Epitaxy of 2D Materials on Gallium Nitride

by Constantin Hilbrunner
Doctoral thesis
Date of Examination:2025-10-09
Date of issue:2025-12-17
Advisor:Prof. Dr. Angela Rizzi
Referee:Prof. Dr. Angela Rizzi
Referee:Prof. Dr. Stefan Mathias
Persistent Address: http://dx.doi.org/10.53846/goediss-11722

 

 

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Abstract

English

In this thesis, the epitaxial growth of 2H-TaS$_2$ few layers onto GaN(0001) is demonstrated. A growth study is presented, in which the influence of substrate temperature and reactant fluxes is investigated via electron diffraction and scanning probe microscopy techniques. The best structural properties are found at a high growth temperature of $825^\circ$C and are attributed to the observed transition from a screw dislocation driven growth mode to layer-by-layer growth with increasing substrate temperature. From the first monolayer, the 2H-TaS$_2$ grows unstrained with an epitaxial relation of 2H-TaS$_2$ ($11\bar20$) $\parallel$ GaN ($11\bar20$). The growth conditions can be directly transfered to hexagonal boron nitride and epitaxial graphene substrates. The composition as well as the structural and electronic properties within the grown overlayers and at the interface to the GaN substrate are further studied by x-ray photoemission spectroscopy and scanning transmission electron microscopy. The latter reveals a layered growth of 2H-TaS$_2$, but also the formation of pits in the substrates, which most probably form due to the thermal decomposition of GaN at the elevated growth temperatures. However, no indication of a chemical reaction between the substrate and overlayer is found. Observed core level shifts in photoelectron spectra are related to an electron transfer from the unintentionally doped n-GaN to the 2H-TaS$_2$ and to the formation of nitrogen vacancies at the interface. With low temperature low-energy electron diffraction and temperature-dependent resistivity measurements, no clear evidence of the charge density wave transition expected in bulk TaS$_2$ below $75$ K is found. Whether this is due to the GaN induced electron doping remains to be cleared. In conclusion, these findings illustrate that GaN(0001) is a promising substrate for the MBE growth of uniform, substrate-scale transition metal dichalcogenide layers with high orientation.
Keywords: molecular beam epitaxy; tantalum disulfide; gallium nitride; x-ray photoemission spectroscopy
 
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