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

光电子器件是光学和电子学的桥梁，它们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)和探测器(可以将光转换回电信号)。许多现代光电子器件依赖于量子限制效应，更一般地依赖于介观量子结构，例如，可以通过生长一系列薄层半导体(厚度在几纳米量级)来实现。这些结构具有大块半导体所缺乏的新的电学和光学性质，被广泛应用于激光器(例如，在量子级联激光器和量子井激光二极管中)、调制器(例如，在电吸收调制器中)和探测器(例如，在量子点和基于量子井的探测器中)。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子器件，而在硅上集成通常是电信和成像应用的必备条件。2D和van der Waals材料引起了光电子学的极大关注，因为它们具有新的物理效应，并且通常可以通过栅氧化物在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令人望而却步的，因为它受到用于图案化2D材料或电栅的光刻和刻蚀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通过异质结构实现的，包括实现莫尔图案。但这些方法对大规模生产有限制，通常效果较弱。这一迫切的研究项目旨在探索一种在2D材料中产生大规模量子限制效应的新技术，并展示基于这一新技术的器件。该方法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，当在栅极上施加电压时，这些氧化物用于在2D材料上产生可变的电势。与大块的3D对应结构不同，这些量子受限结构是广泛可调的，因为量子阱的深度与所施加的栅成正比。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井(也称为超晶格)阵列来实现，这些阵列根据施加的电压以不同的方式吸收光。2D材料中的带间吸收和子带间吸收都可以使用这种新方法进行工程设计。由于这些结构可以用高导电性电极选通，调制速度预计将比今天基于2D材料的调制器高至少一个数量级。此外，这些装置将受益于对2D材料中的激子进行量子工程的可能性，这些激子在室温下出现在可见光和近红外范围内，即使在没有使用量子限制的情况下也是如此。除了其明显的技术相关性，该项目还将促进对2D材料和相关量子现象的理解，并提供机会将这些新知识整合到哈佛大学的几门材料和设备课程中。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.