Jiakui Huang

The growth and investigation on Ga-doped ZnO single crystals with high thermal stability and high carrier mobility

ZnO:Ga (GZO) materials with high thermal stability and high carrier mobility are essential for the development of transparent conductive electrodes (TCE). Theoretically, only the substitution doping of Ga could result in the main donor with very high thermal stability. For achieving this goal, we adopted a hydrothermal method to acquire GZO single crystals, based on the fact that the hydrothermal method can possibly provide an approximate thermodynamic equilibrium growth environment to grow GZO materials with perfect substitution of Ga for Zn. A centimetre-sized GZO single crystal with Ga dosage amount of 0.053 wt% (2.19 × 1019 cm−3) was grown by the hydrothermal method. A precise measurement of the lattice constants reveals that the cell volume of the GZO crystal slightly shrinks by 0.09% after doping with Ga, indicating that the Ga atoms have substituted for Zn atoms, instead of existing as interstitial sites to expand the lattice parameters. A sharp X-ray rocking curve with the full width at half-maximum of the (002) reflection around 46.4 arc sec shows the high crystallinity of the GZO crystal. As expected, the as-grown GZO crystal exhibits a high RT carrier concentration (1.07 × 1019 cm−3) and the highest value of Hall mobility (81.5 cm2/(V s)) among current GZO materials. High thermal-stability is indicated by the little variation of these two parameters after being annealed at 1100 °C for 24 h. The carrier concentration is nearly independent of temperature (77–300 K).

Longitudinal optical phonon-plasmon coupled modes of degenerate Al-doped ZnO films

We have investigated the interaction between carriers and polar phonons by using Raman scattering spectroscopy in highly conductive Al-doped ZnO films grown by metalorganic chemical vapor deposition. Different from the longitudinal optical phonon-plasmon coupled modes (LOPPCM) observed in nondegenerate ZnO, an A1(LO)-like mode appears at the low frequency side of the uncoupled A1(LO) mode, and it monotonically shifts to higher frequencies and approaches to the uncoupled A1(LO) mode as Al composition increases. Based on line shape calculations, the A1(LO)-like mode is assigned to the large wave-vector LOPPCM arising from nonconserving scattering dominated by the Al impurity-induced Frohlich mechanism. Benefiting from the nonmonotonic Al composition dependence of the electron density, it is revealed that the LOPPCM depends mainly on the doping level but not the carrier concentration.