Molybdenum impurity states in amorphous silicon and related materials

The role of molybdenum as an electronically active impurity in amorphous silicon and other tetrahedrally bonded amorphous semiconductors is discussed. The presence of Mo can be detected via the paramagnetic Mo5+ ion, which gives rise to an electron spin resonance with a g value of 1.92-1.93. The formation of this ion is enhanced by the presence of oxygen during deposition of thin films. We present experimental evidence that molybdenum in amorphous silicon can form an impurity donor level and thus can have an influence on the electronic properties of nominally undoped samples. Potential sources of Mo during thin-film deposition are also discussed.