HgCdTe Material Research Articles

Growth of high quality CdZnTe (133) epilayers on GaAs (211) substrate with Zn1−xCdxTe/ZnTe buffer layer by close spaced sublimation

The Cd0.96Zn0.04Te (211) crystal is widely recognized as the optimal substrate for epitaxy growth of HgCdTe material. However, its further application is significantly hindered by size and cost limitations. CdZnTe films grown on GaAs substrates by close spaced sublimation may present a promising substrate for epitaxial HgCdTe. In this work, we successfully prepared high quality CdZnTe (133) films on GaAs (211) substrate. The results of scanning transmission electron microscope and selected-area electron diffraction reveals different crystal orientation between GaAs and CdZnTe, which can effectively accommodate the large lattice mismatch. The crystalline quality of the CdZnTe film was improved by introducing a ZnTe buffer layer, resulting in reduction in the etch pit density from 3.2 × 105 to 2.6 × 105 cm−2; a decrease in the full width at half maximum from 57 to 34″, and a lower surface roughness from 9.88 to 3.69 nm. The incorporation of the ZnTe buffer layer prompt the formation of CuPt-type ordered CdZnTe regions at the interface between the CdZnTe film and ZnTe, efficiently relieving the misfit strain originating from GaAs and ZnTe interface. Additionally, a two-step growth method was employed to adjust the Zn content of the film surface by controlling the growth temperature. Diverse lattice constant CdZnTe film is ready to further epitaxial growth of HgCdTe material with different bandgap.

Influence of annealing on the surface structure evolution of intrinsic p-type HgCdTe films.

In this study, scanning electron microscopy, atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and the Hall effect were used to characterize the microscopic morphology, structural features and electrical properties of a HgCdTe film surface before and after annealing. The effect of annealing on the surface quality of the films was studied in detail. The results showed that the annealing treatment reduced the roughness, improve the denseness and uniformity and ameliorate the surface crystallinity. It also eliminated the defects, such as stress and stacking layer errors, caused by lattice distortion during the growth of the HgCdTe films. An increase in surface carrier concentration and mobility after annealing and improved electrical properties of the intrinsic p-type HgCdTe materials were also observed. It is therefore shown that annealing had a significant effect on improving the surface quality of HgCdTe films.

Lead Chalcogenide Colloidal Quantum Dots for Infrared Photodetectors.

Infrared detection technology plays an important role in remote sensing, imaging, monitoring, and other fields. So far, most infrared photodetectors are based on InGaAs and HgCdTe materials, which are limited by high fabrication costs, complex production processes, and poor compatibility with silicon-based readout integrated circuits. This hinders the wider application of infrared detection technology. Therefore, reducing the cost of high-performance photodetectors is a research focus. Colloidal quantum dot photodetectors have the advantages of solution processing, low cost, and good compatibility with silicon-based substrates. In this paper, we summarize the recent development of infrared photodetectors based on mainstream lead chalcogenide colloidal quantum dots.

Impact of nitrogen annealing on the electrical properties of HgCdTe epitaxial films

The nitrogen annealing of HgCdTe materials grown by molecular beam epitaxy (MBE) was carried out to manipulate their electrical properties. The results show that the annealing temperature, annealing time and cooling process all have significant influences on the electrical properties of HgCdTe materials. Excessive annealing temperature or long annealing time can make voids emerge on the surface of the CdTe passivation layer. Carrier concentration and mobility vary exponentially with annealing time and they reach an equilibrium value determined by annealing temperature over a long annealing duration. Moreover, time constants are given and a longer time is needed for mobility to reach an equilibrium value than carrier concentration. The relationship between equilibrium carrier concentration and annealing temperature is given and the activation energy under nitrogen annealing is calculated as 0.63 eV. For a long cooling duration, Hg vacancies are annihilated by Hg atoms diffusion, which makes carrier concentration lower and mobility higher. In addition, some outlier data were found in this experiment and explained by the combination between Te antisites and Hg vacancies.

