Optical and electrical properties of epitaxial (Mg,Cd)_xZn_(1-x)O, ZnO, and ZnO:(Ga,Al) thin films on c-plane sapphire grown by pulsed laser deposition

摘要

A consistent set of epitaxial, n-type conducting ZnO thin films, nominally undoped, doped with Ga or Al, or alloyed with Mg or Cd, was grown by pulsed laser deposition (PLD) on single-crystalline c-plane sapphire (0001) substrates, and characterized by Hall measurement, and UV/VIS optical transmission spectroscopy. The optical band gap of undoped ZnO films at nearly 3.28 eV was shifted by alloying with Mg up to 4.5 eV and by alloying with Cd down to 3.18 eV, dependent on the alloy composition. In addition, highly doped ZnO:Al films show a blue-shifted optical absorption edge due to filling of electronic states in the conduction band. The Hall transport data of the PLD (Mg,Zn,Cd)O:(Ga,Al) thin films span a carrier concentration range of six orders of magnitude from 3 x 10~(14) to 3 x 10~(20) cm~(-3), which corresponds to a resistivity from 5 x 10~(-4) to 3 x 10~3 Ω cm. Structurally optimized, nominally undoped ZnO films grown with ZnO nucleation and top layer reached an electron mobility of 155 cm~3/V s (300 K), which is among the largest values reported for heteroepitaxial ZnO thin films so far. Finally, we succeeded in combining the low resistivity of ZnO:Ga and the band gap shift of MgZnO in MgZnO:Ga thin films. This results demonstrate the unique tunability of the optical and electrical properties of the ZnO-based wideband gap material for future electronic devices.