Hg Vacancies Research Articles

New insight into defect energy levels in HgCdTe

The paper reports on a summary of the experimental data on the defect energy levels in HgCdTe, that have been collected in our experiments and from articles available over several decades. We have determined the location of the defect levels in HgCdTe for material composition from 0.17 to 0.6. It has been shown that that all specified trap levels are tied to the valence band and do not change (or change slightly) with temperature, and have the slopes with respect to the Cd molar content somewhat smaller than the HgCdTe energy gap.Two levels are formed by mercury vacancies – the first defect level is a shallow acceptor formed by neutral vacancy, the second deep level can be formed by singly negatively charged Hg vacancy. Contrary to earlier assumptions, it does not lie in the middle of the energy gap for the entire range of molar composition, but it is an acceptor-like for the wide-gap HgCdTe and a donor-like for the narrow gap, and may even be located in the conduction band forming a resonant state. Above this main defect level, two additional trap states can be found, most likely formed by oxygen atoms replacing Hg vacancies – positively charged and the neutral oxygen states.

Mid-infrared modulated photoluminescence mapping to investigate in-plane distributions of bandedge transitions in As-doped HgCdTe

In-plane distribution of band edge electronic structure of the narrow-gap arsenic (As)-doped HgCdTe is a crucial topic fundamentally and technically for mid-infrared detector array but hard to be investigated for long experimentally. This work reports a demonstration of the modulated photoluminescence (PL) mapping carried on As-doped HgCdTe, with the spatial resolution of a typical array pixel scale and with the elaborate band edge transitions being distinguished. Curve fittings related to the bandgap and to the impurities of Hg vacancy (VHg), As donor (AsHg), and AsHg–VHg pair are confirmed for PL mapping assisted by a temperature-dependent PL analysis. The spatial non-uniformity of Cd composition and the distributions of impurity relative concentrations are derived. Correlation analysis suggests that the As atom directly replacing the Hg atom in the complete lattice and the As atom occupying the already-existing VHg are the two approaches for the AsHg formation during As doping. The non-uniform band edge properties may lead to the in-plane conductivity fluctuation detrimental to the array performance. The results indicate the significance of probing the band edge impurity uniformity of As-doped HgCdTe for arrays fabrication, and the modulated PL mapping serving as a desirable and effective tool.

Ferromagnetism in two-dimensional metal dibromides induced by hole-doping

Using spin-polarized first-principles calculations based on density functional theory, we study the stability, electronic properties and magnetic behavior induced by hole-doping of two-dimensional (2D) PbBr2 and HgBr2. Although inherently nonmagnetic, these materials can exhibit stable ferromagnetic order when hole-doped at densities above a few 1013 cm-2. We also examined the impact of intrinsic and extrinsic defects on inducing hole-doping and subsequent ferromagnetism. Our findings suggest that p-type doping can be achieved by Pb and Hg vacancies and Br antisites, but the latter behaves as deep acceptors. Among the possible dopants we considered, Li substituting Pb or Hg, and S replacing Br in 2D HgBr2, can produce shallow acceptor states near the valence band edges and potentially result in a stable ferromagnetic order in these 2D dibromides.

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.

Research progress on p-type doping mechanism and low frequency noise of long wave HgCdTe devices

<p indent=0mm>In order to meet the needs of the development of long wave HgCdTe infrared focal plane devices, this paper summarizes the related research progress from two aspects: the p-type doping mechanism of materials and the low-frequency noise of devices. It includes: (1) the classical p-type activation model of as doping is modified theoretically, and a new activation model is proposed, that is, as migration dominated by composite defect Te_Hg-V_Hg leads to the formation of AsTe₂ complex, which shows p-type. (2) It is theoretically explained that the fast diffusion characteristics of Au doped HgCdTe are caused by the interstitial diffusion mechanism; a method is proposed to control the spatial distribution of Au impurities through the interaction between Au impurities and Hg vacancies, which can realize controllable p-type doping and is verified by experiments. (3) A high sensitivity and low frequency noise test system is built. Combined with the numerical calculation model, the evaluation method of low-frequency noise is established. At the same time, the origin of low-frequency noise of long wave HgCdTe device is analyzed.