Structural properties of MBE-grown CdTe (133)B buffer layers on GaAs (211)B substrates with CdZnTe/CdTe superlattice-based dislocation filtering layers

The ever-present demand for high-performance HgCdTe infrared detectors with larger array size and lower cost than currently available technologies based on lattice-matched CdZnTe (211)B substrates has fuelled research into heteroepitaxial growth of HgCdTe and CdTe buffer layers on lattice-mismatched alternative substrates with a (211)B orientation. Driven by the large lattice mismatch, the heteroepitaxial growth of (Hg)CdTe can result in (133)B-orientated material, which, however, has been less explored in comparison to (211)B-oriented growth. Herein, we report on the structural properties of heteroepitaxially grown single-crystal II–VI CdTe (133)B-oriented buffer layers on III–V GaAs (211)B substrates. Azimuthal-dependent x-ray double-crystal rocking curve measurements for the CdTe buffer layers show that the full-width at half-maximum value obtained along the GaAs [1¯11] direction is narrower than that obtained along the GaAs [011¯] direction, which is presumably related to the in-plane anisotropic structural characteristics of the grown CdTe layers. By incorporating strained CdZnTe/CdTe superlattice-based dislocation filtering layers (DFLs), a significant improvement in material quality has been achieved in (133)B-orientated CdTe buffer layers, including a reduced etch pit density in the low-105 cm−2 range and improved surface roughness. These results indicate that the CdTe (133)B DFL buffer layer process is a feasible approach for growing high-quality CdTe and HgCdTe materials on large-area, low-cost alternative substrates.

Effect of Annealing on the Structure of Composite Passivation Films Prepared by Magnetron Sputtering Deposition on the Surface of HgCdTe

In this study, the structural evolution of the CdTe/ZnS composite passivation films on the surface of infrared HgCdTe material before and after annealing was comparatively explored. The surface morphology and microstructure of the passive films before and after annealing were characterized by scanning electron microscope, X-ray diffraction, atomic force microscopy, and Raman and X-ray photoelectron spectroscopy to determine the effects of the annealing process on the surface crystal defects of the films. The results show that annealing at 320 °C for three hours can improve crystal quality, ameliorate the surface density, and reduce the dislocation density of grains, while the Hg grains inhibited by tellurium-rich growth during the annealing process return to the normal growth state, and the S signal can be detected near the surface after annealing. This finding indicates that high-temperature annealing promotes the inhibited grain growth and also causes diffusion of surface elements, thus smoothing the compositional gradient at the passivation interface of the films.

Study of HgCdTe (100) and HgCdTe (111)B Heterostructures Grown by MOCVD and Their Potential Application to APDs Operating in the IR Range up to 8 µm.

The trend related to reach the high operating temperature condition (HOT, temperature, T > 190 K) achieved by thermoelectric (TE) coolers has been observed in infrared (IR) technology recently. That is directly related to the attempts to reduce the IR detector size, weight, and power dissipation (SWaP) conditions. The room temperature avalanche photodiodes technology is well developed in short IR range (SWIR) while devices operating in mid-wavelength (MWIR) and long-wavelength (LWIR) require cooling to suppress dark current due to the low energy bandgap. The paper presents research on the potential application of the HgCdTe (100) oriented and HgCdTe (111)B heterostructures grown by metal-organic chemical vapor deposition (MOCVD) on GaAs substrates for the design of avalanche photodiodes (APDs) operating in the IR range up to 8 µm and under 2-stage TE cooling (T = 230 K). While HgCdTe band structure with molar composition xCd < 0.5 provides a very favorable hole-to-electron ionization coefficient ratio under avalanche conditions, resulting in increased gain without generating excess noise, the low level of background doping concentration and a low number of defects in the active layer is also required. HgCdTe (100) oriented layers exhibit better crystalline quality than HgCdTe (111)B grown on GaAs substrates, low dislocation density, and reduction of residual defects which contribute to a background doping within the range ~1014 cm–3. The fitting to the experimentally measured dark currents (at T = 230 K) of the N+-ν-p-P+ photodiodes commonly used as an APDs structure allowed to determine the material parameters. Experimentally extracted the mid-bandgap trap concentrations at the level of 2.5 × 1014 cm−3 and 1 × 1015 cm−3 for HgCdTe (100) and HgCdTe (111)B photodiode are reported respectively. HgCdTe (100) is better to provide high resistance, and consequently sufficient strength and uniform electric field distribution, as well as to avoid the tunneling current contribution at higher bias, which is a key issue in the proper operation of avalanche photodiodes. It was presented that HgCdTe (100) based N+-ν-p-P+ gain, M > 100 could be reached for reverse voltage > 5 V and excess noise factor F(M) assumes: 2.25 (active layer, xCd = 0.22, k = 0.04, M = 10) for λcut-off = 8 μm and T = 230 K. In addition the 4-TE cooled, 8 μm APDs performance was compared to the state-of-the-art for SWIR and MWIR APDs based mainly on III-V and HgCdTe materials (T = 77–300 K).

