Shallow Boron Emitters in Crystalline Silicon through in-Diffusion by Flash Lamp Annealing


Shallow Boron Emitters in Crystalline Silicon through in-Diffusion by Flash Lamp Annealing

Riise, H. N.; Schumann, T.; Azarov, A.; Hübner, R.; Skorupa, W.; Svensson, B. G.; Monakhov, E.

Flash Lamp Annealing (FLA) is a technique in which Si can be heated to temperatures close to and above its melting point within a few milliseconds [1, 2] and it has been shown to be suitable for annealing of implantation-induced damage [2, 3] and for activation of implanted dopants [4]. Recently, FLA was also proved to be effective in forming shallow Phosphorous (P) emitters in Si through diffusion from a P surface source deposited by spin coating [5].
In this work, it is demonstrated that shallow Boron (B) emitters can be formed in crystalline Silicon (Si) by spin coating and subsequent in-diffusion using FLA. A 300 μm Float Zone mono-crystalline Si wafer was spin-coated at 6000 rpm for 30 seconds by a polyboron spin-on diffusant (Filmtronics B155 SOD) before being processed with FLA. After heat treatment by FLA, the film was oxidized in HNO3:H2SO4 (1:1) before being removed by HF. Secondary Ion Mass Spectrometry (SIMS), sheet resistance measurements and Transmission Electron Microscopy (TEM) analysis were performed to determine the B diffusion profile, the sheet resistance and crystal quality of the samples, respectively.
Annealing for 10 and 20 ms with an energy density of 93-105 J/cm2 leads to B emitter depths of 140- 200 nm and peak B concentrations of 1-3·1020cm−3. Sheet resistance values below 200 Ω/ indicate high dopant activation. These values are well suited for e.g. emitters in crystalline Si solar cells as the shallow emitters will only absorb photons with a wavelength below 420 nm [6] and most of the available sunlight will be absorbed in the base of the cell while the low sheet resistance gives a low series resistance. High-resolution TEM images of the surface and junction regions did not show any crystal defects demonstrating that the FLA treatment does not induce high defect concentrations in the samples. TEM did however reveal a rough surface resulting from the etching treatment to remove the SOD.
Annealing for 10 and 20 ms with energy densities below 90 J/cm2 produce even shallower profiles with a maximum B extension of <100 nm while the peak concentration still remains above 1·1020cm−3 whilst the sheet resistance increases to 300-3000 Ω/. In conclusion, spin-coating with subsequent in- diffusion by FLA is thus a versatile technique with possibility to tailor the emitter depth in Si while still keeping the peak concentration high.
References
[1] H. A. Bomke, H. L. Berkowitz, M. Harmatz, S. Kronenberg, R. Lux, Applied Physics Letters 33, 955 (1978).
[2] J. T. Lue, Applied Physics Letters 36, 73 (1980).
[3] R. Klabes, et al., Physica Status Solidi A: Applications and Materials Science 66, 261 (1981).
[4] T. Ito, et al., Japanese Journal of Applied Physics, Part 1: Regular Papers, Brief Communications & Review Papers 41, 2394 (2002).
[5] H. B. Normann, et al., Applied Physics Letters 102, 132108 (2013).
[6] M. A. Green, Solar Energy Materials & Solar Cells 92, 1305 (2008).

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