Pulse wave monitoring is essential for evaluating cardiovascular health, as monitoring results reflect the periodic fluctuations in blood volume within the cardiovascular system caused by cardiac systole and diastole. Conventional pulse monitoring methods, such as extracting signals from electrocardiograms (ECGs) and measuring blood pressure, often limit patient mobility and engender discomfort. Photoplethysmography (PPG) technology offers a non-invasive and adaptable alternative. In this paper, we aim to develop a monolithic integrated pulse monitoring sensor based on an AlGaInP yellow light-emitting diode (LED) and photodetector (PD). The sensor is designed for low power consumption, high integration, and compact size, making it especially suitable for wearable devices and telemedicine applications. Instead of using existing sensors, we employ an optimized concentric ring structure to enhance photocurrent response sensitivity, making it applicable for health monitoring, athletic diagnostics, and remote healthcare solutions.

The sensor is fabricated using metal organic chemical vapor deposition (MOCVD) on a 4-inch (1 inch=2.54 cm) GaAs substrate. The epitaxial structure consists of an n-type GaAs ohmic contact layer, an n-type AlGaInP current spreading layer, an n-type AlGaInP confinement layer, an active region, a p-type AlGaInP confinement layer, and a p-type GaP current spreading layer. The GaAs substrate is removed by wet etching using a mixture of NH${}_{\mathrm{4}}$OH and H${}_{\mathrm{2}}$O${}_{\mathrm{2}}$ with the ratio of V(NH${}_{\mathrm{2}}$OH) to V(H₂O₂) of 1∶3, where V(NH${}_{\mathrm{4}}$OH) and V(H${}_{\mathrm{2}}$O${}_{\mathrm{2}}$) are the volumes of NH${}_{\mathrm{4}}$OH and H${}_{\mathrm{2}}$O${}_{\mathrm{2}}$, respectively, and the epitaxial layer is bonded to a sapphire substrate using a SiO₂ interlayer. The final device consists of an inner circle (diameter: 400 μm) and an outer ring (width: 200 μm) separated by an electrically insulating trench (depth: 6.15 μm, width: 112 μm). The fabrication process involves several key steps: 1) defining mesa structures for the inner circle and outer ring diodes using photolithography and ICP etching (depth: 6.15 μm) to expose the p-GaP current spreading layer (mesa etch); 2) etching isolation trenches down to the SiO₂ insulating layer for complete electrical isolation (slot etch); 3) depositing 1.3 μm metal alloy layers on the n-GaAs and p-GaP layers by physical vapor deposition (PVD) to form ohmic contacts (p/n ohmic); 4) depositing a Si₃N₄ passivation layer by plasma-enhanced chemical vapor deposition (PECVD) and creating electrode openings and metal pad areas by reactive ion etching (isolation); 5) connecting the electrodes to the metal pads by PVD and metal lift-off (pad). The sapphire substrate is thinned to ~150 μm and diced by laser cutting. Finally, the chip is wire-bonded to a printed circuit board (PCB) using ultrasonic ball bonding for subsequent performance evaluation. The sensor's performance is evaluated by measuring the photocurrent response under different LED drive currents and comparing the inner ring emission/outer ring detection mode with the outer ring emission/inner ring detection mode (Figs. 1 and 2). The physiological signal detection capability of the sensor is validated by PPG experiments.

The optimized concentric ring structure significantly improves the photocurrent response sensitivity of the AlGaInP-based sensor. Experimental results show that the inner ring emission and outer ring detection mode achieve the highest sensitivity due to the larger detection area of the outer ring, which provides better light signal response, the photocurrent increases from 10^-9 A to 10^-5?10^-4 A, representing an increase of four orders of magnitude, mainly due to LED illumination (Fig. 3). Electroluminescence spectra show stable emission characteristics, with the full width at half maximum (FWHM) varying by only 1.2 nm, indicating uniform indium distribution and low defect density. In addition, the overlap of approximately 50 nm between the normalized spectral response (SR) curve of the PD and the emission spectrum of the LED allows the monolithically integrated MQW devices to function as both emitter and detector (Fig. 4). In pulse monitoring experiments, the sensor effectively detects periodic changes in blood volume caused by cardiac activity and accurately identifies key features of the PPG waveform, such as the systolic peak, diastolic peak, and dicrotic notch. Long-term stability tests over 16.5 hours confirm the sensor’s reliability, with consistent photocurrent output throughout the period (Fig. 6). Compared to other sensors, such as GaN-based green-light sensors and organic PPG sensors, the AlGaInP-based sensor demonstrates superior performance in terms of low power consumption, high sensitivity, miniaturization, and integration.

In this paper, we develop a monolithically integrated AlGaInP yellow light sensor that combines emission and photodetection functionalities through an optimized concentric ring design. The sensor exhibits low power consumption, high integration, and enhanced sensitivity. Its ability to detect PPG signals in reflective mode by interfacing with the skin enables the detection of periodic blood volume changes associated with cardiac activity. Experimental results confirm its capability to accurately identify key blood pressure waveform features, including systolic peaks, diastolic peaks, and dicrotic notches. The sensor's adaptability to real-time pulse monitoring, wearable devices, and remote healthcare applications highlights its potential for precision medicine and health management.