CdTe Buffer Layer Research Articles

MBE-grown MCT hetero- and nanostructures for IR and THz detectors

We present an overview of our technological achievements in the implementation of detector structures based on mercury cadmium telluride (MCT) heterostructures and nanostructures for IR and THz spectral ranges. We use a special MBE design set for the epitaxial layer growth on (013) GaAs substrates with ZnTe and CdTe buffer layers up to 3” in diameter with the precise ellipsometric monitoring in situ . The growth of MCT alloy heterostructures with the optimal composition distribution throughout the thickness allows for the realization of different types of many-layered heterostructures and quantum wells to prepare the material for fabricating single- or dual-band IR and THz detectors. We also present the two-color broad-band bolometric detectors based on the epitaxial MCT layers that are sensitive in 150–300-GHz subterahertz and infrared ranges from 3 to 10 μm, which operate at the ambient or liquid nitrogen temperatures as photoconductors, as well as the detectors based on planar HgTe quantum wells. The design and dimensions of THz detector antennas are optimized for reasonable detector sensitivity values. A special diffraction limited optical system for the detector testing was designed and manufactured. We represent here the THz images of objects hidden behind a plasterboard or foam plastic packaging, obtained at the radiation frequencies of 70, 140, and 275 GHz, respectively.

Low dislocation density MBE process for CdTe-on-GaSb as an alternative substrate for HgCdTe growth

This work demonstrates a low dislocation density molecular beam epitaxial process (average etch pit density ∼1.4 × 105 cm−2) for the growth of CdTe buffer layers on GaSb (211)B alternative substrates for subsequent growth of HgCdTe infrared materials. This dislocation density is much lower than that for CdTe layers grown on other alternative substrates (mid-106 to low-107 cm−2 range for Si, Ge and GaAs), is well below the critical level required for fabricating high performance long-wave infrared HgCdTe detectors (5 × 105 cm−2), and is close to that achieved on lattice-matched CdZnTe substrates (mid-104 to low-105 cm−2 range). The low dislocation density is achieved by inserting a ZnTe/CdTe-based transitional buffer layer between the GaSb substrate and the CdTe buffer layer. The main purpose of this transitional buffer layer is to better accommodate the 6.1% lattice mismatch between the GaSb substrate and the CdTe epitaxial layer, which is evidenced by X-ray diffraction reciprocal space mapping. Additional benefits of this transitional buffer layer include possible blocking/filtering of misfit dislocation propagation, as well as gettering of defects and impurities. More importantly, an even lower dislocation density can be expected by increasing the thickness of the CdTe epitaxial layer and implementing a thermal annealing cycle for more efficient gettering. The results of this study indicate the great potential of GaSb as an alternative substrate for growing next generation HgCdTe infrared materials to meet the focal plane array requirements of higher device yield, lower cost and larger array format size.

X‐Ray Reciprocal Space Mapping of MBE Grown HgCdTe on Alternative Substrates

AbstractX‐ray reciprocal space mapping is used to yield quantitative information on strain, strain‐relaxation, shear, as well as dislocation density and mosaicity in MBE‐grown HgCdTe heterostructures. Triple axis x‐ray diffractometry is used to obtain the data, and analysis of the isointensity contours of scattered x‐ray intensity around reciprocal lattice points in HgCdTe epitaxial layers grown on (211)‐oriented substrates. Three different substrates, GaSb, GaAs, and CdZnTe, are used in this investigation. The in‐plane and out‐of‐plane lattice parameters, and strain of the HgCdTe epilayer and underlying CdTe buffer layer, are determined from x‐ray reciprocal space maps about the symmetric (422), and asymmetric (333 & 511) reciprocal lattice points. Analysis of X‐ray scattering about the lattice mismatched reciprocal lattice points show approximately an order of magnitude increase in shear, dislocation density, and strain for growth on alternative substrates in comparison to HgCdTe grown on lattice matched CdZnTe. HgCdTe on GaSb exhibits less shear and the layers are found to be relaxed within the buffer layer. In comparison, higher density of misfit dislocations were found in epilayers grown on GaAs. The analysis of strain distribution, and misfit dislocation density clearly indicate the potential application of GaSb as an alternative substrate for MBE growth of HgCdTe.