Optical and structural studies of Hg0.7Cd0.3Te samples grown by various methods

Photoluminescence and X-ray diffraction (XRD) were used for the studies of the properties of HgCdTe samples with CdTe molar fraction x=0.3 grown by various methods. According to the results of photoluminescence studies, all samples possessed a considerable degree of alloy disorder, yet the scale of the disorder seemed not to be directly related to the structural quality of the material as revealed using XRD. Prospects of using HgCdTe material grown by various methods in optoelectronic devices are discussed.

Impact of conductivity type change in InAs/GaSb superlattice on low frequency noise of photoconductive long-wavelength infrared detectors

This Letter focuses on the 1/f noise properties of InAs/GaSb superlattice (SL), which is a promising material for infrared radiation detection and represents one of the alternatives to well-established bulk HgCdTe material. The InAs/GaSb SL material changes the conductivity type at temperature T ≈ 190 K, which has been correlated with measured 1/f noise. It was shown that 1/f noise comes from resistance fluctuations of linear noise sources. According to the electronic transport and 1/f noise models, the observed 1/f noise is connected with the hole conductivity component rather than the electron conductivity component, which is absent or at least immeasurable, even though electron conductivity governs the total conductivity of the InAs/GaSb SL. In the high-temperature region, the 1/f noise of InAs/GaSb SL is significantly smaller than that of InAs/InAsSb SL. The results favor InAs/GaSb SL material over InAs/InAsSb SL for photoconductive infrared detectors operating at room temperature.

Investigating the influence of CdZnTe and HgCdTe material quality on detector image performance

High-quality tellurium zinc cadmium (CZT) crystal materials and tellurium cadmium mercury (MCT) epitaxial materials are the key materials for the preparation of high-performance MCT-cooled detectors. Studying the relationship between material defects and device imaging is essential for improving the material quality and device performance. In this work, we summarize the defects of the CZT single crystal and the surface defects of the MCT film. Furthermore, we demonstrate the effect of CZT and MCT material defects and lattice matching on the imaging performance of the chips, and summarize the material quality improvement and the standard control methods. Through processes such as heat treatment, the size of the inclusions is controlled to be less than 20 μm and the density is less than 20 cm−2. A focal plane array (FPA) with good uniformity imaging (NETD 14.9 mK, defective pixel rate 0.03%) can be obtained by improving the material preparation level and formulating suitable material selection standards. Based on the continuous improvement of yield of FPA preparation process, MCT detectors can be fabricated in batches based on the optimized material quality.

