Ammonium tetraborate tetrahydrate as a new boron source for the fabrication of p-type emitters in silicon solar cell

Ammonium tetraborate tetrahydrate (ATT, (NH4)2B4O5(OH)4·2H2O) is proposed as a new, inexpensive, and direct boron source that can achieve uniform diffusion at low temperatures. It is prepared by spin coating method to improve the uniformity and decrease the diffusion temperature (T). The effects of ATT solution concentration (CATT) and T on the boron doped-layer are systematically investigated. An excellent uniformity is achieved, and the standard deviation coefficient (σ) of mean sheet resistance (Rsheet) is ∼ 1% at T ≥ 900 ℃ (CATT = 3%), which is better than the previously reported σ (>4.8%). A variety of doping profiles with mean Rsheet ranging from 14.5 to 1170 Ω/sq are obtained by controlling T and CATT for a diffusion time of 45 min. The mean Rsheet is 80–300 Ω/sq for T less than 850 ℃. Furthermore, the effect of surface hydrophilicity treatment on σ and Rsheet is examined by immersing the wafers in oxidation reagents before spin coating. Better uniformity of σ ∼ 0.5% is realized by improving the surface hydrophilicity through nitric acid oxidation for 1–10 min at 120 ℃. However, if the immersion time is longer than 20 min, the mean Rsheet increases due to the barrier effects of thicker oxidization layer.