HgCdTe Films Research Articles

Noncontact Characterization of Carrier Mobility in Long-Wave Infrared HgCdTe Films With Terahertz Time-Domain Spectroscopy

Noncontact Characterization of Carrier Mobility in Long-Wave Infrared HgCdTe Films With Terahertz Time-Domain Spectroscopy

Optical properties and disorder of HgCdTe films grown by molecular beam epitaxy

Optical properties and disorder of HgCdTe films grown by molecular beam epitaxy

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.

Growth of CdZnTe (2 1 1) epilayers on GaAs by close spaced sublimation as an alternative substrate for HgCdTe growth

Cd0.96Zn0.04Te crystals are perfect substrates for epitaxial growth of HgCdTe films due to good lattice matching, but they are limited by cost factors. To grow a replacement substrate for the subsequent growth of low-cost, high-yield HgCdTe IR materials, CdZnTe epitaxial films have been fabricated by close-spaced sublimation on GaAs substrates, and the effect of substrate temperature on the crystalline quality of CdZnTe films has been investigated. The results of EBSD and TEM indicate that the surface is oriented in (211) and the low temperatures enhance the crystalline quality of the film. The results of dislocation density (average etch pit density ∼3.6 × 105 cm−2) and energy dispersive spectrometer (Zn content ∼5 %) indicate that CdZnTe films with appropriate Zn content and excellent crystal quality can be obtained by the close spaced sublimation method, which is expected to be applied for molecular beam epitaxy growth of HgCdTe films.

Effect of sub-grain boundaries in CdZnTe on HgCdTe film grown by molecular beam epitaxy

HgCdTe films grown by molecular beam epitaxy (MBE) are essential for creating high-performance infrared focal plane arrays (IRFPAs) like dual band detectors, high operating temperature (HOT) detectors, and avalanche photodiode (APD) detectors. CdZnTe is recognized as the optimal substrates for growing high crystal quality HgCdTe due to its lattice matching, which is adjusted by selecting the Zn mole fraction. However, defects like grain boundaries, twins, inclusions, dislocations, and sub-grain boundaries that often occur in CdZnTe substrates can significantly reduce the crystal quality of HgCdTe. HgCdTe grown on CdZnTe with these defects usually no longer suit for high quality device fabrication. Although the impact from various defects have been extensively studied in HgCdTe grown on CdZnTe, but the sub-grain boundaries is still not yet involved. In our study, to better understand the effect of sub-grain boundaries in CdZnTe on HgCdTe films, CdZnTe substrates with and without sub-grain boundary were selected to growth MWIR HgCdTe films under optimized condition. Results indicated that etch pit density (EPD) of CdZnTe substrates with sub-grain boundary is approximately 3–5 times higher than boundary free areas. But HgCdTe grown on these substrates bears more than an order of magnitude increase in dislocation proliferation compared to the boundary free region. Dislocation generation resulting from dislocation penetration from substrates, mismatch between HgCdTe and CdZnTe, and sub-grain boundaries were investigated and compared. The intense dislocation proliferation in HgCdTe may be mainly due to twins formed during the initial growth stage of HgCdTe films, rather than dislocation penetration from substrates, as revealed by reflection high-energy electron diffraction (RHEED) pattern monitoring. Therefore, screening for sub-grain boundaries in CdZnTe before HgCdTe growth is necessary.

Review of defects of HgCdTe films grown by LPE

Review of defects of HgCdTe films grown by LPE

Hydrostatic deformation potentials of narrow-gap HgCdTe

Close to the semimetal-to-semiconductor topological phase transition, the band structure of HgCdTe is represented by massive Kane fermions that, in a semirelativistic approach, are characterized by two parameters: rest mass and velocity. Using terahertz magneto-optical spectroscopy, we explore the band structure evolution of HgCdTe films with hydrostatic pressure in the vicinity of the band-gap collapse. By analyzing the energies of interband optical transitions as a function of magnetic field, we have determined the rest mass of Kane fermions at different hydrostatic pressures. The pressure dependence of the rest mass allows us to obtain the hydrostatic deformation potential ${a}_{c}\ensuremath{-}{a}_{v}$ at low temperature, where ${a}_{c}$ and ${a}_{v}$ are the deformation potentials of the conduction and valence bands, respectively.

