In advanced linear-mode infrared avalanche photodiodes (APDs), microscopic doping distributions and geometric dimensions fundamentally dictate avalanche gain and excess noise. This research investigates the internal space-charge dynamics of Mercury Cadmium Telluride (HgCdTe) APDs, introducing the design strategy of Electric Field Sculpting to elucidate non-equilibrium carrier transport. Employing a rigorous microscopicmesoscopic Monte Carlo framework, this study quantifies the impact of doping differentials across independent functional zones (p, i, n + ) alongside the geometric width of the multiplication zone (i-region). Unregulated doping discrepancies sculpt precipitous local field gradients, violently ejecting electrons into low-field zones post-ionization. This induces catastrophic polar optical phonon (POP) emission, shattering the deterministic periodicity of the dead-space effect and drastically inflating the macroscopic excess noise factor. Conversely, a smoothly sculpted electrostatic gradient tightly suppresses statistical gain variance. Furthermore, passively extending the multiplication width beyond maximum depletion limits creates an electrically neutral dead zone. This geometric miscalibration introduces a diffusion tail, severely bottlenecking high-frequency bandwidth. By systematically evaluating multidimensional physical trade-offs-including temporal dynamic response, tunneling dark current, and noise factor-this research architectures an optimally sculpted structure characterized by full depletion and gentle electrostatic gradients. These insights establish a robust theoretical foundation for designing next-generation, ultra-low-noise, high-bandwidth infrared photonic sensors.

This paper presents a quantitative study on the electrical properties of HgCdTe epitaxial materials with ultra-low background carrier concentrations, to support the development of fully depleted infrared structure. Conventional Hall measurements at 77 K reveals a distinct thickness-dependent carrier concentration in undoped mid-wavelength HgCdTe (Cd composition approximate to 0.29-0.32): the measured Hall concentration decreases from >1x10(14) cm(-3) to similar to 5x10(13) cm(-3) as the epilayer thickness increased from 5 mu m to 13 mu m. This phenomenon is attributed to surface states induced by oxidation and dangling bonds, which distort the standard single-layer Hall effect analysis and lead to inaccurate bulk parameter extraction. To decouple surface and bulk contributions, a double-layer Hall model is developed, where the effective Hall concentration neff is the combined response of a uniform bulk layer and a near-surface mixed layer. Assuming equal carrier mobilities in both layers, the model simplifies to n(eff) = n1 + n(surface)/d, predicting a linear correlation between n(eff) and 1/d. Differential Hall measurements with similar to 1 mu m stepwise etching precision are performed on four ultra-low-background HgCdTe samples, and the experimental results confirm this linear relationship, validating the model. The intrinsic bulk background concentrations extracted from fitted line intercepts ranges from 8x10(12) to 2x10(13) cm(-3), comparable to international state-of-the-art values (e.g., Teledyne). Slope variations among samples reflect surface microstate differences, associated with chemical etching, dislocation density, and compositional uniformity. The model is further verified by In-doped HgCdTe samples, with bulk concentrations derived from the model matching secondary ion mass spectrometry (SIMS) results within experimental error. Two-dimensional numerical simulations of mid-wavelength fully depleted HgCdTe devices show that a 5 mu m depletion width is achieved at reverse bias >0.1 V for 2x10(13) cm(-3) doping, and >0.5 V for 5x10(13) cm(-3). These results confirm that the HgCdTe materials, with reproducible ultra-low background concentrations, provide a material basis for fabricating HgCdTe fully depleted structures.

Surface recombination of HgCdTe limits the carrier lifetime and hence the photoresponse performance of the infrared detector. Understanding the contribution of surface effects to carrier lifetime has long been a topic of interest. In this work, we investigate the injection-dependent carrier lifetime of mid-infrared HgCdTe with different layer thicknesses by photoluminescence decay measurements. A pronounced carrier lifetime reduces as the injection level lowers in the surface-unpassivated HgCdTe, while the lifetime reduction is absent in the surface-passivated sample. This behavior cannot be attributed to the bulk recombination but is primarily caused by the surface effect. A semiquantitative model is established to separate the contributions of surface and bulk recombination, and therefore, the surface recombination velocity and bulk lifetime are successfully achieved. The results may contribute to the surface optimization of narrow-gap HgCdTe for the performance enhancement of an infrared detector.

