Incorporation of Al in ZnO by reactive pulsed magnetron sputtering: electrical properties and dopant activation

Al-doped ZnO (AZO) films which combine maximum carrier mobility (μe), moderate free electron densities (Ne) and high surface roughness are of special interest for application as transparent front electrode in thin film solar cells. They posses high transmission in the near infrared region, close to the bandgap energy of absorber materials like Si (Eg =1.11 eV), and enable a superior light trapping behaviour. A key to tailor AZO film properties is understanding the mechanisms and effects of the Al-dopant incorporation into the ZnO matrix. The present work focuses on investigation of the influence of Al concentration on the electrical properties of AZO and on establishing performance limits with respect to carrier mobility and resistivity (ρ). Polycrystalline and epitaxial AZO films are grown on fused silica and c-axis oriented sapphire substrates, respectively, by reactive pulsed magnetron sputtering using several sets of Zn/Al alloy targets with an Al concentration (cAl) between 0.7 and 8.7 at%. A systematic variation of process parameters such as substrate temperature (Ts) and oxygen partial pressure results in polycrystalline films with μe>45 cm2V-1s-1 and
<2.3x10-4 Ωcm at optimum conditions, whereas μe~55 cm2V-1s-1 could be obtained for epitaxial films. It is observed that cAl has a strong influence on the optimum value of Ts, the maximum μe and Ne values and also on film structure and surface roughness. The observed dependence of carrier mobility Ne in AZO is discussed in the framework of ionized impurity scattering and clustering as well as grain boundary limited transport which predicts a fundamental physical limit of μe.
Rutherford Backscattering (RBS) and elastic recoil detection analysis (ERDA) confirm a considerable Al enrichment in the films when Ts is increased above its optimum value which correlates with the deterioration of their electrical properties. Combining ion beam analysis and Hall-effect measurements allows to estimate the fraction of electrically active Al dopants, which is rarely reported in a quantitative and systematic manner. The influence of intrinsic defects and charge compensation complicate the interpretation of the results. Nevertheless the target Al concentration as well as the substrate temperature are shown to have an impact on the dopant activation.