X-ray photogeneration in amorphous selenium: Geminate versus columnar recombination

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Abstract
Amorphous selenium (a-Se) is a photoconductor used for both optical and x-ray imaging. Its optical photogeneration efficiency has been understood in terms of the Onsager theory of geminate recombination. However, x-ray photogeneration in a-Se has not been understood well. A previous argument has dismissed the Onsager theory of geminate recombination in the context of carrier generation in a-Se by high-energy radiation. Instead, columnar recombination has been proposed. In this paper we examine this argument and the predictions of the two recombination mechanisms. We find that the argument which dismissed the Onsager theory of geminate recombination is unwarranted, and that the Onsager theory is generally consistent with available experimental data. On the other hand, the columnar recombination theory is inconsistent with experiment. We have extended the Onsager theory in a form appropriate for x-ray photogeneration in a-Se to reflect that there should be a broad distribution of electron-hole separations.