(Invited) Epitaxial Electrodeposition of Wide Bandgap Semiconductors for Transparent and Flexible Electronics

Abstract

Single-crystal silicon is the bedrock of semiconductor devices due to the high crystalline perfection that minimizes electron-hole recombination, and the dense SiOx native oxide that minimizes surface states. One issue with the material is that it is both brittle and opaque. There is interest in moving beyond the planar structure of conventional Si-based chips to produce flexible and transparent electronic devices such as wearable solar cells, sensors, and flexible displays. In this talk we will discuss the electrodeposition of transparent, wide bandgap semiconductors such as ZnO, CuI, and CuSCN that can be produced as epitaxial films on single-crystal and single-crystal-like substrates. These epitaxial films have an orientation that is controlled by the substrate, with electronic properties that mimic those of single crystals. The talk will emphasize the electrodeposition of the wide bandgap hole conductors CuI and CuSCN. They are electrodeposited by electrochemically reducing Cu(II)EDTA to Cu(I) in the presence of either iodide or thiocyanate anions. Cubic CuI deposits as dense films on Si(111), and CuSCN deposits as nanowires on Au(111). The CuI is epitaxial on the Si(111) in spite of a 2.4 nm thick interfacial SiOx layer. The rectifying p-CuI/SiOx/n-Si heterojunction diode shows an ideality factor of 1.5, a built-in voltage of 0.67 V, and a barrier height of 0.91 eV. The crystal structure and morphology of the CuSCN nanowires can be controlled by varying the SCN-/Cu(II) ratio in solution (see Fig. 1). We also show that the epitaxial films can be removed by a simple lift-off procedure to produce single-crystal-like flexible foils of transparent semiconductors.Fig. 1 – Controlling the crystal structure and morphology of CuSCN nanowires through the SCN-/Cu(II) ratio in solution. Top micrographs are of rhombohedral CuSCN deposited using SCN-/Cu(II) = 4, and the bottom micrographs are of hexagonal CuSCN deposited using SCN-/Cu(II) = 2. The nanowires were both on Au(111), and were deposited at a potential of -0.4 V vs. Ag/AgCl. Figure 1