KEYWORDS: Mercury cadmium telluride, Dark current, Signal to noise ratio, Avalanche photodetectors, Passivation, Fabrication, Etching, Design and modelling, Mid-IR
Mid-Wavelength Infrared (MWIR) HgCdTe electron-initiated avalanche photodiodes (e-APDs) have presented excellent performances to resolve and count photons. Aiming at low flux, the Readout Integrated Circuit (ROIC) noise can be extremely reduced by certain device gain, and very low excess noise of HgCdTe e-APDs makes opportunity for noise equivalent photon (NEPh) to be one. Therefore, the main issue for Signal-to-Noise Ratio (SNR) of HgCdTe APD is Gain Normalized Dark Current Density (GNDCD) at high reverse bias. In this work, the electric field distribution is optimized by designing the mesa device structure to suppress the tunneling current at high operating voltage. Furthermore, new combining dry/wet etching technology and passivation technology based on Plasma Atomic Enhanced Layer Deposition (PEALD) were used to reduce surface leakage current. Finally, 20 μm pitch 128×128 array HgCdTe APDs for cut-off wavelengths 4.32 μm @80K corresponding to compositions xcd 0.326 were fabricated, the measured GNDCD of test unit is about 1.5~50×10-9 A/cm2 at 0~13V
HgCdTe electron avalanche photodiodes (e-APDs) with single-carrier multiplication hold great promise for weak signal detection. This work investigates the key metrics that affect the signal-to-noise ratio of mid-wavelength infrared (MWIR) Hg0.7Cd0.3Te e-APD: current, gain, and excess noise factor. The gain is over 1000 at -10 V, but the maximum useful gain is limited by the generation of band-to-band tunneling current at higher bias voltages. The gain dispersion obtained by characterizing the focal plane array is 4.7% at -7 V, indicating a relatively homogeneous gain from pixel to pixel. The excess noise factors evaluated by the noise power spectral density and the gain fluctuation methods both range from 1 to 1.32 at gains below 400, demonstrating the property of near excess-noise-free amplification of HgCdTe e-APDs.
HgCdTe avalanche photodiodes (APD) have been demonstrated to be one of the most promising paths for low flux and high speed applications. The bandwidth of HgCdTe e-APD has been theoretically predicted to be independent of the gain, owed to its strongly dominant electron multiplication. However, when the photocurrent is high, a large number of electrons exists in the depletion region, and the electrical field in the depletion region might collapse due to the space charge effect, thus limiting the increase of the gain-bandwidth product. In this work, the structure of the device was optimized by simulation, and the effect of the light injection dose on the electric field and bandwidth of the device was studied. Finally, a mid-wavelength infrared HgCdTe e-APD device whose bandwidth almost doesn't decrease with the increase of gain is fabricated. The response bandwidth of the APD is about 480MHz @ gain=625, corresponding to a gain-bandwidth product of 300GHz.
The HgCdTe avalanche photodiode (APD) with built-in gain mechanism has great application prospects in the field of weak light signal detection. Any dark current will be converted into noise affecting the work efficiency of photodetectors. Therefore, the study of dark current mechanism is an important way to obtain high performance HgCdTe APD. In this paper, the photoelectric detection mechanism of planar junction electron injection HgCdTe APD is systematically studied, focusing on two aspects of structural optimization design and performance improvement. The dependence of device performance parameters on structure parameters is obtained by comparing the simulation results and experimental results. The lower the trap concentration, the smaller the dark current under small reverse bias voltage is, and the higher the operation voltage is. The dark current density of HgCdTe APD array is about 10-8 A/cm2 under small bias voltage. In addition to the material parameters, the dark current is mainly related to the internal electric field, which is affected by the width and doping concentration of the depletion region. The results show that device structure is the major determinant for the performance difference of HgCdTe APD. The structural optimization direction of high-performance HgCdTe APD is to further suppress the local electric field and reduce the dark current by adjusting the process.
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