Mid-wave infrared (MWIR) HgCdTe electroninitiated linear-mode avalanche photodiodes (e-APDs) employing a planar structure enable rapid carrier collection, making them promising candidates for high-bandwidth applications. However, in addition to vertical carrier transport, planar structures also involve lateral carrier transport, which can degrade the response speed of the devices. Furthermore, a surface leakage channel introduced by non-ideal passivation can lead to lateral electron injection, thereby prolonging the response time. In this work, the impact of lateral effects on response time has been investigated and clarified based on both simulation and experimental results. To mitigate lateral carrier transport and reduce the adverse impact of non-ideal surfaces on high-speed performance, devices with vertically opposed electrodes are designed and fabricated, achieving a 1.35 GHz bandwidth at 8 V reverse bias. The nearly unity excess noise factor and optimized bandwidth performance offer a perspective on realizing ultra-low noise and high gain-bandwidth product (GBP) MWIR photodetectors.

HgCdTe avalanche photodiodes (APDs) exhibit unique gain stability, making them advantageous for weak signal detection. However, research on neutral impurities (NIs) scattering in HgCdTe remains limited, and the non-parabolic electron dynamics pose significant error in current device scale models. These limitations hinder the theoretical and the industrial development of HgCdTe APD. Here, we designed Si-substrate devices and CdZnTe-substrate devices with high and low NI density, respectively, to clarify the role of NIs. Combining first-principle calculations with Monte Carlo transport simulations, we find that the avalanching electrons could be categorized into four types (I-IV below) with distinct behavior. Furthermore, we proposed a step-solving Monte Carlo (SSMC) simulation framework to resolve accurately electron trajectories with non-parabolic dispersion. We then clarify different contemporary models of impact ionization and polar optical phonon scattering with SSMC and experimental results. Notably, our findings also show that Si-substrate HgCdTe APDs show nearly the same avalanche performance as the classical CdZnTe ones, suggesting that they could play a pivotal role in high-performance infrared detection in the future.

Linear-mode HgCdTe avalanche photodiodes (APDs) have attracted significant attention due to their exceptionally low excess noise, operational bias ranging from-12 to 0 V, and linearly tunable gain. These characteristics suggest promising future applications in optical observations. The design of the multiplication region based on the PIN structure is critical, as it directly influences the photoelectric properties of the device, including gain and excess noise. This paper primarily investigates the effect of implanted areas on the multiplication region and examines area-related gain and noise characteristics of mid-wavelength infrared (MWIR) HgCdTe APDs at a temperature of 80 K. Studies conducted on diodes with varying implanted areas indicate that those with larger implanted areas create a wider multiplication region, which leads to lower gain and higher excess noise factors. Finally, 30 mu m pitch 640 x 512 array HgCdTe APDs for cut-off wavelengths 4.8 mu m at 80 K corresponding to compositions xCd = 0.31 were fabricated. The number of noise bad pixels and the band-to-band tunneling (BBT) current have been significantly reduced due to the optimized the multiplication region width. Consequently, high-definition imaging under short integration times has been successfully achieved.

In this paper , we investigated the crystal quality of Mercury Cadmium Telluride ( HgCdTe ) materials in the key avalanche region of avalanche photodiode detectors ( APDs ) with MWIR PIN structure. We simulated the entire process of the PIN APD device on the experimental materials and used differential Hall testing and differential minority lifetime testing to evaluate the material's real crystal quality in the key avalanche area. It is found that the crystal quality of the avalanche region under the optimized process is good. Its Shockley -Read -Hall ( SRH ) lifetime is 20. 7 mu s , which can be comparable to that of the primary HgCdTe materials. This meets the development requirements for high -quality MWIR HgCdTe avalanche devices. Additionally , based on the obtained SRH lifetime in the avalanche region , we conducted corresponding two-dimensional numerical simulations on HgCdTe APD structural devices to determine the theoretically optimal dark current density of 8. 7x10(-10) A/cm(2) .

