Dependence of the electrical properties of the ZnO thin films grown by atomic layer epitaxy on the reactant feed sequence

Abstract

Nitrogen-doped, p-type ZnO films have been grown successfully on sapphire (0001) substrates by atomic layer epitaxy (ALE) using Zn(C2H5)2 [diethylzinc (DEZn)], H2O, and NH3 as a precursor for zinc, an oxidant, and a doping source gas, respectively. The main process parameters for the growth of p-type ZnO were the NH3 flux and the reactant feed sequence in ALE. An annealing process was performed after the ALE process. The lowest electrical resistivity of the p-type ZnO films grown by ALE was 17.9Ωcm with a hole concentration of 1.59×1017cm−3. The results indicated that the carrier concentrations and carrier mobilities in the as-grown and annealed ZnO thin films depend strongly on the NH3 pulse time and on which step of an ALE cycle the NH3 doping is conducted. For both reactant feed sequences of DEZn–NH3–H2O and H2O–NH3–DEZn, n-type ZnO was changed to p-type ZnO by annealing, but the latter led to p-type ZnO with a carrier concentration after annealing higher than that in sequence 1 for the same NH3 flow rate. In addition, a model is proposed which can explain the difference between the two feed sequences in carrier concentration and carrier mobility before and after annealing. Time-dependent Hall effect measurement results also support that the nitrogen-doped ZnO film grown by ALE and then annealed at 1000°C for 1h is a p-type semiconductor.