Paper 2D+EM+MG+NS+SE+SM+SS+TF-ThM5
Molecular Beam Epitaxy of Large area HfSe2(ZrSe₂)/MoSe₂ van der Waals Heterostructures on AlN(0001)/Si substrates

Thursday, October 22, 2015, 9:20 am, Room 212C

Two dimensional (2D) semiconductor van der Waals heterostructures (HS) made of group IVB (Zr, Hf) and group VIB (Mo, W) metal dichalcogenides are predicted [1] to have type II or type III band alignments mainly because of a large difference in their workfunctions and band gaps, which makes them candidates for novel 2D staggered, or broken gap tunneling field effect transistors (TFET). We use molecular beam epitaxy (MBE) to grow high quality large area HfSe₂ [2,3], ZrSe₂ [4] and MoSe₂ [5] films directly on AlN(0001)/Si(111) substrates. We confirm by RHEED and HRTEM that atomically thin layers (1-6 ML) are grown in single crystal form with a well-defined in-plane orientation on AlN. The films are continuous with smooth surface morphology (0.6 nm RMS roughness) and abrupt interfaces with no detectable reaction as verified by in-situ XPS and HRTEM. Micro Raman mapping for all layers confirms their structural integrity down to one monolayer and reveals very good uniformity on a cm-scale wafer and excellent stability of MoSe2 over a period of at least two weeks in air. Strong room temperature PL signal of 1 ML MoSe₂ indicate high quality direct gap semiconductor in agreement with valence band structure details imaged by our in-situ ARPES [3, 5]. In a second step, MoSe2/HfSe₂ [3] and MoSe₂/ZrSe₂ [4] HS were grown. Despite the large lattice mismatch, all layers are grown epitaxially as evidenced by RHEED with no detectable defects at the interfaces as confirmed by HRTEM suggesting good quality VdW epitaxy [6]. Using UPS the workfunctions (WF) were estimated to be 5.2, 5.5 and 5.4 eV for MoSe₂, HfSe₂ and ZrSe₂ respectively [3,4]. The last two differ substantially from theoretical values (~ 6 eV). Based on our STM and DFT calculations [3], we conclude that this difference is due to an ordered Se adlayer which lowers the HfSe₂ and ZrSe₂ WF bridging the WF gap between them and MoSe2. As a result, small valence band offsets of 0.13 and 0.58 eV were found for the HfSe₂/MoSe₂ and ZrSe₂/MoSe₂ HS, respectively leading to type II band alignments. The availability of low cost wide-gap-AlN/Si wafers in 300 mm wafer sizes defines a manufacturable route for single crystal 2D semiconductor technology.

We acknowledge financial support from ERC Advanced Grant SMARTGATE-291260. We thank IMEC for providing the AlN/Si substrates.

[1] C. Gong et al., APL. 103, 053513 (2013)

[2] R. Yue et al., ACS Nano9, 474 (2014)

[3] K. E. Aretouli et al., APL106, 143105 (2015)

[4] P. Tsipas et al., Microelectron. Eng. (2015), http://dx.doi.org/10.1016/j.mee.2015.04.113

[5] E. Xenogiannopoulou et al, Nanoscale 7, 7896 (2015)

[6] F.S. Ohuchi et al., JAP68, 2168 (1990)