Photoelectric Characteristics of Multielement Photodiode-Based IR FPAs by the Monte Carlo Method on Epitaxial HgCdTe Films

The calculations of photocharacteristics, such as spatial resolution, threshold sensitivity with photosensitive elements sized from 5 µm × 5 µm to 20 µm × 20 µm under uniform illumination, for multielement IR FPAs and also for point objects are presented. The obtained results are based on the simulation of a three-dimensional diffusion process of photo-excited minority charge carriers in the photosensitive layer of photodiode elements by the Monte Carlo method. Numeric estimates are presented for IR FPAs based on photodiodes for epitaxial HgCdTe films, but the model is applicable for the calculations of photoelectric characteristics for photodiodes on other substrates. The requirements for the main photoelectric and design parameters of photosensitive elements and optical system (spot diameter from a point source), which provide reaching IR FPA maximal sensitivity and spatial resolution values, are formulated.

Microstructure and Optical Characterization of Mid-Wave HgCdTe Grown by MBE under Different Conditions

The mid-wave single-crystal HgCdTe (211) films were successfully grown on GaAs (211) B substrates by molecular beam epitaxy (MBE). Microstructure and optical properties of the MBE growth HgCdTe films grown at different temperatures were characterized by X-ray diffraction, scanning transmission electron microscopy, Raman and photoluminescence. The effects of growth temperature on the crystal quality of HgCdTe/CdTe have been studied in detail. The HgCdTe film grown at the lower temperature of 151 °C has high crystal quality, the interface is flat and there are no micro twins. While the crystal quality of the HgCdTe grown at higher temperature of 155 °C is poor, and there are defects and micro twins at the HgCdTe/CdTe interface. The research results demonstrate that the growth temperature significantly affects the crystal quality and optical properties of HgCdTe films.

Nano-scale structural studies of defects in arsenic-implanted n and p-type HgCdTe films

Bright–field and high-resolution transmission electron microscopy were used for nano-scale structural studies of defects induced by implantation of arsenic ions with 190 keV energy and 1014 cm–2 fluence in n and p-type Hg0.78Cd0.22Te films grown by molecular-beam epitaxy. A similarity in defect pattern formed by arsenic implantation in n and p-type material was observed. The electrical properties of the implanted layers in n and p-type films also appeared to be similar, confirming the results of microscopic observations.

Influence of As+ Ion Implantation on Properties of MBE HgCdTe Near-Surface Layer Characterized by Metal–Insulator–Semiconductor Techniques

The effect of As+ ion implantation on the electrical properties of the near-surface layer of n-HgCdTe films grown by molecular beam epitaxy (MBE) on Si (310) substrates was experimentally studied. A specific feature of MBE n-Hg0.78Cd0.22Te films is the presence of near-surface graded-gap layers with a high CdTe content, formed during epitaxial growth. The properties of as-grown films and films after As+ ion implantation with ion energy of 200 keV and fluence of 1014 cm−2 were studied. Post-implantation activation annealing was not performed. Test metal–insulator–semiconductor (MIS) structures were created based on as-grown and as-implanted samples by plasma-enhanced atomic layer deposition of Al2O3 insulator films. The admittance of the fabricated MIS structures was measured over a wide range of frequencies and temperatures. When determining the parameters of MIS structures, we used techniques that take into account the presence of near-surface graded-gap layers and series resistance of the HgCdTe film bulk, as well as the high density of slow surface states. It was found that, in as-implanted samples, the donor center concentration in the near-surface layer exceeds 1017 cm−3 and increases with distance from the HgCdTe-Al2O3 interface (at least up to 90 nm). After implantation, the conductivity of MBE HgCdTe film bulk increases markedly. It was shown that, for as-implanted samples, the generation rate of minority charge carriers in the MBE HgCdTe surface layer is significantly reduced, which indicates the appearance of a low defect layer with a thickness of at least 90 nm.

