Effect of Solid-Phase-Epitaxy Si Layers on Suppression of Sb Diffusion from Sb-Doped n þ -BaSi 2 /p þ -Si Tunnel Junction to Undoped BaSi 2 Overlayers Weijie Du 1 , Takanobu Saito 1 , Muhammad Ajmal Khan 1 , Kaoru Toko 1 , Noritaka Usami 2;3 , and Takashi Suemasu 1;3 1 Institute of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan 2 Institute of Materials Science, Tohoku University, Sendai 980-8577, Japan 3 JST–CREST, Chiyoda, Tokyo 102-0075, Japan Received September 26, 2011; revised December 12, 2011; accepted December 28, 2011; published online April 20, 2012 A new method is proposed for the growth of undoped BaSi 2 overlayers on a Sb-doped n þ -BaSi 2 /p þ -Si tunnel junction with reduced Sb diffusion. Samples with the structure of undoped-BaSi 2 /Si/Sb-doped n þ -BaSi 2 /p þ -Si were prepared; the inserted Si layer was grown by solid phase epitaxy and used to prevent Sb diffusion during the growth of undoped BaSi 2 overlayers. Secondary ion mass spectrometry measurements indicated that Sb diffusion was effectively suppressed when the growth temperature of the undoped BaSi 2 overlayers was 500  C and lower. The X-ray diffraction (XRD) rocking curves revealed that the full width at half maximum for the BaSi 2 (600) intensity increased significantly for BaSi 2 grown at 440  C, indicating that the growth temperature should be higher than this temperature. # 2012 The Japan Society of Applied Physics 1. Introduction Recently, CuInGaSe (CIGS) and CdTe thin-film solar cells have been attracting increasing attention owing to their high efficiency and low cost. Barium disilicide (BaSi 2 ) is a new semiconductor material for thin-film solar cells that has many advantages over other materials. The band gap of BaSi 2 is approximately 1.3 eV. 1–3) In addition, the replace- ment of half of the Ba atoms with isoelectric Sr atoms realizes the ideal value of approximately 1.4 eV for the band gap of BaSi 2 , which matches the solar spectrum better than crystalline Si. 4,5) Both theoretical and experimental research has revealed that BaSi 2 has a very large absorption coefficient of approximately 3  10 4 cm 1 at 1.5 eV. 2,6) We have adopted Si(111) substrates for the formation of high- quality BaSi 2 layers, on which BaSi 2 can be grown epitaxially. 7–11) Recent reports on the photoresponse proper- ties of BaSi 2 epitaxial layers on Si(111) and polycrystalline BaSi 2 layers on h111i-oriented Si films deposited on SiO 2 using an Al-induced crystallization method have shown that BaSi 2 is an interesting and useful alternative material for solar cell applications. 12–14) However, there are large conduction and valence band discontinuities at the BaSi 2 / Si heterointerface, due to the much smaller electron affinity of BaSi 2 compared with that of Si. 15) Thus, even when light is incident on the BaSi 2 /Si structure, and photoexcited carriers are generated in the BaSi 2 layer, they will be blocked at the BaSi 2 /Si interface, thereby significantly decreasing the photocurrent. For good electrical contact between BaSi 2 and Si, a heavily doped n þ /p þ junction that functions as a tunnel junction can be employed to overcome this problem. 16,17) We have previously reported the growth of undoped BaSi 2 layers on Sb-doped n þ -BaSi 2 /p þ -Si tunnel junctions by molecular beam epitaxy (MBE) at 600  C (where Sb was used as an n þ -type dopant). 18) The highest photoresponsivity ever reported in semiconducting silicide films was obtained for the undoped BaSi 2 layers on the tunnel junction. 16) Thus, the remaining process required to fabricate the solar cell is the formation of p-BaSi 2 layers on the undoped n-BaSi 2 . However, detailed examination of the Sb concentration in the undoped BaSi 2 layers revealed the diffusion of significant amounts of Sb atoms from the n þ -BaSi 2 underlayer into the undoped BaSi 2 layer, which then segregated in the surface region. The diffusion of Sb atoms into the undoped BaSi 2 layer may increase the electron concentration in the undoped BaSi 2 layers, which would lead to a reduction in photoresponsivity owing to a shorter minority-carrier diffusion length. Although Sb is not an ideal dopant, it is regarded as the most practical n-type impurity available for BaSi 2 . Therefore, a new growth method by which Sb diffusion can be suppressed is required that does not change the crystalline quality of the BaSi 2 . The surface segregation of Sb has been widely studied in Si MBE. 19–27) Sb atoms are known to diffuse into the Si overlayers of Si/Sb/Si structures formed by MBE. 26) Nakagawa et al. overcame this problem by employing solid phase epitaxy (SPE), 27) that is, the deposition of amorphous SiGe layers onto the Sb-adsorbed SiGe layer with sub- sequent annealing to transform the amorphous layers into the crystalline phase. However, there has been no report on a growth method to prevent Sb segregation in semiconducting silicides such as BaSi 2 and -FeSi 2 . In this study, we have developed a new growth method that involves the insertion of intermediate Si layers by SPE between the undoped BaSi 2 and n þ -BaSi 2 layers in the n þ - BaSi 2 /p þ -Si tunnel junction structure. The effect of this SPE Si layer on the suppression of Sb diffusion into the undoped BaSi 2 overlayers was investigated. 2. Experimental Procedure An ion-pumped MBE system equipped with standard Knudsen cells for Ba and Sb and an electron-beam evaporation source for Si was used. After cleaning the p þ - Si(111) substrates at 850  C for 30 min in ultrahigh vacuum, an approximately 1-nm-thick BaSi 2 template layer was grown by Ba deposition on Si substrates at 550  C for 1 min (reactive deposition epitaxy: RDE), which was used to control the crystal orientation of the overlayers. An approximately 25-nm-thick Sb-doped n þ -BaSi 2 layer was then grown by codeposition of Si, Ba, and Sb at 500  C for 10 min. 12) An undoped BaSi 2 layer more than 300 nm thick was then grown by MBE. 24–27) The preparation of samples A–C is summarized in Table I. Figure 1(a) shows a schematic of the growth processes used to prepare samples A–C. Samples D–I were prepared as shown in Fig. 1(b). After the growth of the 25-nm-thick Sb doped n þ -BaSi 2  E-mail address: suemasu@bk.tsukuba.ac.jp Japanese Journal of Applied Physics 51 (2012) 04DP01 04DP01-1 # 2012 The Japan Society of Applied Physics REGULAR PAPER DOI: 10.1143/JJAP.51.04DP01