Correlation of Etch Pits and Dislocations in As-grown and Thermal-Cycle-Annealed HgCdTe(211) Films

This paper reports observations of the different types of etch pits and dislocations present in thick HgCdTe (211) layers grown by molecular beam epitaxy on CdTe/Si (211) composite substrates. Dislocation analysis for as-grown and thermal cycle-annealed samples has been carried out using bright-field transmission electron microscopy. Triangular pits present in as-grown material are associated with a mixture of Frank partials and perfect dislocations, while pits with fish-eye shapes have perfect dislocations with $$ \frac{1}{2}[0\bar{1}1] $$ Burgers vector. The dislocations beneath skew pits are more complex as they have two different crystallographic directions, and are associated with a mixture of Shockley partials and perfect dislocations. Dislocation analysis of samples after thermal cycle annealing (TCA) shows that the majority of dislocations under the etch pits are short segments of perfect dislocations with $$ \frac{1}{2}[0\bar{1}1] $$ Burgers vector while the remainder are Shockley partials. The absence of fish-eye shape pits in TCA samples suggests that they are associated with mobile dislocations that have reacted during annealing, causing the overall etch pit density to be reduced. Very large pits with a density ∼2×103 cm−2 are observed in as-grown and TCA samples. These defects thread from within the CdTe buffer layer into the upper regions of the HgCdTe layers. Their depth in as-grown material is so large that it is not possible to locate and identify the underlying defects.

CdTe Nanowires by Au-Catalyzed Metalorganic Vapor Phase Epitaxy.

We report on the first Au-catalyzed growth of CdTe nanowires by metalorganic vapor phase epitaxy. The nanowires were obtained by a separate precursors flow process in which (i) di-isopropyl-telluride (iPr2Te) was first flowed through the reactor to ensure the formation of liquid Au-Te alloy droplets, and (ii) after purging with pure H2 to remove unreacted iPr2Te molecules from the vapor and the growth surface, (iii) dimethylcadmium (Me2Cd) was supplied to the vapor so that Cd atoms could enter the catalyst droplets, leading to nanowire self-assembly. CdTe nanowires were grown between 485 and 515 °C on (111)B-GaAs substrates, the latter preliminary deposited with a 2 μm thick (111)-oriented CdTe buffer layer onto which Au nanoparticles were provided. As-grown CdTe nanowires were vertical ([111]-aligned) straight segments of constant diameter and showed an Au-rich nanodroplet at their tips, the contact angle between the droplets and the nanowires being ∼130°. The nanowire axial growth rate appeared kinetics-limited with an activation energy ∼57 kcal/mol. However, the growth rate turned independent from the nanowire diameter. Present data are interpreted by a theoretical model explaining the nanowire growth through the diffusion transport of Te adatoms under the assumption that their growth occurs during the Me2Cd-flow process step. Low-temperature cathodoluminescence spectra recorded from single nanowires showed a well-resolved band-edge emission typical of zincblend CdTe along with a dominant band peaked at 1.539 eV.

Correlation of Etch Pits and Dislocations in As-grown and Thermal Cycle-Annealed HgCdTe(211) Films

This paper reports observations of the different types of etch pits and dislocations present in thick HgCdTe (211) layers grown by molecular beam epitaxy on CdTe/Si (211) composite substrates. Dislocation analysis for as-grown and thermal cycle-annealed samples has been carried out using bright-field transmission electron microscopy. Triangular pits present in as-grown material are associated with a mixture of Frank partials and perfect dislocations, while pits with fish-eye shapes have perfect dislocations with $$ \frac{1}{2}[0\bar{1}1] $$ Burgers vector. The dislocations beneath skew pits are more complex as they have two different crystallographic directions, and are associated with a mixture of Shockley partials and perfect dislocations. Dislocation analysis of samples after thermal cycle annealing (TCA) shows that the majority of dislocations under the etch pits are short segments of perfect dislocations with $$ \frac{1}{2}[0\bar{1}1] $$ Burgers vector while the remainder are Shockley partials. The absence of fish-eye shape pits in TCA samples suggests that they are associated with mobile dislocations that have reacted during annealing, causing the overall etch pit density to be reduced. Very large pits with a density ∼2×103 cm−2 are observed in as-grown and TCA samples. These defects thread from within the CdTe buffer layer into the upper regions of the HgCdTe layers. Their depth in as-grown material is so large that it is not possible to locate and identify the underlying defects.

