EAGER: Combining van der Waals heterostructures and superlattices: new approach to 2D tunable optoelectronic devices

光电子器件是光学与电子的桥梁，在我们的社会中无处不在。它们包括固态光源，如发光二极管和激光二极管，以及调制器（可以将电信号或射频信号编码到光上）和检测器（可以将光转换回电信号）。许多现代光电器件依赖于量子限制效应，更一般地，依赖于介观量子结构，其可以例如通过生长一系列薄的半导体层（厚度在几纳米的量级）来实现。这些结构具有体半导体所缺乏的新的电学和光学性质，并且被广泛用于激光器（例如，在量子级联激光器和量子阱激光二极管中）、调制器（例如，在电吸收调制器中）和检测器（例如，在基于量子点和量子阱的检测器中）。科学界非常积极地研究新材料和新技术，以创造更快、更小和低成本的光电器件，而硅上的集成通常是电信和成像应用的必需品。2D和货车德瓦尔斯材料已经吸引了光电子学的大量关注，因为它们具有新的物理效应，并且通常可以通过经由栅极氧化物在2D材料和衬底之间施加电压来进行电调谐。然而，在这些材料中产生大规模的量子限制是不允许的，因为它受到用于图案化2D材料或电栅极的光刻和蚀刻工艺的分辨率的限制。到目前为止，2D材料的量子工程主要是使用异质结构实现的，包括实现莫尔图案。但是这些方法对于大规模生产具有限制，并且通常显示出较弱的效果。EAGER研究项目旨在探索一种新技术，在二维材料中产生大规模量子限制效应，并展示基于这种新技术的设备。该方法是基于一种新的栅氧化层制造技术。栅极由不同氧化物的交替层组成，当电压施加在栅极上时，这些氧化物用于在2D材料上产生可变电势。与它们的体3D对应物不同，这些量子限制结构是广泛可调的，因为量子威尔斯的深度与所应用的栅极成比例。PI计划利用这些新效应来实现新型调制器和光电探测器。调制器可以通过创建耦合的二维量子威尔斯（也称为超晶格）阵列来实现，其根据所施加的电压以不同的方式吸收光。二维材料中的带间和子带间吸收都可以使用这种新方法来设计。由于这些结构可以用高导电电极进行选通，因此调制速度预计比当今基于2D材料的调制器大至少一个数量级。此外，这些设备将受益于量子工程的可能性，在2D材料中的激子，出现在室温下在可见光和近红外范围内，即使没有使用量子限制。除了其明显的技术相关性外，该项目还将促进对二维材料和相关量子现象的理解，并为将这些新知识整合到哈佛大学的材料和设备课程中提供机会。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Optoelectronic devices bridge optics to electronics and they are ubiquitous in our society. They include solid state light sources such as light emitting diodes and laser diodes, as well as modulators (that can encode an electrical or radiofrequency signal onto light) and detectors (which can convert light back into an electrical signal). Many modern optoelectronic devices rely on quantum confinement effects, and more in general on mesoscopic quantum structures which can be realized, for instance, by growing a sequence of thin layers of semiconductors (with thicknesses in the order of few nanometers). These structures possess new electrical and optical properties which the bulk semiconductors lack and are widely used for lasers (e.g. in quantum cascade lasers and quantum well laser diodes), modulators (e.g. in electro-absorption modulators) and detectors (such as in quantum dots and quantum well based detectors). New materials and technologies are very actively investigated by the scientific community to create faster, smaller and low-cost optoelectronic devices, and integration on silicon is often a must for telecommunications and imaging applications. 2D and van der Waals materials have attracted a lot of attention for optoelectronics, since they host new physical effects and can often be tuned electrically, by applying a voltage between the 2D material and a substrate via a gate oxide. However, the creation of large-scale quantum confinement in these materials is prohibitive since it is limited by the resolution of the lithographic and etching processes used to pattern either the 2D materials or the electrical gates.So far, quantum engineering of 2D materials has been achieved mostly using heterostructures, including the realization of Moiré patterns. But these approaches have limits for large scale production and typically show weak effects. This EAGER research project aims to explore a new technique to create large scale quantum confined effects in 2D materials and to demonstrate devices based on this new technology. The approach is based on a new gate oxide fabrication technology. The gate consists of alternating layers of different oxides that are used to create a variable electrical potential on 2D materials when a voltage is applied on the gate. Unlike their bulk 3D counterparts, these quantum confined structures are widely tunable since the depth of the quantum wells is proportional to the applied gate. The PI plans to use these new effects to realize new types of modulators and photodetectors. Modulators can be realized by creating arrays of coupled two-dimensional quantum wells (also known as superlattices), which absorb light in different ways accordingly to the applied voltage. Both interband and intersubband absorptions in the 2D materials can be engineered using this new approach. Because these structures can be gated with highly conductive electrodes, the modulation speed is expected to be at least one order of magnitude greater than today’s modulators based on 2D materials. Furthermore, these devices will benefit from the possibility of quantum-engineering the excitons in 2D materials which appear at room temperature in the visible and near infrared ranges even when no quantum confinement is used. In addition to its obvious technological relevance, this project will advance understanding of 2D materials and associated quantum phenomena and offer opportunities for integrating this new knowledge in several courses on materials and devices at Harvard University.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.