Влияние имплантации ионов As-=SUP=-+-=/SUP=- и последующего отжига на электрические свойства приповерхностных слоев варизонных пленок n-Hg-=SUB=-0.78-=/SUB=-Cd-=SUB=-0.22-=/SUB=-Te

Films of n-Hg0.775Cd0.225Te with near-surface wide-gap layers were grown by molecular beam epitaxy on Si (013) substrates. To measure the admittance, metal – insulator – semiconductor (MIS) structures were fabricated on the basis of the as-grown HgCdTe film, films after implantation, and films after implantation and annealing. Using techniques that take into account the presence of graded-gap layers and slow interface states, the main parameters of the near-surface layers of HgCdTe films have been determined after the technological procedures used to create photodiodes.

Optical and Structural Properties of HgCdTe Solid Solutions with a High CdTe Content

The results of studying the optical transmission, photoconductivity, photoluminescence, and X-ray diffraction of HgCdTe solid-solution samples with a high content of CdTe (molar fraction 0.7–0.8), grown by molecular-beam epitaxy (MBE) and liquid-phase epitaxy (LPE), are presented. It is shown that the material under study shows a significant degree of disorder of the solid solution, which is greater in structures grown by MBE on GaAs substrates, compared with the LPE-grown films. A study of the photoluminescence reveals states in the band gap, previously considered untypical of HgCdTe films grown on GaAs substrates, but typical only of films grown on Si. On the whole, the high quality of the material with a high CdTe content, grown by MBE and used to create the HgTe/HgCdTe nanostructures currently in high demand, is confirmed.

Admittance studies of modification of HgCdTe surface properties with ion implantation and thermal annealing

Metal–insulator–semiconductor (MIS) structures based on HgCdTe were fabricated after various stages of pn junction formation using As+ implantation and activation annealing. The energy of As+ ions was 200 keV with the fluence of 1014 cm−2. Heteroepitaxial HgCdTe films with near-surface graded-gap layers were grown by molecular beam epitaxy (MBE) on silicon substrates. It was shown that the electron concentration in the near-surface semiconductor layer increases after implantation to values of about 1017 cm−3, and after implantation and annealing in the near-surface semiconductor layer, a p+ layer appears with a hole concentration of more than 1.5 × 1018 cm−3. The generation rate of minority charge carriers in the space charge region after implantation is low, which indicates a low defectiveness of the thin near-surface MBE HgCdTe layer. After implantation and annealing, the generation rate increases significantly, which may be due to high defectiveness near the interface between Al2O3 and MBE HgCdTe. Dopant profiles were constructed in the near-surface HgCdTe layers after various technological procedures. It was shown that after implantation in films, the dopant concentration increases with distance from the interface to the depth of 0.1 μm.

Hall-effect studies of modification of HgCdTe surface properties with ion implantation and thermal annealing

Results of the Hall-effect studies of surface properties of n–type HgCdTe films modified with arsenic ion implantation and thermal annealing are reported on. A complete annihilation of implantation-induced extended defects (dislocation loops), quasi-point defects and related donor centers was observed as a result of a two-stage arsenic activation annealing. A high degree of activation of implanted arsenic was achieved with the annealing. In some cases, the annealing was found to lead to the modification of the properties of the ‘base’ layers not affected by implantation due to activation of uncontrolled acceptor defects and resulting changes in the degree of electrical compensation.

The relaxation of electrophysical properties HgCdTe epitaxial films affected by plasma of high frequency nanosecond volume discharge in atmospheric-pressure air

In this work the results of the experimental investigation of the influence of the high-frequency nanosecond volume discharge in atmospheric pressure air on the electrophysical properties of epitaxial HgCdTe films are presented. Analysis of magnetic-field dependences of the Hall coefficient have shown that in the near-surface region of the material a high-conductivity n-type layer is formed as a result of irradiation. It is shown that relaxation of the values of the electrophysical parameters of the epitaxial films is observed after the irradiation. The supposition is made that the obtained experimental results may be explained by the formation of thin dielectric oxide film at the surface of the irradiated material, containing built-in fixed and mobile positive charge.

