In this article the doping of the oxidic compound semiconductor ZnO is reviewed with special emphasis on n-type doping. ZnO naturally exhibits n-type conductivity, which is used in the application of highly doped n-type ZnO as a transparent electrode, for instance in thin film solar cells. For prospective application of ZnO in other electronic devices (LEDs, UV photodetectors or power devices) p-type doping is required, which has been reported only minimally. Highly n-type doped ZnO can be prepared by doping with the group IIIB elements B, Al, Ga, and In, which act as shallow donors according to the simple hydrogen-like substitutional donor model of Bethe (1942 Theory of the Boundary Layer of Crystal Rectifiers (Boston, MA: MIT Rad Lab.)). Group IIIA elements (Sc, Y, La etc) are also known to act as shallow donors in ZnO, similarly explainable by the shallow donor model of Bethe. Some reports showed that even group IVA (Ti, Zr, Hf) and IVB (Si, Ge) elements can be used to prepare highly doped ZnO films—which, however, can no longer be explained by the simple hydrogen-like substitutional donor model. More probably, these elements form defect complexes that act as shallow donors in ZnO. On the other hand, group V elements on oxygen lattice sites (N, P, As, and Sb), which were viewed for a long time as typical shallow acceptors, behave instead as deep acceptors, preventing high hole concentrations in ZnO at room temperature. Also, ‘self’-compensation, i.e. the formation of a large number of intrinsic donors at high acceptor concentrations seems to counteract the p-type doping of ZnO. At donor concentrations above about 10 ²⁰ cm ⁻³ , the electrical activation of the dopant elements is often less than 100%, especially in polycrystalline thin films. Reasons for the electrical deactivation of the dopant atoms are (i) the formation of dopant-defect complexes, (ii) the compensation of the electrons by acceptors (O _i , V _Zn ) or (iii) the formation of secondary phases, for instance Al ₂ O ₃ , Ga ₂ O ₃ etc. The strong influence of the different deposition methods and annealing conditions on the doping of ZnO is discussed. This review shows that, though it is one of the best-investigated oxide compound semiconductors over many decades, understanding of the details of the doping properties and mechanisms of zinc oxide is still in its infancy. Based on this review, prospective research opportunities are devised.

Photoactive bismuth vanadate (BiVO₄) thin films were deposited by reactive co-magnetron sputtering from metallic Bi and V targets. The effects of the V-to-Bi ratio, molybdenum doping and post-annealing on the crystallographic and photoelectrochemical (PEC) properties of the BiVO₄ films were investigated. Phase-pure monoclinic BiVO₄ films, which are more photoactive than the tetragonal BiVO₄ phase, were obtained under slightly vanadium-rich conditions. After annealing of the Mo-doped BiVO₄ films, the photocurrent increased 2.6 times compared to undoped films. After optimization of the BiVO₄ film thickness, the photocurrent densities (without a catalyst or a blocking layer or a hole scavenger) exceeded 1.2 mA/cm² at a potential of 1.23 V_RHE under solar AM1.5 irradiation. The surprisingly high injection efficiency of holes into the electrolyte is attributed to the highly porous film morphology. This co-magnetron sputtering preparation route for photoactive BiVO₄ films opens new possibilities for the fabrication of large-scale devices for water splitting.

Highly (001)-textured, photoactive WSe₂ thin films have been prepared by an amorphous solid-liquid-crystalline solid process promoted by palladium. By increasing the thickness of the Pd promoter film (≥10 nm) the structure and texture of the WSe₂ films can be improved significantly. However, these as-crystallized WSe₂ films are only weakly photoactive in a 0.5 М H₂SO₄ electrolyte under AM 1.5 solar irradiation which we attribute to an inefficient photogenerated charge transfer across the WSe₂/electrolyte interface via the prevailing van der Waals planes of the WSe₂ crystallites. In this work photochemically deposited platinum on the p-type WSe₂ photocathode is used for an efficient electron transfer thus inducing the hydrogen evolution reaction. Upon illuminating the WSe₂ photocathodes in a Pt-ion containing electrolyte, the photogenerated electrons reduce Pt⁺ to Pt leading to the precipitation of Pt islands, preferentially at edge steps of the WSe₂, i.e. at the grain boundaries of the WSe₂ crystallites. The increasing amount of Pt islands at the grain boundaries linearly enhances the photocurrent density up to 2.5 mA cm⁻² at 0 V_RHE in sulfuric acid, the highest reported value up to now for WSe₂ thin films.

The efficiencies of transition metal dichalcogenide thin films and single crystals (MoS ₂ , WS ₂ , MoSe ₂ , WSe ₂ ) are evaluated for photoelectrochemical hydrogen evolution.