Electrical and Optical Properties of n-Type Indium-Doped CdTe/Mg0.46Cd0.54Te Double Heterostructures

CdTe/Mg0.46Cd0.54Te double heterostructures with n-type In doping concentrations, varied from 1 × 1016 to 7 × 1018 cm−3, have been grown on InSb substrates using molecular beam epitaxy. Secondary ion mass spectroscopy measurements show strong diffusion of In from the InSb substrate to the CdTe buffer layer, while the In concentration is constant in the CdTe layer between the two Mg 0.46Cd0.54Te barrier layers. Capacitance–voltage measurements show that the dopants are 100% ionized for the doping concentration range from 1 × 1016 to 1 × 1018 cm −3. The carrier lifetime decreases with increasing doping concentration (from 0.73 μs for an unintentionally doped sample to 0.74 ns for a 1 × 1018 cm−3 doped sample) due to the decrease of both radiative and nonradiative lifetimes. Decent carrier lifetimes are achieved (∼100 ns) between 1 × 1016 and 1 × 1017 cm−3 doping levels, which is beneficial for developing n-type monocrystalline CdTe solar cells, photodetectors, and other optoelectronic devices. The strongest photoluminescence intensity is observed when the doping concentration is 1 × 1017 cm −3, which corresponds to the highest internal quantum efficiency.

High operating temperature SWIR p+–n FPA based on MBE-grown HgCdTe/Si(0 1 3)

The characteristics of SWIR (1.6–3μm) 320×256 and 1024×1024 focal plane arrays (FPA’s) based on n-type In-doped HgCdTe heteroepitaxial layers are reported. The HgCdTe layers were grown by molecular beam epitaxy on silicon substrates with ZnTe and CdTe buffer layers. p–n junctions were formed by arsenic ion implantation into HgCdTe film. Reverse current in the temperature range from 210 to 330K was found to be limited by the diffusion mechanism. At the same time in the temperature range from 140 to 210K the reverse current was dominated by the thermal generation of charge carriers through deep traps located in the middle of the band gap. At 170K NETD was less than 40mK.

Recent Developments in Mercury Cadmium Telluride IR Detector Technology

In spite of many attempts to find better material for Infrared Detectors, the II-VI compound HgCdTe still dominates the high performance end of the market. Since its invention in 1959, HgCdTe technology evolved from bulk crystal growth and crude single infrared detectors to very sophisticated, epitaxially grown material and large format IR imaging arrays of photovoltaic devices. Today, the applications driven research and development in this field are directed towards high pixel density beyond diffraction limit, high yield, reduced cooling and multiband and hyperspectral operation. In order to meet these goals there is an urgent need for still better quality HgCdTe material and new band engineered devices with reduced dark current and noise levels. In terms of HgCdTe material, the most challenging is development of large area, high quality epi-ready substrates allowing for growth of HgCdTe with low dislocation defect density – a major source of dark current and noise. Traditionally, lattice matched CdZnTe with 4% Zn content has been used. However, the cost of CdZnTe substrates is prohibitive, their size is limited, and hence not adequate for today’s large size detector arrays. The readily available large size Si and GaAs substrates have been investigated for some time but due to large lattice mismatch (19% and 14.3% respectively) few microns thick buffer layers of CdTe are needed to reduce the high lattice dislocations density in HgCdTe layers. But even with these CdTe buffer layers the best reported dislocation density in HgCdTe is still about two orders of magnitude higher than in HgCdTe epi-layers grown directly on lattice matched CdZnTe. Recently low-doped large epi-ready GaSb substrates become available and have been investigated as an alternative for HgCdTe growth. With relatively low lattice mismatch of only 6.1%, these substrates require much thinner buffer layer, and, as our studies indicate, even un-optimised growth results with CdTe buffer layer quality matching the best reported results for CdTe buffers grown on GaAs, consequently leaving much room for improvement. In parallel with development of substrates a large effort is directed towards new detector device structures with the main goals focused on reduction of dark current and noise levels, as well as simplified processing leading to higher yield. In recent years so called barrier structures attracted much attention within HgCdTe research community. The concept based on the device consisting of the n-type absorber, undoped barrier, and thin n-type contact (nBn) comes from III-V technology where it was successfully implemented in InAs/GaSb system. Theoretically device like this does not have built in voltage region (reduced dark current and noise) and if externally biased allows for a free flow of minority photo-electrons while blocking majority photo-holes. Implementation of this concept in HgCdTe material system is not straightforward as its band structure is less favourable than that of InAs/GaSb system resulting in a small barrier in the valence band as well. However, this undesirable effect may be compensated by superior HgCdTe electron mobility and photo-generated carrier lifetime. Moreover, few promising concepts for elimination of the valence band barrier in HgCdTe nBn structure have been proposed. They are based on simultaneous modulation of material composition and doping within the barrier, or alternatively, the growth of the HgTe/CdTe superlattice barrier with energy structure optimised by band engineering.