Оптические и структурные свойства твердых растворов HgCdTe с большим содержанием CdTe

This work presents the results of studying the optical transmission, photoconductivity, photoluminescence, and X-ray diffraction of HgCdTe solid solution samples with a high (molar fraction 0.7–0.8) CdTe content, grown by molecular beam (MBE) and liquid-phase (LPE) epitaxy. It was shown that the studied material had a significant degree of disordering of the solid solution, which was greater in structures grown by MBE on GaAs substrates than in films grown by LPE. Photoluminescence studies have revealed states in the band gap, which were previously considered not typical of HgCdTe films grown on GaAs substrates, but only of films grown on Si. On the whole, the high quality of the material with a high CdTe content, grown by MBE and used to create the currently widely demanded HgTe / HgCdTe nanostructures, was confirmed.

Improved Passivation Effect Due to Controlled Smoothing of the CdTe-HgCdTe Interface Gradient by Thermal Annealing

HgCdTe films grown by liquid phase epitaxy were passivated with CdTe grown by molecular beam epitaxy. A series of annealing tests with different temperatures and durations were then carried out in order to reach a two-fold goal: to smooth the gradients in Hg and Cd content across the HgCdTe film-passivation interface, and to create a Hg vacancy concentration of ∼ 1–2 × 1016 cm−3. We want the composition gradient to prevent minority carrier electrons from reaching the actual interface, as this may contain recombination centers that trap the electrons. Four different temperature–time combinations were tried for the first part of the annealing procedure (the smoothing of gradients), while they were all followed by a Hg vacancy procedure of 250°C for 18–24 h. Then some samples were cleaved, and energy-dispersive x-ray spectroscopy (EDS) line profiles were recorded on the cross sections to monitor the composition gradients. We could clearly see differences between the profiles from samples with the higher-temperature annealing, the lower-temperature annealing and no annealing. Secondary ion mass spectroscopy profiles on two samples verified the EDS results. On another subset of similar samples, planar diodes were processed and resistance-area (RA) values determined. The current–voltage (I–V) curves from the high-temperature annealed samples seemed dominated by leakage current and did generally not display diode characteristics. This could be explained by their EDS profiles, which showed a lot of Hg in the entire passivation layer. The EDS profiles from the three low-temperature annealing procedures were very similar, but planar diode I–V curves revealed diodes of varying quality, and their RA values were used to differentiate among them. The optimal annealing procedure was 300°C for 4 h, followed by 250°C for 24 h.

Arsenic-ion implantation-induced defects in HgCdTe films studied with Hall-effect measurements and mobility spectrum analysis

The Hall-effect/electrical conductivity measurements and mobility spectrum analysis (MSA) have been used for the study of the profiles of different electron species and corresponding defects induced in a HgCdTe film by implantation of arsenic ions. Two specific donor defects were revealed as a result of extracting parameters of different carrier species found with MSA. Identification of the defects was performed with the use of the data acquired with transmission electron microscopy and the literature data on Rutherford backscattering. It was found that low-mobility (∼5000 cm2/(V·s)) electrons originated from donor centers formed when mercury interstitials were captured by implantation-induced dislocation loops, while middle-mobility (∼20,000 cm2/(V·s)) electrons were due to donor centers formed when the interstitials were captured by quasi-point radiation defects. For comparison, profiling was also performed with the single-field differential Hall-effect measurements. The results of the study suggest that only MSA-assisted carrier profiling is capable of retrieving actual defect profiles in ion-implanted HgCdTe.

Electrical profiling of arsenic-implanted HgCdTe films performed with discrete mobility spectrum analysis

The results from the electrical profiling of an n-on-p junction formed by 190-keV arsenic ion implantation in an indium/vacancy–doped Hg0.78Cd0.22Te film are presented. Mobility spectrum analysis in combination with wet chemical etching has been employed for the profiling. After the implantation, a typical n+-n-р structure was observed and three electron species were detected: (a) low-mobility electrons in the 400–500 nm-thick top radiation-damaged n+-layer, (b) mid-mobility electrons also originating from radiation damage and spreading down to 700–900 nm, and (c) high-mobility electrons located in the n-region extending beyond 700–900 nm and down to the p-n junction. A comparison of the extracted electron parameters with the arsenic profile obtained with secondary-ion mass spectroscopy as well as with the defect pattern obtained with transmission electron microscopy allowed for the identification of the origin of all three electron species.