LWIR HgCdTe Detectors Grown on Ge Substrates

Long-wavelength infrared (LWIR) HgCdTe p-on-n double-layer heterojunctions (DLHJs) for infrared detector applications have been grown on 100 mm Ge (112) substrates by molecular beam epitaxy (MBE). The objective of this current work was to grow our baseline p-on-n DLHJ detector structure (used earlier on Si substrates) on 100 mm Ge substrates in the 10 μm to 11 μm LWIR spectral region, evaluate the material properties, and obtain some preliminary detector performance data. Material characterization techniques included are X-ray rocking curves, etch pit density (EPD) measurements, compositional uniformity determined from Fourier-transform infrared (FTIR) transmission, and doping concentrations determined from secondary-ion mass spectroscopy (SIMS). Detector properties include resistance-area product (RoA), spectral response, and quantum efficiency. Results of LWIR HgCdTe detectors and test structure arrays (TSA) fabricated on both Ge and silicon (Si) substrates are presented and compared. Material properties demonstrated include X-ray full-width of half-maximum (FWHM) as low as 77 arcsec, typical etch pit densities in mid 10⁶ cm⁻² and wavelength cutoff maximum/minimum variation <2% across the full wafer. Detector characteristics were found to be nearly identical for HgCdTe grown on either Ge or Si substrates.     

p-on-n HgCdTe Infrared Focal-Plane Arrays: From Short-Wave to Very-Long-Wave Infrared

This paper reports on recent developments made at the DEFIR joint laboratory on fabrication of planar p-on-n arsenic (As)-ion-implanted HgCdTe photodiodes. Our infrared focal-plane arrays (IRFPAs) cover a wide spectral range, from the short-wave infrared (SWIR) to the very-long-wave infrared (VLWIR). Our planar p-on-n technology is a classical one based on ion implantation followed by diffusion and activation. The p-type doping is obtained by As implantation, and n-type indium (In) doping is achieved during the epilayer growth. Our p-on-n IRFPAs show state-of-the-art performance from the SWIR to VLWIR spectral range. Mid-wave infrared (MWIR) and long-wave infrared (LWIR) FPAs have been designed with a television (TV) format and 15 μm pixel pitch. Preliminary results of high-operating-temperature detectors obtained in the MWIR (λ _c = 5.3 μm at 80 K) have shown highly promising electrooptical performance above 130 K. For space applications, imagers dedicated to low-flux detection have first been produced as TV/4 focal-plane arrays, with 15 μm pitch in the SWIR range (2 μm). Finally, TV/4 arrays with 30 μm pixel pitch have been manufactured for the VLWIR range. The measured dark current fits the “Rule 07,” with homogeneous imagers.     

Fabrication and Characterization of Small Unit-Cell Molecular Beam Epitaxy Grown HgCdTe-on-Si Mid-Wavelength Infrared Detectors

Small 15 μm unit-cell mid-wavelength infrared (MWIR) detectors have been fabricated and characterized at Raytheon Vision Systems (RVS) to enable the development of high resolution, large format, infrared imaging systems. The detectors are fabricated using molecular beam epitaxy (MBE) grown 4-in. HgCdTe-on-Si wafers with a p-on-n double layer heterojunction (DLHJ) device architecture. Advanced fabrication processes, such as inductively coupled plasma (ICP) etching, developed for large format MBE-on-Si wafers and 20 μm unit-cell two-color triple layer heterojunction (TLHJ) focal plane arrays (FPAs) have been successfully extended and applied to yield high performance 15 μm unit-cell single color detectors that compare favorably with state-of-the-art detectors with larger pitch. The measured 78 K MWIR cut-off wavelength for the fabricated detectors is near 5.5 μm, and the current–voltage characteristics of these devices exhibit strong reverse breakdown and RoA performance as a function of temperature with diffusion limited performance extending to temperatures down to 120 K.     

