Tuning the electrical properties of the p-type transparent conducting oxide Cu1−xCr1+xO2 by controlled annealing

Abstract

Off-stoichiometric copper chromium oxide delafossite received lately a great interest due to its high p-type electrical conductivity and adequate optical transmittance in the visible range. However, for a suitable integration in active devices such as p-n junctions, transistors or optoelectronic devices, the electronic properties must be efficiently tailored. Here, post-deposition thermal treatment is proven as an adequate approach for finely controlling the electrical properties of this former degenerate semiconducting material. The energetics of the annealing process are investigated using two different approaches, as a function of the annealing temperature and as a function of the annealing time, allowing the accurate determination of the activation energy of the annealing of defects. By using this method, the electrical carrier concentration was varied in the 1021 – 1017 cm−3 range while the recorded changes in the drift mobility covered three orders of magnitude. Moreover, we demonstrate the ability to accurately manipulate the Fermi level of such materials, which is of great importance in controlling the carrier injection and extraction in optoelectronic active layers.

Transparent p-Cu0.66Cr1.33O2/n-ZnO heterojunction prepared in a five-step scalable process

“…Films present high electrical conductivities, that leads to an ohmic behaviour of the junction, showing a non-rectifying response as later described. The annealing effects on the delafossite have been reported in our previous work [17]. A decrease of the carrier concentration for temperatures above 650 °C was observed.…”

“…This model explains thus the higher holes mobility (above 1 cm 2 V −1 s −1 ) achieved in this type of materials when comparing with other TCOs [18]. Among copper delafossites, CuCrO 2 presents a high density of states of 3d cations (Cr 3+ ) near the valence band maximum allowing thus the covalent mixing between the chromium and oxygen atoms and a good dopability [17]. The interest in these particular materials has grown since Nagarajan et al [19] reported the highest conductivity (220 S cm −1 ) of delafossite compounds for Mg doped CuCrO 2 whilst they measured an optical transparency of 30% in the case of 270 nm thin films.…”

“…By using annealing temperatures within 650-900 °C range the carrier concentration was diminished from 10 21 to 10 17 cm −3 while the electrical conductivity and drift mobility were varied few orders of magnitude. More important, the manipulation of Fermi level and the improvement of optical transmittance from 38% to values above 60% were reported [17].…”

“…With the controlled tunability of carrier concentration and good increase of optical transmittance over the annealing processes [17] this particular material might became a viable option for the introduction and study of a transparent p-n junction [18]. As-deposited films present high carrier concentration, up to 10 22 cm −3 , attributed to chains of missing copper [19].…”

Crystallographic Characterisation of Ultra-Thin, or Amorphous Transparent Conducting Oxides—The Case for Raman Spectroscopy

“…Alternative methods, with larger scattering cross-section and sensitivity to local bond structures, rather than long-range order, are hence sought for. Particularly for recently developed amorphous or non-crystalline p-type TCOs gaining additional information on the local bond structure will be helpful to gain insights on the doping mechanisms [21][22][23][24][25][26][27].…”