Loose crystalline lifts and overconvergence of étale (φ, τ)-modules

Let $p$ be a prime, $K$ a finite extension of $\Bbb{Q}_p$, and let $G_K$ be the absolute Galois group of $K$. The category of \'etale $(\varphi,\tau)$-modules is equivalent to the category of $p$-adic Galois representations of $G_K$. In this paper, we show that all \'etale $(\varphi,\tau)$-modules are overconvergent; this answers a question of Caruso. Our result is an analogue of the classical overconvergence result of Cherbonnier and Colmez in the setting of \'etale $(\varphi,\Gamma)$-modules. However, our method is completely different from theirs. Indeed, we first show that all $p$-power-torsion representations admit loose crystalline lifts; this allows us to construct certain Kisin models in these torsion representations. We study the structure of these Kisin models, and use them to build an overconvergence basis.