A new technique of the simultaneous excitation of a magnetron sputtering discharge by rf and dc was used for the deposition of undoped ZnO- and Al-doped ZnO (ZnO:Al) films. By this technique, it was possible to change the ion-to-neutral ratio ji/jn on the substrates during the film growth by more than a factor of ten, which was revealed by plasma monitor and Langmuir probe measurements. While for a pure dc discharge the ions impinging onto a floating substrate have energies of about Ei≈17 eV, the rf discharge is characterized by Ar-ion energies of about 35 eV. Furthermore, the ion current density for the rf excitation is higher by a factor of about five, which is caused by the higher plasma density in front of the substrate. This leads to a much higher ion-to-neutral ratio ji/jn on the growing film in the case of the rf discharge, which strongly influences the structural and electrical properties of the ZnO(:Al) films. The rf-grown films exhibit about the three times lower specific resistances (ρ≈6×10−4 Ω cm), due to lower mechanical stress, leading to higher charge carrier concentrations and mobilities. Undoped ZnO films exhibited the largest compressive stress values up to 2.8 GPa. The aluminium-doped films have a better (001) texture and larger grains (dg≈38 nm), which can be attributed to the beneficial role of Al as a surfactant. The better crystalline film quality of the ZnO:Al films is the reason for the much lower compressive stress of <0.5 GPa in these layers.

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Doped and undoped zinc oxide fims are widely used as transparent window and contact layers in thin film solar cells. Up to now the role of film stress on the layer and solar cell properties was not investigated in detail. In this paper the influence of the energy input from the magnetron sputtering plasma on the structural and electrical layer properties was investigated. The energy input during the deposition could be varied by using a simultaneous RF-and DC-excitation at a fixed sputtering pressure. The energy and the density of the ions bombarding the substrate at floating potential have been measured by a plasma monitor and by a Langmuir probe. Plasma monitor measurements displayed the different plasma potentials and ion energies at a floating substrate. While the DC-discharge is characterized by low plasma potentials and low ion energies, the RF-mode shows up to 2.5 times higher ion energies of EAr+ = 50 eV.The structural and electrical properties of doped and undoped ZnO-films, deposited with different RF/DC-power ratios can be explained by taking into account the different ion energies and densities during the deposition. Higher ion energies (RF-excitation) improve the structural quality and reduce the film stress, which decreases the specific resistance, both by a higher charge carrier mobility as well as higher carrier concentrations. The lowest resistivities and film stresses were = 6.10-4 Qcm and 0.5 GPa, respectively. Layers with different film stresses have been used to prepare CuInS2/CdS/ZnO-solar cells. Lower stress in the ZnO-films leads to higher cell efficiencies. For the first time, these results can explain why RF-sputtering is almost exclusively used for the window preparation of thin film solar cells.

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