Kerfless layer-transfer of thin epitaxial silicon foils using novel multiple layer porous silicon stacks with near 100% detachment yield and large minority carrier diffusion lengths

Abstract

Two important aspects for the success of the porous silicon-based layer transfer method in producing kerfless thin (<50µm) silicon foils for future silicon solar modules are addressed in this work: achieving high detachment yield and high minority carrier diffusion lengths. The detachment characteristics of the porous silicon-based lift-off process is studied using finite element modeling as well as experiments. It is shown that for easy detachment and high detachment yield, a low density of thin silicon pillars must be attained in the high porosity detachment layer (HP-DL) after high temperature sintering. This is elegantly achieved by increasing the thickness of the low porosity template layer (LP-TL) which acts as the vacancy supply to increase the post-anneal porosity of the HP-DL. In this way, near 100% detachment yield has been achieved. However, a thicker LP-TL results in a poorer quality epitaxial growth surface. To circumvent this trade-off, novel triple and quadruple layer porous silicon stacks are introduced which decouple the function of the LP-TL that acts as both the template for epitaxy and as the vacancy supply for the HP-DL. In these new stacks, a surface zone of very low void size and density (nearly void-free) is created which allows high quality epitaxy on easily-detachable porous silicon stacks. Minority carrier lifetime measurements on epitaxial foils grown on such a triple layer stack has resulted in an effective lifetime of ~350µs at the injection level of 1015cm−3 which corresponds to a minimum minority carrier diffusion length of ~670µm (> 16 times the silicon thickness). With such high quality epitaxial foils combined with high detachment yield, very high efficiency solar devices on thin silicon substrates would be a reality in the near future.