Atomically thin p-n junctions with van der Waals heterointerfaces

Abstract: Semiconductor p-n junctions are essential building blocks for modern electronics and optoelectronics. In conventional semiconductors, a p-n junction produces depletion regions of free charge carriers at equilibrium and built-in potentials associated with uncompensated dopant atoms. Carrier transport across the junction occurs by diffusion and drift processes defined by the spatial extent of this region. With the advent of atomically thin van der Waals (vdW) materials and their heterostructures, we are now able to realize a p-n junction at the ultimate quantum limit. In particular, vdW junctions composed of p- and n-type semiconductors each just one unit cell thick are predicted to exhibit completely different charge transport characteristics than bulk junctions. Here we report the electronic and optoelectronic characterization of atomically thin p-n heterojunctions fabricated using vdW assembly of transition metal dichalcogenides (TMDCs). Across the p-n interface, we observe gate-tuneable diode-like current rectification and photovoltaic response. We find that the tunnelling-assisted interlayer recombination of the majority carriers is responsible for the tunability of the electronic and optoelectronic processes. Sandwiching an atomic p-n junction between graphene layers enhances collection of the photoexcited carriers. The atomically scaled vdW p-n heterostructures presented here constitute the ultimate quantum limit for functional electronic and optoelectronic components.

• The paper demonstrates atomically thin TMDC heterojunctions that exhibit gate-tunable diode rectification and tunneling-assisted interlayer recombination.
• It employs unit-cell thick WSe₂ and MoS₂ layers to create clean, dangling-bond-free vdW interfaces that enable rapid charge separation with a photovoltaic response of ~2 mA/W.
• Graphene-sandwiched structures further enhance device performance by improving vertical charge transfer and achieving an external quantum efficiency up to 34%.

Atomically Thin p-n Junctions with Van der Waals Heterointerfaces

The paper discussed herein presents a detailed exploration of atomically thin p-n heterojunctions created using van der Waals (vdW) bonded materials, particularly transition metal dichalcogenide (TMDC) semiconductors. The research stands at the forefront of electronic and optoelectronic innovations by leveraging vdW heterostructures to reach the quantum mechanical limit of p-n junction scaling.

Overview and Experimentation

The authors engineered heterojunctions using unit-cell thick layers of p-type tungsten diselenide (WSe₂) and n-type molybdenum disulfide (MoS₂). The fundamental aspect of this study is the ability of these atomically thin materials to form a high-quality heterointerface due to their intrinsically clean surfaces devoid of dangling bonds, as opposed to traditional semiconductors, which rely on covalent bonding.

The electronic characterization reveals gate-tunable diode-like rectification and photovoltaic response, with tunneling-assisted interlayer recombination processes manifested in both electronic and optoelectronic behaviors. The research further elucidates how an atomic p-n junction, when sandwiched between graphene layers, enhances the collection efficiency of photogenerated carriers, thereby promising advancements in designing nanoscale photovoltaic and optoelectronic devices.

Charge Transport and Recombination

The unconventional behavior observed in the atomically thin p-n junction was studied under various biases. Unlike bulk p-n junctions, the absence of a depletion region allows for significant tunneling-mediated interlayer recombination of carriers, key to understanding the charge transport. Theoretical models consider Shockley-Read-Hall (SRH) and Langevin recombination mechanisms to explicate these tunneling phenomena, indicating that interlayer recombination at forward bias is primarily mediated by majority carriers.

Optoelectronic Performance

Optoelectronic analysis under illumination portrays a distinct photovoltaic response, with a maximum photoresponsivity of approximately ~2 mA/W at 532 nm under optimal gating conditions. Photoluminescence (PL) measurements of the heterojunction display substantial quenching, attributed to rapid charge separation, highlighting the significance of type II band alignment in facilitating charge transfer.

Graphene-Sandwiched Structures

Further improving upon the device performance, the integration of graphene electrodes enables direct vertical charge transfer as opposed to lateral diffusion, reducing interlayer recombination losses significantly. The resulting devices exhibit increased photoresponsivity, with external quantum efficiency (EQE) peaking at 34% for thick multilayer junctions, presenting a viable pathway for higher performance optoelectronic applications.

Implications and Future Directions

The findings illustrate the capability of vdW heterostructures in realizing atomically scaled semiconductor interfaces with properties markedly different from traditional devices. This study opens avenues for the refinement of band alignments and the engineering of material interfaces, crafting a versatile platform for high-efficiency nano-optoelectronics.

Future work may focus on exploring the effects of lattice mismatches and defect indentation on device efficiency or on integrating various TMDCs to optimize bandgap tuning and energy transfer processes. As the demand for lower-dimensional electronic components rises, continuing this line of research could significantly broaden the spectrum of semiconductor technologies, incorporating sophisticated device architectures composed of entirely new material classes.