Surface-illuminated photon-trapping high-speed Ge-on-Si photodiodes with improved efficiency up to 1700 nm
Lateral optical modes excited in a microstructure-hole photodiode by surface illuminated optical signal for enhanced external quantum efficiency with a thin semiconductor layer.
Fiber optical receiver, as a vital part of the optical communication system, needs to be cost-effective and power-efficient to satisfy the growing demand of high speed high traffic volume data transfer in data centers. Commercially available optical receivers often contain photodiodes (PDs) based on III-V materials such as InGaAs/InP, which incur additional cost of packaging with electronics since it is incompatible with complementary metal-oxide-semiconductor/bipolar and complementary metal-oxide-semiconductor (CMOS/BiCMOS) process. Germanium/Si PDs, which are compatible with BiCMOS process, can be monolithically integrated with all electronics on a single chip, fully hermetic and without ceramic multichip carriers for the receiver end, which can reduce the cost dramatically.
Unlike resonant cavity type Ge/Si PDs, where the detection is sensitive to both wavelength and temperature; or waveguide type Ge/Si PDs, where the fiber alignment to the waveguide cost is high and the optical loss is also undesirable, a surface-illuminated Ge-on-Si PDs with photon-trapping microholes is proposed to achieve both high efficiency and high speed at the same time. This work is carried out in Prof. Saif Islam's group at University of California, Davis, USA，and related results are published in Photonics Research, Vol. 6, Issue 7, 2018 (H. Cansizoglu et al., Surface-illuminated photon-trapping high-speed Ge-on-Si photodiodes with improve efficiency up to 1700 nm).
As shown in the figure, the photon-trapping microholes enable predominantly lateral modes in the Ge layer when nominally normal incidence photons are impinged onto the surface. In other words, the light travels in the lateral direction in this structure which enhances the absorption of the photons. This allows the use of thinner Ge layer which can have a high efficiency and high speed for optical interconnect applications. In addition, unlike gratings, photon-trapping is insensitive to angular dependence of incident light and has a relatively smooth spectral response from 1200 to 1700 mm.
Dr. Shih-yuan Wang, Silicon Valley executive, believes that this work has significance for monolithic integration of PDs in one or two-dimension arrays with electronics, and has great potential to drive cost down for both data center optical interconnect applications and autonomous vehicle/robotic applications using light direction and range (LIDAR) sensors.
PR封面故事：可用于1700 nm波长的基于光子囚禁效应的表面入射高速硅基锗光电探测器
用表面光信号激发微结构-空穴光电二极管中的横向光模以提高半导体层的外部量子效率
随着数据中心对数据通信的带宽和速率需求的不断增长，作为光通信系统中重要一环的光接收器正朝着高效率和低成本的方向不断发展。目前的商用光接收器通常由使用III-V族材料比如InGaAs/InP的光电探测器组成。这类光电探测器由于和现行互补金属氧化物半导体/双极-互补金属氧化物半导体（CMOS/BiCMOS）集成电路制备工艺不兼容，往往需要付出额外的封装成本。而Ge/Si光电探测器由于和BiCMOS的工艺具有天然的兼容性，可以实现和其他集成电子元器件的气密性封装和单片集成，并且不需要在接收端使用陶瓷晶片载体，可以显著降低制备成本。
不同于传统的谐振腔型Ge/Si光电探测器对波长和温度变化的敏感性，或者波导型Ge/Si光电探测器由于光纤和波导对接而导致的高成本和信号损耗，基于光子囚禁效应的表面入射硅基锗光电探测器同时具有高速和高量子效率的特点。这项研究工作在美国加州大学戴维斯分校的Saif Islam教授组开展，相关结果在Photonics Research 2018年第6卷第7期上发表（H. Cansizoglu et al., Surface-illuminated photon-trapping high-speed Ge-on-Si photodiodes with improve efficiency up to 1700 nm）。
如图所示，微米尺寸的光子囚禁孔洞结构使得从表面垂直入射的光子在锗材料层形成横模。换句话说，由于这种结构使得入射光在锗材料内部沿横向传播，因此光子的吸收得到提高。这种现象令更薄的锗材料层应用成为可能，使得光电探测器同时具有高量子效率和高速性能，从而能在在光电通信系统中可以得到更好的应用。另外，不同于光栅，光子囚禁结构型光电探测器对光源的入射角没有较大的依赖性，在1200~1700 nm波段都有很平稳的响应。
来自硅谷的高级总裁Shih-yuan Wang（王士元）博士认为，这项研究对于光电探测器在一维和二维方向和现有电子元器件的单片集成具有重要意义。并且由于这项技术在降低光电探测器制造成本方面的巨大潜力，它会对光通信系统在数据中心和激光雷达探测器在自动驾驶和自动机器人领域的应用产生广泛的影响。
接下来的工作将会集中在通过优化锗材料生长和改善钝化处理加工方法来进一步提高光电探测器的高速性能和灵敏度。