. We discovered that the blue PL of a-SiN_x:O films with a last radiative recombination rate of~108 s-1, which can be compared with that in direct band gap CdSe nanocrystals, and can also help us to understand the high PLIQE in a-SiN_x:O films. Moreover, by combining τmeas(T) with the relative PL IQE(T) which were calculated from TD-TTPL intensities, the temperature dependence of radiative and nonradiative lifetime can be determined. The relevant results above are published online in AppL Phys. Lett.108,111103 (2016).
Keywords/Search Tags:a-SiN_x:O films, luminescent N-Si-O defect states, defect radiative recombination mechanism, PL quantum efficiency, PL transient dynamics, PL lifetimes
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