Mid-wavelength infrared HgCdTe electron-initiated avalanche photodiodes (e-APDs) have presented excellent performances on resolving and counting photons. Aiming at low flux, the readout integrated circuit noise can be significantly reduced by certain device gain, and very low excess noise of HgCdTe e-APDs gives the opportunity for noise equivalent photon (NEPh) to be 1. Therefore, the main issue for signal-to-noise ratio 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, etching technology combining dry etching and wet etching and passivation technology based on plasma enhanced atomic layer deposition were used to reduce surface leakage current. Finally, 20 μm pitch 128 × 128 array HgCdTe APDs for cutoff wavelength 4.32 μm at 80 K corresponding to compositions xCd = 0.326 were fabricated, the measured GNDCD of the test unit is about 1.5–10 × 10−9 A/cm2 at 0–13 V, and very low excess noise guaranteed the NEPh to be 2.0 at gain = 93 and 1.6 at gain = 193.

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, own to its strongly dominant electron multiplication. However, when the photocurrent is high, a large number of electrons exist 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 hardly decreases with the increase of gain is fabricated. The response bandwidth of the APD is about 480 MHz @ gain = 625, corresponding to a gain-bandwidth product of 300 GHz.

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

Significance Single-photon counting has great application prospects in weak signal detection and time ranging.Since the first photon counting system in the visible spectrum was developed in the 1970s, in order to fully amplify the photon signal and reduce the readout noise of electronic equipments, many groups in the research field are constantly developing and improving the photon counting techniques. Electron multiplying charge coupled devices（EMCCDs） can replace the traditional visible light photon counting system and have higher quantum efficiency. While due to large avalanche noise, accurate acquisition of incident photon number under multiplication is difficult. The excess noise factor of mercury cadmium telluride avalanche photodiode（HgCdTe APD） is close to 1, there is almost no excess noise. Compared with the Geiger mode avalanche photodiodes, the linear mode HgCdTe APD has no dead time and after pulse, does not need to quench the circuit, has ultra-high dynamic range and adjustable spectrum with wide response range. Its detection efficiency and false count rate can be independently optimized. It opens up a new infrared photon band counting imaging application. It is of great value in astronomical exploration, laser radar, free space communication and other applications.Progress Raytheon and DRS Technologies in the United States, CEA/LETI Laboratory and Lynred in France, and Leonardo in the United Kingdom have successively realized single photon counting of linear HgCdTe APD detectors. This paper summarizes the technical routes and research status of linear mode photon counting HgCdTe APD detectors in Europe and America. The performance of HgCdTe APDs, photon counting ability and the advantages and disadvantages of detector preparation with three structures, namely, separation of absorption and amplification（SAM）, planar PIN type and high density vertically integrated photodiode（HDVIP）, are analyzed.Raytheon Company has prepared SAM short-wave HgCdTe APD detectors with hole multiplication mechanism by molecular beam epitaxy（MBE）, with gain of 350, photon detection efficiency of more than 95% and operating temperature of more than 180 K. DRS Technologies has prepared an electron-multiplication HDVIP medium wave HgCdTe APD detector using liquid phase epitaxy（LPE） material. The detector can respond in the visible to mid-infrared band from 0.4 μm to 4.3 μm, with the highest gain up to 6 100 and the photon detection efficiency greater than 70%. It can realize free space communication of 110 Mbps data transfer. CEA/LETI Laboratory and Lynred Company have prepared PIN-type short-wave and medium-wave HgCdTe APD detectors with electron multiplication mechanism by molecular beam epitaxy or liquid phase epitaxy. The gain of short-wave detector is up to 2 000, the maximum gain of medium-wave is up to 13 000, the internal photon detection efficiency is up to90%, the free space communication of 80 Mbps data transfer is realized, and bandwidth up to 10 GHz is achieved at 300 K and gain of 1. British Leonardo Company has prepared SAM type HgCdTe APD detector with electron multiplication mechanism by metal organic vapor deposition（MOVPE）. The detectors were named Selex Avalanche Photodiode HgCdTe Infrared Array（SAPHIRA）, the device gain can reach 66@14.5 V, single photon detection efficiency is more than 90%. A 24 μm pitch 320×256 array SAPHIRA detectors were supplied to First Light Imaging Company in France to develop a C-RED ONE camera. The C-RED ONE camera was successfully applied to the Michigan Infrared Combiner（MIRC） for astronomical exploration in the United States, which reduced the system noise of MIRC by 10 to 30 times and greatly improved the signal-to-noise ratio of fringe detection. The research on HgCdTe APD detectors started relatively late in China. The main research institutions include Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Kunming Institute of Physics and North China Research Institute of Electro-Optics. Limited by chip preparation technology and circuit technology of HgCdTe APDs, the ability of photon counting has not been realized at present, but some progress has been made in the development of focal plane at home. The single element, 128×128 array and 320×256 array medium wave HgCdTe APD detectors with PIN structure are developed by Shanghai Institute of Technical Physics,Chinsese Academy of Sciences. The gain of the detectors can reach more than 1 000, the gain normalized dark current density is less than 1×10 -7 A/cm~2 within the gain of 100, and the excess noise factor is less than 1.5 within the gain of 400. At the gain of 133, the noise equivalent photon number is 12, and the short integration time fast imaging is demonstrated. Bandwidth of single element detector is up to 300-600 MHz. The single element and256×256 array medium wave HgCdTe APD device with PIN structure are developed in Kunming Institute of Physics. The gain of the single element detector can reach more than 1 000. When the bias voltage is less than 8.5V, the average gain normalized dark current of focal plane is 9.0×10 -14 -1.6×10 -13 A, and the excess noise factor F is between 1.0 and 1.5.Conclusions and Prospects In China, HgCdTe APD devices with planar PIN structure are mainly developed, and the technical path is basically the same as that of France. Therefore, our country can learn from the successful experience of CEA/LETI Laboratory and the business model of Lynred Company, and continue to promote research on HgCdTe APD detectors in order to reach the international advanced level as soon as possible, and realize single-photon detection and photon counting application.

