Surface Absorption Below the Band Gap in a-Si:H Using Photoluminescence Absorption Spectroscopy
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SURFACE ABSORPTION BELOW THE BAND GAP IN a-Si:H USING PHOTOLUMINESCENCE ABSORPTION SPECTROSCOPY S.Q. GU, S. NITTA' AND P.C. TAYLOR Department Physics, University of Utah, Salt Lake City, UT 84112 *Departmentof of Electronic and Computer Engineering, Gifu University, Gifu, 501-11 Japan
ABSTRACT Photoluminescence absorption spectroscopy or PLAS has been used to measure the below gap absorption in a-Si:H at 77 K. As the absorption probed by this technique is composed of contributions from the bulk and the surfaces of the a-Si:H, we have developed an improved sample structure to separate these two contributions. Two kinds of interface have been investigated: a-Sil5 xNx:H/a-Si:H and a-Sil_xOx:H/a-Si:H. For the studies of~a-Sil_xN):H/a-Si:H, two samples have been employed. The first sample consisted of an a-Sil-xNx:H/a-Si:H/a-Si 1 -xNx:H/NiCr layered structure; the second one had a similar structure except that the a-Si:H layer was interrupted periodically by two thin (100 A) a-Si1 5xNx:H layers to increase the contribution of the interface absorption. A shoulder of the absorption around 1.2 eV for the second sample, which was not found in the first one, is probably due to the interfaces between a-Si:H and a-SilxN.:H. All samples were light-soaked using an Ar' laser (5145 A). The increases in the absorption measured by PLAS at 77 K are essentially the same as the results of PDS at 300 K, which suggests that the interfaces do not contribute to the light-induced absorption. INTRODUCTION Amorphous silicon has many applications [1] such as solar cells and thin film transistors; however, all these applications are affected by the defect states in this material, both in the bulk and at interfaces with other materials. One useful technique to study these defect states is subband absorption [2], but the conventional transmission technique is not applicable to measuring the subgap absorption in a-Si:H as the optical path length is too small. Many unconventional techniques have been devised to measure this weak absorption; i.e., photoconductivity [3], photothermal deflection spectroscopy (PDS) [4] and photoacoustic spectroscopy (PAS) [5]; however, all these techniques require specific assumptions such as the assumption that the nonradiative quantum efficiency of the excited carriers, or their lifetimes, are not sensitive to the exciting photon energy. Many of these assumptions are still controversial. If we use the length of the film as the absorption length instead of the thickness, then the optical path will increase by 3 orders of magnitude, and the absorption will be measurable. To achieve this goal, photogenerated luminescence can be used as the built-in light source. This technique, called photoluminescence absorption spectroscopy (PLAS), was used to measure the subgap absorption in a-Si:H by Ranganathan et al. [6]. There is often a built-in electric field associated with an asymmetric charge distribution near an interface of a-Si:H with other materials [7]. Using the PLAS technique, the absorption will reflect the effects
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