Atomic Layer Deposition of Zn(O,S) Alloys Using Diethylzinc with H2O and H2S: Effect of Exchange Reactions

Zn(O,S) thin films have a tunable band gap and are useful as conduction and valence band buffers in various types of solar cells. Previous growth of Zn(O,S) thin films by atomic layer deposition (ALD) has utilized alternating cycles of ZnO ALD and ZnS ALD. Controlling the composition of the Zn(O,S) alloys using alternating cycles is complicated because of an efficient exchange reaction between ZnO and gaseous H2S given by ZnOH* + H2S -> ZnSH* + H2O. This facile exchange reaction leads to a higher than expected sulfur content in the Zn(O,S) films. In this study, the effect of this exchange reaction on the composition of Zn(O,S) films was examined by varying the reaction conditions. For growth using alternating cycles, the Zn(O,S). film composition was strongly affected by the temperature and the size of the H2S exposure. An alternative method that avoids alternating cycles of ZnO ALD and ZnS ALD was also employed to grow Zn(O,S) thin films. This alternative method uses codosing of H2O and H2S at 100 degrees C. Codosing, allows the composition of the Zn(O,S) film to be controlled by the mole fraction of the dosing mixture. The relative magnitudes of the exchange reaction rate (k(1)) for ZnOH* + H2S -> ZnSH* + H2O and the competing exchange reaction rate (k(2)) for ZnSH* + H2O -> ZnOH* + H2S could be estimated using this method. Through the study of the composition of the Zn(O,S) films for different H2O and H2S partial pressures during codosing, the ratio of the exchange reaction rates, k(1)/k(2), was determined to be 71-231. Band gaps were measured for the Zn(O,S) thin films grown using the alternating cycle method and the codosing method. The band gaps could be produced with the most control by varying the mole fraction of H2S in the H2S/H2O codosing mixture.