The Structure of InGaAs Photodetector

The Structure of InGaAs Photodetector
Since the 1980s, researchers have been studying the structure of InGaAs photodetectors, which can be summarized into three main types: InGaAs metal semiconductor metal photodetectors (MSM-PD), InGaAs PIN photodetectors (PIN-PD), and InGaAs avalanche photodetectors (APD-PD). There are significant differences in the production process and cost of InGaAs photodetectors with different structures, and there are also significant differences in device performance.
The schematic diagram of InGaAs metal semiconductor metal photodetector structure is shown in the figure, which is a special structure based on Schottky junction. In 1992, Shi et al. used low-pressure metal organic vapor phase epitaxy (LP-MOVPE) technology to grow epitaxial layers and prepare InGaAs MSM photodetectors. The device has a high responsivity of 0.42 A/W at a wavelength of 1.3 μ m and a dark current of less than 5.6 pA/μ m ² at 1.5 V. In 1996, researchers used gas-phase molecular beam epitaxy (GSMBE) to grow InAlAs InGaAs InP epitaxial layers, which exhibited high resistivity characteristics. The growth conditions were optimized through X-ray diffraction measurements, resulting in a lattice mismatch between InGaAs and InAlAs layers within the range of 1 × 10 ⁻ ³. As a result, the device performance was optimized, with a dark current of less than 0.75 pA/μ m ² at 10 V and a fast transient response of 16 ps at 5 V. Overall, the MSM structure photodetector has a simple and easy to integrate structure, exhibiting lower dark current (pA level), but the metal electrode reduces the effective light absorption area of the device, resulting in lower responsivity compared to other structures.

The InGaAs PIN photodetector has an intrinsic layer inserted between the P-type contact layer and the N-type contact layer, as shown in the figure, which increases the width of the depletion region, thereby radiating more electron hole pairs and forming a larger photocurrent, thus exhibiting excellent electronic conductivity. In 2007, researchers used MBE to grow low-temperature buffer layers, improving surface roughness and overcoming lattice mismatch between Si and InP. They integrated InGaAs PIN structures on InP substrates using MOCVD, and the responsivity of the device was approximately 0.57 A/W. In 2011, researchers used PIN photodetectors to develop a short-range LiDAR imaging device for navigation, obstacle/collision avoidance, and target detection/recognition of small unmanned ground vehicles. The device was integrated with a low-cost microwave amplifier chip, significantly improving the signal-to-noise ratio of InGaAs PIN photodetectors. On this basis, in 2012, researchers applied this LiDAR imaging device to robots, with a detection range of over 50 meters and a resolution increased to 256 × 128.
InGaAs avalanche photodetector is a type of photodetector with gain, as shown in the structure diagram. Electron hole pairs obtain sufficient energy under the action of the electric field inside the doubling region, and collide with atoms to generate new electron hole pairs, forming avalanche effect and doubling the non-equilibrium charge carriers in the material. In 2013, researchers used MBE to grow lattice matched InGaAs and InAlAs alloys on InP substrates, modulating carrier energy through changes in alloy composition, epitaxial layer thickness, and doping, maximizing electroshock ionization while minimizing hole ionization. Under equivalent output signal gain, APD exhibits low noise and lower dark current. In 2016, researchers constructed a 1570 nm laser active imaging experimental platform based on InGaAs avalanche photodetectors. The internal circuit of the APD photodetector received echoes and directly output digital signals, making the entire device compact. The experimental results are shown in Figures (d) and (e). Figure (d) is a physical photo of the imaging target, and Figure (e) is a three-dimensional distance image. It can be clearly seen that the window area in Zone C has a certain depth distance from Zones A and B. This platform achieves a pulse width of less than 10 ns, adjustable single pulse energy (1-3) mJ, a field of view angle of 2 ° for the transmitting and receiving lenses, a repetition rate of 1 kHz, and a detector duty cycle of approximately 60%. Thanks to the internal photocurrent gain, fast response, compact size, durability, and low cost of APD, APD photodetectors can achieve a detection rate that is one order of magnitude higher than PIN photodetectors. Therefore, currently the mainstream laser radar mainly uses avalanche photodetectors.