Optoelectronic Properities of A-Ge:H/A-Si:H Superlattic Structures
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OPTOELECTRONIC PROPERITIES OF A-GE:H/A-SI:H SUPERLATTIC STRUCTURES
C. R. WRONSKI, P. D. PERSANS, B. ABELES, T. TIEDJE AND M. HICKS Exxon Research and Engineering Company Clinton Township, Route 22 East Annandale, New Jersey 08801 ABSTRACT The optoelectronic properties of multilayer a-Ge:H/a-Si:H superlattices with bandgaps between - 1.4 and 1.1eV are presented. The dependence of the electronic properties on the band alignment and the layer thicknesses is established and quantified. Particular emphasis is given to properties relevant to practical device applications and which involve carrier transport perpendicular to the layers. Introduction There is currently a great deal of interest in a-Si:H based semiconductors having bandgaps less than 1.5eV. Such materials are capable of having high photosensitivity at wavelengths greater than 0.7 Um and could be utilized in photoreceptor and tandem solar cell applications. Development has been focused on a-Si:Ge:H and a-Si:Ge:H(F) alloys where bandgaps below 1.5eV are obtained when the Ge content is in excess of ~50%. However, this lowering of the gap is generally accompanied by a corresponding deterioration in the electronic properties. We report here on the optoelectronic properties of a-Ge:H/a-Si:H multilayered superlattice In these structures which have bandgaps from- 1.4 to 1.1 eV (1). superlattice materials the bandgaps are determined by the individual Si and Ge layer thicknesses rather than their chemical composition as in the case of Si-Ge alloys. The a-Ge:H/a-Si:H superlattices studied were multilayered films, 0.5 to 1.0um thick, which were prepared as described earlier (1,2) by plasma-assisted CVD in which the individual a-Ge:H and a-Si:H layer thicknesses (da, ds) ranged from- 5 to 100 A. The electronic properties of these filmsgwere investigated using both Ohmic and blocking, metal Schottky barrier, contacts for electrical measurements parallel and The results presented in transverse (perpendicular) to the layers (3). this paper deal with the properties relevant to practical device applications; optical absorption, transverse electron transport, and the transverse photogenerated carrier transport. Optical Properties The near-edge optical absorption, a, in a-Ge:H/a-Si:H superlattices varies from that of the homogeneous alloy (dg < 5B)to that of a-Ge:H (d9 > 100 A). This is illustrated in Fig. 1 where the values of a between 0.6 and 0.95 pm are shown for a series of a-Ge:H/a-Si:H superlattice with layer repeat distance, dr = d5 + dn, from 16 to 16(A and fixed composition (ds/dg = 1.13). Included for compa ison are the values of a for a-Si:H and an aSi:Ge:H alloy (55% Si, 45% Ge) having the same composition as the superlattice films. The large changes in a for wavelengths > 0.7 Pm that can be obtained by changing the sub-layer thicknesses allows the optical absorption to be tuned to values that are the most suitable for specific device applications. However, the performance of these films also depends on their electronic properties and in particular on the transver
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