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.     

Thermoelectric Characteristics in MBE-Grown HgCdTe-Based Superlattices

We present a study on the thermoelectric properties of n-type Hg_0.75Cd_0.25 Te/Hg_0.7Cd_0.3Te superlattices (SLs). This material system was chosen because HgCdTe is the primary material used in high-performance infrared imaging applications. HgCdTe-based devices can be directly grown on Hg_1−x Cd _x Te/Hg_1−y Cd _y Te SL coolers using advanced growth methods such as molecular-beam epitaxy (MBE), making the monolithic integration of infrared sensors and thermoelectric elements possible. Also, the thermoelectric figure of merit ZT for Hg_0.75Cd_0.25Te/Hg_0.7Cd_0.3Te SLs is predicted to reach values of 2.09, more than two times greater than that achieved in the best thermoelectric devices based on bulk Bi₂Te₃. This large ZT is due to the unique and superior electrical and thermal properties of the HgCdTe system, which has not yet been experimentally explored in any great depth as a thermoelectric material. We used a Riber 32P MBE system equipped with a Hg valved cell, reflection high-energy electron diffraction, infrared pyrometer and in situ spectroscopic ellipsometry to grow the thermoelectric structures. MBE was chosen as a growth technique since it allows for the lowest growth temperature compared with other methods, which limits interdiffusion at the interfaces, thereby allowing for a precise control over electrical and thermal properties. Thermal devices were fabricated using standard photolithography and etching techniques. Thermal properties were evaluated using a differential technique. A thermal conductivity of 0.82 ± 0.07 W/m K and a Seebeck coefficient of 811 ± 150 μV/K were measured. Using a measured value of 0.017 Ω cm for the resistivity, an upper bound ZT of 1.4 is estimated. An erratum to this article can be found at     

Planar n-on-p ion milled mid-wavelength and long-wavelength infrared diodes on molecular beam epitaxy vacancy-doped CdHgTe on CdZnTe

Planar mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) photodiodes were fabricated by ion milling molecular beam epitaxy (MBE) Cd_xHg_1−xTe (CMT) layers with and without compositional grading in the layer. Linear arrays with 32 and 64 diodes, as well as test diodes of varying size, were fabricated. Good quantum efficiencies were measured, and MWIR diodes, with cutoff wavelength λ_CO=4.5 μm, had zero-bias resistance-area values (R₀A) in excess of 1×10⁷ Ωcm², whereas LWIR diodes with λ_CO=8.9−9.3 μm had R₀A=3×10² Ωcm² at 77 K. Comparison between a limited number of layers indicates that in layers with a gradient the RA values are a factor of ∼10 larger, and possibly more uniform, than in layers without a gradient.     

Avalanche Mechanism in p +-n −-n + and p +-n Mid-Wavelength Infrared Hg1−x Cd x Te Diodes on Si Substrates

Hg_1−x Cd _x Te mid-wavelength infrared (MWIR) p ⁺-n ⁻-n ⁺ and p ⁺-n ⁻ avalanche photodiodes (APDs) with a cut-off of 4.9 μm at 80 K were fabricated on Si substrates. Diode characteristics, avalanche characteristics, and excess noise characteristics were measured on two devices. Temperature-dependent diode and avalanche characterization was performed. Maximum 3 × 10⁶ Ω cm² and 9 × 10⁵ Ω cm² zero-bias resistance times active area (R ₀ A) products were measured for the p ⁺-n ⁻-n and p ⁺-n devices at 77 K, respectively. Multiplication gains of 1250 and 410 were measured at −10 and −4 V for the p ⁺-n ⁻-n ⁺ and p ⁺-n APDs at 77 K, respectively, in the front-illumination mode with the help of a laser with an incident wavelength of 632 nm. The gains reduce to 200 and 50 at 120 K, respectively. The excess noise factor in all APDs was measured to be in the range of 1 to 1.2.     

