## Carrier Multiplication

##### By Fatemeh Barati

##### (video included at bottom of page)

Carrier multiplication is a phenomenon in which a single, high-energy excited electron can promote multiple additional electrons into the conduction band of a semiconductor. In conventional photocells, an absorbed photon can only excite one electron. However, by utilizing the flexibility afforded by atomic layer materials, we can engineer electronic energy scales that allow carrier multiplication to take place.

Figure 1: Schematic of energy levels in a MoSe_{2}-WSe_{2} system that can give rise to carrier multiplication.

Highly efficient carrier multiplication has been demonstrated in several important nanoscale systems, including nanocrystal quantum dots, carbon nanotubes and graphene. However, such e–h pair multiplication has not been observed in 2D devices. In our research we studied electron–hole (e–h) pair multiplication in 2D heterostructures composed of monolayer MoSe_{2} and bilayer WSe_{2} integrated into field-effect heterojunction devices.

In plain language we can describe an energy diagram of the carrier multiplication in WSe_{2}-MoSe_{2} device like so: when a photon strikes the WSe_{2} layer, it knocks loose an electron, freeing it to conduct through the WSe_{2}. At the junction between the two materials, the electron drops down into MoSe_{2}. The energy given off in the drop catapults a second electron from the WSe_{2} into the MoSe_{2}, where both electrons are free to move and generate electricity.

Speaking more technically, in the e–h pair excitation process requiring the lowest excess energy, an electron in WSe_{2} gains the combined energy of the interlayer potential energy ΔE_{c} and the kinetic energy of the source–drain electric field to create a low-energy electron in MoSe_{2} plus an interlayer e–h pair (shown in Figure 1), satisfying this energy relation:

**e _{W} +K_{e}(V_{SD}) = e_{Mo} +(e_{Mo} +h_{W}) **

An optically excited e_{W} electron in the conduction band of WSe_{2} undergoes efficient multiplication into an e_{Mo} electron and additional e–h pairs. By exploiting this highly efficient interlayer e–h pair multiplication process, we demonstrate near-infrared optoelectronic devices that exhibit 350% enhancement of the optoelectronic responsivity at microwatt power levels.

For more info, see our brief video summary: