Using ferroelectric domain walls for active control of heat flow at the nanoscale

为了满足社会对微电子器件性能不断改进的需求，晶体管小型化是一个持续的驱动力，这样空间封装密度就可以最大化。然而，相关的工作功率密度的增加会导致热量的增加和片上温度的上升，从而影响设备的可靠性能。这代表了一个巨大的技术挑战，显然需要识别和表征具有新型热性能的材料，这些材料将在纳米尺度上实现卓越的热能管理。特别是，通过外部刺激（例如电压）主动控制热流的能力可能对下一代微电子的热管理需求和寿命产生重大影响。在这方面，氧化物铁电材料提供了一个令人兴奋的机会。在铁电材料中，存在称为“畴壁”（DWs）的原子尖锐结构界面，已知其通过破坏热振动来阻碍热流。DWs的独特之处在于，通过施加电压或压力，它们可以以完全可逆的方式在材料内部创建、擦除或重新定位。这种特性提供了一种前所未有的主动控制热流的方法，可以通过改变给定时间内材料中存在的dw的数量和它们的排列方式。然而，为了实现使用dw的热流控制，必须首先确定dw在不同材料中呈现的热界面阻力的明确估计。因此，该项目的主要目标之一是通过直接导热系数测量来量化DW热阻。然后将确定具有有效抑制热流的DWs的铁电材料系统。在此之后，将制造原型热器件，其中通过施加电压可逆地改变DW模式来改变热流通过材料的相对容易程度。这也将为一个更长期的研究愿景提供基础，以创造一个更奇特的纳米结构“热镜”装置。在这种情况下，可以设想DWs可以被设计成热波的周期性反射器，以最大限度地拒绝热能，就像光如何被介电镜中的多层高效反射一样。在过去的十年中，人们已经清楚地认识到，DWs可以被视为一种新型的片状功能材料，具有与块状材料显著不同的性能。例如，DWs内部的导电可以是金属的，甚至是超导的，当主体相对绝缘时。原型有源器件已经制造出来，其中功能完全来自导电dw的部署。然而，一个互补的观点，即狭窄的DW区域可能完全具有其自身的热特性，是全新的和未被探索的。在导电DWs中，由于额外的热载体（例如移动电子）的可用性，热流可能会增强，并且将进行导热性测量来证实这一点。导电DWs还将用于将废热转化为电能，因为最近的预测表明，与散装DWs相比，DWs内的热电功率可提高100%。总的来说，铁电DWs作为热器件中的有源元件是令人兴奋的候选者，因为DWs可以在功能上增强或限制热流。然而，这两种情况目前都没有得到很好的描述和理解。这些DWs固有的可重构性意味着基于铁电材料设计和制造新型有源热器件的真正潜力尚未得到充分利用。

In order to satisfy societal demand for continual improvements in microelectronic device performance, there is an ongoing drive for transistor miniaturisation so that spatial packing densities can be maximised. However, the associated increases in operational power density leads to increased heat generation and rises in on-chip temperature that can prevent reliable device performance. This represents a tremendous technological challenge and there is a clear need to identify and characterise materials with novel thermal properties that will enable superior thermal energy management at the nanoscale. In particular, the ability to actively control heat flow with an external stimulus (e.g. voltage) could have dramatic implications for the thermal management demands and lifetimes of next generation microelectronics. In this regard, oxide ferroelectric materials present an exciting opportunity.In ferroelectric materials, there exist atomically sharp structural interfaces called 'domain walls' (DWs) that are known to impede heat-flow by disrupting thermal vibrations. What is unique about DWs is their remarkable ability to be created, erased or repositioned inside the material in a fully reversible way by using applied voltages or pressure. This property provides an unprecedented means to actively control heat flow by being able to alter the number of DWs present in the material at a given time and the way in which they are arranged. However, to realise heat flow control using DWs, definitive estimates for the thermal interfacial resistance presented by DWs in different materials must first be determined. Therefore, one of the main goals of this project is to quantify DW thermal resistances through direct thermal conductivity measurements. Ferroelectric material systems having DWs that effectively inhibit heat flow will then be identified. Following this, prototype thermal devices will be fabricated where the relative ease of heat flow through the material will be changed by using applied voltages to reversibly alter the DW pattern. This will also provide the foundation for a longer-term research vision to create a more exotic nanostructured 'thermal mirror' device. In this case, it is envisaged that DWs can be engineered to behave as periodic reflectors of thermal waves in order to maximise the rejection of thermal energy, much like how light is reflected with high efficiency by the multiple layers in a dielectric mirror. Over the last decade, it has become clear that DWs can be considered as a new type of sheet-like functional material with properties that can be remarkably different than bulk. For example, electrical conduction within DWs can be metallic, or even superconducting, when the bulk is comparatively insulating. Prototype active devices have been fabricated where functionality is derived entirely from deployment of electrically conducting DWs. However, the complementary idea that the narrow DW region may have thermal properties entirely of its own is completely new and unexplored. Within conducting DWs, it is likely that heat flow will be enhanced, due to the availability of extra heat carriers (e.g. mobile electrons), and thermal conductivity measurements will be carried out to confirm this. Conducting DWs will also be explored for conversion of waste heat into electricity since recent predictions indicate that the thermoelectric power can be enhanced by up to 100% within DWs, compared to bulk.Overall, ferroelectric DWs are exciting candidates for use as the active elements in thermal devices since the DWs may behave functionally to either enhance or restrict heat-flow. However, neither case is currently well characterised nor understood. The innate reconfigurability of these DWs means there is real potential to design and build new types of active thermal devices based on ferroelectric materials that has yet to be capitalised upon.