MBE Growth of Mid-wave Infrared HgCdTe Layers on GaSb Alternative Substrates

GaSb has been studied as a new alternative substrate for growing HgCdTe via molecular beam epitaxy (MBE). Cross-sectional transmission electron microscopy (TEM) studies indicate that MBE-grown CdTe buffer layers on GaSb have much lower misfit dislocation density than comparable layers grown on GaAs. The MBE-grown mid-wave infrared (MWIR) HgCdTe layers on GaSb substrates present material quality comparable to those grown on GaAs substrates, which is one of the state-of-the-art alternative substrates currently used to grow HgCdTe for the fabrication of MWIR detectors and focal plane arrays. Typically, HgCdTe materials grown on GaSb are found to have a rocking curve (double crystal x-ray diffraction) full width at half maximum of ~122 arcsec and an etch pit density of ~mid-106 cm–2. Electron backscatter diffraction mapping shows that the lattice misorientation/misfit dislocations near the HgCdTe/CdTe interface are negligible for GaSb substrates in comparison to GaAs substrates, and that the material quality of the HgCdTe layer on GaSb is determined primarily by the material quality of the CdTe buffer layer. These preliminary results are very encouraging considering that this is a relatively recent research effort, and higher quality MBE-grown HgCdTe materials are expected on GaSb substrates with further optimization of HgCdTe growth conditions as well as further improvements in the growth conditions for CdTe buffer layers.

Characterization of CdTe Growth on GaAs Using Different Etching Techniques

CdTe buffer layers which were grown on (211)B GaAs by molecular beam epitaxy were subjected to two different etch treatments to quantify the crystal quality and dislocation density. The optical properties and thicknesses of the samples were obtained by ex situ spectroscopic ellipsometry. The surface morphologies of the CdTe epilayers were analyzed by atomic force microscopy, scanning electron microscopy, and Nomarski microscopy before and after chemical etching. We compare the triangle- and trapezoid-shaped etch pits due to the Everson and Nakagawa etch solutions, respectively. Measured etch pit density (EPD) values of triangle etch pits were found in the 8 × 107 cm−2 to 2 × 108 cm−2 range, and trapezoid-shaped etch pits were found in the 1 × 107 cm−2 to 7 × 107 cm−2 range for samples with thicknesses <2 μm.