Design and simulation of Mg&lt;sub&gt;2&lt;/sub&gt;Si/Si avalanche photodiode

InGaAs and HgCdTe materials are widely used in short wave infrared photodetectors, which contain heavy metal elements. The massive use of the heavy metal elements naturally results in their scarcity, and the nonnegligible environmental pollution. Searching for other suitable materials for infrared devices becomes a key to solving the above problems. As a kind of abundant and eco-friendly material, Mg&lt;sub&gt;2&lt;/sub&gt;Si has a high absorption coefficient in the near-infrared band. Its application in infrared detector makes it possible to replace the infrared devices containing toxic elements on the market in the future. The Mg&lt;sub&gt;2&lt;/sub&gt;Si/Si avalanche photodiode(APD) with separation structure of absorption layer, charge layer and multiplication layer, with Mg&lt;sub&gt;2&lt;/sub&gt;Si serving as the absorption layer, is constructed by using the Atlas module in Silvaco software. The effects of the thickness and doping concentration of the charge layer and multiplier layer on the distribution of internal electric field, punch-through voltage, breakdown voltage (&lt;i&gt;V&lt;/i&gt;&lt;sub&gt;b&lt;/sub&gt;), &lt;i&gt;C-&lt;/i&gt;&lt;i&gt;V&lt;/i&gt; characteristics, and transient response of Mg&lt;sub&gt;2&lt;/sub&gt;Si/Si SACM-APD are simulated. The effects of bias voltage on the &lt;i&gt;I-&lt;/i&gt;&lt;i&gt;V&lt;/i&gt; characteristics and spectral response are analyzed. The punch-through voltage, breakdown voltage, dark current density, gain coefficient (&lt;i&gt;M&lt;/i&gt;&lt;i&gt;&lt;sub&gt;n&lt;/sub&gt;&lt;/i&gt;) and the current amplification factor (&lt;i&gt;M&lt;/i&gt;) after avalanche effect of APD are obtained after the structure optimization. According to the simulation results, the spectral response wavelength of the device is extended to 1.6 μm, so the selection of Mg&lt;sub&gt;2&lt;/sub&gt;Si as the absorption layer effectively extends the spectral response band of Si based APD. When the wavelength of incident light is 1.31 µm and the optical power is 10 mW/cm&lt;sup&gt;2&lt;/sup&gt;, the obtained punch-through voltage is 17.5 V, and the breakdown voltage is 50 V. When the bias voltage is 47.5 V (0.95&lt;i&gt;V&lt;/i&gt;&lt;sub&gt;b&lt;/sub&gt;), the peak value of spectral response is 25 A/W at a wavelength of 1.1 μm, a density of dark current is about 3.6 × 10&lt;sup&gt;–5&lt;/sup&gt; A/cm&lt;sup&gt;2&lt;/sup&gt;, a multiplication factor &lt;i&gt;M&lt;/i&gt;&lt;i&gt;&lt;sub&gt;n&lt;/sub&gt;&lt;/i&gt; is 19.6, and &lt;i&gt;M&lt;/i&gt;&lt;i&gt;&lt;sub&gt;n&lt;/sub&gt;&lt;/i&gt; achieves a maximum value of 102 when the device is broken down. Meanwhile, the current amplification factor &lt;i&gt;M&lt;/i&gt; after avalanche effect is 75.4, and the current gain effect of the SACM structure is obvious. The peak value of spectral response for the pin-type photodiode in the previous study is only 0.742 A/W. Comparing with the pin-type photodiode, the spectral response of Mg&lt;sub&gt;2&lt;/sub&gt;Si/Si SACM-APD is greatly improved. In this work, the structure parameters of the device are optimized, which lays a nice foundation for fabricating the high-performance devices.

Infrared MBE-Grown HgCdTe Focal Plane Arrays and Cameras After High Energy Neutron Irradiation

HgCdTe is one of the most important materials for the fabrication of infrared detectors and focal plane arrays (FPAs) deployed in environments where high-energy particles, such as protons and neutrons, are present. We designed and fabricated HgCdTe-based FPAs that can be used in high neutron radiation environments and we measured their characteristics. The influence of the radiation on the infrared FPAs and cameras is investigated. HgCdTe material and devices are capable of maintaining high performances in a high energy neutron irradiation environment. For MWIR FPA directly facing a 2.59 × 108 n/cm2 s neutron flux beam (with the highest energy 66 MeV) for 1 h, the noise equivalent differential temperature (NEDT) increased ∼ 8 times after irradiation. However, NEDT decreased to 33 mK (compared to the original value of 21 mK) after one warming-up (to room temperature) and cooling-down cycle. The NEDT for the MWIR FPAs mounted parallel to the beam did not degrade (16 mK and 28 mK before irradiation, changed to 18 mK and 26 mK after irradiation, respectively).

