In this work, HfO2 was proposed as a passivation material for n-type and n+ surfaces of high efficiency crystalline silicon solar cells. The potential for passivation is evaluated through extensive characterization of HfO2 layers, using a variety of methods, which have provided insight into the physical, chemical, and electrical properties of the material.HfO2 layers were deposited via an atomic layer deposition (ALD) process, using two main precursors, the first being HyALD, a metal-organic precursor, containing a cyclopentadienyl ring, which improves the stability of the molecule, and the second being HfCl4, a metal-halide precursor, used extensively for deposition of HfO2. The HyALD precursor was used to deposit films using a plasma enhanced ALD process, at 250oC, whereas hfCl4 was used in a thermal ALD process, at 200oC and 300oC. Initial simulations, using a Monte Carlo algorithm were used to compare the optical properties of HfO2 with the standard passivation materials, Al2O3 and SiO2, showing that similar performance can be expected from the three oxides. Transmission electron microscopy (TEM) was used to reveal the internal structure of HfO2 under different annealing conditions, and chemical analysis, via energy-dispersive X-ray spectroscopy (EDS) and elastic recoil detection (ERD) have provided information regarding the foreign atomic species contained in the deposited layers, such as Cl and H. Electrical characterization was done via capacitance-voltage (C-V) and conductance-voltage (G-V) measurements of metal-oxide-semiconductor (MOS) structures, using HfO2 as the dielectric. The oxide fixed charge (Qf) and the density of interface traps (Dit) were extracted, revealing the potential of HfO2 for field effect and chemical passivation, respectively. Layer analysis was carried out for as-deposited samples, as well as after 30 minute anneals, at 300oC, 400oC, and 500oC. The HyALD precursor showed very high Dit for as-deposited layers, which was gradually reduced with annealing. The HfCl4 precursor produced structures with a lower Dit, which contained either negative or positive fixed charge, depending on the deposition and annealing temperatures. The negative charge was attributed to the segregation of Cl impurities towards the interface between the HfO2 layer and the silicon wafer.Effective lifetime measurements via photoconductance decay in silicon wafers passivated with HfO2 were used to calculate the surface recombination velocity (Seff), in order to directly evalute the overall passivation quality. High quality, n-doped, high resistivity (15.5 Ocm) Cz silicon wafers with a mirror-polished surface were used for initial lifetime measurements. Good passivation was achieved after annealing at 300oC and 400oC with the HfCl4 precursor, due to the presence of sufficient negative fixed charge to create an inversion layer, and the low Dit, which ensured good chemical passivation. The very high Dit obtained with the HyALD precursor resulted in very high Seff values as-deposited and after annealing at 300oC. Higher temperature anneals provided some improvement, but overall passivation quality remained low. Lifetime measurements were also carried out for PV-grade, lower resistivity (4 Ocm), n-type Cz silicon, textured wafers, with a 120 O/sq n+ front surface field (FSF), in order to evaluate oxide performance in situations corresponding to solar cell applications. The investigation results have led to the conclusion that HfO2 is a material which can provide a wide range of properties, depending on deposition and post-processing conditions, showing potential for passivation of n-type and n+ silicon surfaces. Further research into this topic is porposed, in order to optimize the performance of HfO2 as a passivation material for high efficiency solar cell applications.
| Date of Award | 31 Oct 2015 |
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| Original language | English |
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| Supervisor | W.M.M. (Erwin) Kessels (Supervisor 1), E. Cornagliotti (External coach) & S.A.F. (Bas) Dielissen (External coach) |
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