Atomic scale study of
C
u2
O
/
Z
n
O
heterojunction interfaces by
TEM
,
STEM
and
DFT
Authors: Gorantla Sandeep , Gan Jiantuo , Løvvik Ole Martin , Diplas Spyros , Bergum Kristin , Svensson Bengt , Monakhov Edouard , Nguyen Phuong , Gunnaes Anette 
Published: December 2016

Cuprous oxide (Cu
2
O) is a low‐cost, nontoxic semiconductor interesting for photovoltaic applications. Together with ZnO it forms a p‐n junction diode. With a theoretically estimated conversion efficiency of 18 %
[1]
it is envisaged as a potential material system for the next generation thin‐film based solar cells. However, experimentally reported efficiencies are only 2‐5 %. In order to better understand the microstructural factors that limit their practical achievable efficiency, Cu
2
O/ZnO model systems were grown and studied by conventional TEM and C
s
probe corrected scanning TEM (STEM) methods and interfaces evaluated using Density Functional Theory (DFT). For the model systems, c‐axis single crystal ZnO substrates (both Zn‐ and O polar) were used and the Cu
2
O films were grown by both ceramic and reactive RF magnetron sputtering methods
[2]
. Cross section specimens were prepared in order to study the films and Cu
2
O/ZnO heterojunction interfaces.

Regardless of the polarity, in reactive sputtering case, the (111) [1‐10] Cu
2
O || (001) [110] ZnO orientation relation was observed consistent with an epitaxial relationship across the Cu
2
O\ZnO interface. However, only in O‐polar ceramic sputtering case, additional (110) Cu
2
O || (001) ZnO orientation relationship was also observed. The STEM investigations revealed the presence of an unexpected ~ 5 nm thick polycrystalline interfacial CuO layer between the ZnO substrate and Cu
2
O film, as shown in figure 1. The presence of CuO is unexpected because the deposition conditions were optimized for the Cu
2
O film growth. Repeated growth experiments have, however, confirmed the findings. Interestingly, despite the presence of this interfacial layer, epitaxial twinning of the Cu
2
O films was observed in all the specimens except for the case with O‐polar ZnO, ceramic grown Cu
2
O film. Also notable, the films grown by reactive sputtering were more dense relative to films grown by ceramic sputtering
[2]
.The CuO grains in the interfacial layer were observed to have certain preferred orientations with respect to the substrate i.e. textured growth. Geometric Phase Analysis evaluation of the CuO/ZnO STEM images clearly showed an array of misfit dislocations along their interface reducing the strain in the epitaxial CuO film, as shown in figure 2. DFT calculations were carried out based on the experimentally observed Cu
2
O/CuO/ZnO interfaces. It was found that the formation of CuO on ZnO and Cu
2
O on CuO is energetically favorable compared to the Cu
2
O/ZnO interface even when dislocations are not taken into consideration on the CuO/ZnO interface. The total lattice strain of (111) Cu2O/ (100) CuO/ (001) ZnO is lower than the calculated lattice strain between (111) Cu
2
O and (001) ZnO. It can be concluded that the driving force for formation of the ~ 5 nm thick CuO interface layer is strain induced. Such a CuO interfacial layer is detrimental for Cu
2
O/ZnO p‐n heterojunction diode efficiency and its existence can explain the large gap between the experimental and theoretical reported conversion efficiencies.