No abstract

We study ion implantation for patterned doping of back-junction back-contacted solar cells with polycrystallinemonocrystalline Si junctions. In particular, we investigate the concept of counterdoping, that is, a process of first implanting a blanket emitter and afterward locally overcompensating the emitter by applying masked ion implantation for the back surface field (BSF) species. On planar test structures with blanket implants, we measure saturation current densities J 0 ,p oly of down to 1.0 ± 1.1 fA/cm 2 for wafers passivated with phosphorusimplanted poly-Si layers and 4.4 ± 1.1 fA/cm 2 for wafers passivated with boron-implanted poly-Si layers. The corresponding implied pseudofill factors pF F im p l . are 87.3% and 84.6%, respectively. Test structures fabricated with the counterdoping process applied on a full area also exhibit excellent recombination behavior (J 0 ,p oly = 0.9 ± 1.1 fA/cm 2 , pF F im p l. = 84.7%). By contrast, the samples with patterned counterdoped regions exhibit a far worse recombination behavior dominated by a recombination mechanism with an ideality factor n > 1. A comparison with the blanket-implanted test structures points to recombination in the space charge region inside the highly defective poly-Si layer. Consequently, we suggest introducing an undoped region between emitter and BSF in order to avoid the formation of p + /n + junctions in poly-Si.

Boron diffusion is commonly associated with the formation of an undesirable boron-rich layer (BRL), which is often made responsible for degradation of the carrier lifetime in the bulk. We investigate the phenomenology of the BRL formation, which results from BBr 3 boron diffusion processes, and its impact on sheet resistance and bulk lifetime. Our measurements show that boron silicate glass (BSG) and BRL thicknesses vary between 50 and 600 nm and 0 and 80 nm respectively within the two-dimensional wafer surface of one sample for one diffusion process. Both thicknesses strongly depend on the gas composition during composition and deposition time. Further results show that BRL formation is favored by high concentrations of BBr 3 vapor and of oxygen during B 2 O 3 deposition. Also, high drive-in temperatures promote the growth of the BRL. We find that a BRL of more than 10 nm thickness causes a degradation of the carrier lifetime in the bulk of the silicon wafer. In particular, we show that this bulk lifetime degradation occurs during the cool-down ramp after the diffusion process. We show that carrier lifetime degradation can be avoided either by limiting the process temperature to 850 • C and thus preventing BRL formation or through reconverting the BRL by a drive-in step in oxidizing atmosphere at 920 • C.

No abstract

In semiconductors, the effective excess carrier lifetime, τeff, measured in dependence on the injection density, Δn, is an important parameter. It is frequently observed that τeff decreases with decreasing Δn at low-level injection conditions (where Δn is smaller than the dopant density Ndop), which has been difficult to explain. We compare measurements with numerical device simulations to demonstrate that this observed reduction of τeff is caused by a combination of (i) Shockley-Read-Hall (SRH) recombination at the edges of the sample and (ii) transport effects of the carriers toward the edges. We measure τeff(Δn) of boron-diffused and surface-passivated p+np+ and p+pp+ silicon wafers with the commonly applied photo-conductance decay technique, and we vary the sample size. The photo-conductance is probed by inductive coupling within a sample region of about 3 × 3 cm2; hence, the measurements yield an average value of both τeff,av and Δnav within that region. For a detailed analysis, we determine τeff with a high spatial resolution using the dynamic infrared lifetime mapping technique, which shows a strong decrease of τeff toward the edges of the p+np+ samples at low-level injection. We analyze the measurements by numerical device modeling and circuit simulation. We conclude that the sample size should be at least 6 × 6 cm2 for reliable τeff(Δn) measurements at low injection conditions. However, at high-injection conditions, the recombination usually dominates at the dopant-diffused surfaces. Therefore, the saturation current-density, J0, can be extracted from the τeff(Δn) measurements in samples as small as 3 × 3 cm2, with a measurement error due to edge recombination below 10%.