Study of LWIR and VLWIR Focal Plane Array Developments: Comparison Between p-on-n and Different n-on-p Technologies on LPE HgCdTe

The very long infrared wavelength (>14 μm) is a very challenging range for the design of mercury cadmium telluride (HgCdTe) large focal plane arrays (FPAs). The need (mainly expressed by the space industry) for very long wave FPAs appears very difficult to fulfil. High homogeneity, low defect rate, high quantum efficiency, low dark current, and low excess noise are required. Indeed, for such wavelength, the corresponding HgCdTe gap becomes smaller than 100 meV and each step from the metallurgy to the technology becomes critical. This paper aims at presenting a status of long and very long wave FPAs developments at DEFIR (LETI-LIR/Sofradir joint venture). This study will focus on results obtained in our laboratory for three different ion implanted technologies: n-on-p mercury vacancies doped technology, n-on-p extrinsic doped technology, and p-on-n arsenic on indium technology. Special focus is given to 15 μm cutoff n/p FPA fabricated in our laboratory demonstrating high uniformity, diffusion and shot noise limited photodiodes at 50 K.     

Planar p-on-n HgCdTe FPAs by Arsenic Ion Implantation

This paper reviews recent developments in the characterization of planar p-on-n photodiodes fabricated from long- and mid-wavelength Hg_1−x Cd _x Te at␣the Electronics and Information Technology Laboratory (LETI). The Hg_1−x Cd _x Te epitaxial layers were grown by both liquid-phase and molecular-beam epitaxy. Planar p-on-n photodiodes were fabricated by arsenic implantation into an indium-doped Hg_1−x Cd _x Te base layer. Electro-optical characterization on these p-on-n photodiodes showed low leakage currents (shunt resistance > 10 GΩ) and mean R ₀ A values comparable to the state of the art, i.e., equal to 5000 Ω cm² at λ _c = 9.3 μm (λ _c: cutoff wavelength). Results of focal-plane arrays operating in both the long-wavelength infrared (IR) and middle-wavelength IR bands are reported, with noise equivalent delta temperature and responsivity values at λ _c = 9.3 μm in excess of 99.64%. These results demonstrate the viability and technological maturity of both material growth and device processing.     

Improvement of RTS Noise in HgCdTe MWIR Detectors

Random telegraph signal (RTS) noise is present in all bands of the infrared spectrum from λ _c = 2.5 μm (short-wavelength infrared) to λ _c = 15.75 μm (very long-wavelength infrared) and decreases the performance of infrared photodetectors. The main features of RTS noise such as the jump amplitude and RTS frequency are defined, and their dependence as a function of focal-plane array (FPA) temperature was measured for all bands of the infrared spectrum. Both of these features comply with a Boltzmann activation law \( \left( { \propto {\hbox{e}}^{{\frac{{ - E_{\rm{a}} }}{k_{\rm B} T}}} } \right) \), and their activation energies scale with the bandgap. Comparison of three different HgCdTe mid-wavelength infrared photodetector technologies was also performed, showing that the optimized n-on-p improvement of operability (AOP) and p-on-n high-operating-temperature technologies show a reduced number of pixels exhibiting RTS noise (by about two decades) in comparison with standard n-on-p technology.     

Numerical Estimations of Carrier Generation–Recombination Processes and the Photon Recycling Effect in HgCdTe Heterostructure Photodiodes

An enhanced computer program has been applied to explain in detail the photon recycling effect which drastically limits the influence of radiative recombination on the performance of p-on-n HgCdTe heterostructure photodiodes. The computer program is based on a solution of the carrier transport equations, as well as the photon transport equations for semiconductor heterostructures. We distinguish photons in two energy ranges according to p ⁺ and n region with unequal band gaps. As a result, both the distribution of thermal carrier generation and recombination rates and spatial photon density distribution in photodiode structures have been obtained. The general conclusion, similar to our earlier work concerning 3-μm n-on-p HgCdTe heterostructure photodiodes, confirms the previous assertion by Humphreys that radiative recombination does not limit HgCdTe photodiode performance.     