Evaluation of Mn Uniformity in CdMnTe Crystal Grown by the Vertical Bridgman Method

Cd_1−x Mn _x Te is a typical diluted magnetic semiconductor, as well as substrate for the epitaxial growth of Hg_1−x Cd _x Te. In this paper, the homogeneity of a Cd_1−x Mn _x Te (x = 0.2) single-crystal ingot grown by the vertical Bridgman method was studied. The crystal structure and quality of the as-grown ingot were evaluated. Near-infrared (NIR) transmission spectroscopy was adopted to develop a simple optical determination of the Mn concentration in the as-grown ingot. A correlation equation between cut-off wavelength λ _co from NIR transmission spectra and Mn concentration by inductively coupled plasma atomic emission spectrometry (ICP-AES) was established. Using this equation, we investigated the Mn concentration distribution in both the axial and radial directions of the ingot. It was found that the segregation coefficient of Mn in the axial direction of the ingot was 0.95, which is close to unity. The Mn concentration variation in the wafers from the middle part of the ingot was 0.001 mole fraction. All these results proved that homogeneous Cd_0.8Mn_0.2Te crystals can be grown from the vertical Bridgman method.     

Impulse Response Time Measurements in Hg<Subscript>0.7</Subscript>Cd<Subscript>0.3</Subscript>Te MWIR Avalanche Photodiodes

The response time of front-sided illuminated n-on-p Hg_0.7Cd_0.3Te electron avalanche photodiodes (e-APDs) at T = 77 K was studied using impulse response measurements at λ = 1.55 μm. We measured typical rise and fall times of 50 ps and 800 ps, respectively, at gains of M ≈ 100, and a record gain-bandwidth (GBW) product of GBW = 1.1 THz at M = 2800. Experiments as a function of the collection width have shown that the fall time is strongly limited by diffusion. Variable-gain measurements showed that the impulse response is first-order sensitive to the level of the output amplitude. Only a slight increase in the rise time and the fall time was observed with the gain at constant output amplitude, which is consistent with a strongly dominant electron multiplication. Comparisons of the experimental results with Silvaco finite element simulations confirmed the diffusion limitation of the response time and allowed the illustration of the transit time and RC effects.     

Spectroscopic ellipsometry for monitoring and control of molecular beam epitaxially grown HgCdTe heterostructures

A systematic study of the effect of measurement perturbation on in situ monitoring of the composition of molecular beam epitaxially (MBE) grown Hg_1−xCd_xTe using spectroscopic ellipsometry was carried out. Of the five variables investigated, which included angle of incidence, wavelength of the light beam, modulator rotation, analyzer rotation, and modulator amplitude, the angle of incidence and the modulator rotation had the strongest effect on the in situ Hg_1−xCd_xTe composition monitoring process. A wobble-free sample manipulator was installed to reduce the impact of these two variables. With these improvements, the spectroscopic ellipsometer is now routinely used to monitor Hg _1−xCd_xTe compositions during MBE growth of heterostructures and is a useful tool in diagnosing growth-related problems. Examples are included for both application areas, that include the control of the interface between Hg_1−xCd_xTe layers of different compositions, i.e. device engineering.     

Investigation of 1/<Emphasis Type="Italic">f</Emphasis> Noise Mechanisms in Midwave Infrared HgCdTe Gated Photodiodes

In this work, gated midwave infrared (MWIR) Hg_1–x Cd _x Te photodiodes are used to investigate the physical origin of 1/f noise generation. Gated photodiodes were fabricated on liquid-phase epitaxy p-type HgCdTe MWIR material with a vacancy-doped concentration of 1.6 × 10¹⁶ cm⁻³ and x = 0.31. CdTe was thermally deposited and used as both a passivant and a mask for the plasma-based type conversion, and ZnS was used as an insulator. Fabricated devices show a R ₀ A of 1–5 × 10⁴ Ωcm² with zero gate bias. Application of 2 V to the gate improves the R ₀ A by more than two orders of magnitude to 6.0 × 10⁶ Ωcm², which corresponds to the p-type surface being at transition between depletion and weak inversion. Trap-assisted tunneling (TAT) current was observed at negative gate biases and reverse junction biases. For gate biases greater than 3 V, a field-induced junction breakdown was observed. An I _n = α I ^β f ^−0.5 trend was observed above 200 pA reverse bias dark current, with α = 3.5 × 10⁻⁵ and β = 0.82, which corresponds to the TAT dominated region. Below 200 pA, junction generation-recombination (GR) current starts to dominate and this previously mentioned trend is no longer observed. Junction GR current was not seen to be correlated with 1/f noise in these photodiodes.     