CdHgTe heterostructures on large-area Si(310) substrates for infrared photodetector arrays of the short-wavelength spectral range

Heteroepitaxial structures n-CdxHg1 − xTe for the near-infrared spectral range (x ≈ 0.4) are grown by molecular beam epitaxy on Si(310) substrates 72.6 and 100 mm in diameter. High composition homogeneity over the structure area is attained; the variation in x for 100-mm wafers is 0.015–0.025. During growth, the mercury-cadmium telluride (MCT) layers are doped with In with the concentration (0.5–3) × 1015 cm−3. The magnetic-field dependences of the Hall effect are studied in the range of magnetic fields 0.05–1.0 T at liquid-nitrogen and room temperatures. The experimental values of the electron mobility at room temperature are close to the calculated ones, while at liquid-nitrogen temperature, they are lower than the calculated mobilities. The possible causes of this phenomenon such as the influence of the MCT transition layer at the interface with the CdTe buffer layer and lattice defects of MCT are discussed. The variation in the concentration and mobility of charge carriers in MCT structures after activation annealing are studied.

GaSb: A New Alternative Substrate for Epitaxial Growth of HgCdTe

In this work, GaSb is proposed as a new alternative substrate for the growth of HgCdTe via molecular beam epitaxy (MBE). Due to the smaller mismatch in both lattice constant and coefficient of thermal expansion between GaSb and HgCdTe, GaSb presents a better alternative substrate for the epitaxial growth of HgCdTe, in comparison to alternative substrates such as Si, Ge, and GaAs. In our recent efforts, a CdTe buffer layer technology has been developed on GaSb substrates via MBE. By optimizing the growth conditions (mainly growth temperature and VI/II flux ratio), CdTe buffer layers have been grown on GaSb substrates with material quality comparable to, and slightly better than, CdTe buffer layers grown on GaAs substrates, which is one of the state-of-the-art alternative substrates used in growing HgCdTe for the fabrication of mid-wave infrared detectors. The results presented in this paper indicate the great potential of GaSb to become the next generation alternative substrate for HgCdTe infrared detectors, demonstrating MBE-grown CdTe buffer layers with rocking curve (double crystal x-ray diffraction) full width at half maximum of ∼60 arcsec and etch pit density of ∼106 cm−2.

HgCdTe structures for dual-band photodetectors operating in the 3–5 and 8–12 µm spectral ranges

A bilayer cadmium-mercury-tellurium (CMT) heterostructure was designed consisting of photosensitive layers of compositions xCdTe = 0.29–0.32 and xCdTe = 0.220–0.230, sensitive in the spectral ranges of 3–5 and 8–12 µm, a barrier layer between them, and wide-band variable-gap layers on the heterojunction and the surface grown on a GaAs substrate with ZnTe and CdTe buffer layers. The molecular beam epitaxial (MBE) growth of the heteroepitaxial structure (HES) was controlled by real time ellipsometry. After the growth, the composition distribution throughout the thickness was measured by reflection spectra with layer-by-layer chemical etching. There is good agreement between the results of composition measurements using ellipsometry and reflection spectra. P-type conductivity of bilayer MBE CMT HESs was obtained after thermal annealing at 220–240 °C in an inert gas (helium) for 24 h. The concentration of holes in the photosensitive layers is (4–10) ·1015 cm−3 and (8–20) · 1015 cm−3 at 78 K.

Development of MBE II–VI Epilayers on GaAs(211)B

Large-area, low-cost substrates are envisioned for next-generation HgCdTe infrared focal-plane arrays (IRFPA). Si, GaAs, Ge, and InSb have been previously examined as potential candidates. Fabrication of IRFPAs based on these substrates is limited by fundamental materials properties that potentially lead to lower detector performance and operability. Lattice and thermal mismatch between the substrate and epilayer are just two of several material factors that must be considered. We have reviewed these factors in the context of more recent data, and determined it worthwhile to revisit the use of GaAs substrates for epitaxial growth of HgCdTe. Our study starts with an evaluation of the surface quality (epireadiness) of commercially available (211) B-oriented GaAs substrates. Molecular beam epitaxial growth of CdTe buffer layers and subsequent HgCdTe absorber layers are performed in separate vacuum-interconnected chambers. The importance of optimization of the CdTe buffer layer growth for high-quality HgCdTe is detailed through surface morphology and x-ray studies. x-Ray diffraction rocking-curve full-width at half-maximum values as low as 52 arcsec have been obtained. Long-wave infrared Hg1−xCdxTe (x = 0.23) has been grown on these buffer layers, producing cross-hatch-dominated surface morphologies, with dislocation densities as low as ∼3 × 106 cm−2. We have also obtained (for optimized layers), 80-K Hall-effect n-type carrier concentration and electron mobility of approximately ~1.5 × 1015 cm−3 and 1 × 105 cm2 V−1 s−1, respectively. Finally, we briefly compare GaAs and Si in light of our preliminary investigation.