HgCdTe high operation temperature infrared detectors

Based on the current development direction of infrared detector technology. The advantages of HgCdTe high operation temperature (HOT) infrared detector were analyzed in terms of module weight, shape size, power consumption, environmental adaptability and reliability from the perspective of application requirements of HOT infrared detector. The technical route and research status of HgCdTe HOT infrared detector in Europe and America were summarized. From the perspective of device dark current and noise mechanism, the change of dark current and noise at different operating temperatures and their effects on device performance were analyzed. The basic principles of the process optimization HgCdTe devices based on Hg vacancy n-on-p structure, extrinsic doped HgCdTe HOT devices based on in-doped p-on-n structure and Au doped n-on-p structure, nBn based HOT devices based on barrier structure and non-equilibrium mode HgCdTe HOT devices based on thermally excited carrier auger suppression in absorption layer were summarized. The performance of HgCdTe HOT devices with different technology routes and the technical difficulties in the preparation of detectors were compared and analyzed. Based on the comprehensive analysis of the performance and technical difficulties of HOT devices with different technology routes, the future development direction of HgCdTe HOT device technology was prospected. It is considered that the fully depleted device based on low concentration doping absorption layer has better development potential.

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.

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.

Defect Antiperovskite Compounds Hg3Q2I2 (Q = S, Se, and Te) for Room-Temperature Hard Radiation Detection.

The high Z chalcohalides Hg3Q2I2 (Q = S, Se, and Te) can be regarded as of antiperovskite structure with ordered vacancies and are demonstrated to be very promising candidates for X- and γ-ray semiconductor detectors. Depending on Q, the ordering of the Hg vacancies in these defect antiperovskites varies and yields a rich family of distinct crystal structures ranging from zero-dimensional to three-dimensional, with a dramatic effect on the properties of each compound. All three Hg3Q2I2 compounds show very suitable optical, electrical, and good mechanical properties required for radiation detection at room temperature. These compounds possess a high density (>7 g/cm3) and wide bandgaps (>1.9 eV), showing great stopping power for hard radiation and high intrinsic electrical resistivity, over 1011 Ω cm. Large single crystals are grown using the vapor transport method, and each material shows excellent photo sensitivity under energetic photons. Detectors made from thin Hg3Q2I2 crystals show reasonable response under a series of radiation sources, including 241Am and 57Co radiation. The dimensionality of Hg-Q motifs (in terms of ordering patterns of Hg vacancies) has a strong influence on the conduction band structure, which gives the quasi one-dimensional Hg3Se2I2 a more prominently dispersive conduction band structure and leads to a low electron effective mass (0.20 m0). For Hg3Se2I2 detectors, spectroscopic resolution is achieved for both 241Am α particles (5.49 MeV) and 241Am γ-rays (59.5 keV), with full widths at half-maximum (FWHM, in percentage) of 19% and 50%, respectively. The carrier mobility-lifetime μτ product for Hg3Q2I2 detectors is achieved as 10-5-10-6 cm2/V. The electron mobility for Hg3Se2I2 is estimated as 104 ± 12 cm2/(V·s). On the basis of these results, Hg3Se2I2 is the most promising for room-temperature radiation detection.

Numerical Modeling of SRH and Tunneling Mechanisms in High-Operating-Temperature MWIR HgCdTe Photodetectors

A combined experimental and numerical simulation study is presented on two sets of nominally identical \(\hbox {Hg}_{1-x}\hbox {Cd}_{x}\hbox {Te}\) single-color back-illuminated midwave-infrared n-on-p photodetectors grown by liquid-phase epitaxy, p-doped with Hg vacancies and with Au, respectively. The present numerical model includes a novel formulation for band-to-band tunneling, which overcomes the intrinsic limitations of the classical Kane description without introducing numerical issues typical of other approaches. Our study confirms that adopting n-on-p architectures, avoiding metal vacancy doping, and reducing the acceptor density in the absorber region are prerequisites for obtaining high-operating-temperature photodetectors. A significant contribution to the dark current in both sets of devices is attributed to impact ionization, crucial to obtain a satisfactory explanation for the measured characteristics also at low to intermediate bias.

