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boron doped silicon is one of the most important materials for semiconductor solar cells. It increases the p-type doping efficiency of silicon and helps to separate holes and electrons efficiently at a p-n junction, thereby generating electric energy from light.
boron doped silicon is made by a variety of methods, including B2H6 oxidation and ex-situ boron deposition. The first method, called B2H6 oxidation, involves depositing a boron-rich oxide on the silicon surface to form shallow p-n junctions, followed by high-temperature drive-in anneal [109].
The second boron doping method consists of growing both boron and silicon layers simultaneously in gaseous precursors. The precursors are made by mixing boron, a boron source (B2H6), and a silicon carrier gas. During this process, the gas molecules are diffused into the silicon through a silicon-to-boron exchanger and subsequently activated during high-temperature drive-in anneal.
Using boron doped silicon for solar cell applications has several advantages, such as increased conduction and lower production expense due to growing surface-to-volume ratios and p-n junction dimensions. However, boron doping of silicon can be challenging to achieve.
A defect-free boron doping technique is needed to enable reliable application of boron doped silicon. Unlike phosphorus dopants, majority hole traps in boron doped silicon prevent a high activation rate. Therefore, a non-damaged surface is required to effectively use the self-assembled molecular monolayer (SAMM) doping technique for boron doping.
In order to achieve defect-free boron doping, we have developed a new approach that is based on Hall measurements and secondary ion mass spectrometry (SIMS). The boron dopants are deposited in the presence of oxygen, which acts as a minority trap state that only captures electrons. This defect-free doping technique can be used to achieve high boron doping levels by SAMM and has shown the ability to deliver excellent activation rates.