Effects of Ar vs. O2 ambient on pulsed-laser-deposited Ga-doped ZnO

Ga-doped ZnO films were deposited by pulsed laser deposition (PLD) at 200 °C and 10 mTorr in either pure argon (Ar films) or in oxygen (O2 films). The bulk resistivity of the Ar films is <2×10−4 Ω cm at 300 K, two orders of magnitude lower than that of the O2 films. In the Ar films, the donor concentration ND as determined by a detailed Hall-effect analysis is close to 100% of the total Ga concentration [Ga] measured by secondary ion mass spectrometry (SIMS), while in the O2 films ND is less than 50% of [Ga]. Furthermore, the compensation ratio K=NA/ND is >90% for the O2 films and <60% for the Ar films. Yet, when the oxygen pressure is reduced to 0.2 mTorr, the O2 films have resistivities of about 5×10−4 Ω cm, approaching those of the Ar films. These results suggest that oxygen-rich environments produce Ga/O complexes that reduce the dopant activation efficiency and thus decrease ND and increase K. Some of these complexes may also contribute to the increase in deep centers observed in photoluminescence.