UC Berkeley

Binary oxide materials exhibit a wide range of technologically relevant behaviors (i.e. magnetism, superconductivity, and ferroelectricity). For this reason, there is much interest in oxides due to their potential in applications ranging from batteries and solar cells to power electronics and more. Unfortunately, oxide materials are hindered by experimental and physical challenges such as sample growth, control of the bandgap, and doping —processes that are well understood and realized in traditional semiconductors (i.e., Si, Ge). Band structure engineering is one method that modifies semiconductor properties providing a means to control the performance of a wide range of electronic materials to meet device requirements.

Highly mismatched alloys are semiconductor alloys formed through isoelectronic substitution of anions with very different ion size and electronegativity, which allows drastic band structure modification with dilute alloy content. In this work, ZnO and Ga2O3 were alloyed with S to study the drastic band structure modification with alloy content.

Alloys from ZnO and ZnS were synthesized by radio-frequency magnetron sputtering and pulsed-laser deposition over the entire alloying range. Sputtering is a favorable deposition technique for an industrial production line due to better process integration with currently used methods. Pulsed-laser deposition is suitable for the growth of materials with large miscibility gaps arising from the large differences in atomic size and electronegativity due to the potential for both stoichiometric transfer of target materials to the substrate and the use of non-equilibrium growth conditions. Using X-ray diffraction, the ZnO1-xSx films were found to be highly textured with a columnar-like structure that remains throughout the entire composition range (determined by transmission electron microscopy). The optical absorption edge of these alloys decreases rapidly with small amount of added sulfur (x ~ 0.02) and continues to red shift to a minimum of 2.6eV at x=0.45. At higher sulfur concentrations (x > 0.45), the absorption edge shows a continuous blue shift. The strong reduction in the bandgap for O-rich alloys is the result of the upward shift of the valence-band edge with x as observed by X-ray photoelectron spectroscopy. As a result, the room temperature bandgap of ZnO1-xSx alloys can be tuned from 3.7 eV to 2.6 eV. The observed large bowing in the composition dependence of the energy bandgap arises from the anticrossing interactions between (1) the valence-band of ZnO and the localized sulfur level at 0.30 eV above the ZnO valence-band maximum for O-rich alloys and (2) the conduction-band of ZnS and the localized oxygen level at 0.20 eV below the ZnS conduction-band minimum for the S-rich alloys. The ability to tune the bandgap and knowledge of the location of the valence and conduction-band can be advantageous in applications, such as heterojunction solar cells, where band alignment is crucial.

Stoichiometric gallium oxide sulfide Ga2(O1-xSx)3 thin-film alloys were synthesized by pulsed-laser deposition with x≤0.35. One challenge has been synthesizing Ga2(O,S)3 crystalline films, as these samples have been determined to be amorphous through X-ray diffraction and transmission electron microscopy measurements. Despite the amorphous structure, the films have a well-defined, room-temperature optical bandgap tunable from 5.0 eV down to 3.0 eV. Similar to the amorphous GaN1-xAsx system, the band structure behavior of amorphous Ga2(O,S)3 alloys is in agreement with the predictions of the band anticrossing model. In the case for amorphous Ga2(O,S)3 alloys, the addition of sulfur at a merely 0.013 ratio shows a reduction in bandgap of about 1 eV suggesting that the localized sulfur level is located roughly 1 eV above the valence band of Ga2O3, a value that is comparable to the sulfur level location found in ZnO1-xSx alloys. The optical absorption data are interpreted using a modified valence-band anticrossing model that is applicable for highly mismatched alloys. The model provides a quantitative method to more accurately determine the bandgap as well as insight to how the band edges are changing with composition.