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Silicon Resistivity: Key to Solar Efficiency
By Peter Brown, Lattice Materials
In the quest to improve efficiency and reduce costs associated with solar energy, what goes into a solar cell at the very beginning is just as important as the energy that comes out. When the process begins, highly purified silicon is paramount.
Highly purified silicon (Si, >99.999% purity) is a well-known material for applications such as semiconductors, solar cells, and infrared optical components, both transmissive (lenses) and reflective (mirrors). The physical characteristics of the silicon are important, but they vary from one application to another. Physical characteristics include optical transmission, resistivity, type (N or P), orientation, carrier lifetime, index of refraction and purity.
Looking at the table below, resistivity is the one characteristic that’s of primary importance across the board (with the exception of reflective optics).
Electronics | Solar Power Generation | Transmissive Optics | Reflective Optics | |
Optical Transmission | N | N | P | N |
Resistivity | P | P | P | N |
Type (N or P) | P | P | S | N |
Orientation | P | S | S | S |
Carrier Lifetime | P | P | N | N |
Index of refraction | N | N | P | N |
Purity | P | P | P | P |
Relevance of common silicon specifications, by application
P=primary. S=secondary. N=no importance
P=primary. S=secondary. N=no importance
Resistivity is important in solar power generation because it shows the degree to which a material tends to impede the flow of electrical current. (Note that it is related to, but not equivalent with, resistance, expressed in Ohms.) Resistance is measured for an individual electrical component; i.e., the voltage applied across the component, divided by the amount of current conducted with that voltage applied. Resistivity is a bulk characteristic of the material itself, regardless of how that material is eventually processed into an electrical component.
Low resistivity materials conduct electricity better than high resistivity materials. For example, silver, an excellent conductor, has a resistivity of 1.59×10-8 Ohms·cm, whereas glass, an insulator, has a resistivity of 1010 to 1014 Ohms·cm. The middle contains pure silicon, a semiconductor with a theoretical resistivity of 6.40×104Ohms·cm when no dopants are added.
Dopants and resistivity
When Czochralski growth is used in solar wafer manufacturing, it must contain the proper dopant at the right quantity in order to produce silicon that can be used in solar wafers. Without the dopant, the silicon is useless for the solar industry.
Nearly all applications for silicon require the addition of dopants to produce the particular resistivity required. Dopants are either P type (typically boron) or N type (phosphorous, arsenic, or antimony). Dopants are added to the silicon as part of the crystal growth process, most commonly Czochralski growth (Cz). The amount of dopant added is very small, typically in the parts per billion to parts per thousand range. The following table lists some values of resistivity, and the dopant concentrations that produce these values:
P (Boron) | N (Phosphorous) | |||||
|---|---|---|---|---|---|---|
Resistivity ohm-cm | ppba | atoms/cm3 | atoms/gm Si | ppba | atoms/cm3 | atoms/gm Si |
100 | 2.6 | 1.35 EE+14 | 5.6 EE+13 | 0.86 | 4.2 EE+13 | 1.8 EE+13 |
10 | 27 | 1.35 EE+15 | 5.6 EE+14 | 8.8 | 4.4 EE+14 | 1.9 EE+14 |
1 | 290 | 1.45 EE+16 | 6.2 EE+15 | 96 | 4.8 EE+15 | 2.1 EE+15 |
0.1 | 5500 | 2.8 EE+17 | 1.2 EE+17 | 1550 | 7.8 EE+16 | 3.4 EE+16 |
0.005 | 410000 | 2.0 EE+19 | 8.6 EE+18 | 240000 | 1.2 EE+19 | 5.3 EE+18 |
The actual amount of dopant added to the silicon is very small. For example, a typical solar application calls for P type material, with resistivity > 1 ohm-cm. From the table, we can see that this resistivity corresponds to 6.2 x 1015 atoms of boron per gram of silicon. For a 50 kg charge of silicon in the Cz furnace, this doping level equals about 5.5 milligrams of boron dopant.
The way it works
In the Cz process, the silicon and dopant are loaded into a quartz crucible, melted, and stabilized at about 1400°C. A seed crystal is then dipped into the melt, and as it is slowly pulled upward out of the melt the silicon/dopant mixture freezes onto the seed in perfect crystalline structure. Since the solubility of the dopant is higher in the molten silicon than the solid crystal ingot, the dopant is more concentrated in the melt as the crystal is pulled and the melt consumed. The result is that the bottom of the ingot contains a higher dopant concentration than the top of the ingot, and therefore resistivity decreases from the top to the bottom of the ingot.
If the dopant concentration, and thus resistivity, varies beyond the required specifications over the length of the ingot, it will have a negative impact on process yield. For this reason, techniques have been developed to allow for controlling silicon and dopant amounts in the melt over the course of the run, and thus create a more uniform resistivity profile.
Thermal donors, and the affects of annealing on resistivity
The Cz growth process introduces a small amount of free oxygen atoms into the silicon crystal. This oxygen diffuses out of the quartz (silicon dioxide) crucible that contains the molten silicon. These oxygen atoms are also known as thermal donors, and behave as an N-type dopant. That is, they counteract the dopant effect of a P-type ingot, or magnify the dopant’s effects in an N-type ingot.
In wafer processing, a typical process is to eliminate these thermal donors by annealing the wafers at temperatures above 1000 °C, and then quenching the wafers quickly through the critical temperature range (400-750 °C) where thermal donors are created. By removing the thermal donors, the targeted resistivity is better controlled.
Silicon ingots are often annealed after growth for stress relief. If an ingot is not grown slip-free (i.e., it has dislocations in the crystal lattice), it will have inherent stresses that contribute to fractures and chips during the manufacturing process. This is especially true of polycrystalline silicon, in which there is no single crystal lattice in the ingot. These stresses can be relieved through annealing of the ingot prior to processing. This annealing can also have the effect of reducing local variations in resistivity across the ingot.
As new techniques for controlling the uniformity of resistivity across an ingot are discovered, it will be crucial for wafer manufacturers to adopt those techniques. The solar industry is evolving very quickly and only those manufacturers who stay on the cusp of technology will maintain a competitive advantage.
About the Author
Peter Brown is Manager of Sales and Marketing at Lattice Materials. Specialists in precisely controlling resistivity in silicon ingots for over twenty years, Lattice Materials routinely provides Czochralski crystal growth seeds for the solar industry. Lattice Materials also manufactures polish-ready infrared and reflective optics and provides a wide range of thin-film deposition materials.
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