Characteristics of Au Migration and Concentration Distributions in Au-Doped HgCdTe LPE Materials

Annealing techniques and secondary ion mass spectrometry have been used to study the characteristics of Au migration and concentration distributions in HgCdTe materials grown by liquid phase epitaxy. Secondary ion mass spectrometry measurements showed that Au concentrations had obvious positive correlations with Hg-vacancy concentration and dislocation density of the materials. Au atoms migrate toward regions of high Hg-vacancy concentration or move away from these regions when the Hg-vacancy concentration decreases during annealing. The phenomenon can be explained by defect chemical equilibrium theory if Au atoms have a very large migration velocity compared with Hg vacancies. Au atoms will also migrate toward regions of high dislocation density, leading to a peak concentration in the inter-diffusion region of HgCdTe materials near the substrate. By use of an Hg and Te-rich annealing technique, different concentration distributions of both Au atoms and Hg vacancies in HgCdTe materials were obtained, indicating that Au-doped HgCdTe materials can be designed and prepared to satisfy the requirements of HgCdTe devices.

Structural and Energetic Analysis of Group V Impurities in p-Type HgCdTe: The Case of As and Sb

Although molecular beam epitaxy technology-based arsenic-doped Hg1−x Cd x Te has been extensively studied, according to the newly proposed framework of the defect-complex-based p-type doping mechanism, heavier group V elements such as antimony (Sb) should have a different doping behavior because of their larger radius which can cause larger lattice distortion. In this work, we performed first-principles calculations and took As and Sb as examples to study this issue. The substitutional doping, interstitial doping (including split, tetrahedral, and hexagonal interstitial sites), and defect complex doping forms for arsenic and antimony are all investigated. A significant lattice distortion is found in hexagonal and split-site interstitial-Sb-doped Hg0.75Cd0.25Te due to the larger covalent radius of Sb. Compared with As, Sb can lead to a more complicated configuration change in the case of SbHg-V Hg-SbHg tridoping, and the interstitial Sb is found to be stable even with the coupling of Hg vacancies through detailed energetic calculations, indicating that the interstitial Sb has greater ability to form stable defect complexes, and thus great potential to be a more appropriate p-type dopant. This study provides more complementary understanding of the behaviors of group V impurities in HgCdTe.

Composition dependence of the mercury vacancies energy levels in HgCdTe: Evolution of the “negative-U” property

HgCdTe films grown by liquid phase epitaxy with different Cd compositions were post-annealed to control the Hg vacancy concentration. Then temperature-dependent Hall measurements and photoluminescence measurements allowed us to study the evolution of the Hg vacancy acceptor levels with the cadmium composition. For Cd compositions below 33% the Hg vacancies in HgCdTe present a negative-U property with the ionized state V− stabilized compared to the neutral state V0. For Cd compositions higher than 45%, the Hg vacancies in HgCdTe present a more standard level ordering with the ionized state V− at higher energy than the neutral state V0.

Experimental evidence for the third level (А+) of Hg vacancy in Hg1−xCdxTe

Mercury vacancy in Hg1-xCdxTe is not a two-level (as it was supposed until now), but a three-level acceptor. A third, most shallow (1–1.5meV) level (А+ state) appears due to a capture of a third hole by a neutral acceptor, after the two deeper vacancy levels (A− and А0 states) are already occupied by holes. Due to a capture of nonequilibrium holes by neutral mercury vacancies (under radiation) a positive space charge region arises near an irradiated surface. This causes the anomalies of photoelectromagnetic effect, observed in р-Hg1−xCdxTe at T<10–12K.

The important features of VHg-related defects in arsenic-doped HgCdTe

The as-grown arsenic-doped HgCdTe typically exhibits compensated n-type conductivity. Based on first-principles calculations, we identify Hg vacancies (VHg) as a dominant defect species involved in the compensation. Arsenic donor (AsHg) and VHg acceptor no doubt tend to combine into complex defects but little has been recognized about the features of the complex defects in precious studies. These features outlined in this work enable us to provide a consistent explanation for arsenic activation mechanism proposed by Zandian et al. [7] and Berding et al. [8].