Real-time control of HgCdTe growth by organometallic vapor phase epitaxy using spectroscopic ellipsometry

The use of spectroscopic ellipsometry for monitoring the vapor phase epitaxial growth of mercury cadmium telluride (Hg_1−xCd_xTe) in real-time is demonstrated. The ellipsometer is used to perform system identification of the chemical vapor deposition reactor used for the growth of CdTe and to measure the response of the reactor to different growth conditions. The dynamic behavior of the reactor is also studied by evaluating the gas transport delay. The optical constants of Hg_1−xCd_xTe are determined at the growth temperature for different compositions.In-situ real-time composition control is performed during the growth of Hg_1−xCd_xTe. The required target compositions are attained by the ellipsometer and appropriate corrections are also made by the controller when a noise input in the form of a temperature variation is introduced.     

Electrical activity,mode of incorporation and distribution coefficient of group V elements in Hg1−xCdxTe grown from tellurium rich liquid phase epitxial growth solutions

Hg_1−xCd_xTe films were grown liquid phase epitaxially from tellurium rich solutions containing up to 10 at. % of the group V elements P, As, Sb, and Bi. Chemical analysis of the Te growth solutions and the films was carried out in conjunction with extensive Hall effect measurements on the films subsequent to various annealing treatments under Hg rich and Te rich conditions. Despite the presence of a large concentration of the group V elements in the Te source solution, the maximum concentration of these elements incorporated into the liquid phase epitaxially grown Hg_1-xCd_xTe appears to vary from <10¹⁵cm⁻³ for Bi up to 10¹⁷cm⁻³ for phosphorus and As implying a distribution coefficient varying from <10⁻⁵ for Bi up to 10⁻³ for P at growth temperature of ∼500° C. This low value of the distribution coefficient for group V elements for growths from Te rich solutions contrasts with the moderately high values reported in the literature to date for growth from Hg rich solutions as well as pseudobinary solutions (Bridgman growth). The widely differing distribution coefficients and hence the solubility of the group V elements for Hg rich and Te rich liquid phase epitaxial solutions is explained on the basis that the activity coefficient of the group V elements in Te rich solutions is probably orders of magnitude lower than it is in Hg rich solutions. Finally, the results of the anneals at 200° C under Hg saturated conditions with and without a 500° C Hg saturated preanneal have indicatedn top conversion in many of the films attesting to the amphoteric behavior of the group V elements in LPE grown Hg_1−xCd_xTe(s) similar to the previously reported behavior of P in bulk grown Hg_0.8Cd_0.2Te.     

Photoelectric characteristics of diodes in prototype photosensitive pixels for a monolithic array infrared photodetector

Test photodiodes in the form of mesa structures with different areas from 30 × 30 to 100 × 100 μm in size are fabricated based on a Cd _x Hg_{1 − x} Te/Si structure at x = 0.235, grown by molecular-beam epitaxy (MBE). The current-voltage characteristics of the diodes are measured in the dark and under background light conditions. The experimental results are compared with theoretical calculations. It is found that the dependence of the photodiode photocurrent and dark current on the mesa structure size appears in the mesa size ranges from 30 × 30 to 80 × 80 μm. The dark current decreases and the photocurrent increases with decreasing mesa size. The mechanisms affecting the behavior of current-voltage characteristics are discussed.     

Automated compositional control of Hg1−xCdxTe during MBE, using in situ spectroscopic ellipsometry

The implementation of a feedback control system for maintaining a desired compositional value in Hg_1−xCd_xTe epilayers is reported. An 88-wavelength ellipsometer monitored the Cd content (x) of a Hg_1−xCd_xTe film during molecular beam epitaxy, and deviations from a pre-determined set-point were automatically corrected via adjustments in the CdTe effusion cell temperature. The accuracy of this system (Δx∼0.002) was confirmed by Fourier transform infrared transmission measurements made ex situ on the epilayers.     