Progress of MCT Detector Technology at AIM Towards Smaller Pitch and Lower Dark Current

We present our latest results on cooled p-on-n planar mercury cadmium telluride (MCT) photodiode technology. Along with a reduction in dark current for raising the operating temperature (T _op), AIM INFRAROT-MODULE GmbH (AIM) has devoted its development efforts to shrinking the pixel size. Both are essential requirements to meet the market demands for reduced size, weight and power and high-operating temperature applications. Detectors based on the p-on-n technology developed at AIM now span the spectrum from the mid-wavelength infrared (MWIR) to the very long wavelength infrared (VLWIR) with cut-off wavelengths from 5 μm to about 13.5 μm at 80 K. The development of the p-on-n technology for VLWIR as well as for MWIR is mainly implemented in a planar photodetector design with a 20-μm pixel pitch. For the VLWIR, dark currents significantly reduced as compared to ‘Tennant’s Rule 07’ are demonstrated for operating temperatures between 30 K and 100 K. This allows for the same dark current performance at a 20 K higher operating temperature than with previous AIM technology. For MWIR detectors with a 20-μm pitch, noise equivalent temperature differences of less than 30 mK are obtained up to 170 K. This technology has been transferred to our small pixel pitch high resolution (XGA) MWIR detector with 1024 × 768 pixels at a 10-μm pitch. Excellent performance at an operating temperature of 160 K is demonstrated.     

Numerical Analysis of Current–Voltage Characteristics of LWIR nBn and p-on-n HgCdTe Photodetectors

We performed numerical analysis of the current–voltage characteristics of long-wavelength infrared unipolar HgCdTe nBn photodetectors and compared those results with those of conventional p-on-n HgCdTe photodiodes. A computer program was applied to explain in detail the impact of the charge carrier generation and recombination processes on current densities. In our model the carrier diffusion, thermal generation–recombination, band-to-band tunneling, trap-assisted tunneling (via states located at mercury vacancies as well as dislocation cores), and impact ionization are included as potential limiting mechanisms. To validate the model, we compared the theoretical predictions with experimental data of high-quality p-on-n photodiodes published in the literature.     

Effect of Dislocations on VLWIR HgCdTe Photodiodes

The effects of dislocations on very-long-wavelength infrared (VLWIR) HgCdTe photodiodes (cutoff wavelength >14 μm at 40 K) have been determined experimentally and analyzed. The photodiodes are in the back-illuminated configuration, fabricated from HgCdTe p-on-n double-layer heterostructure (DLHJ) films grown at BAE Systems by liquid phase epitaxy (LPE) onto lattice-matched (111) CdZnTe substrates. Arrays were hybridized to silicon ROICs to form focal plane arrays (FPAs). After characterization for dark current and response, the arrays were dehybridized and stripped of their metals and passivation layers. Dislocations were revealed using a Hähnert and Schenk (H&;S) etch. Pixel traceability was maintained throughout the analysis, permitting one-to-one correlation between photodiode performance and dislocation density measured within that photodiode. We found that response and dark current were correlated to etch pit density (EPD), which we assumed to be equal to dislocation density. Our results support earlier dislocation studies on larger-bandgap HgCdTe, which showed response was only weakly impacted by EPD, while dark current was strongly affected by EPD. Measured EPD values ranged from low 10⁵ to low 10⁷ cm^?2. Potential causes for this range in EPD are discussed.     

Low-Noise Mid-Wavelength Infrared Avalanche Photodiodes

Mid-wavelength infrared (MWIR) p ⁺–n ⁻–n ⁺ avalanche photodiodes (APDs) were fabricated using two materials systems, one with mercury cadmium telluride (HgCdTe) on a silicon (Si) substrate and the other with an indium arsenide/gallium antimonide (InAs/GaSb) strained layer superlattice (SLS). Diode characteristics, avalanche characteristics, and excess noise factors were measured for both sets of devices. Maximum zero-bias resistance times active area (R ₀ A) of 3 × 10⁶ Ω cm² and 1.1 × 10⁶ Ω cm² and maximum multiplication gains of 1250 at −10 V and 1800 at −20 V were measured for the HgCdTe and the SLS, respectively, at 77 K. Gains reduce to 200 in either case at 120 K. Excess noise factors were almost constant with increasing gain and were measured in the range of 1 to 1.2.     

