A research team from the Australian National University in Canberra and Chinese solar module manufacturer Longi has utilised high-resolution steady-state photoluminescence (PL) imaging to assess doping concentration in Czochralski (RCz) silicon wafers and has found that antimony (Sb), phosphorus (P), and gallium (Ga) all exhibit highly uniform radial concentration profiles, a key factor in achieving high solar cell efficiencies.
“Our findings indicate that the lateral, radial dopant distributions across the wafers are well within the tolerances required for high-efficiency solar cells,” the research’s lead author, Afsaneh Kashizadeh, told pv magazine. “The results also show that antimony is acceptable in terms of lateral dopant uniformity across a wafer and is superior in terms of axial uniformity along the ingot length. Our previous work has demonstrated that antimony-doped wafers exhibit excellent axial uniformity and outstanding bulk quality. These characteristics could make antimony a strong candidate to become the dominant dopant in industry in the future.”
The research team used wafers taken from the central sections of ingots grown by the RCz method during separate growth runs and doped Sb, P, Ga. To ensure a fair comparison among dopant types, wafers were taken from comparable relative positions along each ingot. The pseudo-square wafers were then laser-cut into quarters to facilitate laboratory-scale processing and measurements.
PL imaging was used to assess dopant concentration distributions under low-injection conditions and high surface recombination, with PL intensity being proportional to the wafer’s doping concentration and enabling spatially resolved analysis. To prepare the samples, wafers were chemically treated with tetramethylammonium hydroxide (TMAH) and hot deionized water to remove saw damage and enhance surface recombination.
PL images were acquired using a BT Imaging LIS-R1 system under 808 nm laser illumination and detected with a CCD camera. Multiple images were captured and averaged to improve the signal-to-noise ratio. For quantitative analysis, PL intensities were calibrated against doping concentrations measured with a Sinton WCT-120 eddy-current tester.
Radial dopant uniformity was evaluated through multiple center-to-edge line scans across quarter wafers. Additional annealing experiments were also conducted to determine whether the observed striations originated from dopant concentration variations or from thermal donor effects, and to test the sensitivity of PL imaging to thermal donor formation and annihilation.
The PL images revealed distinct structural asymmetries near the wafer centers, while the remaining areas showed largely uniform circular dopant patterns. These central irregularities are most likely linked to buoyancy-driven convection in the melt during crystal growth.
During seed pulling, interacting toroidal convection cells in the melt can create flow instabilities that produce swirl-like dopant asymmetries in RCz-grown silicon wafers. Sb-doped wafers show the strongest central inhomogeneities, likely due to Sb’s high evaporation rate and low segregation coefficient
To verify whether the observed variations were caused by dopants or by oxygen-related thermal donors (TDs), additional annealing experiments were conducted. Benchmark wafers were annealed at 1,050 C, a temperature high enough to eliminate TDs. The absence of resistivity changes confirmed that the dopant variations observed in the PL images originated from the dopants themselves rather than from thermal donors.
Further experiments intentionally generated TDs in a representative Sb-doped wafer through annealing at 450 C for 72 hours, which increased the majority carrier concentration. A subsequent annealing step at 650 C removed the thermal donors and restored the original carrier concentration, confirming that the temporary changes resulted solely from TD formation and subsequent annihilation.
“Radial dopant concentrations had standard deviations of less than 10% from the wafer center to edge, a level of variation expected to have negligible impact on solar cell performance,” the academics explained. “Among the dopants examined, P-doped wafers demonstrated the highest radial uniformity, with standard deviations below approximately 4%, compared to about 5% for Sb-doped and 8% for Ga-doped samples.”
“In contrast, axial profiles along the ingot length revealed a stronger dependence on dopant type,” they went on to say. “Sb-doped wafers maintained relatively stable axial distributions of less than 10%, whereas P-doped wafers showed a marked increase of toward the ingot tail.”
The researchers concluded that the RCz technique can produce silicon wafers with very low radial dopant variation. In particular, Sb-doped wafers show a more uniform doping profile along the ingot, highlighting their potential for next-generation high-efficiency photovoltaic devices.
In November, the same research group published another paper showing why antimony-doped n-type silicon ingots can achieve uniform resistivity distribution despite antimony’s very low segregation coefficient.
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