美国工程院院士John Bowers团队在PR发表重要成果: 直接调制的硅上量子点微型激光器同时具备毫安级阈值电流及高温稳定性

近日,美国工程院院士、加州大学圣芭芭拉分校John Bowers教授带领的光电研究团队研发出可直接调制的硅上量子点微型激光器。该激光器通过外延生长,集成在与CMOS工艺兼容的硅晶圆上。利用量子点特有的衬底缺陷影响、侧壁非辐射复合影响被减小的优异性能,并通过缓冲层的优化以减少III-V族材料与硅晶圆界面的位错密度,该课题组在存在着反向畴、晶格失配和热膨胀系数不同等巨大差异的异质生长材料体系上实现了优异的激光器性能:1.3 μm通讯波段的单模激射、同时兼具103 K特征温度的高温工作环境稳定性及3 mA的低阈值电流、6.5 GHz的3 dB带宽。 相关成果发表在Photonics Research 2018年第6卷第8期上 (doi: 10.1364/PRJ.6.000776)

图1 器件外延结构示意图。插图:量子点的原子力显微镜形貌

摩尔定律预测,高密度集成电路中的晶体管数量大约每两年翻一番。相应的,在硅片阵列上的大型计算机和光通信应用方面,对微型化和大规模集成光子元件持续增长的需求,亦是集成光电子学科永恒的旋律。量子点激光器由于量子点中载流子在材料中的运动受到三维限制,具有独特的衬底缺陷影响被减小的性能。这使得基于量子点的硅上外延生长器件在近年来的研究中取得了巨大的突破。为进一步降低片上连接的光电器件能耗,具有单模激射特性的小尺寸微环激光器的研究引发了极大关注。

利用量子点具备的侧壁非辐射复合影响被减小的优异性能,该研究基于将微环谐振腔与量子点相结合的新型激光器构架的设想,在硅上外延生长小型电抽运量子点激光器,并通过复杂的工艺流程有效解决了电极金属化受到微型尺寸腔限制的问题、回音壁模式(WGM)在工艺过程中的缺陷引发的光学损耗问题。与之前的实验结果相比,此次研究结果 (1)同时实现了高温工作环境稳定性(103 K特征温度)及低阈值电流(3 mA);(2)报道了硅上外延生长量子点微型激光器的调制特性(6.5 GHz的3 dB带宽);(3)系统分析了器件性能随尺寸减小的性能,验证了硅上外延生长的量子点微型器件密集大规模集成的可能性。

图2 微环量子点激光器的(a)器件结构示意图;(b)扫描式电子显微镜图;(c)器件在激射状态下的红外显微镜图

美国集成光子制造创新中心副主任John Bowers教授表示,该项工作对在硅衬底上直接生长III-V元素的外延工艺向替代传统的晶圆接合工艺的发展迈出了重要一步,有望实现大规模制造的同时降低成本,缩小尺寸,减小功耗。第一作者万雅婷博士表示其课题组接下来将致力于集成耦合波导,以有效导出激光。同时,已完成分布反馈式(DFB)激光器、可调谐激光器等器件设计,该课题组将结合波导、调制器、光开关、探测器等硅光子器件,组成光子链路并将其集成在同一材料平台,实现高速、大容量的片上光通信。

 

Directly modulated quantum dot lasers on silicon with a milliampere threshold and high temperature stability

In analogy to the historical scaling of silicon microelectronics governed by Moore’s law, miniaturization and large-scale integration of photonic components on a silicon platform represents an eternal melody for applications of integrated photonics, such as high performance computing and optical communications. By utilizing the in-plane carrier confinement properties of quantum dots (QDs) to combat heteroepitaxial defects, the performance of monolithically grown lasers on Si is improving rapidly. To further achieve the goal of attojoule optoelectronics for on-chip interconnects, small-footprint micro-ring lasers, with their inherent single wavelength operation, are particularly attractive.

Recently, the Optoelectronics Research Group led by Prof. John Bowers from the University of California, Santa Barbra demonstrated direct modulation of electrically-driven quantum dot micro-ring lasers epitaxially grown on a CMOS compatible Si substrate. The lasers leverage a QD gain region, whose discrete nature reduces the lasers' sensitivity to threading dislocations and sidewall recombination. Together with an optimized III-V/Si growth template to minimize the density of dislocations, exceptional lasing performance was achieved even relative to lattice-matched material systems: laser emission near 1.3 µm with few unique modes in spectrally distant locations that enable e?ectively single-mode operation, high temperature stability with T0 ~103 K, low threshold of 3 mA, and a 3 dB bandwidth of 6.5 GHz. Related results are published in Photonics Research, Vol. 6, Issue 8, 2018 (Y. Wan et al., Directly modulated quantum dot lasers on silicon with a milliampere threshold and high temperature stability).

Prof. John Bowers, Deputy Chief Executive Officer of AIM Photonics, believes that this work represents a major step towards using III–V/Si epitaxy to form efficient, easily manufacturable on-chip silicon light sources having dense integration and low power consumption. Dr. Yating Wan, et al. are continuing work to integrate coupled waveguides for efficient extraction/coupling of the lasing light. In the meantime, a number of device designs for DFB lasers and tunable lasers, have been completed for integration on the same material platform.

Schematic of the electrically pumped micro-ring laser