Hg1-xCdxTe is considered as the preferred material for high performance infrared photodetectors and imaging focal plane array (FPA) detectors. One of the technical challenges of multi-dimensional integrated HgCdTe epitaxy by molecular beam epitaxy (MBE) lies in the in-situ extraction, characterization and precisely control of a series of parameters such as alloy composition, surface roughness, substrate temperature and film thickness at a relatively low substrate temperature of about 180°C. Therefore, an in-situ, nondestructive spectroscopic ellipsometry (SE) method is needed to characterize the performance of HgCdTe films. In this paper, real time optical property characterization of short-wave Hg1-xCdxTe epitaxial grown by MBE is reported. Run to run feasibility and stability of in-situ SE is confirmed by buffer layer thickness verification in multiple growth runs. Lorentz oscillator parametric model provides a new approach to describe optical dispersion property of HgCdTe over spectral range of 1.5-4.1 eV. The absorption peaks show blue shift with the increase of HgCdTe Cd composition (x). Under this circumstance, the longitudinal x value for HgCdTe during epitaxy process can be obtained in real time without any surface damage by successfully building a composition-dependent optical constant library, with routine run-to-run reproducibility measurement accuracy Δx of ~ 0.0015. This work will facilitate the fabrication of HgCdTe heterojunctions with complex component distribution and doping profiles.

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.

HgCdTe has been shown to be the first semiconductor exhibiting single-carrier multiplication in short-wavelength, medium-wavelength, long-wavelength avalanche photodiodes detectors for cut-off wavelengths from 1.3 µm to 11 µm corresponding to compositions xcd from 0.7 to 0.2, which has the remarkable characteristics of high gain, high bandwidth and almost no excess noise. These results have opened a new horizon in photon starved and high-speed applications, such as active imaging and free space optical communications. In this paper, we report the latest results at SITP of HgCdTe eFAPDs using LPE-grown absorption layers in the SW and MW wavelength bands. The gain of single element short-wavelength HgCdTe APD for 2.57 μm cut-off wavelength is about 100 at 25V reverse bias, and GNDCD is about 1.47×10- 7A/cm2 at gain of 100 at 130K. For MW HgCdTe APDs, increase the P region doping concentration will reduce the overall dark current density and eliminate sudden rise of dark current at large bias and high temperature, and lower Cd composition could be a trade-off way for GNDCD suppression. 50 μm pitch 128×128 array HgCdTe APDs for cut-off wavelengths 4.88 µm corresponding to compositions xcd 0.307 were fabricated, whose GNDCD is less than 1×10-7A/cm2 at 8V reverse bias, gain is over 1000 at 11V reverse bias. A 50 μm pitch 128×128 array HgCdTe APDs with xcd=0.29 was manufactured, whose gain reaches 1570 at 9.8V reverse bias, the average excess noise factor is 1.25 at average gain of 133, noise equivalent photon is about 12 at average gain of 113. By thinning the absorption region thickness, the response bandwidth of Hg0.79Cd0.31Te APD reaches 635MHz under 1V reverse bias. Moreover, the medium-wavelength focal plane of 320×256 array is demonstrated the imaging, and the low noise, high sensitivity and fast imaging characteristics of HgCdTe APDs under linear avalanche gain are verified.

