In a thin film such as a-Si: H, surface and interface effects can exert an influence throughout the entire material. The surface has different electrical and structural properties from the bulk for various reasons. There are chemical reactions which change the composition, such as oxidation or reactions with metals. There is also transfer of electrical charge across the interface which causes band bending. Possible origins of the charge transfer are the different work functions of the materials in contact or charged species attached to the surface. This chapter describes metal contacts, the free surface, semiconductor and dielectric interfaces, and, lastly, multilayer structures in which a series of very thin layers is grown which has properties which differ markedly from those of the bulk material.

Metallic contacts

When a metal is brought into contact with a semiconductor, there is a transfer of charge across the interface to bring the two Fermi energies into alignment. The space charge in the metal remains very close to the contact, but extends much farther in the semiconductor because of the low density of states in the band gap. The resulting Schottky contact has rectifying electrical properties. A similar barrier is formed between doped and undoped a-Si: H layers. The nature of the metal contact is important in virtually all electrical measurements, as it determines whether charge can flow easily across the contact. Schottky contacts are used in transient capacitance techniques to measure the defect states (see Chapter 3) and in photosensing devices, where the blocking contact reduces the dark current and minimizes noise.