Epitaxial growth of silicon thin films by low temperature RF-PECVD from SiF4/H2/Ar chemistry

Ronan Léala,b,c, Jean-Christophe Dornstettera,b, Farah Haddada, Gilles Poulainb, Pere Roca i Cabarrocasa,c

a LPICM, CNRS, Ecole Polytechnique, 91128 Palaiseau Cedex, France b TOTAL – New Energies, 24 cours Michelet, 92078 Paris La Défense

c IPVF (Institut Photovoltaïque d’Ile-de-France) - 8 rue de la renaissance – F-92160 Antony – France

Motivation

Intrinsic epitaxy

Doped epitaxy

Diffused emitter drawbacks: • Difficult to achieve a sharp junction • Too highly doped regions → Auger recombination • Additional step of PSG/BSG removal by wet etching Suggested solution: Epitaxial emitter • Good doping control (by PH3/B2H6 flow rate) • Better VOC and reduction of J0 expected [1],[2] by doping

profile optimization

Advantages of low temperature epitaxy by PECVD: • Low dopants diffusion (200°C process) → sharp junction • Low thermal stress • Easy scale-up and integration in industry

References : [1] T. Rachow et al., Potential and limitations of epitaxial emitters, 28th EUPVSEC, 2013 [2] B. Hekmatshoar et al., Characterization of thin epitaxial emitters for high-efficiency silicon heterojunction solar cells, APL, 2012 [3] M. Labrune et al., Ultra-shallow junctions formed by quasi-epitaxial growth of boron and phosphorous-doped silicon films at 175 oC by RF-PECVD, Thin solid films, 2010 [4] R. Cariou et al., Silicon epitaxy below 200°C: Towards thin crystalline solar cells, Proceeding SPIE Optics and Photonics, 2012 [5] J.-C. Dornstetter, Understanding the amorphous-to-microcrystalline silicon transition in SiF4/H2/Ar gas mixtures, The Journal of Chemical Physics, 2014 [6] A. Abramov and P. Roca i Cabarrocas: “Addition of SiF4 to standard SiH4+H2 plasma: an effective way to reduce oxygen contamination in µc-Si:H films”. Phys. Stat. Sol., 2010

[7] Y. Djeridane, A. Abramov and P. Roca i Cabarrocas: “Silane versus silicon tetrafluoride in the growth of microcrystalline silicon films by standard radio frequency glow discharge”. Thin Solid Films , 2008 [8] J.-C. Dornstetter et al., Microcrystalline Silicon Deposited from SiF4/H2/Ar Gas Mixtures: Material Properties and Growth Mechanisms Studies, ICANS, 2013 [9] J.-C. Dornstetter et al., Material and growth mechanism studies of microcrystalline silicon deposited from SiF4/H2/Ar gas mixtures, Canadian Journal of Physics, 2014 [10] M. Moreno, Fine-tuning of the interface in high-quality epitaxial silicon films deposited by plasma-enhanced chemical vapor deposition at 200 °C, MRS Review, 2013 [11] Masetti et al., Modeling of carrier mobility against carrier concentration in Arsenic-, Phosphorus-, and Boron-doped Silicon, Electron Devices, IEEE Transactions, 1983

Thick high quality epitaxial layers achieved

wafer

epi

1,5 2,0 2,5 3,0 3,5 4,0 4,5-505

101520253035404550

ε i

E (eV)

Sample B c-Si with 5A thick roughness

2,5µm thick epitaxy with a very smooth interface has been achieved and diffraction patterns show no differences.

wafer

epi

Test cell architecture to assess electrical properties of epitaxy. Similar architecture reached 14,2% efficiency with SiH4/H2/B2H6 chemistry

1,0 1,5 2,0 2,5 3,055

60

65

70

75

80

85

uc-Si on glass/poly,uc-Si on wafer

H2 d

eple

tion

(%)

H2 flow rate (sccm)

a-Si on glass (required for epitaxy)

𝑆𝑆𝑆4 ⇔ 𝑆𝑆𝑆3 + 𝑆

𝐻2 + 𝑆 ⇔ 𝐻𝑆 + 𝐻

Process window for epitaxy

H2+PH3=3sccm (40W, 2,5T) H2+PH3=4sccm (60W, 3T)

n doping of epitaxial layers p doping of epitaxial layers

→ Strong effect of PH3 on epitaxy even for low concentration (0,1%)

→ Lower effect of B2H6 on epitaxy even for higher concentration (1%)

Optimized doping profile

Diffused emitter doping profile SiF4/H2/Ar plasma chemistry advantages: • Better understanding based on a phenomenological model [5] • Lower amount of oxygen incorporated in the layers [6] • Better crystallinity and lower defects density expected [7],[8],[9] • Very smooth interface between the wafer and the epitaxial layer [10]

Source diffused emitter profile: Iftiquar et al., 2012.

The plasma chemistry is led by two reactions:

Solar cell and perspectives

B2H6 flow rate 1sccm 3sccm Doping concentration

(cm-3) 1016 1019

Mobility μ (cm2.V-1.s-1) 280 40 Theoretical μ

(cm2.V-1.s-1) [11] 425 73

Hall effect measurement

PP-TOFMS measurement for B2H6 =3sccm

Authors would like to thanks Agnès Tempez from HORIBA for using their Plasma Profiling Time of Flight Mass Spectrometer

Next steps: • Manufacturing of solar cells

• n-type doped epitaxy with

higher dilution of PH3 gas bottle

• XRD study of doped epitaxial thin films

• Improvement of uniformity

PH3↓

Crystallinity and uniformity assessment h=(Max –Min)/2𝐦�

epi-layers doping>2.1019

bulk (n-type wafer)