HgCdTe avalanche photodetector is one of the most promising candidates in capturing ultra-weak signal or even single photon in mid-wavelength infrared. However, the limited breakdown voltage and the unresolved underlying physics still lay an inevitable obstacle to achieve high-performance photodetectors. Here, for the first time we propose a collapse breakdown mechanism to thoroughly demonstrate the physics that underpins the relatively low-voltage breakdown in traditional HgCdTe positive-intrinsic-negative structures both theoretically and experimentally. Temperature-change measurement of the homemade devices is conducted to reveal the competitive behavior in the breakdown phenomenon. Below 180 K, the collapse breakdown dominates the overall device breakdown; while above 180 K, the avalanche breakdown contributes the majority to the device breakdown. With optimized structure, we achieve photoelectric gain and dark current 1700 and 10 −9 A @11.3V, respectively. Remarkably, a semi-empirical formula is derived to predict the optimal prescription of doping concentrations of different layers which are capable of realizing high gain and low dark current simultaneously. Reported results in literature are included to further confirm the flexibility and universality of the mechanism. This work represents a steady step forward to recognizing the breakdown mechanism and achieving high -performance avalanche single-photon photodetectors.

As diffusion control caused by the thermal annealing of in-situ As-doped HgCdTe grown by molecular beam epitaxy(MBE) was studied. HgCdTe with controllable As diffusion length is obtained at a lower annealing temperature,which is easy to form a PN junction profile that meets the design parameters. It provides a basis for the subsequent development of new HgCdTe FPA devices. It is found that the longitudinal distribution of As concentration of the in-situ As-doped HgCdTe changed under different Hg pressures during the thermal annealing process. And through theoretical calculations,As diffusion coefficients under different Hg pressures are obtained. Meanwhile,the dark current simulation of HgCdTe P-on-N structure with different As diffusion lengths was carried out through numerical simulation, which verified the importance of deep-advancing process for As-doped HgCdTe PN junction.

Mid-wavelength infrared (MWIR) HgCdTe electron-initiated avalanche photodiodes (e-APDs) have presented excellent performances to resolve and count photons with linear mode. Aiming at low flux, the ROIC noise can be extremely reduced by certain gain, and very low excess noise makes opportunity for noise equivalent photon (NEPh) to be 1. Therefore, the main issue for SNR of HgCdTe APD is gain normalized dark current density (GNDCD) at high reverse bias. In this work, the architecture of multiplication region is modeled and studied. The depth and width of multiplication region are controlled by regulating the p-type doping concentration, ion implantation and post thermal annealing conditions as well. Proper processes can keep the peak electric field away from the implantation damage region, effectively increase the Shockley-Read-Hall (SRH) lifetime, reduce the multiplication region concentration and finally increase the operating voltage. Considered with dark current and gain, depletion region (I region) width is optimized and characterized to be 3-3.6 mu m when I region concentration is -1 x 1015 cm-3 in our case. The GNDCD of MW APD (cut off wavelength -5.16 mu m @80 k) is less than 10-6 A/cm2@<=-10 V, with avalanche gain of -1570@-9.8 V. The excess noise factor (F) is measured to be 1-1.4 by noise power spectral density (PSD). The NEPh value is less than 5 photons with gain up to -280 for MW 128 x 128 HgCdTe APD array. Simulation results anticipate that GNDCD can be further reduced by decreasing the doping concentration of I region to below 5 x 1014 cm-3. Furthermore, increasing the p-type doping concentration and band gap will significantly reduce GNDCD below to -10-10 A/cm2@-10 V for 4.22 mu m Hg1-xCdxTe (x = 0.332) APD.

The control of Hg vacancy concentration in HgCdTe grown by MBE with different passivation layer struc???tures was studied.Higher Hg vacancy concentration in HgCdTe was obtained???which provides a basis for the subsequentresearch and development of new focal plane devices.It was found that the change of Hg vacancy concentration inHgCdTe varies with the structure of passivation layer during thermal annealing.The change is because the existence ofthe passivation layer of the HgCdTe surface layer changes the equilibrium process of the original thermal annealing.Atthe same time???the secondary ion mass spectrometry???SIMS???test and the corresponding theoretical fitting were verified the results