ZnO, as a Ⅱ-Ⅵ semiconductor, is one of the promising transparent conducting oxides for optoelectronic applications such as LED and LD due to its wide band gap(3.4 eV) and large exciton binding energy(~60 meV).
Undoped ZnO generally exhibits n-type conductivity due to intrinsic point defect such as oxygen vacancy(VO) and Zinc interstitial(Zni). So it is difficult to make p-type ZnO films due to self-compensating effect, low solubility of acceptor dopants and deep acceptor level. In order to develop ZnO-based optoelectronic applications, both n-type and p-type conduction is necessary.
In many cases of acceptor doping mechanism, A model for large-sized-mismatched group-Ⅴ dopant such as As and Sb is believed to be the most possible acceptor in ZnO. It suggested that Sb would substitute for Zn instead of oxygen and then produce two corresponding Zn vacancy, which is a SbZn-2VZn cluster.
Also, The co-doping of donor and acceptor contribute to enhance the solubility of acceptor and lower the energy level of the acceptor and increase that of the donor due to the strong interaction between donors and acceptor with forming an acceptor-donor-acceptor cluster.
In this study ZnO:Al0.01Sb0.02 thin films were deposited on c-plane sapphire substrates by PLD at 500℃ and at various oxygen partial pressure in the range of 1-500 mTorr. The structural and electrical properties of the thin films were measured by XRD, FE-SEM, AFM, Hall effect measurement, XPS and UPS.
ZnO:Al0.01Sb0.02 thin films deposited at low oxygen partial pressure (1-60 mTorr) were grown with c-axis preferred orientation and 30°in-plane rotated orientation. At higher oxygen partial pressure, ZnO:Al0.01Sb0.02 thin films mainly had a c-axis oriented domain and partially had domain with 32° tilted and 90° tilted c-axis.
ZnO:Al0.01Sb0.02 thin films deposited at low oxygen partial pressure (1-60 mTorr) exhibited n-type conduction, while the films deposited high oxygen partial pressure(100-500 mTorr) had p-type conduction. The XPS results showed p-type thin films had more Sb5+ than Sb3+. It is estimated that p-type conduction is contributed to Sb5+ ion which forms SbZn-2VZn complex.