Van der Waals epitaxy of cadmium telluride thin films on 2D substrates using metalorganic chemical vapor deposition

Abstract

Mica is a readily available, low-cost 2D material. We used it as 2D substrate to grow epitaxial CdTe films. The XRD data showed the growth of a single-crystal CdTe film with [111] out-of-plane orientation. The XRD rocking curve was very narrow with FWHM of ~ 0.1o that is comparable to single crystal CdTe grown by conventional epitaxy. The pole figure showed 6 {111} poles from primary and twin domains with very little dispersion in azimuthal direction. EBSD data showed very large tens of microns sized grains and Raman data showed the stoichiometric nature of the film. The ~100 nm film suffered from low lifetime resulting from high surface recombination that can be improved with proper surface passivation. Thus, both mica and graphene provide low-cost ways to grow high quality epitaxial films, that can potentially help to bring down the cost and increase the efficiency at the same time.

Polycrystalline CdTe solar cells have achieved ~ 22% power conversion efficiency and their market continues to grow, as they become more efficient [1]. But, the low carrier lifetimes resulting from large surface/grain boundary recombination velocities and low doping concentrations are the major culprits that affect their open circuit voltage. In recent studies, single crystal CdTe solar cells have been reported to produce open circuit voltage beyond 1 V [2], [3]. Single-crystal CdTe helps to solve the low carrier lifetime issue and does not require any post growth treatment (e.g. CdCl2 heat treatment). However, for conventional epitaxy, single crystal CdTe can only be grown on a few selected lattice matched substrates, but they are expensive and not feasible for large scale, low-cost manufacturing. If these lattice matched substrates are not used, generated defects act as recombination centers at the interface and grain boundaries to reduce the charge collection efficiency. A new epitaxial growth technique called ‘van der Waals epitaxy’ may be the solution to these problems.

Van der Waals epitaxy uses the dangling bond free surfaces of 2D materials as substrates to grow strain-free, highly epitaxial films. It has been well studied for 2D/2D heterostructures. The 2D materials are less expensive and can be reused as templates for growing these epitaxial films. Hence, it provides a low-cost alternative to grow epitaxial films and may solve CdTe solar cells issues. However, owing to the low surface energy of these 2D surfaces, nucleation of 3D materials has been quite challenging. Many efforts at growing 3D materials using van der Waals epitaxy have resulted in nonlayered nanostructures. Only a handful of attempts (e.g. GaN/graphene) have resulted in blanket 3D films, but many of them used deliberately induced surface defects to aid nucleation. In this dissertation, we have attempted to grow blanket CdTe thin films on two 2D substrates: graphene and mica without using any surface modifications.

In the first part, we investigated CdTe growth on commercial polycrystalline graphene as 2D buffer layer. The inclusion of graphene buffer layer produced strong [111] oriented CdTe films on amorphous substrates. The FWHM of XRD rocking curve reduced by 10 times to 0.7o. However, the polycrystalline nature of the graphene film resulted in a fiber textured epi CdTe film with two rotational orientations. XRD pole figure revealed presence of 12 {111} poles resulting from primary and twin domains. DFT first principle calculations supported the experimental results. PL peak shift revealed that the epilayer is strain relaxed. To improve the epitaxial film, single crystal graphene grown on single-crystal Cu(111) film was used as 2D buffer layer. This helped to grow CdTe(111) film with a single orientation. XRD pole figure revealed presence of only 6 {111} poles from primary and twin domains. Annealing at 550 oC post-growth showed improvement in grain size of the epi film. HRTEM, RHEED and SAED images confirmed the highly crystalline and epitaxial nature of the CdTe film. Initial results also suggest that multilayer graphene may help to further improve the CdTe film quality.