Doping effect of boron in Hg0.75Cd0.25Te: first-principles study

Using the first-principles method based on the density functional theory, we study the doping effect of B impurity in HgCdTe (MCT).We find that the most stable configuration of the impurity is at the B hexagonal interstitial position, rather than at the in-situ substitution. The electronic structures and the density of states of B hexagonal interstitial doped MCT are systematically investigated. Near neighbour (NN) and next-near-neighbor (NNN) atoms around the B impurity are obviously relaxed. The relaxation induces the breaking of NN Te-Hg covalent bond. Moreover, B hexagonal interstitial behaves as triple n-type dopant. The charged state analysis indicates that Bih(2Hg1Cd) with three positive charges is most stable and forms an effecient donor. However, as long as the Hg vacancy exists, complex impurity between Hg vacancy and B impurity can be easily formed, its binding energy reaches up to 0.96 eV. Such complex behaves as single n-type dopant. Considering radiation damage of B ion implantation, the complex is a main factor restricting the activation of B ion in MCT.

Hydrogen passivation and multiple hydrogen–Hg vacancy complex impurities (nH–VHg, n = 1,2,3,4) in Hg0.75Cd0.25Te

Using first-principles method within the framework of the density functional theory, we study the formation energies and the binding energies of multiple hydrogen–mercury vacancy complex impurities (nH–VHg, n = 1,2,3,4) in Hg0.75Cd0.25Te. We find that, when mercury vacancies exist in Hg0.75Cd0.25Te, the formation of the complex impurity between H and VHg (1H–VHg) is easy and its binding energy is up to 0.56 eV. In this case, the deep acceptor level of mercury vacancy is passivated. As the hydrogen concentration increases, we find that the complex impurity between VHg and two hydrogen atoms (2H–VHg) is more stable than 1H–VHg. This complex passivates both the two acceptor levels introduced by mercury vacancy and neutralizes the p-type dopant characteristics of VHg in Hg0.75Cd0.25Te. Moreover, we find that the complex impurities formed by one VHg and three or four H atoms (3H–VHg, 4H–VHg) are still stable in Hg0.75Cd0.25Te, changing the VHg doped p-type Hg0.75Cd0.25Te to n-type material.

Planar n-on-p HgCdTe FPAs for LWIR and VLWIR Applications

Mainly driven by space applications, mercury cadmium telluride (MCT) focal-plane arrays (FPAs) have been successfully developed for very long wavelengths (λCO > 14 μm at 55 K). For this purpose, the standard n-on-p technology based on MCT grown by liquid-phase epitaxy (LPE) and involving vacancy doping has been modified to extrinsic doping by a monovalent acceptor. Due to the planar diode geometry obtained by ion implantation, most of the carrier generation volume is located in the p-type region with a thickness of approximately 8 μm. According to our understanding, the Shockley–Read centers connected with the Hg vacancies are thus significantly reduced. This situation should lead to longer minority-carrier lifetimes and smaller generation rates under equilibrium conditions, therefore yielding lower dark current. We indeed observe a reduction by a factor of approximately 15 by using extrinsic doping. Recent dark current data obtained in the temperature range from 55 K to 85 K on 288 × 384 FPAs with λCO(60 K) = 12 μm, either intrinsically or extrinsically doped, corroborate this finding. These data, new results on a 112 × 112 pixel demonstrator array with λCO(55 K) = 14.4 μm, and earlier measurements are compared with Tennant’s Rule 07 established for p-on-n technology.

Photoluminescence Studies of HgCdTe Epilayers

We studied the photoluminescence of Hg1−xCdxTe (x ≈ 0.3) layers grown by molecular-beam epitaxy on CdZnTe. The investigations were carried out on unintentionally doped, In-doped, and As-doped samples, and the effect of different annealing procedures was examined. Excitation-power-dependent and temperature-dependent comparative studies were performed. The transition mechanisms are discussed, and optical signatures for arsenic site transfer are found. The contributions due to As2Te3, As in an amphoteric site, Hg vacancies, and AsHg complexes were identified, and their ionization energies measured. The results are consistent with results found by other techniques. Thus, it is shown that photoluminescence applied to Hg1−xCdxTe can resolve characteristics corresponding to native and intentional donors/acceptors, and offers a nondestructive, fast tool for material characterization.

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 Hg1� xCdxTe 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 Hg1� xCdxTe 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.