Impurity Gettering by Silicon Nitride Films: Kinetics, Mechanisms, and Simulation

Abstract

Metallic impurities in the silicon wafer bulk are one of the major efficiency-limiting factors in silicon solar cells. Gettering can be used to significantly lower the metal concentrations. Although gettering by silicon nitride films has been reported in literature, much remains unknown about its gettering behaviors and mechanisms. In this study, the gettering kinetics and mechanisms of silicon nitride films, from both plasma-enhanced chemical vapor deposition (PECVD) and low-pressure chemical vapor deposition (LPCVD), are investigated. By monitoring the kinetics of iron loss from the silicon wafer bulk, it is confirmed that silicon nitride gettering takes place mainly via segregation, even at a low annealing temperature of 400 °C. Simulation of the gettering kinetics suggests the presence of an interfacial diffusion barrier in some cases, which slows down the transport of iron impurities from the silicon wafer bulk to the silicon nitride gettering regions. The activation energy of the segregation gettering process is estimated to be 0.9 ± 0.1 eV for the investigated PECVD silicon nitride film at 400–900 °C and 1.6 ± 0.5 eV for the investigated LPCVD silicon nitride film at 400–700 °C.

Industrial Czochralski n‐type Silicon Wafers: Gettering Effectiveness and Possible Bulk Limiting Defects

“…Additional n-type FZ-Si control wafers, of 1.9 AE 0.2 Ω cm and 290 μm thickness were prepared. One set of the FZ-Si wafers were ion implanted with Fe and annealed, resulting in ð1.1 AE 0.2Þ Â 10 12 cm À3 Fe uniformly distributed in the bulk (see [35] for the ion implantation and annealing details), while the other set had no Fe implantation. Both of the Cz-Si and FZ-Si samples (with and without bulk Fe) were subject to depositions of plasma-enhanced atomic layer deposition (PE-ALD) of AlO x films on both sides.…”

“…One set of the FZ‐Si wafers were ion implanted with Fe and annealed, resulting in Fe uniformly distributed in the bulk (see [ 35 ] for the ion implantation and annealing details), while the other set had no Fe implantation. Both of the Cz‐Si and FZ‐Si samples (with and without bulk Fe) were subject to depositions of plasma‐enhanced atomic layer deposition (PE‐ALD) of AlO x films on both sides.…”

External Gettering of Metallic Impurities by Black Silicon Layer

“…Gettering is especially widely used in Si-based solar cell production, for which low-cost solar-grade Si is increasingly used. [3][4][5][6][7][8][9][10] Such material contains various undesirable metal impurities (Au, Cu, Fe, Ni, etc.) and structural defects, such as grain boundaries, dislocations, second-phase precipitates, oxidation-induced stacking faults (OISF), etc.…”

“…Since solar cells are wholesubstrate devices, it is preferable to use external gettering. External gettering is carried out by various methods, including phosphorus or boron diffusion, [4,5] ion implantation, [6] deposition of thin films, [7,8] laser processing, [9] grooving, [10] and others.…”