Optical studies of wide-bandgap HgCdTe material used in potential-and quantum-well structures

Optical transmission and photoluminescence were used for the study of wide-bandgap (0.8-1.1 eV) HgCdTe (MCT) material grown by molecular-beam epitaxy. The material, including layers used as spacers and barriers in potential-and quantum-well structures, showed a considerable degree of alloy disorder similar to narrow-gap MCT grown by the same method. In some samples, defect states in the bandgap were found. Optimization of the growth technology for wide-bandgap material should help improving the quality of MCT-based potential-and quantum-well structures designed for various applications.

Barrier engineering for HgCdTe unipolar detectors on alternative substrates

Delta-doped layers together with compositionally grading have been utilized to get nBn configurations for the HgCdTe material system in all the short-wave (SWIR), medium-wave (MWIR) and long-wave (LWIR) infrared bands. Shockley Read Hall (SRH), trap-assisted tunneling (TAT), Auger and radiative recombination mechanisms have been included in the analyses and strong suppression of SRH and TAT currents have been demonstrated with the designed structures. This methodology is especially useful when the carrier lifetime is limited due to alternative substrate usage. No degradation in photo-response has been observed as adjusting the valence band offset is quite flexible with the delta-doped nano-layers and the valence band barrier can be completely removed. Calculations have been performed for 1–3 μs lifetime targeting the alternative substrate applications and up to 60 degrees of increase in the operation has been shown to be possible.

Enhancing the Performance of Commercial Infrared Detectors by Surface Plasmons

The feasibility to improve the response of quantum type (photonic) infrared (IR) detectors by applying surface plasmons is investigated. The HgCdTe material system is used as the detector platform of interest for which selected plasmonic structures and materials are applied and the influence studied by full-fielded electromagnetic simulations. It is shown that even for the several-micrometers-thick detector structures, similar to the commercial ones, broadband absorption enhancements of 30–40% can be achieved. The results suggest that improved, or new, pixel-level functionalities can be created for commercial IR detectors by relatively simple means. Additionally, high potential for cost reduction in high-performance IR imaging systems with multicolour capabilities is foreseen.

Type-II superlattice detectors for free space optics applications and higher operating temperature conditions

The utmost limit performance of interband cascade detectors optimized for the longwave range of infrared radiation is investigated in this work. Currently, materials from the III–V group are characterized by short carrier lifetimes limited by Shockley-Read-Hall generation and recombination processes. The maximum carrier lifetime values reported at 77 K for the type-II superlattices InAs/GaSb and InAs/InAsSb in a longwave range correspond to ∼200 and ∼400 ns. We estimated theoretical detectivity of interband cascade detectors assuming above carrier lifetimes and a value of ∼1–50 μs reported for a well-known HgCdTe material. It has been shown that for room temperature the limit value of detctivity is of ∼3–4×1010 cmHz1/2/W for the optimized detector operating at the wavelength range ∼10 μm could be reached.

