Thesis Type: Doctorate
Institution Of The Thesis: Middle East Technical University, Turkey
Approval Date: 2016
Thesis Language: English
Student: Mehmet Karaman
Supervisor: RAŞİT TURANAbstract:
In this PhD study, crystallization of amorphous silicon on glass and its photovoltaic applications have been investigated. The crystallization of amorphous silicon (a-Si) was studied in two parts; Metal Induced Crystallization (MIC) and Laser Induced Crystallization (LIC). MIC method was first implemented by gold nanoparticle (AuNP) fabricated by dewetting technique by which gold thin ﬁlms deposited on aluminium doped zinc oxide (AZO) coated glass were annealed for nanoparticle formation. A-Si was then deposited by e-beam evaporation onto metal nanoparticles. Silicon ﬁlms were annealed for crystallization at different temperatures between 500 oC and 600 oC. It was observed that inclusion of AuNPs provide the crystallization at lower temperatures with higher rates. Raman and XRD results showed that the crystallization starts at temperatures as low as 500 oC and an annealing at 600 oC for a short process time provides sufﬁciently good crystallinity. Then, MIC process was studied by Aluminium Induced Crystallization (AIC) of a-Si. Firstly, AIC study was started with the silicon nitride (SiNx) buffer layer optimization by depositing different types of SiNx films with varied NH3/SiH4 content during the PECVD film deposition. Furthermore, the effect of buffer layer content on final poly-Si properties was investigated by this way. AIC process was started with Al film evaporation onto SiNx and AZO layers. Then a-Si deposition was carried out by e-beam evaporation. The crystallization, in other words the layer exchange of Al and Si, was provided by furnace annealing at 500 oC. Based on Raman, EBSD, XRD results, the best buffer layer was chosen in terms of Si crystallinity and grain size for the further AIC experiments. The next AIC experiments were followed by basic characterizations. The crystallization of AIC process at different temperatures and durations was monitored by optical microscopy (OM) and the activation energy of the process was calculated. The Al content of the poly-Si layer was detected by Secondary Ion Mass Spectroscopy (SIMS). The effect of AlOx membrane on the kinetics of crystallization was monitored through optical microscopy by changing the formation conditions of AlOx. Then solid phase epitaxy (SPE) experiments were carried out. Raman and SEM analysis showed well-established SPE layer. To investigate the improvement of the final AIC poly-Si layer quality, some modifications on the process was introduced. The effect of Al annealing in a vacuum environment on the AIC kinetics and final poly-Si layer properties was investigated. The layer exchange process was monitored by optical microscopy and it was observed that Al annealing reduces the crystallization rate. XPS measurements showed that annealing of Al creates more stable and denser AlOx layer compared to Al layer with no annealing. EBSD results indicate that Al annealing notably increases the grain size of Al layer and also improves the grain structure of final poly-Si layer surface. Another improvement on the AIC poly-Si layer quality is seen by comparing the different a-Si deposition methods of E-beam evaporation and PECVD. Two techniques are compared for their effect on the overall AIC kinetics as well as the properties of the final poly-crystalline (poly-Si) silicon film. Raman and FTIR spectroscopy results indicate that the PECVD grown a-Si films has higher intermediate-range order, which is enhanced for increased hydrogen dilution during deposition. EBSD analysis showed that increasing intermediate-range order of the a-Si suppresses the rate of AIC, leading larger poly-Si grain size. In the second part of this work, laser assisted crystallization was carried out. Three layered stack of SiOxNy/SiOx/SiNx was deposited by PECVD onto borosilicate glass as the buffer layer. 10 µm of intrinsic and 10 nm of p and n-type a-Si:H (doping layer) was deposited by PECVD. The crystallization was carried out by 808 nm continuous wave line laser. Moderately doped absorber layer was obtained during laser crystallization by intermixing of doping layer and intrinsic layer. After crystallization the homojunction solar cells were obtained by spin-on dopant coating with following laser doping and the mesa cells were constructed. Two different laser doping velocities of 1 and 5 mm/s were applied. For comparison some of the cells were hydrogen passivated. SunsVoc measurement was accomplished and up to 579 mV of Voc was measured. EQE and solar simulator analysis were carried out both for substrate and superstrate conditions. EQE measurements show that H2 passivation decreases the emitter diffusion length (short wavelenths) due to the increase in surface recombination whereas the absorber diffusion length increases.