Accurate Simulation of Temperature-Dependent Dark Current in HgCdTe Infrared Detectors Assisted by Analytical Modeling

Resistance–voltage curves of n ⁺-on-p Hg_1−x Cd _x Te infrared photodiodes were measured in the temperature range of 60 K to 120 K. Characteristics obtained experimentally were fitted by an improved simultaneous-mode nonlinear fitting process. Based on the extracted parameters, an efficient numerical sim- ulation approach has been developed by inserting trap-assisted and band-to-band tunneling models into continuity equations as generation–recombination processes. Simulated dark-current characteristics were found to be in good agreement with the experimental data, demonstrating the validity of the nonlinear fitting process. Our work presents an efficient method for dark-current simulations over a wide range of temperatures and bias voltages, which is important for investigating mechanisms of carrier transport across the HgCdTe junction.     

High-Operating-Temperature MWIR Detector Diodes

The high-operating-temperature (HOT) midwave infrared (MWIR) n-on-p detector has been pursued using the high-density vertically integrated photodiode (HDVIP^®) architecture. In this device, arsenic-doped HgCdTe grown by liquid-phase epitaxy (LPE) is used, passivated on both surfaces with interdiffused CdTe. Dark current densities on these diodes as low as 2.5 mA/cm² normalized to a 5 μm cutoff at 250 K have been demonstrated. 1/f noise at 1 Hz, measured at 250 K, is found to be 6 × 10^?11 A/rHz-cm measured on a cutoff of 4.43 μm. These results agree with the theoretical predictions for the devices made.     

HgCdTe <Emphasis Type="Italic">p</Emphasis>-on-<Emphasis Type="Italic">n</Emphasis> Focal-Plane Array Fabrication Using Arsenic Incorporation During MBE Growth

Extrinsic p-type doping during molecular-beam epitaxy (MBE) growth represents an essential generic toolbox for advanced heterostructures based on the HgCdTe material system: PiN diodes, mesa avalanche photodiodes (APD) or third-generation multispectral focal-plane arrays. Today, arsenic appears to be the best candidate to fulfill this role and our group is actively working on its incorporation during MBE growth, using an original radio frequency (RF) plasma source for arsenic. Such a cell is supposed to deliver a monatomic As flux, and as expected we observed high As electrical activation rates after annealing short-wave (SW), mid-wave (MW), and long-wave (LW) layers. At last, a couple of technological runs have been carried out in the MW range in order to validate the approach on practical devices. p-on-n focal-plane arrays (FPA) have been fabricated using a mesa delineated technology on an As-on-In doped metallurgical heterojunction layer grown on a lattice-matched CdZnTe layer (320 × 256, 30 μm pitch, 5 μm cutoff at 77 K). Observed diodes exhibit very interesting electro-optical characteristics: large shunt impedance, high quantum efficiency, and no noticeable excess noise. The resulting focal-plane arrays were observed to be very uniform, leading to high operabilities. Noise equivalent temperature difference (NETD) distributions are very similar to those observed with the As ion-implanted p-on-n technology, fabricated in our laboratory as well. In our opinion, those excellent results demonstrate the feasibility of our MBE in situ arsenic doping process. Good electrical activation rates and high-quality layers can be obtained. We believe that such an approach allows precise control of the p-doping profile in the HgCdTe layer, which is necessary for advanced structure designs.     

Mid-Wavelength Infrared nBn for HOT Detectors

Recently, new strategies to achieve high-operating-temperature (HOT) detectors have been proposed, including barrier structures such as nBn devices, unipolar barrier photodiodes, alternative materials such as superlattices, and multistage (cascade) infrared devices. In the case of nBn detectors, the barriers must be correctly engineered and correctly located in the device structure to achieve optimal performance. This paper presents the limitations of barrier unipolar devices and the progress in their development for HOT operation in the mid-wavelength infrared range. Their performance is compared with state-of-the-art HgCdTe photodiodes.     

