Hydrogenation of defects in edge-defined film-fed grown aluminum-enhanced plasma enhanced chemical vapor deposited silicon nitride multicrystalline silicon

Abstract

Gettering of impurities and hydrogen passivation of defects in edge-defined film-fed grown (EFG) multicrystalline silicon were studied by low-cost manufacturable technologies such as emitter diffusion by a spin-on phosphorus dopant source, back surface field formation by screen-printed aluminum, and a post-deposition anneal of plasma enhanced chemical vapor deposited (PECVD) silicon nitride antireflection coating. These processes were carried out in a high-throughput lamp-heated conveyor belt furnace. PECVD silicon nitride-induced hydrogenation of defects in EFG silicon was studied in conjunction with screen-printed aluminum back surface field formation to investigate the synergistic effect of aluminum gettering and silicon nitride hydrogenation of bulk defects. It was found that post-deposition anneal of PECVD silicon nitride at temperatures ranging from 450 to 850 °C, without the coformation of aluminum back surface field on the back, does not provide appreciable passivation or hydrogenation of bulk defects in EFG material. However, simultaneous anneal of PECVD silicon nitride and formation of aluminum back surface field at 850 °C significantly enhanced the hydrogenation ability of the PECVD silicon nitride film. PECVD silicon nitride deposition and a subsequent anneal, after the aluminum back surface field formation, was found to be less effective in passivating bulk defects. It is proposed that aluminum-enhanced hydrogenation from a PECVD silicon nitride film is the result of vacancy generation at the aluminum-silicon interface due to the alloying process. The affinity of hydrogen to react with vacancies provides a chemical potential gradient that increases the flux of atomic hydrogen from the silicon nitride film into the bulk silicon. In addition, vacancies can dissociate hydrogen molecules, increasing the atomic hydrogen content of the bulk silicon. This enhances defect passivation and improves the minority carrier lifetime.

Influence of the high-temperature “firing” step on high-rate plasma deposited silicon nitride films used as bulk passivating antireflection coatings on silicon solar cells

“…In the literature, it has been reported that the simultaneous firing process step of the a-SiN x :H and the formation of an Al back surface field on the back of solar cells enhances the hydrogen passivation of the bulk defects. 13,15 This effect might have taken place during the production of the solar cells ͑containing an Al back surface field͒ 17 discussed in this section, but has not been studied in Sec. III in which the influence of the hightemperature step on the a-SiN x :H properties has been investigated.…”

“…10,11 Furthermore, vacancy-hydrogen complex generation and Alenhanced void generation have been suggested to occur dur- ing the firing process of screen-printed Al-based contacts. [12][13][14][15][16] Furthermore, the retention of the atomic hydrogen at defect sites during the rapid cooling of the cells immediately after the firing process to obtain enhanced passivation effects has been considered. 15,16 An important issue for the application of a-SiN x :H films in the photovoltaic industry is the deposition rate of the a-SiN x :H. Increasing the deposition rate is a real challenge in large-scale solar cell production because at higher deposition rates investments in equipment can be kept relatively low reducing the costs per wafer processed.…”

“…A number of studies have been devoted to elucidate the underlying mechanisms of surface and bulk passivation of Si solar cells as induced by a-SiN x :H. [6][7][8][9][10][11][12][13][14][15][16][17][18] Bulk passivation can be achieved by a short high-temperature step, the socalled firing process, which is used for the application of screen-printed metallization of a-SiN x :H coated mc-Si solar cells. During this firing process, defect passivation in Si is obtained by hydrogen species that are released from the a-SiN x :H film and that diffuse into Si.…”

Bulk and surface passivation of silicon solar cells accomplished by silicon nitride deposited on industrial scale by microwave PECVD

“…This hydrogenation induces passivation of defects and impurities in the bulk and can significantly increase the lifetime of minority charge carriers in the bulk of multicrystalline wafers. [20][21][22][23][24][25][26][27] The physical mechanisms of bulk passivation by hydrogenation are not yet completely understood. For instance, the hydrogen diffusion mechanism 28 and the role of an aluminum back surface field (Al-BSF) in this process [29][30][31][32][33][34] as well as the interaction between hydrogen and oxygen complexes in silicon 35,36 are still the subject of discussion.…”

Opto-Structural Properties of Silicon Nitride Thin Films Deposited by ECR-PECVD

“…This is in agreement with FTIR results: hydrogen diffuses-out mainly by breaking Si-H bonds (3.1 eV) for Si-rich SiN x films with a low stoichiometry x and less by breaking N-H bonds (4.1 eV) for N-rich SiN x films. The released hydrogen is of importance for deactivating the electrically active defects in solar-grade silicon substrates [35] [36]. The bulk passivation efficiency is not necessarily determined by the amount of desorbed hydrogen during annealing.…”