Large piezoelectric responses and ultra-high carrier mobility in Janus HfGeZ₃H (Z = N, P, As) monolayers: a...

Breaking structural symmetry in two-dimensional layered Janus materials can result in enhanced new phenomena and create additional degrees of piezoelectric responses. In this study, we theoretically design a series of Janus monolayers HfGeZ₃H (Z = N, P, As) and investigate their structural characteristics, crystal stability, piezoelectric responses, electronic features, and carrier mobility using first-principles calculations. Phonon dispersion analysis confirms that HfGeZ₃H monolayers are dynamically stable and their mechanical stability is also confirmed through the Born-Huang criteria. It is demonstrated that while HfGeN₃H is a semiconductor with a large bandgap of 3.50 eV, HfGeP₃H and HfGeAs₃H monolayers have narrower bandgaps being 1.07 and 0.92 eV, respectively. When the spin-orbit coupling is included, large spin-splitting energy is found in the electronic bands of HfGeZ₃H. Janus HfGeZ₃H monolayers can be treated as piezoelectric semiconductors with the coexistence of both in-plane and out-of-plane piezoelectric responses. In particular, HfGeZ₃H monolayers exhibit ultra-high electron mobilities up to 6.40 × 10³ cm² V^-1 s^-1 (HfGeAs₃H), indicating that they have potential for various applications in nanoelectronics.