Understanding the Evolution of CdTe Buffer Layer Surfaces on ZnTe/Si(211) and GaAs(211)B During Cyclic Annealing

We present the results of a detailed study of the changes that occur on CdTe buffer layer surfaces grown on ZnTe/Si(211) and GaAs(211)B during the routine thermal cyclic annealing (TCA) process. Observations indicate that CdTe buffer layer surfaces are Te saturated when the TCA is performed under Te overpressure. In the absence of Te flux during the TCA step, the CdTe surface loses CdTe congruently and the typical CdTe nanowires show the presence of nodules on their surfaces. The observed changes in reflection high-energy electron diffraction patterns during TCA are explained in terms of surface chemistry and topography observations. Overall, the Te overpressure is necessary to maintain a smoother and pristine surface to continue the molecular beam epitaxy (MBE) growth.

Recent Progress in MBE Growth of CdTe and HgCdTe on (211)B GaAs Substrates

Alternate substrates for molecular beam epitaxy growth of HgCdTe including Si, Ge, and GaAs have been under development for more than a decade. MBE growth of HgCdTe on GaAs substrates was pioneered by Teledyne Imaging Sensors (TIS) in the 1980s. However, recent improvements in the layer crystal quality including improvements in both the CdTe buffer layer and the HgCdTe layer growth have resulted in GaAs emerging as a strong candidate for replacement of bulk CdZnTe substrates for certain infrared imaging applications. In this paper the current state of the art in CdTe and HgCdTe MBE growth on (211)B GaAs and (211) Si at TIS is reviewed. Recent improvements in the CdTe buffer layer quality (double crystal rocking curve full-width at half-maximum ≈ 30 arcsec) with HgCdTe dislocation densities of ≤106 cm−2 are discussed and comparisons are made with historical HgCdTe on bulk CdZnTe and alternate substrate data at TIS. Material properties including the HgCdTe majority carrier mobility and dislocation density are presented as a function of the CdTe buffer layer quality.

Defects in HgCdTe grown by molecular beam epitaxy on GaAs substrates

AbstractThe Hall effect and photoluminescence measurements combined with annealing and/or ion milling were used to study the electrical and optical properties of HgCdTe films grown by molecular-beam epitaxy on GaAs substrates with ZnTe and CdTe buffer layers. Unintentional donor doping, likely from the substrate, which resulted in residual donor concentration of the order of 1015 cm−3, was observed in the films. Also, acceptor states, possibly related to structural defects, were observed.

Electron probe microanalysis of HgCdTe heteroepitaxial structures with CdTe buffer layers

Some results of electron probe microanalysis of HgCdTe heteroepitaxial structures with wide-gap semiconductor CdTe buffer layers are described. It is shown that these structures may be used for manufacturing high-performance photodetectors for registration of infrared radiation in the ranges of 3–5 and 8–12 μm.

Si Wafer Thinning Techniques Compatible With Epitaxy of CdTe Buffer Layers

Reduction of threading dislocation density is critical for improving the performance of HgCdTe detectors on lattice-mismatched alternative substrates such as Si. CdTe buffer layers grown by molecular beam epitaxy (MBE), with thicknesses on the order of 8 l mt o 12lm, have helped reduce dislocation densities in HgCdTe layers. In this study, the reduction of threading dislocation densities in CdTe buffer layers grown on locally thinned Si substrates was examined. A novel Si back-thinning technique was developed that maintained an epiready front surface and achieved Si thicknesses as low as 1.9 lm. Threading dislocation densities, acquired by defect decoration techniques, were reduced by as much as 60% for CdTe buffer layers grown on these thinned regions when compared with unthinned regions. However, this reduction is inconsistent with prior notions that threading dislocation propagation is dominated by image forces. Instead, the thickness gradient of thinned Si may play a larger role.