GEMINATE AND NON-GEMINATE RECOMBINATION IN AMORPHOUS SEMICONDUCTORS

One of the characteristic features of amorphous semiconductors is the low carrier mobilities which result from small carrier mean-free-paths due to disorder. When the mean-free-path becomes comparable to the Coulomb radius of an electronhole pair. both photogeneration and recombination processes can be affected. The photogeneration process can be controlled by geminate or initial pair recombination which results in a field-dependent photogeneration efficiency. The non-geminate recombination of previously created free carriers can become diffusion-controlled. This paper reviews the evidence for the occurrence of such processes in amorphous semiconductors including recent studies in a-Si:H and a-AszSe3. Introduction.Intense technological interest exists in hydrogenated amorphous silicon, a-Si:H, for applications as photovoltaic cells [1,2], thin film electronic devices [3,4] and electrophotographic photoreceptors [5,6]. Optimization and refinement of the properties of the material for such applications requires some fundamental understanding of their photoelectronic properties. The considerable progress which has been made in this respect is the subject of a recent excellent review[7]. The purpose of this paper is to discuss the photogeneration and recombination processes in a-Si:H in relation and comparison to other amorphous materials such as amorphous selenium a-Se, amorphous arsenic triselenide, a-As2Se3 and amorphous molecular solids. The unifying characteristic of these materials is that they possess low carrier mobilities. In view of this fact the photogeneration and recombination of carriers may be fundamentally different from similar and more familiar processes in crystalline semiconductors [8]. An understanding of such differences and their manifestation, in particular amorphous solids, is obviously an important precursor to any device improvement. Low carrier mobilities are a consequence of small carrier mean-free-paths associated with strong carrier scattering due to the disorder. The best estimates indicate microscopic mobilities in amorphous solids are limited to < 10 cm2/vsec [9,10] while shallow trapping leads to still lower drift mobilities [9]. Such small carrier mean-free-paths can influence both the initial photogeneration step and the subsequent recombination of previously created carriers. If the photoexcited electron-hole pair thermalize at a distance r,, (the themalization radius) which is comparable to the Coulomb radius, rc, (r, is the distance at which the Coulomb energy is -kT) then recombination can occur even before free carrier creation occurs [see Figure l(a)]. This is the so-called geminate or initial recombination. Its primary consequence is to result in a field, temperature abd, in the case of a-Se, a wavelength dependent photogeneration efficiency. The relative value of r, with respect to r, and the quantum efficiency of production of initially thermalized pairs Q, determine the quantum efficiency of photogeneration in zero field @(o). Such a process can be analytically described by a theory due to Onsager [ll]. The characteristic features of a geminate process are a finite quantum efficiency at zero field with a value determined by Q,, r, and r,. Up to fields of -104v/cm this quantum efficiency is essentially constant beyond which it rises with increasing field to ultimately saturate at the value Q,. Thus for a higher value of Q(o) (for given values of Q, and rc this is determined by r,) the weaker the field dependence in the field-dependent regime. Article published online by EDP Sciences and available at http://dx.doi. org/10.1051/jphyscol:1981491