Correlation between visual defects and increased dark current in large-area Hg1−xCdxTe photodiodes

The Cross-Track Infrared Sounder (CrIS) program [an instrument on the National Polar-Orbiting Operational Environmental Satellite System (NPOESS)] requires photodiodes with spectral cutoffs denoted by short-wavelength infrared [γ_c(98 K) ∼5.1 μm], midwavelength infrared [γ_c(98 K) ∼9.1 μm], and long-wavelength infrared (LWIR) [γ_c(81 K) ∼15.5 μm]. The CrIS instrument also requires large-area (850-μm-diameter) photodiodes with state-of-art performance. Molecular beam epitaxy (MBE) is used to grow n-type short-wavelength infrared, midwavelength infrared, or LWIR Hg_1−xCd_xTe on latticematched CdZnTe. Detectors with p-type implants 7 μm in diameter are used to constitute the 850-μm-diameter lateral collection diodes (LCDs). The photodiode architecture is the double-layer planar heterostructure architecture. Quantum efficiency, I-V, R_d-V, and 1/f noise in photovoltaic Hg_1−xCd_xTe detectors are critical parameters that limit the sensitivity of infrared sounders. These are some of the parameters used to select photodiodes that will be part of the CrIS focal plane module (FPM). During fabrication of the FPM, the photodiodes are subject to a significant amount of handling while transitioning from part of newly processed Hg_1–xCd_xTe wafers to individual photodiodes mounted in a CrIS FPM ready to be flown on NPOESS. Quantum efficiency, I-V, noise, and visual inspections are performed at several steps in the detector’s journey. Initial I-V and visual inspections are conducted at the wafer level followed by I-V, noise, and quantum efficiency after dicing and mounting the photodiodes in leadless chip carriers (LCCs). A visual inspection is performed following removal of the detectors from the LCCs. Finally, the individual photodiodes are precision mounted on an FPM base, and I-V, noise, quantum efficiency, and visual inspections are performed again. Each step in the FPM fabrication process requires handling and environmental conditioning that can result in detector dark current and noise increase. Some photodiodes on the first flightlike FPMs fabricated exhibited an increase in dark current and noise characteristics at the FPM level as compared to the measurements performed when the photodiodes were in LCCs prior to integration into the FPM. The degradation observed resulted in an investigation to discern the cause of the performance degradation (baking at elevated temperatures, mechanical handling, electrical stress, etc.). This paper outlines the results of the study and the corrective actions that led to the successful manufacture of LWIR large detectors from material growth to insertion into flight FPMs for the CrIS program.     

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.     

HgCdTe Research at FFI: Molecular Beam Epitaxy Growth and Characterization

This paper presents results from recent work on molecular beam epitaxy growth of HgCdTe at the Norwegian Defence Research Establishment (FFI), including studies of material properties and fabrication of photodiodes and nanostructures. Systematic studies of defect morphology in HgTe and Hg_1−x Cd _x Te have revealed that there is a minimum in the area covered by defects just below the onset of Te precipitation. The shape and density of microvoids in HgTe can be used to determine the deviation from the optimal growth temperature. This can be further related to the optimal growth temperature of Hg_1−x Cd _x Te with any Cd mole fraction by thermodynamic calculations. A mechanism for the formation of microvoids and needles has been presented. Photoluminescence (PL) has been used to study layers without doping and with Hg vacancy, Ag, and In doping. Planar photodiodes with high dynamic resistance and good quantum efficiency were fabricated by ion-milling vacancy-doped mid-wave and long-wave infrared layers. Quantum wells (QWs) with good crystallinity and high PL light output have been grown. Surface patterning has been found to enhance light emission from HgCdTe thin-film and QW samples by ∼30%. Single-crystal HgTe and segmented HgTe/Te nanowires have been grown, and the resistivity of the nanowires has been measured by conductive atomic force microscopy (AFM), where the AFM tip has been used as a mobile electrode.     

Effect of Hydrogen Free Radicals on Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te

The effects of atomic hydrogen (H) and Br/methanol etching on Hg_1−x Cd _x Te films were investigated using x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Exposure of an as-received Hg_1−x Cd _x Te sample to H + H₂ resulted in H-induced TeO₂ reduction. The oxide reduction was first order with respect to H + H₂ exposure. Exposure to H + H₂ after etching the Hg_1−x Cd _x Te film in a Br/methanol solution induced Hg and C depletion. Hg and C removal was also observed after completely reducing the TeO₂ on the as-received sample. The removal process was hindered by the formation of a Cd-rich overlayer on both etched and unetched surfaces.     

