Zinc Oxide Grown by ALD - from Heavily $n$-type to $p$-type Material

Because of a wide band gap, high exciton binding energy and feasibility to form a wide range of nanostructures, ZnO is perspective material for many applications. Polycrystalline ZnO films are of considerable importance for hybrid organic/inorganic electronics and 3D memory devices, that posted severe restrictions on deposition temperature, which is required not to exceed 150°C. Conductivity control is crucial for most of these applications. Atomic Layer Deposition provides an unique opportunity to tune electron concentration in ZnO in a wide range, between 10
¹⁶
and 10
²⁰
cm
^-3
, by varying the growth conditions from O-rich to Zn-rich. This can be realized by changing the deposition temperature between 100 and 200°C. Electrical, structural and optical properties of ZnO films can be modified by post-growth annealing, however also in the annealed ZnO films growth temperature clearly influences the binding energy of existing donors as well as the relative intensity of donor- to acceptor-related luminescence. Scaling of electron concentration with the growth temperature is accompanied by a variance of the bandgap because of the Burstein-Moss effect. Hydrogen seems to play an ambiguous role in conductivity of ZnO grown at low temperature limit. High amount of hydrogen (~10
²⁰
cm
^-3
) is observed in as grown ZnO films. It decreases by 2 orders of magnitude after annealing, but both in as grown and annealed samples, the hydrogen content is higher than free electron concentration, which point out that hydrogen does not always plays a role of donor in this material. Acceptor conductivity can be achieved by nitrogen doping under O-rich conditions. The ZnO-ALD-based homojuction with rectification ratio of 10
⁶
(at ± 2V) has been obtained based on ZnO-ALD films deposited at 100°C.