Stability and Crystallization of Amorphous Semiconductor Multilayers
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STABILITY AND CRYSTALLIZATION OF AMORPHOUS SEMICONDUCTOR MULTILAYERS
P.D.Persans*, A.F.Ruppert# and B.Abeles# * Physics Department and Center for Integrated Electronics
Rensselaer Polytechnic Institute, Troy, NY 12180-3590 # Exxon Research and Engineering Company Annandale, NJ 08801
ABSTRACT We discuss recent measurements of relaxation, interdiffusion and crystallization in amorphous hydrogenated semiconductors and insulators prepared as periodic multilayers. The stability of multilayer structures depends upon temperature, repeat distance and the nature of the materials. Crystallization of amorphous silicon in amorphous silicon/amorphous silicon dioxide layers is inhibited when the silicon thickness is reduced below 20 nm. INTRODUCTION Alternating deposition of hydrogenated amorphous silicon (a-Si:H) with other amorphous semiconductors and insulators such as germanium (a-Ge:H), silicon nitride (aSiN,:H) and silicon oxide (a-SiO.,) under the proper conditions yields compositionally modulated solids (amorphous superlattices) with a high density of reproducible layers and interfaces [1-41. Sub-layer thicknesses down to 1 nm with compositionally abrupt (1 monolayer transitional composition) interfaces have been demonstrated [2,5,6]. These structures hold promise for applications in photovoltaic, photoreceptor and electrophotographic technologies and therefore their electronic properties are of intense current interest. Structural and electronic stability under thermal and optical stress must also be understood in order to exploit amorphous thin films for these applications. The high density of interfaces has two important ramifications. Interface and layer thickness become increasingly important in determining the structure and stability of each layer. For example composition and strain gradients near the interface could lead to enhanced atomic diffusion. Additionally the interfaces make up a significant and experimentally controllable fraction of the deposited structure and therefore probes which are normally used for thick film and bulk measurements (i.e.- optical absorption, Raman scattering, IR absorption ) can be applied to study the nature of the interface region[5].In this paper we focus on the stability of ultrathin layers against crystallization. We report observations of crystallization of thin amorphous silicon sub-layers in hydrogenated amorphous silicon (a-Si:H)/hydrogenated amorphous silicon oxide (a-Si:H/a-SiO•) periodic multilayer structures. EXPERIMENTAL DETAILS
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Samples for the present study were prepared by plasma assisted chemical vapor deosition from pure Sil 4 and SiH 4:N20 in the ratio 1:50 for a-Si:H and a-SiO, respectively -3]. Periodic multilayers were deposited onto Corning 7059 glass substrates by periodically changing the composition of the plasma gases. Typical growth conditions were: substrate temperature, 525 K, total gas pressure, 30 mT; plasma rf power, 5 W at 13.56
Mat. Res. Soc. Syrmp. Proc. Vol. 103. 01988 Materials Research Society
180
M1tz; growth rate, 0.1 nms-1. The re
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