The transport and annealing properties of phosphorus-doped (Zn,Mg)O thin films grown via pulsed laser deposition
(PLD) are studied. The electron carrier concentration for (Zn,Mg)O:P films decreases with increasing deposition and Ar
annealing temperature. All the films exhibit good crystallinity with c-axis orientation. This result indicates the
importance of activation of the P dopant in (Zn,Mg)O:P films. The as-deposited ZnO:P film properties show less
dependence on the deposition growth temperatures. The resistivity of the (Zn,Mg)O:P films is significantly higher than
the ZnO:P films grown under similar conditions, indicating separation of the conduction band edge relative to the defect
donor state. The annealed ZnO:P films are n-type with resistivity dependent on annealing temperature.
Zn<sub>0.9</sub>Mg<sub>0.1</sub>O/ZnO heterostructures were grown on both sapphire and bulk ZnO substrates via pulsed laser deposition (PLD). Electron-beam deposited 100nm Au and Ti/Au (20nm/80nm) were used as the <i>p</i>-Ohmic contact and <i>n</i>-Ohmic contact, respectively. Post-annealing at above 450°C of the contacts showed improved ohmic characteristics. I-V dependences showed good rectifying diode-like behaviors with threshold voltage of 1.36V and 2.16V for the devices fabricated on sapphire and ZnO substrates, respectively. 0.01at%Al-doped n-ZnO (n ~10<sup>19</sup> cm<sup>-3</sup>) was deposited on MgO buffer layer via PLD. The electrical and optical properties strongly depend on the growth temperature, working pressure and laser energy. Room temperature photoluminescence showed band edge emission at ~377nm with very low deep level emission. The intensity of the band edge emission increased with growth temperature and deposition laser energy. Atomic force microscopy (AFM) results also showed that the root-mean-square (RMS) roughness increases with growth temperature and oxygen partial pressure. The full-width-at-half maximum (FWHM) for the ZnO (0002) peak is of 0.26-0.64°.
We achieved <i>p-</i>(Zn,Mg)O by doping with phosphorous and the conduction type was confirmed by capacitance-voltage properties of metal/insulator/<i>p-</i>(Zn,Mg)O:P diode structures as well as Hall measurements. The <i>p-</i>(Zn,Mg)O:P/n-ZnO junction was grown by pulsed laser deposition on bulk ZnO doped with Sn. Without post-growth annealing, the phosphorous-doped ZnMgO showed <i>p</i>-type conductivity (hole density ~10<sup>16</sup> cm<sup>-3</sup>, mobility ~6 cm<sup>2</sup>V<sup>-1</sup>s<sup>-1</sup>) in the as-grown state. The metal contacts in top-to-bottom <i>p-n</i> junctions were made with Ni/Au as the <i>p-</i>ohmic and Ti/Au as the backside <i>n-</i>ohmic contact. The <i>p-</i>contacts showed improved characteristics after annealing up to 350 - 400 °C, but the <i>n-</i>contacts were ohmic as-deposited. The simple, low temperature growth (≤500 °C) and processing sequence (≤400 °C) shows the promise of ZnO for applications such as low-cost UV light emitters and transparent electronics.
The room temperature transport and optical properties of phosphorus-doped ZnO and (Zn,Mg)O thin films are studied. Pulsed laser deposition (PLD) has been employed to grow epitaxial and polycrystalline layers on c-plane (0001) sapphire substrate. The ZnO:P film properties show a strong dependence on the deposition ambient at different growth temperatures. The resistivity of the samples deposited in O<sub>3</sub>/O<sub>2</sub> mixture is two orders of magnitude higher than the films grown in oxygen and O<sub>2</sub>/Ar/H<sub>2</sub> mixture. The photoluminescence (PL) spectra of the as-deposited films are composed of both the near band-edge and broadband visible emission, which peak at 3.29 and 1.87 eV, respectively. It has been shown that growing in the O<sub>2</sub>/Ar/H<sub>2</sub> mixture ambient significantly increases the band edge emission while inhibiting the visible emission. The opposite effect on the PL emissions is shown for the films grown in pure oxygen and O<sub>3</sub>/O<sub>2</sub> mixture. There is an inverse correlation between the intensity of the visible broadband emission and the carrier density. The enhanced UV emission in the films grown in O<sub>2</sub>/Ar/H<sub>2</sub> mixture may result from hydrogen passivation of the deep level emission centers. For the P-doped (Zn,Mg)O grown at 500°C, increasing the oxygen partial pressure from 20 to 200 mTorr yields a carrier type conversion from n-type to p-type without post-annealing. The films grown at 150 mTorr oxygen partial pressure are p-type and exhibit a hole concentration of 2.7 x 10<sup>16</sup> cm<sup>-3</sup>, a mobility of 8.2 cm<sup>2</sup>/Vs and a resistivity of 35 Ω-cm. All the films exhibit good crystallinity with c-axis orientation. These results indicate the importance of oxidation conditions in realizing p-type (Zn,Mg)O:P films.