Rational Design of Perylenediimide‐Substituted Triphenylethylene to Electron Transporting Aggregation‐Induced Emission Luminogens (AIEgens) with High Mobility and Near‐Infrared Emission

Organic materials with both high electron mobility and strong solid‐state emission are rare although for their importance to advanced organic optoelectronics. In this paper, triphenylethylenes with varying number of perylenediimide (PDI) unit (TriPE‐nPDIs, n = 1?3) are synthesized and their optical and charge‐transporting properties are systematically investigated. All the molecules exhibit strong solid‐stated near infrared (NIR) emission and some of them exhibit aggregation‐enhanced emission characteristics. Organic field‐effect transistors (OFETs) using TriPE‐nPDIs are fabricated. TriPE‐3PDI shows the best performance with maximum quantum yield of ≈30% and optimized electron mobility of over 0.01 cm² V^?1 s^?1, which are the highest values among aggregation‐induced emission luminogens with NIR emissions reported so far. Photophysical property investigation and theoretical calculation indicate that the molecular conformation plays an important role on the optical properties of TriPE‐nPDI, while the result from film microstructure study reveals that the film crystallinity influences greatly their OFET device performance.     

Multi-heterojunction large area HgCdTe long wavelength infrared photovoltaic detector for operation at near room temperatures

This paper describes a new multi-heterojunction n ⁺pp photovoltaic infrared photodetector. The device has been developed specifically for operation at temperatures of 200–300K in the long wavelength (8–14 μm) range of the infrared spectrum. The new structure solves the perennial problems of poor quantum efficiency and low dynamic resistance found in conventional long wavelength infrared photovoltaic detectors when operated near room temperature. Computer simulations show that devices with properly optimized multiple heterojunctions are capable of achieving the performance limits imposed by the statistical nature of thermal generation-recombination processes. In order to demonstrate the technology, multiple heterojunction devices have been fabricated on epilayers grown by isothermal vapor phase epitaxy of HgCdTe and in situ As p-type doping. The detector structures were formed using a combination of conventional dry etching, angled ion milling, and angled thermal evaporation for contact metal deposition. These multi-junction n ⁺pp HgCdTe heterostructure devices exhibit performances which make them useful for many applications. D* of optically immersed multiple heterostructure photovoltaic detectors exceeding 10⁸cmHz^1/2/W were measured at λ=10.6 μm and T=300K.     

Van der Waals Integration Based on Two-Dimensional Materials for High-Performance Infrared Photodetectors

Infrared photodetectors have been widely applied in various fields, including thermal imaging, biomedical imaging, and communication. Van der Waals (vdW) integration based on 2D materials provides a new solution for high-performance infrared photodetectors due to the versatile device configurations and excellent photoelectric properties. In recent years, great progress has been made in infrared photodetectors based on vdW integration. In this review, recent progress in vdW integration-based infrared photodetectors is presented. First, the working mechanisms and advantages of photodetectors with different structures and band alignments are presented. Then, the recent progress of vdW integration-based infrared photodetectors is reviewed, focusing on 2D/nD (n  =  0, 1, 2, 3) vdW integration, and the band engineering as well as the performance of the photodetectors are discussed in detail. Finally, a summary is delivered, and the challenges and future directions of vdW integration-based infrared photodetectors are provided.     

From Long Infrared to Very Long Infrared Wavelength Focal Plane Arrays Made with HgCdTe n + n ?/ p Ion Implantation Technology

This paper aims at studying the feasibility of very long infrared wavelength (VLWIR) (12–18 μm) focal plane arrays using n-on-p planar ion-implanted technology. To explore and analyze the feasibility of such VLWIR detectors, a set of four Cd _x Hg_1−x Te LPE layers with an 18 μ cutoff at 50 K has been processed at Defir (LETI/LIR–Sofradir joint laboratory), using both our “standard” n-on-p process and our improved low dark current process. Several 320 × 256 arrays, 30-μm pitch, have been hybridized on standard Sofradir readout circuits and tested. Small dimension test arrays characterization is also presented. Measured photonic currents with a 20°C black body suggest an internal quantum efficiency above 50%. Typical I(V) curves and thermal evolution of the saturation current are discussed, showing that standard photodiodes remain diffusion limited at low biases for temperatures down to 30 K. Moreover, the dark current gain brought by the improved process is clearly visible for temperatures higher than 40 K. Noise measurements are also discussed showing that a very large majority of detectors appeared background limited under usual illumination and biases. In our opinion, such results demonstrate the feasibility of high-performance complex focal plane arrays in the VLWIR range at medium term.