Silicon (Si) photodiodes play a crucial role in complementary metal-oxide-semiconductor (CMOS) image sensors, particularly in visible cameras, and are increasingly in demand for infrared or short-wavelength (SWIR) cameras in modern autonomous vehicles operating under various weather conditions. However, the bandgap energy of 1.12eV in Si limits its capability to detect light in the infrared range, only allowing visible light detection. In this study, we propose transparent, quantum-thickness, Schottky-junction photodiodes on Si for light detection from visible to SWIR wavelengths. We employ an atomically thin TiO2 interfacial layer between an n-type Si substrate and a nanometer-thick metallic layer, which is positioned beneath a transparent conductive oxide (TCO) layer, to create n-Si/TiO2/TiN/ITO multilayered Schottky-junction photodiodes. Without the typical p-n junction in Si, we observed photocurrents through interband transitions by incident photons in the wavelength range of 400 ~ 1,100nm. Additionally, small but noticeable amounts of photocurrent were also measured by internal photoemission (IPE) via hot carrier generation even at the wavelength of 1,310nm. The embedded TiO2 layer significantly reduced dark current by two orders of magnitude with little change in photocurrent or quantum efficiency. This can be attributed to the low conduction band offset of the TiO2 semiconductor, which contributes to a quantum tunneling barrier without changing the Schottky barrier height and disturbing the internal photoemission process.
Wearable devices often employ optical sensors, such as photoplethysmography sensors, for detecting heart rates or other biochemical factors. Pulse waveforms, rather than simply detecting heartbeats, can clarify arterial conditions. However, most optical sensor designs require close skin contact to reduce power consumption while obtaining good quality signals without distortion. We have designed a detection-gap-independent optical sensor array using divergence-beam-controlled slit lasers and distributed photodiodes in a pulse-detection device wearable over the wrist’s radial artery. It achieves high biosignal quality and low power consumption. The top surface of a vertical-cavity surface-emitting laser of 850 nm wavelength was covered by Au film with an open slit of width between 500 nm and 1500 nm, which generated laser emissions across a large divergence angle along an axis orthogonal to the slit direction. The sensing coverage of the slit laser diode (LD) marks a 50% improvement over nonslit LD sensor coverage. The slit LD sensor consumes 100% more input power than the nonslit LD sensor to obtain similar optical output power. The slit laser sensor showed intermediate performance between LD and light-emitting diode sensors. Thus, designing sensors with multiple-slit LD arrays can provide useful and convenient ways for incorporating optical sensors in wrist-wearable devices.
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