优化APD和SPAD中的光穿透深度，实现高增益带宽和超宽光谱响应
Optimizing the light penetration depth in APDs and SPADs for high gain-bandwidth and ultra-wide spectral response
摘要：
控制雪崩光电二极管（APD）和单光子雪崩二极管（SPAD）中的光穿透深度在通过在电场最强的倍增区域附近递送光来实现高倍增增益方面起主要作用。对于特定波长的光的穿透深度的这种控制先前已经使用集成的光子捕获纳米结构来证明。在本文中，我们表明，一个优化的周期性纳米结构的设计可以控制的穿透深度，同时在很宽的范围内的可见光和近红外波长。传统的硅APD结构由于在表面区域附近吸收较短波长的复合而遭受高的光生载流子损耗，而硅对于较长波长具有低的吸收效率。这种优化的纳米结构设计允许较短波长的光更深地穿透到器件中，绕过复合位点，同时通过将垂直传播的光弯曲成水平模式来捕获薄硅器件中的较长波长。这种对穿透深度的操纵改善了设备中的吸收，增加了光灵敏度，同时纳米结构降低了来自顶表面的反射率。而在倍增区附近传递光减少了光生载流子损耗并缩短了渡越时间，导致APD和SPAD在宽光谱范围内的高倍增增益。这些高增益APD和SPAD将在飞行时间正电子发射断层成像（TOF-PET）、荧光寿命成像显微镜（FLIM）和脉搏血氧仪中找到它们的潜在应用，其中在多个波长上需要高检测效率和高增益带宽。
英文摘要：
Controlling light penetration depth in Avalanche Photodiodes (APDs) and Single Photon Avalanche Diodes (SPADs) play a major role in achieving high multiplication gain by delivering light near the multiplication region where the electric field is the strongest. Such control in the penetration depth for a particular wavelength of light has been previously demonstrated using integrated photon-trapping nanostructures. In this paper, we show that an optimized periodic nanostructure design can control the penetration depth for a wide range of visible and near-infrared wavelengths simultaneously. A conventional silicon APD structure suffers from high photocarrier loss due to recombination for shorter wavelengths as they are absorbed near the surface region, while silicon has low absorption efficiency for longer wavelengths. This optimized nanostructure design allows shorter wavelengths of light to penetrate deeper into the device, circumventing recombination sites while trapping the longer wavelengths in the thin silicon device by bending the vertically propagating light into horizontal modes. This manipulation of penetration depth improves the absorption in the device, increasing light sensitivity while nanostructures reduce the reflectance from the top surface. While delivery of light near the multiplication region reduces the photogenerated carrier loss and shortens transit time, leading to high multiplication gain in APDs and SPADs over a wide spectral range. These high gain APDs and SPADs will find their potential applications in Time-Of-Flight Positron Emission Tomography (TOF-PET), Fluorescence Lifetime Imaging Microscopy (FLIM), and pulse oximetry where high detection efficiency and high gain-bandwidth is required over a multitude of wavelengths.
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