Paper TF+AS-WeA8
Epitaxial Growth of Zirconium Diboride Thin Film on Ge(111) Wafer

Wednesday, October 31, 2012, 4:20 pm, Room 11

GaN-based semiconductors are widely used in optoelectronic devices. To grow these films, substrates such as sapphire, SiC and Si are used. However, recently, Lieten et al. [1] have proposed to grow GaN on Ge(111) substrate to have a more closely matching thermal expansion coefficient and to decrease the lattice mismatch. Despite the good quality of the film, misoriented domain and voids can be found in it. While the domains can be suppressed, the voids cannot, as they come from a diffusion of Ge atoms in the film. This is a problem for the growth of p-type or semi-insulating GaN layers.

Using ZrB₂ as a buffer layer on Ge substrate could help by providing a diffusion barrier. Moreover, ZrB₂ substrate has already been used as a conductive growth template for GaN and has proven to be interesting thanks to the low lattice mismatch and close in-plane thermal expansion coefficient [2] . Therefore, ZrB₂ has been used as a buffer layer for the growth of GaN films on Si wafer [3] and those films were shown to be promising.

It was also demonstrated that on top of ZrB₂(0001) thin film on Si(111) substrate, silicene, which has a similar structure to graphene was present [4]. In the periodic table, C, Si and Ge are in the same column. Therefore, we can envisage the possibility of the formation of germanene in the same manner as silicene on top of the ZrB₂ layer grown on germanium substrate.

Here, we report on the epitaxial growth of ZrB₂ thin films on Ge(111) by thermal decomposition of Zr(BH₄)₄ in a dedicated UHV-chemical vapour deposition system. The growth was monitored in situ by RHEED, and the samples were further analysed by XRD and TEM. The film grows with epitaxial relationship of ZrB₂(0001)//Ge(111). Under slow growth conditions (substrate temperature, Ts=750˚C), two types of in-plane orientations, which are rotated by 30˚ can be observed, while under faster growth condition (Ts=650˚C), the layer is monocrystalline. The single-crystalline film has in-plane orientation of ZrB₂[11-20]//Ge[-110], similar to the case of single-crystalline ZrB₂ film on Si(111) [4], but with a different s urface reconstruction of (√3x√3) when cooled down under 450˚C. There is a good epitaxy between the layer and the substrate with the presence of a second phase at the interface, which tends to disappear when the growth was carried out at 550˚C.

[1] R.R. Lieten et al., J. Cryst. Growth, 314, 71 (2011).

[2] H. Kinoshita et al., Jpn. J. Appl. Phys., 42, 2260 (2003).

[3] Y. Yamada-Takamura et al., Phys. Rev. Lett., 95, 266105 (2005).

[4] A. Fleurence et al., Phys. Rev. Lett., accepted for publication.