Nonlinear Amorphous Silicon Photodetectors for Distance Ranging

Imaging is a leading instrumentation technique for a vast range of applications. Among others, it is used for scene recognition in autonomous driving, robotics, the medical sector and in smartphones. Besides classical color imaging of two-dimensional (2D) planes, precise and accurate distance ranging is one advancing imaging modality, expanding imaging towards three dimensional (3D) depth sensing. Beyond image quality, high density integration of these image sensors into miniaturized systems, even on a single chip, is a major driver of ongoing development.
State-of-the-art distance ranging and 3D imaging techniques are Time-of-Flight (ToF) and Triangulation. Both techniques face challenges in terms of small scale integration capabilities. Typical triangulation-based systems require a specific baseline distance in the cm-range, which is not suitable for monolithic chip integration. ToF-systems can be integrated on a single chip, but its principle requires high precision timings in the sub-ns range, advanced beam steering or complex circuity.
Here, developments and achievements on photodetectors based on hydrogenated amorphous silicon (a-Si:H) capable for distance ranging and future 3D imaging are presented. The a-Si:H manufacturing technology at temperatures < 300°C is compatible with the standard CMOS. This approach enables the integration of sensors directly on top of a chip in the back end of line with high geometrical fill factors.
The distance ranging operation exploits the nonlinear device behavior with respect to the optical excitation scenario. Based on simulations and subsequent experimental validations, two nonlinear mechanisms and their respective physics are systematically investigated, analyzed and developed towards individual distance ranging techniques.
The a-Si:H Focus-Induced Photoresponse (FIP) measures nonlinear current response variations originating from an amplitude-modulated monochromatic optical excitation. The distance is acquired by a frequency selective readout of the nonlinear, focus-dependent sensor signal. Distance resolutions down to 540 μm are achieved, allowing for precise ranging at low measurement and integration effort.
The a-Si:H Intrinsic Photomixing Detector (IPD), utilizes an envelope mixing process of two amplitude modulated optical excitations. Thereby, the Time-of-Flight information is transferred to a lower frequency domain, enabling a read-out. This
mixing takes place intrinsically within the detector and decreases the measurement complexity. Distance measurements are shown at > 70 m with depth resolutions down to 23 mm, which is comparable with commercial systems.
Developments on advanced device concepts are studied and discussed. These concepts systematically improve the sensitivity, spectral bandwidth and speed of the detectors.