Ultrafast Carrier Transport and Space-Charge Field Formationin Semiconductors

例如，现在的材料生长和制造技术允许引入屏障，井掺杂和应变控制的方式，并允许生长的所有二进制有序材料（作为一个替代三元合金）。此外，超快激光技术已经发展到可以在皮秒和飞秒的时间分辨率下测量微米和纳米尺度上的电荷输运。简单地说，这项提议的目的是利用这些在材料生长和超快激光光谱方面的进步来研究、测量、优化和控制纳米尺度半导体结构中的电荷输运和空间电荷场形成。其目的不仅是改善传输，而且同时提高由这种传输引起的光学非线性的大小和速度。光子和电子系统的未来发展，以及两者之间的互连，很可能依赖于测量和控制电荷形成的能力，也可能受到限制。滤光：伴随这种传输的光学特性的筛选和变化在这些维度上的载波传输将在亚皮秒时间尺度上发生，甚至可能是弹道传输。此外，即使在如此小的尺寸上施加适度的电压也会产生巨大的场，并可能导致热载流子。虽然我们对能带内非平衡载流子的弛豫了解很多，在这些条件下的输运对非线性光学响应的影响知之甚少。因为我们的想法是设计结构来改善传输，并增强由这种传输引起的光学非线性，这些研究将需要增长，研究和优化几种独特的非传统结构和材料，这些结构和材料的生长技术还没有得到广泛的发展。此外,我们将参与一个积极的迭代过程，在这个过程中，基于光学的传输信息将与结构特征相关联，并反馈到结构设计和生长中获得具有改进的输运和与输运相关的光学性质的材料和结构。通过这种方式，结构可以确定地设计和制造，同时对我们控制的特性(应变，压电场、掺杂剂、界面、势垒、有序等)对所得结构的有用性质的影响。

Materials growth and fabrication techniques now allow, for example, the introduction of barriers, wells dopants and strain in a controlled fashion and allow the growth of all- binary ordered materials (as an alternative to ternary alloys). Moreover, ultrafast laser techniques have been developed that now allow the measurement of charge transport over micron and nanometer dimensions with picosecond and femtosecond temporal resolution. The objective of this proposal, simply stated, is to take advantage of these advances in materials growth and in ultrafast laser spectroscopy to study, measure, optimize and control charge transport and space-charge field formation in semiconductor structures over nanometer spatial dimensions. The intent is not only to improve the transport, but simultaneously to enhance the magnitudes and speeds of the optical nonlinearities that result from such transport. Future advances in the development of photonic and electronic systems, and interconnects between the two, may well rely on, and be limited by, the ability to measure and control charge formation, screening and changes in optical properties that accompany such transport. Carrier transport over these dimensions will occur on sub-picosecond time scales and may even be ballistic. Moreover, even modest voltages applied across such small dimensions will produce huge fields and may result in hot carriers. While much is known about the relaxation of nonequilibrium carriers in energy within the bands, little is known about transport under these conditions of the effects of that transport on the nonlinear optical response. Since the idea is to engineer the structure to improve the transport and to enhance the optical nonlinearities that result from such transport, these studies will require the growth, investigation and optimization of several unique and non-traditional structures and materials for which growth techniques have not been extensively developed. Moreover, we will be engaged in an active iterative process in which the optical-based transport information will be correlated with structural characterization and fed back into the structural design and growth to obtain material and structures with improved transport and transport- related optical properties. In this way, structures can be deterministically designed and fabricated with a firm fundamental understanding of the influence of the characteristics under our control (strain, piezo-electric fields, dopants, interfaces, barriers, order, etc.) on the useful properties of the resulting structure.