MBE growth of high quality HgCdSe on GaSb substrates

This paper demonstrates MBE growth of high quality HgCdSe infrared materials on GaSb (211)B substrates. The as-grown Hg1−xCdxSe samples have a range of x-values (x = 0.37–0.18) and cut-off wavelengths (λc = 3.9–10.4 μm at 80 K), and show typical n-type semiconductor behaviour. At a measurement temperature of 80 K, the as-grown HgCdSe samples with x = 0.18 present a cut-off wavelength of 10.4 μm, an electron mobility as high as 1.3 × 105 cm2 V−1 s−1, a background electron concentration as low as 1.6 × 1016 cm−3, and a minority carrier lifetime as long as 2.2 μs. These values of electron mobility and minority carrier lifetime represent a significant improvement on previous studies of MBE-grown HgCdSe reported in the open literature, and are comparable to those of corresponding HgCdTe materials grown on lattice-matched CdZnTe substrates. This high material quality is primarily due to the nearly lattice-matched epitaxial growth of HgCdSe on GaSb, as well as an optimised growth temperature. These preliminary results indicate that HgCdSe materials grown on GaSb can meet the basic material quality requirements for the fabrication of high performance infrared detectors, although further effort is required in order to reduce the background electron concentration to <1015 cm−3. Furthermore, even higher quality HgCdSe materials on GaSb are expected by further optimization of the growth conditions, using higher purity Se source material, and implementing post-growth thermal annealing in a Se environment. The results of this study demonstrate the great potential of HgCdSe infrared materials grown on large-area commercially-available substrates for meeting the requirements of next generation infrared imaging focal plane arrays with features of lower cost and larger array format size.

Slow Auger Relaxation in HgTe Colloidal Quantum Dots.

The biexciton lifetimes in HgTe colloidal quantum dots are measured as a function of particle size. Samples produced by two synthetic methods, leading to partially aggregated or well-dispersed particles, exhibit markedly different dynamics. The relaxation characteristics of partially aggregated HgTe inhibit reliable determinations of the Auger lifetime. In well-dispersed HgTe quantum dots, the biexciton lifetime increases approximately linearly with particle volume, confirming trends observed in other systems. The extracted Auger coefficient is three orders of magnitude smaller than that for bulk HgCdTe materials with similar energy gaps. We discuss these findings in the context of understanding Auger relaxation in quantum-confined systems and their relevance to mid-infrared optoelectronic devices based on HgTe colloidal quantum dots.

Inductively Coupled Plasma-Induced Electrical Damage on HgCdTe Etched Surface at Cryogenic Temperatures

Plasma etching is a powerful technique for transferring high-resolution lithographic patterns into HgCdTe material with low etch-induced damage, and it is important for fabricating small-pixel-size HgCdTe infrared focal plane array (IRFPA) detectors. P- to n-type conversion is known to occur during plasma etching of vacancy-doped HgCdTe; however, it is usually unwanted and its removal requires extra steps. Etching at cryogenic temperatures can reduce the etch-induced type conversion depth in HgCdTe via the electrical damage mechanism. Laser beam-induced current (LBIC) is a nondestructive photoelectric characterization technique which can provide information regarding the vertical and lateral electrical field distribution, such as defects and p–n junctions. In this work, inductively coupled plasma (ICP) etching of HgCdTe was implemented at cryogenic temperatures. For an Ar/CH4 (30:1 in SCCM) plasma with ICP input power of 1000 W and RF-coupled DC bias of ∼ 25 V, a HgCdTe sample was dry-etched at 123 K for 5 min using ICP. The sample was then processed to remove a thin layer of the plasma-etched region while maintaining a ladder-like damaged layer by continuously controlling the wet chemical etching time. Combining the ladder etching method and LBIC measurement, the ICP etching-induced electrical damage depth was measured and estimated to be about 20 nm. The results indicate that ICP etching at cryogenic temperatures can significantly suppress plasma etching-induced electrical damage, which is beneficial for defining HgCdTe mesa arrays.

Terahertz photoconductivity of double acceptors in narrow gap HgCdTe epitaxial films grown by molecular beam epitaxy on GaAs(013) and Si(013) substrates

The energy spectra of the mercury vacancy, the most common acceptor in HgCdTe material, is studied via numerical calculations and low temperature photoconductivity (PC) measurements of ‘vacancy-doped’ HgCdTe films with low cadmium content. Since the Hg vacancy is known to be a double acceptor, the model for the helium atom was adopted for degerate valence band of zinc blende semiconductors to classify the observed PC bands. This approach provides a fairly good description of the photoionization of both neutral and singly-ionized vacancy when the central cell potential is taken into account.