Defect reduction in Hg1−xCdxTe grown by molecular beam epitaxy on Cd0.96Zn0.04Te(211)B

A study on preparation of Cd_0.96Zn_0.04Te(211)B substrates for growth of Hg_1−xCd_xTe epitaxial layers by molecular beam epitaxy (MBE) was investigated. The objective was to investigate the impact of starting substrate surface quality on surface defects such as voids and hillocks commonly observed on MBE Hg_1−xCd_xTe layers. The results of this study indicate that, when the Cd_0.96Zn_0.04Te(211)B substrates are properly prepared, surface defects on the resulting MBE Hg_1−xCd_xTe films are reduced to minimum (size, ∼0.1 m and density ∼500/cm²) so that these MBE Hg_1−xCd_x Te films have surface quality as good as that of liquid phase epitaxial (LPE) Hg_1−xCd_xTe films currently in production in this laboratory.     

Growth of HgTe Quantum Wells for IR to THz Detectors

We zone-engineered HgCdTe/HgTe/HgCdTe quantum wells (QWs) using the molecular-beam epitaxy (MBE) method with in situ high-precision ellipsometric control of composition and thickness. The variations of ellipsometric parameters in the ψ–Δ plane were represented by smooth broken curves during HgTe QW growth with abrupt composition changes. The form of the spiral fragments and their extensions from fracture to fracture revealed the growing layer composition and its thickness. Single and multiple (up to 30) Cd _x Hg_1−x Te/HgTe/Cd _x Hg_1−x Te QWs with abrupt changes of composition were grown reproducibly on (013) GaAs substrates. HgTe thickness was in the range of 16 nm to 22 nm, with the central portion of Cd _x Hg_1−x Te spacers doped by In to a concentration of 10¹⁴ cm⁻³ to 10¹⁷ cm⁻³. Based on this research, high-quality (013)-grown HgTe QW structures can be used for all-electric detection of radiation ellipticity in a wide spectral range, from far-infrared (terahertz radiation) to mid-infrared wavelengths. Detection was demonstrated for various low-power continuous-wave (CW) lasers and high-power THz pulsed laser systems.     

Flexibility of p–n Junction Formation from SWIR to LWIR Using MBE-Grown Hg(1–x)CdxTe on Si Substrates

In this paper, we show the versatility of using molecular-beam epitaxy (MBE) for the growth of the mercury cadmium telluride (HgCdTe) system. Abrupt composition profiles, changes in doping levels or switching doping types are easily performed. It is shown that high-quality material is achieved with Hg_(1–x)Cd _x Te grown by MBE from a cadmium mole fraction of x = 0.15 to x = 0.72. Doping elements incorporation as low as 10¹⁵ cm⁻³ for both n-type and p-type material as well as high incorporation levels >10¹⁸ cm⁻³ for both carrier types were achieved. X-ray curves, secondary-ion mass spectrometry (SIMS) data, Hall data, the influence of doping incorporation with cadmium content and growth rate, etch pit density (EPD), composition uniformity determined from Fourier-transform infrared (FTIR) transmission spectro- scopy, and surface defect maps from low to high x values are presented to illustrate the versatility and quality of HgCdTe material grown by MBE. All data presented in this work are from layers grown on silicon (112) substrate.     

Molecular beam epitaxial growth of HgCdTe on CdZnTe(311)B

A series of n-type, indium-doped Hg_1−xCd_xTe (x∼0.225) layers were grown on Cd_0.96Zn_0.04Te(311)B substrates by molecular beam epitaxy (MBE). The Cd_0.96Zn_0.04Te(311)B substrates (2 cm × 3 cm) were prepared in this laboratory by the horizontal Bridgman method using double-zone-refined 6N source materials. The Hg_1−xCd_xTe(311)B epitaxial films were examined by optical microscopy, defect etching, and Hall measurements. Preliminary results indicate that the n-type Hg_1−xCd_xTe(311)B and Hg_1−xCd_xTe(211)B films (x ∼ 0.225) grown by MBE have comparable morphological, structural, and electrical quality, with the best 77 K Hall mobility being 112,000 cm²/V·sec at carrier concentration of 1.9×10⁺¹⁵ cm⁻³.