Fabrication of Low B-doped P-Basi 2 /n + -Si Heterojunction Solar Cells

[Introduction] Semiconducting BaSi2 has attracted attention as a future absorber-layer material for thin-film solar cells. It has an indirect band gap of approximately 1.3 eV, matching the solar spectrum, and has large absorption coefficients, reaching 3.0 ́10 cm at 1.5 eV [1,2]. We have successfully fabricated n-Si/Bdoped p-BaSi2 heterojunction solar cells that achieved a conversion efficiency η of 9.9% [3]. In the work mentioned, B-doped p-BaSi2 (p = 2.2 × 10 cm) with an optimum thickness of 20 nm acts as a hole transport layer [3]. The deterioration of η in thicker p-BaSi2 layers is suspected due to the small minority-carrier lifetime τ of p-BaSi2. In previous work, we confirmed that τ strongly depends on the hole concentration p of p-BaSi2. We measured that low doped p-BaSi2 with p=1.4 ́ 10 cm has a τ of 2.0 μs, two orders higher than sample with p=3.9 ́10 cm [4]. In order to utilize B-doped pBaSi2 as an active layer, we need to employ n-Si with lower resistivity (higher electron concentration n), so that the depletion region stretches toward the p-BaSi2 layer, and that the device has a sufficient built-in potential at the junction. In this work, we fabricated 300-nm-thick lowdoped p-BaSi2 on the Si substrates with various resistivities and examined the electrical properties using J-V characteristics. We then evaluated the result from the crystallinity point of view. [Experiment] Briefly, a 5-nm-thick template layer was grown by Ba deposition on a hot n-Si(111) substrates (Tsub = 500°C, TBa = 543°C) with resistivity ρ varied from 0.01-0.1 to 0.1-1.0 and 1.0-4.0 Wcm. Next, Ba, Si, and B were coevaporated to form approximately 300-nm-thick a-axis-oriented B-doped p-BaSi2 epitaxial films by MBE (Tsub = 600°C, TBa = 569°C, RSi = 2.0 Å/s). The boron k-cell temperature TB was set to 1000°C which correspond to a p of 1.4 × 10 cm. Finally, the samples were capped with a 5 nmthick a-Si as a passivation layer. The crystallinity of BaSi2 was investigated by RHEED and q-2q X-ray diffraction (XRD). The current density versus voltage (J-V) curves were measured under standard AM1.5, 100 mW/cm illumination at 25°C. We also measured the FWHM of the X-ray rocking curve using BaSi2(600) diffraction. [Results and discussion] Figure 1 shows the J-V characteristic of sample with different substrate ρ values. In Fig. 1, we see that the rectifying characteristics in the J-V curves gradually disappears as ρ decreases. The J-V characteristic shows a clear ohmic-like behavior when the substrate ρ is less than 0.1 Wcm even though the depletion region in the p layer are calculated to be less than 200 nm, thinner than the pBaSi2 layer thickness. The FWHM values obtained from an w-scan xray rocking curve using a BaSi2(600) diffraction peak of sample with different substrate ρ values are plotted in Fig. 2. In Fig. 2, the degree of preferred aaxis orientation degraded as we decreased the substrate ρ. The decrease of the crystalline quality is probably one possible driving forces behind the degradation of rectifying characteristics in J-V curves. We plan to conduct TEM observations to see what happens around the p-BaSi2/n-Si interfaces to gain further information. [Acknowledgments] This work was financially supported in part by JST-CREST and JSPS (15H02237).