UC Berkeley

Semiconductor alloying is a common method for tailoring material properties such as band gap and lattice constant for specific applications. The most common semiconductor alloys are composed of elements that are relatively well matched in terms of atom size and electronegativity. However, there is a class of semiconductors known as highly mismatched alloys (HMAs), which contain elements with very different properties. These alloys are generally difficult to synthesize due to large miscibility gaps, but can have useful and interesting properties. Recently, the HMA GaN_1-xAs_x has been grown across the entire composition range by low-temperature molecular beam epitaxy. Over a large composition range (0.15 < x < 0.75) these alloys are amorphous with band gaps spanning from 0.7 to 3.4 eV for the entire alloy system. Here, the thermal stability, electrical properties, and local structure of a-GaN_1-xAs_x are investigated. In addition, the extensibility of low-temperature molecular beam epitaxy is tested in the context of the significantly more mismatched GaN_1-xBi_x system.

Thermal annealing experiments show that the a-GaN_1-xAs_x alloys demonstrate remarkable stability. Control of the electronic properties was difficult to attain without creating a mixed phase material. The disordered local structure of a-GaN_1-xAs_x determined by extended x-ray absorption fine structure analysis revealed a significant concentration of As dangling bonds compared to N dangling bonds. The extremely mismatched alloy GaNBi has been grown and a significant monotonic band gap shift occurred with increasing Bi content. Soft x-ray absorption and emission reveal the hybridization of N states with Bi resulting in a density of states modification suggesting that an alloy has indeed been synthesized.