Extension of the Constant Photocurrent Method to Determine Densities of Occupied and Unoccupied Localised States
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A9.5.1
Extension of the Constant Photocurrent Method to Determine Densities of Occupied and Unoccupied Localised States Charlie Main, Steve Reynolds, Ivica Zrinšćak and Amar Merazga1, University of Abertay Dundee, School of Computing and Advanced Technologies, Bell Street, Dundee DD1 1HG 1 Faculté des Sciences et Sciences de l’ingénieur, Université Mohammed Khidir, Biskra, Algéria ABSTRACT
This paper examines the use of Constant Photocurrent (CPM) measurements on thin film semiconductors, employing steady (DC) and modulated (AC) sub-gap illumination, to determine the density of localised states (DOS) in the bandgap. AC and DC measurements often result in different apparent absorption spectra. It is demonstrated that it is possible not only to extract information from the respective 'absorption' spectra, on the DOS below the Fermi level - i.e. occupied states, but also on the density of unoccupied states above the Fermi level. The ability to discriminate between these two groups of states by using DC and AC modulated sub-gap light arises from the frequency dependence of the different excitation pathways by which free electrons can be produced. AC modulated excitation will reveal absorption associated with transitions from occupied states into the conduction band, while DC excitation will include transitions from the valence band into unoccupied defect states, followed by slow thermal emission to the conduction band. We examine the temperature dependence of the CPM spectra and present a simple analysis of the DC and AC absorption spectra which allows the two regions of the DOS, above and below the Fermi level, to be determined.
INTRODUCTION In a previous paper [1], we outlined a model offering a simple explanation for the differences often observed between sub-gap absorption spectra in a-Si:H as measured by DC- and AC – CPM techniques, respectively[2,3,4]. Essentially, the DC measurement monitors the photoconductivity arising from free electrons which reach the conduction band in two ways; (a) by optical transition into the conduction band from occupied gap-states below the Fermi energy EF, and (b) by optical transition from valence band states into unoccupied gap-states above EF, followed by a thermal emission step, into the conduction band. On the other hand, an AC measurement made with modulated sub-gap excitation, even at frequencies as low as 1 Hz, may not register the (b) component if the thermal transition rates involved are much lower than the frequency of modulation. The two processes are illustrated in Figure 1. Thus there is the perception that the DC method is to be preferred in measurement of optical absorption, since it does not exclude path (b). In the present paper, we examine the validity of these assertions, using experimental data and computer-modeled predictions of the temperature dependence of AC and DC – derived sub-gap absorption spectra, since thermal processes are suggested to play a part in the observed differences.
A9.5.2
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