Structural, optical, and electronic stability of copper sulfide thin films grown by atomic layer deposition

Copper sulfide films of nanometer thickness are grown by atomic layer deposition (ALD) and their structural and optoelectronic properties investigated as a function of time and storage environment. At temperatures as low as 80 °C polycrystalline thin films are synthesized, which index to the stoichiometric (Cu₂S) chalcocite phase. As-prepared and prior to exposure to room ambient, conductive films are obtained as a result of a high mobility (4 cm² V^-1 s^-1) and a relatively moderate p-type doping of 10¹⁸ cm^-3. However, exposure to air results in a rapid rise in conductivity due to heavy p-type doping (>10²⁰ cm^-3). The evolving electronic properties in air are correlated with a change in both crystalline phase and optical constants. Surface analysis corroborates a copper deficiency induced by room temperature oxidation in air. Surprisingly, storage in a <0.1 ppm oxygen and water atmosphere significantly slows but does not halt the rise in conductivity with time. However, an Al₂O₃ overlayer - also grown by ALD - results in significantly lower carrier concentrations as well as dramatically slower carrier addition with time, even under ambient conditions. The implications for future use of Cu₂S in more efficient (p/n ⁺) and stable thin film photovoltaics are discussed.