Growth morphology and electrical/optical properties of Al-doped ZnO thin films grown by atomic layer deposition J. Vac. Sci. Technol. A 30, 021202 (2012); 10.1116/1.3687939 Structural, electrical, and optical properties of atomic layer deposition Al-doped ZnO films

We report on the homoepitaxial growth of ZnO thin films by chemical vapor deposition techniques. The preparation of the ZnO substrates after mechanical polishing employed a high temperature annealing step which produced atomically flat surfaces and removed all of the surface and subsurface damage. Two dimensional epitaxial growth was achieved without an additional buffer layer. The substrate had a rocking curve with a full width at half maximum of 27″ which can be compared with that of the film of 17″. The films had superior band edge luminescence as compared with the substrate for which the green luminescence band is dominating. The impurity content in the substrates especially Li is reduced by the high temperature annealing step and drops further close to the detection limit in the films. The low substrate temperatures around 660 °C allow for the incorporation of nitrogen on oxygen site as a shallow acceptor.

Two series of SnO thin films, one doped with N and one doped with H, were deposited on c-plane sapphire by reactive ion beam sputter deposition starting from growth parameters optimized for stoichiometric SnO. The amounts of dopants incorporated into the SnO:H and SnO:N samples were quantified by secondary ion mass spectroscopy. The influence on the structural and electrical properties of SnO thin films was studied as a function of dopant concentration. In the case of N doping, all N incorporated, probably as NO, are active as the acceptor and exhibit long-term stability. We assign an acceptor activation energy of 100 to 150 meV to NO. However, we observe a change in the film morphology at a critical N concentration of about 7⋅1017cm−3, which deteriorates the structural properties of the films. In the case of SnO:H, the situation is different. We observe an outdiffusion of H after growth, i.e., the samples are not stable in the long term. Nevertheless, all H incorporated up to a H-content of 1019cm−3 seem to be electrically active and exhibit an activation energy between 150 and 250 meV, likely corresponding to Hi. Furthermore, at H contents above 1019cm−3, we observe molecular H2 inside the SnO:H thin films. We conclude that N doping of SnO is better suited for tuning the p-type conductivity of SnO. However, it will be essential to overcome the morphology change observed at the critical N concentration to fully explore the tunability of the p-type conductivity in device applications.

As-doping of zinc oxide has been approached by ion implantation and chemical vapor deposition. The effect of thermal annealing on the implanted samples has been investigated by using secondary ion mass spectrometry and Rutherford backscattering∕channeling geometry. The crystal damage, the distribution of the arsenic, the diffusion of impurities, and the formation of secondary phases is discussed. For the thin films grown by vapor deposition, the composition has been determined with regard to the growth parameters. The bonding state of arsenic was investigated for both series of samples using x-ray photoelectron spectroscopy.

For the homoepitaxial growth of ZnO it is inevitable to obtain a regular crystalline single crystal surface prior to growth. Commercially available, hydrothermally grown ZnO single crystals show amorphous surfaces due to mechanical cutting and polishing. Here we present the results of a thermal treatment on these ZnO single crystals. After annealing, a regular crystalline oxygen terminated surface can be obtained. Changes in surface roughness, residual defect concentration and electrical properties can be shown. The bulk crystallinity though was not affected.